Editorial Feature

Polystyrene(PS)(C8H8) Plastic Recycling

Polystyrene (PS) is an aromatic polymer of the monomer styrene. Polystyrene is an extensively used plastic with a production scale of several billion kilograms per year. Polystyrene is colorless and is used in plastic models, disposable cutlery, smoke detector housings and CD and DVD cases. Polystyrene can be white, transparent or be dyed using a range of colors.

  • Molecular formula - (C8H8)n
  • Density - 1.06 g/cm3
  • Melting point - 240°C (464°F)
  • MFR (Melt Flow Rate) – 5-25
  • IUPAC ID - Poly(1-phenylethane-1,2-diyl)

Chemical and Physical Properties of PS

The most significant properties of polystyrene are its flow properties. Polystyrene’s melt flow index is controlled normally by adjusting the molecular weight of the material and adding lubricants like mineral oil. Polystyrene has visco-elastic properties.

The impact strength of crystal polystyrenes is considerably low. Adding lubricants such as mineral oil reduces the tensile strength. Flexural strengths of 5000 to 18000 psi are possible, which are decreased when rubber or other additives are added to the polystyrene. For unmodified polystyrene, the glass transition temperature is 373 K, and for polybutadienes it is 161-205 K subject to the trans, cis and vinyl content.

A commercially important phenomenon of polystyrene is solvent crazing. Solvent crazing may occur in high impact polystyrenes at the interface between the polystyrene phase and the rubber particles. Environmental stress crack resistance (ESCR) is the resistance of polystyrene to this crazing. Crystal polystyrene is a colorless and transparent polymer whereas high impact polystyrene is normally opaque due to the rubber particles. Gloss is a key optical property for high impact polystyrene. Polystyrene and its copolymers can be degraded with sunlight. It has less toxicity, hence is approved for food packaging applications.

Applications of PS Virgin Material

Polystyrene is extensively used and the applications of polystyrene are:

  • Extruded polystyrene foam sheet is thermoformed into carryout food containers or egg cartons.
  • Expandable polystyrene beads can be molded to produce ice chests, hot drink cups, or foam packaging. These can also be molded in large blocks that can then be cut into sheets for thermal insulation.
  • Biaxial orientation of the extruded crystal polystyrene sheet can be carried out for use in cap layers for glossy sheet, envelope windows or thermoforming into food packaging applications.
  • Due to its optical characteristics, polystyrene is used in the manufacture of unbreakable glasses for gauges, lenses and windows, as well as in a number of novelties and specialties and also for edge lighting of indicators and dials
  • Polystyrene parts can be subjected to high quality printing and direct printing of labels on the polystyrene part is possible to produce attractive containers.
  • Polystyrenes are also used in furniture, appliances, automobiles, radios, construction, toys, televisions, luggage and house ware items

Manufacturing Process of PS

The styrene monomer is polymerized to produce polystyrene. Styrene with CAS number 100-42-5 is a volatile colorless liquid. A volatile blowing agent is introduced producing expandable polystyrene that causes the expansion of the polymer while heating.

Physical blowing agents are used for the foaming of polystyrene. Blowing agents are liquids or gases that can be dissolved in the molten polymer under pressure. Depressurization causes volatilizing of the blowing agent resulting in the polymer foaming by forming gas cells. Conventionally volatile hydrocarbon blowing agents were used, which due to being highly inflammable, were replaced by nonflammable chlorofluorocarbons.

Environmental Impacts of PS

Certain environmental impacts of PS are detailed below:

  • During manufacture, the amount of air pollution caused is quite high and a large amount of solid and liquid waste is formed. In terms of product manufacture and also usage and disposal of products, greenhouse gas effect, energy gas consumption, the environmental impact of PS was second only to aluminum
  • Waste management of polystyrene foam is a challenge and it pollutes waterways.
  • Polystyrene is presently manufactured with HCFC-22; although not as destructive as CFC-11 and CFC-12, HCFC-22 is a greenhouse gas and depletes the ozone layer.
  • Polystyrene is slow to degrade and may cause a threat to wildlife
  • If burnt at a temperature below 900°C (1652°F), up to 90 hazardous compounds may be released

Recycling Process

During recycling, it is important to determine which specific plastic material has been used for a particular product. The types of plastics are normally recognized by the numerical coding system that was decided by the Society of the Plastics Industry in the late 1980s.

Polystyrene that is used in meat packing throwaway utensils and protective packaging is coded at 6, which is normally not recyclable. However, researchers are trying to find ways to make this PS also recyclable.

Certain challenges associated with recycling of PS are that bulky foams occupy a large amount of space with a need for multiple trips to hall away a minimal amount of waste. Its light polymer structure makes molding and regrinding into PS components tough without degradation. Direct recycling is prevented due to additives and fillers.

Considering the above, it is still possible to recycle polystyrene though that is not being widely done. Some of the methods are discussed below:

  • Burying polystyrene can be done by first using a melting technology. After melting it into a solid form, PS can be buried stably without contamination
  • PS can be incinerated at temperatures above 1000°C (1832°F)effectively and cleanly. Chemicals are disintegrated by burning with excess oxygen. The byproducts obtained are water vapor, soot, CO2, heat and soot.
  • It is wise to make foam packaging components into generic shapes that can be repeatedly used such as blocks or peanuts.
  • Disposable products can be reused as PS is easy to serialize/clean and is nonporous. It is possible to redesign disposable products so that they are used several times before they are actually disposed.
  • Contaminant removal from PS is done using UV, heat or chemicals.
  • PS is broken down in the absence of oxygen by a technique known as pyrolysis. Base monomers are formed for reprocessing and light crude oil is used for virgin stock or fuel reprocessing.
  • Condensers are used on unusable, compact EPS. This enables more PS to be carried away causing cost reduction related to PS recycle. This method involves applying force mechanically to crush the foam to a dense block.
  • PS contains high energy content and hence is a good fuel

Applications of Recycled PS

Recycled PS can be used for the following applications:

By using colorants, PS can be reground and used in new PS products

  • Expanded polystyrene scrap or EPS can be cut into building materials, used as a filler for structures such as lamp posts and park benches, and can be remolded and used in the casting industry
  • Recycled PS sheet and molded components can be used as chop filler for non PS materials and if near virgin, can be reground into PS materials
  • Recycled polystyrene can be used in insulation, light switch, egg cartons, plates, rulers, vents, carry-out containers and foam packing.

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