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Cross-linked soy-based wood adhesives for plywood
International Journal of Adhesion & Adhesives 50 (2014) 199–203
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Cross-linked soy-based wood adhesives for plywood H. Lei n, G. Du, Z. Wu, X. Xi, Z. Dong Materials and Engineering College, Southwest Forestry University, Kunming 650224, Yunnan, PR China
art ic l e i nf o
a b s t r a c t
Article history: Accepted 22 January 2014 Available online 6 February 2014
To improve the water resistance of soy-based adhesive for wood panels, three kinds of cross-linkers, namely, epoxy resin (EPR), melamine–formaldehyde (MF) and their mixture EPR þMF were used in this paper. The results indicated that all the three cross-linkers improved the water resistance of soy-based adhesive and the hybrid cross-linker EPRþ MF, was the best. With press temperature 160 1C and press time 8 min, type II and even type I plywood could be prepared when 6.4%EPR þ6.4%MF is used as crosslinker of soy-based adhesive. FT-IR indicated that the great improvement of water resistance of soy-based adhesive modified with EPR and MF might be caused by the reaction between epoxy and –OH, and that between MF and –NH. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Soy adhesive Cross-linker Water resistance Plywood
1. Introduction Formaldehyde-based adhesives, such as urea–formaldehyde (UF), melamine–formaldehyde (MF), phenol–formaldehyde (PF), are widely used for the preparation of wood panels. But formaldehyde emission caused by these resin adhesives has confused wood industry and has been a topic of concern for many years. Some standards or requirements were given to define the acceptable formaldehyde emission levels. The newest requirement on formaldehyde emission comes from California Air Resources Board (CARB) formaldehyde emissions regulation of U.S. It was passed in 2008 and got effective in 2009. Today, the mounting interest in formaldehyde emission is still driving major changes in wood panel industry as well as the resin industry that supplies the wood panel industry. In response to the need on environment-friendly adhesives, great attention has been given to adhesives from natural materials, such as starch [1,2], soy-based adhesives [3–5], and so on, although most of these adhesives have almost been pushed out of market in wood panel industry during the past 30 years. Soy-based adhesive was once a major adhesive for preparation of plywood. But it has been replaced by synthetic resins since the 1960s. Till the 1990s, it returned to the study area as a wood adhesive. It is reported that one of the soy-based formaldehyde-free adhesives has been used for production of interior plywood panels since 2004 [6]. However, the application of soy-based adhesives is rather limited. Now, most of the efforts on soy-based adhesive are given to the improvement of its bad water resistance. To resolve this problem, some methods could be employed to modify the soy adhesive, such
n
Corresponding author. E-mail address: [email protected] (H. Lei).
http://dx.doi.org/10.1016/j.ijadhadh.2014.01.026 0143-7496 & 2014 Elsevier Ltd. All rights reserved.
as hydrolysis, chemical denaturation, cross-linking, enzyme modification, and so on. Cross-linking modification is a comparatively acceptable method for the modification of soy-based adhesive. Cross-linkers can be either mixed with soy adhesive directly before its application or added during the preparation of soy adhesive. For the latter method, acrylates [7,8], maleic anhydride [9], etc. are used and in most cases graft polymerization will occur. Commonly, some complex preparation procedures are involved in this method. Therefore, it is much easier in handling to mix cross-linkers directly with soy adhesive. An effective cross-linker is the key for this method. Since there are many reactive groups in soy proteins, such as –OH, –SH, –COOH, and –NH2, many chemicals could be used for the cross-linking of soy adhesive. Epoxy [6], aldehyde and its deratives [10] have already been proved effective cross-linkers for soy-based adhesive. With different cross-linkers, the mechanism for the improvement of performance of soy adhesive is different. Epoxy groups are thought to react with all of the aforementioned functional groups in soy proteins [11]. Huang J et al. proposed the possible curing mechanisms of the soy–polyepoxide adhesives [12]. For cross-linker aldehyde and its derivatives, such as urea–formaldehyde, hydroxymethyl phenol, the main reaction group, comes from –NH2 in soy protein. Besides choosing a suitable cross-linker, its addition amount is very important for the application of soy adhesive. Because of the usage of some expensive cross-linkers, such as epoxy, the cost of soy-based adhesive is greatly dependent on the addition amount of cross-linker. Wood composites bonded with soy protein isolate and Kymene are reported to show shear strengths comparable to or higher than those of composites bonded with phenol–formaldehyde [13]. As a wet-strength agent for paper, Kymene is an aqueous solution of cationic polyamidoamine–epichlorohydrin
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(PAE). However, because of the high dry weight ratio SPI/Kymene 1.33:1, cross-linker Kymene is the most expensive component of the soy-based adhesive. A new curing agent was developed by the same research group from epichlorohydrin and ammonium hydroxide to decrease the cost of soy-based adhesive. But there was still about 14% on dry soy flour weight of this curing agent needed for the preparation of soy-based adhesive and interior plywood [6]. When melamine–urea–formaldehyde (MUF) resin is added to soy-adhesive as a cross-linker, the addition amount of solid MUF is about 40% on solid soy flour or even higher [14,15]. For phenol–formaldehyde, the weight ratio of phenol–formaldehyde to soy adhesive was 30:70 in public reports [16]. In this formulation phenol–formaldehyde had a solid content of 55% and soy solution of 42%. In this paper, the effects of different cross-linkers on the water resistance of soy-based adhesive were studied. In order to transfer all of the possible hydrophilic groups of soy protein to hydrophobic ones, a hybrid cross-linker was used for the modification of soy-based adhesive. The objectives of this work were as follows: (1) to optimize the proportion of components of hybrid crosslinker; (2) to decrease the addition amount of cross-linker; and (3) to prepare plywood with good water resistance.
2. Materials and methods 2.1. Materials Defatted soy flour (53.4% protein content) was obtained from Yuxin Soybean Protein Co., Ltd, China. Poplar veneer with a thickness of 1.5 mm and moisture content of 8–10% was purchased for the preparation of plywood. Epoxy resin (EPR) was a commercial product with the name of E-44, whose epoxide number was 0.41–0.47 and softening point was 12–20 1C. All other chemicals mentioned in this work were all of reagent grade. 2.2. Preparation of soy-based adhesive Soy-based adhesive was prepared according to a method already reported [17]. A three-neck round-bottom flask equipped with a mechanical stirrer, a thermometer and a condenser was charged with water (187 g), sodium dodecyl benzene sulfonate (0.8 g), CaO( 1.9 g) and NaOH (3.7 g) to 70 1C. Soy flour (80 g) was then charged to the rapidly stirring solution. The mixture was heated to 90 1C over 15 min, with rapid agitation, and held between 88 1C and 92 1C for 3 h. The mixture was cooled to 35 1C in an ice bath. The solid content of the resulting soy-based adhesive was 3071%. 2.3. Preparation of melamine–formaldehyde (MF) resin Formaldehyde 37% (150 g), melamine (80 g) and water (107 g) were charged into a three-neck flask equipped with a mechanical stirrer, thermometer and condenser and then the pH was adjusted to 9.0 with NaOH 30%. The mixture was heated to 85 1C during 20–30 min, and held the temperature for another 30 min. The mixture was cooled to room temperature and kept at pH 9.0 and room temperature. 2.4. Preparation of plywood samples bonded with soy-based adhesive The soy-based adhesive was mixed well with different crosslinkers just before the preparation of three-layer plywood of dimensions 300 mm 220 mm 4 mm. The double sides glue loading was 360 g/m2. Before sending into the press, the veneers with adhesives were allowed to rest at room temperature for 15 min and were then assembled. The plywood was pressed under
pressure of 2 MPa at 160 1C for 8 min. Other press times and press temperatures were also used in this paper. 2.5. Test of dry and wet shear strength of plywood samples After conditioning in the laboratory for 1 day, the plywood panel was then cut into shear specimens with dimension of 100 mm 25 mm to determine its shear strength and water resistance. Each specimen has a bonded area of 25 mm 25 mm. Both dry and wet shear strength of plywood specimens were tested on a WDS-50KN mechanical testing machine. For wet shear strength, the specimens were soaked in (6373) 1C water or in boiling water for determined time. The mean result of 8–10 specimens was considered as the final shear strength. The testing method was referred to Chinese national standard GB/T 9846.3-2004. In this standard, for type II plywood, the specimens were soaked in (6373)1C water for 3 h. For type I plywood, the specimens should pass water–dry–water cycle. That is to say, the specimens were firstly soaked in boiling water for 4 h, then dried in (6373) 1C for 20 h, and lastly re-applied in boiling water for another 4 h. For both type II and type I plywood, before the measuring by the testing machine, the specimens were taken out of water and left at room temperature for 10 min. 2.6. FT-IR analysis The oven was preheated to 160 1C. Liquid soy adhesives with or without cross-linkers were put in the oven to a constant weight. The cured soy adhesives were ground into fine powder. 1 g KBr and 0.001 g soy adhesive samples were mixed well for the preparation of KBr pills. The FT-IR spectra were obtained on a Varian 1000 infrared spectrophotometer.
3. Results and discussion 3.1. Effect of cross-linker on performance of soy adhesive-based plywood Table 1 shows the performance of plywood specimens with soy based-adhesive with or without different amounts of cross-linkers. The cross-linkers used in this work included EPR, MF and their mixture. The dry shear strength for all of the soy-based plywood specimens was good enough to satisfy the requirement of relative Chinese national standard (GB/T 9846.3-2004, Z0.70 MPa). But considering that wood failure for almost all of the specimens is 100%, which meant the measured dry strengths were mainly determined by the strength of wood, it was difficult to see the effects of cross-linker on the dry shear strength of plywood. However, the water resistance of plywood specimens showed big differences with or without cross-linkers. Soy adhesive without cross-linking had no water resistance at all, which was indicated by the 100% delamination of specimens when soaked in 63 1C water for 3 h. All of the three cross-linkers in this work improved the water resistance of soy-based adhesive more or less. When cross-linked with different amounts of EPR, although the wet strength of plywood at 60 1C for 3 h could be measured, it was not enough to meet the requirement of relative standard. More than 50% of the specimens cross-linked by EPR failed the soak test in 63 1C water for 3 h and 100% of the specimens delaminated when the test condition became more severe as in boiling water cycles. With the increase of the addition amount of EPR, the percentage of number of specimens that failed in the soak test to the total number of specimens decreased from 50% to 20%, which indicated the improvement of the water resistance of soy-based adhesive, although their water resistance was not enough to meet the relative standard. Epoxy was assumed to improve the water resistance of soy adhesive because of the reaction between epoxy groups
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Table 1 Effect of different kinds of cross-linker on the performance of soy adhesive-based plywood. Cross-linker
Addition amount of cross-linker (%)
Without cross-linker 0 EPR 6 8 10 12 14 MF 6 8 10 12 14 EPR þ MF 10(5 þ5)n n
Dry shear 63 1C 100 1C strength(MPa) Wet shear Number of specimens failed Wet shear Number of specimens failed strength (MPa) in the soak test/total specimens (%) strength (MPa) in the soak test/total specimens (%) 1.02 0.85 0.80 0.86 0.77 0.85 0.80 1.06 0.88 0.86 1.32 1.05
– 0.41 0.38 0.39 0.48 0.46 1.03 0.80 0.94 0.92 0.94 0.91
– – – – – – 0.47 0.51 0.51 0.52 0.52 0.85
100 50 50 30 20 20 0 0 0 0 0 0
– 100 100 100 100 100 40 30 20 10 20 100
The weight ratio of solid EPR to MF on the solid weight of soy adhesive was 1:1.
Table 2 Effect of addition amount of cross-linker EPRþ MF on the performance of soy adhesive-based plywood. Cross-linker
3.2%EPR þ2.6%MF 3.2%EPR þ3.9%MF 3.2%EPR þ5.1%MF 3.2%EPR þ6.4%MF 6.4%EPR þ6.4%MF 6.4%EPR þ7.7%MF
Dry shear strength (MPa)
0.96 1.02 1.01 1.21 0.95 0.93
63 1C
100 1C
Wet shear strength (MPa)
Number of specimens failed in the soak test/total specimens (%)
Wet shear strength (MPa)
Number of specimens failed in the soak test/total specimens (%)
0.49 0.44 0.73 0.72 1.08 0.76
0 0 0 0 0 0
0.45 0.49 0.65 0.65 0.81 0.93
0 0 0 0 0 0
with high reactivity at the opposite ends of resin molecule and reactive groups in soy protein. In this work, just epoxy was used as itself without excess solvent or even curing agent being added. It is wellknown that a curing agent is necessary for the curing of epoxy. The most commonly used curing agent for epoxy is amine. Here, soy protein could be regarded as the curing agent of epoxy. Once there is some curing reaction between epoxy and protein, the water resistance and mechanical performances of the final soy-based system would be improved with the increase of its molecular weight. Although EPR could react with a wide variety of functional groups in soy protein, high addition amount of EPR could not be used as an ideal cross-linker for three reasons. (1) EPR is not soluble in water. Simple mixing of soy flour and an epoxy resin in water cannot generate a homogeneous mixture. High addition amount EPR tended to aggregate in the presence of water and got a poor cross-linking with soy. This was the reason why the soy adhesive could not resist the boiling water cycle test. (2) EPR will increase the viscosity of soy-based adhesive a lot with a high addition amount, which will greatly affect the application of the adhesive. (3) EPR is too expensive. It was unacceptable to the industry when addition amount of EPR was higher than 10% of the solid soy. Once the addition amount of EPR was higher than 10%, the cost of EPR would be almost 30% of the soy powder. MF is a widely-used adhesive with good water resistance in wood industry. Soy adhesive cross-linked with water-soluble MF shows better water resistance than that with the same amount of EPR, as seen from Table 1. All of the specimens with MF passed the soak test at 63 1C for 3 h. No specimen delaminated and the wet strength for all the specimens was higher than the standard requirement 0.70 MPa. This meant that interior type II plywood could be prepared with soy adhesive cross-linked with as low as 6% of MF on solid soy adhesive. But when the test conditions were changed to those of boiling water cycle, although most of the specimens did not
delaminate, the wet shear strength decreased greatly. Even when the addition amount of MF increased to 14% of solid soy, the wet shear strength could not satisfy the relative standard. This meant that only with MF, the hydrophilic groups in soy adhesive of this paper could not be totally hidden or reacted with hydrophobic groups. For wet shear strength in 63 1C water and in boiling water, the effects of addition amount of MF on wet strength were not clear. To make use of the high reactivity of EPR and the water solubility of MF, a hybrid cross-linker EPRþ MF was used in this work. As seen from Table 1, the hybrid cross-linker MFþ EPR indeed showed the best water resistance. With 10% EPRþ MF, even type I plywood could be prepared. 3.2. Effect of hybrid cross-linker EPR þMF on the performance of soy adhesive -based plywood Hybrid cross-linker EPR þMF was proved to be the best crosslinker for soy-based adhesive as seen from Table 1. To optimize the proportion of components of hybrid cross-linker and decrease the addition amount of soy adhesive for plywood, in the hybrid crosslinker, different amounts of EPR and MF were used for preparation of plywood (Table 2). As in Table 1, when using hybrid EPRþMF as a cross-linker of soy adhesive, all of the dry shear strength of plywood specimens cross-linked with different amounts of EPRþMF satisfied the requirements of the relative standard. In the soak test, either in 63 1C water for 3 h or in boiling water cycle, no specimen delaminated. It indicated that soy-based adhesive cross-linked with EPR þMF had good water resistance even with a very low addition amount of cross-linker, that is, 3.2% EPR and 2.6% MF on solid soy adhesive. In laboratory condition, during the preparation of plywood, lower addition amount of cross-linker than 3.2% EPR or 2.6% MF was difficult to handle because of the
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Table 3 Effect of press procedure on the performance of soy adhesive-based plywood. Press variable
Dry shear strength (MPa)
63 1C Wet shear strength (MPa)
100 1C Number of specimens failed in the soak test/total specimens (%)
Wet shear strength (MPa)
Number of specimens failed in the soak test/total specimens (%)
Press temperature (1C) 120 1.12 140 0.94 160 0.95 180 0.85
0.84 1.03 1.08 1.12
0 0 0 0
0.61 0.83 0.81 0.76
0 0 0 0
Press time (min) 2 4 6 8
– – – 1.08
100 100 100 0
– – – 0.81
100 100 100 0
0.45 0.83 0.89 0.95
small amount. When the addition amount of EPR was fixed to 3.2% on solid soy adhesive, with the increase of the addition amount of MF, the wet shear strength of plywood increased. However, at least 5.1% MF should be used together with 3.2% EPR to get the modified soy-based adhesive for the preparation of type II plywood. For type I plywood, the minimum addition amount of cross-linker of soy based-adhesive was 6.4% MF with 6.4% EPR in Table 2. Considering the results of Table 1, the total amount of hybrid cross-linker MF þEPR should be 10–12.8% for preparation of type I plywood. The increase of wet shear strength caused by the increase of addition amount of EPR could also be seen in Table 2. 3.3. Effect of press procedure on the performance of soy-based adhesive with hybrid cross-linker EPR þMF Soy adhesives with 6.4%EPR þ 6.4%MF were used for the preparation of plywood in the laboratory. Effects of press temperature and press time on the performance of plywood with soy-based adhesive are shown in Table 3. The results show that even with press temperature 120 1C, type II plywood could be obtained. With press temperature 140 1C, the water resistance of plywood was good enough to meet the requirement of type I plywood and did not show much difference with those when higher temperatures 160 1C and 180 1C were used. 2–6 min at 160 1C was too short to cure soy adhesive with EPRþ MF, which indicated by the poor water resistance shown in Table 3. With 4–6 min, the dry shear strength of plywood was good. This meant that 4–6 min was enough for curing of soy adhesive but not enough for its cross-linking. Hot-press time as long as 8 min may be a problem for the commercialization of the cross-linked soy adhesive. 3.4. FT-IR analysis of soy adhesive modified with cross-linkers The FTIR spectra of cured soy-based adhesive, soyþ6.4% EPR, soyþ6.4%MF and soyþ6.4% EPRþ6.4%MF are given in Fig. 1. For soy adhesive without cross-linkers, the broad band observed in the range of 3500–3000 cm 1 was assigned to the free and bound O–H and N– H groups. The absorption bands of amide, the characteristic group of protein, were observed at 1654, 1539, and 1238 cm 1, which were assigned to CQO stretching, N–H bending, C–N stretching and N–H bending vibration, respectively. The COO– and –C–NH2 absorptions were seen at 1390 and 1058 cm 1, respectively. The main difference between spectra of soy adhesive with or without EPR came from the absorptions at 1249 and 833 cm 1, which were assigned to C–O–C stretching. There were three possibilities for the C–O–C stretching: (1) epoxy ether; (2) phenol–O–C; and (3) the resulting ether from the reaction between epoxy and –OH of soy protein [18]. The former two groups came from EPR itself. The reaction between EPR and –OH of
4000
3500
3000
2500
2000
Wave numbers/cm
1500
1000
500
-1
Fig. 1. FT-IR of the soy adhesive with or without cross-linker: (a) soy adhesive; (b) soyþ6.4%EPR; (c) soyþ6.4%MF; and (d) soyþ 6.4%EPR þ 6.4%MF.
soy protein might be the main reason for the improvement of water resistance of soy-based adhesives caused by epoxy. However, the possible reaction between EPR and –NH2, –SH, –COOH groups of soy is not clearly observed in Fig. 1. In soy adhesive cross-linked with MF, the absorption at 1238 cm 1 got weaker than that in pure soy adhesive, which might be a result of the reaction between methylol group of MF resin and N–H groups of soy adhesive [11]. The increase of the absorption at 1539 cm 1 was caused by –CQN ring vibration of melamine. Bending vibration of triazine ring was found at 829 cm 1.The spectra of soy cross-linked with EPR and MF were almost the multiplicity of soy adhesive cross-linked by EPR or MF alone. Although a thorough mechanism on the soy-based mixing system is still needed, the fact that epoxy and MF could react with different groups in soy protein might be the main reason for the good water resistance of soy-based adhesive with hybrid cross-linker. On one hand, the mixing of epoxy and MF transferred the hydrophilic groups of soy protein to hydrophobic ones as much as possible. On the other hand, the epoxy was used as less as possible to avoid aggregation and the large increase of viscosity.
4. Conclusions To develop a soy-based adhesive with good water resistance and acceptable cost, it was modified with cross-linkers, namely, EPR, MF and their mixture EPR þMF in this paper. All the three
H. Lei et al. / International Journal of Adhesion & Adhesives 50 (2014) 199–203
cross-linkers improved the water resistance of soy-based adhesive more or less. The soy-based adhesive cross-linked with hybrid cross-linker EPRþ MF showed the best water resistance. With press temperature 160 1C and press time 8 min, type II and even type I plywood could be prepared when 6.4%EPR þ6.4%MF was used as the cross-linker of soy-based adhesive. FT-IR indicated that the great improvement of water resistance of soy-based adhesive modified with EPR and MF might be caused by the reaction between epoxy and –OH, and that between MF and –NH. Acknowledgments This research was supported by the National Natural Science Foundation of China (31170530) and New Century Excellent Talents of Ministry of Education of China (NCET-10-0972). References [1] Wang Z, Li Z, Gu Z, Hong Y, Cheng L. Preparation, characterization and properties of starch-based wood adhesive. Carbohyd Polym 2012;88 (2):699–706. [2] Tondi G, Wieland S, Wimmer T, Schnabel T, Petutschnigg A. Starch–sugar synergy in wood adhesion science: basic studies and particle board production. Eur J Wood Wood Prod 2012;70(1–3):271–8. [3] Lin Q, Chen N, Bian L, Fan M. Development and mechanism characterization of high performance soy-based bio-adhesives. Int J Adhes Adhes 2012;34:11–6.
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[4] Nordqvist P, Khabbaz F, Malmstrom E. Comparing bond strength and water resistance of alkali-modified soy protein isolate and wheat gluten adhesives. Int J Adhes Adhes 2010;30:72–9. [5] Huang J, Li K. A new soy flour-based adhesive for making interior type II plywood. J Am Oil Chem Soc 2008;85:63–70. [6] Jang Y, Huang J, Li K. A new formaldehyde-free wood adhesive from renewable materials. Int J Adhes Adhes 2011;31:754–9. [7] Gao Q, Shi S, Li J, Liang K. Soybean meal-based wood adhesives enhanced by modified polyacrylic acid solution. Bioresources 2012;7(1):946–56. [8] Steinmetz L, Krinski T. Modified protein adhesive binder and process for producing. US patent 4687826. 1987. [9] Liu Y, Li K. Development and characterization of adhesives from soy protein for bonding wood. Int J Adhes Adhes 2007;27:59–67. [10] Lambuth AL. Protein adhesives for wood. In: Pizzi A, Mittal KL, editors. Advanced wood adhesive technology. New York: Marcdel Dekker Inc.; 1994259–81. [11] May CA. Epoxy resins—chemistry and technology. New York: Marcel Dekker; 1988. [12] Huang J, Li C, Li K. A new soy flour–polyepoxide adhesive system for making interior plywood. Holzforschung 2012;66:427–31. [13] Li K, Peshkova S, Geng X. Investigation of soy protein–Kymene adhesive systems for wood composites. J Am Oil Chem Soc 2004;81(5):487–91. [14] Fan D, Qin T, Chu F. A soy flour-based adhesive reinforced by low addition of MUF resin. J Adhes Sci Technol 2011;25(1–3):323–33. [15] Gao Q, Shi SQ, Zhang S, Li J, Wang X, Ding W, et al. Soybean meal-based adhesive Enhanced by MUF resin. J Appl Polym Sci 2012;125:3676–81. [16] Yang I, Kuo M, Myers D. Bond quality of soy-based phenolic adhesives in southern pine plywood. J Am Oil Chem Soc 2006;83(3):231–7. [17] Wu Z, Lei H, Du G. Disruption of soy-based adhesive treated by Ca(OH)2 and NaOH. J Adhes Sci Technol 2013;3:1–7. [18] Rogers J, Geng X, Li K. Soy-based adhesives with 1,3-dichloro-2-propanol as a curing agent. Wood Fiber Sci 2004;36(2):186–94.
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Cross-linked soy-based wood adhesives for plywood H. Lei n, G. Du, Z. Wu, X. Xi, Z. Dong Materials and Engineering College, Southwest Forestry University, Kunming 650224, Yunnan, PR China
art ic l e i nf o
a b s t r a c t
Article history: Accepted 22 January 2014 Available online 6 February 2014
To improve the water resistance of soy-based adhesive for wood panels, three kinds of cross-linkers, namely, epoxy resin (EPR), melamine–formaldehyde (MF) and their mixture EPR þMF were used in this paper. The results indicated that all the three cross-linkers improved the water resistance of soy-based adhesive and the hybrid cross-linker EPRþ MF, was the best. With press temperature 160 1C and press time 8 min, type II and even type I plywood could be prepared when 6.4%EPR þ6.4%MF is used as crosslinker of soy-based adhesive. FT-IR indicated that the great improvement of water resistance of soy-based adhesive modified with EPR and MF might be caused by the reaction between epoxy and –OH, and that between MF and –NH. & 2014 Elsevier Ltd. All rights reserved.
Keywords: Soy adhesive Cross-linker Water resistance Plywood
1. Introduction Formaldehyde-based adhesives, such as urea–formaldehyde (UF), melamine–formaldehyde (MF), phenol–formaldehyde (PF), are widely used for the preparation of wood panels. But formaldehyde emission caused by these resin adhesives has confused wood industry and has been a topic of concern for many years. Some standards or requirements were given to define the acceptable formaldehyde emission levels. The newest requirement on formaldehyde emission comes from California Air Resources Board (CARB) formaldehyde emissions regulation of U.S. It was passed in 2008 and got effective in 2009. Today, the mounting interest in formaldehyde emission is still driving major changes in wood panel industry as well as the resin industry that supplies the wood panel industry. In response to the need on environment-friendly adhesives, great attention has been given to adhesives from natural materials, such as starch [1,2], soy-based adhesives [3–5], and so on, although most of these adhesives have almost been pushed out of market in wood panel industry during the past 30 years. Soy-based adhesive was once a major adhesive for preparation of plywood. But it has been replaced by synthetic resins since the 1960s. Till the 1990s, it returned to the study area as a wood adhesive. It is reported that one of the soy-based formaldehyde-free adhesives has been used for production of interior plywood panels since 2004 [6]. However, the application of soy-based adhesives is rather limited. Now, most of the efforts on soy-based adhesive are given to the improvement of its bad water resistance. To resolve this problem, some methods could be employed to modify the soy adhesive, such
n
Corresponding author. E-mail address: [email protected] (H. Lei).
http://dx.doi.org/10.1016/j.ijadhadh.2014.01.026 0143-7496 & 2014 Elsevier Ltd. All rights reserved.
as hydrolysis, chemical denaturation, cross-linking, enzyme modification, and so on. Cross-linking modification is a comparatively acceptable method for the modification of soy-based adhesive. Cross-linkers can be either mixed with soy adhesive directly before its application or added during the preparation of soy adhesive. For the latter method, acrylates [7,8], maleic anhydride [9], etc. are used and in most cases graft polymerization will occur. Commonly, some complex preparation procedures are involved in this method. Therefore, it is much easier in handling to mix cross-linkers directly with soy adhesive. An effective cross-linker is the key for this method. Since there are many reactive groups in soy proteins, such as –OH, –SH, –COOH, and –NH2, many chemicals could be used for the cross-linking of soy adhesive. Epoxy [6], aldehyde and its deratives [10] have already been proved effective cross-linkers for soy-based adhesive. With different cross-linkers, the mechanism for the improvement of performance of soy adhesive is different. Epoxy groups are thought to react with all of the aforementioned functional groups in soy proteins [11]. Huang J et al. proposed the possible curing mechanisms of the soy–polyepoxide adhesives [12]. For cross-linker aldehyde and its derivatives, such as urea–formaldehyde, hydroxymethyl phenol, the main reaction group, comes from –NH2 in soy protein. Besides choosing a suitable cross-linker, its addition amount is very important for the application of soy adhesive. Because of the usage of some expensive cross-linkers, such as epoxy, the cost of soy-based adhesive is greatly dependent on the addition amount of cross-linker. Wood composites bonded with soy protein isolate and Kymene are reported to show shear strengths comparable to or higher than those of composites bonded with phenol–formaldehyde [13]. As a wet-strength agent for paper, Kymene is an aqueous solution of cationic polyamidoamine–epichlorohydrin
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(PAE). However, because of the high dry weight ratio SPI/Kymene 1.33:1, cross-linker Kymene is the most expensive component of the soy-based adhesive. A new curing agent was developed by the same research group from epichlorohydrin and ammonium hydroxide to decrease the cost of soy-based adhesive. But there was still about 14% on dry soy flour weight of this curing agent needed for the preparation of soy-based adhesive and interior plywood [6]. When melamine–urea–formaldehyde (MUF) resin is added to soy-adhesive as a cross-linker, the addition amount of solid MUF is about 40% on solid soy flour or even higher [14,15]. For phenol–formaldehyde, the weight ratio of phenol–formaldehyde to soy adhesive was 30:70 in public reports [16]. In this formulation phenol–formaldehyde had a solid content of 55% and soy solution of 42%. In this paper, the effects of different cross-linkers on the water resistance of soy-based adhesive were studied. In order to transfer all of the possible hydrophilic groups of soy protein to hydrophobic ones, a hybrid cross-linker was used for the modification of soy-based adhesive. The objectives of this work were as follows: (1) to optimize the proportion of components of hybrid crosslinker; (2) to decrease the addition amount of cross-linker; and (3) to prepare plywood with good water resistance.
2. Materials and methods 2.1. Materials Defatted soy flour (53.4% protein content) was obtained from Yuxin Soybean Protein Co., Ltd, China. Poplar veneer with a thickness of 1.5 mm and moisture content of 8–10% was purchased for the preparation of plywood. Epoxy resin (EPR) was a commercial product with the name of E-44, whose epoxide number was 0.41–0.47 and softening point was 12–20 1C. All other chemicals mentioned in this work were all of reagent grade. 2.2. Preparation of soy-based adhesive Soy-based adhesive was prepared according to a method already reported [17]. A three-neck round-bottom flask equipped with a mechanical stirrer, a thermometer and a condenser was charged with water (187 g), sodium dodecyl benzene sulfonate (0.8 g), CaO( 1.9 g) and NaOH (3.7 g) to 70 1C. Soy flour (80 g) was then charged to the rapidly stirring solution. The mixture was heated to 90 1C over 15 min, with rapid agitation, and held between 88 1C and 92 1C for 3 h. The mixture was cooled to 35 1C in an ice bath. The solid content of the resulting soy-based adhesive was 3071%. 2.3. Preparation of melamine–formaldehyde (MF) resin Formaldehyde 37% (150 g), melamine (80 g) and water (107 g) were charged into a three-neck flask equipped with a mechanical stirrer, thermometer and condenser and then the pH was adjusted to 9.0 with NaOH 30%. The mixture was heated to 85 1C during 20–30 min, and held the temperature for another 30 min. The mixture was cooled to room temperature and kept at pH 9.0 and room temperature. 2.4. Preparation of plywood samples bonded with soy-based adhesive The soy-based adhesive was mixed well with different crosslinkers just before the preparation of three-layer plywood of dimensions 300 mm 220 mm 4 mm. The double sides glue loading was 360 g/m2. Before sending into the press, the veneers with adhesives were allowed to rest at room temperature for 15 min and were then assembled. The plywood was pressed under
pressure of 2 MPa at 160 1C for 8 min. Other press times and press temperatures were also used in this paper. 2.5. Test of dry and wet shear strength of plywood samples After conditioning in the laboratory for 1 day, the plywood panel was then cut into shear specimens with dimension of 100 mm 25 mm to determine its shear strength and water resistance. Each specimen has a bonded area of 25 mm 25 mm. Both dry and wet shear strength of plywood specimens were tested on a WDS-50KN mechanical testing machine. For wet shear strength, the specimens were soaked in (6373) 1C water or in boiling water for determined time. The mean result of 8–10 specimens was considered as the final shear strength. The testing method was referred to Chinese national standard GB/T 9846.3-2004. In this standard, for type II plywood, the specimens were soaked in (6373)1C water for 3 h. For type I plywood, the specimens should pass water–dry–water cycle. That is to say, the specimens were firstly soaked in boiling water for 4 h, then dried in (6373) 1C for 20 h, and lastly re-applied in boiling water for another 4 h. For both type II and type I plywood, before the measuring by the testing machine, the specimens were taken out of water and left at room temperature for 10 min. 2.6. FT-IR analysis The oven was preheated to 160 1C. Liquid soy adhesives with or without cross-linkers were put in the oven to a constant weight. The cured soy adhesives were ground into fine powder. 1 g KBr and 0.001 g soy adhesive samples were mixed well for the preparation of KBr pills. The FT-IR spectra were obtained on a Varian 1000 infrared spectrophotometer.
3. Results and discussion 3.1. Effect of cross-linker on performance of soy adhesive-based plywood Table 1 shows the performance of plywood specimens with soy based-adhesive with or without different amounts of cross-linkers. The cross-linkers used in this work included EPR, MF and their mixture. The dry shear strength for all of the soy-based plywood specimens was good enough to satisfy the requirement of relative Chinese national standard (GB/T 9846.3-2004, Z0.70 MPa). But considering that wood failure for almost all of the specimens is 100%, which meant the measured dry strengths were mainly determined by the strength of wood, it was difficult to see the effects of cross-linker on the dry shear strength of plywood. However, the water resistance of plywood specimens showed big differences with or without cross-linkers. Soy adhesive without cross-linking had no water resistance at all, which was indicated by the 100% delamination of specimens when soaked in 63 1C water for 3 h. All of the three cross-linkers in this work improved the water resistance of soy-based adhesive more or less. When cross-linked with different amounts of EPR, although the wet strength of plywood at 60 1C for 3 h could be measured, it was not enough to meet the requirement of relative standard. More than 50% of the specimens cross-linked by EPR failed the soak test in 63 1C water for 3 h and 100% of the specimens delaminated when the test condition became more severe as in boiling water cycles. With the increase of the addition amount of EPR, the percentage of number of specimens that failed in the soak test to the total number of specimens decreased from 50% to 20%, which indicated the improvement of the water resistance of soy-based adhesive, although their water resistance was not enough to meet the relative standard. Epoxy was assumed to improve the water resistance of soy adhesive because of the reaction between epoxy groups
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Table 1 Effect of different kinds of cross-linker on the performance of soy adhesive-based plywood. Cross-linker
Addition amount of cross-linker (%)
Without cross-linker 0 EPR 6 8 10 12 14 MF 6 8 10 12 14 EPR þ MF 10(5 þ5)n n
Dry shear 63 1C 100 1C strength(MPa) Wet shear Number of specimens failed Wet shear Number of specimens failed strength (MPa) in the soak test/total specimens (%) strength (MPa) in the soak test/total specimens (%) 1.02 0.85 0.80 0.86 0.77 0.85 0.80 1.06 0.88 0.86 1.32 1.05
– 0.41 0.38 0.39 0.48 0.46 1.03 0.80 0.94 0.92 0.94 0.91
– – – – – – 0.47 0.51 0.51 0.52 0.52 0.85
100 50 50 30 20 20 0 0 0 0 0 0
– 100 100 100 100 100 40 30 20 10 20 100
The weight ratio of solid EPR to MF on the solid weight of soy adhesive was 1:1.
Table 2 Effect of addition amount of cross-linker EPRþ MF on the performance of soy adhesive-based plywood. Cross-linker
3.2%EPR þ2.6%MF 3.2%EPR þ3.9%MF 3.2%EPR þ5.1%MF 3.2%EPR þ6.4%MF 6.4%EPR þ6.4%MF 6.4%EPR þ7.7%MF
Dry shear strength (MPa)
0.96 1.02 1.01 1.21 0.95 0.93
63 1C
100 1C
Wet shear strength (MPa)
Number of specimens failed in the soak test/total specimens (%)
Wet shear strength (MPa)
Number of specimens failed in the soak test/total specimens (%)
0.49 0.44 0.73 0.72 1.08 0.76
0 0 0 0 0 0
0.45 0.49 0.65 0.65 0.81 0.93
0 0 0 0 0 0
with high reactivity at the opposite ends of resin molecule and reactive groups in soy protein. In this work, just epoxy was used as itself without excess solvent or even curing agent being added. It is wellknown that a curing agent is necessary for the curing of epoxy. The most commonly used curing agent for epoxy is amine. Here, soy protein could be regarded as the curing agent of epoxy. Once there is some curing reaction between epoxy and protein, the water resistance and mechanical performances of the final soy-based system would be improved with the increase of its molecular weight. Although EPR could react with a wide variety of functional groups in soy protein, high addition amount of EPR could not be used as an ideal cross-linker for three reasons. (1) EPR is not soluble in water. Simple mixing of soy flour and an epoxy resin in water cannot generate a homogeneous mixture. High addition amount EPR tended to aggregate in the presence of water and got a poor cross-linking with soy. This was the reason why the soy adhesive could not resist the boiling water cycle test. (2) EPR will increase the viscosity of soy-based adhesive a lot with a high addition amount, which will greatly affect the application of the adhesive. (3) EPR is too expensive. It was unacceptable to the industry when addition amount of EPR was higher than 10% of the solid soy. Once the addition amount of EPR was higher than 10%, the cost of EPR would be almost 30% of the soy powder. MF is a widely-used adhesive with good water resistance in wood industry. Soy adhesive cross-linked with water-soluble MF shows better water resistance than that with the same amount of EPR, as seen from Table 1. All of the specimens with MF passed the soak test at 63 1C for 3 h. No specimen delaminated and the wet strength for all the specimens was higher than the standard requirement 0.70 MPa. This meant that interior type II plywood could be prepared with soy adhesive cross-linked with as low as 6% of MF on solid soy adhesive. But when the test conditions were changed to those of boiling water cycle, although most of the specimens did not
delaminate, the wet shear strength decreased greatly. Even when the addition amount of MF increased to 14% of solid soy, the wet shear strength could not satisfy the relative standard. This meant that only with MF, the hydrophilic groups in soy adhesive of this paper could not be totally hidden or reacted with hydrophobic groups. For wet shear strength in 63 1C water and in boiling water, the effects of addition amount of MF on wet strength were not clear. To make use of the high reactivity of EPR and the water solubility of MF, a hybrid cross-linker EPRþ MF was used in this work. As seen from Table 1, the hybrid cross-linker MFþ EPR indeed showed the best water resistance. With 10% EPRþ MF, even type I plywood could be prepared. 3.2. Effect of hybrid cross-linker EPR þMF on the performance of soy adhesive -based plywood Hybrid cross-linker EPR þMF was proved to be the best crosslinker for soy-based adhesive as seen from Table 1. To optimize the proportion of components of hybrid cross-linker and decrease the addition amount of soy adhesive for plywood, in the hybrid crosslinker, different amounts of EPR and MF were used for preparation of plywood (Table 2). As in Table 1, when using hybrid EPRþMF as a cross-linker of soy adhesive, all of the dry shear strength of plywood specimens cross-linked with different amounts of EPRþMF satisfied the requirements of the relative standard. In the soak test, either in 63 1C water for 3 h or in boiling water cycle, no specimen delaminated. It indicated that soy-based adhesive cross-linked with EPR þMF had good water resistance even with a very low addition amount of cross-linker, that is, 3.2% EPR and 2.6% MF on solid soy adhesive. In laboratory condition, during the preparation of plywood, lower addition amount of cross-linker than 3.2% EPR or 2.6% MF was difficult to handle because of the
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Table 3 Effect of press procedure on the performance of soy adhesive-based plywood. Press variable
Dry shear strength (MPa)
63 1C Wet shear strength (MPa)
100 1C Number of specimens failed in the soak test/total specimens (%)
Wet shear strength (MPa)
Number of specimens failed in the soak test/total specimens (%)
Press temperature (1C) 120 1.12 140 0.94 160 0.95 180 0.85
0.84 1.03 1.08 1.12
0 0 0 0
0.61 0.83 0.81 0.76
0 0 0 0
Press time (min) 2 4 6 8
– – – 1.08
100 100 100 0
– – – 0.81
100 100 100 0
0.45 0.83 0.89 0.95
small amount. When the addition amount of EPR was fixed to 3.2% on solid soy adhesive, with the increase of the addition amount of MF, the wet shear strength of plywood increased. However, at least 5.1% MF should be used together with 3.2% EPR to get the modified soy-based adhesive for the preparation of type II plywood. For type I plywood, the minimum addition amount of cross-linker of soy based-adhesive was 6.4% MF with 6.4% EPR in Table 2. Considering the results of Table 1, the total amount of hybrid cross-linker MF þEPR should be 10–12.8% for preparation of type I plywood. The increase of wet shear strength caused by the increase of addition amount of EPR could also be seen in Table 2. 3.3. Effect of press procedure on the performance of soy-based adhesive with hybrid cross-linker EPR þMF Soy adhesives with 6.4%EPR þ 6.4%MF were used for the preparation of plywood in the laboratory. Effects of press temperature and press time on the performance of plywood with soy-based adhesive are shown in Table 3. The results show that even with press temperature 120 1C, type II plywood could be obtained. With press temperature 140 1C, the water resistance of plywood was good enough to meet the requirement of type I plywood and did not show much difference with those when higher temperatures 160 1C and 180 1C were used. 2–6 min at 160 1C was too short to cure soy adhesive with EPRþ MF, which indicated by the poor water resistance shown in Table 3. With 4–6 min, the dry shear strength of plywood was good. This meant that 4–6 min was enough for curing of soy adhesive but not enough for its cross-linking. Hot-press time as long as 8 min may be a problem for the commercialization of the cross-linked soy adhesive. 3.4. FT-IR analysis of soy adhesive modified with cross-linkers The FTIR spectra of cured soy-based adhesive, soyþ6.4% EPR, soyþ6.4%MF and soyþ6.4% EPRþ6.4%MF are given in Fig. 1. For soy adhesive without cross-linkers, the broad band observed in the range of 3500–3000 cm 1 was assigned to the free and bound O–H and N– H groups. The absorption bands of amide, the characteristic group of protein, were observed at 1654, 1539, and 1238 cm 1, which were assigned to CQO stretching, N–H bending, C–N stretching and N–H bending vibration, respectively. The COO– and –C–NH2 absorptions were seen at 1390 and 1058 cm 1, respectively. The main difference between spectra of soy adhesive with or without EPR came from the absorptions at 1249 and 833 cm 1, which were assigned to C–O–C stretching. There were three possibilities for the C–O–C stretching: (1) epoxy ether; (2) phenol–O–C; and (3) the resulting ether from the reaction between epoxy and –OH of soy protein [18]. The former two groups came from EPR itself. The reaction between EPR and –OH of
4000
3500
3000
2500
2000
Wave numbers/cm
1500
1000
500
-1
Fig. 1. FT-IR of the soy adhesive with or without cross-linker: (a) soy adhesive; (b) soyþ6.4%EPR; (c) soyþ6.4%MF; and (d) soyþ 6.4%EPR þ 6.4%MF.
soy protein might be the main reason for the improvement of water resistance of soy-based adhesives caused by epoxy. However, the possible reaction between EPR and –NH2, –SH, –COOH groups of soy is not clearly observed in Fig. 1. In soy adhesive cross-linked with MF, the absorption at 1238 cm 1 got weaker than that in pure soy adhesive, which might be a result of the reaction between methylol group of MF resin and N–H groups of soy adhesive [11]. The increase of the absorption at 1539 cm 1 was caused by –CQN ring vibration of melamine. Bending vibration of triazine ring was found at 829 cm 1.The spectra of soy cross-linked with EPR and MF were almost the multiplicity of soy adhesive cross-linked by EPR or MF alone. Although a thorough mechanism on the soy-based mixing system is still needed, the fact that epoxy and MF could react with different groups in soy protein might be the main reason for the good water resistance of soy-based adhesive with hybrid cross-linker. On one hand, the mixing of epoxy and MF transferred the hydrophilic groups of soy protein to hydrophobic ones as much as possible. On the other hand, the epoxy was used as less as possible to avoid aggregation and the large increase of viscosity.
4. Conclusions To develop a soy-based adhesive with good water resistance and acceptable cost, it was modified with cross-linkers, namely, EPR, MF and their mixture EPR þMF in this paper. All the three
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cross-linkers improved the water resistance of soy-based adhesive more or less. The soy-based adhesive cross-linked with hybrid cross-linker EPRþ MF showed the best water resistance. With press temperature 160 1C and press time 8 min, type II and even type I plywood could be prepared when 6.4%EPR þ6.4%MF was used as the cross-linker of soy-based adhesive. FT-IR indicated that the great improvement of water resistance of soy-based adhesive modified with EPR and MF might be caused by the reaction between epoxy and –OH, and that between MF and –NH. Acknowledgments This research was supported by the National Natural Science Foundation of China (31170530) and New Century Excellent Talents of Ministry of Education of China (NCET-10-0972). References [1] Wang Z, Li Z, Gu Z, Hong Y, Cheng L. Preparation, characterization and properties of starch-based wood adhesive. Carbohyd Polym 2012;88 (2):699–706. [2] Tondi G, Wieland S, Wimmer T, Schnabel T, Petutschnigg A. Starch–sugar synergy in wood adhesion science: basic studies and particle board production. Eur J Wood Wood Prod 2012;70(1–3):271–8. [3] Lin Q, Chen N, Bian L, Fan M. Development and mechanism characterization of high performance soy-based bio-adhesives. Int J Adhes Adhes 2012;34:11–6.
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