Note: Descriptions are shown in the official language in which they were submitted.
1339026
DEGRADABLE THERMOPLASTIC FROM LACTIDES PF 2795 PCT
The present application is derived from and claims priority of the
following eight U.S. applications: The application entitled PLIABLE
BIODEGRADABLE PACKAGING THERMOPLASTICS FROM LACTIDES having Serial No.
07/229,896, filed August 8, 1988; and the application entitled
BIODEGRADABLE REPLACEMENT OF CRYSTAL POLYSTYRENE having Serial No.
07/229,939, filed August 8, 1988; and the application entitled BLENDS
OF POLYLACTIC ACID having Serial No. 07/229,894, filed August 8, 1988;
and the application entitled DEGRADABLE IMPACT MODIFIED POLYLACTIDES
having Serial No. 07/317,391, filed March 1, 1989; and the application
PLIABLE BIODEGRADABLE PACKAGING THERMOPLASTICS FROM LACTIDES having
attorney docket number PF 2767-1, filed July 31, 1989; and the
application entitled BIODEGRADABLE REPLACEMENT OF CRYSTAL POLYSTYRENE
having attorney docket number PF 2771-1, filed July 31, 1989; and the
application entitled BLENDS OF POLYLACTIC ACID having attorney docket
number PF 2772-1, filed July 31, 1989; and the application entitled
DEGRADABLE IMPACT MODIFIED POLYLACTIDES having attorney docket number
PF 2781-1, filed on July 31, 1989. All of the above applications
having Battelle Memorial Institute as assignee.
FIELD OF THE INVENTION
In a first embodiment, the present invention relates to
plasticized polymers of L-lactide, D-lactide, D,L-lactide and mixtures
thereof suitable for packaging applications conventionally served by
nondegradable plastics (e.g. poiyethylene). The first embodiment
further relates to a method for producing pliable films and other
packaging items and to the unique product thereof. The invention has
utility in producing a product that has the characteristics of the
usual plastics yet is biodegradable.
In a second embodiment, the invention discloses a material and
process of preparing it which is an offset, that is a replacement for
crystal polystyrene, sometimes known as orientable polystyrene or OPS.
The material is an offset for the OPS but is composed of a polyester
1339026
capable of biodegrading in the environment over approximately 1 years
time. The material is a polyester, comprised of polymerized lactic
acid, prepared from either D-lactic acid or L-lactic acid, and D,L-
lactic acid. The ratio of the two polymerized monomer units, the
process treatment and in some cases certain adjuvants, determine the
precise physical properties required for the exacting requirements of
an OPS offset. Thus, at approximately a ratio of 90/10, L-lactictDlL-
lactic acid, the polymerized lactic acid (PLA) is a well behaved
thermoplastic that is clear, colorless, and very stiff. As such it is
very suitable for preparing films, foams, and other thermoformed items
of disposable or one-way plastic. Having served its purpose as a
packaging plastic, the PLA slowly environmentally biodegrades to
innocuous products when left on or in the environment. This harmless
disappearance can help alleviate the mounting problems of plastic
pollution in the environment.
In a third embodiment, the invention relates to the blending of
conventional thermoplastics with polylactic acid. This provides novel,
environmentally degradable thermoplastics. The environmentally
degradable thermoplastics are useful in a wide variety of applications.
The third embodiment further relates to a method for producing
pliable films and other packaging items and to the unique product
thereof. The invention has utility in producing a product that has the
characteristics of the usual plastics yet is environmentally
degradable.
A fourth embodiment of the invention relates to the blending of
compatible elastomers with polylactides. This provides impact-
resistant modified polylactides that are useful in a wide variety of
applications including those where impact-modified polystyrene would be
used.
The fourth embodiment further relates to a method for producing
packaging items and to the unique product thereof. The invention has
utility in making a product that has the characteristics of the usual
impact-resistant plastics yet is environmentally degradable.
1~39026
BACKGROUND OF THE INVENTION
There is a need for an environmentally biodegradable packaging
thermoplastic as an answer to the tremendous amounts of discarded
plastic packaging materials. U.S. plastic sales in 1987 were 53.7
billion pounds of which 12.7 billion pounds were listed as plastics in
packaging. A significant amount of this plastic is discarded and
becomes a plastic pollution that is a blot on the landscape and a
threat to marine life. Mortality estimates range as high as 1-2
million seabirds and 100,000 marine mammals per year.
A further problem with the disposal of plastic packaging is the
concern for dwindling landfill space. It has been estimated that most
major cities will have used up available landfills for solid waste
disposal by the early 1990's. Plastics comprise approximately 3
percent by weight and 6 percent of the volume of solid waste.
One other disadvantage of conventional plastics is that they are
ultimately derived from petroleum, which leaves plastics dependent on
the uncertainties of foreign crude oil imports. A better feedstock
would be one that derives from renewable, domestic resources.
However, there are good reasons for the use of packaging plastics.
They provide appealing aesthetic qualities in the form of attractive
packages which can be quickly fabricated and filled with specified
units of products. The packages maintain cleanliness, storage
stability, and desirable qualities such as transparency for inspection
of contents. These packages are known for their low cost of production
and chemical stability. This stability, however leads to very long
life of plastic, so that when its one time use is completed, discarded
packages remain on, and in, the environment for incalculably long
times.
The polymers and copolymers of lactic acid have been known for
some time as unique materials since they are biodegradable, biocom-
patible and thermoplastic. These polymers are well behaved thermoplas-
tics, 100 percent, truly biodegradable in an animal body via hydrolysis
over a time period of several months to a year. In a wet environment
they begin to show degradation after several weeks and disappear in
about a year's time when left on or in the soil or seawater. The
1339026
degradation products are lactic acid, carbon dioxide and water, all of
which are harmless.
In practice, lactic acid is converted to its cyclic dimer,
lactide, which becomes the monomer for polymerization. Lactic acid is
potentially available from inexpensive feedstocks such as cornstarch or
corn syrup, by fermentation, or from petrochemical feedstocks such as
ethylene. Lactide monomer is conveniently converted to resin by a
catalyzed, melt polymerization, a general process well-known to
plastics producers. By performing the polymerization from an inter-
mediate monomer, versatility in the resin composition is permitted.Molecular weight can be easily controlled. Compositions can be varied
to introduce specific properties.
Homopolymers and copolymers of various cyclic esters such as
glycolide, lactide, and the lactones have been disclosed in numerous
patents and scientific publications. Early patents disclosed processes
for polymerizing lactic acid, lactide, or both, and did not achieve
high molecular weight polymers with good physical properties, and the
polymer products were frequently tacky, sticky materials, without good
physical properties. See, for example, U.S. Patents 1,995,970;
2,362,511; and 2,683,136. The Lowe patent, U.S. Patent 2,668,162 first
teaches the use of pure glycolide and lactide to achieve high molecular
weight polymers and copolymers of lactide. Lactide is the dilactone of
lactic acid and is an internal ester of lactic acid. When lactide is
formed, byproduct water is eliminated, permitting the lactide
subsequently to be ring-opened polymerized to linear polyester of high
molecular weight without tedious condensation methods.
Copolymerization of lactide and glycolide imparted toughness and
improved thermoplastic processability as compared to the homopolymers.
Polymers and copolymers of excellent physical properties were obtained
by using the intermediate, lactide, to form PLA. Copolymers of lactide
and glycolide are disclosed by the Lowe patent which are tough, clear,
cold-drawable, stretchable, and capable of forming at 210 C into self-
supporting films.
Similar disclosures in the patent and other literature developed
the processes of polymerization and copolymerization of lactide to
13~9026
produce very strong, crystalline, orientable, stiff polymers which were
fabricated into fibers and prosthetic devices that were biodegradable
and biocompatible, sometimes called absorbable. The polymers slowly
disappeared by hydrolysis. See, for example, U.S. Patents 2,703,316;
2,758,987; 3,2g7,033; 3,463,158; 3,531,561; 3,620,218; 3,636,956;
3,736,646; 3,797,499; 3,839,297; 3,982,543; 4,243,775; 4,438,253;
4,496,446; European Patent Application 0146398, International Applica-
tion WO 86/00533, and Offenlegundsschrift 2,118,127.
Other patents teach the use of these polymers as stiff surgical
elements for biomedical fasteners, screws, nails, pins, and bone
plates. See, for example, U.S. Patents 3,739,773; 4,060,089; and
4,279,249.
Controlled release devices, using mixtures of bioactive substances
with the polymers and copolymers of lactide and/or glycolide, have been
disclosed. See, for example, U.S. Patents 3,773,919; 3,887,699;
4,273,920; 4,419,340; 4,471,077; 4,578,384; 4,728,721; R.G. Sinclair,
Environmental Science & Technology, 7 (10), 955 (1973). R.G. Sinclair,
Proceedings, 5th International Symposium on Controlled Release of
Bioactive Materials, 5.12 and 8.2, University of Akron Press, 1978.
These applications of lactide polymers and copolymers required tough,
or glassy materials, that were grindable and did not disclose physical
properties for obvious use in thermoplastic packaging materials.
Some mention has been disclosed in the prior art for use of
lactide copolymers for obvious packaging applications. Thus, in the
aforementioned patent to Lowe, clear, self-supporting films are noted
of a copolymer of lactide and glycolide. In U.S. Patent 2,703,316
lactide polymers are described as film formers, which are tough and
orientable. "Wrapping tissue" was disclosed that was tough, flexible,
and strong, brittle, or pliable. However, to obtain pliability the
polylactide must be wet with volatile solvent, otherwise, stiff and
brittle polymers were obtained. The lactide monomer is specified as
having a melting point greater than 120 C. L-lactide monomer melts at
95 C and D,L-lactide melts at 128 C. This is an example of the prior
art which teaches special modifications of lactide polymers to obtain
pliability. Thus, in U.S. Patent 3,021,309, lactides are copolymerized
1~9026
with delta valerolactone and caprolactone to modify lactide polymers
and obtain tough, white, crystalline solids. Soft, solid copolymer
compositions are mentioned only with the copolymer of caprolactone and
2,4-dimethyl-4-methoxymethyl-5-hydroxypentanoic acid lactone, not with
lactide compositions. U.S. Patent 3,284,417 relates to the production
of polyesters which are useful as plasticizers and intermediates for
the preparation of elastomers and foams. This patent excludes lactides
and uses compositions based on 7 to 9 membered ring lactones, such as
epsilon caprolactone, to obtain the desired intermediates. No tensile
strength, modulus, or percent elongation data are given. U.S. Patent
3,297,033 teaches the use of glycolide and glycolide-lactide copolymers
to prepare opaque materials, orientable into fibers suitable for
sutures. It is stated that "plasticizers interfere with crystallinity,
but are useful for sponge and films". Obvious in these disclosures is
that the lactide polymers and copolymers are stiff unless plasticized.
This is true also of U.S. Patent 3,736,646, where lactide-glycolide
copolymers are softened by the use of solvents such as methylene
chloride, xylene, or toluene. In U.S. Patent 3,797,499 copolymers of
L-lactide and D,L-lactide are cited as possessing greater flexibility
in drawn fibers for absorbable sutures. These fibers have strengths
greater than 50,000 psi with elongation percentages of approximately 20
percent. Moduli are about one million psi. These are still quite
stiff compositions compared to most flexible packaging compositions,
reflecting their use for sutures. U.S. Patent 3,844,987 discloses the
use of graft and blends of biodegradable polymers with naturally
occurring biodegradable products, such as cellulosic materials, soya
bean powder, rice hulls, and brewer's yeast, for articles of manufac-
ture such as a container to hold a medium to germinate and grow seeds
or seedlings. These articles of manufacture are not suitable for
packaging applications.
U.S. Patent 4,620,999 discloses a biodegradable, disposable bag
composition comprised of polymers of 3-hydroxybutyrate and 3-hydroxy-
butyrate/3-hydroxyvalerate copolymer. Lactic acid, by comparison, is
2-hydroxy propionic acid. U.S. Patent 3,982,543 teaches the use of
volatile solvents as plasticizers with lactide copolymers to obtain
1339026
pliability. U.S. Patents 4,045,418 and 4,057,537 rely on copolymeriza-
tion of caprolactone with lactides, either L-lactide, or D,L-lactide,
to obtain pliability. U.S. Patent 4,052,9~3 teaches the use of poly
(p-dioxanone) to obtain improved knot tying and knot security for
absorbable sutures. U.S. Patents 4,387,769 and 4,526,695 disclose the
use of lactide and glycolide polymers and copolymers that are deform-
able, but only at elevated temperatures. European Patent Application
0108933 using a modification of glycolide copolymers with polyethylene
glycol to obtain triblock copolymers which are taught as suture
materials. As mentioned previously, there is a strong consensus that
pliability is obtained in lactide polymers only by plasticizers which
are fugitive, volatile solvents, or other comonomer materials.
Copolymers of L-lactide and D,L-lactide are known from the prior
art, but citations note that pliability is not an intrinsic physical
property. U.S. Patent 2,758,987 discloses homopolymers of either L- or
D,L-lactide which are described as melt-pressable into clear, strong,
orientable films. The properties of the poly-L-lactide are given as:
tensile strength, 29,000 psi; percent elongation, 23 percent, tensile
modulus 710,000 psi. The poly-D,L-lactide properties were: 26,000 psi
tensile strength; 48 percent elongation; and a tensile modulus of
260,000 psi. Copolymers of L- and D,L-lactide, that is copolymers of
L- and D,L-lactic acid, are disclosed only for a 50/50 by weight
mixture. Only tack point properties are given (Example 3). It was
claimed that one antipodal (optically active, e.g., L-lactide) monomer
species is preferred for the development of high strength. The
homopolymers of L-lactide and D,L-lactide, as well as the 75/25, 50/50,
and 25/75, weight ratio, of L-/D,L-lactide copolymers are exampled in
U.S. Patent 2,951,828 that discloses the bead polymerization of alpha-
hydroxy-carboxylic acids. The copolymers have softening points of 110-
135 C. No other physical property data are given relating to stiffnessand flexibility except for physical properties relating to bead size
and softening points in the 110 - 135 C range. The 95/5, 92.5/7.5,
90/10, and 85/15, weight ratio, of L-lactide/D,L-lactide copolymers are
cited in U.S. Patents 3,636,956 and 3,797,499. They are evaluated as
filaments from drawn fibers and have tensile strengths in excess of
1339 02 6
- 8 -
50,000 psi, moduli of about one million, and percent elongations of
approximately 20 percent. Plasticizers, the same as in U.S. Patent
3,636,956, above, were used to impart pliability. A snow-white,
obviously crystalline polymer, is cited in Offenlegundsschrift
2,118,127 for a 90/10, L-lactide/D,L-lactide copolymer. No physical
properties were given for this copolymer. The patent teaches the use
of surgical elements.
U.S. Patents 3,297,033; 3,463,158; 3,531,561; 3,636,956;
3,736,646; 3,739,773; and 3,797,499 all disclose lactide polymers and
copolymers that are strong crystalline, orientable polymers suitable
for fibers and suture materials. These disclosures teach the use of
highly-crystalline materials, which are oriented by drawing and
annealing to obtain tensile strengths and moduli, typically, greater
than 50,000 psi and 1,000,000 psi, respectively. Although formability
is mentioned into a variety of shaped articles, physical properties of
unoriented extrudates and moldings are not mentioned. For example,
U.S. Patent 3,636,956 teaches the preparation of a 90/10 weight ratio
- of L-lactide/D,L-lactide and drawn, oriented fibers are cited.However, it is preferred in this disclosure to use pure L-lactide
monomer for greater crystallinity and drawn fiber strength.
U.S. Patent 3,797,499 teaches the copolymerization of 95/5 weight
ratio, of L-lactide/D,L-lactide (Example V); however, the material is
formed into filaments. In column 5, line 1 Schneider teaches against
enhanced properties in the range provided in the present invention.
Plasticizers such as glyceryl triacetate, ethyl benzoate and diethyl
phthalate are used.
U.S. Patents 3,736,646; 3,773,919; 3,887,699; 4,273,920;
4,471,077; and 4,578,384 teach the use of lactide polymers and
copolymers as sustained-drug release matrices that are biodegradable
and biocompatible. Again, physical properties of the polymers from
ordinary thermoforming methods such as film extrusion or molding are
not mentioned.
Of particular interest, U.S. patent 4,719,246 teaches the blending
of homopolymers of L-lactide, D-lactide, polymers or mixtures thereof;
and copolymers of L-lactide or D-lactide with at least one nonlactide
1339026
comonomer. The blending is intended to produce compositions having
interacting segments of poly(L-lactide) and poly(D-lactide).
Canadian Patent 808,731 cites the copolymers of L- and D,L-lactide
where a divalent metal of Group II is part of the structure. The
90/10, L-/D,L-lactide copolymer (Example 2) and the L-lactide homopoly-
mer were described as "suitable for films and fibers". The 90/10
copolymer is described as a snow-white copolymer and the homopolymer of
L-lactide can be molded to transparent films. (The more crystalline
polymer should be the opaque, or snow-white material, which is the
homopolymer.) The patent discloses "the fact that the novel polylac-
tides of the present invention contain the metallic component of the
catalyst in the form of a lactate is believed to be of significance".
Furthermore, "the polylactides find utility in the manufacture of films
and fibers which are prepared by conventional thermoplastic resin manu-
facturing methods". No physical property data are given on thestrength and flexibility of the films.
Canadian Patent 863,673 discloses compositions of L-lactide and
D,L-lactide copolymers in the ratios of 97/3, 95/5, 92.5/7.5, 90/10,
and 85/15 ratios of L-/D,L-lactide, respectively. These were all
characterized as drawn filaments for surgical applications. Tensile
strength, approximately 100,000 psi, was high, elongation was approxi-
mately 20 percent and plasticizers were mentioned to achieve pliabil-
ity. D,L-lactide compositions of less than 15 weight percent are
claimed.
Canadian Patent 923,245 discloses the copolymers of L- and D,L-
lactide (Example 15). The 90/10 copolymer is described as a snow white
polylactide. The polylactides prepared by the methods of the patent
are stated to have utility in the manufacture of films or fibers
prepared by conventional thermoplastic resin fabricating methods.
U.S. Patent 4,719,246 teaches the use of simple blending of poly
L-and poly (D-lactide), referred to as poly (S-lactide) and poly (R-
lactide), polymers of mixtures thereof; and copolymers of L-lactide or
D-lactide with at least one nonlactide comonomer. The examples are all
physical mixtures. The special properties of the "interlocking" stem
from racemic compound formation (cf. "Stereochemistry of Carbon
1339026
- 10 -
Compounds", E. L. Eliel, McGraw-Hill, 1962, p. 45). Racemic compounds
consist of interlocked enantiomers, that is, the D and L forms (or R
and S) are bonded to each other by polar forces. This can cause a
lowering, or raising, of the crystalline melting points, depending on
whether the D to D (or L to L) forces are less, or greater, than the D
to L forces. Required of polymer racemic compounds to enhance the
effect (and stated in U.S. Patent 4,719,246, Column 4, line 48) are
homopolymers, or long chain lengths, of both D and L. The great
symmetry or regularity of these structures permit them to fit together,
or interlock, by very regular polar forces, either because they are the
same, or mirror images. This leads to considerable crystallinity. The
art of racemic compounds has a long history that goes back to classical
chemistry.
U.S. patent 4,661,530, discloses the mixtures of a poly (L-lactic
acid) and/or poly (D,L-lactic acid) and segmented polyester urethanes
or polyether urethanes. Biodegradable materials are formed that are
useful in synthetic replacements of biological tissues and organs in
reconstructive surgery.
Nowhere in the prior art is it disclosed that lactide polymers,
are capable of pliable, highly-extensible compositions by the use of
lactide monomers, lactic acid, or oligomers of lactic acid or lactide
as the plasticizer. None of the prior compositions are suitable for
well-defined packaging needs of the thermoplastic polymers' industry.
It will be appreciated by those skilled in the art that duplicat-
ing the properties of one thermoplastic with another is not predict-
able. Thus, with crystal polystyrene, or OPS, there are exacting
requirements for satisfactory performance of the polystyrene, which has
been developed over many years to meet manufacturing and end-use
specifications of OPS grades.
BRIEF DESCRIPTION OF THE INVENTION
The general teaching of the invention, and first embodiment, is
that homopolymers of L-lactide, D-lactide, and D,L-lactide and
copolymers of mixtures thereof that have been plasticized with lactide
monomer(s), lactic acid or oligomers of lactide or of lactic acid have
I339026
utility as well behaved thermoplastics which can mimic properties of
the usual environmentally nondegradable plastics, (e.g., the properties
of polyethylene and the like). This composition has the formula:
C~3 oo
~ xlC - C - --~n
H
and is intimately plasticized with a plasticizer selected from the
group consisting of lactide, lactic acid, oligomers of lactic acid, and
mixtures thereof. The oligomers of lactic acid further are preferably
represented by the formula II, where m is an integer: 2 c m < 75.
However, m is preferably 2 < m < 10. The plasticizer preferably
comprises from 2 to 60 weight percent of the polymer. The polymers may
be derived from monomers of lactide selected from the group consisting
of L-lactide, D-lactide, meso D,L-lactide and mixtures thereof.
Preferably n is 150 < n ~ 20,000.
C~3
~ ` ~ - ~ --~m II
Lactide monomer can be present in an amount of from 5 to 40
weight percent of the polymer while lactide oligomer or lactic acid and
its oligomers may be present in an amount of from 2 to 60 weight
percent. This composition allows many of the desirable characteristics
of polyethylene such as pliability, transparency, and toughness.
Further provided is a process for producing the biodegradable
composition. The process includes the steps of mixing, heating, and
melting one or more lactide monomers and catalyst; polymerizing the
monomers of the solution to form a polymer at a temperature
sufficiently low to allow the polymerization reaction to be stopped
prior to complete polymerization; monitoring the level of monomer; and
stopping the reaction prior to complete polymerization at an amount of
monomer determined by the monitoring; so that unreacted monomer is
trapped in association with the polymer.
Further provided is a process for producing a plasticized polymer
of polylactic acid that comprises mixing, heating, and melting one or
1339026
- 12 -
more lactide monomers and a catalyst; polymerizing the monomers of the
solution to form a polymer without stopping the reaction; and
incorporating plasticizer into the polymer whereby the plasticizer is
selected from the group consisting of D-lactide, L-lactide, meso D,L-
lactide, lactic acid, oligomers of lactic acid, and mixtures thereof.
A second embodiment of the invention includes a process for
producing an environmentally biodegradable composition and an
environmentally biodegradable composition useful as a replacement for
polystyrene comprising polylactic acid units of the formula I where n
is an integer between 75 and 10,000 and the alpha carbon is a mixture
of L- and D-configurations with a preponderance of either D- or L-
units, wherein the polymer is prepared from L-lactide or D-lactide, at
85 to 95 parts by weight, and D,L-lactide at 15 to 5 parts by weight,
where the unoriented polymer has a tensile strength of at least 5000
psi and tangent modulus of at least 200,000 psi and dispersed
plasticizer of 0.1 - 5 weight percent.
A third embodiment of the invention teaches the process for
producing an environmentally degradable composition and an
environmentally degradable composition comprising blends of a physical
mixture of polylactic acid; and one or more polymers selected from the
group consisting of a polymer of ethylene terephthalate, a polymer or
copolymer of styrene, ethylene, propylene, vinyl chloride, vinyl
acetate, alkyl methacrylate, alkyl acrylate, and physical mixtures
thereof.
A fourth embodiment teaches the process for producing an
environmentally degradable composition and an environmentally
degradable composition is disclosed comprising blends of a physical
mixture of polylactic acid and blend-compatible elastomers that provide
improved impact resistance to the blended composition. Such an
elastomer may be, for example, a Hytrel~, a segmented polyester which
is a block copolymer of hard crystalline segments of polybutylene
terephthalate and soft long chain segments of polyether glycol. One
example is known by the trade name as Hytrel~ 4056 (DuPont) segmented
polyester.
1~39o26
BRIEF DESCRIPTION OF THE FIGURES
Figure 1 is a graph showing the relationship between percent
lactide in the composition as plasticizer and tensile strength.
Figure 2 is a graph showing the relationship between percent
lactide in the composition as plasticizer and elastic modulus.
Figure 3 is a graph showing the relationship between percent
oligomer in the composition as plasticizer and tensile strength where
curve A is for a 90/10 copolymer and curve B is for a 92.5/7.5
copolymer.
Figure 4 is a graph showing the relationship between percent
oligomer in the composition as plasticizer and the elastic modulus
where curve A is for a 90/10 copolymer and curve B is for a 92.5/7.5
copolymer.
Figure 5 illustrates the differential scanning calorimetry (DSC)
plot of unannealed 90/10, L-/D,L-lactide copolymer of Example 5B.
Figure 6 illustrates the DSC of the material of Example 5B after
remaining at 70 C for 100 minutes.
Figure 7 illustrates the DSC of the material of Example 5B after
annealing in 185 F overnight.
Figure 8 illustrates the DSC of the material of Example 5B that
has been blended with 5 percent calcium lactate.
Figure 9 compares the melt viscosity versus shear rate charac-
teristics of polystyrene and the lactide polymer prepared as in
Example 8B.
Figure 10 illustrates a DSC for the copolymer of Example 8B.
Figure 11 illustrates a DSC for the L-lactide homopolymer that is
added to the copolymer of Example 8B.
Figure 12 illustrates a DSC for the blended composition of Example
23 of the copolymer of Example 8B and a homopolymer of L-lactide.
Figure 13 illustrates a differential scanning calorimetry (DSC)
plot of 90/10, L-/D,L-lactide copolymer blended with 5 weight percent
polystyrene.
1339026
- 14 -
DETAILED DESCRIPTION 0~ THE INVENTION
AND PREFERRED EMBODIMENTS
First General Embodiment
The environmentally biodegradable compositions disclosed herein
are completely degradable to environmentally acceptable and compatible
materials. The intermediate products of the degradation: lactic acid
and short chain oligomers of lactide or lactic acid are widely
distributed naturally occurring substances that are easily metabolized
by a wide variety of organisms. Their natural end degradation products
are carbon dioxide and water. Contemplated equivalents of these
compositions such as those that contain minor amounts of other
materials, fillers, or extenders can also be completely environmentally
degradable by proper choice of materials. The compositions herein
provide environmentally acceptable materials because their physical
deterioration and degradation is much more rapid and complete than the
conventional nondegradable plastics that they replace. Further, since
all or a major portion of the composition will be polylactic acid,
and/or a lactic acid derived lactide or oligomer, no residue or only a
small portion of more slowly degrading residue will remain. This
residue will have a higher surface area than the bulk product and an
expected faster degradation rate.
The general application of the invention results in the first and
general embodiment of the invention. The homopolymers of D-lactide, L-
lactide, D,L-lactide as well as copolymers of D-lactide, L-lactide; D-
lactide, D,L-lactide; L-lactide, D,L-lactide; and D-lactide, L-lactide,
and D,L-lactide all produce materials useful in the invention when
plasticized by lactide monomers, lactic acid, oligomers of lactide,
oligomers of lactic acid, and mixtures thereof. The plasticizer may be
produced by stopping the reaction before polymerization is completed.
Optionally additional plasticizer consisting of lactide monomers (D-
lactide, L-lactide, D,L-lactide, or mixtures thereof), lactic acid,
oligomers lactide or oligomers of lactic acid, and mixtures thereof can
be added to the polymer. The polymer is defined by the formula:
1339026
- 15 -
C~3
( C - : - O--~n
al
where n is the degree of polymerization (number of repeating units),
and is plasticized with a plasticizer derived from incomplete polymer-
ization of the monomers used to produce the polymer. The more
intimately the plasticizer is integrated with the polymer the better
are its characteristics. If desired additional monomer or oligomer can
be added to any residual monomer or oligomer remaining in the
composition. The oligomers of lactic acid useful for a plasticizer are
defined by the formula II, where m is an integer: 2 < m < 75, however,
the preferable range is: 2 < m < 10.
C~3
~ C - : - O--~m II
H
The proportions of L-lactide, D-lactide, and D,L-lactide are not
critical to obtaining flexible thermoplastics. The parts of L-lactide,
D-lactide, and D,L-lactide can vary over a wide, weight-ratio to form a
homopolymer or copolymer. The lactide monomers employed in accordance
with the invention are available commercially so that neither the
monomeric reactant per se nor the method by which it is prepared
constitute any portion of the invention.
D-lactide is a dilactone, or cyclic dimer, of D-lactic acid.
Similarly, L-lactide is a cyclic dimer of L-lactic acid. Meso D,L-
lactide is a cyclic dimer of D- and L-lactic acid. Racemic D,L-lactide
comprises a mixture of and L-lactide. When used alone herein, the term
"D,L-lactide" is intended to include meso D,L-lactide or racemic D,L-
lactide.
One of the methods reported in the literature for preparing a
lactide is to dehydrate lactic acid under high vacuum. The product is
distilled at a high temperature and low pressure. Lactides and their
preparation are discussed by W. H. Carothers, G. L. Dorough and M. J.
Johnson (J. Am. Chem. Soc. 54, 761-762 [1932]); J. Gay-Lussac and J.
Pelouse (Ann. 7, 43 [1833]); C. A. Bischoff and P. Walden (Chem. Ber.
1339026
- 16 -
26, 263 [1903]; Ann. 279, 171 [1984]); and Heinrich Byk (Ger. Pat.
267,826 [1912]); through Chem. Abstr. 8, 554, 2034 [1914]).
The optically active acids can be prepared by direct fermentation
of almost any nontoxic carbohydrate product, by-product or waste,
utilizing numerous strains of the bacterial genus Lactobacillus, e.g.
Lactobacillus delbrueckii, L. salivarius, L. casei, etc. The optically
active acids can also be obtained by the resolution of the racemic
mixture through the zinc ammonium salt, or the salt with alkaloids,
such as morphine. L-lactide is a white powder having a molecular
weight of 144. If an impure, commercially-available product is
employed in accordance with the present invention, it is preferable to
purify it by recrystallization from anhydrous methyl isobutyl ketone.
The snow-white crystals of L-lactide melt at 96-98 C.
D,L-lactic acid which is used in the preparation of D,L-lactide is
available commercially. The D,L-lactic acid can be prepared syn-
thetically by the hydrolysis of lactonitrile (acetaldehyde cyanohydrin)
or by direct fermentation of almost any nontoxic carbohydrate product,
by-product or waste, utilizing numerous strains of the bacterial genus
Lactobacillus. D,L-lactide is a white powder having a molecular weight
of 144. If an impure, commercially-available product is employed in
accordance with the present invention, I prefer to purify it by
recrystallization from anhydrous methyl isobutyl ketone. One such
commercially available product comprising a mushy semisolid melting at
90-130 C was recrystallized from methyl isobutyl ketone and
decolorized using charcoal. After three such recrystallizations, the
product was tumble-dried in vacuo under a nitrogen bleed for 8 to 24
hours at room temperature. The snow white crystals thus obtained
comprise a D,L-lactide mixture melting from 115-128 C.
In preparing the compositions in accordance with the invention, it
is preferred to carry out the reaction in the liquid phase in a closed,
evacuated vessel in the presence of a tin ester of a carboxylic acid
containing up to 18 carbon atoms. The compositions however, can also
be prepared at atmospheric pressure with the polymerization system
blanketed by an inert gas such as, for example, nitrogen. If
polymerization is conducted in the presence of oxygen or air, some
1339026
- 17 -
discoloration occurs with a resulting decrease in molecular weight and
tensile strength. The process can be carried out at temperatures where
the polymerization is sluggish in its later stages so as to trap
residual monomer in the viscous polymer melt. Preferred temperatures
for this purpose are generally between the melting points of pure L-
lactide and pure D,L-lactide, or between 95 to 127 C. While in no way
wishing to limit the scope of the invention it is presently believed
that below about 129 C, the following occurs:
1. The lactide monomer mixture of L- and D,L-lactide monomers
melt to form a eutectic mixture, which melts to a mobile fluid
that is an intimate solution of one, two, or three monomers.
2. The fluid melt is polymerized by catalyst to form an increas-
ingly viscous solution and eventually unreacted monomer is
trapped in association with the polymer as a solution, rather
than as a distinct heterogeneous phase. The monomer no
longer can react since the reaction is extremely diffusion
controlled and cannot contact the low concentration of active
end-groups of the polymer.
3. The polymerization ceases or slows considerably so that at
room temperature the blend of monomer and polymer are a solid
solution that imparts plasticization, clarity, and flexibility
to the composition.
4. The catalyst deactivates so that subsequent melt-fabrication
does not reinitiate the polymerization.
5. The plasticized composition is quite stable since the residual
monomer is very high boiling, e.g., lactide boiling point is
142 C at 8 torr, and is tightly associated with its open-chain
tautomer, polylactide.
Alternatively, the process can be carried out at any temperature
between the melting point of the L-lactide and 200 C and lactic acid or
lactide is subsequently melt or solvent-blended into the polymer as a
further processing step. Temperatures above 200 C are undesirable
because of the tendency of the copolymer to be degraded. Increasing
the temperature within the range of 95 to 200 C generally increases the
speed of the polymerization. Good results are obtained by heating a
1339026
- 18 -
mixture of L-lactide and D,L-lactide at a temperature between about
110 C and 160 C.
The catalysts employed in accordance with the invention are tin
esters of carboxylic acids containing up to 18 carbon atoms. Examples
of such acids are formic, acetic, propionic, butyric, valeric, caproic,
caprylic, pelargonic, capric, lauric, myristic, palmitic, stearic and
benzoic acids. Good results have been obtained with stannous acetate
and stannous caprylate.
The catalyst is used in normal catalytic amounts. In general, a
catalyst concentration in the range of about 0.001 to about 2 percent
by weight, based on the total weight of the L-lactide and D,L-lactide
is suitable. A catalyst concentration in the range of about 0.01 to
about 1.0 percent by weight is preferred. Good results were obtained
when the catalyst concentration is in the range of about 0.02 to about
0.5 percent by weight. The exact amount of catalyst in any particular
case depends to a large extent upon the catalyst employed and the
operating variables including time and temperature. The exact
conditions can be easily determined by those skilled in the art.
The reaction time of the polymerization step, per se, is governed
by the other reaction variables including the reaction temperature, the
particular catalyst, the amount of catalyst and whether a liquid
vehicle is employed. The reaction time can vary from a matter of
minutes to a period of hours, or days, depending upon the particular
set of conditions which are employed. Heating of the mixture of
monomers is continued until the desired level of polymerization is
detected. The level of polymerization can be determined by analysis
for residual monomers. As discussed previously, the reaction
temperature can be chosen to enhance the incorporation of monomer and
provide plasticized compositions coming directly out of the
polymerization reactor. The reaction can be halted at such time that
the composition has attained the conversion of monomer to polymer that
is desired to achieve the desired plastization. In the preferred
embodiment of the invention, approximately 2 to 30 percent lactide is
left unreacted, depending on the plasticization to be achieved.
- 1339026
- 19 -
In general it is preferred to conduct the polymerization in the
absence of impurities which contain active hydrogen since the presence
of such impurities tends to deactivate the catalyst and/or increase the
induction time. It is also preferred to conduct the polymerization
under substantially anhydrous conditions.
The copolymers of the invention can be prepared by bulk polymer-
ization, suspension polymerization or solution polymerization. The
polymerization can be carried out in the presence of an inert normally-
liquid organic vehicle such as, for example, aromatic hydrocarbons,
e.g., benzene, toluene, xylene, ethylbenzene and the like; oxygenated
organic compounds such as anisole, the dimethyl and diethyl esters of
ethylene glycol; normally-liquid saturated hydrocarbons including open
chain, cyclic and alkyl-substituted cyclic saturated hydrocarbons such
as hexane, heptane, cyclohexane, alkylcyclohexanes, decahydronaph-
thalene and the like.
The polymerization process can be conducted in a batch, semi-
continuous, or continuous manner. In preparing the lactide monomeric
reactants and catalyst for subsequent polymerization, they can be
admixed in any order according to known polymerization techniques.
Thus, the catalyst can be added to either of the monomeric reactants.
Thereafter, the catalyst-containing monomer can be admixed with the
other monomer. In the alternative, the monomeric reactants can be
admixed with each other. The catalyst can then be added to the
reactant mixture. If desired, the catalyst can be dissolved or
suspended in an inert normally-liquid organic vehicle. If desired, the
monomeric reactants either as a solution or a suspension in an inert
organic vehicle can be added to the catalyst, catalyst solution or
catalyst suspension. Still further, the catalyst and the monomeric
reactants can be added to a reaction vessel simultaneously. The
reaction vessel can be equipped with a conventional heat exchanger
and/or a mixing device. The reaction vessel can be any equipment nor-
mally employed in the art of making polymers. One suitable vessel, for
example, is a stainless steel vessel.
The environmentally biodegradable compositions produced in
accordance with the present invention depending upon the L-lactide, D-
1339026
- 20 -
lactide, meso D,L-lactide ratios, find utility in articles of manu-
facture, such as films, fibers, molding and laminates, which are
prepared by conventional fabricating methods. These articles of
manufacture are contemplated for nonmedical uses i.e. outside the body
where they can substitute for the common environmentally nondegradable
plastics.
Filaments, for example, are formed by melt-extruding the copolymer
through a spinneret. Films are formed by casting solutions of the
biodegradable compositions and then removing the solvent, by pressing
solid biodegradable compositions in a hydraulic press having heated
platens, or by extrusion through a die.
Various techniques including slow cooling and rapid cooling can be
employed in preparing moldings from the copolymers of the invention.
Contemplated equivalents of the compositions of the invention are
those that contain minor amounts of other materials. The copolymers
produced in accordance with the present invention can be modified, if
desired, by the addition of a cross-linking agent, other plasticizers,
a coloring agent, a filler and the like.
Cross-linking can be effected by compounding the compositions with
free-radical initiators such as cumene hydroperoxide and then molding
at elevated temperatures. This can improve heat-and solvent-resist-
ance. Curing can also be effected by compounding the copolymers with
multifunctional compounds such as polyhydric alcohols and molding, or
thermoforming under heat and vacuum. Graft-extruder reactions to
effect curing of the polyesters is an obvious method of cross-linking
and chain-extending the copolymers.
In preparing moldings, a filler can be incorporated in the
compositions prior to curing. A filler has the function of modifying
the properties of a molding, including hardness, strength, temperature
resistance, etc. Known filler materials include aluminum powder,
powdered calcium carbonate, silica, kaolinite (clay), magnesium
silicate and the like. Of particular advantage is starch, which blends
well with the compositions to obtain a blend which is totally
environmentally biodegradable. Other property modifications can be
1339026
- 21 -
effected by melt blending the compositions with other polymers and
copolymers of the lactides, glycolides, and caprolactone.
The compositions prepared according to the present invention can
be used in producing reinforced laminates according to known
procedures. In general, laminates are made from a fibrous mat or by
assembling a multiplicity of sheets of material to form a matrix which
is consolidated into a unitary structure by flowing molten precursor or
composition through the fibrous material and curing it while in a mold
or hydraulic press to form the polymer. Fibers which are used in
forming the matrix include natural and synthetic fibers such as
cellulose derived from wood, cotton, linen, hemp, and the like, glass,
nylon, cellulose acetate and the like.
The compositions of the invention and their preparation are
further illustrated by the following specific examples.
Example 1
80/20, L-lactide/racemic D,L-lactide
160 grams of L-lactide and 40 grams of racemic D,L-lactide, both
of high purity (Purac, Inc., triply recrystallized), were charged into
a 500 ml, round-bottom flask and purged with dry nitrogen overnight.
10 ml of stannous octoate is dissolved in 60 ml of anhydrous toluene,
and 10 ml of the solvent is distilled to a Dean-Stark trap to effect
dryness of this catalyst solution by azeotropic distillation. From the
10 ml of stannous octoate in 50 ml of dry toluene a 0.20 ml portion is
removed with a syringe and injected into the lactides in the reaction
flask. The nitrogen purge is continuous via a syringe needle connec-
tion that enters the reaction flask through a rubber septum and vents
via a piece of tubing that connects to a bubbler. The nitrogen flow is
maintained at 1-3 bubbles per second. The flask was heated in an oil
bath maintained at 123-127 C. During the first part of the heating the
lactides melt and are mixed thoroughly by swirling. Thereafter, the
products become quite viscous. After 20 hours of heating, the flask
and the colorless, transparent products are removed from the heating
bath, cooled, the flask broken, and shocked with liquid nitrogen to
remove glass from the product. The copolymer was molded in a heated
- 13~9026
- 22 -
hydraulic press. Compression molding to 5 to 10 mil thick films was
possible at 20,000 lb pressure, at 170 C, in a time period of 2
minutes. The films were evaluated for their tensile properties on a
Instron tester, and the results are listed in Table 1. Samples 1/8
inch thick were also molded for impact strength testing. A thermal
gravimetric analysis (TGA) of the product was performed, noting the
weight loss upon heating the sample to 150 C in 4 minutes and holding
the temperature at 150 C for 60 minutes. The weight loss of the sample
was 19.5 percent and nearly complete in 60 minutes. The weight loss is
attributed to loss of lactide monomer. Results of differential
scanning calorimetry (DSC) reveal that the composition has an endotherm
beginning about 110 C, becoming more pronounced as the temperature
increases to 200 C. No melting point was observed. Specimens were
annealed at 185 F overnight and reexamined. They remained transparent,
colorless and pliable. Samples of the copolymer could be remolded 6
times without any discoloration or obvious loss of strength. Thin
films were clear, transparent, colorless, and quite flexible, despite
the repeated molding.
1339026
TABLE 1. PROPERTIES OF COPOLYMERS(a) OF L-LACTIDE AND
D,L-LACTIDE WHEN PLASTICIZED BY LACTIDE
Example No. 1 2 3
Film thickness, mil 8 8 10
Tensile strength, 1000 psi,
ASTM D638 3.9 1.7 7.9
Elongation, percent 28 806 3.5
100 percent modulus, 1000 psi 0.74 -- --
200 percent modulus, 1000 psi 1.20 -- --
Tangent modulus, 1000 psi 36.6 -- 289
Izod impact strength, ft-lb/in.(b) 0.63 -- 0.4
Mw, 1000's 540 281 341
Mn, 1000's 270 118 97.5
Residual lactide,(C) percent 19.5 27.8 2.7
(a) 80/20, weight ratio, of L-/racemic D,L-lactide.
(b) 1/8 inch, notched samples.
(c) By isothermal TGA weight loss at 150 C.
Example 2
In a 3-liter, round-bottom flask was charged 1.84 Kg of L-lactide,
0.46 Kg of racemic D,L-lactide and 2.3 ml of the stannous octoate
solution, similar to Example 1. The mixture was purged with argon for
3 hours, then heated isothermally in a 125 C oil bath. The mixture
melts, was mixed thoroughly by swirling, and forms a homogeneous,
transparent, colorless fluid whose viscosity increases substantially
after several hours. After 64 hours the flask was removed from the
heating bath, cooled, and the glass removed from the clear,
transparent, solid product. The rubbery composition was guillotined
133902~
- 24 -
into slices and ground to 1/8 inch, or smaller, size in a grinder with
dry ice. The grind was dried in an air circulating oven at 100 F for
several hours, then vacuum dried overnight at ambient temperature.
Compression-molded films were prepared as described in Example 1 and
the films were examined for their tensile properties and weight loss by
TGA as shown in Table 1.
Example 3
In a 250-ml, round bottom flask was placed 79.98 9 of L-lactide,
20.04 9 of racemic D,L-lactide, and 0.20 ml of stannous octoate
solution, similar to Example 1. The flask was swept by nitrogen
through inlets and outlets and heated in a 125 C oil bath. The mixture
melted to a colorless and fluid liquid that was thoroughly mixed by
swirling the flask. After 2 hours, the oil bath temperature was
increased to 147 C, and after 14 hours total heating time, the
temperature was decreased to 131 C. Total heating time was 18 hours.
The product is transparent, colorless, and glassy. It was evaluated,
similar to the preceding examples and the results are recorded in Table
1.
Examples 1 to 3 reveal the effect of reaction temperature on the
properties of the copolymers as occasioned by the resulting
compositlon.
Example 4
Films of the copolymers of Examples 1 and 3 were immersed in water
for several months. After 3 weeks, the copolymer of Example 1 became
hazy while that of Example 3 remained clear for approximately 2 months;
after 3 months the film of Example 3 became noticeably hazy and the
film of Example 1 is white and opaque. The water that had been in
contact with the film of Example 1 tastes acidic while that of Example
3 is tasteless.
Inspection of the data of Table 1 reveals that the copolymer of
Example 1 is an environmentally biodegradable replacement for
polyethylene. Those skilled in the art will recognize that the
physical properties of the copolymer are an excellent combination
13~9026
- 25 -
useful for many packaging applications. Its tensile strength and
initial tangent modulus compare favorably with polyethylene
compositions used, for example, in plastic trash bags, general film
wrap, plastic shopping bags, sandwich wrap, six pack yokes and the
like. The shape of the stress-strain curves are approximately the same
for both the copolymer and that for a linear low density polyethylene
composition commonly used in trash bag compositions. A comparison of
properties are shown in Table 2.
TABLE 2. COMPARISON OF POLYETHYLENE TO POLYLACTIC ACID POLYMERS
LDPE-(a) b Lactide
Property NA 272 LLDPE( ) Copolymer(C)
Tensile strength, 2.18 2.9 3.90
1000 psi,
ASTM Standard C
Elongation, % 261 500 280
Tangent modulus, 54.9 51.0 36.6
1000 psi
100% modulus, 1.77 -- 0.74
1000 psi
200% modulus 1.82 -- 1.20
HDT,(d) 264 psi, F 95 99 122
(a) Linear low density polyethylene, 5-10 mil, 2-in./min.,
our experiments.
'b` Linear low density polyethylene, data from computer file.
c Copolymer of L-lactide/racemic D,L-lactide, Example 1.
;d, Heat deflection temperature.
The lactide polymerization can be stopped at incomplete monomer-
to-polymer conversion in a controllable fashion. This is illustrated
in Examples 1 and 2. The lactide monomer binds very intimately with
polymers of lactides. Alternatively, the compositions can be derived
1339026
- 26 -
by mixing of lactide with preformed polymer. In that case, the lactide
added can be the same or different with respect to stereochemistry,
i.e., L-, D-, or D,L-lactide to that used to make the polymer.
The compounding can be accomplished by blending the molten polymer
with lactide monomer in conventional processing equipment such as a
mill roll or a twin screw compounder. The normally stiff, glassy,
lactide polymers are flexibilized by the lactide and remain
transparent, colorless, and very nearly odorless. The lactide is not
very fugitive, requiring heating, and a nitrogen sweep, typically, 170-
200 C for 20-60 minutes to remove the lactide in a gravimetric
analysis. Neither is the lactide visible in films under an optical
microscope. The lactide domains are submicron in size. This
flexibilizing of the polylactic acid suggests its use as a
environmentally biodegradable replacement for polyolefin, disposable,
packaging films.
Examples 5-16
A series of experiments were performed in which copolymers of L-
and racemic D,L-lactide were prepared, melt blended with variable
amounts of lactide, and the physical properties of the blends evaluated
as a function of the lactide composition. Monomer lactide content was
assayed by a previously developed isothermal, thermogravimetric
analysis (TGA). The lactide contents were measured before and after
compounding and molding into films.
It was observed that open roll, 2 roll, milling tended to
volatilize the lactide at temperatures required for the very high,
molecular weight lactide copolymers. These losses could be minimized
by masterbatching or by using lower molecular weight lactide copolymers
(and their lower attendant mixing temperatures). A better mixing and
blending method was a conventional, twin screw extruder, which
minimized volatile losses. Some results are shown in Table 3. The
blends of polylactide and lactide plasticizer are quite pliable,
becoming increasingly so with increasing lactide content. They are
colorless and transparent. Only a very faint (pleasant) odor of
lactide is detectable and no discernable taste of lactide was
- 1339026
- 27 -
noticeable. The Table 3 plasticized film samples were tear resistant,
easily foldable, and can be punctured without shattering or tearing.
They stiffen somewhat when placed in a cooler (5 C, 40 F), but remain
flexible and creasible without breaking. These films noticeably soften
in the hand, indicating a glass transition temperature below 37 C.
When the lactide content is less than 20 percent, the films will have a
rattle typical of a polyolefin film. At greater lactide contents the
films have the drape and "warm" feel of a PVC.
As shown in Table 3, the elastic moduli (initial tangent moduli)
can be relatively high, similar to a linear low density polyethylene
(LLDPE). This is an indication of potential form stability. Lower
moduli and tensile strengths are similar to low density polyethylene
(LDPE). Physical properties, as a function of lactide content, were
plotted as shown in Figures 1 and 2. Referring to Table 3, at
approximately 17-20 percent lactide content, the tensile properties are
similar to polyethylenes used in trash bags and shopping bags.
At lower lactide contents, the blends have a similarity to
polypropylene. Some data can be compared in Table 3. Table 4 defines
the conventional plastics used in the comparisons.
1339026
- 28 -
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1339026
- 29 -
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TABLE 4. MANUFACTURERS' DATA
Recommended Elastic
Trade Name Melt Tensile Modulus in
and/or Density, Temperature, StrengthFlexure, Melt Index
Supplier Grade gm/cu cm F at Yield 35 psi gm/10 min
LDPE (USI) Petrothene 0.924 360-550 1820 0.37 8.0
No. 213
LLDPE (Exxon) 6202.57 0.926 425 1700 0.53 12.0
HDPE (Phillips) HMN 5060 0.950 425-525` 3600 1.75 6.0
80% LLDPE (Exxon) LPX 86 0.927 260 -- -- 0.8
20% HDPE (Proces- (Octene
sing oil) base)
Polypropylene XF1932 0.91 450-500 5872 3.05 0.52
(PP-Chisso)
Polystyrene RI 1.05 400 7900 4.50 1.8
(Amoco)
c~
c~
, ,~
o
- 13~9026
- 31 -
Table 3 reveals some data for lactide and polylactide mixtures.
The results do not differ remarkably from similar compositions of
Examples 1 and 2, prepared by other means. However, those skilled in
the art will recognize that the precise physical properties will vary
somewhat depending on the intimacy of the mixture, the tensile testing
conditions, and the fabrication technique for preparing the films.
Comparisons from Table 3 reveal that the lactide-polymer mixtures have
a broad range of controllable compositions that mimic many
conventional, nondegradable plastic types.
Example 17
An oligomeric polylactic acid (OPLA) was prepared for mixing with
polylactides as follows. An 88 percent solution of L-lactic acid (956
g) was charged to a 3-neck flask (1 liter) fitted with a mechanical
stirrer and a pot thermometer. The reaction mixture was concentrated
under a nitrogen purge at 150-190 C at 200 mm Hg for 1 hour until the
theoretical water of dilution was removed. No catalyst was used except
for lactic acid and its oligomers. This temperature and vacuum were
maintained and distillation continued for 2 hours until 73 percent of
the theoretical water of dehydration was removed.
The total time required was 3 hours. At this time the reaction
was stopped. The water samples and the pot oligomer were titrated with
0.5N NaOH. Some lactic acid, 26.2 9, was found in the water
distillate. The pot oligomer (OPLA) was also refluxed with excess 0.5N
NaOH, then back titrated with standard H2S04. The data are recorded
in Table 5. The OPLA flows well when hot, and shows some cold flow.
It has a degree of polymerization of 3.4. It was used in Example 20
where it was melt blended with the polymer of Example 19.
- 32 - 1339 02 6
TABLE 5. CHARACTERIZATION OF OPLA OF EXAMPLE 1
Total
Percent Titratable Titratable Expressed as
5Dehydrated, Acid, Ester, Lactic Acid Degree of
Theoretical percent percent percent Polymerization
58 34.4 82.4 116.8 3.4
Example 18
The procedure of Example 17 was repeated except the distillation
was conducted more slowly. After 8 hours of heating during which the
temperature was slowly advanced from 63 to 175 C at 200 mm Hg, a sample
of the pot was titrated to reveal 62.2 percent of theoretical water
removal. Titration revealed a degree of polymerization of 4.3. The
molecular weight of the OPLA was further advanced over 2 hours by
heating at 179 C and using a vacuum pump. The OPLA was no longer
soluble in 0.1N NaOH, was water white, and would cold flow. This
material is a second example of an OPLA preparation with somewhat
higher degree of polymerization as compared to Example 1. It was mixed
with polylactide in Examples 22 and 25. It is estimated that the
degree of polymerization was about 6-10.
Example 19
A polymer of lactide was prepared by methods similar to Example 3.
A 90/10, weight percent L-/racemic D,L-lactide copolymer was melt
polymerized using 0.02 parts per hundred, anhydrous stannous octoate
catalyst. In a similar manner a 100 percent L-lactide homopolymer (L-
PLA) was prepared. The copolymer was melt blended with the homopolymer
at 350 F in a twin-screw extruder at a weight ratio of 90/10,
copolymer/homopolymer. Gel permeation chromatography (GPC) of the
blend reveals a weight-average molecular weight (Mw) of 182,000 and a
1339026
- 33 -
number-average molecular weight (Mn) of 83,000. Residual lactide
monomer by thermogravimetric analysis (TGA) was 1.7 weight percent.
This blend was mixed with the oligomeric polylactic acid of (OPLA) of
Example 17 to provide Example 20. The tensile properties are listed in
Table 6.
Example 20
The polymer of Example 19 was melt blended with the OPLA of
Example 17 on an open, 2-roll, mill for 20 minutes at 325 F. The mix
was compression molded into films and tested as shown in Table 6. The
GPC molecular weights were smooth, monomodal distributions (MW/Mn =
2.6) with Mw = 192,000 and Mn = 73.000-
-- 1339026
- 34 -
a~
a
C ~ _
~ ~! E
O ~ _
C ^
_ V ~ C
tY ~ ~-- >,_
LLI ~ m c ~ o
~ _ ~ ~ o
o a~
O E
._ --O ~
O
O C O ~I
~ 0 1~ C~ C~l -- ~ O~ ~ ~ C
z ~ c _ ^ o _ I~ If~ -- ~ a~ ~ ~ -- ~
~ ~ 7 N -- cn 3
:~m ~ c C ~
J ~ ---- a~
C F ~
c ~, F
i~ ^-_ ~ ~ O
CL O
_ ~ Q ^O Q
v~ ~ o ~ ~ o ~ o o~ F
o ~a ~ o _
0 0 .
o ) ~
o ~ - ~
a~ ^ _ ~_, _
~ ~ E~
O --~ ~ 00 D et O O O I
~ ~ ~ -- -- -- O O O O O D ~ ~
~' ~ t~ a~ _ I~ _ co
a~ ~ ^
m ~ o ~ _ a~
Eo a~ ~ E o E
~ 3 Eo ~ ~ ~ _ _ E X x
o _ o a~ o o o o o --
c,~ ~ u~ ~o E C o O o O
:~ o o o
~ ' -- o _ ~ c
~ ~ ~ o _ o _
o E
~o_ooooo ^~o~_o--
E _ o a o ~ ~o ~ d C~
O O _ _ a~ ~ a)~ ~
C_ E c E
a~ ~o ~ o
vl _ E ~_ v~ ~)
~ _ x _ _ E
-- ~C ~ ~ ~ o ~: o a~
E ~ a- o _ c~
m ~ E -- ~ ~ c~ J N ~I ~ ~^^
~: X ~ 8 ~ ~J a~ ~
~ _ ~ 2
1339026
- 35 -
Example 21-25
The copolymer of Example 19 was melt blended with 20 percent of
the L-PLA described in Example 19. The blend is listed as Example 21
in Table 6, where its analyses and tensile properties are listed.
Example 21 was, in turn, melt blended with various amounts of the OPLA
of Example 18 and these were tested as before and listed in Table 6,
Examples 22 to 25. Table 7 lists the GPC molecular weights of these
compositions. The tensile strengths and moduli are compared to the
weight percentages of OPLA in Figures 3 and 4 (Lower Curves).
TABLE 7. MOLECULAR WEIGHTS AND GLASS TRANSITION TEMPERATURES
OF 90/10 POLYLACTIDES AND OLIGOMERIC POLYLACTIC ACID
Res.(a)
5 Example Composition, wt % Mon., GPC x IO-3(b) Tg,
Number Copolymer Oligomer % Mn M~ Mz M~/Mn C
21 100( ~ O 1.6 76 175 410 2.3 58
22 70") 30'f' 0.4 67'9) 136 299 2.0 42
23 60(e) 40~f~ 0.0 61(9) 112 211 1.8 38
24 50~') 50(f) 0.0 62(9) 114 223 1.8 35
4o~e~ 60~f~ 0.0 69(9) 120 207 1.7 35
'a' Residual monomer by TGA.
b Molecular weight by GPC.
c Glass transition temperature by DSC.
d, A blend of 90% of 90/10, L-/D,L-lactide copolymer with 10% L-Pla.
,e, Example 21.
f Example 18
;9, After blending; melt-blending on an open mill roll at 325 F.
All D,L-lactide is racemic.
o
cr~
1339026
Examples 26-30
A second series of copolymers was blended with the OPLA. A
92.5/7.5, L-/D, L-lactide copolymer was prepared by methods similar to
Examples 19 and 21. This is Example 26 of Tables 8 and 9. It was melt
blended with the OPLA of Example 18 on an open, 2-roll mill at 325 F
for approximately 20 minutes. The blends were compression molded into
3-5 mil thick films and their tensile properties and GPC molecular
weights measured. The properties are recorded in Tables 8 and 9, and
plotted in Figures 3 and 4. The second series of blends revealed
significantly higher values for the tensile properties although the
molecular weights were lower. This may be due to lower residual
lactide monomer and/or the change in high polymer composition. All of
the OPLA polylactide blends could be easily molded into tack free,
transparent films.
- 133902S
- 38 -
n _ ~ o
Z ~ ~ _ ~ E
~ m ~ ~
-- E
s ~
,_ _,~ o
m ~ CL-- ^ ^ ^ ^ ^ ~
u~ ~ o a~
._
_ ~
._ u
._ ~ G
O ~ ~O ~ ~ ~ ~ ~ -- ^
_ o o E
L~
_ ~ a~
m~ ~
I~J _ --1-- N t~) C~l ~o Lt~ O E
^ o o O O _ ~E 3
~o ~ ~ o
o ~ _ o
~C '~ E ^ c~
J O ~ O ~ E
~ 3 ~-- o o o o o o E ~ --
r~ ~_ ._ N ~ ~ m ~ a) ~ E
O O ._
~ E o
v~ a ' '-- ' '~
J o E~ o o o o o ~ v~_ .
Q >, ~ o co
m E_~
O O N 1- '--
Co ~ _
a~ ~ _ N X ~
E 5:~ ~ 1~ oo o O
~ E N N N N
X
Z ~~~~
~n o m o
133g~2~
- 39 -
TABLE 9. MOLECULAR WEIGHTS OF 9.25/7.5,
L-/RACEMIC D,L-LACTIDE COPOLYMERS
% GPC x 10-3(a)
Example No. OPLA Mn Mw Mz Mw/Mn
26 0 63 124 228 1.95
27 20 60 108 189 1.81
28 30 48 80 125 1.66
29 40 59 96 151 1.65
56 92 141 1.64
(a) GPC
Examples 31 and 32
Film specimens with, and without plasticizer were exposed to
seawater at Daytona, Florida from March through May. The pH of the
water varied from 7.3 to 7.6 and the salinity from 33.2 to 38.4 ppt.
The water gradually warmed in the test from 15 to 27 C. The specimens
were cut into strips and tensile tested before, and after, periodic
intervals in the seawater. The results are shown in Table 10. All of
the samples showed whitening and physical degradation, which became
progressive with time. Without plasticizer the samples showed
whitening and degradation after six weeks in the seawater. The OPLA
polylactide blend degraded faster, revealing clear evidence of
degradation after 3 weeks. The incorporation of 20 percent lactide
provoked immediate whitening and obvious degradation after one week of
exposure.
TABLE 10. PHYSICAL PROPERTIES AFTER SEAWATER EXPOSURE
Tensile ProPerties. 1000 ~si
Seawater
5 Example Exposure Elastic 1% Secant Yield Break Strain, %
Number Composition Weeks Modulus Modulus Strength Strength Yield Break
31 90/10 copolymer 0 305 292 -- 7.6 -- 4.7
5% L-PLA 3(b) 315 301 -- 7.1 3.1
6~C) 317 317 -- 7.3 -- 3.0
9~ ~ 228 230 -- 6.2 -- 3.0
12(e) 355 343 -- 3.9 -- 1.0
90/10 copolymer 0 275 275 -- 6.1 -- 2.0
with 10% 3(b) 291 281 -- 6.8 -- 2.9
oligomer 6(C~ 246 246 -- 3.9 -- 2.0
9( ) 211 105 2.2 1.4 3 2.0
12(e) 103 103 -- 1.7 -- 1.0
32 90/10 copolymer 0 300 298 -- 7.0 -- 3.0 0
with 1% fumaric3(b) 292 291 -- 6.5 2.5
acid 6(c) 318 318 -- 6.9 2.0
9~ ~ 226 223 -- 6.1 -- 3.
12(e) 70 122 -- 0.8 -- 1.~
9 92.5/7.5 co- 1(e) - - - - - - - - - Too brittle to test - - - - - - - - -
polymer with
20% lactide
'a' 0.5 x 5 in. strips of film, 12-17 mil; strain rate 1 in./in./min.
b 15-21 C, saline seawater, regularly exchanged.
c 20-22 C, saline seawater, regularly exchanged.
30 d 22-23 C, saline seawater, regularly exchanged. ~_~
,e, 22-27 C, saline seawater, regularly exchanged. c~
o
2~
1339026
- 41 -
The above examples establish that an all lactic acid composition
can be a pliable thermoplastic useful for flexible, plastic containers.
By way of comparison, nonplasticized homopoly (L-lactide) is a highly
crystalline polymer with a tensile strength of about 7000 psi with an
elongation of 1 percent and an initial modulus of 500,000 psi. It is
very brittle, opaque, and crazes easily. It is not a well behaved
thermoplastic, nor is it transparent. Poly (racemic D,L-lactide) is an
amorphous, glassy, polymer with a glass transition temperature of
approximately 50 C, a tensile strength of about 6300 psi, an elongation
of approximately 12 percent, and an initial modulus of 160,000 psi. It
is also very brittle although transparent. In stark contrast, a
polymer of L-lactide/racemic ~,L-lactide copolymer that is plasticized
with lactide monomer is remarkably different. For example, the
plasticized polymers can have a tensile strength of approximately 3900
psi, an elongation of 431 percent, and an initial modulus of 56,000
psi. The plasticized polymer is clear and colorless and the blend must
be heated to above 100 C to remove the plasticizer.
Although theory would predict a more amorphous structure as a
result of plasticization, what is surprising is the pliable, transpar-
ent, stable compositions that can arise, and, secondly, the nearlyexact fit of properties needed for certain packaging applications, such
as polyethylene. This invention comes at a time when there is a need
for such initial properties in a material that is slowly
environmentally biodegradable since it could alleviate plastic
pollution problems.
It will be apparent to those skilled in the art that extremely
intimate blends of high polymers and plasticizers are a rarity.
Plasticization allows a wide latitude in the initial physical prop-
erties and the time for environmental biodegradation.
The amount of plasticizer in the polymer depends on the composi-
tional characteristics desired. If lactide is used as plasticizer the
range is preferably 5 to 40 weight percent whereas if only oligomers of
lactide or lactic acid are used the range may be from 2 to 60 weight
percent. Surprisingly, oligomer may be added at up to 30 weight
1339026
- 42 -
percent without substantially affecting the tensile strength or
modulus. See Figures 3 and 4. Addition of 30 to 60 weight percent
oligomers produces significant plasticization and attenuation of
physical properties. This adds great economy to the composition since
oligomeric lactic acid is cheaper than high polymer lactic acid.
Oligomer may be prepared from lactic acid or any lactide. It is
important to note that the oligomer of lactic acid normally contains
significant amounts of lactic acid unless removed. This is an
important consideration in tailoring compositions having specific
properties. Those skilled in the art and knowing the teachings of this
invention will be able to select reaction conditions to obtain
appropriate chain lengths for the polymer, and the proportions of
polymer and plasticizer so as to obtain fabricated compositions having
physical properties similar to commonly used packaging thermoplastics
and yet degrade comparatively rapidly. For example, higher amounts of
plasticizer result in polymers having increased flexibility and
increasingly tough physical properties, however, an increasing
degradation rate will also be obtained. Further, shorter chain lengths
for the polymer will require less plasticizer to obtain the same
properties as with longer lengths.
Further provided by the first embodiment of the invention is a
process for producing a environmentally biodegradable composition that
is a plasticized polymer of polylactic acid having the formula (I).
The process comprises preparing one or more lactide monomers and
catalyst; polymerizing the monomers to form a polymer at a temperature
sufficiently low to allow the polymerization reaction to be stopped
prior to complete polymerization; monitoring the level of monomer to
determine the amount of remaining monomer; and stopping the reaction
prior to complete polymerization at a determined amount of monomer so
that unreacted monomer of a predetermined amount is trapped in
association with the polymer. The lactide monomers of the process are
selected from the group consisting of D-lactide, L-lactide, meso D,L-
lactide, racemic D,L-lactide, and mixtures thereof. Optionally one may
incorporate additional plasticizer into the polymer whereby the
plasticizer is further selected from the group consisting of L-lactide,
- 43 - 133.9026
D-lactide, racemic D,L-lactide, meso D,L-lactide, lactic acid,
oligomers of lactic acid, oligomers of lactide, and mixtures thereof.
Preferably polymerization of the monomers is at a temperature less than
129 C. Further processing of the plasticized polymer into a final
product is preferably at a temperature sufficiently low to retain the
plasticizer in the polymer. This temperature may be above 129 C. If
additional monomer and/or oligomer are added the retention of monomer
is of course not as critical.
Further provided by the first embodiment of the invention is a
process for producing a polymer of formula I comprising preparing one
or more lactide monomers and catalyst; polymerizing the monomers to
form a polymer; and incorporating plasticizer in a separate step into
the polymer whereby the plasticizer is selected from the group
consisting of D-lactide, L-lactide, D,L-lactide, oligomers of lactic
acid, and mixtures thereof.
The compositions of the invention should have a tensile strength
of 300 to 20,000 psi, an elongation to failure of 50 to 1,000 percent
and a tangent modulus of 20,000 to 250,000 psi. Preferably for a
polyolefin replacement the compositions have a tensile strength of at
least 3000 psi, an elongation to failure of at least 250 percent, and a
tangent modulus of at least 50,000 psi.
The homopolymers and copolymers of the present invention are
insoluble in water but upon constant contact with water are slowly
degradable. However, degradation is fast when compared to polyolefin
compositions that are replaced by the invention. Thus, throwaway
objects made from the polymers are environmentally attractive in that
they slowly degrade to harmless substances. If objects made from
polymers of the invention are incinerated, they burn with a clean, blue
flame.
Yet further provided by the first embodiment of the invention is a
method for replacing a thermoplastic composition with the
biodegradable composition of the invention where the thermoplastic
composition comprises first orientable polymer units, by replacing the
first polymer units with a second orientable polymer having an
35 unoriented tensile strength of 300 to 20,000 psi, an elongation to
1339026
- 44 -
failure of 50 to 1,000 percent, and a tangent modulus of 20,000 to
250,000 psi; wherein the second polymer comprises polylactic acid units
of the structure in formula I, wherein n is the number of repeating
units and n is an integer, 150 < n < 20,000 and is plasticized with a
plasticizer selected from the group consisting of lactide, oligomers of
lactic acid, oligomers of lactide and mixtures thereof. The method is
useful for polyolefin compositions and particularly polyethylene and
polypropylene as well as polyvinyl chlorides and polyethylene
terephthalate. In addition to the above list the method is useful for
replacement of polymers of styrene, vinyl acetate, alkyl methacrylate,
alkyl acrylate. It is understood that copolymers made from mixtures of
the monomers in the listed group and physical mixtures of the polymers
and copolymers of the above group are likewise replaceable.
Second General Embodiment
The environmentally biodegradable compositions disclosed as a
second embodiment herein are completely degradable to environmentally
acceptable and compatible materials. The intermediate products of the
degradation: lactic acid is a widely distributed naturally occurring
substance that is easily metabolized by a wide variety of organisms.
Its natural end degradation products are carbon dioxide and water.
Contemplated equivalents of these compositions such as those that
contain minor amounts of other materials, fillers, or extenders can
also be completely environmentally degradable by proper choice of
materials. The compositions herein provide environmentally acceptable
materials because their physical deterioration and degradation is much
more rapid and complete than the conventional nondegradable plastics
that they replace. Further, since all or a major portion of the
composition will be polylactic acid, and/or a lactic acid derived
lactide or oligomer, no residue or only a small portion of more slowly
degrading residue will remain. This residue will have a higher surface
area than the bulk product and an expected faster degradation rate.
The preferred composition of the present invention comprises poly-
merized lactic acid units with the repeating unit of formula I, wherein
n is an integer with a value between 75 and 10,000 and the alpha carbon
133902C
- 45 -
is a random mixture of D and L (or R and S) with a preponderance of one
of the pure enantiomeres. When n is low, the polylactic acid, PLA, is
easily processible, but is considerably weaker than when n is larger.
When n is quite large, e.g., 7000 or greater, the PLA is quite strong
but difficult to injection mold. Preferably n is approximately 500 to
3000 for the best balance of melt-processibility and end-use physical
properties. The monomers are selected in L (or D)/D,L-ratios of
polymerized lactic acid or their cyclic dimer, lactide, as further
discussed below. Both lactic acid and lactide achieve the repeating
PLA unit, shown above, but lactide is preferred since it more easily
obtains the higher molecular weights necessary for good physical
properties. Since lactide, which has the structure:
ICH3
0 - C - H
0 = C / - /C = 0
H-C 0
CH3
has two alpha carbons which are assymetric, there are three types of
lactide, viz., D,D- (or D-); L, L- (or L-); and meso D,L-lactide.
D-lactide is a dilactide, or cyclic dimer, of D-lactic acid.
Similarly, L-lactide is a cyclic dimer of L-lactic acid. Meso D,L-
lactide is a cyclic dimer of D- and L-lactic acid. Racemic D,L-lactide
comprises a 50/50 mixture of D-, and L-lactide. When used alone
herein, the term "D,L-lactide" is intended to include meso D,L-lactide
or racemic D,L lactide. The term dispersed as used herein means the
material is homogeneously and intimately mixed with the polymer.
Pure L-PLA has poor processing characteristics, easily crazes and
becomes opaque. Pure D,L-PLA processes easily but is not sufficiently
rigid to be an adequate OPS offset. The copolymer ratio of between
85/15 to 95/5, and preferably 90/10, L-lactide/D,L-lactide is a
preferred embodiment of the invention. At higher ratios than 95/5, the
copolymer is difficult to thermoform without crazing and easily becomes
opaque at room temperature. At lower ratios than 85/15, the lactide
copolymers exhibit lower than desirable moduli for OPS offsets. In
1339026
- 46 -
between these limits the copolymers are quenched from the melt in
typical manufacturing/processing equipment of plastics technology to
achieve films and moldings which are clear, colorless, and extremely
rigid. Their properties as formed, above, are closely matched to those
properties of an OPS.
Another advantage of this invention is that the all-lactic acid
copolymer can utilize inexpensive feedstocks. Corn syrup via starch
and corn can be fermented to either L- or racemic D,L-lactic acid,
depending on the microorganism. Racemic D,L-lactic acid is cheaply
obtainable via ethylene which can be oxidized to acetaldehyde, which is
reacted with hydrogen cyanide to form lactonitrile, which is hydrolyzed
to racemic D,L-lactic acid. Lactide is simply obtained by distillation
of lactic acid. No change of the stereochemistry of the asymmetric
carbon occurs in transforming lactic acid to lactide by ordinary
distillation/condensation methods.
While the reaction of L-lactide and D,L-lactide is discussed
herein, it is to be understood that the reactions specifying L-lactide
may also use D-lactide. Thus the reaction of D-lactide and D,L-lactide
according to the method described herein will give an equivalent
product; the only difference being that it rotates light in a different
direction.
The copolymers of the present invention are preferably formed by
heating the mixture of monomers to form a homogeneous melt and adding a
catalyst to cause the lactides to undergo a ring-opening polymeriza-
tion. The polymerization is preferably carried out in an inert,anhydrous, atmosphere, such as nitrogen or argon, or in a vacuum.
Suitable catalysts include divalent metal oxides and organo-metallic
compounds such as stannous octoate, zinc acetate, cadmium acetate,
aluminum acetate or butanoate, tin chloride, tin benzoate, and antimony
oxide. Stannous octanote is the preferred catalyst because of its high
solubility in the monomers, ease of preparation in anhydrous form, and
low toxicity. The amount of catalyst required can vary from approxi-
mately 0.02 to 2 percent by weight, based on monomers and is
preferably about 0.2 percent. The molecular weight and melt vis-
cosities of the copolymers are controllable by the amount of catalyst
1339026
- 47 -
andtor chain-transfer agents such as glycolic acid. The reaction
temperature of the polymerization is between approximately 100 to 200
C. The least color formation occurs below 140 C and the rate of
polymerization is best above 135 C. Since racemic D,L-lactide melts at
127 C it is best for conversion of monomer to polymer to polymerize at
a temperature above 127 C.
Where clarity and transparency is required, as with OPS offsets,
the copolymers of this invention are polymerized in an inert atmosphere
above their melting points, which are generally in the 125 to 150 C
range. The molten lactide copolymer can be extruded from the polymer-
izer in strands and rods, quenched, pelletized and stored in bags for
use in subsequent molding and extrusion operations.
Similarly, clarity of thermoformed packaging films and shaped
articles is achieved by molding and extruding above the copolymer's
melting points and fast cooling the fabricated item. Thereafter, the
copolymers remain transparent unless heated for several hours above
their glass transition temperature, Tg, and below the melting point,
Tm. Slow cooling of thermoformed sheets, slabs, films, and molded
items can induce spherulite crystallinity in the copolymers which gains
improvement in the heat stability of the fabricated item, but causes
some loss of transparency. Nucleating agents such as sodium benzoate,
calcium lactate, and the like, can also induce rapid and substantial
crystallinity. A modest amount of drawing of the copolymer, between
its Tg and Tm, induces orientation of the polymer molecules and can
substantially improve physical properties without loss of transparency.
Blending of different types of lactide polymer or copolymer can
substantially change the physical properties. As an example, the melt-
blending of the high-melting L-lactide polymer with a lower melting
lactide copolymer can provide a transparent material which has a
sufficient amount and type of crystallinity to remain transparent.
Those skilled in the art will recognize that transparency in molded
films, great stiffness, elevated heat distortion temperature, thermo-
processibility, and environmental biodegradability are a rare
combination of properties. Thus, the polymers can be blended, as well
as nucleated, oriented, and controlled by molecular weight to provide a
1339026
- 48 -
great deal of latitude in the processibility and final properties in
the final compounded thermoplastic.
The copolymers of the present invention will hydrolyze back to
lactic acid in the presence of moisture. In the presence of ambient
air and humidity the hydrolysis becomes evident in about 12 to 18
months time. The copolymers then become sticky, somewhat opaque, and
very brittle. When immersed in water the copolymers show obvious
hydrolysis effects in 1 to 4 months time, depending on the composition,
molecular weights, the ambient temperature, their surface-to-volume
ratio, and the particular, aqueous environment the copolymers are
placed in. Microorganisms can further reduce the lactic acid to carbon
dioxide and water. As an approximate measure, the copolymers have a
shelf life of several months, but disappear within about a year when
thoroughly wet.
The following examples are merely illustrative of the present
invention. In Examples lB to 7B, a composition series was prepared and
evaluated. It was discovered, in contrast to the prior art, that there
are distinct differences in the processing behavior and physical
properties of the L-lactide/D,L-lactide copolymers.
Example 1B
In a dry, 500 ml, round-bottom flask was charged 160 g of L-
lactide (Purac, Inc., "triple-star" grade) and 40 g of racemic D,L-
lactide (Purac, Inc., "triple star" grade). This mixture was heated
for approximately 1 hour at 123-129 C under a stopper with a continuous
nitrogen purge through a stopper inlet and outlet. The monomers form a
clear melt, which is mixed thoroughly by swirling the melt. Catalyst
solution was prepared and dried by azeotropic distillation, that is, 10
ml of stannous octoate (Polysciences, Inc.) was dissolved in 60 ml of
toluene; 10 ml of toluene, with trace water, was distilled to a Dean-
Stark trap that was vented via a drying tube. A 0.20 ml quantity ofthe stannous octoate solution was pipetted into the melt and mixed
thoroughly. The nitrogen sweep continues and the melt becomes
increasingly viscous over the next 3 hours. Heating continues at 123-
127 C for 20-24 hours. The mixture was allowed to cool to room
- 1339026
- 49 -
temperature and the flask cooled further with liquid nitrogen behind a
shield. The glass shatters and is removed from the polymer by tapping.
The copolymer is clear and colorless and is evaluated in a series of
tests shown in Table lB. Films were compression molded at 170 C in a
heated hydraulic press for later tensile testing. Slabs, 1/8 inch
thick were molded for impact testing by notched Izod, ASTM, D256 and
heat deflection temperature, ASTM, D648. Glass transition temperature
(Tg) and melting point (Tm, center of the endotherm) were evaluated by
differential scanning calorimetry (DSC).
Examples 2B-7B
The procedures of Example lB were repeated except that the ratio
of L- and racemic D,L-lactide were changed as shown in Table lB with
the test results. The pure L-lactide polymer, Example 7B, would not
always mold well at 170 - 200 C since it frequently crazed badly on
cooling in the mold. Frequently, on cooling, it opacified.
- 1339026
.
- 50 -
o m ~,
o ~ cr ~ ~ I ~
o
c ~,
o
O N ~
C C
,_ O --I E
O o U
t~ E
c~
C~ O
J _ el ~ o ~ o -- II ~ ~ C
æ
O _ ~
C
_ .~ ~,r~,~, O ~ ~ O
a~ ~o ~ _~ ~., c
O ~
a~ ~ ~ c
J ~ a~ 0 ~ I~ c ~
C~J IS) C~ ~ N~ E
O a~ ~ ~ o ~ O Lo
~ L.
O ~ ~ O
.~ N ~-- C~ II I ~ I O ~ CJ~
O a~ O, ~ C`~ _ E---
C~ C _ ~ ~
~ C E '~ ~ ----
U ~ _ ~ ..~. ",
, ~ ._ ~ L. 01Cn~ o~ ~ ~ C 01 ~
LlJ c J ~ ._ ~IJ~ C 1-- _ u C
a I ~ E ~ ~~ ~ - ~ ~ ~
ClI ~ ~ U~ C ~ C ~ ~ ~ Q~ _
~_~ J ~ o C-- ~ o o ~ ~ s
' -- E n~ c_ _ ,~ ,,"_
J ~ L. _ _ ~ _ ~--- O O ---- ~ ~ '--
o _ ~ s~ v~ 7 E _ o o u ~ L- O _
~ _ _1-- _ ~ ~ c ~ _8 o o ~ 2
E ~ ~ L. _ c
O ~: E o Ev~ o O CJ~o ~ I
~ ~ -- -- c o -- c o o ~ . - . .___
x ~ ~ ~ o ~ ~ o ~ ~c ~ ~E
o u7 o
C~J
1339026
- 51 -
Example 8B
Similar to Examples 4B and 5B, a 90/10 weight ratio copolymer of
L-lactide/racemic D,L-lactide was prepared. Into a dry, nitrogen-
swept, 2-liter flask was placed 1045.8 9 L-lactide and 116.4 g of
racemic D,L-lactide. A 1.0 ml quantity of anhydrous stannous octoate
(0.2 ml per ml of toluene) solution was added. The flask was swept
with nitrogen overnight, then heated in a 141 C oil bath until the
monomers are melted and well mixed, and the heating decreased slowly to
125 C and continued for 72 hours. The polymer slowly whitens on
cooling. After removing the glass, the cloudy, colorless, glassy
copolymer was evaluated. Gel permeation chromatography obtains a
weight-average molecular weight (Mw) of 522,000, and a number-average
molecular weight (Mn) of 149,000.
A DSC of the lactide polymer reveals a strong Tm at 145 C, see
Figure 10. The lactide polymer was melted, quenched, and examined
again by DSC to reveal no crystallization or melting points. However,
a Tg appears at approximately 50-55 C. The results show the polymer
can be crystalline or amorphous, depending on its heat history.
Examples 9B - 12B
The composition series was extended, using the procedures of
Example lB except other L- and racemic D,L-lactide ratios were used and
heating was 2 hours 125 C, 14 hours 125-147 C, then 2 hours 147-131 C.
The results are shown in Table 2B.
1339026
- 52 -
TABLE 2B. TENSILE AND MODULUS PROPERTIES OF L-LACTIDE
AND D,L-LACTIDE COPOLYMERS
Composition, weight
Ratio, L-Lactide/
D,L-Lactide
(Racemic) 70/30 60/40 20/80 0/100
Example No. 9B 10B 11B 12B
0 Color/transparency Colorless/
clear
Film thickness, mil 6-9 4-6 4-5 5-7
Tensile strength,(a)
1000 psi, ASTM
D638(a) 6.9 6.7 5.8 5.6
Elongation, % 3.2 3.0 2.7 2.8
Tangent modulus,
1000 psi 287 293 275 278
(a) Films were pulled at a jaw separation of 0.2"/min. and chart
speed of 5"/min.
1339026
The results of the above examples reveal that only certain com-
positions have the required properties for an OPS offset. The main
requirements for an OPS-like material are clarity and colorlessness,
tensile strength greater than 7000 psi, tangent modulus (a measure of
stiffness) greater than 400,000 psi and well-behaved thermoplasticity.
Table 3B lists some side-by-side comparisons of a crystal polystyrene
(OPS) and a 87.5 weight percent L-lactide and 12.5 weight percent
racemic D,L-lactide random copolymer.
TABLE 3B. PHYSICAL PROPERTY COMPARISONS
Poly(lactic acid), Crystal
Property Example 3B Polystyrene
Impact strength, notched 0.4 0.4
Izod, ft-lb/in.
Ultimate tensile 8300 7400
strength, psi
Elongation, % 6.0 4.0
Elastic modulus, psi 694,000 450,000
20 Deflection temperature, F
under load, 264 psi (a) 200
Specific gravity 1.25 1.05
Rockwell hardness (b) M75
Vicat softening point, F (c) 225
Melt flow rate, D1238(G) 40-46(d) 1.7 9/10 min.(e)
1.6 9/10 min.(f)
'a' Depends on heat history.
'b' Shore D = 97.
c DSC, Tm = 125 C (257 F) at 10 degree/min.
'd' Flow rate decreases at lower temperature.
e Listed by manufacturer.
;f; By our experiment.
-
1339o26
- 54 -
Example 13B
The copolymer of Example 2B was molded and remolded several times
to determine if color would develop in the films and the molecular
weights remained high. This determines if the copolymer can be
recycled, an important consideration for manufacturing practices. The
results of Table 4B show that the copolymer remained completely
transparent and colorless after repeated heating and molding despite
the fact that the copolymer was repeatedly exposed to air at elevated
temperatures.
TABLE 4B. EFFECT OF MOLDING ON LACTIDE COPOLYMER
Mw, Mn,
Example No. History Appearance 1000's 1000's MWlMn
Example 2B(a) Not molded, Completely 928 218 --
directly from transparent
polymerization and colorless
Example 13B(a) Ex. 2B after " 301 135 2.22
molding(b)
Example 13B(a) Ex. 2B after " 137 56.7 2.42
molding 6
times(~)
(a) 85/15, L-lactide/racemic D,L-lactide copolymer.
(b) Compression molding at 167 C (333 F) for 7 minutes to 5-mil film.
Examples 14B-18B
The copolymers of Examples 2B, 3B and 6B were compression molded
into films of approximately 20 to 30-mil thickness and were placed in a
heated Instron tester where the films were drawn 5 times their length
at 83 C at a rate of 0.5 inch per minute. The films were cooled
quickly upon removal from the Instron, and found to be approximately 5-
mil in thickness. They were clear and colorless. Tensile properties
were evaluated and are listed in Table 5B. When drawn 8 to 10 times
1339~26
- 55 -
their length, the films show evidence of crystallinity formation by
virtue of haze development and some loss of transparency.
The results demonstrate that very thin films can be made with
adequate stiffness and transparency for an OPS offset. Thus, despite
the higher density of the lactide copolymers compared to polystyrene,
less material can be used for stiff OPS offsets.
TABLE 5B. PROPERTIES OF L-LACTIDE/RACEMIC D,L-LACTIDE
COPOLYMERS AFTER ORIENTATION(a)
Composition, weight
Ratio, L-Lactide/
D, L-Lactide
(Racemic) 85/15 85/15 85/15 87.5/12.5 95/5
Example Number 14B 15B 16B 17B 18B
Film thickness, mil 5.5 5.0 6.5 5.0 4.0
Tensile strength,
1000 psi 14.0 14.7 15.0 13.0 16.0
Elongation, % 31.5 15.4 30.0 23.8 37.4
Tangent modulus,
1000 psi -- 564 419 432 513
(a) 5X oriented at 83 C using a draw down speed of 0.5
in./min. on Instron.
Example 19B
Films of the copolymers of lactide of Table lB were immersed in
water for several months interval. The copolymers remained clear for
approximately 2 months; after 3 months a slight haziness developed.
Upon setting on the shelf in humid air and with frequent handling, the
films remain virtually unchanged for approximately 1 year although
Instron data will show a slow decrease in the strength and elongation
after several months. In a landfill, the buried films disappear in 6
months to 2 years, depending on the moisture, pH, temperature,
1339026
- 56 -
composition, surface-to-volume ratio, and biological activity of the
landfill. All of the films burn with a clean, blue flame.
Example 20B
The lactide copolymer of Example 5B (quenched, compression-molded
film) was examined by DSC and found to have less than 2 percent
crystallinity, see Figure 5, in the vicinity of 130 C. A 1/8 inch
thick sample of the copolymer of Example 5B was annealed in a 185 F
oven for 16 hours. The sample turned hazy and the DSC of the sample,
see Figure 7 revealed a pronounced increase in the crystallinity. The
sample showed a 264 psi heat deflection temperature (HDT) of 90 to 95
C. A similar sample without annealing exhibited a heat deflection
temperature of 50 to 55 C, which corresponds to its Tg.
Example 21B
Calcium lactate, 5 weight percent, was blended on a heated mill
roll with the lactide copolymer of Example 5B at 170 C for
approximately 5 minutes. The blend was stripped off the roll as a
sheet and examined. It was stiff, strong, and hazy. Optical
microscopy at 82X reveals heterogeneous domains in the size range of
from a few microns to 30 microns. DSC reveals a substantial increase
in crystallinity in the vicinity of 145 C, see Figure 8, which remain
on quenching and reheating. The results, above, comparing Examples 8B,
20B, and 21B, show that nucleating agents are more prompt and efficient
in inducing crystallinity in lactide copolymers. Nucleating agents
such as salts of carboxylic acids may be used, salts of lactic acid are
preferred.
Example 22B
In a 500-ml, 3-neck, round bottom flask, equipped with a mechan-
ical stirrer and a nitrogen inlet and outlet, was placed 180.7 g of L-
lactide and 40.2 9 of racemic D,L-lactide (both Boehringer and
Ingelheim, grade S). The contents of the flask were heated to 110 C
under a nitrogen sweep to melt the lactides and 20.1 9 of polystyrene
(Amoco R3, melt index 3.5 9/10 min.) was added. The polystyrene
- 57 1 339 ~26
swelled highly and partially dissolved with stirring overnight while
advancing the heat to 185 C. The temperature was decreased to 141 C
and 0.2 ml of anhydrous stannous octoate solution (0.2 ml/ml of
toluene) was added. The stirrer was turned off and the lactides
allowed to polymerize at 141 C over 3 days time. The highly swollen,
polystyrene floats to the top after turning off the stirrer. The
lower, polylactide phase was cooled and examined by DSC. The sample
has a low Tg, approximately 35 C, and is otherwise lacking in apparent
temperature transitions. Compression-molded films are clear,
colorless, and very pliable. These results indicate that the
polystyrene thoroughly interrupts crystallinity formation.
Example 23B
The lactide copolymer of Example 8B was mill-roll blended with 20
weight percent of the homopolymer of L-lactide produced in Example 7B.
A sample of the homopolymer was analyzed by DSC, see Figure 11. The
blended sample was examined by DSC and found to have a Tg of 59-63 C
and strong Tm's at 150 and 166 C, see Figure 12. Films were clear to
slightly hazy, depending on their cooling rate after pressing.
Quenched samples easily crystallize on heating to approximately 80 - 90
C. As a result the heat deflection temperature of the blend is now
quite high. The blend becomes hazy at 80-90 C but does not deflect
with heat as does the unblended 90/10 copolymer. Tensile data as shown
in Table 6B were obtained on unoriented, compression-molded films and
compared to similarly obtained data for polystyrene.
1339o26
- 58 -
TABLE 6B. COMPARISON OF BLEND OF POLYLACTIDE OF EXAMPLE 23B
WITH CRYSTAL POLYSTYRENE
Example 23B(a) Crystal Polystyrene(a'b)
Film thickness, mil 8 14
Tensile strength, ASTM 7.7 6.0
D882, 1000's psi
Elongation, %, to yield 6.5 3.2
Tangent modulus, 323 267
lOOO,'s psi
(a) Thin films, unoriented, compression-molded specimens
(b) Melt Index 1.7
This example illustrates that melt blending is an excellent way to
improve the properties of the copolymer so that advantageous properties
similar to polystyrene are realized. The higher the amount of
homopolymer based on L-lactide (or D-lactide) blended with the polymer
the higher will be the heat deflection temperature, however, haziness
will also increase. Thus addition of homopolymer may be combined with
other methods of increasing polystyrene like properties while still
retaining clarity
As a further example orienting films produced from the polymer
increases the tensile properties. At eight to ten times the draw the
physical properties are still increasing but the material becomes hazy.
The degree of orientation will thus need to be controlled and combined
with the other property changing methods to achieve optimum polystyrene
like characteristics.
Examples 24B-27B
Examples 24B to 27B were polymerizations of lactide with
controlled amounts of chain transfer agents, demonstrating that
molecular weights can be controlled using transfer agents such as
glycolic acid. The results are shown in Table 7B. A nearly straight
1339026
- 59 -
line relationship exists between the amount of transfer agent and the
reciprocal of the weight average molecular weight. A preferred chain
transfer agent is lactic acid.
TABLE 7B. MOLECULAR WEIGHT CONTROL USING CHAIN TRANSFER AGENTS
Example No. PPH of(a) Mn(b) Mw(b) MwlMn
24B 0.22 13,500 107,300 8.0
25B 0.45 12,800 66,700 5.2
26B 0.90 7,300 29,900 4.1
27B 1.80 4,700 13,900 2.9
(a) Parts of glycolic acid chain transfer agent (CTA) per hundred
parts of lactide in polymerization recipe.
(b) Gel permeation chromatography in tetrahydrofuran solvent,
23 C, with 106, 105, 10 , and 103 anhstrom columns, number
average, Mn~ and weight average, Mw, molecular weights are
calculated compared to monodisperse polystyrene standards.
Example 28B
A 4.0 mil, compression-molded film of the lactide copolymer of
Example 2B was evaluated as a barrier film by ASTM methods. The
results are shown in Table 8B. The lactide copolymer is a much better
barrier to carbon dioxide and oxygen than is polystyrene. By
comparison to some other polymer barrier films, the lactide copolymer
is an adequate barrier film for many packaging applications.
1339026
- 60 -
TABLE 8B. EXAMPLE 28B PERMEABILITY T0 GASES(a)
Vinyldiene(b)
Chloride-
Lactide (b) vinyl
Copolymer, Crystal Chloride
Units Example 2B Polystyrene Terephthalate Copolymer
cc/100 sq. in./
24 hr/atmos
C2 32.1 900 15-25 3.8-44
2 19.9 350 6-8 0.8-6.9
(a) ASTM D1434-75, Example 2B was a 4.0 mil, compression-molded film.
(b) Values from Modern Plastics Encyclopedia.
Example 29B
Sheets, 1/8 inch thick of the lactide copolymers of Examples lB-6B
were immersed overnight in a mixture of petroleum ether and methylene
chloride. At ratios of 70/30 to 60/40, petroleum ether/methylene
chloride, the copolymers would foam when placed in boiling water.
Irregular, but well expanded, foams would form.
Example 30B
A comparison was made of the melt viscosities of a commercial,
crystal polystyrene (Type 201, Huntsman Chemical Corp.) and the lactide
polymer of Example 8B. The melt index, ASTM D1238 (G), of the poly-
styrene was 1.6 9/10 min. at 200 C using the standard 5 Kg weight. The
melt index of the lactide polymer was 40-46 9/10 min. under the same
conditions, however, at 160 C the value was 8.0 9/10 min. A more
detailed comparison of melt viscosities was obtained by observing the
melt viscosities of the two polymers in an Instron Capillary Visco-
meter. The comparative results are shown in Figure 9. The shear
rates normally encountered during extrusion and injection molding are
approximately 100 to 1000 reciprocal seconds. Inspection of the data
- 1339~26
- 61 -
of Figure 9 shows that the melt viscosity of the lactide polymer at 160
C is very similar to that of the polystyrene at 200 C.
The above results illustrate that lactide polymers can be melt-
processed, at lower temperatures than polystyrene, by very similar5 methods.
Examples 31B-34B
Small, test polymerizations of purified (recrystallized and dried)
mesolactide (meso D,L-lactide) were carried out as the homopolymer and
the copolymer. The molecular weights were evaluated by GPC and
compared to analogues of D,L-lactide. The results are presented in
Table 9B. The polymers were melt pressed into films and their physical
properties evaluated and compared as shown in Table 10B. Within
experimental differences of sheet thickness and molecular weight, the
copolymers are similar within experimental error. The homopolymer of
mesolactide is somewhat weaker.
TABLE 9B. GPC MOLECULAR WEIGHT COMPARISONS OF MESO-AND
RACEMIC LACTIDE POLYMERS AND COPOLYMERS
GPC x 10 3
Res.
Example Nos. Composition Mon., % Mn Mw Mz MwlMn
31B D,L-PLA -- 97.5 341 757 3.49
32B Meso PLA 2.76 62.5 152 264 2.42
33B 90/10, L-/meso 1.67 29 142 301 1.67
34B 90/10, L-/D,L -- 91.3 201 350 2.20
1339026
- 62 -
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- 63 -
An overall description of the composition of the second embodiment
of the invention includes an environmentally biodegradable polymer
comprising polymerized lactic acid units of the structure of formula I,
where n is an integer between 75 and 10,000 and the alpha carbon is a
mixture of L- and D-configurations with a preponderance of either D- or
L-units, wherein the polymer is suitable for replacement of
polystyrene. The D- and L-units of the polymer may preferably be
prepared from L-lactide or D-lactide, at 85 to 95 parts by weight, and
D,L-lactide at 15 to 5 parts by weight.
An environmentally biodegradable composition with improved
properties more like those of polystyrene comprises blends of a
physical mixture of polymerized lactic acid units of the structure of
formula I, where n is an integer between 75 and 10,000 and the alpha
carbon is a random mixture of L- and D-configurations with a
preponderance of either D- or L-units, a lactide hompolymer of D-
lactide or L-lactide. Compositions having n equal to an integer
between 150 and 10,000 have a good balance between strength and melt
processability.
A general description of the process for producing the composition
of the second embodiment comprises mixing with a catalyst, heating, and
melting L-lactide or D-lactide monomer and D,L-lactide monomer whereby
the L-lactide monomer or D-lactide monomer is in an amount of 85 to 95
parts by weight and D,L-lactide monomer is in an amount of 15 to 5
parts by weight, to form an intimate solution; polymerizing the
solution; and treating the polymer to modify its properties so as to
make the polymer suitable as a replacement for polystyrene. The
properties of the composition may be adjusted by adding a nucleating
agent; adding a D-lactide or L-lactide homopolymer by blending to
produce a physical mixture; orienting the polymer; adding a nucleating
agent and a D-lactide or L-lactide polymer by blending; adding a
nucleating agent and a D-lactide or L-lactide polymer by blending and
orienting the polymer; adding chain transfer agents to the polymeriza-
tion step so as to adjust the characteristics to a polystyrene
replacement, annealing at an elevated temperature, and adding
.
1339d26
- 64 -
additional plasticizer where the plasticizer is selected from the group
consisting of D-lactide, L-lactide, meso D,L-lactide, lactic acid,
lactide oligomer, lactic acid oligomer, and mixtures thereof. If a
monomer is selected as a plasticizer a unique composition may be
obtained by adding monomer that is stereochemically different from that
used to obtain the polylactide in the composition. Similarly, addition
of oligomer stereochemically different from that which may be obtained
during polymerization of the polymer gives a unique product. Color
bodies can be excluded by performing the polymerization in an inert
atmosphere and at reaction temperatures preferably at 140 C or below.
Various combinations of the above treatments can be employed to obtain
the optimum characteristics as those skilled in the art will
appreciate, once knowing the teachings of the invention.
As can be noted in the aforementioned first embodiment, a higher
amount of monomer or oligomer can have significant effect. In the
present second embodiment lower amounts of monomer and oligomer are
preferred to impart stiffness. Plasticizer present in an amount of 0.1
to 5 percent is preferred. The composition usually contains
plasticizer in an amount that depends on polymerization conditions or
on the amount added after polymerization. The additional monomer used
as plasticizer may be selected from the group: D-lactide, L-lactide,
meso D,L-lactide, racemic D,L-lactide, and mixtures thereof. Oligomers
of lactide or lactic acid may also be added. Unique compositions may
be obtained by addition of monomer or oligomer stereochemically
different from those selected for the polymers in the composition.
Further provided by the second embodiment of the invention is a
method for replacing a thermoplastic composition with the
biodegradable composition of the invention where the thermoplastic
composition comprises first orientable polystyrene units, by replacing
the first polymer units with a second orientable polymer having an
unoriented tensile strength of at least 5,000 and a tangent modulus of
at least 200,000; wherein the second polymer comprises polylactic acid
units of the structure in formula I, wherein n is the number of
repeating units and n is an integer, 75 < n < 10,000, and the alpha
carbon is a mixture of L- and D- configurations with a preponderance of
. 133gO26
- 65 -
either D- or L- units, wherein the polymer is prepared from L- or D-
lactide at 85 to 95 parts by weight, and D,L-lactide at 15 to 5 parts
by weight and is plasticized with a plasticizer selected from the group
consisting of lactide, oligomers of lactic acid, oligomers of lactide
and mixtures thereof between 0.1 and 5.0 weight percent.
Contemplated equivalents of the compositions of the invention are
those that contain minor amounts of other materials. The compositions
produced in accordance with the present invention can be modified, if
desired, by the addition of a cross-linking agent, other plasticizers,
a coloring agent, a filler and the like.
The compositions herein can be processed by melt fabrication into
useful articles of manufacture having a self supporting structure such
as disposable containers, eating utensils, trays, plates, drinking
cups, single serving trays, syringes, medical trays, packaging films
and the like. The compositions are useful in that they can have the
characteristics of the usual polystyrenes and therefore substitute for
them yet degrade in the environment. The compositions are especially
useful for articles having only a one time use or a short life span in
use before disposal.
Third General Embodiment
A third embodiment discloses the blending of polylactic acid
(PLA) with polystyrene (PS), polyethylene (PE), polyethylene
terephthalate (PET), and polypropylene (PP). The embodiment discloses
that polylactic acid is melt compatible with these conventional
thermoplastics and the effect on their physical properties.
The environmentally degradable compositions disclosed herein are
at least partially degradable. That is the polylactic acid portion of
the composition will decompose relatively rapidly compared to the more
stable portions of the blend and cause a physical deterioration of the
blended material. For example, when the compositions are intimate and
homogeneous blends with small domain sizes the physical deterioration
will destroy the original formed product. The compositions herein
provide environmentally acceptable materials because their physical
deterioration and degradation is much more rapid than conventional
1339026
- 66 -
nondegradable plastics. Further, since a significant portion of the
composition can be polylactic acid, and/or a lactic acid derived
lactide or oligomer only a small portion of more slowly degrading
thermoplastic residue will remain (e.g. polystyrene). This residue
will have a high surface area and is expected to decompose faster than
a bulk formed product.
D-lactide is a dilactone, or cyclic dimer, of D-lactic acid.
Similarly, L-lactide is a cyclic dimer of L-lactic acid. Meso D,L-
lactide is a cyclic dimer of D- and L-lactic acid. Racemic D,L-lactide
comprises a 50/50 mixture of D-, and L-lactide. When used alone
herein, the term "D,L-lactide" is intended to include meso D,L-lactide
or racemic D,L-lactide. Polylactic acid may be prepared from one or
more of the above.
Example lC
Polystyrene was solvent blended with polylactic acid and solvent
cast from CH2Cl2 to determine optimum compatibility. The solvent cast
films were translucent and apparently "noncheesy". A sample, appears
homogeneous to the naked eye and resists folding and handling without
shredding apart. Optical microscopy at 310X reveals heterogeneous
domains of 3 microns and less. The blend is apparently very com-
patible. It exhibits no change over 2 years with regard to "blooming"
of fugitive material nor does its physical properties show evidence of
degradation.
Example 2C
Polypropylene 8525, Hercules, was similarly melt blended in the
Brabender with polylactic acid at 400 F. Ratios of PP/PLA prepared
were 100/0 for the control, 90/10, and 75/25.
Examples 3C-5C
Melt blends were prepared of polylactic acid with polystyrene.
Both a high molecular weight polystyrene (Piccolasti~, E-125, Hercules)
and a low molecular weight polystyrene (Piccolastic, D-100) were
investigated. Also used was a general purpose polystyrene, (Huntsman
1339026
67
polystyrene 208), a crystal polystyrene. These were mlxed ln
a Brabender at 325F at different ratlos with polylactic
acld.
The polystyrene/polylactic acld ratlos used were
100/0 for the control, and 90/10, and 75/25 for the Huntsman
208, general purpose polystyrene.
Examples 6C-7C
Two types of polyethylene terephthalate were used.
(Goodyear's "Clearstuff " and Eastman's Kodapak TN-0148).
These were drled overnlght at 90C and melt blended at 525F
ln a ~rabender wlth polylactlc acld for only a few mlnutes.
The polylactlc acld reduced the melt vlscoslty.
Examples 8C-16C
The controls and blends for polypropylene, general
purpose polystyrene, and polyethylene terephthalate
(Eastman's) from Examples 2C-7C were ground ln an Abbey grln-
der and compresslon molded lnto approxlmately 5 mil fllms.
Polypropylene-polylactlc acld fllms were molded at about
400F; polystyrene-polylactlc acld fllms were obtalned at 250-
300F; polyethylene terephthalate-polylactlc acld fllms were
molded at about 525F. After condltlonlng at 50 percent r.h.
and 23C for 24 hours they were tested on the Instron. The
controls were slmllarly treated. Samples of the compresslon
molded fllm were placed ln an Atlas Weather-O-Meter for
*
Trade-mark
22628-29
1~3gO26
67a
weatherabillty evaluatlon (cycles of 102 mlnutes of sunshlne
and 18 minutes of raln). The results for these Examples are
shown ln Table lC.
Examples 17C-19C
Three samples of 100 percent polylactlc acld using
poly (D,L-lactlc acid) were prepared as above but with fllm
thlcknesses of 10-15 mll. Tests were performed as ln Examples
20C-27C below except that the second sample was tested after
82 hours of exposure to 50 percent relatlve humldity at 72F.
22628-29
1339026
- 68 -
Examples 20C-27C
High density polyethylene, HDPE, (0.960 g/cc)~Lwas melt blended
with polylactic acid in the Brabender Plasticorderr~at 151 C for 10
minutes. Blend ratios of high-density polyethylene/polylactic acid of
` 5 100/0 for the controls, 90/10, 80/20, and 50/50 were used. Two samples
of each were prepared. The blends were ground in an Abbey grinder and
compression molded into 10-15 mil films. The films were tested in an
Atlas Weather-0-Meter set for 51 minutes of carbon arc light and 9
minutes of water spray. Temperature was varied from ambient to 140 F.
Tensile strengths, elongation to yield tests and classification of the
tensile failure were performed for the samples as shown in Table 2C.
Examples 28C-33C
Low density polyethylene, LDPE, (0.917 g/cc) was melt blended with
polylactic acid in the Brabender Plasticorder at 151 C for 10 minutes.
Blend ratios of low density polyethylene/polylactic acid of 100/0 for
the controls 90/10 and 50/50 were used. Two samples of each were
prepared. The samples were treated and evaluated as in the case of
Examples 20C-27C. Results are shown in Table 2C.
Example 34C
In a 500-ml, 3-neck, round bottom flask, equipped with a mechan-
ical stirrer and a nitrogen inlet and outlet, was placed 180.7 9 of L-
lactide and 40.2 9 of racemic D,L-lactide (both Boehringer and
Ingelheim, grade S). The contents of the flask were heated to 110 C
under~ nitrogen sweep to melt the lactides and 20.1 9 of polystyrene
(Amoco~vR3, melt index 3.5 9/10 min.) was added. The polystyrene
swelled highly and partially dissolved while stirring overnight and
advancing the heat to 185 C. The temperature was decreased to 141 C
and 0.2 ml of anhydrous stannous octoate solution (0.2 ml/ml of
toluene) was added. The stirrer was turned off and the lactides
allowed to polymerize at 141 C over 3 days time. The highly swollen,
polystyrene floated to the top after turning off the stirrer. The
lower, polylactide phase was cooled and examined by differential
scanning calorimetry (DSC). The sample has a low Tg, approximately 35
~h~
1339026
- 69 -
C, and is otherwise lacking in apparent temperature transitions.
Compression-molded films are clear, colorless, and very pliable. These
results indicate that the polystyrene thoroughly interrupts
crystallinity formation under these conditions.
Example 35C
Polylactic acid was mill roll blended with crystal polystyrene.
The blend revealed excellent compatibility of polystyrene dispersed in
polylactic acid. Thus 5 weight percent of polystyrene was dispersed in
a 90/10 ratio of L-/racemic D,L-lactide copolymer in a two roll mill at
170 C. The material became hazy and exhibited considerable
crystallinity by thermal analysis. This example demonstrates that
under these conditions polystyrene easily induces crystallinity in
polylactic acid. A thermal analysis of the material, see Figure 13,
reveals that the material remains crystalline even when heated and
cooled.
The Examples 34C and 35C illustrate that polylactic acid blended
with the environmentally nondegradable plastics herein can produce
final properties in the mixture depending on the mixing or blending
technique used.
Brabender melt-blends of all types exhibited small heterogeneous
particle sizes of 10 microns or less. The tensile strengths were
evaluated before, and after, simulated weathering. After 1248 hours
(52 days) in the Atlas Weather-0-Meter all of the polypropylene samples
were whitened, extremely brittle and were not able to be tested. The
polypropylene samples were retested at shorter intervals as shown in
Table lC. At approximately 300 hours of weathering in the Atlas
Weather-0-Meter, the samples exhibited significant environmental
degradation.
The polystyrene blends with polylactic acid exhibited
environmental degradation that was apparent after 300 hours of
simulated weathering. The polyethylene terephthalate blends were also
visibly environmentally degraded in approximately 300 hours.
I339026
Table lC
TENSILE STRENGTH OF FILMS BEFORE, AND AFTER
ACCELERATED WEATHERING(a)
Tenslle Strenqth(b)/% Elonqatlon
Blend Ratlo After,hrs
and Materlal Before 310 400
100/0 PP/PLA(C) 1665/61585/1.6 4g4/1.7
90/10 PP~PLA 1568/51954/3.2 346/--
75/25 PP/PLA 1124/14370/1.1 254/1.0
100/0 PP/PLA(d) 3200/2.01066/1.0 --
90/10 PS/PLA 2350/2.0582il.0 --
75/25 PS/PLA 1493/1.6484/1.0 --
100/0 PET/PLA(e) 3036/--3509/3.0 --
90/10 PET/PLA 2147/--1378/3.0 --
75/25 PET/PLA 2743/--2041/3.0 --
a) Weather-o-meter, cycle of 102 mlnutes of sunshlne, 18
mlnutes of raln.
b) 0.05 ln./mln., on the Instron .
c) Hercules polypropylene 825.
d) Huntsman 208.
e) Tennessee Eastman, Kodapak TN 0148.
The polylactlc acid, high denslty polyethylene, low
denslty polyethylene, and thelr blends were evaluated for
physlcal strength, before, and after slmulated weatherlng and
the results are 8hown ln Table 2C.
Trade-mark
22628-29
' ~ .
TABLE 2C. PHYSICAL PROPERTIES OF POLYETHYLENE (PE), POLYLACTIC ACID (PLA),
AND THEIR BLENDS, BEFORE, AND AFTER, WEATHER-O-METER EXPOSURES
Material (b ( ) (d) Type of
Blend Ratio ) Weather-O-Meter c TensileElongdtion Tensile
5 Material(a) Polymer/PLAExposure, hours Strength, psito Yield, ~ Failure
100% pLA(e) 0/100 0 6,030 2.2 Brittle
100~ PLA 0/100 o(f) 5,670 2.1 Brittle
100% PLA 0/100 82 (too brittle to test) -- Brittle
100% HDPE(9) 100/0 0 3,540 8 Ductile
100~ HDPE 100/0 233 1,400 1 Brittle
HDPE/PLA 90/10 0 3,480 7 Ductile
HDPE/PLA 90/10 233 1,720 1 Brittle
HDPE/PLA 80/20 0 3,180 4 Brittle
HDPE/PLA 80/20 125 2,150 2 Brittle
HDPE/PLA 50/50 2,720 2 Brittle
HDPE/PLA 50t50 233 (too brittle to test) -- Brittle
a' Compression-molded films, 10-15 mil thickness.
b Melt-blended in Brabender Plasticorder for 10 minutes, 151 C.
c, 51 minutes of carbon arc light and 9 minutes of water spray
for each 1 hour cycle. Temperature varies from ambient to 140 F.
d' Elonqation at maximum in strain curve.
e Poly~D,L-lactic acid), { n} = 1.16 dl/g, 25 C, THF.
f After 82 hours exposure to 50% R.H., 72 F.
g High density polyethylene, density 0.960 g/cc, melt index 0.6 9/10 minutes. c~
o
TABLE 2C. PHYSICAL PROPERTIES OF POLYETHYLENE (PE), POLYLACTIC ACID (PLA),
(CONTINUED) AND THEIR BLENDS, BEFORE, AND AFTER, WEATHER-O-METER EXPOSURES
Material (b)( ) d Type of
Blend RatioWeather-O-Meter c TensileElongation( ) Tensile
5 Material(a) Polymer/PLAExposure, hours Strength, psito Yield, % Failure
100% LDPE(h) 100/0 0 1,320 80 Ductile
100~ LDPE 100/0 125 1,250 67 Ductile
LDPE/PLA 90/10 0 1,190 31 Ductile
LDPE/PLA 90/10 125 855 14 Ductile
LDPE/PLA 50/50 0 1,160 4 Ductile
LDPE/PLA 50/50 125 (too brittle to test) -- Brittle
a' Compression-molded films, 10-15 mil thickness.
b Melt-blended in Brabende-r Plasticorder for 10 minutes, 151 C.
c, 51 minutes of carbon arc light and 9 minutes of water spray
for each 1 hour cycle. Temperature varies from ambient to 140 F.
d) Elongation at maximum in strain curve.
h) Low density polyethylene, density 0.917 g/cc, melt index 0.25 9/10 minutes.
1~9026
- 73 -
The polylactic acid and its blends were much more environmentally
degradable than the pure low density or high density polyethylene. The
high density polyethylene samples degraded substantially without weight
loss while the high density polyethylene-polylactic acid blends
exhibited weight loss, particularly where microscopy revealed
polylactic acid was exposed at the surface of the films. The high
density polyethylene degraded by exposure to actinic light as shown by
microscopy.
With all of the samples, increasing the percentage of polylactic
acid decreased the tensile strength before, and after, simulated
weathering. The incorporation of polylactic acid introduced a faster
degradation in blends of polypropylene, polystyrene, polyethylene
terephthalate, and high and low density polyethylene. Presumably, the
actinic light as well as hydrolysis of the polyesters degrades the
polymer. The small size of the spherical, microheterogeneous, domains
of the blend are undoubtedly polylactic acid, which is mostly buried.
Therefore, polylactic acid hydrolysis is slow. Faster degradation via
hydrolysis can be achieved by controlling the location of the
polylactic acid. This, in turn, is related to the rheology of the
blend during melt blending. The small size of the dispersed, heteroge-
neous domains indicates good compatibility of the mixed polymers.
In a simulated landfill, where light is excluded, the controls and
the blends show much slower rates of degradation. With hydrolysis,
alone, the polylactic acid samples slowly whiten, while the blends are
qualitatively unchanged for the time period tested.
Conversely, addition of minor amounts of nondegradable
thermoplastics to polylactic acid to form compatible blends, using, for
example, polypropylene, polystyrene, polyethylene terephthalate and
high and low density polyethylene will retard the degradation rate of
the polylactic acid. A preferred compositional range is from 80-99
weight percent polylactic acid.
A general description of the environmentally degradable
composition comprises blends of a physical mixture of polylactic acid
(polylactide), and a polymer selected from the group consisting of a
polymer of ethylene terephthalate, a polymer or copolymer of styrene,
1339026
- 74 -
ethylene, propylene, vinyl chloride, vinyl acetate, alkyl methacrylate,
alkyl acrylate, and physical mixtures thereof. Other possible
compositional blends are listed below in the discussion of process
embodiments of the invention.
The blends preferably use a physical mixture of polylactic acid of
formula I, where n is an integer between 75 and 10,000; and a polymer
selected from the group consisting of polystyrene, polyethylene,
polyethylene terephthalate, and polypropylene and other compositions
further discussed below. A preferred composition is that where the
polylactic acid comprises 5 to 50 weight percent of the composition. A
preferred composition has a polylactic acid content of 10 to 20 weight
percent.
The polymers selected from the group above, deemed the added
polymer, can be used alone or in combination. The group is not
restricted to those cited above since other polymer types are noted as
compatible with polylactic acid. These include the polymers and
copolymers comprised from the group of ethylene, propylene, styrene,
vinyl chloride, vinyl acetate, alkyl methacrylates, and alkyl
acrylates. It should be understood that the term copolymers as used
herein includes polymers made from mixtures of the monomers in the
listed group. Physical mixtures of the polymers and copolymers of the
above group are likewise useful in the invention.
The third embodiment further provides for a process for producing
the composition includes providing a polylactic acid; selecting a
polymer from the group consisting of a polymer of ethylene
terephthalate, a polymer or copolymer of styrene, ethylene, propylene,
vinyl chloride, vinyl acetate, alkyl methacrylate, alkyl acrylate, and
physical mixtures thereof; and blending the polymers. The blending may
be by melt blending on a mill roll or by compounding in an extruder or
by other mechanical means. The polylactic acid provided preferably has
the formula I.
It further provides for a process for producing the composition
of the invention includes providing a lactide selected from the group
consisting of D-lactide, L-lactide, meso D,L-lactide, racemic D,L-
lactide, and mixtures thereof; selecting a polymer from the group
1339~26
- 75 -
consisting of the polymers or copolymers of styrene, ethylene,
ethylene terephthalate, propylene, vinyl chloride, vinyl acetate, alkyl
methacrylate, alkyl acrylate, and physical mixtures thereof. The
selected lactide and polymer are mixed and heated to melt the lactide
and at least partially dissolve the polymer. Finally, the lactide, is
at least partially polymerized to obtain a blend of polylactide,
unpolymerized lactide monomer and the selected polymer. The
polymerization is preferably controlled by monitoring the amount of
lactide remaining and stopping the polymerization at the desired level.
If desired, the polymerization can be carried to completion.
Additional lactide monomer, lactic acid, lactide oligomer, lactic acid
oligomer, and mixtures thereof in pliable producing amounts can be
added as plasticizers to obtain desired characteristics as taught in
the first general embodiment.
It will be obvious to those skilled in the art that the
proportions of polylactic acid and the added polymer can vary widely
depending on their mutual solubilities. Solubilities, in turn, vary
with the thoroughness of mixing and the mixing temperature. While
placing both the polylactic acid and the added polymer into a mutual
solvent solution will obtain intimacy, the use of solvent is
impractical for many commercial processes. Physical mixing, such as
melt blending on a mill-roll or extruder is more practical, but must be
controlled to achieve intimacy, that is, high shear is required to
achieve the desired intimacy. Even with intimate mixing different
polymers may not be compatible, that is, they may still separate into
relatively large heterogeneous domains of, for example, 10 to 100
micron size, or larger. This results in a "cheesy" mixture, or a blend
with poor properties. What is surprising is that polylactic acid is
easily blend compatible with a wide variety of other polymers,
including both polar and nonpolar polymers.
The temperature of the melt blending of the polylactic acid with
other polymers may be varied to adjust the proportions of the
polylactic acid with one, or more, added polymers. At lower
temperatures, the solubilities may not be adequate, while too high a
temperature will cause decomposition of the mixture. A general
1339~26
- 76 -
temperature range is 100 - 220 C, and the preferred range is 130 - 180
C. Equally significant is the melt viscosities of the different
polymer components. With increasing molecular weight, the viscosities
increase sharply. By controlling the proportions of the polylactic
acid and the added polymer, or polymers, the temperature, the mixing
type and time, and the molecular weight, a wide range of mixtures can
be obtained. Thus, for example, the polylactic acid can be dispersed
into the added polymer, or polymers, or vice versa, and the size and
geometry of the dispersed phase varied greatly, ranging from discrete
spheres to strands of different diameters or lengths. This results in
a wide latitude of physical properties and degradation times in the
environment. The weight percent ratio of polylactic acid to the
selected polymer can be between 99:1 to 1:99.
Where the lactide monomer is used to dissolve the added polymer
and the lactide is subsequently polymerized, the temperature of mixing
and polymerizing must be balanced between the mutual solubilities and
the reactivity of the lactide. Higher temperatures generally produce
lower molecular weight polylactic acid. A further embodiment of the
invention is to mix at one temperature and polymerize at another
temperature to achieve variations in the geometry of the dispersed
phase, as discussed above.
The compositions herein can be processed by melt fabrication into
useful articles of manufacture having a self supporting structure such
as disposable containers, eating utensils, trays, plates, drinking
cups, single serving trays, syringes, medical trays, packaging films
and the like. The compositions are useful in that they can have the
characteristics of the usual plastics and therefore substitute for them
yet degrade in the environment. The compositions are especially useful
for articles having only a one time use or a short life span in use
before disposal.
Fourth General Embodiment
Within the fourth embodiment of the invention are included those
impact modifiers which are elastomeric and melt compatible with
polylactic acid. By "melt compatible", it is meant all those polymers
133902~
- 77 -
which can be intimately mixed with polylactic acid as discussed in the
third general embodiment. The mix would result in a substantially
homogeneous blend.
The environmentally degradable compositions disclosed herein are
at least partially degradable. That is the polylactic acid portion of
the composition will decompose relatively rapidly compared to the more
stable portions of the blend and cause a physical deterioration of the
blended material. For example, when the compositions are intimate and
homogeneous blends with small domain sizes the physical deterioration
will destroy the original formed product. The compositions herein
provide environmentally acceptable materials because their physical
deterioration and degradation is much more rapid than conventional
nondegradable plastics. Further, since a major portion of the
composition will be polylactic acid, and/or a lactic acid derived
lactide or oligomer only a small portion of more slowly degrading
elastomer residue will remain (e.g. segmented polyester). This residue
will have a high surface area and is expected to decompose faster than
a bulk formed product.
The examples below show the blending of polylactic acid (PLA) with
a Hytrel~, a segmented polyester which is a block copolymer of hard
crystalline segments of polybutylene terephthalate and soft long-chain
segments of polyether glycol. It is shown that polylactic acid is melt
compatible with this elastomer and the effect on its physical
properties.
D-lactide is a dilactide, or cyclic dimer, of D-lactic acid.
Similarly, L-lactide is a cyclic dimer of L-lactic acid. Meso D,L-
lactide is a cyclic dimer of D- and L-lactic acid. Racemic D,L-lactide
comprises a 50/50 mixture of D-, and L-lactide. When used alone
herein, the term "D,L-lactide" is intended to include meso D,L-lactide
or racemic D,L-lactide. Polylactic acid may be prepared from one or
more of the above.
Example lD
A polylact;de copolymer without Hytrel~ segmented polyester was
prepared using the procedure from Example lB of the second general
- 1339026
- 78 -
embodiment of Serial No. 229,939 and tested for Izod impact strength.
Results are shown in the Table lD. For further comparison, Table lB of
the second general embodiment lists the Izod impact strength of other
ratios of L-lactide to D-L lactide.
Example 2D
Into a 3-neck, 250 ml, round-bottom flask is weighed 10.96 g of
D,L-lactide, 108.86 9 of L-lactide, and 5.27 9 of Hytrel~ 4056
segmented polyester (Du Pont, a thermoplastic elastomer). Hytrel~ 4056
segmented polyester is a polyester elastomer with a Shore D durometer,
low flexural modulus, high melt viscosity, a melt index of 7, a sp. gr.
of 1.17, a m.p. 334 F, a vicat softening temperature of 234 F, and an
extrusion temperature of 340 F - 400 F. The flask is fitted with a
mechanical stirrer and a nitrogen inlet and outlet. The contents are
heated by means of an oil bath. The Hytrel~ segmented polyester dis-
solves in the molten lactides at 170 C. A catalyst solution isprepared by dissolving 10 ml of stannous octoate in 60 ml of toluene
and distilling 10 ml into the toluene. A 100 microliter portion of the
catalyst solution is injected into the solution of lactide and Hytrel~
segmented polyester. The mixture is stirred under nitrogen at 155 C
for approximately 64 hours.
The viscosity increases sharply and the mixture turns cloudy. The
product is tough and opaque. Films of 8-9 mil thickness were compres-
sion molded at 155 C and the tensile properties measured, as shown in
Table lD.
Slabs, 1/8 inch thick, were compression molded and their Izod
impact strength measured using a 2 pound pendulum. The results are
recorded in the Table lD where the data are compared to a similar
polylactide copolymer of Example lD without Hytrel~ segmented
polyester, and ~ data for so-called medium-impact polystyrene, Example
7D.
Example 3D
800.0 9 of L-lactide and 202.3 9 of racemic D,L-lactide are
copolymerized using 1.0 ml of the catalyst solution by methods similar
1339026
- 79 -
to Example 2D, omitting the Hytrel~ segmented polyester. The lactide
copolymer is clear and colorless. In a separate polymerization 104.0 9
of L-lactide is melt polymerized using 100 microliters of catalyst.
The polymer (L-PLA) is white, crystalline, and crazes easily when
struck.
An electrically-heated, 2-roll mill is heated to 375 F, then 8.4 9
of Hytrel~ segmented polyester and 19.2 9 of L-PLA are banded on the
roll. To this was added 172.4 of the lactide copolymer. The mixture
blends easily and is removed from the rolls, molded, and tested as in
Example 2D. The data are recorded in Table lD.
Example 4D
The lactide copolymer of Example 3D, 80 9, the L-PLA of Example
3D, 10 9, and 10 9 of Hytrel~ 4056 segmented polyester are 2-roll,
mill-blended as described previously in Example 3D. The blend was
tested as before and the data are recorded in Table lD.
Example 5D
100 9 of the blend of Example 3D was further blended with 20 9 of
Hytrel~ 4056 segmented polyester. The mixture easily mixed on the roll
and was apparently quite compatible. The physical properties were
measured as described previously and recorded in Table lD.
Examples 6D and 7D
Typical crystal polystyrene and medium-impact polystyrene were
tested and used for comparative controls.
The above results clearly indicate that polylactides can be
impact-modified. The blends provided significantly higher Izod impact
strengths than the crystal polystyrene control and gave slightly lower
or equivalent impact strengths compared to medium-impact polystyrene.
Those skilled in the art will recognize that the data on impact-
strength in Table 1~ can be improved further by optimizing the amount
and type of impact modifier.
- 1339026
- 80 -
Since polylactides have been shown previously in the third general
embodiment above, to be blend-compatible with numerous other compounds
and thermoplastics, the process of impact-modifying polylactides is
generic to mixtures of polylactides and elastomers that are blend-
compatible. Also, those skilled in the art will recognize that thedata of the Table lD will improve as the blends are injection-molded,
as opposed to compression-molded, since the former often induces
orientation of the specimens and, consequently, a profound improvement
in impact strength.
~, 1339026
-- 81 --
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1339026
The compositions are useful thermoplastics that can be melt
fabricated by conventional processes such as extrusion and molding.
The blends preferably use a physical mixture of polylactic acid of
formula I, where n is an integer between 75 and 10,000; and a polymer
comprising a segmented polyester. A useful composition is that where
the polylactic acid comprises 50 to 99 weight percent of the
composition. A preferred composition has a polylactic acid content of
70 to 80 weight percent.
Two embodiments of the general process for producing the
composition include (1) melt blending of PLA with a blend compatible
polymer that provides improved impact resistance such as a segmented
polyester and (2) solution blending during PLA polymerization as in
Example 2D where Hytrel~ segmented polyester is dissolved in the PLA.
The polylactic acid provided preferably has the formula I. If desired
plasticizer in pliable forming amounts may be added to the blend that
is selected from the group consisting of lactide monomer, lactic acid
oligomer, lactic acid, and mixtures thereof. Addition of the
plasticizer will provide additional unique physical properties as
discussed in the first, second, and third general embodiments discussed
above.
Microscopic examination of the Hytrel~ segmented
polyester/polylactic acid mixture revealed that the dispersed Hytrel~
segmented polyester is present in small spherical domains a few microns
or less in size. These domain sizes can be adjusted by the mixing
conditions such as time, speed of mixing, and temperature.
Therefore, for example, the polymer, or polymers, added to the
polylactic acid, should be generally of small, heterogeneous domain
size, less than 10 microns, and can be submicroscopic, or dissolved, in
the polylactic acid. In addition, this impact modifier must be
elastomeric.
While not wishing to be held to any particular theory, it is
believed that the present invention provides a continuous matrix of
polylactic acid containing intimately mixed microscopic domains of
Hytrel~ segmented polyester that act as crack arrestors since the
latter is a thermoplastic elastomer compatible with polylactic acid.
- 1339026
- 83 -
For this purpose, the impact modifier must be elastomeric and
intimately bound into the polylactic acid as a discrete heterogeneous
phase. The added polymer, the impact modifier, can be a thermoplastic
elastomer, or a crosslinked rubber, to achieve this elastic behavior.
Examples are natural rubber and styrene-butadiene copolymers.
In a test of material placed in water for five months, the
material embrittled compared to a material not exposed to water. In
addition the water turned acidic indicating breakdown of polylactic
acid to lactic acid. It was further apparent that polylactic acid
alone degraded faster than the Hytrel~ segmented polyester/polylactic
acid mixture. Thus Hytrel~ segmented polyester can also be used to
retard the degradation rate of polylactic acid.
A third component can be added which is compatible with the other
components discussed above to achieve improved compatibility. Thus,
where the polylactic acid and the impact modifier have poor
compatibility, a third component can be added to improve the
compatibility. This third component is usually added where it is
compatible with the other two, individually, and where the other two,
polylactic acid and impact modifier are not very compatible. This
works by increasing the interfacial bonding between polylactic acid and
elastomeric impact modifier. However, what is surprising is the wide
latitude of compatibility of polylactic acid with other polymer types,
both polar and nonpolar. This can be referred to in the third general
embodiment.
The compositions herein can be processed by melt fabrication into
useful articles of manufacture such as containers, eating utensils,
trays, plates, drinking cups, single serving trays, syringes, medical
trays, and the like. The compositions are especially useful for
articles having only a one time use or a short life span in use before
disposal.
While the invention has been described above with reference to
various specific examples and e~bodiments, it will be understood that
the invention is not li~ited to s~ch illustrated examples and embodi-
ments and may be variously practiced within the scope of the claims
hereinafter made.
