Boiling Water Resistant Fully Bio-Based Adhesive Made from Maleated Chitosan and Glucose with Excellent Performance

Preparing green wood adhesives using renewable biomass resources is signi�cant. A three-component biomass adhesive CSC-G was prepared by combining chitosan, maleic anhydride, and glucose. Characterization using Fourier transform infrared (FTIR), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD) demonstrated that dehydrated malic acid serves as a bridge for connecting chitosan and glucose. The introduction of glucose enriches the crosslinking density, thereby improving the bonding strength. In addition, the effects of the mass ratio of CSC to glucose, hot pressing temperature, and hot-pressing time on CSC-G adhesive were also investigated. The excellent performance of CSC-G is mainly shown in that when pressed at 160 ℃ , the shear strength of three-layer plywood remains at 0.88 MPa after soaking in boiling water for 3 hours, and the curing temperature is better than the previously reported polyester adhesive. As an all-biomass polyester adhesive, CSC-G adhesive has a lower curing temperature and excellent performance and provides a new approach to developing high-performance chitosan-based wood adhesives.


Introduction
The development of the wood industry cannot be separated from the wide application of adhesives [1,2].Urea formaldehyde, phenolic aldehyde, and melamine are currently the most widely used wood adhesives [3,4], and will still occupy a dominant position in wood adhesives in the future [5,6].The use of these adhesives may be accompanied by potential formaldehyde pollution issues and oil resource consumption issues [7,8].Therefore, developing non-toxic and renewable green adhesives is very important from a long-term perspective.
Preparing high-performance wood adhesives using biomass materials has been a hot research topic [9][10][11].The preparation of bio-based adhesives to alleviate the potential pollution of formaldehyde-based adhesives and reduce dependence on non-renewable petroleum resources is signi cant and challenging [12].Many scholars have overcome the problems of poor adhesive strength and water resistance of biomass adhesives by modifying and modifying lignin, tannin, cellulose, protein, etc., and have shown promising potential in the preparation of wood adhesives [13][14][15][16].Like other biomass materials, chitosan has the potential to be used as an adhesive in the wood industry [17].
Chitosan (CS) is a product of partial acetylation removal from chitin [18].It is a widely sourced and renewable biomacromolecule polymer that can be extracted from shery wastes such as shrimp shells or crab shells [19,20].Based on the fact that chitosan is the only basic polysaccharide among natural polysaccharides, it has excellent biological properties and can be chemically modi ed [21].Chitosan has been chemically modi ed to improve its utilization, making it widely used in medicine, nanomaterials, food packaging, and other elds [22][23][24][25].Like other biomass materials, chitosan has the potential to be used as an adhesive in the wood industry.However, there are relatively few studies on the application of chitosan in wood adhesives.
The poor solubility of chitosan and the short amino molecular chain limit its application [26].Many scholars have converted chitosan into corresponding derivatives through grafting, cross-linking, and degradation methods [27,28].These modi cations include acylation, esteri cation, etheri cation, and degradation, which improve the solubility and mechanical strength of chitosan while maintaining its unique properties [29,30].Inspired by the above modi cation and modi cation methods, we promote the functionalization of chitosan and its derivatives through the grafting of functional groups to improve the inherent defects of chitosan.
In this work, rstly, maleic anhydride was used to carboxylate chitosan to prepare carboxy chitosan (CSC).Characterization have demonstrated that maleic anhydride acts as a "bridge" for connecting chitosan and glucose.Compared with CS and CSC, the water solubility of CSC-G has been dramatically improved and can be used as a wood adhesive (Fig. 1).CSC has a longer molecular chain and can effectively cross-link with glucose, resulting in better adhesive strength and water resistance.As an allbiomass adhesive, CSC-G adhesive not only meets the requirements of sustainable development, but also provides a reference for modifying chitosan and preparing chitosan-based biological adhesive.
2 Materials and methods

Materials
Chitosan (CS; deacetylation degree ≥ 85%), glucose, and maleic anhydride (MA) were purchased from Shanghai Adamas Reagent Co., Ltd.(Shanghai, China).Poplar veneers were purchased from a local lumber mill.The solvent used in the experiment was deionized water.

Preparation of CSC
15 g of maleic anhydride was heated to melting at 90 ℃, then 2.5 g of chitosan was slowly added.After 6 h of reaction, hot deionized water was added to quench the reaction.The mixture is ltered, washed, and dried at room temperature to obtain carboxylated chitosan (CSC).

Preparation of CSC-G adhesives
The synthesis of CSC-G adhesive is shown in Scheme 1. Add the dried CSC and glucose into the ask according to different mass ratios (the mass of CSC and glucose is 1:2, 1:1, 2:1, respectively), and add deionized water to make the solid content 40% (Table 1).Reaction at 90 ℃ for 5 h to obtain yellow-brown CSC-G adhesive.

Fourier-transform infrared spectroscopy (FTIR) analysis
The samples were scanned by attenuated total re ection infrared spectrometer (Thermo Scienti c Nicolet iS50) with wavenumbers ranging from 400 to 4000 cm − 1 for 32 scans with a resolution of 4 cm − 1 .

X-ray photoelectron spectroscopy (XPS) analysis
XPS was conducted on a Thermo Scienti cTM K-AlphaTM + spectrometer equipped with a monochromatic Al Kα X-ray source (1486.6 eV) operating at 100 W. All peaks would be calibrated with C1s peak binding energy at 284.8 eV for adventitious carbon.

Thermal performance analysis
The thermal properties of CS, CSC, and CSC-G adhesive were determined by differential scanning calorimetry (DSC; NETZSCH DSC204F1) and thermogravimetric analysis (TGA; NETZSCH TG209F1) in a nitrogen protected atmosphere.

Gel permeation chromatography (GPC) determinations
The molecular weight determinations of CS and CSC-G adhesive were carried out on a Waters 1525 gel permeation chromatography (GPC) (Table 2).

Test and analysis of adhesive properties
The prepared CSC-G adhesives were used to prepare three-layer plywood to evaluate adhesive properties.Three-layer plywood panels were prepared by spreading the adhesive on double sides of the central layer at 200 g/m 2 .Hot-pressing parameters were 200 ℃, 1.0 MPa, and 5 min.After the plywood is allowed to stand for 12 hours at room temperature, the three-layer plywood is sawn into test pieces (100 × 25 mm), and the adhesive area remains 25 × 25 mm.The shear strength of plywood specimens was evaluated according to GB/T 17657 − 2013 standard.
3 Results and discussion

FTIR analysis
Figure 2 shows the FTIR spectra of CS, CSC, and CSC-G adhesive.The broad absorption band of CS at 3450 cm − 1 attributed to N-H and O-H tensile vibrations [31,32], with 1090 cm − 1 being the tensile vibration of CH 2 -O-CH 2 .In addition, the peaks at 1598 and 1320 cm − 1 belong to the typical N-H bending and C-N tensile vibrations of chitosan [17], respectively.The peak of 1660 cm − 1 is attributed to the C = O stretching vibration of the residual amide bond in chitin.Compared with CS, the infrared spectrum of CSC shows signi cant changes.Firstly, due to the consumption of -NH 2 and -OH, the peak located at 3450 cm − 1 was signi cantly weakened.A peak attributed to C = O was generated at 1706 cm − 1 , proving that MA reacted with chitosan and produced amide or ester groups.Secondly, compared to CSC, the absorption peak in CSC-G shifted from 1706 cm − 1 to 1714 cm − 1 , and the peak width was signi cantly enhanced, indicating that the carboxyl groups in CSC reacted with the hydroxyl groups on glucose to generate more esters [33], forming a cross-linked structure.In addition, a new broad peak appeared at 3600 cm − 1 , which belongs to the hydroxyl group on glucose [34], indicating that the adhesive contains many hydrogen bonds, which is also why CSC-G has good bonding performance.The above results indicate that maleic anhydride acts as a "bridge" to connect chitosan and glucose, forming a densely cross-linked network structure. 13C NMR analysis In addition, the chemical structures of CS, CSC and CSC-G were further analyzed by 13 C NMR.As can be seen from Fig. 3, both CSC and CSC-G, as derivatives of chitosan, have obvious unit absorption peaks of D-anhydroglucopyranose units (AGUs) [32].Chemical shift peaks less than 100 ppm are attributed to C1-6 or C11-16, and have different chemical shift values in different environments, which is consistent with previous reports [32].In the spectra of CSC, three new absorption peaks were detected at 169.8, 162.5, and 131.8 ppm, which were attributed to C7, C10, C8, and C9, respectively.These three new signals prove that CS and MA react successfully to obtain CSC.Compared with CSC, the peak of CSC-G at 162.5 ppm disappeared and moved to 169.2 ppm, indicating that the carboxyl and glucose reactions on CSC were consumed to form ester groups [34].The above results provided strong evidence for the successful preparation of CSC-G adhesive.

XPS analysis
To better understand the changes in the adhesive before and after curing, XPS analysis was conducted on CS, CSC-G adhesive, and cured CSC-G adhesive (Fig. 4).Upon comparing Fig. 4a and c, it is evident from the high-resolution spectra of C1s in both gures that a new peak at 288.7 eV has emerged, which can be attributed to C = O [35,36].The results indicate that maleic anhydride acts as a bridge to connect CS and glucose.The content of O atom in uncured CSC-G adhesive is 14.96%, and the content of O bonds in cured CSC-G adhesive is 26.47%, indicating a decrease in C-O bonds after resin curing (Fig. 4f).This may be better than the thermal pressing process, where glucose is dissociated from the cross-linked network and undergoes pyrolysis or dehydration to form anhydride [34].High temperature is bene cial for the generation of esters while also leading to a decrease in -OH content and an increase in C = O bonds in the adhesive.These analysis results are consistent with infrared spectroscopy, indicating the composition of the cross-linking network and the successful preparation of CSC-G adhesive.
Thermal performance analysis

DSC
The curing behavior of CSC-G adhesive was studied through DSC analysis.Compared to CS and CSC, CSC-G adhesive has signi cant endothermic peaks.The curing temperatures of CSC-G, CSC-2G, and 2CSC-G are 135.9℃, 151.5 ℃, and 155.6 ℃, respectively (Fig. 5).The addition of too much CSC or glucose is not conducive to solidi cation.In addition, no other exothermic peaks were found in the 170-230 ℃ range, indicating that the reaction between CSC and glucose is complete at 5 hours.Among them, the CSC-G adhesive has the lowest curing temperature, which indicates that its curing degree is more complete and its performance is more excellent than other adhesives under the same curing conditions.According to the DSC curve, the optimal curing temperature for CSC-G adhesive is 160 ℃.However, according to previous reports, polyester adhesives require a higher curing temperature [33,34].Therefore, we rst set the hot-pressing temperature at 200 ℃ to select the optimal CSC-G adhesive ratio and optimize the hot-pressing parameters in subsequent experiments.

TG
To explain the cross-linking network of CSC-G and explore the thermal stability of CS derivatives, TG testing was conducted.For all test samples, the mass loss at 30-120 ℃ is attributed to the evaporation of polymer moisture (Fig. 6).CS (T max =300 ℃) due to the destruction or degradation of molecular chains [35].The decomposition temperature of CSC-G tends to increase, because the high density of covalent bonds in the CSC-G network structure is conducive to the thermal stability of CSC-G adhesive [36].The rapid decomposition of the CSC-G adhesive began at 345.98 ℃, indicating that the CSC-G adhesive has satisfactory thermal stability.At 200 ℃, the mass loss of CSC-G is only 7.77%, indicating that CSC-G is more stable due to the formation of cross-linking networks.It also indicates that short-term hot pressing at 200 ℃ will not cause signi cant decomposition of cured CSC-G.

XRD analysis
As shown in Fig. 7, CS has a strong diffraction peak at 20.3°, which is caused by the semi-crystalline nature of chitosan itself [37], resulting from the free amino groups on the chitosan molecule and internal hydrogen bonds.Compared with CS, the diffraction curve of CSC is relatively more tortuous, and a new diffraction peak appears near 12.3°, indicating that carboxylation of chitosan through maleic anhydride increases the crystallinity of chitosan [38].After the addition of glucose, the peak located at 12.3°d isappeared.The peak at 20.3° showed slight displacement and signi cant weakening, indicating that the multiple interactions of esteri cation reaction and spatial rearrangement formed more coordination bonds and intermolecular hydrogen bonds, resulting in a certain decrease in crystallinity.The emergence of new crystallization peaks at 28.72° may be the reason for the excellent water resistance of CSC-G adhesive [39].

Mechanical properties
CS and CSC have poor solubility in water, so they cannot be used as wood adhesives in this experiment.
But CSC-G exhibits good water solubility, which may be related to adding glucose structure, which brings more hydroxyl groups.The ratio of CSC to glucose has a signi cant impact on the water resistance of plywood.As shown in Fig. 8a, CSC-2G adhesive did not exhibit superior boiling water resistance under 200 ℃ hot-pressing conditions.The addition of excessive glucose results in insu cient CSC to react with it, leading to the presence of a large amount of glucose or byproducts free from the cross-linking network in the adhesive system, resulting in a decrease in the performance of the plywood.The high cross-linking degree helps improve the cured adhesive's shear strength (Table 2).CSC-G adhesive is prepared by reacting carboxyl groups on CSC and hydroxyl groups on glucose, a dynamic equilibrium reaction of ester product synthesis and hydrolysis.Therefore, a higher hot-pressing temperature is conducive to the esteri cation reaction and the curing of CSC-G adhesive and to increasing the adhesive's cohesion and adhesion to the wood surface.Overall, C = O, -OH, and hydrogen bonding endow the adhesive with excellent properties.Considering that reducing temperature and hot-pressing time can reduce energy consumption in the actual production process, and CSC-G adhesive with a CSC/G mass ratio of 1.0 has better water resistance, we took CSC-G adhesive as the experimental object and optimized the hotpressing conditions by changing the hot pressing temperature and time.As shown in Fig. 8b, CSC-G adhesive showed good heat resistance (0.88 MPa) even when the temperature was reduced to 160 ℃.
The effective way to reduce the pressing temperature and obtain satisfactory bond strength is to prolong the pressing time with a lower temperature [40].To our delight, after hot pressing at 180 ℃ for 3 min and soaking in hot and boiling water for 3 h, the wet shear strength was 0.83 MPa and 0.71 MPa, respectively, which was higher than the relevant standards (Fig. 8c).Considering that the DSC peak of CSC-G is 135.9 ℃ (Fig. 5), CSC-G adhesive was hot pressed at 140 ℃ for different times, and the results are shown in Fig. 8d.Under the hot pressed condition, CSC-G adhesive still has certain performance.Different types of wood fracture can be observed in Fig. 8e, indicating that with the increase of temperature, the wood loss rate increases, the permeability of the adhesive to the wood increases, and the mechanical interlocking behavior of the wood glue interface is enhanced.Compared with adhesives with similar structures or similar preparation mechanisms, it can be found that the CSC-G adhesive prepared in this study has better curing temperature and mechanical properties (Table 3).In addition, the raw materials of the prepared adhesive are all from biomass and have no formaldehyde release.As a green and environmentally friendly biomass polyester adhesive, it has a good application prospect in the wood industry.

Conclusions
In this work, a three-component all-biomass wood adhesive (CSC-G) with high water resistance was prepared based on chitosan.Maleic anhydride acts as a "bridge" to connect chitosan and glucose, and covalent and hydrogen bonds improve the inherent defects of biomass adhesives and increase the bonding strength.Carboxylated chitosan prolongs the chain of chitosan molecules, making it more fully cross-linked to glucose.Due to the increased molecular weight, the thermal stability of the CSC-G adhesive was also enhanced.CSC-G adhesives exhibited lower curing temperature than other polyester adhesives.CSC-G showed good water resistance even when hot pressed at 160 o C.This strategy expands the use of shery waste chitosan and provides a new way to modify chitosan and prepare chitosanbased adhesives with excellent performance.

Table 1
Compositions of the CSC-G adhesives.

Table 3
Comparison of wet shear strength between CSC-G adhesive and adhesive with similar synthesis mechanism or structure under hot-pressing temperature for three-plywood.