WO2002084028A1

WO2002084028A1 - Novel coating color composition

Info

Publication number: WO2002084028A1
Application number: PCT/FI2002/000310
Authority: WO
Inventors: Anna-Liisa Tammi; Päivi MIETTINEN
Original assignee: Raisio Chemicals Oy
Current assignee: Ciba Specialty Chemicals Oy
Priority date: 2001-04-12
Filing date: 2002-04-12
Publication date: 2002-10-24
Anticipated expiration: 2003-10-12
Also published as: FI20010773L; EP1381735A1; FI116536B; FI20010773A0

Abstract

The invention relates to coating of paper and paperboard and is especially directed to a novel pigment coating composition for coating of paper and paperboard. The novel coating composition contains pigment, latex, a small amount of a polymer made of hydrophobic monomers in the presence of starch, optionally also other binders and, if necessary, known additives used in coating compositions.

Description

Novel coating color composition

The present invention relates to coating of paper and paperboard and is especially directed to a novel pigment coating composition for coating of paper and paper- board. The novel coating composition comprises pigment, latex, a small amount of a polymer which is made of hydrophobic monomers in the presence of starch, optionally also other binders and, if necessary, known additives used in coating compositions. The invention is also directed to paper or paperboard coated with the novel coating composition and by all coating methods.

The purpose of coating is to improve the surface properties of paper or paperboard, such as gloss, smoothness and printability. The most commonly used surface treatment is pigment coating, wherein color, in other words pigment-binder mixture, is applied on the surface of base material at a coating unit or at a separate coating machine, the thickness of the coating layer is adjusted, and finally the coating is dried. A usual pigment coating mixture comprises pigment agents, of which there may be one or several present, binder or binders, water, and optionally additives, such as hardeners, viscosity and water retention regulating agents and lubricants. The ratios of the coating mixture components can vary rather widely, depending on the purpose of use and quality requirements of the end product, on the coating method and on the quality of the base paper.

US patent No. 5,536,764 discloses a coating composition, which comprises a starch-polymer dispersion formed in the reaction of cationic starch and vinyl ace- tate. According to the publication the polymer dispersion is intended to replace protein, which is used as a binder in a board coating composition. The starch- polymer dispersion has been used in large amounts (7.2 parts), in addition to which latex (11.4 parts) has been used as a co-binder. The dry solids content of the coating composition is low (48%). US patent No. 5,003,022 discloses a polymer dispersion, which comprises a graft polymer formed in the reaction of starch and vinyl monomers. The dispersion is said to be suitable for use as a binder in a coating color composition, wherein it was used either as the only binder or as a co-binder with latex. In the coating composi- 5 tions that contained latex, the proportion of polymer dispersion was high (6 or 10 parts, when the proportion of pigment was 77 or 90 parts, respectively).

From US patent No. 4,301,017 it is known to use a graft polymer of starch and vinyl monomers in pigment coatings either as the only binder or as a co-binder to- o gether with starch. When both plain starch and a graft polymer were used as a binder, both were used in equal amounts (9 parts + 9 parts). However, due to the high amount of polymer the resulting coating is expensive and its dry solids content is too low.

5 From the point of view of the coating result, an essential component of the pigment coating mixture is the pigment. As coating pigments are used La. kaolin, calcinated kaolin, calcium carbonate, titanium dioxide, satine white, talc, gypsum, chalk, aluminium hydroxide, barium sulphate and sodium aluminium silicate, of which the most commonly used are kaolin, calcinated kaolin, talc and calcium carbonate. Pig- 0 ments which contain calcium provide the end product with high brightness, and that is the reason why they are often desired to be used. However, a problem with coating colors, which comprise calcium-containing pigment, is dusting at the printing stage and a low gloss of the printed surface. This problem has previously been tried to be solved for example by treating the binder a. by changing the 5 monomer composition, the particle size of the latex or the reaction conditions during its manufacture. However, by said means the dusting problem and the problem of the low gloss of the printed surface have not been totally solved.

The novel coating color composition according to the invention which comprises in o addition to latex as a binder a small amount of a polymer made of hydrophobic monomers in the presence of starch, surprisingly binds calcium-containing pig- ments extremely well and its dry solids content is sufficiently high from the point of view of the success of the coating process. Further, its price is advantageous, as only a small amount of the expensive starch-stabilized latex is needed. The composition according to the invention is also especially suitable for coating that is based on spray technique.

Consequently, the present invention is directed to a novel coating color composition, which comprises as its components a) pigment, b) latex, c) if necessary, known additives used in coating compositions, and d) a small amount of a polymer made of hydrophobic monomers in the presence of starch, in other words starch- stabilized latex. A preferred coating color composition according to the invention contains no more than 5 parts, preferably no more than 3 parts, of starch-stabilized latex. In addition, the coating color composition according to the invention may contain also other binders than latex and starch-stabilized latex, such as for exam- pie starch and/or CMC. The amount of these may be 0 - 7 parts, preferably 0.5 - 3 parts.

When the composition of coating mixtures is given, the proportions of the components are usually calculated so that the total amount of pigments is marked as 100 and the amounts of the other components are given as relative to the amount of the total pigment. The proportions of all components are given as dry parts. In this disclosure the amounts of the components of the coating composition according to the invention are given as described above.

Suitable pigments for a coating color composition according to the invention are coating pigments known in the art, such as kaolin, calcinated kaolin, gypsum, chalk, calcium carbonate, titanium dioxide, satine white, talc, aluminium hydroxide, barium sulphate and sodium aluminium silicate. Especially suitable for a coating color composition according to the invention are all calcium-containing pig- ments, such as calcinated kaolin, gypsum, calcium carbonate (ground or precipitated). Suitable binders are latexes known in the art, such as styrene butadiene latex (SB), acrylate latexes and polyvinyl acetate latexes (PNAc), especially SB latexes. A coating color composition according to the invention is characterized in that it in addition to latex contains a small amount of starch-stabilized latex. In addition to these, also other binders, such as for example starch and/or CMC, may be used.

Known additives used in coating compositions are La. dispersants, agents which change flow properties, antifoam and foam-removing agents, preservatives, lubricants, hardeners, and optical brighteners. These are often added to coating mixtures in order to improve the use properties of the coating mixture or in order to provide the coating with certain special features.

Within the scope of this invention, starch-stabilized latex means a copolymer, which is formed when hydrophobic vinyl monomers are polymerized in the pres- ence of native or modified starch. Modified starch can mean for example degraded (for example oxidized), anionized, cationized, and/or amphoteric starch. Starch may be any of the known starches, such as potato, corn, waxy corn, barley, wheat, or tapioca starch, preferably potato starch. In the place of starch also any other poly- saccharide, which contains a free hydroxyl group, such as amylose, amylopectine, carrageen, cellulose, chitosan, chitin, dextrines, guar gum (guarane) and other ga- lactomannans, arabic gum, hemicellulose components, and pullulan, can be used, but starch is the most preferred polysaccharide for the purposes of the invention. Processes for the preparation of starch-polymer dispersions have been disclosed for example in the publications US 4,301,017, US 5,003,022 and WO 00/46264. Monomers used in the preparation of a suitable polymer that is included in the coating composition according to the invention, are hydrophobic monomers, such as for example styrene, alpha-methyl styrene, acrylates, acrylonitrile etc.

A preferred coating color composition according to the invention comprises a) 100 parts of pigment, b) 8 - 18 parts of latex, c) 0 - 3 parts of additives and d) 0.1 - 5 parts of starch-stabilized latex. More preferably, the amount of latex is 8 - 15 parts, even more preferably 9 - 14 parts, and most preferably 11 - 12 parts. The amount of additives is preferably 0.2 - 2 parts, more preferably 0.2 - 1.5 parts, for example about 1 part.

In the coating color composition according to the invention, the amount of a polymer made of hydrophobic monomers in the presence of starch is 0.1 - 5 parts, more preferably 0.2 - 4 parts, even more preferably 0.3 - 3 parts, and most preferably about 0.5 - 1.5 parts.

A preferred coating color composition according to the invention which comprises also another binder than latex and starch-stabilized latex, contains a) 100 parts of pigment, b) 9 - 14 parts of latex, c) 0.2 - 1.5 parts of additives and d) 0.3 - 3 parts of a polymer made of hydrophobic monomers in the presence of starch, and e) 0.5 - 3 parts of starch.

An even more preferred coating color composition according to the invention which comprises also another binder than latex and starch-stabilized latex, contains a) 100 parts of pigment, b) 10 - 12 parts of latex, c) 0.5 - 1.5 parts of additives, d) 0.5 - 1.5 parts of a polymer made of hydrophobic monomers in the presence of starch, and e) 1 - 2 parts of starch.

A preferred coating color composition according to the invention comprises a) 100 parts of pigment, b) 12 parts of latex, c) 1 part of additives, d) 1 part of starch- stabilized latex, e) 1 part of plain starch.

An even more preferred coating color composition according to the invention comprises a) 100 parts of pigment, b) 11 parts of latex, c) 1 part of additives, d) 1 part of starch-stabilized latex, e) 2 parts of plain starch. The dry solids content of a ready-for-use coating color composition according to the invention is usually over 50%, for example 50 to 60%, preferably about 55%, and at its most about 62%.

5 Due to the coating color composition according to the invention, pigment, especially calcium-containing pigment, binds better to the base paper or paperboard. Consequently, the dusting problem of especially such coating colors, which comprise calcium-containing pigment, can be avoided. At the same time also the surface of the coated paper becomes more dense, and the surface strengths and the 0 gloss and other properties of the printed surface are improved. The colour setting profile is also changed, which is advantageous in certain applications. The coating color composition according to the invention contains a smaller amount of the expensive starch-stabilized latex than the prior known products, and its price is thus more advantageous. In the coating color compositions according to the invention 5 also the amount of latex can in certain applications be reduced due to the higher surface strengths.

The following examples describe the invention. Examples 1 to 7 describe different coating color compositions, of which the compositions of examples 2 to 7 are com- o positions according to the invention. Examples 8 to 9 present the test results obtained.

Example 1 Basic color

5 100 parts of calcium carbonate

13 parts of SB latex 0.4 parts of hardener 0.5 parts of calcium stearate (lubricant) The basic color was prepared by mixing latex and additives into carbonate slurry. The dry solids of the color were adjusted to 55% by adding water. The pH of the color was adjusted to pH 8.

Example 2

100 parts of calcium carbonate 11 parts of SB latex 2 parts of starch 0.5 parts of starch-stabilized latex

0.3 parts of hardener 0.4 parts of lubricant

Boiled starch and starch-stabilized latex were slurried with the basic color. Other- wise the preparation was in accordance with Example 1.

Example 3

100 parts of calcium carbonate 10 parts of SB latex

2 parts of starch 1 part of starch-stabilized latex 0.3 parts of hardener 0.4 parts of lubricant

The color according to the invention was prepared by first mixing the basic color as in Example 1. The boiled starch and starch-stabilized latex were slurried with the basic color. Example 4

100 parts of calcium carbonate 10 parts of SB latex 3 parts of starch-stabilized latex

0.3 parts of hardener 0.4 parts of lubricant

The color according to the invention was prepared by first mixing the basic color as in Example 1. The starch-stabilized latex was slurried with the basic color.

Example 5

100 parts of calcium carbonate 12 parts of SB latex

1 part of starch-stabilized latex

0.3 parts of hardener

0.4 parts of lubricant

0.5 parts of optical brightener

Example 6

100 parts of calcium carbonate 12 parts of SB latex 0.5 parts of starch-stabilized latex 0.3 parts of hardener 0.4 parts of lubricant The color according to the invention was prepared by first mixing the basic color as in Example 1. The starch-stabilized latex was slurried with the basic color.

Example 7

5

100 parts of calcium carbonate 11 parts of SB latex 1 part of starch

1 part of starch-stabilized latex 0 0.5 parts of optical brightener

0.5 parts of lubricant 0.4 parts of hardener

The color according to the invention was prepared by first mixing the basic color as 5 in Example 1. The boiled starch and starch-stabilized latex were slurried with the basic color.

Example 8

o The base paper was coated with different colors with a pilot-scale coating apparatus. Both sides a and b of the paper were coated. The papers were printed in a laboratory scale.

Figure 1 shows the results that were obtained when the amount of organic compo- 5 nents, i.e. binders, was measured from a sample taken after coating unit from a wet, coated paper. The amount of organic components represents the amount of binder, which is applied from the coating. A higher amount of organic components shows that the pigment is better bound to the paper.

Figure 1. The amount of organic components measured from a wet paper coating

The results of Figure 1 have been obtained with the following coating compositions. Parallel measurements from both sides a and b of the paper were made from the reference color and from the test colors 1 and 2. The dry solids content of all three coatings was adjusted to 55% by weight by adding the required amount of water.

Reference color: 100 parts of carbonate

13 parts of SB latex

0.5 parts of optical brightener

0.4 parts of hardener

0.5 parts of lubricant

Test color 1 : composition according to the invention, which contains

100 parts of carbonate 13 parts of SB latex

1 part of starch-stabilized latex 1

2 parts of starch 0.5 parts of optical brightener

0.3 parts of hardener 0.4 parts of lubricant The starch-stabilized latex 1 is an anionic latex stabilized with anionic starch.

Test color 2: composition according to the invention, which contains 100 parts of carbonate 13 parts of SB latex

1 part of starch-stabilized latex 2

2 parts of starch

0.5 parts of optical brightener 0.3 parts of hardener 0.4 parts of lubricant

The starch-stabilized latex 2 is an amphoteric latex stabilized with cationic starch.

From the results of Figure 1 it is seen that the coating compositions according to the invention bind pigment extremely well compared to the reference coating. A good binding of the polymer into the coating pigment is seen as a good water retention capacity compared to the reference. This enables running of the color without other water retention agents.

Edry solids content

Figure 2. Dry solids content of color (% by weight) The dry solids contents were also measured from the same color samples. From Figure 2 it is clearly seen that the dry solids of the reference color have risen from 55% to nearly 59%, while the dry solids contents of the colors according to the invention rose only slightly, to about 56.5%. It can be concluded that the binder of the reference color has been absorbed into the paper which fact has an adverse effect on the binding of the pigment particles. Instead, in the test colors the pigment particles were better bound to the base paper and to each other.

Example 9

Table 1 shows the measurements from papers coated with different coating color compositions. All the papers had the same base paper. The reference and test colors 1 and 2 were the same as in Example 8.

Table 1.

The improvement in the surface strength of the papers coated with the colors according to the invention is shown both as a better picking strength and as better delamination strength of the coating, when measured with an IGT apparatus. The improvement in the properties of the printed surface is shown by the clear improvement of the gloss of the printed surface and especially by the clear improvement of the gloss contrast (delta gloss) of the paper. These advantageous properties of both coated and printed paper are achieved without adversely affecting the other important, measured properties of the paper, such as brightness/opacity -ratio and bulk. A good paper-technical quality is achieved also with lower amounts of the coating compared to the reference.

Claims

1. A coating color composition which comprises as its components a) pigment, b) latex, and c) if necessary, known additives used in coating compositions, characterized in that it also comprises d) no more than 5 parts of a polymer made of hydrophobic monomers in the presence of starch.

2. The coating color composition according to claim 1, characterized in that it comprises no more than 3 parts of a polymer made of hydrophobic monomers in the presence of starch.

3. The coating color composition according to claim 1 or 2, characterized in that it also comprises e) another binder than latex and a polymer made of hydrophobic monomers in the presence of starch.

4. The coating color composition according to claim 3, characterized in that it comprises starch and/or CMC.

5. The coating color composition according to claim 1, which contains as its com- ponents a) 100 parts of pigment, b) 8 - 18 parts of latex, c) 0 - 3 parts of additives, and d) 0.1 - 5 parts of a polymer made of hydrophobic monomers in the presence of starch.

6. The coating color composition according to claim 1, which contains as its com- ponents a) 100 parts of pigment, b) 8 - 15 parts of latex, c) 0.2 - 2 parts of additives, and d) 0.2 - 4 parts of a polymer made of hydrophobic monomers in the presence of starch.

7. The coating color composition according to claim 1, which contains as its components a) 100 parts of pigment, b) 9 - 14 parts of latex, c) 0.2 - 1.5 parts of additives, and d) 0.3 - 3 parts of a polymer made of hydrophobic monomers in the presence of starch.

8. The coating color composition according to claim 1, which contains as its com- ponents a) 100 parts of pigment, b) 11 - 12 parts of latex, c) 1.0 parts of additives, and d) 0.5 - 1.5 parts of a polymer made of hydrophobic monomers in the presence of starch.

9. The coating color composition according to claim 3, which contains as its com- ponents a) 100 parts of pigment, b) 9 - 14 parts of latex, c) 0.2 - 1.5 parts of additives, d) 0.3 - 3 parts of a polymer made of hydrophobic monomers in the presence of starch, and e) 0.5 - 3 parts of starch.

10. The coating color composition according to claim 3, which contains as its com- ponents a) 100 parts of pigment, b) 10 - 12 parts of latex, c) 0.5 - 1.5 parts of additives, d) 0.5 - 1.5 parts of a polymer made of hydrophobic monomers in the presence of starch, and e) 1 - 2 parts of starch.

11. The coating color composition according to any one of claims 1 to 10, characterized in that the pigment is a calcium-containing pigment.

12. The coating color composition according to claim 11, characterized in that the pigment is ground or precipitated calcium carbonate, calcinated kaolin and/or gypsum.

13. The coating color composition according to any one of claims 1 to 11, characterized in that the latex is styrene butadiene latex.

14. The coating color composition according to any one of claims 1 to 13, characterized in that the polymer used in the composition has been obtained by polymerizing hydrophobic monomers in the presence of native or modified starch.

15. Paper or paperboard, characterized in that it has been coated with a coating color composition according to any one of claims 1 to 14.