Electron-beam irradiation can be used to influence the properties of polymers. Electron beams cause cross-linking that enhances the molecular mass of the polymer; this leads to branched chains until, ultimately, a 3-dimensional network is formed. The aim of this study was to evaluate the effect of electron-beam irradiation on polymer bracket materials.
Since polymers are commonly used materials for brackets, polyoxymethylene, polycarbonate, and polyurethane were chosen for this study. The acceleration voltage of the electron-beam device was 10 MeV, and the energy dose was 100 kGy with an electron accelerator (BGS beta gamma service, Rhodotron, Bruchsal, Germany). Three-medium wear, fracture toughness, and Vickers hardness tests were performed. The irradiated samples were compared with untreated control groups.
Polycarbonate and polyurethane bracket materials have enhanced fracture toughness and Vickers hardness after electron-beam irradiation of 100 kGy and 10 MeV. Polyoxymethylene bracket materials showed significantly lower fracture toughness values after irradiation compared with the untreated control group. Polyoxymethylene had the best mechanical properties, followed by polycarbonate and polyurethane. Almost the same effects could be measured during the 3-medium wear test.
Electron-beam postcuring improved Vickers hardness and fracture toughness of polymers with lower mechanical properties (polycarbonate and polyurethane). Polyoxymethylene, with sufficient hardness and fracture toughness, could not be improved with electron-beam postcuring.
Even though recently developed polymer brackets show better mechanical properties for resisting oral loads, they still have some disadvantages. They show low elastic modulus, decreased fracture toughness, and inability to withstand the torqueing forces generated by rectangular wires. Additionally, a plasticizing effect caused by water sorption of the polymeric structures was described. Therefore, current research on reinforcement methods of plastic brackets has encompassed several areas, including searching for an alternative polymer, reinforcement of the polymer by fillers (so-called “composites”) or fibers, or the use of metallic inserts on the slot of the brackets. However, polymer brackets still compete with ceramic brackets with their increased toughness and low abrasion.
The objectives of this investigation were to modify the chemical structure and improve the mechanical properties of polymer brackets by using high-energy electron-beam irradiation. Electron-beam postcuring is widely used to improve the mechanical properties of polymers, especially polyethylene, polystyrene, and polycarbonate in the industry. Generally, there are 2 types of reactions with electron-beam irradiation competed during radiation: chain breakage and chain linkage. Whether chain linkage or chain breakage dominates during irradiation depends on several parameters: irradiation dosage, polymeric structure, temperature and storage during irradiation, and functional groups of the polymer. The underlying principle of electron-beam postcuring of polymers is that the polymer structure could contain chain entanglements. During irradiation, chain breakage might occur at this entanglement and relax the polymer structure. As a result, there could be more dense packing of the polymer, leading to improved mechanical properties. Nevertheless, in dentistry, only a few studies were found concerning electron-beam postcuring.
Behr et al stated that the mechanical properties of dental composites could be influenced by electron-beam irradiation. Haque et al described UDMA as a polymer that showed improved hardness and reduced abrasiveness after heavy ion irradiation (640 Gy, 290 MeV). Thompson et al showed that electron-beam irradiation of Bis-GMA increases the degree of crosslinking of the polymer and that the residual double bond content decreased as dosages increased during irradiation. However, it was described that polymers could suffer chemical degradation after electron-beam postcuring. Behr et al reported decreased fracture toughness and color changes using electron-beam irradiation as the postcuring method.
The aim of this study was to investigate the influence of electron-beam postcuring on the mechanical properties of various polymer bracket materials. Since polymers are commonly used for brackets, polyoxymethylene, polycarbonate, and polyurethane were chosen for this study. Fracture toughness, Vickers hardness, and wear resistance were measured after electron-beam postcuring by using an energy dosage of 100 kGy and an acceleration voltage of 10 MeV.
Material and methods
Rectangular beams were manufactured with the dimensions of 36 × 8 × 4 mm (length × width × thickness) for electron-beam irradiation. Three polymer bracket groups were investigated (60 specimens per group). Group 1 consisted of polyoxymethylene, the samples of group 2 were made of polycarbonate, and the specimens of group 3 were manufactured by using polyurethane. The surface of the beams was ground with sandpaper (800 grit) at first. Then all bars were polished with a polishing machine (Renfert, Hilzingen, Germany) at 1500 rpm.
Thirty samples of each group were the untreated controls, and the other 30 specimens were irradiated by using an electron beam with an energy dose of 100 kGy in a vacuum with the temperature controlled. The definition of “gray” is the absorption of 1 J of energy, in the form of ionizing radiation, by 1 kg of matter. The acceleration voltage of the electron-beam device was 10 MeV, by using an electron accelerator (BGS beta gamma service, Rhodotron, Bruchsal, Germany). Immediate irradiation of the specimens after they were produced was impossible because of the distance between the manufacturing place and the electron-beam accelerator. Therefore, the irradiated specimens were stored before and after electron-beam irradiation for 7 days in distilled water at 37°C before measurements. The control group was kept in distilled water at 37°C for 14 days to ensure identical storage conditions for all groups.
Fracture toughness was tested as follows. At the midspan of the specimens, a 3-mm deep and 0.5-mm wide notch was sawn. This cut was lenghthened to a notch of 0.2 to 0.5 mm by using a razor blade device (Ivoclar-Vivadent, Schaan, Liechtenstein).
After preparation of the bars, a 3-point bending test was performed with a universal testing machine (1446, Zwick, Ulm, Germany). The load was axially applied in the center of the bars directly above the notch (v = 1 mm/min). The fracture toughness (K1c) was determined according to the following formula.
K 1 c ( max ) = ( P max × S ) B × H 3 2 f ( x )
f ( x ) = 3 x 1 2 [ 1.99 − x ( 1 − x ) ( 2.15 − 3.93 x + 2.7 x 2 ) ] 2 ( 1 + 2 x ) ( 1 − x ) 3 2