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Dental Materials Y3 > Acrylics 1 - Heat Cured PMMA > Flashcards

Acrylics 1 - Heat Cured PMMA Flashcards

1
Q

What is an Acrylic Denture base aka

A

A saddle

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2
Q

What are the dental uses of Acrylic resin PolyMethyl Methacrylates (PMMA)

A
  • Denture Bases
  • Artificial Teeth
  • Orthodontic appliances
  • Soft liners?
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3
Q

What is Denture Base Acrylic Material Made of

A

PMMA and MMA monomer (liquid)

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4
Q

What is the Dough Technique

A

This is the technique of mixing ad Heat Curing PMMA and MMA to form the denture base acrylic material

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5
Q

Why do we mix in PMMA powder in the MMA liquid in the dough technique

A

It acts as a filler

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6
Q

Describe the Dough technique

A

The PMMA powder mixes with the MMA liquid and forms a dough, which is then heat cured

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7
Q

Why can’t we just use MMA liquid to make the denture base

A

It would have a 21% polymerisation shrinkage

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8
Q

How much of the monomer liquid is replaced with PMMA powder in the dough technique

A

2/3

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9
Q

Name some general physical properties of MMA monomer liquid

A
  • V volatile
  • V flammable
  • Distinct smell
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10
Q

What are the constituents of the PMMA powder

A
  • PMMA spherical beads
  • Benzoyl peroxide
  • 1-2% pigments
  • Opacifiers - Ti/Zn oxides
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11
Q

What are the constituents of the MMA monomer liquid

A
  • MMA monomer liquid
  • Cross linking monomer
  • Hydroquinone
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12
Q

Why is Hydroquinone added in the MMA monomer liquid

A

Its an inhibitor that extends half life of the liquid by reacting with any free radicals that form so no polymerisation reaction can cascade

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13
Q

Whats required to activate the polymerisation reaction between PMMA and MMA

A
  • Initiator - benzoyl peroxide in PMMA powder

- Heat

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14
Q

How does the mixing take place in the dough technique

A
  • 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
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15
Q

What will too much monomer when doing the dough technique result in

A

Excessive polymerisation shrinkage

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16
Q

What will too much powder cause when carrying out the dough technique

A

Causes a weak product with porosity as not all powder particles will be wetted by the monomer

17
Q

What is the volume shrinkage of the final product

A

7%

18
Q

Some internal strains in denture bases are caused by thermal contraction - how can we minimise these

A

Using acrylic teeth and allowing the denture flask to cool slowly

19
Q

What is a craze in dentures

A

This is a form of damage on the denture that wasnt enough to crack or fracture the ting but has affected the structural integrity

20
Q

What can cause crazes to form in denture bases

A
  • Heat (polishing)
  • Differential contraction around porcelain teeth (reduced by using acrylic teeth)
  • Attack by solvents (eg alcohol)
21
Q

What can be the causes of porosity in denture bases

A
  • 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
22
Q

What is the Glass Transition Temperature (Tg) of PMMA

A

Around 105C

23
Q

What is the Glass Transition Temperature

A

AKA the softening temperature and is when the plastic becomes much more soft and flexible

24
Q

How common are midline fractures in dentures

A

Happens in about 30% of cases

25
Q

What is the main piece of advice for patients trying to take care of their Heat Cured PMMA dentures

A

Advised to keep their dentures in water when not in the mouth (overnight)

26
Q

Why are patients advised to keep their dentures in water when not in the mouth

A

Denture materials initially absorb ~2% of water from the storage solution/mouth and if allowed to dry out it’ll reabsorb more than 2% when placed in the mouth again eventually causing crazes, mechanical compromisation and denture failure

Dental Materials Y3 (9 decks)

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