GB2398073A

GB2398073A - Polyester manufacturing process

Info

Publication number: GB2398073A
Application number: GB0302369A
Authority: GB
Inventors: Martin Graham Partridge
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2003-02-03
Filing date: 2003-02-03
Publication date: 2004-08-11
Also published as: GB0302369D0

Abstract

A particulate solid additive for a polyester manufacturing process comprises an intimate mixture of solid bishydroxyethyl terephthalate and an active ingredient. In a preferred embodiment, the active ingredient is a polycondensation catalyst such as a compound of antimony.

Description

Polvester Drocess and catalyst The present invention relates to the

manufacture of polyesters, in particular to the manufacture of polyethylene terephthalate (PET) and copolymers thereof, and to catalyst compositions for use in such manufacture.

The manufacture of PET is conventionally carried out in a two stage reaction. In the first stage terephthalic acid or a low-alkyl ester thereof (typically dimethyl terephthalate) is esterified or transesterified with ethylene glycol to form bis-hydroxyethyl terephthalate (BHET). This stage is normally performed at elevated pressure and at temperatures in the range of 260 - 290 C. The resulting BHET product is then reacted under vacuum with the removal of glycol to form PET. In the first (esterification) stage, a catalyst may be added, although when using a terephthalic acid starting material a catalyst may not be necessary.

In the second (polycondensation) stage it is necessary to add a catalyst to enable polycondensation to proceed satisfactorily.

A typical and very widely used polycondensation catalyst is based on antimony, normally in the form of an oxide or an acetate or other salt. Other metals or combinations of metals may also be used; germanium, titanium, manganese, cobalt and zinc being amongst the most common.

Sb2O3, the most commonly used catalyst, is typically supplied as a dry powder. Antimony compounds are toxic. The difficulty of handling toxic powders requires strict precautions to be taken to ensure that plant workers are not exposed to high levels of the compounds.

This normally entails the use of full protective suits and breathing apparatus to avoid contact with or inhalation of the powder.

When antimony-based catalysts are used, there can be great problems in dissolving the catalyst in the reaction medium in order for it to react and deliver benefits in the process.

For example, Sb2O3 is only sparingly soluble in the reaction medium and dissolves in ethylene glycol at only 1-2 wt % at 1 60 C. The Sb203 is typically added in most processes as a slurry in ethylene glycol at the beginning of the process, i.e. in the esterification stage, to ensure that the solid is well mixed and dissolved and forms part of a homogeneous mobile liquid of low viscosity for catalysing the polycondensation stage.

Sb2O3 due to its manufacturing methods quite often contains small quantities of higher oxidation states of Sb which are not soluble in ethylene glycol even at high temperatures.

These insoluble Sb species can cause major problems for polyester producers, creating fouling of in-line polymer filters and down-stream processing equipment such as spin-packs - i: for example, and also residues which may cause fibre breakages in spinning lines or lumps and irregularities in films produced from the polyester.

Catalyst producers have tried to overcome these inherent difficulties with Sb203 by carrying out a chemical purification of the Sb2O3 and also by pre-forming the proposed reactive Sb catalyst species, by manufacturing and supplying the Sb(glycollate)3 species to the industry.

This species overcomes some of the problems as it dissolves more readily in the polyester manufacturing process more quickly and at a lower temperature.

Various other additives are also added to a polyester mixture during the manufacturing process. Typically these additives include stabilisers, which are commonly phosphorus species to prevent the catalysis of the hydrolysis reaction which tends to degrade the polyester. Also colourants may be added to change the optical properties of the polymer.

These are commonly used when Sb catalysts are used because the Sb species tends to impart a grayish tone to the resulting polyester.

We have found that the addition of the catalyst and/or other additives in a different form may provide benefits to the polyester manufacturers and result in a product containing fewer Sb residues.

According to the invention we provide a particulate additive for a polyester manufacturing process comprising an intimate mixture of (i) at least one active ingredient and (ii) solid bishydroxyethyl terephthalate The active ingredient may be a catalyst for the polycondensation of a polyester prepolymer such as BHET derived from the esterification or transesterification of terephthalic acid or its alkyl ester with ethylene glycol. Suitable catalyst active ingredients include compounds of antimony, germanium, cobalt, titanium, manganese and zinc. Preferred ingredients are oxides of antimony, antimony triacetate, antimony glycollate, cobalt acetate tetrahydrate, germanium dioxide. Alternatively the active ingredient may be a polyester stabiliser such as a phosphorus-containing compound, e.g. an inorganic or organic phosphate, phosphonate or phosphite species, or a colourant or colour-correction compound such as a cobalt compound or a dye.

The solid bishydroxyethyl terephthalate is a polyester prepolymer, made during the first stage of a typical PET manufacturing process, as explained above. BHET is a solid at room temperature and atmospheric pressure, having a melting point of about 106 C. The BHET may contain some polyester oligomers which are formed by the occurrence of some polycondensation of the BHET during its manufacture. The BHETis used as a solid matrix in which the active ingredient is dispersed.

The dispersal of the active ingredient into a particulate solid provides several benefits over the use of the ingredient in its raw form. First the use of a particulate composition of the invention to supply the required dosage of antimony catalyst to the polyester reaction in a form which is easily handled involves less risk to health because the antimony compound is encapsulated in the polyester prepolymer so making handling more straightforward and safe. This is because the dust associated with handling antimony compounds as powders is greatly reduced making control of the hazards easier. Also, particularly when the active ingredient is a catalyst, the provision of the catalyst in the BHET particles allows the catalyst to be added to a polyester manufacturing process in an active and disperse form.

This may reduce the process time taken to disperse or dissolve the catalyst during the polyester manufacturing process and lead to shorter batch preparation times.

The processes for making BHET are well-known to persons skilled in the art of polyester manufacture.

BHETis normally made by the esterification of terephthalic acid with ethylene glycol with the addition of a catalyst if desired and then heating to 260 - 270 C under a pressure of about 0.3 MPa. Reaction commences as the acid dissolves at about 230 C and the water formed in the reaction is removed.

Alternatively the BHET may be made by transesterification of dimethyl terephthalate with ethylene glycol - with removal of methanol formed in the reaction between 150 and 230 C at atmospheric pressure. Typical catalysts used are manganese, zinc and/or cobalt salts.

The active compound may be added to the BHET during or after the process for manufacturing the BHET. The active ingredient of the present invention may be added to the raw materials at the start of the reaction or alternatively may be added and mixed with the reaction mixture during the reaction or added to the BHET product at the end of the reaction, preferably whilst the BHETis still molten. When the active ingredient is a catalyst, such as an antimony compound, it may be added at the start of the reaction as is usual in the manufacture of PET. In this way the presence of the antimony compound in the reaction medium during the reaction period allows for the formation of the active antimony species which is useful to catalyse a polycondensation reaction. Without being bound by the theory, it is believed that the dissolution of the antimony compound in the reaction medium forms a soluble antimony glycollate species which then acts as a homogeneous catalyst during subsequent processing.

The process preferably comprises a filtration step so that insoluble compounds, such as insoluble compounds of antimony for example, are removed from the product. In this way, the presence of these insoluble species in the polyester product which is made using the catalyst of the invention can be avoided, which provides cleaner polyester with fewer down- stream processing problems. It has been found that filters of 5 micron are suitable for this operation.

The BHET containing the active ingredient is then formed into solid particles or blocks. The particles may be in the form of pellets, shaped units such as tablets, solidified droplets, flakes, powders, granules or other handleable form. The particles may be formed by Frilling, spraying, extruding or moulding the hot molten or semi-solid BHET/active ingredient mixture or by forming particles from cooled, solid mixture e.g. by granulation, flaking, stamping, tabletting etc. The amount of active ingredient in the particulate additive may vary between about 1% and about 70% by weight, depending on the nature of the active ingredient. When the active ingredient is an antimony compound it is preferred to incorporate between about 20 wt% and about 50 wt% in the particulate additive. Other additives, such as dyestuffs for example may be required in much lower concentrations. The concentration of the active ingredient in the additive is normally considerably greater than would be required for addition to the BHET component of the particulate additive itself if that BHET were intended to be polycondensed into a polyester product. The additive serves instead as a masterbatch so that for a given concentration of active ingredient in the particulate additive, a certain weight of the particulate additive may be dispensed into a polyester manufacturing process depending on the required concentration of the active ingredient in the polyester manufacturing process. When, for example, a certain amount of an active ingredient is required for polyester manufacture, the desired effect may be achieved by adding a larger quantity of the particulate additive containing a relatively low concentration of active ingredient, or alternatively by adding a smaller quantity of a particulate additive which contains a higher concentration of the required additive. Clearly it may be more economical to provide the particulate additive with relatively high concentrations of active ingredient to reduce the amount of additive which must be handled. The particulate additive may be added to the polyester manufacturing process either before or during the first (esterification/transesterification) stage or in the second (polycondensation) stage.

The particulate additive of the invention may contain more than one type of active ingredient. In this way combinations of actives, such as a catalyst and a stabiliser or a colourant and brightener, may be added together to provide consistency of the additive ratios between batches of polyester. Alternatively a cocktail of more than one type of catalyst may be provided in a single additive. An additional benefit is the reduction of pre- weighing and addition steps required for each polyester batch.

The invention is further described in the following examples.

ExamDIe 1 45559 terephthalic acid and 20399 of ethylene glycol containing 12809 Sb2O3, sufficient to provide about 20% by weight of Sb in the final BHET product, is charged to an autoclave.

The mixture is heated to 270 C under a vacuum of about 0.3 Mpa for about 2 hours or until evolution of water ceases. The molten product is then discharged through a filter to form droplets of the product particulate additive. The droplets are allowed to cool to form the solid product.

Example 2

BHET was made by the process described in Example 1, with the omission of the So.

10Og of this BHET was charged to an autoclave together with 0.15 9 of the solid catalyst product prepared in Example 1, i.e. sufficient to provide about 350ppm by weight of Sb in the final polyester. The mixture is heated to 292 C under a vacuum of about 1.5 mbar for 2 hours or until a set torque value is obtained on the agitator. The molten polyester product was then discharged into a water bath and collected.

Claims

Claims 1. A particulate additive for a polyester manufacturing process

comprising an intimate mixture of (i) at least one active ingredient and (ii) solid bishydroxyethylterephthalate
2. A particulate additive as claimed in claim 1, wherein the at least one active ingredient comprises a catalyst suitable for catalysing the esterification or transesterification or polycondensation stages of a polyester manufacturing process. i
3. A particulate additive as claimed in claim 2, wherein the catalyst comprises a compound of antimony, germanium, zinc, titanium, manganese or cobalt or a mixture of more than one of the aforementioned compounds.
4. A particulate additive as claimed in any of the preceding claims, wherein the active ingredient comprises a polyester stabiliser or a colourant.
5. A particulate additive as claimed in any of the preceding claims, wherein the active ingredient is present in an amount of from 1% to 70% by weight of the total additive.
6. A process for the manufacture of a an additive useful in the manufacture of a polyester, comprising the steps of: (i) Forming a mixture of molten bishydroxyethyl terephthalate and an active ingredient (ii) Forming solid particles from said mixture.
7. A process as claimed in claim 6, further comprising the step of filtering the mixture of molten bishydroxyethyl terephthalate and an active ingredient before forming the solid particles.
8. A process as claimed in claim 6 or 7, wherein the solid particles are formed by grilling, spraying, extruding or moulding molten mixture or by forming particles from cooled, solid mixture by granulation, flaking, stamping or tabletting.