Polystyrene (BOPS, OPS)
Polystyrene (PS) is a crystal clear, strong, and durable
inexpensive thermoplastic. Films made from this material have
excellent transparency, spark and gloss. They also have very good
antistatic properties, high tear resistance and are generally
printable. These characteristics and its low processing cost makes
polystyrene an attractive material for many film applications. Films
made of polystyrene also have very high moisture vapor, oxygen and CO2 transmission rates.
On the downside, PS has poor impact strength, i.e. it is rather brittle and stiff, and is more difficult to convert to film than common high volume polyolefin thermoplastics. To lower its brittleness and stiffness, it is often blended with styrene-butadiene copolymer (SBC). However, a larger level of SBC makes the film hazy. Another major drawback of PS is its poor chemical resistance. For example, PS is atacked by hydrocarbons, esters, ethers, ketones and the like.
Polystyrene can be converted into high-quality films and sheets which are used for a number of high-value applications. The two most important grades are cast oriented polystyrene (OPS) and blown biaxially oriented polystyrene (BOPS)1 which are sold with either matte or glossy surface. PS films have high clarity, unless they are blended with too much styrene-butadiene copolymer which makes the film hazy. Important manufacturers and distributors of BOPS and OPS films include Transcendia (Trycite), Tekra, Cheever Specialty, Dow Packaging (Trycite) and Interfilm.
Polystyrene films are used for a variety of packaging, laminating, and printing applications. Some grades are also approved for direct food contact. Important applications include windows in various sized envelops and cartons, poster and sign laminates, high gloss packaging, protective films for display screens and dash board automotive components. The probably biggest use of SBC modified PS is in food packaging. PS is known for its high permeability to water vapor and gases (breathability) which makes it suitable for packaging fresh food products. In general, the gas transmission rate increases with increasing SBC content.
1”Biaxially oriented" means that the PET film is stretched in two different directions which increases strength and modulus (lower elongation and higher stiffness).