Pyrolysis of polyamide 6 under catalytic conditions and its application to reutilization of carpets
Introduction
Carpets are often composite materials composed of nylon fibers or nylon containing fibers and backbones, e.g. on a polypropylene basis. The chemical reutilization of polyamide 6 offers some economic benefit because of the high value of polyamide 6, which is produced in a multi-stage process via the monomer ϵ-caprolactam. The recovery of ϵ-caprolactam from waste polyamide 6 has, therefore, the potential to be economically competitive with the traditional synthesis process and has a significant positive environmental impact. A presently applied recycling process is the Zimmer AG process [1], [2], [3], which performs the depolymerization of polyamide 6 with the help of steam and liquid catalysts such as phosporic acid. This process is applicable only for pure polyamide 6 materials, so that a thorough separation of polyamide 6 from other polymers contained in carpet wastes is required. A disadvantage of this process is the high yield of salts and traces of phosporic acid in the recovered ϵ-caprolactam, which is a drawback for the production of fibers. Therefore, a thermal process based on the pyrolysis of polyamide 6, possibly catalyzed by recoverable catalysts, might be more competitive.
The functional amide group of polyamide 6 enables catalysis in the thermal decomposition. Czernik et al. [4] investigated the catalysis of the thermal degradation of polyamide 6 with α-alumina supported KOH in a fluidized bed reactor. In this work, the catalyzed degradation of polyamide 6 is carried at 330°C and 360°C with a yield of 85% ϵ-caprolactam and higher. Mukherjee and Goel [5] investigated the catalysis under vacuum with 1% NaOH at 250°C and determined a yield of 90.5% ϵ-caprolactam. In both cases the reaction takes place under heterogeneous conditions (melting point of NaOH 318.4°C). Another approach is the catalysis of liquefied polyamide 6 with liquid catalysts. In our previous work [6], the kinetics of polyamide 6 degradation have been investigated by means of isothermal and dynamic kinetic measurements. Formal kinetic parameters of the non-catalyzed thermal degradation of polyamide 6 have been measured in comparison with that of the catalyzed reaction. Two types of catalysts – NaOH/KOH and H3PO4 – have been employed. In order to use catalysts that are liquid under degradation conditions a eutectic mixture of sodium hydroxide and potassium hydroxide (melting point of the mixture of 60 mol% NaOH and 40 mol% KOH is 185°C) has been used. In case of acid catalysis H3PO4 has been employed according to the Zimmer procedure. The isothermal measurements have been performed with a gradient free reactor with online gas analysis by means of a mass spectrometer. The dynamic measurements have been carried out with a thermobalance coupled to a mass spectrometer.
The basic catalysis of polyamide 6 degradation offers the highest reaction rates compared to that of the non-catalyzed degradation and that of the catalysis with H3PO4. Therefore, the basic catalysis with NaOH/KOH is used in this work for the design of a stepwise pyrolysis procedure in order to separate the polyamide 6 by degradation from other plastics contained in carpet wastes, e.g. polypropylene. For this purpose mixtures of pure polyamide 6 and pure polypropylene have been pyrolyzed in a cycled spheres reactor at a low temperature level. The temperature is adjusted in a way that polyamide 6 degradation is quantitative, whereas polypropylene remains unchanged in the residue. The same process with identical temperatures is used for experiments with real carpet fibers containing a mixture of polyamide 6 and polypropylene.
Section snippets
Experimental
The used polyamide 6 is a product of the BASF AG, Ludwigshafen, with the proprietary name Ultramid B3. Some data for characterization of the used polyamide 6 (Ultramid B3) are as follows: melting point, 170–260°C; density, ⩾1 g cm−3; mean molar mass 180 000 g mol−1; water content, 4–8 wt.%.
In the following acid catalysis means the addition of 10 wt.% orthophosphoric acid (85%) and basic catalysis means the addition of 10 wt.% of a eutectic mixture of sodium hydroxide and potassium hydroxide
Dynamic measurements with coupled thermogravimetry/mass spectrometry
The non-catalyzed thermal degradation of pure polyamide 6 leads to the monomer ϵ-caprolactam in high yields, i.e. ⩾92 wt.%. The monomer yield raises by addition of 10 wt.% orthophosphoric acid to about 97.4 wt.%. In case of basic catalysis by means of a eutectic mixture of KOH/NaOH the yield of ϵ-caprolactam increases to about 98.4 wt.% [6]. The major byproduct in all cases is the cyclic dimer of ϵ-caprolactam, 1,8-diazacyclotetradecane-2,9-dione. This dimer is formed by 4 wt.% in case of the
Conclusion
The kinetics of the thermal degradation of polyamide 6 have been investigated under non-catalyzed and catalyzed conditions with dynamic and isothermal methods. By means of TG/MS, an apparent energy of activation Ea of 211.3 kJ mol−1, a decimal logarithm of the pre-exponential factor log k0 of 15.03 min−1 and an apparent order of reaction of 0.81 is determined for the non-catalyzed reaction. These values excellently agree with those from the isothermal method. From that an apparent energy of
Acknowledgements
We gratefully acknowledge the support of this work by the BASF AG and the financial support by the European Commission.
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