The invention is also directed towards solid-state polymerization apparatus operating in accordance with the process.

Background art The invention relates in particular but not exclusively to the field of the transformation of polyester resins which are used in the production of articles for which a high degree of polymerization of the resin is required. Amongst these, the production of PET bottles is certainly of considerable importance.

It is in fact known that, in this field, the PET with which the bottles are made must have a greater degree of polymerization than is required in other fields of use of the same resin such as, for example, the production of textile fibres.

It is also known to produce resin with a high degree of polymerization (known as"upgraded resin") from a resin with a low degree of polymerization (known as"amorphous resin") by means of a process in which the amorphous resin is kept at a temperature of between 200°C and 215°C in a stream of hot inert gas so as to trigger a solid-state polymerization reaction (also known as SSP or post-polycondensation process).

However, this process requires the use of a substantially oxygen-free gas, normally nitrogen, carbon dioxide, noble gases or mixtures thereof, in which the

oxygen fraction is less than 100 ppm. At the above- mentioned temperatures, polyester resin in fact reacts relatively quickly with oxygen, giving rise to undesired polymer degradation phenomena which are made evident by a decline in the final optical properties and, in particular, by yellowing of the resin (evidence of which is given, for example, by measurement of the"Colour b"parameter).

To ensure that the quality of the resins treated is maintained, the known process provides for operation with very pure inert gases which are kept in closed circuit, and also provides for expensive systems for their catalytic purification.

Moreover, because of this problem, in order to be economically sustainable, apparatus that is constructed on the basis of this process is typically constructed on a large scale with average production of between 10 and 20 tonnes/hour and can satisfy the requirements of a plurality of resin-transformation plants which are often of medium- small dimensions and are normally located at sites remote from the place where the solid-state polymerization treatment takes place.

The fact that the upgraded resin produced by the solid- state polymerization process cannot be used immediately leads, in the specific case of PET which, as is well known, is a hygroscopic material, to the need to subject it to a dehydrating treatment prior to its admission to the moulding unit, naturally with an increase in the overall cost of the production cycle.

Disclosure of the invention The problem underlying the present invention is that of providing a process for the polymerization of polyester resins in the solid state, as well as apparatus operating in accordance with the process, which are designed structurally and functionally to overcome the limitations set out above with reference to the prior art mentioned.

Within the scope of this problem, a primary object of the present invention is to provide a process and apparatus which can advantageously be disposed immediately upstream of a unit for moulding the resins.

This problem is solved by the present invention by means of a process for the polymerization of polyester resins in the solid state and apparatus according to the appended claims.

Brief description of the drawings The characteristics and the advantages of the invention will become clearer from the detailed description of a preferred embodiment thereof which is described by way of non-limiting example with reference to the appended drawing in which Figure 1 is a process diagram of solid-state polymerization apparatus operating in accordance with the process of the present invention.

Best mode of carrying out the invention In the appended drawing, the apparatus is generally indicated 1 and comprises a polymerization reactor 2 as well as means for supplying the resin and means for blowing in gas, which are indicated 3 and 4, respectively.

The resin-supply means 3 comprise a hopper 5 for storing the amorphous resin (indicated R in the drawing) which is supplied to the plant 1, and a crystallizer 6 for the pre-treatment of the amorphous resin in accordance with an arrangement known per se in SSP processes.

The crystallizer 6 is preferably of the fluid-bed type, but crystallizers of the"spouted bed"type, or of other types known commercially as Solidarise or TorusdiscTM (both by Hosokawa-Bepex) or also Pulsed BedTM (by Buehler) may be used.

The amorphous resin can be supplied from the hopper 5 to the crystallizer 6 and from there to the reactor 2 by gravity with suitable metering by respective rotary valves,

or pneumatic transport means, indicated 8 in the drawing, may be used.

A circuit for the gas for supporting the fluid bed of the crystallizer is subservient to the crystallizer 6 and comprises first means 10 for removing dust from the gas that is output from the crystallizer 6 and for recovering the gas, as well as second means 11 for admitting and heating the gas as treated by the first means 10. The gas that is present in the circuit may also be restored by means of a reserve tank comprising air which has previously been enriched, for example, by means of an in situ generator of the membrane type or of the PSA (Pressure Swing Adsorption) type, and dehumidified.

The means 4 for blowing in gas (indicated G in the drawing) may comprise a tank of the gas under pressure, connected to a dehumidifier 16 and, downstream, to a heating unit 17 for heating the gas to the desired process temperature, which is typically between 190°C and 230°C.

Alternatively, the gas required for the process may be supplied to the pressurized tank by a generating unit which can produce the gas directly from the ambient air.

The apparatus 1 may also advantageously comprise means for regulating the dehumidifier 16 in order to vary the moisture content of the gas in dependence on the fraction of oxygen that is present in the gas, as will become clearer from the following description.

The reactor 2 is of the moving-bed type, preferably cylindrical with a vertical axis, and suitably insulated from the exterior; the supply of amorphous resin coming from the supply means 3 is arranged in the upper portion of the reactor 2 and the hot gas coming from the blowing means 4 is admitted in a lower region thereof, as a counter-flow relative to the resin.

The dimensions of the reactor 2 are related to the flow-rate of amorphous resin in a manner such that the

average time spent by the resin in the reactor 2 is between 5 and 30 hours, according to the extent of the required increase in the degree of polymerization.

The output of the resin from the base of the reactor 2 may be regulated by a rotary valve controlled by the level of resin inside the reactor 2.

Preferably, a moulding unit, which is conventional per se and is not shown in the drawing, is advantageously installed downstream of the reactor 2 for the immediate treatment of the upgraded resin (indicated P in the drawing). The unit installed downstream may be intended, for example, for the production of PET bottles.

The hot gas output from the reactor 2 is advantageously admitted to the circuit for supplying the gas for supporting the fluid bed of the crystallizer 6, upstream of the dust- removal means 10.

The apparatus 1 is preferably intended for the polymerization treatment of PET in the solid state but can also be used for a similar treatment of any polyester resin which can be polymerized in the solid state. The most common polyesters which can be used in the process of the present invention have at least 75% in moles of their acid fractions constituted by an aromatic dicarboxylic acid such as terephthalic acid, isophthalic acid, or a naphthalene dicarboxylic (preferably 2,6-) acid, and the diol fractions are preferably constituted by glycols such ethylene glycol, butylene glycol, 1,2-dimethyl cyclohexane and the like, or aromatic diols such as hydroquinone and catechol. These polyesters may comprise other dicarboxylic acids such as adipic acid, sebacic acid, and the like. Examples of polyester resins of the type indicated are, in addition to PET which has already been mentioned, polyethylene isophthalate, polyethylene naphthalate, and polybutylene terephthalate.

As stated above, the apparatus 1 is preferably used for the process for the solid-state polymerization of PET which is intended for the production of bottles, for which the PET is required to have an intrinsic viscosity (a physical parameter which is directly correlated with the average molecular weight of a polymer and is therefore commonly used in the technical field in question for the determination of its degree of polymerization) of at least 0.72 dl/g.

The amorphous resin supplied to the apparatus 1, on the other hand, typically has an intrinsic viscosity value of about 0.60 dl/g. Naturally, for different values of the intrinsic viscosity of the amorphous resin or of the upgraded resin, it will suffice to regulate the parameters of the reaction (time spent and temperature in the reactor) in a manner known per se. The amorphous resin can advantageously be supplied in the form of granules of cylindrical or prismatic cross-section or in flake form. In the former case, the granules have a length of less than 10 millimeters (mm) and a cross-section of less than 5 mm, whereas in the latter case, the flakes have a diameter greater than 3 mm and a thickness of less than 3 mm.

According to a principal characteristic of the present invention, the gas admitted to the reactor 2 comprises a mixture of oxygen and one or more gases selected from the group constituted by nitrogen, hydrogen, noble gases (such <BR> <BR> as helium, argon, etc. ), carbon monoxide, and carbon dioxide.

In particular, the oxygen fraction in the gas admitted is such as to be less than a maximum permissible oxygen fraction which is determined in dependence on the moisture content of the gas, which is typically expressed in the art by means of its dew point.

It has in fact been found that the rate of PET oxidative degradation reactions is closely correlated with the presence of water in the gaseous mixture and that the

elimination or at least drastic reduction of this component therefore greatly reduces the reactivity of oxygen with PET.

It is pointed out that, in conventional SSP processes, there is a tendency to prevent these reactions occurring principally by eliminating the oxygen from the gas that is put into contact with the resin, whereas the process of the invention also takes appropriate account of the activity of the water.

This advantageously permits the use of a gas that is composed predominantly of an inert gas, for example, nitrogen, but in which there is a relatively high fraction of oxygen, well above the 100 ppm normally permitted in known processes.

Tests carried out by the Applicant have shown that PET treated in accordance with the process of the invention does not undergo appreciable oxidative degradation phenomena, even in the presence of relatively high oxygen fractions, when the moisture content of the gas is sufficiently low., The results of these tests are summarized in the following table in which the maximum permissible oxygen fractions in the gas are given, related to the moisture content of the gas. The maximum oxygen fractions (Os max) are expressed as percentages of the moles of gas admitted, and the moisture content is expressed in terms of the dew point (D. P. ) of the gas, in degrees centigrade. 02 max (%) 1,0 2,0 3,0 4,0 5,0 5,5 6,0 6,5 7,0 D. P. (°C)-57-60-65-72-80-85-89-91-93 The maximum permissible oxygen fraction was determined, for successive tests, in a manner such that a PET resin subjected to a solid-state polymerization process at a temperature of 195°C and for a period of 17 hours maintained a value of the"Colour b"parameter (indicated by

measurement of the yellow toning of the plastics material) of less than zero.

A PET resin having a starting intrinsic viscosity of about 0.58 dl/g, in cylindrical granular form, with a length of about 2 mm and an elliptical cross-section with axes of 1.4 and 2.4 mm was used in the tests, giving a final intrinsic viscosity of about 0.80 dl/g.

The values given in the table have an operative error margin of 2%.

The use of nitrogen having an oxygen fraction of 5% (enriched air) and a dew point of-80°C is particularly preferred. This condition of use has in fact been found to be the optimal condition for minimizing the running costs of the apparatus. In fact, smaller oxygen fractions would require enriched air generators having considerable membrane surfaces, without taking into account the greater energy consumption for the compression of the air supplied to the generators, whereas larger oxygen fractions would lead to a greater cost of the dehumidification of the enriched air.

The present invention thus solves the problem discussed above with reference to the prior art mentioned, at the same time offering many other advantages, amongst which is the fact that it is possible to use enriched air as the gas (that is, nitrogen with a considerable oxygen component) rather than pure nitrogen, with a consequent saving in running costs. This advantageously also permits the production of solid-state polymerization apparatus with low production capacity for insertion in lines immediately upstream of a moulding unit for producing the finished product, avoiding the stage of dehydration of the upgraded resin which comes from conventional, large solid-state polymerization apparatus. Moreover, the resin is thus supplied to the moulding unit at a higher temperature, increasing production capacity thereof.

A further consequent advantage is the improved quality of the finished product which, since a heating cycle is avoided, has less degradation and a lower dust content.