Mechanical characterization of recycled polymers

Background and motivation for the research

Plastics are made up of polymers and addictive. The percentage of polymer contained can vary from 20% to 100% depending on the application of which it is intended [3]. For simplicity purpose, plastics can be subdivided into two main categories; thermoplastics and thermosets. This distinction relates on the processing route as well as which recycling route can be applied and the basic molecular structure. A virgin plastic/material refer to the first use of the plastic [3]. In simple term, thermoplastics can melt at high temperature and can cool to gain back solid form, in other words are of high molecular weight, the polymer chains are linked together via intermolecular forces. While thermosets ‘thermosetting plastics’ on other hand can withstand high temperatures without melting. This property can be obtained by toughening or hardening the soft and various prepolymer through the introduction of cross-links between polymer chains (they will decompose rather than melt), however they have longer service lives [3]. This work will focus more on thermoplastics.

The amount of plastics ending up in the waste stream seems to be ever increasing. They have found use in various range of products such as construction, automotive, consumer goods and packaging. It is estimated that 35% plastics are used in packaging [3] Sogancioglu [1] explained that thermoset plastics are polymeric materials including number of straight or branched cross-links between chains. Consecutive melting and reshaping availabilities make them economical products. Polyethylene (PE), polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS) are among the most common thermoplastics.

Plastics thermal processing is of increasing importance for both feedstock recycling and energy conversion to liquid (oil) gas and solid products each of which can be used as energy sources and/or chemical feedstock [1].  In this study the focus will be on PET ([-CO-C6H4-COO(CH2)2O-]n) is a transparent or opaque polyester produced by condensation from ethylene glycol and terephthalic acid monomers and are used for transparent drink bottles [1,3].

Thermoplastics processing can be simplified into three sages: melting, forming and solidifying. There are various methods used to process plastic using these simple steps for example extrusion, injection moulding blow moulding and film blowing of which each method is used to manufacture specific type of products. The feed stock for all these processes is usually plastic pellets of similar sizes [3]. During all these processes, the plastic will undergo heating and heat transfer, deformation etc.

Despite increasing techniques used in to find a technical solution to separate plastics, it is not yet economical as a wide range of plastics still end up in waste stream. A typical household waste stream may contain a variety of plastics in form of carrier bags, bottles, packaging, plastics lids and food containers. Most of these plastics are mixed together in our households and are been contaminated by food waste, residues, glue, labels and also mixing with other waste materials like metal cans and paper [3,4] PET bottles collected for recycling are ground into flake. The outcome of this process is in the range of 40% to 75% depending on the amount of contamination in the waste stream [4].

In 2018 rolling to 2019 period, the local authority managed waste remained at 25.6 million tonnes. Only 10.9 million tonnes of waste was sent for recycling by local authority [6]. This calls for more attention for everyone to put effort to recycle plastics. Household wastes are difficult to sort, separate and process, so most end up exported overseas and used for land fill. This work will help increase plastics recyclability by providing proper recycling methods, retrieving the waste plastics for recycling and encouraging student to recycle or bring their wastes for easy recycling. The Society of the Plastics Industry (SPI) established a classification system to allow consumer and recycles to identify different types of plastics.

A classification system called the Resin Identification Code which is number printed on the bottom of most plastics bottles and food containers, describes what kind of plastic resin the product is made out of [ ]. A typical Mechanical recycling route may be divided into two processes [3]

  • Physical methods to homogenize the waste (i.e storage, shredding, washing and sorting).
  • Melt processing (i.e regranulation and reprocessing)\n Plastics recovery can be divided into two groups, recycling and energy recovery.

The choice of plastics depends on the types of plastics, the separation technique or difficulty involve in separation from other waste materials, ecological and cost implication of the process [2] The UK recycling rate for waste from households (WFH; including incinerated bottom ash, IBA metal) 44.7% in 2018, down 0.5 percentage points from 45.2 percent in2017. There is an EU target for the UK to recycle at least 50% of household waste by 2020. The UK generated 222.9 Million tonnes of total waste in 2016, with England responsible for 85% of the UK total. As at the end of March 2019, the recycling rate was 45.1% showing an increase of 0.3 percentage points compared with the previous 12-month period [6].

Approximately 40 per cent of plastics are used for single-use disposable applications, such as packaging, agricultural films and disposable consumer items, between 20 and 25% for long-term infrastructure [5]. Plastics are segregated by the type and colour, granulated, washed and extruded and chopped back into pellets ready for reuse.