Method for non-destructive detection of the presence of recycled pet in a preform

Abstract

The invention relates to a method for non-destructive detection of the presence of recycled polyethylene terephthalate in a preform (12) intended to be formed into a container (11) by forming, in particular by stretch-blow moulding, characterized in that the method consists in determining the presence of an absorption peak (P1, P2) at at least one given wavelength (λ1, λ2) of between 580 nm and 690 nm that is characteristic of the presence of recycled polyethylene terephthalate (rPET).

Description

NON-DESTRUCTIVE METHOD FOR DETECTING THE PRESENCE OF RECYCLED PET IN A PREFORM Technical field of the invention

[0001] The invention relates to a non-destructive method for detecting the presence of recycled polyethylene terephthalate in a preform intended to be formed into a container by forming, in particular by stretch blow molding. Technical background

[0002] It is known to manufacture containers by forming, particularly by stretch blow molding, preforms made of thermoplastic material. The material forming the preforms is generally in an amorphous state that is not suitable for cold forming. Prior to the forming operation, the preforms are therefore heated to a temperature equal to or greater than their glass transition temperature, which allows them to be shaped into the final container.

[0003] More specifically, preforms generally have a substantially cylindrical body of revolution with thick tubular walls, closed at one of its axial ends by a thick-walled base, and extended at its other end by a neck, also tubular. The neck is formed to its final shape and dimensions, while the body of the preform is intended to undergo relatively significant deformation to form it into a container during a forming step.

[0004] For this reason it is preferable that only the body of the preform be heated to a set temperature which is higher than the glass transition temperature, the neck remaining at a temperature lower than said glass transition temperature to avoid its deformation during the manufacture of the container.

[0005] Furthermore, the set temperature must not exceed a crystallization temperature that is higher than the glass transition temperature. Above this crystallization temperature, the thermoplastic material crystallizes and no longer possesses the mechanical properties required for high-quality forming.

[0006] The mass production of containers is carried out in a production facility where preforms move along a predetermined production path. Each preform is handled by conveyors. Such conveyors can be rail conveyors along which the preforms are free to come into contact with each other, or conveyors equipped with gripping devices for each preform individually.

[0007] The production facility includes a heating station which, during a heating step, makes the preform body malleable by heating it to the set temperature. During this heating step, each preform is exposed to radiant heat as it moves. The power of the radiant heat is controlled to ensure that the preform body is heated to the set temperature.

[0008] The production plant also includes a forming station, which is arranged downstream of the heating station according to the direction of preform flow within the production plant. During the forming stage, the hot preform is placed in a forming unit, for example, in a mold of the forming station that has a molding impression corresponding to the desired container. A pressurized fluid, such as air, is then injected into the malleable body of the preform to press its wall against the mold impression. Generally, the injection of pressurized fluid is preceded and / or accompanied by axial stretching of the preform, notably by means of a stretching rod inserted into the preform. As is known, the body is thus subjected to biaxial stretching.

[0009] During its passage through the heating station, each preform is typically exposed to continuous-spectrum heating radiation, which heats the thermoplastic material. The temperature to which the preforms are heated generally depends on the preform's absorption coefficient (A) for the wavelengths of the heating radiation. The absorption coefficient (A) is sometimes referred to by its English name, "absorptance."

[0010] The absorption factor (A) is defined as the ratio of absorbed heating radiation flux to incident heating radiation flux. The absorbed heating radiation flux causes a temperature rise in the thermoplastic material composing the preform. The absorption factor (A) of the preform thus allows us to determine the temperature rise of a preform as a function of the intensity of the infrared radiation and the duration of exposure to that radiation. Less energy is required to heat a preform with a high absorption factor (A) compared to a preform with a lower absorption factor (A).

[0011] The absorption factor (A) can be deduced from two other parameters called the transmission factor (T) of the preform and its reflection factor (R).

[0012] The transmission factor (T), sometimes called "transmittance", is defined as the ratio between the heat radiation flux transmitted through at least one wall of the preform and the incident heat radiation flux.

[0013] The reflectance factor (R), also called reflectance, is defined as the ratio of the heat radiation flux reflected by the preform to the incident heat radiation flux. Reflectance (R) is calculated, for example, as a function of the refractive index (n) of the thermoplastic material using the following formula:

[0014] R = [(n-1) / (n+1)]²

[0015] in which the value "1" corresponds to the refractive index of air.

[0016] These three factors depend on the wavelength of the radiation in question.

[0017] The heating radiation extends at least partially into the near-infrared range, as thermoplastic materials, such as PET, are known to absorb this type of radiation very readily. Therefore, the wavelength used to define the preform's transmission factor is generally taken from the near-infrared range.

[0018] These three factors are linked by the following equation:

[0019] A=1-RT

[0020] The transmission factor depends on the wavelength of the radiation passing through it. Thus, a perfectly transparent preform exhibits a transmission factor close to 100% for wavelengths in the visible light range, while it exhibits a significantly lower transmission factor for radiation with wavelengths in the near-infrared range. This reflects the fact that the preform can be heated by radiation in the near-infrared range, whereas radiation in the visible light range will have only a negligible influence on the preform's temperature, even at high power levels.

[0021] Sometimes the preforms are made of a material with a color.

[0022] This tint can be intentionally obtained by adding dye to color the preform in a desired color in order to obtain a colored container.

[0023] This discoloration can also occur unintentionally and uncontrollably, for example, when the preform is made of a plastic material composed at least partially of recycled plastic. This is the case, for instance, when the preform is made with a certain amount of mechanically recycled polyethylene terephthalate (rPET). In this case, the preform will have a more or less dark tint depending on the quality and quantity of recycled material incorporated into the composition of the thermoplastic material from which it is made.

[0024] Such a preform comprises at least a portion of rPET. To produce such a preform, rPET granules are, for example, mixed with virgin PET granules. The mixed granules are melted together before being injected into a mold to obtain the preform. Thus, the rPET as defined in the invention is melted and used directly to form preforms, possibly mixed with virgin PET. The recycled rPET granules are obtained mechanically by grinding previously used PET objects, which have been sorted by color and cleaned, without chemically decomposing them. The virgin PET granules are obtained chemically by monomer esterification. The ratio between the weight of rPET and the weight of virgin PET determines the rPET content in the preforms.

[0025] The inventor has observed that the presence of rPET is likely to influence the temperature of the preform wall when exposed to heating radiation.

[0026] Indeed, the heating radiation has a spectrum that extends, at least in part, into the visible light range. However, in the presence of rPET, the preform has a dark tint. Consequently, this part of the heating radiation spectrum is likely to significantly heat the thermoplastic material.

[0027] Furthermore, when preforms are delivered to the production site, they may contain a different percentage of rPET than that indicated by the supplier. Therefore, some preforms that are supposed to contain rPET may not actually contain any. This can be explained, for example, by the fact that rPET can be more expensive to produce than virgin PET.

[0028] It is therefore important to be able to determine the presence of rPET, or even the percentage of recycled PET, in order to properly order the thermal conditioning of the preforms.

[0029] Until now, determining the composition of the material used in the preforms of a batch required performing a destructive test on a randomly selected preform from the delivered batch. This test is carried out offline in a laboratory using chemical analysis. However, besides being time-consuming, such a test cannot guarantee that all preforms in the batch actually contain rPET.

[0030] Therefore, there is a need to be able to determine that each preform that feeds the manufacturing facility contains rPET in order to adjust the heating power of the facility accordingly.

[0031] The invention proposes a non-destructive method for detecting the presence of recycled polyethylene terephthalate in a preform intended to be formed into a container by forming, in particular by stretch blow molding, characterized in that the method consists of determining the presence of an absorption peak for at least one determined wavelength between 580 nm and 690 nm which is characteristic of the presence of recycled polyethylene terephthalate.

[0032] According to another optional feature of the process according to the teachings of the invention, said determined wavelength is equal to 680 nm.

[0033] According to another optional feature of the method according to the teachings of the invention, said determined wavelength is equal to 590 nm.

[0034] According to another optional feature of the method according to the teachings of the invention, it comprises: - a first step of measuring a representative value of a transmission factor) of a wall of said preform for at least one determined wavelength located between 580 nm and 690 nm; - a second step of comparing the representative value of the transmission factor) measured relative to the thickness of the polyethylene terephthalate wall being measured with a determined threshold, the preform being considered as containing recycled polyethylene terephthalate when said representative value of the transmission factor) measured is less than said threshold.

[0035] According to another optional feature of the process according to the teachings of the invention, the threshold is less than or equal to 80%.

[0036] According to another optional feature of the process according to the teachings of the invention, the threshold is determined experimentally using a sample of virgin polyethylene terephthalate having the same composition as the polyethylene terephthalate composing said preform but devoid of recycled polyethylene terephthalate, the threshold being lower than the representative value of the transmission factor) measured for said sample at said determined wavelength.

[0037] According to another optional feature of the method according to the teachings of the invention, the representative value of the transmission factor is measured by a measuring device comprising: - a light source which emits a light beam having at least the determined wavelength, - a measuring element measuring the intensity for at least the determined wavelength of said light beam after passing through at least one thickness of the wall of the preform.

[0038] According to another optional feature of the process according to the teachings of the invention, it is implemented in an installation comprising means for conveying preforms in a line along a predetermined production path passing successively through a heating zone of a body of the preforms and through a preform forming station into a container, in particular by stretch-blowing, the installation comprising a device for measuring the representative value of the transmission factor of at least one wall of the preforms when they pass through an associated measuring zone.

[0039] According to another optional feature of the process according to the teachings of the invention, the detected recycled polyethylene terephthalate is mechanically recycled polyethylene terephthalate. Brief description of the figures

[0040] Other features and advantages of the invention will become apparent during the reading of the detailed description which follows, for the understanding of which reference should be made to the attached drawings briefly described below.

[0041] This is a top view which schematically represents a container manufacturing installation made according to the invention comprising a heating station.

[0042] This is a side view that represents a preform intended to be processed by the installation of the.

[0043] Laest is a vertical cross-sectional view along the cutting plane 3-3 of laqui which represents a device for measuring a transmission spectrum in the visible light range of the walls of a preform circulating in the installation made according to a first embodiment.

[0044] Laest is a diagram representing wavelengths on the x-axis and light intensities on the y-axis, schematically representing the transmission spectrum of a preform without recycled PET in solid line and the transmission spectrum of a preform containing recycled PET in dotted line.

[0045] Laest is a block diagram which represents the process carried out according to the teachings of the invention.

[0046] This is a diagram similar to that of the one that represents a measurement spectrum of a measurement beam in dotted line and a transmission spectrum of the light beam after its passage through two walls of a preform containing recycled PET in solid line. Detailed description of the invention

[0047] In the following description, the terms "top", "bottom", and the derived terms "high", "low", are used for clarity in reference to the orientation of the figures without this having any limiting scope.

[0048] In the following description, the wavelengths of light radiation will be expressed in nanometers, indicated by the abbreviation "nm".

[0049] In theory, a monochromatic source is an ideal source emitting a sinusoidal wave of a single frequency. In other words, its frequency spectrum consists of a single line of zero spectral width (Dirac delta function).

[0050] In practice, such a source does not exist; a real source has a frequency emission spectrum that extends over a band of small but non-zero spectral width, for example a few tens of nanometers, centered on a principal frequency where the intensity of the radiation is maximum.

[0051] In the following description, such a real source is considered to be monochromatic.

[0052] Subsequently, a transmission spectrum is defined as a curve that represents the value of the intensity of a continuous spectrum light beam as a function of the wavelength after its passage through a medium to be analyzed, in particular the wall of a preform.

[0053] Figure 10 schematically illustrates an installation for manufacturing containers from thermoplastic preforms made of virgin PET (polyethylene terephthalate) and / or recycled rPET (recycled polyethylene terephthalate). The thermoplastic material may, but does not necessarily, contain additives that artificially increase the preform's absorption coefficient, or a dye to change its color.

[0054] As shown in Figure 1, each preform 12 comprises a cylindrical body 14 with axis "X". The body 14 has a transparent lateral wall 16 that delimits an internal volume. The wall 16 has a shape of revolution about the axis "X" such that, at a given height, the wall 16 has a constant thickness. However, the thickness of the wall 16 is likely to vary depending on the height. The wall 16 is delimited by an external face 18 that faces outwards from the preform 12 and by an internal face 20 that faces into the internal volume.

[0055] An upper end of the body 14 opens through a neck 22. The neck 22 has the final shape of the neck of the container 11 to be produced. Consequently, the neck 22 does not undergo any deformation during the manufacture of the container 11. The body 14 has a base 24 that closes its lower end and is generally hemispherical in shape. The neck 22 has a collar 26 arranged at its junction with the body 14. The lower face of the collar 26 is designed to form a support surface 28 to support the preform 12 during its molding and / or transport.

[0056] At the end of their injection molding process, the preforms 12 are rapidly cooled to give the thermoplastic material an amorphous state. This makes it possible to render the thermoplastic material malleable again by heating it above a glass transition temperature.

[0057] Referring again to Figure 10, the manufacturing installation includes a heating station 30 and a forming station 32. The preforms 12 move in a line along a production path 34 that passes through the heating station 30 and the forming station 32. The direction of movement of the preforms 12 is indicated by the arrows "F1" in Figure 10. During normal operation of the manufacturing installation 10, the preforms 12 are in constant motion along the production path 34.

[0058] The heating station 30 is designed to heat the body 14 of the preforms 12 to a setpoint temperature that is greater than or equal to the glass transition temperature of the constituent material, for example, greater than 70°C when this material is PET, rPET, or a mixture of PET and rPET. The heating station 30 includes a conveyor 36 (shown schematically) for transporting the preforms 12 while rotating them.

[0059] The conveyor 36 typically includes mandrels (not shown) which are fitted to the neck 22 to transport the preforms 12. The mandrels move along a closed circuit. For example, the mandrels are carried by the links of a chain or by independent shuttles moving along a rail.

[0060] The circuit here comprises two parallel straight sections connected by 180° turning sections. The conveyor 36 also includes two wheels 38A, 38B for guiding the mandrels in the turning sections of the closed circuit.

[0061] The heating station 30 also includes heating means 40 for heating the preforms 12. These include, for example, lamps facing reflectors that emit continuous spectrum heating electromagnetic radiation.

[0062] The heating radiation extends, at least in part, into the near-infrared range, which spans wavelengths from 780 nm to 3000 nm. The intensity of the heating electromagnetic radiation is particularly high in the higher intensity wavelength range between 780 nm and 1600 nm.

[0063] The heating radiation also extends, at least in part, into the visible light spectrum, which is generally between 380 nm and 780 nm.

[0064] The spectrum of heating radiation extends, for example, between 300 nm and 3000 nm.

[0065] The heating means 40 are arranged along a heating zone 42 of the preform production path 34. In the example shown in the figure, the heating station 30 comprises a heating zone 42 divided into two parts arranged upstream and downstream of the bend section guided by the guide wheel 38B.

[0066] The preforms 12 entering the heating station 30 are individually handled by the conveyor 36, on which they travel a U-shaped section of their production path 34, passing through the heating zone 42. They are heated as they pass by the heating means 40, which, where appropriate, are positioned to one side or on either side of the preforms 12 relative to their direction of travel. The hot preforms 12 are extracted from the heating station 30 after passing through the heating zone 42 and transferred into molds in the forming station 32 by a first transfer device 44, such as a transfer wheel, interposed between the heating station 30 and the forming station 32.

[0067] The transfer wheel includes arms (not shown, as they are known in themselves) which successively grasp the preforms 12, as they exit the heating station 30, at the level of their neck 22, to introduce each one in turn into a mold 46 of the forming station 32. The forming station 32 includes a rotating carousel 48 around the periphery of which several blowing stations 50 are arranged.

[0068] Each blowing station 50 includes at least one mold 46 which defines a cavity having the imprint of the container 11. Each blowing station 50 includes means (not shown) for forming / deforming the body 14 of the preform 12 and pressing it against the imprint of the mold 46, for example by stretch-blowing.

[0069] Each hot preform 12 coming out of the heating station 30 is introduced into a mold 46 of the blowing station 50 to be blown and transformed into a container 11. Once completed, the container 11 is extracted from the blowing station 50 by a second transfer device 52, similar to the first transfer device 34.

[0070] Generally, the heating means 40 emit heating radiation at a power determined solely by the absorption factor of the preforms 12 in the near-infrared range in order to reach the setpoint temperature. This power is calculated assuming that the preform 12 has perfectly transparent walls 16 in the visible light range.

[0071] Furthermore, the presence of rPET in the material constituting preform 12 has a great influence on the heating of preform 12. To enable the heating of preforms 12 to be as close as possible to the setpoint temperature, it is therefore preferable to know if preform 12 contains recycled PET.

[0072] The inventor observed that the preforms 12 made of PET containing at least some rPET exhibit a specific spectral signature. Thus, spectrographic analysis makes it possible to quickly determine, without destroying the preform 12, whether it contains rPET, and to adjust the heating power accordingly. The speed of this rPET detection allows this spectrographic analysis to be performed on the fly for all the preforms 12 without slowing down the production line 10, that is, while the preforms 12 are moving along their production path 34.

[0073] As shown in the figure, these spectrographic analyses consist of emitting a light beam 55 through the walls of a preform 12, and then measuring the light spectrum of the beam 55 after it has passed through the walls 16 of the preform 12, for example by means of a spectrometer 62.

[0074] For the rest of the description and in the drawings, a light transmission spectrum will be represented by a graph including: - on the abscissa the wavelengths; - on the ordinate, the transmission factor for each wavelength, given here as a percentage of the transmitted intensity relative to the light intensity of the light beam 55 before it passes through the walls 16 of the reference preform 12.

[0075] As shown in solid line in Figure 1, for a reference preform 12 made entirely of virgin PET, measuring the light beam 55 after it passes through the wall 16 of the preform 12 yields a first reference spectrum "σr" measured by the spectrometer 62. In the reference spectrum "σr", it is observed that visible light, indicated by the interval "Dv", is transmitted at approximately 90%, meaning that the reference preform 12 is essentially transparent. Furthermore, it is observed that in the near-infrared range, there are some absorption peaks corresponding to the portion of infrared radiation absorbed by the virgin PET, which contributes to its heating.

[0076] The dashed line represents the transmission spectrum "σ1" for a preform containing 75% virgin PET and 25% rPET. This "σ1" spectrum was measured under the same conditions as the reference "σr" spectrum. An overall attenuation of the transmission factor is observed in the visible and near-infrared ranges, particularly between 380 nm and 1600 nm, compared to the reference "σr" spectrum. This overall attenuation manifests as a downward shift of the transmission spectrum relative to the transmission spectrum of the reference preform.

[0077] This attenuation is not found in the mid-infrared range, beyond 1600 nm, which is absorbed in roughly the same way by preforms containing rPET and preforms not containing rPET.

[0078] Furthermore, in addition to this overall attenuation, which does not alter the overall shape of the "σ1" spectrum compared to the reference "σr" spectrum, an absorption peak "P1" appears at a specific wavelength. This absorption peak manifests as a marked dip in the transmission "σ1" spectrum. The determined wavelength is between 580 nm and 690 nm. The absorption peak at this specific wavelength is characteristic of the presence of recycled polyethylene terephthalate.

[0079] In particular, a significant absorption peak “P1” was observed for the determined wavelength of 680 nm.

[0080] Another “P2” absorption peak, although less pronounced, was also observed for the determined wavelength of 590 nm.

[0081] After many tests on different PET compositions and for different rPET levels, these "P1, P2" absorption peaks were observed on all 12 preforms containing rPET and it was also observed that they were absent for all 12 preforms not containing rPET.

[0082] To detect the presence of rPET in a preform 12 in a non-destructive manner, a non-destructive detection method for the presence of recycled polyethylene terephthalate in a preform intended to be formed into a container by forming, in particular by stretch blow molding, is proposed, characterized in that the method consists of determining the presence of an absorption peak for at least one wavelength between 580 nm and 690 nm.

[0083] This can, for example, be achieved by analyzing the transmission or absorption spectrum obtained after passing a continuous spectrum light beam through the wall of the preform in the visible range.

[0084] The invention more particularly proposes a detection method, represented in the figure, which comprises: - a first step E1 of measuring a representative value of a transmission factor T(λ1) of a wall 16 of said preform 12 for at least one determined wavelength λ1 located between 580 nm and 690 nm; - a second step E2 of comparing the representative value of the transmission factor T(λ1) measured with a determined threshold S1, the preform 12 being considered as containing recycled polyethylene terephthalate (rPET) when said representative value of the transmission factor T(λ1) measured is less than said threshold S1.

[0085] As explained previously, the determined wavelength λ1 is equal to the wavelength at which one of the characteristic peaks indicating the presence of rPET appears. Preferably, this is equal to 680 nm because the absorption peak "P1" is very pronounced at this specific wavelength λ1.

[0086] However, in certain cases, for example for preforms 12 made with tinted PET which would attenuate the transmission of light too much at this determined wavelength λ1 even in the absence of rPET, a determined wavelength λ2 equal to 590 nm corresponding to the second "P2" absorption peak can be used.

[0087] It has been observed that for a reference preform, as shown in Figure 1, the transmission factor T(λ1) at the determined wavelength λ1 is approximately 90%. Thus, for a preform 12, a threshold S1 of 80% or less can be adopted, for example, an S1 threshold of 80%.

[0088] Alternatively, the threshold S1 can be determined experimentally, depending on the composition of the PET used. For example, the threshold S1 is determined experimentally using a sample of virgin polyethylene terephthalate with the same composition as the virgin polyethylene terephthalate constituting the preform 12 to be measured, but without recycled polyethylene terephthalate (rPET). The experimental measurement is preferably carried out with a device identical to that used to measure the transmission factor T(λ1) of the preform 12 to be measured. The threshold S1 is determined to be lower, for example by 10%, than the representative value of the transmission factor T(λ1) measured for the sample at the determined wavelength λ1. Thus, if the sample has a transmission factor of 85%, the threshold S1 will be set at 75%.

[0089] Furthermore, the wall thickness 16 of the preform 12 also influences the transmission factor T(λ1). To allow measurements to be performed independently of the wall thickness 16 of each preform 12 being measured, the representative value of the transmission factor T(λ1) is preferably related to the wall thickness 16 of the preform 12 being measured. Similarly, the threshold S1 is normalized by relating it to the wall thickness used for the reference sample being measured.

[0090] The representative value of the transmission factor "T(λ1)" is weighted to correspond to a transmission factor that would be measured for a wall of predetermined thickness "sn" made of the same material as the preform 12. The predetermined thickness "sn" is, for example, equal to 4 mm. This transmission factor "T(λ1)" relative to the thickness is calculated using the following formula derived from the Beer-Lambert formula:

[0091]

[0092] in which "Tn" represents the transmission factor related to the thickness, "T" represents the transmission factor T(λ1) measured during the first step "E1", "sn" represents the predetermined thickness, "s" represents the thickness of the wall 16 of the preform which is measured for example.

[0093] This transmission factor related to the thickness is automatically calculated by the electronic control unit 54 from the measurement carried out during the first step "E1".

[0094] Preferably, the measurement is carried out for all preforms 12 in a measurement zone “Z” crossed by the production path 34 upstream of the heating zone 42.

[0095] In an unrepresented variant of the invention, the measurement zone "Z" is arranged downstream of the heating zone 42.

[0096] In the examples shown in the figures, the measurement is taken on the fly when the preforms 12 are transported by conveyor 36 from the heating station 30. This conveyor 36 transports the preforms 12 through to the heating zone 42.

[0097] In an unrepresented variant of the invention, the measurement can be carried out when the preforms are transported by another conveyor, for example a transfer wheel.

[0098] This process is implemented automatically here by an electronic control unit 54, visible in particular at the.

[0099] This process is implemented when the preform 12 is transported by a conveyor along the production path 34 in the container manufacturing facility 10. Preferably, the preform 12 is in constant motion during the implementation of the process.

[0100] In the examples shown in the figures, the process is implemented when the preforms 12 are transported by conveyor 36 from the heating station 30. This conveyor 36 transports the preforms 12 through to the heating zone 42.

[0101] The process is preferably implemented on the preforms 12 before they are heated, therefore upstream of the heating zone 42.

[0102] The first step "E1" is carried out here by measuring the attenuation of the intensity "I" of a light beam 55 containing the said determined wavelength λ1.

[0103] For this purpose, the installation 10 includes a measuring device 56 for a representative value of the transmission factor "T(λ1)" for said wavelength λ1 determined, shown in the figure. As shown in the figure, this device 56 is arranged to measure the representative value of the transmission factor "T(λ1)" of a preform 12 passing through said measurement zone "Z".

[0104] The measuring device 56 includes a light source 58 that emits a light beam 55 with a determined wavelength λ1. This may be a monochromatic light source, but this is not mandatory. The light beam 55 may have a continuous spectrum σ0 in the visible range.

[0105] The light source 58 comprises, for example, a light-emitting diode (LED), a superluminescent diode (SLED), or a lamp. The light beam 55 is emitted perpendicularly to the wall 16 of the preform 12. The light beam 55 is emitted so as to pass through the preform 12 via its principal "X" axis.

[0106] The light beam 55 is emitted more particularly along an axis which is perpendicular to a tangent to the path 34 of production of the preform 12 in the measurement “Z” zone.

[0107] The 55 light beam can be guided in the correct direction by an optical guiding means such as an optical fiber.

[0108] The measuring device 56 further includes an element for measuring the intensity "I" of the second light beam 66 after it has passed at least once through the wall 16 of the body 14 of the preform 12 at least for the determined wavelength "λ1".

[0109] The intensity measuring device "I" is arranged here so as to measure the intensity "I" of the light beam 55 on the other side of the preform 12 with respect to the axis "X" of the preform 12 after it has passed twice through the wall 16 at two diametrically opposite points, at the same height "h" of the preform 12.

[0110] The first step "E1" of measuring a representative value of the transmission factor "T(λ1)" for the determined wavelength λ1 is carried out here by measuring the attenuation of the intensity "I" of the light beam 55 which includes said determined wavelength "λ1", here 680 nm. The determined wavelength "λ1" is within the range of visible light.

[0111] More specifically, the luminous intensity "I0" at which the light beam 55 is emitted as a function of the wavelength "λ1" is a known value. The luminous intensity "I0" at which the light beam 55 is emitted can also be measured directly by the spectrometer 62 when no preform 12 passes through the measurement zone "Z", as represented by the dashed curve in the figure.

[0112] After passing twice through the wall 16 of the preform 12, the light intensity of the light beam 55 is attenuated for the determined wavelength "λ1". The light beam 55 then has a light intensity called "transmitted light intensity "I1", as represented by the point "P1" of the dashed curve.

[0113] Thus, it is easy to deduce the transmission factor of the wall 16 of the preform as a function of the wavelength in the visible light range by calculating the ratio between the light intensity "I0" of emission and the light intensity "I1" transmitted.

[0114] Without limitation, the transmission factor T(λ1) and the threshold S1 are expressed here in terms of percentage, of the transmitted light intensity relative to the emitted light intensity for the determined wavelength "λ1".

[0115] Optionally, to calculate precisely the representative value of the transmission factor "T(λ1)", it is possible, but not mandatory, to weight this ratio here by taking into account the quantity of the light beam 55 which is reflected at each entry into a wall 16. This reflected quantity is calculated as a function of the reflectance factor "R" which is a quantity known beforehand.

[0116] The weighting factor for the passage of the light beam 55 through a wall thickness 16 is calculated according to the following equation:

[0117]

[0118] The weighting factor for the passage of the light beam 55 through two wall thicknesses 16 is calculated according to the following equation:

[0119]

[0120] The transmission factor "T(λ1)" when passing through two wall thicknesses 16 is thus calculated by applying the following equation:

[0121]

[0122] The transmission factor "T(λ1)" is calculated by the electronic control unit 54 which receives the measurements made by the measuring device.

[0123] In the embodiment shown in the figure, the light beam 55 is here a beam of so-called "white" light which emits in the visible light range, that is to say globally between 380 nm and 780 nm. For example, the spectrum "σ0" of the light beam 55 extends in a range of wavelengths which extends between a lower bound "λmin", for example equal to 400 nm, and an upper bound "λmax", for example equal to 780 nm.

[0124] The measuring instrument here is a spectrometer 62 which measures the intensity "I" as a function of the wavelength "λ1" of the spectrum "σ0" of the light beam 55 after passing at least once through the wall 16 of the body 14 of the preform 12, as shown in the.

[0125] Spectrometer 62 allows obtaining the measured transmission spectrum “σ1” shown in the image.

[0126] Thus, the detection process makes it easy, quick, and reliable to know for all preforms 12 that pass through the installation 10 whether they contain rPET.

[0127] It is thus possible to determine the presence of rPET contained in each preform 12 circulating in the installation 10. This makes it possible to further improve the quality of heating the preforms 12 by precisely adapting the heating power to the rPET rate.

[0128] In addition, the process makes it possible to verify that all 12 preforms of each batch are compliant in terms of rPET content with what is advertised without having to resort to a destructive and lengthy test.

Claims

Method for non-destructive detection of the presence of recycled polyethylene terephthalate in a preform (12) intended to be formed into a container (11) by forming, in particular by stretch blow molding, characterized in that the method consists of determining the presence of an absorption peak (P1, P2) for at least one determined wavelength (λ1, λ2) between 580 nm and 690 nm which is characteristic of the presence of recycled polyethylene terephthalate (rPET). Method according to the preceding claim, characterized in that said wavelength (λ1) determined is equal to 680 nm. Method according to claim 1, characterized in that said determined wavelength (λ2) is equal to 590 nm. A method according to the preceding claim, characterized in that it comprises: - a first step (E1) of measuring a representative value of a transmission factor (T(λ1)) of a wall (16) of said preform (12) for at least one determined wavelength (λ1) located between 580 nm and 690 nm; - a second step (E2) of comparing the representative value of the transmission factor (T(λ1)) measured relative to the wall thickness (16) in polyethylene terephthalate which is the subject of the measurement with a determined threshold (S1), the preform being considered as containing recycled polyethylene terephthalate when said representative value of the transmission factor (T(λ1)) measured is less than said threshold (S1). A method according to any one of the preceding claims, characterized in that the threshold (S1) is less than or equal to 80%. A method according to any one of the preceding claims, characterized in that the threshold (S1) is determined experimentally using a sample of virgin polyethylene terephthalate having the same composition as the polyethylene terephthalate composing said preform (12) but devoid of recycled polyethylene terephthalate, the threshold (12) being lower than the representative value of the transmission factor (T(λ1)) measured for said sample at said determined wavelength (λ1). A method according to any one of the preceding claims, characterized in that the representative value of the transmission factor (T(λ1)) is measured by a measuring device (56) comprising: - a light source (58) which emits a light beam (58) having at least the determined wavelength (λ1), - a measuring element (62) measuring the intensity for at least the determined wavelength (λ1) of said light beam (55) after passing through at least one thickness of the wall (16) of the preform (12). A method according to any one of the preceding claims, characterized in that it is implemented in an installation comprising means (36) for conveying preforms (12) in a line along a predetermined production path (34) passing successively through a heating zone (42) of a body of the preforms (12) and through a station (32) for forming the preforms (12) into a container (11), in particular by stretch-blowing, the installation (10) comprising a device (56) for measuring the representative value of the transmission factor (T(λ1)) of at least one wall (16) of the preforms (12) when they pass through an associated measuring zone (Z). A process according to any one of the preceding claims, characterized in that the recycled polyethylene terephthalate detected is mechanically recycled polyethylene terephthalate.

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