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Article

Effect of Thermal Aging and Chemical Disinfection on the Microhardness and Flexural Strength of Flexible Resins

by
Inês Nascimento
1,
Nuno Rodrigues dos Santos
2,
Vitor Anes
3,4,*,
Cristina Bettencourt Neves
1,2,* and
Virgínia Santos
1,2
1
Faculdade de Medicina Dentária, Universidade de Lisboa, 1600-277 Lisboa, Portugal
2
Dental Biomaterials Research Group (BIOMAT), Biomedical and Oral Sciences Research Unit (UICOB), Faculdade de Medicina Dentária, Universidade de Lisboa, 1600-277 Lisboa, Portugal
3
Instituto Superior de Engenharia de Lisboa, 1959-007 Lisboa, Portugal
4
Mechanical Engineering Institute (IDMEC), Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal
*
Authors to whom correspondence should be addressed.
Appl. Sci. 2024, 14(1), 361; https://doi.org/10.3390/app14010361
Submission received: 19 November 2023 / Revised: 23 December 2023 / Accepted: 28 December 2023 / Published: 30 December 2023
(This article belongs to the Section Applied Dentistry and Oral Sciences)
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Abstract

:
This article examines the effects of thermal aging and chemical disinfection on the microhardness and flexural strength of flexible resins. The influence of the resin type on the mechanical properties was also investigated. Two flexible resins, Deflex Classic SR and Deflex Supra SF, produced by the injection method, and a thermopolymerizable acrylic resin—ProBase Hot, produced by the flasking method, were subjected to 1000 cycles of thermal aging and three chemical disinfection protocols (n = 8), with daily immersion and during a recommended time, in Corega Whitening, Corega Oxygen Bio-Active, 2.5% sodium hypochlorite, and distilled water (control). Knoop microhardness and three-point flexural strength were evaluated. Data were analyzed using Wilcoxon, Mann–Whitney and Kruskal Wallis tests (α = 0.05). The results varied between 14.5 KHN and 80.1 MPa for ProBase Hot and 7.3 KHN for Deflex Classic SR and 52.5 MPa for Deflex Supra SF. Thermal aging reduced the microhardness of the flexible resins, but not their flexural strength. The microhardness of Deflex Classic SR was influenced by chemical disinfection with Corega Bio-active (p < 0.001). The flexural strength of Deflex Supra SF was influenced by chemical disinfection with Corega Whitening (p < 0.05). It can be concluded that chemical disinfection led to changes in the flexible resins and should be used with caution to maintain the mechanical properties of the resins. Flexible resins showed reduced resistance to physical and chemical environmental influences, which can affect their longevity.

1. Introduction

The loss of teeth remains relevant today, as it has a particularly negative impact on the self-esteem and quality of life of older patients. Removable dentures have emerged as a vital solution for their oral rehabilitation due to their cost-effectiveness and simplicity of design [1,2,3,4]. Although the polymer polymethyl methacrylate (PMMA) has been used for decades for the fabrication of removable dentures [5,6,7], its potential to cause allergic reactions due to the release of residual methyl methacrylate (MMA) monomers, its mechanical impairments due to the large dimensional changes and low impact and fatigue resistance [6,8,9,10], and its esthetic impairments due to low color stability have led researchers to investigate the use of nylon-based thermoplastic polymers [5,6]. These polyamide materials offer a high degree of esthetics due to their translucent color that mirrors the natural gingiva. Their high flexibility, high impact strength and the absence of MMA in their composition show that they offer a good alternative to PMMA [5,6,7,8,11,12,13,14,15,16,17].
Among other disadvantages of flexible resins, such as the low modulus of elasticity and low tensile strength [18], they can lead to a greater accumulation of microorganisms [19] and to stains on the material caused by the pigmentation of food or after prolonged use of denture cleansers [5,13,16,20].
The cleaning habits of the oral mucosa and the remaining teeth as well as denture cleaning are particularly important for the longevity of dentures and the maintenance of oral health of denture wearers as they reduce the adhesion of biofilm to both the denture and the oral tissues [21,22,23]. Biofilm is responsible for the development of diseases such as denture stomatitis, especially in elderly or immunocompromised patients [22,23].
A denture hygiene protocol can be established using mechanical methods, chemical methods or a combination of both [16,19]. Although mechanical brushing of dentures is recognized as a simple and efficient method to remove biofilms, it should be combined with chemical methods for better disinfection [23,24].
In the chemical method, the denture is immersed in chemical agents for a certain period of time. Available chemical agents include sodium hypochlorite solutions and alkali peroxides in the form of effervescent tablets [5,9,25].
Although chemical disinfection procedures are essential, they may alter the physical or mechanical properties of the prosthesis if repeated frequently or over a long period of time [22,26]. In the oral cavity, dentures are exposed to constant temperature fluctuations due to the ingestion of food and beverages. Nevertheless, there is little information on the effects on flexible resins, i.e., the literature on the influence of thermal aging and disinfection on the microhardness and flexural strength of these resins is scarce [9,17,27,28,29].
This work aims to contribute to knowledge in this field and the following research hypotheses were formulated:
(1)
Does thermal aging affect the microhardness and flexural strength of resins?
(2)
Does the resin type influence the microhardness and flexural strength of resins after aging?
(3)
Do the chemical disinfection protocols affect the microhardness and flexural strength of resins?
(4)
Does the resin type affect the microhardness and flexural strength of resins exposed to disinfection protocols?
This work contributes to the knowledge of the mechanical stability of flexible resins after exposure to oral biodegradation processes and the chemical challenges of frequent disinfection with denture cleansers, which ultimately determines their clinical longevity. It can also help define the disinfection protocol that dentists should recommend to their patients to perform adequate denture hygiene without compromising the mechanical properties of the dentures.

2. Materials and Methods

2.1. Material Selection

Three thermoplastic resins were used in the present study: two flexible resins consisting of nylon particles molded by injection (Deflex Classic SR and Deflex Supra SF) (Table 1), and a thermo-polymerizable conventional PMMA-based acrylic resin molded by flasking (ProBase Hot) (Table 2).

2.2. Specimen Preparation

For each resin, 32 test specimens with the dimensions specified in ISO 20795-1:2013 (64 mm × 10 mm × 3.3 mm) were produced. The test specimens were produced using the lost wax technique from pink wax models (Dentaurum, Ispringen, Germany, batch 87647) constructed in stainless steel molds.
For the flexible specimens, the models were transferred to a flask and coated with plaster type III Pro-Solid (Saint-Gobain Formula, Newark, UK). Preformed blue wax channels were applied to the wax models. After removing the wax with a wax remover (Mestra), insulating liquid (Ivoclar Vivadent AG, Liechtensein, batch X18541) was applied to both sides of the flask to ensure separation between the plaster and the resins. The flexible resins were injected through an injection machine (Nuxen SRL, Buenos Aires, Argentina, Series 1300). The flask was then removed from the injection machine and allowed to cool at room temperature. The samples were removed, and the channels were cut with a 0.7 mm thick glass fiber reinforced cutting wheel on a handpiece at high speed and without cooling.
For the acrylic resin samples, type III Pro-Solid plaster (Saint-Gobain Formula, Unit-ed Kingdom) was placed on one side of a flask and the counter flask was filled with silicone putty (Zetalabor 5 kg + Indurent Gel 60 mL, Zhermack S.P.A., Badia Polesin, Italy) in which the wax samples were placed. After removing the wax, the laboratory procedures for fabricating the dentures were followed—inclusion, pressure polymerization, with a standard polymerization cycle (Table 2). After deflasking, the specimens were left at room temperature until they had cooled. To regulate the size of the specimens according to ISO 20795-1:2013, a small grinding process was performed. For this purpose, an 80 grit silicon carbide wheel was used in a polishing machine with constant cooling (DAP-U, Struers, Copenhagen, Denmark).
The flexible resin samples were polished with 600 grit water abrasive paper in 15 cm × 2 cm strips at low speed and without cooling for 15 s. Then a conventional gray polishing rubber, also at low speed, and a pumice stone were used for another 15 s. Finally, the final gloss was applied with Deflex gloss paste (Nuxen SRL, Buenos Aires, Argentina).
A pumice stone was used to polish the acrylic resin samples, followed by a universal polishing paste (Ivoclar Vivadent AG, Schaan, Liechtenstein, batch LWL4112). All polishing steps were carried out according to the manufacturer’s instructions and performed by the same person for 30 s on each surface.
The samples were stored in an oven (Ehret TK/L 4105, Emmendingen, Germany) at 37 °C for 48 h to minimize possible dimensional changes and facilitate the release of residual monomer.

2.3. Thermal Aging Process

All samples were thermally aged in a thermocycling device (Refri 200-E, Aralab, Cascais, Portugal) and subjected to 1000 temperature cycles between 5 °C and 55 °C (20 s in each bath) at 5-s intervals, corresponding to a 6-week thermal alteration of the oral cavity, thus simulating clinical use [30].

2.4. Chemical Disinfection Protocol

The samples of each resin were then randomly divided into 4 groups (n = 8) according to which disinfectant solution was used for immersion at 30-day intervals:
  • A commercial Corega Whitening tablet (alkaline peroxide based) was diluted in 200 mL distilled water once daily for 15 min as recommended by the manufacturer GSK;
  • One commercially available Corega Bio-Active Oxygen tablet (alkaline peroxide based) diluted once daily for 5 min in 200 mL distilled water as recommended by the manufacturer GSK;
  • 15 mL of 2.5% sodium hypochlorite diluted in 200 mL of distilled water for 10 min once a week;
  • 200 mL distilled water for 10 min, once a day (control).
After each immersion, the samples were removed from the solution, washed under running water for 10 s, wiped with absorbent paper, and stored at room temperature for 8 h to simulate the time the patient rests at night. After this time, the samples were stored in distilled water at 37 °C (Ehret TK/L 4105, Emmendingen, Germany).

2.5. Microhardness Test

The microhardness of the 96 samples was tested at three different times—before thermal aging, after thermal aging and after chemical disinfection—using a microhardness indenter (Duramin, Struers DK 2750, Ballerue, Denmark) with a Knoop diamond indenter in the shape of an elongated pyramid. The microhardness test was performed with a force of 98.12 mN for 30 s. Twelve indentations were made in each sample and the values were automatically converted into Knoop hardness numbers (KHN) by the device.

2.6. Flexural Strength Test

Since the flexural strength tests are destructive tests, the flexural strength values were calculated on samples that were not disinfected: 8 were tested before and 8 after thermal aging. The remaining flexural strength tests were performed on the 96 thermally aged and chemically disinfected samples.
A servo-hydraulic testing machine (Instron, Model 4502, Norwood, MA, USA) with a three-point bending fixture was used for the flexural strength tests. Width and thickness were measured for each specimen using a digital micrometer with an accuracy of ±0.01 mm (Mitutoyo Digimatic, MFG. Co., Ltd., Tokyo, Japan) and the average values were entered into the software before testing.
A crosshead speed of 5 mm per minute was selected, as described in ISO standard 20795-1: 2012, and the distance between the supports was 50 mm. The load was applied until failure and the ultimate load was specified in Newtons (N). The flexural strength was calculated using the following Equation (1):
F S = 3 W l 2 b d 2
where FS is the flexural strength in megapascals (MPa), W is the maximum load before fracture (N), I is the distance between the supports (50 mm), b is the width of the sample (mm) and d is the thickness of the sample (mm).

2.7. Statistical Analysis

Data analysis of the results was performed with the Statistical Package for the Social Sciences, Version 25 (IBM, Armonk, NY, USA). Descriptive analysis of microhardness and flexural strength included the mean, standard deviation, median and interquartile range. The normality of the data distribution was checked using the Shapiro–Wilk test. Since data for the analyzed variables did not show a normal distribution, the results were checked with the non-parametric Wilcoxon test for paired samples and the Mann–Whitney and Kruskal–Wallis tests with Bonferroni corrections for independent variables. All statistical tests were based on a significance level of 5% (p = 0.05).

3. Results

A descriptive analysis of the microhardness and flexural strength data before and after thermal aging was performed for each resin (Table 3 and Table 4).
The median value of microhardness is 7.3 KHN for Deflex Classic SR samples after thermal aging and 14.6 KHN for Probase Hot samples before thermal aging. Regarding the effects of thermal aging, Deflex Classic SR and Deflex Supra SF showed lower values for microhardness after thermal aging (p = 0.001 and p < 0.001 respectively). However, no differences in microhardness before and after thermal aging were observed for the ProBase Hot samples (p = 0.599) (Table 3). There were no statistically significant differences in microhardness (p > 0.05) between the three resins, either before or after aging (Table 3).
The median flexural strength values ranged from 52.5 MPa for Deflex Supra SF samples that were thermally aged to 80.1 MPa for Probase Hot samples prior to thermal aging.
For all resins, no differences were found between the flexural strength values before and after thermal aging (p > 0.05) (Table 4). It was found that the ProBase Hot specimens had statistically significantly higher flexural strength values than the flexible resins (p < 0.001) and that there were no statistically significant differences between the flexible resins (p = 0.056) (Table 4).
Following the chemical disinfection protocol, the median microhardness value ranged from 7.0 KHN for Deflex Classic SR samples subjected to the Corega Oxygen Bio-Active disinfection protocol to 14.6 KHN for ProBase Hot samples subjected to the Corega Whitening disinfection protocol (Figure 1).
Regarding the effect of chemical disinfection, the ProBase Hot samples subjected to the Corega Whitening protocol showed higher microhardness values than the other subgroups (vs. Corega Oxygen Bio-Active p = 0.023; vs. 2.5% sodium hypochlorite p = 0.04; vs. water p < 0.001). For Deflex Classic SR, the samples treated with Corega Oxygen Bio-Active showed lower microhardness values than the other subgroups (vs. Corega Whitening p < 0.001; vs. 2.5% sodium hypochlorite p = 0.04; vs. water p < 0.001). The Deflex Supra SF resin showed statistically significantly higher values when the samples were treated with Corega Whitening compared to 2.5% sodium hypochlorite (p = 0.025) (Figure 1).
When comparing the resins, it was found that the microhardness of the ProBase Hot resins was statistically significantly higher compared to the flexible resins for all subgroups (p < 0.001). Overall, there were no differences in the microhardness of the two flexible resins, with the exception of the lower values of Deflex Classic SR when the samples were treated with Corega Oxygen Bio-Active (p < 0.001) (Figure 1).
After the chemical disinfection protocol, the median flexural strength ranged from 39.4 MPa for the Deflex Supra SF specimens subjected to the Corega Whitning disinfection protocol to 83.2 MPa for the water-treated ProBase Hot specimens (Figure 2).
No statistically significant differences were found in any of the subgroups of ProBase Hot and Deflex Classic SR resin (p > 0.05). However, significant differences were found in Deflex Supra SF, as the Corega Whitening subgroups showed lower values compared to Corega Oxygen Bio-Active (p = 0.007) and 2.5% sodium hypochlorite (p = 0.027) (Figure 2).
Overall, the Deflex Supra SF samples showed lower flexural strength values than the Probase Hot (p < 0.001) and Classic SR resins (p = 0.001); no statistically significant differences between ProBase Hot and Deflex Classic SR resins (p = 0.401) (Figure 2).

4. Discussion

The aim of the present study was to investigate the effects of thermal aging and chemical disinfection protocols on the microhardness and flexural strength of two flexible nylon-based resins used as a prosthetic base for removable dentures. In addition, the mechanical behavior of the two flexible resins exposed to physical or chemical environmental influences was compared with a commonly used PMMA-based resin.
It was found that thermal aging only reduced the microhardness of the flexible resins, and the flexural strength of all resins was maintained. However, compared to the acrylic resins, the flexible resins exhibited lower flexural strength. Chemical disinfection with alkaline peroxides affected the mechanical properties of the resins, depending on their chemical composition.
Regarding the effects of thermal cycling on the mechanical properties of the resins, a decrease in microhardness was observed in the flexible resins Deflex Classic SR and Supra SF after thermal aging, which is consistent with previous studies showing that water absorption can plasticize resins [13,29,30,31,32]. During the thermal aging process, the test specimens are immersed in water, which can act as a plasticizer that changes the dimensions and mechanical properties of the resins and reduces the forces between the polymer chains. Some authors claim that acrylic resins have lower water absorption compared to flexible resins and argue that water absorption is greater in polyamide resins due to the amide groups in the main polymer chain [32,33,34,35]. This fact could explain the differences in flexible resins, because the more water molecules are absorbed, the more flexible the material becomes, and consequently the lower its microhardness [11,33,36,37,38]. In contrast, in the present study, the flexural strength values in all groups of specimens did not change after thermal aging. This is probably because only 1000 thermal aging cycles were performed, which corresponds to 6 weeks of biodegradation in the oral cavity and does not affect the core of the materials, which is responsible for flexural strength [30,35].
These results allow us to partially reject the first hypothesis, which states that thermocycling has no effect on mechanical properties, since it was found that thermal aging affects the microhardness of flexible resins, but not their flexural strength. Regarding the second hypothesis, which refers to the influence of the type of resin tested, the ProBase Hot specimens showed higher flexural strength values than the flexible resins both before and after thermal aging. The results are consistent with other studies that have also found higher flexural strength values for conventional resins [10,12,18,31,32,38,39,40,41,42].
This can be attributed to the different composition of the resins, as conventional acrylic resins consist mainly of PMMA with a higher monomer/polymer ratio as well as a higher number of crosslinking agents, in contrast to the flexible resins, which not only have a lower number of these agents but also a higher fiber content [8,38].
The two flexible resins tested had a microhardness of less than 15 KHN, a value recommended by the American Dental Association (ADA) in its Specification No. 12. On the other hand, they had a flexural strength of less than 65 MPa, a value mentioned in the specifications of the ISO standards as the limit for clinical acceptability. However, it should be noted that there are no standards for any of these properties for polyamide resins, so we cannot make direct comparisons with these reference values, which leads us to the need to establish standards that include this family of resins.
Since no differences in microhardness were found when comparing the three resins, the second hypothesis can be partially rejected, as the type of resin influences the flexural strength of the resins but not the microhardness.
Regarding the effects of the disinfection processes on the resins, there is agreement that ideally, they should not change the properties of the materials. However, certain components of the chemical solutions can alter the surface properties of the resins [43]. Several studies [1,13,27,29,30,31,40,41] have investigated the effects of disinfectants on the mechanical and physical properties of acrylic resins; however, there is little information on this relationship with the microhardness and flexural strength of thermoplastic resins.
The present study showed that the microhardness of ProBase Hot was higher after treatment with Corega Whitening than with the other immersion methods. These results are not consistent with the literature, which states that immersion in alkaline peroxides reduces microhardness [3,11,13]. This can be explained by a longer immersion time in aqueous medium (15 min), which could allow a greater release of residual monomers [42,43] and thus a lower plasticizing effect on the superficial part of the resins. Neppelenbroek et al. also considered the possibility of additional polymerization of the material to explain their results, which are to some extent comparable to the present study [41].
In the samples of Deflex Classic SR resin, there was a statistically significant decrease in microhardness in the Corega Oxygen Bio-Active subgroup. This is consistent with several studies in which a decrease in microhardness was observed in samples immersed in alkaline peroxides [6,24]. In addition to their strong alkalinity, alkaline peroxides promote the release of oxygen, which damages the interchain forces of the polymers and leads to a degradation of the surface of the resins and a decrease in their microhardness [6,9,10,13,29,30,44,45].
The microhardness of Deflex Classic SR resin samples immersed in Corega Whitening was also lower than that of samples immersed in Corega Oxygen Bio-Active. According to the manufacturer, there are no differences between the chemical compositions of the two types of alkaline peroxide tablets. However, it is believed that Corega Oxygen Bio-Active has a higher alkalinity because the solution contains a higher content of peroxides, which can be harmful to the surface of the resins [13].
Regarding the effect of chemical disinfection protocols on flexural strength, statistically significant differences were only observed with Deflex Supra SF resin when Corega Whitening was used. These results allow us to partially reject the third null hypothesis, which states that only Corega Whitening affects the flexural strength of Deflex Supra SF resin. This can also be explained by the fact that the test specimens were immersed for a longer period.
A comparison of the resins tested showed higher values for the microhardness of the ProBase Hot specimens compared to the flexible resins. These results seem plausible considering the differences in the composition of the two resin types [8,38]. In terms of flexural strength, the Deflex Supra SR resin showed lower values than the other resins. This can be explained by the fact that the Deflex Supra SR resin has a higher flexural strength and is more susceptible to the effects of alkaline peroxides. In addition, the difference between the two flexible resins could be due to the different nylon content of the two materials, as described by Abhay et al. [7] The Supra SF resin seems to be more susceptible to the plasticizing effect of chemical solutions due to its lower stiffness, thus affecting its surface more. When comparing the effects of sodium hypochlorite on the three resins, the flexural strength values changed statistically significantly. These results are consistent with the literature because although sodium hypochlorite is recommended by the American Dental Association for the disinfection of dentures, it is known to cause changes in the physical and mechanical properties of resins regardless of chemical composition [45,46,47,48].
The fourth hypothesis is therefore accepted, as it has been demonstrated that the effects of disinfection methods on microhardness and flexural strength depend on the chemical composition of the resins.
As this is an in vitro study, there may be limitations if the results are to be transferred to the oral cavity since the biodegradation of the materials is also caused by chemical alterations (like acidic beverages that changes the pH) and mastigatory forces. Also, the present results should be confirmed by microstruture tests like SEM, TEM and XPS.

5. Conclusions

Considering the experimental design used in this study, it can be concluded that:
  • Flexible resins showed lower values for microhardness when thermally aged, but this effect did not occur with the acrylic resin. In contrast, thermal aging did not change the flexural strength of any of the resins. Overall, thermal aging only affected the microhardness of the flexible resins and had no effect on the flexural strength of any of the resins.
  • The type of resin has no influence on the microhardness of the resins after thermal aging. In contrast, the acrylic resin retained a higher flexural strength compared to the flexible resins, which proves that the type of resin influences the flexural strength of the resins after aging.
  • Since chemical disinfection with Corega Oxygen Bio-Active significantly reduced the microhardness of Deflex Classic SR and Corega Whitening reduced the flexural strength of Deflex Supra SF, it can be concluded that the chemical disinfection protocols influence the microhardness and flexural strength of the resins.
  • It was found that the effects of disinfection methods on microhardness and flexural strength depend on the type of resin.
The implications of this study for clinical advice on denture cleaners must include that wearers of removable dentures based on ProBase Hot resin can disinfect them with all tested disinfectant solutions. Flexible dentures based on Deflex Classic SR acrylic should not be disinfected with Corega Oxygen Bio-Active as it has been shown to be particularly harmful. Flexible dentures based on Deflex Supra SF, on the other hand, should not be disinfected with Corega Whitening.
Acrylic resins have also been shown to be more stable against physiological and chemical influences, which suggests that these resins last longer and retain their mechanical properties. In this context, flexible resins should be avoided in patients with poor hygiene habits.

Author Contributions

Conceptualization, C.B.N. and V.S.; methodology, C.B.N.; software, I.N.; validation, C.B.N., V.S. and V.A.; formal analysis, N.R.d.S.; investigation, I.N.; resources, N.R.d.S.; data curation, V.A.; writing—original draft preparation, I.N.; writing—review and editing, V.A. and C.B.N.; visualization, I.N.; supervision, C.B.N.; project administration, C.B.N.; funding acquisition, C.B.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is contained within the article.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 14 00361 g001
Figure 1. Boxplots of the microhardness (KHN) data of the individual resins according to the chemical disinfection protocols. Circles represent outliers.
Figure 1. Boxplots of the microhardness (KHN) data of the individual resins according to the chemical disinfection protocols. Circles represent outliers.
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Applsci 14 00361 g002
Figure 2. Boxplots of the flexural strength data (MPa) of the individual resins according to the chemical disinfection protocols. Circles represent outliers.
Figure 2. Boxplots of the flexural strength data (MPa) of the individual resins according to the chemical disinfection protocols. Circles represent outliers.
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Table 1. Characteristics of the flexible resins used in the study.
Table 1. Characteristics of the flexible resins used in the study.
Deflex Classic SRDeflex Supra SF
ManufacturerNuxen SRL, Buenos Aires,
Argentina		Nuxen SRL, Buenos Aires,
Argentina
Batch Number31318CL3036217SU03
Injection Temperature280 °C260 °C
Plastification Time15 min12 min
Support Time1 min1 min
Air Pressure6.0 Kg/cm2 = 6.0 Bar = 86 PSI3.5 Kg/cm2 = 3.5 Bar = 50 PSI
Table 2. Characteristics of the acrylic resin used in the study.
Table 2. Characteristics of the acrylic resin used in the study.
ProBase Hot
ManufacturerIvoclar Vivadent AG, Schaan, Liechtenstein
Batch NumberWT0763 (Powder)
N46447 (Liquid)
Powder/Liquid ratio (g/mL)22.5/10
CompositionPowder: Polymethylmethacrylate, Softening Agent, Benzoyl Peroxide, Pigments
Liquid: Methylmethacrylate, Dimethacrylate (Linking agent), Catalyst
Standard Polymerization Cycle100 °C for 45 min
Table 3. Values of mean, standard deviation, median and interquartile range of microhardness (KHN) of the tested materials before and after thermal aging. SD—standard deviation; IQR—interquartile range.
Table 3. Values of mean, standard deviation, median and interquartile range of microhardness (KHN) of the tested materials before and after thermal aging. SD—standard deviation; IQR—interquartile range.
MaterialMicrohardness (KHN) before
Thermal Aging		Microhardness (MPa) after Thermal Aging
Mean (±SD)Median (IQR)Mean (±SD)Median (IQR)
ProBase Hot14.6 (±3.13)14.6 (3.3)14.5 (±3.24)14.5 (3.3)
Deflex Classic SR7.8 (±1.60)7.5 (1.5)7.5 (±1.37)7.3 (1.4)
Deflex Supra SF8.6 (±2.10)8.6 (2.7)8.0 (±1.79)7.9 (2.3)
Table 4. Values for mean, standard deviation, median and interquartile range of flexural strength (MPa) of the tested materials before and after thermal aging. SD—standard deviation; IQR—interquartile range.
Table 4. Values for mean, standard deviation, median and interquartile range of flexural strength (MPa) of the tested materials before and after thermal aging. SD—standard deviation; IQR—interquartile range.
MaterialFlexural Strength (MPa) before Thermal AgingFlexural Strength (MPa) after Thermal Aging
Mean (±SD)Median (IQR)Mean (±SD)Median (IQR)
ProBase Hot83.2 (±12.31)80.1 (23.22)71.0 (±18.39)66.9 (14.49)
Deflex Classic SR63.0 (±3.53)62.7 (5.35)69.4 (±3.82)61.2 (3.70)
Deflex Supra SF54.8 (±4.14)54.5 (8.13)52.6 (±2.70)52.5 (2.04)
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Nascimento, I.; Rodrigues dos Santos, N.; Anes, V.; Neves, C.B.; Santos, V. Effect of Thermal Aging and Chemical Disinfection on the Microhardness and Flexural Strength of Flexible Resins. Appl. Sci. 2024, 14, 361. https://doi.org/10.3390/app14010361

AMA Style

Nascimento I, Rodrigues dos Santos N, Anes V, Neves CB, Santos V. Effect of Thermal Aging and Chemical Disinfection on the Microhardness and Flexural Strength of Flexible Resins. Applied Sciences. 2024; 14(1):361. https://doi.org/10.3390/app14010361

Chicago/Turabian Style

Nascimento, Inês, Nuno Rodrigues dos Santos, Vitor Anes, Cristina Bettencourt Neves, and Virgínia Santos. 2024. "Effect of Thermal Aging and Chemical Disinfection on the Microhardness and Flexural Strength of Flexible Resins" Applied Sciences 14, no. 1: 361. https://doi.org/10.3390/app14010361

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