HK1071327A - Processes for producing pet preforms and containers such as food bottles, containers and intermediate preforms obtained - Google Patents

Description

Process for producing PET preforms and containers, such as food bottles, containers and intermediate preforms obtained

Technical Field

The present invention relates to the field of synthetic materials, commonly known as plastics, and to a process for the conversion production of products for external or internal packaging. The invention relates in particular to the field of the production of polyester containers for foodstuffs, such as bottles, in particular bottles for water.

Background

Polyesters, in particular polyethylene terephthalate (PET) and copolymers thereof are well known for the production of all types of external or internal packaging materials such as bottles, tubes, boxes, films, bags and the like.

In particular in the food sector, PET and its derivatives are widely used for the production of bottles, in particular for bottles containing distilled or carbonated water, fruit juices and the like.

At present, the PET polymers used for the production of bottles for food use have properties of use, in particular a so-called "barrier" effect against various gases (water vapour, oxygen, carbon dioxide, etc.) which increases with the Intrinsic Viscosity (IV) of the materials used, in other words with the increase in the chain length of the polymers used.

Therefore, materials with good gas impermeability properties are generally of high viscosity (IV of 0.72 to 0.83dl/g) and therefore difficult to use industrially, with high production and use costs. In addition, considerable acetaldehyde is formed during the conversion of these materials, which is another problem if the bottles are to contain certain beverages, such as mineral water. In fact, even minute amounts of acetaldehyde (about 20ppm) in the material forming the bottle are sufficient to impart an unpleasant fruity taste to the beverage contained therein.

In addition, existing processes for the production of high-polymerized PET involve the presence of a solid state polymerization or post-polymerization (PCS or SSP) stage, making these processes more costly and more complex.

Various solutions have been considered and some have been tried to solve the problems encountered.

In view of these problems, the object of the present invention is to overcome the above drawbacks and to provide a process for producing PET preforms and containers which does not require the SSP stage and which results in containers having physical and chemical properties which meet the present and future requirements.

Disclosure of Invention

The present invention relates to a first process for the production of PET resin preforms and to preforms obtained by this process.

In practice, in the field of container production, the synthetic resins used are generally converted by injection moulding into preforms, in other words into substantially tubular hollow bodies (possibly provided with a bottleneck at their open ends) which are then blown or double-drawn in the production of the actual containers.

The invention also relates to a second process for the production of PET resin containers, such as bottles for foodstuffs, from the above preform and to the PET containers obtained.

Finally, the invention also relates to a PET container, in particular a food bottle, preferably a food bottle for containing water, and to a third method for producing a PET container, which combines the above-mentioned separate first and second methods.

The process for producing PET resin preforms of the invention is characterized in that a PET resin having an Intrinsic Viscosity (IV) of less than 0.65dl/g is used.

The preform obtained by this first process is characterized in that the Intrinsic Viscosity (IV) of the polyester forming the walls of said preform is between 0.45 and 0.65 dl/g.

The second process for producing containers of PET resin, such as bottles for food use, from the preforms of the invention or obtained by the first process of the invention is characterized in that it substantially comprises the following steps:

-feeding the preform to a blowing or double-drawing plant comprising in particular heating means,

-heating the preform by means of a heating device,

pre-blowing the heated preform by injecting a high-pressure gas at a first pressure for a first period of time,

blowing at a second higher pressure the preform that has been subjected to pre-blowing by injecting a high-pressure gas for a second period of time,

-discharging the obtained container.

The third method for producing the PET container of the invention, in particular the food bottle, is characterized in that it substantially comprises the following steps:

-producing preforms from said PET resin by the first process of the invention and/or using the preforms of the invention, and

-producing said container by means of the second process of the invention from the preform obtained previously.

The PET container obtained from the preform according to the invention or obtainable by the second process described above is characterized in that the Intrinsic Viscosity (IV) of the polyester forming the walls of said container is between 0.45dl/g and 0.65 dl/g.

Detailed Description

The invention will be better understood from the following description relating to preferred embodiments, which are non-limiting examples.

The process for producing PET resin preforms of the invention is characterized in that a polyester resin having an Intrinsic Viscosity (IV) of less than 0.65dl/g is used. The term "lower than 0.65 dl/g" is strictly lower than 0.65 dl/g.

Advantageously, the Intrinsic Viscosity (IV) 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 polyester of the present invention, the Intrinsic Viscosity (IV) is expressed in dl/g as calculated by the following formula (I):

IV＝-10^-1VI²+0.94VI+0.0122 (I)

the process of the invention makes it possible to omit the PCS or SSP steps known in the prior art, in particular for increasing the Intrinsic Viscosity (IV). This omission allows considerable savings in energy, equipment and time, since such solid-state post-condensation steps are generally carried out at temperatures above 200 ℃ in a nitrogen stream, which may last for 10-30 hours.

The present invention not only enables to increase productivity and save a large amount of materials on an industrial scale, but also to obtain end products having technical properties equal to or better than those of the existing products described below.

PET resins suitable for use in the process of the present invention include polyethylene terephthalate Polyesters (PET), i.e., saturated thermoplastic polyesters whose generic name includes polymers of varying degrees of copolymerization.

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

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

According to a characteristic of the invention, the polyester contains at least one crystallization retarder for slowing down or delaying the crystallization of the polyester, in particular during the cooling of the moulded or injected articles, such as preforms, with the aim of crystallizing them into very small crystals, while avoiding spherical crystallization, so as to be able to form 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 polyester 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 preferably less than 7.5 mol%, provided that, if other crystallization retarders are present, the DEG content is subtracted from this value. In other words, the total molar concentration of the crystallisation retarder should be below 7.5%, as described in european patent 41035.

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, except for 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 4% diethylene glycol, the content here representing the mol% of crystallization retarder based on the moles of all diacid monomers. According to a preferred embodiment, the polyester does not contain a crystallization retarder, but it contains DEG generated during the production of the polyester.

The main synthetic steps of the resin are as follows. The first is "mixing", i.e., mixing the diacid in powder form with the liquid glycol. The mixture is then esterified by conventional heating methods while the water formed is extracted. In the actual polymerization, the reaction mixture was heated under vacuum at a temperature of about 260 ℃ and 280 ℃ while extracting the diol, wherein the polymerization can be carried out by the first stage of prepolymerization under similar conditions. The reaction was stopped when the resulting viscous mixture had the appearance of honey or molasses.

The viscous mixture is then fed into a well-known bar die. The resulting rods are then fed into a cold water tank and then cut in a granulator into small solid particles, for example, cubes having sides of 2-3 mm. The resulting PET resin particles were amorphous and transparent.

In order to enable the use of the previously synthesized resin in industrial fields, it is also necessary to crystallize it.

To this end, the present invention provides a method for spherical crystallization of PET resin, which is characterized in that the resin is heated to a temperature of 120-200 ℃.

According to a particularly suitable variant, in order to prevent the particles from sticking together, spherocrystallization is carried out while stirring. The transparent, sticky amorphous granules are thus transformed into opaque, white crystalline granules (milky appearance) which are not sticky to each other and have an Intrinsic Viscosity (IV) lower than 0.65dl/g, preferably between 0.45dl/g and 0.65 dl/g.

Preferably, the spherical crystallization is carried out in a fluidized bed. This ensures a better heat exchange and also minimizes the phenomena of cohesion between the particles.

The first method of the invention is also characterized in that it substantially comprises the following steps:

-drying the resin in the presence of a drying agent,

melting the resin by heating in a barrel of a plasticizing extruder equipped in particular with an endless screw,

-transferring the molten resin from the extruder barrel into a hot distributor or slide equipped with at least one nozzle and at least one heating plug,

-injecting a moulding compound and a resin into at least one mould of the preform to be obtained,

-compensating for shrinkage of the injected material by: during cooling of the injected resin, when the preform is formed in one or more molds, the molten resin is reinjected, and

-ejecting the preform obtained from the mould or moulds, allowing the external cooling thereof.

As a non-limiting example, the drying process may be carried out by passing the particles through a hot (about 175 ℃ C.) dry (dew point below 35 ℃ C.) air stream for a period of time typically ranging from 4 hours to 6 hours.

According to a feature of the invention, the drying is carried out until the residual moisture content is less than 20 ppm.

According to another feature, the heating is carried out at a temperature of 275 ℃ and 285 ℃. This allows melting of the resin with a melting point of about 245-. It is important that this temperature not be exceeded in order to limit the formation of decomposition products such as acetaldehyde.

The plastification/melting is carried out in an extruder barrel, temperature-adjusted to about 280 ℃, in which an annular screw with equidistant or leaky flights (leakage thread) is rotating. The compression ratio of the ring screw, i.e. the ratio of the cross-sectional area of the screw inlet to the cross-sectional area of the screw outlet, is 2.5-3, preferably about 2.7.

This prevents, at least reduces, the penetration of external bubbles into the molten polymer mixture, which can form voids in the final extruded or moulded product. This range may also reduce the shear effect (shearing phenomena) which can cause local overheating which may exceed 300 ℃, thereby producing decomposition products including acetaldehyde.

The preform obtained is also characterized in that it is composed of a material having a crystallinity or degree of crystallinity lower than 10%, preferably lower than 5%, and good transparency, similar to the crystallinity and transparency of preforms obtained with polyesters having an intrinsic viscosity higher than 0.65 dl/g.

A check valve is fixed to the end of the ring screw, and in some cases, a rear jack (rear jack) is provided so that the screw can be contracted when the molten material is plasticized and can be advanced when the molten material is discharged or transferred.

The molten material is transferred using a conventional hot slide. According to another feature, the one or more nozzles and the one or more plugs are heated to a temperature of 260 ℃ and 275 ℃.

The preform production process is further characterized in that the pressure at which the molten resin is injected into the mould or moulds is 2.5X 10⁷Pa(250bar)-5×10⁷Pa (500bar) and a temperature of 260 ℃ to 275 ℃. Such low temperature and pressure values can reduce acetaldehyde formation.

The pressure value is about half of the pressure used in the conventional process using the high (IV) resin, i.e., the pressure used in the conventional process is about 0.72 to 0.83dl/g or more. This also enables considerable savings in the required installation costs or maintenance costs thereof, and also in energy.

With respect to the cooling step, it has been found particularly advantageous to cool the mould or moulds to a temperature of from 0 to 10 ℃. This may be done using any cooling method and apparatus known to those of ordinary skill in the art.

The period commonly known as the "hold time" for compensating the shrinkage of the solidified plastic by reinjection of the plastic is about 6 seconds at constant pressure.

When a PET resin preform is obtained by the above method, the Intrinsic Viscosity (IV) of the polyester forming the wall of the preform is 0.45 to 0.65 dl/g.

The invention also relates to a process for producing containers of PET resin, such as bottles for food use, from the preform according to the invention or obtained by the first process according to the invention, characterized in that it substantially comprises the following steps:

-feeding the preform to a blowing or double-drawing plant comprising in particular heating means,

-heating the preform by means of a heating device,

pre-blowing the heated preform by injecting a high-pressure gas at a first pressure for a first period of time,

blowing at a second higher pressure the preform that has been subjected to pre-blowing by injecting a high-pressure gas for a second period of time,

-discharging the obtained container.

The blowing or double stretching device may be chosen from those commonly used in the art. As a non-limiting example, the apparatus can substantially comprise a preform feeder, for example in the form of a guide rail produced in the direction of preform feed, means for thermal conditioning said preforms, such as an assembly of radiant elements, at least one metal mold for axial blowing or double stretching by injection of a suitable gas, such as compressed air, and means for ejecting the blown product or products obtained.

The heating temperature of the preform is preferably 80-100 c, which is about 20 c lower than that of the conventional preform. As mentioned above, the heating can be carried out by any suitable means, preferably by means of an oven or furnace equipped with a set of short infrared radiation lamps directed towards the external surface of the preforms to be heated. The above temperatures are measured at the outlet of the electric furnace, which corresponds to the average longitudinal temperature gradient required to heat the preform wall.

Preblowing of preforms at 1X 10⁵Pa-10×10⁵Pa (1bar-10bar) for a pre-blowing time of 0.15-0.6 seconds.

Blowing of preblown preforms at 3X 10⁶Pa-4×10⁶Pa (30-40bar) for a blowing time of 0.8-2 seconds. It is known that, in order to start the operations and to check the perfection of these operations, it is also possible to introduce tie rods in the preforms during the pre-blowing and/or blowing operations.

The invention also relates to a PET container, characterised in that it is made of a resin having an intrinsic viscosity lower than 0.65 dl/g.

The invention also relates to a PET container obtained from the preform according to the invention or by the second process described above, characterized in that the Intrinsic Viscosity (IV) of the polyester forming the walls of said container is between 0.45 and 0.65 dl/g.

Advantageously, the container is further characterized in that the density of the polyester forming the container wall is 1.36-1.37.

According to another feature, the average crystallinity of the polyester forming the walls of the container is between 25 and 35%.

The container of the present invention may be of any shape and size commonly encountered in the industry.

The container of the invention is also characterized in that its carbon dioxide permeability is at least 5% better than that of a container obtained with a PET resin having an Intrinsic Viscosity (IV) higher than or equal to 0.65 dl/g.

The permeabilities given as examples in the tables of the specification are determined by the "Mocon PermatranC 4/40" test. In this test, samples of the bottles were made in duplicate. The samples were prepared according to a precise procedure, 2 at a time (stagger)ed 2by 2) to have the same CO₂The saturation time. The bottles were carbonated using a conventional chemical carbonation process.

The actual tests were carried out in the following manner:

in the first phase, all units are in a so-called "idle" mode (no activity). If no chamber is connected, the units are removed and the nitrogen inlet and outlet are secured in a similar manner. If a nitrogen inlet is connected to the top of chamber A, this operation is repeated for chamber B. The connector is greased and held in the hand in a quarter turn. The bottles to be tested are then placed in the chambers and the fittings on each chamber cover are greased prior to closure.

The carbon dioxide inlet and outlet must be plugged into one unit. The CO has to be produced for other units₂Bridge to CO₂Can be recycled to other reference assemblies. A flexible tube, each end of which is greased, is used for this purpose.

The individual information items relating to the test to be carried out are then collected in the workstation of the testing device.

In the case of very high barrier values, for which the equilibrium value is difficult to evaluate, it is advisable to carry out a preliminary convergence test.

The corresponding correction value is then added, fixing the flow rate. The actual test is started until the recorded measurement is obtained, displayed or stored. The test conditions used within the scope of the invention are as follows:

time of saturation	10 days
		Flow rate of nitrogen	150 standard cm3/min
CO2Flow rate of flow	300 standard cm3/min

The invention also relates to a third method for producing the PET container according to the invention, in particular a food bottle, characterized in that it substantially comprises the following steps:

producing preforms from PET resin by the first process of the invention and/or using the preforms of the invention, and

-producing said container by means of the second process of the invention from the preform obtained previously.

Finally, the invention also relates to PET food containers, in particular food bottles, preferably for water, which, if applicable, are obtained by the above-described production method.

Non-limiting examples of water or aqueous liquids contained in said container or said bottle may be distilled or carbonated water, mineral water, spring water, treated water (purified, disinfected, supplemented with minerals, flavoured, etc.), carbonated beverages ("sodas"), fruit juices, milk, etc.

The advantages of the invention can be shown in more detail below with reference to the information of the examples given in the table. Copolymer 1 is a prior art copolymer and copolymers 2-6 are copolymers of the present invention.

These polyesters were used to make 50cl bottles by the following method:

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 volume of 0.5l, the neck of which was shaped as designated by the standard designation 28 PCO 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 the table 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 obtained and the blowing process are shown in the table below for each polyester used.

Properties studied for polyesters	1	2	3	4	5	6
							Intrinsic viscosity (dl/g)	0.66	0.57	0.57	0.64	0.54	0.44
Isophthalic acid (wt%)	2.3	2.3	0	0	0	0
							Diethylene glycol (wt%)	1.3	1.4	2.0	2.15	2.05	2.05
CO2Permeability (Mocon) (cm)3Bottle, sky)	4.32	4.24	4.64	4.48	4.06	4.56
							Free blown volume (ml)	1201.46	1423.74	972.30	1146.63	1176.91	1168.57
Preform temperature (. degree. C.)	96	94	89	89	87	83
							Burst pressure (bar)	15.68	13.44	13.84	14.18	13.84	10.64
Young's modulus in the transverse direction (kPa)	96.27	73.21	66.68	77.06	72.95	59.90
							Pressure (daN) deflection (mm)	21.971.84	21.11.72	19.621.64	20.991.74	19.721.74	19.771.68

Comparison table of various physicochemical properties

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

It will be clear that the invention is not limited to the described embodiments. Numerous variations, in particular variations in the composition, or technical equivalents may be made without departing from the scope of the invention.

Claims (10)

1. Use of a PET resin having an Intrinsic Viscosity (IV) lower than 0.65dl/g for the production of PET resin preforms from which containers are then produced.

2. Use according to claim 1, characterized in that the Intrinsic Viscosity (IV) is from 0.45 to 0.65 dl/g.

3. Use according to claim 1 or 2, characterized in that the preform is made of a material having a crystallinity lower than 10%, preferably lower than 5%.

4. PET resin preform obtained with any one of claims 1 to 3, characterised in that the Intrinsic Viscosity (IV) of the polyester forming the walls of the preform is between 0.45 and 0.65 dl/g.

5. A PET resin container characterized in that it is composed of a PET resin having an intrinsic viscosity of less than 0.65 dl/g.

6. Container according to claim 5, characterized in that the Intrinsic Viscosity (IV) of the PET forming the wall of the container is between 0.45 and 0.65 dl/g.

7. Container according to claim 5 or 6, characterized in that the density of the polyester forming the wall of the container is 1.36-1.37.

8. Container according to any of claims 5-7, characterized in that the average degree of crystallinity of the polyester forming the walls of the container is 25-35%.

9. Container according to any of claims 5 to 8, characterized in that the permeability to carbon dioxide is at least 5% better than the permeability to carbon dioxide of a container obtained with a PET resin having an Intrinsic Viscosity (IV) higher than or equal to 0.65 dl/g.

10. Food container, in particular food bottle, according to any of claims 5 to 9, preferably a food bottle for holding water.