Comparative Study of the Flexural Strength and

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Materials and Methods: Three formulae of Thai local made resins and a ... Statistical analysis was performed using the Kruskal-Wallis test followed by Dunn's.
Advanced Materials Research Vol. 746 (2013) pp 303-307 Online available since 2013/Aug/30 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.746.303

Comparative Study of the Flexural Strength and Flexural Modulus of Local Made Orthodontic Resins: A Pilot Study Tassanaporn Saen-isara1,a, Sirasa Yodmongkol2,b, Surachai Dechkunakorn3,c, Niwat Anuwongnukroh3,d, Toemsak Srikhirin2,e, Siriporn Tanodekaew5,f, Theeralaksna Suddhasthira6,g, Wassana Wicha7,h 1

Student in Orthodontics, Department of Orthodontics, Faculty of Dentistry Mahidol University, Thailand

2

Materials Science and Engineering Program, Faculty of Science, Mahidol University, Thailand 3

Department of Orthodontics, Faculty of Dentistry, Mahidol University, Thailand 4 Department of Physics, Faculty of Science, Mahidol University, Thailand 5 Biomedical Engineering Research Unit, National Metal and Materials Technology Center, Thailand 6 Department of Oral and Maxillofacial Surgery, Faculty of Dentistry, Mahidol University, Thailand 7

Research Office, Faculty of Dentistry, Mahidol University, Thailand [email protected], [email protected], [email protected], d [email protected], [email protected], [email protected], [email protected], h [email protected] Corresponding Author: Surachai Dechkunakorn a

Keywords: Flexural strength, Flexural modulus, Orthodontic resin

Abstract. Objective: The aim of this in vitro study was to compare the flexural strength and flexural modulus of three formulae self-curing acrylic resin with commercial orthodontic resin. Materials and Methods: Three formulae of Thai local made resins and a commercial self-curing acrylic resin (Orthocryl) were tested. Thai resin powder was produced by varying amounts of initiator (Asobisisobutyronitrile; AIBN); formula 1, 2, and 3. To investigate the flexural properties, test specimens (3.3 mm x 10 mm x 64 mm) were fabricated, five specimens for each group. Flexural strength and flexural modulus were measured by three-point bending test according to ISO 20795-2: 2010. Statistical analysis was performed using the Kruskal-Wallis test followed by Dunn’s multiple comparison test. Result: The Kruskal-Wallis test revealed significant (p < 0.05) differences in flexural strength and flexural modulus among some groups. Orthocryl had the highest flexural strength and formula 1 Thai local made resin had the highest flexural modulus according to Dunn’s test (p< 0.05). Conclusion: From the results of this study, the amount of initiator in Polymethylmethacrylate (PMMA) beads synthesis had no effect on flexural strength, but it was consistent with flexural modulus of Thai local made resin. However, the molecular weight of each group of Thai local made resin powder needs to be investigated to find the optimal molecular weight distribution in a subsequent study. Introduction Polymethylmethacrylate (PMMA) based polymer is a common material used to fabricate denture bases and polymeric parts of removable orthodontic appliances. Many companies in Thailand market were import self-curing orthodontic acrylic resins. In 2005, through the cooperation between the National Metal and Materials Technology Center (MTEC) and Faculty of Dentistry, Mahidol University (MU), a formula Thai self-curing orthodontic acrylic resins was first produced and the properties were tested according to ISO 1567:1999. However, the physical and mechanical properties of this resin was inferior to Orthocryl® [1]. Flexural strength is an important mechanical property of acrylic denture base materials, because flexural behavior is a guide to service performance. The flexural strength of acrylic is generally sufficient to resist fracture caused by the application of a high masticatory load. Denture fractures All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 202.28.179.5-11/12/14,04:23:54)

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Chemical, Material and Metallurgical Engineering

occur as a result of fatigue. Acrylic resin has a relatively poor resistance to fatigue fracture, which is mainly responsible for the large number of denture repairs carried out annually [2]. Moreover, the tendency for orthodontic appliances to fracture is caused by the occlusal force and the mechanical strain concentrated on the clasp sides [3]. Several factors that influence flexural properties of acrylic resins, including cross-linking agent [4], residual monomers [5], powder to liquid ratio [6], porosity [7], curing process [8], water sorption [9] and molecular weight [10]. The molecular weight distribution is one of the important factors that can be used to determine the properties of acrylic resins [11]. In general, higher molecular weight result in an increase of strength, hardness, and stiffness, which affects the resistance to creep along with increased brittleness [12]. PMMA beads with different molecular weights have an effect on the mechanical properties of polymers [13]. Increasing the amount of initiator increases the initial number of active chains but reduces their mean lengths at the end of the reaction [14]. This study is to investigate different amounts of initiator in PMMA bead synthesis affects the flexural properties of Thai local made resins. The results of this study will be used for developing Thai self-curing orthodontic resins. Materials and Methods PMMA bead synthesis The PMMA beads were synthesized by free radical polymerization. The initiator (azobisisobutyronitrile ; AIBN) (Sigma-Aldrich, Singapore) was dissolved in Methylmethacrylate; (MMA) monomer (PTT Global Chemical Public Company Limited, Thailand) and stabilizer (polyvinyl alcohol; PVA)(Sigma-Aldrich, Singapore) was dissolved in deionized (DI) water (Milipore, USA). Thai local made resins differed in the amount of initiator: 0.17% (1x; formula 1), 0.85% (5x; Formula 2), and 1.7% (10x; formula 3) of MMA monomer, while PVA 1.7 g was dissolved in DI water 400 ml. These solutions were mixed in round-bottom flasks, then was heated to 80°C and stirred at 500 rpm for 3 hours. Specimen preparation Specimens were fabricated from three Thai formulae self-curing acrylic resins and a commercial (Table 1). Acrylic resins were mixed using a powder/liquid ratio of 1.96:1 g/ml for Orthocryl and fomula1 and 0.95:1 g/ml for formula 2 and formula 3 Thai local made resin group, then poured into a rectangular-shaped mould (3.3 mm x 10mm x 64mm), five specimens in each group. Dimension of the specimens were measured by a digital calipers (Mitutoyo Model No. CD-15CW, Mitutoyo Corp., Tokyo, Japan). All surfaces were wet-grinded with metallographic grinding papers which having a grain size of 30µm (P500), 18 µm (P1000), and 15µm (P1200) until edges were smooth and flat. Specimen strips were stored in water containers at 37 + 1°C for 50 + 2 hours before flexural testing [15]. Flexural properties testing A specimen strip was taken from water storage and the flat surface was immediately laid symmetrically on the supports of the flexural test rig (Universal Testing Machine; Instron model 5566, Instron Corp., Buckinghamshire, UK) immersed in the water bath at 37 + 1°C then the force was increased on the loading plunger from zero, uniformly, using a constant displacement rate of 5 + 1 mm/min until specimens broke. The ultimate flexural strength (σ) was calculated using the formula below: 3Fl σ= 2bh 2 Where F is the maximum applied load (N) exerted on the specimen, l is the distance (mm) between the supports, b is the width (mm) of the specimen and h is the thickness (mm) of the specimen. The flexural modulus (E) was calculated using the formula below: E =

F1 l 3 4 bh 3 d

Where F1 is the load (N) at the point in the straight line portion (with the maximum slope) of the load/deflective curve, d is the deflection (mm) at load F1 [15].

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Statistical analysis The data were analyzed by one-way analysis of variance (ANOVA) using the Kruskal-Wallis test followed by Dunn’s multiple comparison post hoc test with statistical software (SPSS version 13) at a significance level of p< 0.05. Table 1. Material tested Types of resin

Manufacturer

Orthocryl Formula1 (1x) Formula 2 (5x) Formula 3 (10x)

Dentaurum, Germany Thai Thai Thai

Powder(P):Liquid(L)ratio (g.ml-1) 1.96/1 1.96/1 0.95/1 0.95/1

Results The mean and standard deviation of flexural strength and flexural modulus are shown in Table 2. The Kruskal-Wallis test revealed significant differences in flexural strength and flexural modulus among some groups (p