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.
ADDITION SILICONE: POLY (VINYL
SILOXANE) OR VINYL POLYSILOXANE
¢ Part A: Siloxane with terminal silane groups
¢ Part B: Siloxane with vinyl terminal groups
¢ Many will Contain surfactants to make this material
hydrophilic.
¢ Cross linking Initiated by chloroplatinic acid.
¢ For extensive impressions, working time of addition
silicones may be lengthened by the addition of a retarder without adversely affecting the other properties.
HYDROGEN GAS FROM ADDITION SILICONE INTO GYPSUM
¢ If there is residual hydroxyl groups present from the mucosa, a secondary reaction occurs which produces hydrogen gas. Although finely divided palladium or platinum will absorb some of this the hydrogen, pouring of the model should be delayed until all of this hydrogen is liberated.
ADDITION REACTION SILICONE IMPRESSION MATERIALS
Advantages
¢ Excellent accuracy and dimensional stability.
¢ Does not stain clothing, skin or other surfaces.
¢ No odor or taste.
¢ Paste/paste systems—easy mixing, homogeneous setting. ¢ Various viscosities are available.
Disadvantages
¢ Difficult to vary the setting time.
¢ Expensive.
¢ May produce bubbles in the final cast.
PROPERTIES OF POLYETHERS
Many of the Same Characteristics of
Polysiloxanes ¢ Differences
Hydrophillic Bad Taste
Low molecular weight polyether oligomer with terminal ethylene imine reactive ring groups will react
¢ with aromatic sulfonic ester by dual ring opening, with the resultant polymer being cross-linked.
¢ Contact of the skin or mucous membranes with the unmixed ester may cause contact dermatitis, but irritation has not been observed with a thoroughly mixed material.
¢ This material requires a rigid custom tray.
¢ This impression material may be available in one
viscosity with a thinner (diluent); in this case there is control of viscosity with adjustment in the thinner/ impression material ratio.
POLYETHER IMPRESSION MATERIALS
Advantages ¢ Excellent stability.
¢ Slightly hydrophilic. ¢ Adhesives work well. ¢ Fast setting
Disadvantages
¢ Extremely stiff—difficult to remove from
undercuts.
¢ Relatively low tear strength.
¢ Slightly hydrophilic.
¢ Taste objectionable to some patients.
¢ May cause hypersensitivity reactions in some patients.
GYPSUM MATERIALS
Dental gypsum products are available in 5 forms (ADA types I to V): ¢ Impression plaster (Type I) –Mounting Plaster
Model plaster (Type II)- Lab Plaster
¢ Dental Stone (Type III) - Quickstone High-strength dental stone (Type IV)-Silky Rock
¢ High-strength, high-expansion stone (V)
What is gypsum
Gypsum is the dihydrate form of calcium sulfate, CaSO4 . 2H2O, found in a compact mass in nature. Dental gypsum products are manufactured by driving off part of the water of the calcium sulfate dihydrate to form calcium sulfate hemihydrate. This process is referred to as calcination.
¢ The gypsum components of these materials are identical chemically; differences in these materials is attributed to calcination.
REVERSE REACTION
When calcium sulfate hemihydrate (dental plaster, stone, etc.) is mixed with water, the reverse reaction takes place, and the hemihydrate is converted back to the dihydrate: - exothermic, heat is released.
GYPSUM MATERIALS
While setting, growth and subsequent interlocking of gypsum crystals occur. Interlocking contributes to strength and dimensional change of the gypsum.
¢ Physical and mechanical properties of the gypsum mass can be influenced by manipulative procedures that influence the difference in solubility and growth of the dihydrate crystals.
