Thermoplastic compositions
Introduction :
Thermoplastic compositions belong to the group of non-elastic imprint materials.
They are close to the wax family, they are materials which, under the influence of temperature, see their viscosity change.
With the advent of elastic impression materials, thermoplastic compositions have seen their use limited to well-defined clinical situations.
The most or less well-known expression of thermoplastic compositions is Kerr paste.
- Classification of thermoplastic compositions:
Thermoplastic compositions have been classified into two groups:
- Old thermoplastic compositions; example: Stents.
- Modern thermoplastic compositions; example Kerr paste.
- Properties
- Setting reaction: These materials are thermoplastic, their melting range varies from 40 to 46%, they are heated to 45% then left to cool to oral temperature (37%) before introducing them into the mouth.
- Creep: It can vary from 2 to 6% at oral temperature and from 70 to 85% at 45 degrees.
It should be remembered that thermoplastic compositions soften when heated without any chemical change occurring, this phenomenon is therefore purely physical and perfectly reversible.
- Conductivity: These materials are not heat conductors, which is why they must be heated for a long time without burning them to obtain a uniform temperature throughout the mass.
- The qualities of thermoplastic compositions
- The material must be characterized by uniform softening and suitable for taking impressions in the oral cavity.
- After softening to between 60 and 70%, the material should be completely homogeneous.
- The material must soften at a temperature that does not cause any damage to the covering fabrics of the supporting surfaces.
- After exposure to flame, the material should have a smooth, shiny surface.
- After cooling and shrinkage, the thermoplastic compositions must exactly reproduce the support surface of the future prosthesis.
- Thermoplastic compositions must not be overheated to avoid any alterations by loss of the most volatile constituents.
- It is preferable to fill thermoplastic compositions by immersion in a water bath or on a thermostatically controlled stove rather than passing the material over the flame.
- Thermoplastic compositions remain plastic at 37° and allow a dynamic and functional impression to be made in a complete prosthesis.
- composition of thermoplastic compositions
Thermoplastic pastes have a composition that is kept secret, just like other imprint materials.
The basic elements that compose them are:
-Commercial stearic acid whose melting point is between 50 and 70°C. This acid results from the mixture:
-Stearic acid which ensures plasticity.
-Palmitic acid on which hardness depends.
-Oleic acid which reduces hardness and melting temperature.
A barite chalk charge which constitutes the inert mass of the product and prevents its liquefaction.
-A resin derived from KAURI, this resin can be synthetic and derived from coumaric acid.
Thermoplastic pastes that meet this composition are called old thermoplastic pastes, they are characterized by mixtures of beeswax and resin.
According to PEYTON the composition of modern thermoplastic pastes would be as follows:
-35 to 40% representing the resins:
.Rosin.
.Copal.
-carnauba wax: 10%.
-Stearic acid (paraffin): 5% fatty acid.
-Inert charge, approximately 45%:
.Talc or chalk or barium sulfate.
.Coloring ligaments.
- indications and contraindications
- Directions
In fixed prosthesis: impression of single elements on pulpless teeth or teeth with retracted pulp using a copper ring.
In removable prosthesis: creation of the peripheral joint.
- Contraindications
Whenever there are undercuts because it is a rigid and brittle material .
After taking the impression, removal of the impression tray imposes major disinsertion forces resulting in fracture of the material or irreversible distortions.
- commercial presentation of thermoplastic compositions
Thermoplastic compositions can come in two forms; plates or rods.
Among the most commercialized thermoplastic compositions, we can cite:
-Red Kerr pasta plates.
-The black plates of BAYER XANTYGEN.
-Kerr white pasta.
-Grey Kerr pasta.
-Green Kerr paste.
-Stents.
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