Such a method is well known, e. g. from Kohan, Nylon Plastics Handbook, Carl Hanser Verlag, Munich, 1995. According to this publication the hydrolytic polymerisation of £-caprolactam in a first reaction step involves the ring opening, under the influence of water, of part of the caprolactam to the corresponding amino caproic acid which then, under different, preferably anhydrous reaction conditions, continues to polymerise, by polycondensation and polyaddition of caprolactam, to produce polyamide-6.

On a large industrial scale, polyamide-6 is obtained from E-caprolactam in accordance with the VK- method (VK = Vereinfacht Kontinuerlich). This involves molten caprolactam which contains some-1-4 weight%- water, being fed in at the top of a vertical tubular reactor or a series of tubular reactors at a temperature of about 265°C and approximately atmospheric or, if required, reduced pressure. Under these conditions, the polymerisation is initiated in a first section of the column by ring opening of the lactam under the influence of the water still present there, the polycondensation and polyaddition then taking place in the following section of the column.

In general, this requires residence times of 10-20 hours, to achieve a sufficiently high degree

of polymerisation. This degree of polymerisation is generally expressed in terms of the relative viscosity, Prel, measured in formic acid, and is generally in the order of 2.0-2.8, when measured in formic acid. If polyamide-6 having a higher degree of polymerisation is required, a solid-phase postcondensation is employed afterwards in an inert gas atmosphere or in a vacuum.

This solid-phase postcondensation process generally requires at least another 12 hours.

During the long residence time in the VK- column, the equilibrium is established between polyamide, monomer and oligomers, as a result of which the reaction mixture leaving the polymerisation column contains in the order of 10 weight% of monomer and 2 weight% of leachable oligomers. This low molecular- weight material must be removed from the polyamide by means of leaching using water. After processing of the extract, these low molecular-weight compounds can be recycled into the process together with the fresh lactam. Separation and work-up of these low molecular- weight compounds requires large installations, large amounts of water and high energy consumption.

It proved possible to shorten the total polymerisation time by a few hours by making use of a prepolymerisation reactor upstream of the VK-column, this prepolymerisation being carried out likewise in a tubular reactor, at elevated pressure and under otherwise comparable conditions. A drawback of this polymerisation carried out in two steps is, however, that the level of cyclic dimer, (CD), in the polyamide which ultimately leaves the VK-column is considerably higher than in the case of the VK-column being used on its own.

It is an object of the invention to provide

a method which does not exhibit the above mentioned drawbacks or does so to a much lesser degree. More particularly, the novel method should in a short time lead to a high molecular-weight polyamide-6 having a low level of low molecular-weight compounds in the polyamide in a reproducible process suitable for large- scale (continuous) use.

Most surprisingly, the inventors have succeeded in this by carrying out the step in which the polymerisation of s-caprolactam to polyamide-6 is initiated in the presence of water, in a reactor or reactor zone in which water is supplied via the gas phase.

The feature of supplying the water in the gas phase to a polyamide melt has been disclosed for a melt of polylaurinelactam in FR-2,736,645 (Huls) in order to reduce the number of gels in the melt as a result of overheating and for a melt of an omega-lactam when the initial caprolactam contains less than 1 % of moisture in US-2,562,796 (Koch).

Preferably, the step is carried out in a reactor or reactor zone provided with means by which a self-renewing interface between the molten phase and the gas phase is effected with a large surface/volume ratio of the molten phase. For example, the surface/volume ratio of the molten phase is 5 m-1, preferably greater than 10 m-1, more preferably 40 m-1, most preferably greater than 100 m-1. The volume ratio molten phase/gas phase is generally less than 1, preferably less than 0.5, more preferably less than 0.2.

Reactors that has means by which a self- renewing interface between the molten phase and the gas phase is effected are known per se and inter alia

comprise stirred gas bubble scrubbers, packed column reactors and horizontal scraped-surface reactors.

Horizontal scraped-surface reactors in particular are potentially suitable, since relatively strong mixing of the molten phase can be achieved in these and the molten phase is present in a thin layer, and a large gas volume is present having a relatively high partial pressure of the water vapour. Moreover, it has been found that a thin layer of which the composition is constantly renewed by shear forces, as is the case in scraped-surface reactors, is most preferred. Examples of such scraped-surface reactors are described, inter alia, in DE-A-4126425 and BE-A-649023. Found to be particularly suitable was a reactor of the turbulent- mixer type, in which axial and radial mass transfer is promoted by stirring paddles which at the same time are provided with scrapers by means of which the product is smeared over the entire internal surface of the horizontal reactor vessel. Such a type of reactor is commercially available up to a total capacity of 50,000 litres, for example from Drais, Mannheim, DE.

The amount of water which is introduced into the reactor/reactor zone in the gas phase can vary within wide limits and is preferably chosen to be between 1 and 400 g of H20 per kg of lactam.

The pressure in the reactor (zone) can be either atmospheric or elevated. Preferably, the first reaction step will be carried out at elevated pressure.

The atmosphere above the melt is generally a mixture of an inert gas and steam. Oxygen must be excluded as far as possible to prevent discoloration of the polyamide.

The water in the gas atmosphere can be supplemented, for example, by recirculation of the gas phase, water consumed outside the reactor (zone) being supplemented, or by injection of water in the gas phase

into the reactor (zone). The first method is preferable. The second method has the additional drawback that the energy required for the evaporation of the water must be supplied by means of relatively costly facilities in the reactor (zone).

The invention will now be explained in more detail with reference to the following examples.

Example I-V In a 100 1 DRAIS TR 100 test reactor fitted with a gas inlet and outlet and pressure controller, 25 kg of caprolactam were melted under a nitrogen gas stream and then, at a pressure of 0.5 MPa, heated to 267 3°C, and 10 kg/h of steam were supplied to the reactor. Steam was then passed over the reaction mixture for the periods specified in the table. The output gas stream was condensed in a vessel containing 520 1 of water. Then nitrogen was passed through for the period specified and with the flow rate specified, while the temperature and pressure remained unchanged.

The polymerisation was terminated by the reactor pressure being slowly released and the molten polyamide then being allowed to flow from the reactor and cool.

The polyamide obtained was assayed in terms of the lactam level, (CL), the cyclic dimer level (CD), and the relative viscosity, measured in formic acid, after extraction of lactam and oligomer.

In all the experiments, the reactor agitator, which consisted of a central shaft with 4 arms at fixed distances at an angle of 90°, was used at a constant speed of 45 revolutions per minute. Each arm of the agitator is provided with a specially shaped paddle which enables mixing of the material by friction with the reactor wall and smears it out over the wall.

Table 1 Reaction conditions and results of Examples I-V Steam N2 rel. CL CD vise. Ex. Flow Time Flow Time Time weight weight rate rate total % % Kg/h [h] kg/h h I 10 0.5 4.5 3 3.5 3.20 3.53 0.42 II 10 0. 5 4. 5 4 5. 0 4.02 2.64 0.48 III 10 0. 5 25 1 4.5 2 3.5 3.52 2.82 0.59 IV 10 0. 5 4. 5 3* 3. 5 3.51 2.08 0.58 V 10 0. 5 4. 5 3 3. 5 3.24 3.49 0.48

* The pressure was set to 0.1 MPa.

These experiments very clearly show that this method, in very short polymerisation times, affords a polyamide having a high relative viscosity.

If the conventional methods are employed on an industrial scale, this requires many hours.

The comparison of Examples I and V, the experiment in duplicate, shows good reproducibility.

The caprolactam level and cyclic dimer level of the polyamide are low, which means that a reduced extraction capacity is sufficient.

If an Tjj-ei in the order of 2.5 is desired, it is sufficient to employ just the first step.

It will be evident to those skilled in the art that the reaction times in the different reaction steps can vary within wide limits and are determined, inter alia, by practical considerations connected to the type of reactor. The residence time in the first reactor (zone) in this context can vary between 0.01 and 5 hours, and the residence time in the second

reactor (zone), if present, can vary between 0.1 and 8 hours.

As with the known methods according to the prior art, the temperature in the reactors or reactor zones can vary within wide limits, for example between 200 and 290°C, preferably between 220 and 275°C.

The method can simply be implemented as a continuous process, for example by the Drais reactor after start-up being continuously fed with lactam, and product being discharged, and this product if required being fed to a second reactor and the polymerisation being continued under different conditions.

The other reactor types mentioned in the introduction to the description likewise lend themselves to continuous operation.