Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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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,
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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
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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 $n some circumstances, with homogeneous
PAN solut~on being injected as the core layer.
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Polyacrylonitrile polymers containing very 1arge
amounts of homopolymeric units do not melt at practical
heat-seallng temperatures. ~he homopolymer can be cast
or extruded by solution techniques; but, once
coagulated, the resulting articles cannot be fused
effectlvely by heat alone. When it is desired to impart
h~at sealing properties to PAN film, this may be
accomplished by introducing a sultable comonomer wlth
acrylonitrile; such as an interpolymer of C4-C8
alkylacrylate and/or other thermoplastic component with
acrylonitrile. Unfortunately, significant amounts of
such comonomers as butylacrylate degrade the gas barrier
characteristics of polyacrylon$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 $s a flaked ~ilm state is admixed
with a metered amownt of hot DMS0, which may be
introduced at various points along the compress{on path
o~ the extruder. Sinc~ the scrap PAN is a low bulk
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mater~al 3 it is sometimes des$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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