Effect of Aspergillus Niger on Mechanical, Diffusion and Thermal Properties of High Density Polyethylene/Cellulose Blend
Waswa, Michael Nakitare
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Plastic materials, particularly polyethylene (PE) are the major source of environmental pollution. High density polyethylene (HDPE) is an essential industrial material in Kenya which is hard and non-biodegradable and hence persists in the environment for longer times. Wider application of HDPE in packaging and agriculture has raised serious issue of waste disposal and its pollution. In order to mitigate this, structural modification through blending with biopolymers is deemed to provide a solution. HDPE/starch blends have been found to posses undesirable properties; they have high diffusivity, less rigid and high thermal stability. Conversely, cellulose (CELL) is a linear polysaccharide with enhanced rigidity and higher thermal stability. This study focused on the effect of inoculating HDPE/CELL blends with Aspergillus niger on their mechanical, diffusion and thermal properties. HDPE granules and CELL powder from acacia cell-sap were used to prepare the samples by hot pressing their molten mixture. The samples were then exposed to a strain of A. niger for 60 days. Dynamic mechanical analysis (DMA) was carried out in single cantilever mode in frequency range from 1 to 30 HZ and temperature range from -30 to 120 oC. Creep measurements were performed at 30 oC, 40 oC, 50 oC and 60 oC. A sample was displaced for 12 minutes and allowed to recover for another 12 minutes. Thermo-gravimetric analysis (TGA) measurements were done within a temperature range of 25 to 550 oC at a heating rate of 5oC/minute. Diffusion measurements were done at 25 oC by dipping the samples in distilled water and mass increase monitored after every 7 days in a time span of 42 days. Viscoelastic behavior of HDPE/CELL blends showed two relaxation processes which were greatly affected by inoculation. The α process is assigned to glass transition while β process is assigned to inter-lamellar shear. The intensity of both α and β peaks significantly reduced on inoculation due to the degradation of the blends by A.niger. The β-relaxation was heavily affected by inoculation suggesting that microbial attack started in the crystalline-amorphous interface. Creep performance of the HDPE/CELL blends improved with CELL loading and decreased on inoculation due to destroyed matrix. William-Landel-Ferry (WLF) model offered a better long-term prediction based on the short-term creep data. Time-temperature superposition technique produced smooth master creep curves through horizontal shifts which was used to predict creep behavior of the samples extending to 105 s. Thermo-gravimetric thermo-grams provided information about the mass loss, thermal decomposition and thermal stability of the blends. The non isothermal kinetics of the decomposition processes were analyzed using the Broido integral method. Thermal stability of the blends decreased with cellulose intake and on inoculation. The activation energies related to the correspondent reactions were calculated. Fick’s second law was used to ascertain the diffusivity of water in the blends which increased with CELL intake and on inoculation. Cellulose thus lowers the thermal stability, improves hydophilicity of HDPE and reinforces it structural rigidity. Inoculation of the blends enhances biodegradability, lowers thermal stability and rigidity. In order to curb environmental pollution caused by plastics especially those used in packaging, use of CELL as a bio-filler and inoculating with A.niger at the pre-disposal stages is thus encouraged.