PMMA (acrylic resin) Flashcards Preview

BDS2 Dental Material Science > PMMA (acrylic resin) > Flashcards

Flashcards in PMMA (acrylic resin) Deck (38)
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1
Q

what usage errors can cause a denture to fail?

A
  • fractures
  • uncomfortable
  • becomes warped
  • surface suffers wear
  • fits patient only for short period
2
Q

what are the ideal properties of PMMA?

A
  • dimensionally accurate and stable in use
  • high softening temperature
  • unaffected by oral fluids
3
Q

what does stress/strain measure?

A

determines a materials stiffness

4
Q

what determines the stress/strain of a material?

A

Young’s (elastic) Modulus

5
Q

what are the ideal MECHANICAL properties of PMMA?

A
  • high Young’s (elastic) modulus

* high proportional limit/elastic limit

6
Q

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

A
  • material is rigid (stiff)

* applying a large stress produces a small strain !

7
Q

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

A

• only large stresses will cause permanent deformation

8
Q

what should the thermal expansion of PMMA match & why?

A

the artificial teeth to avoid internal stresses on cooling during manufacture

9
Q

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

A

to aid retention of upper denture

10
Q

how long has PMMA been around?

A

since about 1937

11
Q

what type of setting reaction does PMMA undergo?

A

a free radical addition polymerisation

12
Q

what is meant by a free radical addition polymerisation reaction?

A

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

13
Q

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

A

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

14
Q

how many stages are their in the acrylic polymerisation process?

A

4 stages

15
Q

what are the stages in the polymerisation of acrylic?

A
  1. activation
  2. initiation
  3. propagation
  4. termination
16
Q

what is the activation stage of polymerisation?

A

activation of initiator to form free radicals

17
Q

what is meant by the initiation stage of polymerisation?

A

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

18
Q

what is meant by the propagation stage of polymerisation?

A

growing polymer chain

19
Q

what is the initiator in the acrylic polymerisation reaction?

A

benzoyl peroxide

20
Q

how is the polymerisation of PMMA activated?

A

(a) a heat > 72°C

(b) self cured

21
Q

what does activation give?

A

two free radicals

22
Q

what is the heat cured acrylic mixture made up of?

A
  • powder

* liquid

23
Q

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

A
  • initiator (benzoyl peroxide)
  • PMMA particles
  • plasticiser
  • pigments
  • co-polymers
24
Q

what is the function of plasticiser?

A

allows quicker dissolving in monomer liquid

25
Q

what is the function of co-polymers?

A

to improve mechanical properties & enhance performance

26
Q

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

A
  • methacrylate monomer
  • inhibitor
  • co-polymers
27
Q

what is the function of methacrylate monomer?

A

dissolves the PMMA particles

28
Q

what is the function of the inhibitor?

A

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

29
Q

why is the powder/liquid mixture used?

A
  • to produce dough-like material that can be handled/mixed easily and customised to desired shape
  • reduce heat of reaction
  • minimise polymerisation shrinkage
30
Q

what proportions should the powder to liquid mixture be?

A
  • 3 to 3.5 / 1 (P/L) by volume

* 2.5 / 1 (P/L) by weight

31
Q

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

A

it is CLAMPED & ready for heat curing

32
Q

what are the heating schedules of PMMA?

A

(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

33
Q

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

A

NO

34
Q

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

A

internal stresses

35
Q

what are the consequences of internal stresses?

A
  • lower compressive and tensile strength
  • lower fatigue strength
  • warping
  • repair problems
36
Q

what can occur if PMMA is under cured?

A
  • free monomer (irritant to oral tissues)

* low molecular weight (poor mechanical)

37
Q

what can occur if PMMA is cured too fast?

A

possible gaseous porosity (compromises denture strength)

38
Q

why is porosity undesirable?

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