1. Introduction
Poplar, eucalyptus, Chinese red pines, and other fast-growing species that are widely distributed in China are the main sources of OSB raw materials [
1]. Meanwhile, OSB is widely utilized in fields, like packaging, construction, furniture manufacturing, etc., because of its characteristics, such as light weight, high strength, and water resistance [
2]. However, in China, the OSB production cost is high, with poor product quality. Additionally, it lacks market competitiveness due to its low visibility, infrequent use as compared with plywood and particleboard, small production scale, and lack of state-of-the-art equipment in China [
3].
Improving the availability of wood is the primary goal of the furniture manufacturing industry [
4]. Researchers are making an effort to improve the original characteristics of wood [
5,
6,
7]. Sandberg et al. [
8] introduced an eco-friendly method, called Thermo‒Hydro‒Mechanical (THM) treatment, which combines the use of temperature, moisture, and mechanical action. They claimed that THM treatment can improve the intrinsic properties of wood. New materials could be produced and the new functionality that is desired by engineers could be achieved without changing its eco-friendly characteristics. Furthermore, Sandberg et al. [
9] studied the effect of this method on reducing the damage that is caused by veneer stretching and buckling.
There are many types of OSB, of which bamboo-oriented strand board (BOSB) is widely studied [
10]. Fu et al. [
11] used bamboo from Hunan province (
Phyllostachys pubescens) as a material for developing BOSB and explored the best process conditions for manufacturing BOSB. Sun et al. [
12] compared two types of BOSB with different orientation distributions, namely, parallel aligned strand orientation (LVSL) and orthogonally oriented strands (BOSB). The results showed that, when compared with LVSL, BOSB provides a larger maximum bearing capacity and shows excellent impact performance. Sun et al. [
13] then studied the bending properties of bamboo strand board I-beams. The bending test confirmed that the rigidity and strength characteristics of the bamboo I-beams exceeded the requirements of APA-EWS in PRI-400-2012.
The impact of processing, such as hot pressing on veneers, needs to be modeled to assist in the further optimization of veneer processes [
14]. Ormarsson et al. [
15] carried out numerical simulations on forming, springback, and deformation, and established a model of veneer hot-pressing technology. In addition, plasticizers, adhesives, etc. also play an important role in particleboard veneering. Tom et al. [
16] studied the application of phenolic resin in veneer molding. In addition, Ozarska conducted in-depth research on veneer decoration technology, and published a monograph,
A Manual for Decorative Wood Veneering Technology [
17].
Due to the low cost and simple production process, adhesives such as phenolic resins and urea‒formaldehyde resins have been widely utilized in the wood industry. However, the widespread utilization of these adhesives has caused serious formaldehyde pollution problems [
18,
19]. Biomass adhesives are natural polymer materials that can be used as adhesives. It is sometimes used more broadly to describe adhesives that are formed from biomonomers, such as sugar, or to mean synthetic materials that are designed to adhere to biological tissue. Adhesives that are based on soy protein are widely studied [
20]. Su et al. [
21] believe that, when compared with traditional adhesives, biomass adhesives have the following advantages: degradable, nontoxic pollution-free, and environmentally friendly; renewable raw materials that are easy to handle; and, suitable to be produced by cold- or hot-pressing technologies. However, due to the low bonding strength, low water resistance, poor product stability, and high price of soybean protein adhesive, its application is limited. In comparison, cornstarch adhesive has better application prospects [
22,
23].
3. Results and Evaluation
3.1. The Comparison between Cold Pressing and Hot Pressing
To compare the performance of two different technologies, cold pressing and hot pressing, we first set the aging time to 30 min., the adhesive amount to 160 g/m2, and the unit pressure to 0.8 MPa. Note here that the hot-pressing temperature is 110 °C and the hot-pressing time is 240 s, while the cold-pressing temperature is room temperature, and the cold-pressing time is 1 h. All of the experiments were repeated three times. The average surface bonding strength of hot pressing was 0.84 MPa, while that of the cold press was 0.63 MPa. Obviously, the hot pressing requires less time for pressing and obtains a better surface bonding strength. This is because the high temperature during the pressing increases the movement of the molecules, which, in turn speeds, up the improvement of the bonding strength. Accordingly, in the following experiments, we are going to use hot pressing to study the veneering of OSB.
3.2. Analysis of Surface Bonding Strength and Aging Time
From the five different situations that are mentioned in
Section 2.2.3, the aging time is set as the abscissa and the surface bonding strength as the ordinate. Each value of the point in
Figure 4 is the average of three experiments. The average and standard deviation of these experiments are 0.758 ± 0.0026, 0.784 ± 0.0050, 0.856 ± 0.0021, 0.821 ± 0.0031, and 0.703 ± 0.0046, respectively. The relationship between the aging time and the surface bonding strength is obtained, as illustrated in
Figure 4.
From
Figure 3, the veneering experiment using cornstarch adhesive shows that the aging time has a great effect on the surface bonding strength. The surface bonding strength increases slowly and then decreases with increasing aging time. When the aging time is 30 min, the surface bonding strength reaches a maximum. The effect of aging time on the surface bonding strength ultimately depends on the effect of aging time on the viscosity of the adhesive. Within a certain aging time range, the cornstarch adhesive has good initial viscosity and good precompression. The adhesive layer can be fully cured within a certain aging time. However, if the aging time is too long, then the degree of curing will become stronger, the viscosity will become smaller, and the surface bonding strength will be smaller.
3.3. Analysis of Surface Bonding Strength with Hot Press
The results are shown in
Table 5.
Ki is the sum of the total strength that is related to level
i. The average
ki reflects the influence of the level
i on the strength. Range
R is the difference between the maximum and the minimum number in
ki. The range R reflects the influence of various factors on the experimental results. Range represents the magnitude of the numerical fluctuation. The column with the largest difference is the factor that has the most influence on the experimental results, which is the most important factor. From
Table 5, some conclusions can be intuitively drawn: from the relationship of
RA >
RB >
RC, we can draw the conclusion that factor A (unit pressure) has the largest influence on the bonding strength, followed by factor B (hot-pressing temperature), and finally factor C (hot-pressing time).
Figure 5 shows a visual analysis of each factor. A larger surface bonding strength indicates better performance, as seen in
Figure 5. Therefore, the optimal process solution in the experiment is A
3B
1C
2, that is, a unit pressure of 1.0 MPa, a hot-pressing temperature of 90 °C, and a hot-pressing time of 240 s.
Figure 5 shows that, when the unit pressure rises from 0.6 to 1.0 MPa, the surface bonding strength first decreases and then increases rapidly. This might be related to the characteristics of the substrate. The OSB in this experiment is made of poplar wood shavings. When the pressure is moderate, the adhesive penetrates into the inside of the substrate. The contact area between the adhesive and the substrate is large, the forming force is large, and the surface bonding strength is large. Meanwhile, when the unit pressure is large, the poplar particleboard has a greater compression degree, and the shavings are compacted. Under these circumstances, the adhesive does not easily penetrate into the interior, the force is small, and the surface bonding strength is small.
3.4. Analysis of Veneer Penetration Rate with Hot Press
Table 6 shows the results. GB/T 15104-2006 [
31] stipulates that top-quality and first-class products of thin wood veneer decorative boards are not allowed to have penetration, and the penetration area of qualified products must not exceed 1% of the board area. Hot-pressing pressure, hot-pressing time, and hot-pressing temperature all affect the veneer penetration rate, as shown in
Table 6.
From the relationship of RA > RB > RC, we can see that factor A (unit pressure) has the greatest influence on the veneer penetration rate, followed by factor B (hot-pressing temperature) and factor C (hot-pressing time).
Figure 6 directly represents their relationship. The larger value of the veneer penetration rate indicates the worse quality of the finished product. It is not difficult to see that the optimal process ratio in the experiment is A
1B
2C
2, which is, a unit pressure of 0.6 MPa, a hot-pressing temperature of 100 °C, and a hot-pressing time of 180 s.
It can be seen from
Figure 6 that the unit pressure has a significant effect on the penetration level. When the unit pressure rises from 0.6 to 1.0 MPa, the penetration rate first increases rapidly and then gradually decreases. The permeation rate is the best when the unit pressure is 0.6 MPa, and then the permeation rate increases with the increase in the unit pressure. When the unit pressure increases, the compression of the wood increases, which increases the permeability. Therefore, the unit pressure should be controlled within a certain range.
The hot-pressing temperature and time are closely related to the penetration rate. The moisture evaporated by heating is increased when the temperature is constant and the time is prolonged. Increasing the hot-pressing temperature will also increase the evaporation of water. Increasing the heating temperature can shorten the curing time and hot-pressing time of the adhesive layer. The hot-pressing temperature is increased, the curing time and hot-pressing time of the glue layer are shortened, the glue liquid has not penetrated to the thin wood surface layer and has been cured, and the adhesive penetration rate has decreased.