Factors Affecting Surfactant Distribution in Pressure-Sensitive Adhesives during Film Formation and Ageing

压敏胶（PSA）在施加轻微压力的情况下几乎可以粘附在任何表面上。它们用于日常生活中，例如胶带、标签、图形、汽车涂层和一些医疗设备。PSA历来是由溶于有机溶剂的粘弹性（软）聚合物制造的。在流延PSA溶液时，有机溶剂蒸发掉，留下膜。如今，为了避免有机溶剂排放到大气中，这可能对环境产生有害影响，PSA越来越多地使用胶体聚合物在水中的分散体（称为胶乳）来制备。为了合成乳胶，使用乳液聚合，这需要使用皂类分子，称为表面活性剂。表面活性剂有助于胶乳分散体的稳定性，并且对于该方法是必不可少的。不幸的是，表面活性剂已被证明会对PSA的性能造成不利影响，例如由于不需要的水吸附和表面活性剂迁移到PSA的表面时的粘附力的损失而导致的光散射导致的光学透明度的损失。本研究的目的是了解影响PSA中表面活性剂的非均匀分布的因素，并确定是否可以控制该机制。由于表面活性剂可以在成膜（干燥）阶段积累，或者在PSA干燥状态下老化期间积累，因此研究将集中在这两种情况下。最近的数学模型预测，几个关键因素对整个成膜阶段的表面活性剂的分布有影响。水蒸发速率、温度、颗粒尺寸、膜厚度和粘度（聚合物和水相）都被预测起作用。在本项目中，将设计实验以系统的方式测试模型，并试图提供明确的作用，每个确定的因素对最终的表面活性剂的分布。该研究还将调查是否诱导电场从离子表面活性剂梯度，在PSA干燥过程中开发可以通过电泳机制驱动颗粒运动。目前膜形成过程中表面活性剂分布的模型没有考虑电泳的影响，但它可能是重要的。表面活性剂的深度分布将在萨里离子束中心通过卢瑟福背散射（RBS）测定重元素，如阳离子，并通过弹性反冲检测（ERD）测定氘和氢。表面活性剂的分布将与离子浓度梯度的贡献增加进行比较。在PSA膜老化方面，以前的实验研究表明，在干燥状态下，在老化研究过程中，湿度会对表面活性剂的分布产生影响。不幸的是，这些实验范围有限，并且结果与PSA的机械性能不相关。在这项工作中，我们将老化两个商业PSA在0%，42%和85%的相对湿度为6个月，并确定表面组成和粘合剂性能的变化。将薄膜浇铸到PET片材（或玻璃板）上，并覆盖硅树脂剥离衬垫进行储存。将独立测量水蒸气吸附。将在老化初期和老化后进行粘附力测量（剥离、环和探针）。老化过程中的薄膜的表面结构和组成的定性调查将使用原子力显微镜（AFM）。将使用西姆斯分析进行表面成分的补充测量。粘合剂性能的任何变化都将与纳米结构和表面成分的变化相关联。该项目将与Synthomer plc合作进行。

Pressure-sensitive adhesives (PSAs) adhere to nearly any surface under the application of light pressure. They are used in everyday life with applications such as tapes, labels, graphics, coatings on cars and in some medical devices. PSAs have historically been manufactured from viscoelastic (soft) polymers dissolved in organic solvents. Upon casting the PSA solution the organic solvent evaporates away, leaving the film behind. Nowadays, in order to avoid the emission of the organic solvents into the atmosphere, which can have detrimental effects on the environment, PSAs are increasingly being made using colloidal polymer dispersions in water, known as latex. In order to synthesise latex, emulsion polymerisation is used, which requires the use of soap-like molecules, called surfactants. Surfactants aid the stability of the latex dispersions and are essential to the process. Unfortunately, surfactants have been shown to cause detrimental effects on the properties of PSAs, such as a loss of optical clarity due to light scattering from unwanted water sorption and a loss of adhesion when surfactants have migrated to the surface of the PSA.The aim of this research is to understand the factors that affect the non-uniform distribution of surfactants in PSAs, and to determine whether the mechanisms can be controlled. Since surfactants can accumulate either during the film formation (drying) stage, or during the ageing of the PSA once in the dry state, the research will focus on both of these two scenarios. Recent mathematical models have predicted that several key factors have an effect on the distribution of surfactants throughout the film formation stage. The water evaporation rate, temperature, particle size, film thickness and viscosity (polymer and water phase) have all been predicted to play a role. In this project, experiments will be designed to test the models in a systematic way, and attempt to provide clarity with respect to the role that each of the identified factors plays on the final surfactant distribution.The research will also investigate whether induced electric fields from ionic surfactant gradients that develop in the PSAs during the drying process could drive particle motion via the mechanism of electrophoresis. Current models of surfactant distribution during film formation do not consider the effects of electrophoresis, but it might be important. Depth profiles of the surfactants will be determined via Rutherford Back Scattering (RBS) for heavy elements, such as the cations, and Elastic Recoil Detection (ERD) for deuterium and hydrogen at the Surrey Ion Beam Centre. The surfactant distributions will be compared with increasing contributions of ionic concentration gradients.In relation to PSA film ageing, previous experimental studies have suggested that humidity can have an effect on surfactant distribution during the dry state, during ageing studies. Unfortunately these experiments had limited scope, and the results were not correlated with mechanical performance of the PSAs. In this work we will age two commercial PSAs at 0%, 42% and 85% relative humidity for 6 months and determine changes to the surface composition and adhesive properties. The films will be cast onto PET sheets (or glass plates) and covered with a silicone release liner for storage. Water vapour sorption will be measured independently. Adhesion measurements (peel, loop, and probe) will be, performed initially and after ageing. Qualitative investigations into the surface structure and composition of the films during ageing will be made using atomic force microscopy (AFM). Complementary measurements of surface composition will be made using SIMS analysis. Any changes in adhesive properties will be correlated with changes in nanostructure and surface composition.The project will be conducted in collaboration with Synthomer plc.