Packing procedure for acrylic resin:
1. Make sure that the hands and all mixing equipments are clean. Acrylic resin can become contaminated easily by foreign body that may change the color of the material and its physical properties.2. Mixing the acrylic resin:Measure and mix the polymer and monomer according to the manufacturer’s direction usually 10 cc of monomer and 30 cc of polymer are used.First pour the monomer into a clean porcelain jar then add the polymer.Stir with a clean spatula till the monomer and polymer are thoroughly combined.Place the lid on the jar and allow the mixture to stand till it reaches the dough stage indicated by which the mix can cleanly be separated from the sides of the jar. During this standing period a lid should be placed on the mixing vessel to prevent evaporation of monomer. Loss of monomer during this stage could produce granular porosity in the set material. This is characterised by a blotchy, opaque surface.On mixing the polymer and monomer the mix passes through 4 stages:
A sandy stage : where a fluid mass occur due to the settling of the polymer into the monomer. A stringy stage: where the monomer starts to attack the polymer. In this stage the mix is tacky, sticky and adheres to the sides of the mixing jar. Smooth dough like stage: where the monomer diffuses into the polymer. Here, the material is more cohesive and has lost much of its ‘tackiness’. It can be moulded like plasticine and does not adhere to the sides of the mixing vessel.The time taken to reach the dough stage is called the doughing time whilst the time for which the material remains at the dough stage and is mouldable is termed the working time.The time to reach stage 3 (dough forming time) depends on:1. Solubility of the polymer into the monomer 2. Size of the polymer particles. The smaller the size the shorter the time. Rubber like stage: further penetration of the monomer into the polymer. In this stage the acrylic resin cannot be packed or molded being too stiff.The transitions from ‘sandy’ to ‘stringy’ to ‘dough’ and eventually rubbery and hard stages are due to physical changes occurring within the mix. Smaller polymer beads dissolve in monomer causing a gradual increase in viscosity of the liquid phase. Larger beads absorb monomer and swell, thus depriving the liquid phase of monomer and causing a further increase in viscosity. During this period the monomer remains unpolymerised.
3. Pack the material in the upper half of the flask being sure to press it well around the teeth. 4. Use enough material to insure overpacking on the first closure. 5. Place two pieces of wet cellophane over the acrylic resin. 6. Put the lower half of the flask in position and press the flask together using hand pressure. 7. Place the flask in a bench press and close it very slowly to give the acrylic resin time to flow. 8. Remove the flask from the press and open it carefully. Trim off the excess acrylic (flash) with a sharp knife. This is called trial closure. If insufficient flash is observed , it indicates insufficient amount of resin was placed in the mold. So remove the cellophane and add more resin.
9-Trial closure is repeated until all excess material is removed and the metal edges of the flask are in complete contact. 10-Final closure: open the flask, paint the cast with separating medium, remove the cellophane sheets and close the flask under the press until metal to metal contact is reached. 11- Flasks Are Secured In Clamp And Placed Aside For 30mins (bench curing).
The applied pressure has three important functions:It ensures that dough flows into every part of the mould.It enables excess dough to be used, thus causing an effective reduction in the polymerisation shrinkage. It prevents the formation of a ‘raised bite’ on the denture by giving a base which is too thick.The use of insufficient dough in the mould or the application of insufficient pressure during curing can lead to porosity voids dispersed throughout the whole mass of the denture base. This is known as contraction porosity.
Processing of the Denture (polymerization)
the processing or polymerization of acrylic resin: is the process of conversion of the monomer to polymer when a mixture of the two is subjected to heat.Types of curing methods: Heat curing which microwave water bath (heat cure acrylic resin) Chemical cure (cold cure acrylic resin ) Light curing (visible cure acrylic resin )
Heat curing cycles
Stages of Addition Polymerization:Activation: This involves decomposition of the peroxide initiator using thermal activation (heat),to produce free radicals. activators: Initiation: When an initiator such as benzoyl peroxide is decomposed and free radicals are produced. The polymerization reaction is initiated when the radical, formed on activation, reacts with a monomer molecule. Propagation: Following initiation, the new free radical is capable of reacting with further monomer molecules. Each stage of the reaction produces a new reactive species capable of further reaction . Termination: Occurs when monomer units are used up, or free radical is tied up by a reaction.
Porosity:
types of porosity in acrylic resin denture base: Contraction porosity. Gaseous porosity. Granular porosity.Contraction porosity: occurs due to monomer contraction by 20% during processing. Insufficient amounts of resin packed in the flask may lead to voids or porosity. Also resin should be packed in the dough stage. Prior to that the resin would flow too rapidly and pressure is lost. Insufficient pressure during curing.
Granular porosity: due to loss of monomer while resin mix is left to stand until dough stage is reached. Also if the resin mix is dry.
Before deflasking the processed denture the flask is cooled to room temperature. To minimize internal stresses and subsequent warpage of the denture. Since the coefficient of thermal expantion of acrylic resin is about 10 times greater than that of the gypsum mould material.