PMMA

Question 1

What are the components of heat cured acrylic?

Answer 1

Powder- initiator (benzoyl peroxide), PMMA particles, plasticiser, pigments, co-polymers

Liquid- methacrylate monomer (dissolves PMMA particles), inhibitor, co-polymers

Question 2

What is the aim of heat curing?

Answer 2

Cross linking of MMA monomer

Question 3

What are some advantages of heat cured PMMA?

Answer 3

Can withstand impact, low density (light weight), high softening temp (not boiling though), dimensionally accurate and stable

Question 4

What are some disadvantages of heat cured PMMA?

Answer 4

Mismatch expansion contraction with porcelain teeth (porcelain expands less)
Low thermal conductivity

Mechanical properties are overcome by increasing thickness of denture

Question 5

What is the difference between heat cured and self cured PMMA?

Answer 5

Self cured PMMA has a tertiary amine in liquid which activates benzoyl peroxide (opposed to heat activation).

Little thermal contraction as no heating stage (more dimensionally accurate), no porosity caused by heating stage
Poorer mechanical properties - chemical activation yields less efficient polymerisation

Question 6

Why may self cure PMMA be an irritant?

Answer 6

Unreacted monomer may leach out

Question 7

What may cause colour change in self cure PMMA?

Answer 7

Tertiary amines are susceptible to oxidisation

Question 8

What is a consequence of fast cooling?

Answer 8

Regions of gaseous porosity

Question 9

What is gaseous porosity?

Answer 9

Monomer boiling, producing a gas that is relieved as a bubble close to surface of acrylic resin, occurs most in bulkier areas.
This is due to the exothermic reaction- cure slowly.

Question 10

What is contraction porosity?

Answer 10

Caused by too much monomer, insufficient excess material or insufficient clamp pressure.
Due to large polymerisation shrinkage of monomer (21%)