Polymer Basics for Polystyrene

Polystyrene is one of the most versatile plastic resin materials and is found in many widely used products. This strong, solid plastic is useful in many manufacturing and medical applications due to its durability and light weight.

Polystyrene can also be manufactured into a foam material for other uses. This expanded polystyrene (EPS) or extruded polystyrene (XPS) has effective insulation and cushioning characteristics. In this form, it can be used in home insulation as well as lightweight, protective packaging.

Characteristics and Properties of Polyamides

There are two main types of polystyrene. Polystyrene crystal, also known as oriented crystal polystyrene, is a rigid, hard material that offers flexural strength. It is usually injection molded and can be colored or formed into many finished products. Some applications of polystyrene crystal include packaging, optics, and medical products.
High impact polystyrene (HIPS) is an amorphous polymer used in medical device packaging due to its rigid nature. HIPS shrink uniformly and are less prone to shrinkage and warping than semi-crystalline polymers. HIPS is used in lower heat applications no higher than 185° F and contains 5-10% more butadiene rubber.

Production Methods

Polystyrene is produced by heating styrene monomer, resulting in a reaction in which the styrene units attach to form this long-chain polymer.
Commercially used polystyrene is produced by free-radical polymerization using initiators such as benzoyl peroxide, redox systems, and azo compounds. Since this produces an exothermic reaction, the monomer-polymer mixture has to then be cooled.
Mass or bulk polymerization uses a two-stage process. The styrene is first polymerized in a stirred tank reactor to achieve a low conversion. The dispersed polymer is then transferred to a tubular thin-film reactor. The finished granulate is produced after the molten polystyrene is pumped through spinnerets or through an extruder. The final result is granulated pellets, approximately 2-5 mm in size. Mass polymerization produces general-purpose polystyrene (GPPS), a thermoplastic that is fully transparent and low-cost.
Polystyrene can also be prepared by solution polymerization, where the styrene is dissolved in ethylbenzene or other suitable solvents. While temperature control is more manageable, the solvent reduces the molecular weight, and impurities lower the appearance of transparency. Another method is the suspension process which is often used to produce expandable polystyrene (EPS) and HIPS.

Common Uses in Medical Devices and Life Sciences

Polystyrene is used extensively in the medical field, specifically for manufacturing implants, devices, and disposables such as gloves and vials. It is also used to manufacture laboratory ware such as Petri dishes, sterilization trays, and pipettes. With its broad optical clarity, HIPS is also widely used in pharmaceutical containers. It is low cost and ease of processing make this material ideal for these uses.
Due to an increased demand for single-use medical devices, the need for medical polymers has never been greater and continues to grow significantly.

Best Sterilization Methods for Polystyrene

Polystyrene products are best sterilized using ethylene oxide (EO) gas or by gamma irradiation. 

There are three steps in the typical EO sterilization procedure.

  1. Preconditioning: This step ensures the ideal temperature, pressure, and humidity in the chamber itself. 
  2. Sterilization: In the actual sterilization process, the EO gas is used with the proper amount of steam to ensure the appropriate humidity levels are met. The process begins once the required concentrations are achieved.
  3. Aeration or Degassing: This is the longest but most critical part of the cycle, as removing any remaining EO gas from the objects is necessary. HEPA filtered air is circulated at a temperature of 30°C to 50°C for approximately 48 hours. Special ventilation piping to the rooftop is required since the sterilizer cannot use the facility's ventilation system due to the toxicity of the gas. 

While this process is extensive, it effectively destroys microorganisms and resistant spores. 

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