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BDS2 - dental material science > PMMA > Flashcards

PMMA Flashcards

1
Q

why is manufacturing a denture challenging

A

it is a complex 3D customised object

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2
Q

how do you measure the success of a denture

A

based on patients feedback - they will either say it fits or it doesn’t fit

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3
Q

what are the error sources in the production of dentures

A

○ Initial impression is flawed
§ Material and method not 100% accurate
§ If these aren’t perfect then the denture cannot be perfect

○ Curing process
§ Several potential error sources

○ May not achieve perfect fit at first attempt
§ Refinements at the chairside may be required
[Can make minor adjustments but the overall fit has to be close enough to the desired fit for this to work]

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4
Q

what are the error sources in the usage of dentures

A

○ Fits patient only for short period

○ Fractures
§ During use
§ Accident

○ Uncomfortable

○ Becomes warped
§ The way the denture is handled

○ Surface suffers wear
§ Gets worn away or the surface becomes rough and becomes uncomfortable for the patient

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5
Q

list ideal properties of dentures (not including mechanical or thermal properties)

A
  • Unaffected by oral fluids
  • Replaces function of natural teeth
  • Is seen by other people
  • Dimensionally accurate and stable in use
  • Must fit patient’s mouth and be retained
  • Low density - aids retention of upper denture as the heavier the material, the greater the gravity pull
  • Colour / translucency matching natural tissues
  • Non toxic, non irritant
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6
Q

list ideal mechanical properties of dentures

A

○ High young’s (elastic) modulus
§ Rigid (stiff)
§ Large stress produces small strain
○ High proportional limit / elastic limit - Only large stresses will cause permanent deformation

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7
Q

list ideal thermal properties of dentures

A

• High softening temperature (Tg)
○ Must not distort during ingesting of hot fluids or during cleaning ie won’t change with high temperatures

• Thermal expansion
○ Must be the same as the artificial tooth
§ Avoid internal stresses on cooling during manufacture

• High thermal conductivity
○ Transmission of thermal stimuli to mucosa to avoid scalding the back of throat or oesophagus

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8
Q

what are the available materials for making a denture

A

acrylic / polymethylmethacrylate (PMMA)

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9
Q

explain free radical addition polymerisation

A
  • Chemical union of 2 molecules either the same or different to form a larger molecule without the elimination of a smaller molecule
  • Involves molecules with (C=C bonds)
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10
Q

explain the free radical addition polymerisation of methacrylate monomer

A

double bond is what we are looking to break down so it links to other molecules = so material becomes rigid, heavy molecule
want cross linking happen

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11
Q

explain acrylic polymerisation

A

• Activation
○ Of initiator to provide free radicals

• Initiation
○ Free radicals break C=C bond in monomer and transfer free radical

• Propagation
○ Growing polymer chain

• Termination
○ Of polymerisation
○ Polymerisation stage just stops

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12
Q

what is the initiator in acrylic polymerisation

A

Benzoyl peroxide
(NB symmetrical)
C6H5COO-OOCH5C6

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13
Q

what activates acrylic polymerisation

A

A) Heat > 72°C
- Different heat curing cycles

B) Self cured
- Not in this lecture

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14
Q

what does activation of polymerisation give

A

○ Activation gives 2 free radicals R*
§ C6H5COO*
§ Breaking down benzoyl peroxide so it is split in the middle
§ Electrical charge

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15
Q

what happens in propagation stage in acrylic polymerisation

A 
R* + M --> R-M*
R-M* + M --> R-M-M*
R-M-M* + M --> R-M-M-M*
…. And so on
Initial monomer grows and cross links with other molecules
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16
Q

what is found in the powder of heat cured acrylic

A

○ Initiator
§ Benzoyl peroxide (0.2-0.5%)

○ PMMA Particles
§ Pre-polymerised beads
§ Speeds things up
□ Introduce into the powder PMMA that has been produced before and then it is ground into little beads which then react with the liquid

○ Plasticiser
§ Allows quicker dissolving in monomer liquid
§ Eg dibutyl phthalate
§ Speeds up the process

○ Pigments
§ To give “natural” colour

○ Co-polymers
§ To improve mechanical properties
§ Eg ethylene glycol dimethacrylate

17
Q

what is found in the liquid of heat cured acrylic

A

○ Methacrylate monomer
§ Dissolves PMMA particles - polymerises

○ Inhibitor (hydroquinone, 0.006%)
§ Prolongs shelf life - reacts with any free radicals produced by heat, UV light
§ Prevents any free radicals that might be produced inadvertently as the material is being stored somewhere
§ Could be triggered by heat or light
§ Stops it from reacting before it gets to the powder

○ Co-polymers
§ Improve mechanical properties - particularly cross-linking of polymers

18
Q

why are the powder and liquid mixed in a heat cured acrylic

A

○ To produce 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 will shrink by 21%
§ Mix it with powder and it will shrink by 7%
§ Substantial amount of shrinkage during heat curing process

19
Q

what are the proportions of powder to liquid in heat cured acrylic

A

○ 3 to 3.5 / 1
§ P / L by volume
§ Follow instructions for ratios

○ 2.5 / 1
§ By weight

20
Q

what are the mixing stages of heat cured acrylic

A

Sandy - tacky

Dough - packing

21
Q

how is the shape of the acrylic denture acquired

A
  • Metal vessel filled with mould material that is the shape of the patient’s mouth
  • Individual teeth placed within the mould material
  • Brush allows separation later in the process
  • Pack dough like material in - important to pack in
  • Allow excess material besides
  • Take flasks and clamp them together
  • Apply an appropriate pressure
22
Q

why do you need efficient polymerisation in acrylics heat curing

A

• Need efficient polymerisation to give high molecular weight polymer ie good mechanical properties
= Better the polymerisation then the better the material you are going to get

Hence high temperature but gaseous porosity limits
○ Ideally want a high temperature
○ But if you have the wrong heat curing cycle / temperature then this will cause porosity

23
Q

what are examples of heating schedules for heat cured acrylic

A

1) 7 hours to 70°C + 2 hours to 100°C + slow cool

2) 72°C for at least 16 hours
- Simplest

3) 20-20-20 reverse curing
- Place flask in boiling H2O
- Remove heat for 20 minutes
- Heat to 70°C for 20 minutes
- Then heart to 100°C for 20 minutes

Don’t be too concerned about the details of these

24
Q

what happens when the temperature is raised about 100 degrees C in heat cured acrylic

A

If temperature peaks above 100°C then gases are produced which lead to porosity which leads to a weak material

Raise temp to 70°C - exothermic reaction of the polymerisation will raise it to 100°C
Then it is dropped and 70°C is maintained

Stay clear of exceeding 100°C within the acrylic

25
Q

how should heat cured acrylic be cooled

A

Mould material and acrylic have different thermal expansion coefficients
this can create Internal stresses within the acrylic

§ Cool slowly - allow relief
Cool flasks down gradually over many hours

26
Q

what is the problem with internal stresses being present in the acrylic resin

A

• Decrease strength
• Decrease fatigue strength
• More prone to warping at a later stage
○ Eg during finishing
• Repair problems later on
○ If the acrylic resin needs repaired (eg there is a crack on the surface) then the technician cannot fix it as easily - Will lead to fracture
○ Won’t find out at time of production or fitting but this problem will be revealed during the later stage

• Internal stresses are related to
○ Shape and size eg notches
○ Curing pressure
○ Curing cycle
○ Thermal expansion
○ Cooling rate
[Complexity of the whole process]
27
Q

what is the problem if the acrylic is under cured

A

○ Free monomer (irritant)
§ Monomer that is not cured that can leak out of the denture base
§ If patient complains of irritant then it is likely to be this problem

○ Low molecular weight
§ Poor mechanical properties
§ Easily fractured

28
Q

what is the problem with fast curing the acrylic

A

○ Possible gaseous porosity
§ Creates bubbles within the material
§ Weakens the material

29
Q

what are the problems with incorrect powder / monomer ratios

A

• Too much monomer
○ Contraction porosity

• Too little monomer
○ Granularity

30
Q

what problems does porosity cause in acrylic

A 
• Affects strength 
• Affects appearance 
	○ Certain roughness to the surface
	○ Roughness attracts plaque - doesn’t look attractive
• Rough sensation to tongue 
	○ Uncomfortable for the patient
• Absorbs saliva - poor hygiene
31
Q

where and how does gaseous porosity occur

A
  • Monomer boiling (100°C) produces bubbles
  • Occurs in bulkier parts
  • Be careful NB exothermic reaction

Hence cure slowly

32
Q

what is contraction porosity caused by

A

too much monomer

33
Q

what is the polymerisation shrinkage in acrylic

A

○ Monomer alone = 21% (by volume)
§ Too much monomer is going to cause more shrinkage than you would expect

○ Powder + monomer mix = 7%

34
Q

what causes polymerisation shrinkage in acrylic

A 
○ Too much monomer 
○ Insufficient excess material
	§ Not enough dough packed in
○ Insufficient clamp pressure
	§ Need pressure to force the flasks together

• Occurs anywhere where dough is not sufficiently packed

35
Q

list acrylic properties (not including thermal and mechanical properties)

A

• Non toxic, Non irritant
○ Provided no monomer released - may cause problems at a later stage
○ some patients allergic to the monomers

• Unaffected by oral fluids
○virtually insoluble in fluids taken orally

• good, natural colour / translucency

• Low density
○ Low is good but the need to increase in bulk to overcome poor mechanical properties offsets this advantage so don’t usually get the benefit of this property

• Dimensionally accurate and stable in use
○ Linear contraction = 0.5% and this is acceptable

36
Q

what are the thermal properties of acrylic

A

• Thermal expansion same as artificial tooth
○ Ok with acrylic teeth
○ Porcelain teeth cause a problem as they have different thermal expansions

• High thermal conductivity
○ Low - poor, doesn’t perform well

• High softening temperature
○ 75°C = ok for ingested hot fluids (tea / coffee no problem)
○ Don’t use boiling water for cleaning as it will cause the denture to warp and not fit

37
Q

what are the mechanical properties of acrylic

A

• Mechanical properties (ie YM and PL & EL)
○ Poor, increase in bulk to compensate
○ Bulk it up to 1.5mm thickness for the acrylic resin denture to make up for the weaker mechanical properties

38
Q

what is the most commonly used denture base material

A

acrylic resin (heat cured)

BDS2 - dental material science (13 decks)

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