Resin Content and Board Density Dependent Mechanical Properties of One-Layer Particleboard Made from Willow ( Salix viminalis )

The paper presents the results of studies on the mechanical properties of one-layer particleboards made from willow (Salix viminalis). Since the particleboards were to simulate the core layer of typical furniture three-layer particleboards, they were made from coarse particles of the size from 1 to 4 mm using urea-formaldehyde resin as a binder. The effects of board density and resin content were evaluated. Three levels of resin content: 8, 9 and 10 %, and board density: 0.57, 0.60 and 0.63 g/cm3, were assumed. The effects of both factors, for their assumed ranges of variation, on the mechanical properties of particleboard were statistically signifi cant. The modulus of elasticity (MOE), modulus of rupture (MOR), internal bond (IB) and screw holding strength (SHS) were determined. The effect of board density on MOE and MOR was greater than that of resin content, whereas the effect of board density on IB and SHS was less than that of resin content.


INTRODUCTION 1. UVOD
One of the possible alternative raw materials in particleboard manufacturing can be fast growing shrubs of willow (Salix viminalis).They are cultivated in Poland for energy purposes, and their suitability for particleboard industry was confi rmed by a number of studies (Frąckowiak, 2007;Frąckowiak et al., 2008;Warmbier et al., 2013Warmbier et al., , 2014aWarmbier et al., , 2014b)).Sean and Labrecque (2006) found the usefulness of Quebec clones of this willow.Kowaluk and Fuczek (2010), and Kowaluk et al. (2008) used non-standard, specially prepared, particles from the willow (Salix viminalis).These particles, named fi brous chips, were produced on a Pallmann defi brator using a span of 1.2 mm between the hammer and milling disc.The authors showed the suitability of these particles for particleboard manufacturing.
Mechanical properties of particleboards depend on many factors, with the resin content and density of particleboard being the major ones.It is known that an increase in resin content and board density leads to improved mechanical properties of particleboards (Rackwitz 1963; Maloney 1993).However, the quantitative effects of these factors on the mechanical properties of particleboards made from willow (Salix viminalis) are not suffi ciently known.
A typical particleboard for furniture purposes is a three-layer board, which consists of a core layer and two face layers.It was assumed that willow particles as a substitute for industrial wood particles would be used for manufacturing only the core layer.Therefore, it was decided to investigate a one-layer particleboard as simulating the core layer of a three-layer particleboard.The objective of this study was to evaluate the effects of resin content and board density on the mechanical properties of particleboards made from willow (Salix viminalis).

MATERIJALI I METODE
Three-year-old stems of willow (Salix viminalis) with a diameter at the base ranging from 28 to 35 mm came from the Wielkopolska Region of Poland.The mean density of the stems was 0.51 g/cm 3 at a moisture content of 12 %, and bark percentage was about 15 % of stem weight.The stems were chipped in a hammermill, and then screened by an analytical sieve shaker using 4 mm and 1 mm sieves.Particles that passed through the 4 mm sieve and remained on the 1 mm sieve were used as furnishes for experimental particleboards that simulated the core layer of three-layer particleboard.The bulk density of these particles was 0.20 g/cm 3 .Prior to pressing, the particles were dried in an air-circulation oven to achieve a moisture content of less than 3 %.
Urea-formaldehyde (UF) resin was used as a binder.It had a density of 1.26 g/cm 3 at 60 % solids, pH value of about 7, a viscosity of 400-600 mPa•s at 20 ºC, and gel time of 40 s at 100 ºC.Three levels of resin content: 8, 9, and 10 %, and board density: 0.57, 0.60, and 0.63 g/cm 3 , were assumed.The board manufacturing parameters are listed in Table 1.The particleboards were prepared in the laboratory.The target board thickness was 10 mm.After spraying the adhesive on particles in a drum blender, a particleboard mat was manually formed inside a 40 x 40 cm box.The pressing conditions were the temperature of 180 ºC, maximum pressure of 2.5 MPa and pressing time of 3 min.Nine types of boards of different density and resin content were made.Four experimental boards were produced for each board type.The boards were not sanded.
Prior to testing, all the boards were stored in controlled conditions (50 % relative humidity and 20 ºC) for two weeks.Test specimens were cut from the boards to determine the following mechanical properties according to appropriate EN standards: modulus of elasticity (MOE) and modulus of rupture (MOR) (EN 310, 1993), internal bond (IB) (EN 319, 1993), and screw-holding strength (SHS) (EN 13446, 2002) by using screws with a diameter of 3.5 mm, length of 45 mm and a hole diameter of 2.5 mm.Twenty specimens were prepared for each test and board type.Test specimens for IB and SHA were prepared from the specimens that were formerly tested for MOE and MOR.
The obtained data were statistically analyzed using the Statistica version 10.The analysis of variance (ANOVA) was conducted to determine the signifi cance of the effects of resin content and board density on mechanical properties of particleboards.

REZULTATI
The results of the ANOVA test are shown in Table 2.Both variables, resin content and board density, signifi cantly affected the mechanical properties of particleboard.The interaction between the variables was not signifi cant for each property.
The effects of board density and resin content on the mechanical properties of particleboards are shown in Figure 1.MOE and MOR increased with increasing board density and resin content.The increases in MOE and MOR, with increasing board density from 0.57 to 0.63 g/cm 3 , were on average 18.5 and 29.2 %, respectively.The increases in MOE and MOR, with increasing resin content from 8 to 10 %, were on average 12.9 and 15.7 %, respectively.Thus, MOR was most af-   3 .The effects of board density and resin content on IB and SHS of particleboards are shown in Figure 1.IB and SHS increased with increasing board density and resin content.The increases in IB and SHS with increasing board density from 0.57 to 0.63 g/cm 3 were on average 7.9 and 6.9 %, respectively.The increases in IB and SHS with increasing resin content from 8 to 10 % were on average 11.0 and 13.7 %, respectively.Therefore, unlike the cases of MOE and MOR, the effect of board density on IB and SHS was less than that of resin content.The explanation of these increasing properties as a function of increasing board density and resin content is the same as for MOE and MOR.Similar positive effects of resin content or board density on IB and/or SHS were mentioned in previous studies: Grigoriou and Ntalos (2001) for particleboards made from castor stalks (Ricinus communis), with increasing board density from 0.66 to 0.72 g/cm 3

ZAKLJUČAK
One-layer experimental particleboards were manufactured from willow (Salix viminalis) as a substitute for industrial wood particles.Since the particleboards were to simulate the core layer of typical furniture threelayer particleboards, they were made from coarse particles of the size from 1 to 4 mm.The board density and resin content, for their assumed ranges of variation, af-fected signifi cantly the mechanical properties of particleboards.The modulus of elasticity (MOE), modulus of rupture (MOR), internal bond (IB) and screw holding strength (SHS) were determined.The effect of board density on MOE and MOR was greater than that of resin content, whereas the effect of board density on IB and SHS was less than that of resin content.