HK1152518B - Process for lightening the colour of polyol esters - Google Patents

Description

Method for lightening the color of polyol esters

Technical Field

The present invention relates to a process for lightening the color of polyol esters formed from linear or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms by treating the polyol esters with a peroxy compound.

Background

Esters of polyols, also known as polyol esters, can be used in industry on a large scale for a variety of purposes, such as plasticizers or lubricants. The selection of suitable raw materials enables control of physical properties, such as boiling point or viscosity, and takes into account chemical properties, such as hydrolysis resistance or stability to oxidative degradation. Polyol esters can also be tailored to the solution of specific performance issues. A detailed review of the use of polyol esters can be found, for example, in Ullmann's Encyclopaedia of Industrial Chemistry, 5 th edition, 1985, VCH Verlagsgesellschaft, volume A1, page 305-319; 1990, volume A15, page 438-; 1981, Vol.14, pp.496-498.

The use of polyol esters as lubricants is very important industrially, especially for those applications in which the mineral oil-based lubricants do not fully meet the requirements set. Polyol esters are particularly useful as turbine engine oils and instrument oils. Polyol esters for lubricant applications are generally based on 1, 3-propanediol, 1, 3-butanediol, 1, 4-butanediol, 1, 2-hexanediol, 1, 6-hexanediol, neopentyl glycol, trimethylolpropane, pentaerythritol, 2, 4-trimethylpentane-1, 3-diol, glycerol or 3(4), 8(9) -dimethylol tricyclo [ 5.2.1.0%^2，6]Decane (also known as TCD alcohol DM) was used as the alcohol component.

Polyol esters are also used to a considerable extent as plasticizers. Plasticizers have many uses in plastics, coatings, sealants and rubber products. They interact physically with high molecular weight thermoplastics without reacting chemically, preferably by virtue of their swelling and dissolving capacity. This results in a homogeneous system, the thermoplastic range of which is shifted to lower temperatures than the original polymer, one result being to optimize its mechanical properties, such as increased deformability, elasticity and strength and reduced hardness.

In order to develop the widest possible field of application as plasticizers, they must meet a series of criteria. They should ideally be odorless, colorless and light-resistant, cold-resistant and heat-resistant. Furthermore, they are expected to be insensitive to water, relatively non-flammable and not very volatile, and not harmful to health. Furthermore, the production of the plasticizer should be simple and, in order to comply with ecological requirements, waste products, such as by-products which cannot be further utilized and waste water containing contaminants, should be avoided.

A particular class of polyol esters (which are referred to simply as G-esters) contain glycols or ether glycols as the alcohol component, for example ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1, 2-propanediol or higher propylene glycols. They can be prepared in different ways. In addition to the reaction of the alcohol and acid (optionally in the presence of an acidic catalyst), other methods are used in practice to obtain the G ester, including the reaction of the diol with an acid halide, the transesterification of a carboxylic ester with a diol, and the addition of ethylene oxide to a carboxylic acid (ethoxylation). In industrial manufacture, only direct reaction of diols with carboxylic acids and ethoxylation of carboxylic acids have been identified as production methods, and esterification of diols and acids is generally preferred. This is because it is possible to carry out the process with conventional chemical apparatuses without particular complexity and to provide a chemically homogeneous product. In contrast, ethoxylation requires large and expensive technical equipment. Ethylene oxide is a very reactive chemical. Which can polymerize explosively and form an explosive mixture with air over a very wide mixing range. Ethylene oxide stimulates the eyes and respiratory tract, causes chemical burns and causes liver and kidney damage, and is carcinogenic. Its operation therefore requires a great deal of safety measures. Furthermore, strict cleanliness of the storage and reaction devices must be ensured to avoid the formation of unwanted impurities due to side reactions of ethylene oxide with foreign matter. Finally, the reaction with ethylene oxide is not very selective, since it produces a mixture of compounds of different chain lengths.

Direct esterification of alcohols with carboxylic acids is one of the basic operations in organic chemistry. In order to increase the reaction rate, the conversion is usually carried out in the presence of a catalyst. The use of an excess of one of the reactants and/or the removal of the water formed during the reaction ensures that the equilibrium is shifted towards the side of the reaction product (i.e. the ester) according to the law of mass action, which means that high yields are achieved.

Comprehensive information on the preparation of polyol Esters (also including Esters of ethylene glycol and Fatty Acids) and on the properties of selected representatives of these classes of compounds can be found in Goldsmith, polyhydroaliphatic Esters of Fatty Acids, chem. Rev.33, 257 ff. (1943). For example, esters of diethylene glycol, triethylene glycol and polyethylene glycol are prepared at temperatures of 130 to 230 ℃ over reaction times of 2.5 to 8 hours. To remove the reaction water, carbon dioxide was used. Suitable catalysts for the esterification of polyols mentioned are mineral acids, acid salts, organic sulfonic acids, acetyl chloride, metals or amphoteric metal oxides. The water of reaction is removed by means of an entrainer, for example toluene or xylene, or by introducing an inert gas, for example carbon dioxide or nitrogen.

Johnson (ed.), fat Acids in Industry (1989) chapter 9, Polyoxydehtylene esters of fat Acids, discusses the production and properties of Fatty acid esters of polyethylene glycol and sets forth a series of preparation details. Higher diester concentrations are achieved by increasing the carboxylic acid/diol molar ratio. Suitable measures for removing the water of reaction are azeotropic distillation in the presence of a water-immiscible solvent, heating while passing an inert gas or carrying out the reaction under reduced pressure in the presence of a drying agent. If the addition of the catalyst is omitted, a longer reaction time and a higher reaction temperature are required. The use of a catalyst allows both reaction conditions to be milder. In addition to sulfuric acid, organic acids, such as p-toluenesulfonic acid and polystyrene-type cation exchangers, are also preferred catalysts. The use of metal powders, such as tin or iron, is also described. According to the teaching from US2,628,249, the color problem in the case of catalysis with sulfuric acid or sulfonic acid can be alleviated when operating in the presence of activated carbon.

A procedure for the preparation of esters of diethylene glycol and triethylene glycol and octanoic acid without addition of a catalyst is known from US2,469,446. The esterification temperature is from 270 to 275 ℃ and the reaction water is driven off by means of a carbon dioxide stream.

In reaction schemes where no catalyst is added, a molar excess of the particular carboxylic acid is typically used, which, due to its acidity, also acts as a catalyst.

Various methods are known for removing the water of reaction formed when esters are formed from polyols and carboxylic acids. For example, the reaction water formed is distilled out of the reaction vessel together with the excess carboxylic acid and fed to a downstream phase separator, where the carboxylic acid and water are separated according to their solubility properties. In some cases, the carboxylic acids used also form azeotropes with water under the reaction conditions and are capable of removing the water of reaction as an entrainer. Other methods used include azeotropic distillation in the presence of added water-immiscible solvent, heating the reaction mixture while passing an inert gas, reaction of the polyhydric alcohol and carboxylic acid starting materials under reduced pressure or in the presence of a drying agent. It has been found in particular that removal of water by azeotropic distillation helps to establish an equilibrium during the polyol ester production process. According to the procedure known from DE 19940991a1, a water-immiscible solvent which acts as an entrainer and must have a boiling point of less than 112 ℃ is added to the reaction mixture only when a temperature of at least 140 ℃ is reached.

In industrial processes, the mixture of water and carboxylic acid removed is separated in a phase separator into an organic phase and an aqueous phase, the aqueous phase is discharged and the carboxylic acid is recycled back to the esterification reaction. With regard to the work-up of the crude ester, for example, U.S. Pat. No. 5,324,853A1 proposes the removal of excess carboxylic acid by passing nitrogen or steam, adding an adsorbent, neutralizing the residual organic acid with a base and filtering off the resulting solid. The residual amount of acid present in the filtrate is removed by passing steam or nitrogen while applying reduced pressure and recycled back to the esterification reaction. The solids obtained in the vacuum treatment were removed in a final fine filtration. One task of the added adsorbent, e.g. activated carbon, is to improve the color of the polyol ester.

According to the procedure known from US2,469,446 a1, the crude ester obtained after removal of the reaction water and excess unconverted starting material (e.g. carboxylic acid) is first treated with an alkaline reagent, for example with an aqueous solution of sodium carbonate or sodium hydroxide, to remove the last residue of acidic components. After washing with water and treatment with bleaching earth and activated carbon, the last traces of odorous substances can be removed by applying reduced pressure at elevated temperature. In some cases, the treatment with bleach and activated carbon must be repeated more than once to produce polyol esters with satisfactory color properties.

Measures for improving the color of the crude esters, such as oxidation, for example with hydrogen peroxide or ozone, and adsorption on activated carbon, are known from the general prior art, for example from h.suter,und seine Verwendung[Phthalic anhydride and use thereof]d, dr. dietichsteinkopf Verlag, Darmstadt 1972. In order to improve the color of polyol-based ester compounds, WO 94/18153a1 proposes a subsequent treatment with aqueous hydrogen peroxide. The effect of ozone on lightening is discussed, for example, in DE2729627a 1.

Due to the quality criteria of the polyol esters described at the outset, the process steps in the esterification stage with removal of the reaction water and in the work-up of the crude esters are very important process characteristics, since the adjustment of these process steps significantly affects the organoleptic and optical properties of the end product. More particularly, there are high demands on the color properties of polyol esters, such as low color values and high color stability. In contrast, the structure of the raw materials, polyol and acid, is critical to the mechanical and thermal properties of the polymeric material plasticized with the polyol ester and affects the hydrolytic and oxidative stability of the lubricant.

It is a conventional process to treat with adsorbents, for example activated carbon, high surface area polysilicic acids, such as silica gel (silica xerogel), diatomaceous earth, high surface area alumina and hydrated alumina, or mineral materials, such as clays or carbonates, to improve the colour during the work-up of the crude polyol ester, but it requires an additional filtration step, which means a considerable degree of complexity in the process carried out industrially. The valuable product can also stick in the filter and on the adsorbent, so that the valuable product is lost in a further filtration step.

Treatment with hydrogen peroxide to improve colour has also been found to be problematic as it can lead to the formation of organic peroxides during polyol ester treatment. Trace amounts of peroxide reduce the ester quality and performance of plasticized polymer products and lubricants made based on polyol esters. Trace amounts of peroxide also impair the storage properties of the polyol esters, although oxidizing agents, such as air, are excluded, but an increase in the peroxide value is also observed during storage. In order to reduce the peroxide number, the prior art proposes an additional treatment with a reducing agent. Although this method enables a reduction of the peroxide number, this operation implies an additional operation step in which the reducing agent must be supplied and removed after its use.

Disclosure of Invention

It has now been found that in the treatment of a crude polyol ester with a peroxy compound, if the treatment with the peroxy compound and immediately thereafter, without further intermediate steps, the treatment with steam and the final drying of the polyol ester, it is possible to obtain a light-coloured product without the use of an adsorbent, the conditions during the treatment, such as the temperature to be used, the duration or the pressure to be applied, being adjusted to the particular polyol ester.

Surprisingly, in this procedure, light-colored polyol esters are obtained which have exceptionally low peroxide values which remain stable and do not increase even over long storage periods.

The invention therefore comprises a process for lightening the color of polyol esters by reacting a polyol with a linear or branched aliphatic monocarboxylic acid having 3 to 20 carbon atoms and subsequently working up the reaction mixture without using an adsorbent. The process is characterized in that, after removal of unconverted starting compounds, the reaction product is treated with a peroxide compound, immediately thereafter, without further intermediate steps, with steam treatment and drying of the residual polyol ester.

The new procedure is notable for a high degree of reliability not only in laboratory and experimental operations, but also in industrial installations in particular. Even in a continuous form, it is easy to implement and provides polyol esters of high purity. Treatment of the crude ester with a peroxy compound followed by steam treatment and further drying results in excellent color properties and significant color stability of the polyol ester, which in addition has only a low peroxide value. The peroxide value also remains stable at low values over extended storage times.

For the treatment of the crude esters obtained after removal of unconverted starting compounds, suitable peroxy compounds are hydrogen peroxide, organic percarboxylic acids, such as peracetic acid or perpropionic acid, organic hydroperoxides, such as cumene hydroperoxide or tert-butyl hydroperoxide, alkali metal or alkaline earth metal perborates, alkali metal or alkaline earth metal percarbonates, alkali metal or alkaline earth metal peroxodisulfates or alkali metal or alkaline earth metal perphosphates.

Particularly suitable are aqueous hydrogen peroxide solutions, liquid organic percarboxylic acids or organic hydroperoxides which can be removed by distillation in a simple manner. The use of alkali metal or alkaline earth metal peroxide compounds in solid form or in the form of salts in aqueous solution is not excluded, but is limited only to a few exceptions, since they and their reaction products are present in solid form or precipitate during the crude ester work-up and have to be removed by an additional filtration step.

Particularly suitable is hydrogen peroxide in the form of an aqueous solution having a hydrogen peroxide content of more than 10% by weight, preferably from 30 to 50% by weight. Hydrogen peroxide solutions with lower active contents are not recommended because too high amounts of water are introduced, which must then be removed again. In the case of excessive hydrogen peroxide concentrations, inconvenient and expensive safety precautions must be observed during operation.

The amount of peroxy compound added to the crude ester to be treated is such that its active content in the total mixture is from 0.03 to 1.0% by weight, preferably from 0.08 to 0.3% by weight. At too low an active concentration, the decolorization force is no longer sufficient to obtain light-colored polyol esters of sufficient quality. In the case of too high an active concentration, uncontrolled degradation reactions of the ester compound are to be expected.

This treatment with the peroxy compound is generally carried out at elevated temperatures, preferably at temperatures of from 70 to 160 c, preferably from 100 to 120 c, although low temperatures, for example room temperature or lower, are not excluded. The processing time can be selected within a wide range. It should not be too short nor too long and can be determined by simple preliminary experiments. Typically, the treatment time is from 0.5 to 4 hours. With shorter processing times, no positive effect on the color values was observed; in the case of excessively long treatment times, there is a risk of increased ester cleavage and uncontrolled degradation of the polyol ester structure due to the presence of water and oxidizing agents. Likewise, in the case of excessively long treatment times, the reactor volume is unnecessarily occupied.

The specific conditions of the treatment with the peroxy compound should be adapted to the specific polyol ester in order to achieve an optimum decolorization on the one hand and to prevent degradation reactions of the polyol ester as far as possible on the other hand. Especially in the case of polyol esters based on ether glycols, such as triethylene glycol or tetraethylene glycol, the degradation of the ether structure is increased if the conditions, such as temperature, duration of action and concentration, in the treatment with the peroxy compound are not precisely adjusted for the particular polyol ester.

After the oxidation treatment, the crude ester is immediately subjected to a steam treatment without further intermediate steps, which can be carried out, for example, in simple form by introducing steam into the crude product. One advantage of the steam treatment is that excess peroxide compound is destroyed in the process and residual starting compounds are removed with the steam. This steam treatment also drives out a relatively large amount of water still present. At the same time, this measure improves the color number and the color stability of the crude ester.

The steam treatment is generally carried out at standard pressure, but the use of a slight reduced pressure, suitably as low as 400hPa, is not excluded. The steam treatment is generally carried out at temperatures of from 100 to 250 ℃, preferably from 150 to 220 ℃, in particular from 170 to 200 ℃, and is also governed by the physical properties of the polyol ester to be prepared in each case.

In the steam treatment process step it was found appropriate to proceed in a very gentle manner during heating up to the working temperature to heat the mixture of crude ester and added peroxy compound to the desired steam treatment temperature.

The duration of the steaming treatment can be determined by routine experimentation and is typically carried out for 0.5 to 5 hours. Too long a steaming causes an undesirable increase in the color of the polyol ester and should therefore be avoided. Increased degradation of the polyol ester to acidic compounds is also observed, as evidenced by an increase in the neutralization or acid number, for example, as measured according to DIN EN ISO 3682/ASTM D1613. In case the treatment time is too short, the destruction of excess peroxy compound and the formed trace organic peroxide is incomplete and the desired polyol ester still has an undesirable peroxide value which is too high, expressed in milliequivalents of oxygen per kilogram of product and measured according to ASTM E298. Another observation in the case of too short a treatment time is only a very small advantageous effect on the polyol ester color.

As in the case of treatment with peroxy compounds, the conditions in the subsequent steam treatment, such as temperature, pressure and duration, must also be precisely adjusted according to the particular polyol ester in order to achieve the best results in terms of polyol ester color value and to minimize as far as possible the residual content of traces of starting compounds, water and peroxide and at the same time suppress degradation reactions. Especially in the case of polyol esters based on ether glycols, such as triethylene glycol or tetraethylene glycol, the conditions in the steam treatment have to be precisely adapted to the particular polyol ester in order to suppress the undesired degradation of the ether chains.

Notably, the vapor distillate that has been removed from the desired polyol ester and obtained after condensation of the vapor removed from the reaction zone has a relatively high peroxide value. At industrial scale, the presence of large amounts of vapor and vapor distillates with high peroxide values has been found to be problematic for safety reasons, since organic and possibly inorganic peroxides can be concentrated in the connected column and distillate receiver. It has been found appropriate to contact this removed vapour of the loaded water and unconverted starting compound, in which peroxide is also present, with a noble metal of groups 9 to 11 of the periodic table of the elements (according to IUPAC recommendations, 1985), for example with palladium or platinum. This measure may destroy the peroxide compounds present in the steam. The contacting is carried out in the vapor phase at the temperature of the removed vapor in the presence of the noble metal by, for example, passing the vapor over a commercial noble metal catalyst in the form of a fixed bed, which may be supported or unsupported. For example, in the column section connected to the reactor section, a solid inner member having a woven or porous structure, such as a rectangular, honeycomb, circular or other conventional structure, to which the noble metal has been applied and through whose channels the gaseous and loaded steam which has passed through the crude ester and is removed at this time can be installed. If the noble metal has been applied to the support, suitable supports are those conventionally used in industry for noble metal catalysts, such as silica, alumina, activated carbon, titania or zirconia, in their various manifestations.

It is also possible to provide solid structures consisting of noble metals, such as fabrics, nets, braids, threads, coils or sponges, in the column section to destroy peroxide compounds driven out with the steam.

The removed condensate distillate, which may be enriched in peroxides, may also be treated with a noble metal from groups 9 to 11 of the periodic table of the elements to destroy peroxide compounds still present, for example with a commercially supported or unsupported noble metal catalyst which may be used in the form of a fixed bed or in the form of a suspension at autogenous temperature. Conventional solid structures of precious metals, such as fabrics, braids or wires, e.g., platinum mesh, may also be contacted with the removed liquid distillate.

After the steaming, the polyol ester is dried, for example by passing an inert gas over the product at elevated temperature. It is also possible to apply reduced pressure and optionally to pass an inert gas over the product at the same time at elevated temperature. Even without the action of an inert gas, it is possible to operate only at elevated temperatures or only under reduced pressure. Specific drying conditions, such as temperature, pressure and time, can be determined by simple preliminary tests and should be designed to accommodate a particular polyol ester. In general, the working temperature is from 80 to 250 ℃, preferably from 100 to 180 ℃, and the working pressure is from 0.2 to 500hPa, preferably from 1 to 200hPa, especially from 1 to 20 hPa. After the end of the drying, the polyol ester is obtained as a residue with a light color, and no filtration step is required to obtain a product that meets specifications. In several exceptional cases, a filtration step may be required after the steam treatment or after drying, if, for example, the solid catalyst residues are not completely removed after the end of the esterification reaction and after unconverted starting compounds have been removed, and therefore before the work-up of the reaction mixture.

In a particular configuration of the process of the invention, the drying of the residual polyol ester follows the steam treatment without further intermediate steps.

The reaction of the polyol and the aliphatic monocarboxylic acid may be carried out without using a catalyst. This reaction variant has the advantage of avoiding the addition of foreign substances to the reaction mixture which would cause undesirable contamination of the polyol ester. However, it follows that it is generally necessary to maintain higher reaction temperatures, since only then is it ensured that the reaction proceeds at a sufficient, i.e. economically acceptable, rate. It should be noted herein that elevated temperatures can cause thermal damage to the polyol ester. It is not always possible to avoid the use of catalysts that promote the reaction and increase the reaction rate. The catalyst may often be an excess of aliphatic monocarboxylic acid, which is at the same time a reaction component of the polyol, so that the reaction proceeds autocatalytically. In addition, conventional esterification catalysts are suitable for influencing the reaction rate, such as sulfuric acid, formic acid, polyphosphoric acid, methanesulfonic acid or p-toluenesulfonic acid, and combinations of such acids. It is likewise possible to use metal catalysts, such as titanium-containing, zirconium-containing or tin-containing catalysts, for example the corresponding alkoxides or carboxylates. Catalytically active compounds which are insoluble in the reaction system and are solid under the reaction conditions, such as alkali metal or alkaline earth metal hydrogen sulfates, for example sodium hydrogen sulfate, can also be used, although the use of solid catalysts is restricted to a few exceptions, since the solid catalyst has to be filtered off from the reaction mixture after the esterification has ended. In some cases, additional fine filtration is required during the work-up of the crude polyol ester to remove the last residue of the solid catalyst. The amount of catalyst used can be within wide limits. 0.001 wt.% or 5 wt.% of catalyst, based on the reaction mixture, can be used. However, owing to the higher amounts of catalyst with hardly any advantage, the catalyst concentration is generally from 0.001 to 1.0% by weight, preferably from 0.01 to 0.5% by weight, based in each case on the reaction mixture. Suitably, it may be decided by preliminary experiments whether to operate at a higher temperature without catalyst or at a lower temperature with catalyst for each case.

Esterification can be carried out with stoichiometric amounts of polyol and aliphatic monocarboxylic acid. However, it is preferred to react the polyol with an excess of monocarboxylic acid without adding a catalyst so that the excess monocarboxylic acid itself acts as a catalyst. The excess monocarboxylic acid can also be removed from the crude ester by distillation in a simple manner, which generally has a lower boiling point than the polyol used and, since solid catalysts are avoided, the filtration step can be omitted. The aliphatic monocarboxylic acids are used in an excess of from 10 to 50 mol%, preferably from 20 to 40 mol%, per mole of hydroxyl groups to be esterified in the polyol.

The water of reaction formed is distilled off from the reaction vessel during the reaction together with the excess monocarboxylic acid and passed to a downstream phase separator, where the monocarboxylic acid and water are separated off according to their solubility properties. The monocarboxylic acids used may also form azeotropes with water under the reaction conditions and can serve as entrainers for removing the water of reaction. The progress of the reaction can be monitored by the water obtained. The separated water is removed from the process while the monocarboxylic acid from the phase separator is returned to the reaction vessel. The addition of other organic solvents which assume the role of entrainers, such as hexane, 1-hexene, cyclohexane, toluene, xylene or xylene isomer mixtures, is not excluded, but is subject to a few exceptions. The entrainer may be added as early as the start of the esterification reaction or at a relatively high temperature. The reaction is terminated by cooling the reaction mixture when the desired theoretical amount of water has been obtained or the hydroxyl number, as measured, for example, according to DIN 53240, has fallen below the set value.

The reaction between the polyol and the aliphatic monocarboxylic acid starts in the range of about 120 to 180 ℃ depending on the starting materials and can proceed to completion in different ways.

One configuration of the process of the invention involves first heating from room temperature to at most 280 c, preferably at most 250 c, and, with the temperature kept constant, reducing the pressure in stages starting from the standard pressure to facilitate the removal of the reaction water. The choice of pressure stage (whether one, two or more stages) and the choice of pressure to be established in a particular stage can be varied within wide limits and adjusted to specific conditions. For example, in the first stage, the pressure may be reduced from the standard pressure to 600hPa, and then the reaction may be carried out to completion at a pressure of 300 hPa. These pressure values are guiding values for proper adherence.

In addition to the pressure change, the temperature can likewise be changed in one, two or more stages from room temperature during the esterification reaction, so that at constant pressure the temperature rises in stages, generally to a maximum temperature of 280 ℃. However, it has been found appropriate to heat up to 280 ℃ with a stepwise increase in temperature and also to reduce the pressure stepwise. For example, the esterification reaction can be carried out in the first stage at a temperature starting from room temperature up to 190 ℃. Reduced pressure down to 600hPa was also applied to accelerate the expulsion of the reaction water. When the temperature stage of 190 ℃ is reached, the pressure is again reduced to 300hPa and the esterification is carried out to completion at a temperature of at most 250 ℃. These temperature and pressure values are the appropriate guidelines for compliance. The temperature and pressure conditions to be established at a particular stage, the number of stages and the particular rate of temperature rise or pressure decrease per unit time may vary within wide ranges and are adjusted according to the physical properties of the starting compounds and the reaction products, the temperature and pressure conditions of the first stage established from standard pressure and room temperature. It has been found that a two-stage increase in temperature and a two-stage decrease in pressure is particularly suitable.

The lower pressure limit to be established depends on the physical properties of the starting compounds and the reaction products formed, such as boiling point and vapor pressure, and also on the plant equipment. Starting from the standard pressure, it is possible to operate in stages within these limits with a pressure reduction from stage to stage. An upper temperature limit of typically 280 ℃ should be observed to prevent the formation of decomposition products, which adversely affect color and other properties. The lower limit of the temperature stage depends on the reaction rate, which still has to be high enough to complete the esterification reaction within an acceptable time. Within these limits, it is possible to operate in stages with a temperature increase from stage to stage.

The reaction mixture obtained after the end of the reaction comprises, in addition to the polyol ester as the desired reaction product, possibly unconverted starting materials, especially if an excess of acid has been used according to the preferred configuration of the process of the invention, still an excess of aliphatic monocarboxylic acid. For the working-up, the excess and unconverted starting material are distilled off, it being appropriate to apply reduced pressure. In order to remove the acidic catalyst, such as dissolved sulfuric acid or solid potassium bisulfate (if added in the esterification stage), and in order to remove the final residue of the acidic components, it is also possible to provide a treatment with an alkaline agent, for example with an aqueous solution of sodium carbonate or sodium hydroxide, or in exceptional cases, a filtration.

Thereafter, the crude ester, freed from unconverted starting compounds and any catalyst present, is worked up according to the measures of the invention, including treatment with the peroxo compound, subsequent steam treatment and final drying, omitting the use of conventional adsorbents, such as activated carbon, high surface area polysilicic acids, such as silica gel (silica xerogel), diatomaceous earth, high surface area alumina and hydrated alumina, or mineral materials, such as clays or carbonates, in the working-up. Without the use of an adsorbent, a light-colored polyol ester with a sufficiently low peroxide value is obtained, which also meets the remaining specifications, such as water content, residual acid content and residual content of the monoester. The purified polyol ester remains in the reaction vessel during drying as a residue of excellent quality, with the addition of a filtration step generally not required and limited to a few exceptions.

The polyols used as starting materials for the process according to the invention correspond to the general formula (I)

R(OH)_n (I)

Wherein R is an aliphatic or alicyclic hydrocarbon group having 2 to 20, preferably 2 to 10 carbon atoms, and n is an integer of 2 to 8, preferably 2, 3, 4, 5 or 6.

Suitable polyols are likewise compounds of the formula (II)

H-(-O-[-CR¹R²-]_m-)_o-OH (II)

Wherein R is¹And R²Each independently of the others is hydrogen, alkyl having 1 to 5 carbon atoms, preferably methyl, ethyl or propyl, or hydroxyalkyl having 1 to 5 carbon atoms, preferably hydroxymethyl, m is an integer from 1 to 10, preferably from 1 to 8, especially 1, 2, 3 or 4, o is an integer from 2 to 15, preferably from 2 to 8, especially 2, 3, 4 or 5.

Suitable polyols which can be converted into the light-coloured polyol esters by the process of the invention are, for example, 1, 3-propanediol, 1, 3-butanediol, 1, 4-butanediol, neopentyl glycol, 2-dimethylolbutane, trimethylolethane, trimethylolpropane, ditrimethylolpropane, trimethylolbutane, 2, 4-trimethyl-pentane-1, 3-diol, 1, 2-hexanediol, 1, 6-hexanediol, pentaerythritol or dipentaerythritol or 3(4), 8(9) -dimethylol tricyclo [5.2.1.0 ]^2，6]Decane.

Other polyols which may be used include ethylene glycol and 1, 2-propylene glycol, and oligomers thereof, especially ether glycols di-, tri-and tetraethylene glycol or dipropylene glycol, tripropylene glycol or tetrapropylene glycol. Ethylene glycol and propylene glycol are industrially produced chemicals. The base materials used for their preparation are ethylene oxide and propylene oxide, from which 1, 2-ethanediol and 1, 2-propanediol are obtained by heating under pressure with water. Diethylene glycol is obtained by ethoxylation of ethylene glycol. Like tetraethylene glycol, triethylene glycol is obtained as a by-product in the hydrolysis of ethylene oxide to ethylene glycol. Both compounds can also be synthesized by reacting ethylene glycol with ethylene oxide. Dipropylene glycol, tripropylene glycol, tetrapropylene glycol and higher propoxylated products are obtained by multiple additions of propylene oxide to 1, 2-propanediol.

In order to obtain light-colored polyol esters by the process of the present invention, linear or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms in the molecule are used. Although saturated acids are preferred in many cases, it is also possible to use unsaturated carboxylic acids as reaction components for the ester synthesis, depending on the particular field of application of the plasticizer or lubricant. Examples of monocarboxylic acids as components of the polyol esters are propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, 2-methylbutyric acid, 3-methylbutyric acid, 2-methylpentanoic acid, n-hexanoic acid, 2-ethylbutyric acid, n-heptanoic acid, 2-methylhexanoic acid, cyclohexanecarboxylic acid, 2-ethylhexanoic acid, n-nonanoic acid, 2-methyloctanoic acid, isononanoic acid, 3, 5, 5-trimethylhexanoic acid, 2-propylheptanoic acid, 2-methylundecanoic acid, isoundecanecarboxylic acid, tricyclodecanecarboxylic acid and isotridecanoic acid. The novel process has been found to be particularly useful for the preparation of monoethylene glycol or oligoethylene glycol and 1, 2-propylene glycol or oligopropylene glycol with C₄-to C₁₃-or C₅-to C₁₀Polyol esters of monocarboxylic acids and for the preparation of polyols based on 1, 3-butanediol, neopentyl glycol, 2, 4-trimethylpentane-1, 3-diol, trimethylolpropane, ditrimethylolpropane, pentaerythritol or 3(4), 8(9) -dimethylol tricyclo [5.2.1.0^2，6]Polyol ester of decane.

Polyol esters of ethylene glycol and its oligomers are outstandingly suitable as plasticizers for all customary high molecular weight thermoplastic materials. They have been found to be particularly useful as additives to polyvinyl butyrals, which are used in combination with glycol esters as interlayers in the manufacture of multilayer or composite glass. They can likewise be used as coalescents or film-forming assistants in aqueous polymer dispersions which find various uses as coatings. The preparation process of the present invention makes it possible to prepare polyol esters having excellent color properties in a simple manner without using conventional adsorbents, which also meet further quality requirements, such as low odor or low acid number. The process of the invention is particularly suitable for the preparation of triethylene glycol di-2-ethylhexanoate (3G8 ester), tetraethylene glycol di-n-heptanoate (4G7 ester), triethylene glycol di-2-ethylbutyrate (3G6 ester), triethylene glycol di-n-heptanoate (3G7 ester) or tetraethylene glycol di-2-ethylhexanoate (4G8 ester).

The process of the invention can be carried out continuously or batchwise in reaction apparatuses which are typically used in chemical technology. Useful devices have been found to be stirred tanks or reaction tubes equipped with heating devices and connected column sections.

Detailed Description

The process of the present invention is illustrated in detail in the following examples, but the invention is not limited to the embodiments.

Examples

For the lightening test, crude triethylene glycol di-2-ethylhexanoate having a color number of 131Hazen units was used, which was obtained by esterification of triethylene glycol with 2.3 molar amounts of 2-ethylhexanoic acid without catalyst and without addition of entrainer. The content (% by weight) of triethylene glycol di-2-ethylhexanoate was 97.7%, triethylene glycol mono-2-ethylhexanoate was 1.2%, and the balance to 100% was 1.1% as measured by gas chromatography.

A workup of the crude triethylene glycol di-2-ethylhexanoate was carried out in a heatable 1 l four-necked flask equipped with stirrer, internal thermometer and dropping funnel with in each case 300 g of crude product. After addition of aqueous hydrogen peroxide and stirring under the reaction conditions described below, the dropping funnel was replaced for subsequent steam distillation by a distillation apparatus with a 1-liter receiver and the 1-liter four-necked flask was equipped with an immersion tube for passage of steam. A platinum net was placed in the distillation column and the expelled peroxide-laden vapor was passed through the platinum net.

After steam distillation was performed under the following conditions, the supply of steam was stopped and reduced pressure was applied to the distillation apparatus for final drying. Without the use of an adsorbent, the resulting residue was a light colored, in-spec polyol ester.

Example 1:

the crude triethylene glycol di-2-ethylhexanoate was treated with an aqueous hydrogen peroxide solution under the following conditions:

H2O2concentration of aqueous solution	30% by weight
		H2O2Based on the absolute amount of the total reaction mixture	0.10% by weight
Reaction temperature	120℃
		Reaction time	1 hour

The subsequent steam distillation was carried out using a platinum gauze under the following conditions:

operating temperature of steam distillation	180℃
		Time of treatment	1 hour

Subsequently, the following drying conditions were established:

pressure of	10hPa
		Drying temperature	140℃
Drying time	0.5h

On completion of the work-up, a light-colored polyol ester is obtained, having the following contents, determined by gas chromatography:

triethylene glycol di-2-ethylhexanoate content	97.9% by weight
		Triethylene glycol Mono-2-ethylhexanoate content	0.9% by weight
Excess material	1.2% by weight

And the following indices:

hazen colour number (DIN ISO 6271)	29
		Neutralization number (mg KOH/g, DIN EN ISO 3682/ASTM D1613)	0.05
Water content (wt.%, DIN 51777Part1)	0.05
		Peroxide content (meq O/kg, ASTM E298)	1.15

In the distillate of the steam distillation, a peroxide content of 3.0meq O/kg was measured.

Example 2:

example 2 was carried out according to example 1, with the sole exception that the steam distillation was carried out without the use of a platinum gauze. The resulting distillate had a peroxide content of 13meq O/kg. The index of the purified polyol ester corresponds to the value shown according to example 1.

Example 3:

preparation of NPG di-2-ethylhexanoate and subsequent treatment with hydrogen peroxide

The esterification of neopentyl glycol with 2-ethylhexanoic acid was carried out in a heatable 1 l four-necked flask equipped with stirrer, internal thermometer and water separator.

A flask was initially charged with 312.75 grams (3.00 moles) of neopentyl glycol and 966.89 grams (6.70 moles) of 2-ethylhexanoic acid. The mixture was heated to 200 ℃ while stirring and a reduced pressure of 600hPa was applied, and reacted under these conditions for 2 hours. Subsequently, the pressure was gradually reduced to 500hPa, and the reaction water formed was removed from the water separator. The progress of the reaction was monitored by continuously weighing the water discharged via the water separator and by variation of the hydroxyl number. After a total reaction time of 8 hours, the reaction is terminated at a residual hydroxyl number of 4.2mgKOH/g (according to DIN 53240).

Subsequently, the excess 2-ethylhexanoic acid is distilled off at a temperature of 190 ℃ and a pressure of 95hPa for 2 hours and additionally at a temperature of 130 ℃ and a pressure of 6hPa for a further 30 minutes. After which it was treated with a 30% by weight hydrogen peroxide solution (pure hydrogen peroxide in an amount of 0.1% by weight based on the reaction mixture) at a temperature of 120 ℃ for 1 hour.

The subsequent steam distillation was carried out at a temperature of 180 ℃ for 1 hour at standard pressure with steam being passed in. Subsequently, the steam distillation was stopped and a pressure of 10hPa was applied to the distillation apparatus for final drying. The drying was carried out at 140 ℃ for 30 minutes. The residue obtained is a pale-coloured neopentyl glycol di-2-ethylhexanoate having the following indices:

content by gas chromatography:

neopentyl glycol bis-2-ethylhexanoate content	92.7% by weight
		Neopentyl glycol Mono-2-ethylhexanoate content	6.2% by weight
Excess material	1.1% by weight

Index:

hazen colour number (DIN ISO 6271)	49
		Neutralization number (mg KOH/g, DIN EN ISO 3682/ASTM D1613)	0.26
Water content (wt.%, DIN 51777Part1)	0.02
		Peroxide content (meq O/kg, ASTM E298)	1.59

Example 4 (comparative):

the esterification of neopentyl glycol with 2-ethylhexanoic acid and the subsequent removal of unconverted and excess 2-ethylhexanoic acid was carried out according to example 3. Without hydrogen peroxide treatment, followed by steam treatment at 180 ℃ for 30 minutes, followed by drying at 120 ℃ for 15 minutes. The neopentyl glycol di-2-ethylhexanoate obtained in the residue showed the following index:

content by gas chromatography:

neopentyl glycol bis-2-ethylhexanoate content	92.9% by weight
		Neopentyl glycol Mono-2-ethylhexanoate content	6.2% by weight
Excess material	0.9% by weight

Index of refraction

Hazen colour number (DIN ISO 6271)	140
		Neutralization number (mg KOH/g, DIN EN ISO 3682/ASTM D1613)	0.17
Water content (wt.%, DIN 51777Part1)	0.02

The measures according to the invention of treating the crude esterification mixture with hydrogen peroxide after removal of unconverted starting compounds and then immediately steam treatment without further intermediate steps lead to light-colored polyol esters with high color stability without the use of adsorbents. In a further configuration of the method of the invention, the steam driven off during the steam treatment can be brought into contact with the platinum gauze. This measure makes it possible to significantly remove the peroxide content in the removed distillate, which avoids safety problems that have to be dealt with in the case of amounts of distillate with a high peroxide content.

Claims (38)

1. By reacting a polyol having the general formula:

H-(-O-[-CR¹R²-]_m-)_o-OH

wherein R is¹And R²Each independently hydrogen, alkyl having 1 to 5 carbon atoms, or hydroxyalkyl having 1 to 5 carbon atoms, m is an integer from 1 to 10, o is an integer from 2 to 15,

process for lightening the colour of polyol esters by reaction with linear or branched aliphatic monocarboxylic acids having 3 to 20 carbon atoms and subsequent workup of the reaction mixture without the use of adsorbents, characterized in that after removal of unconverted starting compounds, the reaction product is treated with a peroxide compound, after which immediately without further intermediate steps a steam treatment is carried out at a temperature of 100 to 250 ℃ for 0.5 to 5 hours and the residual polyol ester is dried, and wherein the active content of the peroxide compound is 0.03 to 1.0% by weight, based on the total mixture.

2. A process according to claim 1, characterized in that the active content of the peroxy compound is from 0.08 to 0.3% by weight, based on the total mixture.

3. Process according to claim 1 or 2, characterized in that the peroxy compound is selected from the group consisting of hydrogen peroxide, organic percarboxylic acids, organic hydroperoxides, alkali or alkaline earth perborates, alkali or alkaline earth percarbonates, alkali or alkaline earth peroxodisulfates and alkali or alkaline earth perphosphates.

4. A process according to claim 3, characterized in that the hydrogen peroxide is used in the form of an aqueous solution having a hydrogen peroxide content of more than 10% by weight.

5. A process according to claim 4, characterized in that the hydrogen peroxide is used in the form of an aqueous solution having a hydrogen peroxide content of from 30 to 50% by weight.

6. A process according to claim 1 or 2, characterized in that the treatment with the peroxy compound is carried out at a temperature of 70 to 160 ℃.

7. A process according to claim 6, characterized in that the treatment with the peroxy compound is carried out at a temperature of from 100 to 120 ℃.

8. A process according to claim 4, characterized in that the treatment with the peroxy compound is carried out at a temperature of from 70 to 160 ℃.

9. A process according to claim 1 or 2, characterized in that the steam treatment is carried out at a temperature of 150 to 220 ℃.

10. A process according to claim 9, characterized in that the steam treatment is carried out at a temperature of 170 to 200 ℃.

11. A process according to claim 4, characterized in that the steam treatment is carried out at a temperature of 150 to 220 ℃.

12. The process according to claim 1 or 2, characterized in that the vapors removed in the vapor treatment are brought into contact in gaseous form with noble metals of groups 9 to 11 of the periodic table of the elements.

13. A process according to claim 4, characterized in that the vapors removed in the vapor treatment are brought into contact in gaseous form with noble metals of groups 9 to 11 of the periodic Table of the elements.

14. The process according to claim 1 or 2, characterized in that the vapors removed in the vapor treatment are first condensed and the condensate distillate is brought into contact with a noble metal from groups 9 to 11 of the periodic Table of the elements.

15. Process according to claim 4, characterized in that the vapors removed in the vapor treatment are first condensed and the condensate distillate is brought into contact with a noble metal from groups 9 to 11 of the periodic Table of the elements.

16. Process according to claim 12, characterized in that the noble metal of groups 9 to 11 of the periodic table of the elements is in the form of a fixed bed.

17. The process according to claim 14, characterized in that the noble metal of groups 9 to 11 of the periodic Table of the elements is in the form of a fixed bed.

18. Process according to claim 17, characterized in that the noble metal of groups 9 to 11 of the periodic table of the elements has been applied to a support.

19. The process according to claim 18, wherein the support used is silicon dioxide, aluminum oxide, activated carbon, titanium dioxide or zirconium dioxide.

20. The process according to claim 17, characterized in that the noble metal of groups 9 to 11 of the periodic table of the elements is arranged in the form of a fabric.

21. The process according to claim 17, characterized in that the noble metals of groups 9 to 11 of the periodic table of the elements are arranged in the form of a net.

22. Process according to claim 17, characterized in that the noble metal of groups 9 to 11 of the periodic table of the elements is arranged in the form of a braid.

23. The process according to claim 17, characterized in that the noble metals of groups 9 to 11 of the periodic table of the elements are arranged in the form of wires.

24. The method according to claim 17, characterized in that the noble metal of groups 9 to 11 of the periodic table of the elements is arranged in the form of a coil.

25. The process according to claim 17, characterized in that the noble metal of groups 9 to 11 of the periodic table of the elements is arranged in the form of a sponge.

26. A process according to claim 12, characterized in that the noble metal of groups 9 to 11 of the periodic table of the elements used is palladium or platinum.

27. A process according to claim 4, wherein the noble metal of groups 9 to 11 of the periodic Table of the elements used is palladium or platinum.

28. A process according to claim 1 or 2, characterised in that the polyol ester is dried at a temperature of 80 to 250 ℃ and a pressure of 0.2 to 500 hPa.

29. A process according to claim 28, characterised in that the polyol ester is dried at a temperature of 100 to 180 ℃ and a pressure of 1 to 200 hPa.

30. A process according to claim 29, characterised in that the polyol ester is dried at a temperature of 100 to 180 ℃ and a pressure of 1 to 20 hPa.

31. A process according to claim 4, characterised in that the polyol ester is dried at a temperature of 80 to 250 ℃ and a pressure of 0.2 to 500 hPa.

32. A process according to claim 1 or 2, characterized in that the residual polyol ester is dried immediately after the steam treatment without further intermediate steps.

33. Process according to claim 4, characterized in that the residual polyol ester is dried immediately after the steam treatment without further intermediate steps.

34. A process according to claim 1 or 2, characterized in that the polyol used is a compound of the formula

H-(-O-[-CR¹R²-]_m-)_o-OH

Wherein R is¹And R²Each independently of the otherIs hydrogen, methyl, ethyl, propyl, or hydroxymethyl, m is an integer from 1 to 8, and o is an integer from 2 to 8.

35. A process according to claim 34, wherein the polyol used is a compound of the formula

H-(-O-[-CR¹R²-]_m-)_o-OH

Wherein R is¹And R²Each independently hydrogen, methyl, ethyl, propyl, or hydroxymethyl, m is an integer of 1, 2, 3, or 4, and o is an integer of 2, 3, 4, or 5.

36. A process according to claim 35, characterized in that the polyol used is ditrimethylolpropane, dipentaerythritol, diethylene glycol, triethylene glycol, tetraethylene glycol, dipropylene glycol, tripropylene glycol or tetrapropylene glycol.

37. A process according to claim 1 or 2, characterized in that the aliphatic monocarboxylic acid converted is propionic acid, n-butyric acid, isobutyric acid, n-valeric acid, 2-methylbutyric acid, 3-methylbutyric acid, 2-methylpentanoic acid, n-hexanoic acid, 2-ethylbutyric acid, n-heptanoic acid, 2-methylhexanoic acid, 2-ethylhexanoic acid, n-nonanoic acid, 2-methyloctanoic acid, isononanoic acid, 3, 5, 5-trimethylhexanoic acid or 2-propylheptanoic acid.

38. The process according to claim 1 or 2, which is used for preparing triethylene glycol di-2-ethylhexanoate, tetraethylene glycol di-n-heptanoate, triethylene glycol di-2-ethylbutyrate, triethylene glycol di-n-heptanoate or tetraethylene glycol di-2-ethylhexanoate.