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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is concerned with oriented polyacrylonitrile films
which have been coated on one or both sides with a vinylidine chloride
multipolymer coating composition.
2. Description of the Prior Art
In the past thermoplastic and thermoset packaging films such as for example
polyolefin films including polyethylene and polypropylene and polybutene
and copolymers thereof; polyester films such as for example polyethylene
terephthalate and the like have been coated with a variety of coating
compositions including vinylidine chloride multipolymers to improve the
heat sealability thereof as well as improving resistance of such films to
oxygen and moisture vapor penetration thereof. In the past when such film
materials have been coated with saran latexes having high concentrations
of vinylidine chloride contained therein, and when such films were sealed
and exposed to high moisture concentrations such as for example immersion
in water at room temperature or above, there was a tendency of the seal to
fail by delamination of the coating from the film substrate. This is
particularly true in the case of saran coated polyacrylonitrile films.
Prior art saran coated film structures have been described in the prior
art, see for example U.S. Pat. Nos. 3,617,368 and 4,058,649.
In general, saran coating systems which have been used in the past comprise
saran coatings which are commercially available in the form of latexes
which are to be applied to the particular film substrate from an aqueous
media. A latex, by definition, is a dispersion of spherical, polymeric
particles in a range from about 0.01 up to about 1.0 micron (0.004-0.04
mils) diameter in water. Latex coating systems employed for coating
applications to film substrates generally consist of from about 50% to
about 60% solids with about 2% to 3% of the solids being conventional,
wetting and suspending agents. On drying the applied coating, the water
evaporates and the particles fuse together at their contacting surface.
SUMMARY OF THE INVENTION
The present invention provides a biaxially oriented polyacrylonitrile film
having coated on at least one surface thereof a multipolymer containing a
predominant amount of vinylidine chloride copolymerized with minor amounts
of other monomer moieties such as for example alkyl acrylates, alkyl
methacrylates, acrylonitrile, lower alkyl alpha-beta ethylenically
unsaturated carboxylic acids and the like.
DESCRIPTION OF SPECIFIC EMBODIMENTS
The biaxially oriented polyacrylonitrile film utilizable herein can be
prepared by casting a film from a solution of polyacrylonitrile and
biaxially stretching it, using known prior art techniques. One procedure
is described in U.S. Pat. No. 3,437,717, the disclosure of which is also
incorporated herein by reference. A particularly preferred technique is
described in U.S. Pat. No. 4,066,731, which also is incorporated herein by
reference.
Briefly, this technique involves pouring a hot, concentrated solution of
polyacrylonitrile in a solvent that is water-soluble, e.g., dimethyl
sulfoxide, onto a film supporting surface and stripping the resulting film
from said surface. The resulting film containing solvent is subjected to
contact with water, preferably under constrained conditions, thereby
removing the solvent from the film and replacing it with water. The film
composed essentially of acrylonitrile and water is then stretched to about
1.5 to about 4 times its original dimension in both the machine direction
and the transverse direction. A suitable method is by use of the
drafter-tenter frame arrangement (see U.S. Pat. No. 3,437,737). While
still under restrained conditions, the film is dried, thus producing
biaxially oriented polyacrylonitrile contemplated for use in this
invention. Such film is available commercially.
Preferably, the biaxially oriented film is prepared from polyacrylonitrile
homopolymer. It is contemplated, however, to use films made from
copolymers of acrylonitrile with other ethylenically unsaturated monomers,
such as alkyl acrylates. Film thickness can be between about 1/4 mil and
about 4 mils.
As hereinbefore noted the biaxially oriented polyacrylonitrile films of the
present invention are coated with vinylidine chloride multipolymer
coatings containing a predominant amount of vinylidine chloride in the
multipolymer structure. Such coatings serve to enhance the heat seal
characteristics by increasing the heat seal temperature range and
additionally, they provide barrier characteristics to the film whereby the
film's resistance to the transmission of oxygen and water vapor is
increased. These latter characteristics are important, particularly in
packaging applications whereby the sealed film package may be exposed to
conditions of relatively high humidity and in some instances the sealed
film containing product may be actually immersed in water including
boiling water for cooking the commodity contained in the sealed film
package. Obviously there are less severe conditions of exposure to
moisture such as when such coated film products are used to contain
moisture emitting comestibles including such products as fresh meat and
produce.
It has been found, as more fully discussed hereinafter, that particular
packaging films such as for example polyethylene terephthalate which have
been coated with saran coating materials exhibit excellent resistance to
coating adhesion failure when immersed in liquids including water at room
temperature and above and to the boiling point of water. However, in the
case of oriented polyacrylonitrile films which have a surface coated with
saran-type coating materials it has been found that the adhesion of normal
commercially available sarans to polyacrylonitrile film surfaces,
including for example sarans identified by their manufacturer as SL-112,
delaminate causing seal failure when the coated substrates are immersed in
water for periods of time at room temperature or are immersed in boiling
water for shorter periods.
Normally, saran latexes are supplied from the manufacturer having a pH
which is acidic, e.g., on the order of 1.5 to 2.0 for example. It has been
normal practice in the prior art to adjust such an acidic pH upwardly to
an alkaline condition for example pH=9.0.
It has now been found, however, that when the pH of the saran latex which
is to be employed for top-coating oriented polyacrylonitrile films is
adjusted downwardly, i.e., down to about less than 1.0 and preferably to
about 0.95 and below, excellent resistance to coating delamination, when
the coated films are immersed in water, results by such pH modification.
If the pH of the latex is adjusted below about 0.90, adhesion of the saran
coating to the polyacrylonitrile base film begins to deteriorate.
The following Table I illustrates saran coating adhesion variations when
saran type materials are coated onto the surface of films such as
polyacrylonitrile and compared to saran coated upon the surface of
polyester, specifically, polyethylene terephthalate. The saran coating
composition, referred to in the following Table I, comprises a
multipolymer of about 80-85% by weight of vinylidine chloride; and 15-20%
by weight comprising acrylonitrile, methyl methacrylate, and methacrylic
acid; and 1.8% sodium sulfoethyl methacrylate.
In the following Table an adhesive coated cellophane is employed to test
the adhesion of the saran latex coating to the film. In the testing
procedure the coated film is cross-hatched with a sharp edged instrument
by making three parallel lines approximately one quarter inch apart and
approximately one inch long intersected at 90.degree. with three
additional lines similarly spaced. A strip of adhesive coated cellophane
approximately three inches long is pressed diagonally across the scribed
squares. The tape should be pressed down firmly with the finger. The tape
is then removed from the film. The removal of the tape should be a peeling
back rather than a direct lifting motion, that is the angle of pull should
be approximately 150.degree. rather than 90.degree.. The tape should be
removed with a quick pull. Adhesion is rated on the basis of how much of
the coating in the scribed squares is removed. No coating removal is rated
10 (No Peel) as being perfect and complete removal of the coating is rated
0 (Total Separation) as being complete failure.
TABLE I
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Dry vs Wet Adhesion of Saran-Coated Films
Saran-Coated.sup.1
Saran-Coated.sup.1
Polyacrylonitrile Film
Polyethylene
Exposure (0.65 mil) Terephthalate Film (0.5 mil)
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Air at room temperature
No Peel No Peel
Water at room temperature
for 16 hours Coating Peels
No Peel
Water at 100.degree. C. for one
Total separation of
hour coating from base film
No Peel
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.sup.1 Dow Saran SL112 Coating 1 lb/Ream.
As can be seen from the foregoing Table I the problem of saran coating
adhesion on the surface of electronically treated polyacrylonitrile film
are quite severe when contrasted to saran coated onto the surface of
polyethylene terephthalate. The saran coating latex employed in the
foregoing tests in Table I had a pH as received from the manufacturer of
about 1.5. Attempts to overcome the saran coating adhesion loss on the
polyacrylonitrile following the water exposure, described in Table I, by
predrying the film or by 2-side saran coating were unsuccessful.
However, it was found that downward adjustment of the pH of the saran latex
prior to coating, such adjustment being made by dilution with dilute
hydrochloric acid, prevented coated delamination as shown in the following
Table II.
TABLE II
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Effect of Saran Latex pH Upon
Saran Adhesion to Polyacrylonitrile
Coating - Base Film After
Coating - Base Film After
Water Immersion at Room
Exposure to Boiling Water
Latex pH
Temperature for 16 Hours
for 1/2 Hour
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0.9 No Peel No Peel
1.5 Total Separation Total Separation
4.0 Total Separation Total Separation
9.0 Total Separation Total Separation
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It will be apparent from the foregoing Table II that the adhesion of saran
multipolymers to polyacrylonitrile base films, when such laminates are
exposed to immersion in water, is definitely improved by maintaining the
pH of the latex used for coating below 1.0.
Although the present invention has been described with preferred
embodiments, it is to be understood that modifications and variations may
be resorted to, without departing from the spirit and scope of this
invention, as those skilled in the art will readily understand. Such
variations and modifications are considered to be within the purview and
scope of the appended claims.
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