INTRODUCTION: Synthetic resins, often called plastics are polymers composed of very large molecules. Modern living has been greatly influenced by the synthetic plastics. They can be molded into various forms and then hardened for commercial use (e.g. clothing, electronic equipment, building materials and household appliances. Plastics have had enormous impact on dentistry, and they are now used as dentures, sealants, bonding materials, restorative materials, veneering materials and impression materials. Most resin systems used currently and widely in dentistry are based on acrylic resins called Methyl Methacrylic acid. However, because the field is dynamic and new types of resins are being developed on a regular basis, the dentist’s knowledge must include basic concepts of resin chemistry so that new developments in the field can be critically evaluated.
HISTORY: The first organic polymers used in orthodontics were rubber and its sulfur crosslinked derivative, vulcanite (Goodyear, 1840). Although other polymers such as cellulose (J.W.Hyatt, 1869), Bakelite (L. Baekland, 1907), Polyvinyl chloride (W.L.Semon) and synthetic rubber (W.H. Carothers, 1925) were launched earlier, the polymers truly adequate for dental purposes were discovered only in the late 1930s. Poly methyl methacrylate (Plexiglass) was synthesized by O. Rohrn in 1936. Previously materials such as vulcanite, nitrocellulose, phenol formaldehyde, Vinyl plastics and Porcelain were used for various dental uses. The acrylic resins were so well received by the dental profession that by 1946 98% of all Denture bases were constructed from methyl methacrylate polymers or copolymers. Self cure resins were used first for dental purposes in Germany during World War II. During 1970’s Fluid resin was introduced into the Dental profession. Various other polymers have been developed since that time which includes vinyl acrylic, polystyrene, epoxy, nylon, polycarbonate, polyurethane ,silicon etc. Recently materials such as glass fibers, carbon fiber’s graphite, polyaramid fibers are being added to acrylic resins to increase its mechanical properties. Recently rubber reinforced and butadiene reinforced acrylics were introduced into dentistry.
CLASSIFICATION OF RESINS: Due to their heterogenous structure and complex nature, it is difficult to classify them. The quantitative determination of its composition and structure is possible only by the analytical techniques.
Based on thermal behavior, they are classified as: (a) Thermoplastic Refers to resins that are softened and moulded under heat and pressure without any chemical change occurring. They are cooled after moulding, they are fusible and are usually soluble in organic solvents, e.g. polymethyl methacrylate, polyvinyl acrylics and polystyrene. (b) Thermoset Refers to resins in which a chemical reaction takes place during the molding process, so that final product is chemically different from the original substance. They are generally infusible and insoluble. E.g. Cross linked poly methyl methacrylate, silicones, etc. IDEAL REQUIREMENTS OF DENTAL RESINS 1. Should be tasteless, odorless, non-toxic and non-irritant to the oral tissues. 2. Aesthetically satisfactory, i.e. should be transparent or translucent and easily pigmented. The color should be permanent. 3. Should be dimensionally stable. It should not expand, contract or warp during processing and subsequent use by the patient. 4. Have enough strength, resilience and abrasion resistance. 5. Be insoluble and impermeable to oral fluids. 6. Have a low specific gravity (light in weight) 7. Tolerate temperatures well above the temperature of any hot foods or liquids taken in the mouth without undue softening or distortion. 8. Be easy to fabricate and repair. 9. Have good thermal conductivity. 10. Be radio-opaque (so that a denture or fragments of a broken denture can be detected by x-rays if accidentally inhaled or swallowed and also to examine the dimensions of the resin restoration in a tooth.) 11. Coefficient of thermal expansion should match that of tooth structure. USES OF RESINS IN DENTISTRY: 1. Preparation of dentures 2. Artificial teeth 3. Tooth restoration 4. Cementation of crowns and bridges 5. Orthodontic and pedodontic appliances 6. Crown and bridge facings 7. Maxillofacial prostheses 8. Inlay and post core patterns 9. Implants 10. Dies 11. Temporary crowns and bridges 12. Endodontic and core filling material 13. Athletic mouth protectors 14. Impression trays 15. Splints and stents 16. Models
NATURE OF POLYMERS – TERMS
Polymer Denotes a molecule that is made up of many parts. Polymers are long molecules composed principally of non metallic elements (e.g. C, O, N, H) that are chemically bonded by covalent bonds. Their principal distinction from other common organic materials is their large size, and thus molecular weight. The mer ending represents the simplest repeating chemical structural unit from which the polymer is composed. A common commercial and dental example is the polymerization of methyl methacrylate monomer (100 gram/mole) into methyl methacrylate polymer (typically 300000 gram/mole). Most polymers are named by adding poly as a prefix to the word for the major monomer in the polymer chain (polymethyl methacrylate) or by adding poly to the description of chemical links formed between monomer units (polyamide, polyester, polyether, polyurethane). In other cases, the original commercial brand name has become the common name (Nylon, Teflon, Dacron, Plexiglass). Usually any chemical pocessing a molecular weight higher than 5000 is considered a polymer. The word polymer means ‘many units’.
Monomer The molecules from which the polymer is constructed are called monomers (one unit). Polymer molecules may be prepared from a mixture of different types of monomers and they are called copolymers.
Molecular weight The molecular weight of polymer molecule equals the molecular weight of the various mers multiplied by the number of mers. They may range from thousand to millions of molecular weight units depending on preparation conditions. The molecular weight plays important role in determining its physical properties.
Degree of polymerization Defined as total number of mers in a polymer. The higher the molecular weight of the polymer made from a single monomer, the higher the degree of polymerization. The strength of the resin increased with increase in the degree of polymerization until a certain molecular weight is reached. Above this there is no change.
Molecular weight distribution A narrow molecular weight distribution gives the most useful polymers. However, most polymers have a single wide range of molecular weights and so vary widely in their properties, e.g. the higher the molecular weight, the higher the softening and melting points and the stiffer the plastic.
STRUCTURE OF POLYMERS The physical structure of the polymer molecule is also important in determining the properties of the polymer. There are three basic structures.
Linear Here the mer units are connected to each other in a linear sequence. They can be further divided into:
Linear homopolymer: It has mer units of the same type
…- M – M – M – M – M – M – M – M – M - … . Random copolymer of linear type: It has two mer units randomly distributed along the chain
…- M – M – M –Y – M – Y – M – M – Y – Y – M – M –
Block copolymer: It has two types of mers distributed in segments or blocks. …-M – M – M -…- M – M – Y – Y - …- Y – Y – M – M -…
The mer units are arranged in a branched fashion.
Branched homopolymer: The mer units are of the same type.
…- M – M – M – M – M – …- M – M – M – M – M . . . . M M . . . . M M . . . .
Random copolymer of branched type: It has two types of mer units distributed randomly.
…- M – M – M – M – M – …- M – M – M – M – M – … . . . . M Y . . . . Y M . . . . M Y . . . . Graft Co-Polymer Of Branched Type: it has one type of mer unit on the main chain and another mer for the branches. …-M – M – M – M – M - …- M – M – M – M – M -… . . . . Y Y . . . . Y Y . . . . Cross linked polymer
It is made up of a homopolymer cross linked with a single cross linking agent. It is a network structure.
…- M – M – M – Y – M – M – M – M – Y … . . Y . . Y . . …- M - Y – M – M – M – M – Y -.. . . . . Y Y . . . . Y Y . . . . … - M -Y – M – M – M – M – Y -… POLYMERIZATION
The process of forming a polymer from identifiable subunits, monomers is called polymerization.
(I) According to the kinetics of polymerization reaction
(b) Stepwise Reaction Polymerization: occurs slowly by random addition of monomers to any growing chain ends.
(II) According to the type of polymerization, (a) If the polymerization occurs by condensation reaction, the process is known as condensation polymerization. (b) If the polymerization is brought out by an addition reaction an addition polymerization takes place.
Condensation polymerization (step-growth polymerization) The primary compounds react with the formation of byproducts such as water, halogen acids, and ammonia. The structure of the monomers is such that the process can repeat itself and build macro molecules. The principal resin employed was a phenol formaldehyde resin known as Bakelite.
So condensation resins are those in which polymerization is accompanied by (a) repeated elimination of small molecules (b) functional groups are repeating in the polymer chain. E.g. Polysulfide rubber impression material, Polyurethane
Addition polymerization The macro molecules are formed from smaller units or monomers without change in composition, and no by-products are formed. The structure of the monomer is repeated many times in the polymer. All resins employed extensively in dental procedures are produced by addition polymerization. Eg: polyethylene
Stages in polymerization The polymerization can be described in four stages.
(a) Induction or intitation is the time during which the molecules of the initiator become energized or activated and start to transfer their energy to the monomer molecules. I1* + M I1M* I1* is the initiator species M is the monomer
Free radical A free radical is a compound with an unpaired electron usually a fragment of a larger molecule that has been split by heating. This unpaired electron makes the radical very reactive. The induction period is greatly influenced by the purity of the monomer. The initiation energy for the activation of each monomer molecular unit is 16,000 to 29,000 calories per mole in the liquid phase.
There are three induction systems for dental resins (1) Heat activated: the free radicals liberated by heating benzoyl peroxide will initiate the polymerization of methyl methacrylate monomer. (2) Chemically activated: this system consists at least two reactants, when mixed they undergo chemical reaction and liberate free radicals. E.g. the use of benzoyl peroxide and an aromatic amine (dimethyl-p-toluidine) in self cured dental resins. (3) Light activated: in this system, photons of light energy activate t;he initiator to generate free radicals, e.g. camphoroquinone and an amine will react to form free radicals, when they are irradiated with visible light. (b) Propagation 5000 - 8000 calories per mol are required once the growth has started. The process continues with considerable velocity. This can be described as
I1 M* + M I1 MM* I1MM* + M I1MMM*
Theoretically the chain reactions should continue with the evolution of heat.
(c) Termination The chain reaction can be terminated by direct coupling or by the exchange of a hydrogen atom from one growing chain to another.
I1Mn* + I1Mm* I1Mm+n
Both molecules become deactivated by an exchange of energy.
(ii) Disproportionation Another means by which such energy exchange can be effected by means of the transfer of a hydrogen atom from one growing chain to another. H H H H H H H H
I1Mn C C* + I1Mm C C* I1Mn C = C + C C H
H R H R R H R
(d) Chain transfer Chain termination can result from chain transfer. But the process differs from the termination reactions in that the active site is transferred from an activated radical to an inactive molecule and a new nucleus for further growth is created.
H H H H H H H
I1Mn C C* + C = C I1Mn C = C + H C C*
H R H H R H H R
Inhibition of polymerization The polymerization reactions are not likely to result in a complete exhaustion of the monomer nor do they always form polymers of high molecular weight. Impurities in the monomer often inhibit such reactions. The impurities can react with free radicals it can react with activated initiator or any activated nucleus or an activated growing chain to prevent further growth. Eg: Hydroquinone (0.006 %). Presence of oxygen causes retardation of the polymerization.
Co-polymerization The macro molecules may be formed by the polymerization of single type structural unit. In order to improve the physical properties it is often advantageous to use two or more chemically different monomers as starting materials. The polymer thus contains units of all the monomers, such a polymer is called co-polymer. Co-polymers are of three different types (a) Random: The different monomers are randomly distributed along the chains - M – Y – M – Y – M – Y – M – M – Y – M – M – (b) Block: Monomer units occur in relatively long sequences along the main polymer chain M – M – M – M —…… M – M – Y – Y – Y – Y – Y -…….M - M………
(c) Graft: One of the monomers are grafted into a backbone of the second monomer M M M …. M M M ….
Y Y . . . .
Importance of Copolymerization Copolymerization is used to improve the physical properties of resins. Many useful resins are manufactured by copolymerization. Small amounts of ethyl acrylate may be copolymerized with methyl methacrylate to alter the flexibility. Block and graft polymers show improved impact strength. In small amounts they modify the adhesive properties of resins as well as their surface characteristics.
Cross linking In some cases, the liner polymers may be joined on bridged through certain reactive side chains to form molecular networks. It forms a three-dimensional network. Cross linking increases their resistance to solvents and strength, solubility, water sorption, surface stresses.
Plasticizers Plasticizers are often added to resins to reduce their softening temperature or fusion temperature. Plasticizers are added to dental resins to increase the solubility of the polymer in the monomer and to decrease the brittleness of polymer. There are two types of plasticizers (a) External plasticizer: it penetrates between macromolecules and neutralizes the secondary bonds or intermolecular forces. It is not so widely used as it may evaporate or leach out during normal use of the resin. (b) Internal plasticizer: eg. Butyl acrylate. Here, the plastisizing agent is part of the polymer. It is done co-polymerization with a suitable co-monomer.
Types of Resins
The various dental resins used in dentistry are (a) Vinyl resins The vinyl resins are derivatives of ethylene. Ethylene is the simplest molecule capable of addition polymerization. Two of the derivatives of Vinyl resins are: Poly Vinyl chloride, Poly Vinyl Acetate
(b) Polystyrene When a benzene radical is attached to the vinyl grouping we will get styrene. When it polymerizes we get polystyrene. It is a clear resin of the thermoplastic type.
(c) Epoxy resins Thermo setting. They posses reactive expoxy on oxirane group. Adhesion to metals, wood and glass, cross linkage is easily accomplished.
(d) Multifunctional acrylate resins The functional groups are acrylic. The resin is known by the acronym BISGMA and is an aromatic ester of a dimethacrylate. The matrices commercial materials are usually mixtures isomeric compounds. The most prevalent of which are Bis GMA and iso Bis GMA. Another resin used in density is urethane dimethacrylate. The functional reactive groups are again acrylic. Both monomers polymerize by the addition mechanism. Both these resins are highly cross linked because of the presence of more than one polymerizable double bond.
(e) Acrylic Resins The acrylic resins are derivatives of ethylene and contain a vinyl group in their structural formula. The acrylic resins used in dentistry are the esters of acrylic acid and methacryclic acid. The carboxyl group in the acrylic acids causes them to imbibe water. The water tends to separate the chains and causes a general softening and loss of strength.
Types of Acrylic Resins Based on the method used for its activation: 1. Heat activated resins 2. Chemically activated resins 3. Light activated resins
HEAT ACTIVATED ACRYLIC RESINS Composition
Principal Ingredients of Acrylic Resins: Powder and Liquid Powder Liquid Poly (methyl methacrylate) and other co-polymers (5%) Initiator (benzoyl peroxide) Pigments Dyes Opacifiers (Zn/Ti oxide) Plasticizer (dibutyl phthalate) Dyed organic fibers Inorganic particles (glass fibres or beads) Monomer (methyl methacrylate) Inhibitor (hydroquinone) Accelerator Plasticizer (dibutyl phthalate) Cross-linking agent (glycol dimethacrylate)
Acrylic resin is usually supplied in the form of powder and liquid or gel form (sheets and cakes). Commercial names: Stellon (DPI), Lucitone (Bayer), Trevelon (Dentsply).
Powder The powder consists of mainly poly (methyl methacrylate). Its solubility in monomer is very slow. So an additive usually in the form of a co-polymer of methyl methacrylate and ethyl acrylate added to the powder. The % of ethyl acrylate is limited to 5% or less.
An initiator in the form of benzoyl peroxide is added in of about 0.5-1.5%.
Dyes and pigments are usually added to the polymer to obtain tissue like shades e.g., are Mercuric sulfide, cadmium sulfide, cadmium selenide, ferric oxides or carbon black. Opacifiers such as zinc or titanium oxide are used so that they are visible on the radiographs. Barium acrylate and dibromo phenyl methacrylate can also be added. Plasticizers can be added to either the polymer or to the monomer. E.g., Dibutyl phthalate concentration should be limited to 8-10%.
Dyed synthetic fibres made from nylon or acrylic are added to the acrylic resin material to simulate minute blood vessels underlying the oral mucosa.
Sometimes inorganic particles are used to increase the coefficient of thermal expansion. E.g., Whiskers of alumina, silicon carbide, Borax nitride these particles are treated with some coupling agents like vinyl diethoxy silane to improve the wetting properties.
Liquid The liquid is supplied in tightly sealed amber colored bottles to prevent premature polymerization by light or ultraviolet radiation on storage. The liquid is mainly methyl methacrylate. The inhibitor used is usually Hydroquinone which aids in the inhibition of polymerization during storage. (0.006%)
Cross linking agent used is glycol dimethacrylate (1-2%) to achieve greater resistance to minute surface cracking or crazing. This has little effect on tensile strength or hardness of acrylic plastics.
Monomer polymer ratio The monomer polymer ratio is of considerable importance to the final structure of the resin. In general the more polymer used the shorter the action time and the shrinkage will be less. But the monomer should be sufficient enough to wet all polymer well.
The usual ratio of polymer to monomer is generally 3 to 1 by volume or 2 to 1 by weight when the powder is fluffed rather than packed down. Failure to blend properly the powder and liquid can result in low strength, porosities and poor colour in the denture.
Monomer polymer interaction Polymerization is achieved by application of heat and pressure. The simplified reaction is Powder Liquid (polymer) (monomer) + + + + Heat -► Polymer + Heat (initiator) (inhibitor) (external) (reaction) The function of the monomer in the polymer is to produce a plastic mass. Four stages are identified during the physical interaction of the powder and the liquid.
Stage 1 (wet sandy stage): The polymer gradually settles into the monomer to form a somewhat fluid incoherent mass.
Stage 2 (sticky stage): The monomer attacks the polymer surface. The layer of polymer penetrated sloughs off and eihter goes into solution or is dispersed in the monomer. This stage is characterized by a stringiness and adhesiveness if the mixture is touched or pulled apart.
Stage 3 (dough or gel stage): As the monomer diffuses into the polymer the mass becomes smooth and dough like. It is no longer tacky and does not adhere to the walls of the mixing jar. It consists of undissolved polymer particles suspended in a plastic matrix of monomer and dissolved polymer. During this stage it is packed into the mold.
Stage 4 (rubbery stage): The monomer disappears by evaporation or by farther penetration into the polymer. The mass becomes more cohesive and rubber like. It is no longer completely plastic and it cannot be molded by dental techniques.
Dough forming time Time required to reach stage 3 and depends upon the solubility of the polymer pearls in this monomer. ADA No. 12 it should be less then 40 min. Most resins reach within 10 min. Working time Is the time elapsing between stage 2 and the beginning of stage 4, that is the time the material remains in the dough form. According to the ADA Sp No. 12 the dough should be moldable for at least 5 minutes. The lower the temperature the longer the working time.
Compression molding technique This is the most commonly used technique in the fabrication of acrylic resin dentures. Preparation of the mold The pattern is invested in a dental flask with dental stone or plaster (flasking). After stone or plaster sets, it is dewaxed by placing the flask in boiling water for not more than 5 minutes. After dewaxing the two halves of the flask are separated and the molten wax is flushed out with clean hot water.
Separating media During processing the resin should be carefully protected from the gypsum surfaces surrounding the mold space for two reasons.
(a) Any water incorporated into the resin from the gypsum during processing will definitely affect the polymerization rate and the colour of the resin.
(b) Dissolved polymer and free monomer must be prevented from soaking into the cast surface. If the liquid resin penetrates into the cast the gypsum material will be joined to the denture after polymerization.
One of the most widely used material was tin foil. Other separating media commonly used are cellulose lacquers and solutions of alginate compounds, soap, sodium silicate and starches. The most popular separating agents are water soluble alginates that produce a very thin water and organic solvent insoluble calcium alginate film on the gypsum surface. (1) The film applied should be uniform and continuous throughout the cast. (2) Waxes or oils remaining on the cast surface will not permit a coating in the area. (3) Excessive thick layers of film will create discrepancies in reproduction of detail. Packing Introduction of acrylic resin into the mold cavity is termed as packing. The packing process should be performed while the resin is in dough like state. The resin is removed from its mixing container and rolled into a rope like form and placed into the mold cavity. A polyethylene sheet is placed over the resin and the flask is reassembled. Pressure is applied incrementally. Excess resin is found on the relatively flat areas that surrounding the mold cavity. This excess resin is called flash. The flash is carefully teased away using a round instrument. The mold sections are properly oriented and placed in the flask. Again pressure is applied incrementally. Curing After final closure, the flasks are kept at room temperature for 30 to 60 minutes (bench curing). Purpose of bench curing are (1) permits an equalization of pressure throughout the mold (2) it allows time for a more uniform dispersion of monomer throughout the mass of dough, since the last material added is usually drier than the first added to the flask.. (3) if resin teeth are used, it provides a longer exposure of resin teeth to the monomer in the dough, producing a better bond of the teeth with the base material.
Polymerization (curing) cycle The polymerization or curing cycle is the technical name for the heating process employed to control the initial propagation of polymerization. In one curing cycle the flask is immersed in H2O at 65°C and allowed to remain for 90 min to polymerize the thick areas of the resin without causing porosity. It is then boiled for 60 min to cure the thinner areas.
Another curing cycle which is long low temperature one in which the resin is processes for 9 hours at 74°C with no terminal boil
Polymerization The dental acrylic resins usually contain benzoyl peroxide. When the temperature of the dough increases above 60°C the molecules of benzoyl peroxide decomposes to form free radicals which in turn becomes attached to another monomer mol.
The principal factor that governs the rate of polymerization is the rate at which free radicals of the benzoyl peroxide are released. In heat activated acrylic resin it is determined largely by the temperature.
Generally the lower the temperature the greater the molecular wt. of the polymer.
Temperature rise The polymerization reaction is exothermic. When the temperature of the H2O and the plaster is increased from 0 – 100°C, the temperature of the acrylic resin increased at the same rate until about 70°C after which the temperature of the resin began to rise rapidly. At this temperature a sufficient number of benzyl peroxide, were activated to produce the chain reaction. Cooling The flask should be cooled slowly from the final water bath temperature (bench cooling). If the flask is placed directly into tap water warpage of the resin may occur. Cooling overnight is ideal. Removing the flask from the water bath bench cooling it for 30 min. and placing it in cold tap water for 15 min is satisfactory. Deflasking has to be done carefully to avoid flexing and breaking of the acrylic denture. The denture is smoothened with sand paper. Finely ground pumice in water is commonly used for polishing.
INJECTION MOLDING TECHNIQUE It requires special equipment. The mold space is filled by injecting the resin under pressure. A sprue hole and a vent hole are formed in the gypsum mold. The soft resin is contained in the injector and is forced into the mold space as needed. It is kept under pressure until it has hardened. No trial closure is required with this technique. There is no difference in accuracy or physical properties as compared to compression molding technique. Advantages of this technique are good dimensional accuracy, low free monomer content and good impact strength. The disadvantages are high cost of the equipment, difficult mold design problems, less craze resistance, less creep resistance and requirement of special flask.
Other sources of heat (1) Steam (2) Dry air oven (3) Dry heat (electrical) (4) Infrared heating (5) Induction or dielectric heating (6) Microwave radiation Microwave energy can be used for the polymerization of acrylic resin. Microwaves are used to generate heat inside the resin by a generator called magnetron.
Advantage: Cleaner Faster than heat cure technique Less prone to porosity Initiator used is benzoyl peroxide
CHEMICALLY CURED ACRYLIC RESINS Instead of using heat to activate the benzoyl peroxide, a chemical activator can be employed so that polymerization can be completed at room temperature. A small amount of NN dimethyl p-toluidine is added to the monomer before the mixing. After mixing free radicals are formed from the benzoyl peroxide by a reaction with NN dimethyl p-toludine. These resins were first used in dentistry during World War II and are variously known as self curing, cold curing or auto polymerizing resins. The degree of polymerization is less than heat activated. The concentration of P-toludine is approx 0.75 %. The max concentration of the peroxide is 2 %. The smaller the particle size of the polymer the more rapid is the polymerization. The colour stability is less than the heat cure type because of oxidation of the tertiary amine. It can be minimized by adding stabilizing agents. Better initial fit, which is because the curing is carried out at room temperature, and thus there is less thermal contraction. It possesses inferior properties because degree of polymerization of self curing resins is less than that of heat cured ones. For repairing dentures, self curing resins are preferred to heat cured resins as heat curing causes warpage.
Mixing techniques for self cure resins
(a) Salt and pepper method (b) Knead – on method (c) Compression molding technique (d) Injection molding technique (e) Fluid resin technique
Salt and pepper method (sprinkle on technique) After application of separating media the polymer is uniformly sprinkled over the cast. Then the monomer is introduced form a dropper so that no excess monomer is splashed over the powder. This process is continued until the entire powder reacts with the liquid to form the acrylic resin with the metallic components incorporated in the resin mass for retention. Soon after acrylization the acrylic resin is immersed in a bowl of water to prevent monomer evaporation and thus preventing granular porosity.
Knead - on Method The powder and the liquid are mixed and when dough stage is reached the mix is molded on to the surface of the cast and shaped accordingly. Disadvantages: (1) Fit of the base to the cast is not as good as that obtained with salt and pepper method. (2) The finger of the operator should be protected as it comes into more contact with the monomer.
Fluid resin technique (pour-type acrylic resins)
The chemical composition of this type of resins is the same as that of polymethyl methacrylate resins. The principal difference is that the pour-type of denture resins have high molecular weight powder particles that are much smaller and when they are mixed with monomer, the resulting mix is very fluid. They are used with significantly lower powder-liquid ratio, i.e. it ranges from 2:1 to 2.5:1. Agar hydrocolloid is used for the mold preparation in place of usual gypsum. The fluid mix is quickly poured into the mold and allowed to polymerize under pressure at 0.14 M Pa. The advantages include better tissue fit, fewer open bites, less fracture of porcelain teeth during deflasking, reduced material cost and simple laboratory procedure. The disadvantages are air inclusion (bubbles), shifting of teeth during processing, closed bites (infraocclusion), occlusal imbalance due to shifting of teeth, incomplete flow of the material over neck of anterior teeth, formation of films of denture material over cervical portions of plastic teeth that had not been previously covered with wax, poor bonding to plastic teeth and technique sensitivity.
Some acrylic resins instead of the amine peroxide system utilize the sulfonate on sulfinic acid system for polymerization. The colour stability has improved over the amine peroxide system. The sulfinic acid and its salts are unstable in the presence of moisture.
LIGHT ACTIVATED DENTURE BASE RESINS Light activated resins were introduced into Dentistry by Douglas et al. The first light activated system utilized u.v. light to initiate free radicals. Subsequently visible light activating systems were developed with a greatly improved ability to polymerize thicker increments. They have totally displaced the u.v. light systems. It consists of a urethane dimethacrylate matrix with an acrylic copolymer, microfine silica fillers, and a camphoroquinone- amine photo-initiator system. The amine accelerator used is DEAEMA (Diethyl-amino-ethyl methacrylate) at a concentration of about 0.15% or less. It is supplied in premixed sheets having clay like consistency. It is provided in opaque light tight packages to avoid premature polymerization. The denture base material is adapted to the cast while it is in a plastic state. The denture base can be polymerized without teeth and used as a base plate. The teeth are added to the base with additional material and the anatomy is sculptured while the material is still soft. It is polymerized in a light chamber (curing unit) with blue light of 400 – 500 nm from high intensity quartz halogen bulbs. The denture is rotated continuously in chamber to provide uniform exposure to the light source.
PHYSICAL PROPERTIES OF ACRYLIC RESINS
(a) Methyl Methacrylate Methyl methacrylate is a clear transparent liquid at room temperature with the following physical properties. Melting point – 48°C Boiling point – 100.8°C Density – 0.945 gm / ml at 20°C Heat of polymerization – 12.9 cal/mol
The degree of polymerization of methyl methacrylate depends upon
(b) Temperature (c) Method of activation (d) Type of initiator used (e) Its concentration (f) Purity of chemicals
During polymerization a volume shrinkage of 21% occurs. (b) Polymethyl methacrylate Transparent resin hard with a knoop hardness no. of about 18-20. Its tensile strength is approximately 59 Mpa and its specific gravity 0.19, modulus of elasticity is approx. 2400 Mpa. Heat stable, softens at 125°C. At about 200°C depolymerization takes place. It is thermoplastic, takes up water by a process called imbibition, shows water sorption (both adsorption and absorption), soluble in organic solvents such as acetone and chloroform. Although it is a thermoplastic resin, it is not usually molded by thermoplastic means in dentistry. Rather the liquid monomer is mixed with the polymer which is in the powdered form. The monomer partially dissolves the polymer to form a dough. The monomer is polymerized by either heat, chemicals or light.
(c) Strength The strength of the acrylic resin denture base materials may fluctuate considerably depending on the composition of the resin, the technique of processing and the subsequent environment of the denture.
The lower the degree of polymerization of a given solid polymer the lower will be its strength. The stress properties of the resin are generally measured by means of a transverse test described by the ADA sp no. 12 Transverse strength is a combination of tensile strength and compressive strength. The transverse strength varies from 78 to 92 MPa. Owing to the lower degree of polymerization, the maximal strength and stiffness of the self cure resins are lower than that of the heat cured type. The mean flexural modulus for heat cured resin is 2500 MPa and that for self cured resin is about 2200 MPa.
The modulus of elasticity, the proportional limit and tensile strength of heat cure acrylic resin is about 2350 Mpa, 27 Mpa, Tensile strength 52 Mpa, respectively. The properties of the resin are reduced by the heat generated from polishing with abrasive and polishing agents. The excess heat generated may cause partial depolymerization with resulting decrease in strength and rigidity.
The bulkier portion of the denture may show grater strength than the thinner portion because of more degree of polymerization.
Strength Characteristics of Acrylic Resin Plastics
Property Poly(methyl methacrylates) Tensile strength (MPa) Compressive strength (MPa) Elongation (%) Elastic modulus (GPa) Proportional limit (MPa) Impact strength, Izod (kg m/cm notch) Transverse deflection (mm) At 3500 g At 5000 g Fatigue strength (cycles at 17.2 MPa) Recovery after indentation (%) Dry Wet KHN (kg/mm2) Dry Wet 48.3 - 62.1 75.9 1-2 3.8 26.2 0.011
2.0 4.0 1.5 x 106
(d) Impact strength It is a measure of the energy absorbed by a material when it is broken by sudden blow. The plasticizing ingredients may increase the impact strength of acrylic resin but they decrease the hardness, proportional limit, elastic modulus and compressive strength.
The charpy impact strength of heat cured acrylic resin is 0.98-1.27 joules and that of self cured resin is 0.78 joules. The Hounsfield impact strength of heat cured acrylic is 455 Nmx10-4
(e) Hardness The knoop hardness no for self cure resin is approximately 16-18 where as that for a resin cured under heat may be as high as 20. The low hardness no. of acrylic resin indicates that these materials may be scratched easily and abraded.
(f)Modulus of elasticity They have sufficient stiffness (2400 M Pa) for use in complete and partial dentures. Self cured acrylic resins have slightly lower values.
Mechanical properties of acrylic resin (a comparison with certain alloys) Modulus of elasticity (GPa) Tensile strength (MPa) Hardness (VHN) Acrylic resin Co-Cr Stainless steel 2.5 220 220 85 850 1000 20 420 400
(g) Abrasion Resistance This has been evaluated by abrading specimens against 600 grit silicon carbide paper for 1 hour under a stress of 0.76 MPa in water at 37°c and measuring loss of material. For Heat cure it is 530 mmx10-3 and self cure it is 611mmx 10-3.
(h) Density: The acrylic resin has densities ranging from 1.16 to 1.36g/cc.
(i) Polymerization shrinkage When methyl methacrylate monomer is polymerized the density changes from 0.94 gm/cm2 to 1.19 gm/cm3. This change in density results in a volumetric shrinkage of 21% usually called polymerization shrinkage. But this shrinkage is distributed uniformly throughout the surfaces so that the fit of the denture to the tissues is not seriously affected.
Another shrinkage called linear shrinkage varies from 0.2% to 0.69% for acrylic resins heat cure. The processing shrinkage has been measured as 0.53% for heat cured acrylic resin as compared with only 0.26% for a self cured resin.
The fit of the acrylic resin produced from heat cure resin is lower than that of self cure resin.
(j) Porosity There are a number of causes of porosity that can occur during the processing of the acrylic resin. If the porosity appears on the surface of the resin cleansing will be difficult. If the porosity is internal the cured resin will be weakened. More over such area of internal pore or bleb is an area of stress concentration; the resin may warp as the stresses relax. (a) Internal porosity of the resin occurs as a result of the vaporization of the monomer or of the low molecular weight polymers when the temperature of the resin increases above the boiling point of the monomer (100.8º C). It is confined to the thick portions of the denture base and it may not occur uniformly. It can be avoided by curing dentures with excessive thickness using long , low temperature curing cycle. (b) Second type of porosity (external porosity) is due a lack of homogeneity in the dough at the time of the polymerization. It is seen that some regions will contain more monomer than other and these regions will shrink more during polymerization than the adjacent regions and such a localized shrinkage will tend to produce voids. It is avoided by using (a) Proper P:L ratio (b) Favorable mixing procedures (c) Pack during the dough stage
External porosity can also occur due to lack of adequate pressure during polymerization or by a definite lack of dough or gel at the time of final closure. It is avoided by using the required amount of dough.
(k) Water absorption Poly methyl methacrylate absorbs water slowly over a period of time. The absorption is undoubtedly due to the polar properties of the resin molecules. The diffusion coefficient of a typical heat cured denture acrylic resin is 1.08 to 10-12 m2/sec of 35°C where the temperature drops to 23°c it is reduced by one half. For self curing resin D is 2.34 x 10-12 m2/sec.
The diffusion presumably occurs between the macro molecules which are forced slightly apart. For each 1% increase in the weight due to the water absorbed the acrylic resin expands linearly 0.23%.
The water sorption of acrylic can be measured by an increase in the weight of the resin per unit of surface area exposed to the water. According to ADA sp. No. 12 a disc of material with specified dimensions is prepared the disk is first dried to constant weight and then it is stored in water for seven days. According to the specification the gain in weight by the resin during this treatment must not be greater than 0.8mg/cm2. (l) Solubility The acrylic resins are soluble in many solvents but they are virtually insoluble in most fluids with which they will come in contact in the oral cavity. They are soluble in ketones, esters, and aromatic and chlorinated hydrocarbons, e.g. chloroform and acetone. Alcohol cause crazing in some resins.
(m) Creep Acrylic resins are viscoelastic when they are subjected to a constant load so that strain can be observed as a function of time, they show primary and secondary creep. Creep rate increases with increase in temperature, stress, residual monomer, plasticizers and cross linking agents. The creep rate for self cure resins is more than heat cure resins.
(n) Colour stability The colour stability is usually tested by exposure to u.v. light. The colour stability of heat cure acrylic resin is more when compared to self cure acrylic resin. According ADA sp. No. 12 when a specimen is exposed for 24 hours to an u.v. light source shall not show more than a slight change in colour when compared with an original specimen.
(o) Dimensional stability and accuracy The dimensional stability of the resin during processing and in service is important in the fit of the denture and the satisfaction of the patient. If the denture is properly processed the original fit and dimensional stability of the various denture base plastics is good. However excess heat generated during finishing can easily distort a denture base by releasing residual stresses.
(p) Processing stresses Whenever a natural dimensional change is inhibited the structure involved will be stressed with the result that a distortion or warpage may occur if such stresses are relaxed. During polymerization shrinkage tensile stresses are actually induced in acrylic resin. The total dimensional change that occurs in a typical resin denture during processing and in service is in the range of only 0.1 to 0.2 mm. (q) Crazing After processing relaxation of surface stresses may result in the formation of cracks or crazing. Crazing of the resin actually consists of small cracks that may vary in size from microscopic dimensions to a size that is readily visible. Crazing may occur under mechanical stress or as a result of an attack by a solvent. Crazing occurs only when a tensile stress is present. The cracks appear at right angles to the direction of the tensile stress.
The modern concept is that crazing is an actual mechanical separation of the polymer chains or groups of chains under tensile stress. Crazing can be avoided by using cross linked acrylics, tin foil separating medium and metal molds. (r) Stability to heat polymethyl methacrylate is chemically stable to heat. It softens at 125º C. However above this temperature it begins to depolymerize. At 450º C, 90 % of the polymer will depolymerize to monomer.
(s) Thermal conductivity Acrylic resins are poor thermal and electrical conductors. The coefficient of thermal conductivity for acrylic resin is 5.7x10-4 compared to 1.3x10-3 for dentin. Low thermal conductivity results in plastic resin bases serving as an insulator between the oral tissues and hot or cold materials placed in the mouth.
(t) Coefficient of thermal expansion They have high co-efficient of thermal expansion of the range of 81x10-6 when compared to tooth having only 11.4 x 10-6. Thermal expansion is important for the fit of the acrylic bases because an acrylic base that fits a cast accurately of room temperature will not fit the same at mouth temperature.
Thermal Characteristics of Acrylic Resin Plastics Property Poly (methyl methacrylates) Thermal conductivity (cal/sec/cm2) (º C/cm) Specific heat (cal/º C/g) Thermal coefficient of expansion (/º C) Heat distortion temperature (º C) 5.7 x 10-4 0.35 81 x 10-6 71-91 (u) Toxicology There is no indication that these dental resins could produce any systemic effects upon the patient. The quantity of methyl methacrylate monomer that might enter the circulation by passing through the oral mucosa would be extremely low. The half life of methyl methacrylate in blood at 37°c is said to range between 20-40 minutes. Clearance being by hydrolyses to methacryclic acid.
(v) Tissue compatibility and allergic reactions True allergic reactions to acrylic resins are rarely seen in the oral cavity. Completely polymerized acrylic resins are biocompatible. Chemical irritation can occur either from the polymer, the residual monomer, the benzoyl peroxide, the Hydroquinone, the pigment or some reaction product. One such product is formaldehyde. Self cure resins release more formaldehyde than heat cure acrylics. Increasing polymer monomer ratio will reduce formaldehyde release.
The residual monomer approximately 0.4% in a well-processed denture is the usual component singled out as an irritant. If residual monomer were the cause, its effect is expected to appear rapidly. However, most of the cases reported occur after months or years. Careful clinical evaluation revealed that either unhygienic conditions under the denture or ill fitting denture that is traumatizing the tissue are the causative factors. A true allergy to acrylic resin can be recognized by a patch rest.
Direct contact of the monomer over a period of time can cause dermatitis. The high concentration of monomer in the dough may produce a local irritation and serious sensitization of the fingers. Inhalation of the monomer vapor can produce toxic reaction. So, the use of the monomer should be restricted to well ventilated areas. The plasticizer phthalates can also produce dermatitis.
Growth of Candida albicans on the surfaces of acrylic resin is a concern for patients. This can be avoided by treating with nystatin or chlorhexidine gluconate. In addition to Candida other microorganisms such as streptococcus oralis, Bacteroides gingivalis, Bacteroides intermedius and streptococcus sanguis also adhere to acrylic resin bases especially on rough surfaces.
BARDAY S.C and FORSYTH. A published a case report on British Dental Journal 1999 October in which they have seen that the colouring agents added to acrylic resin can cause hypersensitivity reactions. (w) Residual monomer The highest residual monomer level is observed with chemically cured resins at 1 to 4 % shortly after processing. When they are processed in less than one hour in boiling water the residual monomer is 1 to 3%. If it is processed for 7 hours at 70º C and then boiled for 3 hours the residual monomer content may be less than 0.4%. In heat cured acrylic, before the start of curing the residual monomer is 26.2%. In one hour at 70ºC it decreased to 6.6% and at 100ºC, it was 0.29%. To reduce the residual monomer in heat cured resins it should be processed for a longer time in boiling water. The temperature should be raised to boiling after most of the polymerization is completed otherwise porosity may result. Infection control Care should be taken to prevent cross contamination between patients and dental personnel. New appliances should be disinfected after construction. Appliances can be sprayed with disinfectants before they leave the operatory. Since the polymeric materials can absorb liquids, toxic agents such as phenolics and glutaraldehyde should not be used. Ethylene oxide gas is a suitable method for sterilization.
SHENEE and JAVID (Journal of Prosthetic Dentistry, June 1989) conducted a study to find out the effect of disinfectants on acrylic resins. They concluded that Glutaraldehyde does not affect the flexural strength and surface morphology of acrylic resins. However, if acrylic resins were immersed in phenolic compounds for more than 12 hours pitting on the surface of acrylic resin may occur. Another study conducted by Watkinson A.C (J. Prost. Dent. 1992 July) shows that transverse strength of acrylic resin is affected by alcohol based disinfectants. CARE OF ACRYLIC DENTURES Dentures should be stored in water when not in use, since dimensional changes occur on drying. Abrasive dentrifices should not be used, since plastic is soft and can be easily scratched and worn away. The tissue surface should be brushed carefully with a soft brush, since any material removed alter the fit of the denture. Acrylic dentures should not be cleaned in hot water, since processing stresses can be released and can result in distortion. DISPOSAL The best method of disposal is incineration. Disposal in landfills must be thoroughly checked with the authorities and should be practiced only as a last resort.
Acrylic Resin Cleansers A wide variety of agents are used by patients for cleaning artificial dentures. The most common commercial denture cleansers used are the immersion type which includes, alkaline compounds, detergents, flavoring agents, and sodium per borate. When the powder is dissolved in water, the perborate decompose to form an alkaline peroxide solution, which in turn decomposed to liberate oxygen. The oxygen bubbles then act mechanically to loosen the debris. Sodium hypochlorite can effectively remove certain type of stains. But they are not preferred to use with metals. Tooth brush has very little effect on the surface of the resin. Salt, soap, soda and most common dentifrices can also be used. But their prolonged use may affect the fit and makes the surface rough which is difficult to maintain clean.
Properties of Acrylic Resin Plastics
Property Poly(methyl methacrylates) Density (g/cc) Polymerization shrinkage (% by volume) Dimensional stability Water sorption (mg/cm2; ADA Test) Water solubility (mg/cm2) Resistance to weak acids Resistance to weak bases Effect of organic solvents
Processing ease Adhesion to metal and porcelain Adhesion to acrylics Colorability Color stability Taste or odor Tissue compatibility Shelf life 1.16-1.18 6
Good 0.69 0.02 Good Good Soluble in ketones, esters, and aromatic and chlorinated hydrocarbons Good Poor Good Good Yellows very slightly None Good Powder and liquid, good; gel, fair
USES IN ORTHODONTICS Chemically activated acrylic resins are mainly used in orthodontics. The accuracy of fit and convenience of molding without the necessity of flasking make this material is useful to the specialty of orthodontics.
Some of the uses of acrylic resins in orthodontics are
(a) For the construction of removable and functional appliances. (b) Used for making impression trays. (c) Used for the fabrication of occlusal splints and as temporary space maintainers. (d) For the fabrication of extraoral chin caps. (e) Bonding materials (f) In Stereolithography
Acrylics for bonding Acrylic resins were the first material to be used as orthodontic adhesive. The acrylic resins used for bonding can be classified as: (a) Acrylic based systems – PMMA systems (b) Diacrylate systems – BISGMA systems
The Acrylic resins are (e.g., Orthomite, Genie) are based on self curing acrylics and consists of methyl methacrylate monomer and ultra fine powder. This was introduced into orthodontics by Miura et al in 1970 when he described an acrylic resin orthomite using a modified trialkyl borome catalyst that proved to be particularly successful for bonding plastic brackets and for enhanced adhesion in the presence of moisture. Acrylic resins possess good flow and wettability, but lack sufficient bond strength. High degree of polymerization shrinkage and great difference in linear coefficient of thermal expansion between tooth and resin further affect bond strength.
Most diacrylate resins are based on the acrylic modified BISGMA or Bowen’s resin. A fundamental difference is that the resins of the acrylic system form linear polymers only where as those of the diacrylate system can be polymerized by cross linking into a 3-D network.
The diacrylate resins have the best physical properties and are the strongest adhesives for metal brackets. Acrylic or combination resins have been most successful with plastic brackets. Buzzitta et al found that a highly filled diacrylate resins with large filler particles gave the highest values of in vitro body strength for metal brackets. The clinical implication is that adhesives with large particle fillers are recommended for extra bond strength but careful removal of the excess is mandatory because such adhesives accumulate plaque more easily. Diacrylate resins do not bond with plastic brackets. Either an acrylic monomer as a primer to enhance bonding between the diacrylate resin and the polycarbonate bracket is necessary or an acrylic resin adhesive must be used.
Other alternatives to chemically autopolymerizing paste-paste systems.
No-mix adhesives: These materials set (e.g., Rely a bond) when paste under light pressure is brought together with a primer fluid on the itched enamel or bracket backing or when another paste on the tooth is to be bonded. Curing occurs within 30 - 60 seconds.
Visible light polymerized adhesives These materials (e.g., Transbond) may be cured by transmitting light through tooth structure and ceramic brackets. Light activated resins are now preferred in some cases because of the ease of processing and elimination of methyl methacrylate monomers.
This technology uses a laser to cross link specific areas as a pool of acrylic is being filled. Polymerization occurs only in the areas the laser activates. This technology has been used to produce copies of dental models and models of craniofacial complex to visualize and plan difficult surgeries.
Newer advances in acrylic Resin
(1) High impact strength materials
These materials are butadiene-styrene rubber – reinforced poly methyl methacrylate. The rubber particles are grafted to methyl methacrylate so that they will bond to the heat polymerized acrylic matrix. These materials are supplied in a powder – liquid form and are processed in the same way as other heat-accelerated methyl methacrylate materials. Indicated for patients who drop their dentures repeatedly, e.g. senility, parkinsonism.
(2) Vacuum adapted plates
A quick and easy method of making a usable base plate is to vaccum mold a sheet of thermoplastic resin (eg: acrylic, polycarbonate, polyvinyl acetate thickness control and good adaption to the cast particularly when using thinner sheets. More intimate adaptation is obtained compared to manual adaptation.
(3) Rapid heat polymerized Resin
These are hybrid acrylics that are polymerized in boiling water immediately after being packed into the flask. After being placed in boiling water, the water is brought back to full boil for 20 min. (reverse cure). After the usual bench cooling to room temperature the denture is deflasked. The initiation is formulated from both chemical and heat activated initiators to allow rapid polymerization without porosity.
(4) Carbon Fiber and polyaramid reinforcement of acrylic resins
Early experiments with glass fiber have resulted in failure because of the irritant nature of the fibers. Carbon fibers have no such irritancy and greatly increased impact strength and flexural stiffness of the denture base. But the black colour of carbon fiber and the potential toxicity at coupling agents silane 174 prevent their use for reinforcement of acrylic resins. Their use is restricted only to lingual aspects of denture. Eg: Polyethylene, Graphite
Polyaramid or Kevlar fibres (poly p Phenylene terephthalmide) have stiffness of 90 GPa, are straw coloured and greatly enhance the mechanical properties of the denture. They do not require treatment with coupling agent. Their soft yellow colour is masked by the pink resin.
Acrylic resins were introduced into dentistry about 70 years back. After its introduction into dentistry, it has revolutionized the way we are practicing Dentistry. So many new materials were introduced into Dental profession after that like epoxy resin, poly styrene and poly urethane etc. to name a few. It has some disadvantages in the form of water absorption, low abrasion resistance low colour stability etc. But, the advantages definitely weigh over disadvantages. It remains as one of the main stay materials used in dentistry for making bases, removable appliances, obturators crown and bridge material etc.
1. Anusavice KJ: Phillips Science of Dental Materials 10th edn., Saunders, 1996 2. Combe EC: Notes on Dental Materials 6th edn., Churchill Livingstone, 1992 3. Craig RG: Restorative Dental Materials 9th edn., Mosby 1993 4. Craig, Obrien, Powers: Dental Materials, Properties and Manipulation 5th edn., Mosby 1992 5. McCabe JF: Andersons Applied Dental Materials 6th edn., Blackwell 1988 6. Miles DA (ed.): Applications of Digital Imaging Modalities for Dentistry, Dental Clinics of North America, April 2000 7. Graber TM, Vanarsdall RL: Orthodontics, Current Principles and Techniques 3rd edn., Mosby 2000 8. Pogonoion: Post Graduate Student Convention Manual 9. Shenee, Javid: Effect of Disinfectants on Acrylic Resins, Journal of Prosthetic Dentistry, June 1989 10. Watkinson AC: Effect of Alcoholic Disinfectants on Acrylic Resins, Journal of Prosthetic Dentistry, June 1992 11. Herserk N, Tincer: Strength Characteristics of Fibre Reinforced Acrylic Resins, Journal of Prosthetic Dentistry, 1999
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