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nanoclay composites reinforced with carbon and glass fibers
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Effect of electron beam irradiation on mechanical properties of unsaturated polyester/nanoclay composites reinforced with carbon and glass fibers Seyed Mohammad Razavi MethodologyInvestigationWriting - Original DraftWriting - Review & Editing , Seyed Javad Ahmadi ConceptualizationMethodologyWriting - Review & EditingSupervision , Peyman Rahmani Cherati MethodologyInvestigation , Mina Hadi , Seyed Amir Reza Ahmadi PII: DOI: Reference:
S0167-6636(19)30866-X https://doi.org/10.1016/j.mechmat.2019.103265 MECMAT 103265
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Mechanics of Materials
Received date: Revised date: Accepted date:
6 October 2019 23 November 2019 27 November 2019
Please cite this article as: Seyed Mohammad Razavi MethodologyInvestigationWriting - Original DraftWriting - Revi Seyed Javad Ahmadi ConceptualizationMethodologyWriting - Review & EditingSupervision , Peyman Rahmani Cherati MethodologyInvestigation , Mina Hadi , Seyed Amir Reza Ahmadi , Effect of electron beam irradiation on mechanical properties of unsaturated polyester/nanoclay composites reinforced with carbon and glass fibers, Mechanics of Materials (2019), doi: https://doi.org/10.1016/j.mechmat.2019.103265
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Highlights
Mechanical properties of UP/nanoclay and UP/nanoclay/fiber composites are studied.
Effect of EB irradiation on mechanical properties of composites is investigated.
Effect of irradiation on composites is studied using FTIR spectroscopy.
Nanoclay enhanced both mechanical and radiation resistance properties of polymer.
The composites had desirable properties for applications with radiation exposure.
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Effect of electron beam irradiation on mechanical properties of unsaturated polyester/nanoclay composites reinforced with carbon and glass fibers Seyed Mohammad Razavia, Seyed Javad Ahmadia,*, Peyman Rahmani Cheratib, Mina Hadib, Seyed Amir Reza Ahmadic a
Materials and Nuclear Fuel Research School, Nuclear Science and Technology Research Institute, P.O. Box 113658486, Tehran, Iran
b
Department of Chemical and Petroleum Engineering, Sharif University of Technology, P.O. Box 11365-9465, Tehran, Iran
c
Mechanical Engineering Department, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran
*
Corresponding Author: Email: [email protected] (S.J. Ahmadi); Tel: +98-21-82064343; Fax:+98-21-88221116
Abstract Following our previous work on morphology, corrosion resistance and thermal properties of unsaturated polyester (UP)/nanoclay composites, the effect of electron beam (EB) irradiation on mechanical properties of UP/nanoclay composites with various nanoclay contents is investigated in this work. Moreover, influence of EB irradiation with doses of 100, 500 and 1000 kGy on tensile properties and hardness of nanocomposite was examined. FTIR spectroscopy was employed to examine the effect of irradiation on chemical structure of polymer. EB irradiation up to 500 kGy led to the improvement of tensile strength and hardness of the samples. The best mechanical properties were observed for the nanocomposite with 1 wt.% nanoclay content. As the selected nanocomposite, UP/nanoclay(1wt.%) composite was reinforced by carbon and glass fibers and the effect of EB irradiation on the mechanical properties of the prepared UP/nanoclay/fiber composites was studied. When compared to the fiberreinforced UP without nanoclay, presence of nanoclay particles in UP/nanoclay/fiber composites resulted in the better mechanical properties and higher radiation resistance. Hence, the UP/nanoclay/fiber composite with proper amount of nanoclay has been shown to be a dependable choice for structural applications in which the material is exposed to ionizing radiations. Keywords: Unsaturated polyester; Nanoclay; Carbon fiber; Glass fiber; Composite; Electron beam irradiation; Mechanical properties; FTIR
1. Introduction 2
Polymer composites are proper materials for the production of structural components because of their high strength-to-weight ratios. Unsaturated polyester (UP) resins are among the most extensively used thermosetting polymers due to their cheapness and processibility [1]. UP polymers reinforced with fibers or micro-sized and nano-sized particles are widely utilized in marine industries, structural materials, manufacturing of chemical containers, etc. [2]. Montmorillonite is one of the most common fillers that is used for the improvement of polymer properties [3,4]. It is a type of layered silicate nanoclay that has a laminated structure formed by the layers which have the thickness of 1 nm. The montmorillonite nanoparticles are usually modified by exchanging metal cations in their structure with an organic intercalating agent such as alkyl ammonium ions. In this way, the interactions between the nanoclay platelets are diminished and the interaction between the polymer chains and the silicate layers are enhanced. Thus, the distribution of clay nanolayers in the polymer matrix is improved. The effect of various nanoclay types on the properties of UP polymers has already been studied extensively. Jo et al. [5] showed that the addition of montmorillonite nanoclay up to 5 wt% enhanced the mechanical and thermal properties of UP. Among the studied nanoclays, Cloisite 30B had the best performance. The enhancement of the mechanical properties and the thermal resistance of the UP resin by the incorporation of different types of nanoclay are also reported by other researchers [6–17]. Although addition of nanoclay improves the mechanical properties of UP polymer, it cannot fulfill the required properties for some structural usages yet. Further improvement in mechanical properties of the polymer could be achieved by reinforcement with continuous fibers such as carbon and glass fibers. It has been shown that the incorporation of woven glass fibers into UP resin reduces the maximum moisture absorption [18], enhances the thermal stability [19], and improves the mechanical properties of the composites [20]. A Combination of glass and carbon fibers was also successfully used to improve mechanical properties of UP [21]. Kusmono and Ishak [22] studied the effect of clay loading on the morphological and mechanical properties of UP/glass fiber composites, showing the improving effect of clay on tensile strength, stiffness, and toughness of UP/glass fiber composites. Enhancement of various mechanical properties of UP/glass fiber composite by the incorporation of clay into the polymer matrix is also reported by others [23,24]. Studies that have examined the chemical resistant properties of UP/nanoclay composites are scarce. In our previous work [25], the morphological structure, density, water absorption, thermal properties, and corrosion resistance of UP/nanoclay composites with various nanoclay contents were investigated. Moreover, the influence of electron beam (EB) irradiation on corrosion resistance of the nanocomposites was studied. Study of the effect of ionizing radiations on the properties of a nanocomposite is vital when the material is aimed to be used in applications such as the production of nuclear waste disposal 3
containers. The completely exfoliated structure was observed for the UP nanocomposites with 1 wt.% and 3 wt.% nanoclay contents. Thermal and corrosion resistance properties of the UP polymer were improved when an appropriate amount of nanoclay was added to the polymer matrix. The nanoclay particles reduced the water absorption of the polymer. Moreover, EB irradiation up to 500 kGy enhanced the water absorption resistance and corrosion resistance of the UP/nanoclay composites. Results of the work [25] that are described above demonstrate the improving effects of both nanoclay addition and EB irradiation on the thermal and chemical resistance of UP polymer. However, investigation of the mechanical properties has not been included in that work. Studying the mechanical properties of the composite at various nanoclay contents and irradiation doses is very important to ensure its reliability for some specific usages such as the production of radioactive disposal containers. Following the previous work [25], in the present work, mechanical properties of UP/nanoclay composites before and after EB irradiation are investigated. Additionally, resulted optimum nanoclay content was used to prepare UP/nanoclay/glass fiber and UP/nanoclay/carbon fiber composites and their tensile properties were studied. To the best of our knowledge, the mechanical properties of UP/nanoclay composites reinforced with continuous fibers and the effect of ionizing radiations on them are not reported elsewhere. Thus, the capability of those composites for the production of structural parts that are exposed to ionizing radiations, such as nuclear waste disposal containers, may be confirmed based on the results of the present work. 2. Experimental 2.1. Materials Unsaturated Polyester resin (UP-770IS) with the viscosity of 500 (mPa.S) at 25oC, density of 1.15 (g/cm3), and Nonvolatile content of 63%–66% was purchased from Resitan Co. (Iran) [26]. The UP resin was based on isophthalic acid and neopentyl glycol. Tert-butyl peroxybenzoate initiator with the commercial name of TRIGONOX C was prepared from AkzoNobel (Netherlands) [27]. The organically modified Cloisite 30B montmorillonite nanoclay was supplied by Southern Clay Products Inc. (USA). Carbon and glass fibers that were already woven into the plain weave cloths with a surface density of 200 (g/m2) were used. The thickness of woven carbon fibers and glass fibers was 0.28 mm and 0.26 mm, respectively. 2.2. Preparation of UP/nanoclay composites UP/nanoclay composites were prepared using the method detailed in our preceding work [25]. Briefly, appropriate amounts of nanoclay were mixed with the UP resin to produce the nanocomposite samples 4
with 0, 1, 3, and 5 wt.% nanoclay contents (named UP0, UP1, UP3, and UP5, respectively). The crosslinking reaction was initiated by the addition of 2 phr of Tert-butyl peroxybenzoate to the resin/nanoclay mixture. The mixture was then cast into the silicon molds with the shapes and sizes appropriate for intended testing methods. The molded samples were cured at 120 oC for 15 minutes and were post-cured at 140oC for 5 minutes. 2.3. Preparation of UP/nanoclay/fiber composites The fiber-reinforced nanocomposites were prepared via hand lay-up method using carbon and glass fibers. A mold formed from two steel plates and a steel frame was utilized. To prevent the sticking of the composite to the surface of the steel mold, a Teflon film was placed on the surface of the plate. The carbon or glass fibers in the form of woven mats were cut regarding the mold size. The first fiber layer was placed on the mold surface covered by the Teflon film. Then, the mixture of UP resin, nanoclay (if required) and TBPB initiator was uniformly spread on the mat surface using a brush. The second fiber layer was then placed on the resin surface and the trapped air bubbles were removed with a roller. The described process was repeated for each layer until eight layers of mat were stacked. Finally, after putting a Teflon film, the top plate of the mold was placed on the stacked layers. The mold was exposed to 10 MPa pressure using a mechanical press equipped with a heating system. The curing and post-curing of the molded composites were carried out under the applied pressure at 120 oC for 15 minutes and at 140oC for 10 minutes, respectively. Afterward, the mold was opened and the completely cured composite was taken out. The composite sheet was cut to produce the specimens required for different tests. Compositions of the prepared UP/nanoclay and UP/nanoclay/fiber composites are shown in Table 1.
Sample UP0 UP1 UP3 UP5 UP0C UP1C UP0G UP1G
Table 1. Prepared UP/nanoclay and UP/nanoclay/fiber composites Nanoclay content (wt.%) 0 1 3 5 0 1 0 1
Type of fiber Carbon fiber Carbon fiber Glass fiber Glass fiber
2.4. Irradiation of composites The prepared composite samples were exposed to EB irradiation with overall doses of 100, 500, and 1000 kGy. A Rhodotron type electron accelerator machine (TT200) with a 10 MeV electron beam and a maximum of 8 mA beam current was utilized at room temperature and air atmosphere under 5
homogeneous oxidation condition. More details about the irradiation process are provided in the previous paper [25]. 2.5. Tensile testing Tensile properties of the composites were studied using a SANTAM STM50 (Iran) tensile testing machine. The cross-head speed was set to 1 (mm/min). Five specimens were tested for each polymeric composite. The shape and size of UP/nanoclay and UP/nanoclay/fiber composite specimens were selected in accordance with ASTM D638M and ASTM D3039M standard methods for tensile testing, respectively. 2.6. Hardness tests Surface hardness of nanocomposites was measured according to ASTM D2240 using Bareiss HP-D (Germany) Shore D durometer. Five specimens were tested for each nanocomposite. 2.7. FTIR Fourier transform infrared (FTIR) spectroscopy was employed to examine the influence of irradiation on the chemical structure of samples. FTIR spectra were recorded at ambient temperature using a MB-100 FTIR spectrophotometer of ABB BOMEM (Canada). |The spectra were obtained in the wavenumber range of 400 to 4000 cm-1 with the rate of 22 scans/minute and resolution of 4 cm-1. 3. Results and Discussion Morphology and structure of the UP/nanoclay composites prepared in the present work have already been studied using XRD and TEM methods and are reported in the previous work [25]. Moreover, complete conversion of UP resin to the polymer at the employed curing and post curing condition was demonstrated in that work. The following sections include the investigation of the effect of irradiation on mechanical properties and chemical structure of the prepared nanocomposites. 3.1. UP/nanoclay composites 3.1.1. Tensile properties Tensile strength and tensile modulus of UP/nanoclay composites are listed in Table 2. Addition of nanoclay to the polymer increased its tensile modulus, since nanoclay particles are more rigid than the polymeric matrix. The improvement in stiffness of composites is the result of proper dispersion of nanoclay particles and strong interaction between the organically modified nanoclay and the polymer 6
chains that reduce their mobility. The tensile strength of polymer was enhanced with nanoclay content of 1 wt.% and 3 wt.%, because of the efficient reinforcement provided by the exfoliated nanoclay particles. The best tensile strength was achieved for UP1 nanocomposite. When the nanoclay content was higher than 3 wt%, the tensile strength was reduced, since the non-exfoliated nanoclay particles may intensify the stress at specific points and reduce the tensile strength of the material. Flocculation of nanoclay particles causes to the lower interaction between the nanoclay particles and polymer chains, which results in the interfacial detachment during the tensile testing. Moreover, the decrease of tensile strength can be attributed to the reduction of crosslinking density at high nanoclay contents [13].
Nanocomposite UP0 UP1 UP3 UP5
Table 2. Tensile properties of UP/nanoclay composites Tensile Modulus (GPa) Tensile Strength (MPa) Mean±SD Mean±SD 0.961±0.017 53.35±0.97 1.033±0.020 57.69±1.05 1.160±0.026 56.71±0.76 1.215±0.034 50.53±0.94
The effect of EB irradiation on the tensile properties of the UP polymer and UP/nanoclay composites is illustrated in Figures 1 and 2. Both tensile strength and tensile modulus of samples were improved by irradiation up to 500 KGy. However, radiation dose of 1000 kGy deteriorated the tensile properties of the samples. Depending on the radiation intensity and the type of polymer, ionizing radiation can cause crosslinking and/or chain scissioning reactions in the polymer structure [28]. The observed changes in the mechanical properties of polymer nanocomposites can be attributed to the predominance of crosslinking reactions in the UP polymer when the EB irradiation dose was lower than 500 kGy, followed by the significant effect of chain scissioning reactions at a radiation dose of 1000 kGy that causes the degradation of the polymer chains and reduction of tensile strength of the nanocomposites. Similar phenomena are reported for the effect of ionizing radiations on different polymeric nanocomposites [29– 32].
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Tensile Strength (MPa)
70 65 60 55 50
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Irradiation Dose (kGy) Figure 2. Effect of EB irradiation on tensile modulus of UP/nanoclay composites
Organically modified nanoclay particles can influence the radiation-induced changes in the polymer properties in different ways. The inorganic structure of nanoclay results in a high resistance against 8
ionizing radiation [33,34]. Nanoclay particles can act as barriers that simultaneously decrease the oxidative degradation of the polymer via reduction of oxygen diffusion through the polymer structure and decrease the cross-linking between polymer chains. On the other hand, the organic modifier of the nanoclay degrades by the ionizing radiations (Hofmann degradation), which leads to the formation of free radicals. Depending on the irradiation dose, the produced free radicals can enhance the cross-linking reactions or accelerate the oxidative degradation of the polymer [35]. Thus, the final consequence of nanoclay addition on the properties of irradiated polymer depends on the nature of the polymer (e.g. critical dose of the Hofmann reaction), nanoclay content, and irradiation dose. As shown in Figure 1, presence of nanoclay particles in the UP polymer matrix reduced the effect of EB radiation on mechanical properties of the polymer. High nanoclay content in UP5 nanocomposite caused the deterioration of the tensile strength of polymer after irradiation. This can be due to the non-exfoliated structure of UP5 nanocomposite as well as its high organic modifier content available for Hofmann degradation reaction. However, incorporation of proper amounts of nanoclay into the polymer moderated the destructive effect of high-dose EB radiation. Although, the highest tensile strength among the irradiated samples was observed for UP0 pure polymer sample after 500 kGy irradiation, addition of 1 wt.% nanoclay into the polymer matrix enhanced its resistance against the degradation at 1000 kGy radiation dose. Therefore, UP1 nanocomposite is a suitable choice for applications that involve the long-term exposure of material to ionizing radiation. 3.1.2. Hardness Data of Shore D hardness of UP/nanoclay composites are listed in Table 3. When compared to the pristine polymer, UP0, the hardness of UP/nanoclay composites were enhanced. Clay nanolayers that were efficiently distributed in the nanocomposites with exfoliated structure hinder the development of indentation. A decrease in the hardness of nanocomposites with high clay content was demonstrated, which was due to the formation of intercalated or flocculated structure at high clay contents. In the composites with non-exfoliated structure, clay nanolayers were not distributed uniformly in the polymer structure. Consequently, the polymeric matrix with low hardness remained empty of hard nanoclay particles in some sections.
Nanocomposite UP0 UP1 UP3 UP5
Table 3. Hardness of UP/nanoclay composites Shore D Hardness Mean±SD 83.0±0.6 85.0±0.5 84.6±0.5 83.6±0.6
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The effect of EB irradiation on UP0 polymer as well as the UP1 nanocomposite that had the highest hardness among the studied nanocomposites is shown in Figure 3. Similar to the trend reported for tensile strength of samples, hardness of UP0 and UP1 materials was increased for radiation doses up to 500 kGy, followed by a decrease at 1000 kGy. It is observed that well-distributed nanolayers of clay reduced the destructive effects of high-dose irradiation on UP polymer in some extent. 90 88
Shore D Hardness
86 84 82 80 UP0 78
UP1 76 0
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Irradiation Dose (kGy) Figure 3. Effect of EB irradiation on Shore D hardness of UP/nanoclay composites
3.1.3. FTIR spectra Figure 4 illustrates the FTIR spectra of UP0 and UP1 samples before and after exposure to 1000 kGy EB irradiation. The spectra related to the lower irradiation doses were not reported since no detectable radiation-induced change was observed in them. Also, FTIR spectra of irradiated specimens were almost identical to those of non-irradiated materials. The phthalic component of the polymer is indicated by characteristic sharp peaks at 1722 cm-1 (C=O stretching) and at 1274 cm-1 (C-O-C). Intensity of the peak at 1722 cm-1 is reduced after irradiation as the result of both crosslinking and degradation reactions which involve unsaturated C=O bands. The presence of nanoclay moderates the effect of irradiation, which is in agreement with the results of the mechanical tests. The slight increase in the intensity of the broad band in 3200-3600 cm-1 range (O-H stretching) after irradiation of both UP0 and UP1 materials can be due to the oxidative degradation induced by the high-dose irradiation.
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Figure 4. FTIR spectra of a) non-irradiated UP0, b) UP0 exposed to1000 kGy EB irradiation, c) non-irradiated UP1, d) UP1 exposed to1000 kGy EB irradiation
3.2. UP/nanoclay/fiber composites 3.2.1. Tensile properties Following the study of the mechanical properties of UP/nanoclay composites, tensile properties of the fiber-reinforced UP composite materials are studied in the present work. As it is described in previous section, UP/nanoclay composites with 1 wt.% of nanoclay showed the best mechanical properties among the studied nanocomposites materials. Additionally, the high viscosity of UP resin at high nanoclay contents makes it difficult to prepare the flawless UP/nanoclay/fiber composites via hand lay-up method. Thus, In the current work, UP/nanoclay/glass fiber and UP/nanoclay/carbon fiber composites with 0 and 1 wt.% of nanoclay were fabricated and examined. Tensile properties of the fiber-reinforced composites, along with the effect of fiber-reinforcement on the properties of each material are reported in Table 4. The effect of fiber reinforcement is determined as the improvement percentage of each property after incorporation of fibers. Obtained results demonstrate the considerable effect of both glass and carbon fibers on tensile properties of UP0 and UP1 materials. In this case, influence of carbon fiber on tensile strength and tensile modulus of polymer and its nanocomposite was superior. When compared to glass fiber, carbon fiber has higher specific strength and lower density that make it a suitable choice for the production of high strength light structures. However, glass fiber is 11
more cost effective [36]. Hence, the suitable fiber may be selected for each specific application considering the required performance and economical aspects. Table 4. Tensile properties of UP/nanoclay/fiber composites Tensile Modulus Tensile Strength Composite Mean±SD (GPa) UP0C UP1C UP0G UP1G
11.696±0.921 13.964±0.542 5.361±0.245 5.573±0.273
Effect of fiber reinforcement (%) 1117 1252 458 439
Mean±SD (MPa) 624.2±59.4 745.7±50.5 286.6±22.8 297.3±20.4
Effect of fiber reinforcement (%) 1070 1193 437 415
It is well known that the mechanical properties of fiber reinforced polymers are mostly governed by the mechanical properties of the fiber. However, incorporation of nanoclay particle into the matrix may influence the properties of fiber-reinforced polymers in some extent. Table 4 shows that the tensile strength and tensile modulus of both UP/glass fiber and UP/carbon fiber composites are improved by nanoclay particles. It can be attributed to the improving effect of nanoclay on UP matrix that is previously discussed. Nanoclay particles with exfoliated structure enhanced the polymeric matrix of the fiberreinforced composite and inhibited the propagation of cracks in the matrix. Moreover, clay nanolayers contribute to the augmentation of the mechanical properties through the enhancement of interfacial interaction between long fibers and polymer chains [37]. The strong interaction between polymeric matrix and long fibers led to the proper transfer of stress between the components and prevented the formation of interfacial cracks between matrix and fibers. The synergic influence of nanoclay particles and long fibers on the tensile properties of UP matrix resulted in the significant improvement of mechanical properties of the material. Effect of EB irradiation on tensile strength and tensile modulus of UP/nanoclay/fiber composites is illustrated in Figures 5 and 6. Considering the effect of irradiation on UP/nanoclay matrices that has previously been discussed, observed improvement in tensile properties of both UP0C and UP1C composites by the irradiation doses up to 500 kGy was anticipated. Further irradiation (up to 1000 kGy) had no significant effect on tensile properties of the composites. Regarding the improving effect of irradiation on tensile properties of pristine carbon fiber [38], the enhancement of UP polymer by carbon fiber provides it with high radiation resistance, which makes the prepared composite a suitable choice for the applications that involve exposure to the ionizing radiation. Although addition of nanoclay into the UP/carbon fiber composite increased its tensile strength and tensile modulus, it had negligible influence on the effect of EB irradiation on UP/carbon fiber composite. It seems that the effect of nanoclay on the
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radiation-induced changes of mechanical properties of samples was eclipsed by the tremendous effect of carbon fiber on mechanical properties. 1000 UP0C UP1C UP0G UP1G
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Irradiation Dose (kGy) Figure 5. Effect of EB irradiation on tensile strength of UP/nanoclay/fiber composites
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Irradiation Dose (kGy) Figure 6. Effect of EB irradiation on tensile modulus of UP/nanoclay/fiber composites
EB irradiation generally increased the tensile strength and tensile modulus of UP/nanoclay/glass fiber composites. For those composites, gamma irradiation caused the formation of a crosslinked structure in the polymer matrix. On the other hand, applied irradiation doses make no substantial effect on the properties of neat glass fibers [39]. Desirable effect of nanoclay on the radiation resistance of the material is observable at the highest applied irradiation dose. Incorporation of nanoclay into the UP/glass fiber composite prevented the deterioration of its tensile strength at 1000 kGy. The robust structure of exfoliated nanoclay layers can act as barriers that block the diffusion pass of oxygen into the polymer structure and can decrease the oxidative degradation of polymer-polymer and polymer-fiber bonds. 4. Conclusions Mechanical properties of UP/nanoclay composites with different nanoclay contents were examined. It was shown that proper amount of nanoclay can improve the tensile properties and hardness of UP polymer. The best mechanical properties was observed when 1wt.% of nanoclay was added to the polymer. EB irradiation up to 500 kGy improved the mechanical properties of the composite, while higher irradiation doses resulted in the degradation of polymer. Moderating effect of nanoclay particles on the destructive influence of high-dose radiation was demonstrated. Obtained results were confirmed using FTIR spectroscopy. As the UP/nanoclay composite with the optimum properties, UP1 nanocomposite was selected to be further reinforced by carbon and glass fibers. Reinforcement of composites with long fibers 14
resulted in the significant improvement in their mechanical properties. In this case, effect of carbon fiber was superior. When compared to the UP/fiber composites, presence of nanoclay particles in UP/nanoclay/fiber composites led to the improvement of mechanical properties of the material after irradiation. Nanoclay particles mitigated the reduction of tensile strength at high irradiation doses by hindering the oxidative polymer degradation. Thus, prepared UP1G and UP1C composites are reliable choices for the applications that include exposure to ionizing radiations. Those materials show desirable mechanical properties even when they are exposed to high-dose ionizing radiation. References [1] Chirayil CJ, Mathew L, Hassan PA, Mozetic M, Thomas S, (2014), Rheological behaviour of nanocellulose reinforced unsaturated polyester nanocomposites. Int J Biol Macromol 69, 274–281 [2] Pereira CMC, Herrero M, Labajos FM, Marques AT, Rives V, (2009), Preparation and properties of new flame retardant unsaturated polyester nanocomposites based on layered double hydroxides. Polym Degrad Stab 94, 939–946 [3] Pavlidou S, Papaspyrides CD ,(2008), A review on polymer–layered silicate nanocomposites. Prog Polym Sci 33, 1119–1198 [4] de Azeredo HMC, (2009), Nanocomposites for food packaging applications. Food Res Int 42, 1240– 1253 [5] Jo BW, Park SK, Kim DK, (2008), Mechanical properties of nano-MMT reinforced polymer composite and polymer concrete. Constr Build Mater 22, 14–20 [6] Tsai TY, Kuo CH, Chen WC, Hsu CH, Chung CH, (2010), Reducing the print-through phenomenon and increasing the curing degree of UP/ST/organo-montmorillonite nanocomposites. Appl Clay Sci 49, 224–228 [7] Kornmann X, Berglund LA, Sterte J, Giannelis EP, (1998), Nanocomposites based on montmorillonite and unsaturated polyester. Polym Eng Sci 38, 1351–1358 [8] Bharadwaj RK, Mehrabi AR, Hamilton C, Trujillo C, Murga M, Fan R, Chavira A, Thompson AK, (2002), Structure–property relationships in cross-linked polyester–clay nanocomposites. Polymer 43, 3699–3705 [9] Dhakal HN, Zhang ZY, Richardson MOW, (2006), Nanoindentation behaviour of layered silicate reinforced unsaturated polyester nanocomposites. Polym Test 25, 846–852 [10] Chieruzzi M, Miliozzi A, Kenny JM, (2013), Effects of the nanoparticles on the thermal expansion and mechanical properties of unsaturated polyester/clay nanocomposites. Compos A 45, 44–48 [11] Ollier R, Rodriguez E, Alvarez V, (2013), Unsaturated polyester/bentonite nanocomposites: influence of clay modification on final performance. Compos A 48, 137–143
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[12] Sen S, Gundem HB, Ortac B, (2013), Property enhancement in unsaturated polyester nanocomposites by using a reactive intercalant for clay modification. J Appl Polym Sci 129, 3247–3254 [13] Inceoglu AB, Yilmazer U, (2003), Synthesis and mechanical properties of unsaturated polyester based nanocomposites. Polym Eng Sci 43, 661–669 [14] Jawahar P, Gnanamoorthy R, Balasubramanian M, (2006), Tribological behaviour of clay– thermoset polyester nanocomposites. Wear 261, 835–840 [15] Nazare S, Kandola BK, Horrocks AR, (2006), Flame-retardant unsaturated polyester resin incorporating nanoclays. Polym Adv Technol 17, 294–303 [16] Martin James, Manoj George K., Cijo Mathew, Dr. K. E. George, Reji Mathew, Modification of Fiber-Reinforced Plastic by Nanofillers, International Journal of Engineering and Innovative Technology (IJEIT) Volume 3, Issue 4, October 2013 234-240 [17] Fekri Laatar, Manuel Gomez, Mohamed Ramzi Ben Romdhane, Ezzeddine Srasra, (2016), Preparation of nanocomposite by adding of Tunisian nanoclay in unsaturated polyester matrix: mechanical and thermal study, J Polym Res 23, 239 [18] Mohd Hafiz Zamri, Hazizan Md Akil, Azhar Abu Bakar, Zainal Arifin Mohd Ishak, Leong Wei Cheng, (2012), Effect of water absorption on pultruded jute/glass fiber-reinforced unsaturated polyester hybrid composites, Journal of Composite Materials 46, 51–61 [19] K. Laoubi, Z. Hamadi, A. Ahmed Benyahia, A. Serier, Z. Azari, (2014), Thermal Behavior of EGlass fiber-Reinforced Unsaturated polyester Composites, Composites: Part B 56, 520-526 [20] K. John, S. Venkata Naidu, (2004), Tensile Properties of Unsaturated Polyester-Based Sisal Fiber– Glass Fiber Hybrid Composites, Journal of Reinforced Plastics and Composites 23, 1815-1819 [21] Abdul Awal, Masud Rana, (2013), Mechanical Properties of Unsaturated Polyester-Based Plain Structured Glass Fiber-Carbon Fiber Hybrid Composites, Polymer-Plastics Technology and Engineering 52, 1376–1380 [22] Kusmono, Zainal Arifin Mohd Ishak, (2013), Effect of Clay Addition on Mechanical Properties of Unsaturated Polyester/Glass Fiber Composites, International Journal of Polymer Science Volume 2013, Article ID 797109, 7 pages [23] M. Somaiah Chowdary, M.S.R Niranjan Kumar, (2015), Effect of Nanoclay on the Mechanical properties of Polyester and S-Glass Fiber (Al), International Journal of Advanced Science and Technology 74, 35-42 [24] Javad Moftakharian Esfahani, Masoud Esfandeh, Ali Reza Sabet, (2012), High-Velocity Impact Behavior of Glass Fiber-Reinforced Polyester Filled with Nanoclay, Journal of Applied Polymer Science 125, E583–E591
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[25] Salehoon E, Ahmadi SJ, Razavi SM, Parvin N (2017) Thermal and corrosion resistance properties of unsaturated polyester/clay nanocomposites and the effect of electron beam irradiation. Polym Bull 74, 1629–1647 [26] https://www.resitan.net/en/ (accessed 23 November 2019). [27] https://www.akzonobel.com/en/ (accessed 23 November 2019). [28] Miller A., (1959), Effects of high-energy radiation on polymers. Ann NY Acad Sci. 82, 774–781 [29] Shahrajabian H, Ahmadi-Brooghani SY, Ahmadi SJ, Saeb MR, Moghri M, (2016), Study on the physical and mechanical properties of electron-beam-irradiated Vinyl ester/TiO2 nanocomposites. J Vinyl Addit Technol. 22, 110–116 [30] Khaksari M, Ahmadi SJ, Moosaviyan SMA, Nazeri M, (2013), Effect of electron beam irradiation on thermal and mechanical properties of epoxy/clay nanocomposites. J Compos Mater. 47, 3517–3524 [31] Razavi SM, Dehghanpour N, Ahmadi SJ, Rajabi Hamaneh M, (2015) Thermal, mechanical, and corrosion resistance properties of vinyl ester/clay nanocomposites for the matrix of carbon fiberreinforced composites exposed to electron beam. J Appl Polym Sci. 132, 42393 [32] Kosari M, Mousavian SMA, Razavi SM, Ahmadi SJ, Izadipanah M, (2017), Polyurethane/clay nanocomposites reinforced with carbon and glass fibres: study of mechanical and thermal properties, and the effect of electron beam irradiation. Plastics, Rubber and Composites 46, 413–420 [33] Negron A, Ramos S, Blumenfeld AL, Pacheco G, Fripiat JJ, (2002), On the structural stability of montmorillonite submitted to heavy γ-irradiation. Clays Clay Minerals 50, 35-37 [34] Pushkareva R, Kalinichenko E, Lytovchenko A, Pushkarev A, Kadochniko V, Plastynina M, (2002), Irradiation effect on physico-chemical properties of clay minerals. Appl. Clay Sci. 21, 117-123 [35] Lu H, Hu Y, Kong Q, Chen Z, Fan W, (2005), Gamma irradiation of high density poly (ethylene)/ethylene-vinyl acetate/clay nanocomposites: possible mechanism of the influence of clay on irradiated nanocomposites. Polym Adv Technol. 16, 688–692 [36] Christopher Wonderly, Joachim Grenestedt, Göran Fernlund, Elvis Cěpus, (2005), Comparison of mechanical properties of glass fiber/vinyl ester and carbon fiber/vinyl ester composites. Composites: Part B 36 417–426 [37] Kornmann X, Rees M, Thomann Y, Necola A, Barbezat M, Thomann R, (2005) Epoxy-layered silicate nanocomposites as matrix in glass fibre-reinforced composites. J Compos Sci Technol 65, 2259– 2268 [38] McKague, JR EL, Bullock RE, Head JW, (1973) Improved Mechanical Properties of Composites Reinforced with Neutron-Irradiated Carbon Fibers, J. Composite Materials 7, 288-297
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Author Contribution Statement
Seyed Mohammad Razavi: Methodology, Investigation, Writing - Original Draft, Writing Review & Editing Seyed Javad Ahmadi: Conceptualization, Methodology, Writing - Review & Editing, Supervision Peyman Rahmani Cherati: Methodology, Investigation Mina Hadi: Investigation Seyed Amir Reza Ahmadi: Investigation
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Effect of electron beam irradiation on mechanical properties of unsaturated polyester/nanoclay composites reinforced with carbon and glass fibers Seyed Mohammad Razavi MethodologyInvestigationWriting - Original DraftWriting - Review & Editing , Seyed Javad Ahmadi ConceptualizationMethodologyWriting - Review & EditingSupervision , Peyman Rahmani Cherati MethodologyInvestigation , Mina Hadi , Seyed Amir Reza Ahmadi PII: DOI: Reference:
S0167-6636(19)30866-X https://doi.org/10.1016/j.mechmat.2019.103265 MECMAT 103265
To appear in:
Mechanics of Materials
Received date: Revised date: Accepted date:
6 October 2019 23 November 2019 27 November 2019
Please cite this article as: Seyed Mohammad Razavi MethodologyInvestigationWriting - Original DraftWriting - Revi Seyed Javad Ahmadi ConceptualizationMethodologyWriting - Review & EditingSupervision , Peyman Rahmani Cherati MethodologyInvestigation , Mina Hadi , Seyed Amir Reza Ahmadi , Effect of electron beam irradiation on mechanical properties of unsaturated polyester/nanoclay composites reinforced with carbon and glass fibers, Mechanics of Materials (2019), doi: https://doi.org/10.1016/j.mechmat.2019.103265
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Highlights
Mechanical properties of UP/nanoclay and UP/nanoclay/fiber composites are studied.
Effect of EB irradiation on mechanical properties of composites is investigated.
Effect of irradiation on composites is studied using FTIR spectroscopy.
Nanoclay enhanced both mechanical and radiation resistance properties of polymer.
The composites had desirable properties for applications with radiation exposure.
1
Effect of electron beam irradiation on mechanical properties of unsaturated polyester/nanoclay composites reinforced with carbon and glass fibers Seyed Mohammad Razavia, Seyed Javad Ahmadia,*, Peyman Rahmani Cheratib, Mina Hadib, Seyed Amir Reza Ahmadic a
Materials and Nuclear Fuel Research School, Nuclear Science and Technology Research Institute, P.O. Box 113658486, Tehran, Iran
b
Department of Chemical and Petroleum Engineering, Sharif University of Technology, P.O. Box 11365-9465, Tehran, Iran
c
Mechanical Engineering Department, Amirkabir University of Technology, P.O. Box 15875-4413, Tehran, Iran
*
Corresponding Author: Email: [email protected] (S.J. Ahmadi); Tel: +98-21-82064343; Fax:+98-21-88221116
Abstract Following our previous work on morphology, corrosion resistance and thermal properties of unsaturated polyester (UP)/nanoclay composites, the effect of electron beam (EB) irradiation on mechanical properties of UP/nanoclay composites with various nanoclay contents is investigated in this work. Moreover, influence of EB irradiation with doses of 100, 500 and 1000 kGy on tensile properties and hardness of nanocomposite was examined. FTIR spectroscopy was employed to examine the effect of irradiation on chemical structure of polymer. EB irradiation up to 500 kGy led to the improvement of tensile strength and hardness of the samples. The best mechanical properties were observed for the nanocomposite with 1 wt.% nanoclay content. As the selected nanocomposite, UP/nanoclay(1wt.%) composite was reinforced by carbon and glass fibers and the effect of EB irradiation on the mechanical properties of the prepared UP/nanoclay/fiber composites was studied. When compared to the fiberreinforced UP without nanoclay, presence of nanoclay particles in UP/nanoclay/fiber composites resulted in the better mechanical properties and higher radiation resistance. Hence, the UP/nanoclay/fiber composite with proper amount of nanoclay has been shown to be a dependable choice for structural applications in which the material is exposed to ionizing radiations. Keywords: Unsaturated polyester; Nanoclay; Carbon fiber; Glass fiber; Composite; Electron beam irradiation; Mechanical properties; FTIR
1. Introduction 2
Polymer composites are proper materials for the production of structural components because of their high strength-to-weight ratios. Unsaturated polyester (UP) resins are among the most extensively used thermosetting polymers due to their cheapness and processibility [1]. UP polymers reinforced with fibers or micro-sized and nano-sized particles are widely utilized in marine industries, structural materials, manufacturing of chemical containers, etc. [2]. Montmorillonite is one of the most common fillers that is used for the improvement of polymer properties [3,4]. It is a type of layered silicate nanoclay that has a laminated structure formed by the layers which have the thickness of 1 nm. The montmorillonite nanoparticles are usually modified by exchanging metal cations in their structure with an organic intercalating agent such as alkyl ammonium ions. In this way, the interactions between the nanoclay platelets are diminished and the interaction between the polymer chains and the silicate layers are enhanced. Thus, the distribution of clay nanolayers in the polymer matrix is improved. The effect of various nanoclay types on the properties of UP polymers has already been studied extensively. Jo et al. [5] showed that the addition of montmorillonite nanoclay up to 5 wt% enhanced the mechanical and thermal properties of UP. Among the studied nanoclays, Cloisite 30B had the best performance. The enhancement of the mechanical properties and the thermal resistance of the UP resin by the incorporation of different types of nanoclay are also reported by other researchers [6–17]. Although addition of nanoclay improves the mechanical properties of UP polymer, it cannot fulfill the required properties for some structural usages yet. Further improvement in mechanical properties of the polymer could be achieved by reinforcement with continuous fibers such as carbon and glass fibers. It has been shown that the incorporation of woven glass fibers into UP resin reduces the maximum moisture absorption [18], enhances the thermal stability [19], and improves the mechanical properties of the composites [20]. A Combination of glass and carbon fibers was also successfully used to improve mechanical properties of UP [21]. Kusmono and Ishak [22] studied the effect of clay loading on the morphological and mechanical properties of UP/glass fiber composites, showing the improving effect of clay on tensile strength, stiffness, and toughness of UP/glass fiber composites. Enhancement of various mechanical properties of UP/glass fiber composite by the incorporation of clay into the polymer matrix is also reported by others [23,24]. Studies that have examined the chemical resistant properties of UP/nanoclay composites are scarce. In our previous work [25], the morphological structure, density, water absorption, thermal properties, and corrosion resistance of UP/nanoclay composites with various nanoclay contents were investigated. Moreover, the influence of electron beam (EB) irradiation on corrosion resistance of the nanocomposites was studied. Study of the effect of ionizing radiations on the properties of a nanocomposite is vital when the material is aimed to be used in applications such as the production of nuclear waste disposal 3
containers. The completely exfoliated structure was observed for the UP nanocomposites with 1 wt.% and 3 wt.% nanoclay contents. Thermal and corrosion resistance properties of the UP polymer were improved when an appropriate amount of nanoclay was added to the polymer matrix. The nanoclay particles reduced the water absorption of the polymer. Moreover, EB irradiation up to 500 kGy enhanced the water absorption resistance and corrosion resistance of the UP/nanoclay composites. Results of the work [25] that are described above demonstrate the improving effects of both nanoclay addition and EB irradiation on the thermal and chemical resistance of UP polymer. However, investigation of the mechanical properties has not been included in that work. Studying the mechanical properties of the composite at various nanoclay contents and irradiation doses is very important to ensure its reliability for some specific usages such as the production of radioactive disposal containers. Following the previous work [25], in the present work, mechanical properties of UP/nanoclay composites before and after EB irradiation are investigated. Additionally, resulted optimum nanoclay content was used to prepare UP/nanoclay/glass fiber and UP/nanoclay/carbon fiber composites and their tensile properties were studied. To the best of our knowledge, the mechanical properties of UP/nanoclay composites reinforced with continuous fibers and the effect of ionizing radiations on them are not reported elsewhere. Thus, the capability of those composites for the production of structural parts that are exposed to ionizing radiations, such as nuclear waste disposal containers, may be confirmed based on the results of the present work. 2. Experimental 2.1. Materials Unsaturated Polyester resin (UP-770IS) with the viscosity of 500 (mPa.S) at 25oC, density of 1.15 (g/cm3), and Nonvolatile content of 63%–66% was purchased from Resitan Co. (Iran) [26]. The UP resin was based on isophthalic acid and neopentyl glycol. Tert-butyl peroxybenzoate initiator with the commercial name of TRIGONOX C was prepared from AkzoNobel (Netherlands) [27]. The organically modified Cloisite 30B montmorillonite nanoclay was supplied by Southern Clay Products Inc. (USA). Carbon and glass fibers that were already woven into the plain weave cloths with a surface density of 200 (g/m2) were used. The thickness of woven carbon fibers and glass fibers was 0.28 mm and 0.26 mm, respectively. 2.2. Preparation of UP/nanoclay composites UP/nanoclay composites were prepared using the method detailed in our preceding work [25]. Briefly, appropriate amounts of nanoclay were mixed with the UP resin to produce the nanocomposite samples 4
with 0, 1, 3, and 5 wt.% nanoclay contents (named UP0, UP1, UP3, and UP5, respectively). The crosslinking reaction was initiated by the addition of 2 phr of Tert-butyl peroxybenzoate to the resin/nanoclay mixture. The mixture was then cast into the silicon molds with the shapes and sizes appropriate for intended testing methods. The molded samples were cured at 120 oC for 15 minutes and were post-cured at 140oC for 5 minutes. 2.3. Preparation of UP/nanoclay/fiber composites The fiber-reinforced nanocomposites were prepared via hand lay-up method using carbon and glass fibers. A mold formed from two steel plates and a steel frame was utilized. To prevent the sticking of the composite to the surface of the steel mold, a Teflon film was placed on the surface of the plate. The carbon or glass fibers in the form of woven mats were cut regarding the mold size. The first fiber layer was placed on the mold surface covered by the Teflon film. Then, the mixture of UP resin, nanoclay (if required) and TBPB initiator was uniformly spread on the mat surface using a brush. The second fiber layer was then placed on the resin surface and the trapped air bubbles were removed with a roller. The described process was repeated for each layer until eight layers of mat were stacked. Finally, after putting a Teflon film, the top plate of the mold was placed on the stacked layers. The mold was exposed to 10 MPa pressure using a mechanical press equipped with a heating system. The curing and post-curing of the molded composites were carried out under the applied pressure at 120 oC for 15 minutes and at 140oC for 10 minutes, respectively. Afterward, the mold was opened and the completely cured composite was taken out. The composite sheet was cut to produce the specimens required for different tests. Compositions of the prepared UP/nanoclay and UP/nanoclay/fiber composites are shown in Table 1.
Sample UP0 UP1 UP3 UP5 UP0C UP1C UP0G UP1G
Table 1. Prepared UP/nanoclay and UP/nanoclay/fiber composites Nanoclay content (wt.%) 0 1 3 5 0 1 0 1
Type of fiber Carbon fiber Carbon fiber Glass fiber Glass fiber
2.4. Irradiation of composites The prepared composite samples were exposed to EB irradiation with overall doses of 100, 500, and 1000 kGy. A Rhodotron type electron accelerator machine (TT200) with a 10 MeV electron beam and a maximum of 8 mA beam current was utilized at room temperature and air atmosphere under 5
homogeneous oxidation condition. More details about the irradiation process are provided in the previous paper [25]. 2.5. Tensile testing Tensile properties of the composites were studied using a SANTAM STM50 (Iran) tensile testing machine. The cross-head speed was set to 1 (mm/min). Five specimens were tested for each polymeric composite. The shape and size of UP/nanoclay and UP/nanoclay/fiber composite specimens were selected in accordance with ASTM D638M and ASTM D3039M standard methods for tensile testing, respectively. 2.6. Hardness tests Surface hardness of nanocomposites was measured according to ASTM D2240 using Bareiss HP-D (Germany) Shore D durometer. Five specimens were tested for each nanocomposite. 2.7. FTIR Fourier transform infrared (FTIR) spectroscopy was employed to examine the influence of irradiation on the chemical structure of samples. FTIR spectra were recorded at ambient temperature using a MB-100 FTIR spectrophotometer of ABB BOMEM (Canada). |The spectra were obtained in the wavenumber range of 400 to 4000 cm-1 with the rate of 22 scans/minute and resolution of 4 cm-1. 3. Results and Discussion Morphology and structure of the UP/nanoclay composites prepared in the present work have already been studied using XRD and TEM methods and are reported in the previous work [25]. Moreover, complete conversion of UP resin to the polymer at the employed curing and post curing condition was demonstrated in that work. The following sections include the investigation of the effect of irradiation on mechanical properties and chemical structure of the prepared nanocomposites. 3.1. UP/nanoclay composites 3.1.1. Tensile properties Tensile strength and tensile modulus of UP/nanoclay composites are listed in Table 2. Addition of nanoclay to the polymer increased its tensile modulus, since nanoclay particles are more rigid than the polymeric matrix. The improvement in stiffness of composites is the result of proper dispersion of nanoclay particles and strong interaction between the organically modified nanoclay and the polymer 6
chains that reduce their mobility. The tensile strength of polymer was enhanced with nanoclay content of 1 wt.% and 3 wt.%, because of the efficient reinforcement provided by the exfoliated nanoclay particles. The best tensile strength was achieved for UP1 nanocomposite. When the nanoclay content was higher than 3 wt%, the tensile strength was reduced, since the non-exfoliated nanoclay particles may intensify the stress at specific points and reduce the tensile strength of the material. Flocculation of nanoclay particles causes to the lower interaction between the nanoclay particles and polymer chains, which results in the interfacial detachment during the tensile testing. Moreover, the decrease of tensile strength can be attributed to the reduction of crosslinking density at high nanoclay contents [13].
Nanocomposite UP0 UP1 UP3 UP5
Table 2. Tensile properties of UP/nanoclay composites Tensile Modulus (GPa) Tensile Strength (MPa) Mean±SD Mean±SD 0.961±0.017 53.35±0.97 1.033±0.020 57.69±1.05 1.160±0.026 56.71±0.76 1.215±0.034 50.53±0.94
The effect of EB irradiation on the tensile properties of the UP polymer and UP/nanoclay composites is illustrated in Figures 1 and 2. Both tensile strength and tensile modulus of samples were improved by irradiation up to 500 KGy. However, radiation dose of 1000 kGy deteriorated the tensile properties of the samples. Depending on the radiation intensity and the type of polymer, ionizing radiation can cause crosslinking and/or chain scissioning reactions in the polymer structure [28]. The observed changes in the mechanical properties of polymer nanocomposites can be attributed to the predominance of crosslinking reactions in the UP polymer when the EB irradiation dose was lower than 500 kGy, followed by the significant effect of chain scissioning reactions at a radiation dose of 1000 kGy that causes the degradation of the polymer chains and reduction of tensile strength of the nanocomposites. Similar phenomena are reported for the effect of ionizing radiations on different polymeric nanocomposites [29– 32].
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Irradiation Dose (kGy) Figure 2. Effect of EB irradiation on tensile modulus of UP/nanoclay composites
Organically modified nanoclay particles can influence the radiation-induced changes in the polymer properties in different ways. The inorganic structure of nanoclay results in a high resistance against 8
ionizing radiation [33,34]. Nanoclay particles can act as barriers that simultaneously decrease the oxidative degradation of the polymer via reduction of oxygen diffusion through the polymer structure and decrease the cross-linking between polymer chains. On the other hand, the organic modifier of the nanoclay degrades by the ionizing radiations (Hofmann degradation), which leads to the formation of free radicals. Depending on the irradiation dose, the produced free radicals can enhance the cross-linking reactions or accelerate the oxidative degradation of the polymer [35]. Thus, the final consequence of nanoclay addition on the properties of irradiated polymer depends on the nature of the polymer (e.g. critical dose of the Hofmann reaction), nanoclay content, and irradiation dose. As shown in Figure 1, presence of nanoclay particles in the UP polymer matrix reduced the effect of EB radiation on mechanical properties of the polymer. High nanoclay content in UP5 nanocomposite caused the deterioration of the tensile strength of polymer after irradiation. This can be due to the non-exfoliated structure of UP5 nanocomposite as well as its high organic modifier content available for Hofmann degradation reaction. However, incorporation of proper amounts of nanoclay into the polymer moderated the destructive effect of high-dose EB radiation. Although, the highest tensile strength among the irradiated samples was observed for UP0 pure polymer sample after 500 kGy irradiation, addition of 1 wt.% nanoclay into the polymer matrix enhanced its resistance against the degradation at 1000 kGy radiation dose. Therefore, UP1 nanocomposite is a suitable choice for applications that involve the long-term exposure of material to ionizing radiation. 3.1.2. Hardness Data of Shore D hardness of UP/nanoclay composites are listed in Table 3. When compared to the pristine polymer, UP0, the hardness of UP/nanoclay composites were enhanced. Clay nanolayers that were efficiently distributed in the nanocomposites with exfoliated structure hinder the development of indentation. A decrease in the hardness of nanocomposites with high clay content was demonstrated, which was due to the formation of intercalated or flocculated structure at high clay contents. In the composites with non-exfoliated structure, clay nanolayers were not distributed uniformly in the polymer structure. Consequently, the polymeric matrix with low hardness remained empty of hard nanoclay particles in some sections.
Nanocomposite UP0 UP1 UP3 UP5
Table 3. Hardness of UP/nanoclay composites Shore D Hardness Mean±SD 83.0±0.6 85.0±0.5 84.6±0.5 83.6±0.6
9
The effect of EB irradiation on UP0 polymer as well as the UP1 nanocomposite that had the highest hardness among the studied nanocomposites is shown in Figure 3. Similar to the trend reported for tensile strength of samples, hardness of UP0 and UP1 materials was increased for radiation doses up to 500 kGy, followed by a decrease at 1000 kGy. It is observed that well-distributed nanolayers of clay reduced the destructive effects of high-dose irradiation on UP polymer in some extent. 90 88
Shore D Hardness
86 84 82 80 UP0 78
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Irradiation Dose (kGy) Figure 3. Effect of EB irradiation on Shore D hardness of UP/nanoclay composites
3.1.3. FTIR spectra Figure 4 illustrates the FTIR spectra of UP0 and UP1 samples before and after exposure to 1000 kGy EB irradiation. The spectra related to the lower irradiation doses were not reported since no detectable radiation-induced change was observed in them. Also, FTIR spectra of irradiated specimens were almost identical to those of non-irradiated materials. The phthalic component of the polymer is indicated by characteristic sharp peaks at 1722 cm-1 (C=O stretching) and at 1274 cm-1 (C-O-C). Intensity of the peak at 1722 cm-1 is reduced after irradiation as the result of both crosslinking and degradation reactions which involve unsaturated C=O bands. The presence of nanoclay moderates the effect of irradiation, which is in agreement with the results of the mechanical tests. The slight increase in the intensity of the broad band in 3200-3600 cm-1 range (O-H stretching) after irradiation of both UP0 and UP1 materials can be due to the oxidative degradation induced by the high-dose irradiation.
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Figure 4. FTIR spectra of a) non-irradiated UP0, b) UP0 exposed to1000 kGy EB irradiation, c) non-irradiated UP1, d) UP1 exposed to1000 kGy EB irradiation
3.2. UP/nanoclay/fiber composites 3.2.1. Tensile properties Following the study of the mechanical properties of UP/nanoclay composites, tensile properties of the fiber-reinforced UP composite materials are studied in the present work. As it is described in previous section, UP/nanoclay composites with 1 wt.% of nanoclay showed the best mechanical properties among the studied nanocomposites materials. Additionally, the high viscosity of UP resin at high nanoclay contents makes it difficult to prepare the flawless UP/nanoclay/fiber composites via hand lay-up method. Thus, In the current work, UP/nanoclay/glass fiber and UP/nanoclay/carbon fiber composites with 0 and 1 wt.% of nanoclay were fabricated and examined. Tensile properties of the fiber-reinforced composites, along with the effect of fiber-reinforcement on the properties of each material are reported in Table 4. The effect of fiber reinforcement is determined as the improvement percentage of each property after incorporation of fibers. Obtained results demonstrate the considerable effect of both glass and carbon fibers on tensile properties of UP0 and UP1 materials. In this case, influence of carbon fiber on tensile strength and tensile modulus of polymer and its nanocomposite was superior. When compared to glass fiber, carbon fiber has higher specific strength and lower density that make it a suitable choice for the production of high strength light structures. However, glass fiber is 11
more cost effective [36]. Hence, the suitable fiber may be selected for each specific application considering the required performance and economical aspects. Table 4. Tensile properties of UP/nanoclay/fiber composites Tensile Modulus Tensile Strength Composite Mean±SD (GPa) UP0C UP1C UP0G UP1G
11.696±0.921 13.964±0.542 5.361±0.245 5.573±0.273
Effect of fiber reinforcement (%) 1117 1252 458 439
Mean±SD (MPa) 624.2±59.4 745.7±50.5 286.6±22.8 297.3±20.4
Effect of fiber reinforcement (%) 1070 1193 437 415
It is well known that the mechanical properties of fiber reinforced polymers are mostly governed by the mechanical properties of the fiber. However, incorporation of nanoclay particle into the matrix may influence the properties of fiber-reinforced polymers in some extent. Table 4 shows that the tensile strength and tensile modulus of both UP/glass fiber and UP/carbon fiber composites are improved by nanoclay particles. It can be attributed to the improving effect of nanoclay on UP matrix that is previously discussed. Nanoclay particles with exfoliated structure enhanced the polymeric matrix of the fiberreinforced composite and inhibited the propagation of cracks in the matrix. Moreover, clay nanolayers contribute to the augmentation of the mechanical properties through the enhancement of interfacial interaction between long fibers and polymer chains [37]. The strong interaction between polymeric matrix and long fibers led to the proper transfer of stress between the components and prevented the formation of interfacial cracks between matrix and fibers. The synergic influence of nanoclay particles and long fibers on the tensile properties of UP matrix resulted in the significant improvement of mechanical properties of the material. Effect of EB irradiation on tensile strength and tensile modulus of UP/nanoclay/fiber composites is illustrated in Figures 5 and 6. Considering the effect of irradiation on UP/nanoclay matrices that has previously been discussed, observed improvement in tensile properties of both UP0C and UP1C composites by the irradiation doses up to 500 kGy was anticipated. Further irradiation (up to 1000 kGy) had no significant effect on tensile properties of the composites. Regarding the improving effect of irradiation on tensile properties of pristine carbon fiber [38], the enhancement of UP polymer by carbon fiber provides it with high radiation resistance, which makes the prepared composite a suitable choice for the applications that involve exposure to the ionizing radiation. Although addition of nanoclay into the UP/carbon fiber composite increased its tensile strength and tensile modulus, it had negligible influence on the effect of EB irradiation on UP/carbon fiber composite. It seems that the effect of nanoclay on the
12
radiation-induced changes of mechanical properties of samples was eclipsed by the tremendous effect of carbon fiber on mechanical properties. 1000 UP0C UP1C UP0G UP1G
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Irradiation Dose (kGy) Figure 6. Effect of EB irradiation on tensile modulus of UP/nanoclay/fiber composites
EB irradiation generally increased the tensile strength and tensile modulus of UP/nanoclay/glass fiber composites. For those composites, gamma irradiation caused the formation of a crosslinked structure in the polymer matrix. On the other hand, applied irradiation doses make no substantial effect on the properties of neat glass fibers [39]. Desirable effect of nanoclay on the radiation resistance of the material is observable at the highest applied irradiation dose. Incorporation of nanoclay into the UP/glass fiber composite prevented the deterioration of its tensile strength at 1000 kGy. The robust structure of exfoliated nanoclay layers can act as barriers that block the diffusion pass of oxygen into the polymer structure and can decrease the oxidative degradation of polymer-polymer and polymer-fiber bonds. 4. Conclusions Mechanical properties of UP/nanoclay composites with different nanoclay contents were examined. It was shown that proper amount of nanoclay can improve the tensile properties and hardness of UP polymer. The best mechanical properties was observed when 1wt.% of nanoclay was added to the polymer. EB irradiation up to 500 kGy improved the mechanical properties of the composite, while higher irradiation doses resulted in the degradation of polymer. Moderating effect of nanoclay particles on the destructive influence of high-dose radiation was demonstrated. Obtained results were confirmed using FTIR spectroscopy. As the UP/nanoclay composite with the optimum properties, UP1 nanocomposite was selected to be further reinforced by carbon and glass fibers. Reinforcement of composites with long fibers 14
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Author Contribution Statement
Seyed Mohammad Razavi: Methodology, Investigation, Writing - Original Draft, Writing Review & Editing Seyed Javad Ahmadi: Conceptualization, Methodology, Writing - Review & Editing, Supervision Peyman Rahmani Cherati: Methodology, Investigation Mina Hadi: Investigation Seyed Amir Reza Ahmadi: Investigation
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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