Investigation On The Relationship Between The Stability Of Waterborne UV-curable Coatings And The Structure Of Emulsifiers

Waterborne UV-curable coatings with the advantages of waterborne coatings and UV-curable technology,which reduce the amount of active diluents and VOC emissions,have become one of the major development directions of coatings in the future.However,it is difficult to control the adsorption and desorption process of emulsifier in waterborne UV-curable coatings,which will decrease the stability of emulsion and affect the performance of paint film.Due to the variety of structure of the oil phase components,the relationship between the chemical structure of emulsifier and the stability of emulsion becomes extremely complicated.Therefore,in-depth study of the relationship between the structure of emulsifiers and the stability of waterborne UV coatings is of great significance for the selection of emulsifier system in the process of emulsion preparation,reducing the blindness of emulsifier system selection,and improving production efficiency and product stability.In order to solve the above problems,a company in Zhejiang Province proposed a project to study the relationship between the stability of waterborne UV-curable coatings and the structural properties of emulsifiers.In this paper,a series of non-ionic and ionic emulsifiers were used to emulsify the polyurethane acrylate resin(M-1).The screening method of emulsifier system and the evaluation method of emulsion stability were studied.The relationship between the structural properties of emulsifiers and the stability of emulsion was discussed.The main conclusions of this paper are as follows:(1)A method for screening emulsifiers,called conductivity method,was developed.The emulsifier could be screened according to the critical water content(Rf)of the opposite turning point during the emulsification process.The smaller the Rf is,the better the stability of emulsion have.A parameter-K value was found to evaluate the stability of emulsion.K value represents the sedimentation rate of the emulsion.The smaller K value means that the emulsion have better stability.By studying the relationship between Rf and K values in different emulsifier systems,it was confirmed that the conductivity method can be effectively used to screen emulsifier systems.(2)During the emulsification process of polyurethane acrylate resin(M-1),the effect of different non-ionic and ionic emulsifiers on the critical water content Rf was studied by conductivity method,which was then related with the emulsion stability.The results are obtained as follows:i)For non-ionic emulsifiers,it is found that when Rf ≤0.25,the stability of the emulsion is relatively good;when Rf>0.3,the stability of the emulsion is poor.Octylphenol polyoxythylene ether(OP-40)emulsifier has the minimum Rf(Rf=0.19)and K value of 42.6%,and the stability of emulsion is the best.The reason is attributed to that OP-40 emulsifier has a good adsorption on the oil-water interface and an elastic film is formed on the oil-water interface,leading to the strongest interface film.ii)For the ionic emulsifiers,the Rf value are all higher than 0.3,and the emulsion stability is poor.During the emulsification process,polyurethane acrylate sodium sulfonate(M-3)has the minimum Rf(Rf=0.31)and K value of 75.1%,and the stability of emulsion is the best.The reason is attributed to that structure of M-3 is the most similar to the oil-phase resin,which is favorable for forming a dense and rigid interfacial membrane and obtaining a good emulsification effect.iii)For compounding emulsifier system,the Rf of OP-40 and M-3 compounding emulsifier system is 0.5,and the K value is only 3.2%,and the emulsion has the best stability.The emulsion stability of the compound system of anionic and non-ionic emulsifier is better than that of the single emulsifier system.The reason is attributed to that addition of anionic emulsifier in the compound emulsifier system increases the electrostatic repulsion between the latex particles,prevents the flocculation of the latex particles,and effectively improves the stability of the emulsion.(3)The suitable emulsifier systems for acrylic resin(M-5)and epoxy resin(M-6)was screened by conductivity method,and waterborne UV-curable coating products with good emulsion stability was successfully prepared.