GARField magnetic resonance of ultraviolet cured acrylic adhesives

• Main Author: Pitts, Simon A.
• Published: University of Surrey 2009
• Subjects: 620.112
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.507068

Pressure sensitive adhesives are a category of adhesives which can be used for both permanent and removable applications. The use of radiation to cure these materials prior to application is becoming more widespread due in part to the recent drive to reduce the release of volatile organic compounds and hazardous air pollutants produced from more traditional solvent or thermal curing methods. For the manufacturer, a greater understanding of how these adhesives cure under near to real life conditions is highly desired. This thesis presents the first use of one-dimensional magnetic resonance imaging to provide information on the molecular mobility as revealed by spin relaxation times as a function of depth in cross-linking acrylic adhesives during photoinitiated curing by ultraviolet light. Imperfect curing in these adhesives may lead to crosslink density inhomogeneities and consequently, to variations in the adhesion and cohesion properties throughout the depth of the adhesive. The GARField magnet developed at the University of Surrey, along with a new high temperature sample mounting probe developed during this work allowed thin film samples to be measured at various temperatures with a spatial resolution better than 16 mum. Two separate components were identified in the exponential decay of the NMR signal from the bulk adhesive. These were attributed to the inter-crosslinked network and to lower molecular weight dangling chain ends. Curing was seen to increase the fraction of the shorter component indicative of an increase in crosslink density. GARField profiles showed that formulations with additional multifunctional acrylates produced the highest cure owing to the increased crosslink density afforded by the multifunctional acrylates. However, the addition of these groups also led to a reduced homogeneity in the depth of cure. The inclusion of further photoinitiating species, sensitive to a wider range of wavelengths was shown to reduce this effect. Measurements at 60°C showed that UV irradiation was more efficient at producing crosslinks at this temperature. However, the contrast between cured and uncured samples on GARField was reduced due to the additional, thermally driven molecular motion. The activation energy for this thermal motion was calculated to be 0.34 +/- 0.04 eV and 0.28 +/- 0.05 eV for the uncured and cured adhesives, respectively. The irradiation regime only became important when small doses separated by more than 20 minutes were used. This regime led to sufficient build up of oxygen in the upper surface of the adhesive to inhibit the curing reaction.