In situations where heat stabilizers could not provide sufficient radiation resistance, specialized radiation stabilizers are needed to maintain color appearance and mechanic strength for sterilization of medical PVC by Gamma or E-beam.
Medical consumables and devices made from PVC are widely sterilized by irradiation with Gamma ray or E-beam (electron beam). But PVC somewhat degrades when exposed to such irradiation, as observed through decrease in mechanical strength and deterioration in color appearance.
Fortunately, medical grade PVC can be stabilized by additives against this type of damage. Even though some commercial-grade PVC compounds and end-products have already claimed radiation sterilizability to certain extent, their Gamma & E-beam resistance still need to be further enhanced by use of stabilizing additives so as to meet more stringent requirements.
Flexible medical PVC
For flexible medical PVC, use of anti-oxidants can serve to boost resistance against Gamma radiation and E-beam. Certain high-performance heat stabilizers also help enhance radiation resistance in addition to heat stability. However, in some cases neither anti-oxidants nor heat stabilizers could provide sufficient Gamma stabilization or E-beam stabilization for sterilization by irradiation in terms of mechanical strength, color & appearance and so on. Therefore a specialized Gamma stabilizer such as RS-08, a clear liquid product provided by Limax Chemical, needs to be used. In a liquid form, RS-08 shows good dispersion in a liquid plasticizer and hence particularly efficient in use as a radiation stabilizer for flexible PVC, which often contains 50 phr or more liquid plasticizer.
The dosages for RS-08, which is in transparent liquid form, are recommended as follows.
- Flexible PVC (plasticizer – 30.0~60.0 phr): 0.05 ~ 0.30 phr.
Rigid or semi-rigid medical PVC
Gamma sterilization or E-beam sterilization could pose much more severe adverse effects for rigid or semi-rigid medical PVC than flexible PVC. Rigid or semi-rigid PVC experiences a pronounced discoloration as well as severe deterioration in mechanical properties after exposure to Gamma radiation or E-beam. In addition, pH shifts and extractables are highly likely to occur. Selection of heat stabilizers is especially important for rigid or semi-rigid PVC regarding Gamma resistance. So far Organotin stabilizers are the only choice for rigid or semi-rigid in this aspect whereas even using high-performance Ca/Zn still ends up with unacceptable Gamma resistance. Anyway, both anti-oxidants and heat stabilizers are far from efficient in stabilizing rigid or semi-rigid PVC against Gamma or E-beam. Therefore the greatest challenge for radiation sterilization of medical PVC actually lies in rigid or semi-rigid applications. As a highly specialized radiation stabilizer, RS-06, a white fine powder, is particularly efficient in protecting rigid PVC against Gamma or E-beam sterilization while contributing to excellent transparency.
The dosages for RS-06, which is in fine powder form, are recommended for rigid PVC as follows.
- Rigid PVC (plasticizer – 0~5.0 phr): 0.30 ~ 0.50 phr (RS-06).
However, for semi-rigid PVC where a relatively large amount of liquid plasticizer exists as compared with rigid PVC, RS-08, a liquid radiation stabilizer, is recommended thanks to excellent transparency. The dosages here suggested here.
- Semi-rigid PVC (plasticizer – 10.0~15.0 phr): 0.10 ~ 0.30 phr (RS-08).
Examples for Gamma & E-Beam Resistant PVC Formulations
- PVC compound for flexible transparent medical tubing: PVC K66 ― 100 phr; plasticizer ― 50 phr; Ca/Zn heat stabilizer ― 2.0 phr; RS-08 ― 0.15 phr; antioxidant ― 0.20 phr
- PVC compound for semi-rigid transparent medical membrane: PVC K66 ― 100 phr; plasticizer ― 10 phr; organotin heat stabilizer ― 1.5 phr; RS-08 ― 0.20 phr; antioxidant ― 0~0.20 phr
- PVC compound for Pharma blister packaging: PVC K66 ― 100 phr; organotin heat stabilizer ― 1.0 phr; plasticizer ― 2.0 phr; RS-06 ― 0.30 phr; anti-oxidant ― 0.20 phr; lubricants ― 0.60 phr; processing aid ― 1.0 phr
- PVC compound for Yankaeur Suction tube: PVC K66 ― 100 phr; organotin heat stabilizer ― 2.0 phr; RS-06 ― 0.40 phr; anti-oxidant ― 0.20 phr; lubricants ― 0.50 phr
There are already some commercial-grade Gamma or E-beam stable medical PVC compounds or end-products in the markets. RS-06 and RS-08 can be added into them to further boost stability against Gamma or E-beam in terms of retention of color appearance and mechanic properties.
Radiation Stabilizer vs PVC Heat Stabilizer
Strong PVC heat stabilizers, such as organotin stabilizers and certain high-performance Ca/Zn stabilizers, can boost radiation resistance, especially for rigid or semi-rigid PVC. However, even those very strong heat stabilizers alone still can not provide sufficient protection for Gamma or E-beam exposure in radiation sterilization in terms of color appearance and mechanic strength. Thus specialized radiation stabilizers are needed on top of PVC heat stabilizers in order to overcome the problems like yellowish discoloration and loss of tensile strength after sterilization exposure at relatively high radiation dose, say, more than 2.5 Mrad for Gamma ray.