Note: Descriptions are shown in the official language in which they were submitted.
737
Berta Case 2
This invention relates to film compositions and biax-
ially oriented polypropylene films having good heat sealability
and good blocking characteristics as well as good clarity. More
specifically~ it relates to film compositions and biaxially ori-
ented polypropylene films including 2 to 20% of unhydrogenated
styrene polymers having specified molecular weights.
It is known in the prior art to blend conventional high
molecular weight polystyrene in small amounts with polypropylene
to obtain homogeneous but incompatible blends, for example, see
U.S~ patent 3,018,263. Uniaxially oriented films and monofil-
aments have been prepared from such blends; see U.S. patent
3,173,163 and German patent 1,813,652. Japanese Publication No.
Sho 49-19698 discloses biaxially oriented polypropylene films con-
taining up to 7% of conventional polystyrene (which is unhydrogen-
ated and has high molecular weight), however, such films have
very poor heat sealability. Furthermoee, due to the known lack
of compatibility of polystyrene with polyolefins, it has not here-
tofore been possible to obtain polypropylene films having good
clarity with over about 5% polystyrene. Thus, lack of compatibil-
ity results in undesirable opaque film in the cast film state.Hydrogenated copolymers of styrene and alpha-methylstyrene or
vinyl toluene having molecu]ar weight distributions between about
600 and 20,000 have been used as additives to polypropylene in
the preparation of biaxially oriented film; see U.S. patents
3,666,836 and 3,361,849. However, films from such blends have a
great tendency to block. B7ocking is the condition wherein suc-
cessive layers of film wound on a reel adhere to one another.
When unwound from the reel, tearing of the film may occur. Un-
hydrogenated low molecular weight styrene polymers such as alkyl
substituted styrenes have not been used in biaxially oriented
films.
In accordance with this invention, it has been deter-
mined that useful polypropylene compositions and biaxially ori-
ented films comprise about 80 to 98 percent by weight of said
-- 2 --
37
composition of a stereoregular polypropylene and 2 to 20 percent
by weight of said composition of an unhydrogenated styrene poly-
mer, said unhydrogenated styrene polymer being a blend of 25-100%
by weight of low molecular weight styrene polymer, and 0-75% by
weight of high molecular weight polystyrene, said low molecular
weight unhydrogenated styrene polymer having a molecular weight
averaging between about 600 and h,000 and having a drop softening
point of between about 70C. and 145C., and said high molec-
ular weight polystyrene having a molecular weight averaging above
6,000. Preferably, the compositions include 90 to 96% polypro-
pylene and 4 to 10% styrene polymers.
The term "styrene polymers" as used herein means homo-
polymers of styrene and alkyl substituted styrenes, copolymers of
styrene and alkyl substituted styrenes, copolymers of alkyl sub-
stituted styrenes with each other, copolymers of styrene and alkyl
substituted styrenes with monoterpenes such as dipentene, carene,
pinene, terpinene, limonene, turpentine, allo-ocimene, and terpin-
olene and blends of the aforementioned homopolymers and copoly-
mers with each other. Preferred alkyl substituted styrenes are
20 alpha-methylstyrene and vinyl toluene. The term "styrene poly-
mers" also includes copolymers of one of the styrenes with an
aliphatic alpha-olefin or a diolefin such as ethylene, propylene,
isobutylene, butene-l, indene, butadiene, isoprene or the like.
The term "stereoregular polypropylene" includes any iso-
tactic polypropylene having an intrinsic viscosity in the range of
about 1.5 to about 4. Such polymers contain a substantial degree
of crystallinity. While the homopolymer is preferred, random co- r
polymers of propylene containing up to about 5% of the second
alpha-olefin, e.g., ethy~ene or butene-l, can be used, or block
30 copo]ymers containing up to 25% of the second alpha-olefin can
likewise be used.
It has been found that polypropylene film containing
low molecular weight unhydrogenated styrene polymers and blends
with high molecular weight unhydrogenated polystyrene in the
-- 3 --
~7 37
specified proportions, although opaque in the cast film state,
becomes clear when biaxially oriented. The use of such unhydro-
genated styrene polymers in accordance with this invention offers
the advantages of low blocking characteristics, good heat stabil-
ity and clarity. It is to be noted that oriented films containing
hydrogenated resins block severely.
Preferably, the unhydrogenated styrene polymers should
have either a low molecular weight or a specified combination of
low and high molecular weights. The low molecular weight unhydro-
genated styrene polymers are characterized by their softeningpoint and molecular weight distribution. They should have a
weight average molecular weight of between 600 and 6,000 and a
drop softening point of between 70 and 145C. The softening
point indicates the temperature at which the polymer changes from
a rigid to a soft state. The drop softening points as used herein
are determined by the Hercules Drop Method (Wood Rosins, Modified
Rosins and Related Resins, Hercules Technical Bulletin, Form
400-44A, Hercules Incorporated, 1965).
Also useful in this invention are blends of unhydrogen-
ated high molecular weight polystyrene, characterized by a molec-
ular weight over 6,000 and generally between about 100,000 and
250,000, in combination with the unhydrogenated low molecular
weight styrene polymers. These polymers may be blended in propor-
tions of 0-75% by weight of high molecular weight polystyrene and
25-1~0% by weight of low molecular weight styrene polymers. These
polymer blends may be added to polypropylene such that the amount
of polypropylene constitutes between about 80 and 98 percent of
the total amount, and the low molecular weight or the blend of low
and high molecular weight styrene polymers constitutes between
about 2 and 20 percent of the total.
In another embodiment of the invention up to 25% by
weight, based on polypropylene, of a compatible hard resin is
employed in combination with the unhydrogenated styrene polymers
or with the polystyrene blends. Compatible hard resins are those
-- 4 --
resins which, when added to polypropylene by themselves at up to
25% concentration, produce clear to translucent cast films. Suit-
able compatible hard resins are characterized by drop softening
points which are preferably in the range of 70C. to 170C.
The halrd resins which may be employed in practicing the invention
inclucle unhydrogenated or hydrogenated terpene polymers, hydro-
genated low molecular weight styrene and alkyl substituted styrene
polymers and copolymers, hydrogenated and unhydrogenated rosin
derivatives, terpene-isoprene or piperylene copolymers and their
hydrogenated counterparts, pentadiene polymers and copolymers and
their hydrogenated counterparts.
The terpene polymers which can be employed in this in-
vention are the polymeric, resinous materials made by polymeriza-
tion and/or copolymerization of terpene hydrocarbons such as the
alicyclic, monocyclic, and bicyclic monoterpenes and their mix-
tures, including allo-ocimene, carene, isomerized pinene, pinene,
dipentene, terpinene, terpinolene, limonene, turpentine, a terpene
cut or fraction and various other terpenes. Particularly useful
materials are terpene mixtures containing at least 20% beta-pinene
and/or limonene or dipentene (racemic limonene), and the sulfate
turpentine obtained as a by-product in the sulfate pulping
process.
The polymerization of the terpene or mixture of terpenes
can be carried out in known manner with or without solvent and
utilizing a known catalyst such as sulfuric acid, phosphoric acid,
fuller's earth, boron trifluoride, amphoteric metal chlorides such
as zinc chloride or aluminum chloride, and so on. The polymeriza-
tion is preferably carried out under conditions which cause sub-
stantially all of the monoterpenes to react with minimum dimer
formation. Any of the polymers prepared by methods known to the
art having average molecular weights of about 500 and abo~e and
drop softening points above 70C. may be employed herein.
The hydrogenated terpene polymers can be any of the
above-described polymers hydrogenated in a well-known manner,
7~7
such as, for example, by the techniques described in U.S. patent
3,361,849. It is preferred that the terpene polymers utilized
herein be hydrogenated because of the improved ultraviolet light
stability and color obtained by hydrogenation and the resultant
improvements in polymer stability during handling at elevated tem-
peratures.
The hydrogenated styrene polymers which are employed as
hard resins in this invention can be hydrogenated low molecular
weight homopolymers of styrene or of alkyl substituted styrenes,
copolymers of styrene and alkyl substituted styrenes, copolymers
of alkyl substituted styrenes with each other and copolymers of
styrene and alkyl substituted styrenes with other hydrocarbons
having non-aromatic carbon-to-carbon unsaturation. The hydrogen-
ated styrene polymers have a drop softening point above about
70C. and are compatible with the polypropy1ene. The preferred
hydrogenated styrene polymer are characterized by having a drop
softening point from between about 70 and 170C., by having at
least about 50% of their aromatic unsaturation hydrogenated, and
by having a number average molecular weight distribution such
that no more than about 15% of the polymer has a molecular weight
outside the range of 600 to 20,000. The styrene polymers can be
prepared in known manner as by polymerization in the presence of
; a Lewis acid such as BF3 etherate or aluminum chloride and
- hydrogenation can be accomplished in accordance with standard and
well known techniques for the hydrogenation of aromatic rings
utilizing a nickel catalyst. The degree of hydrogenation of the
aromatic ring is determined by measuring the decrease in inten-
sity of the ultraviolet absorption band due to aromatic unsatur-
ation at 266 millimicrons.
The rosin derivatives which can be employed as hard
resins in this invention are amorphous, hard, brittle, solid
resins at room temperature, having a drop softening point above
about 70C., and are compatible with the polypropylene. Such
rosin derivatives can be prepared from gum rosin, wood rosin, or
-- 6 --
tall oil rosin, all of which are commercially available. The
nature and chemistry of rosin and rosin derivatives are well de-
scribed in "Rosin and Rosin Derivatives", by Herman I. Enos, Jr.,
George C. Harris and Glenn W. Hedrick, Encyclopedia of Chemical
Technology, Volume 17, pages 475-508, copyright 1968, by
Interscience Publishers, New York.
The rosin derivatives which are suitable hard resins for
the purposes of this invention comprise rosins which have been
modified by hydrogenation, disproportionation, polymerization,
condensation with unsaturated carbocyclic compounds to form resin-
ous condensation adducts, or combinations of such modifying treat-
ments. Some typical representative members of this class include
hydrogenated rosin, disproportionated rosin, polymerized rosin,
specifically dimerized rosin, hydrogenated dimerized rosin, ~on-
densation adduct of rosin and styrene, hydrogenated condensation
adduct of rosin and styrene, condensation adduct of rosin and di-
vinyl benzene, hydrogenated condensation adduct of rosin and di-
vinyl benzene, condensation adduct of rosin and diisopropenyl
benzene, condensation adduct of rosin and alpha-methyl-para-methyl
styrene, condensation adduct of rosin and cyclopentadiene, hydro-
genated condensation adduct of rosin and cyclopentadiene, and the
like. Suitable methods for hydrogenating, disproportionating and
polymerizing rosin are disclosed in the above referenced encyclo-
pedic article. The condensation products of rosin with unsatur-
ated carbocyclic compounds may be prepared by the method described
in U.S. patent 2,532,120 to Alfred L. Rummelsburg, and these con-
~; densation products can be hydrogenated by the same methods used to
hydrogenate rosin.
Hydrogenated rosin for the purposes of this invention
may be partially hydrogenated rosin which has been hydrogenated tothe so-called "dihydrol' stage, where one of the two ethylenic un-
saturated linkages in the naturally occurring abietic-type and
pimaric-type resin acids present in rosin has been substantially
saturated with hydrogen, or fully hydrogenated rosin which has
~ 473~
been hydrogenated to the so~called "tetrahydro" stage, where both
of the two ethylenic unsaturated linkages in the naturally occur-
ring abietictype and pimaric-type resin acids present in rosin
have been substantially saturated with hydrogen. The term
"hydrogenated rosin", therefore, is used to denote any hydrogen-
ated rosin in which at least one of the two ethylenic unsaturated
linkages in the naturally occurring abietic-type and pimaric-type
resin acids present in rosin has been substantially saturated
with hydrogen. Also particularly applicable are esters of parti-
ally or completely hydrogenated rosin prepared by reaction withpolyhydric alcohols such as glycerol and pentaerythritol.
The polypropylene film compositions of this invention
containing the unhydrogenated styrene polymers, or the polystyrene
blends, are hazy and sometimes milky white when extruded or cast
into unoriented film. However, such Eilms when biaxially ori-
ented to a level of 4.0X or higher in each direction yield clear
films. Such films combine good low temperature heat sealing prop-
erties, good clarity and low blocking tendencies. Combination
with hard resins as described above provides even lower heat seal
temperatures without adversely affecting the clarity and blocking
characteristics.
Films of this invention are made by conventional meth-
ods by blending the components, which may if desired be extruded
and cut into pellets, extruding through a die to prepare the un-
oriented film or sheet, and then biaxially orienting the film at
an elevated temperature either simultaneously by a blowing pro-
cess, or sequentially using differential speed rolls and a tenter.
Having set forth the general nature of the invention,
the following examples illustrate some specific embodiments of
the invention. It is to be understood, however, that this inven-
tion is not limited to the examples since the invention may be
practiced by the use of various modifications. In the examples,
the molecular weights of the resins were determined by means of
B gel permeation chromotography using a porous glass (Porasil 60)
;~ f r c~ ~ - 8 -
.. .. .
737
column 20 feet long by 5/16 inch in diameter calibrated with
styrene homopolymer standards of known molecular weight.
Examples 1-9
Examples 1 to 9 are compared with controls A to G in
Table 1. All of the examples and controls include stereoregular
polypropylene flake having a birefringent melting point of
167C. and contain 0.1~ antioxidant and 0.1% calcium stearate
antacid. Each blend was melt-extruded at 450F. into strands
which were chilled and chopped into uniform molding powder gran-
ules. The molding powder was then converted into cast film byextruding the molding powder through a die having a 6 in. length
and a 0.04 in. width onto a roll internally cooled with water at
80F. and rotating at 7 ft./min. giving a film with 0.028 in.
thickness. This cast film was then made into biaxially oriented
film by cutting a suitable size sample and placing it in the jaws
of a T. M. Long Co. stretcher. Each sample was then heated for
35 seconds at 135C. and stretched 6X in both the longitudinal
and transverse directions, giving an oriented film having a thick-
ness of 0.00078 in. Each such biaxially oriented film was subject
to discharge treatment to a sufficient level that the wetting ten-
sion was at least 36 dynes per centimeter.
The heat seal characteristics of each film was deter-
mined by folding the film over onto itself and placing the folded
film between the jaws of a Sentinel sealer and sealing with 20
lbs. pressure for 5 seconds at the desired temperature. The
strength of the seal was measured by clamping the unsealed portion
of the film in ~he clamps of an Instron tensile tester and measur-
ing the force required to pull the seal apart. Very strong seals
caused the film to tear before seal failure occurred. Note in
Table 1 that Examples 1 through 9 had very good seal strength,
with each example tearing before seal failure in at least one or
more of the sealing temperatures noted. In contrast, controls A,
E, F and G gave no film tears and had almost zero seal strength.
Controls B, C and D had very strong seal strength, but had
g _
737
extremely high blocking.
The tendency of a film to block, which occurs when films
are wound on a core, was determined by placing one film on top of
any other and applying a pressure of 150 p.s.i. by means of a
metal plate. After being held in an oven at 40C. for three
hours and the pressure was released, the force required to pull
the pressed oriented films past a cylindrical bar so as to separ-
ate the two pressed films was determined. Very poor blocking is
characterized by the film tearing as the film is pulled past the
bar. It can be seen in Table l that controls B, C and D each
tore during the determination of the blocking characteristics.
..
The control films B, C and D blocked so severely that their com-
mercial use is limited. Examples 1 to 9, however, had very good
blocking characteristics. In addition, Examples 1 to 9 had
acceptable optical characteristics. For the optical character-
istics, haze was tested in accordance with ASTM D-1003 and clarity
was tested in accordance with ASTM D-1746.
Examples 10 to 16
The procedures of Examples l to 9 were utilized in mak-
ing Examples 10-16, which use compatible hard resins in combina-
tion wlth the unhydrogenated styrene polymers. Example 10 uses 4%
unhydrogenated alpha-methylstyrene-styrene, 6% hydrogenated co-
polymer of alpha-methylstyrene and vinyl toluene and ~0% of
stereoregular polypropylene. The hydrogenated copolymer of alpha-
methylstyrene and vinyl toluene used in the controls and examples
hexein has a molecular weight of approximately 3,000, a softening
point of 146C., and a molecular weight distribution such that
more than 85% has a molecular weight between 600 and 20,000. This
copolymer was hydrogenated to a 50% level of aromatic ring satura-
tion. Example 11 includes a combination of 4% hydrogenated alpha-
methylstyrene and vinyl toluene copolymer with a blend of 2% low
molecular weight polystyrene and 2% high molecular weight poly-
styrene. The other E~amples, 12 to 16, include other compatible
hard resins such as pentadiene polymers, hydrogenated and
-- 10 --
J~ 37
unhydrogenated terpenes, hydrogenated rosin esters and hydrogen-
ated low molecular weight polystyrene. It is to be noted that
Examples 10 to 16 all have very good seal strength with the seal
strength being strong enough that the film tears in most
lnstances. Addi~ionally, the blocking characteristics and optic-
als are quite satisfactory. By comparing Examples 10 to 16 with
Controls B, C and D in Table 1, it is evident that improved block-
ing characteristics along with good seals are obtained. Addition-
ally, by comparing Examples 10 to 16 with Examples 1 to 9, it can
be seen that a better balance of sealability and blocking is
achieved by using hard resins in combination with unhydrogenated
styrene polymers. .
37
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