Why is a medical silicone molded part more stable than a thermal plastic part?

Question: Why is a medical silicone molded part more stable than a thermal plastic part?

Answer: Molded medical silicone parts are actually more stable in three different respects of the word, so I would like to provide three different answers to your question.

In the first respect silicone molded parts are more stable than plastic parts due to the fact that molded plastic parts have inherent stresses molded into them.  These high stress regions can be viewed by holding a molded plastic part up to a light source and looking through a polarizing filter.  It is not an uncommon practice to anneal plastic parts after molding them to remove these molded in stresses and create a stronger, more stable part that is less likely to crack under pressure.  Silicone parts do not exhibit these inherent molded in stresses due to the way that silicone parts are molded.   Silicone parts are molded by injecting “cold” (55°F – 70°F) liquid silicone material into a heated mold, this allows the silicone to already be in its desired shape before curing.

In the second respect silicone is more stable as a medical device due to the bio-inert nature of silicone material.  In fact, molded silicone parts are nonreactive with just about everything; it takes some pretty aggressive cleaning agents to remove dissolve fully cured silicone, chemicals designed to remove paint, corrosion, and powerful industrial adhesives.  On the other hand, plastics are frequently in the media headlines and shrouded in scares regarding toxicity in materials that people eat off of and interact with every day, BPA, Teflon, and PVC to name a few.  These polymers have received attention for leeching into foods and even drinking water, under the right conditions.

And in the third respect of the word silicone is more thermally stable than thermoplastic.  While thermoplastics and TPEs have come a long way in terms of temperature resistance, to put it simply, they will eventually reach glass transition point at elevated temperatures and at further elevated temperatures they will melt.  Once silicone has been fully cross-linked it can’t be returned toliquid form.  Molded silicone rubber behaves more like wood when exposed to extreme temperatures and will eventually smolder and burn, however these temperatures are considerably greater than those at which plastic will reach liquid state.