Question: We are bonding a Makrolon PC adapter to a low-durometer polyurethane tube with a medical grade light-curable adhesive. The bond gives great strength initially, but after accelerated aging at 50oC, 60% RH for two weeks, we notice a drop in the bond strength of the adhesive. When we repeated the accelerated aging under dry heat conditions at 60oC, we found the same drop off in bond strength. We just tested the real-time aged parts, and found the same drop off in adhesive strength. It only happens after 4-6 weeks real time and within a few days at higher temperatures. Any ideas on how to solve this?
Answer: Polyurethane tubing is often softened by using a plasticizer. This plasticizer can either be a short-chain or long-chain polymer, where the short chain is highly mobile and comes to the surface easily at both room temperature or higher temperatures, and where the long-chain polymer is not as mobile, and gets tangled in the polyurethane backbone of the tubing. I would recommend evaluating the polyurethane tubing by itself in a high-temp oven to see if an oily film comes to the surface (can it be rubbed off onto a Kimwipe or tissue, or detected by a Gas Chomatograph after a solvent wash/extraction). If an oily film is detected, it can act as a surface contamination and even attack an adhesive bond line during accelerated aging.
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