Effects of solvent drying time on mass change of three adhesives.

AIM: Adhesives may change their mass due to water sorption or dilution of components after curing. The aim of this study was to evaluate the influence of air-drying time and water storage on mass changes (MC) of three adhesives; Adper Single Bond2 (ASB), One-step plus (OSP), Clearfil S(3) Bond (CSB).
MATERIALS AND METHODS: Rectangular-shape samples from each adhesive were prepared and cured for 120 s with a halogen light curing unit. Prior to curing, their solvent was evaporated by means of three different procedures depending on the passive air-drying time (i.e., no air drying, equal to active air drying, complete evaporation after 3 h). Each group was further divided into two subgroups based on the time of water storage (1-day, 7-days), prior to measurement of MC (n = 10). The data were analyzed using a three-way ANOVA.
RESULTS: Adhesives showed different patterns of MC in relation to air drying and water storage; (P < 0.05). In OSP and CSB with increasing water storage and air drying, the MC increased significantly (P < 0.01).
CONCLUSION: The highest MC in the etch-and-rinse adhesives was observed when the adhesive was not dried, while in the self-etch adhesive the highest changes were observed when the adhesive was completely dried.

INTRODUCTION

In contemporary dentin adhesives, hydrophilic resin monomers are often dissolved in evaporable solvents, such as acetone, ethanol, water and combinations. Acetone is frequently used as one of the solvents since it can efficiently remove water from the substrate.[1] However, if acetone is used over a dry dentin cannot prevent the consequences of collagen fibers collapse.[2] The presence of water is also essential for both etch-and-rinse and self-etch adhesives. In etch-and-rinse adhesives, water has a plasticizing effect on the collagen fibrils and decreases the stiffness of the collapsed fibril, which is important for the expansion of the dried dentin collagen.[3] In self-etch adhesives, it provides an ionization medium to facilitate self-etching activity to occur.[4] Moreover, ethanol helps to displace water from the dentinal surface and the moist collagen network.[5]

Ideally, all solvents should be completely eliminated from the adhesive before light-curing, as they may have an adverse effect on polymerization of the adhesive resin monomers.[6] Insufficient air-drying of solvents has been reported to result in micro crack formation, loss of mechanical strength and water sorption of the polymer[5] and lower degree of conversion of polymer.[7]

The major effect of water on polymer matrices is a depression of glass transition temperature (Tg), that results in a decrease in thermal stability and polymer plasticization. These changes occur by different mechanisms, depending on the level of interaction of absorbed water molecules with the polymer matrix. Water sorption may deteriorate polymer mechanical properties, such as the modulus of elasticity, yield strength and produce changes in yield/deformation mechanisms. The water may also result in hygrothermal degradation during aging (such as the formation of swelling stresses, micro crack and craze), degradation of the matrix/fiber or matrix/filler interfaces, and polymer chain scission through hydrolytic cleavage[89] and some clinical failures such as marginal discoloration, microleakage of restorations.[7] Furthermore, several studies have reported that hydrophilic components in adhesives may play a role in water sorption, and loss of mechanical integrity.[101112] For example, a high percentage (20-30% by weight) of 2-hydroxyethyl-methacrylate (HEMA) that acts as a wetting agent, contributes to water sorption after polymerization.[13] It was also reported that functional hydrophilic monomers such as biphenyl-dimethacrlylate (BPDM) or 10-methacryloyloxydecyldihydrogenphosphate (MDP) induce polarity and water sorption.[1415]

Given this background, measurement of MC of adhesives after water storage is a simple way to measure water sorption of adhesives, and clarify degradation mechanisms in relation to various factors. In this regard, the aim of this study was to investigate the MC of three adhesives after different drying time and water storage, and the relationships between the variables. The null hypothesis tested was that MC is not affected by drying time and water storage.

MATERIALS AND METHODS

Three adhesives were used in this study. These adhesives comprised three categories: Two two-step etch-and-rinse adhesives: A water/ethanol-based (Single Bond2, 3M ESPE, St. Paul, MN, USA) and an acetone-based filled adhesive (One-step plus, Bisco, Schaumburg.USA) and a single-step self-etching adhesive (Clearfil s3 Bond, Kuraray Medical, Tokyo, Japan).

The weight of three drops of each adhesive that were poured into the Silicone rectangular-shape molds (15 mm long × 5 mm wide × 0.65 mm depth), was measured with a digital scale (AL-104; Acculab, Mountville, PA, USA) and registered as initial weight (IW). The active solvent evaporation was done by using a dental air-syringe for 10 s from 10 cm distance with 45° angle of the tip at 4 Kg/cm2 pressure to resemble the clinical situation of evaporation and the weight of adhesive was measured after drying (DW). The percent of active evaporation degree (active ED) was calculated as:

For measuring of passive evaporation degree, the weight of another three drops of each adhesive after each minute (DWt) of storage in a dark and closed environment at 37°C was measured until 180 min, where DWt almost plateaued. The percent of passive evaporation degree (passive EDt) at each time (t) was calculated as:

The time when the value of passive EDt in the mold was equivalent to active ED on flat surface was selected as passive drying time equivalent to the clinical or active air-drying time. The procedure was repeated for three samples in each adhesive. This was done to make sure a standardized drying method was followed and the adhesive was homogenously air-dried, without any adverse effects of sever air blowing on the bulk of adhesive.

In order to make samples, three drops of adhesive were dispensed in a Silicone rectangular-shape mold. The mold was maintained in a dark and closed environment at 37°C. Prior to curing, three passive air-drying procedures were followed for each adhesive; no air-drying (No-drying), equal to active air-drying as described above (Active-drying) and drying for 3 h (3 h-drying). The 3 h time was selected to ensure a complete solvent evaporation. The samples were then cured for 120 s with a halogen light curing unit (Optilux 501; Kerr, Danbury, CT, USA) (power output: ~650 mW/cm2). The curing time was selected to ensure adequate light curing of the bulk sample, which needs extra irradiance for curing when compared to a thin film. After curing, the samples were carefully removed from the mold, and the weight of adhesive was measured and recorded as cured weight (CW). The specimens were observed under a stereo microscope (SMZ10; Nikon, Tokyo, Japan) at 10× magnification, and samples with voids, cracks or fractures were withdrawn prior to the storage [Figure 1]. Samples in each of the groups were randomly divided into two subgroups based on the time of water storage; those in the first subgroup (1d) were stored for one day and those in the other (7d) were stored one week in distilled water at 37°C prior testing. In total, 270 samples were included in the analysis with 10 specimens in each of the 27 subgroups.

Figure 1

Complete air-drying

After completing the storage periods (1 day, 7 days), the samples were removed from water. Any visible moisture was removed with a paper towel and the weight of each sample was measured and recorded as storage weight (SW). The MC was expressed in grams, as derived from subtraction of SW from CW.

Kolmogorov-Smirnov test was used to confirm the normality of data. The data were analyzed using a three-way ANOVA, with the material, air-drying and water storage as three factors. Comparison for main effects of each factor, and comparisons between different subgroups within each material were performed using the Bonferroni's correction. The significance level was set in advance at α = 0.05. Statistical analysis was performed using SPSS software (ver. 16, SPSS, Chicago, IL, USA).

RESULTS

The average time for the passively air-dried adhesive to reach the evaporation degree of actively dried adhesive was 20 min for ASB, 15 min for OSP and 17 min for CSB to lose approximately 17, 38 and 7 wt% due to solvent evaporation.[16] Mean and standard deviations of all subgroups are presented in Table 1.

Table 1

Summary of MC values† for all subgroups in this study§

The three-way ANOVA revealed that all the possible interactions were not significant but the effect of all factors was significant in ASB (P < 0.05). The effect of drying time and water storage was not significant statistically in OSP and CSB, respectively. In the pair-wise comparisons of the estimated marginal means, CSB showed a higher value (0.007 g) compared to ASB and OSP (0.006 g and 0.0057 g, respectively) and the difference was not statistically significant (P > 0.05).

Further comparison of subgroups within each material showed different trends for each material; the highest MC of ASB was seen in the group that was not air-dried and the mean of MC in samples with 1 day water storage were significantly higher than the samples that was storage in water for 7 days (P < 0.05).

For OSP, no statistically significant differences were found among any three groups (P > 0.05) but the mean of MC in the subgroups with 7 days water storage was higher significantly than 1 day storage.

For CBS, 3 h-drying showed a significantly higher MC value compared to Active-drying groups (P < 0.05) but the difference between 1 day and 7 days subgroups was not significant (P > 0.05).

DISCUSSION

The adhesives that were used in this study were suitable considering specific properties that we need. These adhesives have different solvent and passive air-drying time. All adhesives were used in a dark and closed environment at 37°C in order to prevent quickly evaporation and early curing.[17] The importance of this research is the simulation of solvent evaporation with clinical situation that was done by using a dental air-syringe for 10 seconds at 10 cm distance with 45° angle of the tip at 4 Kg/cm2 pressure. Using air-syringe for 10 seconds was recommended by several studies for establishment of a monotonous adhesive layer and completely solvent evaporation and creation the best mechanical properties.[10]

MC has been frequently reported as a relevant variable to evaluate the performance of an adhesive. MC was investigated in relation to solvent drying and storage of adhesives. In general, residual solvent testing can be conducted by a number of analytic techniques such as gravimetric analysis (weight loss on drying) which is a simple but effective method to evaluate the solvent evaporation from dental adhesives in relation to time under a clinically relevant setup.[18]

It is expected that adhesive blends containing ethanol and water (44 mmHg and 17.5 mmHg vapor pressure, respectively) present greater difficulty in displacing the remaining solvent and water than those blends containing acetone (184 mmHg vapor pressure) in addition to water.[19] This expectation was also confirmed in this study, where OSP showed the steepest initial weight loss slope under passive solvent evaporation.[16] Naturally, the initial water/solvent content of adhesive determines the final weight loss of adhesive after complete solvent removal. In this regard, from the passive EDt plot it appears that OSP had the highest content, followed by ASB and CSB. Furthermore, the effect of air-drying on MC only in ASB and CSB was significant. It should be noted that acetone has a higher vapor pressure than ethanol, and therefore evaporates more rapidly.[6]

In addition to the solvent concentration, factors such as the efficacy of the photoinitator system, curing parameters and adhesive composition can be effective on MC.

In camphorquinone/tertiary amine photoinitiator system commonly used in current adhesives, an acid–base reaction between the amine coinitiator and acidic monomers in the composition of some adhesives may occur. This adverse reaction can lead to decreased availability of amine to form free radicals, and thereby delayed or inferior polymerization.[20] Insufficient polymerization may also be related to such attributes of curing light as wavelength peak and intensity while an intensity between 400 and 600 mW/cm2 is generally recommended, the actual intensity that an adhesive may receive in the clinical situation is likely lower owing to factors such as distance of the light curing tip from the adhesive, tip design and other parameters related to the curing unit.[21] It was reported that prolonged curing times (20-40 s) for the polymerization of simplified adhesives such as ASB and OSP resulted in an increase in the quality of the polymer network and resin–dentin bonds through removal of residual solvent from the adhesive layer during polymerization.[22] In this study, a long curing time of 120s was selected as shorter times resulted in very fragile samples, especially in ASB.

In current study, by increasing the time of water storage, MC was increased except ASB. A possible explanation for this finding is relaxation of macro molecular polymer chains, swelling and plasticizing of polymer network, reducing of friction forces between polymerized chains and leaching out of unreacted monomer strapped in the polymer network via nanometric spaces because of water sorption. Therefore, desorption of monomers from polymer network can reduce the polymer's weight. In fact, polymer network shows heterogeneity; it has densely cross-linked in some parts and some parts are loosed. This heterogeneity is seen from the beginning of polymerization with unreacted monomers inside the microgel. When this semi-polymerized structure is maintained in water, MC occurs and the weight is increased while unreacted monomers and low molecular weight polymer are released and cause weight loss of polymer.[23] The highest nominal MC in this adhesive was achieved in No-drying group [Table 1], while there were no statistically significant difference among other groups of this adhesive. In line with this finding, some studies suggested that pores formation because of water, acts as channels for water passing and increased amount of solvent promote ionization of acidic monomers, leading to entrapment of water and solvent, thus affecting the degree of conversion of polymer and water sorption.[7] Furthermore, essence of Polyalkenoic acid copolymer in their composition that is reported to have a moisture-stabilizing effect and may not dissolve in the adhesive's solution, leading to phase separation producing many globules within the polymer of the adhesive layer resulting in water sorption and MC of the adhesive layer.[24]

Most studies have reported that increasing the time of water storage could result in higher WS, in both self-etch and total-etch adhesives.[25] In this study, the effect of water storage on MC in total-etch adhesives was significant vice versa self-etch adhesive. Perhaps in the first 24 hours, CSB was saturated by water. An explanation for this was the effect of denser polymer network and the relaxation of polymer of CSB which could result in resistance to water penetration.[25] Furthermore, water could hydrogen bond to polar groups in CSB and formed bound water in the first 24 h of water storage. Also, CSB was made by “Molecular Dispersion Technology” that maintains this adhesive in a homogenous status; so despite of presence of hydrophilic and hydrophobic monomers, phase separation could not occur.[24]

The highest nominal MC was seen in No air-dried groups of ASB, OSP. Possibly, the presence of solvent result in pores and droplets in polymer structure could be act as channels for water passing. The presence of these droplets was approved by stereomicroscope. For CSB, maximum MC was seen in the groups that complete evaporation was done maybe because of hydrophilicity of adhesive and presence of noticeable amount of HEMA in its composition that act, even after polymerization, as semipermeable members.[7] Furthermore, this adhesive has hydrophilic and hydrophobic phase so with increasing air-drying, phase-separation occurred and at the interface of two phases MC happened. Another explanation is the presence of acidic monomers with molecular polarity and the dispersion technology that result in homogeneity of this adhesive. Therefore, even in No air-dried group, droplet formation was not occurred. This was in line with several studies that concluded in hydrophilic adhesives which had Bis-GMA and TEGDMA, MC was higher because of small molecular size and high molar concentration of water that could penetrate into nanometer-size free volume spaces between polymer chains,[24] or cluster around functional groups that are capable of hydrogen bonding.[26] Furthermore, increasing MC in more hydrophilic adhesives could be related with increased bound water in polymer matrices that cannot be evaporated.[24]

CONCLUSION

Maximum MC in two-step etch-and-rinse adhesives was observed in No air dried samples whereas self-etch adhesive showed its maximum mass change when it was completely dried.