Objectives: To assess Parylene coated NdFeB magnets for use in orthodontic applications.

Methods: The permeability of the parylene coatings and subsequent corrosion of the underlying magnets in Ringer's solution, artificial saliva and a number of other environments have been studied using weight gain, SEM, XRD, IR and magnetic measurements. Infrared measurements have been found particularly useful for simultaneously studying the progress of water through the parylene and identifying early corrosion products on NdFeB.

Results: Parylene coatings merely delay onset of corrosion of NdFeB magnets in Ringer's solution and artificial saliva. Visual observation, SEM and IR measurements all confirmed that corrosion of the magnet material occurs in less than three weeks in these solutions at body temperature. This agrees with previously reported diffusion data of water through parylene. For 12mm parylene films, as used in the present work, moisture would be expected to have fully penetrated the film after 2 weeks.

Once moisture makes contact with the underlying NdFeB surface, corrosion accelerates and loss of adhesion of the parylene film occurs. Corrosion rates and corrosion products were found to be dependent on the specific test environment. Artificial saliva was the most aggressive environment used in the present work, probably due to the high content of chloride ions.

Using iron and neodymium controls, identification and build up of corrosion products on NdFeB in the different environments has been tracked with IR and XRD. The IR spectra revealed an early corrosion product on NdFeB to be an amorphous iron oxy-hydroxide, FeOOH, which has not been reported previously.

Conclusions: These results imply that parylene is only suitable for short-term use as a NdFeB encapsulant in orthodontic applications. The permeability of the parylene leads to corrosion of underlying magnet in a matter of days, producing voluminous corrosion products rich in Nd, Fe, and B, which may find their way into the body system.

Back to the Dental Materials: VIII - Others-Non-metallic Program
Back to the IADR/AADR/CADR 80th General Session (March 6-9, 2002)

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