Abstract
The influence of exposure to UV light and moisture on the durability of a multiwalled carbon nanotube(MWCNT)/epoxy nanocomposite was investigated. Samples of epoxy resin based on diglycidyl ether of bisphenol A (DGEBA) cured with 2,2,4-trimethylene-1,6-hexadiamine (TMDA), and epoxy nanocomposite containing 0.5% MWCNT were exposed to different accelerated weathering times between one and six months. Changes in surface chemistry, mechanical properties (tensile tests), thermal properties (thermogravimetric analysis and differential scanning calorimetry), and morphology were evaluated before and after exposure to accelerated weathering for a period of up to six months. Epoxy nanocomposite (DGEBA–TMDA/0.5%MWCNT) samples had improved thermal stability and resistance to degradation, compared to epoxy resin (DGEBA–TMDA). The effect of MWCNT at reducing degradation was more pronounced than previously found for resins prepared with hydrogenated DGEBA.
Keywords
Introduction
Timber composites are increasingly being utilized outdoors in applications such as buildings and bridges where their service life depends to a large extent on the durability of the adhesive used in the composite.1–3 This use of timber composite materials incorporating epoxy resins (glue-laminated4,5 timber and cross-laminated6,7 timber) makes it necessary to obtain a better understanding on the properties of these materials under the effects of environmental conditions such as UV irradiation, elevated temperature, and moisture.8–10 Discoloration and chalking of epoxy in the presence of UV have been the primary cause of concern limiting the use of epoxies for outdoor applications;11–15 however, for structural applications, degradation disrupting the integrity of the epoxy has graver consequences.16–18 Multiwalled carbon nanotubes (MWCNTs) display unique mechanical, electrical, and optical properties so that they have been considered as a nanofiller for epoxy or polymer as nanocomposites.19–21 As UV absorbers, they can inhibit the degradation of cured epoxy resin. 22 The addition of epoxy to MWCNTs has been optimized by four principal factors: good dispersion, a large aspect ratio, alignment, and interfacial stress transfer.23,24 MWCNTs are one nanofiller of potential concern for human health impacts due to their high aspect ratio. Several studies to date have addressed the release of MWCNTs due to mechanical stresses such as sanding and polishing, but fewer have investigated the potential of their release due to environmental stresses. 25 Recent studies on the potential toxicological impacts of materials released from MWCNT nanocomposites have not shown raised toxicity compared to particles released from the polymer matrix under the environmental conditions tested.26–30
In a previous work, 31 we used a cured aliphatic epoxy hydrogenated diglycidyl ether of bisphenol A (HDGEBA) and 2,2,4-trimethylene-1,6-hexadiamine (TMDA) with and without MWCNT as a filler. The samples were exposed to artificial weathering conditions for up to six months and evaluated by chemical, thermal, and mechanical techniques. Greater resistance to accelerated weathering was found with the incorporation of 0.5%MWCNT into epoxy resins.
In the present study, MWCNT were incorporated into epoxy resins based on diglycidyl ether of bisphenol A (DGEBA) to see if improvement of the UV resistance of epoxy-based resins could be obtained. The results of epoxy-based resin and epoxy composites were compared and evaluated by chemical tests (Fourier transform infrared (FTIR)), thermal analysis (thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA)), mechanical tests (tensile strength), and microscopy (scanning electron microscopy (SEM)).
Experimental
Materials
The epoxy resins used were Sikadur®
Sample preparation
Oxidized MWCNT treatment
Pristine MWCNT was mixed with a mixture of 3:1 concentrated sulfuric and nitric acid and magnetically stirred (Figure 1). The mixture of MWCNTs and 3:1 concentrated sulfuric and nitric acid was sonicated for 4 h at 40℃ in an ultrasonic bath to create carboxylic acid groups on the surface of MWCNT. After sonication, the mixture was added dropwise to cold distilled water and the oxidized MWCNT were filtered and dried in a vacuum oven at 80℃ for 5 h. 32
Epoxy nanocomposite preparation
Acid-treated MWCNT filler (0.5 wt%) was mixed with neat Sikadur® 330A for 10 min and then heated for 1 h at 50℃. The curing agent was added directly to the pre-dispersed epoxy resin–MWCNT mixture and mixed by hand for 10 min. After degassing for 1 h at room temperature, the epoxy-amine/MWCNT mixture was poured into PTFE molds to cast testing samples in a dog-bone shape (ISO 37) with dimensions of 50 mm length, 20 mm width, and 2 mm thickness (Figure 2). All samples were cured at ambient conditions (25℃ for 7 days), followed by post-curing at 50℃ for 5 h in a vacuum oven.
Accelerated weathering
UV irradiation of epoxy resin and epoxy nanocomposite films was performed using commercial UV-A lamps. The UV lamps produced a collimated and highly uniform UV flux of approximately 140 W/m2 in the UV-A (295–400 nm) range. The specimens were exposed to UV irradiation for one week, then turned over and exposed for a further weeks after which they were exposed to 100% relative humidity at elevated temperature (50 ± 5℃) for two weeks. The experiments were run at 50℃ to increase the degradation rate; the temperature on composite surfaces during summer outdoor exposure often reaches this level when the ambient temperature in the environment is much lower than 50℃. Specimens were tested after this procedure. This process was then repeated, to give total accelerated weathering times of approximately 1, 2, 3, 4, and 6 months for each set of specimens.
Characterization
Fourier-transform infrared
Changes in the chemical structure of epoxy resin and epoxy composites before and after accelerated weathering were studied by Fourier-transform infrared spectrometry (ATR-FTIR, Perkin-Elmer Spectrum 2 with diamond/ZnSe ATR, Llantrisant, UK). All samples were dried in a vacuum oven at 40℃ for 2 h before measurement. The spectra were measured in the range of 700–4000 cm−1 as the principal changes in the functional groups of interest were within these regions. Five samples were assessed for each set of experimental conditions. For each sample, ten spectra were obtained and the results averaged by the instrument software. A baseline was applied setting the signal at 4000 cm−1 as 100% transmission, after which the carbonyl and hydroxyl indexes (equations (1) and (2)) were used to assess the degree of oxidation in the epoxy resins before and after exposure to accelerated weathering
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Tensile strength tests
Tensile strength was measured using a universal testing machine (UTM) (Industrial Series DX 300KN, Instron Ltd., Bayswater VIC). The specimens were tested according to American Society for Standards and Materials (ASTM D638). The testing speed was 10 mm/min. Specimens were positioned vertically between the grips of the testing machine, and stress-strain curves were plotted during the test. Five specimens were tested for each set of conditions.
Dynamic mechanical analysis
Dynamic mechanical analysis was carried out at the Australian National Fabrication Facility (ANFF), using a Mettler Toledo DMA/SDTA861 at a heating rate of 5℃/min in the air and a frequency of 1 Hz. DMA samples were prepared with dimensions of 40 mm length, 10 mm width, and 2.5 mm in thickness.
Thermogravimetric analysis
The thermogravimetric instrument (TA Instruments Hi-Res TGA 2950) was calibrated with calcium oxalate. Specimens of 5–10 g mass were placed in an aluminum pan and then heated from 30℃ to 600℃ under nitrogen at a rate of 10℃/min.
Differential scanning calorimetry
DSC testing was carried out using a TA instrument Q 1000 in a dynamic mode at a heating rate of 2℃/min.
Scanning electron microscopy
A JEOL 6020 scanning electron microscope (JEOL Pty. Ltd., USA) was used to investigate structural changes in epoxy resin and epoxy nanocomposites before and after accelerated weathering. The specimens were coated with gold using a Neo Coater (MP-19020NCTR, Japan).
Results and discussion
Fourier-transform infrared analysis
FTIR analysis was performed to monitor the changes in chemical composition changes for epoxy resins (DGEBA–TMDA) and epoxy nanocomposite (DGEBA–TMDA/0.5%MWCNT) before and after exposure to different accelerated weathering times. Previous studies have shown that carbonyl and hydroxyl groups are generated by degradative oxidation reactions following chain scission and hydrogen abstraction from the polymer backbone. The area between 1600 and 1800 cm−1 arises from carbonyl groups while the area between 3200 and 3700 cm−1 (area 2) corresponds to hydroxyl groups (with potentially some amine groups).34,35 FTIR spectroscopy of the surface of specimens exposed to different durations (1, 2, 3, 4, and 6 months) detected an increasing relative intensity of a hydroxyl group in the 3300 cm−1 band and a carbonyl group in 1654 cm−1 (Figures 3 and 4). For both resins, there is an initial reduction in the C–H stretching region at about 2900 cm−1, after which little change occurs in this region. The carbonyl index of untreated epoxy (DGEBA–TMDA) specimens was much higher than for epoxy nanocomposites (Figure 5); the hydroxyl index was also higher for the untreated epoxy resin specimens (Figure 6). These results indicate that on a molecular level, epoxy nanocomposites have better resistance than DGEBA-derived resins to the UV, temperature, and moisture conditions employed.
Scheme for the synthesis of carboxylated functionalized MWCNT. Epoxy nanocomposite (DGEBA/TMDA/0.5%MWCNT) prepared samples. FTIR comparison for epoxy resin (DGEBA–TMDA) before and after exposure to different accelerated weathering times. ATR–FTIR spectrum of epoxy nanocomposites (DGEBA/TMDA/0.5%MWCNT) before and after exposure to different accelerated weathering times. Carbonyl indexes of epoxy resin (DGEBA–TMDA) (▪) and epoxy nanocomposites (DGEBA/TMDA/0.5%MWCNT) (•) before and after exposure. Hydroxyl indexes of epoxy resin (DGEBA–TMDA) (▪) and epoxy nanocomposites (DGEBA/TMDA/0.5%MWCNT) (•) before and after exposure.
The impact of accelerated weathering exposure of cured DGEBA epoxy with and without nano clay as a stabilizer on the chemical changes has previously been investigated by Woo et al. 9 They followed the chemical changes of DGEBA resin with and without of 5% clay and found no significant improvement in rate of oxidation over 360 h of accelerated weathering (UV exposure), the value of area under the normalised carbonyl region of the spectrum of the curve of 16.5 for DGEBA resin and 19 for the epoxy composite after this time. 13
Another study was followed the chemical changes (carbonyl and hydroxyl groups ) over a six-month period of accelerated weathering (UV and moisture) of DGEBA resin with 4% fly ash, the carbonyl index increased from 90 to 440 in epoxy and only from 100 to 130 for epoxy composites. Equivalent values for the change in the hydroxyl index were 36 to 127 without fly ash and 40 to 46 with fly ash filler. 36
For both the carbonyl index (Figure 5) and hydroxyl index (Figure 6), the initial change was similar for both DGEBA and DGEBA/MWCNT resins, suggesting that resistance to surface oxidation was not significantly improved by incorporation of the MWCNT. After four months, however, both indices rose much more rapidly for DGEBA than for DGEBA/MWCNT, suggesting that MWCNT can reduce penetration of ultraviolet light and/or oxygen into the subsurface. In a comparison of the results of the current work with our previous work for another epoxy resin based on the hydrogenated diglycidyl ether of bisphenol A (epoxy resins and epoxy nanocomposites with MWCNT), 31 the aromatic resin based on DGEBA exhibited lower resistance to chemical degradation, which is reasonable given the presence of a UV-absorbing chromophore in DGEBA.
Tensile tests
Figures 7 and 8 show replicate stress–strain curves of the epoxy resin and epoxy nanocomposites before and after exposure to six months of accelerated weathering, respectively. Note the relatively high degree of reproducibility in the shape of the curves obtained and the maximum stress and strain. Curves for the other data points presented in Figures 9 and 10 are given in the Supplementary Material. Before exposure to accelerated weathering conditions, the epoxy nanocomposite specimens had the significant higher tensile strength (57.5 MPa) compared with the epoxy resin specimens (42 MPa) as is shown in Figure 9. However, the effect of six months of accelerated weathering was only a small loss of tensile strength of epoxy nanocomposite samples (18%), in contrast to epoxy resin specimens (39%) whose strength significantly decreased over this time.
Stress–strain curves of epoxy resin (solid line) and epoxy nanocomposite (dotted line) samples before accelerated weathering. Stress–strain curves of epoxy resin (solid line) and epoxy nanocomposite (dotted line) specimens after six months accelerated weathering. Tensile strength of epoxy resin and epoxy nanocomposite samples before and after exposure to different accelerated weathering times. Strain at break of epoxy resin and epoxy nanocomposite samples before and after exposure to different accelerated weathering times.
Specimens exposed to accelerated weathering consistently give more strain at break than untreated specimens for both epoxy resin types (Figure 10), and this was consistently greater for epoxy resin than for the epoxy composite containing MWNCT. Before exposure, the strain at break of DGEBA–TMDA epoxy had a higher strain at break (1.8%) than the epoxy DGEBA–TMDA/0.5%MWCNT samples, while the DGEBA–TMDA/0.5%MWCNTs showed fewer strain at break (1.75%), compared to that of DGEBA–TMDA epoxy (2.5%). An increase in strain in the break was not linear with time, with the bulk of the increase on weathering seen in the first month for epoxy resin but between the third and fourth months for epoxy nanocomposites, suggesting a protective effect of the MWCNT. In previous work on the effect of the accelerated weathering on the mechanical properties of epoxy composites with fly ash over six months, a decrease in tensile strength was also observed in all the composite matrices after exposure. 36 On extension of exposure period to six months, the control matrix showed a significant reduction in tensile strength (24%) by comparing with that of before exposure, while epoxy matrix with 4 wt% fly ash filler showed a much smaller reduction in tensile strength for the same period of exposure (15%). 36
Another study showed that the tensile strength and elongation at break of epoxy resins containing UV absorber (Tinuvin 1130) exhibited only a slight reduction after 800 h of UV irradiation. While tensile strength and elongation at break of epoxy resin were decreased by ∼30% and ∼35%, respectively, for a control resin after 800 h of UV exposure, in the presence of UV absorber the tensile strength and the elongation at break were decreased only by 12% and 9%, respectively. 37
In similar experiments we have carried out previously with an epoxy resin based on the hydrogenated diglycidyl ether of bisphenol A, 31 we saw a significantly lower reduction in relative tensile strength over the same regimen of treatment, though the initial mechanical properties were poorer.
DMA analysis of epoxy resin and epoxy nanocomposites
The storage modulus represents the elastic properties or the energy storage ability of the composites, and the loss modulus reflects the viscous behavior or the energy dissipation ability of the composites.
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Typical DMA analysis curves for epoxy resins (DGEBA–TMDA) and epoxy nanocomposite (DGEBA–TMDA/0.5%MWCNT) before and after exposure to six months accelerated weathering are shown in Figures 11 and 12, where storage modulus and tan δ for each sample are plotted as a function of exposure time. Before exposure to accelerated weathering, the storage modulus for epoxy nanocomposites (DGEBA–TMDA/0.5%MWCNT) was 13500 MPa, while epoxy resin (DGEBA–TMDA) was 7000 MPa. Before exposure, the tan δ peak of the epoxy nanocomposites (DGEBA–TMDA/0.5%MWCNT) shifted to a higher temperature with a lower value of tan δ (0.62) compared that of epoxy resin (DGEBA–TMDA) that was tan δ (0.75) as shown in Figures 11 and 12.
Storage modulus and tan δ curves for samples of epoxy resin samples before accelerated weathering (solid line) after six months accelerated weathering (dotted line). Storage modulus and tan δ curves for samples of epoxy nanocomposite samples before accelerated weathering (solid line) after six months accelerated weathering (dotted line).
After exposure to six months of accelerated weathering, there was a significant reduction in storage modulus for epoxy resin (37.4%) with a lower tan δ value of 0.82, compared to epoxy nanocomposite (8.6%) with a higher tan δ value of 1.5. In the tan δ curves of Figures 11 and 12 can be seen that the epoxy nanocomposites (DGEBA–TMDA / 0.5% MWCNTs) present lower amplitude peaks than the epoxy resin (DGEBA–TMDA). By comparing the results obtained from our paper with another work, in 2015, Shanmugam et al. 36 reported that the storage modulus of control epoxy displayed a higher reduction than the 4 wt% OFA-filled epoxy system after exposure to six months accelerated weathering.
Thermogravimetric analysis
Typical thermograms of the samples investigated are given in Figures 13 to 16. The thermal stability of samples explains the onset temperature (Ti), the temperature at the maximum decomposition rate (Tmax), and the residual mass for the weathered and control specimens of (epoxy resin) and (epoxy nanocomposite). The results of thermal degradation are summarized in Table 1. All the specimens show a one-stage decomposition process independent of the exposure (Figures 14 and 16), both before and after exposure. Before accelerated weathering, epoxy nanocomposites had a higher Ti (389℃) than epoxy resin specimens (365℃) (Table 1). After exposure times of one month and two months, there were no observed changes in the weight loss of both two epoxies. The Tmax of the two epoxies also decreased with six months of exposure (405℃ for epoxy nanocomposite) and (375℃ for an epoxy resin). The initial epoxy nanocomposite sample had more residue at 500℃ (42.8%) than that of the epoxy resin sample before weathering (25.3%), and this trend continues to be evident after six months of accelerated weathering, although the absolute amount of residue drops as are shown in Table 1. As this change in the residual mass is vastly higher than the amount of MWCNT present in the modified sample, it is clear that incorporation of MWCNT is providing a significant inhibition of thermal degradation. While a similar increase in the amount of residue was observed in our previous work with an analogous aliphatic epoxy resin,
31
the increase in percent residue on the incorporation of MWCNT is significantly higher in this study. In an earlier study of accelerated weathering of DGEBA epoxy resin incorporating fly ash, the percentage residue of DGEBA epoxy resin after six months accelerated weathering was 4.3%, compared to 6.1% residue on incorporation of 4% fly ash—a decrease in the amount of epoxy resin-derived residue,
36
suggesting the reduction in the extent of pyrolysis of epoxy resin observed with MWCNT is not a general phenomenon of the incorporation of fillers.
TGA comparison of epoxy resin before (dotted line) and after (solid line) exposure to six months accelerated weathering. DTG comparison of epoxy resin before (solid line) and after (dotted line) exposure to six months accelerated weathering. TGA comparison of epoxy nanocomposite before (dotted line) and after (solid line) exposure to six months accelerated weathering. DTG comparison curves of epoxy nanocomposite before (dotted line) and after (solid line) exposure to six months accelerated weathering. TGA results of epoxy resin and epoxy nanocomposite samples before and after exposure to different accelerated weathering times.
Glass transition temperature (Tg) evaluation by differential scanning calorimetry
Tg results of epoxy resins and epoxy nanocomposite before and after exposure to different accelerated weathering times.
MWCNT: multiwalled carbon nanotube.
Scanning electron microscopy
Micrographs of epoxy resin and epoxy nanocomposite specimens before and after exposure to accelerated weathering are shown in Figure 17. Before exposure to accelerated weathering, the untreated samples for epoxy resin and epoxy nanocomposite showed a homogeneous surface, with no cracks found on the surface (Figure 17(a) and (c)). After six months accelerated weathering, epoxy resin sample showed long cracks on the surface and obvious voids (Figure 17(b)), compared to epoxy nanocomposites that exhibited less cracking after six months, with shorter cracks that did not propagate as readily through the heterogeneous matrix (Figure 17(d)). SEM tests confirmed that after six months accelerated weathering, the epoxy nanocomposite (DGEBA–TMDA/0.5%MWCNT) sample exhibited limited crack formation on the surface in comparison with the epoxy resins (DGEBA–TMDA) sample. The weathering reported here appears to be more significant than the surface degradation we have previously observed with composites prepared from the hydrogenated analog of DGEBA.
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Conclusions
The durability of an epoxy resin used in engineering timber composite applications, with and without the incorporation of MWCNT was evaluated by exposure to artificial accelerated weathering. The trends observed across a wide range of tests not only suggest that the addition of 0.5 wt% MWCNT has a significant positive impact on the mechanical properties of the composite, but also it introduces appreciable additional resistance to the accelerated weathering regime used. The differences by various methods of assessing degradation between the epoxy resin and composite were relatively minor over the first three months of processing, which is reasonable considering that from the dimensions of the MWCNT they are likely to be excluded from volumes near the surface. Later in the testing, the performance of the unmodified resin decreased more rapidly by all assessments of performance, while the degradation of the epoxy/MWCNT composite continued at the same slow and steady rate. This is attributed to the impact of the MWCNT in absorbing UV radiation, which would otherwise induce photoreaction in the aromatic chromophore and providing greater structural integrity to retard the permeation of the composite by oxygen and water.
Footnotes
Acknowledgements
The authors gratefully acknowledge Andrew Wallace, Wayne Dillon, Malcolm Lambert, John Pesor, and Tony Mackinnon for instrumental assistance. We would also like to thank the Australian National Fabrication Facility (ANFF) for DMA measurements. Mr Awad’s studies are supported by the University of New England in Australia, the Iraq Higher Committee for Education Development (HCED), and the University of Anbar in Iraq.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors received financial support for the research reported in this article from School of Science and Technology at the University of New England (A$ 3000). Mr Awad’s PhD studies were supported by the Higher Committee for Education Development in Iraq (HCED).