Silicone Based Impression Materials

Silicone Based Impression Materials

Write in detail about the manipulation and properties of silicone rubber base impression material.
Answer:

Silicone Rubber Base Impression Materials:

There are two silicone rubber base impression materials, i.e.

• Condensation silicone
• Addition silicone

Manipulation of Silicone Rubber Base Impression Materials:

Following are the methods of manipulating silicone rubber base impression materials, i.e.

1. Hand mixing
2.  Static auto mixing system
3. Dynamic mechanical system

1. Hand Mixing:

• High viscosity material, i.e. putt is usually supplied in small tubs with different colored scoops for identification and dispensing.
• Equal quantities of both pastes are taken.
• Mixing is done by hand by kneading the material together to obtain a uniform mix without any streaks.
• Manufacturers also supply base and catalyst in separate tubes with a much smaller diameter opening for catalyst to enable dispensing equal lengths of base and catalyst, though the volume is different.
• For addition, silicone mixing of low-viscosity material is accomplished by taking equal lengths of base and catalyst on a paper pad and manipulating by stiff bladed spatula
  in a circular motion for 45 seconds till the mix is free from streaks.
• For condensation silicone, the quantity of reactor paste needed is very little.
• So the base and reactor paste is extruded on a mixing pad with unequal lengths of base and catalyst on a paper pad and manipulated by stiff bladed spatula in a circular motion for 45 seconds till the mix is free from streaks.

2. Static Automixing System:

• These days many of the materials are now dispensed by an extruder gun with a mixing tip.
• The base and catalyst are stored or supplied in separate cylinders of the plastic cartridge.
• The extruder gun contains two plungers on which these cylinders are placed.
• Now the base and catalyst are forced through the mixing tip, which consists of a stationary plastic internal spiral that brings together the correct volume of both base and catalyst and dispenses it premixed.
• Apply the mixed material directly over the prepared teeth and/or the impression tray.
• The main benefits of the mixing tip includes dispensing of the correct ratio of base and catalyst and uniform mixing without the incorporation of air bubbles.
• Both addition and condensation silicones are available in these systems.

Dynamic Mechanical System:

It is also known as an automated mixing system.

• It consists of a separate mixing machine with cartridges for the base and catalyst and a plastic mixing tip.
• The base and catalyst are supplied in collapsible plastic bags.
• After housing of the base and catalyst in the cartridge, the mixing tip is placed in the front of the machine.
• On pressing the button on the top, the machine automatically mixes the base and catalyst by the movement of the plungers pushing against the collapsible plastic bags.
• Here in this system, the internal spiral is a motor drive that rotates. A uniform, thorough mixing can be accomplished even for higher-viscosity materials.
• In this system, benefit is of uniform mixing and the speed of mixing, and the disadvantage is the equipment is expensive.
• Additional silicones are available in these systems.

Properties of Condensation Silicone:

• It has pleasant color and odor.
• Its setting time is 6 to 9 minutes, while the mixing time is 45 seconds.
• It is non-toxic but direct skin contact should be avoided.
• Excellent reproduction of surface details.
• Dimensional stability is comparatively less because of high curing shrinkage (0.4 to 0.65).
• Tear strength (3,000) g/cm is lower than the polysulfides.
• It is stiffer than polysulfide.
• It is hydrophobic, so field should be dried before making an impression.
• It can be plated with silver/copper.
• Shelf-life is slightly less than polysulfide.

Properties of Addition Silicone:

• Its odor and color is pleasant.
• It can lead to allergic reactions.
• Reproduction of surface details is excellent
• The setting time is 5 to 9 minutes. The mixing time is 45 seconds.
• Working time is extended by chilling the tubes.
• Dimensional stability is good. Curing shrinkage is low (0.17%) and permanent deformation is very low, i.e. 0.05 to 0.3%
• Tear strength is good, i.e. 3,000g/cm.
• Material is hydrophobic so care should be taken while pouring impression.
• It can be electroplated from silver or copper
• Flexibility is low. Extra spacing should be provided in the impression tray.
• Care should be taken at the time of removing stone cast from the impression to avoid breakage.
• Its shelf-life ranges from 1 to 2 years.