Abstract
Objective
The aim of this work is the study of the dynamic mechanical thermal properties (viscoelastic properties) of a current dental commercial light-cured nanohybrid resin composite, Kalore, GC (GC Corporation, Tokyo, Japan) along with the study of the effect of some food/oral simulating liquids (FSLs) on these properties.
Methods
Dynamic mechanical thermal analysis (DMTA) tests were performed on a Diamond Dynamic Mechanical Thermal Analyzer in bending mode. A frequency of 1 Hz and a temperature range of 25–185 °C were applied, while the heating rate of 2 °C/min was selected to cover mouth temperature and the material’s likely T g . The properties were determined after storage in air, distilled water, heptane, ethanol/water solution (75% v/v) or absolute ethanol at 37 °C for up to 1 h, 1, 7 or 30 days.
Results
Storage modulus, loss modulus and tangent delta (tan δ ) were plotted against temperature. The glass transition temperatures are taken from the peak of the tangent tan δ versus temperature curves. Moreover, some factors indicating the heterogeneity of the polymer matrix, such as the width (Δ T ) at the half of tan δ peak and the “ ζ ” parameter were determined. All samples analyzed after storage for 1 h or 1 day in the aging media showed two T g values. All samples analyzed after storage for 7 or 30 days in the ageing media showed a unique T g value.
Significance
Storage of Kalore GC in dry air, water or heptane at 37 °C for 7 days caused post-curing reactions. Storage in air or water for 30 days did not seem to cause further effects. Storage in heptane for 30 days may cause plasticization and probably some degradation of the filler–silane bond and polymer matrix. Storage in ethanol/water solution (75% v/v) or ethanol for 7 days seems to cause post-curing reactions and degradation reactions of the matrix–filler bonds. Storage in ethanol for 30 days caused a strong change of the sample morphology and the DMTA results were not reliable.
1
Introduction
The composition of resin-based composite materials has evolved significantly since they were firstly introduced in Dentistry in the mid-1960s. Current changes are more focused on the polymeric matrix of the material, mainly to develop systems with reduced polymerisation shrinkage and, more importantly, reduced polymerisation shrinkage stress or to make them self-adhesive to tooth structure as well . Recently, a low-shrinkage resin composite has developed and launched in the market (Kalore, GC, Tokyo, Japan). Kalore GC is a visible-light curable radiopaque nanohybrid resin composite. Its unique composition enables the creation of anterior and posterior direct composite restorations (i.e. Class I, II, II, IV and V cavities) with high polish, high wear resistance, low polymerisation shrinkage (stress) and durability. It can be also used for direct restorations for wedge-shaped defects and root surface cavities or direct restorations for veneers and diastema closure .
Dental resin-composites are characterized generally as viscoelastic materials, since they behave neither as perfectly elastic solids, nor as completely viscous materials, due to the presence of the resin matrix (polymer). The degree to which a material behaves either viscously or elastically depends on environmental temperature, vibrations’ frequency, dynamic strain rate, time effects (creep, relaxation), aging and other irreversible effects . Since denture-based polymers display viscous and elastic properties, both storage ( E ʹ) and loss moduli ( E ″) are necessary to completely describe deformation response . Storage modulus, which is measured by the slope of the elastic region of the stress–strain diagram, describes the relative stiffness or rigidity of the material . Dental composites with low storage modulus will more readily deform elastically under functional stresses, which may result in catastrophic fracture of the surrounding tooth structure. Ideally, the storage modulus should fit to those of the dental tissues, which are supposed to be replaced .
Dynamic mechanical tests, such as Dynamic Mechanical Thermal Analysis (DMTA) are particularly well-suited for viscoelastic materials, since they determine both the storage and the viscous response of the materials . Also, dynamic tests mimic better the cyclic masticatory loading to which dental composites are clinically subjected . This might be extremely valuable to predict the clinical performance of bio-materials when working under the cyclic oscillations generated by the human body’s physiological movements . In our previous works we have studied the dynamic thermomechanical properties of several commercial dental light-cured resin-based composites .
In oral environment, it can be stated that saliva, food components, beverages and interactions among these materials can degrade and age dental restorations. The food simulating liquids used for the aging of the materials in this study were chosen according to FDA guidelines. Water simulates the oral environment provided by saliva; heptane simulates vegetable oils, butter and fatty meats whereas the aqueous ethanol solution and ethanol simulate beverages, including alcoholic drinks, vegetables, fruits and candy .
This work is concerned with the study of viscoelastic properties and parameters, such as the storage modulus ( E ʹ), the loss modulus ( E ″), the tangent delta (tan δ ) and the glass transition temperature ( T g ) of the Kalore, GC using DMTA. Moreover, some factors indicating the heterogeneity of the polymer matrix, such as the width (Δ T ) at the half of tan δ peak, the “ ζ ” parameter were determined. These properties were determined after storage of the composite in air, water, heptane, ethanol/water solution (75% v/v) or ethanol for several time intervals. It was found interesting to examine the viscoelastic properties of this composite, because of its unique composition of the organic resin matrix, filler and interphase between them.
2
Materials and methods
2.1
Material composition
The light-cured nanohybrid composite studied was the Kalore, GC (GC Corporation, Tokyo, Japan), shade A 3 , Lot. 1107201.
2.1.1
Organic resin matrix
Kalore consists of an organic matrix, fillers, photoinitiation system and pigments. The matrix contains a mixture of dimethacrylates . It contains a new dimethacrylate monomer the DX-511, (CAS: 1026782-73-9, 5–10% by weight) which is recently developed by DuPont (DuPont monomer) and is based on urethane dimethacrylate chemistry ( Fig. 1 ). This monomer has a long rigid molecular core and flexible arms in the structure. The long rigid core prevents monomer’s deformation and reduces polymerisation shrinkage. On the other hand, if the molecular core is flexible, the monomer may fold and will occupy less space, causing a loss in dimension. The molecular weight of this monomer is 895 which is twice that of Bis-GMA or UDMA. Generally, the short chain monomers with lower molecular weight have the greatest polymerisation shrinkage and inferior physical characteristics than the long chain monomers. A high molecular weight monomer reduces polymerisation shrinkage because it contains only a small number of double bonds C C, which is a factor in polymerisation shrinkage. However, if the monomer chain becomes too long, then reactivity decreases. To overcome this challenge, flexible arms were created on the new DuPont monomer, thus increasing the potential for reactivity. The manufacturer has reported volumetric shrinkage values of 1.72%, claiming that the shrinkage stress values are the lowest of any composite resin system . The matrix of Kalore GC also contains Urethane Dimethacrylate (UDMA) (CAS: 72869-86-4, 5–10% by weight) and Bisphenol A polyethoxymethacrylate (CAS: 41637-38-1, 1–5% by weight). The latter is also called Bisphenol A ethoxylate dimethacrylate and its form has 2–4 units of ethoxylation, sometimes referred as Bis-EMA.
2.1.2
Filler
At the core of the Kalore GC filler system the newly developed high-density radiopaque prepolymerised fillers exist (30–35% by weight) . These prepolymerised fillers (average: 17 μm) contain 60 wt% of 400 nm modified strontium glass and 20% by weight of 100 nm lanthanoid fluoride (resin: 20% by weight). The modified strontium glass reinforces the filler’s strength and surface hardness, provides high polishability and matches the refractive index of the UDMA, thereby offering improved aesthetics . Kalore GC contains also inorganic filler, i.e. strontium glass particles (700 nm, 30–33% by weight), fluoroaluminosilicate glass (700 nm, 20–30% by weight) and monodispersed silica (nanofiller: 16 nm, 1–5% by weight) which are dispersed between the prepolymerized fillers. The fluoroaluminosilicate and strontium glasses used are silanated. The silica surfaces are treated hydrophobically with dimethyl constituents to attract the silica and matrix to each other and increase their intimate contact. Dimethyl-treated silica is also more stable than silica treated with methacryloxysilane, resulting in an improved shelf life with less risk of material softening during storage .
The total content in fillers of Kalore GC is 82% by weight. Taking into account that these fillers contain 30–35% by weight prepolymerised fillers and that the prepolymerised fillers contain 20% resin, then the total content in inorganic filler is 75–76% by weight.
Kalore GC contains also a combination of camphorquinone/amine as photoinitiation system (<1% by weight) and pigments (<1% by weight).
2.2
Preparation of specimens
For DMTA tests, bar specimens of rectangular geometry (sticks) were prepared by filling a Teflon mold (2 mm × 2 mm × 40 mm, as recommended by DMTA manufacturer’s instructions) with unpolymerised material, taking care to minimize entrapped air. The upper and lower surface of the mold was overlaid with glass slides covered with a Mylar sheet (thickness 0.05 mm, Stripmat Polydentia SA) to avoid adhesion with the uncured material. The completed assembly was held together with spring clips and irradiated by overlapping, using a XL 3000 dental photocuring unit (3 M Company, St. Paul, USA). This source consisted of a 75 W tungsten halogen lamp, which emits radiation between 420 and 500 nm and lighting power at 700 mW/cm 2 measured by Hilux curing light meter (Benlioglu Dental INC, Serial No. 9080935). This unit was used without the light guide in direct contact with the glass sides. The samples were irradiated for 60 s on each side, as it was applied in our previous works ; this irradiation time is also consistent to that suggested by the manufacturers of Kalore GC. Twenty bar-shaped specimens were prepared by the above procedure which subdivided into five groups of four specimens each. The first group consisted of specimens that had been stored in an oven (Memmert Model 200) at 37 ± 0.1 °C, for 1 h, 1, 7 or 30 days-time. The other four groups consisted of specimens which had been stored in distilled water, heptane (99+% biotech grade solvent, Aldrich, Lot: S41495-177) ethanol/water solution (75% v/v) or ethanol (absolute for analysis, Carlo Erba, Batch: 9M275279M) at 37 ± 1 °C for 1 h, 1, 7 or 30 days-time. The procedure repeated once more.
2.3
Dynamic mechanical thermal analysis (DMTA)
A dynamic mechanical thermal analyzer (DMTA) imposes a sinusoidal deformation to a sample of known geometry. The sample can be subjected to a controlled stress or a controlled strain. For a known stress, the sample will then deform a certain amount that is related to its stiffness. DMTA measures stiffness and damping, which are reported as moduli and tan δ . Since a sinusoidal force is applied, the modulus can be expressed as an in-phase component, the elastic modulus, and an out-of-phase component, the loss modulus.
DMTA tests were performed on a Diamond DMTA dynamic mechanical thermal analyzer (Perkin-Elmer) in bending mode. A frequency of 1 Hz was applied (approximately average chewing rate), bending force of 4000 mN and amplitude of 10 μm. A temperature rate of 25–185 °C and a heating rate of 2 °C/min were selected to cover mouth temperature and the materials’ likely glass transition temperature ( T g ). Storage modulus ( E ′), loss modulus ( E ″) and tangent delta (tan δ ) were plotted against temperature over this period. After the DMTA run was completed, the sample was allowed to cool naturally at room temperature and the values of E ʹ, E ″, tan δ at 37 °C and the T g were noted. This method was used for each of the samples and the mean values were calculated ( n = 2). Determination of peak width at half its height of the tan δ curve (Δ T , °C) and the determination of “ ζ ” parameter (K/Pa) from the storage modulus curve are analytically described in our previous work .
2
Materials and methods
2.1
Material composition
The light-cured nanohybrid composite studied was the Kalore, GC (GC Corporation, Tokyo, Japan), shade A 3 , Lot. 1107201.
2.1.1
Organic resin matrix
Kalore consists of an organic matrix, fillers, photoinitiation system and pigments. The matrix contains a mixture of dimethacrylates . It contains a new dimethacrylate monomer the DX-511, (CAS: 1026782-73-9, 5–10% by weight) which is recently developed by DuPont (DuPont monomer) and is based on urethane dimethacrylate chemistry ( Fig. 1 ). This monomer has a long rigid molecular core and flexible arms in the structure. The long rigid core prevents monomer’s deformation and reduces polymerisation shrinkage. On the other hand, if the molecular core is flexible, the monomer may fold and will occupy less space, causing a loss in dimension. The molecular weight of this monomer is 895 which is twice that of Bis-GMA or UDMA. Generally, the short chain monomers with lower molecular weight have the greatest polymerisation shrinkage and inferior physical characteristics than the long chain monomers. A high molecular weight monomer reduces polymerisation shrinkage because it contains only a small number of double bonds C C, which is a factor in polymerisation shrinkage. However, if the monomer chain becomes too long, then reactivity decreases. To overcome this challenge, flexible arms were created on the new DuPont monomer, thus increasing the potential for reactivity. The manufacturer has reported volumetric shrinkage values of 1.72%, claiming that the shrinkage stress values are the lowest of any composite resin system . The matrix of Kalore GC also contains Urethane Dimethacrylate (UDMA) (CAS: 72869-86-4, 5–10% by weight) and Bisphenol A polyethoxymethacrylate (CAS: 41637-38-1, 1–5% by weight). The latter is also called Bisphenol A ethoxylate dimethacrylate and its form has 2–4 units of ethoxylation, sometimes referred as Bis-EMA.
2.1.2
Filler
At the core of the Kalore GC filler system the newly developed high-density radiopaque prepolymerised fillers exist (30–35% by weight) . These prepolymerised fillers (average: 17 μm) contain 60 wt% of 400 nm modified strontium glass and 20% by weight of 100 nm lanthanoid fluoride (resin: 20% by weight). The modified strontium glass reinforces the filler’s strength and surface hardness, provides high polishability and matches the refractive index of the UDMA, thereby offering improved aesthetics . Kalore GC contains also inorganic filler, i.e. strontium glass particles (700 nm, 30–33% by weight), fluoroaluminosilicate glass (700 nm, 20–30% by weight) and monodispersed silica (nanofiller: 16 nm, 1–5% by weight) which are dispersed between the prepolymerized fillers. The fluoroaluminosilicate and strontium glasses used are silanated. The silica surfaces are treated hydrophobically with dimethyl constituents to attract the silica and matrix to each other and increase their intimate contact. Dimethyl-treated silica is also more stable than silica treated with methacryloxysilane, resulting in an improved shelf life with less risk of material softening during storage .
The total content in fillers of Kalore GC is 82% by weight. Taking into account that these fillers contain 30–35% by weight prepolymerised fillers and that the prepolymerised fillers contain 20% resin, then the total content in inorganic filler is 75–76% by weight.
Kalore GC contains also a combination of camphorquinone/amine as photoinitiation system (<1% by weight) and pigments (<1% by weight).
2.2
Preparation of specimens
For DMTA tests, bar specimens of rectangular geometry (sticks) were prepared by filling a Teflon mold (2 mm × 2 mm × 40 mm, as recommended by DMTA manufacturer’s instructions) with unpolymerised material, taking care to minimize entrapped air. The upper and lower surface of the mold was overlaid with glass slides covered with a Mylar sheet (thickness 0.05 mm, Stripmat Polydentia SA) to avoid adhesion with the uncured material. The completed assembly was held together with spring clips and irradiated by overlapping, using a XL 3000 dental photocuring unit (3 M Company, St. Paul, USA). This source consisted of a 75 W tungsten halogen lamp, which emits radiation between 420 and 500 nm and lighting power at 700 mW/cm 2 measured by Hilux curing light meter (Benlioglu Dental INC, Serial No. 9080935). This unit was used without the light guide in direct contact with the glass sides. The samples were irradiated for 60 s on each side, as it was applied in our previous works ; this irradiation time is also consistent to that suggested by the manufacturers of Kalore GC. Twenty bar-shaped specimens were prepared by the above procedure which subdivided into five groups of four specimens each. The first group consisted of specimens that had been stored in an oven (Memmert Model 200) at 37 ± 0.1 °C, for 1 h, 1, 7 or 30 days-time. The other four groups consisted of specimens which had been stored in distilled water, heptane (99+% biotech grade solvent, Aldrich, Lot: S41495-177) ethanol/water solution (75% v/v) or ethanol (absolute for analysis, Carlo Erba, Batch: 9M275279M) at 37 ± 1 °C for 1 h, 1, 7 or 30 days-time. The procedure repeated once more.
2.3
Dynamic mechanical thermal analysis (DMTA)
A dynamic mechanical thermal analyzer (DMTA) imposes a sinusoidal deformation to a sample of known geometry. The sample can be subjected to a controlled stress or a controlled strain. For a known stress, the sample will then deform a certain amount that is related to its stiffness. DMTA measures stiffness and damping, which are reported as moduli and tan δ . Since a sinusoidal force is applied, the modulus can be expressed as an in-phase component, the elastic modulus, and an out-of-phase component, the loss modulus.
DMTA tests were performed on a Diamond DMTA dynamic mechanical thermal analyzer (Perkin-Elmer) in bending mode. A frequency of 1 Hz was applied (approximately average chewing rate), bending force of 4000 mN and amplitude of 10 μm. A temperature rate of 25–185 °C and a heating rate of 2 °C/min were selected to cover mouth temperature and the materials’ likely glass transition temperature ( T g ). Storage modulus ( E ′), loss modulus ( E ″) and tangent delta (tan δ ) were plotted against temperature over this period. After the DMTA run was completed, the sample was allowed to cool naturally at room temperature and the values of E ʹ, E ″, tan δ at 37 °C and the T g were noted. This method was used for each of the samples and the mean values were calculated ( n = 2). Determination of peak width at half its height of the tan δ curve (Δ T , °C) and the determination of “ ζ ” parameter (K/Pa) from the storage modulus curve are analytically described in our previous work .
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