# The Influence of Copper Oxide Particle Size on the Properties of Epoxy Resin
## 3 Results and Discussion  
### Mechanical Properties  
Table 3 shows a comparison of density and flexural properties for all tested materials. The highest density value was achieved by a composite reinforced with copper particles with a size of 40-60 nm. A general increase in density can be seen for all materials compared to the unreinforced material. The density values of the composites with the addition of copper oxide ranged from 1.26 to 1.30 g/cm\({}^{3}\). The additive content for each was 10% by weight; therefore, the composites have similar density values, and the differences that appear in the results were most likely caused by the different shapes of the particles because particles with a size of 40-60 nm showed a dendritic character, which could result in a greater ability to create clusters in sample areas. Impact strength tests focus on the amount of energy needed to dynamically fracture a sample. The higher the impact strength value, the more energy the material absorbs, and the more plastic cracking occurs. The highest value was obtained for particles in the range of 10-30 nm. This effect was caused by the even dispersion of copper in the matrix, thus reducing the possibility of particle agglomeration, which directly reduces the mechanical properties of the material. For materials with reinforcements in the range of 40-60 nm, we notice a decrease in impact strength which may be caused by microcracks in the matrix structure. The decrease in impact strength is most often associated with the poor adhesion of particles to the polymer matrix, irregularity in the composite structure, the formation of microcracks, for example, during the manufacturing process related to matrix shrinkage, and the presence of inclusions or defects in the form of voids and air bubbles [28; 29]. Table 3 also lists the results from the static bending test. An analysis of the results showed increases in bending strength and the bending modulus for each of the tested materials. The best bending properties were obtained when copper oxide with a particle size in the range of 40-60 nm was used. Both bending strength and the bending modulus increased by over 30% compared to the unmodified material. The remaining composite materials were also characterized by increased strength properties, but the increases were slightly smaller. The addition of copper oxide with the smallest grain size resulted in an increase in bending strength of approximately 25%; however, this material was characterized by the lowest modulus of elasticity compared to the other composites, which is most likely related to the increased plasticity of the material. [PERSON] examined the \begin{table}
\begin{tabular}{c c c c} \hline  & **Function** & **Intensity, W/mw/nm** & **Temperature, \({}^{\circ}\)C** & **Time, min** \\ \cline{2-4}  & UV radiation & 1.55 & 60 & 08:00 \\ \cline{2-4}  & Shower & - & - & 00:15 \\ \cline{2-4}  & Condensation & - & 50 & 03:45 \\ \hline \end{tabular}
\end{table} 
Table 2: Parameters of one thermal cycle. influence of TiO\({}_{2}\) nanoparticles on the mechanical properties of epoxy resin. The particle sizes were 17 nm, 50 nm, and 220 nm. The modification of the epoxy resin with a small percentage of 1% TiO\({}_{2}\) particles resulted in a marked increase in bending stresses. As the particle size decreases, the bending strength increases, and these changes are explained by the fact that smaller particles have better mechanical properties than larger particles and adhere better to the matrix [30]. In a study conducted by [PERSON] et al., the impact of Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and fly ash on the properties of epoxy resin was examined. The researchers observed a decline in the flexural strength of the composites as the age of the particles at the nanoscale decreased. This reduction in strength was attributed to the agglomeration of nanofillers at higher concentrations and the presence of weak adhesion between the filler and matrix materials at high microfiller contents.