Surfactant migration on polymeric substrates
Creators: name: Ramadani, Jona
Publisher: name: Imperial College London
Year: 2022

description: Many industrial nonwoven polymeric fabrics are coated with surfactants to provide improved wettability which is an essential attribute for disposable hygiene products, like facemasks, wipes, absorbent materials and baby nappies. These surfactant coatings on polyolefinic nonwovens appear to be typically not permanent and this fugitive nature of the surfactants is a concern for the industry. However, the interaction between organic species and complex semi-amorphous polymers as used in nonwoven products is an industrially important but poorly understood research area. Experimental studies reported here have established the mechanisms by which surfactants interact with polyolefinic surfaces, provide visualisation of 3D surfactant distributions on these nonwovens as well as their wettability, and report on the processes responsible for surfactant migration/loss from polyolefins. A novel confocal microscopy method is reported here for the non-invasive imaging of the 3D distributions of surfactants on polymeric nonwovens. Optical contrast was achieved by introducing a fluorescent dye via vaporisation at elevated temperatures, which preferentially dissolves into the hydrophilic surfactant regions of the nonwoven sample. The method is quantitative and allows the patch wise heterogenic distribution of surfactant coatings on complex 3D nonwoven materials to be visualised. To understand the interaction between surfactants and nonwoven polyolefins, several chemical properties and physicochemical descriptors of nonwoven materials were determined including wettability, specific surface area, surface energy, solvent sorption kinetics, and their surface elemental composition. Specific surface area BET measurements demonstrated that industrial nonwovens are characterised by generally low specific surface area values, in the range 0.1 - 4 m2/g and that inverse gas chromatography (IGC) offered best sensitivity and precision. The wettability of polyolefin surfaces is well described by the dispersive contribution of surface free energy γsD. Alkane probes are normally used for measuring γsD but dissolve in polyolefins invalidating the method. A new method using a series of normal alcohols was developed as part of this work, yielding γsD values in the range 20 - 40 mJ/m2. XPS analysis confirmed the hydrocarbon nature of polyolefinic nonwoven materials and the polar elements present responsible for the hydrophilic nature when the nonwovens were coated with surfactants, confirming surfactant treatment was not permanent. The solubility interactions between organic solutes and a range of amorphous and semicrystalline PP and PE were investigated by DSC, pycnometry, dynamic vapour sorption (DVS) and ellipsometry. The work confirmed that the presence of crystalline regions decreased the sorption of organic solutes in polyolefins. DVS studies of the sorption and desorption kinetics for small organic molecules in polyolefin films demonstrated that temperature increased diffusion rates and the amounts of solutes sorbed. However, increasing molecular size, or polarity, of the solute decreased the solubility. DVS combined with ellipsometry was used to determine the processes responsible for surfactant loss in thin polyolefin films. The amount of water sorbed by a polyolefin material was used here for the first time as a proxy for the amount of surfactant present on the polyolefin surface. DVS studies confirmed very slow mass losses due to surfactant evaporation from the surfactant coated polymers. However, the total rate of surfactant mass loss from the polymer surface was 10 times higher than the evaporative losses. The significant solubility of the non-polar surfactants and organic solutes in different polymer analogues was experimentally estimated. Based on these studies the hypothesis was that there are two processes causing surfactant loss from the polyolefin surface: • slow surface evaporation of the surfactants into the surrounding environment • a faster concurrent dissolution of the surfactant into the bulk polyolefin In summary, this thesis, provides new experimental insights into the interaction between liquid solutes, including surfactants, with semi-amorphous polyolefin materials including nonwoven fabrics.