Chapitre 4 Physical and mechanical properties of particleboard made from extracted black spruce and trembling aspen bark

4.1 Résumé

L’écorce essentiellement utilisée pour la production d’énergie pourrait être davantage valorisée dans la fabrication de panneaux de particules à base d’écorce. L’écorce est riche en extractibles utilisables dans plusieurs domaines comme la cosmétologie, la pharmacologie ou la production des adhésifs. Cette étude vise à analyser les effets de l’extraction à l’eau chaude des particules d’écorce sur les propriétés mécaniques et physiques des panneaux de particules comme le module d’élasticité (MOE), le module de rupture (MOR), la cohésion interne (CI), la dureté Janka (DJ), le gonflement en épaisseur (GE) et la dilatation linéaire (DL). En outre, ces propriétés sont comparées à la fois à celles des panneaux témoins (100% particules de bois) et à celles de leurs homologues faits d’écorce non extraites. Les résultats ont montré que les propriétés mécaniques des panneaux de particules à base d’écorce extraite d’épinette noire et de peuplier faux-tremble diminuent avec l’augmentation de la proportion d’écorce. Parallèlement et dans les mêmes conditions, la DL augmente. En règle générale, des valeurs élevées de CI et basses de GE avec des panneaux constitués de fines particules ont été obtenues. L’extraction à l’eau chaude appliquée aux particules d’écorce a détériorée toutes les propriétés physiques et mécaniques des panneaux de particules fabriqués à l’exception de la dureté. Cependant, le MOE et le MOR des panneaux ayant 50% d’écorce extraite d’épinette noire et de peuplier faux-tremble ont respecté les exigences de la norme ANSI A208.1-1999 pour les panneaux de particules à moyenne densité à usage commercial (M-1) et de sous-plancher (PBU). En revanche, aucun des panneaux fabriqués n’a respecté les exigences de la norme en matière de stabilité dimensionnelle (GE et DL).

4.2 Abstract

Bark residues are mostly used for thermal energy production. However, a better utilization of that resource could be as raw material for particleboard manufacturing. Bark is also a source of numerous extractives used for several applications including pharmacology and adhesive production. This study aims at analyzing the effect of hot water extracted bark particle content and size on the mechanical and physical properties of bark particleboards including the modulus of elasticity (MOE), modulus of rupture (MOR), internal bond (IB), Janka hardness (HJ), thickness swelling (TS) and linear expansion (LE). Moreover, these properties were compared both to a control (100% wood particles) and to particleboard made from unextracted bark. The results showed that, while the mechanical properties of the particleboard made from extracted black spruce and trembling aspen bark decreased with increasing bark content, LE increased. Particleboard made of fine particles often showed higher IB and lower TS values. Hot water extraction of the bark had a detrimental effect on all the physical and mechanical properties of the particleboards produced except for the Janka hardness where no significant decrease was found. The MOE and MOR of the particleboards made from 50% black spruce and trembling aspen bark met the requirements of the ANSI standard for commercial (M-1) and underlayment (PBU) grades. In contrast, the dimensional properties (TS and LE) of all the boards did not fulfill the minimum requirements of the ANSI standard.

4.3 Introduction

Large quantities of bark are produced by the forest industry and are mostly used for thermal energy production. In the Province of Quebec, Canada, more than 3.5 million tons of anhydrous bark was produced in 2005 (Anonymous 2007). Research projects are carried out in order to foster the use of bark for higher value added products such as alternative raw material for particleboard manufacturing (Blanchet 1999, Blanchet et al. 2000, Villeneuve 2004, and Ngueho Yemele et al. 2007).

Bark is also an important source of extractives used in several fields including pharmacology and adhesive production. Experimental studies support the significant role of the polyphenols as anti-inflammatory, anti-allergic, and antiviral agents (Middleton 1998) as well as their effect in cancer prevention (Savouret and Quesne 2002) and cardiovascular diseases (Frankel et al. 1993). Recent developments in tannin-based adhesives were presented by Pizzi (2006).

Two major approaches to manufacture bark particleboards can be identified in the literature. The first one is based on bark plasticization and extractives polymerization for the self bonding of the bark particles (Burrows 1960, Chow and Pickles 1971, Wellons and Krahmer 1973, Chow 1975, Troughton and Gaston 1997). The second one focuses more on bark particles for their physical properties rather than their chemical properties. Synthetic adhesives including urea-formaldehyde, phenol-formaldehyde, isocyanates and extractives based adhesives were used to bond bark particles (Dost 1971, Deppe and Hoffman 1972, Maloney 1973, Lehmann and Geimer 1974, Anderson et al. 1974ab, Place and Maloney 1977, Wisherd and Wilson 1979, Muszynski and McNatt 1984, Suzuki et al. 1994, Blanchet et al. 2000, Villeneuve 2004, Nemli et al. 2004b, Nemli and Colakoglu 2005, Ngueho Yemele et al.2007). 

The presence of extractives in the raw material impacts the particleboard in both negative and positive ways. Moslemi (1974) reported that extractives can have adverse effects on the setting of adhesives, thereby lowering the particle-particle bond strength. Extractives may cause blows and severely reduce the internal bond strength. In the other hand, phenolic extractives can react with formaldehyde and limit water absorption as well as improve thickness swelling resistance of the board (Moslemi 1974, Anderson et al. 1974abc, Plackett and Troughton 1997, Nemli et al. 2004ab and 2006, Nemli and Colakoglu 2005). For instance, Nemli et al. (2004a, 2006) found a significant improvement of thickness swelling, decay resistance and formaldehyde emission of particleboard made from wood particles impregnated with bark extractives. However, the mechanical properties of these boards were lower than for those made from unimpregnated particles. Similar results were reported by Nemli et al. (2004b) and Nemli and Colakoglu (2005) with addition of black locust and mimosa bark particles to the furnish. Anderson et al. (1974a, c) found that paraformaldehyde added to wood sprayed with concentrated ponderosa pine and white fir bark extract reacted with phenolic compounds present in the extract and formed a waterproof bonding agent which improved the board water absorption resistance and thickness swelling. Therefore, extracted bark particles may lead to high moisture absorption and thickness swelling. The high content of condensed polyphenol present in bark and able to react with formaldehyde was pointed out as the main raison of the aforementioned improvement (Nemli et al. 2004b).

Nevertheless, the use of bark in wood particleboard manufacturing is currently viewed negatively due to the fact that excessive bark content in the raw material produces significant adverse effects on strength and dimensional properties. Several examples given in the literature demonstrate a decrease of the modulus of elasticity (MOE), modulus of rupture (MOR) and internal bond (IB) with addition of bark while the linear expansion (LE) increased (Dost 1971, Lehmann and Geimer 1974, Wisherd and Wilson 1979, Muszynski and McNatt 1984, Blanchet et al. 2000, Ngueho Yemele et al.2007). Muszynski and McNatt (1984) indicated that particleboards suitable for furniture manufacturing could be made from up to 30% spruce bark content. Suzuki et al. (1994) found 35% as the tolerable limit of bark substitution for particleboards. Xing et al. (2006) included up to 40% bark fibers in medium density fiberboard and found its effect on the mechanical and physical properties more detrimental for the MOE, MOR, IB, and LE than for thickness swelling (TS) and water absorption.

Particle geometry is a prime parameter affecting both board properties and its manufacturing process (Moslemi 1974). Suchsland and Woodson (1990) pointed out the high significance of particle geometry in the development of board properties. It has a definite relationship with the compression ratio, and thus it will influence the density of the composite (Brumbaugh 1960, Bhagwat 1971, Hoglund et al. 1976, Kelley 1977).

The type of particle, its geometry and the combination of particles of different type and geometry have significant impacts on board quality (Maloney 1993). The variation of particle geometry results in different fiber surface areas which have a direct impact on the adhesive content per unit fiber surface area (kg/m²) (Moslemi 1974). Generally, the specific surface area (m²/kg) of longer fibers is lower than that of shorter fibers of the same species and thickness due to the higher surface of the fiber cross sections. Thus, the adhesive content per unit fiber surface area is higher for long fibers than for short fibers at a given adhesive content per unit oven-dry mass of particles.

Only few studies examined the effect of bark particle geometry on the properties of particleboard. Gertjejansen and Haygreen (1973) compared properties of wafer and flake- type particles and found that the IB, the LE and the TS were higher on waferboard made from 13 mm wide (1/2 inch) flakes than those made from 38 mm wide (1-1/2 inch) wafers. Blanchet (1999) found that the substitution of wood particles by wood fibers in the surface layer improved the MOE and the MOR of particleboards made from black spruce bark. Ngueho Yemele et al.(2007) found that the MOE, MOR, and IB of particleboard made from black spruce and trembling aspen unextracted bark decreased with increasing bark content. They also found that the IB of the boards often decreased with increasing bark particle size. 

The objectives of this study were 1) to determine the effect of extracted bark particle content and size on the physical and mechanical properties of particleboard made from black spruce and trembling aspen bark; 2) to compare the physical and mechanical properties of particleboard made from extracted bark to those of unextracted bark.

4.4 Material and methods

4.4.1 Bark extraction and particle production

Fresh black spruce (Picea mariana (Mill.)) and trembling aspen (Populus tremuloides (Michx.)) bark was collected respectively from Arbec Forest Products Inc. softwood sawmill located in L’Ascension, Québec, Canada and from the Louisiana Pacific Canada, oriented strandboard mill located in Chambord, Québec, Canada. The raw bark was taken directly from the debarking units in each mill. Bark anhydrous density, wood content of bark residues, and the effective bark content ratio (ebacr) of the panels were determined according to the procedure described in Ngueho Yemele et al. (2007).

Hot water extraction was performed at a large scale in order to produce extracted particles for particleboard manufacturing. Bark was soaked in warm water at the initial temperature of 55°C water. The mean concentration was 35 and 28 g of dried weight per litre of hot water for black spruce and trembling aspen, respectively. The system was heated with water vapor and the average temperature maintained at 100°C for 3 hours. The weight loss ratio was determined gravimetrically and the results were reported as a percentage of the dry raw material weight. Weight loss comprises the extracts and undesirable materials like sand, stone, etc. A weight loss after extraction and air-drying of 16.6 and 10.8% was obtained for black spruce and trembling aspen, respectively.

The air-dried extracted bark was crushed in a hammer mill and sieved in four groups: one for the surface layer and three for the core layer. Wood particles were added to the bark particles to produce mixed wood and bark particleboards. The particle size distribution of each raw material type (extracted bark and wood) was determined with a CE Tyler testing sieve shaker.

4.4.2 Bark organic and inorganic extractive content

The bark was sampled and prepared according to Tappi T 257 cm-02. Total extractive contents, for both unextracted and extracted bark, were determined by successive extraction of bark flour with organic solvents (first with hexane and after with denatured ethanol) and then with hot water, according to Tappi T 204 cm-07 and T 207 cm-99 standards. The ash content was determined according to Tappi T 211 om-07. Two replicates were used for each sample. 

4.4.3 Raw material pH value and buffering capacity measurement

Knowledge of pH and buffering capacity of the raw material (bark and wood) is important for the understanding of the adhesive curing process. For each particle size, the aqueous extract was prepared by refluxing 25 g of dry powdered bark in 250 ml of distilled water for 20 min. Two replicates for each sample were prepared. After refluxing, the mixture was filtered through a filter paper using a vacuum. The aqueous extract was cooled to room temperature in a 25 ml pipette and diluted to 50 ml before titration. After recording the initial pH, 50 ml of extract solution was titrated potentiometrically to a pH of 3 (for alkaline buffering capacity) and 7 (for acid buffering capacity) with nominal 0.025 N H₂SO₄ and 0.025 N NaOH solution respectively. 

4.4.4 Particleboard production

Particleboards measuring 560 x 460 x 8 mm with a target density of 800 kg/m³ were manufactured using a 1000 x 1000 mm Dieffenbacher hot press equipped with a PressMAN control system manufactured by Alberta Research Council. A liquid phenol-formaldehyde adhesive from Dynea Company Ltd was used. The adhesive content shown in Table 27 was determined by the procedure described by Ngueho Yemele et al.(2007) in order to maintain the adhesive per unit specific surface (kg. m^-2) constant. Panels were pressed at a platen temperature of 200 ± 0.1°C for a press closing time of 20 seconds, curing time of 200 seconds and an opening time of 60 seconds which resulted in a total press cycle of 280 seconds. One and 0.5% of wax were added respectively to the surface and core layer particles in the blender. 

4.4.5 Determination of mechanical and physical properties

The panels were conditioned at 20 ± 3°C and 65 ± 1% for one week. Mechanical and physical properties were determined according to the American National Standards Institute (ANSI) standard A.208.1-1999. The properties determined were the modulus of elasticity (MOE) and modulus of rupture (MOR) in static bending, internal bond (IB), Janka hardness (HJ), thickness swelling (TS), and linear expansion (LE). However, in order to consider the impact of the sample density on the mechanical and physical properties of particleboards, the obtained values were adjusted (Garcia et al. 2005, Xing et al. 2007). Thus, other dependent variables such as the specific modulus of elasticity (MOE_spec = MOE/sample density), the specific modulus of rupture (MOR_spec = MOR/sample density), the specific internal bond (IB_spec = IB/sample density), the specific hardness (HJ_spec= HJ/sample density), the specific thickness swelling (TS_spec = TS/sample density), and the specific linear expansion (LE_spec = LE/sample density) were used to perform statistical analyses.

4.4.6 Experimental design and data analyses

The factorial design used in this work is presented in Table 27. The factors chosen were species (black spruce and trembling aspen), extracted bark content (50 and 100%), and bark particle size of the core layer (fine, medium and coarse). For mixed extracted bark and wood particleboards, a bark and wood content of 50% was used in both surface and core layers. The mixture of extracted bark and wood particles was made in a cylindrical rotary blender in order to obtain a homogeneous mixed furnish material. This led to 12 combinations with 3 replicates resulting in a total of 36 panels (Table 27). Moreover, 3 control panels were manufactured in the same laboratory conditions with wood particles obtained from a particleboard mill.

The Statistical Analysis System (SAS) software 9.1 was used for statistical analyses. The analysis of variance (ANOVA) was performed at 13 levels (12 levels of treatments and 1 level of control). Contrasts were performed to determine interactions between the factors studied. Comparisons between treatments and control were performed in order to identify the best treatments following the method of Scott and Knott (1974). Finally, properties of particleboard made from extracted bark were compared to those made from unextracted bark from a previous study (Ngueho Yemele et al.2007).

Table 27 Factorial experimental design used for each species and adhesive content of core layer

4.5 Results and discussion

4.5.1 Raw material characteristics and properties

Relevant characteristics and properties of raw material are summarized in Figure 1 and Tables 28, 29 and 30. Figure 29 shows the size distribution of extracted bark and wood particles used in the surface layer. The size classes and distributions of extracted black spruce and trembling aspen bark particles as well as the industrial wood particles used in the core layer are presented in Table 28. They indicate that the sizes of more than 77% of the industrial wood particles used in the core layer are below 2.60 mm. Therefore, only the bark particles of size 1.5-2.6 mm fit that optimum size range used in wood particleboard manufacture.

Bark organic and inorganic extractive contents presented in Table 29 show significant differences between the denatured ethanol solubility of unextracted and extracted bark of both species. There was also a significant difference between the hot water solubility of unextracted and extracted trembling aspen bark. In contrast, the hexane solubility indicated that there was no significant effect of the hot water extraction on the lipophilic extractives of the two species (Table 29). Nevertheless, both extracted and unextracted trembling aspen bark exhibited higher amount of lipophilic substances than the black spruce bark.

Results on the pH value and buffering capacity of raw materials are presented in Table 30. Because of their important role for the understanding of the adhesive curing process, discussion is carried out on this topic in the last section.

Figure 29 Size distribution of extracted bark and wood particles used in surface layer (BSB= extracted black spruce bark; TAB= extracted trembling aspen bark; W SL= industrial wood particles of surface layer). 

Table 28 Extracted bark particle size classes for the core layer and distributions

Table 29 Bark organic and inorganic extractive content

4.5.2 Mechanical and physical properties of particleboards

The ANOVA results are summarized in Table 5. Detailed analyses of the effect of extracted bark content and particle size on each mechanical and physical property as well as comparison with results obtained for unextracted bark particles of the same species (Ngueho Yemele et al. 2007) are discussed in the following sections.

Table 30 pH and buffering capacity of raw materials

Table 31 Results of the analysis of variance (F values) for mechanical and physical properties of particleboard made from extracted bark of black spruce and trembling aspen

4.5.1 Bending properties

ANOVA results presented in Table 31 show a significant effect of extracted bark content on the static bending properties (MOE_spec and MOR_spec) at the 0.01 probability level and a significant effect of species and bark particle size on the MOE_spec at the 0.01 and 0.05 probability level respectively. Figures 30 and 31 show that for both species, the MOE_spec and the MOR_spec obviously decreased with increasing extracted bark content. Likewise for particleboards with 100% bark content, Figure 30 shows an increase of MOE_spec with increasing particle size. All the boards produced with 50% extracted bark content exhibited higher values of MOE and MOR than that obtained with 100% bark content. Moreover, there was no significant difference of MOE and MOR among the particleboards made from 50% extracted bark content of both species. In fact, Ngueho Yemele et al. (2007) reported a lower cellulose content of black spruce and trembling aspen bark compared to wood particles. Because of its degree of polymerization and linear orientation, cellulose is responsible for strength in the wood fibers (Winandy and Rowell 1984). This involves lower bending properties of particleboard made from 100% bark content than that of those made from 50% bark content. In addition, Blanchet et al. (2000) also found that the tack of the bark particle furnish and the rate of heat transfer through bark particles furnish were lower than for a wood particles furnish. This may result in an incomplete adhesive cure and could explain the decrease noticed for MOE and MOR with increasing bark content. Table 31 also shows a significant effect of the interaction between extracted bark content and bark particle size on the MOE_spec at the 0.01 probability level and a significant effect of the interaction between species and extracted bark content on both MOE_spec and MOR_spec at the 0.01 probability level. This may suggest that the effect of extracted bark content on the MOE_spec and MOR_spec depends on bark particle size and species. The MOE and MOR values of the boards made from 50% extracted bark content of black spruce and trembling aspen were 33 and 50% lower than the control.Nevertheless, those values of MOE and MOR still exceeded the minimum requirements for the commercial (M-1) and the underlayment (PBU) grades (Figures 30 and 31). In contrast, no boards made from 100% bark content of both species met these requirements.

Figure 30 Effect of extracted bark content and particle size on the specific modulus of elasticity (BSB= black spruce bark; TAB= trembling aspen bark. M-1, PBU: ANSI standard particleboard grades).

Figure 31 Effect of extracted bark content and particle size on the specific modulus of rupture (BSB= black spruce bark; TAB= trembling aspen bark. M-1, PBU: ANSI standard particleboard grades).

4.5.2 Internal bond

Table 31 indicates a significant effect of species, extracted bark content and bark particle size on the specific internal bond (IB_spec) at the 0.01 probability level. Figure 32 shows that IB_spec of the particleboard made from extracted black spruce bark obviously decreased with increasing extracted bark content. For those panels made from extracted trembling aspen bark, the decrease is observed merely on fine and medium particle size classes. For coarse particles of trembling aspen bark, no significant difference of IB was noticed between 50 and 100% bark content. The highest value of IB was found on the particleboard made from 50% extracted fine bark particle of both species (Figure 32). In fact, those particles showed a low slenderness (length-thickness) ratio. Table 31 also shows a significant effect of all the interactions of the factors species, extracted bark content and bark particle size on the specific internal bond (IB_spec) at the 0.01 probability level. However, the F value of the factors extracted bark content and species were respectively twenty and six times higher than that of bark particle size (Table 31). Thus, the variation observed on the IB_spec could be explained more by the difference of extracted bark content and species than on bark particle size. This may be due to a decrease of pH or/and a decrease of reactive materials like polyphenols, particularly bark tannin which can positively react with the adhesive and could have been extracted by hot water treatment. Although, the IB of the 50% bark boards of fine particles was 65% lower than the control, they met the requirement for M-1 and PBU grades of the ANSI standard as shown in Figure 32.

Figure 32 Effect of extracted bark content and particle size on the specific internal bond (BSB= black spruce bark; TAB= trembling aspen bark. M-1, PBU: ANSI standard particleboard grades).

4.5.3 Hardness (Janka)

Table 31 shows a significant effect of the extracted bark content on specific Janka hardness (HJ_spec) at the 0.01 probability level. Figure 33 obviously shows that HJ_spec decreased with increasing bark content. There is also a significant effect of the triple interaction among species, extracted bark content and bark particle size on specific Janka hardness (HJ_spec) at the 0.05 probability level. Moreover, the factor extracted bark content exhibited the highest F value (Table 31). Particleboard made from 100% extracted black spruce bark showed higher HJ than the particleboard made from 100% extracted trembling aspen bark. In contrast, all the boards made from 50% extracted trembling aspen bark showed higher HJ than the 50% extracted black spruce bark except those of medium size particles which was found to be not significantly different to those made from 50% trembling aspen bark (Figure 33).The HJ of those boards were 9% higher than the control. All the particleboards produced met the requirements of the ANSI A208.1-1999 standard except for those made from 100% extracted fine particles of trembling aspen bark (Figure 33).

Figure 33 Effect of extracted bark content and particle size on the specific Janka hardness (BSB= black spruce bark; TAB= trembling aspen bark. M-1, PBU: ANSI standard particleboard grades).

4.5.4 Thickness swelling

Table 31 indicates a significant effect of species, extracted bark content and bark particle size on specific thickness swelling (TS_spec) at the 0.01 probability level. Figure 33 obviously shows that TS_spec of particleboard made from trembling aspen bark is lower than for particleboard made from black spruce bark. In fact, trembling aspen bark contains much more lipophilic extractives (Table 31) than black spruce bark which can increase thickness swelling resistance. A similar trend was reported for unextracted bark particles (Ngueho Yemele et al. 2007). The TS_spec value of particleboard of medium bark particle size was often higher than the two other size classes. Table 31 also indicates significant interactions between species and bark particle size as well as species and extracted bark content on specific thickness swelling (TS_spec) at the 0.01 probability level. This means that the effect of species on TS depends on the extracted bark content and bark particle size. Boards made from 100% fine and coarse extracted trembling aspen bark particles exhibited the lowest TS value which was 29% higher than the control and also exceeded the maximum value required by the ANSI standard for manufactured home decking (D-2 and D-3) grades as shown in Figure 34.

4.5.5 Linear expansion

Table 31 shows a significant effect of extracted bark content on the specific linear expansion (LE_spec) at the 0.01 probability level. Figure 35 indicates that the LE_spec increased with increasing bark content for both species. There are also a significant effect of the interactions between extracted bark content and bark particle size on the one hand, and between species and bark particle size on the other hand on the specific linear expansion (LE_spec) at the 0.01 and 0.05 probability level, respectively. Thus, the LE_spec of the particleboard made from 50% extracted bark content, increased with increasing bark particle size. The trend seems to be opposite for boards made from 100% extracted trembling aspen bark content (Figure 35). In contrast, no significant difference was found between the LE of the particleboards made from 100% black spruce and the LE of particleboards made from other extracted bark contents and species. Low LE value was obtained for the boards made from 50% of fine and medium extracted black spruce bark particle, which was 52% higher than the control. Some of the boards produced fulfilled the LE requirements of the ANSI A208.1 standard but not all of them as shown in Figure 35.

Figure 34 Effect of extracted bark content and particle size on the specific thickness swelling (BSB= black spruce bark; TAB= trembling aspen bark. M-1, PBU: ANSI standard particleboard grades).

Figure 35 Effect of extracted bark content and particle size on the specific linear expansion (BSB= black spruce bark; TAB= trembling aspen bark. M-1, PBU: ANSI standard particleboard grades).

4.5.6 Effect of extracted versus unextrated bark particles on the properties of particleboards

Physical and mechanical properties of particleboard made from extracted bark were compared to those obtained by Ngueho Yemele et al. (2007) for particleboards made from unextracted bark. Table 32 shows that the hot water extraction applied in this study had a detrimental effect on the physical and mechanical properties of particleboard made from black spruce and trembling aspen bark. However, the effect of the extraction was light on the bending properties (MOE and MOR) of boards made from 50% trembling aspen bark. The IB of the boards made from extracted bark was significantly reduced (from 16 to 67%) except for particleboard made from 50% of coarse particles of black spruce bark probably due to its low effective bark content ratio. The TS of boards made from extracted bark was higher than that of those made from unextracted bark except for those made from 100% trembling aspen bark as shown in Table 32. In fact, for those boards, the high lipophillic content of both extracted and unextracted trembling aspen bark acts as a barrier to reduce water absorption and thickness swelling. No significant difference was found between the LE of furnish made from extracted and unextracted bark of both species except for those made from coarse black spruce raw material which showed an increase of 30%. No significant decrease of the extraction process implemented was found on the HJ of the boards (Table 32). Furthermore, an improvement of 22% was observed on the HJ value of the particleboard made from 50% extracted trembling aspen bark.

He and Riedl (2004) reported that pH and buffering capacity are important factors influencing PF adhesive curing. A decrease of the PF/particle system pH led to a decrease of the adhesive functional group reactivity. Therefore, both the quality of the interactions (PF adhesive/particles) and the mechanical properties of the boards decrease. Significant differences were found between the average pH values of extracted and unextracted bark of both species presented in Table 29. In addition, the values of acid and alkaline buffering capacity increased and doubled. This suggests a positive correlation between the alkaline buffering capacity and the mechanical properties (MOE, MOR and IB) of the particleboards. In fact, the higher the alkaline buffering capacity, the longer the delay for PF acidification. A decrease of the pH observed on extracted bark particles of the two species led to the alteration of the adhesive reticulation conditions and its interaction with bark particles.

The decrease of the mechanical properties of particleboard made from extracted (or hydrothermally treated) bark could also be explained by the kind of interactions between particles and PF adhesive. Previous studies have shown that the interactions between PF adhesive and wood are of secondary nature and mainly based on hydrogen bonds (He and Riedl 2004, Laborie and Frazier 2006). Hot water extractives are essentially polyphenols including tannins that can react with formaldehyde, free sugars and ash. Extracted bark particles after hot water treatment exhibit less secondary interactions than unextracted bark due to a decrease of the functional groups (hydroxyl, carbonyl, carboxylic). These groups involved in the hydrogen bonds were removed together with the hydrophilic compounds during the hot water extraction process. Therefore, the mechanical properties (MOE, MOR and IB) of the boards made from extracted bark particles should be lower than that of those made from unextracted bark. This is confirmed by the results obtained in the current study as presented in Table 32.

Table 32 Comparison of particleboards made from extracted bark versus those of unextracted bark

Table 32 also shows an increase of TS value of particleboard made from extracted bark particles as compared to that made from unextracted bark. In fact, during the extraction process, extractives soluble in alcohol and water are released from the rhytidome cells (Srivastava 1964, Martin and Crist 1970). Thus, the porosity of extracted bark particles and the whole furnish might increase and they became less resistant to water absorption and thickness swelling.

The significant impact of bark acidity on the curing of PF adhesive suggests that the properties of particleboard made from bark could be further improved by using an appropriate adhesive formulation as well as more specific manufacturing parameters for each kind of raw material. 

4.6 Conclusions

In this study, the physical and mechanical properties of particleboard made from extracted black spruce and trembling aspen bark were investigated. The effect of the hot water extraction on these properties was evaluated. The following conclusions can be drawn:

1. The mechanical properties including modulus of elasticity (MOE), modulus of rupture (MOR), internal bond (IB) and Janka hardness (HJ) decreased with increasing extracted bark content. In contrast, an increase in extracted bark content resulted in an increase in linear expansion (LE) and a slight effect on thickness swelling (TS).
2. The effect of particle size was observed mostly on IB and TS. Particleboard made from fine particles often showed higher IB and lower TS values. 
3. The hot water extraction applied on black spruce and trembling aspen bark had detrimental effect on all the physical and mechanical properties of the particleboards produced except for the Janka hardness.
4. Bark extracts are highly desired in pharmacology and for adhesive production. Therefore, it becomes necessary to carry out studies in order to manufacture value added products with the residues remaining after the extraction process. Although, boards made from extracted bark showed lower physical and mechanical properties than unextracted ones, the high hardness exhibited by extracted bark boards suggests that some of them could be suitable for flooring products where hardness is the main property required.

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