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Physico-mechanical properties of resol phenolic adhesives derived from liquefaction of oil palm empty fruit bunch fibres
Industrial Crops and Products 62 (2014) 119–124
Contents lists available at ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Physico-mechanical properties of resol phenolic adhesives derived from liquefaction of oil palm empty fruit bunch fibres Rasidi Roslan a , Sarani Zakaria a,∗ , Chin Hua Chia a , Ricarda Boehm b,c , Marie-Pierre Laborie b,c a
School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia University of Freiburg, Chair of Forest Biomaterials, Werthmannstr. 6, 79085 Freiburg, Germany c Freiburg Materials Research Center (FMF), University of Freiburg, Stefan-Meier Str. 21, 79104 Freiburg, Germany b
a r t i c l e
i n f o
Article history: Received 14 April 2014 Received in revised form 12 August 2014 Accepted 14 August 2014 Keywords: Phenol–formaldehyde Liquefaction Shear strength 13 C NMR Thermal analysis
a b s t r a c t Utilization of oil palm empty fruit bunch (EFB) fibres in the production of phenolic (PF) resin is an alternative way to reduce the dependency of petroleum-based phenol. In this study, resol-type phenolic resin was synthesized from oil palm EFB fibres via liquefaction process using phenol and sulfuric acid, followed by resinification reaction with formaldehyde in alkaline condition. The increase of the ratio of formaldehyde/liquefied EFB (F/LEFB ) results in the increase in the viscosity and molecular weight of the produced PF resin. The obtained FTIR spectra confirmed that the chemical functionalities of the produced PF resin are almost similar to that of commercial PF resin. The NMR analyses indicated that the phenol para position was favoured for the reaction over the ortho position in both commercial resin and resin synthesized due to the existence of steric hindrance effect from the hydroxyl phenol and phenol derivatives of liquefied EFB. The shear strength of the produced PF resins is fulfilling the requirement as specified in JIS K-6852. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Thermosetting resins play a vital role in various industries due to their excellent properties including modulus, strength, durability, thermal and chemical resistance (Raquez et al., 2010). Phenolic resins (PF) remain of high commercial and industrial interest despite the emergence of new thermosets and high performance polymers (Langenberg et al., 2010). PF resins are widely used as an adhesive for wood-based panels and other engineering products (Schmidt and Frazier, 1998). However, there are concerns regarding the chemicals used in the production of phenolic resins, i.e., phenol and formaldehyde, which are expensive and harmful, leading to increasing interest for alternatives derived from lignocellulosic biomass (Jang et al., 2011). PF resins are produced via polycondensation of phenol with formaldehyde. It is the first synthetic polymer developed commercially in 1908 (Pizzi, 2003). It can be divided into two types, i.e. novolac and resol. Chemical properties of PF depend on various key factors, such as formaldehyde to phenol molar ratio and catalyst, during the synthesis process. Acid catalysis produces
∗ Corresponding author. Tel.: +60 3 8921 3261; fax: +60 3 8921 3777. E-mail address: [email protected] (S. Zakaria). http://dx.doi.org/10.1016/j.indcrop.2014.08.024 0926-6690/© 2014 Elsevier B.V. All rights reserved.
thermoplastic phenolic resins called novolacs, while resol type PF resins are produced in the presence of alkaline catalysts. Resol PF is classified as thermosetting resins because it is sensitive to heat (Pan, 2011). There are three differences between the reaction of formaldehyde with phenol in acidic and alkaline conditions. The first one is the rate of aldehyde attacks on phenol, second is the subsequent condensation of phenolic alcohols, and third is in the nature of the condensation reaction (Pizzi, 2003). Lignocellulosic biomass is well recognized as the world’s most abundant and promising material to substitute petroleum-based chemicals due to its similarity in chemical structure. It comprises three main components i.e. cellulose, hemicelluloses and lignin. Over the past decade, several attempts to use lignocellulosic biomass such as wood (Kunaver et al., 2010), bark (Yuan et al., 2009), lignin and tannins (Vázquez et al., 2012) have been done to reduce the consumption of petroleum-based phenol in the production PF resins. By the utilizing of biomass, the issues related to petroleum sustainability can be prevented and bio-products which meet the needs of the present generation without compromising the needs of future generations can be obtained. Apart from the fluctuation of petroleum prices, there are also other problems that arise, such as the management of waste from wood and agricultural industries. Utilization of these wastes to produce highvalue products is capable to reduce the impact of pollution on the
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environment and also enhance the well-being of future development (Langenberg et al., 2010). Malaysia is one of the world largest plywood manufacturers, which consumes great amount of PF adhesives. Hence, the introduction of phenolic derivatives from lignin in biomass for the preparation of phenolic resins can help reduce the dependency on petroleum-based phenol. One of the most important agricultural industries in Malaysia is palm oil refinery. According to the Malaysian Palm Oil Board (MPOB), palm trees were cultivated on 5.2 million hectares in 2013. After palm oil extraction process, waste by-product such as empty fruit bunch (EFB), amount to 70–80 million tons per annum (MPOB, 2013). Therefore, several approaches have been developed to utilize EFB fibres for value added products, such as pulp (Jiménez et al., 2009), composite board (Chai et al., 2009), activated carbon (Alam et al., 2009), bio-oil (Fan et al., 2011), bio-adsorbent (Sajab et al., 2013), etc. The aim of this work is to study the utilization of oil palm EFB fibres to produce resol-type PF resins adhesive for plywood application. The EFB fibres were liquefied using phenol as a liquefying reagent in the presence of sulfuric acid as a catalyst and then reacted with formaldehyde under alkaline condition to produce resol PF resin at a moderate temperature. The produced PF resin was applied on plywood to study the shear strength of the products. 2. Experimental 2.1. Materials Oil palm EFB fibres were purchased from Szetech Engineering Sdn. Bhd. The commercial phenolic (PF Com) resin used as a comparison in this study and industrial grade phenol was supplied by Malayan Adhesives and Chemicals (MAC) Sdn. Bhd. Analytical grade sulfuric acid (98%), methanol, formaldehyde (37%), NaOH, pyridine, and acetic anhydride were purchased from Sigma-Aldrich. All chemicals were used without purification.
2.4. Characterizations The Fourier Transform IR (FTIR) spectra of the PF samples were recorded using a Perkin-Elmer FTIR-ATR spectrometer with a resolution of 1 cm−1 . The chemical shifts of the PF produced were characterized with a liquid-state 13 Carbon Nuclear Magnetic Resonance (13 C NMR) (advance 111, 600 MHz, Bruker) in DMSO-d6 . The molecular weight of the PF Com and PF produced were analyzed on Gel Permeation Chromatography (GPC) (RI detector at 30 ◦ C; UV detector 254 nm; PSS SDV column at 30 ◦ C) using THF as the eluent with flow rate of 1 ml/min. Before GPC analysis, the PF resins were acetylated with 1:1 pyridine and acetic anhydride. Thermogravimetric analysis (TGA) was performed using a Mettler-Toledo (SDTA 851e) thermal analyzer. Approximately 15 mg of PF resin was placed in a 30 l aluminium crucible and subjected to heating in a nitrogen atmosphere at a heating rate of 10 ◦ C/min from 25 to 600 ◦ C. 2.5. Shear strength Shorea sp. plywood was used as wood testing panel. The sample was cut into strips (90 mm × 25.4 mm × 2.5 mm) and conditioned at 23 ± 1 ◦ C for at least seven days before used. After that, the PF Com and PF produced were spread on one side of the sample at amount of 0.035 g/cm2 (on solid basis) in an area of 25.4 mm × 90 mm. Then, the adhesives coated plywood sample was overlapped with uncoated plywood with the length direction parallel to the wood grain. The resulting two layered panel was then clamped with Gclamp for three days and followed by conditioning at temperature of 23 ± 1 ◦ C at a relative humidity of 50% either for seven days, or until they attain a constant mass, whichever is the longer period. The evaluation of bonding strength was carried out according to Japanese Industrial Standard (JIS K-6852) using a Universal Testing Machine (Testometric M500-50CT) until failure at a force of 9.7 MPa of shear area per minute (approximately 1.3 mm/min crosshead speed).
2.2. Liquefaction of EFB fibres
3. Results and discussion
The liquefaction of EFB fibres was conducted according to method reported previously with modification (Ahmadzadeh et al., 2009; Zakaria et al., 2014). Firstly, the EFB fibres were dried in an oven at 105 ◦ C for 24 h. After that, liquefaction of the EFB fibres was carried out at weight ratio of phenol to EFB (P/E) 3:1 in the presence of sulfuric acid (3% based on phenol weight) as the catalyst. This reaction was conducted at 150 ◦ C for 120 min in an oil bath. The liquefied mixture was then diluted in 400 ml methanol and filtered with filter paper (Whatman No. 1) to separate the un-liquefied fractions. Evaporation with rotary evaporator was conducted to recover the methanol added.
3.1. Characterizations of the PF resin
2.3. Resin synthesis Resinification was carried out by mixing the liquefied EFB (LEFB ) mixture and formaldehyde in a round bottom flask at different weight ratios of formaldehyde to LEFB (1.8:1, 2.0:1, and 2.2:1) under alkaline condition. Alkaline condition in the reaction was controlled by two steps addition of 40% aqueous NaOH. The first addition of NaOH (0.5 g) is to neutralize the acid catalyst added during the liquefaction process and the second addition of NaOH (4.5 g) is to maintain the reaction in alkaline condition. The temperature was set at 65 ◦ C for the first 60 min and then increased to 85 ◦ C after the second addition of NaOH solution for another 120 min. After the reaction completed, the product (designated as PF 1.8, PF 2.0 and PF 2.2) were cooled to room temperature and kept in the refrigerator for further characterization.
The physical properties of the PF produced from the liquefied EFB are shown in Table 1. The pH value of the PF resin is lower as compared to the PF Com resin. This may be attributed to the presence of residual H2 SO4 , which was used as a catalyst during the liquefaction process has reduced the pH of the resinification mixture. The solid content of each PF resin produced is almost similar, but slightly lower than that of the commercial PF resin, which may probably due to the existence of urea in the commercial PF resin (Zhao et al., 2010). Meanwhile, the viscosity and molecular weight of the PF produced resins is higher as compared to the commercial PF resin. The results show that the amount of formaldehyde used in the reaction has influenced the viscosity and molecular weight of the PF produced tremendously. It is well known that resin viscosity and molecular weight of PF resin are influenced by non-volatile chemicals, temperature and F/P ratio (Haupt and Sellers, 1994). Higher formaldehyde content might result in the increase of viscosity for each resin produced because higher content of formaldehyde tends to speed up the polymerization process. Besides, the presence of larger molecule in the liquefied EFB in the resin also will increase the viscosity and molecular weight of the PF resin Fig. 1. 3.2. Chemical functionality analysis by FTIR Fig. 1 illustrates the FTIR spectra of the PF Com and PF samples produced using different ratios of F/LEFB (PF 1.8, PF 2.0 and PF 2.2).
R. Roslan et al. / Industrial Crops and Products 62 (2014) 119–124
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Table 1 The physical properties of the produced PF resin.
PF 1.8 PF 2.0 PF 2.2 Com. PF
pH
Solid content (%)
Viscosity (cP)
Mw (Da)
Mn (Da)
Mw /Mn
9.43 9.59 9.75 11.16
49.50 50.28 50.46 64.37
193.3 311.5 441.7 200.0
1796.1 3066.5 3159.4 327.3
965.1 1150.5 1166.5 212.1
1.86 2.66 2.99 1.82
Table 2 Functional groups and observed wavenumbers for commercial phenolic resins compared to the synthesized PF.
Fig. 1. FTIR absorption spectrum for a commercial phenolic resin and for bio-based phenolic resins synthesized at different weight ratios.
Commercial PF [cm−1 ]
Synthesized PF [cm−1 ]
Functional group
3300 2850
3301 2877
1620 1470
1626 1482
1446
1447
OH Out of phase stretching vibration of CH2 alkane C C aromatic ring Methylene bridges, ortho-para Methylene bridges, para-para
1266 1202
1223
1155 1051
1153 1108
1010
1009
964
981
882
884 755
The IR absorbance bands obtained from the PF resins are identical to those of the commercial PF resin, as summarized in Table 2. The IR band at around 1447 cm−1 is assigned to the methylene bridges at the para-para position, while the small absorbance band at 1482 cm−1 is assigned to the methylene bridges at the orthopara position. The intensity of these bands increased when the F/LEFB weight ratio increased, suggesting the greater crosslinking degree at higher formaldehyde concentration. It has been proposed that, there are two types of condensation reactions that occur during the resinification reaction, i.e., condensation reaction between hydroxymethyl groups themselves forming dimethylene ether linkages and at aromatic free positions to produce methylene bridges (Chen et al., 2012). In this case, the second reaction become dominant as can be observed from the increase of the intensities of
Fig. 2.
13
Asymmetric stretch of phenolic C C OH C O stretch Dimethylene ether C O stretch O H deformation stretch in hydroxymethyl groups 1,2,4-trisubstituted benzene ring 1,2,4,6 – tetrasubstituted benzene ring 1,2 – disubstituted benzene ring 1,2,6 – trisubstituted benzene ring
the band of the methylene bridges, while no changes to the band of the dimethylene ether peak at 1108 cm−1 and 1153 cm−1 . 3.3.
13 C
NMR analysis
The liquid-state 13 C NMR spectra of the PF Com and produced PF resin are shown in Figs. 2–5. The chemical shift assignment is made according to results presented in the previous studies (Werstler, 1986; Zhao et al., 2012). The chemical shift observed at around 167–168 ppm represents the carbonyl groups in the PF resins. The chemical shift in the region of 54–55 ppm is attributed to
C NMR spectra of commercial phenolic resin.
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Fig. 3.
13
C NMR spectra of PF 1.8.
Fig. 4.
13
C NMR spectra of PF 2.0.
the methoxyl groups in the liquefied EFB component. The result was in good agreement with those obtained by Zhao et al. (2012) and it is also confirmed when no peak appears in this region for PF Com sample. The un-reacted formaldehyde chemical shift is detected at 82.3 ppm. According to Werstler (1986), the peak around
Fig. 5.
13
84.6–85.2 ppm is related to hemiacetals from paraformaldehyde or oligomers of oxymethylene. As the PF reaction proceeds, these oligomers will break down as free formaldehyde. The chemical shift at 89.8 ppm and at 63–64.4 ppm is attributed to the addition reaction product of ortho-hemiformal (carbon connected to
C NMR spectra of PF 2.2.
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Fig. 6. TGA weight loses curves for the phenolic resins produced at different weight ratios.
the hydroxyl Ph-CH2 OCH2 OH) and para-hemiformal (carbon connected to the hydroxyl Ph-CH2 OCH2 OH) respectively. In addition, the chemical shift of ortho- and para- Ph- CH2 OCH2 OCH2 OH are also observed at 92.2–93.5 ppm. Some peaks are not observed in these spectra, for example from 69–73 ppm and 29–30 ppm which corresponding to the methylene ether bridge and ortho-ortho methylene groups respectively. The para-para methylene bridges are clearly visible at 38–40 ppm while the ortho-para chemical shifts appear only as a small shoulder at 35.10 ppm. It is comprehensible that the ortho-ortho methylene links are not observed because the reactions are not favoured in the NaOH system (Zhao et al., 2012). Meanwhile, in the case of para-para and ortho-para methylene bridges, phenol para position was dominant for reaction over the ortho position on a per site basis most likely due to steric hindrance at the ortho position. In the spectra of the produced PF resins using different ratio of formaldehyde, the chemical shift at 115–117 ppm and 120–124 ppm, which corresponds to the unsubstituted ortho phenol aromatic and para phenol aromatic respectively, are absent, suggesting that all the reactive sites are completely reacted. Meanwhile, the intensity of the chemical shift for substituted ortho and para phenol aromatic at around 126–130 ppm is decreased as the ratio of formaldehyde increased. Since all the unsubstituted ortho and para phenol aromatic are absent, it is expected that the peak intensities in these region would have increased but conversely. This could probably be due to higher amount of water in the formalin added during the resinification process. The higher water content might slow down the condensation reaction in which higher energy is required for the resinification process. 3.4. Thermal analysis Fig. 6 shows thermogravimetric curves of the PF resins with the data summarized in Table 3. There are three main thermal decomposition regions. The first decomposition occurred from 80 to 150 ◦ C due to the removal of low molecular weight components, such as water, free phenol, and formaldehyde (Hu et al., 2011). The second decomposition occurred from 150 to 350 ◦ C can be attributed to the release of carbon monoxide, carbon dioxide, and methylene, suggesting the elimination of carbonyl, diphenyl ether and hydroxymethyl groups as the result of the cleavage of methylene bridges. As the temperature increased, the ratio of the concentration of diphenyl ether link structure of phenol also increased. The structure of the diphenyl ether link will form
123
Fig. 7. Shear strength of plywood bonded with resol-type adhesives from liquefied EFB at different weight ratios of F/LEFB .
an intermediate structure generated for phenolic resins during thermal degradation. The third decomposition at the temperature ranging from 350 to 600 ◦ C is due to thermal pyrolysis of methylene bridges in ortho-para and para-para position. After the curing process, PF structures were mainly methylene bridged phenolic units. These crosslinking networks begin to degrade with further temperature increment (Chen et al., 2012). There is no significant changes occurred on the decomposition temperature when the PF ratio increased. However, the char yield at the end of the analysis show interesting results. When the PF ratio increased from 1.8 to 2.2, the char yield were also increases. This results shows that the addition of formaldehyde are able to produce a product that has a high thermal stability.
3.5. Adhesives shear strength test results The shear test results are shown in Fig. 7. The specimens bonded with PF Com resin gives the highest shear strength value of 2.93 MPa, while the shear strength of the produced PF resin increased from 1.70 to 2.53 MPa as the weight ratio of F/LEFB increased from 1.8 to 2.2. There are many factors that will influence the shear strength of the adhesive, including the formaldehyde to phenol (F/P) molar ratio and the viscosity of the adhesive. By altering the F/P molar ratio, it will affect the crosslinking degree or curing reaction. At higher F/P molar ratio, the crosslinking rate will increase because there are high amounts of phenolic components which having functional groups more than three (Alma and Basturk, 2006). Meanwhile, as for the viscosity, several aspects will be affected such as wetting, flow and penetration of the adhesive. Adhesive at higher viscosity may not flow well to spread on the surface area in a reasonable time frame. At a lower viscosity, the adhesive flow better and wet more surface area, but again have a tendency to over-penetrate into the wood specimens. Overpenetration of adhesive will lead to insufficient adhesive in the bond-line hence resulting in a starved joint (Alma and Basturk, 2006; Baldan, 2012). In Table 1, we can see that the increase of the F/LEFB weight ratio has resulted in the increase of the adhesive viscosity, which increased the shear strength. Although the shear strength of the PF resin produced from the liquefied EFB is slightly lower than that of the PF Com, the strength values exceed the minimum strength as specified in JIS K-6852 (Standard for the tension shear strength of the resol-type adhesive), i.e., 1.18 MPa.
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Table 3 TGA thermal decomposition, weight loss, and char yield of phenolic resins produced at different weight ratios. PF samples
PF 1.8 PF 2.0 PF 2.2
Decomposition temperature (◦ C)
Weight loss (%)
Char yield (%)
1st region
2nd region
3rd region
1st region
2nd region
3rd region
80–150 80–150 80–150
150–350 150–350 150–350
350–600 350–600 350–600
7.5 7.4 6.1
22.3 21.2 20.0
40.1 37.4 34.9
4. Conclusions Resol type PF resins were successfully produced from oil palm EFB fibres via liquefaction and resinification processes. The increasing of weight ratio of F/LEFB resulted in the increase of viscosity and higher molecular weight of the resins produced. The functional groups of the bio-based PF resins are identical to those of a commercial PF resin. Based on the results obtained from the 13 C NMR spectroscopy, the phenol para position was favoured for the reaction over the ortho position due to the existence of steric hindrance effect from the hydroxyl phenol and phenol derivatives of liquefied EFB. The shear strength of all the liquefied PF resins fulfils the requirement of JIS K-6852. Acknowledgements We acknowledge the financial support from the project UKM-ST-07-FRGS0232-2012 and UKM-DIP-2012-034. The authors would also like to acknowledge Dr. Nor Yuziah Mohd. Yunus from MAC for kindly supply the commercial phenolic resins. Acknowledgement also is given to the Centre for Research and Instrumentation (CRIM) for providing the instruments for analysis. We also cordially thank E. Stibal for technical assistance throughout the project. References Ahmadzadeh, A., Zakaria, S., Rashid, R., 2009. Liquefaction of oil palm empty fruit bunch (EFB) into phenol and characterization of phenolated EFB resin. Ind. Crops Prod. 30, 54–58. Alam, M.Z., Ameem, E.S., Muyibi, S.A., Kabbashi, N.A., 2009. The factors affecting the performance of activated carbon prepared from oil palm empty fruit bunches for adsorption of phenol. Chem. Eng. J. 155, 191–198. Alma, M.H., Basturk, M.A., 2006. Liquefaction of grapevine cane (Vitis vinisera L.) waste and its application to phenol–formaldehyde type adhesive. Ind. Crops Prod. 24, 171–176. Baldan, A., 2012. Adhesion phenomena in bonded joints. Int. J. Adhes. Adhes. 38, 95–116. Chai, L.-L., Zakaria, S., Chia, C.H., Nabihah, S., Rashid, R., 2009. Physico-mechanical properties of PF composite board from EFB fibres using liquefaction technique. Iran. Polym. J. 18, 917–923. Chen, Z., Zeng, W., Chen, Y., Li, W., Liu, H., 2012. Influence of F/P on structure and thermal property of phenolic resin. Key Eng. Mater. 500, 98–103.
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Fan, S.-P., Zakaria, S., Chia, C.-H., Jamaluddin, F., Nabihah, S., Liew, T.-K., Pua, F.-L., 2011. Comparative studies of products obtained from solvolysis liquefaction of oil palm empty fruit bunch fibres using different solvents. Bioresour. Technol. 102, 3521–3526. Haupt, R.A., Sellers Jr., T., 1994. Characterizations of phenol–formaldehyde resol resins. Ind. Eng. Chem. Res. 33, 693–697. Hu, L., Zhou, Y., Zhang, M., Liu, R., 2011. Characterization and properties of a lignosulfonate-based phenolic foam. BioResources 7, 554–564. Jang, Y., Huang, J., Li, K., 2011. A new formaldehyde-free wood adhesive from renewable materials. Int. J. Adhes. Adhes. 31, 754–759. Jiménez, L., Serrano, L., Rodríguez, A., Sánchez, R., 2009. Soda-anthraquinone pulping of palm oil empty fruit bunches and beating of the resulting pulp. Bioresour. Technol. 100, 1262–1267. ˇ ˙ E., Poljanˇsek, I., Strnad, T., 2010. AppliKunaver, M., Medved, S., Cuk, N., Jasiukaityte, cation of liquefied wood as a new particle board adhesive system. Bioresour. Technol. 101, 1361–1368. Langenberg, K.V., Grigsby, W., Ryan, G., 2010. Green Adhesives: Options for the Australian Industry – Summary of Recent Research into Green Adhesives from Renewable Materials and Identification of Those that are Closest to Commercial Uptake. Forest & Wood Products, Australia. MPOB, 2013. Malaysian Oil Palm Statistics. Department of Statistics Malaysia http://www.mpob.gov.my Pan, H., 2011. Synthesis of polymers from organic solvent liquefied biomass: a review. Renew. Sustain. Energy Rev. 15, 3454–3463. Pizzi, A., 2003. Phenolic Resin Adhesives. In: Pizzi, A., Mittal, K.L. (Eds.), Handbook of Adhesive Technology. , 2nd ed. Marcel Dekker, New York, pp. 541–571. Raquez, J.-M., Deléglise, M., Lacrampe, M.-F., Krawczak, P., 2010. Thermosetting (bio)materials derived from renewable resources: a critical review. Prog. Polym. Sci. 35, 487–509. Sajab, M.S., Chia, C.H., Zakaria, S., Khiew, P.S., 2013. Cationic and anionic modifications of oil palm empty fruit bunch fibers for the removal of dyes from aqueous solutions. Bioresour. Technol. 128, 571–577. Schmidt, R.G., Frazier, C.E., 1998. Network characterization of phenol–formaldehyde thermosetting wood adhesive. Int. J. Adhes. Adhes. 18, 139–146. Vázquez, G., Santos, J., Freire, M., Antorrena, G., González-Álvarez, J., 2012. DSC and DMA study of chestnut shell tannins for their application as wood adhesives without formaldehyde emission. J. Therm. Anal. Calorim. 108, 605–611. Werstler, D.D., 1986. Quantitative 13C n.m.r. characterization of aqueous formaldehyde resins: 1. Phenol–formaldehyde resins. Polymer 27, 750–756. Yuan, J., Gao, Z., Wang, X.-M., 2009. Phenolated larch-bark formaldehyde adhesive with various amounts of sodium hydroxide. Pigment Resin Technol. 38, 290–297. Zakaria, S., Roslan, R., Amran, U.A., Chia, C.-H., Bakaruddin, S.B., 2014. Characterization of residue from EFB and kenaf core fibers in the liquefaction process. Sains Malaysia. 43, 429–435. Zhao, Y., Yan, N., Feng, M., 2010. Characterization of phenol–formaldehyde resins derived from liquefied lodgepole pine barks. Int. J. Adhes. Adhes. 30, 689–695. Zhao, Y., Yan, N., Feng, M.W., 2012. Biobased phenol formaldehyde resins derived from beetle-infested pine barks—structure and composition. ACS Sustain. Chem. Eng. 1, 91–101.
Contents lists available at ScienceDirect
Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Physico-mechanical properties of resol phenolic adhesives derived from liquefaction of oil palm empty fruit bunch fibres Rasidi Roslan a , Sarani Zakaria a,∗ , Chin Hua Chia a , Ricarda Boehm b,c , Marie-Pierre Laborie b,c a
School of Applied Physics, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia University of Freiburg, Chair of Forest Biomaterials, Werthmannstr. 6, 79085 Freiburg, Germany c Freiburg Materials Research Center (FMF), University of Freiburg, Stefan-Meier Str. 21, 79104 Freiburg, Germany b
a r t i c l e
i n f o
Article history: Received 14 April 2014 Received in revised form 12 August 2014 Accepted 14 August 2014 Keywords: Phenol–formaldehyde Liquefaction Shear strength 13 C NMR Thermal analysis
a b s t r a c t Utilization of oil palm empty fruit bunch (EFB) fibres in the production of phenolic (PF) resin is an alternative way to reduce the dependency of petroleum-based phenol. In this study, resol-type phenolic resin was synthesized from oil palm EFB fibres via liquefaction process using phenol and sulfuric acid, followed by resinification reaction with formaldehyde in alkaline condition. The increase of the ratio of formaldehyde/liquefied EFB (F/LEFB ) results in the increase in the viscosity and molecular weight of the produced PF resin. The obtained FTIR spectra confirmed that the chemical functionalities of the produced PF resin are almost similar to that of commercial PF resin. The NMR analyses indicated that the phenol para position was favoured for the reaction over the ortho position in both commercial resin and resin synthesized due to the existence of steric hindrance effect from the hydroxyl phenol and phenol derivatives of liquefied EFB. The shear strength of the produced PF resins is fulfilling the requirement as specified in JIS K-6852. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Thermosetting resins play a vital role in various industries due to their excellent properties including modulus, strength, durability, thermal and chemical resistance (Raquez et al., 2010). Phenolic resins (PF) remain of high commercial and industrial interest despite the emergence of new thermosets and high performance polymers (Langenberg et al., 2010). PF resins are widely used as an adhesive for wood-based panels and other engineering products (Schmidt and Frazier, 1998). However, there are concerns regarding the chemicals used in the production of phenolic resins, i.e., phenol and formaldehyde, which are expensive and harmful, leading to increasing interest for alternatives derived from lignocellulosic biomass (Jang et al., 2011). PF resins are produced via polycondensation of phenol with formaldehyde. It is the first synthetic polymer developed commercially in 1908 (Pizzi, 2003). It can be divided into two types, i.e. novolac and resol. Chemical properties of PF depend on various key factors, such as formaldehyde to phenol molar ratio and catalyst, during the synthesis process. Acid catalysis produces
∗ Corresponding author. Tel.: +60 3 8921 3261; fax: +60 3 8921 3777. E-mail address: [email protected] (S. Zakaria). http://dx.doi.org/10.1016/j.indcrop.2014.08.024 0926-6690/© 2014 Elsevier B.V. All rights reserved.
thermoplastic phenolic resins called novolacs, while resol type PF resins are produced in the presence of alkaline catalysts. Resol PF is classified as thermosetting resins because it is sensitive to heat (Pan, 2011). There are three differences between the reaction of formaldehyde with phenol in acidic and alkaline conditions. The first one is the rate of aldehyde attacks on phenol, second is the subsequent condensation of phenolic alcohols, and third is in the nature of the condensation reaction (Pizzi, 2003). Lignocellulosic biomass is well recognized as the world’s most abundant and promising material to substitute petroleum-based chemicals due to its similarity in chemical structure. It comprises three main components i.e. cellulose, hemicelluloses and lignin. Over the past decade, several attempts to use lignocellulosic biomass such as wood (Kunaver et al., 2010), bark (Yuan et al., 2009), lignin and tannins (Vázquez et al., 2012) have been done to reduce the consumption of petroleum-based phenol in the production PF resins. By the utilizing of biomass, the issues related to petroleum sustainability can be prevented and bio-products which meet the needs of the present generation without compromising the needs of future generations can be obtained. Apart from the fluctuation of petroleum prices, there are also other problems that arise, such as the management of waste from wood and agricultural industries. Utilization of these wastes to produce highvalue products is capable to reduce the impact of pollution on the
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environment and also enhance the well-being of future development (Langenberg et al., 2010). Malaysia is one of the world largest plywood manufacturers, which consumes great amount of PF adhesives. Hence, the introduction of phenolic derivatives from lignin in biomass for the preparation of phenolic resins can help reduce the dependency on petroleum-based phenol. One of the most important agricultural industries in Malaysia is palm oil refinery. According to the Malaysian Palm Oil Board (MPOB), palm trees were cultivated on 5.2 million hectares in 2013. After palm oil extraction process, waste by-product such as empty fruit bunch (EFB), amount to 70–80 million tons per annum (MPOB, 2013). Therefore, several approaches have been developed to utilize EFB fibres for value added products, such as pulp (Jiménez et al., 2009), composite board (Chai et al., 2009), activated carbon (Alam et al., 2009), bio-oil (Fan et al., 2011), bio-adsorbent (Sajab et al., 2013), etc. The aim of this work is to study the utilization of oil palm EFB fibres to produce resol-type PF resins adhesive for plywood application. The EFB fibres were liquefied using phenol as a liquefying reagent in the presence of sulfuric acid as a catalyst and then reacted with formaldehyde under alkaline condition to produce resol PF resin at a moderate temperature. The produced PF resin was applied on plywood to study the shear strength of the products. 2. Experimental 2.1. Materials Oil palm EFB fibres were purchased from Szetech Engineering Sdn. Bhd. The commercial phenolic (PF Com) resin used as a comparison in this study and industrial grade phenol was supplied by Malayan Adhesives and Chemicals (MAC) Sdn. Bhd. Analytical grade sulfuric acid (98%), methanol, formaldehyde (37%), NaOH, pyridine, and acetic anhydride were purchased from Sigma-Aldrich. All chemicals were used without purification.
2.4. Characterizations The Fourier Transform IR (FTIR) spectra of the PF samples were recorded using a Perkin-Elmer FTIR-ATR spectrometer with a resolution of 1 cm−1 . The chemical shifts of the PF produced were characterized with a liquid-state 13 Carbon Nuclear Magnetic Resonance (13 C NMR) (advance 111, 600 MHz, Bruker) in DMSO-d6 . The molecular weight of the PF Com and PF produced were analyzed on Gel Permeation Chromatography (GPC) (RI detector at 30 ◦ C; UV detector 254 nm; PSS SDV column at 30 ◦ C) using THF as the eluent with flow rate of 1 ml/min. Before GPC analysis, the PF resins were acetylated with 1:1 pyridine and acetic anhydride. Thermogravimetric analysis (TGA) was performed using a Mettler-Toledo (SDTA 851e) thermal analyzer. Approximately 15 mg of PF resin was placed in a 30 l aluminium crucible and subjected to heating in a nitrogen atmosphere at a heating rate of 10 ◦ C/min from 25 to 600 ◦ C. 2.5. Shear strength Shorea sp. plywood was used as wood testing panel. The sample was cut into strips (90 mm × 25.4 mm × 2.5 mm) and conditioned at 23 ± 1 ◦ C for at least seven days before used. After that, the PF Com and PF produced were spread on one side of the sample at amount of 0.035 g/cm2 (on solid basis) in an area of 25.4 mm × 90 mm. Then, the adhesives coated plywood sample was overlapped with uncoated plywood with the length direction parallel to the wood grain. The resulting two layered panel was then clamped with Gclamp for three days and followed by conditioning at temperature of 23 ± 1 ◦ C at a relative humidity of 50% either for seven days, or until they attain a constant mass, whichever is the longer period. The evaluation of bonding strength was carried out according to Japanese Industrial Standard (JIS K-6852) using a Universal Testing Machine (Testometric M500-50CT) until failure at a force of 9.7 MPa of shear area per minute (approximately 1.3 mm/min crosshead speed).
2.2. Liquefaction of EFB fibres
3. Results and discussion
The liquefaction of EFB fibres was conducted according to method reported previously with modification (Ahmadzadeh et al., 2009; Zakaria et al., 2014). Firstly, the EFB fibres were dried in an oven at 105 ◦ C for 24 h. After that, liquefaction of the EFB fibres was carried out at weight ratio of phenol to EFB (P/E) 3:1 in the presence of sulfuric acid (3% based on phenol weight) as the catalyst. This reaction was conducted at 150 ◦ C for 120 min in an oil bath. The liquefied mixture was then diluted in 400 ml methanol and filtered with filter paper (Whatman No. 1) to separate the un-liquefied fractions. Evaporation with rotary evaporator was conducted to recover the methanol added.
3.1. Characterizations of the PF resin
2.3. Resin synthesis Resinification was carried out by mixing the liquefied EFB (LEFB ) mixture and formaldehyde in a round bottom flask at different weight ratios of formaldehyde to LEFB (1.8:1, 2.0:1, and 2.2:1) under alkaline condition. Alkaline condition in the reaction was controlled by two steps addition of 40% aqueous NaOH. The first addition of NaOH (0.5 g) is to neutralize the acid catalyst added during the liquefaction process and the second addition of NaOH (4.5 g) is to maintain the reaction in alkaline condition. The temperature was set at 65 ◦ C for the first 60 min and then increased to 85 ◦ C after the second addition of NaOH solution for another 120 min. After the reaction completed, the product (designated as PF 1.8, PF 2.0 and PF 2.2) were cooled to room temperature and kept in the refrigerator for further characterization.
The physical properties of the PF produced from the liquefied EFB are shown in Table 1. The pH value of the PF resin is lower as compared to the PF Com resin. This may be attributed to the presence of residual H2 SO4 , which was used as a catalyst during the liquefaction process has reduced the pH of the resinification mixture. The solid content of each PF resin produced is almost similar, but slightly lower than that of the commercial PF resin, which may probably due to the existence of urea in the commercial PF resin (Zhao et al., 2010). Meanwhile, the viscosity and molecular weight of the PF produced resins is higher as compared to the commercial PF resin. The results show that the amount of formaldehyde used in the reaction has influenced the viscosity and molecular weight of the PF produced tremendously. It is well known that resin viscosity and molecular weight of PF resin are influenced by non-volatile chemicals, temperature and F/P ratio (Haupt and Sellers, 1994). Higher formaldehyde content might result in the increase of viscosity for each resin produced because higher content of formaldehyde tends to speed up the polymerization process. Besides, the presence of larger molecule in the liquefied EFB in the resin also will increase the viscosity and molecular weight of the PF resin Fig. 1. 3.2. Chemical functionality analysis by FTIR Fig. 1 illustrates the FTIR spectra of the PF Com and PF samples produced using different ratios of F/LEFB (PF 1.8, PF 2.0 and PF 2.2).
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Table 1 The physical properties of the produced PF resin.
PF 1.8 PF 2.0 PF 2.2 Com. PF
pH
Solid content (%)
Viscosity (cP)
Mw (Da)
Mn (Da)
Mw /Mn
9.43 9.59 9.75 11.16
49.50 50.28 50.46 64.37
193.3 311.5 441.7 200.0
1796.1 3066.5 3159.4 327.3
965.1 1150.5 1166.5 212.1
1.86 2.66 2.99 1.82
Table 2 Functional groups and observed wavenumbers for commercial phenolic resins compared to the synthesized PF.
Fig. 1. FTIR absorption spectrum for a commercial phenolic resin and for bio-based phenolic resins synthesized at different weight ratios.
Commercial PF [cm−1 ]
Synthesized PF [cm−1 ]
Functional group
3300 2850
3301 2877
1620 1470
1626 1482
1446
1447
OH Out of phase stretching vibration of CH2 alkane C C aromatic ring Methylene bridges, ortho-para Methylene bridges, para-para
1266 1202
1223
1155 1051
1153 1108
1010
1009
964
981
882
884 755
The IR absorbance bands obtained from the PF resins are identical to those of the commercial PF resin, as summarized in Table 2. The IR band at around 1447 cm−1 is assigned to the methylene bridges at the para-para position, while the small absorbance band at 1482 cm−1 is assigned to the methylene bridges at the orthopara position. The intensity of these bands increased when the F/LEFB weight ratio increased, suggesting the greater crosslinking degree at higher formaldehyde concentration. It has been proposed that, there are two types of condensation reactions that occur during the resinification reaction, i.e., condensation reaction between hydroxymethyl groups themselves forming dimethylene ether linkages and at aromatic free positions to produce methylene bridges (Chen et al., 2012). In this case, the second reaction become dominant as can be observed from the increase of the intensities of
Fig. 2.
13
Asymmetric stretch of phenolic C C OH C O stretch Dimethylene ether C O stretch O H deformation stretch in hydroxymethyl groups 1,2,4-trisubstituted benzene ring 1,2,4,6 – tetrasubstituted benzene ring 1,2 – disubstituted benzene ring 1,2,6 – trisubstituted benzene ring
the band of the methylene bridges, while no changes to the band of the dimethylene ether peak at 1108 cm−1 and 1153 cm−1 . 3.3.
13 C
NMR analysis
The liquid-state 13 C NMR spectra of the PF Com and produced PF resin are shown in Figs. 2–5. The chemical shift assignment is made according to results presented in the previous studies (Werstler, 1986; Zhao et al., 2012). The chemical shift observed at around 167–168 ppm represents the carbonyl groups in the PF resins. The chemical shift in the region of 54–55 ppm is attributed to
C NMR spectra of commercial phenolic resin.
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Fig. 3.
13
C NMR spectra of PF 1.8.
Fig. 4.
13
C NMR spectra of PF 2.0.
the methoxyl groups in the liquefied EFB component. The result was in good agreement with those obtained by Zhao et al. (2012) and it is also confirmed when no peak appears in this region for PF Com sample. The un-reacted formaldehyde chemical shift is detected at 82.3 ppm. According to Werstler (1986), the peak around
Fig. 5.
13
84.6–85.2 ppm is related to hemiacetals from paraformaldehyde or oligomers of oxymethylene. As the PF reaction proceeds, these oligomers will break down as free formaldehyde. The chemical shift at 89.8 ppm and at 63–64.4 ppm is attributed to the addition reaction product of ortho-hemiformal (carbon connected to
C NMR spectra of PF 2.2.
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Fig. 6. TGA weight loses curves for the phenolic resins produced at different weight ratios.
the hydroxyl Ph-CH2 OCH2 OH) and para-hemiformal (carbon connected to the hydroxyl Ph-CH2 OCH2 OH) respectively. In addition, the chemical shift of ortho- and para- Ph- CH2 OCH2 OCH2 OH are also observed at 92.2–93.5 ppm. Some peaks are not observed in these spectra, for example from 69–73 ppm and 29–30 ppm which corresponding to the methylene ether bridge and ortho-ortho methylene groups respectively. The para-para methylene bridges are clearly visible at 38–40 ppm while the ortho-para chemical shifts appear only as a small shoulder at 35.10 ppm. It is comprehensible that the ortho-ortho methylene links are not observed because the reactions are not favoured in the NaOH system (Zhao et al., 2012). Meanwhile, in the case of para-para and ortho-para methylene bridges, phenol para position was dominant for reaction over the ortho position on a per site basis most likely due to steric hindrance at the ortho position. In the spectra of the produced PF resins using different ratio of formaldehyde, the chemical shift at 115–117 ppm and 120–124 ppm, which corresponds to the unsubstituted ortho phenol aromatic and para phenol aromatic respectively, are absent, suggesting that all the reactive sites are completely reacted. Meanwhile, the intensity of the chemical shift for substituted ortho and para phenol aromatic at around 126–130 ppm is decreased as the ratio of formaldehyde increased. Since all the unsubstituted ortho and para phenol aromatic are absent, it is expected that the peak intensities in these region would have increased but conversely. This could probably be due to higher amount of water in the formalin added during the resinification process. The higher water content might slow down the condensation reaction in which higher energy is required for the resinification process. 3.4. Thermal analysis Fig. 6 shows thermogravimetric curves of the PF resins with the data summarized in Table 3. There are three main thermal decomposition regions. The first decomposition occurred from 80 to 150 ◦ C due to the removal of low molecular weight components, such as water, free phenol, and formaldehyde (Hu et al., 2011). The second decomposition occurred from 150 to 350 ◦ C can be attributed to the release of carbon monoxide, carbon dioxide, and methylene, suggesting the elimination of carbonyl, diphenyl ether and hydroxymethyl groups as the result of the cleavage of methylene bridges. As the temperature increased, the ratio of the concentration of diphenyl ether link structure of phenol also increased. The structure of the diphenyl ether link will form
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Fig. 7. Shear strength of plywood bonded with resol-type adhesives from liquefied EFB at different weight ratios of F/LEFB .
an intermediate structure generated for phenolic resins during thermal degradation. The third decomposition at the temperature ranging from 350 to 600 ◦ C is due to thermal pyrolysis of methylene bridges in ortho-para and para-para position. After the curing process, PF structures were mainly methylene bridged phenolic units. These crosslinking networks begin to degrade with further temperature increment (Chen et al., 2012). There is no significant changes occurred on the decomposition temperature when the PF ratio increased. However, the char yield at the end of the analysis show interesting results. When the PF ratio increased from 1.8 to 2.2, the char yield were also increases. This results shows that the addition of formaldehyde are able to produce a product that has a high thermal stability.
3.5. Adhesives shear strength test results The shear test results are shown in Fig. 7. The specimens bonded with PF Com resin gives the highest shear strength value of 2.93 MPa, while the shear strength of the produced PF resin increased from 1.70 to 2.53 MPa as the weight ratio of F/LEFB increased from 1.8 to 2.2. There are many factors that will influence the shear strength of the adhesive, including the formaldehyde to phenol (F/P) molar ratio and the viscosity of the adhesive. By altering the F/P molar ratio, it will affect the crosslinking degree or curing reaction. At higher F/P molar ratio, the crosslinking rate will increase because there are high amounts of phenolic components which having functional groups more than three (Alma and Basturk, 2006). Meanwhile, as for the viscosity, several aspects will be affected such as wetting, flow and penetration of the adhesive. Adhesive at higher viscosity may not flow well to spread on the surface area in a reasonable time frame. At a lower viscosity, the adhesive flow better and wet more surface area, but again have a tendency to over-penetrate into the wood specimens. Overpenetration of adhesive will lead to insufficient adhesive in the bond-line hence resulting in a starved joint (Alma and Basturk, 2006; Baldan, 2012). In Table 1, we can see that the increase of the F/LEFB weight ratio has resulted in the increase of the adhesive viscosity, which increased the shear strength. Although the shear strength of the PF resin produced from the liquefied EFB is slightly lower than that of the PF Com, the strength values exceed the minimum strength as specified in JIS K-6852 (Standard for the tension shear strength of the resol-type adhesive), i.e., 1.18 MPa.
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Table 3 TGA thermal decomposition, weight loss, and char yield of phenolic resins produced at different weight ratios. PF samples
PF 1.8 PF 2.0 PF 2.2
Decomposition temperature (◦ C)
Weight loss (%)
Char yield (%)
1st region
2nd region
3rd region
1st region
2nd region
3rd region
80–150 80–150 80–150
150–350 150–350 150–350
350–600 350–600 350–600
7.5 7.4 6.1
22.3 21.2 20.0
40.1 37.4 34.9
4. Conclusions Resol type PF resins were successfully produced from oil palm EFB fibres via liquefaction and resinification processes. The increasing of weight ratio of F/LEFB resulted in the increase of viscosity and higher molecular weight of the resins produced. The functional groups of the bio-based PF resins are identical to those of a commercial PF resin. Based on the results obtained from the 13 C NMR spectroscopy, the phenol para position was favoured for the reaction over the ortho position due to the existence of steric hindrance effect from the hydroxyl phenol and phenol derivatives of liquefied EFB. The shear strength of all the liquefied PF resins fulfils the requirement of JIS K-6852. Acknowledgements We acknowledge the financial support from the project UKM-ST-07-FRGS0232-2012 and UKM-DIP-2012-034. The authors would also like to acknowledge Dr. Nor Yuziah Mohd. Yunus from MAC for kindly supply the commercial phenolic resins. Acknowledgement also is given to the Centre for Research and Instrumentation (CRIM) for providing the instruments for analysis. We also cordially thank E. Stibal for technical assistance throughout the project. References Ahmadzadeh, A., Zakaria, S., Rashid, R., 2009. Liquefaction of oil palm empty fruit bunch (EFB) into phenol and characterization of phenolated EFB resin. Ind. Crops Prod. 30, 54–58. Alam, M.Z., Ameem, E.S., Muyibi, S.A., Kabbashi, N.A., 2009. The factors affecting the performance of activated carbon prepared from oil palm empty fruit bunches for adsorption of phenol. Chem. Eng. J. 155, 191–198. Alma, M.H., Basturk, M.A., 2006. Liquefaction of grapevine cane (Vitis vinisera L.) waste and its application to phenol–formaldehyde type adhesive. Ind. Crops Prod. 24, 171–176. Baldan, A., 2012. Adhesion phenomena in bonded joints. Int. J. Adhes. Adhes. 38, 95–116. Chai, L.-L., Zakaria, S., Chia, C.H., Nabihah, S., Rashid, R., 2009. Physico-mechanical properties of PF composite board from EFB fibres using liquefaction technique. Iran. Polym. J. 18, 917–923. Chen, Z., Zeng, W., Chen, Y., Li, W., Liu, H., 2012. Influence of F/P on structure and thermal property of phenolic resin. Key Eng. Mater. 500, 98–103.
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