GB2063896A - Insolubilisers for binders for paper coating compositions - Google Patents

Abstract

Reaction products of substantially stoichiometric amounts of glyoxal and cyclic urea, and the partially or wholly alkylated derivatives thereof, are useful as insolubilisers in paper coating compositions comprising binder and pigment.

Description

SPECIFICATION Insolubilisers for binders for paper coating compositions This invention relates to paper coating compositions and to products for insolubilising the binders in such compositions.

A paper coating composition is generally a fluid suspension of pigment, such as clay with or without titanium dioxide, calcium carbonate, or the like, in an aqueous medium which includes a binder, such as starch, protein, or latex, to adhere the pigment to the paper.

The hydrophilic nature of the binder requires the presence of an insolubilising material which crosslinks the binder, making it hydrophobic and thus improving the characteristics of the surface of the coated paper.

The most widely-used crosslinking materials are glyoxal and formaldehyde-donor agents such as melamine formaldehyde, ureamelamine, formaldehyde, and partially or wholly methylated derivatives thereof.

Glyoxal is a highly reactive monomer which cures quickly and has excellent insolubilising properties. As a result of this rapid crosslinking of glyoxal and binder, however, the viscosity of the coating composition increases so rapidly and is so great that the composition cannot be used. Frequently glyoxal-insolubilised coatings gel completely, particularly in high solids formulations; gelling can occur also in moderate or low solids formulations if they are not used promptly. Thus in situations where it is required that the viscosity remain stable for many hours, for example when high solids coatings are to be applied by blade coating techniques, a glyoxal system is unsuitable.

Melamine formaldehyde resins do not build viscosity in the coating compositions, but they have the disadvantage of having an unpleasant odor and of releasing free formaldehyde. Curing with such resins involves the crosslinking of the binder molcule with the methylol or methylated methylol group of the melamine resin, usually in an acid or neutral coating, and full insolubilisation of the binder takes place slowly over a period of several days. Free formaldehyde can be released either directly from the coating mixture or when the coating is cured on the drying machine.The presence of even less than one percent of free formaldehyde, based on the total weight of the product, is undesirable, not only because of its objectionable odor, but because it is an allergen and an irritant, causing severe reactions in the operators who manufacture the coatings and who treat and handle the coated paper.

The use of the reaction product of urea and glyoxal as an insolubiliser is known (U.S. Patent No. 3,869,296).

It has been found that the products of the reaction of glyoxal and cyclic ureas are excellent crosslinking resins for binders for paper coating compositions. They do not build viscosity as does glyoxal; they do not contain or evolve free formaldehyde; and, in smaller amounts, they have insoiubilising effects similar two those of the previously known agents.

A product according to the invention suitable for use as an insolubiliser for binders in paper coating compositions therefore is a reaction product of substantially stoichiometric amounts of glyoxal and at least one cyclic urea, or a partially or wholly alkylated derivative thereof.

Preferred novel compounds are the partially or wholly alkylated derivatives.

A paper coating composition according to the invention comprises pigment, binder and, as an insolubiliser for the binder, a reaction product as defined above.

The reaction products are prepared by condensing in approximately stoichiometric quantities one or more cyclic ureas and glyoxal. The cyclic ureas which may be used have the following general formulas:

wherein R1, R2, R3, R4, R5 and R6 may be the same or different and each may be H, OH, COOH, R, OR, or COOR wherein R is an alkyl or a substituted alkyl group having 1 to 4 carbon atoms, and X may be C, O, or N; and when X is 0, then R3 and R4 are both absent and when Xis N then Rg or R4 iS absent.

Typical examples of such compounds include for instance ethylene urea, propylene urea, uron, tetrahydro-5-(2-hydroxyethyl)-1 ,3,5-tirazin-2-one, 4,5-dihydroxy-2-imidazolidinone, and mixtures of these.

The cyclic urea and the glyoxal are generally reacted in stoichiometric amounts, although a slight excess of either of the reactants may be employed. The general range of glyoxal:cyclic urea is about 0.5-2:1 and preferably is about 0.8-1.2:1. The reaction may be carried out within the temperature range of room temperature up to reflux, but preferably is run at about 50 to 600C. for about two hours. The pH may range from about 2 to 7.0, and preferably it is within the range of about 5.0 to 7.0. The product is a water-soluble oligomer.

It is within the scope of this invention to modify these glyoxal/cyclic urea condensates by partially or wholly alkylating them, e.g. with an alcohol such as methanol, ethanol, n-propanol, isopropanol, a butanol, and mixtures thereof.

The treating agent of this invention has reactive sites capable of reacting with the cellulosic hydroxyl group, thus binding the starch or protein molecules in the coating composition. Because of its polymeric nature, the product is capable of reacting with several binder molecules, leading to greater insolubilising efficiency and so requiring less of the product of this invention than of glyoxal or of a melamine formaldehyde resin to accomplish the same degree of insolubilisation. The polymer is not excessive reactive, as is glyoxal, so no viscosity build-up takes place in the coating composition. Since there is no formaldehyde in the system, the problems found with free formaldehyde are avoided.

The binders used in the paper coating compositions of this invention include, but are not limited to, unmodified starch; oxidised starch; enzyme-converted starch; starches having functional groups such as hydroxyl, carbonyl, amido, and amino groups; proteins, such as casein; and latexes, such as styrenebutadiene resin; and mixtures.

The pigments may for instance be clay with or without titanium dioxide and/or calcium carbonate, and mixtures thereof.

In addition to the binder, the pigment material, and the insolubiliser described above, paper coating compositions may also include conventional materials such as lubricants, defoamers, preservatives and coloured pigments, in conventional amounts. The compositions are normally aqueous.

In the paper coating compositions described herein, the amount of binder is based upon the amount of pigment; the ratio varies with the amount of bonding desired and with the adhesive characteristics of the particular binder employed. In general the amount of binder is about 10 to 25, and preferably about 12 to 18, percent, based on the weight of the pigment.

The amount of insolubiliser varies with the amount and properties of the binder and the amount of insolubilisation desired; in general it is about 2 to 10, and preferably about 3 to 7, percent, based on the weight of the binder.

The total solids content of the composition generally is within the range of about 40 to 70 percent, depending upon the method of application and the product requirements.

The composition of this invention can be applied to paper or paper-like substrates by any known and convenient means.

In order that the present invention may be more fully understood, the following examples are given by way of illustration.

Example 1 176 parts (2.0 moles) of ethylene urea was added to 290 parts (2.0 moles) of a 40% aqueous solution of glyoxal, and the pH was adjusted to 6.5 with sodium bicarbonate. The mixture was heated to 55+5"C. and the temperature was held for two hours, maintaining the pH between 6.0 and 7.0. A steady increase in viscosity was observed. At the end of two hours, 187 parts of water was added to adjust the solids to 45%, and the mixture was cooled to 30 C.

The produce was a clear viscous material, water-white to light straw coloured, with negligible odor. The reaction was essentially complete, as determined by IR and NMR analyses.

Example 2 290 parts (2 moles) of a 40% aqueous solution of glyoxal was adjusted to pH 6.5 with sodium bicarbonate.

176 parts (2 moles) of ethylene urea was added and the temperature raised to 55+5 C. The mixture was stirred at this temperature for two hours, maintaining the pH between 6.0 and 7.0. After two hours 200 parts (6.25 moles) of methanol was added and the pH adjusted to about 3.0 with concentrated sulphuric acid. The reaction was held at reflux for three hours to effect methylation, the resin solution cooled to 300C., and the pH adjusted to about 7.0 with a 25% solution of caustic soda.

The product was a clear viscous liquid, pale yellow, with negligible odor. The reaction was essentially complete, as determined by IR and NMR analyses. IR analysis indicated that methylation had occurred.

Example 3 360 parts (2.5 moles) of a 40% aqueous solution of glyoxal was added to 905 parts (2.5 moles) of a 44% methanol solution of dimethyl methoxy propylene urea. The mixture was heated to 55t50C. for two hours, the pH being maintained between 6.0 and 7.0. After cooling to 300there was obtained a 45% solids, slightly viscous water-white solution with no odor of formaldehyde. The reaction was essentially complete, as determined by IR and NMR analyses.

Example 4 The procedure of Example 1 was repeated except that the glyoxal was reacted with each of the following instead of ethylene urea: propylene urea, uron, tetrahydro-5-(2-hydroxyethyl)-1 ,3,5; tirazin-2-one, and 4,5-dihydroxy-2-imidazolidinone. The results were comparable.

Example 5 (A) A clay slip was prepared as follows: To 600 parts of water in a 2-litre steel beaker were added 2.5 parts of tetrasodium polyphosphate and 2.0 parts of sodium polyacrylate with agitation which was continued until the ingredients were dissolved. With slow agitation and using a high shear mixer, 1400 parts of No. 1 clay was sifted into the mixture, and agitation was increased and continued for about 10 minutes until a smooth slurry was obtained. minutes until a smooth slurry was obtained.

(B) 168 parts of starch Penford gum 280, (Penick & Ford's hydroxyethylated starch) was dispersed in 504 parts of water, and the dispersion was heated to boiling. The solution was then cooled for about 15 minutes, added to the clay slurry of part (A), and calcium stearate added as a lubricant.

The resultant slurry was then used in aliquots with the various insolubilisers. The starch binder was used as a 25% solution, expressed as solids based on clay solids. The latex was used as a 50% emulsion, expressed as solids based on clay solids. The calcium stearate lubricant was used as a 50% emulsion.

The coating compositions were applied to No. 46/ream paper with an No. 8 Meyer applicator, using a draw-down technique.

viscosity was measured with a Brookfield Viscometer after the coating had remained at room temperature for a specified time.

Insolubilisation was measured with an Adams Wet-Rub Tester (Montague Machine Co., Turners Falls, Massachusetts). The results are reported as the weight in grams of coating removed from the substrate; the less the removal of the coating solids, the better the degree of insolubilisation.

The results are tabulated below.

TABLE 1 (1) (2) (3) (4) (5) (6) No.1 Clay 100 100 100 100 100 100 Starch 12 12 12 12 12 12 Latex 6 6 6 6 6 6 Calcium stearate 1 1 1 1 1 1 Insolubilizer A 1.9 1.3 - - - B - - 1.9 2.4 - - C - - - - 1.0 Viscosity (cps after 6hours) 7200 7100 6300 6100 14100 7500 Adams wet-rub 1 day .0067 .0072 .0068 .0064 .0065 .0101 3 days* .0076 .0082 .0081 .0079 .0076 .0120 * Adams wet-rub 3-day figures should be compared to each other, not to the 1-day figures; inconsistencies in sheet drying result in apparent reversal of the 1- and 3-day values.

Starch is Penford Gum 280 Latex is Dow 620, a styrene-butadiene resin latex solution.

Insolubiliser A is the product of Example 1 (45% solids) B is melamine formaldehyde (76% solids) C is glyoxal (40% solids).

From these data it can be seen that glyoxal (5) is the most effective insolubiliser, but the viscosity build-up after 6 hours would make it unsuitable for use in a paper coating composition. The product of this invention (1) and (2) is a more effective insolubiliser than melamine formaldehyde (3) and (4) on an as-is basis and is approximately twice as effective on a solids basis. After 6 hours the product of this invention produced no significant viscosity build-up. Column (6) represents a coating containing no insolubiliser.

Example 6 The procedure of Example 5 was repeated with products of this invention other than that of Example 1.

The results are tabulated below.

TABLE II (1) (2) (3) (4) (5) (6) (7) No. 1 Clay 88 88 88 88 88 88 88 TiO2 2 2 2 2 2 2 2 CaCO3 10 10 10 10 10 10 10 Starch 20 20 20 20 20 20 20 Calcium stearate 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Insolubilizer D 1.2 - - - - - E - 1.2 - - - - F - - 1.2 - - G - - - 1.2 - - H - - - - 2.4 I - - - - - 2.0 - Viscosity (cps after) 15900 17100 22500 26750 13900 77000 16300 6 hours) Adams Wet-rub 1 day .0065 .0081 .0046 .0052 .0131 .0031 .0190 3 days .0099 .0100 .0088 .0088 .0117 .0076 .0190 Starch is Penford Gum 280, Insolubiliser D is the product of the reaction of 176 parts of ethylene urea and 290 parts of glyoxal in the presence of 0.42 part of sodium bicarbonate and diluted with 183 parts-of water; the product was a clear solution having a viscosity of 42.5 cps (45% solids) E is the same as D except that the amount of glyoxal was 320 parts; the product was a clear solution having a viscosity of 35 cps (45% solids) F is the same as D except that the amount of glyoxal was 435 parts; the product was a clear solution having a viscosity of 25 cps (45% solids).

G is the same as D except that the amount of glyoxal was 580 parts; the product was a clear solution having a viscosity of 17.5 cps (45% solids) H is melamine formaldehyde (76% solids) I is glyoxal (40% solids).

From these data it can be seen that glyoxal (6) is the most effective insolubiliser, but its viscosity after 6 hours is unsatisfactory. The products of this invention (1), (2), (3) and (4) are more effective insolubilisers than melamine formaldehyde (5) even at reduced use levels. After 6 hours the products of this invention produced no significant viscosity build-up. Column (7) represents a formulation that does not contain an insolubiliser.

Example 7 The procedure of Example 5 was repeated with the products of Examples 2,3 and 4 instead of that of Example 1. The results were comparable.

The insolubilisers used in the invention do not contain or evolve free formaldehyde, as do the conventional malamine formaldehyde and urea-melamine formaldehyde crosslinking agents. Smaller amounts of the reactants of this invention produce insolubilising effects comparable to those of conventional materials. They satisfactorily insolubilise the pigment binders, but do not build viscosity as does glyoxal.

Claims (14)

1. A product suitable for use as an insolubiliser for binders in paper coating compositions and which is a reaction product of substantially stoichiometric amounts of glyoxal and at least one cyclic urea, or a partially or wholly alkylated derivative thereof.

2. A product according to claim 1 in which the cyclic urea has the formula:

wherein R1, R2, R3, R4, R5 and R5 may be the same or different and each is H, OH, COOH, R, OR or COOR wherein R is an alkyl or a substituted alkyl group having 1 to 5 carbon atoms, and Xis C, O, or N and when X is 0 then R3 and R4 are both absent and when Xis N then R3 or R4 is absent.

3. A product according to claim 1 or claim 2 in which the ratio of glyoxal :cyclic urea is 0.8-1.2:1.

4. A product according to any preceding claim which is a partially or wholly alkylated derivative of the reaction product.

5. A product which is an insolubiliser for binders of paper coating compositions and which is a product as defined in any preceding claim.

6. A product according to claim 1 substantially as herein described with reference to any of Examples 1 to 4.

7. A paper coating composition comprising pigment, binder and, as insolubiliser for the binder, a product according to any preceding claim.

8. A composition according to claim 7 in which the amount of insolubiliser is from 2 to 10% based on the weight of binder.

9. A composition according to claim 8 in which the amount of insolubiliser is from 3 to 7% based on the weight of binder.

10. A composition according to any of claims 7 to 9 in which the amount of binder is from 10 to 25% based on the weight of pigment.

11. A composition according to any of claims 7 to 10 in which the binder comprises starch.

12. A composition according to claim 7 substantially as herein described with reference to any of Examples 5 to 7.

13. A cellulosic substrate coated with a composition according to any of claims 7 to 12.

14. A process for insolubilising the binder in a paper coating composition which comprises including in the composition from 2 to 10%, based on the weight of binder in the composition, of a product according to any of claims 1 to 6.