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BDS2 Dental Materials Sciences > PMMA > Flashcards

PMMA Flashcards

1
Q

what is the measure of success of a denture fit

A

patient feedback

- natural appearance

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2
Q

3 error sources in denture production

A

initial impression is flawed – material and method not 100% accurate

curing process - several potential error sources
may not achieve perfect fit at first attempt

REFINEMENTS at the chairside required
- Little adjustments is feasible
initial impression is flawed – material and method not 100% accurate
curing process - several potential error sources
may not achieve perfect fit at first attempt
REFINEMENTS at the chairside required
Little adjustments is feasible

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3
Q

5 error sources in denture usage

A

fits patient only for short period
- with time and wear fit fails

fractures

uncomfortable

becomes warped – implications
on shape

surface suffers wear

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4
Q

13 ideal properties of denture base material

A

replaces function of natural teeth

goes into patient’s mouth

needs to be aesthetically pleasing

Dimensionally accurate and stable in use
- must fit patient’s mouth AND be retained

High Softening Temperature (Tg)
- must not distort during ingesting of hot fluids, or during cleaning

Unaffected by Oral Fluids
over time

High Young’s (Elastic) Modulus
rigid (stiff) – large stress produces small strain

High Proportional Limit / Elastic Limit
- only large stresses will cause permanent deformation

Thermal Expansion
= Artificial tooth
- Match denture base and artificial teeth
- avoid internal stresses on cooling during manufacture (more on internal stresses in Metals & Alloys)
take into account internal stresses when manufacturing

High Thermal Conductivity
transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus from hot food/drink

Low Density
- aid retention of upper denture
the heavier the material, greater the gravity pull

Colour / Translucency
to match natural tissues

Non Toxic, Non Irritant

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5
Q

what is the free radical addition polyermisation reaction which occurs in PMMA

A

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

INVOLVES molecules with (C=C bonds)

  • Break C=C
  • Become a larger, longer molecule so more rigid
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6
Q

what reaction occurs in PMMA on setting

A

free radical addition polyermisation reaction

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7
Q

4 stages in acrylic polyemrsation

A

activation

initiation

propagation

termination

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8
Q

what occurs in activation in acrylic polymerisation

A

of initator to provide free radicals

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9
Q

what occurs in initiation of acrylic polymerisation

A

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

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10
Q

what occurs in propagation acrylic polymerisation reaction

A

growing polymer chain

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11
Q

what occurs in termination of acrylic polymerisation reaction

A

final molecule made

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12
Q

what is the initiator in acrylic polymerisation

A

benzoyl peroxide

C6H5COO-OOCH5C6
symmetrical

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13
Q

what are the 2 types of acrylic addition polymerisation activations

A

heat above 72 degrees C

or self cured

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14
Q

what does activation of the initiation addition polymerisation reaction achieve

A

two free radicals C6H5COO*

electrically charged

react with another molecule/monomer
- grows and cross links with other methacrylate monomer

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15
Q

monomer in PMMA

A

methacrylate

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16
Q

2 components of heat cured accylic

A

powder

liquid

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17
Q

5 parts of powder in heat cured acrylic

A

Initiator
- (Benzoyl Peroxide, 0.2 - 0.5%)

PMMA Particles

  • pre-polymerised beads
  • produced before, ground into beads, mixed into powder, reacting with liquid

Plasticiser
- allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
- improves dissolution of monomer liquid
speed up process

Pigments
- to give “natural” colour

Co-polymers

  • to improve mechanical properties e.g. ethylene glycol dimethacrylate
  • PMMA monomer cannot do on its own
18
Q

what is the role of the PMMA particles/beads in the powder of PMMA?

A

pre-polymerised beads

- produced before, ground into beads, mixed into powder, reacting with liquid

19
Q

what is the role of the plasticiser in the PMMA powder

A
  • allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
  • improves dissolution of monomer liquid
    speed up process
20
Q

what is the role of the co-polymers in the PMMA powder

A

improve mechanical properties e.g. ethylene glycol dimethacrylate
- PMMA monomer cannot do on its own

21
Q

3 components of the liquid in PMMA

A

Methacrylate Monomer
- dissolves PMMA particles – polymerises

Inhibitor (Hydroquinone, 0.006%)
- prolongs shelf life
reacts with any free radicals produced by heat, UV light
- Prevent any free radicals made inadvertently when material stored somewhere

Co-polymers
- improve mechanical properties - particularly cross-linking of polymers

22
Q

what is the role of the inhibitor in liquid part of PMMA?

A
  • prolongs shelf life
    reacts with any free radicals produced by heat, UV light
  • Prevent any free radicals made inadvertently when material stored somewhere
23
Q

what is the role of the co polymers in the liquid part of PMMA?

A

improve mechanical properties - particularly cross-linking of polymers

24
Q

what is the role of the methacrylate monomer in the liquid part of PMMA?

A

dissolves PMMA particles – polymerises

25
Q

what is the result of mixing the liquid and powder components of PMMA?

A

dough-like material that can be handled/mixed easily and customised to desired shape

reduce heat of reaction

minimise polymerisation shrinkage

  • monomer on its own shrink by 21% but with liquid only 7%
  • substantial shrinkage factored into heat curing process
26
Q

2 stages of PMMA mixing

A

sandy - tacky

dough - packing

27
Q

how to use PMMA clamp flask

A

Filed with investment/mould material

  • Shaped to pt mouth
  • Individual teeth placed to mould material

Brush applied to palate area

Dough packed into investment materials
- Allow excess material at the sides

Clamp together
- Apply sufficient pressure

28
Q

why is sufficient heat curing required for PMMA

A

Need efficient polymerisation to give high molecular weight polymer

i.e. good mechanical properties
better the polymerisation the better the material

use a high temperature ideally to make efficient
- but the wrong temperatures can cause porosities in the acrylic

hence high temperature but gaseous porosity limits
different heating schedules depending on the material used
- Stay clear of 100 o C in acrylic to avoid weakness

29
Q

how should PMMA be cooled

A

Cool slowly in flask over many hours to allow relief and internal stresses

30
Q

what can internal stresses in acrylic lead to

A

Decreased strength

Decreased fatigue

Warping e.g. during finishing

Repair problems

31
Q

what are some causes internal stresses in acrylic

A

Shape and size e.g. Notches

Curing pressure

Curing cycle

Thermal expansion

Cooling rate

32
Q

2 problems if PMMA undercured

A

Free monomer leak out of denure base
- Pt irritant

Low molecular weight
- Poor mechanical properties

33
Q

problem if PMMA cured too fast

A

possible gaseous porosity

34
Q

if too much monomer in PMMA

A

see contraction porosity

35
Q

if too little monomer in PMMA

A

see granularity of surface material

36
Q

4 effects of porosity

A

Affects strength

Affects appearance
- Roughness - Plaque trap

Rough sensation to tongue

Absorbs saliva - poor hygiene

37
Q

gaseous porosity caused by

A

monomer boiling

occurs in bulkier parts
- inspect bulky areas for subsurface imperfections

38
Q

what type of reaction is the free radical addition polymerisation of acrylic

A

exothermic

so cure slowly to avoid gaseous poroisty

39
Q

what are 3 causes contraction porosity

A

excess monomer causing polymerisation shrinkage

insufficient excess material

insufficient clamp pressure

40
Q

what are the softening temperatures of acrylic like

A

high
- OK for ingested hot fluids
DON’T use boiling water for cleaning
Will warp acrylic denture so no longer fits

BDS2 Dental Materials Sciences (13 decks)

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