The effects of low-intensity UV light irradiation on tensile strength of high-modulus (HM) type poly-p-phenylene benzobisoxazole (PBO) fiber improved tensile modulus by heat-treatment were investigated in a monofilament tensile test. Irradiation time was set to 0 h, 1 h, 10 h, 100 h and 1,000 h, while radiance were arranged to become 2, 4 and 8 W/m2. Standard modulus (As spun: AS) type PBO fiber that authors reported its properties before was employed as standard specimen. It was found that the tensile strength distribution of UV irradiated HM type fiber could be approximated to a normal distribution. The relation between tensile strength and radiation time depend on the degree of radiance. However, the relation between tensile strength and radiation dosage was independent of the degree of radiance, and the radiation dosage was most suitable parameter to describe the tensile strength at the different radiance. The tensile strength of HM type fiber was higher than that of AS type fiber over all radiation dosage. It was found by AFM observation that amount of the surface roughness on the UV irradiated HM type fiber with increasing the radiation dosage was smaller than that of the UV irradiated AS fiber. The reason for that poor crystallite orientation region in HM fiber was less than that in AS fiber and an increase in roughness causes autoxidation occur predominantly in the poor crystallite orientation region. Thus the decrease in tensile strength of fibers under UV light irradiation was attributable to not only cutting an internal molecule by the exposure to UV radiation but also the roughness on the surface of the fiber. Effect of roughness on the tensile strength of the UV irradiated HM type fiber was smaller than that of AS type fiber. However, the effect of roughness on the tensile strength of both fibers was not very large.
