• Chairat Wiwatwarrapan B.Sc., M.S.
• Natthawadee Liamcharoen D.D.S., M.S.

Objective To study the effect of ascorbic acid (AA) on compressive strength, setting time and
setting expansion of an experimental polyacrylamide-modified type IV gypsum.

Materials and methods The experimental type IV gypsum powder was composed of 97.5% a
commercial type IV gypsum and 2.5% acrylamide. There were a control group (commercial type IV
gypsum) and 5 experimental groups which contained constant potassium persulfate initiator (0.005 M),
and various amounts of AA activator, including 0.005, 0.013, 0.026, 0.052 and 0.105 M. Compressive
strength test was performed by preparing 15 cylindrical specimens of each group with 10 mm in
diameter, and 20 mm in height. The compressive strength of 5 specimens were measured by UTM
(Instron 8872) at a crosshead speed of 1 mm/min at 1, 24, and 48 hours after mixing. The data were
analyzed with One-way ANOVA and Bonferroni statistic method at the 95% confidence level.
Setting time test was performed by using Vicat needle apparatus for 5 specimens per group, the data
were analyzed with Chi-square with Monte Carlo approach at the 95% confidence level. The
Extensometer was utilized for the setting expansion test for 5 specimens per group. The changes of
the length were determined at 120 min after the start of mixing. The setting expansion was calculated
in percentages of the original length, and then the data were analyzed with the Independent-Samples
T test and One Sample T test at the 95% confidence level.

Results The 0.026 M AA group had the highest average compressive strength in each time interval of
testing. However, this group exhibited significant higher compressive strength at 1 and 24 hours than
the control group (p < 0.05). In addition, the 0.026 M AA group showed less setting time than the
control group (p < 0.05). Although in the setting expansion testing, this group did not show significant
difference from the control group, its mean expansion value was lowest among other groups.

Conclusion Within the limitations of this study to improve the quality of type IV gypsum material,
adding 0.026 M AA is the most suitable method to activate the polymerization reaction.

(CU Dent J. 2010;33:197-206)