In recent years, calcium carbonate has been widely used in the rubber and plastic filling industry because of its excellent properties. The color of calcium carbonate itself is whiter than that of other inorganic mineral powder materials, with better stability and better plasticization. However, in order to manufacture high-grade rubber and plastics, there are certain requirements for calcium carbonate for filling. The following editor will introduce the requirements for calcium carbonate in processing high-grade rubber and plastics!
good chemical stability
Calcium carbonate cannot have harmful reactions with resins and various additives, that is to say, calcium carbonate used in rubber and plastics and its surface organic treatment agents cannot participate in various chemical reactions. After calcium carbonate is calcined at high temperature, its chemical stability is very good, and it basically does not chemically react with various components in the rubber and plastic system, but the organic modifier on its surface may participate in the chemical reaction. For example, calcium carbonate surface-modified by acidic substances such as stearic acid and its derivatives will affect the color of calcium powder due to the reaction between iron in calcium carbonate and stearic acid, and it is easy to turn yellow. particularly evident in.
good temperature resistance
Since the molding and processing of rubber and plastic products are carried out in a high temperature environment (generally greater than 180 ° C), it is required that calcium carbonate should not decompose and discolor under the temperature conditions of heating molding. The thermal decomposition temperature of calcium carbonate is above 800 °C, while the processing temperature of rubber and plastics will not exceed 350 °C, so calcium carbonate itself will not undergo thermal decomposition.
However, traditional organic modifiers (such as stearic acid, titanate, silane coupling agent) are not resistant to high temperature, and are prone to yellowing and decomposition in high temperature environments, resulting in an increase in VOCs. Many calcium carbonate manufacturers are actually The “yellow change” problem has been encountered in the application. Therefore, the traditional organic modifier is not suitable for the organic modification of calcium carbonate for high-grade rubber and plastic products.
good dispersion
For filling rubber and plastics, it is a necessary prerequisite for calcium carbonate particles to be uniformly dispersed in the rubber-plastic matrix, that is, calcium carbonate in the form of a single particle is dispersed in the ocean of matrix rubber and plastics like an island to achieve the effect of reinforcement. The smaller the particle size of the filler used, the better the mechanical properties of the filled rubber and plastic material when the filling ratio is the same. The dispersibility of calcium carbonate is mainly affected by intermolecular forces, polar adsorption, particle fineness and other factors.
Calcium carbonate used for rubber and plastic processing generally has a particle size of about 1 to 10 microns. This fine particle has a very large specific surface area and Gibbs free energy, and is in a thermodynamically unstable state. When calcium carbonate is added to the resin system, due to the polar adsorption and van der Waals force between adjacent particles, the particles tend to approach each other, reducing the total specific surface area and surface free energy, so that it is difficult for the calcium carbonate particles to separate from the agglomerate. Conversion to high dispersion. For calcium carbonate without surface organic modification treatment, it is difficult to disperse calcium carbonate when it is kneaded with resin, and it is difficult to disperse it uniformly even when kneading at high temperature and high speed. In order to improve the dispersion uniformity and stability of calcium carbonate, its surface polarity must be eliminated or weakened and its surface free energy must be reduced.
low oil absorption
The lower the oil absorption, the less resin or plasticizer is required to wet an equivalent amount of calcium carbonate. For most rubber and plastic products, such as soft polyvinyl chloride, artificial leather, cable materials, etc., a plasticizer is required to help plasticize the mixture of the resin system. The higher the oil absorption value of calcium carbonate, the easier it is to adsorb the plasticizer to the filler. , so that it loses the effect of plasticizing resin. In order to achieve a certain softness of the resin system mixture, it is necessary to increase the amount of plasticizer, resulting in an increase in cost. By modifying the surface of calcium carbonate and organically coating the surface of calcium carbonate particles, its oil absorption value can be greatly reduced.
good hydrophobicity
Calcium carbonate is a very hygroscopic substance. Once it absorbs moisture during storage or processing, the following problems will occur:
(1) Calcium carbonate agglomerates and the fluidity of dry powder becomes poor, and it is easy to block the sieve holes, resulting in difficulty in adding;
(2) When rubber and plastic products are formed at high temperature, due to the thermal precipitation of water vapor, defects such as “bulging” or “holes” appear in the products, which seriously affect the appearance and physical strength of rubber and plastic products.
low mobility
Due to the large temperature changes in the molding process of rubber and plastic products, room temperature (raw material) – high temperature (processing process) room temperature (cooling molding). Calcium carbonate must ensure that no migration occurs, otherwise it will cause uneven color or loss of gloss on the surface of rubber and plastic products. The migration of calcium carbonate is mainly due to its poor wetting compatibility with the resin, and when the temperature decreases and the rubber and plastic products shrink, the moving speeds of the resin and calcium carbonate are inconsistent.
To reduce the migration, it is necessary to improve the compatibility of calcium carbonate and resin. According to the similar compatibility principle, the organic modifier on the surface of calcium carbonate preferably has the same or similar functional groups as the applied resin. For the most commonly used polyolefin rubber and plastics, it is more suitable to use organic modifiers containing long-chain alkanes.