High-gloss coatings are loved by consumers for their bright colors and beautiful gloss, thus have been popular in the market for quite a long time. However, with continuous economic improvement, consumers are more and more in favor of leisure and personalized styles for daily life. This, coupled with the booming demand of automobile and home appliance industries, has led to a dramatic increase in the demand for low-gloss coatings that has a soft appearance. Coating designers should further consider how to effectively produce coatings with a matte finish.
What is Gloss Level
Gloss refers to the ability of an object’s surface to reflect light. The measurement of the ability to reflect light is called gloss level expressed as a percentage. The higher the gloss of an object’s surface, the more it reflects light and the higher the brightness. The gloss level is measured by a photoelectric gloss meter. According to different gloss level, the coatings can be divided into high gloss, semi gloss, satin, matte, etc.
|Matte||Satin||Semi Gloss||High Gloss|
|Gloss level (%)||<10||10 – 40||41 – 69||>70|
What Influence Gloss Level
1. Surface Roughness of the Coating Film
The surface gloss level of an object is closely related to the roughness of the object’s surface. When light hits the surface, some parts of it will be absorbed by the object, some parts will be reflected and scattered, and other parts will be refracted. The smaller the roughness of the object’s surface, the more light is reflected, and the higher the gloss level. On the contrary, if the object’s surface is rough, more light will be scattered, resulting in a lower gloss level.
2. Film Forming Process
The coating film is cured by the evaporation of solvents after being applied to the object’s surface. The process of coating film formation is critical to its surface roughness and gloss level.
In the wet film process, the volatilization rate of the solvent is influenced by the diffusion of the solvent on the film surface. When the volatilization rate of each solvent component does not differ much, it is possible to obtain a high-gloss surface. On the contrary, when the volatilization rate of each solvent component is not the same, the polymer molecules tend to curl or even precipitate out into particles or agglomerates of different sizes, resulting in an uneven film surface. In the dry film process, the volatilization rate of the solvent is mainly influenced by its diffusion in the coating instead of just the surface, which also affects the surface roughness of the coating film.
In addition, the coating film will shrink and become thinner as the solvent evaporates, and some suspended heavy particles in the coating will rearrange on the surface of the coating film, causing unevenness on the surface.
3. Particle Size/Distribution of Fine Particles
The particle size and particle size distribution of fine particles (pigments, fillers, etc) in the coating is one of the important factors affecting the gloss of the coating film. It is found in the study that only when the diameter of fine particles is less than 0.3um can you get a high-gloss coating film. The reason is that after making the coating film of a certain thickness and drying, only the uppermost layer of fine particles is partially protruding upwards. The surface roughness caused by fine particle ions less than 0.3 um does not exceed 0.1 um. When the average particle size of fine particles is between 3-5 um, a coating film with good matting effect can be obtained.
In addition to the above three factors, the volume concentration of pigment (PVC), the dispersion of pigment, and the surface structure and surface reflective properties of the coating film can also affect the gloss level of the coating film. As the PVC of pigment increases, the gloss level first decreases and hits a minimum at the limit volume concentration of pigment (CPVC). Then the gloss level increases with the increase of PVC. When the pigment type and amount are determined, the better the dispersion, the higher the gloss level.
What is Matting
Matting is the use of various methods to reduce the surface gloss of the coating film.
Considering the mechanism of gloss reflection and the factors affecting the gloss, it is believed that matting is the use of various means to destroy the smoothness of the coating film, increase the surface’s microscopic roughness, and reduce the light reflection.
The main matting methods are physical matting and chemical matting. Physical matting refers to the addition of matting agents to make the finish uneven in the process of film formation, increasing light scattering and reducing light reflection. Chemical matting refers to adding some structures or groups to the coating that can absorb light, such as grafted polypropylene, to achieve low gloss.
How to Achieve Matting in Coatings
The commonly used method is to add matting agents. There are mainly the following types:
1. Matting Agents
(1) Metal Soap
Metal soap is a common matting agent in the early days, mainly metal stearate including aluminum stearate, zinc stearate, calcium stearate, magnesium stearate, etc. Aluminum stearate is the most used one. The matting principle of metal soap is based on its incompatibility with the coating components. Its fine particles are suspended in the coating and dispersed on the film surface, leading to microscopic roughness of the surface.
Wax is a widely-used organic suspension matting agent. After the evaporation of the solvent, the wax in the coating film separates out as micro crystals and suspends on the surface of the coating film, forming a rough layer that can scatter light.
Silicious earth, kaolin, synthetic silica, etc. are inorganic filling matting agents. When the coating film is dry, its tiny particles form a micro-rough surface on the finish.
Their matting effect is subject to many factors. Take silica as an example. When used as a matting agent, its matting effect will be affected by the particle pore volume, average particle size and particle size distribution, dry film thickness, and surface treatments. Experiments show that silica with larger pore volume, uniform particle size distribution, and matched particle size with dry film thickness has better matting performance.
2. Matting Resins
Since resin is an essential ingredient for coatings, it is possible to use matting resins to achieve low gloss, thus avoiding the use of matting agents and reducing costs. Its principle is to utilize the difference in curing temperature of functional groups and curing agents in the matting resin and other resins to complete curing at different times so that the finish shrinks unevenly and thus destroys the surface smoothness.
Hoyonn Matting Agent Series
Hoyonn Matting Agent Series contains new compound matting agents for indoor, outdoor, and HAA powder coating systems. They are featured in anti-yellowing, good leveling, good impact resistance, excellent gloss stability and many more properties. Learn more about formulas for reference. Or ask us about the products.