① Usually requires:
(1) The surface of the reinforcing filler particles must have strong chemical activity, which can have a good combination with the rubber, and can improve the mechanical properties, aging resistance and adhesion properties of the vulcanizate. The surface of the non-reinforcing filler particles is chemically inert, does not produce chemical bonding with the rubber, and does not affect the mechanical properties, weather resistance, acid and alkali resistance, and water resistance of the vulcanizate.
(2) It has high chemical purity, uniform fineness, and good wettability and dispersibility to rubber.
(3) Non-volatile, odorless, tasteless, non-toxic, and has good storage stability.
(4) The filler used for white, light-colored and colored rubber products also requires no pollution and no color change.
(5) Cheap and easy to obtain.

 ② Performance requirements:
(1) Fineness: Generally speaking, the finer the particles of the reinforcing filler, the larger the specific surface area, and the larger the contact area with the rubber, the better the reinforcing effect. The finer the non-reinforcing filler particles, the better the mixing effect after adding rubber. However, it must be uniformly dispersed. If the dispersion is not uniform, even if the particles are very fine, the mixing effect is not good.
 
(2) Surface properties: The powder filler is mixed with rubber, and its particles are surrounded by rubber molecules, and the degree of particle surface wetted by rubber has a great influence on the reinforcement performance. The particles that are not easy to wet are not easy to disperse in the rubber, and are easy to agglomerate, reducing their reinforcing effect. This condition can be improved by adding certain substances that help increase wetting. For example, calcium carbonate, which has a small reinforcing effect, reduces the surface tension, increases the degree of wetting, and improves the reinforcing effect after adding fatty acids.

(3) Particle shape and crystal form: The shape of the filler particles is preferably spherical, and the flaky or needle-shaped fillers tend to be oriented when the vulcanizate is stretched, resulting in increased permanent deformation and reduced tear resistance. Carbon black and white carbon black in the reinforcing fillers are amorphous, and other fillers are also crystalline. For example, although the silicon powder and silica are chemical components of silica, the former is crystalline and the latter is amorphous.