The role of wax additives in coatings

Wax products have many uses in coating materials, but they can be roughly divided into the following categories:

Change the surface properties of the coating, protect the coating, adjust the gloss and other functions.

The role of waxes in most water-based surface coating formulations is to change the surface properties of the coating, such as improving coating slip or anti-slip, anti-wear, anti-blocking, changing gloss, water repellency, rust prevention, etc. In use, the wax particles occupy the surface of the coating during the drying process of the coating to form an isolation layer, thereby playing a protective role.

The drying process of water-based coatings is divided into three stages: ① water evaporation ② emulsion particles close to deformation and become dense ③ mutual diffusion and finally film formation.

Wax particles migrate out in two ways depending on their particle size:

1. Migration. The so-called floating effect, with the force brought by the evaporation of water during the film formation process, the tiny wax particles overcome the force of the system due to their low surface energy, avoid the entanglement of the macromolecular resin, and migrate to the coating surface for uniform distribution. This process Occurs in the ① and ② stages of coating drying, providing isolation for the coating surface and reducing mutual adhesion.

2. Ball axis effect. The dry film thickness of the coating is smaller than the particle size of the solid wax particles. During the drying process, the wax solid particles protrude from the coating to form a bearing-like structure and achieve surface isolation protection.

The protective effect of wax

01 Anti-adhesion

Adhesion mainly refers to the adhesion between the coating films and the adhesion between the coating films and external substances (such as atmospheric dust) when the coating is not completely dried. Such as high-speed printing in the water-based printing industry, textile printing paste, water-based plastic ink, etc. Generally speaking, drying speed and anti-sticking resistance are inherent problems of water-based coating materials.

The wax particles are suspended in the coating in the form of solid particles, and float to the surface of the coating film during the drying process to form isolation and play a good anti-sticking effect. Through practice, factors such as particle size of wax particles, degree of polarity, compatibility of emulsification system, and density of wax particles all have an impact on the anti-adhesion of the system. We have found in practice that low-melting waxes sometimes have better anti-sticking effect than high-melting waxes.

02 Changes in surface friction coefficient

The wear resistance and scratch resistance of the coating film are related to the surface friction coefficient. Hard waxes such as polyethylene wax can reduce the friction coefficient of the coating surface, so that when the object touches the coating surface, the tendency to slide is greater than the tendency to scratch. Such as printing inks, wood lacquers, etc., the low friction coefficient of the coating surface is required to resolve the impact of the outside world and prevent scratches. Polyethylene waxes; Fischer-Tropsch waxes and even Teflon waxes are good choices. However, in some fields, both coating protection is required and the friction coefficient is not too low. For example, in the packaging industry, if the packaging cartons are too smooth, it is easy to cause the stacking to be unsteady and overturn; another example is the flooring industry, which requires both anti-friction and not too slippery to prevent people from slipping. Polypropylene wax has the two advantages of large friction coefficient and high hardness at the same time, and has been used in the above fields.

03 Wear resistance

In addition to the friction coefficient of the surface, the abrasion resistance of the coating film is also closely related to the toughness of the wax layer. Of course, the protruding thickness of the wax coating also has a great influence, which is also the reason why the wear-resisting effect of the large-particle micropowder wax is obviously better than that of other wax products. However, the relationship between the particle size of wax particles and the gloss of the coating is very large. If the gloss of the coating film is high, the particle size and particle shape of the wax have strict requirements. To a certain extent, the gloss and the wear resistance of the coating are contradictory.

04 Adjustment of glossiness

Generally speaking, even wax emulsion will not improve the gloss of the coating in the high-gloss system, and the effect on the gloss can be ignored at most. But in practice, wax emulsion often plays the role of improving the lubricity of the coating, improving the uniformity and leveling of the coating, thereby improving the gloss of the coating.

Waxes with a particle size larger than 0.2um will definitely affect the gloss, and the degree of extinction is related to the particle size, particle shape and dispersion mode of the wax particles. The presence of the emulsifier will help the wax particles on the surface of the coating film to be evenly distributed. Reduces the roughness of the coating, thereby reducing the effect on gloss.

05 Water repellency

The n-alkane structure of paraffin wax makes it have good water repellency, and the contact angle of water droplets on the coating film is very small, forming a bead shape, that is, the lotus leaf effect. Building exterior wall coatings use wax emulsions as water repellents, which can be made into low-cost antifouling coatings. There are similar demands for wood lacquer and metallic lacquer.