Hydrocolloid Impression Materials

• A solution = liquid consisting of homogenous mixture of solute and solvent - only one phase exists. For example, saline = salt dissolved in water.
• A suspension = liquid continuous phase with solid discrete phase - two phases exist. For example, flour in water - the flour and water will eventually separate.
• A composite = at least two distinct phases which can be individually recognised. For example, a resin composite - the glass particles and resin are clearly visible microscopically.
• A colloid = somewhere between a suspension and a solution. Particles are dispersed in another material (continuous phase) but form a homogenous solution. In general the particles do not "settle out" like a suspension. When continuous phase is water, the material is called a hydrocolloid.
• There are 4 types of colloid solutions:

  • Aerosols
  • A solid or liquid dispersed in a gas - eg fog (liquid water in air)
  • Emulsion
  • A liquid dispersed in a liquid - eg oil and water (note: this is unstable and can separate out)
  • Sols
  • A solid dispersed in a liquid - eg paint (solid pigment in a solvent) - sols behave like liquids, phases are randomly distributed, have no 3-dimensional structure
  • Gel
  • A liquid dispersed in a solid - eg fuit jelly - gels
  • behave like solids, they have a 3-dimensional structure

  Only sols and gels are relevant for hydrocolloid impression materials.
• Dental hydrocolloids are based on polysaccharides:

  • Long chains of saccharide groups
  • Obtained from seaweed
  • Can exist as sols and gels
• The impression taking process for reversible and irreversible hydrocolloids uses the transmission from sol to gel:

  • In sol form (a):
  • Random arrangement of fibrils
  • In gel form (b):
  • Crosslinks form between the fibrils
  • Can be either:
  • Hydrogen bonding (weak) - reversible
  • Covalent/ionic bonding (strong) - irreversible

  So reversible or irreversible refers to how easy it is to break the crosslinks.

  
• Agar is a reversible hydrocolloid. It is supplied as gel in a tube:

  • Agar - polysaccharide from seaweed
  • Borax - to strengthen gel
  • K₂SO₄ - to compensate for borax
  • Remember what borax and K₂SO₄ do to gypsum
  • When K₂SO₄ added to gypsum, the setting goes too fast, so borax is added to slow the reaction down
  • But if K₂SO₄ wasn't added then the agar and borax may not set
  • Filler concentrations related to viscosity
• Procedure for using agar:

  • Convert gel in tube using hot water in a special water bath
  • 100°C converts gel to sol
  • 65°C maintains as sol
  • 45°C converts sol to gel
  • Place in mouth
  • Use special cooling tray - water circulates around tray
  • Returns to gel at mouth temperature
  • Material adjacent to tissues sets last
• Properties of agar:

  • Naturally a hydrophilic material
  • Gel form is a crosslinked polymer - flexible so easy to remove past undercuts using low force
  • In gel form it's viscoelastic
  • Low tear resistance - inspect for tearing in thin sections
• Properties of agar - naturally a hydrophilic material:

  • It contains water so it wets moist oral tissues
  • Not repelled by water so does not require a dry field
  • So it can be used to record impressions from moist tissues
• Properties of agar - in gel form it is viscoelastic:

  • Technique dependent
  • A good technique means that very close to elastic behaviour is seen
  • Remove impression using a single rapid motion
  • Ensure fully set - otherwise material's accuracy is a problem
  • Distortion depends on
  • Depth of undercut - smaller depth means closer to elastic behaviour
  • Thickness of material - greater impression bulk means more elastic behaviour
  • Time under stress - quick, snap release means more elastic behaviour
• Agar impressions have poor dimensional stability:

  • Initially have high water content
  • Can lose water over time
  • Polymer chains collapse in towards each other -syneresis
  • Water on the surface evaporates
  • Leads to a rapid change in dimensions
  • Can gain water - imbibition

  So either pour cast quickly or store in a soaked tissue, not a long term solution!
• Can the reversible nature of agar be used:

  • Crosslinks formed by weak bonds
  • Can be easily broken using heat
  • In theory they can be reused
  • Take gel impression then convert to sol by re-heating
  • Use again and again, possibly
  • However:
  • Contamination
  • Changes composition: model making materials can contaminate the agar
  • Biohazards: requires decontamination between patients
  • So in general: clinical/laboratory procedures
  • Clinical use agar - used on pts, so single use
  • Laboratory use agar - typically duplication of models, so no patients involved, can be re-used
• Agar applications:

  • Clinical use:
  • Not widely used in the UK and Europe
  • Very popular in Japan
  • Partial denture impressions:
  • Alternative to alginate, elastomers
  • Crown and bridge:
  • Alternative to elastomers
  • Laboratory duplicating procedures:
  • Very common use worldwide
• Alginate is an irreversible hydrocolloid:

  • Extracted from seaweed as alginic acid
  • Dental material adapted
  • Sodium alginate (can have potassium too)
  • Impressions made by mixing with water
• Alginate - setting:

  • Mix powder and water - initially mix to form sol (solid dispersed in a liquid)
  • Setting reaction to form gel (liquid dispersed in a solid)
  • Ca²⁺ replace Na⁺ ions in the alginate
  • Divalent ions means that crosslinking is possible
  • Ionic bonding

  
• Control of working time and setting time with alginate:

  • Working time and setting time controlled by CaSO₄ and Na₃PO₄
  • CaSO₄ is sparingly soluble
  • Na₃PO₄ is readily soluble
  • Main reaction:
  • 2Na (alginate) + Ca²⁺ -> Ca (alginate)₂ + 2Na⁺
  • This can be a very fast reaction
  • So Na₃PO₄ is added as a retarder
  • 3CaSO₄ +2Na₃PO₄ -> Ca₃(PO₄)₂ + 3Na₂SO₄
  • This reaction happens first, delays main reaction
  • Distribution of reactive ingredients throughout powder is essential - so shake box before use!
• Alginate - manipulation:

  • Pre-mix powder ingredients in a container
  • Measure powder/water using scoop/cylinder
  • Provided by manufacturer
  • Beware, different manufacturers have different sizes
  • Use water at room temperature
  • Temperature controls ionic reactions
  • Hot water = faster setting
  • Cold water = slower setting
  • Spatulate rapidly to form sol (solid dispersed in liquid) - technique sensitive
  • Retain in tray with adhesive (+perforations)
  • Hold still during setting - material next to tissues sets first
• Properties of alginate:

  • Similar to agar
  • Hydrophilic material - water-based so does not require a dry field
  • Flexible - undercuts ok
  • Elastic/viscoelastic
  • Care on removal from mouth
  • Allow to fully set
  • Low strength - tears easily, use sufficient bulk -> 3-5mm of material between tray and tissues
  • Poor dimensional stability:
  • Water evaporates, syneresis, imbibition
  • Pour cast as soon as possible
  • If stored, cover with damp gauze in sealed container
• Alginate - applications:

  • Alginate is used as a general purpose material
  • Can be used for:
  • Study models
  • Models for constructing special trays
  • Impressions for removable dentures
  • But is NOT used where great accuracy and dimensional stability is required - eg not for crown and bridge work