The influence of wax-sizing on dimension stability and mechanical properties of bagasse particleboard
Introduction
Sugar cane is an important agricultural crop in the southern U.S. (Rowell, 1995, FAO, 2006). The cane stalk consists of an inner pith that contains most of the sucrose, and an outer rind with lignocellulosic fibers. Cane processing crushes the entire stalk to extract the sucrose, from which refined sugar is produced. Large quantities of the bagasse, containing both crushed rind and pith fibers, remain after sugar extraction. Disposal of this byproduct from the sugar industry is so far still inefficient (Rowell and Keany, 1991, Han and Wu, 2004). For instance, approximately 85% of the bagasse produced in Louisiana is currently used in-house as fuel in mill processes and for other low value applications such as mulch and inexpensive ceiling tiles. The remaining 15% is waste that is allowed to decay or is landfilled (Paturau, 1989, Han and Wu, 2004). Transforming bagasse into high quality industrial panel products, such as bagasse particleboards (BPBs) or bagasse–polymer composites (Talavera et al., 2007), provides a prospective solution for more effective bagasse utilization.
One of the potential markets for bagasse-based composites is core material for laminated floors, where high-density wood fiberboard is currently being used. Dimensional stability is one of the key performance characteristics that must be satisfied, which may otherwise induce internal stresses and out-of-plane distortion to floor. Unfortunately, dimensional stability is one of the most difficult problems to overcome for agrifiber composites. With relatively low bulk density, higher compression ratios are usually used to make boards at similar density levels as wood-based composite boards. An early research (Wu, 2001) demonstrated that particleboards derived from bagasse with 5% polymeric methylene diphenyl diisocyanate (pMDI) resin had evident thickness swelling (TS) and linear expansion (LE), which failed to meet the minimum TS (8%) and LE (0.35%) specifications in the ANSI A 208.1 standard (ANSI, 1999). A higher level of pMDI (e.g., 8%) can be effective in controlling the stability properties, but the manufacturing cost correspondingly increases. It was suggested that methods other than increasing resin content levels were necessary to be considered in reducing the stability problems.
Acetylation, steaming, and wax-sizing are three basic methods to improve board dimensional stability for wood-based composites. Acetylation generally shows negative effects on mechanical strength of boards, although it can help improve dimensional stability. For example, Bueso et al. (2000) showed that acetylation could improve both mechanical strength and dimensional stability. Softwood (spruce and pine), mixed waste wood, beech, wheat straw, and recycled paper were pulped into fibers to make dry-process medium density fiberboard (MDF). The fibers were acetylated before mat-forming. Results showed significant decreases of TS, water absorption (WA), and LE. Simultaneously, the mechanical properties of modified boards were statistically better than the control group with wheat straw boards as an exception. For steam treatment of particles or fibers, Chow et al. (1996) showed thathemlock fibers were pretreated at a 1.55 MPa steam pressure for 10 min, and were subsequently made into hardboards. Both optimized dimensional stability and better mechanical properties were achieved. Wax-sizing has long been used for dimensional stabilization of wood and ag-fiber composites (Nada and Hassan, 1999, Cai et al., 2004, Lin et al., 2008). As a hydrophobic substance, it can be effective in water and humidity repellency (Heebink, 1967). Two types of wax are utilized in industrial applications, i.e., molten- and emulsified-wax. Molten-wax is comparatively pure but is difficult to spread evenly on particles or fibers. Emulsified-wax, however, brings extra water into wood particles and may exert potential burdens to the hot-pressing process. Suzuki et al. (1976) investigated the water absorption of dry-process fiberboards, and found lower WAs and TSs with increased content of paraffin wax and board density. However, the effectiveness of wax-sizing was impaired with long-term water immersion.
Although acetylation and steaming are helpful to modify the dimensional stability of boards, their industrial application is often cost-prohibitive. Comparably, wax-sizing is more promising to be widely used in the agro-fiber composites industry. The objective of this research was to investigate effects of wax on dimensional stability and mechanical properties of bagasse particleboard in combination with MDI resin.
Section snippets
Raw material preparation
Bagasse was collected from former Acadia Board Corporation in New Iberia, Louisiana. The fibers had been open-field stored and naturally dried for several months prior to collection. Bulk bagasse collected was then milled into small particles with a PHM3 Pullman hammermill. The particles were manually screened to separate fines, and then kiln-dried to about 3% moisture content (MC) before use.
Polymeric methylene diphenyl diisocyanate, ISOBIND-1088 from DOW CHEMICAL Company, was used in the
Mechanical properties
In general, the introduction of wax did not bring evident negative effects to bagasse particleboard on bending strength, MOR (Table 1). On the other hand, certain positive effects on MOR were observed, especially for the 3% resin content system. For example, a 60% increase (from 16.0 to 25.5 MPa) on MOR was obtained after introducing 1% wax to the system. However, further regression analysis showed that increasing wax content did not change MOR linearly for both 3 and 5% resin content systems (P =
Conclusions
Dimensional stability is one of the key factors that determine in-service performance bagasse particleboard. A factorial experiment was conducted to measure the effects of wax content and pMDI resin content on particleboard dimension stability and mechanical properties.
Wax-sizing had no evident negative effects on the bending properties of MDI-bonded bagasse particleboards under both pMDI resin contents. It brought slightly negative effects on IB, but the negative effects could be controlled if
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