Surface Crack Occurrence and Resistance During Moisture Content Changes in MF-Resin-Impregnated Paper-Decorated Blockboard

Abstract

In multi-layered wood materials, varying rates of dimensional changes can easily lead to cracking, which can have a negative impact on their structure and functionality. This study focuses on cracking issues of decorated blockboard caused by moisture content changes. First, surface cracks on the decorated blockboard were observed and classified using optical microscopy and scanning electron microscopy (SEM). Second, from modeling perspectives, the critical tensile strength of the surface of the decorated blockboard was predicted to be 16.93 MPa, providing guidance for crack-resistant modification. Subsequently, halloysite nanotubes (HNTs) were incorporated into MF-resin-impregnated paper, achieving a Grade 5 crack resistance for decorated blockboard. The interaction between HNTs and MF resin forms a multiscale stress–dispersion system, as confirmed by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), indicating hydrogen and covalent bonding between HNTs and the MF resin. With a 5% HNTs addition, the tensile strength and strain break of the MF-resin-impregnated paper reached 36.60 MPa and 1.12%, respectively, representing increases of 97.39% and 60.00%, respectively, effectively preventing surface cracking. This has significant implications for improving the durability and performance of decorated blockboard in practical applications.

1. Introduction

Small-diameter plantation timber is difficult to process into sawn timber due to size limitations. Additionally, processing these logs into fiber products or pulp for paper production incurs high costs. Therefore, utilizing small-diameter trees for laminated panels represents a promising application [1,2]. Blockboard is a type of multi-layered wood material consisting of a core of wood strips which is covered with veneers on both sides. This structure enables a more efficient utilization of small-diameter wood and wood waste [3]. To enhance its performance, blockboard is typically decorated with melamine formaldehyde (MF)-resin-impregnated paper, which provides resistance to water, stains, wear, and scratches. Therefore, impregnated-paper-decorated blockboard has wide applications in interior decoration and holds great potential for situations that demand high stiffness and lightweight structures [4,5]. However, the complex composition and structure of wood make decorated blockboard susceptible to cracking under humidity changes. Moreover, there have been limited studies conducted on the cracking phenomenon caused by humidity stress in multi-layered wood materials.

Shrinkage is a typical physical characteristic of wood with a moisture content below the fiber saturation point [6,7]. Cracking and deformation of wood may occur when there are frequent fluctuations in relative humidity [8,9]. Many researchers have conducted moisture simulations in timber elements [10,11,12] and have concluded that estimating the moisture content of timber elements exposed to different climates is essential to assess moisture-induced stresses in structural elements [13]. ABAQUS is a finite element model (FEM) software that has a wide range of applications in mass and heat transfer research, providing valuable insights into moisture content, stress, and deformation [11,14]. Currently, many studies use FEM to investigate the dimensional instability of wood materials caused by changes in humidity or temperature [15]. However, there is limited research on moisture stress in multi-layered wood materials, especially in decorated blockboard with complicated structures.

Enhancing the amount of MF resins in impregnated papers has been recognized as a way to inhibit surface cracking in decorated blockboard. Researchers have utilized a variety of nanomaterials to enhance the mechanical strength of MF resins, such as sodium montmorillonite, carbon nanotubes, and cellulose nanotubes [16]. However, high-performance MF resins that can be used in impregnated papers remain a challenge. Halloysite nanotubes (HNTs) are an environmentally friendly natural aluminosilicate mineral with a multi-layer tubular structure similar to that of carbon nanotubes [17]. The outer surface of HNTs is composed of siloxane groups, while the inner surface consists of alumina hexahedron groups [18]. The unique structure of HNTs, characterized by a high specific surface area and a large aspect ratio, endows it with excellent mechanical properties and thermal stability [19,20]. Additionally, HNTs are more economical and readily available compared to carbon nanotubes, making their use highly promising for enhancing the strength and toughness of polymers [21,22]. The addition of 4.8% HNTs has been shown to significantly enhance the mechanical performance of epoxy resin, with improvements in tensile strength, Young’s modulus, and fracture toughness of 23.4%, 31.4%, and 61.4%, respectively [23]. However, the toughening effect and mechanism of HNTs on MF resin are unclear.

In this paper, the cracking behavior of MF-resin-impregnated paper-decorated blockboard under varying moisture contents is investigated. The morphology of cracks is captured by using optical microscopy and a scanning electron microscope (SEM). A finite element model of the decorated blockboard is established and validated to predict the moisture stress, with HNTs used to inhibit the occurrence of cracks on the surface of decorated blockboard and the toughening mechanism between HNTs and MF explored. This work provides crack formation mechanisms and proposes an effective strategy for crack resistance in decorated blockboard.

2. Materials and Methods

2.1. Materials

The blockboard was sampled from commercial products manufactured by Shenghua Yunfeng Co., Ltd. (Huzhou, China). The board consisted of Chinese fir (Cunninghamia lanceolata (Lamb.)) and poplar (Populus × euramericana cv. ‘74/76’) veneers, and its structural arrangement is shown in Figure 1a and

Table 1. Structural dimensions of the blockboard.

	Material	Thickness/mm	Angle/°
L1	Poplar (Populus × euramericana cv. ‘74/76’)	0.40	90
L2	Poplar (Populus × euramericana cv. ‘74/76’)	2.00	0
L3	Chinese fir (Cunninghamia lanceolata (Lamb.))	11.68	90
Total	-	16.48	-

. The sample was cut into specimens of 100 mm × 100 mm.

The poplar used for the study was 10-year-old timber harvested in Jinan, Shandong Province, with a diameter at breast height of 25 cm and a width of the annual rings of 14.75 ± 0.21 mm. Three different loading angles relative to the face grain were tested alongside tensile properties and the Poisson’s ratio, with 10 replicates for each test group. The sample size was 30 mm × 20 mm × 20 mm, as shown in Figure 1b. The Poisson’s ratio was measured by attaching 120-3AA strain gauges (purchased from Chengdu Electro-Mechanical Sensor Technology Co., Ltd., Chengdu, China) to the side of the compressive modulus samples. Prior to the tests, the mechanical samples were placed in a climatic chamber at a temperature of 20 °C and 65% relative humidity (RH) until their constant masses were reached.

Raw paper and impregnated paper were a commercial product by Tianyuan Huibang Co., Ltd. (Guangzhou, China). The raw paper mass, 80 g/m², was impregnated with urea-formaldehyde resin, with a weight gain of 80 g/m², then it was impregnated with MF resin, with a weight gain of 80 g/m². The impregnated paper and blockboard were hot-pressed for 7 min to obtain the decorated blockboard; the pressing conditions were 120 °C, 0.9 MPa, the same as in industrial production.

Melamine (99%) and a formaldehyde solution (37% by weight) were purchased from Sinopharm Chemical Reagent Co. The HNTs (Al₂Si₂O₅(OH)₄) were obtained from Gina New Material Technology Co. (Guangzhou, China), and their inner diameter, outer diameter, and length were in the range of 10–20 nm, 50–65 nm, and 100–550 nm, respectively.

2.2. Methods

2.2.1. Coefficient of Shrinkage and Elasticity

There are several methods available for testing the mechanical properties of wood under compression, such as ASTM D3500 and EN 789 [24,25]. The specimen preparation requirements in these standards vary and are often adapted by academics for their specific test purposes [26,27]. In this paper, the modulus of elasticity in the radial, tangential, and longitudinal direction, denoted by E_R, E_T, and E_L, respectively, were obtained by testing 30 mm × 20 mm × 20 mm specimens with a universal mechanical testing machine, Instron 5582 (Instron, Norwood, MA, USA), at a loading rate of 0.1 mm/min. The data logger TDS 530 (Tokyo Measuring Instruments Lab., Tokyo, Japan) was used to record the strains along and perpendicular to the loading direction of the sample to calculate the Poisson’s ratios for poplar wood. The modulus of elasticity of the sample was calculated based on the slope of the elastic part of the stress–strain curve, as shown in Equation (1):

$$E={\displaystyle \frac{\sigma }{\epsilon }}={\displaystyle \frac{\Delta F/A}{\Delta u/l}}$$

(1)

where $E$ is the modulus of elasticity, $\sigma$ is the compressive stress, $\epsilon$ is the compressive strain, $\Delta F$ is the increment of load between 15% and 35%, $\Delta u$ is the increment of deflection corresponding to $\Delta F$, $A$ is the cross-sectional area of the specimen, and $l$ is the original length of the strain gauges.

The relationship between the shear modulus of wood and the longitudinal modulus of elasticity can be estimated approximately [28] by Equation (2):

$${\displaystyle \frac{G_{RT}}{E_L}}\approx 0.018;{\displaystyle \frac{G_{LT}}{E_L}}\approx 0.06;{\displaystyle \frac{G_{RL}}{E_L}}\approx 0.075$$

(2)

where $G_{RT}$, $G_{LT}$, and $G_{RL}$ are the shear moduli in the RT (radial–tangential), LT (longitudinal–tangential), and RL (radial–longitudinal) plane.

In order to obtain the shrinkage coefficient, ten clear wood cubes (20 mm × 20 mm × 20 mm) were soaked in water until the green status. Then, the samples were placed in an environment at 20 °C and 65% RH for air drying; after that, their masses and dimensions were measured. Afterward, the samples were dried at 60 °C for 6 h and then at 108 °C until their masses were constant. The masses and dimensions of the samples were tested. The dry shrinkage strain of the wood can be calculated using Equation (3):

$${\epsilon }_{u,i}={\displaystyle \frac{L_{2,i}-L_{1,i}}{L_{1,i}}}\times 100\%$$

(3)

where i represents the direction, ${\epsilon }_{u,i}$ is the strain in the i direction, and $L_{1,i}$ and $L_{2,i}$ are the dimensions in the i direction. The average shrinkage per 1% reduction in moisture content in wood is known as the coefficient of shrinkage, as shown in Equation (4):

$${\alpha }_u={\displaystyle \frac{{\epsilon }_u}{\Delta W}}$$

(4)

where ${\epsilon }_u$ is the shrinkage strain and $\Delta W$ is the change in the moisture content.

The impregnated paper was individually hot pressed at a temperature of 120 °C and a pressure of 0.9 MPa for 7 min. The cured impregnated paper was cut into 120 mm × 10 mm strips with a thickness of 0.15 mm to test the tensile strength. Tensile testing was carried out with a universal mechanical testing machine, MTS E450 (MTS Systems, Eden Prairie, MN, USA), at a loading rate of 2 mm/min over 10 repetitions. The tensile strength of the modified H-MF-impregnated paper was measured using the same method.

2.2.2. Deformity of Decorated Blockboards

To facilitate the comparative analysis, the test temperature for the cracking resistance test was determined in accordance with the standard GB/T 17657-2013 [29]. Decorated blockboards with dimensions of 100 mm × 100 mm were dried at 70 °C. The dimensional and mass changes were recorded every 4 h until the change in the moisture content was less than 0.1%.

After drying, the samples were further dried at 103 ± 2 °C until the weight difference between two consecutive measurements (taken 6 h apart) was less than 0.1% of the sample’s weight, indicating a stable mass. The moisture content during the drying process was calculated using Equation (5):

$$H_i={\displaystyle \frac{m_i-m_0}{m_0}}\times 100$$

(5)

where H_i is the moisture content (%) at the i-th recorded time, m_i is the mass (g) of the sample at the i-th recorded time, and m₀ is the mass (g) of the sample after drying.

The 20 points on each sample were marked evenly, as shown in Figure 2, and the deformation of the sample was measured during drying using a spiral micrometer, Mitutoyo 293-821 (Mitutoyo, Kawasaki, Japan).

A 10 mm × 10 mm × 10 mm specimen was collected in the vicinity of the crack and the specimen cross-section was cut flat using a slide-walk slicer. A microscope Leica DM2000 (Leica Microsystems, Wetzlar, Germany), combined with a digital camera Leica DFC295 (Leica Microsystems, Wetzlar, Germany), was used to characterize the distribution of the cracks on the surface of the specimen and the structural features of the cracks in the cross-section. A high-resolution field emission scanning electron microscope (SEM) Hitachi SU8020 (Hitachi, Tokyo, Japan) was used to observe the morphology of the cracks in the cross-section.

2.2.3. Finite Element Model

Numerical simulations were carried out in the commercial software ABAQUS 6.14-1, since the modeling and analysis of heat transfer is well incorporated in the software and the moisture transfer and heat transfer are similar. Also, the expansion and contraction of material due to a change in temperature and moisture content are similar; therefore, the moisture transfer behavior can be modeled by analogy with heat transfer [30]. It is assumed that wood follows Fick’s second law for moisture transfer and heat transfer. The one-dimensional moisture transfer equation is described in Equation (6).

$${\displaystyle \frac{\partial u}{\partial t}}={\displaystyle \frac{\partial }{\partial x}}\left(D{\displaystyle \frac{\partial u}{\partial x}}\right)$$

(6)

where u is the moisture content of the wood and D is the moisture diffusion coefficient.

Similarly, the one-dimensional heat transfer differential equation can be described as in Equation (7).

$${\displaystyle \frac{\partial T}{\partial t}}={\displaystyle \frac{\partial }{\partial x}}\left({\displaystyle \frac{\lambda }{c\rho }}{\displaystyle \frac{\partial T}{\partial x}}\right)$$

(7)

where T is the temperature, c is the specific heat, ρ is the density of the wood, and λ is the thermal conductivity. The parameters were transformed as in

Table 2. Analogy of humidity field and temperature field parameters.

Humidity Field	Temperature Field
Moisture content: u	Temperature: T
Moisture diffusion coefficient: D	Thermal conductivity: λ
Equilibrium moisture content: We	External temperature: Te

.

To analyze the cracking behavior of the decorated blockboard under moisture changes, the finite element model (FEM) was developed. In the model, the following assumptions were made. (1) The defects in each component of the blockboard substrate were not taken into account, such as knots, holes, twill, and other defects. (2) The thickness of glue lines was ignored and it was assumed that the glue lines had no effect on the moisture transfer in a steady-state analysis. The bond among all components of the blockboard was strong, and the deformation between adjacent layers at the glue line was consistent. (3) The initial moisture content and temperature distributions within the blockboard were uniform and consistent.

2.2.4. Preparation of the MF Resin with HNTs

The preparation of MF was based on a previous paper [31]. The ratio of melamine to formaldehyde was 1:1.8. The pH of the formaldehyde solution was adjusted to 8.5–9.0 using a 5% NaOH solution. Melamine and water were added while stirring for 5 min at room temperature. The reactants were heated to 90 °C until the mixture became a clear solution. Then, the MF resin was obtained. HNTs were added to the MF resin with weights of 0.5 wt%, 1 wt%, 3 wt%, 5 wt%, and 7 wt%. Then, the mixtures were subjected to ultrasonic dispersion for 10 min. They were marked as H-MF 0.5%, H-MF 1%, H-MF 3%, H-MF 5%, and H-MF 7%, respectively.

2.2.5. Characterization of H-MF Resin

The changes in functional groups and space structure of H-MF before and after curing were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and NMR. All the resin samples were dried at 0 °C for 48 h in a freeze dryer and then cured at 120 °C in the oven for 10 min. The FTIR analysis was performed in a Nicolet iS10 (Waltham, MA, USA). Spectra were recorded in the wavenumber of 4000 to 500 cm⁻¹ by signal averaging of 32 scans at a resolution of 4 cm⁻¹. The XRD test was performed by a Bruker D8 Advance X-Ray (Bruker, Billerica, MA, USA) diffractometer in the range of 5–70°, with a tube current of 40 mA, a tube voltage of 40 kV, and a Cu target wavelength of 1.5406 Å. The NMR ¹³C was measured by a Bruker Avance III 600 M NMR spectrometer (Bruker, Karlsruhe, Germany). Particle size distribution and the zeta potential of HNTs were obtained by dynamic light scattering (DLS) and electrophoretic light scattering (ELS) using a zeta potential analyzer (Malvern Panalytical, Malvern, UK).

The structure of the H-MF resin was investigated using a field emission transmission electron microscope (TEM) with Tecnai G2 F20 (Thermo Fisher Scientific, Hillsboro, OR, USA). Scanning electron microscopy (SEM), using Hitachi S4800 (Hitachi, Tokyo, Japan), was conducted to investigate the fracture characteristics of the H-MF resin. The H-MF resin was poured into a 70 mm × 10 mm × 2 mm PTFE mold and the samples were obtained by curing at room temperature for 7 days. Observation of the fracture section was conducted after liquid nitrogen embrittlement of the resin.

2.2.6. Cracking Resistance Test of the H-MF-Resin-Decorated Blockboard

The surface cracking test was carried out according to GB/T 17657-2013 [29]. The H-MF-impregnated paper-decorated blockboard was obtained in the manner described in 2.1. The 250 mm × 250 mm samples were heated in a 70 °C drying oven for 24 h. The specimens were taken out and placed in an environment at 25 °C and 50% relative humidity for 24 h. Surface cracking was observed with a 30× electronic magnifying glass and evaluated against standard requirements.

3. Results and Discussion

3.1. Cracking Characteristics of the Decorated Blockboard

The moisture content change of the decorated blockboard during drying is shown in Figure 3. Its equilibrium moisture content was 8.95% at 25 °C and 58% RH. Initially, within the first 4 h, the moisture content of the blockboard decreased significantly by 4.31%. During this stage, irregular craquelures appeared on the surface of the board, and craquelures gradually increased with time, as shown in Figure 3. When the moisture content was 3.70%, some large cracks in a consistent direction appeared on the decorated blockboard. The cracks teared through the impregnated paper and the substrate was exposed.

To investigate the reason behind the craquelures, the cured impregnated paper was dried at 70 °C. The results, as shown in Figure 4, reveal irregular craquelures on the surface of the paper. This proved that the craquelures were not directly related to the substrate but were instead influenced by the properties of the MF resin.

The MF resin was cured to form a transparent coating. The structure of the MF resin involves the linking of triazine rings through methylene, a process which contributes to the high hardness but poor toughness properties of the material [32]. MF resin curing consists of the condensation reaction of the hydroxymethylated melamine, where the triazine rings form methylene bridges or ether bond links directly [33]. Generally, the curing process of the MF resin speeds up as the temperature is raised. Compared to the particleboard, the compression resistance of the blockboard is lower. When finishing it with impregnated paper, it is necessary to choose a lower temperature and a longer pressing time for blockboard compared with those for particleboard and fiberboard. Kandelbauer et al. [34] predicted the degree of MF resin curing at different temperatures by using the isoconversional model-free kinetic approach and observed that the curing rate of the MF resin at 180 °C was 40 times higher than that at 120 °C [35,36]. As a result, the MF resin on the surface of the decorated blockboard was not cured as effectively as that of the fiberboard or particleboard. After curing, some hydroxyl groups remained that were not involved in the condensation. The crosslinking of these hydroxyl groups can form internal stresses and lead to the cracking of the MF resin coating.

The surface crack distribution and deformation of the decorated blockboard specimen are shown in Figure 5a. It is apparent that the distribution of cracks correlated with the core board. The binarized images of the cracked parts are shown in Figure 5b,c. The cracks were mainly concentrated along the axis of the fir core board, with a greater coherence of cracks closer to the axis. At the same time, there was minimal variation in deflection at the shaft center. The change in deflection of the decorated blockboard was mainly influenced by the radial shrinkage of the core board. However, away from the axis, the deflection of the decorated blockboard was influenced by the radial and tangential shrinkage of the core board. The anisotropy of wood results in different shrinkages in the three directions. Typically, the tangential shrinkage is twice as high as the radial shrinkage and about 6–12% [37,38]. This suggests that the crack distribution was related to the radial and tangential shrinkage rates of the fir wood.

The cross-sectional SEM of the crack in Figure 5d,e indicates that the crack occurred in the impregnated paper and in L1. Cracks can be divided into three types based on their stress and fracture characteristics: open (type I) when the applied positive stress is perpendicular to the normal direction of the crack, slip (type II) when there is a shear stress parallel to the direction of the crack, and tear (type III) when the stress makes the material misaligned, resulting in cracking [39,40,41]. The cracks on the surface of L1 were open cracks. The uneven deformation of the core block during the drying process caused the cross-grain tension in L1 on the surface, providing stress for the formation of cracks.

The shrinkage cracks that occurred on the decorated blockboard could be classified into two categories: craquelures, which occurred only on the surface of the impregnated paper and exhibited a distribution pattern independent of the substrate structure, and large cracks, which extended through the L1 on the surface of the blockboard. Craquelures were caused by the incomplete curing of the MF resin, while differences in radial and tangential shrinkage of the core board were the main causes of the cracks.

3.2. Mechanical Properties of the Components

The mechanical properties of the poplar wood and impregnated paper were tested to provide parameters for the FEM (Figure 6). The material property parameters for the poplar wood and the Chinese fir wood are shown in

Table 3. Summary of poplar wood properties.

Elastic Modulus (MPa)	Shear Modulus (MPa)	Poisson’s Ratio	Coefficient of Shrinkage	Density (g/cm3)
EL = 10,653 ± 649	GTL = 799	νTL = 0.032 ± 0.008	0.031 ± 0.013	0.45 ± 0.02
ER = 1134 ± 93	GRT = 639	νRT = 0.789 ± 0.121	0.320 ± 0.059
ET = 465 ± 66	GRL = 191	νRL = 0.050 ± 0.024	0.669 ± 0.029

and

Table 4. Summary of Chinese fir wood properties [30].

Elastic Modulus (MPa)	Shear Modulus (MPa)	Poisson’s Ratio	Coefficient of Shrinkage	Density (g/cm3)
EL = 12,888	GTL = 773	νTL = 0.020	0.019	0.35
ER = 1048	GRT = 232	νRT = 0.430	0.139
ET = 594	GRL = 967	νRL = 0.029	0.255

. It is evident that the elasticity modulus of the various poplar species was of the same order of magnitude, and the Young’s modulus in the radial direction was approximately twice as large as that in the tangential direction. The values for the elasticity modulus of the Chinese fir wood were obtained from the references [30]. E_R, E_T, and E_L represent the moduli of elasticity in the radial, tangential, and longitudinal directions, respectively. G_RT, G_TL, and G_RL denote the shear moduli in the RT, TL, and RL plane. ν_RT, ν_RL, and ν_TL correspond to Poisson’s ratios. Poplar wood had a tensile strength of 127.94 ± 13.50 MPa in the longitudinal directions and of 2.36 ± 0.36 MPa in the tangential direction. As shown in Figure 6d, the average tensile strength of the impregnated paper was 18.50 ± 1.89 MPa, and the strain at break was 0.61 ± 0.04%. The modulus of elasticity was 32.10 ± 2.62 MPa.

Based on these parameters, a humidity response model for the decorated blockboard can be established to investigate the stress–strain distribution in the humidity field.

3.3. Finite Element Analysis

The three-dimensional eight-node coupled temperature–displacement element C3D8T was employed in the model. The surface stress generated is depicted in Figure 7a. To verify the accuracy of the model, it was compared with experimental results. During the drying process, the moisture content of the decorated blockboard decreased from 8.95% to 1.35%, inducing warping and deformation. Twenty points were selected on the surface of the specimen to compare the deflection before and after drying, as illustrated in Figure 7b. The average deflection at the core axis was 0.07 mm, while, at the glued edge of the core, it was 0.20 mm. This significant difference caused warping and deformation. Since the shrinkage coefficient in the tangential direction is nearly twice that in the radial direction, the deformation at the block was small, as it was dominated by the radial shrinkage [7]. As shown in Figure 7c, the deformation at different locations on the samples was simulated and compared with the test results. There was good agreement between the experimental and FEM results, with an average relative error of 12.55%, confirming the validity of the modeling approach.

The moisture stress generated by the decorated blockboard is shown in Figure 7d. In the Y direction, the moisture stress demonstrated an increasing and then a decreasing trend from the L3 layer to the surface. The stress was concentrated in the L1 and L2 layers, with a maximum stress of 38.14 MPa occurring in the L2 layer. After the buffering through the orthogonal structure, the stress transferred to the upper surface was 15.91 MPa. The distribution of surface stress in the decorated blockboard was related to the arrangement of the L3 core block. The stress at the core axis in the L3 core block was 16.93 MPa, which is higher than the stress at the bonding position of the two core blocks (12.64 MPa). The location of the maximum stress in the model was consistent with the actual cracking position. The maximum stress on the surface was 16.93 MPa, while the tensile strength of the impregnated paper was 16.62 MPa, indicating that it cannot withstand the maximum stress, thus leading to surface cracking.

Based on the results of the FEM analysis, enhancing the mechanical strength of the impregnated paper may effectively address the surface cracking issues of the decorated blockboard.

3.4. The Structure and Morphology of the H-MF Resin

The HNTs were incorporated into the MF resin to construct a multiscale stress dispersion system, thereby improving the mechanical strength of H-MF-resin-impregnated paper. Initially, the structural characteristics of the H-MF resin were characterized.

Figure 8a shows FTIR spectra from 400 to 4000 cm⁻¹ for MF, HNTs, and H-MF 5% before and after curing. The peaks observed at 3696, 3618, and 911 cm⁻¹ in the HNTs correspond to the stretching vibrations of the inner Al-OH surface and Al-OH interlayer and the deformation vibration of the inner O-H surface, respectively. The peaks at 3442 and 1630 cm⁻¹ are related to the stretching vibrations of hydroxyl groups (-OH) from the adsorbed water on the outer surface and the interlayer of HNTs, respectively [42]. The peaks at 1090, 470, and 436 cm⁻¹ correspond to the symmetric stretching vibration of in-plane O-Si-O, Si-O-Si, and Si-O, respectively. Additionally, the peaks at 790 and 535 cm⁻¹ are typical of the symmetric stretching vibration of O-Si and the deformation vibration of Al-O-Si in HNTs. In the MF resin, the peak at 3343 cm⁻¹ correspond to the stretching vibrations of -OH and -NH groups. The peak at 2952 cm⁻¹ is related to the asymmetric stretching vibration of C-H [43,44]. The peaks of the triazine ring are represented at 1556 and 810 cm⁻¹. The major bands at 1124 and 2364 cm⁻¹ represent aromatic primary amines. The peaks at 1001 and 1163 cm⁻¹ represent the C-O stretching vibration in hydroxymethyl groups and the C-O-C stretching vibration of ether bonds, respectively [33,45].

After curing, the MF resin peak at 1364 cm⁻¹ shifted to 1334 cm⁻¹, indicating the transformation of hydroxymethyl groups into methylene ether or methylene bridges. The disappearance of shoulder peaks at 1124 cm⁻¹ suggests a reduction in aromatic primary amines, signifying increased crosslinking in the MF resin [46].

In the uncured and cured H-MF 5% samples, the HNTs’ characteristic peak at 3697 cm⁻¹ indicates their presence. Additionally, the broadening of the 3500–3200 cm⁻¹ band may be due to hydrogen bonding between the -OH groups on the outer layer of the HNTs and MF resin. The partial blue shift of the 1364 cm⁻¹ peak of the uncured H-MF 5% suggests the formation of hydrogen bonds involving hydroxymethylated melamine [47]. After curing, a new peak appeared at 1077 cm⁻¹, attributed to the characteristic Al-O-C bond, possibly resulting from the formation of covalent bonds between the hydroxyl groups on the inner surface of HNTs and the hydroxymethyl (-CH₂OH) groups in the MF resin through a condensation reaction [48]. This indicates that the MF resin penetrated into the HNT tubes. The persistence of the 1124 cm⁻¹ peak in the cured H-MF 5% indicates that some aromatic primary amines may form hydrogen bonds with the HNT surface, inhibiting their full reaction, as noted with the 1364 cm⁻¹ peak. The 1630 cm⁻¹ HNT peak shifted to 1606 cm⁻¹, indicating interactions between MF and HNTs. In conclusion, the infrared results clearly confirm the presence of covalent and hydrogen bonds between HNTs and MF.

The zeta potential of pure HNTs was −37.4 mV, which was measured by laser Doppler electrophoresis within a ±200 mV voltage range, and its particle size distribution is shown in Figure 8b. After curing, the MF resin exhibited weak acidity. The amino groups on the surface of the MF resin can protonate (–NH₂ to –NH³⁺) under acidic conditions, resulting in a positively charged surface [34]. Consequently, the oppositely charged HNTs easily adsorb onto the MF resin, leading to a stable structure after curing.

The SEM and TEM images of HNTs and H-MF are shown in Figure 9. HNTs exhibited a smooth tubular structure. Figure 9b,c displays the morphology of H-MF observed through SEM and TEM, clearly showing the intercalation and anchoring of MF at the ends of the HNTs.

Figure 10a shows the XRD spectra of the cured H-MF 5% resin. It can be observed that the crystalline structure of the HNTs remained largely unchanged during the curing process of the MF resin. The typical XRD peaks at 2θ = 12.05°, 19.95°, and 24.56° correspond to the (001), (020), (110), and (002) reflection planes, with d-values of 7.34 Å, 4.45 Å, and 3.62 Å, respectively. The fundamental reflection at 7.31 Å corresponds to the multi-walled structure of HNTs. The d-spacing of the (001) reflection plane in the H-MF 5% was 7.41 Å, indicating no significant change [43]. This demonstrates that the triazine ring network in the MF resin did not intercalate into the HNT interlayers and that HNTs retained their original multi-layered tubular structure. This may be due to the crosslinking and curing of the triazine ring structure through methylene bridges, which hindered the penetration of the HNT layers. Additionally, the strong hydrogen bonds between the interlayers of HNTs may be another reason preventing the intercalation effect.

The solid-state NMR ¹³C profiles of MF and H-MF are shown in Figure 10b. The peak at 166 ppm represents the carbon in the triazine nucleus of the MF resin, and 46 ppm represents the carbon in -NHCH₂NH- [49]. The downfield shifts of the peaks at 86 and 60 ppm indicate a decrease in the electron cloud density of the -NHCH₂OCH₂OH and -NHCH₂OH, respectively, a phenomenon which may be the result of the penetration of MF into the inner ends of the HNTs. The new peak at 61.84 ppm is likely attributed to the carbon atom in the Al-O-C bond formed at the interface between MF and HNTs [50]. The results evidenced that MF was inserted into the HNTs and reacted to form an anchoring effect.

Based on the above characterization results, it is validated that multiple interactions, including hydrogen bonds, electrostatic adsorption, covalent bonds, and anchoring effects, are formed between HNTs and MF resin.

3.5. The Mechanical Property of H-MF-Impregnated Paper

Figure 11 presents the mechanical properties of the impregnated paper with different H-MF contents. Tensile strength initially increased with HNT incorporation, peaking at 5 wt% HNTs, and then decreased. At 5 wt%, the tensile strength reached 36.61 MPa, a 97.39% increase over the control. The strong bonds between the hydroxyl groups on the HNTs and the free hydroxyl groups within the MF resin molecular chains enhanced the strength of the impregnated paper. During resin fracture, the disruption of hydrogen bonds allows energy absorption, thereby increasing tensile strength. At 5 wt% HNTs, the strain at break was 1.12%, indicating a 60.00% improvement compared to the control. This enhancement is due to the plasticizing effect of HNTs, which improved the ductility of the paper. The sliding effect of HNT particles may also contribute to this phenomenon. However, when the HNT content reached 7%, the excessive amount of HNTs was unevenly distributed in the MF resin and agglomerated, subsequently affecting the crosslinking and curing of the resin. Although the high HNT content increased the elastic modulus of the impregnated paper by 22.24%, it compromised the smoothness of the paper and led to slight decreases in both tensile strength and elongation at break. This made the paper weaker and less stretchy.

Table 5. The mechanical property of H-MF-impregnated paper.

	MF	H-MF 0.5%	H-MF 1%	H-MF 3%	H-MF 5%	H-MF 7%
Stress (MPa)	18.50 ± 2.24	24.36 ± 2.54	28.00 ± 2.85	29.82 ± 3.03	36.60 ± 3.03	34.24 ± 3.15
Strain (%)	0.58 ± 0.06	0.83 ± 0.08	0.84 ± 0.07	0.93 ± 0.10	1.12 ± 0.10	0.88 ± 0.09
Elasticity modulus (GPa)	3.18 ± 0.06	2.93 ± 0.03	3.33 ± 0.06	3.21 ± 0.02	3.27 ± 0.02	3.89 ± 0.04

shows that a small amount of HNTs (≤5 wt%) makes MF-resin-impregnated paper tougher without affecting its elastic modulus.

In contrast, reinforcing the epoxy resin with carbon nanotubes increased the tensile strength and elongation at break by only 40%, even with the introduction of a coupling agent [51]. Additionally, the tensile strength of epoxy resin was increased by 39.6% with 3% HNTs [52]. In this study, HNTs were able to form a multiscale stress dispersion system with MF, leading to a simple and efficient way to enhance the toughness and strength of MF.

Figure 12 presents SEM images of the H-MF fracture surfaces. The fracture surface of the pure MF exhibited a smooth and uniform morphology, reflecting its brittle nature (Figure 12a). With the addition of HNTs, rod-like particles were interspersed within the resin, resulting in a rougher surface with nanoscale rod-bridged microcracks.

The presence of rigid HNT particles in the thermosetting polymer introduced energy dissipation mechanisms prior to crack formation [28]. These mechanisms, including plastic deformation near the crack tip, helped reduce stress concentrations and inhibit crack propagation (Figure 12c,d). The tubular structure of HNTs created an anchoring effect within the MF resin, making it more resistant to breakage compared to rod or fiber particles.

MF resin with a 5% HNT addition had a rough fracture surface and scale-like steps, showing that the HNTs hindered crack movement (Figure 12e). However, excessive amounts of HNTs led to agglomeration, reducing the dispersion and strength of the MF resin due to electrostatic interactions.

3.6. Cracking Resistance of the Decorated Blockboard

The results of the cracking resistance test of the decorated blockboard before and after modification are shown in Figure 13. As depicted in Figure 13a,b, the blockboard whose surface was covered with ordinary MF-impregnated paper exhibited numerous surface cracks, meeting only the Grade 1 crack resistance standard specified in GB/T 17657-2013 [29]. This is due to the lower strength of ordinary MF-impregnated paper compared to the surface drying shrinkage stress of the blockboard, making it difficult to withstand deformation caused by drying and shrinkage. In contrast, Figure 13c,d presents the blockboard decorated with H-MF-5%-impregnated paper which remained intact without any surface cracks, meeting the Grade 5 crack resistance standard. This improvement is attributed to the significantly enhanced mechanical properties of H-MF-5%-impregnated paper, an approach which increases the tensile strength to 36.61 MPa and elongation at break by 60%, far exceeding the surface drying shrinkage stress of the blockboard, thereby effectively preventing crack formation.

4. Conclusions

This study systematically investigates the surface cracking mechanisms of decorated blockboards under moisture content variations and proposes an effective prevention strategy through FEM quantitative analysis, confirming stress concentration as the physical origin of cracking. On the basis of this mechanistic understanding, a multiscale stress dispersion system was developed through the incorporation of HNTs into the MF resin. With a 5 wt% HNT addition, the tensile strength and elongation at break of the impregnated paper increased by 97.39% and 60.00%, respectively. When applied to decorative blockboards, the crack resistance was enhanced to Grade 5 according to national standards. The research not only advances the theoretical understanding of moisture–stress cracking in decorative blockboards, but also provides a framework of FEM-based crack prediction–control technology for decorative blockboards. It provides a scientific foundation for industrial development of high-performance decorative panels and sustainable building materials.