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.
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
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