Preparation of composites from natural rubber and oil palm empty fruit bunch cellulose: Effect of cellulose morphology on properties

Abstract

Rubber tree and oil palm are industrial crops cultivated in the same climate and environment. These plants are used to prepare nanocomposites of natural rubber and cellulose from empty fruit bunches, an abundant residue in the palm oil industry. For this study, the cellulose particles were extracted from the bunches and subjected to enzymatic hydrolysis or microfibrillation to produce nanostructured particles. The nanoparticles were blended with natural rubber latex in an aqueous medium, and the mixture was dried. The properties of the nanocomposites were compared to those of pure natural rubber and unprocessed cellulose composites. The mechanical properties of the natural rubber can be modified by the cellulose content and morphology. As a consequence, it is possible to modulate the material properties by changing only the filler morphology. The use of microfibrillated cellulose had stronger reinforcement effects. The thermal properties of natural rubber were not affected by the addition of cellulose.

Full Article

Preparation of Composites from Natural Rubber and Oil Palm Empty Fruit Bunch Cellulose: Effect of Cellulose Morphology on Properties

José Antonio Fiorote,^a Alair Pereira Freire,^a Dasciana de Sousa Rodrigues,^a Maria Alice Martins,^b Larissa Andreani,^a and Leonardo Fonseca Valadares ^a,*

Rubber tree and oil palm are industrial crops cultivated in the same climate and environment. These plants are used to prepare nanocomposites of natural rubber and cellulose from empty fruit bunches, an abundant residue in the palm oil industry. For this study, the cellulose particles were extracted from the bunches and subjected to enzymatic hydrolysis or microfibrillation to produce nanostructured particles. The nanoparticles were blended with natural rubber latex in an aqueous medium, and the mixture was dried. The properties of the nanocomposites were compared to those of pure natural rubber and unprocessed cellulose composites. The mechanical properties of the natural rubber can be modified by the cellulose content and morphology. As a consequence, it is possible to modulate the material properties by changing only the filler morphology. The use of microfibrillated cellulose had stronger reinforcement effects. The thermal properties of natural rubber were not affected by the addition of cellulose.

Keywords: Composite mechanical properties; Natural rubber; Cellulose fibres nanostructure; Transmission electron microscopy; Oil palm empty fruit bunches

Contact information: a: Embrapa Agroenergy, Parque Estação Biológica S/N, Av. W3 Norte (final), 70770-901, Brasília, DF, Brazil; b: Embrapa Instrumentation, P.O. Box 741, 13560-970 São Carlos, SP, Brazil; *Corresponding author: leonardo.valadares@embrapa.br

INTRODUCTION

Composites and nanocomposites based on renewable resources have attracted great interest due to their environmental friendliness, biocompatibility, and biodegradability (Visakh et al.2012). These two classes are differentiated by the dimensions of the components that constitute the disperse phase of the material (Ray and Okamoto 2003).

Natural rubber (NR) is a renewable polymeric matrix used for the composites and nanocomposite preparations. Natural rubber is a biopolymer with elastic properties derived from latex; it is found in the sapwood of Hevea brasiliensis (Bras et al. 2010). It is a highly valuable commercial biopolymer used to manufacture industrial and medical products and is essential for the tire and anti-vibration industries (Rolere et al. 2016).

Elastomers such as NR can be reinforced by the addition of fillers. Furthermore, an increase in elastic modulus is typically obtained along with a reduction in the strength and elongation of the materials (Angellier et al. 2005). Carbon black is commonly used as a filler because of its good interactions with NR (Martins et al. 2003). However, due to environmental concerns, reinforcing NR with natural fibres is attractive because they have low density, are readily available, and can be derived from a variety of renewable resources (Dufresne 2006).

Cellulose is a renewable organic material composed of repeating glucose units, and it is one of the main components of plant cell walls. As such, it is the most abundant polysaccharide on Earth and a potential raw material for reinforcing NR composites and nanocomposites. The interactions at the interface of NR and cellulose fibre composites have not been thoroughly explored (Hamed and Li 1977; Flink et al. 1988; Yano et al. 1992). Yano et al. (1992) observed that the orientation of the fibres caused substantial anisotropy in the mechanical properties of composites with higher loadings of cellulose fibres. However, the adhesion between the fibres and the rubber matrix needs improvement. Studies have reported the development of nanocomposites with extracted cellulose nanofibres from different sources (Bendahou et al. 2009; Bras et al. 2010; Pasquini et al. 2010; Siqueira et al. 2010; Visakh et al. 2012). Generally, formulations with higher nanofibre contents improve Young’s moduli and tensile strength. Furthermore, the presence of cellulose nanofibres increase the rate of degradation of the composites in soil (Abraham et al. 2012).

Oil palm (Elaeis guineensis) and rubber trees are industrial crops that can be cultivated in the same regions of the tropics. During palm oil and kernel oil extraction, a large amount of residual biomass is generated in the form of empty fruit bunches and hulls (Law et al. 2007). Palm oil empty fruit bunches can be used to produce long and thin cellulose nanofibres (Fahma et al. 2010).

There are different mechanical, chemical, chemo-mechanical and enzymatic methods for obtaining nanostructures from purified cellulose (Visakh et al. 2012). In general, cellulose is purified first and then subjected to a controlled hydrolysis process. Under these conditions, the amorphous regions around and between the crystalline cellulose nanofibres preferentially undergo hydrolysis because the hydrolysis kinetics of the amorphous domains are faster than in the crystal region (Silva et al. 2009).

The present work described the production of nanostructures from oil palm empty fruit bunches (OPEFB) by mechanical and enzymatic treatments, followed by the preparation of NR composites and nanocomposites using fibres and nanofibres extracted from OPEFB, respectively. The mechanical and thermal properties of those materials was evaluated. The morphology and crystallinity of the fibres and nanofibres were investigated using transmission electron microscopy (TEM) and X-ray diffraction (XRD). The thermal, dynamic mechanical, and mechanical properties of the composites and nanocomposites were evaluated by thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and tensile tests.

EXPERIMENTAL

Materials

The centrifuged natural rubber (NR) latex was supplied by QR Borrachas Quirino Ltda (Cedral, Brazil). The latex was collected from RRIM 600 clones in São José do Rio Preto, São Paulo, Brazil, and stabilized with ammonia. The sample presented a dry rubber content of 61.86% and a pH of 9.

The palm oil bunch, belonging to 2301 cultivar Tenera hybrid, was collected in Planaltina, Distrito Federal, Brazil, and autoclaved to remove the fruits.

The following chemicals were used for cellulose purification: ethanol (Vetec, Duque de Caxias, Brazil), petroleum ether (Vetec, Duque de Caxias, Brazil), sodium chlorite (Sigma-Aldrich, St. Louis, USA), and acetic acid (Dinâmica, Indaiatuba, Brazil). Trichoderma reesei cellulase enzyme (≥ 700 units/g) (Sigma-Aldrich, St. Louis, MO, USA) was used to hydrolyse the cellulose.

Methods

Cellulose purification

The cellulose pulp was obtained as described by Fahma et al. (2014) with modifications. The OPEFB were ground using a Willey mill (Fortinox, Piracicaba, Brazil). Accelerated solvent extraction (ASE 350, Dionex, Waltham, MA, USA) was used to remove the extractives with petroleum ether:ethanol (2:1) solution at 105 °C. The resulting material was soaked four times in a sodium chlorite (NaClO₂) solution, acidified to pH 4 with acetic acid, and soaked in 6% potassium hydroxide (KOH) solution for 24 h. The extraction procedures with NaClO₂ and KOH were repeated to ensure the purity of the cellulose. After each extraction, the fibres were sedimented, the supernatant was exchanged for distilled water, and the fibres were stored in an aqueous medium.

Cellulose nanostructure production using enzymatic hydrolysis

Nanostructures were produced by enzymatic hydrolysis of the purified cellulose in a shaker (TE-420, Tecnal, Piracicaba, Brazil) at 5 Hz using cellulase from Trichoderma reesei at a concentration of 15 FPU/g for 48 h in a citric acid/sodium citrate buffer solution (pH 5.0) at 50 °C. To inactivate the enzyme and stop the reaction, the sample was heated at 98 °C for 1 h. Finally, the sample was centrifuged and washed with distilled water to remove the buffer and enzymes. The obtained sample was referred to as “hydrolysed cellulose”. The conditions to produce the nanostructures with cellulose were based on preliminary studies.

Microfibrillated cellulose production

To produce the microfibrillated cellulose, the cellulose pulp from the OPEFB was diluted to 1% in distilled water. The dispersion was sheared with an IKA T25 disperser (Staufen, Alemanha) at 24,000 RPM for a total of 120 min. During the shearing process, the temperature and the viscosity of the sample increased, and for this reason, the shearing time was divided in 12 periods of 10 min, and the samples were cooled between the sessions.

Natural rubber/cellulose composite preparation

The composites and nanocomposites were prepared by blending NR latex with purified aqueous cellulose, hydrolysed cellulose nanostructures, or microfibrillated cellulose. The solid contents of the pulp and latex were used to determine the amount of each dispersion required to yield composites with 0.5 per hundred rubber (PHR), 1.0 PHR, 2.5 PHR, or 5.0 PHR. A sample of pure NR was also prepared. The mixtures were stirred for 1 h, poured on glass Petri dishes, and oven-dried at 50 °C for seven days. The samples were dried for one additional day in a vacuum oven.

Characterization

For the TEM analyses, the aqueous dispersion was first diluted and decanted under the action of gravity. Samples were prepared by depositing a droplet of the dispersion on a covered microscope grid (Ted Pella, Redding, CA, USA). After drying, the samples were analysed using a Carl Zeiss TEM 109 microscope (80 kV) (Jena, Germany). Pure cellulose nanostructures and a mixture of cellulose and NR latex (1:1 based on the solid content) were analysed.

X-ray diffraction measurements were performed in a Shimadzu XRD-6000 diffractometer (Kyoto, Japan) using the reflection mode at a scan rate of 0.5°/min with Cu K radiation (1.54 10^-10 m). Purified cellulose, hydrolysed cellulose, and microfibrillated cellulose were freeze dried before the analyses.

Tensile tests were performed using an Arotec WDW-201 universal testing machine (Beijing, China). The specimens were prepared by casting, cut, and then stored at 23 °C in 50% relative humidity for 15 days before the measurements. At least eight specimens were tested for each sample.

The dynamic mechanical properties were measured as a function of temperature using a dynamic mechanical analyser DMA Q800 (TA Instruments, New Castle, USA). The measurements were performed under tension in the temperature range from -120 °C to 120 °C, at a heating rate of 2 °C/min and a frequency of 1 Hz.

Thermogravimetric (TG) analyses were conducted in a Q500 instrument (TA Instruments, New Castle, DE, USA) in the temperature range from 25 °C to 700 °C at a heating rate of 10 °C/min under an inert atmosphere (nitrogen). Approximately 10 mg of sample was used for each analysis.

RESULTS AND DISCUSSION

Typical TEM images of the cellulose after enzymatic hydrolysis are presented in Fig. 1. The cellulose is in the form of needles with size in the micrometer-scale. The isolated fibres presented an average thickness of 6.8 nm with a standard deviation of 2.2 nm based on the measurement of 300 individual particles. The images indicate the fibres were separated, although aggregated particles forced to adhere during water evaporation were also observed.