Biodegradation and Recycling of Polyethylene into Composite Building Materials

Biodegradation and Recycling of Polyethylene into Composite Building Materials.

ABSTRACT

This work presents the results of experimental and theoretical studies of the biodegradation of polyethylene, recycling of polyethylene into strong and tough earth-based composite building materials and the statistical distribution and particle size analysis of the polyethylene composites.

Serratia marcescens subsp. marcescens and its supernatants are able to biodegrade linear low density polyethylene (LLDPE). The results show that the cell-free extracts degrade LLDPE  faster than the bacterium.

The mechanisms of degradation are also elucidated using Scanning Electron Microscopy (SEM), Differential Scanning Calorimetry (DSC) and Fourier Transform Infra-Red Spectroscopy (FTIR). These methods show that the bacterium and its supernatant both degrade LLDPE.

There was also an increase in the concentrations of the carbonyl groups (new peaks) after the microbial degradation of LLDPE. Waste PE can be recycled and used as reinforcement in laterite bricks for sustainable building materials. The bricks are produced with different volume percentages (0-30 volume percent) of PE.

The flexural/compressive strengths and fracture toughness values of the composite blocks were compared with those of mortar (produced from river sand and cement). The composite containing 20 vol. % of PE had the best combination of flexural/compressive strength and fracture toughness.

The flexural/compressive strengths and fracture toughness values then decreased, respectively, to minimum values for 30 vol. % of PE. The trends in the measured strengths and fracture toughness values are explained using composites and crack bridging models.

Different particle sizes of the PE composites were shown to have statistical variations in flexural/compressive strengths and fracture toughness. The statistical variations in the flexural/compressive strengths and fracture toughness are shown to be well characterized by the Weibull distributions.

INTRODUCTION

1.1 Background

Plastics are synthetic organic heteroatomic polymers which originate from oil, coal and natural gas [1, 2]. Plastics are generally non-biodegradable in the presence of enzymes or microbes [3] and their pile up leads to prolonged environmental changes [4].

This has led to environmental concerns as approximately 140 million tons of man-made polymers produced worldwide are found in food packaging, detergents, clothing, shelter, transportation and chemical substances with an increasing annual rate of 12% [5, 6].

These plastics include polyethylene (LLPE, LDPE, MDPE and HDPE), polypropylene (PP), polystyrene (PS), Polyvinyl Chloride (PVC) and polyethylene terephthalate (PET) [5, 7, 8, 9].

Plastics have been used extensively due to their attractive combination of stability [10], thermal properties [6] and mechanical properties [11].

PE consumption is the largest among polymers produced worldwide, recording a total of over 90 million metric tons per annum (The Plastics Portal). The high production of PE hinders the implementation of an efficient disposal system [6].

However, PE can be degraded by chemical, thermal, photo and biological means.  The current mechanisms that are used in the biodegradation of PE include hydro-degradation [7] and oxo-biodegradation [7, 12].

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