Eliminating volatile organic compounds (VOCs) and leachables from silicone rubber using thorough post-curing processes

DATE: 29th-May-2019

Silicones are widely used in food, medical and pharmaceutical industries due to their excellent stability over a wide range of temperatures, mechanical strength, elastic properties and non-toxic material characteristics. However, such industries who typically use silicone for sensitive applications have extremely strict tolerances on leachables and volatile organic compounds (VOCs). Without a dedicated post-curing process, outgassing of silicone elastomers can occur under high temperature and low pressure environments1. At Jehbco, silicone items are subject to a thorough post-curing process to ensure that leachable substances present in the extrusions are eradicated.

Jehbco Silicones medical grade tubing manufactured in Australia

Jehbco Silicones medical grade tubing manufactured in Australia

 

Without post-curing, there are two major constituents that can be present in the silicone rubber. The first category encompasses volatile organic compounds2. These carbon containing compounds are typically only present in silicones cured with a peroxide catalyst. During the extrusion process, the peroxide catalyst facilitates cross-linking of silicone by joining methyl and vinyl groups between silicone chains. However, when the elastomer is fully cross-linked, the organic peroxide migrates to the surface of the extrusion. This process is extremely slow, and is accelerated at higher temperatures. The residual peroxide and peroxide by-products form the majority of VOC content found in silicone rubber. These are eliminated by using a platinum catalyst. Here, the platinum catalyst stays is embedded strongly in the rubber without forming any volatile compounds, and does not leach into its surroundings. The only VOCs found in platinum are derived from degradation of carbon based side-groups on the siloxane chains.

The second category of leachables are uncrosslinked low molecular weight siloxanes. Silicone rubber consists of long-chain molecules made by repeating units of methylsiloxanes and methylvinylsiloxanes2. While most of these are cross-linked completely, some short-chain molecules are not crosslinked, and their small molecular size means they are free to move within the silicone bulk. Thus under the proper conditions, these molecules can leach into their surroundings. Typically, more low molecular weight compounds are found in platinum curing materials due to the curing mechanism requiring a Si-H containing siloxane to be added in excess, as such these compounds do not fully cure into the material. It should be noted that although present, low molecular weight siloxnaes have no known negative biological effects2. Low molecular weight siloxanes are present in both peroxide and platinum catalysed compounds, which is why even platinum compounds require thorough post-curing to facilitate to pass strict regulations required for products used in sensitive industries. A rough quantitative analysis of the extractables in non post-cured silicone can be found in Table 1.  

Table 1: Quantitative analysis of extractables found in platinum and peroxide cured silicone2

Platinum Cured Silicone
(50° ShA)
Peroxide Cured Silicone
(50° ShA)
Organic Carbon 2.63 62.73
Low Mw Siloxanes 10241 3837

Typical guidelines for post-curing processes specify 4 hours at 200°C3 or for the product to be post-cured at least 10°C above the specified operating conditions for 4 hours4. The first method ensures that all of the possible leachables are extracted, whereas the second method ensures that only compounds that are likely to leach into the environment under standard operating conditions are removed. In addition to specified post-curing temperatures, post-curing also requires a high airflow to ensure that the leachables are properly vented3,4. To ensure peace of mind for our customers, all of our products are post-cured at 200°C for 4 hours (excluding heating up and cooling down time) to ensure all of the leachables are eradicated from our silicone profiles. This process has some small effects on the mechanical properties of the silicone, typically raising the hardness while lowering the material’s elastic properties5. The compression set of the material is typically improved by this process5. At Jehbco, platinum and peroxide products are never post-cured in the same unit, to avoid the peroxide leachables damaging the platinum cured silicone3.

Due to our thorough extraction process, Jehbco silicone products exhibit extremely low levels of leachables, far lower than required under food grade standards such as FDA 21 CFR 177.2600 Rubber Articles intended for repeated use. The extraction tests specified in this standard require leachable levels below 4 mg/in2 after 2 hours of continuous extraction in hexane at reflux temperature, or 1 mg/in2 after 2 hours of continuous extraction in water at reflux temperature. These are in addition to further limits at different time intervals. Jehbco products sent for leachable testing have returned results significantly lower than required, a testament to our processing capabilities.

If you would like to learn more please contact us or browse our articles.

References

  1. Rothka, J., Studd, R., Tate, K. and Timpe, D. (2002). Outgassing of Silicone Elastomers. ISC, pp.1-9.
  2. Malczewsk, R., Jahn, D. and Schoenherr, W. (2003). Peroxide or Platinum? Cure System Considerations for Silicone Tubing Applications. Dow Corning Healthcare.
  3. Wacker Chemie AG (2016). SOLID AND LIQUID SILICONE RUBBER MATERIAL AND PROCESSING GUIDELINES. Munich, pp.66-67.
  4. Xiameter (2009). Fabricating with XIAMETER High Consistency Silicone Rubber. USA, pp.45-47.
  5. Inman, W. (2016). Post-Curing of Silicone Elastomers: When is it Necessary?. Technical Service & Development Silicone Elastomers.