Microencapsulation of Metal Catalyst and its Application to Impression Materials

Objectives:

Currently-available dental impression materials are two-paste type materials. This type of materials is based on the mechanisms that include addition cure systems and peroxide-activated cure systems. These two major mechanisms involve the reactivity of vinyl-functional polymers. Addition cure systems require vinyl-terminated polymers, whose chemical bond forming reaction involves platinum-catalyzed hydrosilylation (See Scheme1). Peroxide-activated cure systems, where vinylmethylsiloxane copolymers and/or vinyl t-structure fluids are normally used, involve peroxide-induced free-radical coupling between vinyl and methyl groups. Its initial cross-linking reaction is shown in Scheme2. An addition cure system, which is employed in most of dental impression materials nowadays, consists of two parts; part "A" containing metal catalysts, and part "B" containing hydride-functional siloxane. When not in use, the two parts should be kept separately to avoid a curing reaction between them. Thus the preparation of dental impression material requires mixing of the two parts. This process, however, is prone to introducing air bubbles into the mixture, and obtaining homogeneous mixture requires certain proficiency of an operator. The purpose of this study was to investigate the possibility of producing microcapsules containing rare metal catalysts by Liquid Droplets Coalescence Method (LDCM), and to evaluate the performance of such microcapsules used for one-paste dental impression materials.

Methods:

Schematic images of reaction are shown in Scheme3. O/W emulsion¹, which contains vinyl-terminated siloxane and platinum catalyst, was prepared in advance. Then, sufficiently-emulsified O/W emulsion², which contains trimethylsiloxy-terminated siloxane, was added into O/W emulsion¹ to synthesize microcapsules.

Results:

Microcapsules, having a Core(Pt catalyst with vinyl-terminated siloxane)/Shell(polysiloxane) structure, where small O/W emulsion² droplets were coalesced on the surface of each droplet of O/W emulsion¹, were synthesized. Optical microscopic images of the obtained microcapsules are shown in Fig.1. These microcapsules were easily broken down by pressure.

Conclusion: The results suggested the possibility of producing one-paste dental impression materials.