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# Tannin-based non-isocyanate polyurethane wood adhesive rich in complicated cross-linked networks with ultra-strong bonding property and excellent water resistance by introducing poly-urea structure

Haizhu Wu, Zhigang Duan, Heming Huang, Ruotong Yang, Mengchang Hong, Guanben Du, Bertrand Charrier, Hisham Essawy, Antonio Pizzi, Xiaojian Zhou, et al.

► To cite this version:

Haizhu Wu, Zhigang Duan, Heming Huang, Ruotong Yang, Mengchang Hong, et al. Tannin-based non-isocyanate polyurethane wood adhesive rich in complicated cross-linked networks with ultra-strong bonding property and excellent water resistance by introducing poly-urea structure. Industrial Crops and Products, 2025, 223, pp.120177. 10.1016/j.indcrop.2024.120177. hal-05365175

HAL Id: hal-05365175

https://hal.science/hal-05365175v1

Submitted on 18 Nov 2025

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# Tannin-based non-isocyanate polyurethane wood adhesive rich in complicated cross-linked networks with ultra-strong bonding property and excellent water resistance by introducing poly-urea structure

Haizhu Wu<sup>a,b</sup>, Zhigang Duan<sup>b</sup>, Heming Huang<sup>c</sup>, Ruotong Yang<sup>a,b</sup>, Mengchang Hong<sup>b</sup>,
Guanben Du<sup>a,b</sup>, Bertrand Charrier<sup>d</sup>, Hisham Essawy<sup>e</sup>, Antonio Pizzi<sup>f</sup>, Xiaojian Zhou<sup>a,b,*</sup>,
Xinyi Chen<sup>a,b,*</sup>

## ARTICLE INFO

**Keywords:**

Tannin-based non-isocyanate polyurethane
(TNIPU)

Deamination reaction

Polyurea

Bonding performance

Water resistance

## ABSTRACT

Developing of bio-based wood adhesives is an inevitable common topic in the development at top speed of wood industry. Herein, a bio-based tannin wood adhesive, named as tannin-based non-isocyanate polyurethane-urea (TNIPU-U), was designed which showing a ultra-strong adhesion and water resistance performance by two-step approach. A plausible mechanism was proposed, which implied that urea acted as the crosslinker to connect with derived intermediates containing free amino groups rooted from the reaction of tannin, dimethyl carbonate (DMC), and HMDA, to introduce urea linkages in traditional TNIPU. Furthermore, addition of urea to act as a trapping agent to capture excessive free HMDA in TNIPU adhesive, promoting additional polyurea formation, leading to avoidance of its emission during the hot-pressing process. This will convert the imperfection of TNIPU to a superiority by a eco-friendly strategy. The complicated cross-linked networks will be obtained that are rich in physical entanglement, covalent bonds, and hydrogen bonds. Compared with the TNIPU, the shear strength of TNIPU-U adhesive, after soaked in hot water and boiling water for 3 h, exhibited increase by 17.88 % and 34 %, respectively, indicating the potential for use under harsher conditions. Moreover, the TNIPU-U adhesive exhibits an excellent bonding performance (1.93 MPa) that greatly exceeds the China National Standard (GB/T 9846-2015, ≥ 0.7 MPa) requirement for exterior-grade plywood type I. It is comparable to commercial phenol-formaldehyde resin on the aspect of water tolerance. It can be applied to particle-board preparation and bamboo bonding as well, showing a well-adopted feature. This study provides an efficient, low-cost, and sustainable strategy for high performance bio-based wood adhesives.

## 1. Introduction

Polyurethanes (PUs) has been extensively utilized in several applications, such as foams, elastomers, composite materials, coating, adhesives, and other fields due to their excellent properties(Beran et al., 2021; Guo et al., 2023; Han et al., 2023; Shi et al., 2023; Wang et al.,

2021). However, most of PU products are synthesized using commercial non-renewable petroleum-derived products. Typically, costly materials such as polyols and isocyanates, are impacted by international oil price seriously, which results in an unstable price of commercial PUs. Therefore, in recent years, a renewable bio-based strategy for production of PUs at a low price, was proposed and attracted wide attention of

<footnote><sup>a</sup> Key Laboratory of Forest Resources Conservation and Utilization in the Southwest Mountains of China Ministry of Education, Southwest Forestry University, Kunming 650224, China</footnote>

<footnote><sup>b</sup> Yunnan Provincial Key Laboratory of Wood Adhesives and Glued Products, Southwest Forestry University, Kunming 650224, China</footnote>

<footnote><sup>c</sup> Kunming Feilin wood-based panel Group Co., LTD, Kunming 650033, China</footnote>

<footnote><sup>d</sup> CNRS/Univ Pau & Pays Adour, Institut des Sciences Analytiques et de Physico-Chimie pour l'Environnement et les Matériaux-Xylomat, UMR5254, Mont-de-Marsan 40004, France</footnote>

<footnote><sup>e</sup> National Research Centre, Department of Polymers and Pigments, Cairo 12622, Egypt</footnote>

<footnote><sup>f</sup> LERMAB, University of Lorraine, 27 rue Philippe Seguin, Epinal 88000, France</footnote>

<footnote>* Corresponding authors at: Key Laboratory of Forest Resources Conservation and Utilization in the Southwest Mountains of China Ministry of Education, Southwest Forestry University, Kunming 650224, China.</footnote>

<footnote>E-mail addresses: xiaojianzhou@swfu.edu.cn (X. Zhou), chen_xinyi_csuft@126.com (X. Chen).</footnote>
numerous researchers (Furtwengler and Avérous, 2018; Tran and Lee, 2023).

Some classical studies followed by the concept of sustainable
development, turned their focus on the preparation of bio-based PUs
from some renewable resources, including plant oil, cellulose, lignin,
starch, etc., to replace the petroleum-based polyols (Wang et al., 2022;
Kerche et al., 2021; Mahmood et al., 2016; Ashjari et al., 2018).
Although this could solve the resources sustainability issue during the
preparation of PUs, the use of expensive isocyanates, which are another
raw materials employed for synthesizing polyurethanes, are well-known
to be a toxic volatile compounds that are not conducive to the physical
health of operators, a problem that remains unresolved (Miao et al.,
2023).

Consequently, a totally novel approach, i.e., non-isocyanate poly-
urethanes (NIPUs), to ensure eco-friendly PUs with low/non-toxicity,
has been suggested, and became another research hotspot in the field
of PUs in the past decade (Błažek and Datta, 2019; Quinsaat et al., 2022;
Sessini et al., 2021; Zhang et al., 2023). However, its complicated
preparation and strict process conditions, normally high temperature
and pressure, is still the focal point of consideration for facilitation of
commercial PUs fabrication.

In a previous work, we reported a novel one-pot, two-steps strategy for the synthesis of NIPUs under mild conditions using bio-based ingredients, usually glucose, sucrose, tannin, and dimethyl carbonate (Chen et al., 2022a, 2024; Gholami et al., 2021; Chen et al., 2020; Xi et al., 2018). Dimethyl carbonate is frequently applied with hydroxy-containing bio-based materials to form an intermediate under a low temperature (50–70°C). After that, the amino compounds are introduced to react with the obtained intermediate in the first step under 90–95°C, promoting to compose carbamate structure (Chen et al., 2024, 2022a). This pathway was successfully acted to solve some existing issues either for typical PUs or classical NIPUs preparation. It is generally necessary to add excessive amino compounds to promote the formation of carbamate bonds. These kinds of NIPUs have been utilized as adhesives for bonding plywood or particleboard, allowing an excellent adhesion strength, especially under wet conditions (Chen et al., 2024, 2022a).

Whereas the main shortage refers to the excessive liberation of free HMDA as a result of volatilization that happens when the TNIPU adhesive serves as adhesive medium under such higher hot-pressing temperature. This is absolutely affecting the environment as well as the health of operators. Therefore, glycerol diglycidyl ether (GDE) and branched polyethyleneimine (PEI) were suggested as candidates to solve these problems (Chen et al., 2022a, 2024). Some studies verified that it is an efficient strategy to introduce additional reaction that can capture free hexamethylenediamine (HMDA), promoting construction of the network structure of NIPU via participation of the derived product in this step. Among the suggested materials for this purpose, the commercial GDE and PEI are relatively expensive. Therefore, the best approach to solve this typical problem of TNIPU adhesive is to find out cheap, eco-friendly materials.

As we all know, polyurea (PUr) is a polymer containing urea bonds,
normally obtained by the reaction between diisocyanates and diamines
(Leventis, 2022; Ma et al., 2022; Zhang et al., 2022). It exhibits typically
high molecular polarity, melting point, crystallinity, and hardness so
that it was applied to multiple fields, such as vehicle, aviation, and
building materials (Ma et al., 2023). Normally, PUr displays a fast,
catalyst-free, solvent-free curing, owing to the high reactivity between
its raw materials (W. Liu et al., 2020). It has been extensively utilized for
application in the field of adhesives. However, a typical shortcoming in
case of PUr was similar to that of PUs, namely, the isocyanate as a
starting material, brought the issue of non-economically and
non-eco-friendly. Therefore, many formulations have been proposed for
preparing non-isocyanate polyurea-based adhesives, via deamination
condensation (Jiang et al., 2022; Li et al., 2020; Yang et al., 2021). Urea,
as a classical low-cost and easily available commercial raw material, was

applied to the synthesis of wood adhesive in context its low-cost, which
caused to bring excellent properties to the adhesive. It was used as a
starting material to react with amino-containing compounds for pre-
paring a branched structure and isocyanate-free polyurea (NIPUr) for
further application in wood bonding, leading to an outstanding adhesion
performance (Jiang et al., 2022; Li et al., 2020; Yang et al., 2021).
Inspired by the low-cost of urea and the excellent bonding performance
of NIPUr, an economical and effective strategy, to circumvent the
traditional drawback of TNIPU wood adhesive, was proposed.

In this study, the urea served as an economic, eco-friendly chemical,
which was applied as an additional crosslinker for preparation of tannin-
based non-isocyanate polyurethane-urea (TNIPU-U) adhesive, expecting
to mitigate the typical drawbacks of TNIPU adhesive. The deamination
reaction was easily triggered in presence of urea to capture free-amino
groups, either emerging from HMDA or its derived oligomers. The as-
resultant products with urea bonds were introduced to the adhesive
network so that the bonding performance could be enhanced, which was
verified by the results of the shear strength. Additionally, a possible
mechanism for the strengthening effect was proposed and affirmed by a
series of investigations, including ¹³C NMR, FTIR, XPS, and GPC.
Furthermore, the application of TNIPU-U on particleboard and bamboo
bonding were carried out as well. The current study realizes to provide a
potential strategy for preparing economic, eco-friendly approach to
overcome the problematic issues of typical TNIPU adhesive.

## 2. Materials and methods

2.1. Materials

Urea (U, AR) was provided from the Chengdu Chron Chemical Reagent Co., LTD. Hexamethylenediamine (HMDA, AR) and dimethyl carbonate (DMC, AR) were provided from the Shanghai Macklin Biochemical Co., LTD. Mimosa tannin (MT) was obtained from Changrong Chemical Co. LTD., Haining, Zhejiang Province, China. All other ingredients utilized in this study were applied directly without further purification. The utilized poplar veneers in this study to fabricate plywood panels were obtained from Zhiwei veneer factory, Langfang, Hebei Province, China.

## 2.2. A series of TNIPU-U adhesives prepared via two-step approach

The TNIPU resin was prepared according to our previous report, with slight modification (Chen et al., 2024, 2022a). In brief, 70 % aqueous solution of HMDA was prepared in advance. MT (20 g) and distilled water (16.67 g) were weighed into a three-necked flask, stirred thoroughly, and the temperature was increased gradually. DMC (13.5 g) was added into the former mixture when the temperature reached at 60°C, and the reaction was kept under this temperature for 1 h. Thereafter, the aqueous solution of HMDA (38.8 g) was added while the temperature achieved 90°C, and the reaction was kept for another 2 h. The as-resulted TNPU adhesive was dropped to environment temperature and stored directly in the flask for standby.

The series of TNIPU-U wood adhesives was designed according to the molar ratio of HMDA and Urea. Therefore, the as-obtained flask with TNIPU adhesive was transferred to the equipment with oil bath, heated to the target reaction temperature (140°C). Various amounts of urea ($n_{(HMDA)}$: $n_{(U)}$ = 0.4, 0.6, 0.8) were added into TNIPU adhesive at this moment. The reaction was kept for 12 h to ensure it reached completeness. The obtained TNIPU-U adhesives were labeled TNIPU-U0.4, TNIPU-U0.6, and TNIPU-U0.8 based on the different urea additions. For instance, the TNIPU-U0.4 adhesive expresses a molar ratio 0.4 of HMDA to urea. The produced TNIPU-U adhesive samples were kept properly for subsequent application.
## 2.3. Characterizations

### 2.3.1. Fourier transform infrared (FT-IR) spectroscopy

The obtained adhesive samples in liquid form were kept in an oven at a temperature of 220°C for 2 h to ensure that it is entirely cured. Then, they were further ground into powder and mixed well with KBr before pressed into discs. FT-IR (Nicolet iS50 spectrometer, USA) was utilized for recording the absorbance from 500 cm⁻¹ to 4000 cm⁻¹ at a resolution of 4 cm⁻¹.

### 2.3.2. ¹³C Nuclear magnetic resonance (NMR)

The adhesive samples were prepared for ¹³C NMR measurements using Bruker AVANCE III 600, Germany. Simply, 50 mg of freeze-dried samples were dissolved in 500 µL DMSO-d6 for testing to obtain the ¹³C NMR spectra of HMDA, U, and HU (the product obtained by direct reaction between HMDA and U according to the ratio in the TNIPU-U0.6 formula at 140°C for 12 h).

### 2.3.3. X-ray photoelectron spectroscopy (XPS)

XPS measurements were accomplished on the adhesive samples using X-ray photoelectron spectrometer (XPS) (Thermo Scientific K-Alpha (hv = 1486.6 eV), USA) and the data was collected in full and narrow spectrum scans, with a pass energy of 100 eV and 50 eV, respectively, whereas the step length was 1.0 eV and 0.05 eV, respectively.

### 2.3.4. Gel permeation chromatography (GPC)

Waters 1515 gel permeation chromatography (GPC) (Waters, USA) was used to obtain the molecular weight distribution of the TNIPU-U adhesive samples. The experiment was conducted on a PLgel MINED-BLS300 × 7.5 mm gel column, while deionized water was used as a fluent at a 1 mL/min flow rate.

### 2.3.5. Differential scanning calorimeter (DSC)

NETZSCH DSC 3500 (Germany) was used to investigate the curing behavior of TNIPU and TNIPU-U0.6 adhesives. Before testing, the adhesive samples were freeze-dried for 72 h in advance. The experiment was conducted within the temperature range of 30°C to 250°C at a heating of 10 K/min.

### 2.3.6. Thermogravimetric analysis (TGA)

Thermogravimetric analyzer (Netzsch TG 209 F1, Germany) was employed to check the thermal stability of the TNIPU-U adhesive samples. The run was conducted under nitrogen atmosphere at a heating rate of 10 °C/min within a temperature range of 30°C to 800°C. All adhesive samples were cured at a curing temperature for 2 h before testing.

### 2.3.7. Water resistance test

To investigate the water resistance of TNIPU-U adhesive, the TNIPU-U0.6 adhesive was selected as a delegate to conduct the water resistance. The specimen was first placed in a 50°C oven for overnight to removed moisture from the adhesive and prevented excessive foaming during the subsequent curing process. Then, the temperature was raised to 220°C and remained for 2 h. Eventually, the specimen was dropped to the environment temperature, the weight was recorded, then soaked in boiling water for 3 h before dried and the weight was re-recorded. The weight loss was calculated based on the percentage of the remaining weight after drying compared to the original weight.

### 2.3.8. Preparation of plywood

Three-layer plywood was prepared to investigate the bonding performance of as-obtained TNIPU-U wood adhesives. Poplar veneers with a size of 190 × 120 × 2 mm³ were employed for the plywood preparation. The veneers were stacked with the direction of middle layer veneer perpendicular to the surface. The TNIPU-U wood adhesives were

spread out evenly on both sides of the middle layer using a load of 280–300 g/m². The assembled semi-finished three-layer plywood was stored under light load at the surrounding environment temperature for 15–20 min, then placed on the heating plate of a thermocompress or (BD-8820-C-32T) at 220°C for 5 min for hot-pressing using a pressure of 1.25 MPa. Plywood samples with a final size of 190 × 120 × 5 mm³ were obtained, and then stored at the environment temperature for at least 24 h before testing.

### 2.3.9. Evaluation of plywood

Plywood samples were cut to a size of 100 × 25 × 5 mm³ for evaluation of their bonding performance using universal testing machine according to China National Standard GB/T 17657–2022. For wet shear strength of plywood, the mechanical performance testing was conducted after the specimens were pre-soaked in hot water at 63 ± 2°C for 3 h, and before conducting testing, placed the samples for 10 min at ambient temperature.

In addition, a further investigation was conducted to obtain the bonding strength of the specimens after being pre-soaked in boiling water (around 95°C) for 3 h. For the boiling water cycle test, the plywood was first drowned in boiling water (around 95°C) for 4 h, and then kept in a 60°C oven for 20 h, followed by another soaking in boiling water for 4 h. Eventually, the plywood samples were soaked in water at 25°C for 1 h, taken out, and placed at ambient temperature for 10 min before being used for shear strength evaluation. The measurements were repeated six times for each specimen to obtain the average value.

### 2.3.10. Preparation of particleboard

Particleboard was prepared to investigate the applicability of TNIPU-U adhesive. The shavings were pre-dried to moisture of 10 %, and the mass amount of adhesive added was 10 % of the dry mass of the shavings. The shaving and adhesive were mixed evenly and placed in wooden mold. Then, pre-pressed to form a billet, and finally exposed to hot-pressing at 220°C for 10 min to produce 12 mm thick particleboard with an average density of 0.7 g/cm³.

### 2.3.11. Evaluation of particleboard

Internal bonding strength and 24-h water absorption thickness swelling rate tests of the particleboard were conducted on according to China National Standard GB/T 17657–2022. Each specimen was tested three times and averaged.

### 2.3.12. Preparation of bamboo overlapping specimens

Bamboo overlapping specimens were made by spreading adhesive on the surface of two bamboo pieces with the size of 80 × 20 × 3 mm³ (bonding area of 20 × 20 mm²), and then overlapping them. Afterwards, the assembled semi-finished overlapping samples were stored under weight at the environment temperature for 15–20 min. Subsequently, placed on the heating plate of a thermocompress or (BD-8820-C-32T) at 220°C for 5 min for hot-pressing using a pressure of 1.25 MPa.

### 2.3.13. Evaluation of bamboo overlapping specimens

The dry lap shear strength of the overlapping specimens was determined directly under the dry. The wet shear strength test was conducted according to the following steps: The specimens were pre-soaked in hot water at 95 ± 2°C for 3 h then put at environment temperature for 10 min before testing. The lap shear strength was obtained by taking the average of six repeated measurements for each specimen.

### 2.3.14. Optical microscope analysis

Before proceeding with optical microscope, the three-layer plywood was cut into specimens of 25 × 25 × 5 mm³. The samples were softened in boiling water, and the end surfaces were then sectioned to a thickness of 30 µm using a Leica slicer (LEICA SM 2000R). The glue lines were subsequently examined using an optical microscope (Nikon eclipse 80i DM 2000), with digital images captured for analysis.
# 3. Results and discussion

## 3.1. The design and fabrication of TNIPU-U adhesive

Tannin-based NIPU has broken through the bonding property (normally can endure water erosion, even under boiling water pretreatment) of tannin adhesives that were obtained by some traditional approaches, such as tannin modified phenol-formaldehyde resin (TPF), tannin-hexamine adhesive, and tannin formaldehyde adhesive, etc. (Akaranta and Wankasi, 1999; J. Liu et al., 2020; Sari et al., 2023). This kind of adhesive preparation pathway changed the classical way to acquire urethane bonds in polymers, especially, decreased the consumption of petroleum-based resources. Nevertheless, a typical shortcoming, i.e., excessive HMDA utilization, has adverse effect on the environment and operators during the plywood or particleboard manufacture (Chen et al., 2024, 2022a). Converting its imperfection to a superiority with an eco-friendly strategy would be a highly recommended practice. To perform this target and enhance the performance of TNIPU adhesive, urea, which is economically beneficial, was introduced into the system.

Representatively, TNIPU-U adhesive was obtained by the synthetic route presented in Fig. 1(a). Some derived structures from reaction of tannin, DMC, and HMDA were formed under the employed conditions, leading to some free amino groups as HMDA was linked on the tannin molecule by DMC. A cross-linked adhesive network would be formed by undergoing hot-pressing in absence of an addition cross-linker (Fig. 1(b)) (Chen et al., 2024, 2022a). Following, TNIPU-U adhesives were fabricated by deamination condensation via reaction of free amino groups of TNIPU or HMDA with U (Fig. 1(a), polymer 1 and 2), which ensures decreasing the evaporation of HMDA during hot-pressing. Additionally, the physical entanglement that occur within the network structure of TNIPU-U adhesive, can enhance the bonding properties of the TNIPU-U adhesives (Fig. 1(b), because of the formed structure with long chains through the condensation between HMDA and urea. Moreover, some hydrogen bonds could be formed inevitably between the obtained polymers inevitable, promoting the construction of a complicated cross-linked networks.

## 3.2. Reaction between HMDA and urea

The formed adduct due to reaction between HMDA and urea was investigated using FT-IR and ¹³C NMR, to verify the efficiency of the approach to capture HMDA using urea. The corresponding NMR results of reactants (U and HMDA) and adduct (HU) are shown in the Fig. 2. A peak at 160–164 ppm is attributed to carbonyl region as clear in Fig. 2(a). The peak at 162 ppm in case of U was shifted to 161 ppm in case of HU after the deamination reaction occurred. Additionally, the FTIR spectra showed dual-peaks corresponding in case of U and HU as illustrated in Fig. 2(b), at around 3400–3500 cm⁻¹, which can be assigned to primary amino group (-NH₂) of different environments. This demonstrates that a portion of primary amino groups remained in the sample HU, suggesting adduct formation via urea linkage with HMDA. Therefore, the as-obtained HU provides the possibility for further reaction, either with itself or with TNIPU. A small side peak close to 161 ppm in the NMR spectrum of HU indicates that an allophanamide structure was possibly obtained in case of HU. Likewise, the spectrum of HU contains new peak that appeared at around 30.5 ppm, which can be attributed to methylene carbon (β'' carbon) in the skeleton of HU, revealing a deamination reaction proceeded between urea and HMDA. On the other hand, the peak at 1678 cm⁻¹ in the FTIR spectrum of U (Fig. 2(b)) can be allocated to C=O stretching of urea, which emerged at 1651 cm⁻¹ in case of HU. This can serve further as evidence of coupling between urea and HMDA.

## 3.3. Characterizations of the prepared adhesives

TNIPU, TNIPU-U0.4, TNIPU-U0.6, and TNIPU-U0.8 adhesives were

characterized using FT-IR and the relevant spectra are shown in Fig. 3. A peak at 3336 cm⁻¹ can be seen in all adhesive samples, which is attributed to overlapped peaks pertaining to -OH and -NH₂ groups in the structure (Chen et al., 2022b). The double peaks at 2864 cm⁻¹ and 2933 cm⁻¹ are attributed to the anti-symmetric and symmetric stretching vibrations of the -CH₂- group in HMDA or its derivative products (Li et al., 2014). Some characteristic peaks are also present in the FT-IR spectra. For instance, a peak at 1684 cm⁻¹ corresponds to the stretching vibration of the C=O bond of the amide bond while the peak at 1644 cm⁻¹ is related to the bending vibration of the -NH₂ of the amide group. Moreover, the peak at 1530 cm⁻¹ can be assigned to the bending vibration of the C-N bond in the amide band. The characteristic peaks indicate that the main polyurethane structure of the TNIPU-U adhesives has been maintained even after coupling of (Chen et al., 2022a). Furthermore, the supposed polyurea formation in this system confirm that urea can serve to capture free HMDA in the adhesive system, meanwhile, it can act to connect some products, obtained during the reaction between tannin, DMC, and HMDA. Thus, a dual integrative cooperative effect is produced by the polyurea derivative products resulting in denser network structure of the adhesive with possibility to enhance the bonding performance of adhesive. It can be predicted that some free HMDA could be fixed in the adhesive network leading to avoidance of its emission under pressing at high temperature. Additionally, a peak at 1141 cm⁻¹ is attributed to the stretching vibration of the C-N bond that comes from derivative products of HMDA, or even HMDA. This presumably indicates some unreacted NH₂ groups are still present in the TNIPU-U, which can provide prospect for further cross-linking in the subsequent hot-pressing step.

XPS is an effective technique to examine the elemental composition and reflect the chemical bond transformation on the surface of materials, in spite of it could not accurately quantify the elemental composition. Based on that, some ongoing reactions between reactants can be speculated following XPS investigation. Here, the control group TNIPU and urea modified adhesive, TNIPU-U0.6, were selected for this analysis. The specimens were obtained via air-drying in an oven at the same temperature (220°C) and utilized for the preparation of plywood samples. The obtained XPS spectra of TNIPU and TNIPU-U adhesives are shown in the Fig. 4. Some unreacted species such as unreacted HMDA or U remained free in the TNIPU adhesive system. However, HMDA can do evaporation at high temperature conditions. Obviously, it is clear from the XPS survey (Fig. 4(a)) that there is an increase in the N element content in the cured form of TNIPU-U0.6 adhesive. This can be explained by the reaction that proceeded between U and HMDA, leading to formation of a relatively thermally stable structure, causing reduction in the volatilization of free HMDA as a result of the introduction of U into the TNIPU adhesive system. This occurs because of employing a temperature of 220°C during plywood preparation, which is above the volatilization temperature of HMDA and the thermal degradation temperature of urea. Three peaks were observed in the high-resolution C 1 s spectra of TNIPU and TNIPU-U0.6 adhesives (Fig. 4(b)). Specifically, at 284.6, 285.5, and 287.8 eV, respectively, in the TNIPU adhesive, which can be assigned to C1 (C-C/C-H), C2 (C-O/C-N) and C3 (C=O). In case of TNIPU-U0.6 adhesive, these peaks appeared at positions of 284.6 eV, 285.8 eV, and 288.4 eV. This may be due to the formation of urea bonds in the molecular structure of TNIPU-U adhesive, resulting in some slight shifts in the binding energy. The content of C-O/C-N increased in the adhesive increased from 33.26 % to 60.15 % while U was introduced into TNIPU adhesive for modification. This can further prove that the U became connected to the free amino groups of the tannin-based polyurethane products, as well as the free HMDA in the adhesive system. Moreover, the content of N element of the TNIPU-U0.6 was increased to 15.75 % from 12.38 %, which also indicates that urea can indeed react with free amino groups in the system to reduce the volatilization of HMDA during hot pressing at high-temperature, thereby reducing the impact on the environment and operator health.

The molecular weights of three TNIPU-U adhesives prepared with
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

Some foreseeable main reaction products

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /></figure>

Lay-up

<figure><img src="image_9.png" /><figcaption>**Fig. 1.** The synthetic route of TNIPU-U adhesive (a), the proposed cross-linked network after curing of TNIPU-U adhesives (b).</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 2. ¹³C NMR spectra of HMDA, U, and HU (a), FT-IR spectra of HMDA, U, and HU (b).</figcaption></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 3. FT-IR spectra of TNIPU-U0.4, TNIPU-U0.6, and TNIPU-U0.8 (a), and partial enlarged images (b).</figcaption></figure>

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 4. XPS spectra (a) and high-resolution C1s spectra (b) of TNIPU and TNIPU-U0.6 adhesives.</figcaption></figure>
different molar ratios between HMDA and urea were determined through GPC measurements. The molecular weight of the pre-polymer can reflect the extent of pre-reaction during the preparation, guiding to suggest a proper curing time of the adhesive in the subsequent plywood manufacturing process. The corresponding molecular weight and polydispersity index (PDI) values for the prepared adhesives are displayed in Fig. 5 and Table 1. The molecular weight data (Mn and Mw) of the as-prepared adhesive show an increase associated with the urea addition. Nevertheless, it declined with further increase in the molar ratio of HMDA to urea. This indicates that coupling reaction occurred with urea addition, resulting in initial increase in molecular weight, while the subsequent drop in molecular weight reveals reaching a high branching state. The obtained branched constituents can still maintain reactivity to each other, then further forming a dense adhesive network under high temperature conditions. However, the molecular weight begins to decrease as the coupling between HMDA and urea develops. Urea works on large amount of amino groups, including those from HMDA or those derived from tannin-based NIPU structures, causing increased amount of urea-terminated products. The condensation between urea-terminated structures to form biuret requires a higher reaction temperature, therefore at a given pre-reaction temperature of 140°C the condensation could not progress to yield a higher molecular weight polymer. This result will lead to: (i) The Mn and Mw of TNIPU-U0.8 adhesive were decreased while the pre-polymerization process; (ii) The bonding performance of TNIPU-U0.8 adhesive could go down, which was verified by the shear strength measurements. This is due to the further cross-linking of the urea-terminated products in TNIPU-U0.8 adhesive becomes relatively difficult. As obvious from Table 1, polymers obtained with Mn greater than 13000 and Mw greater than 35000 reveal that polymerization between various raw materials could happen effectively. In addition, the PDI of the three adhesives were 2.8, 2.3, and 2.7, respectively, demonstrating that the molecular weight distribution of the prepared TNIPU-U adhesive was relatively sustaining after preparation.

### 3.4. Curing behavior, thermal stability and water resistance of TNIPU-based adhesive

The thermal curing characteristics of TNIPU and TNIPU-U0.6 adhesives were investigated via DSC. As shown in Fig. 6a, TNIPU and TNIPU-U0.6 adhesives indicated substantial endothermic and exothermic peaks that are similar to most thermosetting resins. The thermal curing behavior of TNIPU-U0.6 was significantly influenced by the urea introduction. From Fig. 6a, the TNIPU-U0.6 adhesive displayed a sustained exothermic peak between 130 and 150°C, which is attributed to the deamination condensation of the unreact urea and free HMDA

<table><caption>**Table 1**
The Mn, Mw, and PDI values of TNIPU-U0.4, TNIPU-U0.6, and TNIPU-U0.8.</caption><thead><tr><td>Adhesives</td><td>Mn</td><td>Mw</td><td>PDI</td></tr></thead><tbody><tr><td>TNIPU-U0.4</td><td>13960</td><td>39184</td><td>2.8</td></tr><tr><td>TNIPU-U0.6</td><td>17898</td><td>41484</td><td>2.3</td></tr><tr><td>TNIPU-U0.8</td><td>13474</td><td>35290</td><td>2.7</td></tr></tbody></table>

within the TNIPU-U0.6 adhesive. Moreover, a pronounced exotherm emerged between 180 and 210°C, suggesting that the primary curing behavior of TNIPU-U0.6 adhesive occurred within this temperature range. Additionally, a remarkable endothermic peak was observed between 189 and 204°C in the curve of the TNIPU adhesive, which is attributed to the endothermic volatilization of free HMDA within the TNIPU system. In contrast, this peak is absent in TNIPU-U0.6 adhesive, indicating urea introduction can efficient mitigates the volatilization of free HMDA during the hot-pressing process.

The thermal decomposition behavior of TNIPU, TNIPU-U0.4, TNIPU-U0.6, TNIPU-U0.8 adhesives was studied by thermogravimetric analysis, and the corresponding TG and DTG traces are exhibited in Fig. 6b and c. The thermal decomposition behavior of TNIPU adhesives is showing a multiple respond steps. The first stage shows a small amount of mass loss that occurred between 50 and 110°C, which is attributed to the evaporation of adsorbed water in the adhesive samples. The losses generated within the range of 110–280°C are allocated to the initial degradation behavior of adhesive samples. Typically, some small molecules (sugars in commercial tannins) and unstable chemical bonds in adhesives, such as ether bonds, will undergo breakage within this temperature range. Additionally, as the boiling point of HMDA and U decomposition temperature of urea proceed within this temperature range, the mass loss of this part may also comprise volatilization of HMDA and degradation of urea. The degradation occurred within the temperature range of 280–420°C is related mainly to the degradation of carbamate and urea bonds in TNIPU and TNIPU-U adhesives. It was found that with the addition of urea, the degradation peak of TNIPU-U adhesive shifted towards higher temperature range during this stage. This can be illustrated by formation of some thermally stable products during the adhesive preparation. As an example, the polymer obtained from the deamination reaction between urea and the amino-containing ingredients. Therefore, it can be speculated that urea act as a crosslinker and could contribute to reducing free HMDA during the plywood preparation process. The final weight loss stage occurred in the temperature range of 420–560°C, signifying the breakage of the covalent C–C bonds, whereas some pyrolysis species produced in the previous stages could also undergo further decomposition within this temperature range.

<figure><img src="image_16.png" /><figcaption>**Fig. 5.** GPC profiles of TNIPU-U0.4, TNIPU-U0.6, and TNIPU-U0.8.</figcaption></figure>
<figure><img src="image_17.png" /><figcaption>**Fig. 6.** DSC profiles of TNIPU, TNIPU-U0.6 wood adhesives (a), TG profiles of TNIPU, TNIPU-U0.4, TNIPU-U0.6, and TNIPU-U0.8 wood adhesives (b), DTG profiles of TNIPU, TNIPU-U0.4, TNIPU-U0.6, and TNIPU-U0.8 wood adhesives (c), test results of boiling water resistance for the cured TNIPU-U0.6 adhesive (d).</figcaption></figure>

To further investigate the hydrolytic stability of the TNIPU-U adhesive, the adhesive was exposed to boiling water treatment as shown in Fig. 6(d). After being treated in boiling water for 3 h, fortunately, compared with the cured adhesive without boiling water treatment, the weight loss caused by adhesive hydrolysis was less than 2 %. Approximately 13.28 % weight loss after water extraction for TNIPU without urea addition has been reported in our previous work (Chen et al., 2022a). The adhesive exhibited excellent water resistance, which is related to the highly stable chemical structure of the adhesive, especially the formation of urea bonds confirmed by structural characterizations.

### 3.5. Bonding performance of TNIPU-based adhesives

The standard three-layer plywood specimens were fabricated according to the China National Standard GB/T 17657–2022 for dry and wet shear strength testing to evaluate the effect of introducing urea on the bonding performance of TNIPU adhesive. Compared with the initial TNIPU adhesive, the dry shear strength of TNIPU-U0.4 increased from 1.70 MPa to 1.84 MPa (Fig. 7a). Although the results only improved the bonding strength by around 10%, it can be demonstrated that the strategy of increasing the bonding strength of TNIPU adhesive by adding urea is effective. Therefore, the bonding strength was further evaluated by warm water and boiling water treatments (soaked in warm or boiling water for 3 h, respectively), encouragingly, the wet strength of TNIPU-U0.4 reached 1.74 MPa and 1.80 MPa after the warm-water and boiling-water treatment, respectively, demonstrating 15% and 25%

increase. The improved bonding performance by urea addition was attributed to the role of urea that took part in the TNIPU adhesive network construction, causing densification of it and providing an enhanced bonding performance provoked by the amino groups of HMDA that could react easily with urea. In addition, the physical entanglement associated with TNIPU-U adhesive network was supplied by the derived products, which rooted from the interaction between excessive HMDA and urea. Furthermore, due to the inherent steric hindrance, MT molecules were still containing a significant number of phenolic hydroxyl groups in their structure after reacting with DMC, which due to their hydrophilic nature, are not conducive to support water resistance of the adhesive. In contrast, the deamination condensation of HMDA and U yields a considerable amount of urea-bonds. It is well known that urea-bonds possess strong polarity, enabling them to readily form inter or intra molecular hydrogen bonds with the phenolic hydroxyl groups, which could enhance the cohesive forces of TNIPU-U adhesive network. This can be clearly illustrated by the proposed glue mechanisms shown in Fig. 7f and a. The synergism contributed by them motivated a well bonding performance for TNIPU-U adhesive, even excellent water repellency.

Furthermore, the addition amount of urea was investigated to determine its effect on the bonding performance of the final obtained TNIPU-U adhesive. The as-obtained adhesive achieved optimal performance when the molar ratio between urea and HMDA was further increased to 0.6 (i.e. TNIPU-U0.6). The corresponding results showed 1.88 MPa for dry shear strength, 1.78 MPa for wet shear strength after
<figure><img src="image_18.png" /><figcaption>**Fig. 7.** Bonding performance of TNIPU, TNIPU-U0.4, TNIPU-U0.6, and TNIPU-U0.8 wood adhesives for three-ply plywood production (a), bonding performance of three-ply plywood prepared by TNIPU-U0.6 adhesive in boiling water cycle test (b), wood failures model of plywood test samples prepared by TNIPU-U0.6 adhesive without water treatment and with different temperature conditional water treatment (c), optical microscope digital image of the glue lines of TNIPU (e1), TNIPU-U0.4 (e2), TNIPU-U0.6 (e3), and TNIPU-U0.8 (e4) after boiling water treatment, proposed enhancement mechanism of TNIPU-U0.6 adhesive (f), rough comparison of bonding performance between this work and common formaldehyde-based adhesive, previous reported polyurea adhesive and previous reported bio-based adhesives (the relevant literature sources was provided in Table S1) (g).</figcaption></figure>

warm water treatment, and 1.96 MPa for wet shear strength after boiling water treatment for 3 h, respectively. Meanwhile, all samples with dry and wet strength were fractured from the wood matrix (close to 100% wood failure) (Fig. 7c). This indicates that the bonding strength of TNIPU-U0.6 adhesive was higher than the tensile strength of wood substrate. Nevertheless, the shear strength data in Fig. 7a indicated that the adhesion property decreased when the molar ratio of HMDA to urea reached 0.8. However, they still located in a higher value range, over 1.60 MPa, which is far beyond the requirements of China National Standard GB/T 9846-2015 (over 0.7 MPa). A conceivable reason for this trend is presumably when the condensation products in the polymer system became terminated by urea as a result of the increase in urea proportion, the condensation between urea-containing products required a higher reaction temperature or longer time. It seems that an evidence for this result would be provided by the shear strength data. An interesting point is that different from some classic bio-based adhesive that showed a decrease in bonding performance after water soaking treatment, all TNIPU-U adhesives showed varying degrees of

improvement in bonding performance after boiling water treatment for 3 h. This was probably related to the post-cure reactions that are invoked by the high temperature (close to 100°C) and long time (3 h). The reaction can further promote while the specimens were exposed to boiling water, thereby the adhesive solidified to form a denser cross-linked network (Li et al., 2020). The plywood hot-pressing conditions given herein cannot support those urea-containing products for further polymerization, most likely due to absence of water which pushes the reaction. Considering the superior bonding strength of adhesives and high wood failure ratio after 3 h boiling water treatment and acknowledging the significant role of mechanical interlocking in bond strength, we examined the mechanical interlocking effects of the cured adhesive in three-layer plywood using an optical microscope (Fig. 7e). All TNIPU-U adhesives exhibited deeper wood penetration and formed more stable physical cross-linking (mechanical interlocking) after boiling water treatment and curing compared to the TNIPU adhesive, resulting in enhanced bonding strength.

Moreover, further research was conducted on the water resistance of
plywood fabricated by TNIPU-U0.6 adhesive. A more rigorous boiling water cycle experiment was conducted in the light of China National Standard GB/T 17657-2022 (for exterior-grade plywood type I). After boiling water treatment, the wet strength of plywood specimens reached 1.93 MPa (normally 1.7-1.9 MPa for PF resin) and still exhibited exceptional water resistance. Meanwhile, consistent with the treatment with warm water and boiling water for 3 h, the samples were mainly fractured from the wood matrix, indicating that the bonding strength of TNIPU-U0.6 adhesive was still higher than that of wood matrix after boiling water cycle treatment.

The discussion above proved that TNIPU-U prepared by introducing urea had great potential in the field of plywood manufacturing. Thus, to further highlight the excellent bonding performance of TNIPU-U adhesives, we selected some common petroleum-based and bio-based adhesives for bonding performance comparison (Fig. 7g). Furthermore, TNIPU-U adhesive still had other advantages that it did not have in other aspects, such as sustainability and no formaldehyde emission. Meanwhile, its bonding performance was much higher than that of the currently synthesized biomass adhesives.

### 3.6. The applicability of TNIPU-U adhesive

With the rapid development of the custom-home industry, particleboard products had been widely used in the fields of furniture and interior decoration. Generally, because of the preparation of particleboard using branch materials and fine wood particles, the performance

requirements for adhesive are higher. Therefore, the TNIPU-U0.6 adhesive was selected for the preparation of particleboard to evaluate its potential for wider application in board preparation.

The standard experimental sample for testing the 24-h water absorption thickness swelling rate and internal bonding strength of particleboard was fabricated according to China national standard GB/T 17657-2022. The internal bonding strength and 24-h water absorption thickness swelling rate of particleboard prepared by the TNIPU-U0.6 adhesive were around 0.6 MPa and 25%, respectively. This result met China national standards GB/T 4897-2015 for P1 type particleboard (Fig. 8a, b).

Additionally, because of the shortage of wood resources and their long renewal cycle, bamboo, which has a short growth cycle and exceptional performance of obtained products, has received widespread attention in the construction and furniture industries. However, its multi-layered cell and low porosity make it difficult to adhere. Therefore, to verify the applicability of TNIPU-U adhesives, the TNIPU-U0.6 adhesive was chosen to bond bamboo and the dry and wet strength (pre-soaked in boiling water for 3 h) of the specimens were tested to evaluate its bonding performance.

The test results of bamboo overlap samples are shown in Figs. 8c and d. The dry and wet strength reached 4.55 MPa and 3.13 MPa, respectively. Meanwhile, all samples fractured from the bamboo matrix, indicating that the bonding performance of the adhesive was higher than that of bamboo itself (Fig. 8d). Further, the bonding strength of TNIPU-U0.6 adhesive, which reached 3.13 MPa after boiling water treatment

<figure><img src="image_19.png" /><figcaption>**Fig. 8.** Internal bonding strength of particleboard prepared by the TNIPU-U0.6 adhesive (a), digital photo and thickness swelling rate of particleboard fabricated by the TNIPU-U0.6 adhesive (b), dry (green) and wet (pre-soaked in boiling water for 3 h) (orange) lap shear strength of the bamboo overlapping test specimens (c), substrate failure model of the bamboo overlapping test specimens prepared by TNIPU-U0.6 adhesive (d).</figcaption></figure>
for 3 h, also indicates that TNIPU-U adhesive has great potential in the field of bamboo bonding.

## 4. Conclusions

Slightly modified tannin-based non-isocyanate polyurethane (TNIPU) adhesive can be prepared in an upgraded form to present an outstanding adhesion performance by using urea in accompany with HMDA. The distinct chemical structure of urea allows formation of a branched product *via* deamination reaction between urea and HMDA or its derived products to form additional urea bonds, especially when the molar ration between urea and HMDA is optimized. This can be accounted for by a linkage between urea and free amino groups of the macromolecular skeleton formed involving reaction between tannin, DMC, and HMDA. This modification is thought to offer a sustainable strategy to establish a complicated cross-linked networks with ultra-strong bonding property and excellent water resistance. This can also help to cope with the traditional shortcoming of this adhesive type, which is the excessive emission of hexamethylenediamine (HMDA) during hot-pressing step employed for particleboard/panel-board preparation. The improvement related to strength, as a result of the structure modification, could also be confirmed by evaluation of the shear strength, in particular, under wet conditions. This efficient strategy removes an annoying obstacle for expansion of this adhesive in industry. The high bonding performance in case of particleboard and bamboo bonding application was further verified which supports the effectiveness of this strategy.

## CRediT authorship contribution statement

**Mengchang Hong:** Validation, Investigation, Formal analysis. **Bertrand Charrier:** Writing – review & editing, Supervision, Methodology. **Guanben Du:** Writing – review & editing, Supervision. **Xinyi Chen:** Writing – review & editing, Supervision, Conceptualization. **Xiaojian Zhou:** Writing – review & editing, Supervision, Funding acquisition, Conceptualization. **Antonio Pizzi:** Validation, Methodology, Formal analysis. **Hisham Essawy:** Writing – review & editing, Validation, Formal analysis. **Ruotong Yang:** Validation, Investigation, Formal analysis. **Heming Huang:** Validation, Investigation, Formal analysis. **Zhigang Duan:** Software, Investigation, Formal analysis. **Haizhu Wu:** Writing – original draft, Methodology, Investigation, Formal analysis.

## Declaration of Competing Interest

The authors declare no conflict of interest.

## Acknowledgments

The study was supported by the Key Laboratory of Forest Resources Conservation and Utilization in the Southwest Mountains of China Ministry of Education, Southwest Forestry University (LXXK-2023M10), The work was supported by the National Natural Science Foundation of China (NSFC 32460364, 31971595), the Key Program of Applied and Basic Research in Yunnan Province (Grant No. 202101AS070008). This work is also supported by the Major Science and Technology Project of Yunnan Province (202402AE090027), the “Xingdian Talents Support Plan” Yunling Scholar and Youth talent, National High-end foreign expert program (Grant No. G2023039003L), the 111 project (D21027), the Foreign Expert Workstation (202305AF150006), and supported by the Scientific Research Foundation of Education Department of Yunnan Province (Grant No. 2024Y593).

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.indcrop.2024.120177.

## Data availability

Data will be made available on request.

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