It has been used as a paper coating adhesive and surface sizing agent in the paper making industry for several decades.

PVA is also widely used in the preparation of thermal paper coatings. The main function of PVA in thermal coatings is to provide wetting and dispersion with adhesive properties, that is, to have the effect of polymer surface activity. PVA used in base coat, back coat, and top coat mainly acts as a adhesive, and the selected PVA types are also mostly non surface active.

1. Introduction to the coating layer of thermal paper

Thermal paper is a special type of coated processing paper, mainly used for supermarket cash receipts, queuing machine call strips, and fax machines. The main chemicals inside the coating are colorless dyes, color developers, and sensitizers. At temperatures below 70 ℃, the coating does not show color. When the fax machine receives a scanning signal, the heat head of the fax machine generates an electric pulse in an instant, heating the thermal coating layer. The colorless dye and the color developing agent undergo a chemical reaction by melting under heat, causing the lactone ring structure of the colorless dye to crack and change from colorless to colored. The transmitted image and text are displayed.

To produce high-quality three proof thermal paper, the first step is to prepare the coating chemicals through grinding equipment. When producing three-proof thermal paper, it is generally necessary to apply four coating stations: base coat, thermal coating, top coating, and back wetting.

2. Classification of PVA

According to the influence of PVA structure on surface activity, it can be divided into the following types: ① fully alcoholized PVA without surface activity used as an adhesive; ② Partial alcoholysis type PVA, block type PVA, modified PVA, and multi branched PVA used as wetting and dispersing agents.

3. The mechanism of action of PVA

The control objective of the preparation of thermal coatings is to disperse the particles of colorless dye, color developer, and sensitizers to below 1 μm, and make a coating with certain viscosity and adhesive properties for use in coating machines. The basic process of preparing thermal layer coatings is: wetting → grinding → stabilization.

Wetting is a prerequisite for the dispersion of solid thermal pigments, followed by grinding assistance, colloidal stability, and viscosity control. The mechanism of PVA polymer surfactants in the preparation of thermosensitive coatings is analyzed as follows.

(1) Wetting and permeation of PVA

Polymer PVA based surfactants serve as wetting and penetrating agents in the preparation of thermal coatings. The wetting and penetrating of thermosensitive particles are important conditions for ensuring uniform and good dispersion and grinding of pigments, while also shortening the grinding time. At present, in the coating preparation of thermal paper produced in the factory, if other small molecule surfactants are not added, the wetting of thermal particles is mainly achieved using polymer based surfactants PVA.

(2) The grinding aid of PVA

Polymer PVA based surfactants are often used as grinding agents in the preparation of thermal coatings. Colorless dyes, color developer, and sensitizers are insoluble in water and are applied as primary particles in the layer of thermal paper, and show an excellent application performance. The primary fine particles of colorless dyes, colorants, and sensitizers sold in general commodities will aggregate to varying degrees. The presence of excessively large aggregated particles not only reduces the storage stability of the coating system, but also affects the color strength, gloss, and other performance of colorless dyes. The use of mechanical external force to disperse thermosensitive raw materials is the process of breaking down the agglomerates of substances into primary particles that meet the requirements. During grinding, the more PVA adsorbs particles on the surface, the more the surface tension of the pigment decreases. This type of surfactant can be directionally arranged on the surface of the particles, making the surface smooth and easy to roll, improving the grinding efficiency.

(3) Colloidal protection of PVA

PVA can be used as a colloid protector for thermal coatings.

Thermal paper coatings are highly dispersed multiphase systems with enormous interfacial energy. In the preparation of thermal coatings, PVA plays a colloidal protective role to prevent pigment aggregation during the grinding process. Experimental results have shown that both partially alcoholized and fully alcoholized PVA have the colloidal protective effect of thermal paper coatings.

(4) The bonding effect of PVA

At present, PVA is widely used as an adhesive in the production of thermal paper in China. PVA, as an adhesive in papermaking coatings, has the strongest bonding strength; The bonding strength is three times that of casein and four times that of starch. When PVA is used in thermal coatings, it is required that it has no effect on the color of thermal paper, and the viscosity range of PVA is wide. In addition to serving as a binder, PVA can also act as a microcapsule to prevent direct contact between thermal dyes and color developer agents, resulting in discoloration.

(5) The regulatory effect of PVA on the viscosity of coatings

Different forms of coating heads have different viscosity requirements for coatings in production. PVA products have a series of products with different viscosity specifications available for production, which is very convenient for adjusting the viscosity of thermal coatings.

A key process in the production process of thermal coatings is grinding in a grinder, where PVA plays a wetting, dispersing, and grinding assisting role in the grinding of thermal coatings; And play a colloidal stabilizing role in the prepared coating; After the coated paper dries, PVA plays a bonding role.

Conclusion:

In the preparation of thermal paper coatings, the main function of polyvinyl alcohol (PVA) is to disperse first and then increase viscosity. Therefore, PVA that plays a dispersing role in production, has high surface activity and low surface tension, mainly consists of block type PVA and modified PVA; The PVA that plays a role in increasing viscosity is mainly the fully alcoholized PVA.