Sue, forgive me if you know some of this information, but I will try to share some of the context of your family's work. Most U.K. rubber came from Malaya, and this source became unavailable after the Japanese occupation from Dec 1941. Rubber was a critical material, not only for high performance aircraft tyres, but in myriad other critical applications : seals for castor oil based hydraulic fluid systems, such as the Mosquito. This was designed for high altitude reconnaissance, and mineral based fluids could freeze, so no rubber, no Mosquito. The aircraft also used solid rubber shock absorbers, and rubber in self sealing fuel tank coatings, fuel hoses, hydraulic hoses, instrument mountings, Merlin engine seals. All absolutely critical. This criticality was common across all aeroplane designs, even those using castor oil based oleo undercarriage struts. The entire Air Force was exposed by the cutting off of Malayan rubber supply, only later met by South American rubber, convoyed at great cost across the Atlantic, and competitively demanded by the US defense industry. It was a dire situation. Note the Germans, already exposed to a 'rubber vulnerability' were dealing with this issue in the 30's, developing polyurethane as a rubber alternative. Once rubber is vulcanised, or heat set, it cannot be 'unvulcanised' or reused as a material with the characteristics of virgin material. This is the modern predicament of recycling millions of used automotive tyres today. It can be shredded and the crumb may be incorporated as a filler, with obvious compromises in engineering performance. So most of the wartime work was exploring what virgin rubber products could be replaced with 'agglomerations' of recycled crumb, without compromising performance, freeing virgin material for critical applications such as hydraulic seals. One that comes to mind are solid rubber wheels used under tank treads. My understanding is that aeroplane tyres, a critical performance item, stayed using virgin material, but I would be curious to see paperwork exploring the topic. Retreading was developed to economise tyre demand, as well as standardisation of wheel and tyre types. I would always be interested to see copies of technical reports on rubber tyre or component production.