疏水作用对可再分散聚合物乳胶粉水分散性的影响机理研究

摘要

通过改变壳层单体中苯乙烯(St)与甲基丙烯酸甲酯(MMA)的比例,合成一系列疏水作用递减的核壳结构聚合物乳液,并将它们干燥制备成聚合物乳胶粉.研究壳层疏水作用对乳液碱中和过程、乳胶粉微观形貌和水分散性能的影响,并探讨其对水分散性的作用机理.研究结果表明,乳胶粒径随羧基中和而逐渐增大,壳层疏水作用对乳胶粒的中和膨胀有显著抑制作用；疏水性增强使乳胶粉颗粒聚集融合更加严重,水分散性和分散稳定性也随之降低.完全中和使乳胶粒壳层羧基电离生成强亲水性羧酸根负离子,乳胶粒因聚合链间电荷排斥作用形成“绒毛结构”和厚水化层,粒径因而显著增大.随着壳层疏水性的增强,疏水缔合作用与电荷排斥作用相抵消,阻碍疏松“绒毛结构”的生成,碱扩散难度因而增大,壳层羧基中和不完全,导致乳胶粒亲水基团减少,电荷密度降低,水分散过程中乳胶粒之间的溶剂渗透压和电荷排斥作用减弱,颗粒难以迅速分离,水分散性变差；疏水缔合作用导致乳胶粒在喷雾干燥过程中凝聚融合成大颗粒乳胶粉,分散时水分向胶粉内部扩散难度增大,但沉降速度加快,导致分散稳定性降低.%Core-shell structured polymer latexes with different hydrophobic interactions were synthesized by altering shell monomer ratio of styrene(St) to methylmethacrylate(MMA), and the corresponding polymer powders were prepared by spray drying. The effects of hydrophobic interaction on the neutralization of latexes, the micromorphology and water-redispersibility of the prepared polymer powders were studied, and the mechanism of the influence of hydrophobic interaction on water dispersion was investigated. The results show that the latex particle size increases in the neutralization process, but the neutralization and swelling are evidently restrained due to hydrophobic interaction. With increasing hydrophobicity of shell layer, the agglomeration of latex particle is more aggravated in the spray drying, and water-dispersibility and dispersion stability of polymer powder decrease. In the case of full neutralization by base, carboxyl groups in the shell polymer dissociate into negative carboxylate ions with high hydrophilicity. The swelling of latex particle is caused by the formation of "hairy structure" and thicker hydrated shell because of electrostatic repulsion. Stronger hydrophobic interaction counteracts the electrostatic repulsion, thus hinders the swelling of latex particle and the formation of loose "hairy structure". Carboxyl groups in shell layer can not be fully neutralized in that the diffusion of base solution is impeded by tighter shell structure. Incomplete neutralization results in relatively less hydrophilic groups, lower charge density, weaker osmotic pressure and electrostatic repulsion, which show negative influences on water-dispersibility. Meanwhile, hydrophobic interaction results in more agglomeration of polymer particle in the drying process. For bigger polymer powders, the diffusion of water is more difficult and sedimentation velocity increases, all these indicate that the polymer powders will have lower dispersion stability.