A ) Heat-Cured Resin

Dr. Waseem Bahjat Mushtaha

Specialized in prosthodontics

I-The composition of heat-cured acrylic resin

Powder:

1 )Bead or granules of polymethyle methacrylate )PMMA). The granules dissolve more rapidly and hence shorten the dough time.

2 )Benzoyl peroxide – initiator.

3 )Dibutyl phthalate – plasticizer to produce a less brittle polymer.

4 )Pigments to give the natural tissue like appearance.

5 )Synthetic fibers to simulate blood vessels of the oral mucosa.

6 )Bismuth salt – opacifires to be seen in x-ray if swallowed.Liquid:

1 )Methyl methacrylate monomer )MMA).2)Hydroquinon – inhibitor, to react with any free

radical that may form in the liquid.3 )Ethylene – glycol dimethacrylate – cross linking

agent, to improve the mechanical properties and increase the resistance of the denture base to crazing.

II-Manipulation and Processing

1 )proportioning: the polymer\monomer ratio is 3:1 by volume or 2.5:1 by weight. Using this ratio, polymerization shrinkage will be decreased from 21% to 6-7%.

a. If less monomer is used, there will be insufficient wetting of the polymer and weaker and granular denture is obtained.

b. If more monomer is used, there will be increase in polymerization shrinkage and possibility of porosity.

2 )polymer- monomer mixture stages

The P-M mixture goes through the following 5- stages:

a) Sandy stage : a wet sand-like mixture is formed .

b) Sticky or fibrous stage: The polymer begins dissolve in the monomer.

c) Dough stage: The material becomes smooth and does not stick to the mixing jar or hands and it is easily manipulated and shaped. This is ideal stage for packing.

d) Rubber stage : The material becomes rubbery i.e. soft but can’t be shaped.

e) Stiff stage : If the material is left longer it becomes stiff.

3 )Packing

When the material reaches the dough stage, it is ready to be packed the denture mold in the flask.

a) If packed in sandy or sticky stages, the material will be of high fluidity and will flow out of the flask resulting in denture porosity.

b) If packed in rubbery or stiff stage, the material will not flow under pressure and there will be no metal to metal contact. This results in dentures without details and usually with moved or fracture teeth.

N.B.1) sufficient pressure should be applied on the mold after packing because inadequate packing pressure will lead to irregular voids on the surface of the denture )shrinkage porosity).

2 )Acrylic dough should be packed in excess to decrease polymerization shrinkage from 7% to 0.2%

4 )Curing

After packing, the flask under pressure is placed in a heated water bath at 72C using either:

a. The long cycle : Heating at 72C for 8-16 hours.

b. The short cycle: Heating at 72C for 1.5 hours, the temperature is then raised to

100C for one hour further .

I- Under curing : ) less time and\or temperature) will result in a denture with high residual monomer )irritant to tissues and of inferior properties).

II- Over curing )more time and\or temperature)

Will result in boiling of the monomer )monomer boils at 100.3C) due to the added heat of polymerization )polymerization reaction is exothermic) and as a result the monomer will vaporise leaving pores inside the denture )gaseous porosity)

Polymerization reaction:

PMMA )polymer) + benzoyl peroxide )initiator)

Activation by heat polymer )PMMA)+ Heat

) exothermic reaction )

Addition polymerization.

5 )Cooling:

The flask should be cooled slowly. There is a difference in the coefficiency of thermal contraction between the mold material and acrylic resin. This difference induces stresses in the denture. Slow cooling

allows the release of these structures .

6) Deflasking : Should be done with care to avoid breaking of the denture.

7) Finishing and polishing.

III- Properties:

a) Residual monomer:

Even in a properly cured acrylic denture, 0.2 - 0.5% of the monomer remains. Curing at too low temperature or for too short time results in higher residual monomer. This should be avoided because:

1-Free monomer may be released from the denture and irritate the oral tissues.

2-Residual monomer will act as a plasticizer and make the denture weaker and more flexible.

B) Porosity: porosity has undesirable effects on both strength and optical properties of the denture.

Contraction or shrinkage porosity Gaseous or internal porosity

Site on the surface of the denture Inside the denture specially in

thick parts.

shape Appears as irregular voids Appears as fine uniform

rounded voids

Causes 1) Underpacking: Over or prolonged curing

It is therefore important that temperature:

sufficient dough is packed in On polymerization, there is

the mould to ensure that the an exothermic reaction

material is under pressure which causes the temp .

during curing. This will cause of the resin to rise above

any voids in the mix to collapse its boiling temp. which is

and also help to compensate 100.3 C . If this temp. is

for the polymerization exceeded, gaseous mono -

contraction. mer will be formed – this is

the cause of gaseous

porosity.

2 )Packing before dough

stage: became prior to this

stage, the high flow

causes rapid loss of

pressure.

c) Water sorption:

Acrylic denture absorb water by 2% of this weight, each 1% increase in weight causes a linear expansion of 0.22%. Similarly, during out the acrylic dentures is associated with shrinkage . In practice, water sorption is an advantage to compensate for the processing shrinkage which is 0.44% )the polymerization shrinkage and the thermal shrinkage occurring when cooling from 72 C to 20 C). Therefore , denture should be

kept wet all the time when not in service .

D) Crazing:

Cracks may appear on the surface of the denture due to the tensile stresses that cause separation of the polymer chains. Crazing has a weakening effect on the denture.

Causes: I- Continuous stresses due to repeated drying and wetting of the denture which cause alternate contraction and expansion

II- Difference in coefficiency of thermal expansion between acrylic denture base and porcelain teeth.

III- Action of solvents e.g. alcohol acetone, chloroform, or monomer e.g. when a denture is repaired, some monomer comes in contact with the resin and may cause crazing.

N.B: Cross linked denture are less likely to craze.

E) Thermal conductivity:

This is very low. From the patient’s point of view, the problem with a low thermal conductivity is that denture isolates the oral tissues from any sensation of temperature. This can lead to a patient consuming a drink that is too hot without realising it, which may lead to the back of the throat and possibly even the oesophagus being scalded

F) Coefficient of thermal expansion:

The coefficiency of thermal expansion is quite high. In general, this does not present a problem, except that there is a possibility that porcelain teeth in denture base may gradually loosen and be lost.