Ask Dr. Dave
July 1, 2011
Adhesives and sealants troubleshooting tips from Dr. Dave Dunn. Question: We are using an ultraviolet (UV) adhesive to bond and seal a polycarbonate medical pump. We get a large reject rate when pressure testing the seal with air. Could you suggest some possible problem areas and improvements?
Answer: Part design could be contributing to the complication. Make sure you have good molded flat parts with sufficient bond area. I have seen some problem designs where sealing issues have been solved merely by increasing the seal area.
In order to get a strong bond and seal, you need to ensure that your adhesive is applied well and is cured completely. It is also crucial to achieve good adhesion to the surface. Many early UV adhesives had problems in bonding plastics because certain plastics don’t transmit UV light very well (particularly those that contain UV absorbers to prevent UV degradation). Remember that if a plastic is transparent, it can transmit the visible part of the electromagnetic spectrum, but it will not necessarily transmit UV light. You can easily investigate this on your plastics by using a UV radiometer. Make sure you measure the intensity of the UV either in the bondline or underneath the plastic.
Some modern adhesives cure using both UV and visible light. When you seal a component this way, you need good adhesion to the surface because-unlike a conventional pre-formed gasket-the seal will not be compressed.
It is also important to ensure that there is no contamination of the plastic surface that might preclude good adhesion. Make sure that mold release agents, which are actually designed to prevent adhesion, are not causing surface contamination.
Some plastics have internal release agents added to them. In particular, silicones can often cause adhesion problems. Silicones are extremely valuable materials in medical device manufacturing as sealants or lubricants. However, I have seen numerous instances where the use of silicones in a clean room has caused adhesion problems with other adhesives. Do not forget that silicones can “creep” over surfaces due to the handling of parts, and many silicones contain volatile cyclic siloxanes that can move through the air and condense on nearby parts.