Accelerated ageing due to moisture absorption of thermally cured epoxy resin/polyethersulphone blends. Thermal, mechanical and morphological behaviour
Introduction
Epoxy resin based materials are widely used in structural applications for their advanced properties such as chemical resistance and high glass transition temperatures.
Furthermore the possibility to tailor a wide number of physical, mechanical and processing properties by acting on the resin blend formulation makes these systems very interesting from an engineering point of view [1].
In the design of these materials the evaluation of their durability, that is the time beyond which the structural properties go under the acceptable threshold value, is of primary importance. In fact when they are exposed to environmental agents, such as temperature, moisture, solvents, etc., a modification in their properties occurs whose severity depends on several factors, concerning both the materials and the environment. All these phenomena are generally known as ageing processes whose complexity, with respect to the molecular changes in the materials, still needs a deeper understanding and often makes the discussion about these phenomena rather limited and sometimes even contradictory.
It is well known that moisture absorption by epoxy resins leads to physico-chemical effects such as plasticization and to molecular modification, including degradation and cross-linking, which generally cause a change of their properties [2], [3], [4], [5].
Like all polymeric materials, epoxy resin systems undergo to physical ageing phenomena, consisting in the decreasing of free volume during time due to the tendency of the material to reach the equilibrium condition. This last effect is responsible of an increase of the brittleness in the material as a consequence of the increase of its density [6]. The severity of physical ageing strongly depends on the thermodynamic status of the system just after the cure process, when it is cooled to a temperature below its glass transition, but it is also strongly affected by the environmental conditions to which the system is exposed after the curing. In particular when moisture is absorbed by the resin, plasticization can both accelerate and emphasize physical ageing effects, due to the induced increase of mobility of the molecular chains. On the other hand physical ageing can affect in turn the moisture absorption by the resin, because a decrease of the free volume induces a less water absorption. Therefore there is a mutual influence of these two ageing causes, increase of mobility and decrease of free volume, so that the resulting effect is not easy to predict. Furthermore the environmental temperature plays a key role in both the kinetics and the thermodynamics of these phenomena, acting as another parameter to be taken into account in the already complex situation.
In advanced applications the intrinsic highly brittle nature of epoxy resins generally needs the introduction of toughening agents [7], [8], [9], [10]. This is usually done in order to obtain a more damage tolerant material toward all those failure mechanisms generally involving brittle fracture (e.g. impact, fatigue, composites delamination, etc.). A very effective way for the toughening of these systems without worsening other properties, such as the rigidity and the thermal resistance, consists in the introduction of an engineered thermoplastic having both high modulus and high Tg [7], [11], [12]. The toughening mechanism induced in this way is due to the occurrence of phase separation between the epoxy and the thermoplastic [13], [14]. In particular the formation of a well distributed thermoplastic rich phase in the epoxy matrix at a nano-micro scale determines an inter-penetrating network (IPN) that is very effective in increasing the epoxy fracture toughness [8].
It is reasonable that the morphology of the epoxy–thermoplastic system can markedly affect the water absorption behaviour of the material because the distribution of the two phases is involved in the water–polymeric matrix interactions and in turn the morphology can be modified by absorbed water [15]. The modification of the morphology by moisture absorption is attributable to the increase of the mobility, due to plasticization/degradation, which allows the morphology to develop toward the equilibrium state, consisting of two separated phases in the corresponding phase diagram [13]. This equilibrium condition provides the distribution of one phase within the other in the form of particles (sea-island morphology) having dimensions dependent on the initial morphological structure and on the environmental conditions (temperature).
The change of the morphology during ageing may cause significant modification of the mechanical performance of the fracture, since it is strongly related to the characteristics of the separated phases. Then it is evident that all these effects can induce in the materials, during their life, changes in the toughness levels that have to be known and taken into account during their design.
This work is part of our wider study regarding the relationship between structure–morphology and ageing phenomena in epoxy based systems for structural composites. To this purpose different curing methodologies have been adopted in order to tune the desired structures and morphologies.
In our previous work the hydrothermal ageing of epoxy based systems cured by means of low temperature ionizing radiation process has been studied [15]. The results indicated that the formation of clusters at different cross-linking densities, typical of low temperature radiation curing process, specifically characterized the ageing response of the material, which gave rise to a “homogenization” of the structure, with degradation and plasticization phenomena that caused significant modification of both thermal and mechanical properties.
In this work in order to emphasize the influence of the moisture on the ageing phenomena a model system with very high water affinity was studied. The system consisted in an epoxy/anhydride blend [16], [17], modified with a polyethersulphone toughening agent. In order to produce crosslinked materials thermal curing by a thermo-programmable hydraulic press was used. Two different concentrations of the toughening agent were considered and the relative systems were compared to the un-toughened one. The effect of accelerated hydrothermal ageing on the thermal and mechanical properties was investigated through DMTA, fracture toughness test and SEM microscopy.
Section snippets
Experimental
The epoxy resin monomer was 2,2-Bis[4-(glycidyloxy)phenyl]propane (DGEBA) and the curing agent was 2,2-Cyclohexanedicarboxylic anhydride (HHPA), both supplied by Sigma Aldrich (Italy). The toughening agent was an OH terminated polyethersulfone engineering thermoplastic, SUMIKA EXCEL 5003P, Mw 25,000, produced by Sumitomo Chemicals (Japan). The catalyst was Cumyltolyliodonium tetra(pentafluorophenyl) borate (RH 2047), by Rhodia Siliconi Italia (Italy). The formulas of the materials are reported
Results and discussion
Thermal properties of the synthesised systems have been investigated, through dynamic mechanical thermal analysis (DMTA).
In Fig. 5 DMTA results are reported as loss factor and storage modulus vs temperature for all systems. In tan δ/T curves only one relaxation peak can be observed for all materials, with a slightly lower temperature value for 0PES with respect to 10PES and 20PES which show almost the same value. For the 0PES system the peak can be associated to the glass transition of the cured
Conclusions
In this work a model epoxy–anhydride system was modified with two concentrations of toughening agent and thermally cured. The characterization was performed by DMTA, SEM and fracture toughness measurements and the behaviour after two procedures of accelerated hydrothermal ageing was studied.
DMTA of as-cured samples showed an increase of the glass transition temperature of about 5 °C and an increase of the fracture toughness of toughened systems compared to the neat resin systems. After
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