Silicone Rubber Impression Materials
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These materials were developed to overcome some of the disadvantages of polysulfide materials, such as their objectionable odor, the staining of linen and clothing by the lead dioxide, the amount of effort required to mix the base with the accelerator, the rather long setting times, the moderately high shrinkage on setting, and the fairly high permanent deformation.
Silicone Rubber Impression Materials in Dentistry Types
Two types of silicone impression materials are available based on the type of polymerization reaction occurring during its setting.
- Condensation silicones
- Addition silicones
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Both silicones are available in a variety of colors, such as pastel pinks, purples, blues, greens, oranges, etc. Different viscosities may be identified by their color.
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Condensation Silicone
This was the earlier of the two silicone impression materials. It is also referred to as conventional silicones.
Condensation Silicone Supplied As
Paste Supplied as two pastes in unequal sized collapsible tubes. The base paste comes in a larger tube while the catalyst paste is supplied in a much smaller tube.
Putty The putty is supplied in a single large plastic jar. The catalyst may be in paste form or sometimes it may be supplied as a liquid.
They come in a variety of colors. The base and accelerator are typically in contrasting colors (which aids mixing).
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Condensation Silicone Available in three viscosities
- Light bodied
- Medium bodied
- Putty
Commercial names Sil 21, Coltex, Dent-a-scon, etc.
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Condensation Silicone Composition
Base
Reactor paste/accelerator
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Condensation Silicone Chemistry And Setting Reaction
It is a condensation reaction. Polymerization occurs as a result of crosslinking between the orthoethyl silicate and the terminal hydroxy group of the dimethyl siloxane, to form a three dimensional network. Stannous octoate acts as the catalyst. The reaction is exothermic (1 °C rise).
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The ethyl alcohol formed as a by-product evaporates gradually from the set rubber leading to shrinkage.
Tray adhesive The adhesive for silicones contain poly (dimethyl siloxane) or a similar reactive silicone, and ethyl silicate. Hydrated silica forms from the ethyl silicate to create a physical bond with the tray, and poly (dimethyl siloxane) bonds with the rubber.
Condensation Silicone Properties
- Pleasant color and odor. Although nontoxic, direct skin contact should be avoided to prevent any allergic reactions.
- Setting time is 6–9 minutes. Mixing time is 45 seconds.
- Excellent reproduction of surface details.
- Dimensional stability is comparatively less because of the high curing shrinkage (0.4–0.6%), and shrinkage due to evaporation of the ethyl alcohol by-products. To avoid this the cast should be poured immediately. The permanent deformation is also high (1–3%).
- Tear strength (Box 16.2) (3000) g/cm is lower than the polysulfides.
- It is stiffer and harder than polysulfide. The hardness increases with time. The spacing in the tray is increased to 3 mm to compensate for the stiffness.
- It is hydrophobic. The field should be well-dried before making an impression. Care should also be taken while pouring the cast to avoid air entrapment.
- Can be plated with silver/copper. Silver-plating is preferred.
- Shelf life is slightly less than polysulfides due to the unstable nature of the orthoethyl silicates.
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Addition Silicones (Polyvinyl Siloxane)
These materials were introduced subsequent to the introduction of the condensation silicones. These new materials had better properties when compared to the condensation silicones. It is also known as polyvinyl siloxane. Currently, the addition silicones are very popular and is perhaps the most widely used elastomeric impression material worldwide.
Addition Silicones Supplied As
- Tubes The base and catalyst pastes come in equal sized tubes (unlike condensation silicones). The different viscosities usually come in different colors like orange, blue, green, etc.
- Cartridge form with static mixing tips For use with a dispensing gun.
- Putty jars Two equal sized plastic jars—containing the base and catalyst.
- A larger electric driven auto dispenser and mixing device is also available (Pentamix—ESPE). This machine stores larger quantities. At the press of the button, it dispenses and mixes the material.
Addition Silicones Available in four viscosities
- Light bodied
- Medium bodied
- Heavy bodied
- Putty
Representative Commercial Products Reprosil, Provil, President (Coltene), etc.
Composition
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Addition Silicones Chemistry And Setting Reaction
It is an addition reaction. In this case, the base polymer is terminated with vinyl groups and is crosslinked with silane (hydride groups). The reaction is activated by the platinum salt.
There are no by-products as long as, there is balance between the vinyl siloxane and the silane siloxane. If unbalanced, hydrogen gas is produced causing air bubbles in the stone models. To avoid this, palladium is added to absorb the hydrogen.
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Addition Silicones Properties
- Pleasant odor and color.
- This may also cause allergic reaction so direct skin contact should be avoided.
- Excellent reproduction of surface details. Polyvinyl siloxanes are currently considered to reproduce the greatest detail of all the impression materials. The international standard for dental elastomeric impression materials states that a type 3 (light bodied) impression material must reproduce a line 20 μm in width. With the exception of the very high viscosity putty materials, all polyvinyl siloxanes (light, medium and heavy body) achieve this. Very low viscosity materials can reproduce lines 1–2 µm wide.
- Setting time ranges from 5 to 9 minutes. Mixing time is 45 seconds. Working time may be extended by chilling the tubes. Gains of up to 90 seconds have been reported when the materials are chilled to 2 °C.
- It has the best dimensional stability among the elastomers. It has a low curing shrinkage (0.17%) and the lowest permanent deformation (0.05–0.3%).
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- Early materials had the problem of hydrogen gas formation. If hydrogen gas is liberated pouring of stone is delayed by 1–2 hours to prevent formation of air bubbles in the stone cast. Current materials do not have this problem because of the addition of palladium.
- It has good tear strength (3000 g/cm).
- It is extremely hydrophobic, so similar care should be taken while making the impression and pouring the wet stone. Some manufacturers add a surfactant (detergent) to make it more hydrophilic.
- It can be electroplated with silver or copper. However, hydrophilic silicones are more difficult to electroplate because of the surfactant added.
- It has low flexibility and is harder than polysulfides. Extra spacing (3 mm) should be provided in the impression tray. Care should also be taken while removing the stone cast from the impression to avoid any breakage.
- Shelf life ranges from 1 to 2 years.
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