The object of the present invention is to provide a process of the above indicated type which will allow the production of an expanded product of adequate characteristics for the above listed uses. This involves, in particular, obtaining a final product of density at least to 40kg/m3, uniform cellular structure of fine cells, and without degradations evidenced by reductions in molecular weight and viscosity with respect to the starting product, which are due for example to possible hydrolysis reactions.

According to the invention this object is achieved by a process and apparatus for its performance having the characteristics specifically set out in the following claims.

Further advantages and characteristics of the present invention will become apparent from the following detailed description, provided purely by way of non-limitative example, with reference to the attached drawings, in which; Figure 1 is a schematic side view of apparatus for performing the process of the invention; Figure 2 is a sectional view on an enlarged scale of a detail of the extruder of the apparatus of the invention, and Figure 3 is a section on an enlarged scale of the nozzle of the extruder of the apparatus of the invention.

Apparatus for the production of expanded polyester comprises (Figure 1) a drier 10 of polyester starting material and a conveyor device 12 for conveying this latter to an extruder 14. Both the drier 10 and the conveyor device 12 are known per se and therefore will not be described in detail herein.

The extruder 14 has a tubular body 16 with an internal cavity in which there can be distinguished, in succession, a polyester feed and melting zone 18 and a cooling and homogenisation zone 20 comprising a static heat exchanger 22 and a homogeniser 24.

In the feed and melting zone 18 the body has apertures not shown in the drawings for the introduction of polyester starting material and an expanding agent.

In the cavity of the feed zone 18 are located two interpenetrating and co-rotating screws 26 with a ratio of axial separation (i.e. the distance between the respective central lines) to diameter lying between 0.6 and 0.8, and preferably between 0.69 and 0.78, and a length to diameter ratio lying between 20 and 26.

Each screw 26 comprises (Figure 2) three conveyor sections 28a, 28b, 28c separated from one another by two back mixing sections 30a, 30b.

Downstream from the cooling and homogenisation section 20 of the extruder 14 is located a nozzle 32 which (Figure 3) has two facing lips 34 having a rectilinear profile in a direction transverse that of the extrusion so as to define a rectangular extrusion orifice which causes the formation of a laminar extruded product. Moreover the lips 34 have, in a direction parallel to that of the extrusion, a profile with a protuberance 36 facing the corresponding protuberance 36 of the opposite lip 34 and joined to the exit edge 38.

Downstream from the nozzle 32 (Figure 1) is located a calibration device 40 for calibrating the thickness of the extruded product, comprising a pair of belts 42 rotating in closed loops and having respective facing sides which define a space of adjustable height through which the product passes.

Advantageously the belts 42 are of glass wool clad in fluorated polymer and can be temperature regulated.

In a manner known per se, downstream of the calibration device 40 there are further located a roller table 44 passing over which the extruded product can cool, a cutting device 46 and various finishing and preparation tools 48 for storage of the cut extruded product.

The operation of the apparatus just described is as follows.

A polyester starting material having an intrinsic viscosity of at least 0.8dl/g, preferably at least 0.9dl/g, is transferred by means of the conveyor device 12 from the drier 10 to the feed zone 18 of the extruder 14 at the first conveyor section 28a of the screws 26.

Advantageously the polyester starting material is obtained by polycondensation of an aromatic bicarboxylic acid, such as terephthalic acid, isophthalic acid or naphthalene dicarboxylic acid, with a diol such as ethylene glycol, tetramethylene glycol, cyclohexane dimethanol or 1,4- butanediol, in the presence of various possible additives (chain extenders, stabilisers, nucleating agents and fireproofing agents) known in the art. The preferred starting material is in particular PET.

It is intended that the intrinsic viscosity is determined according to the ASTM D 4603-86 regulations on solutions of 0.5g of granular polyester in 100 ml of a 60/40 mixture by weight of phenol and tetrachloroethane at 250c.

An expanding agent is fed in at the second conveyor section 28b of the screws 26. This latter agent can be any of those commonly used in the production of expanded plastics materials. For example it can be chosen from the group comprising inert gases (in particular carbon dioxide and nitrogen), aliphatic, cycloaliphatic and aromatic hydrocarbons, and partially or completely halogenated hydrocarbons (CFC, HCFC, HFA).

The temperature of the polyester with added expanding agent in the feed and melting zone lies between 280 0c and 310 0c whilst in the cooling and homogenisation zone it lies between 220 0c and 2800c. The screws 26 are made to rotate at a relatively low speed lying between 15 and 40rpm.

After a residence time of between 15 and 30 minutes in the extruder, the polyester with added expanded agent is extruded through the orifice of the nozzle 32 in the form of a laminar product having a density lying between 40 and 200 kg/m3 and an intrinsic viscosity substantially unchanged with respect to that of the starting material.

These characteristics are optimal in view of the uses to which the extruded product is intended to be put and are obtained thanks in particular to the shape and the manner of rotation of the screws which do not impart damaging stresses to the melt which could compromise its physical and chemical properties.

The thickness of the extruded product can then be regulated to the desired value following passage through the calibration device 40. The extruded product, translating on the roller table 44, then cools until it assumes a consistency such that manipulations to which it is subsequently subjected do not damage it.

It can thus be cut into the desired dimensions by the device 46 and finally be finished and prepared for storage and transport by the various tools 48.

Naturally, the principle of the invention remaining the same, the details of construction and embodiments can be widely varied with respect to what has been described and illustrated in the drawings, without by this departing from the ambit of the present invention.