Hydrophilic nature of epoxy polymers can lead to both reversible and irreversible/permanent changes in epoxy upon moisture absorption. The permanent changes leading to the degradation of mechanical properties due to combined effect of moisture and elevated temperature on EPON 862, Nanomer I.28E, and Somasif MAE clay-epoxy nanocomposites are investigated in this study. The extent of permanent degradation on fracture and flexural properties due to the hygrothermal aging is determined by drying the epoxy and their clay-epoxy nanocomposites after moisture absorption. Significant permanent damage is observed for fracture toughness and flexural modulus, while the extent of permanent damage is less significant for flexural strength. It is also observed that permanent degradation in Somasif MAE clay-epoxy nanocomposites is higher compared to Nanomer I.28E clay-epoxy nanocomposites. Fourier transform infrared (FTIR) spectroscopy revealed that both clays retained their original chemical structure after the absorption-desorption cycle without undergoing significant changes. Scanning electron microscopy (SEM) images of the fracture surfaces provide evidence that Somasif MAE clay particles offered very little resistance to crack propagation in case of redried specimens when compared to Nanomer I.28E counterpart. The reason for the observed higher extent of permanent degradation in Somasif MAE clay-epoxy system has been attributed to the weakening of the filler-matrix interface.
Epoxy polymers are very important class of advanced materials. Their main distinction from other types of polymers lies in their densely crosslinked molecular structure. This crosslinking leads to a number of favorable thermal and mechanical properties including high strength and modulus, high creep resistance, high glass transition temperature, low shrinkage, and better chemical resistance. These properties in conjunction with ease of processing have made epoxy resins an attractive choice for use in many engineering components and structures. They have found huge applications in aerospace, automotive, packaging, coating, and microelectric industries. In recent years, researchers have developed and investigated polymer nanocomposites based on a wide variety of micro-/nanoscale fillers including clay particles [
Epoxy polymers are characteristically hydrophilic, which means that they have strong affinity towards water. This nature of epoxy resins makes them susceptible to high moisture absorption; in general, depending on the nature of the epoxy resin, the equilibrium moisture uptake can be in the range of 1–7% [
Water absorption into a polymer matrix leads to change in both chemical and physical characteristics and affects the mechanical properties through different mechanisms such as plasticization, crazing, hydrolysis, and swelling. Plasticization is the most important physical change that occurs through the interaction of the water molecules with polar groups in the matrix, which can severely depress the glass transition temperature [
In recent years, effect of moisture absorption on the mechanical properties of neat epoxy and clay-epoxy nanocomposite has been frequently studied. Zhao and Li reported that tensile strength and modulus decreased for both neat epoxy and nanocomposites upon moisture absorption, while the tensile strain increased significantly for moisture absorbed samples. Although failure occurred in brittle manner, effect of plasticization was found in SEM images, which showed shear yielding for both neat epoxy and nanocomposite samples after being exposed to moisture [
Most of the research on polymer-clay nanocomposites has been mainly focused on investigating the effect of moisture absorption on mechanical properties such as elastic modulus and tensile strength. Although fracture toughness is a very important property for these nanocomposites as these are used in various structural applications, the effects of moisture absorption on fracture toughness of polymer-clay nanocomposites have not been studied extensively. Durability of polymer/clay nanocomposites is still needed to be studied in depth, particularly for hygrothermal aging in which the degradation of the mechanical properties and loss of integrity of these nanocomposites occur from the simultaneous action of moisture and temperature. This study on clay-epoxy nanocomposites was designed to investigate the effect of hygrothermal aging on mechanical properties of these nanocomposites. Two different clay particles were used to investigate the effect of clay structure on the permanent property changes due to hygrothermal aging. A drying cycle was employed to quantify the recovery of the properties after hygrothermal aging. This was helpful to understand the extent of permanent degradation that occurred by the combined application of elevated temperature and moisture. Mechanical properties in terms of fracture toughness, flexural strength, and flexural modulus are the properties that were studied. Scanning electron microscopy and Fourier transform spectroscopy were conducted to further elucidate the underlying fracture mechanisms of these preconditioned specimens.
The epoxy resin used for this study is EPON 862, which is a diglycidyl ether of bisphenol F. The curing agent used for this resin system was a moderately reactive, low viscosity aliphatic amine curing agent, Epikure 3274. Both of these chemicals were supplied by Miller-Stephenson Chemical Company, Inc., Dunbury, Connecticut. Two structurally different clay particles were used as reinforcement. Nanomer I.28E is a surface modified montmorillonite based layered silicate (Nanocor, Inc., Arlington Heights, IL) modified with a quaternary amine (trimethyl stearyl). Somasif MAE (Co-Op Chemical Co., Japan), which was the other clay particle used for this study, is a synthetic mica modified with dimethyl dihydrogenated tallow ammonium chloride. Table
Structure of the studied clay particles.
Clay | Structure |
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Nanocor I.28E |
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Somasif MAE |
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Epoxy was preheated to 65°C before desired amount of clay was introduced and mixed using mechanical mixer for 12 hours. To reduce the viscosity of the mixture and to facilitate mixing, temperature was maintained at 65°C using a hot plate for the entire duration of mixing. The mixture was then degassed for around 30 minutes to remove any entrapped air bubbles. Bubble-free mixture of clay and epoxy was then shear-mixed using a high-speed shear disperser (T-25 ULTRA TURRAX, IKA Works Inc., North Carolina, USA) for 30 minutes. During this process, temperature was maintained at 65°C using an ice bath. Subsequently, the mixture was then degassed until it was completely bubble-free. Curing agent was added to the mixture at 100 : 40 weight ratio and carefully hand-mixed to avoid introduction of any air bubble. After it was properly mixed, the final slurry containing epoxy and clay was poured into an aluminum mold and cured at room temperature for 24 hours followed by postcuring at 121°C for 6 hours. The final sample had a nominal dimension of
After specimens were cut into the final required dimension according to the ASTM standards D5045 and D790, they were subjected to degradation. Specimens from each nanocomposite were taken and submerged in purified deionized boiling water for 24 hours. Water saturated specimens were dried in an oven at 110°C for 6 hours to remove moisture from the samples leaving only permanent degradation in the form of bonded water.
Critical stress intensity factor,
Flexural strength and flexural modulus were determined using three-point bend (3PB) test according to ASTM D790 on universal testing machine (Instron 5567, Norwood, MA). The nominal dimension for the flexural test specimens was
The flexural strength and flexural modulus were calculated using the following equations, respectively:
Surface morphology of the fractured specimens from SENB tests was observed using scanning electron microscopy (Hitachi S-4800 FESEM, Dallas, TX). As polymer materials are nonconductive to electrons, all fracture surfaces were sputtered with gold-palladium alloy before SEM imaging.
FTIR spectroscopy measurements were performed using ATR-FTIR spectrometer (Nicolet iS10, Waltham, MA) using 64 scans at a resolution of 2.0
Table
Weight changes in samples after preconditioning.
Specimens | Absorption, 24 hr (%) | Desorption, 6 hr (%) | Removal (%) |
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Neat EPON 862 | 2.10 | 0.14 | 93.35 |
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0.5 wt% Nanomer I.28E | 2.09 | 0.40 | 83.65 |
1.0 wt% Nanomer I.28E | 2.15 | 0.42 | 80.33 |
1.5 wt% Nanomer I.28E | 2.13 | 0.44 | 79.56 |
2.0 wt% Nanomer I.28E | 2.20 | 0.45 | 79.35 |
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0.5 wt% Somasif MAE | 2.14 | 0.32 | 85.18 |
1.0 wt% Somasif MAE | 2.12 | 0.35 | 83.70 |
1.5 wt% Somasif MAE | 2.11 | 0.40 | 81.21 |
2.0 wt% Somasif MAE | 2.13 | 0.45 | 79.07 |
The critical stress intensity factors as a function of clay loading percentage for Nanomer I.28E and Somasif MAE clay-epoxy nanocomposites are shown in Figure
Critical stress intensity factor,
Critical stress intensity factor,
For the as-prepared and moisture saturated samples, Somasif MAE nanocomposites showed comparable trend in fracture toughness data; clay reinforcement successfully improved the baseline epoxy properties and moisture absorption degraded the mechanical properties for all clay percentages. However, the permanent degradation after absorption-desorption cycle was found to be more prominent in the case of Somasif MAE clay nanocomposites compared to Nanomer I.28E clay nanocomposites. The recovery of property after 6 hours of drying was negligible for Somasif MAE clay nanocomposites, whereas Nanomer I.28E nanocomposites showed significant recovery of property after drying. The difference in property recovery of these two clay-epoxy nanocomposites can be attributed to the structural differences of the two clay particles and has been further investigated through SEM and FTIR technique.
Flexural modulus for the neat epoxy and clay-epoxy nanocomposites was determined from 3PB test and has been plotted against clay loading percentage in Figure
Flexural modulus as a function of clay loading: (a) Nanomer I.28E and (b) Somasif MAE clay-epoxy nanocomposites.
For the hygrothermally conditioned specimens, the modulus is lower compared to the as-prepared specimens. This behavior observed is mostly due to the presence of water inside the epoxy system, which increases the ductility of the epoxy system. Water acts as an effective plasticizer and can diffuse into the nanofiller-polymer interface and weaken the bonding between them [
For Nanomer I.28E clay-epoxy samples conditioned at 110°C for 6 h, recovery of flexural modulus was observed. Once redried, free water residing in the microvoids was evaporated, and the effect of plasticization was not prominent anymore. As a result, the ductility of the polymer reduced and the recovery of mechanical properties from moisture absorbed state occurred. Nevertheless, in case of Somasif MAE clay-epoxy samples, modulus recovery was negligible after the desorption cycle. Due to the structural difference between the two clay particles, it is possible that the interface of Somasif MAE clay-epoxy is being more affected by the hygrothermal degradation than the Nanomer I.28E clay-epoxy interface.
Flexural strengths for the epoxy and clay-epoxy nanocomposites were determined using three-point bend (3PB) test and are plotted against clay loading percentage in Figure
Flexural strength as a function of clay loading: (a) Nanomer I.28E and (b) Somasif MAE clay-epoxy nanocomposites.
Almost similar trend was observed for Somasif MAE clay-epoxy nanocomposites, where addition of clay did not change the property significantly, and after 24 h of hygrothermal aging property decreased to a lower value compared to the as-prepared samples. However, the severity of degradation was much less in both clay-epoxy systems compared to neat epoxy system. Well dispersed high aspect ratio clay platelets have the capability of crack deflection and crack arresting, which can lead to the observed higher flexural strength in wet clay-epoxy samples in comparison to neat epoxy samples [
The scanning electron micrographs of the fracture surface of tested neat epoxy and clay-epoxy nanocomposites are shown in Figures
Scanning electron micrographs of neat epoxy polymer: (a) as-prepared and (b) redried.
Scanning electron micrographs of the crack growth region for 0.5 wt% Nanomer I.28E clay-epoxy nanocomposites: (a) as-prepared, (b) moisture absorbed, and (c) redried.
Scanning electron micrographs of the crack growth region for 0.5 wt% Somasif MAE clay-epoxy nanocomposites: (a) as-prepared, (b) moisture absorbed, and (c) redried.
Scanning electron micrographs of the crack growth region for 1.5 wt% Nanomer I.28E clay-epoxy nanocomposites: (a) as-prepared, (b) moisture absorbed, and (c) redried.
Scanning electron micrographs of the crack growth region for 1.5 wt% Somasif MAE clay-epoxy nanocomposites: (a) as-prepared, (b) moisture absorbed, and (c) redried.
The SEM micrographs of clay incorporated epoxy systems showed significantly rougher fracture surface compared to the neat polymer. Clay, if present in a system, physically blocks and slows down the crack propagation and the resulting fracture surface of the nanocomposite shows river-markings instead of smooth fracture surface found in the neat polymer. These river-markings provide clear indication of the enhanced toughening mechanism in polymeric materials due to clay incorporation and support the observed higher fracture toughness for clay-epoxy nanocomposites compared to the neat epoxy polymer. Fracture surfaces of nanofiller reinforced polymer nanocomposites showing higher surface roughness have been reported in prior studies [
Fracture surface of moisture absorbed Nanomer I.28E clay nanocomposites (Figures
Fracture surface micrographs of redried Nanomer I.28E nanocomposites (Figures
FTIR spectra of the as-prepared and redried neat epoxy are shown in Figure
FTIR spectra for neat epoxy polymer before and after absorption-desorption cycle.
Figure
FTIR spectra for clay particles before and after absorption-desorption cycle: (a) Nanomer I.28E and (b) Somasif MAE.
Property deterioration due to moisture absorption has been one of the most important areas of interest in polymer research for the last few years. To be used as structural materials, it is of utmost importance to understand the reversible and irreversible changes occurring in polymeric materials as a result of moisture absorption. This study was conducted to elucidate the effect of moisture absorption on the mechanical properties of clay reinforced epoxy polymers. Fracture toughness, flexural strength, and flexural modulus were determined for two different clay-epoxy nanocomposites following the ASTM standards. The effects of hygrothermal aging and subsequent redrying on the mechanical properties of these polymer nanocomposites were investigated. After removing the free water by drying, the irreversible effect or the permanent damage due to hygrothermal aging on the clay-epoxy nanocomposite systems was determined. Irrespective of the clay reinforcement type, all the studied properties were degraded due to hygrothermal aging. Several physical and chemical changes, such as interface weakening, hydrolysis, and chain scission, are responsible for the observed effect. The permanent damage or degradation was severe in case of fracture toughness and flexural modulus. Flexural strength of both systems was relatively unaffected by the absorption-desorption cycle. Permanent damage was found to be the highest for Somasif MAE clay reinforced specimens between two clay-epoxy nanocomposite systems. After studying the SEM micrographs of the fracture surfaces, it was speculated that moisture absorption had higher negative impact on the interface of Somasif MAE clay and epoxy matrix compared to the other clay-epoxy system. The hydrophobic nature of the Somasif MAE clay due to the presence of –F (fluorine) in the structure may have created additional tension between the polymer crosslinks in presence of moisture. FTIR spectra of the clay particles treated with the same absorption-desorption cycle provided proof that both nanoparticles undergo minimal chemical change and retain their respective original chemistry. This observation makes the aforementioned speculation more plausible. Although incorporation of clay in epoxy matrix did not fully stop the degradation, it had positive effects to some extent. It was observed that the studied properties in general were less severely degraded for clay-epoxy nanocomposites compared to neat epoxy samples. Therefore, clay particles could be successfully used to reinforce polymer materials to reduce the severity of property deterioration caused by the moisture absorption. However, the chemistry between the clay particles and polymer matrix and more specifically the chemical structure of the clay particles should be carefully considered to attain the best possible resistance against the property deterioration.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors gratefully acknowledge that this work is funded in part or fully by a Grant through the Oklahoma Nanotechnology Applications Project (ONAP) (Grant no. O9-20) and NASA Experimental Program to Stimulate Competitive Research (EPSCOR) (Grant no. NNXO9AP68A).