As an inorganic superfine filler, nano calcium carbonate is often used in the formula of silicone sealant through physical filling in order to reduce the production cost. However, inorganic ultrafine particles have hydrophilicity, so it is necessary to improve their compatibility with silicone sealant and their dispersion and stability in colloid through surface treatment, so as to improve the bonding and mechanical properties of silicone sealant products.
Silane coupling agent is a kind of common surface treatment agent, widely used in the surface coating modification of talc powder, aluminum hydroxide, magnesium hydroxide and other inorganic powders. However, the small number of hydroxyl groups on the surface of calcium carbonate presenting weakly alkaline leads to the poor binding of silane coupling agents on the surface of calcium carbonate and weak effect of application performance improvement.
Pretreatment: put the nano calcium carbonate suspension in the reactor, add 3% stearic acid at 70 ℃ for 30-minute surface modification; Then, continue to add 0.5% phosphate for 30-minute modification, then press, filter and dry the semi-finished products; When the moisture is dried to 0.8%, shatter and sieve the sample.
Coupling agent modification: The pretreated nano calcium carbonate is modified by different amino silane coupling agents through dry modification, and then shattered and sieved to obtain active nano calcium carbonate. After modification with different amino silane coupling agents, the specific surface area of the sample will decrease slightly, while the particle size does not change significantly with good particle dispersion, which may be related to the secondary shattering and depolymerization of the sample.
Due to the small number of hydroxyl groups on the surface of calcium carbonate presenting weakly alkaline, it is difficult to form a good chemical coating modification of silane coupling agents on the surface of calcium carbonate through conventional treatment process. Adding phosphate in the pretreatment process of nano calcium carbonate can effectively reduce the pH value of the product and provide a suitable reaction environment; furthermore, the phosphate group can also be used as the living site for the reaction with silane coupling agents to strictly control moisture in a certain reasonable range during the drying process. In this way, silane coupling agents can be effectively coated on the surface of nano calcium carbonate through the chemical action of hydrolysis and polycondensation.
(1) The viscosity of the base material tends to increase after dry modification with amino silane coupling agents; Moreover, with the increasing number of amino functional groups, the viscosity and thixotropic index of the base material also increased, and the extrusion rate of the base material decreased.
(2) After modification with different amino silane coupling agents, the mechanical and bonding properties of the corresponding silicone sealant are improved, especially the elongation increased significantly. In the case of the same particle size, the improvement of mechanical properties is mainly due to the good dispersion of nano calcium carbonate particles in 107 adhesive system and the improvement of inorganic organic interface adhesion.
(3) With the increase of the amount of active amino groups in the modifier, the water resistance of silicone sealant gets better.
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