The Impact of Nanoparticles and Moisture Content on Bonding Strength of Urea Formaldehyde Resin Adhesive

Wood and wood products have been used in different environmental conditions. Moisture content (MC) and relative humidity (RH) are key parameters for these conditions and bonding strength. Nanotechnology has paved the way to more durable adhesives. An experimental study was conducted to examine the effects of various nanoparticles and moisture content on bonding strength of urea formaldehyde (UF) resin adhesive. In this study, nanosilicon dioxide (SiO2) and titanium dioxide (TiO2) were blended with UF. Nanoparticle reinforced adhesives were processed at different nano fi llers concentrations (0.5 % and 1 %) and each adhesive was tested at the moisture content of 0 %, 12 %, 18 % and 25 %. According to the results of bonding strength tests, contained nano-SiO2 adhesives showed better bonding strengths as compared to the control (pure UF) and contained nanoTiO2 adhesives. The highest bonding strength has been determined at 12 % wood moisture in all specimens. Increasing the moisture content has decreased bonding strength of all samples including control samples. This study showed that nano (SiO2 and TiO2) particles have improved the bonding strength of pure UF. Besides, the addition of nano-SiO2 and nano-TiO2 changed the physicochemical properties of UF adhesive by XRD test. The novelty of this study was to demonstrate that nanoparticles (SiO2 and TiO2) could be benefi cial for the bonding strength of UF adhesive in harsh environmental conditions.


INTRODUCTION 1. UVOD
The application of nanotechnology in wood industry is on the rise.Adhesive bonding is generally used in various applications due to its several benefi ts such as the large area of bonding, possibility of joining different materials (Katnam et al., 2011;Ghosh et al. 2016).Bonding technology has been developing for a long time.Urea-formaldehyde (UF) resins are the most generally used adhesives in the forest industry (Lubis et al., 2017).UF resin adhesive has a lot of advantages such as good performance in the panel, solubility in water and lower cost.However, there are disadvantages to using UF resin adhesives as well; they have lower resistance to excess moisture and formaldehyde emission from the panels (Park et al., 2009).Moisture absorption is indicated as one of the important causes of impairing the mechanical properties of polymers because physical changes occur at microscopic levels in composites (Mieloszyk and Ostachowicz, 2017).
For many years, nanomaterials have received much interest for application in adhesives due to high specifi c surface areas and unique mechanical properties (Heon Kwon et al.,2015).Nanocomposites are formed by the dispersion of nanoparticles into the polymer.They have unique properties, such as high heat resistance, toughness and stiffness (Bauer and Mehnert, 2005;Salla et al., 2012).Scientifi c and industrial interest has focused on polymer nanocomposites (Dorigato and Pegoretti, 2011).There are many studies that considered the development of nanocomposites (Yang et al., 2006;Wang et al., 2011).However, studies on the assessment of nanoparticles in the adhesive industry are limited.Adhesives have an important place in wood industry.The quality and durability of a wooden product primarily depend on the quality of its adhesive bonding.Nanoparticles can help to produce enhanced wood products (Bardak et al., 2017).

MATERIJALI I METODE
Urea formaldehyde adhesive was obtained from SFC Integrated Forestry Products Company (Turkey).It has a solid content of 60 %, pH 8.0, and density 1.260 g/cm 3 .Nano-SiO 2 and nano-TiO 2 were supplied by MkNANO (Canada).Properties of nanoparticles used in the research are presented in Table 1.In the study, oak (Quercus robur), with the specifi c gravity of 0.65 g/cm 3 , was used as wood material.Modulus of rapture (MOR) and modulus of elasticity (MOE) were determined in bending test according to the procedures described in TS 2474 (1976).MOR and MOE values of wood material were found to be 87 MPa and 12350 MPa, respectively.Nano-SiO 2 and nano-TiO 2 were added to urea formaldehyde adhesive at the rate of 0.5 % and 1 % (according to solid content), separately.The UF resins reinforced with nanoparticles were mixed with a 30 min mechanical stirrer.Then, 1.5 % ammonium chloride was added as a hardener to UF resins reinforced with nanoparticles.Sample codes, nanoparticle types and rates are given in Table 2.

Bonding strength 2.2. Čvrstoća lijepljenja
The obtained UF mixings were applied in the amount of 180-190 g/m 2 to one surface of the samples with the dimensions of 200 x 50 x 5 mm.Then, samples were pressed at 115±5 ºC temperature and 1 N/ mm 2 pressure for 7 minutes by using a hydraulic test machine.Specimens were then removed from press, and they were cut to dimensions of 150 x 20 x 5 mm according to EN 302-1 Norm. Figure 1 shows the dimensions of shear test samples.
The samples were tested in a device with maximum load capacity of 100 kN.Shear strength was calculated from Equation (1).
Where τ is shear strength (N/mm 2 ), Fmax is maximum load, and A is bonding surface of sample (10 x 20=200 mm 2 ).All shear strength data were analyzed by using SPSS software.ANOVA was used for establishing the differences between the groups.
XRD graph was made on a Rigaku Smartlab (Chech Republic) with Ni-fi ltered Cu Ka (1.540562 Å) radiation source operated at 45 kV voltage and 40 mA electric current.The viscosities of the adhesive were measured by Brookfi eld CAP 2000 + viscometer, according to the ASTM D2256-11 standard.All gel-time measurements were made at a temperature of 100 °C in a trace of water.

REZULTATI I RASPRAVA
Table 4 shows that the addition of nanoparticles to UF adhesive generally yields a signifi cant change in bonding strength at different moisture contents.
The results clearly show that the value of the bonding strength of UF adhesive increases with nanoparticles -0.5 % and 1 % SiO 2 at different moisture content of 0, 12, 18 and 25 %.For example, the highest bonding strength value (8.01 N/mm 2 ) was acquired from the samples with wood moisture of 12 % at 0.5 % SiO 2 .At the same time, the addition of 0.5 % TiO 2 was determined to have a good effect on adhesion strength for wood moisture of 12, 18 and 25%.On the other hand, 1 % TiO 2 nanoparticles did not provide significant changes in adhesion strength for wood moisture of 0 % and 25 %.It was reported that the poly(vinyl acetate) adhesive with 0.3 and 1.0 % nanoparticles content showed a higher bonding strength compared to that with pure adhesive in dry state (Peruzzo et al.,2014).
The high surface area of nanoparticles increases the contact surface area with the adhesive resulting in strong adhesion between matrix and nanomaterial (Younesi-Kordkheili, 2017).Duncan test results obtained with the SPSS program are showed in Table 5.
With the rise of moisture content, the bonding strength of all adhesives increased to certain moisture content (12 %) and then started to decrease.Urea formaldehyde absorbs moisture when exposed to harsh environmental conditions (Biswas et al., 2011).Generally, the XRD analysis is used to determine the distribution of nanoparticles in the polymer matrix (Kaboorani and Riedl, 2011).Figures 4 and 5 show the XRD outcomes of UF resins reinforced with nanoparticles and nanoparticles (TiO 2 , SiO 2 ) prepared in this study.
Nanocomposites have three types of morphology: immiscible (conventional or micro composite), intercalated and exfoliated or miscible (Paul and Robeson, 2008).Exfoliated nanocomposites -In an exfoliated system, the individual nano-material layers are separated in a continuous polymer matrix by an average distance that depends on nanomaterial loading (Sinha Ray and Okamoto, 2003; Silvestre et al., 2016).Nano-SiO 2 and nano-TiO 2 signifi cant peaks are reduced in intensity or even disappeare, showing a high degree of intercalation and/ or exfoliation of the nanoparticle layers in UF polymer matrix (Hedayati and Arefazar, 2009;Zabarjad Shiraz et al., 2013).According to XRD test results, the nanoparticles dispersed in the polymer.This dispersion can be the reason for the increase in bonding strength of UF (Bardak et al., 2017).
Table 6 shows the effect of nanoparticles content on the viscosity of UF adhesives.Nano-SiO 2 and nano-TiO 2 rise the viscosity of the UF adhesive in contrast with the pure UF adhesive.This can be explained by strong agglomeration tendency of nanomaterials (Veigel et al., 2011).
Studies have shown that, as the viscosity of UF adhesive increases, so does the adhesion bonding strength.This situation is explained by the increase of molecular weight and crosslink density of the nano/UF (Osemeohan et al., 2010).Table 7 shows the variation of the gel time with nanoparticles.It is clear that the gel time values of the UF adhesive did not change signifi cantly with nano-SiO 2 and nano-TiO 2 .This is in accordance with the literature (Dukarska and Czarnecki, 2016).

CONCLUSIONS 4. ZAKLJUČAK
The purpose of this study was to investigate the bonding performance of UF adhesives reinforced with nanoparticles.The results showed that the bonding strength of UF adhesive with 0.5-1 % nano-SiO 2 and 0.5 % nano-TiO 2 increase signifi cantly when compared to that of pure UF adhesive at all moisture contents.On the other hand, there was no signifi cant difference in 1 % nano-TiO2 in 0 % and 25 % moisture contents.
All samples with moisture content of 12 % resulted in the bonding strength.Besides, the viscosity of UF adhesive increased with the nano-SiO 2 and nano-TiO 2 content.However, the addition of nanoparticles did not change gel time of the UF adhesive signifi cantly.Consequently, it can be stated that nanoparticles could be benefi cial in the development of more moisture-resistant UF adhesives.

Table 1
Properties of nanoparticles used in the research Tablica 1. Svojstva nanočestica upotrijebljenih u istraživanju