Investigation of the Materials Properties of Polyvinyl Alcohol for Soluble Unit Dose Laundry Pods

Abstract

Soluble unit dose (SUD) laundry pods are a recent innovation in the home and laundry care sector, offering enhanced consumer convenience, reducing excess consumption, and enabling compartmentalised reagents for improved laundry performance. Polyvinyl alcohol (PVA) forms the film used to encapsulate laundry products, but the processes involved in pod manufacture and film adhesion are not well understood. This lack of understanding can lead to product failures, such as poor dissolution in low-temperature wash cycles or premature seal failure, which negatively impacts consumer satisfaction. This thesis investigates the polymer properties—specifically chain dimensions, aggregation behaviour, and phase stability—in model PVA systems similar to those used in commercial products. Techniques including small angle neutron scattering (SANS), dynamic light scattering (DLS), and rheology were employed to study these properties.

Key findings from this work include solution state SANS which revealed that partially hydrolysed PVA shows aggregation behaviour and provided chain dimension data across concentrations ranging from 0.25 to 30 w/w%. Solid state SANS further demonstrated that the interaction parameter between polymers in a blend increases with the difference in hydrolysis levels between the two polymers, while also characterising the chain dimensions of PVAc in the solid state. Fluorescence recovery after photobleaching (FRAP) analysis indicated that the degree of hydrolysis significantly influences the diffusion rate of PVA in thin plasticised films, with polymers of lower degree of hydrolysis diffusing faster than those with higher hydrolysis. Diffusion coefficients were extracted for use in diffusion models. Ion beam analysis, including Particle Induced X-ray Emission (PIXE) and Rutherford Backscattering Spectrometry (RBS), was used to visualise and quantify the interfaces in two-layer PVA films, simulating the seal structure of an SUD pod. These results showed that the interface was sharper for higher degrees of hydrolysis, with less diffusion at the interface, while greater interdiffusion was observed in less hydrolysed polymers, which aligns with FRAP findings.
This thesis makes contributions to the understanding of PVA film properties, including chain dimensions and the Flory-Huggins interaction parameter, in well-defined PVA series with consistent degree of polymerisation. Additionally, it demonstrates the novel application of the FRAP technique for observing diffusion in synthetic polymers, extending its typical use from biomolecules. The results offer insights that could improve computational models of interface behaviour in PVA film systems, advancing both material properties and manufacturing processes for SUD laundry pods.