Development of Multi-Layered Kenaf (Hibiscus Cannabinus L.) Board Using Core and Bast Fibres

Kenaf (Hibiscus cannabinus L.) is relatively a new crop in Malaysia. This fast growing species has been choosen and introduced in Malaysia to ensure a continuous supply of raw material for the composite industry, and as alternative solution for the shortage in rubberwood supply. An attempt was made to develop a multi-layered kenaf board (MLKB) utilising different parts of kenaf stem: bast, core and a combination of bast and core fibres. The objectives of this study were: 1) to evaluate methods of retting and separating of kenaf fibres (bast and core fibres) for kenaf board production, 2) to determine the effects of resin content and bast to core proportion on the physical and mechanical properties of MLKB, and 3) to determine the properties of kenaf board with woven-bast mat as core. The kenaf bast and core were separated manually. The retting process (to separate bast fibres from the pectic materials) was carried out by submerging the bast in either cold water or alkali (NaOH) at certain duration. Three levels of alkali concentration were used: 1, 3, and 5%. The crude bast fibres were than combed and washed several times until straight and silky fibres were produced. The core portion was chipped into ≤ 2 cm size particles. Both bast fibres and core particles were then dried to about 5% moisture content. Multi-layered kenaf boards were fabricated using urea formaldehyde (UF) and melamine urea formaldehyde (MUF) resins as binders. Four types of 0.50 g/cm3 density MLKB were made with varying bast : core proportions. Homogenous particleboards utilising 100% rubberwood particles were used as control. Since bast fibres have low wettability, a low molecular weight phenol formaldehyde (LPF) resin was used to pretreat the bast fibres prior to normal blending with either UF or MUF resin. An attempt was also made to produce a kenaf board with woven bast fibre mat as core. The properties of boards were tested using MS standards 1737: 2005. Data were subjected to Analysis of Variance (ANOVA) and the effects were further analysed by means separation using Least Significant Difference (LSD) at p ≤ 0.05. The study indicates that treatment of kenaf bast with different alkali concentrations significantly affected the properties of kenaf bast fibres such as fibre and lumen diameter, cell wall thickness and chemical components. Kenaf bast fibres that have been treated with 5% NaOH gave the lowest amount of holocellulose, hemicellulose, α-cellulose, and lignin (48.7%, 29.7%, 19.0%, and 8.5% respectively.) High yield of holocellulose was obtained for treatment with water alone. Both the kenaf core and rubberwood have similar buffering capacity which is more sensitive towards acid. Bast fibre, on the other hand is more sensitive towards alkali. Due to its morphological properties, kenaf core inner surface exhibited higher wettability than outer surface. Kenaf board comprising bast materials in the middle layer were stiffer than that of homogeneous 100% rubberwood. The incorporation of LPF resin in the fibres of MUF-bonded board comprising 70% kenaf core on the surface and 30% bast in the middle layer produced boards of reasonably good strength and dimensional stability. The modulus of elasticity (MOE) was 873 MPa, modulus of rupture (MOR) 8.9 MPa, internal bonding (IB) 0.32 MPa, thickness swelling (TS) 12.6%, and water absorption (WA) 118.9%. The presence of bast long fibres had improved the linear expansion (LE) length-wise by about 16.2%. All the kenaf board have higher MOR than that of 100% rubberwood. The woven technique applied to improve the performance of the MLKB was found to be effective, producing 8.2% stronger and 22.3% stiffer board. The IB was also improved by 61.9%. The dimensional stability of these boards was superior to that of MLKB. Boards having 100% kenaf core consistently gave superior performance in mechanical strength but relatively poor in TS and WA due to the high porosity and absorbent of the core itself. The linear regression between IB and strength showed higher R2 values were obtained for all boards containing bast fibres compared to those having 100% core particles. The lack of fibre bonding among the bast fibres was found to be the dominant factor affecting the performance of woven-layered kenaf board (WLKB)

Bonding properties and performance of multi-layered kenaf board

Kenaf (Hibiscus cannabinus) has recently been introduced to the Malaysian bio-composite industry. Based on their basic properties, both the bast fibres and core material of kenaf are distinctly different. While bast fibres are stiffer and low in wettability, the core material of kenaf is weaker and has excellent absorbing properties. This study evaluated the properties of kenaf board made from a combination of bast fibres and core material. The bast fibres were separated first from the core, followed by pre-treatment with NaOH, then combing until the fibres became loose. The properties of kenaf board were tested using MS standards 1787: 2005. An analysis of variance was carried out to study the effects of resin types and bast to core proportion on the boards. The buffering capacity study revealed that kenaf bast, kenaf core and rubberwood behaved similarly in alkali but differently in an acidic condition. Both the kenaf bast and core were relatively less stable in acid compared with rubberwood. Due to its morphological characteristics, the kenaf core inner surface exhibited higher wettability than the outer surface. There was significant interaction between resin type and the proportion of bast:core at p < 0.01. Generally, boards made from 100% kenaf core and bonded with urea formaldehyde (UF) resin had superior performance. The mechanical properties [modulus of elasticity (MOE), modulus of rupture (MOR), internal bond (IB)] of the boards were significantly influenced by the amount of bast fibre in the board––the higher the amount, the poorer the strengths. This effect, however, was reversed for thickness swelling (TS). Only UF-bonded kenaf-based boards had comparable water absorption (WA) property to that of the control (100% rubberwood). The incorporation of low molecular weight phenol formaldehyde (LPF) resin in the fibres had mixed effects on board properties. The effects varied based on the resin used; it improved the MOE and MOR of the board but not the IB, TS and WA when used with UF resin. It improved the IB only when used with melamine urea formaldehyde (MUF) resin. The best performance was given by boards made from 100% kenaf core irrespective of the type of resin used. All kenaf boards in this study had higher MOR than that of 100% rubberwood. Insufficient curing of LPF resin was identified as the main factor for the poor performance of LPF-bonded boards

Adhesion characteristics of phenol formaldehyde pre-preg oil palm stem veneers

The purpose of this study was to evaluate the adhesion properties of phenol formaldehyde-prepreg oil palm veneers that have potential for plywood manufacture. Phenol formaldehyde (PF) resin of three different molecular weights (i.e. 600 (low), 2,000 (medium), and 5,000 (commercial)) were used to pre-treat the veneers. The veneers were soaked in each type of PF resin for 20 seconds, pressed between two rollers, and pre-cured in an oven maintained at 103 ± 2 °C for 24 hours. The volume percent gain (VPG), weight percent gain (WPG), pH, buffering capacity, and contact angle of the phenolic pre-preg veneers were determined. The bonding shear was also evaluated according to British Standard European Norm BS EN 314. The results show that veneers from both inner and outer layers treated with low molecular weight PF (LMwPF) resin had significantly higher VPG and WPG compared to the other PF resins. The pH values of all of the veneers were slightly acidic (6.5 to 6.8) except for those that were treated with commercial molecular weight PF resin (7.8). A buffering capacity study revealed that untreated veneer had a greater resistance toward alkali, but was unstable under acidic conditions, while the phenolic pre-preg veneer behaved differently. This effect was more prominent as the molecular weight of the PF resin increased. An examination of the veneer surfaces demonstrated that phenolic treatment had increased the contact angle of the OPS veneer surfaces significantly. The bonding properties of plywood made from pre-preg palm veneers were found to be superior to those of commercial palm plywood

Influence of resin molecular weight on curing and thermal degradation of plywood made from phenolic prepreg palm veneers

The present work evaluates curing and the thermal behavior of different molecular weight phenol formaldehyde (PF) resins used to prepare PF prepreg oil palm stem veneers. The physical properties (solid contents, gelation time, pH, and viscosity) of PF resins were determined. The molecular weight of resins was characterized by gel permeation chromatography, whilst thermal properties were determined by differential scanning calorimetry and thermogravimetric analyses. The average molecular weight of PF resins were 526 g/mole (low), 1889 g/mole (medium), and 5178 g/mole (control - commercial). Among the resins, medium (MMwPF) gives better thermal stability with a retained weight of 48.9% at 300°C. High (Commercial PF) had a low decomposition temperature (109.3°C) which occurred within 11 min. Both low (LMwPF) and MMwPF started to melt at ≥120°C. Based on strength and shear values, phenolic prepreg palm veneers can be prepared using either low or medium molecular weight PF but with varying results. In all cases, the mechanical properties of palm plywood made from PF prepreg veneers were superior to those made from PF-bonded plywood using the commercial process