There are three main problems in the application of nano-calcium carbonate fillers in sealants: one is the blending effect of fillers and colloids; the other is the effect of fillers on thixotropic properties; the third is the enhancement of sealants. The factors that affect the blending performance of the special nano-calcium carbonate for sealant in the sealant are mainly particle size, crystal form, oil absorption value, specific surface area, pH value, sieve residue, moisture, and the factors affecting the alternating thixotropy are mainly Particle morphology, oil absorption value, surface modification and pH value, the main factors affecting the enhancement are: particle size and dispersibility.
1. pH value
The lower the pH value, the better. Calcium carbonate reversion is a common phenomenon in the production of light calcium. The existing alkali will generate water with the acidic components in the rubber compound. Water can easily hydrolyze the siloxane to generate inorganic particles. In the rubber compound, densely packed tiny particles are formed, and the stability of the colloid decreases, resulting in a “gel” phenomenon, which affects the surface properties of the product. However, the high pH value also has an advantage, that is, although the calcium carbonate has undergone surface treatment, due to the incompleteness of the surface treatment, the exposed calcium carbonate molecules provide more -OH groups, and these -OH groups The existence of agglomerates is helpful for the formation of hydrogen bond network at the interface between calcium carbonate molecules and the sealant base material, which is beneficial to the improvement of colloidal thixotropy.
2. Oil absorption value
Oil absorption value is a very important indicator of calcium carbonate for sealant, and it is a factor that affects the wettability of calcium carbonate in the glue. After compounding the glue, it can be found that the thixotropy and various mechanical properties of calcium carbonate with higher oil absorption value are ideal, but the glue has high viscosity and is not easy to adjust. Different sealant systems have different requirements for the oil absorption value of nano-calcium carbonate, and it is difficult to identify with a specific index data, depending on the specific situation.
3. Specific surface area
Consistent with the particle size requirement of 60-100nm for nano-calcium carbonate in adhesives and sealants, the specific surface area of nano-calcium carbonate in adhesives and sealants is 20-25㎡/g, which can achieve the best results. Ideal slump resistance and crushability. When the specific surface area is too large, the extrudability of the sealant appears poor, and the tensile strength increases with the increase of the specific surface area, but the dispersion performance decreases.
4. Moisture
As a sealant filler, the lower the moisture content of nano calcium carbonate, the better, generally it must be less than 0.5%. If the moisture is too high, the pinhole honey refining takes a long time during the processing, the power consumption is large, and the quality of the subsequent process of the glue is affected at the same time; the moisture may also hydrolyze some components in the formula to generate inorganic particles, thus affecting the mixing of fillers in the glue. And or make the sealant produce particles, etc. For example, there are many isocyanate groups in the polyurethane adhesive that are prone to hydrolysis. If there is too much water in the nano-calcium carbonate filler, part of the isocyanate will be consumed, and CO2 will be generated to cause the product to foam.
5. Particle size
60-100nm is suitable, the particle size is too large, the thixotropic properties of the glue are poor, it is easy to sag, and it will affect the mechanical properties of the product. cause the surface of the glue to be rough,
6. Crystal form
The crystal form of nano-calcium carbonate should be compatible with the production technology formula of silicone sealant, the processing technology and equipment conditions of the product. Generally, the adaptability of the cubic or rhombic hexahedron, and the cubic part presents a lock-shaped crystal form.
7. Surface treatment
The modification effect of nano-calcium carbonate will affect the blending effect of its particles on the colloid and the alternating thixotropy. The good modification effect will make the nano-calcium carbonate have good affinity in the polymer, and it is found that a small amount of incompletely covered calcium carbonate particles (<2%) provides a place for adsorbed water, which is helpful for the nano-carbonic acid. A network of hydrogen bonds is created between the calcium particle family, which gives the sealant gel characteristics. Although this hydrogen bond network is easily destroyed under the action of shear stress generated after the viscosity is reduced, it can also be quickly recovered, so that the sealant using nano-calcium carbonate as a filler has thixotropic properties.
8. Sieve residue
The sieve residue should be less than 0.15% (45um sieve). The main reason for the excessive sieve residue to produce surface particles must be strictly controlled.