Effects of Coffee Temperature and Immersion Duration on the Physicochemical and Mechanical Properties of Orthodontic Elastomeric Chains

1. Introduction

Orthodontic elastomeric chains are widely used to deliver continuous force for orthodontic tooth movement, particularly during space closure and anterior tooth retraction [13]. Although these materials are designed to maintain their elastic properties during clinical use, their performance may be affected by exposure to the oral environment, such as saliva, temperature changes, and dietary substances [4,5,6]. Among commonly consumed beverages, coffee is of particular clinical interest because of its widespread consumption and its acidic and pigment-rich composition [7,8]. In addition, coffee may be consumed at different ranges of temperature, from chilled beverages to freshly prepared hot coffee that results in repeated thermal exposure of elastomeric chains during treatment. This exposure may influence the polymeric structure of elastomeric chains, and their physicochemical and mechanical properties.

Temperature and immersion duration may influence the physicochemical stability of polyurethane (PU)-based elastomeric chains by affecting water uptake, molecular mobility, and degradation of the polymer matrix [9,10,11]. In an aqueous environment, water can diffuse into the PU network and act as a plasticizing agent by increasing polymer-chain mobility and reducing intermolecular interactions [12]. Prolonged exposure may further promote hydrolytic degradation of some chemical bonds within the polymer matrix, thereby compromising its molecular structure and mechanical integrity [13,14]. These processes may be enhanced at elevated temperatures because increased thermal energy can facilitate molecular diffusion and accelerate temperature-dependent chemical reactions [11]. Consequently, changes in water uptake and polymer degradation may be reflected in specimen mass, tensile properties, and surface morphology [15].

The effects of environmental exposure on orthodontic elastomeric chains have been reported in previous studies. Force degradation was observed under different environmental conditions and simulated oral conditions [5,6,16,17,18,19,20,21]. In our previous study, coffee temperature and exposure conditions were also found to affect colour change, surface roughness, and force remaining of orthodontic elastomeric chains [22]. These findings indicate that coffee temperature and exposure conditions can affect the properties and force delivery of elastomeric chains. However, our previous study did not evaluate changes in specimen weight or tensile properties, and the relationship between these changes and force degradation was not investigated. Therefore, further study is needed to determine whether changes in the physicochemical properties of elastomeric chains are related to their mechanical behavior during coffee exposure.

The present study extends our previous work by examining a different and complementary set of outcomes. The effects of coffee temperature and immersion duration were evaluated in terms of percentage weight change (WC), force degradation, modulus at 300% strain, strain at break, and surface morphology using scanning electron microscopy (SEM) over a 21-day period. In addition, the relationship between WC and force degradation was investigated to determine whether changes in the physicochemical properties were associated with deterioration in force delivery.

2. Material and Methods

2.1. Study Design

This study employed an in vitro experimental design to evaluate the effect of temperature and immersion duration on percentage WC , force degradation, strain at break, and 300%

modulus of orthodontic elastomeric chains. This study used latex-free closed-type orthodontic elastomeric chains (American Orthodontics, WI, USA; lot number [P0485010]. Sample size calculation was performed using data from a previous study by Nobahari et al. (2024) [20]. The power of the study was set at 80%, and the confidence interval (CI) was set at 95%. Sample size was calculated using G*Power software (version 3.1.9.4) based on force decay data. The required sample size was estimated to be 10 specimens per group as illustrated in Figure 1A.

2.2. Specimen Preparation

A rectangular acrylic plate was fabricated with ten rows of paired holes, into which lingual buttons were bonded using BracePaste^® Adhesive (American Orthodontics, WI, USA). The paired buttons were spaced 25 mm apart to reproduce the clinical distance between the canine and first molar during canine distalization. Each elastomeric chain was comprised of ten loops in total: six central loops served as the active segment, while two loops at each end were reserved for gripping to protect the active segment from damage during mounting and pre-stretching. Each chain was then stretched across the fixed 25-mm span that generated an orthodontic force of 300 gf.

2.3. Immersion Protocol

Specimens were randomly allocated into three experimental groups according to coffee immersion temperature: 4 °C (cold), 25 °C (room temperature), and 65 °C (hot). A coffee solution of 1 g/mL was prepared by dissolving instant black coffee (Nescafé Classic, Nestlé, Switzerland) in deionized water, and the pH was adjusted to 5.0. Based on a previous study by Dimić et.al, 2014 [23], artificial saliva was prepared, which consisted of KCl, NaCl, MgCl₂·6H₂O, CaCl₂·2H₂O, KH₂PO₄, methyl parahydroxybenzoate, and 70% sorbitol dissolved in deionized water. The pH was adjusted to 6.8. The coffee and artificial saliva solutions were freshly prepared on a daily basis.

The temperature of each coffee solution was continuously monitored throughout the immersion period using a calibrated thermometer. Specimens were continuously immersed in coffee solution, which was replaced regularly to maintain consistent temperature and concentration across all experimental groups. Following each immersion session, all specimens were thoroughly rinsed with deionized water and subsequently stored in artificial saliva at 37 °C to simulate intraoral conditions. The artificial saliva was refreshed daily throughout the duration of the study.

2.4. Weight Change Measurement

At each designated time point (days 1, 3, 7, 14, and 21), specimens were removed from the immersion medium and rinsed with deionized water and blotted dry with absorbent paper to

remove surface moisture prior to weighing. Specimens were weighed using an analytical balance with a precision of 0.0001 g. Percentage WC was calculated using the following formula: WC = (final weight − initial weight) / initial weight × 100.

2.5. Force Degradation Measurement

The force degradation of the elastomeric chains was measured after immersion using a digital push and pull force gauge (HF-500 Digital force Gauge Push-Pull Force Gauge, Shenzhen Graigar Technology, China). Each sample was measured three times under identical testing conditions, and the mean force value was calculated to minimize measurement error.

2.6. Strain at Break and Modulus at Strain 300% Measurement

Following the force decay measurement at each designated time point (days 1, 3, 7, 14, and 21), the same specimens were subsequently subjected to destructive tensile testing using a universal testing machine (Shimadzu EZ-SX , Shimadzu Corporation, Kyoto, Japan), equipped with a load cell of 500 N capacity, in accordance with ISO 37:2017. Each elastomeric chain was mounted onto the testing machine by engaging the two end loops (previously used for gripping during mounting) into custom-fabricated hooks, ensuring that only the six central active loops were subjected to the tensile load. Specimens were pulled at a crosshead speed of 100 mm/min until rupture occurred. The force–displacement curve generated by the testing machine was used to determine two outcome measures: 1) strain at break (%), defined as the maximum percentage elongation of the chain, relative to its original gauge length, recorded immediately prior to rupture and 2) 300% modulus (N/mm²), defined as the secant modulus (stress divided by strain) calculated at 300% elongation of the original gauge length.

2.7. Scanning Electron Microscopy

Surface morphological changes of the elastomeric chains following coffee immersion were examined using SEM. Specimens were sputter-coated with gold prior to imaging. SEM examination was performed using a JEOL JSM-7800F field emission scanning electron microscope (FESEM; JEOL Ltd., Tokyo, Japan) equipped with a digital camera and an INCAx-sight (Oxford Instruments, Abingdon, UK) element analyzer, at an accelerating voltage of 5.0 kV. Surface images were captured at ×2000 magnification from the ruptured areas of each specimen (Figure 1B) to qualitatively evaluate surface morphological changes and texture among the experimental groups.

2.8. Statistical Analysis

Data are presented as mean ± standard deviation (SD) with 95% CI. The effects of immersion temperature and duration on WC, force degradation, modulus at 300% strain, and strain at break were analyzed using two-way ANOVA, followed by Tukey’s HSD post hoc test.

One-way ANOVA with Tukey’s HSD test was used for comparisons among temperature groups at each time point. Pearson’s correlation was used to assess the relationship between WC and force degradation. Statistical significance was set at p < 0.05.

3. Results

3.1. Effect of Different Coffee Temperatures and Immersion Times on Weight Change of Elastomeric Chains

WC was evaluated under four conditions (control, cold, room temperature, and hot) at five immersion durations (days 1, 3, 7, 14, and 21). Two-way ANOVA demonstrated significant main effects of immersion temperature (F(3, 180) = 5.698, p < 0.001) and immersion duration (F(4, 180) = 12.456, p < 0.001) on WC. However, the temperature and duration interaction was not significant (F(12, 180) = 0.549, p = 0.880), which indicated the effect of immersion duration on WC did not differ significantly among the temperature groups (Table 1).

Post hoc comparisons of the estimated marginal means (EMMs), averaged across all immersion durations, showed that the hot group exhibited significantly greater overall WC than the control group (EMM, 1.61% vs. 0.87%, respectively). No significant differences were observed among the remaining temperature-group comparisons (all p > 0.10).

Since immersion duration showed a significant main effect, simple-effects analyses were performed within each temperature group. A significant effect of immersion duration was observed only in the hot-temperature group. In this group, WC at days 14 (2.457% ± 1.01%) and 21 (2.332% ± 1.167%) was significantly greater than at days 1 (0.733% ± 0.667%) and 3 (1.036% ± 0.863%), whereas WC at day 7 (1.487% ± 1.023%) did not differ significantly from either the earlier or later time points. No significant changes in WC over time were detected in the control, cold-temperature, or room-temperature groups (all p > 0.05).

When temperature groups were compared separately at each immersion duration, no significant between-group differences were detected at any time point (all p > 0.05). Thus, the significant main effect of immersion temperature was observed in the EMMs averaged across the observation period, whereas temperature-group differences were not statistically significant at any individual measurement day (Table 2).

3.2. Effect of Coffee Temperature and Immersion Duration on Mechanical Properties

The mechanical properties of the elastomeric chains varied according to coffee temperature and immersion duration. For strain at break, there were significant main effects of immersion temperature (F(3, 180) = 301.670, p < 0.001) and immersion duration (F(4, 180) = 98.179, p < 0.001), together with a significant temperature and duration interaction (F(12, 180) = 13.458, p < 0.001). Similarly, for modulus at 300% strain, there were significant main effects of immersion temperature (F(3, 180) = 311.258, p < 0.001) and immersion duration (F(4, 180) = 82.578, p < 0.001), together with a significant temperature and duration interaction (F(12, 180) = 14.377, p < 0.001) (Table 3).

Given the significant temperature and duration interactions, comparisons among temperature groups were examined at each immersion duration. The hot-temperature group exhibited a higher modulus at 300% strain and a lower strain at break than the control group throughout the observation period. At day 21, for example, the modulus at 300% strain was 10.35 N/mm² in the hot-temperature group compared with 9.26 N/mm² in the control group, whereas strain at break was 428.20% and 464.78%, respectively.

The cold- and room-temperature groups showed distinct time-dependent patterns relative to the control group. For modulus at 300% strain, both groups exhibited higher mean values than the control group at days 3 and 7 followed by lower mean values at days 14 and 21. For strain at break, both groups showed lower mean values than the control group at days 3 and 7. At day 14, strain at break remained lower than the control group in both the cold- and room-temperature groups. By day 21, however, strain at break in the cold-temperature group (479.40%) exceeded that of the control group (464.78%), whereas the room-temperature group (459.66%) remained slightly lower.

Overall, exposure to hot coffee was associated with a consistently higher modulus at 300% strain and lower strain at break than the control condition throughout the observation period. In contrast, cold- and room-temperature exposure produced time-dependent changes, particularly in modulus at 300% strain, which showed an early increase followed by lower values at later immersion durations (Figure 2).

SEM revealed broadly comparable surface morphology among the four experimental groups at day 1 with all specimens exhibiting a relatively even, finely granular surface similar to the unaged negative control. With increasing immersion duration, the hot group progressively developed a more irregular surface, characterized by prominent ridge-like folds and a coarser, fibrous texture that became most apparent on days 14 and 21. In contrast, the control, cold-, and room-temperature groups retained a comparatively more even surface texture throughout the 21-day observation period, although mild progressive roughening was also noted in these groups at the later time points. These findings indicate that hot-coffee immersion produced the most pronounced physicochemical alteration of the elastomeric chain surface. As detailed in the following section, this progressive surface roughening in the hot group paralleled its greater and more sustained force degradation that is consistent with a link between surface-level deterioration and the loss of mechanical force retention.

3.3. Effects of Coffee Temperature and Immersion Duration on Force Degradation of Elastomeric Chains

Immersion temperature had a significant effect on force degradation (F(3, 180) = 10,380.29, p < 0.001, partial η² = 0.994) (Table 4). Post hoc comparisons showed that the hot-temperature group exhibited significantly greater force degradation than the control, cold-temperature, and room-temperature groups at every immersion duration (all p < 0.001). Among the control, cold-temperature, and room-temperature groups, force degradation was generally greater in the cold- and room-temperature groups than in the control group from day 1 through day 14, although the statistical significance of these pairwise differences varied across immersion durations. By day 21, no significant differences in force degradation were detected among the control, cold-temperature, and room-temperature groups (Table 5).

Immersion duration also had a significant effect on force degradation (F(4, 180) = 563.78, p < 0.001, partial η² = 0.926), and the temperature × duration interaction was significant (F(12, 180) = 9.37, p < 0.001, partial η² = 0.385),which indicated that the pattern of force degradation over time differed among temperature groups (Table 4). Simple-effects analyses showed that force degradation increased significantly through day 14 in both the control and cold-temperature groups with no further significant increase between days 14 and 21. The room-temperature group showed a similar pattern with force degradation increasing significantly through day 7 and remaining statistically unchanged thereafter. In contrast, the hot-temperature group exhibited a significant increase in force degradation at each successive immersion duration throughout the 21-day observation period with no evidence of a plateau (Table 5).

Overall, hot-temperature exposure resulted in consistently greater force degradation than the other temperature conditions throughout the observation period. Although force degradation increased over time in all temperature groups, the temporal pattern differed substantially among the conditions. The hot-temperature group showing a continuous increase throughout 21 days, whereas the control, cold-temperature, and room-temperature groups showed evidence of stabilization at later immersion durations (Table 5).

3.4. Correlation Between Weight Change and Force Degradation

WC was significantly and positively correlated with force degradation across all individual observations (Pearson’s r = 0.332, R² = 0.110, N = 200, p < 0.001) (Figure 3), which indicated a modest positive association between the two variables. When analyzed separately by immersion temperature, significant positive correlations were observed in all four temperature groups: control (r = 0.442, N = 50, p = 0.001), cold (r = 0.388, N = 50, p = 0.005), room temperature (r = 0.415, N = 50, p = 0.003), and hot (r = 0.633, N = 50, p < 0.001). The correlation coefficient was numerically highest in the hot-temperature group.

When the analysis was performed using group × day mean values (N = 20), WC was also significantly and positively correlated with force degradation (Pearson’s r = 0.590, p = 0.006). Because this analysis was based on aggregated group-by-day means rather than individual observations, the resulting correlation represents an association at the group-by-day level and should not be interpreted as equivalent to the individual-level correlation. Overall, the findings indicate a positive association between WC and force degradation at both the individual-observation and group-by-day levels (Figure 4).

4. Discussion

The present study demonstrated that coffee temperature and immersion duration influenced WC and the mechanical properties of orthodontic elastomeric chains. The effects were most evident with hot-coffee exposure, which resulted in greater WC, increased modulus at 300% strain, reduced strain at break, and greater WC, force degradation compared with the control. In contrast, cold- and room-temperature coffee produced smaller changes with the effects varying according to immersion duration. These findings indicate that higher coffee temperature may accelerate changes in the elastomeric chains, particularly with prolonged exposure.

The increase in WC observed in the hot group over the 21-day period indicates a temperature-dependent change in specimen mass. Previous studies of thermoplastic polyurethane (TPU) revealed that immersion temperature and aging duration can affect water uptake and mechanical properties [9]. Differences between these findings and the present study may be related to differences in polymer composition, temperature, immersion medium, and exposure duration. Although WC represents the net change in specimen mass and may reflect both fluid uptake and the loss of soluble or leachable components, the observed increase in WC does not necessarily indicate water sorption alone. Direct measurements of water sorption and solubility would be needed to distinguish these processes.

The hot group showed a distinct mechanical response with a consistently higher modulus at 300% strain, lower strain at break, and greater force degradation than the control group throughout the 21-day period. A higher modulus indicates greater resistance to deformation, whereas a lower strain at break indicates reduced extensibility. Together, these findings suggest that hot-coffee exposure resulted in a stiffer and less extensible elastomeric chain while reducing force retention. This pattern is consistent with previous findings in TPU, in which thermal aging was associated with increased modulus and reduced extensibility [9]. In contrast, the cold- and room-temperature groups showed more variable changes over time. Modulus at 300% strain increased at days 3 and 7 but decreased at days 14 and 21, while strain at break showed a similar early decrease followed by less consistent differences at later time points. These time-dependent changes suggest that the mechanical response of elastomeric chains may change during prolonged immersion and may depend on both temperature and exposure duration. The significant temperature-duration interactions further support this finding, indicating that the effect of coffee temperature cannot be considered independently of immersion duration.

Furthermore, the hot group exhibited significantly greater force degradation than the control, cold-, and room-temperature groups at every immersion duration. This sustained difference suggests that continuous exposure to hot coffee produced a greater loss of retained orthodontic force under the conditions tested. The magnitude of this effect was greater than that reported in previous studies using intermittent hot-beverage exposure. The magnitude and persistence of force degradation observed in the present study differed from those reported in previous studies using intermittent beverage exposure. Braga et al.(2019) exposed elastomeric chains to hot coffee, hot green tea, or hot water at 70 ± 1 °C for 30 seconds twice daily and found greater force degradation during the early observation period [24], whereas the coffee group was no longer significantly different from the control at day 21. Similarly, Nobahari et al. (2024) reported no clinically concerning adverse effect of hot beverages on force decay following exposure at 65.5 °C for 90 seconds four times daily [20]. In contrast, Torres-Rosas et al.(2023) exposed elastomeric chains to beverages for 15 minutes daily and reported time-dependent force decay and elongation, with responses varying between chain products [19]. In the present study, the coffee groups were also exposed for 15 minutes per day and stored in artificial saliva at 37 °C between exposure cycles. Differences in force degradation among studies may therefore be related to coffee temperature, exposure duration and frequency, material characteristics, and other experimental conditions. Clinically, these findings suggest that repeated exposure to hot coffee may contribute to greater force loss from orthodontic elastomeric chains. However, the present in vitro exposure model cannot fully reproduce the complex conditions of beverage consumption in the oral environment.

The significant positive correlation between WC and force degradation at the individual-observation level (Pearson’s r = 0.332, R² = 0.110, p < 0.001) indicates a modest association between changes in specimen mass and loss of orthodontic force. Positive correlations were also observed within each temperature group (r = 0.388–0.633), with the highest correlation observed in the hot group, although the correlations between temperature groups were not statistically compared. Overall, greater changes in specimen mass tended to accompany greater force degradation, although the modest overall correlation indicates that WC alone is unlikely to predict force degradation across different temperature conditions.These findings complement our previous study [22], which evaluated colour change, surface roughness, and residual force of the same type of orthodontic elastomeric chain under different coffee temperatures. While the previous study demonstrated greater changes in colour, surface roughness, and force retention following hot-coffee exposure, the present study extended these findings by showing that hot-coffee exposure was also associated with greater WCTogether, the two studies showed that hot-coffee exposure affected not only appearance and surface characteristics but also mass change, mechanical behavior, and force retention.

From a clinical perspective, the present findings suggest that repeated exposure to hot coffee may adversely affect the mechanical performance of orthodontic elastomeric chains under the experimental conditions investigated. However, the present in vitro protocol, involving 15 minutes of daily coffee exposure followed by storage in artificial saliva at 37 °C, cannot fully reproduce the complexity of beverage consumption in the oral environment. The findings should therefore not be interpreted as a direct estimate of the effect of routine coffee consumption. This study was also limited to a single elastomeric-chain product, which may limit the generalizability of the findings to other commercially available products. Further studies using different elastomeric-chain products and clinically representative exposure protocols are needed to confirm the present findings and better establish their clinical relevance.