GB1593582A - Process for the production of viscosity-stable aqueous dispersions for paper coating compositions - Google Patents

Abstract

Pigment dispersions for paper coating compositions are prepared by adding maleic anhydride copolymers of the formula I given in Claim 1 as dispersants. The pigment dispersions are stabilised over a wide viscosity range, independently of the pH, by this addition. An example of a particularly suitable maleic anhydride copolymer is propylene/maleic acid half-amide. <IMAGE>

Description

(54) PROCESS FOR THE PRODUCTION OF VISCOSITY-STABLE AQUEOUS DISPERSIONS FOR PAPER COATING COMPOSITIONS (71) We, VEBA-CHEMIE AKTIENGESELLSCHAFr, a German Body Corporate of Pawikerstrasse 30, 4660 Gelsenkirchen-Buer, Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the methods by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a process for the production of viscosity-stable aqueous dispersions of pigments or fillers for use as paper coating compositions.

Paper coating compositions generally comprise fillers or pigments dispersed in water, together with binders and dispersing agents. The pigments used are for example calcium carbonate. china clay, and calcium sulphoaluminate. Aqueous dispersions of these pigments, which in practice may have a solids content of from 30 to 70coo by weight, have certain disadvantages. Thus. for example. the solid matter of the dispersion may settle out within a relatively short time or. in the case of china clay. the dispersions tend to solidify as a result of the so-called "house of cards" effect caused by the different electric charges of the face and edge of the pigment particles. The addition of dispersing agents to the pigment suspension is therefore indispensable in order to maintain a constant desired processing viscosity. In addition, in practice it is frequently desirable to slightly increase the viscosity of coating colours for correct machine processing. so that it is desirable for the dispersing agent to have a viscosity-increasing effect. It is however important that the viscosity once adjusted should remain constant as long as possible.

Dispersing agents effect the necessary rheological stabilisation of the dispersion. The dispersing action depends on the increasing of the zeta potential of the pigment particles; it is based on the fact that the pigment particles receive a surface charge. repel one another, and are thus maintained in the state of a stable dispersion.

It is known to use complex phosphates. salts of polyphosphoric acid. or salts of polycarboxylic acids, for example, as dispersing agents for paper coating compositions.

When these known substances are used as dispersing agents in accordance with the prior art it is found that the viscosity of the pigment suspension is often reduced - in which case low viscosity values are obtained in the low shear range while however undesirable dilatant flow behaviour may occur in the high shear range. It is also found that a controlled increase of viscosity is possible only to a limited extent. while the risk of progressive thickening of the coating composition when stored for a lonp time cannot be cxcluded. Moreover. the effectiveness of the dispersion is relativelv greatly dependent On its pH value.

The present invention relates to substances which are suitable for forming dispersions of pigments or fillers and which are effective over a wide viscosity range irrespective of the pH value, without the dispersing action declining when the dispersions are stored for a long time.

It has now surprisingly been found that pigment or filler dispersions not having the above described disadvantages are obtained hv using. as dispersing agent. special maleic anhydride copolymers, which hereinbelow will he referred to is MA copolvmers.

This finding is surprising particularlv because the MA copolymers used in the invention have been extensively investigated in respect of their use in the paper industry. but have onlv been found suitable for use as paper sizing agents or binders.

The present invention accordinglv provides a viscositv-stable aqueous dispersion suitable for use as a paper coating composition. including a filler or pigment. and ().()5 to 5.t))(' by weight, preferably ().2 to ().5';Si by weight. based on the weight of the filler or pigment. of a dispersing agent which comprises I maleic anhydride copolymer of the general formula

wherein n is an integer of from 2 to 500,preferably 5 to 300, R1 and R2, which may be identical or different, each represents -H or -C113: and M, and M either are identical and represent'-O-alkali metal, preferably -ONa, or -ONH4. or are different and M, represents -NH2 or -N R3R4 wherein each of R3 and R4 independently represents a C1-17 hydrocarbyl group and M2 represents -OH. -0- alkali metal, preferably -ONa, or -ONH4.

Examples of suitable copolymers of formula (I) are maleic acid-propylene-sodium salt, maleic acid-propylene-ammonium salt. maleic a & id-ethylene-sodium salt, maleic acidethylene-ammonium salt, maleic acid-propylene-half amide and maleic acid-ethylene-half amide. Mixtures of maleic anhydride copolymers of formula (1) can also be used, as well as mixtures of one or more copolymets of formula (1) with one or more conventional dispersing agents.

The MA copolymers of formula (I) can be prepared by methods known per se..

The aqueous dispersions according to the invention can be prepared in a simple manner.

Thus, for example the MA copolymers of formula (1) are first dissolved in water, this solution is adjusted to the suitable pH value with alkalis, for example sodium hydroxide solution or ammonia, the pigments or filler is added over a period of It) minutes with stirring and then stirring is continued for 10 more minutes.

The aqueous dispersions obtained in this manner are if necessary mixed with customary additives. such as binders dyestuffs and defrothing agents.

The aqueous dispersions produced by the process of the invention can be used for coating or surfacing fibrous substrates, such as pauper and card.

An advantage of the aqueous dispersions of the invention is that because of the presence of the MA copolymers the viscosity of the dispersion remains constant at the desired value for a long time.

In addition. it has been found that the pH value of the dispersion has no substantial influence on the effectiveness of the MA copolymers.

The invention will now be illustrated by the following Examples.

Example 1 Dispersion I.

Calcium carbonate known as "Millicarb" was stirred over a period of 1() minutes into an aqueous solution (adjusted with NaOH to a pH value of 9) of a maleic anhydride copolymer corresponding to the formula:

and with a propylene: MA ratio of 1: 1, and homogenised for a further period of 10 minutes. The dispersion prepared in this manner had a solids content of 55% by weight and contained 0.3% by weight of the MA copolymer dispersing agent.

Dispersion 11 (comparison).

This dispersion was prepared in the same way as Dispersion I. with the exception that a dispersing agent based on MA-styrene was used instead of the maleic acid-propylene-sodium salt according to the invention.

Dispersion 111.

China clay known as "SPS" ' was dispersed similarly to Dispersion I. 'l'hc maleic anhvdride copolymer used was a copolymer of the formula:

with a propylene: MA ratio of 1:1, in an aqueous solution adjusted with NaOH to a pH value of 9. The dispersion prepared in this manner had a solids content of 55% by weight and a dispersion agent content of 0.3% by weight.

Dispersion IV (comparison).

This dispersion was prepared with china clay SPS, but otherwise in the same way as Dispersion II.

The viscosity of dispersions I to IV was measured at 20 C. Measurement was made immediately after preparation. after I hour, after 2 hours, and after 24 hours. The results are shown in Table 1 below.

TABLE 1 Viscosity D i s p e r s i o n at 20 C (cP) 1 II III IV After preparation 6 15 48 380 After I hour 7 16 50 557 After 2 hours 7 16 50 > 4000 After 24 hours 7 solid 50 solid It can be seen that the dispersions I and Ill prepared in accordance with the invention retained their viscosity during storage. whereas the comparison dispersions II and IV show a sharp increase in viscosity.

Example 2 Dispersions similar to III and IV were prepared. The solids content amounted to 50, 55, and 60% by weight. The viscosities measured for these dispersions are shown in Table 2: TABLE 2 Dispersion Ill IV Solids content (%) 50 55 60 50 55 60 Viscosity at 20 C 26 4X 82 25() 380 not (cP) measurable.

Example 3 Table 3 shows the considerable degree of independence of the dispersions of the invention on pH value. Dispersions were prepared similarly to Ill ind IV and adjusted with NaOH to pH values of 8. 9, and 1().

TABLE 3 Dispersion Ill IV pH value 8 9 10 8 9 10 Viscosity at 2()"C. (cP) 18 18 18 l()7 38() 7')3 Example 4.

Dispersion Ill containing 55'/s bv weight ot china clay and (1.3('( hv weight of the prop- ylene-maleic acid-half nmicle dispersion anent of formula I was adjusted to I4H 9 with NaOH and stored for 52() hours at room temperature. The graph shown ill the accompanying drawing shows the uniform value of the viscosity of this dispersion throughout the storage period, and thus illustrates the effectiveness of the MA copolymers used in the present invention.

Example 5.

Dispersion V.

Calcium carbonate was stirred over a period of 10 minutes into an aqueous solution (adjusted to a pH value of 9 by means of NH4OH) of a maleic-anhydride copolymer of the formula:

and in which the ethylene: MA ratio is 1:1, and the mixture was then homogenised for 10 minutes more. The dispersion prepared in this manner had a solids content of 55% by weight and contained 0.2% by weight of the MA copolymer dispersion agent.

Dispersion VI This dispersion was prepared similarly to dispersion V and contained 0.3% by weight of the MA copolymer dispersion agent.

Dispersion VII This dispersion was prepared with 0.4% of the MA copolymer dispersion agent, but otherwise similarly to dispersion V.

The viscosity of dispersions V to VII was measured at 20"C. The measurements were made immediately after preparation. after I hour, after 2 hours, and after 24 hours. The results are shown in Table 4.

TABLE 4 Dispersion V VI VII MA copolymer content (wt. %) ().2 ().3 0.4 Viscosity at 20 C (cP) After preparation 6 6 6 After I hour 7 6 6 After 2 hours 7 6 6 After 24 hours 8 6 6 Example 6.

Dispersion WILL.

This dispersion was prepared similarly to dispersion l. with the exception that instead of 0.3% by weight of maleic acid-propylene-so(liulll salt use was made of a mixture of 0.2% by weight of maleic acid-ethvlene-sodium salt with ().2% by weight of maleic acid-propylenesodium salt )making a total of t),4(4 by weight of MA copolymer dispersion agent).

Viscosity was measured as in Example I. the results being given in Table 5.

lABLE 5 Dispersion VIII Viscosity at 2() C (cP) After production After 1 hour 7 After 2 hours 7 After 24 hours 8 WHAT WE CLAIM IS: 1. A viscosity-stable aqueous dispersion suitable for use as I paper coaling composition.

including a filler or pigment. and 0.05 to 5.0% by weight. based on the weight of the filler or

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (13)

**WARNING** start of CLMS field may overlap end of DESC **. drawing shows the uniform value of the viscosity of this dispersion throughout the storage period, and thus illustrates the effectiveness of the MA copolymers used in the present invention. Example 5. Dispersion V. Calcium carbonate was stirred over a period of 10 minutes into an aqueous solution (adjusted to a pH value of 9 by means of NH4OH) of a maleic-anhydride copolymer of the formula: and in which the ethylene: MA ratio is 1:1, and the mixture was then homogenised for 10 minutes more. The dispersion prepared in this manner had a solids content of 55% by weight and contained 0.2% by weight of the MA copolymer dispersion agent. Dispersion VI This dispersion was prepared similarly to dispersion V and contained 0.3% by weight of the MA copolymer dispersion agent. Dispersion VII This dispersion was prepared with 0.4% of the MA copolymer dispersion agent, but otherwise similarly to dispersion V. The viscosity of dispersions V to VII was measured at 20"C. The measurements were made immediately after preparation. after I hour, after 2 hours, and after 24 hours. The results are shown in Table 4. TABLE 4 Dispersion V VI VII MA copolymer content (wt. %) ().2 ().3 0.4 Viscosity at 20 C (cP) After preparation 6 6 6 After I hour 7 6 6 After 2 hours 7 6 6 After 24 hours 8 6 6 Example 6. Dispersion WILL. This dispersion was prepared similarly to dispersion l. with the exception that instead of 0.3% by weight of maleic acid-propylene-so(liulll salt use was made of a mixture of 0.2% by weight of maleic acid-ethvlene-sodium salt with ().2% by weight of maleic acid-propylenesodium salt )making a total of t),4(4 by weight of MA copolymer dispersion agent). Viscosity was measured as in Example I. the results being given in Table 5. lABLE 5 Dispersion VIII Viscosity at 2() C (cP) After production After 1 hour 7 After 2 hours 7 After 24 hours 8 WHAT WE CLAIM IS:

1. A viscosity-stable aqueous dispersion suitable for use as I paper coaling composition.

including a filler or pigment. and 0.05 to 5.0% by weight. based on the weight of the filler or

pigment, of a dispersing agent which comprises a maleic anhydride copolymer of the general formula

wherein n is an integer of from 2 to 500, R1 and R2, which may be identical or different, each represents -H or -CH3; and Ml and M2 either are identical and represent -0-alkali metal or -ONH4. or are different and M1 represents -NH2 or -NR3R4 wherein each of R3 and R4 independently represents a CI-17 hydrocarbyl group and M2 represents -OH, -0- alkali metal or -ONH4.

2. An aqueous dispersion according to Claim 1. including 0..2 to 0.5coo by weight, based on the weight of the filler or pigment, of a maleic anhydride copolymer of formula (I).

3. An aqueous dispersion according to Claim I or Claim 2, wherein the alkali metal is sodium.

4. An aqueous dispersion according to any one of Claims I to 3. wherein n is an integer from 5 to 300.

5. An aqueous dispersion according to any one of the preceding claims, wherein the copolymer of formula I is propylene-maleic acid-half amide.

6. An aqueous dispersion according to any one of the preceding claims, wherein the dispersing agent includes a mixture of copolymers of formula (I).

7. An aqueous dispersion according to any one of the preceding claims, wherein the dispersing agent includes one or more copolymers of formula (I) together with one or more conventional dispersion agents.

8. An aqueous dispersion according to Claim I, substantially as described in any one of the foregoing Examples 1 to 6.

9. A process for the preparation of a viscosity-stable aqueous dispersion according to any one of the preceding claims. which process comprises dissolving in water a maleic anhydride copolymer of general formula (1) according to Claim 1, and adding to the resultant aqueous solution while stirring one or more fillers or pigments.

10. A process according to Claim 9 substantiallv as hereinbefore described.

11. A process according to Claim 9 substantiallv as described in any one of the foregoing Examples 1 to 6.

12. An aqueous dispersion according to any one of Claims I to 8 when prepared by a process as claimed in any one of Claims 9 to II.

13. A fibrous substrate when coated with an aqueous dispersion as claimed in any one of Claims I to 8 and 12.