Ghani 2018

Building and Environment 142 (2018) 188–194

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Reducing formaldehyde emission of urea formaldehyde-bonded particleboard by addition of amines as formaldehyde scavenger

T

Aizat Ghania, Zaidon Ashaaria,b, Paiman Bawona, Seng Hua Leea,b,∗ a b

Department of Forest Production, Faculty of Forestry, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

A R T I C LE I N FO

A B S T R A C T

Keywords: Particleboard Formaldehyde catcher Methylamine Ethylamine Propylamine

Particleboard is one of the building materials that contribute to the emittance of formaldehyde in enclosed area. In order to reduce the formaldehyde emission from particleboard, amines were added into the urea formaldehyde (UF) resin as formaldehyde scavenger. The amines used were methylamine, ethylamine and propylamine. 0.5, 0.7 and 1% of each type of amine were added into UF resin and the mixtures were used to produce particleboard from rubberwood particles. The properties of the UF resin after addition of amines such as gelation time, viscosity, pH, free formaldehyde content and thermal stability were evaluated. The physical, mechanical properties and formaldehyde emission of the produced boards were also assessed. The results revealed that fully cured amine-containing UF resin possesses higher thermal stability compared to control UF resin. Amine-containing UF resin also had longer gelation time due to higher pH value. Nevertheless, both physical and mechanical properties of the resultant particleboard were negatively affected. Particleboard made from aminecontaining UF resin had higher thickness swelling and water absorption. In addition, lower bending strength and internal bonding strength were also recorded. Insufficient pressing time for fully cured of resin might be the reason for such phenomenon. Particleboard with F*** emission level (0.5 ≤ x ≤ 1.5 mg/L) as specified in Japanese Industrial Standard (JIS) or European's E0 class equivalent were achieved when ethylamine and propylamine were added, regardless of dosage used. This study showed the feasibility of using amines as formaldehyde scavenger. However, optimisation of processing parameters is needed to enhance the physico-mechanical properties of the particleboard.

1. Introduction As one of the wooden materials for buildings applications, particleboard is classified as reconstructed panels that are mainly used to manufacture furniture as well as for thermal and acoustic insulation [1]. Particleboard is one of the important major timber products in Malaysia. In the year 2017, the total revenue from the exportation of Malaysian major timber products was RM 23.2 billion [2]. Particleboard has contributed 1.88% of the total export value in 2017, which accounted for RM 437 million. The local production line in Asian countries, particularly Malaysia, is continuously influenced by the Japanese trends as Japan is a main and vital for demand of particleboard. Japanese Industrial Standard (JIS) has the most stringent standards in the world where only wood panels with emission level of F**** (≤0.3 mg/L) could be used unrestrictedly within the room, while the F*** (≤0.5 mg/L) and F** (≤1.5 mg/L) panels are only allowed provided that the room is spacious and have good ventilation [3]. According to Athanassiadou and Ohlmeyer [4], the respective emission ∗

level of F****, F*** and F** are more or less equivalent to European standard's SE0, E0 and E1. Sick House Syndrome, a term derived from Sick Building System that was first recognised in the year of 1983 by World Health Organization as a medical condition, has been reported in residential houses and educational facilities throughout the world. The occupants experience various symptoms such as headache, nose and throat irritation and fatigue [5]. Formaldehyde, acetaldehyde, acetone and 2ethyl-1-hexanol are the main indoor pollutants that were detected in buildings and are closely related to the occurrence of mucosal symptoms among users [6]. The formaldehyde levels present in indoor air are highly dependent on the formaldehyde sources, temperature, humidity and air exchange rate in the building. The main sources of indoor formaldehyde emission in the residential houses and educational facilities nowadays include wood floor finishes, wood-based products such as plywood, particleboard and medium density fiberboard, wallpaper and paints as well as cigarette smoke [7]. Urea formaldehyde is a major aminoplastic resins used for the

Corresponding author. Institute of Tropical Forestry and Forest Products, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia. E-mail addresses: [email protected], [email protected] (S.H. Lee).

https://doi.org/10.1016/j.buildenv.2018.06.020 Received 25 April 2018; Received in revised form 10 June 2018; Accepted 12 June 2018 Available online 12 June 2018 0360-1323/ © 2018 Elsevier Ltd. All rights reserved.

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different types of amines, namely, methylamine, ethylamine and propylamine which were used as formaldehyde scavenger om this study were purchased from Evergreen Engineering & Resources. The hardener used in this study was ammonium sulphate and wax was applied as water repellent.

fabrication of interior wood-based products due to its low cost and high reactivity [8]. A study by He et al. [9] revealed that urea formaldehyde (UF) resin is the main source that contributes to the formaldehyde emitted from wood-based panels. Urea and formaldehyde are highly reactive and could react rapidly to form a strong bond. Nevertheless, the reaction is reversible and therefore provides potential for long-term formaldehyde release [10]. Formaldehyde emits from formaldehydebased adhesive bonded particleboard is mainly caused by the existence of unreacted free formaldehyde in the board. However, this type of release lasts only for a short period of time after manufacture. Another release mechanism that could continue throughout the entire working life of the board is through the hydrolysis of the aminoplastic bond when exposed to elevated temperature and relative humidity [11]. In the past decades, great effort in reducing formaldehyde emission from particleboard such as lowering the formaldehyde to urea (F/U) molar ratio in UF resin has been made. However, lowering F/U ratio inevitably lower the UF reactivity and subsequently, reduced the properties of the resulted panels [12]. In addition, lowering of F/U ratio has reached its limit when Maminski et al. [13] reported that the strength of joints made with UF resin with F/U ratio of 0.85 is 20% lower than the resin with an F/U ratio of about 1.1. To make matters worse, no significant reduction of formaldehyde emission was recorded. An additional amount of 15–20% of resin is needed in order to fulfill the performance standards. Although lowering F/U ratio is the most direct and economic method, other methods known to reduce formaldehyde emission including incorporation of formaldehyde catcher or scavenger, optimisation of processing parameters, and coating with nanoparticles modified water based varnish have also been adopted by several researchers [14–17]. Recently, a study by Jiang et al. [18] has proved that particleboard thermally treated at mild temperature (50 or 60 °C) displayed significant reduction in formaldehyde emission. Ayrilmis et al. [19] incorporated microfibrillated cellulose (MFC) into different grades of urea formaldehyde (UF) resins (SE0, E0 and E1) and the formaldehyde emission of produced laminated veneer lumber (LVL) were determined. The results revealed that the modification by MFC only showed significant effect on SE0 grade UF resin in terms of formaldehyde emission reduction, while E0 and E1 grade UF resin did not indicate the same observation. Various amine-based compounds such as urea, ammonia, melamine, dicyandiamide, and polyamides have been incorporated into formaldehyde-based resin to reduce its formaldehyde emission [20]. Nevertheless, studies on the addition of primary alkyl amines as formaldehyde scavenger are very limited. A study by Boran et al. [21] reported on the effectiveness of adding different amine compounds in the reduction of formaldehyde emission of medium density fiberboard bonded with urea formaldehyde (UF) resin. Another study by Ghani et al. [22] revealed that the addition of 1% propylamine into UF resin could reduce the formaldehyde emission of the particleboard from 0.7 mg/L to around 0.3 mg/L. Nevertheless, physical and mechanical properties of the produced particleboard were adversely affected. This study aims to produce UF-bonded particleboard with lower formaldehyde emission using three primary alkyl amines, namely methylamine, ethylamine and propylamine. The effects of incorporating different amines and dosages on the properties of urea formaldehyde resin were investigated. In addition, the mechanical, physical properties and formaldehyde emission of the resultant particleboard were also evaluated.

2.2. Resin properties after addition of amines Several properties such as acidity (pH), viscosity of the resin, gelatin time and free formaldehyde content were tested in the UF resin after addition of different dosage of amines. The Mi105 pH/temperature professional portable meter was calibrated with buffer solutions at pH 4 and 10 before testing begins. The resin/amine mixtures were cooled to 30 °C. Following that, the pH meter electrode was immersed into the mixtures and pH reading was recorded. For viscosity measurements, 75 ml UF resin was poured into a 100 ml beaker. The viscosity of the mixture was measured with an AMETEK Brookfield rotational viscometer & rheometer at 20 °C with a spinning rate of 1 rpm. As for pH determination, mixtures of UF resin and amines were poured into a beaker and stirred well. Then, 6.5 g of the mixture was poured into a test tube which was immersed (below water line) in a 100 °C water bath. Immediately, the content was stirred continuously and the time (in seconds) required for resin mixtures to cure was recorded. For free formaldehyde content determination, 10 g UF resin was weighed and poured into a 250 ml Eerlenmyer flask and 50 ml of dimethyl sulphoxide solution was added. Rapidly, within 5 s whilst stirring, 30 ml of 0.1 M HCl and Na2SO3 were added. Next, to ensure complete reaction of the formaldehyde with sulphite, the mixture solution was cooled in ice cubes for 3 min. Then, 1 ml 0.1% thymolphthalein solution was added. The excess acid was immediately titrated with 0.1 M NaOH solution until it changed to blue color. Volume of the 0.1 M NaOH used was recorded as V1. The blank test under the same condition but without the UF resin was also carried out and the volume of 0.1 M NaOH used was recorded as V2. The free formaldehyde content was calculated using Equation (1). Free formaldehyde (%) = ((V1eV2) x M x 3.002) / W

[1]

where; V1 = volume of 0.1 M NaOH solution for resin, mlV2 = volume of 0.1 M NaOH solution for blank, mlM = molarity of NaOH solutionW = weight in grams for resin, g. The experiment was repeated for the UF resin admixed with different dosage of amines. Two replicate measurements for each sample were made.

2.3. Fourier transform infrared (FT-IR) spectroscopy analysis A FT-IR spectrometer was used to determine any differences occurring to the functional group on pure formaldehyde sample and after the formaldehyde was mixed with different amounts of amine compounds. FT-IR spectra tests were run at ambient temperature using pure samples within the wave number range of 4000 to 400 cm−1 and at a resolution of 4 cm−1. The infrared spectra of the samples were measured on a Perkin-Elmer FT-IR (model spectrum 100 series, USA).

2.4. Thermal stability of UF resin and amine-containing UF resins 2. Materials and methods Samples of cured control UF resin and amine-containing (methylamine, ethylamine and propylamine) UF resins were tested for thermogravimetric analysis (TGA) using Thermal Gravimetric Analyzer, TA Instrument Q500 model. About 8 mg samples were placed in alumina crucible. An empty alumina crucible was used as reference. All the samples were heated from ambient temperature to 600 °C in a 50 mL min −1 flow of nitrogen at 10 °C min −1 heating rate.

2.1. Preparation of materials Rubberwood particles were obtained from a local particleboard plant, HeveaBoard Berhad, which is located in Gemas. The binding agents used in this study, urea formaldehyde (UF) resin type E1, was supplied by Aica Chemicals (M) Sdn. Bhd from Senawang. Three 189

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2.5. Particleboard fabrication

Table 1 Physical properties of UF resin admixed with different dosage of amines.

Single-layer particleboard (340 mm width x 340 mm length x 12 mm thickness) with targeted density of 650 kg/m3 was fabricated. 8% UF resin based on the dry weight of rubberwood particles were prepared. 0.5, 0.7 and 1.0% of methylamine, ethylamine and propylamine based on the solid weight of UF resin were added into the UF resin, respectively. 1% of ammonium sulphate (NH2SO4) based on the solid weight of UF resin was used as hardener. 0.5% of wax emulsion based on the dry weight of rubberwood particles was mixed together into the UF resin. The admixture was then sprayed evenly on the wood particles and blended for 3 min. After blending, the resinated particles were formed manually into a mat using a wooden forming box. Next, the mat was placed into a hot press and pressed at 180 °C for 270 s with pressure of 4 Mpa. After hot pressing, the particleboard was conditioned in a conditioning room to acquire constant weight before properties evaluation. A set of particleboards was produced using UF resin without addition of amines for comparison purpose.

UF type

Gelation time (s)a

Viscosity (cp)

pH

FFCb (%)

M0.5 M0.7 M1.0 E0.5 E0.7 E1.0 P0.5 P0.7 P1.0 Control

240 256 264 268 271 283 267 304 306 65

222 265 286 270 282 314 278 279 308 217

9.2 8.9 9.1 9.3 9.5 9.6 9.4 9.6 9.8 5.5

0.21 0.20 0.19 0.18 0.16 0.14 0.15 0.13 0.11 0.22

***M = methylamine, E = ethylamine, P = propylamine. a Gelation time at 100 °C. b Free formaldehyde content.

value, the initial pH value of control UF resin is 5.5 and turned into basic (pH around 9) after the incorporation of amines. The higher pH values were reportedly to have slowed down the gelation time of the amines containing UF resin as UF resin is an acid-catalyzed resin which requires an acidic condition to cure [25]. The critical pH point for UF resin is 7, where above this point, the resin curing rate begins to increase dramatically [26]. It can be observed that the application of amines has reduced the free formaldehyde content in the UF resin. When 0.5%, 0.7% and 1.0% of methylamine were incorporated into the UF resin, a reduction of 4%, 9% and 14%, respectively, in free formaldehyde content were observed. Meanwhile, addition of 0.5%, 0.7% and 1.0% of ethylamine into UF resin were found to be able to reduce 18%–36% of the free formaldehyde content from the UF resin. On the other hand, propylamine portrays a relatively higher effectiveness in the reduction of free formaldehyde in UF resin, where the addition of 0.5%, 0.7% and 1.0% propylamine has successfully reduced the free formaldehyde content by 31%, 42% and 50%, respectively. Effectiveness of propylamine in the reduction of free formaldehyde content compared to ethylamine and methylamine might be due to its higher reactivity. Differences in reactivity could be attributed to differences in CeH bond dissociation energies where methylamine, ethylamine and propylamine were reported to have dissociation energies of 103 kcal/ mol, 98 kcal/mol and 95 kcal/mol, respectively [27]. Propylamine with the least bond dissociation energies have the weakest bond and broke more easily compared to methylamine and ethylamine. From methyl to propyl, the electron pushing effects increases on the C1 carbon atom and hence, electron density increases on C1 carbon to which NH2 group is attached and hence, faster cleavage. Therefore, propylamine is the most reactive and able to bind with higher amount of free formaldehyde in the resin.

2.6. Properties evaluation of particleboard 2.6.1. Physical and mechanical properties Physical properties such as thickness swelling (TS) and water absorption (WA) was conducted according to the procedure specified in Japanese Industrial Standard (JIS) A 5908: 2003 [23]. A 50 mm × 50 mm section was cut from each board. The samples were weighed and the thickness was measured before immersion in water at 25 ± 2 °C for 24 h. After soaking, the weight and thickness of the samples were remeasured to determine and express the TS and WA values in percentage. Mechanical properties of particleboard such as modulus of rupture (MOR), modulus of elasticity (MOE) and internal bonding (IB) were tested according to procedure specified in JIS A 5908: 2003 [23]. 2.6.2. Formaldehyde emission by desiccator method Formaldehyde emission from the produced particleboards was tested using desiccator method based on JIS A 1460: 2001 [24]. 300 ml of distilled water was filled into a glass crystalizing dish, which is centrally located at the bottom of the desiccator. Nine pieces of samples (50 mm width x 150 mm length x 12 mm thickness) having surface area approximately 1800 cm2 were placed into a desiccator, right above the water-filled glass crystalizing dish. After 24 h, the water in the crystalizing dish was collected and the concentration of formaldehyde in the solution was measured by acetylacetone molecular absorption spectrometry. 2.7. Statistical analysis

3.2. Characterisation of formaldehyde with different amine compound using FT-IR spectroscopy

The data were analysed statistically to verify the significance of the variables studied using Statistical Package for the Social Science (SPSS). The collected data were analysed using one-way analysis of variance (ANOVA) and the means were separated using Tukey's HSD test.

Fig. 1 illustrates the FT-IR spectrum of pure formaldehyde solution before and after the addition of amine compounds. The pure formaldehyde solution showed band at 3307 cm−1 for the OeH stretch, 2986 and 2915 cm−1 for the CH2 symmetric stretch, 1644 cm−1 for the C]O aldehyde saturated aliphatic and 1429 cm−1 for the CH2 methylene bonds [28]. After the addition of amines, the formation of imine bond linkages was confirmed by the FT-IR spectra where all the amine compounds exhibit stretching bending imine (C]N) absorption at about 1640 cm−1 to 1690 cm−1 in the spectrum as shown in Fig. 1b [29,30]. These bands verified the formation of C]N in all amine compound where the formaldehyde reacted with methylamine, ethylamine and propylamine to produce methylmethanimine, ethylmethanimine and propylmethanimine, respectively. Another absorbance band at 1446 cm−1, 1453 cm−1 and 1425 cm−1 were observed from the spectrum which correspond to the CeH bending alkane of CeH in CH3

3. Results and discussion 3.1. UF properties after addition of amines Gelation time, viscosity, pH and free formaldehyde content of the UF resin after addition of different dosage of amines are listed in Table 1. The UF resin with the addition of 0.5, 0.7 and 1.0% of methylamine (M), ethylamine (E) and propylamine (P) were denoted as M0.5, M0.7, M1.0, E0.5, E0.7, E1.0, P0.5, P0.7 and P1.0, respectively. The gelation time of the control UF resin was 65 s. After the addition of amines, the gelation time increased to a range of 240–306 s. Similarly, the viscosity of the UF resin also increased by a range of 2%–41% compared to control after the addition of amines. As for pH 190

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A. Ghani et al. Formaldehyde Formaldehyde + Methylamine Formaldehyde + Ethylamine Formaldehyde + Propylamine

100

amine-containing UF resins which ranged from 11.42 to 20.87% at the same temperature. UF resin admixed with propylamine had a higher thermal stability as it has higher residual weight at 600 °C (16.05–16.56%) compared to ethylamine (15.14–16.05%) and methylamine (14.29–14.87). This phenomenon might be attributed to the higher reactivity of propylamine with the free formaldehyde that was present in the UF resin.

90

Transmittance (%)

80 C=N, stretching imines

70

60

3.4. Physical and mechanical properties of particleboard

C=O, aldehyde saturated aliphatic

The density of the particleboard produced ranged from 640 to 685 kg/m3. Table 3 exhibits the thickness swelling (TS) and water absorption (WA) values of the particleboard made from UF resin and amines-containing UF resin. The thickness swelling and water absorption of the particleboard bonded with control UF resin were 28.01% and 46.07%, respectively. However, after the addition of amines, the TS values increased within a range of 59.96%–72.43%. Similarly, WA values were increased within a range of 92.94%–117.43%. Regardless of the types of amines used, both TS and WA values of the particleboards increased with increasing dosage of amines. This behaviour might be correlated to the reduction in IB strength of the particleboard produced with amine-containing UF resin. Higher IB strength should be positively correlated to the TS and WA of the particleboard as better bonding between particles could hold them tightly and resulted in lower TS and WA [38]. Table 4 displays the mechanical properties of the particleboard made from UF resin and amines containing UF resin. The modulus of rupture (MOR) and modulus of elasticity (MOE) for control UF-bonded particleboard were 15.13 N/mm2 and 1772 N/m2, respectively. After the addition of amines, both MOR and MOE decreased. A reduction of 33.6% in MOR was observed when 1% methylamine was added while MOR of the 1% propylamine-added samples reduced 42.4%. Similar trends were also observed for MOE. Internal bonding (IB) strength of the particleboard made with amines containing UF were 0.29–1.38 N/ mm2, whereas the control UF-bonded particleboard was 1.40 N/mm2. When 1% of methylamine, ethylamine and propylamine were added, the IB strength decreased to 1.30, 1.05 and 0.29 N/mm2, respectively. The findings agree with Roffael et al. [39] who reported addition of tannin as formaldehyde scavenger markedly reduced the IB strength of the resulted particleboard. IB strength of particleboard is the most sensitive physical characteristics that is highly dependent on the crosslinking density of cured resin network [40]. The results suggest that the crosslinking of the cured resin network was interfered by the addition of amines. The reduction in mechanical properties might be due to the pre-consumption of formaldehyde by amines that dispersed in the UF resin [41]. As the free formaldehyde content decreased, the degree of the crosslinking in the cured network also decreased correspondingly and resulted in poorer mechanical properties. In addition, high pH values after the addition of amines also contributed to the reduction in mechanical properties. The rate and degree of the curing reaction of the amine-containing UF resin decreased as well as the numbers of the cross-linked structures in the cured resin. As a result, the mechanical properties of the particleboard were adversely affected.

50

40 1800

1600

1400

1200

1000

800

600

Fig. 1. A comparison between the FT-IR spectrum (1800–600 cm−1) of pure formaldehyde solution and formaldehyde admixed with different amines.

and CH2 resulting from CeC and CeCH interaction from the structure [31,32]. The two bands near 1387 cm−1 and 1351 cm−1 might be attributed to CO2− symmetric stretch of carboxylic acid salts [33], probably due to the occurrence of Cannizzaro reaction that yields formic acid salts and methanol [34]. Addition-elimination reaction occurs when amine reacts with aldehyde. This mechanism happens in two parts. In the first part, the amine nitrogen attacks the carbonyl carbon to form a hemiaminal (RNHC(OH)R2) after the transfer of a proton from nitrogen to oxygen. In the second part, the hemiaminal is protonated and H2O is eliminated, followed by deprotonation, forms the imine (RN = CR2) [35,36]. Based on the FT-IR analysis, the possible reaction between formaldehyde and methylamine, ethylamine and propylamine amine compound are shown in Fig. 2. For methylamine, the amine compound reacts with free formaldehyde in the UF adhesive to produce N-methylmethanimine and water. As for ethylamine, the amine compound will react with free formaldehyde, which will then liberate N- ethylmethanimine and water. By using propylamine as a formaldehyde catcher, the reaction will produce N-popylmethanimine and water as by product.

3.3. Thermogravimetric analysis (TGA) Thermogravimetric analysis curves for the UF resin and resin added with different dosage of methylamine, ethylamine and propylamine are presented in Fig. 3. The thermal stability of the cured amine-containing UF resins are higher compared to that of the control UF resin as higher residues were recorded in the cured amine-containing UF resins as shown in Table 2. As the amines react with free formaldehyde during the curing process in the UF resin system, lower weight lost was observed compared to the control UF resin [37]. Table 2 shows the weight loss of the control UF resin and amine-containing UF resins in the temperature range from 100 to 600 °C. The control UF resin has lost up to 30.39% of its weight at the temperature of 200 °C, almost two-folds compared to that of the

Fig. 2. Possible reaction between free formaldehyde and amine. 191

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Fig. 3. Temperature dependent mass loss (TG) of control UF resin and UF admixed with (a) methylamine (M), (b) ethylamine (E) and (c) propylamine (P). Table 2 Percentage of weight loss of cured UF resin and amine-containing UF resin by Thermogravimetric analysis (TGA). Type of amine

UF M0.5 M0.7 M1.0 E0.5 E0.7 E1.0 P0.5 P0.7 P1.0

Table 4 Mechanical properties of particleboard produced from UF and amine-containing UF resin (n = 5).

Residuea (%)

Percentage weight loss (%) 100 °C

200 °C

300 °C

400 °C

500 °C

25.54 11.32 12.95 12.25 13.33 13.38 11.54 12.27 11.67 11.42

30.39 18.62 20.87 17.95 19.48 20.18 17.92 18.50 17.32 11.42

71.08 64.60 65.70 65.96 65.49 67.65 67.24 66.48 68.23 56.16

80.65 77.00 77.33 76.83 77.01 78.97 78.82 77.97 78.99 68.55

84.14 81.09 81.33 80.85 81.12 82.37 82.54 81.75 82.75 77.84

13.65 14.87 14.29 14.56 15.14 15.68 16.05 16.37 16.05 16.56

Board type

Modulus of rupture (N/mm2)

Modulus of Elasticity (N/mm2)

Internal bonding (N/mm2)

M0.5 M0.7 M1.0 E0.5 E0.7 E1.0 P0.5 P0.7 P1.0 Control

12.74ab (0.98) 12.22ab (1.04) 10.05bcd (1.05) 12.06bcd (1.51) 11.45abc (1.71) 10.26bcd (1.16) 10.47bcd (0.67) 9.74cd (0.43) 8.71d (0.89) 15.13a (1.9)

1697a (165) 1592a (197) 1542a (134) 1591a (193) 1545a (334) 1487a (427) 1317a (575) 1267a (166) 1144a (431) 1772b (231)

1.38a (0.15) 1.34a (0.16) 1.30a (0.16) 1.22ab (0.46) 1.13ab (0.22) 1.05ab (0.36) 0.74bc (0.23) 0.72bc (0.16) 0.29c (0.05) 1.40a (0.31)

*number in the parentheses are standard deviation; Means followed by the same letter is not significant at p ≤ 0.05. **M = methylamine, E = ethylamine, P = propylamine.

**M = methylamine, E = ethylamine, P = propylamine. a Residue at 600 °C.

0.8

Table 3 Physical properties of particleboard produced from UF and amine-containing UF resin (n = 5). Thickness swelling (%)

Water absorption (%)

M0.5 M0.7 M1.0 E0.5 E0.7 E1.0 P0.5 P0.7 P1.0 Control

59.96b (4.99) 60.27b (6.17) 60.53b (9.00) 60.84ab (4.89) 60.99ab (5.24) 61.19ab (2.37) 63.18ab (2.74) 64.54ab (4.03) 72.43a (7.86) 28.01c (2.9)

92.94a (6.91) 98.69a (17.58) 101.02a (10.90) 101.20a (26.30) 106.65a (2.75) 111.37a (7.26) 111.85a (9.15) 114.40a (9.98) 117.43a (18.43) 46.07b (1.5)

Formaldehyde emisison (mg/L)

Board type

0.7 0.6

a b c

0.5

c

d

de

F*** e

0.4

f

0.3

g

g

F****

0.2 0.1 0 Control M0.5

M0.7

M1.0

E0.5

E0.7

E1.0

P0.5

P0.7

P1.0

UF resin type

*number in the parentheses are standard deviation; Means followed by the same letter is not significant at p ≤ 0.05. **M = methylamine, E = ethylamine, P = propylamine.

Fig. 4. Formaldehyde emission of particleboard from UF and amine-containing UF resins. *Means followed by the same letter is not significant at p ≤ 0.05. **M = methylamine, E = ethylamine, P = propylamine.

3.5. Formaldehyde emission of particleboard The formaldehyde emission values of the particleboard panels produced from UF added with different dosage of amines are tabulated in Fig. 4. Statistically significant reduction in formaldehyde emission was 192

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by Cadan and Akbulut [49] revealed that the addition of 1% and 3% nanosilica, nanoalumina and nanozinc oxide particles had significantly reduced the emission of formaldehyde from the UF-bonded particleboards. On the other hand, decrement of 53% and 21% in the formaldehyde emission was observed in the UF-bonded plywood added with 3-aminopropyltriethoxysilane- and 3-methacryloxypropyltrimethoxysilane-modified nanocrystalline cellulose [50]. Despite their superiority in scavenging the formaldehyde emission from wood composites, production of nanomaterials is very costly and therefore is not favourable in terms of economic sense. By comparison, primary amines are preferable due to its higher effectiveness in reducing formaldehyde emission and cheaper price.

observed. The formaldehyde emission of the control board produced using E1 UF resin was 0.7 mg/L, which is F** class according to JIS standard (0.5 ≤ x ≤ 1.5 mg/L). The value was approximately equivalent to European E1 class [4]. After the addition of amines, the formaldehyde emission values of all the particleboard panels were ranging from 0.31 mg/L to 0.56 mg/L. All the particleboard achieved at least F*** or European E0 class equivalent (0.3 ≤ x ≤ 0.5 mg/L) emission level with the exception of particleboard bonded with 0.5% methylamine added-UF resin. As the formaldehyde emission of particleboard continuously decreased with the increasing dosage of the three amines, addition of propylamine has resulted in the most reduction in formaldehyde emission among the three amines used. The highest decrement in formaldehyde emission values were 50%, 54% and 56% after the addition of 0.5%. 0.7% and 1.0% propylamine, respectively. The emission limit for F**** class is ≤ 0.3 mg/L and the emission levels of the board produced with 0.7% and 1% propylamine is on the threshold of this limit value (0.32 and 0.31 mg/L). The results are in agreement with Boran et al. [21] who reported that propylamine is more effective in reducing the formaldehyde emission of the MDF panels compared to other amines used. Ghani et al. [22] also reported that the addition of 1.0% propylamine has successfully reduced 33% of the formaldehyde emission from UF-bonded particleboard. The effectiveness of amines could be attributed to the fact that incorporation of amines led to diminishing density of aminomethylene bonds along the chain of the resin and successively reduced the amount of formaldehyde emittance. The reduction in formaldehyde emission could also be caused by the Cannizzaro reaction, a chemical reaction that involves the base-induced disproportionation of an aldehyde when reacts with amine molecules [34]. As a result, the formaldehyde simultaneously oxidized and reduced in the Cannizzaro reaction to form methanol and formic acid [42]. Superficially, effectiveness of propylamine in formaldehyde reduction compared to ethylamine and methylamine might be due to its higher reactivity as discussed earlier. Apart from amines, other scavengers such as tannin, urea, sodium metabisulfite and nanomaterials also proved effective in reducing the formaldehyde emission from wood composite. Boran et al. [43] used tannin solution extracted from white oak bark as formaldehyde scavenger in the production of UF-bonded medium density fibreboard (MDF). The results revealed that, at the rate of 15% UF resin dosage and 1% tannin solution, the formaldehyde content of the MDF determined by perforator method decreased 28% compared to that of the control MDF. Correspondingly, IB, MOR and MOE experienced reduction of 12%, 28% and 18%, respectively. However, the obtained perforator value (20–40 mg/100 g) is far from the E1 limit of ≤6.5 mg/100 g [44]. Sodium metabisulfite and urea were reported to be effective in reducing the formaldehyde emission of particleboard [45,46]. In comparison to urea, sodium metabisulfite was found to be a much superior formaldehyde scavenger while the reduction in mechanical properties is still within acceptable value [41]. Nevertheless, the dosage needed to achieve F**** class or SE0 class is 5% based on the solid resin, which is much higher compared to dosage of amines used in this study (0.5%–1%). In addition, the initial formaldehyde emission of the control board (0.45 mg/L) in the study by Costa et al. [47] is much lower than the control board in this study (0.7 mg/L). The practice of using nanomaterials in the production of low formaldehyde emission wood composites have been widely adopted by several researchers. Nanoclay, organo-modified montmorillonite (MMT), has been used as formaldehyde scavenger for the production of oriented strand board (OSB). Four levels of MMT, namely 0, 1, 3 and 5% were added into 10% UF resin in the production of paulownia OSB. Incorporation of MMT was found to have significantly decreased the formaldehyde emission of the OSB. The maximum decrement of 39% was recorded when 5% MMT was added (0.69 mg/L) compared to that of the control OSB (1.13 mg/L). Unlike the other scavengers, improvement in mechanical properties was observed [48]. Another study

4. Conclusion This study examined the effects of incorporating of different amines into UF resin and its relation to the properties of the resultant particleboard. From the current study, amine-containing UF resin had higher pH values, gelation time and viscosity compared to the control UF resin. When the added amines react with the existing free formaldehyde in the UF resin, free formaldehyde content of the resin is reduced. Although TGA results showed that a fully cured amine-containing UF resins possess better thermal stability than control UF resin, it did not reflect on the physical and mechanical properties of the resultant particleboard. Addition of amines negatively affected both physical and mechanical properties of the particleboard, with higher TS and WA values but lower MOR and MOE as well as IB values were recorded. The pressing parameters (180 °C and 270 s) used in this study were adopted from the particleboard plant. However, it seems not enough to fully cure the amine-containing UF resins and resulted in poor physical and mechanical properties of the particleboard. UF is an acid-catalyzed resin and therefore longer pressing time is needed for the amine-containing UF resin having higher pH values. However, low emission particleboard (F*** class) were successfully produced when ethylamine and propylamine were added, regardless of dosage used. This study shows great potential in producing particleboard with F**** level had the processing parameters been optimised. Optimisation of processing parameters such as longer pressing time or higher pressing temperature is needed for future study. Acknowledgement This work was supported by the Research University Grant Scheme (RUGS) [grant numbers 9444600 & 9575500] and the Higher Institution Centre of Excellence (HICoE) [grant number 6369107]. References [1] F. Asdrubali, B. Ferracuti, L. Lombardi, C. Guattari, L. Evangelisti, G. Grazieschi, A review of structural, thermo-physical, acoustical, and environmental properties of wooden materials for building applications, Build. Environ. 114 (2017) 307–332. [2] Malaysian Timber Industry Board (MTIB) Malaysia, Export of timber & timber products, Jan – Dec 2017 http://www.mtib.gov.my/index.php?option=com_ content&view=article&id=2034&Itemid=65&lang=en, (2018) , Accessed date: 24 April 2018. [3] I.L. Eastin, D.E. Mawhinney, Japanese F-4Star formaldehyde rating process for value-added wood products, Working Paper 120, Center for International Trade in Forest Products (CINTRAFOR), Washington, 2011, pp. 1–9. [4] E. Athanassiadou, M. Ohlmeyer, Emissions of formaldehyde and VOC from woodbased panels, in: M. Fan (Ed.), COST Action WG3 (E49) - Performance in Use and New Products of wood Based Composites, Brunel University Press, London, 2009, pp. 219–240. [5] A. Kanazawa, I. Saito, A. Araki, M. Takeda, M. Ma, Y. Saijo, R. Kishi, Association between indoor exposure to semi-volatile organic compounds and building-related symptoms among the occupants of residential dwellings, Indoor Air 20 (2010) 72–84. [6] R. Kishi, R.M. Ketema, Y.A. Bamai, A. Araki, T. Kawai, T. Tsuboi, I. Saito, E. Yoshioka, T. Saito, Indoor environment pollutants and their association with sick house syndrome among adults and children in elementary school, Build. Environ. 136 (2018) 293–301. [7] D. Campagnolo, D.E. Saraga, A. Cattaneo, et al., (2017) VOCs and aldehydes source

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