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

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.