The invention also relates to a plant for dryly producing a non-woven fibre web by means of two air-lay stations placed one after the other and each comprising at least one fibre feed duct to feed fibres to the station from a fibre source, at least one air-lay forming head connected to the fibre feed duct and at least partly defined by a perforated screen, and a suction box for successively sucking fibres in the forming head through the screen.

The invention moreover relates to a non-woven fibre web produced by means of the method and the plant for dryly producing a non-woven fibre web.

Finally, the invention relates to the use of Cotton Linters Pulp (CLP), which is a relatively inexpensive by-product derived from the seed coats of the cotton plant, for dryly producing a relatively inexpensive non-woven air laid cotton fibre web.

The air-lay technique is known from e. g. GB Patent No. 1 499 687 which describes a plant for dryly producing a non-woven fibre web. This plant has an air-lay forming head in form of a box which is defined by a perforated base at the bottom. Above the base is rows of rotating wings which distribute the fibres during operation into flows across the perforated base. Below this is placed an air-permeable forming wire which is running endlessly during operation for accommodating fibres which are drawn through the openings of the perforated base by the negative pressure in a suction box placed under the forming wire.

This plant is well suited for producing a non-woven fibre web of like fibres or at least of fibres on the whole having the same configuration. If the fibre web is to be made of fibres having substantially different lengths, the plant can however not function optimally, and the fibre web produced cannot obtain a completely satisfactory quality.

These disadvantages can be assigned to the fact that it is not possible to dimension the openings of the base so that they are suited for both the relatively short and the relatively long fibres. It is therefore necessary to choose openings having a size which is between the optimum sizes for each of the two types of fibres. The openings will then easily be too large for the short fibres and too small for the long ones.

GB No. 2 031 970 describes an air-lay plant having several forming heads of the above-mentioned type placed in a row one after another above a joint forming wire. By means of this arrangement, the plant is able to operate with both short and long fibres with good results. Fibres of one specific length are then distributed from a forming head having base openings which are suited for exactly this fibre length. Layered fibre webs can be produced by means of this technique as it also is desired in some cases. The known plant is however not suited for producing homogeneous fibre webs of fibres of different lengths.

The last-named problem has been tried solved by placing the forming heads on top of each other in stead of in a row one after the other. Such a plant is described in the patent specification WO 96/10663. In this case, the plant has three forming heads. The uppermost one is supplied with SAP (Super Absorbent Powder) and has a base with openings that exactly fit the size of the SAP. The middle forming head is supplied with short cellulose fibres and has a base that exactly fits the size of these short fibres, and the lowest forming head is supplied with long thermobinding fibres and has a base that exactly fits the size of these long fibres.

The base of the upper forming head forms an upper delimitation to the middle forming head, and the base of this head forms an upper delimitation to the lowest forming head.

The openings in the base of the lowest forming head are so large that the SAP and the cellulose fibres can pass through the openings simultaneously with the thermobinding fibres, and the openings in the base of the middle forming head are so large that the SAP can pass through these openings simultaneously with the cellulose fibres.

Below the base of the lowest forming head is an air-permeable forming wire which is running endlessly during operation, and below this a suction box which is joint to all three forming heads.

When the plant is running, the SAP is driven via the openings in the base of the upper forming head down into the middle forming head, the SAP and the cellulose fibres are driven via the openings in the base of the middle forming head down into the lowest forming head, and the SAP, the short cellulose fibres and the long thermobinding fibres are driven via the openings in the base of the lowest forming head down in a layer onto the forming wire.

The SAP and the short cellulose fibres are first mixed in the middle forming head and then the long thermobinding fibres in the lowest forming head. By means of this mixing, a homogenous fibre web should be obtainable.

However, it is difficult to control the different air flows of the process such that the resulting fibre web obtains an acceptable homogeneity.

This is among other things due to the fact that the air flows inevitably will interfere with each other. Thus, air is simultaneously blown in on both sides of the base of both the upper and the middle forming head, and the negative pressure from the suction box furthermore has to propagate to the base of the middle forming head via the openings in the base of the lowest forming head and to the base of the upper forming head via the openings in the middle forming head. At the same time, the SAP, the short cellulose fibres and the long thermobinding fibres are moving in the opposite direction through the openings of the respective bases, said openings are thereby randomly blocked to the flow of the air to a greater or smaller extent.

When the forming heads are placed on top of each other instead of in a row after each other, the plant will furthermore be disproportionately high. Therefore, the plant known from the patent specification WO 96/10663 can rarely find room in existing buildings, and new buildings for the plant will normally look quite out of place in an otherwise harmonious group of buildings. Furthermore, the great height means that it is difficult to inspect the plant and control the process in this plant.

Today, dryly-produced fibre webs are used extensively for many different purposes, of which napkins, toilet paper, diapers, sanitary napkins and products for people suffering from incontinence can be mentioned.

In order to be able to keep the prices of these products at a commercially acceptable level, a large part of the fibres that form part of the air-laid fibre product are normally inexpensive cellulose fibres which can be mixed with e. g. thermobinding fibres depending on the application purpose just as the products also can contain SAP (Super Absorbent Powder) to increase the ability of the products to absorb liquids.

However for some purposes, products are wanted that are substantially softer than the products that are based on cellulose fibres.

Such a very soft, non-woven fibre product can be produced by utilising cotton fibres instead of cellulose fibres. Due to the high price of the cotton fibres, the resulting products will however be to expensive for practical application.

The production of cotton leaves a by-product called Cotton Linters Pulp (CLP) and consisting of short residual fibres from the seed coats of the cotton plants.

This by-product is far less expensive than common cotton fibres. It is therefore possible to use it for producing a very soft, non-woven cotton fibre web which, considering the quality, would be able to compete on price with fibre webs based on cellulose fibres.

However, CLP consists of fibres that are very fine and very short and also has a large content of unopened fibres or nits.

The above known plants and methods can therefore not be used with a satisfactory result to produce a nits-free, homogenous, non-woven fibre web of CLP-fibres and thermobinding fibres.

The object of the invention is to provide a method and a plant of the kind mentioned in the opening paragraph, by means of which a homogenous, nits-free, non-woven fibre web can be produced on the basis of CLP-fibres and thermobinding fibres.

A second object of the invention is to provide a non-woven fibre web of the kind mentioned in the opening paragraph, that has a homogenous and substantially nits-free structure.

A third object of the invention is to provide a use of Cotton Linters Pulp (CLP) as base material for dryly producing a non- woven cotton fibre web.

The novel and unique features of the method according to the invention are the fact that a first air flow with short fibres from a fibre source is generated, that the first air flow is made to pass a first perforated screen having openings of a size that mainly allow opened, but not unopened short fibres to pass, that the first air flow is made to pass an air- permeable first forming wire running endlessly during operation and during this retaining a non-woven layer of short fibres, that a second air flow with short fibres from the fibre layer on the first forming layer is generated, that a third air flow with long fibres from a fibre source of long fibres is generated, that the second and third air flow are made to pass a second perforated screen having openings of a size that allow both short and long fibres to pass, that the second and third air flow are made to pass an air-permeable second forming wire running endlessly during operation, and are made to settle as a fibre layer on the second forming wire, and that a non-woven fibre web is made of this fibre layer upon further treatment.

By using this method, a non-woven fibre web of short and long fibres obtains a homogenous structure which at the same time is nits-free when the unopened short fibres or nits are extracted from the first perforated screen.

The novel and unique features of the plant according to the invention are the fact that a separate fibre source belongs to each station, that the first station has a fibre collector placed under the screen of its at least one forming head, that the second station has a forming wire placed under the screen of its at least one forming head, and that the two stations are connected by at least one conveyor for conveying fibres from the fibre collector of the first station to the forming head of the second station.

When the screen openings of the first station have a size that fit the short fibres which e. g. can be CLP-fibres, and the screen openings of the second station have a size that fit the long fibres which e. g. can be thermobinding fibres, the plant can be utilised for producing a non-woven fibre web having a homogenous structure.

The first station is then fed short fibres whereas the second station is fed both long fibres and short fibres having passed the screen openings of the first station. The short and long fibres thereby obtain being carefully mixed in the second station and can thereby form a fibre layer having a homogenous structure on the forming wire of the second station.

By means of the above arrangement, unopened fibres or nits can selectively be extracted from the first station so that the resulting fibre web also becomes at least essentially free of nits.

The conveyor connecting the first and the second station together can be a belt conveyor but can in an advantageous embodiment be a fibre feed duct connecting the fibre collector of the first station to the forming head of the second station. By means of a fan generating an air flow through the fibre feed duct, the fibres are sucked from the fibre collector of the first station up into the fibre feed duct in order to then via this being blown into the forming head of the second station.

The fibre collector of the first station can be of any expedient kind.

In an advantageous embodiment the fibre collector can quite simply be the suction box of the first station whereby the extraction from the suction box takes place by means of the fibre feed duct which connects the two stations.

In a second advantageous embodiment the fibre collector can be a forming wire which then acts as a travelling filter for filtering off the short fibres from the air flow through the screen openings of the first station.

The novel and unique features of the fibre web according to the invention are the fact that it comprises a mixture of thermobinding fibres and CLP-fibres screened of unopened fibres or nits, and that it has a homogenous and nits-free structure. Such a web has an extraordinarily great softness which is wanted for many purposes. Furthermore, fibre webs on cotton basis are good for the skin and non-allergenic.

The novel and unique features of the use according to the invention are to use CLP as base material to produce a non- woven air-laid cotton fibre web together with thermobinding fibres.

A cotton fibre web based on conventional cotton fibres will be to expensive for practical application due to the relatively high price of these fibres. By using CLP as base material instead, an inexpensive product is obtained having the same advantages of the cotton fibre web based on conventional cotton fibres.

However, the CLP fibres are relatively short and weak. The strength of the web is therefore obtained by binding the fibres together by means of thermobinding fibres using the method and plant according to the invention.

An especially good strength is obtained according to the invention when the thermobinding fibres are bicomponent fibres of the kind that each has a core consisting of a rather strong material and that is surrounded by an outer coating having a lower melting point than the core.

During the production of the fibre web, the outer coating will melt readily and effectively together with both each other and the short and weak CLP fibres, and especially in nodal points whereby the advantageous properties of the CLP cotton fibres are completely maintained whereas the core of the bicomponent fibres transfers its great strength to the cotton fibre web.

The invention will be explained in greater detail below, describing only exemplary embodiments with reference to the drawing, in which Fig. 1 is a diagrammatic view of a first embodiment of a plant according to the invention, fig. 2 is a diagrammatic view of a second embodiment of a plant according to the invention, fig. 3 is a diagrammatic view of a third embodiment of a plant according to the invention, and fig. 4 is a diagrammatic view of a fourth embodiment of a plant according to the invention.

In the following the invention is described on the assumption that a homogenous and nits-free cotton fibre web of CLP-fibres and thermobinding fibres is to be made.

CLP has very fine and short fibres and also a large content of unopened fibres or nits. Therefore, it has so far not been possible in practice to use CLP for producing a cotton fibre web of satisfactory quality.

The fine and short CLP-fibres are bound to the thermobinding fibres in a manner known per se in order to thereby give the fibre web the necessary strength. The resulting cotton fibre web is relatively inexpensive to produce and has a very soft structure. There is thus a considerable market for such a product.

The plant according to the invention in fig. 1 has a first air-lay station 1 for CLP-fibres and a second air-lay station 2 for both thermobinding fibres and CLP-fibres which have been screened of nits in the first station. If the thermobinding fibres are mixed with unscreened CLP-fibres, the result is a fibre web of unsatisfactory quality.

The main components of the first station 1 is a hammer mill 3 for defibrating CLP from a roller 4, and a first forming head 5 having a first perforated base 6, a first set of rotatable wings 7 placed in rows above the base 6, and a first suction box 8 placed under the base 6.

When the first station is operating, the forming head 5 is supplied with defibrated fibres from the hammer mill 3 via first feed duct 9 having a first feed fan 10.

The wings 7 are sweeping the supplied CLP-fibres across the base 8 in continuous flows. During this, the fibres are sucked successively down into the suction box 8 via the openings 11 of the base 8 by a second feed fan 12 which is connected to the suction box via the second feed duct 13.

The openings 11 of the base 8 are arranged with a size that allows the fine, short CLP-fibres to pass but not the coarser nits.

A return fan 14 serves for returning the nits to the hammer mill 3 via a return duct 15 where the nits can be opened.

The screened fibres are collected in the suction box which thus simultaneously acts as fibre collector.

The main components of the second station 2 are a fibre source 16 with a bale breaker 17 for breaking up the thermobinding fibres from a bale of thermobinding fibres 18, and a second forming head 19 having a second perforated base 20, a second set of rotatable wings 21 placed in rows above the base 20, a forming wire 22 placed under the base 20, and a second suction box 23 placed under the forming wire 22 and via a suction duct 24 connected to a fan 25 for creating a negative pressure in the suction box.

When the second station is operating, the second forming head 19 is supplied with screened CLP-fibres from the first suction box 8 via the second feed duct 13 by means of the second feed fan 12.

The second forming head 19 is simultaneously supplied with thermobinding fibres from the fibre source 16 via a third feed duct 26 with a third feed fan 27.

In the second forming head 19, the supplied with CLP-fibres are mixed with the supplied thermobinding fibres. The second set of wings 21 are sweeping the now mixed fibres across the second perforated base 20 in continuous flows. During this, the fibres are sucked successively down into a layer on the forming wire 22 via the openings 28 of the base by means of the fan 25 which is connected to the suction box 23 via the suction duct 24.

The openings 28 of the second base 20 are arranged with a size that allow both CLP-fibres and thermobinding fibres to pass.

Thereby, a fibre layer of CLP-fibres and thermobinding fibres is made on the forming wire 22, said fibre layer has a homogenous and nits-free structure. In a known subsequent process (not shown), the fibre layer is finally converted into the desired fibre web which among other things can be dust bonded by means of of a layer of latex.

The plant according to the invention shown in fig. 2 corresponds essentially to the one shown in fig. 1 and referred to above. Like parts are therefore designated by the same reference numerals.

In this case, the fibre collector of the first station is however a forming wire 29 acting as a travelling filter for filtering off the CLP-fibres from the air flow through the base openings 11 of the first station 1.

The CLP-fibres are sucked successively down onto the forming wire 29 via the openings 11 of the first base by means of a fan 30 which is connected to the suction box 8 via a suction duct 31. An evenly distributed CLP-fibre layer 32 settling on the forming wire 29 is successively conveyed with this wire towards the inlet openings 33 of the second feed duct 13 and supplied in a continuous flow into the second forming head 19 via the second feed duct 13 by means of the second feed fan 12.

This solution has especially the advantage that the feeding of screened CLP-fibres in the second forming head 19 is controlled very accurately, and that the resulting cotton fibre web therefore can obtain a very uniform structure in the longitudinal direction.

The plant according to the invention shown in fig. 3 corresponds essentially to the one shown in fig. 2 and referred to above. Like parts are therefore designated by the same reference numerals.

In this embodiment both stations use a known forming head in form of a drum-shaped screen 41 having horizontal brushes 42 for distributing the fibres over the screen. Each station has two forming heads.

Moreover, the functioning of the embodiment shown in fig. 3 of a plant according to the invention corresponds to the one shown in fig. 2 and referred to above and will therefore not be mentioned any further here.

Fig. 4 is a very diagrammatic view of a plant according to the invention having a first station 34 with one forming head 35 for CLP-fibres placed above a forming wire 36, and a second station 37 with three forming heads 38a, b, c for screened CLP- fibres and other fibres placed above a joint forming wire 39.

The two stations are connected to a fibre conveyor 40 for, as indicated by the arrows, conveying screened CLP-fibres from the forming wire 36 of the first station 34 into the forming heads 38a, b, c of the second station.

This plant can be used for producing cotton fibre webs having a composition that varies in dependence of the fibres with which the different forming heads of the second station are supplied with.

For example, the plant can be used for producing laminated cotton fibre webs, and cotton fibre webs that also comprises a powder for example SAP, and synthetic fibres such as PET.

Two different, known forming heads are mentioned above and shown in the drawing that can form part of a plant according the invention. However, this is only to be taken as an example as the plant within the scope of the invention can function with any other kind of forming head just as two or several different types of forming heads can form part of the same plant.

The invention is furthermore described on the assumption that the short fibres forming part of the fibre web according to the inventions are CLP.

This is also only to be taken as an example as the method and the plant according to the invention advantageously can be used to produce fibre webs having a content of other types of short fibres, for example cellulose fibres.

The first air-lay station of the plant can furthermore have more than one forming head for short fibres.

The first station can thus have one forming head for short cellulose fibres and a second for CLP-fibres whereby a homogenous, nits-free, non-woven cotton fibre web can be produced that has an acceptable softness and is relatively inexpensive.

A web having the advantageously soft properties of the relatively short and weak CLP fibres and the advantageously great strength of the thermobinding fibres can for example be obtained with the following compositions. Example 1. A fibre web consisting of 60 weight percentage CLP fibres and 40 weight percentage thermobinding fibres.

Example 2. A fibre web consisting of 95 weight percentage CLP fibres and 5 weight percentage bicomponent fibres.

Example 3. A fibre web consisting of a multilayer; for example a three-layer product where the top and base layer consist of 60 weight percentage CLP-fibres and 40 weight percentage bicomponent fibres. The middle layer consists of 95 weight percentage CLP-fibres and 5 weight percentage bicomponent fibres but can just as well comprise SAP, SAF or pulp within the scope of the invention.