Study on the Thermal Condensation Mechanism of Dehydrogenated Polymer (DHP) and Glucuronic Acid

Abstract

The preparation of traditional wood-based panels mostly uses adhesives such as urea-formaldehyde resin and phenolic resin, which not only consumes petrochemical resources but also releases formaldehyde, posing potential health risks to the human body. Lignin, a natural adhesive in plant cells, is characterized by high reactivity, and it is expected to aid in the development of a new generation of green formaldehyde-free adhesives. However, current studies of lignin adhesives have revealed that while strides have been made in reducing formaldehyde emissions, its residual presence remains a concern, an issue which is compounded by inadequate water resistance. Dehydrogenated Polymer (DHP) has a lignin-like structure and good water resistance, offering a new option for the development of formaldehyde-free adhesives. In this paper, DHP and glucuronic acid were reacted with each other in a simulated hot-pressing environment to obtain DHP-glucuronic acid complex, and then the structure of the complex was characterized by infrared nuclear magnetic resonance to verify whether DHP can be efficiently connected with hemicellulose components under hot-pressing conditions. The results showed that the thermal condensation reaction of DHP and glucuronic acid can generate ester bonds at the Cα position in a simulated hot-pressing environment. This paper explores the thermal condensation mechanism of DHP and glucuronic acid, which is helpful for understanding the bonding process between adhesives and components of wood-based panels in the hot-pressing process, and provides key theoretical support for the design of more sustainable lignin adhesives.

1. Introduction

Wood-based panels are widely used in furniture, decoration, packaging, and other industries due to their ease of processing, strong functionality, and high-cost performance [1]. Traditional wood-based panels comprise plates or molded products derived from wood or grass plants which are subjected to mechanical processing to yield various constituent materials, and subsequently bonded and hot-pressed using adhesives. At present, the global annual consumption of wood-based panel adhesives exceeds 30 million tons [2]. Among them, “trialdehyde” adhesives (phenolic resin, urea-formaldehyde resin, and melamine-formaldehyde resin), prepared with formaldehyde as a primary precursor, command prominence within this field. However, harmful substances such as formaldehyde and phenol are released during the production and use of “trialdehyde” adhesives, which cause harm to the environment and human health. The release of low concentrations of formaldehyde (0.6–1.9 ppm) can irritate the nasal cavity and eyes, which causes neurological effects and increases the risk of asthma and allergies. At high concentrations (1.9–10.9 ppm), changes in lung function occur and irritation occurs in the eyes, throat, and skin. Therefore, the development of formaldehyde-free adhesives has become an urgent problem to be solved.

In response to the above problems, researchers have proposed many methods for reducing formaldehyde emissions, e.g., the use of formaldehyde scavengers, modifying the adhesives, post-treatment of the finished panels, etc. [3,4]. However, one of the best reported methods is the utilization of biomass materials in adhesive formulations to circumvent the emission of free formaldehyde at the source. Among the many biomass resources, lignin is rich in reserves and contains functional groups such as aliphatic hydroxyl, phenolic hydroxyl, and methoxy groups. It can be modified to synthesize a variety of new polymers, which can effectively replace petroleum-based raw materials for the preparation of adhesives. Today, there are two primary ways of preparing lignin adhesives. One is the blending modification of lignin and trialdehyde resin as a wood-based panel adhesive [5]. This is because lignin has numerous phenolic hydroxyl groups, which leads to its high reactivity, and can be condensed with formaldehyde under certain conditions. Therefore, blending industrial lignin or modified industrial lignin with trialdehyde resin offers the dual benefit of mitigating formaldehyde and phenol content within the resin, thereby diminishing the release of hazardous compounds [6,7,8,9,10,11]. Additionally, this approach enhances the mechanical characteristics of wood-based panels [12,13]. However, while this method effectively reduces the emission of formaldehyde, it falls short of complete elimination of the compound. Another method is to directly use lignin as an adhesive after modification. Related studies have shown that industrial lignin can produce a large number of free radicals after being modified by peroxidase and laccase, and promote its own condensation through free radical coupling to improve its adhesive properties [14,15]. However, peroxidase and laccase enzymes are primarily effective against water-soluble industrial lignin. The wood-based panels prepared by this method exhibit increased susceptibility to water absorption and deformation, thereby compromising their practical utility in real-world applications. Based on this study, the researchers found that DHP with a lignin-like structure can be obtained by treating small-molecule phenols using peroxidase and laccase [16]. The phenoxy radicals on DHP can be coupled with the free phenolic hydroxyl groups on the surface of thermomechanical pulp (TMP) fibers under the catalysis of enzymes [17], thereby effectively improving the wet tensile strength of paper [18,19]. Inspired by these studies, we sought to investigate the potential utilization of lignin-like DHP as a substitute for conventional lignin in order to evaluate its efficacy as a binding agent in the manufacturing process.

In natural wood, lignin and hemicellulose fill in the gaps between cellulose fibrils and form a three-dimensional network structure with fibers to make the fibers intricately connected. In addition, lignin can also form lignin-carbohydrate complexes (LCC) with carbohydrates (mainly hemicellulose) through covalent bonds [20]. The above two traits together give natural wood good physical and mechanical properties [21]. To effectively glue wood-based panel fibers, DHP must satisfy two essential conditions inspired by the structural properties of natural wood. First, it must be able to self-condense to synthesize a polymer during the hot-pressing process, so as to soften at hot temperatures and fill the gaps between the fibers, thus resulting in the physical curing effect of the adhesive [22]. The second is that the DHP and hemicellulose must be able to form an LCC-like covalent bond connection, which connects the fibers more closely. At present, our research group has conducted a simulation experiment on whether DHP can self-condense to synthesize polymers in a hot-pressing environment. The experimental results show that DHP can self-condense into polymers in a simulated hot-pressing environment, indicating the feasibility of DHP glue for use in wood-based panels [23].

Furthermore, this paper will verify whether DHP can react with hemicellulose to form LCC-like polymer compounds under hot-pressing conditions and explore its reaction mechanism. The structure of the reaction product resulting from DHP’s interaction with the glucuronic acid of xylan under hot-pressing conditions was analyzed by infrared spectroscopy and nuclear magnetic resonance. In summary, this paper provides a theoretical basis for the development of green and safe adhesives with high bonding performance.

2. Results and Discussion

The chemical structure of DHP-glucuronic acid complex (DHP-GlcA), alkali-treated DHP-glucuronic acid complex (AT DHP-GlcA), and self-condensed DHP (DHP SC) was detected by various techniques, including FT-IR, CP/MAS ¹³C-NMR, ¹³C NMR, and 2D-HSQC NMR. The infrared spectra of DHP SC, DHP-GlcA, and AT DHP-GlcA are shown in Figure 1. All samples had characteristic absorption peaks of the benzene ring at 1510 cm⁻¹, 1455 cm⁻¹, and 1424 cm⁻¹, which indicated the preservation of the benzene ring scaffold across all three samples [24]. Notably, in comparison to DHP SC, the emergence of a new weak absorption peak at 1780 cm⁻¹ in DHP-GlcA suggests the presence of an ester bond, which is in accordance with the findings of Aurore et al. [25]. Furthermore, the disappearance of the 1780 cm⁻¹ absorption peak in AT DHP-GlcA confirmed the formation of an ester bond between DHP and glucuronic acid during the thermal condensation reaction.

The CP/MAS ¹³C NMR spectra of DHP SC and DHP-GlcA are shown in Figure 2. Compared to DHP SC, DHP-GlcA showed a new absorption peak at 101.7 ppm. This additional peak likely originated from the C₁ signal of glucuronic acid [26], indicating the occurrence of a thermal condensation reaction between DHP and glucuronic acid within a 10% volume fraction acetic acid environment at 140 °C. This interpretation is consistent with the above infrared spectrum analysis.

Given the less conspicuous nature of the CP/MAS ¹³C-NMR signal, ¹³C-NMR was employed to further characterize the structure of the DHP-GlcA. The ¹³C-NMR spectra of DHP SC and DHP-GlcA are shown in Figure 3. Compared with DHP SC, DHP-GlcA showed new absorption peaks at 103.8 ppm, 77.6 ppm, and 69.6 ppm, corresponding to the C₁, C₃, and C₅ signal peaks within the glucuronic acid, respectively [26]. These signal peaks disappeared after alkali treatment, which indicated the formation of ester bonds between DHP and glucuronic acid. It was concluded that the thermal condensation reaction of DHP and glucuronic acid could generate ester bonds under the above reaction conditions.

To substantiate the capability of DHP and glucuronic acid to manifest thermal condensation and form ester bonds under the prescribed reaction conditions, 2D-HSQC NMR analysis was conducted on the ball-milled samples. The 2D-HSQC NMR spectra of DHP- GlcA are shown in Figure 4. The main connective structures of the DHP-glucuronic acid complex are shown in Figure 5. The functional groups of the main signals in the 2D-HSQC NMR spectra were assigned as shown in

Table 1. Analysis of the 2D-HSQC NMR spectra of the DHP–glucuronic acid complex.

Label	pH = 4	pH = 4 (Alkali Treatment)	Assignments
	δC/δH (ppm)	δC/δH (ppm)
Cβ	52.96/3.49	52.93/3.46	Cβ–Hβ in phenylcoumaran (C)
Bβ	53.48/3.06	53.38/3.04	Cβ–Hβ in β-β (resinol) (B)
OCH3	55.49/3.77	55.32/3.76	C–H in methoxyls
Aγ	59.89/3.62 59.99/3.28	59.84/3.25 59.74/3.69	Cγ–Hγ in β–O–4 substructures (A)
Fγ	60.90/4.10	61.37/4.08	Cγ–Hγ in cinnamyl alcohol end-groups (F)
Cγ	62.66/3.73	62.56/3.71	Cγ–Hγ in phenylcoumaran (C)
A′γ	62.74/4.27	ND	Cγ-Hγ in γ-acylated β-O-4 substructures(A’)
Bγ	70.78/3.75 70.83/4.51	70.61/3.74 70.68/4.13	Cγ–Hγ in β-β resinol (B)
Aα	71.19/4.76	71.03/4.73	Cα–Hα in β–O–4 unit (A)
BE	73.74/5.94	ND	α-ester
Aβ	84.00/4.32	84.02/4.29	Cβ–Hβ in β–O–4 substructures (A)
A′β	80.81/4.55	ND	Cβ–Hβ in β–O–4 linked to G (A)
Bα	84.96/4.63	84.83/4.62	Cα–Hα in β-β resinol (B)
B′α	83.25/4.72	82.68/4.55	Cα–Hα in β-β (B’, tetrahydrofuran)
Cα	86.89/5.48 87.89/5.64	86.73/5.46	Cα–Hα in phenylcoumaran (C)
U1	97.86/5.31	ND	C1-H1 in 4-O-methyl-α-D-GlcUA
G2	108.51/6.94 110.23/6.93	108.31/6.92 110.03/6.90	C2–H2 in guaiacyl units (G)
G5	114.65/6.76 115.25/6.99	114.51/6.74 115.17/7.06	C5–H5 in guaiacyl units (G)
G6	118.40/6.78 120.51/6.77	118.25/6.16 120.55/6.74	C6–H6 in guaiacyl units (G)
G′6	118.52/7.29	118.70/7.27	α C6-H6 in G-type structural units with oxidized sites
Eβ	125.99/6.79	ND	Cβ–Hβ in cinnamyl aldehyde end-groups (E)
FA6	123.25/7.21	ND	C6–H6 in ferulate (p-FA)

[26,27]. Figure 4a distinctly presents the side chain (β-O-4, β-5, and β-β) and benzene ring (G₂, G₅, and G₆) structures of DHP within DHP-GlcA. This suggests that ball milling enhances the solubility of the complex while preserving its core structure. In Figure 4a, the C₁-H₁ signal of glucuronic acid and the ester bond signal at the C_α position appeared at 103.40/5.23 ppm (U1) and 73.74/5.94 ppm (BE), respectively, revealing the linkage between DHP and glucuronic acid and the formation of ester bonds during the thermal condensation reaction. After alkali treatment, these two signals disappeared. Based on the ¹³C-NMR analysis, it was inferred that DHP and glucuronic acid are linked via esterification at the C_α position during the thermal condensation reaction (Red dotted line in Figure 6). Alkali treatment disrupted the linkage between DHP and glucuronic acid, causing the glucuronic acid to dissociate from the complex. Subsequently, the glucuronic acid was removed during the following washing and centrifugation steps, which resulted in the disappearance of the signal. The related mechanism is shown in Figure 6.

3. Methods and Materials

3.1. Materials and Reagents

DHP was synthesized based on the previous report [23]. D-glucuronic acid was purchased from Aladdin Industrial Corporation (Shanghai, China). Acetate, toluene, and dimethyl sulfoxide were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemical reagents were analytically pure.

3.2. Thermal Condensation of DHP with Glucuronic Acid

The thermal condensation reaction of DHP with glucuronic acid was conducted in accordance with the self-condensation reaction of DHP reported previously [23]. Specifically, 800 mg of glucuronic acid, 400 mg of DHP, and 600 μL of acetic acid solution (10 vt%) were loaded into a closed reactor. The reactor was heated to 140 °C in an oil bath and maintained at that temperature for 25 min. After the reaction, the product was collected by centrifugation, followed by washing with water until the pH value of the filtrate reached neutral and drying under a vacuum.

3.3. Ball Milling Treatment of DHP–Glucuronic Acid Complex

The above samples were subjected to milling with ZrO₂ balls at 600 rpm for 15 h in a planetary ball mill. After the ball milling, the sample powder was flushed with toluene, followed by collection with centrifugation and drying under a vacuum.

3.4. Alkali Treatment of the DHP–Glucuronic Acid Complex Powder after Ball Milling

The appropriate amount of the above sample powder and 30 mL of 0.5 mol/L NaOH solution were loaded into a centrifuge tube. After fully stirring and allowing the mixture to stand for 10 h, the product was collected by centrifugation, followed by washing with water until the pH value of the filtrate reached neutral and drying under a vacuum.

3.5. Structural Characterization of DHP–Glucuronic Acid Complex

The infrared spectra of the DHP self-condensation product (DHP SC), DHP–glucuronic acid complex (DHP–GlcA), and alkali-treated DHP–glucuronic acid complex (AT DHP–GlcA) were acquired with a Fourier-transform infrared (FTIR) spectrometer (NICOLET 6700, Waltham, MA, USA) ranging from 4000 cm⁻¹ to 400 cm⁻¹. The Cross Polarization/Magic Angle Spinnin Carbon 13-Nuclear Magnetic Resonance (CP/MAS ¹³C-NMR, Bruker Corp., Karlsruhe, Germany) spectra of the DHP self-condensation and DHP–glucuronic acid complex were determined by superconducting nuclear magnetic resonance spectrometer (AVANCE AV 400 MHz, Bruker, Germany). The Carbon 13-Nuclear Magnetic Resonance (¹³C-NMR) spectra and 2-Dimensional Heteronculear Single Quantum Coherence Nuclear Magnetic Resonance (2D-HSQC NMR) spectra of the DHP–glucuronic acid complex and DHP–glucuronic acid complex after DHP self-condensation and alkali treatment were determined by superconducting nuclear magnetic resonance spectrometer (AVANCE III, Bruker, Germany).

4. Conclusions

This work was undertaken to verify whether DHP can react with hemicellulose under hot-pressing conditions to form LCC-like polymer compounds and to explore its reaction mechanism. We prepared DHP–glucuronic acid complexes using DHP and glucuronic acid at 140 °C to simulate the hot-pressing environment of synthetic board molding. Through infrared and CP/MAS ¹³C-NMR, we preliminarily confirmed the occurrence of a thermal condensation reaction between DHP and glucuronic acid under the above reaction conditions, resulting in the formation of ester linkages. Subsequently, a comparison of the ¹³C-NMR spectra of DHP-GlcA before and after alkali treatment further supported this conclusion. In summary, this paper explored the reaction mechanism of DHP with glucuronic acid under simulated hot-pressing conditions, which is helpful for understanding the bonding process between adhesives and components of wood-based panels. This paper lays a theoretical foundation for the application of DHP in formaldehyde-free adhesives.