【作者】 田和保；

【导师】 张超灿；

【作者基本信息】 武汉理工大学 ， 材料物理与化学， 2010， 博士

【摘要】 研究了作为单组分涂料基料的硅溶胶与硅酸钾(钠)混合的混合物的室温放置稳定性,并用粒径测量、pH测量和等温热导微量热法对其作了表征。结果表明,含小的纳米(粒径在19.0 nm以下)胶体二氧化硅粒子的混合物以及当混合化学反应完全时的总焓变(总焓变为1.6234-3.3882 J)大的混合物稳定性好,稳定性受硅酸钾(钠)模数、硅溶胶在混合物中占的相对重量百分比(硅溶胶占53、65、75、85 wt%)、混合操作条件、原材料规格、温度、pH值、二氧化硅浓度、钾和钠离子、有机硅和硅烷偶联剂之类的稳定剂、高分子乳液、增稠剂与分散剂的合理搭配等等因素影响。加了适当稳定剂的该混合物稳定存放时间大大延长。最后再加入苯丙乳液配制成的基料在室温下可存放至少7个月以上。选择了助剂和颜填料,初步配制成的涂料所形成的膜具有光滑坚硬的特点。等温热导微量热法对于二氧化硅聚合反应是一种全新的表征方法,在25°、35°和45℃及搅拌条件下,采用该方法研究了硅溶胶与硅酸钾(钠)的混合过程。结果表明,硅溶胶与硅酸钾(钠)混合时立刻发生了不是酸碱中和而是二氧化硅溶解和聚合的化学反应并产生了热效应,热效应受温度、硅溶胶所占的相对重量百分比、钾和钠离子等因素的影响。其聚合反应的特征是反应级数从低到高、时刻都在快速不断变化；焓变随着温度升高而增大；当硅溶胶和硅酸钾里的二氧化硅低聚化反应处在3.0的反应级数时,反应速率常数在25.0℃最大,为1.22×10-4mol-2dm6s-1；二氧化硅单体低聚化可以分为两个温度区,在25.0°-35.0℃温度区,聚合反应快,在35.0°-45.0℃温度区,聚合反应慢。这两个温度区反映了两个阶段的低聚化反应,并且以两步阴离子机理形成了不同的低聚物。高温有利于环状和大的胶体二氧化硅粒子的形成,而低温所产生的是线性和支链的低聚物,低温形成的硅溶胶与硅酸钾的混合物更稳定。纳米胶体二氧化硅粒子的形成分为三个阶段。第一个阶段,硅溶胶与硅酸钾(钠)混合,pH值发生改变,硅溶胶和硅酸钾(钠)中的胶体粒子溶解,此为热谱曲线上的第1段。第二个阶段,混合物中的二氧化硅单体(原硅酸)聚合,不断生成二聚体、三聚体等等低聚物以及增长的二氧化硅胶体粒子,反应速率不断变小,直至反应完成,此为热谱曲线上的第2段。在这一阶段,硅溶胶的两个粒径分布(由强度)峰发生改变,硅酸钾(钠)里的“活性硅”再沉积到硅溶胶重新排列的粒子上,形成与原硅溶胶和硅酸钾(钠)都不同的新的粒径分布。硅溶胶所占的份额越多,热谱曲线峰越高,焓变越大,其混合物所形成的胶体二氧化硅粒子是小粒径的,反之则是大粒径的。小粒子形成增强的粒子间硅氧键,发展成凝结程度高的纤维状的并具有大的拉伸强度和好的耐水性的结构网络,但是,所形成的凝胶干燥后会开裂。第三阶段,粒子之间进行聚集,热效应很小,此为热谱曲线上的第3段。粒径测量观察到的是热谱曲线第2段的后面部分和第3段全部,这些部分在粒径分布统计图(由强度)上可以定性地指定为粒径在100 nm以下的基本粒子、100 nm以上至几百nm的由基本粒子增长而成的大的胶体粒子和1000 nm左右及以上的二氧化硅单体和低聚物等成分三个部分。微量热法、粒径和pH值测量结合,可以全面完整地观察硅溶胶和硅酸钾(钠)的混合及其陈化过程,为涂料配方设计和涂料研制提供了理论指导。更多还原

【Abstract】 The stability (when aged at ambient temperature) of mixtures (as a one-component coating binder) of silica sol and potassium silicate (or potassium sodium silicate) was investigated. Some of the mixtures were characterized by using isothermal heat conduction microcalorimetry and by measuring their particle size and pH value. Experimental results showed better stability for the mixture containing small nano-sized colloidal silica particles (less than 19.0 nm in diameter) and for that in a larger total enthalpy change (1.6234-3.3882 J) for a mixing reaction when it has gone to completion. And the stability was affected by the molar ratio of silica to alkali metal oxide in the potassium silicate (or potassium sodium silicate), the percentage by weight of the silica sol (53,65,75, and 85 wt% fraction) in the mixture, operating conditions when mixing, raw material specifications, temperature, pH value, SiO2 concentration, K+ and Na+ ions, stabilizers such as organic silicon and silane coupling agent, polymer emulsion, a reasonable collocation between thickener and dispersing agent, etc. When appropriate stabilizers were added into a mixture, the mixture exhibited a significantly longer shelf life. Moreover, the shelf life of the binder which was prepared by mixing the mixture and stabilizers as well as styrene-acrylic emulsion can reach more than 7 months at ambient temperature. Selected additives and pigments and fillers, the coating from preliminary preparation formed a smooth and hard film.Isothermal heat conduction microcalorimetry is a novel characterization method for silica polymerization, and was adopted to investigate the polymerization processes of silica when the combination of silica sol and potassium (sodium) silicate was stirred at 25.0°,35.0°, and 45.0℃. Microcalorimetric results indicated that chemical reactions occurred immediately in the mixed silica sol and potassium (or sodium) silicate were not a acid-base neutralization reaction but the dissolution and complex polymerization of silica with heat evolved, which was affected by temperature, the percentage by weight of the silica sol in the mixture, and K+ and Na+ ions. The silica polymerization was characterized by reaction orders which were rapidly and continued changing from low to high all the time. And when the reaction order for the oligomerization of silica in the mixed silica sol and potassium silicate was 3.0, the maximum rate constant occurred at 25.0℃(k = 1.22 x 10﹣4 mol﹣2 dm6 s﹣1). The two temperature regions (25.0°-35.0℃region with a faster rate and 35.0°-45.0℃region with a lower rate) reflected a two-stage oligomerization of silica monomers with different oligomers formed in a two-step anionic mechanism. The enthalpy change was greater at each higher temperature. The formation of circular and large silica particles was favored at high temperature, and the formation of linear and branched-chain oligomers was done at low temperature. The mixture of the silica sol and potassium silicate was more stable at the low temperature than that at the high temperature. The formation of nano-sized colloidal silica particles can be divided into three stages. In the first phase, when the silica sol was mixed with the potassium (sodium) silicate, the pH value of both silica sol and potassium (sodium) silicate changed, both the small and large colloidal particles in the silica sol and potassium (sodium) silicate dissolved, this was reflected at sectionl in power-time curve in microcalorimetric experiments. The second stage, followed by silica monomer polymerization, continuously created dimers, trimers, etc. oligomers and growing particles of silica, became smaller reaction rate continuously until the reaction completed, this is section2 in the power-time curve. At this stage, the two original peaks of the silica sol on particle size distribution by intensity changed, "active silica" in the potassium (sodium) silicate then redeposited onto the particles of rearrangement of the silica sol to form a distinct particle size distribution from original that of the silica sol and potassium (sodium) silicate. In the mixture of the silica sol and potassium silicate, the more the silica sol fraction by weight in the mixture, the higher the peak height of power-time curve, the greater the enthalpy change, the smaller size the silica particles to form, otherwise the larger size the silica particles to do. Strengthening interparticle siloxane bonds resulted from small size particles can develop into greater coalescence and stronger and more fibrillar Si-O-Si chain structure network with great tensile strength and good water resistance, but gels formed by the small size particles will be cracking during drying. The third stage, that is section3 in the power-time curve, was the process of aggregation between silica particles occurred with much less heat evolved. The size and distribution observed by particle size measurement is the back section2 and the all section3 in the power-time curve. Size statistics reports by intensity or volume can be qualitatively designated as three parts for elementary particles in less than 100 nm size in diameter, large colloidal silica particles grown from the elementary particles in more than 100 nm to several hundred nm sizes and silica monomers and oligomers in 1000 nm around and above sizes. Microcalorimetric experiments, particle size and pH value measurements can be combined to observe comprehensively and completely processes in the mixed and aged silica sol and potassium (sodium) silicate, and to provide a theoretical guidance for the formulation and development of coatings.更多还原