PMMA 1 (Polymethylmethacrylate) Flashcards
What are some ideal properties of denture materials?
- Dimensionally accurate and stable in use
- must fit patients mouth and be retained - High softening temperature (Tg)
- must not distort during ingesting of hot fluids, or during cleaning - Unaffected by oral fluids
- over time - Low density
- aid retention of upper denture (the heavier the material, greater the gravity pull) - Colour/translucency
- to match natural tissues - Non toxic, non irritant
What is the elastic limit in relation to stress strain diagrams
Stress beyond which deformation of the material occurs
What is compressive strength in relation to stress strain diagrams?
Where the curve ends
What is the proportional limit in relation to stress/strain diagrams?
Maximum stress where stress and strain are linearly related
How is elastic modulus calculated?
Stress divided by strain (in linear/elastic region)
What mechanical properties are ideal for denture base materials?
- High Young’s (elastic) modulus
- rigid (stiff)
- applying a large stress produces small strain - High proportional limit/elastic limit
- only large stresses will cause permanent deformation
What thermal properties are ideal for denture base materials?
- Thermal expansion
= artificial tooth
- avoid internal stresses on cooling during manufacture (more internal stresses in metals and alloys) - High thermal conductivity
- transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus
What is 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)
What are the 4 stages of acrylic polymerisation?
- Activation - of initiator to provide free radicals
- Initiation - free radicals break C=C bond in monomer and transfer free radicals
- Propagation - growing polymer chain
- Termination - of polymerisation
What is the initiator molecule in polymerisation of acrylic?
Benzoyl peroxide:
C6H5COO-OOCH5C6
What temperature is the initiator (benzoyl peroxide) activated at?
How is it cured?
> 72 degrees Celsius
Self cured
What components make up heat cured acrylic resin powder?
- Initiator (benzoyl peroxide, 0.2-0.5%)
- PMMA Particles - pre-polymerised beads
- Plasticiser - allows quicker dissolving in monomer liquid e.g. dibutyl phthalate
- Pigments - to give “natural” colour
- Co-polymers - to improve mechanical properties e.g. ethylene glycol dimethacrylate
What components make up heat cured acrylic liquid?
- 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
Why is heat cured acrylic powder and liquid mixed together?
- 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 powder and liquid that are mixed together?
- powder to liquid ratio = 3 to 3.5 - 1 (by volume)
- 2.5-1 by weight
What are 3 heating schedules used for acrylic?
- 7hrs. To 70 degrees C + 2hrs to 100 degrees C + slow cool
- 72 degrees C for at least 16 hrs
- 20-20-20 reverse curing
- place flask in boiling water
- remove heat for 20 mins
- heat to 70 degrees C for 20 mins
- then heat to 100 degrees C for 20 mins
Why is it best to avoid heating cycles that rise over 100 degrees C?
Gaseous porosity will result in voids occurring in the cured material
Why is it better to allow acrylic to cool slowly?
Mould material and acrylic have different thermal expansion coefficients, so will contract at different rates, if cooled too quickly; this can result in internal stresses within the acrylic
What problems do internal stresses in acrylics cause?
- Decreased compressive strength
- Decreased fatigue strength (low level stresses applied repeatedly over a long time, causing fractures)
- Warping e.g. during finishing
- Repair problems
What factors are related to internal stresses?
- Thermal expansion
- Cooling rate
- Shape and size e.g. notches
- Curing pressure
- Curing cycle
What are problems caused by under curing and fast curing?
- Under cured:
- free monomer (irritant)
- low molecular weight = poor mechanical properties - Fast curing:
- possibly gaseous porosity
What are problems caused by porosity in dental materials?
- Affects strength
- Affects appearance
- Rough sensation to tongue
- Absorbs saliva - poor hygiene
Briefly describe gaseous porosity
- Monomer boiling (100 degrees C)
- Occurs in bulkier parts
- Be careful (exothermic reaction)
- hence cure slowly
Where does contraction porosity most often occur?
Anywhere where dough is not sufficiently packed
How much polymerisation shrinkage (%) occurs in monomer alone vs powder + monomer mix?
- Monomer alone = 21% (by volume)
2. Powder + monomer mix = 7%
What causes contraction porosity?
- Too much monomer
- Insufficient excess material
- Insufficient clamp pressure
What problems are caused by incorrect powder to monomer ratios?
- Too much monomer
- contraction porosity - Too little monomer
- granularity
When can acrylic cause irritation in the mouth?
If monomer is left unreacted it can be released and cause irritation
Briefly describe some general properties of acrylic resin
- Non toxic - yes
- Non irritant
- provides no monomer is released
- some patients are allergic - Unaffected by Oral fluids
- water absorption, virtually insoluble in fluids taken orally - Colour/translucency = good
What thermal properties are of concern in acrylic resin?
- Thermal expansion = artificial tooth
- Ok if acrylic teeth are used, significantly higher than porcelain teeth
- acrylic resin:- 80ppm/degrees C - High thermal conductivity
- low (poor)
List the following mechanical properties of acrylic resin (elastic modulus, tensile strength, hardness)
- Elastic modulus = 2.5GPa
- Tensile strength = 85MPa
- Hardness = 20VHN
How would you describe the density of acrylic resin?
- Density low (good)
- but need to increase in bulk to overcome poor mechanical properties offsets this advantage
What is the softening temperature of acrylic?
75 degrees C
- ok for ingested hot fluids
- don’t use boiling water for cleaning
Is acrylic dimensionally accurate and stable in use?
Yes
- linear contraction 0.5% = acceptable
Summarise the acceptability of PMMA properties
Most properties (mechanical and thermal) are satisfactory, but dentures
- may not fit the patient first time
- may not last long
Alternative materials being sought
