Thermal and mechanical properties of biocomposites based on a cashew nut shell liquid matrix reinforced with bamboo fibers

Abstract

Cardanol–formaldehyde thermoset resin was reinforced with raw and chemically modified fibers of bamboo (Bambusoideae). Modified fibers were treated with alkali solutions of NaOH (5% and 10%) and bleached with sodium hypochlorite NaClO/H₂O (1:1) at 60℃–75℃. The biocomposites were fabricated with a hand lay-up technique. This study investigated the fibers microstructure before and after alkali treatment, as the interaction between matrix and reinforce by scanning electron microscopy. The Fourier transform infrared spectra allowed the identification of characteristic stretching frequencies attributed to the methyl groups of lignin. A considerable increase in thermal stability was observed in the materials studied, which was verified by thermogravimetric analysis and confirmed by X-ray diffraction. The tensile tests showed that the mechanical properties (tensile modulus, tensile strength, and elongation at break) of bamboo fibers improved after alkali treatment, as well as the increase in the biodegradation in simulated soil, showing that the alkaline treatment acted removing the macro components of the fibers (lignin and hemicellose), making them more susceptible to the action of microorganisms.

Keywords

Bamboo fibers and biocomposites cashew nut shell liquid resin biodegradability thermomechanical properties

Introduction

The use of polymer matrix biocomposites reinforced by natural lignocellulosic fibers has had a growing development in recent years.^1,2 The great interest of these materials is directly related to their physical, chemical, and mechanical properties such as low density, flexibility, biodegradability, non-abrasivity, porosity, and viscoelasticity, besides the fact that they are a renewable natural resource of low cost and result in less wear on equipment during their processing when compared with synthetic fibers.^3,4

Due to these excellent properties, natural fibers have been used in the textile, paper, biocomposites, and technological applications.⁵ Because the lignocellulosic fibers have accessible hydroxy groups, it can react with other polar functional groups, such as those present in phenolic resins. Among the most important vegetable fiber studied and used worldwide as reinforcement in polymer matrix biocomposites are sisal,⁶ coir,⁷ jute,¹ and banana.⁸

Lignocellulosic fibers are constructed of microfibrils composed of a cellulose crystalline fraction and an amorphous matrix around the hemicellulose, lignin, pectin, wax, and water-soluble compounds. These microfibrils are oriented at different angles forming the various layers that make up the microfiber.

Bamboo is an abundant natural resource in many regions such as Asia, India, and South America^9–11 and has been used traditionally for construction of houses, bridges, and home utensils.¹² However, in recent years, bamboo has been prominent on the international scene as a sustainable material, being used in engineering, structural supports, biocomposite design, being incorporated in all production chain.¹³

The bamboo fibers are long with high resistance and considered ideal for the manufacture of paper duplex board. They are also suitable for food packaging, pharmaceuticals, and reinforcements for the ideal biocomposite.¹⁴ Their high mechanical strength is guaranteed by fibers that represent about 60%–70% of the total mass of stem. Bamboo is considered as a natural resource that takes less time to be renewed, and there is no species that can compete in speed of growth and utilization by area. It is an excellent scavenger of carbon that can be used in reforestation, riparian vegetation, such as protective and regenerating environment, besides being used as raw material in many applications.

Bamboo fibers were selected in the present study as reinforcement materials in resol resin matrix for the following reasons. First, bamboo has the fastest growth rate among the various types of renewable natural fibers. Furthermore, the cellulose fibers in bamboo are aligned along the length of the bamboo providing maximum tensile strength, flexural strength, and rigidity in that direction. Another factor considered decisive in the choice of bamboo as reinforcement material is the high content of cellulose (60%), responsible for the rigidity and resistance, see Table 1.

Table 1.

Composition of some natural fibers.

Fiber	Density (10⁻³kg m⁻³)	Microfibril angle (°)	Cellulose (%)	Hemicellose (%)	Lignin (%)
Bamboo	800	2–10	60	5	32
Banana	1350	11	65	19	5
Coir	1150	30–49	43	0.25	45
Jute	1450	8.1	63	20	12
Sisal	1450	20–25	70	14	12

Chemical modifications on the surfaces of natural fibers have been widely used to improve the interaction between fiber and matrix. Alkali treatment is currently used in the stage of surface modified fibers. The process consists of immersing the material in an alkaline solution at different concentrations.

In this work, we report the use of bamboo fibers as reinforcement and cardanol as polymeric matrix in the preparation of biocomposites (Figure 1). They were evaluated in terms of their thermal and mechanical properties, as were also submitted to biodegradability tests in simulated soil, in order to gather more information on these composites to produce materials less harmful to the environment.

Cardanol, the major component of technical cashew nut shell liquid (CNSL), which is obtained as by-product in the industrial processing of cashew (Anacardium occidentale L.), is a phenolic compound with a long unsaturated chain substituted in the meta position to the hydroxyl.¹⁵ This hydrocarbon chain acts as an internal plasticizer giving flexibility to phenolic resin. The polymeric matrix is result of the polycondensation reaction between cardanol (natural phenol) and formaldehyde.

Experimental procedure

Materials

Bamboo fibers of the family Bamboo vulgaris (100 mm length, diameter 0.04 mm, and density 0.763 g/cm³) were collected at the University Federal of Ceará (Brazil, 3°44′46″ latitude S and 38° 34′33″ longitude W) in September 2011. The CNSL used in this work was kindly provided by Amêndoas do Brasil Ltda located in Fortaleza-Ceará-Brazil. Sodium hydroxide (99%), sodium hypochlorite (97%), formaldehyde (37%), ammonium hydroxide (30%), 2 -(Dimethylamino) ethyl benzoate (99%), and sodium sulphate (99%) were obtained from Sigma-Aldrich and used without prior treatment. Resin epoxy was obtained from Epoxiglass in Brazil.

Chemical treatment of bamboo fibers

Dried bamboo strips were dipped in NaOH solutions 5% and 10% at temperature intervals of 60℃–75℃ for 4 h, for partial removal of lignin, hemicellulose, and other residues (Figure 2). After this period, the fibers were washed with distilled water several times to remove sodium hydroxide excess from the surface until the water no longer indicated any alkalinity reaction. Subsequently, the fibers were dried in oven at 60℃ for 24 h.

Bamboo fibers obtained after alkaline treatment were dipped in NaClO 1% solution under heating (60℃–75℃) for 1 h. At this point, the fibers presented strong bleaching effect, being subsequently washed with distilled water to eliminate remaining chemicals and dried in oven at 60℃ for 24 h. The active bleaching agent is the hypochlorite ion and the reactions are as follows:⁶

NaOCl + H_{2} O \to NaOH + HOCl

(1)

HOCl \to H + + - OCl

(2)

HOCl + H + + Cl - \to {Cl}_{2} + H_{2} O

(3)

Obtainment cardanol and synthesis of polymer resol

Cardanol used in the preparation of the resin was obtained from technical CNSL using the following methodology: Technical CNSL (100 mL) was dissolved in methanol (320 mL), and ammonium hydroxide (170 mL) was added and stirred for 30 min. Then, the solution was extracted with hexane (1 × 250 mL). The organic phase was washed with 2.5% NaOH (2 × 100 mL). To this solution was added HCl (37%, 10 mL) until pH 5. The cardanol obtained was conditioned at a temperature of approximately 15℃.

The polymer resol was obtained by adding cardanol to a solution of formaldehyde (37%) and ammonium hydroxide (30%), in proportions of 1:2:1, under strong magnetic stirring. The mixture remained in refluxing at 80℃ for 3 h in order to complete the polycondensation.

Preparation of biocomposite

The processes of biocomposites manufacturing were reported elsewhere.^1,6 The technique used in the preparation of the biocomposites was a simple hand lay-up. An aluminum mold (160 × 80 × 5 mm) was used. A release agent (silicone) was used to facilitate easy removal of the biocomposite from the mold after curing. The resol resin was mixed with epoxy resin and catalyst 2 -(Dimethylamino)ethyl benzoate (Aldrich, 99%) in the proportion 0.93:1:0.03, respectively.

Epoxy resin acts as a modifier agent in the polymer, giving it greater flexibility and adhesion. The epoxy resin hardened with the aid of a catalyst improving the thermosetting properties of material. Bamboo fibers were cut into uniform length (35 mesh) and added to the polymer matrix, under stirring. This mixture was transferred to an aluminum mold previously waxed with a desmoldant agent. A complete cure of the material was done in oven at 120℃ for 3 h.

Study of biodegradability

The biodegradability measurements of the fibers and biocomposite were obtained by the mass retention technique. The fibers and the biocomposites were weighed and buried in simulated soil at room temperature (24℃). The simulated soil consisted of 23% of loamy silt, 23% of organic matter (cow manure), 23% of sand, and 31% of distilled water (all wt/wt). Biodegradation was monitored for 210 days by measuring the mass retention. The buried specimens were recovered, washed with distilled water, and dried at a room temperature until there was no further variation in weight, after which they were then weighed. The experiments were done in triplicate.

Structural characterization of the jute fibers and biocomposites

Infrared measurements

The infrared measurements were performed using a Spectrum 200 FTIR Spectrometer, Perkin Elmer, in order to determine changes in functional groups chemistry have been caused by the treatments. Prior to the analysis, 10 mg fibers were ground and mixed with KBr. The resultant powder was pressed into transparent pellets and analyzed in transmittance mode within the range of 4000–400 cm⁻¹.

Crystallinity index

X-ray diffraction (XRD) patterns were obtained from bamboo disks at room temperature (300 K) using a Rigaku powder diffractometer model DMAXB. The samples were pulverized and their particles were compacted in a cylindrical mold into disc format (Ø 1.7 cm) and submitted to the pressure of 111 MPa. Co-Kα tube was operated at 40 kV and 25 mA, and the measurements were performed using the Bragg–Bretano geometry in continuous mode with a scan speed of 0.5°/min and step size of 0.02° (2θ) in the angular range of 5°–40° (2θ). The crystalline index of cellulose, C_Ir, was determined based on the Segal empirical method¹⁶

C_{Ir} = (I_{200} - I_{am}) / I_{200} \times 100

(4)

where I₂₀₀ is the peak intensity corresponding to cellulose I, and I_am is the peak intensity of the amorphous fraction.

Mechanical analysis of bamboo

Elasticity module (GPa), tensile strength (MPa), and rupture elongation (%) were the mechanical properties measured in the tensile strength. The tensile tests were carried out for five samples to each kind of fiber (untreated and chemically treated) and biocomposites, using the INSTRON machine model 4433. The fibers were defibrillated and their wires were submitted to mechanical measurements. The conditions for analysis were a speed of displacement of 2.0 mm/min, load of 100 N, and length of the fiber of 5.0 cm. It was preceded by an adaptation of the norm ASTM D3039, and the fibers were located in a rectangular window, with 2.0 cm of width and 4.0 cm of length. The norm ASTM D3039 and ASTM 638 describes the test.

Mechanical analysis of biocomposites

Five samples of biocomposites (160 × 10 × 3 mm) were tested with an INSTRON, model 4433 tensile tester according to ASTM D638 and D3039 with a crosshead speed of 2 mm/min and a span length of 4 cm.

Scanning electronic microscopy

The morphological characterization of the bamboo fibers and the biocomposites was done by scanning electron microscopy (SEM). This technique was used to observe the surface morphology and make analysis of the microstructure in both natural and chemically treated forms. The micrographs were obtained by an electronic microscope DSM 960 Zeiss, with 20 kV electron beam. The samples were coated with Au and “sputter counted” with argon plasma model BALZERS 5CD50.

Results and discussion

The physical and mechanical properties of biocomposites reinforced with natural fibers depend on the interaction of components present in the region of the interface reinforcement/matrix. The magnitude of this interaction can be improved by surface modification, whose purpose is to increase the hydrophilicity, dispersion, mobility, and adherence of the fibers.

The chemical treatment with NaOH was chosen based on satisfactory results obtained for other fibers, where the physical and mechanical properties of fibers were optimized to be used as reinforcement in biocomposites.^1,17 Bamboo fiber as reinforcement in biocomposites has improved mechanical properties after different treatments.¹⁸ The removal of non-cellulosic components of the surface layer increases adhesion between fibers and resin, providing greater stabilization of the fiber–matrix bond.

Figure 3 shows the SEM images obtained for analyses of the surface of bamboo fibers treated, non-treated, and their biocomposites. Figure 3(a) illustrates the morphology of the untreated fiber surface, which presents no cavities and it is covered by a layer of residues responsible for fibrils compression. These residues protect fibers and prevent the penetration of the resin into it. This material that covers fibrils is named cuticle and it was identified as an aliphatic wax,¹⁹ which is incompatible with most polymers and prevents a greater adhesion between reinforcement/matrix. Figure 3(d) shows the biocomposites with natural fibers of bamboo, which presented a low interaction with the matrix, evidenced by limited microfibrillar exposure.

Figure 3(b) shows the fibers after chemical treatment of NaOH 5%, indicating a slight difference due to the partial removal of non-cellulosic component as waxes, pectin, hemicellulose, lignin, etc., providing more exposed fibrils, increasing the surface area provided and thereafter the interaction and adhesion between fiber and resin. Figure 3(c) shows a slight modification of the fiber surface after treatment with NaOH 10%. At this concentration of alkali, a greater degree of leaching of the surface layer of the fibers was observed, making them more resilient to produce a greater adherence to the polymer matrix. The loss of the surface layer occurs by breaking the links that bind the polymer chains present in lignin and hemicelluloses due to sensitivity to alkali treatment, leaving the fiber surface more exposed.^20,21 Based on these results, it is expected that the fibers subjected to a treatment with NaOH 10% would have a greater interaction with the polymer matrix.

Studies by Barreto et al.¹ showed a greater interaction between the reinforcement and matrix after chemical treatment with 10% NaOH. Similar behavior was observed in coconut fibers when used as reinforcement of biocomposites derived from natural rubber.²⁰ The same behavior was observed for Rosa et al.⁷ who investigated the interaction between starch/ethylene vinyl alcohol copolymers/coir biocomposites. As a consequence, greater reinforcement/matrix interaction was observed due to improvement of the surface adhesion. Figure 3(e) and (f) illustrate this behavior for the biocomposites reinforced with bamboo fibers undergoing treatment with NaOH 5% and 10%, respectively.

The XRD patterns of the bamboo fibers untreated and treated with NaOH are shown in Figure 4. The major peaks observed for all fiber samples are at 2θ diffraction angles of 18.2° and 26.0°. They are represented by 101 and 002 planes, attributed to the crystalline phase of cellulose type I (ICDD PDF50–2241).²² Table 2 shows the crystallinity index calculated according to the Segal et al.¹⁶ empirical method.

Table 2.

Mechanical properties and crystallinity index of treated and untreated bamboo fibers.

Fiber	Maximum tensile (MPa)	Breaking tensile (MPa)	Breaking load (N)	Elongation (%)	Elasticity modulus (GPa)	Crystallinity index (%)
Raw	84	84	164	2.05	4.1	22.97
NaOH 5%	174	174	250	4.82	4.6	45.53
NaOH 10%	212	212	358	3.98	5.3	48.28

The crystallinity index increased for all treated samples, especially for the ones treated with NaOH 10% (48.28%). This increase could be related to the partial removal of non-cellulosic compounds: lignin, hemicellulose, and waxes. The surface modification of the bamboo fiber after alkaline treatment was also observed by SEM, as shown in Figure 3.

Figure 5 shows Fourier transform infrared spectra (FTIR) spectra for untreated and treated bamboo strip. Bands frequencies and their corresponding functional assignments are consistent with the literature values for lignocellulosic fibers.^23,24 The spectrum shows the main absorption modes between 400 and 2000 cm⁻¹. A band between 1660 cm⁻¹ to 1740 cm⁻¹, characteristic of the stretching frequency of C = O groups, was observed, which can be attributed to the presence of carboxylic ester in pectin and wax.²⁵ The disappearance of the peak around 1253 cm⁻¹, which represents the C–O stretching in lignin, probably indicates its degradation after alkaline treatment.²⁶

The spectral region between 1050 cm⁻¹ and 1500 cm⁻¹ (between the dashed lines) represented in Figure 5(a) is relative to lignin. In this spectral range was observed a slight change in raw bamboo, when compared with chemically modified fibers (Figure 5(b) and (c)). This variation in the spectra suggests that, despite the alkali treatment does not completely remove lignin, it still caused changes in the groups on fibers surface. These results agreed with those observed in the analysis by XRD analysis, where the amorphous area (I_am) corresponds mainly to the presence of this component in the fibers. The peak around 1735 cm⁻¹, which represents the C = C axial stretching belonging to hemicellulose, pectin and waxes, disappeared after treatment with NaOH 5% and 10%, suggesting that these components were partially removed.^6,27

Table 2 shows the results of mechanical properties of the bamboo fibers before and after chemical treatment. All fibers subjected to tensile tests showed higher resistance and percentage of elongation than other lignocellulosic fibers such as jute, sisal, and banana, whose average stretch is 1.5%, 0.7%, and 0.6%, respectively.²⁸

According to data showed in Table 2, there was an increase in the mechanical properties of fibers after the chemical treatment. An increase in the failure stress of 107% and 152% for the fibers treated with NaOH 5% and 10%, respectively, was observed when compared to raw fibers. The results are associated to cellulose content of bamboo fiber (60%) and confirmed by the XRD data, which shows an increase of crystallinity index after alkali treatment.²⁹

The mechanical properties of biocomposites were evaluated for the influence of the chemical treatment employed in the reinforcement. Similarly, the biocomposites that showed the best performances were those reinforced with fibers treated with NaOH 10%, see Figure 6. According to Gomes et al.,³⁰ upon alkaline treatment, there was a partial removal of the fiber components due the rupture of ester linkages between polyuronic acid and lignin, increasing the index crystallinity after alkali treatment. Accordingly, the fiber becomes less dense and more rigid, increasing the fibrils strength as observed for fibers with lower lignin content and hemicelluloses contents.

The results obtained from these tests are consistent with the literature.³⁰ After chemical treatment, a significant increase in the mechanical properties of fibers was observed.³¹ XRD results corroborated the behavior demonstrated in this test, which revealed an increase in the crystallinity of the fiber after chemical treatment, providing higher resistance of the material due to the high content percentage of cellulose. Therefore, mechanical properties of fibers depend on the chemical composition and their internal structure.

The thermogravimetric analyses (TG) and its derivatives (DTG) curves under nitrogen atmospheres for bamboo fibers untreated and treated with NaOH solutions are shown in Figure 7. In both samples, a progressive loss of mass at a temperature below 100℃ was observed, which was associated with the release of water.³² The decomposition of lignocellulosic compounds (hemicellulose, cellulose, and lignin) for all bamboo fibers showed only one distinct peak to raw fibers and two peaks to treated fibers with 5% to 10% in the DTG curves.

At temperatures between 100℃ and 400℃, mass losses of 64.90, 67.38, and 67.95% in samples of raw bamboo, treated with NaOH 5%, and 10%, respectively, were observed. These corresponded to the thermal decomposition stage of the fiber macrocomponents. Lignocellulosic fibers degrade in several steps.³³ The hemicellulose part degrades at about 240℃–310℃, whereas the cellulose part degrades in the range of 310℃–360℃ and the lignin has been shown to degrade in wide temperature interval (200℃–550℃).⁶

In Figure 7, it is also noted that the chemical treatment improved thermal stability of the fiber, characterized by an increase in initial temperatures of degradation. An increase in the initial degradation temperature (284.0 and 291.0℃) in samples treated with NaOH 5% and 10% respectively, compared with the fibers in their natural state (275.5℃), can be seen. Similar results were found by Maheswari et al.,³⁴ who investigated the thermal behavior of the tamarind fruit fibers.

Initial temperature of degradation, the maximum temperatures of the thermal decomposition, mass loss involved, and the residual mass at 800℃ were determined from TG/DTG (Figure 7) and are shown in Table 3. The bamboo fiber treated with NaOH 10% began to decompose at higher temperature than the other fibers samples analyzed. Despite the increase in alkali concentration, it caused a small increase in the initial temperature of pyrolysis (from 283℃ to 299℃) under these conditions, in addition to a greater mass loss, indicating that chemical treatment was more appropriate with NaOH 10%. This result can be justified if we associate the degradation to the amorphous regions of the fibers as a function of crystallinity. Bamboo fibers treated with NaOH 10% showed a higher crystallinity index, resulting in a greater amount of waste degradation process, thus justifying a higher mass loss.³⁵

Table 3.

Parameters obtained from TG/DTG curves of white and colored cotton fibers at N₂ atmospheres: temperature range of thermal degradation, mass losses, and mass residue at 800℃.

Fiber	Atmosphere (10℃ min⁻¹)	Temperature maximum (℃)	Temperature range (℃)	Mass loss/(%)	Residue (%)
Raw	N₂	315	275–400	55	22
NaOH 5%	N₂	330	283–400	67	15
NaOH 10%	N₂	340	291–400	68	13

The thermal analysis is an important technique for the characterization of polymers, since the good thermal stability observed in these materials allows several technological applications. The TG and DTG curves of the biocomposites, under inert atmosphere (N₂), are shown in Figure 8. Weight loss associated with the moisture present in the fibers, which is justifiable considering the temperature used in the curing process, was not observed in all samples. The thermogravimetric curves of the biocomposites showed two main degradation stages. The first event occurred in the temperature range between 250℃ and 380℃, and this process corresponds to the simultaneous degradation of pectin, hemicellulose, and cellulose.

A second stage occurred at intervals between 400℃ and 450℃, characteristic of the degradation of crosslinking between the polymer chains. At temperatures above 450℃ occurred the decomposition of the final products of the fibers and the polymeric matrix. During these stages, a slight increase in the maximum degradation temperature (DTG curve) from 400℃ to 421℃ for the biocomposite reinforced with raw and chemically modified (10% NaOH) fibers, respectively, was also observed. These results demonstrate that the thermal properties of the bamboo biocomposites improved considerably after the chemical treatment of the fibers.

Figure 9 shows the loss of weight of the biocomposites and fibers (non-treated and treated with alkaline solution) exposed to microorganisms in simulated soil during 210 days. The results showed that the biodegradability of the lignocellulosic fiber is dependent on the applied treatment. It was observed that raw bamboo fiber and treated with NaOH 5% presented an increase of mass at the initial stages of the experiment (3.03% and 1.05% for raw and treated fibers, respectively), which could be attributed to the absorption of water.^32,36 This behavior was not observed for the sample treated with NaOH 10% due to significant decrease of macroconstituents (lignin and hemicellulose) that present hydrophilic hydroxyl groups.

In general, the fibers treated with NaOH (5% and 10%) demonstrated higher weight loss caused by the biodegradation induced by the action of microorganisms. This loss of weight was caused by enzymes produced by microbial population in the simulated soil, where the bamboo fiber treated with NaOH 10% solution was more affected than others.³⁷ This happened due to an increase in surface area, as observed by SEM, caused by the partial removal of the fiber constituents such as hemicellulose, pectin, fat, and lignin.

Similar behavior was also observed for the biocomposite supported with bamboo fiber (treated and untreated). In 30 days, there was an increase in the mass of the biocomposites of 6.09%, 5.10%, and 3.04% for the samples reinforced as raw fiber and treated with NaOH solution of 5% and 10%, respectively. The biocomposite samples reinforced with raw bamboo fiber and treated showed different degree of biodegradation with weight loss after 30 days of analyses.

In general, the biodegradability studies showed that the raw bamboo fiber and the biocomposite reinforced with this fiber was more resistant to the action of the microorganisms due to the presence of more lignin and hemicelluloses contents. In the plant tissue, lignin acts as a reinforcing reinforcement, like cement between the fibers. The association between lignin and hemicellulose forms a protective barrier against microorganism’s attackers.

Conclusions

The chemical treatment directly affected the thermal and mechanical properties and consequently the adhesion between matrix and fiber, as observed in SEM, where the alkaline treatment caused exposure of the cellulose structures. The XRD showed that chemical treatment increased the crystalline fraction due to partial removal of amorphous components. The FTIR was important to observe the main groups function and its variations as function of treatment. This modification was important to improve the adhesion fiber matrix in the biocomposites and resulted in materials more homogeneous, resistant to thermal decomposition and higher mechanical resistance.

Beyond these results, the biodegradation tests showed that alkaline treatment rendered a composite more susceptible the microorganism’s action, especially the composite reinforced with fibers treated with NaOH 10%, which revealed a higher mass loss in simulated soil. In general, we can conclude that the composites produced in this study, obtained from a raw material derived from biomass (CNSL) have a greater biodegradability compared to composites reinforced with synthetic fibers.

Footnotes

Funding

This work was supported by FUNCAP, CAPES, and CNPq (Brazilian agencies).

Conflict of interest

None declared.

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