Gypsum Products used in Dentistry


Introduction

Gypsum products used in dentistry are based on calcium sulphate hemihydrate (CaSO4.2H20)
The current ISO Standard for Dental Gypsum Products identifies five types of material as follow:
  • Type 1 Dental plaster, impression
  • Type 2 Dental plaster, model
  • Type 3 Dental stone, die, model
  • Type 4 Dental stone, die, high strength, low expansion
  • Type 5 Dental stone, die, high strength, high expansion
Composition

Dental plasters(plaster of Paris): Dental plaster is produced by a process known as calcinations.
Plaster remains the most important model material for many dental restorations. As the precision of fit of restorations and orthodontic appliances depends on the quality of the dental plaster/stone, they must comply with the high quality demanded in dental technology.
Dental stone
Manipulation and setting characteristics

Plaster and stone are mixed with water to produce a workable mix. Hydration of the hemihydrate then occurs producing the gypsum model or die.

Water/powder ratios for gypsum model and die materials.


Water (ml)
Powder(g)
W/P ratio (ml/g)
Plaster
50-60
100
0.55
Stone
20-35
100
0.30
Theoretical ratio
18.6
100
0.186
For hand mixing a clean, scratch free rubber or plastic bowl having a top diameter of about 130 mm is normally recommended.
Flexible Mixing Bowls

The material should be used as soon as possible after mixing since its viscosity increases to the stage where the material is unworkable within a few minutes.
            The setting reaction is exothermic, the maximum temperature beibg reached during the stage when final hardening occurs.

Control of setting times:
Factor which control the settings of gypsum products can be divided into those controlled by manufactures and those controlled by the operator.
The manufacture can control the concentration of nucleating agents in the hemihydrated powder
Operator controls temperature and W/P ratio and mixing time.

Properties of the set material
The strength gypsum depends, primary, on the porosity of the set material and the time for which the materials is allowed to dry out after settings.
Since stone is always mixed with at a lower W/P ratio than plaster it is less porous and consequently much stronger and harder. (When porous increased, strength decreased).
The ability of dental gypsum products to reproduce surface details of hard or soft tissues either directly or from impression is central to their suitability as model or die materials. This ability is judged by measuring the extent to which accurately machined lines in a block of stainless steel can be reproduced in a sample of the material.

Applications
When strength , hardness and accuracy are required dental stones are normally used in preference to dental plaster.
The stone materials are less likely to be damaged during the playing down .
Thus , these materials are used when any work is to be carried out on the model or die as would be the case when constructing a denture on a model or a cast alloy crown on a die.
                When mechanical properties and accuracy are not of primary importance the cheaper dental plaster is used.

Advantages and disadvantages

Advantages
Inexpensive and easy to use
Accuracy and dimensional  stability

Disadvantages:
Mechanical properties are not ideal.
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