13: Elastic impression materials

13

Elastic impression materials

Figure 13.1 Compression of impression material at the bulbosity of the tooth as tray is withdrawn from the mouth.

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Figure 13.2 Approximate permissible distortions for 100% recovery of elastomerics.

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Figure 13.3 Contact angle of water and castability of dental gypsum into impression materials.

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Table 13.1 Approximate composition of alginate impression material

Component Amount (%) Function
Sodium/potassium alginate 15 Water soluble, reacts with Ca2+
Calcium sulfate 14 Reacts with potassium alginate to form insoluble gel
K2SO4, silicates, borates 10 Counterbalance gel effect on gypsum
Sodium phosphate 2 Retarder for Ca2+/alginate reaction
Diatomaceous earth 70 Filler
Colorant, flavoring, disinfectant Minor/trace

Table 13.2 Problems with alginate impressions

Grainy material
Improper mixing
Prolonged mixing
Excessive gelation
Incorrect W/P ratio
Tearing
Inadequate bulk
Moisture contamination
Premature tray removal
Prolonged mixing
Irregular voids
Moisture/debris
Poor mixing
Rough/chalky cast
Impression not clean
Impression too wet
Premature cast removal
Delayed cast removal
Incorrect mix of gypsum material
Distortion
Delayed pouring
Tray movement during set
Premature tray removal
Incorrect tray removal
Excessive gelation
Bubbles
Overmixing of material
Poor mixing technique

Table 13.3 Polysulfide elastomeric impression material

Base paste Catalyst
Polysulfide polymer, 80–85%
Filler (TiO2, ZnSO4, CuCO3, or silica), 16–18%
Lead dioxide, 60–68%
Dibutyl phthalate, 30–35%
Sulfur, 3%
Minor ingredients, 2%

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Jan 1, 2015 | Posted by in Dental Materials | Comments Off on 13: Elastic impression materials
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