*3.1.2 Chemical recycling of nylon*

The most widely used nylons are Nylon 6,6 (derived by the polymerization of adipic acid and hexamethylenediamine) and Nylon 6 (obtained from caprolactam). Chemical recycling of nylon includes a depolymerization process followed by distillation to obtain and recover their monomeric constituents: caprolactam (for Nylon-6), and HMDA and adipic acid (for Nylon-6,6). Nylon can be efficiently depolymerized to monomer using chemical and thermal approaches [56, 61, 64, 66].

While chemical recycling, the molecular structure of the polymer is broken down using chemical reactions. After the reaction the products obtained can be purified and used to produce either the same or a related polymer.

During thermal recycling, breaking down of the polymer chemical structure relies on a reaction triggered by heat.

Various chemical processes demonstrated and developed for recycling Nylon 6 and Nylon 6,6 are:


High costs, challenging materials issues, multiple processing steps requiring high operational knowledge are among the barriers to widespread adoption of chemical recyling methods for nylons.

### *3.1.3 Chemical recycling of cotton*

For fiber to fiber recycling in textiles, the chemical recycling of synthetic materials is not new but studies on natural fiber chemical recycling is relatively new. Although energy products, bio-based products, bacterial cellulose, glucose, aerogels etc. are among the potential applications of chemically recycled cotton, chemical recycling of cotton waste is widely used to produce new fibers that can be used in a variety of products.

#### *An Evaluation of Recycled Polymeric Materials Usage in Denim with Lifecycle Assesment… DOI: http://dx.doi.org/10.5772/intechopen.99446*

The two main routes for chemical recycling of cotton is based on the dissolution of cellulose [61, 70, 71]. Accordingly, either glucose monomers are depolymerized for use in other applications or a polymer dissolution route is followed where cellulosic fibers separate and regenerate by use of solvents. Via the latter process, chemically modified or pure cellulosic fiber products, which can be used as feedstock for regenerated man-made cellulosic fibers (MMCF), may be recovered. A simplified process diagram for chemical recycling of cotton is given in **Figure 5**.

In the Lyocell method, cotton fibers are dissolved (dissoving pulp) using N-methylmorpholine N-oxide (NNMO). The regenerated MMCF are produced by processing the dissolved pulp and and blending with other plant-derived pulp products such as wood, flax, hemp, etc.

Ionic Liquids (ILs) are organic salts in liquid state with low melting point, have chemical and thermal stability, and are non-flammable. They are less aggressive to the environment and considered to substitute organic solvents. The ionic liquid process, yet not commercially available, can be applied to all cotton and also to blends of polyester-cotton.

Another checical recycling approach for cotton is the Ioncell-F cellulose spinning process which uses ionic liquid DBNHOAc without co-solvents or stabilizers [62, 71].
