HK1071386A - Polyesters having a low iv and a low acetaldehyde content, performs and containers obtained from such a polymer - Google Patents

Description

Low IV and low acetaldehyde polyester, preforms and containers therefrom

Technical Field

The present invention relates to polyester resins, in particular polyethylene terephthalate resins, used in the production of hollow containers, examples being bottles for the packaging of liquid substances, in particular carbonated beverages and natural or mineral water.

Background

For decades, polyester resins, in particular polyethylene terephthalate, abbreviated as PET, have found increasingly widespread use in the production of hollow containers, in particular bottles.

Among the constraints on PET applied in this field, the transparency of the resulting container is one of the most important elements. Copolymers containing predominantly recurring ethylene glycol terephthalate units, but also other recurring units derived from monomers other than terephthalic acid and ethylene glycol present, have been proposed for many years, particularly in european patent 41035. These monomers, known as crystallization retarders, are present in the polyester at various concentrations, for example, 3.5 to 7.5% of all diacid monomers.

Another constraint that must be met by polyesters is the acetaldehyde content in the container walls. Acetaldehyde is, in fact, a decomposition product of the polyester, which is formed during the high temperature production of the polyester and during the bottle production, in particular during the preform injection molding stage. Various methods are described in various patents for obtaining polyesters with low acetaldehyde content or limiting their formation during molding.

The industrially applied technique described in particular in European patent 41035 relates to the polycondensation of polyesters in a molten medium, limiting the degree of polymerization thereof, in order to avoid the formation of significant decomposition products, which generally form acetaldehyde, for example when the polyester is heated to temperatures above 200 ℃. The polyester thus obtained is generally polymerized again in the solid phase in a nitrogen atmosphere at relatively low temperatures of about 200-220 ℃ which is known as post-condensation in the solid phase. In this operation, the polyester is polycondensed to a sufficiently high degree of polycondensation or intrinsic viscosity to give the desired mechanical properties, in particular to eliminate the formation of acetaldehyde and the formation of a large number of decomposition products during polycondensation in the molten medium.

In this way polyester resins with acetaldehyde concentrations of less than 2ppm, even less than 1ppm, can be obtained.

However, the polycondensation stage in the solid phase requires special equipment and consumes energy. In addition, the use of a polyester resin having a high intrinsic viscosity leads to low fluidity in a molten medium, which is fatal to an injection molding cycle (a stage from the start of injection molding to the ejection of a part from a mold). However, it seems difficult to provide a polyester resin which can satisfy the standards regarding acetaldehyde without passing through the polycondensation stage in the solid phase, and therefore it is difficult to produce preforms and bottles having a specified acetaldehyde content during injection molding with a polyester having a low degree of polymerization with a low acetaldehyde content and the lowest possible acetaldehyde formation rate.

It is a main object of the present invention to overcome these drawbacks and to provide a polyester resin with a low degree of polycondensation with low acetaldehyde content for the production of bottles particularly suitable for the packaging of liquid foods for daily use such as carbonated beverages, natural waters or carbonated or non-carbonated mineral waters.

To this end, the invention provides a polyester comprising at least 92.5% in number of recurring units derived from terephthalic acid and an aliphatic diol, characterized in that it has an intrinsic viscosity of from 0.45dl/g to 0.70dl/g and an acetaldehyde content of less than 3ppm, preferably less than 1.5 ppm.

To prevent the polyester from binding or sticking at temperatures below about 200 ℃, the polyester is preferably crystallized. Thus, the polyester can be treated to obtain a crystallinity advantageously higher than 15%.

Advantageously, the intrinsic viscosity of the polyester is between 0.45dl/g and 0.65 dl/g.

The term Intrinsic Viscosity (IV) denotes the viscosity of the polymer solution at zero concentration. The Viscosity Index (VI) in dl/g of a polymer solution containing 0.5g of the polymer in 100ml of a solvent consisting of o-dichlorobenzene and phenol (50wt/50wt) was determined at 25 ℃ according to ISO standard 1628/5 determined on 6/15/1986, and the intrinsic viscosity value was calculated using the following formula (I). For the polyesters of the invention, the Intrinsic Viscosity (IV) in dl/g is calculated by the following formula I:

IV＝-10^-1VI²+0.94VI+0.0122 (I)

advantageously, the polyesters are obtained from terephthalic acid, esters thereof or mixtures thereof and aliphatic diols such as ethylene glycol, 1, 3-propanediol or 1, 4-butanediol.

Preferred monomers are terephthalic acid and ethylene glycol, which will yield the polyethylene terephthalate known above for short as PET.

In this application, the term PET also includes homopolymers obtained from terephthalic acid or its esters such as dimethyl terephthalate alone and copolymers containing at least 92.5% by number of repeating ethylene terephthalate units.

According to a preferred feature of the invention, the polyester contains at least one crystallisation retarder for slowing down or delaying the crystallisation of the polyester, in particular during cooling of moulded or injected articles such as preforms, with the aim of allowing them to crystallise into very small crystals, while avoiding spherical (spherulitic) crystallisation, thereby enabling the formation of transparent products whose walls are not hazy and which have acceptable mechanical properties.

These crystallization retarders are bifunctional compounds such as diacids and/or diols added to the monomer mixture before or during polymerization of the polyester.

Examples of diacids suitable for use as crystallization retarders include isophthalic acid, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, cyclohexanediacetic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, and sebacic acid, and examples of suitable diols include aliphatic diols having 3 to 20 carbon atoms, cycloaliphatic diols having 6 to 20 carbon atoms, aromatic diols having 6 to 14 carbon atoms, and mixtures thereof, such as diethylene glycol, triethylene glycol, isomers of 1, 4-cyclohexanedimethanol, 1, 3-propanediol, 1, 4-butanediol, 1, 5-pentanediol, 3-methyl-2, 4-pentanediol, 2-methyl-1, 4-pentanediol, 2, 4-trimethyl-1, 3-pentanediol, 2-ethyl-1, 3-hexanediol, 2, 2-diethyl-1, 3-propanediol, 1, 3-hexanediol, 1, 4-bis (hydroxyethoxy) benzene, 2-bis (4-hydroxycyclohexyl) -propane, 2, 4-dihydroxy-1, 1, 3, 3-tetramethylcyclobutane, 2-bis (3-hydroxyethoxypentyl) propane, 2-bis (4-hydroxypropoxyphenyl) propane and mixtures thereof.

Diethylene glycol is typically inherently present in polyesters because it is formed during the synthesis of the condensation of two ethylene glycol molecules. Depending on the desired concentration of repeating units containing diethylene glycol (DEG) groups in the final polyester, diethylene glycol is added to the monomer mixture or the polymer synthesis conditions are controlled to limit the formation of diethylene glycol.

Advantageously, the molar concentration of diethylene glycol in the polyester is less than 3.5 mol%, preferably less than 2 mol%, relative to the moles of diacid monomers.

As for the other crystallization retarders, their molar concentration, based on the moles of all diacids in the monomer mixture and the polyester thus obtained, is advantageously lower than 7.5 mol%, provided that, if other crystallization retarders are present, the DEG content is subtracted from this value. As described in european patent 41035, the total molar concentration of crystallization retarders should be below 7.5%.

It is clear that the polyester may contain a mixture of crystallization retarders of the acid and/or glycol type.

In particular, if the preform injection molding process and the hollow container blowing process are capable of controlling the cooling rate to avoid spherical crystallization of the resin, or if the bottle to be produced is not translucent, such as a baby bottle, the total concentration of the crystallization retarder may be very low, for example, about 1% or even 0, regardless of the DEG formed during the polyester synthesis.

According to a preferred embodiment of the invention, the polyester of the invention preferably contains less than 4% isophthalic acid and less than 3.5% diethylene glycol, where the content denotes the mol% of crystallization retarder based on the moles of all diacid monomers.

The invention also relates to a process for the production of the polyesters of the invention. The method comprises the following steps: in the first step, the polyester is produced by polymerization in a molten medium, and the intrinsic viscosity of the resulting polyester is preferably from 0.45 to 0.8dl/g, more preferably from 0.45 to 0.75 dl/g.

In the second step, the polyester is pelletized by any suitable method known in the art of polymer production. The particles are subjected to a crystallization stage to prevent agglomeration of the particles at temperatures below 200 c, such as 130-200 c. In other words, the crystallization stage should form at least a skin or outer surface that is sufficiently crystalline to prevent the particles from sticking at a temperature of about 200 ℃. The facility method at this stage may be: the particles are maintained at a temperature of 120-170 c or treated with hot water, such as boiling water, at 80-100 c, or any other suitable method that will achieve the above-described effect.

In a third step, the polyester granules are heat treated by maintaining their temperature at 200 ℃ at 130-.

According to a characteristic of the invention, the heat treatment is carried out in the presence of a gas, preferably with a gas having a dew point higher than-60 ℃, preferably between-60 ℃ and 20 ℃, and in a preferred embodiment between 10 ℃ and 20 ℃.

The gas purge is carried out either through a fixed bed of particles or through a fluidized bed of particles, the purge gas preferably being a particulate fluidizing gas.

Advantageously, when the intrinsic viscosity of the polyester to be treated is between 0.7 and 0.8dl/g, the dew point of the purge gas is between-10 ℃ and 20 ℃ (both endpoints included). In this embodiment, the treatment process of the invention may comprise two successive steps, the first being a treatment with a gas having a high moisture content, even with steam, to rapidly reduce the intrinsic viscosity, and then a purge with a dry gas to remove acetaldehyde while limiting the change in intrinsic viscosity.

Advantageously, the gas suitable for use in the present invention is air, nitrogen, carbon dioxide or any other gas that does not damage the polyester.

In another preferred embodiment of the invention, the polyester obtained by polycondensation in the molten medium has a low intrinsic viscosity, for example ranging from 0.45 to 0.65dl/g, and can be subjected to a heat treatment with a gas having a low dew point, for example ranging from-60 ℃ to 0 ℃. During such treatment, the Intrinsic Viscosity (IV) increases slightly, but nevertheless advantageously remains below a value of 0.70dl/g, preferably below 0.65 dl/g.

Advantageously, the heat treatment is carried out at a temperature of 130-200 ℃ for a period of several hours to several tens of hours. The treatment time is determined according to the desired acetaldehyde content and intrinsic viscosity.

As mentioned above, the treated polyester is obtained by polycondensation in the melt phase, generally in the reduced pressure melt phase as described in the literature for more than 50 years.

This process comprises a first stage of esterification or transesterification in the presence or absence of a catalyst. The resulting hydrogenated or esterified material is then polycondensed under reduced pressure in the presence of a catalyst such as an antimony, titanium or germanium compound. At this stage, the alcohol or water is removed in order to carry out the polycondensation reaction.

According to the invention, the polycondensation is stopped when the degree of polycondensation or the intrinsic viscosity has reached the desired value of less than 0.8dl/g, preferably less than 0.75 dl/g.

The polyester obtained is poured into moulds to give bars which are then converted into granules by cutting. However, in order to form these rods, the polyester preferably must have a sufficiently high intrinsic viscosity, for example above 0.45 dl/g.

According to another embodiment of the invention, the polyester may contain monofunctional monomers, preferably monoacids, in order to limit the development of the degree of polycondensation during the heat treatment carried out to reduce the acetaldehyde content. The molar content of monofunctional monomers, based on all diacid monomers, is from 0.5 to 3 mol%. Thus, suitable monoacids for use in the present invention include benzoic acid, naphthalenedicarboxylic acid, fatty acids having boiling points compatible with the polyester synthesis process, in other words having boiling points at least higher than that of ethylene glycol or its esters or fatty alcohols such as cyclohexanol or fatty alcohols, and preferably also having boiling points higher than that of ethylene glycol.

The low acetaldehyde-content particles obtained by the process of the present invention have the acetaldehyde content and intrinsic viscosity specified in the present invention, and such particles are preferably used as a raw material for producing hollow containers such as bottles.

Various additives such as brighteners, dyes or other additives such as light or heat stabilizers or antioxidants may be added to the polyester of the present invention during polymerization, and these additives may also be added to the molten polyester prior to injection molding.

The granules are preferably dried to a moisture content of less than 50ppm, preferably less than 20 ppm. Such a drying step is not necessary if the moisture content of the polyester is sufficiently low.

The pellets are then introduced into an injection molding/blowing process to produce hollow containers such as bottles. These processes are described in various publications and are widely used in industry, which includes a first step of injection molding to form a preform. In a second step, the cooled or uncooled preform is heated and then optionally blown into the desired bottle shape by a double-drawing process.

For example, the polyester may also be plasticized by melting the resin in a single or twin screw injection molding machine to obtain a preform, which is then fed under pressure at a temperature such as 260 ℃ and 285 ℃ into a distributor equipped with a heated nozzle and a gate rod.

The resin is injected into at least one preform mould equipped with cooling means capable of controlling its cooling rate, so as to prevent spherical crystallization, the walls of the resulting preform preferably being not hazy and not opaque (if such a result is desired).

After cooling, the preforms are discharged and cooled at ambient temperature or are introduced directly into the blowing apparatus described below without cooling.

In the process of producing the preform, the polyester is melted at a temperature of about 280 ℃, such as 270-. In order to limit the formation of acetaldehyde, and in particular to reduce the rate of acetaldehyde formation, as low an injection molding temperature as possible is used.

In the preform production method, the pressure at which the molten resin is injected into the mold is preferably 2.5X 10⁷Pa(250bar)-5×10⁷Pa (500bar) and the temperature is preferably 260 ℃ and 270 ℃. If the pressure is below this range, the intrinsic viscosity of the polyester resin is high, i.e., above 0.70 dl/g.

Another advantage is that the mould is cooled to 0-10 c. The cooling may be carried out with any suitable coolant, such as an aqueous solution of ethylene glycol. Preferably about 10-20 seconds, for one injection molding and cooling cycle.

The intrinsic viscosity of the polyester forming the wall of the parison obtained in this way is between 0.45 and 0.70dl/g, preferably between 0.45 and 0.65 dl/g.

The acetaldehyde content of the preform is less than 10ppm, preferably less than 6 ppm.

The preforms thus obtained are generally used in the blowing process for producing bottles. These blowing processes have been widely used and are described in various publications.

These processes generally comprise the introduction of the preform into a blowing device equipped or not with an over-drawing device (over-drawing) comprising heating means.

The preform is heated to a temperature at least above the Tg (glass transition point) of the polymer and then pre-blown by injecting high pressure gas at a first pressure in a first stage.

A second gas injection at a second pressure will produce a bottle of final shape which is then discharged after cooling.

The heating temperature of the preform is preferably 80 to 100 ℃. The heating can be carried out by any suitable means, for example, infrared radiation directed at the outer surface of the preform.

Advantageously, the pre-blowing of the preform is carried out at 4X 10⁵Pa-10×10⁵Pa (4bar-10bar) for a pre-blowing time of 0.15-0.6 seconds.

The second blowing operation is at 3X 10⁶Pa-4×10⁶Pa (30-40bar) for a blowing time of 0.3-2 seconds.

It is known to introduce tie rods into the preform during the pre-blowing and/or blowing operations in order to partially stretch the preform.

The use of the polyesters of the invention makes it possible to produce bottles or hollow containers having a low acetaldehyde content. The invention therefore relates to a bottle whose wall is formed from polyester having an intrinsic viscosity of 0.45 to 0.70dl/g and an acetaldehyde content of less than 6 ppm.

In addition, the material enables the production of bottles having the same contents, the mechanical properties of which are at least equal to those of bottles obtained with polyesters having a higher intrinsic viscosity.

The invention also relates to hollow containers such as bottles obtained by the injection molding/blowing process described herein using the polyesters of the invention. However, hollow containers are obtained by molding the polyester of the present invention, and such molding methods are not those described in the present application, and such molding methods are also part of the present invention.

The bottle of the invention is intended for the packaging of any liquid product, in particular for the packaging of liquid daily food products such as sweet carbonated or non-carbonated beverages known as soda, various natural waters, spring waters, carbonated or non-carbonated mineral waters.

Further advantages of the present invention will be detailed in the following examples, which are intended to be illustrative only.

In the following examples, the acetaldehyde concentration was determined by the following analytical method:

-crushing a sample of particles (about 4g) in a cryogenic crusher under the protection of liquid nitrogen, producing a powder that can pass through an 800 μm mesh sieve (for example, in a Spex 6700 brand cryogenic crusher, the frequency regulator is crushed for 2 minutes at the minimum position and the frequency regulator is crushed for 5 minutes at the maximum position), the particle size of which is preferably 200-400 μm (crushing should be carried out without damaging the polymer).

-weighing 500mg of this powder substantially exactly and then placing it in a sealed end tube (head-space tube).

Placing this end tube in a gas chromatograph (Perkin Elmer HS 40 end tube, VARIAN 3500 chromatograph with PORAPLOTQ column, carrier gas consisting of helium at a flow rate of 5 ml/min), the temperature being raised from 60 ℃ to 230 ℃ (the rate of rise is +10 ℃/min).

-holding this end tube at 145 ℃ for 90 minutes.

The chromatographic correction was carried out with a solution of acetaldehyde in dimethylacetamide.

Example 1

A copolyester containing 2.3 mol% of isophthalic acid (as crystallization retarder) based on the total diacid monomer content and terephthalic acid and ethylene glycol as further monomers was formed in a polycondensation apparatus comprising a 7.5l reactor in the presence of 250ppm of an antimony oxide catalyst, expressed as antimony.

The polymerization was stopped when the intrinsic viscosity of the polyester was 0.48dl/g and the DEG content based on the moles of diacid monomers was 3.4 mol%. The polymer was extruded into rods and then cut to form pellets averaging 13mg per pellet weight.

The residual acetaldehyde content in the amorphous particles determined by the above method was 140 ppm.

The particles were crystallized in an electric furnace at 160 ℃ for 30 minutes and then placed in a column. The dry air stream (dew point below-60 ℃ C.) at 0.39Nm³The flow rate/h through the column was 8 hours, and the particle temperature was maintained at 180 ℃.

After 8 hours of treatment, the intrinsic viscosity of the polyester was 0.56dl/g and the acetaldehyde content was 0.75 ppm. This polyester is referred to as polyester A.

Examples 2 to 5

A polyester containing 2.3 mol% of isophthalic acid was produced in the same manner as in example 1. However, the polycondensation is stopped when the intrinsic viscosity is equal to 0.64 dl/g.

The acetaldehyde content in the amorphous particles was 45ppm and the DEG content was 3.4 mol%.

The particles were crystallized by holding them in an electric furnace at 160 ℃ for 30 minutes.

The particles were treated with various gases at different temperatures to remove acetaldehyde, in the same manner as in example 1.

The process conditions, acetaldehyde content and intrinsic viscosity are compiled in table I below:

TABLE I

Examples	Gas (es)	Temperature of	Time (h)	AA content (ppm)	IVdl/g	Polymer and method of making same
							2(1)	Drying air	183℃	10.30	0.8	0.60	B
3	Air (dew point-10 deg.C)	185℃	8	1.3	0.63	C
							4	Air (dew point +18 deg.C)	185℃	8	1.5	0.60	D
5	Air (dew point +18 deg.C)	200℃	8	1.6	0.56	E

(1) The method begins by pre-treating with steam instead of air for half an hour

The polymers B to E obtained in these examples improve the dyeability. This improvement is evidenced by a decrease in the yellowness index. The presence of moisture limits the deterioration of the yellowness index of the polyester.

Example 6

The homopolyester was formed by adding 2.6 mol% of benzoic acid based on terephthalic acid using the operating mode described in example 1.

When the intrinsic viscosity was 0.47dl/g, the polycondensation was stopped.

After pelletization, the resulting polymer contained 70ppm of acetaldehyde and 3.5 mol% of DEG.

After crystallization using the mode of operation of example 1 and treatment with dry air at 180 ℃ for 8 hours (dew point-60 ℃), the intrinsic viscosity of the granules was 0.53dl/g and the acetaldehyde content was 1.6 ppm. This polymer is referred to as polymer F.

Examples 7 to 9

Bottles were produced using certain polyesters obtained in the above examples by the process described below:

the polyester pellets were melted in an annular single-ended screw with a jacket temperature of 285 ℃. The molten polyester was fed to a preform injector under the trade designation "HUSKY 48 preform machine with XL 300 capsules" injection molding temperature of 262-. The preforms were cooled with circulating water at 8.5 ℃. The total cycle time for injection molding was 15.7 seconds.

The cooled preform was fed to a blowing device for producing bottles having a capacity of 0.5l, the neck of which was shaped as designated by the standard designation 28PCO and the bottom of which was shaped as a petal with 5 petals. The apparatus is sold under the trade name "SIDEL SBO 1F 2 Lab". The preforms were heated to the temperatures shown in table 2 below. Pre-blowing was carried out for 0.19 seconds at a blowing pressure of 8.5bar and blowing was carried out for 1.78 seconds at a blowing pressure of 38 bar. The speed of drawing the tube was 1.2 m/s.

The properties of the bottles and the blowing process obtained with each of the polyesters shown in the following table are shown in table II below.

TABLE II

Examples		7	8	9
					Polyester		A	B	F
Preform temperature		87℃	96℃	89℃
					Burst pressure		13.84bar	15.68bar	13.84bar
In-plane draw ratio		11.9	11.9	11.9
					Density (g/cm)3)	Shoulder-shaped part	1.363	1.3625	1.364
Middle part	1.3615	1.3632	1.363
				Bottom part		1.3612	1.3643	1.3622
Acetaldehyde content		2.85ppm	3.05ppm						3.00ppm

The viscosity of the polyester forming the bottle wall is approximately equal to the viscosity of the polyester used.

Claims (27)

1. Polyester comprising at least 92.5% by number of units derived from terephthalic acid and ethylene glycol, characterized in that it has an intrinsic viscosity of 0.45dl/g to 0.70dl/g and an acetaldehyde content of less than 3 ppm.

2. Polyester according to claim 1, characterized in that it has an intrinsic viscosity of 0.45dl/g to 0.65dl/g

3. Polyester according to any of the preceding claims, characterized in that the acetaldehyde content is below 1.5 ppm.

4. Polyester according to any of the preceding claims, characterized in that it contains at most 7.5 mol% of at least one crystallization retarding compound, based on the total number of moles of diacid.

5. Polyester according to any of the preceding claims, characterized in that it comprises 0.5% to 3% of monofunctional compounds, based on the moles of diacid.

6. Polyester according to claim 5, characterized in that the monofunctional compound is selected from the group comprising benzoic acid containing monocarboxylic acids, naphthoic acids, fatty acids or esters thereof or alcohols comprising cyclohexanol or fatty alcohols.

7. Polyester according to any of claims 4 or 5, characterized in that the crystallization retarder is a diacid compound and/or a diol compound.

8. Polyester according to claim 7, characterized in that the diacid crystallization retarders are selected from the group consisting of isophthalic acid, naphthalenedicarboxylic acid, cyclohexanedicarboxylic acid, cyclohexanediacetic acid, succinic acid, glutaric acid, adipic acid, azelaic acid and sebacic acid.

9. A polyester according to claim 7, characterized in that the diol crystallization retarders are selected from the group consisting of diethylene glycol, triethylene glycol, isomers of 1, 4-cyclohexanedimethanol, 1, 3-propanediol, 1, 4-butanediol, 1, 5-pentanediol, 3-methyl-2, 4-pentanediol, 2-methyl-1, 4-pentanediol, 2, 4-trimethyl-1, 3-pentanediol, 2-ethyl-1, 3-hexanediol, 2-diethyl-1, 3-propanediol, 1, 3-hexanediol, 1, 4-bis (hydroxyethoxy) benzene, 2-bis (4-hydroxycyclohexyl) -propane, 2, 4-dihydroxy-1, 1, 3, 3-tetramethylcyclobutane-ylcyclobutane, ethylene glycol, propylene, 2, 2-bis (3-hydroxyethoxyphenyl) propane, 2-bis (4-hydroxypropoxyphenyl) propane and mixtures thereof.

10. Process for the production of a polyester according to any one of the preceding claims, characterized in that it comprises carrying out a polymerization in a molten medium of a polyester having an Intrinsic Viscosity (IV) lower than 0.80dl/g, forming granules of said polyester, crystallizing said granules, and heat-treating said granules by heating to a temperature of 130 ℃ and 200 ℃ so as to reduce the acetaldehyde concentration value to a value lower than 3ppm, the intrinsic viscosity IV of the polyester obtained being comprised between 0.45 and 0.65 dl/g.

11. A method according to claim 10, characterized in that the heat treatment is carried out by purging the particulate material with a gas having a dew point above-60 ℃.

12. The method according to claim 11, characterized in that the dew point of the gas is-10 ℃ to 20 ℃ (both endpoints inclusive).

13. Process according to any one of claims 10 to 12, characterized in that the gas is selected from air, nitrogen and carbon dioxide.

14. Process according to any one of claims 10 to 13, characterized in that the polyester is crystallized by heating it to a temperature of 120-170 ℃.

15. A process according to any one of claims 10 to 13, characterised in that the polyester is crystallised by treating the particles with water at 80 to 100 ℃.

16. Process according to any one of claims 10 to 15, characterized in that the intrinsic viscosity IV of the polyester obtained by polycondensation in the melt phase is between 0.45 and 0.75 dl/g.

17. Process according to any one of claims 11 to 16, characterized in that the thermal treatment is carried out with a fixed bed of polyester particles traversed by a purge gas.

18. A process according to any one of claims 11 to 16, characterised in that the heat treatment is carried out with a fluidized bed of polyester particles, the particles being fluidized by a purge gas.

19. Process for the production of preforms from the polyester according to any of claims 1 to 9, characterized in that it comprises:

a step of injecting molten polyester into at least one mold to be formed into a preform,

the preform is rapidly cooled to prevent spherical crystallization.

20. Process according to claim 19, characterized in that the polyester is dried before injection molding to reduce the moisture content value to below 50 ppm.

21. The process according to any of claims 19 or 20, characterized in that the molten polyester is injected into at least one heated nozzle and at least one heated gate pin, which are heated to a temperature of 260 ℃ and 285 ℃ before being fed into the mould.

22. A method according to any of claims 19-21, characterized in that the pressure at which the molten resin is injected into the mould is 2.5 x 10⁷Pa(250bar)-5×10⁷Pa (500bar) and a temperature of 260 ℃ and 270 ℃.

23. A method according to any of claims 19-22, characterized in that the mould is cooled to 0-10 ℃.

24. Preform obtainable by the process according to any one of claims 19 to 23, characterized in that the intrinsic viscosity of the polyester is from 0.45 to 0.70 dl/g.

25. Preform according to claim 24, characterized in that the acetaldehyde content is lower than 6 ppm.

26. Hollow containers such as bottles obtained by blowing from preforms according to any one of claims 24 or 25.

27. Hollow container or bottle, characterized in that the intrinsic viscosity of the polyester forming the walls of the container is between 0.45 and 0.65dl/g, the acetaldehyde content of the polyester being lower than 6 ppm.