Microgroove discs (“LPs”, “vinyls”)

From the late 1940s onward a new material was used for replicated pressed discs: a co-polymer of polyvinyl chloride (PVC) and polyvinyl acetate (PVA) was introduced for two different new formats. The RCA record company launched a seven-inch (= 17 cm) disc that runs at 45 rpm for three minutes per side — in terms of duration, a continuation of the old shellac disc format. Columbia started the 10-inch (= 25 cm) LP, later enlarged to 12 inch (= 30 cm), both of which run at 331/3 rpm. Playing times are 15 and 25 minutes per side, respectively. This new material with its almost amorphous structure allowed much finer mechanical signal representation, which made narrower grooves, lower speeds and, therefore, longer playing times possible. The amorphous structure of the plastic also produced considerably less surface noise than shellac discs. PVC/PVA co-polymer, colloquially termed “vinyl”, is chemically very stable. Except for a few very early discs, an average vinyl disc is chemically in good shape. The material is comparatively soft, however, and therefore vulnerable to damage by scratching or abrasion.

In the early days of microgroove disc, small numbers were produced by injection moulding using styrene. These discs can be identified by their light weight and their relatively matte surface in relation to the shiny surfaces of “vinyls”. In reproduction, they have a higher surface hiss level than vinyl records. No systematic stability problems have been observed with this type of LP.