This study investigated the reinforcing effect of hollow and solid discontinuous glass fiber fillers with two different loading fractions on select mechanical properties of conventional and resin modified glass ionomer cements (GICs).
Experimental fiber reinforced GIC was prepared by adding discontinuous glass fiber (hollow/solid) of 0.5 mm in length to the powder of commercial GICs (GC Fuji IX and II LC) with two different weight ratios (5 and 10 wt%) using a high speed mixing machine. Fracture toughness, work of fracture, flexural strength, flexural modulus, compressive strength and diametral tensile strength were determined for each experimental and control material. The specimens ( n = 7) were wet stored (37 °C for one day) before testing. Scanning electron microscopy was used to evaluate the microstructure of the experimental fiber reinforced GICs. Fiber length analysis was carried out to investigate the fiber length distribution of experimental GICs. The results were analyzed statistically using ANOVA followed by Tukey’s post hoc test. Level of significance was set at 0.05.
An increase in fracture toughness (280 and 200%) and flexural strength (170 and 140%) of hollow discontinuous glass fiber reinforced (10 wt%) conventional and resin modified GICs respectively, were achieved compared to unreinforced materials ( p < 0.05). Compressive strength did not show any significant differences ( p > 0.05) between the fiber reinforced and unreinforced GICs.
The use of hollow discontinuous glass fiber fillers with conventional and resin modified GIC matrix is a novel reinforcement. It yielded superior toughening and flexural performance compared to the particulate GICs used.
An increasing demand for direct filling materials in dentistry has been supported by changes in restorative techniques. The trend of adhesively bonded restorations saves sound tooth structure and is compatible with prophylactic concepts. Preserving and stabilizing hard tooth tissues by direct filling techniques are favored over macromechanically styled, destructive preparations with amalgam or indirect restorative materials .
Nowadays, glass ionomer cements (GICs) are used in many dental applications due to several unique advantages among restorative materials . Their benefits include fluoride ion hydrodynamics, biocompatibility, favorable thermal expansion and contraction, and chemical bonding to tooth structure . In addition, GICs have shown their potential in other medical areas, such as orthopedic surgery . On the other hand, poor mechanical properties, such as low flexural strength, fracture toughness and wear, limit their wider use in dentistry as a permanent filling material in stress-bearing areas . In the posterior dental region, GICs are mostly used as a temporary filling material or base . Reinforcement of glass ionomer restorative materials is essential and many researchers have focused on improving the mechanical properties by adding various filler types to the GIC powder component. The fillers used included metallic powders, hydroxyapatite powders, bioactive glass particles, nanoclay and discontinuous glass fibers .
In dentistry, fiber reinforcement has become a modern and clinically interesting technology that could offer a new affordable option for both patients and clinicians. Discontinuous and continuous fibers, i.e. reinforcement with high aspect ratio, are utilized in many fields of technical application as well as in dentistry and medicine . However they have not been studied to a larger extent with GICs . Although, little information about the use of discontinuous glass fibers with GICs are available, thus further investigation is required in order to produce a material with improved properties . Hollow glass fibers have been investigated as a suitable reinforcement material for manufactured composites . With their unique properties which combine low weight, high specific strength and energy absorption capabilities, hollow glass fibers became a very attractive engineering material for use in advanced composites and aerospace industries . They have some documented reinforcing efficiency compared to solid glass fibers . Hollow glass fiber reinforced composites have a structural performance niche in a class of their own. They offer increased flexural rigidity compared to solid glass fiber reinforced composites . Interestingly, analytical and finite element modeling has shown that gains in rigidity and strength could be made available by tailoring the combinations of fiber hollowness and matrix . This might reveal improved adhesion and interlocking between matrix and hollow glass fibers. To the author’s best knowledge, the idea of using hollow discontinuous glass fiber in dentistry is a novel reinforcing approach. Therefore, the aim and hypothesis of this study, was to investigate the reinforcing effect of hollow discontinuous glass fiber fillers on the mechanical properties of conventional and resin modified GICs, in comparison with solid discontinuous glass fiber reinforcement.
Materials and methods
Production of experimental discontinuous fiber reinforced GICs
Hollow (JSC, Polotsk-PSV, Plarussia) and solid (Ahlstrom, Finland) continuous E-glass fibers, 10–12 μm in diameter as-received silanized, were cut manually with shears (Fiskars Brands, Inc., Madison, WI, USA) into 0.5 mm (±0.3 mm) long segments ( Fig. 1 ). Experimental fiber reinforced GICs were prepared by adding discontinuous glass fiber (hollow/solid) to the glass powder of commercial GICs (conventional: GC Fuji IX and resin modified: GC Fuji II LC, shade A3, Tokyo, Japan) with two different weight ratios (5 and 10 wt%). The mixing was performed by using a high speed mixing machine for 3 min until a homogenous powder mixture was obtained (Hauschild Speed Mixer DAC 400.1, 3500 rpm). Finally, the materials produced were reinforced powders with 5 wt% and 10 wt% of either hollow (HFR) or solid (SFR) discontinuous glass fibers which were compared with unmodified materials and other commercial conventional GICs (Ketac Universal, shade A2, 3 M ESPE, Germany; Vivaglass, Ivoclar Vivadent, Germany). The reinforced glass powder and the cement liquid were mixed and manipulated according to the manufacturers’ instructions.
Flexural strength (FS) and modulus (FM) were determined by conducting a 3-point bending specimens (2 × 2 × 25 mm 3 ) from each tested material. The recommended powder/liquid ratio was dispensed on a glass plate. Then this powder was mixed into the liquid using a plastic spatula. The mixing time did not exceed 1 min and the working time was in the range of 2–3 min. Bar-shaped specimens were made in a half-split stainless steel mold between transparent Mylar sheets and a glass slide. For conventional GICs, specimens were kept in their molds for 30 min before they were carefully removed.
Polymerization of resin modified GICs specimens were made using a hand light-curing unit (Elipar S10, 3M ESPE, St. Paul, MN, USA) for 20 s in five separate overlapping portions from both sides of the metal mold. The wavelength of the light was between 430 and 480 nm with a light intensity of 1600 mW/cm 2 . The specimens from each group ( n = 7) were stored wet at 37 °C for 24 h before testing. The three-point bending test was conducted according to the ISO 4049 (test span: 20 mm, cross-head speed: 1 mm/min, indenter: 2 mm diameter). All specimens were loaded in a material testing machine (model LRX, Lloyd Instruments Ltd., Fareham, England) and the load-deflection curves were recorded with computer software (Nexygen 4.0, Lloyd Instruments Ltd., Fareham, England).
Flexural strength ( ơ f ) and flexural modulus ( E f ) were calculated from the following formula (ISO 1992):