d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 1273–1279
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Reinforcement of denture base resin with short vegetable fiber Jie Xu a , Yan Li a,∗ , Tao Yu a , Lei Cong b a b
School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai, China School of Dental Medicine, Tongji University, Shanghai, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Short ramie fibers were selected to investigate the effect of fiber length and volume
Received 31 March 2013
fraction on the flexural properties of ramie fiber reinforced denture base PMMA. With the
Received in revised form
aid of measured interfacial shear strength and theoretical prediction values, experimental
19 July 2013
results were well interpreted.
Accepted 25 September 2013
Methods. Interfacial properties between denture base PMMA and ramie fibers were evaluated by single fiber pull-out test. Then, chopped ramie fibers were pre-stirred with PMMA powder by a mechanical blender and then mixed with MMA liquid to fabricate composites. Two
Keywords:
crucial influencing factors, fiber volume fraction and fiber length, were studied to clarify
Denture base PMMA
their effects on flexural properties of composites.
Vegetable fiber
Results. With 1.5 mm fibers addition, flexural modulus of denture base PMMA rose from 2.50
Composite materials
to 3.46 GPa with 10 vol.% fibers, while flexural strength declined steadily with increment of
Interfacial shear strength
fiber content. If fiber length was 3.0 mm, the modulus showed a growth to 3.5 GPa at 4 vol.%
Flexural properties
fiber content followed by a drop to 3.00 GPa at 10 vol.%, whereas fluctuation in strength
Dispersion
was experienced. Experimental results were discussed by comparison with two theoretical models. Significance. Short ramie fiber reinforced denture base PMMA had higher flexural modulus than neat resin, while strength was lowered due to the weak interfacial adhesion. The potential of vegetable fibers as reinforcing agents for denture base should be further investigated by strengthening the interface between cellulose and denture base PMMA. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Reinforcing fibers were introduced to the family of denture base materials, in order to fulfill their requirement on higher mechanical properties against occlusal overload. Among the wide varieties of used reinforcing agents, glass fibers [1–5] and several kinds of polymeric fibers, e.g. ultra-high molecular
weight polyethylene fiber (UHMWPE) [6,7], have attracted more attentions due to their high tensile properties and acceptable esthetic appearance. However, when fiber came out from the surface of the resin as a result of degradation of resin or mechanical failure, clinical problems such as mucosal irritation might happen [8,9]. Thus, not only reinforcing capacity but also clinical applicability of fibers should be considered during selections. As a result of this fact,
∗ Corresponding author at: School of Aerospace Engineering and Applied Mechanics, Tongji University, 1239 Siping Road, Shanghai 200092, China. Tel.: +86 021 65985919; fax: +86 021 65983950. E-mail address: [email protected] (Y. Li). 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.09.013
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d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 1273–1279
Table 1 – Chemical composition of ramie fiber [16]. Component Cellulose Hemicellulose Pectin Lignin Fat/wax
Percentage by mass 67–99 13–14 1.9–2.1 0.5–1.0 0.3
Table 2 – Physical and mechanical properties of ramie fiber. Property Density (g/cm3 ) Tensile strength (MPa) Tensile modulus (GPa) Fractural elongation (%)
Valuea 1.50 310 61.4–128 [17] 3.0
Tensile modulus was referred. From laboratorial tests, averaged in normal distribution.
a
investigations on the potential of alternative reinforcing agents were ongoing. In the range of naturally obtainable fibers, vegetable fiber is one of the promising biomaterials due to its biocompatibility [10–13]. More importantly, they occupy much higher Young’s modulus and ultimate tensile strength than denture base resin. Therefore, these two inherent properties of vegetable fibers make it worthwhile to characterize their potential as new reinforcing agents for denture base. To authors’ best knowledge, Kondo et al. [14] first introduced short sisal fiber to denture base PMMA. Their results showed that with the increment of fiber content, flexural modulus of composites showed a slight growth at 2.5 wt% and then a continuous decline up to 10 wt%, whereas flexural strength decreased apparently and fluctuated versus fiber content. On top of this, they suggested that further studies should regard fiber aspect ratio and surface treatment. Ramie is one of the oldest textile fibers known as “china grass”, which are referred as bast fibers and come from the phloem tissue of the plant [15]. It has significantly smaller diameter (10–60 m) and consequently higher aspect ratio than sisal fiber (200–300 m) at the same length. In addition, its whiteness might also fulfill the requirement of denture base on esthetical appearance. Due to these facts, the present work aimed to investigate the potential of ramie fiber as a reinforcing agent for denture base resin. In order to disperse short ramie fiber homogeneously in high-viscous denture base PMMA, the approach of mechanical stirring was applied during the fabricating stage. Specifically, effects of fiber volume content and length as well as dispersion and interfacial property on flexural properties of composites were detailedly discussed.
2.
Materials and methods
2.1.
Materials
Heat-polymerizing denture base materials (Rapid Simplified® ) were purchased from Vertex Dental, Netherlands and ramie fibers from Hunan province, China. Chemical composition, physical and mechanical properties of ramie fibers were listed in Tables 1 and 2, respectively. Long ramie fibers were pre-impregnated in 5 wt% sodium hydroxide solution, in order to remove wax and other impurities from ramie fiber [18], which was followed by washing and then drying to drive out residual waters. 100 samples of mercerized ramie fibers were randomly selected and their diameters were measured with the aid of an optical microscope.
2.2.
Fabrication of single fiber pull-out specimens
In order to insure the single fiber located centrally throughthickness direction of the resin, two pieces of plastics with rectangular grooves were cut off from the same plate. Afterwards plastic mold and ramie fibers were positioned with double-sided adhesive tape onto a stainless steel plate, then mixed resin were injected into the overlapped groove with a syringe. When the resin reached dough stage, plastic mold was covered with another stainless steel plate and then pressed under 1 MPa and heated at 100 degrees for 20 min, according to manufacturer’s instruction of curing denture base PMMA. After demolding, specimens for single fiber pull-out test were chopped carefully from the groove and 31 samples with matrix in regular shape were obtained out of 40. Finally, diameters of fibers and their embedded lengths in the matrix were measured with an optical microscope.
2.3.
Fabrication of short fiber reinforced composites
Mercerized ramie fibers were carefully chopped with a scissor, desiring to the length of 1.5 and 3.0 mm. 100 samples for both lengths were randomly selected and measured with a vernier caliper. The majority for the former proved to be 1.5 ± 0.1 mm and for the latter 3.0 ± 0.2 mm. Each group was divided into 4 sub-groups of different weight. Then they were mixed with PMMA powder and stirred with a blender at a rotating speed of 400 rpm (Fig. 1). After 5 min, fibers randomly oriented in PMMA powder and MMA was afterwards added to their mixture at a ratio of 0.6 ml/g (liquid: powder). When the resin reached dough stage, it was poured into a mold and hot-pressed under the same pressure and temperature as in Section 2.2. After demolding, no visibly seen bubbles existed in the composites by making the samples following the manufacturer’s (Vertex Dental) instruction. Then, weight fractions of fiber in composites were calculated into volume fractions.
2.4.
Mechanical testing
Single fiber pull-out test was conducted at a crosshead speed of 1 mm/min based on ASTM STP 452. According to ASTM D7264, specimens for flexural test were cut from molded composites to the dimension of 64 mm × 12.7 mm × 3 mm and the crosshead speed was also 1 mm/min.
d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 1273–1279
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Fig. 1 – Schematic procedures of dispersion by mechanical stirring.
2.5.
Calculation
Apparent Interfacial Shear Strength IFSS was calculated from Eq. (1) [19]: IFSS =
Fmax · d · le
(1)
where Fmax is the peak load, d the diameter of fiber and le the embedded length of fiber in the matrix. Critical fiber length lc was then calculated from Eq. (2):
3.
Results
3.1.
Statistics of diameters of ramie fibers
Scattering of ramie fiber diameters was illustrated in Fig. 2.a. It was obvious that most diameters fell in the range from 25 to 45 m and the statistics normally distributed at 31.2 m, which was one of the necessary parameters for later calculation of theoretical flexural modulus.
3.2. f
lc =
2IFSS
(2)
d
where f is the ultimate tensile strength of ramie fiber. Analytical flexural modulus was expressed by Cox’s model (analytical) in Eq. (3): Ecomp = l Vf Ef + (1 − Vf )Em
(3)
where , l and Vf denote fiber orientation factor, which is 0.375 for random-in-plane distribution [20], fiber length factor and volume fraction. Ef and Em are Young’s modulus of fiber and matrix, respectively. Specifically, l was expressed by Eq. (4): l = 1 −
tanh(L/2) L/2
3.3.
1/2 2Em rf2 Ef (1 + m ) ln(1/Vf )
(5)
Semi-empirical flexural modulus was expressed by HalpinTsai’s model (HT) in Eq. (6) [21]: Ec =
3 5 3 1 + 2sL Vf E11 + E22 = Em 8 8 8 1 − L Vf +
5 1 + 2sT Vf Em 8 1 − T Vf
(random-in-plane)
(6)
where s denoted fiber aspect ratio and L , T could be calculated from Eq. (7): L =
Ef /Em − 1 Ef /Em + 2s
,
Typical single fiber pull-out process (Fig. 2.b) should include (1) elastic deformation (intact interfacial bonding), (2) partial debonding (crack was initiated at “debond force” Fd and propagated until complete debonding after “peak force” Fmax ) and (3) sliding friction [22]. In the test, 16 samples out of 31 underwent the similar process and thus were considered valid data. After being calculated, interfacial shear stress averaged 2.35 MPa with the standard deviation of 0.84, which implied an evident scattering of the values. It was observed that the larger the diameters of ramie fibers were, the more apparently IFSS dispersed. However, the effect of ramie fiber diameter on the interfacial shear strength should be further investigated. Employing the strength of ramie fiber (in Table 2), critical fiber length of 2.1 mm was obtained following Eq. (2).
(4)
and could be deducted by Eq. (5):
=
Interfacial shear strength and critical fiber length
T =
Ef /Em − 1 Ef /Em + 2
(7)
Flexural properties of composites
Flexural modulus of composites versus fiber volume fraction with fiber length of 1.5 and 3.0 mm were compared in Fig. 2c. When fiber length was 1.5 mm, flexural modulus increased from 2.50 GPa of neat resin to 3.46 GPa of composites with 10 vol.% fibers. When fiber length was 3.0 mm, the modulus showed a more dramatic growth and peaked at 3.5 GPa with 4 vol.% fiber, which was followed by a drop to 3.00 GPa at the maximal fiber content. Flexural strengths of composites were shown in Fig. 2d. With 1.5 mm fibers, the strength declined gradually from 90.5 MPa of neat resin to 78.7 MPa with 10 vol.% fibers. By contrast, with 3.00 mm fibers, strength of composites experienced a deviant increment versus fiber content.
4.
Discussion
Problems of unsatisfied dispersion of short fiber in denture base PMMA were widely discussed in previous studies [23–27].
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d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 1273–1279
Fig. 2 – (a) Scattering of ramie fiber diameters; (b) a typical force-to-displacement-curve of single fiber pull-out test; (c) flexural modulus and (d) flexural strength of composites vs. fiber volume fraction.
Initially, since the resin is high-viscous at the processing stage, it has great difficulty in permeating into the areas between fibers, which might introduce voids and porosity in composites and consequently reduces its strength in most cases. Furthermore, the resin under pressure can shift the impregnated fibers apart due to even higher viscosity, which could also cause inhomogeneous dispersion of fibers in the matrix [23,24]. These problems will be more apparent especially when concentration of fibers reaches a level of around 10% by mass [25]. Therefore, it was concluded by many researchers that in order to disperse short fibers homogeneously, concentration of added fibers should be limited within a low portion, among which a critical content of 4% by mass was mostly suggested [26,27]. Although low amount of fibers agglomerate hardly and voids in fabricated composites can be reduced to an ideal level as a result of this, from micromechanical aspect of short fiber reinforcement, fiber content contributes positively to their bearing capacity [19]. Therefore, increasing fiber content and achieving optimal dispersion need to be realized at the same time. In this study, experimental flexural modulus was compared with two kinds of theoretical models. All other parameters for calculation were tested, as listed in Table 2. Due to laboratorial
limitation, Young’s modulus of ramie fiber was referred to the literature [17], where a range from 61.4 to 128 GPa was commonly used. When fiber length was 1.5 mm, growth of flexural modulus had a similar trend with Cox’s and Halpin-Tsai’s models (Fig. 3a, overlapped experimental data were pointed by arrows), despite the fact that gentle agglomeration at higher contents slowed the rising rate (Fig. 4a). With fiber in length of 3.0 mm, flexural modulus also fixed well with models at initial stage (Fig. 3b), while it then fell steadily due to the fact that fibers in higher aspect ratio moved more difficultly and thus agglomerated more easily (Fig. 4b). Moreover, considering the theory of “Maximum Packing Fraction” for randomly oriented short fibers, expressed in Eq. (9) [21]: f
Vmax = kd/l
(9)
the maximum fiber volume contents, above which short fibers no longer have any rotational freedom, were 8% and 4% for fiber in length of 1.5 and 3.0 mm, respectively, which also agree well with this study. However, according to enhanced flexural modulus at initial stage, it could be concluded that mechanical pre-stirring
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d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 1273–1279
5000
5000
length=1.5mm HT~Max HT~Min Cox~Max Cox~Min
4500
Flexural modulus (MPa)
Flexural modulus (MPa)
4500
4000
3500
3000
a
4000
3500
3000
2500
2500
0
length=3.0mm HT~Max HT~Min Cox~Max Cox~Min
0
2%
4%
6%
8%
0
10%
b
Volume fraction
0
2%
4%
6%
8%
10%
Volume fraction
Fig. 3 – Comparison of flexural modulus and theoretical models: (a) fiber length = 1.5 mm; (b) fiber length = 3.0 mm.
Fig. 4 – Status of dispersion of 10 vol.% short ramie fiber in denture base PMMA: (a) fiber length = 1.5 mm; (b) fiber length = 3.0 mm.
1.0
1.0
0.8
0.8
0.6
0.4
E=128 GPa f Vf=0.103 Vf=0.066 Vf=0.037 Vf=0.012
0.2
0.0 0.0
a
factor l in Fig. 5. Apparently, with increment of fiber length up to 1.5 mm, its factor increased exponentially to 0.8. After fiber length exceeded 3.0 mm, it plateaued above 0.9, which was usually considered as a sufficient value for modulus of
Fiber length factor
Fiber length factor
of PMMA powder with short ramie fibers had a positive contribution to their dispersion, which did not introduce any other material and operated easily within a few minutes. Effect of fiber length on modulus was represented by its corresponding
1.5
3.0
Length of fibers (mm)
0.6
0.4
E=61.4 GPa f Vf=0.103 Vf=0.066 Vf=0.037 Vf=0.012
0.2
4.5
b
0.0 0.0
1.5
3.0
Length of fibers (mm)
Fig. 5 – Fiber length factor l vs. increment of fiber length: (a) Ef = 128 GPa; (b) Ef = 61.4 GPa.
4.5
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d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 1273–1279
The effect of ramie fiber on PMMA is quantitatively comparable with that of glass fiber, when both fibers are short and randomly distributed. For instance, as reported in [3], flexural modulus of PMMA was enhanced by 48%, and flexural strength by 31% with 6 mm, 5% E-glass fiber addition. Therefore, by comparison with the reinforcing effects of ramie fibers, glass fibers showed better effect on strength, but similar effect on modulus. The difference of reinforcing effects of ramie and glass fiber mainly lay in their inherent properties. Glass fiber occupies higher tensile strength, reaching 2000–3500 MPa [17], which is three to six times that of ramie fiber. Further research could be devoted to find suitable surface modification for ramie fiber, to increase the efficiency of stress transfer in the composites. However, the biocompatibility and environmental friendly advantages of ramie fibers were much superior to glass fibers. Therefore, there should be some potential using of ramie fibers in the dental applications. Technically, ramie fiber is usually offered in long form and extra work is needed to cut it into the desired length. Therefore, finding an efficient way to make short ramie fibers is important for the bulk production.
5.
Fig. 6 – (a) Fractography of single fiber pull-out specimen: (a) schematic construction of outer layer and inner part of ramie fiber.
short fiber composites. In other words, ramie fiber in length of 3.0 mm should be a reasonable choice for denture base PMMA. Other than flexural modulus, flexural strength is sensitive to the interfacial adhesion. Single fiber pull-out is one of the most commonly used micromechanical tests for investigating interfacial properties and further calculation of critical fiber length lc [19]. Although lc is an essential factor for design of short fiber reinforced composites, assumption of good interfacial adhesion and uniform dispersion should not be neglected. For the reason that the environment in which individual fiber located in a real composite varies from that in a single fiber pull-out specimen, especially at the content when interaction between fibers lowers the quality of wetting. Thus, exceeding the critical fiber length does not simply guarantee the enhancement of ultimate strength, when dispersion becomes an issue, which was also reflected in this study. With 3.0 mm fibers, exceeding critical length of 2.1 mm, the composites had lower strengths than expected values, which revealed that the interface was not good. Supportingly, the weakness of interface was confirmed by Fig. 6a, which pictured the peeling of outer layer from ramie fiber after single fiber pull-out test. It revealed the bonding between the outer layer and inner part of ramie fiber was deconstructed by the shear stress along their interface I2 (Fig. 6b). So after I2 began to fail, the stress from the matrix could not be sufficiently transferred to the inner part of fiber. Thus, the ultimate strength of ramie fiber was not utilized to enhance the strength of composites.
Conclusion
For 1.5 mm ramie fibers reinforced denture base PMMA, due to good dispersion, flexural modulus of composites increased considerably with fiber content approaching 10 vol.%. Although adding 3.0 mm fibers increased the flexural modulus more rapidly up to 4 vol.%, they reduced the modulus at higher contents as a result of apparent agglomeration. Flexural strength of short ramie fiber denture base PMMA declined due to the weak interfacial adhesion. From both experimental and theoretical studies, the critical fiber length for ramie fiber reinforced denture base PMMA was 2.1 mm. With better interfacial bonding, higher flexural strength should be achieved.
references
[1] Nitanda J, Matsui H, Matsui A, Kasahara Y, Wakasa K, Yamaki M. Denture base materials reinforced with glass fibres. Part 1. The application of industrial glass fibres distributed at random. Journal of Materials Science 1987;22:1875–8. [2] Nitanda J, Matsui H, Matsui A, Kasahara Y, Wakasa K, Yamaki M. Denture base materials reinforced with glass fibres. Part 2. The effect of glass fibre contents on bending properties. Journal of Materials Science 1987;22:1879–81. [3] Karacaer Ö, Polat TN, Tezvergil A, Lassila LVJ, 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. Journal of Prosthetic Dentistry 2003;90:385–93. [4] C¸ökeliler D, Erkut S, Zemek J, Biederman H, Mutlu M. Modification of glass fibers to improve reinforcement: a plasma polymerization technique. Dental Materials 2007;23:335–42. [5] Nakamura M, Takahashi H, Hayakawa I. Reinforcement of denture base resin with short-rod glass fiber. Dental Materials Journal 2007;26:733–8. [6] Taner B, Dogan A, Tinc¸er T, Akinay AE. A study on impact and tensile and strength of acrylic resin filled with short
d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 1273–1279
[7]
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[11]
[12]
[13]
[14]
[15]
ultra-high molecular weight polyethylene fibres. Journal of Oral Science 1999;41:15–8. Xu DX, Cheng XR, Zhang YF, Wang J, Cheng HT. The flexural behaviour of denture base reinforced by different contents of ultrahigh-modulus polyethylene fiber. Journal of Wuhan University of Technology (Materials Science Edition) 2003;18(3). Hao F, Terry JL, Steven AA, Robert LS, Derrick LW, Daniel BB. Effect of polyaramid fiber reinforcement on the strength of 3 denture base polymethyl methacrylate resins. Journal of Prosthodontics 2001;10:148–53. Gulsen B, Ozlem D, Canan B, Bora G. Effects of water storage of E-glass fiber reinforced denture base polymers on residual methyl methacrylate content. Journal of Biomedical Materials Research Part B: Applied Biomaterials 2004;70B:161–6. Cheung HY, Ho MP, Lau KY, Cardona F, Hui D. Natural fibre-reinforced composites for bioengineering and environmental engineering applications. Composites Part B 2009;40:655–63. Yussuf AA, Massoumi A, Hassan A. Comparison of polylactic acid/kenaf and polylactic acid/rise husk composites: the influence of the natural fibers on the mechanical, thermal and biodegradability properties. Journal of Polymers and the Environment 2010;18:422–9. Keil CK, Gredes T, Meyer A, Kwiatkowska MW, Dominiak M, Gedrange T. The survival and proliferation of fibroblasts on biocomposites containing genetically modified flax fibers: an in vitro study. Annals of Anatomy 2012;194:513–7. Bashar MM, Khan MA. An overview on surface modification of cotton fiber for apparel use. Journal of Polymers and the Environment 2013;21:181–90. Kondo S, Nodasaka Y, Shimokoube H. Flexural strength of PMMA/MMA-based material reinforced with sisal. In: IADR 86th general session & exhibition, Toronto. 2008. http://www.oriamiknits.com/blog/?p=287
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[16] Wurl and Vetter, Faserinstitut Bremen, 1994. [17] Bledzki AK, Gassan J. Composites reinforced with cellulose base fibres. Progress in Polymer Science 1999;24:221–74. [18] Goda K, Sreekala MS, Gomes A, Kaji T, Ohgi J. Improvement of plant based natural fibers for toughening green composites – effect of load application during mercerization of ramie fibers. Composites Part A 2006;37:2213–20. [19] Hull D. An introduction to composite materials. Cambridge University Press; 1982. [20] Lu YK. Mechanical properties of random discontinuous fiber composites manufactured from wetlag process. Dissertation. Virginia Polytechnic Institute and State University; 2002. [21] Fu SY, Lauke B, Mai YW. Science and engineering of short fibre reinforced polymer composites. Woodhead Publishing Limited; 2009. [22] Pristavok J. Mikromechanische Untersuchungen an Epoxidharz-Glasfaser-Verbundwerkstoffen unter zyklischer Wechselbelastung. Dissertation. Technische Universität Dresden; 2006. [23] Vallittu PK, Lassila VP, Lappalainen R. Transverse strength and fatigue of denture acrylic-glass fiber composite. Dental Materials 1994;10:116–21. [24] Vallittu PK. The effect of void space and polymerization time on transverse strength of acrylic-glass fibre composite. Journal of Oral Rehabilitation 1995;22:257–61. [25] Stipho HD. Effect of glass fiber reinforcement on some mechanical properties of autopolymerizing polymethyl methacrylate. Journal of Prosthetic Dentistry 1998;79:580–4. [26] Gutteridge DL. The effect of including ultra-high-modulus polyethylene fibre on the impact strength of acrylic resin. British Dental Journal 1998;19:177–80. [27] Clarke DA, Ladizesky NH, Chow TW. Acrylic resins reinforced with highly drawn linear polyethylene woven fibres. 1. Construction of upper denture bases. Australian Dental Journal 1992;37:394–9.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Reinforcement of denture base resin with short vegetable fiber Jie Xu a , Yan Li a,∗ , Tao Yu a , Lei Cong b a b
School of Aerospace Engineering and Applied Mechanics, Tongji University, Shanghai, China School of Dental Medicine, Tongji University, Shanghai, China
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Short ramie fibers were selected to investigate the effect of fiber length and volume
Received 31 March 2013
fraction on the flexural properties of ramie fiber reinforced denture base PMMA. With the
Received in revised form
aid of measured interfacial shear strength and theoretical prediction values, experimental
19 July 2013
results were well interpreted.
Accepted 25 September 2013
Methods. Interfacial properties between denture base PMMA and ramie fibers were evaluated by single fiber pull-out test. Then, chopped ramie fibers were pre-stirred with PMMA powder by a mechanical blender and then mixed with MMA liquid to fabricate composites. Two
Keywords:
crucial influencing factors, fiber volume fraction and fiber length, were studied to clarify
Denture base PMMA
their effects on flexural properties of composites.
Vegetable fiber
Results. With 1.5 mm fibers addition, flexural modulus of denture base PMMA rose from 2.50
Composite materials
to 3.46 GPa with 10 vol.% fibers, while flexural strength declined steadily with increment of
Interfacial shear strength
fiber content. If fiber length was 3.0 mm, the modulus showed a growth to 3.5 GPa at 4 vol.%
Flexural properties
fiber content followed by a drop to 3.00 GPa at 10 vol.%, whereas fluctuation in strength
Dispersion
was experienced. Experimental results were discussed by comparison with two theoretical models. Significance. Short ramie fiber reinforced denture base PMMA had higher flexural modulus than neat resin, while strength was lowered due to the weak interfacial adhesion. The potential of vegetable fibers as reinforcing agents for denture base should be further investigated by strengthening the interface between cellulose and denture base PMMA. © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Reinforcing fibers were introduced to the family of denture base materials, in order to fulfill their requirement on higher mechanical properties against occlusal overload. Among the wide varieties of used reinforcing agents, glass fibers [1–5] and several kinds of polymeric fibers, e.g. ultra-high molecular
weight polyethylene fiber (UHMWPE) [6,7], have attracted more attentions due to their high tensile properties and acceptable esthetic appearance. However, when fiber came out from the surface of the resin as a result of degradation of resin or mechanical failure, clinical problems such as mucosal irritation might happen [8,9]. Thus, not only reinforcing capacity but also clinical applicability of fibers should be considered during selections. As a result of this fact,
∗ Corresponding author at: School of Aerospace Engineering and Applied Mechanics, Tongji University, 1239 Siping Road, Shanghai 200092, China. Tel.: +86 021 65985919; fax: +86 021 65983950. E-mail address: [email protected] (Y. Li). 0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.dental.2013.09.013
1274
d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 1273–1279
Table 1 – Chemical composition of ramie fiber [16]. Component Cellulose Hemicellulose Pectin Lignin Fat/wax
Percentage by mass 67–99 13–14 1.9–2.1 0.5–1.0 0.3
Table 2 – Physical and mechanical properties of ramie fiber. Property Density (g/cm3 ) Tensile strength (MPa) Tensile modulus (GPa) Fractural elongation (%)
Valuea 1.50 310 61.4–128 [17] 3.0
Tensile modulus was referred. From laboratorial tests, averaged in normal distribution.
a
investigations on the potential of alternative reinforcing agents were ongoing. In the range of naturally obtainable fibers, vegetable fiber is one of the promising biomaterials due to its biocompatibility [10–13]. More importantly, they occupy much higher Young’s modulus and ultimate tensile strength than denture base resin. Therefore, these two inherent properties of vegetable fibers make it worthwhile to characterize their potential as new reinforcing agents for denture base. To authors’ best knowledge, Kondo et al. [14] first introduced short sisal fiber to denture base PMMA. Their results showed that with the increment of fiber content, flexural modulus of composites showed a slight growth at 2.5 wt% and then a continuous decline up to 10 wt%, whereas flexural strength decreased apparently and fluctuated versus fiber content. On top of this, they suggested that further studies should regard fiber aspect ratio and surface treatment. Ramie is one of the oldest textile fibers known as “china grass”, which are referred as bast fibers and come from the phloem tissue of the plant [15]. It has significantly smaller diameter (10–60 m) and consequently higher aspect ratio than sisal fiber (200–300 m) at the same length. In addition, its whiteness might also fulfill the requirement of denture base on esthetical appearance. Due to these facts, the present work aimed to investigate the potential of ramie fiber as a reinforcing agent for denture base resin. In order to disperse short ramie fiber homogeneously in high-viscous denture base PMMA, the approach of mechanical stirring was applied during the fabricating stage. Specifically, effects of fiber volume content and length as well as dispersion and interfacial property on flexural properties of composites were detailedly discussed.
2.
Materials and methods
2.1.
Materials
Heat-polymerizing denture base materials (Rapid Simplified® ) were purchased from Vertex Dental, Netherlands and ramie fibers from Hunan province, China. Chemical composition, physical and mechanical properties of ramie fibers were listed in Tables 1 and 2, respectively. Long ramie fibers were pre-impregnated in 5 wt% sodium hydroxide solution, in order to remove wax and other impurities from ramie fiber [18], which was followed by washing and then drying to drive out residual waters. 100 samples of mercerized ramie fibers were randomly selected and their diameters were measured with the aid of an optical microscope.
2.2.
Fabrication of single fiber pull-out specimens
In order to insure the single fiber located centrally throughthickness direction of the resin, two pieces of plastics with rectangular grooves were cut off from the same plate. Afterwards plastic mold and ramie fibers were positioned with double-sided adhesive tape onto a stainless steel plate, then mixed resin were injected into the overlapped groove with a syringe. When the resin reached dough stage, plastic mold was covered with another stainless steel plate and then pressed under 1 MPa and heated at 100 degrees for 20 min, according to manufacturer’s instruction of curing denture base PMMA. After demolding, specimens for single fiber pull-out test were chopped carefully from the groove and 31 samples with matrix in regular shape were obtained out of 40. Finally, diameters of fibers and their embedded lengths in the matrix were measured with an optical microscope.
2.3.
Fabrication of short fiber reinforced composites
Mercerized ramie fibers were carefully chopped with a scissor, desiring to the length of 1.5 and 3.0 mm. 100 samples for both lengths were randomly selected and measured with a vernier caliper. The majority for the former proved to be 1.5 ± 0.1 mm and for the latter 3.0 ± 0.2 mm. Each group was divided into 4 sub-groups of different weight. Then they were mixed with PMMA powder and stirred with a blender at a rotating speed of 400 rpm (Fig. 1). After 5 min, fibers randomly oriented in PMMA powder and MMA was afterwards added to their mixture at a ratio of 0.6 ml/g (liquid: powder). When the resin reached dough stage, it was poured into a mold and hot-pressed under the same pressure and temperature as in Section 2.2. After demolding, no visibly seen bubbles existed in the composites by making the samples following the manufacturer’s (Vertex Dental) instruction. Then, weight fractions of fiber in composites were calculated into volume fractions.
2.4.
Mechanical testing
Single fiber pull-out test was conducted at a crosshead speed of 1 mm/min based on ASTM STP 452. According to ASTM D7264, specimens for flexural test were cut from molded composites to the dimension of 64 mm × 12.7 mm × 3 mm and the crosshead speed was also 1 mm/min.
d e n t a l m a t e r i a l s 2 9 ( 2 0 1 3 ) 1273–1279
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Fig. 1 – Schematic procedures of dispersion by mechanical stirring.
2.5.
Calculation
Apparent Interfacial Shear Strength IFSS was calculated from Eq. (1) [19]: IFSS =
Fmax · d · le
(1)
where Fmax is the peak load, d the diameter of fiber and le the embedded length of fiber in the matrix. Critical fiber length lc was then calculated from Eq. (2):
3.
Results
3.1.
Statistics of diameters of ramie fibers
Scattering of ramie fiber diameters was illustrated in Fig. 2.a. It was obvious that most diameters fell in the range from 25 to 45 m and the statistics normally distributed at 31.2 m, which was one of the necessary parameters for later calculation of theoretical flexural modulus.
3.2. f
lc =
2IFSS
(2)
d
where f is the ultimate tensile strength of ramie fiber. Analytical flexural modulus was expressed by Cox’s model (analytical) in Eq. (3): Ecomp = l Vf Ef + (1 − Vf )Em
(3)
where , l and Vf denote fiber orientation factor, which is 0.375 for random-in-plane distribution [20], fiber length factor and volume fraction. Ef and Em are Young’s modulus of fiber and matrix, respectively. Specifically, l was expressed by Eq. (4): l = 1 −
tanh(L/2) L/2
3.3.
1/2 2Em rf2 Ef (1 + m ) ln(1/Vf )
(5)
Semi-empirical flexural modulus was expressed by HalpinTsai’s model (HT) in Eq. (6) [21]: Ec =
3 5 3 1 + 2sL Vf E11 + E22 = Em 8 8 8 1 − L Vf +
5 1 + 2sT Vf Em 8 1 − T Vf
(random-in-plane)
(6)
where s denoted fiber aspect ratio and L , T could be calculated from Eq. (7): L =
Ef /Em − 1 Ef /Em + 2s
,
Typical single fiber pull-out process (Fig. 2.b) should include (1) elastic deformation (intact interfacial bonding), (2) partial debonding (crack was initiated at “debond force” Fd and propagated until complete debonding after “peak force” Fmax ) and (3) sliding friction [22]. In the test, 16 samples out of 31 underwent the similar process and thus were considered valid data. After being calculated, interfacial shear stress averaged 2.35 MPa with the standard deviation of 0.84, which implied an evident scattering of the values. It was observed that the larger the diameters of ramie fibers were, the more apparently IFSS dispersed. However, the effect of ramie fiber diameter on the interfacial shear strength should be further investigated. Employing the strength of ramie fiber (in Table 2), critical fiber length of 2.1 mm was obtained following Eq. (2).
(4)
and could be deducted by Eq. (5):
=
Interfacial shear strength and critical fiber length
T =
Ef /Em − 1 Ef /Em + 2
(7)
Flexural properties of composites
Flexural modulus of composites versus fiber volume fraction with fiber length of 1.5 and 3.0 mm were compared in Fig. 2c. When fiber length was 1.5 mm, flexural modulus increased from 2.50 GPa of neat resin to 3.46 GPa of composites with 10 vol.% fibers. When fiber length was 3.0 mm, the modulus showed a more dramatic growth and peaked at 3.5 GPa with 4 vol.% fiber, which was followed by a drop to 3.00 GPa at the maximal fiber content. Flexural strengths of composites were shown in Fig. 2d. With 1.5 mm fibers, the strength declined gradually from 90.5 MPa of neat resin to 78.7 MPa with 10 vol.% fibers. By contrast, with 3.00 mm fibers, strength of composites experienced a deviant increment versus fiber content.
4.
Discussion
Problems of unsatisfied dispersion of short fiber in denture base PMMA were widely discussed in previous studies [23–27].
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Fig. 2 – (a) Scattering of ramie fiber diameters; (b) a typical force-to-displacement-curve of single fiber pull-out test; (c) flexural modulus and (d) flexural strength of composites vs. fiber volume fraction.
Initially, since the resin is high-viscous at the processing stage, it has great difficulty in permeating into the areas between fibers, which might introduce voids and porosity in composites and consequently reduces its strength in most cases. Furthermore, the resin under pressure can shift the impregnated fibers apart due to even higher viscosity, which could also cause inhomogeneous dispersion of fibers in the matrix [23,24]. These problems will be more apparent especially when concentration of fibers reaches a level of around 10% by mass [25]. Therefore, it was concluded by many researchers that in order to disperse short fibers homogeneously, concentration of added fibers should be limited within a low portion, among which a critical content of 4% by mass was mostly suggested [26,27]. Although low amount of fibers agglomerate hardly and voids in fabricated composites can be reduced to an ideal level as a result of this, from micromechanical aspect of short fiber reinforcement, fiber content contributes positively to their bearing capacity [19]. Therefore, increasing fiber content and achieving optimal dispersion need to be realized at the same time. In this study, experimental flexural modulus was compared with two kinds of theoretical models. All other parameters for calculation were tested, as listed in Table 2. Due to laboratorial
limitation, Young’s modulus of ramie fiber was referred to the literature [17], where a range from 61.4 to 128 GPa was commonly used. When fiber length was 1.5 mm, growth of flexural modulus had a similar trend with Cox’s and Halpin-Tsai’s models (Fig. 3a, overlapped experimental data were pointed by arrows), despite the fact that gentle agglomeration at higher contents slowed the rising rate (Fig. 4a). With fiber in length of 3.0 mm, flexural modulus also fixed well with models at initial stage (Fig. 3b), while it then fell steadily due to the fact that fibers in higher aspect ratio moved more difficultly and thus agglomerated more easily (Fig. 4b). Moreover, considering the theory of “Maximum Packing Fraction” for randomly oriented short fibers, expressed in Eq. (9) [21]: f
Vmax = kd/l
(9)
the maximum fiber volume contents, above which short fibers no longer have any rotational freedom, were 8% and 4% for fiber in length of 1.5 and 3.0 mm, respectively, which also agree well with this study. However, according to enhanced flexural modulus at initial stage, it could be concluded that mechanical pre-stirring
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5000
5000
length=1.5mm HT~Max HT~Min Cox~Max Cox~Min
4500
Flexural modulus (MPa)
Flexural modulus (MPa)
4500
4000
3500
3000
a
4000
3500
3000
2500
2500
0
length=3.0mm HT~Max HT~Min Cox~Max Cox~Min
0
2%
4%
6%
8%
0
10%
b
Volume fraction
0
2%
4%
6%
8%
10%
Volume fraction
Fig. 3 – Comparison of flexural modulus and theoretical models: (a) fiber length = 1.5 mm; (b) fiber length = 3.0 mm.
Fig. 4 – Status of dispersion of 10 vol.% short ramie fiber in denture base PMMA: (a) fiber length = 1.5 mm; (b) fiber length = 3.0 mm.
1.0
1.0
0.8
0.8
0.6
0.4
E=128 GPa f Vf=0.103 Vf=0.066 Vf=0.037 Vf=0.012
0.2
0.0 0.0
a
factor l in Fig. 5. Apparently, with increment of fiber length up to 1.5 mm, its factor increased exponentially to 0.8. After fiber length exceeded 3.0 mm, it plateaued above 0.9, which was usually considered as a sufficient value for modulus of
Fiber length factor
Fiber length factor
of PMMA powder with short ramie fibers had a positive contribution to their dispersion, which did not introduce any other material and operated easily within a few minutes. Effect of fiber length on modulus was represented by its corresponding
1.5
3.0
Length of fibers (mm)
0.6
0.4
E=61.4 GPa f Vf=0.103 Vf=0.066 Vf=0.037 Vf=0.012
0.2
4.5
b
0.0 0.0
1.5
3.0
Length of fibers (mm)
Fig. 5 – Fiber length factor l vs. increment of fiber length: (a) Ef = 128 GPa; (b) Ef = 61.4 GPa.
4.5
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The effect of ramie fiber on PMMA is quantitatively comparable with that of glass fiber, when both fibers are short and randomly distributed. For instance, as reported in [3], flexural modulus of PMMA was enhanced by 48%, and flexural strength by 31% with 6 mm, 5% E-glass fiber addition. Therefore, by comparison with the reinforcing effects of ramie fibers, glass fibers showed better effect on strength, but similar effect on modulus. The difference of reinforcing effects of ramie and glass fiber mainly lay in their inherent properties. Glass fiber occupies higher tensile strength, reaching 2000–3500 MPa [17], which is three to six times that of ramie fiber. Further research could be devoted to find suitable surface modification for ramie fiber, to increase the efficiency of stress transfer in the composites. However, the biocompatibility and environmental friendly advantages of ramie fibers were much superior to glass fibers. Therefore, there should be some potential using of ramie fibers in the dental applications. Technically, ramie fiber is usually offered in long form and extra work is needed to cut it into the desired length. Therefore, finding an efficient way to make short ramie fibers is important for the bulk production.
5.
Fig. 6 – (a) Fractography of single fiber pull-out specimen: (a) schematic construction of outer layer and inner part of ramie fiber.
short fiber composites. In other words, ramie fiber in length of 3.0 mm should be a reasonable choice for denture base PMMA. Other than flexural modulus, flexural strength is sensitive to the interfacial adhesion. Single fiber pull-out is one of the most commonly used micromechanical tests for investigating interfacial properties and further calculation of critical fiber length lc [19]. Although lc is an essential factor for design of short fiber reinforced composites, assumption of good interfacial adhesion and uniform dispersion should not be neglected. For the reason that the environment in which individual fiber located in a real composite varies from that in a single fiber pull-out specimen, especially at the content when interaction between fibers lowers the quality of wetting. Thus, exceeding the critical fiber length does not simply guarantee the enhancement of ultimate strength, when dispersion becomes an issue, which was also reflected in this study. With 3.0 mm fibers, exceeding critical length of 2.1 mm, the composites had lower strengths than expected values, which revealed that the interface was not good. Supportingly, the weakness of interface was confirmed by Fig. 6a, which pictured the peeling of outer layer from ramie fiber after single fiber pull-out test. It revealed the bonding between the outer layer and inner part of ramie fiber was deconstructed by the shear stress along their interface I2 (Fig. 6b). So after I2 began to fail, the stress from the matrix could not be sufficiently transferred to the inner part of fiber. Thus, the ultimate strength of ramie fiber was not utilized to enhance the strength of composites.
Conclusion
For 1.5 mm ramie fibers reinforced denture base PMMA, due to good dispersion, flexural modulus of composites increased considerably with fiber content approaching 10 vol.%. Although adding 3.0 mm fibers increased the flexural modulus more rapidly up to 4 vol.%, they reduced the modulus at higher contents as a result of apparent agglomeration. Flexural strength of short ramie fiber denture base PMMA declined due to the weak interfacial adhesion. From both experimental and theoretical studies, the critical fiber length for ramie fiber reinforced denture base PMMA was 2.1 mm. With better interfacial bonding, higher flexural strength should be achieved.
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