To determine the diffusion coefficient, water sorption and solubility of various types of restorative dental composites and to evaluate the effect of acidic media (orange juice and coke) on their characteristics.
Resin composite specimens (Filtek™ Z350, Spectrum ® TPH ® 3 and Durafill ® VS) were prepared in a stainless steel mold of 1 mm thickness and 10 mm diameter ( n = 5) and light-cured. All samples were dried at 37 °C, immersed in media (distilled water, orange and coke) at 37 °C and weighed at suitable time intervals (15, 35, 155, 320, 785, etc. min) until 40 days of immersion and then were dried again for 40 days. Diffusion coefficient (m 2 s −1 ) was determined according to Fick’s second law while water sorption and solubility (μg/mm 3 ) were calculated based on BS EN ISO 4049:2000. Data of water sorption and solubility were analyzed with One-Way ANOVA and post hoc Scheffe test at p = 0.05.
The experimental data obtained were obeyed and nearly fitted to the diffusion theoretical data plot. The highest values of diffusion coefficients were presented by Durafill ® VS (32.23–45.25 × 10 −13 m 2 s −1 ). Diffusion coefficients of Filtek™ Z350 and Spectrum ® TPH ® 3 were the highest when immersed in coke media followed by distilled water and orange juice. The water sorption of most composites was significantly increased after immersion in coke and orange ( p < 0.05). Meanwhile only Spectrum ® TPH ® 3 showed an increase in solubility when immersed in coke media. Z350 presented the highest water sorption after immersion in distilled water and coke (16.13 and 18.22 μg/mm 3 ) while Durafill ® VS presented the highest solubility (7.20–9.27 μg/mm 3 ).
The exposure of restorative dental composites to acidic drinks can cause an increase in diffusion coefficient, water sorption and solubility parameters which may accelerate the degradation process and thus reduce the life span of composite restoration.
Restorative dental composites are becomingly more popular and extensively used in dentistry due to their esthetic and good in physical and mechanical properties. However, in a wet oral environment the composites may absorb water or other liquids such as saliva, food components or beverages which can have an appreciable influence on the degradation of dental composite. Excessive of liquids uptake may produce deleterious effects on the structure and function of the resin, as these can reduce the mechanical and physical properties that lead to a shortened service life of dental restoration .
ISO 4049 is a standard method which is commonly used by researchers to determine water sorption and solubility of restorative dental composites. The standard limits for the water sorption and solubility are 40 μg/mm 3 and 7.5 μg/mm 3 , respectively . However, the immersion time of 7 days in the standard is not sufficient for most composites, which is normally to be saturated within 7–60 days . Ferracane et al. have shown that the reduction in mechanical properties was predominantly related to water uptake of composites. The mechanical properties were continuously decreased until the polymer network was completely saturated, and then there was no further reduction. This shows that immersion time is very crucial in determining the right properties of resin composites. Asaoka and Hirano also suggested that diffusion coefficient is important in determining the time-dependent mechanical properties and time-dependent hydroscopic expansion of resins for clinical use. However, the measurement of diffusion coefficient is not stated in the standard. Diffusion coefficient which represents the rate of water diffused into polymer network could be used to relate with composite degradation. It is assumed that the higher the diffusion coefficient, the faster degradation process would occur in the composite. The sorption phenomenon in composite is a diffusion-controlled process that causes chemical degradation due to the residual monomer release and filler matrix debonding .
Carbonated soft drinks and fruit juices are common beverages consumed by people worldwide. Statistics showed that the consumption of carbonated soft drink has increased dramatically over the past 50 years, representing about 25% of the recommended daily fluid intake of 67 ounces . This upward trend also occurred on fruit juices consumption, in which children and adolescents are the majority group consuming the drinks . Consumption of acidic drinks can degrade the teeth and restorative materials. Aliping-McKenzie et al. reported a significant reduction on surface hardness of dental restorative materials after immersion in coke and fruit juices. Coke and orange juice contain large quantities of phosphorus acid and citric acid respectively which may lead to increase the water uptake and reduce the properties of the dental restorative composites. The chemical activity of these acids could result in promoting surface erosion . In addition, it is hypothesized that these acids may also increase the sorption rate, water sorption and solubility of dental composites.
Recently, many studies have been carried out on water sorption using distilled water , artificial saliva and ethanol as immersion media, however, study on soft drinks and juices has not yet reported. In our study, we emphasized in determination of diffusion coefficient, water sorption and solubility of various types of restorative dental composites and the effect of acidic media (orange juice and coke) on their characteristics.
Materials and methods
Three different dental composites, Filtek™ and Z350 (Z350; lot no. N108290; shade A2), Spectrum ® TPH ® 3 (TPH3; lot no. 0903000531; shade A2) and Durafill ® VS (VS; lot no. 010214; shade A2) were used in this study. These composites were selected based on various filler sizes as in the classification of commercially available dental composites is based on filler size. The compositions of composite resins are presented in Table 1 . The immersion media used are cola soft drink (coke), orange juice and distilled water. The details of the drinks are shown in Table 2 . The pH of each media was measured using microprocessor pH meter (HANNA Instruments ® , Inc, Woonsocket, RI, USA).
|Material||Manufacturer||Classification||Resin||Filler content||Filler type and size|
|Filtek™ Z350 (Z350)||3M ESPE, St. Paul, MN, USA||Nanofilled||BisGMA
|Zirconia and silica clusters: 0.6–1.4 μm; silica: 5–20 nm|
|Spectrum ® TPH ® 3 (TPH3)||Dentsply, York, PA, USA||Microhybrid||BisGMA
|BABS: <1 μm; BAFG: <1 μm; silica: 10–20 nm|
|Durafill ® VS (VS)||Heraeus-Kulzer, South Bend, IN, USA||Microfilled||UDMA||50.5 wt%
|Prepolymerized silica: 10–20 μm; silica: 0.02–2 μm|
|Orange juice||Rio™ Fiesta||Milk Specialities Distribution Sdn. Bhd., Subang Jaya, Selangor, Malaysia||3.9||Water, sugar, orange pulp, orange juice concentrate, citric acid, sodium citrate, pectin, ascorbic acid, beta-carotene, vitamin E and vitamin A, acidity regulators, stabilizer, flavors and natural coloring substance|
|Cola soft drink (coke)||Coca-Cola ®||F&N Beverages Manufacturing Sdn. Bhd., Shah Alam, Selangor, Malaysia||3.1||Carbonated water, sugar, caramel color, conditioners and flavors|
Water sorption and solubility
Disc-shaped specimens with a dimension of 1 mm thickness and 10 mm diameter ( n = 5) were prepared in a stainless-steel mold between two glass slides covered with matrix strips. All specimens were irradiated with light curing unit (Elipar™ Freelight 2 LED, 3M ESPE, St. Paul, MN, USA) at an intensity of 1000 mW/cm 2 on each side for 40 s. The diameter of the light tip is 5 mm. During irradiation, the light tip was slowly moved around the specimen top surface and all specimens were assumed to have an equal polymerization. The specimens were stored in oven drying at 37 °C for 24 h, and their masses were measured using analytical balance with a precision 0.001 g (GR-200, A&D Company Limited, Toshima-ku, Tokyo, Japan). This cycle was repeated until a constant mass ( m 1 ) was obtained. For determining the volume of specimen, the diameter and thickness of each specimen was measured by digital caliper with a precision 0.001 mm (Mitaka, Japan) at three points and the mean value was obtained. The specimens were then individually placed in a sealed plastic container containing about 10 ml of immersion media (distilled water, coke and orange juice) at 37 °C. The bottles of coke and orange juice were shaken prior to pouring into the containers to ensure that all the ingredients inside were evenly distributed. The media used were changed twice a week to avoid microbial activity. At fixed time intervals (15, 35, 155, 320, 785, etc. min) the specimens were removed, blotted dry to remove excess liquid, weighed and returned to the liquid bath. The time intervals were more frequent during the first few days, and as the sorption slowed, the interval time was extended. The sorption equilibrium assumed to be achieved after constant mass ( m 2 ) observed at 30 days, hence the sorption was taken at day 40 to ensure the composite completely absorbed. The specimens were then reconditioned in drying oven and weighed during desorption period of 40 days to obtain a constant mass, m 3 .
Water sorption and solubility of composites were calculated according to BS EN ISO 4049:2000 using the following formula:
Water sorption = m 2 − m 3 V
Solubility = m 1 − m 3 V
where m 1 is the conditioned mass, m 2 is the mass of specimen after 40 days immersion and m 3 is the mass of the reconditioned specimen.
A diffusion coefficient was obtained by using Stefan’s approximation of the appropriate solution of Fick’s second law for disc-shaped geometry . At earlier stages of uptake <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='(whenMt/M∞≤0.5)’>(whenMt/M∞≤0.5)(whenMt/M∞≤0.5)
( when M t / M ∞ ≤ 0.5 )
sorption follows the equation:
M t M ∞ = 4 L D t π 1 / 2
where M t is the accumulate mass of the diffusing liquid at time <SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='t’>tt
, <SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='M∞’>M∞M∞
is the mass of the sorbed liquid at the equilibrium, L is the thickness of the disc, and D is the diffusion coefficient of the liquid during the sorption process.
For longer time of diffusion, the solution to this differential equation is expressed as follows:
M t M ∞ = 1 − 8 π 2 ∑ n = 0 ∞ 1 ( 2 n + 1 ) 2 exp − ( 2 n + 1 ) 2 π 2 D t L 2