Formaldehyde Release from Plywood Manufactured with Two Types of Urea Formaldehyde Resins after Fire Retardant Treatment of Veneers

In the present study, it was aimed to investigate the formaldehyde release of plywood panels manufactured from beech, poplar, alder and scots pine veneers treated with 5 % aqueous solutions of commonly used fi re retardants: zinc borate, boric acid, monoammonium phosphate and ammonium sulfate. Two types of urea formaldehyde (UF) resin with different free formaldehyde ratios (0.16 % and 0.20 %) in adhesive were used as adhesive. Formaldehyde release of plywood panels was determined according to fl ask method described in EN 717-3 standard. As a result of this study, it was found that formaldehyde release from panels produced by beech, poplar, alder and scots pine veneers treated with zinc borate and boric acid were higher than those of control panels, while lower formaldehyde release was obtained for panels treated with monoammonium phosphate and ammonium sulfate. This is valid for all four wood species. Treatment of monoammonium phosphate and ammonium sulfate caused considerable reduction in formaldehyde emission from manufactured plywood panels. In some usage areas, where high strength properties are not expected, plywood panels manufactured from veneers treated with monoammonium phosphate and ammonium sulfate may be used for reducing formaldehyde release.

Ključne riječi: oslobađanje formaldehida, ploča od uslojenog drva, furnir, usporivač gorenja, urea-formaldehid Plywood is preferred as constructional material and has conventionally played an important role in light frame construction.Plywood and other woodbased materials are extensively used in the production of furniture, engineered fl ooring, housing, and other industrial materials (Bohm et al., 2012).However, the usage and application areas of plywood are limited since the plywood is a fl ammable material.Therefore, there has been much interest in the fi re-retardant-treatment of wood-based panels (Cheng and Wang, 2011).The plywood panels treated with fi re retardant chemicals are extensively used.Especially, they are generally preferred in furniture industry and construction applications (Tanritanir and Akbulut, 1999;Winandy, 2001;Ayrilmis et al., 2006).
The wooden materials treated with fi re retardant chemicals enable an applicable alternative to conventional non-combustible products, where a higher level of fi re safety is necessary or desirable (White and Mitchell, 1992).Boron compounds are known as one of the best fi re retardant chemicals due to their benefi cial effects like neutral pH, protective effi ciency, and less effect on mechanical strength than the others (Levan and Tran, 1990).Also, phosphorus-containing compounds like mono-and di-ammonium phosphates are considered very effective fi re retardant chemicals, so they have been preferred for wooden and wood-based products for quite a long time (Grexa et al., 1999).
Formaldehyde has a high risk level and it is a potential human carcinogen, so it is categorized more distinctly than most other pollutants (Salem et al., 2013).Also, this chemical has adverse health effects such as eye and respiratory irritation, irritability, inability to concentrate and sleepiness (Milota, 2000;Colak and Colakoglu, 2004).The formaldehyde, one of the most signifi cant sources, is made by people, e.g.releases from automotive exhaust not fi tted with catalytic converters.The formaldehyde is also released industrially in large quantities and utilized in many processes.The products containing formaldehyde like resins, glues, insulating materials, oriented strand board (OSB), plywood, and fabrics are the major anthropogenic sources with a serious impact on people in the indoor environment (Uchiyama et al., 2007).During recent years, many studies have evaluated the effects of press conditions like press temperature and time, mat moisture content, lower-mo-lecular-weight UF resins, and formaldehyde scavengers for formaldehyde release, because wood-based panels are one of the sources of the possible formaldehyde release, and investigated the production of various woodbased panels bonding with low-formaldehyde and nonformaldehyde resins (Minemura, 1976;Hao and Liu, 1993;Grigoriou, 2000;Wiglusz et al., 2002;Wang et al., 2003;Aydin et al., 2006;Wang et al., 2007Wang et al., , 2008)).Moreover, it was stated that plywood manufactured by adding borax (Na 2 B 4 O 7 10-H 2 O) (Colak and Colakoglu, 2004) and polyvinyl acetate (Kim and Kim, 2005) to the glue mixture caused the reduction of formaldehyde release.It was also determined that formaldehyde release decreased by laminating wood-based composite panel surfaces with decorative vinyl fi lm and melamine-impregnated paper (Groah et al., 1984;Nemli and Colakoglu, 2005).Also, studies have been made with tannin extracted from the bark of wattle (Vazquez et al., 2000;Kim et al., 2003;Santana et al., 1995;Pizzi and Scharfetter, 1978), acacia (Pizzi, 2000;Jahanshahee et al., 2010;Jahanshahee et al., 2012) and starch (Farag, 1995;Yoshida et al., 2005;Turunen et al., 2003;Basta et al., 2006), mangrove (Sowunmi, 2000) in co-condensed resins with phenol and formaldehyde.
The cost was a signifi cant issue in evaluating a wood adhesive in specifi c implementations, its technical properties and gluing behaviour in the past.However, the environmental and health aspects of the adhesive itself have gained more signifi cance during recent years (Aydin et al., 2006).However, few published papers describing techniques for reducing formaldehyde release as an environmental pollutant from woodbased panels, such as plywood, are available.In this study, the effect of fi re retardant chemicals on reducing formaldehyde release from plywood was investigated.

MATERIJALI I METODE
In this experimental study, 2 mm-thick rotary cut veneers with the dimensions of 500 mm by 500 mm were obtained from beech (Fagus orientalis Lipsky), poplar (Populus deltoides I-77/51 clone), alder (Alnus glutinosa subsp.barbata) and scots pine (Pinus sylvestris L.) logs.A laboratory scale rotary type peeler (Valette&Garreau -Vichy, France), with a maximum horizontal holding capacity of 800 mm, was used for manufacturing veneer.While the alder and poplar veneers were manufactured from freshly cut logs, beech and spruce logs were steamed for 12 h before veneer production.The main function of steam heating is to soften the veneer log temporarily, making it more plastic, pliable, more readily peeled, and improving the quality and quantity of the material recovered from the log (Baldwin, 1995).Therefore, beech and spruce logs were steamed to make easier the rotary cutting, whereas alder and poplar veneers were manufactured from freshly cut logs because alder and poplar logs can be peeled freshly.The horizontal opening between knife and nosebar was 85 % of the veneer thickness, and the vertical opening was 0.5 mm in rotary cutting process.The veneers were then dried to 6-8 % moisture content in a veneer dryer.After drying, veneer sheets were treated with some fi re retardant chemicals.For this aim, 5 % aqueous solutions of zinc borate, boric acid, monoammonium phosphate (MAP) and ammonium sulfate were used.The veneers were subjected to re-drying process at 110 °C after they being immersed in the fi re retardant solution for 20 min.The retention level for each treatment solution was calculated with the following equation, and they are presented in Table 1.
Where R -Retention level, kg/m 3 ; G -grams of treatment solution absorbed by the sample; C -grams of preservative or preservative solution in 100 g treatment solution; V -volume of sample in cm 3 .
Three-ply-plywood panels, 6 mm thick, were manufactured by using two types of urea formaldehyde resin with different free formaldehyde ratios.The formulations of adhesive mixtures used for plywood manufacturing are given in Table 2. Veneer sheets were conditioned to approximately 5-7 % moisture content in a climatization chamber before gluing.The glue mixture was applied at a rate of 160 g/m 2 to the single surface of veneer by using a fourroller glue spreader.Hot press pressure was 12 kg/ cm 2 for alder and beech and 8 kg/cm 2 for scots pine and poplar panels, while hot pressing time and temperature were 6 min and 110 ºC, respectively.Two replicate panels were manufactured for each test group.
Formaldehyde release of plywood panels was determined according to fl ask method described in EN 717-3 standard.This is a simple and inexpensive method for testing formaldehyde release and suitable for testing of uncoated boards (Aydin et al., 2006).In this method, test pieces of known mass are suspended over water in a closed container at constant temperature (40 °C).The formaldehyde released from the test pieces is absorbed by the water and determined photometrically (Sundman et al., 2007).The temperature and time were 40 °C and 3 h, respectively, in determining the formaldehyde release.The test apparatus shown in Figure 1 was used for the determination of formaldehyde release from plywood panels (in milligrams per 100 gram of oven-dry panel).

REZULTATI I RASPRAVA
Formaldehyde release test results are presented in Fig. 2 and Fig. 3.As can be seen, the poplar plywood panels had the highest formaldehyde release for both UF types.The highest formaldehyde release of poplar plywood panels may be due to high permeability of veneers.The lowest formaldehyde release was found with alder for UF1 and beech for UF2.Also, the formaldehyde release results obtained from UF2 were higher than those obtained from UF1.In literature, it was stated that formaldehyde release increased with increasing free formaldehyde ratio in adhesive (Roffael, 1982).
The treatment processes with fi re retardant chemicals evidently affected the formaldehyde release of the panels.Monoammonium phosphate and ammonium sulfate showed a decreasing effect on the formaldehyde release, whereas zinc borate and boric acid showed an increasing effect for both UF types.The lowest formaldehyde release values were obtained for plywood treated with ammonium sulfate.Treatment with ammonium sulfate decreased the formaldehyde release values of the panels produced from treated veneers by 69.53 % and 71.65 % for beech, 18.41 %, and 74.43 % for poplar, 55.94 % and 74.74 % for alder, 19.61 % and 68.70 % for Scots pine panels bonded with UF1 and UF2, respectively.
During the hot pressing, ammonium sulfate and monoammonium phosphate were partially decomposed and produced ammonium in gas.Gao et al., (2015) stated that the released ammonium gas would react with free formaldehyde to produce hexamethylenetetramine, which would be stable in cured glue line and was probably related to the reduction of formaldehyde release levels.In their study, they observed that ammonium pentaborate caused decreasing of free formaldehyde content and formaldehyde release levels, which were mostly reduced by 79.0 % and 81.4 %, respectively (Gao et al., 2015).In literature, it is also stated that the released ammonium could produce N-H functional groups on the surface of the veneer sheets, which contributes to reducing the formaldehyde release (Zhang et al., 2013;Schroder et al., 2001;Wen et al., 2006).Also, Zhang et al., (2013) found for plywood panels that formaldehyde release values decreased with cold-ammonia plasma pretreated veneer sheets and stated that this could result from some etching effect and a large number of free radicals generated after the cold-ammonia plasma treatment, which could develop the wetting and interfacial contact between the UF resin and the veneer sheets (Rehn et al., 2003;Wolkenhauer et al., 2008;Blanchard et al., 2009).The development of the wetting and contact increased the shear strength and obstructed the channels preventing the release of formaldehyde (Zhang et al., 2013).Some studies have also indicated that, when wood was treated with ammonium acetate solution, the formaldehyde release from the wood composites was reduced (Colak et al., 2002;Myers, 1986).Aydin (2004) stated that the ammonium acetate behaves as a formaldehyde scavenger especially when urea-formaldehyde glue was used as adhesive in the manufacturing of wood composites.In addition, Junyou and Shengyou (2010) found that the formaldehyde release from poplar plywood was signifi cantly decreased with the addition of ammonia (Junyou and Shengyou, 2010).Wang et al., (2010) stated that combinations of ammonia and sodium sulfi te as formaldehyde scavengers had positive effects on the formaldehyde release of plywood panels.
The boron compounds used in this study increased the formaldehyde release of all wood species for both UF types.Colak and Colakoglu (2004) found that the boric acid increased the formaldehyde release of the panels and explained it by the fact that acetic acid arisen from boric acid reacted with free formaldehyde in the resin.Due to the increase of pH values of veneers, the ability of UF resins to undergo hydrolyses in acidic environment may have decreased (Colak and Colakoglu, 2004;Pizzi, 1989).Also, Demir et al., (2014) found that the zinc borate increased the formaldehyde release of the panels.

ZAKLJUČAK
This study investigated the effect of fi re retardant chemicals on the formaldehyde release of plywood panels.As a result of this study, for both UF types, formaldehyde release contents of the panels produced from veneers treated with zinc borate and boric acid were higher than those of the control panels, while lower formaldehyde release values were obtained for the panels treated with monoammonium phosphate and ammonium sulfate when compared to the control panels.UF2 type resin, which had a high free formaldehyde ratio, gave higher release values than UF1 type resin.Treatment of monoammonium phosphate and ammonium sulfate caused considerable reduction in formaldehyde release from manufactured plywood panels.In some usage areas, where high strength properties are not expected, plywood panels manufactured from veneers treated with monoammonium phosphate and ammonium sulfate may be used for reducing formaldehyde release.