Mechanical recycling of plastics: associated issues and innovative methods for reducing the environmental impact of plastic consumption

Compared to other materials, plastic is found to have one of the lowest recycling rates, with estimates ranging from 14 to 18%. Although recycling rates are low, the plastic recycling industry is a vital element in the transition to a circular economy, in which any resource has the ability to be recycled or repurposed. To increase recycling rates and drive this transition, it is therefore necessary to first understand what issues are affecting the plastic recycling industry and to provide plausible and practicable solutions to these problems. The focus of this research is on the issues that are affecting the mechanical recycling of plastic, as this is the most popular method for recycling plastic waste. This research is split into two components; issues affecting the quality of the recycled product and the environmental issues associated with the mechanical recycling process.

In order to facilitate the transition to a circular economy, it is important to investigate the possibility for recycling littered plastics, as this may add value to a material that would otherwise be disposed of in a landfill or incinerated. To determine an appropriate recycling pathway for littered plastics, it is essential to understand the changes that occur to the physical, mechanical, and chemical properties of a plastic after it undergoes environmental degradation. In Chapter 3, four types of plastics (polyethylene terephthalate (PET), polypropylene (PP), polycarbonate (PC) and polylactic acid (PLA)), were subjected to simulated terrestrial, ocean, and river environments to determine the levels of degradation that would occur and to propose plausible recycling pathways for the plastics if collected from the respective environments. The recycling pathways were split into two categories based upon the plastic guidelines for the recovery and recycling of plastic wastes (ISO 15720-2008). The two categories were defined as upcycling (conversion of plastic waste into a material of equal value) and downcycling (conversion of plastic waste into a material of lesser value). The results obtained from a scanning electron microscopy (SEM) analysis, colour change analysis, tensile strength analysis and a Fourier-transform infrared (FTIR) analysis showed PET, PP, and PLA to have significant changes to their physical, mechanical, and chemical properties. Due to this, it was concluded that the appropriate recycling pathway for PET, PP and PLA waste collected from the environment would be downcycling. No significant degradation was noted for PC samples in any of the simulated environments due to the plastic having a high resistance to UV exposure. As such, the appropriate recycling pathway for PC waste collected from the environment was upcycling.

The input waste stream at a plastic recycling facility plays a critical role on determining the quality of the recycled product, therefore, it is important to minimise contamination and ensure a homogenous input waste stream. However, current material separation techniques are not capable of ensuring a pure input waste stream and as such, polymer contamination is still highly likely to occur. Chapter 4 explored the scenario of polylactic acid (PLA) contaminating the input waste stream of high-density polyethylene (HDPE) recycling and what impact this had on the quality of the recycled product. PLA contamination levels of 0%, 1%, 2.5%, 5% and 10% by weight were examined and subjected to UVA radiation to assess the long-term impact of PLA contamination on the recycled HPDE. At 10% PLA contamination, the ultimate tensile strength was reduced by 50%. After UVA exposure, the ultimate tensile strength was reduced by 51% when PLA contamination was only at 2.5%. PLA contamination increased the hydrophilicity of the HDPE, which will generate issues if the recycled product was intended to be used for liquid storage. Physical deformations on the surface of the recycled HDPE were noted at all contamination concentrations. PLA contamination was visibly noticed to impact the colour of the recycled HDPE sheets, therefore decreasing the value of the recycled product. Based on the findings of this study, when HDPE is contamination with PLA at concentration of just 1%, the physical, mechanical, and chemical properties can significantly be impacted.

Emerging research has highlighted the plastic recycling industry as potentially being a significant source of microplastic pollution, however, due to the uniqueness of this field of study, there is a dearth of knowledge surrounding the factors that affect the generation of the contaminant during the process. Chapter 5 showed the size reduction phase in the plastic recycling process to generate significant amounts of microplastic particles. An investigation into whether the plastic type and environmental exposure would affect the amount of microplastics being generated was performed. Significant variations in microplastics generation rates between polypropylene (PP), polyethylene terephthalate (PET), polycarbonate (PC), and high-density polyethylene (HDPE) were noted. A microplastic count of the particles generated in the size range of 0.212 – 1.18 mm, showed PC to generate the most and PP to generate the least amount of microplastics (28,600 Å} 3,961 and 6,807 Å} 393 particles/kg of plastic material shredded, respectively). The variations in microplastic generation rates between plastic types was correlated to the hardness of the plastic (R=0.88). When PP, PC, HDPE, and PET were exposed to environmental weathering conditions for a six-month period, the microplastic generation rates increased by 121-185%, compared to their virgin counterparts. The findings from this study confirmed the significant amounts of microplastics that are unintentionally being generated during the plastic recycling process and prompts further research into how to implement microplastic minimisation strategies into the current process.

Recent investigations have discovered that plastic recycling facilities are unintentionally generating large amounts of microplastics during the size reduction process. Chapter 6 proposes a systematic change to the current plastic recycling process by introducing a sieving stage in between the shredding and washing units to capture the microplastics being generated. The benefit of adding the sieving stage to minimise microplastics release to wash water was highlighted by comparing the findings with the case where microplastics are released to wash water and a conventional coagulation process is used to remove microplastics from water. Two coagulants, aluminium sulphate (Al₂(SO₄)₃.18H₂O) and aluminium chloride (AlCl₃.6H₂O), were used to remove polyethylene terephthalate (PET) and polycarbonate (PC) from water. The size of the microplastic particles played a significant role on the removal efficiency. The maximum removal efficiency of PET by AlCl₃.6H₂O was 99.2% for the particles in 1.18 – 5 mm range, whereas the average removal efficiency over the whole tested size range of 0.15- 5.00 mm was 76.1% for the same plastic-coagulant combination. By contrast, the addition of a 5 mm sieve between the shredding and the washing units was found to capture 96-97% of the microplastics generated. The findings of this innovative experiment demonstrate the beneficial impact that this strategy has on capturing microplastics prior to entering water matrix. This study is the first of its kind to propose a process change to the current procedure used in the mechanical plastic recycling process.