Contents lists available at ScienceDirect

International Journal of Adhesion and Adhesives

journal homepage: www.elsevier.com/locate/ijadhadh

Combinations of soy protein and polyacrylate emulsions as wood adhesives

Fapeng Wanga,b,c,d, Jifu Wangb, Fuxiang Chub, Chunpeng Wangb, Chunde Jinc, Siqun Wangd,⁎,Jiuyin Panga,⁎

aWood Material Science and Engineering Key Laboratory of Jilin Province, Beihua University, Jilin 132013, Chinab Institute of Chemical Industry of Forestry Products, CAF; Key Laboratory of Biomass Energy and Material, Nanjing, Jiangsu Province 210042,Chinac Faculty of Engineering, Zhejiang A&F University; Wood Material Science and Engineering Key Laboratory of Zhejiang Province, Hangzhou 311300, ChinadUniversity of Tennessee, 2506 Jacob Dr., Knoxville, Tennessee, 37996, USA.

A R T I C L E I N F O

Keywords:Polyacrylate emulsionSoy proteinWood adhesiveShear strength

A B S T R A C T

A series of soy protein-polyacrylate emulsions (SPCE) used for wood adhesives were synthesized by blending soyprotein in aqueous solution and polyacrylate emulsion with various mass fractions. The chemical compositionand performance properties of these blends were then characterized by FT-IR viscometry, contact angle analysis,scanning electron microscopy (SEM), thermogravimetric analysis (TGA) and shear strength. It was found that theapparent viscosity of these blended emulsions decreased with higher levels of polyacrylate emulsion, whichcould facilitate the wetting and penetrating of wood. FT-IR analysis suggested that hydrogen bonds were formedin the blends. The introduction of polyacrylate copolymers containing neutralized poly(acrylic acid), as well as asmall amount of emulsifiers, can enhance the wettability of blended emulsions and facilitate their application aswood adhesives. The shear strength of plywood prepared using these adhesives and their thermal stability wereimproved after the introduction of the polyacrylate copolymers.

1. Introduction

Adhesives used to joint wood with wood or other substances havereceived considerable attention since the first advent of phenolic resinsfor the manufacture of plywood in 1912 [1]. To date, the frequentlyused wood adhesives are formaldehyde-based amino resins includingurea-formaldehyde resin, melamine-formaldehyde resin, etc, whichhave resulted from their low cost and high performance in bonding.However, the application of formaldehyde-based amino resins in theproduction of wood composites can release free formaldehyde, al-though the emission levels are low [2]. As a result, a regulation onlimiting formaldehyde emission form wood-based products was issuedby the California Air Resources Board, USA, in 2007. Subsequently,formaldehyde emission standards for composite wood products was putinto practice in 2010. Health and environmental concerns relatingformaldehyde release from such adhesives has led to urgent require-ments for formaldehyde free wood adhesives [3–5].

Soy protein, a main byproduct of agriculture, has recently been usedas an adhesive for wood composites due to its sustainability, afford-ability and ready availability. The functional groups in soy protein suchas primary amine, carboxyl and hydroxyl, which can interact with polargroups of wood composites, allow soy protein to be a competitive

formaldehyde free wood adhesives [6,7]. However, the poor water re-sistance and degradability of unmodified soy protein has limited itsapplication in wood adhesive products. In order to overcome thesedisadvantages, many efforts from both industrial and academic in-stitutions have been made to modify soy protein with chemical agents,resins, enzymes, etc, to achieve a water resistant soy protein basedadhesive. These attempts can be summarized as: (a)The denature of soyprotein by alkali [8], urea [9], guanidine hydrochloride [10], sodiumdodecyl sulfate [11].(b)Protein macromolecule modification bygrafting [12], acetylation [13], protein enzyme and biomimetic [14], orby crosslinking agents such as maleic anhydride [4] and glutaraldehyde[15].

Compared with chemical modification, the blending of soy proteinwith other materials with the aim to improve the water resistance isregarded as a facile method. The mixing of phenol-formaldehyde [16],melamine-urea-formaldehyde [17], epoxy resin with melamine-for-maldehyde resin [18], polyethylenimine [4], or polyamidoamineepi-chlorohydrin [3], resins with soy protein and then applying the mix-tures as adhesives, were reported to improve the water resistance.Additionally, sorghum lignin [19], carbohydrate [20], and nano-scalemontmorillonite [21] have been reported to blend with soy protein.However, some of these materials, when mixed with soy protein can

https://doi.org/10.1016/j.ijadhadh.2018.01.002Received 1 June 2015; Accepted 29 December 2017

⁎ Corresponding author.E-mail addresses: wfp880808@163.com, fwang21@utk.edu (F. Wang), wjf118@126.com (J. Wang), Chufuxiang@caf.ac.cn (F. Chu), wangcpg@126.com (C. Wang),

15688958411@163.com (C. Jin), 15688958411@163.com (S. Wang), pangjiuyin@163.com (J. Pang).

International Journal of Adhesion and Adhesives 82 (2018) 160–165

Available online 06 January 20180143-7496/ © 2018 Elsevier Ltd. All rights reserved.

T

http://www.sciencedirect.com/science/journal/01437496https://www.elsevier.com/locate/ijadhadhhttps://doi.org/10.1016/j.ijadhadh.2018.01.002https://doi.org/10.1016/j.ijadhadh.2018.01.002mailto:wfp880808@163.commailto:fwang21@utk.edumailto:wjf118@126.commailto:Chufuxiang@caf.ac.cnmailto:wangcpg@126.commailto:15688958411@163.commailto:15688958411@163.commailto:pangjiuyin@163.comhttps://doi.org/10.1016/j.ijadhadh.2018.01.002http://crossmark.crossref.org/dialog/?doi=10.1016/j.ijadhadh.2018.01.002&domain=pdf

also result in formaldehyde emission.Polyacrylate emulsion is an aqueous dispersion of an acrylic

polymer which can exhibit excellent corrosion resistance, good waterand alkali resistance and good transparency. Due to these attributesthey have been extensively applied in coating and adhesive formula-tions [22,23]. Because of the facile manufacture and commercialavailability, soy protein has been used as a stabilizer in polyacrylateemulsions which can be grafted onto polyacrylate during the emulsionpolymerization process [24]. However, in spite of this potential blendsof soy protein with polyacrylate emulsion in the manufacture of ply-wood has not, to date, been reported.

Herein, we report facile preparations of soy protein-polyacrylateblended emulsions(SPCE) used for wood adhesives by blending soyprotein in aqueous solution with a polyacrylate emulsion containingcopolymers of methyl methacrylate (MMA), butyl acrylate (BA) andacrylic acid (AA). Polymers with acid moieties can interact with otherfunctional groups in the environment which can endow the polymerwith unique properties [25–27]. These blended emulsions could beemployed to glue plywood with protein using diphenyl methane dii-socyanate (MDI) as a crosslinking agent. The combination of poly-acrylate copolymers with soy protein could endow the prepared ply-wood with high strength, high resistance to water as well as no releaseof free formaldehyde and phenol. In addition, hot pressing in the pre-paration of plywood with these blended emulsions would facilitate theiruse in large scale industrial applications.

2. Materials and methods

2.1. Materials

Soy protein (soy protein≥90%, moisture≤7.0, fiber≤1, ash≤6,and fat≤1) was obtained from Harbin Hi-tech Soy Protein Co. ltd,Heilongjiang, China. Methyl methacrylate (MMA), butyl acrylate (BA)and acrylic acid(AA), were purchased from Eastern PetrochemicalCompany, Harbin, Heilongjiang, China. Ammonium persulfate (APS)was obtained from Tianyun Chemical Company, Harbin, Heilongjiang,China. Dodecyl sulfonic acid sodium salt (SLS) and alkylphenol ethox-ylates (OP-10) were purchased from ACROS Chemical ReagentCompany. All chemicals were used as received.

2.2. Synthesis of polyacrylate emulsion

Emulsion polymerization was carried out by semi-continuous seedemulsion copolymerization in a 1000-mL four-neck round-bottomedflask equipped with an overhead Teflon stirrer, a reflex condenser, athermometer, and a calibrated dropping funnel. Prior to polymeriza-tion, a mixture of 148.5 g MMA, 147 g BA, 4.5 g AA was added drop-wise to the 1000 ml round bottomed flash containing 91 g distilledwater, 1.8 g OP, and 0.3 g SLS under the condition of vigorously stirringto prepare the pre-emulsion. Then 1.14 g OP, 0.114 g SLS, 230 g dis-tilled water and 27 g pre-emulsion were charged into the flask placed ina water bath set at 88 °C and stirred at a speed of 300 revolutions perminute (RPM). When the temperature of the mixture in the flaskreached 65 °C, 0.53 g APS (0.18 wt.% of total monomer mass) dissolvedin 9 g distilled water, was added to the flask and polymerization in-itiated. After the reaction temperature had reached 85 °C, 363.1 g ofresidual pre-emulsion and 1.84 g APS in 20 g distilled water were addeddropwise within 4 h. This mixture was continuously stirred for 1 hfollowing addition of the pre-emulsion, and then cooled to room tem-perature. The polyacrylate emulsion was then neutralized to pH7 byammonia and diluted to 10% solid content with distilled water. Thefeature of this polyacrylate emulsion containing poly (MMA-co-BA-co-AA) was described as: 10% solid content, pH=7.0, apparentviscosity=87.8 cP, glass transition temperature (Tg) of copolymer=2 °C.

For comparative purposes, a polyacrylate emulsion (P(MMA-co-BA)) without poly(acrylic acid) was synthesized using the same ap-proach. These polyacrylate emulsions have the same MMA/BA/OP/SLSmass content as the polyacrylate emulsions containing poly (MMA-co-BA-co-AA). And the feature of these (poly(MMA-co-BA) emulsion wasdescribed as: 10% solid content, pH=7.0, apparentviscosity=103.7 cP, glass transition temperature (Tg) of copolymer=2 °C.

2.3. Preparation of soy protein-polyacrylate blended emulsions (SPCE)

SPCE was prepared in a 500-ml three-neck round-bottomed flaskequipped with an overhead Teflon stirrer, a reflex condenser, and athermometer, by blending polyacrylate emulsion and soy protein so-lution (10% solid content in water). The recipe is shown in Table 1. The

Table 1Shear strength of plywood glued by SPCE.

0(%)b 0.1(%)b 0.5(%)b 1(%)b 0(%)b

Sample Namea Shear strength(MPa) Shear strength(MPa) Shear strength(MPa) Shear strength(MPa) Shear strength(MPa)

c d c d c d c d c d

S100 0.82± 0.07 0.44± 0.08 0.86± 0.10 0.50± 0.07 0.91± 0.1 0.55± 0.11 0.93± 0.07 0.58± 0.08 0.94± 0.08 0.63± 0.06A10S90 0.84± 0.04 0.51± 0.07 0.89± 0.03 0.55± 0.06 0.94± 0.04 0.62± 0.08 0.95± 0.06 0.67± 0.08 0.99± 0.07 0.71± 0.09A20S80 0.85± 0.1 0.59± 0.07 0.91± 0.08 0.63± 0.07 0.95± 0.05 0.69± 0.06 0.98± 0.07 0.71± 0.07 1.01± 0.1 0.76± 0.06A30S70 0.88± 0.06 0.64± 0.07 0.93± 0.1 0.67± 0.1 0.97± 0.08 0.71± 0.07 1.01± 0.06 0.73± 0.08 1.01± 0.05 0.76± 0.09A40S60 0.90± 0.07 0.67± 0.06 0.93± 0.12 0.71± 0.1 0.97± 0.11 0.72± 0.1 0.99± 0.06 0.77± 0.06 1.04± 0.1 0.81± 0.12A50S50 0.93± 0.09 0.69± 0.1 0.95± 0.06 0.72± 0.07 1.00± 0.02 0.74± 0.04 1.03± 0.05 0.78± 0.03 1.09± 0.04 0.88± 0.07A60S40 0.94± 0.07 0.71± 0.06 0.96± 0.1 0.74± 0.11 1.02± 0.08 0.78± 0.1 1.08± 0.06 0.79± 0.05 1.11± 0.05 0.88± 0.06A70S30 0.96± 0.11 0.73± 0.09 0.98± 0.06 0.78± 0.08 1.03± 0.06 0.79± 0.06 1.07± 0.13 0.83± 0.09 1.13± 0.08 0.91± 0.08A80S20 0.98± 0.1 0.76± 0.08 1.01± 0.1 0.79± 0.06 1.03± 0.08 0.81± 0.07 1.09± 0.07 0.85± 0.09 1.15± 0.06 0.96± 0.05A90S10 0.99± 0.12 0.79± 0.1 1.02± 0.09 0.80± 0.08 1.05± 0.1 0.84± 0.1 1.10± 0.06 0.87± 0.1 1.10± 0.11 0.95± 0.08A100 1.04± 0.11 0.89± 0.1 1.09± 0.03 0.94± 0.05 1.14± 0.02 1.01± 0.04 1.19± 0.1 1.07± 0.12 1.23± 0.17 1.12± 0.15A110S90 0.83± 0.1 0.49± 0.11 0.87± 0.06 0.53± 0.08 0.92± 0.08 0.58± 0.06 0.94± 0.08 0.61± 0.08 0.96± 0.04 0.68± 0.06A150S50 0.85± 0.11 0.54± 0.1 0.87± 0.07 0.57± 0.07 0.89± 0.04 0.61± 0.07 0.93± 0.08 0.64± 0.06 0.96± 0.1 0.72± 0.12A160S40 0.87± 0.08 0.57± 0.11 0.90± 0.06 0.59± 0.06 0.92± 0.08 0.64± 0.06 0.95± 0.11 0.69± 0.09 0.98± 0.08 0.73± 0.1A170S30 0.84± 0.11 0.52± 0.08 0.87± 0.1 0.56± 0.08 0.89± 0.08 0.62± 0.07 0.94± 0.08 0.65± 0.1 0.97± 0.04 0.71± 0.06A180S20 0.82± 0.08 0.51± 0.07 0.85± 0.05 0.55± 0.08 0.88± 0.07 0.57± 0.08 0.92± 0.08 0.63± 0.1 0.95± 0.08 0.68± 0.08A190S10 0.80± 0.11 0.49± 0.08 0.82± 0.08 0.51± 0.07 0.85± 0.11 0.55± 0.08 0.89± 0.05 0.61± 0.08 0.92± 0.08 0.65± 0.06A1100 0.77± 0.07 0.45± 0.1 0.79± 0.08 0.47± 0.09 0.82± 0.08 0.51± 0.1 0.86± 0.07 0.57± 0.11 0.9±0.09 0.62± 0.1

a A and S in the sample names represent polyacrylate emulsion and soy protein solution respectively, the numbers behind A and S are the feed mass ratio. A1 represents polyacrylateemulsion without the copolymer of poly(acrylic acid), which only contained the copolymers of Methyl methacrylate and butyl acrylate. b MDI dosage based SPCE volume. c measuredbefore the submersion in water. d measured after the submersion in water according to China National Standards GB/T9846-2004

F. Wang et al. International Journal of Adhesion and Adhesives 82 (2018) 160–165

161

pH of the blending emulsion was modulated to 7.0 with ammoniawater. The blended emulsion was continuously stirred at a temperatureof 60 °C for 1.5 h, and then cooled to room temperature. After filteringthe liquid phase to remove the coagulum formed by the blend, SPCEwas obtained.

2.4. Preparation of emulsion film

Film samples of SPCE (Table 1) were prepared by casting theblended emulsions onto a polytetrafluoroethylene dish and dried at50 °C under vacuum for 24 h.

2.5. Apparent viscosity

The apparent viscosity of composite emulsions was determined witha BROOKFIELD DV - II + Pro (USA) type rotary viscosity meter at 25 °Cwith spindles set S64 at 60 revolutions per minute (rpm).

2.6. FT-IR spectrum

FT-IR spectra, after the drying of specimens, were determined usinga Nicolet (USA) IS10 Fourier infrared spectrometer by an attenuatedtotal reflectance method.

2.7. Contact angle

Contact angle measurements were performed at 25 °C with a re-lative humidity of 50% on a DSA100 (KRUSS, Germany) instrument,using 10 μL droplets of demonized water.

2.8. Preparation of plywood

Plywood samples with three layers (400 mm x 400 mm x 5.1 mm)(Table 1) were prepared according to the following procedure: 200 g/m2 SPCE pre-mixed with methylene bisphenyl isocyanate (MDI) wasmanually spread on a single poplar veneer in the middle and two eu-calyptus veneers on the top and bottom. Then, the SPCE-coated ply-wood samples were pressed at 120 °C under 1.0 MPa for 5 min.

2.9. Shear strength measurement

The plywood samples were stored under ambient conditions (22 °C)for 8 h before testing. The determination of shear strength was con-ducted according to China National Standards GB/T9846-2004 for theⅡ plywood. Twelve plywood specimens (2.5 cm × 10 cm) cut from twoplywood panels were immersed in water at 63 °C for 3 h, and then dried

at 22 °C for 10 min before testing.

2.10. Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) images were collected with aS-4800 scanning electron microscope (Hitachi chemical co.ltd. Japan).Film samples of SPCE were sputter coated with an alloy of 60% goldand 40% palladium to minimize charging effects.

2.11. Thermogravimetric analysis (TGA)

Thermogravimetric analysis (TGA) was performed on a NETZSCHSTA 409 PC instrument ranging from 35 to 800 °C at a rate of 10 °C/minunder nitrogen gas at a flow rate of 100 mL/min.

2.12. Differential scanning calorimetry (DSC)

DSC was performed on a Perkin Elmer diamond differential scan-ning calorimeter with 3–4 mg samples employed in the presence of anitrogen flow. The temperature was increased at a rate of 20 K min−1

from −60 °C to 90 °C. The second scan of the sample was used formeasurement of glass transition temperature (Tg).

3. Results and discussion

3.1. Apparent viscosity of SPCE

The soy protein was dissolved in water to form an aqueous solutionwith a maximum 10% solid content due to the intermolecular interac-tions of protein molecules at the higher protein content. The apparentviscosity of 10% solid content of soy protein was determined to be1180 cP at 25 °C. To facilitate full wetting and permeation of the ad-hesive into the wood surface, low viscosity (87.8 cP) polyacrylateemulsions with elaborately synthetic design, were used to modify thesoy protein aqueous solution with different mass fractions. As shown inFig. 1, the apparent viscosity of SPCE decreased with increasing poly-acrylate emulsion content. The introduction of polyacrylate emulsioncontaining the neutralized poly(acrylic acid), weakened the electro-static interaction between protein molecules and resulted in a decreaseof apparent viscosity. In addition, the low viscosity of the polyacrylateemulsion also decreased the apparent viscosity of the SPCE.

3.2. FT-IR of SPCE films

The soy protein-polyacrylate composite emulsions were cast intofilms in order to investigate their properties. To simulate applications asplywood adhesives, 3% wt of MDI based on the solid content of SPCEwas added as a crosslinking agent during the film formation.

Fig. 2 shows the FT-IR spectra of the films derived from SPCE. Forthe soy protein, the stretching vibration of N-H bonds in amide groupsand –OH in carbohydrates appeared and overlapped at 3420 cm−1. Thepeaks at 1650 cm−1, 1573 cm−1and 1314 cm−1 were assigned to thestretching vibration of amide bonds in soy proteins. With the in-troduction of the polyacrylate copolymer containing the neutralizedpoly(acrylic acid), the –OH stretching vibration bands at 3420 cm−1

broadened and shifted to a lower wavenumber (3400 cm−1), indicatingthe formation of intermolecular hydrogen bonds in the blends. It wasalso found that with increasing polyacrylate copolymer content in thefilms, the intensity of the peaks at 1720 cm−1, corresponding to estergroups, also increased.

3.3. Wettability of SPCE films

Fig. 3 shows contact angle images of water droplets on films of soyprotein-polyacrylate blended emulsions. Contact angle values can re-flect the wetting characteristics of adhesives on the surface of wood

Fig. 1. Viscosity curve of different solid-liquid ratios of SPCE with A equally mass % ofacrylic emulsion and S indicating the mass % of soy emulsion.

F. Wang et al. International Journal of Adhesion and Adhesives 82 (2018) 160–165

162

[28]. As shown in Fig. 3, the soy protein had a contact angle (static) of77.95°. After the introduction of the polyacrylate copolymers with masscontents of 10%, 30%, 50%, 70%, 90% and 100%, the contact angledecreased to 73.78°, 69.89°, 64.55°, 62.25°, 54.85° and 52.85°, re-spectively, due to the existence of neutralized poly(acrylic acid) andemulsifiers (OP-10 and SLS) due to both being hydrophilic in nature.The introduction of emulsifier into the soy protein solution, when usedat the same concentrations as employed in the polyacrylate emulsion,resulted in decreases in contact angle from 77.95° to 46.75° (Fig. 3S100+emulsifier). These results suggested that the introduction ofpolyacrylate coploymers, as well as small amounts of emulsifiers, canenhance the wettability of SPCE.

3.4. Shear strength of plywood

The shear strengths of plywood bonded by SPCE were investigatedand the results summarized in Table 1. For comparative purposes, theshear strengths of plywood before and after being submerged in waterat 63 °C for 3 h, were measured. It was found that without the cross-linking agent MDI, the bond strengths of plywood samples (S100) gluedby soy protein decreased dramatically after immersion in water, whichwas due to the existence of hydrophilic groups such as primary amine,carboxyl and hydroxyl. These hydrophilic groups are sensitive to water.As a result, the crosslinking agent MDI has often been employed tomodify soy protein when used as a wood adhesive [29]. The shearstrengths of plywood samples before and after immersion in water

increased with an increase in MDI content. However, the strength va-lues of these samples (S100) were both below 0.7 MPa, a importantfactor in China National Standards GB/T9846-2004 for Ⅱ plywood,indicating that soy protein without modification can't be used as anadhesive for the manufacture of Ⅱ plywood [30]. When the dosage ofMDI for modification of soy protein was above 5% (not shown), theshear strength could surpass 0.7 MPa.

As shown in Table 1, the shear strengths of plywood samples beforeand after water immersion were found to increase with an increase inpolyacrylate emulsion content in SPCE. The strength values of thesesamples (A60S40, A70S30, A80S20, A90S10 and A100) were found to behigher than 0.7 MPa when the polyacrylate emulsion content was above60%. The improvement in strength, as well as the water resistance ofplywood, was probably due to the formation of a gel structure inducedby SPCE containing the neutralized poly(acrylic acid) [31]. To furtherconfirm this hypothesis, P(MMA-co-BA) polyacrylate emulsions withoutpoly(acrylic acid) were synthesized and diluted to 10% solid content.The P(MMA-co-BA) emulsions were then blended with soy protein toprepare plywood. As shown in Table 1, the strength values of thesesamples (A110S90, A150S50, A160S40,A170S30, A180S20, A190S10 and A1100) wereall lower than the S100. These results further verified the possible for-mation of a gel structure induced by neutralized poly(acrylic acid).

MDI was used to further crosslink the composite emulsions. Resultsrevealed that the shear strengths of plywood before and after waterimmersion were increased. 0.1% MDI in SPCE can promote the bondstrengths of plywood glued by A40S60 from 0.67 MPa to 0.71 MPa. Andall the plywood samples bonded by the SPCE containing 3% MDI ex-hibited shear strengths above 0.7 MPa, further confirming that poly-acrylate copolymer can enhance the shear strength of soy protein.

3.5. Scanning electron microscopy (SEM)

Fig. 4 shows SEM images of films of SPCE. As expected, the film ofsoy protein without plasticizer shows a relatively rough, protruding,cracked surface (Fig. 4 S100) due to film casting at 50 °C being lowerthan its glass transition temperature (Tg) at ~150 °C [32]. After theintroduction of polyacrylate copolymers into the matrix of the film, itwas noted that the surface of the films gradually turned to smooth. Theimprovement in the film formation of these blended emulsions can bedescribed as follows: During film formation, the neutralized poly(ac-rylic acid) in the polyacrylate copolymer can interact with soy proteinand improve the compatibility between the two as evidenced by bothFT-IR (Fig. 2) and SEM (Fig. 4) analysis. In particular, the Tg ( 2 °C) ofpolyacrylate copolymer can lower the Tg of SPCE which can result in animprovement in film-forming properties. These results further

Fig. 2. FT-IR spectra of the films of SPCE.

Fig. 3. contact angle images of SPCE films.

F. Wang et al. International Journal of Adhesion and Adhesives 82 (2018) 160–165

163

confirmed that the polyacrylate copolymer containing the neutralizedpoly(acrylic acid) can improve the shear strength of plywood glued bySPCE.

3.6. Thermogravimetric analysis (TGA)

TGA and DTG curves of different solid-liquid ratios of SPCE areshown in Fig. 5. For pure soy protein (S100), a small weight loss attemperatures below 200 °C was observed, which was due to the loss ofadsorbed and bound water. The initial temperature of degradation (Td)of S100 occurred at 292 °C and gave a maximum degradation rate (Tmax)at 320 °C and 28.55% solid residue. Degradation of the pure poly-acrylate copolymer (A100) was found to occur in a single process ex-hibiting a Td of 332 °C and a Tmax at 410 °C, thus showing good thermalstability in comparison with soy protein. When combining polyacrylatecopolymer and soy protein, the characteristic degradation parameters(Tmax= 320 °C for soy protein and Tmax= 410 °C for polyacrylate co-polymer) were both observed. Since SPCE emulsions were prepared byphysical blending, the TGA signal at Tmax, as well as the solid residue,were in proportion to the mass ratio of soy protein and polyacrylatecopolymer. Thus an increase in polyacrylate copolymer content canlead to an increase in the thermal stability of the SPCE.

4. Conclusions

In conclusion, a combination of polyacrylate copolymers and soyprotein with different mass ratios was achieved by a simple blending ofaqueous soy protein and polyacrylate emulsions, with the latter pre-pared via emulsion polymerization of MMA, BA and AA. The in-troduction of polyacrylate emulsion decreased the apparent viscosity ofSPCE. FT-IR analysis confirmed the formation of intermolecular hy-drogen bonds in composites. Static contact angle measurements re-vealed that the introduction of polyacrylate copolymers containingneutralized poly(acrylic acid), as well as a small amount of emulsifier,can enhance the wettability of SPCE and facilitate its application as awood adhesive. The neutralized poly(acrylic acid) in polyacrylate wasfound to induce interaction with soy protein. As a result, the compat-ibility of soy protein and polyacrylate copolymers was enhanced, which

Fig. 4. SEM images of SPCE films.

Fig. 5. (A) TGA and (B) DTG curves of different solid-liquid ratios of SPCE.

F. Wang et al. International Journal of Adhesion and Adhesives 82 (2018) 160–165

164

therefore led to an improvement in the shear strength of plywoodbonded by SPCE. Additionally, the water resistance of plywood can befurther enhanced by the use of MDI as a crosslinking agent. TGA ana-lysis confirmed that the introduction of polyacrylate copolymer resultedin increasing levels of thermal stability in the resultant composites. Thisstudy may open an avenue to modify soy protein with polymer emul-sions with enhanced properties.

Acknowledgments

We would like to acknowledge support from the National NaturalScience Foundation of China (31270589), and Science and TechnologyFoundation of Jilin Province (20140204054NY), Jiangsu ProvincialNatural Science Foundation of China (BK20131070).

References

[1] Nicholson C, Abercrombie J, Botterill W, Brocato R, Detzel J, Fitzpatrick Jr J, et al.History of adhesives. ESC Rep 1991;1.

[2] Pizzi A, Mittal KL., Wood Adhesives, IDC, Hotei; 2011.[3] Li K, Peshkova S, Geng X. Investigation of soy protein-kymene® adhesive systems for

wood composites. J Am Oil Chem' Soc 2004;81:487–91.[4] Liu Y, Li K. Development and characterization of adhesives from soy protein for

bonding wood. Int J Adhes Adhes 2007;27:59–67.[5] Jang Y, Huang J, Li K. A new formaldehyde-free wood adhesive from renewable

materials. Int J Adhes Adhes 2011;31:754–9.[6] Kumar R, Choudhary V, Mishra S, Varma IK, Mattiason B. Adhesives and plastics

based on soy protein products. Ind Crop Prod 2002;16:155–72.[7] Mo X, Sun X. Soy proteins as plywood adhesives: formulation and characterization.

J Adhes Sci Technol 2012;27:2014–26.[8] Hettiarachchy NS, Kalapathy U, Myers DJ. Alkali-modified soy protein with im-

proved adhesive and hydrophobic properties. J Am Oil Chemists’ Soc1995;72:1461–4.

[9] Zhang Z, Hua Y. Urea-modified soy globulin proteins (7S and 11S): effect of wett-ability and secondary structure on adhesion. J Am Oil Chem' Soc 2007;84:853–7.

[10] Zhong Z, Sun XS, Wang D, Ratto J. Wet strength and water resistance of modifiedsoy protein adhesives and effects of drying treatment. J Polym Environ2003;11:137–44.

[11] Huang W, Sun X. Adhesive properties of soy proteins modified by sodium dodecylsulfate and sodium dodecylbenzene sulfonate. J Am Oil Chem' Soc 2000;77:705–8.

[12] Xi D, Yang C, Liu X, Chen M, Sun C, Xu Y. Graft polymerization of styrene on soy

protein isolate. J Appl Polym Sci 2005;98:1457–61.[13] Foulk JA, Bunn JM. Properties of compression-molded, acetylated soy protein films.

Ind Crop Prod 2001;14:11–22.[14] Heck T, Faccio G, Richter M, Thöny-Meyer L. Enzyme-catalyzed protein cross-

linking. Appl Microbiol Biotechnol 2013;97:461–75.[15] Wang Y, Mo X, Sun XS, Wang D. Soy protein adhesion enhanced by glutaraldehyde

crosslink. J Appl Polym Sci 2007;104:130–6.[16] Zhong Z, Sun X. Plywood adhesives by blending soy protein polymer with phenol-

formaldehyde resin. J Biobased Mater Bioenergy 2007;1:380–7.[17] Gao Q, Shi S, 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.[18] Lei H, Du G, Wu Z, Xi X, Dong Z. Cross-linked soy-based wood adhesives for ply-

wood. Int J Adhes Adhes 2014;50:199–203.[19] Xiao Z, Li Y, Wu X, Qi Gn, Li No, Zhang K, et al. Utilization of sorghum lignin to

improve adhesion strength of soy protein adhesives on wood veneer. Ind Crop Prod2013;50:501–9.

[20] Chen N, Lin Q, Rao J, Zeng Q. Water resistances and bonding strengths of soy-basedadhesives containing different carbohydrates. Ind Crop Prod 2013;50:44–9.

[21] Zhang Y, Zhu W, Lu Y, Gao Z, Gu J. Nano-scale blocking mechanism of MMT and itseffects on the properties of polyisocyanate-modified soybean protein adhesive. IndCrop Prod 2014;57:35–42.

[22] Smith WV, Ewart RH. Kinetics of emulsion polymerization. J Chem Phys1948;16:592–9.

[23] Odian G. Principles of polymerization [etc.]. New York: Wiley; 1991.[24] Cui Z, Kong X, Chen Y, Zhang C, Hua Y. Effects of rutin incorporation on the

physical and oxidative stability of soy protein-stabilized emulsions. Food Hydrocoll2014;41:1–9.

[25] Wang L, Cole M, Li J, Zheng Y, Chen YP, Miller KP, et al. Polymer grafted recyclablemagnetic nanoparticles. Polym Chem 2015;6:248–55.

[26] Wang L, Chen YP, Miller KP, Cash BM, Jones S, Glenn S, et al. Functionalised na-noparticles complexed with antibiotic efficiently kill MRSA and other bacteria.Chem Commun 2014;50:12030–3.

[27] Wang L, Benicewicz BC. Synthesis and characterization of dye-labeled poly(me-thacrylic acid) grafted silica nanoparticles. ACS Macro Lett 2013;2:173–6.

[28] Tavisto M, Kuisma R, Pasila A, Hautala M. Wetting and wicking of fibre plant strawfractions. Ind Crop Prod 2003;18:25–35.

[29] Amaral-Labat GA, Pizzi A, Gonçalves AR, Celzard A, Rigolet S, Rocha GJM.Environment-friendly soy flour-based resins without formaldehyde. J Appl PolymSci 2008;108:624–32.

[30] Luo J, Luo J, Gao Q, Li J. Effects of heat treatment on wet shear strength of plywoodbonded with soybean meal-based adhesive. Ind Crop Prod 2015;63:281–6.

[31] Schosseler F, Ilmain F, Candau SJ. Structure and properties of partially neutralizedpoly(acrylic acid) gels. Macromolecules 1991;24:225–34.

[32] Ogale AA, Cunningham P, Dawson PL, Acton JC. Viscoelastic, thermal, and mi-crostructural characterization of soy protein isolate films. J Food Sci2000;65:672–9.

F. Wang et al. International Journal of Adhesion and Adhesives 82 (2018) 160–165

165

http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref1http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref1http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref2http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref2http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref3http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref3http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref4http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref4http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref5http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref5http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref6http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref6http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref7http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref7http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref7http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref8http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref8http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref9http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref9http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref9http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref10http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref10http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref11http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref11http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref12http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref12http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref13http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref13http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref14http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref14http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref15http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref15http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref16http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref16http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref17http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref17http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref18http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref18http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref18http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref19http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref19http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref20http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref20http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref20http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref21http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref21http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref22http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref23http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref23http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref23http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref24http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref24http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref25http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref25http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref25http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref26http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref26http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref27http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref27http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref28http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref28http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref28http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref29http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref29http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref30http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref30http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref31http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref31http://refhub.elsevier.com/S0143-7496(18)30002-2/sbref31

本文献由“学霸图书馆-文献云下载”收集自网络，仅供学习交流使用。

学霸图书馆（www.xuebalib.com）是一个“整合众多图书馆数据库资源，

提供一站式文献检索和下载服务”的24 小时在线不限IP

图书馆。

图书馆致力于便利、促进学习与科研，提供最强文献下载服务。

图书馆导航：

图书馆首页 文献云下载 图书馆入口 外文数据库大全 疑难文献辅助工具

http://www.xuebalib.com/cloud/http://www.xuebalib.com/http://www.xuebalib.com/cloud/http://www.xuebalib.com/http://www.xuebalib.com/vip.htmlhttp://www.xuebalib.com/db.phphttp://www.xuebalib.com/zixun/2014-08-15/44.htmlhttp://www.xuebalib.com/

Combinations of soy protein and polyacrylate emulsions as wood adhesivesIntroductionMaterials and methodsMaterialsSynthesis of polyacrylate emulsionPreparation of soy protein-polyacrylate blended emulsions (SPCE)Preparation of emulsion filmApparent viscosityFT-IR spectrumContact anglePreparation of plywoodShear strength measurementScanning electron microscopy (SEM)Thermogravimetric analysis (TGA)Differential scanning calorimetry (DSC)

Results and discussionApparent viscosity of SPCEFT-IR of SPCE filmsWettability of SPCE filmsShear strength of plywoodScanning electron microscopy (SEM)Thermogravimetric analysis (TGA)

ConclusionsAcknowledgmentsReferences

学霸图书馆link:学霸图书馆