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Mechanical properties and water absorption of glass fibre reinforced bio-phenolic elastomer (BPE) composite
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Mechanical properties and water absorption of glass fibre reinforced bio-phenolic elastomer (BPE) composite Umar Adli Amran, Sarani Zakaria ∗ , Chin Hua Chia, Sharifah Nabihah Syed Jaafar, Rasidi Roslan School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
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Article history: Received 29 August 2014 Received in revised form 27 January 2015 Accepted 29 January 2015 Available online xxx Keywords: Liquefaction Bio-phenolic composite Compatibility Mechanical properties Water absorption
a b s t r a c t In this study, bio-phenolic resin (BPR) was synthesised using liquefied oil palm empty fruit bunch (EFB) fibres. Epoxidised natural rubber (ENR) was added to provide elastomeric properties and improve the phenolic resin brittleness. Three different types of NR/glass fibre prepregs using SMR-L, ENR-25 and ENR50 were prepared and used in the fabrication of the bio-phenolic elastomer (BPE) glass composites. The composites were made by using hand lay-up technique. The ENR-50 showed the best compatibility with the BPR. The impact strength of the composites increased as much as 77.7% with the addition of 35 wt% of ENR-50. Flexural test results indicated that the BPE composites possessed elastomeric properties. The ENR acted as a compatibiliser that helped in improving the wettability of the BPR on the surface of the glass fibres. Furthermore, approximately 47% of water absorbed by the BPE composites was reduced as the ENR-50 composition increased up to 35 wt%. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Phenolic resin is a thermosetting resin produced from the polycondensation of phenol and formaldehyde (Pilato, 2010). The resins are divided into two types, novolac and resol, based on the concentration of phenol to formaldehyde ratios and type of catalyst used. For novolac resin, the formaldehyde to phenol (F/P) ratio is less than one and the catalyst used is acidic, whereas the resol resin is produced with F/P ratio greater than one in the presence of a basic catalyst (Effendi et al., 2008). Phenolic resins are widely used in many applications due to their high strength, modulus, thermal and chemical resistance. However, phenolic resin is known to be brittle, which limits its use in high-performance applications. Concerns about the depletion of fossil fuels, environmental pollutions, and fluctuations in the prices of petroleum-based chemicals have led to numerous studies devoted to replacing phenol with renewable feedstocks, such as lignocellulose biomass and woods. Lignocellulose biomass and woods are abundantly available throughout the world, and they can serve as a continuous supply of raw materials. Lignocellulose material is composed of three main compounds; cellulose, hemicellulose, and lignin. The presence of lignin in lignocellulose materials enables it to be liquefied in phe-
∗ Corresponding author. Tel.: +60 3 8921 3261; fax: +60 3 8921 3777. E-mail address: [email protected] (S. Zakaria).
nol, as lignin contains phenolic derivatives, such as sinapyl alcohol, coumaryl alcohol, coniferyl alcohol, etc (Effendi et al., 2008). Over the last few decades, several studies have been carried out to utilise wood and biomass as raw material in the synthesis of phenolic resin via a liquefaction process (Lin et al., 1994; Alma et al., 1998a,b; Lee et al., 2002). Liquefaction of biomass, as well as cellulose and wood, in the presence of phenol and acid (organic and inorganic) as catalysts, has been intensively studied (Alma et al., 1995a,b). Most biomass liquefaction reactions are carried out in the presence of an acid catalyst, which can significantly increase the conversion yield. Liquefaction under alkaline conditions indeed provides lower biomass residue content; however, it is not an effective catalyst to achieve a high amount of combined phenol (Alma et al., 1998a). During the liquefaction process,by using organic solvent, macromolecule compounds in biomass decompose into reactive small molecules via several processes, such as dehydration, dehydrogenation, deoxygenation, and decarboxylation (Lin et al., 2001, 2004). However, these small molecules will then re-polymerise through condensation, cyclisation, and polymerisation. Investigation into the liquefaction of oil palm empty fruit bunch (EFB) fibres with phenol in the presence of sulphuric acid has been carried out by Ahmadzadeh et al. (2008). In their study, they found that high liquefaction yield can be obtained with the phenol to EFB fibre weight ratio of 3:1. The study allowed them to obtain optimum parameters for satisfactory yield as follows: liquefaction
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time 60–90 min, temperature range of 130–150 ◦ C and catalyst concentration of 3–5%. In addition, the liquefied EFB (LEFB) was used as a phenolic precursor to prepare novolac type resin for moulding application. Meanwhile, Roslan et al. (2014) used the LEFB to prepare resol type phenolic resin and study its physical, chemical, and mechanical properties. From the chemical analysis, they found that LEFB at para position is more favoured for the reaction with formaldehyde when compared to the ortho position due to the steric effect. The plywood shear strength of the resol resin is also in compliance with the JIS K-6852 standard. Epoxidised natural rubber (ENR) is a chemically modified natural rubber which has epoxide groups grafted onto its polymeric chains by the epoxidation process (Ruamcharoen et al., 2011). These epoxides make ENR susceptible to various chemical reactions that involve hydrogen-donor molecules, such as alcohols, carboxylic acids, and amines (Phinyocheep et al., 2005). Commercially, there are two types of ENR, ENR-25 and ENR-50. The designated integers represent the mole percentage (%) epoxidation of the natural rubber. Miscibility of blends or compounds is attributed by molecular interactions, for example a donor–acceptor type of weakly acidic hydrogen or dipole–dipole interaction. Theoretically, ENR should be compatible with proton-donating polymers, such as phenolic resin (Kallitsis and Kalfoglou, 1989). Phenolic resin requires a toughening mechanism to improve its brittleness. An example of a phenolic resin toughening mechanism is the incorporation of rubber. The most common rubber used is nitrile rubber due to its compatibility (Kaynak and Cagatay, 2006). It is expected that the introduction of ENR will provide elastomeric properties and improve the impact resistance of the phenolic composites. However, little literature has reported on the topic of the utilisation of natural rubber as toughening agent in phenolic composites. Hence, the aim of this study is to investigate the mechanical properties and water absorption of ENR-toughened glass fibre reinforced bio-phenolic resin composites, which are referred to as bio-phenolic elastomer (BPE) composites. 2. Experimental 2.1. Materials Oil palm EFB fibres and E-glass fibres (woven and non-woven) were supplied by Szetech Engineering Sdn. Bhd and RTG Intech Sdn. Bhd., respectively. Natural rubbers with three different grades, SMR-L (Standard Malaysian Rubber Light), ENR-25 and ENR-50, were purchased from the Malaysian Rubber Board. The rubber grades represent 0%, 25% and 50% mole epoxidation, respectively. SMR-L (0% epoxidation) is a natural rubber before epoxidation was used as a control to study the effect of rubber epoxidation. Industrial grade phenol (98%) was supplied by Chemzone Sdn. Bhd. Analytical grade sulphuric acid (98%), formalin (37% formaldehyde), sodium hydroxide (NaOH), methanol and toluene were procured from Sigma–Aldrich.
paper (Whattman no. 1) to remove the residues. Later, the methanol added was evaporated from the mixture by using a rotary evaporator to obtain liquefied EFB (LEFB). 2.3. Resinification process A resinification reaction was carried out by reacting formalin with the LEFB to prepare BPR. The LEFB was reacted with formalin (37% formaldehyde solution) as the crosslinking agent with the formalin/LEFB weight ratio of 1.2 in an alkaline medium. Sodium hydroxide (40 wt%) aqueous solution, used to obtain alkaline medium, was added in two-steps process. During the first 60 min of the reaction, 5 wt% (based on LEFB weight) of NaOH solution was added, and the reaction temperature was set at 65 ◦ C. Following this, the reaction temperature was increased to 85 ◦ C and 20 wt% (based on LEFB weight) of NaOH solution was added, where it was held for another 60 min. All of the reactions were conducted in a reflux system. 2.4. Preparation of natural rubber (NR) and epoxidised natural rubber (ENR) solutions Ten grams of natural rubber (SMR-L) and epoxidised natural rubber (ENR-25 and ENR-50) were dissolved in 100 ml of toluene, separately, to produce rubber solutions with 10% solid rubber content. The process was conducted in a 1000 ml round bottom flask attached to a reflux condenser system equipped with an overhead stirrer to stir the mixture. The process was carried out at 80 ◦ C for 6 h in an oil bath. 2.5. Preparation of rubber/glass fibre prepregs To study the effects of the natural rubber degree of epoxidation on the properties of the composite, three prepregs with 5 wt% (from the weight of the woven glass fibres) of each SMR-L, ENR-25 and ENR-35 were prepared. An additional three prepregs with 15, 25 and 35 wt% (from the weight of woven glass fibres) of ENR-50 were prepared to study the effect of different rubber compositions on the composite properties. The rubber solution was applied on both sides of the woven glass fibres. During the application of the rubber solution, care was taken to match the exact required weight and thickness on each side of the prepregs to maintain a balanced composite design. Later, the rubber-coated glass fibres were dried in an oven at 60 ◦ C overnight to remove toluene. The thickness of the prepreg was measured and controlled by using an electronic micrometre. 2.6. Preparation of bio-phenolic elastomer (BPE) composites Composite boards were prepared by using a hand lay-up technique, followed by compression moulding. Total glass fibre loading
2.2. Liquefaction of EFB fibres Liquefaction of the EFB fibres was performed according to procedures reported upon in the previous studies (Ahmadzadeh et al., 2009 Zakaria et al., 2014) with slight modification. First, the EFB fibres were dried in an oven at 105 ◦ C for 24 h. The weight ratio of phenol to EFB fibres (P/EFB) of 3:1 and sulphuric acid (3% based on the phenol weight) were used in the liquefaction reaction. Liquefaction was carried out in an oil bath at a temperature of 150 ◦ C for 90 min. Black and viscous liquid was produced at the end of the reaction. This liquid was then diluted with 400 ml methanol. The mixture of black liquid and methanol was filtered with filter
Fig. 1. Lay-up arrangement.
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Table 1 Effects of natural rubber types on the mechanical properties of BPE composites. Types of rubber
Impact strength (kJ/m2 ) (±SD)
Ref. SMR-L ENR-25 ENR-50
72.40 56.01 71.59 74.04
± ± ± ±
2.12 4.06 3.59 2.64
Flexural strength (MPa) (±SD) 50.87 21.32 21.69 22.55
± ± ± ±
Flexural modulus (MPa) (±SD)
1.59 1.13 1.59 1.07
6212.30 1259.28 2949.03 1475.24
± ± ± ±
325.97 58.70 402.62 126.19
used was 40 wt% of total composite weight. The other 60 wt% comprised the total weight of resin and rubber solid content used. Each composite consisted of five layers of GF mats and two layers of NR/GF prepregs, as shown in Fig. 1. These lay-ups were kept in an oven at 60 ◦ C for 14 h to reach semi-cure stage. The semi-cured lay-ups were then hot-pressed at 105 ◦ C for 15 min in a rectangle mould with a dimension of 150 mm × 150 mm × 3 mm. The samples with 5% SMR-L, 5% ENR-25, and 5% ENR-50 were later referred to as composites SMR-L, ENR-25 and ENR-50, respectively. The same arrangement of composite without NR was fabricated as a reference sample and labelled as “Ref.”.
ically, the specimens were taken out and wiped with absorbent paper to remove the water on the specimen surfaces and then reweighed. The water absorptions of the samples were calculated by using Eq. (1):
2.7. Mechanical tests
3.1. Effects of types of natural rubber
Izod impact and flexural tests were carried out in accordance with ASTM D256 and D790, respectively. The impact test was performed on an Instron CEAST 9050 machine using notched specimens (notch depth: 2.5 mm) with an impact speed of 3.46 m/s and incident energy of 5.0 J. For the flexural test, three-point bending with a nominal specimen dimension of 90 mm × 13 mm × 3 mm was carried out by using a Universal Testing Machine (Testometric M500-50CT). The span between supports was 50.0 mm and the crosshead speed was 10 mm/min. Each value obtained represents the average of five specimens.
Table 1 demonstrates the effects of natural rubber types (SMRL, ENR-25, and ENR-50) on the mechanical properties of the BPE composites. In this case, the rubber composition was fixed at 5 wt% based on the weight of woven glass fibres. From the result, the impact strength of the Ref. sample is 72.40 kJ/m2 . After 5 wt% of SMR-L loaded, the impact strength of the SMR-L composite decreased to 56.01 kJ/m2 . This phenomenon occurred due to the incompatibility between the resin (polar material) and SMR-L natural rubber (non-polar materials). Similar results were observed in a study conducted by Thongnuanchan et al. (2007), when NR latex adhesives used as a binder for particleboards demonstrated that a transetherification reaction of methoxymethyl groups from hexamethoxymethylmelamine did not occur in the non-polar NR system. After the introduction of ENR-25 and ENR-50, the impact strength of the composites increased to 71.59 kJ/m2 and 74.04 kJ/m2 , respectively. Although the results have no significant effect if compared to the Ref. sample due to the large standard deviation, they show that the presence of the epoxides group allows the reaction between OH from the BPR and the ENR to take place. Again, this result is in a strong agreement with the previous finding in the literature on ENR-based adhesives (Thongnuanchan et al., 2007). Phenolic resin consists of an aromatic ring and two ortho- and parapositions for reactions in every phenol molecule. Besides, abundance of OH groups in BPR is capable to crosslink with the oxirane ring of the ENR molecules. Fig. 2 shows the scheme of probable crosslinking mechanism between BPR and ENR.
2.8. Morphological analysis The morphological structure of the BPE composites was investigated by using a scanning electron microscope (SEM Supra 55 VP Zeiss) to observe the fractography of the surface of the BPE composites and the interaction between glass fibres, ENR, and BPR. The impacted specimens were cut cross-sectionally at the notched parts using a sample cutter and coated with a thin gold layer prior to SEM analysis. 2.9. Water absorption Composite specimens with a dimension of 15 mm × 15 mm × 3 mm were used for the water absorption test. The specimens were then immersed in distilled water at a temperature of 25 ◦ C. Period-
Water absorption(%) =
W2 − W1 × 100 W1
(1)
where W1 is the initial weight of the sample before the immersion (g) and W2 is the weight of the sample after the immersion (g). 3. Results and discussion
Fig. 2. Scheme of probable crosslinking between BPR and ENR.
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Fig. 3. Fractographs of composites with different types of NR (a) Ref. (b) SMR-L (c) ENR-25 and (d) ENR-50.
Composites at different amounts (15%, 25%, and 35%) of SMR-L, ENR-25 and ENR-50 were prepared to study the effects of different rubber compositions. Unfortunately, as the composition of SMRL and ENR-25 increase, delamination occurs before the composite specimens are tested (there is no result shown). Delamination happens as SMR-L and ENR-25 show incompatibility with the resin as their compositions increase. Thus, both rubbers are not suitable to be used in the prepreg application. Nevertheless, ENR-50 showed strong compatibility with resin and formed rigid samples as its composition increased. The effects of ENR-50 composition are discussed in Section 3.2. SEM fractographs of composites with different types of natural rubber are shown in Fig. 3. For the Ref. sample in Fig. 3(a), it is observed that there is no BPR wetted on the glass fibre surface. Fig. 3(b) shows that the SMR-L remains on the glass fibre surface, but the resin does not attach to the rubber after impact force is applied. This is due to the incompatibility between SMR-L and BPR. It is noted that, the resin begins to wet the ENR-25 and ENR-50, as shown in Fig. 3(c) and (d), respectively. The compatibility between rubber and resin is solely dependent on the epoxides present in the ENR. As the mole % of epoxidation increases, the ability of the resin to wet the glass fibre prepreg also increases. Table 1 shows the flexural strength and modulus of the BPE composites produced with different types of rubber. From the results,
all BPE composites show reduction in both flexural strength and modulus in comparison with the Ref. sample. Flexural strength indicates an ability of a material to resist deformation under bending force whereas the modulus is heavily dependent on the ratio of stress over strain of that material. Elastomers possess elasticity that enables them to absorb forces while deforming. The results indicate that the BPE composites have turned out to be less rigid. As the composition of the rubber is constant, the flexural strength does not differ much. Nonetheless, there is a slight difference in modulus of the ENR-25 sample. Chemical and physical properties of the ENR change depending on the extent of the mole % of epoxidation. Flexural strength of ENR is decreased as the mole % of epoxidation increases (Baker et al., 1985). 3.2. Effects of rubber composition Table 2 shows the effects of the ENR-50 compositions on the mechanical properties of BPE composites. From the table, it can be seen that the impact strength of the BPE composite increases from 72.40 kJ/m2 to 128.65 kJ/m2 with the addition of 35 wt% of ENR-50. A significant increment on the impact strength can be observed at 15 wt% of ENR-50. The increase in the ENR-50 composition allowed more impact force to be absorbed through stress distribution by rubber particles present in the composite. In addition, more epox-
Table 2 Effects of ENR-50 compositions on the mechanical properties of BPE composites. ENR-50 composition (%)
Impact strength (kJ/m2 ) (±SD)
Flexural strength (MPa) (±SD)
Flexural modulus (MPa) (±SD)
Ref. 5 15 25 35
72.40 ± 2.12 74.04 ± 2.64 96.51 ± 1.72 114.27 ± 2.95 128.65 ± 5.07
50.87 ± 1.59 22.55 ± 1.07 29.59 ± 1.11 25.65 ± 0.82 24.64 ± 1.15
6212.30 ± 325.97 1475.24 ± 126.19 1555.13 ± 98.11 1235.12 ± 111.57 1386.36 ± 110.07
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Fig. 4. Fractographs of composites with different composition of ENR-50 (a) Ref (b) 5 wt% (c) 15 wt% (d) 25 wt% and (e) 35 wt%.
ides exist in the composites when the rubber composition increases which allows for greater interaction between the ENR-50 and BPR, resulting in greater rubber-resin compatibility. Fractographs in Fig. 4 portray the condition of the BPE composites at different ENR-50 compositions after being impacted. From the images, it can be clearly seen that more resin wets the surface of the glass fibre as the ENR-50 composition increases. This has proven that the ENR-50 has acted as a compatibiliser between the glass fibre and BPR. Furthermore, higher amounts of ENR-50 cause more rubber particles to squeezed into the voids formed through solvent evaporation during hotpressing, thereby, reducing the formation of voids. Lesser void content in the composites improves their impact strength. Flexural strength and modulus of the BPE composites at different compositions of ENR-50 are shown in Table 2. The flexural strength and modulus of all BPE composites samples are decreased when compared to Ref. sample when the ENR composition increases. The elasticity and resilient properties of rubber have altered the overall composite rigidity and provide an elastomeric property. This leads to a reduction in the composites’ flexural strength and modulus. There is no pattern or significant dif-
ference in flexural properties of the composites associated with the addition of different amounts of ENR because they are anisotropic material. So, the flexural load applied to the samples is unequal at different axes where it is measured. However, the results show that 15% ENR composition possesses the highest flexural strength and modulus, which is 29.59 MPa and 1555.13 MPa, respectively. It may be at this point that the amount of rubber affords the highest rigidity and optimum compatibility for the ENR-50 BPE composites. 3.3. Water absorption test Fig. 5 presents the water absorption of BPE composites at different ENR-50 compositions. Curing of phenolic resin leaves microvoids that contribute to the sites for water absorption in composite material. Absorbed water trapped in a composite body may deter its performance for a period of time. It is very crucial to reduce the water intake of the composite to prevent this problem from occuring. In the Ref. composite sample, total water uptake is 11.25%. The water absorption by the BPE composites decreases as the ENR-50 composition increases. At 35 wt% of ENR-50, the total water absorbed by the composite sample is 5.95%. Approximately
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Acknowledgements We acknowledge the financial support from research grants PHI-2013-001, UKM-ST-07-FRGS0232-2012 and UKM-DIP-2014013. References
Fig. 5. Water absorption of composites with different composition of ENR-50.
47% water uptake has been reduced by the addition of 35 wt% of ENR-50, when compared to the Ref. sample. This happen as many microvoids are lost due to the formation of a compact composite structure and the ENR-50 prepregs have also protected some areas from allowing water to diffuse in the composite sample. 4. Conclusions In this study, BPE composites were prepared by using BPR with the incorporation of natural rubber and different types of epoxidised natural rubber (ENR) prepregs. ENR was added to provide an elastomeric property to the brittle thermosetting resin. The results demonstrated that the addition of SMR-L into the composite decreased its impact strength due to the incompatibility between the rubber and the resin. However, after the introduction of ENR-25 and ENR-50, the impact strength of the composites was increased. In addition, investigation into the effects of different ENR-50 compositions showed similar results as the impact strength increased from 72.40 kJ/m2 to 128.65 kJ/m2 . These findings proved that the epoxides group in the ENR backbone can provide compatibility by allowing the rubber-resin interaction. This was also supported by the results obtained via SEM images. Flexural properties of the BPE composites were found to be decreased when rubber was introduced. This indicated that the fabricated composites possessed elastomeric properties. Meanwhile, the water absorption test showed that the addition of ENR has a great effect in reducing the total percentage of water absorbed by the BPE composites.
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Mechanical properties and water absorption of glass fibre reinforced bio-phenolic elastomer (BPE) composite Umar Adli Amran, Sarani Zakaria ∗ , Chin Hua Chia, Sharifah Nabihah Syed Jaafar, Rasidi Roslan School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia
a r t i c l e
i n f o
Article history: Received 29 August 2014 Received in revised form 27 January 2015 Accepted 29 January 2015 Available online xxx Keywords: Liquefaction Bio-phenolic composite Compatibility Mechanical properties Water absorption
a b s t r a c t In this study, bio-phenolic resin (BPR) was synthesised using liquefied oil palm empty fruit bunch (EFB) fibres. Epoxidised natural rubber (ENR) was added to provide elastomeric properties and improve the phenolic resin brittleness. Three different types of NR/glass fibre prepregs using SMR-L, ENR-25 and ENR50 were prepared and used in the fabrication of the bio-phenolic elastomer (BPE) glass composites. The composites were made by using hand lay-up technique. The ENR-50 showed the best compatibility with the BPR. The impact strength of the composites increased as much as 77.7% with the addition of 35 wt% of ENR-50. Flexural test results indicated that the BPE composites possessed elastomeric properties. The ENR acted as a compatibiliser that helped in improving the wettability of the BPR on the surface of the glass fibres. Furthermore, approximately 47% of water absorbed by the BPE composites was reduced as the ENR-50 composition increased up to 35 wt%. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Phenolic resin is a thermosetting resin produced from the polycondensation of phenol and formaldehyde (Pilato, 2010). The resins are divided into two types, novolac and resol, based on the concentration of phenol to formaldehyde ratios and type of catalyst used. For novolac resin, the formaldehyde to phenol (F/P) ratio is less than one and the catalyst used is acidic, whereas the resol resin is produced with F/P ratio greater than one in the presence of a basic catalyst (Effendi et al., 2008). Phenolic resins are widely used in many applications due to their high strength, modulus, thermal and chemical resistance. However, phenolic resin is known to be brittle, which limits its use in high-performance applications. Concerns about the depletion of fossil fuels, environmental pollutions, and fluctuations in the prices of petroleum-based chemicals have led to numerous studies devoted to replacing phenol with renewable feedstocks, such as lignocellulose biomass and woods. Lignocellulose biomass and woods are abundantly available throughout the world, and they can serve as a continuous supply of raw materials. Lignocellulose material is composed of three main compounds; cellulose, hemicellulose, and lignin. The presence of lignin in lignocellulose materials enables it to be liquefied in phe-
∗ Corresponding author. Tel.: +60 3 8921 3261; fax: +60 3 8921 3777. E-mail address: [email protected] (S. Zakaria).
nol, as lignin contains phenolic derivatives, such as sinapyl alcohol, coumaryl alcohol, coniferyl alcohol, etc (Effendi et al., 2008). Over the last few decades, several studies have been carried out to utilise wood and biomass as raw material in the synthesis of phenolic resin via a liquefaction process (Lin et al., 1994; Alma et al., 1998a,b; Lee et al., 2002). Liquefaction of biomass, as well as cellulose and wood, in the presence of phenol and acid (organic and inorganic) as catalysts, has been intensively studied (Alma et al., 1995a,b). Most biomass liquefaction reactions are carried out in the presence of an acid catalyst, which can significantly increase the conversion yield. Liquefaction under alkaline conditions indeed provides lower biomass residue content; however, it is not an effective catalyst to achieve a high amount of combined phenol (Alma et al., 1998a). During the liquefaction process,by using organic solvent, macromolecule compounds in biomass decompose into reactive small molecules via several processes, such as dehydration, dehydrogenation, deoxygenation, and decarboxylation (Lin et al., 2001, 2004). However, these small molecules will then re-polymerise through condensation, cyclisation, and polymerisation. Investigation into the liquefaction of oil palm empty fruit bunch (EFB) fibres with phenol in the presence of sulphuric acid has been carried out by Ahmadzadeh et al. (2008). In their study, they found that high liquefaction yield can be obtained with the phenol to EFB fibre weight ratio of 3:1. The study allowed them to obtain optimum parameters for satisfactory yield as follows: liquefaction
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time 60–90 min, temperature range of 130–150 ◦ C and catalyst concentration of 3–5%. In addition, the liquefied EFB (LEFB) was used as a phenolic precursor to prepare novolac type resin for moulding application. Meanwhile, Roslan et al. (2014) used the LEFB to prepare resol type phenolic resin and study its physical, chemical, and mechanical properties. From the chemical analysis, they found that LEFB at para position is more favoured for the reaction with formaldehyde when compared to the ortho position due to the steric effect. The plywood shear strength of the resol resin is also in compliance with the JIS K-6852 standard. Epoxidised natural rubber (ENR) is a chemically modified natural rubber which has epoxide groups grafted onto its polymeric chains by the epoxidation process (Ruamcharoen et al., 2011). These epoxides make ENR susceptible to various chemical reactions that involve hydrogen-donor molecules, such as alcohols, carboxylic acids, and amines (Phinyocheep et al., 2005). Commercially, there are two types of ENR, ENR-25 and ENR-50. The designated integers represent the mole percentage (%) epoxidation of the natural rubber. Miscibility of blends or compounds is attributed by molecular interactions, for example a donor–acceptor type of weakly acidic hydrogen or dipole–dipole interaction. Theoretically, ENR should be compatible with proton-donating polymers, such as phenolic resin (Kallitsis and Kalfoglou, 1989). Phenolic resin requires a toughening mechanism to improve its brittleness. An example of a phenolic resin toughening mechanism is the incorporation of rubber. The most common rubber used is nitrile rubber due to its compatibility (Kaynak and Cagatay, 2006). It is expected that the introduction of ENR will provide elastomeric properties and improve the impact resistance of the phenolic composites. However, little literature has reported on the topic of the utilisation of natural rubber as toughening agent in phenolic composites. Hence, the aim of this study is to investigate the mechanical properties and water absorption of ENR-toughened glass fibre reinforced bio-phenolic resin composites, which are referred to as bio-phenolic elastomer (BPE) composites. 2. Experimental 2.1. Materials Oil palm EFB fibres and E-glass fibres (woven and non-woven) were supplied by Szetech Engineering Sdn. Bhd and RTG Intech Sdn. Bhd., respectively. Natural rubbers with three different grades, SMR-L (Standard Malaysian Rubber Light), ENR-25 and ENR-50, were purchased from the Malaysian Rubber Board. The rubber grades represent 0%, 25% and 50% mole epoxidation, respectively. SMR-L (0% epoxidation) is a natural rubber before epoxidation was used as a control to study the effect of rubber epoxidation. Industrial grade phenol (98%) was supplied by Chemzone Sdn. Bhd. Analytical grade sulphuric acid (98%), formalin (37% formaldehyde), sodium hydroxide (NaOH), methanol and toluene were procured from Sigma–Aldrich.
paper (Whattman no. 1) to remove the residues. Later, the methanol added was evaporated from the mixture by using a rotary evaporator to obtain liquefied EFB (LEFB). 2.3. Resinification process A resinification reaction was carried out by reacting formalin with the LEFB to prepare BPR. The LEFB was reacted with formalin (37% formaldehyde solution) as the crosslinking agent with the formalin/LEFB weight ratio of 1.2 in an alkaline medium. Sodium hydroxide (40 wt%) aqueous solution, used to obtain alkaline medium, was added in two-steps process. During the first 60 min of the reaction, 5 wt% (based on LEFB weight) of NaOH solution was added, and the reaction temperature was set at 65 ◦ C. Following this, the reaction temperature was increased to 85 ◦ C and 20 wt% (based on LEFB weight) of NaOH solution was added, where it was held for another 60 min. All of the reactions were conducted in a reflux system. 2.4. Preparation of natural rubber (NR) and epoxidised natural rubber (ENR) solutions Ten grams of natural rubber (SMR-L) and epoxidised natural rubber (ENR-25 and ENR-50) were dissolved in 100 ml of toluene, separately, to produce rubber solutions with 10% solid rubber content. The process was conducted in a 1000 ml round bottom flask attached to a reflux condenser system equipped with an overhead stirrer to stir the mixture. The process was carried out at 80 ◦ C for 6 h in an oil bath. 2.5. Preparation of rubber/glass fibre prepregs To study the effects of the natural rubber degree of epoxidation on the properties of the composite, three prepregs with 5 wt% (from the weight of the woven glass fibres) of each SMR-L, ENR-25 and ENR-35 were prepared. An additional three prepregs with 15, 25 and 35 wt% (from the weight of woven glass fibres) of ENR-50 were prepared to study the effect of different rubber compositions on the composite properties. The rubber solution was applied on both sides of the woven glass fibres. During the application of the rubber solution, care was taken to match the exact required weight and thickness on each side of the prepregs to maintain a balanced composite design. Later, the rubber-coated glass fibres were dried in an oven at 60 ◦ C overnight to remove toluene. The thickness of the prepreg was measured and controlled by using an electronic micrometre. 2.6. Preparation of bio-phenolic elastomer (BPE) composites Composite boards were prepared by using a hand lay-up technique, followed by compression moulding. Total glass fibre loading
2.2. Liquefaction of EFB fibres Liquefaction of the EFB fibres was performed according to procedures reported upon in the previous studies (Ahmadzadeh et al., 2009 Zakaria et al., 2014) with slight modification. First, the EFB fibres were dried in an oven at 105 ◦ C for 24 h. The weight ratio of phenol to EFB fibres (P/EFB) of 3:1 and sulphuric acid (3% based on the phenol weight) were used in the liquefaction reaction. Liquefaction was carried out in an oil bath at a temperature of 150 ◦ C for 90 min. Black and viscous liquid was produced at the end of the reaction. This liquid was then diluted with 400 ml methanol. The mixture of black liquid and methanol was filtered with filter
Fig. 1. Lay-up arrangement.
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Table 1 Effects of natural rubber types on the mechanical properties of BPE composites. Types of rubber
Impact strength (kJ/m2 ) (±SD)
Ref. SMR-L ENR-25 ENR-50
72.40 56.01 71.59 74.04
± ± ± ±
2.12 4.06 3.59 2.64
Flexural strength (MPa) (±SD) 50.87 21.32 21.69 22.55
± ± ± ±
Flexural modulus (MPa) (±SD)
1.59 1.13 1.59 1.07
6212.30 1259.28 2949.03 1475.24
± ± ± ±
325.97 58.70 402.62 126.19
used was 40 wt% of total composite weight. The other 60 wt% comprised the total weight of resin and rubber solid content used. Each composite consisted of five layers of GF mats and two layers of NR/GF prepregs, as shown in Fig. 1. These lay-ups were kept in an oven at 60 ◦ C for 14 h to reach semi-cure stage. The semi-cured lay-ups were then hot-pressed at 105 ◦ C for 15 min in a rectangle mould with a dimension of 150 mm × 150 mm × 3 mm. The samples with 5% SMR-L, 5% ENR-25, and 5% ENR-50 were later referred to as composites SMR-L, ENR-25 and ENR-50, respectively. The same arrangement of composite without NR was fabricated as a reference sample and labelled as “Ref.”.
ically, the specimens were taken out and wiped with absorbent paper to remove the water on the specimen surfaces and then reweighed. The water absorptions of the samples were calculated by using Eq. (1):
2.7. Mechanical tests
3.1. Effects of types of natural rubber
Izod impact and flexural tests were carried out in accordance with ASTM D256 and D790, respectively. The impact test was performed on an Instron CEAST 9050 machine using notched specimens (notch depth: 2.5 mm) with an impact speed of 3.46 m/s and incident energy of 5.0 J. For the flexural test, three-point bending with a nominal specimen dimension of 90 mm × 13 mm × 3 mm was carried out by using a Universal Testing Machine (Testometric M500-50CT). The span between supports was 50.0 mm and the crosshead speed was 10 mm/min. Each value obtained represents the average of five specimens.
Table 1 demonstrates the effects of natural rubber types (SMRL, ENR-25, and ENR-50) on the mechanical properties of the BPE composites. In this case, the rubber composition was fixed at 5 wt% based on the weight of woven glass fibres. From the result, the impact strength of the Ref. sample is 72.40 kJ/m2 . After 5 wt% of SMR-L loaded, the impact strength of the SMR-L composite decreased to 56.01 kJ/m2 . This phenomenon occurred due to the incompatibility between the resin (polar material) and SMR-L natural rubber (non-polar materials). Similar results were observed in a study conducted by Thongnuanchan et al. (2007), when NR latex adhesives used as a binder for particleboards demonstrated that a transetherification reaction of methoxymethyl groups from hexamethoxymethylmelamine did not occur in the non-polar NR system. After the introduction of ENR-25 and ENR-50, the impact strength of the composites increased to 71.59 kJ/m2 and 74.04 kJ/m2 , respectively. Although the results have no significant effect if compared to the Ref. sample due to the large standard deviation, they show that the presence of the epoxides group allows the reaction between OH from the BPR and the ENR to take place. Again, this result is in a strong agreement with the previous finding in the literature on ENR-based adhesives (Thongnuanchan et al., 2007). Phenolic resin consists of an aromatic ring and two ortho- and parapositions for reactions in every phenol molecule. Besides, abundance of OH groups in BPR is capable to crosslink with the oxirane ring of the ENR molecules. Fig. 2 shows the scheme of probable crosslinking mechanism between BPR and ENR.
2.8. Morphological analysis The morphological structure of the BPE composites was investigated by using a scanning electron microscope (SEM Supra 55 VP Zeiss) to observe the fractography of the surface of the BPE composites and the interaction between glass fibres, ENR, and BPR. The impacted specimens were cut cross-sectionally at the notched parts using a sample cutter and coated with a thin gold layer prior to SEM analysis. 2.9. Water absorption Composite specimens with a dimension of 15 mm × 15 mm × 3 mm were used for the water absorption test. The specimens were then immersed in distilled water at a temperature of 25 ◦ C. Period-
Water absorption(%) =
W2 − W1 × 100 W1
(1)
where W1 is the initial weight of the sample before the immersion (g) and W2 is the weight of the sample after the immersion (g). 3. Results and discussion
Fig. 2. Scheme of probable crosslinking between BPR and ENR.
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Fig. 3. Fractographs of composites with different types of NR (a) Ref. (b) SMR-L (c) ENR-25 and (d) ENR-50.
Composites at different amounts (15%, 25%, and 35%) of SMR-L, ENR-25 and ENR-50 were prepared to study the effects of different rubber compositions. Unfortunately, as the composition of SMRL and ENR-25 increase, delamination occurs before the composite specimens are tested (there is no result shown). Delamination happens as SMR-L and ENR-25 show incompatibility with the resin as their compositions increase. Thus, both rubbers are not suitable to be used in the prepreg application. Nevertheless, ENR-50 showed strong compatibility with resin and formed rigid samples as its composition increased. The effects of ENR-50 composition are discussed in Section 3.2. SEM fractographs of composites with different types of natural rubber are shown in Fig. 3. For the Ref. sample in Fig. 3(a), it is observed that there is no BPR wetted on the glass fibre surface. Fig. 3(b) shows that the SMR-L remains on the glass fibre surface, but the resin does not attach to the rubber after impact force is applied. This is due to the incompatibility between SMR-L and BPR. It is noted that, the resin begins to wet the ENR-25 and ENR-50, as shown in Fig. 3(c) and (d), respectively. The compatibility between rubber and resin is solely dependent on the epoxides present in the ENR. As the mole % of epoxidation increases, the ability of the resin to wet the glass fibre prepreg also increases. Table 1 shows the flexural strength and modulus of the BPE composites produced with different types of rubber. From the results,
all BPE composites show reduction in both flexural strength and modulus in comparison with the Ref. sample. Flexural strength indicates an ability of a material to resist deformation under bending force whereas the modulus is heavily dependent on the ratio of stress over strain of that material. Elastomers possess elasticity that enables them to absorb forces while deforming. The results indicate that the BPE composites have turned out to be less rigid. As the composition of the rubber is constant, the flexural strength does not differ much. Nonetheless, there is a slight difference in modulus of the ENR-25 sample. Chemical and physical properties of the ENR change depending on the extent of the mole % of epoxidation. Flexural strength of ENR is decreased as the mole % of epoxidation increases (Baker et al., 1985). 3.2. Effects of rubber composition Table 2 shows the effects of the ENR-50 compositions on the mechanical properties of BPE composites. From the table, it can be seen that the impact strength of the BPE composite increases from 72.40 kJ/m2 to 128.65 kJ/m2 with the addition of 35 wt% of ENR-50. A significant increment on the impact strength can be observed at 15 wt% of ENR-50. The increase in the ENR-50 composition allowed more impact force to be absorbed through stress distribution by rubber particles present in the composite. In addition, more epox-
Table 2 Effects of ENR-50 compositions on the mechanical properties of BPE composites. ENR-50 composition (%)
Impact strength (kJ/m2 ) (±SD)
Flexural strength (MPa) (±SD)
Flexural modulus (MPa) (±SD)
Ref. 5 15 25 35
72.40 ± 2.12 74.04 ± 2.64 96.51 ± 1.72 114.27 ± 2.95 128.65 ± 5.07
50.87 ± 1.59 22.55 ± 1.07 29.59 ± 1.11 25.65 ± 0.82 24.64 ± 1.15
6212.30 ± 325.97 1475.24 ± 126.19 1555.13 ± 98.11 1235.12 ± 111.57 1386.36 ± 110.07
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Fig. 4. Fractographs of composites with different composition of ENR-50 (a) Ref (b) 5 wt% (c) 15 wt% (d) 25 wt% and (e) 35 wt%.
ides exist in the composites when the rubber composition increases which allows for greater interaction between the ENR-50 and BPR, resulting in greater rubber-resin compatibility. Fractographs in Fig. 4 portray the condition of the BPE composites at different ENR-50 compositions after being impacted. From the images, it can be clearly seen that more resin wets the surface of the glass fibre as the ENR-50 composition increases. This has proven that the ENR-50 has acted as a compatibiliser between the glass fibre and BPR. Furthermore, higher amounts of ENR-50 cause more rubber particles to squeezed into the voids formed through solvent evaporation during hotpressing, thereby, reducing the formation of voids. Lesser void content in the composites improves their impact strength. Flexural strength and modulus of the BPE composites at different compositions of ENR-50 are shown in Table 2. The flexural strength and modulus of all BPE composites samples are decreased when compared to Ref. sample when the ENR composition increases. The elasticity and resilient properties of rubber have altered the overall composite rigidity and provide an elastomeric property. This leads to a reduction in the composites’ flexural strength and modulus. There is no pattern or significant dif-
ference in flexural properties of the composites associated with the addition of different amounts of ENR because they are anisotropic material. So, the flexural load applied to the samples is unequal at different axes where it is measured. However, the results show that 15% ENR composition possesses the highest flexural strength and modulus, which is 29.59 MPa and 1555.13 MPa, respectively. It may be at this point that the amount of rubber affords the highest rigidity and optimum compatibility for the ENR-50 BPE composites. 3.3. Water absorption test Fig. 5 presents the water absorption of BPE composites at different ENR-50 compositions. Curing of phenolic resin leaves microvoids that contribute to the sites for water absorption in composite material. Absorbed water trapped in a composite body may deter its performance for a period of time. It is very crucial to reduce the water intake of the composite to prevent this problem from occuring. In the Ref. composite sample, total water uptake is 11.25%. The water absorption by the BPE composites decreases as the ENR-50 composition increases. At 35 wt% of ENR-50, the total water absorbed by the composite sample is 5.95%. Approximately
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Acknowledgements We acknowledge the financial support from research grants PHI-2013-001, UKM-ST-07-FRGS0232-2012 and UKM-DIP-2014013. References
Fig. 5. Water absorption of composites with different composition of ENR-50.
47% water uptake has been reduced by the addition of 35 wt% of ENR-50, when compared to the Ref. sample. This happen as many microvoids are lost due to the formation of a compact composite structure and the ENR-50 prepregs have also protected some areas from allowing water to diffuse in the composite sample. 4. Conclusions In this study, BPE composites were prepared by using BPR with the incorporation of natural rubber and different types of epoxidised natural rubber (ENR) prepregs. ENR was added to provide an elastomeric property to the brittle thermosetting resin. The results demonstrated that the addition of SMR-L into the composite decreased its impact strength due to the incompatibility between the rubber and the resin. However, after the introduction of ENR-25 and ENR-50, the impact strength of the composites was increased. In addition, investigation into the effects of different ENR-50 compositions showed similar results as the impact strength increased from 72.40 kJ/m2 to 128.65 kJ/m2 . These findings proved that the epoxides group in the ENR backbone can provide compatibility by allowing the rubber-resin interaction. This was also supported by the results obtained via SEM images. Flexural properties of the BPE composites were found to be decreased when rubber was introduced. This indicated that the fabricated composites possessed elastomeric properties. Meanwhile, the water absorption test showed that the addition of ENR has a great effect in reducing the total percentage of water absorbed by the BPE composites.
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Please cite this article in press as: Amran, U.A., et al., Mechanical properties and water absorption of glass fibre reinforced bio-phenolic elastomer (BPE) composite. Ind. Crops Prod. (2015), http://dx.doi.org/10.1016/j.indcrop.2015.01.054