Facial moulage: The effect of a retarder on compressive strength and working and setting times of irreversible hydrocolloid impression material

Statement of problem. Irreversible hydrocolloid is widely used as an impression material for fabrication of extraoral maxillofacial impressions. A disadvantage of irreversible hydrocolloid, however, is its limited working time. Purpose. This study tested the compression strength (elastic recovery) and working and setting times for an irreversible hydrocolloid impression material after a retarder was added. Material and methods. The irreversible hydrocolloid (Jeltrate) was mixed with water in a standard water/powder mixture of 18 mL (I unit) of water and 7 g (I unit) of the impression material. Test specimens (n = 3) were prepared by adding 2, 4, 6, and 8 drops of monobasic sodium phosphate (the retarder) to each. Three other specimens, to which no retarder was added, served as the control. Specimens were prepared according to the American National Standards Institute/American Dental Association's specification no. 18 guidelines for irreversible hydrocolloid impression material. Immediately after the specimens were prepared, the flat end of a polished rod of poly(methyl methacrylate) was placed in contact with its exposed surface and quickly withdrawn. The working-time experiment was a pass/fail test conducted 30 seconds before the initial setting time. The initial setting time was established as extending from the start of the mix to the time when the impression material no longer adhered to the end of the rod. To determine how the compressive strength of the modified irreversible hydrocolloid (with retarder added) compared with that of the control, the mean stress at maximum load (Mpa) was analyzed. The compressive strength (MPa) was calculated. Statistical analysis consisted of descriptive statistics and regression analyses. Results. The results of this experiment demonstrated that the elastic recovery of the irreversible hydrocolloid did not change with the addition of sodium phosphate (2 to 8 drops). The percent recovery with was 95.95% +/- .42%, 96.33% +/- .82%, and 96.28% +/- .53% for 0 (control), 2 and 8 drops, respectively. Average setting times for control specimens were 2 minutes 20 seconds +/- 0 seconds for the control specimen (0 drops) to I I minutes 20 seconds -_ I minute 50 seconds for the test specimen with 8 drops. There was a direct relationship (R-2 = 0.85) between the number of drops added and the setting time. All control specimens and test specimens containing 2, 4, 6, and 8 drops of monobasic sodium phosphate produced acceptable levels of elastic recovery (greater than or equal to95%). Conclusion. Within the limitations of this study, predictable longer working and setting times were demonstrated for the irreversible hydrocolloid specimens with I to 8 drops of the sodium phosphate solution tested. The specimens with 8 drops of retarder solution exhibited variable setting times and would not be suitable for clinical use. The compressive strength of the modified irreversible hydrocolloid material tested was compromised because of the addition of sodium phosphate solution; however, recovery from deformation remained satisfactory as retarder solution was added.