Acrylic Flashcards
ideal properties of denture base (4)
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)
physical properties of denture base needs to be (11)
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
mechanical properties of acrylic resin need to be (6)
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
transverse strength
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
possible ways dentures deal with impacts (2)
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
free radical addition polymerisation
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)
monomer in PMMA free radical addition polymerisation
methacrylate
4 stages in acrylic polymerisation
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
activation
free radical acrylic polymerisation
of initiator to provide free radicals
initiation
free radical acrylic polymerisation
free radicals break C=C bond in monomer and transfer free radical
propagation
free radical acrylic polymerisation
growing polymer chain
termination
free radical acrylic polymerisation
of polymerisation, chain growth stops
2 components of heat cured acrylic
liquid
powder
powder constituents of heat cured acrylics (5)
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
liquids in heat cured acrylics (3)
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
role of co-polymers in heat cured acrylic liquid
improve mechanical properties - particularly cross-linking of polymers
role of inhibitor in heat cured acrylic liquid
Hydroquinone, 0.006%
prolongs shelf life - reacts with any free radicals produced by heat, UV light
role of methacrylate monomers in heat cured acrylic liquid
dissolves PMMA particles – polymerises
technique of making acrylic resin denture base
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
acrylic heat curing
Need efficient polymerisation to give high molecular weight polymer
i.e. good mechanical properties
Hence high temperature but gaseous porosity limits
acrylic properties
non toxic
yes
acrylic properties
non irritant
if no monomer released
few pts allergic
acrylic properties
unaffected by oral fluid
water absorption
virtually insoluble in fluids taken orally
acrylic properties
mechanical properties
poor
increase bulk to compensate
acrylic properties
fatigue strength/impact strength
‘fairly resistant’ but can be cause of failure
acrylic properties
hardness/abrasion resistance
high
retains good polish
some wear over time
acrylic properties
thermal expansion
= artificial teeth
OK if acrylic teeth used
significantly higher than porcelain teeth
acrylic properties
thermal conductivity
low
key disadvantage
acrylic properties
density
low - good
but increase in bulk to overcome poor mechanical properties offsets advantgae
acrylic properties
softening temperatire
75 degrees
Ok for ingested hot fluids
DON’T use boiling water for cleaning
acrylic properties
dimensional accurate and stable in use
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.
heat cured acrylic resin dimensional accuracy and stability
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
self curing acrylics
As heat cured, except benzoyl peroxide, is activated by promoter* (tertiary amine) in liquid
eg * dimethyl-para-toluidine
not heat
promoter in self cured acrylics
tertiary amine in liquid
eg * dimethyl-para-toluidine
why use self curing acrylics
lower temperature thus less thermal contraction (no heating stage)
Hence Better Dimensional Accuracy (better fit)
chemical activation of self curing acrylics
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
issue of less efficient self cured acrylic chemical activation compared to heat cured
- 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)
chemical activation of heat cured acrylic Vs Self cured acrylic
Chemical cure :
- 3 to 5% unreacted monomer (risk of dimensional instability)
Heat cure :
- 0.2 to 0.5% unreacted monomer MORE EFFECTIVE
heat cured Vs Self Cured acrylic
properties
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
acrylic resin dentures issues
poor strength & toughness
10% fracture within 3 years
improved acrylic - why
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
improved acrylic options (2)
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
ultra-Hi - heat cure denture base product
High Impact heat cure acrylic resin - that exudes quality and gives the technician confidence”
new ingredients
ultra-Hi - heat cure denture base product
properties (2 key)
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
pour n cure resins
similar to SC
smaller powder particles
fluid mix pour into mould
good fitting but poor mechanical properties
light activated denture resins (4 components)
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
use of light activated denture resins
used mostly as customised impression tray material & for repair of fractured
radiopaque polymers (4 options)
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
reason for radiopaque polymers
if any fragments break off, and there’s a risk they’ve been swallowed, a radiograph could be taken to confirm this
alternative polymers - used when
PROVEN allergy to acrylic?
TRY
- Nylons
- Vinyl polymers
- Polycarbonates
nylon issue as alternative polymer
Water absorption
- Swelling
- Softening
vinyl polymer as alternative polymer
e.g. polyvinyl acetate, polyvinylchloride, styrene
injection moulding
- expensive
softening in use
- Tg = 60 C
polycarbonates as alternative polymer
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
most commonly used denture base material
acrylic resin (hear cured)
