Characteristics of active calcium carbonate and modified calcium carbonate and common modifiers

Calcium carbonate used as a filler, if  it's with not surface treatment, will have poor affinity with organic polymers, easy to cause uneven dispersion in polymers, resulting in the interface defects of the two materials, so it is necessary to improve the application performance of calcium carbonate fillers.

Active calcium carbonate (also known as modified calcium carbonate) is made of ordinary calcium carbonate powder (heavy calcium carbonate  and light calcium carbonate ) as the base material, and the surface of inorganic powder is modified and activated by using multi-functional surfactants and compound high-efficiency processing AIDS. The modified calcium carbonate powder forms a special coating structure on its surface, which can significantly improve the dispersibility and affinity in polymer matrix such as polyolefin, and can produce interface interaction with polymer matrix, so as to improve the impact strength of the product. It is a kind of incremental filler with excellent performance.

When calcium carbonate is treated with a surfactant, for calcium carbonate is an inorganic substance, it has a great affinity with the hydrophilic group of the surfactant, and the chemical combination between them is similar to chemical bonding, and the oleophilic group is directed to the surface of the calcium carbonate particles to form a single molecular film. This is the basic principle of the production of active calcium carbonate, so that the treated filler has changed from hydrophilic to lipophilic, and has a good affinity for organic compounds such as resins. It must be pointed out that in addition to surfactants, organic coupling agents and various modifiers developed in recent years can be used for surface treatment of calcium carbonate. All calcium carbonate treated with these substances can be generally referred to as active calcium carbonate.

Active calcium carbonate on general rubber, plastic products have a certain reinforcement, improve the compatibility of inorganic filler and resin, thereby improving the mechanical properties of products, processing properties, improve the thermal stability of composite materials, to achieve high filling performance. Used for PVC pipes, plates, cable materials, etc., can improve the thermal stability of composite materials, surface finish, filling amount, reduce the amount of resin, reduce costs. Can also be used for pp, pe, rubber and so on.

Light calcium was the main base material in the early production of active calcium carbonate. Because the production process of light calcium is complex, the price is high, and the production of light calcium causes pollution to the environment, the trend in the past decade is to gradually replace light calcium with heavy calcium. The ratio of heavy calcium to light calcium in developed countries is about 14-18:1, which is significantly higher than that in China. With the development of related industries, in the above application fields, the dosage ratio of heavy calcium to light calcium in China has gradually reached 8-12 times.

Commonly used modifiers and treatment methods

For heavy calcium carbonate , the commonly used surface modifiers are hard ester acid and its salts, coupling agents, etc.

1.Stearic acid and its salts 

The commonly used modification method is to dry calcium carbonate, remove water, and carry out in the modification equipment. The dryness is controlled at 100-110℃, and the time is 0.5-1.0h or less. The dried heavy calcium is added to the modification equipment, and the hard ester acid or its salt is added while stirring. The amount of stearic acid is about 0.8-1.0% of the quality of calcium carbonate. The reaction temperature is controlled at about 100℃, the time is 15-30min, and the product can be packaged after completion.

Calcium carbonate modified by stearic acid or stearate has quite good reinforcing effect and can replace carbon black and white carbon black fillers. The modified calcium carbonate filler can improve the impact resistance of plastic products and partially replace titanium dioxide in the coating industry.

2. Coupling agent

2.1 Titanate coupling agent: According to the molecular structure and the configuration of the mineral surface coupling agent, the titanate coupling agent can be divided into three types: monoalkoxy, chelate and coordination.

Monoalkoxy type: In addition to pyrophosphate ester, due to poor water resistance, the coupling agent can only be diluted with organic solvent or inert anhydrous plasticizer, the mass ratio of solvent and coupling agent should be greater than 1, after being fully stirred and dissolved, drop or spray on the surface of pre-dried heavy calcium powder to mix well, and stir in modified equipment at 90-100℃ for more than 15min. Calcium carbonate can also be put into the solvent containing coupling agent to stir evenly, coupling agent will be heavy calcium wrapped, the excess solvent removed.

Chelating type: good water resistance, can be coated with heavy calcium powder in the water phase, but most of them are insoluble in water, water-based solvents, surfactants or high-speed stirring can be used to promote emulsification and dispersion in water. Titanates containing phosphate, pyrophosphate and sulfonic acid groups can be quaternary aminated with amine reagents and dissolved in water. For example, when modifying calcium carbonate powder with KR-138S chelating coupling agent, mix 1 part KR-138S and 0.5 part triethylamine to make quaternary amine salt, disperse 600 parts of water, add 200 parts of calcium carbonate, stir well, dehydrate and dry.

Coordination: The use of coordination coupling agents and chelate types is similar, because the water resistance is good and most of them are insoluble in water, so they can be dissolved in organic solvents and then coated with calcium carbonate powder.

The amount of titanate coupling agent is about 0.5-3.0% of the mass of calcium carbonate. The heavy calcium powder treated with titanate coupling agent has good compatibility with polymer molecules. Because it can form a molecular bridge between carbonate molecules and polymer molecules, it increases the interaction between organic polymers or resins and calcium carbonate, and correspondingly improves the mechanical properties of thermoplastic composites, such as impact strength, tensile strength, bending strength and elongation.

2.2 Aluminate coupling agent: The modification method of aluminate coupling agent is similar to the modification method of other coupling agents. The heavy calcium powder is placed in the modified equipment at 100-130 ° C, dried while stirring, for about 10 minutes, and then added to the measured coupling agent in three times. Each time interval of 2-3min, after adding, then stir for 5min. The amount of aluminate coupling agent is about 1%-2% of the mass of calcium meganate. The improvement effect of aluminate coupling agent on calcium production is comparable to that of titanate coupling agent, and superior to stearic acid and its salt. In addition, aluminate coupling agent is cheap, light color, does not affect the whiteness of the product.

Through comparison of experimental data, it is found that the modification effect of coupling agent is better than that of Stearic acid , especially titanate coupling agent. Due to the chemical bond between the coupling agent and the hydroxyl group on the surface of calcium carbonate, a single molecular film of coupling agent is coated on the surface of calcium carbonate. And the other end of the titanate coupling agent is miscible with the polymer, so that the calcium carbonate is well combined with PVC, and the product has good elasticity and impact resistance.