Acrylics 1 - Heat Cured PMMA Flashcards
What is an Acrylic Denture base aka
A saddle
What are the dental uses of Acrylic resin PolyMethyl Methacrylates (PMMA)
- Denture Bases
- Artificial Teeth
- Orthodontic appliances
- Soft liners?
What is Denture Base Acrylic Material Made of
PMMA and MMA monomer (liquid)
What is the Dough Technique
This is the technique of mixing ad Heat Curing PMMA and MMA to form the denture base acrylic material
Why do we mix in PMMA powder in the MMA liquid in the dough technique
It acts as a filler
Describe the Dough technique
The PMMA powder mixes with the MMA liquid and forms a dough, which is then heat cured
Why can’t we just use MMA liquid to make the denture base
It would have a 21% polymerisation shrinkage
How much of the monomer liquid is replaced with PMMA powder in the dough technique
2/3
Name some general physical properties of MMA monomer liquid
- V volatile
- V flammable
- Distinct smell
What are the constituents of the PMMA powder
- PMMA spherical beads
- Benzoyl peroxide
- 1-2% pigments
- Opacifiers - Ti/Zn oxides
What are the constituents of the MMA monomer liquid
- MMA monomer liquid
- Cross linking monomer
- Hydroquinone
Why is Hydroquinone added in the MMA monomer liquid
Its an inhibitor that extends half life of the liquid by reacting with any free radicals that form so no polymerisation reaction can cascade
Whats required to activate the polymerisation reaction between PMMA and MMA
- Initiator - benzoyl peroxide in PMMA powder
- Heat
How does the mixing take place in the dough technique
- PMMA powder added slowly to MMA liquid, with no mixing until powder no longer dissolves in the monomer and a fine crust of powder is left on the surface of the liquid
- Mixture stirred and left to stand in a screw top jar
What will too much monomer when doing the dough technique result in
Excessive polymerisation shrinkage
What will too much powder cause when carrying out the dough technique
Causes a weak product with porosity as not all powder particles will be wetted by the monomer
What is the volume shrinkage of the final product
7%
Some internal strains in denture bases are caused by thermal contraction - how can we minimise these
Using acrylic teeth and allowing the denture flask to cool slowly
What is a craze in dentures
This is a form of damage on the denture that wasnt enough to crack or fracture the ting but has affected the structural integrity
What can cause crazes to form in denture bases
- Heat (polishing)
- Differential contraction around porcelain teeth (reduced by using acrylic teeth)
- Attack by solvents (eg alcohol)
What can be the causes of porosity in denture bases
- Gaseous - incorrect curing cycle - monomer vaporises and leaves voids
- Contraction - flasks not kept under constant spring pressure during curing cycle - internal or on denture surface
- Granular - Incorrect mixing/packing - structural deficiency due to loss of monomer
What is the Glass Transition Temperature (Tg) of PMMA
Around 105C
What is the Glass Transition Temperature
AKA the softening temperature and is when the plastic becomes much more soft and flexible
How common are midline fractures in dentures
Happens in about 30% of cases
