Сraig. Dental Materials

75%. Systems used to cement restorations often use both photoand chemical-initiation (dual curing) because it is often difficult to expose regions of the material to sufficient light to reach the maximum degree of conversion and thus maximum strength. With these dual-curing materials, maximum degrees of conversion of 80% have been reported.

Ring-Opening Polymerization Two important ring-opening polymerizations in dentistry are the epoxy and ethylene imine reactions. The former is used to produce dies from rubber impressions, and the latter is used in the setting reaction of polyether rubber impression materials.

The reactants for the epoxy system are a difunctional epoxide oligomer and a difunctional amine, as shown in the following simplified equation:

The amine opens the ring and crosslinking results in a rigid polymer. Water interferes with the setting reaction because it reacts with the epoxide. Therefore agar and alginate impressions are incompatible with this die material.

The polyether oligomer has a threemembered ring containing nitrogen shown as R in formula VI above. The ring is opened by the sulfonium fluoborate catalyst, and polymerization results in a cross-linked rubber. The oligomer and setting reaction are further discussed in Chapter 12, Impression Materials.

Hydrosilylation One final example of an addition reaction is with a vinyl-terminated silicone and a silane (-H)-containing siloxane, as shown above. In this instance the platinum catalyst attacks the hydrogen in the silanecontaining dimethyl siloxane, and this complex reacts with the vinyl-terminated dimethyl siloxane to form a cross-linked silicone rubber. Compounds used in the vulcanization of latex surgical gloves interfere in the polymerization of addition silicones, and thus contact should be avoided.

0CH2CH3

0CH2CH3

byproducts. Polysulfide rubbers are formed by a condensation reaction, the most general reaction being between low-molecular-weight polysulfide polymers having mercaptan (-SH) groups and lead dioxide, as shown above by the simplified reactions.

Water and lead sulfide are byproducts of the reaction. Mercaptan groups are also along the chain and thus crosslinking occurs. The rate of the reaction is proportional to -SH, PbO,, and H,O.

Condensation polymerization of the mercaptan groups can also be accomplished by use of a Cu(OH),, as shown by the following simplified reaction:

+ CH3CH20H,etc. +Cross-linked

rubber polymer

Metal esters used have been stannous octoate and dibutyl tin dilaurate. The ortho-ethyl silicate is used as a crosslinking agent and is more stable if not combined with the metal ester. Ethyl alcohol is the byproduct, and its evaporation from the set rubber accounts for a significant portion of the shrinkage of condensation silicones after setting. Organosilicon compounds with only two ethoxy groups can be substituted for the ortho-ethyl silicate, thus reducing the byproduct and shrinkage on setting.

Polymer acids are used successfully in dentistry to react with hydrated metal ions such as znf2, ~ a ' ~or, ~ l +A~copolymer. of acrylic acid and itaconic acid in water is reacted with zinc oxide in an acid-base reaction to form a cement called zinc polyacylate, as outlined:

The polymerization with Cu(OH), avoids the dark color of PbO, and the staining of fabric by the Pb0,-containing polysulfide impression materials.

Silicones may be polymerized by a condensation reaction if they contain terminal hydroxy groups, as shown by the reaction above.

194 Chapter 7 POLYMERS AND POLYMERIZATION

or simplified:

The copolymer acid can also be freeze-dried and included with the ZnO powder, and then only water is mixed with the powder.

A similar reaction with a copolymer of acrylic and itaconic acid in water solution is used with an aluminosilicate glass to form a glass ionomer. The copolymer is used rather than polyacrylic acid because the presence of itaconic acid prevents thickening of the water solution observed when only polyacrylic acid is used. Tartaric acid is also present in the formulation to increase the strength of the set ionomer by crosslinking from the difunctional acid groups.

I R

HO-C-C-OH

I

H

Tartaric acid

A stronger diacid, maleic acid, has been used to increase the rate of reaction, resulting in a higher early strength and allowing finishing at the placement appointment.

H-C-C-OHP

II P

H-C-C-OH

Maleic acid

The copolymer acid reacts first with caC2and then ~ l dissolved+ ~ out of the glass by an ionic reaction to form metal esters. The material is called an ionomer and is used as both a restorative and a cement.

Materials called compomers use a combination of the ionomer reaction and free-radical polymerization. The polyacrylic acid molecules containing pendant methacrylate groups are dissolved in water along with 2-hydroxyethyl- methacrylate and tartaric acid. The powder contains a glass and microencapsulated potassium persulfate and ascorbic acid catalyst. On mixing the powder and liquid, an acid-base ionomer reaction accompanied by a free-radical methacrylate reaction occurs.

Some compomers are made by reacting a polyfunctional organic acid with hydroxyethyl-

methacrylate to form a compound that will undergo both a free-radical and an acid-base reaction (see bottom of p. 194).

As a result, compomers of the resin-modified ionomer type and ionomer-modified composites type are available. Also, some products are polymerized by chemical and light initiation and by an acid-base reaction; these products are called triple-cured materials. The greater the number polyacid groups, the more the material is like a glass ionomer; the fewer the number of polyacid groups, the more it is like a resin composite.

OTHER POLYMERS

A variety of polymers are used in fully polymerized form without any polymerization reaction carried out by the dentist or laboratory technician. Polyisoprene is available in two forms, cis and trans, and both are natural rubbers. These structures are shown as follows:

Notice that for the cis type the CH, and H are on the same (cis) side, whereas for the trans type they are on opposite (trans) sides. cisPolyisoprene is cross-linked by the process of vulcanization with sulfur or other chemicals such as peroxides. In the vulcanized form it is very flexible and is used in surgical gloves, rubber dams for restorative procedures, and elastics for orthodontic applications. trans-Polyisoprene is rigid; it is compounded mainly with zinc oxide (but also with some waxes and zinc silicate) and is used for endodontic points as part of the filling material for root canals. For this application it is called gutta-percha.

A copolymer of ethylene and vinyl acetate has the following structural formula:

Copolymers containing 18%to 33% vinyl acetate are sold in fully polymerized sheets, which are heated to about 90" C and vacuumor handformed over gypsum dental models to produce athletic mouth protectors. The higher the percentage of vinyl acetate, the softer the copolymer. The manufacturer uses a free-radical polymerization method to produce the polymer, which is then molded into sheets. No polymerization occurs in the dental processing of the mouth protector; they are processed as thermoplastics.

1 SELECTED PROBLEMS

Problem 1

Two samples of poly(methy1 methacrylate) were listed by the manufacturer to be 100% pure, which was true, yet one had a significantly lower softening temperature than the other. Why?

Solution

The two samples could have had different average molecular weights, different molecularweight distributions, or different spatial structures (linear, branched, or cross-linked).

Problem 2

The hardness and stiffness of two samples of ethylene-vinyl acetate copolymer used to fabricate athletic mouth protectors were found to be substantially different at body temperature. What is the most likely cause of the difference?

Solution

It is probable that the ratio of ethylene to vinyl acetate in the samples was different, with the softer and less stiff sample containing more vinyl acetate. It is also possible that the average molecular weights or their distributions were different.

Problem 3

Two denture-base poly(methy1 methacrylate) products were heated. It was found that one sample softened and flowed, whereas the second decomposed rather than melted. What is the most likely reason for this observation?

Solution

The first poly(methy1 methacrylate) sample was most likely a linear polymer and thus thermoplastic, whereas the second was a cross-linked poly (methyl methacrylate) that was not thermoplastic.

Problem 4

An experimenter determined the degree of polymerization of a poly(methy1 methacrylate) material and used this information to calculate the degree of conversion. Would this procedure give a correct result?

Solution

No. The degree of polymerization measures the number of mer units in the polymer molecule, whereas the degree of conversion measures the number of unreacted carbon double bonds.

Problem 5

The dimensional change during polymerization of condensation-silicone rubberimpression material is significantly greater than during polymerization of additionsilicone rubber-impression materials. Why?

Solution

During the polymerization of condensation silicones and the rearrangement of chemical bonds, ethyl alcohol is released as a byproduct, but during the polymerization of addition silicones, the hydrogen of one silicone polymer adds to the carbon double bond of the second silicone polymer and no byproduct is formed.

Problem 6

Polyisoprene is available in two forms, one highly elastic and the other brittle. Why?

Solution

The cis-form has a spatial structure, with the methyl and hydrogen on the same side of the carbon double bond, that allows less intramolecular attraction than the trans-form, with the methyl carbon and the hydrogen on opposite sides of the carbon double bond.

Problem 7

What is the function of (1) itaconic acid,

(2) tartaric acid, and (3) maleic acid in glass ionomer restorations?

Solution

(1)Itaconic acid is used to make a copolymer with acrylic acid, which prevents the water solution from becoming more viscous on storage by interfacing with the inter polymer chain attraction.

(2)Tartaric acid, being difunctional, provides crosslinking and improved strength.

(3)The setting reaction, first with ~ a + and' then ~ l +takes~, several days to reach completion; final finishing of the ionomer restoration should not take place at the first appointment. The addition of maleic acid, a stronger acid that is also difunctional, results in faster setting and allows finishing at the first appointment.

Problem 8

Why should epoxy die materials not be used with alginate impressions but may be used with addition-silicone impressions?

Solution

The epoxide group is very reactive; it will react with water in the alginate impression, thus interfering with the reaction with the amine. The addition-silicone impression is a highly cross-linked rubber and does not contain molecules that interfere with the epoxideamine reaction.

Allen JG, Dart EC, Jones E et al: Photochemistry. In Jones DG: ICI Corporate Laboratoy, Chemist? and Industy, no 3, p 79, Feb 7, 1976, p 86.

Asmussen E: NMR-analysis of monomers in restorative resins, Acta Odontol Scand 33:129, 1975.

Braden M: Characterization of the setting process in dental polysulfide rubbers, J Dent Res 45:1065, 1966.

Braden M, Elliott JC: Characterization of the setting process of silicone dental rubbers, J Dent Res 45:1016, 1966.

Braden M, Causton B, Clarke RL: A polyether in~pressionrubber, J Dent Res 51:889, 1972.

Brauer GM, Antonucci JM: Dental applications. In Encyclopedia of polymer science and engineering, vol 4, ed 2, New York,

1986, Wiley.

Cook WD: Rheological studies of the polymerization of elastomeric impression materials. I. Network structure of the set state,J Biomed Mater Res 16:315, 1982.

Cook WD: Rheological studies of the polymerization of elastomeric impression materials. 11. Viscosity measurements, J Bionzed

Mater Res 16:331, 1982.

Cook WD: Rheological studies of the polymerization of elastomeric impression materials. 111. Dynamic stress relaxation modulus,

J Biomed Mater Res 16345, 1982.

Cook WD: Photopolymerization kinetics of dimethacrylates using camphoroquinone/ amine initiator system, Polymer 33:600, 1992.

Craig RG: Chemistry, composition, and properties of composite resins, Dent Clin North Am 25:219, 1981.

Craig RG: Photopolymerization of dental composite systems. In Leinfelder KF, Taylor DF, editors: Postem'or composites: Proceedings

of the International Symposium on Posterior Composite Resins, Chapel Hill, NC,

1984, Taylor DF.

Darr AN, Jacobsen PH: Conversion of dual cure luting cements, J Oral Rehabil22:43, 1995.

Harashima I, Nomata T, Hirasawa T: Degree of conversion of dual cured composite luting agents, Dent Mater 1033, 1991.

Higashi S, Yasuda S, Horie K et al: Studies on rubber base impression materials. Discussions on the setting mechanism of

polysulfide rubber as the dental impression material, chiefly viewed from variations of viscosity and molecular weight, J Nihon Univ Sch Dent 13:33, 1971.

198 Chapter 7 POLYMERS AND POLYMERIZATION

Joos RW, McCue EC, Nachtsheim HG: Polymerization kinetics of two paste resin composites, Int Assoc Dent Res Program and Abstracts 160, 1971.

Kitian RJ: The application of photochemistry to dental materials. In Gebelein CG, Koblitz FF, editors: Polymer science and technology, vol 14, Biochemical and dental applications of polymers, New York,

1981, Plenum.

McCabe JR, Wilson HJ: Addition curing silicone rubber impression materials, Br Dent J 145:17, 1978.

Phillips D: Polymer photochemistry. In BryceSmith D, editor: Photochemistry, vol 1, London, 1970, The Chemical Society, Burlington House.

Ruyter IE: Monomer systems and polymerization. In Vanherle G, Smith DC, ecls:

International Symposium on Posterior Composite Resin Dental Restorative Materials, Peter Szulc Pub1 6:109, Netherlands, 1985.

Ullmann's Encyclopedia of Industrial Chemisty, ed 6, 2000, Chapter 6.1.2 Elastomers, electronic release.

Williams JR, Craig RG: Physical properties of addition silicones as a function of composition, J Oral Rehabil 15:639, 1988.

200 Chapter 8 PREVENTIVE MATERIALS

Ever since fluoride was documented as a chemotherapeutic measure providing resistance in tooth enamel to in vivo demineralization and the development of active carious lesions, prevention has been the foundation for clinical restorative dentistry. The first major advance came with the administration of low-level fluorides into urban water supplies to ensure systemic ingestion during early life, when tooth structure is forming. Fluoride can also be provided systemically as a dietary supplement to inhibit caries where drinking water is not intentionally fluoridated. For patients who are at high risk for the development of caries in spite of svstemic fluoride administration, various means of topical application have been developed to increase caries protection, such as toothpastes, mouthrinses, gels, and varnishes. For effective application of the fluoride ion and uptake by the enamel surface, a vehicle material must be used to carry the active ingredient in the right concentration and place it in apposition to the tooth surface. It must then be held there for a sufficient, yet clinically practical period, to allow a high absorption rate. The vehicle must be nontoxic and easily disposed from the oral cavity after the therapy is completed. With the combination of systemic and topical fluoride applications, the prevalence of smooth surface caries has greatly diminished over the past 50 years. Pits and fissures on the occlusal surfaces of posterior teeth are more resistant to fluoride uptake because the morphology of the surface structure is irregular and there is opportunity for food retention and caries initiation. These surfaces can be dealt with by applying an adhesive resin coating to obtund the irregularities and create a non-retentive

smooth surface that is less likely to decay.

CHEMOTHERAPEUTIC AGENTS. 810zfig- ,

TOOTHPASTE

The major function of a toothpaste (Fig. 8-1) is to enhance cleaning of the exposed tooth surfaces and removal of pellicle, plaque, and debris left from salivary deposits and the mastication of

Fig. 8-1 A selection of toothpastes with a variety of active ingredients, including whitening agents, tartar control, total protection, and baking soda.

food. As a secondary function, toothpaste can be used as a carrier for fluorides, detergents, abrasives, and whitening agents to improve the quality and esthetics of erupted teeth. The use of toothpaste is a continually growing, vital part of home health care throughout the world, resulting in a multi-billion dollar industry. A practicing dentist should have a sound knowledge of the ingredients in most toothpastes and the therapeutic value, if any, in recommending them to patients for general and specific needs.

The general composition of most toothpastes includes the following:

Colloidal binding agent. This agent acts as a carrier for the more active ingredients. Sodium alginate or methylcellulose will thicken the vehicle and prevent separation of the components in the tube during storage.

Humectants. An example is glycerin, which is used to stabilize the composition and reduce water loss by evaporation.

Preseruatives. Preservatives are used to inhibit bacterial growth within the material.

Fig. 8-2 A variety of toothpaste labels highlighting the abrasive material in each paste; (top) hydrated silica, (center) dicalcium phosphate, (bottom) sodium bicarbonate.

Flavoring agents. Peppermint, wintergreen, and cinnamon are added to enhance consumer appeal and to combat oral malodors.

Abmsives. Abrasives are incorporated into all pastes to aid in the removal of heavy plaque, adhered stains, and calculus deposits. Calcium pyrophosphate, dicalcium phosphate, calcium carbonate, hydrated silica, and sodium bicarbonate are used in varying amounts to obtain this effect (Fig. 8-2).

Detergents. An example is sodium lauryl sulfate, which is used to reduce surface tension and enhance the removal of debris from the tooth surface.

Therapeutic agents. Therapeutic agents are added to most toothpastes marketed in North America and Europe. The use of stannous fluorides has been demonstrated effective in the uptake of the fluoride ion and improved resistance

of fluorapatite to acid demineralization in the initiation of carious lesions. Other chemicals. Minor miscellaneous ingredients are included to reduce tube

corrosion, stabilize viscosity, and provide

pleasing coloration. Minor amounts of peroxides are included in some pastes, with marketing claims that they will remove innate discolorations and improve esthetics.

From a materials standpoint, abrasivity is one of the most important characteristics of toothpaste. Abrasion is a very important functional property that can have widespread destructive effects in the oral environment. Toothbrushing introduces three-body abrasion in areas of the mouth that are not normally subject to that type of stress. The toothbrush bristle is one factor, as it is moved across the softer dentin surface of the root in teeth with gingival recession, and the paste becomes the third body, with its abrasive particles interposed.

Abrasivity of toothpaste has been measured with two different methods. One method is to use a radioactive surface as the substrate and to measure the loss of substance after brushing by measuring radioactivity in the abraded material. The second method uses a profilometer to measure the substrate before and after a brushing experiment; the loss of contour is measured and compared. Studies have generally confirmed that the radiotracer methods are similar and more reliable. The ADA, British Standards Institute, and IS0 have standardized tests for abrasivity that differ slightly in methodology. The factors that have been associated with increased abrasion are larger particle sizes, more-irregular particle shapes, harder mineral composition of the particles, amount of particles in a given volume of the paste, and toothbrush bristle stiffness. Fig. 8-3 illustrates the various abrasive particle sizes and shapes incorporated into common commercial toothpastes.

Chemical agents have been placed in various toothpastes to control tartar formation, reduce caries risk, and whiten tooth surfaces. A new group of pastes have been marketed in recent years to control calculus deposition (Fig. 8-41, These pastes incorporate tetrasodium or tetrapotassium pyrophosphates, which act as inhibitors to hydroxyapatite crystal growth. Their efficacy