Physical and chemical characterization of Mauritia flexuosa and Eucalyptus spp.
The particles of Eucalyptus spp. had a basic density of 0.40 g/cm³ while the particles of Mauritia flexuosa had a 0.11 g/cm³ one. A lower basic density of particles results in a greater quantity of particles to obtain the same mass, thus increasing the compression ratio that influences the physical and mechanical properties of the particleboards produced.
Biomass with lower densities is required to produce high quality particleboards, as is the case of agro-industrial waste. However, there is a minimum limit for the density of the raw material in order to obtain the ideal compression ratio and, consequently, the production of particleboards with satisfactory properties for use in civil construction and the furniture industry. As the basic density of the particles of Mauritia flexuosa was 3.6 times lower than those of Eucalyptus spp. the panels produced may present a reduction in several physical and mechanical properties, due to the greater volume of particles in the panels (Scatolino et al. 2019).
Several researchers have studied the replacement of wood with these wastes in the production of particleboards. In general, lignocellulosic waste usually have low basic density, as verified for soybean pods - 0.200 g/cm³ (Faria et al. 2020) and coffee parchment - 0.100 g/cm³ (Scatolino et al. 2017).
As observed in Table 2, the particles obtained from non-wood biomass had higher total extractives contents in relation to the particles of Eucalyptus spp. The physical and chemical properties of the raw materials used in the production of particleboards are directly related to the final characteristics of the panels produced. Therefore, such properties can vary according to cultivation methods, environmental conditions such as rainfall and soil fertility.
Table 2 Basic density and chemical composition of Eucalyptus spp. and Mauritia flexuosa
Raw material
|
Total extractives (%)
|
Insoluble lignin (%)
|
Ash (%)
|
Eucalyptus spp.
|
3.59 ± 0.06*
|
27.27 ± 0.53
|
0.51 ± 0.02
|
Mauritia flexuosa
|
12.43 ± 0.23
|
20.88 ± 0.28
|
5.25 ± 0.08
|
* Standard deviation.
For total extractives, Mauritia flexuosa presented higher mean values. It has been reported in the literature that non-wood lignocellulosic biomasses have a higher content of extractives in their chemical composition. Scatolino et al. (2017) determined for the coffee parchment 26.24% of total extractives, while the Eucalyptus wood presented 3.59%. The higher content of extractives in non-wood biomass can influence the physical-mechanical performance of particleboards. Iwakiri (2005) reports that the higher the content of total extractives in lignocellulosic raw materials, the greater their chances of not generating quality reconstituted panels, due to their propensity to cause problems with adhesive consumption, reduced mechanical strength and water absorption, in addition to the occurrence of air bubbles during pressing.
Non-wood lignocellulosic materials stand out with lower lignin contents and higher proportions of extractives in their compositions when compared to wood lignocellulosic materials. Pereira et al. (2016) reported that lignin increases the cohesion and adhesion forces of particles, which consequently can improve the quality of adhesiveness in particleboards of lignocellulosic materials with higher amounts of lignin. Thus, lignin is directly related to the stiffness of the panel.
The ash content observed in Mauritia flexuosa particles was 10 times higher than the mean value found for Eucalyptus spp. Ash values above 0.5% negatively interfere with the panel's adhesiveness, by affecting its pH and hindering material processing, increasing the wear of cutting tools (Iwakiri 2005).
Physical properties of particleboards
There was no significant effect of the replacement of Mauritia flexuosa particles on the bulk density of the particleboards produced (Fig. 2). This result is considered positive, as it allows us to precisely understand the consequence of the addition of Mauritia flexuosa particles on the physical and mechanical behavior of chipboards.
It is observed that the averages of the bulk density values varied between 0.48 and 0.55 g/cm³ (Fig. 2), with the panels classified as having low density (Iwakiri 2005). The average bulk densities obtained were lower than the nominal density of 0.60 g/cm³, a fact that can be justified due to the loss of particles during the formation of the mattress and the return in thickness of the panels after removal from the press and packaging. These results do not compromise the interpretations of this research, as the variation in bulk density of the chipboards produced was statistically similar. In addition, there was standardization of the production process at the laboratory level. Thus, the results observed for the physical and mechanical properties of the boards refer exclusively to the effect of the raw materials used and not to the production process.
It is observed that the addition of Mauritia flexuosa particles insertion causes a linear increase in the compression ratio (Fig. 3). The results demonstrate that for each 1% insertion of Mauritia flexuosa in the composition of the panel, there is an increment in the compression ratio in the order of approximately 0.02. Such results are due to the low density of particles of Mauritia flexuosa (0.11 g/cm³) in comparison with wood of Eucalyptus spp. (0.40 g/cm³). Maloney (1993) established that the ideal range for the compression ratio is between 1.3 to 1.6. Replacing these values recommended by these authors in the regression equation obtained, it is observed that the amount of Mauritia flexuosa added to panels that meet this requirement ranged between 5.6 and 24.2%.
In the case of low-density particleboards, it is observed in the literature that the compression ratio increases as lignocellulosic waste are inserted into the panels to replace wood, as observed by Guimarães et al. (2019), with an increase in the compression ratio from 1.5 to 2.2 in particleboards produced with 0 and 50% of soybean hulls in substitution of Eucalyptus grandis wood, respectively.
It is observed that the increase in the insertion of Mauritia flexuosa in the panel in the order of 1% provides an increase of 0.99 and 0.90% in water absorption after 2 and 24 hours of immersion, respectively (Fig. 4). This can be explained by the increase in the compression ratio of the panels, as those produced with greater amounts of Mauritia flexuosa showed an increase in this property, which leads to a greater number of compacted particles in the same volume and, consequently, a greater amount of hydroxylic sites and greater affinity of this material with water. Another justification for the increase in water absorption values is due to the addition of empty spaces in the panels, a fact caused by the greater number of particles.
For water absorption levels, a similar behavior was described by Guimarães et al. (2019) in a study with low density particleboards. The authors evaluated the replacement of Eucalyptus grandis wood by soybean crop waste and found an increase in water absorption levels, ranging from 30.15 to 125.1% for water absorption after 2 hours of immersion and from 50 to 145% for water absorption after 24 hours of immersion. In this sense, the results suggest that Mauritia flexuosa provided the panels with dimensional behavior similar to that reported in the literature. Thus, the behavior observed for water absorption can be associated with pressing conditions, occurrence of empty spaces, compression ratio of particleboards and chemical composition of the lignocellulosic material used.
In Fig. 5, the properties of thickness swelling can be seen after 2 and 24 hours of immersion in water. The smallest coefficients of determination (R²) of the regression models indicate greater variation in thickness swelling values in relation to the adjusted line. In general, the percentage of Mauritia flexuosa explained from 56% to 60% the swelling in thickness after 2 and 24 hours of immersion in water.
Despite the increase in thickness swelling contents, the coefficients of variation for this property reduced as higher percentages of Mauritia flexuosa particles were inserted into the panels. For thickness swelling after 2 hours of immersion in water, the coefficient of variation decreased from 20.89 to 7.38% for panels produced with 0 and 40% of Mauritia flexuosa particles, respectively. On the other hand, for thickness swelling after 24 hours of immersion in water, the variation was from 11.43 to 6.0%. The homogeneity of the physical properties is a fundamental criterion for the application of chipboards in the production of furniture, especially in conditions where the furniture will be exposed to variations in ambient relative humidity.
It can be noted that the increase in the insertion of particles of Mauritia flexuosa in the panel in the order of 1% provides an increase of 0.16 and 0.10% of swelling in thickness after 2 and 24 hours of immersion, respectively. This trend can be explained by the fact that compositions with greater amounts of Mauritia flexuosa present higher values of compression ratio. According to Mendes et al. (2012), a higher compression ratio leads to a lower amount of adhesive per particle, which causes poorer quality of bonding and greater swelling in thickness. A higher compression ratio can be related to the increase in the amount of particles, and, consequently, greater densification of the panel, thus promoting greater hygroscopic swelling of the Mauritia flexuosa particles and the release of greater compression tensions generated during the high temperature pressing process.
Scatolino et al. (2019) produced low density particleboards with Eucalyptus urophylla x Eucalyptus grandis wood and cotton waste and observed mean values ranging between 11.34 and 23.93% for thickness swelling after 2 hours of immersion and 14.17 and 27.45% after 24 hours of immersion, therefore, values close to those observed in this work. The Commercial Standard - CS 236-66 (1968) stipulates maximum thickness swelling values after 24 hours of immersion of 30% for low density particleboards produced with urea-formaldehyde adhesive. In summary, despite the trends found, all panels met this regulatory requirement.
Mechanical properties of particleboards
In Fig. 6, the property of internal bonding for the particleboards produced is verified. The linear regression was the one that best represented the relationship between the inclusion percentage of Mauritia flexuosa particles and the internal bonding values. There was a reduction in the mean values of this property with an increase in the proportion of Mauritia flexuosa in the composition of low-density particleboards.
For internal bonding, the observed trend can be explained by the increase in the proportion of extractives in the panels, with the increase in the insertion of Mauritia flexuosa particles. Marra (1992) states that lignocellulosic materials with high extractive contents present bonding difficulties, resulting in low resistance of the adhesive bond between the particles. Another factor to which this result can be attributed to is the low density of Mauritia flexuosa particles, which increases the compression ratio. This causes lower availability of adhesive per particle, which could have harmed bonding (Iwakiri 2005).
Guimarães et al. (2019) reported similar behavior in a study carried out with soy hulls for the production of low-density particleboards, in which the authors observed a tendency to reduce the mean values of internal bonding as particles of the soybean hulls were inserted, varying between 0.85 and 0.10 MPa for panels produced with 0 and 100% waste, respectively.
The CS 236-66 (1968) commercial standard establishes a minimum value of 0.14 MPa for internal bonding strength in low density panels produced with urea-formaldehyde adhesive. In this sense, by equating the norm value with the equation observed in the linear regression, it is noted that the maximum insertion of Mauritia flexuosa in the panel to meet the normative requirements is 31.6%.
Fig. 7 shows the behavior of the mean values of MOE and MOR to the static bending test for the particleboards produced with particles of Mauritia flexuosa. It can be seen that the 1% increase in the proportion of Mauritia flexuosa in the panel promotes a decrease in MOE in the order of 10.5 MPa and in MOR of 0.1 MPa.
For the MOE and MOR values, the observed trend can be explained due to the reduced lignin content (20.9%) in the Mauritia flexuosa particles when compared to Eucalyptus spp. (27.3%). This is due to lignin's intrinsic characteristics of increasing the rigidity of the cell wall and contributing to the consolidation of particles in function of functioning as a natural adhesive (Mani et al. 2006). Another fact that justifies the decrease in MOE and MOR as the insertion of Mauritia flexuosa particles increases is probably linked to the compression ratio. Maloney (1993) emphasize that higher compression ratios can lead to an increase in the specific surface area of the particles. Under these conditions, the application of the same adhesive content reduces its availability per unit of surface area of the particles, which can result in panels with lower values in mechanical properties. Some extractives can result in a decrease in mechanical properties due to the inhibition of reactions between adhesive and raw material (Iwakiri 2005).
Martins et al. (2018) evaluated different percentages of soybean pod particles in replacement of Eucalyptus sp. in low-density particleboards and observed the same trend as in this work. The values obtained by these authors ranged from 435.21 to 1297.68 MPa for MOE and 2.41 to 7.57 MPa for MOR. The authors justify that the observed decreasing trend would have occurred due to the low basic density of the waste, which increased the compression ratio of the panels, which varied between 1.21 and 2.95. The results indicate that the compression ratio is a more relevant factor to explain the mechanical properties of the panels compared to the chemical composition. The low density of lignocellulosic waste can overcome the effect of the chemical composition on the quality of adhesion and, consequently, the high compression ratio values can result in a decrease in the mechanical properties of the chipboards. However, as previously reported, the ideal compression ratio values can vary significantly according to the lignocellulosic biomass used in the production process of the particleboards. According to Guimarães Junior et al. (2011), the traction strength and the modulus of elasticity of lignocellulosic materials are directly proportional to the cellulose content. In addition, the carbohydrate content of biomass is related to a higher occurrence of interfibrillar bonds, because of the chemical functionality for the formation of hydrogen bonds. This can affect, for example, the mechanical properties of particleboards.
The CS 236-66 (1968) commercial standard establishes as a minimum value for low density and urea-formaldehyde adhesive panels the value of 1052 MPa for MOE and 5.6 MPa for MOR. When the normalized value is equaled in the linear regression generated for this property, it is observed that the limit point for insertion of Mauritia flexuosa particles in the panel to meet the normative requirements is 17.5% for MOE. As for MOR, all treatments presented values higher than the requirements of the standard.
Based on the results presented, Mauritia flexuosa becomes a possible raw material with potential to replace the traditional species used in the production of particleboards, Pinus and Eucalyptus. The use of alternative biomass proves to be viable not only in terms of economic factors, but also ecological and social (Mendes et al. 2010), thus being able to add value to the clean production chain associated with extractivism in Mauritia flexuosa.