METHOD FOR MAKING VESSELS FOR IMPROVING THE MECHANICAL STRENGTH THEREOF

Abstract

Manufacturing process of a container (2) in a mold (5) having a cavity (8) defining the final shape of the container (2), from a polymer blank (3) heated in advance, this process comprising the following operations: introduction of the blank (3) into the mold (5) heated to a predetermined temperature, introduction into the blank (3) of a pressurized explosive gaseous mixture, ignition of the gaseous mixture.

Description

The invention concerns the manufacture of containers, by blowing or stretch-blowing from polymer blanks.

Hereinafter, the term “blank” will not only cover the notion of a preform, but also that of an intermediate container, that is to say an object having undergone a first blowing (which can be free) and intended to undergo a second to form the final container.

Among the polymers currently most used for manufacturing containers are the saturated polyesters and particularly PET (poly(-ethylene terephthalate)).

We would like to briefly point out that stretch-blowing of containers consists of taking a polymer blank that has been pre-heated, introducing this blank into a mold with the shape of the end container to be formed, then stretching the blank by means of stretch rod (also called a cane) and blowing pressurized gas into it (generally air) to pin the material flat against the wall of the mold.

Remarks Concerning PET

Here, we would like to make a few remarks concerning the physical and mechanical properties of PET, so as to better grasp what follows.

The invention is certainly not limited to the PET alone; however, given that this material is, at present, that most currently used for the stretch-blowing of containers, it would appear to be appropriate to study this example carefully.

Here following, the figures between the square brackets will refer to the documents listed in the bibliography attached to this description.

PET is a polyester obtained by polycondensation from terephtalic acid and from ethylene glycol. Its structure can be amorphous or partially crystalline (without exceeding 50% however). The possibility of going from one phase to another depends largely on the temperature: below its vitreous transition (T_g≈80° C.), the micromolecular chains are quite mobile and the material is solid, with a congealed ((solidified)) microstructure; above the fusion temperature T_f (about 270° C.), the bonds between the chains are destroyed and the material is liquid. Between these two temperatures, the chains are mobile and their conformation can change (See [2]).

More precisely, above T_g the chain movements made possible in the amorphous disordered zones permit greater and easier deformations of the matter (the Young module, rigidity characteristic, abrupt drop from T_g) (See [1]).

The PET chemical formula (See [1]) is as follows:

The crystallization of polymer occurs between the vitreous transition temperature T_g and the fusion temperature T_f. If the polymer is subjected to a deformation or a flowing, the kinetic of crystallization is accelerated. It is important to note that the crystalline texture obtained in this case is complex and anisotropic (See. [3]).

The energy contribution necessary to modify the structure of the macromolecular chains can be thermal (which is then referred to as natural or static crystallization), or mechanical, by permanent deformation of the material. This energy contribution by deformation exhibits the following advantages, among others:

• • It is fast (it induces the chains to stretch and to orientate, facilitating their transition from a disordered amorphous state to a crystallized structure),
  • the induced orientation increases the mechanical resistance of the material,
  • the crystals formed retain the translucidity of the material, contrary to crystals generated by thermal crystallization (the lamellas formed remaining small in size in relation to the wave lengths or the visible spectrum)

In this crystallized and orientated state, the PET exhibit numerous qualities: very good mechanical properties (high rigidity, good resistance to traction and to tearing), good optical properties and barrier properties to CO₂. (See [2])

The mechanical properties of PET are essentially a function of the crystalline texture, the crystalline volumic fraction and the molecular dimension and orientation. It is known that these parameters are particularly affected by the thermal history of the material (See.[4]).

The long-term structural hardening of the polymer is exclusively associated with the crystallization; however, recent studies show that this hardening appears even without complete crystallization and can be attributed to the orientation of the macronuclear chains and their organization (See [5]).

These mechanical properties justify the use of PET in the stretch-blowing of containers, and particularly bottles. Stretch-blowing causes a bi-orientation of the polymer, that is, on the one hand, an axial orientation of the macromolecules at the time of the stretching by means of a stretch rod and, on the other hand, a radial orientation of the macromolecules at the time of the blowing.

More precisely, the stretching of the PET causes a warped or trans type change of conformation of the molecular chains, leading to a partial crystallization of the polymer. In microstructure terms, the benzenic cores tend to orient in a parallel plane to the main directions of the stretching. As we remarked above, the PET does not crystallize 100%; the maximum rate noted being about 50%. The containers manufactured within the industry, and particularly bottles, generally exhibit a rate close to 35%. (See [2]).

We will see here following, for certain particular operating applications, the rate of crystallinity can be increased (up to 40% and, some assert, even more than that (See [6])).

Several known methods make it possible to measure the crystallinity rate of a polymer. The two most prevalent methods are densimetry and differential calorimetric analysis (better known by its English acronym DSC—Differential Scanning Calorimetry). These methods are described in [2] and summarized here following.

Densimetry is based on the determination of the density of the material: when the material crystallizes, its density increases due to the compacter organization of the chains in the crystalline phase. Assuming that the specific volumes of the two phases follow a mixture law, one can then calculate the crystallinity rate by the following relationship:

$X_c=\frac{d}{d_c}\frac{d_a-d}{d_a-d_c}$

The density d of the sample is measured by successive weighings in the air and in water. The density d_a=1,333 g/cm³ of the amorphous phase is a relatively well-established value. The density d_c of the crystalline phase varies between 1.423 et 1.433 g/cm³ for an oriented PET having undergone a tempering between 60° C. and 100° C. The generally permissible value is 1.455 g/cm³.

The DSC analysis itself consists of establishing a thermogram for the available polymer sample. Traditionally, this thermogram was traced by implementing a heating of the material at a speed of 10° C./min.

An example of a thermogram is reproduced in FIG. 1, for an industrial PET (PET 9921W EASTMAN), generally used for the stretch-blowing of bottles (See [3]) and the molecular mass of which is M_n=26 kg/mol. The development of the vitreous transition temperature (to T_g=78° C.) results in an inflexion of the curve. One then observes a first exothermic peak centered around 135° C., characteristic of the crystallization of the initially amorphous material (See [2]), then a second peak, endothermic this time, at T_f=250° C., corresponding to the fusion of the previously formed crystals.

The calculation of the initial crystallinity rate can be done by comparing the enthalpy difference (area under the peaks) between fusion and crystallization, that is noted ΔH, with the fusion enthalpy H_ref of a PET assumed to be perfectly crystalline, the value of which is generally chosen at around 100 J.g⁻¹. The crystallinity rate is given by the following equation:

$X_C=\frac{\Delta H}{H_{\mathrm{ref}}}$

DSC analysis appears to be frequently preferred to densimetry, which is reputed to be less precise.

Context of the Invention

Having made these remarks, we will now focus on the context in which the invention was realized.

As we have seen, the crystallinity and the molecular orientation of the polymer have an affect on the mechanical properties. Manufacturers have long applied this knowledge in order to increase the rigidity of containers, for example in order to enable them to sustain significant pressures, possibly reaching several bars (in the case of carbonated drinks).

Numerous manufacturers have likewise sought to apply the supposed consequences of the crystallization of materials to the mechanical properties of the containers manufactured from these materials. In particular, it is universally asserted in scientific and technical literature that increasing the crystallinity reduces the shrinkage of the container during the hot filling of it, that is to say at a temperature greater than the vitreous transition temperature T_g. (The shrinkage is the result of the release of internal stresses accumulated by the material at the time of its macromolecular orientation during the stretch-blowing (See [7]).)

Increased crystallinity has customarily been obtained by a process known as heat setting, which consists, at the end of the blowing, of keeping the formed container against the wall of the mold, which is heated to a preset temperature that can range up to 250° C. The container is thus kept pinned flat against the wall of the mold for several seconds. There are numerous recommendations, varying from manufacturer to manufacturer both with respect to the temperature and the duration of the heat setting. [8] and [9], specifically, propose a range of temperatures between 130° C. and 250° C., and time (6 sec, 30 sec and 120 sec), specifically intended to reduce the shrinkage of the container during a hot filling. The containers having undergone heat setting to make them resistant to deformation at the time of a hot-filling are, in current manufacturing language, called HR (heat resistant).

[6] proposes to circulate in the container, at the end of the blowing, a gas (of air) at a so-called high temperature between 200° C. and 400° C., so as to bring the inside wall of the container to a temperature of at least 120° C. in order to increase its crystallinity. It presumed in this document that the total duration of the manufacturing of the container can be less than 6 sec, while the crystallinity rate obtained varies from 34.4% to 46.7%. (It should be noted that it concerns average container crystallinity rates, measured by a densimetric method akin to that presented above.) According to [6], a crystallinity rate greater than 30% must be considered to be characteristic of a high crystallinity.

It has likewise been proposed (See [10]) to implement the blowing of the container in two stages: a first one during which an intermediate container of greater volume than the container to be obtained is formed, then a second during which the container is molded to its final dimensions. Between these two stages, the intermediate container is heated at a temperature between 180° C. and 220° C. for a period between 1 min et 15 min. According to [10], the variation of the volume of the container during its hot filling (i.e. at a temperature from 93° C. to 95° C.) is less than 5%, in these conditions. One proposed explanation is that the two successive molding operations provide two crystalline bioorientations, instead of one alone in the classical processes.

In addition, it has been proposed (See [11]) to produce containers with a crystallinity on the inside greater than the crystallinity on the outside, in order to minimize the dispersion of the aromas of the liquid.

An analysis of the existing manufacturing techniques, and particularly those that have just be briefly remarked upon, demonstrate that an essential concern, in the proposed solutions, is to find the maximizer crystallinity for the final container, so as to reduce its shrinkage as much as possible when it is hot filled. Manufacturers run up against a recurrent problem: the contradiction between the crystallinity rate (which they want to maximize, as we have just seen) and the cycle time (which they want to minimize). It is in fact currently accepted that the crystallization of the polymer becomes easier as the deformation speed decreases (See [1] in particular). The cycle times presented in the cited documents are relatively long (greater 5 sec., some exceeding one minute) and necessitating multiple machines to meet current requirements in terms of rates (up to 50,000 containers per hour), which puts a burden on production costs.

Presentation of the Invention

The invention seeks to propose an alternative solution for the manufacturing of polymer containers, which specifically makes it possible to obtain good performance when they are hot filled.

For this purpose, the invention proposes, according to a first aspect, a manufacturing process for a container in a mold having a cavity defining the final form of the container, from a polymer blank heated beforehand; this process comprises the following operations:

• • introduction of the blank into the mold heated to a predetermined temperature,
  • introduction into the blank of a pressurized explosive gaseous mixture,
  • ignition of the gaseous mixture.

Measurements have demonstrated that a container obtained by this process exhibits a negative gradient of the inside wall of the container.

Hence, surprisingly, the inventors noted that the container obtained by this process exhibits, with hot filling, performances at least equivalent to those of the classical processes utilizing heat setting of the container. Under certain operating conditions the performances are even better; the shrinkage of the container being very low (less than 1%, on average). This is clearly in contradiction of the widespread idea (we remarked on in the introduction) that it is necessary to maximize the crystallinity rate of the container in order to increase its mechanical stability (that is to say, in practice, to reduce its shrinkage) at the time of its hot filling.

The following are provided, furthermore, depending on the method of execution:

• • a pre-blowing operation during the course of which the blank is stretched, while introducing the explosive gaseous mixture into it;
  • an ignition operation consistent with igniting the explosive gaseous mixture;
  • a degassing operation during the course of which the inside of the container is brought to a pressure close to atmospheric pressure.

As a variant, the process includes:

• • a pre-blowing operation during the course of which the blank is stretched, while introducing the explosive gaseous mixture into it;
  • an ignition operation consistent with igniting the explosive gaseous mixture;
  • a blowing operation consistent with introducing high pressure air into the container;
  • a degassing operation during the course of which the inside of the container is brought to a pressure close to atmospheric pressure.

Following the ignition operation, the process can include a stabilization operation, during which the residual gas deriving from the ignition is maintained in the container.

Moreover, a sweeping operation is provided prior to the degassing operation during which air is circulated in the container.

According to another embodiment, the process comprises:

• • a pre-blowing operation, during the course of which the blank is stretched, while introducing the explosive gaseous mixture into it;
  • a primary ignition operation consistent with igniting the explosive gaseous mixture;
  • a primary degassing operation during the course of which the inside of the container is brought to a pressure close to the atmospheric pressure,
  • a secondary pre-blowing operation during the course of which an explosive gaseous mixture is introduced into the container;
  • a secondary ignition operation consistent with igniting the explosive gaseous mixture;
  • possibly a stabilization operation, during which the residual gas deriving from the ignition is maintained in the formed container.
  • possibly a blowing operation consistent with introducing high pressure air into the container;
  • possibly a sweeping operation during which air is circulated into the container; and
  • a secondary degassing operation during the course of which the inside of the container is brought to a pressure close to atmospheric pressure.

According to yet another embodiment, the process comprises:

• • a pre-blowing operation, during the course of which the blank is stretched, while introducing the pressurized air into it;
  • a blowing operation consistent with introducing high pressure air into the container;
  • a primary degassing operation during the course of which the inside of the container is brought to a pressure close to the atmospheric pressure,
  • a secondary pre-blowing operation during the course of which an explosive gaseous mixture is introduced into the container;
  • an ignition operation consistent with igniting the explosive gaseous mixture;
  • possibly a stabilization operation, during which the residual gas deriving from the ignition is maintained in the formed container;
  • possibly a sweeping operation during which air is circulated into the container.
  • a secondary degassing operation during the course of which the inside of the container is brought to a pressure close to atmospheric pressure.

The mold is preferably heated to a temperature greater than or equal to 100° C. This temperature is about 130° C. according to the method of execution. As a variant, this temperature is about 160° C. environ.

The explosive gaseous mixture can comprise air and hydrogen, for example with a volumetric proportion of hydrogen of about 6%, to obtain a deflagration upon ignition.

The pre-blowing pressure is greater than or equal to 10 bars, according to the method of execution.

The blowing pressure is, itself, preferably greater than or equal to 30 bars.

According to a second aspect, the invention proposes a machine for manufacturing containers from polymer blanks heated in advance, comprising:

• • a mold having a cavity defining the final form of the containers,
  • the means of heating the mold;
  • the means of injection of a pressurized explosive gaseous mixture into the mold,
  • the means of ignition of the gaseous mixture.

According to one embodiment, the machine comprises a nozzle suitable for communicating with the inside of the container and for introducing gas into it, the means of ignition comprising a spark plug leading into a nozzle or, when the machine includes a stretch rod, within it.

Other objects and advantages of the invention would appear to be the light for the description made here following in reference to the drawings attached in them.

FIG. 1 is a differential calorimetric analysis (DSC) thermogram illustrating the thermal capacity variations of an initially amorphous PET;

FIG. 2 is a schematic view of a cross-section elevation view showing a machine for manufacturing containers by stretch-blowing;

FIGS. 3A to 3F are schematic cross-section elevation views showing different successive stages of a manufacturing process by stretch-blowing of containers, according to a first example of execution;

FIG. 4A is a graph illustrating the development over time of the pressure prevailing in the container at the time of its stretch-blowing in accordance with the process illustrated in FIGS. 3A to 3F;

FIG. 4B is a chronogram illustrating the opening and closing of the electromagnetic valves as well as the lighting operation within the machine illustrated on FIG. 2, for the implementation of the process illustrated in FIGS. 3A to 3F and in FIG. 4A;

FIGS. 5A to 5H are schematic cross-section elevation views showing different successive stages of a manufacturing process by stretch-blowing of containers, according to a second example of execution;

FIG. 6A is a graph illustrating the development over time of the pressure prevailing in the container at the time of its stretch-blowing in accordance with the process illustrated in FIGS. 5A to 5H;

FIG. 6B is a chronogram illustrating the opening and closing of the electromagnetic valves as well as the lighting operation within the machine illustrated in FIG. 2, for the implementation of the process illustrated in FIGS. 5A to 5H and in FIG. 6A;

FIGS. 7A to 7L are schematic cross-section elevation views showing different successive stages of a manufacturing process by stretch-blowing of containers, according to a third example of execution;

FIG. 8A is a graph illustrating the development over time of the pressure prevailing in the container at the time of its stretch-blowing in accordance with the process illustrated in FIGS. 7A to 7L;

FIG. 8B is a chronogram illustrating the opening and closing of the electromagnetic valves as well as the lighting operation within the machine illustrated on FIG. 2, for the implementation of the process illustrated in FIGS. 7A to 7L and in FIG. 8A;

FIGS. 9A to 9K are schematic cross-section elevation views showing different successive stages of a manufacturing process by stretch-blowing of containers, according to a fourth example of execution;

FIG. 10A is a graph illustrating the development over time of the pressure prevailing in the container at the time of its stretch-blowing in accordance with the process illustrated in FIGS. 9A to 9K;

FIG. 10B is a chronogram illustrating the opening and closing of the electromagnetic valves as well as the lighting operation within the machine illustrated in FIG. 2, for the implementation of the process illustrated in FIGS. 9A to 9K and on FIG. 10A;

FIGS. 11A to 11F are schematic cross-section elevation views showing different successive stages of a manufacturing process by stretch-blowing of containers, according to a fifth example of execution;

FIG. 12A is a graph illustrating the development over time of the pressure prevailing in the container at the time of its stretch-blowing in accordance with the process illustrated in FIGS. 11A to 11F;

FIG. 12B is a chronogram illustrating the opening and closing of the electromagnetic valves of a machine implementing the process illustrated in FIG. 12A;

FIG. 13 is a top view showing a typical example of a container obtained by a manufacturing process according to any one of the examples illustrated on the preceding figures;

FIG. 14 is an enlarged scale sectional view of a detail of the body of the container of FIG. 13, taken in the inset XIV;

FIG. 15 is a thermogram of differential calorimetric analysis (DSC) illustrating the variations of the mass thermal capacity of a container manufactured according to a manufacturing process according to one of the examples illustrated in the preceding figures, for five successive sections of the wall of the container; and

FIG. 16 is a thermogram similar to that of FIG. 14, for a container manufactured according to a manufacturing process according to another of the examples illustrated in the preceding figures.

FIG. 2 partially illustrates a machine 1 for manufacturing containers 2 by stretch-blowing from polymer blanks 3. There following, the blanks 3 are the preforms: this term is used for purposes of consistency.

Machine 1 comprises a plurality of molding units 4, mounted on a carousel (not shown), each comprising one mold 5 (as illustrated in FIG. 2). This mold 5, made of steel or aluminum alloy, comprises two die halves 6 and a mold base 7 that together define an internal cavity 8, intended to receive a preform 3 previously heated to a temperature greater than the vitreous transition temperature (Tg) of the matter comprising the preform 3, the shape of which corresponds to the final desired shape of the container 2 manufactured from this preform 3. Whatever its shape, container 2 (as illustrated in FIG. 13) generally comprises a neck 9, a body 10 and a bottom 11.

The molding unit 4 comprises:

• • a stretch rod 12 equipped with holes 13 and mounted sliding in relation to the mold 5 along the main axis 14 (generally revolving) thereof, between an upper position permitting the introduction of the preform 3 and a lower position where, at the end of the stretching of the preform 3, the rod 12 reaches the bottom of the mold 7, pinning it flat on the bottom of it 11,
  • a casing 15 defining a nozzle 16 into which the rod 12 slides and which, at the time of the manufacture of the container 2, engages with a neck 17 of the preform 3 (joined with the neck 9 of the container and not sustaining deformation during the manufacture),
  • several fluidic circuits run into the nozzle 16 via casing 15, that is:
  • a medium pressure pre-blowing air circuit 18 (between 5 and 20 bars), this circuit 18 comprising a pre-blowing air source 19 (for example a first compressed air line supplying several machines) and a conduit 20 (that can be established at least partially in the casing 15) connecting this source 19 to the nozzle 16 with the interposition of a first electromagnetic valve EV1,
  • a reactive gas circuit 22 (gas examples will be given here following) comprising a reactive gas source 23 and a conduit 24 (which can be formed at least partially in the casing 15) connecting this source 23 to the nozzle 16 with the interposition of a second electromagnetic valve EV2,
  • a high pressure blowing air circuit 26 (between 30 and 40 bars), comprising a blowing air source 27 (for example a second compressed air line supplying several machines) and a conduit 28 (that can be established at least partially in the casing 15) connecting this source 27 to the rod 12 with the interposition of a third electromagnetic valve EV3,
  • a degassing circuit 30 comprising a vent orifice 31 (possibly doubled from a pump) and a conduit 32 connecting the nozzle to this orifice with the interposition of a fourth electromagnetic valve EV4.

The reactive gases can be hydrogen (H₂), methane (CH₄), propane (C₃H₈) or acetylene (C₂H₂).

Hydrogen is preferred due to the non-polluting character of its oxidation reaction (2H₂+O_2→2H₂O), the product of which is pure water. Hydrogen can either be produced on demand, directly upstream of the machine 1 (for example by electrolysis of the water), or stored in containers from which it is drawn for the needs of the process.

The conduits 20, 24, 28, 32 can be established at least partially in the casing 15, as is illustrated in FIG. 2. As for the electromagnetic valves EV1, EV2, EV3, EV4, they are connected electrically to a control unit 34 that controls the opening and closing of them (duly taking into account the response time of the electromagnetic valves). These electromagnetic valves EV1, EV2, EV3, EV4 can be arranged at a distance from the casing 15 or, integrated within it for greater compactness. To make such a casing 15, a person skilled in the art could refer to patent application FR 2 872 082 (Sidel) or to the equivalent international patent application WO 2006/008380.

The state of electromagnetic valves EV1, EV2, EV3, EV4 (closed/open) and of the spark plug 36 (out/lit) is illustrated in the chronograms of FIGS. 4B, 6B, 8B, 10B, 12B, each consisting of five lines, numbered 1 to 5 and respectively representing:

• • line 1, the state (0: closed/1: open) of the first electromagnetic valve (pre-blowing air supply),
  • line 2, the state (0: closed/1: open) of the second electromagnetic valve (reactive gas supply),
  • line 3, the state (0: out/1: lit) of the spark plug, line 4, the state (0: closed/1: open) of the third electromagnetic valve (blowing air supply),
  • line 5: the state (0: closed/1: open) of the fourth electromagnetic valve (degassing).

As seen in FIG. 2, the molding unit 4 is in addition equipped with a device for its own ignition 35, at a pre-set given instant and actuated by the control unit 34, to produce within the nozzle 16 (or of the container 2) a spark for igniting the air and reactive gas mixture in the container 2.

According to an embodiment, this ignition device 35 comprises a spark plug 36 having a center electrode 37 and an earth electrode 38 both leading into the nozzle 16 (that communicates with the inside of the container 2)—or, as a variant, in the rod 12—and between which, upon actuation by control unit 34, an electrical arc can be produced, causing the ignition of the mixture.

As illustrated in FIG. 2, molding unit 4 is in addition equipped with a circuit 39 for heating mold 5, comprising a pressurized coolant source 40 (by oil or water, for example) and conduits 41 arranged in the thickness of the mold 5 (half dies 6 and bottom 7 included), in which the coolant fluid coming from source 40 circulates to keep mold 5 at a temperature above the ambient temperature (20° C.). In practice, the temperature of the mold is regulated at an average value between 20° C. and 160° C. (measured on the inside wall of mold 5), depending on the applications—examples of temperatures are provided here following.

Here following we describe five specific examples of manufacturing processes for containers 2 of the HR (heat resistant) type having for example a shape such as that illustrated in FIG. 13, by means of machine 1, which has just been described.

For each example described, DSC is used to measure the crystallinity of a container 2 obtained by the corresponding process. More precisely, the crystallinity of body 10 of container 2 is measured, at least on the side of an inside wall 42 and of an outside wall 43. For this purpose a sample is taken in the body 10 and is cut (by microtomic cutting, for example) into serial segments at its thickness, the respective crystallinity of which is then measured. According to one embodiment, the sample is cut into five approximately equal segments. For example, for a container 2, the thickness of which is approximately 360 μm, each cut segment exhibits a thickness of 50 μm (the cutting blade forming, with each pass between two successive segments, shavings of a thickness of approximately 25 μm). A, B, C, D and E show the five successive segments of material, from the insider of container 2.

We would like to point out that the DSC analysis makes it possible to quantify the thermal phenomena (endo or exothermic) accompanying the transformations (crystallization, fusion) of a material.

The procedure used here is as follows.

A differential microcalorimeter with power compensation is used. This microcalorimeter includes two ovens under a neutral atmosphere (generally in nitrogen). A reference (generally an empty cup) is placed into the first. The sample on which the DSC measurements are to be made is placed into the second.

Each oven is equipped with two platinum resistances, one of which serves for the heating and the other for measuring the temperature.

The exchanged heat fluxes are measured, on the one hand between the reference and the outside medium, and on the other hand between the sample and the outside medium as the temperatures is increased at a constant heating velocity from the ambient temperature (about 20° C.) up to a temperature greater than the known fusion temperature of the studied material (in this case, for the PET it is heated up to about 300° C., it being assumed that the material fuses at approximately 250° C.).

The mass thermal capacity is deduced from the studied sample by comparison with the reference, by the following relationship:

$C_p=\frac{1}{m}\frac{Q}{T}$

Where:

m is the mass of the sample in grams,

T the temperature,

Q the amount of heat in J.g⁻¹.K⁻¹ necessary to cause an increase of the temperature in the sample of the value dT.

If the heating velocity q is introduced, kept constant at the time of the measurement (and in the selected case equal to 10 K⁻¹), defined by the relationship

$q=\frac{Q}{t},$

then the mass thermal capacity can be written as follows:

$C_p=\frac{1}{m·q}\frac{Q}{t}$

The variations of the mass thermal capacity of the sample in relation to the temperature are traced from the measurements of the heat flux performed in the microcalorimeter. The curve of these variations is called a thermogram. Such a thermogram is shown in FIG. 1 for a sample of the PET of the manufacturer EASTMANN mentioned in the introduction.

Such a thermogram makes it possible to differentiate the exothermic phenomena (oriented downwards) from the endothermic phenomena (oriented upwards).

For any initially amorphous PET sample, such as the above sample mentioned, one finds two peaks: a first exothermic peak (in this case around 135° C.), corresponding to the crystallization of the material, and an endothermic peak (in this case around 250° C.) corresponding to the fusion of the material.

The rate of crystallinity of the initial material can be calculated from the thermogram, from the difference ΔH of the enthalpies exchanged during the fusion phenomena on the one hand, and the crystallization on the other.

The fusion enthalpy ΔH_f is defined by the area under the fusion peak:

$\Delta H_f={\int }_{\mathrm{pic}\mathrm{fusion}}C_p\left(T\right)T$

The crystallization enthalpy ΔH_c, is itself defined by the area under the crystallization peak:

$\Delta H_c=-{\int }_{\mathrm{pic}\mathrm{cristallisation}}C_p\left(T\right)T$

The differences of the fusion and the crystallization enthalpy are deduced from: ΔH=ΔH_f−ΔH_c, then the crystallinity rate from the following relationship:

$X_C=\frac{\Delta H}{H_{\mathrm{ref}}}$

Where H_ref is the enthalpy of fusion of a presumed completely crystalline sample. Here a value of 140 J.g⁻¹ is selected, which corresponds to the most value most commonly used in plastic materials laboratories.

EXAMPLE 1

FIGS. 3A to 3F, 4A and 4B)

In this example, mold 5 is heated such that such that it exhibits on the side of its inside wall a temperature of approximately 160° C. The material of the preform 3 is a PET. The reactive gas is hydrogen (H₂). The air/hydrogen gaseous mixture is made while maintaining a hydrogen proportion in volume between 4% and 18%, preferably 6%.

Following introduction of the hot preform 3 into the mold 5, the process comprises a first operation, known as pre-blowing, consisting of stretching preform 3 by sliding rod 12, and simultaneously opening the electromagnetic valves EV1 and EV2 to introduce into preform 3 an air and reactive gas mixture at a pressure between approximately 5 and 20 bars (FIGS. 3A to 3C, lines 1 and 2 in FIG. 4B). This first operation of a predetermined durational α1 (less than 250 ms) ends by the closing of the electromagnetic valves EV1 and EV2 after rod 12 has ended its travel having reached the bottom of mold 7, and the threshold for the plastic outflow of the material has been exceeded (which is indicated on the pressure curve by a first pressure peak, at approximately 8 to 10 bars).

A second operation, known as ignition, consists of igniting the gaseous mixture by ignition of a spark plug 36 (FIG. 3D, line 3 in FIG. 4B). Taking into account the proportion of hydrogen in the gaseous mix, an explosion occurs in container 2 that is being formed, which is accompanied by a sudden increase of the temperature (which reaches hundreds of degrees Celsius) and of the pressure (which exceeds 40 bars—on the curve of FIG. 4A; the corresponding peak pressure is clipped due to reasons of scale). The duration of the α2 of the ignition of the mixture is extremely brief (less than 25 ms), but the increased pressure that accompanies it is sufficient to pin the substance flat against the wall of the mold, thus forming container 2.

A third operation, known as stabilization, consists of maintaining in container 2 a residual gas (essentially a mixture of water vapor coming from the combustion of hydrogen, with possible traces of NO_x), for a predetermined duration α3 (between 1000 and 1500 ms) while keeping all the electromagnetic valves EV1, EV2, EV3, EV4 closed, so as to permit the reduction of the temperature and the pressure in container 2 (FIG. 3E).

A fourth operation, known as degassing, consists of degassing container 2, by closing third electromagnetic valve EV3 (FIG. 4B, line 4) and leaving fourth electromagnetic valve EV4 for a preset duration α4 (between 100 and 500 ms), to allow the air to escape (FIG. 3F) until the pressure prevailing inside container 2 has about attained the atmospheric pressure (FIG. 4B, line 5). At the end of this operation, the fourth electromagnetic valve EV4 is closed, the mold 5, opened, and container 2, evacuated, to enable repetition of the cycle with a new preform 3.

DSC was used, in the above described conditions to measure the crystallinity of a container 2 obtained by this process, on the side of inside wall 42 of body 10 (segment A) and of the side of its outside wall 43 (segment E). The results of the measurements are presented in the following table:

Segment          Crystallinity (%)
A (inside wall)  22
E (outside wall) 32

It is apparent that container 2 exhibits a negative gradient of crystallinity in the area of its inside wall 42. The crystallinity measured from the side of inside wall 42 is in this case much less (about 30%) than the crystallinity measured from the side of outside wall 43.

It may also be noted that the mechanical resistance to deformation of a container 2, at the time of a hot filling (with a liquid, the temperature of which is between 85° C. and 95° C.), is greater than that of a container obtained by a process without ignition (See, the comparative example). In fact, for a liquid temperature between 85° C. and 95° C., the retraction rate of the container is less than or equal to 1%.

This phenomenon can be explained as follows. The extreme temperature conditions (that reach several hundred degrees Celsius) and the pressure prevailing within container 2 over the course of the formation at the time of the deflagration induced by the ignition of the explosive gaseous mixture cause the fusion of the material at least of the side of the inside wall 42 of container 2.

Following the deflagration, formed container 2 undergoes a heat setting, from the side of its outside wall 43 in contact with the heated wall of mold 5. It thus benefits, over a certain thickness of its outside wall 43, from a contribution of crystallinity by thermal means, while its inside wall 42, which becomes completely (or almost completely) amorphous following its fusion, retains a high proportion of amorphous matter, with a comparatively lower rate of crystallinity.

Despite the low rate of crystallinity of the side of inside wall 42, the high rate of crystallinity of the side of outside wall 43 gives container 2 a rigidity equivalent to that of a container with constant crystallinity (such as a simply thermoset container, obtained by the process described in the comparative example, that is to say without igniting an explosive gaseous mixture), the portion of high crystallinity matter (comprising outside wall 43) acting in the manner of a brace with respect to the portion of low crystallinity matter (comprising inside wall 42).

In this example, the thickness of inside wall 42 corresponds to the thickness of segment A, as cut out for the needs of the DSC analysis (See above). Measurements have shown that the gradient of crystallinity does not extend beyond segment C. Consequently, inside wall 42 affected by the negative gradient of crystallinity exhibits a thickness less than 100 μm, and more likely less than approximately 50 μm.

During the hot filling, the residual stresses stored by container 2 at the time of its formation are largely released into the amorphous matter present in a large proportion of the side of inside wall 42, which thus acts as a buffer versus the portion of high crystallinity matter, preventing the propagation of deformations in the container.

EXAMPLE 2

FIGS. 5A to 5H, 6A and 6B)

In this example, mold 5 is heated such that such that it exhibits on the side of its inside wall a temperature of approximately 160° C. The material of the preform 3 is a PET. The reactive gas is hydrogen (H₂). The air/hydrogen gaseous mixture is made while maintaining a hydrogen proportion in volume between 4% and 18%, preferably 6%.

Following introduction of the hot preform 3 into mold 5, a first pre-blowing operation, consisting of stretching preform 3 by sliding rod 12, and to simultaneously pre-blow it by opening the electromagnetic valves EV1 and EV2 to introduce into preform 3 an air and reactive gas mixture at a pressure between approximately 5 and 20 bars (FIGS. 5A to 5C, lines 1 and 2 on FIG. 6B). This first operation of a predetermined duration β1 (less than 250 ms) ends by the closing of the electromagnetic valves EV1 and EV2 after rod 12 has ended its travel having reached the bottom of mold 7, and the threshold for the plastic outflow of the material has been exceeded (which is indicated on the pressure curve by a first pressure peak, at approximately 8 to 10 bars).

A second operation, known as ignition, consists of igniting the gaseous mixture by ignition of spark plug 36 (FIG. 5D, line 3 in FIG. 6B). Taking into account the proportion of hydrogen in the gaseous mix, a deflagration occurs in container 2 that is being formed, which is accompanied by an abrupt increase of the temperature (which reaches hundreds of degrees Celsius) and of the pressure (which exceeds 40 bars—on the curve of FIG. 6A the corresponding peak pressure is clipped due to reasons of scale). The duration β2 of the ignition of the mixture is extremely brief (less than 25 ms), but the increased pressure that accompanies it is sufficient to pin the matter flat against the wall of mold 5, thus forming container 2.

A third operation, known as stabilization, consists of maintaining in container 2 the residual gas (essentially a mixture of water vapor coming from the combustion of hydrogen, with possible traces of NO_x), for a predetermined duration β3 (between 200 and 300 ms) while keeping all the electromagnetic valves EV1, EV2, EV3, EV4 closed, so as to permit the reduction of the temperature and the pressure in container 2 (FIG. 5E).

A fourth operation, known as blowing, consists of opening third electromagnetic valve EV3 to introduce into container 2, via holes 13 arranged in rod 12, high pressure air (between approximately 30 and 40 bars) at ambient temperature and thus keep pinned flat against the wall of mold 5 container 2 formed at the time of the ignition operation (FIG. 5F, line 4 on FIG. 6B). During this blowing operation, of a predetermined duration β4 (preferably less than 1000 ms), the fourth electromagnetic valveEV4 is kept closed.

A fifth operation, known as sweeping, consists of making an air sweep of container 2, while keeping third electromagnetic valve EV3 open to continue introducing high pressure air (FIG. 6B, line 4) at ambient temperature into container 2, while simultaneously opening fourth electromagnetic valve EV4 to permit pressurized air to be evacuated (FIG. 6B, line 5). An air circulation is thus generated in container 2 while maintaining it under pressure (between 10 and 15 bars), which continues to pin it flat against the wall of mold 5 while cooling it (at least on the same side of its inside wall 42), so that upon emerging from mold 5 it retains the shape that the latter gives it (FIG. 5G). This sweep operation is performed for a predetermined duration β5, between 200 et 2000 ms.

A sixth operation, known as degassing, consists of degassing container 2, by closing third electromagnetic valve EV3 (FIG. 6B, line 4) and leaving fourth electromagnetic valve EV4 open for a preset duration β6 (between 100 and 500 ms), to allow the air to escape (FIG. 5H) until the pressure prevailing inside container 2 has about attained the atmospheric pressure (FIG. 6B, line 5). At the end of this operation, the fourth electromagnetic valve EV4 is closed, mold 5 opened and container 2 evacuated to enable repetition of the cycle with a new preform.

DSC was used, in the above described conditions to measure the crystallinity of a container 2 obtained by this process, on the side of inside wall 42 of body 10 (segment A) and of the side of its outside wall 43 (segment E). The results of the measurements are presented in the following table:

Segment          Crystallinity (%)
A (inside wall)  22
E (outside wall) 32

It is apparent that container 2 exhibits a negative gradient of crystallinity in the area of its inside wall 42. The crystallinity measured from the side of inside wall 42 is in this case much less (about 30%) than the crystallinity measured from the side of outside wall 43.

It may also be noted that the mechanical resistance to deformation of a container 2, at the time of a hot filling (with a liquid, the temperature of which is between 85° C. and 95° C.), is greater than that of a container obtained by a process without ignition (See, the comparative example). In fact, for a liquid temperature between 85° C. and 95° C., the retraction rate of the container is less than or equal to 1%.

This phenomenon can be explained as follows. The extreme temperature conditions (that reach several hundred degrees Celsius) and the pressure prevailing within container 2 over the course of the formation at the time of the deflagration induced by the ignition of the explosive gaseous mixture cause the fusion of the material at least of the side of the inside wall 42 of container 2.

Following the deflagration, formed container 2 undergoes a heat setting, from the side of its outside wall 43 in contact with the heated wall of mold 5. It thus benefits, over a certain thickness of its outside wall 43, from a contribution of crystallinity by thermal means, while its inside wall 42, which becomes completely (or almost completely) amorphous following its fusion, retains a high proportion of amorphous matter, with a comparatively lower rate of crystallinity.

Despite the low rate of crystallinity of the side of inside wall 42, the high rate of crystallinity of the side of outside wall 43 gives container 2 a rigidity equivalent to that of a container with constant crystallinity (such as a simply thermoset container, obtained by the process described in the comparative example, that is to say without igniting an explosive gaseous mixture), the portion of high crystallinity matter (comprising outside wall 43) acting in the manner of a brace with respect to the portion of low crystallinity matter (comprising inside wall 42).

In this example, the thickness of inside wall 42 corresponds to the thickness of segment A, as cut out for the needs of the DSC analysis (See above). Measurements have shown that the gradient of crystallinity does not extend beyond segment C. Consequently, inside wall 42 affected by the negative gradient of crystallinity exhibits a thickness less than 100 μm, and more likely less than approximately 50 μm.

During the hot filling, the residual stresses stored by container 2 at the time of its formation are largely released into the amorphous matter present in a large proportion of the side of inside wall 42, which thus acts as a buffer versus the portion of high crystallinity matter, preventing the propagation of deformations in the container.

EXAMPLE 3

FIGS. 7A to 7L, 8A and 8B)

In this example mold 5 is heated such that it exhibits on the side of its inside wall a temperature of approximately 130° C. The material of the preform 3 is a PET. The reactive gas is hydrogen (H₂). The air/hydrogen gaseous mixture is made while maintaining a hydrogen proportion in volume between 4% and 18%, preferably 6%.

Following introduction of the hot preform 3 into mold 5, a first operation, known as pre-blowing, consists of stretching preform 3 by sliding rod 12, and simultaneously pre-blowing it by opening electromagnetic valves EV1 and EV2 to introduce into preform 3 an air and reactive gas mixture at a pressure between approximately 5 and 20 bars (FIGS. 7A to 7C, lines 1 and 2 on FIG. 8B). This first operation of a predetermined duration γ1 (less than 250 ms) ends by the closing of the electromagnetic valves EV1 and EV2 after rod 12 has ended its travel having reached the bottom of mold 7, and the threshold for the plastic outflow of the material has been exceeded (which is indicated on the pressure curve by a first pressure peak, at approximately 8 to 10 bars).

A second operation, known as primary ignition, consists of igniting the gaseous mixture by ignition of spark plug 36 (FIG. 7D, line 3 in FIG. 8B). Taking into account the proportion of hydrogen in the gaseous mix, a deflagration occurs in container 2 that is being formed, which is accompanied by an abrupt increase of the temperature (which reaches hundreds of degrees Celsius) and of the pressure (which exceeds 40 bars—on the curve of FIG. 7A; the corresponding peak pressure is clipped due to reasons of scale). The duration of γ2 of the ignition of the mixture is extremely brief (less than 25 ms), but the increased pressure that accompanies it is sufficient to pin the substance flat against the wall of mold 5, thus forming container 2.

A third operation, known as primary stabilization, consists of maintaining in container 2 a residual gas (essentially a mixture of water vapor coming from the combustion of hydrogen, with possible traces of NO_x), for a predetermined duration γ3 (between 200 and 300 ms) while keeping all the electromagnetic valves EV1, EV2, EV3, EV4 closed, so as to permit the reduction of the temperature and the pressure in container 2 (FIG. 8E).

A fourth operation, known as degassing, consists of degassing container 2, by closing the fourth electromagnetic valve EV4 for a preset duration 4 (between 100 and 200 ms), to allow the air to escape (FIG. 7F) until the pressure prevailing inside container 2 has attained about the atmospheric pressure (FIG. 8B, line 5).

A fifth operation, known as secondary pre-blowing, consists of re-opening electromagnetic valves EV1 and EV2 to introduce into the container an air and reactive gas mixture at a pressure between approximately 5 and 20 bars (FIG. 7G, lines 1 and 2 in FIG. 8B). This fifth operation, of a predetermined duration γ5 (less than 250 ms) ends by the closing of the electromagnetic valves EV1 et EV2 after the pressure in container 2 has reached a value between 5 and 20 bars.

A sixth operation, known as ignition, consists of igniting the gaseous mixture by ignition of spark plug 36 (FIG. 7H, line 3 in FIG. 8B). Taking into account the proportion of hydrogen in the mixture, a deflagration is produced in container 2, which is accompanied by an abrupt increase in the temperature (which again reaches several hundred degrees Celsius) and the pressure (which again exceeds 40 bars, the corresponding peak pressure likewise being clipped in FIG. 8A). As in the primary ignition operation, the duration γ6 of the ignition of the mixture is extremely brief (less than 25 ms).

A seventh operation, known as secondary stabilization, consists of maintaining in container 2 a residual gas (essentially a mixture of water vapor coming from the combustion of hydrogen, with possible traces of NO_x), for a predetermined duration γ7 (between 200 and 300 ms), while keeping all the electromagnetic valves EV1, EV2, EV3, EV4 closed, so as to permit the reduction of the temperature and the pressure in container 2 (FIG. 7I).

An eighth operation, known as blowing, consists of opening the third electromagnetic valve EV3 to introduce into container 2, via holes 13 arranged in rod 12, high pressure air (between approximately 30 and 40 bars) at ambient temperature and thus keep pinned flat against the wall of mold 5 container 2 formed at the time of the ignition operation (FIG. 7J, line 4 on FIG. 8B). During this blowing operation, of a predetermined duration γ8 (preferably less than 300 ms), the fourth electromagnetic valve EV4 is kept closed.

A ninth operation, known as sweeping, consists of making an air sweep of the container, while keeping the third electromagnetic valve EV3 open to continue introducing high pressure air (FIG. 8B, line 4) at ambient temperature into container 2, while simultaneously opening the fourth electromagnetic valve EV4 to permit pressurized air to be evacuated (FIG. 8B, line 5). An air circulation is thus generated in container 2 while maintaining it under pressure (between 10 and 15 bars), which continues to pin it flat against the wall of mold 5 while cooling it (at least on the side of its inside wall 42), so that upon emerging from mold 5 it retains the shape that the latter gives it (FIG. 7K). This sweep operation is performed for a predetermined duration γ9, between 200 et 2000 ms.

A tenth operation, known as secondary degassing, consists of degassing container 2, by closing the third electromagnetic valve EV3 (FIG. 8B, line 4) and leaving the fourth electromagnetic valve EV4 for a preset duration γ10 (between 100 and 500 ms), to allow the air to escape (FIG. 7L) until the pressure prevailing inside container 2 has about attained atmospheric pressure (FIG. 8B, line 5). At the end of this operation, the fourth electromagnetic valve EV4 is closed, mold 5 opened and container 2 evacuated to enable repetition of the cycle with a new preform.

DSC was used, in the above described conditions, to measure the crystallinity in the thickness of a container 2 obtained by this process. The results of the measurements are presented in the following table:

Segment          Crystallinity (%)
A (inside wall)  15
B                30
C                26
D                27
E (outside wall) 28

It is apparent that container 2 exhibits a negative gradient of crystallinity in the area of its inside wall 42. The crystallinity measured from the side of inside wall 42 is in this case much less (about 50%) than the crystallinity measured from the side of outside wall 43.

It may also be noted that the mechanical resistance to deformation of such a container 2, at the time of a hot filling (with a liquid, the temperature of which is between 85° C. and 95° C.), is greater than that of a container obtained by a process without ignition (See the comparative example). In fact, for a liquid temperature between 85° C. and 95° C., the retraction rate of the container is less than or equal to 1%.

This phenomenon can be explained as follows. The extreme temperature conditions (that reach several hundred degrees Celsius) and the pressure prevailing within container 2 over the course of the formation at the time of the deflagration induced by the ignition of the explosive gaseous mixture cause the fusion of the material at least on the side of the inside wall 42 of container 2.

Following the deflagrations, formed container 2 undergoes a heat setting, of the side of its outside wall 43 in contact with the heated wall of mold 5. It thus benefits, over a certain thickness of its outside wall 43, from a contribution of crystallinity by thermal means, while its inside wall 42, which becomes completely (or almost completely) amorphous following its fusion, retains a high proportion of amorphous matter, with a comparatively lower rate of crystallinity.

Despite the low rate of crystallinity of the side of inside wall 42, the high rate of crystallinity of the side of outside wall 43 gives container 2 a rigidity equivalent to that of a container with constant crystallinity (such as a simply thermoset container, obtained by the process described in the comparative example, that is to say without igniting an explosive gaseous mixture), the portion of high crystallinity matter (comprising outside wall 43) acting in the manner of a brace with respect to the portion of low crystallinity matter (comprising inside wall 42).

In this example, the thickness of inside wall 42 corresponds to the thickness of segment A, as cut out for the needs of the DSC analysis (See above). Measurements have shown that the gradient of crystallinity does not extend beyond segment C. Consequently, inside wall 42 affected by the negative gradient of crystallinity exhibits a thickness less than 100 μm, and more likely less than approximately 50 μm.

During the hot filling, the residual stresses stored by container 2 at the time of its formation are largely released into the amorphous matter present in a large proportion of the side of inside wall 42, which thus acts as a buffer versus the portion of high crystallinity matter, preventing the propagation of deformations in the container.

In order to verify the amorphous character of layer A, a thermal analysis was performed on this container 2 by means of DSC, by taking a sample similar to that used for measuring the crystallinity, and by cutting it in the same manner to obtain five similar segments A, B, C, D and E. The DSC curves of the five segments are consolidated on the thermogram of FIG. 15. It is apparent that the curves of segments B, C, D and E all exhibit a single peak endothermic fusion around 250° C., while the curve of segment A exhibits, in addition to this same endothermic peak around 125° C., an exothermic peak around 250° C.

This endothermic peak, found between the vitreous transition temperature (occurring around 80° C.) and the fusion peak, is a crystallization peak, attesting to the amorphous character of the matter of segment A, of the side of inside wall 42 of container 2. Conversely, the absence of such a crystallization peak on the curves of the other segments B to E attests to the semi-crystalline character of the matter in particular of the side of the outside wall 43. In other words, container 2 can be considered, at the end of its manufacture, to be amorphous on the side of its inside wall 42.

By comparison, the same analysis was performed on a sample taken from a container obtained by the process described in the comparative example (that is to say without igniting an explosive gaseous mixture). It is apparent (FIG. 16) that the DSC curves of the five segments A to E of the sample exhibit only one peak, endothermic, corresponding to the fusion of the matter at about 250° C., indicating that the container is semi-crystalline throughout its thickness, including on the side of its internal wall.

EXAMPLE 4

FIGS. 9A to 9K, 10A and 10B)

In this example mold 5 is heated such that it exhibits on the side of its inside wall a temperature of approximately 130° C. The material of the preform 3 is a PET. The reactive gas is hydrogen (H₂). The air/hydrogen gaseous mixture is made while maintaining a hydrogen proportion in volume between 4% and 18%, preferably 6%.

Following introduction of the hot preform 3 into mold 5, a first operation, known as primary pre-blowing, consists of stretching preform 3 by sliding rod 12, and simultaneously pre-blowing it by opening first electromagnetic valve EV1 to introduce into preform 3 an air and reactive gas mixture at a pressure between approximately 5 and 20 bars (FIGS. 9A to 9C, line 1 on FIG. 10B). This first operation of a predetermined duration δ1 (less than 250 ms) ends by the closing of electromagnetic valve EV1 and after rod 12 has ended its travel, having reached mold bottom 7, and the threshold for the plastic outflow of the material has been exceeded (which is indicated on the pressure curve by a first pressure peak, at approximately 8 to 10 bars).

A second operation, known as blowing, consists of blowing container 2 by opening the third electromagnetic valve EV3 to introduce into container 2 being formed, via holes 13 arranged in rod 12, high pressure air (between approximately 30 and 40 bars) at ambient temperature, so as to keep container 2 pinned flat against the wall of mold 5 (FIG. 9D, line 4 on FIG. 10B). During this blowing operation, of a predetermined duration δ2 (preferably less than 300 ms), fourth electromagnetic valve EV4 is kept closed.

A third operation, known as degassing, consists of degassing container 2, by opening the fourth electromagnetic valve EV4 for a preset duration δ3 (between 100 and 200 ms), to allow the air to escape (FIG. 9E) until the pressure prevailing inside container 2 has attained about the atmospheric pressure (FIG. 10B, line 5).

A fourth operation, known as secondary pre-blowing, consists of re-opening electromagnetic valves EV1 and EV2 to introduce into container 2 an air and reactive gas mixture at a pressure between approximately 5 and 20 bars (FIG. 9F, lines 1 and 2 on FIG. 10B). This fourth operation, of a predetermined duration δ4 (less than 250 ms) ends by the closing of electromagnetic valves EV1 and EV2 after the pressure in container 2 has reached a value between 5 and 20 bars.

A fifth operation, known as ignition, consists of igniting the gaseous mixture by ignition of spark plug 36 (FIG. 9G, line 3 in FIG. 10B). Taking into account the proportion of hydrogen in the gaseous mixture, a deflagration occurs in container 2, accompanied by an abrupt increase of the temperature (which reaches several hundred degrees Celsius) and of the pressure (which exceeds 40 bars—on the curve of FIG. 10A; the corresponding peak pressure is clipped due to reasons of scale). The duration δ5 of ignition of the mixture is extremely brief (less than 25 ms).

A sixth operation, known as stabilization, consists of maintaining in container 2 a residual gas (essentially a mixture of air and water vapor coming from the combustion of hydrogen, with possible traces of NO_x), for a predetermined duration δ6 (between 200 and 300 ms), while keeping all the electromagnetic valves EV1, EV2, EV3, EV4 closed, so as to permit the reduction of the temperature and the pressure in container 2 (FIG. 9H).

An seventh operation, known as secondary blowing, consists of opening the third electromagnetic valve EV3 to introduce into container 2, via holes 13 arranged in rod 12, high pressure air (between approximately 30 and 40 bars) at ambient temperature and thus keep pinned flat against the wall of mold 5 container 2 formed at the time of the ignition operation (FIG. 9I, line 4 on FIG. 10B). During this blowing operation, of a predetermined duration δ7 (preferably less than 300 ms), fourth electromagnetic valve EV4 is kept closed.

An eighth operation, known as sweeping, consists of making an air sweep of container 2, while keeping third electromagnetic valve EV3 open to continue introducing high pressure air (FIG. 10B, line 4) at ambient temperature into container 2, while simultaneously opening the fourth electromagnetic valve EV4 to permit pressurized air to be evacuated (FIG. 10B, line 5). An air circulation is thus generated in container 2, while maintaining it under pressure (between 10 and 15 bars), which continues to pin it flat against the wall of mold 5, while cooling it (at least on the same side of its inside wall), so that upon emerging from mold 5 it retains the shape that the latter gives it (FIG. 9J). This sweep operation is performed for a predetermined duration δ8, between 200 et 2000 ms.

A ninth operation, known as secondary degassing, consists of degassing container 2, by closing third electromagnetic valve EV3 (FIG. 10B, line 4) and leaving fourth electromagnetic valve EV4 open for a preset duration δ9 (between 100 and 500 ms), to allow the air to escape (FIG. 9K) until the pressure prevailing inside container 2 has about attained atmospheric pressure (FIG. 10B, line 5). At the end of this operation, fourth electromagnetic valve EV4 is closed, mold 5 opened and container 2 evacuated to enable repetition of the cycle with a new preform.

DSC was used, in the above described conditions to measure the crystallinity of a container 2 obtained by this process, on the side of inside wall 42 of body 10 (segment A) and of the side of its outside wall 43 (segment E). The results of the measurements are presented in the following table:

Segment          Crystallinity
A (inside wall)  25
E (outside wall) 31

It is apparent that container 2 exhibits a negative gradient of crystallinity in the area of its inside wall 42. The crystallinity measured on the side of inside wall 42 is in this case much less (about 20%) than the crystallinity measured on the side of outside wall 43.

It may also be noted that the mechanical resistance to deformation of such a container 2, at the time of a hot filling (with a liquid, the temperature of which is between 85° C. and 95° C.), is greater than that of a container obtained by a process without ignition (See the comparative example). In fact, for a liquid temperature between 85° C. and 95° C., the retraction rate of the container is less than or equal to 1%.

This phenomenon can be explained as follows. The extreme temperature conditions (that reach several hundred degrees Celsius) and the pressure prevailing within container 2 over the course of the formation at the time of the deflagration induced by the ignition of the explosive gaseous mixture cause the fusion of the material at least of the side of the inside wall 42 of container 2.

Following the deflagration, formed container 2 undergoes a heat setting, of the side of its outside wall 43 in contact with the heated wall of mold 5. It thus benefits, over a certain thickness of its outside wall 43, from a contribution of crystallinity by thermal means, while its inside wall 42, which becomes completely (or almost completely) amorphous following its fusion, retains a high proportion of amorphous matter, with a comparatively lower rate of crystallinity.

Despite the low rate of crystallinity of the side of inside wall 42, the high rate of crystallinity of the side of outside wall 43 gives container 2 a rigidity equivalent to that of a container with constant crystallinity (such as a simply thermoset container, obtained by the process described in the comparative example, that is to say without igniting an explosive gaseous mixture), the portion of high crystallinity matter (comprising outside wall 43) acting in the manner of a brace with respect to the portion of low crystallinity matter (comprising inside wall 42).

In this example, the thickness of inside wall 42 corresponds to the thickness of segment A, as cut out for the needs of the DSC analysis (See above). Measurements have shown that the gradient of crystallinity does not extend beyond segment C. Consequently, inside wall 42 affected by the negative gradient of crystallinity exhibits a thickness less than 100 μm, and more likely less than approximately 50 μm.

During the hot filling, the residual stresses stored by container 2 at the time of its formation are largely released into the amorphous matter present in a large proportion of the side of inside wall 42, which thus acts as a buffer versus the portion of high crystallinity matter, preventing the propagation of deformations in the container.

COMPARATIVE EXAMPLE

FIGS. 11A to 11F, 12A, 12B, 16)

In this example, mold 5 is heated such that such that it exhibits on the side of its inside wall a temperature of approximately 160° C. The material of the preform is a PET.

Following introduction of the hot preform 3 into mold 5, a first operation, known as pre-blowing, consists of stretching preform 3 by sliding rod 12, and simultaneously pre-blowing it by opening the first electromagnetic EV1 to introduce into preform 3 an air and reactive gas mixture at a pressure between approximately 5 and 20 bars (FIGS. 11A to 11C, line 1 on FIG. 12B). This first operation of a predetermined duration ε1 (less than 250 ms) ends by the closing of electromagnetic valve EV1 after rod 12 has ended its travel having reached the bottom of mold 7, and the threshold for the plastic outflow of the material has been exceeded (which is indicated on the pressure curve by a first pressure peak, at approximately 8 to 10 bars).

A second operation, known as blowing, consists of blowing preform 3 by opening the third electromagnetic valve EV3 to introduce into preform 3, via holes 13 arranged in rod 12, high pressure air (between approximately 30 and 40 bars) at ambient temperature, so as to pin container 2 flat against the wall of mold 5 (FIG. 11D, line 4 on FIG. 12B). During this blowing operation, of a predetermined duration ε2 (between 500 and 1200 ms), fourth electromagnetic valve EV4 is kept closed.

A third operation, known as sweeping, consists of making an air sweep of container 2, while keeping third electromagnetic valve EV3 open to continue introducing high pressure air at ambient temperature into container 2 via holes 13 arranged in rod 12 (FIG. 12B, line 4), while simultaneously opening the fourth electromagnetic valve EV4 to permit pressurized air to be evacuated (FIG. 12B, line 5). An air circulation is thus generated in container 2 while maintaining it under pressure (between 10 and 15 bars), so as to pin it against mold 5 (FIG. 12E). This sweep operation is performed for a predetermined duration ε3, between 500 et 800 ms.

A fourth operation, known as degassing, consists of degassing container 2, by closing the third electromagnetic valve EV3 (FIG. 12B, line 4) and leaving the fourth electromagnetic valve EV4 open for a preset duration ε5 (between 200 and 500 ms), to allow the air to escape (FIG. 11F) until the pressure prevailing inside container 2 has attained about the atmospheric pressure (FIG. 12B, line 5). At the end of this operation, the fourth electromagnetic valve EV4 is closed, mold 5 opened and container 2 evacuated to enable repetition of the cycle with a new preform.

DSC was used, in the above described conditions to measure the crystallinity of a container obtained by this process, from the side of the inside wall of the body of the container (segment A) and in the area of its outside wall (segment E). The results of the measurements are presented in the following table:

Segment          Crystallinity
A (inside wall)  31
B                24
C                28
D                35
E (outside wall) 30

It is apparent that the crystallinity is more or less constant from segment A (inside wall 42) up to segment E (outside wall 43), demonstrating the more or less uniform character, in the thickness of container 2, of the thermosetting realized by this process.

A thermal analysis of this container 2 is performed by DSC, by taking a sample similar to that used for measuring the crystallinity, and by cutting it in the same manner to obtain five similar segments A, B, C, D and E. The DSC curves of the five segments are consolidated on the thermogram of FIG. 16. It is apparent that the curves all exhibit a single endothermic peak of fusion around 250° C., attesting to the semi-crystalline character of the matter throughout the thickness of container 2.

It may also be noted that the mechanical resistance to deformation of such a container 2, at the time of a hot filling (with a liquid, the temperature of which is between 85° C. and 95° C.), is less than that of a container obtained by a process with ignition. For a liquid temperature consisting of 90° C. for example, the retraction rate of the container is 2%. Moreover, for a liquid temperature of 95° C., filling is impossible unless container 2 is deformed (the container is pumped like a barrel) beyond what is commercially permissible.

BIBLIOGRAPHY

• [1] J. P. Trotignon, J. Verdu, A. Dobraczynski, M. Piperaud, “Matières plastiques: structure-propriétés, mise en oeuvre, normalisation [Plastic materials: structure-properties, implementation, standardization]”, Ed. AFNOR/NATHAN, Paris 1996, ISBN Nathan 2-09-176572-4
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• [8] U.S. Pat. No. 4,512,948 (Owens Illinois, Inc.)
• [9] U.S. Pat. No. 4,476,197 (Owens Illinois, Inc.)
• [10] French patent FR 2 595 EV34 (Sidel) and its American equivalent U.S. Pat. No. 4,836,971
• [11] European patent EP 1 305 EV18 (Amcor)

Claims

1. Manufacturing process of a container (2) in a mold (5) having a cavity (8) defining the final shape of the container (2), from a polymer blank (3) heated in advance, this process comprising the following operations:

introduction of the blank (3) into the mold (5) heated to a predetermined temperature,

introduction into the blank (3) of a pressurized explosive gaseous mixture,

ignition of the gaseous mixture.

2. Process according to claim 1, comprising:

a pre-blowing operation during the course of which the blank (3) is stretched, while introducing the explosive gaseous mixture into it;

an ignition operation consistent with igniting the explosive gaseous mixture;

a degassing operation during the course of which the inside of the container (2) is brought to a pressure close to atmospheric pressure.

3. Process according to claim 1, comprising:

a pre-blowing operation during the course of which the blank (3) is stretched, while introducing the explosive gaseous mixture into it;

an ignition operation consistent with igniting the explosive gaseous mixture;

a blowing operation consistent with introducing high pressure air into the container (2);

a degassing operation during the course of which the inside of the container (2) is brought to a pressure close to the atmospheric pressure.

4. Process according to claim 2, comprising, following the ignition operation, a stabilization operation, during which the residual gas deriving from the ignition is maintained in the formed container (2).

5. Process according to claim 2, comprising, prior to the degassing operation, a sweeping operation during the course of which air is circulated in the container (2).

6. Process according to claim 1, comprising:

a pre-blowing operation, during the course of which the blank (3) is stretched, while introducing the explosive gaseous mixture into it;

a primary ignition operation consistent with igniting the explosive gaseous mixture;

a primary degassing operation during the course of which the inside of the container (2) is brought to a pressure close to atmospheric pressure,

a secondary pre-blowing operation during the course of which an explosive gaseous mixture is introduced into the container (2);

a secondary ignition operation consistent with igniting the explosive gaseous mixture;

a secondary degassing operation during the course of which the inside of the container (2) is brought to a pressure close to the atmospheric pressure.

7. Process according to claim 6, comprising, following the secondary ignition operation, a blowing operation consisting of introducing high pressure air into the container (2).

8. Process according to claim 6, comprising, following each ignition operation, a stabilization operation, during which the residual gas deriving from the ignition is maintained in the formed container (2).

9. Process according to claim 6, comprising, prior to the degassing operation, a sweeping operation during the course of which air is circulated in the container (2).

10. Process according to claim 1, comprising:

a pre-blowing operation, during the course of which the blank (3) is stretched, while introducing air into it;

a blowing operation consistent with introducing high pressure air into the blank (3);

a primary degassing operation during the course of which the inside of the container (2) is brought to a pressure close to the atmospheric pressure,

a secondary pre-blowing operation during the course of which an explosive gaseous mixture is introduced into the container (2);

an ignition operation consistent with igniting the explosive gaseous mixture;

a secondary degassing operation during the course of which the inside of the container (2) is brought to a pressure close to the atmospheric pressure.

11. Process according to claim 10, comprising, following the ignition operation, a stabilization operation, during which the residual gas deriving from the ignition is maintained in the formed container (2).

12. Process according to claim 10, comprising, prior to the secondary degassing operation, a sweeping operation during the course of which air is circulated in the container (2).

13. Process according to claim 1, in which the mold (5) is heated to a temperature greater than or equal to 100° C.

14. Process according to claim 1, in which the mold (5) is heated to a temperature greater than or equal to about 130° C.

15. Process according to claim 1, in which the mold (5) is heated to a temperature greater than or equal to about 160° C.

16. Process according to claim 1, in which the explosive gaseous mixture comprises air and hydrogen.

17. Process according to claim 16, in which the volumetric proportion of hydrogen is about 6%.

18. Process according to claim 1, in which the explosive gaseous mixture is introduced into the blank (3) or the container (2) at a pressure greater than or equal to 10 bars.

19. Process according to claim 3, in which the pressure of the air introduced into the blank (3) or the container (2) at the time of the blowing operation is greater than or equal to 30 bars.

20. Machine (1) for the manufacture of containers (2) from polymer blanks (3), heated in advance, comprising:

a mold (5) having a cavity (8) defining the final form of the containers (2),

the means (39, 40, 41) of heating the mold (5);

the means (18-24, EV2) of injection of a pressurized explosive gaseous mixture into the mold (5),

the means (35, 36) of ignition of the gaseous mixture.

21. Machine according to claim 20, in which the means (35, 36) of ignition comprise a spark plug (36).

22. Machine according to claim 21, in which the spark plug (36) leads to a nozzle (16), suitable for communicating with the inside of the container (2) in order to introduce a gaseous mixture into it.

23. Machine (1) according to claim 21, in which the spark plug (36) leads into a stretch rod (12).