Siloxane-modified Polyester: Synthesis And Features Of Its Powder Coating

Siloxane-modified polyester for powder coating was synthesized with the silicone intermediate Z-6018 obtained from Dow Corning company as well as the silicone intermediates synthesized in my own laboratory. The effects of synthesis process, siloxane content and structure of siloxane intermediate on the properties of modified polyester and its powder coating was discussed.The structure of the modified polyester were analyzed by IR and GPC which showed the siloxane had chemically graft on to polyester by Si -O-C bond. The composite of modified resin system was considered as follows: the ABA block copolyester was formed which composed of different amount of the siloxane intermediate segment in the polyester, the copolyester was also mixed with pure polyester. The block copolyester in the modified polyester effected the Tg, melt viscosity, soft point and surface property of modified resin.The synthesis route and technique for the modified polyester affected the siloxane content in the block copolyester. The research showed that the siloxane intermediate was reacted with organic composition more easily and efficiency by the synthesis route one(siloxane intermediate reacts with neopentyl glycol first) , resulted in the improved compatibility so that the Tg and melt viscosity of modified resin was decreased more effectively and the impact-resistance of the coating film was good.The tendency to self-condensation of siloxane intermediate was more obvious as increasing the siloxane content which effected the structure of formed block copolyester. It was found that the addition of 1 wt% siloxane composition decreased the Tg and melt viscosity of modified resin most effectively. At the same time, the surface properties was changed. As compared to the neat polyester, the surface tension of the modified polyester was decreased from 49.5 to 28.4 mN/m due to the enrichment of siloxane containing polyester in the surface layer, the enrichment would reach a balance when siloxane content exceeded 1%.The addition of siloxane improved the weather-resistance of the coating film which was decided by the enrichment of siloxane composition in the surface layer and cure property of surface layer. The research showed that the gloss retention ratio of the coating film was increased from 81% to 91.3% when exposed to medium ultraviolet wave for 216 hours as compared to the pure polyester. The structure of siloxane intermediate was also effect the weather-resistance of the coating film. It was found that the weather-resistance was best when R/Si was 1.3 keeping Ph/R a const value.The effect of the synthesis of siloxane intermediate was researched and discussed.It was found that the diffusion of silanol and siloxane oligomer from aqueous phase to solvent phase was a key step which effected the molecule weight and hydroxyl content of siloxane intermediate.