High vinylidene chloride content interpolymer coating resins and method of preparation

A process for preparing, in aqueous emulsion, controlled interpolymer compositions. The interpolymer compositions have polymerized therein from about 86 to 92 mole percent, vinylidene chloride, from about 14 to 8 mole percent of a comonomer mixture of methyl methacrylate and a comonomer selected from the group consisting of acrylonitrile, methacrylic acid and methacrylonitrile in a ratio of the comonomer to methyl methacrylate of from about 0.5:1 to 2:1, and from about 0.5 to 1.0 weight percent itaconic acid, based on weight of vinylidene chloride plus comonomer mixture. The interpolymers are prepared in a three step process. The third step, or Step III, wherein monomer feeding is step-wise reduced for a portion of the step and a polymerization initiator is added at generally the same constant rate as the rate of addition in Steps I and II, generally reduces the amount of unreacted mixed monomers remaining after monomer addition is completed. As such, the detrimental results produced when a large amount of polymer composition variation in the direction of high vinylidene chloride fractions are markedly reduced.

BACKGROUND OF THE INVENTION 
The homopolymer of vinylidene chloride is generally insoluble at room 
temperature in conventional organic solvents, e.g., pure tetrahydrofuran. 
However, when vinylidene chloride is interpolymerized with one or more 
monomers copolymerizable therewith, useful materials result which may be 
soluble at room temperature in conventional organic solvents. Typical 
copolymerizable monomers include acrylonitrile, methacrylonitrile, methyl 
acrylate (or other C.sub.2 -C.sub.15 alcohol esters of acrylic or 
methacrylic acid, e.g., methyl methacrylate), acrylic acid, itaconic acid, 
chloroacrylonitrile, vinyl chloride, vinyl bromide, vinylidene bromide, 
and the like. Useful organic solvents include mixtures of toluene and 
tetrahydrofuran. 
These materials find particular use in the cellophane industry where a very 
thin (usually about 0.05-0.15 mil) coating of a so-called "soluble" 
vinylidene chloride copolymer resin is deposited from a solvent, or 
mixture of solvents, on both sides of a cellophane film. This thin coating 
serves several purposes. It causes the cellophane film to be a water vapor 
barrier film for packaging purposes where either a loss of or gain in 
water content of the product would render the product less attractive for 
sale. In addition, the coating promotes retention of water (which acts as 
a plasticizer usually along with some glycol or glycerine) in the 
cellophane, thus helping to prevent brittle fracture of the film. Finally, 
such coatings provide a heat sealable outer coating whereas uncoated 
cellophane, by its very nature, cannot be heat sealed. 
For manufacturers concerned with applying such thin coatings to substrates 
such as cellophane, the solubility of a polymer in a solvent and the water 
vapor transmission rate (WVTR) of a coated substrate are particularly 
important properties. Usually the desired properties include as low a WVTR 
as possible coupled with low temperature solubility of the polymer in an 
inexpensive solvent or solvent mixture. These of course represent only a 
few criteria. For example, the decision of which resin to use may depend 
upon other factors such as heat seal temperature, appearance, adhesion to 
the cellophane, etc. As a starting point, however, WVTR and solubility are 
of primary importance. 
The WVTR of vinylidene chloride interpolymers is directly related to the 
mole percent of vinylidene chloride in the interpolymer. Therefore, it is 
generally advantageous to get the mole percent of vinylidene chloride as 
high as possible consistent with solubility in one or more desired organic 
solvent systems. A high vinylidene chloride content generally means that 
strong crystallizing forces are present in the interpolymer. It is known 
that highly crystalline polymers are poorly soluble. Therefore, these two 
factors, low solubility and high vinylidene chloride content, are 
diametrically opposed. 
As the mole percent of vinylidene chloride is raised in an interpolymer 
series, it is obvious that a mole percentage range is reached where the 
interpolymers rapidly change from interpolymers which crystallize slowly, 
are amorphous and dissolve readily, to interpolymers which crystallize 
rapidly, are highly crystalline and are much more difficult to dissolve. 
It also becomes obvious that, in the mole percentage range where the 
interpolymers rapidly change, a small amount of interpolymeric composition 
variation in the direction of a higher vinylidene chloride content in the 
interpolymer than intended can result in interpolymers unsuitable for 
coating purposes. Such interpolymers are unsuitable either because they 
fail to dissolve adequately in a solvent system or because they 
crystallize out of solution after initially dissolving in a solvent 
system. The beginning of a tendency to be unsuitable may be measured by 
light transmission measurements of lacquer solutions of an interpolymer in 
a solvent system using an instrument such as a visible light spectrometer. 
Careful control of solution turbidity, hereinafter called "haze", as 
measured in terms of percent light transmission is essential to use of 
these interpolymers for coating purposes. Careful control is especially 
critical when preparing high mole percent vinylidene chloride 
interpolymers. It is believed that tiny insoluble crystals which remain 
after dissolving act as nuclei for formation of overall crystal structures 
once the interpolymer has been deposited as a thin coating on a substrate. 
The rate of crystallization is affected by the number of nuclei present. 
It has been found that excessively slow crystallization rates result in 
"blocking" during machine operations when a coated film is wound in large 
rolls before crystallization is generally complete. "Blocking", as used 
herein, is the tendency of the interpolymer coating to adhere to another 
coated layer. When blocking is particularly severe, it is generally not 
possible to unwind a roll of coated film. 
It is possible by judicious choice of comonomers and proper reaction 
methods to optimize the WVTR-solubility relationship and to approach an 
ideal composition for use. A great amount of work has been done in the 
past to find such compositions. Substantial effort has also been expended 
to find the best method of preparing these compositions to give the best 
combination of WVTR and solubility. A combination of choice of monomers 
and method of polymerization thereof has now been discovered which is 
believed to be superior to anything known heretofore for attainment of 
optimum combined barrier and solubility properties. 
The primary object of the present invention is to provide an improved 
method for preparing interpolymers having polymerized therein vinylidene 
chloride, methyl methacrylate, itaconic acid and a comonomer selected from 
the group consisting of acrylonitrile, methacrylic acid and 
methacrylonitrile. The interpolymers prepared in accordance with the 
improved method should have a high vinylidene chloride content, a narrow 
composition range, and enhanced solubility in organic solvents. The 
interpolymers should also provide a low permeability to moisture and to 
oxygen. The interpolymers should further provide a good crystallization 
rate for a film cast from an interpolymer solution. 
Still further objects and advantages will appear in the more detailed 
description and examples set forth below. It is to be understood, however, 
that the more detailed description and examples are given by way of 
illustration only, and not by way of limitation. Various changes may be 
made by those skilled in the art without departing from the scope and 
spirit of the present invention. 
SUMMARY OF THE INVENTION 
It has now surprisingly been found that interpolymers, having polymerized 
therein vinylidene chloride, methyl methacrylate, itaconic acid and a 
comonomer selected from the group consisting of acrylonitrile, methacrylic 
acid and methacrylonitrile, which have a high vinylidene chloride content 
result from a process for preparing, in aqueous emulsion, controlled 
interpolymer compositions having polymerized therein (a) from about 86 to 
about 92 mole percent vinylidene chloride; (b) from about 14 to about 8 
mole percent of a mixture of methyl methacrylate and a comonomer selected 
from the group consisting of acrylonitrile, methacrylic acid and 
methacrylonitrile, said mixture of methyl methacrylate and the comonomer 
having a molar ratio of the comonomer to methyl methacrylate of from about 
0.2:1 to 5:1; and (c) from about 0.5 to about 1.0 weight percent itaconic 
acid based on weight of (a) plus (b); said interpolymer compositions 
having a relative viscosity of from about 1.3 to about 1.7 at 25.degree. 
C. as a 1 percent solution in tetrahydrofuran; said interpolymer 
compositions being capable of forming generally haze-free solutions when 
present in an amount of about 20 percent solids in a solvent mixture, said 
solvent mixture comprising about 65 weight percent tetrahydrofuran, based 
on solvent mixture weight, and about 35 weight percent toluene, based on 
solvent mixture weight, said process comprising the sequential steps of: 
(A) initiating emulsion polymerization by forming a seed latex, the seed 
latex being formed in a batch emulsion polymerization process which 
comprises: 
(1) adding a first monomer charge to an aqueous emulsion polymerization 
medium, the first monomer charge comprising from about 3 to about 8 weight 
percent of a principal mixture of monomers, the principal mixture of 
monomers comprising from about 86 to about 92 mole percent vinylidene 
chloride and from about 14 to about 8 mole percent of a comonomer mixture 
of methyl methacrylate and a comonomer selected from the group consisting 
of acrylonitrile, methacrylic acid and methacrylonitrile, said comonomer 
mixture having a molar ratio of the comonomer to methyl methacrylate of 
from about 0.2:1 to about 5:1, the polymerization medium comprising water, 
an emulsifier and from about 0.5 to about 1.0 weight percent itaconic acid 
based on the principal mixture of monomers; 
(2) adding a polymerization initiator to said emulsion polymerization 
medium, the initiator being added at a generally constant rate; 
(3) continuing seed latex formation under autogenous pressure until a 
pressure drop in vapor pressure of monomers in the polymerization medium 
of from about 1.8 to about 2.2 pounds per square inch occurs; 
(B) continuing polymerization under autogenous pressure by adding to the 
emulsion polymerization medium: 
(1) a second monomer charge in an amount between about 85 and 92 weight 
percent of the principal mixture of monomers as in (A)(1), said second 
monomer charge being added at a generally constant rate, the rate being 
sufficient to continually provide an excess of unreacted monomers in the 
polymerization medium, said excess being generally from about 2 to about 
10 weight percent based on weight of the polymerization medium; and 
(2) the polymerization initiator at generally the same constant rate of 
addition as in (A)(2); and 
(C) generally immediately following addition of all of the second monomer 
charge, finishing polymerization in a predetermined time interval T, the 
time interval T having at least a first portion and a second portion, by: 
(1) continuing to add the polymerization initiator at generally the same 
constant rate of addition as in (A)(2) and (B)(2), the initiator being 
added throughout the time interval T; and 
(2) adding a third monomer charge in an amount between about 2 and 12 
weight percent of the principal mixture of monomers as in (A)(1), said 
third monomer charge being added over the first portion of the time 
interval T in such a manner that the vapor pressure of monomers in the 
emulsion polymerization medium is reduced smoothly as reflected by a 
time-pressure curve wherein time is plotted on the abscissa and pressure 
is plotted on the ordinate, the time-pressure curve being generally free 
of discontinuities from the beginning of the time interval T to the end 
thereof and having a shape, the shape being that of a line which is 
generally concave downward. 
Also within the scope of the present invention are the polymers prepared in 
accordance with the aforementioned process. 
Further contemplated as being within the scope of the present invention is 
a crystalline, controlled composition interpolymer having polymerized 
therein (a) from about 86 to about 92 mole percent vinylidene chloride; 
(b) from about 14 to about 8 mole percent of a comonomer mixture of methyl 
methacrylate and a comonomer selected from the group consisting of 
acrylonitrile, methacrylic acid and methacrylonitrile, said comonomer 
mixture having a molar ratio of the comonomer to methyl methacrylate of 
from about 0.5:1 to 2:1; and (c) from about 0.5 to about 1.0 weight 
percent itaconic acid based on weight of (a) plus (b); said interpolymer 
having a relative viscosity of from about 1.3 to about 1.7 at 25.degree. 
C. as a 1 percent solution in tetrahydrofuran; said interpolymer being 
capable of forming generally haze-free solutions when present in an amount 
of about 20 percent solids in a solvent mixture as measured in terms of at 
least 80 percent transmission of the solution using an ultraviolet 
spectrophotometer operating at a wavelength of 640 nanometers against a 
reference of pure solvent mixture after aging the solution at 25.degree. 
C. for a period of 24 hours, said solvent mixture comprising about 65 
weight percent tetrahydrofuran, based on solvent mixture weight, and about 
35 weight percent toluene, based on solvent mixture weight.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIGS. 1 and 2 are time-pressure curves which reflects vapor pressure of 
monomers in the reactor as a function of time. FIG. 1 is a partial 
reproduction of a chart of pressure transducer readings taken during the 
polymerization of monomers in Example 1, Experiment A. FIG. 2 is a partial 
reproduction of a chart of pressure transducer readings taken during the 
polymerization of monomers in Example 2, Experiment D. In FIGS. 1 and 2, 
the vertical axis represents pressure and the horizontal axis reflects 
time in hours. The horizontal axis in FIGS. 1 and 2 is not a straight 
linear progression but contains a portion where the time scale is 
expanded. Steps I, II and III of the polymerization processes represented 
by FIGS. 1 and 2 are so labeled on FIGS. 1 and 2. The charts of pressure 
transducer readings of FIGS. 1 and 2 are generally smooth lines. The lines 
do, however, contain minor variations which result in a "sawtooth" 
appearance. Persons skilled in the art will recognize that such variations 
are usually normal for polymerization apparatus wherein there is a time 
lag between a temperature increase and a response by a cooling medium 
control. Persons skilled in the art will also recognize that more 
sophisticated control apparatus will minimize temperature variation which 
will, in turn, minimize variations in pressure readings as reflected by 
the lines. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As previously indicated, the monomeric materials are polymerized in an 
aqueous emulsion to form interpolymers having polymerized therein from 
about 86 to about 92 mole percent vinylidene chloride, from about 14 to 
about 8 mole percent of a mixture of methyl methacrylate and a comonomer 
selected from the group consisting of acrylonitrile, methacrylic acid and 
methacrylonitrile, and from about 0.5 to about 1.0 weight percent itaconic 
acid based on combined weight of vinylidene chloride, methyl methacrylate 
and the comonomer. The monomeric materials are polymerized using a three 
step process. Step I focuses on the formation of a seed latex. A first 
monomer charge of from about 3 to about 8 weight percent of a principal 
mixture of monomers is added during Step I. The principal mixture of 
monomers comprises from about 86 to about 92 mole percent vinylidene 
chloride, and from about 14 to about 8 mole percent of a comonomer mixture 
of methyl methacrylate and a comonomer selected from the group consisting 
of acrylonitrile, methacrylic acid and methacrylonitrile. The comonomer 
mixture has a molar ratio of the comonomer to methyl methacrylate of from 
about 0.2:1 to 5:1. Step II focuses on the simultaneous and continuous 
addition of monomers. A second monomer charge of from about 85 to about 92 
weight percent of the principal mixture of monomers is added during Step 
II. A portion of the principal mixture of monomers must remain after the 
Steps I and II to be added as a third monomer charge. Step III focuses on 
finishing polymerization. The third monomer charge, or from about 2 to 
about 12 weight percent of the principal mixture of monomers, is added 
according to a monomer feeding schedule. The schedule calls for the third 
monomer charge to be added in at least two incremental steps. A 
polymerization initiator is added at a generally constant rate throughout 
Steps I, II and III. 
Step I is essentially a small batch polymerization. In a batch 
polymerization, polymer composition is likely to vary because of unequal 
reactivity ratios among the monomers. Therefore, it is essential that Step 
II be started as soon as the seed latex shows any signs of polymer 
composition variation. Persons skilled in the art will recognize that an 
indicator of such a variation is a pressure drop in the free space above 
such a reaction mixture when the reaction mixture is contained in a 
polymerization apparatus. The pressure drop is a drop in vapor pressure of 
monomers in the reaction mixture. If the transition from Step I to Step II 
is late, e.g., after a pressure drop of more than about 1.8 to about 2.2 
pounds per square inch, enough polymer composition change may occur to 
produce an unsatisfactory interpolymer. Unsatisfactory interpolymers are 
those which are either insoluble, or which, although soluble, yield 
solutions having unsatisfactory haze values. 
With regard to Step II, prior known processes for preparing interpolymers 
of this general type (e.g., the process of U.S. Pat. No. 3,879,359) 
require that the monomers be added to the polymerization reaction medium, 
in the proper ratios, as fast as they are polymerized. As such, there is 
generally very little excess monomer present at any time within the 
polymerization medium. The processes are disclosed as being critical for 
preventing polymer composition change. The composition change is generally 
accompanied by a resultant loss in WVTR and solubility characteristics. In 
another process, the monomers must be added in two stages during the 
second step. In a first stage, the monomers must be added at a rate 
sufficient to continually provide an excess of monomers in the 
polymerization medium. In a second stage, remaining monomers must be added 
at a rate sufficient to continually ensure an absence of excess monomers 
in the polymerization medium. The latter process is disclosed as being 
critical for providing coatings having adequate flexibility. 
It has now been unexpectedly found that interpolymers having the 
combination of properties required by the present invention are formed 
when from about 85 to about 92 percent of the total monomeric materials, 
excluding the itaconic acid, is added in a generally constant ratio and at 
a generally constant rate throughout Step II. The generally constant rate 
must be sufficient to provide an excess, or a reservoir, of unreacted 
monomers in the polymerization medium of no more than about 10, and no 
less than about 1, weight percent based on weight of the polymerization 
medium. Persons skilled in the art will recognize that the reservoir of 
unreacted monomer is generally greatest near the beginning of Step II and 
least near the end of Step II. 
With regard to Step III, prior known processes for preparing interpolymers 
of this general type (e.g., the process of U.S. Pat. No. 3,879,359) 
require that the polymerization reaction be continued after all monomers 
have been added until about a 50 percent drop in reaction pressure occurs. 
Generally, immediately thereafter the reaction medium must be cooled and 
the unreacted monomer removed. This process is disclosed as providing the 
closest known control of copolymer composition change in a batch system. 
Such process is further disclosed as providing the minimum WVTR and 
maximum resin solubility when properly carried out. A second process 
requires the addition of two components to the polymerization medium 
generally immediately after the addition of all of the monomer mixture. 
The two components are an ethylenically unsaturated comonomer and a 
polymerization initiator. This process is disclosed as preventing 
undesirable interpolymer composition change. 
It has now been unexpectedly found that interpolymers having the 
combination of properties required by the present invention are formed 
when a portion of the principal mixture of monomers is added during Step 
III. The portion is from about two to about twelve weight percent based on 
the principal mixture of monomers. Persons skilled in the art will 
recognize that in a polymerization apparatus used in an emulsion 
polymerization, there is generally a free space above the polymerization 
medium. Such persons will also recognize that at the end of the second 
step (Step II), an amount of unreacted monomers remains. Those skilled in 
the art will further recognize that said amount of unreacted monomers 
exerts a pressure upon the free space above the polymerization medium. It 
is believed that it is generally desirable to reduce the pressure in the 
free space above the polymerization medium prior to opening the 
polymerization apparatus to extract the interpolymer. One technique which 
may be used to reduce the pressure in the free space is to polymerize a 
large portion of the remaining monomers. It has been found that, by 
stopping monomer addition at the end of Step II and thereafter allowing 
the remaining monomers to polymerize by adding additional polymerization 
initiator, sufficient polymer composition variation occurs to render an 
interpolymer recovered after the pressure is reduced by consumption of 
monomers generally insoluble at high levels of vinylidene chloride. By 
monitoring the vapor pressure of monomers in the free space above the 
polymerization medium, it has been found that the vapor pressure decreases 
very rapidly after monomer addition is stopped at the end of Step II. It 
has now unexpectedly been found that, by incrementally reducing the rate 
of monomer addition of the third monomer charge from the rate of monomer 
addition during Step II in at least two incremental steps while 
maintaining the rate of initiator addition, the vapor pressure of monomers 
in the free space decreases gradually. 
An interpolymer produced by incrementally reducing the rate of monomer 
addition during Step III is surprisingly soluble when compared to an 
interpolymer of generally identical composition prepared in the same 
manner except that monomer addition is stopped at the end of Step II. A 
time-pressure curve representing the polymerization wherein monomer 
addition is stopped at the end of Step II has a characteristic shape 
representing Step III. Said characteristic shape is that of a line which 
is generally concave upward when pressure is plotted on the ordinate and 
time is plotted on the abscissa. By way of contrast, a time-pressure curve 
for Step III when monomer addition is incrementally reduced is generally 
concave downward when pressure is plotted on the ordinate and time is 
plotted on the abscissa. See FIG. 1 for an illustration of the latter 
time-pressure curve. See FIG. 2 for an illustration of the former 
time-pressure curve. 
Step III continues for a time interval T. The time interval T varies in 
accordance with (1) the interpolymer which is being polymerized and (2) 
the amount and ratio of mixed monomers remaining at the end of the second 
step. Desirable results have been obtained with a time interval T of about 
11/2 hours. Persons skilled in the art will recognize that satisfactory 
results are obtained when the time interval T is other than 11/2 hours. As 
such, the time interval of 11/2 hours is not to be taken by way of 
limitation. 
It has been found that suitable results are obtained when a first 
incremental step and a second incremental step are used to reduce the rate 
of monomer addition during Step III from that of Step II. In the first 
incremental step, a first reduced rate of addition is maintained for a 
first fractional part of the time interval T. The first reduced rate of 
addition must be less than the rate of monomer addition during the second 
step, or Step II. In the second incremental step, a second reduced rate of 
addition is maintained for a second fractional part of the time interval 
T. The second reduced rate of addition must be less than the first reduced 
rate of addition. Persons skilled in the art will recognize that more than 
two incremental reductions in the rate of monomer addition from the rate 
of monomer addition during Step II may be made. As such, two incremental 
steps are not to be taken by way of limitation. Beneficial results are 
obtained when the time interval T also has a third fractional part wherein 
only the polymerization initiator is added to the polymerization medium. 
Desirable results are obtained when: (1) the first fractional part of the 
time interval T is about T/6; (2) the second fractional part of the time 
interval T is about T/3; (3) the third fractional part of the time 
interval T is about T/2; (4) the first reduced rate of addition is about 
one-half the rate of monomer addition of Step II; and (5) the second 
reduced rate of addition is about one-fourth the rate of monomer addition 
of Step II. 
The three step polymerization technique is necessary to provide 
satisfactory interpolymers in accordance with the present invention. It 
has now been unexpectedly found that interpolymers having the same general 
composition as those of the present invention but which are produced 
without adding a portion of the principal mixture of monomers during Step 
III are unsatisfactory for purposes of the present invention. Such 
interpolymers are unsatisfactory either because of poor solubility or 
because of excessive haze. It is believed that such unsatisfactory results 
arise from the formation of too much high vinylidene chloride content 
interpolymer during Step III. 
Interpolymers containing from about 86 to about 92 mole percent vinylidene 
chloride may also be produced by a conventional batch emulsion 
polymerization. In a conventional batch emulsion polymerization, all of 
the monomers to be polymerized are added at the start of polymerization. 
It has been found, however, that such interpolymers are generally not 
sufficiently soluble in low cost solvents to provide useful coatings for 
substrates such as cellophane. 
Persons skilled in the art will recognize that each of the monomeric 
materials polymerized to form the interpolymers of the present invention 
has a characteristic reactivity ratio with respect to each of the other 
monomeric materials. Such characteristic reactivity ratios are well known 
in the art. In general, it is believed that methyl methacrylate and the 
comonomer which is selected from the group consisting of acrylonitrile, 
methacrylic acid and methacrylonitrile both enter the interpolymer faster 
than vinylidene chloride. It is further believed that itaconic acid enters 
the interpolymer slower than vinylidene chloride. Accordingly, the present 
invention requires that all of the itaconic acid be added during Step I 
rather than added in the same manner as the primary mixture of monomers. 
Because of the differing reactivity ratios, it is believed that the 
vinylidene chloride content of the interpolymer increases as the emulsion 
polymerization proceeds toward completion. At some point as the vinylidene 
chloride content of the interpolymer increases, the interpolymer ceases to 
be soluble in a given solvent system. A typical solvent system is a 
mixture of tetrahydrofuran and toluene in a weight ratio of 65/35. 
Insolubility of an interpolymer, which initially dissolves in a solvent 
system, manifests itself as crystallization of the interpolymer out of 
solution. The crystallization causes the solution to be hazy. Persons 
skilled in the art will recognize that the interpolymer composition at 
which the onset of insolubility occurs varies with solvent strength. There 
will be, however, a threshold level at which solutions of the interpolymer 
are not stable. This threshold may be called the "haze threshold". 
Beneficially, the interpolymers prepared by the present invention do not 
develop hazy solutions upon standing for 24 hours at 25.degree. Centigrade 
(.degree.C.) after dissolving for a period of about 30 minutes in a 
solvent system which is maintained at a temperature of about 30.degree. C. 
Desirably, the interpolymers prepared in accordance with the present 
invention are capable of forming generally haze-free solutions when 
present in an amount of from about 5 to about 20 weight percent solids 
based on weight of solids plus solvent. The solvent system is desirably a 
blend of tetrahydrofuran and toluene in a weight ratio of tetrahydrofuran 
to toluene of about 65/35. Such generally haze-free solutions desirably 
have a haze value of at least 80 percent transmission. Haze values are 
measured in terms of percent transmission of visible light of the 
solution, after the solution has been aged for a period of 24 hours at a 
temperature of 25.degree. C., against a reference of the solvent system. 
Haze values may be obtained by using an ultraviolet spectraphotometer 
operating at a wavelength of 640 nanometers. Preferably, such generally 
haze-free solutions have a haze value of at least 85 percent transmission. 
The interpolymers prepared in accordance with the present invention 
beneficially have a relative viscosity of from about 1.3 to about 1.7 at 
25.degree. C. as a 1 percent solution in tetrahydrofuran. Said 
interpolymers beneficially have the following properties when deposited as 
a coating with a coating weight of 4 grams per square meter: (a) a minimum 
heat seal temperature of from about 70.degree. C. to about 140.degree. C.; 
(b) a water vapor transmission rate of no greater than about 0.25 
grams/100 square inches/24 hours at 38.degree. C.; (c) an oxygen 
transmission rate of no greater than about 0.30 cubic centimeters of 
oxygen/100 square inches/24 hours/1 atmosphere of oxygen at 25.degree. C.; 
(d) a crystallization ratio at 15 minutes at 80.degree. C. of from about 
0.5 to about 2. The interpolymers desirably have a minimum heat seal 
temperature of from about 95.degree. C. to about 130.degree. C. when 
deposited as a coating with a coating weight of 4 grams per square meter. 
The interpolymers of the present invention are prepared from a mixture of 
monomers. The mixture of monomers desirably comprises a principal mixture 
of monomers and an amount of itaconic acid. The principal mixture of 
monomers comprises from about 86 to about 92 mole percent vinylidene 
chloride and from about 14 to about 8 mole percent of a comonomer mixture 
of methyl methacrylate and a comonomer selected from the group consisting 
of acrylonitrile, methacrylic acid and methacrylonitrile. The amount of 
itaconic acid is and from about 0.5 to about 1.0 weight percent itaconic 
acid based on the principal mixture of monomers. The comonomer mixture has 
a molar ratio of the comonomer to methyl methacrylate of from about 0.2:1 
to 5:1. Interpolymers containing less than about 0.5 weight percent 
itaconic acid are believed to provide inadequate coating adhesion when the 
interpolymer is applied as a lacquer coating to a substrate such as 
cellophane. Amounts of itaconic acid in excess of about 1.0 weight percent 
based on the primary mixture of monomers may be used. However, such 
amounts are not required for sufficient adhesion. Amounts in excess of 1.0 
weight percent are accordingly uneconomical. 
It has been found that over a short time interval during Step II, the 
reservoir of unreacted monomers in the polymerization medium shifts 
rapidly from a high level to a low level. The high level is about 10 
weight percent based on weight of the polymerization medium. The low level 
is from about 1 to about 2 weight percent based on weight of the 
polymerization medium. The short time interval will hereinafter be 
referred to as the shift interval. Placement of the shift interval within 
Step II depends upon a number of factors. Such factors include: (1) amount 
of latex solids; (2) desired interpolymer molecular weight; and (3) 
desired interpolymer solution viscosity. Desirably, the amount of latex 
solids to be produced is in the range of from about 50 to about 56 weight 
percent based on weight of the latex. The amount of latex soids is 
advantageously obtained in an emulsion polymerization which lasts for a 
polymerization interval. The polymerization interval is measured from the 
beginning of Step I to the end of Step III. Desirable polymerization 
intervals are in the range of from about 5 to about 20 hours. Persons 
skilled in the art will recognize that polymerization intervals are 
generally selected based on a number of interrelated factors. Illustrative 
factors include, but are not limited to, economic guidelines and a need to 
remove heat from the polymerization medium. Polymerization intervals of 
less than about 5 hours may be used. However, such intervals may require 
the use of complex cooling apparatus. Polymerization intervals of more 
than about 20 hours are possible. However, such intervals are usually 
uneconomical. 
In Step I, the emulsion polymerization medium is beneficially heated to a 
temperature in the range of from about 30.degree. to about 80.degree. C. 
The emulsion polymerization medium is desirably heated to a temperature in 
the range of from about 50.degree. to about 60.degree. C. Preferably, the 
emulsion polymerization medium is heated to a temperature of about 
50.degree. C. In Steps II and III, polymerization is respectively 
continued and finished at a temperature beneficially within the range of 
from about 30.degree. to about 80.degree. C. Desirably, the temperature 
during Steps II and III is within the range of from about 50.degree. to 
about 60.degree. C. Persons skilled in the art will recognize that 
temperature is but one of many polymerization variables which should be 
controlled to ensure production of a consistent interpolymer product. 
Persons skilled in the art will also recognize that temperature control is 
more critical when using thermal initiation rather than a conventional 
reduction-oxidation emulsion polymerization initiator. Beneficial results 
are obtained when polymerization temperatures are maintained within a 
tolerance of .+-.0.5.degree. C. 
The previously stated temperature limitations for each of the three steps 
of the process of the present invention are believed to be important. It 
is believed that if the shift interval occurs too early during Step II, an 
interpolymer which is too soluble will be produced. That is, the 
interpolymer will not crystallize. It is also believed that if the shift 
interval occurs too late during Step II an interpolymer which is generally 
insoluble will be produced. A desirable time interval for Step II is in 
the range of from about 4 to about 15 hours. 
It is to be understood that conventional amounts and types of emulsifiers 
and other additives may be used in preparing the interpolymers of the 
present invention. Such emulsifiers and other additives must not interfere 
with, or significantly alter, the reaction mechanism or the final 
interpolymer prescribed herein. It is also to be understood that small 
amounts of other monomeric materials may be added before, during, or after 
the prescribed polymerization reaction. Such other monomeric materials 
must not interfere with, or significantly alter, the reaction mechanism or 
the final interpolymer prescribed herein. 
Conventional amounts of reduction-oxidation (hereinafter "redox") 
initiators may be used in preparing the interpolymers of the present 
invention. A preferred redox initiator is t-butyl hydroperoxide/sodium 
formaldehyde sulfoxylate mixture wherein the sodium formaldehyde 
sulfoxylate is generally added in excess. The amount of emulsion 
polymerization initiator which is used varies over a broad range and 
depends largely upon the type and concentration of the initiator as well 
as the desired interpolymer molecular weight. As noted above, the emulsion 
polymerization initiator is desirably added at a generally constant rate 
throughout the emulsion polymerization. The generally constant rate must 
be sufficient to produce interpolymers in accordance with the present 
invention. 
The following examples, wherein all parts and percentages are by weight 
unless otherwise stated, illustrate the present invention. The examples 
are not to be construed as limiting the scope of the present invention. 
EXAMPLE 1 
A. Preparation of Vinylidene Chloride (VDC)/ Methyl Methacrylate 
(MMA)/Methacrylic Acid (MAA)/Itaconic Acid (IA) Interpolymer According to 
the Present Invention 
A VDC/MMA/MAA/IA interpolymer was prepared by emulsion polymerization in a 
3-liter glass pipe reactor equipped with an agitator and metering pumps. 
Step I - Initiating Polymerization 
The initial water phase charged to the reactor was as follows: 
1425 grams of deionized water 
18 grams emulsifier (80% active) (a dihexyl ester of sodium sulfo-succinic 
acid commercially available under the trade designation Aerosol MA from 
American Cyanamid Company. 
13 grams itaconic acid (.70 weight % based on the total weight of the 
VDC/MMA/MAA monomer mixture added during polymerization) 
The initial water phase was added to the reactor. The reactor was then 
evacuated, purged with gaseous nitrogen and evacuated a second time. 
A monomer mixture comprising the following ingredients was prepared: 
1898 grams (90 mole %) VDC 
109 grams (5 mole %) MMA 
92 grams (5 mole %) MMA 
An oxidizing solution was prepared by diluting 11.1 grams of 7 percent 
t-butyl hydroperoxide (TBHP) to 500 grams with deionized water. A reducing 
solution was similarly prepared by diluting 4.1 grams of sodium 
formaldehyde sulfoxylate to 500 grams with deionized water. 
Folowing the second evacuation of the reactor, the reactor was heated to a 
temperature of about 55.degree. C. while stirring at a rate of about 250 
revolutions per minute (rpm). The rate of stirring was maintained at about 
250 rpm throughout all three steps. About 73 grams of the monomer mixture 
was then pumped into the reactor as rapidly as possible. Addition of the 
oxidizing and reducing solutions was then started. The oxidizing and 
reducing solutions were each added by pumping, through separate pumps, at 
a rate of about 10 grams per hour. As such, the oxidizing and reducing 
solutions were mixed in the reactor. The seed atex reaction was allowed to 
proceed until there was a drop in pressure from the maximum pressure 
attained during Step I of about 2 pounds per square inch (psi). Step I 
lasted for an interval, beginning with addition of the oxidizing and 
reducing solutions and ending with the drop in pressure of about 2 psi, of 
about one hour. 
Step II - Continuous Addition 
Generally, immediately following the 2 psi pressure drop, addition of the 
monomer mixture was started at a rate of about 130 grams per hour. The 130 
gram per hour rate of addition was maintained for about 131/4 hours. As 
such, a total of about 1722 grams of the monomer mixture was added during 
Step II. Addition of the oxidizing and reducing solutions, each at a rate 
of about 10 grams per hour, was continued throughout Step II. 
Step III - Finishing 
Generally, immediately following completion of monomer mixture addition in 
Step II, finishing was started. In Step III, the monomer mixture was added 
in accordance with the following schedule: 
(1) addition of monomer mixture at a rate of about 65 grams/hour for 1/4 
hour; and 
(2) addition of monomer mixture at a rate of about 32 grams/hour for 1/2 
hour. Addition of the oxidizing and reducing solutions, each at a rate of 
about 10 grams per hour, was continued throughout Step III. Step III 
lasted for an additional 3/4 hour after completion of monomer mixture 
addition. As such, Step III lasted for about 11/2 hours. 
At the end of Step III, addition of the oxidizing and reducing solutions 
was stopped. The reactor was then cooled to ambient temperature. The latex 
was then recovered as detailed below. 
B. Recovery of the Polymer from the Latex 
The interpolymer was recovered from the latex by coagulation in calcium 
chloride (CaCl.sub.2). 35 Cubic centimeters (cc) of a 32% CaCl.sub.2 
solution were added to 1000 cc of water to form a solution. The solution 
was then heated to a temperature of about 40.degree. C. Thereafter 200 cc 
of the latex produced in (A) above were added to the mixture with stirring 
at a rate of about 250 rpm. An additional amount of 1000 cc of water was 
then added to the mixture. The mixture was then slowly heated to a 
temperature of about 80.degree. C. and held at that temperature for about 
2 hours while continuing stirring at a rate of about 250 rpm. The slurry 
was then quenched with ice. The polymer was collected and washed with a 
spray of water for 10 minutes in a centrifuge. The interpolymer was then 
dried to a powder form containing less than about 2% water. The 
interpolymer was then ready for evaluation. 
C. Test for Solubility of the Interpolymer (Haze Stability) 
The haze or tubidity of 19.5 percent interpolymer solids in a 65/35 weight 
ratio tetrahydrofuran/toluene (THF/Tol) solvent mixture was measured using 
a Beckman Model 25 Ultraviolet Spectrophotometer at a wavelength of 640 
nanometers (nm). Haze values are reported as percent transmission. The 
lower the transmission value, the more turbid or hazy the solution. 
D. Testing Coating Performance 
1. Preparation of coated film 
Coating tests were conducted on an oriented polyester film. The film was 
coated with a 19.5 percent interpolymer solids lacquer solution using a 
65/35 weight ratio THF/Tol mixture as the solvent. The coating weight was 
about 4 grams/square meter. The coated film was aged for 16 hours at a 
temperature of 60.degree. C. to ensure development of crystallinity before 
testing the coating. 
2. Measuring permeability to water vapor 
Permeability to water vapor (WVTR) was measured using an infrared 
diffusometer, commercially available from Reigel-Mocon Modern Controls 
under the designation Model IRD-2. The data are reported as grams of water 
per 100 squares inches per 24 hours at 38.degree. C. for a coating weight 
of 4 grams per square meter. 
3. Testing for minimum heat seal temperature 
A Robot automatic controlled, air operated jaw sealer was used for making 
seals so that minimum heat seal temperature (MHST) could be measured. Heat 
seals were made at 5 degree intervals between 95.degree. C. and 
135.degree. C. using 20 psi jaw pressure and dwell time of 1 second. The 
MHST is the temperature at which coating deformation is first observed 
when the seal is torn apart. 
4. Measuring permeability to oxygen 
Permeability to oxygen (O.sub.2 TR) was measured using an oxygen 
diffusometer commercially available from Riegel-Mocon Modern Controls 
under the designation Oxtran Model 1050. The data are reported as cubic 
centimeters (cc) of oxygen per 100 square inches per 24 hours per one 
atmosphere of oxygen at 25.degree. C. for the coating weight of 4 grams 
per square meter. 
5. Determining the crystallization ratio at 15 minutes at 80.degree. C. 
A 15 percent solution of the interpolymer in a solvent mixture of 
tetrahydrofuran and toluene (THF/tol) in a respective weight ratio of 
65/35 was prepared by stirring the interpolymer in the solvent mixture for 
30 minutes at 30.degree. C. Crystallization of a coating prepared from the 
solution was monitored by casting a film from the solution onto a 0.5 mil 
tetrafluoroethylene film directly in the beam of a recording infrared 
spectrophotometer using a coating apparatus generally identical to that 
described in U.S. Pat. No. 3,220,378. The spectrophotometer was a Beckman 
Model 4240 commerically available from Beckman Instruments, Inc. The 
coated film was maintained at a temperature of about 80.degree. C. 
throughout the procedure described herein. An infrared absorption band of 
1045 cm.sup.-1 is characteristic of crystalline vinylidene chloride 
interpolymers. Intensity of the 1045 cm.sup.-1 absorption band was 
monitored with time to yield a crystallization rate curve and a value for 
change in optical density at 1045 cm.sup.-1. The change in optical density 
at 1045 cm.sup.-1 as a result of crystallization is divided by a 
correction factor. The correction factor is the difference between an 
optical density value for the coated film at 1410 cm.sup.-1 and an optical 
density value for the coated film at 1520 cm.sup.-1. The correction factor 
is an approximate correction factor for coating thickness. Data reported 
in Tables I and II was thus determined in accordance with the following 
formula: 
##EQU1## 
6. Measuring relative viscosity of the interpolymer 
Relative viscosity of the interpolymer was determined by using an Oswald 
viscosimeter. A one percent interpolymer solution was prepared by 
dissolving about 1/2 gram of the interpolymer in 50 milliliters of 
tetrahydrofuran. The one percent solution was compared to a pure sample of 
tetrahydrofuran in a manner wellknown in the art to determine a relative 
viscosity. 
The following Table I sets forth the composition of the interpolymer, the 
relative viscosity thereof, the haze stability thereof and the minimum 
heat seal temperature, the WVTR, the O.sub.2 TR and the crystallization 
ratio at 15 minutes at 80.degree. C. of coatings prepared therefrom. For 
purposes of identification, the interpolymer prepared by the process 
detailed in part A of Example 1 is hereinafter identified as Experiment A. 
Two additional interpolymers were prepared and tested in generally the 
same manner as Experiment A. These interpolymers are identified in Table I 
as Experiments B and C. 
TABLE I 
__________________________________________________________________________ 
Solution Haze 
Crystallization 
Stability as % 
Ratio at 
Experiment 
Composition - Mole % 
Wt % 
Relative 
MHST Light Transmission 
15 Minutes 
No. VDC MMA MAA IA* Viscosity 
.degree.C. 
WVTR 
O.sub.2 TR 
Initial 
24 Hours 
at 80.degree. C. 
__________________________________________________________________________ 
A** 90 5 5 0.7 1.43 120 .11 .21 95 86 1.17 
B** 90 7 3 0.7 1.40 120 .13 .18 98 87 1.28 
C** 90 6 4 0.7 1.46 120 .11 .16 95 80 1.20 
__________________________________________________________________________ 
VDC = vinylidene chloride 
MMA = methyl methacrylate 
MAA = methacrylic acid 
IA = itaconic acid 
*The amount of itaconic acid is expressed in terms of weight percent base 
on combined weight of vinylidene chloride, methyl methacrylate and 
methacrylic acid. 
MHST = Minimum heat seal temperature 
WVTR = Water vapor transmission rate; grams of water/100 square inches/24 
hours at 38.degree. C.; coating weight of 4 grams per square meter 
O.sub.2 TR = Oxygen transmission rate; cubic centimeters of oxygen/100 
square inches/24 hours/one atmosphere of oxygen at 25.degree. C.; coating 
weight of 4 grams per square meter. 
**Illustrative of the present invention. 
EXAMPLE 2 - 
Comparative Vinylidene Chloride (VDC)/Methyl Methacrylate (MMA)/Methacrylic 
Acid (MAA)/Itaconic acid (IA) Interpolymers 
In each of a series of additional experiments, comparative vinylidene 
chloride/methyl methacrylate/methacrylic acid/itaconic acid interpolymers 
were prepared as per Experiment No. A of Example 1, except that monomer 
feed during Step II was for 131/2 hours instead of 131/4 hours and monomer 
feeding during Step III was eliminated. Addition of the oxidizing and 
reducing solutions, each at a rate of 10 grams per hour, was continued 
throughout Steps I-III as in Example I. Step III lasted for 11/2 hours as 
in Example 1. 
The following Table II sets forth the composition of the interpolymer, the 
relative viscosity and haze stability thereof, and the mimimum heat seal 
temperature (MHST), the O.sub.2 TR, the WVTR, and the crystallization 
ratio at 15 minutes at 80.degree. C. of coatings prepared therefrom. For 
purposes of identification, the interpolymers prepared in accordance with 
Example 2 as detailed above are hereinafter identified as Experiments D, 
E, and F. 
TABLE II 
__________________________________________________________________________ 
Solution Haze 
Crystallization 
Stability as % 
Ratio at 
Experiment 
Composition - Mole % 
Wt % 
Relative 
MHST Light Transmission 
15 Minutes 
No. VDC MMA MAA IA* Viscosity 
.degree.C. 
WVTR 
O.sub.2 TR 
Initial 
24 Hours 
at 80.degree. C. 
__________________________________________________________________________ 
D 90 5 5 0.7 1.40 120 .11 .19 84 40 1.02 
E 90 7 3 0.7 1.58 120 .16 .18 88 50 1.08 
F 90 6 4 0.7 1.56 120 .13 .17 95 50 1.28 
__________________________________________________________________________ 
VDC = vinylidene chloride 
MMA = methyl methacrylate 
MAA = methacrylic acid 
IA = itaconic acid 
*The amount of itaconic acid is expressed in terms of weight percent base 
on combined weight of vinylidene chloride, methyl methacrylate and 
methacrylic acid. 
MHST = Minimum heat seal temperature 
WVTR = Water vapor transmission rate; grams of water/100 square inches/24 
hours at 38.degree. C.; coating weight of 4 grams per square meter 
O.sub.2 TR = Oxygen transmission rate; cubic centimeters of oxygen/100 
square inches/24 hours/one atmosphere of oxygen at 25.degree. C.; coating 
weight of 4 grams per square meter. 
EXAMPLE 3 - 
Additional Experiments in which Interpolymer Composition is Varied 
In a series of additional experiments, vinylidene chloride interpolymers 
containing either varying amounts of methacrylic acid or a comonomer other 
than methacrylic acid were prepared. The interpolymers were prepared 
either as per Experiment No. A of Example 1 (Process No. 1) or as per 
Experiment No. D of Example 2 (Process No. 2). The following Table III 
sets forth the composition of the interpolymer, the relative viscosity and 
haze stability thereof, and the minimum heat seal temperature (MHST), 
O.sub.2 TR, the WVTR and the crystallization ratio at 15 minutes at 
80.degree. C. of coatings prepared therefrom. For purposes of 
identification, the interpolymers prepared in accordance with this example 
are hereinafter identified as Experiments G, H, I, J, K, L, M and N. 
TABLE III 
__________________________________________________________________________ 
Solution Haze 
Crystallization 
Experi- Stability as 
Ratio at 
ment 
Composition - Mole % 
Wt % 
Relative 
MHST Light Transmission 
15 Minutes 
No. VDC MMA MAA VCN MAN IA* Viscosity 
.degree.C. 
WVTR 
O.sub.2 TR 
Initial 
24 Hours 
at 80.degree. 
__________________________________________________________________________ 
C. 
G 89.4 
4.7 5.9 0.7 1.48 125 0.13 
0.21 
91 30 2.0 
H** 
89.4 
4.7 5.9 0.7 1.47 110 0.16 
0.22 
95 95 1.9 
I 89 3 8 0.7 1.40 130 0.09 
0.17 
90 36 1.6 
J** 
89 3 8 0.7 1.40 130 0.10 
0.21 
98 83 1.5 
K 90 5 5 0.7 1.40 120 0.11 
0.19 
84 40 1.1 
L** 
90 5 5 0.7 1.43 120 0.11 
0.21 
95 86 1.2 
M 86 9 5 0.7 1.68 120 0.19 
0.28 
96 74 0.9 
N** 
86 9 5 0.7 1.7 120 0.21 
0.27 
98 86 0.7 
__________________________________________________________________________ 
VDC = vinylidene chloride; 
MAA = methacrylic acid; 
MMA = methyl methacrylate; 
VCN = acrylonitrile; 
MAN = methacrylonitrile 
IA = itaconic acid 
*The amount of itaconic acid is expressed in terms of weight percent base 
on combined weight of monomers other than itaconic acid which are 
polymerized into the interpolymer. 
**Illustrative of the present invention. 
MHST = Minimum heat seal temperature; 
WVTR = Water vapor transmission rate; grams of water/100 square inches/24 
hours at 38.degree. C. with a coating weight of 4 grams per square meter; 
O.sub.2 TR = Oxygen transmission ratio: cubic centimeters of oxygen/100 
square inches/24 hours/one atmosphere of oxygen at 25.degree. C. with a 
coating weight of 4 grams per square meter. 
A comparison of the data contained in Tables I, II and III clearly 
demonstrates that interpolymers prepared in accordance with the present 
invention (Examples A, B, C, H, J, L and N) have much better haze 
stability than the interpolymers prepared without following a reduced 
monomer feeding schedule (Examples D, E, F, G, I, K and M). Similar 
results are obtained with other interpolymers prepared in accordance with 
the present invention.