Flexural Properties of Poly(Methyl Methacrylate) Resin Reinforced with Oil Palm Empty Fruit Bunch Fibers: A Preliminary Finding

Transcription

1 Flexural Properties of Poly(Methyl Methacrylate) Resin Reinforced with Oil Palm Empty Fruit Bunch Fibers: A Preliminary Finding Jacob John, BDS, MDS, 1 Shani Ann Mani, BDS, MDS, MFDS RCPS (Glasg), 2 Kalpana Palaniswamy, BDS, 3 Anand Ramanathan, BDS, MDS, 4 & Abdul Aziz Abdul Razak, BDS, MSc, PhD 5 1 Department of Diagnostic and Integrated Dental Practice, Faculty of Dentistry, University of Malaya, Malaysia 2 Department of Children s Dentistry and Orthodontics, Faculty of Dentistry, University of Malaya, Malaysia 3 Primary Care Unit, Faculty of Dentistry, University of Malaya, Malaysia 4 Department of Oral Pathology, Oral Medicine and Periodontology, Faculty of Dentistry, University of Malaya, Malaysia 5 Department of Conservative Dentistry, Faculty of Dentistry, University of Malaya, Malaysia Keywords PMMA; oil palm empty fruit bunch fibers; natural fibers; flexural strength; flexural modulus. Correspondence Jacob John, Department of Diagnostic and Integrated Dental Practice, Faculty of Dentistry, University of Malaya, Kuala Lumpur, Malaysia. jacob_john69@yahoo.com This research was supported by the University of Malaya Research Grant UMRG (Grant no. RG279-10HTM) under the Biomaterials Technology Research Group (BTRG). The authors deny any conflicts of interest. Accepted January 28, 2014 Abstract Purpose: The purpose of this preliminary study was to evaluate the flexural properties of poly(methyl methacrylate) (PMMA) reinforced with oil palm empty fruit bunch (OPEFB) fiber. Materials and Methods: The flexural strength and flexural modulus of three OPEFB fiber-reinforced PMMA were compared with a conventional and a commercially available reinforced PMMA. The three test groups included OPEFB fibers of 0.5 mm thickness, 2.0 mm thickness, and OPEFB cellulose. Results: All test group specimens demonstrated improved flexural strength and flexural modulus over conventional PMMA. Reinforcement with OPEFB cellulose showed the highest mean flexural strength and flexural modulus, which were statistically significant when compared to the conventional and commercially reinforced PMMA used in this study. OPEFB fiber in the form of cellulose and 0.5 mm thickness fiber significantly improved flexural strength and flexural modulus of conventional PMMA resin. Further investigation on the properties of PMMA reinforced with OPEFB cellulose is warranted. Conclusions: Natural OPEFB fibers, especially OPEFB in cellulose form, can be considered a viable alternative to existing commercially available synthetic fiber reinforced PMMA resin. doi: /jopr Poly(methyl methacrylate) (PMMA) is one of the most widely used denture base materials since its introduction in 1937 by Dr. Walter Wright. 1 Due to its favorable working characteristics, processing ease, accurate fit, stability in the oral environment, superior esthetics, and cost effectiveness, PMMA replaced vulcanite. 2 However, it has far-from-ideal mechanical properties due to its unsatisfactory transverse strength, impact strength, and fatigue resistance, 3,4 which has been well documented in clinical reports demonstrating midline fracture of maxillary complete dentures as a common problem. 5,6 Hence there is a need for improvement in PMMA fracture resistance. This has led to three routes of investigation 3 to improve its fracture resistance, namely the search for, or development of an alternative material to PMMA, the chemical modification of PMMA, and the reinforcement of PMMA with other materials. Denture fracture may be due to the mechanical properties of the acrylic resin or a multiplicity of other factors. Alternative polymers such as polystyrene, polyvinyl acrylic, polyamides, polycarbonates, and light-activated urethane dimethacrylate resin have been developed, but these materials failed to produce dentures of greater accuracy or better performance than PMMA. 7 Attempts to improve the properties of PMMA by adding rubber graft copolymer to produce a high impact resin 8,9 have increased impact strength, but at the expense of Young s modulus, resulting in reduced transverse strength. Moreover, these materials are up to 20 times more expensive than conventional resins. 3 Lately, the reinforcement of PMMA with other materials such as carbon fibers, 10 aramid fibers, 11 glass fibers, 12 stainless steel mesh, 13 or ultra-high modulus polyethylene fibers 14 has been attempted. Journal of Prosthodontics 00 (2014) 1 6 C 2014 by the American College of Prosthodontists 1

2 Flexural Properties of OPEFB Fiber Reinforced PMMA John et al Table 1 Materials used in the investigation Group A B C D E Description Conventional PMMA resin (Acron Express, Lot No ) High-impact PMMA resin (Acron HI, Lot No ) Conventional PMMA resin (A) reinforced with OPEFB fibers (0.5 mm thick) Conventional PMMA resin (A) reinforced with OPEFB fibers (2.0 mm thick) Conventional PMMA resin (A) reinforced with OPEFB cellulose Worldwide environmental concerns such as climate change and the need for sustainability are encouraging the development of green materials. Natural fibers (NF) that originate from plants, crops, animals, agro waste, or other sources that are nonhazardous, noncarcinogenic, renewable, and biodegradable after their end use 15 are garnering interest across a wide range of polymer-composite materials. Biocomposites, made by combining NF and petroleum-derived polymers, are likely to deliver an eco-social system change and are termed green composites. 16 A major goal of NF composites is to alleviate the need to use expensive artificial fiber, which has a relatively higher density and is dependent on nonrenewable sources. 17 Few studies have reported on the application of cheaper and sustainable NFs as denture reinforcement material, but none have reported on their mechanical properties compared to existing PMMA. Oil palm (Elaeis guineensis), mainly cultivated in tropical countries like Indonesia and Malaysia, is the highest yielding edible oil crop in the world. Oil palm empty fruit bunch (OPEFB) is the hard and tough, lignocellulosic fibrous mass left behind from the processed fresh fruit bunch (FFB) or the fleshy mesocarp of the fruit to produce the crude palm oil. 21 The chemical composition and physico-mechanical properties of OPEFB as reported by various researchers are summarized by Shinoj et al. 21 The purpose of this study is to explore the feasibility of using OPEFB fibers with conventional acrylic resin (PMMA) by comparing the flexural properties of the conventional acrylic resin and high impact acrylic resin with conventional acrylic resin reinforced with OPEFB fibers. The null hypothesis tested was that OPEFB fibers would not influence the flexural properties of PMMA. Materials and methods Materials The flexural properties of five types of PMMA resin (Table 1) were tested in this in vitro study. The control group (group A) was made of conventional heat-polymerized acrylic resin (Acron Express; Kemdent, Wiltshire, UK) whereas Group B was made of high-impact acrylic resin (Acron HI; Kemdent). The test specimens for groups C, D, and E were made with conventional heat-polymerized acrylic resin (Acron Express) reinforced with OPEFB fibers of 0.5 mm thickness, 2.0 mm thickness, and OPEFB cellulose, respectively. Specimen preparation Fifty specimens (65 long, 10 mm wide, 2.5 mm thick) were fabricated according to the ISO/DIS 1567:1997 specifications 22 ; 10 each for the five groups. Preformed metal dies of the said measurement were invested with Type III dental stone in metal dental flasks for the preparation of the specimens. More details of the fabrication process can be found elsewhere. 2 The specimens in groups A and B were mixed and packed according to the manufacturers recommendations. For groups C and D, the respective fibers, 2% by weight, were chopped into 6 mm length and soaked in monomer for 10 minutes for better bonding with the acrylic resin. 23 Similarly, 2% by weight of cellulose was also soaked in the monomer for the preparation of the Group E specimens. All the fiber specimens for this study were supplied by the Malaysian Palm Oil Board. The dough, made of MMA liquid and PMMA powder, was pressed into the mold to initially fill half the depth. The fibers and cellulose were then placed by evenly distributing them and orienting them along the long axis of the test specimen. A second layer of dough was placed to fill the upper half of the mold such that the fibers/cellulose were sandwiched between the two layers of the acrylic dough. Trial closure was performed with a hydropress (OL57; Manfredi, Nicosia, Cyprus). The flask was clamped, and low pressure was maintained for 30 minutes to allow proper penetration of monomer into the polymer, even flow of the material, and outward flow of excess material. The flask was then immersed in water in an acrylizer (Acrydig 4; Manfredi) at room temperature. The temperature was raised gradually to 73 C and maintained for 7 hours. After completion of the polymerization cycle, the flask was allowed to cool in the water bath to room temperature before deflasking. The acrylic specimens were then retrieved, finished, and polished. If the specimens revealed exposed fibers at the peripheral border after deflasking, they were trimmed using diamond abrasives to avoid delamination of the reinforcement. Specimens were labeled at each end before testing. All specimens were stored in water at room temperature before testing. Flexural strength and flexural modulus test All specimens were tested for flexural strength by three-point bending test on a universal testing machine (Autograph AGX; Shimadzu, Kyoto, Japan) at a 2 mm/min crosshead speed over a two-point support span set at 40 mm. A load was applied by a centrally located rod until fracture. The flexural strength was calculated with the following formula: FS = 3Pl 2bd 2 where FS is flexural strength expressed in MPa; P = the peak load applied at a given point on the load-deflection curve, expressedinn;l= the support span length, expressed in mm; b = the width of the specimen tested, expressed in mm; and d = the depth of the specimen tested, expressed in mm. 2 Journal of Prosthodontics 00 (2014) 1 6 C 2014 by the American College of Prosthodontists

3 John et al Flexural Properties of OPEFB Fiber Reinforced PMMA Table 2 Flexural properties of the specimen groups with and without reinforcement n Mean (SD) F statistics a (df) p-value Flexural strength (MPa) A (4.13) B (3.02) (4,45) C (2.14) D (4.70) E (2.07) Flexural modulus (GPa) A (0.16) 0.12 <0.001 B (0.19) (4,45) C (0.13) D (0.09) E (0.06) a ANOVA test. The flexural modulus was calculated with the following formula: FE = l3 Px 4bh 3 d where l = the support span length, b = the width of the test specimen, h = the thickness of the test specimen, and d = the deflection corresponding to load Px = 9:8 N. Force-deflection curves and a complete stress versus strain history for each test were obtained. Trapezium X (v1.0; Shimadzu) software was used to calculate the flexural strength and modulus of elasticity from the data curves along with the means and standard deviations for each experimental group. Scanning electron microscopy (SEM) analysis The morphology of the fractured surface of the specimens from each experimental group was examined with a scanning electron microscope (Quanta 250 FEGSEM; FEI, Brno, Czech Republic) at 100 magnification power. Statistical analysis For each experimental group, the mean flexural strength and flexural modulus values and standard error were calculated, and the data were analyzed by means of one-way ANOVA followed by post hoc comparison using Scheffe s test to detect the mean difference between pairs of the different groups using SPSS software v.12.0 at 95% confidence level. Results The mean flexural strength and flexural modulus of the tested specimens are summarized in Table 2. Overall, specimens reinforced with OPEFB had higher flexural strength and flexural modulus than the conventional specimens. The flexural strength and flexural modulus of high-impact acrylic resin (group B), though higher than the control (group A), was not statistically significant. Two of the OPEFB (E and C) groups had better flexural properties compared to the high-impact acrylic resin (group B). For flexural strength, acrylic resin reinforced with OPEFB cellulose had the highest value, while conventional Table 3 Multiple comparisons by post hoc test (Scheffe s procedure) of mean flexural strength (FS) and flexural modulus (FM) between the specimen groups A B C D FS a FM b FS FM FS FM FS FM B C D E * Significant at p < a Mean scores of group C and E are significantly different from A by post hoc test (Scheffe s procedure). b Mean scores of group C, D, and E are significantly different from A by post hoc test (Scheffe s procedure). acrylic resin had the lowest strengths. The range of flexural modulus for the OPEFB fiber-reinforced groups tested was between 5.29 and 5.40 GPa, compared to only 2.53 GPa for the conventional group. Statistical analysis (Table 3) showed that OPEFB reinforcement with cellulose and 0.5-mm-thick fibers (groups E and C) significantly increased the flexural strength of conventional acrylic resin. The flexural modulus in all OPEFB-reinforced specimens (groups C, D, E) was higher than the control specimens (groups A and B; p < 0.05). No statistically significant difference was found between the conventional (group A) and high-impact (group B) specimens. Among the OPEFBreinforced specimens, cellulose fibers (group E) processed the highest flexural strength and flexural modulus; however, there was no statistically significant difference between groups C, D, and E. A variation in the morphology of the fractured surface was visualized using SEM analysis. Figure 1 shows the surface characteristics of specimens in different compositions, representing conventional acrylic resin (A), commercially available high-impact acrylic resin (B), and conventional resin reinforced with OPEFB fibers of 0.5 mm thickness (C), 2.0 mm thickness (D), and OPEFB cellulose, respectively. The micrograph of commercially available PMMA (Figs 1A and B) showed a homogenous system, while for those reinforced with OPEFB fibers (Figs 1C E), a dispersed-phase system was observed. Discussion Denture base acrylic resins are subjected to different stresses. Repeated masticatory forces lead to fatigue phenomena intraorally, while extraorally, high-impact forces can occur if the prosthesis is dropped, resulting in fracture. Stipho 24 reported that midline fractures are a common problem in complete maxillary dentures. In the 1960s, such cyclic stress-induced fractures were thought to be caused by flexural fatigue phenomena. Alternatively, flexural strength of a material is a measure of stiffness and resistance to fracture, and it can be measured and interpreted by various tests. PMMA resin is the material of choice for fabrication of a denture base, even though fracture is known to occur during Journal of Prosthodontics 00 (2014) 1 6 C 2014 by the American College of Prosthodontists 3

4 Flexural Properties of OPEFB Fiber Reinforced PMMA John et al Figure 1 SEM micrographs (100 magnification) of (A) conventional heat-polymerized PMMA resin, (B) high-impact PMMA resin, (C) A reinforced with OPEFB fibers of 0.5 mm thickness, (D) A reinforced with OPEFB fibers of 2.0 mm thickness, and (E) A reinforced with OPEFB cellulose. function due to its poor flexural and impact properties. PMMA reinforced with artificial fibers such as glass, aramid, and nylon has been in the market for the last two decades. A few reports exist on the effect of reinforcing PMMA with NF for denture base materials, but this is the first study on the application of NF as PMMA reinforcement. In Malaysia, there is intense research on the use of OPEFB as a source of fuel for renewable energy power generation. 25 In addition, OPEFB fiber has also shown great potential in its use as a reinforcing material with polymers to develop biocomposite materials. 26 Hence, it was appropriate to study the feasibility of improving the flexural properties of PMMA reinforced with these fibers. Three types of OPEFB fibers were used in this study chopped OPEFB fibers of 0.5 mm thickness, 2.0 mm thickness, and OPEFB cellulose form, each 2% by weight. In this study, the null hypothesis that OPEFB fibers would not influence the flexural properties of PMMA was rejected. The mean flexural strength of all three experimental groups (C, D, E) showed improvement when compared to that of conventional acrylic resin (group A), with statistically significant improvement in groups C and E. A reason for the improvement could be because the modulus of elasticity of OPEFB fibers is very high, and thus they can absorb most stresses without deformation. 27 This will also result in a marked increase in the mean flexural modulus of the experimental groups when compared to the control groups. Among the three experimental groups, group E (2% by weight of OPEFB fiber in cellulose form) showed the highest mean flexural strength and flexural modulus followed by group C (2% by weight of 0.5 mm thickness OPEFB fibers), while the least improvement was noted in group D (2% by weight of 2.0 mm thickness OPEFB fibers) specimens. Therefore, the thickness of the OPEFB fibers tends to have an inverse relationship with the improvement of the flexural modulus and flexural strength. It can be assumed that thicker fibers may cause a weak point in the structure, causing failure, while the cellulose and 0.5 mm fibers are thin, thus blending well with the surrounding PMMA and reinforcing it. According to Ouajai et al 28 the acetylation reaction of cellulose, whereby the hydroxyl groups in the fiber are substituted by the acetyl groups, reduces the moisture adsorption and improves interface adhesion with the polymer matrix, thereby increasing the strength of the matrix. Interestingly, this study also found a statistically significant increase in the flexural strength of the conventional acrylic resin reinforced with OPEFB cellulose compared to the commercially available high-impact acrylic resin used in this study. Certain inherent properties of OPEFB fibers make it suitable for composite application. The transverse section of OPEFB shows a lacuna-like portion in the middle surrounded by porous tubular structures. This porous surface facilitates better mechanical interlocking with polymer matrix in composite 4 Journal of Prosthodontics 00 (2014) 1 6 C 2014 by the American College of Prosthodontists

5 John et al Flexural Properties of OPEFB Fiber Reinforced PMMA fabrication. 29 In addition, the silica bodies found on the fiber strand can be dislodged mechanically, leaving behind a perforated silica crater, which can also enhance penetration of the matrix. 21 OPEFB fibers are also reported to have high toughness value. 30 Even though NFs enjoy some specific superior properties compared to synthetic fibers, they also suffer serious problems caused by their polar nature, 31 such as poor moisture resistance, limited processing temperatures, and low dimensional stability. 32 A notable difficulty is that OPEFB fibers are chemically hydrophilic and so absorb water. 33 This can lead to poor adhesion between the polymer matrix and OPEFB fibers after aging, which can adversely affect the mechanical properties and requires further testing. Therefore, there is a need to identify effective surface modifications to reduce water sorption that can lead to dimensional changes. Although in SEM photographs there appeared to be more void space inside the acrylic OPEFB fiber composite, this did not seem to affect the flexural strength of the reinforced test specimens. Impregnated and preimpregnated fiber reinforcements reinforce denture base polymer better than nonimpregnated fiber reinforcements. 34 In this study, the OPEFB fibers were pretreated with monomer prior to inserting into the PMMA mixture. According to Chow et al, 35 this results in improved wetting of the fibers and reduction in the water sorption and dimensional changes in acrylic denture bases. Hence, this preliminary study has established a significant improvement of flexural strength and flexural modulus of conventional acrylic resin when reinforced with OPEFB fibers. Thus, natural OPEFB fibers, a representative of green composites, can be looked into as a viable alternative to the commercially available synthetic fiber reinforced PMMA resin; however, further investigation on other mechanical properties and biocompatibility testing of the OPEFB-reinforced PMMA is necessary. Conclusion This study evaluated the flexural strength and flexural modulus of conventional denture base PMMA resin, reinforced with various OPEFB fibers. Within the limitations of this study, we found an improvement in the flexural strength and flexural modulus of the various OPEFB fiber-reinforced PMMA resin. OPEFB cellulose-reinforced PMMA resin showed statistically higher mean flexural strength and flexural modulus values than the conventional and high-impact PMMA resins used in this study. Based on flexural properties and handling characteristics, OPEFB fibers in the cellulose form may prove to be a useful natural reinforcement for conventional PMMA resins. Acknowledgments The authors would like to record their deepest gratitude to the late Mrs. Rosnah Mat Som from the Engineering and Processing Division of the Malaysian Palm Oil Board for all her support in conducting the research. References 1. Peyton FA: History of resins in dentistry. Dent Clin North Am 1975;19: John J, Gangadhar SA, Shah I: Flexural strength of heat-polymerized polymethyl methacrylate denture resin reinforced with glass, aramid, or nylon fibers. J Prosthet Dent 2001;86: Jagger DC, Harrison A, Jandt KD: The reinforcement of dentures. J Oral Rehabil 1999;26: Yazdanie N, Mahood M: Carbon fiber acrylic resin composite: an investigation of transverse strength. J Prosthet Dent 1985;54: Machado C, Sanchez E, Azer SS, et al: Comparative study of the transverse strength of three denture base materials. J Dent 2007;35: Prombonas AE, Vlissidis DS: Comparison of the midline stress field in maxillary and mandibular complete dentures: a pilot study. 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6 Flexural Properties of OPEFB Fiber Reinforced PMMA John et al 22. ISO/DIS 1567 working draft 2 Dentistry denture base polymers. Geneva, Switzerland, International Organization for Standardization; Tacir IH, Kama JD, Zortuk M, et al: Flexural properties of glass fibre reinforced acrylic resin polymers. Aust Dent J 2006;51: Stipho H: Repair of acrylic resin denture base reinforced with glass fiber. J Prosthet Dent 1998;80: Menon NR, Rahman ZA, Bakar NA: Empty fruit bunches evaluation: Mulch in plantation vs. fuel for electricity generation. Oil Palm Ind Econ J 2003;3: Hassan A, Salema AA, Ani FN, et al: A review on oil palm empty fruit bunch fiber reinforced polymer composite materials. Polym Composite 2010;31: Ibrahim N, Hashim N, Rahman M, et al: Mechanical properties and morphology of oil palm empty fruit bunch polypropylene composites: effect of adding ENGAGETM J Thermoplast Compos Mater 2011;24: Ouajai S, Ruangwilairat P, Ongwongsakul K, et al: Morphology and structure of modified oil palm empty fruit bunch cellulose fibre. Adv Mater Res 2010;93: Sreekala M, Kumaran M, Thomas S: Oil palm fibers: morphology, chemical composition, surface modification, and mechanical properties. J Appl Polym Sci 1997;66: John MJ, Francis B, Varughese K, et al: Effect of chemical modification on properties of hybrid fiber biocomposites. Compos Part A S 2008;39: Ndazi B, Tesha J, Bisanda ETN: Some opportunities and challenges of producing bio-composites from non-wood residues. J Mater Sci 2006;41: Bogoeva Gaceva G, Avella M, Malinconico M, et al: Natural fiber eco composites. Polym Composite 2007;28: Raju G, Ratnam CT, Ibrahim NA, et al: Enhancement of PVC/ENR blend properties by poly (methyl acrylate) grafted oil palm empty fruit bunch fiber. J Appl Polym Sci 2008;110: Narva KK, Lassila LV, Vallittu PK: The static strength and modulus of fiber reinforced denture base polymer. Dent Mater 2005;21: Chow T, Ladizesky N, Clarke D: Acrylic resins reinforced with woven highly drawn linear polyethylene fibres. 2. Water sorption and clinical trials. Aust Dent J 1992;37: Journal of Prosthodontics 00 (2014) 1 6 C 2014 by the American College of Prosthodontists