Membrane from fibrous web and method of producing a membrane from fibrous web

Abstract

A membrane comprises a fibrous web comprising thermoplastic polymer fibers. The web comprises at least one treated region which has been heated and compressed. Said treated region has a transparency of at least 85 %, preferably at least 90 %, more preferably at least 92 %, and a void ratio of 10 % or more and 80 % or less, preferably 20 % or more and 75 % or less, more preferably 30 % or more and 70 % or less.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a membrane from fibrous web and to a method of producing a membrane from fibrous web.PRIOR ART

[0002] In the packaging industry, there is a wide range of applications for membranes made from thermoplastic polymers. Typically, such membranes are used as heat sealable layers on a substrate to form a laminate used in the packaging of foods and beverages, such as in beverage cartons, tea bags, the packaging of loose fruit such as berries, but also in the packaging of products such as sports equipment or similar products. Such membrane can also be used alone, for example, to form a viewing window in a packaging that allows the contents of the packaging to be seen. Hence, it is desired that such membranes have a certain degree of transparency in order to make the content of the packaging visible and / or heat sealing properties in order to easily fuse the membrane to a substrate or neighboring part of a packaging.

[0003] Conventionally, the membranes are manufactured by extruding a film made of a thermoplastic polymer, typically polyesters like polyethylene terephthalate (PET). The use of such conventional membrane in laminates is known from WO 2008 092 328 A1 and US 9,393,763 B2, for example. One main disadvantage of these conventional membranes is a lack of air permeability caused by the extrusion. Furthermore, in the recent past, the sustainability requirements of packaging materials have increased, so it is desired to obtain a membrane that is recyclable, is made from renewable resources, is biodegradable and / or is compostable.

[0004] Hence, it is an object of the present invention to provide a membrane having a certain transparency, air permeability and is environmentally friendly.SUMMARY OF THE INVENTION

[0005] The above object is solved by a membrane according to claim 1 and by a method of producing a membrane according to claim 9. Further preferred embodiments are set out in the dependent claims.

[0006] The present invention provides a membrane comprising a fibrous web comprising thermoplastic polymer fibers. The web comprises at least one treated region which has been heated and compressed. Said treated region has a transparency of at least 85 %, preferably at least 90 %, more preferably at least 92 %, and a void ratio of 10 % or more and 80 % or less, preferably 20 % or more and 75 % or less, more preferably 30 % or more and 70 % or less.

[0007] Defining that the web comprises a treated region includes a case in which the entire web forms the treated region but also includes a case in which the web comprises at least one region which is the treated region and at least one region which is not the treated region.

[0008] The void ratio as used in the present disclosure is a measure of the amount of voids in the treated region of the web and is calculated by the following formula: vr % = ρ polymer − ρ web , h + c ρ polymer × 100 % = 1 − ρ web , h + c ρ polymer × 100 %

[0009] In Formula 1, ρ polymer is the density (mass per volume) of the pure thermoplastic polymer which is a material property that can be taken from the material data sheet or can be determined in the literature. In other words, ρ polymer relates to the density of the polymer itself without any air inclusions or voids. ρ web,h+c is the density of the treated region of the web, i.e. the heated and compressed fibrous web, also called bulk density or apparent density, and can be determined by dividing the mass of the heated and compressed web by the total volume the heated and compressed web occupies, also called bulk volume.

[0010] If the fibrous web comprises thermoplastic polymer fibers of more than one type of polymer, the density of each polymer type is considered for the determination of ρ polymer by weighting the arithmetic mean according to the respective mass ratio of the polymer type in the fibrous web.

[0011] Throughout the present disclosure, all transparency measurements were carried out according to the standard DIN 53147:1993-01 using a test equipment model BCMTS M Type 40605, with Touchscreen M software of the company Frank-PTI. This test equipment uses a Konica Minolta CM-3630 Spectrophotometer.

[0012] By heating and compressing the fibrous web comprising thermoplastic polymer fibers, the transparency of the fibrous web can be increased. By increasing the transparency of the web, the membrane can be used in a packaging to make the content of the packaging visible. Furthermore, by heating and compressing a fibrous web, in contrast to an extruded film, a porosity in the resulting treated region can be achieved which allows for a certain air permeability thereof. Setting the void ratio as described has the technical effect, that a desired air permeability of the material can be achieved.

[0013] In a preferred embodiment, a proportion of thermoplastic polymer fibers by weight of all fibers of the web is 90% or more, preferably 95% or more, more preferably 99% or more.

[0014] The more fibers in the fibrous web are made of thermoplastic polymers, the more uniform the treated region will be when the web is heated and compressed. It may be preferred that all fibers in the web, except for negligible impurities, are made of thermoplastic polymers.

[0015] In a preferred embodiment, the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers which are preferably one or more of Polylactic Acid (PLA) fibers, Polyhydroxyalkanoate (PHA) fibers and Polyhydroxybutyrate (PHB) fibers.

[0016] The term bio-sourced polymer as used in the present disclosure refers to a polymer the monomers of which can be derived or obtained from molecules that are producible by the cells of living organisms. The fibrous web may contain fibers of only one type of polymer, preferably one type of bio-sourced polymer; or the fibrous web may contain a blend of fibers, i.e. fibers of different kinds of polymers, preferably different kinds of bio-sourced polymers.

[0017] Using thermoplastic bio-sourced polymer fibers in the fibrous web has the technical effect that the membrane can be more sustainable because bio-sourced polymers are renewable raw materials rather than fossil ones being based on petroleum. Furthermore, depending on the choice of bio-sourced polymer, the web can be biodegradable and even compostable. When using PLA fibers, PHA fibers and / or PHB fibers, for example, the web can be biodegradable and / or compostable, for example according to standard EN 13432.

[0018] In a preferred embodiment, the web is a non-woven web, preferably a spunbond non-woven web.

[0019] Providing the web as a non-woven web, preferably a spunbond non-woven web has the technical effect that the web can be produced in a continuous process of spinning and dispersing fibers, and thereby in a cost-efficient manner.

[0020] In a preferred embodiment, the web further comprises a region which is not the treated region. The web has in the region which is not the treated region a first air permeability and in the treated region a second air permeability, wherein the second air permeability is 0.1 % or more and 90 % or less, preferably 0.5 % or more and 80 % or less, more preferably 1 % or more and 70 % or less, of the first air permeability.

[0021] There is a certain correlation between the air permeability and the porosity or void ratio of the membrane, but they are not necessarily the same. A certain number of voids is a prerequisite for air permeability. However, the exact arrangement and size of these voids determines how permeable the membrane is to air. The fibrous web is inherently permeable to air and has a certain porosity or void ratio. By applying heat and pressure to the web, this air permeability is reduced, but it is intended and preferred to maintain a certain air permeability.

[0022] Providing a membrane that has a region that is the treated region and a region that is not the treated region has the technical effect that the membrane can have the treated region in places where increased transparency is desired and can have a region that is not the treated region in other places where increased transparency is not required. The region that is not the treated region has a higher strength, in particular a tear resistance, than the treated region and therefore facilitates the handling of the membrane in this region. Preserving part of the original air permeability of the web in the treated region has the technical effect that the membrane in the treated region can have both increased transparency and a certain air permeability. This makes the membrane particularly suitable for packaging food products for which a certain air permeability is desired, like fresh fruit, for example.

[0023] In a preferred embodiment, the web further comprises a region which is not the treated region. This region can be the region which is not the treated region of the preferred embodiment described above, or it can be another region which is not the treated region. The region which is not the treated region has a transparency which is lower than the transparency of the treated region, and which is preferably 80 % or less, more preferably 75 % or less, even more preferably 70 % or less.

[0024] As stated above, providing a membrane that has a region that is the treated region and a region that is not the treated region has the technical effect that the membrane can have the treated region in places where increased transparency is desired and can have a region that is not the treated region in other places where increased transparency is not required. The region that is not the treated region has a higher strength, in particular a tear resistance, than the treated region and therefore facilitates the handling of the membrane in this region. With this preferred embodiment, the transparency of the web in the treated region can be increased by 25 % or more, preferably by 33 % or more, more preferably by 42 % or more. In other words, in the region of the web to which heat and pressure are applied to form the treated region, by applying heat and pressure to the web, the web's transparency can be increased by 25 % or more, preferably by 33 % or more, more preferably by 42 % or more compared to the transparency of the web in this region before the heat and the pressure are applied to form the treated region or compared to a region of the web which is not the treated region.

[0025] In a preferred embodiment, the web further comprises a region which is not the treated region. This region can be the region which is not the treated region of the two previously described preferred embodiments, or it can be another region which is not the treated region. The membrane has, in a sample region comprising at least a part of said region which is not the treated region, a tear resistance of 200 mN or more, preferably 500 mN or more, more preferably 800 mN or more.

[0026] Throughout the present disclosure, all tear resistance values relate to the standard ISO 1974:2012 and hence all tear resistance measurements were carried out according to the standard ISO 1974:2012.

[0027] The technical effect of using a web with such a tear resistance is that the web can be processed when producing the membrane in a roll-to-roll process without the web tearing when unrolling or deflecting the web.

[0028] In a preferred embodiment, the treated region of the web is a contiguous area that has a size of 0.5 cm 2< or more, preferably 1 cm 2< or more, more preferably 2 cm 2< or more, even more preferably 5 cm 2< or more.

[0029] Thus, the treated region of the web is not just one or more dot- or line-like areas that serve merely to attach the membrane to something else, but said treated region is a contiguous area that is large enough to be a functional surface with increased transparency and / or defined air permeability.

[0030] The present invention provides a method of producing a membrane, preferably the membrane according to any one of claims 1 to 8. The method comprises preparing a fibrous web comprising thermoplastic polymer fibers, and applying heat and pressure to at least one region of the web to form a treated region. When applying the heat to the at least one region of the web, the web is heated from ambient temperature to a heating temperature within a heating time period of 0.5 seconds or more and 12 seconds or less, preferably of 1 second or more and 6 seconds or less. The heating temperature is at least as high as a melting temperature of the thermoplastic polymer fibers, preferably at least 5 °C or more but not more than 20 °C above the melting temperature, more preferably at least 10 °C or more but not more than 15 °C above the melting temperature.

[0031] In other words, the heating temperature preferably lies in an interval of 5 °C or more and 20 °C or less above the melting temperature of the thermoplastic polymer fibers. If more than one type of thermoplastic polymer fibers is used, the interval refers to the highest melting temperature of the different types of thermoplastic polymer fibers. The term ambient temperature as used in the present disclosure, refers to a temperature of 15 to 25 °C, preferably 18 to 22 °C, most preferably 20 °C. The ambient temperature as the starting temperature for heating does not have a particularly strong influence on the quality of the treated region, as long as the ambient temperature lies within said interval.

[0032] Both the heating time and the heating temperature have a great influence on whether the fibrous web's transparency can be increased to a desired level without causing the fibrous web to shrink or to get holes or cracks. With the described parameters for the heating time and the heating temperature, it is possible to obtain a membrane having a treated region with increased transparency and without any distortion or shrinkage, cracks or holes.

[0033] In a preferred embodiment, a proportion of thermoplastic polymer fibers by weight of all fibers of the web is 90% or more, preferably 95% or more, more preferably 99% or more. In addition, or alternatively, the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers which are preferably one or more of Polylactic Acid (PLA) fibers, Polyhydroxyalkanoate (PHA) fibers and Polyhydroxybutyrate (PHB) fibers.

[0034] The more fibers in the fibrous web are made of thermoplastic polymers, the more uniform the treated region will be when the web is heated and compressed. It may be preferred that all fibers in the web, except for negligible impurities, are made of thermoplastic polymers. Using thermoplastic bio-sourced polymer fibers in the fibrous web has the technical effect that the membrane can be more sustainable because bio-sourced polymers are renewable raw materials rather than fossil ones being based on petroleum. Furthermore, depending on the choice of bio-sourced polymer, the web can be biodegradable and even compostable. When using PLA fibers, PHA fibers and / or PHB fibers, for example, the web can be biodegradable and / or compostable. For example, the bio-sourced polymer may be made biodegradable according to standard EN 13432.

[0035] In a preferred embodiment, when applying the heat to the at least one region of the web, the heat is applied only from one side of the web, and / or the heat is applied by means of a hot press, a calender or an ultrasonic horn.

[0036] It has been surprisingly found that by applying heat to only one side of the web, the treated region can be formed and the transparency of the web can be increased in that region without damaging the web, in particular without the web getting cracks or holes or shrinking. When using a hot press or a calender, the heat and the pressure can be applied simultaneously and formation of cracks or holes can be prevented.

[0037] In a preferred embodiment, when applying heat and pressure to the at least one region of the web, the heat and the pressure are applied so as to increase transparency of the web to a transparency of 85 % or more, preferably 90 % or more, more preferably 92 % or more.

[0038] Providing the membrane with a high level of transparency makes it possible to use the membrane for applications in which, for example, the inside of a package is to be visible from the outside.

[0039] In a preferred embodiment, the method further comprises cooling the web. The web is cooled from heating temperature to ambient temperature within a cooling time period of 0.5 seconds or more and 120 seconds or less, preferably of 1 second or more and 100 seconds of less.

[0040] The ambient temperature as the target temperature for cooling does not have a particularly strong influence on the quality of the web with increased transparency, as long as the ambient temperature lies within the above-mentioned interval of 15 to 25 °C, preferably 18 to 22 °C, most preferably 20 °C.

[0041] Surprisingly, it turned out that the speed at which the web is cooled has an influence on whether the treated region in which the web's transparency is increased can be formed without forming cracks or holes or shrinking. With the above values, it is possible to form said treated region while preventing it from shrinking, cracking or forming holes.

[0042] In a preferred embodiment, the method further comprises bringing the web in contact with a release surface having a non-stick coating prior to applying heat and pressure to the web, the non-stick coating preferably comprising Polytetrafluoroethylene (PTFE) and / or silicone.

[0043] The release surface can comprise a first release sheet in direct contact with a first side of the web so that the release sheet is sandwiched between the first side of the web and a heating surface. Naturally, the release surface can comprise a second release sheet in direct contact with a second side of the web, opposite to the first side of the web, so that the web is sandwiched between the first and second release sheet. This is preferred when using a hot press to apply heat to the web, for example. Alternatively, the release surface can be the surface of a nip roller. This is preferred when using a calender, for example. It is preferred that heat is applied to the web by passing through the release surface. In other words, when heat is applied to at least one side of the web, the heat is to be applied to that side of the web which is in direct contact with the release surface.

[0044] Bringing the web into contact with a release surface facilitates preventing the formation of cracks or holes in the web or the shrinking thereof when increasing its transparency by applying the heat and the pressure to the web. If the release surface is a release sheet placed on top of the web, the release sheet can prevent the web from cooling down too quickly after applying heat to the web which may lead to shrinkage or the formation of holes and cracks in the web. In other words, a release sheet contacting the web can reduce the cooling rate of the web, preventing shrinking and the formation of holes and cracks in the web. Also, the use of a release sheet prevents the web from sticking to a surface from which heat is applied to the web.

[0045] The present invention further provides a use of a membrane described above for the fabrication of a packaging.

[0046] Using the membranes described above for the fabrication of a packaging has the technical effect that a packaging can be obtained which allows its content to be seen from the outside and which allows for a certain air permeability.BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Fig. 1a is a photograph showing a fibrous web made of PLA fibers that has shrunk and developed holes due to the application of heat and pressure with non-suitable conditions in a hot press. Fig. 1b is a photograph showing a fibrous web made of PLA fibers that has shrunk and developed cracks and holes due to the application of heat and pressure with non-suitable conditions in a calender. Fig. 2 is a photograph showing in the left half thereof a fibrous web made of PLA before it was heated and compressed to form a treated region, and in the right half thereof a membrane according to the present invention which was produced according to the present invention by applying heat and pressure to a fibrous web made of PLA fibers in a hot press. A handwritten note is placed underneath both samples to illustrate the transparency of both samples. Fig. 3 is a schematic representation of the arrangement of the fibrous web and two release sheets when using a hot-press for applying heat and pressure to the web. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0048] Any identical reference signs listed in different figures denote identical, corresponding or functionally similar parts.

[0049] The fibrous web of the membrane according to the present invention comprises thermoplastic polymer fibers. It is preferred that a proportion of thermoplastic polymer fibers by weight of all fibers of the web is 90% or more, preferably 95% or more, more preferably 99% or more. In other words, it may be preferred that all fibers in the web, except for negligible impurities, are made of thermoplastic polymers.

[0050] Thermoplastic polymers suitable to be used as thermoplastic polymer fibers in the fibrous web according to the present invention are, for example, Polyethylene Terephthalate (PET), Polymethyl Methacrylate (PMMA), Copolyester (Co-PET), Polyamides such as Nylon, Polyethylene (PE), Polyether Ether Ketone (PEEK) and Polypropylene (PP), for example. Preferably, the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers. Bio-sourced polymers suitable to be used as thermoplastic bio-sourced polymer fibers in the fibrous web according to the present invention are, for example, Polylactic Acid (PLA), Polybutylene Succinate (PBS), Polybutylene Succinate Adipate (PBSA), Polyhydroxyalkanoates (PHA), Polyhydroxybutyrate (PHB), Polyvinyl Alcohol (PVOH), Poly(tetramethylene adipate-coterephthalate) (PTAT). It is preferred that the fibrous web is a non-woven web, preferably a spunbond non-woven web. If the fibrous web is a spundbond non-woven web, of the above-mentioned suitable polymers, it is preferred to use PET, Co-PET, Polyamides such as Nylon, PE or PP. Of the listed suitable bio-sourced polymers, it is preferred to use PLA.

[0051] By applying heat and pressure to the web to form the treated region, the transparency of the web can be increased. In some embodiments, before applying the heat and the pressure to the web to form the treated region, the web has a transparency of at least 50 %, preferably of at least 60%, more preferably of at least 65%. According to the present invention, the treated region of the web which is formed by applying heat and pressure to the web, has a transparency of at least 85%, preferably at least 90%, more preferably at least 92%.

[0052] When applying heat and pressure to the web to form the treated region, heat and pressure may be applied to the entire web, so that the entire web forms the treated region. In this case the web does not comprise any region which is not the treated region, i.e. to which the heat and the pressure have not been applied. Alternatively, heat and pressure may be applied only to portions of the web so that the web comprises at least one region which is the treated region and at least one region which is not the treated region. Throughout the present disclosure, it is preferred that if the web comprises a region which is not the treated region, the web has in this region which is not the treated region the same properties or at least essentially the same properties as the web had in the treated region before heat and pressure have been applied thereto to form the treated region. Therefore, throughout the present disclosure, if a feature relates to a "region which is not the treated region", this feature may also apply to the treated region of the web before heat and pressure have been applied thereto to form the treated region, even if the entire web forms the treated region. To avoid unnecessary repetition, the expression "region which is not the treated region" is therefore used.

[0053] For illustrative purposes, the test results of measuring the transparency of one sample each of the treated region of a membrane made from two spunbond non-woven-webs of different basis weight and produced according to the present invention are shown below in Table 1. The spunbond non-woven webs of the samples 1 and 2 shown in the tables following below are made of Polylactic Acid (PLA) and have been heated to a heating temperature of 185 °C and compressed at a pressure of 275 kPa. Table 1Spunbond non-woven web made of PLA fibers (before heating and compressing) - sample 1Spunbond non-woven web of from PLA fibers (before heating and compressing) - sample 2Treated region (after heating and compressing) - sample 1.1Treated region (after heating and compressing) - sample 2.2Comparative example - extruded film made of PLABasis weight (g / m 2< )9027902725Transparency (%)65.2984.3694.6294.1197.42

[0054] As can be seen in Table 1 above, spunbond non-woven webs made of PLA fibers of two different basis weights were tested as sample 1 and 2. By applying heat and pressure to the web, a membrane according to the present invention can be obtained having an increased transparency in the treated region. Transparency values which are almost as high as that of an extruded PLA film can be obtained.

[0055] According to the present invention, the treated region of the web, i.e. the region of the web to which the heat and the pressure has been applied, has a void ratio of 10% or more and 80 % or less, preferably of 20 % or more and 75 % or less, more preferably of 30 % or more and 70 % or less. The void ratio is a measure of the amount of voids in treated region, i.e. the heated and compressed web, and is calculated with Formula 1 stated above in the summary of the invention based on a ratio of the apparent density to the theoretical density of the polymer. For example, the theoretical density ρ polymer of the bio-sourced polymer PLA is 1240 kg / m 3< and can be taken from the product data sheet or determined in the literature. The values for the theoretical density ρ polymer of the individual polymers on which the present disclosure is based are given in Table 2 below. Table 2PolymerTheoretical density ρ polymer in kg / m 3< Copolyester (Co-PET)1200-1400Poly(tetramethylene adipate-coterephthalate) (PTAT)1220Nylon1060-1180Nylon 61130Nylon 661140Polybutylene Succinate (PBS)1260Polybutylene Succinate Adipate (PBSA)1230Polyether ether ketone (PEEK)1320Polyethylene (PE)935Polyethylene terephthalate (PET)1560Polyhydroxyalkanoate (PHA)1000-1300Polyhydroxybutyrate (PHB)1250Polylactic acid (PLA)1240Polymethyl methacrylate (PMMA)1185Polypropylene (PP)895Polyvinyl Alcohol (PVOH)1250

[0056] The value of the theoretical density of Copolyester depends on its chemical composition, as Copolyesters (Co-PET, or CoPET) are a polymer family with a range of densities. PTAT is one example, among others, of a Copolyester. The value of the theoretical density of Nylon depends on its chemical composition, as Nylons are a polymer family with a range of densities. Nylon 6 and Nylon 66 are two examples, among others, of Nylons. The exact value of the theoretical density of Polyhydroxyalkanoate (PHA) depends on its chemical composition, as Polyhydroxyalkanoates (PHAs) are a polymer family with a range of densities. PHB is one example, among others, of a PHA Polymer.

[0057] The apparent density ρ web,h+c of the treated region, in which the fibrous web has been heated and compressed, can be measured in a sample and calculated according to the following formula: ρ web , h + c = m V = m A × t = G × 1 t

[0058] In Formula 2, m is the mass of the sample in kg, V ist the volume of the sample in m 3< , A is the area of the sample in m 2< , t is the thickness of the sample in m and G is the basis weight in kg / m 2< , also called Grammage, of the sample. If the Grammage of the fibrous web before heat and pressure has been applied thereto to form the treated region is known, the apparent density ρ web,h+c can be calculated relatively easy by merely measuring the thickness of the treated region, for example according to the standard TAPPI / ANSI T 411 om-21.

[0059] Hence, if the apparent density of a test sample comprising only thermoplastic bio-sourced polymer fibers made from PLA is 630 kg / m 3< , for example, the resulting void ratio of the treated region is 49.19 %.

[0060] For illustrative purposes, the test results of measuring the thickness according to the standard TAPPI / ANSI T 411 om-21 and calculating the apparent density of one sample each of the treated region of a membrane made from two spunbond non-woven webs of different basis weight and produced according to the present invention are shown below in Table 3 along with the respective calculated void ratio. Table 3Spunbond non-woven web made of PLA fibers (before heating and compressing) - sample 1Spunbond non-woven web of from PLA fibers (before heating and compressing) - sample 2Treated region (after heating and compressing) - sample 1.1Treated region (after heating and compressing) - sample 2.2Basis weight G (g / m 2< )90279027Thickness t (µm)378.4136.2142.269.8Apparent Density ρ web,h+c (kg / m 3< )237.8198.2632.9386.8Void ratio vr (%)80.8284.0648.9568.80

[0061] As can be seen in Table 2 above, spunbond non-woven webs made of PLA fibers of two different basis weights were tested as sample 1 and 2. By applying heat and pressure to the web, the initial void ratio of the spunbond non-woven webs is lowered but nevertheless maintained to some extent. The void ratio is a prerequisite for the treated region's permeability to air. In other words, according to the invention, the transparency of the tested samples of spunbond non-woven webs was increased, while the void ratio thereof was reduced in a controlled manner, so that a certain amount of voids remained in the treated region allowing for a permeability to air.

[0062] As stated in the summary of the invention above, there is a certain correlation between the air permeability and the porosity or void ratio of the membrane, but they are not necessarily the same. A certain number of voids is a prerequisite for air permeability. However, the exact arrangement and size of these voids determines how permeable the membrane is to air. The fibrous web is inherently permeable to air and has a certain porosity or void ratio. By applying heat and pressure to the web, this air or gas permeability is reduced, but it is intended and preferred to maintain a certain air permeability.

[0063] The use of a fibrous web in the membrane has, in comparison to the use of an extruded film, as it is known from the prior art, the particular advantage that the air permeability of the membrane can be adjusted in a controlled and targeted manner. The term air permeability, as used in the present disclosure, refers to the measurement result of the determination of air permeability according to the standard ISO 9237:2005. The extruded film, as it is known from the prior art, is typically almost or completely impermeable for air. The fibrous web, on the other hand, inherently has a certain air permeability. When heat and pressure are applied to the fibrous web to form the treated region, the air permeability of the web in that region is reduced but may be maintained to some extent. This allows the air permeability in this area to be specifically adjusted. For example, by increasing either temperature, pressure or time, the air permeability of the treated region can be reduced. Incidentally, it may be possible to conclude that the treated region in the membrane originates from a fibrous web and not from an extruded film. For example, by observing under a microscope, void areas can be detected in the treated region which originate from voids in the structure of the fibrous web before applying heat thereto. Such voids originating from a fibrous web are typically irregular in size, shape and distribution, whereas voids obtained by perforating an extruded film, for example, are more regular at least in shape, and often also more regular in size and distribution as they are typically provided in patterns.

[0064] As described above, in a particular embodiment, the web further comprises a region which is not the treated region. The web has in the region which is not the treated region a first air permeability and in the treated region a second air permeability, wherein the second air permeability is 0.1 % or more and 90 % or less, preferably 0.5 % or more and 80 % or less, more preferably 1 % or more and 70 % or less, of the first air permeability. In a first non-limiting example, for some specific applications, the second air permeability may be 0.1 % or more and 5 % or less of the first air permeability or even 0.1 % or more and 2 % or less of the first air permeability. A specific application in which such limited but nevertheless secured air permeability is desired is the packaging of fruits and vegetables. These goods require a little exchange of air, but the exchange may not be too high to prevent the fruit or vegetables from spoiling prematurely. In a second non-limiting example, for other specific applications, the second air permeability may be 40 % or more and 60 % or less of the first air permeability. A specific application in which such reduces but still relatively great air permeability is desired is the performance of filtrations, and / or when it is needed to keep regions wherein an air passage is much higher than in other regions.

[0065] In regions of the web, which are not the treated region, the web can have an air permeability of 200 L / m 2< / s at 200 Pa or more and 4000 L / m 2< / s at 200 Pa or less, preferably of 500 L / m 2< / s at 200 Pa or more and 3000 L / m 2< / s at 200 Pa or less, more preferably of 800 L / m 2< / s at 200 Pa or more and 2000 L / m 2< / s or less. Depending on the temperature and the pressure at which the heat and the pressure are applied to the web to form the treated region, the air permeability of the treated region can be reduced by 20 % and up to 99.9 %, depending on the desired target air permeability. The treated region of the web preferably has an air permeability of 20 L / m 2< / s at 200 Pa, or more and 3200 L / m 2< / s at 200 Pa, or less. Preferably, the air permeability of the treated region is 1000 L / m 2< / s at 200 Pa, or more and 3200 L / m 2< / s at 200 Pa, or less; more preferably 1200 L / m 2< / s at 200 Pa, or more and 2400 L / m 2< / s at 200 Pa, or less. In other words, the air permeability of the treated region of the web can be adjusted so as to lie within a specific interval, wherein the treated region of the web has an air permeability of 5 L / m 2< / s at 200 Pa or more, preferably of 20 L / m 2< / s at 200 Pa or more, more preferably of 50 L / m 2< / s at 200 Pa or more, even more preferably of 100 L / m 2< / s at 200 Pa or more, still more preferably of 200 L / m 2< / s at 200 Pa and most preferably of 500 L / m 2< / s at 200 Pa or more. Yet, the air permeability of the treated region of the web is 3200 L / m 2< / s at 200 Pa or less, preferably 2500 L / m 2< / s at 200 Pa or less, more preferably 2000 L / m 2< / s at 200 Pa, even more preferably 1500 L / m 2< / s at 200 Pa and most preferably 1000 L / m 2< / s at 200 Pa.

[0066] For illustrative purposes, the test results of measuring air permeability of three samples each of the treated region of a membrane made from two spunbond non-woven webs of different basis weight and produced according to the present invention are shown below in Table 4. Table 4Spunbond non-woven web of from PLA fibers (before heating and compressing) - sample 1Treated region (after heating and compressing) - sample 1.1Treated region (after heating and compressing) - sample 1.2Basis weight (g / m 2< )909090-Air permeability (L / m 2< / s at 200 Pa)9102032-Spunbond non-woven web made of PLA fibers (before heating and compressing) - sample 2Treated region (after heating and compressing) - sample 2.1Treated region (after heating and compressing) - sample 2.2Treated region (after heating and compressing) - sample 2.3Basis weight (g / m 2< )27272727Air permeability (L / m 2< / s at 200 Pa)36122368126563

[0067] As can be seen in Table 4 above, spunbond non-woven webs made of PLA fibers of two different basis weights were tested as sample 1 and 2. By applying heat and pressure to the web, the initial air permeability of the spunbond non-woven webs is lowered but nevertheless maintained to a desired extent.

[0068] In the membrane, the web may further comprise a region which is not the treated region. The region of the web, which is not the treated region, usually has a tear resistance which is higher than the tear resistance of the treated region. In other words, by applying the heat and pressure to the web to form the treated region, the strength of the web in the treated region is lowered, particularly the tear resistance is lowered. It can therefore be advantageous to provide the membrane with a region of the web that is not the treated region and thus has a comparatively high tear resistance. This facilitates handling of the membrane, and particularly the production of the membrane in a roll-to-roll process.

[0069] For illustrative purposes, the test results of measuring tear resistance of two samples of the treated region of a membrane made from two spunbond non-woven webs of different basis weight and produced according to the present invention is shown below in Table 5. Table 5Spunbond non-woven web made of PLA fibers (before heating and compressing) - sample 1Spunbond non-woven web of from PLA fibers (before heating and compressing) - sample 2Treated region (after heating and compressing) - sample 1.1Treated region (after heating and compressing) - sample 2.2Basis weight (g / m 2< )90279027Tear resistance (mN)5111865.620345

[0070] As can be seen in Table 4 above, spunbond non-woven webs made of PLA fibers of two different basis weights were tested as sample 1 and 2. By applying heat and pressure to the web, the initial tear resistance of the spunbond non-woven webs is lowered.

[0071] In the membrane according to the present invention, the tear resistance in a region of the web which is not the treated region is preferably 200 mN or more, more preferably 500 mN or more and even more preferably 800 mN. In exceptional cases, it might be preferred that the tear resistance in a region of the web which is not the treated region is 1000 mN or more.

[0072] According to the present invention, a membrane, preferably the membrane described above, can be produced by preparing a fibrous web comprising thermoplastic polymer fibers, and applying heat and pressure to at least one region of the web to form a treated region. When applying the heat to the at least one region of the web, the web is heated from ambient temperature to a heating temperature within a heating time period of 0.5 seconds or more and 12 seconds or less, preferably of 1 second or more and 6 seconds or less. The heating temperature is at least as high as a melting temperature of the thermoplastic polymer fibers, preferably at least 5 °C or more but not more than 20 °C above the melting temperature, more preferably at least 10 °C or more but not more than 15 °C above the melting temperature.

[0073] Both the heating time and the heating temperature have a great influence on whether the fibrous web's transparency can be increased to a desired level without causing the fibrous web to shrink or to get holes or cracks. With the described parameters for the heating time and the heating temperature, it is possible to obtain a membrane having a treated region with increased transparency and without any distortion or shrinkage, cracks or holes.

[0074] Figs. 1a and 1b each exemplarily show a spunbond non-woven web to which heat and pressure has been applied but with non-suitable conditions causing the web to shrink and to develop cracks and holes therein. Fig. 2, on the other hand, shows a membrane which was produced according to the present invention. The depicted membrane was produced by applying heat and pressure to a fibrous web made of PLA fibers in a hot press.

[0075] Generally, the features and properties described above regarding the membrane as such also apply to the method of producing a membrane. They are not described again for the sole reason of avoiding unnecessary repetition and to improve the comprehensibility of the present disclosure. For example, in the method of the present disclosure, it may be preferred that all fibers in the web, except for negligible impurities, are made of thermoplastic polymers. Likewise, it may be preferred that the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers, as mentioned above.

[0076] In the method of producing a membrane, when heat is applied to the web for forming the treated region, it is preferred that heat is applied only from one side of the web. It may be preferred that heat is applied by means of a hot press or a calender or an ultrasonic horn. When heat is applied by means of a hot press, it may thus be preferred that only one surface of the two surfaces of a hot press is heated. Similarly, when heat is applied by means of a calender, it may thus be preferred that only one nip roller of a pair of nip rollers is heated.

[0077] It has been surprisingly found that by applying heat to only one side of the web, the treated region can be formed and the transparency of the web can be increased in that region without damaging the web, in particular without the web getting cracks or holes or shrinking. When using a hot press or a calender, the heat and the pressure can be applied simultaneously and formation of cracks or holes can be prevented.

[0078] The membrane can be produced in a plurality of ways. It can be produced in a discontinuous process, in a continuous process or, as kind of a hybrid of the two, in a stop-and-go process.

[0079] If the membrane is manufactured in a discontinuous process, the membrane can be manufactured in a hot press, for example. This may be done by bringing the web into contact with a release surface having a non-stick coating and placing the release surface in contact with one of the two surfaces of the hot press, so that at least a part of the release surface is sandwiched between the web and said one of the two surfaces of the hot press. The non-stick coating preferably comprises Polytetrafluoroethylene (PTFE) and / or silicone. The release surface may comprise a release sheet. Preferably two release sheets are used which are preferably arranged so that the web is interposed between the two release sheets. This is exemplarily shown in Fig. 3. In Fig. 3, the web 10 is interposed between two release sheets 23. The two release sheets 23 and the web 10 interposed therebetween are arranged between the two surfaces 21, 22 of the hot press 20. Preferably, only one surface 21 of the hot press 20 is heated. The heated surface 21 of the hot press 20 comes into non-direct contact with the web 10 by having a release sheet 23 interposed between the heated surface 21 and the web 10. As a next step, the other of the two surfaces of the hot press is brought into non-direct contact with the web and pressure and heat are applied to the web to form the treated region. When heat and pressure are applied to the web by means of a hot press, the pressure may be relatively low. Preferably, the pressure which is applied to the web when forming the treated region is 60 kPa or more and 1500 kPa or less; more preferably 120 kPa or more and 900 kPa or less, even more preferably 170 kPa or more and 600 kPa or less.

[0080] If the membrane is manufactured in a stop-and-go process, the membrane can still be manufactured using a hot press as described above, for example. In this case, the web may be supplied to the hot press by unwinding the web from a roll. While heat and pressure are applied to the web by means of the hot press, the conveying of the web from the roll to the hot press is stopped. After heat and pressure have been applied to the web, the web is further conveyed, and heat and pressure can be applied to a further region of the web.

[0081] If the membrane is manufactured in a continuous process, the membrane can be manufactured by means of a calender, for example. When heat is applied to the web by means of a calender, the release surface is preferably formed on the surface of a heated nip roller of a pair of nip rollers. By using a release surface, unwanted sticking of the web to the nip roller can be prevented. If the membrane is manufactured by means of a calender, the web may be supplied to the calender by unwinding the web from a roll. Generally, three different types of calenders may be used. As a first example, a calender may be used in which the web is passed between a pair of nip rollers of which preferably one has a hardness higher than the hardness of the other and which apply a pressure or linear load onto the web. In this case, the linear load which is applied to the web by the pair of nip rollers when forming the treated region is 150 kN / m or less, preferably 100 kN / m or less, more preferably 50 kN / m or less, even more preferably 30 kN / m or less and most preferably 20 kN / m or less but preferably no less than 100 kN / m, more preferably no less than 15 kN / m. As a second example, a calender may be used which provides a temperature gradient in the passing web, as described below. Also in this case, the linear load which is applied to the web by the pair of nip rollers when forming the treated region is 150 kN / m or less, preferably 100 kN / m or less, more preferably 50 kN / m or less, even more preferably 30 kN / m or less and most preferably 20 kN / m or less but preferably no less than 10 kN / m, more preferably no less than 15 kN / m. As a third example, a belt-calender system may be used in which the web is passed between one nip roller and a belt. In this case, the nip roller has a comparatively high hardness and the belt may be made of metal, fabric or latex. In this case, the linear load which is applied to the web by the nip roller and the belt when forming the treated region is 30 kN / m or less, preferably 20 kN / m or less, more preferably 10 kN / m or less and even more preferably 5 kN / m or less.

[0082] As described further above, it was surprisingly found that the speed at which the web is heated or cooled has an influence on whether the web's transparency can be increased to a desired level without causing the fibrous web to shrink or to get holes or cracks. The speed at which the web is heated or cooled can be adjusted by various measures, depending on the way the membrane is produced. For example, if the membrane is produced in a discontinuous process using a hot press, the speed at which the web is heated may be controlled by adjusting the temperature of the heated surface of the hot press and by adjusting the contact time of that surface and the web. Alternatively, the speed may be controlled by varying the temperature of the heated surface of the hot press during the contact time instead of heating the heated surface of the hot press to a constant temperature. As another example, if the membrane is produced in a continuous process using a calender, a plurality of pairs of nip rollers may be used and the web is passed successively through each of the pairs of nip rollers. In each pair of the plurality of nip rollers, at least one nip roller may be heated. By heating the heated roller of a first pair of nip rollers to a temperature which is higher or lower than the temperature of the heated roller of a second pair of nip rollers, a temperature gradient in the web along the conveying direction of the web can be reached and the speed at which the web is heated or cooled can be adjusted. Alternatively, if the membrane is produced in a continuous process using a calender only one pair of nip rollers may be used of which at least one is heated. By arranging additional heaters such as infrared heaters or hot air blowers before or after the pair of nip rollers, the speed at which the web is heated or at which it cools down may be adjusted.

[0083] In the process of manufacturing a membrane, preferably the membrane described above, it may be generally preferred that after applying heat to the web, the web is held under tension to prevent shrinkage of the web. In other words, the edges of the web to which heat is applied can be held in place after the application of heat so that the web does not shrink when it cools. It may be preferable to keep the web under tension and / or hold its edges also during the application of heat. It may also be preferred that the web is held under tension until the web has cooled from the heating temperature to essentially room temperature. The web can be held under tension by various means. If a hot press is used to apply heat to the web, the web can be held under tension by applying an appropriate pressure to the web by means of the hot press. If a calender is used to apply heat to the web, the web can be held under tension by arranging or controlling the rollers accordingly or by providing additional deflection means or guide means.

[0084] The membrane of the present disclosure and the method of producing a membrane of the present disclosure is particularly advantageous when used for the fabrication of packaging for foods and beverages, such as in beverage cartons, tea bags, of packaging for loose fruit such as berries, but also of the packaging of products such as sports equipment or similar products. For example, the membrane can be used to make various kinds of packaging with tailored properties to serve different requirements, especially regarding a high transparency, a controlled air and / or gas permeability or air and / or gas tightness, a heat-sealability, some patterns, and a minimal mechanical resistance.

Examples

Embodiment Construction

[0048]Any identical reference signs listed in different figures denote identical, corresponding or functionally similar parts.

[0049]The fibrous web of the membrane according to the present invention comprises thermoplastic polymer fibers. It is preferred that a proportion of thermoplastic polymer fibers by weight of all fibers of the web is 90% or more, preferably 95% or more, more preferably 99% or more. In other words, it may be preferred that all fibers in the web, except for negligible impurities, are made of thermoplastic polymers.

[0050]Thermoplastic polymers suitable to be used as thermoplastic polymer fibers in the fibrous web according to the present invention are, for example, Polyethylene Terephthalate (PET), Polymethyl Methacrylate (PMMA), Copolyester (Co-PET), Polyamides such as Nylon, Polyethylene (PE), Polyether Ether Ketone (PEEK) and Polypropylene (PP), for example. Preferably, the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers. Bio-sourced...

TECHNICAL FIELD

[0001] The present disclosure relates to a membrane from fibrous web and to a method of producing a membrane from fibrous web.PRIOR ART

[0002] In the packaging industry, there is a wide range of applications for membranes made from thermoplastic polymers. Typically, such membranes are used as heat sealable layers on a substrate to form a laminate used in the packaging of foods and beverages, such as in beverage cartons, tea bags, the packaging of loose fruit such as berries, but also in the packaging of products such as sports equipment or similar products. Such membrane can also be used alone, for example, to form a viewing window in a packaging that allows the contents of the packaging to be seen. Hence, it is desired that such membranes have a certain degree of transparency in order to make the content of the packaging visible and / or heat sealing properties in order to easily fuse the membrane to a substrate or neighboring part of a packaging.

[0003] Conventionally, the membranes are manufactured by extruding a film made of a thermoplastic polymer, typically polyesters like polyethylene terephthalate (PET). The use of such conventional membrane in laminates is known from WO 2008 092 328 A1 and US 9,393,763 B2, for example. One main disadvantage of these conventional membranes is a lack of air permeability caused by the extrusion. Furthermore, in the recent past, the sustainability requirements of packaging materials have increased, so it is desired to obtain a membrane that is recyclable, is made from renewable resources, is biodegradable and / or is compostable.

[0004] Hence, it is an object of the present invention to provide a membrane having a certain transparency, air permeability and is environmentally friendly.SUMMARY OF THE INVENTION

[0005] The above object is solved by a membrane according to claim 1 and by a method of producing a membrane according to claim 9. Further preferred embodiments are set out in the dependent claims.

[0006] The present invention provides a membrane comprising a fibrous web comprising thermoplastic polymer fibers. The web comprises at least one treated region which has been heated and compressed. Said treated region has a transparency of at least 85 %, preferably at least 90 %, more preferably at least 92 %, and a void ratio of 10 % or more and 80 % or less, preferably 20 % or more and 75 % or less, more preferably 30 % or more and 70 % or less.

[0007] Defining that the web comprises a treated region includes a case in which the entire web forms the treated region but also includes a case in which the web comprises at least one region which is the treated region and at least one region which is not the treated region.

[0008] The void ratio as used in the present disclosure is a measure of the amount of voids in the treated region of the web and is calculated by the following formula: vr % = ρ polymer − ρ web , h + c ρ polymer × 100 % = 1 − ρ web , h + c ρ polymer × 100 %

[0009] In Formula 1, ρ polymer is the density (mass per volume) of the pure thermoplastic polymer which is a material property that can be taken from the material data sheet or can be determined in the literature. In other words, ρ polymer relates to the density of the polymer itself without any air inclusions or voids. ρ web,h+c is the density of the treated region of the web, i.e. the heated and compressed fibrous web, also called bulk density or apparent density, and can be determined by dividing the mass of the heated and compressed web by the total volume the heated and compressed web occupies, also called bulk volume.

[0010] If the fibrous web comprises thermoplastic polymer fibers of more than one type of polymer, the density of each polymer type is considered for the determination of ρ polymer by weighting the arithmetic mean according to the respective mass ratio of the polymer type in the fibrous web.

[0011] Throughout the present disclosure, all transparency measurements were carried out according to the standard DIN 53147:1993-01 using a test equipment model BCMTS M Type 40605, with Touchscreen M software of the company Frank-PTI. This test equipment uses a Konica Minolta CM-3630 Spectrophotometer.

[0012] By heating and compressing the fibrous web comprising thermoplastic polymer fibers, the transparency of the fibrous web can be increased. By increasing the transparency of the web, the membrane can be used in a packaging to make the content of the packaging visible. Furthermore, by heating and compressing a fibrous web, in contrast to an extruded film, a porosity in the resulting treated region can be achieved which allows for a certain air permeability thereof. Setting the void ratio as described has the technical effect, that a desired air permeability of the material can be achieved.

[0013] In a preferred embodiment, a proportion of thermoplastic polymer fibers by weight of all fibers of the web is 90% or more, preferably 95% or more, more preferably 99% or more.

[0014] The more fibers in the fibrous web are made of thermoplastic polymers, the more uniform the treated region will be when the web is heated and compressed. It may be preferred that all fibers in the web, except for negligible impurities, are made of thermoplastic polymers.

[0015] In a preferred embodiment, the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers which are preferably one or more of Polylactic Acid (PLA) fibers, Polyhydroxyalkanoate (PHA) fibers and Polyhydroxybutyrate (PHB) fibers.

[0016] The term bio-sourced polymer as used in the present disclosure refers to a polymer the monomers of which can be derived or obtained from molecules that are producible by the cells of living organisms. The fibrous web may contain fibers of only one type of polymer, preferably one type of bio-sourced polymer; or the fibrous web may contain a blend of fibers, i.e. fibers of different kinds of polymers, preferably different kinds of bio-sourced polymers.

[0017] Using thermoplastic bio-sourced polymer fibers in the fibrous web has the technical effect that the membrane can be more sustainable because bio-sourced polymers are renewable raw materials rather than fossil ones being based on petroleum. Furthermore, depending on the choice of bio-sourced polymer, the web can be biodegradable and even compostable. When using PLA fibers, PHA fibers and / or PHB fibers, for example, the web can be biodegradable and / or compostable, for example according to standard EN 13432.

[0018] In a preferred embodiment, the web is a non-woven web, preferably a spunbond non-woven web.

[0019] Providing the web as a non-woven web, preferably a spunbond non-woven web has the technical effect that the web can be produced in a continuous process of spinning and dispersing fibers, and thereby in a cost-efficient manner.

[0020] In a preferred embodiment, the web further comprises a region which is not the treated region. The web has in the region which is not the treated region a first air permeability and in the treated region a second air permeability, wherein the second air permeability is 0.1 % or more and 90 % or less, preferably 0.5 % or more and 80 % or less, more preferably 1 % or more and 70 % or less, of the first air permeability.

[0021] There is a certain correlation between the air permeability and the porosity or void ratio of the membrane, but they are not necessarily the same. A certain number of voids is a prerequisite for air permeability. However, the exact arrangement and size of these voids determines how permeable the membrane is to air. The fibrous web is inherently permeable to air and has a certain porosity or void ratio. By applying heat and pressure to the web, this air permeability is reduced, but it is intended and preferred to maintain a certain air permeability.

[0022] Providing a membrane that has a region that is the treated region and a region that is not the treated region has the technical effect that the membrane can have the treated region in places where increased transparency is desired and can have a region that is not the treated region in other places where increased transparency is not required. The region that is not the treated region has a higher strength, in particular a tear resistance, than the treated region and therefore facilitates the handling of the membrane in this region. Preserving part of the original air permeability of the web in the treated region has the technical effect that the membrane in the treated region can have both increased transparency and a certain air permeability. This makes the membrane particularly suitable for packaging food products for which a certain air permeability is desired, like fresh fruit, for example.

[0023] In a preferred embodiment, the web further comprises a region which is not the treated region. This region can be the region which is not the treated region of the preferred embodiment described above, or it can be another region which is not the treated region. The region which is not the treated region has a transparency which is lower than the transparency of the treated region, and which is preferably 80 % or less, more preferably 75 % or less, even more preferably 70 % or less.

[0024] As stated above, providing a membrane that has a region that is the treated region and a region that is not the treated region has the technical effect that the membrane can have the treated region in places where increased transparency is desired and can have a region that is not the treated region in other places where increased transparency is not required. The region that is not the treated region has a higher strength, in particular a tear resistance, than the treated region and therefore facilitates the handling of the membrane in this region. With this preferred embodiment, the transparency of the web in the treated region can be increased by 25 % or more, preferably by 33 % or more, more preferably by 42 % or more. In other words, in the region of the web to which heat and pressure are applied to form the treated region, by applying heat and pressure to the web, the web's transparency can be increased by 25 % or more, preferably by 33 % or more, more preferably by 42 % or more compared to the transparency of the web in this region before the heat and the pressure are applied to form the treated region or compared to a region of the web which is not the treated region.

[0025] In a preferred embodiment, the web further comprises a region which is not the treated region. This region can be the region which is not the treated region of the two previously described preferred embodiments, or it can be another region which is not the treated region. The membrane has, in a sample region comprising at least a part of said region which is not the treated region, a tear resistance of 200 mN or more, preferably 500 mN or more, more preferably 800 mN or more.

[0026] Throughout the present disclosure, all tear resistance values relate to the standard ISO 1974:2012 and hence all tear resistance measurements were carried out according to the standard ISO 1974:2012.

[0027] The technical effect of using a web with such a tear resistance is that the web can be processed when producing the membrane in a roll-to-roll process without the web tearing when unrolling or deflecting the web.

[0028] In a preferred embodiment, the treated region of the web is a contiguous area that has a size of 0.5 cm 2< or more, preferably 1 cm 2< or more, more preferably 2 cm 2< or more, even more preferably 5 cm 2< or more.

[0029] Thus, the treated region of the web is not just one or more dot- or line-like areas that serve merely to attach the membrane to something else, but said treated region is a contiguous area that is large enough to be a functional surface with increased transparency and / or defined air permeability.

[0030] The present invention provides a method of producing a membrane, preferably the membrane according to any one of claims 1 to 8. The method comprises preparing a fibrous web comprising thermoplastic polymer fibers, and applying heat and pressure to at least one region of the web to form a treated region. When applying the heat to the at least one region of the web, the web is heated from ambient temperature to a heating temperature within a heating time period of 0.5 seconds or more and 12 seconds or less, preferably of 1 second or more and 6 seconds or less. The heating temperature is at least as high as a melting temperature of the thermoplastic polymer fibers, preferably at least 5 °C or more but not more than 20 °C above the melting temperature, more preferably at least 10 °C or more but not more than 15 °C above the melting temperature.

[0031] In other words, the heating temperature preferably lies in an interval of 5 °C or more and 20 °C or less above the melting temperature of the thermoplastic polymer fibers. If more than one type of thermoplastic polymer fibers is used, the interval refers to the highest melting temperature of the different types of thermoplastic polymer fibers. The term ambient temperature as used in the present disclosure, refers to a temperature of 15 to 25 °C, preferably 18 to 22 °C, most preferably 20 °C. The ambient temperature as the starting temperature for heating does not have a particularly strong influence on the quality of the treated region, as long as the ambient temperature lies within said interval.

[0032] Both the heating time and the heating temperature have a great influence on whether the fibrous web's transparency can be increased to a desired level without causing the fibrous web to shrink or to get holes or cracks. With the described parameters for the heating time and the heating temperature, it is possible to obtain a membrane having a treated region with increased transparency and without any distortion or shrinkage, cracks or holes.

[0033] In a preferred embodiment, a proportion of thermoplastic polymer fibers by weight of all fibers of the web is 90% or more, preferably 95% or more, more preferably 99% or more. In addition, or alternatively, the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers which are preferably one or more of Polylactic Acid (PLA) fibers, Polyhydroxyalkanoate (PHA) fibers and Polyhydroxybutyrate (PHB) fibers.

[0034] The more fibers in the fibrous web are made of thermoplastic polymers, the more uniform the treated region will be when the web is heated and compressed. It may be preferred that all fibers in the web, except for negligible impurities, are made of thermoplastic polymers. Using thermoplastic bio-sourced polymer fibers in the fibrous web has the technical effect that the membrane can be more sustainable because bio-sourced polymers are renewable raw materials rather than fossil ones being based on petroleum. Furthermore, depending on the choice of bio-sourced polymer, the web can be biodegradable and even compostable. When using PLA fibers, PHA fibers and / or PHB fibers, for example, the web can be biodegradable and / or compostable. For example, the bio-sourced polymer may be made biodegradable according to standard EN 13432.

[0035] In a preferred embodiment, when applying the heat to the at least one region of the web, the heat is applied only from one side of the web, and / or the heat is applied by means of a hot press, a calender or an ultrasonic horn.

[0036] It has been surprisingly found that by applying heat to only one side of the web, the treated region can be formed and the transparency of the web can be increased in that region without damaging the web, in particular without the web getting cracks or holes or shrinking. When using a hot press or a calender, the heat and the pressure can be applied simultaneously and formation of cracks or holes can be prevented.

[0037] In a preferred embodiment, when applying heat and pressure to the at least one region of the web, the heat and the pressure are applied so as to increase transparency of the web to a transparency of 85 % or more, preferably 90 % or more, more preferably 92 % or more.

[0038] Providing the membrane with a high level of transparency makes it possible to use the membrane for applications in which, for example, the inside of a package is to be visible from the outside.

[0039] In a preferred embodiment, the method further comprises cooling the web. The web is cooled from heating temperature to ambient temperature within a cooling time period of 0.5 seconds or more and 120 seconds or less, preferably of 1 second or more and 100 seconds of less.

[0040] The ambient temperature as the target temperature for cooling does not have a particularly strong influence on the quality of the web with increased transparency, as long as the ambient temperature lies within the above-mentioned interval of 15 to 25 °C, preferably 18 to 22 °C, most preferably 20 °C.

[0041] Surprisingly, it turned out that the speed at which the web is cooled has an influence on whether the treated region in which the web's transparency is increased can be formed without forming cracks or holes or shrinking. With the above values, it is possible to form said treated region while preventing it from shrinking, cracking or forming holes.

[0042] In a preferred embodiment, the method further comprises bringing the web in contact with a release surface having a non-stick coating prior to applying heat and pressure to the web, the non-stick coating preferably comprising Polytetrafluoroethylene (PTFE) and / or silicone.

[0043] The release surface can comprise a first release sheet in direct contact with a first side of the web so that the release sheet is sandwiched between the first side of the web and a heating surface. Naturally, the release surface can comprise a second release sheet in direct contact with a second side of the web, opposite to the first side of the web, so that the web is sandwiched between the first and second release sheet. This is preferred when using a hot press to apply heat to the web, for example. Alternatively, the release surface can be the surface of a nip roller. This is preferred when using a calender, for example. It is preferred that heat is applied to the web by passing through the release surface. In other words, when heat is applied to at least one side of the web, the heat is to be applied to that side of the web which is in direct contact with the release surface.

[0044] Bringing the web into contact with a release surface facilitates preventing the formation of cracks or holes in the web or the shrinking thereof when increasing its transparency by applying the heat and the pressure to the web. If the release surface is a release sheet placed on top of the web, the release sheet can prevent the web from cooling down too quickly after applying heat to the web which may lead to shrinkage or the formation of holes and cracks in the web. In other words, a release sheet contacting the web can reduce the cooling rate of the web, preventing shrinking and the formation of holes and cracks in the web. Also, the use of a release sheet prevents the web from sticking to a surface from which heat is applied to the web.

[0045] The present invention further provides a use of a membrane described above for the fabrication of a packaging.

[0046] Using the membranes described above for the fabrication of a packaging has the technical effect that a packaging can be obtained which allows its content to be seen from the outside and which allows for a certain air permeability.BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Fig. 1a is a photograph showing a fibrous web made of PLA fibers that has shrunk and developed holes due to the application of heat and pressure with non-suitable conditions in a hot press. Fig. 1b is a photograph showing a fibrous web made of PLA fibers that has shrunk and developed cracks and holes due to the application of heat and pressure with non-suitable conditions in a calender. Fig. 2 is a photograph showing in the left half thereof a fibrous web made of PLA before it was heated and compressed to form a treated region, and in the right half thereof a membrane according to the present invention which was produced according to the present invention by applying heat and pressure to a fibrous web made of PLA fibers in a hot press. A handwritten note is placed underneath both samples to illustrate the transparency of both samples. Fig. 3 is a schematic representation of the arrangement of the fibrous web and two release sheets when using a hot-press for applying heat and pressure to the web. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0048] Any identical reference signs listed in different figures denote identical, corresponding or functionally similar parts.

[0049] The fibrous web of the membrane according to the present invention comprises thermoplastic polymer fibers. It is preferred that a proportion of thermoplastic polymer fibers by weight of all fibers of the web is 90% or more, preferably 95% or more, more preferably 99% or more. In other words, it may be preferred that all fibers in the web, except for negligible impurities, are made of thermoplastic polymers.

[0050] Thermoplastic polymers suitable to be used as thermoplastic polymer fibers in the fibrous web according to the present invention are, for example, Polyethylene Terephthalate (PET), Polymethyl Methacrylate (PMMA), Copolyester (Co-PET), Polyamides such as Nylon, Polyethylene (PE), Polyether Ether Ketone (PEEK) and Polypropylene (PP), for example. Preferably, the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers. Bio-sourced polymers suitable to be used as thermoplastic bio-sourced polymer fibers in the fibrous web according to the present invention are, for example, Polylactic Acid (PLA), Polybutylene Succinate (PBS), Polybutylene Succinate Adipate (PBSA), Polyhydroxyalkanoates (PHA), Polyhydroxybutyrate (PHB), Polyvinyl Alcohol (PVOH), Poly(tetramethylene adipate-coterephthalate) (PTAT). It is preferred that the fibrous web is a non-woven web, preferably a spunbond non-woven web. If the fibrous web is a spundbond non-woven web, of the above-mentioned suitable polymers, it is preferred to use PET, Co-PET, Polyamides such as Nylon, PE or PP. Of the listed suitable bio-sourced polymers, it is preferred to use PLA.

[0051] By applying heat and pressure to the web to form the treated region, the transparency of the web can be increased. In some embodiments, before applying the heat and the pressure to the web to form the treated region, the web has a transparency of at least 50 %, preferably of at least 60%, more preferably of at least 65%. According to the present invention, the treated region of the web which is formed by applying heat and pressure to the web, has a transparency of at least 85%, preferably at least 90%, more preferably at least 92%.

[0052] When applying heat and pressure to the web to form the treated region, heat and pressure may be applied to the entire web, so that the entire web forms the treated region. In this case the web does not comprise any region which is not the treated region, i.e. to which the heat and the pressure have not been applied. Alternatively, heat and pressure may be applied only to portions of the web so that the web comprises at least one region which is the treated region and at least one region which is not the treated region. Throughout the present disclosure, it is preferred that if the web comprises a region which is not the treated region, the web has in this region which is not the treated region the same properties or at least essentially the same properties as the web had in the treated region before heat and pressure have been applied thereto to form the treated region. Therefore, throughout the present disclosure, if a feature relates to a "region which is not the treated region", this feature may also apply to the treated region of the web before heat and pressure have been applied thereto to form the treated region, even if the entire web forms the treated region. To avoid unnecessary repetition, the expression "region which is not the treated region" is therefore used.

[0053] For illustrative purposes, the test results of measuring the transparency of one sample each of the treated region of a membrane made from two spunbond non-woven-webs of different basis weight and produced according to the present invention are shown below in Table 1. The spunbond non-woven webs of the samples 1 and 2 shown in the tables following below are made of Polylactic Acid (PLA) and have been heated to a heating temperature of 185 °C and compressed at a pressure of 275 kPa. Table 1Spunbond non-woven web made of PLA fibers (before heating and compressing) - sample 1Spunbond non-woven web of from PLA fibers (before heating and compressing) - sample 2Treated region (after heating and compressing) - sample 1.1Treated region (after heating and compressing) - sample 2.2Comparative example - extruded film made of PLABasis weight (g / m 2< )9027902725Transparency (%)65.2984.3694.6294.1197.42

[0054] As can be seen in Table 1 above, spunbond non-woven webs made of PLA fibers of two different basis weights were tested as sample 1 and 2. By applying heat and pressure to the web, a membrane according to the present invention can be obtained having an increased transparency in the treated region. Transparency values which are almost as high as that of an extruded PLA film can be obtained.

[0055] According to the present invention, the treated region of the web, i.e. the region of the web to which the heat and the pressure has been applied, has a void ratio of 10% or more and 80 % or less, preferably of 20 % or more and 75 % or less, more preferably of 30 % or more and 70 % or less. The void ratio is a measure of the amount of voids in treated region, i.e. the heated and compressed web, and is calculated with Formula 1 stated above in the summary of the invention based on a ratio of the apparent density to the theoretical density of the polymer. For example, the theoretical density ρ polymer of the bio-sourced polymer PLA is 1240 kg / m 3< and can be taken from the product data sheet or determined in the literature. The values for the theoretical density ρ polymer of the individual polymers on which the present disclosure is based are given in Table 2 below. Table 2PolymerTheoretical density ρ polymer in kg / m 3< Copolyester (Co-PET)1200-1400Poly(tetramethylene adipate-coterephthalate) (PTAT)1220Nylon1060-1180Nylon 61130Nylon 661140Polybutylene Succinate (PBS)1260Polybutylene Succinate Adipate (PBSA)1230Polyether ether ketone (PEEK)1320Polyethylene (PE)935Polyethylene terephthalate (PET)1560Polyhydroxyalkanoate (PHA)1000-1300Polyhydroxybutyrate (PHB)1250Polylactic acid (PLA)1240Polymethyl methacrylate (PMMA)1185Polypropylene (PP)895Polyvinyl Alcohol (PVOH)1250

[0056] The value of the theoretical density of Copolyester depends on its chemical composition, as Copolyesters (Co-PET, or CoPET) are a polymer family with a range of densities. PTAT is one example, among others, of a Copolyester. The value of the theoretical density of Nylon depends on its chemical composition, as Nylons are a polymer family with a range of densities. Nylon 6 and Nylon 66 are two examples, among others, of Nylons. The exact value of the theoretical density of Polyhydroxyalkanoate (PHA) depends on its chemical composition, as Polyhydroxyalkanoates (PHAs) are a polymer family with a range of densities. PHB is one example, among others, of a PHA Polymer.

[0057] The apparent density ρ web,h+c of the treated region, in which the fibrous web has been heated and compressed, can be measured in a sample and calculated according to the following formula: ρ web , h + c = m V = m A × t = G × 1 t

[0058] In Formula 2, m is the mass of the sample in kg, V ist the volume of the sample in m 3< , A is the area of the sample in m 2< , t is the thickness of the sample in m and G is the basis weight in kg / m 2< , also called Grammage, of the sample. If the Grammage of the fibrous web before heat and pressure has been applied thereto to form the treated region is known, the apparent density ρ web,h+c can be calculated relatively easy by merely measuring the thickness of the treated region, for example according to the standard TAPPI / ANSI T 411 om-21.

[0059] Hence, if the apparent density of a test sample comprising only thermoplastic bio-sourced polymer fibers made from PLA is 630 kg / m 3< , for example, the resulting void ratio of the treated region is 49.19 %.

[0060] For illustrative purposes, the test results of measuring the thickness according to the standard TAPPI / ANSI T 411 om-21 and calculating the apparent density of one sample each of the treated region of a membrane made from two spunbond non-woven webs of different basis weight and produced according to the present invention are shown below in Table 3 along with the respective calculated void ratio. Table 3Spunbond non-woven web made of PLA fibers (before heating and compressing) - sample 1Spunbond non-woven web of from PLA fibers (before heating and compressing) - sample 2Treated region (after heating and compressing) - sample 1.1Treated region (after heating and compressing) - sample 2.2Basis weight G (g / m 2< )90279027Thickness t (µm)378.4136.2142.269.8Apparent Density ρ web,h+c (kg / m 3< )237.8198.2632.9386.8Void ratio vr (%)80.8284.0648.9568.80

[0061] As can be seen in Table 2 above, spunbond non-woven webs made of PLA fibers of two different basis weights were tested as sample 1 and 2. By applying heat and pressure to the web, the initial void ratio of the spunbond non-woven webs is lowered but nevertheless maintained to some extent. The void ratio is a prerequisite for the treated region's permeability to air. In other words, according to the invention, the transparency of the tested samples of spunbond non-woven webs was increased, while the void ratio thereof was reduced in a controlled manner, so that a certain amount of voids remained in the treated region allowing for a permeability to air.

[0062] As stated in the summary of the invention above, there is a certain correlation between the air permeability and the porosity or void ratio of the membrane, but they are not necessarily the same. A certain number of voids is a prerequisite for air permeability. However, the exact arrangement and size of these voids determines how permeable the membrane is to air. The fibrous web is inherently permeable to air and has a certain porosity or void ratio. By applying heat and pressure to the web, this air or gas permeability is reduced, but it is intended and preferred to maintain a certain air permeability.

[0063] The use of a fibrous web in the membrane has, in comparison to the use of an extruded film, as it is known from the prior art, the particular advantage that the air permeability of the membrane can be adjusted in a controlled and targeted manner. The term air permeability, as used in the present disclosure, refers to the measurement result of the determination of air permeability according to the standard ISO 9237:2005. The extruded film, as it is known from the prior art, is typically almost or completely impermeable for air. The fibrous web, on the other hand, inherently has a certain air permeability. When heat and pressure are applied to the fibrous web to form the treated region, the air permeability of the web in that region is reduced but may be maintained to some extent. This allows the air permeability in this area to be specifically adjusted. For example, by increasing either temperature, pressure or time, the air permeability of the treated region can be reduced. Incidentally, it may be possible to conclude that the treated region in the membrane originates from a fibrous web and not from an extruded film. For example, by observing under a microscope, void areas can be detected in the treated region which originate from voids in the structure of the fibrous web before applying heat thereto. Such voids originating from a fibrous web are typically irregular in size, shape and distribution, whereas voids obtained by perforating an extruded film, for example, are more regular at least in shape, and often also more regular in size and distribution as they are typically provided in patterns.

[0064] As described above, in a particular embodiment, the web further comprises a region which is not the treated region. The web has in the region which is not the treated region a first air permeability and in the treated region a second air permeability, wherein the second air permeability is 0.1 % or more and 90 % or less, preferably 0.5 % or more and 80 % or less, more preferably 1 % or more and 70 % or less, of the first air permeability. In a first non-limiting example, for some specific applications, the second air permeability may be 0.1 % or more and 5 % or less of the first air permeability or even 0.1 % or more and 2 % or less of the first air permeability. A specific application in which such limited but nevertheless secured air permeability is desired is the packaging of fruits and vegetables. These goods require a little exchange of air, but the exchange may not be too high to prevent the fruit or vegetables from spoiling prematurely. In a second non-limiting example, for other specific applications, the second air permeability may be 40 % or more and 60 % or less of the first air permeability. A specific application in which such reduces but still relatively great air permeability is desired is the performance of filtrations, and / or when it is needed to keep regions wherein an air passage is much higher than in other regions.

[0065] In regions of the web, which are not the treated region, the web can have an air permeability of 200 L / m 2< / s at 200 Pa or more and 4000 L / m 2< / s at 200 Pa or less, preferably of 500 L / m 2< / s at 200 Pa or more and 3000 L / m 2< / s at 200 Pa or less, more preferably of 800 L / m 2< / s at 200 Pa or more and 2000 L / m 2< / s or less. Depending on the temperature and the pressure at which the heat and the pressure are applied to the web to form the treated region, the air permeability of the treated region can be reduced by 20 % and up to 99.9 %, depending on the desired target air permeability. The treated region of the web preferably has an air permeability of 20 L / m 2< / s at 200 Pa, or more and 3200 L / m 2< / s at 200 Pa, or less. Preferably, the air permeability of the treated region is 1000 L / m 2< / s at 200 Pa, or more and 3200 L / m 2< / s at 200 Pa, or less; more preferably 1200 L / m 2< / s at 200 Pa, or more and 2400 L / m 2< / s at 200 Pa, or less. In other words, the air permeability of the treated region of the web can be adjusted so as to lie within a specific interval, wherein the treated region of the web has an air permeability of 5 L / m 2< / s at 200 Pa or more, preferably of 20 L / m 2< / s at 200 Pa or more, more preferably of 50 L / m 2< / s at 200 Pa or more, even more preferably of 100 L / m 2< / s at 200 Pa or more, still more preferably of 200 L / m 2< / s at 200 Pa and most preferably of 500 L / m 2< / s at 200 Pa or more. Yet, the air permeability of the treated region of the web is 3200 L / m 2< / s at 200 Pa or less, preferably 2500 L / m 2< / s at 200 Pa or less, more preferably 2000 L / m 2< / s at 200 Pa, even more preferably 1500 L / m 2< / s at 200 Pa and most preferably 1000 L / m 2< / s at 200 Pa.

[0066] For illustrative purposes, the test results of measuring air permeability of three samples each of the treated region of a membrane made from two spunbond non-woven webs of different basis weight and produced according to the present invention are shown below in Table 4. Table 4Spunbond non-woven web of from PLA fibers (before heating and compressing) - sample 1Treated region (after heating and compressing) - sample 1.1Treated region (after heating and compressing) - sample 1.2Basis weight (g / m 2< )909090-Air permeability (L / m 2< / s at 200 Pa)9102032-Spunbond non-woven web made of PLA fibers (before heating and compressing) - sample 2Treated region (after heating and compressing) - sample 2.1Treated region (after heating and compressing) - sample 2.2Treated region (after heating and compressing) - sample 2.3Basis weight (g / m 2< )27272727Air permeability (L / m 2< / s at 200 Pa)36122368126563

[0067] As can be seen in Table 4 above, spunbond non-woven webs made of PLA fibers of two different basis weights were tested as sample 1 and 2. By applying heat and pressure to the web, the initial air permeability of the spunbond non-woven webs is lowered but nevertheless maintained to a desired extent.

[0068] In the membrane, the web may further comprise a region which is not the treated region. The region of the web, which is not the treated region, usually has a tear resistance which is higher than the tear resistance of the treated region. In other words, by applying the heat and pressure to the web to form the treated region, the strength of the web in the treated region is lowered, particularly the tear resistance is lowered. It can therefore be advantageous to provide the membrane with a region of the web that is not the treated region and thus has a comparatively high tear resistance. This facilitates handling of the membrane, and particularly the production of the membrane in a roll-to-roll process.

[0069] For illustrative purposes, the test results of measuring tear resistance of two samples of the treated region of a membrane made from two spunbond non-woven webs of different basis weight and produced according to the present invention is shown below in Table 5. Table 5Spunbond non-woven web made of PLA fibers (before heating and compressing) - sample 1Spunbond non-woven web of from PLA fibers (before heating and compressing) - sample 2Treated region (after heating and compressing) - sample 1.1Treated region (after heating and compressing) - sample 2.2Basis weight (g / m 2< )90279027Tear resistance (mN)5111865.620345

[0070] As can be seen in Table 4 above, spunbond non-woven webs made of PLA fibers of two different basis weights were tested as sample 1 and 2. By applying heat and pressure to the web, the initial tear resistance of the spunbond non-woven webs is lowered.

[0071] In the membrane according to the present invention, the tear resistance in a region of the web which is not the treated region is preferably 200 mN or more, more preferably 500 mN or more and even more preferably 800 mN. In exceptional cases, it might be preferred that the tear resistance in a region of the web which is not the treated region is 1000 mN or more.

[0072] According to the present invention, a membrane, preferably the membrane described above, can be produced by preparing a fibrous web comprising thermoplastic polymer fibers, and applying heat and pressure to at least one region of the web to form a treated region. When applying the heat to the at least one region of the web, the web is heated from ambient temperature to a heating temperature within a heating time period of 0.5 seconds or more and 12 seconds or less, preferably of 1 second or more and 6 seconds or less. The heating temperature is at least as high as a melting temperature of the thermoplastic polymer fibers, preferably at least 5 °C or more but not more than 20 °C above the melting temperature, more preferably at least 10 °C or more but not more than 15 °C above the melting temperature.

[0073] Both the heating time and the heating temperature have a great influence on whether the fibrous web's transparency can be increased to a desired level without causing the fibrous web to shrink or to get holes or cracks. With the described parameters for the heating time and the heating temperature, it is possible to obtain a membrane having a treated region with increased transparency and without any distortion or shrinkage, cracks or holes.

[0074] Figs. 1a and 1b each exemplarily show a spunbond non-woven web to which heat and pressure has been applied but with non-suitable conditions causing the web to shrink and to develop cracks and holes therein. Fig. 2, on the other hand, shows a membrane which was produced according to the present invention. The depicted membrane was produced by applying heat and pressure to a fibrous web made of PLA fibers in a hot press.

[0075] Generally, the features and properties described above regarding the membrane as such also apply to the method of producing a membrane. They are not described again for the sole reason of avoiding unnecessary repetition and to improve the comprehensibility of the present disclosure. For example, in the method of the present disclosure, it may be preferred that all fibers in the web, except for negligible impurities, are made of thermoplastic polymers. Likewise, it may be preferred that the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers, as mentioned above.

[0076] In the method of producing a membrane, when heat is applied to the web for forming the treated region, it is preferred that heat is applied only from one side of the web. It may be preferred that heat is applied by means of a hot press or a calender or an ultrasonic horn. When heat is applied by means of a hot press, it may thus be preferred that only one surface of the two surfaces of a hot press is heated. Similarly, when heat is applied by means of a calender, it may thus be preferred that only one nip roller of a pair of nip rollers is heated.

[0077] It has been surprisingly found that by applying heat to only one side of the web, the treated region can be formed and the transparency of the web can be increased in that region without damaging the web, in particular without the web getting cracks or holes or shrinking. When using a hot press or a calender, the heat and the pressure can be applied simultaneously and formation of cracks or holes can be prevented.

[0078] The membrane can be produced in a plurality of ways. It can be produced in a discontinuous process, in a continuous process or, as kind of a hybrid of the two, in a stop-and-go process.

[0079] If the membrane is manufactured in a discontinuous process, the membrane can be manufactured in a hot press, for example. This may be done by bringing the web into contact with a release surface having a non-stick coating and placing the release surface in contact with one of the two surfaces of the hot press, so that at least a part of the release surface is sandwiched between the web and said one of the two surfaces of the hot press. The non-stick coating preferably comprises Polytetrafluoroethylene (PTFE) and / or silicone. The release surface may comprise a release sheet. Preferably two release sheets are used which are preferably arranged so that the web is interposed between the two release sheets. This is exemplarily shown in Fig. 3. In Fig. 3, the web 10 is interposed between two release sheets 23. The two release sheets 23 and the web 10 interposed therebetween are arranged between the two surfaces 21, 22 of the hot press 20. Preferably, only one surface 21 of the hot press 20 is heated. The heated surface 21 of the hot press 20 comes into non-direct contact with the web 10 by having a release sheet 23 interposed between the heated surface 21 and the web 10. As a next step, the other of the two surfaces of the hot press is brought into non-direct contact with the web and pressure and heat are applied to the web to form the treated region. When heat and pressure are applied to the web by means of a hot press, the pressure may be relatively low. Preferably, the pressure which is applied to the web when forming the treated region is 60 kPa or more and 1500 kPa or less; more preferably 120 kPa or more and 900 kPa or less, even more preferably 170 kPa or more and 600 kPa or less.

[0080] If the membrane is manufactured in a stop-and-go process, the membrane can still be manufactured using a hot press as described above, for example. In this case, the web may be supplied to the hot press by unwinding the web from a roll. While heat and pressure are applied to the web by means of the hot press, the conveying of the web from the roll to the hot press is stopped. After heat and pressure have been applied to the web, the web is further conveyed, and heat and pressure can be applied to a further region of the web.

[0081] If the membrane is manufactured in a continuous process, the membrane can be manufactured by means of a calender, for example. When heat is applied to the web by means of a calender, the release surface is preferably formed on the surface of a heated nip roller of a pair of nip rollers. By using a release surface, unwanted sticking of the web to the nip roller can be prevented. If the membrane is manufactured by means of a calender, the web may be supplied to the calender by unwinding the web from a roll. Generally, three different types of calenders may be used. As a first example, a calender may be used in which the web is passed between a pair of nip rollers of which preferably one has a hardness higher than the hardness of the other and which apply a pressure or linear load onto the web. In this case, the linear load which is applied to the web by the pair of nip rollers when forming the treated region is 150 kN / m or less, preferably 100 kN / m or less, more preferably 50 kN / m or less, even more preferably 30 kN / m or less and most preferably 20 kN / m or less but preferably no less than 100 kN / m, more preferably no less than 15 kN / m. As a second example, a calender may be used which provides a temperature gradient in the passing web, as described below. Also in this case, the linear load which is applied to the web by the pair of nip rollers when forming the treated region is 150 kN / m or less, preferably 100 kN / m or less, more preferably 50 kN / m or less, even more preferably 30 kN / m or less and most preferably 20 kN / m or less but preferably no less than 10 kN / m, more preferably no less than 15 kN / m. As a third example, a belt-calender system may be used in which the web is passed between one nip roller and a belt. In this case, the nip roller has a comparatively high hardness and the belt may be made of metal, fabric or latex. In this case, the linear load which is applied to the web by the nip roller and the belt when forming the treated region is 30 kN / m or less, preferably 20 kN / m or less, more preferably 10 kN / m or less and even more preferably 5 kN / m or less.

[0082] As described further above, it was surprisingly found that the speed at which the web is heated or cooled has an influence on whether the web's transparency can be increased to a desired level without causing the fibrous web to shrink or to get holes or cracks. The speed at which the web is heated or cooled can be adjusted by various measures, depending on the way the membrane is produced. For example, if the membrane is produced in a discontinuous process using a hot press, the speed at which the web is heated may be controlled by adjusting the temperature of the heated surface of the hot press and by adjusting the contact time of that surface and the web. Alternatively, the speed may be controlled by varying the temperature of the heated surface of the hot press during the contact time instead of heating the heated surface of the hot press to a constant temperature. As another example, if the membrane is produced in a continuous process using a calender, a plurality of pairs of nip rollers may be used and the web is passed successively through each of the pairs of nip rollers. In each pair of the plurality of nip rollers, at least one nip roller may be heated. By heating the heated roller of a first pair of nip rollers to a temperature which is higher or lower than the temperature of the heated roller of a second pair of nip rollers, a temperature gradient in the web along the conveying direction of the web can be reached and the speed at which the web is heated or cooled can be adjusted. Alternatively, if the membrane is produced in a continuous process using a calender only one pair of nip rollers may be used of which at least one is heated. By arranging additional heaters such as infrared heaters or hot air blowers before or after the pair of nip rollers, the speed at which the web is heated or at which it cools down may be adjusted.

[0083] In the process of manufacturing a membrane, preferably the membrane described above, it may be generally preferred that after applying heat to the web, the web is held under tension to prevent shrinkage of the web. In other words, the edges of the web to which heat is applied can be held in place after the application of heat so that the web does not shrink when it cools. It may be preferable to keep the web under tension and / or hold its edges also during the application of heat. It may also be preferred that the web is held under tension until the web has cooled from the heating temperature to essentially room temperature. The web can be held under tension by various means. If a hot press is used to apply heat to the web, the web can be held under tension by applying an appropriate pressure to the web by means of the hot press. If a calender is used to apply heat to the web, the web can be held under tension by arranging or controlling the rollers accordingly or by providing additional deflection means or guide means.

[0084] The membrane of the present disclosure and the method of producing a membrane of the present disclosure is particularly advantageous when used for the fabrication of packaging for foods and beverages, such as in beverage cartons, tea bags, of packaging for loose fruit such as berries, but also of the packaging of products such as sports equipment or similar products. For example, the membrane can be used to make various kinds of packaging with tailored properties to serve different requirements, especially regarding a high transparency, a controlled air and / or gas permeability or air and / or gas tightness, a heat-sealability, some patterns, and a minimal mechanical resistance.

Examples

Embodiment Construction

[0048]Any identical reference signs listed in different figures denote identical, corresponding or functionally similar parts.

[0049]The fibrous web of the membrane according to the present invention comprises thermoplastic polymer fibers. It is preferred that a proportion of thermoplastic polymer fibers by weight of all fibers of the web is 90% or more, preferably 95% or more, more preferably 99% or more. In other words, it may be preferred that all fibers in the web, except for negligible impurities, are made of thermoplastic polymers.

[0050]Thermoplastic polymers suitable to be used as thermoplastic polymer fibers in the fibrous web according to the present invention are, for example, Polyethylene Terephthalate (PET), Polymethyl Methacrylate (PMMA), Copolyester (Co-PET), Polyamides such as Nylon, Polyethylene (PE), Polyether Ether Ketone (PEEK) and Polypropylene (PP), for example. Preferably, the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers. Bio-sourced...

Claims

1. A membrane comprising: a fibrous web comprising thermoplastic polymer fibers; wherein the web comprises at least one treated region which has been heated and compressed; wherein said treated region has a transparency of at least 85%, preferably at least 90%, more preferably at least 92%, and a void ratio of 10% or more and 80% or less, preferably 20% or more and 75% or less, more preferably 30% or more and 70% or less.

2. The membrane according to claim 1, wherein a proportion of thermoplastic polymer fibers by weight of all fibers of the web is 90% or more, preferably 95% or more, more preferably 99% or more.

3. The membrane according to claim 1 or 2, wherein the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers which are preferably one or more of Polylactic Acid (PLA) fibers, Polyhydroxyalkanoate (PHA) fibers and Polyhydroxybutyrate (PHB) fibers.

4. The membrane according to any one of claims 1 to 3, wherein the web is a non-woven web, preferably a spunbond non-woven web.

5. The membrane according to any one of claims 1 to 4, wherein the web further comprises a region which is not the treated region; the web has in the region which is not the treated region a first air permeability and in the treated region a second air permeability, wherein the second air permeability is 0.1 % or more and 90 % or less, preferably 0.5 % or more and 80 % or less, more preferably 1 % or more and 70 % or less, of the first air permeability.

6. The membrane according to any one of claims 1 to 5, wherein the web further comprises a region which is not the treated region; said region which is not the treated region has a transparency which is lower than the transparency of the treated region, and which is preferably 80 % or less, more preferably 75 % or less, even more preferably 70 % or less.

7. The membrane according to any one of claims 1 to 6, wherein the web further comprises a region which is not the treated region; the membrane has, in a sample region comprising at least a part of said region which is not the treated region, a tear resistance of 200 mN or more, preferably 500 mN or more, more preferably 800 mN or more.

8. The membrane according to any one of claims 1 to 7, wherein the treated region of the web is a contiguous area that has a size of 0.5 cm2 or more, preferably 1 cm2 or more, more preferably 2 cm2 or more, even more preferably 5 cm2 or more.

9. A method of producing a membrane, preferably the membrane according to any one of claims 1 to 8, the method comprising: preparing a fibrous web comprising thermoplastic polymer fibers, and applying heat and pressure to at least one region of the web to form a treated region; wherein when applying the heat to the at least one region of the web, the web is heated from ambient temperature to a heating temperature within a heating time period of 0.5 seconds or more and 12 seconds or less, preferably of 1 second or more and 6 seconds or less; wherein the heating temperature is at least as high as a melting temperature of the thermoplastic polymer fibers, preferably at least 5 °C or more but not more than 20 °C above the melting temperature, more preferably at least 10 °C or more but not more than 15 °C above the melting temperature.

10. The method according to claim 9, wherein a proportion of thermoplastic polymer fibers by weight of all fibers of the web is 90% or more, preferably 95% or more, more preferably 99% or more; and / or the thermoplastic polymer fibers are thermoplastic bio-sourced polymer fibers which are preferably one or more of Polylactic Acid (PLA) fibers, Polyhydroxyalkanoate (PHA) fibers and Polyhydroxybutyrate (PHB) fibers.

11. The method according to claim 9 or 10, wherein when applying the heat to the at least one region of the web, the heat is applied only from one side of the web, and / or the heat is applied by means of a hot press, a calender or an ultrasonic horn.

12. The method according to any one of claims 9 to 11, wherein when applying heat and pressure to the at least one region of the web, the heat and the pressure are applied so as to increase transparency of the web to a transparency of 85 % or more, preferably 90 % or more, more preferably 92 % or more.

13. The method according to any one of claims 9 to 12, further comprising cooling the web, wherein the web is cooled from heating temperature to ambient temperature within a cooling time period of 0.5 seconds or more and 120 seconds or less, preferably of 1 second or more and 100 seconds of less.

14. The method according to any one of claims 9 to 13, further comprising bringing the web in contact with a release surface having a non-stick coating prior to applying heat and pressure to the web, the non-stick coating preferably comprising Polytetrafluoroethylene (PTFE) and / or silicone.

15. Use of a membrane according to any one of claims 1 to 8 for the fabrication of a packaging.

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