d e n t a l m a t e r i a l s 3 1 ( 2 0 1 5 ) 855–863
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A systematic approach to standardize artificial aging of resin composite cements Lukas Blumer a , Fredy Schmidli a , Roland Weiger b , Jens Fischer a,c,∗ a
Department of Dental Materials and Engineering, University Hospital of Dental Medicine, University of Basel, Basel, Switzerland b Department of Periodontology, Endodontology and Cariology, University Hospital of Dental Medicine, University of Basel, Basel, Switzerland c VITA Zahnfabrik, Bad Säckingen, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. The aim of the investigation was to contribute to the ongoing discussion at the
Received 1 February 2015
international standardization committee on how to artificially age dental resin composite
Received in revised form
cements.
20 March 2015
Methods. Indirect tensile strength (n = 30) of a dual-cured resin composite cement (Panavia
Accepted 27 April 2015
F2.0) was measured to evaluate the effect of water storage at 37 ◦ C or thermal cycling (5 ◦ C/55 ◦ C/1 min) for up to 64 days. The influence of water temperature (5–65 ◦ C) after 16 days and the effect of 1 day water storage at 37 ◦ C prior to aging were assessed. Storage in
Keywords:
air at 37 ◦ C served as control.
Resin composite cement
Results. Thermal cycling affected the indirect tensile strength most, followed by water stor-
Compressive strength
age at 55 ◦ C, whereas water storage at 37 ◦ C had only little influence. Major deterioration
Indirect tensile strength
occurred before day 4 (≈6000 cycles). A 1-day pre-treatment by water storage at 37 ◦ C prior
Aging
to thermal cycling attenuated the effect of aging.
Thermal cycling
Significance. For the material investigated, thermal cycling for 4 days is the most efficient
International standardization
aging procedure. A 1-day water storage at 37 ◦ C prior to thermal cycling is recommended to allow complete polymerization. A 4-day water storage at 55 ◦ C may be considered as a viable alternative to thermal cycling. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
With the increasing number of ceramic restorations in dentistry, the use of adhesive resin composite cements has steadily increased. Compared with conventional cements based on acid–base reactions, they have better mechanical
properties, higher bond strength, reduced solubility and improved esthetics [1,2]. In general, these materials consist of three components: the polymer matrix, the fillers and the silanes as binder between the organic and the inorganic phase [3,4]. The properties of resin composite cements such as elasticity, plasticity, hardness, strength and thermal as well as chemical stability are determined by the properties of
∗ Corresponding author at: Department of Dental Materials and Engineering, University Hospital of Dental Medicine, University of Basel, Hebelstrasse 3, 4056 Basel, Switzerland. Tel.: +41 61 267 26 26. E-mail address: jens.fi[email protected] (J. Fischer).
http://dx.doi.org/10.1016/j.dental.2015.04.015 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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these single components and the respective microstructure [4–6]. Polymerization is catalyzed either by autocatalysis (“selfcuring”) or photo-initiation (“light-curing”). The autocatalytic polymerization has the disadvantage that the reaction starts immediately after mixing the catalyst (initiator) with the polymerizable component. Thus, the processing time is limited. In contrast, the photo-initiation allows determining the start of the polymerization. However, areas that are not reached by the light will not polymerize adequately. To avoid the disadvantages of each, cements were developed comprising both initiator systems (“dual-curing”). The idea was to provide a material of which the processing time is adapted to the clinical requirements and, independent of the influence of light, a high degree of conversion will be achieved [3,7]. The conversion level is an important factor for the mechanical strength of luting cements because composites with a high degree of conversion also have better mechanical properties [3,7]. Most cements show a higher degree of conversion by dual-curing compared with self-curing alone [8–10]. Additional catalysts are beneficial to improve polymerization. For instance, the application of ED Primer (Kuraray-Noritake, Kurashiki, Japan) increases the degree of conversion of Panavia F2.0 (Kuraray-Noritake) by 15% after self-curing and by 24% after dual-curing [11,12]. The conversion degree in self-curing is influenced not only by the concentration of monomer and catalyst, but also by the ambient temperature [13–15]. An increase in temperature from 3 to 60 ◦ C resulted in an increase in the degree of conversion of 36% [16]. Cements in an aqueous medium such as saliva are exposed to a long-term aging process, which might significantly reduce the mechanical properties [17]. The effects are wide-ranging, but typically include the leaching of unreacted components and the degradation of the polymer network [17,18]. At present, no standardized procedure for artificial aging of resin composite cements is available. Thermal cycling simulates the effect of varying temperatures in the oral cavity, which might range from 0 ◦ C (melting ice) to 60 ◦ C (hot beverages). For that reason, thermal cycling is usually performed between 5 and 55 ◦ C with cycle times of 1 min [19–21]. The suggested duration of thermal cycling differs from 3000 to 100,000 cycles [22–26]. It is proposed that 10,000 cycles might represent 1 year of service [27,28]. After the placement of the restoration, however, the cement is setting at 37 ◦ C and polymerizes for another 24 h [29]. During this time, thermal stress is rare. Thus, to mimic the clinical situation, the effect of a pre-treatment of the cement at 37 ◦ C prior to artificial aging should be analyzed. Composite resin cements are brittle materials and therefore tolerant to compressive stress but susceptible to tensile loading [30]. Hence, a three-point bending test for the characterization of the strength of luting materials is recommended in the respective standard ISO 4049. A 1-day water storage at 37 ◦ C prior to the test is specified to allow a complete polymerization. However, the preparation of specimens for bending tests is demanding and time-consuming. Especially during development of a new product and with the increasing number of products available, the quantity of such tests will increase. Therefore, simplification of these tests would be attractive. Compressive or indirect tensile strength tests provide reliable information on the mechanical strength of
Table 1 – Composite resin cement and primer used in the study (material compositions according to the manufacturer’s product specification). Material Panavia F 2.0
ED Primer
Composition Base paste: hydrophobic aromatic dimethacrylate, hydrophilic and hydrophobic aliphatic dimethacrylate, sodium aromatic sulphinate, silanated barium glass filler, surface-treated sodium fluoride, catalysts, accelerators, pigments Catalyst paste: 10-methacryloyloxydecyl dihydrogen phosphate (MDP), hydrophobic aromatic dimethacrylate, hydrophilic and hydrophobic aliphatic dimethacrylate, silanated silica filler and colloidal silica, dl-camphorquinone, catalysts, initiators Liquid A: 2-hydroxyethyl methacrylate (HEMA), MDP, N-methacryloyl-5-aminosalicylic acid (5-NMSA), water, accelerators Liquid B: N-methacryloyl-5-aminosalicylic acid (5-NMSA), catalysts, accelerators, water
the investigated material with the advantage of easy handling [31,32]. To the knowledge of the authors, an impact of aging on indirect tensile strength or compressive strength of resin composite cements is not yet investigated and therefore worth to be analyzed. Further, water storage instead of thermal cycling could be an option to economize the test procedure, because thermal cycling requires a special device with a limited capacity. Against that background, the appropriate international standardizing committee (ISO/TC 106/SC 1/WG 9) is currently in the process of evaluating an adequate test design for assessing the effect of aging on the mechanical strength of resin composite cements. The aim of the present study was to contribute to the discussion by a systematic analysis of the effect of different aging procedures on composite resin cements by compressive and indirect tensile strength tests.
2.
Materials and methods
In the present study, Panavia F2.0 (Kuraray-Noritake) was selected as test material (Table 1). The specimens were produced in a customized Teflon mold 3 mm in height with five cylindrical holes each 3 mm in diameter. Depending on the curing mode, an opaque Teflon plate (self-curing) or a transparent glass plate covered with a Mylar strip (Hawe Transparent Strips, KerrHawe, Bioggio, Switzerland) (dualcuring) was fixed at the bottom of the Teflon plate. All surfaces of the mold coming in contact with the specimens were first coated with ED Primer (Kuraray-Noritake). One drop each of primer liquids A and B was mixed for 20 s, applied to the mold and gently dried for 5 s with air as recommended by the manufacturer. Base and catalyst pastes of the cement were mixed according to the manufacturer’s instructions in a 1:1 ratio for 20 s with a plastic spatula on a mixing tray and at the latest 2 min after start of mixing filled into the molds with slight excess. For self-curing, the top of the Teflon mold was also covered with an opaque Teflon plate and left for 20 min under a load of 1 kg. For “dual-curing,” the top of the Teflon mold was
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Fig. 1 – Effect of different aging procedures on mechanical parameters of the cement investigated (mean values and standard deviations): (a) compressive strength after self-curing, (b) compressive strength after dual-curing, (c) indirect tensile strength after self-curing and (d) indirect tensile strength after dual-curing.
covered with a Mylar strip and a glass plate. As the diameter of the light emission window was greater than that of the specimens, only one light exposure for 20 s on each side to an LED lamp (Elipar S10, 3M ESPE, Seefeld, Germany) was performed. The tip of the lamp was centered in a distance of 5 mm perpendicular to the specimens. Subsequently, the specimens were carefully removed from the mold, deflashed and stored, depending on the intended aging test as follows. Air storage: Specimens were stored in a sealed glass container at the respective temperature in an incubator (T-40/25, CTS, Hechingen, Germany). Water storage: Specimens were subjected to the same conditions as for the storage in air; however, the glass container was filled with distilled water. Thermal cycling: In a customized device, the specimens were immersed alternately in water baths of 5 and 55 ◦ C, using a sieve for storage and transportation. The cycle duration was 1 min with a dwell time in each water bath of 28 s and a transfer time between baths of 2 s. Self- and dual-cured specimens were tested in a screening test both for compressive and for indirect tensile strength (n = 5) immediately after preparation and after 1, 4, 9, 16,
25, 36, 49 and 64 days in a universal testing machine (Z010, Zwick/Roell, Ulm, Germany) with a cross-head speed of 1 mm/min. Prior to the measurements, the specimens were sized in diameter and height, using a digital caliper (Cal IP 67, Tesa, Renens, Switzerland). For compressive strength, the load was applied axially for indirect tensile strength radially. Strength values were calculated using the following equations: Compressive strength :
c =
F (d/2)
Indirect tensile strength : t =
2
2F dh
where F is the load at fracture, d the specimen diameter and h the specimen height. Based on the obtained results, test conditions were selected in order to obtain more reliable results and to determine the Weibull modulus (n = 30). Statistical analysis was performed with a one-way ANOVA (˛ = 0.05).
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Indirect tensile strength (MPa)
60 50 40 30 20
37°C air 37°C water
10
TC light-cured
0 0
10
20
30 40 Time (d)
50
60
70
Fig. 3 – Indirect tensile strength of dual-cured specimens after aging by different aging procedures (mean values and standard deviations).
After water storage at 37 ◦ C, values were similar or slightly lower. After self-curing, no correlation between compressive and indirect tensile strength values was observed (Fig. 2a). After dual-curing, however, a correlation between compressive and indirect tensile strength values was evident (Fig. 2b), which indicates that the results are more reliable. As the indirect tensile strength was more sensible to aging than the compressive strength and as dual-curing produced more reliable data, all further tests were performed by measuring the indirect tensile strength, using dual-cured specimens. As five specimens per measuring point did not provide solid data, all further tests were performed with 30 specimens per measuring point. Fig. 2 – Correlation of compressive strength and indirect tensile strength (mean values): (a) after self-curing and (b) after dual-curing.
3.
Results
3.1.
Specification of the test method
Compressive strength reached a level of about 250 MPa after self-curing and storage in air or water at 37 ◦ C (Fig. 1a), which seems to be stable over time. After thermal cycling, a value slightly below 250 MPa was reached on day 4. However, after longer periods of aging, an irregular curve was observed. After dual-curing (Fig. 1b), compressive strength showed slightly higher values compared with self-curing, which remained constant from the first day over the entire observation period. Indirect tensile strength values of self-cured specimens reached a plateau slightly above 40 MPa when stored in air at 37 ◦ C (Fig. 1c). After water storage, indirect tensile strength values initially ranged in the same order but seemed to decrease after 25 days. By thermal cycling, an initial strength of nearly 40 MPa after 1 day was distinctly reduced to less than 30 MPa. After dual-curing (Fig. 1d), corresponding results were obtained. Both compressive and indirect tensile strength values showed the highest values after storage in air at 37 ◦ C.
3.2.
Effect of aging procedures and duration
Indirect tensile strength of dual-cured specimens after different periods of aging (Fig. 3) showed results analog to the screening test (cf. Fig. 1d). Immediately after curing, a value of 22.0 ± 3.2 MPa was measured. Storage in air at 37 ◦ C for 1 day resulted in an indirect tensile strength of 41.8 ± 3.9 MPa. After 4 days, a value of 44.0 ± 3.9 MPa and after 16 days, a value of 44.5 ± 5.0 MPa were measured. After 64 days of storage, there was still a slight increase up to 45.0 ± 4.7 MPa. After 1 day of water storage at 37 ◦ C the indirect tensile strength reached 40.0 ± 3.4 MPa and slightly decreased to 39.2 ± 4.6 MPa after 4 days and 36.8 ± 6.0 MPa after 16 days. After 64 days of storage, a further decrease to 34.3 ± 5.4 MPa was observed. Although the values of the indirect tensile strength after 1 day were similar for both aging procedures (P = 0.077), further aging in water significantly affected the strength (4 days: P = 0.000; 16 days: P = 0.000; 64 days P = 0.000). After 1 day of thermal cycling, the value increased to 36.7 ± 4.5 MPa and subsequently decreased to 32.1 ± 5.6 MPa at day 4 and 29.2 ± 5.1 MPa at day 16. A slightly declining plateau was reached up to day 64 (27.4 ± 4.1 MPa). After thermal cycling, all values were significantly lower than those obtained after water storage (1 day: P = 0.002; 4 days: P = 0.000; 16 days: P = 0.000; 64 days: P = 0.000). The Weibull modulus (m) was 7.98 directly after dual-curing (Fig. 4). After the first day of storage in air at 37 ◦ C, the Weibull
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50 Indirect tensile strength (MPa)
16 14 Weibull modulus
12 10 8 6 37°C air
4
37°C water
37°C water 1d 37°C water/TC
10
TC light-cured
2
4
6
8
10
12
14
16
18
Time (d)
0 0
10
20
30 40 Time (d)
50
60
70
Fig. 4 – Weibull modulus of dual-cured specimens after aging by different aging procedures.
modulus increased to 13.04, reached a highest value of 13.71 after 4 days but dropped to 10.86 at day 16. After 64 days of storage, an increase to 11.49 was measured. A similar but more pronounced effect was observed after water storage at 37 ◦ C. After 1 day, the Weibull modulus increased to 14.05 but dropped to 7.27 after 16 days of storage and showed a very slight increase at day 64 (m = 7.65). After 1 day of thermal cycling, the Weibull modulus increased to 9.73 and then decreased, reaching a minimum at about 6.7 between days 4 and 16. Between days 16 (m = 6.67) and 64 (m = 7.96), a slight increase was observed.
Effect of water temperature
To evaluate the influence of water storage at varying temperatures on the indirect tensile strength, specimens were stored in water at 5, 25, 37, 45, 55 or 65 ◦ C for 16 days. As control, additional specimens were stored in air at 5, 37 or 65 ◦ C for 16 days (Fig. 5). Between 5 ◦ C (32.2 ± 3.6 MPa) and 25 ◦ C (32.8 ± 5.7 MPa) of water storage, indirect tensile strength ranged in the same order. It steadily increased up to 36.8 ± 6.0 MPa at 37 ◦ C and 38.4 ± 4.5 MPa at 45 ◦ C, then distinctly dropped to 34.6 ± 5.2 MPa at 55 ◦ C and remained constant at 65 ◦ C 60 Indirect tensile strength (MPa)
20
0
light-cured
3.3.
30
0
TC
2
40
50
Fig. 6 – Indirect tensile strength of dual-cured specimens after thermal cycling with or without preceding water storage for 1 day at 37 ◦ C (mean values and standard deviations). The mean values obtained after aging by water storage at 37 ◦ C are added for comparison.
(33.5 ± 5.9 MPa). After storage in air at 5 ◦ C 32.9 ± 4.4 MPa, at 37 ◦ C 44.5 ± 5.0 MPa and at 65 ◦ C 44.2 ± 9.1 MPa were measured. The differences between the two values at 37 and 65 ◦ C were significant (37 ◦ C: P = 0.000; 65 ◦ C: P = 0.000).
3.4. Effect of 1-day water storage at 37 ◦ C prior to thermal cycling The previous tests demonstrated that curing of the cement is basically completed after 1-day storage in water at 37 ◦ C (40.0 ± 3.4 MPa), while immediate thermal cycling impeded to reach the maximum strength (36.7 ± 4.5 MPa after 1 day). After placing a restoration in the oral cavity, immediate thermal stress as applied by thermal cycling is not expected to that extent during the first day. Subject of the following analysis, therefore, was to evaluate the effect of 1-day water storage at 37 ◦ C prior to thermal cycling to allow complete polymerization. The results show that after the pre-treatment the effect of thermal cycling is attenuated (Fig. 6): although after 1 day of thermal cycling, the results without pre-treatment and pre-treatment were similar (36.7 ± 4.5 and 37.6 ± 6.6 MPa, respectively; P = 0.519), after 16 days, the difference was significant (29.2 ± 5.1 and 33.1 ± 5.9 MPa, respectively; P = 0.007). For comparison, the mean strength values of the specimens aged by water storage at 37 ◦ C are provided in Fig. 6 as well.
Aging by water storage at 55 ◦ C
40
3.5.
30
As water storage at 55 ◦ C showed a strong effect of aging, further tests were performed under these conditions, and the results were compared to the effect produced by thermal cycling (Fig. 7). After 1-day storage at 55 ◦ C, indirect tensile strength values ranged at 41.5 ± 4.6 MPa (without 37 ◦ C pre-treatment) and 41.3 ± 4.6 MPa (with 37 ◦ C pre-treatment). After 4 days of aging, the values strongly decreased (37.3 ± 5.2 and 35.6 ± 4.4 MPa, respectively). Between days 4 and 16, there was a further decrease in the strength of specimens without pre-treatment (33.5 ± 5.9 MPa), whereas specimens with pre-treatment did
20 16d, air
10
16d, water
0 0
10
20
30
40
50
60
70
Temperature (°C)
Fig. 5 – Influence of temperature on indirect tensile strength after dual-curing and storage for 16 days (mean values and standard deviations).
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Indirect tensile strength (MPa)
50 40
Discussion
4.1.
Specification of the test method
30 20
1d 37°C water/55°C water 55°C water 1d 37°C water/TC
10
TC light-cured
0 0
2
4
6
8 10 12 Time (d)
14
16
18
Fig. 7 – Indirect tensile strength of dual-cured specimens after water storage at 55 ◦ C with or without preceding water storage for 1 day at 37 ◦ C (mean values and standard deviations). The mean values obtained after thermal cycling are added for comparison.
not lose strength (36.2 ± 4.9 MPa). No significant differences were found between values obtained without pre-treatment and those obtained with pre-treatment (1 day: P = 0.832; 4 days: P = 0.170; 16 days: P = 0.060). All these values significantly differed from the corresponding values after thermal cycling except for day 4, when a pre-treatment was performed (without pre-treatment: 1 day: P = 0.000; 4 days: P = 0.000; 16 days: P = 0.004; with pre-treatment: 1 day: P = 0.014; 4 days: P = 0.113; 16 days: P = 0.032). The Weibull modulus after water storage at 55 ◦ C strongly increased from 7.98 directly after light-curing to 10.98 after 1 day of water storage, but dropped to 7.32 after 16 days (Fig. 8). When pre-treatment for 1 day by water storage at 37 ◦ C was performed, the corresponding Weibull moduli were 14.05 directly after pre-treatment and 10.79 and 8.68 after 1 and 16 days of storage at 55 ◦ C, respectively. For comparison, the Weibull moduli after thermal cycling are also represented in Fig. 8. After thermal cycling without pre-treatment, a similar characteristic with generally lower values was observed. When a pre-treatment at 37 ◦ C was performed, the Weibull modulus after 1 day of thermal cycling strongly decreased, then increased again and finally, after 16 days, ranged in the same order as without pre-treatment. 50 Indirect tensile strength (MPa)
4.
40 30 20 37°C water 1d 37°C water/TC
10
TC light-cured
0 0
2
4
6
8
10
12
14
16
18
Time (d)
Fig. 8 – Effect of a 1-day pre-treatment in water at 37 ◦ C on the Weibull modulus.
Despite the high level of uncertainty, it was learned from the preliminary tests that the compressive strength test in contrast to the indirect tensile strength test is a rather insensitive test method. This may be explained by the degradation mechanism. Diffusion of water molecules into the material leads to a superficial decomposition of chemical bonds, resulting in a reduction of mechanical strength at the surface of the specimens [33]. The indirect tensile strength test is more sensitive to surface defects than the compressive strength test. Hence, the effect of surface degradation is more apparent in a tensile test than in a compressive test [30]. Self-cured specimens in these preliminary tests showed a stronger variation in compressive and indirect tensile strength than dual-cured specimens, which might be explained by an incomplete or more irregular polymerization [8,10]. These findings are supported by the fact that only with dual-cured specimens, a correlation between indirect tensile strength and compressive strength was observed. In summary, the general conclusion can be drawn that the indirect tensile strength test is more sensible than the compressive strength test and dual-curing provides more credible results than self-curing.
4.2.
Effect of aging procedures and duration
Indirect tensile strength values obtained with a reasonable number of specimens substantiate the conclusions derived from the preliminary results. The values obtained after 1-day storage in air or water at 37 ◦ C were similar, but with longer storage time, a significant divergence of both measuring curves occurred, indicating a certain aging effect by water storage at 37 ◦ C. In contrast, thermal cycling had a considerable effect on strength. Some studies suggest that the temperature change and the associated dimensional changes of the two phases—resin and fillers—generate internal stress which explains the more intensive aging effect of thermal cycling [34–37]. For example, the coefficient of thermal expansion of glass and ceramic fillers is about 8–12 × 10−6 and 76 × 10−6 K–1 for acrylic resin. Thus, if cement in the oral cavity is exposed to temperature changes, this may lead to fatigue and wear [38]. Weibull modulus in every case increased after 1 day of storage, indicating a homogenization of the material. The increase was less with thermal cycling, which corresponds to the effects observed in the indirect tensile strength tests. Obviously, polymerization is more impeded and thus less homogeneous in those specimens which underwent thermal cycling compared with the specimens after water storage at 37 ◦ C. When stored in air, the Weibull modulus showed a maximum only at day 4 followed by a decrease up to 16 days and a further increase after 64 days. Residual moisture in the containers might be an explanation. It could also explain the delayed decrease in the Weibull modulus compared with the specimens stored in water or aged by thermal cycling as the concentration of water at the specimen surface is significantly
d e n t a l m a t e r i a l s 3 1 ( 2 0 1 5 ) 855–863
less than that in a water bath. After a longer period, the aging has homogeneously affected the surface of the specimens, thus raising the Weibull modulus. It has to be kept in mind that only the Weibull modulus decreased, and the indirect tensile strength after storage in air slightly increased over time. When stored in water at 37 ◦ C, a more pronounced decrease in Weibull modulus was observed, compared with storage in air. Together with the results of the indirect tensile strength test, the findings indicate a water attack at the surface and subsequently a heterogeneous surface with reduced Weibull modulus. Between days 1 and 4, a strong decrease in the Weibull modulus occurred after thermal cycling. Surface degradation obviously caused a strongly heterogeneous surface and probably surface defects [17]. Consequently, the Weibull modulus declined. An increase in the Weibull modulus at day 64 of thermal cycling might again be explained by a more homogeneously affected surface as stated earlier. The decrease in Weibull modulus was more pronounced after thermal cycling than after water storage. In contrast, there was no increase in Weibull modulus after water storage for 64 days, indicating that the effect of aging in water is decelerated in comparison to aging by thermal cycling.
4.3.
Effect of water temperature
Thermal cycling revealed to be the most effective aging method. The thermal cycling is a combination of hydrolysis and thermal degradation and is intended to simulate an increased stress by repeated sudden temperature changes. In order to better understand the effect of thermal cycling, indirect tensile strength was measured after storage in water for 16 days at temperatures between 5 and 65 ◦ C. As control some strength values after storage in air were added. Interpolation of the results after storage in air showed that with higher temperatures, the indirect tensile strength steadily increases with a maximum gain of 34.3%. These findings are consistent with the results of Daronch et al. [16], who observed an increase in the conversion rate of 36% when the temperature was increased from 3 to 60 ◦ C. The indirect tensile strength of specimens aged by storage in water reached a maximum between 37 and 45 ◦ C and distinctly dropped at a storage temperature of 55 ◦ C. The effects may be explained as follows: at lower temperatures the polymerization is slow and thus the strength is reduced. With increasing temperatures, the polymerization and thus the strength increase, but the water attack is also more efficient, impeding the polymerization at the surface and thus reducing the strength. Above 45 ◦ C the degradation effect is more pronounced than the polymerization effect. Therefore, the strength is strongly reduced.
4.4. Effect of 1-day water storage at 37 ◦ C prior to thermal cycling After storage in water, in every case, the values were higher than those obtained after thermal cycling, which indicates that thermal cycling interferes with polymerization, if applied immediately after light-curing. In clinical use, after placing a restoration, the cement is setting at 37 ◦ C and completely polymerizes over a period of 24 h [29]. Thermal stress is rare during
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this early stage. Hence, the procedure for artificial aging has to allow complete polymerization, which may be obtained by storing the resin composite cement for 1 day at 37 ◦ C in a water bath prior to thermal cycling. The present investigations have shown that after a pre-treatment at 37 ◦ C in water for 1 day the strength was less reduced after long-term thermal cycling. The different behaviors might be explained by the different conversion rates obtained after the different test set-ups. As specified earlier, thermal cycling leads to dimensional changes of the composite phases, thus providing better access of water molecules, which in turn impede the polymerization [17,18]. With a 37 ◦ C pre-treatment, a complete polymerization may be obtained before thermal stress is applied.
4.5.
Aging by water storage at 55 ◦ C
Water storage at 55 ◦ C had strongly affected the mechanical strength of the specimens and thus may be considered as an economical procedure to age composite resin cements. However, during thermal cycling, specimens were only half the time exposed to 55 ◦ C but the loss of strength was more distinct compared with those specimens stored permanently at 55 ◦ C, which proves that not only the temperature and the duration of its exposure but also the sudden temperature change during thermal cycling has an impact on the strength. Thus, when balancing between both procedures, the aging mechanisms have to be completely understood. The strength values after short-term aging at 55 ◦ C did not significantly differ, whether a pre-treatment for 1-day water storage at 37 ◦ C was performed or not. An explanation might be that at 55 ◦ C the polymerization is faster than at 37 ◦ C and thus the kinetics of polymerization and degradation are shifted toward the polymerization. In the long term, the specimens with pre-treatment provided higher strength values, observation of which is identical to the results after thermal cycling, proving that a pre-treatment by water storage at 37 ◦ C is beneficial to allow complete polymerization and to obtain reliable results.
4.6.
Specification of the test design
Based on the present results, an aging procedure should include a 1-day pre-treatment by water storage at 37 ◦ C to allow complete polymerization and a 4-day thermal cycling with water baths of 5 and 55 ◦ C and cycle duration 1 min.
4.7. Limitations of the study and further investigations 4.7.1.
Restriction to one material
The major shortcoming of the present study is the use of only one resin composite cement. Long-term measurements with various resin composite cements have to be performed to cover the whole material variations and thus to confirm the observations as a generally valid result.
4.7.2.
Specimen geometry
The indirect tensile test revealed to be a reliable method for evaluating the effect of aging on the mechanical strength of resin composite cements. The fabrication of the specimens
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is much easier compared with the flexural strength test. However, it has to be proven that flexural strength test and indirect tensile strength test provide similar results.
4.7.3.
Sample size
It has been shown that a sample size of five specimens is not sufficient to provide reliable results. In contrast, it seems that the number of 30 specimens for each measuring point is exaggerated. The appropriate sample size has to be evaluated.
4.7.4.
Water storage at 55 ◦ C
Aging by 55 ◦ C water storage was less effective than thermal cycling. Nevertheless, water storage is much easier than thermal cycling, because there is no special equipment required and thus the capacity is unlimited. At least for screening tests during the development of new materials aging by water storage might be a cost- and time-saving method. Water storage at 55 ◦ C as a potential aging method has to be further investigated.
5.
Conclusion
Within the limitations of this study, particularly in view of the restriction to one material, the following can be concluded. (1) The compressive strength test is not suitable for evaluating aging effects in resin composite cements. (2) The indirect tensile test is an appropriate test to assess the influence of artificial aging on resin composite cements. (3) Dual-cured specimens showed less variance of strength values than self-cured specimens. (4) Thermal cycling (5 ◦ C/55 ◦ C, 1 min) is the most efficient aging procedure. (5) Water storage at 37 ◦ C for 1 day should be performed prior to thermal cycling. (6) Water storage at 55 ◦ C might be considered as an economical aging procedure. (7) The findings have to be verified with additional cements.
Acknowledgment The authors are grateful to Dr Heinz Schuh, Kuraray-Noritake Dental Inc., Europe for providing the cement.
references
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[5] Peutzfeldt A. Resin composites in dentistry: The monomer systems. Eur J Oral Sci 1997;105:97–116. [6] Wendt Jr SL. The effect of heat used as secondary cure upon the physical properties of three composite resins. I. Diametral tensile strength, compressive strength and marginal dimensional stability. Quintessence Int 1987;18:265–71. [7] Braga RR, Cesar PF, Gonzaga CC. Mechanical properties of resin cements with different activation modes. J Oral Rehab 2002;29:257–62. [8] Caughman WF, Chan DC, Rueggeberg FA. Curing potential of dual-polymerizable resin cements in simulated clinical situations. J Prosthet Dent 2001;86:101–6. [9] Braga RR, Condon JR, Ferracane JL. In vitro wear simulation measurements of composite versus resin-modified glass ionomer luting cements for all-ceramic restorations. J Esthet Restor Dent 2002;14:368–76. [10] Fonseca RG, Gomes dos Santos J, Adabo LG. Influence of activation modes on diametral tensile strength of dual-curing resin cements. Braz Oral Res 2005;19:267–71. [11] Faria-e-Silva AL, Moraes RR, Ogliari FA, Piva E, Martins LRM. Panavia F: the role of the primer. J Oral Sci 2009;51:255–9. [12] Jose E, Prieto LT, Soares GF, Santos Dias CT, Aguiar FHB, Paulillo LAMS. The effect of curing light and chemical catalyst on the degree of conversion of two dual cured luting cements. Lasers Med Sci 2012;27:145–51. [13] Daronch M, Rueggeberg FA, De Goes MF, Giudici R. Polymerization kinetics of pre-heated composite. J Dent Res 2006;85:38–43. [14] Cantoro A, Goracci C, Papacchini F, Mazzitelli C, Fadda GM, Ferrari M. Effect of pre-cure-temperature on the bonding potential of self-etch and self-adhesive resin cements. Dent Mater 2008;24:577–83. [15] Cantoro A, Goracci C, Carvalho CA, Coniglio I, Ferrari M. Bonding potential of self-adhesive luting agents used at different temperatures to lute composite onlays. J Dent 2009;37:454–61. [16] Daronch M, Rueggeberg FA, De Goes MF. Monomer conversion of pre-heated composite. J Dent Res 2005;84:663–7. [17] Medeiros IS, Gomes MN, Loguercio AD, Filho LER. Diametral tensile strength and Vickers hardness of a composite after storage in different solutions. J Oral Sci 2007;49:61–6. [18] Ferrance JL. Hygroscopic and hydrolytic effects in dental polymer networks. Dent Mater 2006;22:211–22. [19] Barclay CW, Spence D, Laird WR. Intra-oral temperatures during function. J Oral Rehabil 2005;32:886–94. [20] Ernst CP, Canbek K, Euler T, Willershausen B. In vivo validation of the historical in vitro thermocycling temperature range for dental materials testing. Clin Oral Invest 2004;8:130–8. [21] Palmer DS, Barco MT, Billy EJ. Temperature extremes produced orally by hot and cold liquids. J Prosthet Dent 1992;67:325–7. [22] Braga Pereira SM, Almeida Castilho A, Salazar-Marocho SM, Costa Oliveira KM. Thermocycling effect on microhardness of laboratory composite resins. Braz J Oral Sci 2007;6:1372–5. [23] Hahnel S, Henrich A, Bürgers R, Handel G, Rosentritt M. Investigation of mechanical properties of modern dental composites after artificial aging for one year. Oper Dent 2010;4:412–9. [24] Assunc¸ão WG, Gomes EA, Barão VA, Barbosa DB, Delben JA, Tabata LF. Effect of storage in artificial saliva and thermal cycling on Knoop hardness of resin denture teeth. J Prost Res 2010;54:123–7. [25] Hirabayashi S, Nomoto R, Harashima I, Hirasawa T. The surface degradation of various light-cured composite resins by thermal cycling. Shika Zairyo Kikai 1990;9:53–64.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
A systematic approach to standardize artificial aging of resin composite cements Lukas Blumer a , Fredy Schmidli a , Roland Weiger b , Jens Fischer a,c,∗ a
Department of Dental Materials and Engineering, University Hospital of Dental Medicine, University of Basel, Basel, Switzerland b Department of Periodontology, Endodontology and Cariology, University Hospital of Dental Medicine, University of Basel, Basel, Switzerland c VITA Zahnfabrik, Bad Säckingen, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. The aim of the investigation was to contribute to the ongoing discussion at the
Received 1 February 2015
international standardization committee on how to artificially age dental resin composite
Received in revised form
cements.
20 March 2015
Methods. Indirect tensile strength (n = 30) of a dual-cured resin composite cement (Panavia
Accepted 27 April 2015
F2.0) was measured to evaluate the effect of water storage at 37 ◦ C or thermal cycling (5 ◦ C/55 ◦ C/1 min) for up to 64 days. The influence of water temperature (5–65 ◦ C) after 16 days and the effect of 1 day water storage at 37 ◦ C prior to aging were assessed. Storage in
Keywords:
air at 37 ◦ C served as control.
Resin composite cement
Results. Thermal cycling affected the indirect tensile strength most, followed by water stor-
Compressive strength
age at 55 ◦ C, whereas water storage at 37 ◦ C had only little influence. Major deterioration
Indirect tensile strength
occurred before day 4 (≈6000 cycles). A 1-day pre-treatment by water storage at 37 ◦ C prior
Aging
to thermal cycling attenuated the effect of aging.
Thermal cycling
Significance. For the material investigated, thermal cycling for 4 days is the most efficient
International standardization
aging procedure. A 1-day water storage at 37 ◦ C prior to thermal cycling is recommended to allow complete polymerization. A 4-day water storage at 55 ◦ C may be considered as a viable alternative to thermal cycling. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
With the increasing number of ceramic restorations in dentistry, the use of adhesive resin composite cements has steadily increased. Compared with conventional cements based on acid–base reactions, they have better mechanical
properties, higher bond strength, reduced solubility and improved esthetics [1,2]. In general, these materials consist of three components: the polymer matrix, the fillers and the silanes as binder between the organic and the inorganic phase [3,4]. The properties of resin composite cements such as elasticity, plasticity, hardness, strength and thermal as well as chemical stability are determined by the properties of
∗ Corresponding author at: Department of Dental Materials and Engineering, University Hospital of Dental Medicine, University of Basel, Hebelstrasse 3, 4056 Basel, Switzerland. Tel.: +41 61 267 26 26. E-mail address: jens.fi[email protected] (J. Fischer).
http://dx.doi.org/10.1016/j.dental.2015.04.015 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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these single components and the respective microstructure [4–6]. Polymerization is catalyzed either by autocatalysis (“selfcuring”) or photo-initiation (“light-curing”). The autocatalytic polymerization has the disadvantage that the reaction starts immediately after mixing the catalyst (initiator) with the polymerizable component. Thus, the processing time is limited. In contrast, the photo-initiation allows determining the start of the polymerization. However, areas that are not reached by the light will not polymerize adequately. To avoid the disadvantages of each, cements were developed comprising both initiator systems (“dual-curing”). The idea was to provide a material of which the processing time is adapted to the clinical requirements and, independent of the influence of light, a high degree of conversion will be achieved [3,7]. The conversion level is an important factor for the mechanical strength of luting cements because composites with a high degree of conversion also have better mechanical properties [3,7]. Most cements show a higher degree of conversion by dual-curing compared with self-curing alone [8–10]. Additional catalysts are beneficial to improve polymerization. For instance, the application of ED Primer (Kuraray-Noritake, Kurashiki, Japan) increases the degree of conversion of Panavia F2.0 (Kuraray-Noritake) by 15% after self-curing and by 24% after dual-curing [11,12]. The conversion degree in self-curing is influenced not only by the concentration of monomer and catalyst, but also by the ambient temperature [13–15]. An increase in temperature from 3 to 60 ◦ C resulted in an increase in the degree of conversion of 36% [16]. Cements in an aqueous medium such as saliva are exposed to a long-term aging process, which might significantly reduce the mechanical properties [17]. The effects are wide-ranging, but typically include the leaching of unreacted components and the degradation of the polymer network [17,18]. At present, no standardized procedure for artificial aging of resin composite cements is available. Thermal cycling simulates the effect of varying temperatures in the oral cavity, which might range from 0 ◦ C (melting ice) to 60 ◦ C (hot beverages). For that reason, thermal cycling is usually performed between 5 and 55 ◦ C with cycle times of 1 min [19–21]. The suggested duration of thermal cycling differs from 3000 to 100,000 cycles [22–26]. It is proposed that 10,000 cycles might represent 1 year of service [27,28]. After the placement of the restoration, however, the cement is setting at 37 ◦ C and polymerizes for another 24 h [29]. During this time, thermal stress is rare. Thus, to mimic the clinical situation, the effect of a pre-treatment of the cement at 37 ◦ C prior to artificial aging should be analyzed. Composite resin cements are brittle materials and therefore tolerant to compressive stress but susceptible to tensile loading [30]. Hence, a three-point bending test for the characterization of the strength of luting materials is recommended in the respective standard ISO 4049. A 1-day water storage at 37 ◦ C prior to the test is specified to allow a complete polymerization. However, the preparation of specimens for bending tests is demanding and time-consuming. Especially during development of a new product and with the increasing number of products available, the quantity of such tests will increase. Therefore, simplification of these tests would be attractive. Compressive or indirect tensile strength tests provide reliable information on the mechanical strength of
Table 1 – Composite resin cement and primer used in the study (material compositions according to the manufacturer’s product specification). Material Panavia F 2.0
ED Primer
Composition Base paste: hydrophobic aromatic dimethacrylate, hydrophilic and hydrophobic aliphatic dimethacrylate, sodium aromatic sulphinate, silanated barium glass filler, surface-treated sodium fluoride, catalysts, accelerators, pigments Catalyst paste: 10-methacryloyloxydecyl dihydrogen phosphate (MDP), hydrophobic aromatic dimethacrylate, hydrophilic and hydrophobic aliphatic dimethacrylate, silanated silica filler and colloidal silica, dl-camphorquinone, catalysts, initiators Liquid A: 2-hydroxyethyl methacrylate (HEMA), MDP, N-methacryloyl-5-aminosalicylic acid (5-NMSA), water, accelerators Liquid B: N-methacryloyl-5-aminosalicylic acid (5-NMSA), catalysts, accelerators, water
the investigated material with the advantage of easy handling [31,32]. To the knowledge of the authors, an impact of aging on indirect tensile strength or compressive strength of resin composite cements is not yet investigated and therefore worth to be analyzed. Further, water storage instead of thermal cycling could be an option to economize the test procedure, because thermal cycling requires a special device with a limited capacity. Against that background, the appropriate international standardizing committee (ISO/TC 106/SC 1/WG 9) is currently in the process of evaluating an adequate test design for assessing the effect of aging on the mechanical strength of resin composite cements. The aim of the present study was to contribute to the discussion by a systematic analysis of the effect of different aging procedures on composite resin cements by compressive and indirect tensile strength tests.
2.
Materials and methods
In the present study, Panavia F2.0 (Kuraray-Noritake) was selected as test material (Table 1). The specimens were produced in a customized Teflon mold 3 mm in height with five cylindrical holes each 3 mm in diameter. Depending on the curing mode, an opaque Teflon plate (self-curing) or a transparent glass plate covered with a Mylar strip (Hawe Transparent Strips, KerrHawe, Bioggio, Switzerland) (dualcuring) was fixed at the bottom of the Teflon plate. All surfaces of the mold coming in contact with the specimens were first coated with ED Primer (Kuraray-Noritake). One drop each of primer liquids A and B was mixed for 20 s, applied to the mold and gently dried for 5 s with air as recommended by the manufacturer. Base and catalyst pastes of the cement were mixed according to the manufacturer’s instructions in a 1:1 ratio for 20 s with a plastic spatula on a mixing tray and at the latest 2 min after start of mixing filled into the molds with slight excess. For self-curing, the top of the Teflon mold was also covered with an opaque Teflon plate and left for 20 min under a load of 1 kg. For “dual-curing,” the top of the Teflon mold was
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Fig. 1 – Effect of different aging procedures on mechanical parameters of the cement investigated (mean values and standard deviations): (a) compressive strength after self-curing, (b) compressive strength after dual-curing, (c) indirect tensile strength after self-curing and (d) indirect tensile strength after dual-curing.
covered with a Mylar strip and a glass plate. As the diameter of the light emission window was greater than that of the specimens, only one light exposure for 20 s on each side to an LED lamp (Elipar S10, 3M ESPE, Seefeld, Germany) was performed. The tip of the lamp was centered in a distance of 5 mm perpendicular to the specimens. Subsequently, the specimens were carefully removed from the mold, deflashed and stored, depending on the intended aging test as follows. Air storage: Specimens were stored in a sealed glass container at the respective temperature in an incubator (T-40/25, CTS, Hechingen, Germany). Water storage: Specimens were subjected to the same conditions as for the storage in air; however, the glass container was filled with distilled water. Thermal cycling: In a customized device, the specimens were immersed alternately in water baths of 5 and 55 ◦ C, using a sieve for storage and transportation. The cycle duration was 1 min with a dwell time in each water bath of 28 s and a transfer time between baths of 2 s. Self- and dual-cured specimens were tested in a screening test both for compressive and for indirect tensile strength (n = 5) immediately after preparation and after 1, 4, 9, 16,
25, 36, 49 and 64 days in a universal testing machine (Z010, Zwick/Roell, Ulm, Germany) with a cross-head speed of 1 mm/min. Prior to the measurements, the specimens were sized in diameter and height, using a digital caliper (Cal IP 67, Tesa, Renens, Switzerland). For compressive strength, the load was applied axially for indirect tensile strength radially. Strength values were calculated using the following equations: Compressive strength :
c =
F (d/2)
Indirect tensile strength : t =
2
2F dh
where F is the load at fracture, d the specimen diameter and h the specimen height. Based on the obtained results, test conditions were selected in order to obtain more reliable results and to determine the Weibull modulus (n = 30). Statistical analysis was performed with a one-way ANOVA (˛ = 0.05).
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Indirect tensile strength (MPa)
60 50 40 30 20
37°C air 37°C water
10
TC light-cured
0 0
10
20
30 40 Time (d)
50
60
70
Fig. 3 – Indirect tensile strength of dual-cured specimens after aging by different aging procedures (mean values and standard deviations).
After water storage at 37 ◦ C, values were similar or slightly lower. After self-curing, no correlation between compressive and indirect tensile strength values was observed (Fig. 2a). After dual-curing, however, a correlation between compressive and indirect tensile strength values was evident (Fig. 2b), which indicates that the results are more reliable. As the indirect tensile strength was more sensible to aging than the compressive strength and as dual-curing produced more reliable data, all further tests were performed by measuring the indirect tensile strength, using dual-cured specimens. As five specimens per measuring point did not provide solid data, all further tests were performed with 30 specimens per measuring point. Fig. 2 – Correlation of compressive strength and indirect tensile strength (mean values): (a) after self-curing and (b) after dual-curing.
3.
Results
3.1.
Specification of the test method
Compressive strength reached a level of about 250 MPa after self-curing and storage in air or water at 37 ◦ C (Fig. 1a), which seems to be stable over time. After thermal cycling, a value slightly below 250 MPa was reached on day 4. However, after longer periods of aging, an irregular curve was observed. After dual-curing (Fig. 1b), compressive strength showed slightly higher values compared with self-curing, which remained constant from the first day over the entire observation period. Indirect tensile strength values of self-cured specimens reached a plateau slightly above 40 MPa when stored in air at 37 ◦ C (Fig. 1c). After water storage, indirect tensile strength values initially ranged in the same order but seemed to decrease after 25 days. By thermal cycling, an initial strength of nearly 40 MPa after 1 day was distinctly reduced to less than 30 MPa. After dual-curing (Fig. 1d), corresponding results were obtained. Both compressive and indirect tensile strength values showed the highest values after storage in air at 37 ◦ C.
3.2.
Effect of aging procedures and duration
Indirect tensile strength of dual-cured specimens after different periods of aging (Fig. 3) showed results analog to the screening test (cf. Fig. 1d). Immediately after curing, a value of 22.0 ± 3.2 MPa was measured. Storage in air at 37 ◦ C for 1 day resulted in an indirect tensile strength of 41.8 ± 3.9 MPa. After 4 days, a value of 44.0 ± 3.9 MPa and after 16 days, a value of 44.5 ± 5.0 MPa were measured. After 64 days of storage, there was still a slight increase up to 45.0 ± 4.7 MPa. After 1 day of water storage at 37 ◦ C the indirect tensile strength reached 40.0 ± 3.4 MPa and slightly decreased to 39.2 ± 4.6 MPa after 4 days and 36.8 ± 6.0 MPa after 16 days. After 64 days of storage, a further decrease to 34.3 ± 5.4 MPa was observed. Although the values of the indirect tensile strength after 1 day were similar for both aging procedures (P = 0.077), further aging in water significantly affected the strength (4 days: P = 0.000; 16 days: P = 0.000; 64 days P = 0.000). After 1 day of thermal cycling, the value increased to 36.7 ± 4.5 MPa and subsequently decreased to 32.1 ± 5.6 MPa at day 4 and 29.2 ± 5.1 MPa at day 16. A slightly declining plateau was reached up to day 64 (27.4 ± 4.1 MPa). After thermal cycling, all values were significantly lower than those obtained after water storage (1 day: P = 0.002; 4 days: P = 0.000; 16 days: P = 0.000; 64 days: P = 0.000). The Weibull modulus (m) was 7.98 directly after dual-curing (Fig. 4). After the first day of storage in air at 37 ◦ C, the Weibull
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50 Indirect tensile strength (MPa)
16 14 Weibull modulus
12 10 8 6 37°C air
4
37°C water
37°C water 1d 37°C water/TC
10
TC light-cured
2
4
6
8
10
12
14
16
18
Time (d)
0 0
10
20
30 40 Time (d)
50
60
70
Fig. 4 – Weibull modulus of dual-cured specimens after aging by different aging procedures.
modulus increased to 13.04, reached a highest value of 13.71 after 4 days but dropped to 10.86 at day 16. After 64 days of storage, an increase to 11.49 was measured. A similar but more pronounced effect was observed after water storage at 37 ◦ C. After 1 day, the Weibull modulus increased to 14.05 but dropped to 7.27 after 16 days of storage and showed a very slight increase at day 64 (m = 7.65). After 1 day of thermal cycling, the Weibull modulus increased to 9.73 and then decreased, reaching a minimum at about 6.7 between days 4 and 16. Between days 16 (m = 6.67) and 64 (m = 7.96), a slight increase was observed.
Effect of water temperature
To evaluate the influence of water storage at varying temperatures on the indirect tensile strength, specimens were stored in water at 5, 25, 37, 45, 55 or 65 ◦ C for 16 days. As control, additional specimens were stored in air at 5, 37 or 65 ◦ C for 16 days (Fig. 5). Between 5 ◦ C (32.2 ± 3.6 MPa) and 25 ◦ C (32.8 ± 5.7 MPa) of water storage, indirect tensile strength ranged in the same order. It steadily increased up to 36.8 ± 6.0 MPa at 37 ◦ C and 38.4 ± 4.5 MPa at 45 ◦ C, then distinctly dropped to 34.6 ± 5.2 MPa at 55 ◦ C and remained constant at 65 ◦ C 60 Indirect tensile strength (MPa)
20
0
light-cured
3.3.
30
0
TC
2
40
50
Fig. 6 – Indirect tensile strength of dual-cured specimens after thermal cycling with or without preceding water storage for 1 day at 37 ◦ C (mean values and standard deviations). The mean values obtained after aging by water storage at 37 ◦ C are added for comparison.
(33.5 ± 5.9 MPa). After storage in air at 5 ◦ C 32.9 ± 4.4 MPa, at 37 ◦ C 44.5 ± 5.0 MPa and at 65 ◦ C 44.2 ± 9.1 MPa were measured. The differences between the two values at 37 and 65 ◦ C were significant (37 ◦ C: P = 0.000; 65 ◦ C: P = 0.000).
3.4. Effect of 1-day water storage at 37 ◦ C prior to thermal cycling The previous tests demonstrated that curing of the cement is basically completed after 1-day storage in water at 37 ◦ C (40.0 ± 3.4 MPa), while immediate thermal cycling impeded to reach the maximum strength (36.7 ± 4.5 MPa after 1 day). After placing a restoration in the oral cavity, immediate thermal stress as applied by thermal cycling is not expected to that extent during the first day. Subject of the following analysis, therefore, was to evaluate the effect of 1-day water storage at 37 ◦ C prior to thermal cycling to allow complete polymerization. The results show that after the pre-treatment the effect of thermal cycling is attenuated (Fig. 6): although after 1 day of thermal cycling, the results without pre-treatment and pre-treatment were similar (36.7 ± 4.5 and 37.6 ± 6.6 MPa, respectively; P = 0.519), after 16 days, the difference was significant (29.2 ± 5.1 and 33.1 ± 5.9 MPa, respectively; P = 0.007). For comparison, the mean strength values of the specimens aged by water storage at 37 ◦ C are provided in Fig. 6 as well.
Aging by water storage at 55 ◦ C
40
3.5.
30
As water storage at 55 ◦ C showed a strong effect of aging, further tests were performed under these conditions, and the results were compared to the effect produced by thermal cycling (Fig. 7). After 1-day storage at 55 ◦ C, indirect tensile strength values ranged at 41.5 ± 4.6 MPa (without 37 ◦ C pre-treatment) and 41.3 ± 4.6 MPa (with 37 ◦ C pre-treatment). After 4 days of aging, the values strongly decreased (37.3 ± 5.2 and 35.6 ± 4.4 MPa, respectively). Between days 4 and 16, there was a further decrease in the strength of specimens without pre-treatment (33.5 ± 5.9 MPa), whereas specimens with pre-treatment did
20 16d, air
10
16d, water
0 0
10
20
30
40
50
60
70
Temperature (°C)
Fig. 5 – Influence of temperature on indirect tensile strength after dual-curing and storage for 16 days (mean values and standard deviations).
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d e n t a l m a t e r i a l s 3 1 ( 2 0 1 5 ) 855–863
Indirect tensile strength (MPa)
50 40
Discussion
4.1.
Specification of the test method
30 20
1d 37°C water/55°C water 55°C water 1d 37°C water/TC
10
TC light-cured
0 0
2
4
6
8 10 12 Time (d)
14
16
18
Fig. 7 – Indirect tensile strength of dual-cured specimens after water storage at 55 ◦ C with or without preceding water storage for 1 day at 37 ◦ C (mean values and standard deviations). The mean values obtained after thermal cycling are added for comparison.
not lose strength (36.2 ± 4.9 MPa). No significant differences were found between values obtained without pre-treatment and those obtained with pre-treatment (1 day: P = 0.832; 4 days: P = 0.170; 16 days: P = 0.060). All these values significantly differed from the corresponding values after thermal cycling except for day 4, when a pre-treatment was performed (without pre-treatment: 1 day: P = 0.000; 4 days: P = 0.000; 16 days: P = 0.004; with pre-treatment: 1 day: P = 0.014; 4 days: P = 0.113; 16 days: P = 0.032). The Weibull modulus after water storage at 55 ◦ C strongly increased from 7.98 directly after light-curing to 10.98 after 1 day of water storage, but dropped to 7.32 after 16 days (Fig. 8). When pre-treatment for 1 day by water storage at 37 ◦ C was performed, the corresponding Weibull moduli were 14.05 directly after pre-treatment and 10.79 and 8.68 after 1 and 16 days of storage at 55 ◦ C, respectively. For comparison, the Weibull moduli after thermal cycling are also represented in Fig. 8. After thermal cycling without pre-treatment, a similar characteristic with generally lower values was observed. When a pre-treatment at 37 ◦ C was performed, the Weibull modulus after 1 day of thermal cycling strongly decreased, then increased again and finally, after 16 days, ranged in the same order as without pre-treatment. 50 Indirect tensile strength (MPa)
4.
40 30 20 37°C water 1d 37°C water/TC
10
TC light-cured
0 0
2
4
6
8
10
12
14
16
18
Time (d)
Fig. 8 – Effect of a 1-day pre-treatment in water at 37 ◦ C on the Weibull modulus.
Despite the high level of uncertainty, it was learned from the preliminary tests that the compressive strength test in contrast to the indirect tensile strength test is a rather insensitive test method. This may be explained by the degradation mechanism. Diffusion of water molecules into the material leads to a superficial decomposition of chemical bonds, resulting in a reduction of mechanical strength at the surface of the specimens [33]. The indirect tensile strength test is more sensitive to surface defects than the compressive strength test. Hence, the effect of surface degradation is more apparent in a tensile test than in a compressive test [30]. Self-cured specimens in these preliminary tests showed a stronger variation in compressive and indirect tensile strength than dual-cured specimens, which might be explained by an incomplete or more irregular polymerization [8,10]. These findings are supported by the fact that only with dual-cured specimens, a correlation between indirect tensile strength and compressive strength was observed. In summary, the general conclusion can be drawn that the indirect tensile strength test is more sensible than the compressive strength test and dual-curing provides more credible results than self-curing.
4.2.
Effect of aging procedures and duration
Indirect tensile strength values obtained with a reasonable number of specimens substantiate the conclusions derived from the preliminary results. The values obtained after 1-day storage in air or water at 37 ◦ C were similar, but with longer storage time, a significant divergence of both measuring curves occurred, indicating a certain aging effect by water storage at 37 ◦ C. In contrast, thermal cycling had a considerable effect on strength. Some studies suggest that the temperature change and the associated dimensional changes of the two phases—resin and fillers—generate internal stress which explains the more intensive aging effect of thermal cycling [34–37]. For example, the coefficient of thermal expansion of glass and ceramic fillers is about 8–12 × 10−6 and 76 × 10−6 K–1 for acrylic resin. Thus, if cement in the oral cavity is exposed to temperature changes, this may lead to fatigue and wear [38]. Weibull modulus in every case increased after 1 day of storage, indicating a homogenization of the material. The increase was less with thermal cycling, which corresponds to the effects observed in the indirect tensile strength tests. Obviously, polymerization is more impeded and thus less homogeneous in those specimens which underwent thermal cycling compared with the specimens after water storage at 37 ◦ C. When stored in air, the Weibull modulus showed a maximum only at day 4 followed by a decrease up to 16 days and a further increase after 64 days. Residual moisture in the containers might be an explanation. It could also explain the delayed decrease in the Weibull modulus compared with the specimens stored in water or aged by thermal cycling as the concentration of water at the specimen surface is significantly
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less than that in a water bath. After a longer period, the aging has homogeneously affected the surface of the specimens, thus raising the Weibull modulus. It has to be kept in mind that only the Weibull modulus decreased, and the indirect tensile strength after storage in air slightly increased over time. When stored in water at 37 ◦ C, a more pronounced decrease in Weibull modulus was observed, compared with storage in air. Together with the results of the indirect tensile strength test, the findings indicate a water attack at the surface and subsequently a heterogeneous surface with reduced Weibull modulus. Between days 1 and 4, a strong decrease in the Weibull modulus occurred after thermal cycling. Surface degradation obviously caused a strongly heterogeneous surface and probably surface defects [17]. Consequently, the Weibull modulus declined. An increase in the Weibull modulus at day 64 of thermal cycling might again be explained by a more homogeneously affected surface as stated earlier. The decrease in Weibull modulus was more pronounced after thermal cycling than after water storage. In contrast, there was no increase in Weibull modulus after water storage for 64 days, indicating that the effect of aging in water is decelerated in comparison to aging by thermal cycling.
4.3.
Effect of water temperature
Thermal cycling revealed to be the most effective aging method. The thermal cycling is a combination of hydrolysis and thermal degradation and is intended to simulate an increased stress by repeated sudden temperature changes. In order to better understand the effect of thermal cycling, indirect tensile strength was measured after storage in water for 16 days at temperatures between 5 and 65 ◦ C. As control some strength values after storage in air were added. Interpolation of the results after storage in air showed that with higher temperatures, the indirect tensile strength steadily increases with a maximum gain of 34.3%. These findings are consistent with the results of Daronch et al. [16], who observed an increase in the conversion rate of 36% when the temperature was increased from 3 to 60 ◦ C. The indirect tensile strength of specimens aged by storage in water reached a maximum between 37 and 45 ◦ C and distinctly dropped at a storage temperature of 55 ◦ C. The effects may be explained as follows: at lower temperatures the polymerization is slow and thus the strength is reduced. With increasing temperatures, the polymerization and thus the strength increase, but the water attack is also more efficient, impeding the polymerization at the surface and thus reducing the strength. Above 45 ◦ C the degradation effect is more pronounced than the polymerization effect. Therefore, the strength is strongly reduced.
4.4. Effect of 1-day water storage at 37 ◦ C prior to thermal cycling After storage in water, in every case, the values were higher than those obtained after thermal cycling, which indicates that thermal cycling interferes with polymerization, if applied immediately after light-curing. In clinical use, after placing a restoration, the cement is setting at 37 ◦ C and completely polymerizes over a period of 24 h [29]. Thermal stress is rare during
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this early stage. Hence, the procedure for artificial aging has to allow complete polymerization, which may be obtained by storing the resin composite cement for 1 day at 37 ◦ C in a water bath prior to thermal cycling. The present investigations have shown that after a pre-treatment at 37 ◦ C in water for 1 day the strength was less reduced after long-term thermal cycling. The different behaviors might be explained by the different conversion rates obtained after the different test set-ups. As specified earlier, thermal cycling leads to dimensional changes of the composite phases, thus providing better access of water molecules, which in turn impede the polymerization [17,18]. With a 37 ◦ C pre-treatment, a complete polymerization may be obtained before thermal stress is applied.
4.5.
Aging by water storage at 55 ◦ C
Water storage at 55 ◦ C had strongly affected the mechanical strength of the specimens and thus may be considered as an economical procedure to age composite resin cements. However, during thermal cycling, specimens were only half the time exposed to 55 ◦ C but the loss of strength was more distinct compared with those specimens stored permanently at 55 ◦ C, which proves that not only the temperature and the duration of its exposure but also the sudden temperature change during thermal cycling has an impact on the strength. Thus, when balancing between both procedures, the aging mechanisms have to be completely understood. The strength values after short-term aging at 55 ◦ C did not significantly differ, whether a pre-treatment for 1-day water storage at 37 ◦ C was performed or not. An explanation might be that at 55 ◦ C the polymerization is faster than at 37 ◦ C and thus the kinetics of polymerization and degradation are shifted toward the polymerization. In the long term, the specimens with pre-treatment provided higher strength values, observation of which is identical to the results after thermal cycling, proving that a pre-treatment by water storage at 37 ◦ C is beneficial to allow complete polymerization and to obtain reliable results.
4.6.
Specification of the test design
Based on the present results, an aging procedure should include a 1-day pre-treatment by water storage at 37 ◦ C to allow complete polymerization and a 4-day thermal cycling with water baths of 5 and 55 ◦ C and cycle duration 1 min.
4.7. Limitations of the study and further investigations 4.7.1.
Restriction to one material
The major shortcoming of the present study is the use of only one resin composite cement. Long-term measurements with various resin composite cements have to be performed to cover the whole material variations and thus to confirm the observations as a generally valid result.
4.7.2.
Specimen geometry
The indirect tensile test revealed to be a reliable method for evaluating the effect of aging on the mechanical strength of resin composite cements. The fabrication of the specimens
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is much easier compared with the flexural strength test. However, it has to be proven that flexural strength test and indirect tensile strength test provide similar results.
4.7.3.
Sample size
It has been shown that a sample size of five specimens is not sufficient to provide reliable results. In contrast, it seems that the number of 30 specimens for each measuring point is exaggerated. The appropriate sample size has to be evaluated.
4.7.4.
Water storage at 55 ◦ C
Aging by 55 ◦ C water storage was less effective than thermal cycling. Nevertheless, water storage is much easier than thermal cycling, because there is no special equipment required and thus the capacity is unlimited. At least for screening tests during the development of new materials aging by water storage might be a cost- and time-saving method. Water storage at 55 ◦ C as a potential aging method has to be further investigated.
5.
Conclusion
Within the limitations of this study, particularly in view of the restriction to one material, the following can be concluded. (1) The compressive strength test is not suitable for evaluating aging effects in resin composite cements. (2) The indirect tensile test is an appropriate test to assess the influence of artificial aging on resin composite cements. (3) Dual-cured specimens showed less variance of strength values than self-cured specimens. (4) Thermal cycling (5 ◦ C/55 ◦ C, 1 min) is the most efficient aging procedure. (5) Water storage at 37 ◦ C for 1 day should be performed prior to thermal cycling. (6) Water storage at 55 ◦ C might be considered as an economical aging procedure. (7) The findings have to be verified with additional cements.
Acknowledgment The authors are grateful to Dr Heinz Schuh, Kuraray-Noritake Dental Inc., Europe for providing the cement.
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