Use of Populus deltoides wood in the Production of MDP and HDP

The aimed to evaluate the properties of MDP and HDP panels, produced with pure Populus deltoides or in a mixture with Pinus taeda, together with urea-formaldehyde resin. The density and chemical properties of this wood with 8 and 12 years of age were determined, which were used for the production of panels with nominal density of 0.65 and 0.90 g.cm-3. Water absorption and thickness swelling tests were performed after 2 and 24 hours of immersion, as well as static bending and perpendicular tensile. Populus deltoides wood was classified as low-density in both ages and presented chemical properties similar to Pinus taeda. Mechanical properties obtained for the panels produced reached the minimum requirements values established by the standards NBR 14810-2 and ANSI A208.1, demonstrating the potential use of wood species for the production of MDP and HDP panels, at both ages studied and used pure or mixed with Pinus taeda wood.


INTRODUCTION AND OBJECTIVES
For particleboard production, companies use Pinus taeda, Eucalyptus grandis and Eucalyptus urograndis species as raw material, which the plantations are located mainly in the south of the country. For this purpose, the trees are cut shallowly at age of 15 for pine and close to 7 for eucalyptus (Arauco Forest Brasil SA, 2018;Berneck SA, 2019).
The high demand and competitiveness for these wood species in order to make other types of panels, such as fiberboard and plywood, in addition to sawn wood, create the need to increase planting areas or seek alternative species of fast growth . According to Trianoski et al. (2011) and Bianche et al. (2012), the use of new species, pure or mixed with traditional species, in the production of particleboard is characterized as a fast, low cost, and sustainable alternative.
According to the Forest Stewardship Council (FSC, 2020), in countries as China, Slovenia, Taiwan, Austria, Poland, India, and Spain, there are records of the use of Populus deltoides wood for particleboard production. Populus deltoides is a fast-growing species naturally occurring in the United States and Canada. In Brazil, they have been cultivated in the states of Paraná and Santa Catarina, aiming mainly at making matchsticks (Medeiros & Hope, 2002;Lorenzi et al., 2003).
The Populus genus has a wide variety of species, which highlight for their rapid growth, chemical composition (absence of resins), whitish color, straight fiber, hybridization potential, ease of regrowth and adaptation to different edaphoclimatic conditions (Medeiros & Hope, 2002;Arce, 2004;Kuzovkina & Vietto, 2014). [2][3][4][5][6][7][8]Hillig E,Marchesan R,Ferro FS 2 For Mercosul countries, Populus spp. has a great economic importance, especially in Argentina, where it is the third most planted forest genus (SBS, 2006). In Argentina and in the vast majority of plantations worldwide, the species Populus nigra L. and Populus deltoides W. Bartram ex Marshall and their hybrids (P. nigra x P. deltoides) are cultivated (Borodowski, 2006). Lau (2017) and Javarini (2011) studied Populus deltoides wood at 12 and 11 years old in Santa Catarina, and reported that it was a low density wood (0.33 g.cm -3 ) with potential for making edge glued panels (EGP), particleboard, and plywood. Nourbakhsh (2010) assessed the technical feasibility of producing particleboard with Populus deltoides wood for a 3-year-old planting grown in Iran. The author reported that the panels had adequate physical and mechanical properties for non-structural application as interior decoration (furniture, walls, and ceiling panels), demonstrating the possibility of using this species in the first years of planting.
The age of the tree is an extremely important factor when it comes to the end use of wood, as several transformations in the anatomical and chemical composition, as well as physical and mechanical characteristics occur as it ages, interfering in the quality of the wood and consequently in the age of cutting (Vidaurre et al., 2012;Carneiro et al., 2014;Soares et al., 2015).
Regarding this scenario, this study aimed to evaluate the quality of homogeneous particleboards, with medium and high density, produced with pure Populus deltoides wood or mixed with Pinus taeda, together with the urea-formaldehyde resin.

MATERIALS AND METHODS
The commercial plantations of Populus deltoides with 8 and 12 years old, located in the cities of Porto União and Irineópolis, both located in Santa Catarina state, were implanted by means of cuttings with approximately 4.5 m in length and 5 cm in diameter at breast height (DBH). It was used the spacing of 5.2 m x 6 m, and silviculture methods as fertilizing, weed control, disease and pest control, and pruning were adopted.
For the study, three Populus deltoides trees were ramdomly selected by age. Trees were harvested, delimbed, and sectioned in 25%, 50%, 75%, and 100% of commercial height. It was adopted as a tip diameter of 16 cm, the last log being used to make medium density particleboard (MDP) and highdensity particleboard (HDP).
The logs of Populus deltoides were unfolded on a band saw and processed in a disc chipper and forage chipper/shredder, until obtaining "sliver" particles. Pinus taeda wood with a bulk density of 0.41 g.cm -3 in the form of particles, as well as resin, catalyst and paraffin, were obtained in a particleboard industry in the region of Guarapuava-PR, Brazil. For adhesion of the particles, urea-formaldehyde resin was used in the proportion of 10% solids in relation to the dry weight of the particles, with the following properties: 66.5% solids content, 8.2 pH, 324 cPs of Brookfield viscosity, 40 s of gel time, and 1.30 g.cm -3 of density.
Paraffin of 1% was used to reduce the hygroscopicity of the panels either, and 2% ammonium sulfate as a catalyst to accelerate the curing time of the resin. For the agglutination of the panels, a specific pressure of 40 Kgf.cm -2 , a temperature of 160°C, and a pressing time of 10 minutes were used. Homogeneous panels were produced with five combinations of wood and two densities, totaling 10 types of panels with three repetitions each (Table 1). After pressing, the panels remained in an air-conditioned chamber with a relative humidity of 65 ± 5%, and a temperature of 20 ± 3°C until they reached the equilibrium moisture content. After this period, it were obtained specimens for physical properties determination as water absorption and thickness swelling after 2 and 24 hours of immersion in water (WA2; WA24; TS2; TS24). Mechanical properties as static bending, to determine the modulus of elasticity (MOE) and modulus of rupture (MOR), and tensile strength perpendicular to the panel surface to check the internal bond (IB) were also determined. These test were performed according to recommendations of NBR 14810-2 standard (ABNT, 2013). From each produced panel, 10 specimens were obtained for physical and mechanical test. The compaction ratio of the panels was also determined using values density and composition density of the panel, as described by Moslemi (1974).
Particle geometry of Populus deltoids at 8 and 12 years old, and of Pinus taeda obtained from the industry, was evaluated using the slenderness ratio, the flatness ratio, and the surface area as described by Moslemi (1974). A sample of 100 particles was used for each species.
To verify difference in physical and chemical characteristics of the wood, values were compared by means of a completely randomized design, which was considering as treatments the species Populus deltoides -8 year and Populus deltoides -12 years old. It were considered 3 repetitions for chemical properties and 6 repetitions for physical properties. For particle geometry, values obtained were compared using a completely randomized design considering as treatments Pinus taeda, Populus deltoides species -8 years old and Populus deltoides -12 years old, with 100 repetitions. For compaction ratio values and for those obtained in the physical and mechanical tests for MDP and HDP panels, the comparison was made through a completely randomized design considering the five wood combinations (100% Pt; 100% Pd8 ; 50% Pt + 50% Pd8; 100% Pd12; 50% Pt + 50% Pd12) as treatments. It were considered 10 repetitions for each property analyzed.
Thus, the values obtained for the physical and chemical properties of wood, particle geometry, compaction ratio and physical and mechanical properties of MDP and HDP panels were subjected to statistical tests. Kolmogorov-Smirnov test were used to analyze the normality of the data; Levene's test to analyze the homogeneity of variances. Once the normality and homogeneity of the variances was confirmed, the Analysis of Variance (ANOVA) was applied. If the equality hypothesis was rejected by ANOVA, the averages were compared using Tukey test at of 5% significance level. Pearson's correlation analysis was also performed between the average values obtained for the physical and mechanical properties of the MDP and HDP panels. Table 2 shows the average values of basic density, extracts in cold water, extracts in hot water, extracts in sodium hydroxide, extracts in ethanol-toluene, insoluble lignin, ash and pH. With the exception of the basic density and the content of extractives in sodium hydroxide, for the other chemical properties analyzed, significant statistical differences were found where higher values were obtained with increasing age of Populus deltoides wood.

Wood properties
The increase in the extractives levels according to the age of tree, soluble in water and/or organic solvent may be attributed to the process of wood cernification (Soares et al., 2015;Fonte et al., 2017). Already the increase in ash content with the age may be linked to the influence of habitat and the fertilization received by trees. The presence of higher values of non-process minerals in wood could affects the quality of its by-products, such as charcoal and paper, in addition to being able to compromise the performance of machinery due to its wear by this component (Garcia et al., 2016;Benites et al., 2018). For basic density, average values of 0.38 g.cm -3 were obtained for wood from trees with age of 8 years and 0.40 g.cm -3 of 12 years old. Results obtained in this study are compatible with 0.38 g.cm -3 of basic density reported by Klasnja et al. (2003) for Populus deltoides clones wood of 4 year.
Basic density obtained for Populus deltoides woods at the ages of 8 and 12 years, is close to 0.48 g.cm -3 found by Trianoski et al. (2016) for Pinus taeda wood, a species traditionally used in particleboard manufacture in Brazil.
Low density of Populus deltoides wood in both ages, indicate ease of bonding of wood. That characteristic are associated with wood with thin -walled cells and with large lumens. Thus, consequently, a greater amount of resin may be absorved, thus not requiring the application of high pressure during the pressing process (Frihart & Hunt, 2010). Moslemi (1974) adds that besides to the influence on gluing, the use of low-density wood in the production of particleboard leads to obtaining adequate values for compaction ratio.
Results obtained in this study for extracts in ethanol-toluene, insoluble lignin and ash are in line with those reported by Klasnja  Klasnja et al. (2003) obtained the content of 22.20% insoluble lignin and 2.20% of extracts in ethanol-toluene for the wood of clones of Populus deltoides with 4 years of age. Jin & Kamdem (2009) found the contents of 22.55% insoluble lignin, 4.00% extracts in ethanol-toluene and 0.69% ash for the wood of the hybrid Populus deltoides x Populus nigra var. Caudina.
The levels of extracts soluble in cold water, hot water, NaOH and ethanol-toluene, as well as the results obtained for insoluble lignin, ash and pH obtained for the wood of Populus deltoides in the three ages under study, are close to those presented by Lau (2017) for the wood of Populus deltoides at 11 years. The author obtained an average of 2.62% for extracts in cold water, 3.39% for extracts in hot water, 17.74% for extracts in NaOH, 2.45% for extracts in ethanol-toluene, 27.01% for insoluble lignin, 0.98% for ash and 6.72 for pH.
The values of 3.14% of total extracts, 27.67% of lignin and 4.56 of pH obtained by Iwakiri et al. (2017) and Pereira et al. (2017) for Pinus spp. industrial particles are close to the average values found for Populus deltoides woods, demonstrating the potential of the species under study as a raw material for the particleboards production.

Particle geometry
For the characteristics, slenderness ratio, flatness ratio, and surface area determined for Pinus taeda and Populus deltoides particles 8 and 12 years shown in Table 3, a statistical difference was found. The difference, mainly between species, may be attributed to the equipment used for particles generation. Particles from Pinus taeda used in this study were produced on an industrial level using automated equipment, while particles from Populus deltoides were obtained using less sophisticated equipment, besides that its classification being performed manually with the aid of sieves. The long, fine particles of Pinus taeda led to a higher rate of slenderness and flatness ratio and a smaller surface area, tending to obtain panels with greater strength to static bending and greater dimensional stability. In contrast, the shorter and thicker particles obtained for Populus deltoides with 8 and 12 years provided a greater surface area and lower slimness ratio and flatness ratio. Consequently, panels produced with these type of particles presented a greater of availability of resin, which may provided improvement in tensile strength perpendicular due to its more efficient bonding (Vital et al., 1992).

Physical and mechanical properties of MDP and HDP panels
MDP panels presented density close to the established nominal density of 0.65 g.cm -3 , classified, according to the ANSI A208.1 standard (ANSI, 2009), as medium density (0.64 g.cm -3 to 0.8 g.cm -3 ) ( Table 4). The average values of compaction ratio ranged from 1.54 to 1.67, considering the range from 1.3 to 1.6, reported by Moslemi (1974) as ideal for particleboard.
A statistical difference was found between the studied properties, highlighting that the addition of Populus deltoides wood in the composition with Pinus taeda provided an improvement in the dimensional stability of the panels.
For MOR, MOE, and IB (Table 4), a reduction in average values was found when Pinus taeda was mixed with Populus deltoides at both ages. In general, it appears that the panels produced with the wood of Populus deltoides at the age of 8 resulted in better physical-mechanical properties.
Regarding the fulfillment of the minimum requirements required by NBR 14810-2 (ABNT, 2013) for non-structural panels intended for indoor use in dry conditions, none of the panels manufactured met the maximum requirement of 18% thickness swelling after 24 hours immersion. Better results may be obtained with the increase in the proportion of urea-formaldehyde resin in the production of the panels. 58%, and 72%. Authors also pointed out that the use of wheat straw in greater proportion provided improvement in water resistance of produced panels. This result is related to the greater silica content in this material and to the geometry of the particles. Compared to populus, wheat straw particles are longer and thinner. The values obtained by these authors are lower than those of this study for the mechanical properties are and higher for the physical properties of the panels. The improvement in water resistance coupled with the particle dimensions was also evidenced for the thickness swelling property for panels produced with the mixture of Populus deltoides and Pinus taeda in both ages.
Nourbakhsh (2010) used wood particles of Populus deltoides with 3 years of age and urea-formaldehyde resin in the proportion of 10% in the production of MDP panels with a density of 0.70 g.cm -3 . For mechanical properties, values of 16.81 MPa, 1890 MPa, and 0.86 MPa, for MOR, MOE, and IB were obtained, while for physical properties of thickness swelling after 2 and 24 hours of immersion, 19.07% and 21.84% were obtained. Results obtained by the author are close to this study (Table 4), confirming the feasibility of Populus deltoides wood in the early years for non-structural uses as interior decoration (furniture, walls, and ceiling). Legend: Pt -Pinus taeda; Pd8 -Populus deltoides 8 years; Pd12 -Populus deltoides 12 years; Cd -composition density; Pd -panel density; CR -compression ratio; WA2 -water absorption after 2 hours of immersion; WA24 -water absorption after 24 hours of immersion; TS2 -thickness swelling after 2 hours of immersion; TS24 -thickness swelling after 24 hours of immersion; MOR -modulus of rupture; MOE -modulus of elasticity; IB -internal bond; averages followed by the same letter in the column do not differ by Tukey test at 5% probability of error; value in parentheses refers to the percentage variation coefficient.
As shown in Table 5, the HDP panels presented a density close to the nominal density established at 0.90 g.cm -3 , being classified according to the criteria of the ANSI A208.1 standard (ANSI, 2009) as high density (> 0.80 g.cm -3 ).
For other properties analyzed shown in Table 5, a statistical significant difference was found. It is noteworthy that panels made with Populus deltoides wood in both ages, pure or mixed with Pinus taeda, presented lower average values than the Pinus taeda panels for water absorption and thickness swelling after 2 and 24 hours of immersion and, higher average values for MOR, MOE and IB.
There´s no Brazilian normative standards for highdensity particleboard (HDP), therefore, obtained results were compared to values established by the American standard ANSI A208.1 (ANSI, 2009). This standard establishes values for MOR, MOE and IB of 14.9 MPa, 2160 MPa and 0.81 MPa, respectively. In this case, only HDP panels produced with Populus deltoides at both ages (8 and 12 years), met the minimum requirement for IB.
The improvement of physical and mechanical properties of the MDP and HDP panels linked to the use of Populus deltoides wood may be attributed to the combination of several factors, such as chemical composition, particle geometry, and composition density.
The use of short and thick particles obtained for the wood of Populus deltoides at ages 8 and 12 in the manufacture of the panels, provided the achievement of greater tensile strength perpendicular for the MDP and HDP panels. Pereira et al. (2017) observed a similar behavior for MDP panels produced with industrial particles of Pinus spp and particles of Eucalyptus badjensis produced under laboratory conditions with wood from the entire shaft. 6 -8 Sozim PCL, Hillig E, Marchesan R, Ferro FS Moslemi (1974), that particleboards produced with low density wood result in better compaction, and consequently, it improve mechanical properties.
The greater compaction rate and better bonding of the MDP particle provided the panels less hygroscopicity. It is observed a significant negative correlation between compaction ration and water absorption after 24 hours of immersion, and internal bond and water absorption after 2 and 24 hours of immersion. According to Frihart and Hunt (2010) and Trianoski et al. (2013), this behavior may be related to the wood chemical composition, mainly the presence of extracts. The presence of extractives may interfere in a physical way in the bonding process, as forming a barrier on the surface through its migration during the wood drying process, as well as in a chemical way, depending on the nature of the extracts they may be more acidic, interfering in the cure of the resin.
Significant positive and negative correlations were obtained for physical properties such as water absorption and thickness swelling of the MDP, in addition to a significant positive correlation between the modulus of rupture in static bending and internal bond. There is a significant positive correlation between thickness swelling and modulus of elasticity in static bending, which may be attributed to the swelling of the wood itself, the return in thickness of the panel after pressing and material separation (Hsu, 1987).
For HDP, Pearson's correlations between physical and mechanical properties are in Table 7. It is noteworthy a significant positive correlation between the compaction ratio and the modulus of elasticity in static bending, and between water absorption and thickness swelling. In addition to significant negative correlation between water absorption after 2 hours of immersion and modulus of elasticity in static bending. Legend: Pd -panel density; CR -compression ratio; WA2 -water absorption after 2 hours of immersion; WA24 -water absorption after 24 hours of immersion; TS2 -thickness swelling after 2 hours of immersion; TS24 -thickness swelling after 24 hours of immersion; MOR -modulus of rupture; MOE -modulus of elasticity; IB -internal bond; * -significant level of 5% probability of error. Legend: Pd -panel density; CR -compression ratio; WA2 -water absorption after 2 hours of immersion; WA24 -water absorption after 24 hours of immersion; TS2 -thickness swelling after 2 hours of immersion; TS24 -thickness swelling after 24 hours of immersion; MOR -modulus of rupture; MOE -modulus of elasticity; IB -internal bond; * -significant level of 5% probability of error. 7 -8

CONCLUSIONS
The wood of Populus deltoides with 8 and 12 years old presented basic density close to the wood of Pinus taeda, a species traditionally used in the manufacture of particulate panels.
The use of short and thick particles of Populus deltoides in the production of MDP and HDP panels, resulted in better tensile strength perpendicular, and when mixed the long and fine particles of Pinus taeda improved the dimensional stability of the panels, mainly for the thickness swelling property.
The mechanical properties obtained met the minimum requirements established by the NBR 14810-2 standard for non-structural medium density particulate panels intended for internal use and, ANSI A208.1 for high-density particulate panels.
Pearson's correlations obtained for the MDP and HDP panels demonstrated that the use of low-density wood together with the compaction and bonding of the panel positively influenced the physical-mechanical properties, and that the dimensional stability of the panel had an influence mainly on the modulus of elasticity to static bending.
In view of the results obtained for the physical and mechanical properties evaluated for wood or panels, it is recommended to use Populus deltoides at 8 years, pure or mixed with Pinus taeda in the manufacture of medium and high-density particulate panels, enabling the conduction of plantings of this species for this purpose in a short rotation system. 8 -8 Sozim PCL, Hillig E, Marchesan R, Ferro FS