Preparation And Modification Of Water-diluted UV-curable Polyurethane Acrylate Oligomers Of High Solid Content

Waterborne UV-curable resins have combined the advantages of both traditional UV-curable coatings and waterborne coatings, such as environmental protection(little VOC and easy cleaning), extensive applications(applicable to various coating equipment and convenient coating), good performances(suitable hard & soft oligomer segments and good adhesions), and will become an inevitable trend of industrial wood coatings. Nowadays, waterborne polyurethane-acrylate(WPUA) has been an important research topic of modern coatings for its excellent performances. However, the aqueous dispersion or emulsion polyurethane acrylate prepared by general methods has such drawbacks as a low solid content(below 50%), slow evaporation of water and increased electric power & application time. The aim of the thesis is to synthesize water-diluted UV-curable polyurethane acrylate oligomers of high solid content, which are simply added a little amount of water to dilute and greatly improved production efficiency, coating purpose, reduced transportation costs, and easy cleaning etc. What’s more, a detailed research of various reaction parameters on viscosity effect was conducted to improve production technology of waterborne resins.In order to further reduce the water content in the UV-curable aqueous oligomers, the WPUA oligomers were synthesized using toluene diisocyanate as hard segment, polyether polyol and polyethyleneglycol as soft segments, dimethylol propionic acid as hydrophilic reactant, hydroxyl propyl acrylate and pentaerythritol triacrylates as end-capping reagent. The solid contents of the oligomer were about 80% or so. The influence of types and molecular weights of soft segments, the molar ratios of NCO/OH, the addition sequence, amount of DMPA, the amount of inner crosslinking agent and COOH on the viscosity and film properties of UV-curable WPUA oligomer was investigated. The prepared WPUA oligomers were characterized by FT-IR, GPC and Malvern ParticleSizer. The cured films were analyzed by DSC and TG analysis. The results showed that the COOH content was not the only factor influencing the oligomers’ viscosity. N210 or PPG 400 as soft segments, diols of small molecular weight, the value of n(-NCO):n(-OH)between 1.7 and 1.8, DMPA concentration in hard segments or soft segments, 15% of trimethylolpropane were more favorable to reduce WPUA viscosity. An optimum synthesis result was obtained when the pencil hardness was H, the bending flexibility 2mm, the fastest curing time 2 s, the largest tensile strength 11.88 MPa, the largest elongation percentage 21.4%, and moderate molecular weight, 100 nm of particle size, good thermal stability, as shown by GPC, DSC and TG analysis, respectively, which are more applied to waterborne furniture wood coatings.The effect of curing time, photoinitiator types & concentration, film thickness and other factors on kinetic behaviors were studied by real-time IR spectroscopy, equipped with UV-LED spot light source and infrared spectrometer technology. It was found that the waterborne photoinitiator-Darocur 4265 had better compatibility and higher initiation efficiency with waterborne resin, which only took two seconds to meet the curing demand. The cured film gained the maximum double bond conversion and polymerization rate when the photoinitiator concentration was 4% with a corresponding curing time 0.72 s at the highest polymerization rate. The results showed that the best film thickness was 120 μm, the maximum conversion and gel content was 65% and 85% or so, respectively.Nano-cellulose whiskers(CNWs) were furtherly used to physically and chemically grafting to modify the synthesized WPUA oligomers. FT-IR, GPC, SEM and TEM were used to characterize the performance of resin and UV-cured film. It was found that the waterborne resin of chemical modification had better compatibility and performance than that of physical modification. After chemical modification, the weight-average molecular weight and number-average molecular weight has increased by 25% and 50%, respectively, while the tensile strength and gel fraction has increased with the increase of cross-linking polymerization degree. The fastest curing time(2s) was obtained when the cellulose percentage was 1% and has good water resistance and adhesion. The SEM images showed that there were lots of small and evenly distributed bumps on the unsmooth coated surface. The TEM images showed the size of nano-cellulose 200 nm, the transverse diameter distributions 40-50 nm, which meets the designed requirement of chemical modification.