Nylon-6 is produced almost exclusively by hydrolytic polymerization of caprolactam, with predominant use of the VK tube (abbreviation for the German expression “vereinfacht kontinuierlich” which means simplified continuous), a heated vertical flow pipe (Fig. 1).
The molten caprolactam, with 0.3-5% of water, chain length regulators, and, if necessary a dulling agent, is fed from above, and the polymer melt is discharged at the reactor bottom. Typically the VK tube is equipped with 3 heat exchangers establishing the temperature profile along the reactor. The VK-tube consists of a plug flow zone in the lower part and a mixing/evaporating zone in the top.
The function of the top part is to heat up the reaction mass and to evaporate excess water thus setting the total water content in the polymer melt. The endothermic caprolactam ring opening reaction is started, followed by exothermal polyaddition and polycondensation. With the central heat exchanger, the temperature is corrected and equalized over the tube cross section. After passing the central heat exchanger, the temperature rises to about 270-280°C due to the heat of reaction. The bottom heat exchanger drops the temperature down to 240 - 250°C, thus reaching a higher degree of polymerization in the equilibrium. Simultaneously a higher degree of caprolactam conversion to Nylon-6 is achieved.
Specially designed inserts are applied evening out the dwell-time over the tube cross section. 16-20 hours are quoted for the mean dwell-time in the tube. Relative solution viscosities from 2.4 to 2.8 are achieved with a single stage process (solvent: 96% sulphuric acid, concentration: 1g/100ml, temperature: 25°C). The maximum capacity is 130 t/day.
In the 2-stage technology (Fig. 2) a prepolymerizer, operated under pressure and with high water content, is followed by a final VK polymerizer operated at atmospheric pressure or vacuum. The 2-stage process allows the production of higher degrees of poly-condensation, as required for example for tire cord. Relative solution viscosities up to 3.5 approx. are obtained. The high reaction rate of the caprolactam ring opening under the conditions in the prepolymerizer yields a low total residence time making the process suitable for very high throughput rates up to 300t/day.
A discontinuous autoclave process is also applied. It can be of importance in producing smaller batches or special grades.
The polymer chips are formed using strand cutters or underwater cutting systems and are conveyed to the extraction unit.
Important for up to date production technology is the recycling of caprolactam and its oligomers, which are extracted from Nylon-6 raw polymer. The standard technology is distillation and depolymerization of the extracts. This has, however, severe drawbacks concerning raw material utilization, energy consumption and waste disposal. In modern recycling processes, the concentrated extract is polymerized directly into Nylon-6 either alone or mixed with virgin caprolactam (Fig. 3, Re-Feeding process).
Due to the equilibrium situation of the polyaddition reaction the conversion of caprolactam to nylon-6 is not complete. The polymer contains a low-molecular proportion of about 8 – 10 % consisting of caprolactam and oligomers. These must be removed by hot water extraction to a very high extent in order to achieve a good polymer quality and high performance in further processing. Water at a temperature between 100 and 120°C is fed to the bottom of the extractor, which is a vertical column, countercurrent to the chips flow (Fig. 4). The rising water is enriched to 8-14% of extract, while the granulate flowing downwards reaches a residual extractable content of £ 0.6%. Integral trays prevent circulation flows of water which otherwise are generated by differences in density.
A further possibility of continuous caprolactam removal from the raw polymer is vacuum demonomerization of the Nylon-6 melt e.g. applying thin-layer evaporators. The process avoids hot water extraction, drying and re-melting and makes direct spinning possible. However, so far vacuum demonomerization could not prevail as the cyclic oligomers can hardly be removed due to their low vapor pressure.
Higher molecular weights, suitable e.g. for film and foil applications, are produced via solid-state post-polycondensation at 150-190°C. The process can be implemented continuously or discontinuously.
A further possibility of continuous caprolactam removal from the raw polymer is vacuum demonomerization of the Nylon-6 melt e.g. applying thin-layer evaporators. The process avoids hot water extraction, drying and re-melting and makes direct spinning possible. However, so far vacuum demonomerization could not prevail as the cyclic oligomers can hardly be removed due to their low vapor pressure.
Higher molecular weights, suitable e.g. for film and foil applications, are produced via solid-state post-polycondensation at 150-190°C. The process can be implemented continuously or discontinuously. |
Dr. Paul Schlack from I.G. Farben on 28 January 1938 was the first to make a spinable polymer from caprolactam. This polymer was given the name “Perlon” and is now commonly known as Nylon-6 (or Polyamide-6). Parachutes, stockings, toothbrushes, tents and ropes represent some of the first applications of Nylon-6. At present this incredible versatile and exciting material is unrivalled in its number of applications, including textiles, carpets, industrial fibres, engineering plastics and film. The first production of caprolactam (literally “goat milk”) on a laboratory scale was reported in the literature almost forty year earlier, in 1899. Later caprolactam was made by heating cyclohexanonoxim in sulphuric acid. This chemical reaction is known as the “Beckman rearrangement”.