Hot extrudable blend, thermoplastic body and its manufacture

HOT EXTRUDABLE BLEND, T~IERMOPLASTIC BODY AND ITS MANUFACTURE FIELD OF THE INVE~TION This invention relates to a hot e~trudable blend comprising vinylidene chloride-methyl acrylate copolymer and an acrylic polymer lubricant-type processing aid, a thermoplastic body having at least a discrete portion formed from the blend, and a method for preparing the thermoplastic body. The latter may for e~ample be a monolayer film formed from the hot e~trudable blend, or a multilayer film having one layer as for e~ample a core barrier formed from the blend.

BACKGROU~D OF THE INVENTION Primal meat cuts, or primals, are large cuts of meat, smaller, for e~ample, than a side of beef, but larger than the ultimate cut that is sold at retail to the consumer.

Primal cuts are prepared at the slaughter house and are then shipped to a retail meat store or an institution such as a restaurant where they are butchered into small cuts of meat called sub-primal meat cuts or sub-primals. Sub-primals may also be prepared at the slaughter house. When primals and sub-primals are prepared at the slaughter house, they are usually packaged in such a way that air (i.e., 02ygen) is prevented from contacting the meat during shipping and handling in order to minimize spoilage and discoloration.

One way to package primals and sub-primals so as to protect them from degradation due to moisture loss and contact with air is to shrink package them with a package material that has good barrier properties. One such shrink packaging D-20102 ~ ~ .

--2- material that has good oxygen and moisture barrier properties is vinylidene chloride-vinyl chloride copolymer film.

One approach to the provision of a film for use in shrink packaging primal and sub-primal meat cuts and processed meats is to employ a multilayer film having oxygen and moisture barrier properties, one layer of which is a vinylidene chloride-vinyl chloride copolymer film. ~he other layer or layers of such a multilayer film are selected so as to provide the requisite low temperature properties and abrasion resistance which are lacking in vinylidene chloride-vinyl chloride film. In providing such a film, however, it must be recognized that good barrier properties, abrasion resistance, and low temperature properties are not the only requirements for a film that is to be used for shrink packaging primal and sub-primal meat cuts. The film must have been biaxially stretched in order to produce shrinkage characteristics sufficient to enable the film to heat shrink within a specified range of percentages, e.g., from about 15 to 60 percent at about 90C, in both the machine and the transverse directions.

The film must also be heat sealahle in order to be able to fabricate bags from the film and in order to heat seal the open mouths of the fabricated bags when the meat cut has been placed within the bag. Additionally, the heat sealed seams of the bags must not pull apart during the heat shrinking operation, the film must resist puncturing by sharp edges such as bone edges during the heat shrinking operation, and there must be adequate adhesion between the several layers of the film so that delamination does not occur, either during the heat shrinking operation or during exposure of the film to the relatively high temperatures that may be reached during shipping and storage of the film in the summer time.

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It has been proposed to prepare multilayer films, one layer of which is a vinylidene chloride-vinyl chloride copolymer and at least one other layer of which is an ethylene-vinyl acetate copolymer. For example, such ~ilms are proposed in McFedries, Jr. et al U.S.

Pat.

No.

3,600,267; Peterson U.S.

Pat.

No. 3,524,795; Titchenal et al U.S.

Pat.

No. 3,625,348, Schirmer U.S.

Pat.

Nos. 3,567,539 and 3,~07,505; and Widiger et al U.S.

Pat. ~o. 4,247,584.

In addition, multilayer films comprising a core layer of a vinylidene chloride copolymer, wherein the layer is a copolymer of a vinylidene chloride monomer and a vinyl chloride monomer, are known, for e~ample, as disclosed in Bra~ et al, U.S.

Pat Nos. 3,741,253 and 4,278,738; Baird et al, U.S.

Pat No. 4,112,181; and Lustig et al Canadian Pat No. 982,983.

Also in the prior art, cross-linking by irradiation has been used to enhance the properties of films employed in packaging operations. However, it has been found that an irradiated multilayer film containing a vinylidene chloridevinyl chloride copolymer layer discolors significantly during storage due to degradation of this layer. It is believed that discoloration of the vinylidene chloride-vinyl chloride copolymer layer is due to radiation-induced cleavage of hydrogen and chloride radicals resulting in the production of double bonds and the associated chromophores.

Discoloration also occurs when this copolymer layer is e~posed to elevated (above ambient) temperatures for sustained periods.

Lustig et al U.S.

Patent No. 4,737,391 teaches that the aforedescribed discoloration problem may be avoided by using vinylidene chloride-methyl acrylate copolymer as the barrier layer in shrinkable multilayer films. The Lustig et al invention is based on the discover~ that this particular D-20102 ~ ' :, , copolymer does not significantly discolor from irradiation or sustained e~posure to high temperatures.

Notwithstanding this important advantage of no significant discoloration, there are processing difficulties associated with vinylidene chloride-methyl acrylate copolymers as compared with the vinylidene chloride-vinyl chloride type. The basic problem is that the methyl acrylate copolymer is very temperature and shear sensitive during extrusion into a film~ This extrusion can only be performed over a narrow temperature range without causing premature degradation of the polymer in the e~truder or die. The premature degradation causes particles or gels of degraded material to e~it from the extruder. These particles cause imperfections in the film and may result in a bubble break and waste of film product.

These limitations on the use of substantially pure vinylidene chloride-methyl acrylate barrier layers in multilayer films are to a considerable e~tent overcome by the improvement described in Schuetz U.S.

Patent No.

4,798,7~1. The patentee describes a blend of vinylidene chloride-methyl acrylate copolymer and vinylidene chloride-vinyl chloride copolymer with between about 2.9 and about 13.5 weight percent methyl acrylate, and between about 2.9 and about 11.6 weight percent vinyl chloride in the ~lend. Even though the major constituent of the barrier layer copolymer is methyl acrylate, such a film may be prepared by the double bubble extrusion method without eYperiencing the processing difficulties associated with methyl acrylate copolymer barrier layer-type multilayer films. Yet the film does not significantly discolor (i.e., yellow) on moderate irradiation dosage or exposure to elevated temperature for sustained periods, even though it contains a substantial quantity o vinyl chloride copolymer.

D-20102 ~' . . . : :, '' ' ' ' ' ' ~,: ' , Notwithstanding these advantages, even the Schuetz blend invention has certain limitations. It has been ~ound that after blending, the two resins tend to separate during processin9 and this tends to diminish the intrinsic advantages of the blend. Moreover, blending requires mixing equipment and to minimize separation by settlin~ during storage, the blending is most effective when done immediately prior to extrusion. This in turn requires additional labor and close coordination of the resin inventories with manufacturing schedules. Accordingly, it would be desirable to eliminate the need for using a substantial percentage of another resin (as for example vinylidene chloride-vinyl chloride copolymer) in the hot extrusion manufacture o~ thermoplastic bodies having at least a discrete portion consisting essentially of vinylidene chloride-methyl acrylate copolymer.

Physical properties considered important in processing of MA saran include its melting point and viscosity, the latter being hereinafter discussed in connection with E~ample 1. Melting point is a function of composition and in the case of MA saran is directly related to the methyl acrylate content. Pure vinylidene chloride is neither stable or e~trudable and the methyl acrylate is copolymerized therewith for stability and extrudability.

Too much methyl acrylate in the copolymer reduces its crystallinity and diminishes the barrier properties of the copolymer.

The prior art has discovered that thermoplastic copolymers such as MA saran cannot be effectively processed in the pure resin form. This is accomplished by the inclusion of several types of additives in very small quantities. These additives include plasticizers - liquids which are absorbed by the resin and serve to reduce its viscosity when heated to the molten state. Suitable plasticizers include dibutyl sebacate and epoxidized soybean oil. Stabilizers are added to improve thermal stability of '~ ' ' ' ' --6- the copolymer in the heated molten condition. As a consequence the copolymer (wi~h the added stabilizer) will take longer to degrade and ideally the time is e~tended beyond that occurring during processing.

The other major type of additive is generally termed a lubricant, and there are two varieties. The internal type reduces heating due to friction within the copolymer during e~ternal heating and physically imparted movement of the individual copolymer molecules against each other. The e~ternal type of processing aid affects the frictional contact between the copolymer's outer surface and the surrounding inner surface of the e~trusion equipment. The lubricant system of this invention is the e~ternal type, and provides lu~rication between the copolymer and the surrounding metal die surface.

Harrop U,S.

Patent No. 4,1S6,703 de~cribe~ the ua~ of acrylic polymers a~ lubricants for copolyethylene. In particular, a copolymer of ethyl acrylate with methyl methacrylate is described as aiding the processing of high density polyethylene, preferably having weight average molecular weight (Mw) of above 600,000. The acrylic polymer preferably has a weight average molecular weight (Mw) of over 100,000.

The Rohm and Hass brochure "ACRYLOID~ K-175 Lubricating Processing Aid for Polyvinyl Chloride" (April, 1985) ~escribes the use of this particular acrylic polymer as both an e~ternal lubricant and processing aid for polyvinyl chloride (PVC).

Unfortunately, these teachings are of little assistance to the practitioner in the processing i.e. e~tr~sion of vinylidene chloride-methyl acrylate copolymer resin. The main reasons are that the polymers are different, i.e.

polyethylene and polyvinyl chloride versus vinylidene chloride-methyl acrylate, and the materials of construction D-20102 ' :' ,' ' ;

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for the e~trusion dies are also different. Chromium plated stainless steel is the preferred PVC e~truder screw and die construction material whereas nickel alloys are preferred for vinylidene chloride-methyl acrylate (MA saran) extruder screws and dies.

Another reason why the PVC and MA saran lubricant/ processing aid requirements are nonanalogous i5 that MA saran is much more heat sensitive tha~ PVC, a~d when it degrades MA saran becomes very tacky and adheres to the surrounding metal surface. Accordin~ly, the effectiveness of additives is more critical in MA saran processing than in PVC processing. A further reason or different processin~ aid requirements is the difference in PVC and MA sara~ reactivities (due to their different compositions?.

For e~ample, zinc stearate is commonly used as a lubricant in PVC processinq, but heavy metals such as zinc react violently with MA saran during processin~. Finally, different types of stabilizers are used for PVC and MA saran processing, again because of their different compositions and reactivities. For e~ample, organo tins are used with PVC as for e~ample butyl tin mercaptide. In contrast, these organo tins cannot be used with MA saran and stabilizers such as epo~idized soy bean oil and tetra sodium pyro phosphate ("TSPP") are commonly used as stabilizers in MA saran processing.

Accordingly, there is no basis for predictin~ the requirements for MA saran e~trusion processing from prior teachings on PVC extrusion processing. This conclusion is supported by the results of the processing aid screening tests of E~ample 1, discussed hereinafter.

An ob;ect of an aspect of this invention is to provide an MA saran resin with improved ext~rnal lubrication during hot extrusion to form a thermoplastic body of desired shape.

An object of an aspect of the invention is to provide an MA saran resin blend characterized by retarded thermal degradation (due to less shear) during hot extrusion.

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An object of an aspect of the invention is to provide an MA saran resin blend chara~terized by reduced adhesion to the surrounding metal scrPw and die wall during hot extrusion.

An object of an aspect of the invention is to provide a hot extrud~d thermoplastic body having at least a discrete ~ saran portion, and characterized by lower metal adhesion and thermal degradation during extrusion.

An object of an aspect of the invention is to provide a hot extruded MA saran biaxially oriented and heat shrinkable flexible film having good physical characteristics.

An ob~ect of an aspect of the invention is to provide a hot extruded MA saran rigid member having good physical characteristics.

An object of an aspect of the invention is to provide an improved method for preparing a thermoplastic body having at least a discrete portion consisting essentially of MA saran, which method is characterized by processing ease and low waste rate at least equivalent to that achieved with MA saran, vinylidene chloride-vinyl chloride blends.

These and other objects and advantages are provided by this invention as described hereinafter.

8~MMARY OF ~E INV~NTION An aspect of this invention is as follows:

A hot extruded thermoplastic body having at least a discrete portion consisting essentially of vinylidene chloride-methyl acrylate copolymer with between about O.5 and 5.O parts acrylic polymer per hundred weight of said vinylidene chloride-methyl acrylate copolymer, said acrylic polymer comprising methylmethacrylate-butylacrylate-ctyrene copolymer and being uniformly dispersed in said discrete portion as a lubricant-type processing aid, and in su~ficient quantity to provide lower metal adhesion and thermal degradation o~ said vinylidene chloride-methyl aarylate copolymer during the hot ex~rusion.

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.: : . ' , ' , ' : ` The hot e~truded thermoplastic body may be either rigid or fle~ible. Rigid bodies include relatively thick walled (e.g. greater than 10 mils) sheets and containers of the type used for extended shelf life food storage. Fle~ible bodies include relatively thin walled (e.g. less than 10 mils) films, especially the bia~ially oriented heat shrinkable type used to fabricate bags for shrink wrapping of fresh red meat and poultry. Since MA saran is an o~ygen and moisture barrier, one common use is in rigid containers or heat shrinkable films for storing food products which deteriorate when e~posed to o~ygen and/or moisture over a sustained period. Accordin~ly, the discrete portion of the hot e~truded thermoplastic body may for e~ample be a layer of a multi-walled rigid container or a core layer of a multilayer film fabricated into an evacuable and sealable bag with an inner heat sealable layer in direct contact with the enclosed food and an outer layer formed of a material selected for its abuse resistance and general toughness.

Alternatively, the MA saran discrete portion may form the ~ntire hot e~truded thermoplastic body, as for e~ample when a rigid container is hot e~truded from a single die processing only essentially 100% MA saran. Also, the thermoplastic body may be a bia~ially oriented heat shrinkable monolayer film such as a meat ca~ing.

other aspects of this invention are as follows:

A hot extrudable blend comprising vinylidene chloride-methyl acrylate copolymer and between about 0.5 and 5.0 parts acrylic polymer lubricant-type processing aid per hundred weight of said vinylidene chloride-methyl acrylate copolymer, said acrylic polymer comprising methylmethacrylatebutylacrylate-styrene copolymer, and being present in su~ficient quantity to provide lower metal adhesion and thermal degradation of said vinylidene chloride-methyl acrylate copolymer during the hot extrusion.

A method for preparing a thermoplastic body having at least a discrete portion consisting essentially of vinylidene chloridemethyl acrylate copolymer, comprising:

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: . ~ -- 10 -a) blending with said copolymer with between about 0.5 and 5.0 parts acrylic polymer per hundred weight o~ said vinylidene chloride-methyl acrylate copolymer, said acrylic polymer comprising methylmethacrylate-butylacrylate-styrene copolymer;

b) heating and extruding at least the blend of a) as the sole material in a particular extrusion flow path bounded by metal walls, to form a thermoplastic body with lower metal adAesion and reduced thermal degradation o~ the polymer mixture during the extrusion.

~ he MA saran-acrylate polymer mi~ture may be simultaneously e~truded, i.e. coe~truded with other thermoplastic polymers in separate flow paths which are adhered to each other in the molten state at the discharge end of the e~truder. For e~ample, a three layer film may be formed by coe~truding an MA saran core barrier layer and separate thermoplastic polymer layers (as for e~ample from ethylene ~inyl acetate) on each side of the MA saran core barrier layer. The three hot extruded layers are then directly adhered to each other~at the extruder discharge end in a multilayer die. Alternatively, the MA saran-acrylate polymer mi~ture may be e~truded as a monolayer film and used in this form. In this embodiment the MA saran monolayer film comprises the entire thermoplastic bag.

With respect to the multilayer film embodiment of the invention, the film may also be manufactured by the coating lamination process as for e~ample described in ~rax et al U.S.

Patent No. 3,741,25~. In this process the first layer is e~truded by itself and then a second layer as for example the MA saran-acrylate polymer mi~ture is coated thereon.

If still another layer is desired, a third layer may be coated on the opposite side of the second layer so that the second layer becomes a core and is enclosed between the first and third layers. Alternatively, a pair of layers may be coextruded and one or more additional layers applied by coating lamination to this substrate.

If the film embodiment of the invention is to be used in shrink wrapping a product as for example food, the film is D-20102 A .

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oriented as by "tenter framing~ or by bubble inflating.

These well-known procedures stretch the film in the transverse or longitudinal direction, or both. The latter (biaxial orientation) is most commonlY achieved by the "trapped bubble~ or "double bubble" procedure as for example described in Pahlke U.S.

Patent No. 3,55~,604.

For increased puncture resistance of thin layer film embodiments of the invention, it may be desirable to cross- link at least the outer layer of a multilayer film or the entire film if the monolayer type. This may be done by adding crosslinking agents to the resin blend as for example pero~ide or siloxane compounds. Alternatively, crosslinking may be accomplished by irradiation as for example with an electron beam, preferably at a level of at least 1 megarad (MR) and most preferably in the range of 2-5MR. Irradiation may be performed on a single layer such as the substrate layer of the multilayer film prepared by the coating lamination method described in the aforementioned Brax et al U.S.

Patent 3,741,253. Alternatively, the entire multilayer film may be irradiated and preferably after biaxial orientation (if such be employed) in accordance with the teachings of Lustig et al U.~.

Patent 4,737,391.

With respect to the rigid thermoplastic body of this invention as for example a sheet, those skilled in the art understand that a typical extrusion line consists of a blender, a dryer, feed hopper, extruder (single or twin screw), die, quenching section, pull rolls, and winder or sheeter. Melt filters and metering pumps improve quality and uniformity. The quench section may be a chilled casting drum, water bath or three-roll stock, depending on the material, sheet thickness and extrusion rate.

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DETAILED DESCRIPTION OF T~E INVENTION The vinylidene chloride-methyl acrylate copol~ner molecular weight in the practice of this invention is preferably at least about 7~,000 to readily achieve biaxial orientation (if d~sired) and film/sheet toughness, and up to about 130,Q00 for ease o~ e~trudability. The most preîerred range is between about 90,000 and about 110,000 molecular weight. Also, the methyl acrylate content in the copolymer is preferably between about 5 weight percent for ease of e~trudability and up to about 15 weight percent which is in the U.S.

Federal Food and Drug Administration's limit. The most preferred range for methyl acrylate content is between about 6 and about 10 weight percent in the copolymer with vinylidene chloride.

In addition to the aforedescribed major types of additives comprising plasticizers, stabilizer and processing aid/lubricants, the hot e~trudable blend of this invention may include other conventional additives such as slip agents, anti-blocking agents and pigments as is well known in the art. The blend preferably contains less than 5 parts per hundred plasticizer based on the copolymer weight, in order to maximize the barrier properties of the vinylidene chloride-methyl acrylate copolymer. Also, the blend preferably contains between about 2 and about 10 parts additives per hundred weight of said copolymer other than the acrylic polymer. Lower concentrations may not provide enough additives to per~orm their intended functions whereas higher concentrations may significantly reduce the overall barrier properties of the copolymer. The aforedescribed perferences for plasticizer and additive (other than acrylic polymer) concentrations also apply to the other aspects of the invention, i.e. the thermoplastic body discrete portion and the blend used to prepare same.

D-20102 Suitable acrylic polymers for the prac~ice of this invention include those described in the aforementioned Harrop U.S.

Patent No. 4,156,703. These acrylic polymers preferably have a weight average molecular weight Mw of at least 100,000. Moreover, they are pre~erably polymeri7ed f rom a monomer system comprising at least 50% by weiqht of at least one ester of acrylic acid or methacrylic acid. The ester may f or e~ample he selected f rom the group consisting of methyl methacrylate, isobornyl methacrylate, methyl acrylate, ethyl acrylate and butyl acrylate. The ester is preferably a mi~ture in the Cl to C35 range. Still more preferably the acrylic polymer is polymerized f rom a monomer system comprising at least 50% by weight methyl methacrylate and the remainder another acrylate.

The preferred acrylic polymer is the aforementioned ACRYLOID (now called PARALOID)~ K-175.

The acrylic polymer is present in a concentration of at least 0.5 parts per hundred w~ight copolymer. Significantly lower concentration do not provide the desired lower metal adhesion and thermal degradation of the vinylidene chloride-methyl acrylate copolymer during hot e~trusion.

The acrylic polymer is preferably below about 5 parts per hundred weight copolymer as e~cessive lubrication reduces the desired friction and shearing efficiency in the e~truder.

For thin fle~ible heat shrinkable three layer films of this invention, the first outer layer is preferahly an ethylene-vinyl acetate copolymer containing from about 9 to about 15 weight percent of vinyl acetate, based on the weight of the copolymer, said copolymer having a melt inde~ of between about 0.1 and about 1.0 decigram per minute, and it may be selected from the group consisting of (a) a single ethylene-vinyl acetate copolymer and (b) a D-20102 -14 blend of ethylene-vinyl acetate copolymers having melt indices and vinyl acetate contents within the aforementioned ranges of values.

The second outer layer of this preferred thin flexible heat shrinkable three layer film comprises an ethylene-vinyl acetate copolymer selected from the group consisting of (a) an ethylene-vinyl acetate copolymer having a melt index of between about 0.1 and about 1.0 decigram per minute, and a vinyl acetate content of from a~out 18 weight percent, most preferably from about 10 to about 15 weight percent, based on the weight of said second ethylene-vinyl acetate copolymer, and (b) a blend of at least two ethylene-vinyl acetate copolymers, wherein one of said ethylene-vinyl acetate copolymers has a melt inde~ of from about 0.1 to about 1.0 decigram per minute and a vinyl acetate content of from about 10 to about 18 weight percent, based on the weight of said copolymer, and the other ethylene-vinyl acetate copolymer has a melt index of from about 0.1 to .

ahout 1.0 decigram per minute and a vinyl acetate content of from about 2 to about 10 weight percent, based on the weight of said copolymer. The blend (b) of said two ethylene-vinyl acetate copolymers has a vinyl acetate content of from about 3 to about 18 weight percent, and preferably from about 10 to about 15 weight percent, based on the weight of said copolymers.

In addition to ethylene-vinyl acetate, one or both of the outer layers may be formed of ethylene-alpha olefin polymers as for example linear low density polyethylene (LLDPE) or very low density (sometimes referred to as ultra low density) polyethylene or mixtures thereof with EVA.

This type of thin flexible multilayer film will generally have a total thickness of from about 1.75 mils to about 4.0mils, and preferably of from about 2.0 mils to about 4.0 mil, because when the thickness of the multilayer D-20102 . ~' .

film i5 more than 4.0 mils, clipping problems are encountered in that it is difficult to gather together the open end of a bag made therefrom. When the thickness of the multilayer film is less than 1.75 mils, the bag will have diminished puncture resistance.

The first outer layer will pre~erably have a thickness of from about 1.1 mils to about 2.0 mils; the core layer will preferably have a thickness of from about 0.20 mil to about 0.45 mil; and the second outer layer will preferably have a thickness of from about 0.4 mil to about 1.5 mils.

The thickness of the first outer layer, which is the inner layer of the bag, is preferably within the aforementioned range because the sealing and processability properties of the film layer would otherwise be diminished. The thickness of the core layer is preferably within the above-indicated range because the film would provide inadequate barrier properties of the core layer thickness is less than about 0.20 mil. The preferred upper limit of 0.45 mil for the core layer is based on the barrier effectiveness needed for intended uses of the multilayer film. The thickness of the second outer layer, which is the outer layer of the bag, is preferably in the aforementioned range to provide desired toughness and puncture resistance and also keep the total thickness of the film in the range from about 1.75 mil to about 4.0 mils.

Bags suitable for the shrin~ packaging of primal and subprimal meat cuts and processed meats are provided from the aforedescribed heat shrinkable multilayer film. The bags may be produced from this film by heat sealing. For instance, if the film is produced in the form of tubular film, bags can be produced therefrom by heat sealing one end of a length of the tubular film or by sealing both ends of the tube, then slitting one edge to form the bag mouth. If the film is made in the form of flat sheets, bags can be D-20102 ..

formed therefrom by heat sealing three edges of two superimposed sheets of film. When carrying out the heat sealing operation, the surfaces which are heat sealed to each other to form seams are the aforedescribed first outer layers of the films. Thus, for e~ample, when forming a bag by heat sealing one end of a length of tube film, the inner surface of the tube, i.e. the surface which will be heat sealed to itself, will be the first outer layer of the film.

Although the fle~ible multilayer film is specifically described in the form of a three layer film, in its broadest conte~t only two layers are required: the barrier layer and one outer layer. More than three layers are also contemplated, ~or e~ample, a five layer film comprising outer layers of polypropylene or ethylene-propylene copolymer, the aforedescribed blend as a barrier layer and an adhesive layer between each outer layer and the barrier layer.

The invention will be more fully understood by a reading of the following e~amples.

EXAMPLE I In this Example the properties of various processing aids were tested in several copolymer resin formulations including 100% MA saran, 75% MA saran - 25% vinylidene chloride-vinyl chloride copolymer, and 100% vinylidene chloride-vinyl chloride copolymer. Two-roll milling tests were performed using 6 inch diameter, 13 inch long counterrotating rolls longitudinally positioned in parallel relation. These rolls were oil heated internally so that the roll surfaces could be maintained at about 325F. The front roll was rotated at S0 RPM and the rear roll was rotated at 68 RPM.

In each instance, 600 gms of resinadditive formulation was added to the heated turning rolls D-20102 .

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and flused. A~ter complete melting had occurred, the evaluation began and observations were made as the test continued. The test was stopped when a high degree of formulation degradation occurred. Adhesion to metals was analyzed on the basis of the roll time for significant adhesion to the metal rolls. Also, the estent of adhesion was evaluated on the basis of visual observation and reported on the basis o~ qualitative ratings between 0 (no adhesion to the rolls) and S (masimum adhesion to the rolls).

Chemical degradation hence tackiness of the formulation was also determined in the two-roll millin~ tests on the basis of the time required for formulation foaming to occur. When degradation oecurs, gases are formed in the melt and their release is visually observed in the form o~ foam. The estent of foaming, i.e. thermal degradation, was also reported on the basis of qualitative ratings ~etween 0 (no foaming) and S (masimum foaming).

The processing aid-containin~ copolymer formulations w~re also evaluated in kne~ding tests using a high intensity Brabender miser, i.e. a Brabender Plasticorder~ torque rheometer maintained at 300~F with a No. 6 mi~er roller head operated at 50 RPM. The test procedure was to add 10 gms of resin-additive formulation to the mi~er. The torque was recorded for lO minutes and the ~ormulation removed. The time for fusion torque to occur was measured along with fusion torque itself (in meter grams/sec.). As understood by those skilled in the art, fusion torque is a ma~imum value and a measure of ma~imum viscosity. As mixing is continued, the torque diminishes to a stable level, referred to as final torque which was measured after the aforementioned lO minutes. If the torque values are too low, there is a tendency towards non-uniform circumferential distrihution of resin melt in the various sections of the estruder and this in turn results in nonstable operation.

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-18 If the torque is too high, e2cessive melt pressure results at the onset of melting in the extruder barrel.

To obtain a visual indication of melt degradation during kneading, after stable operation was reached (lO minutes) the mixer was stopped. The mixed formulation color was determined and rated between 0 (no color development and minimum thermal degradation) and 5 (maximum discoloration and thermal degradation).

Certain of the processing aid-containing polymer formulations performing most favorably in the two-roll mill and kneading tests were then extruded at constant throughput of 18 lbs/hr. MA saran. In these e~trusion tests the dies were ~'Duranickel~ type 201 age hardened (93.7% nickel, 0.5% nickel and a maximum of 0.2% carbon, all by weight). The e~truder screws were stainless steel. The e~trusion parameters measured included screw speed, melt pressure and electric current requirement. During e~trusion the e~trudate was e~amined for dark smears and black particles.

Also, any throughput variation was observed. Based on these parameters, the e~trusion performance was qualitatively rated between l (best performance) to 5 (poorest performance). That is, a rating of l indicates that the waste rate in production would be very low and commercially acceptable based on previous experience, whereas a high rating indicates the likelihood of progressively increasing and unacceptable waste rate.

The results of these tests are summarized in Table A.

It will be noted that sample nos. l through lO are 100% MA saran with various processing aids other than acrylic polymer and sample ll is 75% MA saran - 25% vinylidene chloride, vinyl chloride copolymer with 0.1 pph.

polyethylene wax processing aid. The latter embodies the blend invention of aforementioned Schuetz U.S.

Patent No.

4,798,751 and has proven satisfactory in commercial use.

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--19- Accordingly, a 100% MA saran formulation with processing characteristics equivalent to sample ll would represent a substantial improvement by eliminating the comple~ities and performance limitation of blending two resins.

Sample 12 is a 100~ vinylidene chloride-vinyl chloride, (hereinafter sometimes referred to as "PVDC") 0.5 pph.

magnesium stearate processing aid formulation, and the same has also been used commercially.

However, the PVDC type polymer has the previously discussed limitation of degradation and discoloring from irradiative cross-linking treatment. Accordingly, sample 12 is also a basis for comparison and a 100~ MA saran formulation with equivalent processing characteristics would represent a substantial improvement because it does not significantly degrade and discolor when exposed to moderate irradiation dosage.

Samples 13 through 18 are 100% MA saran ~ormulations employing PARALOID K-175 acrylic polymer as the processing aid in concentrations between 0.5 and 5.0 pph. Accordingly, they are embodiments of the hot e~trudable blend aspect of the invention. Referring now to sample 13, it provides an adhesion time of 10 minutes and an adhesion rating of 2 in the two-roll milling test. Of the other 100% MA saran formulations only sample 3 (5 pph. fluoroelastomer processing aid concentrate) and sample 6 (0.3 pph. high density polyethylene wax processing aid) provided equivalent performance in the two roll milling adhesion test. The foaming-degradation performance of samples 3 and 6 was slightly better than the acrylic polymer processing aid formulation sample 13 (10 minutes versus 7 minutes foaming time).

A comparison of the kneading (viscosity) tests indicates that the performances of the three samples (nos. 3, 6 and 13) were about the same. However, the color rating of sample 6 (0.3 pph. HDPE processing aid) was unacceptably D-20102 .

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- ', ' -20 high at 4.0 compared to 2.5 for samples 3 and 13, indicating substantial degradation.

Because of their superior performance in the two-roll milling and kneading tests, the 100% MA saran formulations of samples 3 and 13 ~along with other 100% MA saran formulations samples 1 and 7) were subjected to an extrusion test. The throughput was the same for each formulation - 18 lbs/hr. MA saran. Inspection of the overall extrusion performance indicates substantial operating problems for all of the 100% MA saran-processing aid formulations with the e~ception of acrylic polymer processing aid sample 13.

That is, the latter has a rating of 1 whereas samples 1, 3 and 7 has ratings o~ 3 or ~. The latter ratings are on the basis of brown smears and black particles exiting the die, indicating chemical degradation. These variations were undoubetly due to accumulation of particles in the extruder passageways which gradually built up and eventually were released as particles which discharged with the e~trudate.

This release resulted in a loss of purity, o~f color appearance and high waste rate. Moreover, any impurities in the form of particles may eventually cause bubble break if the extrudate is in the form of a tube which is inflated for orientation.

It should also be noted that the overall extrusion performance rating of 1 for the acrylic polymer processing aid sample 13 is the same as the e~trusion performance rating for 7S% MA saran-25% PVDC sample 11 and 100% PVDC sample 12.

Inspection of the data for other 100% MA saran-acrylic polymer formulations (samples 14-18) indicates that similar improvement may be obtained over a range of at least 0.5 to 5.0 pph. acrylic polymer and with lower plasticizer levels in the formulation.

D-20102 ' ' : , ~ ' , A comparison of certain individual e~trusion process characteristics (screw speed, melt pressure and required electric current) indicates that the values for the 100% MA saran-acrylic polymer processing aid formulation are similar to those for 75% MA saran-25% PVDC and 100% PVDC with different types of processing aids. Accordingly, Table A indicates that the present invention provides substantial advantages over prior art vinylidene chloride copolymer-processing aid formulations in terms of hot e~trusion manufacturing characteristics.

Several of the processing aids used in the Example 1 tests are advertized by their manufacturers as suitable for inproving the processability of PVC resin. These include polyolefin waxes, calcium stearate, magnesium stearate and OP wa~. AS previously ac~nowledged, acrylic polymers such as PARALOID K-175 are also advertized for this purpose.

E~ample 1 demonstrates that their effectiveness as processing aids for 100% MA saran varies greatly, and in fact PARALOID K-175 is far superior to the others.

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.lo~O.¦o oN O O O_~ oLn~ oLnN oLn- LnLr O 0O 0,_ 0~v 06Ln~ 0-106 ¦-OI-OI_LC _O -ol-ol-oc _OO _OO _OO ~ _C ~N ~3 ~3C ~O ~-1~¦_-~ol_I I l l l l _ ~ZI INI~ ~lLnl~l~ ~ cn O _ Nl~ ~ Ln ~~ D-20 102 -23 E~ample 2 Tests were conducted using commercial equipment wherein 100% MA saran-containing film was manufactured by a double bubble process similar to that disclosed in the aforementioned Pahlke U.S.

Patent No. 3,456,014. The films were the three layer type and prepared by coe~trusion with a saran core layer between two EV~ layers, followed by biaxial orientation. Two different MA saran formulations were used: sample 19 was very similar to sample 13 (Example 1) and contained 2.0 pph. PARALOID K-175 acrylic polymer processing aid. Sample 20 was identical to sample 19 except that it did not include the acrylic polymer processing aid.

The three layer film compositions prepared from samples 19 and 20 were otherwise identical. The same e~truder was used for both formulations. The dies were the aforementioned Duranickel and the extruder screws formed of stainless steel.

Sample 20 was briefly e~truded for comparison with sample 19. This formulation had relatively low throughput rate per e~truder revolution, indicating poor feeding. Melt pressure variation was 12% with the only variation noted being due to screw rotation.

A total of 1515 pounds of commercial quality film were made using the sample 19 formulation. The measured film properties were substantially identical. The MA saran pressure trace was stable over time and an 11% melt pressure variation was noted at the frequency of screw rotation. The bubble break rate and the waste rate for the acrylic polymer-containing sample 19 were about 64-65~ of the otherwise identical sample 20 without the acrylic polymer processing. Resin feeding was much more efficient with sample 19 than with the sample 20 formulation, and there was a 40% increase in the throughput rate per extruder revolution.

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-24 The results of these tests are summarized in Table B.

The following test methods were used in determining the properties of films in the examples. Tensile strength and elongation values were obtained following ASTM Method D-882, procedure A.

Oxygen transmission was tested in accordance with ASTM Method D-39~5-81. Haze was measured in accordance with ASTM Method D-1003, Procedure A, and gloss was measured accordance with ASTM Method D-523, 45 angle.

Non-ASTM test method employed are described in the following discussion. Shrinkage values were obtained by measuring unrestrained shrink at 90C for five seconds.

The dynamic puncture-impact test procedure is used to compare films for their resistance to bone puncture. It measures the energy required to puncture a test sample with a sharp bone end. A Dynamic Ball Burst Tester, Model No.

138, available from Testing Machines, Inc., Amityville, Long Island, NY, is used and a 3/8 inch diameter triangular trip, as aforedescribed, is installed on the tester probe arm and employed in this test procedure. Six test specimens approximately 4 inches square are prepared, a sample is placed in the sample holder, and the pendulum is released.

The puncture energy reading is recorded. The test is repeated until 6 samples have been evaluated. The results are calculate in cm-kg per mil of film thickness.

Film color measurements were obtained using the CIELAB (Commission Internationale de l'Eclairage) uniform color space.

D-20102 Table B Effect of Acrylic Polymer on Extrusion Production _ mPle 19 Sample 20 Good Production, lbs. 1515 259 Screw Speed, rpm. 11.7 16.4 Melt pressure, PSI 520~ 4900 Melt Pressure Variation, PSI 580 600 Total Throughput, lb/hr.115 114 Film ProPerties Shrinkage @90C, % MD/TD51/59 51/58 Haze % 4 7 4-7 Gloss, Hunter Units (45 angle) 84 81 EXAMPLE 3 Tests were conducted using commercial e~trusion equipment (substantially identical to the commercial extruder of Example 2) to compare the physical properties of 100% MA saran core layer, three layer film with the aforedescribed 75% MA saran-25% PVDC core layer, three layer film in commercial use. The outer layers on each side of the core layer were EVA. The 100% MA saran formulation of this sample 21 was very similar to sample 19 e~cept that the formulation contained 0.1 pph inorganic stabilizer and a reduced 3.5 pph plasticizer level.

The manufacturing method was the same coextrusion followed by biaxial orientation using the double bubble process, as previously described.

The physical properties of sample 21 are summarized and compared with a 75% MA saran-25% PVDC core layer, three layer film sample 22 (which is very similar to sample 11) in Table C.

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Table ~ Film Properties with Acrylic Polymer Sample 21 Sample 22 Core layer 100% MA saran 75% MA saran, 25% PVDC Haze, % 4-9 5 Gloss, Hunter Units (45 angle) 79 80 Shrink Force, g/mil MD/TD @90C 130/160 130/140 @ room temp. 135/160 45~55 Shrink at 90C, % MD/TD 45/54 44/53 2 transmission, cc/100 in2/24 hrs~atm 1.53 1.5 Tensile Strength, psi MD/TD g400/10,000 9000/10,000 Elongation at Break, ~ MD/TD 155~155 150/170 Secant Modulus, psi MD/TD at room temp. 25M/27M 25M/25M at 40F 72M/72M 70M/70M Dynamic Puncture cmkg/mil 1.9 1.7 Hot Water Puncture at 90C, sec 20 20 Table C demonstrates that an acrylic pol~ner processing aid-containing 100% MA saran core layer type three layer (EVA outer) film has physical propertiPs fully equivalent to a commercially employed 75~ MA saran-25% PVDC core layer, otherwise identical film. This includes about the same o~ygen transmission despite the addition of acrylic polymer. The reason is that a lower plasticizer content can be employed and the two effects balance each other.

E~ample 4 In another series of tests, the e~truder melting mechanism of the present invention was visually observed and found to be substantially different from the conventional melting mechanism of prior art PVDC-processing aid blend formulations.

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-27 More specifically, these tests were performed in a 2 1/2 in. diameter 20:1 instrumented e~truder operated at a constant screw speed of 13 RPM. The die and screw were constructed of Duranickel. The extruder was capable of being "crashed", an e~pression used in the art to describe a sequence whereby stable e~trusion operation is first achieved, then the resin flow is stopped and coolant is passed through passages surrounding the extruder barrel to "freeze~ the material at various s~ages of heating, melting and plastic flow through the e~truder. By this procedure the condition of material may be observed within the disassembled extruder.

In these tests, formulation sample 12 (PVDC-0.5 mg.

magnesium stearate processing aid) and formulation sample 13 (100% MA saran-2.0 mg. acrylic polymer processing aid) were used. After stable operation was reached (as indicated by constant melt pressure), each formulation was crashed.

Inspection of these PVDC sample 12 indicated that the melt behavior was conventional, that is, a distinct demarcation e~isted between the upstream solids and the progressively increasing melt pool across the entire e~truder cross-section. For this particular extruder, PVDC melting began at screw flight 8 and was complPted at screw flight 18.

The e~truder melting mechanism or behavior for the 100% MA saran-acrylic polymer processing aid formulation was quite different, and in my e~perience unique. Instead of a gradual or progressive transition from solid resin to melt across the entire extruder cross-section, the as-formed thin melt pool initially surrounded the solid bed in ringlike fashion. Then during longitudinal movement towards the discharge end the melt pool became progressively larger, began penetrating the surrounded solids mass which ~ecame progressively smaller until it was completely dissolved in the melt. In this sequence, the MA saran-acrylic polymer D-20102 , ' ' ' ' ' " '' ' ' ' '. ' ~ ~ :. .

-28 blend first formed as a thin ring around the solid mass at flight 8, had penetrated the solid mass at flight 12 and total melting had occurred at flight 17. Although not fully understood, it is theorized that the acrylic polymer processing aid was such an effective lubricant that there was very little shear force between the extruder walls and the e~ternal surface of the flowing mass. Accordingly, most of the malting occurred due to conductive heating from the e~truder barrel and very little heating occurred due to shear. The data from these tests is summarized in Table D.

D-20102 -29 Table D Crash Test Processinq Performance PVDC Sample 12 100 MA saran/ acrylic polymer Sample 13 Throughput lb/hr 48 52 E~truder Load, amps. 40 + 0.4 38 + 0.8 Temperature, F point 1 273 271 point 2 280 280 point 3 289 293 melt-surface 343 333 melt-center 347 336 Pressures, psi point 1 3560 + 500 2610 + 480 point 2 7320 + 450 5000 + 500 point 3 3960 3900 melt 1860 + 15 1970 -~ 20 Inspection of Table D indicates that at constant screw speed, the throughput of this invention was significantly higher and the required electric current was lower. Also, the resultant melt temperature was lower.

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', , , '' ` ', ~ . . ~, . , . ., : . - . . . ~ . ~ -30 Example 5 In still another series of tests a commercial extruder system (substantially identical to that used in Example 2) was ~irs~ operated with a 7S% MA saran-25% PVDC core layer, three layer film of the sample 19 type. Then it was converted without shutdown to a core layer 100% MA saran-25 pph PARALOID K-175 acrylic polymer processing aid - O.25 pph inorganic stabilizer (sample 23). The outer EVA layers were not changed. The conversion proceeded smoothly with no change in melt pressure or e~truder load. The core layer screw tip temperature increased ~F and the MA saran throughput increased 2 lbs./hr. (5.5%). Melt pressure was very stable throughout the conversion and sample 2~ extrusion. A high break rate was e~perienced with sample 23 but this was not believed due to the use of acrylic polymer processing aid.

Then sample 23 was replaced with a blend without the inorganic stabilizer ~sample 24) but otherwise identical to sample 23. The bubble heat rate was reduced to the same level as with sample 19 and processing conditions remained very stable. The throughput rate was 0.75% higher than with sample 19.

This Example 5 along with Example 2 demonstrates that the invention permits higher throughput rates than with the prior art 75~ MA saran-25% PVDC blend system and at stahle operating conditions.

Although preferred embodiments of this invention have been described in detail, it is contemplated that modifications thereof may be made and some preferred features may be employed without others.

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