Expansion of high flow mixtures of gypsum-bonded investments in contact with absorbent liners

Objectives To evaluate the expansion of high flow mixtures of gypsum-bonded investments in contact with two dry absorbent liners since early setting. Methods Cristobalite-Dentsply™ and Cristobalite-Polidental™ gypsum-bonded investments were tested under 10 setting conditions (n=4): Normal and Hygroscopic (controls); four conditions of ‘Dry-Lining’ (set in contact with 3 different amounts of dry cellulose liner: 0.2, 0.4 or 0.8 g and with 0.1 g of a granulated Super Absorbent polymer liner) and four conditions of ‘Dry-Lining with hygroscopic expansion’ (initially setting in contact with dry absorbents, as ‘Dry-Lining’ conditions, followed by water immersion immediately after loss of gloss). The percent of expansion at 2 h was analyzed. Results The ‘Dry-Lining’ condition, with 0.4 and 0.8 g of cellulose, for brand Dentsply (1.41±0.13 and 1.42±0.16) and Polidental (1.14±0.17 and 1.01±0.22) allowed a significant higher expansion than the controls Normal (Dentsply=0.30±0.04, Polidental=0.31±0.05) (P<0.05). The ‘Dry-Lining with hygroscopic expansion’ condition with 0.8 g of cellulose expanded significantly more (3.57±0.55) than the control Hygroscopic (2.63±0.31) in brand Dentsply (P<0.05). Super Absorbent did not produce different expansion (Dentsply=0.86±0.02, Polidental=0.79±0.18) than the controls Normal and Hygroscopic (Dentsply=2.93±0.17, Polidental=2.12±0.40). Significance The water removal with dry cellulose absorbent liner before loss of gloss increased the potential of setting expansion for Normal and Hygroscopic conditions. Thus, when using high fluid mixtures, which are more desirable for investment pouring, it will be possible to obtain enough setting expansion potential in order to adequately compensate for wax and alloy shrinkage in the casting process.