## 1 Introduction  
Polymers are widely used in industry, science, and our daily life, due to a wide range of properties, including light weight and easy processing, chemical resistance, high strength, electrical insulating properties, etc. [1; 2]. Among various types of polymers, epoxy resins are one of the most popular thermosetting polymers. Epoxy resins have excellent chemical and corrosion resistance, high adhesion, good mechanical properties, and low shrinkage [3; 4]. The main disadvantage of polymer materials, including epoxy resins, is their high flammability, making them a fire hazard [5; 6]. The growth in consumption of polymers has led to an increase in the number of fires and, as a result, damage to property and threat to life. Therefore, the problem of reducing the flammability of polymers and giving them heat-resistant properties is of great importance.  
One of the most common and effective ways to achieve enhancement in the properties of polymers, including flame retardant properties and thermal stability, is the incorporation of fillers of an inorganic and organic nature into epoxy resin [7; 8; 9; 10; 11; 12]. In recent years, special attention was paid to the use of nanodispersed materials as fillers. In contrast to conventional coarse flame-retardant additives, the effectiveness of the flame-retarding properties of which is achieved at high loading (50 wt.% or more), nanodispersed fillers can contribute to solving the problem of reducing the flammability of polymers at relatively low loading (<5%) [7; 13; 14; 15]. Generally, the nanodispersed fillers act as barriers against oxygen and combustion gas and produce, during combustion, strong, dense, and crack-free nanocoating surface layers.  
Owing to their unique physical properties, carbon nanomaterials, such as carbon nanotubes [16] and nanofibers [17], graphene [18; 19; 20], graphene nanoplatelets [21], reduced graphene oxide [22], graphene oxide [23; 24], and expanded graphite [25], have attracted great attention in recent years, particularly for the improvement of flame retardancy of epoxy composites [26; 27; 28; 29; 30; 31; 32; 33; 34].  
A comparison of thermal degradation of epoxy composites and multi-wall carbon nanotubes (MWNTs) was studied in [33]. It was found that the preparation technique has an important role in the formation of thermal behavior characteristics. [PERSON] et al. [35] found the synergetic effect of the ratio of MWCNT and graphene nanoplatelets (9:1 ratio MWCNT:graphene nanoplatelets) on the thermomechanical properties of epoxy composites. [PERSON] et al. [36] reported the enhancement of flame retardancy of epoxy resin based on functionalized multi-walled carbon nanotubes obtained by the sol-gel technique. It was found that the activation energy of the degradation reaction increased when increasing carbon nanotubes content, and after reaching 7 wt.%, it began to decrease.  
[PERSON] et al. [37] used the solution-mixing method for preparation of epoxy/expanded graphite composites. They reported an increase in thermal degradation temperature due to the enhanced dispersion of expanded graphite in the epoxy matrix. [PERSON] et al. [38] studied the thermal degradation of epoxy/expandable graphite composites. The weight percentage of EG in the studied epoxy composites was 1, 3 and 5 wt.%. It was found that the time-to-ignition, the critical heat flux, the ignition temperature, the thermal inertia, the smoke yield, and the peak heat release rate decreased with the increasing EG content in the epoxy composites. Despite the presence of a lot of publications devoted to epoxy resin based on multi-walled carbon nanotubes and expanded graphite, there are not enough data on filler concentration effect at a low filler content on thermal behavior and flammability.  
The aim of this work is to study the effects of carbon nanofillers, multi-walled carbon nanotubes and expanded graphite (EG), taken in different concentrations, on the thermal oxidative degradation, hardness, and flammability of the epoxy composites.