Impression Materials
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AGAR
Agar (reversible hydrocolloid) is a “water-based” impression material that can change from a gel to a sol (and back) when heated and cooled. Reversible hydrocolloids are not set by chemical reaction, they simply change state in response to temperature changes. In material science, hysteresis can describe a situation where the melting temperature and freezing temperature do not agree. Agar, for example, melts at 85 °C and solidifies at 32-40°C. Reversible hydrocolloids are composed of Agar (12–15%), Potassium sulfate (1.7%), and water (85.5%). It may contain traces of borax for strength. The high water content makes them unstable.
Agar impression materials require specialized equipment, must be poured immediately, can only be poured once, and exhibit poor tear strength. The conditioning bath for reversible hydrocolloid impression materials contains three compartments. The first bath is used for liquefying the material, starting at 100°C and cooling to 65.5°C. The second bath becomes the storage bath. The third bath is kept at 43°C and is used for tempering the material after it has been placed in the tray. Agar is rarely used as it has been replaced by superior materials.
ALGINATE
Alginate (irreversible hydrocolloid) is another “water-based” hydrocolloid consisting of two main reactors, sodium alginate (18%) and calcium sulfate dihydrate (14%). These chemically react (double decomposition reaction) to form an insoluble calcium alginate gel. Diatomaceous earth or silica (56%) is used as a filler adding flexibility and consistency. Potassium sulfate (10%) counters the inhibiting effect of alginate on dental stone’s surface.
Sodium phosphate or trisodium phosphate (1-2%) acts as a retarder to control the speed of the setting reaction. The reaction with the sodium phosphate retarder is as follows:
The setting time of alginate is controlled by the amount of sodium phosphate that is present. When it is consumed, the following reaction is permitted.
Gelation is the term describing the setting process of alginate impression materials, when it changes from a sol to a gel. Using cold water will increase the setting time. The setting time can be decreased by increasing the water temperature, vigorous mixing, or decreasing the water-to-powder ratio. Decreasing the amount of water will affect the material’s consistency. Once set, alginate is prone to dimensional changes.
Imbibition is the process of absorbing water which will cause irreversible hydrocolloid expansion, and a shrunken cast if the expanded impression material is poured.
Syneresis is the process of losing water which will cause irreversible hydrocolloid shrinkage, and an enlarged cast if the shrunken impression material is poured. Since both imbibition and syneresis are undesirable, alginate impressions should be poured immediately when possible.
A thicker section of alginate will be more resistant to dimensional changes and tearing. Alginate is most accurate when there is 3mm or more between the impression tray and the tissues. Fast removal (one quick movement) of the impression from the mouth increases the compressive strength and resistance to tearing. ADA type III stone is usually used to pour alginate impressions after it’s been rinsed and disinfected with glutaraldehyde or iodophor. Common problems with alginate impressions may include:
- Grainy material – could be from improper mixing, prolonged mixing, or low water to powder ratio.
- Tearing – inadequate material bulk, moisture contamination, removal before the material has set, or prolonged mixing.
- Irregular-shaped voids – contamination from moisture or debri.
- Rough/chalky dental stone – impression may not have been cleaned well, or water left on the impression during the pour. Impression may have been removed too quickly or left on the cast for too long. Dental stone mix could have been improper.
- Distortion – loss or gain of water (imbibition, syneresis), premature or improper removal from the mouth, or movement of the tray in the mouth during gelation.
ZINC OXIDE-EUGENOL
Zinc oxide-eugenol impression paste can record soft tissue at rest, sets within 5 minutes, is dimensionally stable, and is relatively inexpensive. But it is messy, sticky, irritating to tissues, and difficult to manipulate. Zinc oxide-eugenol can be classified as a rigid impression material and is not to be used if there are any undercuts (sets hard). The setting reaction is a typical acid-base reaction to form a chelate (amorphous gel), which subsequently crystalizes. Calcium chloride (CaCl2) functions as an accelerator, and the setting time can further be accelerated by adding a drop of water in the mixture. The reaction can be slowed by adding inert oils (olive oil or mineral oil) during mixing. Oil of cloves (containing 78% eugenol) is sometimes used instead of eugenol to soothe a burning sensation in the mouth. A filler like wax or inert kaolin powder can be added to improve handling properties. Zinc oxide-eugenol impression materials are rarely used.
Impression compound is a thermoplastic material “prepared” using heat, and rehardens in the mouth. It is best used as a preliminary impression material for the fabrication of a custom tray in the edentulous patient and is not suitable where severe tissue undercuts exist or where teeth are retained. Impression compound is not very dimensionally stable and must be poured immediately.
IMPRESSION COMPOUND
Impression compound is a thermoplastic material “prepared” using heat, and rehardens in the mouth. It is best used as a preliminary impression material for the fabrication of a custom tray in the edentulous patient and is not suitable where severe tissue undercuts exist or where teeth are retained. Impression compound is not very dimensionally stable and must be poured immediately.
ELASTOMERS
Elastomers are non-aqueous rubbery polymers with good elastic properties. A rigid custom impression tray with occlusal stops is recommended for all elastomers. A custom tray will minimize permanent distortions and allow for a more uniformly thick (2-4mm) and accurate impressions. Using a custom tray uses less impression material and ensures a good peripheral adaptation. Addition silicones, condensation silicones, polysulfides and polyethers are elastomeric impression materials.
Polysulfide impression materials were the first “rubber” materials used in dentistry. They are non aqueous elastomeric impression materials that undergo an exothermic cross-linking and chain lengthening condensation polymerization reaction, producing water as a by-product. The reaction is accelerated by increasing the temperature or humidity. Polysulfides are more accurate than alginate, but not as accurate as polyethers or silicone materials. The impressions should be poured within several hours after mixing, preferably within 30 minutes. The wettability with gypsum is poor. Polysulfide impression materials have an unpleasant smell and taste, and stains clothing (contains lead oxide). It has the longest working time (4-6 minutes) and longest setting time (12-14 minutes) of any elastomeric material.
The polysulfide base is usually a white paste and contains:
- mercaptan rubber with sulfhydryl groups (-SH).
- low-molecular-weight polysulfide polymers (80%).
- reinforcing fillers like titanium dioxide that modify viscosity and increase strength (20%).
- a plasticizer like dibutyl phthalate.
- an accelerator, usually sulfur.
The polysulfide catalyst is usually a brown paste and contains:
- lead dioxide (brown color, stains clothes) or tert-butyl-hydroperoxide.
- a retarder like oleic or stearic acid to control the setting reaction.
When the two pastes are mixed, the polymer chains are lengthened and cross-linked through oxidized thiol groups. Polysulfide impression materials require a custom tray to ensure uniform thickness and to control polymerization shrinkage.
Polyether impression materials are elastomers that do not produce volatile by-products. They are known for their excellent dimensional stability and low permanent deformation, but are prone to water absorption (imbibition). Polyethers are hydrophilic, tolerating moisture better than any other impression material. Once set, however, polyethers can be very rigid and difficult to remove.
The polyether base contains:
- polyether with ethylene imine groups.
- silica filler particles.
- a plasticizer.
The polyether catalyst contains:
- sulfonate (aromatic sulfonic acid ester).
- a thickening agent.
When the two are mixed a rubber forms via a fast cationic polymerization process which is very similar to addition polymerization, but a cation (positive ion) is the reactive molecule instead of free radicals. Custom trays are needed to maintain uniform thickness.
Condensation silicones are hydrophobic elastomeric impression materials that are set by a cross-linking polymerization reaction. They are hydrophobic, exhibit poor dimensional stability and low tear strength, and must be poured immediately. Condensation silicones do retain some favorable properties, including good strength, relatively low cost, and ease of handling.
The condensation silicone base contains:
- polydimethylsiloxane
- fillers such as calcium carbonate or silica.
- an accelerator like stannous octoate and alkyl silicate.
Condensation silicones release ethyl or methyl alcohol (ethanol) as a by-product causing dimensional contraction. Condensation silicones are rarely used because of their poor dimensional stability.
Addition silicone or Polyvinyl siloxane (PVS) impression materials have become one of the most popular impression materials due to their superior mechanical properties. There are no by-products (like condensation silicones) and they exhibit excellent dimensional stability, very low permanent deformation, great surface detail, and moderate working times. Addition silicones can confidently be poured 1 week after the impression is taken. Hydrogen gas can be released, caused by the reaction between moisture and residual hydrides of the base polymer. Platinum or palladium is added to act as a hydrogen gas scavenger. PVS materials are hydrophobic and temperature sensitive. Increases in temperature will shorten the working time and setting time.
The addition silicone base contains:
- silicone with terminal silane hydrogen groups.
- filler particles to alter the viscosity.
The addition silicone catalyst contains:
- silicone with terminal vinyl groups
- a chloroplatinic acid inhibitor that slows the reaction.
- filler particles to alter the viscosity.
Polyvinyl siloxane impression materials are set by a chain reaction polymerization, not by stepwise polymerization reactions like polysulfides, condensation silicones, or polyethers. Latex gloves should not be worn when mixing PVS because the sulfur in the latex retards the setting reaction.
Light body materials can be referred to as “wash type”, useful due to their low viscosity (ability to flow). A syringe or extruder is used to place light-body impression material directly onto a prepared tooth surface to ensure maximum detail.
Regular and heavy bodied materials can also be referred to as “tray type” and are more viscous compared to light-body material. The stiffness helps to force light-body material into close contact with the tooth and surrounding tissues, providing the maximum amount of detail in an impression. Heavy body impression material contains roughly 60% filler by weight.
Bite registration material is used to make an accurate interocclusal record. The ideal material should present minimal resistance to the patient’s jaw so as not to cause deviation during a closing movement. Addition silicone impression materials are the most commonly used because of their physical properties. Injecting the material between already tightly closed teeth in maximal intercuspation is a sure way to avoid any deviation.
Impression materials may be ranked according to a specific physical property:
- Cost (low to high) – alginate, agar, polysulfide, condensation silicone, PVS, polyether.
- Dimensional stability (low to high) – hydrocolloids, condensation silicones, polysulfides, polyether, PVS.
- Wettability (low to high) – condensation silicones, hydrophobic PVS, polysulfides, hydrophilic PVS, polyether, hydrocolloids.
- Castability (low to high) – condensation silicones, hydrophobic PVS, polysulfides, polyethers, hydrophilic PVS, hydrocolloids.
- Stiffness (low to high) – hydrocolloids, polysulfides, condensation silicones, PVS, polyether, zinc oxide-eugenol.
- Tear strength (low to high) – hydrocolloids, condensation silicones, polyether, PVS, polysulfides.
GYPSUM
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. Calcination is the process of dehydrating gypsum by heating it to 300°F/150°C forming calcium sulphate hemihydrate (gypsum powder). When gypsum powder is mixed with water it rehydrates into gypsum (calcium sulfate dihydrate) in an exothermic reaction.
The setting time of gypsum can be decreased by decreasing the amount of water used or increasing the temperature. Gypsum expands during the exothermic setting reaction due to the calcium sulfate dihydrate crystal growth. Sprinkling powder onto the water is often recommended to enhance mixing and reduce the chance of air bubbles forming. There are five American Dental Association (ADA) classifications for dental stone:
- Type I – impression plaster – can be used for dental impressions (no longer popular).
- Type II – lab plaster or model plaster – used when the strength of a cast is not important, say in orthodontic cases.
- Type III – Class 1 dental stone – commonly used for most dental applications. Also called “Yellow stone” or “Microstone”.
- Type IV – Class 2 dental stone – high strength dental stone used as master casts for die fabrication, for crown and bridge, implants, inlays and onlays. Lowest percentage of expansion. Also called “Densite” or “Improved Dental Stone”.
- Type V – high strength dental stone but also high expansion, used to compensate for the solidification shrinkage of base metal alloys. Used for dies for crowns and bridges. Also called “DieKeen”.
α-calcium sulphate hemihydrate (dental stone) has a dense, prismatic crystalline structure resulting in a stronger, harder material (compared to beta-hemihydrate). Heating gypsum under steam pressure in an autoclave at 120-150°C will produce dental stone, with particles uniformly shaped and less porous.
β-calcium sulphate hemihydrate (Plaster of Paris) has a spongy irregular crystal structure, and requires more water to “float” its powder particles. Heating gypsum in an open pan produces Plaster of Paris.
Boiling gypsum in a 30% aqueous solution of calcium chloride and magnesium chloride produces high strength (improved) die stone, with the least porous and strongest particles.
All gypsum products are weak in tension but exhibit good compressive strength. Gypsum is poured under vacuum to eliminate bubbles. Vibrations can also be used to bring bubbles to the surface. The water-powder ratio is very important. When there is too much water, the powder particles are further apart, resulting in less expansion and a weaker product. Rapid spatulation (mixing) accelerates the setting time. Accelerators include potassium sulfate, sodium chloride, and alum (aluminum potassium sulfate). Retarders include borax and sodium citrate.
To help prevent gagging many techniques can be used. The mandibular alginate impression is usually done first (less gagging). For the maxillary impression, the posterior portion of the tray is first seated, then the anterior portion is rotated into place to prevent material squeezing down the patient’s throat. The patient is seated in an upright position instead of lying down, and asked to breathe through their nose. Placing salt on the tongue and acupuncture are techniques occasionally employed to help with gagging.
