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An evaluation of the Flexural Properties of Meliodent and Acropars Heat Polymerized Acrylic Resins
F. Golbidi 1~, O. Jalali 2
1Assistant Professor, Department of Prosthodontics and Torabinejad Dental Research Center, School of Dentistry, Isfahan University, Isfahan, Iran
2Dentist, Private Practice
Abstract:
Objective: The aim of this study was to measure and compare the transverse strength and deflection of two acrylic resins.
Method and Materials: A total of 30 samples, with dimensions of 2.5×10×65mm, were prepared with Acropars and Meliodent acrylic resins according to ADA specification No. 12. The specimens were divided into two groups of 15 samples each.
A three-point bending test was carried out using a universal testing machine.
Differences between the means of the two groups were analyzed with Student's t-test.
Results: The mean (SD) transverse strength was 81.55 (1.54) Mpa in the Meliodent and 73.58 (0.6) Mpa in the Acropars samples, which showed a significant difference (P<0.001). The mean (SD) transverse deflection was 3.86 (0.09) mm and 4.96 (0.06) mm in the Meliodent and Acropars samples, respectively. A significant difference was found between the two groups (P<0.001).
Conclusion: In the present investigation, the transverse strength of the Acropars specimens was smaller than that of the Meliodent samples and ADA specifications.
However, the mean transverse deflection was larger in the Acropars group as compared to the Meliodent group.
Key Words: Heat polymerized acrylic resin; Transverse strength; Transverse deflection
Journal of Dentistry, Tehran University of Medical Sciences, Tehran, Iran (2007; Vol: 4, No.2)
INTRODUCTION
Poly methyl methacrylate (PMMA) is one of the most widely used denture base materials with numerous advantages [1,2]. However, poor mechanical properties like susceptibility to fracture due to unsatisfactory transverse strength, impact strength or fatigue resistance have also been reported in this acrylic resin [3]. Considerable progress has been made in the development of new acrylic resins with improved qualities. Vallittu [4-7] has estab- lished the usefulness of glass fiber roving as a strengthener of dental resins.
Other attempts have been made to enhance the
mechanical properties of acrylic resins by changing the processing methods [8]; copoly- merization and cross linking [9-11], giving maximum bulk to the material in the regions most heavily stressed [3]; and reinforcement with glass [12-15], carbon [16,3], polyethylene [17] or methyl methacrylate fibers [18,19].
Acropars (Marlic Co. Tehran, Iran) is an Iranian heat polymerized acrylic resin which has become available in recent years. Various factors can affect the physical properties of acrylic resins such as dimensional stability, weight and volume stability, molecular struc- ture of the polymer, color stability, surface Original Article
~ Corresponding author:
F. Golbidi, Department of Pros- thodontics, School of Dentistry, Isfahan University, Isfahan, Iran.
[email protected] Received: 17 March 2006 Accepted: 11 October 2006
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roughness, the amount of porosity, residual monomer and strength of the acrylic resins (i.e., transverse strength, fatigue strength, flex- ural strength, impact strength, tensile strength, etc.) [1,2].
The aim of the present investigation was to determine the transverse strength and deflect- tion of Acropars acrylic resin and to compare it with ADA specification [20] and Meliodent acrylic resin (Bayer Co. Germany).
MATERIALS AND METHODS
Two acrylic resins were examined in this laboratory experimental study, based on Ame- rican Dental Association (ADA) specification (No. 12), with minor modifications. Acropars (Marlic Co. Tehran, Iran), an Iranian acrylic denture base material, and Meliodent (Bayer Co. Germany) were selected for evaluation.
The reason for choosing Meliodent was that it was an ADA-approved and available heat polymerized acrylic resin.
Fifteen samples were tested in each group Three 2.5×10×65mm stainless steel master dies were prepared and invested in conven- tional denture flasks with dental stone (Hinrizit stone, Ernst Hinrichs GmbH, Germany).
Finally, fifteen molds were made for each material and randomly coded.
All samples were prepared using the conven- tional compression molding technique and processed according to the manufacturers’ re- commendations. They were then carefully removed from the metal molds, wet-polished with no. 400 sandpaper and stored in distilled water at37oC for 48 hours prior to testing.
The specimens were randomly selected and subjected to a three-point bending test with a constant crosshead speed of 5mm/min using a universal testing machine (Instron 4301, Instron Corp. Canton, MA, USA). Transverse deflection was recorded at 48 N and fracture force was registered in Newton. All measure- ments were obtained on the same day.
The transverse strength of each specimen was
calculated using the following formula, [1]:
S=3fL/2bd2
S was transverse strength or modulus of rup- ture (MPa), f was fracture force (N), L was the distance between the supports (length of spe- cimen) (mm), b was the width of the specimen (mm) and d was specimen thickness (mm).
Mean values and standard deviations were cal- culated for transverse deflection and strength.
Student’s t-test was used for statistical analy- sis. P-values less than 0.05 were considered to indicate significant differences.
RESULTS
The mean (SD) transverse strength and bending deflection were 81.548 (1.541) Mpa and 3.855 (0.092) mm, in the Meliodent speci- mens and 73.575 (0.604) Mpa and 4.957 (0.059) mm in the Acropars samples, respec- tively. The results indicated that Meliodent samples revealed larger transverse strength and smaller transverse deflection values com- pared to the Acropars samples.
A statistically significant difference in trans- verse strength and bending deflection was observed between the Meliodent and Acropars specimens (P <0.001).
DISCUSSION
Fractures in acrylic resin dentures occur quite often even though metal strengtheners have been incorporated into their designs [3].
Measurement of transverse strength is more commonly used for the evaluation of denture plastics as compared to tensile or compressive strengths. This is due to the fact that transverse strength closely represents the type of loading applied to the denture [1,2].
The aim of this study was to measure and compare the transverse strength and deflection of Acropars and Meliodent acrylic resins.
In the present investigation the mean (SD) transverse strength of Acropars and Meliodent samples was 73.575 (0.604) Mpa and 81.548 (1.541) Mpa, respectively. The mean trans-
Journal of Dentistry, Tehran University of Medical Sciences Golbidi & Jalali
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verse deflection was 4.957 (0.059) mm in the Acropaars and 3.855 (0.092) mm in the Melio- dent group.
According to ADA specification no 12 [20], the deflection in the center of a specimen should be 2 to 5.5mm for a load of 1500 to 5000 g, and the transverse strength of heat polymerized acrylic resins should be 78 to 92 Mpa. These values are close to those obtained for the Meliodent samples in the current investigation.
Acropars revealed a lower transverse strength compared to previous reports, however the transverse strength and deflection of the Meliodent group was similar to those reported by others [11,12,18,19].
Several factors can influence transverse stren- gth and bending such as degree of polymeri- zation, powder particle size, porosity, polymer molecular weight and the amount of residual monomer, fillers and plasticizers [1,2].
Numerous investigators have evaluated dif- ferent reinforcement techniques for streng- thening PMMA. Yazdanie and Mahood [3]
showed that carbon-fiber acrylic resin compo- sites were stronger than unfilled resins. Deboer et al [16] studied the effect of carbon fiber orientation on fatigue resistance and bending of denture resins and demonstrated an impro- vement in bending strength when the carbon fibers were aligned perpendicular to the stress direction. Stipho [12] and Uzun and Keyf [13,14] found a higher transverse strength in samples treated with glass fiber. Improvement of the transverse strength of PMMA has been reported following addition of dibutyl methac- rylate. High esters of this type act as internal plasticizers and prevent release of monomer into the oral cavity [1].
It has been suggested that longer polymeriz- tion times can decrease the porosity and in- crease the polymerization degree and trans- verse strength of acrylic materials. On the other hand, short polymerization cycles have been reported to increase the amount of resi-
dual monomer [21,22]. The plasticizing effects of excess monomer may adversely affect the transverse strength of acrylic resins.
A previous study has shown that the solubility of Acropars acrylic resin was higher than the maximum standard requirements [23]. This may be due to cross-linking agents, plastici- zers, unreacted monomer, initiators or soluble materials [1].
Therefore it seems that evaluation of a longer polymerization cycle on Acropars acrylic resins requires further investigation. This pro- cessing method would increase the molecular weight and degree of polymerization and decrease the amount of porosity and residual monomer in the samples [1,2,22].
Considering that addition of glass fibers and high-ester materials like dibutyl methacrylate have been reported to improve the physical properties of acrylic resins, further evaluation is suggested in order to assess the effects of these factors on Acropars polymers [1,12-14].
CONCLUSION
A significant difference was found between the mean transverse and bending strengths of Acropars and Meliodent acrylic resins (P<0.001). The mean transverse strength of the Acropars specimens did not meet the ADA specification requirements.
REFERENCES
1- Craig RG. Restorative Dental materials.
Eleventh edition, St Louis: Mosby company; 2002.
p.635-72.
2- Anusavice KJ. Phillips science of dental materials. Eleventh edition, St. Louis: Saunders company; 2003. p. 143-70, 721-56.
3- Yazdanie N, Mahood M. Carbon fiber acrylic resin composite: an investigation of transverse strength. J Prosthet Dent 1985 Oct;54(4):543-7.
4- Vallittu PK. The effect of void space and poly- merization time on transverse strength of acrylic- glass fibre composite. J Oral Rehabil 1995 Apr;
22(4):257-61.
Golbidi & Jalali Flexural Properties of Heat Polymerized Acrylic Resins
2007; Vol. 4, No. 2 71
5- Vallittu PK. Acrylic resin-fiber composite, Part II: The effect of polymerization shrinkage of polymethyl methacrylate applied to fiber roving on transverse strength. J Prosthet Dent 1994 Jun;71(6):613-7.
6- Vallittu PK, Lassila VP, Lappalainen R.
Wetting the repair surface with methyl metha- crylate affects the transverse strength of repaired heat-polymerized resin. J Prosthet Dent 1994 Dec;72(6):639-43.
7- Vallittu PK. Flexural properties of acrylic resin polymers reinforced with unidirectional and woven glass fibers. J Prosthet Dent 1999;81(3):318-26.
8- Ward JE, Moon PC, Levine RA, Behrendt CL.
Effect of repair surface design, repair material, and processing method on the transverse strength of repaired acrylic denture resin. J Prosthet Dent 1992 Jun;67(6):815-20.
9- Price CA. The effect of cross-linking agents on the impact resistance of a linear poly (methyl methacrylate) denture-base polymer. J Dent Res 1986 Jul;65(7):987-92.
10- Caycik S, Jagger RG. The effect of cross- linking chain length on mechanical properties of a dough-molded poly(methylmethacrylate) resin.
Dent Mater 1992 May;8(3):153-7.
11- Arima T, Murata H, Hamada T. Properties of highly cross-linked autopolymerizing reline acrylic resins. J Prosthet Dent 1995 Jan;73(1):55-9.
12- Stipho HD. Repair of acrylic resin denture base reinforced with glass fiber. J Prosthet Dent 1998 Nov;80(5):546-50.
13- Uzun G, Keyf F. The effect of woven, chopped and longitudinal glass fibers reinforcement on the transverse strength of a repair resin. J Biomater Appl 2001 Apr;15(4):351-8.
14- Uzun G, Keyf F. The effect of fiber reinforce- ment type and water storage on strength properties
of a provisional fixed partial denture resin. J Biomater Appl 2003 Apr;17(4):277-86
15- Uzun G, Hersek N. Comparison of the fracture resistance of six denture base acrylic resins. J Biomater Appl 2002 Jul;17(1):19-29
16- DeBoer J, Vermilyea SG, Brady RE. The effect of carbon fiber orientation on the fatigue resistance and bending properties of two denture resins. J Prosthet Dent 1984 Jan;51(1):119-21.
17- Dixon DL, Breeding LC. The transverse strengths of three denture base resins reinforced with polyethylene fibers. J Prosthet Dent 1992 Mar;67(3):417-9.
18- Jagger DC, Alshumailin YR, Harrison A, Rees JS. The effect of the addition of poly (methyl methacrylate) fibres on the transverse strength of repaired heat-cured acrylic resin. J Oral Rehabil 2003 Sep;30(9):903-8.
19- Jagger D, Harrison A, Jagger R, Milward P.
The effect of the addition of poly (methyl methac- rylate) fibres on some properties of high strength heat-cured acrylic resin denture base material. J Oral Rehabil 2003 Mar;30(3):231-5.
20- Revised American Dental Association specification no. 12 for denture base polymers. J Am Dent Assoc 1975 Feb;90(2):451-8.
21- Ogawa T, Hasegawa A. Effect of curing environment on mechanical properties and polymerizing behaviour of methyl-methacrylate autopolymerizing resin. J Oral Rehabil 2005 Mar;32(3):221-6.
22- Zarb G A, Bolender C L. Prosthodontics treat- ment for edentulous patients. 12th edition, St.Louis:
Mosby Co; 2004. p. 192.
23- Golbidi F, Taherian A. Comparision of water sorption and solubility of Acropars and Meliodent heat cure resins. J of Dental Medicine 2006;19(1):
55-63.
