The crack resistance of heat-shrinkable tubing during heating was related to the material properties at temperatures higher than its melting point. The crack initiation temperature was derived from the temperature dependence of breaking stress and strain of heat-shrinkable tubing. The oxidative induction time of heat-shrinkable tubing was measured by a differential scanning calorimetry method, and the relationship between its induction time and the thermal oxidative deterioration was discussed. The notch resistance was investigated in the range of various % shrinks of notched heat-shrinkable tubing. When heat-shrinkable tubing is used in the range of 30∼60% shrink and has normal shrink stress (radius direction stress caused by shrink) of about 100g/cm2 in the % shrink, the material properties required for good crack resistance must be as follows:
(1) When the tubing temperature rises up to 350°C, the breaking strain is more than 9 at 130°C.
(2) To prevent the thermal oxidative deterioration, the oxidative induction time at 200°C is more than 90min.
(3) Tangential shrink stress (circumference direction stress caused by shrink, σT) at 30% or 60% shrink is more than 100r/t0g/cm2, where t0 is the thickness of completely shrank tubing and r is the radius at 30% or 60% shrink. In this case, the ratio of breaking stress (σb) to σT is more than the stress concentration factor at notch.
