Tuesday, 25 April 2017

My Top 5 Commodity Plastics for Medical Device Applications – Part 4: PS

Welcome back to the blog series ”My top 5 commodity plastics for medical device applications”. This is part 4 – Polystyrene (PS).
Here you can jump to part 1 –PVC, part 2 – PE and part 3: PP.
Nr. 4 – Polystyrene (PS)
Polystyrene has long found its way in medical device applications and is widely used. PS is an amorphous polymer and is available in two forms: crystal clear polystyrene also referred to as General Purpose Polystyrene (GPPS) and as High Impact Polystyrene (HIPS).
HIPS is usually modified using polybutadiene elastomers. Depending on the added amount a high-impact grade (6-12% elastomers) or a medium-impact grade (2-5% elastomers) may be obtained.
A comparison in properties of GPPS and HIPS can be found in Table 1 [1].
Table 1: overview properties of GPPS and HIPS
Similarly to PP, PS can be found in three different structures: atactic (A-PS), isotactic (I-PS) and syndiotactic (S-PS). The A-PS is the most commercially available structure followed by S-PS. In applications with rather demanding specifications S-PS is usually preferred due to its superior properties i.e. high melting point (270°C), good chemical resistance and very low dielectric constant. Furthermore, S-PS has high flow capability which facilitates processing and enables thin-wall applications. Virgin S-PS is brittle. Thus, when toughness is required S-PS is usually reinforced with glass or alloyed with other polymers.  S-PS is produced in a continuous polymerization process using metallocene-based catalysts similarly to polyolefins.
How does PS perform in terms of sterilization?
Steam and autoclave sterilization are not applicable to PS due to its low heat distortion temperatures (85 °C at 1.85 MPa / 95 °C at 0.46 MPa). These will cause warp and disfigure. On the other hand, PS can be sterilized by Ethylene Oxide. This is valid for both types – GPPS and HIPS. PS shows a great stability to gamma radiation due to its high aromatic content. The aromatic ring has free electron clouds which are able to absorb the radiation inhibiting the generation of free radicals. No significant shift in color is generally observed either.  Therefore, PS can also be sterilized by irradiation.
What about biocompatibility?
PS is usually not used for applications where biocompatibility is required. However, there are biocompatible grades available from specific manufactures [2]. These allow using the versatility of polystyrene under the ISO 10993 compliance of the medical market.
Where is PS used in medical device applications?
GPPS can be processed over injection moulding leading to applications in labware, diagnostic equipment (e.g. petri dishes, test tubes and IVD products), and device components. GPPS processed by extrusion is used for packaging. As for HIPS is rather used in trays, bottles, containers, and medical components. Generally, HIPS is preferred over GPPS when impact resistance is of greater importance. Table 2 gives an overview of medical applications using PS. Since the properties of PP have been improving over the last decade, this becomes more and more competitive with PS, especially due to the relatively lower cost of PP.

Table 2:  Examples of applications using PS adapted from [1]
Where to get PS for your medical device applications?
Table 3 lists suppliers for GPPS and HIPS.
  Table 3: Suppliers of PS [1]

Table 3: Suppliers of PS [1]
Thanks for reading! Have a beautiful day & till part 5: COC!
P.S. New to my blog – check out my ‘start here’ section.
1] Vinny R. Sastri: Plastics in Medical Devices, 2014
[2] Trinseo - STYRON™ 2678 MED Polystyrene Resin: http://www.trinseo.com/News-And-Events/Trinseo-News/2016/June/Trinseo-Introduces-Biocompatible-Polystyrene-for-Medical-Devices

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