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Comparative assessment of Static and Dynamic Mechanical Properties of Glass and PET fiber Reinforced Epoxy Composites
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 18 (2019) 4048–4057
www.materialstoday.com/proceedings
ICMPC-2019
Comparative assessment of Static and Dynamic Mechanical Properties of Glass and PET fiber Reinforced Epoxy Composites a
Manjunath R Na, Vikas Khatkara, B K Beheraa
Department of Textile Technology, Indian Institute of Technology Delhi- New Delhi (INDIA)-110016
Abstract This paper summarizes an experimental study on static and dynamic mechanical behaviour of PET-epoxy unidirectional composites. Under static mechanical conditions, the behaviour of composites was studied under tensile, bending and impact loads. The nature of fiber-matrix interface and fracture surfaces were examined through scanning electronic microscopy. Under dynamic conditions, the behaviour of composites was studied with reference to the effect of temperature and frequency on storage modulus, loss modulus and damping properties. From the overall analysis it was evident that the unique combination of PET based epoxy composites have considerably superior impact resistance and dynamic mechanical behaviour than the Glassepoxy counterparts © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Dynamic mechanical analysis, impact resistance, lightweight constructions
Introduction With environmental issues like carbon emission and global warming getting serious over the years, current researchers have dovetailed their research to counter world’s climatic change [1]. In a drift towards lightweight technology, textile reinforced composites are well established in today’s transportation and constructional sectors due to their high strength to weight ratio and energy efficiency [2]. These composites notably include carbon, glass and aramid reinforced polymeric materials among which, glass reinforced composites are most widely used due to their low cost with remarkable physical and mechanical assets [3-6]. Though being anti-corrosive, fire retardant, impermeable and resistant to weathering, glass fibers do come with certain inadequacies [7,8]. For instance, glass fibers are heavier than most of the textile fibers. Even being inert to the moisture, chemically glass fibers (E-glass) are susceptible to chlorine attack making them unsuitable for marine applications [9,10]. Glass fibers have low resistance to friction and abrasion which would amplify the complexity in handling the high density warp sheets while weaving complex 3D fabrics [7, 11,12]. In addition to this, glass fibers are serious hazardous to health and can cause intense irritation and itching if inhaled or repeatedly exposed [13]. *Corresponding author. E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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Currently, all the material researchers are in search of potential replacement, yet sustainable approach to uphold the principle of lightweight construction. Attempts have been made to introduce natural fibers (like jute, hemp, sisal, banana, kenaf, coir etc) based environment friendly composites in non-structural applications [1, 14, 15]. Poor interfacial adhesion between the natural fiber and most of the polymeric matrices has been improved sufficiently through physical and chemical treatments to fully exploit the mechanical capabilities of natural fibers. Nevertheless, the lower mechanical endurance of natural fibers in comparison to glass fibers can be fairly compensated with their lightweight, less health risk, bio-degradability and comparable stiffness properties. On the other hand, researchers have also reported certain downbeats of the natural fibers; especially about their pitiable performance in the transverse direction [4, 16-18]. Henceforth, a search for novel yet reliable structural materials is everlasting and in this regard, materials having desirable properties in all perspectives to contend with glass fibers are generally invited. Today, most of the plastic recycling industries are involved in reprocessing of waste or scrap plastics to useful products like PET, HDPE, polystyrene etc which can be made abundantly available. It would be necessary to provide a benchmark database to most of the manufacturing companies that utilizes these recycled plastic products as raw materials. Among the range of final products, polyester (PET) based composites with the potential to replace glass counterpart, can be a reliable choice of structural materials; especially when energy absorption property is of prime requisite than the load bearing capabilities. It is a well-known fact that, Polyester (PET) yarns are generally considered to have the best abrasion resistance than glass yarns [19, 20] and can be preferred for weaving complex 3D structures. Further, using polyester yarns which are lighter than glass fibers can reduce weight of the overall structure [21]. In this work, a comparative study between the mechanical properties of polyester and glass reinforced epoxy composites are carried out to explore their potential of using them as structural material. Moreover, it is well known that the polyester is most versatile, abundantly available and easily process-able fibre compared to glass. 2. Experimental 2.1 Materials In this study high tenacity fully drawn polyester yarns and E-glass yarns, both of 300 Tex linear densities were selected and their properties were subsequently analysed in raw form and in reinforced composite form. Polyester filament yarns and glass rovings were supplied by Reliance Industries Pvt. Ltd and Owens Corning respectively. Further details of the reinforcement materials are listed below in the Table 1. Uni-directional (UD) composites were prepared by using epoxy based Thermoset resin systems. The matrix used for the fabricating composite was the mixture of epoxy resin (Grade LY 556) and hardener (Grade HY 917) mixed in the mass ration of 10:1.Epoxy Resin and Hardener were supplied by Northern Polymers Pvt. Ltd as shown in below Table 1. Table 1. Reinforcement material details Sl No. Parameters
Glass
Polyester
1
Density (g/cc)
2.54
1.38
2
Breaking force (N)
321.64
245.25
3
Initial Modulus (N/tex)
17.42
4.38
4
Tenacity (N/tex)
1.04
0.81
5
Elongation (%)
2.53
12.49
2.2 Preparation of UD composites The process of developing Uni-directional composites requires the reinforcement material to be positioned along a particular direction. For this purpose, the yarns were converted to hanks of standard size and placed layer-by-layer inside a rectangular steel mould of length 500mm and width 25mm. The mould is sufficiently wrapped with Teflon sheets to prevent the resin form sticking on to the mould surface during the curing process. The hank was stretched to maintain the linear orientation of yarns and the resin application process is begun. The resin is applied using a brush type applicator and uniformly distributed through the reinforcement within the mould. Following resin application, the mould mould assembly was allowed to cure at room temperature for 24 hours followed by thermal post curing at 100oC for 5 hours in a convection oven to obtain the final composite samples. After curing, the samples were separated from the mould and cut to required specimen sizes for testing.
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2.3 Testing of composite samples 2.3.1 Static mechanical analysis The static mechanical analysis involves testing of composite samples under tensile, bending and impact loads. The tensile and flexural properties of UD composites were tested on [Model details] according to ASTM D638 and ASTM D790 test methods respectively. The samples for tensile testing were prepared in the size of 150mm in length and 25mm in width and tested at 2mm/min cross-head speed and 100mm gauge length. The thickness of the composite samples was found to be in the range of 6.3 - 6.6 mm and they were flexural tested through three-point bending at 2mm/min test speed and 100mm span length. Impact properties of the composite samples were studied by testing them on a Notch Izod impact tester according to ASTM D256-97 test method. The composite samples for Izod notch test were prepared according to the standard size of size 63.5mm in length in fiber direction and 12.7mm in width and a notch of 1.5mm was made at the middle of each sample. Impact resistance of the sample is determined by releasing a pivoting arm from specific height to swing and hit a notched sample and thereby calculating the energy lost in breaking the specimen of unit thickness. The mode of failures at the fractured surfaces was studied using SEM micrographs captured from scanning electron microscope (SEM ZEISS EVO 50). As a part of sample preparation, the fractured samples obtained from tensile and three-point bending test methods were selected and placed on a metallic sample holder to coat them with gold (through spluttering technique) to make their surface conductive for SEM analysis. 2.3.2 Dynamic mechanical analysis Dynamic mechanical analyzer MCR 702 Multidrive was the test instrument used to study the dynamic mechanical properties of the composite samples like storage modulus, loss modulus and damping factor. Samples were tested under three point bending mode at a frequency of 1, 10 and 100 Hz. The testing temperature ranged from room temperature to 180oC. The amplitude was set between 40-50 µm based on the thickness of the samples. 3. Results and discussions 3.1 Static mechanical properties of the UD composites samples 3.1.1 Tensile properties The stress-strain curve for glass- epoxy composite exhibited linearly elastic behaviour until its brittle failure whereas, the curve for polyester-epoxy composite showed insignificant non-linearity before reaching the maximum stress. This non-linearity is mainly due to ductile behaviour of polyester material which has the tendency to undergo extensive deformation under the action of loads before any failure occurs. The tensile modulus of glass and polyester reinforced composites calculated from the slope of its respective stress-strain curves were found to be 15.24 GPa and 5.63 GPa respectively. Tensile energy absorption which measures the total work done to break a sample was mathematically determined by calculating the area beneath their respective stress-strain curve. The resulting values of energy absorption that can accurately guage a material toughness are reported (Figure 1).
Figure 1. Overview of tensile testing results
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The visual investigation of tensile fractured composites reveals that the fiber and matrix in both the specimen cases, work as a single unit and fail altogether in a more or less a straight line. The SEM images of the glass and polyester fractured composite ends are shown in the Figure 2 (a-b) with fiber breakages more prominently sighted in both the cases. In addition, the shorter fiber pull-out length in case of polyester-epoxy composites authenticates the presence of a strong interfacial bonding between the fiber and matrix. From these observations, the properties of polyesterepoxy material combination are found to be in fair comparable limits of glass-epoxy material combination.
Figure 2. SEM micrographs of tensile fracture of (a) Glass-epoxy and (b) Polyester-epoxy composites
3.1.2 Flexural analysis
Figure 3. Overview of three-point bending results
Figure 3 shows the comparison of flexural stress-strain data for glass and polyester reinforced UD composites that were computed from the Load-deflection curves recorded during the 3-point bending test. Subsequently, Flexural modulus was calculated from the slope of Flexural stress-strain curve. The load deflection response for the glass reinforced composites reveals that the material behaves linearly upwards till its maximum stress reaches the peak load. Post peak load, the stress-strain curve of the specimen experienced a sharp fall with its overall load bearing capacity plunging to zero. At the initial stages of loading process, the failure in the glass-epoxy specimen was foreseen by local matrix cracking on its tensile side, followed by its obvious brittle fracture leading to further damage accumulation and complete material destruction. On the other end, the load deflection response for polyester-epoxy specimen demonstrated an impressive ductile behaviour of the composites in which, the curves initially progressed to maximum stress level and thereafter exhibiting longer plateaus. The formation of larger plateaus was greatly attributed to the plastic deformation of the polyester material. Unlike glass-epoxy composites, the complete failure in the structure was never observed even at the mid-span deflection of 50mm. Ahead of this, the specimen were still able to take up 60% of its maximum load.
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Figure 4. Flexural tested samples (a) Glass-epoxy and (b) Polyester-epoxy composites
The morphology of the fractures was observed from the macroscopic point of views in order to investigate the failure mechanism of the composites. During 3-point bending test, the specimen facing the indenter experienced compressive force while, the opposite side experienced the tensile force. The compressive force causes the surface to kink and bulge sidewise while, the tensile force initiates the crack causing the fibers on the outer surface to fail under tensile stress. The crack continues to propagate from tensile face to compressive face (through the thickness) until complete failure of the sample. Figure 4(a-b) depicts the fracture photographs of three-point tested glass-epoxy and polyester-epoxy composite samples in both the faces. The image of the fractured sections confirms the fiber pull-out and brittle failure of the glass-epoxy composite and ductile behaviour of polyester-epoxy composites with no fiber breakage (except few outer fibers due to extreme tensile stress at higher mid-span deflection). From the above mentioned observations it can be concluded that, the polyester-epoxy combination encompass excellent toughness (residual strength) properties which was in contrast to the typical brittle behaviour of glass-epoxy composites. 3.1.3 Izod impact test In Izod impact test method, the kinetic energy required to initiate a crack fracture and propagate it through a notched sample is recorded. The average izod impact values measured for glass and polyester reinforced epoxy composites were 11.917 J and 12.862 J respectively. Interestingly, polyester-epoxy showed better energy absorption than the glass-epoxy composites. From the previous studies, it is evident that the yield strength, ductility and fracture mechanism are the important factors that affect the izod impact energy of a specimen. With the increase in yield strength, the material turns out to be more brittle causing the impact energy of the specimen to plunge. Brittle materials are inclined to absorb lower impact energies due to their inability to undergo plastic deformation. This could be main the reason for glass reinforced epoxy composite to demonstrate relatively low impact properties than the polyester reinforced composites. The ductile properties in polyester dominantly assist in plastic deformation during yielding of the specimen and subsequently improve its ability to withstand higher impact energies prior to fracture.
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Figure 5. Fracture images of Izod impact failed samples (a) G-E and (b) P-E composites
From the Figure 5, the mode of fractures observed in glass-epoxy composites was much different from the polyester-epoxy sample. When the sample of glass-epoxy composite was struck by the hammer, the strike force caused the outer layer of fibers to fracture under tensile stress. The crack imitated at the notch was propagated through the cross-section of the sample but not clearly perpendicular to the fibers. As the specimen continues to bend further, subsequent adjacent layers were fractured in tension causing delamination in the fibers layers and crack growth. But in the case of polyester-epoxy composite, the impact force causes extensive branching of the fibers to the side, without giving any room for the crack to propagate. This property is largely attributed to the ductile behaviour of the polyester material thereby showing better energy absorption capability than the glass-epoxy counterpart. 3.2 Dynamic Mechanical properties 3.2.1 Effect of temperature on dynamic mechanical properties of the composites Storage modulus (E’) measures the stiffness of a material which closely relates to its load bearing capacity and analogous to its flexural modulus [22, 23]. While, loss modulus (E”) of a material is a measure of its energy dissipated as heat per cycle under deformed conditions [24]. Figure 6 graphically enumerates the variation of composite’s storage and loss modulus as a function of temperature and tested at a frequency of 10 Hz.
Figure 6. Effect of temperature on storage modulus, loss modulus and tan delta values of the composites
The E’ value was higher for the glass-epoxy composites, which is obvious due to the fact that the modulus of elasticity of glass fibers is greater than that of polyester filament yarns. The E’ value was slightly lesser for polyester-epoxy composites and the difference was approximately was 10%. In spite of having lower modulus when
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compared to glass fibers, such admirable display of dynamic modulus in P-E composites can be attributed to its excellent interfacial bonding and effective stress transfer taking place between the fiber and matrix. Previous works have reported that storage modulus of the composites, increases with weight fraction of fibers [25, 26]. It is a well known fact that density of polyester and fiberglass being 1.38 g/cm3 and 2.54 g/cm3 respectively, the number fibers available per weight fraction is more in case of polyester. The increase in fiber content could also be one of the reasons for the at par behavior of P-E composites. The loss of storage modulus is gradual at lower temperatures for both the composites. However, the sharp fall in the modulus at the vicinity of glass-transition temperature (Tg) indicates the material’s transition from solid to glassy/rubbery state. The loss in modulus at transition phase is higher in the case of G-E composite which is clearly indicated by sharp vertical fall of the curve to a minimum level which is considerably lower than P-E composites. With matrix material being the same in both the cases, the differences in the Tg is purely dependant on the intrinsic properties of the reinforcement material. The Tg for P-E composites are higher and broader than G-E composites clearly indicating the effect of semi-crystalline polymeric nature of polyester filaments on composite’s behaviour during phase transition. The curves of Loss modulus and tan δ followed similar trend as in the case of storage modulus, with G-E composites competitively showing higher values. Tan δ basically defines dampness and can be easily related to the material’s resistance towards impact loads. Higher the tan δ values of a material associated with glass transition temperature, lower is their load bearing properties and vice versa [23]. The tan δ values for P-E composites were found to be around 0.267 while, for G-E composites the value were 0.532. It is clear from these values that P-E composites have better interfacial bonding and load bearing capacity especially at temperatures where G-E composites tend to lose their properties [27]. Since the peak of E” and tan δ curves occurs in the region of glass transition, the Tg values obtained from both the curves are tabulated in Table 2. Table 2. Glass transition temperatures measured from loss modulus and tan delta curves Composites Temperature (ºC) Tg from E”max curves
Tg from tan δmax curves
Glass-epoxy
76.02
94.14
Polyester-epoxy
119.10
131.03
The Tg obtained from the loss modulus cures are lower and more realistic than that of tan δ curves. Earlier studies have reported the same [25]. Again, with the same matrix system in both the composites, the damping properties are affected by the elastic nature of reinforcing fibers [28]. Therefore, it can be concluded that are mainly that the elastic nature of the polyester fibers are is the main reason for such excellent damping behaviour in P-E composites 3.2.2 Effect of frequency on dynamic mechanical properties of the composites Previous studies have reported the effect of testing frequencies on the dynamic mechanical properties of Glassepoxy composites [29, 30]. The variation storage modulus, loss modulus and tan δ values as a function of frequency are studied for P-E composites and the respective data are enumerated in the Figure 7.It was reported earlier that, modulus measured at higher frequencies are higher than those measured at lower frequencies [28]. In our systems, there was no much significant difference observed in the storage modulus however, increase of frequency has been found to increase loss modulus values clearly depicting its direct impact on the dynamic modulus of the composites. This change in dynamic properties with frequencies may be attributed to morphological rearrangement and improved fiber-matrix interaction. With the increase in frequency, the tan δ peaks shifted to higher temperatures and the Tg values recorded at three consecutive frequencies were found to be 124.99 ºC, 133.88 ºC and 136.87 ºC respectively. The shift in tan δ peaks which is also an indicative of degree of cross-linking is due to the micromechanical transition in a polymeric system.
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Figure 7. Effect of frequency on storage modulus, loss modulus and tan delta values of PET-epoxy composites
3.2.3 Cole-cole plots The curves of loss modulus data plotted against storage modulus are referred as Cole-Cole plots. The nature and shape of the Cole-Cole plots indicated the homogeneity of the material combination in the samples being tested. Ideally, curve following the semi-circular path indicates that the polymeric material combination within the system is homogeneous [31]. The deviation from the regular semi-circular shape to irregular shape of the curve signifies the lack of homogeneity. Figure 8 shows the Cole-Cole plots for glass-epoxy and polyester epoxy material combinations. It can be seen from the figure that the curves of both the composite samples are imperfectly semicircular following the polynomial equation of fourth order. Imperfect semi-circular shape of the curves indicates that, there is heterogeneity in both the material combination which may be due to dissimilarity in the reinforcement and matrix phase especially at lower temperatures. The extent of regularity in the shape observed at higher temperatures (nearing glass transition and beyond) is mainly attributed to the transition of the material from solid state to glassy/rubbery state. When compared with each other, the curves of P-E composites can be observed with greater extent of irregularity from semi-circular shape. This difference is due to the fact that crystalline polymers like PET are characterized with heterogeneous structures due to interspersed amorphous regions while amorphous polymers like glass have homogeneous structure [32-34]. Similar analysis through Cole-Cole plots were made to study the homogeneity in oil palm based epoxy composites [23, 28].
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Figure 8. Cole-Cole plots of Glass and PET reinforced epoxy composites
4. Conclusions The static and dynamic mechanical properties of PET reinforced epoxy composites were investigated and compared with glass-epoxy counterparts. Composites developed by this material combination exhibits unique properties giving a platform to explore the possibilities of using them in lightweight constructions. The moderately lower strength and modulus properties in P-E composites were compensated by their excellent toughness and energy absorbent properties. From the fracture morphological studies, it was evident that there was good fiber-matrix interaction and the breakage type confirms the effective stress transfer between them. It was more interesting to note that, P-E composites performed well even in presence of fractures bearing almost 60% of their maximum limit while, G-E composites exhibited brittle and complete failure at higher level of deformation. DMA analysis highlights the P-E composite’s gradual loss of dynamic modulus properties at the transition phase. The higher glass-transition temperatures and lower tan δ values confirm the suitability of P-E composites to perform well at higher temperatures with better resistance to impact loads. Based on these studies, it can be concluded that the PET-epoxy composites can be a new choice of engineering materials in designing lightweight and impact resistive composites structures. Further, this work can drive the plastic recycling industries to enhance the utilization spectra of the recycled products in the form of polyester-epoxy composites. Acknowledgment Authors are thankful to Ministry of Textile for sponsoring Focus Incubation centre for 3D weaving and structural composites. References 1.
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12. Suresha, B., et al. "Three-body abrasive wear behaviour of carbon and glass fiber reinforced epoxy composites." Materials Science and Engineering: A 443.1-2 (2007): 285-291. 13. Fibrous glass, Hazardous substance fact sheet, New Jersey, Department of Health and Senior services. 14. Dhakal, H. N., Z. Y. Zhang, and M. O. W. Richardson. "Effect of water absorption on the mechanical properties of hemp fibre reinforced unsaturated polyester composites." Composites science and technology 67.7-8 (2007): 1674-1683. 15. Ahmed, K. Sabeel, and S. Vijayarangan. "Tensile, flexural and interlaminar shear properties of woven jute and jute-glass fabric reinforced polyester composites." Journal of materials processing technology 207.1-3 (2008): 330-335. 16. Ku, H., et al. "A review on the tensile properties of natural fiber reinforced polymer composites." Composites Part B: Engineering 42.4 (2011): 856-873. 17. Pickering, Kim L., MG Aruan Efendy, and Tan Minh Le. "A review of recent developments in natural fibre composites and their mechanical performance." Composites Part A: Applied Science and Manufacturing 83 (2016): 98-112. 18. Bar, Mahadev, Apurba Das, and R. Alagirusamy. "Effect of interface on composites made from DREF spun hybrid yarn with low twisted core flax yarn." Composites Part A: Applied Science and Manufacturing (2018). 19. Akgun, M. (2014). Surface roughness properties of polyester woven fabrics after abrasion. The Journal of the Textile Institute, 105(4), 383-391. 20. Manjunath RN, Behera BK. Modelling the geometry of the unit cell of woven fabrics with integrated stiffener sections. The Journal of The Textile Institute. 2017 Nov 2;108(11):2006-12. 21. Manjunath RN, Behera BK, Mawkhlieng U. Flexural stability analysis of composite panels reinforced with stiffener integral woven preforms. The Journal of The Textile Institute. 2018 Jun 1:1-0. 22. Mohanty, S., Verma, S. K., & Nayak, S. K. (2006). Dynamic mechanical and thermal properties of MAPE treated jute/HDPE composites. Composites Science and Technology, 66(3-4), 538-547. 23. Jawaid M, Khalil HA, Hassan A, Dungani R, Hadiyane A. Effect of jute fibre loading on tensile and dynamic mechanical properties of oil palm epoxy composites. Composites Part B: Engineering. 2013 Feb 1;45(1):619-24. 24. Hameed N, Sreekumar PA, Francis B, Yang W, Thomas S. Morphology, dynamic mechanical and thermal studies on poly (styreneco-acrylonitrile) modified epoxy resin/glass fibre composites. Composites Part A: Applied Science and Manufacturing. 2007 Dec 1;38(12):2422-32. 25. Sreekala MS, Thomas S, Groeninckx G. Dynamic mechanical properties of oil palm fiber/phenol formaldehyde and oil palm fiber/glass hybrid phenol formaldehyde composites. Polymer Composites. 2005 Jun;26(3):388-400. 26. Jacob M, Francis B, Thomas S, Varughese KT. Dynamical mechanical analysis of sisal/oil palm hybrid fiber‐reinforced natural rubber composites. Polymer Composites. 2006 Dec;27(6):671-80. 27. Idicula M, Malhotra SK, Joseph K, Thomas S. Dynamic mechanical analysis of randomly oriented intimately mixed short banana/sisal hybrid fibre reinforced polyester composites. Composites Science and Technology. 2005 Jun 1;65(7-8):1077-87. 28. Pothan LA, Oommen Z, Thomas S. Dynamic mechanical analysis of banana fiber reinforced polyester composites. Composites Science and Technology. 2003 Feb 1;63(2):283-93. 29. Saff CR. Effect of load frequency and lay-up on fatigue life of composites. In Long-term behavior of composites 1983 Jan. ASTM International. 30. Epaarachchi JA, Clausen PD. An empirical model for fatigue behavior prediction of glass fibre-reinforced plastic composites for various stress ratios and test frequencies. Composites Part A: Applied science and manufacturing. 2003 Apr 1;34(4):313-26. 31. Devi LU, Bhagawan SS, Thomas S. Dynamic mechanical analysis of pineapple leaf/glass hybrid fiber reinforced polyester composites. Polymer composites. 2010 Jun;31(6):956-65. 32. Demirel B, Yaraş A, Elçiçek H. Crystallization behavior of PET materials. Balıkesir Üniversitesi Fen Bilimleri Enstitüsü Dergisi. 2016 Jun 11;13(1):26-35. 33. Groeninckx G, Berghmans H, Overbergh N, Smets G. Crystallization of poly (ethylene terephthalate) induced by inorganic compounds. I. Crystallization behavior from the glassy state in a low‐temperature region. Journal of Polymer Science: Polymer Physics Edition. 1974 Feb;12(2):303-16. 34. Munk, P., and Aminabhavi, T.M., Introduction to Macromolecular Science, New York, USA, John Wiley & Sons, Inc. (2002).
ScienceDirect Materials Today: Proceedings 18 (2019) 4048–4057
www.materialstoday.com/proceedings
ICMPC-2019
Comparative assessment of Static and Dynamic Mechanical Properties of Glass and PET fiber Reinforced Epoxy Composites a
Manjunath R Na, Vikas Khatkara, B K Beheraa
Department of Textile Technology, Indian Institute of Technology Delhi- New Delhi (INDIA)-110016
Abstract This paper summarizes an experimental study on static and dynamic mechanical behaviour of PET-epoxy unidirectional composites. Under static mechanical conditions, the behaviour of composites was studied under tensile, bending and impact loads. The nature of fiber-matrix interface and fracture surfaces were examined through scanning electronic microscopy. Under dynamic conditions, the behaviour of composites was studied with reference to the effect of temperature and frequency on storage modulus, loss modulus and damping properties. From the overall analysis it was evident that the unique combination of PET based epoxy composites have considerably superior impact resistance and dynamic mechanical behaviour than the Glassepoxy counterparts © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019 Keywords: Dynamic mechanical analysis, impact resistance, lightweight constructions
Introduction With environmental issues like carbon emission and global warming getting serious over the years, current researchers have dovetailed their research to counter world’s climatic change [1]. In a drift towards lightweight technology, textile reinforced composites are well established in today’s transportation and constructional sectors due to their high strength to weight ratio and energy efficiency [2]. These composites notably include carbon, glass and aramid reinforced polymeric materials among which, glass reinforced composites are most widely used due to their low cost with remarkable physical and mechanical assets [3-6]. Though being anti-corrosive, fire retardant, impermeable and resistant to weathering, glass fibers do come with certain inadequacies [7,8]. For instance, glass fibers are heavier than most of the textile fibers. Even being inert to the moisture, chemically glass fibers (E-glass) are susceptible to chlorine attack making them unsuitable for marine applications [9,10]. Glass fibers have low resistance to friction and abrasion which would amplify the complexity in handling the high density warp sheets while weaving complex 3D fabrics [7, 11,12]. In addition to this, glass fibers are serious hazardous to health and can cause intense irritation and itching if inhaled or repeatedly exposed [13]. *Corresponding author. E-mail address: [email protected]
2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of the 9th International Conference of Materials Processing and Characterization, ICMPC-2019
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Currently, all the material researchers are in search of potential replacement, yet sustainable approach to uphold the principle of lightweight construction. Attempts have been made to introduce natural fibers (like jute, hemp, sisal, banana, kenaf, coir etc) based environment friendly composites in non-structural applications [1, 14, 15]. Poor interfacial adhesion between the natural fiber and most of the polymeric matrices has been improved sufficiently through physical and chemical treatments to fully exploit the mechanical capabilities of natural fibers. Nevertheless, the lower mechanical endurance of natural fibers in comparison to glass fibers can be fairly compensated with their lightweight, less health risk, bio-degradability and comparable stiffness properties. On the other hand, researchers have also reported certain downbeats of the natural fibers; especially about their pitiable performance in the transverse direction [4, 16-18]. Henceforth, a search for novel yet reliable structural materials is everlasting and in this regard, materials having desirable properties in all perspectives to contend with glass fibers are generally invited. Today, most of the plastic recycling industries are involved in reprocessing of waste or scrap plastics to useful products like PET, HDPE, polystyrene etc which can be made abundantly available. It would be necessary to provide a benchmark database to most of the manufacturing companies that utilizes these recycled plastic products as raw materials. Among the range of final products, polyester (PET) based composites with the potential to replace glass counterpart, can be a reliable choice of structural materials; especially when energy absorption property is of prime requisite than the load bearing capabilities. It is a well-known fact that, Polyester (PET) yarns are generally considered to have the best abrasion resistance than glass yarns [19, 20] and can be preferred for weaving complex 3D structures. Further, using polyester yarns which are lighter than glass fibers can reduce weight of the overall structure [21]. In this work, a comparative study between the mechanical properties of polyester and glass reinforced epoxy composites are carried out to explore their potential of using them as structural material. Moreover, it is well known that the polyester is most versatile, abundantly available and easily process-able fibre compared to glass. 2. Experimental 2.1 Materials In this study high tenacity fully drawn polyester yarns and E-glass yarns, both of 300 Tex linear densities were selected and their properties were subsequently analysed in raw form and in reinforced composite form. Polyester filament yarns and glass rovings were supplied by Reliance Industries Pvt. Ltd and Owens Corning respectively. Further details of the reinforcement materials are listed below in the Table 1. Uni-directional (UD) composites were prepared by using epoxy based Thermoset resin systems. The matrix used for the fabricating composite was the mixture of epoxy resin (Grade LY 556) and hardener (Grade HY 917) mixed in the mass ration of 10:1.Epoxy Resin and Hardener were supplied by Northern Polymers Pvt. Ltd as shown in below Table 1. Table 1. Reinforcement material details Sl No. Parameters
Glass
Polyester
1
Density (g/cc)
2.54
1.38
2
Breaking force (N)
321.64
245.25
3
Initial Modulus (N/tex)
17.42
4.38
4
Tenacity (N/tex)
1.04
0.81
5
Elongation (%)
2.53
12.49
2.2 Preparation of UD composites The process of developing Uni-directional composites requires the reinforcement material to be positioned along a particular direction. For this purpose, the yarns were converted to hanks of standard size and placed layer-by-layer inside a rectangular steel mould of length 500mm and width 25mm. The mould is sufficiently wrapped with Teflon sheets to prevent the resin form sticking on to the mould surface during the curing process. The hank was stretched to maintain the linear orientation of yarns and the resin application process is begun. The resin is applied using a brush type applicator and uniformly distributed through the reinforcement within the mould. Following resin application, the mould mould assembly was allowed to cure at room temperature for 24 hours followed by thermal post curing at 100oC for 5 hours in a convection oven to obtain the final composite samples. After curing, the samples were separated from the mould and cut to required specimen sizes for testing.
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2.3 Testing of composite samples 2.3.1 Static mechanical analysis The static mechanical analysis involves testing of composite samples under tensile, bending and impact loads. The tensile and flexural properties of UD composites were tested on [Model details] according to ASTM D638 and ASTM D790 test methods respectively. The samples for tensile testing were prepared in the size of 150mm in length and 25mm in width and tested at 2mm/min cross-head speed and 100mm gauge length. The thickness of the composite samples was found to be in the range of 6.3 - 6.6 mm and they were flexural tested through three-point bending at 2mm/min test speed and 100mm span length. Impact properties of the composite samples were studied by testing them on a Notch Izod impact tester according to ASTM D256-97 test method. The composite samples for Izod notch test were prepared according to the standard size of size 63.5mm in length in fiber direction and 12.7mm in width and a notch of 1.5mm was made at the middle of each sample. Impact resistance of the sample is determined by releasing a pivoting arm from specific height to swing and hit a notched sample and thereby calculating the energy lost in breaking the specimen of unit thickness. The mode of failures at the fractured surfaces was studied using SEM micrographs captured from scanning electron microscope (SEM ZEISS EVO 50). As a part of sample preparation, the fractured samples obtained from tensile and three-point bending test methods were selected and placed on a metallic sample holder to coat them with gold (through spluttering technique) to make their surface conductive for SEM analysis. 2.3.2 Dynamic mechanical analysis Dynamic mechanical analyzer MCR 702 Multidrive was the test instrument used to study the dynamic mechanical properties of the composite samples like storage modulus, loss modulus and damping factor. Samples were tested under three point bending mode at a frequency of 1, 10 and 100 Hz. The testing temperature ranged from room temperature to 180oC. The amplitude was set between 40-50 µm based on the thickness of the samples. 3. Results and discussions 3.1 Static mechanical properties of the UD composites samples 3.1.1 Tensile properties The stress-strain curve for glass- epoxy composite exhibited linearly elastic behaviour until its brittle failure whereas, the curve for polyester-epoxy composite showed insignificant non-linearity before reaching the maximum stress. This non-linearity is mainly due to ductile behaviour of polyester material which has the tendency to undergo extensive deformation under the action of loads before any failure occurs. The tensile modulus of glass and polyester reinforced composites calculated from the slope of its respective stress-strain curves were found to be 15.24 GPa and 5.63 GPa respectively. Tensile energy absorption which measures the total work done to break a sample was mathematically determined by calculating the area beneath their respective stress-strain curve. The resulting values of energy absorption that can accurately guage a material toughness are reported (Figure 1).
Figure 1. Overview of tensile testing results
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The visual investigation of tensile fractured composites reveals that the fiber and matrix in both the specimen cases, work as a single unit and fail altogether in a more or less a straight line. The SEM images of the glass and polyester fractured composite ends are shown in the Figure 2 (a-b) with fiber breakages more prominently sighted in both the cases. In addition, the shorter fiber pull-out length in case of polyester-epoxy composites authenticates the presence of a strong interfacial bonding between the fiber and matrix. From these observations, the properties of polyesterepoxy material combination are found to be in fair comparable limits of glass-epoxy material combination.
Figure 2. SEM micrographs of tensile fracture of (a) Glass-epoxy and (b) Polyester-epoxy composites
3.1.2 Flexural analysis
Figure 3. Overview of three-point bending results
Figure 3 shows the comparison of flexural stress-strain data for glass and polyester reinforced UD composites that were computed from the Load-deflection curves recorded during the 3-point bending test. Subsequently, Flexural modulus was calculated from the slope of Flexural stress-strain curve. The load deflection response for the glass reinforced composites reveals that the material behaves linearly upwards till its maximum stress reaches the peak load. Post peak load, the stress-strain curve of the specimen experienced a sharp fall with its overall load bearing capacity plunging to zero. At the initial stages of loading process, the failure in the glass-epoxy specimen was foreseen by local matrix cracking on its tensile side, followed by its obvious brittle fracture leading to further damage accumulation and complete material destruction. On the other end, the load deflection response for polyester-epoxy specimen demonstrated an impressive ductile behaviour of the composites in which, the curves initially progressed to maximum stress level and thereafter exhibiting longer plateaus. The formation of larger plateaus was greatly attributed to the plastic deformation of the polyester material. Unlike glass-epoxy composites, the complete failure in the structure was never observed even at the mid-span deflection of 50mm. Ahead of this, the specimen were still able to take up 60% of its maximum load.
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Figure 4. Flexural tested samples (a) Glass-epoxy and (b) Polyester-epoxy composites
The morphology of the fractures was observed from the macroscopic point of views in order to investigate the failure mechanism of the composites. During 3-point bending test, the specimen facing the indenter experienced compressive force while, the opposite side experienced the tensile force. The compressive force causes the surface to kink and bulge sidewise while, the tensile force initiates the crack causing the fibers on the outer surface to fail under tensile stress. The crack continues to propagate from tensile face to compressive face (through the thickness) until complete failure of the sample. Figure 4(a-b) depicts the fracture photographs of three-point tested glass-epoxy and polyester-epoxy composite samples in both the faces. The image of the fractured sections confirms the fiber pull-out and brittle failure of the glass-epoxy composite and ductile behaviour of polyester-epoxy composites with no fiber breakage (except few outer fibers due to extreme tensile stress at higher mid-span deflection). From the above mentioned observations it can be concluded that, the polyester-epoxy combination encompass excellent toughness (residual strength) properties which was in contrast to the typical brittle behaviour of glass-epoxy composites. 3.1.3 Izod impact test In Izod impact test method, the kinetic energy required to initiate a crack fracture and propagate it through a notched sample is recorded. The average izod impact values measured for glass and polyester reinforced epoxy composites were 11.917 J and 12.862 J respectively. Interestingly, polyester-epoxy showed better energy absorption than the glass-epoxy composites. From the previous studies, it is evident that the yield strength, ductility and fracture mechanism are the important factors that affect the izod impact energy of a specimen. With the increase in yield strength, the material turns out to be more brittle causing the impact energy of the specimen to plunge. Brittle materials are inclined to absorb lower impact energies due to their inability to undergo plastic deformation. This could be main the reason for glass reinforced epoxy composite to demonstrate relatively low impact properties than the polyester reinforced composites. The ductile properties in polyester dominantly assist in plastic deformation during yielding of the specimen and subsequently improve its ability to withstand higher impact energies prior to fracture.
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Figure 5. Fracture images of Izod impact failed samples (a) G-E and (b) P-E composites
From the Figure 5, the mode of fractures observed in glass-epoxy composites was much different from the polyester-epoxy sample. When the sample of glass-epoxy composite was struck by the hammer, the strike force caused the outer layer of fibers to fracture under tensile stress. The crack imitated at the notch was propagated through the cross-section of the sample but not clearly perpendicular to the fibers. As the specimen continues to bend further, subsequent adjacent layers were fractured in tension causing delamination in the fibers layers and crack growth. But in the case of polyester-epoxy composite, the impact force causes extensive branching of the fibers to the side, without giving any room for the crack to propagate. This property is largely attributed to the ductile behaviour of the polyester material thereby showing better energy absorption capability than the glass-epoxy counterpart. 3.2 Dynamic Mechanical properties 3.2.1 Effect of temperature on dynamic mechanical properties of the composites Storage modulus (E’) measures the stiffness of a material which closely relates to its load bearing capacity and analogous to its flexural modulus [22, 23]. While, loss modulus (E”) of a material is a measure of its energy dissipated as heat per cycle under deformed conditions [24]. Figure 6 graphically enumerates the variation of composite’s storage and loss modulus as a function of temperature and tested at a frequency of 10 Hz.
Figure 6. Effect of temperature on storage modulus, loss modulus and tan delta values of the composites
The E’ value was higher for the glass-epoxy composites, which is obvious due to the fact that the modulus of elasticity of glass fibers is greater than that of polyester filament yarns. The E’ value was slightly lesser for polyester-epoxy composites and the difference was approximately was 10%. In spite of having lower modulus when
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compared to glass fibers, such admirable display of dynamic modulus in P-E composites can be attributed to its excellent interfacial bonding and effective stress transfer taking place between the fiber and matrix. Previous works have reported that storage modulus of the composites, increases with weight fraction of fibers [25, 26]. It is a well known fact that density of polyester and fiberglass being 1.38 g/cm3 and 2.54 g/cm3 respectively, the number fibers available per weight fraction is more in case of polyester. The increase in fiber content could also be one of the reasons for the at par behavior of P-E composites. The loss of storage modulus is gradual at lower temperatures for both the composites. However, the sharp fall in the modulus at the vicinity of glass-transition temperature (Tg) indicates the material’s transition from solid to glassy/rubbery state. The loss in modulus at transition phase is higher in the case of G-E composite which is clearly indicated by sharp vertical fall of the curve to a minimum level which is considerably lower than P-E composites. With matrix material being the same in both the cases, the differences in the Tg is purely dependant on the intrinsic properties of the reinforcement material. The Tg for P-E composites are higher and broader than G-E composites clearly indicating the effect of semi-crystalline polymeric nature of polyester filaments on composite’s behaviour during phase transition. The curves of Loss modulus and tan δ followed similar trend as in the case of storage modulus, with G-E composites competitively showing higher values. Tan δ basically defines dampness and can be easily related to the material’s resistance towards impact loads. Higher the tan δ values of a material associated with glass transition temperature, lower is their load bearing properties and vice versa [23]. The tan δ values for P-E composites were found to be around 0.267 while, for G-E composites the value were 0.532. It is clear from these values that P-E composites have better interfacial bonding and load bearing capacity especially at temperatures where G-E composites tend to lose their properties [27]. Since the peak of E” and tan δ curves occurs in the region of glass transition, the Tg values obtained from both the curves are tabulated in Table 2. Table 2. Glass transition temperatures measured from loss modulus and tan delta curves Composites Temperature (ºC) Tg from E”max curves
Tg from tan δmax curves
Glass-epoxy
76.02
94.14
Polyester-epoxy
119.10
131.03
The Tg obtained from the loss modulus cures are lower and more realistic than that of tan δ curves. Earlier studies have reported the same [25]. Again, with the same matrix system in both the composites, the damping properties are affected by the elastic nature of reinforcing fibers [28]. Therefore, it can be concluded that are mainly that the elastic nature of the polyester fibers are is the main reason for such excellent damping behaviour in P-E composites 3.2.2 Effect of frequency on dynamic mechanical properties of the composites Previous studies have reported the effect of testing frequencies on the dynamic mechanical properties of Glassepoxy composites [29, 30]. The variation storage modulus, loss modulus and tan δ values as a function of frequency are studied for P-E composites and the respective data are enumerated in the Figure 7.It was reported earlier that, modulus measured at higher frequencies are higher than those measured at lower frequencies [28]. In our systems, there was no much significant difference observed in the storage modulus however, increase of frequency has been found to increase loss modulus values clearly depicting its direct impact on the dynamic modulus of the composites. This change in dynamic properties with frequencies may be attributed to morphological rearrangement and improved fiber-matrix interaction. With the increase in frequency, the tan δ peaks shifted to higher temperatures and the Tg values recorded at three consecutive frequencies were found to be 124.99 ºC, 133.88 ºC and 136.87 ºC respectively. The shift in tan δ peaks which is also an indicative of degree of cross-linking is due to the micromechanical transition in a polymeric system.
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Figure 7. Effect of frequency on storage modulus, loss modulus and tan delta values of PET-epoxy composites
3.2.3 Cole-cole plots The curves of loss modulus data plotted against storage modulus are referred as Cole-Cole plots. The nature and shape of the Cole-Cole plots indicated the homogeneity of the material combination in the samples being tested. Ideally, curve following the semi-circular path indicates that the polymeric material combination within the system is homogeneous [31]. The deviation from the regular semi-circular shape to irregular shape of the curve signifies the lack of homogeneity. Figure 8 shows the Cole-Cole plots for glass-epoxy and polyester epoxy material combinations. It can be seen from the figure that the curves of both the composite samples are imperfectly semicircular following the polynomial equation of fourth order. Imperfect semi-circular shape of the curves indicates that, there is heterogeneity in both the material combination which may be due to dissimilarity in the reinforcement and matrix phase especially at lower temperatures. The extent of regularity in the shape observed at higher temperatures (nearing glass transition and beyond) is mainly attributed to the transition of the material from solid state to glassy/rubbery state. When compared with each other, the curves of P-E composites can be observed with greater extent of irregularity from semi-circular shape. This difference is due to the fact that crystalline polymers like PET are characterized with heterogeneous structures due to interspersed amorphous regions while amorphous polymers like glass have homogeneous structure [32-34]. Similar analysis through Cole-Cole plots were made to study the homogeneity in oil palm based epoxy composites [23, 28].
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Figure 8. Cole-Cole plots of Glass and PET reinforced epoxy composites
4. Conclusions The static and dynamic mechanical properties of PET reinforced epoxy composites were investigated and compared with glass-epoxy counterparts. Composites developed by this material combination exhibits unique properties giving a platform to explore the possibilities of using them in lightweight constructions. The moderately lower strength and modulus properties in P-E composites were compensated by their excellent toughness and energy absorbent properties. From the fracture morphological studies, it was evident that there was good fiber-matrix interaction and the breakage type confirms the effective stress transfer between them. It was more interesting to note that, P-E composites performed well even in presence of fractures bearing almost 60% of their maximum limit while, G-E composites exhibited brittle and complete failure at higher level of deformation. DMA analysis highlights the P-E composite’s gradual loss of dynamic modulus properties at the transition phase. The higher glass-transition temperatures and lower tan δ values confirm the suitability of P-E composites to perform well at higher temperatures with better resistance to impact loads. Based on these studies, it can be concluded that the PET-epoxy composites can be a new choice of engineering materials in designing lightweight and impact resistive composites structures. Further, this work can drive the plastic recycling industries to enhance the utilization spectra of the recycled products in the form of polyester-epoxy composites. Acknowledgment Authors are thankful to Ministry of Textile for sponsoring Focus Incubation centre for 3D weaving and structural composites. References 1.
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