The new technologies broaden the applicability of Medalist MD-500 Series compounds as the first practical TPE alternative to PVC in medical tubing, according to Elliott Pritikin, senior medical market manager. They enable common-size TPE infusion tubing to achieve bonds exhibiting a retention force significantly greater than the minimum required by device manufacturers, with 99.7% confidence that failures will not occur below the threshold of 35.6 Newton force (8.0 lbf).
“Commercially available adhesive and solvent systems used with PVC tubing either do not enable most TPE tubing to achieve this bonding strength or do not permit sufficient work time for ease of assembly,” Pritikin said. “Teknor Apex has overcome these limitations for TPE tubing in the 65 to 85 Shore A range with development of two patent-pending adhesive systems and one patent-pending solvent bonding system”.
Room temperature (RT)-cured adhesive. This system is designed for slow setting to allow precise control of assembly. Bonds exhibit cohesive rather than adhesive failure, an indication of the high level of bond strength achieved.
Light-cured adhesive. This system exhibits a higher retention force than is achieved with any commercially available light-cured adhesive, according to Pritikin.
Solvent bonding. This system uses commonly available solvents and may eliminate need for multilayer extrusion.
In tests of bonds between traditional connectors and tubing extruded from a 75 Shore A Medalist elastomer, the new adhesive systems exhibited “minus 3 sigma average retention force values” of 50.6 and 41.0 N, respectively, and the solvent bonding system exhibited a force of 43.7 N (see bar graph). These figures reflect a statistical 99.7% confidence level, or subtraction of an average retention force by three times the standard deviation.
The breakthroughs in bonding technology remove remaining barriers to use of Medalist MD-500 Series elastomers in medical tubing. Compared with PVC, the Medalist compounds exhibit comparable crystal clarity and mechanical properties; provide similar clamp resilience and resistance to kinking and necking; have a similar “feel”; and are substantially more flexible and significantly less dense than PVC.
At the same time they undergo minimal color shift upon heat aging after exposure to gamma irradiation, the most severe type of sterilization. A typical compound in the series, Medalist MD-575, actually exhibits 70% less heat-aged color shift than a gamma-stabilized PVC compound of comparable hardness.
Before developing the new systems, Teknor Apex researchers headed by product development manager Dr. Kevin Cai worked to identify more systematically the causes of difficulty in bonding TPE tubing to traditional connectors with conventional bonding systems.
Three causes were identified:
1) the low polarity and low surface energy of TPEs interfere with bonding to traditional connectors, which typically are made of polar materials;
2) the excellent chemical resistance of TPEs causes problems with standard solvent bonding systems;
3) the greater flexibility of TPEs in comparison with PVC can cause TPE tubing to pull away from the wall of a connector when the tubing is stretched.
“This last mechanical problem is often overlooked,” Cai pointed out, “yet it is very important. We refer to it as the ‘poisson effect.’ Our patent pending development has overcome all these hurdles. With one of our newly developed adhesive systems, the bond exhibited cohesive failure, in which the TPE tubing itself actually broke before it could be pulled out from the connector.”