Studies on tyre cords: degradation of polyester due to fatigue Amit K. Naskar a,1 , A. K. Mukherjee a , R. Mukhopadhyay b, * aDepartment of Textile Technology, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India b Hari Shankar Singhania Elastomer and Tyre Research Institute (HASETRI), Jakaygram, Kankroli, Rajasthan 313342, India Received 2 April 2003; received in revised form 30 May 2003; accepted 3 June 2003 Abstract A comparison of physical characteristics of nylon 6, nylon 66 and polyester tyre cords was made. Polyester was found to have excellent dimensional stability in comparison to others. Service conditions of the tyre cords were simulated by applying cyclic compression and tension onto the cord reinforced rubber composite specimens at different strain levels (±10 and ±15%) and time using a Goodrich compression and tension (C-T) fatigue tester. Fatigued cords were taken out from the composites and their ten- sile properties were measured. Polyester was found to have poor fatigue resistance. At very high strain level (±15%) and after 120 h continuous run nylon 66 registered very good fatigue strength retention (80%) while nylon 6 showed 62% retention but polyester cord failed and broke before taking out from the composite. Poor fatigue property of polyester was attributed to the degradation of the cord as observed by decrease in intrinsic viscosity with fatigue time and poor adhesion to matrix. Apart from heat generation, mechanical scission, organic amines present in the rubber compound possibly catalysed the degradation of the polyester cords. Scanning electron microscopy showed a combination of brittle and ductile failure in the polyester filament due to fatigue. Keywords: Tyre cord; Polyester; Nylon; Degradation; Fatigue 1. Introduction Tyres link a vehicle with the ground, and permit the transmission of forces and torques that appear on the contact patch of a stopped or moving vehicle. Tyre reinforcing elements carry the major share of the struc- tural load of the automobiles. For the better service life of a tyre, tyre-reinforcing elements should exhibit excel- lent fatigue resistance, low heat generation, good toughness and strength properties [1-9]. Details about the tyres, their different components, manufacturing procedures and properties have been reviewed in many articles [4,10-14]. Apart from steel, textile cords like nylon 6, nylon 66 and polyester are the most widely used tyre reinforcing materials. Different cords have different mechanical characteristics. A tyre producer has to select a cord depending on the performance of tyre and manufacturing advantages/constrains. In India polyester tyre cords are not yet popular like nylon cords for heavy vehicles. Studies have been made on physico-mechanical char- acteristics of different tyre cords, to predict dynamic performance of the tyre. Prevorsek el al. [3] have shown that a viscoelastic composite under dynamic stresses generates heat, which causes thermo-chemical degrada- tion and changes in molecular structure. Lee et al. [15] have studied on fatigue behaviour of cord rubber com- posite for the prediction of structural durability of air- craft tyres. It has been exhibited that damage accumulation of the composite in the form of debond- ing, matrix cracking and delamination is accompanied by dynamic creep, heat generation and acoustic emis- sion. But the effect on the cord component has not been investigated properly. Bunsell et al. [16] suggested that creep could not be the controlling mechanism in fatigue failure. Sawada et al. [17] have shown that a polyester fibre in the presence of ammonia or amines undergoes degradation by aminolysis. It has been shown that fati- gue resistance of tyre cord is controlled by several fac- tors such as, twist, interfilament friction, and cord rubber adhesion [8,14,18,19].