Effect of Pink Coloring Liquid on Mechanical and Optical Properties of 3Y Zirconia

To evaluate the effect of pink coloring liquid on the mechanical and optical properties of the 3 mol% yttria stabilized zirconia (3Y).

Discs of 1 mm and 2 mm were fabricated from 3Y zirconia (IPS E.max ZirCAD LT). Pre-sintered discs were randomly allocated into four groups (n = 10 per group) including control group with no coat of pink coloring liquid (C0), one coat (C1), three coats (C3), and five coats (C5) of pink coloring liquid. Discs were then sintered according to the manufacturer's recommendation. Translucency parameter (TP) of the 1 mm thick discs was evaluated by measuring L*a*b* values against a black and a white background with a spectrophotometer, and both TP_ab and TP₀₀ were calculated. Surface roughness of the samples after sintering was measured using a profilometer. Biaxial flexural strength of 1 mm thick specimens was measured according to ISO 6872. Surface hardness was measured by Vicker's hardness. One-way analysis of variance (ANOVA) was used for the statistical analyses with the significance level set at 5% (α = 0.05).

Applying different coats of pink coloring liquid resulted in a significant reduction in TP and biaxial flexural strength (p < 0.001). Increasing the number of applied pink coloring liquid coats reduced the TP and biaxial flexural strength further. Application of pink coloring liquid did not alter the surface roughness and hardness of the sample (p > 0.05).

Within the limitations of the present study, applying pink coloring liquid on zirconia in green state altered the translucency and biaxial flexural strength of sintered zirconia.

When applying pink coloring liquid to zirconia restorations in the green state, caution needs to be taken in the number of coats applied as a higher number of coats (3 and 5 coats) of currently tested pink coloring liquid reduce the strength and increase the opacity of the zirconia.