WO2000056510A1

WO2000056510A1 - Method and apparatus for the treatment of materials in the production of hollow bodies of polyethylene terephtalate

Info

Publication number: WO2000056510A1
Application number: PCT/EP2000/001164
Authority: WO
Inventors: Lucia Comper; Luca Stramare; Stefano Porta
Original assignee: Sipa S.P.A.
Priority date: 1999-03-24
Filing date: 2000-02-12
Publication date: 2000-09-28
Also published as: ITPN990031A1; ITPN990031A0; IT1311213B1

Abstract

Process pre-treating a mass of thermoplastic resin in its pelletized state, such as P.E.T., which precedes or anyway integrates the drying treatment and consists in maintaining said resin in a flow of inert gas heated to approx. 170 °C for a period of not less than two to three hours, so as to allow said mass of resin to be thoroughly exposed to said flow of inert gas. The purpose of this process is to remove as much free oxygen as possible from said mass of resin, so that the final moulded products obtained from the same resin, i.e. food-grade containers, are almost free from oxygen that might migrate to the food held in the same containers and contaminate it. In a preferred manner, said inert gases are nitrogen or carbon dioxide. The apparatus that carries out this process comprises a container provided with inert-gas blowing means, inert-gas suction means, inert-gas pre-heating means, as well as means for transferring the material from said container to a feeding hopper, under fully sealed conditions with respect to the outside ambient.

Description

METHOD AND APPARATUS FOR THE TREATMENT OF MATERIALS IN THE PRODUCTION OF HOLLOW BODIES OF POLYETHYLENE TEREPHTALATE

DESCRIPTION

15

The present invention refers to an improved method and a related apparatus for the production of hollow bodies of polyethylene terephtalate or P E T, or P E.N , or a mixture of these thermoplastic materials m their peUetized state, of the generally known type used to produce semi-finished products, of the type commonly known as 20 preforms or pansons, adapted to be subsequently conditioned thermally and blow- moulded for conversion into finished containers, in particular bottles, featuring a very low content of oxygen which might mdesirably be released and migrate to, ana therefore possibly contaminate, the substances that are subsequently filled into said containers

25

In particular, the present invention relates to a method and related apparatus for treating said thermoplastic materials, starting from the moment in which they are still m a state of pelleuzeα mass, in orαer to remove from them the greatest possible amount of oxygen absorbed and present at the moment of the polymerization thereof

The process currently useα to produce containers of the above cited kind is generally known to be of two basic types, le single-stage or two-stage Although the characteristics of ana differences between said two types of processes are generally known in the art, they will anyway be shortly revieweα below, particularly in order to put the emphasis on the differences in their oasic behaviour patterns with particular reference to the effects thereof on the variation in the oxygen content of the final container, which is actually the subject of the present invention

Short review of relevant features of blow-moulding processes

The raw material used in both single-stage and two-stage processes is partially crystalline PET, with a crystaHimty of mass-50% The material is available m pelletized form, typically with a grain size of just a few millimeters, and comes from a storage facility where it is generally kept under ambient temperature and pressure conditions in atmospheric air Before being sent to extrusion, the pelletizeα material is dried m an air current at atmospheric pressure and a temperature of 150°C, for a length of time of approx 5 hours As a result, when it leaves the dryer, the PET has still a crystaHinity of approx 50% and an oxygen content which, as this will be indicated further on in this descnption, turns out to be drastically reduced and close to the equilibrium or steady-state one with air at 1 bar and 150°C

Two-stage process

Two-stage processes consist of two subsequent processing steps, the first one of which produces the so-called preforms, or pansons, while the second one then produces the bottles or containers in their final form and aspect

The first stage starts with the extrusion of the matenal, in which a working temperature of 280° is usually found to prevail m the extruαer screw, along with relatively moαest pressures The overall dwelling time of the PET in the extruder is approx 5 minutes

In this phase, the PET remains substantially m contact with air m the feeding hopper, where the temperature is approximately the material temperature when it leaves the dner, le approx 150°C, and the pressure is the atmosphenc one -n the screw, during both the melting ana the actual extrusion phases, there is no contact with air, so that these phases have no relevance as far as the variation in the oxygen content is concerned

The PET leaving the extruder is then conveyed to the moulds for the production of the preforms by means of an injection moulding operation The latter takes place at a temperature of approx 110°C for a duration of approx 11 seconds

In the so produced preform, the neck of the bottle is already formed in its final shape, le is already capable of ensuring the tight sealing function it is intended to after the resultmg bottle has been filled The remaining body of the preform must on the contrary be subsequently allowed to undergo a blow-moulding operation m view of being convened in a bottle in the final form and aspect thereof The kinetics of the process is such as to produce preforms of PET material which is completely amorphous, as this is on the other hand required m view of the subsequent processability thereof ana the required transparency of the finished bottle

The preform constitutes the product that is obtained m the first stage of the process Before being sent to further processing, the preforms so obtained in the first stage are then stored in contact with atmospheric air and kept in such storage conditions for even quite long penods of time

The seconα stage of the process is then carried out to the purpose of obtaining the finished bottles starting from the preforms These preforms come from the storage facility and are therefore in a state of equilibrium with the ambient air, m particular, the oxygen content is the equilibrium one with a partial 0₂ of 0 21 bar They are pre-heated to 100°C ana conditioned to such a temperature by means of IR radiation, m appropnate heating stations, again in the presence of ambient air This phase has an overall duration of approx 25 seconds

After this phase, the preforms are then transferred to the contiguous blow- moulding tools in which, by means of an appropriate rod being lowered, they are conferreα an axial stretching and, by means of air being appropriately blown, the PET matena-. constituting the bottle is conferred a circumferential stretching In this phase, the average temperature is 70°C and the wall thickness of the part being processed decreases considerably During blow moulding, and the resulting biaxial stretching, the material undergoes a partial crystallization of approx 35% The overall duration of the blow-mouidrng step is approx 7 seconds, of which 0 5 s are spent in contact with pnmary air at 10 bar, and 1 5 s are spent for blowing with secondary air at 38 bar, in the remaining time, the PET matenal is in contact with air at atmospheric pressure

Single-stage process In this kind of process, the production of the preform takes place by first extruding and then injection-moulding the PET matenal, following the same procedure as used in the previously described two-stage methoα The basic difference from the latter lies here in the fact that the so obtained preforms are not cooled and then sent to storage, but are on the contrary exposed immediately to further processing in view of being sent directly to the subsequent conversion phase on the blow-moulding line

Preforms that leave the injection-moulding stage are at a temperature of approx 110°C and are conditioned at a temperature of 100°C for approx 21 seconds They are then conveyed to the blow-moulding tools, where they unαergo the same processing conditions as descnbeα previously for the two-stage process

Identification of salient phases as far as oxygen exchanges are concerned

The two above cited kmαs of processes are characterized m that they include a number of different phases, m which the matenal is exposeα to atmospheric air at different temperatures for different lengths of time As a result, the content of oxygen dissolved in the walls of the resulting bottles will be different for the two processes

To the purpose of providing a comparative assessment of the two different oxygen contents resulting m the PET material from the two processes, the salient phases are considereα m which oxygen exchanges take place successively

Since the dimensions ana, in particular the thicknesses involved are of great importance for the rate of exchange of the oxygen by diffusion reference is maαe to the typical dimensions of a bottle having a volume of 0 500 litres ana a mass of 25 gr Correspondingly, the thickness of the walls m the preform is 3 4 mm, whereas the final bottle has a wall thickness of approx 0 3 mm

Two-stage process

In this process, the preforms of amorphous PET obtained in the first stage thereof are stored m atmospheric air for an undetermined and generally very long period of time As a result, at the beginning of the second stage of the process they are provideα with an oxygen content that is equal to the equilibnum one for the amorphous material m atmospheric air at ambient temperature, le approx 25°C on an average

Unαer such an initial condition they are then exposed to air at 100°C for 25 seconds during the heating/conditioning phase, in this phase, the wall thickness is the one of the original preform, less an amount due to the effect of a possible heat expansion Since oxygen solubility decreases with an increasing temperature, in this phase the matenal will tend to release part of the oxygen contained initially

As indicated earlier in this description, in the subsequent blow-moulding phase, which takes place at an average temperature of 70°C, the preform is stretched and blown unαer exposure to primary air at a pressure of 10 bar for approx 0 5 seconαs ana secondary air at a pressure of 38 bar for approx 1 5 seconαs In this process, the wall thickness undergoes a significant reduction and, at the same time, a partial crystallization

In this particular stage of the process, the phase that most of all contributes to the variation in the oxygen containeα in the bottle is the stretching action induced by the secondary air Such an action takes place by exerting a considerably greater pressure upon a larger exposed surface for a longer length of time than typically experienced with the action brought about by the primary air

For this reason, the mass contribution of oxygen absorbeα in the bottle in this stage of the process is estimateα by considering the exposure of the inner surface of the bottle, in the final dimensions thereof, to a phase of air at 38 bar for a time of 1.5 seconds.

Conclusively, to the purpose of assessing the most significant oxygen exchanges taking place in the process, the latter can be schematically broken down into following phases, for each one of them the main characteristics are indicated below:

Single-stage process This process certainly required a closer, more detailed analysis m this connection, since the process itself is actually fed with pelletized matenal having a 50% of crystallinity and coming from storage and, therefore, saturated with respect to the atmosphenc air. The salient phases to be considered here are:

1) drying for 5 hours m air at 150°C of a pellet having an edge of a few millimetres; conservatively, the pellet is considered as havmg an edge of 3 mm, although it usually has a smaller grain size,

2) extrusion at 280°C for 5 minutes, as already discussed earlier in this description, this phase has no practical relevance as far as oxygen exchange is concerned, since a contact with air takes only place during the short period of time spent in the feeding hopper, and this actually has only minor, even negligible effects as compared with the ones connected to the preceding drying phase,

3) injection moulding of the preforms at 110°C for 11 seconds, with a cross-section size, or wall-thickness, of 3 4 mm, in this phase, the surface that is exposed to atmospheric air is limited to the one of the flow front of the molten polymer moving forwards into the mould, such a surface is negligible if compared with the total surface area of the preform, and it is anyway exposed to air at atmospheric pressure and not at any higher pressure, even in this phase the exchanged mass of oxygen is negligible,

4) thermal conditioning for 21 seconds at 100°C in air at 1 bar;

5) blow moulding at 70°C with air blown at pressures of up to 38 bar, as indicated earlier in this description, the preform is stretched and blown under exposure to primary air at 10 bar for approx 0 5 seconds, and secondary air at 38 bar for approx 1 5 seconds In this process, the wall thickness undergoes a significant reαuction and, at the same time, a partial crystallization To the purpose of calculating the oxygen that is exchanged with the air by the bottle, it is reasonable to assume that the latter essentially absorbs oxygen in the blow moulding phase with air at 38 bar for a duration of 1 5 seconds The action of the air is exerted on the bottle in the by now ultimate form and shape thereof, therefore with a wall thickness amounting to 0 3 mm The surface of the bottle that is exposed to the action of the air at 38 bar is solely the inner one

Conclusively, to the purpose of assessing the most significant oxygen exchanges taking place m the single-stage process, the latter can be schematically broken down into following phases, for each one of them the main characteristics are indicated below:

Mass of oxygen exchanged in each processing phase and mass of oxygen contained at the end of each phase In consideration of the schematizations that have been useα for the geometry of the matnx, the initial conditions and the ambient and general conditions prevailing m the two types of processes considered (Tables 1 and 2 above), the mass of oxygen exchanged in each single phase of said processes can now be assesseα The mass of oxygen contained in the PET matnx can be calculated from a simple material balance M_,_ = M i_mnit_hia_l + M ^.exchanged

Oxygen contents have been therefore found out with this procedure Owing to the calculation procedure being quite long and complex to explain in full detail, this will be omitted here for reasons of greater simplicity and brevity Anyway, the results obtained for the oxygen content in the PET matenal at the end of the single phases included in both single-stage and two-stage processes are indicated in Tables 3 ana 4, respectively (in mass ppm) For a more complete analysis, also the penod of time spent by the just produced bottles in contact with the ambient atmosphere during the phase m which they are collected as they leave the moulding plant has been considered in the calculations, although such a phase is generally known for anyway having an altogether negligible effect in this connection

Table 3 Oxygen content at the end of the most cntical phases of the two-stage process

Table 4 Oxygen content at the end of the most cntical phases of the single-stage process

As it can be noticed, the two-stage process produces bottles with a capacity of 0 500 litres and a mass of 25 g, with an oxygen content that is estimated to amount to 23 8 ppm, whereas the single-stage process produces the same bottles with an oxygen content that is estimated to amount to 8 7 ppm, le an oxygen content that is lower by as much as a factor 2 7 than the one resulting from the two-stage process With reference to the stated aim of limiting the final oxygen content in the PET matenal, such a ratio represents a good measure of the advantageousness of the single-stage process as compared to the two-stage one

It is interesting to notice that, when the mass of oxygen contained m the PET is transferred to the liquiα contained in the bottle, the oxygen content in the nquid undergoes an increase by 1 2 ppm when bottles produced with a two-stage process are used whereas such an increase amounts to just 0 44 ppm when use is maαe of bottles out of a smgle-stage process From the results that have been obtained it is possible to infer that, with the usual operating times and conditions recurring in the afore considered processes, the amount of oxygen contained m the walls of the bottles is substantially affected by the actual oxygen content m the PET matenal being fed to the plant producing the same bottles In particular, the value that is obtained m the two-stage process is practically dictated by the 0₂ content in the preforms that arnve from storage, whereas in smgle-stage processes such a value is on the contrary controlled by the oxygen content in the pellets that are fed to the extruder and come from the dner

As discussed earlier in this description, the assessment of the oxygen content m the preforms that are fed to the two-stage process is made on the basis of values of the coefficient of solubility that do not take into account the rapid cool-down phase provided for obtaining the same preforms, so that they shall for sure be considered as bemg underestimated to a significant extent Therefore, the value 2 7 assigned to the afore cited factor of comparison must correspondingly be regarded as constituting a lower limit, since the actual value of such a factor is most probably much higher than that

An analysis of the afore indicated results and values shows that, if the oxygen content in the PET matenal of the bottles has to be lowereα, the neeα first of ail anses for the oxygen content in the PET material fed to the production plant to be substantially reduced Now, this does not appear as bemg capable of being easily carried out and obtained m a two-stage process, since it does not seem a proposable option for the preforms to be kept in a storage facility with an oxygen- free atmosphere Quite on the contrary, such an aim seems much more easily pursuable m the case of smgle-stage processes, m which the pellets are anyway dned for a penod of approx 5 hours immediately before bemg fed to the extruder

In fact, prior-art solutions, in connection with which following patents are cited by mere way of example

- US 4,820,463 - US 3,862,284 - US 4,820,463

- US 4,764,405 - US 5,244,615 - US 4 880,675 - US 5,401 ,451 - US 5,213,734 - GB 2 053 775 teach that barrier properties of the container can be enhanced, and the extent or amount of oxygen absorbed by it can be reduced, by means of treatments that must be carried out substantially on the preform when the latter has already been moulded and finished, in particular immediately before and during the blow- moulding phase

However, all these solutions share a common drawback m that they are strongly conditioned, le affected by both the limited effectiveness of the results obtained and the complexity of the processes and the plants involved, which further contnbutes to the high costs and, eventually, the poor attractiveness of the same solutions

It is therefore a mam purpose of the present invention to provide a method and an apparatus that are capable of treating these thermoplastic materials in such a manner as to remove from them a significant amount of the oxygen absorbed by them before being melted and compacted, m a simple, reliable and safe manner and with the use of readily available techniques

Such mam aim of the present invention, along with further features thereof, are reacheα in a method and an apparatus that are made and operate as recited m the appended claims

The features ana advantages of the present invention will anyway be more readily understood from the description that is given below by way of non-limiting example, which may be therefore be the subject of a number of variants and modifications, with reference to the accompanying drawings, in which

- Figure 1 is a symbolical, block-diagram view of a smgle-stage plant according to the pnor art,

- Figure 2 is a view of the same plant to which an apparatus according to the present invention, represented symbolically, has however been added, - Figure 3 is a view of a preferred, basic configuration of a portion of the apparatus indicated in Figure 2,

- Figure 4 is a view of the same apparatus shown m Figure 2, in a preferred embodiment thereof, as associated to a feeding hopper of a single-stage or a two- stage plant

If reference is made to Figure 1 , this is shown to schematically illustrate a single- stage plant for the production of plastic bottles, according to the general state of the art This plant can be noticed to essentially consists of a hopper 1, in which the thermoplastic material is introduced m its pelletized state, an extruαer 2, m which said thermoplastic matenal is melted and extruded mto a plurality of multi-cavity moulds 3 m view of obtaining from these a sequence of preforms These preforms are then removed from said moulds and introduced, by means of appropnate handling means, and usually by passmg through appropriate temperature conditioning and levelling-off stations 4, in respective blow-moulding tools 5 m which said preforms are therefore blow moulded and converted mto the desired final product

These types of plants, as well as the processes that are typically carried out in them, are largely known m the art, so that no need anses here to αweil upon them any longer

The present invention applies to both smgle-stage and two-stage processes, wherem it shall be appreciated that, m the case of two-stage processes, it only applies to that part of the process which starts with the filling of the thermoplastic material m its pelletized state m the feeding hopper and ends with the removal of the preforms from the injection-moulding moulds

It will on the other hand be also appreciated that the greatest aαvantage is obtaineα with the application of the present invention to smgie-stage processes and plants, although no actual prejudice seems to exist that might impair or even talk against an application to two-stage processes and plants According to the present invention, the mass of pelletized PET to be treated is initially introduced in a container mto which a current of gas, which will be defined as mert m the following descnption, thereby meaning any type of gas that would not bmd or combine chemically with said thermoplastic material, is insufflated through appropnately arranged mjectors, nozzles or grilles

It has m fact been found expenmentally that ventilating a mass of pelletized thermoplastic material for a certain penod of time has the property of absorbing and/or promoting the removal of the free oxygen being contained in said matenal

In this way, when said matenal is filled in the hopper for being extruded, its content of free oxygen is alreaαy drastically reduced, so that the same material is inherently adapted to be converted first mto preforms and then mto finisheα blow- moulded containers that are m practice almost totally free of oxygen and, therefore, will not release any appreciable amount of oxygen mto the foodstuffs that are eventually filled in said containers

Durmg the expenmentation and assessment of the characteristics of the process used to insufflate saiα stream of mert gas agamst and through said pelletized thermoplastic material, it has been observed that such a process may be earned out according to various modes of implementation and under various operating conditions tending to improve and/or optimize the final results

The first one of these operatmg conditions relates to the temperature of said mert gas, which should be pre-heated to a temperature ranging from 120°C and 180°C before bemg blown mto the container m which the mass of pelletized thermoplastic material to be treated has been collected It has been furthermore been observed that the above cited oxygen-removing process produces an additional advantage durmg stretchmg it m fact contnbutes to a reduction in the moisture content m the PET pellets and this, as anyone skilled m the art generally knows very well, proves to be a considerable aαvantage, since the lower moisture content induceα in the pelletizeα thermoplastic material favours actually the crystallization process in the surface portion of the preform during blow moulding, owing to the fact that the moisture that is present in the matenal actually acts as a lubricant to promote the reassembling of the molecules, thereby reducmg molecular fnction between them and, therefore, reducmg also the heat generated by said friction, and ultimately lowering the extent of crystallization of the bottle

It is therefore an obvious conclusion that the method according to the present invention for the removal of free oxygen from the mass of thermoplastic matenal ideally combines mto the known process used to remove the moisture from the same matenal before extrusion by means of an appropnately heated-up flow of gas blown therethrough for such a long time as required to obtam the desired degree of dryness

The second condition relates to the actual flow rate of the mert gas and the duration of the treatment It has been found experimentally that optimum results are generally obtained when mert gas is insufflated at a specific flow rate (m relation to the weight of the matenal) of 0 5 m /hr kg for an accordingly shorter duration of two hours

Those skilled m the art will at this pomt be fully capable of appreciatmg that similar results can be obtained by suitably combining intermediate values of such parameters, although still acceptable results may be reached under extreme conditions at the boundaries of the above cited ones

The third conditions relates to the nature of the mert gas used to the purpose It has of course been venfied that nitrogen serves very well the purpose However, also carbon dioxide may be used effectively, without any significant contraindications, as a further gas that neither bmds or combines with oxygen nor interacts with the thermoplastic material under the particular treatment conditions

The fourth condition consists m providing for said mass of thermoplastic material to be suitably stirred as it is bemg exposed to such a flow of mert gas, so as to improve both the uniformity of the passage of the mert gas over almost the totality of the surface of the pelletizeα matenal, and the condition according to which the temperature of said mert gas when the same mvests the matenal_, must be as unⁱform and constant as possible In fact, if the openmg through which the gas is ⁱnsufflated m the container is situated on a side of said container_, while the gas exhaust openmg is of course situated on the opposite side thereof_, as this is quite frequently the case, it ensues that the mass of matenal near the gas exhaust openmg ⁱs ⁱnvested by the flow of mert gas after the same has flown through the greatest bulk of the remⁱanmg mass of matenal and, therefore, has cooled down accordingly The result ⁱs that saⁱd mass near the gas exhaust openmg is heated up to a lower level than the requⁱred one, which thing practically means that both oxygen and moisture removal effects are lowereα

As far as the apparatus for carrying out the above descnbed method is concerned^, reference should be made to Figures 2 and 3, which illustrate a container 20 provⁱded with an internal rotating shaft 21 on which there are applied_, by means of approprⁱate arms, a plurality of stirring paddles 22, in which said rotating shaft is dnven by per se known means

However^, any other means for stirring the pelletized contents of the container may prove adequate, provided that it is actually capable of ensuring a continuous uniform stirring of the material

In an appropriate zone m the mtenor of the container there is provided an mert gas dⁱffuser 24^, which diffuses mto the mass of peUetized material 25 a stream of mert gas blown by per se known means (not shown) and appropnately heated up The contaⁱner ⁱs also proviαeα with an openmg 26 through which the mert gas is exhausted upon its havmg so treated said pelletized matenal, wherem said mert gas may of course be recovered at the exhaust view of bemg regenerated and reused

Furthermore^, m view of an improved efficiency and proαuction continuity, there ⁱs provⁱdeα an extruder feeding hopper 27, which is in turn connected_, via an appropnate con^duit 28 to said container 20 m which the process according to the present mvention is carried out and which is adapteα and controlleα so as to cause the therein contameα pelletized matenal to be discharged, after the treatment, mto said hopper To the purpose of preventmg any possible contacts with the atmosphere, and therefore any possible contaminations with the oxygen contained m said atmosphere, both said hopper and said conduit 28 have a sealed, airtight construction

Figure 4 illustrates an improved embodiment of the present mvention, m which the hopper and the container where the pre-heated mert gas is diffused are made and arranged so as to build a smgle body 30

The operation of the apparatus can at this point be quite easily inferred the contamer 20 is filled with the mass of PET pellets and the lmmission of heated mert gas is then activated at the same time with, possibly, the rotation of the paddles for stirring said mass of pelletized material

Such an operatmg condition is maintained for a penod of at least two hours, durmg which the material undergoes a progressive transformation that leads it to assume the afore mentioned and defined properties of dryness and quasi-absence of oxygen It will be appreciated that such an msufflation of mert gas mto the mass of thermoplastic material while the latter is bemg stirred m view of preventmg it from agglomerating may be implemented with any other means suiting the purpose For instance, use can be made of a rotatmg contamer, of the type similar to the mortar or cement mixing machines and mdustnal machines used to mix the most vaned substances, provided with mtemal nbs attached to the inner wall of the contamer for ensuring the mixing or stirring action, while the mert gas diffuser may be implemented so as to be able to be introduced and removed upon appropnate command through the filling mouth of the mixer-like contamer A further option may be a contamer arrangeα to rotate about a horizontal axis and provided with two mouths or openmgs situated in mutually opposed positions in order to enable the mert gas to be blown therem ana exhausted therefrom, respectively

Claims

1 Method for the neutralization of a mass of resin, PET (polyethylene terephtalate), PEN or other blended thermoplastic materials m a pelletized form, charactenzed m that said mass of resm in a pelletizeα form is filled m a contamer m which a flow of mert gas is forcedly blown so as to enable said mert gas to substantially mvest the whole mass of said pelletizeα material by penetreting the interstices thereof

2 Method according to claim 1 , charactenzed m that said mert gas is pre-heated to a temperature ranging from 120°C to 180°C

3 Methoα according to claim 2, charactenzed m that the flow rate of said mert gas is compnsed between 0 5 m³/hr kg and 5 m³/hr kg of pelletized matenal m its original state, and that the duration of said drying and neutralizing treatment is compnszed between 2 and 10 hours

4 Method according to any of the preceding claims, charactenzed m that said mert gas is nitrogen or carbon αioxide

5 Methoα for the production of hollow bodies of thermoplastic matenal, compnsmg the phases in which a mass of thermoplastic material is melteα and is then extruded mto a plurality of intermediate products, as well as a phase m which said intermediate products are blow moulded for conversion mto the desired finished products, charactenzed m that it further compnses a neutralization phase earned out with mert gas m accordance with any of the preceding claims or any combination thereof, and that this neutralization phase is earned out immediately before said phases in which said thermoplastic material is melted and extruded

6. Apparatus for the production of hollow bodies out of a thermoplastic material which is fed m its pelletized state to the same apparatus, charactenzed m that it compnses a contamer (20) along with

- means for continuously stirnng and mixing said pelletized thermoplastic matenal, - and means for blowing m and circulating a continuous flow of mert gas m said contamer

7 Apparatus according to claim 6, charactenzed m that said contamer (20) is provided with at least a rotating shaft (21) carrymg a plurality of stirnng blaαes or paddles (22) applied to said shaft by means of appropnate arms, and appropnately provided with dnvmg means

8 Apparatus according to claim 7, charactenzed m that said means for blowing in and continuously circulating said flow of mert gas comprise - at least an lmmission openmg (24) adapted to diffuse said mert gas within the mass of said thermoplastic material m said contamer,

- at least an exhaust openmg (26) for letting out said mert gas blown mto said container, said exhaust openmg bemg preferably arranged on the opposite side with respect to said gas lmmission openmg

9 Apparatus according to any of the preceding claims 6 to 8, charactenzed in that it is adapted to directly discharge, via an appropriate conduit (28), the pelletized material contained m said contamer mto the feeding hopper (27) of an extruder

10 Apparatus according to claim 9, charactenzed in that said conduit (28) and/or said hopper (27) have an air-tight construction sealing them agamst the outside atmosphere