POLYMERS
- Polymer is a molecule that is made up of many units.
- Oligomer is a short polymer composed of two, three, or four mer( mer = unit) units.
- Monomers (mono= single) are the molecules that unite to form a polymer, and the process by which this occurs is termed polymerization.If monomers of two or more different
- types are joined, copolymers are formed.Copolymers may be either random(mers do not appear in specific order) or block (large numbers of one type of mer appear arranged in sequence).Atoms along the length of any polymer are joined through strong, primary covalent bonds.
- There are three basic spatial structures of polymers: linear, branched, and cross- linked.
- Linear and branched molecules are discrete but are bonded to one another through weak, physical bonds.Upon heating, the weak bonds break and the ability of the chains to then slide past one another results in a softened material.Upon cooling, the bonds reform and hardening occurs.Materials that are able to undergo this process are termed thermoplastic (polyvinylacrylics, poly(methyl methacrylate)).
- Cross- linking results in the formation of a network structure of covalently bonded atoms;primary linkages occur between chains, and the polymer actually becomes a single giant macromolecule.The spatial structure that allows chain sliding upon haeting is not present in cross- linked materials.cross- linked polymers therefore do not undergo softening upon heating and are termed thermosets (sylicones, cross-linked poly(methyl methacrylate), bisphenol A-diacrylate,cis-polyisoprene).
- Longer chains and higher molecular weight result in the polymer‘s increased strength, hardness, stiffness, resistance to creep along with increased brittleness.Small plasticizer molecules, when added to a stiff uncross-linked polymer, reduce its rigidity.When small molecules surround large ones, the large molecules are able to move more easily.A plasticizer therefore lowers the glass-transition temperature of the polymer,so a material that is normally rigid at a particular temperature may become more flexible.The glass-transition temperature is the temperature at which a polymer ceases to be glassy and brittle and becomes rubberlike.
Polymerization
- There are two types of polymerization reactions: additional polymerization, in which no by- product is formed, and condensation polymerization, in which low molecular weight by- product such as water or alcohol is formed.Polymerization has four stages: activation, initiation, propagation, termination.Reaction may be accelerated by light, heat, radiation or small amount of peroxides.Free radical additional polymerization is used for the synthesis of polymers.The free radicals are produced by reactive agents called initiators.The most popular initiator is benzoyl peroxide.Activation-involves decomposition of the peroxide initiatior using special conditions.Initiation-involves production of free radicals, which will encourage a polymer chain to begin growing.Activator allows polymerization to occur at low temperature(aromatic tertiary amines).
- Free- radical molecules have chemical groups with unshared electrones.In chemically activated systems, free radicals are produced by the reaction of an organic poroxide initiator and amine accelerator.In light- activated systems, the scission of camphorquinone results in the production of two molecules with one production, the free radicals attack the double bonds of available monomer molecules, resulting in the shift of the unshered electron to the end of the monomer and the formation of activated monomer molecules.Propagation- Activated monomers attack the double bonds of additional available monomers, resulting the rapid addition of monomer molecules to the free radical.This stage continues as the chain grows and length.Termination-it is posible for the propagation to continue until the supply of monomer molecules is exhausted.These reactions produce dead polymer chains which are not capable of further additions.Small amounts of inhibitors, such as hydroquinone, may be added to the monomer to increase storage life.Hydroquinones react with free radicals, thereby decreasing the rate of initiation.
Classification of denture base, liners and tissue conditioners:
Denture base:
I. Heat –cured (PMMA )
- conventional
- Unfilled
- Re-inforced (Carbon,Polyfiber)
2.high impact
II. Autopolymerized(PMMA)
III.Injection molded 1. PMMA polycarbonate nylon
Soft liners:
I. Acrylic
II.silicone a) room temperature vulcanizing (RTV) heat cured
Tissue conditioners-
plasticized acrylics
Denture base polymers
The polymeric denture base can consist of either a simple stiff base on which the teeth are arranged, or a sandwich of stiff base and a resilient liner to provide greater retention and comfort.When the tissue underlying a loose denture is traumatized due to the constant motion of the hard plastic over the mucosa, a viscoelastic gel known as a tissue conditioner can be molded onto the fitting surface of the denture in situ so the tissue can heal and an accurate impression of the untraumatized fitting surface can be taken prior to making a new, better-fitting denture.
Requirements of denture base polymers:
- Physical properties: good esthetic, thermal conductivity, dimensional stability,“light“, radiopaque.
- Mechanical properties: high value of modulus of elastisity,sufficient flexural strength,sufficient abrasion resistance.
- Chemical properties: materials should be chemically inert, insoluble in oral fluids, should not absorb water.
- Biological properties: should not be harmful to the technician, doctors and patients;should be non- toxic and non- irritant to the patients, should not be able to sustain the growth of bacteria or fungi.
Heat-cured acrylic
It consists of powder and liquid.The major component of powder is polymer (beads of polymethylmethacrylate), pigments(pink pigment-cadmium salts) and initiator-benzoyl peroxide ~0.5%.The liquid: monomer (methylmethacrylate), cross- linking agent(improves the physical properties of the set material), inhibitor(prolongs the shelf life of the liquid components).The inhibitor, which is usually hydroquinone, works by reacting with radicals formed within the liquid to form stabilized radicals which are not capable of initiating polymerization.The ratio of powder to liquid is important since it controls the workability of the mix as well as the dimensional change on setting.A powder/liquid ratio by weight is 2.5:1.The mixture should be lefted for few minutes, and the mixing vessel should be closed to prevent evaporation of monomer.There are few stages of this material: „sandy“ consistency, after the short period of time it becomes „sticky“,which forms strings of material.The „dough“ stage.The material can be moulded like plasticine and does not stick to the mixing vessel.The material is packed in the mould at this stage.Later can be „rubber“ and „hard“ stages.
The dough is packed into a two-part gypsum mould.The excess of dough is removed, then the flask is closed again using the pressure and the heat.The polymerization reaction itself is exothermic, so if the rate of reaction is too high, it cal lead to porosity.Under the pressure the dough flows into every part of mould and you can avoid of porosity.There are few kinds of porosity: granular porosity-it can be then there is insufficient amount of monomer to bind all the polymer beads together,evaporation of monomer.Contraction porosity-the use of insufficient dough to create an excess in the mould or the application of insufficient pressure during curing can lead to this kind of porosity.Gaseous porosity-it can be then the temperature of dough during polymerization is rised significantly above 100.3˚C, the monomer at this temperature will boil and will produce spherical voids.
High- impact acrylic
High-impact acrylic denture base is also made by the hear- cured dough method.Impact resistance arises from the incorporation of a rubber phase into the beads during their suspension polymerization.
Autopolymerizing denture base (cold curing resines)
The autopolymerizing denture base is chemically similar to the heat- cured denture base except that a reducing agent is added to the monomer.The reducing agent is usually a tertiary aromatic amine, although barbituric acid derivates.The reducing agent reacts with the benzoyl peroxide at room temperature to produce peroxy free radicals, which initiate the polymerization of the monomer in the denture base.Autopolymerizing materials are used for repairing and relining of dentures, because their mechanical properties are weak, and there is high residual free- momomer content.
Injection- molded plastic
The injection- molded plastics have the advantage of consistent molecular weight, but the disadvantage of capital equipment costs, and difficulties associated with attachment of teeth to the denture base.The plastics still offered for the use as injection-molded denture base acrylic are polycarbonate and nylon.They represent very small fraction of the market, although they offer a real alternative to metal dentures for patients sensitized to conventional methacrylate or to nickel or cobalt.
The technician has little leeway when using injection-molded plastics.The mold should be dry to prevent the generation of steem during molding.Patience is required to ensure the melt has reached the right temparature and cools sufficiently after molding.Inadequate spruing will lead to underfilled molds, as can underheating the melt;overheating the melt can cause explosions, especially when polycarbonate is injected into moist molds.
Injection moldings rely almost totally on mechanical forces to retain the teeth.Low melt temperatures will cause strong forces to be put on the teeth during the injection phase and may dislodge some molars, even from plaster molds.Depolymerization or oxidation from overheating the melt can result in porosity, loss of strength, color changes, and increased fouling.
Light- activated materials
This material consists of a urethane dymethacrylate matrix with an acrylic copolymer and has a microfine silica filler.It is supplied in premixed sheet or rope form.A base plate is made by adapting the material to a cast and polymerizing in a light chamber at 400 to 500nm(blue light).Teeth are added to the base with additional material followed by a second light exposure.The system eliminates the need for flasks, wax, boil-out tanks, packing presses, and heat processing units required for the construction of the conventional dentures.Light- activated materials contain no methylmetacrylate monomer, they may be considered for use in those patients who have demonstrated a sensitivity.polymerization shrinkage is smaller than conventional systems.
A comparison of denture base materials
1.Heat cured
Advantages:
- good apperance
- high glass-transition temp.
- Easy fabrication
- Low capital costs
- Good surface finish
Disadvantages:
- free monomer content or formaldehyde can cause
- sensitation
- low impact strength
- fatique life too short
- radiolucency
2.Heat cured, rubber
Advantages:
- improved impact strength
- reinferced
Disadvantages:
- reduced stiffness
3.Heat cured, fiber
Advantages:
- high stiffness
- reinforced
- very high impact strength
- good fatique life
- polypropylene fibers make good
- translucency
- good surface finish
Disadvantages:
- carbon and Kevlar fibers make
- poor color
- poor surface
4.Autocured
Advantages:
- easy to deflask
- dimensional accuracy
- capable of flexural strength than heat cured
Disadvantage:
- no cheaper over long term
- increased creep
- increased free-monomer content
- color instability
- reduces stiffness
- tooth adhesion failure
5.Injection molded
Advantages:
- dimensional accuracy
- low free-monomer content
- polycarbonate and nylon make
- good impact strength
Disadvantages:
- high capital costs
- difficult mold design problems
- less craze resistance
- less creep resistance
6.light activated
Advantages:
- no methylmethacrylate monomer
- decreased polymerization shinkage
- possible improved fit compared to
- conventional materials
- requires little equipment
- time savings
Disadvantage:
- decreased elastic modulus
Denture lining materials
Permanent soft lining materials
Permanent soft lining materials are resilient polymers used to replace the fitting surface of a hard plastic denture, either because the patient cannot tolerate a hard fitting surface or to improve retention of the denture.Because the lining is soft, its dimensional stability is important, as are its durability and resistance to fouling.However, because by definition soft lining materials are above their glass-transition temperature when in the mouth, such physical phenomena as water absorbtion, osmotic presence of soluble components, and biodegradability play a greater role in the clinical success of a liner than they do in the glassy polymers used as denture bases.
Acrylic soft liners
The acrylics consist of either highly plasticized intrinsically glassy polymers or soft acrylics that have a natural glass-transition temperature (softening temp.) at least 25˚C less than that of the muoth.The plasticizer used to soften the acrylic can either be unbound to the acrylic and hence free to diffuse out during use, resulting in a loss of resilence, or it can be reacted into the cured matrix of the acrilic.
Silicone soft liners
The silicones used as soft liners can be devided into two types: room temperature vulcanizing (RTV) and heat cured.The resilence of silicones makes them at first seem to be the ideal soft materials.However, silicones have poor tear strength, no intrinsic adhesion to acrylic denture base, and, if not properly cured, a tendency to osmotic pressure effects.RTV silicones‘ geatest drawback is their lack of adhesion, which is especially a problem around the edges of the attachment between acrylic and silicone.Heat cured silicones have in their formulation a silicone methacrylate than can polymerize into curing denture base and into the heat- cured addition silicone.The RTV silicones use a condensation cross-linking system based on organo-tin derivates such as those used in impression rubbers.Their degree of cross-linking is lower and their serviceability is low as a result, with frequent reports in the literature of swelling and buckling during use and excessive sensitivity to denture cleansers.The rupture strength of some RTV silicones is known to deteriorate cosiderably when exposed to waterfor long periods.The heat-cured silicones achieve a greater degree of cross-linking and have much longer clinical lifetime.
Temporal soft lining materials
These materials are similar to tissue conditioners, but they are not so soft as conditioners immediately affter setting but they retain their softness for longer, taking up to a month or two to harden.These materials are viscoelastic.Some kinds of cleansers can cause surface degradation and pitting of material.Temporal soft lining material can be used improving the fit of ill-fitting denture until such a time as a new denture will be maden.These materials will go hard, when this occurs, the surface is rough and increases the risk of trauma.In this state the base can be colonized by Candida,it leads to stimatitis.
Tissue conditioners
Tissue conditioners are soft denture lining material which may be applied to the fitting surface of a denture, to the dentures of patients who have undergo surgery.,they are useful when a tooth is being added to a denture( very shortly after extraction).The material consists of powder (polymer beads-polyethylmethacrylate) and liquid (solvent-ethyl alcohol; + plasticizer- butylphthalyl butylglycolate).
A comparison of soft liners and tissue conditioners
1. Soft liners:
a) Acrylic
Advantages :
- high peel strength to acrylic denture base
- high rupture strength
- some can be polished if cooled
- reasonable resistance to damage by denture cleansers
Disadvantages:
- poor resilience
- loses plasticizer in time
- some buckle in water
b) Silicone(RTV)
Advantage:
- resilience
Disadvantage:
- low tear strength
- low bond strength to dentures
- attacked by cleaners
- buckle in water
- poor abrasion resistance
c) Silicone heat cured
Advantages:
- resilience
- adequate bond strength to acrylic
- more esistant to aqueous environment and
- cleanser than RTV
Disadvantages:
- low tear strength
- poor abrasion resistance
2. Tissue conditioners
Advantages:
- viscoelastic properties almost ideal
- can be applied chairside
- denture fit well
- can record freeway space
Disadvantages:
- low cohesive strength
- affected by cleanser
- alcogol can sting inflamated mucosa.
No comments:
Post a Comment