Polymer architecture via abstraction of a hydrogen, can create a mid-chain growth site. Branching can be quantified by the branching index. An effect related to branching is chemical crosslinking - the formation of covalent bonds between chains. Crosslinking tends to increase T and increase strength and toughness. Among other applications, this process is used to strengthen rubbers in a process known as vulcanization, which is based on crosslinking by sulfur. Car tires, for example, are highly crosslinked in order to reduce the leaking of air out of the tire and to toughen their durability. Eraser rubber, on the other hand, is not
Extreme pressure additive pressure additives are rarely used in motor oils. The sulfur or chlorine compounds contained in them can react with water and combustion byproducts, forming acids that facilitate corrosion of the engine parts and bearings. Extreme pressure additives typically contain organic sulfur, phosphorus or chlorine compounds, including sulfur-phosphorus and sulfur-phosphorus-boron compounds, which chemically react with the metal surface under high pressure conditions. Under such conditions, small irregularities on the sliding surfaces cause localized flashes of high temperature (300-1000 °C), without significant increase of the average surface temperature. The chemical reaction between the additives and the surface is confined to this area.