Physical and mechanical properties of particleboard manufactured from kenaf (Hibiscus cannabinus L.) and rubberwood (Hevea brasiliensis Mull)

This study evaluates the technical feasibility of producing particleboard from kenaf (Hibiscus cannabinus L.) stem. The work comprises evaluation of basic properties of kenaf namely moisture content, specific gravity, and adhesion characteristics through contact angle of wettability study and buffering capacity determination. The evaluation of basic properties, namely moisture content and specific gravity, was done on three sections of stem height (bottom, middle, and top) and also two parts (core and whole stem) of the kenaf stem. Findings show that stem height had no significant differences but the sections do have significant differences in terms of moisture content and specific gravity of kenaf stem, where the kenaf whole stem produced a specific gravity of 22% higher than that of the core. Interestingly, the specific gravity of both kenaf whole stem and kenaf core was found to increase from the bottom to top parts of the stem height; which is opposite of that found in woody tree. This was contributed by the smaller vessel diameter at the top. Meanwhile, kenaf’s inner core had high wettability in both acidic and alkaline condition, and low initial contact angle of 41.2˚ compared to kenaf’s outer bast, with initial contact angle values of 65.6˚. In buffering capacity study, kenaf bast and kenaf core have the highest and lowest buffering capacity in acidic conditions, respectively. Buffering capacity measures the resistance of the wood to change in pH level. A wood with high buffering capacity make it more difficult to cure UF resin, thus requires additional amount of acid catalyst to reduce the pH level to the level required for optimum resin cure. Three types of 100% homogeneous kenaf boards were fabricated from kenaf whole stem (KWS), kenaf core (KC), and kenaf bast (KB), and their mechanical and physical properties were evaluated. For comparison purposes, 100% homogenous rubberwood (RW) particleboard was fabricated and used as a control. Meanwhile, urea formaldehyde resin was used as the binder. Apart from control panel, panel made from 100% kenaf whole stem was found to exhibit the highest results either in MOR, MOE, IB, TS, and WA, with the values of 15.1 N/mm2, 1559 N/mm2, 0.51 N/mm2, 28%, and 77%, respectively. Kenaf whole stem was observed to be the best form of kenaf raw material for particleboard manufacture. Kenaf core can also be used as a raw material in particleboard, but it requires the use of more resin due to high absorbent property. Conversely, kenaf bast is not suitable to be used alone due to the lack of adhesive penetration, resulting in low strength and dimensional stability. After screening, the particle size distribution of KWS, KC, KB and RW were 74%, 62%, 68% and 59%, respectively. Based on the findings, 75% of kenaf core particles were of rectangular or nearly rectangular in shape, in which the study identified the kenaf core (ten classes), kenaf bast (seven classes),and rubberwood (seven classes) of particles geometries. Analysis of the aspect ratio showed that kenaf bast is much more slender as compared to the kenaf core and rubberwood. Particleboards made from combination of 30% KWS and 70% RW (70RW-30KWS) resulted in superior performance in terms of MOR (17 N/mm2), MOE (1756 N/mm2), IB (0.90 N/mm2). The dimensional stability of such panels also increased about 28% and 77% in terms of TS and WA, respectively. In this study, the panels consisting of slender, rectangular or nearly rectangular shape of rubberwood particles were shown to increase the strength, stiffness, and stability properties. Under the SEM observation, the panel made from 70RW-30KWS gave a better compaction and the lowest void compared to others, suggesting the compatibility among the RW, KC, and KB particles. Three-layer particleboards were manufactured to improve the elasticity properties of kenaf particleboard. Six types of three-layer particleboard consist of KWS or KC in the middle layer, and RW particles on the two-face layers. The MOE values of the three-layer (35RW-30KWS-35RW) panel was 56% and 79% higher than those of the panels comprising single-layers of 100% KWS and 100% KC, respectively. The findings show that the panels produced with 70% shelling ratio have higher MOE, MOR and IB, but lower TS and WA than those of the panels with 30% shelling ratio. After taking all into consideration, kenaf whole stem is the preferred material to be used in particleboard manufacture incorporated with rubberwood as admixture or three-layer panels. Kenaf core may be good for producing lightweight panel, while kenaf bast was found to be unsuitable for particleboard manufacture.

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