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Reactive fibre reinforced glass ionomer cements
Biomaterials 24 (2003) 2901–2907
Reactive fibre reinforced glass ionomer cements Ulrich Lohbauera,*, Jurgen . Walkerb,c, Sergej Nikolaenkoa, Jan Wernerc, Alexis Clareb, Anselm Petschelta, Peter Greilc a
Policlinic for Operative Dentistry and Periodontology, University of Erlangen-Nuremberg, Glueckstrasse 11, Erlangen 91054, Germany b New York State College of Ceramics, Alfred University, One Saxon Drive, Alfred, NY 14802, USA c Department of Materials Science and Engineering (Glass and Ceramics), University of Erlangen-Nuremberg, Martensstrasse 5, Erlangen 91058, Germany Received 14 October 2002; accepted 20 February 2003
Abstract The mechanical properties of glass ionomer cements used in restorative dentistry reinforced by chopped glass fibres were investigated. Reactive glass fibres with a composition in the system SiO2–Al2O3–CaF2–Na3AlF6 and a thickness of 26 mm were drawn by a bushing process. The manufacturing parameters were optimized with respect to maximum strength of the glass fibre reinforced ionomer cements. Powder to liquid ratio, pre-treatment of the glass, grain size distribution and fibre volume fraction were varied. Glass fibre and cement were characterized by X-ray diffraction, transmission electron microscopy and energy dispersive spectroscopy techniques, respectively. The highest flexural strength of the reinforced cement (15.6 MPa) was found by compounding 20 vol% reactive fibres and extending the initial dry gelation period up to 30 min. Microscopic examination of the fractured cements indicated a distinct reactive layer at the fibre surface. A pronounced fibre pull out mode gives rise to an additional work-of-fracture contributed by pulling the fibres out of the fracture surface. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Glass ionomer cement; Glass fibres; Reinforcement; Fibre drawing; Mechanical strength
1. Introduction In the late 1970s the glass ionomer cements (GICs) were developed as an outgrowth from the research into dental silicate cements and zinc polycarboxylate cements [1,2]. They consist of a sodium aluminium calcium (or strontium) silicate glass powder and an aqueous polyacrylic or related polymeric acid [3]. The single compounds react as soon as they are mixed together. The polymeric acid attacks the glass and leaches out mainly calcium and aluminium ions. The metal ions in turn catalyse cross linking the polymer chains and cause the cement to set. GICs set with the formation of a siliceous hydrogel as a result of this acid–base reaction [4,5]. During a first phase after mixing a dry storage of the material has to be ensured in order to provide the gelation process from water dilution. After that initial *Corresponding author. Tel.: +49-9131-853-4236; fax: +49-9131853-3603. E-mail address: [email protected] (U. Lohbauer).
dry stage, the specimens have to be stored in water to impede dehydration [1,2]. GIC has several unique advantages among restorative materials. The content of fluorine plays an important role [6–8]. Fluorine disrupts the glass network and lowers the fusion temperature of the glass melt, increases the mechanical strength of the set cement and increases the susceptibility of the glass to acid attack. It allows the matching of the refractive index to that of the tooth for the translucency of the cement. The presence of fluorine as a crystalline phase in the glass has been observed by many authors [9,10]. They reported the presence of a second droplet phase that is rich in calcium and fluoride and assumed the existence of CaF2 crystals in the separated phase [11]. However, the main advantage is a constant fluoride release during the lifetime of the restoration which makes it cariostatic i.e. prevent from secondary caries. Many studies have shown that the fluoride release of the cements can remineralize tooth tissue. This allows a minimal invasive cavity preparation technique, preserving more sound tissue [1,12,13].
0142-9612/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0142-9612(03)00130-3
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Compared to other restorative materials like amalgam or polymer composites, however, GICs suffer from lower strength, wear resistance and fracture toughness [3,14–16]. Reinforcement of GIC has therefore become a matter of increasing research efforts. In former experiments various alumina, zirconia, silicon carbide, carbon fibres, or metal particles were used with encouraging results [2]. Fibre length and volume fraction are thereby key microstructural parameters determining the resulting cement properties [17]. A strengthening effect, due to fibre pull-out or crack bridging during fracture process is mainly influenced by the matrix–fibre interface coupling [18]. Fibre reinforcement with reactive glass fibres is one attempt to achieve proper fibre embedding in the matrix cement. Short glass fibre reinforcement was studied by Kawano et al. [19] and Kobayashi et al. [20] with a glass composition in the system CaO–P2O5– SiO2–Al2O3. They used phosphate glasses, since improved self-adhesion to human dentine is described [21]. The aim of the present work is to incorporate high strength reactive glass fibres into the GIC matrix. Depending on the interfacial coupling of fibre-to-matrix, theoretical models predict a significant increase of the work-of-fracture when continuous fibres are aligned with the primary loading direction so that effective load transfer from the matrix to the fibre can be induced. In the case of short fibres dispersed with random orientation in the matrix the situation is more complex [22,23]. Fibres below a critical length (which is determined by the ratio of the fibre strength to the shear strength of the matrix–fibre interface) are less effective in reinforcement whereas using longer fibres nothing is gained by the extra length [22,24]. For the system under investigation a critical fibre length of approximately 550 mm was estimated from fibre and matrix properties. In this investigation, a model glass frit of the system SiO2– Al2O3–CaF2–Na3AlF6 was used to prepare glass fibres by a bushing process. The study deals with fluoride glasses, since an increased fluorine content was shown to result in a rise of strength [25]. The influence of different processing conditions on the mechanical strength of the fibre reinforced glass ionomer cement (FRGIC) was analysed.
2. Experimental procedure
powders were measured using light scattering with a laser (Cilass 715, Cilas Corp., Marcoussis, France), and the surface area was determined by standard nitrogen adsorption method (BET, ASAPs 2000 Nitrogen, Micromeritics Corp., Norcross, USA). The crystalline phase composition was determined by X-ray diffraction using monochromated CuKa radiation (D 5000s, Siemens Corp., Mannheim, Germany). The microstructure of the quenched glass and the fibre was examined in an analytical transmission electron microscope (Philips CM 30 T, Philips, USA) and by energy dispersive spectroscopy (EDS). The TEM work was performed using 300 KV acceleration voltage. The specimens were prepared by focused ion beam (FIB) technique. Thirty grams of the glass frit were melted in a platinum crucible with a hole in the bottom. The glass melt flowing through the hole with a diameter of 1.5 mm was pulled into a fibre and, after rapid air coolingwound onto a rotating drum with a diameter of 150 mm. The rate of mass flow of the glass melt out of the bushing is dependent on geometrical factors of the bushing and the viscosity of the glass [26,27]. The two parameters that were varied to adjust an optimum thickness of the chopped fibre were temperature and drawing speed. Finally a temperature of 1120 C and a drawing speed of 3 m/s were selected. The glass frit was heated up to 1300 C before drawing and maintained there for 1 h in order to homogenize the melt. The drawn glass fibre was cut into short fibres by a rotating guillotine. The critical fibre length 2xc ; which is supposed to provide maximum load transfer by means of elastic strain and frictional pull out, was calculated by [22] 2xc ¼
d sf ; 4 sm
ð1Þ
where d is the fibre diameter, sf the fibre strength and sm the shear strength at the matrix–fibre interface. Ashby et al. reported a correlation between interface shear strength and matrix yield strength: smðShearÞ E1=2smðyieldÞ [22]. The strength smðyieldÞ was derived from the flexural matrix strength of the used base cement system. The tensile strength of the fibres was measured in an universal testing machine (Z 2.5, Zwick, Germany) according to ENV 1007-4 standard. The fibre length and diameter distributions were measured using light microscopy, Fig. 2.
2.1. Glass fibres 2.2. Glass ionomer cements The following glass composition was selected for fibre processing: SiO2:33.3, Al2O3:16.7, CaO:14, NaF:3.3, AlF3:3.3, Na3AlF6:16.2 [wt%]. The oxide powder mixture was melted at 1400 C for 1 h and subsequently quenched in water of room temperature. The glass frit was milled to obtain different grain sizes of 6.8 and 2.4 mm, respectively. The grain size distributions of the
The GIC matrices were prepared by mixing a glass powder with an aqueous polycarbonic acid. The mean particle size of the glass powders are listed in Table 1. The milled glass powder was treated in various ways to reduce surface reactivity and to increase working time [28,29]. A combination of acid treatment and annealing
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Table 1 Test specifications and results of the cement optimizing tests Glass charge
Grain size [mm] (d10 =d50 =d90 )
G1 G2 G3
1.7/12.2/46.0 1.7/12.2/46.0 1.6/6.8/28.7
2.0 2.0 4.1
G4
1.6/6.8/28.7
4.1
G5 G6 G7
n.m. n.m. 1.6/6.8/28.7
n.m. n.m. 4.1
G8 G9
1.2/3.0/21.1 0.9/2.4/7.3
6.1 10.4
Glass charge
Fibre fraction [vol.%] 20 40 60
BET surface area [m2/g]a 1.624 1.249 0.873
F1 F2 F3 a
BET surface area [m2/g]
Acid washing/heat treatment
Working time [sec]
Powder/liquid ratio
Compressive strength [MPa] (S.D.)
Non Non 7 h, 3% HCl/6 h 360 C 7 h, 3% HCl/6 h 360 C 5 h, 360 C 5 h, 3% HCl 7 h, 3% HCl/6 h 360 C Non 7 h, 3% HCl/ 6 h 360 C
n.m. 45 268
2.0 2.8 2.0
112 (34) 64 (24) 70 (4)
268
2.8
96 (3)
49 111 268
2.8 2.8 2.8
n.m. n.m. n.m.
n.m. n.m.
1.8 1.5
138 (31) 170 (18)
Base glass
Working time [sec]
G4 G4 G4
n.m. n.m. n.m.
Powder/liquid ratio 4 5.4 5.4
Compressive strength [MPa] (S.D.) 134 (4) 98 (4) 82 (6)
Calculated cylindrical fibre surface area: 0.1225 m2/g.
procedure was applied, as shown in Table 1 (G5–G7). The optimum powder to liquid ratio was found for the processing conditions which finally resulted in the GIC with the highest compressive strength, Table 1 (G1–G4). The medium grain size was decreased for G8 and G9. For all cements an aqueous solution of 40–45 wt% polyacrylic–maleic acid solution containing 8–10 wt% tartaric acid was used as suspension liquid. The FRGICs were prepared by adding a percentage of fibres to the glass powder. The optimum fibre loading was determined by measurements of the compressive strength of the composite materials. The GIC and FRGIC were all hand mixed and manufactured in a suitable mould according to ISO standard. The specimens for compressive strength testing had a diameter of 4 mm and a height of 6 mm (ISO 9917-1:2002). The bending bars were produced with the dimensions 25 2 2 mm3 (ISO 4049). The specimens were all stored for 24 h in distilled water at 37 C after an initial dry gelation period. Table 1 summarizes the specimen compositions, test specifications and results of strength measurements.
2.3. Strength measurements and fractography Fracture strength under compressive and bending loading of the GICs and FRGICs was determined by measuring the critical fracture strength of at least 15 specimens each set. All specimens were loaded in an universal testing machine (Z 2.5, Zwick, Germany) with a crosshead speed of 0.75 mm/min. The compressive strength measurement was used for the cement optimiz-
ing tests. The 4-point-bending-test was applied on GICs and FRGICs with optimized parameters. The data were statistically treated by an ANOVA test with a Bonferroni post-hoc routine (po0:05). For fractographic analysis the fractured FRGIC specimens were examined under a SEM (Leitz ISIs SR50, Akashi, Japan).
3. Results and discussion 3.1. Reactive glass fibre Due to a 1 h homogenisation of the glass melt and due to the knowledge of fluorine as a volatile, the loss of fluorine was determined from previous work to 53.4%. Based on a calculated content of 1.26 g fluorine in 10 g oxide powder prior to melting, a remaining content of 0.67 g in 10 g glass fibres was detected, using the ionselective electrode (ISE) method [30]. However, the XRD patterns of the glass still show a distinct, crystalline peak of calcium fluoride (CaF2) as the only crystalline phase. The broadness of the peak suggests the presence of small CaF2 crystals. Fig. 1 shows a TEM image of the glass. TEM and EDS examination exhibit a droplet phase (A) in the glass that is rich in fluorine and calcium. The droplet phase can also be seen to contain nanoscale crystals (B). Other authors assumed the presence of CaF2 crystals in the droplet phase [9,10]. Small crystals (B) may also be seen in the glass. EDS spectra (A–C) were taken and the same elements, calcium, fluorine, aluminium, silicon and sodium were detected in all regions. The amorphous
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(C) (A) (A)
(B) (C)
(B)
500 µm
Fig. 2. Short glass fibres cut in length of 580 mm (F1–F6).
200 nm Fig. 1. TEM image of the glass with different regions. A: separated droplet phase; B: CaF2 crystals; C: matrix glass.
droplet phase (A) was rich in fluorine, while in the other regions the presence of fluorine is not very pronounced. The small crystals in region B were found to be CaF2 which was confirmed by XRD. In the glass (C) the most prominent peaks are aluminium and silicon, with smaller peaks of calcium, fluorine and sodium. According to Wood and Hill an amorphous phase separation in ionomer glasses is effected by fluorine in the glass through a nucleation decomposition process [10]. They stated that the droplet phase contains CaF2 and is much more reactive than the matrix phase. It is the droplet phase that provides the cations for the cross linking of the polyacrylic molecules, while the matrix phase provides a stable interface to which the polyacrylic chains can bond [9]. Fibres were drawn from the melted glass frit and subsequently cut into short fibres. The tensile strength of the drawn fibres attained 420798 MPa. The fibre diameter was determined to 26 mm. The shear strength of the matrix–fibre interface was estimated to be half the yield strength of the unreinforced GIC [22]. The flexural matrix strength smðyieldÞ of the used base cement G9 attained 10 MPa. Following Eq. (1) a resulting critical fibre length 2xc of 546 mm, e.g. a fibre pull-out length xc of 273 mm was calculated. Fig. 2 shows the fibres which were cut by a rotating guillotine to yield an average length of 5807160 mm. 3.2. Glass ionomer cement The optimum powder to liquid ratio for the various glass compositions are shown in Table 1 (G1–G4).
Untreated glass of G1 showed the highest compressive strength of 112 MPa at a powder to liquid ratio of 2.0. The low strength of 64 MPa (G2) at a higher powder to liquid ratio is caused by poor mixing properties of the samples, due to a rapid setting of the cement. In general, the higher the powder to liquid ratio for a cement system might be selected, the stronger the resulting cement will be formed. The maximum powder to liquid ratio that is attainable, however, is limited by the increase in viscosity of the cement [31]. G9 cement showed the highest compressive strength of 170 MPa at a ratio of 1.5 and was finally chosen to serve as the base glass for the fibre reinforced composites (F4–F6). For a clinical acceptability, the minimum compressive strength is limited to 100 MPa, according to ISO 9917-1:2002 standard. However, modern commercial GICs exhibit a compressive strength of 200 MPa or more [3]. Since the system under investigation was created for fibre drawing, a simplified model glass composition was intended, suffering from a reduced strength performance. A low scatter in strength within the treated groups is most likely the result of the prolonged working time, allowing for better homogenisation of the cement paste. Comparable results were published by other authors who investigated the influence of optimum powder to liquid ratio and of the viscosity of the cement paste on the final GIC strength [31–33]. The treatment of the glass with a combination of acid washing and annealing procedure led to reasonable working time. Heat treatment, acid washing or the application of complex fluoride salts might be promising approaches to prolong the working time of the cement system. A deactivation of the reactive glass surface by an ion leaching process or heat treatment that relieves residual stresses from the quenched specimen was reported to be successful in extending the working time [28,29,34]. The particle size and distribution of the glass frit is of great importance to control the working and setting
U. Lohbauer et al. / Biomaterials 24 (2003) 2901–2907
Table 2 Test specifications for producing GIC and FRGIC and their resulting flexural strengths Material
Specification
P/l ratio
Flexural strength [MPa] (S.D.)
G4 G9 F4
d50 ¼ 6:8 mm d50 ¼ 2:4 mm 20 vol% fibres, 5 min initial dry gelation period 20 vol% fibres, 30 min initial dry gelation period 20 vol% treated fibres, 30 min initial dry gelation period
2.8 1.5 1.8
7.8 (1.2)a 9.6 (1.3)b 8.9 (1.8)a,b
1.8
15.6 (1.6)
1.8
10.7 (2.2)b
F5 F6
Data with same superscript letters are not significantly different (ANOVA, Bonferroni, po0:05).
423
420
Flexural tensile strength σ [MP]
characteristics of the cement [35]. The grain size distributions of the glasses before and after acid treatment show a pronounced difference. The reactive surface area of G8 increased from 6.1 to 10.4 m2/g in G9 while the grain size decreased from 3.0 to 2.4 mm by ion leaching with 3% hydrochloric acid for 7 h. The compressive strength could be increased from 138 to 170 MPa by decreasing the glass powder grain size from 3.0 to 2.4 mm (G8–G9). The powder to liquid ratio was thereby reduced from 1.8 to 1.5 with a decreased particle size distribution, due to a higher reactive surface area of 10.4 m2/g. A fibre loading of 20 vol% (F1–F3) finally resulted in a fibre reinforced cement with the highest compressive strength. The powder to liquid ratio was individually adjusted to account for the decreased surface area due to fibre loading. The cements showed an increase in compressive strength of approximately 30% after reinforcement e.g. the compressive strength of 96 MPa for GIC (G4) increased to 134 MPa for FRGIC (F1). The fibre loading is limited by a critical fibre volume that has to be exceeded to result in an increased strength [22] and by a maximum fibre loading above which large microstructural defects are formed [16,17]. Kawano et al. worked with 40 vol% as the highest fraction of reinforcing fibres whereas Kobayashi et al. reported a maximum fibre loading of 60 vol%, respectively [19,20]. The results of flexural strength testing are listed in Table 2. The duration of the dry hardening period, fibre compounding and optional pre-treatment were set as variables. All cements were mixed with their optimum powder–liquid ratio. Non-reinforced control groups were included, G4 and G9. The optimized fibre reinforced cements based on G9 glass powder. Group F5 attained the highest flexural strength of 15.6 MPa. A typical stress–strain curve for FRGIC (F5) and for the GIC matrix (G9) in case of bending loading is shown in Fig. 3. The stress–strain curve of the fibre is given for tension.
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Glass fibre 417
2.7
2.9
15
12
FRGIC composite 9
GIC matrix 6
3
0
0.0
0.2
0.4
0.6
0.8
1.0
Strain ε [mm]
Fig. 3. Stress–strain curve of FRGIC.
Compared to the unreinforced GIC matrix the FRGIC clearly exhibits a higher fracture strength and a non-catastrophic failure. The large area under the stress–strain curve indicates a significant increase of the work-of-fracture in the FRGIC compared to the brittle GIC matrix material. However, compared to strength optimized current commercial materials, a fibre reinforcement of those materials may lead to a further improvement [16]. An increased flexural strength for the FRGIC materials could only be determined by extending the initial dry gelation period from 5 min to 30 min (F5), since extended neutralisation of the acid-based gelation process takes place and the material might set more sufficiently [4,5]. The ion leaching of the glass fibre surface (F6) counteracts an increase in strength since a reduced liberation of the metal ions impedes the neutralisation process. As a consequence, the gelation process on the glass fibre surface is delayed [34]. Both findings suggest that the reaction kinetics at the interface matrix–fibre might be different compared to the reaction at the interface matrix–glass powder. The reactivity of the fibres might also be affected by fibre processing [36]. Fig. 1 shows a fully amorphous surface layer, free of nanocrystalline precipitations. It was caused by rapid cooling during fibre drawing. Since in phase separated ionomer glasses the droplet phase will be highly responsible for its reactivity [10] the proportion of phase separation is reduced in the surface layer of the fibres. The analysis of the fracture surfaces gave indications of the failure origin in the cement. Fig. 4 shows a fractured surface of a FRGIC (F5). The FRGIC surface shows matrix cracks generated by water evaporation. The cracks propagated along the interface of the initial glass particles and fibres.
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Fig. 4. Fracture surface of a FRGIC showing pull-out of fibres (F5).
inforcement of GICs to be applied as a dental restorative material with improved mechanical properties. Compression strength for the GIC material was increased from 64 to 170 MPa. This was reached by refined glass particle sizes and by pre-treatment of the glass surface. A maximum flexural strength of 15.6 MPa was achieved in the FRGIC compared to 8.9 MPa of the GIC matrix. This increase was attained by using a fibre fraction of 20 vol% and by extending the initial dry gelation period of the GIC matrix. Fractographic examination showed a pronounced fibre pull-out mode which gives rise to a resulting increased fracture toughness and work-of-fracture, respectively. Application of FRGIC materials with improved mechanical strength and toughness properties might lead to extended clinical indications, especially in stress bearing areas.
Acknowledgements The authors wish to thank the 3 M ESPE Corporation (Seefeld, Germany) for financially supporting this work. Special thanks to Dr. G. Rackelmann and S. Hoescheler for the fruitful discussions on that project.
References Fig. 5. Reactive surface layer at the fibre–matrix interface (F5).
At the interface between matrix and fibre, a distinct reactive layer has been formed during the setting process. Wilson stated in his work [2] that the interface between the silica gel layer around the unreacted glass core of glass particles in cements tends to be weak. On this site fracture was observed as illustrated in Fig. 5. The fracture surface of the fibre reinforced cements showed clear evidence of extended fibre pull-out. The absorbed energy due to pull-out gives rise for an increase in fracture toughness. A further increase in toughness and failure strength of the FRGIC can be expected by embedding strong fibres in a weak matrix material. Defined interface properties of matrix–fibre lead to a high profit in work-of-fracture and high toughness, respectively [17,37].
4. Conclusions Short reactive glass fibres with a length of 580 mm were prepared from a glass frit of the system SiO2– Al2O3–CaF2–Na3AlF6. The fibres were used for re-
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Reactive fibre reinforced glass ionomer cements Ulrich Lohbauera,*, Jurgen . Walkerb,c, Sergej Nikolaenkoa, Jan Wernerc, Alexis Clareb, Anselm Petschelta, Peter Greilc a
Policlinic for Operative Dentistry and Periodontology, University of Erlangen-Nuremberg, Glueckstrasse 11, Erlangen 91054, Germany b New York State College of Ceramics, Alfred University, One Saxon Drive, Alfred, NY 14802, USA c Department of Materials Science and Engineering (Glass and Ceramics), University of Erlangen-Nuremberg, Martensstrasse 5, Erlangen 91058, Germany Received 14 October 2002; accepted 20 February 2003
Abstract The mechanical properties of glass ionomer cements used in restorative dentistry reinforced by chopped glass fibres were investigated. Reactive glass fibres with a composition in the system SiO2–Al2O3–CaF2–Na3AlF6 and a thickness of 26 mm were drawn by a bushing process. The manufacturing parameters were optimized with respect to maximum strength of the glass fibre reinforced ionomer cements. Powder to liquid ratio, pre-treatment of the glass, grain size distribution and fibre volume fraction were varied. Glass fibre and cement were characterized by X-ray diffraction, transmission electron microscopy and energy dispersive spectroscopy techniques, respectively. The highest flexural strength of the reinforced cement (15.6 MPa) was found by compounding 20 vol% reactive fibres and extending the initial dry gelation period up to 30 min. Microscopic examination of the fractured cements indicated a distinct reactive layer at the fibre surface. A pronounced fibre pull out mode gives rise to an additional work-of-fracture contributed by pulling the fibres out of the fracture surface. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Glass ionomer cement; Glass fibres; Reinforcement; Fibre drawing; Mechanical strength
1. Introduction In the late 1970s the glass ionomer cements (GICs) were developed as an outgrowth from the research into dental silicate cements and zinc polycarboxylate cements [1,2]. They consist of a sodium aluminium calcium (or strontium) silicate glass powder and an aqueous polyacrylic or related polymeric acid [3]. The single compounds react as soon as they are mixed together. The polymeric acid attacks the glass and leaches out mainly calcium and aluminium ions. The metal ions in turn catalyse cross linking the polymer chains and cause the cement to set. GICs set with the formation of a siliceous hydrogel as a result of this acid–base reaction [4,5]. During a first phase after mixing a dry storage of the material has to be ensured in order to provide the gelation process from water dilution. After that initial *Corresponding author. Tel.: +49-9131-853-4236; fax: +49-9131853-3603. E-mail address: [email protected] (U. Lohbauer).
dry stage, the specimens have to be stored in water to impede dehydration [1,2]. GIC has several unique advantages among restorative materials. The content of fluorine plays an important role [6–8]. Fluorine disrupts the glass network and lowers the fusion temperature of the glass melt, increases the mechanical strength of the set cement and increases the susceptibility of the glass to acid attack. It allows the matching of the refractive index to that of the tooth for the translucency of the cement. The presence of fluorine as a crystalline phase in the glass has been observed by many authors [9,10]. They reported the presence of a second droplet phase that is rich in calcium and fluoride and assumed the existence of CaF2 crystals in the separated phase [11]. However, the main advantage is a constant fluoride release during the lifetime of the restoration which makes it cariostatic i.e. prevent from secondary caries. Many studies have shown that the fluoride release of the cements can remineralize tooth tissue. This allows a minimal invasive cavity preparation technique, preserving more sound tissue [1,12,13].
0142-9612/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0142-9612(03)00130-3
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Compared to other restorative materials like amalgam or polymer composites, however, GICs suffer from lower strength, wear resistance and fracture toughness [3,14–16]. Reinforcement of GIC has therefore become a matter of increasing research efforts. In former experiments various alumina, zirconia, silicon carbide, carbon fibres, or metal particles were used with encouraging results [2]. Fibre length and volume fraction are thereby key microstructural parameters determining the resulting cement properties [17]. A strengthening effect, due to fibre pull-out or crack bridging during fracture process is mainly influenced by the matrix–fibre interface coupling [18]. Fibre reinforcement with reactive glass fibres is one attempt to achieve proper fibre embedding in the matrix cement. Short glass fibre reinforcement was studied by Kawano et al. [19] and Kobayashi et al. [20] with a glass composition in the system CaO–P2O5– SiO2–Al2O3. They used phosphate glasses, since improved self-adhesion to human dentine is described [21]. The aim of the present work is to incorporate high strength reactive glass fibres into the GIC matrix. Depending on the interfacial coupling of fibre-to-matrix, theoretical models predict a significant increase of the work-of-fracture when continuous fibres are aligned with the primary loading direction so that effective load transfer from the matrix to the fibre can be induced. In the case of short fibres dispersed with random orientation in the matrix the situation is more complex [22,23]. Fibres below a critical length (which is determined by the ratio of the fibre strength to the shear strength of the matrix–fibre interface) are less effective in reinforcement whereas using longer fibres nothing is gained by the extra length [22,24]. For the system under investigation a critical fibre length of approximately 550 mm was estimated from fibre and matrix properties. In this investigation, a model glass frit of the system SiO2– Al2O3–CaF2–Na3AlF6 was used to prepare glass fibres by a bushing process. The study deals with fluoride glasses, since an increased fluorine content was shown to result in a rise of strength [25]. The influence of different processing conditions on the mechanical strength of the fibre reinforced glass ionomer cement (FRGIC) was analysed.
2. Experimental procedure
powders were measured using light scattering with a laser (Cilass 715, Cilas Corp., Marcoussis, France), and the surface area was determined by standard nitrogen adsorption method (BET, ASAPs 2000 Nitrogen, Micromeritics Corp., Norcross, USA). The crystalline phase composition was determined by X-ray diffraction using monochromated CuKa radiation (D 5000s, Siemens Corp., Mannheim, Germany). The microstructure of the quenched glass and the fibre was examined in an analytical transmission electron microscope (Philips CM 30 T, Philips, USA) and by energy dispersive spectroscopy (EDS). The TEM work was performed using 300 KV acceleration voltage. The specimens were prepared by focused ion beam (FIB) technique. Thirty grams of the glass frit were melted in a platinum crucible with a hole in the bottom. The glass melt flowing through the hole with a diameter of 1.5 mm was pulled into a fibre and, after rapid air coolingwound onto a rotating drum with a diameter of 150 mm. The rate of mass flow of the glass melt out of the bushing is dependent on geometrical factors of the bushing and the viscosity of the glass [26,27]. The two parameters that were varied to adjust an optimum thickness of the chopped fibre were temperature and drawing speed. Finally a temperature of 1120 C and a drawing speed of 3 m/s were selected. The glass frit was heated up to 1300 C before drawing and maintained there for 1 h in order to homogenize the melt. The drawn glass fibre was cut into short fibres by a rotating guillotine. The critical fibre length 2xc ; which is supposed to provide maximum load transfer by means of elastic strain and frictional pull out, was calculated by [22] 2xc ¼
d sf ; 4 sm
ð1Þ
where d is the fibre diameter, sf the fibre strength and sm the shear strength at the matrix–fibre interface. Ashby et al. reported a correlation between interface shear strength and matrix yield strength: smðShearÞ E1=2smðyieldÞ [22]. The strength smðyieldÞ was derived from the flexural matrix strength of the used base cement system. The tensile strength of the fibres was measured in an universal testing machine (Z 2.5, Zwick, Germany) according to ENV 1007-4 standard. The fibre length and diameter distributions were measured using light microscopy, Fig. 2.
2.1. Glass fibres 2.2. Glass ionomer cements The following glass composition was selected for fibre processing: SiO2:33.3, Al2O3:16.7, CaO:14, NaF:3.3, AlF3:3.3, Na3AlF6:16.2 [wt%]. The oxide powder mixture was melted at 1400 C for 1 h and subsequently quenched in water of room temperature. The glass frit was milled to obtain different grain sizes of 6.8 and 2.4 mm, respectively. The grain size distributions of the
The GIC matrices were prepared by mixing a glass powder with an aqueous polycarbonic acid. The mean particle size of the glass powders are listed in Table 1. The milled glass powder was treated in various ways to reduce surface reactivity and to increase working time [28,29]. A combination of acid treatment and annealing
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Table 1 Test specifications and results of the cement optimizing tests Glass charge
Grain size [mm] (d10 =d50 =d90 )
G1 G2 G3
1.7/12.2/46.0 1.7/12.2/46.0 1.6/6.8/28.7
2.0 2.0 4.1
G4
1.6/6.8/28.7
4.1
G5 G6 G7
n.m. n.m. 1.6/6.8/28.7
n.m. n.m. 4.1
G8 G9
1.2/3.0/21.1 0.9/2.4/7.3
6.1 10.4
Glass charge
Fibre fraction [vol.%] 20 40 60
BET surface area [m2/g]a 1.624 1.249 0.873
F1 F2 F3 a
BET surface area [m2/g]
Acid washing/heat treatment
Working time [sec]
Powder/liquid ratio
Compressive strength [MPa] (S.D.)
Non Non 7 h, 3% HCl/6 h 360 C 7 h, 3% HCl/6 h 360 C 5 h, 360 C 5 h, 3% HCl 7 h, 3% HCl/6 h 360 C Non 7 h, 3% HCl/ 6 h 360 C
n.m. 45 268
2.0 2.8 2.0
112 (34) 64 (24) 70 (4)
268
2.8
96 (3)
49 111 268
2.8 2.8 2.8
n.m. n.m. n.m.
n.m. n.m.
1.8 1.5
138 (31) 170 (18)
Base glass
Working time [sec]
G4 G4 G4
n.m. n.m. n.m.
Powder/liquid ratio 4 5.4 5.4
Compressive strength [MPa] (S.D.) 134 (4) 98 (4) 82 (6)
Calculated cylindrical fibre surface area: 0.1225 m2/g.
procedure was applied, as shown in Table 1 (G5–G7). The optimum powder to liquid ratio was found for the processing conditions which finally resulted in the GIC with the highest compressive strength, Table 1 (G1–G4). The medium grain size was decreased for G8 and G9. For all cements an aqueous solution of 40–45 wt% polyacrylic–maleic acid solution containing 8–10 wt% tartaric acid was used as suspension liquid. The FRGICs were prepared by adding a percentage of fibres to the glass powder. The optimum fibre loading was determined by measurements of the compressive strength of the composite materials. The GIC and FRGIC were all hand mixed and manufactured in a suitable mould according to ISO standard. The specimens for compressive strength testing had a diameter of 4 mm and a height of 6 mm (ISO 9917-1:2002). The bending bars were produced with the dimensions 25 2 2 mm3 (ISO 4049). The specimens were all stored for 24 h in distilled water at 37 C after an initial dry gelation period. Table 1 summarizes the specimen compositions, test specifications and results of strength measurements.
2.3. Strength measurements and fractography Fracture strength under compressive and bending loading of the GICs and FRGICs was determined by measuring the critical fracture strength of at least 15 specimens each set. All specimens were loaded in an universal testing machine (Z 2.5, Zwick, Germany) with a crosshead speed of 0.75 mm/min. The compressive strength measurement was used for the cement optimiz-
ing tests. The 4-point-bending-test was applied on GICs and FRGICs with optimized parameters. The data were statistically treated by an ANOVA test with a Bonferroni post-hoc routine (po0:05). For fractographic analysis the fractured FRGIC specimens were examined under a SEM (Leitz ISIs SR50, Akashi, Japan).
3. Results and discussion 3.1. Reactive glass fibre Due to a 1 h homogenisation of the glass melt and due to the knowledge of fluorine as a volatile, the loss of fluorine was determined from previous work to 53.4%. Based on a calculated content of 1.26 g fluorine in 10 g oxide powder prior to melting, a remaining content of 0.67 g in 10 g glass fibres was detected, using the ionselective electrode (ISE) method [30]. However, the XRD patterns of the glass still show a distinct, crystalline peak of calcium fluoride (CaF2) as the only crystalline phase. The broadness of the peak suggests the presence of small CaF2 crystals. Fig. 1 shows a TEM image of the glass. TEM and EDS examination exhibit a droplet phase (A) in the glass that is rich in fluorine and calcium. The droplet phase can also be seen to contain nanoscale crystals (B). Other authors assumed the presence of CaF2 crystals in the droplet phase [9,10]. Small crystals (B) may also be seen in the glass. EDS spectra (A–C) were taken and the same elements, calcium, fluorine, aluminium, silicon and sodium were detected in all regions. The amorphous
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(C) (A) (A)
(B) (C)
(B)
500 µm
Fig. 2. Short glass fibres cut in length of 580 mm (F1–F6).
200 nm Fig. 1. TEM image of the glass with different regions. A: separated droplet phase; B: CaF2 crystals; C: matrix glass.
droplet phase (A) was rich in fluorine, while in the other regions the presence of fluorine is not very pronounced. The small crystals in region B were found to be CaF2 which was confirmed by XRD. In the glass (C) the most prominent peaks are aluminium and silicon, with smaller peaks of calcium, fluorine and sodium. According to Wood and Hill an amorphous phase separation in ionomer glasses is effected by fluorine in the glass through a nucleation decomposition process [10]. They stated that the droplet phase contains CaF2 and is much more reactive than the matrix phase. It is the droplet phase that provides the cations for the cross linking of the polyacrylic molecules, while the matrix phase provides a stable interface to which the polyacrylic chains can bond [9]. Fibres were drawn from the melted glass frit and subsequently cut into short fibres. The tensile strength of the drawn fibres attained 420798 MPa. The fibre diameter was determined to 26 mm. The shear strength of the matrix–fibre interface was estimated to be half the yield strength of the unreinforced GIC [22]. The flexural matrix strength smðyieldÞ of the used base cement G9 attained 10 MPa. Following Eq. (1) a resulting critical fibre length 2xc of 546 mm, e.g. a fibre pull-out length xc of 273 mm was calculated. Fig. 2 shows the fibres which were cut by a rotating guillotine to yield an average length of 5807160 mm. 3.2. Glass ionomer cement The optimum powder to liquid ratio for the various glass compositions are shown in Table 1 (G1–G4).
Untreated glass of G1 showed the highest compressive strength of 112 MPa at a powder to liquid ratio of 2.0. The low strength of 64 MPa (G2) at a higher powder to liquid ratio is caused by poor mixing properties of the samples, due to a rapid setting of the cement. In general, the higher the powder to liquid ratio for a cement system might be selected, the stronger the resulting cement will be formed. The maximum powder to liquid ratio that is attainable, however, is limited by the increase in viscosity of the cement [31]. G9 cement showed the highest compressive strength of 170 MPa at a ratio of 1.5 and was finally chosen to serve as the base glass for the fibre reinforced composites (F4–F6). For a clinical acceptability, the minimum compressive strength is limited to 100 MPa, according to ISO 9917-1:2002 standard. However, modern commercial GICs exhibit a compressive strength of 200 MPa or more [3]. Since the system under investigation was created for fibre drawing, a simplified model glass composition was intended, suffering from a reduced strength performance. A low scatter in strength within the treated groups is most likely the result of the prolonged working time, allowing for better homogenisation of the cement paste. Comparable results were published by other authors who investigated the influence of optimum powder to liquid ratio and of the viscosity of the cement paste on the final GIC strength [31–33]. The treatment of the glass with a combination of acid washing and annealing procedure led to reasonable working time. Heat treatment, acid washing or the application of complex fluoride salts might be promising approaches to prolong the working time of the cement system. A deactivation of the reactive glass surface by an ion leaching process or heat treatment that relieves residual stresses from the quenched specimen was reported to be successful in extending the working time [28,29,34]. The particle size and distribution of the glass frit is of great importance to control the working and setting
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Table 2 Test specifications for producing GIC and FRGIC and their resulting flexural strengths Material
Specification
P/l ratio
Flexural strength [MPa] (S.D.)
G4 G9 F4
d50 ¼ 6:8 mm d50 ¼ 2:4 mm 20 vol% fibres, 5 min initial dry gelation period 20 vol% fibres, 30 min initial dry gelation period 20 vol% treated fibres, 30 min initial dry gelation period
2.8 1.5 1.8
7.8 (1.2)a 9.6 (1.3)b 8.9 (1.8)a,b
1.8
15.6 (1.6)
1.8
10.7 (2.2)b
F5 F6
Data with same superscript letters are not significantly different (ANOVA, Bonferroni, po0:05).
423
420
Flexural tensile strength σ [MP]
characteristics of the cement [35]. The grain size distributions of the glasses before and after acid treatment show a pronounced difference. The reactive surface area of G8 increased from 6.1 to 10.4 m2/g in G9 while the grain size decreased from 3.0 to 2.4 mm by ion leaching with 3% hydrochloric acid for 7 h. The compressive strength could be increased from 138 to 170 MPa by decreasing the glass powder grain size from 3.0 to 2.4 mm (G8–G9). The powder to liquid ratio was thereby reduced from 1.8 to 1.5 with a decreased particle size distribution, due to a higher reactive surface area of 10.4 m2/g. A fibre loading of 20 vol% (F1–F3) finally resulted in a fibre reinforced cement with the highest compressive strength. The powder to liquid ratio was individually adjusted to account for the decreased surface area due to fibre loading. The cements showed an increase in compressive strength of approximately 30% after reinforcement e.g. the compressive strength of 96 MPa for GIC (G4) increased to 134 MPa for FRGIC (F1). The fibre loading is limited by a critical fibre volume that has to be exceeded to result in an increased strength [22] and by a maximum fibre loading above which large microstructural defects are formed [16,17]. Kawano et al. worked with 40 vol% as the highest fraction of reinforcing fibres whereas Kobayashi et al. reported a maximum fibre loading of 60 vol%, respectively [19,20]. The results of flexural strength testing are listed in Table 2. The duration of the dry hardening period, fibre compounding and optional pre-treatment were set as variables. All cements were mixed with their optimum powder–liquid ratio. Non-reinforced control groups were included, G4 and G9. The optimized fibre reinforced cements based on G9 glass powder. Group F5 attained the highest flexural strength of 15.6 MPa. A typical stress–strain curve for FRGIC (F5) and for the GIC matrix (G9) in case of bending loading is shown in Fig. 3. The stress–strain curve of the fibre is given for tension.
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Glass fibre 417
2.7
2.9
15
12
FRGIC composite 9
GIC matrix 6
3
0
0.0
0.2
0.4
0.6
0.8
1.0
Strain ε [mm]
Fig. 3. Stress–strain curve of FRGIC.
Compared to the unreinforced GIC matrix the FRGIC clearly exhibits a higher fracture strength and a non-catastrophic failure. The large area under the stress–strain curve indicates a significant increase of the work-of-fracture in the FRGIC compared to the brittle GIC matrix material. However, compared to strength optimized current commercial materials, a fibre reinforcement of those materials may lead to a further improvement [16]. An increased flexural strength for the FRGIC materials could only be determined by extending the initial dry gelation period from 5 min to 30 min (F5), since extended neutralisation of the acid-based gelation process takes place and the material might set more sufficiently [4,5]. The ion leaching of the glass fibre surface (F6) counteracts an increase in strength since a reduced liberation of the metal ions impedes the neutralisation process. As a consequence, the gelation process on the glass fibre surface is delayed [34]. Both findings suggest that the reaction kinetics at the interface matrix–fibre might be different compared to the reaction at the interface matrix–glass powder. The reactivity of the fibres might also be affected by fibre processing [36]. Fig. 1 shows a fully amorphous surface layer, free of nanocrystalline precipitations. It was caused by rapid cooling during fibre drawing. Since in phase separated ionomer glasses the droplet phase will be highly responsible for its reactivity [10] the proportion of phase separation is reduced in the surface layer of the fibres. The analysis of the fracture surfaces gave indications of the failure origin in the cement. Fig. 4 shows a fractured surface of a FRGIC (F5). The FRGIC surface shows matrix cracks generated by water evaporation. The cracks propagated along the interface of the initial glass particles and fibres.
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Fig. 4. Fracture surface of a FRGIC showing pull-out of fibres (F5).
inforcement of GICs to be applied as a dental restorative material with improved mechanical properties. Compression strength for the GIC material was increased from 64 to 170 MPa. This was reached by refined glass particle sizes and by pre-treatment of the glass surface. A maximum flexural strength of 15.6 MPa was achieved in the FRGIC compared to 8.9 MPa of the GIC matrix. This increase was attained by using a fibre fraction of 20 vol% and by extending the initial dry gelation period of the GIC matrix. Fractographic examination showed a pronounced fibre pull-out mode which gives rise to a resulting increased fracture toughness and work-of-fracture, respectively. Application of FRGIC materials with improved mechanical strength and toughness properties might lead to extended clinical indications, especially in stress bearing areas.
Acknowledgements The authors wish to thank the 3 M ESPE Corporation (Seefeld, Germany) for financially supporting this work. Special thanks to Dr. G. Rackelmann and S. Hoescheler for the fruitful discussions on that project.
References Fig. 5. Reactive surface layer at the fibre–matrix interface (F5).
At the interface between matrix and fibre, a distinct reactive layer has been formed during the setting process. Wilson stated in his work [2] that the interface between the silica gel layer around the unreacted glass core of glass particles in cements tends to be weak. On this site fracture was observed as illustrated in Fig. 5. The fracture surface of the fibre reinforced cements showed clear evidence of extended fibre pull-out. The absorbed energy due to pull-out gives rise for an increase in fracture toughness. A further increase in toughness and failure strength of the FRGIC can be expected by embedding strong fibres in a weak matrix material. Defined interface properties of matrix–fibre lead to a high profit in work-of-fracture and high toughness, respectively [17,37].
4. Conclusions Short reactive glass fibres with a length of 580 mm were prepared from a glass frit of the system SiO2– Al2O3–CaF2–Na3AlF6. The fibres were used for re-
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