In the rubber industry, calcium carbonate is the third largest inorganic filler and reinforcing agent after carbon black and silica. In the past two years, in order to reduce carbon emissions and environmental problems caused by excessive carbon black release, the application of calcium carbonate in rubber has received more attention.
What is the specific role of calcium carbonate in rubber?
As a filler, calcium carbonate can increase the volume of rubber, reduce the cost of products, and improve processing performance without significantly affecting the performance of rubber products.
As a reinforcing agent, calcium carbonate can greatly improve the wear resistance, tear strength, tensile strength, modulus, swelling resistance and other properties of the vulcanizate.
What kind of calcium carbonate is a filler for rubber, and what kind of calcium carbonate is a reinforcing agent for rubber?
Among them, there is no strict or standard division, but the properties of different types of calcium carbonate are indeed biased.
Light calcium is a commonly used filler and extender in the rubber industry. It has large filling amount, easy extrusion, good gloss, low hardness and high elongation. The mechanical properties can exceed the level of clay and semi-soft carbon black.
Active nano-calcium can not only be used as a filler for rubber compounds, but also has high reinforcing properties, which can improve the wear resistance, tear strength, modulus and other properties of rubber, and can also reduce production costs.
Heavy calcium has outstanding filling performance in rubber, easy to mix and disperse, and the compounding amount can reach 100% to 250% of the rubber amount.
Nano calcium carbonate has both filling and reinforcing functions, and has received a lot of attention in recent years. However, the modification work often faces difficulties in practical application.
Nano-calcium carbonate has high surface energy and is easy to form stable aggregates, which makes the distribution of these fillers poor. Additionally, most rubbers are hydrophobic, whereas these fillers are hydrophilic and require surface treatment/modification to reduce the hydrophilicity of the filler, thereby improving the filler-polymer interaction.
Special modification properties of calcium carbonate
1. Effect on tensile and impact properties
Not all calcium carbonate can improve the tensile strength and impact strength of plastic film, which in turn is affected by the particle size of calcium carbonate and the effect of surface treatment.
Effect of particle size: The particle size of calcium carbonate is different, and the modification effect on plastic is different, as shown in the following table. Generally, the particle size is below 1000 mesh, which is mainly used for incremental modification; the particle size is 1000~3000 mesh, and the addition amount is below 10%, which has a certain modification effect; the particle size is above 5000 mesh, which belongs to functional calcium carbonate, with The obvious modification effect can improve the tensile strength and impact strength. Although nano-scale calcium carbonate has a finer particle size, it can only achieve the same modification effect as 8000 mesh calcium carbonate because it is difficult to disperse at present.
It can be seen from the table that as the particle size of calcium carbonate becomes finer, the impact strength, tensile strength and elongation at break increase, and the flexural strength and flexural modulus are basically the same, but the fluidity decreases.
Effect of surface treatment: Calcium carbonate with appropriate particle size can improve the tensile strength and impact strength of composite materials if the surface treatment is good. In recent years, with the continuous development of organic/inorganic composite theory, CaCO3 has changed from a simple filler to a new type of functional filler. For example, the homopolymeric PP/CaCO3 composite material made of calcium carbonate can more than double the notched impact strength of the matrix plastic.
Second, the smoke suppression effect when burning
CaCO3 has excellent smoke suppression effect, the principle is that it can react (capture) with hydrogen halide in smoke to generate stable CaCl2. Therefore, all polymers that generate hydrogen halide during combustion, such as vinyl chloride, chlorosulfonated polyethylene, neoprene, etc., can use CaCO3 as a smoke suppressant. Since the combustion reaction is a solid-gas heterogeneous reaction and can only be carried out on the surface of solid particles, the particle size of CaCO3 particles becomes an important factor for the smoke suppression effect. Only tiny particles have a much larger specific surface area, and the finer the particle size of calcium carbonate, the better the smoke suppression effect.
3. Anti-blocking agent
The blown tubular film containing calcium carbonate has good opening and will not stick when it is curled, and calcium carbonate acts as an opening agent.
Fourth, increase the thermal conductivity
The addition of calcium carbonate improves the thermal conductivity of the film, so that the bubble of the blown film cools faster, improves the production efficiency, and increases the output of the extruder. Taking the addition of 25% light calcium carbonate to the PVC sheet as an example, it only takes 3.5 seconds to heat to 200°C, while the pure PVC sheet needs 10.8 seconds, and the thermal conductivity is increased by as much as 3 times.
5. Improve liquidity
Calcium carbonate can improve the fluidity of composite systems, reduce melt viscosity and extruder torque, increase extruder output, and improve production efficiency. Different varieties of calcium carbonate have different effects on the flow, and the specific order of the fluidity of the composite material is: large calcite calcium carbonate > marble calcium carbonate, dolomite calcium carbonate > small calcite calcium carbonate > light calcium carbonate.
6. Color matching performance
Calcium carbonate with high whiteness can replace some white pigments such as titanium dioxide, thereby saving the content of high-priced titanium dioxide. Large calcite calcium carbonate is the first choice because of its high whiteness and high hiding power. The reason why calcium carbonate can become a white pigment is mainly because it has a certain hiding power.
In heavy calcium carbonate, the background color of calcium carbonate from different origins is different. For example, the background color of calcium carbonate in Sichuan is blue, the background color of calcium carbonate in Guangxi is red, and the background color of calcium carbonate in Jiangxi is blue. In the specific color matching, the color of calcium carbonate should be consistent with the main color. For example, calcium carbonate with blue color will eliminate the tinting strength of yellow pigments. Calcium carbonate with blue color is often used to eliminate the yellow color of the product.
7. Increase breathability
The plastic film filled with calcium carbonate will produce tiny pores in the film when stretched, which can only pass through water vapor but not liquid water, so it can be used to produce breathable plastic products. For example, a typical breathable film uses a large amount of calcium carbonate. Made of padding and film stretching. Generally, only 3000 mesh calcium carbonate can be selected for the production of calcium carbonate for breathable film, and the particle size distribution should be absolutely narrow.
8. Promote product degradation performance
After the polyethylene plastic bag containing calcium carbonate is buried in the ground, the calcium carbonate may react with carbon dioxide and water to form Ca(HCO3)2 that can dissolve in water and leave the film, leaving fine holes on the film, increasing the density of the plastic film. The area where the surrounding air and microorganisms come into contact, thereby promoting the degradation of the product.
Nine, play a role in nucleation
Nano-CaCO3 can induce the crystal nucleation of polypropylene, which can increase the content of β crystals, thereby improving the impact toughness of polypropylene.
10. Reduce the water absorption of PA plastics
The water absorption of PA/calcium carbonate composite is lower than that of PA pure resin. For example, when PA6 is filled with 25% calcium carbonate, the water absorption of the composite is reduced by 56%.
11. Improve surface properties
Calcium carbonate can increase the surface tension of the composite material and has excellent adsorption, so it can improve the electroplating, coating and printing properties of the composite material.
12. The effect of calcium carbonate on foaming
Whether calcium carbonate affects the foaming performance of plastic materials is very complicated, depending on the size and the degree of addition.