GB2310216A

GB2310216A - Coating composition

Info

Publication number: GB2310216A
Application number: GB9520986A
Authority: GB
Inventors: Philip Martin Mcgenity; Janet Susan Preston
Original assignee: ECC International Ltd
Current assignee: Imerys Minerals Ltd
Priority date: 1995-10-13
Filing date: 1995-10-13
Publication date: 1997-08-20
Also published as: GB9520986D0

Description

A COATING COMPOSITION This invention concerns an improved coating composition for use in preparing a coated cellulosic sheet material, for example paper or cardboard, which has special advantages when the coated sheet material is printed by the gravure process.

Coating compositions for cellulosic sheet materials generally comprise an aqueous suspension of one or more pigments and one or more adhesive materials, together with small amounts of other additives such as rheology modifiers, lubricants etc.

The most commonly used pigments are generally natural or synthetic inorganic materials of white colour and of fine particle size, which is generally such that at least 45% by weight of the particles have an equivalent spherical diameter (esd) smaller than 2ym, when measured by a sedimentation method. Examples of pigments which are commonly used in coating compositions for cellulosic sheet materials are kaolin clay, calcined kaolin clay, natural or precipitated calcium carbonate, satin white, calcium sulphate and talc.

The gravure printing process is generally most suited to those applications in which a very large number of copies is required to be printed, because the process by which a gravure printing cylinder is prepared is expensive relative to the cost of preparing printing plates for use in other methods of printing.

A gravure printing cylinder has on its surface a matrix of cells or depressions which vary in depth according to the amount of ink which is required to be transferred to the paper from each individual cell.

The surface of the cylinder is initially smooth and highly polished, and the matrix of cells on the surface is prepared by an expensive photographic etching process. Because of the large number of copies to be printed, it is generally desirable to run the printing press at high speed, and this necessitates the use of a printing ink which dries quickly on the paper to avoid "offset", or the transfer of undried printing ink from one sheet of paper to the next. It is therefore desirable to use a printing ink which has a solvent which is more volatile than water. In addition a solvent-based ink generally gives better print quality than a water-based ink. Such volatile solvents are almost always organic and oleophilic.

It has been discovered in the paper coating industry that, when large reels of paper are wound or unwound at high speed, frequent breakages can occur in the web of coated paper. One cause of this is believed to be the relatively high coefficient of friction of the coated surface of certain types of coated paper, which limits the freedom of movement of one turn of a reel of coated paper relative to an adjacent turn.

In order to reduce or eliminate this problem, some operators include in the coating composition from 10k to 45% by weight of talc in the total dry weight of the pigment in the composition. This reduces the coefficient of friction of the surface of the coated paper prepared using the composition and gives it a "slippery" feel. Talc of the required quality is, however, expensive, and in some cases, the slippery surface of the coated paper can cause problems in printing processes in which the paper is used.

EP-A-0565691 discloses a method whereby a clay coating pigment is treated with a primary, secondary or tertiary amine or with a quaternary ammonium compound, each of which has at least one hydrocarbon group having from 8 to 30 carbon atoms. An example of a dispersing agent which is useful with surface treated kaolin clay is sodium dodecyl sulphate.

According to a first aspect of the present invention there is provided a coating composition for preparing a coated cellulosic sheet material, which composition comprises a particulate paper coating pigment, an adhesive, a dispersing agent for the pigment, and a surfactant.

The coating composition is preferably a paper coating composition.

The particulate paper coating pigment may be, for example, an inorganic pigment such as kaolin or china clay, a calcined kaolin, natural or precipitated calcium carbonate, calcium sulphate, satin white, talc, or a blend of any two or more of these. The pigment will normally be present in the coating composition in an amount of at least about 45 by weight, based on the total weight of the composition, and will normally be present in an amount no greater than about 70% by weight.

The amount of adhesive solids present in the composition will vary according to the type of adhesive used and the type of process by which the coated paper is to be printed, but will generally be in the range of 3% to 20% by weight, based on the dry weight of the pigment. Preferably, the adhesive is a latex in which case it is preferred that the quantity of the latex used is such that the amount of latex solids is in the range from 3% to 6% by weight, based on the weight of dry pigment. The latex may be, for example, a styrenebutadiene latex or an acrylic latex, which may or may not be of the alkali-swelling type. The latex to be employed will normally be one which is in the form of an aqueous emulsion containing about 40k to 60k by weight of latex solids, preferably about 50k by weight of latex solids.Starch is another possible adhesive which may be used in the coating composition of the invention.

The surfactant is preferably anionic. Preferably, it has at least 8 carbon atoms, more preferably at least 12 carbon atoms in its hydrophobic portion.

Examples of suitable surfactants are sodium dodecyl sulphate and sodium slphosuccinate dioctyl ester.

The dispersing agent may be of the type conventionally used in coating compositions for cellulosic sheet material, for example, inorganic dispersing agents such as the water soluble salts of polyphosphoric acid or of polysilicic acid, or polycarboxylate dispersing agents such as the water soluble salts of poly(acrylic acid) or of poly(methacrylic acid). Mixtures of different dispersing agents are also operable. Typically, the amount of the dispersing agent used will be at least 0.05% by weight, based on the weight of dry pigment, and will preferably be no greater than 0.5t by weight, based on the weight of dry pigment, preferably from 0.1k to 0.4W by weight.

It has surprisingly been discovered that the improvement in the print quality obtained by the gravure process on a sheet material coated with a composition in accordance with the invention is at least as good as the improvement which is obtained when a sheet material coated in accordance with the invention disclosed in EP-A-0565691 is used. The present invention provides an alternative coating compositions to those of the type which contain an inorganic pigment which has been surface treated with an amine or quaternary ammonium compound.

It has been found by the inventors that, in some circumstances, the addition of the surfactant to the coating composition will result in the generation of foam in the composition, with resultant poor runnability on the paper coating machine, and poor print performance of the coated material. The addition of an anti-foaming agent to the coating composition has been found to alleviate the foaming problem without impairing the capability of the composition to provide a coated sheet material which will give excellent print quality when printed by the gravure process. Antifoaming agents are well known materials and are exemplified in Chapter 7 (Foaming and Anti foaming by Aqueous Solutions of Surfactants) of the book "Surfactants and Interfacial Phenomena", Second Edition, by Milton J. Rosen, John Wiley & Sons., New York, 1989.Amongst those presently preferred are those comprising finely divided hydrophobic silica dispersed in a silicone oil. Other suitable antifoamers are those comprising an aqueous emulsion of silicone oil together with other additives.

The minimum amount of anti-foaming agent necessary to avoid foaming will normally be used, in view of the cost of this ingredient. Typically, this will be in the range of from 0.01% to 1.0% by weight, based on the weight of dry pigment.

According to a second aspect of the present invention, there is provided a process for preparing a coating composition for use in preparing a coated cellulosic sheet member, which comprises the step of combining, in an aqueous medium, a paper coating pigment, a dispersing agent for the pigment, an adhesive, a surfactant and optionally an anti foaming agent.

Preferably, in this process aspect of the invention, the components of the composition are mixed together with the use of some agitation to ensure a homogenous mixture.

According to a third aspect of the present invention, there is provided a coated cellulosic sheet member prepared by applying a coating composition in accordance with the first aspect of this invention to a surface of a base cellulosic sheet member and permitting said coating to dry.

More details of the manner in which coated sheet members may be prepared can be found in Chapter 1 ("Mineral Pigmented Coatings") of the book "Coating Equipment and processes" by George L. Booth, Lockwood Publishing Co.m Inc., New York, 1970.

Typically, the coated sheet member of the invention has a coat weight in the range of from 4 to 12mlg-1. The coated sheet member of this aspect of the invention will normally possess a coefficient of friction less than 0.3, although this figure can vary somewhat with base paper type, coat weight and other factors.

According to a fourth aspect of the present invention, there is provided a gravure printing process in which an image is printed onto the surface of a coated sheet member in accordance with the third aspect of the present invention. The gravure printing process is described in general terms at pages 42-52 of the book "The Printing Ink Manual", 5th Ed., edited by R.H.

Leach and R.J. Pierce, Chapman and Hall, London, 1993.

According to a fifth aspect of the present invention, there is provided the use of a surfactant as an additive to a coating composition for a cellulosic sheet material, for the purpose of improving the print quality obtained by the gravure process on the sheet material coated with said composition.

The invention will now be illustrated by reference to the following examples.

EXAMPLE 1 Three paper coating compositions (A and C comparative and B being in accordance with the invention) were prepared according to the recipes given in Table 1 below: Table 1

Composition A B C Clay 1 85 100 100 Talc 1 15 0 0 Sodium dodecyl sulphate 0 1.0 0.5 Antifoamer 1 0 0.2 0 Antifoamer 2 0 0 0.2 Styrene butadiene latex 5.0 5.0 5.0 adhesive Sodium hydroxide to give pH 8.5 Water to give 57.0k by weight solids The amount of each pigment is given in parts by weight of the total pigment.The amount of the latex adhesive is given as the percentage by weight of latex solids, based on the weight of the dry pigment. The amounts of sodium dodecyl sulphate and antifoamers is the percentage by weight, based on the weight of the dry pigment.

Clay 1 was a kaolin clay having a platey particle shape (an aspect ratio of about 50) and a particle size distribution such that 65% by weight consisted of particles having an equivalent spherical diameter smaller than 2ym and 40% by weight consisted of particles having an equivalent spherical diameter smaller than lym.

Talc 1 was a paper coating grade talc having a particle size distribution such that 35k by weight consisted of particles having an equivalent spherical diameter smaller than 2ym and 15% by weight consisted of particles having an equivalent spherical diameter smaller than lym.

Antifoamer 1 was a suspension of fine silica in a silicone oil, marketed under the trade name "MSA".

Antifoamer 2 was marketed under the trade name "FOMEGON 212", and comprised a mixture of a hydrotreated naphthenic oil, ethylene bis-stearamide, a chlorinated paraffin and hydrophobic silica, supplied as a 15 active emulsion in water.

The compositions were applied to a rotogravure base paper of weight 39g.m2, using a laboratory coating machine of the type described in British Patent Specification No. 1032536 at a paper speed of 400m.min1 and a blade holder angle of 45 . Coatings were applied to sheets of the base paper at different weights per unit are in the range from about 5 to 10g.m~2 by adjusting the loading on the blade.Each sheet of coated paper was conditioned for 24 hours at 230C and 50% relative humidity, and were then calendered by 10 passes through a Perkins laboratory supercalender at a temperature of 650C, a pressure of 69 bar and a speed of 36m.min~l. The sheets of calendered coated paper were tested for gloss, coefficient of friction, gravure print quality (% missing dots) and print density, the results in each case being plotted graphically against coat weight, and the result corresponding to a coat weight of 7g.m2 being found by interpolation. The results obtained are set forth in Table 2 below: Table 2

Composition Gloss Coefficient % Missing Print of Friction Dots Density A 58 0.235 1.5 1.94 B 58 0.250 1.4 1.90 C 58 0.264 1.5 1.84 EXAMPLE 2 Three paper coating compositions (D and F comparative, E being in accordance with the invention) were prepared according to the recipes given in Table 3 below:: Table 3

Composition D E L F Clay 1 65 65 52 Clay 2 35 35 28 Talc 1 0 0 20 Sodium dodecyl sulphate 0 1.0 0 Antifoamer 3 0 0.1 0 Styrene-acrylic (alkali-swellable) 5.0 5.0 5.0 latex adhesive Sodium hydroxide to give pH 8.5 Water to give 57.0k by weight solids Clay 2 was a kaolin clay having a particle size distribution such that 80k by weight consisted of particles having an equivalent spherical diameter smaller than 2ym and 62k by weight consisted of particles having an equivalent spherical diameter smaller than lym.

Antifoamer 3 was a 30% active emulsion in water of a silicone based compound.

These compositions were coated on to base paper, and the coated sheets conditioned, calendered and tested as described in Example 1 above.

The results of the tests which corresponded to a coat weight of 7g.m2 were found by interpolation, except for the coefficient of friction results which were each found by measurements at a coat weight which was as near as possible to 8g.m2. The results obtained are set forth in Table 4 below:: Table 4

Composition Gloss Coefficient Missing Print of Friction Dots Density D 50 0.263 2.7 1.95 E 53 0.231 1.9 1.81 F I 51 0.243 2.5 1.94 EXAMPLE 3 Four paper coating compositions (G comparative, H, I and J being in accordance with the invention) were prepared according to the recipes given in Table 5 below Table 5

Composition G H I J Clay 1 100 100 100 100 Sodium dodecyl sulphate 0 1.0 1.0 1.0 Antifoamer 3 0 0 0.1 0 Antifoamer 4 0 0 0 0.15 Styrene butadiene latex 5.0 5.0 5.0 5.0 adhesive Sodium hydroxide to give > pH 8.5 Water to give 57.0k by weight solids Antifoamer 4 was a 100% active silicone compound.

The results of the tests which corresponded to a coat weight of 7g.m~2 were found by interpolation and are set forth in Table 6 below: Table 6

Composition Gloss Coefficient k Missing Print of Friction Dots Density G 54 0.274 3.5 1.87 H 56 0.271 2.5 1.81 I 56 0.235 2.3 1.83 J 56 0.246 2.5 1.83 EXAMPLE 4 Two paper coating compositions (K comparative and L in accordance with the invention) were prepared according to the recipes given in Table 7 below:: Table 7

Composition | K | L Clay 1 100 100 Sodium dodecyl sulphate 0 1.0 Antifoamer 1 0 0.2 Low molecular weight 0.25 0.25 acrylic thickener Sodium hydroxide to give pH 8.5 Water to give 57.02 by weight solids The compositions were coated on to a rotogravure base paper of weight per unit area 34g.m2 using the same laboratory coating machine as described in Example 1 at a paper speed of 1400m.min-1 and at a blade holder angle of 450. The coated sheets were conditioned, calendered and tested as described in Example 1 above.

The results of the tests which corresponded to a coat weight of 7g.m2 were found by interpolation and are set forth in Table 8 below: Table 8

Composition Gloss Coefficient k Missing Print of Friction Dots Density K 48 0.262 3.2 1.97 L 50 0.247 2.2 1.83

Claims

CLAIMS:

1. A coating composition for preparing a coated cellulosic sheet material, which composition comprises a particulate paper coating pigment, an adhesive, a dispersing agent for the pigment, and a surfactant.

2. A coating composition according to claim 1, wherein the particulate paper coating pigment is an inorganic pigment selected from a kaolinitic clay, a calcined kaolinitic clay, natural or precipitated calcium carbonate, calcium sulphate, satin white, talc, or a mixture of any two or more thereof.

3. A coating composition according to claim 1 or 2, wherein the pigment is present in the coating composition in an amount of from 45% to 70% by weight, based on the total weight of the composition.

4. A coating composition according to claim 1, 2 or 3, wherein the quantity of adhesive present in the composition is such that the amount of adhesive solids is from 3t to 20k by weight, based on the dry weight of the pigment.

5. A coating composition according to any preceding claim, wherein the adhesive is a latex

6. A coating composition according to claim 5, wherein the quantity of latex used is such that the amount of latex solids is in the range from 3% to 6t by weight, based on the weight of dry pigment.

7. A coating composition according to any preceding claim, wherein the surfactant is an anionic surfactant.

8. A coating composition according to claim 7, wherein the anionic surfactant has form 8 t 12 carbon atoms in its hydrophobic portion.

9. A coating composition according to claim 8, wherein the surfactant is selected from sodium dodecyl sulphate and sodium sulphosuccinate dioctyl ester.

10. A coating composition according to any preceding claim, wherein the dispersing agent is an inorganic dispersing agent or a polycarboxylate dispersing agent.

11. A coating composition according to claim 10, wherein the amount of the dispersing agent is in the range of from 0.05% to 0.5% by weight, based on the weight of dry pigment.

12. A coating composition according to any preceding claim, further comprising an anti-foaming agent.

13. A coating composition according to claim 12, wherein the anti-foaming agent is a finely divided hydrophobic silica dispersed in a silicone oil.

14. A coating composition according to claim 12 or 13, wherein the amount of anti-foaming agent employed is in the range of from 0.01% to 1.0% by weight, based on the weight of dry pigment.

1S. A process for preparing a coating composition for use in preparing a coated cellulosic sheet member, which comprises the step of combining, in an aqueous medium, a paper coating pigment, a dispersing agent for the pigment, an adhesive, a surfactant.

16. A process according to claim 15, further comprising an antifoaming agent.

17. A process according to claim 15 or 16, wherein the components of the composition are mixed together with agitation to ensure a homogenous mixture.

18. A coated cellulosic sheet member prepared by applying a coating composition as claimed in any one or more of claims 1 to 14 to a surface of a base cellulosic sheet member and permitting said coating to dry.

19. A gravure printing process in which an image is printed onto the surface of a coated sheet member as claimed in claim 18.

20. The use of a surfactant as an additive to a coating composition for a cellulosic sheet material, for the purpose of improving the print quality obtained by the gravure process on the sheet material coated with said composition.

21. A coating composition for preparing a coated cellulosic sheet material as claimed in claim 1, substantially as hereinbefore described, with reference to the accompanying examples.

22. A process for preparing a coating composition as claimed in claim 5, substantially as hereinbefore described, with reference to the accompanying examples.