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Dental applications of polymers: a review
Dental applications of polymers: a review
Gerhard M. Brauer, PhD, W ashington, D.C.
M ethacrylates are the most extensively used dental resins; they fulfill the requisites for denture plastics of high strength, outstanding optical properties, low water sorption and solu bility, and dimensional stability. M ost denture bases, the primary application of plastics in dentistry, therefore contain poly (methyl m ethacrylate) as the main ingredient. Prop erties of typical acrylic, polyvinyl-acrylic, and polystyrene denture bases are discussed. Plas tics are also used for a rtific ia r teeth, filling m aterials, and crown and bridge prostheses. The merits and inadequacies of various plas tics in each of these capacities are reviewed.
Plastics have found m any applications in dental practice during the p ast 30 years. E v en before the introduction of acrylic polym ers to dentistry in 1937, such substances as vulcanite, cellulose
nitrate, phenol-form aldehyde, and vinyl resins were used as denture base m aterials. These m a terials, however, did not possess m any of the requisites of a satisfactory dental plastic. These requisites include ( 1 ) adequate strength, resili ence, and abrasion resistance, ( 2 ) dim ensional stability, (3 ) translucence o r transparency such that it can be m ade to duplicate in appearance the m outh tissue th at it replaces, (4 ) good color stability after fabrication, ( 5 ) resistance' to the m outh fluids, ( 6 ) good tissue tolerance, and (7 ) ease of fabrication into a dental appliance. A lthough plastics are used prim arily in the construction of denture bases, they have an ex tensive variety of other dental applications such as artificial teeth, denture relining and repair m aterials, anterior fillings, cem ents, orthodontic splints, im pression trays and m aterials, inlay p a t terns, and obturators fo r cleft palates. A s w ould be expected, plastics function m ore satisfactorily in certain applications than in others. According to the 1 9 5 9 S u rvey of D en ta l P rac tice, 1 5.6 m illion com plete dentures, 3.7 m illion partial dentures, and 166 m illion fillings were m ade during the year for the civilian population in this country. A ccording to another survey,2 50 per cent of the U.S. population older th an 21 years of age (th a t is, about 58 m illion) wore dentures in 1959. This total is m ade up of ap proxim ately 36 p e r cent upper dentures, 24 per cent low er dentures, and 40 per cent partial 1151
Table 1 ■ Estimate of consumption of plastics in dentis try in 1964
Appliance
Units made Total per year W eig ht per resin (million) unit (Gm.) used (kg.)
Complete dentures 5.5 Partial dentures 4.0 Acrylic teeth* 32.0 Denture reliners 3.3 Fillings 10.0 Silicone impression materials Rubber base impression materials Miscellaneous
__ __
29 6 0.5 4 0.5
__
160,000 24,000 16,000 13,000 5,000 19,000
.
50,000 3,000 290,000 (638,000 lb.)
*30 per cent of all teeth used
dentures. O rdinarily, people do not obtain a com plete low er d enture before a com plete upper denture. H ence, the percentage of people w ear ing a com plete low er denture is a close approxi m ation to th e percentage w earing two com plete dentures. By averaging these and oth er figures supplied by the A m erican D ental T rade A ssociation and by taking into account the increased population, the consum ption of plastics in the U nited States for d en tal applications in 1964 is estim ated (T ab le 1 ).
Acrylic resins D espite the introduction of m any new polym ers, m ethacrylates have continued to be th e m ost extensively used dental resins. This fact is not surprising since acrylic resins com bine reason ably high strength w ith outstanding optical p rop erties, particularly in clarity and light transm is sion, low w ater sorption and solubility, and ex cellent dim ensional stability.
Acrylic denture base M ost d enture base m aterials, therefore, contain poly (m ethyl m ethacrylate) as the m ain ingredi ent. A m onom er-polym er slurry is m ost com m only used fo r m aking dentures, b u t gel-type resins th a t cure fully on heating are also avail able. T h e liquid consists of m ethyl m ethacrylate (inhibited by 0.0 0 6 p er cent o r less of hydroquinone or buty lated hydroxy to lu en e), small am ounts of oth er acrylic m onom ers, plasticizers, and 5 to 15 p er cent of a crosslinking agent, 1152
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such as ethylene dim ethacrylate o r divinylbenzene. T he pow dered polym er usually contains suspension-polym erized m ethyl m ethacrylate th at m ay have been modified w ith small am ounts of ethyl o r butyl m ethacrylate o r ethyl acrylate to produce a som ew hat softer product. A catalyst, generally benzoyl peroxide in 0.5 to 1 p er cent concentration, is incorporated into the pow der together with pigm ents, dyes, and opacifiers. These slurries are cured at 165°F. (7 4 °C .) for 8 hours o r at 165°F. (7 4 °C .) fo r 90 m inutes followed by an additional ho u r at 2 1 2 °F . (1 0 0 ° C .). Preform ed gels containing methyl m ethacrylate m onom er and polym er are also com m ercially available. Instead of a heat-curing cycle, the slurry can be cured at room tem pera ture if the m onom er liquid contains an acceler ator, generally a tertiary am ine such as dim ethylp-toluidine in 0.3 to 0.8 per cent concentration. Because of the form ation of am ine oxide the re sulting resin is color stable only on addition of ultraviolet absorbers o r stabilizing agents. A l though polym erization of the cold-curing resins is never as com plete as th at of the heat-cured type, cold-cured dentures have som ew hat better initial fit and reduced internal strain. Table 2 gives the properties of typical acrylic, polyvinyl-acrylic, and polystyrene den ture bases.3’4 T he differences in the tensile, com pressive, and transverse strengths and in the K noop hardness values of the three plastic m aterials are of little clinical significance. Polyvinyl-acrylic resins have a low er elastic m odulus b u t higher im pact strength. Thus, they will deform elastically to a greater extent and require a higher im pact to fracture them th an styrene o r m ethyl m ethacrylate poly mers. T em peratures above 150°F. (6 5 °C .) are rarely encountered in the oral cavity. Therefore, the resins possess adequate heat distortion tem peratures. R epair of dentures by cold-curing tech nics, however, is indicated to avoid the dim en sional distortion th at m ay occur on heating above this tem perature. W ater sorption should be limited since it is invariably accom panied by an expansion of the resin. Polystyrene and the vinyl-acrylic com bina tion exhibit lower w ater sorption th a n the acrylic resins. T he w ater absorption of the acrylic den ture base resins, however, reaches a m axim um and then rem ains constant. I t does not cause them to becom e foul, to w arp, o r to expand excessively, and it does not im pair their serviceability. In fact, the slight expansion of acrylic resins caused by
Property
Table 2 ■ Properties of denture
base plastics3»4
Tensile strength (psi) Compressive strength (psi) Elastic modulus (psi) Im pact strength (ft.-lb./inch of notch) Transverse strength Knoop hardness number Thermal coefficient of expansion (/ °C .) H eat distortion temperature (°C .) Polymerization shrinkage (vol. % ) Dimensional stability W ate r sorption (% in 24 hours for Vb-inch specimen) W a te r solubility (Gm./cm.2) Processing ease Shelf fife; Powder-liquid Gel
w ater absorption com pensates in p a rt for the shrinkage occurring during processing. Polyvinyl-acrylic— m ixtures of vinyl acetatevinyl chloride copolym er and poly (m ethyl m eth acry late)— and polystyrene resins are supplied com pletely com pounded; these require special injection m olding equipm ent for processing. C om parison of 12 types of organic denture base resins have show n that the conventional acrylic resins, processed with the usual technic of com pression m olding, p roduced dentures th at w ere ju st as stable in dim ension and as satisfactory as den tures produced with special resins w ith elaborate processing equipm ent.4'7 C old-curing acrylic resins produce dentures th a t are as satisfactory as those m ade from heat-curing resins.6 8
Poly (methyl methacrylate)
Polyvinylacrylic
7,000-9,000 11,000 5.5 x 105 60 6,000-8,000 16-22 80 x 10-6 70-90 6 good
7,500 10,000-11,000 3.3 x 105 180 6,000-8,000 14-20 70 x 10-6 55-77 6 good
6,000 15,000 5.3 x 105 50-60 8,000 14-20 60-80 x 10-6 70-100
0.3-0.4 0.02-0.06 gopd
0.07-0.4 0.01 good
0.05-0.3 0.01 good
good fair
Polystyrene
....
good
__
__
fair
good
A m ong the advantages claim ed fo r acrylic teeth are a m ore natural appearance, less break age, reduction o r elim ination of clicking, better bond betw een to o th and the resin base, and ease of grinding and polishing. F u rth er im provem ent in the abrasion resistance appears desirable, how ever, and m any heat-annealing technics and crosslinking agents have been investigated. A novel technic is described in a G erm an patent (no. 1,067,976). T he degree of crosslinking at the surface is increased by subjecting the teeth to irradiation such as gam m a rays, neutrons, or roentgen rays in the presence of crosslinking catalysts, for instance Ce o r P b com pounds or peroxides.
Acrylic filling resins Acrylic teeth A pproxim ately 30 p er cent of all artificial teeth sold in the U nited States are plastic; English dentists use plastic teeth in 80 p er cent of their w ork. Excessive w ear, crazing, and blanching of these teeth have been overcom e by better m ethods of m olding, im proved crosslinking agents, and com plete rem oval of residual emulsifiers and sta bilizers em ployed in the suspension polym eriza tion of the resin used in the fabrication of the teeth. T eeth are m anufactured by injection or transfer m olding technics. T o obtain crosslinked teeth, the labial portion is first m ade and p ar tially crosslinked. T he m old is then packed with m onom er-polym er m ixture containing a lower concentration of crosslinking agent and then cured. T hus, the lingual side of the tooth will be only lightly crosslinked and will be receptive to the m onom er of th e denture base.
T he use of acrylic resins as filling m aterials has not been entirely successful. T he poor m echan ical properties, com pared with metals, limit their application to anterior restorations th a t are sub jected to low stresses. T he shrinkage on poly m erization, high coefficient of therm al expansion, poor abrasion resistance, and flow u n der an oc clusal load, however, have prevented them from displacing silicate cem ents as anterior filling m a terials. T he com position of the m onom er-polym er slurries is sim ilar to th a t of cold-curing denture base resins. The pow der particles are often spher ical. T he “pearls” are usually finer th an 300 mesh, th at is, the particle size of the pow der is usually less th an th at for the denture base m a terials. R educing the particle size of the polym er beads speeds up the polym erization. Some pro d ucts contain a m ixture of powders of different particle sizes to influence the packing characterBrauer: D EN T A L A P P L IC A T IO N S O F PO L Y M ER S " 1 1 5 3
I
T able 3 ■ Physical properties of direct filling resins • *
Type of direct filling resin Commercial Silica reinforced
Volu metric Compressive Modulus of shrinkage strength elasticity on harden (psi) (psi x 10-®) ing ( % ) 10,700 23,000
0.26 1.6
6.2 2.8
C oeffi cient of thermal expansion (p pm /°C .) 81-100 22
istics. With optimum particle size distribution, the powder can be wetted by a smaller amount of liquid, and the total polymerization shrinkage is reduced. A number of papers and patents have dealt with initiator-accelerator systems for these resins and the kinetics and mechanism of the peroxideamine decomposition. Most often, benzoyl per oxide or /j,p'-dichlorobenzoyl peroxide are used as catalysts. Dimethyl-p-toluidine and 2,2' - m tolyliminodiethanol are among the most efficient amine accelerators.9 Generally, substitution of dimethylaniline in the para position with elec tron donating groups increases the reactivity of the amine in the decomposition of the benzoyl peroxide, thereby accelerating the rate of poly merization. The effects of varying concentrations of peroxide, amine, and hydroquinone on the polymerization rate of poly (methyl methacry late) slurries have been investigated by Rose, Lai, and Green.10 Since the presence of a tertiary aromatic amine invariably. results in a colorted product, many other accelerators have been suggested. Accelera tors such as p-toluene sulfinic acid or n-lauryl mercaptan will give products with improved color stability. Because of the poor storage stability of sulfinic acids, fairly complex accelerator systems have been developed that are quite satisfactory for use in dental filling resins. Bredereck, Bäder, and Wohnhas1112 recommended oxysulfones, a-aminosulfones, and sulfinic acid salts, and Brau er and Bums13 found that salts of secondary and tertiary aromatic amines are efficient accelerators for the peroxide-initiated polymerization, but form colored products. Fast-setting products may be obtained from preformed polymeric powder containing initiator and salt of a difficultly sol uble sulfinic acid. The powder is mixed with monomer containing 0.5 to 2 per cent of a qua ternary ammonium salt and a few per cent of an acid, such as acrylic, methacrylic, or phos phoric acid.1416 The polymerization is greatly ac celerated in the presence of 0.001 to 0.0Ö01 per cent of Cu++ or iron salt,17 methanol,18 or water. 1154 ■ JA D A , Vol. 72, M ay 1966
Direct-filling resin materials shrink approxi mately 6 to 8 per cent in volume during poly merization. The shrinkage can be made to take place somewhat directionally so that most of the shrinkage will be manifested on the outside sur face of the restoration rather than at the inter face between the filling and the cavity. The hardened material contains around 2 per cent of unreacted monomer. Presence of higher con centration of monomer in the set mass decreases the strength of the filling resin. More serious are the high coefficient of thermal expansion and absence of bonding to tooth structure. The large difference in the coefficients of thermal expansion of the tooth and of the restoration creates a crevice at the margins into which fluids and gases ebb and flow with changes in temperature. The presence of bacteria in these crevices is consid ered a major cause of the recurrent decay com monly observed around or beneath plastic fill ings. To decrease the thermal coefficient of ex pansion, fillers such as fiber glass, silica, alumi num oxide, or finely ground ceramic powder have been incorporated into the resin,19 20 but this addi tion often results in products that have lower im pact resistance and are difficult to polish. Good results have been obtained by using vinylsilanetreated fused silica particles with a monomer consisting of about 80 per cent of an adduct of bisphenol A and glycidyl methacrylate, 10 per . cent tetraethylene dimethacrylate, and 10 per cent me.thyl methacrylate.20* As can be seen from Table 3, such resin fillings shrink less on hard ening, are stronger and stiffer, and have a much lo\yer coefficient of thermal expansion than the commonly used direct resin fillings. Recently, a direct filling resin of a similar chemical com position that contains about 70 per cent inor ganic filler (glass spheres and rods) has been introduced. This material is reported to have a coefficient of thermal expansion of 45. ppm/de gree Celsius. N,N-dimethyl-i>im-xylidene is an efficient accelerator for reinforced acrylic resins and yields products showing improved color stability.2011 Much thought has been given to the develop ment of a restorative filling material that ad heres well to the tooth structure.20“ The inherent difficulty in bonding resin to tooth structure is generally recognized to be caused primarily by the moisture at the cavity walls, which cannot be completely removed, as well as the porosity and nonhomogeneity of the surface. An inter mediate impermeable layer of a cementing me-
dium , which m ust adhere to b o th too th struc ture and the restorative resin, seems the m ost prom ising approach. A n adduct of N -phenylglycine and glycidyl m ethacrylate has shown significant bonding betw een a direct filling resin and dentin and enam el surfaces after soaking in w ater for prolonged periods of tim e.21 M uch basic inform ation is needed, however, regarding the tooth surface and how to w et this surface under clinical conditions w ith a suitable adhesive before significant advances can be m ade.
Acrylic crown and bridge prostheses G reater rigidity is required of an inlay, crown, o r fixed p artial denture to w ithstand th e forces of m astication than is necessary for rem ovable appliances. A crylic and vinyl-acrylic resins are used with varying results in place of fused p o r celain for crown and bridge prostheses. Because of their low strength, their tendency to flow and deform perm anently u nder the forces of m astica tion, and their poor abrasion resistance these resins should be em ployed prim arily as tem porary crow ns or in protected areas. A crylic resins have been used quite successfully for veneers on gold jacket crowns where the m aterial is n o t subject to high occlusal stresses. M ost failures have been caused by differences in the coefficient of therm al expansion of gold alloys and acrylic resins, non adhesion to gold alloys, discoloration, staining, and poor abrasion resistance.
Miscellaneous applications Inlay patterns can be p repared from acrylic plas tics. T he finished casting is n o t superior to one produced from a wax pattern, and the technic has not aroused m uch interest. C old-curing plas tics are used in m aking contoured im pression trays. These resins contain substantial quanti ties of fillers th at increase the rigidity of the m aterial after setting. O rthodontists m ake ex tensive use of plastics in such specialized appli cations as splints, tem porary space m aintainers, and bite planes.
Other plastics The shortcomings of plastic acrylic filling m a terials have led to suggestions th at they be re
placed by other polym eric systems.20’22 T he follow ing discussion will b e lim ited to plastics th at have been investigated in detail and show some prom ise; that is, the polystyrene, epoxy, azirino, polyam ide, polycarbonate, and sem iorganic m a terials. M ost o f these products should be con sidered strictly experim ental m aterials. ■ P o lystyren e: Styrene resins are available com m ercially for m aking dentures by ah injection molding technic. C om parison of the properties of polystyrene and acrylic resin denture m ate rials are given in T able 2. Clinically, they give equally satisfactory results when used in m aking acrylic dentures. ■ E p o x y resins: R esearch to develop dental resins based on epoxy resins was started around 1935. A lthough m any properties of fully cured resins would be desirable for a dental prosthetic m a terial, especially their adhesive strength and lo\4 curing shrinkage, the clinical results have been quite disappointing. T he m ain problem s’ have been the toxicity of the accelerators, which are necessary for rapid hardening. The high w ater sorption and solubility and poor dim ensional stability reported for epoxy denture base m a terial result from incom plete curing of the m a terial. In 1958,23 an epoxy resin filling m aterial was developed th at used a m etal chloride ca ta lyst. T he dry resin possessed good physical p ro p erties. T he apparent advantage of the less than 1 per cent curing shrinkage of the epoxide resins is outweighed by the inferior physical properties obtained after prolonged w ater im m ersion. In particular, the cure of these resins appears to be inhibited by the presence of w ater and, th ere fore, tacky surfaces w ere obtained w hen in direct contact with natu ral tooth dentin. Epoxy resins have been suggested for crow n and bridge w ork. Initially they adhere well to gold and other dental m aterials, and it is claim ed that they reduce seepage and darkening of glue lines between gold and jacket o r crow n. This bond is often dissipated in time by the high w a te r sorption of these resins. Epoxy resins . have slightly higher com pressive strength, stiffness, and abrasion resistance to toothbrushing, b u t their scratch resistance and hardness is not greater than that of acrylic m aterials.24-25 Because o f handling difficulties, the epoxy m aterial gives m ore variable; results than other resins. T he color stability of epoxy resin is not very satis factory, and it is heavily stained by lipstick. Brauer: D E N T A L A P P L IC A T IO N S O F PO L Y M E R S « 1 1 5 5
■ P o ly este r resins: A n interesting experim ental resin fo r anterior restorations is an unsaturated polyester such as an adduct of maleic anhydride and 1,3-butanediol containing aziridino (a,/3ethyleneim ino) groups.19'26’27 This m aterial is supplied as a paste. T o initiate polym erization, a sulfonic acid ester (4 p er cent m ethyl p-toluenesulfonate) is added, also in the form of a paste. T he slurry hardens in two phases in an involved m ultipolym erization process. T he aziridino groups are som ew hat hydrophilic. A n excess of catalyst also produces excessive expansion. T he substance contains n o filler and after hardening is highly crosslinked. T he coefficient of therm al expan sion is about the same as th at of acrylic resins, and m arginal percolation of fluids can be ob served. T h e aziridino polyester is slightly softer th a n acrylic resin and has som ew hat low er com pressive strength. T h e w ater sorption is m uch higher th a n that of acrylics and results in a linear expansion of over 1 p er cent after 35 days. This expansion is m ore than three tim es the linear polym erization shrinkage of this m aterial a t body tem perature. Originally, it was hoped th a t the w ater absorption of the resin w ould im prove the fit in the m outh. W ith therm al ex pansion causing the m arginal percolation of fluids aroun d the cavity edge, however, w hatever ad vantage w ater absorption m ight have offered is lost. U nless the m aterial is used w ith a base, it causes pathological reactions in the pulp. A polyam ide N ylon 11 which is the polym er of am inoundecanoic acid has found lim ited use in partial denture construction. I t appears, how ever, th a t high w ater sorption causes undesirable dim ensional changes. A lso, the m aterial is som e w hat soft. ■ P olycarbon ate: P olycarbonate absorbs only 0 .3 6 p e r cent w ater and hence has good dim en sional stability. Its therm al coefficient of expan sion is 70 x 10'6/° C . I t is claim ed to be highly abrasion resistant and color stable. In G erm any, it has been used as a denture base resin. Because of its high m elting point, it m ust be m olded at tem peratures ranging from 2 8 4 to 3 2 0 °F . (1 4 0 to 1 6 0 °C .). T he m olding shrinkage am ounts to 0.005 to 0.007 in ch /in ch . A postm olding shrink age has not been observed. Because of the tem peratures involved, only porcelain teeth m ay be safely invested into polycarbonate denture base. , Jarb y and A nderson28 have described a technic for using polycarbonate for anterior fillings. T he m aterial, in the form of dry sticks, is inserted .1156
■ JA D A , Vol. 72, M ay 1966
into an electrically heated syringe th a t has a plas tification cham ber, kept betw een 284 to 3 0 2 °F . (1 4 0 to 1 5 0 °C .), and a w ater-cooled jacket. The cavity should, if possible, be prepared in the form of round excavations. A 10 per cen t solu tion o f polycarbonate in chloroform is used as a cavity liner to im prove retention of fillings. D eep cavities are lined w ith phosphate cements. H e at ing of the plastic takes about 30 seconds, and the h o t m aterial is directly injected into the cavity. A lthough the highly plastic m aterial is still ex trem ely hot, it is claimed th at the therm al strain on the pulp by the filling is not greater th a n th at of introducing h o t therm oplastic im pression m a terials. Polishing of the filling can be d o n e at once, after the injection, since the m aterial solidi fies nearly instantaneously. The m arginal accur acy is claim ed to be excellent. Since the coeffi cient of therm al expansion is only slightly low er th an th a t of acrylics and addition of fillers that are n o t free-flowing is impossible, it will b e in teresting to see if the good m arginal accuracy will be m aintained o r if percolation will; eventu ally ru in the m arginal seal. Because of the fnany problem s to be solved, especially the w isdom of injecting a m aterial kept over 300°F . ( 15 0 °C .) into a cavity, the polycarbonate should be. con sidered a highly experim ental filling m aterial. A s has been indicated, all the organic polym ers have coefficients of therm al expansion ranging from 70 to 90 x 10‘6/° C . Incorporation of fillers reduces this value. Perhaps a b etter approach to reduce the therm al expansion w ould be th e use o f a resin containing an inorganic backbone or a sem iorganic polym er. Such a m aterial is likely to be m ore com patible with tooth structure, which contains inorganic hydroxyapatite as the m ain constituent. A great deal of w ork is being done in the field of chelation polym ers, especially to obtain highly heat-resistant resins. Thus far, only low m olecular weight coordination polym ers have been synthesized, which do not possess desirable physical properties. F u rther progress in this field, however, m ay lead to polym ers having properties th at w ould m ake them logical candidates for fu rth er study as filling resins.
Elastomeric materials T here is a great need for a satisfactory resilient liner fo r com plete dentures. M any of th e pres ently available liners are highly plasticized vinyl chloride o r m ethacrylate resins th at do n o t re
m ain resilient because of leaching out of the plasticizer, p oor abrasion resistance, peeling from the denture base, o r high w ater sorption. M ix tures of acrylates o r m ethacrylates th at are poly m erized in the presence o f a vinyl stearate-vinyl acetate copolym er have also been suggested.29 Soft liners m ade from silicone rubber, however, m aintained their resiliency for as long as 5 years in clinical trials. W ith th e use of p ro p er prim ers and im proved form ulations, th eir m ain problem o f lack of perm anent adhesion to denture base appears to have been overcom e.30’31
Silicone rubber base impression materials T he m ain ingredient of th e silicone base m ate rials is a Unear diorganopolysiloxane such as poly (dim ethyl siloxane) containing term inal hydroxyl groups an d a few p e r cen t of a second siloxane, fo r exam ple, trim ethyl siloxane-end-blocked m eth yl hydrogen polysiloxane o r m ethyltriacyloxysilane. Curing is effected w ith an organom etallic com pound, such as 1 to 2 p er cent dibutyltin dilaurate, dioctyltin m aleate, 0.2 p er cent tellu rium diethyldithiocarbam ate, tetram ethylthiuram disulfide, o r sulfur in polyethyl silicate contain ing 50 p er cent ethoxy groups.32’33 Inorganic fillers, such as zinc oxide, m agnesium oxide, or calcium carbonate o r zinc carbonate, also speed up the setting reaction. O rthosilicic acid esters, especially m ethyl or ethyl o rth o polysilicates are used m ost com m only as crosslinking agents, but other organopolysiloxanes, organohydropolysiloxanes, and silanes w ith m ore than tw o reactive functional groups can also b e em ployed for this purpose. T he chief use of the silicone im pression m a terials is fo r crow n an d bridge w ork. T he cur ren t brands of silicone elastom er do not evolve hydrogen gas or exhibit tack and porosity be cause of incom plete polym erization. T he shelf life on storage at elevated tem peratures also has been greatly im proved during the last few years. T h e contraction of silicone elastom ers on cooling from m outh to room tem perature (2 2 °C .) is ab o u t 0.35 p er cent com pared w ith 0.20 p er cent fo r polysulfide ru b b ers.34 T he latter-type m ate rials allow m ore latitude in w orking tim e than the silicone m aterials and appear to be the pre ferred m aterial w hen m ultiple inlay o r crown im pressions are taken. A b o u t 15 p er cent of all single tooth and fixed bridge im pressions and 5
p er cent of all im pressions for dentures are m ade w ith silicone im pression m aterials.
Polysulfide rubber base impression materials Polysulfide ru bber base m aterials are used p ri m arily in inlay and crown and bridge w ork. T he basic ingredient is a polyfunctional m ercaptan th a t form s a rubberlike m ass in the presence of accelerators, such as lead peroxide o r other oxi dizing agents and sulfur. Traces of organic am ines also are claim ed to speed up the reac tion. In ert fillers, such as zinc oxide and calcium sulfate, are added to modify the viscosity of the mix and to give strength and color to the set im pression. T hese m aterials are reliable and give strong, relatively stable im pressions. The m ain disadvantages are the extrem e stickiness of the freshly m ixed paste, its odor, and the stains it will produce if spilled on clothing. It is estim ated th at polysulfide ru b b er is employed in about 35 p e r cent of all fixed bridge w ork and 13 p e r cent of all final im pressions used for m aking dentures.
Summary Plastics are em ployed successfully in m any den tal applications. T heir properties m ake them espe cially valuable in the construction of dentures, plastic teeth, and im pression m aterials. Plastic filling m aterials have not been successful because of their high coefficient of therm al expansion and lack of bonding to tooth structure. P erhaps the biggest im provem ent in filling m aterials w ould be the developm ent of a satisfactory cavity liner th a t bonds to tooth structure and is im pervious to the m outh fluids. If such a liner w ere available, the need for a filling m aterial th a t m atches the therm al expansion o f the tooth w ould be less critical.
This investigation is part of the dental'research pro gram conducted by the National Bureau of Standards, in cooperation with the Council on Dental Research of the American Dental Association; the Arm y Dental Corps; the Dental Sciences Division of the School of Aerospace Medicine, U SA F; the National Institute of Dental Re search, and the Veterans Administration. Doctor Brauer is a chemist in the Dental Research Section, Institute of M aterials Research, National Bureau of Standards, Washington, D.C. 1. Bureau of Economic Research and Statistics, AmerBrouer: D E N T A L A P P L IC A T IO N S O F PO LY M ER S " 1 1 5 7
icon Dental Association. 1959 survey of dental practice. X I. Dental services rendered. JA D A 62:627 M ay, 1961; X II. Summary. JA D A 62:764 June, 1961. 2. Bureau of Economic Research and Statistics, American Dental Association. Survey of public attitudes regarding dentures. III. Dentures worn; source of den tures. JA D A 62:226 Feb., 1961. 3. Peyton, F. A., and others.- Restorative dental materials, St. Louis, C. V. Mosby Co., 1960. 4. W oelfel, J. B., Paffenbarger, G. C., and Sweeney, W . T. Some physical properties of organic denture base materials. JA D A 67;489 Oct., 1963. 5. W oelfel, J. B., Paffenbarger, G. C., and Sweeney, W . T. bimensional changes occurring in dentures dur ing processing. JA D A 61;413 Oct., I960. 6. W oelfel, J. B., Paffenbarger, G. C., and Sweeney, W . T. Changes in dentures during.storage in water and in service. JA D A 62:643 June, 1961. 7. Peyton, F. A., and Anthony, D. H. Evaluation of dentures processed by different techniques. J . Pros. Dent. 13:269 March-Apcil, 1963. 8. M irza, F. D. Dimensional stability of acrylic resin denture's: clinical evaluation. J . Pros. , Dent. 1 1 :848 Sept.-Oct., 1961. 9. Brauer, G, M ., Davenport, R. M.., and Hansen, W . C. Accelerating effect of amines on polymerization of methyl methacrylate. Mod. Plastics 34:no. 3:153 Nov., 1956.."' / ,10. Rose, E. E.; Lai, Joginder, and Green, Richard. Effects of peroxide, amine and hydroquinone in varying concentrations on the polymerization rate of polymethyl methacrylate slurries. JA D A 56:375 March, 1958. 11. Brederick, H., Bäder, E., Wohnhas, A. a-Oxy-, a-Amino-sulfone und sulfinsaure Salze organischer Basen als Polymerisationskatalysatoren. Makromolek. Chem. 12:100 Dec., 1953. ' • 12. Brederick, H:, and Bäder, E. Polymerization prod ucts with accelerators containing at least one aminomethyl sulfone group. U. S. patent 2,750,357 Jun e 12, 1956. 13. Brauer, G. M., and Burns, F. R. Sulfinic acid derivatives as accelerators in the polymerization of methyl methacrylate. J. Polymer Sei. 19:31 1 Feb., 1956. 14. Kulzer and Co. G.m.b.H. (by Carlo Rossetti) Vinyl polymers for dental fillings. German patent 1,123,824 Feb. 15, 1962. 15. Brederick, H., and Bäder, E. Salts of sulfinic acids as polymerization catalysts. U. S. patent 2,846,418 Aug. 5, 1958. 16. Bäder, E., Schweitzer, O., and Querfurth, W . Polymerization of vinylidene compounds with organic peroxide, organic sulfur and quaternary ammonium com pounds as catalysts. U. S. patent 2,946,770 Ju ly 26, 1960. 17. Brederick, H., and others. Polymerization of un saturated compounds with sulfone-contaimng accelera tors. U. S. patent 2,779,751 Jan . 29, 1957. 18. Brederick, H., Bäder, E., and Wohnhas A. Poly merization accelerators or catalysts. U. S. patent 2,758,106 Aug. 7, 1956. 19. McLean, J. W . Anterior filling materials in Europe. D. Progress 2:181 April, 1962. 20. M cLean, J. W . The future of dental restorative materials. Rev. Beige Med. Dent. 20: no. 3:277, 1965.
1158 ■ JA D A , Vol. 72, M ay 1966
20a. Bowen, R. L. Properties of a silica-reinforced polymer for dental restorations. JA D A 66:57 Jan ., 1963. 20b. Bowen, R. L., and Argentar, H. Color forma tion in methacrylate accelerator systems (abstract) . J. D. Res. IA D R Program and Abstracts of papers. July, 1965, p. 83. 1 20c. Phillips, R. W ., and Ryge, G., editors. Adhesive restorative dental materials. Proceedings from workshop sponsored by Dental Study Section, National Institutes of Health. Indianapolis, Indiana University School of Dentistry, 1962. 21. Bowen, R. L. Adhesive bonding of various m ate rials to hard tooth tissues. II. Bonding to dentin pro moted by a surface-active comonomer. J. D. Res. 44:895 Sept.-Oct., 1965; III. Bonding to dentin im proved by pretreatment and the use of surface-active comonomer. J. D. Res. 44:903 Sept.-Oct, 1965; IV. Bonding to dentin) enamel, and fluorapatite improved by the use of a surface-active comonomer. J . D. Res. 44:906 Sept.-Oct., 1965; V. The effect of a surfaceactive comonomer on adhesion to diverse substrates. J . D Res. 44:1369 Nov.-Dee., 1965. 22. Smith, D. C. Recent developments and pros pects in dental polymers. J. Pros. Dent. 12:1068 Nav.Dec., 1962. 23. Ley, J. B. Investigation into epoxy resin systems with dental application. J. D. Res. 43:950 Sept.-Oct., 1964. Abstract. 24. Peyton, F. A., and Craig, R. G. Current evalua^ tions of plastics in crown and bridge resins. J. Pros. Dent. 13:743 July-Aug., 1963. 25. Kafalias, M . C., Swartz, M. L., and Phillips, R. W . Physical properties of selected dental resins. Pact I. J. Pros. Dent. 13:1087 Nov.-Dee., 1963. 26- ESPE Fabrik Pharmazeutischer Präparate G.m.b.H. (by W . Schmitt, R. Purrmann and P. Jo ch u m ). Use of cross-linkable organic polyethyleneimine compounds for the preparation of self-curing plastics suitable for the filling of dental cavities and for attaching dentures. Ger man patent 1,146,617 April 4, 1963. 27. ESPE Fabrik Pharmazeutischer Präparate G.m.b.H. (by W . Schmitt, R. Purrmann, P. Jochum and W . D. Z a h le r ). Curing, molding or coating compositions con taining polymers with ethyleneimine groups. German patent 1,166,471 M arch 26, 1964. 28. Jarby, Sven, and Andersen, Elsebeth. Dental fill ings of polycarbonate. J. D. Res. 41 :214 Jan.-Feb., 1962. 29. Landry, L. A . Soft, flexible resinous compositions for dentures and prosthetic appliances. U. S. patent 3,084,436 April 9, 1963. 30. Barnhart, G. W . Silicone materials for lining dentures. D. Progress 3:246 July, 1963. 31. Silastic 390, Soft liner evaluation report. M ed ical and Pharmaceutical Products Division, Dow-Coming, Midland, Mich. 32. Nitsche, S., and W ick, M. Vulkanisationssysteme des Silikonkautschuks. Kunststoffe 47:431 Aug., 1957. 33. W acker-Chemie G.m.b.H. (by S. Nitzsche and M. W ic k ) . Organopolysiloxane molding materials as dental impression molds. German patent 1,163,021 Feb. 13, 1964. 34. M cLean, J. W . Physical properties influencing the accuracy of silicone and thiokol impression m ate rials. Brit. Dent. J. 110:85 Feb. 7, 1961.
Gerhard M. Brauer, PhD, W ashington, D.C.
M ethacrylates are the most extensively used dental resins; they fulfill the requisites for denture plastics of high strength, outstanding optical properties, low water sorption and solu bility, and dimensional stability. M ost denture bases, the primary application of plastics in dentistry, therefore contain poly (methyl m ethacrylate) as the main ingredient. Prop erties of typical acrylic, polyvinyl-acrylic, and polystyrene denture bases are discussed. Plas tics are also used for a rtific ia r teeth, filling m aterials, and crown and bridge prostheses. The merits and inadequacies of various plas tics in each of these capacities are reviewed.
Plastics have found m any applications in dental practice during the p ast 30 years. E v en before the introduction of acrylic polym ers to dentistry in 1937, such substances as vulcanite, cellulose
nitrate, phenol-form aldehyde, and vinyl resins were used as denture base m aterials. These m a terials, however, did not possess m any of the requisites of a satisfactory dental plastic. These requisites include ( 1 ) adequate strength, resili ence, and abrasion resistance, ( 2 ) dim ensional stability, (3 ) translucence o r transparency such that it can be m ade to duplicate in appearance the m outh tissue th at it replaces, (4 ) good color stability after fabrication, ( 5 ) resistance' to the m outh fluids, ( 6 ) good tissue tolerance, and (7 ) ease of fabrication into a dental appliance. A lthough plastics are used prim arily in the construction of denture bases, they have an ex tensive variety of other dental applications such as artificial teeth, denture relining and repair m aterials, anterior fillings, cem ents, orthodontic splints, im pression trays and m aterials, inlay p a t terns, and obturators fo r cleft palates. A s w ould be expected, plastics function m ore satisfactorily in certain applications than in others. According to the 1 9 5 9 S u rvey of D en ta l P rac tice, 1 5.6 m illion com plete dentures, 3.7 m illion partial dentures, and 166 m illion fillings were m ade during the year for the civilian population in this country. A ccording to another survey,2 50 per cent of the U.S. population older th an 21 years of age (th a t is, about 58 m illion) wore dentures in 1959. This total is m ade up of ap proxim ately 36 p e r cent upper dentures, 24 per cent low er dentures, and 40 per cent partial 1151
Table 1 ■ Estimate of consumption of plastics in dentis try in 1964
Appliance
Units made Total per year W eig ht per resin (million) unit (Gm.) used (kg.)
Complete dentures 5.5 Partial dentures 4.0 Acrylic teeth* 32.0 Denture reliners 3.3 Fillings 10.0 Silicone impression materials Rubber base impression materials Miscellaneous
__ __
29 6 0.5 4 0.5
__
160,000 24,000 16,000 13,000 5,000 19,000
.
50,000 3,000 290,000 (638,000 lb.)
*30 per cent of all teeth used
dentures. O rdinarily, people do not obtain a com plete low er d enture before a com plete upper denture. H ence, the percentage of people w ear ing a com plete low er denture is a close approxi m ation to th e percentage w earing two com plete dentures. By averaging these and oth er figures supplied by the A m erican D ental T rade A ssociation and by taking into account the increased population, the consum ption of plastics in the U nited States for d en tal applications in 1964 is estim ated (T ab le 1 ).
Acrylic resins D espite the introduction of m any new polym ers, m ethacrylates have continued to be th e m ost extensively used dental resins. This fact is not surprising since acrylic resins com bine reason ably high strength w ith outstanding optical p rop erties, particularly in clarity and light transm is sion, low w ater sorption and solubility, and ex cellent dim ensional stability.
Acrylic denture base M ost d enture base m aterials, therefore, contain poly (m ethyl m ethacrylate) as the m ain ingredi ent. A m onom er-polym er slurry is m ost com m only used fo r m aking dentures, b u t gel-type resins th a t cure fully on heating are also avail able. T h e liquid consists of m ethyl m ethacrylate (inhibited by 0.0 0 6 p er cent o r less of hydroquinone or buty lated hydroxy to lu en e), small am ounts of oth er acrylic m onom ers, plasticizers, and 5 to 15 p er cent of a crosslinking agent, 1152
■ JA D A , Vol. 72, M ay 1966
such as ethylene dim ethacrylate o r divinylbenzene. T he pow dered polym er usually contains suspension-polym erized m ethyl m ethacrylate th at m ay have been modified w ith small am ounts of ethyl o r butyl m ethacrylate o r ethyl acrylate to produce a som ew hat softer product. A catalyst, generally benzoyl peroxide in 0.5 to 1 p er cent concentration, is incorporated into the pow der together with pigm ents, dyes, and opacifiers. These slurries are cured at 165°F. (7 4 °C .) for 8 hours o r at 165°F. (7 4 °C .) fo r 90 m inutes followed by an additional ho u r at 2 1 2 °F . (1 0 0 ° C .). Preform ed gels containing methyl m ethacrylate m onom er and polym er are also com m ercially available. Instead of a heat-curing cycle, the slurry can be cured at room tem pera ture if the m onom er liquid contains an acceler ator, generally a tertiary am ine such as dim ethylp-toluidine in 0.3 to 0.8 per cent concentration. Because of the form ation of am ine oxide the re sulting resin is color stable only on addition of ultraviolet absorbers o r stabilizing agents. A l though polym erization of the cold-curing resins is never as com plete as th at of the heat-cured type, cold-cured dentures have som ew hat better initial fit and reduced internal strain. Table 2 gives the properties of typical acrylic, polyvinyl-acrylic, and polystyrene den ture bases.3’4 T he differences in the tensile, com pressive, and transverse strengths and in the K noop hardness values of the three plastic m aterials are of little clinical significance. Polyvinyl-acrylic resins have a low er elastic m odulus b u t higher im pact strength. Thus, they will deform elastically to a greater extent and require a higher im pact to fracture them th an styrene o r m ethyl m ethacrylate poly mers. T em peratures above 150°F. (6 5 °C .) are rarely encountered in the oral cavity. Therefore, the resins possess adequate heat distortion tem peratures. R epair of dentures by cold-curing tech nics, however, is indicated to avoid the dim en sional distortion th at m ay occur on heating above this tem perature. W ater sorption should be limited since it is invariably accom panied by an expansion of the resin. Polystyrene and the vinyl-acrylic com bina tion exhibit lower w ater sorption th a n the acrylic resins. T he w ater absorption of the acrylic den ture base resins, however, reaches a m axim um and then rem ains constant. I t does not cause them to becom e foul, to w arp, o r to expand excessively, and it does not im pair their serviceability. In fact, the slight expansion of acrylic resins caused by
Property
Table 2 ■ Properties of denture
base plastics3»4
Tensile strength (psi) Compressive strength (psi) Elastic modulus (psi) Im pact strength (ft.-lb./inch of notch) Transverse strength Knoop hardness number Thermal coefficient of expansion (/ °C .) H eat distortion temperature (°C .) Polymerization shrinkage (vol. % ) Dimensional stability W ate r sorption (% in 24 hours for Vb-inch specimen) W a te r solubility (Gm./cm.2) Processing ease Shelf fife; Powder-liquid Gel
w ater absorption com pensates in p a rt for the shrinkage occurring during processing. Polyvinyl-acrylic— m ixtures of vinyl acetatevinyl chloride copolym er and poly (m ethyl m eth acry late)— and polystyrene resins are supplied com pletely com pounded; these require special injection m olding equipm ent for processing. C om parison of 12 types of organic denture base resins have show n that the conventional acrylic resins, processed with the usual technic of com pression m olding, p roduced dentures th at w ere ju st as stable in dim ension and as satisfactory as den tures produced with special resins w ith elaborate processing equipm ent.4'7 C old-curing acrylic resins produce dentures th a t are as satisfactory as those m ade from heat-curing resins.6 8
Poly (methyl methacrylate)
Polyvinylacrylic
7,000-9,000 11,000 5.5 x 105 60 6,000-8,000 16-22 80 x 10-6 70-90 6 good
7,500 10,000-11,000 3.3 x 105 180 6,000-8,000 14-20 70 x 10-6 55-77 6 good
6,000 15,000 5.3 x 105 50-60 8,000 14-20 60-80 x 10-6 70-100
0.3-0.4 0.02-0.06 gopd
0.07-0.4 0.01 good
0.05-0.3 0.01 good
good fair
Polystyrene
....
good
__
__
fair
good
A m ong the advantages claim ed fo r acrylic teeth are a m ore natural appearance, less break age, reduction o r elim ination of clicking, better bond betw een to o th and the resin base, and ease of grinding and polishing. F u rth er im provem ent in the abrasion resistance appears desirable, how ever, and m any heat-annealing technics and crosslinking agents have been investigated. A novel technic is described in a G erm an patent (no. 1,067,976). T he degree of crosslinking at the surface is increased by subjecting the teeth to irradiation such as gam m a rays, neutrons, or roentgen rays in the presence of crosslinking catalysts, for instance Ce o r P b com pounds or peroxides.
Acrylic filling resins Acrylic teeth A pproxim ately 30 p er cent of all artificial teeth sold in the U nited States are plastic; English dentists use plastic teeth in 80 p er cent of their w ork. Excessive w ear, crazing, and blanching of these teeth have been overcom e by better m ethods of m olding, im proved crosslinking agents, and com plete rem oval of residual emulsifiers and sta bilizers em ployed in the suspension polym eriza tion of the resin used in the fabrication of the teeth. T eeth are m anufactured by injection or transfer m olding technics. T o obtain crosslinked teeth, the labial portion is first m ade and p ar tially crosslinked. T he m old is then packed with m onom er-polym er m ixture containing a lower concentration of crosslinking agent and then cured. T hus, the lingual side of the tooth will be only lightly crosslinked and will be receptive to the m onom er of th e denture base.
T he use of acrylic resins as filling m aterials has not been entirely successful. T he poor m echan ical properties, com pared with metals, limit their application to anterior restorations th a t are sub jected to low stresses. T he shrinkage on poly m erization, high coefficient of therm al expansion, poor abrasion resistance, and flow u n der an oc clusal load, however, have prevented them from displacing silicate cem ents as anterior filling m a terials. T he com position of the m onom er-polym er slurries is sim ilar to th a t of cold-curing denture base resins. The pow der particles are often spher ical. T he “pearls” are usually finer th an 300 mesh, th at is, the particle size of the pow der is usually less th an th at for the denture base m a terials. R educing the particle size of the polym er beads speeds up the polym erization. Some pro d ucts contain a m ixture of powders of different particle sizes to influence the packing characterBrauer: D EN T A L A P P L IC A T IO N S O F PO L Y M ER S " 1 1 5 3
I
T able 3 ■ Physical properties of direct filling resins • *
Type of direct filling resin Commercial Silica reinforced
Volu metric Compressive Modulus of shrinkage strength elasticity on harden (psi) (psi x 10-®) ing ( % ) 10,700 23,000
0.26 1.6
6.2 2.8
C oeffi cient of thermal expansion (p pm /°C .) 81-100 22
istics. With optimum particle size distribution, the powder can be wetted by a smaller amount of liquid, and the total polymerization shrinkage is reduced. A number of papers and patents have dealt with initiator-accelerator systems for these resins and the kinetics and mechanism of the peroxideamine decomposition. Most often, benzoyl per oxide or /j,p'-dichlorobenzoyl peroxide are used as catalysts. Dimethyl-p-toluidine and 2,2' - m tolyliminodiethanol are among the most efficient amine accelerators.9 Generally, substitution of dimethylaniline in the para position with elec tron donating groups increases the reactivity of the amine in the decomposition of the benzoyl peroxide, thereby accelerating the rate of poly merization. The effects of varying concentrations of peroxide, amine, and hydroquinone on the polymerization rate of poly (methyl methacry late) slurries have been investigated by Rose, Lai, and Green.10 Since the presence of a tertiary aromatic amine invariably. results in a colorted product, many other accelerators have been suggested. Accelera tors such as p-toluene sulfinic acid or n-lauryl mercaptan will give products with improved color stability. Because of the poor storage stability of sulfinic acids, fairly complex accelerator systems have been developed that are quite satisfactory for use in dental filling resins. Bredereck, Bäder, and Wohnhas1112 recommended oxysulfones, a-aminosulfones, and sulfinic acid salts, and Brau er and Bums13 found that salts of secondary and tertiary aromatic amines are efficient accelerators for the peroxide-initiated polymerization, but form colored products. Fast-setting products may be obtained from preformed polymeric powder containing initiator and salt of a difficultly sol uble sulfinic acid. The powder is mixed with monomer containing 0.5 to 2 per cent of a qua ternary ammonium salt and a few per cent of an acid, such as acrylic, methacrylic, or phos phoric acid.1416 The polymerization is greatly ac celerated in the presence of 0.001 to 0.0Ö01 per cent of Cu++ or iron salt,17 methanol,18 or water. 1154 ■ JA D A , Vol. 72, M ay 1966
Direct-filling resin materials shrink approxi mately 6 to 8 per cent in volume during poly merization. The shrinkage can be made to take place somewhat directionally so that most of the shrinkage will be manifested on the outside sur face of the restoration rather than at the inter face between the filling and the cavity. The hardened material contains around 2 per cent of unreacted monomer. Presence of higher con centration of monomer in the set mass decreases the strength of the filling resin. More serious are the high coefficient of thermal expansion and absence of bonding to tooth structure. The large difference in the coefficients of thermal expansion of the tooth and of the restoration creates a crevice at the margins into which fluids and gases ebb and flow with changes in temperature. The presence of bacteria in these crevices is consid ered a major cause of the recurrent decay com monly observed around or beneath plastic fill ings. To decrease the thermal coefficient of ex pansion, fillers such as fiber glass, silica, alumi num oxide, or finely ground ceramic powder have been incorporated into the resin,19 20 but this addi tion often results in products that have lower im pact resistance and are difficult to polish. Good results have been obtained by using vinylsilanetreated fused silica particles with a monomer consisting of about 80 per cent of an adduct of bisphenol A and glycidyl methacrylate, 10 per . cent tetraethylene dimethacrylate, and 10 per cent me.thyl methacrylate.20* As can be seen from Table 3, such resin fillings shrink less on hard ening, are stronger and stiffer, and have a much lo\yer coefficient of thermal expansion than the commonly used direct resin fillings. Recently, a direct filling resin of a similar chemical com position that contains about 70 per cent inor ganic filler (glass spheres and rods) has been introduced. This material is reported to have a coefficient of thermal expansion of 45. ppm/de gree Celsius. N,N-dimethyl-i>im-xylidene is an efficient accelerator for reinforced acrylic resins and yields products showing improved color stability.2011 Much thought has been given to the develop ment of a restorative filling material that ad heres well to the tooth structure.20“ The inherent difficulty in bonding resin to tooth structure is generally recognized to be caused primarily by the moisture at the cavity walls, which cannot be completely removed, as well as the porosity and nonhomogeneity of the surface. An inter mediate impermeable layer of a cementing me-
dium , which m ust adhere to b o th too th struc ture and the restorative resin, seems the m ost prom ising approach. A n adduct of N -phenylglycine and glycidyl m ethacrylate has shown significant bonding betw een a direct filling resin and dentin and enam el surfaces after soaking in w ater for prolonged periods of tim e.21 M uch basic inform ation is needed, however, regarding the tooth surface and how to w et this surface under clinical conditions w ith a suitable adhesive before significant advances can be m ade.
Acrylic crown and bridge prostheses G reater rigidity is required of an inlay, crown, o r fixed p artial denture to w ithstand th e forces of m astication than is necessary for rem ovable appliances. A crylic and vinyl-acrylic resins are used with varying results in place of fused p o r celain for crown and bridge prostheses. Because of their low strength, their tendency to flow and deform perm anently u nder the forces of m astica tion, and their poor abrasion resistance these resins should be em ployed prim arily as tem porary crow ns or in protected areas. A crylic resins have been used quite successfully for veneers on gold jacket crowns where the m aterial is n o t subject to high occlusal stresses. M ost failures have been caused by differences in the coefficient of therm al expansion of gold alloys and acrylic resins, non adhesion to gold alloys, discoloration, staining, and poor abrasion resistance.
Miscellaneous applications Inlay patterns can be p repared from acrylic plas tics. T he finished casting is n o t superior to one produced from a wax pattern, and the technic has not aroused m uch interest. C old-curing plas tics are used in m aking contoured im pression trays. These resins contain substantial quanti ties of fillers th at increase the rigidity of the m aterial after setting. O rthodontists m ake ex tensive use of plastics in such specialized appli cations as splints, tem porary space m aintainers, and bite planes.
Other plastics The shortcomings of plastic acrylic filling m a terials have led to suggestions th at they be re
placed by other polym eric systems.20’22 T he follow ing discussion will b e lim ited to plastics th at have been investigated in detail and show some prom ise; that is, the polystyrene, epoxy, azirino, polyam ide, polycarbonate, and sem iorganic m a terials. M ost o f these products should be con sidered strictly experim ental m aterials. ■ P o lystyren e: Styrene resins are available com m ercially for m aking dentures by ah injection molding technic. C om parison of the properties of polystyrene and acrylic resin denture m ate rials are given in T able 2. Clinically, they give equally satisfactory results when used in m aking acrylic dentures. ■ E p o x y resins: R esearch to develop dental resins based on epoxy resins was started around 1935. A lthough m any properties of fully cured resins would be desirable for a dental prosthetic m a terial, especially their adhesive strength and lo\4 curing shrinkage, the clinical results have been quite disappointing. T he m ain problem s’ have been the toxicity of the accelerators, which are necessary for rapid hardening. The high w ater sorption and solubility and poor dim ensional stability reported for epoxy denture base m a terial result from incom plete curing of the m a terial. In 1958,23 an epoxy resin filling m aterial was developed th at used a m etal chloride ca ta lyst. T he dry resin possessed good physical p ro p erties. T he apparent advantage of the less than 1 per cent curing shrinkage of the epoxide resins is outweighed by the inferior physical properties obtained after prolonged w ater im m ersion. In particular, the cure of these resins appears to be inhibited by the presence of w ater and, th ere fore, tacky surfaces w ere obtained w hen in direct contact with natu ral tooth dentin. Epoxy resins have been suggested for crow n and bridge w ork. Initially they adhere well to gold and other dental m aterials, and it is claim ed that they reduce seepage and darkening of glue lines between gold and jacket o r crow n. This bond is often dissipated in time by the high w a te r sorption of these resins. Epoxy resins . have slightly higher com pressive strength, stiffness, and abrasion resistance to toothbrushing, b u t their scratch resistance and hardness is not greater than that of acrylic m aterials.24-25 Because o f handling difficulties, the epoxy m aterial gives m ore variable; results than other resins. T he color stability of epoxy resin is not very satis factory, and it is heavily stained by lipstick. Brauer: D E N T A L A P P L IC A T IO N S O F PO L Y M E R S « 1 1 5 5
■ P o ly este r resins: A n interesting experim ental resin fo r anterior restorations is an unsaturated polyester such as an adduct of maleic anhydride and 1,3-butanediol containing aziridino (a,/3ethyleneim ino) groups.19'26’27 This m aterial is supplied as a paste. T o initiate polym erization, a sulfonic acid ester (4 p er cent m ethyl p-toluenesulfonate) is added, also in the form of a paste. T he slurry hardens in two phases in an involved m ultipolym erization process. T he aziridino groups are som ew hat hydrophilic. A n excess of catalyst also produces excessive expansion. T he substance contains n o filler and after hardening is highly crosslinked. T he coefficient of therm al expan sion is about the same as th at of acrylic resins, and m arginal percolation of fluids can be ob served. T h e aziridino polyester is slightly softer th a n acrylic resin and has som ew hat low er com pressive strength. T h e w ater sorption is m uch higher th a n that of acrylics and results in a linear expansion of over 1 p er cent after 35 days. This expansion is m ore than three tim es the linear polym erization shrinkage of this m aterial a t body tem perature. Originally, it was hoped th a t the w ater absorption of the resin w ould im prove the fit in the m outh. W ith therm al ex pansion causing the m arginal percolation of fluids aroun d the cavity edge, however, w hatever ad vantage w ater absorption m ight have offered is lost. U nless the m aterial is used w ith a base, it causes pathological reactions in the pulp. A polyam ide N ylon 11 which is the polym er of am inoundecanoic acid has found lim ited use in partial denture construction. I t appears, how ever, th a t high w ater sorption causes undesirable dim ensional changes. A lso, the m aterial is som e w hat soft. ■ P olycarbon ate: P olycarbonate absorbs only 0 .3 6 p e r cent w ater and hence has good dim en sional stability. Its therm al coefficient of expan sion is 70 x 10'6/° C . I t is claim ed to be highly abrasion resistant and color stable. In G erm any, it has been used as a denture base resin. Because of its high m elting point, it m ust be m olded at tem peratures ranging from 2 8 4 to 3 2 0 °F . (1 4 0 to 1 6 0 °C .). T he m olding shrinkage am ounts to 0.005 to 0.007 in ch /in ch . A postm olding shrink age has not been observed. Because of the tem peratures involved, only porcelain teeth m ay be safely invested into polycarbonate denture base. , Jarb y and A nderson28 have described a technic for using polycarbonate for anterior fillings. T he m aterial, in the form of dry sticks, is inserted .1156
■ JA D A , Vol. 72, M ay 1966
into an electrically heated syringe th a t has a plas tification cham ber, kept betw een 284 to 3 0 2 °F . (1 4 0 to 1 5 0 °C .), and a w ater-cooled jacket. The cavity should, if possible, be prepared in the form of round excavations. A 10 per cen t solu tion o f polycarbonate in chloroform is used as a cavity liner to im prove retention of fillings. D eep cavities are lined w ith phosphate cements. H e at ing of the plastic takes about 30 seconds, and the h o t m aterial is directly injected into the cavity. A lthough the highly plastic m aterial is still ex trem ely hot, it is claimed th at the therm al strain on the pulp by the filling is not greater th a n th at of introducing h o t therm oplastic im pression m a terials. Polishing of the filling can be d o n e at once, after the injection, since the m aterial solidi fies nearly instantaneously. The m arginal accur acy is claim ed to be excellent. Since the coeffi cient of therm al expansion is only slightly low er th an th a t of acrylics and addition of fillers that are n o t free-flowing is impossible, it will b e in teresting to see if the good m arginal accuracy will be m aintained o r if percolation will; eventu ally ru in the m arginal seal. Because of the fnany problem s to be solved, especially the w isdom of injecting a m aterial kept over 300°F . ( 15 0 °C .) into a cavity, the polycarbonate should be. con sidered a highly experim ental filling m aterial. A s has been indicated, all the organic polym ers have coefficients of therm al expansion ranging from 70 to 90 x 10‘6/° C . Incorporation of fillers reduces this value. Perhaps a b etter approach to reduce the therm al expansion w ould be th e use o f a resin containing an inorganic backbone or a sem iorganic polym er. Such a m aterial is likely to be m ore com patible with tooth structure, which contains inorganic hydroxyapatite as the m ain constituent. A great deal of w ork is being done in the field of chelation polym ers, especially to obtain highly heat-resistant resins. Thus far, only low m olecular weight coordination polym ers have been synthesized, which do not possess desirable physical properties. F u rther progress in this field, however, m ay lead to polym ers having properties th at w ould m ake them logical candidates for fu rth er study as filling resins.
Elastomeric materials T here is a great need for a satisfactory resilient liner fo r com plete dentures. M any of th e pres ently available liners are highly plasticized vinyl chloride o r m ethacrylate resins th at do n o t re
m ain resilient because of leaching out of the plasticizer, p oor abrasion resistance, peeling from the denture base, o r high w ater sorption. M ix tures of acrylates o r m ethacrylates th at are poly m erized in the presence o f a vinyl stearate-vinyl acetate copolym er have also been suggested.29 Soft liners m ade from silicone rubber, however, m aintained their resiliency for as long as 5 years in clinical trials. W ith th e use of p ro p er prim ers and im proved form ulations, th eir m ain problem o f lack of perm anent adhesion to denture base appears to have been overcom e.30’31
Silicone rubber base impression materials T he m ain ingredient of th e silicone base m ate rials is a Unear diorganopolysiloxane such as poly (dim ethyl siloxane) containing term inal hydroxyl groups an d a few p e r cen t of a second siloxane, fo r exam ple, trim ethyl siloxane-end-blocked m eth yl hydrogen polysiloxane o r m ethyltriacyloxysilane. Curing is effected w ith an organom etallic com pound, such as 1 to 2 p er cent dibutyltin dilaurate, dioctyltin m aleate, 0.2 p er cent tellu rium diethyldithiocarbam ate, tetram ethylthiuram disulfide, o r sulfur in polyethyl silicate contain ing 50 p er cent ethoxy groups.32’33 Inorganic fillers, such as zinc oxide, m agnesium oxide, or calcium carbonate o r zinc carbonate, also speed up the setting reaction. O rthosilicic acid esters, especially m ethyl or ethyl o rth o polysilicates are used m ost com m only as crosslinking agents, but other organopolysiloxanes, organohydropolysiloxanes, and silanes w ith m ore than tw o reactive functional groups can also b e em ployed for this purpose. T he chief use of the silicone im pression m a terials is fo r crow n an d bridge w ork. T he cur ren t brands of silicone elastom er do not evolve hydrogen gas or exhibit tack and porosity be cause of incom plete polym erization. T he shelf life on storage at elevated tem peratures also has been greatly im proved during the last few years. T h e contraction of silicone elastom ers on cooling from m outh to room tem perature (2 2 °C .) is ab o u t 0.35 p er cent com pared w ith 0.20 p er cent fo r polysulfide ru b b ers.34 T he latter-type m ate rials allow m ore latitude in w orking tim e than the silicone m aterials and appear to be the pre ferred m aterial w hen m ultiple inlay o r crown im pressions are taken. A b o u t 15 p er cent of all single tooth and fixed bridge im pressions and 5
p er cent of all im pressions for dentures are m ade w ith silicone im pression m aterials.
Polysulfide rubber base impression materials Polysulfide ru bber base m aterials are used p ri m arily in inlay and crown and bridge w ork. T he basic ingredient is a polyfunctional m ercaptan th a t form s a rubberlike m ass in the presence of accelerators, such as lead peroxide o r other oxi dizing agents and sulfur. Traces of organic am ines also are claim ed to speed up the reac tion. In ert fillers, such as zinc oxide and calcium sulfate, are added to modify the viscosity of the mix and to give strength and color to the set im pression. T hese m aterials are reliable and give strong, relatively stable im pressions. The m ain disadvantages are the extrem e stickiness of the freshly m ixed paste, its odor, and the stains it will produce if spilled on clothing. It is estim ated th at polysulfide ru b b er is employed in about 35 p e r cent of all fixed bridge w ork and 13 p e r cent of all final im pressions used for m aking dentures.
Summary Plastics are em ployed successfully in m any den tal applications. T heir properties m ake them espe cially valuable in the construction of dentures, plastic teeth, and im pression m aterials. Plastic filling m aterials have not been successful because of their high coefficient of therm al expansion and lack of bonding to tooth structure. P erhaps the biggest im provem ent in filling m aterials w ould be the developm ent of a satisfactory cavity liner th a t bonds to tooth structure and is im pervious to the m outh fluids. If such a liner w ere available, the need for a filling m aterial th a t m atches the therm al expansion o f the tooth w ould be less critical.
This investigation is part of the dental'research pro gram conducted by the National Bureau of Standards, in cooperation with the Council on Dental Research of the American Dental Association; the Arm y Dental Corps; the Dental Sciences Division of the School of Aerospace Medicine, U SA F; the National Institute of Dental Re search, and the Veterans Administration. Doctor Brauer is a chemist in the Dental Research Section, Institute of M aterials Research, National Bureau of Standards, Washington, D.C. 1. Bureau of Economic Research and Statistics, AmerBrouer: D E N T A L A P P L IC A T IO N S O F PO LY M ER S " 1 1 5 7
icon Dental Association. 1959 survey of dental practice. X I. Dental services rendered. JA D A 62:627 M ay, 1961; X II. Summary. JA D A 62:764 June, 1961. 2. Bureau of Economic Research and Statistics, American Dental Association. Survey of public attitudes regarding dentures. III. Dentures worn; source of den tures. JA D A 62:226 Feb., 1961. 3. Peyton, F. A., and others.- Restorative dental materials, St. Louis, C. V. Mosby Co., 1960. 4. W oelfel, J. B., Paffenbarger, G. C., and Sweeney, W . T. Some physical properties of organic denture base materials. JA D A 67;489 Oct., 1963. 5. W oelfel, J. B., Paffenbarger, G. C., and Sweeney, W . T. bimensional changes occurring in dentures dur ing processing. JA D A 61;413 Oct., I960. 6. W oelfel, J. B., Paffenbarger, G. C., and Sweeney, W . T. Changes in dentures during.storage in water and in service. JA D A 62:643 June, 1961. 7. Peyton, F. A., and Anthony, D. H. Evaluation of dentures processed by different techniques. J . Pros. Dent. 13:269 March-Apcil, 1963. 8. M irza, F. D. Dimensional stability of acrylic resin denture's: clinical evaluation. J . Pros. , Dent. 1 1 :848 Sept.-Oct., 1961. 9. Brauer, G, M ., Davenport, R. M.., and Hansen, W . C. Accelerating effect of amines on polymerization of methyl methacrylate. Mod. Plastics 34:no. 3:153 Nov., 1956.."' / ,10. Rose, E. E.; Lai, Joginder, and Green, Richard. Effects of peroxide, amine and hydroquinone in varying concentrations on the polymerization rate of polymethyl methacrylate slurries. JA D A 56:375 March, 1958. 11. Brederick, H., Bäder, E., Wohnhas, A. a-Oxy-, a-Amino-sulfone und sulfinsaure Salze organischer Basen als Polymerisationskatalysatoren. Makromolek. Chem. 12:100 Dec., 1953. ' • 12. Brederick, H:, and Bäder, E. Polymerization prod ucts with accelerators containing at least one aminomethyl sulfone group. U. S. patent 2,750,357 Jun e 12, 1956. 13. Brauer, G. M., and Burns, F. R. Sulfinic acid derivatives as accelerators in the polymerization of methyl methacrylate. J. Polymer Sei. 19:31 1 Feb., 1956. 14. Kulzer and Co. G.m.b.H. (by Carlo Rossetti) Vinyl polymers for dental fillings. German patent 1,123,824 Feb. 15, 1962. 15. Brederick, H., and Bäder, E. Salts of sulfinic acids as polymerization catalysts. U. S. patent 2,846,418 Aug. 5, 1958. 16. Bäder, E., Schweitzer, O., and Querfurth, W . Polymerization of vinylidene compounds with organic peroxide, organic sulfur and quaternary ammonium com pounds as catalysts. U. S. patent 2,946,770 Ju ly 26, 1960. 17. Brederick, H., and others. Polymerization of un saturated compounds with sulfone-contaimng accelera tors. U. S. patent 2,779,751 Jan . 29, 1957. 18. Brederick, H., Bäder, E., and Wohnhas A. Poly merization accelerators or catalysts. U. S. patent 2,758,106 Aug. 7, 1956. 19. McLean, J. W . Anterior filling materials in Europe. D. Progress 2:181 April, 1962. 20. M cLean, J. W . The future of dental restorative materials. Rev. Beige Med. Dent. 20: no. 3:277, 1965.
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20a. Bowen, R. L. Properties of a silica-reinforced polymer for dental restorations. JA D A 66:57 Jan ., 1963. 20b. Bowen, R. L., and Argentar, H. Color forma tion in methacrylate accelerator systems (abstract) . J. D. Res. IA D R Program and Abstracts of papers. July, 1965, p. 83. 1 20c. Phillips, R. W ., and Ryge, G., editors. Adhesive restorative dental materials. Proceedings from workshop sponsored by Dental Study Section, National Institutes of Health. Indianapolis, Indiana University School of Dentistry, 1962. 21. Bowen, R. L. Adhesive bonding of various m ate rials to hard tooth tissues. II. Bonding to dentin pro moted by a surface-active comonomer. J. D. Res. 44:895 Sept.-Oct., 1965; III. Bonding to dentin im proved by pretreatment and the use of surface-active comonomer. J. D. Res. 44:903 Sept.-Oct, 1965; IV. Bonding to dentin) enamel, and fluorapatite improved by the use of a surface-active comonomer. J . D. Res. 44:906 Sept.-Oct., 1965; V. The effect of a surfaceactive comonomer on adhesion to diverse substrates. J . D Res. 44:1369 Nov.-Dee., 1965. 22. Smith, D. C. Recent developments and pros pects in dental polymers. J. Pros. Dent. 12:1068 Nav.Dec., 1962. 23. Ley, J. B. Investigation into epoxy resin systems with dental application. J. D. Res. 43:950 Sept.-Oct., 1964. Abstract. 24. Peyton, F. A., and Craig, R. G. Current evalua^ tions of plastics in crown and bridge resins. J. Pros. Dent. 13:743 July-Aug., 1963. 25. Kafalias, M . C., Swartz, M. L., and Phillips, R. W . Physical properties of selected dental resins. Pact I. J. Pros. Dent. 13:1087 Nov.-Dee., 1963. 26- ESPE Fabrik Pharmazeutischer Präparate G.m.b.H. (by W . Schmitt, R. Purrmann and P. Jo ch u m ). Use of cross-linkable organic polyethyleneimine compounds for the preparation of self-curing plastics suitable for the filling of dental cavities and for attaching dentures. Ger man patent 1,146,617 April 4, 1963. 27. ESPE Fabrik Pharmazeutischer Präparate G.m.b.H. (by W . Schmitt, R. Purrmann, P. Jochum and W . D. Z a h le r ). Curing, molding or coating compositions con taining polymers with ethyleneimine groups. German patent 1,166,471 M arch 26, 1964. 28. Jarby, Sven, and Andersen, Elsebeth. Dental fill ings of polycarbonate. J. D. Res. 41 :214 Jan.-Feb., 1962. 29. Landry, L. A . Soft, flexible resinous compositions for dentures and prosthetic appliances. U. S. patent 3,084,436 April 9, 1963. 30. Barnhart, G. W . Silicone materials for lining dentures. D. Progress 3:246 July, 1963. 31. Silastic 390, Soft liner evaluation report. M ed ical and Pharmaceutical Products Division, Dow-Coming, Midland, Mich. 32. Nitsche, S., and W ick, M. Vulkanisationssysteme des Silikonkautschuks. Kunststoffe 47:431 Aug., 1957. 33. W acker-Chemie G.m.b.H. (by S. Nitzsche and M. W ic k ) . Organopolysiloxane molding materials as dental impression molds. German patent 1,163,021 Feb. 13, 1964. 34. M cLean, J. W . Physical properties influencing the accuracy of silicone and thiokol impression m ate rials. Brit. Dent. J. 110:85 Feb. 7, 1961.