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Description : Check out this medical presentation on Dental Acrylic Resin, which is titled "Polymers", to know about the roles of polymers in dentistry.

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Published on : Jul 01, 2021 | Views : 4198 | Downloads : 3

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Slide 1-

POLYMERS
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Done by AMRITA.H(10) 11DO14 ANITTA CYRIAC(11) 11DO16 ANUSREE C.K(13) 11DO18 AIYSHA IRSHANA(15) 11DO20
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OVERVIEW -KEY TERMS -STRUCTURE OF POLYMER -PROPERTIES OF POLYMER -FACTORS AFFECTING PROPERTIES OF POLYMER -CLASSIFICATION OF POLYMER USED IN DENTISTRY -TYPES OF POLYMER
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-SPATIAL ORGANIZATION -TYPES OF POLYMERISATION -CHEMICAL STAGES OF ADDITION POLYMERISATION. -APPLICATION OF RESINS IN DENTISTRY -APPLICATION OF ACRYLIC RESINS IN DENTISTRY
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-PROSTHETIC APPLICATIONS OF POLYMERS -DENTURE BASE RESINS -HEAT ACTIVATED DENTURE BASE RESINS -COMPRESSION MOULDING TECHNIQUE -CHEMICALLY ACTIVATED DENTURE BASE RESINS
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-MISCELLANEOUS RESINS AND TECHNIQUES -RESIN TEETH FOR PROSTHETIC APPLICATIONS -MATERIALS FOR MAXILLOFACIAL PROSTHETICS
Slide 8-

KEY TERMS -MONOMER-simple chemical compounds ,repeating units, reacting to form a polymer. -POLYMER-chemical compounds of huge organic or inorganic molecules formed by large number of repeating smaller chemical structural units. -POLYMERISATION-it is the repetitive process of chemical reactions between the small chemical structural units
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Polymerize and does not soften again on reheating to same temperature. -TERMINATION-stage of polymerization during which polymer chains cease to grow. -VISCOELASTIC-ability of a polymer to behave as an elastic solid and as a viscous liquid. -SETTING-the extent to which polymerization has progressed.
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STRUCTURE OF A POLYMER
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Growing into giant molecules with long chain structures , having millions of mers -THERMOPLASTIC POLYMER-polymeric material made up of linear or branched chains that softens when heated above glass transition temperature , at which molecular motion begins to force the chains apart. -THERMOSETTING POLYMER-polymeric material that becomes permanently hard when heated above the temperature at which it begins to
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GENERAL PROPERTIE OF POLYMES *MECHANICAL PROPERTIES: resistance to deformations are more for higher; -degree of polymerization. -long branched. *RHEOMETRIC PROPERTIES: -causes irreversible strain or plastic flow. -more rigid or less flexible.
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SOLVATION OF POLYMERS
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*SOLVATION OF POLYMERS:- -sorption causes separation of the polymer chains. -this causes dimensional changes in acrylic denture. *PLASTICIZERS: -increases the flexibility -decreases the softening. *THERMAL PROPERTIES: -thermosetting polymers are cross linked hard set materials.
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FACTORS AFFECTING THE PROPERTIES OF POLYMERS -molecular weight and chemical structural units of monomers. -degree of polymerization. - copolymerization and spatial organization. - crystallinity. -temperatures.
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CLASSIFICATION OF POLYMERS USED IN DENTISTRY -CHEMICAL REPEATING UNITS: poly vinyl chloride. -TYPES OF POLYMERISATION: .step growth polymerization .addition polymerization -ACTIVATING ENERGIES: .thermal .chemical
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TYPES OF POLYMERISATIONS
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-PHYSICAL STATES: .liquids .gels .elastomers -INTERNAL STRUCTURE: .crystalline . semi crystalline .noncrystalline
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STRUCTURE OF POLYMERS(SPATIAL STRUCTURE) -LINEAR: linear homopolymer ,random copolymer , block. -BRANCHED-branched homopolymer , random copolymer , graft copolymer. -CROSS-LINKED- homopolymer cross linked with single cross linking agent.
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STRUCTURES OF POLYMERS
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TYPES OF POLYMERIZATION CONDENSATION :slow method , repeated elimination of small molecules. -ADDITION-in dental procedures , no change in chemical composition and no by products . Simple but not easy to control giant molecules.
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TYPES OF POLYMERISATIONS
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INDUCTION -it is the time during which the molecules of the initiator becomes activated and start to transfer the energy to the monomer. Impurity-increases the length of this period. -increases temp : shorter is the length of this period.
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CHEMICAL STAGES OF POLYMERIZATION -INDUCTION -PROPOGATION -CHAIN TRANSFER -TERMINATION
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3 INDUCTION SYSTEMS HEAT ACTIVATION-Free radicals are liberated by heating benzoyl peroxide CHEMICAL ACTIVATION-at least 2 reactants-chem.reaction –liberate free radicals benzoyl peroxide + Aromatic amine LIGHT ACTIVATION-photons of light energy activate the initiator
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-PROPOGATION –After growth has started -CHAIN TRANSFER-Active state is transferred from activated radical to inactive molecules -TERMINATION-direct coupling or exchange of hydrogen atom.
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INHIBITION OF POLYMERIZATION -complete exhaustion of monomer -formation of high molecular weight polymer. -INHIBITED BY: -impurities -hydroquinone -oxygen retards polymerization.
Slide 28-

ACRYLIC RESINS -are derivatives of ethylene and contain vinyl group in their structural formula. -acrylic resin used in dentistry are esters of acrylic acid and meth acrylic acid. -available as methyl methacrylate and poly methyl methacrylate.
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PROSTHETIC APPLICATION OF POLYMERS
Slide 30-

TYPES OF RESINS -ACRYLIC RESINS -VINYL RESINS -POLYSTYRENE -EPOXY RESINS OTHER RESIN SYSTEM: -POLYCARBONATES -POLYURETHANE -CYANOACRYLATES
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DENTURE BASE RESINS COMPLETE DENTURE : is a removable dental prosthesis that replace entire dentition and associated structures of the maxilla or the mandible . This contain artificial teeth attached to the denture base which is supported through the contact with the underlying oral tissues , teeth or implants.
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IDEAL REQUIREMENTS +BIOLOGICAL-CHEMICAL COMPATABILITY -non poisonous ,non toxic , non irritant. -non carcinogenic. -non bad taste +PHYSICAL-MECHANICAL -requirements -low density -low weight
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+THERMAL COMPATABILITY -high thermal conductivity to feel the hotness. -low COTE +AESTHETIC REQUIREMENT - Color stability - Color parameter should be same for all wave length of incident light.
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PROPERTIES OF POLYMETHYLMETHACRYLATE DENTURE BASE RESINS =BIOCOMPATABILITY: .non toxic, non irritant , non carcinogenic. No taste -THERMAL PROPERTIES: .moldable temp .heat distortion temp -OPTICAL PROPERTIES .transparent organic glass .good color stability.
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COMPRESSION MOULDING TECHNIQUE -FLASKING:MULTIPLE MIX METHOD -DEWAXING -SEPARATING MEDIUM -HEAT CURE ACRYLIC DENTURE BASE RESIN -MANIPULATION OF DENTURE BASE MATERIALS -PACKING
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Flasking:
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POLYMERISATION DEFLASKING AND FINISHING
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APPLICATION OF SEPERATING MEDIUM -To prevent water from the mould to enter into acrylic resins. -To prevent monomer penetrating into the mould. TYPES- .tinfoil .tinfoil substitutes-cellulose lacquers. Solution of alginate compounds.Evaporated milk,soap,sodium silicate, starches.
Slide 40-

MIXING OF POWDER AND LIQUID POLYMER:MONOMER RATIO -Accepted ratio 3:1 by volume or 2:1 by weight. IF MORE MONOMER -Greater polymerization shrinkage. -Additional time is requires to reach the packing consistency. -Tendency for porosity. IF LESS MONOMER-less wetting-granular acrylic.
Slide 41-

Compression moulding technic Flasking:multiple mix method The casts with the wax dentures, are removed from the articulator and cleaned . Prepare dental stone or plaster , fill the lower half of the flask, place the cast and wax denture , in the mix and remove the excess mix , without creating undercut in the stone mount.
Slide 42-

PREPARATION OF THE MOuLD Before mold preparation: prosthetic teeth must be selected Arranged in a manner that fulfill both aesthetic &functional requirements Neccessasitates absolute accuracy in: impression making Cast generation Record base fabrication Articulator mounding tooth arrangement Wax contouring At this master cast & complete tooth arrangement are removed from the dental articulator & thin coat of separating medium
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LOWER PORTION : of dental flask is filled with freshly mixed dental stone& Master cast is placed in to this mixture Dental stone contoured to facilitate : Wax elimination Packing And de-flasking procedures Upon reach initial set, apply separating medium on stone cast. UPPER PORTION : of selected denture placed on top of lower portion of flask Apply Vaseline on top of wax surfaces to reduce surface tension & second mix of dental stone prepared
Slide 44-

Pour mix to the dental flask , care is taken to investing stone achieve intimate contact with external surface . Add dental stone to remaining portion till all parts is completely covered Incisal &occlusal surface are minimally exposed to facilitate subsequent de-flasking procedures Stone is coated with separator when permitted to set The lid of flask is gently tapped in to the place & stone is allowed to set . Clamp the flask &press the for lid is properly seated. Allow it to set for 30 mins .[THIS THREE-MIX FLASKING TEHNIC HELPS DEFLASKING without causing damage to the denture & teeth
Slide 45-

Pour mix to the dental flask , care is taken to investing stone achieve intimate contact with all external surface Add dental stone to remaining portion till all parts is completely covered Incisal &occlusal surface are minimally exposed to facilitate subsequent de-flasking procedures Stone is coated with separator when permitted to set The lid of flask is gently tapped in to the place & stone is allowed to set Clamp the flask &press the for lid is properly seated. Allow it to set for 30 mins .[THIS THREE-MIX FLASKING TEHNIC HELPS DEFLASKING without causing damage to the denture & teeth
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Stone is coated with separator when permitted to set The lid of flask is gently tapped in to the place & stone is allowed to set Clamp the flask &press the for lid is properly seated. Allow it to set for 30 mins .[THIS THREE-MIX FLASKING TEHNIC HELPS DEFLASKING without causing damage to the denture & teeth
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DE-WAXING Upon completion the setting process , the record base &wax must be removed from the mold .to accomplish this , denture flask placed in boiling water for 5 mins The flask removed from the water ,& appropriate segments are separated the record base &softened wax remain in the lower portion of denture flask The record base & softened wax are carefully removed from the surface of the mold .residual wax is removed from the mold cavity using wax solvent . The mold cavity subsequently is cleaned with a mild detergent solution & rinsed with boiling water & allowed to dry.
Slide 48-

SEPARATING MEDIUM To separate the plaster surface from acrylic dough ,an impervious separating medium (thin film ) is formed in between them. Thin tin foil was adapted earlier, which was found to be time consuming & inconvenient. Certain paints , cellulose lacquers, soap/starch solutions ,were tried . At present , alginate sol is found to be more convenient & effective , separating medium. The stone surface in the flask except the exposed teeth surfaces ,are applied with one / two layers of thin coatings of separating medium with a painting camel brush . It reacts & forms a thin transparent gel form of coating alginate . This is impervious to water & monomer
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Setting reaction Nan Alg + n/2 Caso4→ Ca n/ 2 Alg(gel film)+n/2 Na2 so4
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Composition of stellon-cold-mold seal Soluble salt of Na/ NH4 alginate = 2- 3 % Alcohol (for evaporation ) =3% Glycerin = 7% - for suitable viscosity Preservative , color =(small amount) Water (balance – 85% - 88% )
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Purpose of application separating medium is to prevent- Diffusion of water from wet plaster in to un polymerized dough which later cause voids , resulting in crazing . Diffusion of monomer from the dough into plaster which polymerize & cause, rough surface & adhesion of plaster to acrylic (plaster free denture)
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Precautions Perform complete de-waxing , Apply carefully two / three layers without trapping air , Do not apply on the exposed teeth surface , as this film prevents chemical bonding of teeth with denture. Apply when the plaster mold is still hot for quick setting
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Manipulation of denture base material PHYSICAL-STAGES OF MIXING;POLYMER-MONOMER-INTERACTIONS Monomer liquid as much as required is taken in a non conducting thick porcelain mixing jar with a lid and the powder is added in the ratio 3:1 by weight or 2.5:1 by volume or until all the powder is added, get wetted completely . It is quickly mixed and lid is closed to prevent evaporation of monomers.
Slide 54-

Physically stages of mixing WET-SANDY STAGE STRINGY-STICKY-TACKY STAGE DOUGH STAGE RUBBERY ELASTIC STAGE STIFF STAGE DOUGH TIME: 5-10 MINUTES According to ADA specification number 12,it should be less than 40o minutes.
Slide 55-

The dough time is shorter if: -Molecular weight of polymer is small. Presence of copolymer Plasticizers High temperatures Low powder/liquid ratio WORKING TIME: more than 5 minutes
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Powder PMMA : fine powder or beads Copolymers Benzoyl peroxide – initiator Dibutyl phthalate-plasticizer Color pigments Liquid Methyl methacrylate- monomer Comonomer N.N Dimethyl para toluidine – activator Ethylene glycol-dimethacrylate- cross-linking Dibutyl phthalate- plasticizer Hydroquinone-inhibitor Composition of cold cure denture base resins
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Packing It is a carefully completed filling of the mould space without air trapping . Under packing , with less dough causes lack of pressure and air trapping porosities . Over packing increases denture thickness and may causes displacement of the teeth. METHOD-As soon as soon as the dough stage is reached , it is kneaded , rolled and bent into U shape which is carefully placed on the tooth surface in the upper half of the flask and is slightly overfilled. Thin polythene sheet is covered and the two halves are assembled
Slide 58-

Injection moulding technique A special flask , with two sprue holes is used. From these holes , two wax sprues is attached to wax denture. After the master cast with the wax denture is settled on the lower half , in stone investment. Then complete investment is done. After dewaxing separating medium is applied and two halves are closed. The dough is injected through one hole using a high pressure injecting device until the excess dough begins to come out from the other hole. After bench curing, it is placed in the water bath and polymerized.
Slide 59-

Polymerization: curing CURING CYCLES: It is the method of complete polymerization of heat cure acrylics by raising the temperature of the packed flask and cooling down to the room temperature . To eliminate boiling monomer porosity due to exothermic polymerization , the temperature of the heating system is to be kept much below the particular value and retained for long time for maximum polymerization and then cooled.
Slide 60-

Slow curing: After bench curing for 30-60 minutes , the flask assembly is placed in water bath and temperature is slowly raised to 70 degree Celsius and kept constant for about 8 hours using a thermostat device . Monomer begins to polymerize releasing large quantity of heat at about 60 degree Celsius and temperature raises. But it cannot go beyond 90-95 degree Celsius, as that of water bath is at lower temperature and excess of heat generated is conducted from acrylic back to water bath. Thus boiling of monomer is avoided.
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BENCH COOLING: After keeping it for 70 degree Celsius for about 8 hours, heating is switched off and it is allowed to cool by itself in the water bath. Sudden cooling of the flask under the tap water causes the outer investment, first contracts producing the internal stresses. The relaxation of internal stresses causes crazing facing result in fracture.
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FAST CURING METHOD During this 90 percent monomer polymerize and corresponding heat produced is conducted back to water bath. It is heated further to boil the water for about 30 minute, when the remaining monomer polymerize with negligible heat. Afterwards bench cooling is done over night.
Slide 63-

Slow curing cycle
Slide 64-

Fast curing cycle
Slide 65-

Powder and liquid are mixed in 3:1 ratio. As soon as these are mixed, the benzoyl peroxide in presence of amine is activated and begins to release free radicals. Polymerization is initiated and gets propagated , thus dough time becomes shorter. The working time is reduced such that only one and closures are possible. Manipulation procedure
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Curing cycle : Temperature of acrylic dough
Slide 67-

Properties Due to lower curing temperature the kinetic energy of polymer is lower. The diffusion becomes less. This causes lower degree of polymerization. Molecular weight becomes less. All the properties become inferior.
Slide 68-

Residual monomer is more , i.e. 5% which may cause greater allergic reactions All mechanical properties ,compressive tensile, flexure, impact, fatigue strengths, modulus of elasticity and resilience, surface hardness etc. are reduced by about 10%.Sorption is lower and solubility is slightly higher The softening glass transition temperature is slightly lower Color stability is poor due to oxidation of amines causes gradually “yellowing” of color. Disadvantages over heat cure acrylic
Slide 69-

Simpler laboratory procedure, which even does not require any heating equipment. Less internal – boiling monomer porosities. Less dimensional change and better fit, due to lower thermal shrinkage. Advantages of chemically cured acrylics
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Denture repairs Special trays Soft and hard denture relining Record base Orthodontic functional removable appliances Athletic mouth protectors Obturators Maxillofacial appliances Applications of cold cure acrylics
Slide 71-

Comparison between heat cure and cold cure acrylics
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Comparison
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This is a chemically activated resin supplied as powder and liquids having similar compositions as the cold cure denture base resin. But, on mixing, it forms a thin dough which can be poured into the mould space prepared in a special flask. Pour and cure – fluid resin technic
Slide 74-

The complete wax denture is attached with two wax rod- sprues. With the cast , it is placed on the lower half of the special flasks, with holes and closed. Agar- agar investment is melted and poured into it and allowed to set. The two haves are separated. The wax denture and sprue rods are pulled out. The teeth are separated, cleaned and fitted in agar in their positions. The thin dough is prepared, and poured through the other hole. It is kept in a pressure chamber for 30 – 60 minutes for polymerization. Easily the smooth denture is recovered and the sprue- excess is removed. Very minimum polishing is needed and can be delivered to the patient in a short time Technic
Slide 75-

Simpler and quicker technic with lower cost Finishing and polishing –minimum procedure Less thermal change- cause better fit Less damage to artificial teeth while deflasking Advantages
Slide 76-

Poor bonding of acrylic teeth to denture Shifting of teeth due to soft investment Larger polymerization shrinkage Air trapping Decrease in vertical dimensions Same disadvantages-higher residual monomer , lower mechanical properties, higher solubility etc. as the cold cure acrylic denture bases. Disadvantages
Slide 77-

This has composite resin like composition High molecular weight acrylic resin monomer Urethene dimethacrylate- cross linking agent- matrix Micro fine silica inorganic filter Acrylic resin particles- organic filler N. N dimethyl paratoluidine- accelerator Light cure denture base - technic
Slide 78-

As visible light of wave length=468mm is used as activator ,opaque investment – flasking technic cannot be used…….
Slide 79-

Finished wax up trial denture is completed. Resin is sheet is adapted on the denture over the occlusal surface of teeth to form a template. This template is exposed to visible light and cured for 10 min. in a light chamber. The heat, during this curing time, softens the wax. The teeth in the template, the wax denture and cast are placed in boiling water and complete dewaxing is done Outline of technic
Slide 80-

2. Separating medium or releasing agent is applied on the cast. Light activated denture base sheet are adapted on the cast and cured. 3.Another strip is placed under the teeth, which are repositioned, and kept in contact with previous base, and anatomical form is carved to develop final shape and cured in a light chamber. Finally denture is removed from the cast and finished. Outline of technic
Slide 81-

High impact strength rubber modified acrylics: Butadiene – styrene rubber is included, to obtain higher resilience, abrasive resistance, impact strength, toughness etc. Rapid processing heat cure acrylics: this can be cured by placing the packed flask, directly immersing in boiling water, immediately after packing for short time, without producing porosities Fibre reinforced acrylics have higher impact strength. Modified denture base materials
Slide 82-

Properties of heat cured acrylics and its suitability for denture base materials Only the aesthetics, biocompatibility and simpler fabrication technics are the plus points, where as, the mechanical and thermal properties are not very satisfactory.
Slide 83-

Completely polymerized acrylic resin is chemically inert and non toxic. Very rarely patients have allergic reactions. It is more in cold cure acrylics as residual monomer (5%) is more. Sensitization of fingers causing dermatitis is more frequently observed. Tasteless, hygienic and not foul smelling. 1.Biocompatibility
Slide 84-

Even though density is low due to poor mechanical properties thick dentures are prepared, which have greater weight and less retention. Proportional limit, compressive strength, tensile strength, flexure strength, modulus of elasticity are not satisfactory. Careless handling, large dynamic masticating forces, accidental falls cause fracture Low surface hardness and abrasive resistance, causes abrasion of denture Large polymerization shrinkage as well as thermal thermal shrinkage during fabrication Low fatigue strength and endurance limit Viscoelastic nature causes dimensional changes. 2.Physical and mechanical
Slide 85-

Low thermal conductivity causes the patient, inability to feel the real taste of food Large coefficient of thermal contraction results, shrinkage while cooling polymerized denture from about 850 c down to oral temperature and adds up total dimensional inaccuracy. 3.Thermal
Slide 86-

Colorless and transparent organic glass Any color pigments, vein – like colored nylon fibers can be added to get natural appearance. Good color stability for heat cure acrylics. Chemically cured acrylics gradually become more yellowish due to oxidation of residual amines Aesthetics
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complicated Simple equipment and cheap auxiliary materials Fabrication technic is time consuming but not Shelf life of monomer liquid is limited Not very expensive Other considerations
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MISCELLANEOUS RESINS AND TECHNICS: DENTURE RELINERS: Hard Liners Soft Resilient Liners Short Term Resilient Liners-tissue conditioners
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HARD LINERS Due to the recessions and shrinkage of the tissues, after the teeth extraction the denture prepared gradually becomes loose or misfitting.This requires an additional layer on the tissue side of the denture for better fit. REQUIREMENTS-same as those for denture base materials MATERIALS USED-usually cold cure acrylics and sometimes heat cure acrylics with compositions similar to denture base resins are used.
Slide 91-

Technics of fabrications of hard liners Using the old misfitting denture as a special tray, alginate impression of the modified oral structure is obtained and a dental stone cast is prepared. The stone cast and old denture with alginate impression are invested in the flask. After setting the two halves are separated and the alginate impression material is completely removed. Separating medium is applied. Polymer and monomer are mixes. The resin dough is packed in a space between the stone cast and the denture.
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COLD CURE ACRYLIC IS PREFERRED DUE TO: Simpler technical procedure Les distortion of the denture. Repeated heating of the old denture cause distortion. Strength of the cold cure acrylic is adequate , for liners.
Slide 93-

Dimensional changes due to inherent properties of the materials during fabrication, by solubility , creep which cause poor retention Fabrication defects such as surface roughness and irregularities, internal porosities and crazing. These cause poor aesthetics unhygienic and week dentures Defects of denture
Slide 94-

1.Polymerization shrinkage - There is a chance for inclusion of slight excess dough material due to high pressure, while packing The temperature during polymerization is around 85-90 oc which is slightly above Tg, and hence initially the polymerized material is in soft condition. While cooling, the investment cool earlier, due to which this soft resin is not allowed to undergo shrinkage. Dimensional changes
Slide 95-

Effective below Tg Large coeff of thermal shrinkage may produce linear contractions when cooled from Tg to 370c. This may adds up the polymerization shrinkage, causing dimensional inaccuracy In cold cure or light cure acrylic dentures, as the range of temperatures of cooling is smaller this inaccuracy is slightly smaller 2.Thermal shrinkage
Slide 96-

When acrylic denture is kept in mouth the polarity of polymethylmethacrylate causes adsorption of water molecules by van der waal’s force. The water thus absorbed enters into the space in between the polymer chains Acts like a plasticizer and reduces the strength Causes an expansion of about 0.2% linear, which slightly compensates the thermal and polymerization shrinkage. On drying later, it causes relaxation of the internal forces. 3.Water sorption
Slide 97-

The solubility of acrylics in water or saliva is very negligible. The cold cure acrylics have slightly higher solubility that heat cure acrylics Both have no clinical significance as they have very low values 4.Solubility
Slide 98-

Time dependent deformations of viscoelastic materials under constant static materials under constant static or dynamic loads. Creep rate is more when temperature is higher, dynamic masticating forces are higher, residual monomer is more, plasticizers are more, degree polymerization is less and cross linking is less 5.Creep
Slide 99-

1.Rough surface Causes unhygienic by collection of food debris, poor aesthetics by scattering of light & irritation of soft tissues Causes Rough wax pattern High water powder ratio and inadequate mixing of plaster stone Incomplete dewaxing Improper application of separating medium. Air trapping Coarse powder particles Lack of pressure during curing or packing Denture fabrication defects and remedies
Slide 100-

Porous dentures lack strength and aesthetic qualities Irregular voids- if monomer and polymer is not mixed properly larger polymerization shrinkage occurs. Bench curing about 30 min allows the monomer to diffuse, homogeneously. Opacity-this is caused due to inadequate pressure during the polymerization period. Micro irregular voids formed causes polymerization shrinkage. This scatters and prevents the refraction of light resulting in poor aesthetics Regular voids-boiling monomer porosity-large amount of heat liberated during polymerization 2.Internal porosities
Slide 101-

Regular voids- sub surface porosity Regular boiling monomer porosity, forming well below the surfaces Heat produced can not get conducted immediately to water bath Sub surface porosity formed at lingual side of the bulkiest posterior parts of denture Remedy- curing at low temperature Regular large void air inclusion porosity Occurs due to trapping of air, while packing the resin dough. Remedy- proper kneading of the resin dough before packing Internal porosities
Slide 102-

Rapid cooling after curing- The outer investment cools earlier and contracts initially This frictional adhesion adhesion of two materials of different coef of thermal contractions and both being nonconductors, inhibits the contraction of acrylics Internal stresses cause separation of polymer chain and micro cracks Remedy-Bench cooling overnight Processing stress
Slide 103-

Porcelain teeth in acrylic dentures Porcelain teeth and acrylic denture have a vast difference in their coeff of thermal expansions When oral temperatures fluctuates by hot or cold food the teeth and denture expand and contract differentially creating internal stress at their interface Crazing is formed in weaker denture around the porcelain teeth Remedy-Select acrylic teeth for acrylic denture
Slide 104-

Cross linked acrylic resin have greater resistance to the separation of polymer chains and hence decrease crazing as well as creep Dimethacrylate system resins have highly cross linked 3D structure resist crazing.
Slide 105-

Separating medium applied improperly: Water from plaster enters into resin dough. This goes out later creating voids which become centers of stress conc and relaxation Internal porosities also act as stress conc regions Chemical attacks such as eugenol,alcohol etc. Solvent action: Increase the size of micro cracks rapidly due to their tendency to push apart the polymer chains Repeated drying and wetting of denture in service accelerates the internal stress relaxation and crazing
Slide 106-

MODIFICATIONS OF HARD LINERS Some special relining fixtures can be used.The method does not require elaborate flasking procedure. Some manufactures supply cold cure acrylic or polysilicone liners- chair side materials which the patient with adequate knowledge ,can use himself. Visible light cured materials are also supplied.
Slide 107-

SOFT RESILIENT LINERS Long term permanent soft liners and short term tissue conditioners are very soft cushion like material . These absorb the large dynamic , masticating energies and distribute, as required by the clinical situation. REQUIREMENTS- Large modulus of resilience Permanently resilient Chemically bond with acrylic denture bases Dimensionally stable Easy to trim and polish
Slide 108-

Materials used for long term permanent soft liners 1-POLYSILICONES- Chemically activated material is similar to condensation impression material supplied as two paste system. Heat cured system is supplied as single paste. ADVANTAGES- Permanent high resilience. DISADVANTAGES- Poor bonding to acrylic denture Difficult to trim or polish
Slide 109-

2-polymer resins These are highly plasticized polymethyl methaacrylate and both chemically and heat cure resins are available. The external plasticizer like dibutyl phthalate are added in large quantity. These plasticizer molecules are quit large. They pushes polymer chain apart, reduce entanglement of chains . So that the chains slip one over the other easily, that is it becomes soft, flexible and very resilient. 3-HIGHLY PLASTICISED polyvinyl chlorides and acetate have been tried. These also become stiffer. 4-PLASTICIZED POLYURETHENES- Polyphosphazines were also tried but, without much success.
Slide 110-

SHORT TERM RESILIENT LINERS-TISSUE CONDITIONERS Denture may obstruct easy blood circulation in some inflammed irritated mucosa of the soft tissues. To enhance the healing of the affected part, some massaging effect is required for few days by a soft resilient liners until the tissue become normal. These short term soft term liners are sometime called as tissue conditioners
Slide 111-

Recent modification of short term resilient liners Recently, a new technic for soft liners is studied A viscous silicone liquid is filled in an envelope of thin polyethylene film. These liner has a advantage of continuous adaptation to the denture base.
Slide 112-

Problems faced in fabrication and use of soft liners Poor mechanical bonding debonding from dentures. No permanent resilience -leeching of plasticizers and stiffening in service. Dimensionally instability, absorption of oral fluid. Difficult to polish and maintains cleanliness. Decrease of strength of denture due to forming of relief spaces
Slide 113-

Denture rebasing technic In certain clinical situations ,the denture base get abraded and damaged , where as the denture teeth remain without any damage. Materials Used-is same as heat cure acrylic resins. Methods-Using the old denture as tray , obtain the alginate impression of oral structure and prepare the stone cast . Mount these in a device known as reline jig .The position of the teeth are carefully indexed with respect to their occlusal surface. The teeth are removed and positioned in the jig. On the cast wax base plate and the occlusal rims are adapted . The teeth are fixed . The finished wax denture is then processed and acrylate in the conventional method.
Slide 114-

Due to poor mechanical properties and low impact strength the denture may fracture easily by mishandling. The broken denture can be repaired Selection of materials Repairing procedure is simpler and less time consuming Thermal shrinkage of the repair cold cure acrylic is smaller Repeated heating and cooling may cause warpage of denture. Denture repair resin
Slide 115-

Fractured pieces are joined using sticky wax and a stone cast is prepared Sticky wax is eliminated by flushing with hot water Sharp corners are trimmed to provide enough space and bonding for the prepare resins The cast is coated with separating medium Repair sites are wetted with monomer and polymer powder sprinkled on it. And forms dough Then allow to cure in pressure chamber. Repaired denture is kept immersed in water until delivery Method of denture repair resins
Slide 116-

Denture repair
Slide 117-

Custom made special trays are prepared Materials used- cold cure acrylic resins, supplied as monomer and polymer Method- preliminary impression is taken and a stone cast is prepared. spacer like wax sheet is adapted. Acrylic dough rolled and adapted over spacer is allowed to self cure. wait for 24 hrs to attain dimensional stability Resin special trays and base plate materials
Slide 118-

Distortions of the impression due to relaxation of internal stresses induced by setting shrinkage and thermal shrinkage are minimized, as impression has uniform thickness Less wastage expensive impression materials Advantages
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Used for correction or replacement of the defects of the craniofacial complex These are fabricated mainly for aesthetics REQUIREMENTS Biocompatible Elastic properties should be permanent Able to withstand thermal fluctuations Aesthetically most suitable ,stable . Fabrication technique should be simple MAXILLOFACIAL RECONSTRUCTIVE MATERIALS
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An alginate impression is taken and stone master cast cast is prepared and suitable wax pattern is carved on the master cast ,invested , dewaxed ,packed with materials-dough , putty or paste consistencies and polymerized . Suitable color pigments are added to the dough before packing . Sometimes the inner part of the semitransparent prosthesis is painted with oil paints , which method is not good due to poor stability. OUTLINE OF FABRICATION
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Polysilicones - both heat curing and room temperature vulcanzing types are used Suitable colour pigments like dry earth ,rayon fibers , oil paints etc , are added to pastes. Plasticized polymethyl methacrylate –fabrication method is simpler ,curing is done at room temperature , using a stone mould. DISADVANTAGES Prosthesis is heavy Low flexibilty MATERIALS USED
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Gradual hardening due to leaching of plasticizer . Hence it is not suitable . Plasticized PVC –Supplied as finely divided PVC particles suspended in a solvent Latex rubber –has skin like soft texture ,low cost. Synthetic rubbers- Tripolymer of butyl acrylate ,methyl methacrylate and methyl methacrylamide Polyurethanes- supplied as 3 component system
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ACRYLIC TEETH: These are selected due to the chemical bonding and higher fracture resistance . These are prepared from heat cure acrylic ,supplied as powder and liquid forms. Cross linking agents , glycol or other dimethacrylates are included mechanical properties . Split brass moulds are used for packing the dough of suitable shades . After , heat curing, polishing and finishing are done. ARTIFICIAL TEETH FOR DENTURES
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Removing the glossy ridge-lap surface of teeth. Roughening the bonding surface Cutting small grooves or holes in the bonding surface MECHANIAL BONDING OF TEETH WITH ACRYLIC DENTURE IS IMPROVED BY,
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Wetting the ridge-lap surface with methyl methacrylate monomer liquid. Coating with a mixture of methylene chloride and MMA liquid to the necks or ridge-lap area, five min before use. Complete dewaxing by flushing with hot water or using a detergent. Avoiding coating of seperating medium on the bonding surface of teeth. CHEMICAL BONDING OF TEETH IS IMPROVED BY:
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PORCELAIN TEETH
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Superior permanent aesthetic quality ,higher resistance to abrasion and biocompatibility . MANUFACTURING METHOD: Split brass moulds of two or three sections are used .Two semi halves have the semi mould spaces corresponding to the labial and lingual sides of the teeth . Porcelain powders of suitable shades corresponding to more opaque dentin , more transparent enamel and incisal porcelain powders are chosen , and mixed with special liquids to form slurries . Enamel porcelain is packed on the labial half , dentin porcelain on the lingual side half and transparent slurry are packed at the incisal tip of the moulds . Small base metal rings are inserted PORCELAIN TEETH
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In the anterior teeth as provisions for fixing gold plated retention pins and also some wax provisions for diatoric or rhetoric holes ,in the posterior teeth . Two parts are pressed together and fired upto the low bisque stage . The teeth are removed, placed in quartz trays and fired under vacuum or in diffusible gases , in an electric furnace, cooled it under pressure, finished and glazed . To overcome the large shrinkages , the mould spaces , should be larger. CONTI-
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Brittleness-low fracture resistance to impact forces. High cost Clicking sound when opposite teeth contact. Abrasion of opposing natural teeth. DISADVANTAGES OF PORCELAIN TEETH
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The procedure of preparing permanent metallic crown and bridges involves several steps and requires few week after teeth preparations . During this interval ,the teeth may decay further or drift ,which will cause misfit of the appliance. Most common material used is chemically cured acrylic resin of similar composition as resin teeth . This is supplied as monomer liquid and polymer powder in various shades for selection to match the tooth colour . TEMPORARY CROWN AND BRIDGE RESIN -RESTORATIONS
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Before preparation of the teeth ,a stone cast is obtained and an acrylic tooth is waxed in the gap . A thin polystyrene sheet is softened by heat , and closely adapted on this to form a template . After the prepartion of teeth ,cold cure acrylic dough is prepared ,the template is packed and placed in position . Patient has to apply the pressure ,with the opposing teeth . During polymerization ,cooling with water spray is done many times . After curing it is taken out ,excess is trimmed ,polished and cemented temporarily .
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