# Multifunctional Modifying Systems Based on Ionic Liquids for Epoxy Resin Systems and Composites
## 1 Introduction  
Composite materials based on epoxy resins and reinforcing fibers are the backbone of polymer materials used widely around the world. It is a valuable construction material in areas such as electronics (equipment housings, electronic components) [1], paints and coatings [2], construction (construction material used both indoors and outdoors) [3], or transportation (construction materials in aviation, automotive, marine or railroad industries) [4]. It owes its popularity due to its high tensile strength, high adhesion to substrates, good corrosion resistance, chemical resistance, and moisture resistance [5, 6]. The biggest disadvantage of this group of materials is their flammability, resulting in the generation of large amounts of heat and the formation of a number of combustion products that are harmful to human health, such as: (i) toxic gases (HBr, HCl), (ii) benzene derivatives and higher aromatic hydrocarbons (PAHs), (iii) non-flammable gases (CO\({}_{2}\)NOx, SOx), (iv) flammable VOCs (methane, ethane) or (v) organic irritants, the composition of which is determined by the components of the resin and the overall composite [6; 7; 8; 9; 10]. Glass and carbon fibers are the most widely used fiber reinforcement in polymer composites. Although both reinforcements can be considered inert materials that significantly reduce the heat release rate compared to the polymer matrix, they often need additional flame retardancy to comply with the relevant fire safety standards. Flame retardant compounds containing halogens are known, but their use is not friendly to health and the environment as their combustion products are mainly toxic gases (HCI, HBr, HCN). Restrictive requirements for flame retardants and their effectiveness and properties (especially in terms of environmental and health impacts) are forcing the search for halogen-free solutions. Organophosphorus compounds used in composites to increase their thermal stability can be described as reactive (such as 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, DOPO [9; 11]) or non-reactive (such as triphenyl phosphate, TPP [12; 13; 14; 15]). Their flame-retardant effect is directly related to the form in which phosphorus is present in them and can sometimes consist, for example, of generating large amounts of foam during combustion, which prevents further ignition (swelling when exposed to fire). Silicon-based flame retardants (e.g., borosilicates) are preferred to reduce flammability and suppress smoke, which is extremely beneficial in the case of epoxy resins [6]. These flame retardants are widely regarded as environmentally friendly and can be combined with each other: (i) synergy between silicon and phosphorus compounds, (ii) synergy between phosphorus and nitrogen compounds, and (iii) synergy of organophosphorus compounds with inorganic filler (metal oxides and hydroxides) [6; 16; 17; 18]. Admittedly, inorganic fillers such as magnesium hydroxide and magnesium oxide can impart flame-retardant properties of epoxy resin, but to meet the flame-retardancy criterion they need to be used in high concentrations, which deteriorates the mechanical properties of the final material. The search for effective flame retardants, using synergistic interaction between elements that are biodegradable or environmentally friendly and easily scalable from the lab to industry, is currently a challenge in many research groups around the world [6]. Ionic liquids, denoted as ILs, constitute a unique category of chemical compounds defined by their distinctive ionic architecture. These compounds consist of a cation and an anion, each endowed with specific and remarkable attributes that set them apart from other chemicals. Typically, the cation component of ionic liquids originates from organic sources and manifests a wide array of structural variability.