Production of an environmentally friendly, pressed composite material from straw using biological binders developed from modified residual beer yeast and spent microbial culture media containing the polysaccharide levan

This study shows the possibility of producing compressed biocomposite materials made from pressed straw without the use of phenol-formaldehyde resins. We used a biological binder consisting of modified residual beer yeast and spent culture media containing a microbial levan polysaccharide, which was obtained from an Azotobacter vinelandii culture. The physical and mechanical properties of the pressed straw depended on the type of biobinder used as well as on the conditions surrounding the preparation of the straw and the biocomposite material pressing. Removal of the fat-wax shell from the straw improved the characteristics of the material. Pressed materials obtained using the spent culture media as a biological binder were more water-resistant than materials obtained using modified residual beer yeast. Swelling, as measured by thickness, was 38% and 62% using the former and latter biobinders, respectively. The ultimate bending strengths of the pressed materials prepared using the modified residual beer yeast and spent culture media were 13.7 and 13.9 MPa, respectively. The addition of paraffin to the biological binder mixtures prior to pressing improved the water resistance of the pressed materials but reduced the ultimate bending strength. The addition of glutaraldehyde to the modified residual beer yeast improved the water resistance of the resulting biocomposite materials and did not affect its ultimate bending strength. All samples did not emit methanol or phenol, but there was emission of ammonia and formaldehyde at concentrations that did not exceed the threshold limit value. Obtained biocomposites could be used as the materials with heat-insulating properties.