ポリカーボネートの光劣化機構

The photodegradation mechanism of polycarbonate by irradiation with ultra violet rays has been investigated using infrared and ultra violet analysis. It has been concluded that the degradation mechanism has two types discribed below depending upon the wave length of irradiating ultra violet rays:<BR>1) When polycarbonate is irradiated with long wave length UV rays (>about 280mμ) contained in sun light at sea level, light absorption takes place at about 287mμ. This absorption is based on the excitation of a p-electron of C=O in the carbonate linkage from a non-bonding oxygen orbital to an antibonding p-orbital (n-π^* transition). This excitation energy breaks down the ether linkage in the carbonate group by innermolecular transfer phenomenon. Then auxochromes of phenols, ethers, esters, acids and so on are formed with liberation of CO and/or CO₂ and depolymerization reaction progresses.<BR>2) Under irradiation with short wave length UV rays (240-about 280mμ), polycarbonate absorbs light at 258-273mμ the fine structure of this absorption spectrum is due to π→π^* forbidden transition of the benzene group. This excitation energy attacks the carbonate linkage with the same mechanism as above mentioned, and also attacks the isopropylidene linkage by innermolecular transfer phenomenon. Presumably there takes place depolymerization reaction with formation of isopropenyl linkage, diphenyl ethylene linkage and so on. On the other hand the cross links are developed and then irradiated polycarbonate becomes insoluble in methylene chloride.<BR>In this study, interesting phenomena were observed, i. e., IR and UV spectra changed with time after irradiation. Moreover the change at long wave length and short wave length UV rays was different from each other.<BR>Those phenomena suggest that the formed radicals and/or reaction intermediates have relative long life and two degradtion mechanisms are different each other.