Fig. 6-5 Radiation induced cross-linking for PCL When PCL is irradiated by ionizing radiation, cross-linking takes place between polymer chains. Cross-linked PCL does not melt and becomes insoluble in common solvents.

Fig. 6-6 Radiation cross-linking of PCL at super-cooled state Gel fraction is defined as the ratio of insoluble part of the polymer weight after irradiation to the initial weight of the polymer, and this value shows the degree of cross-linking achieved. In the ordinary solid state, radiation induced cross-linking proceeds moderately. In the molten state, however, the rate of cross-linking increases but many bubbles appear in PCL during radiation cross-linking (Fig. a). In the irradiation under the super-cooled state, the rate of cross-linking is larger and the thermal resistance of cross-linked PCL elevates by about 60 degrees cent.

Fig. 6-7 Bio-degradability of radiation cross-linked PCL PCL is decomposed in soil, finally to water and carbon dioxide. Inhibition of decomposition by microorganisms was suspected due to formation of cross-links, but the high degree cross-linked PCL (50 - 100% gel fraction) is decomposed by microorganisms in soil at same rate as non-irradiated specimen.

Plastics are indispensable materials for comfortable human life. With the increase in consumption, they have a large effect on the environment due to their non-corrosive character. An approach for their decomposition is the utilization of enzymatic degradable synthetic polymers in soil (biodegradable polymer) in the same manner of such natural high polymeric materials as cotton, wool and silk. Poly (epsilon -caprolactone), PCL (Fig. 6-5), has gained attention as a synthetic biodegradable polymer, but it has a disadvantage in low thermal resistance because its melting point is at 60 degrees cent. The study, intended to extend utilization fields of PCL, in which its biodegradative character is kept and thermal resistance is improved, has been made by using the radiation cross-linking technique in JAERI. When PCL is irradiated above the melting point of 60 degrees cent., efficiency of cross-linking increases, however the probability of radiation induced decomposition increases at the same time and many bubbles are formed in a PCL sheet by gaseous products. It is observed that PCL turns to the non-crystalline state after quenching from the molten state PCL to about 45 degrees cent. and remains in a super-cooled state like inorganic glass. By irradiation at this super-cooled state, the radiation induced cross-linking between polymer chains takes place efficiently and the irradiated PCL is improved to a plastic with thermal resistance up to 120 degrees cent. Most characteristics of PCL such as biodegradation in soil are not inhibited even though there are more cross-links, and 50 to 60% of cross-linked PCL is decomposed by burying it in soil (Fig. 6-7). Studies for the development of various kinds of biodegradable polymers are still in progress.

Reference D. Darwise et al., Heat Resistance of Radiation Crosslinked Poly (epsilon -caprolactone), J. Appl. Polym. Sci., 68, 581 (1998).

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