The Performance Optimization of Oriented Strand Board Veneer Technology

: Oriented strand board (OSB) veneer technology and its performance have been widely studied in order to expand the range of OSB substrates. In this paper, OSB was a modified composite with boards as substrates, Myanmar old mahogany bark as the veneer material, and a cornstarch adhesive. Under such conditions, the optimal veneer technology was studied, and the index of the surface bonding strength and the veneer penetration rate were utilized in order to determine the performance. Two different processing technologies, cold pressing and hot pressing, were experimentally compared and hot pressing showed better performance. Subsequently, experiments were performed on the surface bonding strength and veneer penetration rate. The results show that the veneer performance of OSB is best when the unit pressure is 1.0 MPa, the hot-pressing temperature is 90 °C, and the hot-pressing time is 240 s. Furthermore, the magnitude of influence of the factors affecting the bonding strength is as follows: unit pressure > hot-pressing temperature > hot-pressing time. The research results have prospective significance for the performance optimization of OSB veneer technology.


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 Ibeams. The bending test confirmed that the rigidity and strength characteristics of the bamboo Ibeams 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 ureaformaldehyde 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].

Materials and Equipment
(1) Oriented strand board (OSB): the OSBs utilized in our experiments are from Nanjing Redsun Decoration City. The format size of the utilized OSB is 1220 mm × 2440 mm, the thickness is 18 mm, and the species is poplar, with a thin strip of shavings. According to GB/T 17657-2013 [24], the density of the OSB is measured as 5.88 g/cm 3 and the moisture content is 7.99%.
(3) Adhesive: the prepared cornstarch adhesive has good adhesive performance, and all of the indexes meet the requirements of GB/T 20241-2006 [26] and GB/T 14074-2006 [27]. The solid content is ≥30%, the storage period is ≥100 d, the pH value is 6-7, and the esterification degree is 0.0166-0.0300.

Experimental Process
In this section, the experimental processes are given. Note that all of the experiments in this paper were repeated three times and the average value of all statistical results was taken into consideration.

Cold Pressing of Myanmar Old Mahogany
Cornstarch adhesive was utilized in this experiment. The amount of adhesive and the unit pressure were set to 160 g/m 3 and 0.8 MPa, respectively. Cold-pressing time was the experimental variable, and surface bonding strength was the test index. The cold-pressing time was set to 1, 2, 4, or 6 h, as shown in Table 1. Each experiment was repeated three times. The process is designed as in Figure 1.  Figure 1 illustrates the operations. In detail, after cutting the OSB base material to a size of 50 mm × 50 mm, we sanded the surface to make the surface flat and easy to stick the veneer, then stuck the veneer onto the base material surface, aged it for 30 min., performed the cold-pressing experiment, checked the veneer adhesive quality after cold pressing, placed for 72 h, made the adhesive fully solidified, and finally trimmed the OSB with a paper cutter, polishing the edge with sandpaper. Substrate coating and adhesive utilization were repeated after the first substrate finished overlaying. Note that the shape stability of the two pieces of substrate after coating was ensured. Cold pressing was carried out according to the veneer technology that is required by the experimental process. The unit area pressure was 0.8 MPa and the gauge pressure was set to 1.3 MPa. The coldpressing time was 1 h.

Hot Pressing of Myanmar Old Mahogany
In this experiment, the experimental variables were hot-pressing time, hot-pressing temperature, and unit pressure. Additionally, the test index was the surface bonding strength. The orthogonal experimental design [24] was adopted in the experiment. The orthogonal experimental design is an efficient method for the analysis of multifactor and multi-level problems and it has been widely utilized in much furniture-related research [28,29]. Table 2 shows the factor levels.  The three factors, A, B, and C, are ranked in the orthogonal table. Table 3 shows the experimental scheme. Figure 2 illustrates the experimental process. The process settings are similar to for the cold pressing.
The aging time was 30 min. The main purpose of aging was to make the adhesive infiltrate and disperse water to improve the bonding strength. Hot pressing was performed in this process according to the veneering process. The unit pressure was 0.6 MPa and the gauge pressure was set to 1.1 MPa. The hot-pressing time was 120 s and the temperature was 90 °C. The OSB was put into the press for hot pressing as soon as the aging time was up. We repeated the hot-pressing operation and changed the pressure per unit area to 0.8 or 1.0 MPa. After that, the hot-pressing time was set to 120, 180, or 240 s. The hot-pressing temperature was set to 90, 100, or 110 °C for nine groups of hotpressing experiments.

The Surface Bonding Strength of the Veneer
Single factor analysis was utilized to design the experiment. In hot pressing, the aging time was set to 20, 30, 40, 50, or 60 min. Each experiment with a different aging time was repeated three times and the final result was the average. In the experiment, the adhesive amount was 160 g/m 2 , the unit pressure was 0.8 MPa, the hot-pressing temperature was 100 °C, and the hot-pressing time was 240 s.
This section discusses the surface bonding strength of OSB veneer. The influence of pressure, time, and temperature will be analyzed, and the preferred combination will be chosen.
The surface bonding strength is to determine the ratio of the maximum tensile force of the substrate surface and the veneering material on the vertical specimen surface to the bonding area, as shown in Equation (1) [24]: where Z σ ⊥ is the surface bonding strength of the test piece, max P is the maximum load when the surface of the test piece is damaged, and A is the bonding area. The surface bonding strength is an important index for evaluating the surface adhesive quality of plywood. According to [24], the testing method for surface bonding strength was designed, as follows ( Figure 3): i.
Sawing test piece: cut the veneered OSB into the test piece with a size of 50 mm × 50 mm.
ii. Drawing line: draw a small square 20 mm × 20 mm in the center of the sawed specimen. iii.
Gluing the square iron block: use the hot-melt adhesive to glue the front (the side with the circular groove) of the customized square iron block upward in the center of the test piece. iv.
Placing: place for at least 24 h to decrease the residual thermal stress. v.
Cutting: cut the decorative layer along the periphery of the square iron block with a hand hacksaw to the surface of the base material. vi.
Testing: fix the test piece on the universal mechanical testing machine with the chuck and fixture, load it in the direction perpendicular to the glued surface, and then record the gluing strength of the test piece from the beginning of loading to the failure of the test piece.

The Veneer Penetration Rate
The veneer penetration rate is the test index in this section. The veneer penetration rate is an important index for the study of surface properties of veneer. Different hot-pressing conditions will inevitably have different effects on the penetration. It is necessary to study the penetration rate to avoid affecting the aesthetic appearance and the effect of the later finishing.
It is difficult to calculate the specific area of permeation, so it is necessary to directly observe the situation of permeation of the board when performing the permeation statistics. It is necessary to destroy the whole wood or use a scanning instrument to determine the penetration of the cornstarch adhesive inside the wood because penetration refers to the discoloration of the surface of the base material due to the penetration of the glue layer. In this paper, we measure the ratio of discolored veneer area to total veneer area as the penetration. Equation (2) can describe the average veneer penetration rate (VPR) of tested n boards: where sp is the size of discolored veneer area and Si represents the total size of the ith board. Table 4 gives the permeation area and the corresponding permeation rate [30].

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/m 2 , 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.

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.

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 A3B1C2, 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.  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.

Analysis of Veneer Penetration Rate with Hot Press
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 A1B2C2, 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.

Conclusions
i.
Under laboratory conditions, OSB is pasted with a 0.6 mm Burmese old mahogany veneer, regardless of whether cold or hot pressing is utilized. Its surface bonding strength meets China's requirements for the surface bonding strength of decorative veneer particleboard. ii.
The comparison between hot pressing and cold pressing shows that under the same circumstances, hot pressing produces a better surface bonding strength in a shorter time. The high temperature during the pressing increases the movement of the molecules, which, in turn, speeds up the improvement of the bonding strength. iii.
Under the condition of single-factor analysis, the relationship between the aging time and the surface bonding strength of the veneer is as follows: when the aging time is 30 min., the surface bonding strength performance of the veneer of OSB is the best. iv.
The best performance of the veneer technology is achieved when OSB is veneered with 0.6 mm Burmese old mahogany bark, 160 g/m 2 cornstarch adhesive, 240 s hot pressing with 1.0 MPa unit pressure, and 90 °C pressing temperature. v.
The influence of the factors that affect the surface bonding strength of OSB with 0.6 mm Burmese old mahogany bark and hot pressing are in the following order: unit pressure > hotpressing temperature > hot-pressing time. vi.
All three factors, namely unit pressure, hot-pressing temperature, and hot-pressing time, affect the veneer penetration rate of OSB with 0.6 mm Burmese old mahogany bark and hot pressing. vii.
In the case of a fixed amount of adhesive, the unit pressure has the greatest effect on the surface penetration rate of OSB with 0.6 mm Burmese old mahogany bark. The compression of the wood increases when the unit pressure is increased, which increases the permeability. Therefore, the unit pressure should be controlled within a certain range. The hot-pressing temperature is the second factor that needs to be considered, and finally the hot-pressing time.