GB2043663A - Continuous production of polyethers - Google Patents

Abstract

Polyethers having a uniform and predictable molecular weight distribution and distribution of functional groups may be obtained by this continuous process with the same success that has been possible using batch processes. The apparatus which is necessary is simple and very little auxiliary equipment is required, also a less rigourous control system is required. An alkylene oxide, a polyhydric alcohol and a catalyst are passed through a reaction system having at least six sections in series. The catalyst is an alkali metal alkoxide which is preferably dissolved in the alcohol before being passed to the reaction system. The molar ratio of alkylene oxide to alcohol in the first section of the reaction system is from 1:1 to 1:5. The product from this reaction system may be reacted with a different alkylene oxide to produce a block copolymer.

Description

SPECIFICATION Continuous production of polyothers The present invention relates to a process for the continuous production of polyethers which, after purification are useful as a raw material for producing polyurethanes. Polyurethanes have many uses, for example production of polyurethane foams, and as a component of brake fluid, lubricants, demulsifiers and surfactants.

The present invention consists in a process for the continuous production of a polyether, which comprises passing an alkylene oxide, a polyatomic alcohol, and, as catalyst, an alkali metal alkoxide through a reaction system having at least six sections in series; the molar ratio of the alkylene oxide to the polyatomic alcohol in the first section being from 1:1 to 1: 5. The final product from the reaction system may be reacted with further alkylene oxide to produce a block copolymer. The desired product may, of course, be purified.

The catalyst is preferably supplied as a solution in the polyatomic alcohol, and the alkalinity of this solution is preferably from 1 to 20 weight percent (as calculated for the corresponding metal hydroxide) and the water content of this solution is preferably from 0.05 to 0.5 weight percent.

The reaction system preferably contains from 10 to 15, more preferably from 10 to 12, sections. Alkylene oxide may be added only to the first section or it may be added to the first section and one or more other sections. The amount of alkylene oxide added to each section is preferably sufficient to produce an excess of from 25 to 50 weight percent over the stoichiometric amount of alkylene oxide required in that section.

The process of the present invention allows production of a product having a uniform chemical composition in terms of molecular-weight distribution and distribution of various functional groups. A product very similar in these respects to that obtained by batch processes can be obtained by this new continuous process. This standardization of the properties and principle characteristics of the product means that more effective polyurethane formulations can be developed, and this is especially useful for the production of polyurethane foams.

A further advantage of the process of the present invention is that a single multisection piece of apparatus rather than several single pieces of apparatus may be used, and this means that the number of auxiliary pieces of equipment (for example metering pumps, collectors, and condensers) may be reduced. The size of any condensers may also be reduced due to a decrease by a factor of from 4 to 5 in the amount of unreacted alkylene oxide vapours.

The parameters of the process at the initial step and throughout the remainder of the apparatus may be controlled more simply due to the homogeneity of the reaction mixture throughout the entire reaction system. This homogeneity ensures a steady reaction, improves the quality of the final product, and thereby stabilizes the main quality parameters.

The polyatomic alcohol or alcohols used as initial components in the process of the invention may be any of those alcohols usually used in batch production of polyethers for polyurethane. Specific examples of such alcohols include: diethylene glycol, glycerine, trimethylpropane and hexanetriol. Most alcohols that are liquid at the temperature of the reaction may be used, and the particular alcohol chosen will depend on the particular polyether or polyurethane desired. Of the alkylene oxides that may be used ethylene oxide and propylene oxide are preferred; butylene oxide is also used, but more rarely.

The catalyst is one or more alkali metal alkoxide, for example a potassium, sodium and, more rarely, a caesium alkoxide. Preferably, the alkoxides are produced in the alkylene oxide by anionic polymerization of the alkylene oxide. As a result, the alkali metals are introduced into the reaction system as soluble alkoxides in the initial polyatomic alcohol. This solution of alkoxide may be produced by dissolving metallic potassium, sodium or caesium (or other alkali metal) into a chosen polyatomic alcohol, filtering the resulting solution, and removing any water present. An alternative method is to dissolve an alkali metal hydroxide in the polyatomic alcohol, and in this case it will be necessary to remove water formed from the interaction of the hydroxide with the alcohol. This second method is much less hazardous than the first, and it is therefore more widely used in industry.The amount of alkali metal ions is reckoned as weight percent as calculated for the corresponding hydroxide (KOH, NaOH or CsOH). The preferred upper limit for the alkali metal ion concentration (20 weight percent, as calculated for KOH) is determined firstly by a sharp increase in the viscosity of the resulting solution and by resulting difficulties in drying, transporting, storing and metering the solution, and secondly by the maximum amount of alkaline component at which purification and drying of the final mixture is still economic. (Generally, the reaction mixture delivered for purification contains up to 0.5 to 0.8 weight percent of the alkaline component.) The preferred minimum of alkaline component is determined by the efficiency of the polymerization system which is governed by a direct relationship between the anionic polymerization rate and the concentrations of the alkaline component. When the temperature and pressure in the reaction system are similar to those usually employed in anionic polymerizations, it is not economic to use less than one weight per cent of alkaline component (as calculated for the alkali metal hydroxide). This minimum value is really appropriate for potassium hydroxide, and the minimum value for sodium hydroxide is slightly higher owing to the inferior catalytic activity of sodium ions.

The preferred upper limit to the moisture content of the solution (0.5 weight per cent) is determined by the preferred maximum amount of diol in the final product. The maximum acceptable amount of diol will vary depending on the molecular weight of the final product and on its intended use. The preferred minimum moisture content will depend on the efficiency with which the initial solution can be dried and frequently on the type of equipment used. The preferred minimum value is 0.05 weight per cent.

We prefer that the reaction system used to produce polyethers by the continuous method of the invention is a vertical direct-flow bubbling apparatus. Such apparatus may be of known design, and apparatus used for chlorinating hydrocarbons and for other step wise reactions may be used. This type of apparatus is divided into sections by perforated partitions in such a way that vapour, in this case alkylene oxide vapour, bubbling through the reaction mass causes this mass to be stirred intensively in each section and, after redistribution, to pass through the holes in the partition into the adjacent section. This passage of vapour hinders a counter current of reaction mass through the same holes. This type of apparatus, therefore, comprises several cells in which ideal mixing occurs and through which the reaction mass passes in an upward direction.

The intensity of the stirring in each section depends on the geometrical ratio of the section and on the rate of passage of vapours through the section. The geometrical size of a section for similar types of reaction, the ratio of the height to diameter of the section, should, in general, be from 1.5 to 2.5, and this has been found to be suitable for the process of the present invention. The rate of passage of vapours through the section is preferably from 0.5 to 1.5 miser, as calculated for the total cross-section of the apparatus. Alkylene oxide vapours introduced into the first section of the system partly dissolve in the reaction mixture and react with liquid intermediates (semi-products).The vapours then pass through the subsequent sections and the amount of vapour passing into the final or upper section should be from 20 to 50 weight per cent, depending upon the temperature of the vapour and on the grade of polyether desired, in excess of the stoichiometric amount of alkylene oxide required to produce a product of the desired molecular weight throughout the whole volume of the apparatus.

Chemical reactor theory says that a sufficiently short residence time of an ideally stirred liquid phase in a set of cells can only be achieved when the number of cells is six or more, preferably from 10 to 12. In practice, a system having from 12 to 15 cells has been found to be suitable.

The process of the present invention may be carried out in the following way. Vapours of alkylene oxide are introduced continuously into the first section of the reaction system together with a solution of an alkali metal hydroxide in a chosen polyatomic alcohol. Alternatively, some alkylene oxides, for example propylene oxide, may be introduced into the first section as a liquid and allowed to evaporate. The alkalene oxide vapours stir the alkoxide solution, partly dissolve in it, to an extent depending on the temperature and pressure of the system, and then react to form low molecular weight products. These low molecular weight products are mixed with fresh alkoxide solution which is fed continuously to the sections.Since the viscosity of the initial alkoxide solution may be high and since the solubility of alkalene oxide in the alkoxide is very small, the interaction between the two compounds occurs almost entirely at their interface, and propagates into the bulk of the reactants only when product of a certain molecular weight has been produced. At which particular value of molecular weight this happens will depend on the nature of the initial polyatomic alcohol, on the nature of the alkaline component in the alcohol, and on the nature of the alkalene oxide.A laboratory study has shown that the alcohols, diethylene glycol, glycerine or trimethylpropane, together with oxides of propylene or ethylene (in appropriate relative amounts) produce multi component systems containing polyatomic alcohol, alcoholate (two potassium alkoxides were investigated) and low molecular weight products of alkalene oxide plus alcohol. Such systems are sufficiently compatible with the oxides (no less than 3 to 4% of oxide can be incorporated), whereas pure alcohol and, in particular, solutions of potassium alkoxide in the corresponding alcohols form homogeneous systems with the oxides. The oxide content of such homogeneous systems is no more than 0.1 to 0.2 weight per cent over a temperature 40 to 600C, and this is insufficient to produce a suitable rate of reaction.

This accounts for the induction period which can be observed at the beginning of batch methods for producing polyethers. During this induction period no addition of oxide to the alkoxide solution or solid alkali takes place. It has been found that a uniform molecular weight will be obtained when from 1 to 5 moles of an alkalene oxide are added to each mole of polyatomic alcohol. This ratio of reactant ensures homogeneity of the reaction mixture and therefore uniform interaction between the alkalene oxide and polyatomic alcohol (plus alkali metal ions) throughout the whole volume of the first section of the reaction system. This substantially overcomes the problem of maintaining a particulartemperature in the first and neighbouring sections of the system, thereby obviating the need for complex automatic control and stabilization devices.

Particular parameters of the process, which are to be kept at the initial step of the process (which in this case may be the first section of a direct-flow bubbling reactor), must be determined having regard to data on the kinetics of the process. This involves consideration of the kinetics of the chemical reaction itself and of the process of mass transfer which ensures a certain operating concentration of alkalene oxide in the reaction mixture.

The reaction conditions which ensure the addition of from 1 to 5 moles of alkalene oxide to each mole of polyatomic alcohol can be chosen by solving the following two equations. Equation (1 relates to the rate of chemical reaction, Equation (2) to the rate of mass transfer.

Equations dN AO = Kx [AO] [RO] V (1) dt dG AO = F(CAo-C'Ao) (2) dT The symbols in these equations have the following meanings: NAO - number of moles of alkylene oxide; - time; dNAo - rate of feed of alkylene oxide to the reaction system; dT GAO - weight of alkylene oxide; Kx - the second order rate constant of the reaction of addition [OA] - molar concentration of alkylene oxide in the bulk of the liquid phase; [RO-] - molar concentration of active centre to which alkylene oxide adds.This is practically equal to the concentration of the ions of the alkali metal; V - volume of liquid phase for a given section of the apparatus (here we have been considering the first section); p - the mass transfer coefficient for the transition of alkylene oxide from the vapour to the liquid phase; F - the surface area of contact of alkylene oxide vapours with the liquid phase in which the oxide dissolves: CAO - The concentration of alkylene oxide in the liquid phase equilibrium of alkylene oxide vapour over the surface of the liquid at the particular temperature and pressure: and C'AO - the operating concentration of alklene oxide in the liquid phase as determined by the ratio between the rates of mass transfer and consumption of alkalene oxide due to chemical interaction.

The product formed in the first section of the reaction system, together with any excess vapours of alkylene oxide, passes into adjacent section through, for example, holes in a perforated partition. In this adjacent section intensive stirring of the reaction mixture with vapours of alkalene oxide occur throughout the whole volume of the section, and also part of the vapour dissolves in the reaction mixture and thereby interacts with the polyatomic alcohol that is present. This polyatomic alcohol will, of course, have a molecular weight between that of the initial polyatomic alcohol and the final product. This process is then repeated in all of the subsequent sections of the reaction system.Any alkylene oxide vapours leaving the last section of the system (generally the amount of such excess alkylene oxide will be from 25 to 50. of the stoichiometric requirements of alkylene oxide) may then be transferred to a separate condenser tube. The desired fraction of the product leaving the last section of the system, which has a predetermined molecular weight and residual alkylene oxide content (0.05 to 0.1 weight per cent) determined by the average residence time of the liquid in the apparatus and by the operating temperature and pressure, is delivered for purification, filtering and drying. Purification may be carried out using acids or sorbents. Instead of purifying the product at this stage, the product may be delivered into a similar reaction system to that used for the original polymerization in order to react the polymer further.This further reaction may be with the same or a different alkylene oxide to that originally used, to produce a block copolymer. For example, a polyether may be obtained by reacting glycerine and propylene oxide in a first reaction system and then by additional reaction with ethylene oxide in a second reaction system, thereby producing an activated polyether. Such activated polyethers have been widely used recently. In this case an excess of alkylene oxide from the second reaction system is condensed separately, and the reaction mixture containing the block copolymer is sent for purification, filtration and drying.

The invention is further illustrated by the following examples.

Example 1 A potassium glycerate solution with a total alkalinity of 11.6 weight percent as calculated for KOH and a water content of 0.5 weight per cent according to Fischer is fed into the bottom (first) section of the reactor in the amount 0.9 kg/hr. The total alkalinity value in the initial solution is calculated proceeding from the condition that the final product delivered for purification must have the total alkalinity no more than 0.4 wt.% at an average molecular weight of the purified polyether 3200 + 200 in accordance with specifications for a commercial product obtained by the batch process.The reactor has the following parameters: total number of sections - 16 diameter of the section No.1-6 -100 mm No.7-16 -250 mm height of the section - (2.3 - 2.4) of the diameter surface area of the clear opening of the holes in perforated dividing partitions - 1.0 - 1.2% of the total cross-section surface area of the section.

Propylene oxide is fed in the form of overheated vapours under the distributing grate of the first section in amounts 12.5 kg/hr and into the 7-th section additional amount 41.5 kg/hr is delivered. During the process a temperature of 115 C (in sections 1 - 6) and 1200C (in sections 7 - 16) are maintained and a pressure up to 0.1 at.ga. is kept at the exit of the alkylene oxide vapours from the apparatus. The pressure in the first section was 1.0 - 1.1 at.ga. and remained constant during the experiment. The conditions selected for the first section of the apparatus (the size of the section and parameters of the initial potassium glycerate solution being given) ensure the addition to 1 mole of glycerine of about 4 moles of propylene oxide. An average molecular weight of the intermediate product formed is 320.After purification, filtration and drying the product has the following parameters: molecular weight (with respect to hydroxyl groups as calculated for trion) MoH - 3,400 OH-group content - 1.5 wt.% iodine number -1.1 9 of iodine 100 g polydispersity coefficient MW = 1.03 Mn content of monools - 3 - 4 wt.% content of diols - 6 - 7wt.%.

Distribution with respect to functionality was determined by adsorption chromatography on alumina; distribution with respect to molecular weights by the method of gel-penetrating chromatography.

The product obtained from the similar raw material in the batch apparatus has MOH = 3,200, MW/Mn = 1.02 with monool content 3 - 5 wt.% and diol content 5 - 7 wt.%.

The conditions of processing and properties of flexible slabstock polyurethane foams obtained on the basis of both polyethers were almost identical.

Example 2 Potassium glycerate solution in glycerin is fed into the first section of the reactor, similar to that described in Example 1, at a flow rate of 0.78 kg/hr. Total alkalinity of the solution is 11.7 wt.% as calculated for KOH; water content is 0.39 wt.% according to Fischer.

Ethylene oxide vapours are fed at a flow rate of 5.9 kg/hr under the distributing grate of the first section. A temperature of 85 t 5"C is kept in all sections of the reactor; a pressure at the exit of an excess of ethylene oxide vapours is 0.03 at.ga. In the first section of a homogeneous reaction mixture is formed with an average molecular weight corresponding to the addition of about 3 moles of ethylene oxide to 1 mole of glycerin. A semi-product (alkalinity 2.2 wt.% as calculated for KOH and MOH 560) is discharged from the 6-th section of the apparatus and delivered into the first section of the second apparatus of the similar construction, under the distributing grate of the section the propylene oxide vapours being introduced.At a temperature of 118 + 2"C and a pressure at the exit of an excess of propylene oxide vapours 0.03 - 0.04 at.ga. an alkaline product is obtained of the total alkalinity 0.33 wt.% containing blockpolymer. The parameters of the product after purification, filtration, and drying are as follows: content of OH-groups - 1.64 wt.% -3100 iodine number 1 3 9 of iodine 100 g The parameters characterizing the degree of polydispersity of the product and distribution with respect to the types of functionality were similar to those of the product obtained by the following procedure described in Example 1. The product thus obtained was processed into flexible slabstock polyurethane foam under standard conditions.

Example 3 A solution of potassium alkoxide in glycerin of the total alkalinity 11 wt.% and water content 3.4 wt.% (without distilling off the water from the solution of solid alkali in glycerin) was used under the conditions similar to those described in Example 1. An alkaline product has an alkalinity of about 0.33 wt.% but a lower molecular weight has calculated fortriol (MoH = 2900) since the content of diols was 17-20 wt.%, the content of monools being the same (about 3 wt.%).

Example 4 A solution of potassium glycerate in glycerin with total alkalinity 19.7 wt.% (as calculated for KOH) and water content 0.4wt.% (according to Fischer) is fed in amounts 0.84 kg/hr into the first (bottom) section of the apparatus similar to that described in Example 1. Under the distributing grate of the section the vapours of propylene oxide are introduced in the amounts 24.5 kg/hr.After filling the bottom (No. 1 -7) sections of the apparatus at a temperature of 115 + 2"C and a pressure in the first section 0.8 - 0.9 at.ga., additional amount 45.5 kglhr of propylene oxide are introduced into the 7-th section (in the form of vapours); the process is run upon keeping in the upper sections (No.8 - 16) a temperature of 120 t 1"C and a pressure of 0.05 - 0.07 at.ga at the exit of the excess of propylene oxide vapours.An alkaline product leaving the 16-th section has an alkalinity 0.42 - 0.45 wt.% (as calculated for KOH) and an average molecular weight 4500 - 4600 (the capacity of the apparatus with respect to this intermediate product is about 40 kg/hr) is delivered to the section reactor (for obtaining block-copolymer with ethylene oxide) operating at 90 + 2"C; under the distributing grate of the first section of the second reactor the vapours of ethylene oxide are fed (7.2 kg/hr). An alkaline product with alkaline 0.35 - 0.36 wt.% is discharged from the 6-th section of the apparatus in the amount about 45 kg/hr.

After purification, filtration, and drying the product has the following parameters: OH groups content - 1.04 wt.% MoH (as calculated fortriol) -4900 iodine number - 1.7 g of iodine 100g The polyetherthus obtained was processed into flexible cold molding polyurethane foam.

An attempt to use a solution of potassium alkoxide in glycerine with a KOH content of 20.8 wt.%, in order to obtain polyether with an average molecular weight 5500 was unsuccessful since it was difficult to meter the solution (it was required to heat the reservoir, pipelines, and pump housing up to 85 - 90"C) and impossible to start the apparatus because of a high viscosity of the liquid which hindered the stirring with vapours of propylene oxide, heating up to operating temperature, and mass transfer of the oxide from a vapour to liquid phase. However, it should be noted that the use of elkoxide solutions with an elevated content of an alkaline agent, for example upon producing polyethers of high molecular weight is not typical of the batch process coholation.

Example 5 A solution of potassium alkoxide in diethyleneglycol (0.6 kg/hr) with a total alkalinity 1.2 wt.% as calculated for KOH is introduced into the first section of the apparatus similar to that described in Example 1; under the distributing grate of this section the vapours of propylene oxide in amounts 12.5 kg/hr are fed. 2.8 kg/hr of the product with alkalinity 0.27 wt.% (as calculated for KOH) are discharged from the 6-th section of the apparatus, the temperature in the sections being 120 + 1"C. In the first section 1 mole of propylene oxide is added to 1 mole of diethyleneglycol.After purification, filtration, and drying, polyether has the following parameters: MOH -270 iodine number . - 0.6 g of iodine 100g polydespersity coefficient, Mw/Mn - 1.03 The product is used in formulations for manufacturing rigid polyurethane foams which have the properties similar to those of polyurethane foams obtained from the product produced by the batch process.

Example 6 A solution of sodium glycerate in glycerin with a total alkalinity 11.5 wt.% (as calculated for NaOH) and water content 0.38 wt.% (according to Fischer) is used under the conditions similar to those described in Example 1. The amount of propylene oxide vapours delivered into the 7-th section is 20.5 kg/hr. The product discharged from the 16-th section has the following parameters after purification, filtration, and drying: MOH -2300 iodine number - 1.05 polydispersity coefficient, MwIMn - 1.027 Example 7 A solution of potassium glycerate in glycerin with water content 0.054 wt.% (according to Fischer) was used under the conditions similar to those described in Example 1.

The operation of the apparatus was not changed; the capacity slightly increased which may also be due to the accuracy of keeping the temperature. After purification, filtration, and drying the product has the following parameters: MOH -3560 content of OH-groups - 1.43 wt.% g of iodine iodine number -1.12 g of iodine 100g polydispersity coefficient, MwiMn - 1.026 content of monools - 3 - 4 wt.% content of diols - 5 - 6 wt.%.

As is seen, the content of diols in the final product is somewhat decreased, but the conditions of purification and the presence of water (even within the limits accepted for the high grade propylene oxide) in the initial propylene oxide do not allow for producing the product with the diol content less than 4 - 5 wt.%.

More careful drying of the alkoxide solution also encounters difficulties such as elevated temperature and deep vacuum which are not always obtainable under industrial conditions and adversely affect the properties of the polyatomic alcohol. It should be noted that modern requirements imposed upon the quality of polyethers admit the presence of diols (and monools) in triols in amounts attained when performing the batch and the proposed continuous processes.

Claims (12)

1. A process for the continuous production of a polyether, which comprises passing an alkylene oxide, a polyatomic alcohol, and, as catalyst, an alkali metal alkoxide through a reaction system having at least six sections in series; the molar ratio of the alkylene oxide to polyatomic alcohol in the first section being from 1 :1 to 1: 5.

2. A process according to Claim 1, in which the final product from the reaction system is reacted with an alkylene oxide to produce a block copolymer.

3. A process according to Claim 2, in which the alkylene oxide used to produce the final product and the alkylene oxide reacted with the final product to produce the block copolymer are different alkylene oxides.

4. A process according to any one of the preceding Claims, in which the final product from the reaction system, or the block copolymer is purified.

5. A process according to any one of the preceding Claims, in which the alkali metal alkoxide is passed to the reaction system dissolved in the polyatomic alcohol.

6. A process according to Claim 5, in which the solution of the alkoxide in the alcohol has an alkalinity of from 1 to 20 weight per cent, as calculated for the corresponding alkali metal hydroxide, and a water content of from 0.05 weight per cent.

7. A process according to any one of the preceding Claims, in which the alkalene oxide, polyatomic alcohol and alkali metal hydroxide are passed through a vertical direct-flow bubble column.

8. A process according to any one of the preceding Claims, in which the number of sections in the reaction system is from 10 to 15.

9. A process according to Claim 8, in which the number of sections in the reaction system is from 10 to 12.

10. A process according to any one of the preceding Claims, in which the alkylene oxide is added to at least one section of the reaction system such that the amount of alkylene oxide in that or each of those sections is a 25 to 50 weight per cent excess of the stoichiometric requirement of alkylene oxide.

11. A process according to Claim 1, substantially as herein described with reference to any one of the foregoing examples.

12. A polyether when produced by a process according to any one of the preceding Claims.