Industrial-grade materials commonly find their way into medical designs due to the R&D time crunch. My experience has shown that industrial-grade O-rings are more readily available than their medical- grade equivalents, so engineers will tend to obtain industrial-grade samples to test the functionality of their design.
Testing actual physical prototypes is always a better decision, since designs that appear satisfactory on paper might not always work in actual manufactured parts. The down side is that these industrial-grade samples work their way into the final specification when they are not the best choice for the application. Another down side is that industrial- grade materials may not satisfy regulatory requirements. That is why it is important to select the correct rubber material that will work in the final product as soon as preliminary testing is completed.
It is projected that the global rubber gasket and seal market will reach $30 billion by 2017. Much of that market is automotive uses. This is why the most-stocked material is Nitrile (Buna-N) rubber, ASTM D1418 designation NBR. This material is very good for automotive applications. It provides superior resistance to petroleum-based oil and fuels typically found when sealing in this industry. Many companies keep a large stock of AS568 or ISO 3601 sizes in NBR rubber. Because the parts are in stock, there are very short lead times and easy sample requirements. When the crunch is on, these O-rings are the easiest to obtain and test.
Why Use Medical-Grade Rubber?
Most rubbers exhibit slight variations in physical properties but will perform similarly in most applications. The problem is not performance, but getting the material approved by regulatory agencies. For medical applications, medical grade rubbers (USP Class VI or ISO 10993) should be used, like silicone or ethylene propylene (EPDM). These compound are very stable and can be compounded with only a few ingredients. These materials will not typically have the same regulatory issues that NBR might have, as shown in the regulatory list in Table 1.
Dioctyl phthalate (DOP or DEHP) is commonly used in NBR compounding. Phthalate plasticizers provide exceptional resistance from extraction and very good low-temperature properties for automotive seals. With new regulations globally, many of these low molecular weight phthalates are being banned from medical products, especially when in contact with toys for small children. New regulations are restricting use of Diisononyl phthalate (DINP), which was used to replace DEHP. Without knowing how the NBR samples are formulated can lead to nightmares when going through regulatory approval. This can lead to actions from special markings being required to show the presence of phthalates, up to complete rejection.
Many of the industrial-grade NBR O- rings are not processed in controlled environments. This can lead to bioburden issues or cytotoxicity concerns. Many seal manufacturers have built cleanrooms or controlled environments to process medical-grade materials. This will reduce the chances of foreign material in the finished goods. Also, controlled cleaning centers are used, which control the total organic carbon and pyrogen levels.
Another common failure for NBR O- rings are cracks that form over time in the rubber. This is the result of ozone or UV exposure. Over time, ozone will attack the NBR polymer due to unsaturation in the polymer chain.
Figure 1 shows O-rings composed of three different rubber compounds exposed to 25 pphm ozone for 70 hours. This is considered a very low exposure test. Most ozone resistance grades are exposed at 50 to 100 pphm for 168 hours. The top O-ring is NBR. As you can see, this O-ring has cracks caused by the breakdown of the polymer. The second and third O-rings down are EPDM and silicone, respectively. Both of these polymers are very stable and can handle the ozone exposure. This is very important for when product might be stored or has repeated uses and is exposed to the atmosphere.
Although it might be a quick way to test a design, getting the correct O-ring or seal material for a medical application is critical. It is best to either request the correct material when asking for samples, or follow up with getting correct material after preliminary testing is conducted. This will assure you have the more robust design and an easier experience with regulatory agencies. NBR also has issues with different sterilization techniques. As shown with ozone, gamma and ETO sterilization can also cause cracking or decomposition of the NBR material.
This article was written by John Tranquilli, Materials Manager, Apple Rubber Products, Inc., Lancaster, NY. For more information, visit http://info.hotims. com/49750-164.