Hydrocolloids Flashcards
What is a solution?
Liquid consisting of homogenous mixture of solute and solvent e.g. saline
What is a suspension?
Liquid continuous phase with solid discrete phase (2 phases exist) e.g. flour in water (they will eventually separate)
What is a composite?
At least 2 distinct phases which can be individually recognised e.g. resin composite = glass particles and resin visible microscopically
What is a colloid?
Somewhere between a suspension and a solution - particles dispersed in another material (continuous phase) but form a homogenous solution. The particles do not settle out like suspensions
What is a hydrocolloid?
Colloid - particles dispersed in another material (when the material is water)
List 4 types of colloid solutions?
Aerosols
Emulsion
Sols
Gel
What is an aerosol?
Solid or liquid dispersed in gas
What is an emulsion?
liquid dispersed in liquid
What is a sol?
Solid dispersed in liquid
What is a gel?
Liquid dispersed in solid
What type of hydrocolloids are used for impression materials?
Sols and gels
What process does impression taking using hydrocolloids use during setting?
Transition from sol to gel
Difference in strutcure of sol and gel
Sol = random arrangement of fibrils Gel = cross links between fibrils
Difference between reversible or irreversible hydrocolloids?
How easy it is to break cross links
Name a reversible hydrocolloid
Agar
Properties of agar
a) interaction with water
b) stiffness
c) elasticity
d) strength
e) dimensional stability
a) hydrophilic = doesn’t require dry field
b) flexible - can move past undercuts
c) visco-elastic - will eventually return to original dimensions
d) low tear resistance
e) poor dimensional stability
Why is agar technique sensitive?
Good technique means to is close to elastic rather than visco-elastic
- remove in rapid motion
Ensure fully set
Describe agar dimensional stability, how to avoid?
Has high water content so can lose water over time
Pour cast quickly or store in soaked tissue
Name an irreversible hydrocolloid?
Alginate
effect of temperature of water in setting of alginate?
Hot water = faster setting
Cold water = slower setting
Mixing alginate powder and water forms what type of hydrocolloid?
Sol
Sets into gel
Properties fo alginate
a) water reactions
b) rigidity
c) elasticity
d) strength
e) dimensional stability
a) hydrophilic
b) flexible = undercuts
c) elastic/viscoelastic - allow to fully set
d) low strength = tears easily
e) poor dimensional stability - water evaporates
