Water-absorbent starch copolymerisates and method for their preparation.

Abstract

 O Water-absorbent, starch copolymerisates which comprise a non-linear lattice of starch chains linked together by polymeric linkages represented by the formula

wherein Starch represents a starch chain of D-glucose units, Z represents an organo group which links the

group to the carbon atom of the starch chain by a sulphur or an oxygen atom, R is hydrogen or a monovalent organic radical, M represents a plurality of copolymerised ethylenically unsaturated monomers with "p" representing the number of copolymerised monomeric units in the linkage, (W) is a water-attractant group linked to the polymeric linkage and "n" represents the number of (W) moieties contained within the polymeric linkage of the copolymerised monomers with the number of (W) moieties being sufficient to impart water-absorbency properties to the copolymerisate. The water-absorbent starch copolymerisate may be easily prepared into a desired configuration for a particular end-use or combined with other substrates or carriers. The unpolymerised starch and monomers or precursors thereof may be applied or incorporated into a suitable carrier.or substrate and copolymerised in situ to provide a composite article of unitary construction with the water-absorbent starch copolymerisate permanently bonded or affixed thereto.

Description

[0001] This invention relates to water-absorbent starch copolymerisates.

[0002] Within recent years, certain derivatised starches capable of absorbing and retaining large amounts of water have been developed. These derivatised starches are frequently referred to as "water-absorbent starches". In U.S. Patent Nos. 3,935,099 and 3,997,484 (both by Weaver et al.), starch polymers which reportedly absorb more than 1,000 times their own weight are disclosed. These water-absorbent starches are generally prepared by grafting polyacrylonitrile to starch molecules and then derivatising the polyacrylonitrile chains to anions. The grafting is accomplished by free-radical catalysis (e.g., ceric or irradiation). The starch-grafting process is difficult to control and time consuming. The achievement of a critical grafting level is an essential prerequisite for a water-absorbent, end-product. A series of derivatisation and neutralisation steps are typically used to convert the nitrile group to anions and a water-absorbent starch product. This contaminates the product with salt. The water absorbency properties of these salt-contaminated starches are seriously impaired when they are used in aqueous solutions which contain trace amounts of salts and minerals. It is also difficult to achieve uniform and reproducible water-absorbency results. This apparently arises from difficulties in controlling the reaction. These water-absorbent starch compositions are also deficient in certain other properties which are essential and desirable for many end-usages (e.g., lack adhesiveness, prefabrication and shaping, film-forming, bonding, coating, etc. properties}. This generally restricts their usage to limited areas of application (e.g., separately contained by a water- permeable enclosure or separately added or mixed to another substrate). In addition, these water-absorbent starches

[0003] United States Patent No. 3,661,815 by Smith also discloses analogous water-absorbent starches which are prepared by saponifying starch-polyacrylonitrile graft derivatives with certain alkali metal bases. These water-absorbent starch grafts reportedly absorb more than 50 times their weight of water. The Smith process and products suffer from similar deficiencies as mentioned above with respect to Weaver et al.

[0004] There is a need in the art to more easily and effectively prepare water-absorbent starch compositions under conditions which provide greater uniformity and end-product reproducibility. Greater tolerance and compatibility with aqueous solutions containing salt and mineral contaminants is also required. Even more important, is the development of a water-absorbent starch which can be easily bonded or affixed to a substrate or preformed. Such a water-absorbent starch would considerably expand upon the versatility and usage of water-absorbent starches by the trade.

[0005] An object of the invention is to provide a simple und reproducible method for preparing water-absorbent staren compositions. Another object is to obtain water-absorbent starch compositions which in comparison to existing water-absorbent starches have improved versatility, utility and functional properties. further object is to provide a method for applying or affixing water-absorbent starches to carriers or substrates or preparing preformed products and the products thereof.

[0006] According to the present invention there is provided water-absorbent starch copolymerisate which is capable

several times its own weight of water, the starch copolymerisate oomprising the copolymerisate product of ethylenically unsaturated starch molecules and ethylenically unsaturated monomers with the ethylenically unsaturated monomers forming a connective polymeric linkage between the copolymerised starch molecules to provide a non-linear lattice of a plurality of starch chains linked together by polymeric linkages represented by the formula:

wherein Starch represents a starch chain of D-glucose units, Z represents an organo group which links the

group to the carbon atom of the starch chain by a sulphur atom or an oxygen atom, R is hydrogen or a monovalent organic radical, M represents a plurality of copolymerised ethylenically unsaturated monomers with "p" representing the number of copolymerised monomeric units in the linkage, (W) is a water-attractant group or a hydrophilic moiety such as an anion, cation, non-ion, amphoteric, zwitterion or amphiphilic moiety or a mixture thereof linked to the polymeric linkage and "n" represents the number of (W) moieties contained within the polymeric linkage of the copolymerised monomers with the number of (W) moieties being sufficient to impart water-absorbency properties to the copolymerisate.

[0007] The water-absorbent starches or their precursors may be prepared by a copolymerisation process which comprises copolymerising:

.(a) starch chains containing appendant, terminal ethylenic unsaturated groups represented by the formula:

wherein Starch, Z and R are as defined above and "a" represents the degree of substitution of the terminal unsaturated groups on the starch chain, and

(b) ethylenically unsaturated monomers represented by the formula:

wherein M' represents an ethylenically unsaturated monomer, "(W')" represents at least one water- attractant group or a precursor thereof, and n' is an integer

to provide a cross-linked lattice of a plurality of starch chains linked together by polymeric linkages represented by the formula: (W')_n

wherein the Starch, Z, R, (W') and n are as defined above, M represents a plurality of copolymerised ethylenically unsaturated monomers which contain a sufficient number of (W) or (W') precursors within the polymeric linkage to impart water-absorbency to the copolymerised product, and "P" represents the number of copolymerised ethylenically unsaturated monomers between juxtapositional starch chains.

[0008] In the copolymerisation process, a wide variety of M'-(W' )_n' monomers may be used to prepare the water-absorbent starch of this invention. The value of the n'

others may be free from the "W" or "W" precursor moieties (e.g., n' is 0). Similarly, the copolymerised monomers may be essentially comprised of monomers which contain the "W" or "W" precursors. In the aforementioned formula, M' may be comprised of an ethylenically unsaturated portion of an organic group of the same chemical composition, or a mixture of different copolymerised monomers in which the M' group differs in composition. Likewise, the "W" or "W"' precursors may be the same or different in type. The amount of "W" monomer or "W" monomer precursors copolymerised with the starch is maintained at a level sufficient to impart water-absorbency to the copolymerised starch product. If "W^I" precursors are solely used, then it is necessary to convert a sufficient number of precursors to the water attractant form to achieve the desired water-absorbent, starch copolymerisate product.

[0009] In comparison to existing water-absorbent starches, the present starch copolymerisates are more versatile and useful. They may be prefabricated from water-soluble or water-dispersible, modified or hydrolysed starches into high-molecular-weight and cross-linked, water-absorbent starch copolymerisates. In general, the ethylenically unsaturated starches used herein are most typically provided in a water-soluble form or may be easily converted to such a form. This renders the present invention particularly applicable to prefabricating operations wherein water or aqueous systems are used to disperse, dissolve or plasticise the starch. The invention therefore is ideally suited for most prefabricating operations (e.g., coating, moulding, casting, extrusion, drying, sheeting, printing, bonding, encapsulating, gelling, impregnating, laminating, plasticising, etc.) wherein the starch is initially provided in a form most suitable for prefabrication (e.g., liquid, pliable, mouldable, etc.) and then preformed and converted into a solid object.

pea, amylose or amylopectin fractions, combinations thereof and the like. The starch amylose content affects the temperature at which a starch will convert to a water-dispersible or starch paste form. The high amylose starches typically require elevated temperatures and pressures (e.g., extrusion, jet cooking, etc.), for uniform dispersal into aqueous systems. In contrast, starches of a lower amylose content (e.g., 30 % amylose or less) are more easily dispersed or pasted in water (e.g., 50° to 70°C). Prepasted or pregelled starches of an amylose content of less than 30 % normally disperse

prefabricating applications. Modification, derivatisation or hydrolysis of such starches may be accomplished prior or after its derivatisation to the ethylenically unsaturated form.

[0010] Ethylenically unsaturated starches which contain hydrophobic substituents may be used but will typically require a dispersant. Water-miscible, organo dispersants such as alkanols (e.g. methyl, ethyl, isopropyl, or butyl-alcohol), polyhydric alcohols (e.g. glycerol, ethylene glycol), ethers, (e.g. methyl, ethyl or propyl ethers, etc.), ketones (methyl ethyl ketone, ethyl ketone, etc.), as well as conventional anionic, nonionic and cationic surface active agents or emulsifiers (e.g., see McCutcheon's Detergents and Emulsifiers - North American Edition - 1975) may be used to facilitate their conversion to a more water-dispersible form.

[0011] It is usually advantageous to employ hydrophilic, ethylenically unsaturated starches which will uniformly disperse into water at-temperatures above the starch gelation point without the aid of water-miscible organo dispersants or surfactant systems. Hydrophilic starches characterised as yielding a centrifugal starch residue of less than 25 % (preferably less than 10 %) upon immersion in water (6ne part ethylenically unsaturated starch/100 pbw water) for one hour at temperatures above their gelation point and centrifugation at 103 g's for 10 minutes are most suitably used to coating and prefabricating applications. Hydrophilic ethylenically unsaturated starches containing pendant ethylenically unsaturated groups with polar moieties or substituents to impart hydrophilicity to the unsaturated portion of the starch molecule (e.g. hydroxy, carboxy, amide, carbamyl, sulphoamyl, imido, sulphoamino, thio, thiolamino, oxy, thiocarbonyl, sulphonyl, carbonyl, sulphoamido, quaternary ammonium halides, the alkali or ammonium salts) are especially useful.

[0012] The water-diespersible, ethylenically unsaturated marches perein may be prepared by a variety of starch derivatisation processes. Derivatisation processes which may be used to produce appendant, monoethylenically unsaturated groups include reacting alkali metal starch or hydroxyethylated starch salts with an allyl propiolate to provide carboxylated vinyl starch ether; reacting starch with ethylenically unsaturated organic carboxylic anhydrides (e.g., methacrylic anhydride, etc) or organic allyl halides (e.g., allyl bromides, allyl chloroformates, etc)., or epoxides (e.g., butadiene monoxides, etc.) to provide ethylenically unsaturated starch esters or ethers. The most suitable monoethylenic unsaturated starches are the starch esters of alpha, beta ethylenically unsaturated carboxylic acids (e.g., acrylate, methacrylate, crotonate, citraconate, itaconate starch esters as well as alkali salts and amides thereof, mixtures thereof and the like); N-allyl carbamate starch esters (e.g.,

glycidyl methacrylate and glycidyl acrylate starch ethers (e,g., see U.S. Patent No.3,448,089); allyl starch ethers (e,g. , allyl, isopropenyl, etc.); the allyl alkyl starch ethers (e.g., ethyl, propyl, butyl, etc. starch ethers) and the allyl ethylene oxide starch ethers; aallyloxialkyl starch ethers (e,g., the allyl oxyethyl, oxypropyl and oxybutyl,

starch ethers); allyloxy hydroxyalkyl starch (e.g., allyloxy-2-hydroxy-propyl starch, etc,); starch acrylamides, etc.; combinations thereof and the like,

[0013] In a more limited embodiment of the invention, the ethylenically unsaturated starches comprise those starches which will readily and uniformly copolymerise with the bridging comonomers. Ethylenically unsaturated starches which contain polar groups immediately

to the unsaturated group and which activate the copolymerisability of the double bonds in the presence of free-radical initiating systems are particularly well suited for this purpose. Such ethylenically unsaturated starches may be represented by Formula III:

wherein starch is a starch chain of D-glucose units, E represents an activating polar group juxtapositional to the ethylenic unsaturation, D is sulphur or oxygen, Q.is an organo group which divalently joins the D group to the activating polar group, R represents a monovalent group and "a" represents the D.S. (i.e., the number of appendant ethylenic unsaturated groups per anhydroglucose unit of said starch chain). Typical juxtapositional activating polar groups (i.e., E) include carbonyl

thiocarbonyl

groups and the like. The ethylenically unsaturated portion of the starch chains are most typically comprised of appendant . groups which individually have a molecular weight of less than 500 with those having an appendant molecular weight of greater than 50 but less than 300 (preferably from 75 to about 150 M.W.) being most typical.

[0014] In a more preferred embodiment of the invention, the E group contains a

radical, R' is a hydrogen atom or a mono-organo group which is joined directly to the nitrogen atom by a monovalent bond.

[0015] In Formula III, Q may be any divalent organo group which joins the activating radical to the starch chain e.g., linked to D and acrylamide nitrogen atoms via carbon linkages). The starch oxygen or sulphur atom and activating radical may be directly linked together by a single carbon atom or by an organo group which is

acrylamide wherein R' and R represent monovalent group Substituted acrylamides which contain a reactive N-methylol group linked to the acrylamide nitrogen atoms by intervening divalent Q organo groups end starches containing cationic and anionic or ionic acrylamide substituents may be obtained by etherifying a starch with the appropriate N-methylol acrylamide (e.g. sodium-2-N-methylol -acrylamido-2-methylpropanesulphonate, a N-methylol acrylamide quaternary ammonium halide such as 3-(N-methylol acrylamido)-3-methyl butyl trimethyl ammonium chloride, etc.). Representative R' substituents in V above include hydrogen, N-arylol, the N-alkylamines and N-arylamines such as N-methylol-; N-ethyl -; N-isopropyl -; N-n-butyl-; N-isobutyl-; N-n-dodecyl-; N-n-octadecyl-; N-cyclohexyl-; N-phenyl-; N-(2-hydroxy-1, 1-dimethylpropyl)-; N-p-hydroxybenzyl-; N-(3-hydroxybutyl)-; N-(4-hydroxy-3,5 - dimethylbenzyl)-; N-(3-hydroxy-1-1 dimethyl)-; N- (2-hydroxy-1,1-dimethylethyl); N-(2-hydroxyethyl)-; N-(5-hydroxy-1 -naphthyl)-; combinations thereof and the like. Illustrative acrylamide reactants (B) include N-methylol and N-thiomethyl acrylamides such as N-(hydroxymethyl) acrylamide; N-(hydroxymethyl)-N-[(1-hydroxymethyl) propyl] acrylamide; N-(hydroxymethyl)-2-alkyl acrylamides, (e.g., N-(hydroxymethyl)-2-(methyl-hepthyl) acrylamide; -[(1-hydroxymethyl)-1-nonyl]-2-methyl acrylamide; N- (1- nydroxymethyl)-2-methyl acrylamide; N-( hydroxymethyl) --propyl acrylamide; etc.); N-(mercaptomethyl) acrylamide; N-methylol-N-isopropyl acrylamide; 3-(N-methylol crylamido)-3-methyl butyl trimethyl ammonium chloride (cationic); sodium 2-N-methylol acrylamido-2-methyl (cationic); sodium 2-N-methylol propane sulphonate (anionic -CH₂:C(H)C(:O)(CH₂OH)C-[(CH₃)₂]CH₂SO₃Na⁺), combinations thereof and the like.

[0016] Reaction V may be suitably conducted in the

temperature from about 70°C to about 95°C until the desired D.S. level is achieved. Conventional polymerisation inhibitors (e.g., hydroquinone, its derivatives, 2,5-di-t-butylquinone, etc.) prevent homopolymerisation of the starch acrylamide and acrylamide reactants. The starch acrylamides may be prepared via solution, slurry, dry, semi-dry or other appropriate condensation processes. To prepare a starch- acrylamide having a D.S. level of 0.03 or higher, it is desirable to uniformly disperse the acrylamide, acid or acid-generating catalyst and polymerisation inhibitor throughout the starch reactant. Uniform dispersal of the N-methylol-acrylamide reactant, catalyst and i polymerisation inhibitor throughout the starch may be effectively accomplished by initially forming a starch slurry or treating the starch with an absorbable dispersant media (e.g., water) in which the acrylamide, catalyst and polymerisation inhibitor are soluble or placed in mobile form and thereafter imbibing or absorbing the dispersant and its solutes into the starch granules.

[0017] As more fully explained below, the most appropriate ethylenic unsaturates for optimum water-absorbency will depend upon the starch chain type. An ethylenically unsaturated monoglucoside will typically require at least a D.S. of about 2.0 or more, whereas long chain starch chains (e.g., unhydrolysed starch) typically require a considerably lower D.S.. level (e.g., 0.0002) to be water-absorbent. Moreover, there exists a direct relationship between the D.S. for any given starch chain and the optimum water-absorbency which may be achieved from the starch copolymerisate thereof. An insufficient or excessive ethylenic unsaturation D.S. level will genera result in a copolymerisate having poor water-absorbency properties. A D.S. deficiency will fail to provide the necessary multifunctional.polymerisation sites for the water-absorbency materials. For a majority of starches

copolymerisate which is capable of

several times its own weight can be typically

by copolymerising a starch which has an ethylenic

ranging from about 0.002 D.S. to about 0.10

Higher ethylenic unsaturated D.S. levels (e.g, 0.2 or higher) will usually require more carefully controlled copolymerisation conditions with an appropriate proportion of ethylenically unsaturated monomers and type of monomer. Starch copolymerisates which typically absorb more than 10 times their weight in water are obtained from starches having an ethylenic unsaturation ranging from about 0.005 D.S. to about 0.05 D.S. For applications requiring a more highly water-absorbant. starch (e.g., greater than 100 times the starch dry weight), it is advantageous to use starch substrates which contain appendant ethylenic unsaturation at a level ranging from about 0.005 D.S. to about 0.01 D.S.

[0018] The starch copolymerisate water-absorbency properties are directly related to its lattice (i.e., molecular configuration) and its ionic hydrophilicity. The characteristics of the starch chain and the polymeric linkages formed by the interpolymerised ethylenically unsaturated monomers primarily dictate

copolymerisate lattice structure. Failure to hieve proper polymeric linkage or bridging between

molecules will adversely affect the water-absorbency properties of the starch copolymerisate. accessively long polymeric monomer linkages tend to

in an excessively open structure which adversely affects the water-absorbency character of the starch copolymerisate lattice. Conversely, excessive crosslinking (e.g., high D.S. ethylenically unsaturated

or an insufficient amount of copolymerised

(e.g., very short linkages between starch

tend to create a closed lattice and concomitant

-absorbency. The net ionic charge of the copolymerisate in conjunction with its water porous lattices contributes to its water absorbtion and retention properties. Similarly, achievement of the optimum lattice and an insufficient ionic charge impairs its water-absorbency. The combination of a proper lattice and a sufficient level of ionic charge to attract and absorb water molecules within its porous lattice provides maximum water-absorbency

[0019] In the water-absorbent starch copolymerisate, the copolymerised ethylenic unsaturated monomers (i.e., -[M]_p' of Formula I) contain a sufficient number of hydrophilic substituents (e.g., ←(W)_n of Formula 1) to impart water-absorbency to' the copolymerised starch product. Illustrative hydrophilic substituents include cationic, anionic, nonionic, ampholytic, zwitterionic, amphoteric moieties, mixtures thereof and the like. As mentioned above, it is unnecessary for each copolymerised monomeric unit to be a water-attractant group. Thus, a significant portion of the polymeric chain units may be free from ionic substituents with the balance of the units providing a sufficient level of "W" substitution to render the starch copolymerisate water-absorbent. The degree of "W" substitution necessary to achieve a water-absorbent starch copolymerisate will depend upon a multiplicity of factors. Factors such as the ionic charge and type of ionic substituents, proportions of ethylenically unsaturated starch to monomer, hydrophilicity and polarity of the copolymerised monomer units, etc. affect the required "W" substitution level. For most application, it is advantageous for the starch copolymerisate to contain either anionic or cationic substituents.

[0020] A variety of conventional, ethylenically unsaturated monomers which either contain the water-absorbtive substituents or its precursors may be used to prepare the starch copolymerisates herein. The polymeric linkages may be amphiphilic (i.e., contain both polar

hydrophobic water-insoluble groups). Janie monomers include ethylenically unsaturated

which contain acid groups or acid-salt groups of acid-salt precursors. Exemplary anionic substituents include carboxylates, oxalates, benzoates, phosphonates, maleates, malates, phthalates, succinates, sulphate, sulphonates, tartrates, fumarates, mixtures thereof and the like. Illustrative ethylenically unsaturated cationic monomers include nitrogen-containing cations such as primary, secondary and tertiary and quaternary ammonium compounds; sulphur containing cationssuch as sulphonium salts, halides, etc. phosphorous containing cations such as phosphonium salts; mixtures thereof and the like. Typical nitrogen containing cations include monomers represented by the formula:

wherein M' represents an ethylenically unsaturated organo group, R_a, R_b and R represents at least one hydrogen atom or organo group, and X is an anion (e.g. halide, acetate, CH₃S0₄, C₂H₅S0₄, etc). Exemplary R_a' R_b and R_c mono-organo groups include substituted and unsubstituted alkyl, monoheterocyclic (e.g. piperidine, morpholine, etc.), hydroxyalkyl, aralkyl, cycloalkyl groups as well as cyclic and heterocyclic groups divalently bonded to the nitrogen atom (e.g., R_a and R_b form a cyclic structure. The preferred nitrogen containing ethylenically unsaturated cationic monomers are the water-soluble, monomeric salts such as the lower alkyls of 1-5 carbon atoms (e.g. ethyl, methyl, propyl); polyoxyalkylene (e.g. polyoxyethylene and polyoxypropylene), mixtures thereof and the like: alkoxy (e.g. methoxy, ethoxy, propoxy, etc.); hydroxyalkyl and polyhydroxyalkyl (e.g. hydroxyethyl, hydroxypropyl, dihydroxypropyl, dihydroxybutyl); heterocyclic

morpholine); amines and amides bearing meno- organics; mixtures thereof and the lilce. The sulphur and phosphorus containing cationic monomers are similar to the aforementioned except either the phosphorous atom or sulphur atom replaces the nitrogen atom. The preferred phosphorus and sulphur cations are the phosphonium and sulphonium cationic salts. Water-soluble, "W'" ethylenically unsaturated monomers which contain an activating group adjacent to the ethylenic unsaturation (e.g. wherein M' contains a CH₂=CR_-E-radical with the activating group "E" and the "R" group being as defined above) are preferred.

[0021] Representative cationic monomers include the N- methylol acrylamide reactants mentioned above, dimethylaminoethyl methacrylate; t-butylaminoethyl methacrylate; 2-hydroxy-3-methacryloxypropyl trimethyl ammonium chloride; allyl-trimethyl-ammonium chloride; S-allyl-thiuronium bromide, S-methyl(allyl-thiuronium) methosulphate, diallyl- dibutyl-diammonium chloride, diallyl-dimethyl-ammonium methosulphate, dimethallyl-diethyl-ammonium phosphate, diallyl-dimethyl-ammonium nitrate, S-allyl-(allyl-thiuronium) bromide, N-methyl(4-vinylpyridinium) methosulphate, N-methyl(2-vinylpyridinium) methosulphate, allyl-dimethyl-beta-methacryloxyethyl-ammonium methosulphate, beta-methacryloxymethyl-trimethylammonium nitrate; beta-methacryloxyethyl-trimethylammonium p-toluene-sulphonate, delta-acryloxybutyl-tributylammonium methosulphate, methallyl-dimethyl-0-vinylphenyl-ammoniumchloride, octyldiethyl-m-vinylphenyl-ammonium phosphate, beta-hydroxyethyl-dipropyl-p-vinylphenyl-ammonium bromide, benzyl-dimethyl-2-Biethyl-5-vinyl-phenyl-ammonium phosphate; 3-hydroxypropyl-diethyl-vinyl-phenylammonium sulphate; octadecyl-dimethyl-vinylphenvl-ammonium p-toluene sulphonate, amyl-dimethyl-3-methyl-5-vinylphenyl-ammonium thiocyanate, vinyloxyethyl-triethyl-ammonium chloride, N-butyl-5-ethyl-2-vinylpyridinium iodide, N-propyl-2-vinyl-quinolinium methyl sulphate, N-butyl-5-ethyl-3-

N-propyl-2-vinyl-quinolinium methyl phate, allyl-gamma-myristamidopropyl-dimethyl- moniumchloride, methallyl-gamma-caprylamido-propylmethyl-ethyl―ammonium bromide; allyl-gamma-capryl- midopropyl-methylbenzyl-ammonium phosphate, ethallyl- gamma-myristamido-propyl-methyl-alpha-naphthymethyl- ammonium chloride, allyl-gamma-palmit-amidopropyl-ethylhexyl ammonium sulphate; methallyl―gamma-lauramido- propyl-diamyl-ammonium phosphate, propallyl-gamma- lauramidopropyl-diethyl-ammonium phosphate, methallyl- gamma-caprylamido-propyl-methyl-beta-hydroxyethyl- ammonium bromide, allyl-gamma-stearamido-propyl-methyl- dihydroxypropyl-ammonium phosphate, allyl-gamma- lauramidopropyl-benzyl- beta-hydroxyethyl-amonium chloride and methallyl-gamma-abietamidopropyl-hexyl- gamma'-hydroxy-propyl-ammonium phosphate, vinyl diethylmethyl sulphonium iodide, ethylenically unsaturated nitrogen containing cations having the formula CH₂=CHQN+(R₁R₂R₃)X^- such as disclosed in U.S. Patent No. 3,346,563 by Shildneck et al. with Q, R₁, R₂, and X- groups being defined as above, mixtures thereof and the like.

Maska et al. and 3,946,139 by Bleyle et al.), vinyl phosphonic acid and vinyl phosphonates; alpha, beta-ethylenically unsaturated carboxylic acids, their salts (e.g. acrylic acid, methacrylic acid, ethacrylic acid, prapacrylic acid, butacrylic acid, itaconic acid, monoalkyl esters of itaconic acid, crotonic acid and crotonates, fumaric acid and fumarates, etc.), mixtures thereof and the like.

[0022] The water-absorbent starches may be prepared by initially copolymerising the starch with ethylenically unsaturated comonomers which contain reactive sites (e.g. polar or unpolymerised ethylenic unsaturation) which are then derivatised to "W" moieties. For example, the i ethylenically unsaturated starches herein may be copolymerised with unsaturated precursors and converted to the anionic form such as by saponification to replace the alkyl ester group with a metal salt, and known techniques of derivatising organic compounds to acidic or the neutralised acid-salt form. Preferably the starting monomers contain the hydrophilic structure or one which can be directly converted to its "W" form by neutralisation. This will avoid the derivatisation step as well as the possibility of contaminating the copolymerisate with salts and minerals, and the need to wash and refine to remove such contaminants therefrom.

[0023] The polymeric linkages between copolymerised starch chains may be comprised of interpolymerised ionic monomeric units and monomeric units free from "W" substituents. The interpolymerised monomeric units free from "W" substituents may be selected from a broad range of ethylenically unsaturated monomers. Hydrophilic and/or hydrophobic comonomers may be used for this purpose. Illustrative interpolymerised comonomers include vinyl aromatics (e.g. styrene and styrene derivatives); the alkyl esters of alpha, beta-ethylenically unsaturated acids; the alpha, beta-

unsaturated nitriles, alpha, beta-

unsaturated amides; vinyl halides (e.g, methyl chloride and bromide) , olefins such as mono-and di-olefins; vinylidene halide (e.g. vinylidene chloride and bromide), vinyl esters (e.g. vinyl acetate and derivatives); diesters of alpha, beta-ethylenicaly unsaturated dicarboxylic acids (e.g. dimethyl or diethyl itaconate, dimethyl or diethyl maleate, diethyl or dimethyl fumarate, etc.); alkyl vinyl ethers such as methyl or ethyl vinyl ether, etc.; alkyl vinyl ketones (e.g. methyl vinyl ketone, etc.), mixtures thereof and the like.

[0024] The polymeric linkages are advantageously predominantly comprised of polar or water-soluble monomeric units. Illustrative polar or water-soluble comonomers free from "W" substituents which may be copolymerised with the "W" monomers and the starch include the hydroxyalkyl esters of alpha, beta-ethylenically unsaturated carboxylic acids such as hydroxyethyl, hydroxyethoxyethyl, hydroxysethyl, 2-3-dihydroxypropyl acrylates and methacrylates, di(2,3-dihydroxypropyl) fumarate, di(hydroxyethyl) itaconate, ethyl hydroxyethylmaleate, hydroxyethyl crotonate, nixtures thereof and the like; the lower alkyl esters of alpha, beta-ethylenically unsaturated carboxylic

(e.g. C₁ to C₂ alkyl ester of mono- and dicarboxylic acid such as methyl and ethyl ester of acrylic, methacrylic, itaconic, fumaric, crotonic, maleic,etc.); 7-(3-methylamino) propyl methacrylate: 1-butyl-aminoethyl methacrylate; di-methylaminoethyl methacrylate; beta-(5-butylamino)ethyl acrylate; 2-(1;1,3,5-tetra- methylbutylamino) ethyl methacrylate, etc.); alpha, beta-ethylenically unsaturated nitriles (e.g. acrylenitrile, methacrylonitrile, ethacrylonitrile, etc.); alpha, beta-ethylenically unsaturated amides

[0025] The water-absorbent starch copolymerisates are advantageously prepared under aqueous polymerisation conditions. Homogeneity of the reactants throughout the aqueous phase results in more uniform and reproducible water-absorbent properties. Ethylenically unsaturated starch and ethylenically unsaturated monomer systems which provide homogeneous dispersions essentially free from centrifugal residue and/or supernatant (e.g. heated to a temperature above the starch gelation point to gelatinise the starch and centrifuged for 10 minutes at 103g's) as evidenced by less than 10 % by weight centrifuged residue (preferably less than 5 %) are particularly well suited systems for preparing the water-absorbent starch copolymerisates.

[0026] In thermal fabrication processes (e.g. moulding, calendering, extrusion, etc), a relatively high monomer and starch to water weight ratio (5:1 to 9:1) is typically used. At the reduced water levels and elevated monomer levels, incompatibility of the ethylenic unsaturated monomer starch system can arise. Elevated fabricating temperatures and pressures may be used to improve upon the compatibility of this system. Likewise water-miscible solvents in which the ethylenic unsaturated monomers are soluble (e.g. such as glycerol) or emulsifying agents may be effectively utilised to enhance the water- dispersibility of the monomer-starch system into the aqueous phase. In extrusion cperations, a sufficient amount of water (with or without conventional starch plasticisers) to convert the polymerisable mass to a molten plasticised mass at elevated temperatures (e.g. 80° to 250°C) and pressures is used. The molten mass is then extruded through a die orifice into an atmosphere of reduced pressure and temperature maintained below the boiling point (B.P.) to produce void-free extrudates and above its B.P. to produce puffed extrudates.

[0027] In coating applications, it is particularly advantageous to utilise a gelatinised or pregelatinised starch. Asucous coating compositions containing the low viscosity ethylenically unsaturated starch hydrolysates are particularly useful when it is desired to coat substrates at dry binder weight levels of at least 40 %. Substrates may be uniformly wetted and coated at solids levels ranging from about 50 % to 75 % by weight with stability against syneresis, separation and viscosity changes. Such coatings dry easily at nominal evaporation costs. Depolymerisation of the starch to the appropriate short chain length (e.g. D.E. 0.2-100) for coating applications say be accomplished by conventional saccharification and/or thinning techniques (e.g. acid or enzymatic). The starch chains may be depolymerised to the appropriate chain length prior or after the ethylenically unsaturated derivatives are prepared. Starch chains having a degree of polymerisation comparable to that achieved by alpha-amylase hydrolysis of starch to a D.E. ranging from about 0.1 to 32 advantageously from about 0.25 to about 15 and most preferably less than 10) may be effectively used to coat substrates. The reduced starch chain length will not adversely affect starch-coating permanence provided the ethylenically unsaturated D.S. is sufficiently high enough to provide chains which contain multifunctional unsaturation sites.

[0028] In most coating applications, the water content is typically adjusted to a fluidity most suitable to coat the substrate. The starch coating composition viscosity may vary considerably and depends to a large extent upon the type of coating operation employed (e.g. from about 1 to about 40,000 cps or higher for extrusion coating). The proportions of water, monomer and ethylenic usaturated starch weight ratios may likewise vary considerably (e.g., about 5 to about 10,000 parts by weight, i.e. pbw, water and about 1 to about 5,000 pbw monomer for each 100 pbw ethylenically unsaturated starch). In coating operations conducted under ambient temperatures, it is advantageous to utilise a homogeneous starch coating composition of viscosity greater than about 10 cps but less than 5,000 cps (most typically between about 20 cps to 1,000 cps) and containing from about 25 to about 800 pbw water and about 10 to about 2,000 pbw ethylenically unsaturated monomer for each 100 pbw ethylenically unsaturated starch. Water-miscible organo solvents or surfactants are desirably incorporated into the coating composition for purposes of achieving homogeneity and a uniform monomer dispersion if the starch coating formulation contains a low amount of water and a high monomer concentration. Starch coating compositions which are adapted for use in high-speed coating operations are typically formulated at a viscosity ranging from about 100 cps to about 300 cps (with or without fugitive organo solvents or surfactants at about 30 to about 500 pbw water and about 25 to about 1,000 pbw (preferably between about 50 to about 500 pbw) ethylenically unsaturated monomer for each 100 pbw ethylenically unsaturated starch. In formulations for high-speed coating operations, starch coating homogeneity is more easily achieved by using less than 3 weight parts ethylenically unsaturated monomer for each 2 weight parts of water and preferably at a weight ratio of less than one part monomer for each water part.

[0029] The copolymerisates are copolymerised by conventional polymerisation initiating means. The unpolymerised starch and monomers may be conveniently prefabricated into the desired configuration and then copolymerised in situ via such conventional polymerisation initiating systems. The starch compositions will undergo copolymerisation upon exposure to conventional irradiation processes which generate in situ

unpolymerised starch composition which will then latently copolymerise upon exposure to appropriate initiating conditions (e.g. photochemical, ultra-violet; heating or microwave techniques, etc.).

[0030] Conventional free-radical polymerisation initiators at levels sufficient to copolymerise the ethylenic unsaturated starch and monomer (e.g. about 0.2 % to about 20 % on a starch-monomer weight basis) which may be incorporated into the starch composition include the organic and inorganic peroxides (e.g. hydrogen peroxide, benzoyl peroxide, tertiary butyl hydroperoxide, diisopropyl benzene hydroperoxide, cumene hydroperoxide, caproyl peroxide, methyl ethyl ketone peroxide, etc.), oxidation- reduction initiator systems (ammonium, potassium or sodium persulphates or hydrogen peroxide with reducing agents sach

as sodium bisulphites, sulphites, sulphoxylates, thiosulphates, hydrazine, etc.); azo initiators (e.g. rtiary aliphatic azo compounds which undergo homolytic diss ation) such as azo di-isobutyronitrile, phenylazotriphenyl- methane, 1,1'-azodicyclohexane-carbonitrile, 1,1-dimethi- azoethane; diazoamino compounds (e.g. 3,'3-dimethyl-1-phenyl-triazene and aryldiazo thioethers) and other free-radical generating catalysts such as certain aromatic ketones (e.g. benzoin methyl ether, benzophenone and its derivatives), chlorinated aromatics as well as other free-radical type of polymerisation initiators. Free-radical initiator systems which require externally applied energy (e.g. thermally, photochemical, etc.) for free-radical generation may be used to provide a latently copolymerised system. ; Advantageously the free-radical polymerisation initiators are uniformly dispeised throughes the aqueous phase of the starch composition at levels ranging from about 0.3 % to about 10 % (based on polymerisable starch and monomer dry weight).

[0031] Polymerisation initiation via U.V. and white light sources ('e.g. 200-430 nanometer (nm) range, such as by carbon arc lamps, Zenon lamps, high pressure mercury lamps) is particularly useful in high-speed coating operations. If desired, conventional photosensitisers (e.g. triethanol amine-soluble benzophenones, eosin- sulphonates, methylene blue-sulphinate, combinations thereof, etc.) by active energy transfer may be incorporated into the starch composition to facilitate, the copolymerisation initiation reaction. The ultra-violet polymerisation initiating processes are generally suitable for coatings or films of a thickness of less than about 20 mils (preferably less than about 10 mils). Thicker starch polymerisate articles or films normally require higher penetrating irradiation devices (e.g. X-ray, electron-beam, gamma generation, etc.) or thermal induction. The ultra-violet copolymerisation process is particularly effective for high solids starch coating. applications (e.g. about 55 % to about 73 % dry solids) because it simultaneously dries and copolymerises the starch coating in a single step. Water-dispersible, non-fugitive free-radical initiating systems (e.g. catalysts which evaporate or do not leave catalytic residue in the copolymerisate) such as hydrogen peroxide are preferred.

[0032] The water-absorbent starch copolymerisates have a wide and divergent field of use. A major advantage of the water-absorbent starches-of this invention resides in the ability to apply the unpolymerised product to a substrate or prefabricate it into the desired shape or configuration and then convert it to a water-absorbent, starch copolymerisate. The unpolymerised product can be applied to divergent substrates ranging from natural and man-made products and thereafter polymerised in situ to form an integrated product of unitary construction. This advantage is particularly useful for applications wherein it is desirable to permanently affix or impregnate a natural or synthetic substrate (e.g. films, webbings, fibres, filaments, etc.) with the water-absorbent starch. Illustrative applications for the water-absorbent starches include hygenic pads, bandages, surgical and catamenial tampons, sanitary napkins, diapers, antiperspirant and deodorant pads, sponges, surgical pads, sorptive dental rolls, disinfectants, decorative seedling films, etc. If desired, the water-absorbent starch copolymerisates may be admixed with natural and man-made products for such divergent uses as cosmetics, water scavengers, paint removers, solid humectants, pesticides, improving the water-holding capacity of soils, catalysts or chemical carrier, binders, etc.

[0033] The following examples are merely illustrative and should not be construed as limiting the scope of the invention.

EXAMPLE I

[0034] An aqueous acrylamidomethyl starch hydrolysate (D.S. 0.009) was prepared employing the following proportions of reagents.

[0035] The ingredients were mixed and filtered on a Buchner funnel. The starch cake.was sucked free of excess aqueou: phase and the unwashed cake (with 63 percent retention of non-starch reagents) was air-dried to a ten percent drying loss. The dried reaction premix had the following ratios of reagents (pbw) - 250 starch; 7.95 N-methylolacrylamide, 0.025 methyl hydroquinone; 29 water. The powdered reaction premix was layered onto a stainless steel tray and heated for 2 hours in a forced air oven at 75.5°C. After resuspending in distilled water, filtering and washing free of unreacted reagent impurities, the dried product contained 0.10 percent nitrogen (dry basis), which when corrected for the nitrogen in the STA-TAPE 100 starch (0.022 percent) is equivalent to a D.S. of 0.009. Further information on the preparation of the acrylamidomethyl starches

grams) was homogeneously dispersed into 8.43 grams water (15 minutes at its boiling point) and cooled to ambient temperature in a 50 ml flask. Acrylic acid (0.43 grams) and acrylamide (0.24 grains) were homogeneously dispersed into the acrylamido starch solution followed by the addition of 0.0169 grams (d.s.b.) ammonium persulphate (2.28 % aqueous solution) and 0.0076 gram (d.s.b.) of sodium bisulphide (1.04 % aqueous solution). Then 0.002 gram (d.s.b.) of ferrous sulphate (0.28 wt.% FeSO₄.7H₂O aqueous solution) was added which caused an exothermic copolymerisation of the ethylenic unsaturates. Within 1 minute the entire reaction medium had gelled (12.1/g) into a copolymerisate which could be agitated with a magnetic stirrer. To convert the acrylic moieties to the anionic salt form, 0.42 gram of solid potassium hydroxide was added. The resultant viscous dispersion (12.53 g total) was then stirred for 15 minutes. The sample contained 15.24 wt.% solids. The gel was then diluted to 5 wt.% dry solids with 25.64 g distille : water and allowed to stand for 24 hours. Thereafte. the dispersion (36.84 grams) was further diluted with 55.26 g distilled water to provide a 2 % gel solids dispersion. The viscosity of the dispersion respectively after standing for six and twenty-three hours (No. 4 spindle at 20 rpm) was 500 cps and 3500 cps, The solids was again diluted with 90.52 g of distilled wacer (1 % dry solids dispersion) which after 29 hours standing had a 700 cps viscosity (No. 4

at 20 rpm) and after 58 hours a viscosity of 710 cps. The 1 % gel dispersion was ambiently air-dried (evaporating dish for 11 days). A 0.1526 g sample of the resultant gummy resin was transferred and hydrate with 11,85 grams of distilled water in a 15 ml centrifuge tube. The sample swelled to

volume. The hydrated sample was centrifuged for 15 minutes at 103g's. The supernatant liquid was decanted into a tared aluminum pan. 11.38 grams of the swollen gel was transferred to a 50 ml centrifuge tube and diluted with 11.38 grams of water and allowed to swell for 17 hours followed by centrifugation for 15 minutes at 103g's. The supernatent (pH 6.6) along with the aforementioned supernatant was analysed for water-soluble starch (0.0423 grams or 27.7 % by weight.via evaporation).

[0036] The copolymerisate weight swelling ratio (WSR) was determined by the equation

wherein I, 0 and S respectively represent the weight of swollen insolubles, 9.63 grams; original sample 0.1526 grams and solubles, 0.0423 grams (i.e.,

EXAMPLE II

[0037] A cationic, water-absorbent starch copolymerisate was prepared by copolymerising (in 34.6 pbw distilled water) 8.5 pbw (0.008 moles) acrylamidomethyl starch 0 (d.s. 0.008), 30.9 pbw CH₂=C(CH₃)-C-OCH(OH)CH₂N(CH₃)₃Cl^- (0.0199 moles) and 11.1 pbw acrylamide (0.0241 moles). The copolymerisation reaction was exothermically initiated with 0.1 pbw ammonium persulphate (0.13(NH₄ )₂S₂O₈ + 5 ^pbw water), 0.07 pbw sodium bisulphite (0.07 pbw NaHSO₃ + 5 pbw water) and 0.01 pbw FeS0₄.7H₂0 (0.01 pbw FeS0₄.7H₂0 + 4.7 pbw water). Within 90 seconds the copolymerisation reaction was completed to yield a water-absorbent, hydrated copolymerisate gel. This cationic gel was analysed in accordance with the test procedure of Example I at 25°C. The copolymerisate contained 73 % (by weight) insoluble copolymerisate solids and 27 % (by weight) solubl.es and had a 152 WSR. The insoluble copolymerisate absorbed 152 times its dry weight of water at a pH 4.0 and 25°C.

[0038] (11.25 pbw) and 0.01673 moles

CH₃OSO₃^- (30.82 pbw) with the exothermic initiating system of Example II. The resultant copolymerisate gel (copolymerisation completed within 150 seconds after initiation) was admixed with 2000 ml. water and allowed to swell for 8 days at 25°C. The decanted supernatent liquid portion thereof contained 20.18 % water-solubles. The insoluble copolymerisate (79.82 % of the total copolymerisable reactants) absorbed 86 times it weight of water at pH 3.5 and 25°C.

EXAMPLE IV

[0039] 

EXAMPLE V

[0040] 

starch (0.01 D.S.)², 47 pbw distilled water, 12 pbw acrylic acid, 12 pbw acrylamide, 9 pbw potassium hydroxide and 10 pbw aqueous hydrogen peroxide (30 %).

[0041] Five grams of the copolymerisable composition was placed in an aluminium weighing pan (2" or approx. 5 cm I.D.) and irradiated an inch away from a 275 watt sun lamp for 1 minute to give a firm gel. Another portion of copolymerisable starch composition was applied with a No. 40 wire wound rod to a 4" x 12" (10 cm x 30.5 cm) glass plate and irradiated 6 passes at 20 ft./min. (6.2 m/min) at 1.5" (3.8 cm) under a Hanovia 679A lamp. The copolymerisable starch composition gelled on the first pass (1/6 sec.) and converted to a dry film after the sixth pass through the irradiator (i.e. one second). The WSR for the resultant starch copolymerisates were 150. A 0.056 D.S. acrylamidomethyl starch was used instead of the 0.01 D.S. acrylamidomethyl starch to provide a copolymerisate with a WSR of 30.

[0042] In another test, a 0.014 D.S. acrylamidomethyl starch' was substituted for the 0.01 D.S. acrylamidomethyl starch reactant and applied to the glass plates with a No. 40 wire wound rod (pH 6.2; 25-36 cps viscosity, No. 1 spindle, at 20 rpm at 25°C). After 4 passes through the irradiator, a dry, water-absorbent film coating, 83.51 % insoluble copolymerisate dry solids and a WSR of 120 was obtained. This test was repeated by immersing three cotton cloth pieces (18" x 6" or approx. 45.7 x 15.2 cm) in the 0.014 D.S. copolymerisable starch coating compositions, passing the coated cotton through the rolls of a Birch Brothers Padder, placing the coated cotton pieces on glass plates and then irradiating the three samples for 2 - The 0.01 D.S. acrylamidomethyl starch hydrolysate contained an average of approximately two acrylamidomethyl groups for each starch molecule;

than 100 for the eloth which was exposed to 6 passes.

[0043] Although the aforementioned Examples primarily illustrate the use of relatively high-molecular-weight starch chains, the invention applies to a broad range of ethylenically unsaturated glucose-containing monomers such as those ranging from a completely hydrolysed starch (e.g. dextrose) to an unhydrolysed starch. The glucose-containing monomers which contain multifunctional ethylenically unsaturated groups provide the necessary structure for the porous lyophilic network. The most appropriate D.S. level for a glucose-containing monomer will depend upon the number of glucose units present in its starch chain. To achieve multifunctional copolymerisable groups for a monosaccharide, disaccharide, trisaccharide or tetrasaccharide monomer would respectively require a D.S. of 2.0, 1.0, 0.66 and 0.5 with the oligosaccharides (e.g. D.P. 4⁺) and higher starch chains requiring a correspondingly lesser to achieve multifunctionality. In contrast, the higher molecular weight starches (e.g. unhydrolysed starches) will typically have multifunctional copolymerisable groups at a D.S. of 0.0002 or less.

[0044] Since the starch copolymerisates as described herein possess a porous structure, their lyophilic properties can be altered via the composition and character of unsaturated starches, monomers and lyophilic groups which are used in their preparation. By replacing the polar, water-attractant groups wich

lyophilic and amphophilic starch copolymerisates may be obtained by starch copolymerisates which contain both polar water-soluble and hydrophobic, water-insoluble substituents.

Claims

1. A water-absorbent starch copolymerisate characterised in that it comprises the copolymerisate product of ethylenically unsaturated starch molecules and ethylenically unsaturated monomers with the ethylenically unsaturated monomers forming a connective polymeric linkage between the copolymerised starch molecules to provide a non-linear lattice of a plurality of starch chains linked together by polymeric linkages represented by the formula

wherein Starch represents a starch chain of D-glucoce units, Z represents an organo group which links the

group to the carbon atom of the starch chain by a sulphur atom or an oxygen atom, R is hydrogen or a monovalent organic radical, M represents a

copolymerised ethylenically unsaturated monomers with"p" representing the number of copolymerised monomeric units in the linkage, (W) is a water-attractant group linked to the polymeric linkage and "n" represents one number of (w) moieties contained within the polymeric linkage of the copolymerised monomers with the number of (W) moieties being sufficient to impart water-absorbency properties to the copolymerisate.

2. A water-absorbent starch copolymerisate as claimed in claim 1 characterised in that the Z organe group consists essentially of a

group and R' is at least one hydrogen or mono-organo group joined directly to the nitrogen atom by a monovalers bond.

3. A water-absorbent starch copolymerisate as claimed in claim 1 or 2 characterised in that the ethylenically unsaturated starch molecules consist essentially of starch molecules and appendant ethylenic unsaturated groups of a molecular weight ranging from greater than 50 to less than 300 and the degree of substitution of the ethylenically unsaturated appendant groups ranges from about 0.002 to about 0.1, and "W" is at least one anion, cation, nonion or zwitterion.

4. A water-absorbent starch copolymerisate as claimed in any of claims 1 to 3 characterised in that the weight of ethylenically unsaturated monomer in the copolymerisate ranges from about 10 to about 1000 parts by weight for each 100 parts by weight of copolymerised ethylenically unsaturated starch and from about 25 % to about 100 % of the copolymerised ethylenically unsaturated monomer contains the "W" substituent.

5. A water-absorbent starch copolymerisate as claimed in any of claims 1 to 4 characterised in that the copolymerised ethylenically unsaturated starch consists essentially of amylopectin hydrolysate with a degree of substitution of the starch ethylenic unsaturated groups ranging from about 0.005 to about 0.05.

6. A water-absorbent starch copolymerisate as claimed in any of claims 1 to 5 characterised in that the copolymerised ethylenically unsaturated starch consists essentially of an acrylamide starch having a D.S. ranging from about 0.005 to about 0.05 and a D.E. ranging from about 0.25 to about 15, and in that the copolymerisate contains 100 parts by weight copolymerised ethylenically unsaturated starch hydrolysate, from about 100 to about 750 parts copolymerised ethylenically unsaturated monomer which contain "W" groups and from 0 to about 200 parts by weight of copolymerised ethylenically unsaturated monomers devoid of "W" groups.

7. A method for preparing a water-absorbent starch copolymerisate, characterised in that the copolymerisase comprises a plurality of starch chains linked together by polymeric linkages represented by the formula:

in which Starch represents a starch chain of D-glucose units, Z represents an organo group which links the

group to the carbon atom of the starch chain by a sulphur atom or an oxygen atom, R is hydrogen or a monovalent organic radical, M represents a plurality of copolymerised ethylenically unsaturated monomers with contain a sufficient number of (W) groups to impart water-absorbency τo the copolymerised product, "p" represents the number of copolymerised ethylenically unsaturated monomers linking together the starch

and further characterised in that the method

copolymerising:

(a) starch chains containing appendant, terminal ethylenic unsaturated groups represented by the formula:

wherein Starch, Z and R are as defined above. and "a" represents the degree of substitution of the terminal unsaturated groups on the starch chain, and

(b) ethylenically unsaturated monomers represented by the formula:

wherein M' represents an ethylen

monomer, "(W)" represents at least one water-attractant group or a precursor of a water-attractant group, and n' is an integer

with the proviso that when the copolymerised M'-(W')_n' monomer consists essentially of a precursor of the water-attractant group, a sufficient number of the precursor groups are derivatised to a water-attractant group to impart water-absorbing properties to the starch copolymerisate.

8. A method as claimed in claim 7 characterised in that (W') is at least one water-attractant group or a precursor of a water-attractant group which is an anion, cation, nonion, or zwitterion, and that Z represents an organo group which links the -CRH- group to the starch chain by an oxy moiety.

9. A method as claimed in claim 7 or 8 characterised in that Z comprises an organo group represented by the formula:

and R' is at least one hydrogen atom or a mono-organo group joined directly to the nitrogen atom by a monovalent bond.

10. A method as claimed in any of claims 7 to 9 characterised in that the appendant ethylenic unsaturated groups of the starch chains consist essentially of groups having a molecular weight ranging from about 75 to about 150 and "a" represents a degree of substitution ranging from about 0.002 to about 0.1.

ll. A method as claimed in any of claims 7 to 10 characterised in that from about 25 % to 100 % by weight of ethylenically unsaturated monomers contain the "W" group and that the copolymerised weight of ethylenically unsaturated monomer ranges from about 10 to about 1000 parts by weight ethylenically unsaturated monomer for each 100 parts by weight ethylenically unsaturated starch.

12. A method as claimed in any of claims 7 to 11 characterised in that the ethylenically unsaturated starch consists essentially of an acrylamido starch hydrolysate having a D.S. ranging from about 0.005 to about 0.10 and a D.E. ranging from about 0.25 to about 15, and that 100 parts by weight of the ethylenically unsaturated starch hydrolysate is copolymerised with from about 100 to about 750 parts copolymerised ethylenically unsaturated monomers free from "W" groups and from 0 to about 200 parts by weight ethylenically unsaturated monomers free from "W'" groups.

13. A method as claimed in any of claims 7 to 12 characterised in that the starch chains consist essentially of an amylopectin hydrolysate and "W" represents at least one anion or cation.

14. A method as claimed in any of claims 7 to 13 characterised in that it includes the additional steps of applying the unpolymerised ethylenically unsaturated starch and ethylenically unsaturated monomers to a substrate and thereafter copolymerising in situ the applied starch and monomers.