A Study On Silicone Sealants With Surface-paintability

Organosiloxane sealants have been widely used in many applications. However, painting of these materials is difficult due to low surface tension and surface energy of them, which results in a considerably limitation for their application.In this dissertation, our primary concern aims at the improvement of the surface paintability of organosiloxane sealants, which could be achieved by several routs such as introducing polyurethane prepolymer, filler surface treatment and the incorporation of promoters.Firstly, a novel aminoalkysilane, N-n-butyl-γ-aminopropyl-methyldimethoxy-silane, was synthesized in order to improve the compatibility between polyurethane segments and polysiloxane chains. The chemical structure of the product was well confirmed by IR, 1H NMR, 13~CNMR and elemental analysis.Secondly, different kind of polyurethane terminated with isocyanate group (NCO) or hydroxyl group (OH), was synthesized using different polyethers and blended with the main compositions of organosiloxane sealant. The influence of these prepolymers on paintability revealed that the incorporation of polyurethane prepolymer can significantly increase the surface paintability. Moreover, the factors effecting the surface paintability were discussed. It is found that the surface paintability was dependent on painting time and testing period. Generally, the longer the coating curing time or the quicker the painting, the better paintability appeared. On the contrary, the reduction of testing time and the prolongation of painting time are unfavorable to the surface paintability.Additionally, the results revealed the molecular weight of the polydiorganosiloxane, the addition and the types of the organosilane coupler and the structure of the polyurethane prepolmer could also affect the paintability of the modified organosilicon sealant. The reasons were explained through the surface free energy and surface contact angle results.Furthermore, filler surface treatment and promoters, the effects of different types of calcium carbonate and surface treating agents on paintability of silicone sealants were compared. It is concluded that no significant influence of surface treatment of filler (CaCO3) on the paintability of organosilicone sealants appeared. These unpromising results were considered to be resulted from CaCO3 sedimentation and the soft segments migration of polysiloxanes. Nevertheless, the surface paintability could partially increased by increasing the filler concentration, decreasing filler's specific gravity and increasing average particle size. In the case of the incorporation of two different kind of promoter respectively, namely AN and SA which contains CN group and SH group, the surface printability could be improved considerably. However, the surface paintability was effective only to alkyd resins, but not hydrophilic acrylic coating.Taking increasing coverage of coating and stability into consideration, the main component of sealants, polysiloxane, was further modified through the co-polymerization with the polyurethane prepolymer. Testing outcome confirmed that owning to incorporation of this modified polysiloxanes, improvement of the surface paintability was much better than that of blending method.In summary, by incorporating polyurethane prepolymer, we are able to prepare surface paintable organosilicone sealants, mechanical properties of which almost maintain unchanged as compared with those of unmodified samples. It is found that the products prepared are friendly to environment and more applicable for mass production.