Behaviour of glass fibre reinforced recycled mixed plastic solid waste composites for civil engineering and construction applications

Recycled plastics composites have raised interest for reapplications due to the growing worldwide concern over the disposal of plastic waste as only around one-fourth of it is currently being recycled while the rest of it continues to litter our roadways, clog our landfills and pollute our environment. As plastic solid waste (PSW) are in mixed form and separating the blends of numerous plastics from each other is expensive, therefore this research focuses on converting mixed PSW into reliable products suitable for building and construction industry to increase their recycling rate. This study investigated on the possibility of developing engineered composite materials made from recycled mixed plastic solid waste (PSW) containing a mixture of high density polyethylene (HDPE), low density polyethylene (LDPE) and polypropylene (PP). Coupon and bigger size specimens were produced through injection moulding process and their important structural properties were characterised. This experimental characterization enabled the author to determine the coefficient of variance of the measured properties which was less than 10% indicating the consistent material quality of mixed PSW. The strength properties were found to be 14.8, 19.8, 20, 5.6 MPa in tension, compression, flexure and shear respectively, which is comparable to low grade timber. An experimental study on the effects of the addition of short glass fibres (10-30% by weight), compatibiliser (2.5 wt. % of polypropylene-grafted-maleic anhydride, PP-g-MA), processing stabiliser (1 wt. % of phosphitic antioxidant, Irgafos-168) and light stabiliser (1 wt. % of oligomeric hindered amine, Univul-5050) on the physical, mechanical and durability properties of mixed PSW was conducted. With the addition of 30 wt. % of glass fibre, the strength properties and elastic modulus increased by as much as 141% and 357%, respectively compared to unreinforced PSW. The best improvement was seen in the flexural properties due to the better orientation of glass fibres in the longitudinal direction at the outer layers with moderate improvement in shear. The addition of 2.5 wt. % of PP-g-MA further increased the tensile strength and modulus by 2.7% and 4%, respectively while 1 wt. % of Irgafos-168 resulted in an increment of 5.5% in the tensile strength and modulus. SEM observations on the specimen with chemical additives showed a good bonding of the glass fibre with the mixed PSW. With the improved properties of glass fibre reinforced mixed PSW composites (GMPC), the strength of this material is comparable to medium grade timber. The behaviour of mixed PSW and its composites under different environmental conditions such as elevated temperatures, ultraviolet (UV) radiation, and ingress of moisture was studied for wider acceptance as a civil engineering material. When the test temperature was increased from 23°C to 40°C, the mixed PSW retained 73% and 64% of its tensile strength and modulus, respectively. The addition of 30 wt. % glass fibres improved the tensile strength and modulus retention to 78% and 87%, respectively. Under UV radiation, the mixed PSW and its composites showed weight loss and surface degradation. After 4000 hours of exposure, the mixed PSW lost 1.4% of its weight, which was reduced to 0.6% with the addition of 30 wt. % of glass fibre and 1 wt. % of Univul-5050. However, the addition of 2.5 wt. % of PP-g-MA in the specimens containing 30 wt. % of glass fibre increased the weight loss to 1.2% due to the presence of photo-responsive groups in PP-g-MA that can catalyse the photo-oxidation. Interestingly, the tensile strength and modulus of the PSW and its composites was not affected by UV radiation. After 4000 hours of exposure, the mixed PSW retained 96% and 103% of its tensile strength and modulus, respectively. These properties improved to 116% and 121%, respectively with the addition of 30 wt. % glass fibre and 2.5 wt. % PP-g-MA due to shrinkage of the specimens which increased the bonding between the glass fibre and matrix. Under hygrothermal ageing, the mixed PSW and its composites absorbed moisture and formed microvoids and cracks on the surface. The addition of 30 wt. % of glass fibre and 2.5 wt. % of PP-g-MA reduced the moisture absorption by 25% due to the binding of the matrix by glass fibre and better bonding of the fibre-matrix interface by PP-g-MA. Further to this, the addition of 30 wt. % of glass fibre and 2.5 wt. % of PP-g-MA resulted in the mixed PSW retaining 100% of its strength and stiffness. The simplified fibre model and finite element analyses using the material properties determined from coupon tests were found to reliably predict the non-linear flexural behaviour of beams made from mixed PSW. The enhancement on the mechanical properties due to the addition of short glass fibre were reasonably predicted by accounting for the randomness and length of the glass fibres. On the other hand, the effect of elevated temperatures, UV radiation and hygrothermal ageing was accounted for by introducing a temperature reduction factor, shrinkage factor and strength ratio factor, respectively to the modified rule of mixture equation. Based on Arrhenius model, the mixed PSW and its composites will retain 88% of their strength, which is further improved to 96% with the addition of 30 wt. % glass fibre and 2.5 wt. % PP-g-MA even after 50 years of moisture exposure at an annual temperature of 23°C. From these findings, it can be concluded that engineered composite materials made from recycled mixed plastic solid wastes have the physical, mechanical and durability characteristics suitable for building and construction applications. A detailed understanding of their behaviour under different environmental conditions for potential civil engineering applications is an outcome of this investigation