NON-DESTRUCTIVE METHOD FOR DETERMINING THE RATE OF PET RECYCLED IN A PREFORM

Abstract

The invention relates to a non-destructive method for determining the percentage of recycled polyethylene terephthalate in a preform (12), characterized in that the method comprises: - a first step (E1) of measuring a first representative value of a transmission factor (T(λ1)) of a wall (16) of said preform (12) for at least a first determined wavelength (λ1) located in the visible range; - a second step (E2) of measuring a second representative value of a transmission factor (T(λ2)) of a wall (16) of said preform (12) for at least a second determined wavelength (λ2) located in the near-infrared; - a third step (E3) of calculating a value (rrPET) representative of the percentage of recycled polyethylene terephthalate present in the preform (12) as a function of the first and second values ​​(T(λ1), T(λ2)) measured during the first step (E1) and the second step (E2). Figure for the abbreviation: Figure 5

Description

Title of the invention: NON-DESTRUCTIVE METHOD FOR DETERMINING THE RATE OF PET RECYCLED IN A PREFORM Technical field of the invention

[0001] The invention relates to a non-destructive method for determining the percentage 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, in particular 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 greater than or equal to a glass transition temperature that allows them to be shaped into the final container.

[0003] More specifically, the 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 setpoint 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 setpoint 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 exhibits the mechanical properties required for high-quality forming.

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

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

[0008] The production installation also includes a forming station arranged downstream of the heating station according to the direction of preform flow in the production installation. During the forming step, the hot preform is placed in a forming unit, for example, in a mold of the forming station that has a molding impression conforming to the container to be produced. 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. In a known manner, the body is thus subjected to biaxial stretching.

[0009] During its passage through the heating station, each preform is generally 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 factor (A) for the wavelengths of the heating radiation. The absorption factor (A) is sometimes referred to by its English name, "absorptance".

[0010] The absorption factor (A) is defined as the ratio of the absorbed heating radiation flux to the 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 makes it possible to determine the temperature rise of a preform as a function of the intensity of the infrared radiation and the duration of exposure to said 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 reflection factor (R), also called reflectance, is defined as the ratio between the heating radiation flux reflected by the preform and the flux of incident heating radiation. The reflectance (R) is, for example, calculated as a function of the refractive index (n) of the thermoplastic material according to the following formula:

[0014] R = [(nl) / (n+l)]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 considered.

[0017] The heating radiation extends at least in part into the near-infrared range, since it is known that thermoplastic materials, such as PET, readily absorb this type of radiation. The wavelength considered for defining the preform's transmission factor is therefore generally taken from the near-infrared range.

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

[0019] A=lRT

[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 temperature of the preform, even at high power levels.

[0021] It sometimes happens that the preforms are made of a material having a colour.

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

[0023] This color change 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 quantity of mechanically recycled polyethylene terephthalate (rPET). In this case, the preform has a more or less dark color 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 granules thus mixed are melted together before being injected into a mold to obtain the preform. Thus, the rPET as defined by the invention is melted and directly used to form preforms, possibly mixed with virgin PET. The recycled rPET granules are obtained mechanically by grinding Used PET objects, pre-sorted by color and cleaned without chemical decomposition, are used. Virgin PET granules are chemically obtained 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 partially 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 the preforms are delivered to the production site, they may contain a different percentage of rPET than that indicated by the supplier. It may therefore happen that some preforms that are supposed to contain rPET do not actually contain any. This can be explained, for example, by the fact that rPET may be more expensive to produce than virgin PET.

[0028] It is therefore important to be able to determine the rPET content in each preform in order to properly control the thermal conditioning of the preforms.

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

[0030] There is therefore a need to be able to determine the rPET content in each preform that feeds the manufacturing installation in order to adjust the heating power of the installation accordingly. Summary of the invention

[0031] The invention proposes a non-destructive method for determining the percentage of recycled polyethylene terephthalate in a preform intended to be formed into a container by molding, in particular by stretch blow molding, characterized in that the method comprises: - a first step of measuring a first representative value of a transmission factor of a wall of said preform for at least a first determined wavelength located in the visible domain; - a second step of measuring a second representative value of a transmission factor of a wall of said preform for at least a second determined wavelength located in the near-infrared; - a third step of calculating a representative value of the rate of recycled polyethylene terephthalate present in the preform as a function of the first and second values ​​measured during the first and second steps.

[0032] According to another optional feature of the process according to the teachings of the invention, the representative value of the recycled polyethylene rate is calculated as a function of the following parameter: T(A1>T(A2) Ttxi)

[0033] According to another optional feature of the process according to the teachings of the invention, the representative value of the recycled polyethylene rate obtained during the third step is compared to a reference curve obtained experimentally allowing the recycled polyethylene terephthalate rate to be obtained as a function of the value calculated during the third step.

[0034] According to another optional feature of the process according to the teachings of the invention, the reference curve is obtained by applying the process to several polyethylene terephthalate samples of the same formula, each having a different and known percentage of recycled polyethylene terephthalate.

[0035] According to another optional feature of the method according to the teachings of the invention, the reference curve is obtained by a mathematical regression method from multiple experimental measurements.

[0036] According to another optional feature of the method according to the teachings of the invention, the first determined wavelength is between 580 nm and 690 nm.

[0037] According to another optional feature of the method according to the teachings of the invention, the first determined wavelength is equal to 680 nm.

[0038] According to another optional feature of the method according to the teachings of the invention, the first determined wavelength is equal to 590 nm.

[0039] According to another optional feature of the method according to the teachings of the invention, the second determined wavelength is between 780 nm and 1600 nm.

[0040] According to another optional feature of the method according to the teachings of the invention, the second determined wavelength is equal to 980 nm.

[0041] According to another optional feature of the method according to the teachings of the invention, the first and second representative values ​​of the transmission factor are measured by a measuring device comprising: - a light source that emits a beam of light exhibiting at least the two specified wavelengths, - a measuring device measuring the intensity for at least the said determined wavelengths of said light beam after passing through at least one thickness of the wall of the preform.

[0042] 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 blow molding, 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.

[0043] 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

[0044] 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 will be made to the attached drawings briefly described below.

[0045] Fig. 1 is a top view which schematically represents a container manufacturing installation made according to the invention comprising a heating station.

[0046] The [Fig.2] is a side view which represents a preform intended to be processed by the installation of the [Fig.1].

[0047] Fig. 3 is a vertical cross-sectional view along section plane 3-3 of Fig. 1, which represents a device for measuring a transmission spectrum in the visible light range of the walls of a preform circulating in the installation, carried out according to a first embodiment.

[0048] Fig. 4 is a diagram representing wavelengths on the abscissa and light intensities on the ordinate, Fig. 4 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 dashed line.

[0049] Fig. 5 is a diagram similar to that of Fig. 4, which 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.

[0050] The [Fig.6] is a diagram showing on the abscissa a percentage of recycled PET and on the ordinate a representative value of the recycled PET rate calculated by application of the process of the invention. Detailed description of the invention

[0051] 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.

[0052] In the following description, the wavelengths of the light radiation will be expressed in nanometers, indicated by the abbreviation "nm".

[0053] 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).

[0054] In practice, such a source does not exist, a real source having a frequency emission spectrum which extends over a band of small but non-zero spectral width, for example of a few tens of nanometers, centered on a main frequency where the intensity of the radiation is maximum.

[0055] In the following description, such a real source is considered to be monochromatic.

[0056] Subsequently, a transmission spectrum is defined as a curve which 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.

[0057] Figure 1 schematically illustrates an installation 10 for manufacturing containers 11 from preforms 12 made of thermoplastic material, and more particularly of virgin PET (polyethylene terephthalate) and / or rPET (recycled polyethylene terephthalate). The thermoplastic material may optionally, but not necessarily, contain additives that artificially increase the absorption factor of the preform, or a dye to change its color.

[0058] As shown in [Fig. 2], 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 is oriented towards the outside of the preform 12 and by an internal face 20 that is oriented towards the internal volume.

[0059] 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 obtained. 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 during its transport.

[0060] At the end of their injection molding, the preforms 12 are rapidly cooled to give the thermoplastic material an amorphous state. It is thus possible to make the thermoplastic material malleable again by heating it above a glass transition temperature.

[0061] Referring again to [Fig. 1], the manufacturing installation 10 comprises 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 "Fl" in [Fig. 1]. During normal operation of the manufacturing installation 10, the preforms 12 are in constant motion along the production path 34.

[0062] 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.

[0063] The conveyor 36 generally includes mandrels (not shown) which are fitted with the neck 22 to transport the preforms 12. The mandrels move along a closed circuit. The mandrels are, for example, carried by the links of a chain or by independent shuttles moving along a rail.

[0064] The circuit here comprises two parallel straight sections connected by 180° turning sections. The conveyor 36 further comprises two wheels 38A, 38B for guiding the mandrels in the turning portions of the closed circuit.

[0065] 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.

[0066] The heating radiation extends at least in part into the near-infrared range, which is within a wavelength range between 780 nm and 3000 nm. The intensity of the heating electromagnetic radiation is notably higher in a higher intensity wavelength range between 780 nm and 1600 nm.

[0067] The heating radiation also extends at least partly into the visible light range, which is generally between 380 nm and 780 nm.

[0068] The spectrum of the heating radiation extends, for example, between 300 nm and 3000 nm.

[0069] The heating means 40 are arranged along a heating zone 42 of the preform production path 34. In the example shown in [Fig. 1], 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.

[0070] 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 of 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.

[0071] The transfer wheel includes arms (not shown, as they are known per se) which successively grasp the preforms 12, as they exit the heating station 30, at the level of their neck 22, to introduce them one after the other 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.

[0072] 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.

[0073] 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.

[0074] Generally, the heating means 40 emit the 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.

[0075] Furthermore, the presence of rPET in the material constituting the preform 12 has a significant influence on the heating of the preform 12. To enable the heating of the preforms 12 as close as possible to the set temperature, it is therefore preferable to know if preform 12 contains recycled PET.

[0076] The inventor has observed that preforms 12 made of PET containing at least some rPET exhibit a particular 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 preforms 12 without slowing down the production line 10, that is, while the preforms 12 are moving along their production path 34.

[0077] As shown in [Fig.3], these spectrographic analyses consist of emitting a first light beam 55 through the walls of a preform 12, and then measuring the light spectrum of the beam 55 after its passage through the walls 16 of the preform 12, for example by means of a spectrometer 70.

[0078] For the remainder of the description and in the drawings, a light transmission spectrum will be represented by a graph comprising:

[0079] - the wavelengths are on the x-axis;

[0080] - on the ordinate, the transmission factor for each wavelength, given here in percentage of transmitted intensity relative to the luminous intensity of the light beam 55 before it passes through the walls 16 of the reference preform 12.

[0081] As shown in solid line in [Fig. 4], for a reference preform 12 made entirely of virgin PET, by measuring the light beam 55 after passing through the wall 16 of the preform 12, a first reference "gold" spectrum is obtained, measured by the spectrometer 70. In the reference "gold" spectrum, it can be seen that visible light, indicated by the interval "Dv", is transmitted at approximately 90%, which means that the reference preform 12 is essentially transparent. Furthermore, it can be seen that in the near-infrared range, there are some absorption peaks which correspond to the portion of the infrared radiation that is absorbed by the virgin PET and which contributes to its heating.

[0082] The "ol" transmission spectrum for a preform comprising 75% virgin PET and 25% rPET is shown in dashed lines in [Fig. 4]. This "ol" spectrum was measured under the same conditions as the reference "or" 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 "or" spectrum. This overall attenuation manifests as a downward shift of the transmission spectrum relative to the transmission spectrum of the reference preform.

[0083] This attenuation is not found in the mid-infrared range, beyond 1600 nm, which is absorbed in a very similar way by preforms containing rPET and preforms not containing rPET.

[0084] Furthermore, in addition to this overall attenuation, which does not alter the overall shape of the "ol" spectrum compared to the reference "or" spectrum, an absorption peak "PI" appears at a specific wavelength. This absorption peak results in a marked dip in the transmission "ol" 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.

[0085] In particular, a significant absorption “PI” peak was observed for the determined wavelength of 680 nm.

[0086] Another “P2” absorption peak, however less marked, was also observed for the determined wavelength of 590 nm.

[0087] After many tests on different PET compositions and for different rPET levels, these "PI, P2" absorption peaks were observed on all preforms 12 containing rPET and it was also observed that they were absent for all preforms 12 not containing rPET.

[0088] The specific spectral signature of the preforms 12 containing rPET makes it possible to determine the rate of rPET they contain by taking into account the overall attenuation of the transmission factor in the visible and near-infrared range.

[0089] We therefore propose a non-destructive method for determining the percentage of recycled polyethylene terephthalate (rPET) in a preform 12 intended to be formed into a container 11 by forming, in particular by stretch blow molding.

[0090] This process comprises:

[0091] - a first step El of measuring a first representative value of a transmission factor T(X1) of a wall 16 of said preform 12 for at least a first determined wavelength XI located between 670 nm and 690 nm;

[0092] - a second step E2 of measuring a second representative value of a transmission factor T(X2) of a wall 16 of said preform 12 for at least a second determined wavelength / .2 located in the near-infrared;

[0093] - a third step E3 of calculating a value "rrPET" representative of the rate of recycled polyethylene terephthalate present in the preform depending on the first and second values ​​measured during the first and second steps.

[0094] The first step El and the second step E2 are identical except that the wavelength being measured is not the same.

[0095] For the first step El, the determined wavelength XI is the characteristic wavelength for which the presence of rPET causes the appearance of an absorption peak, here 680 nm.

[0096] The second step E2 is carried out in the same way as the first step but for the second determined wavelength X2. The second determined wavelength "X2" is in the near-infrared range. It preferably belongs to the range of wavelengths of highest intensity of the heating electromagnetic radiation, here between 780 nm and 1600 nm. For example, it is equal to 980 nm.

[0097] Each of the two steps "El, E2" of measurement of the transmission factor "T(X1), T(X2)" is carried out here by measuring the attenuation of the intensity "I" of an associated light beam 55, 66 which includes said determined wavelength "XI, X2".

[0098] For each of these steps, the installation 10 includes a device 56, 64 for measuring the transmission factor "T(X1), T(X2)" shown in [Fig. 3]. While this refers to a single measuring device 56, 64, it will be understood that each step E1, E2, can be implemented by two separate devices 56 and 64.

[0099] As shown in [Fig. 1], this device 56, 64 is arranged to measure the infrared transmission factor “T(X1), T(X2)” of a preform 12 passing through said measurement zone “Z”.

[0100] As shown in [Fig. 3], the transmission factor measuring device 56, 64 “T(X1), T(X2)” comprises two light sources 58, 68, each emitting an associated light beam 55, 66. The first light beam 55 has the first wavelength XI, while the second light beam 66 has the second wavelength “X2”.

[0101] These may be monochromatic light sources, but this is not mandatory.

[0102] The light sources 58, 68 comprise, for example, a light-emitting diode (LED), a superluminescent diode (SLED), or a lamp. Each light beam 55, 66 is emitted perpendicularly to the wall 16 of the preform 12. Each light beam 55, 66 is emitted so as to pass through the preform 12 via its principal "X" axis.

[0103] Alternatively, the measuring device 56, 64 comprises a single light source which emits a light beam simultaneously comprising the two determined wavelengths XI, X2.

[0104] The first light beam 55 and the second light beam 66 pass through the wall 16 of the preform 12 at the same height “h”.

[0105] The light beams 55, 66 are more particularly emitted along an axis which is perpendicular to a tangent to the path 34 of production of the preform 12 in the measurement “Z” zone.

[0106] The light beams 55, 66 can be guided in the correct direction by an optical guiding means such as an optical fiber.

[0107] The transmission factor measuring device 56, 64 “T(X1), T(X2)” further includes an intensity measuring element “I” of the second light beam 66 after passing at least once through the wall 16 of the body 14 of the preform 12.

[0108] The intensity measuring device "I" is arranged here so as to measure the intensity "I" of each light beam 55, 66 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.

[0109] In an alternative embodiment of the invention, the intensity measuring element is arranged to measure the intensity of each light beam after it has passed through the wall of the preform only once. In this case, the measuring element includes a light beam interceptor that is introduced into the preform during the first step, the interceptor guiding each intercepted light beam to an intensity measuring element.

[0110] The measuring element is here formed by a spectrometer measuring the intensity "I" for each wavelength "XI, X2" determined of the second light beam 66 after passing through at least one thickness of the wall 16 of the preform 12.

[0111] The luminous intensity "10" at which the first light beam 66 is emitted for each of the determined wavelengths "XI, X2" is a known value. The luminous intensity "10" at which each light beam 55, 66 is emitted can also be measured directly by the measuring element 70 when no preform 12 passes through the measuring zone "Z", as shown in dashed lines in [Fig. 5].

[0112] After passing twice through the wall 16 of the preform 12, the light intensity of each light beam 55, 66 is attenuated. Each light beam 55, 66 then has a light intensity referred to as "transmitted light intensity II, 12".

[0113] Thus, it is easy to deduce the transmission factor "T(X1), T(X2)" as a function of the ratio between the luminous intensity "10" of emission and the luminous intensity "II, 12" transmitted.

[0114] Optionally, to calculate precisely the transmission factor "T(X1), T(X2)", it is possible, but not mandatory, to weight this ratio here by taking into account the quantity of each light beam 55, 66 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.

[0115] The weighting factor for the passage of the second light beam 66 through a wall thickness 16 is calculated according to the following equation: [Math 1]

[0116] The weighting factor for the passage of the second light beam 66 through two wall thicknesses 16 is calculated according to the following equation: [Math 2] ' — 1 2“ (2-f J

[0117] The transmission factor "T(X1), T(X2)" during passage through two wall thicknesses 16 is thus calculated by application of the following equation: [Math 3]

[0118] Each transmission factor "T(X1), T(X2)" is calculated by the electronic control unit 54 which receives the measurements made by the measuring device.

[0119] The representative value of the transmission factors "T(X1), T(X2)" can also be 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(X1), T(X2)" relative to the thickness is calculated using the following formula derived from the Beer-Lambert formula: [Math 4] Tn = T 2

[0120] in which "Tn" represents the transmission factor related to the thickness, "T" represents the transmission factor T(X1), T(X2) measured during the first step "El" or the second step E2, "sn" represents the predetermined thickness, "s" represents the thickness of the wall 16 of the preform which is measured for example.

[0121] This transmission factor T(X1), T(X2) related to the thickness is automatically calculated by the electronic control unit 54 from the measurement carried out during the first step "El".

[0122] Advantageously, the measurement of the representative value of the infrared transmission factor "T(X2)" and the measurement of the representative value of the visible transmission factor "T(X1)" of the wall 16 are carried out simultaneously in the same measurement zone Z of the production path 34, as shown in [Fig.1].

[0123] The measurement zone “Z” crossed by the production path 34 is preferably arranged upstream of the heating zone 42.

[0124] In an unrepresented variant of the invention, the measurement zone “Z” is arranged downstream of the heating zone 42.

[0125] In the examples shown in the figures, the measurement is carried out on the fly when the preforms 12 are transported by the conveyor 36 from the heating station 30. This conveyor 36 transports the preforms 12 through to the heating zone 42.

[0126] 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.

[0127] To do this, the measuring device 56 and the measuring element of the infrared transmission factor "T(X2)" are arranged here in the common measurement zone "Z", as shown in [Fig.3].

[0128] Most advantageously, the spectrometer of the device 56 for measuring the "o(X1)" transmission spectrum in the visible light range and the spectrometer of the device 64 for measuring the infrared transmission factor "T(X2)" are formed by a single spectrometer 70 common to both devices 56, 64.

[0129] To this end, the first light beam 55 and the second light beam 66 are merged into a common beam 72 before reaching the preform. This merging is achieved, for example, by means of an optical coupler 74.

[0130] The spectrum measured by the common spectrometer 70 after the common beam 72 has passed through the wall 16 is shown as a solid line in [Fig. 5]. It simultaneously shows the transmission spectrum 10 and the intensity peak '12' for the second wavelength ' / .2'.

[0131] The common light beam 72 is emitted perpendicularly to the wall 16 of the preform 12 so as to pass through the wall 16 by passing through its main "X" axis.

[0132] The common light beam 72 is more particularly emitted along an axis perpendicular to a tangent to the trajectory of the preform 12 in the common measurement “Z” zone.

[0133] The common light beam 72 passes through the wall 16 at said height “h”.

[0134] In an unshown variant of the invention, the device 56 for measuring the factor The transmission factor "T(X1)" for the first determined wavelength XI and the device 64 for measuring the infrared transmission factor "T(X2)" are arranged in two separate measurement zones of the production path so that the two corresponding measurements are carried out successively on the same preform. In this case, each of the devices includes a light source and an associated spectrometer.

[0135] During the third calculation step E3, the value “rrPET” representing the recycled polyethylene content is calculated as a ratio of the difference between the first value and the second value to the second value. For example, to obtain the value “rrPET” representing the percentage of recycled polyethylene terephthalate, the following equation is applied: [Math 5] _WMinn 1 rPET T(X1)

[0136] This third step E3 is implemented by the electronic control unit 54.

[0137] During a fourth comparison step E4, this "rrPET" value representing the recycled polyethylene rate obtained during the third step E3 is compared to a reference curve allowing the determination of the rPET rate contained in the preform, as shown in [Fig.6].

[0138] This curve is obtained experimentally, for example, by applying the process to several polyethylene terephthalate samples of the same formula, each with a different and known percentage of recycled polyethylene terephthalate. This yields a scatter plot on a curve where the x-axis represents the percentage of rPET contained in the preform and the y-axis represents the value "rrPET" of the rPET content calculated in step E3.

[0139] The reference curve is obtained by applying to this cloud of points obtained by experimental measurements a mathematical regression method, such as the least squares method.

[0140] It is thus possible to estimate the rPET content in each preform 12 circulating in the installation 10. This makes it possible to further improve the heating quality of the preforms 12 by precisely adapting the heating power to the rPET content.

[0141] In addition, both processes make it possible to verify that all the preforms 12 of each batch are indeed compliant in terms of rPET content with what is advertised without having to resort to a destructive and lengthy test.

Claims

Demands

1. A method for non-destructive determination of the percentage 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 comprises: - a first step (E1) of measuring a first representative value of a transmission factor (T(X1)) of a wall (16) of said preform (12) for at least a first determined wavelength (X1) located in the visible range; - a second step (E2) of measuring a second representative value of a transmission factor (T(X2)) of a wall (16) of said preform (12) for at least a second determined wavelength (X2) located in the near-infrared;- a third step (E3) of calculating a value (rrPET) representative of the rate of recycled polyethylene terephthalate present in the preform (12) as a function of the first and second values ​​(T(X1), T(X2)) measured during the first step (E1) and the second step (E2).;

2. A method according to the preceding claim, characterized in that the value (rrPET) representing the recycled polyethylene content is calculated as a function of the following parameter: [Math 6] T(X1>-TU2) T(X1)

3. A method according to the preceding claim, characterized in that the value (rrPET) representing the recycled polyethylene rate obtained during the third step (E3) is compared to a reference curve obtained experimentally allowing the recycled polyethylene terephthalate rate to be obtained as a function of the value calculated during the third step (E3).

4. A method according to the preceding claim, characterized in that the reference curve is obtained by applying the method to several polyethylene terephthalate samples of the same formula, each having a different and known percentage of recycled polyethylene terephthalate.

5. A method according to the preceding claim, characterized in that the reference curve is obtained by a mathematical regression method from multiple experimental measurements.

6. A method according to any one of the preceding claims, characterized in that the first determined wavelength (XI) is between 580 nm and 690 nm.

7. Method according to the preceding claim, characterized in that the first wavelength (XI) determined is equal to 680 nm.

8. Method according to claim 6, characterized in that the first wavelength (XI) determined is equal to 590 nm.

9. A method according to any one of the preceding claims, characterized in that the second determined wavelength (X2) is between 780 nm and 1600 nm.

10. Method according to the preceding claim, characterized in that the second determined wavelength (X2) is equal to 980 nm.

11. A method according to any one of the preceding claims, characterized in that the first and second values ​​(T(X1), T(X2)) representing the transmission factor are measured by a measuring device (56, 64) comprising: - a light source (58, 68) which emits a light beam having at least the two determined wavelengths (XI, X2), - a measuring element (70) measuring the intensity for at least the said determined wavelengths (XI, X2) of said light beam (55) after passing through at least one thickness of the wall (16) of the preform (12).

12. A method according to any one of the preceding claims, characterized in that it is implemented in an installation (10) 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 blow molding, the installation (10) comprising a device (56, 64) for measuring the representative value of the transmission factor of at least one wall (16) of the preforms (12) as they pass through an associated measuring zone (Z).

13. A process according to any one of the preceding claims, characterized in that the detected recycled polyethylene terephthalate is mechanically recycled polyethylene terephthalate.

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