The so-called compound calcium carbonate product usually refers to heavy calcium carbonate as the main body (>50%), with a certain proportion of light calcium carbonate powder, through compound modification technology, light and heavy calcium are mixed and combined. Modification can well balance the difference between light and heavy calcium, and have the advantages of both, while greatly reducing the cost of the product, expanding the scope of application of the product and improving the quality of the product.
Production process of compound activated calcium carbonate
Based on the dry production process of heavy calcium, the following two processes can be used for compounding activated calcium carbonate:
★One is to mix and grind heavy calcium and light calcium with a certain particle size and proportion → modification → classification;
★The second is to use heavy calcium and light calcium to grind → grade → modify.
Its characteristics are: simple process, small equipment investment, direct packaging after discharge, especially suitable for the modification of calcium carbonate by coupling.
Based on the wet production process of heavy calcium, the following process can be used for compounding activated calcium carbonate: wet mixing and grinding of slurry with heavy calcium and light calcium adjusted to a certain concentration → modification → slurry product → drying → dry powder product.
Its characteristics are: wet activation is to treat calcium carbonate particles in a liquid state, which can evenly coat calcium carbonate, and the surfactant can be fully dispersed on the surface of calcium carbonate particles through steric hindrance effect and electric double layer effect. The distance between the particles is increased, the secondary agglomeration of the particles is reduced in the process of dehydration and drying, and the dried product has a fine particle size, a small bulk density, and good fluidity and dispersibility.
Surface modification method of compound activated calcium carbonate
The correct choice of the modifier should not only consider the affinity and miscibility of the modifier with calcium carbonate and its matching pigment powder, but also consider the compatibility of the modifier with other chemical additives of the matrix, especially for Compatibility effect of modified calcium products and auxiliary materials in plastic systems.
At present, the surface modification methods of compound calcium carbonate mainly include surfactant modification, coupling agent modification, reactive monomer and active macromolecule modification, and in-situ polymerization modification.
(1) Surfactant modification method
At present, there are many types of surfactants used to modify calcium carbonate, with large production capacity and low price. They can be mainly divided into two categories: fatty acid (salt) and titanate.
(2) Coupling agent modification method
One end of the coupling agent molecule is a polar group, which can chemically react with the hydroxyl groups on the surface of calcium carbonate to form a stable chemical bond; the non-polar group at the other end is coated on the surface of the calcium carbonate particles, making the surface hydrophilic. Oiling can reduce the surface free energy of particles, improve the miscibility with resin, and endow the composite material with better physical and mechanical properties. At present, the coupling agents used for calcium carbonate modification mainly include titanate coupling agents and aluminate coupling agents.
(3) Polymer modification method
The polymer can be directionally adsorbed on the surface of calcium carbonate, so that calcium carbonate has electric charge characteristics, and forms a physical and chemical adsorption layer on its surface, preventing the agglomeration and agglomeration of calcium carbonate particles and improving dispersibility. It is generally believed that polymer-coated calcium carbonate can be divided into two categories:
One is to first adsorb the polymerized monomer on the surface of calcium carbonate, and then initiate its polymerization, thereby forming a very thin polymer film layer on its surface;
The other type is to dissolve the polymer in a suitable solvent and then add calcium carbonate. When the polymer is gradually adsorbed on the surface of calcium carbonate, the solvent is excluded to form a coating.
(4) In-situ polymerization modification method
The in-situ polymerization method is to disperse the particles uniformly in the monomer first, and then initiate the polymerization with an initiator, and the particles or molecules are uniformly dispersed on the polymer matrix to form particles with a core-shell structure with an elastic coating layer. In situ heterophasic polymerization ensures a homogeneous dispersion of the filled particles compared to conventional filled polymer materials.
Common problems in application and analysis of causes
The common problems of compound activated calcium carbonate in plastics are mainly reflected in the effective dispersion of the powder, the surface gloss of the product, the extrusion performance during the processing of the product, and the stability of the color and luster.
(1) Powder dispersion problem
★When the surface coating of the active agent is incomplete and the oil absorption value is too high, it is easy to cause the material sticking to the wall during dry powder mixing and poor compatibility with DOP and other plasticizers, resulting in prolonged mixing time;
★The pH value of the product is too high, especially when the pH value is higher than 10, the dielectric properties of the surface of the powder particles will change, the electric charge will increase, and the product will obviously be easy to stick to the wall, easy to dust, and difficult to mix;
★ Heavy calcium can effectively improve the fluidity of processing, but if the heavy calcium is added too much and the particle size is too large, it is easy to cause poor surface flatness of the masterbatch and obvious rough hand.
(2) Product surface gloss problem
★When the particle size of the powder is large, there are obviously many tiny particles on the surface of the extruded pellets after plasticization, the surface reflection is reduced, and the light scattering effect makes the surface of the rubber particles less glossy and dark;
★Due to the formation of latex-like bubbles in the plasticizing process of the rubber, after the rubber is extruded and cooled, the tiny bubbles form cavities, the surface becomes uneven, and the gloss is reduced;
★When the material is extruded, the surface gloss is good, but when the rubber compound is cooled and stored for a period of time, the surface appears “matte”, the main reason is that some stearic acid is precipitated due to the excessive use of stearic acid in the rubber compound;
★The poor surface smoothness of the product is mainly due to high colloidal viscosity and poor fluidity.
(3) The problem of poor extrusion performance
★Slow extrusion speed is due to excessive filling of calcium carbonate powder, which leads to increased viscosity and difficult plasticization, resulting in reduced extrusion speed;
★The poor fluidity of the rubber compound is mainly due to the strong adsorption capacity of the light calcium powder particles to the plasticizer, which affects the normal plasticization of the rubber compound and prolongs the plasticization time.
(4) Color stability problem
★The whiteness of calcium carbonate powder is low and unstable;
★The powder contains a small amount of oxide impurities;
★The pH value of calcium carbonate powder is high;
★The oil absorption value of calcium carbonate powder is high.