CA1163076A - Multilayer film manufacture utilizing scrap resin - Google Patents

Abstract

MULTILAYER FILM MANUFACTURE UTILIZING SCRAP RESIN

ABSTRACT OF THE DISCLOSURE

A novel system for producing film by extruding and/or casting at least two layers of resin, such as polyacrylonitrile homopolymer or interpolymers. The system provides for co-extruding a continuous multi layer film with contiguous or tandem dies from a first supply of homogeneous resin solution and a second supply of non-homogeneous redissolved resin scrap. The film may be cast onto a smooth cooled drum surface to form substantially continuous adjacent layers from two or more resin supplies. By stripping the film from the drum as a continuous film strip and stretching the film an oriented structure is provided. Scrap resin, such as selvage trimmed from the stretched film or mill scrap, is comminuted and redissolved in solvent for recycle to the second supply of the extrusion step. The homogeneous layer prevents film disruption by inhomogeneities present in the contiguous layer containing redissolved scrap, which might cause localized stresses in the film and discontinuities during stretching.

Description

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The present invention involves a method and apparatus for utilizing selvage or scrap from polyacrylonitrile (PAN) film manufacturing operations.
In processing polymeric films of various types for orientation by stretching the film, a certain amount of edge portions, ends and other film scrap is generated.
In some production lines it is not unusual to have lû to 30% or nore of selvage materials, whlch should be recycled into product if the manu~acturing facility is to be economic.
Various processes have been devised for using marginal strlps and waste products from extruded ~ilm.
In U. 5. Patent No. 4,013,745 a typical prior art system reprocesses scrap by severing and recyoling it to an extruder screw inlet, along with virgin polymer. The two materials are dissolved in a common solvent and fed in a unitary stream thrnugh a sheeting die to form a f~lm. While such reprncessing techn1ques are feasible ~or certain products, they are not suitable for recycling PAN resin for use in high-performance films.
In forming a film sheet OI' foil of PAN resin, gas barrier properties and appearance, important qualities o~ the product, are dependent upon the uniformity of composition. Where only virgin PAN resin and pure solYent constitute the extrusion mass, homogeneous solutions can be obtained without undue p~ocessing. PAN resins may be synthesized in the solvent and used without being recovered as discrete solid particles. Also, ~inely divided powders of aorylonitrile homopolymers and interpolymers are relatively easy to dissolve completely, due to their small partiole size, usually 1 to 15 microns. However, when recycling scrap or selvage resin, it is dif~icult ,~

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to obtain such fine dlvision by ordinary chopping, gsinding or other comminution processes. Recycled resln solids may have a size and shape ~hich render the material difficult to handle and present problems in solv@nt penetration during dissolution. Even with the use of auxiliary equipment, such as homogenizers, fi~ters, etc., it is impractical to obtain completely homogeneous solutions of the recycle resin suitable for mixing with the virgln PAN feedstock. Very small amounts of undissolved resin can provide heterogeneities and film discontinuities when cast as a single layer, especially when oasting a thin film. Localized stresses due to such imperfections may result in uneven stretching, pinholes or tears in the film, which are unacceptable for gas barrier service and affect appearance adversely.
It is an object of the present invention to provide a method for the production of multi-layer film comprising~polyacrylonitrile or the like formed o~
contiguous layers of non-homogenPous and homogeneous resin materials having good film integrity. The system provides means for extruding at least two streams uf r slnous material as discrete layers. This may be achieved by multiple die means which extrude the resins in laminar ~lo~ relationship. The extruded re.qin, usually in hot concentrated solution form, is solidifled to form a film, as by ooollng and ooagulation. The ~ilm is ùriented by stretching and dried to remove volatile matter. Cutting means removes the edge trim and produces a ~lnished film product. Resin scrap is recycled by comminuting and dissolving the resin to ~orm a non-homogeneous material fox re-extrusion as a disorete layer, 1 1 8307~

- 2a -The present invention also provides a multilayer oriented polyacrylonitrile film comprising a first layer of substantially homogeneous polyacrylonitrile layer containing undissolved polyacrylonitrile resin particles.

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The present invention is now de.scribed in more detail, with reference to the accompanying drawings, in which:
Fig. 1 ~s a cross-sectional view o~ a film segment.
Fig. 2 is a schematic diagram of a typical system for solvent castlng of multilayer ~ilm.
Fig, 3 is a cross-sectional view o~ a segment of three-layer film.
Fig. 4 is a cross-sectional view of a ~ilm segment having virgin resin, recycled resin and a thermoplastic layer.
Fig, 5 is a schematic drawlng of a preferred process fnr casting polyacrylonitrile ~ilm with organic solvent and aqueous washing medla.
Re~erring to the drawing, in Fig. 1 is shown a typical multilayer film 1 including a laysr 2 of homogeneous resin and a contiguous layer 3 of non-homogeneous resin, containing small particles 4 o~
undissolved material.
This film can be manufactured by employing the casting and scrap recovery system shown in Fig. 2. The initial hot solution of resin and solvent is mixed and homogenized in makeup station 10 and passed through pump means 20 to casting drum 30 via first sheeting die 31, which lays down a solidi~ied layer of virgin resln~
Casting drum 30 is maintained su~ficiently C301 ~o solldify the resin forming a uniform layer. A second layer ls cast from tandem sheeting die 32, spaced apart ~rom the first die 31. The coagulated~film 1 is stripped frnm drum 30 as a self-supporting contlnuous multilayer film strip. Thereafter, the multilayer film is passed through a ~eries o~ operatively connected proc~ssing units, which includP solvent removal means 40, stretching means 50 to provide an oriented structure, drying means 70 to remove volatile components o~ the c~st film, trimming means BO ~or removing excess ~ 1 630'76 ~4-resin from the marginal portions of the stretched f~lm as selvage~ and wlnding means 81 ~or product film. The trimmed selvage and other mill scrap ls then chopped and/or 3round in suitable comminuting means B2 and fed to screening unit 83 or other suitable means for separating out large scrap particles ~or further slze reduction. The smaller particles are admixed and hot solvent in dissolver unit 84 and passed through filter means 85 which retains over-si2ed particles suspended in the non-homogen20us resin solution. The hot scrap solution is then recycled to the second sheeting die 32.
While the sy tem can be adapted to handle a wide variety of scrap materials from various points in a ~ilm production plant, a main source of recycled material is selvage obtained ~rom edge trimming operations and slitting scrap. This is usually in the form o~ a thin sheet material, having a thickness of 12-50 microns (0.5 to 2 mils), typically. 8y chopping, severing or otherwise cutting the film, thin ~lake-like particles oan be obtained having 8 relatively small thickness, buk having rath~r larger planar dimensions of several millimeters. The present system is well-adapted for redissolving these flake film particles by admixing the comminuted material with hot solvent. Although the non-homogeneous resin supply can be more dilute than the homogeneous supply, lt is desirable to optimize the process through the utilization of.the least amount o~
solvent that will produce a satisfactory multilayer film. Hot DMS0 solvent with PAN homopolymer scrap oan be success~ully recycled using only 15 to 30 wt% resin.
If greater quantities of ~olvent are required, solvent removal before extrusion may b~ required to assure ~ilm integrlty. The dissolution step may be per~ormed by high-shear equipment or the like ~o disperse and - .. ~ .. .. .. .... .

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dissolve the scrap resin. Large solids may be further disp~rsed with compression-type equipment or "homogenizers" to provide a non-plugging stream o~
recycled resin. A screen-type filter can be employed to retain oversized particles that might be larger than the ~ilm extrusion thickness.
Certain types of extruders can handle initially-larger rec~cle~ particles and still produce satisfactory extrudate. In some screw-type equipment, dry recycled resin can be compressed to a low bulk feedstock which is redissolved with hot solvent as it advances along the path of the screw means.
The relative thickness o~ layers may be fixed or varied according to available scrap resin being generated and reprocessed. Where the virgin PAN supply is unlimited, the continuous production rate for single-resin ~ilm can be met for a wide range of scrap content from zero to the upper llmit of film ~ntegrity.
About 5 to 25 microns (0.2 to 1 mil) gives satisfactory performance for the individual layexs of typical film used ln wrapping ~ood or other articles. Finished PAN
having a tot21 film thickness of 10 to 20 microns ~orms a good gas barrier for oxygen and water vapor.
It is possible to extrude the PAN homopolymer on both sides and at each edge o~ a multilayer film, as shown in Fig. 3. The inner core layer 3 may comprise the non-homogeneous selvage extrusion oompdsition. A
system ~or co-extruding triple-layer film with beaded edges ~s disclosed in U. S. Patent No. 3,448,183. The ~dge bead ~acilitates stretching the film by tentering and can be trimmed from the product following orientation. It may be ~asible to employ selvage as the outer layer $n some circumstances, with homogeneous PAN solut~on being injected as the core layer.

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7 1 63()~6 Polyacrylonitrile polymers containing very 1arge amounts of homopolymeric units do not melt at practical heat-seallng temperatures. ~he homopolymer can be cast or extruded by solution techniques; but, once coagulated, the resulting articles cannot be fused effectlvely by heat alone. When it is desired to impart h~at sealing properties to PAN film, this may be accomplished by introducing a sultable comonomer wlth acrylonitrile; such as an interpolymer of C4-C8 alkylacrylate and/or other thermoplastic component with acrylonitrile. Unfortunately, significant amounts of such comonomers as butylacrylate degrade the gas barrier characteristics of polyacrylon$trile. However, it may be desirable to incorporate 10-20% alkylacrylate in at least one layer of the film.
In one aspect of the invention shown in Fig. 4, an adhering thermoplastic l~yer 5 is coextruded with the virgin PAN layer 2 and recycled PAN layer 3 to obtain a multilayer orientable ~ilm having heat sealing properties. Advantageously, this is achieved by a three-orifice die by feeding homogeneous PAN solution to an outer orifice, recycled non-hQmogeneous PAN selvage solution to a middle orifice, and a compatible thermoplastic material to the other outer orifice. This results in a three-layer ~ilm having its weaker inside layer protected by the outer layers during stretchlng.
The present invention also provid~s a manufacture of multilayer film from selvage or scrap containing at least one substantially non-thermoplastic polyacrylonitrile component. For instance, if the product film has one layer of PAN homopolymer to prov$de low oxygen and water Yspor permeability and a thermoplastic co-extruded heat seal layer of 20%

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butylacrylate ~ 80% acrylonitrile copolymer, the selvage can be ground and redissolved in DMS0 or suitable cosolvent to provide the non-homogeneous layer.
Numerous variations in materials can be included in ths fllm within the inventive concept.
In a preferred embodiment of the invention, the multilayer film is extruded from an extrusion die having a plurality of manifolds for supplying the different rcsinous streams to a common flow passage from whlch the '~ film-~orming material is extruded at elevated temperature onto an adjacent cold casting roll. Flow control means i~ provided ~or feeding the lndividual resinous streams continuously at predetermined uniform rates, which establish the relative thickness of the discrete layers. At flow rates at which lamlnar flow prevails~ fluld streams combine wlthout substantial intermixing between layers giving a uniform ~ilm.
Suitable multilayer extrusion die assemblies are disclosed in U.S. Patent No. 3,559,239 of W.A. Work et al., granted Feb. 2, 1971, and in u.s. Patent No. 4,165,21Q of H.O. C~rbett, granted August 21, 1979. The layers may be ~ormed sequentially by tandem die means ~herein the layers are extruded individually onto a moving surface, one being oast onto a cold roll and one or more subsequent layers beins cast over the initial layer.
In addition to casting o~ planar films onto drums or the like, multilayer tubular films may be form~d with concentric orifices. FQr instance, in U. 5.
Patent No. 4,144,299, PAN f~lm is produced by extruding an organic solution into an aqueous coagulation bath 3 while wat~r is introduced $nto and withdrawn ~rom the inside o~ the extruded tube. By appropriate modification of the orifice to provide two or more concentric layers, scrap may be utilized in mak~ng tubular film.

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While the inventive concept may be employed $n ordinary solvent-plastici~ed film stretching operations, in recent years an improved aqueous washing system has been developed which gives high quality PAN ~ilm. The details of this system are disclosed in U. S. Patent No.
4,066,731. Thls system, as adapted for use herein, is hown in Flg. 5. The homogeneous solution of PAN in dimethyl sulfoxide (DMS0) is introduced as a hot casting dope containing 30 to 40% PAN through a pump to multiple sheeting die 132, where it is co-extruded as a multilayer film onto the cold casting drum 130, wetted with an aqueous solution of DMS0. The solidified film 101 is then contacted with an aqueous solution of DMSû
123, which is passed countercurrently through a wash tank 140. The ~llm is stripped from the drum continuously and procedes through the wash tank 140 wherein the DMS0 migrates out of the film and ls replaced by water ln the interstices of the f~lm. By stretching the wet Pilm longitudinally in the machine direction with heated differential roll means 150, the film is ~xially oriented. This is followed by transverse hot stretching in a steam or wa~er vapor environment in tenter sect~on 160. Thereafter, the film is dried under constraint by radiant and/or convection means in drier section 170. The marginal areas ar~ cut from the product in slitting line 180, with edge trim being recycled to comm~nuter 182 and fed through hopper 186 and conveyor 188 to screw-type extruder 1~6. The weighted scrap, now $s a flaked ~ilm state is admixed with a metered amownt of hot DMS0, which may be introduced at various points along the compress{on path o~ the extruder. Sinc~ the scrap PAN is a low bulk . . ' '`.:' ' ' ' ` ~ ..

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g mater~al 3 it is sometimes des$rable to lntroduce at least part o~ the DMSO toward the feed section of the extruder 126, from which the mixture is ~ed to multiple sheeting die 1~2 for coextrusion with the v~rgin PAN
501uti on.
Operating temperatures ~or the redissolution step with DMS~ are generally maintained elevated in the range of about 110 to 175C, preferably at about 150C. The ~mount of solvent required will depend upon the scrap composit~on and solubility parameters of the solvents. Rather large amounts of solvent mu~t be employed to obtain complete dissolution of the polymer, requirin~ an expensive evaporation tep to concentrate th~ resin to 30-40X. It is a significant advantage of the present invention that complete solution o~ the resin is not required, resulting in small inhomogeneities. At the point of extursion, the largest undissolved particles for most film applications would be 25 microns or less, depending upon the extursion equlpment and ~ilm dimensions. In addition to DMSO, various organic solvents or co-solvent mixtures, such as dimethyl ~ormamid2, tetramethylene sulfone or other eompatible solvents may be employed. Water miscibility is d~seribed where the aqueous washing step is ~mployed bet~een the casting and orienting steps. The solvent may be recovered from the various processing units and separated for reus~.

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Claims (7)

Claims

1. A multilayer oriented polyacrylonitrile film comprising a first layer of substantially homogeneous polyacrylonitrile homopolymer resin in contact with a second non-homogeneous polyacrylonitrile layer containing undissolved polyacrylonitrile resin particles.

2. The film of Claim 1 wherein said non-homogeneous layer consists essentially of acrylonitrile homopolymer.

3. A three-layer film according to Claim 1 consisting essentially of a non-homogeneous core and continuous, homogeneous homopolymer outer layers.

4. A multilayer film comprising contiguous layers of non-homogeneous polyacrylonitrile resin made with at least one layer containing recycled polyacrylonitrile by co-extruding a continuous multilayer film from a first supply of substantially homogeneous polyacrylonitrile resin solution and a second supply containing non-homogeneous redissolved polyacrylonitrile resin scrap; casting said multilayer film onto a smooth cooled drum surface to form substantially continuous adjacent layers from said first supply and said second supply;
stripping said multilayer film from said drum as a self-supporting continuous film strip; removing at least a portion of the solvent from said multilayer film; stretching said multilayer film to provide an oriented structure; trimming excess polyacrylonitrile resin from said stretched film as selvage resin; and comminuting said selvage resin in solvent for recycle to said second supply.

5. The film of Claim 4, wherein said first supply comprises a dimethyl sulfoxide solution of acrylonitrile homopolymer.

6. The film of Claim 4 wherein said multilayer film consists essentially of polyacrylonitrile.

7. The film of Claim 1 comprising at least one co-extruded layer of thermoplastic resin.