Modified formaldehyde-based resin adhesives for rice hull-wood particleboard.
A study was conducted to develop an effective and economical resin system to improve the physical and mechanical properties of rice hull-wood composites. Boards 0.5 inch (12.5 mm) thick were made from a mixture of rice hulls and pine particles with three main resin types: urea formaldehyde (UF), phenol formaldehyde (PF), and polyisocyanate (ISO), in various adhesive formulations. Results indicated that conventional UF and PF resins did not perform well with rice hull-wood composites. However, a modified resin system with IS0 as minor component significantly improved the boards’ strength properties and dimensional stability. Resin content had a significant effect on the quality of the boards, with the higher resin content resulting in stronger boards. The boards bonded with the l%ISO / 6%UF resin system had higher internal bond strength, and attained the best dimensional stability and the highest bending strength (MOR) and stiffness (MOE). As global population increases and developing countries increase their use of wood and paper products, there is new interest in producing and conducting research on composite board from agricultural fibres. Rice hull is quite fibrous by nature and requires little energy input to prepare, so its suitability for the manufacture of particleboards has been assessed in a number of studies (Vasishth 197 1; Hancock and Chandramouli 1974; Mahanta et al. 1980; Viswanathan et al. 1987). However, particleboards made from rice hulls have not found commercial acceptance because substantially more adhesive is needed for rice hulls than for wood flakes to yield boards with acceptable properties (Vasishth 197 1; Chen 1979). The reason for the higher resin requirement for bonding rice hulls is not completely understood, ‘Southern Research Station, USDA Forest Service, Pineville, Louisiana, 7 1360, USA. School of Forestry, Wildlife, and Fisheries, Louisiana State University Agricuhural Center, Baton Rouge, Louisiana, 70803, USA. Email: chse@fs.fed.us but a comparison between wood and rice hull showed that rice hull has less holocellulose and a much higher ash content (Houston 1972). The predominant component of its ash is silica (Luh 1991). The silica covers almost the entire outer layer of the rice hull surface which also contains the water repellent cuticle (Juliano 1985). This silica layer and the partially hydrophobic surface of rice hull are incompatible with aqueous urea formaldehyde (UF) or phenol formaldehyde (PF) resins and prevent the formation of a good bond between rice hull surfaces. Thus, a new and improved resin adhesive system is needed to produce high quality rice hull particleboards. Urea formaldehyde resins are often fortified with melamine to increase the bond strength and water resistance of particleboard. More recently, highly reactive polyisocyanate (ISO) has been used to modify UF resins (Deppe and Ernst 1971; Deppe 1977; Pizzi 1981; Liu and Binglye 1992) and PF resins (Hse 1978, 1980) for board products. IS0 modified adhesives improve the bond strength and performance of wood particleboards, so they might have similar beneficial effects on the bonding of rice hulls. Therefore,
Bonding of Agrofibers-based t ] omposition Panels
Urea formaldehyde resins were formulated with vatiables of three reaction pHs (1.0, 4.8, and 8.0 5.0) and three catalysts (sodium hydroxide, sodium hydroxide/ammonium mix, and triethanolamine) . The resins were prepared by placing all formaldehyde and water in reaction kettle and pH was adjusted with sulfnric acid and sodium hydroxide, respectively. Urea was added in 1.5 equal parts at 1 minute intervals. The proportion of high molecular weight products in the rosin increased substantially as the reaction pH decreased. The formation of a high percentage of uron derivatives under strong acidic conditions indicated these resins differed considerable from conventional UF resins formulated in the past. Bagasse panels bonded with UF resins formulated with acidic pH resulted in higher IF3 than with those formulated with the conventional alkaline acidic pH system. Similar results were also shown with PMDIAJF bonded Bagasse panels. The apparently higher IB strength of PMDI/UF resin systems indicate that these resin systems would be expected to provide the greatest potential for further improvement in terms of IB/cost basis. It is noted that mixed catalyst systerfLof sodium hydroxide/ammonium and triethanohunine yielded a higher average IB, while sodium hydroxide catalyst resulted in a lower IB than commercial UF resin used as control. Since the cost of UF is substantially lower than that of PMDI, economic gains of PMDI/UF resin could be significant by using UF resin as the major component in the system.
Stability of planar PEMFC in Printed Circuit Board technology
Abstract The use of planar PEMFCs in printed circuit board (PCB) technology with a thickness of less than 3.5 mm are presented. This planar design consists of an open cathode side which allows a completely passive, self-breathing operation of the fuel cell. Power densities of 100 mW/cm 2 at 500 mV with hydrogen as a fuel were achieved. A steady operation with this type of fuel cell was demonstrated over a week without any cell flooding. Since the electrical conducting elements of PCBs usually are made of copper, corrosion in the wet environment of a PEMFC is expected. Thus, a significant degradation in performance of fuel cells made of plain copper PCBs was seen in long-term operation. In order to avoid corrosion, the copper layer has to be coated. Fuel cells in PCB design with diverse coatings were tested in long-term operations up to 1000 h under load. Moreover, corrosion currents have been determined by the use of potentiodynamic scans. Promising coatings are electroplated Cr and Ni and a combination of Ni/Au.