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Questions and Answers
What is the primary purpose of using inert fillers like silica in silicone addition curing?
Which component in the base paste of silicone condensation curing aids in mixing?
What is the function of the dibutyl phthalate in the base paste of polysulphides?
Which consistency classification corresponds to a material with a very high consistency in synthetic elastomers?
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What is the purpose of the chloroplatinic acid in the addition curing of silicone?
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What effect does increasing filler content have on synthetic elastomers?
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Which component in the catalyst paste of silicone addition curing is responsible for polymerization?
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What initiates the cross-linking process in the catalyst paste for polysulphides?
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Which material is primarily involved in the gelation stage of alginate?
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What is the main function of trisodium phosphate in the composition of alginate?
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Which of the following materials is classified as non-elastic?
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At what temperature does agar transition from sol to gel?
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Which component of alginate serves as a filler to increase thickness and strength?
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What is the purpose of potassium sulphate in the formulation of agar?
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What is the function of borax in the composition of agar?
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Which impression material has a physical reaction and is reversible?
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Study Notes
Accuracy
- Accuracy of impression materials is vital for accurate reproduction of tooth preparation and soft tissues in the mouth
- Impression materials vary in accuracy based on their material properties, manipulation technique, and the clinical scenario
Clinical scenarios
- Alginate is commonly used for preliminary impressions for diagnostic casts
- Polyether can be used for final impressions in crowns and bridges
- Silicone is used for impressions in implant dentistry due to its high accuracy and dimensional stability
Synthetic elastomers
- They are widely used in dentistry for taking impressions
- They include materials like silicone, polyether, polysulfide, and agar-agar
Silicone - Addition curing
- Silicone is a versatile elastomer used for dental impressions
- It is available in two types: addition curing and condensation curing
- Addition curing silicone is more popular due to its superior properties like improved dimensional stability and less odor
Composition:
- It consists of two components: a base paste and a catalyst paste
Reaction:
- The addition curing reaction occurs when the base paste and catalyst paste are mixed
- This sets the material into a rubber-like impression
Material
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The base paste consists of:
- Polymethylhydrosiloxane (liquid silicone prepolymer) - this is the component that is polymerised and cross-linked to form the rubber
- Inert fillers - silica is a common filler that controls the viscosity of the paste
- Platinum containing catalyst - these are usually chloroplatinic acid
- These components are combined in the base paste to facilitate the polymerization reaction
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The catalyst paste consists of:
- Divinylpolysiloxane (liquid silicone prepolymer) - this component polymerises and cross-links to form the rubber
- Inert fillers - silica is used to control viscosity
- These components are combined in the catalyst paste
Silicone - Condensation curing
- This type of silicone is less commonly used than addition curing silicone because it produces a by-product (water) which can compromise the accuracy of the impression
Composition:
- It contains a base paste and a catalyst paste
- The base paste typically consists of a silicone prepolymer, pigments, fillers, and a plasticizer
- The catalyst paste typically consists of a silane coupling agent (like an alcohol) which enhances the bond between the filler and the silicone polymer
Reaction
- Mixing the two components initiates a condensation reaction which involves the release of water as a by-product
- As the reaction progresses, the silicone polymer chains link together, forming a cross-linked network (solid rubber)
Polyethers
- These materials are known for their excellent dimensional stability, tear strength, and accuracy
- They have a strong affinity for water which makes them suitable for impressions of areas with moisture
Composition:
- Consists of a base paste and a catalyst paste
Reaction:
- When the two components are mixed, a chemical reaction occurs that forms a very stable and strong rubber-like material
Material
- The base paste typically contains an imine-terminated prepolymer, inert fillers (like silica), and a plasticizer to improve workability
- The catalyst paste typically contains an ester derivative of aromatic sulphonic acid which acts as a catalyst, to initiate crosslinking
- Inert fillers (like silica) are also added to control viscosity
Polysulphides
- These materials are characterized by a long working time, and high tear strength
- They are known for their strong odors
Composition:
- Consists of two components: a base paste and a catalyst paste
Reaction
- When the two components are mixed, a chemical reaction occurs where the polysulfide polymer reacts with the catalyst, forming a strong rubber-like material
Material
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The base paste typically contains a polysulfide prepolymer, dibutyl phthalate (controls viscosity), and inert fillers (like chalk or titanium dioxide)
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The catalyst paste typically contains oxidizing agents (like lead dioxide) which react with the polysulfide polymer, forming a cross-linked network
Impression materials and digital impressions
- These impression materials are used to create an impression of the teeth and surrounding tissues
- The impression is used to create a model of the teeth which will be used for dental treatment planning and fabrication of dental restorations
Mode of Setting
- Impression materials can be categorized based on their mode of setting
- Non-Elastic: these materials set into a rigid material, typically used for creating models
- The reaction is usually chemical and irreversible (Impression plaster)
- Elastic: these materials are flexible, allowing for easy removal from the mouth and accurate reproduction of the oral tissues
- The reaction is usually chemical and irreversible (Alginate, Polysulfide, Polyether)
- Some can be reversible ( Agar) - These materials are heated to a liquid state and then cooled to set into a semi-solid state
Hydrocolloids
- They are based on a colloidal suspension of particles that bind to a liquid
- Their properties are based on the interaction between the solid particles and the liquid medium
Sol
- The particles are dispersed evenly in the liquid, creating a flowing suspension
Gel
- The particles form a network that traps the liquid, giving the material a semi-solid consistency
Alginate
- A highly popular impression material due to its efficiency and low cost
Composition:
- Sodium alginate - this material is the basis of the gel and constitutes 15-20% of the material
- Calcium sulfate dihydrate - This material reacts with the sodium alginate to form a more stable calcium alginate gel (represents 14-20 % of the material)
- Potassium Sulfate - This material ensures that the gypsum material sets properly
- Trisodium phosphate - This component acts as a retarder to control the setting time, it contributes to 2% of the material
- Diatomaceous earth - This component is a filler material that adds strength and thickness to the alginate, representing 55-60% of the material
Reaction:
- The setting reaction of alginate is a chemical reaction, irreversible
- The sodium alginate dissolves in water, forming a sol
- Then, the calcium ions from the calcium sulfate react with the sodium alginate to form a calcium alginate gel
Properties:
- Dimensional Stability: Alginate is known for its good dimensional stability when properly handled
- Tear Strength: This refers to the amount of force that is required to tear the alginate impression material, it is relatively low
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Handling:
- Mixing: The powder and water are mixed quickly and thoroughly to ensure a consistent mix and avoid air bubbles
- Setting time: The setting time is the amount of time it takes for the alginate to set from a liquid to a solid state. Proper mixing and the water to powder ration is crucial
- Accuracy: Alginate is considered an accurate material when handled correctly
- Clinical Scenarios: Alginate is primarily used for preliminary impressions, because it has a low tear strength and can be more prone to distortion than other materials
- The impression can be affected by humidity, temperature, and handling
Agar
- This material is a reversible hydrocolloid commonly used due to its accuracy and versatility
- This material has a long working time and provides precise reproduction of the oral tissues, especially when a good impression tray is used
Reaction:
- Agar undergoes a physical change which is reversible
- The material is heated (above 70 degrees) to a liquid state (sol)
- The sol is then cooled (37-45 degrees) back to a gel state. The sol can also be heated again, turning it back to a liquid state
Composition
- Agar: This is the main component (12-15%) of the material. It is a seaweed extract and the basis for gel formation.
- ** Potassium sulfate**: This material is added to control the setting time (1%)
- Borax: An additional ingredient that strengthens the agar gel (0.2%)
- ** Alkyl Benzoate**: This is an antifungal agent that helps prevent the material from growing mold (0.1%)
- Water: This is the dispersion medium for the agar, representing 85% of total composition
Properties
- Dimensional Stability: Agar is highly dimensionally stable when properly handled
- Tear Strength: Agar has a relatively high tear strength
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Handling:
- Equipment: This material requires special equipment (agar bath) to heat and cool the impression material properly
- Working time: This material has a high working time, which allows for more time to take the impression
- Setting time: The setting time is relatively rapid once the agar impression is cooled (37-45 degrees Celsius).
- As it is a reversible material, it can be reheated to a liquid state and cooled again to produce multiple impressions, making it a good material for crown and bridge impressions.
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