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The reduction of formaldehyde and VOCs emission from wood-based flooring by green adhesive using cashew nut shell liquid (CNSL)
Journal of Hazardous Materials 182 (2010) 919–922
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
The reduction of formaldehyde and VOCs emission from wood-based flooring by green adhesive using cashew nut shell liquid (CNSL) Sumin Kim ∗ Green Building Materials Lab, School of Architecture, Soongsil University, Seoul 156-743, Republic of Korea
a r t i c l e
i n f o
Article history: Received 13 November 2009 Received in revised form 1 March 2010 Accepted 1 March 2010 Available online 7 March 2010 Keywords: Environment-friendly resin Cashew nut shell liquid (CNSL) TVOC Formaldehyde Wood-based flooring
a b s t r a c t To discuss the reduction of formaldehyde and volatile organic compound (VOC) emissions from engineered flooring, cashew nut shell liquid (CNSL)-formaldehyde (CF) resin and CF/PVAc resin were applied for the maple face of the veneer bonding on plywood. The CF resin was used to replace urea-formaldehyde (UF) resin in the formaldehyde-based resin system in order to reduce formaldehyde and VOC emissions from the adhesives used between the plywoods and fancy veneers. For the CF/PVAc resins, 5, 10, 20 or 30% of PVAc was added to the CF resin. The CF/PVAc resins showed better bonding than the commercial natural tannin adhesive with a higher level of wood penetration. The standard formaldehyde emission test and a VOC analyzer were used to determine the formaldehyde and VOC emissions, respectively, from the engineered floorings. The CF resin and CF/PVAc resin systems with UV coating satisfied the E1 and E0 grades of the Korean Standard. TVOC emission was slightly increased by the PVAc addition. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the USA, interior wood-based panel has been adapted to recently enacted regulations by the California Air Resources Board (CARB) that regulate formaldehyde emissions from wood-based panels. The manufacturers in the USA have therefore focused on this issue with most of the attention being directed at UF resin [1]. Various green building rating systems such as the Building Research Establishment Environmental Assessment Method (BREEAM) [2], Green Star from Australia [3], the Comprehensive Assessment System for Building Environmental Efficiency (CASBEE) from Japan [4], the Building and Environmental Performance Assessment Criteria (BEPAC) from Canada [5], and the Leadership in Energy and Environmental Design (LEED) from the United States [6] have addressed ‘low-emitting’ and ‘renewable materials’ for the building materials and adhesives [7]. CNSL, an agricultural by-product of the cashew nut processing industry and a renewable resource, is a source of a long chain, m-substituted phenol which promises to be an excellent monomer for polymer production. CNSL occurs as a reddish brown, viscous fluid in the soft honeycomb structure of the shell
∗ Correspondence address: Tel.: +82 2 820 0665; fax: +82 2 816 3354. E-mail address: [email protected]. 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.03.003
of cashew nuts. Many researchers have investigated its extraction, chemistry and composition [8]. CNSL contains four major components: 3-pentadecenyl phenol (cardanol), 5-pentadecenyl resorcinol (cardol), 6-pentadecenyl salicylic acid (anacardic acid) and 2-methyl, 5-pentadecenyl resorcinol (2-methyl cardol) [9]. The application of CNSL as a full replacement for synthetic resins is of immense interest in these days of diminishing petroleum reserves. About 90% consists of anacardic acid, cardanol and cardol is CNSL [10]. Cardanol, which can be obtained by thermal treatment of CNSL, is a phenol derivative mainly composed of the meta substitute of a C15 unsaturated hydrocarbon chain with one to three double bonds [11,12]. Double vacuum distillation of CNSL yields pure cardanol at 50% yield [13]. CNSL constitutes nearly one-third of the total nut weight. Thus, a large amount of CNSL is formed as a by-product of the mechanical processes used to render the cashew kernel edible and its total production approaches one million tons annually [14]. CNSL has potential industrial applications such as for resins, friction lining materials, and surface coatings. The greatest problem of formaldehyde and TVOC emissions in engineered flooring is caused by the face of the fancy veneer bonding. UF resin is still used in industry in large quantity. The author’s previous work [15] suggested the suitability of Tannin as a renewable resource for surface bonding in engineered flooring. In this study, to reduce formaldehyde and VOC emissions from the resin for bonding the face of the fancy veneer on engineered flooring, the UF resin was replaced by natural CNSL.
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2.2.2. Light microscopy and scanning electron microscopy For light microscopy, small pieces containing a glue line between the maple fancy veneer and plywood were hand sectioned with a sharp razor blade to minimize artificial cell distortions by sectioning with a microtome. The sections were stained with Sudan 4 and toluidine blue to enhance the contrast between the glue line and the wood, respectively, prior to observation with the microscope (DW-BM, Korea) with a photo camera [17].
Fig. 1. Chemical structure of polycardanol.
2. Experimental 2.1. Materials The plywood and decorative veneer used for fabricating the test samples were supplied by Easywood Co., Ltd., South Korea. The decorative veneers were 0.5-mm-thick maple, while 7-mmthick plywoods manufactured in China were used. Their moisture contents were 0.08% and 3.5%, respectively. Polycardanol, which was prepared by enzyme-catalyzed oxidative polymerization, was kindly supplied by Hyundae Paint Co., Ltd., Korea. Fig. 1 depicts the chemical structure of the polycardanol used in this work. The as-received polycardanol was comprised of pre-polymer and thermally curable. Its monomer-to-polymer conversion was 97.7%. Sodium hydroxide pellets and glacial acetic acid were supplied as general laboratory reagents. Acetic anhydride with a density of 1.08 g/cm3 and boiling temperature of 140 ◦ C was used. A formaldehyde solution of 40% weight of solid per unit volume of liquid and hexamine as a hardener were added to make the CF resin. The CF resin was polymerized by adding 1 ml of sodium hydroxide solution to the resin mixture. In the second stage of the experiment, 1 ml of 15% hexamine was also added [10]. Blends of various CF/polyvinyl acetate (PVAc) compositions were prepared. To make the CF/PVAc hybrid resin, 5, 10, 20 or 30 wt % of PVAc was added to CF and the mixture was stirred for 20 min. The engineered floorings for the formaldehyde and VOC emission tests were manufactured using natural tannin adhesive and CF/PVAc hybrid resin, with dimensions of 400 mm × 400 mm × 75 mm (length × width × thickness). After the resin was spread on the plywood, 0.5-mm-thick maple fancy veneer was bonded by cold- and hot-pressing. The pressure-time diagram followed a three-stage schedule, consisting of preliminary cold-pressing at 1 kgf/cm2 for 2 min for assembly, main hot-pressing at 5 kgf/cm2 and 160 ◦ C for 180 s for resin curing, and final cold-pressing at 1 kgf/cm2 for 2 min for resin postcuring.
2.2.3. Formaldehyde and VOC emission The formaldehyde emissions from the engineered floorings bonded with each blend were determined with a desiccator in accordance with the JIS method using a glass desiccator [18]. The VOC analyzer is a portable device to measure the four main aromatic hydrocarbon gases: toluene, ethylbenzene, xylene and styrene. To prepare the samples for the VOC analyzer, the engineered floorings bonded with each blend were conditioned at 25 ◦ C and 50 ± 5% in a thermo-hygrostat for 15 days and then cut into 4 pieces of dimensions 50 mm × 50 mm and placed in a 3 L polyester plastic bag that was sealed with teflon tape, purged 3 times with N2 gas, and then filled with N2 gas by pulling up the plunger. For the blank control, an empty bag with N2 gas was prepared. The gases for the VOC analyzer were collected from the 3 L polyester plastic bag in a gas tight (0.5 cm3 ) manner after 4 days, placed into the VOC analyzer and analyzed. This 4-day period was chosen according to the authors’ previous study for determining the optimum method for VOC emission test by VOC analyzer [19,20]. The measurement procedure comprised three steps. First, the product was inserted into the 3 L polyester plastic bag, after which the plunger was slowly pulled out, pushed in again, and pulled out for the second time before the syringe was removed from the plastic bag. If the top of the syringe was wet, it was wiped dry with a tissue. Second, a dedicated needle was attached, and 0.5 cm3 (1/2 calibration) of the sampled gases was ejected by pushing in the plunger. Finally, the remaining gases were injected into the inlet on the main unit of the VOC analyzer, after which the measurement was automatically started. 3. Results and discussion 3.1. Surface bonding strength The bonding strengths of the engineered flooring samples bonded with CF resin and PVAc at a press temperature and time
2.2. Methods 2.2.1. Surface bonding strength The bonding strength between the fancy veneer and plywood was evaluated with a Universal Testing Machine (UTM, Hounds Co.) in the tensile mode. Samples were cut at 50 mm × 50 mm (length × width) and a knife mark of 20 mm × 20 mm was made on the surface (fancy veneer face) to a depth of 0.5 mm. We bonded the upper device of UTM and the surface knife mark with hot-melt adhesive. Another face was bonded to the underneath of the device like the internal bonding strength test specimen. The tests were performed at a crosshead speed of 2 mm/min [16].
Fig. 2. Bonding strength between the face of the fancy veneer and plywood substrate in engineered flooring: CNSL-formaldehyde (CF) resin and CF/PVAc green adhesives.
S. Kim / Journal of Hazardous Materials 182 (2010) 919–922
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Fig. 3. Glue line between maple fancy veneer and plywood as determined by light microscopy.
Fig. 4. Formaldehyde emission from engineered flooring bonded with CNSLformaldehyde (CF) resin and CF/PVAc green adhesives as determined by the desiccator method.
of 160 ◦ C and 180 s, respectively, are shown in Fig. 2. The bonding strengths of the non-treated (before boiling), engineered flooring samples made using CF/PVAc hybrid adhesives were much higher than those of the CF resin. With increasing PVAc content, the bonding strength was increased until a PVAc content of 20%. With further increase in the PVAc content, the bonding strength plateaued. The use of PVAc introduces reactive sites in the CF resin. This high
reactivity of CNSL towards the carbonyl group is due to the miscible blend formed by the water-soluble CF resin and PVAc. The reaction and miscibility between CNSL and PVAc raised the bonding strength above that of the CF resin. The long chains in CNSL impart flexibility due to internal plasticizing, resulting in the formation of soft resins at elevated temperature [10]. The samples were boiled at 60 ± 3 ◦ C for 4 h, dried for 20 h, boiled again for 4 h and finally dried for 3 h to determine the waterproof property of each adhesive system. All the adhesive systems showed better waterproof adhesion property than the commercial UF resin, which was rated at 12 kgf/cm2 in the author’s previous work [17]. The similarity of the surface bonding strength was confirmed by the vivid glue lines shown in all adhesive systems. The glue lines between the maple fancy veneer and plywood by light microscopy are shown in Fig. 3. The voids in the wood near the glue line were filled with resins. Furthermore, the tracheid walls in the outer few layers were also ruptured in the CF/PVAc hybrid resin. The rays were also greatly distorted, with many broken and often compressed at the point of contact with the glue line, thus blocking the glue penetration. The glue penetration into the tracheids was mainly confined to the outermost layer in direct contact with the glue line. 3.2. Formaldehyde and VOC emission Renewable phenolic compounds from trees and plants such as tannin, lignin and CNSL were successfully applied as a thermosetting wood adhesive for wood panels to reduce formaldehyde
Fig. 5. VOC emission concentrations (toluene, ethylbenzene, xylene and styrene) from engineered flooring bonded with CNSL-formaldehyde (CF) resin and CF/PVAc green adhesives as determined by the VOC analyzer.
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emission [15,18]. These renewable phenolic compounds give excellent adhesive performance, good moisture resistance, and tend to give a lower formaldehyde emission than UF resin [17]. Before surface coating, the formaldehyde emissions from the products glued with CF resin was already less than E1 grade (under 1.5 mg/L) of the formaldehyde emission level in the Korean Standard, even though the CF resin contained formaldehyde, as shown in Fig. 4. The CF/PVAc hybrid adhesives samples showed a similar tendency to that of the CF resin. PVAc was not the cause of the increased formaldehyde emission. When the surface was coated with a UV-curable, urethane acrylate coating for flooring, the emission level was reduced to much less than half. In the CF/PVAc resin system, the role of PVAc was to avoid generating formaldehyde emission, by replacement of CNSL with hexamine, and to increase the initial bonding strength through PVAc’s high viscosity and room temperature curable property. The formaldehyde emission level of the coated sample of the CF/PVAc system appeared to satisfy the requirement of E0 grade when the UV coating was applied. Fig. 5 presents the concentrations of the four indicated VOCs from the engineered flooring bonded with each resin system, as determined by the VOC analyzer [19]. For each substance, the concentrations were measured at 96 h after the start of the test. Three different phases are evident in the figure. In the first phase, the concentration in the 3 L polyester bag increased due to the constant emission of organic compounds and was limited by the given air exchange rate. Xylene was the highest detected compound in all samples, followed by ethylbenzene and toluene. Styrene, however, was not detected at all in any of the systems. However, TVOC emission of the PVAc-added CF resin system was slightly increased compare to that of the CF resin. In Korea, the Ministry of Environment provides TVOC guidelines for building materials. Even natural VOCs from wood are considered to be harmful and are included in the TVOC calculation. TVOC was calculated between C6 and C16 [19]. In the VOC analyzer test, however, we defined TVOC as the sum of the four detected main aromatic hydrocarbon gases: toluene, ethylbenzene, styrene and xylene [21,22]. 4. Conclusion The surface bonding strength of the CF/PVAc resins was higher than that of the CF resin. The bonding strength was maximized for the CF/PVAc resin with a PVAc content of 20%. By desiccator method, the formaldehyde emission, which presented similar emission data for the CF and CF/PVAc resins, was lowered with the PVAc addition, and further reduced when coated with a UVcurable urethane acrylate. The formaldehyde emission level was sufficiently reduced to satisfy the E0 grade of the Korean Standard. In the case of the CF/PVAc resin, the VOC emission measured by the VOC analyzer was slightly increased by the PVAc addition. In conclusion, the CF/PVAc resins were successfully applied as environmentally friendly adhesives of surface bonding for manufacturing engineered flooring.
Acknowledgement This research was supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2009-0081944). References [1] T.C. Ruffing, P.M. Smith, N.R. Brown, North American resin suppliers’ green perspectives, in: Biographies & Abstracts of International Conference on Wood Adhesives 2009, Forest Products Society, 2009, p. 4. [2] R. Baldwin, A. Yates, N. Howard, S. Rao, BREEAM (Building Research Establishment Environmental Assessment Method) 98 for Offices, Watford, UK, 1998. [3] GBCA (Green Building Council of Australia), GBCA Website, 2008, Last accessed from http://www.gbca.org.au. [4] CASBEE (Comprehensive Assessment System for Building Environmental Efficiency), CASBEE Website, 2008, Last accessed from http://www.ibec.or.jp/ CASBEE/english/index.htm. [5] R.J. Cole, D. Rousseau, G.T. Theaker, Building Environmental Performance Assessment Criteria (BEPAC), BEPAC Foundation, Vancouver, Canada, 1993. [6] USGBC (United States Green Building Council), LEED—Leadership in Energy and Environmental Design: Green Building Rating System, Version 1.0, US Green Building Council, 1999. [7] D. Castro-Lacouture, J.A. Sefair, L. Florez, A.L. Medaglia, Optimization model for the selection of materials using a LEED-based green building rating system in Colombia, Build. Environ. 44 (2009) 1162–1170. [8] H.P. Bhunia, G.B. Nando, A. Basak, S. Lenka, P.L. Nayak, Synthesis and characterization of polymers from cashew nut shell liquid (CNSL), a renewable resource. III. Synthesis of a polyether, Eur. Polym. J. 35 (1999) 1713–1722. [9] H.P. Bhunia, G.B. Nando, T.K. Chaki, A. Basak, S. Lenka, P.L. Nayak, Synthesis and characterization of polymers from cashew nut shell liquid (CNSL), a renewable resource. II. Synthesis of polyurethanes, Eur. Polym. J. 35 (1999) 1381–1391. [10] L.Y. Mwaikambo, M.P. Ansell, Cure characteristics of alkali catalysed cashew nut shell liquid-formaldehyde resin, J. Mater. Sci. 36 (2001) 3693–3698. [11] R. Ikeda, H. Tanaka, H. Uyama, S. Kobayashi, A new crosslinkable polyphenol from a renewable resource, Macromol. Rapid Commun. 21 (2000) 496–499. [12] Y.H. Kim, E.S. An, S.Y. Park, B.K. Song, Enzymatic epoxidation and polymerization of cardanol obtained from a renewable resource and curing of epoxide-containing polycardanol, J. Mol. Catal. B: Enzym. 45 (2007) 39–44. [13] H.P. Bhunia, R.N. Jana, A. Basak, S. Lenka, G.B. Nando, Synthesis of polyurethane from cashew nut shell liquid (CNSL), a renewable resource, J. Polym. Sci. Polym. Chem. 36 (1998) 391–400. [14] S.Y. Park, Y.H. Kim, B.K. Song, Polymer synthesis by enzyme catalysis, Polym. Sci. Technol. 16 (2005) 342–353. [15] S. Kim, Environment-friendly adhesives for surface bonding of wood-based flooring using natural tannin to reduce formaldehyde and TVOC emission, Bioresour. Technol. 100 (2009) 744–748. [16] S. Kim, H.-J. Kim, Effect of addition of polyvinyl acetate to melamineformaldehyde resin on the adhesion and formaldehyde emission in engineered flooring, Int. J. Adhes. Adhes. 25 (2005) 456–461. [17] S. Kim, H.-J. Kim, G.Z. Xu, Y.G. Eom, Environment-friendly adhesives for fancy veneer bonding of engineered flooring to reduce formaldehyde and TVOC emissions, Mokchae Konghak 35 (2007) 58–66. [18] S. Kim, H.-J. Kim, Evaluation of formaldehyde emission of pine and wattle tannin-based adhesives by gas chromatography, Holz. Roh. Werkst. 62 (2004) 101–106. [19] S. Kim, J.-A. Kim, J.-Y. An, H.-J. Kim, S.D. Kim, J.C. Park, TVOC and formaldehyde emission behaviors from flooring materials bonded with environmentalfriendly MF/PVAc hybrid resins, Indoor Air 17 (2007) 404–415. [20] J.-Y. An, S. Kim, J.-A. Kim, H.-J. Kim, Development of simple test method using VOC analyzer to measure volatile organic compounds emission for particleboards, Mokchae Konghak 34 (2006) 22–31 (in Korean). [21] C.Y. Chao, G.Y. Chan, Quantification of indoor VOCs in twenty mechanically ventilated buildings in Hong Kong, Atmos. Environ. 35 (2001) 5895–5913. [22] A. Katsoyiannis, P. Leva, D. Kotzias, VOC and carbonyl emissions from carpets: a comparative study using four types of environmental chambers, J. Hazard. Mater. 152 (2008) 669–676.
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
The reduction of formaldehyde and VOCs emission from wood-based flooring by green adhesive using cashew nut shell liquid (CNSL) Sumin Kim ∗ Green Building Materials Lab, School of Architecture, Soongsil University, Seoul 156-743, Republic of Korea
a r t i c l e
i n f o
Article history: Received 13 November 2009 Received in revised form 1 March 2010 Accepted 1 March 2010 Available online 7 March 2010 Keywords: Environment-friendly resin Cashew nut shell liquid (CNSL) TVOC Formaldehyde Wood-based flooring
a b s t r a c t To discuss the reduction of formaldehyde and volatile organic compound (VOC) emissions from engineered flooring, cashew nut shell liquid (CNSL)-formaldehyde (CF) resin and CF/PVAc resin were applied for the maple face of the veneer bonding on plywood. The CF resin was used to replace urea-formaldehyde (UF) resin in the formaldehyde-based resin system in order to reduce formaldehyde and VOC emissions from the adhesives used between the plywoods and fancy veneers. For the CF/PVAc resins, 5, 10, 20 or 30% of PVAc was added to the CF resin. The CF/PVAc resins showed better bonding than the commercial natural tannin adhesive with a higher level of wood penetration. The standard formaldehyde emission test and a VOC analyzer were used to determine the formaldehyde and VOC emissions, respectively, from the engineered floorings. The CF resin and CF/PVAc resin systems with UV coating satisfied the E1 and E0 grades of the Korean Standard. TVOC emission was slightly increased by the PVAc addition. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the USA, interior wood-based panel has been adapted to recently enacted regulations by the California Air Resources Board (CARB) that regulate formaldehyde emissions from wood-based panels. The manufacturers in the USA have therefore focused on this issue with most of the attention being directed at UF resin [1]. Various green building rating systems such as the Building Research Establishment Environmental Assessment Method (BREEAM) [2], Green Star from Australia [3], the Comprehensive Assessment System for Building Environmental Efficiency (CASBEE) from Japan [4], the Building and Environmental Performance Assessment Criteria (BEPAC) from Canada [5], and the Leadership in Energy and Environmental Design (LEED) from the United States [6] have addressed ‘low-emitting’ and ‘renewable materials’ for the building materials and adhesives [7]. CNSL, an agricultural by-product of the cashew nut processing industry and a renewable resource, is a source of a long chain, m-substituted phenol which promises to be an excellent monomer for polymer production. CNSL occurs as a reddish brown, viscous fluid in the soft honeycomb structure of the shell
∗ Correspondence address: Tel.: +82 2 820 0665; fax: +82 2 816 3354. E-mail address: [email protected]. 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.03.003
of cashew nuts. Many researchers have investigated its extraction, chemistry and composition [8]. CNSL contains four major components: 3-pentadecenyl phenol (cardanol), 5-pentadecenyl resorcinol (cardol), 6-pentadecenyl salicylic acid (anacardic acid) and 2-methyl, 5-pentadecenyl resorcinol (2-methyl cardol) [9]. The application of CNSL as a full replacement for synthetic resins is of immense interest in these days of diminishing petroleum reserves. About 90% consists of anacardic acid, cardanol and cardol is CNSL [10]. Cardanol, which can be obtained by thermal treatment of CNSL, is a phenol derivative mainly composed of the meta substitute of a C15 unsaturated hydrocarbon chain with one to three double bonds [11,12]. Double vacuum distillation of CNSL yields pure cardanol at 50% yield [13]. CNSL constitutes nearly one-third of the total nut weight. Thus, a large amount of CNSL is formed as a by-product of the mechanical processes used to render the cashew kernel edible and its total production approaches one million tons annually [14]. CNSL has potential industrial applications such as for resins, friction lining materials, and surface coatings. The greatest problem of formaldehyde and TVOC emissions in engineered flooring is caused by the face of the fancy veneer bonding. UF resin is still used in industry in large quantity. The author’s previous work [15] suggested the suitability of Tannin as a renewable resource for surface bonding in engineered flooring. In this study, to reduce formaldehyde and VOC emissions from the resin for bonding the face of the fancy veneer on engineered flooring, the UF resin was replaced by natural CNSL.
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2.2.2. Light microscopy and scanning electron microscopy For light microscopy, small pieces containing a glue line between the maple fancy veneer and plywood were hand sectioned with a sharp razor blade to minimize artificial cell distortions by sectioning with a microtome. The sections were stained with Sudan 4 and toluidine blue to enhance the contrast between the glue line and the wood, respectively, prior to observation with the microscope (DW-BM, Korea) with a photo camera [17].
Fig. 1. Chemical structure of polycardanol.
2. Experimental 2.1. Materials The plywood and decorative veneer used for fabricating the test samples were supplied by Easywood Co., Ltd., South Korea. The decorative veneers were 0.5-mm-thick maple, while 7-mmthick plywoods manufactured in China were used. Their moisture contents were 0.08% and 3.5%, respectively. Polycardanol, which was prepared by enzyme-catalyzed oxidative polymerization, was kindly supplied by Hyundae Paint Co., Ltd., Korea. Fig. 1 depicts the chemical structure of the polycardanol used in this work. The as-received polycardanol was comprised of pre-polymer and thermally curable. Its monomer-to-polymer conversion was 97.7%. Sodium hydroxide pellets and glacial acetic acid were supplied as general laboratory reagents. Acetic anhydride with a density of 1.08 g/cm3 and boiling temperature of 140 ◦ C was used. A formaldehyde solution of 40% weight of solid per unit volume of liquid and hexamine as a hardener were added to make the CF resin. The CF resin was polymerized by adding 1 ml of sodium hydroxide solution to the resin mixture. In the second stage of the experiment, 1 ml of 15% hexamine was also added [10]. Blends of various CF/polyvinyl acetate (PVAc) compositions were prepared. To make the CF/PVAc hybrid resin, 5, 10, 20 or 30 wt % of PVAc was added to CF and the mixture was stirred for 20 min. The engineered floorings for the formaldehyde and VOC emission tests were manufactured using natural tannin adhesive and CF/PVAc hybrid resin, with dimensions of 400 mm × 400 mm × 75 mm (length × width × thickness). After the resin was spread on the plywood, 0.5-mm-thick maple fancy veneer was bonded by cold- and hot-pressing. The pressure-time diagram followed a three-stage schedule, consisting of preliminary cold-pressing at 1 kgf/cm2 for 2 min for assembly, main hot-pressing at 5 kgf/cm2 and 160 ◦ C for 180 s for resin curing, and final cold-pressing at 1 kgf/cm2 for 2 min for resin postcuring.
2.2.3. Formaldehyde and VOC emission The formaldehyde emissions from the engineered floorings bonded with each blend were determined with a desiccator in accordance with the JIS method using a glass desiccator [18]. The VOC analyzer is a portable device to measure the four main aromatic hydrocarbon gases: toluene, ethylbenzene, xylene and styrene. To prepare the samples for the VOC analyzer, the engineered floorings bonded with each blend were conditioned at 25 ◦ C and 50 ± 5% in a thermo-hygrostat for 15 days and then cut into 4 pieces of dimensions 50 mm × 50 mm and placed in a 3 L polyester plastic bag that was sealed with teflon tape, purged 3 times with N2 gas, and then filled with N2 gas by pulling up the plunger. For the blank control, an empty bag with N2 gas was prepared. The gases for the VOC analyzer were collected from the 3 L polyester plastic bag in a gas tight (0.5 cm3 ) manner after 4 days, placed into the VOC analyzer and analyzed. This 4-day period was chosen according to the authors’ previous study for determining the optimum method for VOC emission test by VOC analyzer [19,20]. The measurement procedure comprised three steps. First, the product was inserted into the 3 L polyester plastic bag, after which the plunger was slowly pulled out, pushed in again, and pulled out for the second time before the syringe was removed from the plastic bag. If the top of the syringe was wet, it was wiped dry with a tissue. Second, a dedicated needle was attached, and 0.5 cm3 (1/2 calibration) of the sampled gases was ejected by pushing in the plunger. Finally, the remaining gases were injected into the inlet on the main unit of the VOC analyzer, after which the measurement was automatically started. 3. Results and discussion 3.1. Surface bonding strength The bonding strengths of the engineered flooring samples bonded with CF resin and PVAc at a press temperature and time
2.2. Methods 2.2.1. Surface bonding strength The bonding strength between the fancy veneer and plywood was evaluated with a Universal Testing Machine (UTM, Hounds Co.) in the tensile mode. Samples were cut at 50 mm × 50 mm (length × width) and a knife mark of 20 mm × 20 mm was made on the surface (fancy veneer face) to a depth of 0.5 mm. We bonded the upper device of UTM and the surface knife mark with hot-melt adhesive. Another face was bonded to the underneath of the device like the internal bonding strength test specimen. The tests were performed at a crosshead speed of 2 mm/min [16].
Fig. 2. Bonding strength between the face of the fancy veneer and plywood substrate in engineered flooring: CNSL-formaldehyde (CF) resin and CF/PVAc green adhesives.
S. Kim / Journal of Hazardous Materials 182 (2010) 919–922
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Fig. 3. Glue line between maple fancy veneer and plywood as determined by light microscopy.
Fig. 4. Formaldehyde emission from engineered flooring bonded with CNSLformaldehyde (CF) resin and CF/PVAc green adhesives as determined by the desiccator method.
of 160 ◦ C and 180 s, respectively, are shown in Fig. 2. The bonding strengths of the non-treated (before boiling), engineered flooring samples made using CF/PVAc hybrid adhesives were much higher than those of the CF resin. With increasing PVAc content, the bonding strength was increased until a PVAc content of 20%. With further increase in the PVAc content, the bonding strength plateaued. The use of PVAc introduces reactive sites in the CF resin. This high
reactivity of CNSL towards the carbonyl group is due to the miscible blend formed by the water-soluble CF resin and PVAc. The reaction and miscibility between CNSL and PVAc raised the bonding strength above that of the CF resin. The long chains in CNSL impart flexibility due to internal plasticizing, resulting in the formation of soft resins at elevated temperature [10]. The samples were boiled at 60 ± 3 ◦ C for 4 h, dried for 20 h, boiled again for 4 h and finally dried for 3 h to determine the waterproof property of each adhesive system. All the adhesive systems showed better waterproof adhesion property than the commercial UF resin, which was rated at 12 kgf/cm2 in the author’s previous work [17]. The similarity of the surface bonding strength was confirmed by the vivid glue lines shown in all adhesive systems. The glue lines between the maple fancy veneer and plywood by light microscopy are shown in Fig. 3. The voids in the wood near the glue line were filled with resins. Furthermore, the tracheid walls in the outer few layers were also ruptured in the CF/PVAc hybrid resin. The rays were also greatly distorted, with many broken and often compressed at the point of contact with the glue line, thus blocking the glue penetration. The glue penetration into the tracheids was mainly confined to the outermost layer in direct contact with the glue line. 3.2. Formaldehyde and VOC emission Renewable phenolic compounds from trees and plants such as tannin, lignin and CNSL were successfully applied as a thermosetting wood adhesive for wood panels to reduce formaldehyde
Fig. 5. VOC emission concentrations (toluene, ethylbenzene, xylene and styrene) from engineered flooring bonded with CNSL-formaldehyde (CF) resin and CF/PVAc green adhesives as determined by the VOC analyzer.
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emission [15,18]. These renewable phenolic compounds give excellent adhesive performance, good moisture resistance, and tend to give a lower formaldehyde emission than UF resin [17]. Before surface coating, the formaldehyde emissions from the products glued with CF resin was already less than E1 grade (under 1.5 mg/L) of the formaldehyde emission level in the Korean Standard, even though the CF resin contained formaldehyde, as shown in Fig. 4. The CF/PVAc hybrid adhesives samples showed a similar tendency to that of the CF resin. PVAc was not the cause of the increased formaldehyde emission. When the surface was coated with a UV-curable, urethane acrylate coating for flooring, the emission level was reduced to much less than half. In the CF/PVAc resin system, the role of PVAc was to avoid generating formaldehyde emission, by replacement of CNSL with hexamine, and to increase the initial bonding strength through PVAc’s high viscosity and room temperature curable property. The formaldehyde emission level of the coated sample of the CF/PVAc system appeared to satisfy the requirement of E0 grade when the UV coating was applied. Fig. 5 presents the concentrations of the four indicated VOCs from the engineered flooring bonded with each resin system, as determined by the VOC analyzer [19]. For each substance, the concentrations were measured at 96 h after the start of the test. Three different phases are evident in the figure. In the first phase, the concentration in the 3 L polyester bag increased due to the constant emission of organic compounds and was limited by the given air exchange rate. Xylene was the highest detected compound in all samples, followed by ethylbenzene and toluene. Styrene, however, was not detected at all in any of the systems. However, TVOC emission of the PVAc-added CF resin system was slightly increased compare to that of the CF resin. In Korea, the Ministry of Environment provides TVOC guidelines for building materials. Even natural VOCs from wood are considered to be harmful and are included in the TVOC calculation. TVOC was calculated between C6 and C16 [19]. In the VOC analyzer test, however, we defined TVOC as the sum of the four detected main aromatic hydrocarbon gases: toluene, ethylbenzene, styrene and xylene [21,22]. 4. Conclusion The surface bonding strength of the CF/PVAc resins was higher than that of the CF resin. The bonding strength was maximized for the CF/PVAc resin with a PVAc content of 20%. By desiccator method, the formaldehyde emission, which presented similar emission data for the CF and CF/PVAc resins, was lowered with the PVAc addition, and further reduced when coated with a UVcurable urethane acrylate. The formaldehyde emission level was sufficiently reduced to satisfy the E0 grade of the Korean Standard. In the case of the CF/PVAc resin, the VOC emission measured by the VOC analyzer was slightly increased by the PVAc addition. In conclusion, the CF/PVAc resins were successfully applied as environmentally friendly adhesives of surface bonding for manufacturing engineered flooring.
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