**4. Conclusions**

In the current study, several experiments using TG-FTIR, MCC, and cone calorimetry were employed for PVC sheaths of new and aged cables. The results show that the pyrolysis behavior between aged cable sheaths and new cable sheaths with varied heating rates is markedly different in nitrogen atmosphere. The onset temperature of mass loss for the aged cable sheath is greater than that of the new one, regardless of the heating rates. In addition, the mass of the pyrolysis residue of the aged cable sheath is slightly greater than that of the new cable sheath. This indicates that the new cable sheath starts to pyrolyze more easily and completely than the aged one. It is also concluded that there is a main DTG peak for new and aged cable sheaths under all conditions. The value of the main DTG peak of the aged cable sheath is clearer than that of the new one. The evolved gas that was measured by FTIR spectra illustrates that the aged cable sheath releases pyrolysis gases slightly later but more quickly than the new cable sheath. The results also show that the values of PHRR and THR for the aged cable sheath are clearly less than those of the new one. However, the duration from onset decomposition to PHRR and time to ignition for the aged cable sheath are significantly greater than those of the new cable sheath. It must be noted that the difference of pyrolysis and combustion between the aged sheath with 30 days and aged sheath with 60 days is slight, which may indicate that there is a critical stage during the thermal aging. The pyrolysis and combustion properties of materials change slightly when the materials is aged in a long enough period. Generally, the pyrolysis and combustion properties depend on the material itself under the same condition. In consequence, the modification of chemical composition, chain structure, and additives might be deduced to be the reason for different pyrolysis and combustion properties of the new and aged cable sheaths, eventually resulting in the change of flammability characteristics. Whereas, the currently available evidence is insufficient for this deduction and more research is needed. This work adds to the understanding of the difference in pyrolysis and combustion performances between new and aged cable sheaths. Finally, pyrolysis and combustion of waste plastic allow the obtainment of valuable chemicals, hydrocarbon compounds, combustible, gases, and energy. Knowledge of the pyrolysis mechanisms and combustion properties of typical aged cable sheath will benefit the recycling of plastic waste and energy conversion, which deserves further examination in future study.

**Author Contributions:** Conceptualization, Z.W. and J.W.; Methodology, Z.W., J.H and J.W.; Investigation, Z.W., R.W. and X.W.; Data Curation, Z.W. and J.H.; Writing-Original Draft Preparation, Z.W.; Writing-Review & Editing, Z.W., R.W., X.W. and J.W.; Supervision, J.W.; Project Administration, J.W.

**Funding:** This work was supported by the National Key R&D Program of China (No. 2016YFC0802500). The authors gratefully acknowledge this support.

**Conflicts of Interest:** The authors declare no conflict of interest.
