摘要
可再分散聚合物乳胶粉是一种加水能重新分散在水中形成分散液,并具有与原乳液相似性能的聚合物粉末。一般是由聚合物乳液经保护胶体等改性后,通过喷雾干燥而制得。少量可再分散乳胶粉添加于水泥基干粉建材中可改善其多种性能,因而广泛应用于多个领域。随着干粉建材的日益发展以及人们对建材性能的要求的提高,高性能的可再分散乳胶粉的需求越来越大。
本文通过粒子设计理论制备出具有“软核硬壳”结构的苯丙乳液,并通过喷雾干燥制备出性能优异的可再分散乳胶粉。重点探讨了喷雾干燥过程对乳胶粉性能的影响。
(1)采用半连续种子乳液聚合工艺,以甲基丙烯酸甲酯(MMA)和苯乙烯(St)为硬单体、以丙烯酸丁酯(BA)和丙烯酸异辛酯(2-EHA)为软单体、以甲基丙烯酸(MAA)和甲基丙烯酸羟丙酯(HPMA)为功能单体,制备了具有核壳结构的苯丙乳液。实验结果表明,当壳层Tg达到65℃～75℃,总Tg为0℃～5℃、乳液粘度为91.5 mPa·s、粒径为114nm、固含量为45%时,乳液较适合于喷雾干燥。
(2)采用保护胶体和强碱对苯丙乳液进行改性。研究结果表明,当选用聚合度为500、水解度为88%的聚乙烯醇(PVA-205)作为保护胶体、PVA用量为4%～6%(占乳胶粉质量百分比,下同)、采用5%的氢氧化钠溶液(质量浓度)、其用量为将喷雾液pH值调节至9、将调碱后的喷雾液放置5小时、再将喷雾液的粘度控制在15～20s时,配制的喷雾液较适合喷雾干燥。
(3)无机抗结块剂在可再分散乳胶粉中能够阻止乳胶粉粒子相互接触而发生结块,提高储存稳定性并且可以降低乳胶粉的成本。经试验表明,采用超细硅酸铝作为无机抗结块剂,并在喷雾干燥后加入,加入量为10%～12%,可显著提高乳胶粉的储存稳定性。
(4)通过调节喷雾干燥过程的各种参数,实验结果表明,当喷雾干燥的进口温度为120～140℃、出口温度为70～80℃、、雾化盘为满负荷运转且转速为24000 r·min-1、进料泵转速为30～35 rpm时,可得到性能较好的可再分散乳胶粉。
通过喷雾干燥法制备的可再分散苯丙乳胶粉,其再分散性能优异、流动性好、储存稳定优良的耐碱性和性价比较高。
Re-dispersible polymer powders is a kind of polymeric powders which can easily re-emulsified in water to reform liquid emulsion with essentially identical properties similar to the original emulsion. It is made by means of spray- drying from the emulsion. modified by protective colloids. It can be added into many cement-based architecture materials to improve its performances, and therefore it has been widely used in many industrial fields. With the development of powdered architecture material and the more rigorously demand of people for the qualities of architecture material, the requirement of high-performance redispersible powdered emulsion became more and more .
A kind of styrene-acrylate emulsion with“soft core and hard shell”structure was synthesized on the basis of“the latex particle design”, and and re-dispersible polymer powders were produced by spray-drying. The effects of spray drying process that influence the properties of re-dispersible styrene-acrylate polymer powders were mainly discussed.
(1) Seeded emulsion polymerization based on semi-continuous process was used to produce styrene-acrylate emulsion with“core and shell”structure. MMA and St were applied as hard monomers, BA and 2-EHA acrylate were applied as soft monomers, MAA and HPMA were applied as functional monomers. The results showed that when shell Tg was 65℃~ 75℃, the total was Tg 0℃~ 5℃, emulsion viscosity was 91.5 mPa ? s, particle size was 114nm, and solid content was 45%, the emulsion was more suitable for spray drying.
(2) Styrene-acrylate emulsion was modified by protective colloid and alkali. The results showed that the polyvinyl alcohol(PVA-205) should be chosen as protective colloid, in that the degree of polymerization was 500 and the degree of hydrolysis was 88%, and the concentration of 5 wt % NaOH solution should be used. When the PVA-content was 4%～6% (mass fraction based on re-dispersible polymer powders, the same below), the pH value of spray liquid was 9, the storage time was 5 hours, and the viscosity of spray liquid was controlled in15～20s, The spray liquid was suitable for spray - drying.
(3) Inorganic anti-caking agents were added into re-dispersible polymer powders in order to avoid coalescence, to improve storage stability and to reduce the cost of re-dispersible powder. The results showed that when 10～12wt% ultra-fine aluminum silicate were added into re-dispersible polymer powders, the storage stability of the powders could be significantly improved.
(4) Various parameters of spray drying tower were adjusted to optimize the drying process. The showed that inlet temperature of 120～140℃, outlet temperature of 70～80℃, atomizing disc running of 24000 r·min-1and feeds of 30～35 rpm were chosen as the optimal process condition.
Re-dispersible styrene-acrylate polymer powders prepared by spray-drying showed superior redispersion, fine fluidity, high-quality storage stability, excellent alkali resistance and high performance-price ratio..
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