Elastocaloric properties of Natural Rubber for Solid-State Cooling Technologies

In recent years, the need to develop sustainable and energy efficient air-cooling technologies has become increasingly urgent due to the environmental and energy-related disadvantages of conventional vapour-compression air conditioning systems. Solid-state cooling technology, based on caloric effects, represents a promising alternative, with elastocaloric cooling identified as one of the most suitable candidates for replacing conventional air-cooling systems. In this framework, natural rubber (NR), a well-known renewable resource, is attracting increasing attention as an elastocaloric material, owing to its low cost, non-toxicity, long fatigue life, and remarkable elastocaloric properties, which arise from the synergistic contribution of thermoelasticity and strain-induced crystallization. Nevertheless, research on the refrigeration potential of natural rubber is still in its infancy, and not all parameters influencing the elastocaloric behaviour of this material are fully understood. In particular, most studies so far have focused primarily on the influence of testing parameters such as temperature, strain rate and maximum deformation. This thesis aimed at investigating the influence of rubber network structure and formulation on the elastocaloric properties of NR-based systems. Specifically, the influence of (i) the crosslinking density, and (ii) the addition of nanofillers was systematically analysed. In the first part, natural rubber samples with varying crosslinking densities (ranging from 2.9 to 5.2 10-4·mol/cm3) were prepared and characterized from a structural, thermal and mechanical point of view. Their elastocaloric properties were then thoroughly examined, revealing that crosslinking density is a crucial structural parameter governing cooling performance. The results showed that reducing the crosslinking density enhanced cooling capacity, primarily due to stronger contributions from strain-induced crystallization and thermoelasticity. In the second part, the effect of adding both natural (MMT) and organo-modified (O-MMT) nanoclays to the most promising formulation identified in the first part of the work was assessed. Two series of NR-based nanocomposites, containing 1, 3 and 5 phr nanofiller, were prepared and characterized in terms of morphology, thermal, mechanical and elastocaloric properties. The results highlighted that the better dispersion of the organoclays within the rubber matrix promoted not only a better mechanical behaviour (in terms of stiffness and strength), but also a significantly enhanced cooling performance with respect to MMT nanofilled systems. The NR/O-MMT samples demonstrated up to a ~45% increase in heat absorbed per refrigeration cycle compared to the unfilled NR. This improvement was ascribed to a synergistic combination of enhanced strain-induced crystallization, improved macromolecular structural homogeneity, and potential strain amplification effects promoted by the organoclays. These findings demonstrated that the rubber network structure and formulation represent the fundamental starting point for future research aimed at maximizing the cooling performance of NR-based systems, given their strong influence on the elastocaloric behaviour. This is therefore a fundamental first step towards scaling up solid-state cooling technologies.