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Ruayruay, W., and Khongtong, S. (2014). "Impregnation of natural rubber into rubber wood: A green wood composite," BioRes. 9(3), 5438-5447.

Abstract

A green wood composite material was developed from the two environmentally friendly substrates natural rubber (cis-1,4-polyisoprene) and rubber wood (Hevea brasiliensis). Natural rubber (NR) was introduced into rubber wood by pressurization of NR latex, followed by the removal of the aqueous phase to allow only dry NR to remain inside the wood structure. Scanning electron microscopy images and the weight increase of the dry impregnated samples revealed the retention of dry NR within the rubber wood. The natural rubber enhanced the water resistance and compressive strength of the treated rubber wood.


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Impregnation of Natural Rubber into Rubber Wood: A Green Wood Composite

Wassa Ruayruay and Sureurg Khongtong*

A green wood composite material was developed from the two environmentally friendly substrates natural rubber (cis-1,4-polyisoprene) and rubber wood (Hevea brasiliensis). Natural rubber (NR) was introduced into rubber wood by pressurization of NR latex, followed by the removal of the aqueous phase to allow only dry NR to remain inside the wood structure. Scanning electron microscopy images and the weight increase of the dry impregnated samples revealed the retention of dry NR within the rubber wood. The natural rubber enhanced the water resistance and compressive strength of the treated rubber wood.

Keywords: Wood impregnation; Wood composites; Rubber wood; Natural rubber

Contact information: Materials Science and Engineering Programs, School of Engineering and Resources, Walailak University, Thasala, Nakhon Si Thammarat 80160, Thailand;

* Corresponding author: ksureurg@wu.ac.th

INTRODUCTION

The development of materials from sustainable sources has become a current challenge in materials science. This offers opportunities for improving material properties as well as producing more environmentally friendly materials. The focus of our research group has been the introduction of composite materials by integrating natural rubber (NR), a product from cultivated rubber trees (Hevea brasiliensis), and rubber wood, a by-product from aged rubber trees felled for replanting due to low latex yield.

Natural rubber and rubber wood are attractive because they are renewable raw materials and, in fact, have become the most consumed biopolymer in industry (Malaysian Rubber Board 2013). The initial form of NR is a colloidal latex, which consists of natural cis-1,4-polyisoprene particles in an aqueous phase collected from the bark of rubber trees (Blackley 1997) and is generally used for the manufacture of latex gloves and condoms. In addition, solidified NR latex is a major raw material for the production of vehicle tires. Rubber wood is currently an important source of timber in Southeast Asia because it is affordable as well as environmentally friendly, as it is a by-product of farm trees and normally thrown away. This wood, however, displays low durability and strength, especially when used in contact with water (Scheffer and Morrell 1998). Therefore, quality improvement is essential for wider applications.

Studies involving wood impregnation with polymeric materials to improve water resistance and strength have been reported. The inclusion of polystyrene maleic anhydride and the derivatives of polyacrylate into wood can upgrade its dimensional stability and water absorption ( Uyttenhove and Tilquin 2005; Grelier et al. 2007). Bakraji and Salman (2003) found that the compressive strength of white poplar, cypress, and white willow can be enhanced by acrylamide and butyl methacrylate. The hardness of beech and spruce were elevated by introducing polyvinyl alcohol and polyacrylate into the wood (Solpan and Guven 1999). Hamdan et al. (2010) improved the stiffness and thermal stability of wood by impregnation with dimethylacetamide. Poplar wood displayed a greater density and degree of crystallinity after impregnation with urea-formaldehyde (Wu et al. 2010). Additionally, rubber wood impregnation with synthetic polymers has been reported, with the result that treated rubber wood provided increased dimensional stability, water resistance, and strength (Devi et al.2003; Mohamad et al. 2007).

Impregnation of a variety of synthetic polymers, as stated previously, is difficult and expensive for industrial-scale applications. Moreover, some monomers used for impregnation are harmful (Caldeira 2010), and thus the hazards of using polymer products have been mentioned (Teoh et al. 2011). Therefore, the system of NR-impregnated rubber wood proposed in this study can overcome these disadvantages. Simple pressurization of NR latex into the rubber wood prior to oven drying allows the retention of dry NR within the wood structure. Furthermore, this procedure is systematically analogous to the method of wood treatment by the impregnation of preservatives currently applied to the processing of rubber wood (Gnanaharan and Dhamodaran 1993), and may provide industrial applications for the manufacture of green wood composites with improved water resistance and strength.

EXPERIMENTAL

Materials

Wood samples with dimensions of approximately 20 mm in width (radial) × 20 mm in thickness (tangential) × 60 mm in length (longitudinal) were randomly cut from lumber of rubber trees (RRIM-600 variety), grown in the area of Nakhon Si Thammarat Province, Southern Thailand, that were about 25 years of age. The cut samples were dried in an oven at 103 ± 2 °C until they reached a constant weight.

It was found that the density of these dry samples was in the range of 0.62 to 0.68 g/cm3, which is about the same as that of the rubber wood generally used in the industry. They were then kept desiccated until use.

Concentrated NR latex (high ammonium grade, 60% dry rubber content; Rubber Estate Organization, Thailand) was used for impregnation. Before use, the NR latex was gently stirred at an ambient (26 to 28 °C) temperature for about 1 h to remove the ammonia. To vary the percentage of dry rubber content (DRC) in the NR latex (12%, 24%, 36%, 48%, and 60%), water was added and the mixture was gently mixed for 20 min. The latex viscosity was monitored by a Brookfield RTV viscometer (Massachusetts, USA), and each value reported in this study (1, 2, 3, 4, 12, and 50 centipoises) is an average of at least three measurements at temperature of 26 to 28 ºC.

Methods

Scanning electron microscopy

Scanning electron microscopy (SEM) was performed on the representatives of treated and untreated wood samples. The samples (10×10×5 mm3) were mounted on an aluminum stub with double-sided tape, sputter-coated with gold, and examined using an FEI Quanta 400 SEM (Oregon, USA) operating at an accelerating voltage of 10 kV.

Laser particle size analysis

The particle sizes of concentrated NR latex used in this study were monitored using a Coulter LS230 (California, USA) laser light scatter particle size analyzer (LPSA). Water (10 mL) was added into 10 mL of NR latex (60% DRC), and 10 mL of a mixer was used for this analysis.

Latex pressurization

Rubber wood samples were placed in a shop-made pressurization chamber (Fig. 1) at a vacuum of 635 mmHg for 15 min to remove the air and moisture from the samples. The chamber was then filled with 0% (water) to 60% DRC of NR latex, as stated in each experiment, and pressurized to 6 bars for 2 h. These conditions were sufficient to achieve the maximum latex embedding in the pressurized samples.

Fig. 1. An illustration of the pressurization chamber

Measurement of NR latex infiltration and dry NR inclusion

To investigate how much NR latex could infiltrate the wood samples under pressure, the excess latex on sample surfaces was removed after retrieving the samples from the pressurization chamber to ensure that the observed weight increase was due only to the embedded latex. The percentages of embedded latex (stated as the weight increase) in treated samples were calculated using Eq. 1,

% weight increase = (W2 – W1) × 100 / W1 (1)

where W1 and W2 are the sample weights before and after pressurization, respectively. The pressurized samples were then dried in an oven at 103 ± 2 °C until they reached a constant weight. The percentage of dry rubber inclusion in treated samples was then calculated using the same equation, where W1 and W2 are the sample weights before pressurization and after drying, respectively.

Water absorption and thickness swelling tests

The weight and thickness (in the tangential direction) of oven-dried specimens were taken before they were soaked in water for either 2, 24, or 48 h. After each time interval, they were retrieved from the water and the excess water on their surfaces was removed using a soft cloth. The weight and thickness of these specimens were then measured again. Water absorption and thickness swelling were calculated for these controls using the following equations,

%water absorption = [(W1-W0) ×100/ W0] (2)

%thickness swelling = [(T1-T0) ×100/ T0] (3)

where W0 and W1 are the weight before and after water soaking, respectively, and T0 and Tare the thickness before and after water soaking, respectively.

Compressive strength testing

Compressive strength parallel and perpendicular to the fiber grain was evaluated using a Lloyd LR150R (Hampshire, UK) universal testing machine with a load cell of 150 kN. This test was performed at ambient temperature with a crosshead speed of 2 mm/min (ISO 3132 1975; ISO 3787 1976). The instrument software (Nexygen 4.5) was used to control the test and analyze the data. Each value reported in this study is an average of at least 10 measurements.

RESULTS AND DISCUSSION

Mechanism of Natural Rubber Impregnation into Rubber Wood

To achieve the goal of this study, cis-1,4-polyisoprene particles in NR latex must move through the pathways in the wood structure, as illustrated in Fig. 2, during latex pressurization. Dry rubber would then remain inside the wood structure after the pressurized samples were dried in an oven.

Fig. 2. Schematic illustration of the pathways for NR latex flow in rubber wood

Consistent with this hypothesis, the images obtained by a scanning electron microscope reveal the microstructure of dry rubber wood (Fig. 3). Figure 3a shows the structure of fibers and vascular tissues with diameters in the approximate range of 20 to 120 m. Vessel pits, mainly oval in shape with the length of the minor axis about 4 to 8 m, are seen in Fig. 3b. However, the narrowest pathway found in this system was the fiber pit, which is also oval in shape with the length of the minor axis about 2 to 4 m (Fig. 3c). In separate experiments, NR latex was examined by a laser particle size analyzer, which found particle sizes in the range of 0.04 to 2.92 m, with the average value of 0.80 ± 0.42m. The results from SEM and LPSA, therefore, suggested the possibility that NR particles should be able to penetrate the structure of rubber wood upon pressurization.

Fig. 3. SEM images of wood structure: (a) vascular tissues and fibers, (b) vessel pits, and (c) fiber pits

To test this hypothesis, wood samples were pressurized in a chamber filled with different dry rubber contents of NR latex. The weight increase after pressurization was monitored; this reflects the amounts of NR latex retained in treated samples. The result of a weight increase on latex pressurized samples (stated as embedded NR latex) is shown in Fig. 4a, suggesting that NR latex was able to penetrate into the rubber wood during pressurization. In addition, the amount of embedded latex approached the upper limit, approximately 90%, when pressurized with NR latex at 12% DRC. This limit was indicated by a comparison of the pressurized water control samples. The amounts of embedded latex, however, declined from about 90% to 50% when the DRC of latex was gradually raised from 12% to 60%. The decrease in the latex-embedded levels observed here is accounted for by an increase in the viscosity of the latex (Blackley 1997). Natural rubber latex with a lower viscosity (and values of DRC) penetrates into the wood structure more easily than NR latex at a higher viscosity.

The same set of pressurized samples was then dried in an oven at 103 ± 2 °C until a constant weight was reached, normally within 72 h. This step was performed to remove the aqueous portion of the NR latex embedded inside the pressurized wood samples. As a result, dry NR particles were retained within the samples and were monitored by the weight gain. As expected, all latex-pressurized samples displayed a weight increase after drying, while that pressurized by water did not (Fig. 4b). The additional weight found here, therefore, inferred that the fixed dry NR was inside the samples and in the range of about 4% to 8% when the DRC of latex was varied from 12% to 60%. The amount of dry NR that remained in the wood samples treated by latex with different DRC values, however, was smaller than what it should have been. For example, wood samples treated by the latex with 12% DRC displayed a value of embedded latex of about 85%, while the ideal retention of dry NR should be about 10%, rather than the 4% shown in Fig. 4b. This may be due to the coagulation of rubber particles in NR latex during pressurization into rubber wood. Basically, each pair of adjacent NR particles in latex was stabilized by the attractive and repulsive forces of the surrounding ionic molecules (Blackley 1997). During pressurization, rubber particles were forced to move through the narrow pathways, e.g., vessel pits and fiber in the wood structure. This brings about the destabilization of these forces from operating between each pair of adjacent particles. Consequently, coagulation of NR particles took place and occluded the pathways inside the wood. Thus, only the aqueous phase of latex was allowed to move through the wood structure.

Fig. 4. Impregnation levels of treated wood samples as a function of DRC of NR latex: (a) embedded latex and (b) dry NR inclusion. The numbers the in parentheses of x axis labels refer to the latex viscosity (in centipoises) at each value of DRC.

To ensure that NR impregnated the oven-dry pressurized rubber wood, representatives of treated samples (~8% of rubber inclusion) were then compared with controls using SEM. The SEM images revealed the substrate layers and accounted for dry NR deposited on the walls of the vascular tissue of the treated samples, while the untreated controls were normal with clearly visible pits (Fig. 5).