Effect of Diameter of Glass Fibers on Flexural Properties of Fiber-reinforced Composites

Transcription

1 Dental Materials Journal 2008; 27(4): Original Paper Effect of Diameter of Glass Fibers on Flexural Properties of Fiber-reinforced Composites Motofumi OBUKURO, Yutaka TAKAHASHI and Hiroshi SHIMIZU Division of Removable Prosthodontics, Fukuoka Dental College, Tamura, Sawara-ku, Fukuoka , Japan Corresponding author, Yutaka TAKAHASHI; This study investigated the effect of the diameter of glass fibers on the flexural properties of fiber-reinforced composites. Bar-shaped test specimens of highly filled fiber-reinforced composites (FRCs) and FRC of 30 vol% fiber content were made from a light-cured dimethacrylate monomer liquid (mixture of urethane dimethacrylate and triethylene glycol dimethacrylate) with silanized E-glass fibers (7, 10, 13, 16, 20, 25, 30, and 45 μm in diameter). Flexural strength and elastic modulus were measured. The flexural strength of the highly filled FRCs increased with increasing fiber diameter. In particular, the strengths of highly filled FRCs with 20-, 25-, 30-, and 45-μm-diameter fibers was significantly higher than the others (p<0.05). The flexural strength of FRC of 30 vol% fiber content increased with increasing fiber diameter, except for the FRC with 45-μm-diameter fibers; FRCs with 20-, 25-, and 30-μm-diameter fibers were significantly stronger than the others (p<0.05). Therefore, it was revealed that the diameter of glass fibers significantly affected the flexural properties of fiber-reinforced composites. Key words: Fiber-reinforced composites, Flexural properties, Glass fiber diameter Received Jun 22, 2007: Accepted Jan 28, 2008 INTRODUCTION Continuous glass fibers have been investigated with a view to applying them to the reinforcement of denture base resins 1-12) and repair of resin dentures 13-16). Recently, fiber-reinforced composite (FRC) materials consisting of glass fibers embedded in a methacrylate resin have been used in dentistry for the frameworks of fixed partial dentures 17-23) and implant-supported overdentures 24), and as post materials 25,26). Their clinical success depends on the strength of the FRCs, which in turn is affected by a combination of factors. The rigidity and strength of such appliances are contingent upon the polymer matrix of the FRCs and the type of fiber reinforcement 27). The reinforcing capacity of the fibers, on the other hand, relies on the orientation of the fibers, the adhesion of fibers to the resin, and the impregnation of the fibers in the resin 28). Furthermore, FRCs derive their strength from the inherent superior mechanical properties of the glass fibers 29). In the fiber glass industry, glass fiber rovings are made by gathering a number of continuous-filament strands and winding them to form cylindrical packages 30). The glass fiber strands are bundled parallel without being twisted 31). Unidirectional glass fibers are fiber rovings or yarns consisting of 1,000 to 200,000 single glass fibers 32). E-glass (electrical glass) fiber rovings are used in most general-purpose fiber-reinforced plastic (FRP) materials 33). Filaments of continuous glass fibers range from 3 to 25 μm in diameter 31), and E-glass fibers range from 5 to 15 μm in diameter 30). A roving consists of strands made of μm-diameter filaments 31). Rovings for chopping are made from 10 μm filaments, and all other rovings are made from filaments that are 13 μm or coarser 31). In the dental field, E-glass fibers, S-glass (high strength glass) fibers and R-glass fibers are used for commercially available FRCs 29,34,35). Glass fibers in commercial FRCs are available in diameters of 11 μm 36), 12 μm 9), 15 μm 37), or μm 38) the range of diameters similar to those of the glass fibers in industrial FRPs. It is possible that glass fibers of a certain diameter will render FRPs with optimal flexural properties for dental use. The mechanical properties of commercially available FRCs have been studied 9,29,35,36,39,40), but little is known about the relationship between the diameter of glass fibers and the flexural properties of FRCs. As the flexural properties of FRCs increase with an increase in their fiber content percentage 27), it is believed that the strength of FRCs with higher fiber content will be higher. At this juncture, two related considerations must be taken into account for a holistic investigation: (1) the fiber content of FRCs may be influenced by the diameter of glass fibers; and (2) the fiber diameter may also influence the strength of FRCs with identical fiber content. It was hypothesized in this study that the diameter of glass fibers affects the flexural properties and fiber content of the FRCs, but does not affect the flexural properties of FRCs with identical fiber content. The purpose of this study was to investigate

2 542 Dent Mater J 2008; 27(4): the effect of the diameter of glass fibers on the flexural properties of FRCs. MATERIALS AND METHODS Materials used Silanized E-glass fibers (7, 10, 13, 16, 20, 25, 30, and 45 μm in diameter; Asahi Fiber Glass Co. Ltd., Tokyo, Japan) and a dimethacrylate monomer liquid (mixture of urethane dimethacrylate and triethylene glycol dimethacrylate) were selected as materials to be used in this study. Urethane dimethacrylate (Lot No. SH-500B, Negami Chemical Industrial Co. Ltd., Ishikawa, Japan) and triethylene glycol dimethacrylate (NK-Ester, Lot No. 0604R, Shin-Nakamura Chemical Co. Ltd., Wakayama, Japan) were mixed at a ratio of 1:1 by weight. As a light initiator, camphorquinone (Lot No. C0014, Tokyo Kasei Co. Ltd., Tokyo, Japan) and 2-dimethylaminoethyl methacrylate (Lot No. M0082, Tokyo Kasei Co. Ltd., Tokyo, Japan) were mixed at a ratio of 1:2 by weight and added to the monomer liquid at 0.7 wt%. FRC specimen preparation The experimental protocol was adapted from ISO 10477:1992(E) 41) for polymer-based crown and bridge materials. Each glass fiber in the bundle was impregnated with the dimethacrylate monomer liquid, and then the impregnated glass fibers were packed in a stainless steel mold (2.0 mm 2.0 mm 25.0 mm) at as high density as possible by hand. Bar-shaped specimens of each glass fiber diameter were initially polymerized in the mold under a glass cover for one minute with a light curing unit (Visio Alfa, 3M ESPE, Seefeld, Germany). After removal from the mold, the specimens were finally polymerized with a light curing unit (UniXS II, Heraeus Kulzer, Wehrheim, Germany) for three minutes. Dimensional accuracy of the specimens was verified with a micrometer at three locations within a 0.05 mm tolerance for each dimension. Ten specimens of each group (Table 1, A groups) were fabricated for the three-point flexural testing and stored in air for 24 hours. Three-point bending flexural test The ultimate flexural strength (MPa) and elastic modulus (GPa) of the specimens were measured. Each specimen was placed on a 20-mm-long support for three-point flexural testing. A vertical load was applied at the mid-point of the specimen at a crosshead speed of 1 mm/min on a load testing machine (AGS-J, Shimadzu Co., Kyoto, Japan). The flexural strength (FS) (MPa) of the specimens was calculated using the following formula: FS = 3PL/2bd 2 Table 1 Group Classification groups of the specimens where P = maximum load, L = span distance (20 mm), b = width of the specimen, and d = thickness of the specimen. The load P was determined from each load-deflection graph. Elastic modulus (E) in GPa was calculated according to the following formula: E = FL 3 /4bd 3 D Diameter of glass fiber (μm) where F is the load (N) at a convenient point in the straight line portion of the load-deflection graph, and D is the deflection (mm) at load F. All tests were performed under uniform atmospheric conditions of 23.0±1ºC and 50±1% relative humidity. Fiber content determination The fiber content (vol%) of each glass fiber diameter of the FRCs was determined with an ashing method 9,27,29,34,39). Five specimens of each FRC were desiccated for 36 hours at 37 C and weighed to an accuracy of 1 mg. The specimens were then ashed for 45 minutes at 700 C. Each specimen was weighed before and after ashing on an electronic scale (A 120 S, Sartorius GmbH, Goettingen, Germany). Fiber content was calculated according to the following formula: V g = W g/ρ g (W g/ρ g + W r/ρ r) 100 Glass fiber content of FRC A-7 7 hightly filled A hightly filled A hightly filled A hightly filled A hightly filled A hightly filled A hightly filled A hightly filled B vol% B vol% B vol% B vol% B vol% B vol% B vol% B vol%

3 Dent Mater J 2008; 27(4): where V g = volume percentage (vol%) of fiber, W g = weight percentage of fiber, ρ g = density of fiber ( 2.55 g/cm 3 ), W r = weight percentage of matrix resin, and ρ r = density of matrix resin (1.227 g/cm 3 ). For comparison, FRCs with 30 vol% fiber content of each glass fiber diameter were investigated. The weight percentages of the fibers and matrix resins were calculated. Bar-shaped specimens of 30 vol% fiber content were fabricated in the same way as the bar-shaped FRC specimens, and their flexural strength and elastic modulus were measured (Table 1, B groups). SEM analysis Representative specimens of the highly filled FRCs and the FRCs of 30 vol% fiber content were embedded in resin and prepared for scanning electron microscopy analysis (SEM; JSM-6330F, JEOL, Tokyo, Japan). Micrographs of the cross-sections of the highly filled FRC specimens were taken at 40 and 500 magnification to study the interfacial quality between the fibers and their matrix resins. Micrographs of the cross-sections of FRC specimens with 30 vol% fiber content were also taken at 40 magnification. Fractured, highly filled FRC specimens were viewed visually. SEM micrographs of the lateral view of representative fractured specimens were taken at 200 magnification and examined for mode of fracture. Statistical analysis Data were analyzed statistically using one-way analysis of variance (ANOVA) (STATISTICA, StatSoft Inc., Tulsa, OK, USA) to examine the effects due to glass fiber diameter. Newman-Keuls post hoc comparison (STATISTICA, StatSoft Inc., Tulsa, OK, USA) was applied when appropriate at 95% confidence level. RESULTS One-way ANOVA revealed significant differences (p<0.05) in flexural strength attributed to the diameter of glass fibers in the highly filled FRCs (Table 2). The flexural strengths of A-20, A-25, A-30, and A-45 were significantly higher than the others (p<0.05). Post hoc analysis showed that flexural strength increased with the increasing fiber diameter Table 2 Flexural strength, elastic modulus and fiber content values of the highly filled FRCs (A groups) Group Flexural strength (MPa) Elastic modulus (GPa) Fiber content (vol%) A (52) a * 16.0 (2.4) 34.2 ( 4.9) a A (40) a, b 20.0 (3.0) a 41.9 ( 1.8) a, b A (33) b 21.9 (1.5) a, b 43.8 ( 2.8) b A (54) 25.0 (1.9) c, d 53.9 ( 6.4) c A (63) c 23.6 (2.9) b, c 54.6 ( 8.8) c A (40) c 28.7 (2.0) e 59.0 ( 2.1) c A (33) c 26.9 (2.5) d, e 59.2 ( 5.5) c A (70) c 29.1 (5.1) e 63.2 (10.9) c * The same alphabet denotes no significant differences (p>0.05) Table 3 Flexural strength and elastic modulus values of FRCs with 30 vol% fiber content (B groups) Group Flexural strength (MPa) Elastic modulus (GPa) B (42) a * 14.8 (1.4) a B (70) b 13.5 (2.1) a B (53) b 13.0 (2.5) a B (61) b 13.1 (2.1) a B (30) c 14.2 (2.1) a B (37) c 15.0 (2.5) a B (45) c 14.4 (1.2) a B (29) a 14.8 (1.3) a * The same alphabet denotes no significant differences (p>0.05)

4 544 Dent Mater J 2008; 27(4): Fig. 1 Scanning electron micrographs (a-h: 40 magnification; i-p: 500 magnification) of the cross-sections of highly filled FRCs: (a) A-7; (b) A-10; (c) A-13; (d) A-16; (e) A-20; (f) A-25; (g) A-30; (h) A-45; (i) A-7; (j) A-10; (k) A-13; (l) A-16; (m) A-20; (n) A-25; (o) A-30; (p) A-45.

5 Dent Mater J 2008; 27(4): Fig. 2 Scanning electron micrographs ( 40 magnification) of the cross-sections of FRCs with 30 vol% fiber content: (a) B-7; (b) B-10; (c) B-13; (d) B-16; (e) B-20; (f) B-25; (g) B-30; (h) B-45. Fig. 3 Scanning electron micrographs ( 200 magnification) of the lateral view of fractured, highly filled FRC specimens: (a) A-7; (b) A-10; (c) A-13; (d) A-16; (e) A-20; (f) A-25; (g) A-30; (h) A-45. The bottom part of each specimen shows the tensile side of three-point flexural testing.

6 546 Dent Mater J 2008; 27(4): of the FRCs. Significant differences (p<0.05) in elastic modulus attributed to the diameter of glass fibers were also found for the FRCs. Post hoc analysis showed that the elastic modulus generally increased with increasing fiber diameter. There were significant differences (p<0.05) in the fiber content attributed to the diameter of some of the glass fibers. The fiber contents of A-16, A-20, A-25, A-30, and A- 45 were significantly higher than the others (p<0.05). Post hoc analysis showed that the fiber content of the FRCs increased with increasing fiber diameter. Significant differences (p<0.05) in flexural strength attributed to the diameter of glass fibers were found for the FRCs with 30 vol% fiber content (Table 3). The flexural strengths of FRCs with 20-, 25-, and 30-μm-diameter fibers were significantly higher than the others (p<0.05). Post hoc analysis showed that flexural strength increased with increasing fiber diameter of the FRCs with 30 vol% fiber content, except for the one with 45-μm-diameter fibers. There were no significant differences (p>0.05) in elastic modulus attributed to the glass fiber diameter of FRCs with 30 vol% fiber content. SEM micrographs of representative highly filled FRC specimens at 40 and 500 magnification showed the distribution of the glass fibers in the resin matrix. Impregnation of glass fibers within the resin matrix was generally good, and that the presence of voids was not observed (Fig. 1). SEM micrographs of representative FRC specimens with 30 vol% fiber content showed that the fibers were not evenly distributed in the resin matrix (Fig. 2). On failure mode analysis, SEM micrographs of the fractured, highly filled FRC specimens revealed similar modes of failure (Fig. 3). Fiber fracture was the predominant mode of failure in all the specimens. Fibers remained adhered to the matrix resin after fracture, indicating good fiber-matrix bonding. Matrix resin fracture appeared to be localized where the fibers fractured. DISCUSSION The flexural strength of FRCs increased with increasing fiber diameter. Flexural strength values of 20-, 25-, 30-, and 45-μm-diameter fibers (ranging from 664 MPa to 700 MPa) were significantly higher than the others (p<0.05). Similarly, the elastic modulus increased with increasing fiber diameter, ranging from 23.6 GPa to 29.1 GPa for the FRCs containing fibers larger than 20 μm in diameter. To date, numerous studies have been undertaken to examine the flexural properties of commercially available FRCs. In a study on the flexural properties of commercially available light-cured FRCs 29), the flexural strengths of FibreKor (Preimpregnated S- glass FRC, Pentron Corporation, Wallingford, CT, USA) and Stick (Impregnated E-glass FRC, Stick Tech, Turku, Finland) ranged from 367 to 405 MPa and from 430 to 460 MPa respectively; the elastic moduli of FibreKor and Stick were 23.8 GPa and 28.0 GPa respectively. Another study 36) showed that the flexural strength and elastic modulus of FibreKor were 567 MPa and 26.5 GPa respectively. In a study that examined the flexural strengths of six commercially available FRCs, the values ranged from 132 MPa to 764 MPa whereby the highest was that of everstick (Preimpregnated E-glass FRC, Stick-Tech, Turku, Finland) 35). The flexural strength values varied among different studies because the fabrication methods were different namely the polymerization and storage conditions. On this ground, we could not compare our results with those found in published literature. Nonetheless, the flexural strengths of the FRCs in the present study were on par with the highest values found for the commercially available FRCs, and the elastic modulus values were at an identical level too. The fiber content of the FRCs in this study increased with increasing fiber diameter. Scanning electron microscopic views of specimens crosssections with different fiber diameters showed good impregnation of the fibers within the resin matrix, and high filling of fiber in FRC. Fiber contents of FRCs with glass fiber diameter greater than 16 μm were significantly higher than the others (p<0.05), ranging from 53.9 to 63.2 vol%. On comparison with commercially available FRCs, two recent studies showed that the fiber contents of Vectris (Preimpregnated E-glass FRC, Ivoclar Vivadent, Schaan, Liechtenstein), FibreKor, and Stick were 46.5 vol%, 45.2 vol%, and 51.8 vol% respectively 29,34). In another study 35), the fiber contents of Stick, everstick, Vectris, and FibreKor were 48 vol%, 48 vol%, 53 vol%, and 38 vol% respectively. Evidently, there were differences in fiber content value between these studies. In the present study, FRCs with fibers larger than 16 μm in diameter yielded higher fiber content values than the commercially available FRCs. At this juncture, it behoves to put into perspective that with barshaped specimens, the number of glass fiber filaments in the FRCs gradually decreased with an increase in fiber diameter. Consequently, thicker fibers were more easily packed into a mold. Furthermore, it was thought that the impregnation of fibers with matrix resin became increasingly difficult with fibers of smaller diameters. This was because the total surface area of the fibers increased with an increase in the number of fibers. Based on the results obtained in this study, it was clearly revealed that FRCs with significantly higher fiber contents also exhibited higher flexural

7 Dent Mater J 2008; 27(4): strength. The fiber content of each FRC group with different diameters was determined by ashing, and that the content values were significantly different. Therefore, it was suggested that the flexural strength of FRCs depended on both the fiber diameter and content. In a previous study, it was found that the flexural properties of FRCs increased with an increase in fiber content percentage 27). Likewise in this study, the fiber content significantly affected the flexural strength of the FRCs. However, it remained to be confirmed if fiber diameter also made a difference in flexural strength among the groups. It was thus proposed that as the fiber content of FRCs increased with increasing fiber diameter, the flexural strength correspondingly increased with an increase in fiber diameter. To investigate this proposal, identical-fiber-content FRCs with different diameters were used as controls. It was reported that the mechanical properties of fiber composites depended on the direction of fibers in the polymer matrix 9). In the present study, the fiber-matrix ratio of cross-sections of identical-fibercontent specimens fabricated with unidirectional glass fibers of various diameters was considered to be uniform among the specimens. During the threepoint bending test, stress was exerted perpendicular to the direction of the fibers. Hence, it was hypothesized that there would be no significant differences in the flexural strength of FRCs with 30 vol% glass fiber content though fabricated with various fiber diameters. However, the flexural strengths of FRCs of 20, 25, and 30 μm diameter were significantly higher than the others. In other words, the fiber diameter significantly affected the flexural strength of FRCs of 30 vol% glass fiber content. Moreover, amongst the FRCs of 30 vol% glass fiber content, FRCs with significantly higher flexural strength were almost identical to those of the highly filled FRCs, except for 45 μm. Therefore, under the conditions of the present experiment, it was concluded that the diameter of glass fibers significantly affected the flexural strength and elastic modulus of FRCs. Apart from fiber content and diameter, the inherent mechanical properties of glass fibers at each diameter also seemed to influence flexural strength. As mentioned earlier, glass fibers with diameters ranging from 10 to 17 μm are generally used for FRP materials. With commercially available FRCs, glass fiber diameters generally range from 11 to 17 μm. Thus, it seems reasonable that commercially available FRCs could be used as FRP materials. However, FRCs for dental use are different from industrial FRPs, which are employed for purposes such as bathtubs and train upholstery. Conversely, prostheses fabricated with FRCs are smaller and finer. Hence, there should be a certain diameter range of glass fibers that is appropriate for FRCs. This study indicated that better flexural properties were found for the μm diameter glass fibers in the unidirectional glass fiber-reinforced composite (FRC) materials. REFERENCES 1) Goldberg AJ, Burstone CJ. The use of continuous fiber reinforcement in dentistry. Dent Mater 1992; 8: ) Vallittu PK, Lassila VP. Reinforcement of acrylic resin denture base material with metal or fiber strengtheners. J Oral Rehabil 1992; 19: ) Vallittu PK, Lassila VP, Lappalainen R. Transverse strength and fatigue of denture acrylic-glass fiber composite. Dent Mater 1994; 10: ) Vallittu PK, Lassila VP, Lappalainen R. Acrylic resin-fiber composite Part I: The effect of fiber concentration on fracture resistance. J Prosthet Dent 1994; 71: ) 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; 71: ) Vallittu PK. The effect of void space and polymerization time on transverse strength of acrylic-glass fiber composite. J Oral Rehabil 1995; 22: ) Vallittu PK, Vojtkova H, Lassila VP. Impact strength of denture polymethyl methacrylate reinforced with continuous glass fibers or metal wire. Acta Odontol Scand 1995; 53: ) Vallittu PK, Narva K. Impact strength of a modified continuous glass fiber poly(methyl methacrylate). Int J Prosthodont 1997; 10: ) Vallittu PK. Flexural properties of acrylic resin polymers reinforced with unidirectional and woven glass fibers. J Prosthet Dent 1999; 81: ) 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: ) Karacaer O, Polat TN, Tezvergil A, Lassila LV, Vallittu PK. The effect of length and concentration of glass fibers on the mechanical properties of an injection- and a compression-molded denture base polymer. J Prosthet Dent 2003; 90: ) Narva KK, Lassila LV, Vallittu PK. The static strength and modulus of fiber reinforced denture base polymer. Dent Mater 2005; 21: ) Polyzois GL, Andreopoulos AG, Lagouvardos PE. Acrylic resin denture repair with adhesive resin and metal wires: effects on strength parameters. J Prosthet Dent 1996; 75: ) Vallittu PK. Glass fiber reinforcement in repaired acrylic resin removable dentures: preliminary results of a clinical study. Quintessence Int 1997; 28: ) Narva KK, Vallittu PK, Helenius H, Yli-Urpo A. Clinical survey of acrylic resin removable denture repairs with glass-fiber reinforcement. Int J Prosthodont 2001; 14:

8 548 Dent Mater J 2008; 27(4): ) Polyzois GL, Tarantili PA, Frangou MJ, Andreopoulos AG. Fracture force, deflection at fracture, and toughness of repaired denture resin subjected to microwave polymerization or reinforced with wire or glass fiber. J Prosthet Dent 2001; 86: ) Freilich MA, Karmaker AC, Burstone CJ, Goldberg AJ. Development and clinical applications of a lightpolymerized fiber-reinforced composite. J Prosthet Dent 1998; 80: ) Vallittu PK. Prosthodontic treatment with a glass fiber-reinforced resin-bonded fixed partial denture: A clinical report. J Prosthet Dent 1999; 82: ) Gohring TN, Mormann WH, Lutz F. Clinical and scanning electron microscopic evaluation of fiberreinforced inlay fixed partial dentures: preliminary results after one year. J Prosthet Dent 1999; 82: ) Vallittu PK, Sevelius C. Resin-bonded, glass fiberreinforced composite fixed partial dentures: a clinical study. J Prosthet Dent 2000; 84: ) Freilich MA, Duncan JP, Alarcon EK, Eckrote KA, Goldberg AJ. The design and fabrication of fiberreinforced implant prostheses. J Prosthet Dent 2002; 88: ) Freilich MA, Meiers JC, Duncan JP, Eckrote KA, Goldberg AJ. Clinical evaluation of fiber-reinforced fixed bridges. J Am Dent Assoc 2002; 133: ) Monaco C, Ferrari M, Miceli GP, Scotti R. Clinical evaluation of fiber-reinforced composite inlay FPDs. Int J Prosthodont 2003; 16: ) Duncan JP, Freilich MA, Latvis CJ. Fiber-reinforced composite framework for implant-supported overdentures. J Prosthet Dent 2000; 84: ) Monticelli F, Grandini S, Goracci C, Ferrari M. Clinical behavior of translucent-fiber posts: a 2-year prospective study. Int J Prosthodont 2003; 16: ) Naumann M, Blankenstein F, Dietrich T. Survival of glass fiber reinforced composite post restorations after 2 years an observational clinical study. J Dent 2005; 33: ) Lassila LV, Nohrstrom T, Vallittu PK. The influence of short-term water storage on the flexural properties of unidirectional glass fiber-reinforced composites. Biomaterials 2002; 23: ) Vallittu PK. The effect of glass fiber reinforcement on the fracture resistance of a provisional fixed partial denture. J Prosthet Dent 1998; 79: ) Chai J, Takahashi Y, Hisama K, Shimizu H. Effect of water storage on the flexural properties of three glass fiber-reinforced composites. Int J Prosthodont 2005; 18: ) Rosato DV. Filament winding: its development, manufacture, applications, and design, Interscience Publishers, New York, 1964, pp.51,67. 31) Loewenstein KL. The manufacturing technology of continuous glass fibers, 3rd ed, Elsevier Scientific Publishing Company, Amsterdam, 1993, pp.29, ) Vallittu PK. Compositional and weave pattern analyses of glass fibers in dental polymer fiber composites. J Prosthodont 1998; 7: ) Mohr JG, Powe WP. Fiber glass, Van Nostrand Reinhold Company, New York, 1978, pp ) Chai J, Takahashi Y, Hisama K, Shimizu H. Water sorption and dimensional stability of three glass fiber-reinforced composites. Int J Prosthodont 2004; 17: ) Alander P, Lassila LV, Tezvergil A, Vallittu PK. Acoustic emission analysis of fiber-reinforced composite in flexural testing. Dent Mater 2004; 20: ) Nakamura T, Waki T, Kinuta S, Tanaka H. Strength and elastic modulus of fiber-reinforced composites used for fabricating FPDs. Int J Prosthodont 2003; 16: ) Le Bell AM, Tanner J, Lassila LV, Kangasniemi I, Vallittu PK. Depth of light-initiated polymerization of glass fiber-reinforced composite in a simulated root canal. Int J Prosthodont 2003; 16: ) Narva KK, Lassila LV, Vallittu PK. Fatigue resistance and stiffness of glass fiber-reinforced urethane dimethacrylate composite. J Prosthet Dent 2004; 91: ) Lastumaki TM, Lassila LV, Vallittu PK. Flexural properties of the bulk fiber-reinforced composite DC- Tell used in fixed partial dentures. Int J Prosthodont 2001; 14: ) Takahashi Y, Chai J, Tan SC. Effect of water storage on the impact strength of three glass fiberreinforced composites. Dent Mater 2006; 22: ) International Standard: ISO for Dentistry Polymer-based crown and bridge materials. Switzerland, Genève: International Organization for the Testing of Materials. 1992(E).