Effect of nano-sized calcium carbonate modified by silane coupling agent on properties of silicone adhesive

As a kind of inorganic ultrafine filler, nano calcium carbonate is often used in the formulation of silicone sealant by physical filling to reduce the production cost.However, inorganic ultrafine particles have hydrophilic, need to improve their compatibility with silicone sealant through surface treatment, improve its dispersity and stability in the colloid, so as to improve the adhesive property and mechanical properties of silicone sealant products.

Silane coupling agent is a common surface treatment agent, widely used in surface coating modification of talc powder, aluminum hydroxide, magnesium hydroxide and other inorganic powder, with good application effect.However, due to the small number of hydroxyl groups on the surface of calcium carbonate, it is usually weakly alkaline, so the silane coupling agent has poor bonding on the surface of calcium carbonate, resulting in weak performance improvement.

Pan Xibao et al. used different aminosilane coupling agents to modify the surface of specially pretreated nano calcium carbonate, and investigated the effect of modified nano calcium carbonate on the rheological properties, mechanical properties and water bond resistance of sealant.

1. Modification of nano calcium carbonate

Pretreatment: The nano-cacO3 suspension was placed in a reaction kettle, and 3.0% (measured by cacO3 dry base) stearic acid was added at 70℃ for surface modification for 30min.Then, 0.5% (based on calcium carbonate dry base) phosphate ester was added to modify for 30min. Finally, the modified semi-finished products were pressed and dried.When the moisture is dried to 0.8%, the sample is crushed and screened.

Coupling agent modification:The pre-treated nano calcium carbonate was modified by different aminopropyl triethoxysilane coupling agents (γ -aminopropyl triethoxysilane KH-550, N- (γ -dimethopropyl) -γ -aminopropyl methyl dimethoxysilane YC-621, new blocked triamine silane QX-618) for dry modification. Finally, the active nano calcium carbonate was obtained by grinding and sieving.After modification with different aminosilane coupling agents, the BET specific surface area of the samples decreased slightly, but the particle size did not change significantly, and the particle dispersion was good, which may be related to the secondary crushing and depolymerization of the samples.

Because the surface of ordinary nano calcium carbonate is weakly alkaline and the surface hydroxyl content is low, it is difficult to form good chemical coating modification by silane coupling agent on the surface of calcium carbonate by conventional treatment process.The addition of phosphate ester in the pretreatment process of nano calcium carbonate can effectively reduce the pH value of the product and provide a suitable reaction environment;On the other hand, the phosphate ester group can also be used as a living site for the reaction with silane coupling agent. In the drying process, the water content is strictly controlled in a reasonable range. In this way, the silane coupling agent can be effectively coated on the surface of nano calcium carbonate through the chemical action of hydrolyzation-polycondensation.