Paper coating compositions

Abstract: The present invention relates to the production and use of protein-containing paper coating compositions in the preparation of cast coated papers where extremely high gloss, smoothness, flexibility, resistance to drum adhesion and pick resistance are required, the paper coating composition comprising soy protein binder which has been modified by copolymerisation to add a synthetic component to form a graft copolymer of the latex variety.

Description: The present invention relates to the production and use of protein-containing paper coating compositions, and particularly to the production of paper coating compositions which find a special utility in cast coated papers where extremely high gloss, smoothness, flexibility, resistance to drum adhesion and pick resistance are required. The following US patents form relevant prior art: US-A-2,274,983, US-A-2,246,466, US-A-2,849,334, US-A-2,881,076, US-A-2,950,214, US-A-3,081,182, US-A-3,411,925, US-A-4,048,380, US-A-4,109,056, US-A-4,277,524, US-A-4,352,692, US-A-4,421,564, US-A-4,474,694, US-A-4,520,048, US-A-4,575,477, US-A-4,581,257, US-A-4,607,089, US-A-4,620,992 and US-A-4,812,550. Information relating to cast coating techniques may be found in: "Soy Protein Latex Interpolymers - Properties and Function," Proceedings of the 1987 Coating Conference TAPPI. pp 133 - 139, Coco, C.E.; "Ultrahigh Finish Coated Papers Cast Coating and Other Processes," Pigmented Coating Processes For Paper and Board. TAPPI Monograph No. 28. pp 74 - 85, Casey, J.P.; "Isolated Soy Protein Binders for Paper and Paperboard Coatings," Protein Binders In Paper and Paperboard Coating. TAPPI Monograph No. , pp. 75-96, Olson, R.A. and Hoelderle, P.T.; and "Pigment Coating," Pulp and Paper Chemistry and Chemical Technology , pp 1551 - 1753, 1961, Casey, J. P. Basic coating techniques for cast coated paper are well known, and the use of protein adhesive binders for coating formulations for these papers is also well known in the art, as is the use of vegetable protein, such as modified soy protein, for adhesive binders. However, the adhesive of choice for these products is typically a casein/latex combination, due to the greater whiteness, smoothness, gloss uniformity and pick release obtainable with such formulations (pick being used herein to refer to the loss of adhesion of the coating composition to the paper substrate during printing). More specifically, pick is believed to result from the segregation or separation of the adhesive and other components of the coating formulation during the coating and drying process itself. To counteract this effect, various dispersants and additives are typically added to coating formulations to keep the coating components uniformly dispersed during coating and drying of the paper web. These efforts have been effective to make casein/latex combination binders the best known in the art to date. Soy protein adhesive binders for use in paper coating are typically prepared from isolated soy protein. The protein is extracted from oil-free soybean flakes in an alkaline solution and recovered by isoelectric precipitation. The recovered soy protein is typically modified by hydrolysis and other chemical treatment to prepare a protein adhesive material which is suitable for use as a binder in paper coating compositions. Binders of this type find many uses in the paper coating industry. Recently, however, a modified adhesive binder, such as disclosed in US-A-4,607,089, US-A-4,620,992 and US-A-4,812,550, has also been used for some paper coating applications. US-A-4,607,089 discloses a conjugated diene such as buta-1,3-diene and a vinyl aryl monomer such as styrene, or α-methylstyrene, graft polymerised or over polymerised on a dissolved or finely divided anionic fully caustic treated soy protein in aqueous alkaline media using minor effective amounts of an oil soluble azo catalyst, a chelating agent and a chain transfer agent to form a latex for use in paper coating compositions, particularly for gravure or offset printing applications. The area in which these compositions are used is in the field of calendaring. In this, the coating composition is applied wet, dried off and then rolled to achieve the final effect. By way of contrast, cast coating comprises laying on the wet coating and using a heated drum to dry and cure the surface to give a smooth, glossy coating of the highest quality. Although soy protein based materials are now recognised as being of use in the preparation of adhesive binders for cast coating, these binders suffer from disadvantages, particularly when compared with casein/synthetic resin combinations. Paper coating compositions for cast coated fancy paper do not usually contain soy binders, because these binders do not provide the high quality necessary for cast coated fancy paper. The resulting surface yielded by binders containing soy protein is of a greyish hue, and has a matt or dull texture. Thus, in this area, the state-of-the-art binder is a combination of casein and a synthetic adhesive, such as latex, which provide a good combination of smoothness, gloss and runability. However, even casein/latex based coating compositions suffer from drum adhesion and pick. Drum adhesion occurs on the casting drum, the coating losing adhesion to the paper substrate in local areas, and adhering to the casting drum in the form of small particles. With time, these particles grow by the accumulation of additional coating material, and cause voids in the paper surface. The resulting coated paper is rough, porous and has diminished gloss. Drum adhesion is particularly associated with higher latex contents of the coating formulations. The problem cannot simply be overcome by reducing levels of latex. The latex component is essential in conventional formulations, since it imparts flexibility to the coated paper produced by the casting process. Accordingly it is desirable to increase the level of latex, if possible but, with conventional formulations it has not been practical to use coating formulations in which the latex component is in excess of 60% of the total binder, and not usually that high. At such high levels of latex, much latex comes into contact with the hot surface of the casting drum. The thermoplastic nature of latex causes it to melt and adhere to the casting drum, resulting in drum adhesion. Thus, drum adhesion, in conventional coating formulations, severely limits the amount of latex which can be used, and limits the flexibility, smoothness and gloss which might otherwise be obtained. We have now found that, by use of particular combinations of coating components, a soy protein based coating composition can be obtained which produces an extremely flexible, smooth and glossy coated paper. The coated paper has exceptional pick resistance and drum release, which properties are comparable to, or better than, the best casein/latex formulations. In particular, the present invention provides the production and use of protein-containing paper coating compositions in the preparation of cast coated papers where extremely high gloss, smoothness, flexibility, resistance to drum adhesion and pick resistance are required, the paper coating composition comprising soy protein binder which has been modified by copolymerisation to add a synthetic component to form a graft copolymer of the latex variety. In a first aspect, the present invention provides a process for producing a cast coated paper having resistance to drum adhesion and to pick, comprising: cast coating a paper web substrate with an aqueous solution of: (i) an adhesive binder comprising soy protein synthetic graft copolymer; and, (ii) appropriate paper coating pigments, and curing the coated paper web substrate to form a finished paper. In an alternative aspect, the present invention provides a cast coated paper having resistance to drum adhesion and to pick, comprising a paper web and a cured coating thereon, the coating comprising: (i) an adhesive binder comprising soy protein synthetic graft copolymer; and, (ii) appropriate paper coating pigments By "appropriate" is meant any pigment necessary or desired. The present invention also provides a process for producing a coating composition as described in claim 7. The technique of the present invention requires the use of a soy/synthetic graft polymer as an adhesive binder and, preferably, includes control of the binder in combination with other coating components. Thus, the present invention allows the preparation of a coating composition and cast coated papers having an extremely high gloss, smoothness, flexibility, pick resistance and drum release. The present invention also allows the preparation of an adhesive binder from soy protein using a soy/synthetic copolymer which has the ability to produce coated papers as above. The adhesive binder and the process for its production can be used in conjunction with conventional cast coating processes, and a coating composition using a protein copolymer adhesive which combines with particular pigments can be prepared to yield a smooth, high gloss, flexible and pick resistant paper when used in accordance with the present invention. The present invention provides the use of soy protein copolymer adhesive binders in combination with particular coating pigments. Preferably, the soy protein copolymer is a graft soy protein copolymer, and may be prepared according to the disclosures of US-A-4,607,089 and US-A-4,812,550, for example. The adhesive binders of the present invention, preferably when used in combination with particular pigment combinations, have been found effective in overcoming the problems associated with the prior art. Cast coated paper having smoothness and gloss comparable to that of the art, and also having improved flexibility, may be produced in accordance with the present invention. In particular, it is possible to produce paper having properties equal to or superior to paper produced using casein/latex adhesives. We have found that, by using a soy/synthetic copolymer, the latex component of the binder, by forming a part of the copolymer, can be increased up to a proportion of as much as 80% of the binder. The resulting coating composition has extraordinary resistance to drum adhesion, and has exceptional gloss, flexibility, smoothness and pick resistance. In fact, the coating compositions of the present invention substantially overcome the limitations of casein/latex based coating formulations, especially with regard to the elimination of coating pick (as determined by standard tests). The unexpected increase in the amount of the synthetic (latex) component which can be incorporated in the binder (and coating composition formulation) may be because the latex moiety is surrounded or protected by the protein shell. As a result, the synthetic moiety is prevented from contacting the casting drum. Thus, to a substantial degree, only the protein moiety contacts the casting drum. Consequently, the synthetic moiety does not overheat, melt nor adhere to the surface of the casting drum. The copolymer may contain up to about 80% synthetic moieties; the 20% soy moiety still being sufficient to protect the synthetic elements from the casting drum surface, while permitting the flexibility enhancement, smoothness, gloss and pick resistance equivalent to the same, or similar, amount of free latex. The graft copolymer preferably comprises a conjugated diene and a vinyl, or vinylidene, aryl monomer polymerised on a soy protein shell, but a general preference is for the synthetic moiety of the copolymer to be a latex moiety, especially a butadiene-styrene copolymer. It is preferred that the synthetic moiety forms up to about 80% by weight of the graft copolymer. The process of the present invention is suitable even to produce the superior aesthetic and optical qualities required for fancy papers, owing to the substantial lack of drum adhesion and pick. The coating compositions of the present invention preferably use clay and calcium carbonate pigments in combination with the soy copolymer binder, although other conventional pigments may be used. The clay and calcium carbonate may each be present at levels between from about 10% to about 90% by weight of the total pigment present in the coating compositions according to the present invention. The copolymer binder may be present at between from about 10% to about 25% by weight of the pigment. The invention will now be further illustrated by reference to the accompanying, non-limiting Examples. EXAMPLE 1 A cast coating composition was prepared with the following formulation: 90 parts by weight clay 10 parts by weight calcium carbonate 0.2 parts by weight dispersant 0.1 parts by weight defoamer 15 parts by weight soy synthetic interpolymer, {US-A-4,607,089 (RP 535)[82% synthetic moiety]} 0.5 parts by weight release agent The above coating composition was suspended in water (58% solids) at room temperature, the pH was adjusted to 9.4 and the aqueous composition used for coating by a cast coating process. The coat weight was 9.1 kg (20 pounds) per ream. The web speed was about 76 m (250 feet) per minute. After coating, the coated paper from the process was subjected to tests of its gloss, colour, smoothness and pick resistance. The results of those tests are given in Table 1. The coated paper was examined and judged to have high flexibility. EXAMPLE 2 For comparison, a casein/latex cast coating composition was prepared with the following formulation: 90 parts by weight clay 10 parts by weight calcium carbonate 0.2 parts by weight dispersant 0.1 parts by weight defoamer 10 parts by weight casein 6 parts by weight latex 0.5 parts by weight release agent The above coating composition was suspended in water (58% solids) at room temperature, the pH was adjusted to 9.4 and the aqueous composition was used for coating by a cast coating process. The coat weight was 9.1 kg (20 pounds) per ream. The web speed was approximately 76 m (250 feet) per minute. The resulting coated paper was then subjected to tests of its gloss, colour, smoothness and pick resistance. The results of those tests are also given in Table 1. The coated paper was examined and judged to have high flexibility. EXAMPLE 3 A cast coating composition was prepared with the following formulation: 10 parts by weight clay 90 parts by weight calcium carbonate 0.2 parts by weight dispersant 0.1 parts by weight defoamer 15 parts by weight soy synthetic interpolymer, {US-A-4,607,089 (RP 535)[82% synthetic moiety]} 0.5 parts by weight release agent The above coating composition was suspended in water (58% solids) at room temperature, the pH was adjusted to 9.4 and the aqueous composition was coated using a cast coating process. The coat weight was 9.1 kg (20 pounds) per ream. The web speed was about 76 m (250 feet) per minute. The resulting coated paper was then subjected to tests of its gloss, colour, smoothness and pick resistance. The results of those tests are again given in Table 1. The coated paper was examined and judged to have high flexibility. EXAMPLE 4 For comparison, a casein/latex cast coating composition was prepared with the following formulation: 10 parts by weight clay 90 parts by weight calcium carbonate 0.2 parts by weight dispersant 0.1 parts by weight defoamer 10 parts by weight casein 0.5 parts by weight release agent The above coating composition was suspended in water (58% solids) at room temperature, the pH was adjusted to 9.4 and the aqueous composition was coated using a cast coating process. The coat weight was 9.1 kg (20 pounds) per ream. The web speed was about 76 m (250 feet) per minute. The resulting paper was then subjected to tests of its gloss, colour, smoothness and pick resistance. The results of those tests are given in Table 1. The coated paper was examined and judged to have high flexibility. Example 1 Comparison Example 2 Example 3 Comparison Example 4 20° Gloss 60 45 43 36 75° Gloss 92 85 85 83 Hunter Color L (Whiteness) 91.5 91 95 95 Hunter Color a (red/green) 0.6 0.6 0.15 0.15 Hunter Color b (blue/yellow) 3.7 3.7 2.8 3.0 Parker Smoothness (S-5) 0.80 0.93 0.80 0.93 Parker Smoothness (S-lO) 0.67 0.73 0.67 0.73 K & N (% loss) 31 37 10 16 Ink Gloss 95 94 88 86 IGT pick (cm/sec) none 175 none 135

Claims: A process for producing a cast coated paper having resistance to drum adhesion and to pick, comprising: cast coating a paper web substrate with an aqueous solution of: (i) an adhesive binder comprising soy protein synthetic graft copolymer; and, (ii) appropriate paper coating pigments, and curing the coated paper web substrate to form a finished paper. A process according to claim 1, wherein the graft copolymer comprises a conjugated diene and a vinyl, or vinylidene, aryl monomer polymerised on a soy protein shell. A process according to claim 1 or 2, wherein the synthetic moiety of the copolymer is a latex moiety. A process according to any preceding claim, wherein the copolymer is a butadiene-styrene copolymer. A process according to any preceding claim, wherein the amount of the synthetic moiety forms up to about 80% by weight of the graft copolymer. A process according to any of claims 1 to 5 for producing a cast coated paper having high gloss, flexibility, smoothness, brightness, resistance to drum adhesion and pick resistance comprising preparing an aqueous paper coating composition containing paper coating pigments and an adhesive binder, the adhesive binder consisting essentially of a soy protein synthetic graft copolymer, the combination of pigment and adhesive binder being effective to produce a high gloss, flexible, smooth, bright and substantially pick free coated paper when coated on a paper web substrate and being effective to substantially reduce drum adhesion, cast coating a paper web substrate with the aqueous paper coating composition and curing the coated paper web substrate to form a finished paper. A process of producing a cast paper coating composition effective to substantially reduce drum adhesion and being effective to produce a high gloss, flexible, smooth, bright and pick resistant coated paper comprising preparing an aqueous suspension of a paper coating adhesive binder and paper coating pigments, the paper coating adhesive binder consisting essentially of a soy protein synthetic graft copolymer, the combination of pigments and adhesive binder being effective to substantially reduce drum adhesion and being effective to produce a high gloss, flexible, smooth, bright and substantially pick free coated paper when cast coated on a paper web substrate, said binder optionally being as defined in any of claims 1 to 5. A cast coated paper having resistance to drum adhesion and to pick, comprising a paper web and a cured coating thereon, the coating comprising: (i) an adhesive binder comprising soy protein synthetic graft copolymer; and, (ii) appropriate paper coating pigments, said binder optionally being as defined in any of claims 1 to 5.

Inventors: COCO CHARLES E, COCO, CHARLES E.
Applicants: RALSTON PURINA CO, RALSTON PURINA COMPANY
Application year: 1990
Main classification: D21H25
IPC codes: D21H19, D21H17, C08F289, C09J151