When a materials is heated the extra energy it absorbs causes the atoms and molecules to vibrate with an increased amplitude. As a result the material expands.
Most common way of measuring this expansion is by taking the length of the material, heating it to a certain temperature and then measure resultant change in length
Most often linear thermal expansion is expressed as coefficient of thermal expansion ( ά ).
Defined as change in length per unit length of the material for 1 ºC change in temperature. Units are ppm/ ºC.
For restorative materials it should be similar or close to the tooth structure, otherwise thermal mismatch can give rise to marginal gap formation, breakdown of adhesive bonds and finally failure of restoration.
The presence of metallic restoration in mouth may cause a phenomenon called as Galvanic action or galvanism.
It is the generation of electrical current that the patient can feel
It results from difference in potential between dissimilar materials in opposing or adjacent teeth
Metals placed in an electrolyte (liquid containing ions) have various tendencies to go into solution. Aluminium the alloys of which are used as temporary crowns has great tendency to go into solution and has electrode potential of +1.33 volts.
Gold on the other hand has little tendency to go into the solution as indicated by the electrode potential of -1.36 volts.
In this situation, two opposing teeth where oral fluids acts as electrolytes. The system is similar to that of an electrical cell. When the two restorations touch the current flows because the potential difference is 2.69 volts and the patient feels pain, discomfort and metallic taste.
The same effect is experienced if an aluminium foil from a baked potato becomes wedged between two teeth and contacts a gold restoration. Temporary plastic crowns are used to prevent this problem since they are poor electrical conductors.
Thixotropy when some fluids are sheared at a steady rate viscosity decreases with time, on standing the fluid regain its original viscosity. It occurs due to structural breakdown and reformation. It is exhibited by many dental materials extremely useful in impression materials