Impression materials Flashcards
Impression
Cast or Die
- dimension, + dimension - balances
Investment
Restoration
+ dimension, - dimension - balances
Rigid or Brittle (Edentulous)
Zinc Oxide & Eugenol Impression Paste
• Dental Impression Plaster
Plastic
• Impression Compound • Wax
“Elastic” – but really
viscoelastic
Hydrocolloids
Reversible (Agar) Hydrocolloid
• Irreversible (Alginate) Hydrocolloid
[Jeltrate® - Dentsply]
Synthetic Elastomers:
Oligomers polymerize into elastomers
Silicone
Condensation Silicone
Addition Silicone – Poly (vinyl siloxane)
[Aquasil Ultra - Dentsply]
Hydrocolloids: Water is bound within a
Colloid matrix
Both systems lack dimensional stability with time and require immediate pouring of models.
Alginate (irreversible): Convenient & relatively inexpensive
• Agar (reversible): Good initial dimensional accuracy, but requires special equipment.
Thermoplastic: No
Cross-linking; Physical state is reversibly controlled by temperature (e.g., wax).
Thermoset: Transformation from
monomer to polymer involves irreversible cross-linking.
Addition polymerization releases no
by- products.
• Free-radical • Ring-opening • Ionic
Condensation polymerization results in a
by-product.
Hydrocolloids
- Reversible (Agar) Hydrocolloid
* Irreversible (Alginate) Hydrocolloid
Synthetic Elastomers: Oligomers polymerize into
elastomers
During the change from liquid or paste into a solid,
Good: advantageous final properties are increased.
Bad3: the inherent density increase causes a
volumetric contraction.
Meh: heat of reaction causes material to heat up (usually minor).
Bad: there is a contraction on cooling.
Polysulfide: Low molecular weight oligomers containing
reactive mercaptan groups
Polysulfide: conform well to a
prepared tooth, resulting in good surface detail, and are
Polysulfide: oxidized by
lead dioxide or copper hydroxide to form a polysulfide rubber with a water condensate.
Polysulfide: To be accurate, this material requires a
rigid custom tray for minimal thickness of impression material.
Polysulfide: They do not recover from
permanent deformation as well as some other impression materials.
Polysulfide: Models should be poured
as quickly as possible.
Silicones (Polysiloxanes) are inherently hydrophobic but the Addition type can be made more
hydrophilic.
Condensation type was first introduced.
Addition type can be made more hydrophilic
by:
• simple addition of surfactant, or
• chemical incorporation of hydrophilic moieties into the silicone backbone.
Contact Angle important for:
Unset impression material over wet oral structures. • Unset gypsum-water mix into set impression.
Condensation Silicone
Low molecular weight oligomers containing reactive terminal hydroxyl groups which
react with ortho-ethyl silicate and stannous octoate to form a three dimensional network rubber with the liberation of ethyl alcohol.
Models should be poured as quickly as possible.
Addition Silicone: Poly (vinyl siloxane) or Vinyl Polysiloxane
Low molecular weight polymers with terminal silane groups and low molecular weight polymers with vinyl terminal groups, are now supplied with a surfactant to make this material hydrophilic.
In the presence of chloroplatinic acid, ionic polymerization produces chain-lengthening and cross linking.
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.
For extensive impressions, working time of addition silicones may be lengthened by the
addition of a retarder without adversely
affecting the other properties.
Contamination of addition silicone impression material with sulfur or dithiocarbamate compounds before setting will either
lengthen the setting time or cause the material to not set.
Addition Silicone: Poly (vinyl siloxane) or Vinyl Polysiloxane
Since sulfur and dithiocarbamate compounds are used in the
vulcanization of latex rubber gloves, latex gloves often contaminate this impression material either directly or indirectly, causing unacceptable impressions.
Thorough washing of these gloves with detergent, water and agitation before handling or manipulating either the impression material or the tissues of which the impression is being made will usually eliminate this problem.
Addition Silicone: Poly (vinyl siloxane) or Vinyl Polysiloxane
Polyether: 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.
Polyether: 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.
Polyether: This material requires a
rigid custom tray.
Polyether: 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.
PolyetherRequirements of Ideal Impression Materials
with Respective Range of Properties of Currently Available Impression Materials.
- Low cost.
- Long shelf life.
- Biocompatibility.
- Pleasant to patient.
(e) Compatibility with die and cast materials.
I. Contact angle of gypsum-water mix.
II. Chemical interference (retardation) effects of impression material to surface of the cast. Alginate materials interfere with the setting of gypsum, but manufactures add ingredients to off-set this interference.
methods using light and heavy body impression systems
reinserted into mouth or add light body first
Oligomers polymerize into elastomers:
already formed into “longer chemistry” has some body to it
Contact angle double count
once in mouth, again when stone is made (not as detailed)
P sulfide for patients with
poor tooth health - very flexible, so not as likely to remove tooth when removing impression material.
Hydrocolloids
tear easily
you want tear with least amount of
deformation. High strain rate*
Alginate has low
compatibility, retards gypsum formation.
Dimesional change happens in
different direcitons. Will always occur with impression trays depending on where the walls are.
