Preparation Technology Optimization And Properties Of UV-curable Waterborne Polyurethane Coating

In recent years,environmental governance has attracted more and more attention all over the world,and many laws and regulations have been set up to protect environment.Coating is the one main source of pollution and most of the diluents are toxic.Non-toxic,less harm to the person and environmentally-friendly coatings have attracted more and more attention.The UV-curable waterborne coatings combine the advantages of UV-curable coatings and waterborne coatings,such as environmental friendly,non-toxicity,pollution-free,low VOC content,fast curable speed,energy saving,etc.It has become the hotspot of environment-protected coating.However,the research on the preparation technology of UV-curable waterborne coatings is not systematic.In this paper,the effect of structure of diol,carboxylic acid content,acrylate functionality,neutralizing agent,neutralizing degree and shear emulsification process on the dispersion particle size,dispersion stability,curing process and cured film’s properties were studied.Furthermore,in order to improve the water resistance,a silane coupling agent diol was used to modify the UV-curable waterborne polyurethane acrylate.The specific research contents as follows:A series of polyurethane acrylate prepolymer with different functionality and carboxylic acid content were prepared from commercialized polyol(PEG1000/PCL1000/PLA1000),isophorone diisocyanate(IPDI),dimethylol propionic acid(DMPA),hydroxypropyl acrylate(HPA)and pentaerythritol triacrylate(PETA).The prepolymer was characterized by Fourier transform infrared spectroscopy(FTIR)and 1H Nuclear Magnetic Resonance(1H-NMR),which showed that the expected product was successfully synthesized.The carboxylic acids was neutralized with triethylamine(TEA),N,N-dimethylethanolamine(DMEA),triethanolamine(TEOA).The effects of neutralizing degree,shear rate,shear time and carboxylic acid content on the dispersion behavior of the resin were studied.The particle size of dispersion was analyzed by laser particle sizer and the stability of the dispersion was studied by high speed centrifugation and freeze-thaw cycles.The results showed that the viscosity increased from 12.0 mPa.s to 17.6 mPa.s and the average particle sized decreased from 45.50 to 10.32 nm with increasing carboxylic acid content from 4wt% to 10wt%.As a result,the dispersion appearance changed from milkiness to transparency.Moreover,with increasing neutralizing degree from 60% to excessive,the average particle sized decreased from 94.20 nm to 39.31 nm and the dispersion appearance changed from milkiness to milky white latex with bluish.The appropriate carboxylic acid content is 4wt% and the appropriate neutralizing agent is DEMA.The best dispersion process is obtained.Such as neutralization degree is 80%~90%,shear time is 40 min,shear rate is 3000 r/min.The dispersion appearance is milkiness with bluish,the average particle size is 36.60 nm and the stability s over six months.The UV curing process was monitored by FTIR.The conversion of double was reached equilibrin after UV irradiation 25 s.The final acrylate conversion was higher than 92%.The gel content,water absorbable index,water soluble index,the solvent resistance and physical properties of the cured film were tested to discuss the influence of the structure of diol,acrylate functionality and carboxylic acid content on the properties of cured film.The gel content increased with increasing functionality.On the contrary,the gel content decreased from 85% to 69% with increasing carboxylic acid content from 4wt% to 10wt%.The water absorbable index and water soluble index of cured film with PCL/PLA as diols were lower than those of the cured film with PEG as diols.The absorbable index the cured film increased from 18.5% to 19.8% and the water soluble index increased from 3.1% to 6.9%,when the content of carboxylic acid increased from 4wt% to 10wt%.All cured samples possessed excellent flexibility of 0.5 mm and adhesion of 0 level on tinplate substrate.The cured films with PCL/PLA as diols had higher pencil hardness of 3H than the cured film with PEG as diol.Moreover,the pencil hardness of the cured film decreased from 2H to 2B with increasing carboxylic acid content from 4wt% to 10wt%.In order to improve the water resistance,a silane coupling agent diol was introduced into the synthesis of the UV curable waterborne polyurethane in different ratios.The modified resin was characterized by using Fourier transform infrared spectroscopy(FTIR)The surface property of the cured film of modified resin is investigated by contact angle test.The water contact angle obviously increased from 54° to 96° with silane coupling agent diol increased from 0wt% to 8wt%.Meanwhile,the gel content of cured film increased from 83% to 94% with PLA as diol.As a result,the water resistance and the solvent resistance were significantly improved with the addition of silane coupling agent,which suitable content was 8wt%.