3. INTRODUCTIONS
• What is a denture base ? :The part of a denture that sits on the foundation parts and to which teeth
are attached.
• FUNCTION of denture base are:
1. Attach the prosthetic teeth to the Removable Partial Denture.
2. Transfers functional forces to supporting oral structures.
3. Improve esthetics by reproducing natural-looking contours.
• The materials used for denture bases are:
1. Plastic e.g. acrylic and Valplast
2. Metal e.g. Gold and Titanium
4. REQUIREMENT OF DENTURE BASE
1. Low cost
2. It should match the appearance of oral tissue for aesthetics
3. Should not toxic or irritant to oral cavity
4. Should have sufficient strength and toughness
5. Good thermal conductivity
6. It should not have a bad taste or smell
5. COMPOSITION OF HEAT-CURED ACRYLIC
RESIN
POWDER:
1. granules of polymethyle methacrylate
2. Benzoyl peroxide as an initiator.
3. Dibutyl phthalate as a plasticizer to produce a less brittle polymer.
4. Pigments to give the denture base the natural appearance of tissues.
5. Synthetic fibers to simulate blood vessels of the oral mucosa.
6. Bismuth salt so it be Radiopaque in x-ray if swallowed.
6. COMPOSITION OF HEAT-CURED ACRYLIC
RESIN
LIQUID:
1. Methyl methacrylate monomer
2. Hydroquinon as an inhibitor to react with any free radical.
3. Ethylene as an cross linking agent, to improve the mechanical
properties and increase the resistance of the denture base
7. MANIPULATION AND PROCESSING
1-PROPORTIONING: the ration of polymer and monomer 3:1 by volume
or 2.5:1 by weight, and this ratio will decrease the polymerization
shrinkage from 21% to 7%.
• If less monomer is used, the polymer will not be adequately wetted,
resulting in a weaker, granular denture.
• If more monomer is used, the polymerization shrinkage will increase,
and the denture base may have porosity.
8. MANIPULATION AND PROCESSING
2-POLYMER-MONOMER MIXTURE STAGES: The P-M mixture passes through
the five stages listed below:
a) Sandy stage: the mixture becomes wet and sand-like.
b) Fibrous or sticky stage: The polymer start to dissolves in the monomer.
c) Dough stage: The mixture becomes smooth, does not adhere to the mixing
jar or hands, and can be controlled and shaped easily. This is the best time
to start packing.
d) Rubber stage: The material becomes rubbery, which means it becomes soft
but cannot be shaped.
e) Stiff stage: If the material is left for a longer period of time, it will become
stiff.
9. 3) PACKING
• The material is ready for packing in the flask as
soon as it reaches the dough stage.
A. When packed in a sandy or sticky stage, the material
will be of high fluidity, flowing out of the flask and
resulting in porosity of the dentures.
B. If the dentures are packed in rubbery or stiff stage,
the material won't flow under pressure and will not be
in contact with the metal. The result is dentures with
no details and usually broken or moved teeth.
N.B : In order to reduce polymerization shrinkage from 7% to 0.2%,
acrylic dough should be packed in excess
10. 4) CURING
• When the flask is packed, it is placed in a hot water
bath at 72°C using either:
A. The long cycle is heating at 72C for a period of 8-16
hours.
B. Short cycle: Heating at 72C for 1.5 hours, and then
raising the temperature to 100C for one hour.
A lower denture made from heat cured poly methylmeth acrylate
(PMMA).
11. I. Under curing : If the denture is made in a short time and/or at a low temperature, it
will have high residual monomer (irritant to tissues and less effective).
II. Over curing : (more time andor temperature)Will result in boiling of the monomer
(monomer bisoils at 100.3C) due to the added heat of polymerization (polymerization
reaction is exothermic) and as a result the monomer will vaporise leaving pores inside
the denture (gaseous porosity)
Polymerization reaction :
PMMA (polymer) + benzoyl peroxide (initiator) Polymer (PMMA) + Heat
(exothermic reaction) Addition polymerization.
5) Cooling : It is important to cool the flask slowly as there is a difference in the
coefficiency of thermal contraction between the mold material and acrylic resin. This
difference induces stresses in the denture. Slow cooling allows the structures to release.
Activation by heat
12. • 6) DEFLASKING : THE DENTURE
SHOULD BE HANDLED CAREFULLY
TO PREVENT BREAKING
• 7) FINSHING AND POLISHING.
