# The Influence of Copper Oxide Particle Size on the Properties of Epoxy Resin
## 3 Results and Discussion
### Accelerated Aging  
The manufactured materials were also subjected to accelerated aging tests. Aging is a picture of all the irreversible physical and chemical changes occurring in a material at a specific time. The tested conditions were temperature, chemical compounds, and mechanical loads. When all these parameters are known, it is worth increasing one factor to speed up the test. We mention two types of aging: internal and external. The internal aging of polymer materials is the result of thermodynamic instability, which manifests itself through recrystallization, the relaxation of residual stresses, the dispersion of phases in multi-component systems, and the migration of plasticizers. On the other hand, external aging refers to processes such as thermo-oxidative degradation, the formation of stress and fatigue cracks, and swelling, which are directly related to environmental chemical or physical impacts on plastic. Aging is also divided into physical and chemical aging, but a given environmental factor can cause both effects at the same time, which significantly complicates the clear affiliation to a given group, e.g., increased temperature can cause both physical (secondary crystallization) and chemical (chain breakdown) effects. Changes in structure, secondary crystallization, and disorientation or the relaxation of residual stresses are physical phenomena typical of aging processes. Physical aging phenomena may occur during the evaporation or migration of volatile components or with mass increases through water absorption or swelling. However, the chemical structure is most often changed under the influence of chemical effects such as the impact of heat in combination with oxygen or other aggressive substances or radiation, with a strong emphasis on solar UV radiation. Increased temperature and, consequently, heat absorbed by the material, can initiate cross-linking, which changes the properties of the material while improving its thermal, mechanical, and physical properties. In the medium term, however, this may contribute to the degradation of the chains and the formation of cross-linking, which has a negative impact on the material. Oxygen has the ability to attach to other polymer chains, resulting in the formation of thermally unstable hydroxides, causing chain breakage. Ozone and radiation support oxidation, which accelerates the breakdown of chains. UV radiation causes the material to crumble, and in the case of condensation polymers, water combined with heat leads to the hydrolysis of the material; an example is polyamide. Four types of chemical degradation can be distinguished: (1) depolymerization, (2) chain breakdown, (3) thermo-oxidation, and (4) radiation. The first one involves the formation of monomers, while the second one can occur at any point in the chain. The last two types are related to cross-linking reactions and the detachment of substituents and their elimination. The type of degradation and the degree of changes during aging depend on the chemical structure of the material, the environmental conditions, wall thickness, and the test sample. The sample's electrical, mechanical, and thermal properties are changed under the influence of the test. In addition, surface changes are noticeable, such as a change in the color of crystallinity or structure [43; 44]. Figures 5-7 show test results from before and after the aging process. The applied aging conditions resulted in a significant decrease in the strength properties of the unmodified material. The introduction of copper oxide particles resulted in an increase in the stability of the tested materials, both in tensile strength and Young's modulus. The strain at break decreased with an increase in the accelerated thermal aging time for all materials, but in this case, copper particles also limited the negative impact of the external environment on the tested materials. The best results were achieved by materials modified with copper nanoparticles.