Development of laser joining method of thermoplastic resin for denture base and dental alloy

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A strong interface between the denture-base resin and the metal frame of removable partial dentures is necessary to prevent fractures. Until now, thermoplastic acrylic resin of the denture base has only been mechanically joined. We investigated the application of a technique that directly joins the metal and resin materials called the Laser-Assisted Metal and Plastic (LAMP) joining method. This technique has been developed at the Joining and Welding Research Institute, Osaka University, for joining thermoplastic resin used in denture bases to a metal frame. We also confirmed that the LAMP joining method can be used to prepare dentures. In this study, we first investigated the strength of single-lap joint samples of thermoplastic acrylic resin for the denture base (acrylic resin) and a cobalt-chromium alloy prepared employing the LAMP joining method. A high-power laser diode was used for LAMP joining, with the irradiation condition set at an output of 160, 170, 180, 190, and 200 W. Five samples were prepared for each condition. The joined samples were subjected to a shear test, and the tensile load (N) was compared. In addition, we observed the cross-sectional and fracture surfaces of the joined region under a scanning electron microscope. To investigate application of the LAMP joining in dentistry we experimentally joined an existing palatal bar (cobalt bar wire) made of a cobalt-chromium alloy to an acrylic resin, and investigated the joint strength. The cobalt bar wire surface was joined to the acrylic resin using LAMP joining, with the laser focal position set at the bar surface. Cobalt bar wires were prepared with and without surface treatment with sandblasting. The irradiation condition was set at 150, 160, and 170 W. Five samples were prepared for each condition. Cobalt bar wires in the joint samples were subjected to a pull-out test, and the loads (N) were compared with those of non-irradiated samples. The fracture surface of the joined region of the cobalt bar wire that was pulled out was observed under a scanning electron microscope. Testing single-lap joint samples revealed that a predictable, high joint strength was obtained at 180 W. Analysis of variance with the irradiation condition as a factor clarified that the irradiation condition significantly influenced the joint strength. Observation of the cross-sectional and fracture surfaces showed that the, acrylic resin filled the uneven surface of the alloy without a break. This experiment of joining the cobalt bar wire encased in acrylic resin showed that the joint strength of samples without sandblasting was low regardless of the use of laser irradiation. Although laser irradiation increased the joint strength in sandblasted samples, variations were large. Observation of the fractured surface of the joined region showed that while only a small amount of acrylic resin was present in the non-sandblasted sample, the sample was tightly joined with the acrylic resin. This suggests that it is possible to have a strong bond of the cobalt bar wire embedded in the acrylic resin.
2013-04-00

Development of laser joining method of thermoplastic resin for denture base and dental alloy

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