Process of manufacturing containers made of polyester resin

The present invention relates to the field of the manufacture of containers made of polymer material, in particular polyester. More particularly, it relates to the field of the manufacture of polyester bottles, preferably of polyethylene terephthalate (PET), containing a liquid, preferably water and in particular mineral water.

The present invention relates to a polyester container, its manufacturing process by injection under pressure of liquid into a preform. It also relates to a device for implementing this method.

For many years, PET bottles usually found on the market have been produced by blow molding or stretch blow molding, using compressed air, of PET preforms.

A preform is usually in the form of a cylindrical tube closed at one of its ends and open at its opposite end. The head of the preform, open, corresponds to the neck of the container. During the conventional process for manufacturing the container from a preform, the preforms are threaded upside down on the cylindrical pads of a continuous conveying chain which thus transports the preforms through an oven, essentially consisting of a rectilinear section bordered on each side by radiant heating means, in order to condition the plastic material in temperature for the subsequent blow-stretching step.

The hot preform is then removed and transported in a mold of a blowing machine. The transport movement carried out by a transfer arm for example, is coordinated with that of the blowing machine which is generally carried out in the form of a rotary carousel rotating continuously around its vertical axis and which carries, at its periphery , a series of identical molds. Thus, the preform is placed in the mold immediately after it has opened and the previously formed container is removed.

The preform is preheated so as to be in the mold at a temperature higher than the glass transition temperature of the PET (approximately 100 ° C.) in order to allow shaping by blow-drawing. The temperature of the preform at the end of the heating stage is slightly higher than that required inside the mold of the snowblower, in order to take into account the cooling which takes place over the distance existing between the place of heating and the place of blowing. Thanks to the rotary movement and the simultaneous presence of several molds, such a blowing machine makes it possible to produce containers at very high rates, of the order of several tens of thousands of units per hour or of the order from 1000 to 2000 bottles per hour and per mold.

The stretch-blow molding is done by stretching using a metal rod, an air injection being made at pressures varying from 3 to 40 bars (3.10⁵ Pa to 4.10⁶ Pa). Air is injected through a nozzle, the end of which is introduced through the opening of the preform head.

Bottles manufactured by injection of pressurized air have a relatively satisfactory lifetime for a given weight and type of material. However, the intrinsic characteristics and properties of PET may suggest even better results by modifying the manufacturing process of the containers.

One of the objectives of the present invention is therefore to provide an improved method of manufacturing a polyester container from a preform.

Another objective is to possibly be able to integrate the container filling step into the latter's manufacturing process. There are already developments in the state of the art according to which the container is blown-stretched-molded by the liquid which will then fill said container. Patent JP 63249616 in the name of Komatsu Ltd. concerns such a development. The material used is polyethylene, polypropylene or polyvinyl chloride. The patent FR 1430316 also relates to such a filling system, but in this case with polytetrafluoroethylene. None of the known documents relates to a filling approach with polyester. There is not in the aforementioned materials the specificity of polyester which is to crystallize during drawing. All the aforementioned materials are amorphous and / or semi-crystalline and remain so during stretching.

The present invention relates to a polyester resin container capable of being obtained by a process comprising heating a preform which is substantially in the form of a cylindrical tube having an opening at the neck at a temperature above the transition temperature. vitreous of said polyester, the introduction of said preform into a mold, drawing said preform by a drawing rod and introducing an incompressible fluid through the opening of said preform to form said container. The polyester of the container obtained has a crystallinity of between 25 and 50%. The object of the present invention is to obtain the highest possible crystallinity, because high crystallinity means better mechanical strength of the container. Without wishing to be bound by a scientific theory, it is known that the polyester preform is amorphous and that stretching induces crystallization and at the same time an exothermic reaction. The release of heat is detrimental to the development of crystallinity. According to the present invention, one therefore operates by a primary stretching with the stretching rod, inducing crystallization and release of heat and at this time, the incompressible fluid is introduced which will very quickly absorb this emitted heat and thus allow move the equilibrium point of the crystallinity obtained in the final container upwards, preferably between 30 and 50%.

The crystallinity measurement is carried out on a Lloyd-Davenport density column according to the following method. The column is filled with a saline solution (calcium nitrate) having a density gradient. The column is calibrated with beads having a known density between 1.335 and 1.455. Then, small pieces of the container according to the invention are immersed in the column and after a certain time, they stabilize at a certain height of the column corresponding to a certain density. The measurements are carried out at 23 ° C. The following table of correspondence with a ρ_vs 1.455 gives crystallinity. In the table, it is therefore necessary to look only at the right part of the crystallinity<tables id="tabl0001" num="0001"><img file="EP1529620A1_D0001.tif" /></tables>

The invention further relates to a method of manufacturing a polyester resin container from a preform which is substantially in the form of a cylindrical tube comprising an opening, preferably at a neck, said method comprising a step heating said preform to a temperature higher than the glass transition temperature of said polyester, a step of introducing said preform into a mold, a step of stretching said preform by a drawing rod and an expansion step carried out inside a cavity (101), characterized in that, during said expansion step, an incompressible fluid under pressure is injected through the opening of said preform to form said container.

The containers obtained by this process have much better characteristics than those obtained with a blow-stretching process of the conventional type with inflation by gaseous fluid. In particular, it has been found that they have a longer lifespan for a given weight and type of material. The degree of crystallinity, that is to say the mass of crystalline phase relative to the total mass of polymer, of a container obtained with the process according to the invention can in particular be much greater.

For example, in the case of PET bottles, the bottles obtained by the process according to the invention can have crystallinity levels of between 30% and 50% which gives them longer lifetimes than the bottles currently obtained for the same weight and the same type of PET having crystallinity levels of between 25% and 30%.

According to another aspect of the present invention, said incompressible fluid is a liquid injected under pressure, at a controlled speed and pressure, (in controlled quantity), preferably the liquid intended to be contained in said final container to be formed. The speed of the drawing rod for drawing the preform is between 1 and 2 m / s. Consequently, the material of the container has the same speed of movement in the mold.

Thus, the liquid used for the manufacture of the containers can be the liquid to be conditioned, for example water and in particular mineral water, which makes it possible to dispense with a subsequent filling step. The steps for manufacturing containers and filling these containers are thus integrated in a single step. This solution obviously has significant economic advantages and limits the risks of contamination, in particular bacterial, of the empty container.

Advantageously, when said container to be formed is a PET bottle, said preform is heated to a temperature higher than the glass transition temperature of said PET, typically around 75 ° C to 85 ° C, during said heating step and the temperature of said incompressible fluid is between 10 ° C and 90 ° C during said expansion step.

According to a preferred embodiment of the invention, said temperature of the incompressible fluid is approximately 15 ° C.

According to another aspect of the present invention, there is also proposed a device for implementing a method of the type which has just been defined, characterized in that it essentially comprises<ul id="ul0001" list-style="dash" compact="compact"><li>a double jack system comprising a mouthpiece for the arrival of incompressible fluid and</li><li>a mold in which is placed the preform comprising an opening,</li></ul> said double jack system being disposed above said mold so as to connect its mouth to the opening of the preform at the time of drawing-filling.

According to the present invention, the double cylinder system comprises<ul id="ul0002" list-style="dash" compact="compact"><li>a first piston allowing the connection of said system with the opening of the preform,</li><li>a second piston co-axial with the first piston and allowing the filling of incompressible fluid in the preform, the two pistons being driven by air in the respective chambers of said pistons and</li><li>a co-axial stretching rod with the first and the second piston.</li></ul>

The method according to the invention can be implemented with an installation specifically built for this purpose and having the various elements mentioned above.

Advantageously, the arrival of incompressible fluid is connected to an inlet of a liquid supply line, preferably in liquid for filling the containers to be formed.

In this way, as explained above, the liquid used for the manufacture of the containers is the liquid to be conditioned, which makes it possible to dispense with an additional filling device. The devices for manufacturing containers and for filling these containers are thus integrated into a single device.

The invention will be better understood thanks to the description below, which relates to a preferred embodiment, given by way of nonlimiting example, and explained with reference to the drawings, in which<ul id="ul0003" list-style="none" compact="compact"><li>Figure 1 is schematic section of the device according to the invention before stretching-expansion-filling and</li><li>Figure 2 is a schematic section of the same device during the stretch-expand-fill phase.</li></ul>

The embodiment given here by way of example relates to a process for manufacturing PET water bottles from a heated preform. As can be seen in the appended figures, a preform is in the form of a cylindrical tube closed at its lower end. The head of the preform, open, corresponds to the neck of the bottle, that is to say, in this case, to the neck onto which a cap must be screwed.

The method is implemented by means of modifications made to an installation for manufacturing PET mineral water bottles, as described previously in the introductory part of this document.

According to the present invention, during the expansion step, is injected through the opening of the preform not pressurized air, but an incompressible fluid. Advantageously, the liquid that this bottle must contain is used as incompressible fluid allowing the bottle to be shaped, so as to eliminate a subsequent filling step.

In order to implement the invention, modifications are made to an installation for manufacturing PET bottles, as can be seen in the appended figures.

The figures schematically represent a mold 1 inside which is enclosed the body of a preform 2. The neck 3 of the preform protrudes outside the mold. The double jack system 4 associated with the mold 1 is designed to be placed above said mold so as to be connected to the opening 3 of the preform 2 at the time of stretching-expansion-filling. The double jack system comprises a body 5 with a first piston 6 and a second piston 7. The first piston 6 comprises a spring 8 arranged in the chamber 9 and a chamber 10. The second piston 7 comprises a chamber 11 and a chamber 12. Throughout the system, seals 13 are provided, assigned the same number reference. The different chambers of the pistons are connected either to an air pressure system or to atmospheric pressure. Line 14 allows the arrival of incompressible fluid 17, namely water allowing the filling of the preform. The piston 6 allows the connection of the double jack with the neck 3 of the preform and the piston 7 allows the entry of incompressible fluid into said preform. Finally, a drawing rod 15 is provided co-axially with the first and the second piston.

The operation of the device according to the invention is as follows. First of all, the preform must be heated to a temperature sufficient to allow it to undergo stretching. The heated preform is then introduced into the mold 1 and the cycle can begin. At the start, the chamber 10 of the piston 6 is put under air pressure of the order of 10 bar, so that the spring 8 is bandaged. To allow the descent of the double jack system 4, the chamber 10 is brought to atmospheric pressure, which lowers the piston 6 and thus press the mouth 16 against the opening 3 of the preform 2. Then the rod d drawing 15 is set in motion to descend into the preform so as to operate a vertical drawing. It is then time to allow the water to arrive in the preform. The chamber 11 of the piston 7 is at a pressure of the order of 10 bar and the chamber 12 at atmospheric pressure. It then suffices to put a pressure of the order of 10 bar in the chamber 12 and leave the chamber 11 to the exhaust so that the piston 7 is released from the seat of the mouthpiece 16 and allows the entry of water into the preform, thereby inducing horizontal stretching. As already mentioned above, the stretch induces a release of heat absorbed by the incompressible fluid, thus allowing the final container to have a higher crystallinity than in conventional blow-stretch devices. Then, the cycle begins again on a new preform. It is understood that use will be made of a conventional molding machine, in which a new stretch-fill head will be available.

The entire system is controlled by servomechanisms controlled by a central servo control unit or automaton (not shown) of the bottle manufacturing installation so that the operation of the injection device water is coordinated with the operation of the forming machine (blowing) and, more generally with the operation of the entire bottle manufacturing facility.

The operation of the water injection device, coordinated with the operation of the forming or blowing machine, is carried out according to an injection cycle of a duration less than the duration of the rotation cycle of a mold around the axis of the forming machine.

The optimization of the process is done for a predetermined stretching rate, a preform temperature higher than the temperature of the glass transition of the polyester used and a fast injection speed, higher than that of cooling of the material used. Another advantage is therefore a very short cycle time, the injection time of the incompressible fluid being considerably less than a second, preferably between 0.02 s and 0.5 s and more preferably between 0.1 s and 0.2 s. The pressure of the incompressible fluid is greater than 1 bar, preferably between 1 and 10 bar.

In order to eliminate any air bubbles contained in the circuit, a purge cycle is also provided. The start cycle is defined via the PLC.

The water temperature can be between 10 ° C and 90 ° C depending on the technical constraints imposed by the bottle that one wishes to produce. In particular, the pressure must be high enough to deform the preform and the low temperatures impose higher pressures. However, when the technical constraints allow it, the temperature of the liquid will advantageously be 15 ° C.

Advantageously, so that the neck 3 of the preform 2 does not risk being deformed during the expansion cycle, the latter is isolated from the liquid by a sealed and / or cooled part.

The containers obtained by this process have much better characteristics than those obtained with a conventional type expansion process using gas fluid blowing. In particular, they have a longer lifespan for a given weight and type of material. The degree of crystallinity, that is to say the mass of crystalline phase relative to the total mass of polymer, of a container obtained with a process according to the invention can in particular be much greater. The use of rapid expansion rates makes it possible to obtain a fairly high drawing rate and an induced crystallization which is also.

For example, in the case of tests carried out with water at water temperatures ranging from 85 ° C to 95 ° C and under pressures ranging from 2 bars to 3 bars (2.10⁵ Pa to 3.10⁵ Pa) in the case of PET containers, the containers obtained by the process according to the invention had crystallinity levels of up to 50%. To obtain this rate, all the non-amorphous phases, that is to say the crystalline phase and the mesophase, were integrated.

Of course, the invention is not limited to the embodiment described and shown in the accompanying drawings. Modifications remain possible, in particular from the point of view of the constitution of the various elements or by substitution of technical equivalents, without thereby departing from the scope of protection of the invention.