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Flexural properties of denture base polymers reinforced with a glass cloth–urethane polymer composite
Dental Materials (2004) 20, 709–716
www.intl.elsevierhealth.com/journals/dema
Flexural properties of denture base polymers reinforced with a glass cloth –urethane polymer composite Takahito Kanie*, Hiroyuki Arikawa, Koichi Fujii, Seiji Ban Department of Biomaterials Science, Kagoshima University Graduate School of Medical and Dental Sciences, Field of Oral and Maxillofacial Rehabilitation, Course for Advanced Therapeutics, 8-35-1 Sakuragaoka, Kagoshima 890-8544, Japan Received 15 July 2003; received in revised form 4 November 2003; accepted 15 November 2003
KEYWORDS Denture base resin; Glass cloth; Flexural property; Light-curing; Reinforcement
Summary Objectives. A newly designed light-cured reinforcement made from urethanemethacrylate oligomer and woven glass cloth has orthotropic anisotropy. This is produced for incorporation into the outermost position under the greatest tension in denture base resins. In this study, the flexural properties of self-, heat-, and light-curing reinforced resins were determined. Methods. The silanized glass cloth was soaked in urethanemethacrylate oligomer containing camphorquinone and 2-(dimethylamino)ethylmethacrylate. It was sandwiched between two pieces of polyethylene film and pressed to form a reinforcement sheet 0.3 mm in thickness, which was light-cured and prepared using four different surface conditions: with or without the polyethylene film and with or without a bonding agent. The reinforcement sheet was fixed in a fluorocarbon resin mold 3 mm in thickness, which was filled with self-, heat-, or light-curing resin and cured. The cured laminated plate was cut for flexural testing (40 £ 7 £ 3 mm3). A three-point flexural test was carried out at a crosshead speed of 2 mm/min and a span length of 30 mm. In this study, the glass fiber content was measured at percentages by weight because it was not possible to determine accurately the volume of the various polymers. Results. The baseline flexural strengths of the self-, heat-, and light-curing resins were 76.2, 68.6, and 55.6 MPa, respectively, and these values were increased to 271.7, 216.4, and 266.5 MPa by the reinforcement sheet. The baseline flexural moduli of self-, heat-, and light-curing resins were 2.0, 2.4, and 2.1 GPa, respectively. These values were increased to 7.2, 5.1, and 6.6 GPa by the reinforcement sheet. SEM photographs revealed good impregnation of the glass fiber within the polymer matrix. Significance. The differences in the flexural strengths and flexural moduli of the control and reinforced specimens were significant ðp , 0:01Þ: Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Introduction *Corresponding author. Tel.: þ81-99-275-6172; fax: þ81-99275-6178. E-mail address: [email protected]
Dentures are commonly made of metal or resin. Metal has isotropic strength, which allows the production of thin palatal plates for dentures.
0109-5641/$ - see front matter Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2003.11.007
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In contrast, resins have good esthetic properties and do not cause the allergic reactions often associated with metals. The selection of metal or resin is often determined according to purpose. In addition to polymethacrylate, urethanedimethacrylate, polycarbonate, and polysulfone are used as base polymers in dentures. However, all of these polymer resins are far weaker than metals. Consequently, denture base resins sometimes crack or break with repeated chewing stress or if accidentally dropped.1 Therefore, it is important to enhance the strength of denture base resins. One approach to resolve this problem is to incorporate some type of reinforcement into the denture base polymer. Though metals of cast dentures are isotropic, the properties of the reinforced denture base polymer vary according to fiber direction. Continuous unidirectional fibers are anisotropic in one direction, woven fibers are an orthotropic anisotropy in a plane, and chopped fibers are isotropic.2 Metal wire or wire mesh is commonly used as a reinforcing material.3 – 7 However, metal does not chemically bond to resins and the resin over the metal appears black. Furthermore, metal rods and wire mesh have disadvantages in that handling and cutting is difficult. To remedy this, polyethylene fiber reinforcement has been used, as it has better esthetic characteristics than other fibers.8 – 9 However, polyethylene fibers do not adhere to the denture base resin and the surface treatment of polyethylene required to improve adhesion is complicated.10,11 Therefore, denture base resins reinforced with polyethylene fibers lack adequate strength. Glass cloth and glass fibers, which are other materials with good esthetic characteristics, can chemically bond to denture base resins with silane treatment. Moreover, glass cloth is easy to cut with scissors, and is readily shaped. However, it is very difficult to incorporate flexible glass cloth accurately into the denture base resin under tension because the oral shape is complex and dentures are custom-made. Vallittu12 reported a new technique to incorporate glass fibers accurately in the desired region of dental prosthetics. The porous preimpregnated polymer was used to obtain a homogeneous polymer matrix for the fiber-reinforced denture base system. Jagger et al.13,14 reported on the selfreinforcement system, which is chemically identical to the matrix holding the fibers in place. Butadiene styrene surface treated poly(methyl methacrylate) fibers in cross-ply arrangement did not produce an improvement in mechanical properties. The preimpregnation could not prevent
T. Kanie et al.
the glass cloth end from fraying or PMMA fibers from asymmetry. Vallittu15 introduced the approach which divided reinforced denture base resins into two groups. These are total fiber reinforcement (TFR) and partial fiber reinforcement (PFR). TFR can entirely reinforce the denture base with the glass cloth, however, some fibers do not contribute to reinforcement. A possible fracture line in a denture base can easily be predicted, PFR is easier to handle than TFR. This approach has been aimed at PFR. Therefore, a means of reinforcing denture base resins was examined that can reproduce complex oral shapes without affecting the esthetic characteristics using a newly produced glass cloth – urethane polymer composite. Then, we evaluated the flexural properties of self-, heat-, and lightcuring denture base resins reinforced with woven glass cloth.
Materials and methods Preparation of the reinforcement for denture base resins The glass cloth (YEA2306, Mie Fabrics, Mie, Japan) used in this study consisted of E-glass (Fig. 1, left). According to manufacturer’s information, the standard thickness of this glass cloth was 0.23 mm, the longitudinal tensile strength was 2048 N/25 mm, and the fiber density of warp and weft were 50 and 25 threads/25 mm, respectively. The glass cloth was cleaned in boiling water for 1 h and then dried in air. The glass cloth was cut into sheets measuring 40 £ 70 mm2 with the warp parallel to the short axis, and soaked in a solution of 2%-g-methacryloxypropyltrimethoxysilane (gMPTS) (Shinetsu Chemicals, Tokyo, Japan) in ethanol for 10 min for silanization. Then, it was dried in air for 1 h, and heated in an oven at 115 8C for 10 min. The silanized glass cloth was soaked in urethanemethacrylate oligomer (Viscosity: 9.85 Pa s at 23 8C) (H-500B, Negami Industry Ltd,
Figure 1 The glass cloth used in this study (left) and the cured reinforcement sheet made with glass cloth and urethane oligomer (right).
Flexural properties of denture base polymers
Ishikawa, Japan) containing 1 wt%-camphorquinone (Wako Pure chemical Industries Ltd, Osaka, Japan) as a photosensitizer and 1 wt%-2-(dimethylamino)ethylmethacrylate (Wako Pure Chemical Industries Ltd, Osaka, Japan) as a reductant. For degassing, the soaked material was placed in an aspirator shielded from light for 3 days. On removal of the glass cloth from the vessel, it was immediately sandwiched between two pieces of polyethylene film (GC Corp., Tokyo, Japan) and then pressed to a sheet 0.3 mm in thickness in a dark room. This reinforcement is designated the reinforcement sheet.
Preparation of test specimens Only one layer of the reinforcement sheet was used in this investigation. Reinforcement sheets were divided into two groups and placed on a fluorocarbon resin board, as shown in step 1 of Fig. 2. The reinforcement sheets in each group were lightcured using an irradiation unit (Triad TCU-1, Dentsply Int., York, USA) for 5 min either as is or after removing the polyethylene film (Fig. 2, step 2). The cured reinforcement sheet, which was
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Table 1 Codes of the surface conditions of the reinforcement sheet before filling of denture base polymers. Code
Polyethylene film
Bonding agent
P P–B N N –B
þ þ 2 2
2 þ 2 þ
0.26 – 0.29 mm thick, is shown in Fig. 1 (right). After removal of all of the polyethylene film, the cured reinforcement sheets were subdivided into two additional groups and then bonding agent (Triad VLC Bonding Agent, Dentsply Int., York, USA) was applied to the illuminated surface of one group (Fig. 2, step 3). The surface conditions are listed in Table 1. The cured reinforcement sheets adhered on the fluorocarbon resin board were left intact to standardize the laminate location of the cured reinforcement sheets. Immediately, a fluorocarbon resin mold 3 mm in thickness was fixed around the cured reinforcement sheets adhered on the fluorocarbon resin board, filled with self-, heat-, or light-curing denture base resin, and then cured (Fig. 2, step 4). The self-curing denture base resin, a PMMA-based slightly cross-linked multiphasic polymer (Pour Resin, Shofu, Kyoto, Japan), was mixed in a powder/liquid ratio of 1.8 g/ml. The fluorocarbon resin mold was filled 5 min after mixing, and heated in an oven at 50 8C for 30 min. The PMMA polymer (Acron, GC, Tokyo, Japan) used for the heat-curing denture base resin was mixed in a powder/liquid ratio of 2.5 g/ml and the mold was filled 20 min after mixing. The mold was pressed at 5 MPa and heated in an oven at 100 8C for 60 min. The light-curing denture base resin, a urethanedimethacrylate-based polymer (Triad VLC Denture Base Materials, Dentsply Int., York, USA), was put in the mold and irradiated for 5 min. The cured reinforcement plates (40 £ 70 £ 3 mm3) were cut into 7-mm strips using a water-cooled diamond blade. The cutting directions of the test specimens and fiber density of the glass cloth are shown in step 5 of Fig. 2. The warp is used toward the longitudinal direction of the test specimen and acts against the tension in flexural testing.
Measurement of flexural properties
Figure 2 Flow chart for preparation of specimens for flexural test (Steps 1– 4), and schematic representation of the cutting direction of the test specimens and fiber density of the glass cloth (Step 5).
The test specimens were polished with wet and dry polishing paper (#600) until they reached 3 mm in thickness and 7 mm in width. After being stored for 24 h in water at 37 8C, a three-point flexural test was conducted with a universal testing machine (TG-50kN, Minebea, Nagano, Japan) using
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Table 2 The flexural strength (MPa) of the self-, heat- and light-cured denture base resin reinforced with the pre-light-cured reinforcement sheet.
PourResin Acron Triad
Control
P
P–B
N
N–B
76.2 (15.3)abc 68.6 (6.6)abcd 55.6 (9.4)abcd
230.7 (22.3)a 197.3 (10.2)a 266.5 (16.9)a
245.1 (12.9)b 216.4 (19.9)b 265.9 (35.6)b
257.8 (25.2)c 210.6 (16.6)c 263.5 (18.7)c
271.7 (15.3)a 209.7 (26.4)d 242.2 (6.9)d
SD in parenthesis. Superscript letters indicate significant differences ðp , 0:01Þ:
a crosshead speed of 2 mm/min and a span length of 30 mm. The flexural strength ðFS Þ and flexural modulus ðFm Þ were calculated using the formulas FS ¼
(Wg =Wa ) was calculated and averaged for three samples.
Statistical analysis
3lPm 2bh2
l3 Px Fm ¼ 4bdh3 where Pm is the maximum load, l is the span length, b is the width of the test specimen, h is the thickness of the test specimen, and d is the linear deflection corresponding to load Px :
Seven test specimens were used for each flexural test. Test specimens lacking glass cloth were used as controls. One-way analysis of variance and Tukey’s test were used to compare the differences in flexural strength and flexural modulus determined in the three-point flexural test.
Results
SEM examination The side surfaces of each reinforced test specimen were polished with wet and dry polishing paper (#1200). SEM (JSM-5510, JEOL, Tokyo, Japan) photomicrographs were then taken with an acceleration potential of 15 kV and the degree of impregnation of the glass fibers with the polymer matrix was analyzed.
Woven glass fiber content Test specimens of the same dimensions as those used in the flexural test were dried in an oven at 37 8C for 24 h, and weighed (Wa ) with an analytical balance. Then, test specimens were put into porcelain crucibles and fired in an electric furnace at 700 8C for 6 h. The glass cloth was reweighed (Wg ) after it had cooled. The glass fiber content
The flexural strengths are summarized in Table 2. The flexural strengths of the reinforced specimens were higher than those of the controls ðp , 0:01Þ: The N –B test specimens made with Pour Resin were significantly stronger than the P specimens ðp , 0:01Þ: The flexural strengths of the self-, heat-, and light-cured controls were 76.2, 68.6, and 55.6 MPa, respectively, and these values were increased to 271.7, 216.4, and 266.5 MPa, respectively, by the reinforcement sheet. The flexural modulus values are summarized in Table 3. Highly significant differences were seen in the flexural modulus values of control and reinforced specimens ðp , 0:01Þ: The flexural modulus of N –B in Pour Resin was significantly greater that those of P, P– B, and N, and the flexural modulus of N in Triad was significantly greater than those of P –B and N– B (all p , 0:05). The flexural
Table 3 The flexural modulus (GPa) of the self-, heat- and light-cured denture base resin reinforced with the pre-light-cured reinforcement sheet.
PourResin Acron Triad
Control
P
P–B
N
N –B
2.0 (0.4)abcd 2.4 (0.1)abcd 2.1 (0.2)abcd
5.8 (0.3)aA 4.9 (0.7)a 6.3 (0.4)a
5.7 (0.4)bB 4.6 (0.4)b 6.0 (0.6)bA
6.1 (0.2)cC 5.0 (0.4)c 6.6 (0.2)cAB
7.2 (0.6)dABC 5.1 (0.6)d 5.8 (0.3)dB
SD in parenthesis. Superscript letters (abcd) indicate significant differences ðp , 0:01Þ: Superscript letters (ABC) indicate significant differences ðp , 0:05Þ:
Flexural properties of denture base polymers
modulus values of the self-, heat-, and light-cured controls were 2.0, 2.4 and 2.1 MPa, and these values were increased to 7.2, 5.1, and 6.6 MPa, respectively, by the reinforcement sheet. SEM photographs of the denture base resins revealed good impregnation of the glass fibers with the polymer matrix (Fig. 3). The glass fiber contents of the self-, heat- and light-cured resin were 8.8, 8.7, and 8.1 wt%, respectively.
Figure 3 SEM photographs of cross-sections reinforced with the cured reinforcement sheet showing impregnation of glass fibers with urethane polymer matrix (top: self-curing type, white bar indicates 10 mm, center: heatcuring type, white bar indicates 50 mm, bottom: lightcuring type, white bar indicates 50 mm).
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Discussion A number of methods of reinforcing denture base resins have been studied. Although some reinforcements have been shown to have stiffening effects, few of these methods have been applied clinically. Vallittu12 mentioned that this is because it is very difficult to place accurately a reinforcement in the desired position in the denture base resin. In the heat-curing method, dough is inserted into a die in a single mass, and the reinforcement can shift when the die is put under pressure. With self-curing resins, the reinforcement may not only shift when the dough is injected, but air bubbles may form near it. It is very difficult to incorporate glass cloth reinforcements into a high-viscosity sheet-like resin without shifting the glass fiber or mixing air bubbles in a light-cured resin. In an attempt to use accurately placed PFR, Vallittu developed a method in which a highly porous polymer reinforced with glass fibers is wetted with a mixture of polymer powder and monomer liquid, and the porous preimpregnation polymer is then plasticized by dissolution. This method makes the denture with the same polymer material. In the present study, another method was used, lamination of the reinforcement, which made it easy to position the glass cloth in the planned position under tension. The greatest tension in a denture is at the outermost position, so the stiffening effect should be greater in this part of the prosthesis. The next problem to overcome is obtaining adhesion between the glass cloth and polymer. Curing dental resins involves radical polymerization, but oxygen inactivates the free radicals formed from an initiator. As a result, polymerization is inhibited and non-polymerized monomer remains in the surface layer.16 An incremental method using this non-polymerized layer is widely used for composite resins and resins for crowns and bridges. Lamination is performed by polymerizing the same monomer as in the polymer, and the reduction in the adhesive strength is small.17 In the present study, no delamination of broken test specimens was observed (Fig. 4). Comparison of the bond strengths of the denture tooth and light-, heat-, and light-cured denture resins showed the light-cured resin to be the weakest.18 In this case, the denture tooth polymer was acrylic, which does not generally have a non-polymerized surface layer. Lastuma ¨ki et al.19 has mentioned that the monomers of the intermediate resin swell PMMA phases in the multi-phase polymer matrix of the fiberreinforced resin substrate. During polymerization, the monomers of the intermediate resin and new
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Figure 4 Fractured test specimens of the denture base polymers reinforced with the cured reinforcement sheet by flexural test (left: self-curing type, center: heatcuring type, right: light-curing type).
composite resin are interlocked into the swelled PMMA phases of the fiber-reinforced resin substrate, and interpenetrating polymer network bonding is formed. On the other hand, monomers, which have a poor dissolving capability, show low bond strength. Although the flexural strengths of controls of the MMA-type Pour Resin and Acron were 76.2 and 68.6 MPa, respectively, the reinforced polymers both showed a flexural strength of 216.4 MPa, which was similar to that of UDMAtype Triad (Table 2). Polymerization appeared to proceed normally even when different types of monomer (MMA and UDMA) were combined. The non-polymerized surface layer of UDMA plays an important role in bonding. This experiment examined four surface conditions. Initially, polyethylene film was used to prevent oxygen from causing formation of a nonpolymerized layer. However, no differences were observed in the flexural strengths (Table 2). The effect of the bonding agent was also examined. This bonding agent is commonly painted on the polished surface when repairing denture base resin. Kallio et al.20 reported that the function of unfilled intermediate resin, which corresponds to the bonding agent in the present study, is to achieve better wetting of the substrate surface, and to some degree dissolve and swell the polymer surface of the substrate. The bonding agent is not useful for increasing the flexural strength because the PUDMA is highly cross-linked and does not swell. No differences were observed in the flexural strengths of the three types of denture base resin confirming that sufficient bonding was obtained without requiring the extra labor involved in using a bonding agent.
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The problem of fraying when glass cloth is cut must be considered. Vallittu resolved this problem by impregnating the porous glass fibers with a polymer. The authors resolved this problem by using a high-viscosity oligomer and the polyethylene film sandwich. Clinically, it is not necessary to cut the reinforcement sheet with scissors if the plate is approximately the shape of the palate, and plates can be prepared in several sizes. Moreover, in this study the reinforcement sheets were stored in an opaque bag which was not opened until necessary. Two factors affect the strength of reinforcement: adhesion between the denture base resin and the reinforcement sheet, and impregnation of the glass cloth with resin. The glass cloth used in this study was E-glass, which has been reported to adhere to methacrylate when silanized.21 – 23 The next problem is whether the glass cloth is sufficiently impregnated with the monomer. This is especially problematic when a high-viscosity oligomer is used. Therefore, the glass cloth was soaked in the oligomer for three days, while the oligomer was absorbed with an aspirator. This period of 3 days was shown to be sufficient in a preliminary experiment. As a result, a transparent polymer composite was obtained with no air bubbles, as shown in Fig. 1 (right). The advantage of using a high-viscosity oligomer is that air bubbles do not form readily once it is degassed. The glass fiber in the SEM image in Fig. 3 shows a unit adhering to the polymer matrix. This specimen is well impregnated. As a polymer reinforcement, the glass fiber content is an important factor affecting the strength of the denture. In the present study, it was not possible to determine accurately the volume of each material as an aggregate of various polymers was used. Therefore, the glass fiber content was measured as percentages by weight. The glass fiber content of the cured resin was 8.1 – 8.8 wt%. The fillers remained in the heat- and lightcured resin, and filler contents were 0.1 and 11.9 wt%. The density of matrix polymer and filler content would influence the result of the glass fiber content. The glass fiber content becomes 4.2 vol%, when the density of the self-cured resin in which a filler is not included is assumed as 1.2 g/cm3. Based on the results of a previous study24 using the same glass cloth and Pour Resin, the glass cloth content was about 4 vol%. Vallittu’s test specimens contained 6 – 6.5 vol% woven glass fiber and their flexural strength was increased by about 1.3fold. 12 The authors obtained a 2.9 – 3.6-fold increase in the flexural strength in the present study. The elastic modulus showed a similar tendency. The weft yarn used in the glass cloth, which differs from a unidirectional fiber, does not
Flexural properties of denture base polymers
directly affect the strength. Although there are some differences in the weave, excellent results were obtained by placing the glass fiber near the surface.25 The denture base resin is used in the wet environment in the oral cavity; when not in use, it is stored under water. Therefore, the effect of water is an important factor. Glass-reinforced denture base resin has many factors such as matrix resins, silane-coupling agents (siloxane bonds) and glass fibers, which affect water absorption. Though the glass cloth and siloxane bond are not directly in contact with the water on the surface of the glassreinforced denture base resin, the absorbency of the matrix resin is affected first. The water which permeates the matrix resin acts as a plasticizer and the strength of the denture base is reduced. Therefore, it is important that a resin with low water absorption should be chosen. According to Arima et al.,26 the type of cross-linking agent greatly affects the water absorption. Lassila et al.27 mentioned that both Bis-GMA and UDMA absorbed a relatively high amount of water and the proportional decrease in the strength was mainly dependent on the type of polymer matrix rather than fiber-volume fraction. It is important to raise the degree of conversion, since the residual monomer is greatly affected by the water absorption of the matrix resin. Secondly, water would act on the siloxane bond between the matrix resin and the glass, and the network is subject to hydrolytic degradation. Finally, the glass is affected by water absorption. Alkali ions, earth alkali ions and B2O3 are reactive with water and the glass containing these substances decreases the hydrolytic stability. The glass cloth used in this investigation contains 5 – 13 wt% B2O3 according to the manufacturer’s information. The reinforced denture base resin is composed of some components possibly degradable by water, therefore further investigation on the effect of water should be undertaken. Theoretically, the methylmethacrylate monomer used in denture base resin contracts by 21 vol% during polymerization. The powder-liquid method is used to reduce the amount of shrinkage. Furthermore, filler is added to composite resin to reduce shrinkage during curing. A technique reducing the absolute amount of shrinkage was adopted to reduce curing shrinkage due to the monomer.28, 29 In this regard, the quantity of monomer removed by putting the reinforcement sheet in the mold reduces the overall amount of shrinkage. In a study30 using a U-shaped test specimen, the method of curing the MMA and the reinforcement materials used was shown to affect the dimensions of the specimen. Nitanda et al.31 reported clear
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differences in the size of model dentures using a base resin and a reinforced resin. The denture base resin is influenced by the position, shape, and type of reinforcement. However, there have been few studies of these effects, and further studies are therefore required. This reinforcement sheet is designed to use in contact with model plasters. The cured reinforcement sheets adhere to the fluorocarbon resin board. This adhesion force will be useful to fix the reinforcement sheet against the filling stress of denture resin. This reinforcement sheet, which is produced for incorporation into the outermost position under the greatest tension in denture base resins, can be placed in the planned position in denture base resin for the so-called PFR, and the resulting denture base resin has good mechanical properties. The glass cloth used to reinforce denture base resins is transparent and can easily be shaped into complex shapes. Furthermore, no new device is required, because a standard lightcuring process is used for this reinforcement sheet. These facts could be useful in clinical application. In conclusion, this newly designed reinforcement sheet improved the flexural properties of self-, heat-, and light-cured reinforced resins.
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11. Samadzadeh A, Kugel G, Hurley E, Aboushala A. Fracture strengths of provisional restorations reinforced with plasmatreated woven polyethylene fiber. J Prosthet Dent 1997;78: 447—50. 12. Vallittu PK. Flexural properties of acrylic resin polymers reinforced with unidirectional and woven glass fibers. J Prosthet Dent 1999;81:318—26. 13. Jagger D, Harrison A, Vowles R, Jagger R. The effect of the addition of surface treated chopped and continuous poly (methyl methacrylate) fibres on some properties of acrylic resin. J Oral Rehabil 2001;28:865—72. 14. Jagger D, Harrison A, Jagger R, Milward P. The effect of the addition of poly(methyl methacrylate) fibres on some properties of high strength heat-cured acrylic resin denture base material. J Oral Rehabil 2003;30:231—5. 15. Vallittu PK. Glass fiber reinforcement in repaired acrylic resin removable dentures: preliminary results of a clinical study. Quintessence Int 1997;28:39—44. 16. Ruyter IE. Unpolymerized surface layers on sealants. Acta Odontol Scand 1981;39:27—32. 17. Boyer DB, Chan KC, Reinhardt JW. Build-up and repair of light-cured composites: bond strength. J Dent Res 1984;63: 1241—4. 18. Clancy JM, Boyer DB. Comparative bond strengths of lightcured, heat-cured, and autopolymerizing denture resins to denture teeth. J Prosthet Dent 1989;61:457—62. 19. Lastuma ¨ki TM, Kallio TT, Vallittu PK. The bond strength of light-curing composite resin to finally polymerized and aged glass fiber-reinforced composite substrate. Biomaterials 2002;23:4533—9. 20. Kallio T, Lastuma ¨ki T, Vallittu PK. Bonding of restorative composite resin to some polymeric composite substrates. Dent Mater 2001;17:80—6. 21. Solnit GS. The effect of methyl methacrylate reinforcement with silane-treated and untreated glass fibers. J Prosthet Dent 1991;66:310—4.
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22. Vallittu PK. Comparison of two different silane compounds used for improving adhesion between fibers and acrylic denture base material. J Oral Rehabil 1993;20: 533—9. 23. Debnath S, Wunder SL, McCool JI, Baran GR. Silane treatment effects on glass/resin interfacial shear strengths. Dent Mater 2003;19:441—8. 24. Kanie T, Arikawa H, Fujii K, Ban S. Impact strength of acrylic denture base resin reinforced with woven glass fiber. Dent Mater J 2003;22:30—8. 25. Kanie T, Arikawa H, Fujii K, Ban S. Mechanical properties of reinforced denture base resin: the effect of position and the number of woven glass fibers. Dent Mater J 2002;21: 261—9. 26. Arima T, Murata H, Hamada T. The effects of cross-linking agents on the water sorption and solubility characteristics of denture base resin. J Oral Rehabil 1996;23: 476—80. 27. Lassila LVJ, Nohrstrom T, Vallittu PR. The influence of shortterm water storage on the flexural properties of unidirectional glass fiber-reinforced composite. Biomaterials 2002;23:2221—9. 28. Cheng YY, Hui OL, Ladizesky NH. Processing shrinkage of heat-curing acrylic resin reinforced with high-performance polyethylene fibre. Biomaterials 1993;14:775—80. 29. Vallittu PK, Lassila VP, Lappalainen R. Transverse strength and fatigue of denture acrylic-glass fiber composite. Dent Mater 1994;10:116—21. 30. Vallittu PK. Dimensional accuracy and stability of polymethyl methacrylate reinforced with metal wire or continuous glass fiber. J Prosthet Dent 1996;75:617—21. 31. Nitanda J, Wakasa K, Matsui H, Kasahara M, Yamaki M, Matsui A. Dental application of glass cloth to denture: dimensional accuracy in reinforced resins. J Mater Sci Mater Med 1992;3: 137—40.
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Flexural properties of denture base polymers reinforced with a glass cloth –urethane polymer composite Takahito Kanie*, Hiroyuki Arikawa, Koichi Fujii, Seiji Ban Department of Biomaterials Science, Kagoshima University Graduate School of Medical and Dental Sciences, Field of Oral and Maxillofacial Rehabilitation, Course for Advanced Therapeutics, 8-35-1 Sakuragaoka, Kagoshima 890-8544, Japan Received 15 July 2003; received in revised form 4 November 2003; accepted 15 November 2003
KEYWORDS Denture base resin; Glass cloth; Flexural property; Light-curing; Reinforcement
Summary Objectives. A newly designed light-cured reinforcement made from urethanemethacrylate oligomer and woven glass cloth has orthotropic anisotropy. This is produced for incorporation into the outermost position under the greatest tension in denture base resins. In this study, the flexural properties of self-, heat-, and light-curing reinforced resins were determined. Methods. The silanized glass cloth was soaked in urethanemethacrylate oligomer containing camphorquinone and 2-(dimethylamino)ethylmethacrylate. It was sandwiched between two pieces of polyethylene film and pressed to form a reinforcement sheet 0.3 mm in thickness, which was light-cured and prepared using four different surface conditions: with or without the polyethylene film and with or without a bonding agent. The reinforcement sheet was fixed in a fluorocarbon resin mold 3 mm in thickness, which was filled with self-, heat-, or light-curing resin and cured. The cured laminated plate was cut for flexural testing (40 £ 7 £ 3 mm3). A three-point flexural test was carried out at a crosshead speed of 2 mm/min and a span length of 30 mm. In this study, the glass fiber content was measured at percentages by weight because it was not possible to determine accurately the volume of the various polymers. Results. The baseline flexural strengths of the self-, heat-, and light-curing resins were 76.2, 68.6, and 55.6 MPa, respectively, and these values were increased to 271.7, 216.4, and 266.5 MPa by the reinforcement sheet. The baseline flexural moduli of self-, heat-, and light-curing resins were 2.0, 2.4, and 2.1 GPa, respectively. These values were increased to 7.2, 5.1, and 6.6 GPa by the reinforcement sheet. SEM photographs revealed good impregnation of the glass fiber within the polymer matrix. Significance. The differences in the flexural strengths and flexural moduli of the control and reinforced specimens were significant ðp , 0:01Þ: Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
Introduction *Corresponding author. Tel.: þ81-99-275-6172; fax: þ81-99275-6178. E-mail address: [email protected]
Dentures are commonly made of metal or resin. Metal has isotropic strength, which allows the production of thin palatal plates for dentures.
0109-5641/$ - see front matter Q 2004 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2003.11.007
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In contrast, resins have good esthetic properties and do not cause the allergic reactions often associated with metals. The selection of metal or resin is often determined according to purpose. In addition to polymethacrylate, urethanedimethacrylate, polycarbonate, and polysulfone are used as base polymers in dentures. However, all of these polymer resins are far weaker than metals. Consequently, denture base resins sometimes crack or break with repeated chewing stress or if accidentally dropped.1 Therefore, it is important to enhance the strength of denture base resins. One approach to resolve this problem is to incorporate some type of reinforcement into the denture base polymer. Though metals of cast dentures are isotropic, the properties of the reinforced denture base polymer vary according to fiber direction. Continuous unidirectional fibers are anisotropic in one direction, woven fibers are an orthotropic anisotropy in a plane, and chopped fibers are isotropic.2 Metal wire or wire mesh is commonly used as a reinforcing material.3 – 7 However, metal does not chemically bond to resins and the resin over the metal appears black. Furthermore, metal rods and wire mesh have disadvantages in that handling and cutting is difficult. To remedy this, polyethylene fiber reinforcement has been used, as it has better esthetic characteristics than other fibers.8 – 9 However, polyethylene fibers do not adhere to the denture base resin and the surface treatment of polyethylene required to improve adhesion is complicated.10,11 Therefore, denture base resins reinforced with polyethylene fibers lack adequate strength. Glass cloth and glass fibers, which are other materials with good esthetic characteristics, can chemically bond to denture base resins with silane treatment. Moreover, glass cloth is easy to cut with scissors, and is readily shaped. However, it is very difficult to incorporate flexible glass cloth accurately into the denture base resin under tension because the oral shape is complex and dentures are custom-made. Vallittu12 reported a new technique to incorporate glass fibers accurately in the desired region of dental prosthetics. The porous preimpregnated polymer was used to obtain a homogeneous polymer matrix for the fiber-reinforced denture base system. Jagger et al.13,14 reported on the selfreinforcement system, which is chemically identical to the matrix holding the fibers in place. Butadiene styrene surface treated poly(methyl methacrylate) fibers in cross-ply arrangement did not produce an improvement in mechanical properties. The preimpregnation could not prevent
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the glass cloth end from fraying or PMMA fibers from asymmetry. Vallittu15 introduced the approach which divided reinforced denture base resins into two groups. These are total fiber reinforcement (TFR) and partial fiber reinforcement (PFR). TFR can entirely reinforce the denture base with the glass cloth, however, some fibers do not contribute to reinforcement. A possible fracture line in a denture base can easily be predicted, PFR is easier to handle than TFR. This approach has been aimed at PFR. Therefore, a means of reinforcing denture base resins was examined that can reproduce complex oral shapes without affecting the esthetic characteristics using a newly produced glass cloth – urethane polymer composite. Then, we evaluated the flexural properties of self-, heat-, and lightcuring denture base resins reinforced with woven glass cloth.
Materials and methods Preparation of the reinforcement for denture base resins The glass cloth (YEA2306, Mie Fabrics, Mie, Japan) used in this study consisted of E-glass (Fig. 1, left). According to manufacturer’s information, the standard thickness of this glass cloth was 0.23 mm, the longitudinal tensile strength was 2048 N/25 mm, and the fiber density of warp and weft were 50 and 25 threads/25 mm, respectively. The glass cloth was cleaned in boiling water for 1 h and then dried in air. The glass cloth was cut into sheets measuring 40 £ 70 mm2 with the warp parallel to the short axis, and soaked in a solution of 2%-g-methacryloxypropyltrimethoxysilane (gMPTS) (Shinetsu Chemicals, Tokyo, Japan) in ethanol for 10 min for silanization. Then, it was dried in air for 1 h, and heated in an oven at 115 8C for 10 min. The silanized glass cloth was soaked in urethanemethacrylate oligomer (Viscosity: 9.85 Pa s at 23 8C) (H-500B, Negami Industry Ltd,
Figure 1 The glass cloth used in this study (left) and the cured reinforcement sheet made with glass cloth and urethane oligomer (right).
Flexural properties of denture base polymers
Ishikawa, Japan) containing 1 wt%-camphorquinone (Wako Pure chemical Industries Ltd, Osaka, Japan) as a photosensitizer and 1 wt%-2-(dimethylamino)ethylmethacrylate (Wako Pure Chemical Industries Ltd, Osaka, Japan) as a reductant. For degassing, the soaked material was placed in an aspirator shielded from light for 3 days. On removal of the glass cloth from the vessel, it was immediately sandwiched between two pieces of polyethylene film (GC Corp., Tokyo, Japan) and then pressed to a sheet 0.3 mm in thickness in a dark room. This reinforcement is designated the reinforcement sheet.
Preparation of test specimens Only one layer of the reinforcement sheet was used in this investigation. Reinforcement sheets were divided into two groups and placed on a fluorocarbon resin board, as shown in step 1 of Fig. 2. The reinforcement sheets in each group were lightcured using an irradiation unit (Triad TCU-1, Dentsply Int., York, USA) for 5 min either as is or after removing the polyethylene film (Fig. 2, step 2). The cured reinforcement sheet, which was
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Table 1 Codes of the surface conditions of the reinforcement sheet before filling of denture base polymers. Code
Polyethylene film
Bonding agent
P P–B N N –B
þ þ 2 2
2 þ 2 þ
0.26 – 0.29 mm thick, is shown in Fig. 1 (right). After removal of all of the polyethylene film, the cured reinforcement sheets were subdivided into two additional groups and then bonding agent (Triad VLC Bonding Agent, Dentsply Int., York, USA) was applied to the illuminated surface of one group (Fig. 2, step 3). The surface conditions are listed in Table 1. The cured reinforcement sheets adhered on the fluorocarbon resin board were left intact to standardize the laminate location of the cured reinforcement sheets. Immediately, a fluorocarbon resin mold 3 mm in thickness was fixed around the cured reinforcement sheets adhered on the fluorocarbon resin board, filled with self-, heat-, or light-curing denture base resin, and then cured (Fig. 2, step 4). The self-curing denture base resin, a PMMA-based slightly cross-linked multiphasic polymer (Pour Resin, Shofu, Kyoto, Japan), was mixed in a powder/liquid ratio of 1.8 g/ml. The fluorocarbon resin mold was filled 5 min after mixing, and heated in an oven at 50 8C for 30 min. The PMMA polymer (Acron, GC, Tokyo, Japan) used for the heat-curing denture base resin was mixed in a powder/liquid ratio of 2.5 g/ml and the mold was filled 20 min after mixing. The mold was pressed at 5 MPa and heated in an oven at 100 8C for 60 min. The light-curing denture base resin, a urethanedimethacrylate-based polymer (Triad VLC Denture Base Materials, Dentsply Int., York, USA), was put in the mold and irradiated for 5 min. The cured reinforcement plates (40 £ 70 £ 3 mm3) were cut into 7-mm strips using a water-cooled diamond blade. The cutting directions of the test specimens and fiber density of the glass cloth are shown in step 5 of Fig. 2. The warp is used toward the longitudinal direction of the test specimen and acts against the tension in flexural testing.
Measurement of flexural properties
Figure 2 Flow chart for preparation of specimens for flexural test (Steps 1– 4), and schematic representation of the cutting direction of the test specimens and fiber density of the glass cloth (Step 5).
The test specimens were polished with wet and dry polishing paper (#600) until they reached 3 mm in thickness and 7 mm in width. After being stored for 24 h in water at 37 8C, a three-point flexural test was conducted with a universal testing machine (TG-50kN, Minebea, Nagano, Japan) using
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Table 2 The flexural strength (MPa) of the self-, heat- and light-cured denture base resin reinforced with the pre-light-cured reinforcement sheet.
PourResin Acron Triad
Control
P
P–B
N
N–B
76.2 (15.3)abc 68.6 (6.6)abcd 55.6 (9.4)abcd
230.7 (22.3)a 197.3 (10.2)a 266.5 (16.9)a
245.1 (12.9)b 216.4 (19.9)b 265.9 (35.6)b
257.8 (25.2)c 210.6 (16.6)c 263.5 (18.7)c
271.7 (15.3)a 209.7 (26.4)d 242.2 (6.9)d
SD in parenthesis. Superscript letters indicate significant differences ðp , 0:01Þ:
a crosshead speed of 2 mm/min and a span length of 30 mm. The flexural strength ðFS Þ and flexural modulus ðFm Þ were calculated using the formulas FS ¼
(Wg =Wa ) was calculated and averaged for three samples.
Statistical analysis
3lPm 2bh2
l3 Px Fm ¼ 4bdh3 where Pm is the maximum load, l is the span length, b is the width of the test specimen, h is the thickness of the test specimen, and d is the linear deflection corresponding to load Px :
Seven test specimens were used for each flexural test. Test specimens lacking glass cloth were used as controls. One-way analysis of variance and Tukey’s test were used to compare the differences in flexural strength and flexural modulus determined in the three-point flexural test.
Results
SEM examination The side surfaces of each reinforced test specimen were polished with wet and dry polishing paper (#1200). SEM (JSM-5510, JEOL, Tokyo, Japan) photomicrographs were then taken with an acceleration potential of 15 kV and the degree of impregnation of the glass fibers with the polymer matrix was analyzed.
Woven glass fiber content Test specimens of the same dimensions as those used in the flexural test were dried in an oven at 37 8C for 24 h, and weighed (Wa ) with an analytical balance. Then, test specimens were put into porcelain crucibles and fired in an electric furnace at 700 8C for 6 h. The glass cloth was reweighed (Wg ) after it had cooled. The glass fiber content
The flexural strengths are summarized in Table 2. The flexural strengths of the reinforced specimens were higher than those of the controls ðp , 0:01Þ: The N –B test specimens made with Pour Resin were significantly stronger than the P specimens ðp , 0:01Þ: The flexural strengths of the self-, heat-, and light-cured controls were 76.2, 68.6, and 55.6 MPa, respectively, and these values were increased to 271.7, 216.4, and 266.5 MPa, respectively, by the reinforcement sheet. The flexural modulus values are summarized in Table 3. Highly significant differences were seen in the flexural modulus values of control and reinforced specimens ðp , 0:01Þ: The flexural modulus of N –B in Pour Resin was significantly greater that those of P, P– B, and N, and the flexural modulus of N in Triad was significantly greater than those of P –B and N– B (all p , 0:05). The flexural
Table 3 The flexural modulus (GPa) of the self-, heat- and light-cured denture base resin reinforced with the pre-light-cured reinforcement sheet.
PourResin Acron Triad
Control
P
P–B
N
N –B
2.0 (0.4)abcd 2.4 (0.1)abcd 2.1 (0.2)abcd
5.8 (0.3)aA 4.9 (0.7)a 6.3 (0.4)a
5.7 (0.4)bB 4.6 (0.4)b 6.0 (0.6)bA
6.1 (0.2)cC 5.0 (0.4)c 6.6 (0.2)cAB
7.2 (0.6)dABC 5.1 (0.6)d 5.8 (0.3)dB
SD in parenthesis. Superscript letters (abcd) indicate significant differences ðp , 0:01Þ: Superscript letters (ABC) indicate significant differences ðp , 0:05Þ:
Flexural properties of denture base polymers
modulus values of the self-, heat-, and light-cured controls were 2.0, 2.4 and 2.1 MPa, and these values were increased to 7.2, 5.1, and 6.6 MPa, respectively, by the reinforcement sheet. SEM photographs of the denture base resins revealed good impregnation of the glass fibers with the polymer matrix (Fig. 3). The glass fiber contents of the self-, heat- and light-cured resin were 8.8, 8.7, and 8.1 wt%, respectively.
Figure 3 SEM photographs of cross-sections reinforced with the cured reinforcement sheet showing impregnation of glass fibers with urethane polymer matrix (top: self-curing type, white bar indicates 10 mm, center: heatcuring type, white bar indicates 50 mm, bottom: lightcuring type, white bar indicates 50 mm).
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Discussion A number of methods of reinforcing denture base resins have been studied. Although some reinforcements have been shown to have stiffening effects, few of these methods have been applied clinically. Vallittu12 mentioned that this is because it is very difficult to place accurately a reinforcement in the desired position in the denture base resin. In the heat-curing method, dough is inserted into a die in a single mass, and the reinforcement can shift when the die is put under pressure. With self-curing resins, the reinforcement may not only shift when the dough is injected, but air bubbles may form near it. It is very difficult to incorporate glass cloth reinforcements into a high-viscosity sheet-like resin without shifting the glass fiber or mixing air bubbles in a light-cured resin. In an attempt to use accurately placed PFR, Vallittu developed a method in which a highly porous polymer reinforced with glass fibers is wetted with a mixture of polymer powder and monomer liquid, and the porous preimpregnation polymer is then plasticized by dissolution. This method makes the denture with the same polymer material. In the present study, another method was used, lamination of the reinforcement, which made it easy to position the glass cloth in the planned position under tension. The greatest tension in a denture is at the outermost position, so the stiffening effect should be greater in this part of the prosthesis. The next problem to overcome is obtaining adhesion between the glass cloth and polymer. Curing dental resins involves radical polymerization, but oxygen inactivates the free radicals formed from an initiator. As a result, polymerization is inhibited and non-polymerized monomer remains in the surface layer.16 An incremental method using this non-polymerized layer is widely used for composite resins and resins for crowns and bridges. Lamination is performed by polymerizing the same monomer as in the polymer, and the reduction in the adhesive strength is small.17 In the present study, no delamination of broken test specimens was observed (Fig. 4). Comparison of the bond strengths of the denture tooth and light-, heat-, and light-cured denture resins showed the light-cured resin to be the weakest.18 In this case, the denture tooth polymer was acrylic, which does not generally have a non-polymerized surface layer. Lastuma ¨ki et al.19 has mentioned that the monomers of the intermediate resin swell PMMA phases in the multi-phase polymer matrix of the fiberreinforced resin substrate. During polymerization, the monomers of the intermediate resin and new
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Figure 4 Fractured test specimens of the denture base polymers reinforced with the cured reinforcement sheet by flexural test (left: self-curing type, center: heatcuring type, right: light-curing type).
composite resin are interlocked into the swelled PMMA phases of the fiber-reinforced resin substrate, and interpenetrating polymer network bonding is formed. On the other hand, monomers, which have a poor dissolving capability, show low bond strength. Although the flexural strengths of controls of the MMA-type Pour Resin and Acron were 76.2 and 68.6 MPa, respectively, the reinforced polymers both showed a flexural strength of 216.4 MPa, which was similar to that of UDMAtype Triad (Table 2). Polymerization appeared to proceed normally even when different types of monomer (MMA and UDMA) were combined. The non-polymerized surface layer of UDMA plays an important role in bonding. This experiment examined four surface conditions. Initially, polyethylene film was used to prevent oxygen from causing formation of a nonpolymerized layer. However, no differences were observed in the flexural strengths (Table 2). The effect of the bonding agent was also examined. This bonding agent is commonly painted on the polished surface when repairing denture base resin. Kallio et al.20 reported that the function of unfilled intermediate resin, which corresponds to the bonding agent in the present study, is to achieve better wetting of the substrate surface, and to some degree dissolve and swell the polymer surface of the substrate. The bonding agent is not useful for increasing the flexural strength because the PUDMA is highly cross-linked and does not swell. No differences were observed in the flexural strengths of the three types of denture base resin confirming that sufficient bonding was obtained without requiring the extra labor involved in using a bonding agent.
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The problem of fraying when glass cloth is cut must be considered. Vallittu resolved this problem by impregnating the porous glass fibers with a polymer. The authors resolved this problem by using a high-viscosity oligomer and the polyethylene film sandwich. Clinically, it is not necessary to cut the reinforcement sheet with scissors if the plate is approximately the shape of the palate, and plates can be prepared in several sizes. Moreover, in this study the reinforcement sheets were stored in an opaque bag which was not opened until necessary. Two factors affect the strength of reinforcement: adhesion between the denture base resin and the reinforcement sheet, and impregnation of the glass cloth with resin. The glass cloth used in this study was E-glass, which has been reported to adhere to methacrylate when silanized.21 – 23 The next problem is whether the glass cloth is sufficiently impregnated with the monomer. This is especially problematic when a high-viscosity oligomer is used. Therefore, the glass cloth was soaked in the oligomer for three days, while the oligomer was absorbed with an aspirator. This period of 3 days was shown to be sufficient in a preliminary experiment. As a result, a transparent polymer composite was obtained with no air bubbles, as shown in Fig. 1 (right). The advantage of using a high-viscosity oligomer is that air bubbles do not form readily once it is degassed. The glass fiber in the SEM image in Fig. 3 shows a unit adhering to the polymer matrix. This specimen is well impregnated. As a polymer reinforcement, the glass fiber content is an important factor affecting the strength of the denture. In the present study, it was not possible to determine accurately the volume of each material as an aggregate of various polymers was used. Therefore, the glass fiber content was measured as percentages by weight. The glass fiber content of the cured resin was 8.1 – 8.8 wt%. The fillers remained in the heat- and lightcured resin, and filler contents were 0.1 and 11.9 wt%. The density of matrix polymer and filler content would influence the result of the glass fiber content. The glass fiber content becomes 4.2 vol%, when the density of the self-cured resin in which a filler is not included is assumed as 1.2 g/cm3. Based on the results of a previous study24 using the same glass cloth and Pour Resin, the glass cloth content was about 4 vol%. Vallittu’s test specimens contained 6 – 6.5 vol% woven glass fiber and their flexural strength was increased by about 1.3fold. 12 The authors obtained a 2.9 – 3.6-fold increase in the flexural strength in the present study. The elastic modulus showed a similar tendency. The weft yarn used in the glass cloth, which differs from a unidirectional fiber, does not
Flexural properties of denture base polymers
directly affect the strength. Although there are some differences in the weave, excellent results were obtained by placing the glass fiber near the surface.25 The denture base resin is used in the wet environment in the oral cavity; when not in use, it is stored under water. Therefore, the effect of water is an important factor. Glass-reinforced denture base resin has many factors such as matrix resins, silane-coupling agents (siloxane bonds) and glass fibers, which affect water absorption. Though the glass cloth and siloxane bond are not directly in contact with the water on the surface of the glassreinforced denture base resin, the absorbency of the matrix resin is affected first. The water which permeates the matrix resin acts as a plasticizer and the strength of the denture base is reduced. Therefore, it is important that a resin with low water absorption should be chosen. According to Arima et al.,26 the type of cross-linking agent greatly affects the water absorption. Lassila et al.27 mentioned that both Bis-GMA and UDMA absorbed a relatively high amount of water and the proportional decrease in the strength was mainly dependent on the type of polymer matrix rather than fiber-volume fraction. It is important to raise the degree of conversion, since the residual monomer is greatly affected by the water absorption of the matrix resin. Secondly, water would act on the siloxane bond between the matrix resin and the glass, and the network is subject to hydrolytic degradation. Finally, the glass is affected by water absorption. Alkali ions, earth alkali ions and B2O3 are reactive with water and the glass containing these substances decreases the hydrolytic stability. The glass cloth used in this investigation contains 5 – 13 wt% B2O3 according to the manufacturer’s information. The reinforced denture base resin is composed of some components possibly degradable by water, therefore further investigation on the effect of water should be undertaken. Theoretically, the methylmethacrylate monomer used in denture base resin contracts by 21 vol% during polymerization. The powder-liquid method is used to reduce the amount of shrinkage. Furthermore, filler is added to composite resin to reduce shrinkage during curing. A technique reducing the absolute amount of shrinkage was adopted to reduce curing shrinkage due to the monomer.28, 29 In this regard, the quantity of monomer removed by putting the reinforcement sheet in the mold reduces the overall amount of shrinkage. In a study30 using a U-shaped test specimen, the method of curing the MMA and the reinforcement materials used was shown to affect the dimensions of the specimen. Nitanda et al.31 reported clear
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differences in the size of model dentures using a base resin and a reinforced resin. The denture base resin is influenced by the position, shape, and type of reinforcement. However, there have been few studies of these effects, and further studies are therefore required. This reinforcement sheet is designed to use in contact with model plasters. The cured reinforcement sheets adhere to the fluorocarbon resin board. This adhesion force will be useful to fix the reinforcement sheet against the filling stress of denture resin. This reinforcement sheet, which is produced for incorporation into the outermost position under the greatest tension in denture base resins, can be placed in the planned position in denture base resin for the so-called PFR, and the resulting denture base resin has good mechanical properties. The glass cloth used to reinforce denture base resins is transparent and can easily be shaped into complex shapes. Furthermore, no new device is required, because a standard lightcuring process is used for this reinforcement sheet. These facts could be useful in clinical application. In conclusion, this newly designed reinforcement sheet improved the flexural properties of self-, heat-, and light-cured reinforced resins.
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22. Vallittu PK. Comparison of two different silane compounds used for improving adhesion between fibers and acrylic denture base material. J Oral Rehabil 1993;20: 533—9. 23. Debnath S, Wunder SL, McCool JI, Baran GR. Silane treatment effects on glass/resin interfacial shear strengths. Dent Mater 2003;19:441—8. 24. Kanie T, Arikawa H, Fujii K, Ban S. Impact strength of acrylic denture base resin reinforced with woven glass fiber. Dent Mater J 2003;22:30—8. 25. Kanie T, Arikawa H, Fujii K, Ban S. Mechanical properties of reinforced denture base resin: the effect of position and the number of woven glass fibers. Dent Mater J 2002;21: 261—9. 26. Arima T, Murata H, Hamada T. The effects of cross-linking agents on the water sorption and solubility characteristics of denture base resin. J Oral Rehabil 1996;23: 476—80. 27. Lassila LVJ, Nohrstrom T, Vallittu PR. The influence of shortterm water storage on the flexural properties of unidirectional glass fiber-reinforced composite. Biomaterials 2002;23:2221—9. 28. Cheng YY, Hui OL, Ladizesky NH. Processing shrinkage of heat-curing acrylic resin reinforced with high-performance polyethylene fibre. Biomaterials 1993;14:775—80. 29. Vallittu PK, Lassila VP, Lappalainen R. Transverse strength and fatigue of denture acrylic-glass fiber composite. Dent Mater 1994;10:116—21. 30. Vallittu PK. Dimensional accuracy and stability of polymethyl methacrylate reinforced with metal wire or continuous glass fiber. J Prosthet Dent 1996;75:617—21. 31. Nitanda J, Wakasa K, Matsui H, Kasahara M, Yamaki M, Matsui A. Dental application of glass cloth to denture: dimensional accuracy in reinforced resins. J Mater Sci Mater Med 1992;3: 137—40.