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Glue wood veneer to wood-fiber–high-density-polyethylene composite
International Journal of Adhesion and Adhesives 95 (2019) 102444
Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Glue wood veneer to wood-fiber–high-density-polyethylene composite ∗
T
Yanan Sun, Limin Guo, Yinan Liu, Weihong Wang , Yongming Song Key Lab of Bio-Based Material Science & Technology of Education Ministry, Northeast Forestry University, Harbin, 150040, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Wood fiber High-density polyethylene Wood veneer Decoration
Wood-fiber-reinforced thermoplastic plastic composite is a type of environmentally friendly material. However, its application in furniture and interior decoration is limited due to its lack of wood texture. In this work, woodveneer-facing decoration was explored to beautify the composite's appearance. Surface planning and slight sanding were applied to wood-fiber–high-density-polyethylene (WF-HDPE) composite. Polyvinyl acetate emulsion was used as adhesive, and the veneer-faced WF-HDPE composite floor was prepared by the hot-pressing method. The bonding line was analyzed with Fourier-transform infrared spectroscopy and scanning electron microscopy. Planning plus sanding treatment removed the HDPE film on the composite surface and exposed the wood fibers to react with the adhesive. The rough surface provided all decorative samples bonding strengths higher than 0.9 MPa. The bond line could stand 63 °C water soaking for 3 h without delamination. The hotpressing temperature exhibited a significant effect on surface bonding strength. The optimized hot-pressing technique included a hot-pressing temperature of 80 °C, a hot-pressing time of 10–20 min, and a hot-pressing pressure of 0.2 MPa. With this combination, the surface-decorative veneer retained the beauty of its original color.
1. Introduction Wood-plastic composite (WPC) is produced by mixing, melting, and usually extruding a mixture of recycled polyolefin plastic and biomass material, such as waste wood and crop straw. It has the advantages of anticorrosion, reuse of recycled materials, excellent mechanical properties, low hydroscopicity, high-dimensional stability, and machinability that is similar to that of wood [1,2]. However, WPCs lack wood texture and have a monotone color. They do not have satisfactory tactile and visual perception of wood. These limitations limit its application in the fields of interior decoration and furniture manufacturing. Thus, the advantages of eco-environmental protection and product performance from this material cannot be fully achieved. Polyethylene (PE) and polypropylene (PP) are commonly used plastic matrices in WPCs. They enrich the surface during the extrusion process. Thus, the crystallinity of the WPC surface is high and the surface energy low. Usually, the excellent wettability and diffusion of the adhesive to the bonded material are pre-conditions of forming highstrength bonding. However, the WPC surface is smooth and compact, and its wettability poor [3,4]. This makes it difficult to bond to itself and other materials, particularly to solid wood materials [3–5]. Related research has focused mainly on small areas of adhesion. For example, Di et al. improved the bonding quality of WPC parts by
∗
Corresponding author. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.ijadhadh.2019.102444
Available online 24 September 2019 0143-7496/ © 2019 Elsevier Ltd. All rights reserved.
applying a plasma, a coupling agent, and a liquid-phase oxidation method to the PE composite [6–8] and explained the principle of action of the epoxy resin. Cheng et al. researched the bond strength of polyvinyl/wood-flour composite in a small area and analyzed the curing reaction of the adhesive. They suggested that the bonding quality of epoxy resin is better than that of acrylic ester [9]. Wu et al. [10] explored the method for bonding 50 × 50-mm [2] veneers to WPC pieces. They pointed out that a special adhesive must be applied. However, these adhesives are expensive and not commonly used in the traditional wood industry for laminating, e.g., laminating wood veneer with particleboard or fiberboard. Researchers have shown that an increase in material surface energy can improve the coating property, such as by surface grinding and corona treatment [11] and chromic-acid plasma treatment [12]. Chromic-acid oxidation and water infiltration can increase the specific surface area of wood flour and enhance the bonding effect of epoxy resin to the composite [13]. Atomic-force-microscopy and contact-angle measurements showed that PE and PP composites are rough on the surface [14] after chromic-acid, flame, and sanding treatment [15]. The surface energy is enhanced, and the shear strength of the coating and bonding is increased [14,15]. The bonding quality is better for the composites containing a high wood-flour content [16]. These methods may contribute to decorating WPCs.
International Journal of Adhesion and Adhesives 95 (2019) 102444
Y. Sun, et al.
Orthogonal experimental design was adopted to explore the impact of hot-pressing temperature, hot-pressing time, and veneer species on the surface bonding strength of the WF-HDPE composite. All the factors and their levels are listed in Table 1, and the combination of these factors is listed in Table 2. In the present study, the highest hot-pressing temperature was fixed at 90 °C because too much vapor would evaporate from the adhesive at high temperature, which would result in the separation of the surface veneer from the substrate. Another reason is that the WF-HDPE composite would deform at high temperature. More importantly, no discoloration occurred on the surface-decoration veneers at these temperatures.
In addition to bonding with adhesive, fusion welding can bond thermoplastic polymer composite parts [17]. Similar to that, adhesion could be achieved by using the thermoplastic as an adhesive. Highdensity-polyethylene (HDPE) plastic film has an adhesive ability that is comparable with that of urea resin. Wang et al. [18–20] pressed plywood by using an HDPE plastic film as the adhesive. Excellent water resistance can be obtained by this method [21]. To obtain a better bond strength, high-temperature and silane-coupling-agent treatment must be applied to enhance the compatibility of the veneer with HDPE. However, for WPCs facing decoration, a continuous high temperature and pressure will lead to pyrolysis and discoloration of wooden decorations on the surface layer and the softening deformation of the WPC substrate. Therefore, it is necessary to establish a convenient bonding technology for the surface decoration of wood-plastic composites. In the present work, we propose a simple method for decorating WPC materials. The WPC surface was first roughened for easy decoration. Polyvinyl acetate emulsion, commonly used in the wood industry, was employed as adhesive. The veneers were laminated with a WPC at a temperature not higher than 90 °C. In this way, the deformation of thermoplastic matrices would be avoided, the discoloration of surfacedecoration materials would not occur, and, moreover, the method would be easy to adopt by the traditional wood industry. Surface bonding strength and water resistance were evaluated. The main purpose of the work reported herein is to develop the feasibility of conventional hot-pressing and adhesion technology to decorating wood plastic composites.
2.3. Performance test 2.3.1. Chemical property of the WF-HDPE composite surface The surface of the WF-HDPE composite was analyzed by Fouriertransform infrared (FTIR, Nicolet 6700, Thermo Scientific, USA) spectroscopy. Two scans were obtained for each sample. Specimens were scanned between 4000 and 400 cm−1 at 4-cm−1 resolution. The tests were performed at a speed of 40 times/min. 2.3.2. Micro-morphology of the veneer-covered WF-HDPE composite The bond line between the WF-HDPE composite substrate and surface veneer was analyzed by scanning electron microscopy (SEM, Quanta200, FEI, The Netherlands). Before testing, the veneer-covered WF/HDPE samples were frozen by liquid nitrogen and then broken. The fracture surface was observed. Additionally, the sanded and un-sanded surfaces of the WF-HDPE composite were also observed by SEM.
2. Materials and methods 2.1. Materials
2.3.3. Surface bonding strength test After curing, the veneer-covered composite was set in room temperature at least 72 h before testing. The surface bonding strength was tested using an electronic universal mechanical testing machine (CMMJ5504, Shenzhen, China) according to the standard GB/T151042006, “Decorative veneered wood-based panel.” Samples were cut to 50 × 50 × 9 mm3 with a middle circle area of 1000 mm2 as shown in the study of Li et al.[21] The loading speed for separating the surface veneer and substrate WF/HDPE composite was 2 mm/s.
Wood-fiber–HDPE (WF-HDPE) composite lumber for decoration was manufactured by the extrusion method. Its width was 152 mm and the thickness 25 mm. It contained a WF content of 70 wt%, an HDPE content of 26 wt%, and a maleic anhydride grated polyethylene (MAPE) content of 4%. Three kinds of wood veneer were used to face the WF-HDPE composite floor. Two were black walnut and Northeast China ash, and the third was technical wood, a man-made teak veneer. All these veneers, provided by Jiangsu Tubaobao novel material company, Suqian, China, were 0.25 mm thick and backed with non-woven cloth. The adhesive used to adhere the wood veneer consisted of two components: The main agent, polyvinyl acetate emulsion, and the crosslinking agent, isocyanate. It was purchased from Jilin Zhenlong Material Company, Jilin, China. This adhesive is often used in the wood industry for covering particleboard or fiberboard with wood veneer.
2.3.4. Water resistance of bond line For evaluating the water resistance of the bond line, samples of size 75 × 75 × 9 mm3 were immersed in water for 3 h at 63 °C and then dried at 63 °C in an oven for 3 h. After treatment, delimitation between the surface veneer and WF/HDPE composite substrate was measured. Usually, the delamination length of each side should not be longer than 25 mm according to the requirement of GB/T15104-2006.
2.2. Preparation of the veneer-faced WF-HDPE composite 3. Results and discussion 2.2.1. Pre-treatment to the WF-HDPE composite Surface roughening is an easy way to change the surface morphology and chemical property in practical production. In this study, the surface of the WF-HDPE composite was first planed with a planer to remove the surface plastic layer. Then, the planed surface was sanded using sandpapers (80 mesh plus 100 mesh) for further polishing. Finally, the sanded surface was cleaned with acetone. The treated substrate was placed in a chemical hood for some time at room temperature for acetone volatilizing.
3.1. Surface bonding strength of the veneer-faced WF-HDPE composite As shown in Table 2, the surface bonding strength of three types of veneers exceeded 0.9 MPa, which met the requirement of at least 0.4 MPa as regulated by GB/T15104-2006. As mentioned in the Introduction, an HDPE-enriched surface of WF-HDPE is not conducive to bonding. The data in Table 2 indicate planing plus sanding treatment could solve this problem and provide good bonding conditions, yielding excellent bonding strength. The range analysis in Table 3 shows that factor A (hot-pressing temperature) is the major factor that affects the bonding strength, and this effect is significant (Table 4). An increase in hot-pressing temperature from 70 °C to 80 °C favored bonding. However, a high temperature (90 °C) reduced the surface bonding strength. This is determined by the characteristics of polyvinyl acetate, which cannot withstand high temperature.
2.2.2. Bonding veneer onto the WF-HDPE composite surface The polyvinyl acetate main agent and isocyanate cross-linking agent were uniformly mixed in a ratio of 100:12. The mixture was then brushed onto the treated surface of the WF-HDPE composite in an amount of 110 g/m2. The veneer was laid onto the coated surface, and then the assembly was hot-pressed at 0.2 MPa in a hot press for adhesive curing. The decorated WF-HDPE composite is shown in Fig. 1. 2
International Journal of Adhesion and Adhesives 95 (2019) 102444
Y. Sun, et al.
Fig. 1. Veneer-faced WF-HDPE composite lumber. Table 1 Level of hot-pressing temperature and time and type of decoration veneer. Level
Table 3 Range analysis of surface bonding strength of the veneer-faced WF-HDPE composite.
Factor
Factor
1 2 3
A
B
C
Hot-pressing temperature (°C)
Hot-pressing time (min)
Type of veneer
70 80 90
10 15 20
Technical wood Black walnut Northeast China ash
A B C
Sum of index
Average of index
Range
K1
K2
K3
k1
k2
k3
2.76 3.19 3.57
3.82 3.32 3.31
3.49 3.56 3.19
0.92 1.06 1.19
1.27 1.11 1.10
1,16 1.19 1.06
0.35 0.13 0.13
Table 4 Variance analysis of surface bonding strength of the veneer-faced WF-HDPE composite.
Although an extension of the hot-pressing time (factor B) increased the surface bonding strength, variance analysis shows that the effect of B is insignificant (Table 4). The type of veneer also presents an insignificant effect. A possible reason for this behavior is that three types of veneers have the same backing, non-woven fabric, and little difference exists between the characteristics of the bonded surface. The best combination is A2B3C1 with a hot-pressing temperature of 80 °C and hot-pressing time of 20 min, which yields the highest surface bonding strength of the technical-wood-veneer-faced WF-HDPE composite.
Sources of variation
Sj
fj
Mean sum of square
F value
Critical value F(fj,fe) (a = 0.05)
Significance
A B C Error Total
0.20 0.02 0.03 0.01 0.25
2 2 2 2 8
0.10 0.01 0.01 0.00
30.23 3.62 3.88
4.46 4.46 4.46
Significant Insignificant Insignificant
3.3. Water resistance of veneer bonding to the WF/HDPE composite
3.2. Morphology of bond line between WF/HDPE composite and wood veneer
As shown in Table 5, four types of hot-pressing conditions presented no delamination. Under a proper hot-pressing condition, the technical wood and Northeast China ash veneer achieved the effect without cracking or with slight cracking. Although black walnut veneer shows higher surface bonding strength (Table 2), it delaminated from the WF/ HDPE substrate after soaking-drying treatment with water. This may be due to the larger shrinkage of the walnut veneer compared with the other two veneers. The facing effect of black walnut is poorer, especially for a low hot-pressing temperature and a short hot-pressing time. At lower temperatures, 70 °C and 80 °C, it took a longer time to finish the reaction and form a strong combination. At 90 °C, the hotpressing duration could be shortened, which will improve production efficiency.
The broken cross-section of the veneer-covered WF/HDPE sample was observed with SEM. As Fig. 2 shows, the veneers were tightly attached on the WF-HDPE composite. No gap exists between the three types of veneers and the WF-HDPE substrate. Massive adhesion was deposited in the veneer because the veneer permeability is better than that of the polished WF-HDPE composite. Therefore, during hot pressing, most resin permeates into the veneer, combining wood fibers extruded from the WF-HDPE composite surface with the wood veneer. In addition, a sanded surface is rougher than an un-sanded surface. This is beneficial to increasing the surface area for bonding and forming mechanical anchoring.
Table 2 Surface bonding strength of the veneer-faced WF-HDPE composite. Number
A Hot-pressing temperature (°C)
B Hot-pressing time (min)
C Type of veneer
Surface bonding strength (MPa)
1 2 3 4 5 6 7 8 9
70 70 70 80 80 80 90 90 90
10 15 20 10 15 20 10 15 20
Technical wood Black walnut Northeast China ash Black walnut Northeast China ash Technical wood Northeast China ash Technical wood Black walnut
0.92 0.93 0.91 1.18 1.19 1.45 1.09 1.2 1.2
3
International Journal of Adhesion and Adhesives 95 (2019) 102444
Y. Sun, et al.
Table 5 Delimitation length between veneer and WF-HDPE composite after soaking. Number
1 2 3 4 5 6 7 8 9
Hot-pressing temperature (°C)
Hotpressing time (min)
Delimitation length (mm)
70 70 70 80 80 80 90 90 90
10 15 20 10 15 20 10 15 20
6
Technical wood
Black walnut
Northeast China ash
35 0 42 22 0 0 0 22
3.4. Effect of surface treatment on surface characteristics of the WF-HDPE composite Fig. 3 shows that the WF-HDPE composite without treatment possesses a smooth and rich HDPE film surface, indicating the difficulty of liquid penetration. Thus, the WF/HDPE composite is not conducive to getting wet and forming adhesion. After planing and sanding treatment, the plastic layer on the surface was removed, and the interior wood fibers exposed. Many ravines also exist on the surface due to sanding. All these characteristics have a positive effect on forming adhesion. Fig. 4 shows that the WF-HDPE composite before treatment has strong characteristic absorption bands at 2914.71 and 2847.51 cm−1, which are attributed to the symmetrical and asymmetrical stretching vibration absorption of –CH2, respectively [22–24]. A strong in-plane bending vibration band exists at ∼1458.12 cm−1. These results indicate that the surface of the WF-HDPE composite is mainly composed of HDPE [25]. After mechanical abrasion, the intensities of the above two characteristic absorption bands are weakened. Meanwhile, characteristic absorption bands of cellulose, hemicelluloses, and lignin became more obvious. For example, the band at ∼3344.78 cm−1 corresponds to the O–H stretching vibration and that at 1729.86 cm−1 to the C]O stretching vibration [22–24]. A vibration band of the benzene-ring carbon skeleton and a stretching vibration band of C–O of the lignin appear at 1592.79 and 1237.66 cm−1, respectively [23]. The polar groups can enhance the wettability and reaction with the adhesive. This suggests that it is necessary to abrade the WF-HDPE composite mechanically before bonding and facing. In a combined surface bond strength and water resistance test, it is proved that planing and subsequent slight sanding can yield a good surface for adhesive bonding. 4. Conclusions Three types of decorative veneer were studied as surface-decoration materials. Facing was conducted on WF-HDPE composite lumber that contained 70% wood flour. The surface bonding strength test, water resistance of the bond line, surface morphology, and chemical characteristic analysis yielded the following conclusions. (1) For WF/HDPE composites, after the surface was planed and sanded, the wood characteristics on the surface increased significantly. Excellent surface bonding strength could be obtained by polyvinyl acetate adhesive with the common aqueous polymer isocyanate. (2) The surface bonding strength of three types of veneer-faced WFHDPE composites was higher than 0.9 MPa. When the hot-pressing temperature was 70 °C and 80 °C, hot-pressing duration should be prolonged to 15–20 s; when hot-pressed at 90 °C, 10–15 min should be optimum to achieve relatively good bonding performance. (3) Suitable surface treatment allowed for wood-plastic composites to
Fig. 2. Adhesion interface of the veneer-faced WF-HDPE composite (hotpressing temperature of 80 °C).
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International Journal of Adhesion and Adhesives 95 (2019) 102444
Y. Sun, et al.
Fig. 3. Surface morphology of the WF-HDPE composite surface before (left) and after (right) abrading and sanding treatment. Plastics, Addit Compd 2005;7:20–6. [2] Huang Z, Cheng XC, Zhao YF, Xue T. The development and application of WPC. Int Wood Ind 2008:38–40. [3] Test methods for characterization of WPC materials and products. European Committee for Standardization; 2007. 40P.;A4. [4] Zhang XM. Low carbon economy will lead the vigorous development of straw board industry. China Wood-Based Panels 2011;18:134–5. [5] Wang WH. Development of wood plastic composites in the United States. Foreign Forest Product Industries; 2003. p. 19–22. [6] Tao Y. Study on durability of bonding joint for plasma treated wood/polyethylene composites. Northeast Forestry University; 2012. [7] Teng XL. Study on durability of bonding joint for liquid oxidation treated wood/ polyethylene composites. Northeast Forestry University; 2012. [8] Wang H. Study on coupling agent coating treatment of polyethylene wood/plastic composites and its adhesion properties. Northeast Forestry University; 2013. [9] Li Y. Study on bonding properties of PVC wood plastic composites. Northeast Forestry University; 2010. [10] Wu Y, Mao ZN, Wang F, Zhang XJ, Wu ZH. Initial study on veneer overlaying technology for wood-plastic composites. China For. Sci. Technol. 2013;27:92–4. [11] Akhtarkhavari A, Kortschot MT, Spelt JK. Adhesion and durability of latex paint on wood fiber reinforced polyethylene. Prog Org Coat 2004;49:33–41. [12] Gupta BS, Laborie MPG. Surface activation and adhesion properties of wood-fiber reinforced thermoplastic composites. J Adhes 2007;83:939–55. [13] Gramlich W, Gardner D, Neivandt D. Surface treatments of wood–plastic composites (WPCs) to improve adhesion. J Adhes Sci Technol 2006;20:1873–87. [14] ArndtWolkenhauer GeorgAvramidis, EvelynHauswald HolgerMilitz. WolfgangViöl: plasma treatment of wood–plastic composites to enhance their adhesion properties. J Adhes Sci Technol 2008;22:13. [15] Oporto G, Gardner D, GeorgeBernhardt Neivandt D. Characterizing the mechanism of improved adhesion of modified wood plastic composite (WPC) surfaces. J Adhes Sci Technol 2007;21:1097–116. [16] Boeglin N, Masson D, Pizzi A. Interfacial mechanical bonding by SEM of wood and plastic composites. Holz Roh Werkst 1996;54:48. [17] Yousefpour A, Hojjati M, Immarigeon J-P. Fusion bonding/welding of thermoplastic composites. J Thermoplast Compos Mater 2004;17:303–41. [18] Wang Z, Guo WJ: The production process of environmental friendly plywood. [19] Chang L, Wang Z, Guo WJ, Gao L, Ren YP, Fan LF: An enviroment friendly bamboo plywood for decoration. [20] Chang L, Wang Z, Guo WJ, Ren YP. Study on hot-press process factors of wood/ plastic composite plywood. Wood Process. Mach. 2009;20:12–5. [21] Guo LM, Wang WH, Wang QW, Yan N. Decorating wood flour/HDPE composites with wood veneers. Polym Compos 2018;39:1144–51. [22] Baeza J, Freer J. Chemical characterization of wood and its components. 2000. [23] Pandey KK. A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy. J Appl Polym Sci 2015;71:1969–75. [24] Stark NM, Matuana LM. Characterization of weathered wood–plastic composite surfaces using FTIR spectroscopy, contact angle, and XPS. Polym Degrad Stab 2007;92:1883–90. [25] Colom X, Carrasco F, Pagès P, Cañavate J. Effects of different treatments on the interface of HDPE/lignocellulosic fiber composites. Compos Sci Technol 2003;63:161–9.
Fig. 4. FTIR spectra of the WF-HDPE composite surface before and after polishing.
apply a cover process similar to a wood-based panel, yielding a high surface bonding strength and high water resistance. The qualified surface decoration provides great possibilities for the potential application of wood–plastic materials in the field of furniture manufacturing and interior decoration. Acknowledgments We thank Natural Science Foundation of China (31670573), Heilongjiang Natural Science Foundation Key Project (ZD2016002), and Central University Fundamental Research Project (2572017ET05) for their supporting this research. We also thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript. References [1] Markarian J. Wood-plastic composites: current trends in materials and processing.
5
Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: www.elsevier.com/locate/ijadhadh
Glue wood veneer to wood-fiber–high-density-polyethylene composite ∗
T
Yanan Sun, Limin Guo, Yinan Liu, Weihong Wang , Yongming Song Key Lab of Bio-Based Material Science & Technology of Education Ministry, Northeast Forestry University, Harbin, 150040, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Wood fiber High-density polyethylene Wood veneer Decoration
Wood-fiber-reinforced thermoplastic plastic composite is a type of environmentally friendly material. However, its application in furniture and interior decoration is limited due to its lack of wood texture. In this work, woodveneer-facing decoration was explored to beautify the composite's appearance. Surface planning and slight sanding were applied to wood-fiber–high-density-polyethylene (WF-HDPE) composite. Polyvinyl acetate emulsion was used as adhesive, and the veneer-faced WF-HDPE composite floor was prepared by the hot-pressing method. The bonding line was analyzed with Fourier-transform infrared spectroscopy and scanning electron microscopy. Planning plus sanding treatment removed the HDPE film on the composite surface and exposed the wood fibers to react with the adhesive. The rough surface provided all decorative samples bonding strengths higher than 0.9 MPa. The bond line could stand 63 °C water soaking for 3 h without delamination. The hotpressing temperature exhibited a significant effect on surface bonding strength. The optimized hot-pressing technique included a hot-pressing temperature of 80 °C, a hot-pressing time of 10–20 min, and a hot-pressing pressure of 0.2 MPa. With this combination, the surface-decorative veneer retained the beauty of its original color.
1. Introduction Wood-plastic composite (WPC) is produced by mixing, melting, and usually extruding a mixture of recycled polyolefin plastic and biomass material, such as waste wood and crop straw. It has the advantages of anticorrosion, reuse of recycled materials, excellent mechanical properties, low hydroscopicity, high-dimensional stability, and machinability that is similar to that of wood [1,2]. However, WPCs lack wood texture and have a monotone color. They do not have satisfactory tactile and visual perception of wood. These limitations limit its application in the fields of interior decoration and furniture manufacturing. Thus, the advantages of eco-environmental protection and product performance from this material cannot be fully achieved. Polyethylene (PE) and polypropylene (PP) are commonly used plastic matrices in WPCs. They enrich the surface during the extrusion process. Thus, the crystallinity of the WPC surface is high and the surface energy low. Usually, the excellent wettability and diffusion of the adhesive to the bonded material are pre-conditions of forming highstrength bonding. However, the WPC surface is smooth and compact, and its wettability poor [3,4]. This makes it difficult to bond to itself and other materials, particularly to solid wood materials [3–5]. Related research has focused mainly on small areas of adhesion. For example, Di et al. improved the bonding quality of WPC parts by
∗
Corresponding author. E-mail address: [email protected] (W. Wang).
https://doi.org/10.1016/j.ijadhadh.2019.102444
Available online 24 September 2019 0143-7496/ © 2019 Elsevier Ltd. All rights reserved.
applying a plasma, a coupling agent, and a liquid-phase oxidation method to the PE composite [6–8] and explained the principle of action of the epoxy resin. Cheng et al. researched the bond strength of polyvinyl/wood-flour composite in a small area and analyzed the curing reaction of the adhesive. They suggested that the bonding quality of epoxy resin is better than that of acrylic ester [9]. Wu et al. [10] explored the method for bonding 50 × 50-mm [2] veneers to WPC pieces. They pointed out that a special adhesive must be applied. However, these adhesives are expensive and not commonly used in the traditional wood industry for laminating, e.g., laminating wood veneer with particleboard or fiberboard. Researchers have shown that an increase in material surface energy can improve the coating property, such as by surface grinding and corona treatment [11] and chromic-acid plasma treatment [12]. Chromic-acid oxidation and water infiltration can increase the specific surface area of wood flour and enhance the bonding effect of epoxy resin to the composite [13]. Atomic-force-microscopy and contact-angle measurements showed that PE and PP composites are rough on the surface [14] after chromic-acid, flame, and sanding treatment [15]. The surface energy is enhanced, and the shear strength of the coating and bonding is increased [14,15]. The bonding quality is better for the composites containing a high wood-flour content [16]. These methods may contribute to decorating WPCs.
International Journal of Adhesion and Adhesives 95 (2019) 102444
Y. Sun, et al.
Orthogonal experimental design was adopted to explore the impact of hot-pressing temperature, hot-pressing time, and veneer species on the surface bonding strength of the WF-HDPE composite. All the factors and their levels are listed in Table 1, and the combination of these factors is listed in Table 2. In the present study, the highest hot-pressing temperature was fixed at 90 °C because too much vapor would evaporate from the adhesive at high temperature, which would result in the separation of the surface veneer from the substrate. Another reason is that the WF-HDPE composite would deform at high temperature. More importantly, no discoloration occurred on the surface-decoration veneers at these temperatures.
In addition to bonding with adhesive, fusion welding can bond thermoplastic polymer composite parts [17]. Similar to that, adhesion could be achieved by using the thermoplastic as an adhesive. Highdensity-polyethylene (HDPE) plastic film has an adhesive ability that is comparable with that of urea resin. Wang et al. [18–20] pressed plywood by using an HDPE plastic film as the adhesive. Excellent water resistance can be obtained by this method [21]. To obtain a better bond strength, high-temperature and silane-coupling-agent treatment must be applied to enhance the compatibility of the veneer with HDPE. However, for WPCs facing decoration, a continuous high temperature and pressure will lead to pyrolysis and discoloration of wooden decorations on the surface layer and the softening deformation of the WPC substrate. Therefore, it is necessary to establish a convenient bonding technology for the surface decoration of wood-plastic composites. In the present work, we propose a simple method for decorating WPC materials. The WPC surface was first roughened for easy decoration. Polyvinyl acetate emulsion, commonly used in the wood industry, was employed as adhesive. The veneers were laminated with a WPC at a temperature not higher than 90 °C. In this way, the deformation of thermoplastic matrices would be avoided, the discoloration of surfacedecoration materials would not occur, and, moreover, the method would be easy to adopt by the traditional wood industry. Surface bonding strength and water resistance were evaluated. The main purpose of the work reported herein is to develop the feasibility of conventional hot-pressing and adhesion technology to decorating wood plastic composites.
2.3. Performance test 2.3.1. Chemical property of the WF-HDPE composite surface The surface of the WF-HDPE composite was analyzed by Fouriertransform infrared (FTIR, Nicolet 6700, Thermo Scientific, USA) spectroscopy. Two scans were obtained for each sample. Specimens were scanned between 4000 and 400 cm−1 at 4-cm−1 resolution. The tests were performed at a speed of 40 times/min. 2.3.2. Micro-morphology of the veneer-covered WF-HDPE composite The bond line between the WF-HDPE composite substrate and surface veneer was analyzed by scanning electron microscopy (SEM, Quanta200, FEI, The Netherlands). Before testing, the veneer-covered WF/HDPE samples were frozen by liquid nitrogen and then broken. The fracture surface was observed. Additionally, the sanded and un-sanded surfaces of the WF-HDPE composite were also observed by SEM.
2. Materials and methods 2.1. Materials
2.3.3. Surface bonding strength test After curing, the veneer-covered composite was set in room temperature at least 72 h before testing. The surface bonding strength was tested using an electronic universal mechanical testing machine (CMMJ5504, Shenzhen, China) according to the standard GB/T151042006, “Decorative veneered wood-based panel.” Samples were cut to 50 × 50 × 9 mm3 with a middle circle area of 1000 mm2 as shown in the study of Li et al.[21] The loading speed for separating the surface veneer and substrate WF/HDPE composite was 2 mm/s.
Wood-fiber–HDPE (WF-HDPE) composite lumber for decoration was manufactured by the extrusion method. Its width was 152 mm and the thickness 25 mm. It contained a WF content of 70 wt%, an HDPE content of 26 wt%, and a maleic anhydride grated polyethylene (MAPE) content of 4%. Three kinds of wood veneer were used to face the WF-HDPE composite floor. Two were black walnut and Northeast China ash, and the third was technical wood, a man-made teak veneer. All these veneers, provided by Jiangsu Tubaobao novel material company, Suqian, China, were 0.25 mm thick and backed with non-woven cloth. The adhesive used to adhere the wood veneer consisted of two components: The main agent, polyvinyl acetate emulsion, and the crosslinking agent, isocyanate. It was purchased from Jilin Zhenlong Material Company, Jilin, China. This adhesive is often used in the wood industry for covering particleboard or fiberboard with wood veneer.
2.3.4. Water resistance of bond line For evaluating the water resistance of the bond line, samples of size 75 × 75 × 9 mm3 were immersed in water for 3 h at 63 °C and then dried at 63 °C in an oven for 3 h. After treatment, delimitation between the surface veneer and WF/HDPE composite substrate was measured. Usually, the delamination length of each side should not be longer than 25 mm according to the requirement of GB/T15104-2006.
2.2. Preparation of the veneer-faced WF-HDPE composite 3. Results and discussion 2.2.1. Pre-treatment to the WF-HDPE composite Surface roughening is an easy way to change the surface morphology and chemical property in practical production. In this study, the surface of the WF-HDPE composite was first planed with a planer to remove the surface plastic layer. Then, the planed surface was sanded using sandpapers (80 mesh plus 100 mesh) for further polishing. Finally, the sanded surface was cleaned with acetone. The treated substrate was placed in a chemical hood for some time at room temperature for acetone volatilizing.
3.1. Surface bonding strength of the veneer-faced WF-HDPE composite As shown in Table 2, the surface bonding strength of three types of veneers exceeded 0.9 MPa, which met the requirement of at least 0.4 MPa as regulated by GB/T15104-2006. As mentioned in the Introduction, an HDPE-enriched surface of WF-HDPE is not conducive to bonding. The data in Table 2 indicate planing plus sanding treatment could solve this problem and provide good bonding conditions, yielding excellent bonding strength. The range analysis in Table 3 shows that factor A (hot-pressing temperature) is the major factor that affects the bonding strength, and this effect is significant (Table 4). An increase in hot-pressing temperature from 70 °C to 80 °C favored bonding. However, a high temperature (90 °C) reduced the surface bonding strength. This is determined by the characteristics of polyvinyl acetate, which cannot withstand high temperature.
2.2.2. Bonding veneer onto the WF-HDPE composite surface The polyvinyl acetate main agent and isocyanate cross-linking agent were uniformly mixed in a ratio of 100:12. The mixture was then brushed onto the treated surface of the WF-HDPE composite in an amount of 110 g/m2. The veneer was laid onto the coated surface, and then the assembly was hot-pressed at 0.2 MPa in a hot press for adhesive curing. The decorated WF-HDPE composite is shown in Fig. 1. 2
International Journal of Adhesion and Adhesives 95 (2019) 102444
Y. Sun, et al.
Fig. 1. Veneer-faced WF-HDPE composite lumber. Table 1 Level of hot-pressing temperature and time and type of decoration veneer. Level
Table 3 Range analysis of surface bonding strength of the veneer-faced WF-HDPE composite.
Factor
Factor
1 2 3
A
B
C
Hot-pressing temperature (°C)
Hot-pressing time (min)
Type of veneer
70 80 90
10 15 20
Technical wood Black walnut Northeast China ash
A B C
Sum of index
Average of index
Range
K1
K2
K3
k1
k2
k3
2.76 3.19 3.57
3.82 3.32 3.31
3.49 3.56 3.19
0.92 1.06 1.19
1.27 1.11 1.10
1,16 1.19 1.06
0.35 0.13 0.13
Table 4 Variance analysis of surface bonding strength of the veneer-faced WF-HDPE composite.
Although an extension of the hot-pressing time (factor B) increased the surface bonding strength, variance analysis shows that the effect of B is insignificant (Table 4). The type of veneer also presents an insignificant effect. A possible reason for this behavior is that three types of veneers have the same backing, non-woven fabric, and little difference exists between the characteristics of the bonded surface. The best combination is A2B3C1 with a hot-pressing temperature of 80 °C and hot-pressing time of 20 min, which yields the highest surface bonding strength of the technical-wood-veneer-faced WF-HDPE composite.
Sources of variation
Sj
fj
Mean sum of square
F value
Critical value F(fj,fe) (a = 0.05)
Significance
A B C Error Total
0.20 0.02 0.03 0.01 0.25
2 2 2 2 8
0.10 0.01 0.01 0.00
30.23 3.62 3.88
4.46 4.46 4.46
Significant Insignificant Insignificant
3.3. Water resistance of veneer bonding to the WF/HDPE composite
3.2. Morphology of bond line between WF/HDPE composite and wood veneer
As shown in Table 5, four types of hot-pressing conditions presented no delamination. Under a proper hot-pressing condition, the technical wood and Northeast China ash veneer achieved the effect without cracking or with slight cracking. Although black walnut veneer shows higher surface bonding strength (Table 2), it delaminated from the WF/ HDPE substrate after soaking-drying treatment with water. This may be due to the larger shrinkage of the walnut veneer compared with the other two veneers. The facing effect of black walnut is poorer, especially for a low hot-pressing temperature and a short hot-pressing time. At lower temperatures, 70 °C and 80 °C, it took a longer time to finish the reaction and form a strong combination. At 90 °C, the hotpressing duration could be shortened, which will improve production efficiency.
The broken cross-section of the veneer-covered WF/HDPE sample was observed with SEM. As Fig. 2 shows, the veneers were tightly attached on the WF-HDPE composite. No gap exists between the three types of veneers and the WF-HDPE substrate. Massive adhesion was deposited in the veneer because the veneer permeability is better than that of the polished WF-HDPE composite. Therefore, during hot pressing, most resin permeates into the veneer, combining wood fibers extruded from the WF-HDPE composite surface with the wood veneer. In addition, a sanded surface is rougher than an un-sanded surface. This is beneficial to increasing the surface area for bonding and forming mechanical anchoring.
Table 2 Surface bonding strength of the veneer-faced WF-HDPE composite. Number
A Hot-pressing temperature (°C)
B Hot-pressing time (min)
C Type of veneer
Surface bonding strength (MPa)
1 2 3 4 5 6 7 8 9
70 70 70 80 80 80 90 90 90
10 15 20 10 15 20 10 15 20
Technical wood Black walnut Northeast China ash Black walnut Northeast China ash Technical wood Northeast China ash Technical wood Black walnut
0.92 0.93 0.91 1.18 1.19 1.45 1.09 1.2 1.2
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International Journal of Adhesion and Adhesives 95 (2019) 102444
Y. Sun, et al.
Table 5 Delimitation length between veneer and WF-HDPE composite after soaking. Number
1 2 3 4 5 6 7 8 9
Hot-pressing temperature (°C)
Hotpressing time (min)
Delimitation length (mm)
70 70 70 80 80 80 90 90 90
10 15 20 10 15 20 10 15 20
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Technical wood
Black walnut
Northeast China ash
35 0 42 22 0 0 0 22
3.4. Effect of surface treatment on surface characteristics of the WF-HDPE composite Fig. 3 shows that the WF-HDPE composite without treatment possesses a smooth and rich HDPE film surface, indicating the difficulty of liquid penetration. Thus, the WF/HDPE composite is not conducive to getting wet and forming adhesion. After planing and sanding treatment, the plastic layer on the surface was removed, and the interior wood fibers exposed. Many ravines also exist on the surface due to sanding. All these characteristics have a positive effect on forming adhesion. Fig. 4 shows that the WF-HDPE composite before treatment has strong characteristic absorption bands at 2914.71 and 2847.51 cm−1, which are attributed to the symmetrical and asymmetrical stretching vibration absorption of –CH2, respectively [22–24]. A strong in-plane bending vibration band exists at ∼1458.12 cm−1. These results indicate that the surface of the WF-HDPE composite is mainly composed of HDPE [25]. After mechanical abrasion, the intensities of the above two characteristic absorption bands are weakened. Meanwhile, characteristic absorption bands of cellulose, hemicelluloses, and lignin became more obvious. For example, the band at ∼3344.78 cm−1 corresponds to the O–H stretching vibration and that at 1729.86 cm−1 to the C]O stretching vibration [22–24]. A vibration band of the benzene-ring carbon skeleton and a stretching vibration band of C–O of the lignin appear at 1592.79 and 1237.66 cm−1, respectively [23]. The polar groups can enhance the wettability and reaction with the adhesive. This suggests that it is necessary to abrade the WF-HDPE composite mechanically before bonding and facing. In a combined surface bond strength and water resistance test, it is proved that planing and subsequent slight sanding can yield a good surface for adhesive bonding. 4. Conclusions Three types of decorative veneer were studied as surface-decoration materials. Facing was conducted on WF-HDPE composite lumber that contained 70% wood flour. The surface bonding strength test, water resistance of the bond line, surface morphology, and chemical characteristic analysis yielded the following conclusions. (1) For WF/HDPE composites, after the surface was planed and sanded, the wood characteristics on the surface increased significantly. Excellent surface bonding strength could be obtained by polyvinyl acetate adhesive with the common aqueous polymer isocyanate. (2) The surface bonding strength of three types of veneer-faced WFHDPE composites was higher than 0.9 MPa. When the hot-pressing temperature was 70 °C and 80 °C, hot-pressing duration should be prolonged to 15–20 s; when hot-pressed at 90 °C, 10–15 min should be optimum to achieve relatively good bonding performance. (3) Suitable surface treatment allowed for wood-plastic composites to
Fig. 2. Adhesion interface of the veneer-faced WF-HDPE composite (hotpressing temperature of 80 °C).
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International Journal of Adhesion and Adhesives 95 (2019) 102444
Y. Sun, et al.
Fig. 3. Surface morphology of the WF-HDPE composite surface before (left) and after (right) abrading and sanding treatment. Plastics, Addit Compd 2005;7:20–6. [2] Huang Z, Cheng XC, Zhao YF, Xue T. The development and application of WPC. Int Wood Ind 2008:38–40. [3] Test methods for characterization of WPC materials and products. European Committee for Standardization; 2007. 40P.;A4. [4] Zhang XM. Low carbon economy will lead the vigorous development of straw board industry. China Wood-Based Panels 2011;18:134–5. [5] Wang WH. Development of wood plastic composites in the United States. Foreign Forest Product Industries; 2003. p. 19–22. [6] Tao Y. Study on durability of bonding joint for plasma treated wood/polyethylene composites. Northeast Forestry University; 2012. [7] Teng XL. Study on durability of bonding joint for liquid oxidation treated wood/ polyethylene composites. Northeast Forestry University; 2012. [8] Wang H. Study on coupling agent coating treatment of polyethylene wood/plastic composites and its adhesion properties. Northeast Forestry University; 2013. [9] Li Y. Study on bonding properties of PVC wood plastic composites. Northeast Forestry University; 2010. [10] Wu Y, Mao ZN, Wang F, Zhang XJ, Wu ZH. Initial study on veneer overlaying technology for wood-plastic composites. China For. Sci. Technol. 2013;27:92–4. [11] Akhtarkhavari A, Kortschot MT, Spelt JK. Adhesion and durability of latex paint on wood fiber reinforced polyethylene. Prog Org Coat 2004;49:33–41. [12] Gupta BS, Laborie MPG. Surface activation and adhesion properties of wood-fiber reinforced thermoplastic composites. J Adhes 2007;83:939–55. [13] Gramlich W, Gardner D, Neivandt D. Surface treatments of wood–plastic composites (WPCs) to improve adhesion. J Adhes Sci Technol 2006;20:1873–87. [14] ArndtWolkenhauer GeorgAvramidis, EvelynHauswald HolgerMilitz. WolfgangViöl: plasma treatment of wood–plastic composites to enhance their adhesion properties. J Adhes Sci Technol 2008;22:13. [15] Oporto G, Gardner D, GeorgeBernhardt Neivandt D. Characterizing the mechanism of improved adhesion of modified wood plastic composite (WPC) surfaces. J Adhes Sci Technol 2007;21:1097–116. [16] Boeglin N, Masson D, Pizzi A. Interfacial mechanical bonding by SEM of wood and plastic composites. Holz Roh Werkst 1996;54:48. [17] Yousefpour A, Hojjati M, Immarigeon J-P. Fusion bonding/welding of thermoplastic composites. J Thermoplast Compos Mater 2004;17:303–41. [18] Wang Z, Guo WJ: The production process of environmental friendly plywood. [19] Chang L, Wang Z, Guo WJ, Gao L, Ren YP, Fan LF: An enviroment friendly bamboo plywood for decoration. [20] Chang L, Wang Z, Guo WJ, Ren YP. Study on hot-press process factors of wood/ plastic composite plywood. Wood Process. Mach. 2009;20:12–5. [21] Guo LM, Wang WH, Wang QW, Yan N. Decorating wood flour/HDPE composites with wood veneers. Polym Compos 2018;39:1144–51. [22] Baeza J, Freer J. Chemical characterization of wood and its components. 2000. [23] Pandey KK. A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy. J Appl Polym Sci 2015;71:1969–75. [24] Stark NM, Matuana LM. Characterization of weathered wood–plastic composite surfaces using FTIR spectroscopy, contact angle, and XPS. Polym Degrad Stab 2007;92:1883–90. [25] Colom X, Carrasco F, Pagès P, Cañavate J. Effects of different treatments on the interface of HDPE/lignocellulosic fiber composites. Compos Sci Technol 2003;63:161–9.
Fig. 4. FTIR spectra of the WF-HDPE composite surface before and after polishing.
apply a cover process similar to a wood-based panel, yielding a high surface bonding strength and high water resistance. The qualified surface decoration provides great possibilities for the potential application of wood–plastic materials in the field of furniture manufacturing and interior decoration. Acknowledgments We thank Natural Science Foundation of China (31670573), Heilongjiang Natural Science Foundation Key Project (ZD2016002), and Central University Fundamental Research Project (2572017ET05) for their supporting this research. We also thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript. References [1] Markarian J. Wood-plastic composites: current trends in materials and processing.
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