DMS - Acrylic Resin

Question 1

list the properties of heat cured acrylic PMMA. (12)

Answer 1

Moderately stable in use: Retained within the mouth.

High softening temperature: withstands hot fluids

Unaffected by oral fluids = yes since the water absorption is counteracted by the contraction on heating.

Moderate thermal expansion: ok if acrylic teeth used but much higher if porcelain teeth used

Impact strength(i.e. dropped) - fairly resistant however can fail

Low/poor thermal conductivity: prevent scalding of the throat/oesophagus.

Low density = good - so upper denture doesn’t fall out. (Aids retention)

Colour matches natural tissues.

non-toxic = yes

Poor mechanical properties - improved by increasing the bulk.

High young elastic modulus: Rigid, large stresses produce small strain.

High elastic limit: Only large stresses cause permanent damage.

Question 2

how is acrylic PMMA made?

Answer 2

free radical addition polymerisation

Question 3

Describe the stages of polymerisation of PMMA - both types (4)

Answer 3

Activation - Initiator broken down to produce free radicals.

Initiation - Free radicals produced from the initiator break the carbon to carbon double bonds in the monomer and transfer the free radicals.

Propagation - Monomers join via free radical addition polymerisation to create a growing polymer chain.

Termination of polymerisation

Question 4

at what temperature is the initiator activated in heat cured acrylics?

Answer 4

> 72 degrees

Question 5

what is the initiator in heat cures acrylic PMMA?

Answer 5

benzoyl peroxide

Question 6

what is the function of the co-polymers in HC PMMA?

Answer 6

improve mechanical properties

Question 7

why do we not heat the HC acrylic PMMA to > 100?

Answer 7

At this temperature gases are produced which will create porosities in the material
= weakening of the material.

Question 8

what occurs if the HC PMMA is under cured? (2)

Answer 8

Unreacted monomer exists that can cause irritation.

Poorer mechanical properties.

Question 9

what are the problems associated with porous HC acrylic PMMA? (4)

Answer 9

Lower strength.
Reduction in aesthetics.
Rough to touch with the tongue, acts as a plaque trap.
Absorbs saliva = reduced oral hygiene.

Question 10

what causes porosity in the HC acrylic PMMA? (2)

Answer 10

heating > 100 degrees = gaseous porosity

dough not packed sufficiently = contraction porosity

Question 11

instead of heat, what is the benzoyl peroxide in self-cure PMMA’s activated by?

Answer 11

promoter - tertiary amine

Question 12

what are the limitations of self cure PMMA? (3)

Answer 12

higher monomer levels = irritant and softening of base

poor colour stability

absorbs water to become oversized.

(made faster and is more efficient so that’s why used in orthodontics baseplates)

Question 13

describe transverse stress.

Answer 13

How well the upper denture copes with stresses that cause deflection

Question 14

where is the pivot point of an upper denture?

Answer 14

the palate

Question 15

where would fracture occur when forces are being applied at either side of the denture?

Answer 15

the pivot point = palate

Question 16

describe the relationship between contraction during setting and water absorption in heat cured acrylic PMMA.

Answer 16

During usage it expands 0.4% due to water absorption which makes up for the contraction it undergoes during the heat curing stage.

Question 17

list the properties of self cured acrylic PMMA. (4)

Answer 17

Little thermal contraction = initially has a better fit and is more accurate.

expands during use due to water absorption = oversized and not as well tolerated.

less efficient polymerisation/chemical activation = lower molecular weight = poorer mechanical properties.

polymerisation is less efficient = more unreacted monomer = which acts as a plasticiser to soften the denture base (more vulnervale to fracture) and also causes more irritation (no longer biocompatible)

Question 18

what are the limitations of light cured resins? (1)

Answer 18

There is a maximum depth of cure so this limits the thickness of the denture.

Question 19

what materials can we use if a patient has an allergy to resin? (3)

Answer 19

Nylons - absorbs water = swelling and softening

Vinyl polymers - soften at 60 degrees C and expensive to produce.

Polycarbonates - expensive production and internal stresses cause distortion = poor fit.