## 1 Introduction  
Polymeric materials have become increasingly attractive to a growing number of industrial sectors due to their unique properties, such as low weight, low or relatively low cost, easy processing and high flexibility to adapt the desired properties through a wide range of molecular changes [1]. Thermosetting polymers are highly crosslinked tridimensional structures that constitute about 20% of global plastic production, being widely used in engineering fields [2, 3]. One of the most versatile classes of thermoset materials is represented by epoxy resins, which are widely used as adhesives for electronic components and circuits, semiconductor capsules, coatings, adhesives, structural materials in civil infrastructure, materials in the automotive and aerospace industries, composites, etc. [3, 4, 5]. This intensive use of epoxylic materials is due to their numerous qualities such as high chemical and mechanical strength, high thermal stability, high stiffness, good creep resistance, low shrinkage and good processability and compatibility with other polymers [2, 3, 4, 5, 6]. The majority of epoxy resins are based on bisphenol A (BPA) as their main compound, which has recently begun to be restricted due to its carcinogenic, mutagenic and reproductaic (CMR) nature [7, 8]. Therefore, to avoid the use of BPA and also to find alternatives to petrochemical resources creating problems of limited availability and volatile costs, academic and industrial studies on the development of new environmentally friendly epoxy materials from cheap, non-toxic, natural and renewable raw materials have intensified. A renewable alternative for fossil derivatives is represented by vegetable oils. These are triglycerides which, due to their numerous reactive groups (double bonds, ester groups, etc. ), can be easily chemically modified by numerous pathways (e.g., epoxidation) to develop new functionalities [9]. One of the main vegetable oils used in the development of thermosets is epoxidized linseed oil (ELO) [10]. Various academic studies have been dedicated to develop thermoset resins based on ELO, with properties comparable to commercial ones [11; 12; 13; 14; 15; 16]. Epoxidized linseed oil was also investigated as a potential matrix for the development of composites reinforced with fibres [17] or filled with micro- or nanoparticles [18; 19]. The wood-based biomass is the major source of lignocellulosic fibres, intensely used for the production of pulp and paper, wood pellets and coal, but these industries also produce large amounts of residues such as, for example, the bark. Bark is the external layer of trees covering the stem, branches and roots, and it represents approximately 10% of the stem volume [20]. After peeling, a large amount of bark waste is produced, of which only a limited amount is used for bioenergy production or soil modification, while most of it is left to rot in the forest or burned to recuperate energy, losing an important source of lignocellulosic fibres [21]. Although there is a wide range of potential uses for this by-product, very few studies have been conducted on the valorisation of this industrial waste [22; 23]. Another feasible valorisation method for this industrial by-product is by converting it, using different thermal methods, into interesting carbonaceous fillers that could replace the carbon nanotubes and graphene fibres [24; 25; 26]. These carbonaceous products, also called hydrochar or biochar, can be obtained through different thermochemical processes such as pyrolysis, gasification or hydrothermal carbonization [27; 28]. The hydrothermal carbonization (HTC) process is considered to be the main process in the field of biomass use due to several advantages, such as being a cheap, environmentally friendly, non-toxic, green and easily applicable process [29]. Compared to other carbonization methods (incineration, pyrolysis and gasification), the HTC process can be operated at low temperatures ranged between 180 and 350 \({}^{\circ}\)C [30].