Production, Modification And Application Of Compound Active Calcium Carbonate For Plastics

The compound calcium carbonate products usually refer to heavy calcium carbonate as the main part (>50%), with a certain proportion of light calcium carbonate powder, through the compound modification technology, the light and heavy calcium mixture, combined modification, can balance the difference between light and heavy calcium, both have the advantages of both, greatly reduce the cost of products at the same time, expand the scope of application of products, improve product quality.

1、Production technology of compound active calcium carbonate

Based on dry calcium production process，the following two processes can be used to compound active calcium carbonate：

One is to mix a certain size, proportion of heavy calcium, light calcium grinding → modification → grading；

The second is to use heavy calcium, light calcium grinding, grading, modification separately.

The characteristics are: simple process, small investment in equipment, can be directly packaged after discharging, especially suitable for coupling of calcium carbonate modification process.

Based on the wet generation process of heavy calcium, the following processes can be used to mix active calcium carbonate: wet mixing of heavy calcium and light calcium adjusted to a certain concentration of slurry → modification → slurry products → drying → dry powder products.

The characteristics are: Wet activation is to deal with calcium carbonate particles in the liquid state, can be evenly coated on calcium carbonate, surfactant by steric hindrance effect and electric double layer effect adequately dispersed in calcium carbonate particle surface, make the distance between particles increases, in the process of dehydration and drying to reduce the secondary coagulation of particles, drying the product fine granularity, bulk density is small, Liquidity and dispersion are good.

2、Surface modification method of compound active calcium carbonate

The correct selection of modifier should not only consider the affinity miscibility of modifier with calcium carbonate and pigment powder, but also consider the miscibility of modifier with other chemical additives of matrix, especially the compatibility effect of modified calcium used in plastic system with auxiliary additives.
At present, the surface modification methods of compound calcium carbonate mainly include surfactant modification, coupling agent modification, reactive monomer and active macromolecule modification, in-situ polymerization modification, etc.

（1）Surfactant modification method

At present, there are many kinds of surfactants used to modify calcium carbonate, with large production capacity and low price, which can be mainly divided into fatty acids (salts) and titanate esters.

（2）Coupling agent modification method

One end of the coupling agent molecule is a polar group, which can chemically react with the hydroxyl group 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 calcium carbonate particles to make the surface oleophilic, which can reduce the surface free energy of particles, improve the mixing ability with resin, and give the composite better physical and mechanical properties. At present, the coupling agents used for calcium carbonate modification mainly include titanate coupling agent and aluminate coupling agent.

（3）Polymer modification

The polymer can be adsorbed on the surface of calcium carbonate directionally, so that calcium carbonate has charge characteristics, and the formation of physical and chemical adsorption layer on its surface, blocking calcium carbonate particles agglomerate, improve the dispersion. It is generally believed that polymer-coated calcium carbonate can be divided into two categories:

One is to attach the polymer monomer to the surface of calcium carbonate, and then induce its polymerization, thus forming a very thin polymer film on the surface;

The other is to dissolve the polymer in a suitable solvent and then add calcium carbonate. When the polymer gradually adsorbs on the surface of calcium carbonate, the solvent is removed and a coating is formed.

（4）In situ polymerization modification method

In situ polymerization is to disperse the particles evenly in the monomer, and then use the initiator to initiate polymerization. The particles or molecules are evenly dispersed in the polymer matrix to form core-shell particles with elastic coating layer. Compared with traditional packed polymer materials, in situ polyphase polymerization ensures the uniform dispersion of packed particles.

3、Analysis of common problems and causes in the application of compound active calcium carbonate

The problems of compound active calcium carbonate in plastics are mainly reflected in effective dispersion of powder, surface gloss, extrusion performance and color stability during processing.

（1）The dispersion of powder

When the surfactant surface coating treatment is incomplete and the oil absorption value is high, it is easy to lead to poor compatibility between the material wall and plasticizer such as DOP during dry powder mixing, resulting in prolonged mixing time;

The pH value of the product is high, especially when the pH value reaches more than 10, the dielectric property of the powder particle surface changes, the electric charge increases, and the product is obviously easy to stick to the wall, easy to dust, difficult to mix and other phenomena;

Heavy calcium can effectively improve the fluidity of processing, but if too much heavy calcium is added and the particle size is too large, it is easy to cause poor smoothness of master grain surface and obvious rough feel.

（2）Product surface gloss problem

When the powder particle size is larger, there are more small particles on the surface of the extruded particle after plasticizing, the surface reflection is reduced, and the light scattering effect makes the surface luster of the colloidal particle is poor and dark;

Due to the formation of latex bubbles in the plasticizing process of the rubber, after the extrusion and cooling of the rubber, the location of tiny bubbles formed holes, the surface became uneven, and the gloss decreased;

When the material is extruded, the surface luster is good, but when the rubber material is cooled and stored for a period of time, the surface appears "fog surface", the main reason is that the use of stearic acid in the rubber material is excessive, resulting in part of stearic acid precipitation;

The poor surface smoothness of products is mainly high colloid viscosity, poor fluidity.

（3）The problem of poor extrusion performance

The slow extrusion speed is due to the calcium carbonate powder filling too much, resulting in the increase of viscosity, difficult to plasticize, resulting in the reduction of extrusion speed;

The poor fluidity of the compound is mainly due to the strong adsorption capacity of the light calcium powder particles to the plasticizer, which affects the normal plasticizing of the compound and prolongates the plasticizing time.

（4）The stability of color

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.

3、Key points of production of compound active calcium carbonate

In order to produce high quality compound active calcium carbonate to meet the needs of downstream users, the following points need to be noted:

（1）Choose high-quality raw materials

Calcite and limestone should be raw materials with high whiteness and low impurities to control the magnesium oxide content in the ore, and the best one is the magnesium oxide content less than 0.5%. Low magnesium oxide content can reduce pH value and oil absorption value of powder.

（2）Optimize the ratio of heavy calcium and light calcium and particle size range

It can effectively improve the surface quality of the product while improving the fluidity of the material.

（3）Strictly control the equipment material used in the processing of powder

Generally, stainless steel should be selected to avoid impurities such as rust mixed into the product. In particular, activation equipment and drying equipment, must use stainless steel material, in order to avoid contact with active agent (generally slightly acidic) rust; Carbon steel equipment is easily corroded by wet materials in contact with drying equipment.

（4）Surface coating technology of calcium carbonate powder

When the surface coating (insufficient amount of coating agent), the powder oleophilic performance is reduced, and the compatibility with plasticizer is reduced, it is necessary to ensure that the calcium carbonate coated by surfactant treatment, high activation degree.