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,
Manufacture of thin rice straw particleboards bonded with various polymeric methane diphenyl diisocyanate/ urea formaldehyde resin mixtures
Reducing particleboard thickness is one of the major approaches to decrease consumption volume of particleboard for furniture manufacture. This study employed an adhesive mixture of polymeric methane diphenyl diisocyanate (PMDI) and urea formaldehyde (UF) to produce single-layer medium density thin rice straw particleboard. The effects of various PMDI/UF formulations as well as board density on mechanical properties and water resistance of rice straw particleboard were studied. The results indicated that the mechanical properties and water resistance of the thin rice straw particleboard were appreciably affected by resin formulation. The panels bonded with PMDI/UF adhesive mixtures had mechanical properties and water resistance far superior to those bonded with UF. Higher PMDI content levels in resin mixtures led to improved mechanical properties and water resistance. Density influenced mechanical properties and water resistance of the thin rice straw particleboard. Increasing the density of the panel could upgrade the mechanical properties of the thin rice straw particleboard. The experimental outcomes showed that PMDI/UF resin systems had potential to substitute for pure PMDI resin in producing thin rice straw particleboard, which could effectively lower manufacturing cost and bring economic efficiencies due to reduced amount of pricey PMDI.
Measuring Sheet Resistances of Dielectrics Using Co‐Planar Hydrogel Electrochemical Cells with Practical Applications to Characterize the Protective Quality of Paints on Sculptures
Electrical properties of thin dielectric films at the solid-liquid phase boundary are an important performance characteristic of many devices, coatings and sensors. In this paper, co-polymeric hydrogels of polyacrylic acid co-sulfonic acid, swollen with a salt solution to act as the solid electrolyte, were used to assess interfaces using electrochemical impedance spectroscopy (EIS) in a co-planar geometry. Silane-modified glasses were characterized by the co-planar hydrogel EIS cell and found to be distinguishable based on their surface monolayer chemistry. EIS measurements were also made on primed and painted metal substrates, using both test panels and an outdoor sculpture, Tony Smith's Stinger. The panels were then exposed to accelerated and outdoor weathering and showed degradation on the surface paint layers, which was observable electrochemically using EIS and confirmed visually by SEM. Electrochemical spectral features were compared between data from a standard paint-test cell versus this co-planar hydrogel cell; both cell types were able to measure coating capacitance, providing useful information about the condition of the bulk coating. Yet, sheet resistance (Rs) was a spectral feature seen only by the co-planar hydrogel cell. Thus, it can be concluded that the use of such co-planar hydrogel cells can provide an earlier warning sign to coating degradation and such cells provide a new type of spectral information that is not assessable by the standard geometry.