JPH0240797B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a dispersant for pigments for coated paper. For more information, see Alkali metal salts of acrylic acid.
A copolymer of 96 to 98% by weight and 4 to 2% by weight of acrylamide, characterized in that the viscosity of a 40% by weight aqueous solution of the copolymer at 25°C is in the range of 2 to 20 poise. This invention relates to a dispersant for pigments for coated paper. Conventionally, coated paper such as art paper and coated paper has been produced by adding a certain type of dispersant to pigments such as kaolin clay, calcium carbonate, titanium oxide, and satin white and dispersing them in water, and then using synthetic rubber latex as a pigment binder. , a coating liquid is prepared by adding synthetic resin emulsion, starch, casein, polyvinyl alcohol, etc., and, if necessary, a water-resistant agent such as melamine resin, urea resin, glyoxal, polyamide polyurea-formaldehyde resin, etc.
It is manufactured by applying the liquid to base paper, then drying it, and then subjecting it to mechanical processing such as supercalendering. In the production of such coated papers, there is a tendency today to use mixtures of various pigments in order to obtain the best printability effect, and in order to produce coated papers as economically as possible. A highly concentrated pigment dispersion with low water content is desired as a coating liquid. Furthermore, regarding coating methods, there is a tendency to use ultra-high-speed coating machines such as blade coaters in addition to conventional coating methods. Based on these trends, the following characteristics are required of dispersants for coated paper pigments. Demonstrates good dispersion effect for a wide variety of pigments. In terms of economy and physical properties of coated paper, a dispersion effect can be obtained with a smaller amount added, and a pigment dispersion with low viscosity and high concentration can be prepared. The viscosity of the pigment dispersion has excellent stability over time. Organic polymer dispersants, such as alkali metal salts or ammonium salts of polyacrylic acid such as sodium polyacrylate or ammonium polyacrylate, are relatively excellent dispersants that meet these required characteristics. It is known to be a dispersant. However, the above-mentioned required characteristics are still not fully satisfied. In other words, sodium polyacrylate does not provide a low-viscosity dispersion for certain pigments, such as heavy calcium carbonate, and the stability of its viscosity over time is also poor.On the other hand, ammonium polyacrylate Although this improves the water resistance of coated paper, there is a problem in that the dispersibility itself is poor when actually used as a dispersant for pigments. In view of this, the present inventors have conducted deeper research into dispersants that can meet the various required characteristics described above, and have found that they are copolymers of alkali metal salts of acrylic acid and acrylamide in a specific ratio. The inventors have discovered that substances whose viscosity in aqueous solution is within a specific range are extremely effective, and have completed the present invention. That is, the present invention is a copolymer of 96 to 98% by weight of an alkali metal salt of acrylic acid and 4 to 2% by weight of acrylamide, and the viscosity at 25°C of the copolymer at a concentration of 40% by weight is 2.
By using a dispersant in the range of ~20 poise as a dispersant for pigments for coated paper, it has an extremely superior dispersion effect than the conventionally known alkali salt of polyacrylic acid alone, that is, it can be applied to a wide range of pigments. It has been found that pigment dispersions and coating liquid compositions with high concentration, low viscosity, and excellent stability without thickening over time can be obtained. The basic structure of the copolymer in the present invention is that a relatively small amount of acrylamide is copolymerized with an alkali metal salt of acrylic acid, and the copolymerization ratio is extremely important to obtain the effects of the present invention. .
When the proportion of acrylamide is less than 2% by weight, no significant improvement in the dispersion effect is seen compared to conventional alkali metal salts of acrylic acid homocopolymers, and when it exceeds 4% by weight, the dispersion Although the stability over time is slightly improved, the viscosity of the dispersion itself tends to be high, which does not meet the purpose of the present invention, which is to obtain a low-viscosity dispersion.
Therefore, the copolymerization ratio of acrylamide in the copolymer must be within the range specified in the present invention. In other words, if the copolymerization ratio is 96 to 98% by weight of the alkali metal salt of acrylic acid and 4 to 2% by weight of acrylamide, a comprehensive balance can be achieved in terms of pigment dispersion effect, viscosity of the dispersion, and its stability over time. An excellent dispersion can be obtained. Another point of the present invention is the viscosity range of the aqueous copolymer solution, which needs to be 2 to 20 poise when measured with a Bruckfield viscometer at 25°C at a concentration of 40% by weight. It is preferably in the range of 4 to 15 poise. In this case, if the viscosity is less than 2 poise, the dispersing effect on the pigment is poor, and if it exceeds 20 poise, the dispersing effect tends to be similarly poor. In the copolymer of the present invention, as the alkali metal salt, a lithium salt, a sodium salt, a potassium salt, etc. may be used alone, or a mixture thereof. The copolymer of the present invention can be produced by a conventional copolymerization method, for example, by dissolving an alkali metal salt of acrylic acid and acrylamide in water, and copolymerizing in water in the presence of a water-soluble radical polymerization initiator such as potassium persulfate. can be manufactured. Hereinafter, the present invention will be explained in more detail with reference to Examples. Example 1 [Synthesis of dispersant] 96.2 g of sodium acrylate and acrylamide
3.8 g was dissolved in ion-exchanged water, and copolymerization was carried out at 90 to 95°C for 5 hours using potassium persulfate as an initiator. Solid content concentration of copolymer is 40% by weight
Thus, an aqueous copolymer solution having a viscosity of 9.8 poise at 25°C was obtained. This is called dispersant A. Copolymer aqueous solutions or homopolymer aqueous solutions having the compositions shown in Table 1 were synthesized in a similar manner.

【table】

[Table] [Preparation of pigment dispersion] Dissolve the dispersants (A to J) in a predetermined amount of water, add calcium carbonate (Hakuenka PZ, Shiraishi Calcium Co., Ltd. product) as a pigment while stirring with a lab mixer, A dispersion liquid with a pigment concentration of 65% by weight was prepared, and the viscosity of these liquids at 25°C was measured using a Bruckfield viscometer using rotor No. 1,
Measured at 60 rpm. Table 2 shows the relationship between the amount of each dispersant added and the viscosity of the pigment dispersion. Note that the amount added in the table is the solid content weight percent of the dispersant relative to the pigment. The pigment dispersion using the dispersant of the present invention had a lower viscosity and was even better.

[Table] Example 2 The same procedure as in Example 1 was carried out using heavy calcium carbonate (Whiten SSB, Shiraishi Calcium Co., Ltd.) as a pigment.
20 parts by weight, kaolin clay (ultra white)
A dispersion liquid having a pigment concentration of 70% by weight, consisting of 80 parts of 90, manufactured by Minelas Chemicals, UK) and 0.1% of a dispersant (based on the pigment), was prepared. In order to examine the viscosity stability of these dispersions, the viscosity was measured at 25° C. immediately after preparation and after standing for 24 hours using a Bruckfield viscometer with rotor No. 3 and 60 rpm. The results are shown in Table-3. The viscosity of the dispersion containing the dispersant of the present invention was low, and the stability over time was also excellent.

[Table] Example 3 To each of the pigment dispersions prepared in the same manner as in Example 2, starch (Oji Ace A, a product of Oji National Co., Ltd.) was separately heated and dissolved in water to give a concentration of 20% by weight, and a solution of styrene was added. A coating composition liquid was prepared using a butadiene latex (SW-307, manufactured by Sumitomo Nogatatsu Co., Ltd.) in the following blending ratio. The blending ratio parts are solid weight parts. [Coating liquid composition] Kaolin clay (Ultra White 90) 85 parts Calcium carbonate (Shiraenka PZ) 15 parts Dispersant (A to J) 0.25 parts Starch (Oji Ace A) 5 parts Styrene-butadiene latex (SW-
307) 10 parts Adjust the solid content concentration of this coating liquid to 60% by weight,
Each coating composition liquid was left at 25°C and its viscosity stability was examined. The results are shown in Table-4. However, the viscosity was measured using a Bruckfield viscometer with rotor No. 1 at 60 rpm. The viscosity of the coating liquid using the dispersant of the present invention was low, and its stability over time was also extremely excellent.

【table】

【table】

Claims (1)

[Claims] 1 A copolymer of 96 to 98% by weight of an alkali metal salt of acrylic acid and 4 to 2% by weight of acrylamide, and the viscosity of an aqueous solution of the copolymer at a concentration of 40% by weight at 25°C is 2 to 20 poise. A dispersant for pigments for coated paper, which is characterized by: