US3624043A - Method for the preparation and use of a catalyst for the production of polyesters, more particularly high molecular weight linear polyesters and catalyst thus obtained - Google Patents

Abstract

Method for the preparation of finely and evenly divided elemental antimony for use as catalyst in polyester manufacturing, wherein powdered antimony is subjected to mechanical size reduction operations, and then is dispersed in a liquid medium.

Description

United States Patent 72 Inventors Francesco Slclarl Cesano Maderno; Giuseppe Messina, Llmbiate; Edgardo Honk, Bnrlassina, all of Italy [2| Appl. No. 784,248

[22] Filed Dec. I6, 1968 [45] Patented Nov. 30, I971 [73 Assignee Sula Viscose Societal Nazlouale Industria Appllcazlonl Viscose S.p.A. Milan, Italy [32] Priority Dec. 23, 1967 133 Italy [54] METHOD FOR THE PREPARATION AND USE OF A CATALYST FOR THE PRODUCTION OF POLYESTERS, MORE PARTICULARLY IIIGII MOLECULAR WEIGHT LINEAR POLYESTERS AND CATALYST THUS OBTAINED 8 Claims, No Drawings [52] U.S. Cl. 260/75, 252/46l, 252/430 [5 1] Int. Cl C08g l7/00 [50] Field ol Search 252/461, 430;260/75;24l/l6 [56] References Cited UNITED STATES PATENTS 2,274,766 3/1942 Zirhc 106/290 2,065,762 l2/l936 Stanley 83/94 FOREIGN PATENTS 740,38 l 2/1953 England 260/75 OTHER REFERENCES Goetzec, Treatise on Powder Metallurgy Vol. I p. 201 Interscience Pub. Inc. New Y ork 1949 Primary Examiner-Daniel E. Wyman Assistant E.raminer-Philip M. French Auomey- B. Edward Shlesinger ABSTRACT: Method for the preparation of finely and evenly divided elemental antimony for use as catalyst in polyester manufacturing. wherein powdered antimony is subjected to mechanical size reduction operations. and then is dispersed in a liquid medium.

METHOD FOR THE PREPARATION AND USE OF A CATALYST FOR THE PRODUCTION OF POLYESTERS, MORE PARTICULARLY HIGH MOLECULAR WEIGHT LINEAR POLYESTERS AND CATALYST THUS OBTAINED This invention relates to a method for preparing finely and evenly divided elemental antimonium which has the physical and general characteristics required for a catalyst to be used in the preparation of linear polyesters.

It is known that, in the preparation of polyesters, and more particularly linear polyesters adapted to be converted into fibers, films, tapes and the like, it is common practice to react a dialkyl ester (in which the alkyl has a low molecular weight) of a bicarboxylic organic acid, or, as an alternative, the bicarboxylic acid itself, with a diol, and more particularly with a diol of the series HO(CH ),,OH wherein n is an integer comprised between two and 10.

As a general rule, the starting materials are dimethylterephthalate, or the terephthalic acid as such, and, among the diols, ethylene glycol is the most commonly adopted one.

When starting from a dialkyl ester and a diol, the first stage of the method is an ester interchange of the two reactants, which is generally called a reesterification reaction. In the most common instance in which dimethylterephthalate and ethylene glycol are used, this first stage gives bis-(2-hydroxyethyl) terephthalate, which is the monomer to be polymerized in the subsequent stage in order to convert it into the end product, that is, the linear polyester. Said reesterification reaction is usually carried out in the presence of a catalyst in order that the reaction in question be accelerated, and the catalysts which are most used to this purpose are, for example, the organic salts of zinc, manganese or salts of other metals.

The product obtained from said ester interchange is subsequently polymerized or polycondensed until a linear polyester having a high molecular weight is obtained, which is adapted to be converted into important commercial products, more particularly fibers or yarns.

It is known that also the polymerization or polycondensation reaction is carried out, as a rule, in the presence of a suitable catalyst. The use of antimony, in different forms and sizes, as a dust, filings and the like, has already been disclosed, among several metals, as a catalyst for carrying out both the initial stage which is conducive to the monomer, and, still more particularly, the second stage, that is to say, the polycondensation.

Also in the case in which the bicarboxylic acid and the diol are the starting materials, more particularly terephthalic acid and ethylene glycol, the monomer is obtained in the first stage and should then be polycondensed to originate the desired end polymer, and it is known that, in both these stages, catalysts are conventionally used to accelerate the process run, such as compounds of bivalent or trivalent metals, for example salts of calcium, zinc and others.

Obviously, the use of powdered antimony gives variable results, which are more or less satisfactory, and are a function, inter alia, of the fineness of the component particles. According to principles and conceptions which are well known to the chemists, a granular catalyst must have such a size as to become finely dispersed within the reaction mass and more particularly, in the present case, in the mass of the monomer, for example bis-(2hydroxyethyl) terephthalate to be condensed.

As a matter of fact, the presence of antimony particles having too coarse a size, for example 80 to I microns, would not allow the metal dispersed in the reaction mass to offer a contact surface with the mass which is high enough for the catalyst action to take place in a most complete and satisfactory manner as far as practicable.

It has now been ascertained that metallic antimony, as obtained with the conventional methods, even in particles having a size less than the one indicated above, for example comprised between a few microns and a few l0s of microns, has the shortcoming of consisting of particles of uneven size which, within the reaction mass, do not produce a uniform dispersion of the metal, the sizes of said particles varying at random within a considerably wide range.

- It has been ascertained that the use of a catalyst formed by said uneven particles, is conducive to a unsatisfactory polymers, or filaments or yams, due to the presence therein of colored or dark particles and/or lumps. It is possible that this phenomenon is due to the fact that, during the treatments, the particles may have become aggregated or clustered to form units having a size greater than the original one, these units then remaining undissolved in the polymer and in the yarns produced thereby.

Several methods are known in the art for preparing elemental antimony as a catalyst for the production of polymers. The conventional methods are concerned both with catalysts having a size between a few microns and a few 10's of microns or greater, as outlined above, and catalysts having a smaller size, whose use is likewise known in the polymer production art.

In general, these methods require chemical and physical means which hardly ensure a satisfactory reproducibility of the results. ln addition, these methods could be conducive to catalysts incapable of affording to the polymer the desirable whiteness rating. Among the several known methods, the most commonly used are the chemical reduction of antimony com pounds and the grinding of powders, while more intriguing and difficult methods are available, such as the electric reduction to powder of a metal dispersed within a liquid, the thermal decomposition of antimony compounds, the grinding of elemental antimony with the aid of airjets and others.

Powdered elemental antimony of commercial grade is formed by particles having a size which is generally over 10 microns, whose average value is between 10 and 1000 microns. The elemental antimony which can be obtained by reduction chemical reactions is formed by particles whose size varies according to the method and the conditions under which the reaction has been carried out, said particles consisting of aggregates of unit particles, having a rather coarse size, for example over microns (for example 250 microns), or unit particles having a lesser size, for example lO-lOO microns.

ln any case, commercial antimony, or antimony which is obtained with chemical reduction methods does not give acceptable results if used as a catalyst in said polycondensation reaction, inasmuch as it is formed by particles, or particle aggregates, having too large a size or having an irregular size distribution.

According to the invention, a simple and quick method has been devised, which is based on physical treatments only, for the preparation of elemental antimony in fine particles, the latter exhibiting a predetermined and constant uniformity of grit size and being adapted to be used as a highly efficient catalyst in the production of polyesters, more particularly high-molecular-weight linear polyesters.

Said physical treatments can be applied to elemental antimony in powder form as obtained from the market, or from conventional chemical reactions, since the elemental or the antimony resulting from conventional reactions is unacceptable as a catalyst in a polycondensation reaction.

The inventive method is characterized in that powdered elemental antimony, formed by particles or particle-aggregates having substantially a diameter over l0 microns, is subjected to physical size-reducing treatments, homogenization and dispersion, such as to permit the obtention of an antimony dispersion formed by particles having substantially a size in the order of 10-20 microns, in a liquid which is compatible with the subsequent use as a catalyst, the elemental antimony thus obtained being a catalyst which has a high efficiency for the production of polyesters, more particularly linear polyesters having a high molecular weight.

According to the invention, the powdered elemental antimony is dispersed within said liquid, usually ethylene glycol,

in the presence of an inert gas to prevent oxidation, preferably with the aid of a mechanical stirrer rotated at a high speed (preliminary dispersion) and the subjected to one or more treatments in a colloid mill, still in the presence of an inert gas, said treatment giving as a result a dispersion of antimony particles having a size in the order of l-20 microns. Said final dispersion is adapted to be directly used as a catalyst in the production of polyesters. It can be introduced in the initial reesterification stage, or, as an alternative, it can be added to the reaction mass at the outset of the polycondensation stage aforementioned.

in a similar manner, when the process involves direct esterification of the bicarboxylic acid and the diol, as outlined above, and the subsequent polycondensation stage, said dispersion can be added to the reaction mass during the first stage or at the beginning of the second stage. As a matter of fact, the elemental antimony obtained according to the invention can be used as a catalyst in both the esterification and polycondensation stages; it exhibits, however, its best properties during progress of the polycondensation stage.

lt is consequently preferable that the antimony prepared according to the invention be associated, in said first stage, with a conventional esterification catalyst, such as a zinc salt, a calcium salt and others.

Preferably, said preliminary dispersion in the liquid by a mechanical stirrer rotated at a high speed is effected on dispersions having a concentration of antimony of from 0.1 to 1 percent.

The final stage in the preparation of the catalyst, that is the treatment in the colloid mill, is preferably carried out as a single step at room temperature, although different temperature might also be used.

Subsequent to said final stage, a settling treatment could be carried out, during a time of from a few minutes, for example minutes, to 6 hours, followed by the drawing off of the supernatant clear layer which contains the finest particles.

The utility of carrying out said settling step is a function of the manner in which the previous treatments have been performed: it can be successfully used whenever the previous treatments have been conducted in such a way as to produce a large number of particles having a diameter which is in the neighborhood of microns.

The settling step can be dispensed with, conversely, when the dispersion obtained has particles whose size distribution is satisfactorily uniform and between 10 and 20 microns, which occurs in the majority ofinstances.

The dispersion of antimony particles obtained according to the inventive method contains particles having an average size within a very narrow range of values, and, more exactly, from ID to 20 microns.

The size of the particles is thus very uniform and the dispersion has a grey color and can be easily admixed with and incorporated in the reaction mass in the process for producing polyesters, and without lump formation.

For checking purposes, the dispersion obtained after the treatment in the colloid mill, is subjected to at least two consecutive filtrations through porous diaphragms, the first of which has pores whose nominal size is from 15 to 40 microns, and the second one has pores with a nominal size of from 5 to 15 microns.

lt has been ascertained that said dispersion passes through the first porous diaphragm entirely, without leaving any appreciable residue on the diaphragm. The dispersion thus filtered, passed through the second porous diaphragm, leaves on the diaphragm a grey residue, which is clearly visible, whereas the filtrate does not show any appreciable amount of antimony particles.

The controls made by passing the dispersions through the two porous diaphragms are a confirmation of the results obtained on samples of said dispersion with microscopical examinations carried out according to the conventional techniques.

When the starting material for performing the method according to the invention consists of powdered antimony, whose particles have, as an average, a size considerably greater than microns, as frequently occurs with commercial grade antimony, it is wiser to carry out two preliminary dry-grinding and wet grinding treatments on said material, so as to reduce at least the major portion of said particles to a size of less than 20 microns, then effecting said preliminary dispersion in a liquid and the subsequent treatment in the colloid mill.

The two preliminary treatments aforementioned are intended to reduce the comparatively great size of said particles to such a value that they can be readily processed by said preliminary dispersion and the colloid mill, to obtain the desired size.

In the case in which the starting material consists of powdered antimony, composed of aggregates of particles having a size in excess of 100 microns, or of particles with a diameter between 10 and 100 microns, as is the case when antimony is the result of chemical reduction operations, then said preliminary treatments are redundant, inasmuch as the particles can be reduced to the desired size without any appreciable difficulty.

The first of said preliminary treatments consists in one or more dry-grinding operations to give particles having an average diameter of less than 35 microns. These dry-grinding operations are preferably carried out by treating the powdered antimony in a ball mill rotated at high speed, and causing the thusly ground product to pass through screens having 16,000- mesh gauzes, so as to permit that particles having a diameter substantially below 35 micron be detected.

The resulting product is then subjected to the second preliminary treatment which comprises one or more grinding operations within a liquid, preferably ethylene glycol, to give particles having an average diameter of less than 20 microns.

Preferably, the wet-grinding operation is carried out by dispersing the antimony powder as obtained from the first treatment, in ethylene glycol at a concentration of l0-50 percent by weight of antimony in the liquid, and grinding said dispersion with ball mills or sandmills.

Said dimensions of the particles can be determined with the conventional analytical tests, for example, microscopical examinations of the relevant samples.

The product obtained is properly dispersed (preliminary dispersion) and then subjected in the colloid mill to a dispersion and homogenization action which leads to the desired end product according to the invention.

It has been ascertained that the dispersion of antimony, as obtained according to the invention, has the necessary and most desirable properties for its use as a catalyst in the production of polyesters, and more particularly of linear polyesters.

In order that said properties may be ascertained, polymerization tests have been carried out, by adding the dispersion obtained according to this invention, to the reaction mass, both in reesterification and polycondensation processes and in those processes which are based on the direct esterification of terephthalic acid with a glycol.

Said tests, a few of which are disclosed in the practical examples to be described later, show that the polymerization reaction has a satisfactory short duration and that it takes place in a uniform manner throughout the whole reaction mass, that both said mass and the end polymer have a clear and uniform color, free of lumps and dark particles. No staining is noted in the chips obtained by extruding the polymer, and the filaments and yarns produced thereby are devoid of undesirable discolorations and have the best whiteness characteristics, even retaining, to a high degree, the known textile and mechanical properties of said yafns, for example a high tensile strength.

The polymer thus obtained also shows the satisfactory characteristic of thermal stability; it withstands temperatures of 280-290 C., during long periods of time without exhibiting any degradation or other alterations liable to impair the polymer itself.

EXAMPLE 1 A dispersion of elemental antimony in monoethylene glycol in the form of a fine and uniform particles having a size between and 20 microns is prepared according to the following procedure:

a. synthesis of elemental Sb by reacting TiCl with SbCl in an aqueous acidic solution.

Filtration, washing until acidity has been removed with secondary reaction products, drying in a vacuo until powdered Sb is obtained having a purity of 99 percent or higher, and whose particles or particle aggregates have a size substantially over 10 microns.

b. Preliminary dispersion of powdered Sb in ethylene glycol in the proportion of one part of Sb per 1,000 parts of glycol, by stirring with an Ultraturrax turbine-dispersing machine rotated at 10,000 r.p.m. for 5 minutes in an inert environment at room temperature.

c. Treatment of the preliminary dispersion in a colloid mill of the Manton-Caulin type. A fraction of the final dispersion obtained is passed for checking purposes, through porous diaphragms of the Gooch G /3 Jena type having pores with a nominal diameter between and 40 microns, without leaving any appreciable residues on the diaphragms. The dispersion is then passed through porous diaphragms of the G4 Jena type, having pores with a nominal size of 5-15 microns and leaves thereon a grey residue which is clearly visible, whereas no particle passes, in visible manner, through said diaphragms.

Another fraction of the final dispersion thus obtained, looked at through a microscope, shows particles having a size substantially of from 10 and microns.

A portion of said dispersion is used to obtain a spinnable polyethylene terephthalate polymer, as follows:

A test-polymerization reactor having a capacity of liters and equipped with a stirrer and distillation columns is charged with the following materials:

dimethyl terephthalate (DMT) 8,000 parts monoethylene glycol (GE) 2,800 parts bihydrated zinc acetate 1.6 parts trielhylsulfate 1.33 parts dispersion of Sb (0. 1% in GE) containing 2.4 parts Sb,

equivalent to 0.031 by weight on DMT 2,400 parts The reaction mass is gradually brought to 220 C., during 7 hours, and, during this time all the methanol formed in the ester interchange reaction of dimethyl terephthalate and ethylene glycol is distilled off virtually entirely from the reactor.

A gradual vacuum is then applied to the reaction system so as to attain, during 2 hours, a value of 0.5 to 1 mms. of mercury, the temperature being raised from 220 C., to 280 C.

Polycondensation is completed by maintaining the reaction mass under a vacuum of 0.5 mms. of mercury at 280 C., during 3 hours. The formed polymer is then extruded under nitrogen pressure in cold water.

Said polymer, converted into granular form, is colorless and is water transparent.

It has, moreover, the following specifications:

[n]=0.673 (intrinsic viscosity, measured at 20 C., in a mixture of phenol and tetrachloroethane in the weight ratio of 60 to 40, the concentration of the solution being 0.58 gr. of polymer in I00 mls.

m.p. 265 C.

melt index 0.60 g./l0 mins. as measured at 270 C. with a spinneret having a diameter of 0.5 mms. and under a load of2 160 gr.

Number of equivalent C0011 32 (referred to 10 gr. of polymer) Filaments or yarns obtained from said granules have a clear color and have no lumps or dark spots.

EXAMPLE 2 A sample of elemental antimony, in the form of an impalpable powder, is obtained by reduction of an aqueous solution of SbC 1,, acidified with hydrogen chloride, with zinc powder. The antimony particles, or particle-aggregates, have a size substantially over 10 microns.

The powdered Sb is then converted into a 0.1 percent dispersion in ethylene glycol according to the procedure indicated under b) of example 1 and is then subjected to a pass in a colloid mill of the Manton-caulin type.

Also this dispersion, viewed through a microscope, is essentially formed by particles having a size between 10 and 20 microns.

An amount of 2,400 parts of said dispersion, containing 2.4 parts of Sb, is subjected to a polycondensation test, by using an implementation and a charge according to the same operative conditions as in example 1.

The polymer obtained upon crystallization and drying up to a moisture contents of less than 0.01 percent has the following specifications [1 =0.668 (measured as in example 1 m.p. 265 C.

Melt index 0.61 gr./ 10mins.

No. of CO0H per 10 grs. of polymer 37 Color white with a slight grey hue.

The filaments or yarns obtained by melt-spinning of the granules exhibit a satisfactory clear color and do not show any staining or other undesirable discoloration.

EXAMPLE 3 A specially provided stainless-steel esterification reactor equipped with a stirrer and a rectification column, containing 200 parts of prepolymerized polyethylene terephthalate having a average degree of polymerization of 2 kept at a temperature of 245 C., is charge during 1 hour with a slurry consisting of:

parts of terephthalic acid 50 parts of monoethylene glycol 0.05 parts of Sb slurried in 25 parts ofGE, obtained as in ex. 2.

The Esterification reaction is carried out under a pressure of 3.5 kgs./sq.cm., at a temperature of 245 C., the water formed during the reaction being continually withdrawn for 1 hour. The balance of the reaction water is removed from the reactor by venting the system from 3.5 kgs./sq. cm. down to atmospheric pressure, during an additional hour. The ester thus obtained is transferred to a polycondensation reaction wherein the temperature of the reaction mass is brought, during 2 hours, to 280 C. while gradually applying a vacuum, from 760 mms. of mercury to 1 mm. of mercury.

The polycondensation reaction is carried out within 5 hours at 280 C. under vacuum of l mm./Hg.

The polymer thus obtained is extruded under nitrogen pressure in water and converted into granular form. The product has the following specifications m.p. 263 C.

N of CO0H per 10 gr. of polymer: 3]

Color virtually colorless, water-transparent. Upon crystallization at C. during 30 mins. under nitrogen, the polymer granules exhibit an excellent whiteness rating.

EXAMPLE 4 A sample of commercial powdered antimony having a grit size of 60 mesh (mesh width 250 microns) and a purity of 99 percent, is ground in a rod mill rotated at 15,000 rpm. then screened through a screen having a 16,000 mesh gauze until a powder is obtained having a grit size of less than 35 microns.

The fine powder thus obtained is admixed with ethylene glycol, up to a percentage of 33.3percent of the whole mixture, until obtaining a uniform slurry which is subjected to grinding in a ball mill during 100 hours under an inert gas blanket. The fine dispersion thus obtained, which contains particles having a diameter substantially less than 20 microns, is diluted to the concentration of 0.1 percent in ethylene glycol by stirring in a turbine dispersing machine for 5 minutes under an inert gas blanket at room temperature, then it is made homogeneous by treatment in a colloid mill of the Manton-Gaulin type.

A portion of the dispersion so prepared is employed for producing a polyethylene terephthalate polymer according to the following procedure:

A polymerization reactor is charged with the following materials:

dimethyl terephthalate monoethylene glycol zinc formate [,000 parts 250 parts 0.2 parts (equal to 0.025 mols per l mols of DMTI 0.l6fi parts 400 arts The ester interchange and polymerization reactions are carried out with the same procedure as in example 2 the exception being that the duration of the high vacuum (0.5 mm of mercury) at 280 C., is as long as 2 hours 30 mins.

The polymer obtained in the form ofgranules exhibits, upon crystallization and drying up to a moisture contents of less than 0.01 percent, the following specifications:

[n] 0.645 (measured as in example 1) Melt index 0.070 gr./ 10 mins.

m.p. 263 C.,

Number ofCOOH per l 0 g. of polymer: 33.5

Color white EXAMPLE For comparison purposes a polymerization test is carried out with a dispersion of elemental antimony in ethylene glycol, prepared as follows: l Synthesis of elemental Sb according to the procedure reported under (a) in example I 2 Dispersion of 2.4parts of Sb in 240 parts ofethylene glycol.

The dispersion thus obtained exhibits, when observed through a microscope, a very uneven grit size for the particles or particle aggregates, characterized by average dimensions between 10 and 80 microns.

Said dispersion is subjected to a polycondensation test in the reactor described in example 1, which is charged with the following materials:

2.4 parts of Sh, equivalent to 0.03% by weight on DMT) The reaction mass is subjected to ester interchange and polycondensation reactions according to the same procedure as reported in example i. The formed polymer is extruded under nitrogen pressure in cold water and converted into granules.

Said polymer contains many black lumps, a few of which have a size of a few hundreds of microns and thus they are clearly visible by the unaided eye.

it has, in addition, the following specifications:

m.p. 263 C.,

Melt index 1.08 gr./l0 mins.

Number ofCOOH per 10 g. 30

What is claimed is:

1. A method for the production of poly-methylene terphalates by reacting a reagent selected from the group consisting of bicarboxylic organic acids and dialkyl esters thereof with ethylene glycol liquid in the presence 0 a catalyst consisting of a dispersion in the liquid glycol of antimony having a substantially uniform particle size in the range of from ID to 20 microns 2. A method for the production of poly-methylene terphalates according to claim 1 wherein the reaction mass is maintained under a vacuum of 0.5 to l mms. of mercury and at a temperature of 220 C., to 280 C., during the reaction.

3. A method according to claim 1, wherein powdered antimony is initially dispersed in ethylene glycol by mechanical stirring, and the dispersion thus obtained is subjected to one or more treatments in a colloid mill, to obtain final dispersion of the antimony particles in the ethylene glycol in a concentration of from 0.1 percent to 1 percent of antimony and in the particles size range of from l0 to 20 microns, both the stirring and the colloid mill treatment being carried out in an inert gas environment.

4. A method for the production of poly-methylene terphalates according to claim 1, wherein the first-named reagent is dimethyl terephthalate, and is in the proportion of 800 to 1000 parts of dimethyl terephthalate to 250to 490 parts of ethylene glycol.

5. A method for the production of poly-methlene terphalates according to claim 1, wherein the first-named reagent is terephthalic acid, and is in the proportion of parts of terephthalic acid to 50 parts of ethylene glycol, and this mixture, mixed with 0.05 parts of antimony per 25 parts of ethylene glycol, is charged into 200 parts of polymerized polyethylene terephthalate.

6. A catalyst for use in the production of poly-methlene terphalates by reacting a compound selected from the group consisting of bicarboxylic organic acids and dialkyl ethers thereof with ethylene glycol consisting of a dispersion in the ethylene glycol of antimony particles, whose sizes are substantially uniform and are in the range offrom 10 to 20 microns.

7. A catalyst as claimed in claim 6, wherein the dispersion has a concentration of from 0.] percent to 1 percent of antimony.

8. A catalyst as claimed in claim 6, wherein the dispersion of the antimony particles in the ethylene glycol has a concentration of 10 percent to 50 percent by weight of antimony in the glycol.

Claims (7)

1. 2. A method for the production of poly-methylene terphalates according to claim 1 wherein the reaction mass is maintained under a vacuum of 0.5 to 1 mms. of mercury and at a temperature of 220* C. to 280* C. during the reaction.
2. 3. A method according to claim 1, wherein powdered antimony is initially dispersed in ethylene glycol by mechanical stirring, and the dispersion thus obtained is subjected to one or more treatments in a colloid mill, to obtain final dispersion of the antimony particles in the ethylene glycol in a concentration of from 0.1 percent to 1 percent of antimony and in the particles size range of from 10 to 20 microns, both the stirring and the colloid mill treatment being carried out in an inert gas environment.
3. 4. A method for the production of polymethylene terphalates according to claim 1, wherein the first-named reagent is dimethyl terephthalate, and is in the proportion of 800 to 1000 parts of dimethyl terephthalate to 250 to 490 parts of ethylene glycol.
4. 5. A method for the production of polymethylene terphalates according to claim 1, wherein the first-named reagent is terephthalic acid, and is in the proportion of 125 parts of terephthalic acid to 50 parts of ethylene glycol, and this mixture, mixed with 0.05 parts of antimony per 25 parts of ethylene glycol, is charged into 200 parts of polymerized polyethylene terephthalate.
5. 6. A catalyst for use in the production of polymethylene terphalates by reacting a compound selected from the group consisting of bicarboxylic organic acids and dialkyl ethers thereof with ethylene glycol consisting of a dispersion in the ethylene glycol of antimony particles, whose sizes are substantially uniform and are in the range of from 10 to 20 microns.
6. 7. A catalyst as claimed in claim 6, wherein the dispersion has a concentration of from 0.1 percent to 1 percent of antimony.
7. 8. A catalyst as claimed in claim 6, wherein the dispersion of the antimony particles in the ethylene glycol has a concentration of 10 percent to 50 percent by weight of antimony in the glycol.