Denture base materials PDF

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Summary

Denture base materials used in dentistry for the fabrication of dentures...

Description

DENTURE BASE POLYMERS 

Denture: an artificial substitute for missing natural teeth and adjacent tissues

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Denture base: the part of a denture that rests on the foundation tissues and to which teeth are attached Initially vulcanite was the most widely used denture base. It is a highly cross-linked rubber which was difficult to pigment and tended to become unhygienic due to uptake of saliva. Now acrylic resin is most widely used as a denture base Incase of allergy alternative: polycarbonate & Vinyl polymers

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Denture base material: any substance of which a denture base may be made

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Metallic denture base materials eg. Cobalt- chromium, gold alloys, aluminum, stainless-steel

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Nonmetallic denture base materials eg. Acrylic resins

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Temporary denture base materials eg. Shellac base plate

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Permanent denture base materials eg. Heat cure acrylic resins, metallic denture base

Ideal properties: •

1.Biological properties: Nontoxic, Nonirritant, Noncarcinogenic, Nonallergenic

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2. Physical Properties: Matching appearance of natural soft tissues Tg= high enough to prevent softening and distortion during use. Dimensional stability Color stable, Odorless, Tasteless Low value of specific gravity= Light weight High value of thermal conductivity Radiopaque

4. Mechanical Properties • High value of modulus of elasticity. • High value of elastic limit (ensuring that stresses during mastication don’t cause permanent deformation). • High modulus and high value of elastic limit will allow denture base to fabricated in relatively thin sections. • Sufficient flexural strength to resist fracture. • Should have adequate fatigue life and a high value of fatigue limit. • High impact strength • High abrasion resistance 5. Chemical Properties • Chemically inert + Should not absorb water or saliva. 6. Miscellaneous properties •

Inexpensive

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Long shelf like

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Easy to manipulate, repair and clean

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Sterilizable

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Nonporous to microorganisms + Not affected by oral environment-bacteria, food, medicines, etc.

Classification of denture base polymers:     

Resin: Natural or synthetic substances that form plastic materials after polymerization Acrylic resin: Thermoplastic resins made by polymerizing esters of acrylic or methyl methacrylate acids. Polymer: Powder Monomer: Liquid Curing: Process of hardening/setting/polymerizing

Composition of acrylic denture base materials:

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Powder

Liquid

polymer

PMMA beads

Initiator

Benzoyl peroxide

Pigments

Salts of Cd, Fe, organic dyes

monomer crosslinking agent inhibitor activator

MMA Ethyleneglycoldimethacrylate Hydroquinone NN’-dimethyl-p-toluidine

Beads of PMMA have a diameter of around 100µm.

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They are produced by a process of suspension polymerization in which MMA containing the initiator is suspended as droplets in water.

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Starch or carboxymethylcellulose can be used as thickeners and suspension stabilizers (disadvantage of contaminating polymer beads).

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The temp is raised to decompose the peroxide and bring about polymerization of MMA to form beads of PMMA, which after drying, form a free-flowing powder at room temp.

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PMMA is a clear, glass-like polymer.

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Pigments and opacifiers are added to produce a ‘life like’ denture base.

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Small fibers coated with pigment are used to give a veined appearance.

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Pink pigments- are salts of Cadmium.

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These pigments have good colour stability.

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Concern for toxicity associated with cadimium.

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MMA (Heat cure liquid): Clear Colorless Low viscosity liquid Boiling point 100.3C Distinct odor High vapor pressure Very susceptible to free radical addition polymerization.

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Liquid contains some crosslinking agent (ethylene glycol dimethacrylate) and is used to improve physical properties of set material.

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Inhibitor is used to prolong shelf life of liquid component & in its absence polymerization would occur slowly

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Inhibitor works by reacting rapidly with radicals formed within the liquid to form stabilized radicals which are not capable of initiating polymerization.

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One way of reducing occurrence of unwanted radicals in liquid is to store the material in a can or in a dark brown bottle and by eliminating the source of radiation.

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Activator is present in products which are self-curing or auto polymerizing materials and its function is to react with peroxide in the powder to create free radicals which can initiate polymerization of the monomer.

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Importance of Powder liquid ratio: Controls workability of the mix Influences dimensional stability High P:L = The monomer undergoes a shrinkage of about 21% that can be controlled by high P/L but if its too high then it produces dry and unmanageable mix that will not flow under pressure when placed in gypsum mould. Structure of the polymer:

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On polymerization of the monomer the bead and matrix phases become inextricably bound together by inter-penetrating networks of polymer Extent to which interpenetrating networks are formed depends on: M.W of bead polymer M.W of bead polymer is high and only surface layers of the larger beads become impregnated with monomer. Beads with highly cross-linked polymer would greatly reduce the extent to which they become impregnated. Inc. monomer content would inc. extent of penetration of beads by matrix material, however setting contraction would offset result. Time during and after mixing during which monomer polymer remain in contact is imp. As in heat cure material remains in dough state before packing; giving adequate time for penetration of bead by monomer In self cure, monomer begins to polymerise immediately after mixing. A rapid rate of polymerization leaves insufficient monomer to cause penetration of polymer beads, hence s/c have slower rate of polymerization to allow more time for monomer penetration.

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Polymer/ monomer ratio

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Time for which polymer and monomer are in contact before polymerization

Mixing: • Mixing of powder and liquid( 2.5:1 by weight) • Decreasing the polymerization shrinkage to 5-6% • When mixed in proper proportions, the resultant mass passes through five distinct stages: 1. 2. 3. 4.

Sandy Stringy Dough like Rubbery 5. Stiff The acrylic denture base is usually fabricated in a two-part gypsum mould. The mould is produced by investing wax trial dentures on which the artificial teeth have been mounted. After boiling out of the wax the gypsum mould is treated with an alginate mould sealing agent. Mould sealing agent: It is a viscous solution of sodium alginate that is rapidly converted to calcium alginate on contact with gypsum. It forms a thin skin over the surface of the mould, preventing monomer in the acrylic dough from entering gypsum. During sandy stage, little or no interaction occurs on a molecular level. Polymer beads remain unaltered. This stage is ideal for compression molding. Hence material is inserted into mold cavity during dough like stage. Later, mixture enters stringy stage. Monomer attacks the surfaces of individual polymer beads. Stage characterized by stringiness. The mass enters a dough like stage. On molecular level increased number of polymer chains are formed.Clinically the mass becomes as a pliable dough. It is no longer tacky (sticky) • •

Time taken to reach dough stage is doughing time Time for which the material remains at dough stage and is mouldable is termed as working time.

AIM: short doughing time and long working time Controlled by factors such as bead size and M.W. of powder.

Small beads with low M.W. dissolve more rapidly in the polymer. Small polymer beads dissolve in monomer causing increase in viscosity of liquid phase Larger beads absorb monomer and swell (depriving the liquid phase of monomer) causing an increase in viscosity. Following dough like stage, the mixture enters rubbery or elastic stage. Monomer is dissipated by evaporation and by further penetration into remaining polymer beads. In clinical use the mass rebounds when compressed or stretched. Upon standing for an extended period, the mixture becomes stiff. This may be due to the evaporation of free monomer. From clinical point, the mixture appears very dry and resistant to mechanical deformation

Packing: • Placement and adaptation of denture base material within the mold cavity is termed packing • Overpacking- leads to excessive thickness and malpositioing of prosthetic teeth • Underpacking- leads to noticeable denture base porosity • Trial packing is done to ensure proper packing of resin mass in the mold. • Excess material to form a flash at the point where 2 halves of the flask meet • After the final closure of the flasks, they should remain at room temperature for 30- 60 min. it is called bench curing. • If packing is delayed the material becomes tough, rubbery and eventually quite hard Pressure applied has 3 functions: • Dough flows in every part of mould. • Allows excess dough to be used causing effective reduction in polymerization shrinkage • Prevents formation of raised bite (thick base) Bench curing • It permits equalization of pressure throughout the mold • Allows more time for uniform dispersion of monomer throughout the mass of dough • If resin teeth are used, it provides a longer exposure of resin teeth to the monomer producing a better bond of the teeth with the base material

Curing: • •

Variety of curing cycles are available for placing the clamped flask in either a water bath or an air oven in order to polymerize the monomer to produce final processed denture. When choosing a curing cycle, following facts need to be considered: 1. Benzoyl peroxide (initiator) begins to decompose rapidly to form free radicals above 65ºC. 2. Polymerization reaction is highly exothermic. 3. Boiling point of monomer is 100.3- spherical voids in hottest part of curing dough= gaseous porosity 4. Aim is to produce high degree of conversion and produce polymer with high M.W (thus good mechanical properties)

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7 hr at 70ºC + 3 hr at 100ºC

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Most of the conversion of monomer to polymer occurs during 7 hr at 70ºC, during which the temp of the dough may approach 100ºC(exothermic)

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Final 3 hours ensure complete conversion of monomer in the thinner areas of denture base where exothermic effect is less pronounced.

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Place flask in bath of cold water and gradually bring to boil over period of 1 hr and allowed to boil for 1 hr and then cooled slowly.

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If placed directly at 100ºC, the exothermic heat of reaction can cause the dough to reach temp of 150ºC.

Polymerization via microwave energy • Resins can also be polymerized by microwave energy • Use of high intensity visible radiation to activate polymerization but using a light box • This technique employs a specially formulated resin and a nonmetallic flasks… FRP Flask [ Fiber Reinforced Plastic flasks] Advantages: • Cleaner and faster polymerization (3 mins) • Minimal color changes • Less fracture of artificial teeth and resin bases • Superior denture base adaptability Light activated polymerization Contains a blend of 1. urethane dimethacrylate monomer 2. Sub-micron particles of silica 3. Polymethylmethacrylate beads (organic filler) 4. Light sensitive initiator (camphoroquinone) 5. Activator (amines) Supplied in form of a flexible sheet in a light-proof sachet They are exposed to activating radiation in a specialist oven at normal atmospheric temp. The surface of the material is coated with a non-reactive barrier compound(Carboxy methyl cellulose) to prevent inhibition of polymerization by oxygen. Useful when trying to produce hollow denture bases used for large obturators after ablative surgery for cancer

Injection molding •

Polymers are made of Nylon or Polycarbonate.

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The material is supplied as a gel in the form of a putty.

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It must be heated and injected into a mold.

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A sprue hole and a vent hole are formed in the gypsum mould and the metal flask is constructed such that it will adapt to the injection moulding equipment.

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The equipment is arranged so a ‘wave’ of curing propagates from the part of the flask which is furthest from the sprue and vents.

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Enabling shrinkage during curing to be compensated by taking up extra material from sprue reservoir. Alternative polymer:

Chemically activated denture base resins • Chemical activators are used to induce polymerization. • Small acrylic beads are dissolved in monomer whereas larger beads absorb monomer and become swollen • Peroxide from powder and chemical activator from liquid meet during mixing, polymerization of monomer is initiated. • Within few mins of attaining dough consistency, rate of polymerization increases rapidly causing large temp rise (material becomes hard and unmanageable) • Does not require thermal energy and therefore may be completed at room temperature. • Hence often referred to as cold curing, self curing or auto polymerizing resins. • Time available for carrying out trial closure of the processing flask is minimal and if the viscosity has increased beyond a certain point at the time of final closure there is a danger of increased vertical height in the denture • Inferior mech properties • Higher residual monomer content (Short curing cycle and high solubility) • Usually used for repair(very fluid mix of cold cure resin is used) and relining of denture • Chemical activation is accomplished through the addition of a tertiary amine such as dimethyl- para- toluidine to the liquid. • Upon mixing, the tertiary amine causes decomposition of benzoyl peroxide. Consequently, free radicals are produced, and polymerization is initiated. Pourable resins: Cold cure resins that are used for denture base construction. They are mixed to a very high fluid consistency using low P/L ratio. The mix is then poured in a hydrocolloid mould and allowed to cure at or just above room temp. Adv: Easy removal of cured denture from hydrocolloid mould + min effort & time Disadv: High residual monomer level, inf mech properties, distortions due to use of flexible mould Use of pour type resin which is heat cured in hydrocolloid mould under dual influence of vacuum(help adaption of denture base to flask) and pressure(increase the degree of conversion and reduce the effects of shrinkage)

Properties: 1. Physical: – Available in variety of shades, opacity, veined/un veined. – Tg is 105ºC lower for self cure – Light activated material have high values of Tg – Factors that decrease Tg • Low M.W material • Increase residual monomer • Improper curing

Low values of specific gravity Radiolucent Good thermal insulator (disadvantage as the oral soft tissues are denied normal thermal stimuli.) Pt. lose protective reflex responses to hot/cold stimuli. Painful experience

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2. Mechanical Soft, weak, flexible. Denture thickness is doubled to provide adequate strength. Sufficient transverse strength. Poor resistance to fatigue fracture Poor impact strength VHN 20 (weaker than CoCr and SS) Poor abrasive resistance 3.Chemical and Biological Absorbs water- crazing and dimensional changes Candidal growth- denture stomatitis MMA allergy + irritation Oxygenating cleanser used in hot water causes whitening of denture base

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Modified acrylic materials Incorporate elastomers to improve impact strength Carbon fiber inserts to improve fatigue resistance Metal inserts/ powdered metals: weaken base and appearance is poor Inorganic salts(barium sulphate): Insufficient radiopacity at low conc Co-monomer containing heavy metals( barium acrylate): Poor mechanical properties Halogen containing co-monomer or additive: May act as plasticizer ad are expensive

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Defects of denture base polymers:

A) Porosity: 1. Granular porosity: Insufficient monomer(liquid component) to bind all beads together= granular effect. Loss of monomer during mixing stage/ proportioning also produces granular porosity (blotchy, opaque surface)  Seen during mixing 2. Contraction porosity: Occurs because the monomer contracts by 21% of its volume during processing resulting in insufficient dough to create an excess in the mould or due to the application of insufficient pressure during curing that leads to porosity voids.  Seen during packing 3. Gaseous porosity: (Overboiling) On polymerization there is an exothermic reaction which causes the temp of the resin to rise above 100C. If the temp raises before the polymerization process is completed, the monomer will boil producing spherical voids in the hottest part of the curing dough It can be avoided by rising the temp in a slow and controlled fashion. 

Seen during curing

B) Crazing: due to development of series of surface cracks that weaken denture. Cracks may arise due to: a. Frequent denture removal + Mechanical stresses b. Constant cycles of water absorption followed by drying + Attack by solvents c. Porcelain teeth (due to difference on coefficient of thermal expansion) d. Metal mold, tin foils and cross linking agents’ function is to reduce crazing

C) Warpage: Deformity or change of shape of denture affecting the fit of the denture. a. Release of stresses incorporated during processing: as a result of curing shrinkage/uneven or rapid cooling b. Packing during rubbery stage c. Stresses incorporated during improper flasking Bench cooling: • Coefficient of thermal expansion is about 10 times greater than gypsum, hence internal stresses are set up which lead to warpage of the denture base if at later stage it is placed in warm water (for cleaning). • Stresses can be reduced by allowing the flask to cool slowly from curing temp. 

Internal stresses caused by polymerization shrinkage are normally eliminated by plastic flow when polymerization takes place at elevated temperatures. If not removed can also contribute to warpage....