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Cite This: ACS Sustainable Chem. Eng. 2019, 7, 10452−10459

Dialdehyde Cellulose as a Bio-Based Robust Adhesive for Wood Bonding Hua Zhang,†,§ Peiwen Liu,†,§ Saleh Md. Musa,† Carsten Mai,‡ and Kai Zhang*,† †

Wood Technology and Wood Chemistry, Georg-August-Universität Göttingen, Büsgenweg 4, D-37077 Göttingen, Germany Wood Biology and Wood Products, Georg-August-Universität Göttingen, Büsgenweg 4, D-37077 Göttingen, Germany



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S Supporting Information *

ABSTRACT: Novel adhesives based on natural biopolymers with high performance are still desired for the wood industry, in order to replace conventional fossil-based adhesives on the market. In this study, aqueous solutions of dialdehyde cellulose (DAC) with various degrees of oxidation (DOs) and distinct concentrations were evaluated as robust adhesives for wood bonding, which has not yet been systematically studied. The adhesion performance of DAC adhesives was investigated using tensile shear strength measurements according to European standard EN302-1. Results showed the DO and the concentration of DACs had a predominant impact on the adhesion performance. The optimal formulation of DAC adhesives was found to be the aqueous solutions of DAC with a DO of 1.75 and a concentration of 40 wt %. The corresponding best bonding performance was represented by the bonding strength of about 9.53 MPa for beech wood specimens and 5.75 MPa for spruce wood specimens. Furthermore, wood specimens in shear strength tests mainly revealed a substrate failure mode rather than adhesive or cohesive failure. This indicates that the DAC adhesives possessed a stronger bonding strength than the wood itself. Therefore, our study demonstrates that DAC is a potential bio-based adhesive for wood bonding, especially under indoor conditions. KEYWORDS: Dialdehyde cellulose, Wood adhesives, Bonding strength, Water-soluble



INTRODUCTION Wood adhesives are widely used for constructions, floorings, and furniture.1−4 Essentially, conventional adhesives for wood products on the market are based on fossil-derived polymers, such as melamine-formaldehyde (MF),5−8 phenol formaldehyde (PF),9−12 urea formaldehyde (UF),13−15 polymeric diphenylmethane diisocyanate (pMDI) resin,16−18 and poly(vinyl acetate).19 Although these synthetic resins possess the advantages of high bonding performance and high water resistance, there are also some drawbacks: (i) Most of them are made of volatile organic compounds (e.g., formaldehyde) or other toxic compounds (e.g., methylene diphenyl diisocyanate and epichlorohydrin), which would cause environmental problems.1,16−18,20 (ii) The production of these resins relies on nonrenewable fossil resources, accompanied by the negative environmental impact.1,21,22 Therefore, there has been a rapidly growing interest in the development of adhesives from a natural renewable source with good bonding performance.23 Over the past few decades, numerous natural biopolymers, such as starch,24,25 lignin,3,26−28 tannin,29−31 proteins,32−34 gums,35−37 and chitin/chitosan,38−42 have been used as promising candidates in the synthesis of bio-based wood adhesives.43,44 However, the development of bio-based adhesives also suffers from some challenges, which greatly limits their practical applications in wood bonding. It is © 2019 American Chemical Society

necessary to distinguish that bio-based wood adhesives in this study refer to purely natural biopolymers or their mixture, without incorporating any synthetic resins. For instance, ligninbased adhesives in most studies still require the incorporation of phenol-formaldehyde or polymeric methylene diphenyldiisocyanate (PMDI) to satisfy the requirements with acceptable mechanical properties.44 Navarrete et al. reported a novel biobased adhesive derived from a low molecular mass lignin and tannin without incorporating any synthetic resin,45,46 but the beech plywood bonded with tannin/lignin adhesives exhibited a relatively low dry tensile strength of about 1.4 MPa according to the interior panel standard (EN 312). Some efforts have also been explored to completely replace fossil fuel-based adhesives by starch- or protein-based bioadhesives, but the bonding strength and water resistance of the ultimate products are still low. Wang et al. developed a novel bioadhesive via grafting copolymerization of the synthetic polymers onto the starch backbone.47 The optimal shear strength with a starch/ monomer ratio of 1:1.2 (w/w) was 4.3 and 2.17 MPa in the dry and wet state, respectively. Wu et al. reported a wood adhesive from grafting glycidyl methacrylate on canola protein via free radical polymerization.48 The enhanced thermal Received: February 8, 2019 Revised: May 7, 2019 Published: May 14, 2019 10452

DOI: 10.1021/acssuschemeng.9b00801 ACS Sustainable Chem. Eng. 2019, 7, 10452−10459

Research Article

ACS Sustainable Chemistry & Engineering stability and water resistance of the protein-based adhesive were attributed to the formation of hydrogen bonding and covalent bonds during the conjugating process. The conjugate with a grafting degree of 82.0% showed a dry shear strength of 8.25 ± 0.12 MPa and a wet strength of 3.68 ± 0.29 MPa according to ASTM standard D1151-00. Further chemical modification of these biopolymers or the incorporation of some additives was needed to enhance their bonding strength and water resistance. In addition, the extraction and fractionation procedures required to isolate bio-based polymers have a great impact on the final properties of bio-based adhesives, as well as the cost. Therefore, the development of new bio-based environmentally friendly wood adhesives with high cost-efficiency and high adhesion performance still needs further exploration. Dialdehyde cellulose (DAC) is one of the cellulose derivatives that is synthesized by selective oxidation on C2 and C3 positions of the anhydroglucose units (AGUs).49,50 The conversion of hydroxyl groups to aldehyde groups not only reduces the crystallinity of initial materials but also gives rise to highly oxidized forms of DAC, which could be easily dissolved in hot water.51 Furthermore, the aldehyde moieties in DAC are predominantly masked as intra- and intermolecular hemiacetal and hemialdal moieties, cross-linking the rather short DAC polymer chains.51,52 These characters would endow DAC with excellent mechanical properties and make it a potential wood adhesive. As far as we know, DAC as an environmentally friendly bio-based wood adhesive has yet never been systematically studied. Herein, the aim of this study is to evaluate adhesive properties of diverse DACs with the objective to develop new bio-based adhesives. Various reaction conditions were performed to obtain aqueous DAC adhesive formulations with various DOs. The effects of DOs and DAC concentrations in their aqueous solutions on dry bonding strength were investigated via tensile shear strength tests and microscopic analysis.



Table 1. Summary of Degrees of Oxidation and Reaction Conditions for Obtained DAC reaction conditions DAC

DOa

reagents

DAC-1.88

1.88 ± 0.002

DAC-1.75

1.75 ± 0.002

DAC-1.25

1.25 ± 0.003

DAC-0.78

0.78 ± 0.008

MCC (1 g), NaIO4 (1.65 g), and NaCl (3.87 g) MCC (1 g) and NaIO4 (1.65 g) MCC (1 g) and NaIO4 (1.65 g) MCC (1 g) and NaIO4 (1.65 g)

temp (°C) time (h) 25

72

25

72

55

4

45

4

a

The degree of oxidation (DO) was determined by a titration method. Each sample was measured with three replicates.

60%, and 70%) were used as DAC adhesives. The concentration in this study is generally the mass percentage of solid DAC with respect to the whole solution, if not mentioned otherwise. The obtained DAC adhesives were denoted as DAC-x, where x represents the DO value of DAC determined by titration. Determination of Degree of Oxidation (DO). To measure the DO values of the obtained DACs, both titration and element analysis methods were performed according to previously reports.50 For titration, the pH value of the DAC solution (10 mL) was first adjusted to 3.5 using HCl solution (1 M) at room temperature. Then, 10 mL of hydroxylamine hydrochloride solution (5 wt %) was added into the solution. After that, a NaOH standard solution (0.05 M) was added drop by drop into the mixture solution under stirring, until the pH value recovered to 3.5. Finally, the DOs of synthesized DACs were calculated by using the following equation DO =

162*C NaOHVNaOH m + 2*C NaOHVNaOH

(1)

where m is the dry weight of DAC, CNaOH and VNaOH represent the concentration and the consumption volume of the aqueous NaOH solution, and 162 is the molecular weight of the anhydroglucose units (AGUs) of cellulose, respectively. By using the elemental analysis, the aldehyde contents were also calculated from the nitrogen content of products according to the ammoniation of hydroxylamine hydrochloride and aldehyde groups. Thus, the DO of DAC can be determined by the given eq 2

EXPERIMENTAL SECTION

Materials. Laboratory grade microcrystalline cellulose (MCC, ∼50 μm), ethylene glycol (anhydrous, 99.8%), and hydroxylamine hydrochloride (NH2OH·HCl, 98.5%) were purchased from SigmaAldrich Chemie GmbH (Steinheim, Germany). Sodium periodate (NaIO4, 99.8%), sodium chloride (NaCl, 99.8%), sodium hydroxide (NaOH, 99.8%), and hydrochloric acid aqueous (HCl, 37% w/w) were purchased from VWR International GmbH. Deionized (DI) water with a conductivity of 0.05 ± 0.03 μs cm−1 was used for all experiments. All chemicals were used as obtained without further purification. Preparation of DAC Adhesives. Typically, MCC (1 g), NaIO4 (1.65 g), and NaCl (3.85 g) were first mixed in 50 mL of DI water to form a suspension. Then the reaction bottle was completely covered with several layers of aluminum foil to avoid the light-induced decomposition of NaIO4. Thereafter, the periodate oxidation was performed at the desired temperature (e.g., 25, 45, and 55 °C) under magnetically stirring for the desired time (e.g., 4 and 72 h). After oxidation, 5 mL of ethylene glycol was added into the mixture to quench the reaction. The obtained products were then purified using dialysis membranes with a molecular-weight cutoff of 3500 Da (Thermo Fisher Scientific GmbH, Germany). After that, the resultant DACs were collected after being dissolved in DI water at 80 °C in an oil bath. To prepare various DACs with distinct degrees of oxidation (DOs), diverse reaction conditions were performed, and corresponding parameters are presented in Table 1. Diverse aqueous DAC solutions with various DOs and concentrations (e.g., 30%, 40%, 50%,

DO =

72 N ×2× 28 C

(2)

where N/C is the mass ratio of the nitrogen and carbon contents of synthesized DAC. The detailed parameters are presented in Table S1. Preparation of Wood Specimens. The wood specimens used in the study were beech (Fagus sylvatica, hardwood) and Norway spruce (Picea abies, softwood), which have been widely used for the purpose of adhesives characterization.53 The average densities for beech wood and spruce wood were measured to be 0.66 ± 0.02 and 0.38 ± 0.02 g cm−3, respectively. For the shear testing, both beech and spruce specimens were manufactured and constructed in accordance with the type of EN 302-1 shear lap joint (European Committee for Standardization 2013). The dimensions (L × W × H) for each shear specimen were 124 × 20 × 20 mm3, and the glued area of each adhesive was about 20 × 20 mm2 (Scheme 1). Thereafter, DACs with diverse DOs and concentrations were applied to glue the specimens. In all cases, the glued direction was aligned with the longitudinal axis of each specimen. The excess adhesive was cleaned off from the edges and surfaces of the specimens using a cleaned dry brush. Subsequently, the obtained specimens were pressed with a uniform pressure of 0.72 MPa and then dried at 80 °C for 24 h. In this study, all specimens for shear strength measurements were referred to as DAC-x-y, where x is the DO value of DAC and y is the concentration of DAC in the formulation. 10453

DOI: 10.1021/acssuschemeng.9b00801 ACS Sustainable Chem. Eng. 2019, 7, 10452−10459

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ACS Sustainable Chemistry & Engineering W = F /A

Scheme 1. Schematic Illustration for (a) the Preparation of Wood Specimens Glued with DAC Adhesives and (b) the Failure Modes That Occurred in Tests

(4)

where W is the shear strength (MPa), F is the force at the failure point (N), and A is the shear area (mm2), respectively. Microscopic Characterization. The surface microstructure of the specimens was investigated using a digital microscope (VHXS500E, Keyence, Neu-Isenburg, Germany). Both 2D and 3D images were recorded to study the adhesive penetration beneath the adhered surface at 30× magnification.



RESULTS AND DISCUSSION Preparation and Characterization of DAC. The periodate oxidation of cellulose has been considered as one of the most facile and effective methods to prepare DAC due to its mild conditions in aqueous solution.49 This methodology bases on the cleavage of the C-C linkage between the C2 and C3 of the AGU of cellulose, followed by the transformation from hydroxyl groups to aldehyde groups (Figure 1a). To prepare DAC with various DOs, diverse reaction conditions were adjusted including reaction temperature and times (Table 1). For instance, DAC-1.75 was prepared at 25 °C for 72 h and the corresponding DO value was determined to be 1.75 ± 0.02 by titration. With the presence of additional NaCl during the reaction, a higher DO value of 1.88 ± 0.02 was reached for DAC-1.88. When the reaction temperature was raised to 45 °C along with the reaction time reduced to 4 h, a DO value of 0.78 ± 0.02 was achieved for DAC-0.78. In comparison, a higher reaction temperature (55 °C) resulted in a higher DO value of 1.25 ± 0.02. The increase of the DO value along with reaction temperature was in accordance with previously reports.49,54 The DOs of produced DAC samples were determined using both hydroxylamine hydrochloride titration and elemental analysis methods (Table S1), which led to comparable DO values. The chemical structure of neat MCC and obtained DACs was characterized by FTIR analysis (Figure 1b,c) of freezedried DAC samples. The broadened signals located at 3600− 3000 cm−1 originated from the O-H stretching vibration (νO‑H).55,56 Compared with the FTIR spectrum of MCC, three

The solid content per unit area (μg mm−2) of DAC adhesives applied on wood specimens was calculated using the following equation

SC = (m2 − m1)/A

(3)

where SC is the solid content, m1 is the dry weight of the wood specimen without DAC adhesive, m2 is the dry weight of the wood specimen applied with DAC adhesive, and A is the shear area of each specimen. Shearing Strength Measurements. Before shearing strength tests, all specimens were reconditioned at 20 °C with a relative humidity of 65% until constant weights. The shear strength of specimens applied of the lap joint specimens glued with DAC adhesives was measured using a Zwick-10 KN tensile testing machine (ZwickRoell GmbH & Co. KG, Ulm, Germany). The specimens at an average moisture content of 11.5 ± 0.3% were tested at a load speed of 1 mm min−1. Ten replicates were used for each adhesive and treatment. The force applied at the failure point was recorded for each specimen and used to calculate the shear strength (W) as follows

Figure 1. Characterization of obtained DACs. (a) Schematic illustrations for the preparation of DAC via periodate oxidation, (b) FTIR spectra (4000−500 cm−1) of neat MCC and obtained DACs, and (c) the detailed view in the range of 2000−1500 cm−1. 10454

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Figure 2. (a) Shear strength of wood specimens glued using DAC with various DOs, and (b) the corresponding solid contents for each DAC adhesive. Ten replicates were used for each sample.

Figure 3. Representative fracture surface morphology of (A) beech wood and (B) spruce wood specimens glued with DAC adhesives with various DO values. A1,3,5,7,9 and B1,3,5,7,9 show corresponding 2D images of the fractured surfaces with the main failure modes that occurred during the shear strength testing. A2,4,6,8,10 and B2,4,6,8,10 show corresponding 3D color images using a VHX-S500E microscope. White arrows refer to the failed structure that occurred on the wood surface after shear strength testing.

characteristic peaks appeared at 1730, 1635, and 880 cm−1 within the FTIR spectra of DACs. The signal at 1635 cm−1 (Figure 1c) assigned to δO‑H vibrations was caused by adsorbed water.57 The signals at 1730 and 880 cm−1 originated from the stretching vibrations of carbonyl groups (νCO) and the vibration of hemiacetal bands (νC‑O) between the dialdehyde groups and their adjacent hydroxyl groups, respectively. These

results confirmed the successful introduction of aldehyde groups into cellulose chains. Tensile Shear Strength Measurements. Effect of DO on the Adhesion Performance. DACs with various DOs and concentrations were further used as wood adhesives, and their dry bonding performance was evaluated using tensile shear strength measurements according to European standard EN302-1. First, we probed the effect of the DO of DAC on 10455

DOI: 10.1021/acssuschemeng.9b00801 ACS Sustainable Chem. Eng. 2019, 7, 10452−10459

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Figure 4. (a) Shear strength of specimens glued with DAC-1.75 adhesives at various concentrations, and (b) the corresponding solid contents of DAC-1.75 adhesives applied on specimens. Ten replicates were used for each sample.

Figure 5. Representative fracture surface morphology of (A) beech wood and (B) spruce wood specimens glued with DAC-1.75 adhesive at various concentrations. A1,3,5,7,9 and B1,3,5,7,9 show corresponding 2D images. A2,4,6,8,10 and B2,4,6,8,10 demonstrate 3D color images. White arrows refer to the failed structure that occurred on the wood surface after shear strength testing.

their dry bonding strengths. After considerable preliminary experimentation, the DAC concentration in the adhesive for this series was set at 40%. In all cases, the shear strength increases with increasing DO until 1.75, while it decreased with further rising DO to 1.88. For instance, along with the increasing DO from 0.78 to 1.75, the bond strength of beech wood specimens increased from 3.18 ± 0.15 MPa to 9.53 ± 0.07 MPa (Figure 2a). However, the bond strength decreased to 4.74 ± 0.03 MPa, when DO further increased to 1.88.

Similarly, the bond strength of spruce wood specimens also exhibited a similar tendency with increasing DO values (see Table S2 for details). For both beech wood and spruce wood specimens, the solid contents of applied DAC adhesives with various DOs were about 40 μg mm−2 (Figure 2b). This shows that, at the given concentration, variation in the amount of adhesive is not the reason for the differences in the shear strength. Thus, the DO of DACs plays a vital role in the shearing strength of wood specimens. The reason for this 10456

DOI: 10.1021/acssuschemeng.9b00801 ACS Sustainable Chem. Eng. 2019, 7, 10452−10459

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DO lower than 75% cannot completely dissolve in water, even with prolonged treatment time at 100 °C.54 DAC-1.75 with the DO of 1.75 was found to be easily dissolved in hot water at a concentration of 40% or lower, but it started to precipitate at concentrations of higher than 50%. When used as wood adhesives, this precipitated DAC in the adhesion layer might induce defects, which result in the slight decrease in the bond strength. The effect of adhesive concentration on adhesion performance was also visible in Figure 5. At a concentration of 30% for the DAC-1.75 adhesive, there is no clear structure wood fracture observed on the fracture surface of DAC-1.75 specimens. All wood specimens mainly exhibited a cohesive failure mode. At higher concentrations of 40%, 50%, 60%, and 70%, there is almost no impact on the glue line but a rupture of wood, displaying a type of substrate failure. This result indicates that the adhesive with 40% DAC-1.75 under dry conditions was strong enough to serve as wood adhesives, as the dry bond strength was higher than the intrinsic strength of the wood. All the above results demonstrated that the DAC adhesive containing 40% of DAC-1.75 exhibited the best adhesion performance with the solid content of ∼40.6 μg mm−2. The dry shear strength (EN302-1) is 9.53 ± 0.15 MPa for beech wood and 5.75 ± 0.05 MPa for spruce wood, respectively. This bonding performance of DAC adhesives is comparable to that of other bioadhesives measured with the European standard, which shows the potential of DACs as wood adhesives. It is worth noting that different measuring methods, wood types, and many other bonding conditions could affect the adhesive strength value.43 For instance, the dry strength of the same washed cottonseed meal reached 10.7 MPa with EN204/ 205,62 but only about 4.0 MPa according to ASTM standard D906-98.63 Thus, a direct comparison of the adhesive strength for wood adhesives should be avoided unless they are measured with exactly the same procedures. As DAC is a water-soluble compound, it suffers from a big challenge as for most natural biopolymers, namely, poor water resistance. As shown in Figure S1, wood specimens glued with DO-1.75 adhesive with a concentration of 30% were very easy to break after soaking in water for 24 h at room temperature. Although the bond was stronger at higher concentrations of more than 40%, the wet resistance of DAC adhesives still needs significant improvements. Further research will be on the improvement of the adhesive performance under wet conditions in order to enlarge the area of application of the DAC adhesive for wooden items used under outdoor conditions.

tendency of adhesion performance can be ascribed as follows. First, the aldehyde groups present in DAC chains may have reacted with the hydroxyl groups on the surface of the wood specimens, and then formed a cross-linking adhesive structure upon heating.37 The increase of the DO values indicates the increasing of aldehyde concentration in DAC, which gives rise to an increased number of covalent bonds. However, the DAC chains become shorter and more fragile with increasing DO values,49,54 leading to a weaker bonding performance for higher DO. The effect of the DO of DAC adhesives on the fracture surface microstructure of wood specimens was investigated using a VHX-S500E microscope, as shown in Figure 3. From 2D images of beech wood specimens, the breaking down of the wood surface was observed only at the fracture surface of specimens glued with DAC-1.75 adhesive, as presented by white arrows. 3D images with the false color show the height difference on wood specimen surfaces, which can be used to detect differences of the main breaking types between the differentially treated samples. For instance, the main breaking types are changed from cohesive failure to substrate failure with increasing DO values of DACs (Figure 3A1−A10). The substrate failure of DAC-1.75 also indicated that the shear strength of beech wood is lower than that of the adhesive, which has also been observed for other bio-based adhesives.58−60 In comparison, spruce wood specimens glued with DAC adhesives exhibited diverse main breaking types varying from cohesive failure to substrate failure (Figure 3B1−B10). The difference of failure modes between beech and spruce wood specimens can be attributed to the lower density of spruce wood (0.38 ± 0.02 g cm−3) compared to beech wood (0.66 ± 0.02 g cm−3) and thus leads to lower shear strength.61 The intrinsic shear strength of spruce wood is lower than that of the DAC adhesives at a concentration of 40%, while beech wood shows a higher strength itself except for the specimens bonded with DAC-1.75 adhesive. Overall, the DAC-1.75 adhesive exhibited the highest adhesion performance (9.53 ± 0.07 MPa) and thus was used for further evaluation. Effect of DAC Concentration on the Adhesion Performance. In addition to the DO value of DAC, the amounts of DAC applied for gluing also strongly influence the adhesion performance. Considering the simplicity, economy, and practicality of the operation, the amount of DAC was adjusted by changing the concentration of DAC in the adhesives. Adhesives containing 30%, 40%, 50%, 60%, and 70% of DAC1.75 were applied to glue wood specimens for shear strength testing. With rising concentrations, the corresponding solid content of DAC-1.75 adhesive increased from 30.4 to 69.4 μg mm−2 (Figure 4b). For both beech wood and spruce wood specimens, their bond strength exhibited a similar tendency along with increasing amounts of DAC, as shown in Figure 4a (see Table S3 for details). For example, the bond strength of beech wood specimens significantly increased from 3.38 ± 0.13 to 9.53 ± 0.15 MPa with rising DAC concentrations from 30% to 40%. However, a slight decrease in the bond strength was observed with further increasing concentrations from 50% to 70%. The bond strength of DAC-1.75-50%, DAC-1.75-60%, and DAC-1.75-70% specimens decreased to 7.84 ± 0.28, 9.34 ± 0.41, and 9.06 ± 0.14 MPa, respectively. The reason for this tendency could be attributed to the solubility of DAC in water. It was reported that DAC with the DO more than 75% can be easily dissolved in hot water (e.g., at 80 °C), while DAC with



CONCLUSION In this study, we presented the potential of dialdehyde cellulose (DAC) as a new bio-based wood adhesive. DACs with various DOs were prepared by periodate oxidation under various reaction conditions. When used as a wood adhesive, DOs and the concentrations of DAC in their aqueous solutions were proved to play pivotal roles on the adhesion performance under dry condition. The shear strength of the DAC adhesive increased with increasing DO of DAC up to 1.75, while further higher DO caused decreasing strength. In a similar manner, rising concentrations of the aqueous solutions of DAC led to higher bonding strength. The optimal adhesive formulation contained DAC with the DO of 1.75 at a concentration of 40%. The highest dry bonding strength was measured to be 10457

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(9) Jin, Y.; Cheng, X.; Zheng, Z. Preparation and characterization of phenol−formaldehyde adhesives modified with enzymatic hydrolysis lignin. Bioresour. Technol. 2010, 101 (6), 2046−2048. (10) Zhang, W.; Ma, Y.; Xu, Y.; Wang, C.; Chu, F. Lignocellulosic ethanol residue-based lignin−phenol−formaldehyde resin adhesive. Int. J. Adhes. Adhes. 2013, 40, 11−18. (11) Zhang, W.; Ma, Y.; Wang, C.; Li, S.; Zhang, M.; Chu, F. Preparation and properties of lignin−phenol−formaldehyde resins based on different biorefinery residues of agricultural biomass. Ind. Crops Prod. 2013, 43, 326−333. (12) Moubarik, A.; Grimi, N.; Boussetta, N.; Pizzi, A. Isolation and characterization of lignin from Moroccan sugar cane bagasse: Production of lignin−phenol-formaldehyde wood adhesive. Ind. Crops Prod. 2013, 45, 296−302. (13) Deka, M.; Saikia, C. N. Chemical modification of wood with thermosetting resin: effect on dimensional stability and strength property. Bioresour. Technol. 2000, 73 (2), 179−181. (14) Dunky, M. Urea−formaldehyde (UF) adhesive resins for wood. Int. J. Adhes. Adhes. 1998, 18 (2), 95−107. (15) Park, B. D.; Kim, J. W. Dynamic mechanical analysis of urea− formaldehyde resin adhesives with different formaldehyde-to-urea molar ratios. J. Appl. Polym. Sci. 2008, 108 (3), 2045−2051. (16) Pan, Z.; Cathcart, A.; Wang, D. Properties of particleboard bond with rice bran and polymeric methylene diphenyl diisocyanate adhesives. Ind. Crops Prod. 2006, 23 (1), 40−45. (17) Simon, C.; George, B.; Pizzi, A. Copolymerization in UF/pMDI adhesives networks. J. Appl. Polym. Sci. 2002, 86 (14), 3681−3688. (18) He, G.; Yan, N. Effect of moisture content on curing kinetics of pMDI resin and wood mixtures. Int. J. Adhes. Adhes. 2005, 25 (5), 450−455. (19) Chiozza, F.; Pizzo, B. Innovation in poly(vinyl acetate) water resistant D3 glues used in wood industry. Int. J. Adhes. Adhes. 2016, 70, 102−109. (20) Dongre, P.; Driscoll, M.; Amidon, T.; Bujanovic, B. LigninFurfural Based Adhesives. Energies 2015, 8 (8), 7897−7914. (21) Pizzi, A. Urea-formaldehyde adhesives. In Handbook of Adhesive Technology; Pizzi, A., Mittal, K. L., Eds.; Taylor & Francis Group, LLC: : New York, 2003; [Revised and expanded]. (22) Bye, C. N. Casein and Mixed Protein Adhesives. In Handbook of Adhesives; Skeit, I., Ed.; Van Nostrand Reinhold: New York, 1990; pp 135−152. (23) Hemmilä, V.; Adamopoulos, S.; Karlsson, O.; Kumar, A. Development of sustainable bio-adhesives for engineered wood panels−A Review. RSC Adv. 2017, 7 (61), 38604−38630. (24) Wang, P.; Cheng, L.; Gu, Z.; Li, Z.; Hong, Y. Assessment of starch-based wood adhesive quality by confocal Raman microscopic detection of reaction homogeneity. Carbohydr. Polym. 2015, 131, 75− 79. (25) Li, Z.; Wang, J.; Li, C.; Gu, Z.; Cheng, L.; Hong, Y. Effects of montmorillonite addition on the performance of starch-based wood adhesive. Carbohydr. Polym. 2015, 115, 394−400. (26) Kunaver, M.; Medved, S.; Č uk, N.; Jasiukaitytė, E.; Poljanšek, I.; Strnad, T. Application of liquefied wood as a new particle board adhesive system. Bioresour. Technol. 2010, 101 (4), 1361−1368. (27) Klašnja, B.; Kopitović, S. Lignin-phenol-formaldehyde resins as adhesives in the production of plywood. Holz als roh-und Werkstoff 1992, 50 (7−8), 282−285. (28) Ghaffar, S. H.; Fan, M. Lignin in straw and its applications as an adhesive. Int. J. Adhes. Adhes. 2014, 48, 92−101. (29) Trosa, A.; Pizzi, A. A no-aldehyde emission hardener for tanninbased wood adhesives for exterior panels. Eur. J. Wood Wood Prod. 2001, 59 (4), 266−271. (30) Pizzi, A.; Scharfetter, H. The chemistry and development of tannin-based adhesives for exterior plywood. J. Appl. Polym. Sci. 1978, 22 (6), 1745−1761. (31) Kim, S.; Kim, H. J. Curing behavior and viscoelastic properties of pine and wattle tannin-based adhesives studied by dynamic mechanical thermal analysis and FT-IR-ATR spectroscopy. J. Adhes. Sci. Technol. 2003, 17 (10), 1369−1383.

about 9.53 MPa for beech wood specimens and 5.75 MPa for spruce wood specimens according to European standard EN302-1. Although the improvement of water resistance of DAC adhesives still needs further studies, our findings demonstrate the great potential of DAC as a new promising bioadhesive for wood bonding, especially under indoor conditions.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b00801. Digital images of wood specimens applied water soaking treatment; summary of the DO determined by elemental analysis and titration; comparison of adhesion performance of DACs (PDF)



AUTHOR INFORMATION

Corresponding Author

*Tel.: 0049 551394505. Fax: 0049 551399646. E-mail: kai. [email protected]. ORCID

Hua Zhang: 0000-0001-7413-3699 Kai Zhang: 0000-0002-5783-946X Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS H.Z. and P.L. appreciate the financial support provided by the China Scholarship Council (CSC) for the Ph.D. grant. K.Z. thanks the Funding for the Promotion of Young Academics of Georg-August-University of Goettingen for the financial support.



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