Method for providing a substrate with a barrier and a substrate comprising a barrier

Abstract

The invention relates to a method for providing a surface of a fiber based substrate with a barrier layer wherein the barrier layer is formed by depositing nanofibers on the surface by the use of electrospinning or meltspinning and wherein the film is formed by post treatment of the substrate. The invention further relates to a substrate comprising such a carrier layer.

Claims

1. A method for producing a fiber based packaging material with a barrier layer, the method comprising providing a fiber based substrate having a surface, depositing ultrafine polymer fibers having a diameter less than 5 m on the surface of the substrate by the use of electrospinning or meltspinning, and post treating the substrate together with the polymer fibers on the surface to cause the polymer fibers to melt and coalesce together into a barrier layer film on the surface of the substrate, wherein the post treating step comprises at least one of increasing the temperature of the ultrafine polymer fibers, applying pressure to the ultrafine polymer fibers, subjecting the ultrafine polymer fibers to an electric field, radiation curing of the ultrafine polymer fibers, and decreasing the temperature of the ultrafine polymer fibers.

2. The method according to claim 1 wherein the post treating step comprises increasing the temperature to or above the glass transition temperature or melting temperature of the deposited ultrafine polymer fibers so that a film is formed.

3. The method according to claim 1 wherein the ultrafine polymer fibers that are deposited comprise at least two components that are spun simultaneously in order to deposit the components to the surface of the substrate in a single step.

4. The method according to claim 1 wherein the barrier layer comprises at least two layers.

5. The method according to claim 1, wherein the barrier layer has a dry weight of 0.1-20 g/m2.

6. The method according to claim 1, wherein the surface of the fiber based substrate is provided with a coating layer before the ultrafine polymer fibers are deposited on the surface.

7. The method according to claim 1, wherein the barrier layer is provided with an additional coating layer that is laminated or extrusion coated to the barrier layer.

8. The method according to claim 1 wherein the ultrafine polymer fibers are formed by electrospinning or meltspinning a polymer selected from the group consisting of polyvinyl alcohol, varnish, polystyrene, polybutadiene, polyurethanes, polyethylene dispersions, polypropylene, PLA, chitosan, starch, sodium carboxymethyl cellulose, acrylate copolymers, polyvinyl acetate, poly ethylene oxide, polyethylene dispersions, polyethylene terephthalate dispersions, mixtures or its modified analogues of any of the mentioned components.

9. The method according to claim 8 wherein the electrospinning is done with a liquid or dispersion comprising the polymer.

10. The method according to claim 1 wherein the barrier layer further is provided with a functional property by addition of a functional additive.

11. The method according to claim 10 wherein the functional additive is spun together with the polymer.

12. The method according to claim 10 wherein the functional additive is spun as a separate layer of the barrier layers.

13. A method for providing a surface of a fiber based substrate with a barrier layer which barrier layer is formed as a film by depositing nanofibers on the surface by the use of electrospinning or meltspinning wherein the film is formed by post treating the substrate together with the nanofibers on the surface, wherein the post treating causes the polymer fibers to melt to form the barrier layer film on the surface of the substrate, and wherein the nanofibers that are deposited comprise at least two components that are spun separately and simultaneously in order to deposit nanofibers of the components to the surface of the substrate in a single step.

14. A method for providing a surface of a fiber based substrate with a barrier layer which barrier layer is formed as a film by depositing nanofibers on the surface of the fiber based substrate by the use of electrospinning or meltspinning wherein the film is formed by post treating the substrate together with the deposited nanofibers on the surface, wherein the post treating causes the polymer fibers to melt to form the barrier layer film on the surface of the substrate, and wherein the surface of the fiber based substrate is provided with a separate coating layer before the nanofibers are deposited on the surface.

15. A method for providing a surface of a fiber based substrate with a barrier layer which barrier layer is formed as a film by depositing nanofibers on the surface of the fiber based substrate by the use of electrospinning or meltspinning wherein the film is formed by post treating the substrate with deposited nanofibers on the surface, wherein the post treating causes the polymer fibers to melt to form the barrier layer film on the surface of the substrate, and wherein the barrier layer further is provided with a functional property by addition of a functional additive.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention relates to a method for providing a surface of a fiber based substrate with a barrier layer wherein the barrier layer is formed by depositing nanofibers on the surface by the use of electrospinning or meltspinning.

(2) It has been found that by depositing nanofibers on a surface of a fiber based substrate by the use of electrospinning or meltspinning it is possible to form a thin layer of nanofibers which will form a barrier layer on the surface. It has surprisingly been found that a thin layer of deposited nanofibers are sufficient in order to completely cover a rough surface, such as a surface of a fiber based substrate. Furthermore, due to the characteristic fiber dimension and fiber properties of the nanofibers produced and deposited by electrospinning or meltspinning, the penetration of the nanofibers into the substrate is insignificant. The nanofibers will thus be deposited on the surface and stay on the surface of the substrate. They will still bond in a satisfactory way to the surface of the substrate due to chemical and/or physico-chemical interactions between the fibers of the substrate and the deposited nanofibers. There might also be some interdiffusion which can create mechanical or physical interlocking of the fibers. The deposited nanofibers will either coalesced or melt on the surface of the substrate to form a film which will work as the barrier. By depositing fibers on the surface before having a complete film a substantially lower amount of coating is needed, this is due to the fact that the penetration into the substrate is much lower or even insignificant compared to other barriers formed on fiber based substrates created with other coating techniques. Consequently, the film formation according to the invention is formed from a fiber network which should be compared to prior art solutions where films are formed from a solution or a dispersion.

(3) The amount of fibers added to the surface of the fiber based substrate depends on different parameters, for example on the roughness of the surface. It is preferred that the barrier layer has a dry weight of 0.1-20 g/m2, preferably 0.1-5 g/m2 or even more preferably 0.2-3 g/m2. If the surface to which the barrier layer is added is rough a higher amount of fibers need to be added and it is the preferred that the dry weight of the barrier layer is between 2-20 g/m2.

(4) By optimizing the spinning conditions it is possible to increase the moisture during the spinning so that the deposited fibers are coalesced into a film. The first deposited fibers will be dry or they will have a high solid content which makes the rheology of the fibers high. This is partly due to that the surface to which they are deposited on is dry. The spinning conditions will thereafter change so that the following deposited fibers are wetter, i.e. semidry. This results in that the fibers are coalesced to a film which will work as an excellent barrier. The dryness and/or solids of the liquid or air used during the electrospinning or meltspinning can be controlled so that the fibers will coalesced to a film. It is also possible to control the moisture, temperature, production rate and spinning distance so that the fibers will form a film. In this way it is possible to produce a fiber based substrate in a very easy way.

(5) It is also possible to post treat the substrate or the surface of the substrate together with the deposited nanofibers in order for the nanofibers to form a film. It is preferred to use a heat treatment which will increase the temperature of the deposited nanofibers so that the properties of the deposited nanofibers changes whereby a film is formed. It is preferred to increase the temperature to or above the glass transition temperature or the melting temperature of the nanofibers. In this way the properties of the deposited nanofibers will change, e.g. they will start to flow or melt, and they will thus form a film which will work as a barrier layer of the fiber based substrate. Depending on the material used for the production of nanofibers and on the time of the treatment, it may not be necessary to increase the temperature to or above the glass transition temperature. For some materials it may be sufficient to increase the temperature some, still being below the glass transition temperature, in order for the properties of the deposited nanofibers to change and form a film. The heating can be done by the use of flame, infra-red dryer, fuser roll, air-dryer, plasma, steam, laser UV, EB or any other known technique. It is also possible to use hot fusing or hot nip in which the deposited fibers are heated and simultaneously formed to a thin film on the substrate. It is also possible post treat the deposited nanofibers by increasing the pressure, preferably in combination with increased temperature. Another possible post treatment is subjecting the nanofibers to an electric field. Other possible post treatments may be radiation curing, e.g. IR, NIR etc. If the deposited fibers are hot it is also possible to cool them and thus form a film by decreasing the temperature.

(6) The substance or substances to be formed into nanofibers by the use of electrospinning or meltspinning may be polymers or a blend of polymers. Suitable polymers may be chosen from e.g. polyolefins, polyvinyls, polyamides, polyimides, polyacrylates, polyesters, and mixtures thereof. It is especially preferred to use polyvinyl alcohol, varnish, polystyrene, polybutadiene, polyurethanes, polyethylene dispersions, polypropylene, PLA, chitosan, starch, sodium carboxymethyl cellulose, acrylate copolymers, polyvinyl acetate, poly ethylene oxide, polyethylene dispersions, polyethylene terephthalate dispersions, mixtures or its modified analogues of any of the mentioned components. The polymer used depends on the end use of the fiber based substrate. The different polymers will form a barrier layer against different properties, for example will PVA form a barrier layer against grease.

(7) The present invention makes it possible to provide a barrier layer comprising at least two components. This can be done by simultaneously spin two or more components which thus will be deposited on the surface of the substrate. The simultaneously spinning of two or more components may be done through different nozzles or other feeding arrangements, so that one component is spun through one nozzle and another component through another. In this way the barrier layer will comprise a mixture of different nanofibers making it possible to provide a single barrier layer with different properties, i.e. making a kind of composite material. For example may one component give barrier properties against one compound, such as water and another component can give barrier properties against another compound, such as grease. In this way it is also possible to spin one component which will melt in order to form a film and one component which will support the melted component and prevent it to penetrate too deep into the base material, i.e. keep them on the surface of the fiber based substrate. It is thus possible that a single barrier layer comprises protection against several different compounds.

(8) A big advantage with the present invention is that it is possible to provide the barrier layer with more than one layer in an easy way. It is thus possible to produce a barrier layer which fulfils different properties which previously has demanded several process steps. In this way it is possible to provide the barrier layer with several layers with different properties, for example one prime layer, a barrier layer, a protective layer, a hot sealable layer and/or a blocking resistant layer. The use of different polymer dispersions gives different properties. For example, a polyurethane dispersion will give a barrier against aroma, fat and grease as well as sealability properties. A fiber based substrate with a polyurethane barrier will thus be easy to seal in order to form a package at the same time as it has great barrier properties. If the polyurethane barrier layer is combined with an ethylene layer the fiber bases substrate will also have a barrier against water.

(9) The surface of the fiber based substrate may be provided with a coating layer. Consequently, the nanofibers will be deposited on the coating layer of the fiber based substrate. In this way the deposition of the coating layer is done on a smooth surface and the amount of nanofibers may be even further reduced. The coating can be of any conventional coating colour, such as calcium carbonate or kaolin.

(10) The barrier layer of the substrate may be provided with a coating layer. It is preferred that the coating layer comprises a polymer that is laminated or extrusion coated to the barrier layer. In this way the barrier layer may comprise a primer which will increase the adhesion between the fiber based substrate and an extrusion coated layer making it possible to increase the speed of the extrusion coating process. The coating layer may also comprise any conventional coating components, both polymer layer as well as pigment coating layer.

(11) It is also possible to provide the barrier layer with functional properties by the addition of a functional additive to the medium which is spun. Possible additives may be fillers which may increase whiteness or provide the substrate with UV protection or absorbents which may trap taste and odor chemicals and thus reduce problems. The functional additive may be mixed with the medium, preferable a polymer dispersion, and thus be spun together with the polymer. The nanofibers formed will thus comprise a mixture of polymer nanofibers and nanofibers from the additive. It is also possible to incorporate an additive to the formed nanofibers. Yet another possibility is to spin the functional additive simultaneously with a polymer, as described above.

(12) It is also possible to use a solid medium, such as a solid polymer or a wax, as a starting medium of the nanofiber forming process. This is normally called meltspinning. A big advantage with this method is that no water or liquid is added to the surface of the substrate and there is thus no need to evaporate the added water off by increased drying. It is thus possible to decrease the amount of drying, saving both energy and time.

(13) The fiber based substrate is preferably a paper or board produced from lignocelluloses. Other fiber based substrate such as non-woven, or textiles may also be used.

(14) The formation of particles is carried out by electrospinning or meltspinning, whereby ultrafine fibers are formed. The diameter of single fibers may, e.g., be less than 5 m or even less than 40 nm. The term electrospinning or meltspinning relates to generation of fibers in the nanosize region due to viscoelastic and electrostatic forces. The medium from which the fibers are formed may be a foam, a melt or a solid material, preferable a polymer.

(15) It is preferred that the formed nanofibers are deposited on a paper or board substrate. The particles can, e.g., be applied to a moving web of paper or paperboard during the paper making process. The method of the invention can thus be used for e.g. coating or sizing of paper or board. The method may be used to incorporate various types of polymer fibers, directly onto or incorporated into the surface of paper or board.

(16) The electrostatic particle formation of the present invention may be carried out by means of a conventional apparatus suitable for electrospinning. The apparatus may comprise a collector, a feed section and a voltage source adapted to provide an electrical potential difference between the collector and the feed section. The collector may be a metal plate for supporting the substrate, although a plate, a roll, a belt, a drum, a cylinder or the like also may be possible to use. The electrostatic voltage is preferably between 10 and 100 kV, more preferably between 40 and 60 kV, and the distance between the medium and the substrate is preferably between 10 and 300 mm, more preferably about 50 mm.

(17) The electrospinning of the particles can be conducted using both direct and/or alternating voltages. In one embodiment of the invention, the electrostatic processing is performed in the presence of an alternating current (AC) electric field. This can be achieved by applying an alternating electrical potential to either of the electrodes forming the electric field, e.g. an alternating electrical potential can be applied either to the feed section or to the collector. The use of AC potentials gives rise to an improved coverage of the deposited surface by the formed particles.

(18) The electrospinning may also be performed using both alternating current and direct current simultaneously. In this way, the form of the particles produced in the process may be varied. According to one embodiment, an alternating electrical potential is applied to the collector and a direct electrical potential is applied to the feed section, whereby particles in the form of quite large fibers may be produced. In another embodiment, an alternating electrical potential is applied to the feed section and a direct electrical potential is applied to the collector, whereby more fine particles may be produced.

(19) The feed section of the apparatus suitable may, e.g. be an opening, one or a number of nozzles or it is also possible to spin from an open surface, i.e. a free flowing liquid surface of roll.

EXPERIMENTAL

(20) The invention is further described with reference to some examples below. It is to be understood that the invention is not limited to the particular process steps and materials disclosed herein. The results are shown in the attached figures.

BRIEF DESCRIPTION OF THE FIGURES

(21) FIG. 1a) shows an uncoated paperboard used as a reference.

(22) FIG. 1b) shows an e-spun paperboard coated with 0.3 g/m.sup.2.

(23) FIG. 1c) shows an e-spun paperboard coated with 1.2 g/m.sup.2.

(24) FIG. 2a) shows an e-spun paperboard with a coat weight (dry) of 1.2 g/m.sup.2 and heated in the oven at 550 C. for 1 s.

(25) FIG. 2b) shows an e-spun paperboard with a coat weight (dry) of 1.2 g/m2 and treated in the oven at 550 C. for 3 s.

(26) FIG. 3a) shows a paperboard with a coating with electrospinning using Cartaseal VGL after post heating.

(27) FIG. 3b) coating with electrospinning using Cartaseal FTU after post heating.

Example 1

(28) Nanofibers produced by the use of electrospinning were deposited on an uncoated paperboard sample. The polymer used was polyvinyl alcohol and the concentration of the e-spun solution was slightly less than 10% by weight, whereas the deposited fibers have high solid content due to evaporation occurring during transfer between nozzle and substrate.

(29) The pictures attached have been taken with a Scanning Electron Microscope (SEM) and FIG. 1a shows a picture of an uncoated paperboard used as a reference. The fibers and the pores between fibers are evident from this picture.

(30) In FIGS. 1b and 1c, the weights of the deposited nanofibers are gradually increased which can be seen as higher amount of nanofibers but also that the local fiber-fiber coalescence of the nanofibers starts.

(31) The even spots shown in FIG. 1c indicate that a film has been formed. These spots were obtained by changing the electrospinning conditions so that the deposited nanofibers were partly wet, i.e. semidry, and hence coalesced into a film. Thus, no external heating was applied in this case to get the film formation.

Example 2

(32) Two paperboard samples which have been coated with e-spun nanofibers to a dry weight of 1.2 g/m2 as described in example 1 were post treated by heating. By heat-treating the deposited e-spun fibers it is possible to melt the deposited fibers in order to create a film. The coat weights in this case were quite low but it still gave an almost complete coverage of the paperboard which demonstrates the advantages of the present invention.

(33) The paperboard samples were treated in an oven at 550 C. for 1 s and 3 s, respectively. FIGS. 2a and 2b demonstrates the effect of post heating of the spun coated samples.

Example 3

(34) Commercial barrier chemicals (Cartaseal, Clariant) were tested and applied onto paperboard using the said deposition or coating method as described above. In this case, the flow properties were adjusted with polyethylene oxide.

(35) Recipe was:

(36) 2000 g Cartaseal VGL respectively FTU of 10% solids

(37) 200 g polyethylene oxide 600 000 of 6%

(38) Hence, e-spun fibers were created onto the substrate. In this particularly case, a rough substrate was used and the targeted dry coat weight was 10 g/m.sup.2. The paperboard was thereafter dried at 115 C. for 10 minutes and SEM images were captured on the spun coated paperboard.

(39) Results shown in FIG. 3a and FIG. 3b demonstrate that full coverage was obtained and that no pinholes exists.

(40) Table I demonstrates the results from the fat/grease resistance tests according to a modified ASTM F 119-82 standard test procedure which includes testes for specified chemicals at given temperature (40 C.).

(41) TABLE-US-00001 TABLE I Grease resistance Grease resistance through board to through board, the TLC plate, Chemical Show trough time Break-through time Cartaseal VGL >52 h >52 h Cartaseal FTU >52 h >52 h