Recycling Plastic Waste as Composite Reinforcement

. Environmental pollution due to improper disposal of plastic waste has greatly degraded the livelihood of both humans and animals. In this article, polyethylene terephthalate (PET) bottles were used as reinforcement in scrap aluminum. The composite material was made through the stir-casting route. The samples were characterized using the microstructure, tensile strength, and hardness. Results show an increase in tensile strength up to 4% addition of PET and steady hardness reduction as reinforcement composition increases. However, the mechanical strength declines as reinforcement goes beyond 8%.


Introduction
Recycling as a way of reusing materials is now a common thing in different countries of the world.Governments and institutions have developed unique and sustainable methods of discarding and recycling waste.The present situation of environmental degradation has further created the need for the adoption of sustainable strategies for recycling.Plastic, metal scraps, and electronic wastes constitute the bulk of materials recycled all over the world, and the methods of disposal have caused havoc to the environment [1].
The common practice of recycling plastics is shredding them into pallets and remelting them to form a new product.In some cases, plastics form parts of composite structures in engineering applications.Metals have been used as reinforcement in plastics; likewise, plastics as reinforcement in metals.A review of reinforced plastic composites by Adeniyi and Ighalo [2] shows that aluminum is the major metal used to reinforce plastics.Also, manual mixing, casting, and hand laying up are the common methods used in producing metal-reinforced plastic composites.Another method reported in the literature is the sandwich method, in which the plastic reinforcement is arranged in interlayer form with the base material [3].A study by Yang et al. [4] on sandwiched hybrid metal-carbon-glass fiber reinforced plastic composite shows about 30% improvement in mechanical strength and reduced weight compared to metals only.In another study by Fonseca et al. [5], a 36% weight reduction was achieved using carbon fiber-reinforced plastic/hybrid metal to design an engine cradle.The ribs of the automotive component were plastic, and other parts were made of metal.
However, there is scarce literature on the reinforcement of metals using plastics through the stir-casting method, which is known to be one of the cheapest production routes [6][7][8].The research also investigated the possible ways of disposing of plastics without constituting further environmental hazards.Therefore, this research seeks an alternative way of using plastic products to reinforce metals.

Experimental procedure
Aluminum-Zinc (Al-Zn) scraps with the chemical composition of base Al, 5.71% Zn, 2.47% Mg, 0.87% Fe, 1.52% Si, 0.095% Cu, and 0.37% others were gathered, crushed using hammers and remelted in a furnace.The PETs were shredded and washed to remove dirt and other organic matter.They were dried in the furnace at 105 °C for 4 h to remove moisture.The furnace was preheated to 500 °C before charging the aluminum cans, and the temperature of the furnace was further increased to 800 °C for 45 minutes, with stirring done at 450 rpm [6].The molten metal was brought out, and the slags were removed and allowed to cool to 500 °C before shredded polyethylene terephthalate (PET) bottles were charged into the molten metal according to the percentage composition shown in Table 1.The composite was thoroughly stirred and poured into the mold.
The cast samples were machined to the ASTM E8 standard [9] and were pulled using the universal testing machine M500-100AT.A Vickers hardness test was carried out on the cast samples using a load of 4.9 N and a dwell period of 15s.Furthermore, the microstructure of the samples was captured using an optical microscope, and the samples' density was also determined.3 Results and discussion

Microstructure
The microstructure of the as-cast sample is shown in Figure 1a.Intergranular eutectic and intergranular crystallization can be observed in the microstructure [10,11].Figure 1b-e shows the intragranular and grain boundary residual phase [12].The presence or agglomeration of the PET was not observed within the microstructure as there is complete melting of the PET.According to Singh et al. [13], PET is expected to degrade at 450-550 °C temperatures.

Tensile strength
Figure 2 shows the ultimate tensile strength of the samples.Samples reinforced with 4% PET had a higher strength when compared to the base material and other samples with PET reinforcement.As percentage reinforcement increases, the reduction in strength could be attributed to the lack of adequate cohesion/bonding of the materials.It is also evident in the microstructure as the percentage increment in PET increases voids/pores within the material.These have been attributed to the lower tensile strength observed in samples with more PET reinforcement [14].

Hardness
Figure 3 shows the microhardness of the samples.An average of three indentations were made on each sample.The results show a reduction in hardness as the percentage of PET increases in the composite.This could be attributed to increased voids/pores within the samples.It could also be attributed to alloying elements such as Mn, Cr, V, and Zr, which have been reported to reduce hardness in aluminum alloys [15].

Conclusion
The feasibility of recycling PET through the stir casting of Al-Zn has been studied.The study results showed that the addition of PET did not significantly change the microstructure of the composite.Still, porosities were observed as the percentage of PET increased in the composite.Furthermore, the tensile strength decreased as the percentage of PET in the composite.Lastly, the hardness decreases as the percentage of PET increases.Recommendations for further studies suggest that the stir casting could be carried out under an inert environment to avoid the burning off or degradation of the PET materials.Further studies on the porosity viz-a-viz the percentage addition of PET should also be studied.

Table 1 .
Composition of composite