PMMA (acrylic resin)

Question 1

What usage errors can cause a denture to fail?

Answer 1

• fractures
• uncomfortable
• becomes warped
• surface suffers wear
• fits patient only for short period

Question 2

what are the ideal properties of PMMA?

Answer 2

• dimensionally accurate and stable in use
• high softening temperature
• unaffected by oral fluids

Question 3

what does stress/strain measure?

Answer 3

determines a materials stiffness

Question 4

what determines the stress/strain of a material?

Answer 4

Young’s (elastic) Modulus

Question 5

what are the ideal MECHANICAL properties of PMMA?

Answer 5

• high Young’s (elastic) modulus

* high proportional limit/elastic limit

Question 6

what is meant by a high Young’s (elastic) modulus?

Answer 6

• material is rigid (stiff)

* applying a large stress produces a small strain !

Question 7

what is meant by a high proportional limit/elastic limit?

Answer 7

• only large stresses will cause permanent deformation

Question 8

what should the thermal expansion of PMMA match & why?

Answer 8

the artificial teeth to avoid internal stresses on cooling during manufacture

Question 9

why is it important that a denture has a low density?

Answer 9

to aid retention of upper denture

Question 10

how long has PMMA been around?

Answer 10

since about 1937

Question 11

what type of setting reaction does PMMA undergo?

Answer 11

a free radical addition polymerisation

Question 12

what is meant by a free radical addition polymerisation reaction?

Answer 12

chemical union of two molecules either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecules

Question 13

what type of bonding is seen in a free radical addition polymerisation reaction?

Answer 13

involves molecules with carbon to carbon double bonds (C=C)

Question 14

what are the stages in the polymerisation of acrylic?

Answer 14

1. activation
2. initiation
3. propagation
4. termination

Question 15

what is the activation stage of polymerisation?

Answer 15

activation of initiator to form free radicals

Question 16

what is meant by the initiation stage of polymerisation?

Answer 16

free radicals break the C=C bond in monomer and transfer free radicals

Question 17

what is meant by the propagation stage of polymerisation?

Answer 17

growing polymer chain

Question 18

what is the initiator in the acrylic polymerisation reaction?

Answer 18

benzoyl peroxide

Question 19

how is the polymerisation of PMMA activated?

Answer 19

(a) a heat > 72°C

(b) self cured

Question 20

what does activation give?

Answer 20

two free radicals

Question 21

what is the heat cured acrylic mixture made up of?

Answer 21

• powder

* liquid

Question 22

what components make up the powder aspect of heat cured acrylic?

Answer 22

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

Question 23

what is the function of plasticiser?

Answer 23

allows quicker dissolving in monomer liquid

Question 24

what is the function of co-polymers?

Answer 24

to improve mechanical properties & enhance performance

Question 25

what components make up the liquid aspect of heat cured acrylic?

Answer 25

• methacrylate monomer
• inhibitor
• co-polymers

Question 26

what is the function of the inhibitor?

Answer 26

• prolongs shelf life (by reacting with any free radicals produced by heat/UV light)

Question 27

why is the powder/liquid mixture used?

Answer 27

• to produce dough-like material that can be handled/mixed easily and customised to desired shape
• reduce heat of reaction
• minimise polymerisation shrinkage

Question 28

what occurs once the dough like acrylic has been packed into mould?

Answer 28

it is CLAMPED & ready for heat curing

Question 29

what are the heating schedules of PMMA?

Answer 29

(1) 7 hrs. to 70°C. + 2 hrs. to 100°C. + slow cool
(2) 72°C for at least 16 hours
(3) 20-20-20 reverse curing

Question 30

do the mould material and the acrylic have the same thermal expansion coefficients?

Answer 30

NO

Question 31

what may occur if the mould & mixture is cooled too quickly?

Answer 31

internal stresses

Question 32

what are the consequences of internal stresses?

Answer 32

• lower compressive and tensile strength
• lower fatigue strength
• warping
• repair problems

Question 33

what can occur if PMMA is under cured?

Answer 33

• free monomer (irritant to oral tissues)

* low molecular weight (poor mechanical)

Question 34

what can occur if PMMA is cured too fast?

Answer 34

possible gaseous porosity (compromises denture strength)

Question 35

why is porosity undesirable?

Answer 35

• affects strength
• affects appearance
• rough sensation to tongue
• absorbs saliva (poor hygiene)