A method for detecting defects in fiber-based materials having barrier properties

Abstract

The present invention relates to a method for assessing the quality of a fiber-based substrate having barrier properties, said method comprising the steps of: f. providing a sample of said fiber-based substrate having a first surface and a second surface; g. providing a chemochromic gas-sensing element including a flexible support structure and an adjacent pigment layer arranged to change color upon contact with a gas; h. placing the gas-sensing element on top of the first surface of the sample of said fiber-based substrate; i. subjecting the second surface of said fiber-based substrate to a test gas whereupon presence of any ruptures and / or pinholes in said fiber-based substrate will lead to leakage of gas and allow gas to contact the gas- sensing element causing a change in color; and j. examine any color change in the pigment layer to detect ruptures and / or pinholes in the fiber-based substrate.

Description

[0001] A METHOD FOR DETECTING DEFECTS IN FIBER-BASED MATERIALS HAVING BARRIER PROPERTIES

[0002] Technical field

[0003] The present disclosure relates to a method for evaluating barrier quality of fiberbased materials.

[0004] Background

[0005] In the board- and papermaking field, it is known to evaluate the material's quality by detecting occurrence of so-called pinholes. Pinholes, i.e. small pin-sized holes, are defects in the fiber-based material often produced by disturbances such as foreign particle contamination, air bubbles and / or fiber flocs during forming. This is obviously an undesirable phenomenon since it leads to problems during coating and to inferior function e.g. in case the material is intended for use as a barrier layer.

[0006] Known methods for detecting pinholes in paper, board or fiber-based films include tests with liquids. An example is disclosed in US5847265A. Herein, the pinhole detection method comprises positioning a sheet of the paper to be tested on top of a backing sheet, applying ink or other viscous liquid to the top of the test sheet with uniform pressure to force some of the liquid through any pinholes in the sheet and onto the backing sheet underneath. The test sheet is then discarded and the backing sheet is allowed to dry. The imprint on the backing sheet is then scanned into a computer for image analysis of the pinholes or analyzed in another manner. Sometimes, pinholes or other types of ruptures or defects in barrier function are tested with the very foodstuff presumed to be in contact with the material upon use, such as coffee, tea, soft drinks etc.

[0007] A problem with known methods is that it is often difficult to select the most suitable test solution regarding the tolerance of the barrier material to the aggressiveness of the test solution. The reagent may also cause swelling of the fibers and the fiber network, and furthermore, known tests may be time-consuming and even take up to 60 minutes depending on the selected reagent.

[0008] There is a need for an improved method for detecting defects such as pinholes and ruptures in fiber-based materials including paper, board and / or films, where such an improved method offers a rapid, reliable, and user-friendly test.

[0009] Summary of the invention

[0010] It is an object of the present disclosure to alleviate at least some of the problems presented above. It is a further object of the present disclosure to provide a method for assessing the quality of a fiber-based sheet material having barrier properties such as paper, board and / or film or laminates thereof, where said method is rapid, reliable, accurate and user-friendly.

[0011] The above-mentioned objects, as well as other objects as will be realized by the skilled person in the light of the present disclosure are achieved by the various aspects of the present disclosure.

[0012] Thus, according to the invention, there is provided a method for assessing the quality of a fiber-based substrate having barrier properties, said method comprising the steps of: a) providing a sample of said fiber-based substrate having a first surface and a second surface; b) providing a chemochromic gas-sensing element including a flexible support structure and an adjacent pigment layer configured to change color upon contact with a gas; c) placing the gas-sensing element onto of the first surface of the sample of said fiber-based substrate; d) subjecting the second surface of said fiber-based substrate to a test gas whereupon presence of any ruptures and / or pinholes in said fiberbased substrate will lead to leakage of gas and allow gas to contact the gas-sensing element causing a change in color; and e) examine any color change in the pigment layer of the gas-sensing element to detect ruptures and / or pinholes in the fiber-based substrate.

[0013] It is to be understood that in the context of the present invention, the assessment of quality of the fiber-based substrate referred to above and in claim 1 means to detect unwanted points of leakage in the substrate impairing the expected barrier function. Examples include pinholes, cracks, ruptures and other defects in the barrier layer leading to such undesirable leakage I permeation. Furthermore, “undesirable leakage” may for instance mean deviations from a specified, targeted or measured barrier mean value. In the following text, the term “pinhole” is frequently used to define such point of leakage, but the skilled person understands that the method of the invention is not limited to detect pinholes only but that other types of defects in the barrier can also be identified in addition to such pinholes.

[0014] It is also to be understood that a material having “barrier properties” herein refers to a material’s ability to resist permeability of e.g. liquids, moisture, grease or oil and / or gases (for instance 02, H2, CO2 or N2 gases). Good barrier properties are important for packaging materials to protect its content and maintain the content’s quality.

[0015] Thanks to the invention, there is provided a method for assessing the quality of paper by measuring the frequency and magnitude of ruptures and / or pinholes in a fiber-based material. Additionally, there is provided a method for measuring e.g. ruptures and / or pinholes in a sheet of fiber-based material in an easy, quick and inexpensive manner. Thanks to the invention, there is also provided a method to detect defect areas in the barrier function of a material, i.e. detect significant deviations in gas permeability based on the targeted mean value. Thus, the method of the invention allows for a better understanding of such deviations.

[0016] According to one aspect of the invention, the gas-sensing element is a flexible gas detection tape comprising an adhesive layer for fixating said gas-sensing element onto the first surface of the sample of said fiber-based substrate such that the gassensing element follows the contour of the substrate when fixated thereto. Thanks to this feature, the tape can be pressed against the substrate to be examined and be adhered thereto and, by means of the adhesive, closely follow any change in surface contour of the substrate. This allows for very precise and accurate detection of the position of e.g. pinholes. The present invention is also suitable for detecting very small pinholes, i.e. as small as 10pm in average diameter. Gases have much lower viscosity compared to liquids, and hence gases enable for detectability of very small pinholes compared to traditional pinhole-tests. Thus, the method according to the invention may detect pinholes in the size range of 5 - 1000pm in average diameter, or surprisingly as small as 6 - 100pm or 20 - 60pm. The most detrimental pinholes are known to be in the size of 30 - 55pm in diameter and it has been found that the current invention allows for very accurate detection also of these dimensions. The method is also suitable for evaluation of microperforated samples and / or for detecting uniformity of holes.

[0017] According to another aspect of the invention, the adhesive layer is arranged to cover the pigment layer so that when placing the gas detection tape onto the first surface of the sample, the adhesive layer is interposed between the sample and the pigment layer. Thus, the adhesive layer is in direct contact with the sample allowing for efficient fixation of the gas detection tape while also protecting the pigment layer from damage. The adhesive layer is made from a gas-permeable polymer, such as an amorphous polymer. In one example, the adhesive layer does not contain any color-shifting pigment, i.e. it is void of color-shifting pigment.

[0018] According to another aspect of the invention, the test gas is a reducing gas, such as hydrogen, hydrogen sulfide, carbon dioxide, carbon monoxide, methane, formaldehyde, acetylene, sulfur dioxide, ammonia, or nitrous oxide. Upon contacting the pigment in the tape, the reducing gas will lead to a detectable change in color, indicating leakage and e.g., that a pinhole is present.

[0019] According to another aspect of the invention, the fiber-based substrate is a cellulose- based sheet- or web-material mainly formed from pulp of wood or other fibrous substances comprising cellulose fibers. In one embodiment, the fiber-based substrate is a film, such as a microfibrillated cellulose (MFC) film. The fiber-based substrate may also be a metallized film. The fiber-based substrate may also be any of a paperboard, extrusion-coated paperboard or dispersion-coated paperboard. Thanks to the invention and possibility to adhere the gas-sensing element onto the substrate, it is possible to examine fiber-based packages with complex 3D shapes, such as converted packages and as an example the seams of a gable top package made from coated paperboard.

[0020] According to another aspect of the invention, the fiber-based substrate has a moisture content between 0-15wt%, such as between 3-15wt% or between 10- 15wt% determined with standard method ISO 287. Samples having a moisture content of >15wt% are not very relevant because at these moisture levels, the fiber network is swollen and physical properties partially lost.

[0021] According to another aspect of the invention, the temperature during step d) is between 0-60°C.

[0022] According to another aspect of the invention, in step e), said examining includes use of a computer-assisted image analysing software. This means that any defects in the fiber-based material can be quantified and analysed digitally to e.g. better analyse pinhole pattern, discover micro-sized holes and distribution of defects. Detection may further include inspection by vizualisation in 2D and / or 3D, for instance by means of creating a 3D model using microtomography. It is also within the ambit of the invention to examine the results of the test method by visual inspection, i.e. observing the color change in the tape.

[0023] The invention also comprises a test kit for performing the method, said test kit comprising:

[0024] - a receiving means for receiving and holding a sample of fiber-based substrate having a first surface and a second surface;

[0025] - a chemochromic gas-sensing element including a flexible support structure and an adjacent pigment layer arranged to change color upon contact with a gas;

[0026] - means for providing a test gas;

[0027] - means for subjecting a surface of said fiber-based substrate to the test gas whereupon presence of pinholes in said fiber-based substrate will allow gas to contact the gas-sensing element causing a change in color; and

[0028] - means for examining any color change in the gas-sensing element to detect pinholes in the fiber-based substrate.

[0029] Brief description of the figures

[0030] Figs. 1a-b show in a schematic way the substrate and the gas-sensing element separate and superimposed respectively, and

[0031] Fig. 2 shows an example of a chemochromic gas-sensing element arranged on top of a fiber-based substrate sample having been subjected to a test method according to the invention.

[0032] Detailed description of the invention

[0033] In order to better understand the present invention, further aspects of the method are now to be described. It is to be understood that the terminology employed herein is used for the purpose of describing particular embodiments only and is not intended to be limiting since the scope of the present invention is limited only by the appended claims and equivalents thereof.

[0034] Referring to the appended Figures, an example of performing the method according to the invention is now to be described, said method serving the purpose of assessing the quality of a sample 2 such as a fiber-based substrate having barrier properties, in particular assessing the quality by means of detecting small-scale points of leakage in such a substrate. Examples of “points of leakage” include pinholes, cracks, ruptures and other leakage-causing defects in the material. The fiber-based substrate 2 is a sheet material made from cellulose fibers having barrier properties. Such barrier properties may be provided by the material itself, e.g. if the material is a film or densified material or having a compact surface, or it may be provided on the fiber-based substrate as a coating as is the case for coated paper or paperboard. Coated paper or paperboard may comprise one or several layers of coating, or contain a film laminated onto the surface. When performing the method of the invention, a sample of a fiber-based substrate 2 having barrier properties is firstly provided, said substrate having a first surface 20 and a second surface 21. The sample of fiber-based substrate may be a fiber film like an MFC film, metallized fiber-based film, paperboard, extrusion-coated paperboard or dispersion-coated paperboard or any other fiber-based material and normally has a moisture content between 3-15wt%. Next, a chemochromic gassensing element 1 is provided, said element including a flexible support structure 10 and adjacent thereto a pigment layer 11 arranged to change color upon contact with a test gas. The gas-sensing element 1 enables passive and efficient gas level detection and is preferably provided as a gas-detection tape. Such gas-detection tape, and in particular the pigment layer thereof, comprises a chemochromic composition arranged to shift color upon contact with a test gas. Examples of chemochromic composition include palladium oxide, palladium hydroxide, and palladium salts. In one embodiment, said chemochromic composition is dispersed within a polymer matrix of a siloxane crosslinked by free radical transfer reaction with a peroxide initiator. The gas-detection tape preferably also includes an adhesive layer 12, which in a preferred example is arranged on top of the pigment layer as seen in Figs. 1a-b, protecting the chemochromic composition. The gas-detection tape 1 is then placed on top of the first surface 20 of the sample 2 of said substrate. In a preferred embodiment, the gas-detection tape is adhered onto the surface of the substrate sample such that it closely follows the surface contour of the sample.

[0035] Wrinkles in the tape may create air pockets that collect test gas, leading to full colour change in such wrinkled area causing detection errors.

[0036] The sample of fiber-based substrate can be covered with an adhesive mask intended to cover and protecting the edges of said sample and / or for exposing only a predetermined area. Such mask eliminates the edge effect, i.e. the edges of the sample do not cause discoloration and also ensures that the measured area has a predefined size.

[0037] The size of the area to be tested is limited to the dimensions of the gas-detection tape. Examples of presently available gas-detection tapes are between 45 - 55mm in width. Depending on the test circumstances, a suitable test area is a circular zone with a diameter around 50mm, or adapted such that the sample edges may be covered with tape.

[0038] The second surface 21 of the sample 2 of fiber-based substrate is then subjected to a test gas G (see Fig. 1 b) whereupon presence of any ruptures and / or pinholes in said fiber-based substrate will lead to leakage of gas and allow gas to contact the pigment layer 11 of the gas detection tape 1 causing a change in color. The test gas may be for instance hydrogen, hydrogen sulfide, carbon dioxide, carbon monoxide, methane, formaldehyde, acetylene, sulfur dioxide, ammonia, or nitrous oxide. The pressure of the gas may for example be around atmospheric pressure. The exposure to the test-gas can be performed in various ways. An example is to enclose the taped substrate in a closed environment filled with test-gas, for instance a minigrip plastic bag that is filled with gas. Other options are to use a test cell arranged to fix the taped substrate in a holding structure and fill the cell with gas whereby gas will contact the non-taped surface of the sample. The time from applying the test gas to a possible reading of color change is 2-3 minutes. In a final detection I evaluation step, any color change in the gas-sensing tape 1 is examined thus detecting ruptures and / or pinholes in the fiber-based substrate. The detection I evaluation may be performed by visual inspection or by analyzing instruments such as scanning electron microscope (SEM) or other electronic equipment. Analysis of the result may further include computer-assisted image analyzing software. Scanning of the color changes may also include spectroscopically for instance using multispectral or hyperspectral imaging systems. The method according to the invention may detect pinholes in the size range of 5 - 1000pm in average diameter, or surprisingly as small as 6 - 100pm or 20 - 60pm.

[0039] The method may be performed on sheet-like samples of fiber-based materials, and also on folded 3D structures. It may for instance be useful to evaluate barrier function on the edges of converted carton such as gable- top packages. In such a case, the gas-detection tape is attached onto the sample while paying extra attention to attachment across folded edges and / or creases. Thus, the method of the invention can be used to evaluate barrier function of edges and corner section, such as at 45- 180° folds by means of covering such sections with the gas-sensing element, i.e. the tape. An example of a chemochromic gas-sensing element on top of a fiber-based substrate sample having been subjected to a test method according to the invention is seen in Fig. 2. Herein is seen a perspective view of a gas-detection tape 1 that is adhered onto the surface of a coated paperboard sample, wherein the subjection to test gas has resulted in spots of color change on said tape. Each color change is an indication that test gas has penetrated the paperboard at the corresponding location, for instance due to a pinhole or rupture in the barrier coating.

[0040] While the invention is described herein with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

CLAIMS1. A method for assessing the quality of a fiber-based substrate having barrier properties, said method comprising the steps of: f) providing a sample (2) of said fiber-based substrate having a first surface (20) and a second surface (21); g) providing a chemochromic gas-sensing element (1) including a flexible support structure (10) with an adjacent pigment layer (11) arranged to change color upon contact with a gas; h) placing the gas-sensing element (1) onto the first surface (20) of the sample of said fiber-based substrate; i) subjecting the second surface (21) of said fiber-based substrate to a test gas (G) whereupon presence of any ruptures and / or pinholes in said fiber-based substrate will lead to leakage of gas and allow gas to contact the pigment layer (11) of the gas-sensing element causing a change in color; and j) examine any color change in the pigment layer to detect ruptures and / or pinholes in the fiber-based substrate.

2. The method according to claim 1, wherein the gas-sensing element is a gas detection tape comprising an adhesive layer for fixating the gas-sensing element onto the first surface of the sample of said fiber-based substrate such that the fixated gas-sensing element follows the surface contour of the substrate.

3. The method according to claim 2, wherein the adhesive layer is a gas- permeable polymer, and is arranged to cover the pigment layer and protecting the pigment layer from outer wear and tear, so that when placing the gas detection tape onto the first surface of the sample, the adhesive layer is interposed between the sample and the pigment layer.

4. The method according to any one of the previous claims, wherein the test gas is a reducing gas, such as hydrogen, hydrogen sulfide, carbon dioxide, carbonmonoxide, methane, formaldehyde, acetylene, sulfur dioxide, ammonia, or nitrous oxide.

5. The method according to any one of the previous claims, wherein the fiberbased substrate is a film, such as an MFC film.

6. The method according to any one of the previous claims, wherein the fiberbased substrate is any of a paperboard, extrusion-coated paperboard, dispersion-coated paperboard or a metallized fiber-film.

7. The method according to any one of the previous claims, wherein the fiberbased substrate has a moisture content between 0-15wt%, preferably between 3-15wt%, preferably between 3-10wt%.

8. The method according to any one of the previous claims, wherein the temperature during step d) is between 0-60°C.

9. The method according to any one of the previous claims, wherein in step e) said examining includes computer-assisted image analysing software.

10. The method according to any one of claims 1-8, wherein in step e) said examining includes visual inspection.

11. A kit for performing the method according to any one of claims 1-10, comprising: a. a receiving means for receiving and holding a sample of fiber-based substrate having a first surface and a second surface; b. a chemochromic gas-sensing element including a flexible support structure with an adjacent pigment layer arranged to change color upon contact with a gas; c. means for providing a test gas; d. means for subjecting a surface of said fiber-based substrate to the test gas whereupon presence of pinholes in said fiber-based substrate willallow gas to contact the gas-sensing element causing a change in color; and e. means for examining any color change in the gas-sensing element to detect pinholes in the fiber-based substrate.

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