WO2014175792A1

WO2014175792A1 - Expandable laminate, expanded laminate, a process for forming the expandable laminate, and uses of the laminates

Info

Publication number: WO2014175792A1
Application number: PCT/SE2013/050442
Authority: WO
Inventors: Kent Malmgren; Görgen NORDLANDER
Original assignee: Sca Forest Products Ab
Priority date: 2013-04-24
Filing date: 2013-04-24
Publication date: 2014-10-30
Also published as: EP2988937A4; EP2988937A1

Abstract

An expandable laminate, comprising at least two paper layers and at least one intermediate expandable layer, said expandable layer comprising expandable particles, which are in an unexpanded state, and each side of the expandable layer being attached to a surface of one of the at least two paper layers by an adhesive coating that has been applied onto the paper layer surface, thus leaving the expandable layer substantially free from adhesive, an expanded laminate, obtained by heat treatment of said expandable laminate, a process for forming the laminate comprising the steps of applying an adhesive to one side of a first paper layer to form an adhesive coating; applying dry expandable particles, which are in an unexpanded state, onto the first paper layer;removing any surplus of expandable particles not being adhesively attached to the first paper layer; applying an adhesive to one side of a second paper layer to form an adhesive coating; assembling the first and second paper layers so that the expandable particles are positioned between the paper layers, thus forming an intermediate expandable layer, and the of the laminate in the manufacture of packages or containers, or in the manufacture of building panels.

Description

EXPANDABLE LAMINATE, EXPANDED LAMINATE, A PROCESS FOR FORMING THE EXPANDABLE LAMINATE, AND USES OF THE LAMINATES

TECHNICAL FIELD

The invention relates to an expandable laminate having at least two paper layers and at least one intermediate expandable layer, in which the expandable layer comprises expandable particles in an unexpanded state. The invention also relates to an expanded laminate obtained from the expandable laminate. Furthermore, the invention pertains to a process for forming the expandable laminate. The expandable laminates, and corresponding expanded laminate, may be used in the manufacturing of packages or containers, or in the manufacture of building panels.

BACKGROUND ART

Expandable laminates may be used in many areas, including packaging and building industry, and are consistently gaining market shares. For example, they may replace corrugated board and paper board, which are well-known products combining low weight with high stiffness through so-called I-beam construction. In such a construction type, two outer strong layers, such as two liners from a unbleached softwood pulp, are connected by a distance layer having low density and lower strength requirements, such as a fluting from a mechanical or sulphite pulp, resulting in a laminate having high stiffness and large bulk. A common problem with corrugated board is that the distancing fluting layer may cause the liners to adapt to a wavy form since the liners only are connected to the fluting at its wave crests. These problems are often referred to as washboard effects.

Building panels of sandwich type typically include an internal foam core and oriented strand board faces, or similar, on the core, forming an I-beam construction type. A commonly used panel is an EPS plate having a foam core of expanded polystyrene (EPS). EPS plates are typically produced in a discontinuous process.

In the manufacture of food and liquid containers, expanded laminates may be used for increasing the rigidity and/or improve the isolating properties of the final packaging. Expandable laminates that can be expanded by simply heating them provide an opportunity to reduce volume, and thus reduce the cost for transportation and storage in the chain from manufacture to final use of the product. Such heat-expandable laminates provide an alternative to traditional laminates having fixed distance layers, arranged at the production site, which require a large amount of space in storage and transportation.

The expandable part in a heat-expandable layer regularly includes expandable particles, such as microspheres of core/shell type consisting of an exterior of a thermoplastic shell that encapsulates a low boiling point hydrocarbon. Upon heating, this outside shell will soften and expand as the hydrocarbon exerts a pressure on the internal shell wall. Such microspheres may expand to more than 20 times their original volume.

It is generally desired that an expandable layer upon heating should expand as much as possible, i.e. increase its thickness plenty fold, for a given amount of expandable material. One way to improve the expansion of the layer is to improve the expansion of the expandable particles upon heating. Consequently, many different types of expandable particles have been proposed to date. The development has also led to that new expandable particle products, especially tailored-made for specific purposes, have reached the market. An recent example is in the series of Expancel^® microspheres, manufactured by Akzo Nobel, where Expancel FG^® recently has been introduced as a thermoplastic expandable microsphere that meets the European requirements for direct food contact. Examples of heat-expandable laminates are found in US 3941634 A, DE 20117662 Ul, and WO 00/00408 Al. In WO 00/00408 Al, a heat-expandable packaging material having high printability and recyclability is disclosed. The distance layer comprises a matrix material, such as expandable starch, and cellulosic reinforcement fibres. Two outer surfaces, such as paper surfaces, are bonded to the distance layer by hydrogen bonds. A problem of the hitherto available expandable laminates is that they do not expand very much in thickness with respect to the loaded amount of expandable particles. Another common problem in the prior art is that the expandable layer may result in an uneven laminate. Moreover, the production methods available at present, beside leading to the problems of the products described, are also often tedious to carry out, for example by lacking the possibility of running a fast continuous process. There is a need for improved paper-based expandable laminate comprising expandable particles that also can be produced efficiently in a simple manner in a continuous process and at a low cost.

The objective of the present invention is thus to provide such an improved expandable laminate, and also provide an efficient process for its production. This object is achieved by the present invention as defined in the appended claims.

SUMMARY OF THE INVENTION

The present invention relates to an expandable laminate, comprising at least two paper layers and at least one intermediate expandable layer, where the expandable layer comprises expandable particles, which are in an unexpanded state, and where each side of the expandable layer is attached to a surface of one of the at least two paper layers by an adhesive coating that has been applied onto the paper layer surface, thus leaving the expandable layer substantially free from adhesive. The absence of adhesive in the expandable layer allows the expandable particles to expand to their full extent. The expandable laminate will show an excellent expansion upon heating, and accomplish an expanded laminate having even surfaces. Due to the excellent expansion, costs for transportation and storage is reduced in comparison to hitherto available expandable laminates.

The expandable layer preferably only consists of expandable particles, without addition of other components, in order to take the full advantage of the inherent expandable potential of the expandable particles used. If other components, especially adhesives, are present in the expandable layer they will in a more or less extent disturb the swelling of the expandable polymers, and hence lower the total expansion of the expandable layer, during heating.

The expandable layer may comprise a monolayer between two paper layers, in which the expandable particles are oriented side-by-side. However, the expandable layer may alternatively be constituted of a bi-layer of expandable particles obtained by the assembly of two paper layer surfaces, each having an attached monolayer of expandable particles, against each other, whereby a certain degree of relative movement of the two paper webs is allowed during production. The adhesive coating on each paper layer may be present in an amount of from 0.1 to 5.0 g/m², and preferably in an amount of from 0.3 to 1.0 g/m², and may be applied so as to cover at least 25% of the paper surface area to which it is applied. An advantage with the invention is that only a small amount of adhesive is necessary, since the adhesive need only be present in the interface between the particles and the paper layer. By utilizing a pattern the adhesive amount can be further decreased when attaching expandable particles of larger sizes.

The paper layers should preferably have a basis weight of 20 to 300 g/m². The basis weight should preferably not exceed 300 g/m², to allow convenient continuous manufacture and give end product having low weight, which is easy to handle as well. The paper layer should preferably have a basis weight above 20 g/m², to give sufficient strength, and avoid web-break during handling.

When the expandable laminate is intended to be used in the manufacture of containers or packages or wrapping foils, for example for holding liquid or food, the average size of the expandable particles is preferably from 5 μιη to 0.5 mm in diameter, and more preferably from 50 μιη to 0.4 mm. These particle sizes will provide laminates that in their expanded form will show thicknesses in the range of 1-3 mm, which is a suitable range for various packaging applications, such as wet and dry food packaging applications. This particle size and thickness is also suitable for various other applications, as an alternative to corrugated board or paperboard. The expandable laminate may, for example when intended as liquid packaging board preferably be provided with a barrier layer, such as a liquid barrier. Barrier layers may be useful also in other applications.

If the expandable laminate is intended to be used in the manufacture of building panels, the average size of the expandable particles may be from 1.0 to 6.0 mm in diameter, preferably 2.0 to 5.0 mm. This size range is adapted to achieve a thick expandable layer between two paper layers. A multilayer laminate comprising several expandable layers of which each surface is adjacent to a paper layer surface may be particularly suitable for building panels.

The present invention also relates to a process for producing the above mentioned expandable laminate. The process comprises the following steps:- applying an adhesive to one side of a first paper layer to form an adhesive coating,

- applying dry expandable particles, which are in an unexpanded state, onto the first paper layer,

- removing any surplus of expandable particles not being adhesively attached to the first paper layer,

- applying an adhesive to one side of a second paper layer to form an adhesive coating, - assembling the first and second paper layers so that the expandable particles are positioned between the paper layers, thus forming an intermediate expandable layer.

The process is preferably operated in a continuous manner meaning that each paper layer is feed from a reel of paper, and the steps above are carried out on the moving sheets of paper, where after the assembled laminate is rolled onto a reel, or cut into sheets that are stacked. By applying expandable particles to each paper surfaces adjacent to an intermediate expandable layer, the intermediate expandable layer obtained will be a bi-layer of expandable particles, whereby a thicker expanded laminate then is achieved, and the laminate is easier to roll. The obtained laminate having one intermediate expandable layer may be provided with additional intermediate expandable layers having a paper layer adjacent to each side. This could be desired when really high basis weight products are targeted.

The invention also relates to the use of the expandable laminate in the manufacture of packages or containers, for example for holding liquid or food, as well as the use of the laminate according to the invention in the manufacture of building panels.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates an unexpanded expandable laminate according to the prior art.

Fig. 2 illustrates an unexpanded expandable laminate according to the present invention.

Fig. 3 illustrates an expanded expandable laminate according to our invention.

DETAILED DESCRIPTION

The present invention is based on the realization that an improved expandable laminate can be produced in a simple and efficient manner resulting in an expandable laminate that shows a improved expansion upon heating, and having even surfaces in the final expanded laminate. With "paper" is meant in the context of the present invention all kinds of webs comprising of hydrogen-bonded cellulosic fibres independent of their basis weight, from very thin paper qualities to thick paperboard/carton board/card box. With "paper layer" is meant a layer consisting of a paper sheet or web or a laminate comprising at least one paper layer. With "substantially dry" is meant near or equal to conditioned climate, i.e. approximately 5-15% moisture content. With "adhesive" is meant any composition capable of, and intended for, binding one component to another by molecular forces. With "expandable layer" is meant a layer, that either may be continuous or discrete, that can be expanded. With "intermediate expandable layer" is meant an expandable layer positioned anywhere between two other, not necessarily expandable, layers. With "expandable particles" is meant any kind of particles that have the ability to expand, and also are intended therefore. Expandable particle thus covers for example the terms expandable bead, sphere, core/shell-particle, powder, granulate, etc. A common trademark for expandable particles is Expancel^®. With "monolayer" is meant a layer of particles lying side-by-side. With "bi-layer of expandable particles" is meant two superpositioned monolayers of expandable particles. With "plastic layer" is meant any kind of sheet or web mainly comprising one or several polymers. With "building panel" is meant all kinds of board used in various constructional contexts, and includes for example thermally and acoustically isolation panels.

The expandable laminate of the present invention comprises an intermediate expandable layer held between paper layers where the expandable layer comprises expandable particles in an unexpanded state. The expandable layer is attached to the paper layers by adhesive applied onto the paper surface, thus leaving the expandable layer substantially free from adhesive. The expandable laminate shows an excellent expansion upon heating, and accomplishes an expanded laminate having even surfaces. The expandable laminate has at least one expandable layer containing expandable particles that are unexpanded. Each side of the expandable layer should be attached to a paper layer surface within the laminate body by an adhesive coating that has been applied before onto the respective paper layer surface. The adhesive is present between the expandable particle surface and the paper surface at the point where they meet, and the expandable layer will be substantially free from adhesive. The word "substantially" here implies that very small amounts of the adhesive coating might accidentally enter the zone referred to as the expandable layer, for example due to that particles might move somewhat on the paper surface before they are fixated.

The expandable layer preferably consists of expandable particles only. However, smaller amounts of various additives, not functioning as adhesives, may also be comprised in the expandable layer.

An advantage of the invention is that an expandable laminate is achieved showing excellent expansion properties and even surfaces, since there is no adhesive that may hinder the expansion, and it can efficiently be produced.

A further advantage is that the construction of the laminate having paper sheets on both sides of the expandable layer will maintain the E-modulus of the paper sheets on the outer layers and create a large bulk in the mid-layer upon expansion resulting in an effective "I-beam construction", which is a well known construction for high bending stiffness. Since adhesive and expandable particles are located separately in the laminate, and not mixed together, this will result in an improved I-beam construction having higher stiffness. Without being bound to any particular theory, this effect might be due to that the strictly separate layers maintain the high elastic modulus in the paper layers, at the same time as the expanded intermediate expandable layer is bulkier, since there is no expansion limiting component in the expandable layer. Prior art technologies involving a glue matrix material with expandable particles are expected to hinder the expansion because of the matrix. The same applies to expandable paper products where expandable particles are added in wet condition (e.g. in partially dewatered webs or in pulp), so that the particles are held by inter-fiber bonds, which restrict the expansion of the particles. The expandable particles in the expandable layer according to the invention are more free to expand, which means that a lower density and higher stiffness can be obtained at the same weight for the expandable laminate product. A better expansion of the expandable laminate makes it possible to obtain thinner unexpanded laminates, thus saving space by having a reduced volume, which cuts costs for transport and storage. Moreover, the assembly of the expandable laminate having separate layers also enables each separate layer to be chosen to meet certain requirements, e.g. regarding barrier properties, friction, softness and isolation (temperature, sound).

The even surfaces of the expandable laminate according to the invention has an important effect of improving the printability properties. This is for example beneficial for graphical display paperboard and in consumer packaging. The even surfaces are also valuable for products intended to replace corrugated boards. This is because the so-called wash board effect related to the wavy intermediate layer in the corrugated board is avoided. The smooth, dense, and even layer of expandable particles, obtained by the process according to the invention, results in the even surfaces of the expandable laminate.

The expandable layer preferably solely consists of expandable particles.

The expandable particles are attached in a dry and an unexpanded form to the paper surface within the laminate by means of an adhesive applied onto the paper surfaces forming an adhesive coating. The expandable particles that are attached to each paper layer will form a monolayer. In the monolayer the particles lie side-by-side and are attached to the paper layer by the adhesive coating. The expandable particles are applied in excess amount to the adhesive coating present on the paper layer, and all particles that do not attach to the adhesive coating are removed from the paper layer by turning the paper layer upside-down or by shaking it. Thus, it can be ensured that an even layer of expandable particles is obtained. By assembling two paper layer surfaces, each having a monolayer of expandable particles attached thereto, a bi-layer of expandable particles from the two monolayers can be achieved. In this case the two monolayers constituting the bi-layer are not attached to each other, and can thus move slightly in relation each other, since the laminate is not consolidated. The tension in the web will keep the laminate together during the production steps. When the unexpanded laminate, containing the particular bi-layer, is ready, it will be kept tightly, either in the form of reels, which is preferred, or as stacks of sheets. Secondly, the laminate will become consolidated in a subsequent heating process when it is intended to be expanded for its final application. During the heating step, the polymer walls of the shells will soften and agglomerate with each other, i.e. sinter, and form a consolidated intermediate expanded layer. An advantage of having bi-layers of expandable particles in the expandable laminate is that the expansion of the final product is further increased, due to the double amount of expandable particles in the layer. Another advantage is that rolls of expandable laminate are easier achieved without having to use excessively large reels. This is due to certain flexibility between the paper webs forming the paper layers of the laminate enabling a slight relative movement during the reeling process, which counteracts the risks for folds, wrinkles, and other damages.

The type of adhesive used for the attachment of the expandable particles needs to be adapted depending on the specific application in which the laminate is to be used. This means that both curable and non-curable adhesives may be suitable. Examples of adhesive types that may be employed include dextrin, starch, other polysaccharides, polyvinyl acetate (PVAc), and hot melt adhesives, animal protein based adhesives, latex-based adhesives, polystyrene adhesives, adhesives based on styrene acrylic copolymers, cyanoacrylate adhesives, polyurethane adhesives, polyvinyl alcohol adhesives and polyvinyl pyrrolidone adhesives. A suitable basis weight of an adhesive coating onto the paper surface is from 0.1 to 5.0 g/m², preferably 0.3 to 1.0 g/m². However, the exact amount applied must be tailored for each intended application. The adhesive coating may be evenly distributed, or partially distributed in a suitable pattern according to the size of the expandable particles. The area of paper web surface coated with adhesive, expressed as the coverage degree, is preferably in the range of from 25 to 100%. If applied in a pattern, the adhesive should preferably cover at least 25% of the paper surface area to which it is applied. An advantage of applying the adhesive in a layer on the paper layer instead of as a matrix containing the expandable particles is that only small amount of adhesive is necessary. By utilizing a pattern the adhesive amount can be further reduced when attaching expandable particles of larger sizes. A low amount of adhesive is advantageous from several aspects, not at least considering material savings, laminate weight.

By first adding the adhesive to the paper layer and then adding the unexpanded expandable particles to the adhesive coating, more evenly distributed particles, lower amounts of adhesive, and lower total weight is obtained. As less adhesive is needed to achieve the laminate, the material cost for the laminate is reduced. The paper layers preferably have a basis weight of 20 to 300 g/m². This means that the paper layers cover several types of paper substrates depending on the end use of the laminate. If the strength requirements are low, for example if the laminate is to be used for production of isolation plates, a low-cost paper, based on recycled pulp or mechanical pulp may be suitable. If, on the other hand, strong laminates are desired, a paperboard, such as Kraft liner would be a more suitable for the paper layers. For display products, where printing quality is important, a high-quality, coated paper grade is preferred. For liquid packaging boards (LPB), both strength and printability are important, since the top layer often also is printed on. A top liner of a bleached Kraft pulp is preferably used in a expandable laminate intended for liquid packaging board, at least for the paper layer forming the outer surface of the container or package. A liquid packaging board preferably includes a barrier layer, for example in the form of a plastic layer, aluminium foil or wax.

The outer paper layer primarily functions as a provider of a high elastic modulus, good printing surface, and carrier for the expandable particles. If there are intermediate paper layers these will have the main function of carrying expandable particles. It should be pointed out that different applications might have different demands regarding the surface properties, such as specific demands regarding printability, barrier for different conditions, gloss, friction, tactile properties, etc.

For example, for achieving good printability a fine paper is preferably used. A publication paper could be used as an alternative in applications where yellowing is not a problem. A liner or sack paper may preferably be used for applications where high tensile strength, burst strength, bending stiffness, etc., is needed. Such applications include for example liquid packaging board or outer paper layers of building panels. The pulps used may be of chemical, semi-chemical, or mechanical type, or mixtures thereof. A paper having high tensile strength is typically made from a chemical pulp, such as an unbleached sulphate (Kraft) pulp. Bleached Kraft pulps have generally somewhat lower strength properties than unbleached Kraft pulps, but are still regarded as a strong pulp quality, and are suitable in applications where both high strength and good printability are required.

All paper layers need not have the same basis weight. For example, in the case of a building panel made from the expandable laminate, comprising several intermediate expandable layers, the intermediate paper layers may consist of paper webs of a lower basis weight than the outermost paper layers of the laminate.

The expandable particles are preferably of the shell/core type, having an outer polymer shell encasing carbon dioxide, or a low-boiling hydrocarbon, which normally is pentane. Suitable polymers for use in the shell of the expandable particles are polystyrene (PS), polylactic acid (PLA), co-polymers of Expancel^®-type, where the main polymers are based on monomers like acrylonitrile, methacrylonitrile, methyl acrylate, methyl methacrylate, styrene, divinylbenzene, and the expandable gases are hydrocarbons such as pentane. In expandable polystyrene (EPS) particles the contained expandable gas is typically pentane. For expandable polylactic acid (E- PLA), on the other hand, carbon dioxide is preferably used as the expanding gas. The E-PLA is a rather new product that has an advantage of being completely biodegradable and the use of E-PLA in the expandable laminate provides an excellent biodegradable expandable laminate, which is environmentally friendly. The adhesives used are preferably biodegradable as well.

New Expancel^®-products are available that might be of certain interest to use, as expandable particles in the laminate of the invention. For example, Expancel FG^® has recently been introduced as a thermoplastic expandable microsphere that meets the European requirements for direct food contact, and may thus be suitable in expandable laminates for manufacture of packages intended for holding food.

Different applications require different thickness of the expanded laminate. This may advantageously be controlled by the type of expandable particles used, the amount and size of the expandable particles, by the amount and distribution (i.e. pattern) of adhesive, and by the number of expanded layers in the laminate.

The amount of expandable particles in the expandable laminate may be in the range of from 10 to 10 000 g/m². For example, in applications for packaging, such as liquid board packaging, the weight of expandable particles is suitably from 5 to 150 g/m². Higher amounts, from 150 g/m² to 10 000 g/m², may be suitable for applications as building materials, such as building insulating panels. The amount added is mainly regulated by the size of the expandable particles, and may also be regulated by the number of intermediate expandable layers used. However, the most preferred expandable laminate will only have one expandable layer due to the simplicity of manufacture. More expandable layers are added if necessary for a certain application.

A suitable average expandable particle size is from 0.005 mm to 6.0 mm referring to the average diameter. The particles typically expand to a diameter of up to four times the original diameter upon a complete expansion by heating. In case the expandable laminate is intended for use in liquid or food packaging or wrapping materials, a preferred average particle size is from 5 μιη to 0.5 mm, more preferably 50 μιη to 0.4 mm. The expanded laminate, having one expandable layer, which is preferred in the case of packaging materials, will then reach a thickness of 1 to 3 mm, which is suitable for packaging purposes. If instead the intended use is building panels, insulating panels, furniture construction, or similar constructional materials, the preferred average expandable particle size is from 2.0 mm to 6.0 mm. Here the final expanded laminate will often have a thicknesses of several centimetres, e.g. 1 to 5 cm, or even more than 10 cm. In order to obtain such thicknesses, larger expandable particles may be used, and several, perhaps two to four intermediate expandable layers may be required. If the expandable laminate is intended to be used in the manufacture of building panels, the average size of the expandable particles may be from 2.0 to 12.0 mm in diameter, preferably 2.0 to 6.0 mm. This size range is adapted to achieve an as thick expandable layer as possible between two paper layers having a suitable thickness. A building construction material will often require a thickness of more than 100 mm. Therefore an expanded laminate containing one intermediate expandable layer might not be thick enough. Consequently, a multilayer laminate comprising several expandable layers of which each surface is adjacent to a paper layer surface is particularly suitable for building panels. The number of suitable intermediate expandable layers depends on the specific product.

The expandable laminate might also find use in various display products, such as posters, signs etc., and for these kinds of applications the whole range of suitable particle sizes may be employed.

The expansion of the expandable laminate is accomplished by a heating to a suitable temperature, which is determined by the specific expandable particle used. Commonly, the temperature would be between 70-220°C, preferably 95-110°C for expandable PS, preferably 70-85°C for expandable PLA, and preferably 80-210 °C for Expancel^®-particles in order to obtain sufficient expansion. The process may involve rolling out the expandable laminate if the laminate is provided as a roll, and heating by IR, hot air, steam, or by hot surfaces for a suitable time until full, or desired, expansion has been achieved. During expansion, the surface of the particles may also soften or partially melt, so that the particles attach to each other. The shape of the particles may change during expansion, as they have to adapt to the space available between each other and the paper layers.

In the unexpanded phase substantially all expandable particles are bonded to the surface of the paper surface in one monolayer by the adhesive, and during expansion when the particles expand to several times their original volume, at least some of the particles will be forced to move in a direction away from the paper surface, since there not will be enough space for all the particles to stay in contact with the paper surface. The force of the expansion is thus large enough to break some of the particle-paper adhesive-mediated attachments between the particles and the paper surfaces, or to delaminate the adhesive coating from the paper layer at certain minor areas. The expandable laminate can be directly expanded with heat as soon as the laminate have been assembled, or the laminate is first formed to a reel, by a reeling-up process, or cut into sheets for later expansion in a separate process. By expanding the laminate in a separate subsequent step, transport is facilitated since the material is in a more compact state.

The present invention also relates to a process for producing the above mentioned expandable laminate. The problem of obtaining a paper-based expandable laminate providing improved expansion ability in a simple and efficient manner, and at the same time having even surfaces in the expanded form of the laminate, is solved by the following process steps:

- applying an adhesive to one side of a first paper layer to form an adhesive coating,

The process is preferably operated in a continuous manner where each paper layer is fed from a roll of paper web, and the steps above are carried out on the moving paper web, and the process preferably includes a step of rolling up the assembled laminate onto a reel, or cutting into sheets that are stacked.

It is advantageous to attach unexpanded expandable particles to substantially dry paper sheets having an adhesive coating, instead of adding them to partially dewatered webs (8-20 wt% dry content) as in US 3941634. In this prior art method in which the expandable particles are held by the interfiber bonds (primarily hydrogen bonds), the way of bonding will reduce the expansion. The application of expandable particles on dry paper will also give a more even distribution of expandable particles, many different expandable materials can be used, for example expandable particles having a relatively low expansion temperature (e.g. below 100°C), which cannot be used in wet addition, since they would expand during drying of the web. Thus, by means of applying the expandable particles to the adhesive coating according to the invention, also low temperature expanding particles can be used, to achieve unexpanded expandable laminates, which can be expanded at any desired point in time, thus providing opportunities to save costs for transportation and storage.

The application of adhesive may be performed by roll-, blade-, curtain-, foam-coating or spraying. The suitable adhesive temperature is dependent on the type of the adhesive; however, the maximum temperature must be below the expansion temperature of the chosen expandable particles in order to avoid premature expansion. The time period from applying adhesive to applying the beads is normally very short; however, some delay for penetration of the adhesive into the paper web might be needed, and when using hot melt adhesives some delay might also be caused by the necessity of achieving a suitable temperature.

The paper layer surface coated with an adhesive may preferably be passed through a chamber containing an excess amount of expandable particles, where the particles attach to adhesive coating. Excess particles are removed, e.g. by directing the coated side of the paper layers downwards so that any particles, which are not in contact with the adhesive coating will fall off. The removed excess particles are preferably re-circulated back to the process step of addition of expandable particles. The excess, i.e. surplus, of expandable particles can be collected, either by gravity into a tray, or by suction, i.e., a negative air pressure. When suction is used the expandable particles will preferably be transported via a cyclone and screw feeding to a tray. A second paper layer surface is coated with an adhesive in the same way as the first paper layer. The second paper layer is preferably of the same paper type as used for the first paper layer. The second paper can also be treated with expandable particles if, for example, a thicker product is desired. Both paper layers having adhesive coating and expandable particles there between are then laminated together by applying a light pressure. The pressure is chosen so that the expandable articles are brought in secure contact with the adhesive but are not substantially compressed. Preferably, a roll nip with light pressure or distance control is utilized for the assembly of the expandable laminate.

Due to the fact that the invention is based on a continuous process, preferably by making rolls in a first step, the production efficiency is higher. By applying expandable particles to each paper surfaces adjacent to an intermediate expandable layer, the intermediate expandable layer obtained will be a bi-layer of expandable particles whereby a thicker expanded laminate can be achieved, as has been described above. During the expansion step the expandable particles will soften and attach to each other, forming a bonded intermediate expanded layer, and thus a fully consolidated expanded laminate product.

The expandable laminate having one intermediate expandable layer may be provided with additional intermediate expandable layers having a paper layer adjacent to each side. This could be desired when products having high thickness or high basis weight products are targeted. In brief, the process of forming the laminate according to the steps above will be repeated by just replacing the first paper layer with the obtained laminate instead. This can then be looped until the desired number of intermediate expandable layers is obtained. As an alternative, several expandable laminates having one intermediate expandable layer may also be glued together, paper-to-paper, without an intermediate expandable layer between the separate expandable laminate elements. The whole process may be described by the following additional process steps to those described above (forming an expandable laminate having one intermediate particular layer):

- providing a laminate having a first paper layer an expandable intermediate layer and a second paper layer, as described above, - applying an adhesive to the other side of the second paper layer,

- applying dry expandable particles, which are in an unexpanded state, onto the second paper layer and removing any surplus of expandable particles,

- optionally applying an adhesive to one or more additional paper layers, applying dry expandable particles, which are in an unexpanded state, onto the one or more additional paper layers, and removing any surplus of expandable particles,

- applying an adhesive to one side of a final paper layer,

- assembling the paper layers so that the expandable particles are positioned between the paper layers, thus forming intermediate expandable layers.

An alternative way to form a thicker laminate could be to simply glue together several obtained laminates in a paper-to-paper fashion without caring for an intermediate expandable layer between the separate expandable laminate elements.

The adhesive is preferably added as in liquid form (melted, solution, dispersion). The adhesive can be applied by roll-, blade-, curtain-, foam-coating or spraying. In case spraying is used, the pattern of the droplets is preferably evenly distributed. The droplet size when applied to the surface should be chosen with regard to the required contact area of each bead. The equipment used may be such that are conventionally used for coating a surface with adhesive by the mentioned techniques. The adhesive applied results in a suitable tackiness for the expandable particle to adhere, and thereby cause an attachment of the adhesive-coated paper surface. A suitable tackiness should preferably be reached within a short period of time. With suitable tackiness is meant that an expandable particle will immediately be adhered and firmly attached to the paper surface upon contact. The temperature during, or after addition, may range from ambient conditions to a temperature below the expansion temperature of the expandable particles, and of the melting point of the polymer shell. An increased temperature above ambient may have the advantage of achieving a suitable tackiness in shorter period of time, and may also be necessary when using hot melt adhesives in order to maintain stickiness long enough for the expandable particles to become attached.

The expandable laminate of the invention can be used in all applications where corrugated board or paperboard is used. An example of an application of the expandable laminate is as containers or packages for various purposes, for example within the food industry, such as liquid packaging board. An unexpanded expandable laminate may be transported, preferably in the form of rolls, or bales of stacked sheets of the laminate, to a factory producing packages. At the packaging unit, the expansion of the material can take place by heating the material, either before converting the board to a package, or after. When the expansion has been accomplished a conventional process for making the package may follow. The unexpanded expandable laminate could also be expanded before transport to the factory; however, this would not give the space saving advantage. As an alternative, the unexpanded expandable laminate is first used for making the package, which is then heated to expand the folded package to its final proportions. A container or package made from an expanded laminate board has several benefits, for example superior mechanical strength and stiffness, i.e. a high flexural rigidity, as well as excellent thermally and acoustically insulating properties. The expandable laminate may, when intended as liquid packaging board preferably be provided with a barrier layer, such as a liquid barrier, for example in the form of a separate plastic layer (e.g. in the form of a polymer film, such as a thermoplastic film), an aluminium foil or a wax layer. Especially polyethylene is suitable as a liquid barrier. The barrier layer may be applied on both sides of the laminate; however, if only used on one side, the side forming the inside of the packaging is preferred.

The expandable laminate according to the invention will offer packaging solutions that can reduce impact from ambient temperature, due to its intermediate expanded layer present in the final packaging product, which can act as an insulating layer. This may be of special interest for liquid packaging board, especially for sensitive liquids such as dairy products such as milk or juice.

The beneficial properties of mechanical strength and insulation also make the expandable laminate suitable for construction materials, in particular such as building panels. Another possible product area is as a material for furniture construction. For these kinds of applications, a thicker laminate having a higher basis weight than in the case of packaging applications is probably necessary. Normally, the finished expanded laminate should be several centimeters in thickness, even more than 10 cm, implicating that several intermediate expandable layers in the laminate may be required. A use of larger expandable particles, for example exceeding a diameter average of 2 mm, may also be preferred. This may be compared with the case of packaging where the average expandable particle size often will be less than 1 mm, however not less than 0.005 mm.

The weight ratio between the paper and the expandable particles in the finished expandable laminate is suitably in the range of 0.004 to 60.

The invention will now be described by reference to Figures 1-3, where Fig. 1 schematically illustrates an unexpanded expandable laminate according to the prior art. The laminate includes a first outer layer 1, onto which an intermediate layer 2 including expandable particles 3 comprised in a matrix 4 of adhesive is applied, and a second layer 5 applied onto the intermediate layer 2. In this laminate, the expandable particles are thus encapsulated by adhesive. When the laminate is subjected to a heat treatment in order to obtain expansion, the particles will expand. However, the adhesive matrix restricts the expansion since the adhesive is not as easily stretched as the shells of the expandable particles, and since the adhesive itself occupies space between the particles. Fig 2. schematically illustrates an unexpanded expandable laminate according to the invention. This laminate includes a first paper layer 6 onto which an adhesive coating 7 has been applied. Unexpanded expandable particles 8 have been applied to the adhesive coating 7, forming an intermediate expandable layer 9 and are thus attached to the paper layer 6 by adhesive attachment at the point where they meet. A second paper layer 10, onto which an adhesive coating 11 has been applied, is applied onto the intermediate layer 9, so as to co come into adhesive attachment to the expandable particles at the points where they meet. The intermediate layer 9 is substantially a mono-layer of expandable particles 8, since any excess particles not adhesively attached to the first paper layer have been removed during the manufacture of the expandable laminate. As illustrated in Fig. 2, adhesive is present only in the interface between the paper layer and the expandable particles 8 of the intermediate layer 9, and there is no adhesive present in the interface between adjacent expandable particles. This means that there is no adhesive matrix that can hinder the expansion of the expandable particles.

Fig. 3 schematically illustrates laminate of Fig. 2 after expansion, i.e. after heat treatment having caused the expandable particles 8 to expand. In this Figure the expanded particles are drawn as spheres, but in practice they will attain a more bulky and uneven appearance and will fill up substantially all space between the paper layers. As shown in Fig. 2, the expandable particles 8 lay side-by-side in a mono-layer before expansion. When the particles expand, they are forced to "climb" on each other as illustrated in Fig. 3 since there isn't room enough for the particles to expand within the monolayer.

Claims

1. An expandable laminate, comprising at least two paper layers and at least one intermediate expandable layer, characterized in that the expandable layer comprises expandable particles, which are in an unexpanded state, and that each side of the expandable layer is attached to a surface of one of the at least two paper layers by an adhesive coating that has been applied onto the paper layer surface, thus leaving the expandable layer substantially free from adhesive.

2. The laminate according to claim 1, in which the expandable layer consists of expandable particles.

3. The laminate according to any of the previous claims, in which the expandable layer is a bi-layer of expandable particles.

4. The laminate according to any of the previous claims, in which the adhesive coating on each paper layer is present in an amount of from 0.1 to 5.0 g/m², preferably 0.3 to 1.0 g/m², and at a coverage degree of at least 25% of the paper layer surface.

5. The laminate according to any of the previous claims, in which the paper layers have a basis weight of 20 to 300 g/m2.

6. The laminate according to any of the previous claims, in which the size of the expandable particles is defined by a mean diameter of from 5 μιη to 0.5 mm, preferably 50 μιη to 0.4 mm.

7. The laminate according to any of the previous claims, further comprising at least one plastic layer, preferably comprising polyethylene, as a liquid barrier, and optionally an oxygen barrier layer, preferably comprising aluminium foil, ethylene vinyl alcohol, polyvinylidene chloride, polysaccharides, or biaxially oriented thermoplastics, or combinations thereof.

8. The laminate according to any of the previous claims, in which the size of the expandable particles is defined by a mean diameter of from 1.0 to 6.0 mm, preferably 2.0 to 6.0 mm.

9. An expanded laminate, obtained by heat treatment of the expandable laminate according to any of the previous claims.

10. A process for forming the laminate according to claim 1 comprising the steps:

- applying an adhesive to one side of a first paper layer to form an adhesive coating, - applying dry expandable particles, which are in an unexpanded state, onto the first paper layer,

11. The process according to claim 10, in which the intermediate expandable layers is a bi-layer of expandable particles obtained by an application of expandable particles to both paper surfaces adjacent to said intermediate expandable layer.

12. The process according to claim 10 or 11, wherein additional intermediate expandable layers are applied having a paper layer adjacent to each side thereof, comprising the steps:

- applying an adhesive to the other side of the second paper layer;

- applying an adhesive to one side of a final paper layer, - assembling the paper layers so that the expandable particles are positioned between the paper layers, thus forming intermediate expandable layers.

13. The process according to any of the claims 10-12, in which the application of adhesive onto the paper layers is performed by spraying.

14. Use of the laminate according to any of the claims 1-9, in the manufacture of packages or containers.

15. The use of claim 14, wherein the laminate is used in the manufacture of containers or packages for holding liquid or food.

16. Use of the laminate according to any of the claims 1-9, in the manufacture of building panels.