Synthesis And Application Research Of Carboxyl-terminated Polyester And Semi-crystalline Polyester For Hybrid Powder Coatings

Hybrid powder coatings,which is a hot research topic, has the dual advantages of zeroVOC and cost-effective. This project is aimed at developing carboxyl-terminated polyesterwith low acid value and a semi-crystalline polyester for hybrid polyester/ epoxy powdercoating. Effects of various factors on the synthesis and properties of polyester were discussed.The polyester was cured with E-12 the epoxy resin. Various factors were found to beinfluential on the curing process and film properties. Experimental formula and curingprocess conditions for the production of polyester/ epoxypowder coating were optimized.Based on formula design and experimental validation, optimal parameters during thesynthesis process of carboxyl-terminated polyester were found as follows: The molar ratio ofethylene glycol (EG) to neopentyl glycol (NPG) was 0.23, trimethylol propane (TMP) dosagewas 2.5% of alcohol component, The molar ratio of terephthalic acid (TPA) to isophthalicacid(IPA) was 4, Adipic acid (AA) dosage was 16% of total acid component, The molar ratio of-OH to -COOH was 1.04~1.05, Trimellitic anhydride (TMA) was selected for the finalacidulation treatment, The dosage of organic tin catalyst was 0.08% of total mass weight,Triphenyl phosphite (TPP) at 0.15% of total mass weight was added thermal stabilizer. Underconditions above, the resultant polyester exhibited the best performances.Optimized synthesis process for semi-crystalline polyester is as follows: gradualprogramed heating strategy was put into practice through direct esterification followed by fritreaction. Reaction kettle temperature was controlled at 230℃~240℃in latter stage. thevacuum degree was controlled at 700~750 mmHg for 2~2.5 h. polyester with appropriatemolecular weight and low free-monomer content could be obtained at optimized synthesisprocess above.1, 4-Cyclohexanedimethanol and Trimethylol propane were selected as polyester diols, atmole fraction of 4.5% and 0.45% respectively. Adipic acid was post added at latter stage. Thecrystallinity degree of resultant polyester is 0.38 at conditions above. As was suggested fromperformance testing results, semi-crystalline polyester possesses favorable physical andchemical properties, such as great stability and good leveling property. Some novel ideas wereproposed about the formula design and synthesis process optimization of semi-crystallinepolyester.In this paper, hybrid powder coating was produced from carboxyl-terminated polyesterand epoxy, with polyester/ epoxy ratio at 70/30. It has excellent performances and priceadvantage. By mixing carboxyl-terminated polyester and semi-crystalline polyester at the weight ratio of 1:1, hybrid powder coating with lower curing temperature, no orangeappearance and better leveling as well as mechanical properties could be prepared. Whenphosphonium promoter as high effect curing catalyst was added at a dosage of 0.2%~0.3%,curing temperature of the hybrid powder coating could be lowered by 30~40℃, in addition, ithas a good film performance.