|The use of lignocellulosic materials as reinforcement in polymer adhesives has increased considerably over the past few years. Lignocellulosics, also known as natural fibers are more attractive to use compared with synthetic reinforcements because they are biodegradable, less costly and have acceptable specific strength. Phenol formaldehyde (PF) adhesives are the most widely utilized binders used in the wood composites industry. Although PF is more moisture resistant and gives acceptable strength properties, it is also known for its significant brittleness. This could be overcome by reinforcement.
The first part of this study aimed at reinforcing Phenol formaldehyde adhesive with microcrystalline cellulose (MCC) at different loading rates (0 – 10% wt). The viscosity of the adhesive system after reinforcement was evaluated. To determine the cure properties and thermal stability of the adhesive/cellulose composite, thermal analysis using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) was conducted. The strength of the cellulose reinforced adhesive system was evaluated using lap shear tests. Fourier transform infrared (FTIR) spectroscopy was used to investigate possible interactions between cellulose and the adhesive matrix.
Results from this portion of the study showed that cellulose reinforcement lowers the cure temperature as well as the thermal stability of the neat PF adhesive. Lap shear tests revealed an increase in shear strength with the addition of cellulose. The necessary cure temperatures and optimal MCC loadings from the first study were then utilized in the design of the second study.
In the second study, two particle sizes, two species, and two MCC loadings were utilized to determine the effects of these factors on the mechanical and physical properties of particleboard. A 15% PF loading was used to accommodate the high surface area of the 10% MCC loading. Sweetgum and southern yellow pine were the two species used and a 0.15 cm mesh sieve was used to separate the large particles from the small particles. The mechanical (Modulus of elasticity and modulus of rupture) and physical (thickness swell) properties of the boards were investigated.
The results revealed that control boards (untreated PF with southern yellow pine and sweetgum wood particles) had better mechanical (both modulus of elasticity and modulus of rupture) properties than boards manufactured with 10% MCC. It was found that addition of MCC to particleboard resulted in debonding of the adhesive from the cellulose matrix as indicated by the lower mechanical properties. This was attributable to the difference in viscoelastic properties and consequent springback of the wood and cellulose components within the matrix. A future study to reduce springback is necessary if MCC is to be used for the improvement of particleboard mechanical properties. On the other hand, increases in particle size had a positive effect on mechanical properties of boards produced and may be a cost effective way of dialing in particleboard mechanical properties to meet industry standards. Finally, sweetgum exhibited better dimensional stability and competitive mechanical properties than southern pine. Since sweetgum is an underutilized hardwood that is abundant in regions that include southern pine, use of sweetgum in composite board production may help to alleviate pressures on natural resources that are likely to occur as other biobased manufacturers compete for southern pine raw material.