Since their appearance in 1937, polyurethanes (PUs) have found rapid and wide acceptance because of their versatility that makes them fit to very different applications. [1] By careful selection of the two main components isocyanate and polyol and adjusting the formulation, which may include several other components or additives, it is possible to obtain materials with quite different properties, elastomers, thermoplastics, rigid thermosets, and foams. [2][3][4][5][6][7] The relatively recent appearance of waterborne polyurethanes (WBPUs) in the academic and commercial sectors is a response to the needs of using less environmentally harmful formulations [8][9][10][11][12] by replacement of traditional organic solutions by aqueous suspensions. The preparation of self-supporting films or the application of coatings from traditional PUs leads to the release of organic volatiles to the environment. To solve this issue, WBPUs are prepared as aqueous suspensions, in which PUs particles, generally in the order of 20-200 nm, are dispersed and stabilized in water. [13,14] The stabilization is most usually achieved by the incorporation of an acid (or basic) diol to the molecular structure of the polymer. This component results in the formation of a polyanionic or a polycationic prepolymer (most frequently the first one), which is balanced by the addition of a counter ion.Different components, organic, such as cellulose nanocrystals, or inorganic, such as silica, silver, clay, or magnetic nanoparticles (MNPs) can be added to polymers to modify their properties or to add new functionalities. [15][16][17][18] In particular, MNPs have been considered widely to generate materials with magnetic or superparamagnetic properties, that can be used in medical applications, sensors, magnetic filters, or incorporated in materials to be applied in anechoic chambers or as antistatic coatings, etc. [19][20][21][22] Magnetite nanoparticles are produced by relatively simple methods that lead to well-controlled nanosizes. One of the most frequently used synthesis is the chemical coprecipitation method using ammonium hydroxide as the precipitating agent. [23][24][25] In the development of magnetic nanocomposites, it is desirable that the MNPs are smaller than 10 nm to exhibit superparamagnetic properties. [26] This type of magnetic behavior translates into the loss of coercivity and remanence, usually presented by these oxides. Among the advantages of using these nanoparticles, it could be mentioned that their nontoxicity and high biocompatibility make them suitable for application in biotechnology and biomedicine. [27] Pursuing a different goal, other
