Packing the mold with Packing the mold with acrylic resin and acrylic resin and

Processing acrylic resinProcessing acrylic resin

Dr.Mohammad Ibrahim Al SayedDr.Mohammad Ibrahim Al Sayed

Packing the resin with acrylic resin:Packing the resin with acrylic resin:

Acrylic resin is a resinous plastic material of various esters Acrylic resin is a resinous plastic material of various esters of acrylic acid. It is used as a denture base material.of acrylic acid. It is used as a denture base material.It is formed of a powder and liquid.It is formed of a powder and liquid. *Powder:*Powder: is polymethyl methacrylate (polymer) + Benzoyl peroxide is polymethyl methacrylate (polymer) + Benzoyl peroxide (initiator) +pigments.(initiator) +pigments.

*Liquid: *Liquid: is methyl methacrylate (monomer) + hydroquinone is methyl methacrylate (monomer) + hydroquinone (inhibitor) (inhibitor)

Powder and liquid are mixed in A ratio of 3 to 1 by volume Powder and liquid are mixed in A ratio of 3 to 1 by volume for an average sized denturefor an average sized denture

On mixing the polymer and monomer the mix passes through 4 stages:

11 . .A sandy stage :A sandy stage : where a fluid mass occur due to the where a fluid mass occur due to the settling of the polymer into the monomersettling of the polymer into the monomer..

22 . .A stringy or fibrous stage:A stringy or fibrous stage: where the monomer starts to where the monomer starts to attack the polymer. In this stage the mix is tacky, sticky attack the polymer. In this stage the mix is tacky, sticky and adheres to the sides of the mixing jarand adheres to the sides of the mixing jar..

33 . .Smooth dough like stage:Smooth dough like stage: where the monomer diffuses where the monomer diffuses into the polymerinto the polymer..

44 . .Rubber like stage:Rubber like stage: further penetration of the monomer further penetration of the monomer into the polymer. In this stage the acrylic resin cannot be into the polymer. In this stage the acrylic resin cannot be packed or molded being too stiffpacked or molded being too stiff..

The time to reach stage 3 (dough The time to reach stage 3 (dough

forming time) depends onforming time) depends on::

11..Solubility of the polymer into the monomerSolubility of the polymer into the monomer 22 . .Size of the polymer particles. The smaller the Size of the polymer particles. The smaller the

size the shorter the timesize the shorter the time..

The denture base material used in denture The denture base material used in denture base construction is base construction is heat cured typeheat cured type..

* *In In heat curedheat cured acrylic resin : The acrylic resin : The polymerization process is activated by polymerization process is activated by heatheat..

* *Autopolymerization Autopolymerization acrylic resin (self cured or acrylic resin (self cured or cold curedcold cured ) has ) has a chemicala chemical activator for the activator for the polymerization processpolymerization process..

Packing procedure for acrylic resinPacking procedure for acrylic resin::

11 . .Make sure that the hands and all mixing equipments Make sure that the hands and all mixing equipments are clean. Acrylic resin can become contaminated are clean. Acrylic resin can become contaminated easily by foreign body that may change the color easily by foreign body that may change the color of the material and its physical propertiesof the material and its physical properties..

22 . .Mixing the acrylic resinMixing the acrylic resin::

- -Measure and mix the polymer and monomer according to the Measure and mix the polymer and monomer according to the manufacturer’s direction usually 10 cc of monomer and 30 cc manufacturer’s direction usually 10 cc of monomer and 30 cc of polymer are usedof polymer are used..

- -First pour the monomer into a clean porcelain jar then add the First pour the monomer into a clean porcelain jar then add the polymerpolymer..

- -Stir with a clean spatula till the monomer and polymer are Stir with a clean spatula till the monomer and polymer are thoroughly combinedthoroughly combined..

- -Place the lid on the jar and allow the mixture to stand till it Place the lid on the jar and allow the mixture to stand till it reaches the dough stage indicated by which the mix can reaches the dough stage indicated by which the mix can cleanly be separated from the sides of the jar.(15 minutes)cleanly be separated from the sides of the jar.(15 minutes)

33 . .Pack the material in the upper half of the flask being Pack the material in the upper half of the flask being sure to press it well around the teethsure to press it well around the teeth..

44 . .Use enough material to insure overpacking on the first Use enough material to insure overpacking on the first closureclosure..

55 . .Place two pieces of wet cellophane over the acrylic Place two pieces of wet cellophane over the acrylic resinresin..

66 . .Put the lower half of the flask in position and press the Put the lower half of the flask in position and press the flask together using hand pressureflask together using hand pressure..

77 . .Place the flask in a bench, press and close it very Place the flask in a bench, press and close it very slowly to give the acrylic resin time to flowslowly to give the acrylic resin time to flow..

88 . .Remove the flask from the press and open it carefully. Remove the flask from the press and open it carefully. Trim off the excess acrylic (flash) with a sharp knifeTrim off the excess acrylic (flash) with a sharp knife..

This is called This is called trial closuretrial closure. If insufficient flash is observed . If insufficient flash is observed , it indicates insufficient amount of resin was placed in , it indicates insufficient amount of resin was placed in the mold. So remove the cellophane and add more the mold. So remove the cellophane and add more

resinresin . .

99--Trial closure is repeated until all excess Trial closure is repeated until all excess material is removed and the metal edges material is removed and the metal edges of the flask are in complete contactof the flask are in complete contact..

1010--Final closure:Final closure: open the flask, paint the open the flask, paint the cast with separating medium, remove the cast with separating medium, remove the cellophane sheets and close the flask cellophane sheets and close the flask under the press until metal to metal under the press until metal to metal contact is reachedcontact is reached..

1111--Remove from the press and place in the Remove from the press and place in the spring press both flaskspring press both flask..

Processing acrylic resin (curing)Processing acrylic resin (curing)

--The processing or polymerization of acrylic The processing or polymerization of acrylic resin is the conversion of the monomer to resin is the conversion of the monomer to the polymer when a mixture of the two is the polymer when a mixture of the two is subjected to heat (in water bath)subjected to heat (in water bath)..

--The amount of heat must be controlled while The amount of heat must be controlled while processing acrylic resin as the reaction is processing acrylic resin as the reaction is exothermic and becomes very rapid at exothermic and becomes very rapid at temperatures between 140temperatures between 140°°C and 160C and 160°°FF..

--Once polymerization has begun the Once polymerization has begun the temperature of the resin may become temperature of the resin may become higher than the temperature of water bathhigher than the temperature of water bath..

--The boiling point of the monomer is 212The boiling point of the monomer is 212° F° F . .

--If the heat is not controlled the exothermic If the heat is not controlled the exothermic reaction will cause the monomer to boil in reaction will cause the monomer to boil in the thick sections of the dentures and result the thick sections of the dentures and result in areas of porosityin areas of porosity..

--For that reason the temperature of the water For that reason the temperature of the water bath should be kept at or below 160°F for at bath should be kept at or below 160°F for at least 1 1/2 hours till the exothermic heat is least 1 1/2 hours till the exothermic heat is conducted away fro the resin to the investing conducted away fro the resin to the investing mediummedium..

The two methods of processing

11--slow processing (long cycle) by placing the slow processing (long cycle) by placing the flask in water bath at 160flask in water bath at 160°°F for 9 hoursF for 9 hours

22--fast processing (short cycle ) by placing fast processing (short cycle ) by placing the flask in water bath at 165the flask in water bath at 165°°F for 2 hours F for 2 hours then raising the temperature to 212 then raising the temperature to 212 °°F for F for a further 2 hoursa further 2 hours

After processing the flasks should be After processing the flasks should be allowed to cool slowly to room allowed to cool slowly to room temperaturetemperature..