Method for acid hydrolysis of pure polycaprolactam

Abstract

The present invention relates to an improved process for depolymerizing laurolactam with sulfuric acid. A particularly suitable starting material is poly-laurolactam, which is intended for recycling and which is characterized by a low laurolactam content.

Description

[0001] The present invention relates to an improved process for the acid hydrolysis of polylaurolactam with sulfuric acid. A suitable starting material is in particular polylaurolactam, which is intended to be recycled and is characterized by a low content of laurolactam. Background Art

[0002] Polylaurolactam (CAS No.: 24937-16-4; alternative name: nylon 12, polyamide 12, hereinafter abbreviated as "PA12") is an industrially important polymer.

[0003] PA12 contains repeating units having the following structure (I), wherein the bond identified by (*) of one repeating unit is bonded to the bond identified by (**) of an adjacent repeating unit.

[0004]

[0005] PA12 is valuable in applications especially because of its hydrolysis stability and is significantly different in this property from short-chain polyamides such as polycaprolactam (CAS No. 25038-54-4; alternative name: polyamide 6; hereinafter abbreviated as "PA6") or poly(N,N'-hexamethyleneadipinediamide) / poly(adipic acid hexamethylene diamide) (CAS No.: 32131-17-2; alternative name: polyamide 66; hereinafter abbreviated as "PA66").

[0006] Due to the higher density of amide groups, PA6 and PA66 absorb several times more water. Additionally, the reaction equilibrium of short-chain polyamides is further from the monomer side, which means that hydrolysis can proceed faster.

[0007] The recycling of these materials requires the cleavage of amide bonds, which is usually carried out under acid or base catalysis to obtain defined monomer units.

[0008] Once these materials are available, the first method for the chemical recycling of nylon 66 was started.

[0009] US 2,872,420 describes a method for recovering nylon 66 from waste using sulfuric acid. According to this document, highly pure sulfuric acid has only a small effect on the molecular weight of polyamides and can therefore be used for purification by dissolution and precipitation.

[0010] The recycling of nylon 6 is a special case because the monomer caprolactam can be separated from the reaction mixture by distillation and therefore the reaction can reach a high conversion even with non-quantitative addition of acid or base.

[0011] In the presence of water, treatment of PA6 and PA66 with an alkali or an inorganic acid at relatively high temperatures causes PA6 and PA66 to degrade into monomer units.

[0012] US 2,407,896 A and US 3,069,465 A describe the hydrolysis of PA66 with sulfuric acid.

[0013] WO 97 / 00846A1 describes the hydrolysis of PA66 with nitric acid.

[0014] US 2,840,606 A describes the alkaline hydrolysis of short-chain AABB-type polyamides. Under alkaline catalysis, diamines and diacids are re-formed into components at temperatures above 160 °C. The addition of fatty alcohols accelerates the reaction.

[0015] During the hydrolysis of PA6 by the methods described in the prior art, the post-treatment of the aqueous solution of this amino acid is difficult because short-chain ω-amino carboxylic acids are easily soluble in the form of salts and free acids, and thus purification requires distilling large amounts of water.

[0016] CN102399363 A describes the hydrolysis of nylon-6 by hydrolysis degradation on a solid acid having an SO 4 2- group. A route for producing a hot-melt adhesive therefrom is described. The reaction does not achieve complete conversion.

[0017] DE1240087B describes the post-treatment of long-chain ω-amino carboxylic acids such as ω-aminolauric acid.

[0018] The hydrolysis recycling of polyamide 12 is technically more demanding due to its relatively high stability.

[0019] JPS55-108453A describes the degradation of polyamide 12 by adding water and phosphoric acid during extrusion. However, this only involves reducing the molecular weight to a value higher than the monomer value. Therefore, this degradation does not proceed to aminolauric acid.

[0020] DE 693 09 709T2 describes the hydrolysis and oxidation of polyamides. As possible polyamides, PA6, PA66 and PA12 can be mentioned. The reaction is based on the fact that nitroso groups are dissolved in the hydrolysis medium.

[0021] DE 3401415 A describes the post-treatment of waste streams from laurolactam production. The waste stream consists of a mixture of laurolactam, oligomeric polyamides and polyamides as well as other impurities. Acid- or base-catalyzed hydrolysis allows conversion to ω-aminolauric acid.

[0022] It has now been found that in the process described in DE 3401415 A, hydrolysis is facilitated by means of laurolactam as a plasticizer. However, in most cases, PA-12 waste does not contain laurolactam.

[0023] Furthermore, US 4,170,588 A describes a process for hydrolyzing polylaurolactam with sulfuric acid at 80 °C.

[0024] Accordingly, it is an object of the present invention to provide an improved process for hydrolyzing polyamide 12 waste.

[0025] Now, a process for achieving this object has surprisingly been found. SUMMARY OF THE INVENTION

[0026] Accordingly, the present invention relates to a process for hydrolyzing polylaurolactam,

[0027] wherein polylaurolactam is cleaved with sulfuric acid at a temperature of 125 °C to 190 °C, preferably 140 °C to 180 °C, more preferably 150 °C to 170 °C, even more preferably 160 °C to 165 °C to obtain ω-aminolauric acid. DETAILED DESCRIPTION

[0028] Surprisingly, it has been found that the hydrolysis of polylaurolactam with sulfuric acid proceeds with a higher yield compared to the processes described in the prior art.

[0029] The polylaurolactam used in the process according to the invention is not further restricted. Preference is given to using materials for recycling (PA12 waste).

[0030] The polylaurolactam is used in particular in the form of a powder, since the increase in surface area thereby can accelerate the hydrolysis. Suitable powdered PA12 is described, for example, in EP 0 911 142 A1. For this purpose, before the PA12 is used in the process according to the invention, it can be ground to a powder by methods known to those skilled in the art.

[0031] The PA12 used according to the invention comprises repeating units having the above structure (I), wherein the bond identified by (*) of one repeating unit is bonded to the bond identified by (**) of an adjacent repeating unit.

[0032] The PA12 used according to the invention preferably has a molar mass of 1000 to 10 6 g / mol, more preferably 3000 to 200 000 g / mol, still more preferably 15 000 to 150 000 g / mol, yet more preferably 25 000 to 120 000 g / mol, even more preferably 40 000 to 95 000 g / mol, still even more preferably 80 000 g / mol.

[0033] The present invention is particularly suitable for hydrolyzing polylaurolactam containing a low content of laurolactam. Preferably, polylaurolactam having a laurolactam content of <4.9% by weight, preferably <4.0% by weight, more preferably <3.0% by weight, still more preferably <1.0% by weight, and even more preferably <0.01% by weight (in each case based on the sum of the masses of PA12 and laurolactam) is used.

[0034] The present invention is particularly suitable for hydrolyzing polylaurolactam containing a low content of ω-aminolauric acid. Preferably, polylaurolactam having an ω-aminolauric acid content of <4.9% by weight, preferably <4.0% by weight, more preferably <3.0% by weight, still more preferably <1.0% by weight, and even more preferably <0.3% by weight (in each case based on the sum of the masses of PA12 and ω-aminolauric acid) is used.

[0035] This particularly differentiates the method from the method in DE 3401415 A, where residues from laurolactam production are hydrolyzed, which results in a high proportion of laurolactam in the reactants to be hydrolyzed.

[0036] In contrast, hydrolysis with sulfuric acid in the method according to the invention is particularly suitable for using recycled material characterized by a low content of laurolactam and ω-aminolauric acid.

[0037] The hydrolysis can be carried out by methods known to those skilled in the art.

[0038] The temperature in the method according to the invention is from 125 °C to 190 °C, preferably from 140 °C to 180 °C, more preferably from 150 °C to 170 °C, and even more preferably from 160 °C to 165 °C.

[0039] The pressure in the method according to the invention is preferably in the range from 1 bar to 100 bar, preferably from 10 bar to 60 bar.

[0040] Generally, the polylaurolactam to be hydrolyzed is first charged into a reaction vessel, such as an autoclave coated with a noble metal such as gold or stainless steel, and sulfuric acid is added thereto.

[0041] In this case, sulfuric acid is usually an aqueous solution, and the content of sulfuric acid in the aqueous solution is not further limited.

[0042] The content of sulfuric acid in the aqueous solution is preferably in the range from 10% by weight to 90% by weight, preferably from 20% by weight to 80% by weight, more preferably from 30% by weight to 50% by weight, and even more preferably from 35% by weight to 40% by weight.

[0043] The ratio of sulfuric acid used to polycaprolactam used is likewise not further restricted. In particular, sulfuric acid is used in an amount such that the weight ratio of sulfuric acid used to PA12 used is in the range of 1:0.1 to 1:1, preferably in the range of 1:0.2 to 1:0.8, more preferably in the range of 1:0.3 to 1:0.7, even more preferably in the range of 1:0.4 to 1:0.6, still even more preferably in the range of 1:0.45 to 1:0.51.

[0044] The process according to the invention is preferably carried out until at least 30% by weight, more preferably at least 50% by weight, even more preferably at least 70% by weight, still more preferably at least 90% by weight, even still more preferably at least 99% by weight of the PA12 used has reacted.

[0045] In an alternative specific embodiment of the process according to the invention, after 1-99% by weight, preferably 30-90% by weight, more preferably 50-70% by weight of the PA12 used has reacted, alkali metal hydroxide is added to neutralize the sulfuric acid (i.e., in particular, the pH in the reaction mixture is set to 7), and then further alkali metal hydroxide is added, which further alkali metal hydroxide reacts with the remaining PA12 as a hydrolysis catalyst.

[0046] The alkali metal hydroxide used is in particular lithium hydroxide, sodium hydroxide or potassium hydroxide, preferably potassium hydroxide.

[0047] This alternative embodiment makes use of the fact that the hydrolysis of PA12 with low caprolactam content or very similar ω-aminocaproic acid proceeds very well with sulfuric acid. When the PA12 has decomposed to a certain extent, the remaining hydrolysis cleavage can be carried out with alkali metal hydroxide in a subsequent step.

[0048] Then, this subsequent step can preferably be carried out as described in DE 3401415 A.

[0049] In particular, the alkali metal hydroxide is used in the subsequent step in such an amount that after the neutralization of the sulfuric acid, the alkali metal hydroxide is present in an amount such that the weight ratio of unreacted polycaprolactam to the remaining alkali metal hydroxide is in the range of 1:0.1 to 1:1, preferably in the range of 1:0.2 to 1:0.8, more preferably in the range of 1:0.3 to 1:0.7, even more preferably in the range of 1:0.4 to 1:0.6, still even more preferably in the range of 1:0.45 to 1:0.51.

[0050] The temperature in the subsequent step is preferably in the range of 160 °C to 280 °C, preferably 180 °C to 250 °C, more preferably 200 °C to 230 °C.

[0051] The subsequent pressure is preferably in the range from 1 bar to 100 bar, more preferably from 10 bar to 60 bar.

[0052] In a subsequent step, the alkali metal hydroxide is preferably used as an aqueous solution, more preferably at a concentration of from 1% to 10% by weight of alkali metal hydroxide.

[0053] If the subsequent step is carried out, the remaining work-up of the depolymerized PA12 can be carried out as described in DE 3401415 A.

[0054] The following examples illustrate the invention without restricting it in any way.

[0055] Examples

[0056] The depolymerization of PA12 having in each case a laurolactam and ω-aminolauric acid content of < 1% was investigated under different catalyst and temperature conditions.

[0057] The following catalysts were tested: KOH, H 2 SO 4 、H 3 PO 4 .

[0058] The conversion was determined by 1H NMR in CDCl 2 + trifluoroacetic anhydride. 1

[0059] The following tests were carried out:

[0060] 1. Comparative test C1: H 3 PO 4 , 320 °C

[0061] 6.66 mg of PA12 powder was placed in a gold-plated autoclave. Thereafter, 6.8 mg of H 3 PO 4 (85%) and 6.30 mg of water were added and the autoclave was sealed. The autoclave was heated in an oven to 320 °C and the reaction mixture was kept at this temperature for 4 h. The mixture was then cooled to room temperature at a rate of 100 K / min. The reaction mixture was removed from the reactor and neutralized by adding 1 g of water and a stoichiometric amount of KOH. The mixture was dried at 80 °C and 200 mbar. The conversion of PA12 was 96%.

[0062] 2. Comparative test C2: H 3 PO 4 , 300 °C

[0063] 5.9 mg of PA12 powder was placed in a gold-plated autoclave. Thereafter, 4.25 mg of H 3PO 4 (85%) and 5.23 mg of water, and seal the autoclave. Heat the autoclave in an oven to 300 °C, and maintain the reaction mixture at this temperature for 4 hours. Then cool the mixture to room temperature at a rate of 100 K / min. Remove the reaction mixture from the reactor and neutralize it by adding 1 g of water and a stoichiometric amount of KOH. Dry the mixture at 80 °C and 200 mbar. The conversion rate of PA12 is 85%.

[0064] 3. Comparative test C3: H 3 PO 4 , 250 °C

[0065] Place 7.17 mg of PA12 powder in a gold-plated autoclave. Thereafter, add 4.98 mg of H 3 PO 4 (85%) and 6.23 mg of water, and seal the autoclave. Heat the autoclave in an oven to 250 °C, and maintain the reaction mixture at this temperature for 4 hours. Then cool the mixture to room temperature at a rate of 100 K / min. Remove the reaction mixture from the reactor and neutralize it by adding 1 g of water and a stoichiometric amount of KOH. Dry the mixture at 80 °C and 200 mbar. The conversion rate of PA12 is 88%.

[0066] 4. Comparative test C4: H 3 PO 4 , 220 °C

[0067] Place 6.95 mg of PA12 powder in a gold-plated autoclave. Thereafter, add 4.6 mg of H 3 PO 4 (85%) and 6.30 mg of water, and seal the autoclave. Heat the autoclave in an oven to 220 °C, and maintain the reaction mixture at this temperature for 4 hours. Then cool the mixture to room temperature at a rate of 100 K / min. Remove the reaction mixture from the reactor and neutralize it by adding 1 g of water and a stoichiometric amount of KOH. Dry the mixture at 80 °C and 200 mbar. The conversion rate of PA12 is 87%.

[0068] 5. Comparative test C5: H 3 PO 4 , 200 °C

[0069] Place 7.27 mg of PA12 powder in a gold-plated autoclave. Thereafter, add 5.43 mg of H 3 PO 4(85%) and 6.20 mg of water, and the autoclave was sealed. The autoclave was heated to 200 °C in an oven, and the reaction mixture was maintained at this temperature for 4 hours. Then the mixture was cooled to room temperature at a rate of 100 K / min. The reaction mixture was removed from the reactor and neutralized by adding 1 g of water and a stoichiometric amount of KOH. The mixture was dried at 80 °C and 200 mbar. The conversion rate of PA12 was 100%.

[0070] 6. Invention Example I1: H 2 SO 4 , 180 °C

[0071] 6.5 mg of PA12 powder was placed in a gold-plated autoclave. Thereafter, 8.62 mg of H 2 SO 4 (35%) was added, and the autoclave was sealed. The autoclave was heated to 180 °C in an oven, and the reaction mixture was maintained at this temperature for 4 hours. Then the mixture was cooled to room temperature at a rate of 100 K / min. The reaction mixture was removed from the reactor and neutralized by adding 1 g of water and a stoichiometric amount of KOH. The mixture was dried at 80 °C and 200 mbar. The conversion rate of PA12 was 100%.

[0072] 7. Invention Example I2: H 2 SO 4 , 160 °C

[0073] 6.61 mg of PA12 powder was placed in a gold-plated autoclave. Thereafter, 8.63 mg of H 2 SO 4 (35%) was added, and the autoclave was sealed. The autoclave was heated to 160 °C in an oven, and the reaction mixture was maintained at this temperature for 4 hours. Then the mixture was cooled to room temperature at a rate of 100 K / min. The reaction mixture was removed from the reactor and neutralized by adding 1 g of water and a stoichiometric amount of KOH. The mixture was dried at 80 °C and 200 mbar. The conversion rate of PA12 was 98%.

[0074] 8. Comparative Test C6 (corresponding to DE 3407415 A1): KOH, 320 °C

[0075] 30.0 g of PA12 powder was placed in a nickel-plated autoclave. Thereafter, 18.95 g of an aqueous KOH solution (50%) was added and the autoclave was sealed. Then nitrogen was injected into the autoclave up to 28 bar and it was heated to 320 °C. The reaction mixture was maintained at this temperature for 4 hours. Then the mixture was cooled to room temperature at a rate of 100 K / min and the pressure was released. The reaction mixture was removed from the reactor, dissolved in water and neutralized by adding 13 g of sulfuric acid. The mixture was dried at 70 °C and 100 mbar. The conversion of PA12 was 94%.

[0076] 9. Comparative test C7 (corresponding to DE 3407415 A1): KOH, 300 °C

[0077] 7.42 mg of PA12 powder was placed in a gold-plated autoclave. Thereafter, 4.03 mg of an aqueous KOH solution (50%) was added and the autoclave was sealed. The autoclave was heated to 300 °C in an oven. The reaction mixture was maintained at this temperature for 4 hours and then cooled to room temperature at a rate of 100 K / min. Subsequently, the mixture was cooled to room temperature. The reaction mixture was removed from the reactor and neutralized by adding 1 g of water and a stoichiometric amount of phosphoric acid. The mixture was dried at 70 °C and 100 mbar. The conversion of PA12 was 78%.

[0078] 10. Comparative test C8 (corresponding to DE 3407415 A1): KOH, 280 °C

[0079] 7.17 mg of PA12 powder was placed in a gold-plated autoclave. Thereafter, 4.42 mg of an aqueous KOH solution (50%) was added and the autoclave was sealed. The autoclave was heated to 280 °C in an oven. The reaction mixture was maintained at this temperature for 4 hours and then cooled to room temperature at a rate of 100 K / min. Subsequently, the mixture was cooled to room temperature. The reaction mixture was removed from the reactor and neutralized by adding 1 g of water and a stoichiometric amount of phosphoric acid. The mixture was dried at 70 °C and 100 mbar. The conversion of PA12 was 78%.

[0080] 11. Comparative test C9 (corresponding to DE 3407415 A1): KOH, 280 °C

[0081] 6.80 mg of PA12 powder was placed in a gold-plated autoclave. Thereafter, 0.95 mg of an aqueous KOH solution (50%) was added and the autoclave was sealed. The autoclave was heated to 320 °C in an oven. The reaction mixture was maintained at this temperature for 4 h and then cooled to room temperature at a rate of 100 K / min. Subsequently, the mixture was cooled to room temperature. The reaction mixture was removed from the reactor and neutralized by adding 1 g of water and a stoichiometric amount of phosphoric acid. The mixture was dried at 70 °C and 100 mbar. The conversion of PA12 was 2%.

[0082] 12. Comparative test C10: Without catalyst, 320 °C

[0083] 7.62 mg of PA12 powder was placed in a nickel-plated autoclave. Thereafter, 3.91 mg of water was added and the autoclave was sealed. The autoclave was heated to 320 °C in an oven. The reaction mixture was maintained at this temperature for 4 h and then cooled to room temperature at a rate of 100 K / min. The reaction mixture was removed from the reactor and dried at 80 °C and 200 mbar. The conversion of PA12 was 6%.

[0084] The results of the tests and comparative tests are summarized in Table 1 below.

[0085] Table 1

[0086]

[0087] It can be clearly seen from the results that at particularly low temperatures, namely as low as 190 °C and 160 °C, the addition of H 2 SO 4 results in almost quantitative conversion.

[0088] 13. Temperature dependence of the hydrolysis of PA12 with sulfuric acid

[0089] To study the temperature dependence of the hydrolysis of PA12 with sulfuric acid, the following series of tests were carried out:

[0090] PA12 powder and H 2 SO 4 (35%) were added in the amounts shown in Table 2 below to a gold-plated autoclave. Then the autoclave was sealed. The autoclave was heated to the corresponding temperature T shown in Table 2 below (heating rate 5 K / min) in an oven and the reaction mixture was maintained at this temperature for 4 h. Then the mixture was cooled to room temperature at a rate of 5 K / min. Subsequently, the residual content of PA12 was determined.

[0091] Table 2

[0092]

[0093] It is clear from the test results shown in Table 2 that the hydrolysis of PA12 with sulfuric acid is unexpectedly effective in the temperature range from 125 °C to 190 °C (I3 to I5; I6 to I8).

[0094] In contrast, the hydrolysis at a temperature of 80 °C carried out in the prior art (US 4,170,588 A) (comparative tests C11, C13) is not beneficial. Comparative tests C12 and C14 additionally demonstrate that at a temperature of 230 °C, the residual content of PA12 increases again; thus, the conversion efficiency decreases again.

Claims

1. A method for hydrolyzing polylaurolactam, wherein the polylaurolactam is cleaved with sulfuric acid at a temperature of 125 °C to 190 °C to obtain ω-aminolauric acid, wherein the sulfuric acid is an aqueous solution, and the content of sulfuric acid in the aqueous solution is in the range of 10% to 90% by weight, wherein polylaurolactam with a laurolactam content of < 4.9% by weight and / or an ω-aminolauric acid content of < 4.9% by weight is used, and wherein sulfuric acid is used in an amount such that the weight ratio of the sulfuric acid used to the polylaurolactam used is in the range of 1:0.1 to 1:

1.

2. The method according to claim 1, wherein the polylaurolactam has a molar mass of from 1000 to 10 6 g / mol.

3. The method according to any one of claims 1 to 2, wherein the pressure is from 1 bar to 100 bar.

4. The method according to claim 1, wherein, after 1 - 99% by weight of the polylaurolactam used has reacted, alkali metal hydroxide is added in a subsequent step to neutralize the sulfuric acid, and then further alkali metal hydroxide is added to react with the remaining polylaurolactam.

5. The method according to claim 4, wherein, the alkali metal hydroxide is used in the subsequent step in an amount such that after neutralization of the sulfuric acid, the alkali metal hydroxide remains in an amount such that the weight ratio of the unreacted polylaurolactam to the remaining alkali metal hydroxide is in the range of 1:0.1 to 1:

1.

6. The method according to claim 4 or 5, wherein the temperature in the subsequent step is in the range of 160 °C to 280 °C.

7. The method according to any one of claims 4 to 5, wherein the pressure in the subsequent step is in the range of 1 bar to 100 bar.

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