A. Revathi

Hygrothermal Effects on RT-Cured Glass-Epoxy Composites in Immersion Environments. Part A: Moisture Absorption Characteristics

A study on the effect of moisture absorption characteristics of RT-cure glass/epoxy composite and epoxy casting specimens immersed in freshly prepared artificial seawater and distilled water maintained at 333º K are presented in this paper. Results showed that the equilibrium moisture absorption value and the diffusion rate (slope of the moisture absorption curve) are higher in specimens immersed in distilled water than those of artificial seawater. The mixture rule for moisture absorption estimates has been verified for specimens immersed in both environments. These theoretically calculated values compared well with those experimentally obtained. The time of saturation for specimens immersed in seawater was found to be higher than that for those immersed in distilled water.

Hygrothermal Effects on Painted and Unpainted Glass/Epoxy Composites—Part A: Moisture Absorption Characteristics

Moisture diffusion behaviour of glass/epoxy composites coated with a polyurethane base paint and those without the paint were studied on immersion in water at 323 K. Neat resin castings of the epoxy resin system were also studied for comparison. The composite specimens with paint showed lower moisture absorption values than those of the unpainted specimens subjected to the same immersion conditions. The diffusion parameters obtained from the experiment conducted up to saturation limits agreed well with the data computed using moisture absorption programme (M8 gain) proposed by G. S. Springer. The mixture rule for moisture absorption was also verified for the weight fraction studied. Further, Fickian correlation was found to exist in all the cases.

Hygrothermal Effects on RT-Cured Glass-Epoxy Composites in Immersion Environments. Part B: Degradation Studies

The degradative effects of artificial seawater and distilled water immersion at 333° K on the mechanical properties (ILSS, compression strength) of BID glass/epoxy composite materials has been studied. Experimental results showed that the composite degradation trends were similar in both the media. It was found that the composite specimens exhibited a higher compressive and ILSS retention when immersed in the artificial seawater than when immersed in distilled water. This is attributed to the moisture absorption levels being higher in specimens immersed in distilled water than those immersed in artificial seawater (Part A). Further, it was found that the measured and predicted mechanical strength retention ratios (compression and ILSS) were in fairly good agreement with each other for both the immersion conditions.

Hot-wet property characterisation of a high-temperature cured glass-epoxy composite in immersion environment

The hygrothermal diffusion and degradation behavior of a high-temperature cured glass–epoxy (Epoxy Novolac – EPN) composite system was studied under two environmental conditions (i.e. 323 K and 343 K immersion in distilled water). Samples immersed at 343 K showed higher diffusivity (Dc) value and lower saturation time (tm) than those of 323 K immersion while the maximum moisture content (Mm) remained the same, good Fickian correlation were observed for the composite system. As regards the degradative effects, the glass transition temperature (Tg) of the composite decreased with increased moisture content showing a maximum drop of 30 C at full saturation, while the mechanical properties (ILSS and IPS) of saturated specimens degraded upto 26 and 33% respectively. Further these mechanical properties obtained at 70 C/85%RH test condition showed good correlations with those predicted by a theoretical equation (Chamis et al. An Intergrated Theory for Predicting the Hydrothermo Mechanical Response of Advanced Composite Structural Components, Lewis Research Center, Cleveland, Ohio, NASA Technical Memorandum 73812).

Post-curing Effects on Hygrothermal Behavior of RT-cured Glass/Epoxy Composites

Studies were carried out on RT-cured glass/epoxy composite specimens, subjected to different post-cure schedules (50, 60, 70, and 85°C and durations) and immersed until saturation in distilled water at 50°C. The effects of different post-cure schedules, on the moisture diffusion characteristics of the composite were studied. Results showed that the saturation moisture levels decreased with increased post curing, a trend attributed to increased matrix cross-linking, as evidenced by the increase in Tg (glass transition temperature) values with extent of post cure. The diffusion parameter-like composite diffusion coefficient (Dc) and time of saturation (t m) showed either insignificant or marginal changes, within the range of post-cure schedule considered in these studies.

Hygrothermal Effects on Painted and Unpainted Glass/Epoxy Composites—Part B: Degradation Studies

Studies were conducted on the degradation of ILSS and compression strength properties of glass/epoxy composite specimens painted and unpainted that were subjected to immersion in a water bath maintained at 323 K. It is found that both ILSS and compressive strength of the composite degraded more in the case of unpainted specimens (coded L 0) compared to the painted ones (L 1 or L 2). Further, the specimens with a thicker paint coat (L 2) degraded less than those with a thinner paint coat (L 1).

Thermo-Mechanical Creep and Recovery of CTBN–Epoxy Shape Memory Polymers Under Saline Environment

In the present study, the creep and recovery experiments of unmodified epoxy SMP and 5% carboxyl-terminated butadiene acrylonitrile (CTBN) modified epoxy SMPs were carried out after being exposed to saline environment (5% NaCl, 35 °C/98% RH). The creep results of the SMP specimens before and after exposure to saline environment, obtained through short-term tensile creep test at different temperatures (i.e., 25, 80, 103 and 120 °C) and loading conditions, were compared. Prior to exposure to saline environment, the creep strain was found to increase with increasing temperature and reached a maximum around the glass transition temperature (i.e., 103 °C). A higher creep recovery ratio was noticed in the CTBN-modified epoxy SMP particularly at high-temperature conditions. The creep strain was higher in saline environment at 25, 80 and 103 °C; however, at 120 °C both the SMPs could not withstand the applied load. The creep recovery ratio was unaffected by the saline environment in both the SMPs.

Investigations on tensile creep of CNT-epoxy shape memory polymer nanocomposites

In this work, epoxy-based shape memory polymer nanocomposites (SMPnCs) were prepared using 0.5 wt% and 1 wt% multi walled carbon nanotubes (MWCNTs) pre-dispersed in epoxy resin. This was cured with triethylene tetraamine (TETA) curing agent. The glass transition temperatures (Tg) were determined using Advanced Rheometric Expansion System (ARES). The creep behaviour of the 0 wt% CNT SMPnC as well as the 0.5 wt% and 1 wt% MWCNT SMPnCs were studied through short term tensile creep test at different temperatures. Master curves of creep compliance were derived using a time-temperature superposition principle (TTSP) based on Williams-Landel-Ferry (WLF) equation. A Findley power-law model was used to predict the creep deformation behaviour of 0, 0.5 and 1 wt% CNT SMPnCs. Good correlations between experimental data and the predictive model were obtained for both SMPs particularly at lower temperature and above Tg.

Moisture Distribution Profiles in RT-Cured Glass/Epoxy Laminates of Different Thicknesses

Investigations were carried out on the moisture absorption behavior of RT-cured glass/epoxy composite specimens subjected to 333 K/Immersion in distilled water. Different laminate thicknesses (i.e., 0.25-2.25 mm) were studied for comparative evaluation of their moisture diffusion and distribution profiles across the thickness of the composite as a function of the exposure period. Moisture absorption data showed that the equilibrium moisture levels (Mm) remained practically the same for different laminates of the same fiber weight fraction (i.e., 0.62). Further, the times of saturation (tm) increased with the laminate thickness, while the absorption rates (slopes of the absorption curves) showed a reverse trend. It was also found that the Diffusion Coefficient (Dc) values of different thicknesses specimens ranged between 0.28 and 7.81(10 7)mm2/sec, indicating a negligible thickness effect on this diffusion parameter. The small variations of Dc (all of the order 10 7 mm2/sec) are attributed mainly to the variations in the edge areas for different thickness laminates.