Silicone tubing has many beneficial properties and has been utilized in medical and pharmaceutical applications for over 50 years. It is made from silicone polymers that are extruded then crosslinked and “cured” in to solid form using an assortment of curing methods. The two most common of these methods are platinum-catalyzed addition polymerization (platinum curing) [see Figure A] or peroxide-initiated free-radical polymerization (peroxide curing) [see Figure B]. Platinum-cured silicone tubing is widely accepted in applications where purity is a concern, where peroxide-cured silicone tubing typically exhibits enhanced mechanical strength.
Byproducts
It is important to note platinum curing has no byproducts. Peroxide curing does result in byproducts, which tend to be volatile organic acids [1]. Although a high-heat post-curing method can be employed to drive out many of these impurities, they are a major reason why platinum-cured silicone is often preferred for medical and FDA applications. In addition to its purity, it is sometimes favored for its inherent optical clarity, as peroxide-cured varieties tend to be a bit hazier in appearance, interfering with a user’s ability to visually inspect the contents of the tubing. The tear strength of platinum-cured silicone is usually higher due to the nature of its crosslinks.
A benefit to peroxide-cured silicones is that they typically have superior mechanical properties. In addition, they are generally less expensive. However, their use in the life-sciences industries are limited because of potential liability due to toxicity. To meet the mechanical-performance characteristics of peroxide-cured products, some manufacturers are using platinum-cured low hysteresis silicone, resulting in pump life similar to that seen with peroxide-cured tubing and high-accuracy dosing for peristaltic-pump applications.
Effect of Sterilization Methods on Mechanical Properties
There are four methods most commonly employed for sterilizing non-reinforced silicone tubing. They are electron beam (e-beam) irradiation, gamma irradiation, autoclave (steam sterilization), and treatment with ethylene oxide (EtO) gas.
Irradiation
With respect to e-beam and gamma irradiation, a study done by Adamchuk et. Al. [2] showed peroxide-cured silicone, in particular, exhibited a very significant drop in tensile strength and increase in hardness and tensile modulus (resulting in decreased flexibility), while there was a relatively small change in these values for the platinum-cured sample. Tear strength decreased significantly for platinum and peroxide cured samples, but much more so for the peroxide cured sample. According to the Second Edition of Effect of Steriliaztion Methods on Plastics and Elastomers, some grades of platinum-cured silicone can withstand up to 9 megarads radiation and not experience a significant change in mechanical properties, while peroxide-cured silicone is usually limited to less than 5 megarads [3].
Ethylene Oxide
In the same study, when treated with EtO gas, both samples actually showed an increase in tensile strength and negligible changes in tensile modulus, hardness, and tear strength.
Autoclave
In a separate study, three platinum-cured silicone samples were autoclaved 25 times using three different methods. The methods: flash autoclave (10 minutes at 132°C, at 30psi), standard gravity autoclave (30 minutes at 121°C, at 15psi), and pre-vacuum high-temperature autoclave (30-35 minutes at 121°C). No significant change in physical properties was noted [3].
Summary
For applications where high purity, critical dosing, or repeated sterilization is required, platinum-cured silicone is often the material of choice. Peroxide-cured silicone is a common choice for less demanding (in terms of purity) applications. It is commonly, but not universally, a less expensive alternative to platinum-cured silicone, and often exhibits longer pump life in peristaltic-pump applications.
Figures:
Figure A: Platinum Curing of a Silicone Polymer
Figure B: Peroxide Curing of a Silicone Polymer
Sources
1. dowcorning.com/content/publishedlit/52-1077-01.pdf
2. medical.saint-gobain.com/en/pdf/Saint-Gobain_Silicone_Sterilization_White_Paper.pdf
3. books.google.com/books?id=cvdMtLVeRq4C&pg=PA265&lpg=PA265&dq=autoclave+silicone&source=bl&ots=lggHkGLafR&sig=2ddh8ENuiOLra2d0EOFUZ9wL57Y&hl=en&sa=X&ei=6u12Ud_8MJLE9gTnioHYCA&ved=0CGQQ6AEwBw#v=onepage&q=autoclave%20silicone&f=false<.p>
Photos: Silicones-Science.com