BDS2 Acrylic Resin Flashcards by Fiona Rankin

What are denture bases often made of?

Acrylic resin (PMMA)

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Often dentures done fit the patient first time, what sources of error might there be?

Production: -initial impression is flawed - material and method not 100% -curing process: several poten tial error sources

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What error sources might lead to an acrylic resin (PMMA) denture not fitting the patient over time?

-fracture due to trauma -uncomfortable -becomes warped -surface suffers wear and becomes uncomfortable

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What are the basic requirements for a denture base?

-replaces the function of natural teeth -goes into the patients mouth -is aesthetically pleasing

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What can be done at chairside if an acrylic resin denture doesn’t fit first time?

minor adjustments/ refinement but must have a reasonable fit to start off with

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What are the ideal properties of PMMA as a denture base?

-Dimensionally accurate and stable in use -high softening temperature (T g ) -unaffected by oral fluids over time

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What does ‘dimensionally accurate and stable in use’ mean?

-must fit the patients mouth AND be retained

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What does a high softening temperature mean?

-must not distort during ingestion of hot fluids, or during cleaning (P’s sometimes use boiling water)

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What is the elastic limit of a material?

represents the stress beyond which the material will become deformed (slightly) - when stress is relieved the material will not return to original shape

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What are the ideal mechanical properties for PMMA as a denture base?

-High Young’s (Elastic) Modulus i.e. - is rigid and requires large stresses to produce small strain (small change in shape) -High Proportional Limit/Elastic Limit i.e. - only large stresses will cause permanent deformation

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What are the ideal thermal properties of PMMA as a denture base?

-The thermal expansion = that of the artificial teeth -thermal expansion allows avoidance of internal stresses on cooling during manufacture -high thermal conductivity

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Why do you want high thermal conductivity?

-for the transmission of thermal stimuli to mucosa - avoid scalding of back of throat or oesophagus

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What other properties are ideal for a denture base?

-low density -colour/translucency to match the natural tissues -non-toxix -non-irritant

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Why do we want a denture base to have a low density?

-lower density aids retention of upper denture (the heavier the material - the greater the gravity pull)

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What type of reaction takes place to forma acrylic resin (PMMA)?

Free-radical addition polymerisation

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What is the definition of a free radical addition polymerisation reaction?

Chemical union of TWO molecules either the same or different to form a larger molecule WITHOUT the elimination of a smaller molecule Note: crucial part = WITHOUT ELIMIN ATION (no small molecules relesased)

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What do free-radical addition polymerisation reactions involve molecules with?

Molecules with C=C bonds

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What monomer is used in the formation of PMMA?

Methacrylate monomer

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What are the steps in acrylic polymerisation?

-activation -initiation -propagation -termination

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What happens in the activation step?

Activation of the initiator to provide free radicals

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What is the initiator in this reaction?

Benzoyl peroxide C 6 H 5 COO-OOCH 5 C 6 Note: it is symmetrical

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How is the initiator (benzoyl peroxide) activated?

a) by heating to >72degrees if heat cured b) can be a self-curing material

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What does activation of the initiator give rise to?

2 free radicals (R*): C 6 H 5 COO*

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What happens in the initiation stage?

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

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What happens in the propagation step?

Growing polymer chain due to cross-linking with other molecules

What happens in the termination step?

The propagation is terminated

What do you end up with at the end of the termination?

https://s3.amazonaws.com/classconnection/589/flashcards/19203589/png/pmma-170074C03FA7DE11F6A.png

What are the 2 components of heat cured acrylic?

Powder and liquid

What is in the powder component of heat cured acrylic?

-initiator (Benzoyl Peroxide, 0.2-0.5%) -PMMA particles (pre-polymerised beads) -Plasticiser -pigments -co-polymers

What is the purpose of the PMMA particles in the powder?

Speeds up the reaction

What is the function of the plasticiser in the powder?

Allows quicker dissolving in monomer liquid (imp roves dissolution process)

What is the function of the pigments in the powder?

To give a natural colour to the material

What is the function of the co-polymers in the powder?

To impoive mechanical properties

What is found in the liquid component of heat cured acrylic?

-methacrylate monomer -inhibitor (hydroquinone, 0.006%)-co-polymers

What is the function of the methacrylate monomer?

-dissolves PMMA particles - polymerises

What is the function of the inhibitor?

-Prolongs shelf-life -reacts with any free-radicals produces by heat, UV light (inadvertently) as these would react with the monomer in the liquid too early

What is the function of the co-polymers in the liquid?

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

Why is a powder/liquid mix produced?

-to produce dough-like material that can be handles/mixed easily and customised to desired shape -reduce heat of reaction -minimise polymerisation shrinkage (monomer on own shrinks 21%, with powder 7%)

What are the proportion of powder to liquid used?

3-3.5/1 (powder to liquid) by volume or 2.3/1 by weight FOLLOW THE INSTRUCTION ON THE PACK

Describe what happens to the powder liquid mix.

Is packed into a metal vessel that has a mould in it The vessel is then clamped together and then heated to cure to then produce the denture base

What are the requirements for acrylic heat curing?

-need a high temp for efficient polymerisation to give high molecular weight polymer (which improves the materials mechanical properties) -but if the temp is too high will have porosity in the materia;

What are some examples of heating schedules for acrylic resin? (3)

1) 7hrs to 70 degrees + 2hrs up to 100degrees and slow cool 2) 72degrees for at least 16 hrs 3) 20-20-20 reverse curing: -flask in boiling water -remove heat for 20misn -heat to 70 for 20 mins -heat to 100 for 20 mins

Over what temp will acrylic begin to get porosity?

100 degrees celcius Want the heat cycle in blue in the pic

Why is how the acrylic is cooled important?

-the mould ad acrylic have different thermal expansion coefficients which can cause internal stresses

How can you minimise internal stresses when cooling the acrylic?

Cool slowly to allow relief

What can internal stresses in the material cause?

-decrease in strength -decrease in fatigue strength (ability to withstand low level stresses over a period of time) -warping -repair problems (minor crack may end up being bigger fracture)

What are internal stresses related to?

-shape and size -curing pressure -curing cycle -thermal expansion -cooling rate (highlighting the complexity of the process)

What will under-curing of the material lead to?

-Free monomers which will act as an irritant to the patient (leeches out) -low molecular weight = poor mechanical properties

What can too fast curing lead to?

Possibly gaseous porosity

What can the incorrect powder monomer ratio cause when there is too much monomer?

Contraction porosity

What can the incorrect powder monomer ratio cause when there is too little monomer?

Granularity on the surface of the material

Why is porosity of the material not ideal?

-affects strength -affects appearance -rough sensation to the tongue -absorbs saliva - poor hygiene -plaque trap

Where and when will porosity occur?

-when monomer bpiling exceeeds 100 degrees celcius -occurs in bulkier parts of the material

Why do you need to cure the acrylic slowly to avoid reaching above 100 degrees?

The reaction is exothermic

What is the polymerisation shrinkage of the monomer on its own?

21% (by volume)

What is the polymerisation shrinkage of the monomer+powder mix?

What can cause contraction porosity?

-too much monomer -insifficient excess materla -insufficient clamp pressure -occurs anywhere where dough is not sufficiently pack ed

Describe acrylic resin properties with relation to toxicity, irritability and oral fluid impact?

-Is non-toxic -Is a non-irritant provided no monomer is released BUT some patients are allergic -will absorb water but is virtually insoluble in fluids taken orally

Describe the thermal properties of acrylic.

Thermal expansion compared to artificial tooth: -is OK if acrylic teeth used -significantly higher than porcelain teeth Has low thermal conductivity which is poor

Describe the mechanical properties (YM, PL & EL) of acrylic?

Are poor (lower than ideal)

How can you compensate for the poor mechanical properties of acrylic?

Increase the bulk of the material used

How is the colour/translucency of acrylic?

Is good - natural looking which is an advantage

Describe the density of acrylic.

-is low which is good BUT -the need to increase the bulk (overcome poor mechanical properties) offsets this advantage

Describe the softening temp for acrylic.

-is 75 degrees -is OK for ingested hot fluid -CANNOT use boiling water to clean (will warp the acrylic)

Is acrylic dimensionally accurate and stable in use?

It is ok Linear contraction 0.5% which is acceptable