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
Spatulaterapidly 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