| Traditional solvent-based coatings contain more than 50% of the organic solvents,and release to pollute the environment and endanger human health in the use process.The traditional solvent-borne two-component polyurethane coatings(2K-PU)possess China’s majority of wood coatings market due to excellent performance.Hydroxyl resins are the main components and the performance decisive factors of the 2K-PU,and have high viscosity at high solid contents,and its volatile organic compound(VOC)contents are too high to meet the increasingly stringent environmental regulations.In this paper,two kinds of high solid content and low viscosity hydroxyl resins,namely hydroxyl acrylic resin and star hydroxyl polyester,were synthesized to prepare low VOC content 2K-PU.High solid contents and low viscosities hydroxyl acrylic resins were synthesized and the effects of polymerization process on the resin properties were studied.The results showed that resin viscosity is low as well as good performance with 4 : 1(mass ratios)of xylene and acetone as solvents,3.5% DTAP as a initiator,2% mercaptoethanol as a chain transfer agent,3% of the hydroxyl contents,and 160 ℃ of the reaction temperature.The acrylic resin modified with tert-carbonate(V10)possess low viscosity,and the film display high gloss and excellent water resistance with 10% to 15% of the amounts.The structures of the resins were characterized with FTIR and GPC,and the GPC analysis shows that the molecular weight of the resin is ranged from 5000 to 8000,and the molecular weight distribution index is between 2.00 and 2.25.The multi-arm structure of the star hydroxyl polyesters were prepared with polyol was used to react with the acid anhydride,and then ring-opened react with the epoxy compound.The effects of the categories and dosages of the raw materials on the properties of the resin were studied.It was found that the more polyol functionality,the greater the viscosity of the resin,the higher the hardness.Improve the molar ratios of anhydride and polyol,the resin viscosities decreased first and then increased,and the hardness of the paints increased first and then decreased.The hydroxyl polyesters were synthesized by used of different glycidyl ether instead of ECH,and found that resin viscosities and film hardness are reduced as well as the reaction conversion rate.It was also found that a small amount of polyether polyol could effectively reduce the viscosity of the resin and improve the flexibility of the film,which has hardly effect on the hardness,and the N210 modification effect is better than N220.The structure of star resin was characterized by FT-IR and 1H NMR spectroscopies.GPC analysis showed that the molecular weight was in the range of 800-2000 and the distribution indexes was about 1.12.The effects of two kinds of hydroxyl resin on the properties of the coating were studied.It was found that the 2K-PU has 60.5% solid contents and 380 g/L of the VOC contents with 25 s of construction viscosity when hydroxyl acrylic resin and the hydroxyl star polyester were completed according to the mass ratio of 6:4,and the curing agent was prepared according to G21 : MDI-50 = 6 : 4(mass ratio)with 1.05 : 1 of the NCO/OH molar ratio,which are lower than the national standard requirements 420 g/L,and the film has excellent mechanical properties and chemical resistance with 0.76 of the pendulum hardness. |
