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ORIGINAL ARTICLE
Year : 2021  |  Volume : 24  |  Issue : 12  |  Page : 1835-1840

Evaluation of physical properties of polyamide and methacrylate based denture base resins polymerized by different techniques


1 Department of Prosthodontics, Faculty of Dentistry, Bursa Uludağ University, Bursa, Turkey
2 Department of Prosthodontics, Faculty of Dentistry, Istanbul Okan University, Istanbul, Turkey
3 Department of Dental Prostheses Technology, Health Services Vocational High School, Hacettepe University, Ankara, Turkey

Date of Submission28-Jul-2020
Date of Acceptance14-Dec-2020
Date of Web Publication09-Dec-2021

Correspondence Address:
Dr. G Deste Gokay
Department of Prosthodontics, Faculty of Dentistry, Bursa Uludağ University, Bursa
Turkey
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/njcp.njcp_469_20

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   Abstract 


Aim: This study aims to comparatively evaluate the flexural strength, internal adaptation, elastic modulus, and maximum deflection of a newly introduced, strengthened injection-molded semi-flexed polyamide resin (Deflex) and a conventional heat-cured resin containing cross-linking polymethyl methacrylate denture base polymers (QC-20). Materials and Methods: A vinyl polysiloxane film replicating the gap between the denture base and the metallic master model of an edentulous maxilla was weighed using an analytical balance with an accuracy of 0.0001 g for the measurement of internal adaptation. The measurements were performed immediately after surface finishing. Seven rectangular test samples measuring 65 × 10 × 3.3 mm3 were set up for flexural strength test. Flexural strength test (three-point bending test) was performed using a universal machine under axial load at a crosshead speed of 5 mm/min. One-way ANOVA (α = 0.05) following by t tests was utilized in statistical analysis. Results: The difference between the flexural strength of the denture base resins of Deflex and QC-20 was found to be statistically significant. The injection-molded resin demonstrated better internal adaptation compared to the conventional heat-polymerized resin. Evaluation of the physical test results revealed that the polyamide samples were more flexible than polymethyl methacrylate and did not break during flexural strength tests. Conclusion: Some properties of denture base resins, such as resin types, internal adaptation, and mechanical strength, may play a significant role in clinical performance of complete dentures and removable partial prostheses. Because of the superior flexural strength properties and internal adaptation characteristics, Deflex may prove to be a useful alternative to conventional denture base resin.

Keywords: Denture bases, nylons, polymethyl methacrylate


How to cite this article:
Gokay G D, Durkan R, Oyar P. Evaluation of physical properties of polyamide and methacrylate based denture base resins polymerized by different techniques. Niger J Clin Pract 2021;24:1835-40

How to cite this URL:
Gokay G D, Durkan R, Oyar P. Evaluation of physical properties of polyamide and methacrylate based denture base resins polymerized by different techniques. Niger J Clin Pract [serial online] 2021 [cited 2022 Jan 20];24:1835-40. Available from: https://www.njcponline.com/text.asp?2021/24/12/1835/332113




   Introduction Top


The maximum bite strength of a patient with a full denture is approximately 16% of that of a dentate patient. Consequently, the acrylic resins used for the full denture play a significant role.[1–3] Some properties of denture base resins, such as internal adaptation/dimensional stability and physical properties, may play a significant role in the clinical performance of complete dentures.[2],[3]

Dentures are mostly constructed from polymethyl methacrylate (PMMA) acrylic resins by using traditional polymers/monomers cured using a water bath system. These resins are heat-polymerized and their physical properties are far from ideal. Thus, other materials and measures are being pursued in order to improve or replace the resins.[4–9]

Many methods can be used to improve the mechanical and physical properties of PMMAs, such as using an alternative material, producing chemical changes in the PMMA, or reinforcing the PMMA by other materials.[10–12] The first method involves reinforcing the PMMA material by altering an ordinary heat-polymerized acrylic resin. This reinforcement is pursued by using graft copolymer or cross-linking agents.[13],[14] Polymers with different structures, such as polystyrene, polycarbonate, poly (vinyl acrylic), polyethyleneterephthlate, polybutiylenterephthalate, urethane dimethacrylate, polyurethane, and polyamides (nylons), have been evaluated for reinforcing denture bases.[13–17] Polyamides and polycarbonates have also been researched as elective elements for people who are sensitive to methyl methacrylate materials; their side effects have been examined as well.[18–21]

Polyamides, first produced in the second half of the 20th century, are created by the buildup responses between a dibasic acid and a diamine. The first type of polyamide demonstrated a few issues, including discoloration, water sorption, bacterial contamination, surface roughness, and trouble with surface cleaning. In recent years, it has been broadly used as a denture base material because of its advantages, including favorable aesthetic properties, nontoxic effects in the case of allergies to resin monomers and conventional metals, higher elasticity than heat polymerized resins, nondegradation by heat and chemical agents, and high flexural strength.[22–27] However, there is no chemical bonding between denture teeth and this resin; thus, mechanical retention is necessary.[23–27]

With new advances in polymer science, advanced initiations and molding procedures have emerged, such as microwave polymerization and injection molding. The advantages of injection molding over the conventional heat-cured method include greater homogeneity of the mixture, careful temperature control throughout the process, excellent dimensional accuracy, and high strength. However, it also carries some disadvantages, including high equipment costs, failures related to the bonding of the teeth to the denture base, and low fracture resistance. Polymethyl methacrylate, polycarbonate, and polyamide (nylon) are resins used for injection-molded denture-based materials.[3],[20],[28]

The composition of the resin and the polymerization methods play an important role in influencing the flexural strength of these materials. The flexure of denture bases is a significant mechanical property, and the lifespan of dentures depends on the flexural quality of the acrylic resin forms.[29],[30]

Another significant characteristic to be assessed in denture base materials is their internal adaptation. Internal adaptation of complete denture bases can be affected by factors such as polymerization cycle, crack propagation, and fracture, which determine the source of stress absorption in the denture base resin.[12]

There is a sufficient amount of research in the literature regarding the analysis of recent polyamide resin systems for denture base construction. In the present study, it was expected that the internal adaptation, flexural strength, elastic modulus, and maximum deflection of the polyamide material would be comparable to those of the conventional polymethyl methacrylate resin. The null hypothesis was that there would be no statistically significant difference in this properties of these materials.


   Materials and Methods Top


Preparation of denture resin samples

A conventional heat-polymerized resin, QC-20 (Dentsply International Inc., Chicago, Il., USA), and a high-impact polyamide resin, Deflex (Nuxen SRL, Buenos Aires, Argentina), were used as two denture base materials for the denture base resins for the transverse strength and internal adaptation tests in the study.

Seven samples from each gathering were set up for the flexural strength test. The flexural strength was assessed by the ISO/DIS 1567 International Standard. To mold the samples from the denture base materials, stainless steel molds measuring 64 × 10 × 3,3 mm3 were set up.[30],[31] The dimensional differences in the flexural test samples were eliminated through grinding. Injection-molded resin samples were invested in flasks and then fabricated according to the manufacturer's suggestions.

For internal adaptation test, Type III dental stone casts were obtained from a vinyl polysiloxane impressions (Elite Double, Zhermack, Rovigo, Italy) of an edentulous Cooper-aluminum maxillary master die containing three pins placed at the incisive papilla and tuberosity regions. These pins served as indexes for future reposition of the processed denture bases in the master die. The thickness of all denture bases was standardized using 2-mm plastic sheets and the vacuum system (Plastivac). This study evaluated bases of approximately 2-mm thickness, which represents the average thickness of the central palatal area in the posterior region.

For conventional heat-polymerized resin, the powder-to-liquid proportion was 23.4 g/10 mL (powder: methyl methacrylate monomer, N-N dimethyl p-toluidine, ethylene glycol dimethacrylate [EGDMA] as a cross-linking agent, terpinolene, and hydroquinone; liquid: methyl methacrylate [methyl-n-butyl] copolymer and atoxic pigments benzoyl peroxide). The PMMA samples were set up in metal denture flasks and polymerized in water (100°C, 30 min).

The samples for polyamide resin (polyamide polymer cartridge) were set up in aluminum denture flasks and were then prepared using a thermoplastic injection machine.

The polyamide cartridge as a solitary segment and the flasks were set into the injection machine (M.A.D.). Then, the temperature was increased up to 280°C for 15 min and injected automatically into the flask at an injection pressure of 6 bars for 30 s. All sample groups were placed idle to cool the flasks. The polyamide samples were kept in boiling water for 15 min as per the manufacturer's directions.

All the samples were ground, finished, and polished on a wet surface. For this cleaning process, an automatic cleaning tool (Grin PO 2V processor polisher, Metkon A.Ş., Bursa, Turkey) was used as per the producer's guidelines. Prior to testing, samples were kept at 37°C for 48–52 h.

Flexural strength test

The samples were exposed to a three-point flexural strength test using the Universal Instron testing device (Lloyd Lr 30 K materials testing tools, Lloyd Instruments Ltd. Segenworth West, UK) with a crosshead speed of 5 mm/min with a bend test jig composed of two equal-tempered steel bars and applied as a heap midway. For the modulus of flexural determination, the machine was adjusted such that the diversion of the sample could be detected.[31] The flexural strength (σ) was determined by calibrating the machine, and the values were automatically computed using the equation:

σ =3 Fl/2 bh2

F = the greatest load enforced (N),

l = dimensions between the supports (range: 50 mm),

b = width (10 mm)

h = thickness of the sample (3 mm)

The flexural strength of denture base components of Type I heat-polymerized polymers should not be under 65 MPa as indicated by ISO 1567. If results obtained for at least four out of five samples comply with this requirement, the material passes. If one or two of the samples comply with the requirement, the material fails. If three of the samples comply with ISO 1567, six new samples must be prepared and test is repeated. If at least five of the second series of samples comply with the requirements, the material passes.[30],[31]

Elastic modulus (E) was determined using the flexural strength tests. Furthermore, the deflection of the samples (mm) and the comparing powers (N) were resolved. The elastic modulus was determined from the formula:

E = Fl3/4bh3d,

where d is the deflection (mm). As per ISO 1567, the base elastic modulus of denture base materials of Type I heat-polymerized resin should not be less than 2000 MPa. If at least three of the samples do not comply with the requirement, the material fails. If three of the elastic modulus results comply with the requirements, six new samples should be prepared. If at least five results of the second series comply with the requirement, the material passes.[31]

Internal adaptation test

At this stage, a metallic edentulous maxilla was copied. The copy is pursued utilizing a form in type III dental stone (Gyproc, Prevest Denpro, Jammu, India) as well as vinyl polysiloxane to create 14 patterns. The denture bases were fabricated from polyamide and heat-polymerized PMMA resin for the internal adaptation test. The master model had delimitation of the denture basal area and three projections (one anterior and two posterior) to allow repositioning of the resin bases during the adaptation test.

Internal adaptation was evaluated immediately after finishing and after 30 days of storage in water at 37°C by weighing a silicon film between the resin base and the metallic master model. A standardized portion of flowable type vinyl polysiloxane (3M ESPE Express, St. Poul, MN) was prepared and used for covering the inner surface of each resin base, which was set onto the master cast under a 5-kg axial load. The force was applied perpendicular to the center of the sample strips. The silicon film demonstrated the gap between the resin base and the master model for the entire basal region. After the polymerization of the silicone material, the resulting silicon film was trimmed at the borderline mark of the master cast and removed from the denture. Then, it was weighed using an analytical balance with an accuracy of 0.0001 g (MODEL ag 204, Mettler Toledo, Switzerland). This weighing procedure gives us information about adaptation as a lower weight of silicone film indicates a more adaptive prosthesis, whereas a higher weight indicates a less adaptive prosthesis. This procedure was repeated twice for each samples, and the average value was recorded to evaluate the internal adaptation of each samples. The mean values and standard deviations were calculated using the SPSS statistical software program (SPSS Inc., Chicago III). Analysis of variance ANOVA at the 95% confidence level was used to examine the effects of variables when significant differences in weights of impression material were detected; moreover, the t test was performed within groups.


   Results Top


In [Table 1], the flexural strength mean, elastic modulus, deflection, and internal adaptation estimations of the two denture base patterns are presented. Results of ANOVA demonstrate differences to be significant in the resin flexural strength of two groups (P < 0.05). Deflex demonstrated the most elevated mean flexural strength followed by QC-20.
Table 1: Mean and standard deviation of flexural strength, elastic modulus, maximum deflection, and internal adaptation for each study material

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Polyamide samples passed the limit of the flexural test tool and fell off the supports of the jigs. Flexural strength value was taken on the grounds that the samples did not break during the test. The samples of PMMA cracked completely as the test progressed. One-way ANOVA uncovered differences among polyamide and PMMA resin (P < 0.05) for the flexural strength test. The mean flexural strength of polyamide resin (111.34 MPa) was higher than that of PMMA resin (72.4 Mpa). Elastic modulus values between the resin groups were additionally seen as significantly different (P < 0.05), while the greatest redirection results were not significant (P > 0.05). Polyamide test samples showed the lower elastic modulus mean values (3705.93 MPa) contrasted with PMMA test samples values (6821.97 MPa). In contrast, the lower deflection mean values were evaluated from the PMMA samples (9.05 mm). Polyamide samples revealed higher outcomes (14.65 mm); however, this result was not statistically significant.

The consequences of the one-way ANOVA uncovered contrasts between the resin groups for the internal adaptation test (P < 0.05). Polyamide samples (1.89 ± 0.16 g) displayed almost 50% of mean internal adaptation value contrasted with PMMA samples (2.20 ± 0.17 g).


   Discussion Top


The null hypothesis in this study (there would be no statistical difference when comparing the physical properties of the two tested denture base materials) was rejected.

In prosthetic dentistry, it is very important that the structure of polymers, its dimensional stability/adaptability in the fitting of denture bases, biocompatibility, esthetics, mechanical properties, and molding or activation procedures must be reliable. Previous investigations have not been in agreement regarding the physical properties and behavior of these resins.[2],[11]

Kimura et al.[28] stated that the resin should be carefully chosen in order to avoid fractures. Additionally, Hayden[27] linked the type of resin to its influence on the resistance to fractures in 1986. The present in vitro study was intended to evaluate two commercially available denture base materials in terms of flexural/transverse strength, elastic modulus, maximum deflection, and internal adaptation testing. The flexural or transverse strength tests include a combination of tensile and compressive strength and modulus of elasticity.[4],[11] Internal adaptation is the main extraordinary component of these two polymeric denture bases introduced to the patients with enhanced characteristics. In the present research, the lower elastic modulus values displayed by polyamide test samples imply that this dental base material is less rigid than PMMA. These results were in agreement with the results presented by Yunus et al.,[15] Stafford et al.,[23] Sasaki et al.,[32] and Kürkçüoğlu et al.,[33] where the flexibility of polyamide was determined to be higher than that of PMMA denture base polymers. The examination empowers the execution of polyamide denture base patterns in some clinical circumstances where flexibility is needed. Polyamide resin patterns permit the manufacturing of removable dentures in mouths with certain degrees of undercut without block-out procedures.[22],[25]

The utilization of injection-molded polyamide denture base resin has been bolstered for displaying worthy strength and dimensional stability or internal adaptation, for their complete polymerization without free monomers, and for simplicity of control and disposal of the typical denture processing armamentarium.[34]

Deflex is a methyl methacrylate–free denture base material that was presented as an option for patients adversely affected by methyl methacrylate and its products.[15],[16],[26]

Ganzarolli et al.[34] reported higher flexural strength for the injection-molding technique. The present study showed that in accordance with this study, Deflex resin presented the highest flexural or transverse strength when compared to a standard heat-cured denture base material, QC-20. Moreover, in this study, the internal adaptation of Deflex was found to be better than that of QC-20. Ganzarolli et al.[34] reported that the injection-molded resin showed better internal adaptation compared with the conventional heat-polymerized resin, particularly after 30 days. This result is in concurrence with Yunus et al.,[15] who revealed that the internal adaptation of injection-molded resin was considerably better when compared with other denture base acrylic resins.

In spite of the fact that in vitro tests may not indicate intraoral conditions and be prescient of clinical execution, they are significant and can be pertinent to clinical conditions.[4],[11] This study has several limitations and it is not possible to directly extrapolate the flexural strength, elastic modulus, maximum deflection and internal adaptation test results of the materials tested in the study to other materials. Further studies are required to explore the effect of such resins by using quicker speed testing patterns to reveal the impacts of water sorption, recoloring, and other physical characteristics of Deflex as possible advantages that methacrylate-free materials can present in prosthetic dentures.


   Conclusion Top


The flexural strength, deflection, internal adaptation, and modulus of elasticity of two distinctive denture base materials, namely Deflex and QC-20, were analyzed in this study. As a result of the research, the following conclusions were drawn:

  1. Deflex indicated the highest mean flexural strength value than QC-20. Because the flexibility of polyamide samples is higher than the flexibility of PMMA, polyamide samples did not break during the flexural strength test. Thus, it can be said that Deflex may end up being a helpful option in contrast to customary denture base resin because of its flexural strength and internal adaptation attributes.
  2. Maximum deflection values of the polyamide test samples were found to be higher than those of the PMMA samples. However, the differences were not statistically significant.


Acknowledgements

The authors thank Dr. Tamer Tüzüner for conducting statistical analysis, and Mr. İbrahim Boran and Mr. Lokman Yılmaz for their help in preparing the resins samples.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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