CS258821B1 - Coating active pigment for the intake layer of pH-written papers - Google Patents

Abstract

The solution relates to an active pigment composition for a color developing layer of direct-copy papers, in which the color-forming reaction takes place on the surface of an active pigment consisting of 50 to 95% acid-activated bentonite and 5 to 50% precipitated silica.

Description

The invention relates to the production of an active pigment for the color developing layer of direct-copy papers, in which the color-forming reaction takes place on the surface of the active pigment.

In the previously known systems of direct-copy papers of the chemical type, the fact is used that the color-forming component (e.g. crystal violet lactone (CVL), benzoyl leucomethylene blue (BLMB), etc.), when in contact with the surface of the active pigment, reacts to form a dye. In the production of direct-copy papers, the solution of the color-forming component is usually enclosed in microcapsules, which are applied in the form of a thin coating to the underside of the sheet (so-called CB-coating) and the active pigment is applied in the form of a coating to the top side of the following sheet (so-called CF-coating).

The active pigment is obtained either by acid activation of sorted clay raw materials (containing kaolinite, montmorillonite, attapulgite, etc.), or synthetically in various forms of aluminosilicates. Activation is usually carried out by heating the raw material in an acid solution, washing, annealing at 300 to 500 °C, grinding and dispersing to prepare a coating paste.

These methods of preparing active pigments are described in the following foreign patent documents:

NSR Pat. No. 2,303,825, 2,512,448, 2,213,130

U.S. Pat. No. 3,963,852

United Kingdom Pat. No. 1,307,319.

When applying the active pigment obtained in the described manner to the receiving CF layer of direct-copy papers, some deficiencies in the properties of the coatings and the coloring of the copy are manifested, such as color shade, poor coloring in the first phase, instability of coloring and yellowing of the coating.

The listed shortcomings are eliminated by the active coating pigment prepared according to the invention, where acid-activated bentonite and precipitated silica are used to produce the pigment. Acid-activated bentonite is usually produced by dissolving bentonite raw material with water in a chain dissolver and sorting the obtained bentonite slurry into 0.3 mm mesh. The resulting suspension is activated with hydrochloric acid at constant boiling in digesters. After activation, the activated suspension is decanted and drained on filter presses. The final product is obtained by drying in a disk dryer and grinding, which is further sorted for application as a coating pigment.

Silicon dioxide is produced by precipitation of water glass with hydrochloric acid. The resulting product is filtered, washed and dried in a spray dryer. The dried product has a surface area greater than 50 m/g and electron-acceptor properties such that the acid-base indicator dimethyl yellow (p-dimethylaminoazobenzene) adsorbed on its surface from a 0.1% solution in ethanol is converted at least partially to its acid form after drying and the original yellow colour of the indicator changes to orange to carmine red (the greater the proportion of the acid form, the redder the shade). A noticeable proportion of red colour corresponds to a pH value of the solution less than 4.

It has been found that a mixture of acid-activated bentonite and silica, where the silica content in the mixture ranges from 5 to 50%, significantly increases whiteness, reduces the decrease in whiteness during heat and light aging, the intensity of the copy coloring is higher and the color of the copy is more brilliant. An important prerequisite for increasing the intensity and brilliance of the copy coloring is that the silica has a surface area greater than 50 m/g and the electron-acceptor properties of the dry surface correspond to a pH of less than 4, as stated above. As documented in the following examples, the optimal pigment composition is precisely in the above-mentioned range. None of the two pigment components (activated bentonite and silica) has by itself a set of the above-mentioned properties at such a level as their mixture.

Example

Preparation of active pigment coatings and their evaluation g of pigment is dispersed in 75 g of a 0.5% dispersant solution (Polysalz CA) at pH = = 8 to 10. Then a dispersion binder (aeronal 320 d) is added in an amount of 17% by weight of the pigment. The suspension prepared in this way is spread on paper so that the coating after drying is 5.5 to 6.5 g/m ² . Drying is carried out with a stream of hot air at a temperature of 80 to 120 °C. The paper prepared in this way is covered with paper coated with a microcapsule coating with a color-forming substance so that the microcapsule coating lies on the active pigment coating, and a copy is made in an electric typewriter. Printing is carried out as standard with type X. With the given type of color-forming substance (mixture of CVL and BLMB 3:1) we obtain a blue copy. The print intensity of the copy is evaluated with a reflection photometer (24 hours after the copy was made) as the reflectance factor R x of red light defined by the CIE filter X. (CIE - International Commission on Illumination).

The value ranges from 0 to 100% and the smaller it is, the more intense the coloring. The whiteness of the coating is evaluated using the standard procedure according to ON 50 0418 as the limiting factor of reflection of blue light defined by a filter with a maximum transmittance of 457 nm. The whiteness value is given in % of the reflection factor of an absolutely white body. The aging of the coating and the copy is carried out by heat at 100 °C for 24 hours. After 24 hours of storage in the dark and at laboratory temperature, the print intensity and whiteness of the coating are measured.

Examples of active pigment composition and its properties:

Example

Active pigment composition

Silicon oxide % Acid activated bentonite %

Note.

100

Silicon dioxide has a surface area of 120 m/g.

The silica used has a surface area of .2. m/g

Example Whiteness % Relative reduction Reflection factor Aging through sweat whiteness due to aging R _x print % heat % 1 75.2 6.9 62.7 it gets dark 2 76.5 6.1 60.1 it gets dark 3 77.2 6.0 58.2 it gets dark 4 77.8 5.5 55.5 it gets dark 5 80.2 4.2 52.8 does not change 6 83.5 1.1 50.2 fades 7 78.1 5.4 64.8 it gets dark It was found that that mixture acid activated bentonite and silicon dioxide , where the content

of silica in the mixture ranges from 5% to 50%, significantly increases whiteness, reduces the decrease in whiteness during heat and light aging, the intensity of the copy coloring is higher and the color is more brilliant. An important prerequisite for increasing the intensity and brilliance of the copy coloring 2 is that the silica has a surface area greater than 50 m /g and the electron-acceptor properties of the dry surface correspond to pH 4, as mentioned above.

Claims (1)

Coating active pigment for the uptake layer of self-copying paper, characterized in that it consists of 50 to 95% acid-activated bentonite and 5 to 50% precipitated silica whose specific surface area is greater than 50 m / g and the acid-base surface properties correspond to pH less than 4.