Straw Pressboard Composites

Abstract

To fully explore the potential of wheat straw and expand its use in composite applications, this thesis aimed to develop a feasible and effective manufacturing method for wheat straw based polymer pressboard composites.

The effects of hot-water and alkaline treatments of wheat straw raw material were evaluated in terms of wettability and binder absorption, surface morphology and thermal stability. Alkaline treatment was proved to significantly increase the wettability and binder absorption of hydrophilic binder system.

The curing kinetics of thermosetting polyester binders (Acrodur) were studied isothermally and non-isothermally using two complementary thermal analysis techniques (DSC and TGA). Onset and peak temperatures, reaction enthalpy, and activation energy were measured and evaluated to determine suitable conditions for curing Acrodur. This was an extensive and systematic study not previously available in the literature. The threshold temperatures for curing Acrodur were determined to be above 180°C for hermetic system for 10 minutes, and above 150°C for non-hermetic system. It was also concluded that the pressure influenced the onset of cure reaction. As a result of this study, a set of conditions of temperature, time and ventilation can be selected for compression molding.

The adhesion strength of Acrodur on wheat straw stem were measured and evaluated according to ASTM D3163 (for plastic) and ASTM D5868 (for fiber reinforced plastic). It was concluded that Acrodur at low concentrations was capable to provide sufficient adhesion in wheat straw based composites and straw was the weak point.

The wheat straw based composite pressboards were compression molded with different binder systems and processing conditions. The binder systems evaluated were polyamide 6, thermosetting Acrodur, CNF reinforced thermosetting Acrodur, thermoplastic Acrodur, The processing conditions evaluated were: straw loading level, compression molding temperature, pressure and time. The flexural strength and modulus of resultant pressboards were measured and evaluated according to ASTM D790; the dielectric breakdown voltage was also measured according to ASTM D149-09, method A.

The effect of wheat straw size, treatment, binder type, compression molding temperature and pressures on the mechanical properties was investigated. Wheat straw particle generally showed higher reinforcing effect than longer wheat straw fiber. The sodium hydroxide treated straw improved mechanical properties, which was attributed to increased compatibility and binder absorption. Binder loading level of 40% exhibited better mechanical properties than 20% and 60%. Using polyamide 6 could result in thermal degradation on wheat straw. Thermosetting Acrodur significantly improved the mechanical properties. Incorporation of CNF led to improvement of mechanical properties of pressboards on untreated wheat straw. The thermoplastic Acrodur achieved the remarkable improvement on mechanical properties with both hot-water and alkaline treatments and both sizes of straw at low binder loading of 20%.

The flexural strength and modulus of pressboard prepared here were at high end in comparison with the results in the literature. The pressboards manufactured in this research met and exceeded the requirements for MDF (medium density fiberboard) and HDF (high density fiberboard) and car interior parts in automotive industry.