# The Influence of Copper Oxide Particle Size on the Properties of Epoxy Resin
## 3 Results and Discussion
### Mechanical Properties
The test results from the static tensile test are presented in Figures 1 and 2. Strength tests have shown that the introduction of copper particles has a beneficial effect on strength properties. As the particle size of the introduced additive decreased, an increasing tendency in strength properties was noticed. The best parameters were obtained for the material modified with copper oxide particles with sizes ranging from 10-30 nm. An increase in tensile strength by approximately 30% compared to the base material, a several percent increase in the elastic modulus, and an almost 40% increase in strain at break were observed. The use of larger copper oxide particles (40-60 nm) causes slight changes in tensile strength while increasing the elastic modulus. Adhesion forces between components are largely responsible for the strengthening effect in polymer composites. Adhesion is caused by forces occurring at the reinforcement/matrix interface. These forces result from the chemical structure of the matrix and the composite reinforcement in the form of Cu particles. Chemical actions can also occur due to the similarity of functional groups. Adhesion additionally depends on the degree of wetting, the magnitude of friction forces at the phase boundary, the direction of shrinkage stress, the compactness and geometry of the filler, as well as defects in the form of voids and air bubbles [34; 35]. The results show that the composite with the smallest particle size has the best properties, which is probably related to the greater number of contact points at the particle-matrix interface. However, the introduction of larger particles with a dendritic structure resulted in decreases in strength and plastic properties, which may be related to the formation of agglomerations and defects in the form of  
temperatures mixing [36]. [PERSON] et al. reached similar conclusions. They investigated the influence of the size of food waste particles on the properties of epoxy resin. The filler content was 15% by weight. The particle size was (i) (100-250) mm, (ii) (350-500) mm, and (iii) (650-800) mm. The tests showed a decrease in mechanical properties with an increase in particle size. These results were explained by the fact that larger particles detach more easily than small particles. This debonding results in more voids, which ultimately reduces the stiffness of the composites [33]. In turn, [PERSON] and others pointed out other potential problems related to the introduction of micro- and nanoscale additives. They examined the influence of Al\({}_{2}\)O\({}_{3}\), TiO\({}_{2}\), and fly ash on the properties of epoxy resin. The filler content ranged from 10 to 30 wt%, and the particle sizes were 10 nm and 40 nm and 45 and 50 um. The results clearly show that the tensile strength of microfiller composites decreases with increasing filler content. This can be explained by the fact that increasing the filler content caused poor adhesion between the matrix and the fillers and led to a decrease in the strength of the epoxy composite. In the case of the nanofiller, the decrease in strength was due to the uneven distribution of fillers at high filler proportions, which led to agglomeration and caused areas of stress concentration, which led to some decrease in strength [31]. [PERSON] and others presented in their work a summary of various studies on polymer matrix composites modified with particles of different sizes. Conclusions can be drawn from their work. In summary, there appears to be a certain critical particle size beyond which no significant effect on the modulus of the composite is observed. When the particle size is smaller than this critical value, the particles' effect on the modulus of the composite becomes more significant. However, the precise value of this critical particle size cannot be predicted in advance because it depends on various factors such as the properties of the particle, the matrix, and their mutual adhesion.