It is known to use depolymerization processes in the treatment of waste plastics. A substantial goal of such treatments to plastic waste is chemical recycling. In such a recycle process the waste plastics are converted into constituent monomers which may be suitable for reforming original plastic. In such recycling processes, it is desirable to improve the efficiency of the depolymerization of the waste polymers into their corresponding monomers. Generally, known depolymerization methods involve high-temperatures, high-boiling solvents and often vessels able to withstand high pressures. Solvents used in depolymerization may also include supercritical water or methanol which may require the use of special vessels and pressure handling apparatus.
Polyamides of substantially aliphatic composition, hereinafter nylon, are known to be depolymerized through acid hydrolysis. Such a depolymerization process uses an excess of sulfuric acid, which also functions effectively as the solvent for the process. In order to recover the monomeric materials, a separation or neutralization step is required from the sulfuric acid solvent in which the reaction takes place. The disadvantages of such processes are that they give rise to the generation of significant amounts of problematic effluent streams, and they are also associated with difficulties in the separation and isolation of the target monomers. The products of acid hydrolysis are amine salts and carboxylic acids. In the case of the commercially important polyamide nylon 6,6 (N66), depolymerization would result in the regeneration of the monomers hexamethylene diamine (HMD) and adipic acid (AA).
Recently, ionic liquids (IL) have achieved renewed interest as solvents due to their unique properties. ILs are known for their nonvolatility and stability at high temperature making them suitable for consideration in a low emission recycling system. A recent report of the use of ILs in the depolymerization of polyamides was published in Org. Lett., 9 (13), 2533-2535, 2007 (Akio Kamimura and Shigehiro Yamamoto; “An Efficient Method To Depolymerize Polyamide Plastics: A New Use of Ionic Liquids”) and reports a process for the treatment of nylon 6 in an ionic liquid at 300° C. resulting in depolymerization of the polyamide to provide the corresponding monomer, caprolactam, which is collected by direct distillation of the reaction mixture. These authors reported that N-methyl-N-propylpiperidinium bistriflimide (bistrifluoromethylsulfonylimide) provided the best results for the depolymerization of nylon 6 and that the IL could be repeatedly used (ca. five times) without significant decomposition. There is no disclosure of the depolymerization of nylon 6,6.