Acrylic

Question 1

ideal properties of denture base (4)

Answer 1

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)

Question 2

physical properties of denture base needs to be (11)

Answer 2

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

Question 3

mechanical properties of acrylic resin need to be (6)

Answer 3

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

Question 4

transverse strength

Answer 4

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

Question 5

possible ways dentures deal with impacts (2)

Answer 5

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

Question 6

free radical addition polymerisation

Answer 6

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)

Question 7

monomer in PMMA free radical addition polymerisation

Answer 7

methacrylate

Question 8

4 stages in acrylic polymerisation

Answer 8

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

Question 9

activation

free radical acrylic polymerisation

Answer 9

of initiator to provide free radicals

Question 10

initiation

free radical acrylic polymerisation

Answer 10

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

Question 11

propagation

free radical acrylic polymerisation

Answer 11

growing polymer chain

Question 12

termination

free radical acrylic polymerisation

Answer 12

of polymerisation, chain growth stops

Question 13

2 components of heat cured acrylic

Answer 13

liquid

powder

Question 14

powder constituents of heat cured acrylics (5)

Answer 14

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

Question 15

liquids in heat cured acrylics (3)

Answer 15

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

Question 16

role of co-polymers in heat cured acrylic liquid

Answer 16

improve mechanical properties - particularly cross-linking of polymers

Question 17

role of inhibitor in heat cured acrylic liquid

Answer 17

Hydroquinone, 0.006%

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

Question 18

role of methacrylate monomers in heat cured acrylic liquid

Answer 18

dissolves PMMA particles – polymerises

Question 19

technique of making acrylic resin denture base

Answer 19

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

Question 20

acrylic heat curing

Answer 20

Need efficient polymerisation to give high molecular weight polymer

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

Question 21

acrylic properties

non toxic

Answer 21

yes

Question 22

acrylic properties

non irritant

Answer 22

if no monomer released

few pts allergic

Question 23

acrylic properties

unaffected by oral fluid

Answer 23

water absorption

virtually insoluble in fluids taken orally

Question 24

acrylic properties

mechanical properties

Answer 24

poor

increase bulk to compensate

Question 25

acrylic properties fatigue strength/impact strength

Answer 25

'fairly resistant' but can be cause of failure

Question 26

acrylic properties hardness/abrasion resistance

Answer 26

high retains good polish some wear over time

Question 27

acrylic properties thermal expansion

Answer 27

= artificial teeth OK if acrylic teeth used significantly higher than porcelain teeth

Question 28

acrylic properties thermal conductivity

Answer 28

low key disadvantage

Question 29

acrylic properties density

Answer 29

low - good but increase in bulk to overcome poor mechanical properties offsets advantgae

Question 30

acrylic properties softening temperatire

Answer 30

75 degrees Ok for ingested hot fluids DON'T use boiling water for cleaning

Question 31

acrylic properties dimensional accurate and stable in use

Answer 31

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.

Question 32

heat cured acrylic resin dimensional accuracy and stability

Answer 32

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

Question 33

self curing acrylics

Answer 33

As heat cured, except benzoyl peroxide, is activated by promoter* (tertiary amine) in liquid eg * dimethyl-para-toluidine not heat

Question 34

promoter in self cured acrylics

Answer 34

tertiary amine in liquid eg * dimethyl-para-toluidine

Question 35

why use self curing acrylics

Answer 35

lower temperature thus less thermal contraction (no heating stage) Hence Better Dimensional Accuracy (better fit)

Question 36

chemical activation of self curing acrylics

Answer 36

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

Question 37

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

Answer 37

- 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)

Question 38

chemical activation of heat cured acrylic Vs Self cured acrylic

Answer 38

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

Question 39

heat cured Vs Self Cured acrylic properties

Answer 39

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

Question 40

acrylic resin dentures issues

Answer 40

poor strength & toughness 10% fracture within 3 years

Question 41

improved acrylic - why

Answer 41

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 ```

Question 42

improved acrylic options (2)

Answer 42

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 ```

Question 43

ultra-Hi - heat cure denture base product

Answer 43

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

Question 44

ultra-Hi - heat cure denture base product properties (2 key)

Answer 44

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

Question 45

pour n cure resins

Answer 45

similar to SC smaller powder particles fluid mix pour into mould good fitting but poor mechanical properties

Question 46

light activated denture resins (4 components)

Answer 46

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

Question 47

use of light activated denture resins

Answer 47

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

Question 48

radiopaque polymers (4 options)

Answer 48

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

Question 49

reason for radiopaque polymers

Answer 49

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

Question 50

alternative polymers - used when

Answer 50

PROVEN allergy to acrylic? TRY - Nylons - Vinyl polymers - Polycarbonates

Question 51

nylon issue as alternative polymer

Answer 51

Water absorption - Swelling - Softening

Question 52

vinyl polymer as alternative polymer

Answer 52

e.g. polyvinyl acetate, polyvinylchloride, styrene injection moulding - expensive softening in use - Tg = 60 C

Question 53

polycarbonates as alternative polymer

Answer 53

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

Question 54

most commonly used denture base material

Answer 54

acrylic resin (hear cured)