Unique Properties of Cord

Cord, also known as cord, is the warp material used in weaving cord fabric. Cord fabric is woven with cord yarn as the warp and medium (or fine) count cotton yarn as the weft. The warp yarns are densely arranged, bearing loads; the weft yarns, which merely stabilize the warp yarns, are sparsely arranged, resembling a cord, hence the name cord fabric. It is used as a reinforcement material, or skeletal material, in rubber products such as tires, conveyor belts, and transmission belts, primarily for tires. The properties and manufacturing techniques of cord vary depending on the rubber product. General requirements include: ① high strength and initial modulus; for example, polyamide and polyester cords must have a strength of at least 79 cN/dtex; ② good heat resistance; ③ excellent fatigue resistance and impact load resistance; ④ good dimensional stability; and ⑤ adhesion to rubber.

Cord materials include cotton yarn, viscose fiber, polyamide fiber, polyvinyl alcohol fiber, glass fiber, aromatic polyamide fiber, and steel wire, each with varying properties. The selection is often based on the performance requirements of the rubber product and other factors, such as cost.

① Viscose cord offers fatigue resistance, heat resistance, and excellent dimensional stability, becoming the dominant type by the mid-1940s. However, it has the disadvantage of being easily hygroscopic, which reduces its strength. With the development of highways, vehicle speeds increased, and trucks, engineering vehicles, tractors, and military vehicles became larger, as did aircraft, which became larger and faster. This placed higher demands on tire strength, heat resistance, and fatigue resistance. In the 1960s, synthetic polyamide cord gradually replaced viscose cord. It is primarily used in radial passenger car tires.

② Polyamide cord offers high strength and excellent impact and fatigue resistance, but suffers from poor dimensional stability and poor adhesion to rubber. It must be impregnated and heat-stretched before use to improve its dimensional stability and adhesion. It is widely used in truck and aircraft tires, as well as in tapes and hoses.

③ Polyester cord offers high strength, high initial modulus, good thermal shrinkage, and dimensional stability. However, it generates high fatigue heat and is easily aminized and hydrolyzed, reducing its strength and making it difficult to bond with rubber. To improve adhesion, it requires surface activation and dipping. It is widely used in passenger car tires.

④ Aromatic polyamide cord, developed in the United States in the 1970s, offers high strength and modulus, improved heat resistance, and dimensional stability. However, it is expensive and exhibits poor adhesion to rubber. It is suitable for tires requiring high performance, such as those for luxury cars, military vehicles, and aircraft.

In addition to the chemical fiber cords mentioned above, there are also cotton cords and steel cords. Cotton cord was first used in tires. It has excellent adhesion to rubber and reached its peak in the 1920s and 1930s. However, due to its requirement for long-staple, high-quality cotton, it has low strength, slow heat dissipation, and poor heat resistance. Since the introduction of viscose cord in 1926, it has gradually been replaced. Tire carcasses made from steel cord offer high strength, low rolling loss, dimensional stability, and excellent impact, wear, and heat resistance. However, fatigue resistance is poor and adhesion to rubber is poor. Steel cord is primarily used in radial truck tires, high-strength conveyor belts, and high-pressure pipes. In recent years, steel cord production has grown rapidly, with its total output now ranking first among all types of cord.

For example, in the manufacturing process of nylon 6 (see polyamide), heat-resistant agents are added to nylon 6 resin to create chips with a relative viscosity greater than 3. These chips are melt-spun into porous, coarse-denier fibers. Two strands are then combined and twisted into 93 tex/2, 140 tex/2, or even thicker cords. These are then woven into cord fabrics, impregnated with a mixture of resorcinol-formaldehyde resin and latex, heat-stretched, and calendered to coat with rubber before being used as the backbone material for rubber products such as tires, hoses, and tapes.