HK1027341B - Method for producing ester plasticizers - Google Patents

Description

Method for preparing ester plasticizer

The invention relates to a method for producing ester plasticizers from dicarboxylic or polycarboxylic acids or their anhydrides and alcohols by reacting starting materials in the presence of a metal catalyst.

Plasticizers are used in many ways in plastics, coating compositions, sealing compositions and rubber articles. They physically interact with the thermoplastic polymers but do not chemically react, preferably with the aid of their solvent and swelling capacity. This forms a homogeneous system whose thermoplastic range has shifted to lower temperatures than the original polymer, with the result that the capacity, for example, to change shape and elasticity, is increased and the hardness is reduced.

In order to open up a very wide field of application of plasticizers, they must also meet a number of requirements. Ideally, they should be tasteless, colorless, light-resistant, freeze-resistant, and heat-resistant. Also, they are expected to be water resistant, non-flammable, have low volatility and be harmless to health. In addition, the preparation process of the plasticizer should be simple and, in order to meet economic requirements, should be carried out without generating waste materials such as unrecoverable by-products and precursors of waste water containing contaminants.

The most important plasticizers are esters of dicarboxylic and polycarboxylic acids with plasticizing alcohols, i.e. unbranched or branched primary alcohols having from about 6 to 13 carbon atoms, which can be used as individual compounds or in mixtures. The preparation of the esters is carried out by the classical method by reaction of an acid or anhydride with an alcohol or a mixture of different alcohols in the presence of an acid, preferably sulfuric acid, as catalyst. The alcohol component is generally used in excess. Attempts have been made to overcome the problems of color and odor of the reaction products by careful selection of the acid used as catalyst, by mild reaction conditions and by complicated purification measures.

Further developments in the preparation of esters suitable for use as plasticizers constitute the use of metal-containing esterification catalysts. Suitable catalysts are, for example, tin, titanium and zirconium, which are used in the form of finely divided metals or, ideally, in the form of their salts, oxides or soluble organic compounds. These catalysts are high temperature catalysts, reaching their full activity only at esterification temperatures above 180 ℃. Examples are tin powder, tin (II) oxide, tin (II) oxalate, titanates such as tetraisopropyl orthotitanate or tetrabutyl orthotitanate and zirconium esters such as tetrabutyl zirconate. Alkyl titanates and titanium chelates, i.e. the titanates of polyols, have achieved particularly important industrial values in industrial production processes. One such method is for example the subject of US patent 5,434,294 and is also described in DE 1945359.

Furthermore, EP-A-0424767 discloses ｃA process for esterifying phthalic anhydride with isodecanol at 230 ℃ in the presence of tetrabutyltitanate as catalyst. After esterification, the reaction mixture was treated with sodium carbonate solution and the excess alcohol was distilled off. The treatment with sodium carbonate solution neutralizes the phthalic acid monoester present in the reaction mixture to form the corresponding salt. These salts are obtained as a pasty precipitate, which is difficult to filter and is expensive in terms of time and apparatus. Obtaining the desired phthalic diester in pure form therefore requires overcoming too many difficulties.

The current processes for preparing ester plasticizers have not yet met all aspects of the various requirements described above with respect to the production process and the reaction products.

The object of the present invention is therefore to improve the known processes by adapting and simplifying the individual successive partial steps of the overall process in order to optimize the process and to simplify the separation of a large number of reaction products, so that the reaction products can be used in as many applications as possible.

The present invention provides a process for preparing ester plasticizers by reacting dicarboxylic or polycarboxylic acids or their anhydrides with alcohols in the presence of a titanium-, zirconium-or tin-containing catalyst. It comprises first reacting a mixture of acid or anhydride and alcohol at 100-160 ℃ while removing any water formed, completing the reaction by adding a catalyst and raising the temperature to about 250 ℃, neutralizing the reaction mixture with an aqueous solution of an alkali metal hydroxide or alkaline earth metal hydroxide, then separating the excess alcohol and drying and filtering the remaining crude ester.

The novel process has high reliability in industrial scale plants. It is easy to perform (including continuous mode) and gives a high purity plasticizer. The easy and complete removal of the monoesters formed as by-products during the reaction, of the catalysts present in the reaction mixture, and of the reagents used for neutralization, etc., is a particularly outstanding advantage. The complete removal of these by-products and auxiliaries leads in particular to excellent color properties and outstanding color stability of the product of the process. Also noteworthy is their very low electrical conductivity, which makes them as widespread as possible for use in plastic articles in the field of cable insulation.

Suitable acids as starting materials for the process according to the invention are aliphatic and aromatic dicarboxylic and polycarboxylic acids. The aliphatic carboxylic acid can be saturated or unsaturated and contain at least 4 carbon atoms. Examples of such compounds are succinic acid, glutaric acid, adipic acid, suberic acid and azelaic acid, maleic acid and fumaric acid. Examples of aromatic carboxylic acids are phthalic acid and 1, 2, 4-trimellitic acid. These acids can be used in pure form or as mixtures. Their anhydrides may also be used successfully in place of the acids.

The alcohols used are, in particular, straight-chain or branched, saturated aliphatic compounds having more than 6 carbon atoms in the molecule. They usually contain primary hydroxyl groups, but secondary hydroxyl groups are generally not the subject of reaction with acids. Examples of such compounds are 1-n-octanol, 2-ethylhexanol-1-ol, n-nonanol, decanol, and the so-called oxo alcohols, i.e. mixtures of straight-chain and branched alcohols of the corresponding molecular size obtained from olefins by oxo processes and subsequently hydrogenated. These alcohols can likewise be used as pure compounds or as mixtures of structurally isomeric compounds or compounds of different molecular sizes.

The novel process has been found to be particularly suitable for the preparation of phthalic acid and C₈-C₁₂Esters of alcohols, in particular for the preparation of di (2-ethylhexyl) phthalate.

The esterification catalysts used in the novel process are titanium, zirconium or tin as the metal in finely divided form or preferably as a compound. Suitable compounds are tin (II) oxide, tin (II) oxalate, phenoxy-, acyl-and chelate compounds of titanium and zirconium and esters of titanium and zirconium, such as tetra (isopropyl) orthotitanate, tetrabutyl orthotitanate and tetrabutyl zirconate. The amount of catalyst can vary within a wide range. It is possible to use 0.01% by weight and also more than 5% by weight of catalyst, based on the reaction mixture. However, since larger amounts of catalyst do not give any advantage, the catalyst concentration is therefore frequently from 0.01 to 1.0% by weight, preferably from 0.01 to 0.5% by weight, based in each case on the reaction mixture.

Although esterification can be carried out using stoichiometric amounts of alcohol and acid, especially when an entrainer is used to remove the water of reaction, it is preferred to use a stoichiometric excess of 0.05 to 0.6 mole of alcohol per mole of dicarboxylic or polycarboxylic acid or anhydride in order to convert the acid as completely as possible.

According to the invention, the esterification is carried out in two stages. In the first stage, the monoester of the dicarboxylic acid is formed without the addition of a catalyst. The temperature used in this stage depends mainly on the starting materials. Satisfactory reaction rates are achieved at temperatures above 100 ℃ and preferably above 120 ℃. It is also possible to complete the formation of the monoester at these temperatures. However, it is more desirable to continue increasing the temperature up to about 160 ℃. When a carboxylic acid (rather than a carboxylic anhydride) is used as the esterification component, the water formed is removed from the reaction system as an azeotrope with the alcohol, provided that the reaction temperature is above the boiling point of the azeotrope (i.e., in the range of 90-100 ℃ at atmospheric pressure). The course and completion of the esterification is in this case observed by the formation of water. The use of a pressure lower than atmospheric pressure or higher than atmospheric pressure is not excluded, but is limited to a specific case. In order to suppress the occurrence of concentration differences, it is advisable to stir the reactor contents from time to time or to mix them, for example by passing an inert gas through the reaction mixture.

In the second stage, the esterification of the dicarboxylic or polycarboxylic acid is completed. It is carried out in the presence of the above-mentioned catalyst at a temperature which is higher than the temperature used in the first stage and which does not exceed about 250 ℃. This temperature is an index value that can be used ideally. Lower temperatures may be sufficient, for example, if a sufficiently high reaction rate is achieved or only partial conversion is achieved under certain circumstances. Higher temperatures can also be used, but should be avoided if the presence of decomposition products adversely affects the color. The reaction is ideally carried out at the boiling point of the alcohol used. It can be carried out in a single reactor or in a first stage reactor. The water formed during the reaction is removed as an azeotrope, with the alcohol acting as an entrainer. It is also advisable to mix the reactor contents at least at times during this reaction stage in order to equalize the concentration differences and to facilitate the removal of reaction water.

After the reaction is complete, the reaction mixture includes not only the desired reaction product, i.e., the diester or polyester, but also the partially esterified di-or polycarboxylic acid, excess alcohol and catalyst.

To work up the crude ester plasticizer, the product from the reactor is first neutralized with an alkali metal hydroxide or alkaline earth metal hydroxide. The alkaline agent is used here as an aqueous solution containing 5 to 20% by weight, preferably 10 to 15% by weight, of hydroxide, based on the solution. The amount of neutralizing agent used depends on the proportion of acid component, free acid and monoester in the crude product. The ratio is determined in the form of the acid number (according to DIN 53169). According to the invention, the alkaline agent is added in excess, which corresponds to the neutralization of the H present therein⁺2-4 times the stoichiometric amount of ions required. The use of the selected hydroxide, of which sodium hydroxide has been found to be particularly useful, in a specific concentration and in an excess of the specified aqueous solution ensures that the acidic components of the reaction mixture precipitate in crystalline, very filterable form. At the same time, the catalyst mostly decomposes to form products which are likewise readily filterable. The base treatment of the crude ester does not necessarily require the maintenance of a specific temperature. It is desirable to carry out the esterification step immediately thereafter, without prior cooling of the reaction mixture.

Excess alkali metal hydroxide or alkaline earth metal hydroxide, which is only in small amounts relative to the reaction product, reacts with the ester to form a carbonate, which is obtained in crystalline form and is easily filtered.

After neutralization, the free alcohol is separated from the reaction mixture. Steam distillation was found to be particularly suitable for this step and can be carried out in a simple manner by passing steam into the crude product. The advantage of steam distillation is that the final residue of the catalyst is destroyed in the process and converted into hydrolysis products which are conveniently filtered. For this reason, it is desirable to add a high surface area adsorbent, e.g., activated carbon, to the reaction mixture prior to distillation in order to remove the catalyst downstream of the product.

The free alcohol is removed followed by drying of the ester. In a particularly simple and efficient example of this step, the product is dried by passing an inert gas through it. The crude ester is then filtered to separate the solid, i.e., the (possibly partially esterified) salt of the carboxylic acid, the catalyst hydrolyzate, and the adsorbent therefrom. The filtration is carried out in conventional filtration equipment at room temperature or at a temperature of at most 150 ℃. Filtration can also be assisted by conventional filter aids such as cellulose, silica gel, diatomaceous earth or wood flour. However, their use is not limited to the exceptional cases.

The process of the invention dramatically reduces the time required for separating the solids present in the crude ester to a value which ensures that the process of the invention is carried out economically on an industrial scale and avoids the formation of ecologically harmful waste waters in terms of quantity and composition.

The esters obtained by the claimed process are of excellent quality and meet even higher requirements in every respect.

Claims (17)

1. A process for the preparation of ester plasticisers by reacting an aliphatic or aromatic dicarboxylic or polycarboxylic acid or anhydride thereof with a straight or branched chain saturated aliphatic alcohol having a carbon number greater than 6 in the molecule in the presence of a titanium, zirconium or tin containing catalyst which process comprises reacting a mixture of the acid or anhydride and the alcohol first at 100-160 ℃ with removal of any water formed, completing the reaction by adding the catalyst and raising the temperature to about 250 ℃, neutralising the reaction mixture with an aqueous solution of an alkali or alkaline earth metal hydroxide, then separating off excess alcohol and drying and filtering the remaining crude ester wherein the alkali metal is goldThe solution of the metal hydroxide or alkaline earth metal hydroxide is used in excess, which corresponds to the neutralization of H present in the reaction mixture⁺2-4 times the stoichiometric amount of ions required.

2. The process according to claim 1, wherein the catalyst is tetra (isopropyl) orthotitanate, tetrabutyl orthotitanate or tetrabutyl zirconate.

3. A process according to claim 1 or 2, wherein a stoichiometric excess of 0.05 to 0.6 mole of alcohol is used per mole of dicarboxylic or polycarboxylic acid.

4. A process according to claim 1 or 2, wherein the water of reaction formed during the reaction is removed from the reaction mixture as an azeotrope with the alcohol used.

5. The process of claim 3, wherein the water of reaction formed during the reaction is removed from the reaction mixture as an azeotrope with the alcohol used.

6. The process according to claim 1 or 2, wherein the reaction is carried out at the boiling point of the alcohol used.

7. The process of claim 3, wherein the reaction is carried out at the boiling point of the alcohol used.

8. The process according to claim 1 or 2, wherein the alkali metal hydroxide or alkaline earth metal hydroxide solution used for neutralizing the reaction mixture contains 5 to 20% by weight of hydroxide based on the solution.

9. The process according to claim 8, wherein the alkali metal hydroxide or alkaline earth metal hydroxide solution used for neutralizing the reaction mixture contains 10 to 15% by weight of hydroxide based on the solution.

10. The process of claim 3 wherein the alkali metal hydroxide and alkaline earth metal hydroxide solution used to neutralize the reaction mixture contains 5 to 20 weight percent hydroxide based on the solution.

11. A process according to claim 1 or 2, wherein the excess alcohol is separated from the reaction mixture by steam distillation, if desired with the presence of an adsorbent.

12. A process as claimed in claim 3, wherein the excess alcohol is separated from the reaction mixture by steam distillation, if desired with the presence of an adsorbent.

13. The process of claim 8, wherein the excess alcohol is separated from the reaction mixture by steam distillation, if desired with an adsorbent.

14. The process according to claim 1 or 2, wherein phthalic acid and 2-ethylhexanol as starting materials are reacted with each other.

15. The process of claim 3 wherein phthalic acid and 2-ethylhexanol as starting materials are reacted with each other.

16. The process of claim 8 wherein phthalic acid and 2-ethylhexanol as starting materials are reacted with each other.

17. The process of claim 11 wherein phthalic acid and 2-ethylhexanol as starting materials are reacted with each other.