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BDS3 DMS > Acrylic > Flashcards

Acrylic Flashcards

1
Q

ideal properties of denture base (4)

A

replaces function of natural teeth

goes into patient’s mouth

aesthetic as is seen by other people

has to give value for money (NHS or patient)

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2
Q

physical properties of denture base needs to be (11)

A

Dimensionally accurate and stable in use

High Softening Temperature (Tg)

Unaffected by Oral Fluids

Thermal Expansion

Low Density

High Thermal Conductivity

Radiopaque
- Ideally in case part of the denture fractures

Non-Toxic, Non Irritant

Colour / Translucency

Easy & inexpensive to manufacture

Easy to repair

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3
Q

mechanical properties of acrylic resin need to be (6)

A

High Young’s (Elastic) Modulus

High Proportional Limit
- able to cope with high stresses, before recovering to its original shape

High Transverse Strength*

High Fatigue Strength

High Impact Strength

High Hardness / Abrasion Resistance

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4
Q

transverse strength

A

2 point loading (Flexural)

How well does upper denture cope with stresses that cause deflection

The pivot point is the palate

Forces on either side are applied, potentially causing fracture at the pivot point
- This is the worst case scenario for acrylic resin

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5
Q

possible ways dentures deal with impacts (2)

A

A denture material has to cope with impacts. Being dropped on a hard floor may generate such an impact force that the acrylic resin may fracture.

A more insidious failure in this same scenario, is where the acrylic seems to withstand the impact.
- But, the impact may create micro-cracks sub-surface, denture fails a little later; say when being fitted

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6
Q

free radical addition polymerisation

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)

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7
Q

monomer in PMMA free radical addition polymerisation

A

methacrylate

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8
Q

4 stages in 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

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9
Q

activation

free radical acrylic polymerisation

A

of initiator to provide free radicals

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10
Q

initiation

free radical acrylic polymerisation

A

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

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11
Q

propagation

free radical acrylic polymerisation

A

growing polymer chain

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12
Q

termination

free radical acrylic polymerisation

A

of polymerisation, chain growth stops

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13
Q

2 components of heat cured acrylic

A

liquid

powder

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14
Q

powder constituents of heat cured acrylics (5)

A

Initiator (Benzoyl Peroxide, 0.2 - 0.5%)

PMMA Particles – pre-polymerised beads

Plasticiser - allows quicker dissolving in monomer liquid eg dibutyl phthalate

Pigments – to give “natural” colour

Co-polymers - to improve mechanical properties eg ethylene glycol dimethacrylate

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15
Q

liquids in heat cured acrylics (3)

A

Methacrylate Monomer
- dissolves PMMA particles – polymerises

Inhibitor (Hydroquinone, 0.006%)
- prolongs shelf life - reacts with any free radicals produced by heat, UV light

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

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16
Q

role of co-polymers in heat cured acrylic liquid

A

improve mechanical properties - particularly cross-linking of polymers

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17
Q

role of inhibitor in heat cured acrylic liquid

A

Hydroquinone, 0.006%

prolongs shelf life - reacts with any free radicals produced by heat, UV light

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18
Q

role of methacrylate monomers in heat cured acrylic liquid

A

dissolves PMMA particles – polymerises

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19
Q

technique of making acrylic resin denture base

A

vessel containing a mould material. Artificial teeth are being placed ready for the
acrylic resin, in dough-like form, to be packed into place.

acrylic in its dough-like form is inserted into the mould recess taking up the shape of the patient’s dentition.
- Now the acrylic needs to be CURED to form a strong solid denture base

2 halves of the vessel are clamped together. Ready to be subjected to the heating cycle required to cause polymerization

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20
Q

acrylic heat curing

A

Need efficient polymerisation to give high molecular weight polymer

i.e. good mechanical properties
Hence high temperature but gaseous porosity limits

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21
Q

acrylic properties

non toxic

A

yes

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22
Q

acrylic properties

non irritant

A

if no monomer released

few pts allergic

23
Q

acrylic properties

unaffected by oral fluid

A

water absorption

virtually insoluble in fluids taken orally

24
Q

acrylic properties

mechanical properties

A

poor

increase bulk to compensate

25
Q

acrylic properties

fatigue strength/impact strength

A

‘fairly resistant’ but can be cause of failure

26
Q

acrylic properties

hardness/abrasion resistance

A

high

retains good polish
some wear over time

27
Q

acrylic properties

thermal expansion

A

= artificial teeth

OK if acrylic teeth used

significantly higher than porcelain teeth

28
Q

acrylic properties

thermal conductivity

A

low

key disadvantage

29
Q

acrylic properties

density

A

low - good

but increase in bulk to overcome poor mechanical properties offsets advantgae

30
Q

acrylic properties

softening temperatire

A

75 degrees

Ok for ingested hot fluids

DON’T use boiling water for cleaning

31
Q

acrylic properties

dimensional accurate and stable in use

A

OK

Linear Contraction 0.5% - Acceptable

Water absorption - expands about 0.4% - approx makes up for the contraction that took place in the heat-curing stage.

32
Q

heat cured acrylic resin dimensional accuracy and stability

A

Manufacture: 0.5% linear contraction

Usage: 0.4% expansion
- Water absorption - expands about 0.4% - approx makes up for the contraction that took place in the heat-curing stage

33
Q

self curing acrylics

A

As heat cured, except benzoyl peroxide, is activated by promoter* (tertiary amine) in liquid

eg * dimethyl-para-toluidine
not heat

34
Q

promoter in self cured acrylics

A

tertiary amine in liquid

eg * dimethyl-para-toluidine

35
Q

why use self curing acrylics

A

lower temperature thus less thermal contraction (no heating stage)

Hence Better Dimensional Accuracy (better fit)

36
Q

chemical activation of self curing acrylics

A

less efficient than heat cured

Chemical cure :
- 3 to 5% unreacted monomer (risk of dimensional instability)
Heat cure :
- 0.2 to 0.5% unreacted monomer MORE EFFECTIVE

37
Q

issue of less efficient self cured acrylic chemical activation compared to heat cured

A
  • Hence lower molecular weight
  • Hence poorer mechanical properties and Tg lower
  • Hence more unreacted monomer
    which acts as plasticiser, softening denture base, reducing transverse strength
  • more vulnerable to failure
  • potential tissue irritant, compromising its biocompatibility (monomer leaking out)
38
Q

chemical activation of heat cured acrylic Vs Self cured acrylic

A

Chemical cure :
- 3 to 5% unreacted monomer (risk of dimensional instability)
Heat cure :
- 0.2 to 0.5% unreacted monomer MORE EFFECTIVE

39
Q

heat cured Vs Self Cured acrylic

properties

A

Heat Cured
- higher molecular weight
- Stronger
Survive longer – better for pt

  • curing process may cause porosity
    technician’s skills invaluable to prevent this
  • check for flaws – bubbles below surface

Self Cured
- higher monomer levels
irritant on pt soft tissue
tell pt of risk and get them to keep you informed

  • fits cast better but water absorption in mouth makes oversized
  • poorer colour stability
    tertiary amines susceptible to oxidation

Neither ideal

40
Q

acrylic resin dentures issues

A

poor strength & toughness

10% fracture within 3 years

41
Q

improved acrylic - why

A

attempts to strengthen acrylic resin

high impact resistant materials

incorporate rubber toughening agent (butadien styrene)

  • stop crack propagation on base upon impact
  • long term fatigue problems
incorporate fibres (carbon, UHMPE – ultra-high molecular wt polyethylene, glass)
- difficult processing – ongoing
42
Q

improved acrylic options (2)

A

incorporate rubber toughening agent (butadien styrene)

  • stop crack propagation on base upon impact
  • long term fatigue problems
incorporate fibres (carbon, UHMPE – ultra-high molecular wt polyethylene, glass)
- difficult processing – ongoing
43
Q

ultra-Hi - heat cure denture base product

A

High Impact heat cure acrylic resin - that exudes quality and gives the technician confidence”

new ingredients

44
Q

ultra-Hi - heat cure denture base product

properties (2 key)

A

exceptional flexural strength
- increasing its chances of surviving for longer.

superior fracture toughness (ductility)
- helps to mitigate the effect of any micro-cracks that may be present

These two key features together gives Ultra-Hi

  • a slight bending aspect which keeps the material from being brittle and subject to cracking and/or breaking.
  • used in GDH production lab
45
Q

pour n cure resins

A

similar to SC

smaller powder particles

fluid mix pour into mould

good fitting but poor mechanical properties

46
Q

light activated denture resins (4 components)

A

urethane dimethacrylate UDMA matrix plus acrylic copolymers
- and microfine silica fillers
(small amounts to control rheology (flow during manufacture))

photoinitiator systems - see Composite lecture

adapted to cast

cured in light chamber
- limited depth of cure thus limiting depth of denture

used mostly as customised impression tray material & for repair of fractured

47
Q

use of light activated denture resins

A

used mostly as customised impression tray material & for repair of fractured

48
Q

radiopaque polymers (4 options)

A

metal inserts added to resin
- weaken, poor aesthetics

inorganic salts (e.g. barium sulphate) -

  • low conc = not radiopaque
  • high conc = weak base

comonomers containing heavy metals e.g. barium sulphate,
- poor mechanical properties

halogen containing comonomers or additives e.g. tribromophenylmethacrylate
- may act as plasticiser
- expensive
? promising – no indication if these are sufficient

49
Q

reason for radiopaque polymers

A

if any fragments break off, and there’s a risk they’ve been swallowed, a radiograph could be taken to confirm this

50
Q

alternative polymers - used when

A

PROVEN allergy to acrylic?

TRY

  • Nylons
  • Vinyl polymers
  • Polycarbonates
51
Q

nylon issue as alternative polymer

A

Water absorption

  • Swelling
  • Softening
52
Q

vinyl polymer as alternative polymer

A

e.g. polyvinyl acetate, polyvinylchloride, styrene

injection moulding
- expensive

softening in use
- Tg = 60 C

53
Q

polycarbonates as alternative polymer

A

injection moulded
- expensive

Good impact strength

Tg = 150C
- Able to withstand large temperatures (more than acrylic resin)

internal stresses develop in use
- distortion and poor fit

54
Q

most commonly used denture base material

A

acrylic resin (hear cured)

BDS3 DMS (9 decks)

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