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

在轻压力作用下，压敏胶粘剂(PSA)几乎可以附着在任何表面上。它们在日常生活中与胶带、标签、图形、汽车涂层和一些医疗设备一起使用。压敏胶历来是由溶解在有机溶剂中的粘弹性(软)聚合物制成的。当浇注PSA溶液时，有机溶剂蒸发掉，留下薄膜。如今，为了避免有机溶剂排放到大气中对环境产生有害影响，越来越多的人使用胶体聚合物分散体在水中制造PSA，这种分散体被称为胶乳。为了合成乳胶，使用乳液聚合，这需要使用类似肥皂的分子，称为表面活性剂。表面活性剂有助于胶乳分散体的稳定性，对该过程至关重要。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而引起的光散射造成的光学透明度的损失，以及当表面活性剂迁移到PSA表面时的粘附性损失。本研究的目的是了解影响表面活性剂在PSA中不均匀分布的因素，并确定其机理是否可控。由于表面活性剂可以在成膜(干燥)阶段积累，也可以在PSA在干燥状态下的老化过程中积累，因此研究将集中在这两种情况下。最近的数学模型预测，几个关键因素对表面活性剂在整个成膜阶段的分布有影响。水分蒸发速率、温度、颗粒大小、膜厚和粘度(聚合物和水相)都被预测起到了作用。在这个项目中，将设计实验来系统地测试模型，并试图澄清每个已确定的因素对最终表面活性剂分布的作用。研究还将研究在干燥过程中在PSA中形成的离子表面活性剂梯度所产生的感应电场是否可以通过电泳机制来驱动颗粒运动。目前的表面活性物质在成膜过程中的分布模型没有考虑到电泳法的影响，但它可能是重要的。表面活性剂的深度分布将通过重元素(如阳离子)的卢瑟福背散射(RBS)和萨里离子束中心的弹性反冲探测(ERD)来确定。表面活性剂的分布将与离子浓度梯度增加的贡献进行比较。关于PSA膜老化，先前的实验研究表明，在干燥状态下，在老化研究期间，湿度可以影响表面活性剂的分布。不幸的是，这些实验的范围有限，结果与PSA的机械性能没有相关性。在这项工作中，我们将在0%、42%和85%的相对湿度下老化两种商用PSA 6个月，并确定其表面组成和粘合性能的变化。这些薄膜将被浇注在PET片材(或玻璃板)上，并用硅胶离型衬垫覆盖以供储存。水蒸气的吸收率将独立测量。将在初始和老化后进行附着力测量(剥离、环路和探头)。使用原子力显微镜(AFM)对老化过程中薄膜的表面结构和成分进行定性研究。表面成分的补充测量将使用SIMS分析。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Synthmer 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.