MXPA01006818A - Use of heterogeneous catalysts in methods for the production of polyamides - Google Patents

Abstract

The invention relates to the use of metal oxides as heterogeneous catalysts in a method for the production of polyamides by polymerization of lactams and possibly other polyamide-forming monomers. According to said method the metal oxides are used in a form which permits their mechanical separation from the reaction mixture and during or at the end of polymerization are separated from the reaction mixture so as to reduce the content of extractable compounds in the polyamide obtained in this way.

Description

USE OF HETEROGENEOUS CATALYSTS IN PROCEDURES FOR THE PREPARATION OF POLYAMIDES.

The invention relates to the use of metal oxides as heterogeneous catalysts in a production process of polyamides by the polymerization of lactams. The polyamides based on e-caprolactam are used in the production of fibers, films and castings. However, the hydrolytic polymerization of e-caprolactane, one of the most important processes for the production of polyamide 6, only takes place incompletely. The crude polymer produced from caprolactam consequently comprises a high content of e-caprolactam and low molecular weight reaction products (oligomers). Since both the oligomer and the caprolactam are soluble and extractable in water, the content of low molecular weight constituents in the polymer is also known as the extractable content. • To avoid any deterioration in product quality and processing properties, for example, during injection or extrusion molding or during spinning, the content of extractables should be decreased. The so-called necessary extraction is usually carried out with water at elevated temperatures, as described in DE-A 2 501 348, for example.

To increase the polymerization yield and avoid any adverse impact on the environment, the aqueous extracts are not disposed of as waste, but are recycled. For this purpose, for example, they are concentrated or extracted in order to recycle the caprolactam obtained during the polymerization. The low solubility of o Ligó me o and especially of the dimer still contained in the granulate is not the only reason why complex and costly processes must be used from the energy point of view for the extraction, so that they can obtain polyamides of satisfactory quality. In connection with this, processes were found. in which caprolactam is used as a solubilizer for the oligomers of caprolactam in the extraction of polyamide 6. The document DE-A-43 24 616 accordingly proposes the incorporation of caprolactam monomer into the water of extraction at the beginning of the extraction. Processes are also known in which e! The content of extractables is decreased by vaporizing the monomer and the oligomer of the polymer. DE-A-29 AQ 865 discloses a process of demonomerization in which the polymer is subjected to a vacuum in the molten state and in the form of thin films. All the aforesaid processes have the disadvantage of requiring in some cases expensive and intense steps from the energy point of view, of multiple stages, for the purposes of demonomerizing the polymer and making the aqueous extract. Consequently, it is convenient to produce polyamides that have a reduced content of extractables and especially of dimers even without being subjected to an extraction process, so that the technical and economic effort necessary to extract the low molecular weight constituents and elaborate the extract watery can be reduced. It is an object of the present invention to provide a process for producing polyamide in which the formation of higher dimer and oligomer and the presence of monomer in the polymer are restricted and the polyamides thus obtained have a low extractable content even before being subjected to an extraction process.

We have discovered that this object is achieved according to the invention by the use of metal oxides as heterogeneous catalysts in a process of preparation of polyamides by the polymerization of lactams and optionally other monomers forming polyamides in which the metal oxides are used in a form that allows the mechanical removal of the reaction mixture being removed from it during or after the polymerization, in order to reduce the extractable content of the obtained polyamide. A similar process is described in document DE-A-198 08 190, which has a priority date prior to the present invention but which was not published at that time. To produce polyamides with low extractables content in the process of the invention, e-caprolactam and optionally other polyamide-forming monomers are reacted, preferably continuously, in the presence of metal oxides which do not remain in the reaction mixture.

Surprisingly, the process provides a marked reduction in the level of dimer, whose extraction is particularly difficult given its water solubility behavior. The advantage of the present invention is that the total amount of low molecular weight constituents and especially of dimers that must be removed from the polymer in the subsequent extraction operation is reduced. This not only reduces the residence time in the extractor and thus increases the yield per time-spade, but also reduces the amount of extract to be processed or discarded. More specifically, the production of dimers as well as trimers and tetramers, which are more critical in the removal and reuse than monomers, can be reduced. On the other hand it is possible to reduce the reaction times. According to the invention, an aqueous lactam solution optionally also contains other film-forming monomers and has a water content ranging from 0.05 to 20% by weight, preferably from 1 to 10% by weight, moreover from 2 to 10% by weight. and 6% by weight, is intermittently reacted and preferably continuous in the melt phase at a temperature ranging between 180 and 290 degrees C. If desired, before proceeding to the subsequent reaction steps, the monomers are fused and mixed with water in a shaping tank. The metal oxide used can be any of the known ones, such as drconium oxide, aluminum oxide, magnesium oxide, cerium oxide, lanthanum oxide and preferably titanium oxides and also beta-zeouettes and laminated silicates. It has been discovered that even silica gel and metal oxides stimulants, examples of these materials being ruthenium, copper or fluoride, can significantly reduce the content of extractables. The use of metal oxides that are Bronsted acids is preferred. Particularly preferred is titanium dioxide in anatase form. Preferably 70% by weight of the titanium dioxide, more still at least 90% by weight, and moreover all of it is "anatase" According to the invention, the bodies of the metal oxides have a shape and size which allow the mechanical separation of the melt polymer from the catalyst, for example by means of sieves or filters The invention proposes the use of the catalyst in the form of extrusion flakes or as a coating applied on garnishes According to another embodiment, the lactam it is subjected to an reaction with homogeneously dissolved acid cocatalysts or a mixture of different catalytically active components in the presence of the heterogeneous catalysts previously mendonated.The cocatalysts used in this case are preferably acid catalysts selected from organic mono- and dicarboxylic acids, inorganic acids. , its salts and mixtures thereof, for example the acid mendonates carboxylic acids, terephthalic acid, adipic acid, propionic acid and isophthalic acid, or phosphorous compounds containing oxygen, in particular phosphoric acid, phosphorous acid, hypophosphorous acid, alkali metal salts and alkaline earth metals and ammonium salts, or sulfur compounds containing oxygen, especially sulfuric acid and sulfurous acid. One-stage process If the mixture of initiator materials is used in a one-stage reaction, this stage preferably contains the fixed or tilting bed of metal oxide bodies within the limits of the reactor where the mixture is present as a single liquid phase . In order to achieve a higher molecular weight, the reactor must allow the elimination of water and an adequate residence time. Preferably, the polymerization in the single-stage embodiment is for that reason carried out in a downward vertical flow tube, a tube VK, which has a gas phase in the upper part. In this embodiment, the melting phase flows through the metal oxide bodies. Reaction temperatures are set within a range between 180 and 270 degrees C, preferably between 190 and 250 degrees C, more still between 200 and 230 degrees C. In a particularly preferred embodiment, the VK tube has a continuous or discontinuous thermal gradient whereby the temperature preferably rises from 180-250 degrees C in the upper part of the tube at 240-280 degrees C above the outlet in the lower portion thereof. Two-stage process Preferably the polymerization is carried out in at least two reaction steps. The first one is developed under high pressure while in the second the gas phase containing condensation products such as water is removed. The metal oxide bodies can be used in all stages according to the invention, although their application in the first stage is particularly preferred. Step 1 The pressure in the first stage of the process is preferably set so that the reaction mixture is found as a single liquid phase. The reaction mixture has a volumetric temperature which generally ranges between 180 and 290 degrees C, preferably between 190 and 250 degrees C, and even more between 200 and 230 degrees C. The residence time in general ranges between 10 minutes and 10 hours , preferably between 0.5 and 8 hours, and even more between 1 and 4 hours. Stage 2"The pressure in the second stage is selected so that it has a liquid phase, which contains the polymer, and a gas phase.The pressure preferably ranges between 0.01 and 10 x 105 Pa. The volumetric temperature of the mixture Reaction in general ranges between 220 and 290 degrees C, preferably between 230 and 270 degrees C, and even more between 240 and 260 degrees C when a catalyst is used.

The removal of the gas phase can be efeduated by the use of separation tanks or tank batteries with or without agitation and also through the use of evaporator devices, for example circulatory evaporators or evaporators of thin films, for example "filmtruders" or ring disk readores, that guarantee a greater interface between the phases. The reservoir pumping of the reaction mixture or the use of a closed-loop reaction can be necessary in order to extend the interface between the phases. In addition, the removal of the gas can be favored by the addition of water vapor or inert gas in the liquid phase. If it is convenient to use a heterogeneous catalyst in the second stage, it is preferably present as a tilting bed - in which case dynamic mixing elements must not be installed - or as a coating for the internal components of the reactor. If the degree of polymerization is sufficient, the obtained polymer melt can be discharged from the second stage, for example with the aid of a gear pump, cooled in a water bath and then further processed by methods known to those skilled in the art. art, for example, by extrusion, extraction, pigmentation, tempering, etc. - The residence time in the reaction stage 2 depends on the content of the mixture, the reaction temperature and the pressure of the reactor and also the degree of polymerization desired for the product at the end of the reaction period. In general it ranges between 0.1 and 15 hours, preferably between 0.5 and 8 hours, and even more between 1 and 4 hours. The volumetric temperature of the reaction mixture in general ranges between 230 and 300 degrees C, preferably between 240 and 280 degrees C, and even more between 250 and 270 degrees C, without the use of a catalyst. Three-stage process The implementation of three stages according to the invention, as well as the steps described in the two-stage process, comprises an adidonal rotor, which is preferably operated without a tilting bed of metal oxides.

According to the properties desired for the product, the reaction mixture in this stage is present as a single liquid phase or as a gaseous / liquid two-phase system: If the molecular weight of the polymer melt that is discharged from the Second stage is suffciently elevated, it will be possible to use, for example, a flow tube in which the reaction mixture is present as a single liquid phase and where static mixers are used to add chain regulators, fillers, stabilizers or other additives and pigments. However, if the molecular weight should be increased, it is convenient to use reactors or devices that allow the removal of water (see previous discussion). This embodiment is preferred when the second stage is also equipped with metal oxide bodies. In this case, the invention avoids an excessive generation of viscosity in the second stage in such a way as to guarantee an effective separation of the reaction mixture from the metal oxide bodies. The residence time in the reaction stage 3 also depends on the water content of the mixture, the reaction temperature and the reactor pressure, as well as the degree of polymerization desired for the product after the reaction lapse. In general it ranges between 0 and 15 hours, preferably between 0.5 and 8 hours, even more between 1 and 4 hours. The pressure is selected such that there is a liquid phase, containing the polymer, and a gas phase, and preferably ranges from 0.01 to 3 x 105 Pa. In the context of the invention process it is also possible to carry out an elongation chain or a branch or a combination of both. To such an effect, known substances are added to branch or elongate the polymer chains to the reaction mixture. Substances can be incorporated into both the initiator mixture as to the reaction mixture that is post-condensed. The usable substances are: trifunctional amines or carboxylic acids as branching agents or crosslinkers. Examples of suitable at least trifundonal amines or carboxylic acids are those described in EP-A-G 345648. At least three-functional amines possess at least three amino groups capable of reacting with carboxylic acid groups. Preferably they do not possess carboxylic acid groups. The at least trifundonal carboxylic acids have at least three carboxylic acid groups capable of reacting with the amines and which are also present, for example, in the form of its derivatives, such as esters. The carboxylic acids preferably do not contain amino groups capable of reacting with the carboxylic acid groups. Examples of suitable carboxylic acids are trimesic acid, trimerized fatty acids, which can be prepared, for example, from oleic acid and which can have between 50 and 60 carbon atoms, and naphthalene polycarboxylic acids, such as naphthalene-1 acid, 3,5,7-tetracathyl) oxyl. The carboxylic acids are preferably defined organic acids and are not polymeric compounds.

Examples of amines possessing at least three amino groups are nitriloalkylamine, in particular nitrilotrietanoamine, dialkylenetriamines, especially diethylenetriamine, trialkylene tetramines and tetralakylenepentamines, and alkylene derivatives, preferably ethylene. On the other hand, it is possible to use dendrimers as amines. The dendrimers preferably have the general formula I. (R2N (CH2) n) 2-N (CH2) xN ((CH2) n-NR2) 2 (I) in which R is H or - (CH2) n-NR12, where R1 is H or - < CH2) n-NR22, where R2 is H or - (CH2) n-NR32, where R3 is H on is an integer ranging from 2 to 6 and x is an integer ranging from 2 to 14. Preferably, n is 3 or 4, particularly 3, and x is an integer ranging between 2 and 6, preferably between 2 and 4, particularly 2. The radicals R can also have the meanings established independently from each other. It is convenient that R is a hydrogen atom or a radical -. { CH2) rrNH2. Suitable carboxylic acids are those having between 3 and 10 carboxylic acid groups, preferably between 3 and 4. Preferred carboxylic acids are those possessing aromatic and / or heterocyclic nuclei. By way of example, the following may be mentioned: benzyl, naphthyl, anthracene, biphenyl, triphenyl radicals or heterocycles such as pyridine, bipyridine, pyrrole, indole, furan, thiophene, purine, quinoline, phenanthrene, porphyrin, phthalocyanine, naphthalocyanine. The following are the preferred compounds: 3,5,3 ', 5'-biphenyltetracarboxylic acid-phthaiocyanin, naphthalocyanine, 3,5,5', 5'-biphenyltetracarboxylic acid, 1,3,5,7-naphthalenetetracarboxylic acid, 2 , 4,6-pyridinetricarboxylic acid, 3,5,3 ', 5'-bipyridyltetracarboxylic acid, 3,5,3', 5'- benzophenonetracarboxylic acid, 1,3,6,8-acridinatetracarboxylic acid, with particular preference being given to the acid 1 , 3,5-benzenetricarboxylic acid (trimesic acid) and 1,2,4,5-benzenetetracarboxylic acid. Such compounds are commercially available or can be prepared by means of the process described in DE-A-43 12 182. If ortho-substituted aromatic compounds are used, the imide form is preferably avoided by the addition of appropriate reaction temperatures. These substances are at least trifunctional, preferably at least tetrafunctional. The number of fundonal groups can vary between 3 and 16, preferably between 4 and 10, more still between 4 and 8. The processes of the invention are conduddos using either at least trifundonal amines or at least trifunctional carboxylic acids, but mixtures of such amines or acids are not used. However, small amounts of at least trifunctional amines may be present in the trifundonal carboxylic acids and vice versa. The substances are present in an amount ranging between 1 and 50 micromol / g of polyamide, preferably between 1 and 35, more still between 1 and 20 micromol / g of polyamide. The substances are preferably present in an amount ranging from 3 to 150, more still from 5 to 500, and even more from 10 to 70 m? Chromols of equ? Valent / g of polyamide. The equivalents are based on the number of functional amino groups or carboxylic acid groups. The difunctional carboxylic acids or difunctional amines are used as chain extenders. These possess two carboxylic acid groups which can react with amino groups, or two amine groups which can be subjected to a reaction with carboxylic acids. The carboxylic acids or drfunctional amines, in addition to the carboxylic or amino acid groups, do not contain other functional groups capable of reacting with amino groups or carboxylic acid groups. Preferably, they do not contain adidonal functional groups. Suitable examples of difunctional amines are those which form salts with difunctional carboxylic acids. They can be aliphatic linear, for example C? _i4-a! Quilendiamine, preferably C2 ^ -alkylenediamine, for example hexylenediamine. They can also be cycloaliphatic. Examples are isophorone diamine and laromine. It is also possible to use branched aliphatic diamines, such as Vestamin TMD (trimethyl hexamethylene diamine, marketed by Hüls AG). The complete amines can be substituted by the C?-Β2 alkyl radicals, preferably C alquilo .14 alkyl, in the carbon skeleton. The difunctional carboxylic acids are, for example, those which form salts with difunctional diamines. They may be linear aliphatic dicarboxylic acids, preferably O 2 or dicarboxylic acids. By way of example, the following acids may be mentioned: adipic, azelaic, sebacic, suberic. They may also be aromatic, for example, isophthalic acid, terephthalic acid, naphthalenedicarboxylic acid, as well as dimerized fatty acids. Additives and fillers, aliphatic and aromatic chain regulators such as mono and bifunctional amines and acids and also thermal stabilizers, photostabilizers, and substances to improve the coloring properties of the polymer can be added to the reaction mixture. Substances that do not dissolve homogeneously in the reaction mixture, such as pigments and fillers, are preferably incorporated therein after the phases of the synthesis occurring in the presence of the fixed-bed catalyst. The pigments and dyes are generally present in amounts of up to 4% by weight, preferably between 0.5 and 3.5% by weight, and even more between 0.5 and 3% by weight. The pigments for coloring thermoplastics are well known, see for example R. Gachter and H. Muller, Taschenbuch der Kunststoffadditive, Cari Hanser Veriag, 1983, pages 494-510. The first preferred group of pigments a -It is mendonados are the white pigments such as zinc oxide, zinc sulphide, lead white (2 PbCO3 Pb (OH) 2), lithopone, antimony white and titanium dioxide. Of the two most common crystal polymorphs (rutile and anatase) of titanium dioxide, the use of the rutile form as a white pigment is preferred for the molding composites of the invention. 15 The black pigments that can be used according to the invention are black iron oxide (Fe3? 4), spinel black (Cu (Cr, Fe) 2 04, manganese black (mixture of manganese dioxide, dioxide silicone and iron oxide) cobalt black, and antimony black and in addition, preferably carbon black, usually used in the form of oven or gas black (see G.

Benzing, Pigmente für Anstrichmittel, Expert-Verlag (1988), page 78ff). It will be noted that inorganic color pigments such as chromium oxide green or pigments of organic colors such as azo pigments and phthalodanins can be used according to the invention in order to obtain certain shades. Usually these pigments are available in the market.

It may be beneficial to use the mentioned pigments or dyes in a mixture, such as, for example, carbon black with copper phthalocyanines, since this generally facilitates the dispersion of the color in the thermoplastic material. The oxidation retarders and thermal stabilizers which can be incorporated into the thermoplastic compositions of the invention include, for example, the metal halides of group I of the periodic table, for example sodium halides, potassium halides, lithium halides, optionally together with copper (I) halides, such as, for example, dorides, bromides or iodides. Halides, in copper spell, may also contain p-ligands rich in eledrons. Examples of such copper complexes are copper halide complexes with triphenylphosphine, for example. It is also possible to use zinc fluoride and zinc doride. Other possibilities are the spherically limited phenols, hydroquinones, substituted representatives of this group, secondary aromatic amines, optionally together with phosphorus-containing acids and salts thereof, and mixtures of these compounds, preferably at a concentration of up to 1% by weight , in relation to the weight of the mezda. Examples of UV stabilizers are various substituted resorcinols, salicylates, benzotriazoles and benzophenones, which are generally used in amounts of up to 2% by weight. Lubricating and demolding agents, which are generally included in the thermoplastic material in amounts of up to 1% by weight, are stearic acid, stearyl alcohol, alkyl stearates and N-alkylstyramides and also esters of pentaerythritol with long-chain fatty acids. It is also possible to use broth, zinc or aluminum salts of stearic acid and also dialkyl ketones, for example distearyl ketone. The polyamides of the invention, in particular nylon-6 and their copolymers, can be used to produce fibers or fabrics and materials for construction. Suitable processes are described in EP-A-0 462 476, for example. In a preferred embodiment, the level of low molecular weight constituents such as caprolactam, linear and cyclic oligomers in the polyamide produced according to the invention can be reduced by subjecting it to an extraction with an aqueous caprolamide solution, firstly, and then with water and / or an extraction with gas phase. Figure 1 schematically illustrates an apparatus for carrying the process, where CL means caprolame, 1 is a supply tank, 2 is a tubular reactor, 3 is a separator and P means polyamide. The following examples illustrate the invention. Examples The illustrative realizations were taken to practice using a two-stage apparatus (see Figure 1). It consisted of a supply tank (1), a tubular reactor (2), containing the catalyst bed, and a separator (3) for the subsequent condensation of the PA prepolymer. The reaction mixture was pumped by means of a HPLC piston pump (high performance liquid chromatography) through a heat exchanger in the tube or prereactor (1 liter without Ti02 bed) and then post-condensed in the separator ( 2 liters) at low pressures after which it was discharged. To ensure adequate exchange of the reactants with the catalyst surface, the pressure in the prereactor was selected such that the reaction mixture formed a single liquid phase. Reactions ended after a short period, ie 4 hours, and resulted in high conversions. The products were processed into flakes, extracted and analyzed in relation to their viscosity and extractable content. The parameters of the process and corresponding values of viscosity and extractable content are summarized in table 1. Analysis The relative viscosity (VR) of the extracted products, a measure of the increase in molecular weight and the degree of polymerization, is measured in a soludón a! 1% by weight in 96% sulfuric acid, at 25 degrees C using an Ubbelohde viscometer. The polyamide samples were extracted with boiling water under reflux for 32 or 16 hours. The dry residue obtained by evaporation of the aqueous extract whose weight corresponds to the total extractables content of the polymer was analyzed by HPLC. - 'The catalyst used was an extrusion product prepared from titanium dioxide marketed by Finnti, Type S150, with a diameter of 4mm and a length ranging between 5 and 20 mm. The titanium dioxide of the S150 catalyst was in anatase form. A water content of 2-2.5% by weight in the initiator mixture resulted in extrados with a very low content of dimers and a low content of trimers and tetramers.

Table 1 CA Water content TR Residence time T Temperature P EC Pressure Removable content Monomer D Dimer Tri Trimer Te Tetramer Pe Pentamer H Hexamer He Hnmer Weight percentages are based on the mass of pre-extraction of the product produced.

Claims (10)

1. A method for reducing the content of extractables of polyamides in a process for producing polyamides by the polymerization of lactams and optionally other polyamide-forming monomers, wherein the polymerization is carried out in the presence of metal oxides as heterogeneous catalysts, wherein the metal oxides are They are used in a form that allows the mechanical separation of the reaction mixture and are removed from the reaction mixture during or after the polymerization, and the polymerization is carried out in two stages, carrying out the first stage at elevated pressure in which the mixture of reaction, with the exception of the heterogeneous catalyst, forms a single liquid phase, and the last stage being a post-condensation under pressure within the range from 0.01 to 10 x 105 Pa, wherein the heterogeneous catalyst is present in either or both stages .

The method of claim 1, wherein the metal oxide catalyst is used in the form of pieces, granules, fixed beds or packaged elements coated with catalyst.

The method of claim 1 or 2, wherein the metal oxide catalysts are selected from the group consisting of zirconium oxide, aluminum oxide, magnesium oxide, cerium oxide, lanthanum oxide, titanium oxide, beta - Zeolites and lamellar silicates.

4. The method of claim 3, wherein the metal oxide catalyst used is titanium oxide which is at least 70% by weight present in anatase form.

The method of any one of claims 1 to 4, wherein the metal oxide catalysts are used in conjunction with acid co-catalysts dissolved in homogeneous form in the reaction mixture.

6. The method of claim 5, wherein the acid cocatalysts are selected from the group consisting of: organic mono- and dicarboxylic acids, inorganic acids, their salts or mixtures thereof.

The method of any of claims 1 to 6, wherein the polymerization of the lactams is carried out in the presence of organic monocarboxylic acids, dicarboxylic acids or mixtures thereof as chain regulators and / or co-catalysts.

The method of claim 1, wherein "the reaction is carried out at temperatures from 180 to 290 ° C when the heterogeneous catalyst is present, and from 230 to 300 ° C when the heterogeneous catalyst is absent.

9. The method of any of claims 1 to 8, wherein the reaction is carried out in the presence of less than 10% by weight of water, based on the total reaction mixture.

10. Metal oxides for use as heterogeneous catalysts in a process for producing polyamides by polymerization of lactams and optionally other polyamide-forming monomers, wherein the metal oxides are used in a form that allows mechanical separation of the reaction mixture and are removed of the reaction mixture during or after the polymerization, and the polymerization is carried out in two stages, the first stage being carried out at elevated pressure in which the reaction mixture with the exception of the heterogeneous catalyst, forms a single liquid phase, and the last stage being a post-condensation under pressure within the range of 0.01 to 10 x 105 Pa, wherein the heterogeneous catalyst is present in either or both stages to reduce the extractable content of the obtained polyamide.