# New Flame Retardant Systems Based on Expanded Graphite for Rigid Polyurethane Foams
## 1 Introduction  
Nowadays, where materials are increasingly demanding, applications are constantly striving to improve their properties. This is why the purpose of this work was to prepare rigid polyurethane foams (RPUF) with improved performance. RPUFs are materials with very large application possibilities. However, a certain limitation in their use is the flammability of RPUF, which results from their porous structure. For this reason, one of the inseparable elements of any RPUF composition intended for foaming is the flame retardant, also known as an anti-pyrene (FR) [1]. To improve fire resistance, it is necessary to reduce the number of flammable products, as well as to reduce the rate of their release and avoid prolonged burning [2]. Smoke retarders are usually used as reactive and additive additives [3]. These additives have very high requirements for thermal resistance and high resistance to alkaline conditions occurring during the formation of foams. Reactive flame retardants have functional groups capable of reacting with components involved in the foaming process of polyurethane foam [4]. Reactive antipyrine contains elements such as phosphorus in their structure, which are incorporated into the polymer structure, thus causing permanent binding so that they are not susceptible to volatilization during further application. Currently, the most commonly used reactive retardants are phosphoric acid derivatives that containhydroxyl groups [5]. Unlike reactive retarders, additive retarders do not have reactive functional groups capable of being incorporated into the polyurethane structure [6]. One of the most popular additive flame retardants includes halogen-derived liquid phosphates, whose task is to limit the combustion of the substance to a strictly insulating carbon layer, which prevents the access of oxygen and heat energy. In the case of such additives, the synergistic effect of phosphorus and halogen atoms is used to delay or diminish the intensity of the smoking process [7]. In line with the principle of sustainable development and environmental protection, great emphasis is placed on the use of halogen-free phosphorus-based flame retardants, including phosphamides and phosphazenes [8]. Expanded graphite is another anti-pyrene that is also very popular in the production of polyurethane foams. Its operation consists in creating a charred layer that acts as an \"insulator\" before further degradation of the polymer matrix due to the emerging small air gaps between individual layers of graphite [9]. The addition of graphite greatly reduces heat dissipation, weight loss and smoke production. In industry, low-temperature expanded graphite is most commonly used [10]. In previous studies, we indicated a positive effect of ionic liquids (IL) on the thermal stability and mechanical strength of porous polyurethane composites due to better dispersion of filler particles in the polymer matrix [11; 12; 13]. Graphite is a commonly used and effective flame retardant for PU foams, but it does not show reinforcing properties. Therefore, it seems advantageous to combine graphite with a reinforcing silica filler, which can improve the mechanical properties of rigid PU foams and with a small amount of ionic liquid, an improvement in the dispersion of low-molecular weight substances in the polyol can be expected. Therefore, in order to obtain foams with increased fire resistance and improved mechanical properties, we have proposed a modification system consisting of expanded graphite, pyrogenic silica and an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate [emim] [BF4]). The effect of the components of the synergistic flame retardant system on morphology, thermal properties, physical and mechanical properties as well as fire behavior of the resulting PU composites have been thoroughly studied. The results presented in the paper show that the application of the proposed modification methods leads to the production of porous polyurethane composites with improved performance parameters.