(1) The tensile strength and η_inh of Kevlar tire cord decreased markedly by steam-heat treatment above about 160°C for 1 hr., but they did not change by dry-heat treatment at 200°C for 1 hr. The specific gravity increased with the increase in treating temperature. From these results, it was concluded that the decrease in the degree of polymerization by hydrolysis and the change of fine structure of the steam-heat treated Kevlar tire cords are the causes for the decrease in the tensile strength and fatigue resistance.
(2) Heat shrinkage of Kevlar tire cords was lower than that of nylon 6, polyester and rayon tire cords. Change of X-ray wide angle diffraction pattern in steam-heat treated (tensionless state) Kevlar filaments was very small as compared with that of nylon 6 filaments. On the other hand, we found by DTA and TGA analysis that the decomposition of Kevlar begins at about 500°C and it decomposes at about 605 C. From these results, it was considered that the change of entropy (ΔS) in Kevlar by heat is very small as compared with that of nylon 6, because the molecular conformation of Kevlar is in trans-form and has extended-chain structure with very little flexibility.
(3) a: We observed the fibrillation of Kevlar filaments at the fracture after fatigue test and also at the broken area of the Kevlar tire cords which were tested after steam- and dry-heat treatments by the tensile strength test.
b: Fatigue resistance increased with increasing twist of Kevlar tire cords, but the tensile strength decreased with increasing twist.
c: During fatigue test, the temperature of the air in rubber tube samples which contain Kevlar tire cords was lower than that of polyester tire cords and was equal to that of nylon 6 tire cords.