Influence of Ketones on Selected Mechanical Properties of Resin Composites A. PEUTZFELDT and E. ASMUSSEN Department of Dental Materials, School of Dentistry, Faculty of Health Sciences, University of Copenhagen, 20 Nbrre All, DK-2200 Copenhagen, Denmark

The present study investigated a concept for additional crosslinking of dental polymers, by which resistance to wear of resin composites might be increased. Bifunctional ketones were added to monomer mixtures, which were then made light-curing and loaded with filler. The monomer mixtures were varied with respect to type and ratio of monomer and ketone. For measurement of possible effects ofthe cross-linking agents added, four mechanical properties ofthe experimental resin composites were determined. Addition of the bifunctional ketone diacetyl resulted in the following increases in mechanical properties: diametral tensile strength, 11%; flexural strength, 29%; modulus of elasticity, 19%; and modulus of resilience, 50%. J Dent Res 71(11):1847-1850, November, 1992

Introduction. Inadequate resistance to wear in areas of occlusal contact constitutes one ofthe main limitations of posterior composite restorations (Lutz et al., 1985; Leinfelder et al., 1986; Roulet, 1988). One possible way the wear resistance of resin composite restorations could be improved is for the degree of conversion of the monomer system to be increased. Thus, in order to minimize the quantity of remaining, unreacted methacrylate groups, M6rmann (1982) and Lutz et al. (1987) suggested that the resin composite be post-cured at elevated temperatures and/or in high-intensity light. Subsequent studies which measured the effects of these methods of additional curing gave both corroborative (Wendt, 1987, 1989; Kullmann, 1988; Reinhardt and Smolka, 1988; Peutzfeldt and Asmussen, 1991a) and invalidating results (Reinhardt and Smolka, 1988; Peutzfeldt and Asmussen, 1991a). Addition of cross-linking agents to the monomer mixture might increase the degree of cross-linking and render the resin composite more wear-resistant. Bifunctional ketones such as diketones and vinyl ketones are one group of potential cross-linking agents and may react in several ways. In so-called Michael additions, ketones react with o43,-unsaturated carbonyl compounds (Solomon, 1988). Because of incomplete conversion, all polymerized resin composites contain such otp,unsaturated carbonyl compounds in the form of unreacted double bonds. As shown in the Fig., a diketone could thus be used to crosslink two polymer chains, each having an unreacted double bond. It is also possible that each carbonyl group of a diketone could react with an aldehyde group of a copolymerized monomer in a ClaisenSchmidt manner (Solomon, 1988), resulting in the cross-linking of two polymer chains. If it is assumed that ketones can react in the same manner as aldehydes, a third cross-linking reaction may be hypothesized if the polymer has pendant amide groups (Temin, 1965). According to this mechanism, the diketone reacts with pendant amide groups of a copolymerized monomer. The hydroxy groups formed by this process might also participate in crosslinkingreactions (Temin, 1965). Finally, otherbifunctionalketones such as methyl vinyl ketone might also be able to act as cross-linking agents by making use of two different addition reactions: The vinyl ketone can react (1) by copolymerization with the growing polymer Received for publication August 26, 1991 Accepted for publication May 8, 1992

chain and (2) by a reaction similar to that depicted in the Fig. The mechanisms described are nucleophilic reactions, but cross-linking reactions involving free radicals are also conceivable, e.g., keto-enol tautomerism forms double bonds (Solomon, 1988), which may give rise to additional cross-linking. Photochemical reactions have been described in which ketones can add to double bonds (March, 1977). Finally, diketones are used as initiators in resin composites in conjunction with reducing agents (Ruyter, 1985). Upon irradiation, radicals are formed which may increase the degree of polymerization. The purpose of this work was to test the cross-linking potential of several bifunctional ketones by measuring the effect of their addition on some selected mechanical properties of experimental resin composites.

Materials and methods. The resin composites used in this investigation were prepared from the compounds listed in Table 1; their compositions (in mol-%) are given in Table 2. The monomers used were UEDMA and HEMA. In an earlier study (Peutzfeldt and Asmussen, 1991b), resins based on these two monomers showed better mechanical properties than did resins based on BISGMA and TEGDMA. Furthermore, UEDMA contains two urethane groups which might take part in the crosslinking reactions. In order for the materials to be made light-curing, 0.2% w/w of CQ and 0.2% w/w of CEMAwere dissolved in each ofthe monomer mixtures. Following addition of the potential crosslinking additives, the resins were loaded with silanized glass filler (Type GM 31 685, Schott-Schleiffer AG, Feldbach, Switzerland) to a content of 80% w/w. The resin composites were tested with respect to (1) diametral tensile strength, (2) flexural strength, (3) modulus of elasticity, and (4) modulus of resilience. Unpolymerized material was applied in molds, covered on both sides with a clear matrix strip, and irradiated with a Visilux 2 unit (3M Company, St. Paul, MN) for 40 s on each side. The specimens were then placed in water at 370C for one wk prior to being tested. Diametral tensile strength.-The unpolymerized material was placed in a cylindrical brass mold (h = 3.0 mm, d = 6.0 mm). The specimens were light-cured and stored as described. After water storage, the specimens were ground on wet silicon carbide (grit No. 1000). Length (1) and diameter (d) of the ground specimens were recorded, and the specimens were fractured in the diametral position in a Universal Testing Machine (Instron Ltd, High Wycombe, England) operating at a cross-head speed of 1 cm/min. The diametral tensile strength T was calculated from T = 2F/(ir.d.1), where F is the force at fracture. Flexural strength.-The unpolymerized material was placed in a rectangular brass mold (1 = 10 mm, h = 2.0 mm, and w = 2.0 mm). The specimens were polymerized, stored, and ground on all four sides. The height (a) and the width (b) of the ground specimens were measured, and the specimens were subjected to three-point loading at a cross-head speed of 1 mm/min, with 6 mm (c) between the supports. The flexural strength S was computed as S = (3.c.F)/ (2.b.a2), where F is the force at fracture. Modulus of elasticity.-The flexural strength specimens were also used for determination of the elastic moduli. A chart paper speed of 500 mm/mi was used in measurements offlexural strength; the relationship between applied force and movement of the cross-

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1848 PMONO


J Dent Res November 1992


h'S Xsoj



Fig.-Michael additions between diacetyl and two methacrylate groups. The two monomer chains are cross-linked by formation of carbon-carbon bonds. PMONO = polymer with pendant methacrylate groups. DIAC = diacetyl.

head was found to be approximately linear. Straight lines were fitted by hand to the curves on the chart paper, and the slopes of these lines were calculated. However, for determination of the relationship between force and deflection of the specimens, the deformation taking place within the measuring cell must be accounted for. Therefore, calibration curves were produced by use of rods of electrolytically pure copper, having a modulus ofelasticity of 129.8 GPa(Kaye and Laby, 1953), asdescribedindetailbyAsmussen and Peutzfeldt (1990). Modulus of elasticity E was calculated from the relationship, E = (a.c3)/(4.b.a3), in which is the slope of the straight-line relationship between force and deflection ofthe specimen, c is the distance between the two supports (6 mm), and a and b are as defined above. Modulus of resilience.-For each specimen, the modulus of resilience was calculated as R = S2/2E. The measurements of T and S were done on eight specimens of each resin composite. The statistical treatment of the results involved one-way analyses of variance (Hald, 1952) and NewmanKeuls' Multiple Range Tests (Bruning and Kintz, 1977), with p = 0.05 as the level of significance. a

Results. The mean values of T, S, E, and R are presented in Table 2. The standard deviations of the four properties were pooled to give SDT =2.9 MPa, SD, = 16.3 MPa, SDE = 1.00 GPa, and SD = 0.42 MJ/m3. Within the first group of comparison (K1-K9), which shows the effects of MAAM and of 4 different ketones, significant differences occurred in the 9 mean values of each mechanical property. From the results of the statistical analyses also shown in Table 2, K4 and K5 had higher T than the other 7 resin composites. With respect to S, K1 gave a lower value than K2, KS, K8, and K9. E was lower for K2, K6, and K8 than forK3, K5, and K9. For R, K1 gave a lowervalue than K2 and K8. In K10-K14, the relative contents of DIAC and MAAL were varied. The 5 mean values of modulus of elasticity and the 5 mean values of modulus of resilience did not differ with statistical


K14 had a lower T than the other 4 resin composites and a lower S than K10. cance.

Discussion. The first part of the study investigated the effects of different ketones on certain mechanical properties of resin composites with orwithoutMAAM as part ofthe monomer system. Addition of8 mol% ketone to monomer mixtures led to significant improvements in mechanical properties in 25% of the cases, but no one ketone gave consistently superior results. Thus, admixture of DIAC improved the diametral tensile strength of both the MAAM and the nonMAAM resin composites; ACET, DIAC, and VINYL improved the flexural stength of one or both types of resin composites; no significant effect of ketone addition was found on modulus of elasticity; and, finally, ACET and VINYL gave higher modulus of resilience of the non-MAAM resin composite. Thus, our assumption that addition of ketones to resin composites might increase the degree of cross-linking was partly corroborated. In only two out of 16 cases did MAAM-containing resin composites show increases in mechanical properties as compared with the correspondingnon-MAAM-containingresins. It consequently seems that PMAAM (defined as a copolymer with pendant amide groups)


Compound Urethane dimethacrylate* 2-Hydroxyethyl methacrylate Camphorquinone N,N-cyanoethyl-methylaniline Methacrylamide




Ivoclar AG, Schaan, Liechtenstein


Merck-Schuchardt, Miinchen, Germany


EGA-Chemie, Albuch, Germany Ivoclar AG Merck-Schuchardt



Aldrich-Chemie, Steinheim, Germany

Acetylacetone Diacetyl 2,5-Hexanedione Methyl vinyl ketone Silanized glass filler *Ruyter (1985).









GM 31685

Schott-Schleiffer AG, Feldbach, Switzerland

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Vol. 71 No. 11

did not, to a significant degree, react as hypothesized in the "Introduction". Therefore, in our further work we did not include MAAM in the monomer mixtures. In the second part of the investigation, the effect of MAAL and ofa possible interplay between PMAAL (defined as a copolymer with pendant aldehyde groups) and DIAC was studied. Resin composites containing only DIAC (K10) or DIAC as well as MAAL (K11-K13) had mechanical

properties of the


magnitude. Thus, replacing

of the ketone by MAAL did not compromise the mechanical properties. This suggests that a cross-linking reaction between DIAC and PMAAL possibly complemented the reaction of the Fig. some

Code K1 K2 K3 K4 K5 K6 K7 K8 K9 K10 K11 K12 K13 K14


between DIAC and PMONO (defined as a copolymer with pendant methacrylate groups). In contrast to the effect of the bifunctional compound DIAC, the polyfunctional PMAAL was not able to increase the degree of cross-linking by itself, as evidenced by formulation K14, which gave a decrease in mechanical properties as compared with the DIAC-containing resin composites. In an effort to clarify the effect of ketone addition further, we increased the content of DIAC from 8 mol-% to 20 mol-%. This change in composition imparted substantial improvements in the mechanical properties. Compared with the control material (K1), K10 showed the following increases in mechanical properties: dia-


20 23 20 20 20 20 20 20


8 8 8


20 15 10 5

5 10 15 20

Results S (MPa) 156a

E (GPa)


6.3a 8.1c 7.3abc



T (MPa) 63ab* 59ab 60ab 67c 65c







2. lab

K7 K8 K9 K10 Kll K12

57a 6 lab 60ab


6.9ab 6.5a 8.0bc 8.7a 8.8a

K13 K14

69b 61a

178bc 184c 201b 189ab 180ab 193ab 172a

1.9ab 2.5b 2.2ab

Code K1 K2

K3 K4

70b 7lb 7 lb

174abe 167abc 178bc


8.8a 7.5a

R (MJ/m3) 1.6k 2,5b



2.1a 1.9a 2.5a

7.7a2"0a *Values with same superscript within each group of comparison (K1-K9 and K10-K14) were not significantly different. Downloaded from at CARLETON UNIV on July 8, 2015 For personal use only. No other uses without permission.



metral tensile strength, 11%; flexural strength, 29%; modulus of elasticity, 19%; and modulus of resilience, 50%. In the "Introduction', several possible mechanisms were outlined by which ketones may bring about an increased degree of cross-linking of resin composites. The first four mechanisms suggested are nucleophilic reactions, which normally require a strong base as catalyst to take place with appreciable rate. The investigated monomers are not basic, but the photoreductant CEMA is a base. It may be argued that the cross-linking reactions should also take place in the uncured resin composite, leading to an increase in viscosity of the paste. This was not observed. It may be speculated that a catalytic effect of the CEMA is more pronounced in an aqueous environment. Therefore, the catalytic effect does not occur in the non-aqueous monomer systems, but may be present in a resin composite imbued with water. However, CEMA is not a strongbase, and although a nucleophilic cross-linking mechanism may be a possibility, it is probably a remote one. The other mechanisms outlined in the 'Introduction" involve free radicals. Ketones undergo keto-enol tautomerism, and certain diketones may exist in the enol form to an appreciable extent (Solomon, 1988). This form, containing a double bond, may participate in the polymerization reactions ofthe methacrylate monomers and conceivably lead to an increased degree of conversion. Further, when irradiated, ketones may form free radicals (March, 1977). These radicals may either initiate polymerization or add to the carbon-carbon double bonds of the methacrylate groups (March, 1977). The wavelengths required for these processes to occur are normally in the UV-visible-light region. By way of example, camphorquinone is a diketone, and, in the presence of a photoreducing agent like CEMA, it produces free radicals when irradiated in the visible-light range. Thus, it is possible that the ketones investigated in the present study may react in a similar manner. Summing up, it may seem that the more likely explanation of the observed results is that they stem not from nucleophilic reactions but from free radical processes. In conclusion, the results of the present study lend support to the hypothesis that certain bifunctional ketones can act as cross-linking agents. Further work is required, however, to determine other properties ofthe experimental resin composites and to elucidate the mechanisms of the proposed cross-linking reactions. REFERENCES Asmussen E, Peutzfeldt A (1990). Mechanical properties of heat treated restorative resins for use in the inlay/onlay technique. Scand JDent Res 98:564-567.

J Dent Res November 1992

BruningJL, KintzBL(1977). Computational handbook of statistics. 2nded. Glenview (IL): Scott, Foresman & Co. Hald A (1952). Statistical theory with engineering applications. New York: John Wiley & Sons, Inc. Kaye GWC, Laby TH (1953). Tables of physical and chemical constants. 12th ed. London (UK): Longmans, Green & Co., Ltd. Kullmann W (1988). Extraorale photopolymerisation zur optimierung physikalisch-technischer merkmale von fiillungskunststoffen. Dtsch Zahndrztl Z 43:383-385. Leinfelder KF, Taylor DF, Barkmeier WW, Goldberg AJ (1986). Quantitative wear measurements of posterior composite resins. Dent Mater 2:198-201. Lutz F, Imfeld T, Phillips RW (1985). P-10-Its potential as a posterior composite. Dent Mater 1:61-65. Lutz F, Krejci I, Mormann W (1987). Die zahnfarbene seitenzahnrestauration. Phillip J 4:127. March J (1977). Advanced organic chemistry: reactions, mechanisms, and structure. 2nd ed. Tokyo (Japan): McGraw-Hill International Book Company, 220, 221, 889. Mormann W (1982). Kompositinlay: forschungsmodell mit praxispotential? Quintessenz 33:1891-1900. PeutzfeldtA,AsmussenE(1991a). Mechanical properties ofthreecomposite resins for the inlay/onlay technique. J Prosthet Dent 66:322-324. Peutzfeldt A, Asmussen E (1991b). Influence of carboxylic anhydrides on selected mechanical properties of heat-cured resin composites. J Dent Res 70:1537-1541. Reinhardt Yd, Smolka R (1988). Kunststoffe in seitenzahnbereich-Fullung oder inlay? Dtsch Zahnarztl Z 43:909-913. Roulet JF (1988). The problems associated with substituting composite resins for amalgam: A status report on posterior composites. J Dent 12:101-113. Ruyter IE (1985). Monomer systems and polymerization. In: Vanherle G, Smith DC, editors. International symposium on posterior composite resin dental restorative materials. St. Paul (MN): Minnesota Mining and Mfg. Co., 109-135. Solomon TWG (1988). Aldehydes and ketones II. Reactions at the a carbon. Aldol reactions. In: Organic chemistry. 4th ed. New York: John Wiley & Sons, Inc., 783-807. Temin SC (1965). Crosslinking. In: Mark HF, Gaylord NG, Bikales NM, editors. Encyclopedia of polymer science and technology. Vol. 4. New York: John Wiley & Sons, Inc., 331-398. Wendt SL (1987). The effect ofheat used as secondary cure upon the physical properties of three composite resins. II. Wear, hardness, and color stability. Quint nt 18:351-356. Wendt SL (1989). Time as a factor in heat curing of composite resins. Quint Int 20:259-263.

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Influence of ketones on selected mechanical properties of resin composites.

The present study investigated a concept for additional cross-linking of dental polymers, by which resistance to wear of resin composites might be inc...
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