Surface modification of thermally expandable microspheres for enhanced performance of disbondable adhesive
Introduction
End-of-life vehicle (ELV) legislations and recycling issues for future multi-material vehicles necessitate the development of new joining solutions that enable rapid disassembly for automotive vehicle maintenance or recycling scenarios [1]. In the search for a disbondable adhesive system suitable for automotive applications, an adhesive system containing thermally expandable microspheres (TEMs) has been identified as one of the most promising approaches with satisfactory disbonding performance observed in previous research conducted within the Joining Technology Research Centre (JTRC) at Oxford Brookes University [2]. TEMs are micron scaled spherical particles comprised of a thermoplastic shell material (2–15 μm thickness) and an encapsulated hydrocarbon core which has a low boiling temperature. At elevated temperature, shell softening and hydrocarbon gasification allows TEMs to expand 30–80 times in volume, which can lead to fracture of the adhesive bond line.
The major limitation of the TEMs approach is the poor compatibility between the TEMs and the adhesive which weakens the mechanical properties and long term durability of the original adhesive system [1], [3]. A suitable surface modification technique was selected to especially improve the TEMs/adhesive compatibility while retaining satisfactory in-service disbonding effectiveness. Surface grafting technology is a successful means of producing strong covalent bonds between TEMs and adhesive matrix. ‘Grafting to’ and ‘grafting from’ are two common approaches [4]. In the ‘grafting to’ approach, end-functionalised groups (e.g. hydroxyls, epoxides and thiols) react with a suitable surface to form a covalent bond. However, limitations such as low grafting density or difficulties in the synthesis of suitable functionalised polymers restrict its application. The ‘grafting from’ approach, in which polymer chains grow from the flat/spherical surface (Surface Initiated Polymerisation ‘SIP’), shows wider applicability. Atom transfer radical polymerisation (ATRP) has been demonstrated as a successful SIP technique in various areas including cell adhesion [5], [6], drug release [7], anti-biocorrosion coatings [8], and silicon wafers [9].
First discovered in 1995 [10], [11], ATRP provides a simple and controlled polymerisation solution with good control of molecular weight, structure and a high degree of chain end functionality. ATRP with activators regenerated by electron transfer or ARGET ATRP offers two major advantages over conventional ATRP technique: a) only ppm amount of catalyst is required which makes the purification or disposal process of toxic components cheaper or sometimes unnecessary, especially for industrial application [12]; b) a higher tolerance to oxygen and a catalyst that can be added in oxidatively stable state [13].
Inspired by the pioneering study conducted by Jonsson and co-workers [14], this study investigated TEMs that were modified by growing poly(glycidyl methacrylate) (PGMA) chains from the surface of TEMs via the ARGET ATRP process. Since each GMA molecule contains one epoxide group, PGMA chains contain an amount of n epoxide groups (n denotes the degree of polymerisation). Modified TEMs containing epoxide groups can react with the hardeners during the curing process of epoxy adhesive forming covalent bonds and thereby strengthening the TEMs/adhesive composite.
Section snippets
Experimental test programme
The experimental test programme was divided into three stages: a) surface modification of TEMs; b) investigation of disbonding performance of TEMs/adhesive joints; c) investigation of bulk properties of TEMs/adhesive composite.
Temperature–time effect on the surface modification of TEMs
FTIR test of the modified TEMs showed major changes occurred at the 7 peaks shown in Fig. 4, Fig. 5, Fig. 6, Fig. 7. These changes can be correlated with the molecular structure of GMA, Fig. 8. Main assignments of these peaks were listed in Table 3. The increases of the three peaks at 904 cm−1, 847 cm−1, and 761 cm−1 confirmed the presence of the grafted epoxy rings on the modified TEMs. These changes indicate that PGMA chains were successfully grafted from the surface of TEMs.
The intensities of
Conclusions
It has been demonstrated that TEMs can be modified using the ARGET ATRP technique whilst retaining the equivalent expansion performance. It has also been shown that temperature can significantly accelerate the rate of polymerisation and hence the length of PGMA chains, which also increased with polymerisation time. Grafted chains, allowed growing beyond a certain length, incurred gel formation. It was suggested for future work that an optimum chain length could be determined. Surface
Acknowledgements
The authors would like to express their sincere gratitude to Professor Fernando Audebert for his invaluable time and kind help with scanning electron microscopy at Begbroke Science Park, Oxford University. AkzoNobel Expancel was also gratefully acknowledged for the supply of TEMs samples.
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