Patent Description:
Packaging containers of the single use disposable type for liquid foods are often produced from a packaging laminate based on paperboard or carton. Such packaging containers are often used for packaging of liquid foods, such as milk or fruit juices, that are sold for long term ambient storage. The packaging material in such known packaging containers is typically a laminate, comprising a bulk or core layer of paper, paperboard or other cellulose-based material and outer liquid-tight layers of thermoplastics. In order to render the packaging container gas-tight, in particular oxygen gas-tight for the purpose of aseptic packaging, the laminate in these packaging containers normally comprises an additional barrier layer, for example an aluminum foil.

The packaging containers are generally produced by means of modern highspeed packaging machines of the type that form, fill and seal packages from a web or from prefabricated blanks of packaging material. Packaging containers may thus be produced by reforming a web of the laminated packaging material into a tube by both of the longitudinal edges of the web being united to each other in an overlap joint created by welding together the inner- and outermost thermoplastic polymer layers, which are heat sealable. The tube is then filled with the intended liquid food product and is thereafter divided into individual packages by repeated transversal seals of the tube at a predetermined distance from each other below the level of the contents in the tube. The packages are separated from the tube by incisions along the transversal seals and are given the desired geometric configuration, by fold formation along prepared crease lines in the packaging material.

Packaging containers for sensitive liquid food, for example milk or juice, can also be produced from sheet-like blanks or prefabricated blanks of the laminated packaging material. From a tubular blank of the packaging laminate that is folded flat, packages are produced by first of all building the blank up to form an open tubular container capsule, of which one open end is closed off by means of folding and heat-sealing of integral end panels. A thus closed container capsule is filled with the food product in question, e.g. juice, through its open end, which is thereafter closed off by means of further folding and heat-sealing of corresponding integral end panels. Such tubular blanks produced from sheet-like blanks or prefabricated blanks of the laminated packaging material may also comprise an overlap joint of the above described type. An example of a packaging container produced from sheet-like and tubular blanks is the conventional so-called gable-top package. There are also packages of this type, which have a molded top and/or screw cap made of plastic.

When any desired structure is formed from prefabricated blanks or a web of packaging material by uniting the longitudinal edges of the laminated packaging material in the overlap joint, a cross-section of the laminated packaging material lies free at the respective edge of the blank or web and reveals the different layers comprised inside the laminated packaging material.

However, upon filling of the packaging container with the liquid food product, the liquid food product may get in contact with these layers inside the laminated packaging material and affect them negatively, as those layers may not be liquid proof. This is for example the case with paper materials. This may lead to problems in terms of durability of the laminated packaging material due to deterioration of the laminate, which may also have an impact on the food hygiene.

It is therefore known to apply a strip of a sealing material, in particular of a polymer material, to the edge that is exposed to the liquid food product in order to prevent the liquid food product from affecting the layers inside the laminated packaging material.

Such strips are often made as laminates themselves and may comprise an inner surface layer, an outer surface layer and center layer. The different layers may be laminated to each other by their own material adhesion properties, for example by extrusion lamination or by additional adhesives applied to them, for example in wet lamination.

As sealing strips need to follow the contour of the respective edge to be sealed to achieve a proper sealing function, they need to feature good flexibility and therefore the respective strips tend to be rather thin. However, the deformation of the material upon application to the edge contour imposes mechanical stress on the strip. If the bonding between the different layers of the strip is not sufficiently strong, the required deformation may cause the layers of the strip to delaminate, which means the layers may be separated from each other at least in sections.

This causes problems with regard to the sealing function and there is a need for an improved sealing strip, accordingly.

Patent publication No. <CIT> relates to manufacturing of a longitudinal sealing strip, involving film blowing and an orientation process including an annealing heat treatment step to anneal the polymers in the stretching operation.

Patent publication No. <CIT> relates to a laminated packaging material for packaging of liquid food, which is improved for transversal heat sealing of a continuous form fill-seal tube of packaging material. Patent publication No. <CIT> relates to a polypropylene film manufacturing method including an annealing step to increase the transparency of the film. The film consists of a mono-material of polypropylene.

Patent publication No. <CIT> relates to the altering of the transparency/opacity in a multilayer material. The transparency is increased by heat treatment.

It is now an objective of the present invention to at least partly overcome one or more of the above-identified limitations of the prior art. In particular, the present invention aims at providing an improved sealing strip, which is resistant against delamination of its layers and that can be manufactured at low technical effort.

To achieve these objectives, a first aspect of the invention refers to a method of manufacturing a strip for sealing a laminated packaging material for liquid food products according to independent claim <NUM>.

In the method of the invention, a pre-manufactured strip comprising a plurality of layers made of polymer material is provided, said layers comprising at least an inner surface layer, an outer surface layer and center layer.

According to the invention, at least one of said inner and outer surface layers is exposed to a heat treatment effecting that a degree of transparency of the manufactured strip is increased with regard to the degree of transparency of the pre-manufactured strip.

This is based on findings by the applicant, according to which the pre-manufactured strip features a rather low degree of transparency due to bonding material inhomogeneity present in the joining interface of the different layers. As known in the art, the degree of transparency indicates the amount of light reaching though the strip and can therefore range from <NUM> (no light reaches through) to <NUM> (all light reaches through), whereas a degree of transparency is rather a theoretical case. The bonding material inhomogeneity of the pre-manufactured strip is for example a result of air entrapments and inhomogeneous allocation of strip material or adhesive created during the lamination process to pre-manufacture the strip. For that reason, the pre-manufactured strip normally appears opaque.

By the heat treatment, an annealing process is performed on the strip. This causes the bonding material inhomogeneity to be reduced, because air entrapments can be resolved and inhomogeneous material allocation can be smoothened. It shall be noted that herein it is referred to the pre-manufactured strip, as long as it has not yet been heat treated. Accordingly, it is referred to the manufactured strip or simply the strip, once the heat treatment has at least begun. As a result of the heat treatment, the degree of transparency is increased, thereby indicating an increase of bonding material homogeneity. This allows for visual inspection to verify that the desired technical effect has been achieved. It shall be noted, that even a little increase of the degree of transparency achieves an improvement according to the described mechanism to some extent. This means, the method of the invention is successfully carried out, once a higher degree of transparency can be measured on the manufactured strip relative the degree of transparency of the pre-manufactured strip, within the measurement accuracy achievable by known measurement methods to detect a degree of transparency. Such measurement methods are well known in the art. It is not necessarily required, but possible of course, to quantify the achieved increase of the degree of transparency. For the method of the invention, it is however sufficient to come up with a qualitative assessment in terms, whether the degree of transparency could be increased or not ("yes or no" decision). Based in that, the required assessment can be done by human eye visual inspection or by using a respective measurement device.

The main influencing factors to perform a successful heat treatment are the material of the pre-manufactured strip, wherein the different layers may comprise different polymer materials and may comprise additional adhesives there between, and further a strip surface temperature during the heat and a heat treatment duration.

A person skilled in the art is, based on the present disclosure, enabled to carry out experiments with different pre-manufactured strips, strip surface temperatures and heat treatment durations to identify preferable combinations of strips and heat treatment parameters in detail, wherein the above described effect to increase the degree of transparency is exploited best.

As a starting point for such experiments, a strip surface temperature of <NUM> can be created and the pre-manufactured strip can be exposed to that temperature for <NUM>. This is a verified set of heat treatment parameters achieving the desired technical effect of the invention for a pre-manufactured strip with an inner and outer surface layer of polyethylene (PE) and a center layer of (polyethylene terephthalate) PET or ethylene vinyl alcohol (EVOH), wherein an adhesive in the form of polyester polyol adhesives is applied between the respective surface layers and the center layer.

From this starting point other specific heat treatment parameters can be explored in experiments to identify preferred variants in detail without any further technical problems. Therein, a higher temperature and/or higher duration of the heat treatment will in principle enhance the desired effect and a lower temperature and/or lower duration of the heat treatment will in principle lower the desired effect. It will be well understood by a skilled person that thermal material stress of the strip due to the heat treatment is limiting the available process window in the direction of higher temperature/duration values and a decrease of the desired effect on the degree of transparency is limiting the available process window in the direction of lower temperatures/duration values. Different temperatures/duration values may be explored for different strip materials.

A major advantage of the increased bonding material homogeneity of the strip after heat treatment is that a resistance of the strip against delamination of its layers is significantly increased, because the bonding of the different layers is significantly improved.

This can be verified in a delamination test, also referred to as peeling test. Therein, a sample strip is provided and immersed into oleic acid (<NUM> ppm) at <NUM> for four weeks to simulate a liquid food product environment. Then, a pulling force is applied in the longitudinal direction of the strip. This causes the material of the strip to stretch and since the different layers of the strip do not feature totally identic mechanical properties, a separation force acts on the joining area between the layers due to a different mechanical stress level in the different layers.

In an example, a reference strip as specified above (made of PE and PET/EVOH) has been pre-manufactured according to the prior art, which means without any heat treatment. The reference strip has been exposed to the peeling test. Then, an identic reference strip has been heat treated according to the method of the invention has been exposed to the peeling test.

In case of the reference strip according to the prior art, the separation force has caused the layers to separate/to peel at least in sections. The delamination has been detected at a pulling force around <NUM> N/m, wherein "m" refers to the width of the sample strip. The width is measured perpendicular the longitudinal direction of the strip, wherein the longitudinal direction is defined by the direction of the pulling force in the peeling test. If the pre-manufactured strip is made in a flow manufacturing process, the longitudinal direction normally coincides with a machine direction of the manufacturing process. In the above example, the width of the sample strip measures <NUM>. In case of the sample strip according to invention, no delamination of the layers could be detected before the strip has been torn apart entirely.

A person skilled in the art is, based on the present disclosure, enabled to carry out further experiments with different sample strips and the above described testing method in order to identify a required degree of transparency that indicates a sufficient resistance against delamination.

With this said, the method of the invention provides a simple way of manufacturing sealing strips with a resistance against delamination tailored to the requirements of a particular product.

The pre-manufactured strip can generally be achieved by any lamination or extrusion process, wherein the inner and outer surface layers are applied to the center layer. Preferably, this happens in a process wherein a web of these layers is manufactured, preferably by lamination. The web may feature relatively large dimensions, for example <NUM> by <NUM>. In principle, a web may be considered a large pre-manufactured strip and the heat treatment according to the invention can be done on the web. However, to achieve a more even and balanced allocation of the heat on the web surface and to reduce the technical complexity of the heat treatment, the dimensions of the pre-manufactured strip are preferably reduced.

Therefore, the large web is preferably slit into lanes. This may also be done in iterations to reduce the dimensions of the lanes further down to a desired final width of the pre-manufactured strip, preferably <NUM>. A width of <NUM> has led to excellent heat distribution in the heat treatment, as found by the applicant. Performing the reduction of the dimensions in iterations is beneficial with regard to handling of the flexible material of the strip and with regard to different tools that may be used.

In an embodiment of the method of the invention the material of the center layer differs from the material of the inner surface layer and/or from the material of the outer surface layer. This typically leads to higher inhomogeneity in the bonding material at the interface between the center layer and the inner surface layer and/or the outer surface layer of the pre-manufactured strip. Consequently, the beneficial effects of the inventive method described above lead to particularly significant improvement of the bonding between the different materials. However, it shall be underlined that these beneficial effects also occur, if all layers are of the same material. This is due to the fact, that bonding material inhomogeneity at a bonding interface is technically inevitable at least to some degree. Here, even if the bonding is generally good in case of the same materials, the method of the invention allows for even further reduction of the remaining bonding material inhomogeneity.

Regarding the materials of the different layers, some preferred examples are provided below. Preferably, the center layer may be made of polyesters, such as polyethylene terephthalate (PET), polyamides (PA), higher density polyolefins, such as polypropylene (PP), or ethylene vinyl alcohol copolymers (EVOH). The inner and/or outer surface layers may be made of ethylene-based polyolefins, such as LLDPE or LDPE. In specifically preferred combinations, the center layer is made of PET and the inner and/or outer surface layers are made of LDPE and/ or LLDPE. It is also preferred to make the center layer of PA and the inner and/or outer surface layers of LDPE and/ or LLDPE. It is also preferred to make the center layer of EVOH and the inner and/or outer layers of LDPE and/or LLDPE. Preferably, the inner and outer layers are made of the same material, because this leads to the same bonding system between the center layer and the inner and outer surface layer, respectively. Thus, the heat treatment leads to equally good improvement of the material homogeneity at the bonding interface on both sides of the center layer of the manufactured strip.

In an embodiment of the method of the invention, the degree of transparency of the pre-manufactured strip is detected by a measurement device prior to the heat treatment and, based on the material of the pre-manufactured strip and the detected degree of transparency, a parameter or set of heat treatment parameters are selected from a plurality of reference heat treatment parameters, the selected parameter(s) lead to a pre-defined increase of the degree of transparency of the pre-manufactured strip during the heat treatment.

Based on this, the required degree of transparency and resistance against delamination, respectively, can be achieved at low technical effort.

There are various known measurement devices and methods, which are suitable to detect the degree of transparency. For example a colorimeter may be used. Another simply example is a light source being arranged on one side of the strip and a photo cell being arranged on an opposite side of the strip, wherein the photo cell detects the light reaching through the strip.

In an embodiment of the method of the invention, the plurality of reference heat treatment parameters are stored in a database and a control unit receives the degree of transparency detected by the measurement device, compares it to the content of the database, selects the heat treatment parameters leading to the pre-defined increase of the degree of transparency and controls the manufacturing process, accordingly.

This way, the process quality can be significantly enhanced and a high efficiency is achieved. The material of the strip may be manually provided to the control unit, for example by selection of a certain program upon setup of the control unit. In principle, the material could also be determined by a material analysis unit as known in the related art.

In an embodiment of the method of the invention, the degree of transparency of the strip is detected by a measurement device during the heat treatment and the heat treatment is stopped by a control unit, if a pre-defined degree of transparency of the strip is achieved.

This allows for tailoring a quantitatively specified degree of transparency of the strip at a high process stability level.

Detecting the degree of transparency of the pre-manufactured strip during and prior to the heat treatment can also be combined, leading to an even increased process quality.

In an embodiment of the method of the invention, the reference heat treatment parameters comprise at least a reference strip surface temperature value and a reference heat treatment duration value.

This means, any equipment used to execute the method of the invention is adjusted and configured according to achieve these heat treatment parameters.

Further exemplary heat treatment parameters that need to be adjusted or configured, accordingly, may comprise a heating power of a heat source used for the heat treatment, a relative speed of the heat source and the strip and a distance between the heat source and the strip. However, it shall be noted that these are secondary parameters that influence the achieved strip surface temperature and the duration this is achieved for.

In an embodiment of the method of the invention, a strip surface temperature value is detected on the inner and/or outer surface layers by a measurement device and is controlled by a control unit to match with a reference strip surface temperature value selected as a heat treatment parameter during the heat treatment.

This can be achieved by a thermal camera, for example, and further increases the process quality.

Preferably, both said inner surface layer and said outer surface layer are exposed to the heat treatment.

In an example, not covered by the claims, the strip surface temperature value is selected from an interval from <NUM> to <NUM>, preferably from °C to <NUM> and the heat treatment duration value is selected so as to achieve a degree of transparency of at least <NUM> after the heat treatment. Preferably, the degree of transparency is at least <NUM>, further preferred at least <NUM>, further preferred at least <NUM> and even further preferred at least <NUM> after the heat treatment.

Even more preferred, the strip surface temperature value is selected from an interval from110 °C to <NUM>, further preferred from120 °C to <NUM> and mostly preferred from130 °C to <NUM>. It shall be noted, that any temperature value covered by the interval from100 °C to <NUM> or any degree of transparency between <NUM> and <NUM>, which is achievable by the use of available technical equipment, is also disclosed with regard to the method of the invention. The same applies for any sub-interval that can be defined in the interval from <NUM> to <NUM> or in an interval of the degree of transparency from <NUM> to <NUM>.

In an embodiment of the method of the invention, the strip is moved during the heat treatment, relative a heat source used for the heat treatment and a measurement device.

This way, a flow manufacturing process can be realized allowing for efficient manufacturing of the strip.

In that case, the process parameters preferably include a speed value of this relative movement. Preferably, the speed value may replace the duration value of the heat treatment, as the duration value is implied by the speed value that determines how long the heat from the heat source is affecting the surface of the strip.

Preferably, one or more measurement devices and/or heat sources used to achieve and measure the temperature on the surface(s) of the strip are moved relative the strip.

Mostly preferred, the strip is moved, while the heat source(s) and heat measurement device(s) are static equipment.

Preferably, the speed value of the movement ranges in an interval [<NUM>/s; <NUM>/s]. It shall be noted, that any speed value covered by that interval, which is achievable by available technical equipment, is also disclosed with regard to the method of the invention. The same applies for any sub-intervals that can be defined in the interval from0. <NUM>/s to <NUM>/s.

In an embodiment of the method of the invention, the strip is moved in a flow manufacturing process at a speed selected from an interval from <NUM>/s to <NUM>/s.

In an embodiment of the method of the invention, the inner and outer surface layers of the pre-manufactured strip are made of PE and the center layer is made of PET or EVOH.

With reference to <FIG> an embodiment of a packaging container <NUM> for a liquid food product <NUM> is illustrated. For example, a not further specified packaging container producing machine <NUM> may deliver a laminated packaging material <NUM>, from which the packaging container <NUM> can be made. The packaging container <NUM> can exemplarily be made by forming the laminated packaging material <NUM> to a tube structure featuring overlapping sections <NUM> (also see <FIG>), which are welded together and which exhibit at least one exposed edge <NUM> (seen in <FIG>), covered and sealed by a strip <NUM> (also see <FIG>) according to the invention.

The thus formed laminated packaging material <NUM> is then filled with the liquid food product <NUM>, sealed and cut off by respective sealing and cutting means <NUM> to close the packaging container <NUM>.

For the purpose of further describing the present invention, a machine direction <NUM> is defined in <FIG>, which reference will be made to in the following figures. The machine direction <NUM> refers to the fact that the laminated packaging material <NUM> is normally manufactured in a flow process that has a material feed direction, which is the machine direction <NUM> in other words.

<FIG> shows the laminated packaging material <NUM>, from which a packaging container <NUM>, for example such as shown in <FIG>, can be formed. The laminated packaging material <NUM> is folded (folded sections not shown) to create overlapping sections <NUM>, in which the laminated packaging material <NUM> is overlapping with two of its edges <NUM>. The sections <NUM> are joined with each other, for example welded together. At the edges <NUM> of said overlapping sections <NUM>, a cross-section <NUM> of the laminated packaging material <NUM> is revealed, wherein different layers <NUM> of the laminated packaging material <NUM> are exposed to the environment (as illustrated in the broken off section in <FIG>). These layers are well known in laminated packaging materials <NUM> of the present type and may for example comprise a main layer made of paperboard, at least one barrier layer and sealing layers made of a polymer material, as well as surface coatings.

One of said cross-sections <NUM> is located on an inside <NUM> of the laminated packaging material <NUM>, wherein the location of that inside <NUM> is merely an example, indicating the side of the laminated packaging material <NUM>, which is intended to form the inside <NUM> of the packaging container <NUM>, which is exposed to the liquid food product <NUM>.

To protect the layers <NUM> of the laminated packaging material <NUM> in said cross-section <NUM> on the inside <NUM>, said cross-section <NUM> is sealed by the strip <NUM>. The location of the strip <NUM> on the laminated packaging material <NUM> and in the packaging container <NUM>, respectively, is exemplarily and schematically shown in <FIG>, as well. Merely as an example and to make reference to <FIG>, the machine direction <NUM> is indicated in <FIG>, too, thus indicating how the view of <FIG> corresponds to that of <FIG>.

<FIG> shows an embodiment of a strip <NUM> in a cross-sectional view. The strip <NUM> comprises the plurality of layers <NUM>, that are made of polymer material. Said layers <NUM> comprise at least an inner surface layer <NUM>, an outer surface layer <NUM> and center layer <NUM>. Merely as an example, the inner surface layer <NUM> is intended to face the liquid food product <NUM> on the inside <NUM> of the laminated packaging material <NUM> and the packaging container <NUM>, respectively, as shown in <FIG>.

In this example, the inner and outer surface layers <NUM>, <NUM> of the strip <NUM> are made of PE and the center layer <NUM> is made of PET or EVOH. Between the layers <NUM> an adhesive may be present, which is not illustrated.

Now turning to <FIG>, an embodiment of an inventive method of manufacturing the strip <NUM> for sealing the laminated packaging material <NUM> is shown, with reference to a device <NUM> being adapted and configured to execute the method.

In a first step of the method, a pre-manufactured strip <NUM> is provided, comprising the plurality of layers <NUM> made of polymer material and comprising at least the inner surface layer <NUM>, the outer surface layer <NUM> and the center layer <NUM>. According to the invention, at least one of said inner and outer surface layers <NUM>, <NUM> is exposed to a heat treatment effecting that a degree of transparency <NUM> (see <FIG>) of the manufactured strip <NUM> is increased with regard to the degree of transparency <NUM> of the pre-manufactured strip <NUM>. As indicated by reference line <NUM>, it is referred to the pre-manufactured strip <NUM> prior to heat treatment and to the manufactured strip <NUM>, once the heat treatment has begun. This means, the pre-manufactured strip <NUM> and the manufactured strip <NUM> are the same part, but feature different material properties before and after the heat treatment has begun.

The device <NUM> comprises a heat source <NUM> to deliver the thermal energy for the heat treatment. Preferably, both said inner surface layer <NUM> and said outer surface layer <NUM> are exposed to the heat treatment. For that purpose, the device <NUM> may comprise a second heat source <NUM>, even though the heat source <NUM> may also be designed to heat up both surface layers <NUM> and <NUM> of the strip <NUM>. For example, the heat source <NUM> may be designed as a heat chamber to locally cover the entire strip <NUM>.

Preferably, the degree of transparency <NUM> of the pre-manufactured strip <NUM> is detected by a measurement device <NUM> prior to the heat treatment. Then, based on the known material of the pre-manufactured strip <NUM> and the detected degree of transparency <NUM>, a selection of heat treatment parameters <NUM> is done from a plurality of reference heat treatment parameters <NUM>, which lead to a pre-defined increase of the degree of transparency <NUM> of the pre-manufactured strip <NUM> during the heat treatment. The manufactured strip <NUM> will then feature that increased degree of transparency <NUM>. It is preferred, that the reference heat treatment parameters <NUM> comprise at least a reference strip surface temperature value and a reference heat treatment duration value.

Preferably, the plurality of reference heat treatment parameters <NUM> are stored in a database <NUM> and a control unit <NUM> receives the degree of transparency <NUM> detected by the measurement device <NUM>, compares it to the content of the database <NUM>, selects the heat treatment parameters <NUM> leading to the pre-defined increase of the degree of transparency <NUM> and controls the manufacturing process, accordingly.

Preferably, the degree of transparency <NUM> of the manufactured strip <NUM> is detected by a measurement device <NUM> during the heat treatment and the heat treatment is stopped by a control unit <NUM>, if a pre-defined degree of transparency <NUM> of the strip <NUM> is achieved. Preferably, the same measurement device <NUM> and control unit <NUM> as described above are used for this purpose.

During the heat treatment, it is beneficial to detect a strip surface temperature value on the inner and/or outer surface layers <NUM>, <NUM> by a measurement device <NUM> and to control it by a control unit <NUM> to match with the reference strip surface temperature value selected as a heat treatment parameter <NUM> during the heat treatment. Preferably, the same measurement device <NUM> and control unit <NUM> as described above are used for this purpose. For multiple measurement purposes, the measurement device <NUM> may comprise respective measurement modules, for example for transparency and temperature measurement.

During the heat treatment, the strip <NUM> may be moved relative the heat source <NUM>, <NUM> used for the heat treatment and/or the measurement device <NUM>, which allows for the heat treatment being embedded in a flow manufacturing process. For the purpose of providing an example, the machine direction <NUM> corresponding to the previous figures is shown in <FIG>, as the inventive method may be combined with the manufacturing of the packaging container <NUM>.

Finally turning to <FIG>, an example for the increased degree of transparency <NUM> of the manufactured strip <NUM> compared to the initial degree of transparency <NUM> of the pre-manufactured strip <NUM> is provided. The shown sample pre-manufactured strip <NUM> and sample manufactured strip <NUM> are exemplarily identic with the pre-manufactured strip <NUM>/strip <NUM> as described above and have been moved in a flow manufacturing process relative the heat source <NUM> at a speed selected from an interval [<NUM>/s; <NUM>/s]. At the same time, the strip surface temperature value has been selected from an interval [<NUM>; <NUM>]. All possible combinations of any of the values covered by these intervals, which are executable by known technical means, have achieved an increase in the degree of transparency <NUM> of the manufactured strip <NUM> with regard to the degree of transparency <NUM> of the pre-manufactured strip <NUM>.

The illustration in <FIG> shows two specifically good test results based on the following set of heat treatment parameters <NUM>, that have both led to the same increased degree of transparency <NUM>.

For a first test, a set of heat treatment parameters <NUM> has been selected as (speed, the strip surface temperature) = (<NUM>,<NUM>/s, <NUM>).

For a second test, a set of heat treatment parameters <NUM> has been selected as (speed, the strip surface temperature) = (<NUM>,<NUM>/s, <NUM>).

A heat influence zone of the heat source <NUM> on the inner surface layer <NUM> has been selected as a circle with a diameter of <NUM>.

In <FIG> it can be seen, that the pre-manufactured strip <NUM> appears opaque, because the black colored background is not significantly shining through the pre-manufactured strip <NUM>. It can further be seen, that the heat treated strip <NUM> appears transparent, as the black colored background is significantly shining through the strip <NUM>.

The respective degrees of transparency <NUM>, <NUM> have been measured with a colorimeter, which is a well-known measurement device in the art that measures three values with the sample part placed in front of a black background. The measured values are:.

Claim 1:
Method of manufacturing a strip (<NUM>) for sealing a laminated packaging material (<NUM>) for liquid food products (<NUM>), wherein a pre-manufactured strip (<NUM>) comprising a plurality of layers (<NUM>) made of polymer material is provided, said layers (<NUM>) comprising at least an inner surface layer (<NUM>), an outer surface layer (<NUM>) and center layer (<NUM>), at least one of said inner and outer surface layers (<NUM>; <NUM>) being exposed to a heat treatment effecting that a degree of transparency (<NUM>) of the manufactured strip (<NUM>) is increased with regard to the degree of transparency (<NUM>) of the pre-manufactured strip (<NUM>), characterized in that the degree of transparency (<NUM>) of the pre-manufactured strip (<NUM>) is detected by a measurement device (<NUM>) prior to the heat treatment and, based on the material of the pre-manufactured strip (<NUM>) and the detected degree of transparency (<NUM>), a parameter or set of heat treatment parameters (<NUM>) are selected from a plurality of reference heat treatment parameters (<NUM>), the selected parameter(s) lead to a pre-defined increase of the degree of transparency (<NUM>) of the pre-manufactured strip (<NUM>) during the heat treatment, wherein the reference heat treatment parameters (<NUM>) comprise at least a reference strip surface temperature value and a reference heat treatment duration value and wherein a strip surface temperature value is detected on the inner and/or outer surface layers (<NUM>; <NUM>) by a measurement device (<NUM>) and is controlled by a control unit (<NUM>) to match with a reference strip surface temperature value selected as a heat treatment parameter (<NUM>) during the heat treatment.