CA1104737A

CA1104737A - Polypropylene film compositions and biaxially oriented polypropylene film containing styrene polymers

Info

Publication number: CA1104737A
Application number: CA290,554A
Authority: CA
Inventors: Dominic A. Berta
Original assignee: Hercules LLC
Current assignee: Hercules LLC
Priority date: 1976-12-15
Filing date: 1977-11-09
Publication date: 1981-07-07
Also published as: BE861862A; AU3154077A; LU78701A1; BR7708324A; JPS5374547A; AU510913B2; NL7713913A; MX3553E; ES465053A1; DE2755446A1; FR2374358A1; GB1550570A; IT1089965B

Abstract

Abstract of the Disclosure Polypropylene film compositions and biaxially oriented polypropylene films containing 80-984 by weight of a stereoreg-ular polypropylene and 2-204 by weight of an unhydrogenated styrene polymer. The unhydrogenated styrene polymer is a blend of 25-100% by weight of low molecular weight styrene polymer hav-ing a drop softening point of between about 70°C. and 145°C., and 0-75% by weight of high molecular weight polystyrene.

Description

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 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 ~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 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 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 -.. .. .

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 _ 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 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. .

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

1. A composition comprising about 80 to 98 percent by weight of said composition of a stereoregular polypropylene and 2 to 20 percent by weight of said composition of an unhydrogenated styrene polymer, 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 6,000 and having a drop softening point of between about 70°C. and 145°C., and said high molecular weight polystyrene having a molecular weight averaging above 6,000.

2. A composition as set forth in claim 1 wherein said unhydrogenated styrene polymer is alpha-methylstyrene-styrene copolymer.

3. A composition as set forth in claim 2 in which the unhydrogenated styrene copolymer contains about 75% alpha-methyl-styrene and about 25% styrene.

4. A composition as set forth in claim 1 wherein said unhydrogenated styrene polymer is a blend of low molecular weight and high molecular weight polystyrene.

5. A composition as set forth in claim 1 wherein said unhydrogenated styrene polymer is low molecular weight poly-styrene.

6. A composition as set forth in claim 1 wherein said unhydrogenated styrene polymer is a copolymer of styrene and an aliphatic alpha-olefin.

7. A composition as set forth in claim 1 wherein said unhydrogenated styrene polymer is a blend of alpha-methylstyrene-styrene copolymer and polystyrene.

8. A composition as set forth in claim 1 including up to 25% by weight of the polypropylene of a compatible hard resin having a drop softening point of between about 70°C. and 170°C.

9. A composition as set forth in claim 8 wherein said hard resin is a pentadiene polymer.

10. A composition as set forth in claim 8 wherein said hard resin is a terpene polymer.

11. A composition as set forth in claim 10 wherein said terpene polymer is hydrogenated.

12. A composition as set forth in claim 8 wherein said hard resin is a partially or completely hydrogenated rosin ester of a polyhydric alcohol.

13. A composition as set forth in claim 8 wherein said hard resin is a hydrogenated low molecular weight polystyrene.

14. A composition as set forth in claim 8 wherein said hard resin is a hydrogenated styrene polymer having an average molecular weight distribution such that no more than about 15% of said hydrogenated styrene polymer has a molecular weight outside the range of about 600 to 20,000 and has been hydrogenated to the point that at least 50% of the aromatic unsaturation is reduced.

15. A biaxially oriented film of the composition of claim 1.

16. A biaxially oriented film of the composition of claim 2.
17. A biaxially oriented film of the composition of claim 5.

18. A biaxially oriented film of the composition of claim 8.

19. A biaxially oriented film of the composition of claim 9.

20. A biaxially oriented film of the composition of claim 10.

21. A biaxially oriented film of the composition of

claim 13.

22. A biaxially oriented film of the composition of

claim 14.