Qinglin Wu
The United States produces about 10 million tons of rice annually, with about 1.4 million tons in Louisiana. With an approximately equal rice-to-straw weight ratio, an equivalent amount of dry rice straw is produced per year. Rice straw consists of more than 60 percent lignocellulosic fibers. In rice farming practice, most straw is left in the field to decompose after rice harvesting. Thus, the use of the straw fiber resources for value-added applications is so far limited. Transforming the straw fiber resources into high quality panel products provides a prospective solution to the straw disposal problem. Straw-based composites offer potential as materials for sub-floors, cabinets, shelving and building products. Technical information on strength and dimensional stability of the product is critical for such an application.
LSU AgCenter researchers conducted studies to develop technical data for manufacturing straw-based particleboard. The objectives were 1) to evaluate straw strength properties in comparison with wood, and 2) to investigate effects of panel density, resin content and wax level on dimensional stability and mechanical properties of the straw particleboard.
Straw Collection, Preparation
Green straw (with the top portion removed during rice harvesting) was hand-harvested from an experimental rice field at the Rice Research Station at Crowley. The collected straws were 51 to 66 centimeters long, and 2.2 to 4.8 millimeters in diameter with a wall thickness of 0.23 to 0.68 millimeters. Moisture content of the straw was approximately 200 percent. The straw was transported to the Engineering Composite Laboratory at the Louisiana Forest Products Development Center at the LSU AgCenter in Baton Rouge in large plastic bags.
Testing Straw Property More than 200 long straw stems were selected from the collected straws for preparing samples for tensile strength testing. They were first dried to about 30 percent moisture content. The straw samples (without containing node portion) were taken from three locations along each straw stem (bottom, middle, and top). The samples were cleaned with water at room temperature and subsequently conditioned for two weeks to a moisture content of about 7 percent. Tensile specimens were prepared by notching the middle portion of each sample to ensure the breakage from there. Tensile tests were conducted using an INSTRON universal testing machine at room temperature. Similar wood samples from southern pine lumber were also tested for comparison.
Producing Straw Panel The rest of the straw stock was dried and hammermilled to pass an 8- millimeter screen. The processed straw particles were re-dried to 2 percent to 3 percent moisture content before panel manufacture. During panel manufacturing, dry straw particles were blended with various levels of polymeric Methylene Diphenyl diisocyanate (pMDI) resin and wax emulsion with a laboratory blending system. The mats were then manually formed and hotpressed into solid panels with target density levels. All boards were conditioned at 68 degrees F and 65 percent relative humidity for two weeks before cutting test samples. Tests including basic mechanical and physical properties were conducted according to the American Society for Testing and Materials Standard. Test results were analyzed and compared with the corresponding values of wood-based particleboards specified by the American National Standard Institute (ANSI 208.1).
Straw Properties Rice straw has a tensile strength four times higher than southern pine wood. Thus, the high tensile strength properties of the straw can be used to improve strength properties of particleboard. Sampling locations from a straw stem had significant influences on the tensile properties of rice straw. Generally, middle nodes of rice straw had the highest average tensile strength, which was followed by the bottom and top parts.
Straw Panel Properties Panel density and resin level played a significant role in controlling mechanical performances of straw particleboard. For panels with densities higher than 0.70 g/cm3, bending modulus of elasticity, modulus of rupture and internal bond strength met the minimum values specified in the ANSI 208.1 standard for the corresponding wood particleboard. Thus, these products can compete directly with wood particleboard in terms of their strength properties in the market place. In general, bending strength of the straw particleboard was higher than its corresponding tensile strength. This was due to the density profile across sample thickness formed during hot-pressing of the panel, which helps increase the bending properties. There was no significant influence of wax on mechanical properties of the particleboard.
Dimensional stability of straw particleboard was also strongly affected by density and resin content. The dimensional instability of the particleboard under water was significantly improved by wax sizing. High-density boards had relatively low short-term (24 to 48 hours) linear expansion and thickness swelling values, but these boards had higher deformation potentials. By increasing resin and wax content (wax sizing), both linear expansion and thickness swell were reduced. In general, linear expansion met the specifications for the corresponding wood-based particleboards in ANSI 208.1. Thickness swelling was also in the range of values specified in the standard.
This study shows that it is technically possible to make rice straw particleboard with pMDI resin as a bonding agent and wax as a dimensional stabilizer. The particleboard developed had mechanical properties that well exceeded the standard requirement for wood particleboards. Panel dimensional stability properties were also in the range of the values for wood particleboards. The study demonstrated an effective way of transforming rice straw into high quality industrial panel products, providing a prospective solution for value-added straw use. Further development of the technology includes bonding the straw with formaldehyde- based resins at reduced cost.
Successful commercialization of straw-based panel products depends on development of a cost-effective manufacturing process on a commercial scale and establishment of a market base for the products. With increasing wood fiber costs and environmental pressure for using agricultural residues, the industry is developing manufacturing facilities for using valuable straw fibers. Current technology includes straw particleboard, whole-straw-based build-ing blocks, and extruded plastic composites reinforced with straw fibers.
Qinglin Wu, Professor, School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, La.
(This article appeared in the summer 2005 issue of Louisiana Agriculture.)
