Note: Descriptions are shown in the official language in which they were submitted.
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Expandable Beads of a Comnostable or Blobased Thermoplastic Polymer
BACKGROUND
FIELD OF THE INVENTION
The present invention relates generally to compostable or biobased material
compositions and to novel methods for producing lightweight, compostable or
biobased
foams for various expandable materials. In particular, the present invention
discloses
methods for producing foams using melt processing techniques to blend
compostable or
biobased materials and blowing agents. The compositions and processes are
useful for
the production of a variety of products.
DESCRIPTION OF THE BACKGROUND
Polymeric foams include a plurality of voids, also called cells, in a polymer
matrix. By replacing solid plastic with voids, polymeric foams use fewer raw
materials
than solid plastics for a given volume. Thus, by using polymeric foams instead
of solid
plastics, material costs can be reduced in many applications.
The material used for expandable polystyrene (EPS) is typically an amorphous
polymer that exhibits a glass transition temperature of about 95 C and a
melting
temperature of about 240 C. The process of converting EPS resins into
expanded
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polystyrene foam articles requires three main stages: pre-expansion,
maturation, and
molding. Expandable beads produced from polystyrene and a blowing agent are
made,
and then expanded by steam in a pre-expander. The purpose of pre-expansion is
to
produce foam particles of the desired density for a specific application.
During pre-
expansion, the EPS beads are fed to a pre-expander vessel containing an
agitator and
controlled steam and air supplies. The introduction of steam into the pre-
expander yields
two effects: the EPS beads soften and the blowing agent that is dispersed
within the EPS
beads heats to a temperature above its boiling point. These two conditions
cause the EPS
beads to expand in volume. The diameter of the particles increases while the
density of
the resin decreases. The density of pre-expanded granules is about 1000 kg/m3,
and that
of expanded beads lies in the range of 20 to 200 kg/m3; depending on the
process, a 5 to
50 times reduction in density may be achieved.
Maturation serves several purposes. It allows the vacuum that was created
within
the cells of the foam particles during pre-expansion to reach equilibrium with
the
surrounding atmospheric pressure. It permits residual moisture on the surface
of the
foam particles to evaporate. And, it provides for the dissipation of excess
residual
blowing agent. Maturation time depends on numerous factors, including blowing
agent
content of the original resin, pre-expanded density, and environmental
factors. Pre-
expanded beads that are not properly matured are sensitive to physical and
thermal shock,
Molding of such beads before maturation may cause the cells within the
particles to
rupture, thereby producing an undesirable molded foam part.
Once the pre-expanded beads have matured, they are transferred to a molding
machine containing one or more cavities that are shaped like the desired
molded foam
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article(s). The purpose of molding is to fuse the foam particles together into
a single
foam part. Molding of EPS may follow a simple sequence: first, fill the mold
cavity with
pre-expanded beads; heat the mold by introducing steam; cool the molded foam
article
within the mold cavity; and eject the finished part from the mold cavity. The
steam that
is introduced to the molding machine causes the beads to soften and expand
even further.
The combination of these two effects in an enclosed cavity allows the
individual particles
to fuse together into a single solid foam part.
There is an increasing demand for many plastic products used in packaging to
be
biodegradable, for example trays in cookie and candy packages. Starch films
have been
proposed as biodegradable alternatives for some time. U.S. Patent No.
3,949,145
describes a starch/polyvinyl alcohol/glycerol composition for use as a
biodegradable
agricultural mulch sheet.
A common approach to creating biodegradable products is to combine polylactic
acid (PLA) with starch to create a hydrolytically degradable composition.
Difficulties
have been encountered in producing starch based polymers particularly by hot
melt
extrusion. The molecular structure of the starch is adversely affected by the
shear
stresses and temperature conditions needed to plasticize the starch and pass
it through an
extrusion die, For most products, foaming has to be avoided, which generally
requires
close attention because of the water content of the. starch. Foaming has been
avoided by
degassing the melt prior to exiting the die as suggested in U.S. Patent Nos.
5,314,754 and
5,316,578. The latter patent also avoids adding water to the starch. As
explained in U.S.
Patent No, 5,569,692, by not drying the starch and avoiding the addition of
water, the
starch can be processed at temperatures between 120 C and 170 C because the
water
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bound to the starch does not generate a vapor pressure such as to require high
pressures.
Another approach to improving the melt processability of starch is to provide
an additive
as in U.S. Patent No. 5,362,777 that reduces the melting point of the starch.
Certain patents or patent applications disclose the use of pentane as a
blowing
agent, However, in those methods utilizing pentane, the PLA is necessarily
combined
with an adjuvant or polymer to create an expandable PLA. U.S. Patent No.
6,593,384 to
Anderson et al. describes expandable particles produced using broad polymer
materials
and a physical blowing agent. U.S. Patent No. 7,226,615 to Yuksel et al.
describes an
expandable foam based on broad disclosure of biomaterials combined with a
bicarbonate
blowing agent. U.S. Published Patent Application No. 2006/0167122 by Haraguchi
et al.
describes expandable particles derived from the combination of PLA, a blowing
agent,
and a polyolefin wax.
It is generally accepted that there is a trade off between small cell size and
optimal material properties as blowing agent levels in microcellular polymeric
material
are altered.
SUMMARY
Accordingly, it is an object of the present invention to provide a compostable
or
biobased expandable bead that avoids the disadvantages of the prior art,
It is another object of the present invention to provide an environmentally
`green'
replacement material for expandable polystyrene (EPS). A related object of the
present
invention is to provide a compostable, expandable bead formulation.
It is another object of the present invention to provide a biobased,
expandable
bead formulation.
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Another object of the present invention is to provide a foamed bead that is
capable
of composting.
It is another object of the present invention to provide a method for
producing
compostable or biobased expandable beads using melt processing techniques.
It is another object of the present invention to provide a compostable or
biobased,
foamed bead that can be processed using conventional molding equipment.
A further object of the invention is to provide a compostable or biobased,
foamed
bead that can be fabricated into a three-dimensional shape.
These and other objects of the present invention are accomplished by providing
a
composition and process for producing expandable beads from a compostable or
biobased thermoplastic polymer. One aspect of this invention is the ability to
incorporate
sufficient amounts of hydrocarbon blowing agent into the matric of the
compostable
polymer, such as PLA. In one embodiment, the melt processable composition
includes
additional hydrophobic additives to increase solubility of the blowing agent
in the
composition. The foamed beads of this invention can be further processed using
conventional molding equipment to provide a lightweight, compostable or
biobased,
foamed article. Articles of this invention have utility in applications where
conventional
expandable polystyrene (EPS) is utilized today, including those applications
relating to
protective packaging, sound dampening, and thermal insulation.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
The invention summarized above and defined by the enumerated claims may be
better understood by referring to the following description. This description
of an
embodiment, set out below to enable one to build and use an implementation of
the
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invention, is not intended to limit the invention, but to serve as a
particular example
thereof, Those skilled in the art should appreciate that they may readily use
the
conception and specific embodiments disclosed as a basis for modifying or
designing
other methods and systems for carrying out the same purposes of the present
invention.
Those skilled in the art should also realize that such equivalent assemblies
do not depart
from the spirit and scope of the invention in its broadest form.
The present invention is directed toward a variety of products that are made
of
compostable or biobased materials. The compostable or biobased materials can
include
either or both of an externally or an internally modified polymer composition,
as those
terms are described below.
DEGRADABILITY
Biodegradability refers to a compound that is subject to enzymatic
decomposition,
such as by microorganisms, or a compound, portions of which are subject to
enzymatic
decomposition, such as by microorganisms. In one instance, for example, a
polymer such
as polylactic acid can be degraded by hydrolysis to individual lactic acid
molecules that
are subject to enzymatic decomposition by a wide variety of microorganisms.
Microorganisms typically can consume carboxylic acid-containing oligomers with
molecular weights of up to about 1000 daltons, and preferably up to about 600
daltons,
depending on the chemical and physical characteristics of the oligomer.
Biobased means materials that are synthesized from biological sources and
refers
to ingredients that reduce the use of non-renewable resources by integrating
renewable
ingredients as a replacement for at least a portion of the petroleum used in
making EPS.
Biobased ingredients can be used in many products without hindering their
performance.
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Composting is the biological process of breaking down organic waste into a
useful substance by various microorganisms in the presence of oxygen.
Preferably, the polymer in the present materials breaks down by composting.
The
degradation characteristics of the polymer in the present materials depend in
large part on
the type of material being made with the polymer. Thus, the polymer needs to
have
suitable degradation characteristics so that when processed and produced into
a final
material, the material does not undergo significant degradation until after
the useful life
of the material.
The polymer of the present materials is further characterized as being
compostable within a time frame in which products made from the materials
break down
after use. The materials of this invention degrade in a time period of a few
weeks to a
few years, whereas similar mass-produced, nondegradable products typically
require
decades to centuries to break down naturally.
The present invention describes a composition and process for producing
expandable beads from a compostable or biobased thermoplastic polymer. The
intended
purpose of such compostable or biobased thermoplastic polymer material is to
replace
expandable polystyrene (EPS) with a foamed bead produced from a compostable or
biobased polymer in the construction of foamed articles. Ideally, one would
substitute
EPS with a compostable or biobased polymer. of the same chemical and physical
properties.
The compostable or biobased polymers of this invention are produced by melt
processing compostable or biobased polymers with a blowing agent and,
optionally,
additives that modify the rheology of the compostable or biobased polymer,
including
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chain extenders and plasticizers. The compostable or biobased polymers may
include
those polymers generally recognized by one of ordinary skill in the art to
decompose into
compounds having lower molecular weights. Non-limiting examples of compostable
or
biobased polymers suitable for practicing the present invention include
polysaccharides,
peptides, polyesters, polyamino acids, polyvinyl alcohol, polyamides,
polyalkylene
glycols, and copolymers thereof.
The expandable beads of this invention are produced using a compound
comprising a compostable or biobased polyester and a blowing agent. Additives
including plasticizers and chain extenders are optionally included in the
compostable or
biobased composition. Expandable beads can be produced using conventional melt
processing techniques, such as single and twin-screw extrusion processes. In
one
embodiment, the compostable or biobased polymer is mixed with a hydrophobic
additive
by melt processing to produce pellets. These pellets are then impregnated with
a blowing
agent to make expandable beads. The expandable beads are then heated to cause
foaming, producing foamed beads. The foamed beads are then molded into
articles. In
another embodiment, melt processing is used to mix compostable or biobased
polymer,
hydrophobic additive, and blowing agent to produce an expandable beads
directly from
the melt processing operation. In this case, extrudate from the die must be
cooled rapidly
to lock in the blowing agent so that it does not escape and foaming does not
occur. It is
desired that foaming occurs at a controlled time in a pre-expander operation
by heating
the expandable beads to produce foamed beads. The foamed beads are then
molded.
Preferably, the resulting foamed bead has a specific gravity less than 0,15
g/cm3. More
preferably, the foamed bead has a specific gravity of less than 0.075 g/cm3,
and most
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preferably less than 0,05 g/cm3. In a preferred embodiment, more than 50 wt%
of the
foam is compostable, as determined by ASTM D6400. More preferably, more than
80%
of the foam is compostable. In a most preferred embodiment, greater than 95%
of the
foam is compostable.
In one aspect, the compostable or biobased polymer is a polyester. Non-
limiting
examples of polyesters include polylactic acids, poly-L-lactic acid (PLLA),
poly-D-lactic
acid (PDLA) and random or stereoregular copolymers of L-lactic acid and D-
lactic acid,,
and derivatives thereof. Other non-limiting examples of polyesters include
polycaprolactone, polyhydroxybutyric acid, polyhydroxyvaleric acid,
polyethylene
succinate, polybutylene succinate, polybutylene adipate, polymalic acid,
polyglycolic acid, polysuccinate, polyoxalate, polybutylene diglycolate, and
polydioxanone.
In this invention, a compostable or biobased polymer is melt processed with
hydrophobic additives, and mixed or impregnated with a hydrocarbon blowing
agent to
produce an expandable bead. The expandable bead can be converted to a foamed
bead by
heating, thus expanding the bead by the volatilization of the blowing agent
and softening
of the material, Blowing agents are materials that can be incorporated into
the melt
processable composition (e.g., the premix of the additives, polymeric matrix,
and/or
optional fillers, either in melt or solid form) to produce cells through the
release of a gas
at the appropriate time during processing. The amount and types of blowing
agents
influence the density of the finished product by its cell structure. Preferred
hydrocarbon
blowing agents for this invention include propane, butane, pentane, hexane,
heptane, and
octane.
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The invention allows for the conversion of an existing EPS manufacturing plant
to
produce a foamed article based on a compostable plastic. The chemical
composition of
the plastic is designed to allow for improved solubility of hydrocarbon
blowing agents.
Preferred polymer resins for this invention include known compostable
materials
derived from biological sources (e.g. biopolymer resins), but biodegradable
synthetic
polymers capable of being composted are also acceptable. The biopolymer
polylactic
acid (PLA) is the most preferred example due to its known compostability and
its origins
from agricultural (e.g. corn) feedstocks. Both amorphous and semi-crystalline
PLA
polymers can be used. Examples of compostable or biobased polymers include
Ingeo
2002D and Ingeo 4060D grade plastics and Ingeo 8051D grade foam from
NatureWorks,
LLC, and Cereplast Compostable 5001.
In some embodiments, the plastic formulation of interest may be compounded, as
required, into a homogeneous material for extrusion. As appropriate, the
plastic will be
pelletized and optionally ground and classified into particles of a
predetermined size, for
example 0.25 mm diameter. The polymer pellets may then be added into a stirred
pressure tank with water to produce a slurry. Solution stabilizers, such as
surfactants or
salts, may be added to inhibit coagulation of pellets and to promote diffusion
of
hydrocarbon blowing agent into the polymer particles. In some embodiments,
hydrocarbon blowing agent will be added to the slurry as a liquid. Preferably,
the amount
of hydrocarbon blowing agent added to the system will be predetermined based
on the
desired degree of hydrocarbon blowing agent in the expandable beads. The
pressure tank
may be temperature controlled, for example by a circulating hot water bath. In
some
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embodiments, the pressure tank will be mechanically sealed and pressurized
using
compressed gas, such as nitrogen.
It was conceived that this invention could replace EPS materials in existing
equipment of production plants, The EPS raw material would be replaced by a
raw
material consisting of the compostable or biobased expandable bead.
Hydrocarbon
blowing agents were conceived as blowing agents because these are already used
in EPS
manufacturing, and processes exist to capture and burn the volatile
hydrocarbons for fuel.
It was desired to minimize the costs required to convert an existing plant
from EPS to the
new compostable or biobased material.
A key aspect of this invention is the ability to incorporate sufficient
amounts of
hydrocarbon blowing agent into the matrix of the compostable or biobased
polymer such
as PLA. For example, PLA does not exhibit the affinity for absorption of
pentane that
polystyrene exhibits to produce EPS. At room temperature, pentane readily
absorbs into
solid polystyrene, but this does not occur with PLA. It is therefore necessary
to produce
a composition of compostable or biobased polymer that allows for the
impregnation of
hydrocarbon blowing agent. To do this, hydrophobic additives are added to the
formulation, although not all hydrophobic additives are favorable. Hydrophobic
additives with low hydrophilic-lipophilic balance (HLB) numbers are preferred.
Examples of low HLB number hydrophobic additives include Span 60 (sorbitan
monostearate, HLB=4.7), Span 80 (Sorbitan oleate, HLB=4), and Span 85
(Sorbitan
trioleate, HLB=1.8). Block copolymer nonionic emulsifiers can also be used as
hydrophobic additives to improve hydrocarbon blowing agent solubility. An
example of
a suitable nonionic emulsifier is Unithox 420 ethoxylate (HLB=4) from Baker
Petrolite,
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which is a low molecular weight block copolymer of polyethylene and
polyethylene
glycol. Biologically derived oils, such as soybean oil or acetylated
monoglyceride
derived from hydrogenated castor oil, can additionally be used to aid in
hydrocarbon
blowing agent solubility.
The composition of the compostable or biobased polymer formulation was
additionally modified by the use of conventional plasticizers, chain extension
agents,
cross-linkers, and blends with other thermoplastics to improve other aspects
of the
processability and foaming capabilities of the resin.
For example, the material compositions listed in Table 1 below were produced
using melt processing by twin-screw extrusion. NatureWorks 2002D polylactic
acid
(PLA), a compostable and biobased polymer, was the main component of all
formulations. Additional raw materials included citric acid ester (Citroflex A-
2,
Vertellus Performance Materials), copolyester elastomer_ (Neostar FN007,
Eastman
Chemical), stearic acid surface treated calcium carbonate (Omyacarb FT, Omya
North
America), sorbitane monostearate (Span 60, Sigma-Aldrich), polyethylene glycol
(Carbowax 8000, Dow Chemical), acetylated monoglyceride derived from
hydrogenated
castor oil Grindsted (Soft-N-Safe, Danisco), dicumyl peroxide (Sigma-Aldrich),
ethoxylated nonionic emulsifier (Unithox 450, Baker Petrolite) and a custom
maleated
PLA. The maleated PLA was produced as a precursor to formulations by melt
processing
of 3% by mass maleic anhydride, 0.5% by mass dicumyl peroxide, and 96.5% by
mass
NatureWorks 2002D PLA in an extruder using a constant temperature profile of
180 C.
Pentane
Composition Content wt%
1 100% NatureWorks 2002D PLA (Control Sample) 2.09
2 100% Maleated PLA 2.29
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it
3 95% 2002D PLA, 5% Grindstead SoftNSafe 4.34
4 90% 2002D PLA, 5% Grindstead SoftNSafe, 5% Span 60 5.55
95% NIA-graft PLA, 5% octadecylamine 4.11
6 80% 2002D PLA, 10% Neostar FN007, 5% Citroflex A-2, 5%
Span 60 5.05
7 65% 2002D PLA, 10% Neostar FN007, 10% Citroflex A-2,
10% Carbowax 8000, 5% Span 60 7.89
8 74% 2002D PLA, 10% Neostar FN007, 1% Citroflex A-2, 5%
Carbowax 8000, 5% Span 60, 5% Omyacarb FT 7.16
9 59% 2002D PLA, 10% Neostar FN007, 1% Citroflex A-2, 5%
Carbowax 8000, 5% Span 60, 20% Omyacarb FT 5.77
90% 2002D PLA, 5% Span 60, 5% Grindstead Soft N Safe 5.55
11 79.5% 2002D PLA, 5% Omyacarb FT, 5% Carbowax 8000,
5% Unithox 450, 5% Span 60, 0.5% dicumyl peroxide 4.77
Table 1. Compositions and Percentage Content of Pentane after Impregnation
The raw materials were fed into the feed throat of a 26mm, co-rotating, twin-
screw extruder (model LTF 26-40 from LabTech Engineering Company, LTD). A
constant temperature profile of 180 C was used. The extrudate was passed
through a die
5 to produce a strand, cooled by water or by air, and pelletized. To
impregnate the
compositions with pentane blowing agent, a pre-weighed sample of pellets were
sealed in
a pressure vessel in contact with liquid pentane at room temperature. The
sample vessels
were heated to 80 C while submerged in a water bath for 2 hours. After two
hours, the
samples were removed and allowed to cool. The pellets were removed, blotted
dry to
10 remove any surface coating of liquid pentane, and weighed. The mass of
pentane
impregnated into the pellets was calculated by the difference in final and
initial mass, and
is expressed as a percentage of the sample mass in Table 1. Control samples of
NatureWorks 2002D PLA and a maleated PLA are included as reference. The
control
samples were measured to contain less than 2.5% pentane by mass after
impregnation,
whereas materials containing the hydrophobic additives greatly increased the
mass of
pentane incorporated by the impregnation process. The materials listed in
Table 1 were
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subsequently expanded to produce foamed pellets by heating on a hot plate,
allowing for
liberation of the pentane blowing agent to the gas phase.
The amount of components in the melt processable, compostable or biobased
composition may vary depending upon the intended end use application. The
compostable or biobased polymer may comprise from about 40 to about 99 percent
by
weight of the final composition.
The melt processable, compostable or biobased composition of the invention can
be prepared by any of a variety of ways. For example, the compostable or
biobased
polymer, hydrophobic additive, hydrocarbon blowing agent, and optional
additives can
be combined together by any of the blending means usually employed in the
plastics
industry, such as with a mixing extruder. The mixing operation is most
conveniently
carried out at a temperature above the melting point or softening point of the
polymer.
The resulting melt-blended mixture can be processed into lightweight strands
and
subsequently cut into pellets using a strand pelletizer. The resulting pellets
can be
molded into a three-dimensional part using conventional equipment utilized in
molding
expandable polystyrene.
it
The invention has been described with references to specific embodiments.
While
particular values, relationships, materials and steps have been set forth for
purposes of
describing concepts of the invention, it will be appreciated by persons
skilled in the art
that numerous variations and/or modifications may be made to the invention as
shown in
the disclosed embodiments without departing from the spirit or scope of the
basic
concepts and operating principles of the invention as broadly described. It
should be
recognized that, in the light of the above teachings, those skilled in the art
could modify
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those specifics without departing from the invention taught herein. Having now
fully set
forth certain embodiments and modifications of the concept underlying the
present
invention, various other embodiments as well as potential variations and
modifications of
the embodiments described herein will obviously occur to those skilled in the
art upon
becoming familiar with such underlying concept. It is intended to include all
such
modifications, alternatives and other embodiments insofar as they come within
the scope
of the invention. It should be understood, therefore, that the invention might
be practiced
otherwise than as specifically set forth herein, Consequently, the present
embodiments
are to be considered in all respects as illustrative and not restrictive,
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