NO133905B - - Google Patents

Description

The present invention relates to pigment dispersions for production

of paper with a high gloss, using a calcium-carbonate pigment based on a natural chalk.

The different types of coated paper produced today are

mainly coated with an aqueous dispersion which essentially comprises an adhesive, also known as a binder, and a pigment. A discussion of the constituents of paper coating compositions and of methods of applying such compositions to paper will be found in Chapter XIX, Volume III, of the 2nd Edition of a book of

James P. Casey entitled “Pulp and Paper, Chemistry and

Technology".

The binders used can e.g. be starch, casein or a synthetic latex, as the particular binder used will likely depend on the printing method to be used. For example

offset lithography requires the binder to be water insoluble. Usually, the pigment will consist of kaolin clay as the main component

as well as smaller amounts, i.e. less than 20 percent by weight of one or more components, e.g. calcium carbonate, zinc white, barium sulphate, titanium pigments, talc or calcium sulphate. The calcium carbonate pigment that is usually used for this purpose is known as field calcium carbonate and is produced by calcining limestone with the formation of carbon dioxide gas and slaked lime, after which the slaked lime is slaked with water to form milk of lime which is treated in various ways, e.g. by recombining with carbon dioxide to give the precipitated calcium carbonate.

A possible source for calcium carbonate pigment is natural chalk. "Natural chalk" means naturally occurring calcium carbonate formed in the geological Cretaceous period from remains of coccoliths. Natural chalk consists of calcite crystals in the form of particles which mainly have an equivalent spherical diameter in the range from 1 to 5 µm, and as it is extracted these particles are loosely bound together during formation

of larger particles which will be different from feldspar calcium carbonate and from other calcium carbonate deposits, such as crushed limestone, marble or oyster shell. The usually commercially available natural chalk tends to produce coatings with reduced gloss. This has been found to be partly due to the fact that it usually contains no more than 35% by weight of particles smaller than 2 µm.

It has now been found that it is possible to obtain a calcium carbonate pigment containing a greater percentage of finer particles, i.e. particles smaller than 2 µm, by treating a natural chalk, and that by using such a treated natural chalk it is possible to produce a paper coating mixture using significant amounts of calcium carbonate pigment in the form of the thus treated natural chalk without adversely affecting the gloss properties of the coated paper.

The present invention thus relates to a pigment dispersion for the production of coated paper with a high gloss, consisting of a calcium carbonate-containing pigment in an aqueous medium containing a binder, and the peculiarity of the pigment dispersion according to the invention is that the calcium carbonate pigment consists of a natural chalk which has been treated so that it contains at least 60% by weight, and preferably 70 to 95% by weight, of particles with an equivalent spherical diameter of less than 2 µm, at most 5 % by weight of particles with an equivalent spherical diameter of more than 10 µm and at most 0.05 % by weight of particles larger than 53 µm.

The significant and surprising feature of the present invention is the fact that natural chalk that has been treated in the specified manner can be mixed in rather large quantities into pigment dispersions based on kaolin clay and/or feldspar calcium carbonate or can be used alone with retention or only an insignificant reduction in gloss in relation to e.g. the gloss achieved by the use of high quality kaolin clay, while at the same time surprising improvements are achieved in the coated paper in terms of ink absorption,

whiteness and reduced binder requirement.

The treated natural chalk used in the present invention

is also significantly cheaper to manufacture than a field calcium carbonate with a similar particle size analysis.

The treated natural chalk used in the pigment dispersions according to the present invention can be produced by finely crushing a natural chalk, e.g. when grinding in a regular ball mill, when grinding with sand or when grinding in a jet mill,

i.e. a micronizer, or by particle size classification of a natural chalk to separate from unwanted coarse material.

The treated natural chalk used in the pigment dispersions according to the present invention will normally be used together with a kaolin clay pigment with retention or achievement of the advantages mentioned above.

The binder used in the pigment dispersions according to the present invention can be a common binder, e.g.

a starch, gelatin, casein or a synthetic latex. The pigment dispersions according to the present invention can also contain common additives and can be applied to a paper substrate using common equipment for the production of coated paper.

After coating, the paper sheets are subjected to a super calendering treatment to achieve the desired surface properties.

The invention will be illustrated by the following examples:

Example 1

Samples of a normal wood-free paper intended for tow knife coating and manufactured by Alex Pirie Ltd. was coated using the drag knife method with pigment dispersions to give a coating weight of 10 g/m 2 in each case. In a test series, the dispersions contained a pigment

comprising 50 percent by weight kaolin clay ("Dinkie A") and 50

weight percent commercially available natural calcium carbonate containing 32 weight percent particles larger than 10 µm and 0.05 weight percent particles larger than 53 µm. In another test series, the dispersion contained a pigment comprising 50% by weight of kaolin clay (Dinkie A") and 50% by weight of a treated natural chalk (in accordance with the present invention) containing 80% by weight of particles having an equivalent sphere diameter of less than 2µm, 2% by weight of particles which had an equivalent sphere diameter greater than 10 µm and 0.02 weight percent particles greater than 53 µm.

The treated natural chalk pigment was produced by deflocculation of a slurry of natural chalk and subjected to particle size classification by direct sedimentation methods and separation of coarser particles.

The binders used in the dispersions are indicated in the heading of Table I below. In each case, the amount of binder used is indicated in percentage by weight on the basis of total pigment used.

After coating, the sheets were super calendered in a laboratory super calender comprising a metal roll and a filled roll. The sheets were attached to the metal roller and subjected to 10 passes at 85°C

and a linear pressure of 45 kg per linear cm. This super calendering treatment corresponds approximately to 10 passes at 85°C and at a linear pressure of 90 kg per linear cm on a normal working super calender.

The numbers in table I below are a measure of the gloss in the coated ones

sheets in TAPPI gloss units.

Example 2

In this example, samples of the same wood-free paper as used in example 1 were coated using an air knife to in each case a o give a coating weight of o 20 g/m 2. In a series of samples, the dispersion contained a pigment consisting of 50 percent by weight of kaolin clay ("Dinkie A") and 50 percent by weight commercially available natural chalk as used in Example 1'. In another series of samples, the dispersion contained 50 weight percent kaolin clay ("Dinkie A") and 50 weight percent natural chalk (treated in accordance with the invention) containing 75 weight percent particles with an equivalent sphere diameter less than 2 µm. 3 weight percent particles with an equivalent spherical diameter greater than 10 µm and 0.03 weight percent particles greater than 53 µm.

The binder used in the dispersions is indicated in the heading of Table II below. In each case, the amount of binder used is given as a percentage by weight based on the total amount of pigment.

After coating, the sheets were super calendered in the manner described in example 1 by 10 passes on a laboratory super calender at 85°C and at a linear pressure of 45 kg per linear cm.

The numbers in Table II below are a measure of the gloss of the coated sheets in TAPPI gloss units.

Example 3

In this example, samples of the same wood-free paper were used

in example 1 coated using a "Massey Coater" to give a coating weight of 10 g/m in each case. In one series of samples the dispersion contained a pigment consisting of 50% by weight kaolin clay ("Dinkie A") and 50% by weight commercially available natural chalk as used in Example 1. In another series of samples the dispersion contained a pigment consisting of 50% by weight kaolin clay ("Dinkie A") and 50% by weight natural chalk (treated according to the invention) containing 82% by weight particles. with an equivalent sphere diameter less than 2µm, 2% by weight particles with an equivalent sphere diameter greater than 10µm and 0.01% by weight particles greater than. 53^, approx. The treated natural chalk was prepared by passing it through a micronizer.

The binder used in the dispersions is indicated in the heading

for Table III below. In each case, the amount of binder used is indicated as a percentage by weight based on the total amount of pigment.

After the printing, the sheets were supercalendered in the manner described in example 1 with 10 passes on a laboratory supercalender at 85°C and at a linear pressure of 45 kg per linear cm.

The numbers in table III below are a measure of the gloss in the coated ones

sheets in TAPPI gloss units.

Example 4

In this example, the same wood-free paper was used as in the example

1 coated using a "Champion" knife to give a coat-

weight in each case of 10 g/m p. In a series of samples, the dispersions contained a pigment comprising 50 percent by weight of kaolin clay

.("Dinkie A") and 50 weight percent commercially available natural chalk

as used in Example 1. In another test series contained

the dispersion 50% by weight kaolin clay ("Dinkie A") and 50% by weight treated natural chalk (according to the invention) containing 70% by weight particles with an equivalent spherical diameter less than 2 µm, 3% by weight particles with an equivalent spherical diameter

larger than 10 µm and 0.03 weight percent particles larger than 53 µm.

The treated natural chalk was produced by painting in an ordinary

ball mill.

The binder used in the dispersions is indicated in the heading

for Table IV below. In each case, the amount of binder used is indicated as a percentage by weight based on the total amount of pigment.

After coating, the sheets were super calendered as is

described in example 1 by 10 passes on a laboratory

supercalender at 85°C and at a linear pressure of 45 kg per linear cm.

The figures in Table IV below are a measure of the gloss in the coated ones

sheets in TAPPI gloss units.

Although the latex used in all the preceding examples was a styrene-butadiene latex manufactured by Dow Chemical Co. Ltd., it should be noted that other types of latex normally used in paper coating, can be used, for example, "those based on .

".acrylic polymers or polyvinyl acetate"

Example 5

In this example, samples of the same wood-free paper "as used" in example 1 were coated using the trailing blade method

with dispers ions -f or _to 'give?--©! coating weight in each case on

2 10 g/m. r ea-pr.practice series. the dispersion contained a pigment comprising: 75% by weight kaolin clay ("Dinkie A") and 25% by weight

commercially available natural chalk as used in example 1.

In another test series...the dispersion contained a .pigment comprising 75 weight percent kadii'nieire •(■"Dinkie'A," ) and 25 weight percent ."-

natural chalk (according to the invention) containing 80% by weight of particles with an equivalent spherical diameter of less than 2 µm, 2 % by weight of particles with an equivalent spherical diameter of more than 10 µm

and 0.02 weight percent particles larger than 53 µm. The treated natural chalk was prepared by deflocculation from a natural chalk slurry and subjected to a particle size classification. • by direct sedimentation methods during the removal of coarser...

particles. ■

The binder used in the dispersions is indicated in the heading of Table V below. In each case, the amount of binder used is indicated in percent by weight based on the total amount of pigment.

After coating, the sheets were supercalendered in the manner described in Example "1 by 10 passes on a 1 laboratory super-calender at 85°C, and at a linear pressure of 45 kg per linear cm.

The numbers in Table V below are a measure of the gloss of the coated sheets in TAPPI units.

Example 6

The conditions and the pigments used are the same as in example 5 with the exception that both the mixtures containing natural chalk and those containing treated natural chalk consisted of 25 weight percent kaolin clay ("Dinkie A") and 75 weight percent chalk.

The binders used and the super calendering conditions were the same as in Example 5.

The numbers in Table VI below are a measure of the gloss of the coated sheets in TAPPI units.

EXAMPLE VII

Experiments were further conducted to compare two calcium carbonate paper coating pigments by mixing each pigment into a paper coating mixture which was then coated on a base paper using a standard method and conditions; The first pigment was a treated natural chalk pigment and the second pigment was a commercially available field calcium carbonate pigment manufactured and sold by John & E. Sturge Ltd. under the trade name "CALOPAKE PC". The properties of those

two calcium carbonate pigments are shown in the following Table VII.

The procedure for preparing the paper coating mixture and coating the paper was as follows: 1 kg of the calcium carbonate pigment was thoroughly mixed with 440 ml of water containing 0.25 percent by weight, based on the weight of dry pigment, of a sodium polyacrylate dispersant with a number average molecular weight of 1650. More water was then added to set the final total solids content of the paper coating mixture to 62% by weight and then 5% by weight, based on the weight of dry pigment, of an oxidized potato starch manufactured and sold by Starch Products Limited under the trade name "VISCOSOL 310" was stirred in. The mixture was then heated to a temperature of 85°C for 10 minutes to boil the starch and then allowed to cool below 40°C. Sufficient of a styrene-butadiene rubber latex containing 50 weight percent solids and known as "Dow Latex No. 620" was then incorporated to yield 7 weight percent latex solids, based on the weight of dry pigment, and the entire paper coating mixture was then sieved through a 100 raesh B.S. sieve (nominal opening 152^um). The paper coating mixture was then tested for viscosity using a Brookfield Viscometer and weight percent solids determined. The mixture was coated on a normal wood-free trailing blade base paper, manufactured by Alex Pirie Ltd., at a temperature in the range of 20-25°C using the laboratory paper coater known as "HELI-COATER" and described in British Patent No. 1,032,536. Speed and blade pressure were adjusted in accordance with the viscosity of the finished mixture1 to give a constant coating weight of approximately 10 g per m 2. The coated paper was dried and calendered in a laboratory calender where the conditions were 10 passes at 70°C and a roller pressure of approx. 90 kg per linear cm. The properties of the coated paper are set forth in the following Table VIII.

In the printing ink microcontour test, an excess of a non-absorbing printing ink is applied to the surface of the paper using a roller applicator so that the surface is completely covered.

The ink-coated paper is then rubbed off with a soft rag or cotton and the ink remains in any surface holes or irregularities that may be present. By measuring the whiteness of the ink-treated sheet in comparison with a non-ink-coated sheet of the same paper, it is possible to obtain a figure for the roughness of the paper. The results of the test represent the difference in whiteness between the non-ink-coated sheet (given a value of 100.0) and the ink-coated sheet. The greater this difference, the greater the roughness of the paper.

At the so-called I.G.T. detachment test, a strip of the coated paper is clamped to move by the action of a falling pendulum over the surface of a freely rotatable roller which is covered with a film of a standard oil and which is pressed against the paper with a standard pressure. The pendulum moves with an ever-increasing speed and the speed of the pendulum at which the coating is first torn off from the base paper by means of the oil film is determined. The speed gives a measure of the binder requirement for the pigment. The higher the speed, the less binder is required.

The felt calcium carbonate "CALOPAKE PC", which is a typical material for paper coating purposes, gives a poorer gloss and uniformity compared to the natural chalk and a somewhat higher binder requirement.

It would be possible to use a calcium carbonate field with a coarser particle size distribution and the gloss would then be much worse, but the binder requirement would be lower.

EXAMPLE VIII

Samples of treated natural chalk were prepared with different particle size distributions as shown in the following Table IX. Paper coating compositions comprising each of the above samples were prepared and each composition was coated on the same base paper using the technique described in Example 7. The gloss of the coated sheet was measured in each case and the results are shown in Tables X, XI and XII.

It is clear from the above table that the gloss increases

fast when the proportion of particles smaller than 2 µm is in the range 70 - 80% and is several units higher than when the proportion of particles smaller than 2 µm is in the range 50 to 60%.

TABLE XI

Effect of volume fraction of particles larger than lOyUm.

It can be seen from the table that when the quantity proportion of particles greater than

lOyUm rises above 4 - 5% there is a clear reduction in gloss.

When the proportion of particles larger than 53 µm is less than 0.05% by weight, the gloss remains approximately constant, but when this proportion is over 0.05%, the gloss drops considerably.

Claims (1)

Pigment dispersion for the production of coated paper with a high gloss, consisting of a calcium carbonate-containing pigment in an aqueous medium containing a binder, characterized in that the calcium carbonate pigment consists of a natural chalk that has been treated so that it contains at least 60 percent by weight, and preferably 70 to 95 percent by weight, particles with an equivalent spherical diameter less than 2 µm, at most 5 percent by weight particles with an equivalent spherical diameter greater than 10 µm and at most 0.05 percent by weight particles larger than 53 µm.