Determination of residual stresses in denture base polymers using the layer removal technique.

In order to study the influence of residual stresses on dimensional accuracy and mechanical properties of denture bases, an experimental model was devised for measurement of residual stresses in acrylic denture base polymers. Rectangular bar coupons were cut from resin plates heat processed by conventional dental methods. They included samples which had been slow and fast cooled after polymerization and had been stored in dry and wet environments. Uniaxial residual stress distributions for each coupon were disclosed by removing layers of known thicknesses and measuring the ensuing deflection of the specimens. Coupons of a commercial acrylic resin were annealed and used as controls. ANOVA and Scheffe's test were used to compare experimental conditions at the 95% confidence level. Dry specimens exhibited residual surface compressive stresses from 0.43-0.83 MPa (62-120 psi). Water-stored specimens showed higher stress levels, 2.86-3.24 MPa (414-470 psi). Slow cooled pigmented acrylic coupons which were dry showed higher residual stresses compared with other dry but nonpigmented specimens. No significant differences were found between pigmented or clear coupons which contained moisture.