Abstract
Epoxy resins are the most widely used systems for structural composite applications; however, they lack fracture toughness, impact strength and peel strength due to high cross-linking densities. Use of conventional toughening agents to combat this can lead to reductions in mechanical, thermal and processability properties desirable for bonded composite applications. In this work, an asymmetric triblock copolymer of poly(styrene)–b–poly(butadiene)–b–poly(methylmethacrylate) was used to modify an epoxy resin system, with the materials processed using both vacuum bag and positive pressure curing techniques. Interlaminar fracture toughness testing showed improvements in initiation fracture toughness of up to 88%, accompanied by a 6 °C increase in glass transition temperature and manageable reductions in gel-time. Shear testing resulted in a 121% increase in ultimate shear strain with only an 8% reduction in shear strength. Performance improvements were attributed to nano-structuring within the toughened resin system, giving rise to matrix cavitation and dissipation of crack front strain energy upon loading.
Highlights
Epoxy resins are the most widely used thermoset systems for structural applications, due to their excellent chemical and mechanical properties, low shrinkage, good compatibility with a range of different reinforcing fibres [1] and relatively high glass transition temperatures (Tg )
Viscosity was found to increase with the addition of the copolymer, and decrease with increasing temperature as shown in the results of Table 1, with a maximum of 16 Pa·s found for the K3600 + E21 at 15 ◦ C
Improvements in initiation values of interlaminar fracture toughness were as great as 88% for autoclave-cured parts, with a slight increase in resin Tg and only a small reduction in resin gel time
Summary
Epoxy resins are the most widely used thermoset systems for structural applications, due to their excellent chemical and mechanical properties, low shrinkage, good compatibility with a range of different reinforcing fibres [1] and relatively high glass transition temperatures (Tg ). This latter property results from the highly cross-linked nature of the polymer, which translates into poor fracture toughness, poor resistance to crack propagation and low impact and peel strengths. Despite the Polymers 2018, 10, 888; doi:10.3390/polym10080888 www.mdpi.com/journal/polymers
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