The rubber seals used in medical devices and personal protective equipment (PPE) are often stored for many years before they are put to use. Choosing the right rubber material and ensuring their proper storage are critical factors in preventing seal failures when these products are needed most. Improper storage can lead to rubber breakdown, causing seals to fail or bands to break when stretched. In some cases, ventilators have required rebuilding after being taken out of storage, and surgical mask straps have broken when pulled on due to inadequate storage practices.
To avoid these crucial failures, consider the following factors when selecting and storing rubber seals for medical devices and PPE:
Selecting the Right Rubber Material
Different types of rubber possess varying properties, and oxidative resistance is a key concern during long-term storage. Oxidative degradation occurs when free radicals from heat, ozone, or UV light attack the rubber polymer chain, leading to deterioration. This can be demonstrated by pulling on an old rubber band that has become brittle.
To combat this issue, rubber compounders can add antioxidants or waxes to improve resistance. However, the polymer itself plays a significant role in determining the level of resistance. Rubbers with more saturated polymer chains, such as EPDM, FKM, HNBR, and silicones, exhibit excellent stability in long-term storage. On the other hand, NBR, natural rubber, and SBR are more susceptible to degradation when not stored properly.
Proper Storage Conditions
The Aerospace Recommended Practice ARP5316, titled “Storage of Aerospace Elastomeric Seals and Seal Assemblies,” provides guidelines for proper storage of assemblies with rubber seals. Some of the key recommendations include:
- Temperature: Maintain storage temperatures below 100°F (38°C)
- Humidity: Keep humidity levels below 75% (AU/EU less than 65%) if not using sealed moisture-proof bags
- Light: Protect seals from light sources, especially direct sunlight or intense artificial light
- Ozone: Ensure storage rooms do not contain any equipment capable of generating ozone
ARP5316 also offers guidance on shelf-life based on the specific polymer type.
Lubrication Considerations
When using lubricants in assemblies, it is crucial to ensure their compatibility with all components. Medical silicone oil is a common lubricant for medical devices, and it is generally compatible with plastic. However, if using a silicone seal, it is important to note that silicone oil can cause swelling due to the “likes dissolve likes” principle. Most standard and medical-grade silicones are dimethyl silicones, so consider using phenyl or fluoro-based silicone oils to prevent excessive swelling that could compromise seal performance after storage.
Alternative lubricants, such as dry lubricant coatings like parylene or cured silicone types can also be considered. These lubricants reduce friction without causing rubber seals to swell and facilitate high-volume automatic assembly by preventing parts from sticking together in vibratory bowl feeders.
When designing medical assemblies for long-term storage, keep these critical considerations in mind. Apple Rubber can assist in designing and selecting the appropriate rubber material for your o-rings, seals, or gaskets to ensure reliable performance when it matters most.
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