US3560318A - Fibrous pulp containing partially hydrolyzed polyvinyl acetate - Google Patents

Abstract

A FIBROUS PULP COMPRISING ORIENTED FIBERS OF AT LEAST THREE INCOMPATIBLE THERMOPLASTIC RESINS, SAID FIBERS HAVING FIBRILLATED SURFACES, AND ONE OF SAID THERMOPLASTIC RESINS BEING A PARTIALLY HYDROLYZED POLYVINYL ACETATE.

Description

United States Patent O 3,560,318 FIBROUS' PULP CONTAINING PARTIALLY HYDROLYZED POLYVINYL ACETATE Walter A. Miller, Somerville, and Ivey Allen, Jr., Neshanic, N.J., assignors to Union Carbide Corporation No Drawing. Filed Dec. 26, 1967, Ser. No. 693,144 Int. Cl. B32b 5/08, 27/02 US. Cl. 16182 8 Claims ABSTRACT OF THE DISCLOSURE A fibrous pulp comprising oriented fibers of at least three incompatible thermoplastic resins, said fibers having fibrillated surfaces, and one of said thermoplastic resins being a partially hydrolyzed polyvinyl acetate.

FIELD OF THE INVENTION This invention relates to plastic paper particularly paper made from fibrous plastic pulp.

THE PRIOR ART Much effort has been directed at producing a paper substitute, particularly plastic paper for reasons including the availability and economy of plastics, their durability, moisture resistance and other suitable properties. Attempts at making paper from plastic fibers have frequently been unsuccessful due to the inability of the fibers to interfelt sufiiciently during pulp preparation, resulting in a paper of low wet strength. Interfelting is primarily due to an intertangling of microscopic fibrillaeon the surface of a fiber with the fibrillae of other fibers. The synthetic fibers that have been proposed, prior to the present invention, for paper making have been extruded or spun through very small orifices either in solution or in the molten state to form long continuous fibers, the surfaces of which are quite smooth and slippery. As such they have no fibrillae to interfelt. Nor are they capable of being easily and uniformly dispersed in a water media or of fibrillating by being beaten while dispersed in water as is the case with natural fibers. A specific example of the difiiculty encountered with this type of fiber was demonstrated when an attempt was made to form a paper sheet from Dynel, a vinyl chloride-acrylonitrile copolymer. When this resin was extruded into fine fibers, chopped into /8 to inch lengths, and sheeted in the conventional manner on a laboratory hand sheeting machine, the sheet produced could not be removed from the screen without tearing and was not self-sustaining.

To achieve a higher degree of interfelting other paper products have been proposed characterized by having synthetic fibers whose surface and ends are frayed into minute tendrils or fibrillae. These fibers are formed by the extrusion of a mixture of two or more incompatible thermoplastic materials to form a composite monofilament product. The monofilament, after being oriented by cold drawing, is fibrillated by mechanical working such as chopping into /8 to 4 inch lengths in water for several hours and beating into frayed fibers having minute fibrillae. The plastic pulp thus formed resembles normal cellulose pulp. However, paper made from this fibrous pulp has heretofore had little or no bonding strength and has been markedly weak and non-coherent. Accordingly, it has been necessary in the past to either bond the fibers by heat or solvent sealing, in which case a stiff, board-like paper of substandard writability was produced or to add a considerable amount of natural pulp, cellulose fibers (which are hydrophilic and bondable in water) to the synthetic pulp, e.g. 20% to 50% by weight cellulose, to produce a paper of satisfactory strength. However, an

3,550,318 Patented Feb. 2, 1971 ice all plastic fibrous paper of satisfactory strength and paperlike properties has heretofore not been known.

Accordingly, there has now been discovered an all plastic paper of high strength that has paper-like properties such as writability, flexibility, and the like. The plastic paper of the invention which contains a hydrophilic polymer as bonding agent, is tear-resistant, durable and does not yellow with age or crumble.

SUMMARY Broadly the invention provides a fibrous plastic pulp and the resulting plastic paper which comprises fibrillated fibers of at least three incompatible polymers, which polymers include a polymerized vinyl ester having a molar degree of hydrolysis of from 1 to 99 percent and at least two other thermoplastic resins having overlapping orientation temperature ranges.

DESCRIPTION Polyvinyl alcohol while highly water soluble is not wholly suitable in the process of the present invention because it is not readily extrudable. Polyvinyl ester while extrudable is not water soluble, However, partially hydrolized polyvinyl ester such as polyvinyl acetate is both extrudable and provides the hydrophilic property so important in paper making. The ester is further highly effective in interfiber bonding and can bond with one or more other polymers to produce high strength plastic paper.

The incompatible polymers are mixed, at an appropriate blending temperature which can be at room temperature but typically is at an elevated temperature as dictated by the physical properties of the material. The polymers are mixed by mechanical mixing, e.g. a kneader or a Banbury mixer, solvent mixing and the like and then extruded as one or more filaments or sheets.

The mixture can be extruded in any conventional extruder operated at a temperature low enough to prevent decomposition and high enough to be consistent with processing viscosity. It is only necessary to heat the mixture hot enough to maintain all components in a melt extrudable state. One or more components are heated only enough to achieve workable plasticity. Hot drawing of the plastic mixture as it emerges from the extruder is not necessary but may be done if a reduction in size of each filament is desired. Cold drawing to induce molecular orientation in the fibrils comprising the filament is desirable however if a product which will fibrillate readily is to be produced. The optimum degree of orientation induced will vary depending on the composition of the filament. Generally 350 to 550 percent orientation is adequate but orientation up to 2,000 percent may sometimes be advantageous. Aside from facilitating fibrillation, molecular orientation imparts improved physical properties to a large number of fiber forming polymers such as polyamides, polyesters, polyurethanes, and vinyl and acrylic type polymers, and as a consequence, improves the paper made from them.

Each filament is composed of individual fibrils having longitudinal axes essentially parallel to each other and are weakly attached in a lateral direction, i.e., contact of one fibril with another is along a line running longitudinally along the outer surface of the fibrils. Fibril groups, comprised of several fibrils of each of the incompatible resin making up the monofilament, when split off from the monofilament are designated as fibers. The number of fibrils comprising each fiber may be in the range of about 2 to 100.

Fibrillation of the thus produced filaments can be advantageously carried out by mechanical beating or working. Chopped filaments /s to inch in length are generally suitable for any of the numerous mechanical methods available.

A typical fibrillation operation may be carried out in a commercial Holland paper beater which consists of a cylinder of knives or bars and an adjustable bed plate. Chopped monofilament in a water media is circulated repeatedly under the beater roll by flowing around a circular trough. The filaments immediately begin to break up and within a few hours the plastic pulp resembles normal cellulose pulp.

The dimensions of the staple fibers, i.e., those fibers relatively short in length produced from chopped filament, depend somewhat on the fibrillation method employed and are normally in the range of .1 to 100 microns in diameter. Fibers with diameter in the range of to 30 microns are easily produced in a Holland paper beater.

A wide variety of polymers which are incompatible with partially hydrolyzed polyvinyl acetate are suitable for the paper of the present invention, i.e. one or more incompatible polymers, copoylmers or mixtures thereof. These include all of the normally solid fiber-forming resins from which conventional size fibers used in the textile and paper industries may be produced by ordinary melt extrusion operations. Typical resins are polyolefins such as polyethylene, polypropylene and the like, the polymers of styrene, both atactic and isotactic, and the alkyl and halogen substituted styrenes, polymers of methacrylic esters such as polymethylmethacrylate, polymers of vinyl esters such as polyvinylacetate and polyvinyl butyrate, vinyl halide polymers such as polyvinylchloride, polyamides such as the nylon, fluoro vinyl polymers such as polytrifiuorochloroethylene (a fluoroethene), polyesters, polyethers, polyurethanes, copolymers of styrene such as styrene and acrylonitrile copolymers, copolymers of vinyl halides and vinyl esters such as a coplymer of vinyl chloride and vinyl acetate, and copolymers of vinylidene halides and vinyl halides such as a copolymer of vinylidene chloride and vinyl chloride.

It is important to the present invention that the pulp and paper composition contain at least two incompatible polymers of similar or overlapping orientation temperature ranges so that when an extruded composite strand of the two polymers is oriented, both polymers will be oriented and result in better and finer fibrillation and fibers.

It is possible to form fibrillated plastic fibers from polymers having non-overlapping orientation temperature ranges, as has been done in the art; however when a. composite strand or film of such polymers is oriented only one of the polymers is oriented, while the other is merely stretched and not significantly oriented and the resulting fibers are coarse and fit only for coarse fabric. Accordingly, the composition of the present invention, as indicated above, includes at least two polymers of overlapping orientation temperature ranges. It is possible that the composition could include partially hydrolyzed polyvinyl ester and one other resin incompatible therewith. However, certain esters such as partially hydrolyzed polyvinyl acetate have an orientation temperature range that the composition could include partially hydrolyzed such as those listed above. And while it is possible, within the scope of the present invention, for the composition of the invention to include partially hydrolyzed polyvinyl ester e.g. polyvinyl butyrate and one other incompatible polymer, provided the two polymers have overlapping orientation temperature ranges, preferably the composition includes the above ester and at least two other incompatible resins, which resins have overlapping orientation temperature ranges. Thus the preferred fiber composition has three or more incompatible polymers, at least two of which can be highly oriented in a composite strand and segmented to produce ultra fine fibers for paper and fabric formation.

Any partially hydrolyzed, polymerized vinyl ester that can be obtained is suitable for use in the present invention, preferably an ester having a degree of hydrolysis of from 30 to 60 weight percent.

However the ploymerized vinyl ester preferred in the present invention is not the type that is presently commercially available in that it has a degree of hydrolysis of from about 1 to about 99 percent by weight, preferably from about 30 to 60 percent by weight, and the individual particles thereof have an outer layer of polymeric, organic interfacial agent described in detail herein.

The hydrolyzed polyvinyl ester preferred herein is prepared by a method, described below, which produces the ester in the form of fine particles each of which has an outer layer of organic interfacial agent as described above. It should be understood that in mixing the polyvinyl ester particles with other polymers as described herein and processing the resulting composition using conventional plastic forming techniques such as extrusion and the like, this outer layer can become partly or completely admixed with the ester but nonetheless remains associated therewith.

The hydrolyzed polyvinyl ester used herein is prepared by a method which comprises contacting a non-aqueous dispersion of the ester having an average particle size of about 0.05 to about 50 microns and up to 20 carbon atoms per vinyl ester monomer unit, dispersed in an inert hydrocarbon diluent by means of a polymeric organic interfacial agent with about 0.01 to 10.0 moles, per mole of the polymerized vinyl ester, of an aliphatic alcohol having from 1 to about 6 carbon atoms and about 0.001 to 0.10 mole per mole of the polymerized vinyl ester, of an alcoholysis catalyst for at least 5 minutes at a temperature of about 15 to about 100 C., preferably from about 20 to about 50 C., and recovering the polyvinyl ester.

The preferred polymerized vinyl ester is polyvinyl acetate although other esters such as polyvinyl formate, polyvinyl propionate, polyvinyl butyrate and the like having up to 20 carbon atoms can be used if desired.

It is preferred to employ as the non-aqueous dispersion of polymerized vinyl ester one which has been obtained by the dispersion polymerization of vinyl ester monomer although polymerized vinyl esters made by other methods such as bead, emulsion or solution polymerization can also be used and then converted to a non-aqueous dispersion with a polymeric organic interfacial agent if desired.

Although the average particle size of the polymerized vinyl ester used in the dispersions can range from about 0.05 to 50 microns it is preferred to use particles in the range of about 0.1-5 microns and it is particularly preferred to use particle sizes in the range of about 0.l1 micron.

The preferred inert organic diluents are aliphatic or cycloaliphatic hydrocarbons having from about 5 to 12 carbon atoms therein with pentane, isopentane, hexane, heptane, and isoctane being preferred aliphatic hydrocarbons and cyclopentane, cyclohexane, and methylcyclohexane being preferred cycloaliphatic hydrocarbons, as well as mixtures of the above. Although aromatic hydrocarbons, ethers, esters, and other polar group containing diluents inert towards free radical initiators cannot be used alone, they may be if mixed with aliphatic or cycloaliphatic hydrocarbons such as those enumerated above. Stated another way, the inert organic diluent or diluent mixture employed must be a solvent for the vinyl ester monomer but a non-solvent for the polymerized vinyl ester.

The polymeric organic interfacial agent used must be one which has a backbone that is soluble in inert hydrocarbon .diluents and which has at least one site for grafting or anchoring to polymerized vinyl esters. The preferred interfacial agents include copolymers and graft copolymers of alpha olefins and vinyl esters, alpha olefins with polar group containing vinyl monomers, polyvinyl alkyl ethers, propylene oxide rubbers, butadiene-styrene rubbers, ethylene-propylene terpolymers, and the like. It is preferred to employ as the copolymer of an alpha olefin and a vinyl ester an ethylene/ vinyl acetate copolymer or vinyl acetate graft polymerized onto ethylene/vinyl acetate copolymer. These ethylene/vinyl acetate copolymers and graft copolymers preferably contain from about 5 to 80 percent vinyl acetate being particularly preferred. The preferred alkyl vinyl ether polymer for the interfacial agent is polyvinyl ethyl ether, although others such as polyvinyl isobutyl ether, polyvinyl propyl ether and the like can also be used.

The preferred oxide rubber polymer for the interfacial agent is polypropylene oxide rubber.

The concentration of interfacial agent should be at least 0.1 percent based on the weight of the dispersed polymer. It is preferred to use at least 0.2 percent, up to about percent and if desired, even higher concentrations can be used.

The minimum alcoholysis time has been given above as about 5 minutes. There is no maximum time since no further reaction occurs once essentially complete alcoholysis has taken place, the actual time used in any one run will depend upon the degree of alcoholysis desired.

The preferred alcoholysis catalysts are basic or acidic alcoholysis catalysts. It is preferred to use a basic alcoholysis catalyst of an alkali metal although other agents such as guanidine carbonate, sodium methyl carbonate, organic amines and the like can also be used. The preferred basic catalysts are hydroxides of alkali metals such as sodium hydroxide, potassium hydroxide, and lithium hydroxide and alkoxides of alkali metals such as sodium methoxide, potassium methoxide, sodium ethoxide, and the like. Acidic alcoholysis catalysts which can be used include sulfuric acid, hydrogen chloride, sulfonic and phosphonic acids, sulfur dioxide and the like.

Other alcoholysis catalysts which can be used include alkyl orthotitanates such as tetraethyl orthotitanate, tetrabutyl orthotitanate and the like as Well as derivatives thereof such as sodium hydrogen titanate, organic silicates, titanium tetrahalides and the like.

Hydrolyzed polymerized vinyl esters made by the dispersion techniques disclosed herein have many advantages over polyvinyl esters made by the prior methods. For example, the esters are easily isolated from the reaction media by such methods as filtration, spray drying, Centrifugation, and the like. Since these are prepared in dispersion there is no increase in viscosity during the alcoholysis step and the product may be obtained in high solids content of approximately 60 to 80 percent. This provides a means of obtaining a wide variety of polyvinyl esters of varying degrees of alcoholysis or hydrolysis, that is to say, products having degrees of hydrolysis of about 1 to 99%. The thus produced polyvinyl esters endow plastic paper produced therefrom with high uniformity, texture and bond strength.

It is preferred that the non-aqueous dispersions of the polymerized vinyl esters be prepared by a non-aqueous dispersion polymerization of the vinyl ester monomer used. The inert organic hydrocarbon diluent and interfacial agents given above as well as the concentrations given may be used for the non-aqueous dispersion polymerization of vinyl ester monomers. The dispersion polymerization polyvinyl ester particles obtained are unique in that they contain an outer coating of polymer organic interfacial agent attached to the polyvinyl ester particle. This is believed to result from the graft polymerization of the vinyl ester to the backbone of the interfacial agent as the vinyl ester homopolymerizes. Thus, for example, the dispersion polymerization of vinyl acetate using ethylene/ vinyl acetate copolymer as interfacial agent affords polyvinyl acetate particles with an outer coating of ethylene/ vinyl acetate grafted thereto.

Any free radical polymerization initiator known in the art may be used to polymerize the vinyl ester monomer including organic peroxides such as, benzoyl peroxide, lauroyl peroxide, capryloyl peroxide, diacetyl peroxide; azo catalysts such as, aZo-bisisobutyronitrile; and dialkylperoxy dicarbonates, such as diisopropylperoxy dicarbonate as well as redox initiators and the like. Although initiator concentrations in the range of about 0.01 to 1 percent, biased on the weight of vinyl ester, can be used, about 0.06 to 0.2 is preferred.

Temperatures for the vinyl ester polymerization of about 0 to 150 C. may be used although a range of about 25 to C. is preferred and a temperature of about 50 to 100 C. is particularly preferred.

Where polyvinyl esters other than those prepared by the non-aqueous dispersion technique are to be used they must be reduced in average particle size to a range of about 0.05 to 50 microns and then dispersed in the aforementioned diluents using one of the aforementioned interfacial agents to stabilize the dispersion.

-No special equipment is required for the alcoholysis of polymerized vinyl esters using the dispersion technique of this invention. Stirred reactors capable of being heated or cooled are well known in the polymerization art are satisfactory. The same equipment is also satisfactory for the polymerization of the vinyl esters in dispersion if it is desired to use such for the preparation of the hydrolyzed polyvinyl esters.

Pressure is not at all critical for any of the processes of this invention so that while atmospheric pressures are preferred for economic reasons, subatmospheric or superatmospheric pressures can also be used if desired.

Although two incompatible polymers can form the composition of the invention, as previously discussed, preferably the plastic paper composition is formed of partially hydrolyzed polyvinyl ester and at least two other polymers e.g. by weight 10 to 60 parts of partially bydrolyzed polyvinyl acetate, 20 to 60 parts polyethylene and 20 to 40 parts polystyrene. Particularly preferred is a pulp composition of by weight, 50 parts of polyethylene, 30 parts polystyrene and 20 parts of partially hydrolyzed polyvinyl acetate. Polystyrene adds considerably to the tensile modulus of the paper and accordingly is preferred as a third component.

As indicated above, fibrillation of the composite polymer filament is brought about by mechanical working, i.e. chopping and beating. Fibrillation is further accomplished by swelling in water of the hydrolyzed polyvinyl ester fiber component which promotes splitting of the strands and by dissolving in the water of a portion of the soluble hydrolyzed polyvinyl ester from the fibers during the pulping process, since voids left in the filbers by the dissolved ester hydroxy groups result in further fibrillation. Up to 30% or more weight loss of ester from extruded filament to paper product has been noted with no adverse effects on paper strength evident.

The following example is illustrative of this invention but is not intended to serve as any limitation or restriction thereof.

EXAMPLE I Nine mixes were prepared from polyethylene (PE), polystyrene (PS), and partially hydrolyzed polyvinyl acetate (PVAc(OH)) of three different degrees of hydrolysis. The acetate included as interfacial agent 0.6% by weight (based on the acetate) of ethylene-vinyl acetate copolymer containing 28% vinyl acetate polymerized therein and having a melt index of 23.8 dg./min. The components for each mix were fiuxed together on hot rolls, extruded, and oriented by being stretched approximately 800% after immersion in a glycerine bath at C. The strands were then pulped in water in a Nobel and Wood cycle beater with a clearance of two mils between roll and bed plate. Hand sheets were prepared from the pulps after dilution with white water from the same pulp, rather than with fresh water. This was done to minimize additional leaching of soluble components from the web during sheet formation. Original mix compositions and sheet density are shown in Table I. The average thickness of the sheets was about 5 mils. Two bleached sulfite cellulose sheet samples were also tested for comparison. The physical properties of the sheets described in Table I, were determined after brief conditioning in the laboratory atmosphere. Tensile (ASTM D 638), burst (Mullen), and tear (Elmendorf), are shown in Table I.

i.e., 40 to 60% of the total composition, give hand sheets with good tensile strength and unusually high tensile elongation, up to 30%. These products should be particularly useful in specialty applications and in combinations with other types of fibers.

In sum, synthetic polymer papers covering a wide range of useful property characteristics can be made by the ex- TABLE I [Physical properties of synthetic polymers and cellulose hand sheets] Pul type, PE PS/PVAe Tensile (OH percent) Sheet (Degree of Burst, Tear, Elongation, density, hydrolysis) p.s.i./mil gins/mil P.s 1 percent g./cc

Sample:

A H 50/40/10(36) 64 42 450 4. 5 0. 37 97 49 510 7. 0. 38 1. 37 59 660 9.0 0. 40 60 29 420 4. O 0. 37 2. 26 80 960 12. 0 0. 4O 1. 71 59 825 9. 0 0. 37 l. 51 740 10. 0 0. 36 2. 24 52 950 12. 0 0. 38 2. 66 64 1, 200 15. 0 0. 37 1. 1. 96 1, 050 l. 3 0. 54 2. 24 2. 1, 500 l. 8 0. 61

It can be seen that the plastic paper samples compared favorably with the cellulose paper samples, particularly those plastic paper samples having increased relative amounts of polyvinylacetate and increased degrees of hydrolysis. Also because of the lower comparative densities of the plastic sheet samples it is clear that the basic weight of a snythetic polymer sheet will be one-third less than that of a comparable cellulose sheet.

Economics considerations made it desirable to limit the amount of hydrolyzed vinyl acetate in a pulp to the minimum amount which will yield the desired properties in a sheet. Present indications are that 20 parts of PVAc(OH) with 80 of polyethylene plus polystyrene gives a reasonable balance of properties for comparison with a good grade of writing paper. There is, however, a wide range of specialty papers to be considered, and some of the compositions containing PVAc(OH) as the major component are of particular interest in this connection. The mechanical properties of three samples containing a high proportion of PVAc(OH) are listed in Table II. Two values are given for each composition. One is the value obtained on hand sheets made from stock which had been been subjected to intensive agitation in a Waring Blendor just before it was placed in a deckle box. The other is for sheets made from stocks which had only gentle agitation by a paddle in the proportioner before the sheets were made. Values from Table I for a 50-3020(48) composition, and for cellulose hand sheets are also included for comparison.

(1) High opacity (2) Low specific gravity (3) High burst strength for a given basis weight (4) Good wet strength (5 Capability for heat bonding (6) Good chemical resistance The fibers or the pulp of the present invention can also serve as or part of, woven and non-woven textile fabrics including garments, drapes and the like. In addition the fibers or the pulp can, within the scope of the present invention, be combined with cellulose to upgrade paper and other cellulose products. Note also US. Pat. 3,097,991 to W. A. Miller et al. issued July 16, 1963.

What is claimed is:

1. A fibrous pulp comprising oriented fibers of at least three incompatible melt-extrudable thermoplastic TABLE II [Properties of hand sheets containing a high proportion of PVAc(OH)] Composition, PS- Stock Sheet Bur t, Tear, Tensile, Elongation PE-PVAc(OH) agitation density p.s.i./rnil gm./rnil p.s.i. percent 5030 20(48) Gentle 40 2. 3 80 960 12 501040(52.5) Waring 49 5. 3 1. 25 2, 450 22 Gentle 48 4. 3 1. 19 l, 633 16 401050(52.5) Waring 51 6. 2 l. 00 2, 632 30 Gentle- 50 4. 6 94 2, 962 29 4()060(52.5) Waring 54 5. 7 1. 09 3, 154 26 Gentle 56 5. 6 94 2, 429 21 Cellulose Waring 61 2. 2 2. 3 1, 500 1. 8

(ble)ached sul- Gentle 54 1. 3 2. 0 1, 050 1. 3 fite PVAc OH) resins, said fibers having fibrillated surfaces, which furr" ther corn rises:

(a) that one of said thermoplastic resins is a partially hydrolyzed polyvinyl acetate having a degree of hydrolysis of from about 30 to 60 percent based on the mole percent of hydroxyl, and

(b) that at least two other incompatible thermoplastic resins have overlapping orientation temperature ranges.

2. The fibrous pulp of claim 1 wherein (b) is polyethylene and polystyrene.

3. A fibrous pulp comprising oriented fibers of a thermoplastic mix comprising at least three incompatible meltextrudable thermoplastic resins, said fibers having fibrillated surfaces and having diameters of from about 0.1 to 100 microns and lengths of from about A; to A inch, which further comprises:

(a) that one of said thermoplastic resins is from about 10 to 30 percent by weight of partially hydrolyzed polyvinyl acetate, having a degree of hydrolysis of from about 30 to 60 percent based on the mole percent of hydroxyl, and

(b) that at least two other incompatible thermoplastic resins are polyethylene and polystyrene; so that said pulp is capable of being formed into a sheet product having a tensile strength of from about 400 to 1200 p.s.1.

4. The fibrous pulp of claim 3 wherein there is from about 20 to 60 percent by weight of partially hydrolyzed polyvinyl acetate, having a degree of hydrolysis from about 45 to 60 percent based on the mole percent of 1O mer is ethylene-vinyl acetate.

8. Claim 6 wherein the alpha-olefin-vinyl ester copolymer is ethylene-vinyl acetate.

References Cited 15 UNITED STATES PATENTS 2,708,617 5/1955 Magat et al. 264-184 3,047,455 7/1962 Holmes et al 162-157 3,097,991 7/1963 Miller et al 162-157 3,236,788 2/1966 Johannsen 162-146UX 3,402,231 9/1968 Bynurn et at 162-146X S. LEON BASHORE, Primary Examiner A. L. CORBIN, Assistant Examiner U.S. C1.X.R.