Synthesis And Properties Of Star Polyesters For High Solid Content Two-component Polyurethane Wood Coatings

Half of the coatings used in China are traditional solvent-based coatings, which release 8 million tons organic solvent per year resulting in severe environmental pollution. Although solvent-based coatings can not be completely replaced, high-solid coatings are the development tendency due to low content of organic solvent. The VOC emission limitation of solvent-based coatings is reduced from 670g/L to 420g/L, 4% of consumption tax for the excess part and high sewage charges for users will be levied, which are enforced by Ministry of Environmental Protection of China in February, 2015. 70% of the wood coatings in China are solvent-based coatings, among which the two-component polyurethane coatings occupy a dominant market share due to the excellent performance. The wood coatings of two-component polyurethane are composed of isocyanate curing agent and hydroxyl resin that have high viscosity and low solid content, which lead to less construction solid content of 20% and out of requirements for new air pollution regulation. Therefore, it is particularly urgent to develop hydroxyl resin with high solid content for the preparation of two-component polyurethane wood coatings. In view of this, we synthesized star hydroxyl polyesters with low viscosity and high solid content through polyol, caprolactone and glycidyl ethers via ring-opening reaction and investigated the rule of relation between structures and properties.A series of four-arm star hydroxy polyesters were obtained by using Di-TMP as the core molecule via ring-opening reaction with caprolactone and glycidyl ethers. Their molecular weight distribution were from 785 g/mol to 1173 g/mol, PDI index was 1.57~1.76. It was characterized by FT-IR and 1H NMR spectroscopy and found that the film gloss, pot life and compatibility between resins and curing agents were improved through modifying partial primary hydroxyl group to secondary hydroxyl group of glycidyl ethers. Aliphatic glycidol ether was used for modification and the product viscosity was lower than that of aromatic products such as benzyl glycidyl ether and unmodified product. In addition, the viscosity reduced with increasing adding amount of aliphatic glycidol ether. The viscosity of the star hydroxyl polyesters prepared by C12 glycidyl ether which possess a long flexible chain was as low as 200 mPa·s when the solid content was 80%, on which the coatings based show an advantage of low viscosity and high solid content.In order to increase the hydroxyl functionality and further enhance the film hardness and chemical resistance, a series of star hydroxyl polyesters with several arm structures were acquired by respectively using polyols trimethylolpropane(TMP) Di-TMP,pentaerythritol(PE) and polyether N303 as core monomer, dimethylol propionic acid(DMPA) as AB2 type monomer, and modified by caprolactone and various glycidyl ethers. The molecular weight distribution of the star hydroxy polyesters were from 1208 g/mol to 1747 g/mol, PDI index was 1.42~1.71. The multi-arm star hydroxyl polyesters were characterized by FT-IR and 1H NMR spectroscopy and it was found that their viscosity and film hardness were significantly enhanced when molecular weight increased. In the same formula system, however, the viscosity and the film hardness decreased with increasing of the molecular weight of nuclear polyols; and the viscosity of the polyesters based on N303 were lower. Additionally, with increasing amount of DMPA; the average hydroxyl functionality of polyesters, film hardness and the resin viscosity increases significantly; and film of the product modified by octyl glycidyl ether showed better transparency than that of C12 glycidyl ether.It was studied the effect of additive amount of hard resin and polymer polyols on the film performance through optimizing hard resin, low viscosity polymer polyols, catalyst, defoamers, diluents and other additives It was able to gain the best two-component coating formula when curing agent ratio was G21: MDI50 = 3 : 2(mass ratio). When hydroxyl component was comprised of G3-C12GE: LTH = 4: 1(mass ratio) and SHP-OGE-1: CO-955 = 4 : 1 and curing agent was composed of G21: MDI50 = 3 : 2 as, the VOC content in construction were 328 g/L and 302 g/L, which is below 420 g/L prescribed by national legislation. The film was of excellent chemical resistance and the pendulum hardness exceeded 0.74.