# A Green Approach for Preparing High-Loaded Sepiolite/Polymer Biocomposites
## 1 Introduction  
The demand for environmental process and products made from renewable and sustainable nonpetroleum-based resources is progressively growing in our society [1; 2]. Among renewable materials, natural rubber latex (NRL) is the main elastomeric material obtained from the sap of the _Hevera Brasiliensis_ tree, used for specific applications in different industry sectors, also combined with other rubbers and thermoplastics [3]. NRL appears as a milky colloid, based on poly(1,_A-cis_-isoprene), a highly unsaturated hydrocarbon polymer, and non-rubber components as water, carbohydrates, proteins, polypeptides, fatty acids, and phospholipids [4]. Natural rubber (NR) polymer has very high structural regularity, providing remarkable properties, i.e., good film-forming ability, highelasticity, low hysteresis and high resilience [5; 6]. Within NRL, the nonrubber components contribute significantly to the excellent mechanical properties of NR [6]. Thus, NR is still indispensable for heavy-duty applications, especially tires [7; 8] and rubber bearings for seismic, and is widely used to produce nanocomposites for protective thin films, adhesives, gloves, coatings, joints, pipes and tubes [9]. In addition, NRL can be easily employed for the preparation of rubber nanocomposites through the latex compounding technique (LCT). This method consists of mixing a stable aqueous elastomer dispersion (latex) with an aqueous dispersion of reinforcing filler. LCT offers several advantages: (i) absence of organic solvents; (ii) use of natural polymers available in latex form; (iii) since water acts as an excellent swelling and exfoliating agent, no expensive surface modification procedures for hydrophilic fillers, such as silica and clay, are required. Moreover, LCT represents an economic and ecofriendly alternative to the commonly employed melt mixing technique, which is based on the introduction of all the solid ingredients into a mixer [10]. In fact, during traditional melt mixing volatile organic compounds are commonly released, in contradiction with the increasing legislative restrictions about emissions. Thus, LCT appears a promising and sustainable approach with clear benefits for the environment and the workers. In particular, LCT seems particularly appropriate for fillers like clays (e.g., talc, mica or phyllosilicates), whose efficient dispersion through melt mixing still remains a challenging issue [11], due to their poor compatibility with the organic matrix [12]. Clays are considered promising candidates for the strengthening of various types of polymers due to their lamellar structure with particles of high aspect ratio (AR) [13]. Besides, they are naturally occurring and readily available, almost ubiquitously, in large quantities compared to traditional silica and carbon black. Among clays, sepiolite (Sep) is an abundant, natural and nontoxic phyllosilicate. Sep is a hydrated magnesium clay with unit cell formula Si\({}_{12}\)O\({}_{30}\)Mg\({}_{8}\)(OH, F)\({}_{4}\)(H\({}_{2}\)O)\({}_{4}\bullet\)SH\({}_{2}\)O [14]. Structurally it is formed by fibrous particles of 0.5-10 \(\upmu\)m length, constituted by alternating rods and tunnel-like cavities which extend throughout the fiber direction. Thanks to its natural abundance and low cost, Sep has been extensively used as filler in various polymeric matrices [15; 16] to enhance their thermal, mechanical and barrier properties. However, the pristine bundle-like form of the Sep fibers inhibits its uniform dispersion within the polymeric matrix and their chemical modification is usually required to ensure interfacial interaction with polymers [17; 18]. To date, there are only a limited number of studies regarding on clay/NR nanocomposites obtained by straightforward LCT [19; 20; 21; 22]. In these reports, nanocomposites are generated from a coagulation process, activated by electrolytes as suitable coagulating agents. However, these compounds are usually nonbiodegradable, and may compromise also the properties and performances of the final materials.