Method for preparing a coating composition

The present invention relates to a method for preparing a coating composition for a web comprising papermaking fibers, the coating composition comprising an aqueous first component whose mineral part comprises calcium carbonate and whose solids content is low, and at least a second component.

A conventional method for preparing a coating paste is to make a slurry containing mineral particles whose dry solids content is as high as possible. The dry solids content is restricted by the fact that the slurry must remain pumpable. When several different mineral particles are used in the coating paste, the principle is the same: each slurry is prepared separately before mixing them together. The dry solids content of a calcium carbonate containing paste is typically approximately 60 wt.-%.

A problem related to the use of calcium carbonate is that the dry solids content must be raised high, which is complicated and expensive.

The method of the invention is characterized in that the coating composition is prepared so that the second component is dispersed in a dry state in the first component in an amount which raises the solids content of the coating composition to a level between 40 and 80 wt.-%.

In the present application, the following expressions are used:

- Aqueous first component: The first component is an aqueous slurry.

- The solids content is low: The solids content is too low to achieve the desired solids content of the coating composition. Typically, the solids content is lower than 40 wt.-%.

- The second component is dispersed in the dry state: The second component is dispersed in the form of a dry powder. However, the moisture content of the powder may typically be from 0 to 20 wt.-%.

In the method of the invention, a slurry having a low solids content and containing calcium carbonate is mixed with at least one second component. The second component is added in a dry state and thus, the solids content of the slurry increases. This method is quite inexpensive compared to the conventional method, and the quality of the coated paper is about the same as by using the conventional method.

The second component may comprise at least one organic or inorganic pigment, or mixtures of organic pigments, or mixtures of inorganic pigments, or mixtures of organic and inorganic pigments. For example, the second component may comprise ground calcium carbonate, kaolin, talcum, precipitated calcium carbonate, gypsum, titanium dioxide, or mixtures of those.

In addition to the pigments, the coating slurry usually contains at least one binder and auxiliary agents, such as optical brighteners or rheological modifiers.

It is possible that the coating composition is prepared so that the second component is dispersed in the dry state in the aqueous first component having the low solids content so that a first slurry forms, and a second slurry is prepared by the conventional method and mixed with the first slurry in order to prepare the final coating composition. It is also possible that there is more than one pigment dose which is dispersed in the dry state. Further, it is possible that more than one conventional slurry is added to the aqueous first component.

Example.

Five coating trials were made in order to find out the effect of the method for preparing the coating composition (coating colour) on the final coating quality.

Colours 1 and 3 were reference samples, and colours 2, 4 and 5 were according to the invention.

When preparing the reference coating compositions, each pigment was in a slurry having a high solids content, and all the slurries were mixed together.

When preparing the coating compositions of the invention, i.e. colours 2, 4 and 5, calcium carbonate was present in a slurry having a low solids content of 35 wt.-%. Dry clay was added to the low solids content slurry, and after that the slurry was mixed for 30 minutes.

As one can see from table 1 , all the colours included 25 wt.-% of GCC. The GCC was a commercial slurry. The target solids content of the ready-made coating composition was 62 wt.-%.

Table 1. Coating compositions.

GCC = ground calcium carbonate, the median particle size is typically from 0.4 to 1 .5 μm (Sedigraphe 5100), and the solids content of the slurry is from 70 to 80 wt.-%.

PCC 1 and PCC 2 = precipitated calcium carbonate having a mean particle size from 0.4 μm to 1.5 μm, and the solids content of the slurry is from 65 to 75 wt.-%.

PCC 1 , low solids and PCC 2, low solids = the same PCCs as above but the solids content of the slurry is typically at the highest 50 wt.-%.

SB latex = styrene/butadiene latex CMC = carboxy methyl cellulose OBA = optical brightener Ca-stearate = calcium stearate

As one can see from table 2 below, there was not much difference in the quality of the paper coated by the coating compositions prepared according to the method of the invention compared to the coating compositions prepared according to the conventional method. Thus, it is possible to apply an easier and more cost-effective method in preparing the coating compositions.

Table 2. Coating trials.

1 ) Mean value of two sides of a paper