Fact Sheet: ChronoFlex® AL
[In pdf format]

ChronoFlex® AL is a family of biodurable thermoplastic polyurethane elastomers developed by us. These medical-grade elastomers have been specifically synthesized to overcome the in vivo formation of stress-induced microfissures. Surface microfissuring is a major weakness of present polyurethane elastomers which may lead to catastrophic failures in long-term implanted devices.

In the early 1980's it became clear that ether-based polyurethane elastomers were susceptible to biologically induced environmental stress cracking. Microcracks formed on the surface of polyurethanes as a result of exposure to the biological environment, particularly where the polymer was subjected to mechanical stress. ChronoFlex® AL is at the forefront of a new generation of aliphatic, ether-free, thermoplastic polymers which do not undergo biologically induced environmental stress cracking. ChronoFlex® AL derives inherent biodurability from its polycarbonate-based structure.

Physical Properties

Handling Extruded Tubing

Following extrusion, ChronoFlex® AL tubing will remain sticky for about 10-15 minutes. The tubing must be protected during this time frame. Afterwards, the tubing surface will become tack-free. Surface tack will diminish as surface crystallization of the hard segments takes place.

Drying Recommendations

ChronoFlex® AL pellets should be dried to a moisture content equal to or less than 0.01% by weight prior to melt processing. Most extrusion problems can be traced to improperly dried pellets.

Since ChronoFlex® AL elastomers are highly hygroscopic (they attract and hold chemisorbed water tenaciously), the pellets must be well dried before extrusion. Unless pellets are properly dried, ChronoFlex® AL polyurethane tubing will exhibit haze, gels, surface craters and, under extreme moisture conditions, will appear "foamy."

We recommend a desiccant-type hopper drier capable of reaching and maintaining a dew point of -40° F. Typically, the pellets should be dried for at least four hours with a drier inlet temperature of 160°-185° F. If the pellets can be pre-dried for extended periods, such as overnight, an inlet temperature of 150° F is generally sufficient.

Melt Processing

Process Equipment Used: Killion Extruder, with a 2" 1/24 (L/D) screw.

Product Advantages

ChronoFlex® AL elastomers are custom synthesized from the reactive monomers. We tailor polymers to specific customer applications. The ChronoFlex® AL family of polymers is aliphatic-based to prevent inadvertent formation of aromatic amines. ChronoFlex® AL polymers can be compounded with radiopacifiers at the prepolymerization stage to ensure maximum dispersion of particulate matter. ChronoFlex® AL polymers are offered in hardnesses ranging from a soft 80 Shore A to a very hard 75 Shore D.

Annealing

ChronoFlex® AL elastomers are exceptionally strong. Even at the recommended extruding temperatures, the polymer retains a large amount of elasticity. This elasticity, or memory, can result in nearly 5% shrinkage of the article when exposed to an aqueous environment. To overcome this shrinkage, we recommend annealing the melt-processed article at 60° C for a minimum of two hours.

This information has been gathered from standard reference sources and is believed to be accurate and reliable. CardioTech makes no other warranty, express or implied, that the results obtained from the use of this product or information contained herein shall be merchantable or fit for any particular purpose, or that the use of this product will not infringe any patent. It is offered solely for your consideration, investigation, and verification.
[In pdf format]