Non-Elastic Materials

• What we mean by non-elastic:

  • Ambiguous term, but general features will be either:
  • Too rigid/stiff to be removed past undercuts (it's difficult to make the material deform
  • OR
  • Plasticity deforms when removed past undercuts (material has a yield point or is never elastic)
• Non-elastic impression materials:

  • Impression plaster
  • Impression compound
  • Impression pastes
• Impression plaster is made from Plaster of Paris:

  • β-calcium sulphate hemihydrate (calcined - dried off some water, therefore hemihydrous)
  • Setting procedure = add powder to water
  • (CaSO₄)₂.H₂O + 3H₂O -> CaSO₄.2H₂O  (calcium sulphate dihydrate is the set material)
  • Setting reaction leads to crystallisation
  • Anti-expansion agents are added
  • Use ratio of 100g powder to 50-60ml water
  • Fluid mix i.e. low viscosity
• Impression plaster is made from Plaster of Paris:

  • β-calcium sulphate hemihydrate (calcined - dried off some water, therefore hemihydrous)
  • Setting procedure = add powder to water
  • Setting reaction leads to crystallisation
  • Can lead to expansion - effect on accuracy
  • Want as little expansion as possible to get the most accurate impression possible
• Impression plaster is made from Plaster of Paris:

  • β-calcium sulphate hemihydrate (calcined - dried off some water, therefore hemihydrous)
  • Setting procedure = add powder to water
  • Anti-expansion agents are added
  • K₂SO₄ - reduces expansion but accelerates setting - makes it basically unusable on its own
  • Borax - retarder - slows down reaction
  • Concentration of K₂SO₄ and borax need to be carefully controlled in order to get the optimal results
  • Manufacturers either add anti-expansion agents to:
  • The powder - setting reaction requires mixing with water
  • OR
  • Supply an anti-expansion solution
• Impression plaster properties:

  • Initial low viscosity
  • Very rigid and brittle when set
  • Compatibility with model materials
  • Small expansion on setting (<0.1%)
  • Dimensionally stable
  • Non-toxic, but drying sensation in the mouth
  • Pts can find it uncomfortable - drying sensation; seems as if the material is drawing liquid out of the soft tissues
• Impression plaster properties - initial low viscosity:

  • Often requires special tray
  • Low viscosity means it is mucostatic (it won't displace and deform the patient's tissues when we place the impression in the patient's mouth, so we're going to get a good representation of the patient's soft tissues during rest)
• Impression plaster properties - very rigid and brittle when set:

  • Difficult to deform
  • Often breaks when removing past undercuts
  • Not suitable for dentate areas but ok for edentulous pts
• Impression plaster properties - compatibility with modern materials:

  • Models mostly made from gypsum products
  • Can be difficult to separate model - separating agent required
• Impression compound - developed by combining:

  • Waxes and resins
  • Traditional - natural materials
  • Modern - synthetic (better consistency)
  • Characterise thermal characteristics
  • Plasticiser (eg stearate)
  • Waxes and resins are brittle
  • Plasticiser will improve flow of material
  • Filler
  • Control viscosity, rigidity, thermal contraction
• Impression compound - typical composition:

  • Rosin, Copal or Shellac
  • Carnauba Wax
  • Stearic acid
  • Talc
  • Pigment
• Impression compound - thermoplastic material:

  • Hard when cool
  • Soft when warm
  • Can be plastically deformed when soft
  • Characterised by softening temperature - typically 55-60°C
• Impression compound - supplied as either sticks or sheets:

  • Supplied hard
  • Soften (by warming)
  • Use
  • Harden (by cooling)
  • Remove
• Two types of impression compound are generally used in dentistry:

  • Type 1: (lower temperature fusing)
  • High flow value just above mouth temp
  • Sheet form used for preliminary impressions for edentulous patients
  • Stick form used for marginal additions to stock trays
  • Type 2: (high temperature fusing)
  • Low flow value at or just above mouth temperature
  • Rigid at mouth temperature - use as tray material
• Issues with compound:

  • High viscosity
  • Material is mucocompressive
  • Has poor detail reproduction
  • Thermal contraction
  • From softening to removal from mouth:
  • Temperature change from 55°C - 23°C
  • After removal stress relief can occur - loss of accuracy - can lead to poor device fit
• Issues with compound:

  • No setting reaction
  • Final material is rigid and non-elastic
  • Applications
  • Edentulous areas
  • Tray extensions
  • NOT appropriate for use with undercuts
• Issues with compound:

  • High viscosity
  • Thermal contraction
  • No setting reaction
  • Limited applications
• Technical aspects of compound use:

  • When using sheets they need to be softened in water (55-60°C)
  • Beware:
  • If water is too cool - compound will not flow sufficiently
  • If water is too hot - compound can be sticky and painful for the patient
  • If softening time is too short - insufficient softening achieved - won't flow
  • If softening time is too long - leaching of plasticiser - won't flow
  • When using sticks they are softened in a Bunsen flame
  • Same technical aspects as above but, also, can catch fire
• Impression pastes:

  • Typically supplied in 2 pastes: most products use ZnO and eugenol (oil of cloves)
  • Similar to zinc oxide eugenol (ZOE) cements
  • Paste 1 - ZnO, Zn-acetate, oils
  • Paste 2 - Eugenol, filler (e.g. Kaolin)
• Impression pastes:

  • Setting occurs via an acid-base reaction
  • Chelate reaction
  • Ionic reaction
  • Good setting characteristics
  • Accelerated by water (as soon as it's placed in an aqueous environment, like the mouth, the reaction will speed up)
  • Some products have added accelerator
• Impression paste properties:

  • Materials have very low viscosity
  • Used in thin section (normally 1mm thick)
  • Good accuracy and dimensional stability - small dimensional change on setting and storing but used as thin section so small overall effect
  • Rigid and non-elastic - cannot record undercuts
  • Eugenol can be an irritant - eugenol-free versions available
• Impression paste properties - materials have very low viscosity:

  • Do not displace soft tissue - mucostatic
  • Can record fine detail
  • Psuedoplastic - viscosity reduces under stress
• Impression paste properties - used in thin section (normally 1mm thick):

  • Low viscosity makes it difficult to produce thick films
  • Typically used in a close-fitting special tray
  • Occasionally dentists can use the existing denture
• An example clinical application:

  • 2 stage impression technique for edentulous arches
  • Stage 1 (primary impression)
  • Use impression compound in stock tray
  • Compound is mucocompressive so will displace soft tissues
  • Allows a record of the full depth of the sulcus
  • Use primary impression to make special tray
  • Stage 2 (major impression)
  • Use impression paste in special tray, extended with compound where needed
  • Record detail
  • Maintain sulcus impression