Impression materials Viva voice questions
- what is syneresis?
fluid exuded from the impression seen as droplets on surface are called syneresis
- What is imbibition?
Absorption of water by the impression when placed in water is called imbibition.
- which impression shows least dimensional change upon disinfection?
Addition silicones have very little residual polymerization, making them the most dimensional you stable of all the dental materials. They are also highly biocompatible impression materials.
- what is suspension?
Larger, visible, non-soluble particles in solvent. It exists as 2-phase system.
- what is Emulsion?
Liquid droplets which are suspended in water.
- Define colloids or Sols?
A solid, liquid, or a gaseous substance made up of large molecules or masses of small molecules that remain in suspension in a surrounding continuous medium of different matter.
- what is Accelerator?
A compound that speeds up the reaction; also refers to the component called the catalyst in the reaction of impression materials.
- what is Cast?
A dimensionally accurate reproduction of a part or parts of the oral cavity or extra oral facial structures produced in a durable hard material.
- Define Addition reaction?
A polymerisation reaction in which each polymer chain grows to a maximum length in sequence and no reaction byproduct is formed.
- Define Condensation reaction?
A polymerisation process in which the polymer chains grow simultaneously and a reaction by-product is formed with associated shrinkage.
- What is Cure?
The reaction process that takes place primarily during the setting of a polymer but continues after setting.
- Define Working time?
The total time from the start of mixing to the final time at which an impression tray can be fully seated without distortion.
- Define Setting time?
The elapsed time from the star of mixing until the impression material becomes firm enough to resist permanent deformation .
- what is permanent Deformation?
Irreversible change in shape that occurs when the polymer responds as a viscous liquid under an applied pressure.
- what is pseudo plastic behaviour?
Characteristic of a material to become more fluid when an applied force is increased; this behaviour involves shear thinning and is strain rate dependent.
- what is dashpot?
An element of the viscoeladtic model describing the viscous response of a polymer.
- what are the ideal properties of an Impression material?
- They should be fluid enough to adapt to the oral tissue.
- they should be viscous enough to be contained in the tray that is seated in the mouth.
- while in the mouth, they should be transform into a rubbery or a rigid solid in a reasonable amount of time. Ideally the total Setting time should be less than 7 minutes
- The set impression should not distort or tear when removed from the mouth
- the impression should maintain itsdimensional stability after removal of a cast so that a second or third cast can be made from the same impression.
- The materials should be biocompatible
- The materials, associated processing equipment, and processing time should be cost-effective.