A Study on Microstructure Characteristics of TEPs-modified Adhesives

Thermally expandable particles (TEPs) were developed by Dow Chemical Co in the early 1970´s [1] and were further developed by others [2,3]. They are particles made up of a thermoplastic shell filled with liquid hydrocarbon. On heating them, two transformations will occur. One is the softening of shell material and the other is the gasification of the hydrocarbon liquid inside it. As a consequence, the shell will expand as the gas inside it will push the softened shell from inside out causing it to grow in size [4]. When fully expanded, the growth in volume of the particle can be from 50 to 100 times [3]. Owing to this unique behaviour, TEPs are used by the industry in a wide variety of applications mainly for weight reduction and appearance improvement for thermoplastics, inks, and coatings. In adhesive bonding, TEPs have been used for recycling purposes. Moreover, TEPs might be used to modify structural adhesives for other new purposes, such as: to increase the joint strength by creating an adhesive functionally modified along the overlap of the joint by gradual heating and/or to heal the adhesive in case of damage. Nevertheless, the success of the concept of using TEPs with adhesives depends on the combination of the TEPs and the adhesive system as the resulting properties of the TEPs-modified adhesives are influenced by the mechanical, physical and chemical properties of the used TEPs and adhesives. However, there are few studies in the literature about the properties of TEPs-modified materials and in particular of TEPs-modified adhesives. In a previous study by the authors, a thermal analysis was performed in order to investigate the behaviour of TEPs while encapsulated in adhesives and it was shown that the TEPs expansion takes place in a range of temperatures, between 90 and 120 ºC [4]. In this work, the fracture surfaces of the unmodified and TEPs-modified thermal specimens, as well as the dispersion and the morphology of the particles, were examined by a scanning electron microscopy (SEM) analysis.