Fouling during emulsion polymerization

该研究项目旨在更深入地了解乳液聚合过程中导致反应器壁和传热表面形成沉积物的机制。因此，存款形成的初始阶段应通过两种互补措施原位跟踪：首先，集成到反应器壁中的石英晶体微量天平（QCM），其次，可以以逐步方式从反应器垂直抽出的加热指。以这种方式，在相应时间的沉积状态被保存以用于稍后的检查。一个全面的时间分辨表征的所有过程中的散装和表面结合这些过程的相互关系，提供了一个全面的观点对所有机制的存款形成。表征应包括化学参数（如污染物的成分）和物理参数（如沉积层的粒度、粗糙度和柔软度）。表征应允许区分颗粒污垢和反应污垢，前者也发生在非反应性分散体中。初步研究表明，在某些条件下，在壁上形成薄的聚合物层（厚度< 1 µm），从而稳定该表面，防止形成更厚的污垢层。哪些系统参数会导致这种钝化，目前尚不清楚，应进行彻底调查。关于材料，重点应放在聚丙烯酸酯和聚醋酸乙烯酯（pVAc）上。pVAc倾向于交联并且具有低于反应温度的玻璃化转变温度。它通常用聚乙烯醇（PVOH）的接枝链稳定。高温下PVOH在水中的溶解度不足可能会促进结垢。丙烯酸酯具有不同的玻璃化转变温度。硬颗粒和软颗粒的不同之处在于球体变形成多面体的程度，而涂层仍在形成（“湿烧结”）。QCM的使用在分析中打开了特殊的机会。特别地，平面层的生长可以与簇生长区分开。根据其中一个工作假设，平面层倾向于比簇更有效地钝化表面以抵抗厚层的沉积。此外，QCM可以估计有效剪切模量。随时间增加的剪切模量表明层的致密化，可能是通过湿烧结。该项目的目的是在所涉机制的描述和定量建模方面开辟新天地。

The research project aims at a deeper understanding of the mechanisms that lead to the formation of deposits on the reactor walls as well as heat transfer surfaces during emulsion polymerization. Therefore, the initial phase of the deposit formation shall be tracked in-situ by two complementary measures: firstly, a quartz crystal microbalance (QCM) integrated into the reactor wall and, secondly, a heating finger that can be withdrawn vertically from the reactor in a stepwise manner. In this way, the state of deposition at the respective time is preserved for later inspection. A comprehensive time-resolved characterization of all processes in the bulk and at the surface combined with the correlation of these processes with each other provides a comprehensive view on all mechanisms contributing to the deposit formation. The characterization shall include both chemical parameters, such as the composition of the fouling material, and physical parameters, such as the particle size, the roughness and the softness of the deposited layer. The characterization shall allow to distinguish between particle fouling and reaction fouling, the former also occurring in non-reactive dispersions. Preliminary investigations have shown that under certain conditions a thin polymer layer (thickness < 1 µm) forms on the wall, which stabilizes this surface against the formation of further, thicker fouling layers. Which system parameters bring about such a passivation is poorly understood and shall be investigated thoroughly. With regard to the materials, a focus shall be on polyacrylates and polyvinyl acetate (pVAc). pVAc tends to crosslink and has a glass transition temperature below the reaction temperature. It is usually stabilized with grafted chains of polyvinyl alcohol (PVOH). Insufficient solubility of PVOH in water at high temperatures may promote fouling. Acrylates are available with different glass transition temperatures. Hard and soft particles differ in the extent to which the spheres deform into polyhedra while the coating is still forming ("wet sintering"). The use of the QCM opens special opportunities in the analysis. In particular, the growth of a planar layer can be distinguished from cluster growth. According to one of the working hypotheses, planar layers tend to passivate the surface against the deposition of thick layers more efficiently than clusters. Furthermore, the QCM can estimate an effective shear modulus. A shear modulus increasing with time indicates compactification of the layer, possibly by wet sintering. The project aims to break new ground in terms of both a description of the mechanisms involved and of quantitative modelling.