New material synthesis by radiation processing at high temperature—polymer modification with improved irradiation technology

Electron irradiation enhanced wear resistance and hardness of polytetrafluoroethylene (PTFE)

The surface of polytetrafluoroethylene (PTFE) bulk materials was modified by irradiation at high temperatures using a 1.2 MeV electron beam. The mass wear rate was decreased from 2.5 × 10-6 g N-1 m-1 to 0.08 × 10-6 g N-1 m-1, and the hardness was increased from 33.2 MPa to 93.9 MPa. The yield strength was increased from 12.1 MPa to 25.8 MPa and Young's modulus was enhanced from 101 MPa to 261 MPa. The use of electron beam irradiation on PTFE at high temperature is an effective modification method to significantly improve its wear resistance and hardness, and increase the elastic response range of PTFE. The chemical reaction induced by electron irradiation at high temperature changes the molecular structure of the PTFE bulk material from highly linear to a network, thus improving the surface mechanical properties of PTFE.

Modification of Polyethylene by RF Plasma in Different/Mixture Gases

Herein, low-density polyethylene (LDPE) films were treated using radio-frequency plasma discharge in the presence of air, nitrogen, oxygen, argon, and their mixtures to introduce new chemical functionalities. The surface properties of treated LDPE were qualitatively and quantitatively characterized using various analytical and microscopic techniques. It was found that the optimum plasma treatment for LDPE occurs in the presence of air plasma at an exposure time of 120 s and 80 W of nominal power. The plasma formed layer had tendency to increasing thickness with increasing treatment time up to 60 s using air and oxygen and even more with inert gases. An aging study of plasma-treated LDPE samples stored in ambient air or water medium revealed the partial hydrophobic recovery.

Effect of ionizing radiation on polymer brackets

OBJECTIVE

The purpose of this study was to investigate whether the mechanical properties of polymer brackets can be improved without discoloration by electron beam (EB) irradiation applied with a Rhodotron electron accelerator using standard high-energy parameters including a 10 MV acceleration voltage and 100 kGy of energy. We analyzed polymer samples and several commercially available brackets.

MATERIALS AND METHODS

The first group included three polymeric base materials (polyoxymethylene, polycarbonate, polyurethane) currently used in various bracket systems. The second group included five bracket types, three of which are on the market (Aesthetik-Line and Brillant by Forestadent; Envision by Ortho Organizers) while the other two were experimental brackets containing urethane dimethacrylate as a monomer matrix and functional silane-treated SiO2 fillers with a filler content of 10 or 40 vol%. Each category included 40 specimens previously irradiated by a commercial provider (Beta-Gamma-Service, Bruchsal, Germany) and another 40 nonirradiated controls. The polymer specimens were analyzed for fracture toughness, Vickers hardness, and wear resistance, and the bracket specimens for Vickers hardness and color stability. The Wilcoxon-Mann-Whitney U test was used for pairwise comparison to identify significant differences (p ≤ 0.05).

RESULTS

Significant increases in fracture toughness and Vickers hardness were observed in polycarbonate and polyurethane after EB irradiation, while EB irradiation of polyoxymethylene resulted in a significant decrease in these parameters. The polyurethane samples demonstrated significantly less postirradiation wear. All the commercially available brackets except for Brillant revealed significant increases in Vickers hardness. Significant discoloration was observed in all brackets after irradiation.

CONCLUSION

Although our evaluation of polymer brackets revealed considerable improvements in mechanical properties after EB irradiation, this benefit was marred by very obvious discoloration. We cannot therefore recommend industrial scale EB irradiation for any polymer bracket currently on the market.

Effects of low-energy electron beam irradiation on flexural properties of self-curing acrylic resin

PURPOSE

The purpose of this study was to confirm the effectiveness of LEB irradiation onto the polymer powder for improving the mechanical properties of self-curing acrylic resin.

METHODS

The polymer powder of self-curing acrylic resin was irradiated with total LEB doses of 25, 50, 75 or 100kGy. Non-irradiated powder was used as a control. After LEB irradiation, ESR measurement, weight-average molecular weight measurement and three-point bending test were performed.

RESULTS

ESR spectrum of control had no peaks. After LEB irradiation, nine peaks were observed in each ESR spectrum, which indicates the presence of free radicals from main polymer chain. The quantity of free radicals increased linearly up to 100kGy. Calibrated weight-average molecular weights were as follows: control, 960,000; 25kGy, 500,000; 50kGy, 440,000; 75kGy, 410,000; and 100kGy, 390,000. Molecular weight decreased with increasing LEB irradiation dose. The mean values of flexural strength (MPa) were as follows: control, 61.5±3.0; 25kGy, 68.1±4.0; 50kGy, 73.0±1.9; 75kGy, 70.4±3.6; and 100kGy, 67.7±2.3. The flexural strength of the specimens cured with the LEB-irradiated powder was significantly higher than that of control (p<0.01). These results indicate that flexural strength of polymer materials cured with the LEB-irradiated powder increases because of increase in cross-linking structure.

CONCLUSION

It is confirmed that LEB irradiation onto the polymer powder of self-curing acrylic resin improves the flexural strength.

The influence of electron beam irradiation on colour stability and hardness of aesthetic brackets

Electron beam irradiation can be used to improve the mechanical properties of polymers. The aim of this study was to investigate the influence of electron beam irradiation with an energy dose of 100 kGy on the mechanical properties and colour stability of conventional polymer brackets and experimental filled composite brackets. The conventional brackets tested were Aesthetik-Line, Brillant, and Envision. The experimental brackets contained urethane dimethacrylate, as a monomer matrix and functional silane-treated SiO(2) fillers with a filler content of either 10 (Exper 1) or 40 (Exper 2) vol per cent. The influence of electron beam post-curing on Vickers hardness (VH) of the polymer brackets was investigated. Additionally, a possible discolouration of the brackets after electron beam irradiation was determined according to the three-dimensional L* a* b* colour space. The irradiated brackets were compared with untreated control groups. Statistical analysis was performed using the Wilcoxon test. With the exception of Brillant brackets, all investigated brackets showed a significant enhancement of VH after electron beam post-curing. However, the brackets suffered a significant increase in discolouration. Aesthetik-Line brackets showed the highest discolouration, ΔE, and Exper 2 brackets the lowest ΔE values. The discolouration of the examined brackets differed significantly. These results demonstrate that the mechanical properties of polymer brackets could be modified by electron beam irradiation. Nevertheless, clinical use of electron beam post-curing might be restricted because of unacceptable colour changes.

Experimental composites made of electron beam irradiated reinforced fillers

This study investigated the influence of electron beam irradiated reinforced fillers on the three body wear and flexural strength of experimental composite blends. Three formulations of reinforced fillers were investigated: (A) high loaded inorganic filler composite with 60 wt.% SiO2, (B) low loaded inorganic filler composite with 40 wt.% SiO2, (C) organic filler composite (precipitated Bis-phenol-A-di-methacrylate). The fillers were assigned to two subgroups of unswollen (A, B, C) and monomer swollen (As, Bs, Cs) fillers. The experimental blends (matrix: Urethane-dimethacrylate) were mixed using un-treated, annealed (90 degrees C), or electron beam irradiated fillers with 30 and 90 kGy, respectively. All specimens were heat-cured for 20 min at 140 degrees C. Three-body abrasion and flexural strength tests were performed. The highest flexural strength was evaluated for composites made of the 30 kGy irradiated type Bs filler. The comparison with annealed fillers showed that the effect was independent of increasing temperatures during the radiation process. Blends with a SiO2 content of 60 wt.% (type A, As) had significantly less wear than blends with 40 wt.% (type B, Bs) or blends with organic fillers (type C, Cs). The flexural strength of the composite could be improved by using pre-irradiated reinforced fillers. However, wear was not affected using this procedure.

Fast neutron irradiation effects in PM-355 nuclear track detector

Samples from sheets of the polymeric material PM-355 have been exposed to neutrons of incident energy in the range 0.8-19.2 MeV. The resultant effect of neutron irradiation on the thermal properties of PM-355 has been investigated using thermo-gravimetric analysis (TGA). The onset temperature of decomposition T(o) and thermal activation energy of decomposition E(a) were calculated, results indicating that the PM-355 detector decomposes in two weight loss stages. The variation of transition temperatures with neutron energy has been determined using differential thermal analysis (DTA). The results indicate the PM-355 thermograms to be characterized by the appearance of an endothermic peak due to melting. Melting temperature was found to be dependent on the neutron energy. In addition, structural property studies using X-ray diffraction and infrared spectroscopy were performed on both non-irradiated PM-355 samples and all of the samples that were irradiated. The results indicate that both the degree of ordering and the absorbance of the PM-355 polymer are dependent on neutron energy.

Electron-beam irradiation of experimental denture base polymers

OBJECTIVE

Since the properties of polymers can be influenced using electron-beam irradiation, the aim of this study was to investigate whether electron-beam post-curing can improve the mechanical properties of experimental denture base polymers.

MATERIAL AND METHODS

Rectangular specimens of different experimental polymeric blends were electron-beam irradiated (post-cured) with 25 kGy and 200 kGy using an electron accelerator of 4.5 MeV. Fracture toughness, work of fracture, Vickers hardness and colour changes were measured and compared in non-irradiated specimens.

RESULTS

The mechanical properties of all the investigated polymers seemed to benefit from low-energy electron-beam irradiation (25 kGy). Using an energy dose of 200 kGy, all blends showed deteriorated mechanical properties resulting from chain breakage. Nevertheless, all investigated polymers had undesirable colour changes after electron-beam irradiation.

CONCLUSIONS

Mechanical properties of experimental polymethyl-methacrylate could be changed using electron-beam irradiation. Because of discolorations caused by the irradiation levels investigated, these levels cannot be recommended for practical applications.

Influence of electron beam irradiation on the alloy-to-resin bond strength

The aim of this study was to investigate whether postcuring using electron beam irradiation had an effect on the bond strength of resin-to-base-metal after priming their surfaces using silicoating methods or functional monomers. Composite cylinders were bonded on a restricted area of 5 mm2 to flat rectangular titanium and cobalt-chromium specimens. Under investigation were the silicoating system Rocatec, the thiol-phosphate system Metal Primer II and the phosphate ester SR Link. Tensile strength and shear bond strength were determined for the three test groups in each case: (i) after 24 h, (ii) after electron beam irradiation (100 kGy), and (iii) after irradiation (100 kGy) + 12,000 cycles of thermal cycling (5 degrees /55 degrees C). The bond strength was highly affected by irradiation and the metal priming method used. However, the tribochemical silicoating method and phosphate-ester group showed no significant statistical change in bond strength. Only the thiol-phosphate system showed considerably higher tensile and shear bond strengths after irradiation. Thermal cycling did not deteriorate this bond and there was a tendency for higher bond strength on titanium. As a result it was determined that thiol-phosphate primers in combination with postcuring using electron beam irradiation can considerably improve the bond strength between resins and titanium or cobalt-chromium alloys.

Effect of surface oxyfluorination on the dyeability of polyethylene film

The effect of surface oxyfluorination on low-density polyethylene (LDPE) film was studied in terms of surface functionality and surface energetics of the film surfaces, which can be attributed to improvement of the dyeability. The growth of functional groups and surface free energy was confirmed by FTIR-ATR, XPS, and contact angle methods. As a result, the total surface free energy was increased with oxyfluorination time, as a progressive increase of the polar component together with a small decrease of the dispersive component of surface free energy. From the dyeability test using the Kubelka-Munk equation, it was found that the oxyfluorination treatment plays an important role in the growth of oxygen-containing functional groups of LDPE film, resulting in improving the dyeability with a basic dyeing agent. A direct linear relationship is shown between the specific component of surface free energy and the K/S value for this work.

Electron beam irradiation of denture base materials

Electron beam irradiation can be used to influence the properties of polymers. It was the aim of this study to investigate whether PMMA denture base materials can benefit from irradiation in order to have increased fracture toughness, work of fracture or hardness. Rectangular specimens of heat-and auto-curing denture base materials were electron beam irradiated (post-cured) with 25, 100 and 200 kGy using an electron acceleration of 10 MeV or 4.5 MeV respectively. Fracture toughness, work of fracture, Vickers hardness and colour changes were measured and compared with not-irradiated specimens. The toughness, work of fracture and hardness increased using 10 MeV with a dose of 25 kGy and with 100 kGy using 4.5 MeV. However, the clinical use may not benefit from the observed small changes. Higher dosage (200 kGy) decreased the values significantly. The colour changes reached a level which was found to be not clinically acceptable.

CONCLUSION

PMMA denture base materials do not benefit from post-curing with electron beam irradiation.