Metrolite, Deccalite, Merco Plastic, and Sav-o-flex! What are these “non-breakable under normal use” 78rpms actually made of? Is it styrene? Is it PVC? I'm fairly sure these are very hard PVC blends. Polyvinyl chloride itself can be made either hard or soft, by adding more or less plasticizer. I've also read that Decca pressed some 789s in styrene. Styrene is more brittle much like the original shellac though it is quieter like the PVC.
As the era of shellac wore on, some labels debuted what appear to be hard PVC plastics. Some of them claimed to be patented by the labels but exactly what varies between the plastic blends is unclear.
These could not be played on the old hand crank phonographs. the weighty tone arms would wear down the groove very quickly. "Deccalite" was used by the Decca label “Metrolite” by the MGM label, “Merco Plastic” by the Mercury label) and “Sav-o-flex” by the Savoy label.It must have been very political internally. The 33 had debuted in 1930 so the 78 had nowhere to go but down. Shellac was scarce because of the war effort and more expensive than PVC and required more care in it's mass production. So labels began transitioning away from Shellac and pressing their first PVC 78s. In 1949 MGM began issuing 78s in "unbreakable" Metrolite.
RCA-Victor paved the way in 1930, launching the first vinyl long-playing record. It was marketed as a "Program Transcription" disc. These discs were designed for playback at 33⅓ rpm and pressed into a 30 cm flexible plastic disc. Consume acceptance was slow initially, but they shipped better. Brittle shellac often cracked or chipped in transit. These first "vinyl" 33s were DJ copies. they shipped well and the surface noise was far superior. They were great for broadcast. And as you know their popularity stayed strong well into the 1980s when they were finally overtaken by CDs.