An association football, or ‘soccer’ ball, essentially consists of an inflatable bag, or bladder, surrounded by a lining and covered with synthetic leather. A synthetic rubber, known as butyl rubber, or just butyl, is impermeable to air and, as such, is the material of choice for bladders in high-quality footballs. Likewise, polyester, a synthetic fabric, has replaced cotton as the material of choice for linings in state-of-the-art footballs, principally because its fibres degrade slower than the natural alternative. Several layers of lining are laminated together and placed between the bladder and the cover to provide shape, strength and consistent bounce characteristics.
The outer cover of high-end footballs typically consists of panels of polyurethane (PU) leather, which is not waterproof, but water-resistant, so does not become waterlogged like old-fashioned full grain leather. Most footballs consists of 32 panels, 20 hexagonal and 12 pentagonal, arranged to form a figure that mathematicians call a truncated icosahedron. Once assembled and inflated, the faces of the football bulge due to the internal air pressure to create a perfect sphere, as required by the Laws of the Game.
Upmarket footballs are usually tightly hand-stitched, with thick, waxed polyester or nylon cord, by turning the ball completely inside out. Hand-stitching is highly labour-intensive and even an experienced stitcher can take three or four hours to complete a single football from start to finish. Neverless, the process creates deep, strong seams that are water- and wear-resistant. By contrast, thermally bonded, or moulded, footballs are not stitched at all. Instead, adhesive is applied to the edges of the component panels, which are assembled in a mould and heated, under pressure, to join them together. This process is obviously less labour-intensive, and cheaper, than hand-stitching the panels together, but the finish is inferior.