EXPERIMENTAL INVESTIGATION OF THE EFFECT OF HYGROTHERMAL AGING ON THE MECHANICAL BEHAVIOR OF CARBON NANOTUBE/PA6 NANOCOMPOSITE K.I. TSERPES * Laboratory of Technology & Strength of Materials Department of Mechanical Engineering & Aeronautics University of Patras, Patras, 26500, Greece kitserpes@upatras.gr E. MOUTSOMPEGKA Laboratory of Technology & Strength of Materials Department of Mechanical Engineering & Aeronautics University of Patras, Patras, 26500, Greece mucobega@mech.upatras.gr O. MURARIU Laboratory of Polymeric and Composite Materials, Materia Nova Research Center, 1 Avenue Nicolas Copernic, 7000 Mons, Belgium Oltea.murariu@materianova.be L. BONNAUD Laboratory of Polymeric and Composite Materials, Materia Nova Research Center, 1 Avenue Nicolas Copernic, 7000 Mons, Belgium Leila.Bonnaud@materianova.be Abstract A. CHANTELI Laboratory of Technology & Strength of Materials Department of Mechanical Engineering & Aeronautics University of Patras, Patras, 26500, Greece aggelikichanteli@mech.upatras.gr The objective of the present work is to investigate experimentally the effect of hygrothermal aging on the mechanical behavior of polyamide 6 (PA6) filled with 10 wt.% carbon nanotubes (CNTs). Neat and CNT-filled PA6 samples were prepared by melt mixing using an internal mixer. The samples were subjected to two different hygrothermal conditions, namely 25 C/85%RH and 40 C/85%RH into an environmental chamber until saturation point. The reference conditions are 25 C/55%RH (RT). Unaged and aged, neat and CNT-filled samples were subjected to tension and 3-point bending tests. The results from mechanical tests show a considerable degradation of the mechanical behavior of both neat PA6 and CNT-filled PA6 specimens due to moisture absorption. The Young s modulus and flexural modulus of the materials are decreased by more than 30% while the flexural strength by 6-17%. The decrease is larger for the conditions of 40 C/85%RH which reveals that the effect of combined elevated temperature and moisture is more critical. On the other hand, the decrease in the properties is smaller for the CNT-filled PA6 specimens than for the neat PA6 specimens, which shows that the presence of CNTs mitigates the effect of hygrothermal aging. Finally, it is observed that the increase in the properties of the neat PA6 material offered by CNTs is so large that it compensates the negative effect of hygrothermal aging. Keywords Carbon nanotubes; Hygrothermal aging, PA6 material; Nanocomposites 1. Introduction Carbon nanotube (CNT)-based polymer nanocomposites find an increasing use in aerospace applications due to their improved mechanical, thermal and electrical properties (Joshi and Chatterjee, 2016; Gohardani et al., 2014) As polymers are highly sensitive to environmental conditions, the understanding of environmental effects on the mechanical behavior of polymer nanocomposites could extend their use in aerospace applications in which severe hygrothermal conditions are present (Barkoula, 2013). Polyamide 6 (PA6) is a thermoplastic material with good mechanical strength, high impact strength, good fatigue strength, very good wear resistance and good
sliding properties. However, these properties might be counterbalanced by water absorption as PA6 is a superabsorbent material (Miri et al., 2009). While the effects of the presence of CNTs and hygrothermal aging on the mechanical behavior of PA6 have been studied extensively e.g. (Miri et al., 2009), the effect of aging on PA6 nanocomposite still remains to be addressed. Existing works on other types of PA6-based nanocomposites (Vlasveld et al., 2005; Vlasveld et al., 2005; Allaoui, 2008; Abacha, 2009; Liu, 2001) have already reported a degradation of mechanical properties due to environmental aging. In the present work, the combined effect of CNT and hygrothermal aging of multi-walled CNT (MWCNT)- reinforced PA6 material is experimentally investigated by means of hygrothermal tests, tension tests, 3-point bending tests and scanning electron microscopy (SEM) tests. 2. Materials and processing The polymer used in this study is the Akulon F132-C1-PA6, which is a commercial product from DSM Engineering Plastics. MWCNTs were provided by Nanocyl (NC7000) and were used without any further purification. According to the supplier, the MWCNTs have an average diameter of 9.5 nm, mean length of 1.5 mm, a surface area ranging from 250-300 m 2 /gr and a purity of ca. 90%. Before processing by melt-blending, PA6 was dried overnight at 80 C under vacuum. An internal mixer (Brabender, Germany) was used to prepare the neat and doped PA6 samples. For melt mixing experiments, the conditions are as follows: 250 C, 3 min at 30 rpm (for introduction of materials) and further 7 min at 60 rpm to homogeneously mix them in the melt. A Thermo-Haake mini-injection machine was used to produce dog-bone specimens and bar specimens. The injection molding parameters were set as follows: temperature of the barrel: 280 C; time for preheating and melting: 120s; temperature of the mold: 80 C for neat PA6 samples and 100 C for doped samples. 3. Experimental 3.1 Hygrothermal aging PA6 and MWCNT/PA6 specimens were subjected to hygrothermal aging using the ESPEC SH-641 environmental chamber shown in Fig.1. The conditions applied until saturation are: 25 ο C/85%RH and 40 ο C/85%RH. During the tests, the specimens weight was periodically measured. (a) Fig.1. (a) The environmental chamber and (b) Bar specimens inside the chamber. (b) 3.2 Mechanical tests Tension tests were conducted on dog-bone specimens according to ASTM D638-03 (ASTM International, 2003) using a Tinius-Olsen H5KT testing machine with a load cell of 5 kn. Strain on the specimens was measured using strain gauges and the digital image correlation system ARAMIS. The applied load rate was 1 mm/min. A photo of a specimen during a tension test is shown in Fig.2a. Three-point bending tests were also conducted on bar specimens according to ASTM D790-03 (ASTM International, 2003) using a Tinius-Olsen H5KT testing machine with a load cell of 5 kn. A photo of a specimen during a three-point bending test is shown in Fig.2b.
(a) Fig.2. (a) A specimen during a tension test and (b) A specimen during a three-point bending test. (b) 3.3 SEM In order to characterize the morphology of the MWCNT/PA6 specimens and try to evaluate the combined effect of the presence of MWCNTs with the aging process of the PA6 material, SEM characterization has been performed using a JEOL JSM-6610 LV Scanning Microscope. 4. Experimental results and discussion 4.1 Water absorption Fig.3 plots the percentage weight gain M(t) of the materials due to water absorption with regard to exposure time. It is observed that at the higher temperature (40 o C) both materials show a larger absorbing rate and the presence of MWCNTs reduces the absorbability of PA6 material. The MWCNT/PA6 material also shows an intermediate plateau between 2 and 4 days. The final weight gain is the same (~4.25%) for all cases except for the MWCNT/PA6 material at 40 o C. Fig.3. Evolution of weight gain with regard to exposure time. 4.2 Young s modulus The measured Young s moduli of unaged and aged neat PA6 and MWCNT/PA6 materials are compared in the diagram of Fig.4. The standard deviation for each case is also included in the diagram. For the neat PA6 material, the standard deviation is rather small and it is larger for the reinforced material, which is probably due to the variation of reinforcement quality. At RT conditions, the results show an extraordinary increase of the Young s modulus of PA6 material due to the addition of MWCNTs which reaches up to 180%. Aging is causing a decrease in the Young s modulus of both materials. The rate of decrease is larger for the MWCNT/PA6 material which indicates a possible interaction between moisture and MWCNTs. In any case, the presence of MWCNTs compensates the negative effect of aging since the Young s modulus of the aged MWCNT/PA6 material is still much higher than that of the aged PA6 material.
Fig.4. Young s moduli of unaged and aged neat PA6 and MWCNT/PA6 materials. 4.3 Flexural properties Fig.5 compare the flexural stress-flexural strain curves of the neat PA6 and MWCNT/PA6 materials for the RT, 25 o C/85%RH and 40 o C/85%RH conditions. A first general comment drawn from the diagrams is that a good correlation between the curves of the same case is obtained. The mechanical properties derived from the diagrams, i.e. the flexural modulus and flexural strength are compared for the different materials and different environmental conditions in Fig.6, respectively. The flexural modulus and flexural strength of PA6 material at RT conditions increase due to the addition of MWCNTs by 146% and 82%, respectively. Aging causes a degradation of both properties for both materials. The degradation rate is larger for the flexural modulus and almost the same for the two materials. Again, the addition of MWCNTs compensates the negative effect of aging on the flexural properties of the PA6 material. Fig.5. Flexural stress-flexural strain curves of the neat PA6 and MWCNT/PA6 specimens: (a) RT, (b) 25 o C/85%RH and (c) 40 o C/85%RH (a) (b) Fig.6. (a) Flexural modulus at different conditions and (b) Flexural strength at different conditions.
4.4 SEM investigation Fig.7 shows representative SEM images of the PA6 material for the different conditions. The comparison of the images reveals a swelling (indicated areas in Figs.7(b) and (c)) of the material due to water absorption. SEM images of the MWCNT/PA6 material, illustrated in Fig.8, show a uniform and a dense dispersion of MWCNTs within the PA6 material and the division of material s surface into two areas with different roughness. The uniform and dense dispersion of MWCNTs is not influenced by water absorption. A careful look at the SEM images of the unaged and aged MWCNT/PA6 samples (Fig.9) reveals that the pull-out length of MWCNTs at the aged sample is significantly larger than at the unaged sample, which reveals that aging reduces the cohesion strength between MWCNT and PA6. Fig.7. SEM images of PA6 material: (a) RT, (b) 25 o C/85%RH and (c) 40 o C/85%RH Fig.8. SEM images of MWCNT/PA6 material: (a) RT, (b) 25 o C/85%RH and (c) 40 o C/85%RH Fig.9. SEM images of MWCNT/PA6 material: (a) RT, (b) 25 o C/85%RH and (c) 40 o C/85%RH 5. Conclusions In the present paper, the effect of hygrothermal aging on MWCNT/PA6 nanocomposite material has been experimentally investigated by means of hygrothermal tests, tension tests, three-point bending tests and SEM investigation. The results show that hygrothermal aging reduces the mechanical properties of PA6 and the nanocomposite but the increase in the properties of the nanocomposite due to the presence of MWCNTs is so large that it compensates the decrease in the properties of PA6 material due to aging. The decrease in the properties is smaller for the MWCNT/PA6 specimens than for the neat PA6 specimens, which shows that the presence of CNTs mitigates the effect of hygrothermal aging.
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