Polymer film coated with aqueous polymer

A system for producing coated, oriented polymer film. Methods and apparatus are provided for forming a continuous film of the polymer with water uniformly dispersed therein and applying an aqueous coating composition to at least one surface of the film. The film is oriented and dried under constraint to remove water from the substrate film and coating.

BACKGROUND OF THE INVENTION 
This invention relates to coating of water-containing plastic sheet and 
production of oriented polymer film therefrom. In particular, it relates 
to methods for applying aqueous polymer coating compositions to 
water-containing substrates during film manufacture. 
A variety of polymers are suitable for making films or foil products coated 
with water-dispersible, film-forming resins, such as vinylidene chloride 
polymers (saran-type), epoxy resins and the like. Film products are often 
manufactured by casting or extruding a continuous web, tubular shape or 
other sheet-like material, which is subsequently stretched to provide 
molecular orientation and treated to remove volatile components. Often 
such films require an additional layer of a different polymer to achieve 
the desired sealing or bonding properties, gas barrier characteristics, 
etc. Conventional systems often apply another coating to the film 
substrate as an aqueous dispersion. 
Various films, such as polyolefins, polyesters, polyamides, acrylics or 
many others are treated by coating with various resin coatings. 
Acrylonitrile polymers and other resinous materials may be treated with 
saran dispersions to obtain the desired product. 
In U.S. Pat. No. 4,197,356 a cast oriented polyacrylonitrile (PAN) is 
treated with an aqueous emulsion containing saran polymer. The film can be 
manufactured by solvent casting with subsequent removal of the solvent. 
After orientation and drying, the PAN film is treated with an aqueous 
vinylidene chloride copolymer dispersion and redried to obtain a coated 
finished product. Saran-type coatings are applied to a variety of 
substrate materials in U.S. Pat. Nos. 3,262,808, 3,271,178, 3,506,751 and 
3,988,157, as subbing layers, heat sealing promoters, barrier layers and 
photographic film support, for instance. 
Prior art film manufacturing techniques usually treat the formation, 
orientation and coating treatment of films as separate and distinct 
production steps, often requiring multiple handling and drying, frequently 
requiring special procedures to assure adequate bonding between the 
coating and substrate. It is an object of the invention to provide a 
simplified method for coating water containing sheets with an aqueous 
dispersion of polymeric coating material. 
SUMMARY OF THE INVENTION 
This invention provides a method for treating water-containing orientable 
film with an aqueous coating composition prior to complete drying, 
advantageously applying the coating between orientation and/or drying 
equipment sections of a continuous film manufacturing line. In one 
embodiment methods are provided for producing biaxially-oriented polymer 
film comprising the sequential steps of (1) forming a continuous 
water-containing film having a self-supporting hydrophilic polymer matrix 
with water uniformly dispersed therein; (2) heating and stretching the 
film longitudinally to orient the polymer uniaxially; (3) applying an 
aqueous polymeric coating to at least one surface of the aquagel film; (4) 
transversely stretching the coated film to provide a biaxially-oriented 
polymer substrate; and (5) drying the coated, oriented polymer film under 
constraint to remove water. This method is especially useful where the 
polymer consists essentially of acrylonitrile interpolymer or homopolymer 
in aquagel form essentially free of volatile organic matter, and the 
aqueous coating comprises a dispersion of thermally-bondable saran-type 
copolymer containing a major amount of vinylidene chloride. 
One of the objects of this invention is to provide a method for applying an 
aqueous polymeric coating to a wetted substrate as a liquid vehicle under 
circumstances which permit good adhesion of the coating composition. By 
employing a wet substrate, drying operations are simplified and handling 
steps are minimized. Advantageously, the longitudinally stretched aquagel 
film is passed through tentering means wherein the aqueous coating is 
applied to a center portion of the film immediately before transverse 
stretching.

DESCRIPTION OF PREFERRED EMBODIMENTS 
Film feedstock to be fed to the orientation/coating system can be derived 
from continuous film casting or extruding equipment. Flat film may be 
solvent cast according to the process of U.S. Pat. No. 4,066,731, wherein 
acrylonitrile homopolymer or interpolymer is cast onto a rotating drum 
from a sheeting die and coagulated as a self-supporting flim. Organic 
solvent, such as dimethyl sulfoxide, can be washed with a water bath to 
obtain an aquagel film typically containing 40 to 60% water, integrally 
bound in the molecular interstices or dispersed in the orientable 
polymeric matrix. A tubular PAN film can be extruded and water-coagulated 
if desired, according to the teachings of U.S. Pat. No. 4,144,299 and the 
unoriented film can be slit and fed to the orientation/coating units as a 
flat strip. Aqueous PAN film can also be made by melt-extrusion of a high 
temperature polymer hydrate in a known manner. 
The preferred film feedstock is a hydrophilic polymeric material containing 
sufficient water to be stretched at low temperatures. Acrylonitrile 
polymers containing at least 5% H.sub.2 O, preferably aquagels containing 
about 40 to 60% H.sub.2 O, are excellent film substrates for use herein. 
The present system is especially valuable for treating non-thermoplastic 
PAN homopolymer, such as duPont Type A resin. 
Referring to FIG. 1, a continuous flow system for manufacturing aquagel 
film is shown. For instance, hot polyacrylonitrile-dimethylsulfoxide 
solution is fed under pressure to sheeting die 1, which extrudes a thin 
film of polymer solution onto cold drum surface 20. After contacting an 
aqueous medium, the self-supporting aquagel film is stripped from drum 20 
and traverses a countercurrent aqueous bath 30, wherein the organic 
solvent is removed and replaced by water, thereby forming the aquagel. The 
film 10 passes through the machine direction orienter (MDO) 40 comprising 
a first heated roll maintained at about 75.degree. C. and thereafter a 
series of orienting rolls 42, 43, 44 which are maintained at a sufficient 
differential speed to longitudinally stretch the web about 2.times. to 
3.times. thereby providing a uniaxially oriented aquagel film. The faster 
rolls are kept at about 50.degree. C. Thereafter the film is transferred 
to the coating and subsequent treatment sections shown in FIG. 2. 
Referring to FIG. 2, the uniaxially oriented aquagel film 10 is passed over 
a series of rolls 45, 46, 48 which provide adequate tension to control the 
coating operation and to guide the film laterally. Dancer idler 45 and web 
guide 48 provide the appropriate tension and position for the film prior 
to coating in unit 50 wherein a liquid coating composition is metered 
through pump 51 to a trough coating unit 50 maintained in contact with 
film strip 10. The position is maintained by roller guide 52 which 
contacts only the underside of film 10 which has a wet top coating applied 
to a center band. 
The device 50 for coating flat film on a continuous line shown in FIG. 2 is 
a trough-type film coater having means for holding film strip 10 taut 
against the open-sided applicator into which a steady stream of liquid 
coating composition is fed at metered rate corresponding to the take up 
rate of the liquid onto the film surface. This coating unit is 
particularly useful for applying a low viscosity aqueous liquid, such as 
saran dispersion. Upward movement of the flat film provides adequate 
pick-up as it passes across the open side of the coating unit. The film is 
held against the open side, thereby forming an upwardly moving fluid 
barrier, with the film providing an effective fluid seal at the bottom and 
side edges of the coater face. The aqueous coating composition is metered 
at a predetermined rate which is coordinated with linear film speed. There 
are several advantages to the trough coater unit, e.g., the amount of 
aqueous coating composition can be varied widely without cracking or 
cratering of the film during drying. With a thickened 55% aqueous saran 
dispersion, 60 microns or more (dry basis) can be applied. Using the same 
coating conditions, but with a relatively thinner 25% saran concentration, 
only about 8 microns (dry basis) is deposited. This system is efficient in 
that it places a wide band of uniformly deposited coating on the film 
strip, leaving the edges uncoated. The trough coater unit is space 
efficient and can be incorporated into existing orientation lines before 
the tentering or drying apparatus to provide closely-coupled coating, 
tentering and/or drying operations. While it is preferred to place the 
coating step before the tentering step, this sequence can be reversed, if 
desired. Design considerations should take into account the change in 
coating thickness during orientation, wherein the film may be stretched 
several times its original linear dimension, reducing the coating 
thickness. 
The coating composition can be applied by calendering, spraying, 
roll-coating, meniscus contact or a variety of other well-known 
techniques. A gravure coater, reverse-roll coater or air knife coater may 
be used effectively for some applications. While the coating is usually 
required on one side only, double-sided coatings are feasible. 
Because most orientation equipment is designed to provide longitudinal 
stretching in the machine direction (MD) to provide uniaxial orientation 
by differential speed rolls, the initial orientation step does not 
increase film width. Thereafter the film can be coated, transversely 
oriented and dried without contacting the film with mechanical handling 
equipment except at the edges. Since the film width is increased 
substantially by the lateral stretching operation, coating equipment size 
can be kept small by installing the coater between MD and TD orienting 
equipment. It may be advantageous to house the coater integrally with the 
tentering unit, preferably connected operatively to receive the uniaxially 
oriented film from the MDO unit. 
As the coated film progresses from the coating section, as shown in FIG. 2, 
it passes through a transverse direction orientation (TDO) unit 60. The 
transverse direction orientation (TDO) step is usually effected by 
attaching edge clips to the film progressively and stretching the film 
perpendicular to its machine direction travel. The edge portions are much 
thicker than the main body of the film and are ordinarily trimmed from the 
finished product. In the TDO unit the film may be contacted with moist hot 
gas to prevent excessive water loss. Means for impinging hot 
water-saturated air or the like at high velocity can be provided in a 
standard tentering apparatus; such as manufactured by Marshall and 
Williams Co. or as disclosed in U.S. Pat. No. 3,611,479. TD stretch ratios 
of 2:1 to 4:1 or higher may be employed, with 3:1 being employed for 
typical PAN aquagel film. 
The biaxially-oriented film is dried under constraint to remove water and 
other volative materials which may be present in the film, either residual 
organic solvent or monomer from the film casting operation or volatile 
components of the coating composition. 
As the film passes through the drier unit 70, it receives energy from a 
bank of radiant heaters 71 and thereafter is completely dried in oven 
section 72, where hot air at about 200.degree. C. is directed toward the 
film at high velocity. Thereafter the film is reduced to handling 
temperature by a stream of cool air at the exit end of drier 70 and 
trimmed by slitting blades 75 to remove the edge portions. The coated, 
biaxially-oriented film may then be wound onto a spool for storage or 
further processed by addition steps or taken directly to a fabrication 
line. 
In FIG. 3 film 10 is shown in plan view as it progresses through the 
coating, TDO and finishing operations. The uniaxially oriented strip 10 
from the MDO unit receives a coating in a center band portion 10C, leaving 
edge portions 10E uncoated. Progressive transverse stretching, for example 
at 3:1, orients the base film and distributes the coating composition as a 
thin layer. The film is held in this configuration by appropriate 
tensioning means while the water is removed. Since the film is dried under 
restrained conditions, no significant shrinkage of the oriented film can 
occur prior to cooling the dry product. 
In the following description and examples, metric units and parts by weight 
are employed unless otherwise stated. 
EXAMPLE 1 
Polyacrylonitrile homopolymer aquagel film is made according to U.S. Pat. 
No. 4,066,731. The sheet weighs about 140 mg/in to 250 mg/in.sup.2 (38 
mg/cm.sup.2) and contains 45-50% water. It is longitudinally stretched 
2.times. on a machine direction orienter with a first heated roll 
(75.degree. C.) and cooler speed differential rolls (50.degree. C.). The 
uniaxially oriented wet film enters the coating apparatus at a linear 
speed of about 9 meters/minute. The film is coated uniformly on its 
topside with a vinylidene chloride-vinyl acetate copolymer (Dow saran 
latex SL-112 containing about 55% solids), sufficient to yield a final dry 
coating thickness of 40-50 microns. The film then enters the TDO tentering 
apparatus for lateral stretching (3.times.). The tentering atmosphere is 
maintained at about 75.degree. C. and high moisture content (90-100% RH) 
by steam injection. As the film is stretched, the coating becomes hazy, 
but becomes water clear upon drying. A radiant heater bank followed by 
convection oven at 205.degree. C. dries the coating and film substrate 
simultaneously during a 12 second oven residence time. The uncoated edges 
are trimmed and the products wound onto a spool for storage. 
The saran-coated film was heat sealed to itself on the coated side and 
tested for bond strength, with good results. A 40 micron coating (1.7 mil) 
produced a seal having strength of greater than 700 grams/inch before film 
failure, with a sealing temperature of at least 160.degree. C. giving 
satisfactory seals in 4 to 6 seconds. By reinforcing the PAN substrate 
with pressure sensitive tape, seal strength of greater than 2500 gm/in is 
obtainable. 
The saran coating was tested for tensile strength alone after delamination 
and had a yield strength of 6600 psi. Adhesion of the cured coating to dry 
PAN film can be improved by exposing the finished dry film to ultraviolet 
radiation, which passes through the U-V transparent PAN material to the 
coating interface. 
Standard water vapor transmission rate (WVTR) for the saran-coated PAN was 
0.03 gms/100 in.sup.2 /day, and standard oxygen transmission rate was 0.04 
cc/100 in.sup.2 /day. 
EXAMPLE 2 
Uncoated PAN film and the coated film of Ex. 1 were laminated to low 
density polyethylene sheet (38 microns), and tested for water vapor 
transmission. Pouches were made from each film, filled with water, sealed 
and placed in a dry room for one month at about 50.degree. C. and 10% RH 
for conditioning. The conditioned samples were weighed periodically to 
measure weight loss. The uncoated PAN WVTR was 0.35 gm/day vs 0.16 gm/day 
for the saran-coated film. At the end of the test the laminations were 
observed to be intact and the films showed no adverse effects from the 
test conditions. The polyethylene layer also improves flex crack 
resistance, especially when bonded to the PAN side. 
In addition to the SL-112 polymer dispersion, other saran-type materials 
such as "Diophan 1850" or Dow XD 30315.11, may be employed with similar 
results. 
While the preferred coating composition comprises vinylidene chloride 
copolymers, the technique is useful for applying a variety of aqueous 
polymeric coating compositions, including solution polymers and/or 
dispersions of acrylic, epoxy, styrene, melamine, modified cellulose, or 
halogenated olefinic resins. Latices of elastomeric polymers, such as 
butadiene copolymer rubber materials, wax dispersions, etc. are useful as 
coatings for various substrates. Polycarboxylic resins made with 
ethylenically unsaturated monomers, carboxymethyl cellulose, or 
polyvinylalcohols comprise a class of water-soluble resins which may be 
applied advantageously. Melamine-formaldehyde ("Accobond") prepolymers 
and, amide resins may also be coatings, etc. 
Application rate or coating thickness may vary considerably depending upon 
the use. Where a saran-type coating is required only for heat sealing 
purposes a coating weight of 1 to 10 grams/m.sup.2 is adequate. However, 
much thicker coatings may be desired for certain purposes. In some cases 
50 to 75 microns or more (dry basis after stretching) may be applied in a 
single pass of polyacrylonitrile aquagel film with a thick concentrated 
saran dispersion. Where the coating step is followed by further 
stretching, the final thickness may be considerably smaller than applied 
prior to orientation and drying. 
The dry, coated, biaxially oriented film may be employed as transparent 
wrap for food or other packaged materials requiring a barrier film. Low 
oxygen and water vapor permeability are obtained. The laminates are heat 
sealable, strong and tear-resistant. A relatively high modulus of 
elastically and elongation at the break during tensile testing provides 
good wrap film properties. 
It is understood that more than one coating layer can be applied, and that 
mixed resins, dyes, pigments, fillers, and other additives may be 
incorporated into the coating composition and/or polymeric film without 
departing from the inventive concept. 
The saran-coated polyacrylonitrile products may be heat-laminated to other 
preformed films, such as polyolefins (e.g., propylene-ethylene copolymer) 
to obtain laminates which are useful in manufacturing containers, 
retortable pouches for food storage and the like.