The need for soft lining material for dentures has been recognized for many years, and numerous materials have been used for clinical purposes. The purpose of the present investigation was to investigate the most favorable thickness of lining materials and the available conditions for clinical application. The tensile and shear strength, the adhesion to acrylic resin, the change of weight after setting, the absorption of water and solution in water using five commercial soft lining materials (Neosnugger, Mollosil, Simpa, Coe-soft, Visco-gel) were examined. The results were as follows : 1. Thickness of more than 2mm for the soft lining material seemed unnecessary. Thick lining with soft material of low elastic modulus was found to have about the same effectiveness as thin lining with hard material of high elastic modulus. 2. The failure stress of each of the four materials from the denture was greater than the tensile strength of the lining materials, except in the case of Mollosil. 3. The soft lining materials of acrylic type were lower in elastic modulus and in coefficient of viscosity than that of silicone type. 4. The variations of mechanical properties of the materials stored in artificial saliva at 37℃ were greater than those of materials left in air at 37℃. 5. The variations of mechanical properties with time were greater in acrylic type than in silicone type. 6. The absorption of water increased with time in all materials. In the absorption of water, Neosnugger of silicone type was 3.2%, others of silicone type less than 1.0%, and acrylic type 2.5∿3.0%. 7. The changes in weight of the materials resulting from the absorption of water and solution of contents coincided in timing with the changes in mechanical properties. 8. The molecular weight of the substances eluted from the five soft lining materials ranged from 90 to 470. In solubility ratio, silicone type was 4.73∿89.17% and acrylic type 0.01∿0.27%.
