POLYMER MODIFICATION BY IMPROVED IRRADIATION TECHNOLOGY T. Seguchi, T. Yagi, S. Ishikawa*, and Y. Sano* MY0001468 Takasaki Radiation Chemistry Research Establishment, Japan Atomic Energy Research Institute, Takasaki, Gunma, 370-1292 Japan, Fax: (81) 27-346-9687, E-mail: [email protected] *The Shiyuukai Foundation, The Medical Machinery and Materials Laboratory, Masago, Niigata 950- 2074 Japan, Fax:(81) 25-233-8041 1. INTRODUCTION Irradiation technology for polymer modification has been widely applied; the processing of heat resistant wire and cables, polymer form, shrinkable tube, and so on. These applications are based on the radiation induced crosslinking among polymer chains, and the most of processing has been carried out at ambient temperature in atmosphere. It is known that the radiation induced chemical reactions are affected by the irradiation conditions such as atmosphere, temperature, but those effects are not so considerable problem on the processing for above applications. Recently, it was found that the chemical reactions are much dependent on the irradiation temperature for some polymers. A typical one is polytetrafluoroethylene(PTFE), which is chain scission by irradiation at ambient temperature, but crosslinking at the high temperature as the molten state(1). Another case is polystyrene(PS), which is crosslinking at ambient temperature, but chain scission become predominant above the glass transition temperature®. For engineering plastics as polycarbonate(PC) and polysulfone(PSF), the irradiation effects were widely studied on the point of irradiation temperature. A remarkable change on hardness for PC and PSF was observed at high temperature irradiation. This is a new technology for polymer modification by radiation application. 2. EXPERIMENTAL 2.1 Material The chemical structure of polycarbonate(PC) and polysulfone(PSF) for sample was shown in Fig.l. PC was supplied from Mitsubishi Engineering Plastics Co.Ltd., the molecular weight is Mn: 2.3 x 104, and glass transition temperature is about 150 C. PSF was from Union o- Carbide Co.Ltd., the molecular weight 2.8 x 104, and glass transition Polycarbonate (PC) temperature is 178C. Both polymers were molded to 4 mm thick sheet. The sheet sample was cut to 20 mm x 20 mm for the hardness and resistant Polysurfone (PSF) to wear test and dumbbell shape (JIS K7162-1B) for tensile test. Fig.l Chemical structure of PC and PSF 496 2.2 Irradiation Polymer sheet was sealed in glass tube after evacuation up to 10"2 Pa. The glass tube was irradiated by ^Co gamma-rays in a thermostat which is controlled at a constant temperature. Irradiation dose was 1-300 kGy with a dose rate of 3 kGy/h. 2.3 Measurement Hardness of samples was measured by Rockwell meter. The resistant to wear was measured by abrasion test. The change of mechanical properties was obtained by tensile test. All of the measurements were carried out at room temperature in air. 3. RESULTS 3.1 Polycarbonate (PC) The original PC sheet sample showed transparency. The sheet sample was colored to yellowish above few hundred kGy by increase of dose, and the coloring was accelerated by increase of irradiation temperature. The Rockwell hardness increased much with dose at high temperature around 150C, and reached the maximum at 3-4 kGy, whereas the hardness decreased gradually with increase of dose at room temperature as shown in Fig.2 (a) and (b). The temperature dependency on Rockwell hardness at 3.6 kGy was plotted in Fig.3. The peak of hardness appeared at 150C. The coloring was scarce in the dose range of several kGy. 120 100 150°C CO 20 50 100 150 200 250 2 3 4 5 Dose (kGy) Dose (kGy) (a) (b) Fig.2 Changes of Rockwell hardness of PC irradiated at 150 C and at 25 C under vacuum. The mass loss by abrasion against dose was plotted in Fig.4. The abrasion by rubbing test decreased sharply with dose and saturated at higher doses above 10 kGy. These data mean that the resistant to wear was improved by about 30% or more by the irradiation of 3-4 kGy at 150C. Improvement of the 497 resistant to wear is closely related to the increase of hardness at 150C irradiation as 90 shown in Fig.2 (b). Dose: 3.6kGy The tensile strength and elongation at break were determined from tensile test 80 after irradiation at 150C and were plotted in Fig.5. The strength was not so changed CO until about 10 kGy, but the elongation at 0) c break decreased to half at around 4 kGy. •a CO 70 As the comparison, the elongation change by irradiation at room temperature was almost constant in these low dose ranges. 8 cc The gel formation was not observed for both polymers in low dose up to lOkGy at high temperature. The density of PC increased by 1% in a 50 case of high temperature with 3.5 kGy, but it was not changed in a case of room 130 140 150 160 170 temperature irradiation. Irradiation temperature (°C) Fig.3 Rockwell hardness of PC irradiated to 3.6 kGy at various temperatures. 1.20 - 80 0.00 5 10 15 2 4 6 8 10 Dose (kGy) Dose (kGy) Fig.4 Resistant to wear of PC irradiated at Fig.5 Tensile strength and elongation at break of PC 150 C under vacuum irradiated at 150 C 498 100 3.2 Polysulfone (PSF) The changes of Rockwell hardness for PSF were shown in Fig.6 when PSF was irradiated at high temperature of 150,170,185, and 200 C. The hardness c "2 increased sharply with dose until 3-4 to kGy, and the maximum was attained at CD temperature around 170 C. Above 185 o o C the hardness decreased. At room DC temperature irradiation it did not increase and tended to decrease with dose. The mechanical properties were 4 6 8 10 12 scarcely changed in these lower doses Dose (kGy) even at high temperature as same as the Fig.6 Changes of Rockwell hardness of PSF reported data(3). irradiated at high temperature 4. DISCUSSION The Rockwell hardness of both PC and PSF increased much by low dose irradiation at high temperature, but it did not increase at lower temperature. The apparent chemical reactions such as crosslinking or chain scission were negligible and also the changes of mechanical properties were scarce or small in dose range less than 3-4 kGy. The temperature at peak of Rockwell hardness corresponds to the glass transition temperature(Tg) for both polymers. The chemical reactions would be much accelerated by the improvement of molecular motion at Tg when the active species were induced by radiation. Although the chemical reactions as crosslinking or chain scission for PC and PSF were not clear at the peak of hardness, a certain amount of chemical reactions should occur. If the chain scission occurred at the molecular entanglement in the polymer matrix, the rearrangement of molecular packing should proceed with the progress of molecular motion at Tg. Then, it is assumed that the main factor of hardness improvement is induced by the change of molecular packing in polymer matrix. The increase of density at the peak of hardness for PC may support the model of condensed molecular packing. A small amount of crosslinking might contribute to improve hardness and also resistant to wear. With increasing the dose more than 10 kGy, the products induced by irradiation should be accumulated according to total dose, so the molecular packing would be loosen to result the decrease of the hardness even if irradiation was carried out at Tg of the polymer. 5. CONCLUSION The hardness of PC and PSF as the useful engineering plastic could be improved by the irradiation at around glass transition temperature. As the dose is so small, the radiation damage to these polymer materials would be negligible. This is a new technique for radiation application to polymer modification. In the present technology, the hardness for engineering plastics was improved by the mixing with 499 inorganic additives such as ceramic powder or glass fiber filler. This radiation hardness technique could be applied for these engineering plastics, the process and the products should become clean. REFERENCES (1) A.Oshima, Y.Tabata, FLKudoh, and T.Seguchi, "Radiation induced crosslinking of polytetrafluoroethylene", Radiat.Phys.Chem. Vol.45, pp.269-273 (1995) (2) Y.Tabata, A.Oshima, KTakashika, and TSeguchi, "Temperature effects on radiation induced phenomena in polymers", Radiat.Phys.Chem., Vol.48, pp.563-568 (1996) (3) D.J.T. Hill, HLKudoh, and T.Seguchi, "The elevated temperature radiation chemistry of some engineering thermoplastics containing aromatic group", Radiat.Phys.Chem., Vol.48, pp.569-571 (1996) 500.