Recycling of polyurethane rubber and foam by the application of high power ultrasound

The use of high power ultrasound to break down chemical crosslinks in thermosets is a recent approach that has been successful in recycling numerous thermosets. The present study involves the application of ultrasound for recycling of thermoset polyurethanes. Sulfur-cured polyurethane rubber (PUR), both unfilled and filled with carbon black was used. Ultrasonic treatment was carried out using the coaxial and grooved barrel reactors designed and developed in our laboratory. Treated and untreated samples were characterized via measurements of molecular weight, gel fraction, crosslink density, cure behavior, dynamic and mechanical properties. In the case of filled rubber, due to polymer filler interactions and reduced mobility of the rubber chains at the surface of the filler particles, more chain breakup and partial deactivation of the filler were seen to occur when subjected to ultrasound. Revulcanization of treated rubbers shows a decrease in properties but on blending with the virgin in various proportions, properties comparable to that of the virgin were obtained.; A similar treatment was applied to polyurethane foams used for the manufacture of car seats. The decrosslinked foam was characterized using similar techniques as the rubber and blended with PUR in various proportions; the blends show properties similar to the virgin rubber but exhibit higher crosslink densities, modulus and hardness.; Molecular mobility and diffusion studies were carried out with the samples using NMR spectroscopy to understand the mechanism of breakage of crosslinks, by ultrasound, on a molecular level. The transverse relaxation decay, for both rubber and foam, was successfully described using a three-component model. The diffusivity spectrum for treated PU foams was bimodal in nature consisting of a fast-moving monodisperse and a slow-moving polydisperse species. However, diffusivity spectrum for PUR could not be obtained due to the proximity of its Tg to the temperature of the experiment causing all spin-spin relaxation times to be too short.; The Dobson-Gordon theory of rubber network statistics was employed to interpret the dependence of gel fraction in devulcanized rubbers on the crosslink density. In case of unfilled PUR, the breakage of crosslinks dominated over main chain breakage but with increase in the amount of filler, the breakage shifts towards main chain breakage.