BDS2 Acrylic Resin

Question 1

What are denture bases often made of?

Answer 1

Acrylic resin (PMMA)

Question 2

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

Answer 2

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

Question 3

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

Answer 3

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

Question 4

What are the basic requirements for a denture base?

Answer 4

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

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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

Question 7

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

Answer 7

-must fit the patients mouth AND be retained

Question 8

What does a high softening temperature mean?

Answer 8

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

Question 9

What is the elastic limit of a material?

Answer 9

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

Question 10

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

Answer 10

-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

Question 11

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

Answer 11

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

Question 12

Why do you want high thermal conductivity?

Answer 12

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

Question 13

What other properties are ideal for a denture base?

Answer 13

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

Question 14

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

Answer 14

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

Question 15

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

Answer 15

Free-radical addition polymerisation

Question 16

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

Answer 16

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)

Question 17

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

Answer 17

Molecules with C=C bonds

Question 18

What monomer is used in the formation of PMMA?

Answer 18

Methacrylate monomer

Question 19

What are the steps in acrylic polymerisation?

Answer 19

-activation -initiation -propagation -termination

Question 20

What happens in the activation step?

Answer 20

Activation of the initiator to provide free radicals

Question 21

What is the initiator in this reaction?

Answer 21

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

Question 22

How is the initiator (benzoyl peroxide) activated?

Answer 22

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

Question 23

What does activation of the initiator give rise to?

Answer 23

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

Question 24

What happens in the initiation stage?

Answer 24

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

Question 25

What happens in the propagation step?

Answer 25

Growing polymer chain due to cross-linking with other molecules

Question 26

What happens in the termination step?

Answer 26

The propagation is terminated

Question 27

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

Answer 27

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

Question 28

What are the 2 components of heat cured acrylic?

Answer 28

Powder and liquid

Question 29

What is in the powder component of heat cured acrylic?

Answer 29

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

Question 30

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

Answer 30

Speeds up the reaction

Question 31

What is the function of the plasticiser in the powder?

Answer 31

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

Question 32

What is the function of the pigments in the powder?

Answer 32

To give a natural colour to the material

Question 33

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

Answer 33

To impoive mechanical properties

Question 34

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

Answer 34

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

Question 35

What is the function of the methacrylate monomer?

Answer 35

-dissolves PMMA particles - polymerises

Question 36

What is the function of the inhibitor?

Answer 36

-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

Question 37

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

Answer 37

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

Question 38

Why is a powder/liquid mix produced?

Answer 38

-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%)

Question 39

What are the proportion of powder to liquid used?

Answer 39

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

Question 40

Describe what happens to the powder liquid mix.

Answer 40

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

Question 41

What are the requirements for acrylic heat curing?

Answer 41

-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;

Question 42

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

Answer 42

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

Question 43

Over what temp will acrylic begin to get porosity?

Answer 43

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

Question 44

Why is how the acrylic is cooled important?

Answer 44

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

Question 45

How can you minimise internal stresses when cooling the acrylic?

Answer 45

Cool slowly to allow relief

Question 46

What can internal stresses in the material cause?

Answer 46

-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)

Question 47

What are internal stresses related to?

Answer 47

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

Question 48

What will under-curing of the material lead to?

Answer 48

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

Question 49

What can too fast curing lead to?

Answer 49

Possibly gaseous porosity

Question 50

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

Answer 50

Contraction porosity

Question 51

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

Answer 51

Granularity on the surface of the material

Question 52

Why is porosity of the material not ideal?

Answer 52

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

Question 53

Where and when will porosity occur?

Answer 53

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

Question 54

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

Answer 54

The reaction is exothermic

Question 55

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

Answer 55

21% (by volume)

Question 56

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

Answer 56

7%

Question 57

What can cause contraction porosity?

Answer 57

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

Question 58

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

Answer 58

-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

Question 59

Describe the thermal properties of acrylic.

Answer 59

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

Question 60

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

Answer 60

Are poor (lower than ideal)

Question 61

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

Answer 61

Increase the bulk of the material used

Question 62

How is the colour/translucency of acrylic?

Answer 62

Is good - natural looking which is an advantage

Question 63

Describe the density of acrylic.

Answer 63

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

Question 64

Describe the softening temp for acrylic.

Answer 64

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

Question 65

Is acrylic dimensionally accurate and stable in use?

Answer 65

It is ok Linear contraction 0.5% which is acceptable