Enhancing strength and rigidity of polymers used for non-medical applications is often achieved by adding glass, carbon, and other fibrous materials to the polymer during melt compounding. These fillers are too large for the extremely small geometries and thin wall sections of minimally invasive medical devices. In such applications, nano clay reinforcement of polymers is far more effective.
The critical factor for the success of a polymer reinforcing additive is the aspect ratio (or length-to-thickness ratio). Additives with aspect ratios of less than 20:1 do not generally provide meaningful strength enhancements to be considered reinforcing agents. Thus, length reduction of traditional fiber reinforcements to accommodate small medical devices is insufficient.
Nano clays are platy minerals which have high aspect ratios, and a thickness dimension measured in nanometers. Â Aspect ratios for these clays range from 300:1 to 1,500:1. Clay particles of this size closely approach the size of the polymer molecules. The result, when melt blended into the polymer, is immobilization of portions of the polymer chain that creates a reinforcement effect.
Because nano clays contains so many individual particles in a small volume, it takes very low loading to obtain a high concentration of constrained areas within the polymer. This leads to noticeable mechanical strength enhancement of the polymer with small amounts of additive. For example, reinforcement of a nylon using 5% nano clay is roughly similar to 10-15% traditional glass fiber.
Two characteristics of nano clay particles that are critical for success as polymer reinforcement are purity and cation exchange capacity. Purity is important in achieving maximum increases in mechanical properties. Impurities act as stress concentrators, resulting in poor impact and tensile properties. Cation exchange capacity provides the surface activity necessary for acceptance of surface treatments to the clay. This treatment is required for the small particles to be effectively dispersed in the polymer matrix.