Mechanical, Thermal and Microstructural Properties of ...

International Conference on Advances in Design and Manufacturing (ICAD&M'14)

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Mechanical, Thermal and Microstructural Properties of Microwave Processed Luffa-Epoxy Natural Fibre Composite S. Javed Syed Ibrahim, K. Rajkumar, A. Gnanavelbabu, R. Panneerdhass  Abstract--- Today’s research ventures highlight a shift of interest from synthetic materials to eco friendly materials and particularly natural fibre composites. The major demerits of natural fibre composites like poor mechanical properties and the high processing time can be addressed by the adoption of an efficient processing/curing technique. Conventional methods involve processing/curing the composites at room temperature or heating at temperatures below the degradation temperature of natural fibres, which is usually carried by a direct convectional application of thermal energy to the matrix resins. The conventional thermal curing poses a number of processing related problems such as long curing times, large temperature gradients and thermal degradation of fibres. Microwave curing has been referred to as an efficient alternative energy source for curing resins and their composites because of its ability to produce faster cure and increased crosslink rate. The heating is achieved by volumetric heating effect which results in more efficient curing, more uniform cure and improved physical/mechanical properties of the materials. In this research work, a series of experiments have been conducted to investigate the effects of microwave curing of natural ﬁber reinforced composites. Composites were prepared with a 40% weight fraction of randomly chopped luffa fibers reinforced in epoxy matrix. The natural fibre reinforced composite was cured in a microwave furnace at a temperature close to the 900C and data of a conventional room temperature cured composite was used for comparison purpose. Thermo gravimetric analysis was performed to compare the thermal characteristics and of conventionally cured and microwave cured composites. Microwave cured samples exhibited higher thermal stability indicating a better extent of cure. Standard tensile, compressive and impact tests were performed on microwave cured luffa-epoxy composites and the results were compared to those of the conventionally cured ones. All the mechanical properties were almost similar for both the composites, but considerably higher impact strength was observed in the microwave cured composite. The microstructure of the MW cured composite was studied using Scanning Electron Microscopy.

Keywords--- Luffa Fiber, Microwave Curing, Thermal Properties Mechanical Properties

S. Javed Syed Ibrahim, PG Scholar, Department of Mechanical Engineering, SSN College of Engineering, Kalavakkam-603 110, India. K. Rajkumar, Associate Professor, Department of Mechanical Engineering, SSN College of Engineering, Kalavakkam-603 110, India. A.Gnanavelbabu, Associate Professor, Department of Industrial Engineering, Anna University, Chennai. Tamil Nadu, India R. Panneerdhass, Assistant Professor, Department of Mechanical Engineering, A.R.Engineering College,Villupuram-605 601, Tamil Nadu, India.

The major limitations of natural FRCs are poor mechanical properties and high processing time involved, and research is underway to devise techniques for overcoming these limitations. In the manufacture of natural fibre reinforced epoxy composites, conventional curing methods which involve addition of curing agents at room temperature and elevated temperatures are widely used and newer methods like

I.

INTRODUCTION

I

N the present age of modern engineering materials, degradability has become an important factor in the material selection process. The non-degradable nature of most of the latest materials like synthetic fibre reinforced composites (FRC) is promoting resurgence into natural FRCs. The environmental and energy issues have created interest in materials derived from more sustainable resources that involve lower energy for processing and are recyclable as well. Since 2000, a huge increase in the use of natural fibres in cars has been recorded, for example in Germany, from around 10000 tonnes in 2000 to 19000 tonnes in 2005 [1]. In production of natural fiber-reinforced transport pallets the energy consumption is 45% lesser and emission of toxic gases (CO2, methane, SO2, and CO) is lower than production of Glass fibre reinforced transport pallets, and natural FRCs contain higher fiber content to satisfy the performance requirements, which lowers the amount of more polluting base polymers [2], and hence increases the degradability level. Natural fibres like hemp, sisal, kenaf.etc. are being successfully exploited as fillers in polymer matrices. A very few researchers have explored the reinforcing potential of luffa fibre, which is obtained from the dried fruit of luffa cylindrica (sponge gourd). This fibre, in the vernacular name of luffa sponge, has been used in domestic applications for several years and can be easily procured at a lower cost. The luffa fibre is composed of cellulose 63.0%, hemicelluloses 14.4%, lignin 1.6%, ash 0.9% and others 20.1%, and this composition renders the fibre, the properties of a good reinforcing material in polymer matrix [3]. In a study on luffa fibre reinforced vinylester matrix, the matrix showed 50% increase in tensile strength after reinforcement by esterified luffa fibres [4]. In another study conducted on luffa fibre reinforced epoxy composite, it was found that the fibrous network setup of luffa paved way for the enhancement of mechanical properties for a double layer laminate composite [5].

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microwave curing/processing have started gaining importance only in the recent past. The salient feature of microwave heating is its ability to produce self-heating of the material by causing intermolecular friction and this is called volumetric heating [6]. MW heating produces an inside-out cure and can greatly reduce the overall processing time. A comparative study of microwave and conventional thermal curing of epoxy DGEBA showed that former produces high reaction rates compared to the latter [7]. Experiments were conducted to study the microwave processing of epoxy adhesive joints and their results stated that curing of epoxies using microwaves reduces curing time and could improve the mechanical properties. Their results also highlighted the higher bond strength exhibited by MW cured epoxies as compared to ambient cured ones [8]. The dielectric properties of the materials involved should be considered before experimenting. Research carried out in the past reveals that in microwave processing of fibre reinforced epoxy composites, the heating of matrix takes place prior to the heating of fibres. This is mainly because, the dielectric loss factors of matrix resins is high compared to most of the fibres [9]. So, unlike the conventional thermal curing in which the matrix and fibre phases are heated at the same time which may cause early degradation of the fibres, microwave curing produces selective heating of the epoxy matrix which avoids the degradation of fibres before the curing of epoxy. The main aim of this research work is to study the effect of microwave curing on the mechanical properties of luffa fibre reinforced epoxy composite, its efficiency in terms of energy and time, and thermal characteristics in comparison with those of conventionally cured composite (cured at room temperature). II.

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Fig.1. luffa fibre

Microwave curing of composite A multimode microwave (MW) furnace with a maximum heating output of 1200W was used. The epoxy DGEBA resin and TETA hardener were mixed in the ratio 10:1 and then chopped luffa fibres were introduced into this mixture such that the fibres accounted for 40% of the weight of the mixture. Initially, epoxy (200 gm) taken in an earthen container was heated in the MW furnace at a power of 220 W for 2 mins. This was followed by addition of hardener and fibres in the proportions mentioned above, to the heated resin in a rectangular PPE container and the mixture was left at room temperature for 4 mins. Then it was heated for 30 mins in the furnace at a power of 110W. The temperature was maintained close to 900C using a thermometer (refer fig. 2) as the hardener belongs to the family of Aliphatic amines, which produce a maximum reaction rate at 90◦ C [13]. Later, the container was removed from the MW furnace and the composite was obtained.

EXPERIMENTAL

Materials used Matrix and Fiber materials Epoxy LY556 DGEBA (Diglycidyl ether of bisphenol-A) resin, and HY951 TETA (triethylene tetraamine) hardener supplied by Hunstman Corp were used to prepare the matrix phase. The luffa fibres were isolated from dried luffa cylindrica fruits supplied by markets. To improve the compatibility of the fibres with the polymer matrix, a chemical treatment using NaOH was carried out on the fibres after isolation. This treatment increases effective fibre surface area which contributes to increased fibre-matrix adhesion [10], and in FRCs a good fibre-matrix adhesion is a key contributor to good mechanical properties. In addition to that, the chemical treatment also improves the thermal stability of the fibres by removing significant amounts of hemicellulose and pectin the luffa fibres [11,12], and in this experiment as heating is involved, the fibre needs to possess higher thermal stability The luffa fibres were immersed in 1N 10% NaOH solution for 2 hours followed by washing in distilled water and drying. Then, the inner core and the outer fibrous layers of the luffa sponge were separated and the outer core was spread (like a mat). This mat was randomly chopped into small rectangular pieces (roughly 30x20 mm) as shown in fig.1.

Fig.2. Experimental Setup for MW curing Characterization Cure and Thermal Thermo gravimetric analysis (TGA) TGA was used to determine the degradation temperatures of natural fibre composites. The decomposition characteristics of the composites were used to compare the extent of cure between the conventionally cured and microwave cured samples. All the samples were heated in a TA Instruments TGA to 6000C at a heating rate of 100C/min in a N2 atmosphere.

ISBN 978-93-84743-19-2 © 2014 Bonfring


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Mechanical

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The ultimate tensile and compressive strengths were determined using a Universal Testing Machine according to ASTM D 3039/D 3039M standard. The impact energy was measured using the charpy impact test. Rectangular specimens of dimension 65x12x3mm were prepared for this test. Five specimens were used for all tests and the average values were recorded.

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RESULTS AND DISCUSSION

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Time and energy consumption

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Energy consumption= Power (W) x Time(s) =220x120 + 110x1800 =198 KJ Therefore, MW curing seems to be energy efficient. More importantly, it is highly timesaving compared to room temperature curing. Cure and Thermal characteristics In this work, TGA was used for cure and thermal characterization. The thermal degradation characteristics can be used to confirm the presence of cured networks. The thermal degradation of cured epoxy-amine networks starts roughly at 390C [9, 13] and the MW cured luffa-epoxy sample, the matrix of which contains the same network started degrading at 3880C, indicating the attainment of cure. A similar approach was adopted in the work done by Amanda L. Higginbotham et al [14]. Table 1: Thermal properties of the composites

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Fig.3. TGA curve for MW cured composite 100.0 80.00 91.6%

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The MW curing proves to be a highly rapid method of curing a polymer resin and in this experiment it consumed 35 minutes for complete curing of the composite. The rapid curing is mainly because of the frictional heat generated at the intermolecular level due to MW heating of the epoxy resin leading to increased reaction with the curing agent. Among the conventionally used methods in the curing of composites, curing at room temp takes 24 hours and thermal curing by autoclave heating involves a minimum of 2 hours [13]. In the MW curing of luffa-epoxy composite, heating was carried out for 2 mins(120s) at a power of 220W and for 30 mins(1800s) at a power of 110W.

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Fig.4. TGA curve for conventionally cured composite In case of the conventionally (room temp) cured and MW cured composites (see fig 2&3), it is seen that the start of decomposition of MW cured composite is prolonged. The delay in the decomposition start by 250C compared to the room temperature cured composite indicates the presence of higher crosslink density or improved cure and also higher thermal stability in the microwave cured sample. Therefore, it is evident that, as a result of volumetric heating, MW curing results in a better extent of cure. The initial degradation temperatures of both the conventionally and MW cured composites fall between the initial degradation temperature of neat epoxy(cured) and the luffa fibre. A similar trend was observed in the experiments carried out on curing of natural fibre composites by Nikki Sgriccia and M.C. Hawley [9]. Mechanical Properties The tensile and compressive property values of the MW cured composites were close to that of conventionally cured composites but the impact strength that showed an increase. In fibre reinforced composites, increase in impact strength results from good fibre-matrix adhesion which hinders fibre pullout. Previous works suggest that, improved resin–fibre adhesion and fibre wetting can be achieved by lowering the resin viscosity [15-17]. In our experiment, MW heating, which produces an inside-out heating, rapidly reduced the resin viscosity at the initial stage. Later, the viscosity started increasing again, during the start of curing and this trend agrees with the observation from the work by D.A. Papargyris et al. [18]. During the MW heating, it was also observed that rapid heating at high power settings resulted in high brittleness of the composite whereas gradual heating at low power settings resulted in good flexibility and low brittleness. The results and practical learning from experiments suggest that the use of a compression moulding setup inside a MW furnace

ISBN 978-93-84743-19-2 © 2014 Bonfring


can help in achieving better mechanical properties. But, that remains a challenge because most of the compression moulds are made of metals, which are reflectors of MW radiation.

Tensile strength in MPa

Table.2. Mechanical properties of the composites

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IV.

MICROSTRUCTURE

The microstructure of the fractured surface of the MW cured composite explains that fibre breakage accompanied by matrix cracking is the failure mechanism involved. The absence of fibre-pullout in the MW cured composite, once again indicates the increased fibre matrix adhesion which results due to increased fibre wetting (caused by reduction of resin viscosity). The individual fibre strands of the MW cured composite are easily distinguishable unlike those of the conventionally cured sample.

18 17 16

Fig.8. Micrograph of fractured surface of MW cured composite

15 14 conventionally cured composite

MW cured composite

Fig.5. Tensile strength

Compressive strength in MPa

106 104 102

Fig.9. Micrograph of fractured surface of conventionally cured composite

100 98 conventionally cured composite

MW cured composite

Impact strength in Joules

Fig.6. Compressive strength 2.5 2 1.5 1 0.5 0 conventionally cured composite

MW cured composite

Fig.10. Micrograph showing fibres of MW cured composite The presence of undamaged fibres (refer fig.10), proves the selective heating capability of microwaves that has left the fibres undamaged in spite of causing a rapid heating of the resin.

Fig.7. Impact strength

ISBN 978-93-84743-19-2 © 2014 Bonfring


V.

CONCLUSION

Therefore, this paper has proved the feasibility of curing luffa-epoxy green composite using MW heating that makes use of the difference in dielectric properties of the fibre and resin, thus avoiding fibre degradation. MW curing has shown high energy and time efficiency, which make it a recommended technique for large scale processes. The usage of polymeric materials in high temp applications has always been challenging and MW curing by way of improving thermal stability makes it less challenging. The increased impact energy achieved by MW curing, recommends this technique for production of impact handling components like bumpers, ballistic shields. etc. Apart from that, the practical experience on MW heating has revealed that, variations in the heating cycles and power settings can help us tailor the composite to desired requirements(because heating at higher power settings produced more brittleness and gradual heating at very low power settings rendered good flexibility) which is impossible by other curing techniques. An overall consideration of all the observations of this work suggests that MW curing is an excellent alternative to any conventional curing method.

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[15]. Liu L, Huang Y, Zhang Z, Yang X, “Effect of ultrasound on wettability between aramid fibers and epoxy resin”,J Appl Polym Sci 2006;99(6):3172. [16]. Davies LW, Day RJ, Bond D, et al., “Effect of cure cycle heat transfer rates on the physical and mechanical properties of an epoxy matrix composite”, Compos Sci Technol 2007;67(9):1892–9. [17]. D.A. Papargyris a,*, R.J. Day a, A. Nesbitt a, D. Bakavos b. “Comparison of the mechanical and physical properties of a carbon fibre epoxy composite manufactured by resin transfer moulding using conventional and microwave heating”, Composites Science and Technology 68 (2008) 1854–1861.

REFERENCES [1]. “Use of Natural Fibres in Composites in the Automotive Sector in Germany from 1999 to 2005”, A survey by Nova-Institut GmbH, Goldenbergstr 2 D-50354 Huerth im Rheinland, Germany, 2006. [2]. Joshi VS, Drzal TL, Mohanty KA, Arora S, “Are natural fibercomposites environmentally superior to glass fiber reinforced composites?” Composites Part A 2004; 35:371–6. [3]. Yoldas Seki, Kutlay Sever, Seckin Erden, Mehmet Sarikanat, Gokdeniz, Neser, Cicek Ozes, “Characterization of luffa cylindrica fibers and the effect of water aging on the mechanical properties of its composite with polyester” Journal of Applied Polymer Science 2012; 123, 2330–2337. [4]. Vagner Roberto Botaro,1 Ka´tia Monteiro Novack,2 E ´ der J. Siqueira2, “Dynamic Mechanical Behavior of Vinylester Matrix Composites Reinforced by Luffa cylindrica Modified Fibers” Wiley Online Library (wileyonlinelibrary.com). [5]. Erik T. Thostenson and Tsu-We1 Chou,"Microwave and Conventional Curing of Thick-Section Thermoset Composite Laminates:Experiment and Simulation” [6]. Wei, M.C. Hawley, J.D. Delong, “Comparison of microwave and thermal cure of epoxy resins” Polym. Eng. Sci. 33 (17) (1993) 1132– 1140. [7]. A.Olofinjana, P.K.D.V. Yarlagadda, A. Oloyede, “Microwave processing of adhesive joints using a temperature controlled feedback system” Int. J. Machine Tools Manuf. 41 (2001) 209–225. [8]. Nikki Sgriccia, M.C. Hawley, “Thermal, morphological, and electrical characterization of microwave processed natural fiber composites" Composites Science And Technology [9]. Joseph PV, Joseph K, Thomas S, Pillai CKS, Prasad VS, Groeninckx G, et al., “The thermal and crystallisation studies of short sisal fibre reinforced polypropylene composites” Compos Part A – Appl Sci Manuf 2003;34(3):253–66. [10]. Sgriccia N, Hawley MC, Misra M, “Characterization of natural fibre surfaces and natural fibre composites” Compos Part A – Appl Sci Manuf 2008;39(10):1632–7. [11]. Taj S, Ali M, Khan S, “Review: natural fibre reinforced polymer composites” Proc Pak Acad Sci 2007;44(2):129–44. [12]. Ha Q. Pham, Maurice J. Marks, Epoxy Resins, pg. 58. [13]. Amanda L. Higginbotham a, Padraig G. Moloney b, Michael C. Waid b, Juan G. Duque a, Carter Kittrell a, Howard K. Schmidt a, Jason J. Stephenson a, Sivaram Arepalli b, Leonard L. Yowell b, James M. Tour Carbon, “Nanotube composite curing through absorption of microwave radiation”Composites Science and Technology 68 (2008) 3087–3092. [14]. Huang YD, Liu L, Qiu JH, Shao L, “Influence of ultrasonic treatment on the characteristics of epoxy resin and the interfacial property of its carbon fiber composites”, Compos Sci Technol 2002;62(16):2153–9.

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Mr.S. Javed syed Ibrahim is a II year ME manufacturing engg student of SSN college of engg, Chennai. He completed his BE in mechanical engg in Noorul Islam college of engg, Kanyakuamari in 2012. He is currently pursuing his ME academic project in Natural fibre composites. Email: [email protected]