Flexural properties and impact strength of denture base polymer reinforced with woven glass fibers

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1 dental materials Dental Materials 16 (2000) Flexural properties and impact strength of denture base polymer reinforced with woven glass fibers T. Kanie*, K. Fujii, H. Arikawa, K. Inoue Department of Biomaterials Science, Kagoshima University Dental School, Sakuragaoka, Kagoshima , Japan Received 16 February 1999; received in revised form 27 August 1999; accepted 12 October 1999 Abstract Objectives: The present investigation was undertaken to determine the reinforcing effect of woven glass fibers on deflection, flexural strength, flexural modulus and impact strength of acrylic denture base polymer. Methods: Three silanized or unsilanized woven glass fibers were used. Specimens were made by heating the denture cure resin dough containing glass fibers, which were sheathed in the dough. Specimens with four different thicknesses and of five different types were made, incorporating the glass fiber. Three-point flexural test and flywheel type impact test were employed to determine the flexural properties and impact strength. Results: When specimens contained unsilanized glass fiber, the flexural strength in specimens of 1 and 2 mm thickness and the impact strength in specimens of 2 mm thickness were higher than those of specimens without glass fiber p 0:01 : On the contrary, the flexural strength and deflection in specimens reinforced with silanized glass fiber of 1 mm thickness were significantly higher p 0:01; p 0:05 than those of unreinforced specimens. Further, the impact strength in specimens reinforced with silanized glass fiber of 2 mm thickness was significantly higher p 0:01 than that of unreinforced specimens. Statistically significant differences were found in the flexural strength p 0:05 and in the impact strength p 0:01 when specimens of 4 mm thickness were reinforced with two or three unsilanized glass fibers. Significance: The reinforcement with glass fiber was effective in thin specimens, and the reinforcing effect increased with the increase of the number of glass fibers in the case of thick specimens Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. Keywords: Denture base polymer; Acrylic; Fiber reinforced composite; Glass fiber; Flexural strength; Impact strength 1. Introduction Although polymethylmethacrylate (PMMA) has been widely used as a main component of denture base polymer for many years, this material is sometimes fractured or cracked in clinical use. One of the factors that causes fracture is considered to be low resistance to impact, flexural or fatigue. There have been many studies on the strength problem and they are summarized in the following two approaches. The first approach is to increase the strength of denture base polymer by adding a cross-linking agent of poly-functional monomer such as polyethyleneglycol dimethacrylate. The second approach is to devise a reinforcement of denture base polymer with fibers or rods such as metal wires or metal nets. According to previous * Corresponding author. Tel.: ; fax: address: einak@dentb.hal.kagoshima-u.ac.jp (T. Kanie). studies [1,2], high-strength metal increases the flexural strength and impact strength of denture base polymer slightly, and its use is limited because of the obvious effect on aesthetics. In addition, the influence of metal wires on flexural fatigue resistance is minor [3]. Recently various types of fibers such as carbon fiber, aramid fiber and ultra high molecular weight polyethylene fiber have been investigated [4 11] as reinforcing materials, and it has been shown that the fibers also increase the flexural strength and impact strength of denture base polymer. However, carbon fiber has a springy nature in handling and is less aesthetic than the other fibers. Some disadvantages of aramid fiber are poor aesthetics and difficulties in polishing. In the case of polyethylene fibers, the surface treatment to improve the adhesion between fibers and denture base polymer is complicated [12,13] and has not resulted in adequate adhesion. More recently, glass fibers or glass rods have been investigated for the reinforcement of denture base polymer [14 28]. Mechanical properties of denture /00/$ Academy of Dental Materials. Published by Elsevier Science Ltd. All rights reserved. PII: S (99)

2 Table 1 Physical properties of three woven glass fibers used (presented by manufacturer) T. Kanie et al. / Dental Materials 16 (2000) Code Trade name Texture Standard thickness (mm) Tensile strength (Longitudinal) (N/25 mm) Density (threads/25 mm) Warp Weft N YCTM13075 Plain weave (falling) D YEM2115 Plain weave (falling) S YEH1836 Plain weave base polymer with glass fibers were compared with those of denture base polymer with carbon fiber, Kevlar fiber and polyethylene fiber for reinforcement, and the polyethylene fiber group produced the highest impact test values, but there were no significant differences in transverse strength [11]. The effect of glass fiber, carbon fiber, aramid fiber and metal wire reinforcement on the fracture resistance of denture base polymer was tested, and a beneficial effect of metal reinforcement was reported [15]. Glass fibers are favorable for denture base polymer when used in lightcuring type resin because they have excellent transparency compared to the other fibers. In dental work, it has been difficult to introduce continuous long glass fibers into the dough of liquid methylmethacrylate (MMA) monomer and polymethylmethacrylate (PMMA) powder, and glass rods are limited to application on thin palatal areas of denture base polymer. Woven glass fiber has a suitable form as a reinforcing material for the so-called partial fiber reinforcement (PFR), because the woven glass fiber is a tape with various widths. Further, glass fiber is easy to cut with scissors, however, glass fibers frayed when the fiber is bent excessively. Polymer-preimpregnation eliminates this problem [28]. Improper impregnation of polymer matrix into the fiber bundle caused reduction in transverse strength of denture base polymer with glass fiber and a lack of the adsorbed monomer liquid in the fiber bundle before polymerization caused a void space inside the test specimens [21]. The polymer-preimpregnated reinforcing glass fibers may considerably enhance flexural properties of the multiphase denture base polymers, which is due to proper impregnation of fibers with polymer matrix [28]. There are some investigations [14,16,23] of the effects of silanation on adhesion of glass fibers to polymer matrix. The SEM photographs showed [16] that silanation of glass fibers enhanced the adhesion between the fibers and polymer matrix, and then the fracture resistance and transverse strength of denture base polymers with silanized glass fibers increased. The direction of glass fibers is a very important point on denture base polymer with glass fibers. Continuous unidirectional fibers gave the highest strength and stiffness, but only in the direction of the fiber [29]. Woven fibers are able to reinforce the denture base polymers in two directions. The reinforcing effect on the flexural properties of woven glass fibers oriented at angles of ^45 to the long axis of the test specimens was minor compared to the unidirectional glass fibers oriented within the long axis of test specimens [28]. Glass fiber reinforcement enhanced the impact strength of denture base polymer with glass fibers, while the use of additional fiber reinforcement in the test specimen did not have an effect on the impact strength [24]. Reinforcing denture base polymer with a low concentration (1 wt%) of loose small cut glass fibers mixed in random form enhanced the fracture resistance [25], but the increased amount of fibers in denture base polymer with glass fibers (up to 14.8 wt%) increased the tensile strength [27]. Although there are a few investigations [4,30 32] using woven carbon fiber and woven aramid fiber as reinforcing materials of denture base polymer, the effect of woven glass fiber reinforcement on flexural strength and impact strength has not been investigated with an exception of a very recent study [28]. The aim of this investigation was to clarify the effect of specimen thicknesses on flexural strength and impact strength of denture base polymer reinforced with three silanized glass fibers or unsilanized glass fibers. Further, the effect of the number and position of glass fibers incorporated into denture base polymer sheets, on flexural strength and impact strength were investigated. 2. Materials and methods A conventional heat-curing powder liquid type of denture base polymer was used in this investigation. M-4005 PMMA powder (Average molecular weight: , Average particle diameter: 0.10 mm, Solid content: 98.0%, Negami Industry Ltd., Ishikawa, Japan) and MMA liquid (Assay: 98.0%, Wako Pure Chemical Industries Ltd., Osaka, Japan) were mixed in a ratio of 2.9:1 by weight. Benzoylperoxide at 0.5 wt% (Wako Pure Chemical Industries Ltd., Osaka, Japan) was added to MMA liquid as a polymerization initiator. Three E-glass fibers (N, D and S), which contained wt% SiO 2, wt% Al 2 O 3, wt% CaO and 5 13 wt% B 2 O 3, according to

3 152 T. Kanie et al. / Dental Materials 16 (2000) Fig. 1. Condition of test specimens used in this study. manufacturer s information, were used in this study. Details of trade name, code and physical properties presented by the manufacturer (Mie Textile, Mie, Japan) are given in Table 1. g-methacryloxypropyltrimethoxysilane (g-mps) (Toray Dow Corning Silicone Ltd., Tokyo, Japan) was used as a silane-coupling agent. The glass fibers were cleaned with boiled water for 10 min After drying in air, the glass fibers were wet for 5 min with g-mps that was not diluted, dried in air for 1 h, and then placed in an oven for 6 h at 115 C. The dough of MMA liquid and PMMA powder was pressed at 2 MPa into a sheet (80 mm in length and 40 mm in width) of about half the thickness of the test specimen. The glass fiber was oriented within the long axis of the test specimen and sandwiched between the two sheets for finishing in five different ways (Fig. 1). The composite incorporating the glass fiber between the two sheets was put into a Teflon mold with a depth of 1, 2, 3 or 4 mm, and then the mold was pressed at 2 MPa to impregnate the dough into the glass fiber. The curing process was 1 h at 70 C and 1 h at 100 C under N 2 -gas pressure of 0.5 MPa. The cured composite was cut to 40 mm in length and 4 mm in width using a diamond blade, and then the test specimens were ground with wet and dry paper (#320). All test specimens were stored in a water cabinet at 37 ^ 1 C for 3 weeks. Test specimens were classified into three groups (Fig. 1): (a) C-type specimens of 1, 2, 3 and 4 mm thickness with unsilanized N, D and S glass fibers; (b) C-type specimens of 1, 2, 3 and 4 mm thickness with silanized N, D and S glass fibers; (c) C-, U-, L-, W- and T-type specimens of 4 mm thickness with unsilanized N glass fibers. A three-point flexural test was carried out with a Techno Graph testing machine (Minebea, Nagano, Japan) using a crosshead speed of 2 mm/min and span length of 30 mm. The flexural strength (F S ) and the flexural modulus (F E ) were calculated from the formula: F S ˆ 3P ml 2bh 2 F E ˆ l3 P x 4 bh 3 d where P m is the maximum load, l the span length, b the width of the test specimen, h the thickness of the test specimen, and d the deflection corresponding to load P x ˆ 9:8 N: The impact test was carried out with a flywheel type impact-testing machine (Thrive Seiko, Kagoshima, Japan) using the flywheel as the force of inertia. The edge speed of the impact test was 543 mm/min and the span was 30 mm. The impact strength (I S ) of the test specimen was calculated by the following equation: ZD f dx 0 I S ˆ ab where a is width of the test specimen, b thickness of the test specimen and D the deflection at the fracture point. The numerator R f dx of the equation corresponds to the shaded area under the curve (Fig. 2), since the impact strength corresponds to the impact energy absorbed by a specimen [33,34]. This area can be accurately read with a Fig. 2. Typical trace of force/deflection curve measured by the impact testing machine.

4 T. Kanie et al. / Dental Materials 16 (2000) Table 2 Flexural properties and impact strength of C-type specimens reinforced with three unsilanized woven glass fibers. The values in parentheses refer to standard deviation Fiber Flexural strength (MPa) Deflection (mm) Flexural modulus (MPa) Impact strength (J/m 2 ) 1 mm thickness Control 95.4 (1.2) 5.5 (0.6) (210.5) N (6.7) a 6.1 (0.2) (247.2) D (7.0) a 6.7 (0.7) (328.5) S (2.7) a 6.4 (0.3) b (140.9) 2 mm thickness Control (4.2) 4.5 (0.5) (40.8) (36.8) N (4.1) a 4.3 (0.8) (76.8) b (152.2) a D (1.6) a 4.5 (0.6) (74.8) (189.5) a S (5.1) a 4.7 (1.1) (114.8) (154.5) a 3 mm thickness Control (5.7) 3.1 (0.2) (31.9) (125.5) N (4.4) 3.0 (0.2) (18.5) (194.5) D (3.8) 4.0 (0.6) b (88.4) b (208.6) S (11.3) 4.0 (0.7) b (125.1) (187.2) 4 mm thickness Control (10.5) 2.5 (0.6) (80.0) (222.2) N (5.2) 2.2 (0.6) (25.2) (230.8) D (7.1) 2.8 (0.6) (152.6) (204.6) S (8.8) 2.5 (0.3) (200.0) (212.7) a Significant difference P 0:01 between control and specimens with N, D or S glass cloth. b Significant difference P 0:05 between control and specimens with N, D or S glass cloth. scanner connected to a computer and calculated with software. The quantity of glass fiber was determined by dissolution of the polymer matrix with tetrahydrofuran (THF). The test specimens were dried for 24 h in an air cabinet at 37 C and then weighed to an accuracy of g. After storing in THF for 24 h, the test specimens eluted by the polymer matrix were carefully picked up from THF and rinsed Table 3 Flexural properties and impact strength of C-type specimens reinforced with three silanized woven glass fibers. The values in parentheses refer to standard deviation Fiber Flexural strength (MPa) Deflection (mm) Flexural modulus (MPa) Impact strength (J/m 2 ) 1 mm thickness Control 95.4 (1.2) 5.5 (0.6) (160.5) N (2.1) a 6.8 (0.4) b (202.8) D (2.8) a 6.8 (0.3) a (247.2) S (3.2) a 7.1 (0.4) a (134.3) 2 mm thickness Control (4.2) 4.5 (0.5) (40.0) (36.8) N (13.8) b 4.0 (1.0) (180.0) (175.6) a D (8.3) a 4.5 (1.3) (130.5) (244.9) a S (21.5) 4.2 (0.9) (63.2) (319.5) a 3 mm thickness Control (5.7) 3.1 (0.2) (31.9) (125.5) N (6.0) 3.5 (0.4) (130.8) (298.3) D (9.7) 4.0 (0.6) b (76.3) (249.9) b S (4.9) b 4.3 (0.5) a (100.2) (260.5) 4 mm thickness Control (10.5) 2.5 (0.6) (80.4) (222.2) N (8.1) 2.2 (0.3) (41.9) b (158.5) D (5.4) 2.9 (0.3) (37.3) (178.1) S (8.7) b 3.2 (0.4) (76.3) (160.3) a Significant difference P 0:01 between control and specimens with N, D or S glass cloth. b Significant difference P 0:05 between control and specimens with N, D or S glass cloth.

5 154 T. Kanie et al. / Dental Materials 16 (2000) Table 4 Flexural properties and impact strength of U-, C-, L-, W- and T-type specimens reinforced with unsilanized N glass fibers. The values in parentheses refer to standard deviation Specimen type Flexural strength (MPa) Deflection (mm) Flexural modulus (MPa) Impact strength (J/m 2 ) Control (10.5) 2.5 (0.6) (80.3) (222.2) U (13.5) 2.1 (0.7) (94.9) (302.0) C (5.2) 2.2 (0.6) (25.2) (230.8) L (3.3) 2.2 (0.3) (158.2) (197.3) a W (4.3) b 2.2 (0.2) (162.9) (436.8) a T (7.8) b 2.0 (0.2) (58.1) a (425.0) a a Significant difference P 0:01 between control and U-, C-, L-, W- or T-type specimens. b Significant difference P 0:05 between control and U-, C-, L-, W-or T-type specimens. several times in new THF. The test specimens eluted by the polymer were weighed and then stored in THF again. This process was repeated until the weight of the test specimen eluted by the polymer ( ˆ the weight of glass fiber) was constant. The glass fiber contents as percentage by volume (V g ) (vol%) was calculated with the following formula [24,26 28]: W g =r g V g ˆ W g =r g W r =r r where W g is the weight proportion of the glass fiber, r g density of glass fiber (2.56 g/cm 3 ), W r the weight proportion of polymer matrix and r r the density of the polymer matrix (1.19 g/cm 3 ). Cross-sections of the test specimens with unsilanized N and silanized N were taken and the surface with exposed glass fiber was polished with wet and dry paper (#1200). The cross-section pieces were examined with a scanning microscope (JSM-35CF, JEOL, Tokyo, Japan) with an accelerating potential of 25 kv. The SEM photographs were used to clarify the degree of impregnation of the glass fiber with polymer matrix. Table 5 Effect of silanation and fiber quantity on the deflection of the test specimens of 1 mm thickness (a), of 3 mm thickness (b) and of 4 mm thickness (c) compared by two-way ANOVA Variables Sum of square Mean square df F value p value (a) Silanation (A) Fiber quantity (B) A B Residual (b) Silanation (A) Fiber quantity (B) A B Residual (c) Silanation (A) Fiber quantity (B) A B Residual Six specimens were used in a group for testing. Unreinforced specimens were made by the same method to act as control. Mann Whitney U-test was carried out to analyze the difference of mechanical properties between the control specimen and the specimens reinforced with glass fiber. Two-way analysis of variance (ANOVA) was conducted with silanation of glass fiber and fiber quantity as the independent variables and mechanical properties as dependent variables. 3. Results The flexural properties and the impact strength of Group (a) are summarized in Table 2. When test specimens of 1 and 2 mm thickness were reinforced with unsilanized fiber, the flexural strengths were enhanced significantly p 0:01 : A similar tendency was not noted in the values of deflection and flexural modulus. The impact strength of all specimens of 1 mm thickness was excluded because ideal force-deflection curves could not be obtained. The impact strengths in all reinforced specimens of 2 mm thickness were higher than those in control specimens p 0:01 : The flexural properties and the impact strength of Group (b) are summarized in Table 3. A highly statistically significant difference p 0:01 in flexural strength was found between control specimens and test specimens reinforced with silanized fiber of 1 mm thickness. There was no statistical difference p 0:05 in the flexural strength of test specimens reinforced with silanized S fibers of 2 mm thickness. The deflection between control specimens and test specimens reinforced with silanized fiber of 1 mm thickness was statistically significant p 0:05 : Impact strengths in all reinforced specimens of 2 mm thickness were higher than those in control specimens p 0:01 : No statistically significant difference p 0:05 was found between unsilanized specimens and silanized specimens in the same condition. The flexural properties and the impact strength of Group (c) are summarized in Table 4. Statistically significant differences were found in the flexural strength of the control

6 T. Kanie et al. / Dental Materials 16 (2000) Table 6 Calculated glass fiber content Fiber quantity Specimen thickness (mm) wt% vol% N D S specimens and W- and T-type specimens p 0:05 : Further, there were highly statistically significant differences in the impact strength of the control specimens and L-, W- and T-type specimens p 0:01 : Two-way ANOVA revealed significant effects of silanation of glass fiber and deflection in the test specimen of 1 mm thickness, and of fiber quantity and Fig. 4. Delamination observed from a side view of each type of specimen fractured by the flexural test. deflection in the test specimen of 3 and 4 mm thickness p 0:05 (Table 5). The quantities of glass fiber are shown in Table 6. The fiber contents as percentage by volume were from 1.13 to 2.13% for N fibers, from 2.05 to 3.95% for D fibers and from 2.20 to 3.80% for S fibers. SEM photographs revealed that polymer matrix impregnated well into the fibers on the test specimen reinforced with silanized fibers and with unsilanized fibers, but cracks were partially observed between the fibers (Fig. 3). Fig. 4 shows the side view of each type of specimen Fig. 3. SEM photographs of cross section of: (a) test specimen with unsilanized glass fiber ( 100 and 1000); (b) test specimen with silanized glass fiber ( 100 and 1000).

7 156 T. Kanie et al. / Dental Materials 16 (2000) fractured in the flexural test. To the left of the test specimens is the fractured portion and the indicatory delamination of fibers from the polymer matrix. The delamination spreads to the center of the control specimens. However, in the reinforced specimens, the delamination is stopped from spreading by the fiber. 4. Discussion Denture fracture in clinical use occurs from a large transitory force caused by an accident or a small force during repeated chewing. The flexural test, impact test and flexural fatigue test were employed to examine these forces. The measurement conditions in this study were designed to simulate clinical conditions, where the thickness of the test specimens stays within the thickness range of actual denture base polymer, and the span of the impact and flexural test approximates to chewing. Jones et al. [31] and Oku [34] measured the impact properties of denture base polymer using a test speed of 135 mm/s for an average chewing speed. Consequently, 135 mm/s seems to be adequate for test speed in this study. However, the test speed of 534 mm/s was employed because this test speed cannot break some test specimens. The test speed of 534 mm/s is larger than that of 135 mm/s. However, Oku [34] who used the same impact testing machine reported that the impact strength was relatively stable and constant at the test speed from 80 to 500 mm/s. Therefore, values of the impact strength measured at the test speed of 534 mm/s seem to be roughly equivalent to values at the average chewing speed (135 mm/s). The results in this study indicate that flexural properties and impact strength are not only examined under constant thickness but varying thicknesses of the test specimen are also important for an incorporated material. In the investigations by Carroll et al. [1] and Vallittu et al. [15], the flexural strength of the denture base polymer reinforced with metal or glass rod was higher than that of the unreinforced denture base polymer. The shape and diameter of the rod directly influenced the flexural strength, but the shape or diameter also limited its use. Shimozato et al. [30] found that the flexural strength of denture base polymer reinforced with carbon fiber increased by 35% compared to that of unreinforced denture base polymer. Inanaga et al. [32] found that the flexural strengths of denture base polymer reinforced with carbon fiber and aramid fiber increased by 6% compared to that of unreinforced denture base polymer. The results (Tables 2 and 3) showed that when test specimens of 1 mm thickness were reinforced with glass fiber, there was a 16 25% increase in flexural strength. These values are located between the values of Shimozato et al. [30] and Inanaga et al. [32]. The quantity of glass fiber in this investigation (Table 6) is less than that of woven glass fibers by Vallittu [28], nevertheless, the flexural strengths (Tables 2 and 3) were larger than the values of denture base polymer with woven glass fibers oriented at angles of ^45 to the long axis of the test specimens [28]. This increase of flexural strength is due to the direction of fibers, which suggests that orientation is a very important factor in denture base polymer with glass fibers. Further investigation is also required to determine the adequate direction of reinforcing materials against the actual intraoral force. When a load is applied in the flexural test, tension occurs within the lower half below the neutral axis of the specimen. Therefore, in general, occurrence of resistance force against tension does not allow one to put the glass fiber near the neutral axis of the specimen. Shimozato et al. [30] reported that chemical bonding or chemical affinity increased the flexural strength of the test specimens reinforced with carbon fiber. This investigation also yields a similar result. The glass fiber controls the flexure of the test specimen because the glass fiber has some degree of thickness and shifts from the neutral axis of the specimen. Fig. 4 shows that the glass fiber prevents the spread of delamination in the specimen. This means that the lengthening of polymer matrix caused by flexural test, stops at the glass fiber. When polymer matrix is laminated with glass fiber, more delamination is seen in the area of outer glass fiber than in the area of inner glass fiber. Finally, a fracture of glass fiber leads the test specimen to a complete fracture. This fact means that the tensile strength of the glass fiber is taking part in the flexural strength of the test specimen. Inanaga et al. [30] also obtained similar results in their investigation using carbon fibers and aramid fibers. The flexural modulus in reinforced specimens of Groups (a) and (b) did not generally differ from that of the control specimens, because in the early stages of flexural test, the lower surface of the test specimen lengthens slightly but the inner glass fiber does not change. Therefore, the flexural modulus measured at this point is not influenced by the quality of glass fiber. Solnit [14] and Vallittu [23] investigated the effects of silane-treatment of glass fiber and glass rod. They reported that when glass fibers were scattered separately in polymer matrix, silane-treatment is effective to bond each fiber with polymer matrix. In the current study, the flexural strength of each test specimen reinforced with unsilanized fiber and silanized fiber shows no statistical difference. The result showed that the bond between the glass fiber and polymer matrix depends on the mechanical retention by polymerization shrinkage and roughness caused by longitudinal threads and transverse threads, that is, on the frictional force between the glass fiber and polymer matrix. Although the woven glass fiber sandwiched between two sheets were pressed at 2 MPa in this investigation, good impregnation of polymer matrix into glass fibers was observed in SEM photographs (Fig. 3). Impregnation of the polymer matrix into fibers was probably caused by decrease of dough viscosity with rising temperature before polymerization and consequent increase of fluidity of the

8 T. Kanie et al. / Dental Materials 16 (2000) polymer matrix. There is an investigation of preimpregnated fiber reinforcement in one attempt to provide the proper impregnation [28], because it is difficult to adjust the impregnation of polymer matrix by controlling the laminating pressure. Longitudinal tensile strength of glass fiber N is 382/25 N/ mm, which is about half compared to 785/25 N/mm in that of glass fiber D and 873/25 N/mm in that of glass fiber S (Table 1). However the flexural strength and impact strength among the test specimens reinforced with three glass fibers showed no statistical difference p 0:05 : The tensile strength acts on the lower half below the neutral axis of the test specimen both in the flexural test and impact test. The tensile strength of autopolymerization denture base polymer is 40.5 MPa according to Vallittu [20]. The tensile strengths calculated from Table 1 are MPa for glass fiber N, MPa for glass fiber D and MPa for glass fiber S, respectively. The tensile strengths of denture base polymers with three glass fibers are higher than those of unreinforced specimens. It appears that the glass fiber works effectively in polymer matrix when the tensile strength of unreinforced specimens is over approximately 40.5 MPa. Therefore, the glass fiber must be put in an area where greater tension can be applied to it than the polymer matrix, to use the glass fiber more effectively. The quantity of glass fiber (Table 6) is under 10% compared with that of the test specimen reinforced with continuous unidirectional glass fibers used by Vallittu [28] and it is considered that the decrease of quantity resulted in a decrease of the bending strengths (Table 4) compared with the values obtained by Vallittu [28]. Increases of 11, 17 and 14% in the flexural strength and 31, 58 and 49% in the impact strength of L-, W- and T-type specimens respectively, compared with those of the control specimens, were observed (Table 4). However, it was found that flexural strength and impact strength did not increase with increasing numbers of glass fibers. When the specimens flex, the glass fiber near the lower surface primarily blocks the lengthening of polymer matrix. Therefore, one possibility for reinforcement of denture base polymer is to increase the tensile strength of the glass fiber. References [1] Carroll CE, Von Fraunhofer JA. Wire reinforcement of acrylic resin prostheses. J Prosthet Dent 1984;52: [2] Ruffino AR. Effect of steel strengtheners on fracture resistance of the acrylic resin complete denture base. J Prosthet Dent 1985;54:75 8. [3] Vallittu PK. Comparison of the in vitro fatigue resistance of an acrylic resin removable partial denture reinforced with continuous glass fibers or metal wires. J Prosthodont 1996;5: [4] Yazdanie N, Mahood M. Carbon fiber acrylic resin composite: an investigation of transverse strength. J Prosthet Dent 1985;54: [5] Ekstrand K, Ruyter IE. Carbon/graphite fiber reinforced poly(methyl methacrylate): properties under dry and wet conditions. J Biomed Mater Res 1987;21: [6] Braden M, Davy KWM, Parker S, Ladizesky NH, Ward IM. Denture base poly(methyl methacrylate) reinforced with ultra-high modulus polyethylene fibres. Br Dent J 1988;164: [7] Gutteridge DL. The effect of including ultra-high-modulus polyethylene fiber on the impact strength of acrylic resin. Br Dent J 1988;164: [8] Berrong JM, Weed RM, Young JM. Fracture resistance of Kevlarreinforced poly(methyl methacrylate) resin: a preliminary study. Int J Prosthodont 1990;3: [9] Gutterridge DL. Reinforcement of poly(methyl methacrylate) with ultra-high-modulus polyethylene fibre. J Dent 1992;20:50 4. [10] Chow TW, Cheng YY, Ladizesky NH. Polyethylene fiber reinforced poly (methyl methacrylate) water sorption and dimensional changes during immersion. J Dent 1993;21: [11] Uzun G, Hersek N, Tinçer T. Effect of five woven fiber reinforcements on the impact and transverse strength of a denture base resin. J Prosthet Dent 1999;81: [12] Ramos VJ, Runyan DA, Christensen LC. The effect of plasma-treated polyethylene fiber on the fracture strength of polymethyl methacrylate. J Prosthet Dent 1996;76:94 6. [13] Samadzadeh A, Kugel G, Hurley E, Aboushala A. Fracture strengths of provisional restorations reinforced with plasma-treated woven polyethylene fiber. J Prosthet Dent 1997;78: [14] Solnit GS. The effect of methyl methacrylate reinforcement with silane-treated and untreated glass fibers. J Prosthet Dent 1991;66: [15] Vallittu PK, Lassila VP. Reinforcement of acrylic resin denture base material with metal or fiber strengtheners. J Oral Rehabil 1992;19: [16] Vallittu PK. Comparison of two different silane compounds used for improving adhesion between fibers and acrylic denture base material. J Oral Rehabil 1993;20: [17] Vallittu PK, Lassila VP, Lappalainen R. 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