**4.2 Recycling of polyimides**

Generally, two recycling technologies apply to PI materials: chemical and mechanical. The chemical recycling is a viable solution since it allows the maximum output of the final products (starting monomers, partially imidized powders) with a high degree of purity. Its main disadvantages are the long duration (multistage process) and energetic intensity.

In recent years, mechano-chemistry (e.g., ball milling) has proved its advantages over conventional chemical methods (fly ash modification, rubber, and plastic recycling), which lead to its intensive application in polymer recycling, PIs included. The process is simpler, economically favorable, and more eco-friendly (no solvents or intermediate fusion are used). Moreover, it allows attaining final products in a metastable state, which is not easily accessible by other conventional recycling methods.

Solid-state mecano-chemistry can be successfully applied in the recycling of PI film wastes with the purpose to develop thermostable blends and multicomponent tribocompositions. To gain applicative potential, these can be prepared in combination with other high-performance polymers, carbon black, diamond powder, and Al-Cu-Fe crystals [167].

The recycling treatment method depends on the physical form of the PI waste. PI-based powder compositions are obtained through low-energy planetary ball mill, PI film wastes are treated by high-energy planetary ball mill, while bulk samples are obtained by compression molding.

Multicomponent systems with a "multiscaled structure" based on PI waste and fluorinated ethylene propylene were obtained by the above methods. The resulting composites displayed an increased wear resistance and a smaller friction coefficient as compared to the raw propylenic material.

As mentioned before, a disadvantage of recycled PI usage in the development of composite materials is the lack of control of the interface and cohesion between composite elements. This results in a composite with unsatisfying integrity and poor mechanical features.

One solution to this issue is the use of polymer wastes of the same nature and proper tuning of the milling process. For example, common PMDA-ODA (pyromellitic dianhydride-4,4<sup>0</sup> -oxydianiline) PI films were grinded to powders and mixed with another commercial PI powder (BTDA [3,3<sup>0</sup> ,4,4<sup>0</sup> -benzophenonetetracarboxylic dianhydride]-ODA) and a liquid, BTDA-based, commercial PI resin [168].

Several adjustments were performed to the high energy ball milling process, especially in terms of duration. The milling time strongly influences the average size and the particle-size distribution of the PI powder and the corresponding mechanical features of the resulting PI composites. Finally, bulk samples were obtained by compression molding at 400°C. The technique provided access to homogeneous materials with a flexural strength of 87 MPa, deformation at failure around 4.2%, and linear elastic modulus between 50and 350°C.

The milling duration also influences the chemical structure of the resulting recycled PI and its cohesion with the other two components of the composite material. Shorter milling times (45 min and 65 min) create larger particles that are

## *New High-Performance Materials: Bio-Based, Eco-Friendly Polyimides DOI: http://dx.doi.org/10.5772/intechopen.93340*

strongly reinforced through links provided by the PI resin during compression molding. On the other side, longer milling times (90 min and 180 min) provide smaller particles that are more compatible with and reinforced by the other powdered component (BTDA-ODA commercial PI).

Another mechanical recycling method is the application of uniaxial stretching on PI films to control and reduce the coefficient of thermal expansion of the recycled material [169]. The resulting PI material is stretchable at a certain operating temperature regime and draw ratios. Increased stretching stress (performed in the machine's direction) leads to recycled films with higher birefringence and Young's modulus and lower thermal expansion, mostly independent of temperature.

A very useful recycling procedure was proposed for gelled PAAs, a common issue for researchers dealing with PI synthesis. PAAs frequently form gels during synthesis or storage due to various intermolecular and intramolecular interactions, the entanglement of long, high mass chains, partial imidization, or even undesired cross-linking. The formed PAA gels are usually discarded both in research and industry, which translates into time and economic loss and, more importantly, to negative environmental impact.

The recycling method was applied for a broad range of conventional PI building blocks and is based on the conversion of PAA gels to homogeneous solutions by using common microwave irradiation for a short time at room temperature. In some random cases, similar results can be obtained by less green conditions: heating the gelled PAA at 135°C for at least an hour [170].

The resulting PAA solutions can be successfully converted to the corresponding PIs. The resulting materials maintain the original film-forming features and show superior thermal (Tg values included), mechanical, and dielectric features as compared to the original PIs (obtained from the ordinary, homogeneous PAA solutions).

Recyclability can be also attained by finding new applications for already established, commercial PI materials, as to ensure a longer product lifetime. For example, the Kapton PI film was used to build an efficient, active particulate matter air filter which, very important, is also recyclable. Patterned through-holes were developed by ion etching on the common film (15 μm hole diameter; 30 μm interhole distance) [171]. These holes are combined with the strong electrostatic forces coming from Kapton®' s high work function to capture particulate matter. The device was tested under real working conditions and proved efficient in long-term filtration of dust particles with dimensions ranging from 0.3 up to 10 μm.

Moreover, the highly flexible, thermally stable filter is easily recyclable and reusable by simple washing (with tap water), which makes it suitable for various air filter-based applications (air purifiers, air conditioners, humidifiers, and industrial filtration systems).

### **4.3 Recyclable PIs by design**

Another strategic approach in the field of recycling PI-based materials is to ensure recyclability through judicious design, a tactic inspired by applications based on cyclic operations.

One example in this direction is the development of highly hydrophilic, composite recyclable PI adsorbents for wastewater treatment and removal of heavy metal ions [172]. These can be easily attained by the common sol-gel process which leads to hybrid PI/silica materials with Cu2+ adsorption yields comparable to those of common activated carbon adsorbents. The innate resilience of PIs affords adsorbents with stable adsorption performance over 50 recycling processes.

A more complex concept for hybrid, recyclable PIs is the development of intricate, green, poly(imide imine) thermosets that combine the mechanical resilience of rigid PIs with the on-demand degradation and recyclability of flexible polyimines through dynamic covalent chemistry [173]. The greenness of the approach comes from the ability of imine units to provide heat- or water-triggered reversibility in the absence of expensive, environmentally unfriendly (transition metal) catalysts. The hybrid architecture is based on various ratios of an aromatic bisimide developed from widely used, commercial PI building blocks, a common aldehyde, and some aliphatic triamines or tetraamines as cross-linkers. The strategy affords thermosets with thermal and mechanical features comparable to common polyimides.

The key characteristic of these hybrid structures resides in the use of the primary amine as a catalyst to generate interchain imine exchange reactions at the interface of ruptured film strips. This feature leads to the conversation of mechanical properties and promotes a high healing output. The mechanical features of the (re) healed material are comparable or slightly superior to the original material up to the third generation.

The dynamic covalent chemistry concept unlocks novel pathways in the design of smart, high-performance PI-based materials able to lay out rehealability, repairability, and recyclability.

Another pathway to develop recyclable PIs is based on the introduction of flexible spacer units within and pendant to the macromolecular backbone. For example, the highly flexible 4,4<sup>0</sup> -diamino-3,3<sup>0</sup> -dimethyldiphenyl methane was polycondensed with BTDA to generate recyclable, PI-based, nanofiltration membranes. These are fabricated by phase inversion and retain a skinned asymmetric architecture [174].

The optimized membranes are efficient in rejecting Rose Bengal, PEG 1000, and typical salts with yields above 91%. They successfully corroborated their recyclability and display stable, longtime performance.
