Study on Preparation Optimization Technology and Surface Properties of Decorated Bamboo Filament Board

: To further improve the manufacturing process and product performance of decorated bamboo ﬁlament board, the Box – Behnken response surface analysis method was used to analyze the correlation between the hot-pressing parameters and surface bonding strength, and the optimal process optimization parameters were obtained. In addition, the wettability and color of each group of samples were tested. The results show that the optimum process parameters of decorated bamboo ﬁlament boards were 130 °C, 165 s and 2.00 MPa, and the surface bonding strength was 1.58 MPa. The relative error between the measured value and the predicted value was less than 5%. The contact angle of the bamboo ﬁlament after hot pressing was higher than without hot pressing. However, there was no correlation between wettability and the hot - pressing parameters. There was no eﬀect on the change in bamboo surface color. This indicates that the temperature range selected in this study meets the requirements of surface color control in production.


Introduction
Bamboos are the fastest-and tallest-growing gramineous plants, and they come in a wide variety and are widely distributed in tropical and subtropical regions around the world [1,2].They have unique biological characteristics and are environmentally friendly plants that integrate ecological characteristics, material characteristics and cultural tourism.They can be used sustainably after planting [3].In terms of physical properties, bamboo has a unique functional gradient structure, with a vascular bundle system and parenchymatous tissues, giving it a specific strength 2-5 times higher than steel [4,5].Compared to wood, bamboo has excellent mechanical strength, good elasticity, flexibility and thermoplasticity [6][7][8].From the perspective of visual perception, bamboo also has a unique aesthetic value and unique materials, and a representative straight texture can show the texture and color of bamboo, thus creating a comfortable and healthy living environment for human beings [9,10].Therefore, bamboo is widely used in the field of furniture manufacturing and interior decoration [11,12].woven from bamboo silk, which can be customized in a diversified and personalized way [13,14].The color of bamboo filament decorative materials is mainly divided into three colors: carbonization, green and yellow.The carbonization color is formed by heat treatment, which can bring people a feeling of elegance [15,16].Green is derived from the color of bamboo skin, giving a fresh feeling.However, without green treatment, the color will change from the original green to yellow in the application [17].Yellow is the original color inside the bamboo, which brings a feeling of warmth.Currently, the common bamboo filament weaving methods include bamboo filaments perpendicular to each other, multi-foot crisscross, horizontal parallel weaving and so on [18].
Based on the changing patterns and rich colors of bamboo filament decorative materials, it has been used by many designers as a decorative material for furniture production [19,20].However, these designs only exist at the creative level and are difficult to achieve in industrial production.The reason for this situation is that the composition unit of bamboo filament decorative material is a single independent structure, and there are gaps between each unit.In production, the resin adhesive is mostly in a fluid state, and improperly controlling the amount of glue and pressure will cause the adhesive to penetrate the bamboo filament gap, thus affecting the visual effect.Therefore, the cumbersome gluing process and strict pressure control are the main problems in producing and applying decorated bamboo filament boards (DBF boards).Previous studies found that the preparation process of impregnated film paper instead of resin adhesive can effectively solve the above problems.This process is an improvement that meets the requirements of environmental protection and process simplification, which are of great significance to the production and application of DBF boards.Initially, a poplar blockboard was used as a wood-based panel substrate, and a three-layer structure of a single-sided DBF blockboard was successfully developed [21].Subsequently, the product structure was innovated, and a five-layer double-sided DBF board was developed with a Chinese fir finger-joint board as the substrate.The finger-joint plate is used as the base material of the decorative board, which can remove the wood veneer layer structure and reduce the production cost compared with the blockboard [22].
In order to adapt to the development of the custom furniture industry and expand the use of base material, the hot-pressing process of eucalyptus finger-joint board as the base material was studied.The Box-Behnken Design (BBD) method was adopted to establish the response surface model of the hot-pressing process [23][24][25].In the correlation analysis and establishment of the response surface model, the relationship between the hot-pressing parameters and the surface properties of the DBF board was obtained.The paper aims to optimize the process parameters of the hot pressing of DBF board, thereby promoting the value of bamboo products and expanding the application of bamboo in the furniture industry.

Materials
A 4-year-old moso bamboo (Phyllostachys edulis (Carr.)J.Houz.) was used as the raw material of the DBFs and was produced in Anji, Zhejiang Province, China.The bamboo culm was cut into a bamboo tube with a length of 1 meter and classified according to the diameter of the bamboo tube.A bamboo tube with a 10-12 cm diameter was selected for this study.The density and average thickness of the bamboo culms were 0.783 g/cm 3 and 10 mm, respectively.Then, the bamboo tube was manufactured into DBFs according to the steps in Figure 1.The moisture content of the DBFs was 7%, and the dimension of a single bamboo filament was 100 mm (length) × 3 mm (width) × 1.2 mm (thickness).The eucalyptus finger-joint plate served as the foundational material for the DBF board, which was purchased from Shandong Laidian Technology Co., Ltd., Liaocheng, China.The moisture content and density of the eucalyptus finger-joint were 10% and 0.485 g/cm 3 , respectively.The glue weight per unit area, pre-curing degree and thickness of the melamine-modified urea formaldehyde resin (MUF)-impregnated adhesive paper were 51 g/m 2 , 45% and 0.3 mm, respectively (purchased from Hubei Honglian Industrial Co., Ltd., Xiaogan, China).

Experimental Design
The DBF, eucalyptus finger-joint, and MUF-impregnated adhesive paper were formed and hot-pressed according to Figure 1.The Box-Behnken response surface methodology was employed to investigate the impact of three factors (hot-pressing time, temperature and pressure) on the response variables, namely, surface bonding strength, wettability and changes in surface color, respectively.Based on the previous research, the range and level of the three factors of hot pressing were set and are shown in Table 1.In the design of the experiment, two repeated experiments were selected as the central point.
The test was conducted in triplicate for each set of test parameters, ensuring parallelism.The experiment was designed using Design Expert software V8.0.6 (Stat-Ease Inc., Minneapolis, MN, USA), and the results were analyzed to establish the data model.A universal mechanical testing machine (WDW-100, Jinan Huaheng Equipment Co., Ltd., Jinan, China) was used to test the surface bonding strength.The test procedure was conducted in accordance with the Chinese National Standard for wood-based panels and surface-decorated wood-based panels (GB/T 17657-2022) [26].The sample size was 50 mm × 50 mm (length × width), and six parallel tests were performed in each group, and the failure location was at the glue bond.Before the test, the specimens were placed at 20 ± 2 °C and relative humidity (RH) of 65 ± 2% for 14 days.The mechanical loading speed was set to 2.0 mm/min.The specimens were tested, as shown in Figure 2. The test result was calculated by Equation ( 1) where σ stands for the surface bonding strength (MPa), Fmax is the maximum failure load (N), and A is a constant of 1000 mm 2 , which represents the joint area of the abrasive tool and the sample.

Contact Angle Test
According to the test requirements, the BDF board was manufactured in small pieces, and the dimensions were 50 mm × 50 mm (length × width).After being placed in an environment with a temperature of 20 ± 2 °C and RH of 65 ± 2% for 14 days, the state of the specimens reached a unified level.Four specimens were selected for each group.Five points were randomly selected for each specimen, and the average value was taken as the test value.The wettability of the DBF board surface was tested with deionized water (polar liquid) using a contact angle measuring instrument (DSA 100, Kruss, Germany).A microsyringe was used to drop the droplet on the test specimen, and the image of the droplet on the test specimen's surface was captured in real time using the video measurement and control system.The water drop was 4 µL, and the temperature was 20 ± 2 °C during the test.The data collection began as soon as the test droplets were detected to have touched the material surface.Typically, 0-3 data frames are collected as test results [27,28].All the data were collected within 5 s from the starting point of measurement.The contact angle was calculated using the system's software after completing the test.

Surface Color Test
A colorimeter (CM-600d, Konica Minolta, Tokyo, Japan) was used to determine the related colorimetric indexes of the DBF board in each group, and the DBF without hot pressing was used as the control group.Before the test, the light source of the color meter was adjusted to a D65 light source and a 10 normal observer.The test aperture and measurement geometry were set to 11 mm and 45°/0, respectively.The sample size was 100 mm × 100 mm (length × width).Three samples were randomly selected; each sample was measured 10 times, and the average value was recorded as the test result.All samples were placed in the same temperature and humidity environment (65 ± 2% 20 ± 2 °C, RH 65 ± 2%) for 2 weeks before testing.The L*, a* and b* components were used for color evaluation in the CIE laboratory system, and described the chromatic coordinates on lightness and the green-red and blue-yellow axes, respectively.The ΔE* represented the color difference value, which was an important indicator to quantify the color change before and after hot pressing.The calculation formula is shown below: where ΔL*, Δa* and Δb* represent the differences between the DBF board and DBF in lightness and the green-red and blue-yellow axes, respectively.

Response Surface Test
The temperature, time and pressure of the hot-pressing parameters were optimized, and the response surface analysis method of three factors and three levels was used to design 14 groups of experiments.The results of the surface bonding strength of the BDF board are shown in Table 2.The group numbers in Figure 3 correspond to those in Table 2. Figure 3a is the comparison of surface bond strength between groups.Figure 3b1   Design-Expert V8.0.6 software was used to regression-fit the response value and various factors, and the following regression equation was obtained: In the formula, Y (surface bonding strength) is the response value, and the three independent variables are the actual value of the hot-pressing temperature (X1), time (X2) and pressure (X3), respectively.The mathematical model was subjected to variance analysis, and the correlation analysis between the independent variables and response values is shown in Table 3.As can be seen from Table 3, the model F-value of 6.34 implies the model is significant.The p-value (p = 0.0455) less than 0.0500 indicates the model terms were significant, and the p-values of X1, X3, X1X2 and X1X3 were 0.0279, 0.0120, 0.0280 and 0.0383, respectively, and were significant model terms.According to the significance analysis, the influence order of the single factors of the hot-pressing element was pressure (X3), temperature (X1) and time (X2) and of the interactive factors was temperature and time (X1X2), temperature and pressure (X1X3), and time and pressure (X2X3).The reason for this result was that the pre-cured MUF resin had good fluidity and could be fully combined with bamboo and eucalyptus in the time range of 150-180 s, so the significance of the hot-pressing time could not be reflected in the model analysis [22,29].In addition, the p-values of all the quadratic terms were greater than 0.05, showing no significance.
The "lack-of-fit F-Value" of 10.61 implies the lack of fit was insignificant relative to the pure error.There was a 22.11% chance that a "lack-of-fit F-Value" this large could occur.The "lack-of-fit F-Value" was insignificant, indicating that the regression equation was simulated well.The correlation coefficient R 2 and adjusted R 2 were 0.9345 and 0.7871, respectively.The results show that the model could be applied to the actual production of BDF board.
The response analysis for the interaction is shown in Figure 4. Figure 4a,c,e are the contours of the hot-pressing temperature and time, hot-pressing temperature and pressure, and hot-pressing time and pressure, respectively.Figure 4b,d,f are response surface 3D diagrams, which correspond to the contour map.The slope and interval of the contour lines in Figure 4a,c change significantly, showing that the interaction factors play a significant role.Similarly, the results are also consistent with the significance analysis in Table 3.The p-values of X1X2 (temperature and time) and X1X3 (temperature and pressure) were 0.028 and 0.0383, respectively, which was less than 0.05.The pvalue of X2X3 (time and pressure) was 0.3136, which is considered insignificant.In contrast, the spacing between contour lines in Figure 4e is relatively flat, which indicates that time and pressure were non-significant on the factors influencing the surface bonding strength.This is also consistent with the non-significant results in Table 3.
From the 3D response surface diagram, it can be seen that the slope in Figure 4b,d is obvious.In contrast, the slope in Figure 4f is relatively gentle, indicating that the DBF board's surface bonding strength value fluctuates greatly with the changes in hot-pressing temperature and time and the interaction factors between hot-pressing temperature and pressure.Therefore, the performance is consistent with the results of the data analysis.

Optimization Process Prediction and Verification by Response Surface Method
In order to reduce energy consumption and improve production efficiency, the conditions of the response surface method optimization process were set to the minimum value of temperature, time and pressure in the range that was the most suitable production optimization scheme.The optimal process parameters obtained using Design Expert software V8.0.6 were as follows: hot-pressing temperature, 130 °C; hot-pressing time, 165 s; and hot-pressing pressure 2 Mpa; and the corresponding predicted surface bonding strength was 1.60 MPa.
The accuracy of the model was measured by verifying the accuracy of the predicted process parameters.According to the predicted parameters, the hot-pressing process operation of the BDF board was carried out, and tests were conducted three times.The average measured value, relative error, actual value and predicted value are listed in Table 4.It can be seen from Table 4 that the average value of the surface bonding strength of the DBF board prepared according to the optimized process parameters was 1.58 MPa, which was rather close to the predicted value of 1.60 MPa.In addition, the comparative result also shows that the relative error between the measured value and the predicted value was less than 5%, and is considered to have the accuracy of the predicted value.Therefore, the model can predict the surface bonding strength of the DBF board well.

Analysis of Surface Wettability
Bamboo is a hydrophilic biomass material; therefore, the BDF board has a hygroscopic property in application [30].Its surface wettability is determined by changes in its chemical composition and is usually expressed in terms of the contact angle.A larger contact angle indicates a greater surface hydrophobicity.The contact angle of liquid on the surface of the BDF board can directly reflect the wettability of the BDF board.The contact angle of water with different groups of hot-pressing parameters is shown in Figure 5, and the corresponding data are presented in Table 5.As seen from the above chart, the contact angle of water on the bamboo surface fluctuated with the change in hot-pressing parameters.The contact angle of the control group was 64.46°.The surface contact angle of the BDF board ranged from 67.32 to 84.01, which was higher than that of the control group.The maximum contact angle was 84°, which resulted from the hot-pressing temperature, pressure and time of 140 °C, 180 S and 4 MPa, respectively.
The variance analysis of the response surface quadratic model was carried out with the test results of Table 5, and the results are shown in Table 6.It can be seen from Table 6 that the model p-value of the contact angle (p = 0.4868) is greater than 0.0500, which indicates that the model is not significant.In addition, it can be seen from Table 6 that the pvalue of single factor and interaction factors are greater than 0.05, which is not significant.Therefore, the response surface model cannot be applied to this study.The contact angle of water on the bamboo surface fluctuated with the change in hotpressing parameters.Meanwhile, the results show that hot pressing could reduce the surface wettability of bamboo filaments.The reasons can be attributed to the bamboo surface's chemical composition and structure.Firstly, the hemicellulose and lignin in bamboo were degraded and crosslinked by the heat energy of the pressing machine, which resulted in the reduction of hydroxyl and carbonyl to different degrees [31,32].Secondly, the bamboo filament was taken from the inner wall of the bamboo tube, mainly composed of fiber cells and parenchyma cells.The fiber cells exhibited superior mechanical attributes.However, the parenchyma cells were different from the fiber cells, akin to porous foam materials, which were easily densified by pressure [33].Consequently, it was more difficult for water to penetrate the bamboo, resulting in an increased contact angle.In order to further explore the correlation between the hot-pressing process and contact angle variation, the response surface method was used to analyze the data.The results show no significant relationship between the hot-pressing parameters and the contact angle in the range of this test parameter.

Analysis of Surface Color
The color of bamboo is one of the important evaluation indicators of decorative material, and it is also the most vivid and active factor in product production and design [34].As the main indicator of color research, the lightness L* ranges from 0 to 100, the smaller value meaning a darker color.The red-green index a* indicates the position of the chromaticity axis from red to green: the smaller the a*, the more pronounced the green, and the larger the a*, the more red the color.The yellow-blue index b* represents the position of the chrominance axis from yellow to blue: the smaller the b*, the more blue the color, and the larger the b*, the more yellow the color [35].In order to investigate the change in surface color before and after hot pressing, the values of L*, a* and b* and chromatic aberration are shown as follows.
As can be seen from Figure 6a, the surface brightness value L* of the BDF board after hot pressing generally decreased, and the value changed slightly.When the hot-pressing parameters were 150 °C, 180 s and 3 MPa, the brightness value changed and decreased by −2.60.In addition, both a* and b* increased to varying degrees.When the hot-pressing parameter was 140 °C, 180 s and 2 MPa, a* increased by 1.01, which was the maximum increase in value.When the hot-pressing parameter was 140 °C, 150 s and 4 MPa, b* increased by 1.84, which was also the maximum.ΔE* is a comprehensive representation index of color change before and after hot pressing.As can be seen from Figure 6b, the color in groups 8, 9 and 13 changed particularly significantly, and the corresponding color difference values were 3.00, 2.47 and 2.35, respectively.According to the changing trend of the test data, the surface of the BDF board was darker than that of the control group, which is due to the degradation of the main chemical composition content after the bamboo was heated.In the process of hot pressing, the main components in the cell wall of the bamboo surface are degraded, and quinone compounds are produced by oxidation.Meanwhile, the degradation of pentosan in the hemicellulose and the change in phenolic substances in the extract also cause a change in color [36][37][38].However, the color changes of the bamboo surface caused by hot pressing cannot be accurately discerned visually, as shown in Figure 6c.It can be seen that the hot-pressing process in this study does not affect a change in bamboo surface color, which indicates that the parameters match the appearance-quality requirements.

Conclusions
The hot-pressing process of BDF board was experimentally designed and analyzed by the response surface method designed by Design Expert V8.0.6 software.The process parameters of the board were optimized and verified.The hot-pressing temperature (X1), time (X2) and pressure (X3) were independent variables, and surface bonding strength was a dependent variable.The quadratic response surface regression equation model of the surface bonding strength of the BDF board was obtained, which was as follows: Y = 0.00055X1 2 + 0.000255556X2 2 + 0.0275X3 2 − 0.000683333X1X2 + 0.00925X1X3 − 0.00233333X2X3 − 0.06175X1 + 0.018583X2 − 0.98125X3 + 5.17750 The p-value was 0.0455 (p < 0.05), indicating that the model was significant in this study.The independent variable of hot-pressing time, all the quadratic terms, and interactions between time and pressure were negligible.The hot-pressing time set in this study is reasonable.The MUF resin with a pre-curing degree of 50% has good fluidity within this interval.
The optimum process parameters of the DBF board were 130 °C, 165 s and 2.00 MPa, and the surface bonding strength was 1.58 MPa.The relative error between the measured value and the predicted value was less than 5%, indicating that the model has excellent predictability and can be applied to the actual production of DBF board.
In order to further study the surface properties of the DBF board, the surface wettability and color tests were also carried out.The contact angle of bamboo filaments after hot pressing was higher than that without hot pressing.However, there was no correlation between wettability and the hot-pressing parameters.Bamboo filaments will degrade and discolor uncontrollably under the influence of heat during hot pressing, destroying the decorative effect.Therefore, the setting area of the hot-pressing temperature in this study was kept at a low level.By comparing the color changes, there was no effect on the change in bamboo surface color.This indicates that the temperature range selected in this study meets the requirements of surface color control in production.
Overall, this study optimized the parameters of the DBF board process to guide production efficiently.These parameters can be used as the research basis of this kind of material in the direction of coating and color control applications.In this study, the position and thickness of bamboo decoration materials were almost kept at the same level, so the influence of density and thickness on surface bonding strength was not considered.The influence of this aspect on the formation and failure behavior of the bonded interface will be researched in the future.

Figure 1 .
Figure 1.The hot-pressing process of DBF board.

Figure 2 .
Figure 2. The surface bonding strength test.
,b2 represent the adhesive interface failure diagram of the same specimen, as do Figure 3c1,c2.

Figure 3 .
Figure 3.The comparison of surface bond strength between groups and display of failure specimens.(a) the results of the surface bonding strength test, (b1-c2) failure specimens.Error bars show standard deviation.

Figure 4 .
Figure 4. Response surface and contour plot on the surface bonding strength.(a,c,e) are contour plots of AB, AC and BC, respectively.And (b,d,f) are the response surface diagrams of AB, AC and BC, respectively.

Figure 5 .
Figure 5. Contact angle of DBF board and bamboo filament.Error bars show standard deviation.

Figure 6 .
Figure 6.The L*, a*, b* and ΔE of DBF board.Error bars show standard deviation.(a) the L*, a*, b* of DBF Board, (b) ΔE of DBF board.Error bars show standard deviation.

Table 1 .
The experimental design of response surface test.

Table 3 .
Analysis of variance for developed regression equation for surface bonding strength.

Table 4 .
The comparison of actual and predicted values.

Table 5 .
Changes in contact angle of different groups of hot-pressing parameters.

Table 6 .
Analysis of variance for developed regression equation for surface wettability.