# The Influence of Copper Oxide Particle Size on the Properties of Epoxy Resin
## 1 Introduction  
Polymer matrix composite materials have been used in construction since the 1990s. They have achieved popularity owing to the large number of combinations of materials used for reinforcement, such as glass, fibrous polyamides, and carbon reinforcements. Their weight is four times lower than that of commonly used construction materials while maintaining similar mechanical properties. The greatest advantage of polymer composites is their corrosion resistance, which translates to their wide use in economic sectors dealing with the processing of chemically active materials. Examples of interesting applications include polymer composites, which can be found as elements supporting the vaults of drilled mining tunnels. Their main advantages include their low weight and electrical insulation. Materials of this type do not produce sparks during grinding and cutting, which is a highly desirable feature [1]. The storage of liquefied petroleum gas (LPG) typically involves the use of transportation and steel tanks, which have been increasingly replaced by composite tanks in recent years. The reason for this is the winding method used to produce tanks made of carbon prepreg. These composite tanks offer a volume-to-weight ratio that is four times higher than conventional tanks, resulting in a significant reduction in vehicle weight, increased fuel efficiency, and decreased pollution. Therefore, these tanks are widely utilized in industries that manufacture land and air vehicles, as well as in rescue operations, for water transport for firefighting helicopters, and as pressure tanks for filling rescue pontoons [2]. The utilization of polymer compositesin the Airbus A380 aircraft accounts for approximately 22% of its overall weight. These composites are typically layered, or laminate, structures that incorporate various materials. Examples of such composites include GFR (\"glass-fiber-reinforced polymer\"), CFRP (\"carbon-fiber-reinforced polymer\"), and GLARE aluminum matrix composites (\"glass-reinforced laminate\"). These materials are favored due to their high stiffness relative to their low weight and are commonly employed in the construction of fuselage plating, wings, and tails. The anticipated demand for new materials for military vehicles in the near future will likely drive the innovation and development of materials utilized in civilian transportation [3; 4]. Thermoplastics, including chemo- and thermosetting resins, are widely utilized as materials for polymer composite matrices, commonly serving as fibrous reinforcements. Notable representatives of these materials include unsaturated polyester and vinyl ester resins, polyamides, bismaleimides, phenyl-formaldehyde resins, and epoxy resins. Among these, epoxy resins are available in various molecular weights. The most commonly utilized epoxy resins are bisphenol resins, which are employed as matrices for glass or carbon reinforcements. These resins are produced through the polyaddition of bisphenol and glycerine epichlorohydrin [5]. Epoxy resins can be described using two key measures: the epoxy number and the hydroxyl number. These figures indicate the quantity of hydroxyl and epoxy groups present in every 100 g of the substance. A greater hydroxyl number signifies a higher viscosity or softening point, as well as a lower proportion of epoxy groups, which can affect the form of the substance. This may take the form of a viscous liquid or semi-solid [6]. The resin hardness is increased and retains its shape during the cross-linking process. The resulting oligomer chains are subjected to various cross-linking processes, most often at high temperatures. The cross-linking of the resin involves a copolymerization reaction involving the double bonds of both components. The main strength properties, including hardness, HDT deflection temperature, elastic modulus, tensile strength, and elongation at break, depend on the cross-linking process [7]. The use of epoxy resins is preferred over polyester resins due to their superior adhesion to fibers.