I am hoping someone can shed some light on a problem I am facing.
I am seeing brittle failure of a Polyether block Amide material (Trade name PEBAX). The material is in the form of an extruded tube, approx .010" wall thickness. The material is heat processed after extrusion by heating the surface to melting and "reflowing" it to create a "glassy" surface. The tiniest imperfection on the surface creates a stress point for brittle failure. What is going on here? How do I prevent it? Is there anything to be done to the material to reverse the brittle characteristics?
I'm not sure how familiar you are with the process, so I'll hit on some basics. Please forgive me if I am too "elementary"....
Two mat'l specs to look at are the impurities and the other is gel count. Both are usually listed on the Atochem/Atofina/Arkema C of A. I believe the polymer made in EU has a lower gel count spec then the grades made in the US. When they dropped the SA00 grades and went exclusive to the SA01 grades, the gel count max spec was raised from 7 to 9 or something like that.
For the process, filtration is important, as is ensuring there are no "dead" spots or hang up areas in the tooling (mating areas are always suspect). Helicoid (if using a cross head), breaker and/or melt filter design are all important as well. If using a melt filter, these are great for reducing the pressure drop across the screens, but are notoriously ill designed for a good streamlined flow and can greatly increase residence time. A flow analysis of your filtration and tool design may help. Also look at the fit of the helicoid (again, if using a crosshead) in the head cartridge. It should be almost to the point where you need to run hot water over the cartridge and cold over the helicoid to put them together. Any slop is an opportunity for degradation (and in our business, we are always just a step away from degradation !) Melt pumps are opportunities for degradation as well.
Pebax grades that are at the lower end of the IV Arkema specification can have issues with brittle fracture (lower Mw). See if there is a correlation to your parts.
What is the residence time of the polymer in the extrusion process? I would think that with an 0.010 wall and guessing that you are using a higher durometer (70D-72D) for proximal catheter shafts of the standard 4-5-6-7 French variety, you should have high line speeds that would put your process up in the 50-60 RPM range minimum (line speed dependent on screw geometry). If you are using a Harrell, get the line speeds up even higher, as these have a high volume of material in the front end of the tooling that greatly increases residence time (never was very fond of them). A distributive/dispersive mixing screw can help, but only if you can put some shear into the material (RPM up!!!).
That's about all I can say without giving up the farm!
Foster Corp is now a N. American distributor of Pebax. You should be able to get some info from them if you are a customer.
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