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Comparing bond strength and water resistance of alkali-modified soy protein isolate and wheat gluten adhesives
ARTICLE IN PRESS International Journal of Adhesion & Adhesives 30 (2010) 72–79
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International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Comparing bond strength and water resistance of alkali-modified soy protein isolate and wheat gluten adhesives ¨ a, Petra Nordqvist a, Farideh Khabbaz b, Eva Malmstrom a b
Department of Fibre and Polymer Technology, Royal Institute of Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden Analyscentrum, Casco Adhesives AB, Box 115 38, SE-100 61 Stockholm, Sweden
a r t i c l e in f o
a b s t r a c t
Article history: Accepted 14 September 2009 Available online 23 September 2009
The tensile strength of beech substrates bonded with dispersions of alkali-denatured soy protein isolate (SPI) and wheat gluten (WG) was measured for comparison of bond strength and resistance to cold water. The proteins were denatured with 0.1 M NaOH (pH 13). Dispersions with different protein concentration and viscosity were investigated. The adhesive properties were studied at different press temperatures (90, 110, and 130 1C) and press times (5, 15, and 25 min). Two types of application methods were used in order to overcome the problem with different viscosity of the dispersions. In addition, SPI was denatured at two different pH levels (approximately 10 and 13) and with two different concentrations of salt (158 mM and 0.1 M), in order to compensate for the different isoelectric points of the proteins. The adhesive properties of WG powder with different particle sizes were also compared. The tensile strengths of the wood substrates were measured according to somewhat simplified versions of the European Standards EN 204 and EN 205. The bond lines were studied with light microscopy. The results indicate that the adhesive properties of SPI are superior, particularly with regard to water resistance. However, the water resistance of WG was to some extent improved when starved adhesive joints could be avoided. Similar tensile strength values were obtained for the dispersions of alkalidenatured SPI regardless of pH or salt concentration. No apparent difference in adhesive strength was observed for the WG dispersions from powder with different particle sizes. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Adhesives for wood Novel adhesives Wood Mechanical properties of adhesives Plant protein
1. Introduction Soybean, blood, and casein have long been the most widely used sources of protein for manufacturing of wood adhesives [1]. Since the 1960s, however, protein adhesives have more or less been replaced by petroleum-based products, most of which are superior in strength and water resistance. Nevertheless, due to the increasing environmental concern, adhesives based on proteins are becoming more attractive since they are derived from renewable resources, while the resources of petroleum-based adhesives are both non-renewable and limited. Protein from soybean has been used as a wood adhesive on its own, but also in combination with other proteins, such as blood and casein, and with synthetic resin polymers [2–5]. Extensive research has been performed in recent years to improve adhesion strength and water resistance of soy protein. Heat treatment and treatment with different denaturation agents, such as alkali, urea, guanidine hydrochloride, trypsin, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and cationic detergents, were found
Corresponding author. Fax: + 46 8 790 82 83.
¨ E-mail address: [email protected] (E. Malmstrom). 0143-7496/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2009.09.002
to be useful in enhancing the adhesive properties of soy protein [6–9]. If this resumed interest in proteins is to result in a growing market for plant-protein-based adhesives, it is required, both from an economic and environmental point of view, that the plant protein sources are grown close to the market. Since the soybean plant is not native worldwide, it is therefore of interest to also study proteins from other plants, such as wheat gluten from wheat. Wheat gluten is an industrial by-product of wheat starch production, and is mainly used in the bakery industry. However, it has successfully been used in making biodegradable films and coatings for food and non-food applications [10]. Some research has also been performed regarding its ability to bond wood [11]. Even though soy protein and wheat gluten are similar in the sense that they both consist mainly of storage proteins, their chemical compositions and physical behaviours are different. Soy protein consists of two protein classes: water-soluble albumins and salt solution-soluble globulins, where globulins are the major fraction [9,12]. However, the globulins can be further divided into glycinin and conglycinin. Glycinin consists of six acidic-basic subunits joined by disulfide bonds, while conglycinin consists of only three subunits. Furthermore, conglycinin is less hydrophobic
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than glycinin and the subunits of conglycinin are mainly joined with hydrogen bonds. The molecular weight of glycinin is approximately 200–400 kDa, while it ranges from 100 to 200 kDa for conglycinin. The pH is approximately 4.5 at the isoelectric point (pI) of soy protein. On the other hand, approximately 80% of wheat gluten consists of wheat storage protein [10]. The rest is composed of traces of polysaccharides, lipids, and minerals. The wheat storage protein can be divided into two protein classes: gliadins and glutenins. Gliadins are alcohol-soluble, while glutenins are dispersible in dilute acid or alkaline solutions. Both protein classes are rich in proline and glutamine. However, the molecular weight of gliadines ranges from 30 to 74 kDa, while the estimated molecular weight of glutenin is over 107 Da. Glutenins consists of polypeptide chains linked together by disulfide bonds. The pH value at the isoelectric point of wheat gluten is about 7.3 [13]. In overall, wheat gluten has a more hydrophobic character than soy protein. In this study, water dispersions of soy protein isolate (SPI) and wheat gluten (WG) were used as wood adhesives, in order to bond together wood substrates of beech. The proteins were denatured with alkali (0.1 M NaOH (aq); pH 13). The adhesive properties were studied at different press temperatures (90, 110, and 130 1C) and press times (5, 15, and 25 min). Two types of application methods were used in order to overcome the problem of different viscosity of the dispersions. Furthermore, SPI was denatured at different pH (approximately 10 and 13) and ion concentrations (158 mM and 0.1 M), in order to compensate for the different isoelectric points of the proteins. The aim of the study was to investigate whether there are any differences in adhesive behaviour between SPI and WG. WG dispersions of powder with different particle sizes were also investigated. The wood substrates were conditioned and the tensile strength measurements were performed according to somewhat simplified versions of the European Standards EN 204 and EN 205 [14,15]. The standards were used for assessment of bond strength and resistance to cold water of the protein dispersions.
2. Experimental 2.1. Materials Commercial soy protein isolate (SPI) Soy Pro 900 (from Qingdao Crown Imp. & Exp. Corp. Ltd, kindly supplied by Roquette), containing approximately 90% protein, and wheat ¨ gluten (WG) Reppe Vital (Lantmannen Reppe AB, Sweden), containing approximately 85% protein, were employed in this study. A fraction of Reppe Vital, with a smaller particle size ( o56 mm) than the regular batch of Reppe Vital, was also used. This fraction was denoted WG fine. According to the supplier, the
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particle size of the regular batch of Reppe Vital has the following distribution: 4100 mm (28%), 50–100 mm (55%), and o50 mm (18%). For Soy Pro 900, at least 96% of the sample has a particle size below approximately 150 mm. Beech wood pieces were purchased from Konrad Bruckeder (Rosenheim, Germany). 2.2. Preparation of dispersions of WG, WG fine, and SPI Table 1 summarizes the conditions used for preparing dispersions of WG, WG fine, and SPI. A sodium hydroxide solution (0.1 M; pH 13) was used to prepare 24% (w/w) dispersions of WG and WG fine, and a 12% (w/w) dispersion of WG. The protein products were added while stirring at room temperature. The dispersions are denoted 24-WG-NaOH, 24-WGfine-NaOH, and 12-WG-NaOH, respectively. SPI was dispersed in four different solutions: de-ionized water, 0.1 M NaOH (pH 13), 158 mM NaOH (pH 10), and a water solution containing 158 mM NaOH and 99.8 mM NaCl. SPI was added to the different solutions while stirring at room temperature. The dispersions are denoted 12-SPI-H2O, 12-SPI-NaOH, 12-SPINaOH(pH 10), and 12-SPI-NaOH(pH 10)/NaCl, respectively. The salt concentration of dispersion 12-SPI-NaOH(pH 10)/NaCl was 0.1 M. Thus, it was the same concentration as for 12-SPI-NaOH, even though that was prepared only from NaOH. Depending on the combination of protein and solvent, different combinations of stirring rate and stirring time had to be used in order to obtain homogeneous dispersions without increasing the temperature of the dispersions as a result of the choice of stirring rate. 2.3. pH and viscosity measurements of the protein dispersions pH was measured with a pH glass electrode (Metrohm 6.0258.000, Metrohm Nordic AB, Bromma, Sweden) connected to a pH meter (704 pH Meter, Metrohm Nordic AB, Bromma, Sweden). It was measured on the same day as the dispersions were manufactured. The viscosity of the dispersions was measured with a Brookfield Viscometer (Model DV-II+ Pro, VWR International AB, Stockholm, Sweden) and computer program Rheocalc V2.5 (Brookfield Engineering Labs, Inc.) the day after they were manufactured. Spindle LV1 and LV4, and speed 100, 3, and 0.3 rpm were used. The viscosity was recorded one minute after starting the measurement. 2.4. Specimen preparation Application Method 1: In Application Method 1 the one-day-old dispersion (24-WGNaOH, 24-WGfine-NaOH, 12-SPI-H2O, and 12-SPI-NaOH) was used
Table 1 Conditions used for preparing dispersions of wheat gluten (WG), wheat gluten fine (WG fine), and soy protein isolate (SPI). Dispersion
Conc. (%, w/w)
Amount of SPI/WG (g)
Dispersing agent
Amount of dispersing agent (g)
Stirring rate (rpm)
Stirring time (min)
24-WG-NaOH 24-WGfine NaOH 12-WG-NaOH 12-SPI-H2O 12-SPI-NaOH 12-SPI NaOH(pH 10) 12-SPI-NaOH(pH 10)/NaCl
24 24 12 12 12 12 12
48 48 30 26 30 30 30
0.1 M NaOH 0.1 M NaOH 0.1 M NaOH De-ionized water 0.1 M NaOH 158 mM NaOH 158 mm NaOH+ 99.8 mM NaCl
152 152 220 190 220 220 220
200–330 200–330 300 370 300–370 300 300
100 100 80 115 110 40 55
The dispersions were prepared at room temperature.
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to bond together two panels of beech, with dimension 5 135 400 mm or 5 126 650 mm (thickness, width, and length), and conditioned and evaluated according to slightly modified versions of the European Standards EN 204 and EN 205 [14,15]. The panels were bonded together with what according to the standards is classified as a thin bond-line. On one side of each panel, 180 g/m2 of dispersion was applied. A pressure of 0.7 MPa, and different combinations of press temperatures and press times were used. The temperatures and times are summarized in Table 2. All combinations were used with the dispersions 24-WG-NaOH, 12SPI-H2O, and 12-SPI-NaOH, while 24-WGfine-NaOH was only used with the combination 110 1C and 15 min. The bonded beech panels were cut into test pieces, which were treated according to the conditioning sequences shown in Table 3. The table also summarizes the minimum values of adhesive strength that must be reached for the classification of thermoplastic adhesives into the durability classes D1 to D3. An Alwetron tensile testing machine (model TCT 50, Lorentzen & Wettre, Sweden) was used. The length of the test pieces was 100 mm instead of 150 mm, which is the standard (EN 205). Five or seven test pieces were tested for each conditioning sequence instead of 10 pieces, which is the standard. Application Method 2: In Application Method 2 the one-day-old dispersions of 12WG-NaOH, 12-SPI-H2O, 12-SPI-NaOH, 12-SPI-NaOH(pH 10), and 12-SPI-NaOH(pH 10)/NaCl were used for bonding. The same type of beech panels with dimensions 5 135 400 mm or 5 126 650 mm (thickness, width, and length) were used. Two panels were bonded together. On one side of each panel, 180 g/m2 of dispersion was applied. The dispersion layer was allowed to dry in a conditioned room ((2072) 1C and (6575)% relative humidity) for 24 h. The procedure was repeated once. The panels were allowed to dry in the conditioned room for an additional 24 h. Prior to bonding, the treated surfaces of the panels were re-wetted with water (approximately 170 g/m2). A pressure of 0.7 MPa, a press temperature of 110 1C, and a press time of 15 min were used. The rest of the evaluation was performed according to Application Method 1. Table 2 Different combinations of press temperatures and press times. Press temperature (1C)
Press time (min)
90 90 110 130 130
5 25 15 5 25
Table 3 Conditioning sequences and minimum values of adhesive strength for thin bondlines. Conditioning sequences Duration and condition
Adhesive strength (MPa)
Durability classes
7 daysa in standard atmosphereb
Z 10
D1, D2, and D3
7 daysa in standard atmosphereb, 3 h in water at (207 5) 1C, 7 daysa in standard atmosphereb
Z8
D2
7 daysa in standard atmosphereb, 4 daysa in water at (20 75) 1C
Z2
D3
a b
1 day= 24 h. (207 2) 1C and (65 7 5)% relative humidity.
2.5. Particle size distribution Five dispersions (24-WG-NaOH, 24-WGfine-NaOH, 12-WGNaOH, 12-SPI-H2O, and 12-SPI-NaOH) were analysed with light scattering/diffraction according to Mie theory [16]. A Mastersizer particle size analyser (Mastersizer 2000, Malvern Instrument Nordic AB, Uppsala, Sweden) with measuring range 0.020– 2000 mm, was used. Dispersion 12-SPI-H2O was diluted with de-ionized water, while the other dispersions were diluted with 0.1 M NaOH. The solutions were stirred for five minutes in the instrument prior to the analysis. The results are based on two samples of each dispersion analysed in triplicate. The analyses were performed the day after the dispersions were manufactured. 2.6. Light microscopy The bonded joints of the wood substrates were examined under a Leica DMRM light microscope (Leica Microsystems AB, Stockholm, Sweden) equipped with a fluorescence filter (Leica filter cube H3) and a CCD camera (Leica DFC 280). The wood substrates were stained with 0.1% aqueous Safranine-O (Basic Red 2, ICN Biomedicals Inc.) prior to the analysis.
3. Results and discussion 3.1. Preparation of protein dispersions, pH and viscosity measurements, and the choice of application techniques WG and SPI can be dispersed in water solutions adjusted to a pH both below and above their isoelectric points (pH approximately 7.3 and 4.5, respectively) [9,10]. In this study, an alkaline condition was chosen for comparing the adhesive properties of the two plant proteins. A water solution of sodium hydroxide (0.1 M; pH 13) was chosen as a dispersing agent, since WG Reppe Vital is dispersible in NaOH (aq) with a concentration above 0.07 M. Nevertheless, since SPI is also dispersible in de-ionized water, it was of interest to compare the adhesive properties of SPI when it is dispersed in water and in 0.1 M NaOH. Moreover, since the pH value at the isoelectric point of WG and SPI is different, SPI was also dispersed in 158 mM NaOH (pH 10). Thus, the difference in pH between the isoelectric point of SPI and the dispersing agent (158 mM NaOH) is similar to the corresponding gap between the isoelectric point of WG and 0.1 M NaOH (pH 13). In order to partly compensate for the lower ion concentration of 158 mM NaOH, SPI was also dispersed in 158 mM NaOH with addition of sodium chloride (99.8 mM). The results from the pH measurement are summarized in Table 4, and the results indicate that the buffer capacity is slightly different for WG and SPI. According to the results, the pH of 12-WG-NaOH is 12.2, while it is 11.5 for 12-SPI-NaOH, even though the dispersing agent is the same (0.1 M NaOH, pH 13). However, this is expected since the amino acid composition of WG and SPI is different. Furthermore, due to the buffer capacity of SPI, the pH of the dispersions 12-SPI-H2O, 12-SPI-NaOH(pH 10), and 12-SPI-NaOH(pH 10)/NaCl is similar (about 7.7–7.9) even though the pH of the dispersing agents ranges from approximately 6–10. There are three factors that an adhesive needs to fulfil for a proper bond to form: it must be able to wet the surface of the wood material, flow over it, and penetrate into the wood [17]. These three factors are partly dependent on the viscosity of the adhesive and thereby the dry content of the adhesive. The dry content of water-borne adhesives is normally about 50–65%, partly in order to prevent a too large extent of water being left in the adhesive joint and wood material after pressing. It is therefore
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of interest to try to maximize the dry content of the adhesives, without the adhesive becoming too viscous and thereby decreasing its ability to properly wet, flow, and penetrate the wood substrate. A highly viscous adhesive is also more difficult to handle. However, in this study, the concentration of WG and SPI in the dispersions could not be increased to more than 24% (w/w) and 12% (w/w), respectively, before the viscosity became too high. Despite the high water content, it was possible to use the dispersions as adhesives. Nevertheless, in order to facilitate the comparison of adhesive properties, it would have been desirable to use the same concentration of protein in the dispersions regardless of protein type. However, as can be seen in Table 4, the viscosity of the 12% dispersion of WG (12-WG-NaOH) was unfortunately too low. This dispersion could therefore only be used with Application Method 2. Overall, the viscosity of the dispersions differs largely, even though similar concentrations of protein and dispersing agent are Table 4 Viscosity and pH of soy protein isolate (SPI) and wheat gluten (WG) dispersions. Dispersion
pHa
Viscosityb (Pas)
Spindle, speedc (rpm)
24-WG-NaOH 24-WGfine-NaOH 12-WG-NaOH 12-SPI-H2O 12-SPI-NaOH 12-SPI-NaOH(pH 10) 12-SPI-NaOH(pH 10)/NaCl
10.6 10.4 12.2 7.9 11.5 7.8 7.7
77 94 0.025 3.1 980 3.5 2.7
LV4, LV4, LV1, LV4, LV4, LV4, LV4,
3 3 100 100 0.3 100 100
a
pH was measured on the manufacturing date of the dispersions. Viscosity was measured the day after the manufacturing date of the dispersions. The viscosity values were recorded one minute after starting the measurement. c Different spindles and speeds had to be used, due to the large difference in viscosity between the dispersions. b
24-WG-NaOH 12-SPI-H2O
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used (Table 4). Due to this difference, different spindles and speeds had to be used while measuring the viscosity, which must be considered when comparing the results. Furthermore, the difference in viscosity also complicates the comparison of the adhesive properties, since the viscosity ought not to be too high or too low when the dispersion is applied on the wood panel prior to the pressing. If the viscosity is too high, it will be very difficult or even impossible to evenly spread the adhesive onto the wood panel. On the other hand, if the viscosity is too low, the dispersion will either drain off or to a considerable extent penetrate the substrate, rendering the adhesive layer too thin. In order to deal with this problem, two different techniques were used to apply the dispersions. In the first case (Application Method 1), the viscosity of the dispersions was similar, but the concentrations of SPI and WG were different. The dispersions 24-WG-NaOH, 12-SPI-NaOH, and 12-SPI-H2O were applied to the wood panel directly prior to pressing. Equal amounts of the dispersions were applied, but also half the amount of 24-WG-NaOH in order to reduce the differences in amount of protein in the joint. In the second case (Application Method 2), the same concentration of SPI and WG was used and the following dispersions were used as adhesives: 12-WG-NaOH, 12-SPI-NaOH, 12-SPINaOH(pH 10), and 12-SPI-NaOH(pH 10)/NaCl. Since the viscosity of 12-WG-NaOH is extremely low, a different application technique had to be used in order to prevent the dispersion draining off or over-penetrating the wood substrate. Therefore, after applying the dispersions onto the wood substrates, the dispersion layer was allowed to dry. This procedure was used twice on each wood substrate. The treated surfaces were remoistened immediately prior to bonding. However, irrespective of application method, the problems of different viscosities and protein concentrations of the dispersions cannot be completely circumvented. Even though the amount of protein is adjusted in Application Method 1, the initial amount of water in the joint will be different. In Application Method 2, this
24-WG-NaOH, glue amount/2 12-SPI-NaOH
20 18
Tensile strength (MPa)
16 14 12 10 8 6 4 2 0 90°C, 5 min
130°C, 5 min
90°C, 25 min
130°C, 25 min
110°C, 15 min (1)
110°C, 15 min (2)
110°C, 15 min (3)
Fig. 1. Tensile strength measurements of dry wood substrates (Application Method 1). The wood substrates were bonded on the same occasion with different combinations of press temperature and press time. The combination 110 1C and 5 min was used in triplicate. The horizontal black line at 10 MPa indicates the limit for passing the test according to the European Standard EN 204.
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problem has been eliminated, but since the viscosity differs, the proteins might behave differently during drying and re-wetting, which could influence the adhesion properties. Therefore, these issues have to be considered when comparing the results.
dispersion. Furthermore, the 24-WG-NaOH dispersion failed to keep the wood pieces together at 90 1C and 5 min. This failure indicates, at least, that for these types of WG dispersions a higher press temperature or a longer press time is needed for improved bonding results. Nevertheless, for all combinations of press temperature and press time except 90 1C and 5 min, and one of the combinations of 110 1C and 15 min, the highest tensile strength value is obtained with 12-SPI-NaOH. Thus, the tensile strength of the sodium hydroxide denaturated SPI seems to be superior to that of the non-modified SPI (12-SPI-H2O). This conclusion agrees with what is reported by Hettiarachchy et al. [6]. The unfolded protein is more available for entanglement, interaction, and reaction. Fig. 2 shows the results from tensile strength measurements of the wood substrates soaked in water for four days. The results distinctly reveal that the adhesive properties of SPI are superior to WG. This is especially evident when the wood substrates are still wet, such as in this case. The wood substrates bonded with the WG dispersions at 90 1C press temperature, and 130 1C and 5 min
3.2. Tensile strength measurements—comparison between 24-WGNaOH, 12-SPI-H2O, and 12-SPI-NaOH (Application Method 1) The tensile strength measurements of the dry wood substrates are presented in Fig. 1. Most of the values are above 10 MPa, which means that the wood substrates have passed the test according to standards. The results do not reveal any clear differences in adhesive strength between the different dispersions. However, the variation in results is relatively high, especially when comparing the triplicates at 110 1C and 15 min originating from the same occasion. This variation in results can be due to inhomogeneous wood. It is noteworthy that similar tensile strength values are obtained for 24-WG-NaOH, regardless of the added amount of
6 24-WG-NaOH 24-WG-NaOH, glue amount /2
5 Tensile strength (MPa)
12-SPI-H2O 12-SPI-NaOH
4 3 2 1 0
90°C, 5 min
-1
130°C, 5 min
90°C, 25 min
130°C, 25 min
110°C, 15 min (1)
110°C, 15 min (2)
110°C, 15 min (3)
Fig. 2. Tensile strength measurements of wood substrates soaked in water for four days (Application Method 1). The wood substrates were bonded on the same occasion with different combinations of press temperature and press time. The combination 110 1C and 15 min was used in triplicate. The horizontal black line at 2 MPa indicates the limit for passing the test according to the European Standard EN 204.
20 12-WG-NaOH
18
Tensile strength (MPa)
16 14
12-SPI-NaOH 12-SPI-NaOH(pH~10) 12-SPI-NaOH(pH~10)/NaCl
12 10 8 6 4 2 0 110°°C, 15 min (1)
110°°C, 15 min (2)
Fig. 3. Tensile strength measurements of dry wood substrates (Application Method 2). The wood substrates were bonded in duplicate on the same occasion at 110 1C and 15 min. The horizontal black line at 10 MPa indicates the limit for passing the test according to the European Standard EN 204.
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press time, even fell apart. On the other hand, those bonded with the SPI dispersions performed well and the majority of them even passed the test according to standards. The results of the WG dispersions are slightly improved at higher press temperature and time, but the tensile strength values are still markedly below the values of SPI.
3.3. Tensile strength measurements—comparison between 12-WGNaOH, 12-SPI-NaOH, 12-SPI-NaOH(pH 10), and 12-SPINaOH(pH 10)/NaCl (Application Method 2). Comparison between results obtained with Application Methods 1 and 2 The tensile strength values of the wood substrates bonded with the three dispersions of SPI are similar between themselves, regardless of the three types of conditioning sequences. The results from the tensile strength measurements of dry wood substrates are shown in Fig. 3 (Application Method 2). Different pH and ion concentration do not seem to affect the adhesive
77
properties of SPI at these high pH levels (pH approximately 10 or 13). According to these results, either the degree of unfolding of SPI due to the salt concentration and/or pH is more or less similar, or the extent of unfolding has reached a limit where it no longer contributes with additional effect on the results. In the comparison between the WG and SPI dispersions, the results from the measurements agree with the results shown for Application Method 1. Hence, the adhesive properties of SPI are superior to those of WG with regard to water resistance. The tensile strength values of the wood substrates bonded with 12-WG-NaOH and soaked in water for four days are much lower than the values obtained with the substrates bonded with the SPI dispersions. However, in a comparison of the strength values from the three-hour-water-soaked and conditioned wood substrates, the relationship between the strength values differs depending on the application method used (Fig. 4a and b). Similar results (approximately 10–12 MPa) are obtained for the wood substrates bonded according to Application Method 2 regardless of dispersion used, while those bonded according to Application
20 18
24-WG-NaOH
12-SPI-NaOH
Tensile strength (MPa)
16 14 12 10 8 6 4 2 0 110°°C, 15 min (method 1, bonding 1)
110°°C, 15 min (method 1, bonding 2)
110°°C, 15 min (method 1, bonding 3)
20 18
12-WG-NaOH
12-SPI-NaOH
Tensile strength (MPa)
16 14 12 10 8 6 4 2 0 110°C, 15 min (method 2, bonding 1)
110°C, 15 min (method 2, bonding 2)
Fig. 4. (a) Tensile strength measurements of wood substrates soaked in water for three hours and conditioned for 7 days (Application Method 1). The dispersions 24-WGNaOH and 12-SPI-NaOH were used as adhesives. The concentrations of protein in the dispersions are different, which results in different protein amounts in the joint. The wood substrates were bonded in triplicate on the same occasion at 110 1C and 15 min. The horizontal black line at 8 MPa indicates the limit for passing the test according to the European Standard EN 204. (b) Tensile strength measurements of wood substrates soaked in water for three hours and conditioned for 7 days (Application Method 2). The dispersions 12-WG-NaOH and 12-SPI-NaOH were used as adhesives. The concentrations of protein in the dispersions are similar, which results in similar protein amounts in the joint. The wood substrates were bonded in duplicate on the same occasion at 110 1C and 15 min. The horizontal black line at 8 MPa indicates the limit for passing the test according to the European Standard EN 204.
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Method 1 and with SPI give a slightly higher value (approximately 10–17 MPa) than those glued with WG (approximately 6–13 MPa). However, the results of bonding no. 3 in Fig. 4a rather resemble those of the duplicates in Fig. 4b than those of no. 1 and 2 in Fig. 4a. Furthermore, the variation in the results for each dispersion presented in Fig. 4a is larger than for those shown in Fig. 4b. There is no evident explanation for either the variation between the replicates or the larger degree of variation between the results from the same dispersion and bonding of Fig. 4a. It can be due to inhomogeneous wood, but the combination of the viscosity of the dispersion and the degree of penetration into the wood material due to the choice of application method might also contribute to less robust results. Nevertheless, it seems as if the strength values are somewhat improved for wood substrates bonded with WG and Application Method 2, while the values from SPI substrates are slightly decreased, even though twice the amount of SPI was used with Application Method 2 as was used with Application Method 1.
3.4. Light microscopy and particle size distribution One explanation to the differences in adhesion behaviour may be found in the microscopy images of the bond lines. The images from wood substrates bonded with 24-WG-NaOH (Application Method 1) and 12-WG-NaOH (Application Method 2) are shown in Fig. 5a and b, respectively. Even though the protein amount in the joints is similar, the joint with 12-WG-NaOH is slightly thicker than that with 24-WG-NaOH (cf. Fig. 5b and a). The 24-WG-NaOH dispersion has penetrated into the wood and filled the pores, even as far as 400 mm from the bond line. Penetration of adhesive into
the wood is important for a strong interaction between adhesive and wood, but there must be a good balance between the amount of adhesive in the bond line and in the wood [17]. In this case, the penetration is probably too extensive, rendering the adhesive layer too thin, with strength loss as a result. There is not enough adhesive in the bond line to properly connect the wood surfaces. In contrast, the microscopy image of the wood bonded with 12-WG-NaOH does not reveal any significant penetration (Fig. 5b). This difference in degree of penetration cannot be an effect of particle size since there are only minor differences in particle size between 24-WG-NaOH and 12-WG-NaOH (Table 5). One can therefore probably conclude that the differences in the appearance and the strength of the joints are related to viscosity and the application method rather than to particle size. The 24-WG-NaOH dispersion was applied onto the wood substrates, which thereafter were directly placed in the press. The 12-WGNaOH, on the other hand, was applied onto the wood substrates and thereafter dried. The dried wood surfaces were re-wetted immediately prior to the pressing. Even though the viscosity of 24-WG-NaOH was markedly higher than the viscosity of 12-WGNaOH, the layer of 24-WG-NaOH was still wet when subjected to pressure. However, the layer of 12-WG-NaOH was probably still dry at the border between wood and protein layer and only wet on the outer surface of the layer. Thus, 12-WG-NaOH only penetrated into the wood to a minor extent, rendering the bond-line thicker and stronger. Fig. 5c shows that the bond line of 12-SPI-NaOH (Application Method 1) is relatively thin, whereas that of 12-SPI-NaOH (Fig. 5d; Application Method 2) is very thick. The difference in bond-line thickness is partly due to the fact that twice the amount of protein was applied with Application Method 2. However, that probably
Fig. 5. Light microscopy images of wood substrates bonded with different dispersions (approximately pH 13): (a) 24-WG-NaOH (Application Method 1), (b) 12-WG-NaOH (Application Method 2), (c) 12-SPI-NaOH (Application Method 1), and (d) 12-SPI-NaOH (Application Method 2). A 50 mm scale bar is shown in each image.
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4. Conclusions
Table 5 Particle size distribution of the dispersions. Dispersion
Average diameter D [4,3] (mm)
79
Median diameter D (0.5) (mm)
24-WG-NaOH
43 40
29 26
24-WGfine NaOH
39 36
28 26
12-WG-NaOH
27 40
22 23
12-SPI-H2O
115 118
100 102
12-SPI-NaOH
262 278
165 158
According to this study, there is a clear difference in performance between SPI and WG. The adhesive properties of SPI are superior, particularly with regard to water resistance. However, the water resistance of WG can be partly improved, if overpenetration into the wood is avoided with the proper choice of application method. At the high pH levels used in this study (pH approximately 10 or 13), different pH and ion concentration do not seem to markedly change the adhesive properties of SPI. Furthermore, according to this study there is no apparent difference in the tensile strength between WG powders with different particle sizes.
Acknowledgments does not account for all the difference. None of the images reveal any large extent of penetration of protein into the wood, but at least some of the dispersion from Application Method 1 ought to have penetrated into the wood in order to be able to account for the thinner bond-line (Fig. 5c). The bond strength of 12-SPI-NaOH, Application Method 2, is slightly lower than that of 12-SPI-NaOH, Application Method 1 (cf. Fig. 4b and a). This bond-strength difference is probably related to the high viscosity of the dispersion, especially since the panels from Application Method 2 were dried and not subjected to pressure immediately after the application. Thus, the dispersion will not wet the wood surface and penetrate the wood to the same extent. Wetting of the surface is also an important factor with regard to bond strength [17]. Moreover, there is also a large difference in bond-line thickness between 12-WG-NaOH (Application Method 2) and 12-SPI-NaOH (Application Method 2) (cf. Fig. 5b and d) even though these joints contain the same amount of protein and were obtained with the same application method. This difference in bond-line appearance is probably also related to the large difference in viscosity between these dispersions (Table 4). Even though none of the dispersions were subjected to pressure immediately after the application, the low-viscous 12-WG-NaOH probably still penetrated the wood substrate to a higher extent during the drying step than the more viscous 12-SPI-NaOH dispersion. Furthermore, the difference in particle size between the dispersions of WG and SPI is relatively large (Table 5). This difference could partially explain why WG seems to penetrate more into the wood (cf. Fig. 5a and c). The bond-line of 12-SPINaOH (Fig. 5c) is also thin, but the joint contains half the amount of protein compared with that of 24-WG-NaOH (Fig. 5a). The particle size results also indicate that the extent of unfolding/ swelling is more pronounced for SPI, which in turn can mean improved cohesion and adhesion due to increased entanglement and interaction. Consequently, this could also to some extent explain why the bond strength and water resistance of SPI are superior. Moreover, there is no apparent difference in adhesive properties between the two qualities of WG. The results are similar and this could partly be due to the fact that there are only minor differences in the size of the particles of the dispersions 24-WGNaOH and 24-WGfine-NaOH (Table 5).
Support for this work was provided by EcoBuild (Sweden). We would like to thank Olle Paulson, Casco Adhesives AB, Sweden, for the preparation of the light microscopy images.
References [1] Lambuth AL. Protein adhesives for wood. In: Pizzi A, Mittal KL, editors. Handbook of adhesive technology. New York: Marcel Dekker, Inc.; 1994. p. 259–81. [2] Frihart CR, Wescott JM. Improved water resistance of bio-based adhesives for wood bonding. In: International conference on environmentally-compatible forest products, Oporto, Portugal, 2004. [3] Kintzley TG, et al. Adhesive compositions for wood composites, methods of production, and methods of manufacture thereof. USA: Hexion Specialty Chemicals Inc., Application: WO; 2006. 24 pp. [4] Huang J, Li K. A new soy flour-based adhesive for making interior type II plywood. Journal of the American Oil Chemists’ Society 2008;85(1):63–70. [5] Breyer RA, et al., Wood composites bonded with protein-modified ureaformaldehyde resin adhesive. USA: Georgia-Pacific Resins, Inc., Application: US; 2005. 6 pp. [6] Hettiarachchy NS, Kalapathy U, Myers DJ. Alkali-modified soy protein with improved adhesive and hydrophobic properties. Journal of the American Oil Chemists’ Society 1995;72(12):1461–4. [7] Kalapathy U, et al. Modification of soy proteins and their adhesive properties on woods. Journal of the American Oil Chemists’ Society 1995;72(5):507–10. [8] Wang Y, Wang D, Sun XS. Thermal properties and adhesiveness of soy protein modified with cationic detergent. Journal of the American Oil Chemists’ Society 2005;82(5):357–63. [9] Sun XS. Soy protein adhesives. In: Sun XS, Wool R, editors. Bio-based polymers and composites. Elsevier, Inc.; 2005. p. 327–68. [10] Guilbert S. Formation and properties of wheat gluten films and coatings. In: Gennadios A, editor. Protein-based films and coatings. CRC Press LLC: Florida; 2002. p. 69–122. [11] El-Wakil NA, et al. Modified wheat gluten as a binder in particleboard made from reed. Journal of Applied Polymer Science 2007;106(6):3592–9. [12] Sun XS. Thermal and mechanical properties of soy proteins. In: Sun XS, Wool R, editors. Bio-based polymers and composites. Elsevier, Inc.; 2005. p. 292–326. [13] Gennadios A, et al. Effect of pH on properties of wheat gluten and soy protein isolate films. Journal of Agricultural and Food Chemistry 1993;41(11):1835–9. [14] European Standard EN 204:2001. Classification of thermoplastic wood adhesives for non-structural applications. European Committee for Standardization, 2001. [15] European Standard EN 205:2003. Adhesives—wood adhesives for nonstructural applications—determination of tensile shear strength of lap joints. European Committee for Standardization, 2003. [16] Bohren CF, Huffman DR. Absorption and scattering of light by small particles. New York: Wiley-Interscience; 1983. [17] Frihart CR. Wood adhesion and adhesives. In: Rowell RM, editor. Handbook of wood chemistry and wood composites. Florida: CRC Press; 2005. p. 215–78.
Contents lists available at ScienceDirect
International Journal of Adhesion & Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Comparing bond strength and water resistance of alkali-modified soy protein isolate and wheat gluten adhesives ¨ a, Petra Nordqvist a, Farideh Khabbaz b, Eva Malmstrom a b
Department of Fibre and Polymer Technology, Royal Institute of Technology, Teknikringen 56-58, SE-100 44 Stockholm, Sweden Analyscentrum, Casco Adhesives AB, Box 115 38, SE-100 61 Stockholm, Sweden
a r t i c l e in f o
a b s t r a c t
Article history: Accepted 14 September 2009 Available online 23 September 2009
The tensile strength of beech substrates bonded with dispersions of alkali-denatured soy protein isolate (SPI) and wheat gluten (WG) was measured for comparison of bond strength and resistance to cold water. The proteins were denatured with 0.1 M NaOH (pH 13). Dispersions with different protein concentration and viscosity were investigated. The adhesive properties were studied at different press temperatures (90, 110, and 130 1C) and press times (5, 15, and 25 min). Two types of application methods were used in order to overcome the problem with different viscosity of the dispersions. In addition, SPI was denatured at two different pH levels (approximately 10 and 13) and with two different concentrations of salt (158 mM and 0.1 M), in order to compensate for the different isoelectric points of the proteins. The adhesive properties of WG powder with different particle sizes were also compared. The tensile strengths of the wood substrates were measured according to somewhat simplified versions of the European Standards EN 204 and EN 205. The bond lines were studied with light microscopy. The results indicate that the adhesive properties of SPI are superior, particularly with regard to water resistance. However, the water resistance of WG was to some extent improved when starved adhesive joints could be avoided. Similar tensile strength values were obtained for the dispersions of alkalidenatured SPI regardless of pH or salt concentration. No apparent difference in adhesive strength was observed for the WG dispersions from powder with different particle sizes. & 2009 Elsevier Ltd. All rights reserved.
Keywords: Adhesives for wood Novel adhesives Wood Mechanical properties of adhesives Plant protein
1. Introduction Soybean, blood, and casein have long been the most widely used sources of protein for manufacturing of wood adhesives [1]. Since the 1960s, however, protein adhesives have more or less been replaced by petroleum-based products, most of which are superior in strength and water resistance. Nevertheless, due to the increasing environmental concern, adhesives based on proteins are becoming more attractive since they are derived from renewable resources, while the resources of petroleum-based adhesives are both non-renewable and limited. Protein from soybean has been used as a wood adhesive on its own, but also in combination with other proteins, such as blood and casein, and with synthetic resin polymers [2–5]. Extensive research has been performed in recent years to improve adhesion strength and water resistance of soy protein. Heat treatment and treatment with different denaturation agents, such as alkali, urea, guanidine hydrochloride, trypsin, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, and cationic detergents, were found
Corresponding author. Fax: + 46 8 790 82 83.
¨ E-mail address: [email protected] (E. Malmstrom). 0143-7496/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2009.09.002
to be useful in enhancing the adhesive properties of soy protein [6–9]. If this resumed interest in proteins is to result in a growing market for plant-protein-based adhesives, it is required, both from an economic and environmental point of view, that the plant protein sources are grown close to the market. Since the soybean plant is not native worldwide, it is therefore of interest to also study proteins from other plants, such as wheat gluten from wheat. Wheat gluten is an industrial by-product of wheat starch production, and is mainly used in the bakery industry. However, it has successfully been used in making biodegradable films and coatings for food and non-food applications [10]. Some research has also been performed regarding its ability to bond wood [11]. Even though soy protein and wheat gluten are similar in the sense that they both consist mainly of storage proteins, their chemical compositions and physical behaviours are different. Soy protein consists of two protein classes: water-soluble albumins and salt solution-soluble globulins, where globulins are the major fraction [9,12]. However, the globulins can be further divided into glycinin and conglycinin. Glycinin consists of six acidic-basic subunits joined by disulfide bonds, while conglycinin consists of only three subunits. Furthermore, conglycinin is less hydrophobic
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than glycinin and the subunits of conglycinin are mainly joined with hydrogen bonds. The molecular weight of glycinin is approximately 200–400 kDa, while it ranges from 100 to 200 kDa for conglycinin. The pH is approximately 4.5 at the isoelectric point (pI) of soy protein. On the other hand, approximately 80% of wheat gluten consists of wheat storage protein [10]. The rest is composed of traces of polysaccharides, lipids, and minerals. The wheat storage protein can be divided into two protein classes: gliadins and glutenins. Gliadins are alcohol-soluble, while glutenins are dispersible in dilute acid or alkaline solutions. Both protein classes are rich in proline and glutamine. However, the molecular weight of gliadines ranges from 30 to 74 kDa, while the estimated molecular weight of glutenin is over 107 Da. Glutenins consists of polypeptide chains linked together by disulfide bonds. The pH value at the isoelectric point of wheat gluten is about 7.3 [13]. In overall, wheat gluten has a more hydrophobic character than soy protein. In this study, water dispersions of soy protein isolate (SPI) and wheat gluten (WG) were used as wood adhesives, in order to bond together wood substrates of beech. The proteins were denatured with alkali (0.1 M NaOH (aq); pH 13). The adhesive properties were studied at different press temperatures (90, 110, and 130 1C) and press times (5, 15, and 25 min). Two types of application methods were used in order to overcome the problem of different viscosity of the dispersions. Furthermore, SPI was denatured at different pH (approximately 10 and 13) and ion concentrations (158 mM and 0.1 M), in order to compensate for the different isoelectric points of the proteins. The aim of the study was to investigate whether there are any differences in adhesive behaviour between SPI and WG. WG dispersions of powder with different particle sizes were also investigated. The wood substrates were conditioned and the tensile strength measurements were performed according to somewhat simplified versions of the European Standards EN 204 and EN 205 [14,15]. The standards were used for assessment of bond strength and resistance to cold water of the protein dispersions.
2. Experimental 2.1. Materials Commercial soy protein isolate (SPI) Soy Pro 900 (from Qingdao Crown Imp. & Exp. Corp. Ltd, kindly supplied by Roquette), containing approximately 90% protein, and wheat ¨ gluten (WG) Reppe Vital (Lantmannen Reppe AB, Sweden), containing approximately 85% protein, were employed in this study. A fraction of Reppe Vital, with a smaller particle size ( o56 mm) than the regular batch of Reppe Vital, was also used. This fraction was denoted WG fine. According to the supplier, the
73
particle size of the regular batch of Reppe Vital has the following distribution: 4100 mm (28%), 50–100 mm (55%), and o50 mm (18%). For Soy Pro 900, at least 96% of the sample has a particle size below approximately 150 mm. Beech wood pieces were purchased from Konrad Bruckeder (Rosenheim, Germany). 2.2. Preparation of dispersions of WG, WG fine, and SPI Table 1 summarizes the conditions used for preparing dispersions of WG, WG fine, and SPI. A sodium hydroxide solution (0.1 M; pH 13) was used to prepare 24% (w/w) dispersions of WG and WG fine, and a 12% (w/w) dispersion of WG. The protein products were added while stirring at room temperature. The dispersions are denoted 24-WG-NaOH, 24-WGfine-NaOH, and 12-WG-NaOH, respectively. SPI was dispersed in four different solutions: de-ionized water, 0.1 M NaOH (pH 13), 158 mM NaOH (pH 10), and a water solution containing 158 mM NaOH and 99.8 mM NaCl. SPI was added to the different solutions while stirring at room temperature. The dispersions are denoted 12-SPI-H2O, 12-SPI-NaOH, 12-SPINaOH(pH 10), and 12-SPI-NaOH(pH 10)/NaCl, respectively. The salt concentration of dispersion 12-SPI-NaOH(pH 10)/NaCl was 0.1 M. Thus, it was the same concentration as for 12-SPI-NaOH, even though that was prepared only from NaOH. Depending on the combination of protein and solvent, different combinations of stirring rate and stirring time had to be used in order to obtain homogeneous dispersions without increasing the temperature of the dispersions as a result of the choice of stirring rate. 2.3. pH and viscosity measurements of the protein dispersions pH was measured with a pH glass electrode (Metrohm 6.0258.000, Metrohm Nordic AB, Bromma, Sweden) connected to a pH meter (704 pH Meter, Metrohm Nordic AB, Bromma, Sweden). It was measured on the same day as the dispersions were manufactured. The viscosity of the dispersions was measured with a Brookfield Viscometer (Model DV-II+ Pro, VWR International AB, Stockholm, Sweden) and computer program Rheocalc V2.5 (Brookfield Engineering Labs, Inc.) the day after they were manufactured. Spindle LV1 and LV4, and speed 100, 3, and 0.3 rpm were used. The viscosity was recorded one minute after starting the measurement. 2.4. Specimen preparation Application Method 1: In Application Method 1 the one-day-old dispersion (24-WGNaOH, 24-WGfine-NaOH, 12-SPI-H2O, and 12-SPI-NaOH) was used
Table 1 Conditions used for preparing dispersions of wheat gluten (WG), wheat gluten fine (WG fine), and soy protein isolate (SPI). Dispersion
Conc. (%, w/w)
Amount of SPI/WG (g)
Dispersing agent
Amount of dispersing agent (g)
Stirring rate (rpm)
Stirring time (min)
24-WG-NaOH 24-WGfine NaOH 12-WG-NaOH 12-SPI-H2O 12-SPI-NaOH 12-SPI NaOH(pH 10) 12-SPI-NaOH(pH 10)/NaCl
24 24 12 12 12 12 12
48 48 30 26 30 30 30
0.1 M NaOH 0.1 M NaOH 0.1 M NaOH De-ionized water 0.1 M NaOH 158 mM NaOH 158 mm NaOH+ 99.8 mM NaCl
152 152 220 190 220 220 220
200–330 200–330 300 370 300–370 300 300
100 100 80 115 110 40 55
The dispersions were prepared at room temperature.
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to bond together two panels of beech, with dimension 5 135 400 mm or 5 126 650 mm (thickness, width, and length), and conditioned and evaluated according to slightly modified versions of the European Standards EN 204 and EN 205 [14,15]. The panels were bonded together with what according to the standards is classified as a thin bond-line. On one side of each panel, 180 g/m2 of dispersion was applied. A pressure of 0.7 MPa, and different combinations of press temperatures and press times were used. The temperatures and times are summarized in Table 2. All combinations were used with the dispersions 24-WG-NaOH, 12SPI-H2O, and 12-SPI-NaOH, while 24-WGfine-NaOH was only used with the combination 110 1C and 15 min. The bonded beech panels were cut into test pieces, which were treated according to the conditioning sequences shown in Table 3. The table also summarizes the minimum values of adhesive strength that must be reached for the classification of thermoplastic adhesives into the durability classes D1 to D3. An Alwetron tensile testing machine (model TCT 50, Lorentzen & Wettre, Sweden) was used. The length of the test pieces was 100 mm instead of 150 mm, which is the standard (EN 205). Five or seven test pieces were tested for each conditioning sequence instead of 10 pieces, which is the standard. Application Method 2: In Application Method 2 the one-day-old dispersions of 12WG-NaOH, 12-SPI-H2O, 12-SPI-NaOH, 12-SPI-NaOH(pH 10), and 12-SPI-NaOH(pH 10)/NaCl were used for bonding. The same type of beech panels with dimensions 5 135 400 mm or 5 126 650 mm (thickness, width, and length) were used. Two panels were bonded together. On one side of each panel, 180 g/m2 of dispersion was applied. The dispersion layer was allowed to dry in a conditioned room ((2072) 1C and (6575)% relative humidity) for 24 h. The procedure was repeated once. The panels were allowed to dry in the conditioned room for an additional 24 h. Prior to bonding, the treated surfaces of the panels were re-wetted with water (approximately 170 g/m2). A pressure of 0.7 MPa, a press temperature of 110 1C, and a press time of 15 min were used. The rest of the evaluation was performed according to Application Method 1. Table 2 Different combinations of press temperatures and press times. Press temperature (1C)
Press time (min)
90 90 110 130 130
5 25 15 5 25
Table 3 Conditioning sequences and minimum values of adhesive strength for thin bondlines. Conditioning sequences Duration and condition
Adhesive strength (MPa)
Durability classes
7 daysa in standard atmosphereb
Z 10
D1, D2, and D3
7 daysa in standard atmosphereb, 3 h in water at (207 5) 1C, 7 daysa in standard atmosphereb
Z8
D2
7 daysa in standard atmosphereb, 4 daysa in water at (20 75) 1C
Z2
D3
a b
1 day= 24 h. (207 2) 1C and (65 7 5)% relative humidity.
2.5. Particle size distribution Five dispersions (24-WG-NaOH, 24-WGfine-NaOH, 12-WGNaOH, 12-SPI-H2O, and 12-SPI-NaOH) were analysed with light scattering/diffraction according to Mie theory [16]. A Mastersizer particle size analyser (Mastersizer 2000, Malvern Instrument Nordic AB, Uppsala, Sweden) with measuring range 0.020– 2000 mm, was used. Dispersion 12-SPI-H2O was diluted with de-ionized water, while the other dispersions were diluted with 0.1 M NaOH. The solutions were stirred for five minutes in the instrument prior to the analysis. The results are based on two samples of each dispersion analysed in triplicate. The analyses were performed the day after the dispersions were manufactured. 2.6. Light microscopy The bonded joints of the wood substrates were examined under a Leica DMRM light microscope (Leica Microsystems AB, Stockholm, Sweden) equipped with a fluorescence filter (Leica filter cube H3) and a CCD camera (Leica DFC 280). The wood substrates were stained with 0.1% aqueous Safranine-O (Basic Red 2, ICN Biomedicals Inc.) prior to the analysis.
3. Results and discussion 3.1. Preparation of protein dispersions, pH and viscosity measurements, and the choice of application techniques WG and SPI can be dispersed in water solutions adjusted to a pH both below and above their isoelectric points (pH approximately 7.3 and 4.5, respectively) [9,10]. In this study, an alkaline condition was chosen for comparing the adhesive properties of the two plant proteins. A water solution of sodium hydroxide (0.1 M; pH 13) was chosen as a dispersing agent, since WG Reppe Vital is dispersible in NaOH (aq) with a concentration above 0.07 M. Nevertheless, since SPI is also dispersible in de-ionized water, it was of interest to compare the adhesive properties of SPI when it is dispersed in water and in 0.1 M NaOH. Moreover, since the pH value at the isoelectric point of WG and SPI is different, SPI was also dispersed in 158 mM NaOH (pH 10). Thus, the difference in pH between the isoelectric point of SPI and the dispersing agent (158 mM NaOH) is similar to the corresponding gap between the isoelectric point of WG and 0.1 M NaOH (pH 13). In order to partly compensate for the lower ion concentration of 158 mM NaOH, SPI was also dispersed in 158 mM NaOH with addition of sodium chloride (99.8 mM). The results from the pH measurement are summarized in Table 4, and the results indicate that the buffer capacity is slightly different for WG and SPI. According to the results, the pH of 12-WG-NaOH is 12.2, while it is 11.5 for 12-SPI-NaOH, even though the dispersing agent is the same (0.1 M NaOH, pH 13). However, this is expected since the amino acid composition of WG and SPI is different. Furthermore, due to the buffer capacity of SPI, the pH of the dispersions 12-SPI-H2O, 12-SPI-NaOH(pH 10), and 12-SPI-NaOH(pH 10)/NaCl is similar (about 7.7–7.9) even though the pH of the dispersing agents ranges from approximately 6–10. There are three factors that an adhesive needs to fulfil for a proper bond to form: it must be able to wet the surface of the wood material, flow over it, and penetrate into the wood [17]. These three factors are partly dependent on the viscosity of the adhesive and thereby the dry content of the adhesive. The dry content of water-borne adhesives is normally about 50–65%, partly in order to prevent a too large extent of water being left in the adhesive joint and wood material after pressing. It is therefore
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of interest to try to maximize the dry content of the adhesives, without the adhesive becoming too viscous and thereby decreasing its ability to properly wet, flow, and penetrate the wood substrate. A highly viscous adhesive is also more difficult to handle. However, in this study, the concentration of WG and SPI in the dispersions could not be increased to more than 24% (w/w) and 12% (w/w), respectively, before the viscosity became too high. Despite the high water content, it was possible to use the dispersions as adhesives. Nevertheless, in order to facilitate the comparison of adhesive properties, it would have been desirable to use the same concentration of protein in the dispersions regardless of protein type. However, as can be seen in Table 4, the viscosity of the 12% dispersion of WG (12-WG-NaOH) was unfortunately too low. This dispersion could therefore only be used with Application Method 2. Overall, the viscosity of the dispersions differs largely, even though similar concentrations of protein and dispersing agent are Table 4 Viscosity and pH of soy protein isolate (SPI) and wheat gluten (WG) dispersions. Dispersion
pHa
Viscosityb (Pas)
Spindle, speedc (rpm)
24-WG-NaOH 24-WGfine-NaOH 12-WG-NaOH 12-SPI-H2O 12-SPI-NaOH 12-SPI-NaOH(pH 10) 12-SPI-NaOH(pH 10)/NaCl
10.6 10.4 12.2 7.9 11.5 7.8 7.7
77 94 0.025 3.1 980 3.5 2.7
LV4, LV4, LV1, LV4, LV4, LV4, LV4,
3 3 100 100 0.3 100 100
a
pH was measured on the manufacturing date of the dispersions. Viscosity was measured the day after the manufacturing date of the dispersions. The viscosity values were recorded one minute after starting the measurement. c Different spindles and speeds had to be used, due to the large difference in viscosity between the dispersions. b
24-WG-NaOH 12-SPI-H2O
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used (Table 4). Due to this difference, different spindles and speeds had to be used while measuring the viscosity, which must be considered when comparing the results. Furthermore, the difference in viscosity also complicates the comparison of the adhesive properties, since the viscosity ought not to be too high or too low when the dispersion is applied on the wood panel prior to the pressing. If the viscosity is too high, it will be very difficult or even impossible to evenly spread the adhesive onto the wood panel. On the other hand, if the viscosity is too low, the dispersion will either drain off or to a considerable extent penetrate the substrate, rendering the adhesive layer too thin. In order to deal with this problem, two different techniques were used to apply the dispersions. In the first case (Application Method 1), the viscosity of the dispersions was similar, but the concentrations of SPI and WG were different. The dispersions 24-WG-NaOH, 12-SPI-NaOH, and 12-SPI-H2O were applied to the wood panel directly prior to pressing. Equal amounts of the dispersions were applied, but also half the amount of 24-WG-NaOH in order to reduce the differences in amount of protein in the joint. In the second case (Application Method 2), the same concentration of SPI and WG was used and the following dispersions were used as adhesives: 12-WG-NaOH, 12-SPI-NaOH, 12-SPINaOH(pH 10), and 12-SPI-NaOH(pH 10)/NaCl. Since the viscosity of 12-WG-NaOH is extremely low, a different application technique had to be used in order to prevent the dispersion draining off or over-penetrating the wood substrate. Therefore, after applying the dispersions onto the wood substrates, the dispersion layer was allowed to dry. This procedure was used twice on each wood substrate. The treated surfaces were remoistened immediately prior to bonding. However, irrespective of application method, the problems of different viscosities and protein concentrations of the dispersions cannot be completely circumvented. Even though the amount of protein is adjusted in Application Method 1, the initial amount of water in the joint will be different. In Application Method 2, this
24-WG-NaOH, glue amount/2 12-SPI-NaOH
20 18
Tensile strength (MPa)
16 14 12 10 8 6 4 2 0 90°C, 5 min
130°C, 5 min
90°C, 25 min
130°C, 25 min
110°C, 15 min (1)
110°C, 15 min (2)
110°C, 15 min (3)
Fig. 1. Tensile strength measurements of dry wood substrates (Application Method 1). The wood substrates were bonded on the same occasion with different combinations of press temperature and press time. The combination 110 1C and 5 min was used in triplicate. The horizontal black line at 10 MPa indicates the limit for passing the test according to the European Standard EN 204.
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problem has been eliminated, but since the viscosity differs, the proteins might behave differently during drying and re-wetting, which could influence the adhesion properties. Therefore, these issues have to be considered when comparing the results.
dispersion. Furthermore, the 24-WG-NaOH dispersion failed to keep the wood pieces together at 90 1C and 5 min. This failure indicates, at least, that for these types of WG dispersions a higher press temperature or a longer press time is needed for improved bonding results. Nevertheless, for all combinations of press temperature and press time except 90 1C and 5 min, and one of the combinations of 110 1C and 15 min, the highest tensile strength value is obtained with 12-SPI-NaOH. Thus, the tensile strength of the sodium hydroxide denaturated SPI seems to be superior to that of the non-modified SPI (12-SPI-H2O). This conclusion agrees with what is reported by Hettiarachchy et al. [6]. The unfolded protein is more available for entanglement, interaction, and reaction. Fig. 2 shows the results from tensile strength measurements of the wood substrates soaked in water for four days. The results distinctly reveal that the adhesive properties of SPI are superior to WG. This is especially evident when the wood substrates are still wet, such as in this case. The wood substrates bonded with the WG dispersions at 90 1C press temperature, and 130 1C and 5 min
3.2. Tensile strength measurements—comparison between 24-WGNaOH, 12-SPI-H2O, and 12-SPI-NaOH (Application Method 1) The tensile strength measurements of the dry wood substrates are presented in Fig. 1. Most of the values are above 10 MPa, which means that the wood substrates have passed the test according to standards. The results do not reveal any clear differences in adhesive strength between the different dispersions. However, the variation in results is relatively high, especially when comparing the triplicates at 110 1C and 15 min originating from the same occasion. This variation in results can be due to inhomogeneous wood. It is noteworthy that similar tensile strength values are obtained for 24-WG-NaOH, regardless of the added amount of
6 24-WG-NaOH 24-WG-NaOH, glue amount /2
5 Tensile strength (MPa)
12-SPI-H2O 12-SPI-NaOH
4 3 2 1 0
90°C, 5 min
-1
130°C, 5 min
90°C, 25 min
130°C, 25 min
110°C, 15 min (1)
110°C, 15 min (2)
110°C, 15 min (3)
Fig. 2. Tensile strength measurements of wood substrates soaked in water for four days (Application Method 1). The wood substrates were bonded on the same occasion with different combinations of press temperature and press time. The combination 110 1C and 15 min was used in triplicate. The horizontal black line at 2 MPa indicates the limit for passing the test according to the European Standard EN 204.
20 12-WG-NaOH
18
Tensile strength (MPa)
16 14
12-SPI-NaOH 12-SPI-NaOH(pH~10) 12-SPI-NaOH(pH~10)/NaCl
12 10 8 6 4 2 0 110°°C, 15 min (1)
110°°C, 15 min (2)
Fig. 3. Tensile strength measurements of dry wood substrates (Application Method 2). The wood substrates were bonded in duplicate on the same occasion at 110 1C and 15 min. The horizontal black line at 10 MPa indicates the limit for passing the test according to the European Standard EN 204.
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press time, even fell apart. On the other hand, those bonded with the SPI dispersions performed well and the majority of them even passed the test according to standards. The results of the WG dispersions are slightly improved at higher press temperature and time, but the tensile strength values are still markedly below the values of SPI.
3.3. Tensile strength measurements—comparison between 12-WGNaOH, 12-SPI-NaOH, 12-SPI-NaOH(pH 10), and 12-SPINaOH(pH 10)/NaCl (Application Method 2). Comparison between results obtained with Application Methods 1 and 2 The tensile strength values of the wood substrates bonded with the three dispersions of SPI are similar between themselves, regardless of the three types of conditioning sequences. The results from the tensile strength measurements of dry wood substrates are shown in Fig. 3 (Application Method 2). Different pH and ion concentration do not seem to affect the adhesive
77
properties of SPI at these high pH levels (pH approximately 10 or 13). According to these results, either the degree of unfolding of SPI due to the salt concentration and/or pH is more or less similar, or the extent of unfolding has reached a limit where it no longer contributes with additional effect on the results. In the comparison between the WG and SPI dispersions, the results from the measurements agree with the results shown for Application Method 1. Hence, the adhesive properties of SPI are superior to those of WG with regard to water resistance. The tensile strength values of the wood substrates bonded with 12-WG-NaOH and soaked in water for four days are much lower than the values obtained with the substrates bonded with the SPI dispersions. However, in a comparison of the strength values from the three-hour-water-soaked and conditioned wood substrates, the relationship between the strength values differs depending on the application method used (Fig. 4a and b). Similar results (approximately 10–12 MPa) are obtained for the wood substrates bonded according to Application Method 2 regardless of dispersion used, while those bonded according to Application
20 18
24-WG-NaOH
12-SPI-NaOH
Tensile strength (MPa)
16 14 12 10 8 6 4 2 0 110°°C, 15 min (method 1, bonding 1)
110°°C, 15 min (method 1, bonding 2)
110°°C, 15 min (method 1, bonding 3)
20 18
12-WG-NaOH
12-SPI-NaOH
Tensile strength (MPa)
16 14 12 10 8 6 4 2 0 110°C, 15 min (method 2, bonding 1)
110°C, 15 min (method 2, bonding 2)
Fig. 4. (a) Tensile strength measurements of wood substrates soaked in water for three hours and conditioned for 7 days (Application Method 1). The dispersions 24-WGNaOH and 12-SPI-NaOH were used as adhesives. The concentrations of protein in the dispersions are different, which results in different protein amounts in the joint. The wood substrates were bonded in triplicate on the same occasion at 110 1C and 15 min. The horizontal black line at 8 MPa indicates the limit for passing the test according to the European Standard EN 204. (b) Tensile strength measurements of wood substrates soaked in water for three hours and conditioned for 7 days (Application Method 2). The dispersions 12-WG-NaOH and 12-SPI-NaOH were used as adhesives. The concentrations of protein in the dispersions are similar, which results in similar protein amounts in the joint. The wood substrates were bonded in duplicate on the same occasion at 110 1C and 15 min. The horizontal black line at 8 MPa indicates the limit for passing the test according to the European Standard EN 204.
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Method 1 and with SPI give a slightly higher value (approximately 10–17 MPa) than those glued with WG (approximately 6–13 MPa). However, the results of bonding no. 3 in Fig. 4a rather resemble those of the duplicates in Fig. 4b than those of no. 1 and 2 in Fig. 4a. Furthermore, the variation in the results for each dispersion presented in Fig. 4a is larger than for those shown in Fig. 4b. There is no evident explanation for either the variation between the replicates or the larger degree of variation between the results from the same dispersion and bonding of Fig. 4a. It can be due to inhomogeneous wood, but the combination of the viscosity of the dispersion and the degree of penetration into the wood material due to the choice of application method might also contribute to less robust results. Nevertheless, it seems as if the strength values are somewhat improved for wood substrates bonded with WG and Application Method 2, while the values from SPI substrates are slightly decreased, even though twice the amount of SPI was used with Application Method 2 as was used with Application Method 1.
3.4. Light microscopy and particle size distribution One explanation to the differences in adhesion behaviour may be found in the microscopy images of the bond lines. The images from wood substrates bonded with 24-WG-NaOH (Application Method 1) and 12-WG-NaOH (Application Method 2) are shown in Fig. 5a and b, respectively. Even though the protein amount in the joints is similar, the joint with 12-WG-NaOH is slightly thicker than that with 24-WG-NaOH (cf. Fig. 5b and a). The 24-WG-NaOH dispersion has penetrated into the wood and filled the pores, even as far as 400 mm from the bond line. Penetration of adhesive into
the wood is important for a strong interaction between adhesive and wood, but there must be a good balance between the amount of adhesive in the bond line and in the wood [17]. In this case, the penetration is probably too extensive, rendering the adhesive layer too thin, with strength loss as a result. There is not enough adhesive in the bond line to properly connect the wood surfaces. In contrast, the microscopy image of the wood bonded with 12-WG-NaOH does not reveal any significant penetration (Fig. 5b). This difference in degree of penetration cannot be an effect of particle size since there are only minor differences in particle size between 24-WG-NaOH and 12-WG-NaOH (Table 5). One can therefore probably conclude that the differences in the appearance and the strength of the joints are related to viscosity and the application method rather than to particle size. The 24-WG-NaOH dispersion was applied onto the wood substrates, which thereafter were directly placed in the press. The 12-WGNaOH, on the other hand, was applied onto the wood substrates and thereafter dried. The dried wood surfaces were re-wetted immediately prior to the pressing. Even though the viscosity of 24-WG-NaOH was markedly higher than the viscosity of 12-WGNaOH, the layer of 24-WG-NaOH was still wet when subjected to pressure. However, the layer of 12-WG-NaOH was probably still dry at the border between wood and protein layer and only wet on the outer surface of the layer. Thus, 12-WG-NaOH only penetrated into the wood to a minor extent, rendering the bond-line thicker and stronger. Fig. 5c shows that the bond line of 12-SPI-NaOH (Application Method 1) is relatively thin, whereas that of 12-SPI-NaOH (Fig. 5d; Application Method 2) is very thick. The difference in bond-line thickness is partly due to the fact that twice the amount of protein was applied with Application Method 2. However, that probably
Fig. 5. Light microscopy images of wood substrates bonded with different dispersions (approximately pH 13): (a) 24-WG-NaOH (Application Method 1), (b) 12-WG-NaOH (Application Method 2), (c) 12-SPI-NaOH (Application Method 1), and (d) 12-SPI-NaOH (Application Method 2). A 50 mm scale bar is shown in each image.
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4. Conclusions
Table 5 Particle size distribution of the dispersions. Dispersion
Average diameter D [4,3] (mm)
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Median diameter D (0.5) (mm)
24-WG-NaOH
43 40
29 26
24-WGfine NaOH
39 36
28 26
12-WG-NaOH
27 40
22 23
12-SPI-H2O
115 118
100 102
12-SPI-NaOH
262 278
165 158
According to this study, there is a clear difference in performance between SPI and WG. The adhesive properties of SPI are superior, particularly with regard to water resistance. However, the water resistance of WG can be partly improved, if overpenetration into the wood is avoided with the proper choice of application method. At the high pH levels used in this study (pH approximately 10 or 13), different pH and ion concentration do not seem to markedly change the adhesive properties of SPI. Furthermore, according to this study there is no apparent difference in the tensile strength between WG powders with different particle sizes.
Acknowledgments does not account for all the difference. None of the images reveal any large extent of penetration of protein into the wood, but at least some of the dispersion from Application Method 1 ought to have penetrated into the wood in order to be able to account for the thinner bond-line (Fig. 5c). The bond strength of 12-SPI-NaOH, Application Method 2, is slightly lower than that of 12-SPI-NaOH, Application Method 1 (cf. Fig. 4b and a). This bond-strength difference is probably related to the high viscosity of the dispersion, especially since the panels from Application Method 2 were dried and not subjected to pressure immediately after the application. Thus, the dispersion will not wet the wood surface and penetrate the wood to the same extent. Wetting of the surface is also an important factor with regard to bond strength [17]. Moreover, there is also a large difference in bond-line thickness between 12-WG-NaOH (Application Method 2) and 12-SPI-NaOH (Application Method 2) (cf. Fig. 5b and d) even though these joints contain the same amount of protein and were obtained with the same application method. This difference in bond-line appearance is probably also related to the large difference in viscosity between these dispersions (Table 4). Even though none of the dispersions were subjected to pressure immediately after the application, the low-viscous 12-WG-NaOH probably still penetrated the wood substrate to a higher extent during the drying step than the more viscous 12-SPI-NaOH dispersion. Furthermore, the difference in particle size between the dispersions of WG and SPI is relatively large (Table 5). This difference could partially explain why WG seems to penetrate more into the wood (cf. Fig. 5a and c). The bond-line of 12-SPINaOH (Fig. 5c) is also thin, but the joint contains half the amount of protein compared with that of 24-WG-NaOH (Fig. 5a). The particle size results also indicate that the extent of unfolding/ swelling is more pronounced for SPI, which in turn can mean improved cohesion and adhesion due to increased entanglement and interaction. Consequently, this could also to some extent explain why the bond strength and water resistance of SPI are superior. Moreover, there is no apparent difference in adhesive properties between the two qualities of WG. The results are similar and this could partly be due to the fact that there are only minor differences in the size of the particles of the dispersions 24-WGNaOH and 24-WGfine-NaOH (Table 5).
Support for this work was provided by EcoBuild (Sweden). We would like to thank Olle Paulson, Casco Adhesives AB, Sweden, for the preparation of the light microscopy images.
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