Plastic recycling is the process of recovering scrap or waste plastics and reprocessing the material into useful products, sometimes completely different in form from their original state. For instance, this could mean melting down soft drink bottles and then casting them as plastic chairs and tables.
Challenges
When compared to other materials like glass and metal, plastic polymers require greater processing (heat treating, thermal depolymerization and monomer recycling) Due to the high molecular weight of their large polymer chains, plastics have a low entropy of mixing. When different types of plastics are melted together, they tend to phase-separate, like oil and water, and set in these layers. The phase boundaries cause structural weakness in the resulting material, meaning that polymer blends are useful in only limited applications.
Another barrier to recycling is the widespread use of dyes, fillers, and other additives in plastics. The polymer is generally too viscous to economically remove fillers, and would be damaged by many of the processes that could cheaply remove the added dyes. Additives are less widely used in beverage containers and plastic bags, allowing them to be recycled more often. Yet another barrier to removing large quantities of plastic from the waste stream and landfills is the fact that many common but small plastic items lack the universal triangle recycling symbol and accompanying number. A perfect example is the billions of plastic utensils commonly distributed at fast food restaurants or sold for use at picnics.The use of biodegradable plastics is increasing. If some of these get mixed in with the other plastics for recycling, the reclaimed plastic is not recyclable because of the variance in properties and melt temperatures.
Before recycling, most plastics are sorted according to their resin type. In the past, plastic reclaimers used the resin identification code (RIC), a method of categorization of polymer types, which was developed by the Society of the Plastics Industry in 1988. Polyethylene terephthalate, commonly referred to as PET, for instance, has a resin code of 1. Most plastic reclaimers do not rely on the RIC now; they use automatic sort systems to identify the resin, such as near infrared (NIR) technology. Some plastic products are also separated by color before they are recycled. The plastic recyclables are then shredded. These shredded fragments then undergo processes to eliminate impurities like paper labels. This material is melted and often extruded into the form of pellets which are then used to manufacture other products.
Plastic identification code
Five groups of plastic polymers, each with specific properties, are used worldwide for packaging applications (see table below). Each group of plastic polymer can be identified by its Plastic Identification code (PIC) – usually a number or a letter abbreviation. For instance, Low-Density Polyethylene can be identified by the number "4" or the letters "LDPE". The PIC appears inside a three-chasing-arrow recycling symbol. The symbol is used to indicate whether the plastic can be recycled into new products.
The PIC was introduced by the Society of the Plastics Industry, Inc., to provide a uniform system for the identification of different polymer types and to help recycling companies separate different plastics for reprocessing. Manufacturers of plastic products are required to use PIC labels in some countries/regions and can voluntarily mark their products with the PIC where there are no requirements. Consumers can identify the plastic types based on the codes usually found at the base or at the side of the plastic products, including food/chemical packaging and containers. The PIC is usually not present on packaging films, as it is not practical to collect and recycle most of this type of waste.
Properties
Clarity, strength, toughness, barrier to gas and moisture.
Common Packaging Applications
Soft drink, water and salad dressing bottles; peanut butter and jam jars
Glass Transition and Melting Temperatures(°C)
Tm = 250;Tg = 76
Young's Modulus (GPa)
2-2.7
Properties
Stiffness, strength, toughness, resistance to moisture, permeability to gas.
Common Packaging Applications
Water pipes, hula hoop rings, five gallon buckets, milk, juice and water bottles; grocery bags, some shampoo / toiletry bottles
Glass Transition and Melting Temperatures(°C)
Tm = 130;Tg = -125
Young's Modulus (GPa)
0.8
Properties
Versatility, ease of blending, strength, toughness.
Common Packaging Applications
Blister packaging for non-food items; cling films for non-food use. Not used for food packaging as the plasticisers needed to make natively rigid PVC flexible are usually toxic. Non-packaging uses are electrical cable insulation; rigid piping; vinyl records.
Glass Transition and Melting Temperatures(°C)
Tm = 240;Tg = 85
Young's Modulus (GPa)
2.4-4.1
Properties
Ease of processing, strength, toughness, flexibility, ease of sealing, barrier to moisture.
Common Packaging Applications
Frozen food bags; squeezable bottles, e.g. honey, mustard; cling films; flexible container lids.
Glass Transition and Melting Temperatures(°C)
Tm = 120;g = -125
Young's Modulus (GPa)
0.17-0.28
Properties
Strength, toughness, resistance to heat, chemicals, grease and oil, versatile, barrier to moisture.
Common Packaging Applications
Reusable microwaveable ware; kitchenware; yogurt containers; margarine tubs; microwaveable disposable take-away containers; disposable cups; plates.
Glass Transition and Melting Temperatures(°C)
Tm = 173;Tg = -10
Young's Modulus (GPa)
1.5-2
Properties
Versatility, clarity, easily formed
Common Packaging Applications
Egg cartons; packing peanuts; disposable cups, plates, trays and cutlery; disposable take-away containers;
Glass Transition and Melting Temperatures(°C)
Tm = 240 (only isotactic);Tg = 100 (atactic and isotactic)
Young's Modulus (GPa)
3-3.5
Properties
Dependent on polymers or combination of polymers.
Common Packaging Applications
Beverage bottles; baby milk bottles. Non-packaging uses for polycarbonate: compact discs; "unbreakable" glazing; electronic apparatus housings; lenses including sunglasses, prescription glasses, automotive headlamps, riot shields, instrument panels;
Glass Transition and Melting Temperatures(°C)
Polycarbonate: Tg = 145;Tm = 225
Young's Modulus (GPa)
Polycarbonate: 2.6; ABS plastics: 2.3
