Kinetic aspects of the silver ion release from antimicrobial polyamide/silver nanocomposites

Spherical silver nanoparticles were grown in situ in different polyamides by a thermal reduction of silver acetate during melt processing of the polymers. Most of the particles have a diameter of about 20 nm. The absolute amount as well as the kinetics of the silver ion release from the various polyamide/silver nanocomposites differ strongly, although the filler content in all materials is the same (1.5 wt. %) and the morphologies of the silver particles are not very different. One result of the investigations was that the absolute amount of the long-term silver ion release increases exponentially with the maximum water absorption of the polymers used as matrix materials, because silver ions are formed from elemental silver particles in the presence of water, only. Moreover, it was also found that the long-term silver ion release increases with a growing diffusion coefficient of water in the polymer. The water absorption properties of the polymers govern the kinetics of the silver ion release, too: for strong hydrophilic polyamides like PA6 or PA6.6, which are plasticized by water, the silver ion release is a zero-order process. For nanocomposites with less hydrophilic polyamides like a cycloaliphatic polyamide or a P12 modified with polytetrahydrofurane (PA12-poly-THF), the silver ion release is governed by diffusion. As expected from the efficacy of the silver ion release, PA6, PA6.6, PA12 and PA12 modified with polytetrahydrofurane and a cycloaliphatic polyamide filled with 1.5 wt. % of silver nanoparticles are active against Escherichia coli. But, only nanocomposites with PA6, PA6.6 and P12-poly-THF as matrix materials are suitable as long-term biocidal materials.