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Carrillo-Varela, I., Valenzuela, P., Gacitúa, W., and Mendonca, R. T. (2019). "An evaluation of fiber biometry and nanomechanical properties of different Eucalyptus species," BioRes. 14(3), 6433-6446.


Wood fibers from seven Eucalyptus species were collected to investigate the relationships among species, fiber biometry, and nanomechanical properties. The results indicated significant differences in wood density, coarseness, fiber length, fiber width, and cell wall thickness among the different Eucalyptus species. The nanomechanical properties of the S2 cell wall layer also showed significant differences among the Eucalyptus species. The elasticity modulus ranged from 16 to 19 GPa, the hardness spanned 0.24 to 0.31 GPa, and the ductility ratio was between 54 and 68. Moreover, significant correlations were observed for hardness versus cell wall thickness (r = 0.87), and elasticity modulus versus crystallinity index (r = 0.80) and crystallite size (r = 0.68). Among the evaluated species, E. dunnii showed the highest elasticity modulus, highest hardness average, and the highest crystallinity index. The range of nanomechanical values indicated that Eucalyptus wood fibers are suitable for the development of new composite materials or engineering products by selecting the most adequate species for each use according to its properties.

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An Evaluation of Fiber Biometry and Nanomechanical Properties of Different Eucalyptus Species

Isabel Carrillo-Varela,a,b Paulina Valenzuela,c William Gacitúa,c,d and Regis Teixeira Mendonça a,b,*

Wood fibers from seven Eucalyptus species were collected to investigate the relationships among species, fiber biometry, and nanomechanical properties. The results indicated significant differences in wood density, coarseness, fiber length, fiber width, and cell wall thickness among the different Eucalyptus species. The nanomechanical properties of the S2 cell wall layer also showed significant differences among the Eucalyptus species. The elasticity modulus ranged from 16 to 19 GPa, the hardness spanned 0.24 to 0.31 GPa, and the ductility ratio was between 54 and 68. Moreover, significant correlations were observed for hardness versus cell wall thickness (r = 0.87), and elasticity modulus versus crystallinity index (r = 0.80) and crystallite size (r = 0.68). Among the evaluated species, E. dunnii showed the highest elasticity modulus, highest hardness average, and the highest crystallinity index. The range of nanomechanical values indicated that Eucalyptus wood fibers are suitable for the development of new composite materials or engineering products by selecting the most adequate species for each use according to its properties.

Keywords: Fiber length; Fiber width; Coarseness; Cell wall thickness; Hardness; Elastic modulus

Contact information: a: Laboratorio de Recursos Renovables, Centro de Biotecnología, Universidad de Concepción, Concepción, Chile; b: Facultad de Ciencias Forestales, Universidad de Concepción, Concepción, Chile; c: Centro de Biomateriales y Nanotecnología, Universidad del Bío-Bío, Concepción, Chile; d: Facultad de Ingeniería, Departamento de Ingeniería en Maderas, Universidad del Bío-Bío, Concepción, Chile; *Corresponding author:


Natural fibers are attractive materials that are widely used for paper, paperboard, textiles, fiberboards, and a variety of other uses. Natural fibers can also replace man-made fibers as reinforcement and fillers to make environmentally friendly products (Gindl et al. 2006b; Cheng et al. 2007; Wu et al. 2009). To make this application possible, it is necessary to fully understand the characteristics of the raw material for the development of new composite materials or engineering products. Accordingly, it is important to assess the chemical features and fiber biometry of wood, and more specifically, test the mechanical properties of the cell wall S2 layer (Tze et al. 2007; Wu et al. 2009). The nanoindentation technique is a helpful tool to better understand the strength properties of the individual fibers at the microscopic level, since it can be used to investigate mechanical behavior of materials at the nanoscale. The test involves the penetration of a sample material using an indenter. The penetration depth and load are recorded, and the elastic modulus and hardness of wood cell walls is calculated (Gindl et al. 2004; Wu et al. 2009). The test can detect the mechanical properties of the S2 cell wall layer, which is the major contributor to the mechanical properties of wood cell walls because it constitutes approximately 80% of the total cell wall thickness (Tze et al. 2007).

For nanoindentation experiments there is no need for chemical pretreatment to isolate individual wood fibers as required in single-fiber tensile tests (Tze et al. 2007), and the measured values are very consistent for elasticity and hardness evaluations (Huang et al. 2012). Thus, this technique has been used to investigate the fiber nanomechanical properties of several wood species. Nanomechanical characterizations have been performed on the fiber cell walls of softwoods species (Wimmer and Lucas 1997; Wimmer et al. 1997; Gindl et al. 2002, 2004; Gindl and Schoberl 2004; Tze et al, 2007; Yu et al. 2011; Huang et al. 2012; Vincent et al. 2014), crops stalks (Wu et al. 2010), and several hardwood species (Gacitúa et al. 2007; Wu et al.2009; Muñoz et al. 2012; Valenzuela et al. 2015; Zanuncio et al. 2016). In addition, regenerated fibers (Gindl et al. 2006a, b), microcrystalline cellulose (Das et al. 2010), and cellulose nanofibers (Yildirim and Shaler 2016) have been studied.

Eucalyptus trees are widely used in commercial plantation as raw material for pulp, paper, and cellulose derivatives production due to several desirable features: fast growth, straight form, valuable wood properties, wide adaptability to soils and climates, and easy management (Turnbull 1999; Gomes et al. 2015; Carrillo et al. 2018b). Several reports have addressed the morphological, anatomical, and chemical features of various Eucalyptus species as well as the respective pulp and derivatives products (Kibblewhite et al. 2000; Ona et al. 2001; Ramírez et al. 2009a, b; Aguayo et al. 2014; Carrillo et al. 2015, 2017, 2018a, b). Nanocharacterization research has been also carried out in some Eucalyptus species, mainly Eucalyptus nitens(Gacitúa et al. 2007; Muñoz et al. 2012; Valenzuela et al. 2015) and E. grandis × E. urophylla (Zanuncio et al. 2016). This work attempts to describe the main anatomical and nanomechanical features of the wood fibers from seven Eucalyptus species using microscopic and indentation techniques in order to elucidate their relationship and variation within the Eucalyptus species. In a previous study (Carrillo et al. 2018a), a comparative evaluation of the cellulose supramolecular structure of the same seven Eucalyptus wood species was made. The results from this previous study support this work and are considered for the results discussion. It is expected that this report will provide valuable information about wood property variations in the Eucalyptus genre, as well as explore the potential use of these species as raw material for the design and development of new forestry products.


Eucalyptus Wood Samples

Six-year-old Eucalyptus trees were provided by a Chilean forestry company located in the Biobío Region of southern Chile. The species provided were Eucalyptus badjensisE. benthamiiE. dunniiE. globulusE. nitensE. smithii, and two hybrids E. nitens × E. globulus, coded En × Eg (1) and En × Eg (2). The seven Eucalyptus species grew under the same field and planting conditions. Wood chips were used for wood density determination, according to the TAPPI Standard T258 om-94 (1996), for nanomechanical evaluation and fiber biometry characterization. Experimental analyses were carried out in duplicate. As shown in Table 1, the chemical composition of the wood chips was determined in a previous study with the same sampled species (Carrillo et al. 2018a).

Table 1. Compositional Analysis of the Eucalyptus Wood Samples (Carrillo et al. 2018a)

Fiber Biometry

Wood chips were treated according to the protocol reported by Mansfield and Weineisen (2007). A chisel was used to obtain matchsticks (0.1 × 0.1 × 0.5 cm) from wood chips. The obtained matchsticks were macerated and treated using Franklin solution (30% H2O2 and CH3COOH, 1:1 v/v) for 8 h at 70 °C. The solution was decanted, and the remaining fibrous material was washed with water until a neutral pH was achieved. Average fiber length and coarseness were determined in a Lorentzen & Wettre Fiber Tester (Kista, Stockholm, Sweden) using 200 mg of sample that was previously disintegrated in 200 mL of distiller water for 10 min. During the suspension analysis, the equipment was set to measure 35,000 fibers of each sample. Fines were characterized as 0 to 0.2 mm in length to ensure that broken fibers and fines were not included in the final averages of fiber measurements (Carrillo et al. 2015, 2017).

Nanomechanical Characterization

Cubes 3 mm × 3 mm in size were cut from wood chips obtained close to the bark section. The wood cubes were impregnated with Spurr epoxy resin (Spurr 1969) to provide mechanical support for cutting in a Leica RM2265 rotary microtome (Leica, Wetzlar, Germany), and to prevent damage during indentation of the fibers cell wall. The transverse surfaces of the samples were leveled with a glass knife and smoothed with a diamond knife. The indentation area obtained was about 1 mm2 with a low and uniform roughness to increase the accuracy of the indenter measurements. The samples were conditioned for at least 24 h at 21 ºC and 60% relative humidity in the room that housed the nanoindenter. Nanoindentations were performed using a Hystron TriboIndenter TI-900 (Hystron Inc., Minneapolis, MN, USA), using a cube corner diamond tip.

The elastic modulus of the secondary cell wall layer was obtained through a load-hold-unload cycle in areas of the S2 cell wall layer. The loading cycle was worked to obtain an accelerated mapping of properties (XPM). For the load cycle, a 5 × 5 array with a separation of 1 µm between each indentation was used (Fig. 1), with a maximum load of 100 µN and a total time of 0.3 s. An area of 5 µmwas analyzed, and at least 25 measurements were taken for the S2 cell wall layer of each Eucalyptus species. The reduced elastic modulus was obtained through Eq. 1,


where Er corresponds to the reduced elastic modulus resulting from the elastic deformation of the diamond tip (i) and sample (s), S is the slope of the discharge curve  when the discharge starts, and A is the contact area between the material and the maximum load of the indenter. The elastic modulus of the sample (Es) was determined using Eq. 2 (Gindl et al.2004),


where Poisson’s ratio vs and vi represent the sample and diamond tip, respectively, and Ei is the elastic modulus of the diamond (1140 GPa).

Hardness (H) is the maximum load divided by the contact area, projected from indentation. Hardness was calculated through the Eq. 3 (Wu et al. 2009),


where Pmax is the maximum load and A is the contact area.

Fig. 1. A 2D image showing the indentations (triangular shapes) on the S2 cell wall layer of Eucalyptus fibers. The indentations located in the middle lamella (ML) and in the S2 border were discarded.

Transversal Anatomical Characterization

The transversal characterization was performed on the same cube sample used for nanoindentation tests. The wood cubes were mounted on stubs to apply a conductive coating using a metallizer (SPI-MODULE sputter-coated). The coating was performed with gold for 60 s. Images were obtained using a JEOL JSM-6380LV (Tokyo, Japan) scanning electron microscope (SEM) connected to a personal computer for image capture. Forty fibers were randomly selected, and their cell wall thickness, fiber width, and lumen width were measured at 1000 times total magnification. All these parameters were measured using the JEOL SEM software.

Data Analysis

Statistical analysis of the anatomical and nanomechanical characteristics were performed using the SAS software system version 9.2 (Cary, USA). To determine significant differences between the Eucalyptus samples, analysis of variance (ANOVA) followed by Tukey’s statistical test were performed at p < 0.5. Correlation analysis between the wood features were performed using Pearson’s correlation coefficient.


Wood Density and Fiber Biometry

Wood density and fiber biometry of the different Eucalyptus trees are shown in Table 2. Wood density values ranged from 420 to 484 kg/m3, with E. globulus and E. smithii as the higher wood density trees and E. badjensis as the lower one. These values agreed with whole-tree average densities reported by McKinley et al. (2002) for 8-year-old E. globulus and E. nitens(476 kg/m3 and 440 kg/m3, respectively).

Regarding fiber biometry, fiber length ranged from 0.59 to 0.70 mm, with E. badjensis and E. smithii exhibiting the lowest and highest values, respectively. Fiber width and lumen width ranged 11 to 15 µm and 4 to 9 µm, respectively. The lowest fiber and lumen widths were found in E. benthamii, while the highest widths were found in E. dunnii. Fiber data results agree with studies of fiber biometry in Eucalyptus species (Muneri and Raymond 2001; Ona et al. 2001; Ohshima et al. 2004; Ramírez et al. 2009b; Carrillo et al. 2015, 2017). Muneri and Raymond (2001) evaluated the fiber length of 5- to 9-year-old E. globulus and E. nitens trees from different sites, reporting values of 0.66 to 0.75 mm and 0.56 to 0.72 mm, respectively. Cell wall thickness values spanned between 1.9 and 2.3 µm, which is a lower range than the values reported by Ramírez et al. (2009b) for 7-year-old E. globulus trees, and by Carrillo et al.(2015) for 15-year-old E. globulus trees. Coarseness is defined as fiber mass per fiber length. Coarseness is a good index for predicting pulp properties and is closely related to the biometric properties of fibers and basic density of wood (Via et al. 2004; Mansfield and Weineisen 2007; Carrillo et al. 2015). The higher coarseness values were observed in E. smithii(8.9 mg/100 m) and E. globulus (8.5 mg/100 m), while the lower values were seen in E. badjensisE. benthamii, and En × Eg (1) species, with approximate values of 7.0 mg/100 m.

Table 2. Wood Density and Fiber Biometry of the Different Eucalyptus Species

Nanomechanical Properties

Nanomechanical properties obtained for the Eucalyptus wood samples are shown in Fig. 2. The Spurr epoxy resin has an influence on mechanical properties of embedded wood cell walls, increasing their hardness (around 20%) (Gindl et al. 2004; Meng et al. 2013). This has prompted the implementation of alternative preparation methods to avoid the influence of embedding mediums on wood cell wall (Meng et al. 2013). In this work, with a comparative purpose, all the samples were subjected to the same impregnation treatment. Thus, the influence of the epoxy resin on nano-mechanical properties is expected to be the same in all the evaluated specimens, which makes it possible to make an adequate comparison among the different Eucalyptus species.

Elastic modulus (E) averages were significantly different among the Eucalyptus species (p-value = 0.0361), ranging from 16 to 19 GPa. The highest average was observed in E. dunnii, while E. badjensis and E. smithii showed the lowest average values (Fig. 2a). The E variation coefficient ranged between 15 and 24%, where E. benthamii showed the highest heterogeneity (Fig. 2a).

Hardness (H) average values ranged from 0.24 to 0.31 GPa, with E. badjensis and E. dunnii being the highest, and E. nitens the lowest (Fig. 2b), displaying significant differences between the different Eucalyptus species (p-value < 0.0001). The H variation coefficient ranged from 4 to 13%, with E. badjensis and En × Eg (2) exhibiting more heterogenous data, while E. dunnii exhibiting the most homogeneous.

The ductility ratio (E/H) spanned from 54 to 68, and significant differences were observed between the different Eucalyptus species (p-value < 0.0001). The highest average value was observed in E. nitens fibers, while the lowest was observed in E. badjensis (Fig. 2c). The E/H variation coefficient spanned from 5 to 17%.

The data obtained were similar to nanomechanical values published elsewhere for hardwoods species (Wu et al. 2009) and for Eucalyptus samples (Muñoz et al. 2012; Valenzuela et al.2012). Muñoz et al. (2012) evaluated 12-year-old E. nitens from different sites. They reported HE, and E/H values for the S2 cell wall layer ranging from 0.23 to 0.43 GPa, 8.95 to 16.99 GPa, and 26.40 to 62.94, respectively. Valenzuela et al. (2012) also evaluated 12-year-old E. nitens from different sites, reporting S2 cell wall layer E and H average values of approximately 10 GPa and 0.29 GPa, respectively. The E/H values for the same study were approximately 40. In addition, the authors suggested a correlation between cracking levels and the E/H ratio. Therefore, Eucalyptus samples with the lowest E/H values subjected to small deformations should be more fragile and more easily weakened by micro-fracture effects (Muñoz et al. 2012). The E/H ratio has been used to describe the stiffness of materials; brittle materials have low E/H values, such as glass (E/H=12), and ductile materials have high E/Hvalues, such as aluminum (E/H=117) (Bolshakov and Pharr 1998). Other studies have reported a correlation between E values and micro-fibrillar angle (MFA), where E decreased with increasing MFA in hardwoods species (Wu et al. 2009) and softwoods (Tze et al. 2007).

Relationship between Eucalyptus Wood Features

Table 3 shows the correlation index between the different Eucalyptus wood properties evaluated in this work. As mentioned previously, coarseness has a close relationship with wood density (Via et al. 2004; Mansfield and Weineisen 2007; Carrillo et al. 2015). However, a significant correlation between both properties was not observed in this study.

Fig. 2. Nanomechanical properties of Eucalyptus wood in the S2 cell wall layer. A: Elastic modulus (E); B: hardness (H); and C: ductility ratio (E/H). Box and whisker plots show the average (x), the median (horizontal line), the 50% interquartile range (box), and the maximum and minimum value (whiskers).

Cell wall thickness versus wood density showed a significant but negative correlation (= -0.69), which was unexpected. Wood density is determined primarily by anatomical structures such as vessel features, fiber width, cell wall thickness, and parenchyma proportion. Chemical composition, especially bulking by extraneous materials, can also play an important role in determining wood density (Carrillo et al. 2015, 2017). Several authors have reported different correlation coefficients between anatomical properties of Eucalyptus (Kube et al. 2001; Wimmer et al. 2002; Ohshima et al. 2004; Carrillo et al. 2015, 2017), which reflect the wide variability in anatomical features presented within Eucalyptus species.

On the other hand, relationships between nanomechanical properties, such as hardness, and fiber morphological features have been suggested (Muñoz et al. 2012; Savva et al. 2010; Vincent et al. 2014), while Wu et al. (2009) suggested an influence of the cell wall thickness during the nanoindentation test. In agreement with these reports, a positive and significant correlation between cell wall thickness and H in Eucalyptus wood samples was observed. These results contradict Huang et al. (2012), who found that in mature conifer wood, nanohardness was not affected by cell wall thickness. Other factors such as the complex cell wall structure and its chemical composition can influence the nanomechanical properties (Wu et al. 2009; Vincent et al. 2014). Wimmer and Lucas (1997) attributed a distinctly reduced elastic modulus in the middle lamella to the absence of cellulose in this region. However, results from this study showed no significant correlation between chemical composition and nanomechanical properties (Table 3). In this sense, Gindl et al. (2002) suggested that the elasticity and stiffness of the wood cell wall is affected by the arrangement, organization, and quantity of the wood components that shape the cell wall architecture.

Table 3. Pearson Correlation Index between Evaluated Variables of the Eucalyptus Wood Species (n=8)

Wu et al. (2013) suggested that in some cellulosic materials, the mechanical properties are dependent on the direction relative to the cellulose crystalline structure and chain arrangement within the crystal structure. In a previous work, the crystalline properties of cellulose from the seven Eucalyptus wood species studied in this work were evaluated (Carrillo et al. 2018a). Table 4 shows the results obtained in the work for the crystallinity degree (CrI) and crystallite size (L) of the same Eucalyptus species evaluated in this study. A significant correlation was observed between the crystallite size and crystallinity degree of cellulose in wood with values (Table 3), which could be related to the arrangement of the cellulose microfibrils that influence the elastic modulus of the wood cell wall (Wimmer and Lucas 1997; Gindl et al. 2004; Muñoz et al. 2012). According to Das et al. (2010), a high hardness could be expected in cellulosic samples due to high crystallinity and large crystallite size. However, those correlations were weak and not significant (Table 3).

Table 4. Crystallinity Index (CrI) and Lateral Crystallite Size (L) of the Different Eucalyptus Wood Species (Carrillo et al. 2018a)

Wu et al. (2010) suggested that the polymerization degree, in addition to crystallization and holocellulose content, contributed to a higher hardness in cotton stalk cell walls, which could be an interesting parameter to include in future work. Additionally, is has been observed in regenerated fibers (Lyocell and viscose fibers) that the degree of orientation of both crystalline and amorphous cellulose, which is an indication of the lateral bonding degree, could influence the hardness of fibers (Gindl et al. 2006a).

Eucalytpus species that grew in the same field and same plantation conditions differ in their chemical composition and fiber biometry. As expected, E. globulus frequently shows the best chemical and fiber features for pulping procedures, such as a low lignin and extractives content, high holocellulose content, high wood density, high coarseness, and high fiber length. However, other Eucalyptus species may also be suitable as raw material for forest products, working as reinforcement and/or filler in the development of new composite or engineering materials (Muñoz et al. 2012), since their native wood fibers provide material with adequate nanomechanical and supra-structural properties. Foresters can discriminate among species and genotypes by applying several non-destructive techniques to predict wood chemical composition (Jones et al. 2006), wood density (Isik and Li 2003; Carrillo et al. 2017), and wood mechanical properties (Kelley et al. 2004). In this work, no significant correlation between these features and nanomechanical properties were stablished. According to Pearson results, XRD analysis might be an alternative for nondestructive estimation of nanomechanical properties in Eucalyptus trees. However, despite that the evaluated samples corresponded to unrelated Eucalyptus trees growing in the same field and plantation conditions, additional analysis of a higher number of trees is required in order to increase the statistical significance of the results.


  1. Eucalyptus species growing in the same field and same plantation conditions developed wood with different chemical composition and different fiber biometry, while the nanomechanical properties of the S2 cell wall layer of native fibers also displayed significant differences. The highest elasticity modulus and hardness averages were observed in E. dunnii, while E. nitens exhibited the highest ductility ratio.
  2. Significant and positive correlations were established between hardness versus cell wall thickness and the elasticity modulus versus crystallinity index and crystallite size.
  3. No significant correlation was observed among nanomechanical properties and the chemical composition of Eucalyptus wood.


The authors are grateful for the financial support from FONDECYT (Grant 1160306) and the provision of facilities and technical support by CESMI-UdeC for SEM analysis. Isabel Carrillo-Varela thanks CONICYT-PFCHA/Doctorado Nacional/2018-21180299.


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Article submitted: December 4, 2018; Peer review completed: March 23, 2019; Revised version received: April 15, 2019; Accepted: May 18, 2019; Published: June 25, 2019.

DOI: 10.15376/biores.14.3.6433-6446