IE940384A1 - Process for the preparation of 2-cyanoacrylic acid - Google Patents

Abstract

Process for the preparation of 2-cyanoacrylic acid from poly(alkyl-2-cyanoacrylates), comprises the steps of: (a) subjecting oligomeric cyanoacrylate material to catalysed pyrolysis in a reactor under polymerisation inhibiting conditions at a temperature greater than 200°C and at a reduced pressure sufficient to vaporise the 2-cyanoacrylic acid formed; (b) cooling the vapours which emerge from the reactor on completion of the pyrolysis; and (c) collecting the 2-cyanoacrylic acid formed. The oligomeric cyanoacrylate material can be selected from crude oligomer, purified oligomer, oligomer produced as a by-product in the conversion of crude oligomer to purified oligomer, oligomer produced as a by-product during the depolymerisation of purified oligomer, oligomer producd during the conversion of crude monomer to purified monomer and polymer produced during storage of purified monomer.

Description

TITLE: Process for the preparation of 2-cyanoacrylic acid ABSTRACT : Process for the preparation of 2-cyanoacrylic acid from poly(alkyl-2-cyanoacrylates), comprises the steps of: (a) subjecting oligomeric cyanoacrylate material to catalysed pyrolysis in a reactor under polymerisation inhibiting conditioris at a temperature greater than 200°C and at a reduced pressure sufficient to vaporise the 2-cyanoacrylic acid formed; (b) cooling the vapours which emerge from the reactor on completion of the pyrolysis; and (c) collecting the 2-cyanoacrylic acid formed. The oligomeric cyanoacrylate material can be selected from crude oligomer, purified oligomer, oligomer produced as a by-product in the conversion of crude oligomer to purified oligomer, oligomer produced as a by-product during the depolymerisation of purified oligomer, oligomer producd during the conversion of crude monomer to purified monomer and polymer produced during storage of purified monomer. ,940384 APPLICATION No Process for the preparation of 2-cvanoacrvIic acid This invention relates to a process for the preparation of 2cyanoacrylic acid, more particularly, a process for the preparation of 2-cyanoacryIic acid from alkyl-2-cyanoacrylale oligomers produced as intermediate products of alkyl-2-cyanoacrylale production and poly(alkyl-2-cyanoacry!ate)-containing products thereof which are normally produced as an unrecoverable by-product in the production of alkyl-2-cyanoacrylale monomers. 2-Cyanoacrylic acid having the formula CN / h2c= c COOH can potentially be used for the preparation of a wide range of cyanoacrylate monomers. The ability of 2-cyanoacrylates to polymerise rapidly under the influence of moisture or nucleophilic substances has led to their exploitation as instantaneous adhesives. However, the inherent ability of 2-cyanoacrylates to undergo rapid anionic polymerisation gives rise to complications as regards the synthesis of free 2-cyanoacrylic acid. Accordingly, whereas esters of 2-cyanoacrylic acid are known and well characterised since 1940, the first reported synthesis of free 2-cyanoacrylic acid was described in 1984 by Henkel KGaA (DE 34 15 181 Al). The only known method of obtaining 2-cyanoacrylic acid is described in DE 34 15 181 Al.

The method described in DE 34 15 181 Al for the preparation of 2-cyanoacrylic acid involves pyrolysis of alkyl esters of 2cyanoacrylic acid or Diels-Alder adducts of such esters. The highest reported yield was 13.4% of a crude product having a melting point of 80°-81°C. Purified 2-cyanoacrylic acid has a melting point of 92°93°C. 940184 Apart from the low yield of 2-cyanoacrylic acid resulting from the method of DE 34 15 181 Al, a further limitation is that the starting material consists of purified alkyl esters of 2-cyanoacrylic acid which are expensive. Accordingly, the method of DE 34 15 181 Al is not a practical means of obtaining new alkyl esters of 2-cyanoacrylic acid, starting from the free acid.

At present the main commercial route for the preparation of cyanoacrylate esters is the Knoevenagel Condensation Method (H. Lee. (Ed.) (1981) Cyanoacrylic Resins - The Instant Adhesives, Pasadena Technology Press, Pasadena, U.S.A.). This method is used commercially for the synthesis of low molecular weight esters. The Knoevenagel Method is a two stage method. In the first stage a cyanoacetate ester and formaldehyde are reacted together in the presence of an amine to give relatively crude oligomers of polyalkylcyanoacrylates. After appropriate purification, the oligomer, in a second stage is thermally depolymerised to yield monomeric ester in accordance with the following reaction scheme: CN / catalyst CH2O + CH \ COOR . oh--ch2 / OH-4-CH2 —C CN \ COOR CN / \ COOR oh + h2o CN / ch2=ch \ COOR In the above scheme R is an alkyl, cycloalkyl or alkenyl moiety of up to ten carbon atoms. The catalyst is normally a secondary amine, a basic salt or a combination thereof. 940364 When the Knocvenagel Condensation Method is applied on an industrial scale four main types of oligo(alkyl-2-cyanoacrylate)containing products result as follows: 1. Oligomer formed at the condensation stage; 2. Purified oligomer formed following purification of crude oligomer which is used as a starting material for the depolymerisation reaction; 3. Oligomer containing by-product from the purification step; and 4. Oligomer containing residue formed during the depolymerisation step, which residue is made up of oligomer which cannot be depolymerised under the conditions used to form esters of 2-cyanoacrylic acid.

Industrial processes for the production of cyanoacrylate adhesives or glues, hereinafter referred to as adhesives, involve the purification of crude cyanoacrylates by vacuum distillation, followed by depolymerisation. Following distillation a residue is obtained as a by-product which contains oligomer of relatively high molecular weight which is normally discarded as a waste product, which must be disposed of.

Furthermore, the shelf life of cyanoacrylate adhesives is dependent on storage conditions. For example, it is usual following the distillation step to add a free radical stabiliser such as hydroquinone to inhibit free radical polymerisation during storage. Free radical polymerisation can be initiated, for example, by exposure to light. Accordingly, under certain conditions, such as when insufficient stabiliser is used, these adhesives may undergo spontaneous polymerisation with the formation of highly viscous liquid or solid products which cannot be used as an adhesive or as a component of an adhesive. The polymer products formed on storage are solutions of high molecular weight poly(alkyl-2-cyanoacrylates) in the monomeric 040384 alkyl-2-cyanoacrylates or, sometimes they are completely polymerised solid poly(alkyl-2-cyanoacrylates). Some monomer can be recovered by distillation of viscous liquid products of the type hereinbefore referred to. However, residual high molecular weight solid polymers cannot be converted into monomer even by thermal depolymerisation and must be disposed of. The oligomers which arise as by-product at various stages during the production of adhesives cannot generally be converted into monomer by depolymerisation.

The following scheme indicates various stages at which 10 oligomers are formed which cannot be further processed according to conventional methods to form adhesives or be converted into adhesives.

Condensation (1) Crude oligomer (2) Purified oligomer Purified monomer by-product (high molecular (weight polymer) (7) (6) Polymer _ (in monomer solvent) Storage polymerisation by-product (3) Purified ^‘rificaiion Crude monomer ) monomer by-product (5) Products (3), (4), (5) and (7) cannot presently be processed to commercially useful materials, more specifically material which can be used directly as an adhesive or as a component of an adhesive. The cost of disposing of such oligomers is high and also there are environmental problems with the disposal of such oligomers. In the case of products (1), (2) and (6) a number of steps are required to obtain a purified monomer which can be used as an adhesive or as a component of an adhesive. 4 0 384 The present invention provides a process for the preparation of high purity 2-cyanoacrylic acid in high yield from polymeric cyanoacrylate materials of the type hereinabove defined.

Accordingly, the invention provides a process for the preparation of 2-cyanoacrylic acid from poly(alkyl-2-cyanoacrylates), which process comprises the steps of: a) subjecting oligomeric cyanoacrylate material to catalysed pyrolysis in a reactor under polymerisation inhibiting conditions at a temperature greater than 200°C and at a reduced pressure sufficient to vaporise the 2-cyanoacrylic acid formed; b) cooling the vapours which emerge from the reactor on completion of the pyrolysis; and c) collecting the 2-cyanoacrylic acid formed.

The oligomeric cyanoacrylate material used as a starting material in the process according to the invention is any type of poly(alkyl-2cyanoacrylate)-containing material.

The term oligomer as used herein is any poly(alkyl-2cyanoacrylate) synthesised by any method, including the Knoevenagel Condensation Method using different catalysts and formaldehyde to alkylcyanoacetate ratios. Typically the oligomers will be those which are formed by the Knoevenagel Condensation Method. However, byproducts of commercial 2-cyanoacrylate monomer production and purification and also oligomers formed by the polymerisation of esters of 2-cyanoacrylic acid can be used as starting material in the process according to the invention.

Thus, the process according to the invention can be used to obtain 2-cyanoacrylic acid from any of the oligomeric materials hereinbefore defined, including the products identified as (1)-(7) in the scheme hereinabove set out. Thus, high molecular weight oligomers which cannot be used to prepare esters of 2-cyanoacrylic acid by depolymerisation can be used as starting material in the process according to the invention. Starting material in accordance with the invention includes also polymer-monomer solutions of the type hereinabove defined and residual polymers obtained in known commercial processes.

Preferably, the oligomeric cyanoacrylate material is fed to the reactor in a carrier gas.

However, the process according to the invention can be carried out without a carrier gas as hereinafter described.

Pyrolysis in accordance with the invention can be carried out over a relatively broad temperature range, typically between 200° and 700°C. However, preferably the pyrolysis is carried out at a temperature in the range 300°-600°C.

Polymerisation inhibiting conditions in the process according to the invention are conditions wherein one or both of an anionic polymerisation inhibitor and a free radical polymerisation inhibitor is added to one or both of the oligomeric cyanoacrylate material and the carrier gas, when a carrier gas is used.

Suitable anionic polymerisation inhibitors are those known in the ait and include sulfur dioxide, carbon dioxide and nitrous oxide.

Suitable free radical polymerisation inhibitors are those known in the art and include hydroquinone and methylhydroquinone.

The reduced pressure which is used during pyrolysis according to the invention is a pressure sufficient to vaporise the 2-cyanoacrylic acid formed and maintain it in the vapour phase. The pressure, temperature and gas flow rate through the reactor are selected so as to provide a sufficient contact time in the reactor of the material being subjected to pyrolysis to achieve complete pyrolysis of vapours formed.

Preferably the pressure is in the range 0.05-100 mm Hg.

The type of reactor used to carry out the process according to the invention is typically a tube-type reactor or a standard pyrolysis reactor in which the pyrolysis is carried out in the environment of a pseudoliquid layer of a catalyst.

When a tube-type reactor is used, the reactor is preferably a quartz reactor. A tube-type reactor for use in accordance with the invention can be a spiral reactor. Such a reactor can be filled with a catalyst as hereinafter defined adsorbed or fixed to a stationary support.

However, the surface of the reactor can also serve as an integral catalyst. For example, quartz can serve as a catalyst in that the surface of the quartz tube reactor serves as a catalyst for pyrolysis. Likewise quartz rings and quartz sand act as catalysts for pyrolysis.

Furthermore, by providing the inner surface of the quartz tube with indentations or other surface configurations which increase the surface area of the quartz tube also increases the catalysing surface. Furthermore, such configured surfaces improve heat transfer and heat exchange.

The catalyst, when such is used, is suitably a metal oxide, more particularly a transition metal oxide. Preferably, the catalyst is bonded to a solid support. For example, the solid support can be in the form of rings in the case of a tube-type reactor and solid quartz sand where pyrolysis is carried out in the environment or conditions of a pseudoliquid layer of a catalyst.

The carrier gas, when such is used, is suitably an inert gas or an acidic gas.

A carrier gas must be used when the pyrolysis reactor is a pseudoliquid layer reactor. However, in the case of a tube-type pyrolysis reactor the use of a carrier gas is optional.

Suitable inert gases include argon and nitrogen.

Suitable acidic gases include sulfur dioxide, carbon dioxide, carbon monoxide and nitrous oxide. Acidic gases are especially useful as carrier gases in accordance with the invention because they also serve as stabilisers which prevent anionic polymerisation of cyanoacrylic acid. The most preferred acidic carrier gas is sulfur dioxide.

The How rate of carrier gas depends on the pressure and the inner diameter of the reactor and is suitably at a rate of 0-100 ml/min for a pyrolysis reactor of the tube-type.

Tor a reactor wherein pyrolysis is carried out in conditions using a pseudoliquid layer, the gas flow rate required to provide a suitable pseudoliquid layer is suitably in the range 0.1-2 1/min.

The carrier gas, when such is used, is preferably heated to the pyrolysis temperature before being injected into the pyrolysis zone.

The oligomeric cyanoacrylate material is preferably injected into the pyrolysis reactor in the form of a melt, solid or solution, including a solution of oligomer in an ester of 2-cyanoacrylic acid as a solvent.

The emergerent vapours can be cooled using a two step cooling process. In a first step, the emergent vapours are cooled by passing through a condenser cooled by cold water and the bulk of the 2cyanoacrylic acid is collected, in a second step the cooled vapours are passed through a condenser cooled by dry ice or by liquid nitrogen and an olefin as a by-product containing the remainder of the 2cyanoacrylic acid is collected. 0 3 8 4 Oligomeric material for use in the process according to the invention can also be specially prepared from a cyanoacrylate monomer for the purposes of demonstrating the process according to the invention under experimental conditions as set forth in Example 1.

The invention will be further illustrated by the following Examples.

Example 1 Preparation of 2-cyanoacrvlic acid from poly(ethyl-2-cvanoacrylate) obtained by anionic polymerisation of ethyl-2-cyanoacrylate Preparation of poly(ethyl-2-cyanoacrvlate) A 500 ml flask arranged for distillation fitted with a mechanical stirrer was charged with a solution containing 100 ml of ethyl-2cyanoacrylate in 95 ml of acetone. 5 ml of water was added with stirring and the mixture was heated with reflux and stirring for 3 hours. The solvent was distilled off to give a yellow transparent solid residue. The residue was dried in vacuo at 100°C under 0.5 mm Hg to drive off monomeric ethyl-2-cyanoacrylate and acetone to give a transparent solid oligo ethyl-2-cyanoacrylate NMR ppm (CD3)2CO: 1.31t CH3; 1.39t CH3;2.71 m CH2; 2.82 m CH2; 4.28 m CH20; 4.31 m CH2O; 4.36 m CH2O; Calculated for C6H7NO2: C 57.6, H 5.6, N 11.2; found: C 57.10 H 5.92 N 10.97.

Preparation of starting composition A 150 ml flask fitted with a mechanical stirrer was filled with lOg of poly(ethyl-2-cyanoacrylate) and 0.2 g of phosphorus pentoxide was added to the melt of polymer with stirring at 80°C. The mixture was stirred at 80°C for 30 min. 1.8 ml of ethyl-2-cyanoacrylate containing 5 mg of hydroquinone was added with stirring and the mixture was cooled to 20°C to give a viscous liquid starting composition. ίο 040384 Pyrolysis of poly(ethvl-2-cyanoacrylate) The dosing funnel of a tube-type quartz pyrolysis reactor fitted with a capillary tube for sparging SO2 and heated to 100°C was filled with 4.63 g (5 ml) of poly(ethyl-2-cyanoacrylate) starting composition. The composition was added dropwise at a rate of 3-4 droplets per min. into the pyrolysis tube heated to 590°- 600°C, with constant sparging of SO2, at a flow rate of 0.5 ml/min., and at a pressure of 0.5-1 mm Hg. Emerging vapours were cooled to give 2.95 (77.3%) of solid crude 2cyanoacrylic acid. The solid was recrystallised from toluene to give 1.55 (43%) of 2-cyanoacrylic acid m.p. = 93-94°C.

Example 2 Preparation of 2-cyanoacrylic acid from high molecular weight polv(ethyl-2-cyanoacrylate) formed during long-term shelf storage of industrial glue Isolation of polymer and preparation of starting composition 100 g of Cyacrin (Cyacrin is a Trade Mark) industrial glue based on ethyI-2-cyanoacrylate monomer stabilized with hydroquinone which had a high viscosity after two years of shelf storage due to partial polymerisation was distilled in vacuo in the presence of phosphorus pentoxide with sparging of SO2 to give 25 g of a solid transparent glass-like residue. The residue was melted and oligomer was separated from residual phosphorus pentoxide.

Pyrolysis of high molecular weight polyfethyl-2-cyanoacrylate) The dosing funnel of a tube-tyre quartz pyrolysis reactor heated to 100°C was filled with 10 g of solid polymer. The polymer was melted and 100 mg of phosphorus pentoxide was added to the melt.

The reactor, which was provided with indentations, was heated in vacuo under 1 mm Hg to 600°C and sparged with SO2 for 30 min. The sparging was stopped and the melt of polymer was added dropwise into II the heated zone of the reactor at a speed of ten droplets per min. 6.2 g of solid was collected in the cooled zone of the reactor. The solid was recrystallised from toluene to give 3.82 g (47%) of pure 2cyanoacrylic acid m.p. 93-94°C.

Claims (23)

1. Claims: 1. A process for the preparation of 2-cyanoacrylic acid from poly(alkyl-2-cyanoacrylates), which process comprises the steps of: a) subjecting oligomeric cyanoacrylate material to catalysed pyrolysis in a reactor under polymerisation inhibiting conditions at a temperature greater than 200°C and at a reduced pressure sufficient to vaporise the 2-cyanoacrylic acid formed; b) cooling the vapours which emerge from the reactor on completion of the pyrolysis; and c) collecting the 2-cyanoacrylic acid formed.

2. A process according to Claim I, wherein the oligomeric cyanoacrylate material is fed to the reactor in a carrier gas.

3. A process according to Claim 1 or 2, wherein the pyrolysis is carried out at a temperature in the range 200°-700°C.

4. A process according to any preceding claim, wherein the pyrolysis is carried out at a temperature in the range 300°-600°C.

5. A process according to any one of Claims 2-4, wherein the polymerisation inhibiting conditions are conditions wherein one or both of an anionic polymerisation inhibitor and a free radical polymerisation inhibitor is added to one or both of the oligomeric cyanoacrylate material and the carrier gas.

6. A process according to any preceding claim, which is carried out at a pressure in the range 0.05 to 100 mm Hg.

7. A process according to any preceding claim, wherein the reactor is a lube-type reactor.

8. A process according to Claim 7, wherein the surface of the reactor serves as an integral catalyst.

9. A process according to Claim 7 or 8, wherein the tube of the reactor is made of quartz.

10. A process according to any one of Claims 1-6, wherein the reactor is a reactor wherein the pyrolysis is carried out in the environment of a pseudoliquid layer of a catalyst.

11. A process according to any one of Claims 1-7, 9 when dependent on Claim 7 and Claim 10, wherein the catalyst is a metal oxide.

12. A process according to Claim 11, wherein the catalyst is a transition metal oxide.

13. A process according to Claim 11 or 12, wherein the catalyst is bonded to a solid support.

14. A process according to any one of Claims 2-13, wherein the carrier gas is an inert gas.

15. A process according to any one of Claims 2-13, wherein the carrier gas is an acidic gas.

16. A process according to Claim 15, wherein the acidic gas is selected from sulfur dioxide, carbon dioxide, carbon monoxide and nitrous oxide.

17. A process according to Claim 16, wherein the acidic gas is sulfur dioxide.

18. A process according to any preceding claim, wherein the oligomeric cyanoacrylate material is in the form of a melt, solid or solution. 940364

19. A process according to Claim 18, wherein the oligomeric cyanoacrylate material is in the form of a solution with a cyanoacrylate as solvent.

20. A process according to any preceding claim, wherein the oligomeric cyanoacrylate material is a product of a Knoevenagel Condensation.

21. A process according to any one of Claims 1-20, wherein the oligomeric cyanoacrylate material is selected from crude oligomer, purified oligomer, oligomer produced as a by-product in the conversion of crude oligomer to purified oligomer, oligomer produced as a by-product during the depolymerisation of purified oligomer, oligomer produced during the conversion of crude monomer to purified monomer and polymer produced during storage of purified monomer.

22. A process according to any preceding claim, wherein the emergent vapours are cooled by passing through a condenser cooled by cold water.

23. A process according to Claim 22, wherein the cooled vapours are passed through a condenser cooled by dry ice or by liquid nitrogen and the 2-cyanoacrylic acid collected.