Abstract
Purpose
To evaluate important material properties of six experimental resin-based restorative materials (EXP) with systematically modified resin matrices using conventional and alternative monomers in comparison with an experimental standard (ST).
Materials and methods Commercially available monomers were selected according to their molecular weight, functionality, viscosity, and polymerization shrinkage. ST, 71 wt% filler, matrix UDMA/Bis-GMA/TTEGDMA and six EXPs with modified organic matrices but the same filler content were manufactured. Flexural strength, flexural modulus, water sorption, solubility and polymerization shrinkage of all EXPs were measured and compared with the results of ST.
Results
ANOVA ( p < 0.05) revealed significant differences among the materials and all investigated properties. Bis-GMA and UDMA were substituted by alternative monomers without losing flexural strength or modulus. Replacing the diluting monomer TTEGDMA with alternative monomers resulted in increased flexural strength. None of the experimental products with modified matrices showed increased water sorption or solubility but some even performed better than ST. Increased hygroscopic expansion and reduced shrinkage were achieved using a very hydrophilic monomer but no significant differences of water sorption and solubility in comparison with ST were found.
Conclusion
The results indicated that there are monomers commercially available providing the same or even better properties than conventional matrices.
1
Introduction
Mechanical properties , water sorption, solubility , hygroscopic expansion and polymerization shrinkage are important factors for characterizing resin-based restorative materials. The influence of the resin matrix on the mechanical properties as well as other material attributes, such as water sorption and solubility or polymerization shrinkage is significant. Water sorption and solubility affect strength , abrasion resistance , color stability , biocompatibility and hygroscopic expansion of resin materials. In particular, polymerization shrinkage is still a challenging problem. Many attempts were made to identify less shrinking or even expanding monomers. Successful approaches were made with the ormocer products and the silorane-based products, shrinking around 1% by volume . Also expandable monomers were reported not to compromise the mechanical properties . Substituting triethyleneglycol dimethacrylate (TEGDMA) with other low viscous monomers or substituting Bis-GMA or UDMA or both of them was tried as well. However, none of these approaches were reported to be successfully used in commercial products. It is assumed that specially synthesized monomers for dental purposes only are very complicated and costly to synthesize and therefore, too expensive as raw materials.
Since a broad range of commercial monomers or oligomers, respectively, are available that have not been reported in scientific literature to have been used until now in dental materials, the question arose if some of these monomers/oligomers could be appropriate not only to replace commonly used ones but also to improve material quality. Identifying appropriate commercially available monomers is also relevant in terms of cost effectiveness. The tested alternative monomers were selected according to their molecular weight, functionality, viscosity, and shrinkage. Six experimental resin-based restorative materials were manufactured by systematically replacing conventional monomers with alternative ones and compared with an experimental standard material with a conventional organic matrix (Bis-GMA/UDMA/TTEGDMA). All experimental materials had the same filler content. Flexural strength, flexural modulus, water sorption, solubility, polymerization shrinkage and hygroscopic expansion were investigated. The working hypothesis was that alternative resin matrices can be identified providing: (a) the same or even better mechanical properties, (b) the same or even lower water sorption, solubility, polymerization shrinkage and (c) high hygroscopic expansion compensating shrinkage.
2
Materials and methods
Seven experimental light-curing resin-based restorative materials ( Table 3 ) were manufactured using a laboratory vacuum planet kneader (Herbst Maschinenfabrik GmbH, Buxtehude, Germany). The raw material specifications were obtained from the manufacturers’ technical data sheets ( Table 1 ) and own measurements ( Table 2 ). The chemical structures and molecular masses of the raw materials M4004, 4215, CN435, and CN965 ( Table 1 ) are not exactly defined because they are statistical oligomer mixtures. They are used in many technical applications such as adhesives, lithographic inks, silk screen inks, and many industrial and specialty coatings.
| Code | Product/properties | Batch | Company |
|---|---|---|---|
| Photoinitiator | α.α-Dimethoxy-α-phenylacetophenone | 0066162S | Ciba Specialities Chemical Inc., Basel, Switzerland |
| Stabilizer | Pentaerythrityl-tetrakis[3-(3,5-di-tert.-butyl-4-hydroxyphenyl)]-propionate | 26099IC3 | |
| TTEGDMA | Tetraethylene glycol dimethacrylate, funcionality = 2, MW = 330 g mol −1 , standard monomer, good chemical and physical properties, diluting | J1620 | Cray Valley, Paris, France |
| CN965 | Craynor CN 965, funcionality = 2, MW≈1600 g mol −1 , aliphatic urethane diacrylate, very elastic, very tough, good chemical resistance | KE146801VSP | |
| CN435 | Craynor CN 435, functionality = 3, MW≈1100 g mol −1 , highly ethoxylated trimethylpropane triacrylate, very elastic, water soluble | LG227220 | |
| UV-stabilizer | 2-Hydroxy-4-methoxy-bezophenone | 411351/143302 | Fluka AG, Buchs, Switzerland |
| UDMA | 7,7,9-Trimethyl-4,13-dioxo-3,14-dioxa-5,12-diaza-hexadecan-1,16-diol-dimethacrylate, functionality = 2, MW = 471 g mol −1 , standard monomer, flexible, tough, very good chemical resistance | 330503057 | Rahn AG, Zürich, Switzerland |
| Bis-GMA | Bis-GMA, functionality = 2, MW= 513 g mol −1 , standard monomer, rigid, very good chemical resistance | 2008218303 | |
| 4215 | Urethane acrylate 03-978, functionality = 2, MW≈1500 g mol −1 , aliphatic polyesterurethane diacrylate, very elastic, very tough, very good chemical resistance | 320502047 | |
| M4004 | Miramer M4004, polyetherpolyoltetraacrylate, funcionality = 4, MW≈560 g mol −1 , polyesterpolyol tetraacrylate, very rigid, very high strength, very good chemical resistance, diluting | 80219445 | |
| CQ | d , l -Camphorquinone | 0148990002 | |
| Amine | Ethyl-4-(dimethylamino)-benzoate | 310170 | |
| Glass | Strontiumborosilicate glass (Glass G0 18-093. 0.7 μm). Silaned (3-methacryloyloxypropyltrimethoxy silane) | Lab14701 | Schott Electronic Packaging GmbH, Landshut, Germany |
| Monomer | Viscosity [Pa s] | Density [g cm −3 ] | Shrinkage [vol.%] | |
|---|---|---|---|---|
| Uncured | Cured | |||
| UDMA | 12 (1.8) | 1.11 (0.02) | 1.192 (0,003) | −7.6 (0.2) |
| Bis-GMA | 850 (15) | 1.15 (0.04) | 1.224 (0.002) | −6.3 (0.4) |
| TTEGDMA | 0.014 (0.002) | 1.08 (0.02) | 1.221 (0.006) | −12.7 (0.4) |
| 4215 | 1500 (120) | 1.11 (0.09) | 1.152 (0.002) | −4.1 (0.5) |
| CN965 | 312 (21) | 1.08 (0.03) | 1.145 (0.003) | −5.7 (0.3) |
| CN435 | 0.19 (0.05) | 1.11 (0.03) | 1.186 (0.002) | −6.9 (0.3) |
| M4004 | 0.17 (0.06) | 1.15 (0.03) | 1.281 (0.004) | −11.2 (0.5) |
| Experimental resin-based filling materials. Formulations [wt%] | |||||||
|---|---|---|---|---|---|---|---|
| ST | A | B | C | D | E | F | Raw material |
| 12.789 | 12.789 | 14.239 | 14.239 | 14.239 | 14.239 | 9.889 | UDMA |
| 8.700 | – | 5.800 | 2.900 | – | – | 8.700 | Bis-GMA |
| 7.250 | 7.250 | – | – | – | – | – | TTEGDMA |
| – | 8.700 | 1.450 | 4.350 | 7.250 | – | – | 4215 |
| – | – | – | – | – | 7.250 | – | CN965 |
| – | – | – | – | – | – | 2.900 | CN435 |
| – | – | 7.250 | 7.250 | 7.250 | 7.250 | 7.250 | M4004 |
The standard ST and all experimental materials had the same filler content to investigate the influence of the matrix on the properties. The matrix of ST was modified by replacing Bis-GMA, UDMA and TTEGDMA stepwise or completely, respectively. Bis-GMA was totally replaced in experimental material A by oligomer 4215 (very high molecular mass and viscosity) to see the influence on polymerization shrinkage. However, TTEGDMA was still needed as diluting monomer to keep the material’s processability. In materials B to D TTEGDMA was totally replaced by oligomer M4004 (tetrafunctional, low viscous) and Bis-GMA was stepwise replaced by oligomer 4215. Due to the very high viscosity of oligomer 4215 and the higher viscosity of M4004 compared to TTEGDMA, the amount of UDMA was increased but kept constant to obtain workable consistencies of materials B to D. Material E was formulated to examine if Bis-GMA and TTEGDMA could be totally replaced by oligomer CN965 (very high molecular mass, much lower viscosity compared to Bis-GMA but higher ones compared to UDMA). In material F TTEGDMA was totally replaced by oligomer M4004 and UDMA was partially replaced by oligomer CN435 (low viscosity, high molecular mass, water soluble) to investigate if oligomer CN435 caused hygroscopic expansion high enough to compensate shrinkage. Oligomer M4004 on the one hand was used as diluting monomer on the other hand it was expected to reinforce the matrix due to its tetrafunctionality.
Flexural strength, water sorption, solubility (ISO 4049 ), flexural modulus , polymerization shrinkage and hygroscopic expansion (Archimedes’ principle ) were determined from all materials and the viscosities (Haake Viscotester V7 Plus, Thermo Electron Corp., Karlsruhe, Germany) and the polymerization shrinkages (Archimedes’ principle) of all monomers were also measured. Curing was done with a quartz–tungsten halogen device (Hilux Ultra Plus, Benlioglu Dental Inc., Ankara, Turkey) performing an irradiance of 800 ± 67 mW cm −2 , which was checked periodically with the Curing Light Meter (Benlioglu Dental Inc.).
2.1
Flexural strength, flexural modulus
Ten specimens (25 ± 2 × 2 ± 0.1 × 2 ± 0.1 mm) were made from each material according to ISO 4049 and cured in five 40 s steps from each side (400 s in total). Testing was done after 24 h of water storage at 37 °C with the three-point-bending test (universal testing machine, crosshead speed of 0.75 mm min −1 , Model 106.L, Test GmbH, Erkrath, Germany). Flexural strength was calculated by σ = (3 FL )/(2 bh 2 ) and flexural modulus by E = ( L 3 /4 bh 3 ) × ( F / Y ) both expressed in MPa with F = maximum strength, L = distance between the rests (20 mm), b = width of the specimen, h = height of the specimen, and F / Y = slope of the linear part of the stress–strain curve.
2.2
Water sorption and solubility
Ten discs (thickness: 1 ± 0.1 mm, diameter: 15 ± 0.1 mm) were made from each material according to ISO 4049 and cured in eight overlapping steps of 40 s on each side (320 s in total). The specimens were stored in a vacuum desiccator and weighed with an exactness of ±0.1 mg (analytical balance, Toledo XS, Mettler Toledo GmbH, Greifensee, Switzerland) until mass m 1 was constant (deviation < 0.1 mg). Their volumes V were determined by measuring the diameters from two perpendicular planes and the thicknesses from five measurements, one at the center and four at equally spaced points on the specimen’s circumference using a mechanical caliper (Special Caliper, accuracy of 0.02 mm, MIB Messzeuge GmbH, Spangenberg, Germany). After 7 days of water storage at 37 ± 1 °C, the specimens were dried with cellulose pads until no water could be seen on their surfaces and mass m 2 was weighed. Then the specimens were re-dried to obtain mass m 3 . Water sorption was calculated by W sp = ( m 2 − m 3 )/ V and solubility by W sl = ( m 1 − m 3 )/ V both expressed in μg mm −3 with m 1 = specimen’s mass prior to water storage, m 2 = specimen’s mass after water storage at 37 °C for 7 days, m 3 = specimen’s mass after water storage and drying, and V = specimen’s volume.
2.3
Method of density measurement according to the Archimedes’ principle
Polymerization shrinkage was calculated from the densities measured according to the Archimedes’ principle with the commercial Density Determination Kit of the analytical balance Mettler Toledo XS (Mettler Toledo GmbH, Greifensee, Switzerland). The specimens were weighed in air and in water and the density was calculated in g cm −3 by the software of the Mettler Toledo XS balance by D = ( A /( A − B )) × ( D 0 − D L ) + D L with D = density of sample, A = weight of sample in air, B = weight of sample in water, D 0 = density of water at the exactly measured temperature in °C according to the density table of distilled water, and D L = air density (0.0012 g cm −3 ). An internal balance correction factor (0.99985) of the Mettler Toledo XS balance software took air buoyancy of the adjustment weight into account.
2.4
Polymerization shrinkage of the monomers/oligomers
The densities of the uncured monomers were determined by weighing 10.00 ml at 23 °C. To evaluate polymerization shrinkage, 0.1% camphorquinone was added to each of the uncured monomers and approximately 1 g was polymerized with the light-curing unit UniXS (Heraeus-Kulzer GmbH, Hanau, Germany) for 6 min to assure complete polymerization. Then the densities were measured and shrinkage was calculated.
2.5
Polymerization shrinkage
From each uncured material, 10 spherical specimens, each of approximately 0.1 g, were carefully formed so that trapped air bubbles were avoided. Each specimen was put on a polyester film (thickness 0.05), fixed on the special holder of the balance, of which the masses in air and in water were known and the masses of the whole assembly in air and in water were weighed. Since the weighing process was very fast (approximately 10 s) there was no water uptake or flow of the material. It was observed that the uncured specimens were optimally wetted. The mass of each specimen was calculated by subtracting the mass of the polyester film from the mass of the whole assembly and the density of the uncured material ( D un ) was computed. Next 10 discs (diameter 10 ± 0.1 mm, thickness 1 ± 0.1 mm) of each material were prepared and polymerized for 40 s from each side. Then the masses in air m 1 and in water and the densities ( D 1 ) were evaluated and the polymerization shrinkage in % was calculated by Δ V = ((1/ D 1 ) − (1/ D un )) × (1/ D un ) × 100.
2.6
Hygroscopic expansion
Ten cylindrical specimens of each material (diameter 10 ± 0.1 mm, thickness 1 ± 0.1 mm) were polymerized for 40 s from each side. The volume V 1 was calculated after 15 min of dry and dark storage at room temperature according to ISO 4049 as described above to avoid any moisture influence (volumes of actually dry specimens). Immediately afterwards the mass m 1 and the density D 1 of each specimen were determined by weighing in air and in water. After 30 days dark storage in water at 37 ± 1 °C the masses m 2 and densities D 2 were measured again and the respective volumes were calculated by V 2 = m 2 / D 2 . The hygroscopic expansion Δ V was calculated by subtracting V 2 from V 1 and the results were expressed in percent. Prior to each measurement, the specimens were tempered to room temperature in a water bath for 10 min. Before weighing the specimens in the air, they were blot-dried with a cellulose pad.
2.7
Statistical analysis
Means and standard deviations were calculated. Normal distribution was tested by Kolmogoroff–Smirnoff test. One-way ANOVA and post hoc Scheffé’s test were made for all the properties (SPSS 15.0, SPSS, Chicago, IL, USA). This was performed separately for each of the different properties. Significant changes of flexural strength, flexural modulus prior to and after thermocycling were calculated with the lowest significant difference ANOVA. The level of statistical significance was set to p < 0.05.
2
Materials and methods
Seven experimental light-curing resin-based restorative materials ( Table 3 ) were manufactured using a laboratory vacuum planet kneader (Herbst Maschinenfabrik GmbH, Buxtehude, Germany). The raw material specifications were obtained from the manufacturers’ technical data sheets ( Table 1 ) and own measurements ( Table 2 ). The chemical structures and molecular masses of the raw materials M4004, 4215, CN435, and CN965 ( Table 1 ) are not exactly defined because they are statistical oligomer mixtures. They are used in many technical applications such as adhesives, lithographic inks, silk screen inks, and many industrial and specialty coatings.
| Code | Product/properties | Batch | Company |
|---|---|---|---|
| Photoinitiator | α.α-Dimethoxy-α-phenylacetophenone | 0066162S | Ciba Specialities Chemical Inc., Basel, Switzerland |
| Stabilizer | Pentaerythrityl-tetrakis[3-(3,5-di-tert.-butyl-4-hydroxyphenyl)]-propionate | 26099IC3 | |
| TTEGDMA | Tetraethylene glycol dimethacrylate, funcionality = 2, MW = 330 g mol −1 , standard monomer, good chemical and physical properties, diluting | J1620 | Cray Valley, Paris, France |
| CN965 | Craynor CN 965, funcionality = 2, MW≈1600 g mol −1 , aliphatic urethane diacrylate, very elastic, very tough, good chemical resistance | KE146801VSP | |
| CN435 | Craynor CN 435, functionality = 3, MW≈1100 g mol −1 , highly ethoxylated trimethylpropane triacrylate, very elastic, water soluble | LG227220 | |
| UV-stabilizer | 2-Hydroxy-4-methoxy-bezophenone | 411351/143302 | Fluka AG, Buchs, Switzerland |
| UDMA | 7,7,9-Trimethyl-4,13-dioxo-3,14-dioxa-5,12-diaza-hexadecan-1,16-diol-dimethacrylate, functionality = 2, MW = 471 g mol −1 , standard monomer, flexible, tough, very good chemical resistance | 330503057 | Rahn AG, Zürich, Switzerland |
| Bis-GMA | Bis-GMA, functionality = 2, MW= 513 g mol −1 , standard monomer, rigid, very good chemical resistance | 2008218303 | |
| 4215 | Urethane acrylate 03-978, functionality = 2, MW≈1500 g mol −1 , aliphatic polyesterurethane diacrylate, very elastic, very tough, very good chemical resistance | 320502047 | |
| M4004 | Miramer M4004, polyetherpolyoltetraacrylate, funcionality = 4, MW≈560 g mol −1 , polyesterpolyol tetraacrylate, very rigid, very high strength, very good chemical resistance, diluting | 80219445 | |
| CQ | d , l -Camphorquinone | 0148990002 | |
| Amine | Ethyl-4-(dimethylamino)-benzoate | 310170 | |
| Glass | Strontiumborosilicate glass (Glass G0 18-093. 0.7 μm). Silaned (3-methacryloyloxypropyltrimethoxy silane) | Lab14701 | Schott Electronic Packaging GmbH, Landshut, Germany |
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