Engineering Materials Used in the Manufacture of the Tire

1.      Engineering Materials Used in the Manufacture of the Tire

The earliest tires were composed of canvas, cotton, and other woven fabrics as reinforcement for rubber tires.  Cotton was eventually replaced by rayon, the first man-made fiber.  The use of rayon did not happen until bonding of rubber to rayon was accomplished; this was done using a resorcinol-formaldehyde latex (RFL) bonding process that promoted adhesion of rubber to the smooth rayon fibers.  During World War II, nylon fibers

became available and were first used in the tires of military aircraft where light weight and toughness were principle design parameters.  Nylon fibers tend to cause ‘flatspotting’ in tires, restricting the popularity of nylon in passenger car tires.  ‘Flatspotting’ is the tendency for the nylon fibers to deform while a tire remains stationary for even short amounts of time.  When the vehicle is driven once again the flat spot on the tire eventually regains a round shape but may produce vibrations and thumping for a short distance.  Steel and polyester cords soon followed in the late 1950’s and early 1960’s.  The use of steel and polyester lead to the decline in popularity of rayon and nylon for use in tire cords for consumer vehicles [5]. 

1.1.   Polymers

The modern tire is a composite product consisting primarily of polymers.  In most tires, a blend of both natural and synthetic polymers are used to obtain a composite that fits the tires performance parameters,  be it a racing application or one for commercial trucking.  The various components of the tire will also contain different blends of polymers.  For example, tire treads used for heavy-duty commercial trucking will utilize higher levels of natural rubber for increased fuel economy and tread wear resistance.  Natural polymers will tend to soften with aging where as other synthetic rubbers will harden with age and exposure to oxidation.

         Table 1                         

                     

Typical Percentages of Synthetic and Natural Rubber Found in Various Applications []

1.1.1.     Natural Polymers

Natural rubbers are formed from the latex extracts from the Hevea Brasiliensis although can be acquired from over 500 various plant species.  The Hevea brasiliensis, originally from Brazil, is widely grown in South and South-East Asia, India, the Philippines and West Africa.  The natural polymer is tapped from the bark of the Hevea brasiliensis tree and must undergo a shearing process called mastication due to the extremely high molecular weight of the raw substance, which is on the range of 1 million grams per mole.  Due to the high molecular weight, there are many entanglements of polymer chains in the microstructure and therefore the raw polymer is unable to be shaped through melting processes and is also highly elastic.  Mastication helps break down the molecular weight of the polymer to a more suitable level at which it can be vulcanized.  Natural rubbers generally demonstrate high resilience and are highly elastic.  Natural rubber has disadvantages which include only moderate wear resistance and quite poor resistance to oils, although penetration from oil is relatively slow.

1.1.2.     Synthetic Polymers

Synthetic polymers include styrene-butadiene rubber (SBR) and polybutadiene rubber (BR), along with others.  SBR holds about 30% of the total new rubber market where as polybutadiene takes 10% of the market share for new rubber.  Styrene-butadiene rubber is a copolymer which generally is comprised of 25% styrene and 75% butadiene.  In unvulcanised form, styrene-butadiene compounds have lower strength compared to that of natural rubber’s green strength (unvulcanized strength).  For this reason, reinforcing additives such as carbon black are necessary not only as filler material but for increased strength characteristics as well.  Syrene-butadiene is advantageous over natural rubbers for abrasion resistance as well as wet grip performance.  Lower rolling resistances may also be achieved when using SBR in a tire application. 

1.2.   Waxes and Antioxidants

Paraffinic and microcrystalline waxes are used on tires to protect from harmful ozone and aging degradation of the tire surface which can lead to product failure over time.  Short-term protection is achieved with the use of paraffinic waxes while microcrystalline waxes account for long term protection.  Antioxidants, such as amine and phenolic trimethylquinoline (TMQ), prevent damage due to oxidation.  The antioxidants fight against oxidation by reacting with broken polymer chains and oxides; preventing the effects of oxidation from spreading [7,9].

Table 2                                                                                                                                 

 Material Properties of Natural and Synthetic Rubbers Commonly Used in Tires [13]

1.3.   Metals and Textiles

Metals and textiles are primarily used as tire reinforcements in the form of ply cords and belts inside the tire.  These belts and cords are necessary to provide strength and rigidity to the tire, as mentioned previously.  Modern tire belts are generally composed of steel wire (which may be coated with brass), PET, nylon-6 or -66.  Plies can also be made from steel, polyethylene terephthalate, and nylon depending on the application.  Cottons and rayon used on earlier generation tires are no longer used to any major extent.  Nylon-6 and nylon-66 have melting points of 265°C and 225°C respectively.  Nylon-66 is usually preferred over nylon-6 due to its higher melting point.  Nylons are used in tire reinforcement components for good durability and fatigue resistance although nylon has poor dimensional stability and can cause the aforementioned ‘flatspotting’ if a tire is left stationary.