Classification of coating adhesives and their specific application

With the advancement of science and technology and the improvement of people’s living standards, the amount of coated paper is continuously increasing, and the range of applications and applications of coated paper are becoming more and more widespread, such as packaging, calendars, fine prints, trademarks, and gifts. This is mainly due to the unparalleled advantages and characteristics of coated papers. The properties of coated papers mainly depend on the coatings. The main components of the paints are pigments and adhesives. Due to the requirements on the coating properties and the different coating equipment, the proportions of the components are also different. The ratio of these two components is generally Pigments account for 75% to 90%, and adhesives account for 10% to 25%. Here are some brief introductions on the classification, function, properties and preparation of adhesives.

1 Classification of Adhesives

Adhesives generally fall into two categories: One is natural products, such as modified starch, casein, casein, and animal glue. The most widely used natural adhesive is modified starch, which has the characteristics of stable quality and low price. The other type is synthetic styrene commonly used styrene butadiene latex, carboxylated styrene butadiene latex, polyvinyl alcohol, polyacrylates and the like. The most widely used synthetic adhesives are carboxylated styrene-butadiene latex and polyacrylate emulsions. The former has poorer emulsion stability and is easy to reverse yellow. The latter is relatively stable, but its relative price is relatively high.

In general, it is difficult for an adhesive to achieve the required viscosity and concentration of the coating. Therefore, two or more adhesives are generally used in combination. When mixing, pay attention to the mutual influence between the adhesives. General adhesives are divided into adhesives for protein systems and adhesives for starch (carbohydrate) systems. They are used within the system and have good adaptability to each other. For synthetic resin latexes, casein is generally more adaptable than starch, but in recent years there has also been latex with good starch compatibility. In common paint formulations, two types of adhesives are used in combination, so-called "two-component" system adhesives, in order to exert their respective advantages and to obtain good coatings that can meet the printing and use requirements.

2 Properties and functions of adhesives and conditions to be used

2.1 Nature

The nature of the adhesive should be strong to the pigment's ability to bond, otherwise it will cause a decrease in whiteness and opacity, followed by a better adaptability to the pigment, and to ensure that the coating has proper ink absorption and proper plasticity to improve the calendering effect .

2.2 Role

1) As a pigment carrier, the pigment has a proper fluidity.
2) As a colloidal protective agent for pigments.
3) As a binder for pigments and pigments as well as pigments and base paper.
4) When the coated paper is printed, the absorptivity of the printed ink is controlled.

3 Commonly used adhesive properties

3.1 Starch Adhesives A white powder, mainly extracted from corn, sweet potato, maroon potato and wheat. It has no adhesive properties. Starch needs to be adhesive and must be dispersed in water to form a granular suspension. The tanning of the colloidal dispersion is carried out, and the tannage form is known as gelatinized starch. Starch used as a paste requires a solids content of 25% to 35%.

3.2 Polyvinyl alcohol with alkali or acid hydrolysis of polyethylene acetate polymer obtained, referred to as PVA, the extent of hydrolysis of carboxypolyacetic acid, known as the degree of hydrolysis. The higher the degree of hydrolysis, the lower the water solubility. According to the degree of hydrolysis, polyvinyl alcohol is divided into two grades of cold-melt and hot-melt polyvinyl alcohol. For the coating, hot-melt products with a degree of hydrolysis of 98% were used. It is a white to pale crumb with a specific gravity of 1.19 to 1.27.

3.2.1 Characteristics of Polyvinyl Alcohol 1) It is easy to film and the ink gloss is good.
2) Strong binding force to the pigment and less adhesive.
3) High whiteness, not easy to fade.
4) It is not easy to be mildewed, and its disadvantage is that it has poor water resistance and poor fluidity.

3.3 The advantage of carboxylated styrene-butadiene latex is that it can reduce the viscosity of the coating, properly reduce the stiffness of the coated paper, and it is favorable for further processing and good printability. It is currently the most widely used adhesive for coated printing paper. It is generally used together with modified starch, casein and other auxiliary adhesives to obtain coatings with satisfactory flowability, sizing, and excellent paper-forming properties.

3.4 Polyacrylate acrylate latex and styrene butadiene latex, polyester acid ethylene are similar in appearance and latex properties, have good shear stability, low viscosity, film folding resistance, flexibility and so on. Its printability is almost unaffected by the amount of coating, and it shows superior printability under each coating, that is, improves the surface smoothness of the coated paper and also improves the compressibility of the coated paper. Visible use of acrylic Ester latex improves print quality, ink absorbency, and increases print gloss.

4 Preparation of adhesives

4.1 Dissolution of casein

The casein is dissolved in a dissolver with stirrer and steam tube heating or jacket heating. In order to reduce the amount of alkaline acid, the casein can be firstly soaked with cold water to make the particles swell and change the water. 2 times to remove the residual acid. The soaked casein is placed in a dissolution pot and slowly aerated with a certain amount of water, so that the temperature reaches 45 to 50° C., the required alkali is added one by one, and the temperature is further increased to 60 to 65° C. until the granules are completely dissolved. The whole dissolution time 40 ~ 60min.

4.2 Dissolution of polyvinyl alcohol

The polyvinyl alcohol is added to the cold water when dissolved. The amount of water is 5 to 10 times its weight. After mixing, the mixture is stirred and heated. The temperature is controlled at 80-90 DEG C. After it is completely dissolved and cooled, the degree of polymerization is too high. A small amount of sodium hypochlorite solution can be added for chlorination degradation.

Reprinted from: 21st Century Fine Chemicals Network

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