My Top 5 Commodity Plastics For Medical Device Applications – Part 3: PP

Welcome back to the blog series, “My top 5 commodity plastics for medical device applications”. This is part 3 – PP. Here you can jump to part 1 – PVC and part 2 – PE.

Nr. 3 – Polypropylene (PP)

Polypropylene commonly known as PP can nowadays fulfill the requirements of a multitude of applications ranging from packaging, automotive to healthcare. It can usually be found under three different structures:

• Isotactic PP (I-PP): all methyl groups are on one side of the polymer chain.
• Syndiotactic PP (S-PP): the methyl groups are alternatively located along the polymer chain.
• Atactic PP (A-PP): the methyl groups are randomly distributed along the polymer chain.

I-PP is the structure most commercially available. S-PP is difficult to manufacture and A-PP has nearly no commercial use due to its poorly defined physical and mechanical properties in comparison to I-PP. This is a result of the random distribution of the methyl groups along the polymer chain. I-PP is semi crystalline and exhibits higher chemical resistance as well as higher tensile strength together with a higher melting point in comparison to A-PP and S-PP. The latter shows thermoplastic elastomeric behavior and is more ductile compared to I-PP.

All PP structures are nowadays produced with the support of either metallocene or Ziegler-Nata type of catalysts. Metallocene catalysts are based on cyclopentadiene or other polyaromatic compounds bonded to a metal element e.g. zirconium or hafnium. Interesting to see is that PP produced in the presence of metallocene catalysts shows a superior transparency compared to PP conventional produced with Ziegler-Nata catalysts. This is due to the formation of crystals which are smaller than the wavelength of light. In terms of processing, metallocene based PP is advantageous due to its narrower molecular weight distribution which results in reduced levels of distortion during injection moulding for example. The narrow molecular weight distribution also facilitates the production of dimensional stable parts.

By adding additives, properties of “virgin” PP can be enhanced. Nucleating agents such as talcum and sorbitols are commonly used to increase the stiffness and chemical resistance of PP. During injection moulding, such nucleating agents reduce the time of the processing cycles. As a consequence, throughput and productivity are increased.

Additionally, we can distinguish between Polypropylene Homopolymer (PP-H) and Polypropylene Copolymer (PP-C). The PP-H has a good strength-to-weight ratio, combined with high stiffness and strength. Also, it has a good chemical resistance which allows it to be used in applications which need to have corrosion resistance. On the other hand, PP Copolymer has an ethylene built in improving the impact performance, as well as toughness, especially low-temperature toughness. Stress cracking performance is improved too. 

Comparing PP to PE, it has a higher processing temperature (220-230°C; softening point: 130°C) and a more narrow processing window compared to PE (LDPE: processing temperature of 160°C). Reason for the high softening point is the CH3 group in the side chain. It takes more energy to move them since they are taking more space and blocking the other chain movements. 

How does PP perform in terms of sterilization?

Steam sterilization (123°C of saturated water vapor) and autoclaving of PP may be applied only in a limited cycle scenario due to the lower heat distortion temperature of PP (100°C in case of I-PP). Ethylene Oxide (EtO) can be applied for sterilizing PP. In case of radiation sterilization, PP needs to be stabilized by free radical scavengers which help preventing degradation and discoloration as a side effect of the high energy concentration. 

What about biocompatibility?

General testing according ISO 10993 showed that PP can be used without influencing basic immunological functions of the human body. No negative physiological, allergic or toxic reactions are expected [2]. PP biocompatible grades are available and suppliers are listed at the end of this blog post.

Where is PP used in medical device applications?

There are three major reasons why PP is very useful in medical device applications as well as in packaging applications:

• high clarity
• good barrier properties
• and radiation resistance.

PP is mostly used in the production of disposable hypodermic syringes. In this case, PP needs to be transparent and most importantly radiation-resistant, because the manufacturing of large quantities of these medical devices require cost-effective sterilization procedures such as radiation [4]. Both, the barrel and plunger of the syringe are made of PP [3].

Further applications are found in medical labware. Herein, PP needs to have high clarity, chemical resistance and toughness. PP is used for labware application such as centrifuge tubes, pipettes, containers and sample cups.

Non-woven fabrics made out of PP play an important role in applications such as surgical gowns, drapes, sterilization wraps, face and surgical masks.

In Europe, parental nutrition devices and dialysis films can be named as an emerging market for PP as well [1].

PP is also used for medical devices requiring biocompatibility. Among those are, for example, the PP meshes used in general surgery, plastics reconstructive surgery as well as in hernial repair operations [5]. In this regard, heart valve structures, wound dressings and catheters are also application examples.Table 1 shows medical device applications of PP.

Table1:  Examples of applications using PP based on [1]

Where to get PP for your medical device applications?

Table 2: HC grade certified thermoplastics suppliers of PP [1]

This was part 3 of the series. The next part will be PS.

Thanks for reading! Have a beautiful day & till next time!
Greetings,
Herwig

P.S. New to my blog – check out my ‘start here’ section.

Literature:

[1] Vinny R. Sastri: Plastics in Medical Devices, 2014

[2] Archita Datta Majumdar, Biocompatible plastics and their importance in the medical device industry, (http://multibriefs.com/briefs/exclusive/biocompatible_plastics_medical_industry.html#.WNmD-bm7qWg)

[3] http://www.madehow.com/Volume-3/Syringe.html

[4] http://en.bgs.eu/services/radiation-sterilization-contamination-control/application-fields/medical-devices-and-diagnostics/
[5] Van Der Velden MA, Klein WR. A modified technique for implantation of polypropylene mesh for the repair of external abdominal hernias in horses: a review of 21 cases. Vet Quart, 1994

[6] https://www.machinedesign.com/community/article/21837192/whats-the-difference-between-polypropylene-types