Evaluation of Water Sorption and Solubility and FTIR Spectroscopy of Thermoplastic Orthodontic Retainer Materials Subjected to Thermoforming and Thermocycling

Featured Application

Thermoplastic orthodontic retainers maintain teeth in their corrected positions after orthodontic treatment. Since they are prescribed to be used indefinitely, this study evaluated the stability and durability of six commercially available thermoplastic retainer materials. This study could be applied in the orthodontic clinic by guiding orthodontists to invest in a durable and stable materials.

Abstract

Due to the fact that retainers are often recommended for a lifetime, their endurance and longevity are directly related to the quality of the materials used in their production. Our study examined the water sorption, water solubility, and Fourier transform infrared (FTIR) spectroscopy of six commercially available thermoplastic retainer materials (Essix Plus, Zendura, Duran Plus, Tru-Tain DX, Imprelon S pd, and Essix ACE). Moreover, this study evaluated the effect of thermoforming and thermocycling on the water sorption and solubility and surface molecular composition of the tested materials. The present study found that the type of retainer material affected water sorption and solubility capabilities. In addition, the aging methods employed significantly affected some retainer materials’ water sorption and solubility. Moreover, the surface molecular composition evaluated by FTIR spectroscopy revealed that most of the evaluated materials had similar FTIR spectra except for Zendura. All materials had a spectrum that resembled polyethylene terephthalate glycol (PETG) while Zendura had a spectrum similar to semi-rigid polyurethane (PU). Only Zendura had relatively unstable surface structural composition evaluated under the effects of (thermoforming and thermocycling) compared to all tested materials.

1. Introduction

Retention is the final stage of orthodontic therapy after achieving the desired teeth position. The primary purpose of any retention appliance is to maintain the obtained results and avoid relapse, as teeth tend to return to their pre-treatment positions [1,2]. In addition, maintaining the results is crucial to determining long-term patient satisfaction following orthodontic treatment [3].

In the absence of scientific evidence, the selection of the retainer material is typically guided by the patient’s and the dentist’s preferences [4]. The aesthetics and invisibility of thermoplastic retainers are favored by many patients. A thermoplastic orthodontic retainer is a removable device that can be made of different types of polymers. These retainer materials are more cost-effective and easier to produce than Hawley or fixed retainers [5,6]. Widespread use of thermoplastic retainers has been linked to increased patient satisfaction and adherence [7].

Retention methods vary greatly across practitioners, and no consensus exists [8]. However, the necessity of lifelong retention is now well recognized. Multiple studies have shown that stability can only be ensured through long-term retention [1,9,10,11,12]. The long-term use of thermoplastic retainers for extended periods can cause the material to deteriorate [13]. The disadvantages of thermoplastic retainers are their short lifespan and frequent replacements [14,15,16]. To lower the cost and preserve the results of orthodontic treatment, thermoplastic retainers should be fabricated from materials that exhibit high durability and stability.

Fabricating thermoplastic retainers involves either vacuum forming or pressure forming that forms the plastic sheets into the patient model [17]. Since the forming procedure involves heating the plastic sheets, changes in their properties are anticipated. So, the effects of thermoforming on the water sorption and solubility and surface molecular analysis of different commercially available thermoplastic retainer materials were evaluated in the current study. Furthermore, thermocycling was employed in the current study to elucidate their clinical endurance and persistence [18]. Moreover, as the water uptake into the polymeric material leads to physiochemical degradation in the material itself, water sorption and solubility were used in the present study to assess the durability and stability of six commercially available thermoplastic retainer materials [19,20,21].

The material’s surface molecular structure can be assessed with the help of Fourier transform infrared (FTIR) spectroscopy. It is possible to assess the effects of various aging methods on the surface molecular structure by comparing the FTIR spectra of the samples taken before and after thermoforming with or without thermocycling. Therefore, this was employed in the present study to identify the surface molecular analysis of the investigated thermoplastic retainer materials and the effects of different aging methods used (thermoforming and thermocycling) [22].

Although many studies evaluated the water sorption and solubility and FTIR spectroscopy of thermoplastic aligner materials, few have evaluated thermoplastic retainer materials. However, since they are prescribed for life, their stability in wet environment and durability are crucial factors determining their cost-effectiveness.

We are unaware of any published research that has studied the water sorption and solubility and FTIR spectroscopy of Imprelon S pd, Essix Plus, Duran Plus, and Tru-Tain DX. Not only that, but we are unaware of any research that has evaluated the effects of thermoforming and thermocycling on the investigated thermoplastic materials’ water sorption and solubility and FTIR spectroscopy.

Therefore, in the current study we evaluated the water sorption and solubility of six commercially available thermoplastic retainer materials before thermoforming, after thermoforming, and after thermoforming and subsequent thermocycling. Moreover, FTIR spectroscopy was conducted to identify the differences between the investigated thermoplastic retainer materials and to compare the effects of thermoforming and thermoforming plus thermocycling on the surface molecular structure. The null hypotheses of this study were:

1.

There are no differences in the water sorption and solubility and surface molecular composition of the tested thermoplastic retainers before thermoforming.

2.

There are no differences in the water sorption, water solubility, and surface molecular composition of the tested thermoplastic retainers after thermoforming.

3.

There are no differences in the water sorption, water solubility, and surface molecular composition of the tested thermoplastic retainers after thermoforming and subsequent thermocycling.

4.

Thermoforming has no statistically significant effect on the water sorption and solubility and surface molecular composition of the tested thermoplastic retainers.

5.

Thermocycling has no statistically significant effect on the water sorption and solubility and surface molecular composition of the tested thermoplastic retainers.

2. Materials and Methods

The thermoplastic retainer materials listed in (

Table 1. Thermoplastic retainer materials used in the current study. The reported composition of each material in this table is according to its material safety and data sheet.

Material Name	Manufacturer	Composition
Zendura Item #9169	Bay Materials LLC, Fremont, CA, USA	Polyurethane (PU)
Essix Plus	Dentsply Raintree Essix, Bradenton, FL, USA	Copolyester
Tru-Tain DX	Tru-Tain, Minnesota, USA	Copolyester
Duran Plus	Scheu-Dental GmbH, Iserlohn, Germany	Polyethylene terephthalate glycol (PETG)
Essix ACE	Dentsply Raintree Essix, Bradenton, Fla, USA	Copolyester (of polyethylene terephthalate)
Imprelon S pd	Scheu-Dental GmbH, Iserlohn, Germany	Copolyester

) were evaluated. The current investigation used the 1-mm thickness, favored by most practitioners [23]. The thermoplastic retainer materials evaluated in the current study were grouped into three groups: The first group (Group 1) consisted of thermoplastic retainer material prior to thermoforming. The second group (Group 2) consisted of thermoplastic retainer materials following thermoforming. The third group (Group 3) consisted of thermoplastic retainer materials following both thermoforming and thermocycling.

2.1. Thermoforming

Following the manufacturer’s instructions, the thermoplastic sheets were heated and vacuumed over a stainless-steel model with a diameter of 50 mm and a height of 10 mm (Figure 1). A Biostar vacuum forming machine (Scheu-Dental GmbH, Iserlohn, Germany) was utilized to form all specimens.

2.2. Thermocycling

This study employed thermocycling to assess the long-term performance of thermoplastic retainer materials. Following thermoforming, thermocycling was performed on the investigated thermoplastic retainer materials (Group 3). The thermocycler 1100 (SD-Mechatronik, Westerham, Germany) was used to perform 10,000 cycles, simulating one year of intraoral use of the thermoplastic retainers [18]. Prior to thermocycling, samples were kept in 37 °C distilled water for one day. Fifteen seconds was the dwelling time in 5 and 55 °C water baths and 10 s was the dripping time [18].

2.3. FTIR Analysis

After thermoforming, as mentioned previously in Section 2.1, a scissor was used to obtain specimens of 10 × 10 mm, and the edges were finished using a Jean Wirtz polishing machine (Dusseldorf, Germany). Fourier-transform infrared spectroscopy (FTIR) was performed to identify the differences between materials in their surface molecular structure and the changes in the surface molecular composition of the specimens for Group 2 and Group 3 compared to Group 1. Spectra acquisitions were carried out using a compact FTIR spectrometer alpha II (Bruker Nano GmbH, Berlin, Germany) (Figure 2). The acquisition settings were as follows: wavenumber range of 4000–650 cm⁻¹, resolution of 4 cm⁻¹, and 20 scans acquired for every single FTIR spectrum. The FTIR reading was repeated five times for each material [24].

2.4. Water Sorption and Solubility

2.4.1. Specimen Preparation

After thermoforming, as mentioned previously in Section 2.1, the specimens of each material were cut using a water-cooled diamond disc (Hager & Mesinger GmbH, Neuss, Germany) to obtain circular specimens with a 50-mm diameter (Figure 3) following the ISO 20795-2 (2013) [25] specifications.

2.4.2. Water Sorption Evaluation

The materials’ water sorption was evaluated following the ISO 20795-2 (2013) [25] specifications for Group 1, Group 2, and Group 3. Specimens were kept in water at 37 ± 1 °C for seven days. To determine water sorption (Wsp), the following formula was used:

$${\mathrm{W}}_{\mathrm{sp}}=\frac{m_2-m_3}{V} \left(\mathsf{\mu }\mathrm{g}/{\mathrm{mm}}^3\right)$$

(1)

where m₂ is the specimen’s post-immersion mass in (μg), m₃ is the specimen’s post-immersion mass of the specimen after drying (μg), and V is the pre-immersion volume of the specimen after drying (mm³).

2.4.3. Water Solubility Evaluation

The materials’ water sorption was evaluated following the ISO 20795-2 (2013) specifications for Group 1, Group 2, and Group 3 [25]. The water solubility (Wsl) was calculated using the following formula:

$${\mathrm{W}}_{\mathrm{sl}}=\frac{m_1-m_3}{V} \left(\mathsf{\mu }\mathrm{g}/{\mathrm{mm}}^3\right)$$

(2)

where m₁ is the specimen’s pre-immersion mass after drying (μg), m₃ and V are as described in Section 2.4.1.

2.5. Statistical Analysis

For the sample size calculation of the water sorption and solubility evaluation at α = 0.05 with a power of 80%, the total sample size should be at least 90 specimens for the water sorption and solubility evaluation. One hundred eight samples were made to evaluate the water sorption and solubility for the six retainer materials. Eighteen samples were made for each type of thermoplastic retainer material to evaluate the water sorption and solubility of three groups. Each group had six samples.

Data were analyzed using IBM^® SPSS^® version 23 (SPSS Inc., IBM, Chicago, IL, USA). The water sorption and solubility data were checked for normality using the Shapiro–Wilk test. The normality was satisfied, and two-way analysis of variance (ANOVA) was carried out with a significance level of p < 0.05, followed by one-way ANOVA with Tukey’s post-hoc test for multiple comparisons.

3. Results

3.1. Water Sorption and Solubility

3.1.1. Water Sorption

The means and standard deviations of water sorption values are shown in Figure 4. Essix Plus showed the lowest water sorption, and Zendura showed the highest water sorption in all three groups. Water sorption (mean ± SD) values varied from 6.9 ± 0.11 µg/mm³ to 15.61 ± 0.24 µg/mm³, 6.87 ± 0.16 µg/mm³ to 19.93 ± 0.63 µg/mm³, and 6.9 ± 0.27 µg/mm³ to 18.55 ± 0.57 µg/mm³ in Groups 1, 2, and 3, respectively (Figure 4).

There was a statistically significant interaction between the effects of the type of aging used and the type of thermoplastic retainer material on water sorption (p < 0.05) (

Table 2. Two-way ANOVA (Tests of Between-Subjects Effects).

Source	Type III Sum of Squares	df	Mean Square	F	Sig.
Corrected Model	1720.8 a	17	101.23	907.82	0
Intercept	10,485	1	10,484.98	94,032.67	0
Group	12.56	2	6.28	56.3	0
Material	1661.6	5	332.32	2980.35	0
Group × Material	46.7	10	4.67	41.87	0
Error	10	90	0.112
Total	12,215.85	108
Corrected Total	1730.87	107

^a. R² = 0.994 (Adjusted R² = 0.993).

).

A statistically significant difference in the water sorption between the three groups was only observed with Zendura (Figure 4). In Group 1, a statistically significant difference in water sorption was found between all the materials except between Essix ACE and Tru-Tain DX (Figure 5). In Groups 2 and 3, a statistically significant difference in water sorption was found between all the materials except between Imprelon S pd and Essix Plus and between Essix ACE and Tru-Tain DX (Figure 5).

3.1.2. Water Solubility

The means and standard deviations of water solubility values are shown in (Figure 6). Zendura showed the lowest water solubility in Groups 1 and 2 and the highest water solubility in Group 3. Essix Plus showed the highest water solubility in Groups 1 and 2, and it was the lowest in Group 3 (Figure 6). Water solubility (mean ± SD) values varied from −3.01 ± 0.19 µg/mm³ to −0.06 ± 0.06 µg/mm³, −1.42 ± 0.19 µg/mm³ to −0.11 ± 0.08 µg/mm³, and −0.01 ± 0.24 µg/mm³ to 0.55 ± 0.11 µg/mm³ in Groups 1, 2, and 3, respectively (Figure 6).

The combined effects of the aging methods used and the type of thermoplastic retainer material on water solubility were evaluated using two-way ANOVA. There was a statistically significant interaction between the type of aging and the type of thermoplastic retainer material on water solubility at (p < 0.05), as shown in (

Table 3. Two-way ANOVA (Tests of Between-Subjects Effects).

Source	Type III Sum of Squares	df	Mean Square	F	Sig.
Corrected Model	58.25 a	17	3.43	199.14	0
Intercept	13.47	1	13.47	782.92	0
Group	9.53	2	4.76	276.78	0
Material	19.21	5	3.84	223.23	0
Group × Material	29.52	10	2.95	171.57	0
Error	1.55	90	0.02
Total	73.27	108
Corrected Total	59.8	107

^a. R ² = 0.974 (Adjusted R² = 0.969).

).

A statistically significant difference in the water solubility between the three groups was only observed in Zendura material (Figure 6). Statistically significant differences between Groups 1 and 3 and Groups 2 and 3 were found in Duran Plus, Essix ACE, and Imperelon S pd. Tru-Tain DX showed a statistically significant difference only between Groups 2 and 3 (Figure 6). The statistically significant difference in water solubility between the materials among the three groups is shown in Figure 7.

3.2. Fourier Transform Infrared Spectroscopy (FTIR)

Figure 8 shows the FTIR spectra demonstrating that Duran Plus, Imprelon S pd, Essix Plus, Tru-Tain DX, and Essix ACE were very similar in their structural composition, and they had the typical infrared bands of PETG, indicating that these materials are formed mainly from PETG polymer. The absorption bands in the range of 2950–2800 cm⁻¹ (at around 2926 and 2855 cm⁻¹) were assigned to the asymmetric and symmetric stretching vibrations of C-H bonds (belonging to the methylene groups of the initial ethylene glycol/ethylene oxide monomers and to the methylene groups of the cyclohexane rings of the polymer main chain) and the bands located within 1715–1696 cm⁻¹ were associated with the stretching vibrations of C=O bonds of the ester groups that is related to the polymer backbone. At the same time, in the fingerprint region, the absorption peaks around 1244 cm⁻¹ were ascribed to the aromatic C-H in-plane bending vibrations, while those located at around 1097–956 cm⁻¹ were attributed to the cyclohexane ring vibrations combined with the aromatic C-H out-of-plane bending vibrations. The FTIR analysis of Zendura showed a spectrum that resembles semi-rigid polyurethane (PU). It exhibits a relatively broad peak at about 3306 cm⁻¹ attributed to the N-H stretching vibrations (with NH involved in hydrogen bonding) and two other absorption bands around 2930 and 2857 cm⁻¹ assigned to the asymmetric and symmetric C-H stretching vibrations of the methylene groups of the polyurethane main chain, respectively. The bands located around 1696–1697 cm⁻¹ and around 1595 and 1514 cm⁻¹, on the other hand, were assigned to the C=O stretching (with CO also implied in hydrogen bonding) and to the C=C-C aromatic ring stretching modes of vibrations, respectively. At the same time, the bands around 1200 cm⁻¹ were attributed to the aromatic C-H in-plane bending vibrations and those at 1067 cm⁻¹ were attributed to the aromatic C-H out-of-plane bending vibrations superimposed on the C-O-C stretching most likely bought by the presence of poly(ethylene glycol) segments in the structure of the polyurethane-based material.

For all the retainer materials, the FTIR spectra of Groups 1, 2, and 3 were almost identical except for the Zendura material (Figure 9). It appears that the aging methods used had little effect on the surface molecular composition for all materials except Zendura. Zendura displayed a change in the intensity of absorbance bands, even though their positions in the spectrum practically have not significantly changed, indicating that the material remained the same throughout the different aging methods used. However, the decrease in the intensity of the absorbance bands following thermocycling denotes the beginning of chemical degradation. The results based on FTIR spectroscopy showed that Zendura had the least stable surface structural composition compared to all tested materials (Figure 9).

4. Discussion

Many studies have evaluated the physical properties of thermoplastic materials as received from the manufacturer, which is unjustifiable since all of them are thermoformed to be used intraorally. However, such data could be used for purposes of material improvement and development. Moreover, it could be used for comparison across the different types of available materials. In this study, the thermoplastic retainer materials were assessed both before and after thermoforming to give a better idea about the materials’ clinical performance and stability. This study focused on evaluating the effect of thermoforming on water sorption and solubility and the molecular structure composition of thermoplastic retainer materials. In addition, since running a clinical study is costly and time-consuming process, in vitro aging (thermocycling) was used in the present study to evaluate the stability and durability of thermoplastic retainer materials in terms of their water sorption and solubility, and surface molecular composition.

Comparisons with the existing literature data are difficult due to the fact that no research has examined the six retainer materials investigated in the current study. Regardless of the type of material used, water sorption and solubility and molecular structure composition were compared with the existing literature results based on the similarities with the aging methods used in the current investigation.

4.1. Water Sorption and Solubility

Essix Plus had the lowest water sorption, and Zendura had the highest water sorption in all three groups. In a previous study, Zendura as received sheets were found to have the highest water sorption value of (15 µg/mm³) (after one week of immersion in water at 37 °C) compared to three different aligner materials (Erkoadur, Ghost Aligner, Essix Plastic) [26], while in the current study, it was found to be (15.61 µg/mm³) on average.

The effect of thermoforming and both thermoforming and subsequent thermocycling on water sorption was only significant in Zendura. The water sorption of Zendura had substantially increased after thermoforming. However, thermocycling significantly decreased the water sorption of Zendura when compared to the water sorption of thermoformed specimens. The results reported by Ryu et al. [27] show that the water absorption capacity increased for all four types of aligner materials after thermoforming compared to before thermoforming. Moreover, Ihssen et al. [28] reported that thermocycled specimens (1000 cycles) absorbed 48% more water compared to their immersed counterparts. Whereas our experimental data showed a negligible effect of thermocycling on the water sorption behavior. This could be due to the high number of cycles used in this study (10,000 cycles) prior to water sorption evaluation which probably consumed the material’s potential to absorb more water.

In the current study, the water sorption of thermoplastic retainer materials before thermoforming (Group 1) was significantly different between all the materials except Essix ACE and Tru-Tain DX. Moreover, in Group 2 and Group 3, there was a significant difference in water sorption between all thermoplastic retainer materials except between Imprelon S pd and Essix Plus and between Essix ACE and Tru-Tain DX. Thus, we rejected the first, second, and third null hypotheses for water sorption. We failed to reject the fourth and fifth null hypotheses for all materials except for Zendura in terms of water sorption.

The negative results of water solubility may indicate that thermoplastic retainer materials absorb more water, leading to an increased weight that masks their true solubility. Before and after thermoforming, Zendura had the lowest water solubility, and Essix Plus showed the highest water solubility. After thermoforming and subsequent thermocycling, Zendura had the highest water solubility, and Essix Plus had the lowest water solubility. Zendura was the only material that showed a statistically significant difference in water solubility between the three groups. As the effects of aging intensified, water solubility increased in Zendura. Daniele et al. [26] found that the water solubility of Zendura as received sheets after 14 days of immersion in water at 37 °C was (−0.6 µg/mm³) [26], while in the current study, it was found to be (−3 µg/mm³) (after one week of water immersion at the same temperature).

In this work, water solubility decreased slightly (statistically not significant) after thermoforming in Imprelon S pd, Tru-Tain DX, Essix ACE, and Essix Plus compared to before thermoforming. Whereas Ryu at al. [27] have reported that the water solubility was significantly greater after thermoforming than before thermoforming for most of their tested aligner materials. This could be because of variations in the experiment methods adopted or the materials used.

Thermocycling significantly increased the water solubility for all the materials except Essix Plus when compared to thermoforming effects only. When we compared the water solubility of thermocycled specimens to raw as received specimens, it increased significantly for all the materials except Essix Plus and Tru-Tain DX.

There was a significant difference in water solubility between the different types of retainer materials among the three groups. Thus, we rejected the first, second, and third null hypotheses regarding water solubility. We failed to reject the fourth null hypothesis for all materials except for Zendura. We rejected the fifth null hypothesis for all materials except for Essix Plus.

The optimal orthodontic retainer must have low water absorption capabilities [29]. The water sorption of all evaluated materials was within the ISO specification, which should not exceed 32 µg/mm³ [25]. Moreover, the water solubility of all evaluated materials was within the ISO specification, which should not exceed 5 µg/mm³ [25]. However, PU-based material (Zendura) performed the poorest in terms of water sorption compared to all tested materials in all groups. Essix Plus had the highest water solubility in Groups 1 and 2, and Zendura was the highest in Group 3.

Water sorption and solubility results can be attributed to the dissimilar polymer molecular weight, chemical composition, density, and crystallinity of the examined materials, which led to different water molecule penetration into the polymeric material. Variations in the water sorption and solubility characteristics of the evaluated PETG polymers could be attributed to differences in their molecular weight (which would go undetected by FTIR spectroscopy) and different additives used during manufacturing.

Depending on the nature of a polymer material and some other operating parameters (temperature or relative humidity), the water uptake and swelling phenomena could lead to initiating and developing mechanical degradation processes, with direct consequences in undesirable changes of the physio-chemical properties of the material [19,20,21]. Thus, an increased water absorption capacity seems to be strongly related to a higher potential degradation of such a material. In fact, the water absorption tests provide a crucial indicator of the material’s interaction with the oral environment, as saliva is composed of more than 99% water [30].

4.2. FTIR Spectroscopy

The FTIR spectra of Imprelon S pd, Duran Plus, Essix Plus, Essix ACE, and Tru-Tain DX exhibited a significant similarity irrespective of the thermal treatment applied. They showed typical features associated with PETG spectra. In contrast, Zendura showed a FTIR spectrum resembling a semi-rigid polyurethane (PU). Therefore, we failed to reject the first, second, and third null hypotheses in terms of surface molecular composition for all materials except Zendura.

It is possible to assess the effects of various aging methods on the chemical composition by comparing the FTIR spectra of the samples taken before and after thermoforming with or without thermocycling. A shift in the intensity of the absorption bands for the relevant groups is a clear indication of irreparable degradation in the material [22]. For all materials except Zendura, the spectra of Group 1, Group 2, and Group 3 appeared almost identical (Figure 9). The results showed that the aging methods used had little effect on the surface molecular structure of all materials except for Zendura. The intensity of absorbance bands changed in Zendura with thermoforming and thermocycling. However, the same peaks were present, indicating that the material remained the same throughout the different aging methods used. As stated earlier, the decrease in the intensity of the absorbance bands following thermocycling denotes the beginning of chemical degradation. According to the results from the FTIR spectroscopy, Zendura had relatively unstable surface structural composition compared to all other materials (Figure 9). Therefore, we failed to reject the fourth and fifth null hypotheses in terms of the surface molecular composition for all materials except for Zendura.

4.3. Limitations and Recommendations

This study’s outcomes shed light on the durability and stability of thermoplastic retainer materials. The differences between clinical situations and the in vitro conditions described in this paper should be considered when analyzing the current study findings. The current investigation illustrated how thermoplastic retainer materials behave in a moist environment and how they may degrade over time. Nevertheless, some limitations must be noted with this study. In this investigation, only 1 mm thickness was analyzed. Future research should compare samples of various thicknesses. This work was able to provide insight into the time-dependent alterations of water sorption and solubility and surface molecular structure of thermoplastic retainer materials. Future research should explore a longer duration of water immersion. Moreover, the effect of saliva immersion on water absorption and surface molecular structure should be evaluated in future studies. In addition, clinical investigations should be performed to assess the impact of in-vivo use on the physical and structural properties of the various types of thermoplastic retainer materials.

5. Conclusions

The findings from the current study showed that the type of retainer material affected its water sorption and solubility capabilities. Moreover, the aging method altered some retainer materials’ water sorption and solubility. Additionally, the surface molecular changes evaluated by FTIR spectroscopy revealed that all of the evaluated materials had similar spectra (resembling PETG) except Zendura (which had a spectrum that resembled semi-rigid PU). At the same time, the aging methods used did not affect the surface molecular composition of all materials with the same exception of Zendura, where its FTIR spectra indicated a decrease in intensities of some absorption bands after thermocycling which could be related to degradation processes on a molecular level.