|Year : 2016 | Volume
| Issue : 2 | Page : 91-94
Comparative Evaluation of Flexural Strength of Heat Polymerised Denture Base Resins after Reinforcement with Glass Fibres and Nylon Fibres: An In vitro Study
Kusum Singh1, Sumit Kumar Sharma2, Pooja Negi3, Manish Kumar1, Divya Rajpurohit1, Priyanka Khobre4
1 Department of Dentistry, S.P. Medical College and Hospital, Bikaner, Rajasthan, India
2 Primary Health Centre, Swaroopganj, Sirohi, Rajasthan, India
3 Department of Dentistry, Government Dental College and Hospital, Shimla, Himachal Pradesh, India
4 Department of Oral Medicine and Radiology, Government Dental College and Hospital, Chennai, Tamil Nadu, India
|Date of Web Publication||12-Sep-2016|
Department of Dentistry, S.P. Medical College and Hospital, Bikaner, Rajasthan
Source of Support: None, Conflict of Interest: None
Aim: This study was an in vitro study done to evaluate and compare the flexural strength (FS) of heat polymerised polymethyl methacrylate (PMMA) denture base resins after reinforcement with nylon fibres and different concentration of glass fibres (GFs). Materials and Methods: Fifty heat-cured PMMA resin samples were fabricated using a die and divided into five groups, having ten samples in each group. All the samples were tested on universal testing machine and three point bending test was done. Then, FS of each sample was calculated. Mean value of FS of each group was used for statistical analysis. One-way analysis of variance test was used for statistical analysis. Results: Results showed that the fibres significantly affected the FS of PMMA. FS increases to the maximum with 5% GFs as compared to 2% glass, 2% nylon and 10% GFs. Conclusion: Polymers used in denture base fabrication, reinforced with GFs have shown to have a positive effect on the fracture resistance of dentures as compared to unreinforced PMMA.
Keywords: Flexural strength, glass fibres, nylon fibres, polymethyl methacrylate, reinforcement
|How to cite this article:|
Singh K, Sharma SK, Negi P, Kumar M, Rajpurohit D, Khobre P. Comparative Evaluation of Flexural Strength of Heat Polymerised Denture Base Resins after Reinforcement with Glass Fibres and Nylon Fibres: An In vitro Study. Adv Hum Biol 2016;6:91-4
|How to cite this URL:|
Singh K, Sharma SK, Negi P, Kumar M, Rajpurohit D, Khobre P. Comparative Evaluation of Flexural Strength of Heat Polymerised Denture Base Resins after Reinforcement with Glass Fibres and Nylon Fibres: An In vitro Study. Adv Hum Biol [serial online] 2016 [cited 2019 May 20];6:91-4. Available from: http://www.aihbonline.com/text.asp?2016/6/2/91/190315
| Introduction|| |
Polymethyl methacrylate (PMMA) resin was introduced by Dr. Walter Wright  in 1937 and since then it is in continuous use because of its multiple advantages such s processing ease, accurate fit, stability in oral environment, superior aesthetics and use with almost inexpensive equipment. A common clinical problem, which is faced by the dentist, is the fracture of denture base made of PMMA. Fracture of the prosthesis can result from impact, fatigue or degradation of base materials., After fracture, the prosthesis generally requires repair until more definitive treatment can be rendered to the patient. If repairs are done with heat cure denture base resin, changes will occur because the older resin softens during the processing of the resin. These changes may introduce a change in occlusion and fit of the denture base. Most of the time repairs are made using autopolymerising PMMA. However, the mean transverse strength of heat polymerised denture base repaired with autopolymerised resin reduces to 57% of the original material before fracture. These fractures can be prevented by improving the strength of the PMMA in one or the other way. Various approaches for the strengthening of intact acrylic resin prosthesis have been suggested in the literature. Chemical modification has been performed to produce graft copolymers or rubber methacrylate which is also known as high impact resins. Reinforcement of denture base has been attempted through incorporation of solid metal forms (meshes and wires, etc.) and various types of fibres in fracture-prone areas., Reinforcement of PMMA with various fibres such as carbon,,, polyethene,, polyaramide fibres, glass fibres (GFs) and nylon fibres  has been studied by various authors. Strengthening by fibre reinforcement is based on the principle that polymer matrix is fully capable of transferring an applied load to fibres via shear forces at the interface. Fibres used for reinforcement act as the main load bearing constituents and the matrix forms a continuous phase to surround and hold the fibres in place.
Flexural strength (bending strength or modulus of rupture)
'It is defined as force per unit area at the point of fracture of a test specimen subjected to flexural loading'.
Flexural (Bending) Stress - these stresses are produced by bending forces in dental appliances in the following two ways:
- By subjecting a structure to three-point loading, whereby the endpoints are fixed, and a force is applied between these endpoints
- By subjecting a cantilevered structure that is supported only at one end to a load along any part of the unsupported section.
Furthermore, when patient bites into an object, the anterior teeth receive forces that are at an angle to their long axes, thereby creating flexural stresses within the teeth.
| Materials and Methods|| |
This study was an in vitro study done to evaluate and compare the flexural strength (FS) of heat polymerised PMMA denture base resins after reinforcement with nylon fibres and different concentration of GFs.
Preparation of stainless steel dies
Stainless Steel dies (S.S. die) having three slots of 65 mm × 10 mm × 3 mm were fabricated. Die was prepared according to American Dental Association specification.
S.S. die was lubricated with thin layer of petrolatum jelly and placed on clean, clear and dry glass slab. Molten base plate wax was poured in the die. After filling all three slots with the molten wax, wax was allowed to cool for 10 min. Patterns were flasked, dewaxed and cured using manufacturer's instructions, by compression moulding technique for packing and using long curing cycles.
Test specimen preparation
Test groups were reinforced with 10–15 µ thick and 6 mm long fibres.
Group A – Control group: Unreinforced PMMA was used for preparing the specimen.
Group B – PMMA denture base resin reinforced with 2% nylon fibres.
Group C – PMMA denture base resin reinforced with 2% GFs.
Group D – PMMA denture base resin reinforced with. 5% GFs.
Group E – PMMA denture base resin reinforced with 10% GFs.
Storage of samples
All the samples were kept in different boxes, designated by Groups A, B, C, D and E in distilled water for 7 days, to remove the remaining unreacted monomer.
Testing of samples
The FS for the specimens was determined by loading the specimen in the same universal testing machine at room temperature. Each specimen was positioned on the bending fixture, consisting of 2 parallel supports, 50 mm apart. The load was applied at a crosshead speed of 2 mm/min, with a third rod placed centrally between the supports. Peak force for each specimen was recorded and used to calculate the FS.
Measuring the flexural strength
The mathematical formula  for computing the FS is as following:
Where σ = FS, L = distance between the supports, b = width of specimen d = thickness P = maximum load at the point of fracture.
| Results|| |
One-way ANOVA test showed a statistically significant difference between all the five groups (P < 0.0001); this suggests that at least one group among these entire five groups is different statistically. The mean FS ranged from 53.6 N to 71.67 N with the highest mean being exhibited by 5% GF reinforced samples [Table 1] and [Figure 1].
|Table 1: The mean and standard deviation of flexural strength of various groups|
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|Figure 1: Flexural strength (in Newton) of heat polymerised denture base resin after reinforcement with nylon and glass fibres.|
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| Discussion|| |
Factors affecting the physical properties  of the composites are stiffness, adhesion, impregnation of fibres and the concentration used. Fibres used should be stiff to reinforce the brittle material otherwise little or no effect on the properties. Adequate adhesion of the fibres to the polymer is the most important variable for the strength of the composite so that stresses can be transferred from matrix to the fibres. Silane coupling agent can be used to improve the adhesion. Effective impregnation allows the resin matrix to come into contact with the surface of every fibre and thus bonding is improved. Concentration of fibre when increased judiciously, considerably affects the properties of the composite, but increased concentration may induce voids in the composite.
GFs are used for reinforcing polymers because of their good aesthetic qualities. The most common type of glass used is called electrical (E) glass claimed to be superior in FS. E-GFs have also shown relatively good long-term stability against water. They have better potential despite the difficulty of achieving adequate impregnation of the fibres.
The use of polymer fibre composite (polymer matrix and reinforcement used) in the fabrication of dentures has not been without problems. Major difficulties in using fibres with polymers include inadequate impregnation  of fibres with polymer resin and the difficult handling of the fibres due to the fraying and spreading of the fibres to undesired regions of the denture. To solve this problem, the fibres were pre-impregnated with the monomers. In addition, the pre-impregnation made the GF easy to use, fibre did not fray and they can be placed in the desired region of the prosthesis. Inadequate impregnation, fraying and spreading of the fibres, difficulty in manipulation and voids formation may be blamed for the decrease in FS after 10% GF reinforcement in this study. According to Nagai, these fibres have been shown to improve the mechanical properties, especially fatigue resistance, impact strength and FS. This is because of good adhesion of the GFs to denture base polymer and low percentage of elongation at breakage of GFs. According to Marei, improvement in strength results because of the fact that the GFs used were bonded to PMMA matrix and the stress applied to the matrix was transferred to the fibres.
According to Vallittu, fibres can be incorporated in chopped, longitudinal and woven form. Longitudinally incorporated fibres may change their position with the applied pressure when the mould is placed in a hydraulic press. The woven form resembles cloth, its contact with the acrylic is problematic. Since both the above-mentioned negative features are not encountered in the chopped form, this form was employed in this study. Since Unidirectional fibres enhance the strength in one direction while randomly oriented fibre improves the mechanical properties in all directions. With regard to the ratio of fibre to resin, the amount of fibres used to improve the properties of fibre reinforced composites is based on the value of fibre content. The theoretically expected elastic modulus of fibre reinforced composites was calculated using the rule of mixtures.
ET = EFVF + EMVM
Where E, V, F and M represent the elastic modulus, volume fraction, fibre and matrix, respectively, provided that there is perfect bonding between them.
Reinforcement with GF is an attractive option to improve the mechanical properties as it can be done without any new equipment outlay. Other advantage is that if the prosthesis fractures catastrophically then the fractured portions are likely to remain in close proximity, held together by the fibres as in this study, FS of 2% nylon reinforced group did not differ significantly from control group although nylon reinforcement has shown slightly higher fractures resistance than the control group. Similar results are observed by John et al. in their study. Vallittu  used continuous, untreated unidirectional fibres and observed that 5% GF increase the transverse strength up to 38%. Knoel et al. (1975) used untreated, randomly distributed fibres and found an increase of 4% in the transverse strength with 2% GF reinforcement. Wright et al. (1979) used untreated randomly organised short fibres and observed 17% increase with 1% GF reinforcement and 24% increase with 4% GF reinforcement. On comparing the results with this study, it was found that there was 17% increase with 2% GF reinforcement and 31% increase with 5% GF reinforcement. This discrepancy can be because of the different material brand and batch or different processing techniques, etc.
| Conclusion|| |
Within the limitations of the study, it was concluded that reinforcement of PMMA is beneficial. All the groups showed higher load required to fracture the specimen as compared to the control group. Reinforcement with 2% and 5% GFs showed statistically significant improvement in the FS of the PMMA. Five percent GFs showed statistically highly significant improvement. As the concentration was increased to a higher level (10%), FS decreased in this study.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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