AS Resin/Styrene Acrylonitrile /SAN (C8H8)n-(C3H3N)m Plastic Recycling

Topics Covered

Introduction
Physical and Chemical Properties of SAN
Manufacturing Process of SAN
Applications of Virgin Material
Environmental Impact of SAN
Recycling Process
Applications of Recycled SAN
Sources

Introduction

Styrene Acrylonitrile resin is a copolymer plastic comprising acrylonitrile and styrene. It is popularly known as SAN. It is extensively used instead of polystyrene due to its high thermal resistance. The polymer chains show alternating styrene and acrylonitrile units.

The relative composition is between 20 and 30% acrylonitrile and 70 and 80% styrene. A higher amount of acrylonitrile content enhances chemical resistance and mechanical properties and also adds a yellow tint to the normally transparent plastic.

Molecular Formula: (C8H8)n-(C3H3N)m

Density: 1.07 g/cm3

Melting Point: 220 -270°C

Physical and Chemical Properties of SAN

SAN is transparent, offers high strength, chemical resistance, rigidity and dimensional stability as well as low impact strength. SAN materials have a high resistance to chlorinated and aliphatic hydrocarbons, various oils, fats and household detergents.

Due to its molecular structure, SAN is yellowish in color and loses transparency after injection molding. Styrene-based materials offer the unique features of high performance, versatile design, simple production and economy.

Manufacturing Process of SAN

SAN resins include both acrylonitrile and styrene and their synthesis is done by suspension, emulsion and continuous mass polymerization, however maximum production is by batch emulsion.

In both batch and continuous emulsion, acrylonitrile and styrene monomers are injected into a monomer makeup tank along with chemical additives and recovered acrylonitrile. The monomer mix is next injected into the polymerization reactor with initiator, emulsifier, deionized water and chain-transfer agent. After being retained in the reactor for a specific time, the SAN copolymer melt is pumped to a steam stripper to recover unreacted monomers. The polymer is subjected to filtration, then washing and drying so that the solid SAN copolymer is formed.

The suspension process also is conducted in both batch and continuous emulsion. Styrene and acrylonitrile monomers are injected into a pressure reactor and mechanical dispersion is done in water that comprises suspending agents and catalysts. Water and monomer are used in equal amounts. The monomer droplets copolymerize when suspended by agitation forming insoluble polymer beads. About 95% of monomers get converted and unreacted monomers are recovered in a later step. The dried SAN is completed by mechanically blending in anti-oxidants, dyes and other additives.

Applications of Virgin Material

SAN finds applications in:

  • Electronic lenses , Automotive instrument lenses
  • Dust covers
  • Housewares
  • Cups, toothbrush handles, trays, containers
  • Industrial battery cases, knobs, dials switches
  • Tractor components, automotive trim, marine instruments
  • Instrument panels, swimming pool components

Environmental Impact of SAN

According to the Environmental Protection Agency, around 10% of all municipal solid waste was recycled, 10% was incinerated and around 80% was sent to landfills in the 1990s. Currently, a significant amount is being recycled and the amount of plastics in landfills is around 20%.

SAN degrades slowly in landfills and the plastic also has hazardous additives. These additives include colorants, stabilizers and plasticizers that may have toxic components such as lead and cadmium. The pollutants leak slowly into the water and soil. According to studies, there are atleast 100 Mt of plastic in oceans posing a danger to marine life.

Recycling Process

As a result of the global recession, SAN consumption saw a setback. World growth in SAN consumption will be driven by consumer good/houseware applications/compounding materials, appliances and medical uses. It is expected that leading up to 2015 there will be a significant increase in the use of SAN as the global economic situation improves.

Around 2.8 Mt of household equipment, 10 million vehicles and lots of other products containing mixed plastics are scrapped annually. Although metal recovery is done, non-metallic material often gets disposed in landfills. Plastic usage has drastically increased over the years and it has become very tough to separate high-purity plastics like SAN from the waste plastic stream.

SAN comes under type 7 plastic and hence is quite difficult to recycle. However, froth floatation can be used to separate highly pure individual plastics from waste streams having a mixture of plastics. SAN has a high energy content that makes it ideal for combustion with energy recovery. However, the combustion temperatures need to be more than 900°C to eliminate dioxins completely. There are health risks associated with lesser combustion temperatures.

Applications of Recycled SAN

The recycled ABS resin is compatible with virgin materials and can be used for the same applications as the virgin material.

Sources

http://www.patterson- rothwell.co.uk/images/services/materialinformation.pdf

http://www.ihs.com/products/chemical/planning/ceh/styrene-acrylonitrile.aspx

http://scialert.net/fulltext/?doi=jest.2010.148.158&org=11

https://www.federalregister.gov/articles/2012/01/09/2011-32934/national-emission-standards-for-hazardous-air-pollutant-em

http://www.epa.gov/ttnchie1/le/styrene.pdf

http://www.dc.engr.scu.edu/cmdoc/rd_doc/RD_11_xF_resin_guide.html

http://engr.bd.psu.edu/rxm61/METBD470/Lectures/PolymerProperties%20from%20CES.pdf

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