# Thermal and Mechanical Behavior of Elastomers Incorporated with Thermoregulating Microcapsules
## 1 Introduction  
Polyurethane (PU) stands out as one of the most demanded polymers, and an exponential growth of its market is expected, which could reach USD 105.2 billion by 2025 [1]. This growth is based on the constant development of enhanced PUs by the improvement of their properties for covering new applications. PU properties can be modified by varying the amount and type of polyols and isocyanates and the rest of the additives employed in their synthesis, which also allows for classifying them into two main groups: foams (flexible or rigid) and coatings, adhesives, sealants and elastomers (CASEs) [2]. Among PUs, elastomers can be also classified as cast elastomers, thermoplastic polyurethane elastomers, millable polyurethane gums and microcellular elastomers [3]. Theroplastic polyurethane elastomers are one of the PUs with the fastest growth, since they are extremely adaptable to a high number of applications such as sports footwear, athletics tracks, auto body side molding, automotive lumbar supports and electronicproducts [3, 4]. In most of these applications, the mechanical and thermal properties are essential to achieve proper human comfort and electronic device performance and durability. As for the rest of the PUs, the enhancement of the elastomers' quality and characteristics is related to the amount and type of their main constituents and additives [5]. The main constituents are polyester or polyether polyols (soft segments) and a diisocyanate whose chain is extended with a low molecular weight diol (hard segment) [5]. Regarding the additives, there has been an increased tendency to synthesize and characterize various clay-containing polymer nanocomposites (CPNs) for improved CASE applications for the last 10-15 years [6, 7]. By focusing on elastomer polymers improved by additives, [PERSON] et al. [6] reported the employment of poly(ethylene-co-octene)-poly(ethylene-co-vinyl acetate) as a hybrid polymer additive for poly(ethylene-co-octene)-elastomers (POEs). The addition of only 0.5 wt.% of this additive into the POE improved the tensile strength, modulus, elongation at break, stress relaxation and hysteresis with respect to the virgin POE. Furthermore, they added organically modified montmorillonite (OMt) to the POE (0.5-1.0 wt.%), resulting in a remarkable improvement in appearance compared with the virgin POE. The addition of cellulose nanocrystals (CNCs) into a styrene-butadiene rubber (SBR) elastomer was assayed by [PERSON] et al. [8], improving the tensile storage moduli (E\({}^{\prime}\)), [PERSON] et al. studied the addition of CaSO\({}_{4}\) as filler into an epoxidized natural rubber (ENR) matrix, observing a significant improvement in the mechanical properties, reaching a 100% increase in the dynamic modulus and the thermal stability. [PERSON] et al. [9] introduced a novel reactive rubber composite made by compounding magnesium oxide (MgO) powder with hydrogenated nitrile butadiene rubber (HNBR) using different MgO concentrations of 0, 14, 28 and 40% \(v/v\). The elastic modulus value tripled, reaching 80 MPa, while doubling in volume for the rubber filled with 40% MgO by volume. Regarding the thermal properties, the enrichment of the thermoregulating capacity of elastomers to enhance the thermal comfort of the end users is garnering important attention. The thermoregulating capacity of PU foams applied in the footwear and mattress industries or automobile and construction sectors has been improved by the incorporation of microcapsules containing phase change materials (PCMs) [10, 11]. PCMs can absorb or release the energy equivalent with their latent heat when the temperature undergoes or overpasses their melting points. Most PCM applications require their previous encapsulation or PCM stabilization before the inclusion in the final system to avoid leakage during the melting process [12]. In the case of microcapsules, their shell type, size and amount of used microencapsulated PCMs determine their final distribution into the polyurethane matrix [13].