GB1579695A - Production of polyesters - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE PRODUCTION OF POLYESTERS (71) We, KOPPERS COMPANY, INC., a corporation organised under the laws of the State of Delaware, one of the United States of America, of the Koppers Building, City of Pittsburgh, Commonwealth of Pennsylvania, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to the production of polyesters from isophthalic acid or terephthalic acid, a polyol, and an aliphatic polycarboxylic acid, When isophthalic acid or terephthalic acid was substituted for phthalic anhydride in the production of polyesters, it was found that the one-stage process used with phthalic anhydride was not as effective with isophthalic acid or terephthalic acid. Therefore, there were developed two-stage processes in which in the first stage the isophthalic acid or terephthalic acid is reacted with a polyol to produce a half-ester mixture, and in the second stage the half-ester mixture is reacted with an aliphatic polycarboxylic acid to produce the desired polyester. The two-stage processes endeavour to increase the effectiveness of using isophthalic acid or terephthalic acid as compared with a one-stage process. However, a two stage process suffers from a serious disability as compared with a one-stage process; more particularly, a two-stage process takes much longer to complete than does a one-stage process.

Various processes have been developed with the aim of overcoming this disadvantage, see for example U.S. Patent Specifications Nos. 2,904,533; 3,196,131; and 3,345,339, but there is still room for further improvement.

According to the present invention there is provided a method of preparing a half-ester mixture which method comprises: (a) contacting isophthalic acid or terephthalic acid with a first portion of a polyol in an amount at least sufficient to form with the isophthalic acid or terephthalic acid an agitatable mixture; (b) heating the agitatable mixture with agitation to raise the temperature of said agitatable mixture to at least 1900C; and (c) thereafter adding the remaining portion of the polyol to the heated, agitatable mixture in a manner such that the heated, agitatable mixture is maintained at a temperature of at least 1900C during said addition, thereby producing the desired half-ester mixture.

The present invention also provides a process for the production of a polyester which process comprises a first stage in which a half-ester mixture is prepared by the method of the invention, and a second stage in which the half-ester mixture is reacted with a polycarboxylic acid to produce the desired polyester.

Herein, the term "polyester" is used to describe the polymer product wherein both of the carboxylic acid groups of isophthalic acid or terephthalic acid and the carboxylic acid groups of an aliphatic polycarboxylic acid have reacted with a polyol to form a polymer; the term "half-ester mixture" is used to describe the mixture obtained upon the reaction of isophthalic acid or terephthalic acid with the polyol, this mixture consisting mainly of the ester wherein both of the carboxylic acid groups of isophthalic acid or terephthalic acid have reacted with the polyol although the mixture may also contain some of the monoester product, formed by esterification of one of the carboxylic acid groups of the isophthalic acid or terephthalic acid and the polyol, some free polyol and of other further reacted compounds; the term "total amount of polyol" is used to mean an amount of polyol which is at least the theoretical amount (which is one equivalent of polyol for each equivalent of acid required to produce the polyester) and is usually, although not essentially, not more than 125 percent of the theoretical amount since the use of more than this excess reduces the effectiveness of the process; the term "agitatable mixture" is used to mean a mixture formed from all or a major part of the isophthalic acid or terephthalic acid, and a portion of the polyol which is sufficient to allow mixing in the equipment used in the production of polyesters from isophthalic acid or terephthalic acid (the agitatable mixture of the isophthalic acid or terephthalic acid and the portion of the total amount of polyol must be mixed in order to provide even temperatures throughout the mixture); and the term "acid" is used to include the corresponding anhydrides where such anhydrides exist.

The present invention provides an improvement in the first stage of a two-stage process for the production of a polyester from isophthalic acid or terephthalic acid, a polyol, and a saturated or unsaturated aliphatic polycarboxylic acid. As outlined hereinbefore, in the first stage of a two-stage process, the isophthalic or terephthalic acid is reacted with at least a theoretical amount of polyol to form a half-ester mixture from the isophthalic acid or terephthalic acid and polyol; and in the second stage this half-ester mixture is reacted with the saturated or unsaturated aliphatic polycarboxylic acid to form the polyester. The theoretical amount of polyol charged to the reactor in the first stage is approximately one equivalent of polyol for each equivalent of acid needed to produce the polyester. The improvement in the first stage of the process, embodying the method of the present invention, is to decrease the initial portion of polyol mixed with the isophthalic acid or terephthalic acid and to heat this mixture until the temperature of the mixture is at least 1900C., and then to add the remaining portion of the polyol to this heated mixture at a rate such that the temperature does not drop below 1900C. More particularly, in the first stage a portion of the total amount of polyol is contacted with all or a major part of the isophthalic acid or terephthalic acid, said portion of the polyol being sufficient to make an agitatable mixture with the isophthalic acid or terephthalic acid, the agitatable mixture is heated with agitation to raise the temperature thereof to at least 1900C., at which temperature the reaction of isophthalic acid or terephthalic acid with the polyol to form a half-ester mixture proceeds at a satisfactory rate, and the remaining portion of the total amount of polyol is then added to the heated, agitatable mixture in a manner such that the temperature of the heated, agitatable mixture remains at or above 1900C. whereby a half-ester mixture is obtained.

In the preparation of the half-ester mixture, an esterification catalyst, or initiator, may be added to the reaction mixture in order to reduce further the processing time. Generally, the catalyst can be any tin, lead or lithium catalyst or initiator or any other catalyst or initiator known in the art to catalyze or initiate the reaction of isophthalic acid or terephthalic acid and polyols. The reaction of the remaining portion of polyol with the agitatable mixture may be accomplished by adding the remaining portion continuously or incrementally. Also, the remaining portion of polyol may be heated and added to the heated agitatable mixture.

All the isophthalic acid or terephthalic acid need not be reacted initially with the first portion of the total amount of polyol. A major part of the isophthalic acid or terephthalic acid should be reacted initially and the remaining part of the isophthalic acid or terephthalic acid should be reacted before the remaining portion of the total amount of polyol is added to the heated agitatable mixture which already contains an amount of the half-ester.

The agitatable mixture is heated at autogenous pressure to a temperature of at least 1 900C.

and maintained at least at this temperature while the remaining portion of polyol is added.

However, the temperature can be as much above 1900C. as the esterification equipment will allow but the polyester resin produced may have undesired characteristics if the temperature is too high. Therefore, the temperature should preferably not be above 230"C. Most preferably, the temperature of the reaction mixture is maintained in the range of 195"C. to 210 C.

If a pressure other than autogenous pressure is used the temperature should still be at least 1900C.

The main problem in reducing the processing time in a two-stage process for the production of a polyester from isophthalic acid or terephthalic acid, a glycol, and a saturated or unsaturated aliphatic polycarboxylic acid, arises because of the first stage reaction rate. This esterification reaction proceeds very slowly until an adequate reaction temperature is obtained. Broadly, the reaction should be conducted at a temperature of at least 1900C., and preferably at a temperature of at least 1950C. The attainment of the desired reaction temperature appears to be limited by the boiling point of the reaction mixture when all of the total amount of polyol is contacted at once with all the isophthalic acid or terephthalic acid. A reaction mixture comprising a portion of the total amount of polyol and all or a major part of the isophthalic acid or terephthalic acid achieves the desired reaction temperature more quickly.

The total amount of polyol is commonly in the range of from about 105 to 110 percent of the theoretical amount needed. The first portion of the total amount of polyol used is at least an amount which, together with all or a major part of the isophthalic acid or terephthalic acid forms an agitatable mixture. The maximum amount of polyol used as the first portion should be slightly less than the theoretical amount of polyol needed to react with the isophthalic acid or terephthalic acid and saturated or unsaturated aliphatic polycarboxylic acid to form the polyester. The closer the first portion comes to this theoretical amount, the smaller the reduction in processing time. The amount of polyol constituting the first portion of polyol is preferably in the range of from 0.5 to 1.7 moles per mole of isophthalic acid or terephthalic acid. Also, a solvent can be used to make the mixture of isophthalic acid or terephthalic acid more mobile. If a solvent is used, it is preferably a non-reactive solvent which forms an azeotrope with water. Typical of such a solvent is xylene.

The amount bf isophthalic acid or terephthalic acid with which the first portion of polyol is contacted need not be all of the isophthalic acid or terephthalic acid that is going to be reacted. The amount of isophthalic acid or terephthalic acid used need be only a major part of the total amount of isophthalic acid to be reacted. This major part is that amount which will form an agitatable mixture with amounts of polyol that are less than the theoretical amount.

This major part is in excess of 50 percent by weight of the total amount of isophthalic acid or terephthalic acid to be used, said total amount depending on the formulation for the particular polyester desired.

The agitatable mixture of isophthalic acid or terephthalic acid and first portion of polyol is heated, while being agitated, to a temperature of at least 1900C. and preferably to a temperature in the range of 195-210"C. While the agitatable mixture is at this temperature, the remaining portion of the total amount of polyol is added to the mixture in such a manner as not to reduce the temperature below 1900C. and preferably not below 195"C. This may be performed by adding the remaining portion of polyol at such a rate that the temperature is not decreased. The rate of addition may be accelerated by heating the remaining portion of polyol, before it is added to the heated, agitatable mixture, to a temperature above ambient temperature but below the boiling temperature of the polyol. The agitatable reaction mixture containing the total amount of polyol continues to react to produce a half-ester mixture. The esterification reaction takes place as soon as the first portion of polyol is contacted with the isophthalic acid or terephthalic acid and the resulting agitatable mixture is heated to an adequate reaction temperature of at least 190"C. The esterification reaction continues until the reaction is complete as indicated by the acid number of the half-ester mixture. If the final desired acid number of the half-ester mixture is reached before all of the remaining portion of polyol is added, the excess polyol that is left may be used as a coolant, to aid in cooling the half-ester mixture before the addition of the aliphatic polycarboxylic acid. The half-ester mixture produced has an acid number of less than 90 milligrams of potassium hydroxide to neutralize 1 gram of the half-ester mixture.

In the process for producing a polyester, a half-ester mixture produced by the method of the invention is reacted with an aliphatic polycarboxylic acid in the second stage of a two-stage process. The aliphatic polycarboxylic acid may be a saturated acid or an unsaturated acid, but is preferably an unsaturated acid. This second stage can be conducted in any manner known the those skilled in the art of two-stage polyester production processes. The product from the second stage is the polyester having a desired acid number.

The polyester produced by the process of this invention can be prepared, for example, from those polyols or a mixture of those polyols utilized in conventional processes. Such polyols include ethylene glycol, propylene glycol, butylene glycol, diethylene glycol, dipropylene glycol, triethylene glycol, neopentyl glycol, trimethylene glycol, polyethylene glycol, polypropylene glycol, 1,6-hexanediol, 1,5-pentanediol, trimethylolethane, trimethylolpropane, glycerol, 1 ,2,6,-hexanetriol, pentaerythritol, sorbitol, mannitol, and methyl glycoside. Other polyols, including unsaturated polyols, may be used alone or in addition to the above mentioned polyols. The foregoing list of polyols is intended to be exemplary of the polyols that may be used, and the list is not exhaustive. Typically, the total amount of polyol up to 125% of theoretical is used, but even greater excesses may be utilized if desired.

The aliphatic polycarboxylic acid used in the production of the polyester by the improved process of this invention may be, for example, any of the polycarboxylic acids conventionally used. Such conventional polycarboxylic acids include maleic acid, fumaric acid, itaconic acid, citraconic acid, glutaconic acid and mesaconic acid. Other polycarboxylic acids, including saturated polycarboxylic acids, may be used in addition to the above-mentioned acids.

The polyester produced by the process of this invention may be obtained with an even greater reduction in processing time when an esterification catalyst or mixture of esterification catalysts is used in the first stage. Catalysts known by those skilled in the art to be effective in the esterification reaction of the first stage include inorganic salts and organic compounds of tin, lead or lithium. Examples of suitable inorganic tin salts include stannous salts, for example stannous halides, such as stannous bromide, stannous chloride, stannous fluoride, stannous iodide and stannous oxychloride, stannous hydroxide, stannous sulphate, stannous oxide and stannous acetate; and stannic salts, for example stannic halides such as stannic chloride, stannic bromide, stannic fluoride and stannic oxychloride. Examples of suitable organic tin compounds include dialkyltin salts of carboxylic acids, for example dibutyltin diacetate, dibutyltin dilaurate and dibutyltin maleate; dialkyltin chlorides, for example dibutyltin chloride; and dialkyktin oxides, for example dibutyltin oxide, and dilauryltin oxide; and trialkyltin hydroxides, for example tributyltin hydroxide and trimethyltin hydroxide. These catalysts are effective in amounts ranging from about 0.01 percent by weight to about 2.0 percent by weight of the total weight of the charge in the first stage.

Higher quantities of catalysts can be used if desired. Other catalysts known to those skilled in the art that may be used in the improved process of this invention are tetrabutyl zirconium and zirconium naphthenate..

In a preferred embodiment of the method and process of this invention there is used isophthalic acid (and not terephthalic acid), maleic acid, and propylene glycol. The total amount of propylene glycol used is the theoretical amount of around 2 moles of propylene glycol per mole of isophthalic acid and maleic acid. Advantageously, there is used a catalyst which preferably is dibutyltin oxide used in an amount of around 0.2 percent by weight, based on the total weight charged to the first stage. The production of the polyester is conducted in the presence of an inert gas at atmospheric pressure. All of the isophthalic acid is added to a reaction vessel suitable for carrying out condensation reactions. This vessel may be constructed of stainless steel or it may be a glass-lined kettle which is insulated to prevent heat loss. The vessel should be equipped with an agitator, heating and cooling coils, a temperature mcasuring device, vapour removal equipment for continuous removal of water of esterification, and it may include vapour columns designed to minimise loss of low boiling polyols. The vessel also contains an inert gas inlet for conducting the reaction in an inert atmosphere of nitrogen. The first portion of the total amount of propylene glycol is added to this vessel which contains all the isophthalic acid to be reacted. The amount of propylene glycol in this first portion is in the range of from 0.8 - 1.4 moles per mole of isophthalic acid, and most preferably is in the range of 0.9 - 1.0 mole per mole of isophthalic acid. The resulting mixture is quite thick but it can be agitated and it becomes more fluid as its temperature increases.

This agitatable mixture of isophthalic acid and the first portion of propylene glycol is agitated and heated until its temperature is at least 1900C. and preferably a temperature in the range of 195"C. - 210 C. at autogenous pressure. This temperature is quickly and easily obtained. When the temperature reaches about 198"C. the remaining portion of the total amount of propylene glycol is added to the mixture of isophthalic acid and the first portion of propylene glycol. This addition is at the fastest practical rate while maintaining a minimum temperature of 1900C. and preferably 195"C. Upon completion of the addition of the remaining portion of propylene glycol, the first stage reaction is continued to completion which is indicated by the acid number of the product. The product of the first stage is a half-ester mixture of isophthalic acid and propylene glycol which has an acid number of less than 90 milligrams and is preferably in the range of 20 - 90 milligrams of potassium hydroxide to neutralize 1 gram of half-ester mixture.

The vessel containing the half-ester mixture is cooled to around 1500C. and maleic anhydride is added. The vessel is heated to a temperature of around 200"C. and held at this temperature for the duration of the polyesterification reaction. The duration of this reaction depcnds on the desired acid number in the final product. After the desired acid number is obtained, the reactor is cooled. The product from the second stage is the unsaturated polyester.

The unsaturated polyesters produced by the improved process of this invention can be thinned in a suitable reactive monomer in a manner known to those skilled in the art.

For a better understanding of the invention, reference should be had to the following Examples: EXAMPLE 1 (COMPARISON) For the purposes of comparison, an unsaturated polyester was prepared from isophthalic acid, propylene glycol and maleic anhydride according to a conventional two-stage process.

All of the propylene glycol, which was the theoretical amount of two moles of propylene glycol to one mole of isophthalic acid (taking into account the one mole of maleic anhydride to ba added to form the polyester) was added to the isophthalic acid. In other words, a weight percent amount of 36.5% of propylene glycol was added to 39.9 wt. % of isophthalic acid, based on a total weight of the components, for the final unsaturated polyester. This mixture was heated to a reaction temperature of 200"C. and held until the acid number of the half-ester mixture produced decreased to 25-30 milligrams. The first stage processing time was 11-1/4 hours. The mixture was cooled and 23.6 wt %ofmaleic anhydride was added to produce the unsaturated polyester. The yield was 96.7% of the theoretical yield.

EXAMPLE 2 A first portion of 7,310 kg of propylene glycol was added to 11,402 kg of isophthalic acid.

This gave a mixture having a 1.4/ 1.0 molar ratio of propylene glycol to isophthalic acid. This mixture was agitated and heated to 1980C. The remaining portion of propylene glycol, viz 3,132 kg equivalent to 0.6 moles per mole of isophthalic acid, was added to the heated, agitatable mixture over a period of 2-3/4 hours, thus holding the reaction temperature at 198"C. The reaction was continued until the half-ester mixture had an acid number of 25 to 30 milligrams at 2000C. without the use of catalyst. The half-ester mixture which contained the esterification product of propylene glycol and isophthalic acid was cooled.

The first stage processing time was 7-1/2 hours. This is a processing time saving of 40% compared to the conventional process described in Example 1.

Maleic anhydride in an amount of 6,732 kg was added to the half-ester mixture. This combination of half-ester mixture and maleic anhydride was heated to a temperature of around 200"C. This temperature was maintained until the desired acid number of the unsaturated polyester was achieved. Then the temperature was decreased and the product, an unsaturated polyester resin, was.removed.

EXAMPLE 3 A first portion of propylene glycol, equalling 4,614 kg, was added to 7,279 kg of isophthalic acid. The resulting mixture had a molar ratio of 1.4/1.0 of propylene glycol to isophthalic acid. 57 kg or 0.2 wt % of dibutyl tin oxide, based on the total weight of components in the charge to the first stage, was added to the mixture. The mixture was agitated and heated to 195"C. at which temperature the mixture became more fluid. An additional amount of 3,919 kg of isophthalic acid was then added to the mixture. This addition brought the molar ratio of propylene glycol to isophthalic acid to 0.9/1.0 in the mixture. The mixture was agitated and heated to 198"C and the balance of the propylene glycol, 6,152 kg (1.20 moles/mole of isophthalic acid), was added over a period of 2-3/4 hours thus maintaining the reaction temperature at 1980C. This addition of the balance of the propylene glycol was at a rate such that the temperature of the reaction did not fall below 198"C, and was achieved by dribbling the remaining portion of the propylene glycol into the heated agitated mixture from a weigh tank. After the addition of all of the balance of propylene glycol the mixture was held at 200"C. until the acid number decreased to 25 to 30 milligrams and the half-ester mixture was cooled to a temperature of around 1500C. The first stage processing time was 7-1/2 hours.

Maleic anhydride in an amount of 6,732 kg was added to the half-ester mixture and the combination of half-ester mixture and maleic anhydride was heated to a temperature of around 200"C. The temperature was held until the desired acid number product was obtained. Then the reactor vessel was cooled and the unsaturated polyester removed.

EXAMPLE 4 Table 1 below summarises the first stage processing time results of several runs conducted by the two-step polyol addition of the process of this invention, and by a conventional two-stage process for the purpose of comparison. The conventional runs were conducted in a manner similar to that described in Example 1 with a few of the conventional runs being performed with a dibutyl tin oxide catalyst. The runs carried out according to the process of this invention were conducted in a manner similar to that described in Example 2, some with and some without the use of a catalyst. The results showed that the process of this invention, even without the use of a catalyst, improves the processing time of a two-stage process more than the use of a catalyst in a conventional two-stage process.

Table I Processing Time Comparison 5S Wt. Processing Time Run No. Process Used DBTO* First Stage hrs 1 Conventional None 12 2 Conventional None 11 3 Conventional None 9 4 Conventional None 9 5 2-Step Glycol Addition None 7-1/2 6 Conventional 0.1 10 7 2-Step Glycol Addition 0.1 6 8 Conventional 0.2 10 9 2-Step Glycol Addition 0.2 4 10 2-Step Glycol Addition 0.2 4 * Dibutyltin Oxide WHAT WE CLAIM IS: 1. A method of preparing a half-ester mixture which method comprises: (a) contacting isophthalic acid or terephthalic acid with a first portion of a polyol in an amount at least sufficient to form with the isophthalic acid or terephthalic acid an agitatible mixture; (b) heating the agitatable mixture with agitation to raise the temperature of said agitatable mixture to at least 1900C; and (c) thereafter adding the remaining portion of the polyol to the heated, agitatable mixture in a manner such that the heated. agitatable mixture is maintained at a temperature of at least 1900C. during said addition. thereby producing the desired half-ester mixture.

2. A method according to claim 1, wherein an esterification catalyst is added to the agitatable mixture.

3. A method according to claim 2 wherein said esterification catalyst is selected from inorganic salts and organic compounds of tin. lead or lithium, mixtures of said salts and compounds, tetrabutyl zirconatc, and zirconium naphthenate.

4. A method according to claim 3 wherein the esterification catalyst is dibutyltin oxide which is used in an amount of from 0.01 to 2.0 percent by weight, based on the total weight of the agitatable mixture.

5. A method according to claim 1, 2, 3 or 4 wherein the polyol is selected from ethylene glycol, propylene glycol, butylene glycol. diethylene glycol, dipropylene glycol, triethylene glycol, ncopentyl glycol, trimethylene glycol, polyethylene glycol and polypropylene glycol.

6. A method according to claim 1, 2. 3, 4 or 5 wherein the amount of polyol present in said first portion of polyol is in the range of from 0.5 to 1.7 moles of polyol per mole of isophthalic acid or terephthalic acid.

7. A method according to claim 6, wherein the amount of polyol present in said first portion of polyol is in the range of from 0.8 to 1.4 moles of polyol per mole of isophthalic acid or terephthalic acid.

8. A method according to any one of claims 1 to 7, wherein the agitatable mixture is heated until its temperature is in the range of from 195"C. to 210"C.

9. A method according to any of claims 1 to 8. wherein said remaining portion of polyol is

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. Table I Processing Time Comparison 5S Wt. Processing Time Run No. Process Used DBTO* First Stage hrs

1 Conventional None 12

2 Conventional None 11

3 Conventional None 9

4 Conventional None 9

5 2-Step Glycol Addition None 7-1/2

6 Conventional 0.1 10

7 2-Step Glycol Addition 0.1 6

8 Conventional 0.2 10

9 2-Step Glycol Addition 0.2 4

10 2-Step Glycol Addition 0.2 4 * Dibutyltin Oxide WHAT WE CLAIM IS: 1. A method of preparing a half-ester mixture which method comprises: (a) contacting isophthalic acid or terephthalic acid with a first portion of a polyol in an amount at least sufficient to form with the isophthalic acid or terephthalic acid an agitatible mixture; (b) heating the agitatable mixture with agitation to raise the temperature of said agitatable mixture to at least 1900C; and (c) thereafter adding the remaining portion of the polyol to the heated, agitatable mixture in a manner such that the heated. agitatable mixture is maintained at a temperature of at least 1900C. during said addition. thereby producing the desired half-ester mixture.

2. A method according to claim 1, wherein an esterification catalyst is added to the agitatable mixture.

3. A method according to claim 2 wherein said esterification catalyst is selected from inorganic salts and organic compounds of tin. lead or lithium, mixtures of said salts and compounds, tetrabutyl zirconatc, and zirconium naphthenate.

4. A method according to claim 3 wherein the esterification catalyst is dibutyltin oxide which is used in an amount of from 0.01 to 2.0 percent by weight, based on the total weight of the agitatable mixture.

5. A method according to claim 1, 2, 3 or 4 wherein the polyol is selected from ethylene glycol, propylene glycol, butylene glycol. diethylene glycol, dipropylene glycol, triethylene glycol, ncopentyl glycol, trimethylene glycol, polyethylene glycol and polypropylene glycol.

6. A method according to claim 1, 2. 3, 4 or 5 wherein the amount of polyol present in said first portion of polyol is in the range of from 0.5 to 1.7 moles of polyol per mole of isophthalic acid or terephthalic acid.

7. A method according to claim 6, wherein the amount of polyol present in said first portion of polyol is in the range of from 0.8 to 1.4 moles of polyol per mole of isophthalic acid or terephthalic acid.

8. A method according to any one of claims 1 to 7, wherein the agitatable mixture is heated until its temperature is in the range of from 195"C. to 210"C.

9. A method according to any of claims 1 to 8. wherein said remaining portion of polyol is

heated to a temperature less than the boiling point of the polyol before being added to the heated, agitatable mixture.

10. A method according to any one of claims 1 to 9 wherein the first portion of polyol is contacted with a major part of the isophthalic acid or terephthalic acid and the mixture thus formed is heated before the remaining part of the isophthalic acid or terephthalic acid is added thereto.

11. A method according to claim 1, which comprises (a) contacting the isophthalic acid or terephthalic acid with a first portion of the polyol in an amount which is in the range of from 0.8 to 1.4 moles of polyol per mole of isophthalic acid or terephthalie acid whereby an agitatable mixture is formed; (b) heating the agitatable mixture with agitation to a temperature of at least 1900 C. in the presence of an esterification catalyst to bring about the reaction of the isophthalic acid or terehthalic acid and the polyol; and (c) adding the remaining portion of the polyol to the heated agitatable mixture in a manner such that the heated, agitatable mixture is maintained at a temperature of at least 1900C. whereby the desired half-ester mixture is produced.

12. A method for producing a half-ester mixture, substantially as described in either one of the foregoing Examples 2 and 3 or in Example 4 with reference to any one of Runs 5, 7, 9 and 10 set out in the foregoing table 1.

13. A half-ester mixture whenever produced by the method claimed in any one of the preceding claims.

14. A process for the production of a polyester which process comprises a first stage in which a half-ester mixture is prepared by a method as claimed in any one of the preceding claims 1 to 12 and a second stage in which the half-ester mixture is reacted with a saturated or unsaturated aliphatic polycarboxylic acid to produce the polyester.

15. A process according to claim 14, wherein, in the second stage, the aliphatic polycarboxylic acid is maleic acid or the anhydride thereof, fumaric acid or the anhydride thereof, itaconic acid or the anhydride thereof, citraconic acid or the anhydride thereof, glutaconic acid or the anhydride thereof, or mesaconic acid or the anhydride thereof.

16. A process according to claim 14 for the production of a polyester, substantially as hereinbefore described.

17. A polyester whenever produced by the process claimed in any one of claims 14, 15 and 16.