Thermal aging of an anhydride-cured epoxy resin

Abstract

Fiber reinforced polymer (FRP) composites with anhydride cured epoxy resin matrices are widely used in civil engineering (e.g., pultruded FRP plates and bars), and their thermal aging behavior is a concern when they are subjected to elevated temperatures (e.g., FRP chimney). In the present article, thermal aging of an epoxy resin matrix at 130 °C–160 °C for 30 days was performed, and the effects on the flexural properties, molecular structures, free volume fraction, and mechanical properties were investigated. FTIR spectroscopy indicated that oxidation and molecular rearrangement occurred in the skin of the epoxy samples during thermal aging. Dynamic mechanical thermal analysis (DMTA) further illustrates the dominant effect of the molecular rearrangement in the sample skin with a thickness less than 100 μm, leading to a new high temperature tan δ peak. The free volume fraction of the skin and the bulk epoxy sample was characterized by positron annihilation lifetime spectroscopy (PALS). The results indicate that a noticeable reduction of the apparent free volume fraction occurred in the sample skin, while the bulk sample was only slightly affected. The flexural results indicate that thermal aging obviously reduced the break strain, while the flexural strength was only slightly affected and the modulus increased.

Introduction

Due to their high mechanical properties, low shrinkage during cure, and ease of processing, epoxy resins have been widely used as resin matrices for fiber reinforced polymer (FRP) composite materials [1], [2], [3], [4]. One of the rapidly increasing applications of FRPs is the rehabilitation and retrofitting of aged concrete or steel structures [5], [6]. Among these applications, the service temperatures may be high and lead the epoxy matrix to be thermally aged, for example, the FRP chimneys or FRP retrofitted concrete chimneys that have been widely applied in recent years [7]. Compared to fibers, the epoxy resin matrix is more susceptible to elevated temperatures. Anhydride cured epoxy resin, one of the widely used resin matrices for fiber reinforced polymer (FRP) composites, is investigated on its thermal aging performances herein.

For an epoxy resin, the evolution of the chemical structures due to elevated temperatures can be categorized in the following stages sequently: post-curing, oxidation of active groups, and chain scission. Generally, the post-curing reaction is occurred during the initial stage of thermal aging [8], [9], [10]. For an anhydride-cured epoxy, the reactive groups to be oxidized include the secondary alcohol and the hydrogen placed on the tertiary carbon at the α position of the ester. The secondary alcohol in the epoxy resin can be oxidized to carbonyl groups [11], [12], [13], [14], [15], [16]. Oxidation of the hydrogen connected to the tertiary carbon at the α position of the ester is associated with the limited chain scission, which can lead to the formation of various carbonyl groups, particularly of ketone and ester groups [17], [18]. Chain scission involves the migration of the liberated segment under the decomposition of the molecular chain by thermo-oxidation [10], [19]. In addition, thermo-oxidation of an epoxy resin is frequently associated with a mass increase due to the incorporation of oxygen to the molecules [20]. A possible correlation was found between the structural degradation and the mechanical degradation for an aged epoxy resin. The degradation process is enormously influenced by several factors, such as the aging temperatures and the aging time [21]. For a cured epoxy resin, generally, thermal aging results in an increased modulus and brittleness [22].

Free volume is the intermolecular space necessary for atoms, molecular segments, and the entire chain of the polymer to undergo thermal motion [23], [24]. The detection of molecular-level micro-structural changes of the free volumes will provide an understanding of the origins, mechanisms, and progression of the thermo-oxidation degradation process [25], [26], [27], which may eventually lead to macroscopic structural changes and the loss of durability. The positron annihilation technique can be used to directly measure the free volume of a polymer sample [24], [28]; the method was used to detect the variation of the free volume of a polymer material due to aging. As reported, ultraviolet aging of a polymer coating led to the decrease of the free volume content and thus the degradation of the mechanical durability [26]. The dependence of the other macro-mechanical properties and physical properties of the polymer materials on the free volume content were also reported [28], [29].

The epoxy resin with a Diglycidylether of bisphenol-A (DGEBA) and MeTHPA (methyl tetrahydrophthalic anhydride) is one of the most common resin systems for pultruded FRPs. Such FRPs are widely used in construction, and in some cases, the FRPs must operate at elevated temperatures, e.g., chimney structures built or strengthened by FRPs. The evolution of the molecular structures and the mechanical properties of the FRPs due to the elevated temperatures, especially under long-term exposure, have rarely been studied. As expected, the resistance of the FRPs to long-term elevated temperature exposure may be mainly dependent on the degradation of the resin matrix as well as the bonding between fiber and the resin rather than the fibers [30]. In view of this background, thermal aging of an anhydride cured epoxy resin is studied as the first step in our series of investigations on the long-term performance of FRPs at elevated temperatures. It is worth noting that thermal aging of the epoxy-anhydride resin systems has been studied in terms of thermal degradation kinetics [31], [32], mechanistic aspects [33], and thermo-mechanical properties [34]. The present work was conducted specially to correlate the variation of the chemical structures, free volume content and the thermo-mechanical properties of a DGEBA/MeTHPA resin system, which was thermally aged for a relatively long term.