denture base resins

1. 1 Denture base resins

2. 2

3. 3 Old materials used to construct dentures

4. 4 The material of choice? Acrylic resin (polymethyl methacrylate) PMMA Why? Desirable qualities �reasons why it is used nowadays- : Good esthetics Cheap Easy to process & use Disadvantages: Not strong enough especially impact strength (denture fall accidentally and it is broken easily) Susceptible to distortion with time Low thermal conductivity leads to the pt frequently burning the mucosa as they not actually feeling the heat immediately. Radiolucent if by a chance, part of it broken or pt is wearing a small partial denture & swallows it, you cant see it in radiographs, so you don�t know if it went to the lung or to the stomach, if it went to the stomach it is fine, its gonna come out but in the lung it is a problem. so they include opcifiers to the composition to make it opaic (radio-opaic)

5. 5 Dental uses Denture base material Denture repair Denture teeth Soft liners Construction of customized trays Soft tissue replication on cast metal framework

6. 6

7. 7 Composition Different types, sometimes it is classified according to the way it sets. Heat cured set by heat Cold cures it doesn�t need heat only chemical rxn cause them to become hard. Light cured harden as subjected to light at certain wavelength. Pour and cure resins very fluid & poured into the model.

8. 8 Continue, The reaction is an addition polymerization reaction to convert MMA (methyl methacrylate) by cross linking to PMMA (polymethyl methacrylate). Mono= single Poly= many No by product not like condensation polymerization reaction.

9. 9 Polymerization & crosslinking Polymer: , large long chain molecules formed by chemically joining smaller molecules called monomers Copolymer: is formed when two or more different types of monomers join together Cross-linked polymers: adjacent polymers that bond together by side chains: Weak bond not highly charged between side chains leads to a easily manipulated and stretched polymer so the end result is soft polymer. If bond or cross-linked or side chains is highly charged, resulting polymer is stiffer rigid

10. 10 Polymerization Addition polymerization: Activation: by producing free radicals very active component & cause the connection between monomers. Activators; needed for activation: Tertiary amine Heat Initiation: when an initiator such as benzoyl peroxide source of free radicals is decomposed and free radicals are produced. Propagation: occurs when free radicals break the bond between carbon atoms in monomer and adjacent monomers bond together, keep going until either there is no more free radicals or no more monomers. Termination: occurs when monomer units are used up, or free radical is tied up by a reaction

11. 11 Polymerization Condensation polymerization: usually more than one type of monomer is used. The reaction produces by-products not like addition polymerization such as water, hydrogen or alcohol.

12. 12 Heat cured resins Powder: Beads or granules of PMMA so they are already polymerized not monomers. Initiator: benzoyl peroxide Pigments Opacifiers: titanium/zinc oxide to make it radio-opac Plasticiser: dibutyl phthalate make the material softer, added flexibility. Synthetic fibers: nylon/acrylic to look like blood vessels �small blood vessels- to give the gingiva a natural appearance.

13. 13 Heat cured resins Liquid: Methyl methacrylate monomer Cross-linking to make polymers at the end agent: ethylene glycol dimethacrylate. This component improves mechanical properties add strength.

14. 14

15. 15 Flasking and Heat curing Flasking Dewaxing Application of separating medium prevent the 2 parts of the flask from sticking to each other (water soluble alginate solution Placing acrylic dough Packing closing the flask together and removing excess Heat curing under pressure so the 2 flasks are placed on top of each other & pressed. Pressure is important to let the dough to flow all over & cover all the necessary components. If pressure was lost from the flask or you pack the flask before dough stage (sandy or sticky stage) the material will not flow & there will be defect. Processing at 74�C for 8 hours or longer Processing at 74�C for 8 hours then increasing temperature to 100�C for 1 hour Processing at 74�C for 2 hours and raise temperature to 100 for 1 hour. Cooling should be slow to avoid warpage so it will not fit in pt�s mouth. Most important is heating gradually & cooling gradually, it should not be over heated & if you cool it down quickly there will be distortion heating should be controlled & cooling should be slow.

16. 16 Heat curing, continue In addition to previously mentioned curing methods (heating of acrylic resin is in water path to raise the temperature), curing may be done in microwave, where the flasks used should be non metallic. But in that case we can�t place it in the metal flask, the flask is a device that contain the model which has the denture in it, so sometimes heat cure resin instead of using warm water to heat it, it can be heated in microwave.

17. 17

18. 18 Setting stages When the powder & liquid mixed, you can�t immediately use that mix to make the denture, you have to wait for sometime for the reaction to occur. So there are stages for the setting of the material or after you mix the material. Sandy stage: immediately after mixing sandy texture (no use) Sticky or stringy stage Dough stage: mix is cohesive and less tacky, easy to manipulate (time to pack in the moulds) , use the material & place it in the flask. Dough stage period is the working time. Time taken to reach dough stage is doughing time Rubbery stage -wait too long- So if you try to place it where it should as it is elastic, it will not remain in its position, it will attend to come back & rebound, so deficiency occur, so you should never wait, it should be used during the dough stage. There is a fifth stage called stiff stage.

19. 19 Continue, The previous composition allows for: Processing by the dough technique Minimal polymerization shrinkage Heat generation is minimal because in the powder we have already PMMA, so you don�t need to polymerize it, it is already polymerized. As the polymerization rxn of acrylic resin is exothermic but because we are placing pre-polymerized beads, they are already polymerized so no excess is necessary so this will minimize heat generation & also shrinkage will be reduced.

20. 20 Dough technique: The powder-liquid mix is used, when it reaches the dough stage, it is used to pack the flask containing the teeth and the set plaster which will then be closed under pressure. Manipulation of the mix in the dough stage is easier and produces better results. If you stretch it, it will stay stretched & not re-bound & go back. So it is placed on the model, excess is removed & at the end the flask is re-putted again, always put excess material. Excess is necessary to compensate for any shrinkage of defect that might occur during setting, so this is what we do after de-waxing. Continue,

21. 21 Continue, Polymerization shrinkage: is reduced due to the presence of pre-polymerized grains or beads rather than the monomer itself. Heat generation occurs due to the polymerization reaction (80 kJ/mol). This is reduced due to the presence of the already polymerized resin.

22. 22 Continue, MMA monomer: Volatile Flammable & also irritant for the skin or the mucosa. Precautions: Kept in a sealed container Dark container away from light source or heat source or flame No contact should be allowed with the polymer beads (carry benzoyl peroxide initiator) Sometimes if the processing is incorrect, there will be residual monomer >>> processing should be accurate, otherwise if there is residual monomer remaining un reactive, it will cause irritant to the skin & the mucosa inside the pt�s mouth.

23. 23 Cold cure resin Composition: identical to heat cure resin except polymerization is initiated by tertiary amine, it will react with benzoyl peroxide & produce free radicals (e.g. sulfinic acid or dimethyl-p- toluidine). They contain an inhibitor (hydroquinone) that destroys free radicals to prolong working time to give us time to work with the dough So no heat but chemical �tertiary amine- produce the free radicals & there is also inhibitors.

24. 24 Cold cure resin used for making special trays

25. 25 Cold VS heat cure Cold cure powder is a little bit different from heat cure powder Smaller polymer beads are used in cold cure resin to ease dissolution (easy to mix) in monomer there is no heating & we need it to mix very well with the liquid& the end result the denture will have a lower Molecular Weight, so it have less strength sometimes there is residual monomer because there is no heat to evaporate residual monomers. Cold curing method results in a polymer with a lower molecular weight leading to: Lower strength Residual uncured monomer Lower color stability color might change with time, more prone to yellowing More prone to creep and therefore distortion so more susceptible to distortion, so the fit will get worse.

26. 26 Pour and cure resin: not used much. Mixed and poured into a mould of hydrocolloid elastic material. Excellent reproduction of details as it is more soluble but inferior to cold and heat cure resin in other aspects but it is not very strong as it is very flowable. Light cure resin (blue light): used as denture hard relining material, repair material, construction of trays. Different components so that they are sensitive to light so when they are subjected to light they become harden & no need of heat. We cover it by a translucence material to allow the light to get in, usually the light cure resin come in form of sheets as one component so we don�t mix the powder & the liquid. You adapt the sheet to the cast (model), you cut the excess, set the teeth & then cure it. & usually in clinics or labs we use this type of resin to make special trays (in our center we don�t use it to make dentures but to make special trays.) Composition: UDMA Colloidal silica to control flow Filler of acrylic beads

27. 27 Manipulation Powder: liquid, THE GOLDEN RULE: always use powder: liquid ratio recommended by manufacturer. (2.0/1.0 wt %, 1.6/1.0 vol.%) to keep polymerization shrinkage at 5-8% Too much powder the mix will be too dry, there will not be good mixing or good wetting of the powder by monomer Too much monomer lead to irritation at the end to the pt & if it is too much it evaporates while the material is still setting & cause porosity like bubbles inside the denture. Porosities inside the denture cause it to become weaker & if you can see it, it is unaesthetic. You don�t need a dry mix or the mix to be too wet but you want a proper proportioning.

28. 28 Manipulation Control of color: pigments responsible for the color are incorporated into the polymer, sometimes on the beads surface and can be washed away if incorporated into monomer too quickly Mould lining: to prevent penetration of resin into plaster, separating medium used e.g. sodium alginate, tin foil, vasline. If the resin go inside the pours of plaster & set & become hard, it will be difficult to separate the 2 flasks from each other, as the flask is containing the model & the model has teeth set on top so between the cast & the flask there is space & this space is filled with plaster which has pours.