摘要
阳离子型乳液的基本特征是乳胶粒表面或聚合物本身带正电荷。由阳离子型苯丙乳液制备的可再分散乳胶粉所成胶膜的耐水性及机械性能比普通的苯丙乳液制备的好,因此合成阳离子型苯丙乳液并制备可再分散乳胶粉具有重要的应用价值。
本研究首先通过粒子设计和工艺优化,合成了阳离子型核壳结构的苯丙乳液,然后对其进行喷雾干燥,制备可再分散乳胶粉。并借助差热分析(DSC)、透射电镜(TEM)等对制备的乳液及所成胶膜、可再分散乳胶粉进行了表征与分析。
以甲基丙烯酸甲酯(MMA)、苯乙烯(St)为硬单体,丙烯酸丁酯(BA)为软单体,丙烯酰胺(AM)为功能性单体,设计了具有“软核硬壳”结构的乳胶粒子,考查了聚合反应温度、乳化剂用量、甲基丙烯酰氧乙基三甲基氯化铵(DMC)用量等因素对聚合反应体系、乳液及所成胶膜、可再分散乳胶粉性能的影响,得到了合成阳离子型苯丙乳液的最佳配方和工艺条件:采用半连续种子乳液聚合法、聚合反应温度为80℃,种子单体用量为10%(占单体总质量的百分数);乳化剂采用十六烷基三甲基溴化铵(CTAB),其用量为2%;选取偶氮二异丁眯盐酸盐(AIBA)为引发剂,其用量为0.53%;DMC用量为2%且DMC:AM=1:1。此条件下得到的乳液主要性能参数为:粒径105nm,粘度225 mPa·s,固含量36.6%,pH值5.0~6.0,体系ξ电位45.1mV。DSC测试显示,合成的阳离子苯丙乳胶粒核层与壳层的玻璃化转变温度(Tg)分别为-17℃和65℃;透射电镜(TEM)照片显示,乳胶粒子呈明显的核壳结构。
用自制的阳离子型苯丙乳液经喷雾干燥制备可再分散乳胶粉,对干燥过程中的预处理工艺、喷雾干燥塔进口温度、进料流量、保护胶用量等因素进行研究。结果表明:对乳液不调pH值、喷雾干燥塔进口温度为120℃,进料泵最佳转速为30 rpm,加入1%(占乳液总质量的百分数)的保护胶体PVA-205,在此条件下干燥能得到含水率低、再分散性较好的可再分散乳胶粉。扫描电镜(SEM)测试表明,可再分散乳胶粉粒子表面光滑,单个可再分散乳胶粉颗粒形状基本呈球型,平均粒径在10-30μm左右,可再分散乳胶粉不粘连,能自由流动,加入水中能再分散。
The essential character of cationic emulsion is that emulsion particle surface or polymer itself is positively charged. Comparing with the film prepared from redispersible emulsion powder of original anionic styrene-acrylic emulsion, the film formed by cationic styrene-acrylic emulsion had better water resistance and mechanical performance. Therefore, preparation of cationic styrene-acrylic emulsion used for redispersible emulsion powder is very important in practical application.
This research was mainly divided into two parts: first, cationic styrene-acrylic emulsion with core-shell structure was synthesized by particle design and process optimization; Next, redispersible emulsion powder was prepared based on the cationic styrene-acrylic emulsion by spray drying process. The emulsions, their films and corresponding redispersible emulsion powder were characterized with Differential Scanning Calorimetry (DSC), Transmission Election Microscopy (TEM) and so on.
The particle structure of“soft core and hard shell”was designed with methyl methacrylate (MMA) and styrene (St) as hard monomers, butyl acrylate (BA) as soft monomer, acrylamide (AM) as functional monomer. The effects of polymerization temperature, the amount of emulsifier, the amount of cationic monomer [2-(methacryloyloxy) ethyl] trimethyl-ammonium chloride (DMC) on polymerization reaction system, performances of emulsion and redispersible emulsion powder were investigated, and then the best formulation and process to prepare cationic styrene-acrylic emulsion were obtained. The best formulation and process were as following: the synthesis reactions were carried on at 80℃by semi-continuous seeded emulsion polymerization. The content of seed monomers was 10 wt% based on total monomers. Cetyl trimethyl ammonium bromide (CTAB) was used as emulsifier and the amount was 2 wt% based on total monomers. Azobisisobutryamide chloride (AIBA) was selected as initiator and the amount was 0.53 wt% based on total monomers. The mass ratio of DMC to AM was 1:1 and the amount of DMC was 2 wt% based on total monomers, and then the uniform and stable emulsion with small particle size and narrow particle size distribution could be obtained. Its main performance parameters were as following: the particle size was 105nm; the viscosity was 225 mPa·s; the solid content was 36.6%; pH value was 5.0 ~ 6.0;ξpotential was 45.1 mV. DSC results showed that glass transition temperature (Tg) of core and shell of the obtained polymer were -17℃(theoretical value -27℃) and 65℃(theoretical value 77℃) respectively. Pictures of Transmission Electron Microscopy (TEM) showed that the emulsion particles had clear core-shell structure.
Redispersible emulsion powder was prepared from cationic styrene-acrylic emulsion by spray drying process, and factors such as pretreatment technology, the inlet temperature of spray drying tower, feeding flow, dosage of protective colloid and so on were considered. The results indicated that: without adjusting pH value of styrene-acrylic emulsion, when inlet temperature of spray drying tower was 120℃, the rotational speed of feeding pump was 30 rpm, the content of protective colloid PVA-205 was 1 wt% based on emulsion, The redispersible emulsion powder with low water content and good redispersibility was successfully prepared. Scanning Electron Microscopy (SEM) pictures showed that redispersible emulsion powder particles had smooth surface and presented globular shape with an average particle size about 10~30μm. Redispersible emulsion powder had good fluidity without agglomerating, and it could be redispersed into water.
引文
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