Physical and Mechanical Properties of Particleboard Made from Palm Tree Prunings

Article Physical and Mechanical Properties of Particleboard Made from Palm Tree Prunings

Clara-Eugenia Ferrández-García 1, Antonio Ferrández-García 1,*, Manuel Ferrández-Villena 1, Juan Fernando Hidalgo-Cordero 2, Teresa García-Ortuño 1 and María-Teresa Ferrández-García 1

1 Department of Engineering, Universidad Miguel Hernández, Carretera de Beniel, Km. 3,2, 03312 Orihuela, Alicante, Spain; [email protected] (C.-E.F.-G.); [email protected] (M.F.-V.); [email protected] (T.G.-O.); [email protected] (M.-T.F.-G.) 2 Construction Unit, Department of Agroforestry Engineering, Universidad Politécnica de Madrid, ETSI Montes, Avda. Ramiro de Maeztu S/N, 28040 Madrid, Spain; [email protected] * Correspondence: [email protected]; Tel.: +34-68-095-7181

Received: 30 October 2018; Accepted: 3 December 2018; Published: 5 December 2018

Abstract: Palm trees are very fast-growing species. Their management produces annually a large amount of biomass that traditionally has been either disposed of at dumping sites or has been burnt onsite. This paper presents an experimental study to obtain particleboard using this biomass in a low energy process (short pressing time and low pressing temperature), using particles of different sizes from the rachis (midrib) of the three palm species most representative of urban gardening in Spain: canary palm (Phoenix canariensis hort. ex Chabaud), date palm (Phoenix dactylifera L.) and washingtonia palm (Washingtonia robusta H. Wendl). Their physical and mechanical properties were tested, and the feasibility of their use as a construction material was evaluated. The results showed that the manufactured particleboard had similar performance to conventional wood particleboard and good thermal insulation properties. Boards made with the canary species showed better mechanical performance. The properties of the particleboard depended on the particle size and species. The use of the pruning waste of palm trees to produce durable materials such as particleboard could be beneﬁcial to the environment since it is a method of carbon ﬁxation, helping to decrease atmospheric pollution and reducing the amount of waste that ends in dumping sites.

Keywords: thermal conductivity; palm rachis; biomass; hot pressing; Phoenix canariensis; Phoenix dactylifera; Washingtonia robusta

1. Introduction Climate change is a real long-term problem that requires a multidisciplinary global approach. One of the proposed strategies for responding to this issue is mitigation, understood as actions to limit the emission of greenhouse gases by capturing and storing atmospheric carbon dioxide (CO2) in sinks such as forests [1]. The appropriate management of carbon sinks is needed in order to conserve and expand their size in a socio-economically sustainable manner. One manner of storing carbon is by transforming the biomass produced within forests into wood-based products. A few studies have been performed to assess the carbon ﬂux through the life cycle of wood panels: Wang et al. [2] conducted a study to assess the contribution of the wood-based panels to CO2 emissions and removal in different panels in China. They estimated the CO2 emissions through panel production and the CO2 stored during their useful lives, concluding that the wood-based industry can potentially contribute to climate change mitigation. Rivela et al. [3] studied the life cycle of particleboard and created a database to identify and characterize the manufacture of particleboard. They reported that environmental,

Forests 2018, 9, 755; doi:10.3390/f9120755 www.mdpi.com/journal/forests Forests 2018, 9, 755 2 of 14 economic and social considerations strengthened the hypothesis that the use of forest residues in particleboard manufacture is more sustainable than their use as fuel. In the south-east of Spain, palm trees are extensively used in urban landscaping, as in most of the countries surrounding the Mediterranean Sea. Among the different species, the most abundant are: Phoenix dactylifera L. (hereafter called date palm), Phoenix canariensis hort. ex Chabaud (canary palm), and Washingtonia robusta H. Wendl (washingtonia palm). Apart from their ornamental use, the date palm can also be found in groves in the Alicante province, in two cities (Elche and Orihuela) forming a unique forest ecosystem. Some palm species can live from 150 to 300 hundred years. As part of their management, they are pruned to remove old leaves (fronds) and inflorescences at least once a year, producing a large amount of biomass that has traditionally been disposed of either at dumping sites or has been burnt onsite. Palm trees are a very fast-growing species. Their management produces an annual average dry mass of 49.34 kg/tree from Phoenix canariensis [4], 72.26 kg/tree from Phoenix dactylifera [4], and 35.70 kg/tree from Washingtonia robusta [5]. This biomass is classified as urban waste according to the European list of waste [6]. Several studies have been conducted focusing on the manufacturing of building materials using different palm residues. Particleboard with synthetic adhesives and different manufacturing procedures have been studied using fibers or chips of date palm [7–15], washingtonia palm [5,16–18], and canary palm [19–21]. Other studies have been performed using date palm pruning waste as a reinforcement in concrete [22–24], in gypsum [25], and in the manufacturing of different composites [25–27]. Most of these works were aimed at the use of palm pruning waste to produce thermal insulating materials [18,21,22,25,28–30]. These investigations showed different results depending on the palm species and the part of the plant used (generally the leaves or trunk). The particle size and production parameters can also affect the physical and mechanical properties of the resulting materials. Particleboard is made by applying pressure and heat to particles of wood or other lignocellulosic biomass with the addition of an adhesive. The temperature in the hot press varies with the type of adhesive employed. For urea–formaldehyde (UF) the hardening occurs at 90 ◦C, but to shorten this process, temperatures over 200 ◦C are employed. Times of 3 to 4 min for a 20 mm thick particleboard are required when using UF at 180 ◦C. The majority of the world’s production of particleboard is made of wood with densities ranging from 0.60 to 0.80 g/cm3. Most of these are three-layered, with outer layers made of finer particles and a central section composed of coarser and cheaper chips, which improves the strength, stiffness, and appearance. In general, 9% and 12% of UF is needed to bond the particles of the core and the particles of the outer layers, respectively [31]. Although it is possible to manufacture particleboard from almost all types of wood, those with a specific weight smaller than 0.60 g/cm3 are more suitable, since denser woods present difficulties when cutting [31]. The utilization of hard wood causes wear to chippers and other tools and requires higher pressures at the hot press [32]. However, the use of soft woods results in lower strength [33]. Several authors have studied the influence of different manufacturing parameters for particleboard. Of these parameters, Nemli et al. [34] studied the humidity of the mat, the amount of resin, the addition of wood powder, and the pressing time. Boonstra et al. [35] studied steam pretreatments at 200 ◦C and 210 ◦C. Han et al. [36] studied the press speed and the humidity of the raw material. Abdalla et al. [37] studied the density of the boards. Blanchet et al. [38] studied the amount of wood particles and UF (12%, 14% and 16%) in the outer layers (12%, 14% and 16%) and in the core (8%). Ashori et al. [9] studied the amount of UF (9%, 10% and 16%) in single-layer particleboards with 4, 5 and 6 min in the hot press, respectively. Wang et al. [39] studied the different adhesives in the board industry and their optimization. Current investigations are aimed at obtaining a new generation of biocomposites with a wide range of materials, such as plastics, plaster, cement, metal, glass and lignocellulosic residues. The aim of this study was to obtain particleboard using a low energy process (short pressing time and low pressing temperature), using particles of different sizes from the rachis (midrib) (Figure1) of Forests 2019, 10 FOR PEER REVIEW 3

Forests 2018, 9, 755 3 of 14 mechanical properties were tested, and the feasibility of their use as a construction material was evaluated. the three mostForests representative2019, 10 FOR PEER REVIEW palm species in urban gardening in Spain. Their physical and mechanical3 properties were tested, and the feasibility of their use as a construction material was evaluated. mechanical properties were tested, and the feasibility of their use as a construction material was evaluated.

FigureFigure 1. Frond1. Frond and and rachis of of a palma palm tree. tree. 2. Materials and Methods 2. Materials and Methods The raw material used was the rachis of the fronds of three different palm species—date, canary, The raw and washingtoniamaterial used palm—that was the the were rachis rachis kindly of of suppliedthe the fron fronds byds the of of municipality three three different different of the Sanpalm palm Anton species—date, species—date, palm grove canary, canary, and washingtonia in Orihuela, palm—that Alicante (Spain). were These kindly rachises supplied were obtained by the by municipality trimming the leaflets of the from San theAnton fronds. palm grove in Orihuela,They Alicante had an initial (Spain). moisture These content rachises of 73% were and obtainedwere air dried by trimmingfor 8 months the to leaﬂetsan approximate from the fronds. in Orihuela,moisture Alicante content (Spain). of 8%. TheseParticles rachises were obtained were obtainedusing a laboratory-scale by trimming ring-knife the leaflets chipper, from after the fronds. They had which an initial they were moisture sieved using content a horizontal of 73% screen and wereshaker airwith dried sieves forof 8, 8 4, months 2, 1 and 0.25 to anmm approximate to moisture contentcontentremove oversize of of 8%. 8%. and Particles Particles undersize were were (dust) obtained particles.obtained using A combinationusing a laboratory-scale a laboratory-scale of the fractions ring-knife retained ring-knife on chipper, each sievechipper, after which after whichthey were theywas sieved were used forsieved using the panel ausing horizontal manufacture: a horizontal screen 0.25 to shakerscreen 1 mm, 1 withshakerto 2 mm sieves andwith 2 oftosieves 4 8, mm. 4, 2,of 18, and 4, 2, 0.25 1 and mm 0.25 to removemm to oversize and undersizeThe particleboard (dust) manufacturing particles. method A combination applied was of the the conventional fractions dry retained process used on eachin the sieve was remove oversizeindustry. and Particles undersize were mixed (dust) by injectionparticles. with A 8%combination UF with a 65% of solidthe fractionscontent in aretained resin blender on each sieve wasused used for the for panel 5the min. panel manufacture:The mixture manufacture: was 0.25placed to0.25 into 1 mm,to iron 1 mm, 1molds to 21 measuring mmto 2 andmm 2600and to ×4 2400 mm.to mm 4 mm. to form the mat. The The particleboard amount of material manufacturing used was variable method since the appliedthickness wasof the the panels conventional was fixed to 10 dry mm. process Mats were used in the industry. ParticlesParticlesthen pressed werewere in a mixed hotmixed plate by bypress injection injection under 2.6 with with MPa 8% of8% pressure UF UF with with for a 5 65% amin 65% at solid 130solid °C. content Acontent total in of a9in types resin a resin of blender blender for 5 min. Theparticleboard mixture was (Figure placed 2) were into made iron moldsusing three measuring different 600particle× 400 sizes mm from to the form 3 different the mat. palm The amount for 5 min.species The mixture studied. Five was replicate placed panels into were iron made molds for eachmeasuring type; therefore, 600 × a 400total mmof 45 particleboardsto form the mat. The amountof material ofwere material used produced. was used variable was variable since the since thickness the thic ofkness the panelsof the panels was ﬁxed was tofixed 10 mm.to 10 Matsmm. Mats were were then ◦ pressedthen pressed in a hotin a platehot plate press press under under 2.6 2.6 MPa MPa of pressureof pressure for for 5 min5 min at at 130 130 C.°C. A A total total ofof 99 typestypes of particleboard (Figure 22)) werewere mademade usingusing threethree differentdifferent particleparticle sizessizes fromfrom thethe 33 differentdifferent palmpalm species studied. Five replicate panels were made for each type; therefore, a total of 45 particleboards were produced.

Figure 2. Particleboards made with rachises of different palm species.

Figure 2. Particleboards made with rachis rachiseses of different palm species.

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After pressing, the particleboards were conditionedconditioned at 2020 ◦°CC and 65% relative humidity for one weekweek inin aa verticalvertical position.position. The ﬁnishedfinished particleboards were trimmed to avoid edge effects to a ﬁnalfinal size ofof 600600 mmmm ×× 400400 mm mm ×× 1010 mm, mm, and and then then cut cut into into various various sizes sizes for for property property evaluation evaluation according toto ENEN 326-1:1994326-1:1994 [[40]40] (Figure(Figure3 3).).

600 300 250

1 = 50

2 = 50

1 4 50 cond 300

2 5 50 400 1T 2T 3T 250

3 6 50

50 3 =

250 50 50 50 50 50 Figure 3. Cutting scheme of particleboards.

Some physical properties were determined in accordance with appropriate EN standards for woodwood products:products: density [[41],41], water absorption (WA), thicknessthickness swellingswelling (TS)(TS) afterafter 22 andand 2424 hh of water immersionimmersion [[42]42] andand the the thermal thermal conductivity conductivity was was measured measured following following the the heat he ﬂowat flow meter meter method method [43]. The[43]. mechanical The mechanical properties properties determined determined were the were modulus the modulus of rupture of rupture (MOR) and (MOR) modulus and modulus of elasticity of (MOE)elasticity [44 (MOE)], and internal[44], and bond internal strength bond (IB)strength [45]. (I EachB) [45]. panel Each was panel cut towas obtain cut to six obtain density six samples density (50samples mm × (5050 mm mm), × 50 three mm), WA/TS three samplesWA/TS samples (50 mm × (5050 mm mm), × 50 six mm), MOR/MOE six MOR/MOE samples samples (different (different lengths, dependinglengths, depending on the thickness, on the thickness,×50 mm ×50 width), mm andwidth), three and IB samplesthree IB (50samples mm × (5050 mm mm). × 50 Table mm).1 shows Table the1 shows relation the between relation thebetween samples the and samples the properties and the properties tested. The tested. tests for The the tests mechanical for the mechanical properties, WA,properties, TS, and WA, density TS, and were density conducted were conducted on an Imal on universal an Imal testinguniversal machine testing (Model machine IB600, (Model Modena, IB600, Italy).Modena, Samples Italy). ofSamples 300 × 300of 300 mm × 300 were mm used were to used test the to test thermal the thermal conductivity conductivity using a using heat ﬂowa heat meter flow (Modelmeter (Model HFM 436/3/0, HFM 436/3/0, NETZSCH NETZSCH Gerätebau Gerätebau GmbH, GmbH, Selb, Germany).Selb, Germany).

Table 1. Name of the samples and tests conducted. Table 1. Name of the samples and tests conducted.

PropertyProperty Name Name of of the the Samples Numb Numberer of SamplesSamples Dimensions (mm) (mm) DensityDensity 1, 1, 2, 2, 3, 3, 4, 5, 66 6 50 50× ×50 50 × TS,TS, WA WA 2, 2, 4, 4, 5 3 50 50 ×50 50 MOR, MOE 1=, 2=, 3=, 1T, 2T, 3T 6 250 × 50 MOR, IBMOE 1=, 2=, 3=, 1, 3,1T, 6 2T, 3T 36 50 250× ×50 50 ConductivityIB 1, cond 3, 6 13 300 50× ×300 50 Conductivity cond 1 300 × 300 Particleboards were classiﬁed in accordance with the European standard [46] considering the requirementsParticleboards for wood were particleboard classified in with accordance a thickness with range the European of 6–13 mm. standard [46] considering the requirementsThe results for were wood analyzed particleboard using the with IBM a thickness SPSS v.24.0 range software of 6–13 (Amonk, mm. NY, USA). Average values and theThe standard results were deviation analyzed of the using properties the IBM of the SPSS panels v.24.0 were software obtained (Amonk, to determine NY, USA). the variability. Average One-wayvalues and analysis the standard of variance deviation was conductedof the properties to identify of the the panels inﬂuence were between obtained the to manufactureddetermine the processvariability. (material One-way and particleanalysis size) of variance and the propertieswas conducted of the particleboard.to identify the influence between the manufactured process (material and particle size) and the properties of the particleboard. 3. Results 3. Results 3.1. Physical Properties 3.1. Physical Properties The experimental particleboard can be considered to be of medium density (Table2). The average valuesThe obtained experimental were 841.55 particleboard kg/m3 with can canary, be consider 813.20ed kg/m to 3bewith of medium washingtonia density and (Table 797.38 2). kg/m The3 average values obtained were 841.55 kg/m3 with canary, 813.20 kg/m3 with washingtonia and 797.38 kg/m3 with date palm rachis. The boards with the highest density were achieved with the smallest

Forests 2019, 10 FOR PEER REVIEW 5

particle size (0.25–1 mm) for the three species. There was no significant deviation in the thickness of the panels since this was a fixed variable of the manufacturing process.

Table 2. Average values of the physical properties of the palm rachis particleboard.

Particle Thickness Density TS 2 h TS 24 h WA 2 h WA 24 h Material (mm) (mm) (kg/m3) (%) (%) (%) (%) 10.42 856.37 37.83 49.74 72.14 85.91 Canary 0.25–1 Forests 2018, 9, 755 (0.24) (28.67) (5.60) (8.11) (5.49) (8.12)5 of 14 10.63 840.31 25.22 38.21 54.31 71.56 Canary 1–2 (0.38) (17.54) (2.65) (2.46) (3.90) (4.64) with date palm rachis. The boards10.68 with the highest818.95 density 32.11 were achieved39.44 with the59.43 smallest 71.15 particle Canary 2–4 size (0.25–1 mm) for the three species.(0.10) There was(7.54) nosigniﬁcant (2.09) deviation (2.43) in the thickness(3.26) of the(4.52) panels 10.53 878.25 31.35 39.95 63,43 77.29 sinceWashingtonia this was a ﬁxed0.25–1 variable of the manufacturing process. (0.31) (47.17) (7.94) (9.93) (9.94) (12.52) 10.61 815.32 29.86 38.33 59.86 79.43 WashingtoniaTable 2. Average1–2 values of the physical properties of the palm rachis particleboard. (0.97) (41.22) (6.46) (5.86) (9.90) (14.02) 3 Material Particle (mm) Thickness10.57 (mm) Density746.30 (kg/m ) TS 226.65 h (%) TS 2438.90 h (%) WA61.35 2 h (%) WA 2472.72 h (%) Washingtonia 2–4 Canary 0.25–1 10.42(0.55) (0.24) 856.37(35.80) (28.67) 37.83(4.40) (5.60) 49.74(5.84) (8.11) 72.14(8.64) (5.49) 85.91(11.83) (8.12) Canary 1–2 10.63 (0.38) 840.31 (17.54) 25.22 (2.65) 38.21 (2.46) 54.31 (3.90) 71.56 (4.64) 10.48 841.73 25.51 31.96 61.90 82.78 CanaryDate 2–40.25–1 10.68 (0.10) 818.95 (7.54) 32.11 (2.09) 39.44 (2.43) 59.43 (3.26) 71.15 (4.52) Washingtonia 0.25–1 10.53(0.35) (0.31) 878.25(32.01) (47.17) 31.35(1.79) (7.94) 39.95(4.79) (9.93) 63,43(9.96) (9.94) 77.29(6.59) (12.52) Washingtonia 1–2 10.6110.68 (0.97) 815.32765.49 (41.22) 29.8623.57 (6.46) 38.3333.03 (5.86) 59.8662.62 (9.90) 79.4374.91 (14.02) Date 1–2 Washingtonia 2–4 10.57(0.51) (0.55) 746.30(23.24) (35.80) 26.65(2.61) (4.40) 38.90(3.83) (5.84) 61.35(4.73) (8.64) 72.72(8.89) (11.83) Date 0.25–1 10.4810.26 (0.35) 841.73784.92 (32.01) 25.5119.99 (1.79) 31.9634.09 (4.79) 61.9047.84 (9.96) 82.7861.26 (6.59) DateDate 1–22–4 10.68 (0.51) 765.49 (23.24) 23.57 (2.61) 33.03 (3.83) 62.62 (4.73) 74.91 (8.89) Date 2–4 10.26(0.25) (0.25) 784.92(25.56) (25.56) 19.99(3.93) (3.93) 34.09(4.91) (4.91) 47.84(2.51) (2.51) 61.26(8.41) (8.41) (( ) Standard) Standard deviation. deviation. TS: Thickness TS: Thickness swelling swelling after 2 and after 24 h2 of and water 24 immersion. h of water WA: immersion. Water absorption WA: afterWater 2 andabsorption 24 h of water after immersion. 2 and 24 h of water immersion.

TheThe TSTS averageaverage valuesvalues afterafter 22 hh ofof waterwater immersionimmersion rangedranged fromfrom 19.9919.99 toto 37.83%;37.83%; afterafter 2424 hh theythey rangedranged fromfrom 31.9631.96 toto 49.74%49.74% (Figure(Figure4 ).4). The The date date palm palm particleboard particleboard showed showed the the lowest lowest TS TS results, results, whilewhile thethe canarycanary palmpalm particleboardsparticleboards showedshowed thethe highesthighest TSTS results.results. AsAs cancan bebe observedobserved inin FigureFigure4 4a,a, thethe TSTS valuevalue diddid notnot dependdepend onon thethe particle particle size. size.

(a) (b)

FigureFigure 4.4. ((aa)) AverageAverage TSTS valuesvalues afterafter 2424 hh ofof waterwater immersionimmersion accordingaccording toto thethe palmpalm speciesspecies andand particleparticle sizesize (b(b)) Average Average TS TS values values after after 2 2 and and 24 24 h h according according to to palm palm species. species.

The average WA results after 2 h of water immersion ranged from 47.48 to 72.14%, and after 24 h from 61.26 to 85.91% (Figure5). Particleboards manufactured with smaller particles (0.25 to 1 mm) absorbed a larger amount of water and had greater TS values.

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The average WA results after 2 h of water immersion ranged from 47.48 to 72.14%, and after 24 h from 61.26 to 85.91% (Figure 5). Particleboards manufactured with smaller particles (0.25 to 1 mm) Forests 2018, 9, 755 6 of 14 absorbed a larger amount of water and had greater TS values.

(a) (b)

FigureFigure 5.5. ((a)) Average Average WA WA values values after after 2 2and and 24 24 h of h ofwa waterter immersion immersion according according to palm to palm species. species. (b(b)) Average Average WA WA values values after after 2 2 and and 24 24 h h of of water water immersion immersion according according to to particle particle size. size.

3.2.3.2. MechanicalMechanical PropertiesProperties 2 CanaryCanary palm palm particleboards particleboards had had an an average average MOR MOR performance performance of of 13.97, 13.97, 19.85, 19.85, and and 12.68 12.68 N/mm N/mm2 fromfrom thethe smallersmaller toto biggerbigger particleparticle sizesize respectivelyrespectively TheThe MORMOR achievedachieved byby thethe washingtoniawashingtonia palmpalm 2 particleboardsparticleboards was was 16.95, 16.95, 12.40, 12.40, and and 7.38 7.38 N/mm N/mm2.. TheThe datedate palmpalm boardsboards hadhad thethe lowestlowest MORMOR valuesvalues 2 withwith 13.51, 13.51, 10.76, 10.76, and and 7.85 7.85 N/mm N/mm2 (Table3 3).).The The highest highest MOR MOR value value was was obtained obtained with with canary canary palm, palm, withwith 1–21–2 mmmm ofof particle size. The The MOE MOE showed showed the the same same tendency tendency asas the the MOR, MOR, and and was was higher higher for forthe the canary canary palm palm boards boards and and lower lower for forthe thedate date palm palm panels. panels. TheThe IB values IB values ranged ranged from from 0.720.72 to 0.99 to 2 0.99N/mm N/mm2, with, withthe canary the canary palm palm boards boards having having the lowest the lowest values. values.

TableTable 3.3. AverageAveragemechanical mechanical properties properties of of particleboards particleboards from from different different palm palm rachis rachis species. species. 2 2 2 Material Particle (mm) (mm) MOR MOR (N/mm (N/mm)2) MOEMOE (N/mm (N/mm)2) IBIB (N/mm (N/mm) 2) Canary 0.25–1 0.25–1 13.97 13.97 (0.45) (0.45) 1567.16 1567.16 (49.12) (49.12) 0.75 0.75 (0.07) (0.07) Canary 1–2 1–2 19.85 19.85 (0.74) (0.74) 2018.63 2018.63 (106.11) (106.11) 0.97 0.97 (0.12) (0.12) Canary 2–4 12.68 (0.41) 1373.56 (87.24) 0.72 (0.06) Canary 2–4 12.68 (0.41) 1373.56 (87.24) 0.72 (0.06) WashingtoniaWashingtonia 0.25–1 0.25–1 16.95 16.95 (1.52) (1.52) 1526.40 1526.40 (161.00) (161.00) 0.95 0.95 (0.15) (0.15) Washingtonia 1–2 12.40 (1.07) 1208.22 (139.39) 0.97 (0.15) WashingtoniaWashingtonia 2–4 1–2 12.407.38 (0.34) (1.07) 1208.22 662.98 (49.40) (139.39) 0.99 0.97 (0.13) (0.15) Washingtonia 2–4 7.38 (0.34) 662.98 (49.40) 0.99 (0.13) Date 0.25–1 13.51 (0.80) 1263.07 (59.93) 0.93 (0.07) Date 0.25–1 1–2 10.76 13.51 (1.07) (0.80) 1263.07988.14 (63.73) (59.93) 0.98 0.93 (0.06) (0.07) Date 2–4 1–2 10.767.85 (0.32) (1.07) 694.96 988.14 (45.01) (63.73) 0.91 0.98 (0.07) (0.06) P1Date [46] 2–4 7.85 10.5 (0.32) 694.96 - (45.01) 0.91 0.28 (0.07) P1P2 [[46]46] 10.511 1800 - 0.40 0.28 P2P3 [[46]46] 15 11 2050 1800 0.45 0.40 P3P4 [[46]46] 16 15 2300 2050 0.40 0.45 ( ) StandardP4 deviation. [46] MOR: Modulus of rupture. MOE: 16 Modulus of elasticity. 2300 IB: Internal bond. 0.40 Pn. Standard reference values [46]. ( ) Standard deviation. MOR: Modulus of rupture. MOE: Modulus of elasticity. IB: Internal bond. Pn. Standard reference values [46]. European standards classify wood particleboard according to its physical and mechanical properties [46], starting with the P1 grade for general indoor uses, to the P7 grade for structural uses.

All three types of particleboard made of canary palm particles exceeded the requirements for grade P1 or higher (Figure6). Panels made with washingtonia and date palm with particle sizes of

0.25–1 and 1–2 mm could be classiﬁed as grade P1. Boards made with canary palm with a particle size Forests 2019, 10 FOR PEER REVIEW 7

European standards classify wood particleboard according to its physical and mechanical properties [46], starting with the P1 grade for general indoor uses, to the P7 grade for structural uses. All three types of particleboard made of canary palm particles exceeded the requirements for Forests 2018, 9, 755 7 of 14 grade P1 or higher (Figure 6). Panels made with washingtonia and date palm with particle sizes of 0.25–1 and 1–2 mm could be classified as grade P1. Boards made with canary palm with a particle ofsize 1–2 of mm1–2 mm reached reached the gradethe grade P2. P2. Washingtonia Washingtonia and and date date palm palm boards boards made made with with a particle a particle size size of 0.25–1of 0.25–1 mm mm met met the the minimum minimum values values of MORof MOR and and IB IB of P2of P2 but but not no thet the required required MOE MOE value. value. None None of theof the particleboards particleboards achieved achieved the the grade grade P3, P3, which which requires requires a minimum a minimum MOE MOE value value of 2050of 2050 N/mm N/mm2. 2.

(a) (b)

(c)

Figure 6. (a) MOR. (b) MOE (c) IB accordingaccording to palmpalm speciesspecies and particleparticle size and compared to the European standards (black horizontalhorizontal lines)lines) [[46].46].

3.3. Thermal Conductivity The average values values of of thermal thermal co conductivitynductivity ranged ranged from from 0.053 0.053 to to0.061 0.061 W/mK W/mK (Figure (Figure 7). No7). Nosignificant signiﬁcant dependence dependence was was observed observed between between the the particle particle sizes sizes or or palm palm species species with thermal conductivity (Figure8 8).).

Forests 2018, 9, 755 8 of 14 Forests 2019, 10 FOR PEER REVIEW 8 Forests 2019, 10 FOR PEER REVIEW 8

FigureFigure 7.7 Average7Average Average thermal thermal conductivity conductivity resultsresults accord accordingaccordinging to to the the palm palm species species and and particle particle size. size.

(a) (b) (a) (b) FigureFigure 8. (8.a )(a Average) Average thermal thermal conductivity conductivity results results accordingaccording to the the palm palm species. species. (b ()b Average) Average thermal thermal conductivityFigureconductivity 8. (a) Average results results according thermal according conductivity to to the the particle particle result size. size.s according to the palm species. (b) Average thermal conductivity results according to the particle size. 4. Discussion4. Discussion 4. DiscussionSeveralSeveral authors authors have have obtained obtained particleboards particleboards mademade of palm rachis rachis with with an an adhesive adhesive following following a a similarsimilarSeveral manufacturing manufacturing authors have process process obtained (Table (Table particleboards4 ).4). Nemli Nemli et et al. al.made [[8]8] manufacturedof palm rachis boards boardswith an from from adhesive date date palm palmfollowing using using a ◦ 33%similar33% of ofmanufacturing UF UF and and applying applying process 150150 °C (TableC for for 55 4).min min Nemli in in a ahot et hot al.press. press.[8] manufacturedThey They obtained obtained boardsboards boards with from witha datedensity a palm density of 650using of 65033%kg/ kg/m of m UF3, 3a , andgood a good applying MOR, MOR, and 150 and a low°C a low for TS TS5due min due to inthe to a thehigh hot high press.amount amount They of adhesive ofobtained adhesive used. boards used. The withboards The boardsa densityobtained obtained of by 650 bykg/Ashori Ashori m3, a goodand and Nourbakhsh NourbakhshMOR, and a [9] [low9 ]achieved achieved TS due ato alower lowerthe high MOR MOR amount and and a ahigher of higher adhesive density density used. than than The the the boardslatter latter when obtained when using using by ◦ 9%Ashori9% of UFof and UF at 160atNourbakhsh 160C, °C, but but they they[9] accomplished achieved accomplished a lower better better MOR mechanical me andchanical a higher properties properties density by by increasing thanincreasing the latter the the amount amountwhen using of of UF UF to 11%. Amirou et al. [13] studied the mechanical properties of boards from date palm to9% 11%. of UF Amirou at 160 °C, et al. but [13 they] studied accomplished the mechanical better propertiesmechanical of properties boards from by increasing date palm manufacturedthe amount of manufactured at a higher temperature and pressing◦ time (195 °C and 7.5 min), and 10% of PF. Their atUF a higherto 11%. temperature Amirou et and al. pressing[13] studied time (195the mechanicalC and 7.5 min), proper andties 10% of of boards PF. Their from results date showed palm results showed a large increase in MOE, satisfactory values of MOR and IB, and a low TS value, amanufactured large increase at in a higher MOE, satisfactorytemperature values and pressing of MOR time and (195 IB, and°C and a low 7.5 TSmin), value, and probably10% of PF. due Their to probably due to the nature of the curing resin. Hegazy and Ahmed and Hegazi et al. [14,15] studied theresults nature showed of the a curing large increase resin. Hegazy in MOE, and satisfac Ahmedtory and values Hegazi of et MOR al. [14 and,15] IB, studied and a the low inﬂuence TS value, of the influence of density on the strength of panels produced with date palm fibers at 140 °C for 10 min density on the strength of panels produced with date palm ﬁbers at 140 ◦C for 10 min and 160 ◦C for probablyand 160 due °C forto the8 min nature with ofthe the same curing resin resin. dose. Hegazy Better strength and Ahmed (MOR, and MOE Hegazi and IB) et al.was [14,15] achieved studied in 8 min with the same resin dose. Better strength (MOR, MOE and IB) was achieved in both cases with theboth influence cases withof density higher on densities. the strength In addition, of panels the produced mechanical with properties date palm achieved fibers wereat 140 greater °C for with10 min higher densities. In addition, the mechanical properties achieved were greater with the increase in andthe 160 increase °C for in8 minthe pressingwith the temperature. same resin dose. In genera Betterl, thestrength results (MOR, of the MOEpresent and study, IB) was which achieved used a in thebothlower pressing cases pressing with temperature. higher temperature, densities. In general, pressing In addition, the time, results theand of mechanicalam theount present of propertiesresin study, were which achievedin accordance used were a lower withgreater pressing those with temperature,thedescribed increase above.in pressing the pressing time, andtemperature. amount of In resin genera werel, the in accordance results of the with present those describedstudy, which above. used a lower pressing temperature, pressing time, and amount of resin were in accordance with those described above.

Forests 2018, 9, 755 9 of 14

Regarding the palm species, several studies have been made with canary palm. Ferrandez-Garcia et al. [22] obtained binderless boards with poor mechanical properties and a high TS value. Garcia-Ortuño et al. [19] substituted the UF with 10% of starch and longer pressing times. They obtained panels with a high density and with a good MOR and IB. These results are similar to the values obtained in the present work. Ferrandez-Garcia et al. [18] made boards from the rachis of washingtonia palm using a pressing temperature of 120 ◦C for 6 min. They obtained better values of MOR and MOE, and similar but lower values of IB and TS than the present study just by lowering the temperature by 10 ◦C and increasing the pressing time by 1 min. In general, using a higher processing temperature and a higher density resulted in the improved mechanical behavior of the particleboard, nevertheless, the best performance was obtained with pressing times between 5 and 7.5 min. The addition of higher amounts of UF was found to improve the mechanical properties and to reduce the TS, however, regulation limits must be taken into account due to the harmful effects of UF on human health. Particleboards with good mechanical properties were produced using pressing temperatures between 120 and 140 ◦C.

Table 4. Average properties of palm rachis particleboards by different authors.

Material: Temp Time Adhesive Density MOR MOE IB TS 24 h Source Rachis Palm (◦C) (min) (%) (kg/m3) (N/mm2) (N/mm2) (N/mm2) (%) Date 150 5 33% UF 650 15.3–18.9 - 0.43–0.83 14.4 [8] Date 160 5 9% UF 750 10.5 1333 0.38 30.1 [9] Date 160 5 11% UF 750 16.6 1861 0.63 30.1 [9] Date 195 7.5 10% PF 700 14.0 2780 0.66 14.9 [13] Date 140 10 10% UF 650 6.7 950 0.67 84 [15] Date 140 10 10% UF 750 8.2 1140 2.43 23.8 [15] Date 160 8 10% UF 670 9.04 1443 0.43 - [14] Date 160 8 10% UF 790 13.3 2018 0.53 - [14] Canary 120 15 None 850 6.0 1022.32 0.39 55.22 [22] Canary 110 240 10% Starch 1100 13.85 1702.67 0.63 28.98 [19] Washing. 120 6 8% UF 866 16.73 1469.78 0.90 41.20 [18] Canary 130 5 8% UF 842 15.93 1696.13 0.83 42.90 This study Washing. 130 5 8% UF 813 12.48 1129.24 0.97 39.44 This study Date 130 5 8% UF 797 10.71 982.06 0.94 33.02 This study (UF) Urea–formaldehyde, (PF) Phenol–formaldehyde.

Industrial companies in Spain traditionally employ pine wood particles (Pinus radiata D. Dom) and other sawmill leftovers in the manufacturing of commercial particleboard. The standard process uses a higher pressing temperature and amount of UF [47,48], resulting in single-layer panels that have a similar MOR but higher MOE than those obtained in this study (Table5). A different study [ 49] that used a hard wood such as eucalyptus (Eucalyptus urophylla S.T. Blake), manufactured particleboards that achieved a greater MOR but had a thickness of 17 mm.

Table 5. Average properties of commercial particleboards tested by different authors.

Thick Temp UF Density MOR MOE IB Manufac- Material Source (mm) (◦C) (%) (kg/m3) (N/mm2) (N/mm2) (N/mm2) turer Pine wood 16 200 15% 660 13.7 1626 1.36 AA [47] Mix of pine, 13 200 12% 665 11.7 2425 0.35 TL [48] oak & beech wood 75% pine wood & 13 200 12% 630 9.68 2094 0.30 EM [48] sawmill rests Eucalyptus 17 190 12% 838 33.0 - 0.45 - [49] Canary Palm 10.58 130 8% 842 15.93 1696.13 0.83 - This study Wash. Palm 10.57 130 8% 813 12.48 1129.24 0.97 - This study Date Palm 10.48 130 8% 797 10.71 982.06 0.94 - This study (AA) Aserraderos Aragón, S.A., (TL) Tableros Losan, S.A., (EM) Unión de Empresas Madereras, S.A. Forests 2018, 9, 755 10 of 14

Wood has always been considered a building material with good insulation properties. For comparison purposes, the thermal conductivity of some woods with a similar density to that of the particleboard in the present study is shown in Table6. The experimental particleboard has lower thermal conductivity values than those made from wood.

Table 6. Average thermal conductivity values of wood with similar densities as rachis particleboards.

Material Density (kg/m3) Thermal Conductivity λ (W/mK) Source Maple wood 750 0.349 [50] Oak wood 850 0.209 [50] Beech wood 800 0.143 [50] Canary palm rachis 842 0.057 This study Washingtonia palm rachis 813 0.059 This study Date palm rachis 797 0.059 This study

In terms of thermal conductivity, the results suggested the possibility of using these particleboards as thermal insulation materials, since they had better thermal properties than commonly-used woods with similar densities. In the literature, some authors have investigated the thermal conductivity of particleboards using ﬁbers from the rachis of palm species. The results of such tests are in accordance with the values achieved in this study (Table7). The board density and palm species did not have a relevant effect on their thermal insulating capacity. The composites shown in the table had the least insulation capacity, as cement and gypsum are not good insulators.

Table 7. Average thermal conductivity values of palm particleboards studied by different authors.

Board Type Material: Rachis Density (kg/m3) Thermal Conductivity λ (W/mK) Source Particleboard Date palm 254 0.042 [12] Particleboard Date palm 273 0.084 [12] Particleboard Date palm 176 0.048 [51] Particleboard Date palm 270 0.070 [51] Composite Date palm/ (PF) 1240 0.160 [28] Composite Date palm/ (PF) 1320 0.200 [28] Composite 20% Date palm/gypsum 736 0.174 [23] Composite 50% Date palm/cement 1217 0.243 [26] Particleboard Washingtonia palm 860 0.084 [21] Particleboard Washingtonia palm 746 0.062 [18] Particleboard Canary palm 880 0.054 [22] Particleboard Canary palm 1030 0.079 [22] Particleboard Canary palm 842 0.057 This study Particleboard Washingtonia palm 813 0.059 This study Particleboard Date palm 797 0.059 This study (PF) Phenol–formaldehyde.

The results showed that the physical and mechanical properties of particleboard in general were inﬂuenced by the palm species in addition to other parameters, such as the particle size, pressing temperature, pressure, pressing time, raw material, amount and type of adhesive used, particle moisture, and particleboard density. In the present study, the manufacturing variables were the palm species and the particle size. A statistical analysis was conducted in order to determine the dependence of the mechanical and physical properties of the experimental particleboards with respect to the variables of the study (Table8). Forests 2018, 9, 755 11 of 14

Table 8. ANOVA of the results of the tests.

Factor Properties Sum of Squares d.f. Half Quadratic F Sig. Particle size Density (kg/m3) 59,221.826 2 29,610.913 15.628 0.000 MOR (N/mm2) 437.186 2 218.593 25.186 0.000 MOE (N/mm2) 4,212,577.867 2 2,106,288.933 20.621 0.000 IB (N/mm2) 0.066 2 0.033 1.825 0.173 TS 24 h (%) 187.647 2 93.823 1.607 0.212 WA 24 h (%) 1103.284 2 551.642 4.946 0.011 Palm species Density (kg/m3) 12,789.499 2 639.749 2.185 0.124 MOR (N/mm2) 181.009 2 90.505 6.297 0.004 MOE (N/mm2) 3,737,773.838 2 1,868,886.919 16.584 0.000 IB (N/mm2) 0.161 2 0.080 5.031 0.011 TS 24 h (%) 631.436 2 315.718 6.508 0.003 WA 24 h (%) 119.859 2 59.929 0.449 0.641 d.f.: degrees of freedom. F: Fisher–Snedecor distribution. Sig.: signiﬁcance.

With a signiﬁcance of <0.05, the density, MOR, MOE, and WA depended on the particle size of the particleboard, whereas the MOR, MOE, IB, and TS depended on the palm species.

5. Conclusions This study was focused on testing the mechanical, physical and thermal behavior of particleboard made of particles from the rachis of canary palm (Phoenix canariensis hort. ex Chabaud), date palm (Phoenix dactylifera L.) and washingtonia palm (Washingtonia robusta H. Wendl). Moreover, the influence of the particle size and the raw material (palm species) of the particleboard was also investigated. It can be concluded that it is feasible to manufacture particleboard using palm rachis that will have similar properties to conventional wood particleboard using a low energy manufacturing process (short pressing time and low pressing temperature). This particleboard could replace the traditional raw materials used in construction, contributing to a reduction of the pressure on forest wood resources. The particleboard made with canary palm rachis had better properties than the washingtonia and date palm rachis boards; therefore, it could be considered that it was the best raw material for the production of particleboard under the conditions tested in this study. In terms of the particle size, the results showed that it affected some of the properties of the particleboard, namely the density, WA, MOR, and MOE. A particle size of 1 to 2 mm of canary palm and 0.25 to 1 mm of washingtonia and date palm achieved the best results. In general, smaller particles had better properties. The TS, MOR, MOE and IB were found to be influenced by the palm species. Canary palm rachis particleboard of any particle size, and boards made of washingtonia and date palm with particle sizes of 0.25 to 1 mm, could be classified as P1 “General purpose boards for use in dry conditions”. Canary palm rachis with particle sizes from 1 to 2 mm could be classified as type P2 “Boards for interior fitments (including furniture) for use in dry conditions”. The particleboard manufactured can be considered a good insulating material. The thermal conductivity did not depend on the particle size or the palm species. The use of the pruning waste of palm trees to produce durable materials such as particleboard could be beneficial to the environment since it is a method of carbon fixation, helping to decrease atmospheric pollution and reducing the amount of waste that ends in dumping sites.

Author Contributions: M.-T.F.-G. and T.G.-O. conceived and designed the experiments; C.-E.F.-G., T.G.-O. and A.F.-G. performed the experiments; M.F.-V. and M.-T.F.-G. analyzed the data; M.F.-V. obtained and selected the raw materials; C.-E.F.G., A.F.-G., and J.F.H.-C. wrote the paper. Funding: This research was funded by the Ministerio de Economía y Competitividad, grant number AGL2013- 41612-R. And the APC was funded by the Department of Engineering at the Miguel Hernandez University. Forests 2018, 9, 755 12 of 14

Acknowledgments: The authors wish to acknowledge the support of the Ministerio de Economía y Competitividad of Spain through the program “Retos”. Conﬂicts of Interest: The authors declare no conﬂict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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