**4. Recycling of polyimides**

Polymer recycling is (or should be) a vital criterion to be taken into account in the life cycle of a product, especially when it comes to PIs. They represent key, reliable, long-life materials for a wide range of high-tech applications, but their recycling is a major challenge. Several technological and economic hurdles hamper the progress in this field.

On one side, recycling should enable the reintegration of discarded PI materials in the technological cycle or the production of new PI materials from recycled ones (especially since PIs represent materials of high-end value). On the other, it implies the quest for real sustainable solutions for the PI wastes issue, even if they represent a very small percent of the total polymer volume in use/to be recycled. Last but not least, it also means cutting down the dependence on oil-based raw materials, this matter being (at least partially) treated in the previous sections.

As in the case of most materials, mechanical and chemical recycling and combustion are the most common polymers' recycling methods, PIs included.

When it comes to the recycling or reuse of PI-based materials, two options are available, depending on whether you are dealing with a thermoplastic or thermoset. Important constraints are met in both cases.

Only a low percent (between 5 and 8%) of thermoplastic PIs is prone to recycling by reuse. There is no clear, general answer to the dilemma regarding the faith of the more than 92% left.

In the case of PI thermosets, the problem is even more complex, since the threedimensional structure impedes easy changes in the physical state, especially to the molten state. Cross-linked PIs cannot be melted, molded, or extruded and therefore suffer from severe limitations in terms of recycling or require complicated or energy-intensive recycling methods. One easy example would be their reuse as additives in composites. If such, the problem of the matrix-reinforcement interface will arise from the homogeneity of the composite.

### **4.1 Degradation of polyimides**

At this point, a separate discussion must be dedicated to the degradation of PI materials. Degradation (commonly known as aging) usually encompasses any modification of the initial polymeric material's features due to one or several external factors like light, heat, or chemicals. This is a major concern when it comes to the application of PI-based materials. Moreover, degradation is a key process when it comes to the reuse or recycling of a PI-based product to meet green environmental criteria. Its importance is also crucial when considering the final disposal or destruction/decomposition of the envisaged material and its environmental impact.

Assisted degradation of polymeric materials is an intricate procedure due to multifaceted mechanisms involving highly reactive macro- and micro-radicals and a plethora of degradation and decomposition products. These mechanisms are strongly dependent on the environment, more precisely on the presence or absence of oxygen. In the particular case of the highly stable PIs, there are also strongly time-dependent mechanisms [139–141].

The degradation of PI-based materials can be the subject of various pathways that will be briefly approached below.
