Plastic armour was a type of vehicle armour

Vehicle armour

Military vehicles are commonly armoured to withstand the impact of shrapnel, bullets, missiles, or shells, protecting the personnel inside from enemy fire. Such vehicles include tanks, aircraft, and ships....

 originally developed for merchant ships by Edward Terrell

Edward Terrell

Edward Terrell OBE was a Liberal politician, a successful barrister and magistrate with a flair for invention; by 1940 he had registered a number of patents relating to pens, ink bottles and peeling knives...

 of the British Admiralty in 1940. It essentially consisted of small, evenly sized rock or stones in a matrix of bitumen.
It was typically applied as a casting in situ to existing ship structure in a layer of about two inches thick or formed in equally thick sections on a half inch thick steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

plate for mounting as gun shields and similar.

It replaced the use of concrete slabs which although expected to provide protection were prone to cracking and breaking up when struck by armour piercing bullets.
Plastic armour was highly effective at stopping armour piercing bullets because the very hard particles would deflect the bullet which would then lodge between plastic armour and the steel backing plate.