Shaped articles from insoluble hydrogels and method of manufacturing same

Substantially water-insoluble, water-swellable crosslinked hydrogels are prepared by polymerizing monomeric material consisting predominantly of acrylonitrile in aqueous salt solution containing zinc chloride as the main component, in absence of added crosslinking agents. The concentration of acrylonitrile is in the range of about 20 to about 40% by weight, related to the weight of the polymerization batch as a whole 100%. The process is carried out by partially hydrolyzing the primary gel thus obtained which consists of polyacrylonitrile plasticized with the aqueous salt solution, whereafter the zinc ions are removed from the gel by treating it with a dilute aqueous solution of at least one electrolyte forming an insoluble precipitate therewith. The partial hydrolysis may be carried out either by increasing the acidity of the solution or temperature or both. Insoluble crosslinked hydrogels are prepared in the form of shaped articles, preferably in either closed or open molds, or in raw castings such as sheets or plates from which the final shaped articles can be manufactured by mechanical processes such as by cutting.

PRIOR ART 
Synthetic hydrogels, i.e. crosslinked or non-crosslinked hydrophilic 
polymers which swell in water and are stable towards chemicals agents, 
have recently been widely applied in the field of optics, such as in 
contact lenses; in surgery and medicine as a substitute or prosthetic 
material for various tissues; in dentistry, and also for multiple 
technical purposes, such as in substitution for gelatin in photography; in 
the production of hydrophilic coatings and lacquers for anti-fogging glass 
protection; in the preparation of ship coatings preventing the growth of 
algae and molluscs; in cosmetics; and the like. Heretofore, hydrogels 
based on polymers and copolymers of glycol methacrylates and glycol 
acrylates have proven to be the most satisfactory. However, the high costs 
of the monomer precursor material has proven to be an obstacle to the wide 
application of the hydrogels. Additionally, when swollen, the above 
hydrogels are not strong enough for many purposes. 
High-elastic gels containing 20-30% of polyacrylonitrile or copolymers of 
acrylonitrile with a predominant component of acrylonitrile and the 
remaining 70-80% consisting of a concentrated aqueous solution of zinc 
chloride, sodium thiocyanate or calcium thiocyanate are known and have 
been suggested for use in the production of oriented acrylic fibers, 
bands, tubes and the like. The uses thereof have, however, been limited by 
the fact that neither zinc nor other ions could be properly washed out. 
Moreover, the copolymers known so far which contained ionogenic 
components, such as ethylene sulfonic acid or its salts and which had been 
prepared in solutions of inorganic salts, contained more than 90% of 
acrylonitrile and were hydrophilic to only a limited extent, so that they 
could not be properly included among the hydrogels. In order to remove 
zinc from the gel-like intermediate fiber, washing in solutions of 
complexones has been suggested, i.e. in the soluble salts of 
ethylenediamine tetra-acetic acid, or in solutions of complex-forming 
organic acids, such as tartaric or citric acid, or in solutions of 
polyphosphoric acids. The foregoing procedure helped to reduce the zinc 
content in fibers and other like products at however increased expense. 
Noteworthy however is that none of the methods described above resulted in 
the complete removal of zinc. 
BACKGROUND OF THE INVENTION 
An object of the present invention is a method of producing crosslinked 
hydrogels of a new type, characterized by increased tensile strength and 
partly also by a rubber-like elasticity, by high adhesion to substrates 
and other useful properties. 
It is another object of the invention to avoid one or more drawbacks of the 
prior art. 
Other objects and advantages of the invention will become more apparent 
from the following detailed disclosure and claims. 
In furtherance of the invention it has been found that the raw materials 
used in production of the new hydrogels are cheaper than glycol 
methacrylates and glycol acrylates of the prior art. 
It has now been established that gels produced from copolymers of 
acrylonitrile containing acrylamide and carboxylic groups, react with 
dilute solutions of carbonates, hydrogen carbonates, phosphates, chromates 
and other inorganic salts, acids and hydroxides, preferably alkali or 
ammonium salts which form insoluble salts with Zn.sup.2+ ions, so that the 
precipitate of the corresponding insoluble zinc compound is formed in the 
surrounding solution, whereas in the gel, the Zn.sup.2+ ions are 
substituted, in the first place, by water and by the ions of the alkali 
metals or by the ammonium ion. On the other hand, anions enter the gel 
much less readily, owing to the anions already present therein and which 
are partially bonded to the copolymers. In contrast to the 
polyacrylonitrile and the copolymers of acrylonitrile with hydrophobic 
monomers, such as the C.sub.1 -C.sub.3 lower alkyl esters of acrylic and 
methacrylic acid, the copolymers according to the present invention do not 
substantially form a surface film of coagulated polymer, which would 
otherwise prevent or slow down the further diffusion of ions, whereby 
instead of the gel of the Zn.sup.2+ salt of the corresponding copolymer, a 
no less elastic and strongly swelling in water hydrogel is formed 
simultaneously with the washing out of the zinc chloride. If the surface 
layer of the hydrogel becomes turbid owing to the formation of a 
precipitate of an insoluble Zn.sup.2+ compound the turbidity can easily be 
removed by using a bath of a dilute mineral acid, such as hydrochloric or 
nitric acid, and by dipping the hydrogel again into a salt bath which 
forms an insoluble salt with the Zn.sup.2+ ions. In this way, 
substantially perfectly clear, transparent crosslinked hydrogels are 
obtained, substantially free from zinc and other metals, which might have 
irritating or other harmful effects when the hydrogels are in contact with 
living tissue or mucous membranes. In the case of technical uses, a slight 
turbidity due to negligible amounts of insoluble Zn.sup.2+ salt does not 
substantially matter in most cases, so that the aforesaid intermediary 
treatment with a dilute mineral acid can generally be omitted. 
Broadly speaking, the instant invention includes the provision of a method 
of producing insoluble crosslinked hydrogels by polymerizing 20-40% by 
weight of acrylonitrile or of a mixture thereof with up to 15 mol percent 
of other monomers copolymerizable therewith in absence of added 
crosslinking agents, in a concentrated aqueous solution of zinc chloride, 
for which some other salt may be partly substituted (i.e. up to about 40% 
by weight CaCl.sub.2, MgCl.sub.2 or NaCl), and thereafter carrying out a 
partial acidic hydrolysis of the polymer plasticized with aqueous 
ZnCl.sub.2 containing solution. The partial hydrolysis in homogeneous 
acidic medium is carried out either by heating the gel or by increasing 
the acidity thereof with or without heating. The copolymers are obtained 
in the form of an elastic gel containing about 20% to about 40% of the 
polymer. The polymers contain mostly nitrilic and amidic side groups, with 
a minor amount of carboxylic and acrylimide groups. The content of 
carboxylic and imidic groups increases with increasing temperature. Zinc 
chloride and other metal cations are then removed from the gel by 
treatment of the gel with a solution of a carbonate, hydrogen carbonate, 
chromate, phosphate, or other compatible agent which forms an insoluble 
precipitate with the zinc ions. 
A minor part of acrylonitrile in the starting monomeric material can be 
replaced, for example, by up to about 15% molar, of monomers capable of 
copolymerizing with acrylonitrile, such as of acrylamide, methacrylamide, 
acrylic acid, methacrylic acid, sodium ethylene sulfonate, mixtures 
thereof or the like. The term partial acid hydrolysis as employed herein 
is meant to include a conversion of a part of nitrilic groups, first to 
amide groups which can be partly further hydrolyzed to carboxylic groups, 
a small amount of imide groups being simultaneously formed. The term 
homogeneous acid medium is meant to include gels of polyacrylonitrile and 
its products of partial hydrolysis, plasticized by aqueous zinc chloride 
containing solutions, acidified with an inorganic acid soluble therein 
without forming a precipitate. The expression "mainly nitrilic and amide 
side groups" means that said groups, both together are present in amounts 
of about 60 to 100%, (molar), preferably about 80 to 99%. The "minor 
amount of carboxylic and acrylimide groups" means said groups are present 
in the product in amounts of about 0 to 40% (molar), preferably about 0.1 
to 20%. 
More specifically the invention includes the provision of preparing 
insoluble crosslinked hydrogel copolymers by polymerizing to form a gel 
about 20 to about 40% by weight of monomeric material consisting 
predominantly of acrylonitrile in absence of added crosslinking agent in 
an aqueous salt solution containing zinc chloride as the main component 
thereof, partially hydrolyzing the thus obtained gel consisting of 
polyacrylonitrile or its copolymers with up to 15% (molar) of other 
monomer, plasticized with said salt solution, and removing salt therefrom 
by treating the gel with a dilute aqueous solution of an electrolyte, the 
anion of which forms a water-insoluble zinc compound until substantially 
all of the zinc ions have been removed from said gel and replaced by 
water, thereby forming an insoluble crosslinked hydrogel. 
The insolubility in any solvent is caused by covalent crosslinking which 
can be explained, in absence of usual cross-linking agents added, only by 
chain transfer onto the monomer and polymer. Soluble, gel-free copolymers 
of this kind can be obtained only if the initial acrylonitrile 
concentration is lower than 15% by weight; at 20% the product is sparingly 
but markedly crosslinked and therefore insoluble. Higher initial 
concentrations than 35% up to about 40% by weight of acrylonitrile are 
feasible, only the dissipation of the heat of polymerization is more 
difficult, and the shrinking is rather high. 
The crosslinking by chain transfer in absence of any added crosslinking 
agents (usually a monomer with more than one polymerizable double bond), 
is caused by increasing the initial acrylonitrile concentration to 20-40% 
by weight, related to the weight of the polymerization batch as a whole. 
Zinc chloride solutions do not cause chain transfer onto the solvent. It 
is understood, naturally, than no compound causing chain transfer is 
present as an impurity or added intentionally, because in such a case the 
chain transfer onto the monomer would be either reduced or fully avoided. 
The degree of crosslinking depends on the initial acrylonitrile 
concentration, increasing proportionally therewith. 
The crosslinking by the chain-transfer onto the monomer posseses 
considerable advantages. First, the crosslinks possess the same chemical 
composition as the main chains, containing no easily hydrolyzable or 
chemically attackable chain links. Second, the crosslinks are, in average, 
considerably longer than in case of using a common crosslinking agent such 
as ethylene glycol dimethacrylate. This can be explained by the fact that 
chain-transfer onto the monomer results in a trifunctional co-monomer 
having one function, namely the free radical, more reactive than the two 
other functions, namely the double bond. Thus, a sufficiently long polymer 
chain has grown on the free radical prior to the reaction of the double 
bond with another growing free radical. Moreover, the crosslinks are 
formed by a recombination of two growing free radicals so that their final 
length is still longer. 
The absence of easily hydrolyzable chain links results in increased 
chemical stability, the presence of long crosslinks improves physical and 
physico-chemical characteristics of the product (elasticity, strength, 
swelling capacity, structural strength and the like). 
The polymerization of acrylonitrile in an aqueous solution of an inorganic 
salt, the main component of which is formed by zinc chloride, followed by 
partial acid hydrolysis has in this case many advantages. Firstly, a 
concentrated aqueous solution of zinc chloride is a good solvent for 
acrylonitrile, as well as for its polymers and copolymers formed following 
the partial hydrolysis. Organic solvents, such as dimethylformamide or 
dimethylsulfoxide, are not well suited for this purpose, since they lead 
to a considerable chain transfer, thus largely reducing the degree of 
polymerization and are further inflammable or poisonous, and still further 
their recovery is comparatively difficult. Inorganic salts, on the other 
hand, exhibit only a negligible chain transfer in free radical 
polymerization, so that chain transfer onto the monomer leads to branching 
and crosslinking. Moreover, said salt solutions have the advantage that 
they substantially do not dissolve molecular oxygen, so that in most cases 
no inhibition is observed, even if the polymerization is not carried out 
under an inert gas. The method according to the present invention provides 
for virtually complete recovery of the solvent in the form of an insoluble 
salt, as almost no zinc is present in the waste water. In this way, the 
most difficult problem of waste water pollution is solved, with the 
additional advantage of the recovery of a metal in a time of shortages. If 
a countercurrent system of washing is employed, only a small amount of the 
salt used for the precipitation is transferred into the waste water; and 
if a harmless salt, such as sodium hydrogen carbonate has been used, the 
waste water pollution is negligible. 
The electrolytes used for the precipitation of zinc cations can be chosen 
in such a way as to yield zinc in the form of a white or coloured pigment 
or raw material suitable for being used in pigment preparation. This again 
increases the economy of the process. 
PREFERRED EMBODIMENT 
The copolymers of acrylonitrile with acrylamide and acrylic acid can be 
produced with special advantage by preparing an elastic crosslinked gel 
homopolymer of acrylonitrile by polymerizing 20-40% by weight of 
acrylonitrile at low temperatures and in absence of added crosslinking 
agents using effective redox initiators in a solution containing zinc 
chloride as its main component; the elastic crosslinked gel thus obtained 
is then subjected to partial hydrolysis prior to the removal of zinc. For 
this partial hydrolysis, heating to about 80.degree. to about 130.degree. 
C. for several hours is to be recommended. Temperatures ranging from about 
50.degree. to 80.degree. C. may also be employed with good results. In 
this way, one part of the nitrile groups is transformed into amidic and 
carboxylic groups, and the distribution of the amidic and carboxylic 
groups in the copolymer is as a rule, more advantageous as in the case of 
copolymerization. Heating is advantageously carried out in such a way as 
to prevent excessive dehydration, that is, preferably in a closed vessel 
in the presence of superheated water vapor. The vapor should not be 
permitted to condense on the gel, thereby avoiding an irregular partial 
leaching of the salts from the surface. Temperatures of about 50.degree. 
to about 150.degree. C. are acceptable. 
Another way to accelerate the partial hydrolysis is to increase the acidity 
of the homogeneous gel, consisting mainly of polyacrylonitrile plasticized 
with the zinc chloride-containing aqueous solution. The acification can be 
performed by adding a small amount of a mineral acid, preferably sulfuric 
acid, into the polymerizing mixture. It is better, however, to expose the 
zinc chloride-plasticized (primary) gel to a gaseous hydrogen halide which 
diffuses with sufficient velocity into the gel. The resultant increase in 
acidity is very high and the partial hydrolysis takes place at any 
convenient temperature, with or without heating. Operative hydrogen 
halides include HF, HCl, HBr and the like. 
The amount of mineral acid employed may vary from about 0.01 to 40%, 
preferably about 0.1 to 5% by weight. The concentration of gaseous 
hydrogen halide employed is variable, and may range from about 0.01 to 
100%, preferably about 0.1 to 50%. It is generally exposed to the gel 
until such time as the desired effect is obtained. The treatment with 
gaseous hydrogen halide can be advantageously carried out at decreased 
temperatures and increased pressure, to delay the partial hydrolysis until 
the whole cast article is homogeneously penetrated by the hydrogen halide. 
Then, if desired, the temperature can be increased to increase the rate of 
hydrolysis. 
The achieved degree of hydrolysis can be followed either by chemical or 
spectral analysis, or, most simply, by determining the swelling capacity 
of the thoroughly washed hydrogel. 
Crosslinked hydrogels prepared according to the invention are chemically 
and physically stable, and can be sterilized in neutral or weakly acid 
medium by boiling without substantial damage. They are also 
physiologically inert. 
Washing of the partly hydrolyzed gel in a solution of an agent which 
precipitates the ions of zinc or other metals to be removed from the gel 
can be carried out in such manner that in the first washing, the 
concentration is at its lowest and in the last one, at its highest, that 
is, the process is a countercurrent one, either continuous or stepwise. In 
the case of a continuous process the precipitate formed can be removed 
continuously or intermittently by passing the liquid suspension through a 
filter or a centrifuge.

The following Examples are illustrative in accordance with the invention. 
It will be understood that numerous modifications can readily be made in 
light of the guiding principles and teachings of the present invention 
disclosed above. The Examples given are, therefore, by way of illustration 
and not by way of limitation. All parts, proportions and ratios listed in 
the Examples and appended claims are in terms of weight unless otherwise 
noted. The percentages of concentrations are related to the weight of the 
batch as a whole which corresponds to 100%. 
EXAMPLE 1 
27 Parts by weight of acrylonitrile are dissolved in 73 parts by weight of 
a mixture of concentrated aqueous solution of zinc chloride and calcium 
chloride in a volume ratio of 3:2. The resulting solution is degassed and 
cooled to -15.degree. C.; a redox initiator is added to the solution and 
the mixture is poured into a glass mold, plate distance 1 mm. The plates 
are 30.times.30 cm in size. The onset of the polymerization is observed on 
the basis of the viscosity of the rest of the solution. As soon as the 
viscosity begins to increase markedly, the glass mold is placed in a 
cooling box at -25.degree. C. The contents of the mold, slightly turbid 
immediately after the solution has been poured into it, become clear in 
the course of polymerization. After 2 hours the mold is removed from the 
cooling box and left in the air at room temperature for another hour. Then 
the mold is put into a larger dish, placed in a drying oven and heated to 
125.degree. C. for two hours. After dismounting, the strong, 
rubberlike-elastic, yellowish, transparent foil is cut into strips, 3.9 cm 
in size. 
The individual strips are freed from zinc chloride and calcium chloride by 
immersing them in dilute aqueous solutions of sodium hydrogen carbonate, 
sodium carbonate, potassium chromate, and ammonium hydrogen phosphate at 
pH 7 to 11. The solutions are changed as long as a precipitate is formed. 
After this, the strips are washed in a 2% hydrochloric acid and immersed 
for 24 hours into distilled water, which is changed four times during that 
time. 
The hydrogel thus obtained is very elastic. If transformed into the sodium 
salt by reaction with weak alkali, it is quite clear and considerably 
swollen in water. A strip freed from zinc chloride and calcium chloride in 
a solution of alkali chromate is transparent and bright yellow in color. 
If the samples are in the hydrogen form, they can be sterilized by boiling 
without change. Boiling in the sodium form leads to enormous swelling and 
final disintegration and dissolution of the samples by breaking covalent 
crosslinks. The sterilization of the articles from the crosslinked 
copolymer of acrylonitrile with acrylamide and acrylic acid (whose units 
are formed additionally by a partial hydrolysis on heating of the original 
gel) can therefore be carried out in a hydrogen form preferably at pH 4 to 
pH 7; the copolymer is then placed into a sterile physiological solution 
or a dilute solution of sodium hydrogen carbonate; when it is again 
transformed into the sodium form, swells to equilibrium and becomes 
perfectly transparent. It is highly swellable in highly polar solvents of 
polyacrylonitrile such as dimethyl formamide, but absolutely insoluble 
therein, provided that covalent bonds are not broken by a too long heating 
at temperatures higher than 100.degree. C. 
EXAMPLE 2 
30 Parts by weight of acrylonitrile are dissolved in 70 parts of aqueous 
70% zinc chloride solution, the mixture is cooled down to -15.degree. C. 
and reaction initiated with a redox polymerization catalyst consisting of 
equal parts of potassium metabisulfite and ammonium persulfate (0.1% each) 
in the form of a 5% aqueous solution. The solution is then poured into a 
flat glass mold (thickness 2 mm) precooled to -25.degree. C. to 
-30.degree. C. The filled mold is then kept at -25.degree. C. After 6 
hours the mold is removed from the refrigerator and kept for another two 
hours at room temperature. Then the mold is dismounted and the tough, 
rubbery sheet of zinc chloride-plasticized polyacrylonitrile fastened with 
one of its edges onto the inner side of a tight lid covering a 5 liter 
glass vessel on the bottom of which 150 ml of 37% hydrochloric acid have 
been poured. The exposure of the gel to the hydrogen chloride lasts for 36 
hours at 15.degree. C. Then the sheet of the partly hydrolyzed 
polyacrylonitrile is immersed into 0.5% aqueous sodium chromate solution, 
the bath being renewed several times until no further yellow precipitate 
is formed. After thorough washing in water, in 0.5% sulfuric acid and in 
water again, a transparent, tough, rubbery sheet is obtained, containing 
53% water at swelling equilibrium. The dry substance is a copolymer 
containing approximately 40% acrylonitrile, 55% acrylamide and 5% acrylic 
acid units, with a negligible amount of acrylimide groups. 
The hydrogel is fully insoluble in any solvent, including highly polar 
solvents capable of dissolving polyacrylonitrile such as dimethyl 
formamide, dimethyl sulfoxide or a 60% aqueous sodium thiocyanate 
solution. The hydrogel only swells in said solvents, and if the solvent is 
removed by washing the gel thoroughly in water, the article regains its 
original size. The general shape of the article, e.g. of a strip of said 
sheet, remains unchanged during the whole process of swelling and 
de-swelling. Its edges remain sharp even at the highest swelling degree. 
This is a positive proof of a covalently crosslinked structure. 
When implanted in rats, the hydrogel causes no irritation or inflammation 
of the surrouding tissue and is encapsulated by a thin layer of ligament 
tissue only. 
The precipitated zinc chromate can be used in pigment manufacture. 
EXAMPLE 3 
The process described in Example 1 is repeated except that 2% by weight of 
sodium ethylene sulfonate and 0.5% of 96% sulfuric acid are added to the 
monomer mixture. The obtained primary gel is similar to that of Example 1, 
but the increased acidity caused mainly by the presence of strongly acidic 
sulfonic groups greatly accelerates the hydrolysis so that either the time 
of heating or the temperature can be substantially reduced, or at the same 
conditions, the degree of hydrolysis can be increased. 
Similar results to those set out in the preceding Examples are obtained if 
up to 15% (mol) of acrylonitrile is replaced by acrylic or methacrylic 
acid or by their respective amides. 
The insoluble copolymer in the H-form can be washed with hot water and 
further treated, for example, for additional crosslinking. As crosslinking 
agent for the polymer any bifunctional or trifunctional compound reacting 
with the side groups can be used, incl. formaldehyde or trivalent cations 
of aluminium, iron and chromium. 
A part of the zinc chloride can be substituted by cheaper salts in a known 
manner, e.g. by calcium, sodium or magnesium chloride, and the like, as is 
known in the art. 
Since it is obvious that numerous changes and modifications can be made in 
the above-described details without departing from the spirit and nature 
of the invention, it is to be understood that all such changes and 
modifications are included within the scope of the invention. 
The term "shaped articles" means various objects which can be prepared by 
polymerization casting as described above, or by mechanical processing of 
the raw castings such as sheets or rods. It is obvious that the insoluble 
elastic material, containing covalent crosslinks, cannot be used for 
spinning fibers or similar processes such as extrusion in which either 
dissolution or melting of the material is necessary. 
The term "insoluble" means incapable of purely physical dissolution in any 
solvent including solvents of non-crosslinked polyacrylonitrile such as 
dimethyl sulfoxide, dimethyl formamide or aqueous solutions of sodium or 
caleium thiocyanate.