WO1991018941A1

WO1991018941A1 - PROCESS AND APPARATUS FOR THE PREPARATION OF NYLON-6 FROM ε-CAPROLACTAM USING ULTRASOUND TREATMENT

Info

Publication number: WO1991018941A1
Application number: PCT/EP1991/000987
Authority: WO
Inventors: Vittorio Ragaini
Original assignee: Consorzio Per La Costituzione Di Milano Ricerche Centro Per L'innovazione In Città Studi
Priority date: 1990-05-30
Filing date: 1991-05-30
Publication date: 1991-12-12
Also published as: JPH05505640A; IT9020478A0; IT9020478A1; IT1248835B

Abstract

Process for the production of Nylon-6 starting from ε-caprolactam containing an initial water quantity of between 0.03 % and 15 % in weight of the total mass, in which this mass is subjected to ultrasound treatment for a time of from 1 to 180 minutes at a temperature between 95° and 270 °C, and achieving a total reaction time, including that taken by the ultrasound treatment, of from 3 to 8 hours and the relative apparatus comprising a batch reactor (1) fitted with an ultrasound device (3) whose tip (4) forms the area from which ultrasound is emitted, a recycling pump (7), a filler container (6), and a probe (5) for continually detecting the viscosity of the product.

Description

PROCESS AND APPARATUS FOR THE PREPARATION OF NYLON-6 FROM ∈- CAPROLACTAM USING ULTRASOUND TREATMENT

Field of the invention

The present invention refers to a process and relative equipment using ultrasound for obtaining Nylon-6 from e-caprolactam containing an initial quantity of water from 0.03 to 15% in weight of the total mass.

Existing techniques

It is known in the art that the industrial preparation of Nylon-6 is based on the hydrolytic polymerization of 6-caprolactam (CPL), as in the following expression:

(H.K. Reimschuessel: Lactims in Ring-Opening Polymerization. Volume 2, Chapter 7, page 303. - M. Dekker (1969).

This reaction is continuously or discontinuously carried out at temperatures between 215° and 270°C, with total reaction times between 15 and 25 hours.

The production cycle includes the following operations:

- Stage one: Hydrolysis of the CPL and start of poly-addition;

- Stage two: Poly-addition and poly-condensation of CPL and oligomers;

- Stage three: Final polymerization for poly-condensation.

The reaction (1) is made easier by an excess of water and the presence of acids.

In the first stage, the CPL is converted, with water and, preferably , in the presence of acids or alkalis , into ∈-aminocaproic acid (AAC) : + H₂O

H₂N - (CH₂)₅ - COOH (2 )

(AAC)

The AAC can later yield a dimer (D) via a poly-condensation reaction:

H₂N - (CH₂)₅-COOH + H₂N - (CH₂)₅- COOH

H₂N - (CH₂)₅ - C - N - (CH₂)₅ - COOH + H₂O (3)

This reaction can be generalized to obtain a polymer P_m+n from units

P_m and P_n:

H₂N-[(CH₂)₅-C-N-(CH₂)₂]m-COOH+H₂N-[(CH₂)₅-C-N-(CH₂)₅]n COOH

(4)

H₂N—R_m+n—COOH + H₂O where R is: (CH₂)₅ - C - N - (CH₂)₅

While there are CPL molecules present, a poly-addition reaction between CPL and unit P_n is possible, thus obtaining P_n+1: + H₂N-R_n-COOH H₂N(CH₂)₅-C-R_n-COOH (5)

(CPL) (Pn) (P_n+1)

The initial H₂O/CPL molar ratio is of fundamental importance for pointing the reaction towards the poly-condensation (3 and 4) or poly-addition reactions (5) shown above. An excess of water favours the CPL hydrolysis reaction (2), but moves the equilibria (3 and 4 ) to the left and reduces the product polymerized by polycondensation.

Although the water does not form part of the equilibrium (5), it can be seen that the P_n+1 polymer concentration obtained from the polyaddition reaction (5) depends on the water concentration in the reaction system. By combining the equilibrium constants K₂ and K₅ in reactions (2) and (5) respectively, the following is obtained:

[P_n+1] = (K₅/K₂) [P_n] [AAC] / [H₂O] (6)

[from(5)]

Naturally, the poly-addition mechanism ceases when the CPL molecules are exhausted, and thus this mechanism cannot ensure the high molecular weights required for using the product in the manufacture of yarn.

For these reasons, it is advisable to carry out a first phase that is favourable to the AAC (equilibrium 2), by feeding a mixture with an initial H₂O/CPL molar ratio ≥ 0.2 (approx. 3% in weight water of the total mass), and then reduce the quantity of water in a second phase, generally using a vacuum.

The total poly-condensation reaction (4) produces a P_m+n polymer whose concentration is inversely proportional to the water content. Thus, unless the excess of water used in the hydrolysis reaction phase (2) is removed, high molecular weights cannot be obtained, as the following expression shows:

[P_m+n] = K₄ [P_m] [P_n] / [H₂O] (7)

In any case, the water cannot be entirely removed from the reagent system in these processes, and thus a total conversion of the initial product into a high molecular weight polymer can never be obtained.

This is howver odd, as in theory the reaction (1) only requires a trace of water.

Lastly, Russian patent 670582 describing a process which uses ultrasound to polymerize CPL containing 1% water is already known in the art.

Summary of the invention

A process has now been found for preparing Nylon-6 from ∈- caprolactam (CPL), comprising treatment with ultrasound, that makes it possible to operate within the wide limits of the process parameters, obtain high yields, and also obtain a product with a high molcular weight and narrow distribution of molecular weights. This process is characterized by the use of CPL with an initial water percentage of 0.03 to 15% in weight in respect of the total mass, and the fact that this mass, to which a hydrolysis and polymerization promoter is added, undergoes ultrasound treatment for a time of from 1 to 180 minutes at a temperature between 95° and 270°C, and at the end of this ultrasound treatment if it has been conducted at a temperature lower than 260°C and for less than 3 hours, the mass undergoes heat treatment at 250° to 270°C for a time of between 3 and 8 hours.

These conditions make it possible not only to set off the first phase of the process [opening of the CPL ring, as per reaction (2)], but also to cause the poly-condensation phase [reaction (4)] to take place producing 'in situ' water that is useful for the CPL hydrolysis reaction (2).

The apparatus for realizing the process according to the invention consists of a recycling system comprising a batch reactor (1), a recyling pump (7), a filler container (6), and a probe (5) for continuously detecting the viscosity of the product, or in a continuous system using one or more tubular ultrasound reactors (13, 14, and 15), a filler container (11) in which the CPL is melted, a pump (12) for feeding the tubular reactors, and a polymerization reactor (17) mounted downstream of the tubular reactors.

Detailed description of the invention

The characteristics and advantages of the process and apparatus for the production of Nylon-6 from ε-caprolactam (CPL) according to the present invention, are better illustrated in the following detailed description.

Two methods of construction of the apparatus are shown in figures 1 and 2.

Figure 1 shows a recycling system with an ultrasound batch reactor. Figure 2 shows a continuous production system with tubular ultrasound reactors.

The reactor shown in figure 3. which is already known in the art, can be used as a tubular reactor for a continuous production system. The apparatus shown in figure 1 above all comprises a reactor (1), with interspaces (2) for heating, and an ultrasound device (3) the tip (4) of which forms the ultrasound emission area.

Vacuum creation and nitrogen pressurizing operations for conditioning the reactor are carried out via the tubing (9) , valve (VI ) , and pressure gauge (M).

Caprolactim is loaded into the first container (6) with a heating skirt which melts the material. The recycling pump (7) is used to feed the caprolactam to the reactor (1), and from the reactor to the second container (10) with a probe (5) for continuously detecting the viscosity of the product.

From the second container (10), the material is recycled to the first container (6), and from there to the reactor (1) via the pump (7), and thus the cycle continues. The final product is let out of the outlet (10').

A thermal fluid operated thermostat (8) for medium to high temperatures controls the supply of hot fluid to the insulated apparatus.

The apparatus shown in figure 2 comprises three tubular reactors in series (13. 14, and 15) with ultrasound systems (18), designed in such a way that there is a heating fluid circulating in the central tubing (19), and the reagent fluid circulating in the interspaces (20).

The first container (11) is used for melting the reagent material, whereas the second container (17) is used for the polymerization stage of the reagent material following ultrasound treatment.

A thermal fluid operated thermostat (16) controls the supply of heating fluid to the reactors (13, 14, and 15), and a second thermostat (not shown) controls the supply of heating fluid to the containers (11 and 17).

Figure 3 shows a known type of tubular ultrasound reactor (indicated in its entirety by 21) that could be used in the system shown in figure 2. The figure shows the ultrasound treated interspaces (22) in which the reagent material circulates, the duct (23) in which the heating fluid circulates, the reagent fluid inlet/outlets (24), the heating fluid inlet/outlets (25), the tips of the ultrasound devices (26), further reagent fluid inlet/outlets (27), and further heating fluid inlet/outlets (28).

In the process according to the invention, caprolactam with the desired water content undergoes ultrasound treatment, possibly in the presence of a hydrolysis and polymerization promoter, and under certain conditions that are later explained, polymerization is completed via heat treatment which follows the ultrasound treatment. Regarding the initial water percentage in the caprolactam: percentages lower than 1% can be used in the process according to the present invention, down to 0.03% in weight of the total, as can very high percentages, up to 15% , without the water requiring removal between the CPL hydrolysis and polymerization phases, to obtain a polymer with a molecular weight of around 10,000 in approximately 6 hours.

If a polymer with a molecular weight of over 10,000 is required in a time of approximately 6 hours, the aforementioned water removal can take place.

The use of intitial water percentages lower than 1% in weight of the total mass is also extremely useful for running a continual and thus high yield process, without the need to include a water removal phase following CPL hydrolysis.

The removal of the water is not without problems, including that of recovering raw materials or useful substances from the vapour produced, particularly if the water is removed at temperatures above 100°C.

Ultrasound treatment takes place at temperatures between 95° and 270°C, for times between 1 and 180 minutes, at frequencies between 15 to 25 KHz, and at a power of between 2 and 15 KW/Kg of CPL.

The CPL is not only hydrolysed during the ultrasound treatment phase, but the poly-addition reaction also takes place.

This is demonstrated by the fact that polymers with a molecular weight of around 10,000 are obtained, even when using approximately 14% by weight of initial water and treating the mass with ultrasound for 180 minutes at 260°C without removing the initially added water. An amino acid is used as a hydrolysis and polymerization promoter, preferably ∈-aminocaproic acid (6-aminohexanoic), in quantities in weight of 1.10^-3 at 1.0 meq/g of CPL.

The addition of this promoter is especially useful when the initial water percentage in the capolactim is between 0.03% and 0.8% in weight.

The heat treatment following the ultrasound treatment is carried out at temperatures between 250° and 270°C and for times between 3 and 8 hours, or if the ultrasound treatment was conducted at a temperature lower than 260°C, for a time of less than 3 hours.

The process is preferably conducted by initially pressurizing the reactor with 3 to 4 atm. of nitrogen.

The process according to the invention has several advantages, particularly including:

- An improvement in yield, especially when working with low initial water contents;

- A reduction in CPL hydrolysis times; - The possibility of continuous operation without degassing the reactors to remove excess water; - The possibility of using commercially available CPL, which contains approximately 0.05% water, without having to add water, when running a continuous production cycle;

- If CPL with an initial water content of between 0.05% and 0.8% is used, the polymerization mechanism for poly-addition is far more relevant than that of poly-condensation, and this means that a polyamide polymer with a very narrow distribution of molecular weights is produced.

Aside from the polymerization of ∈-caprolactam, the process can also be used to polymerize other lactams.

In order to illustrate the process according to the present invention, the following examples of the preparation of Nylon-6 using the apparatus shown in figure 1 are given. Unless expressly. otherwise stated, the relative viscosities in these examples

were determined at 30°C in respect of sulphuric acid at 96% in weight. The viscosities were determined on a 1% in weight solution of the product in sulphuric acid at 96%. The average molecular weights (M.W.) are approximate and calculated on the basis of a line of regression, x vs. y, where y = M.W. and x = and the equation

was y = 11049.6 x -10773.6 .

Example 1

The tip of a commercially available ultrasound device with a maximum power of 150 Watts was immersed in the reactor shown in figure 1; the ultrasound waves were emitted from the end part of the tip of the device.

The volume of the reactor was approximately 50 cm³.

Before fitting the ultrasound device to the reactor and sealing it, the CPL (∈-caprolactam) with the desired water content was loaded into the reactor. The air was then sucked out of the reactor using a vacuum pump, and the reactor was pressurized with anyhdrous nitrogen to a pressure of

3 Kg/cm².

The reactor was then heated, and once the desired temperature was reached, the ultrasound device was powered.

A mixture was used of CPL and water with an initial molar ratio of H₂O/CPL = 1 (13-7% in weight of the total mass), and subjected to ultrasound treatment at 260°C for 3 hours at 20 KHz at a specific power of 5 kW/kg of CPL. The reactor was kept sealed for the entire ultrasound treatment time.

The product obtained was a white polymer with a relative viscosity of (1.87) and a M.W. of 9,900.

Example 2 (for comparison)

Example 1 was repeated, with the sole difference that ultrasound treatment was not used.

The product obtained had a relative viscosity of 1.64 and a M.W. of 7,350.

Example 3

A mixture was used of CPL and water with an initial molar ratio of H₂O/CPL = 1 (13.7% in weight water), and subjected to ultrasound treatment at 250°C for 3 hours at 20 KHz at a specific power of 8.5 kW/kg of CPL.

The product obtained had a relative viscosity of 1.78 and a M.W. of 8,900, without degassing the reactor during the experiment.

Example 4 (for comparison)

Example 3 was repeated, with the sole difference that ultrasound treatment was not used. The product obtained had a relative viscosity of 1.25 and a M.W. of

3.050.

Example 5

A mixture was used of CPL and water with an initial molar ratio of H₂O/CPL = 0.048 (0.76% in weight water), with the addition of ∈- aminocaproic acid (1.10^-3 meq/g CPL), and subjected to ultrasound treatment at 260°C for 5 minutes at 20 KHz, and then treated for 6 hours at 260°C.

The product obtained had a relative viscosity of 2.06 and a M.W. of 12,000.

Example 6

A mixture was used of CPL and water with an initial molar ratio of

H₂O/CPL = 0.038 (0.6% in weight water), with the addition of ∈- aminocaproic acid (2 x 10^-2 meq/g CPL), and subjected to ultrasound treatment at 110°C for 5 minutes at 20 KHz, and then treated for 6 hours at 260°C.

The product obtained had a relative viscosity of 2.10 and a

M.W. of 12,450.

Example 7 (for comparison)

A mixture was used of CPL and water with an initial molar ratio of

H₂O/CPL = 0.05 (0.8% in weight water), with the addition of acetic acid (strength 99%) (5.2 x 10^-2 meq/g CPL), and subjected in two separate experiments to ultrasound treatment at 260°C at 20 KHz at a specific power of 10 kW/kg of CPL, for times of 60 and 180 minutes repectively.

The products obtained had relative viscosities and M.W.s of repectively: 1.27; 3.260 (60 minute experiment); and 1-50; 5.800 (180 minute experiment).

Example 8

A commercially obtained quantity of CPL containing 0.05% water in weight (molar ratio of H₂O/CPL = 0.003) was used, with the addition of ∈-aminocaproic acid (1.8 x 10^-2 meq/g CPL), and subjected to ultrasound treatment at 110°C for 5 minutes at 20 KHz, after which the mass was heated to 260°C and kept at that temperature for 6 hours.

The product obtained had a relative viscosity of 2.12 and a M.W. of 12,650.

Example 9 (for comparison)

Example 8 was repeated, with the sole difference that ultrasound treatment was not used.

The product obtained had a relative viscosity of 2.0 and a M.W. of 11,300.

Example 10 (for comparison)

Example 9 was repeated, with the sole difference that ∈-aminocaproic acid was not added.

A powdery mass was obtained in which polymerization had not taken place.

Example 11 (for comparison)

Example 9 was repeated, with the sole difference that acetic acid

(strength 99%) for a total of 2 x 10^-2 meq/g CPL

was used.

A powdery mass was obtained in which polymerization had not taken place.

Example 12 Example 8 was repeated, with the sole difference that the heat treatment at 260°C, following ultrasound .reatment, lasted 7 hours. The product obtained had a relative viscosity of 2.20 and a M.W. of 13.550.

Example 13

A commercially obtained quantity of CPL containing 0.05% water in weight (molar ratio of H₂O/CPL = 0.003) was used, with the addition of e-aminocaproic acid (1.6 x 10^-2 meq/g CPL), and subjected to ultrasound treatment at 150°C for 5 minutes at 20 KHz, after which the mass was heated to 260°C and kept at that temperature for 7 hours.

The product obtained had a relative viscosity of 2.00 and a M.W. of

11,320.

Example 14 (for comparison)

Example 8 was repeated, with the sole difference that ultrasound treatment took place at 70°C.

The product obtained had a relative viscosity of 1.95 and a M.W. of

10,770.

Example 15

Example 8 was repeated, with the sole difference that the heat treatment at 260°C, following ultrasound treatment, lasted 8 hours.

The product obtained had a relative viscosity of 2.35 and a M.W. of

15,200.

The product of the polymerization process was subjected to 10 extraction cycles using methyl alcohol at its boiling temperature in a Soxlet apparatus. 8% in weight of the original product was extracted by the methyl alcohol. The polymer residue had a relative viscosity of 2.45 and a M.W. of

16,300.

Example 16 (for comparison)

The preceding example (without the Soxlet extraction phase) was repeated, with the sole difference that ultrasound treatment was not used.

The product obtained had a relative viscosity of 2.0 and a M.W. of

11,320.

Example 17

Example 8 was repeated, with the sole difference that the heat treatment at 260°C, following ultrasound treatment, lasted 9 hours.

The product obtained had a relative viscosity of 2.24 and a M.W. of

13,970

Example 18

A mixture was used of CPL and water with an initial molar ratio of

H₂O/CPL = 0.013 (0.2% in weight water), with the addition of ∈- aminocaproic acid (8 x 10^-2 meq/g CPL), and subjected to ultrasound treatment at 110°C for 5 minutes at 20 KHz in a nitrogen atmosphere

(3 atm.), and then treated for 6 hours at 260°C.

The product obtained had a relative viscosity of 2.12 and a M.W. of

12,650.

The experiment was repeated in order to ascertain that the results could be repeated, and the product obtained had a relative viscosity of 2.11 and a M.W. of 12,540.

Example 19

The preceding example was repeated, with the sole difference that the heat treatment at 260°C, following ultrasound treatment, lasted 7 hours .

The product obtained had a relative viscosity of 2.33 and a M.W. of

14,970.

Example 20 (for comparison)

The preceding example was repeated, with the sole difference that ultrasound treatment was not used.

The product obtained had a relative viscosity of 2.07 and a M.W. of

12,100.

Example 21

Example 18 was repeated, with the sole difference that the heat treatment at 260°C, following ultrasound treatment, lasted 8 hours.

The product obtained had a relative viscosity of 2.30 and a M.W. of

14,640.

Example 22

A mixture was used of CPL and water with an initial molar ratio of

H₂O/CPL = 0.063 (1% in weight water), and subjected to ultrasound treatment at 110°C for 5 minutes at 20 KHz in a nitrogen atmosphere

(3 atm.), and then treated for 5 hours at 260°C.

The product obtained had a relative viscosity of 1.98 and a M.W. of 11.100.

Example 23

The preceding example was repeated, with the sole difference that the heat treatment at 260°C, following ultrasound treatment, lasted

6 hours.

The product obtained had a relative viscosity of 2.22 and a M.W. of

13.760.

Example 24 (for comparison) The preceding example was repeated, with the sole difference that ultrasound treatment was not used.

The product obtained had a relative viscosity of 2.11 and a M.W. of

12,540.

Example 25

Example 22 was repeated, with the sole difference that the heat treatment at 260°C, following ultrasound treatment, lasted 7 hours.

The product obtained had a relative viscosity of 2.19 and a M.W. of

13,420.

Example 26

Example 22 was repeated, with the sole difference that the heat treatment at 260ºC, following ultrasound treatment, lasted 8 hours.

The product obtained had a relative viscosity of 2.18 and a M.W. of

13,310.

Example 27

Example 22 was repeated, with the difference that ∈-aminocaproic acid (8 x 10^-2 meq/g CPL) was added to the reaction mixture, and the difference (also in respect of example 22) that the heat treatment at 260°C, following ultrasound treatment, lasted 6 hours (as in example 23).

The product obtained had a relative viscosity of 2.14 and a M.W. of

12,870.

Example 28

Example 19 was repeated, with the difference that, unlike all the preceding examples, the reactor was pressurized with helium at 3 atm., instead of with nitrogen, the pressure being the same as that in all the preceding examples. The product obtained had a relative viscosity of 2.19 and a M.W. of

13,420.

Example 29

Example 19 was repeated, with the difference that the reactor was pressurized with Krypton gas, as in example 28.

The product obtained had a relative viscosity of 2.01 and a M.W. of

11,440.

Example 30

Example 19 was repeated, with the difference that the reactor was pressurized with Argon gas, as in example 28.

The product obtained had a relative viscosity of 2.25 and a M.W. of

14,090.

Example 31

A commercially obtained quantity of CPL containing 0.05% water in weight was used, with the addition of ∈-aminocaproic acid (8 x 10^-2 meq/g CPL), and heated to 70°C and subjected to a vacuum of 0.1 mmHg

(Torr) for 1 hour at that temperature, thus obtaining CPL with an

H₂O content of 0.03% in weight. After this treatment , the reactor was pressurized with nitrogen to 3 atm., and the CPL was subjected to ultrasound treatment for 5 minutes at 20 KHz. Ultrasound treatment was begun at 70°C (after around 1 minute the reactor temperature was raised to 110°C by the effect of the ultrasound treatment), after which the CPL was heat treated at 260ºC for 9 hours.

The product obtained had a relative viscosity of 1.70 and a M.W. of

8,010.

Example 32 Example 31 was repeated, with the sole difference that the heat treatment at 260°C lasted 12 hours.

The product obtained had a relative viscosity of 1.91 and a M.W. of

10,330.

Example 33

Example 31 was repeated, with the sole difference that the heat treatment at 260°C lasted 14 hours.

The product obtained had a relative viscosity of 2.32 and a M.W. of

14,860.

Example 34

Example 31 was repeated, with the sole difference that the heat treatment at 260°C lasted 16 hours.

The product obtained had a relative viscosity of 2.55 and a M.W. of

17,400.

The polymerization product was subjected to extraction using methyl alcohol in a Soxlet apparatus, as described in example 15. The product obtained had a relative viscosity of 2.86 and a M.W. of

20,830.

Example 35

Example 31 was repeated, with the sole difference that the heat treatment at 260°C lasted 18 hours.

The product obtained had a relative viscosity of 2.21 and a M.W. of

13,645.

Example 36 (for comparison)

Example 34 was repeated, with the differences that neither the ultrasound treatment nor the methyl alcohol extraction phases were used. The product obtained had a relative viscosity of 1.96 and a M.W. of

10,880.

The products obtained in the experiments in examples 12 and 23 were subjected to molecular weight distribution analysis using the 'gel permeation' technique conducted at 50°C using trifluoroethanol as a solvent at a rate of 1 ml/min. The results for examples 12 and 23 are shown in figures 5 and 6 respectively.

In the graphs, the UV detector signal is shown on the X axis, and the elution time in minutes is shown on the Y axis. When figure 5 is compared with figure 6 (width of the curve detected at various heights), it can be seen that the distribution of molecular weights obtained by polymerizing CPL containing 0.05% water is narrower than that obtained when polymerizing CPL with 1% water.

The following comments can be made about the above results:

The comparison of the results obtained in examples 1 and 2, 3 and 4, 8 and 9, 15 and 16, 19 and 20, 23 and 24, and 34 and 36 shows that the ultrasound treatment improves the polymerization of CPL over a wide range of initial water contents in the CPL (from almost 15% to less than 0.05% in weight).

The results of examples:

A) 8, 12, 15, 17, and that of comparison experiment 16 (without ultrasound);

B) 18, 19, 21, and that of comparison experiment 20 (without ultrasound);

C) 22 , 23, 25, 26, and that of comparison experiment 24 (without ultrasound);

D) 31, 32, 33, 34, 35, and that of comparison experiment 36 (without heat treatments are shown on the Y axes. The curves show that the benificial effect of ultrasound varies with the initial water content of the CPL. At the highest molecular weight and both with and without ultrasound, the increase in fact varies from 10% to 24% to 35% to 60%, as the CPL initial water content passes from 1% to 0.2% to 0.05% to 0.03%. Figures 4 and 7 show that decidedly better results are obtained when using CPL with an initial water content lower than 1% in weight, and especially with CPL (in its original state) with an initial water content equal to or lower than 0.05% in weight.

The possibility of polymerizing CPL into Nylon-6 in a single stage process, and obtaining high molecular weights, is especially suited, given its simplicity, to industrial production.

As can be seen from figures 4 and 7. this is possible when using CPL in its original state, or with less than 0.05% water in weight, but is not possible when CPL contains 1% in weight water.

However, the polymerization of CPL in its original state requires different conditions than CPL with 1% in weight water.

In fact, only when a promoter (∈-aminocaproic acid) is added to CPL in its original state does polymerization occur (see examples 9, 10, and 11), whilst the addition of this promoter to CPL with 1% in weight water is not useful, and in fact has a slightly negative effect (see examples 23 and 27). The oligomer percentage obtained when using CPL in its original state and with 0.03% water at the highest molecular weight, is 8% in weight. This percentage is of some interest, and takes the M.W. in the polymer residue to 16,300 (example 15), and to 20,830 (example 34).

The pressurization of the reactor with nitrogen gives better results than when other gasses are used (see examples 19. 28, 29, and 30).

This result could not be foreseen from literature on the subject

(T.J. Mason and J.P. Lorimer: Sonochemistry. Page 47 - Ellis Horwood Ltd, Chichester (1988).

Claims

1. Process for the preparation of Nylon-6 from ∈-caprolactam (CPL), characterized by the fact that CPL with an initial water percentage between 0.03% and 15% in weight in respect of the total mass is used, and that this mass, to which an amino acid may be added as a hydrolysis and polymerization promoter, is subjected to ultrasound treatment for from 5 to 180 minutes at a temperature from 95° to 270°C, and that when the ultrasound treatment is completed, provided it is conducted at a temperature of less than 260°C and for a time of less than 3 hours, the mass is subjected to heat treatment at a temprature between 250° and 270°C for a time of from 3 to 8 hours.

2. Process as in Claim 1, characterized by the fact that the aforementioned ultrasound treatment is carried out at frequencies between 15 and 25 KHz and at a power from 2 to 15 KW/Kg of CPL.

3. Process as in Claim 1, characterized by the fact that the aforementioned addition of an amino acid hydrolysis and polymerization promoter takes place when the aforementioned initial water percentage in the CPL is between 0.03% and 0.8% in weight.

4. Process as in Claim 1, characterized by the fact that the aforementioned amino acid hydrolysis and polymerization promoter is ∈-aminocaproic acid in quantities from 1.10 to 1.0 meq/g of CPL.

5. Process as in Claim 1, characterized by the fact that the reactor is initially pressurized with nitrogen to a pressure of 3 to 4 atmospheres.

6. Process as in Claim 1, characterized by the fact that it is carried out without removing water between the CPL hydrolysis and polymerization phases, in the case that the aforementioned initial H₂O percentage in the CPL is between 0.03% and 0.8% in weight.

7. Process as in Claim 1, characterized by the fact that it is carried out with the removal of water between the CPL hydrolysis and polymerization phases.

8. Apparatus for the preparation of Nylon-6 from ∈-caprolactam, comprising a batch reactor (1) fitted with an ultrasound device (3) whose tip (4) forms the area from which ultrasound is emitted, a recycling pump (7), a filler container (6), and a probe (5) for continually detecting the viscosity of the product.

9. Apparatus for the preparation of Nylon-6 from ∈-caprolactam in a continuous cycle, comprising one or more tubular ultrasound reactors (13, 14, and 15), a filler container in which caprolactam is melted, a pump (12) for feeding the aforementioned tubular reactors, and a polymerization reactor (17) downstream from the aforementioned tubular reactors.

10. Polyamide product with a very narrow distribution of molecular weights obtained through the polymerization of CPL with an H₂O content of between 0.03% and 0.8% in weight produced using the process described in Claim 1.