Materials Science 6 Flashcards
Impressions
Used to make a Negative Mold of Dental
Structures
¢ Transfer to a Gypsum Model
¢ Visual Methods becoming more Widely used.
¢ Irreversible Hydrocolloid (alginate)
Making an impression
¢ Gypsum Products (dental plaster & stone)
Making Diagnostic casts
REQUIREMENTS OF IDEAL IMPRESSION MATERIALS
- Low cost.
- Long shelf life.
- Biocompatibility.
- Pleasant to patient.
- Dimensionally Stable
- Good Handling Properties
WATER BASED IMPRESSION MATERIALS
¢ Alginates
¢ Agar-Agar
¢ Hydrophilic – Wets Tooth Structure and Gypsum
IRREVERSIBLE HYDROCOLLOID IMPRESSION MATERIAL
Also called “alginate impression material” Used to make dental impressions for:
- removable partial dentures
- preliminary impressions for complete dentures - orthodontic models and study models
Not used for:
crown and bridge impressions (lack of accuracy)
¢ Alginic acid is soluble in
in water and forms a “sol”
sol: resembles a solution, but is made up of colloidal particles dispersed in a liquid;
Setting RXN
Sol to Gel transformation
¢ Temperature controlled (reversible) ¢ Chemically controlled (irreversible)
gel
“gel” entangled framework of solid colloidal particles in which liquid is trapped in the interstices and held by capillary forces (Jello)
PROBLEMS WITH ALGINATE IMPRESSIONS
Dimensional stability:
Because it is a gel, it undergoes shrinkage or expansion upon loss or gain of water.
Syneresis
Loss of water to the surroundings
Imbibition
Pick up of water from the surroundings
For least dimensional change/avoid distortion: Store impressions in 100% relative humidity Pour quickly after removal from mouth
HYDROPHOBIC IMPRESSION MATERIALS
¢ Hydrophobic Impression Materials are Designed to Provide Better Stability
Polysulfides Polysiloxanes Polyethers
¢ Does Not Wet Tooth Structure or Gypsum Well
WETTABILITY
Important for:
Unset impression material over wet oral structures. Unset gypsum-water mix into set impression.
POLYSULFIDE
¢ Low molecular weight oligomers containing reactive mercaptan groups
¢ Conform well to a prepared tooth, resulting in good surface detail, and are
¢ Oxidized by lead dioxide or copper hydroxide to form a polysulfide rubber with a water condensate.
¢ To be accurate, this material requires a rigid custom tray for minimal thickness of impression material.
¢ They do not recover from permanent deformation as well as some other impression materials.
¢ Models should be poured as quickly as possible.
POLYSULFIDE IMPRESSION MATERIALS
Advantages
¢ Among the least expensive of the elastomers.
¢ Good accuracy and stability.
¢ Adhesives retain the material in the tray optimally.
¢ Additional layers bond to each other.
¢ Shrinkage reduced due to attachment of the material to the tray.
Disadvantages
¢ Can stain clothing, skin and other surfaces.
¢ Objectionable odor. (Patient Satisfaction)
¢ Slow setting. (Patient Satisfaction)
¢ Should be poured within 1 hour after use. (Poor Dimensional Stability)
SILICONES (POLYSILOXANES) ARE INHERENTLY HYDROPHOBIC BUT ADDITION TYPE CAN BE MADE HYDROPHILIC.
Condensation type was first introduced.
n Hydrophobic but Chemically and Dimensionally Stable n Addition type can be made hydrophilic by:
simple addition of surfactant, or
chemical incorporation of hydrophilic moieties into the silicone backbone.