PMMA Flashcards
Why is manufacturing a denture challenging?
As it is complex, customized 3D object
Often dentures don’t fit a patient first time. What problems in the production process could cause this? (2 points)
- Initial impression is flawed (material and method not 100% accurate)
- Curing process - several potential error sources
- May not achieve perfect fit at fist attempt but REFINEMENTS at chairside can be made
Often dentures don’t fit a patient first time. What problems in the usage of the denture could cause this? (5 points)
- Fits patient only for a short period
- Fractures
- Uncomfortable
- Becomes warped
- Surface suffers wear
What are the basic requirements for a denture material (PMMA)? (6 points)
- Replaces function of natural teeth
- Goes into patient’s mouth
- Is seen by other people
- Dimensionally accurate and stable in use (must fit patient’s mouth and be retained)
- High softening temperature (must not distort during ingestion of hot fluids, or during cleaning)
- Unaffected by oral fluids (over time)
What does the linear slope of a stress/strain diagram indicate?
- The rigidity of a material (young’s modulus)
What is the elastic limit of a material?
- Represents the stress beyond which the material will become deformed
What do you want the young’s modulus to be for denture material?
- High Young’s modulus
- Rigid (Stiff) - large stress produces small strain
What do you want the proportional/elastic limit of a denture material to be?
- High limit
- Only large stresses will cause permanent deformation
What do you want the thermal expansion of a denture material to be?
Want it to match an artificial tooth
- Avoid internal stresses on cooling during manufacture
What do you want the thermal conductivity of a denture material to be?
- High (can conduct heat readily)
- Transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus
What do you want the density of a denture material to be?
- Low
- Air retention of upper denture (the heavier the material, greater the gravity pull)
What do you want the colour/translucency of a denture material to be?
- To match the natural tissues
Do you want denture materials to be non-toxic and non-irritant?
- Yes
PMMA is produced by a free radical addition polymerisation. What is this?
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
What is the ‘activation’ stage of acrylic polymerisation?
- Activation of initiator to provide free radicals
What is the ‘initiation’ stage of acrylic polymerisation?Term
- Free radicals break C=C bond in monomer and transfer free radical
What is the ‘propagation’ stage of acrylic polymerisation?
Growing polymer chain
What is the ‘Ternimation’ stage of acrylic polymerisation?
End of polymerisation
What is the initiator in the polymerisation reaction of PMMA?
- Benzoyl peroxide (it is symmetrical)
What is the polymerisation reaction of PMMA activated by?
Can be heat at >72 degrees
or can be self cure
What is the free radical that is produced in the polymerisation reaction of PMMA?
C6H5COO*
What is included in the powder of heat cured acrylic? (5 points)
- Initiator
- PMMA particles
- Plasticiser
- Pigments
- Co-polymers
What is the initiator in heat cured acrylic?
Benzoyl Peroxide, 0.2-0.5%
What are PMMA particles already in heat cured acrylic?
To speed up the reaction
What does the plasticiser in heat cured acrylic do?
Allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
What do the pigments in heat cured acrylic do?
- Give the heat cured acrylic a natural colour
What does the co-polymer in heat cured acrylic do?
- Included to improve mechanical properties e.g. ethylene glycol dimethacrylate (because PLL methacrylate monomer will not do this by itself)
What does the methacrylate monomer in the liquid of heat cured acrylic do?
- Dissolves PMMA particles - polymerises
What does the inhibitor (Hydroquinone, 0.006%) in the liquid of heat cured acrylic do?
- Prolongs shelf life - reacts with any free radicals produced by heat, UV light
What do the co-polymers in the liquid of heat cured acrylic do?
Improve mechanical propertied - particularly cross-linking polymers
What is PMMA made of a powder/liquid mix? (3 points)
- To produce dough-like material that can be handled/mixed easily and customised to desired shape
- Reduce heat of reaction
- Minimise polymerisation shrinkage
What are the proportions of the powder/liquid mix of the PMMA?
- 3 to 3.5/1 (P/L) by volume
- 2.5/1 (P/L) by weight
What are the mixing stages of the powder/liquid mix of PMMA?
Sandy - tacky
Dough - packing
Why do you need efficient polymerisation of PMMA?
- To give a high molecular weight polymer i.e. good mechanical properties
- Hence high temperature but gaseous porosity limits
What are 3 examples of heating schedules of PMMA?
1) 7hrs. to 7C + 2hrs. to 100C + slow cool
2) 72C for at least 16hrs (easiest one)
3) 20-20-20 Reverse curing
- Place flask in boiling H2O
- Remove heat for 20 mins
- Heat to 70C for 20 mins
- Then heat to 100*C for 20 mins
When cooling the acrylic material, the mould and the acrylic have different thermal expansion coefficients which could cause internal stresses. How can this be avoided?
- Cool slowly - allow relief
- Cool gradually over many hours to reduce stresses
What are the disadvantages of having internal stresses? (4 points)
- Decreased strength
- Decreased fatigue strength (reduced ability to withstand low level stresses over a long time)
- Warping e.g. during finishing
- Repair problems e.g. if needs repair over time - if internal stresses, that’s when it really shows - will lead to fracture of the denture
What are internal stresses related to (what can cause them)? (5 points)
- Shape and size e.g. notches
- Curing pressure
- Curing cycle
- Thermal expansion
- Cooling rate
What happens if the PMMA is under-cured?
- Free monomer is an irritant
- Low molecular weight - poor mechanical properties
What happens if the PMMA is cured to quickly?
- Possible gaseous porosity
What happens if there is too much monomer in the PMMA?
Causes contraction porosity
What happens if there is too little monomer in the PMMA?
- Causes granularity of the material
What does porosity affect? (4 points)
- Affects strength
- Affects appearance
- Rough sensation to tongue
- Absorbs saliva - poor hygiene
Where does gaseous porosity occur?
- In bulkier parts
How is gaseous porosity caused?
- Monomer boiling (100*C)
How much polymerisation shrinkage will there be of the monomer on its own?
21% by volume
How much polymerisation shrinkage will there be of the powder + monomer mix?
7%
What are possible cases of contraction porosity? (3 points)
- Too much monomer
- Insufficient excess material
- Insufficient clamp pressure
(occurs anywhere where dough is not sufficiently packed)
Is acrylic non-toxic?
Yes
Is acrylic non-irritant?
- Yes, provided there is no monomer release
- But, some patients are allergic
Is acrylic affected by oral fluids?
- No
- Water absorption, virtually insoluble in fluids taken orally
Is the thermal expansion of acrylic okay with artificial teeth?
- Yes, OK if acrylic teeth are used, significantly higher than porcelain teeth
Does acrylic have a high thermal conductivity?
- No, which is poor (ideally look for a high one)
Are the mechanical properties of acrylic good?
- No, they are poor
- Increase in bulk to compensate
Is the colour/translucency of acrylic appropriate?
- Yes, it is good (natural looking)
Is the density of acrylic good?
- Yes, it is low which is good
- BUT need increase in bulk to overcome the poor mechanical propertied offsets this advantage
Acrylic has a high softening temperature (75*C) is this good?
- OK for ingested hot fluids
- DON’T use boiling water for cleaning
Is acrylic dimensionally accurate & stable in use?
- Pretty much
- Linear contraction 0.5% - acceptable
