Preparation Of Siloxane End-capped Polyurethane And Its Application On Coatings

Siloxane end-capped polyurethane (SPU) is a modified polymer which based onurethane chains and terminated with alkoxysilane. SPU can be obtained by end-modifyingpolyurethane (PU) prepolymer with silane coupling agent containing functional group. SPUpolymer has a special structure with both organosilicone and polyurethane chains. Thus SPUproducts generally present excellent adhesion property, aging resistance, water resistance andheat resistance properties. In this paper, new silane end-capping agents were synthesised toprepare SPU, in order to further optimize the preparation process and expand the applicationfields of SPU polymer. The detailed research contents and results are summarized as follows：Five kins of new silane end-capping agents were synthesized, which all terminated withalkoxysilane. Silane end-capping agent A contains6alkoxy groups, silane end-capping agentB contains5alkoxy groups, while silane end-capping agent C, D, and E all contain3alkoxygroups.A series of SPU polymers were prepared through a two-step method. First, isocyanateterminated polyurethane prepolymers were synthesized, second, SPU polymers were preparedby end-modifying polyurethane prepolymer with silane end-capping agent agent A, B, C, D orE. No gel was appeared through the preparation process. The50%weight loss temperature ofSPUA5film is22.6°C higher than that of PU5film, and the char yield shows an enhancementratio of367%, which prove that the thermal stability of SPU film is better than PU film. Thethermal stability of SPU film increases with the increase of NCO/OH molar ratio or thecontent of alkoxy silane. In general, with the increase of NCO/OH molar ratio, the tensilestrength of SPU films firstly increase and then decrease, and the tensile strength reaches themaximum (8.0MPa) when NCO/OH molar ratio is1.5. The tensile strength of SPUA5filmincreases by300%compared with PU5film, showing that after modified by silane, themechanical strength of polyurethane increased. Tensile strength of SPU films increases, whileelongation at break decreases, with the increase of alkoxy number in the silane end-cappingagent. Tg of SPU film is higer than PU film. Tg of SPUA5film is41.7°C, presenting anenhancement of37.6°C than PU5, which is4.1°C. Phase separation is observed in SPU films.The degree of phase separation increases with the increase of NCO/OH molar ratio or thenumber of alkoxy group of silane end-capping agent.SPU coatings were prepared using SPU as base polymer, and filler, plasticizer, andcuring catalyst as additives. Mechanical property, chemical resistance, water permeability andwater vapor permeability of SPU coatings were analyzed. The results show that nano calcium carbonate can well improve the strength of SPU coating, while rubber oil can increase theelasticity of SPU coatings. After immersed in water,5wt%NaOH aqueous solution, ortoluene, the retention ratio of tensile strength and elongation at break of SPU coatings are keptmore than80%, which shows that SPU coatings present good chemical resistance. Waterpermeability test result of SPU coatings is all0mL, proving that SPU coatings have goodwaterproof performance. Water vapor permeability of SPUA coating is704g/(m2·24h), andincreases15%than that of PU5coatings, which shows that SPU coating exhibts excellentwater vapor permeability.