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Polyolefins are finding increased usage
in medical applications because of their inertness and excellent
chemical resistance. Some of the medical products are expected to
be sterilized by either of the two processes : *Autoclaving; where
the material is expected to withstand temperatures of more than
121 degree C for at least 20-30 minutes or
Gamma irradiation; generally gamma irradiation involves dosage of
about 50kGy.
For the products that are autoclaved, Polypropylene(PP), because
of higher heat deflection temperatures, is more suitable than Polyethylenes.
Among Polypropylenes, homopolymer grades have the highest heat deflection
and are therefore more suitable compared to copolymers. On the other
hand, PP, because of its tertiary Methyl group, is less resistant
to high-energy gamma irradiation. Polythylenes, particularly HDPE
and LDPE are thus more suitable. When PP is exposed to gamma irradiation
it develops a yellowish colour and also tends to become brittle.
Resistance can be enhanced by incorporation of Ethylene groups in
PP. In fact, both random PP copolymer and impact copolymer provide
better gamma resistance compared to homopolymer. PP copolymers alone
are not able to provide sufficient resistance to gamma irradiation
and are formulated with special additives for improvement of resistance.
PP is required to be stabilized with antioxidants at higher levels
to prevent chain scission.
>Generally antioxidants are based on hindered Phenols. These Phenols
develop yellowish colour when exposed to the Gamma irradiation.
These hindered Phenols should therefore not be used. Instead, HALS
and Phosphites/Phosphonites should be used to improve the Gamma
irradiation resistance. While monomeric HALS may be suitable, polymeric
HALS with FDA approval perform better and should be used to improve
the performance of PP. To further enhance resistance, a blend with
PE or advanced Polyolefins like Metallocene based PE with higher
content of Octene comonomer should be considered. Higher molecular
weight of the advanced Polyolefins would provide better resistance.
However, typically, material with 1 MFR at about 5-20% levels can
enhance the gamma irradiation resistance. Some of the leading suppliers
of PP like Borealis supply PP grades for the Gamma irradiation sterilization
techniques.
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