NO134995B - - Google Patents

Description

The present invention relates to a method for producing a web with a net structure that has trunk fibers and between these thinner and shorter branch fibers that connect the trunk fibers, where a film with alternating thick ribs and thin bands is split into the bands to form the branch fibers.

Thin, strong and cheap webs are previously known

with mesh structure, especially for reinforcing disposable fabrics, as wrapping material where air can pass through, as a base for carpets, filters and the like. The structural material produced by the method according to the invention is suitable for such applications and the production is simple and cheap.

Admittedly, it is stated in some patent documents and in other literature that such a web can best be produced from a foil, but due to fundamental disadvantages and complications during the development of such methods, the product in question or the method has so far not been developed on a technical scale. far that the production is in the right relation to the assumed market for the product in question.

In British patent no. 1,128,274, for example, a uniaxially oriented foil of uniform thickness is described, and this foil is split into continuous main fibers and into branch fibers that connect these, by passing the foil in its direction of orientation over a spiked roller whose spikes stand in a similar pattern and tears up the foil, forming areas of branch fibers. In addition, the foil is pulled out across the longitudinal direction. With such a web with a net structure, however, a stronger split than was intended can easily be obtained because the cracks that occur when the foil is pulled across the direction of orientation can easily expand into open slits.

An attempt has therefore been made to avoid an unwanted large splitting by forming a large number of nubs in the foil which limit the splitting of the cracks at the end, so that further tearing is prevented. See U.S. Patent No. 3,137,746. Even with such webs of material with a net structure, the tensile strength across the orientation is relatively small even when the foil has nubs because the slits present can continue by going around the nubs.

It is also known that the tendency to excessive splitting of the foil can be somewhat reduced when the splitting takes place by stretching the foil across its direction of orientation and at increased temperature because a certain reorientation then occurs at the ends of the split. See British patent no. 1,103,141. However, since such a measure still does not lead to the desired result, according to the known proposals, excessive tearing of the foil is prevented by two uniaxially oriented sub-foils with orientation directions that cross each other being put together and connected with each other. When the foil is split, intersecting and essentially continuous fibers occur which prevent an increase in the gaps at the crossing points. However, such a method is generally not suitable for the production of cheap webs with a net structure.

In another known method for the production of

the product in question here (see U.S. patent no. 3,495,752) uses a foil which, after extrusion, is embossed in a pattern with row-shaped, parallel, thick foil areas and intermediate thinner foil or film areas. When such a foil or film area is oriented uniaxially in the longitudinal direction of the foil area and split up, e.g. with a spike roller or similar, the splitting is, due to the greater thickness of the thicker foil areas, limited to the thin areas. Since the foil or film is oriented in its longitudinal direction and this direction corresponds to the longitudinal direction of the strip-shaped thick foil areas, the branch fibers that occur during the splitting are initially also_parallel to the trunk fibers formed by the thick foil areas. If the foil or film is then pulled out across the longitudinal direction, the formed branch fibers are bent so that in areas where the bent branch fibers join the stem fibers, lines with greater tensile strength appear which

asymptotically passes into the longitudinal direction of the stem fibers. Here, at the transitions between the branch fibers and the trunk fibers, a reinforced notching effect is obtained so that the branch fibers can easily be torn off by pulling out the formed foil or film across the longitudinal direction of the trunk fibers.

With the present invention, this task is solved

making it possible to produce a web-shaped material with a net structure in a simple way, while at the same time obtaining a significant increase in the tensile strength at the transitions between branch fibers and trunk fibers, so that it becomes more difficult to tear the branch fibers from the trunk fibers. This is achieved according to the method using the features specified in the requirements.

With this method, it is achieved that the branch fibers are oriented uniaxially in their middle area, but at the transition points to the stem fibers they are oriented biaxially. The orientation of the branch fibers in their longitudinal direction takes place by stretching before splitting the thin bands in the film. The further orientation of the branch fibers across their longitudinal direction at the points of connection with the trunk fibers is achieved by the stretching of the trunk fibers that takes place after the splitting. In this way, the orientation of the trunk fibers also takes place in their longitudinal direction. With the method according to the invention, the notch effect at the transition points for the branch fibers is surprisingly small. This results in an increased tear strength for web-shaped material with a net structure across the trunk fibres.

In addition, by stretching the thin strip areas of the film before splitting, at an angle to the longitudinal direction of the thicker film areas, the effect is that the later branch fibers go flat into the trunk fibers, which further contributes to increasing the strength at the transition points. As it is important for the web structure of the web to have sufficient strength in the direction of the trunk fibers and as the strength of the branch fibers across the trunk fibers is small, a sufficient strength across the trunk fibers is of great importance for several reasons. Before and during a lamination, the handling of the net structure shaped web is thus greatly facilitated. In the final product, the tear strength is thereby increased and the need for bonding points between the net structure and the material in which it is embedded is reduced. When you have sufficient strength also across the trunk fibres, the end product can be made softer and have a greater flexibility in the diagonal direction and in the case of layered materials as the end product, point gluing can be carried out.

The film can be produced by extrusion, casting from a solution or emulsion or in some cases by casting a prepolymer which is cured. The embossing can take place either as an independent process or as part of the film formation. In the first case, the film is rolled with an embossing roller or embossing with a vibrating knife can be used, while in the latter case the film can be extruded through a profiled matrix, or cast on an embossing surface or using a vibrating scraper blade.

One embodiment of the method is characterized in that the strip-shaped parts are made predominantly perpendicular to the longitudinal direction of the film, and another embodiment is characterized in that the strip parts are designed obliquely, e.g. at an angle of between 25 and 65° with the longitudinal direction of the film. In any of the two embodiments, it is possible to obtain products whose strength is primarily in the longitudinal direction, but which still has a strength in the longitudinal direction which is at least sufficient to handle the sheet before lamination . This can be achieved by means of an embossing roller helical embossing, or a vibrating knife or scraper blade placed at an angle, or a rotating, profiled, annular extrusion die.

In most cases, it is important to avoid any significant orientation of the length and width of the strip portions during orientation of the intermediate portions. Difference in thickness is one way of achieving this, but difference in resistance to stretching can be further enhanced by keeping the intermediate parts, while these are oriented, at a temperature that is higher than the temperature of the strip parts. This can preferably be done by embossing the film on only one surface and feeding the flat film surface over a heated element while the element's temperature and contact time are suitably regulated. In this way, the intermediate parts can be heated to the most suitable temperature for stretching, while the thicker strips remain at a temperature where they have a high resistance to stretching.

At the same time as an orientation of the strip-shaped parts is avoided, at least to some extent, they can preferably be moved relative to each other in their longitudinal direction while the intermediate parts are oriented. This achieves a more uniaxial orientation and, consequently, a higher stretchability than with a straight forward orientation. The distance between the strip parts can be increased at the same time as they are moved further apart. It is within the idea of the invention/ but usually not appropriate, to carry out the orientation by a single spreading of the strip parts, that is to say without the above-mentioned displacement of the strips relative to each other. Furthermore, care must be taken that the device for holding back and the device for pulling the strip parts do not take hold of one and the same strip, whereby the strips would be stretched in their longitudinal direction.

The above conditions can be suitably satisfied by effecting the relative movement of the strips and the orientation of the intermediate portions within a predominantly linear zone by changing the direction of advance of the film. As an alternative, if the strips are placed at an angle, it is possible to carry out the stretching in an oven without a change of direction, but with a relatively long distance between the retaining and the pulling device. The strips will then gradually move along each other while the intermediate parts are oriented, and the film will become narrower while stretching of the strips can be avoided. Under these conditions, however, it is difficult to maintain the temperature difference between the strips and the intermediate portions, and consequently stretching under a change of direction is usually preferable.

When the intermediate parts have been oriented uniaxially during mutual displacement of the strips, the relative movement of the strips is preferably reversed, during or after splitting. This step will produce and/or spread the branch fibres, and can also preferably be carried out by changing the direction of the film's progress. A transverse stretching method used for splitting the oriented film with a change of direction is described in U.S. Patent No. 3,350,491, but the process can be simplified in connection with the present invention as the non-splittable strips enable control of the development of the splits in the film. However, it is also possible to initiate or carry out the entire splitting process with a set of brushes or the like and use another change of direction to spread the branch fibers that are thus formed.

For the final stretching step where the strips (that is, the stem fibers) are oriented in their longitudinal direction, a stretching frame or similar is used. However, it may be appropriate to use a modified stretching device which will be described in connection with fig. 7.

As can be seen from what has been described above, accurate embossing is important, and to achieve this, an embodiment of the method is characterized by only one surface of the film being embossed and this surface being kept melted or semi-melted while the surface which should remain smooth solidifies immediately before embossing. The surface which is to remain smooth is preferably brought into contact with a cooling roll which can be resisted for the embossing, and which is cooled for a certain period so that the predominant part of the film remains melted or semi-melted, and immediately afterwards embossed on the side which can easily be deformed. The strong cooling, on the other hand, serves to control the depth of the embossing.

With the aim of producing the two- or multi-layer sheets described above, another embodiment of the method is characterized by the fact that the film is formed from at least two layers of different polymer material and that the ratio between the materials is changed during embossing mainly by pressing one or some of the components in in the film strips from the intermediate parts of the film. Such pressing is achieved automatically with the aid of the embossing tool when the fluidity of the layers is sufficiently different.

Such different flowability can be achieved by using materials of different melt viscosities. The most effective way, however, is to keep one material solid and another semi-melted to molten during embossing. The strong cooling of a surface can thus be used as described immediately above, and/or the material that must resist pressing can be chosen so that it has the highest melting point.

In the description of the layered product, it is further specified that the branch fibers can be formed mainly of a highly fissionable polymer, while the trunk fibers are formed of a less fissionable material. In order to achieve this, the embodiment of the method which consists in forming more than one layer and pressing the layers is differently adapted so that the material which is mainly pressed is a material which, in the directional state, is relatively resistant to splitting, while the other material is a material that in a directionally oriented state easily splits. Thus, one of the materials can be adapted for cleavage resistance by the addition of a predominantly compatible elastomer or other plasticizer, or by suitable copolymerization. And/or another material can be adapted to become particularly fissile by mixing in an incompatible, predominantly rigid polymer. In order to produce very fine cleavage fibers from the intermediate parts, an added, predominantly incompatible polymer can be completely or partially dissolved before or after the cleavage, as described in British patent no. 1,148,382. In the intermediate parties, there would like to remain a remnant of the team that is sought to be pushed out as much as possible. This residue can also be dissolved and removed or made brittle by swelling, without significantly affecting the strength of the stem fibres. The above-mentioned dissolution or swelling process will not significantly complicate the method, as the parts to be dissolved or swollen will generally constitute very small proportions of the entire sheet.

The invention will be described below with reference to drawings and photos.

Fig. 1-4 are photomicrographs at sixteen times magnification

of different samples of the sheet product.

Fig. 1 is an intermediate product after splitting the thin zones into "branch fibers", but before stretching the "stem fibers".

fig. 2-4 are final products produced from the product shown in fig. 1, as the difference between the three samples is due to different stretching conditions,

fig. 5 is a perspective view, greatly enlarged, of a trunk fiber and adjacent portions of branch fibers,

fig. 6 is a process diagram of a suitable process line,

fig. 7 is a schematic drawing of the process line on

fig. 6 showing the direction of the trunk fibers (strips) by thick arrows, and the direction of the branch fibers (orientation of the thin zones) by thin arrows,

fig. 8 shows the step by which the oblique or vertical embossing of the film takes place,

fig. 9 shows the embossed film in perspective and high magnification,

fig. 10 is a schematic but more detailed view of the transformation which leads to the stretching of the thin sections of

the embossed film, while the thicker strips remain unstretched.

and slide past each other,

fig. 11 is similar to fig. 8, but applies -a sheet in two

layer with the aim of giving a different composition of the trunk fibers and the branch fibers,

fig. 12 is a greatly enlarged perspective view showing the embossed two-layer film produced by the process of FIG. 11,

fig.- 13 is a greatly enlarged perspective view which very schematically shows the final product resulting from the process in fig. 11,

fig. 14 is a schematic representation of the longitudinal orientation of the thin intermediate parts.

The samples shown in the photo in fig. 1-4 are produced from an embossed polypropylene film where the thick strips and the thin zones have had the same width, and the thin zones have been oriented at an angle of 15° to the strips. The cleavage, shown in fig.

1, is produced by pulling the strips apart, whereby the film is broken up into "trunk fibers" and "branch fibers".

In fig. 2, the trunk fibers are directional while the branch fibers were kept stretched. The latter thereby remains predominantly fiber-shaped over the greater part of its length, although the fiber shape is ribbon-like, but forms biaxially oriented film sections where they are connected to the stem fibers. This product has high strength in the direction of the trunk fibers and is very flexible. It is thus suitable e.g. for reinforcement in disposable clothing.

The product in fig. 3 is produced by stretching the product in fig. 2 almost in the direction formed by the branch fibres, while the stem fibers have been kept in a relatively slack state so that they have been able to deviate from the straight line. This form is generally preferable when the product is to be used as a single layer, e.g. for packaging.

The product in fig. 4 has also been produced from the product in fig. 2 by stretching the trunk fibers. In this case, however, the cleaved film portions that form the branch fibers have been allowed to contract and remain more film-shaped, but cleaved into a substructure of very fine fibers in the central portion. In this form, where the stem fibers are close to each other, the product is very suitable as a layer in a substrate for a carpet.

In fig. 5 shows a trunk fiber 1 and a branch fiber 2 -v. which is integrally connected with the stem fiber. The parts 3 close to the stem fibers have remained in film form and are biaxially oriented by two stretches in different directions, namely first the stretch that takes place before the splitting, and then the stretch caused through directional orientation of the stem fibers.

In fig. 7 which further explains the process diagram on

fig. 6, the film 4 is extruded from a flat die 5 and fed between a set of rollers 6 where one is smooth and the other is embossed with a fine spiral line pattern at an angle of 45°. The film is embossed while it is predominantly semi-melted and cooled

down before it leaves the rollers 6. Embossing and cooling are further explained in connection with fig. 8. The film 4, when it leaves the embossing station 6, has thicker strip-shaped portions at an angle of approx. 45°, as shown by arrow 7, and thinner, predominantly unoriented intermediate parts.

The next station of the process is an oblique stretching which takes place between a heated stretching rod 8 which is parallel to the direction 7 and a set of drawing rollers 9. During the stretching which takes place on the heated rod 8 and within a few centimeters thereafter, the film is deflected to the side which, to illustrate the point, can produce a considerable reduction in width even if the draw in the longitudinal direction is very small. In the actual process, however, the drawing ratio in the longitudinal direction should be relatively high, e.g. between 1.5:1 and 5:1. The stretching is explained in more detail in connection with fig. 10. An edge guide device (not shown) is provided. The stretching leaves the strips predominantly unoriented, but strongly orients the intermediate parts. The direction of the strips after deflection of the film and stretching is indicated by arrow 10, while the direction of the orientation which is not exactly longitudinal is indicated by arrow 11. The splitting takes place by means of another deflection in the direction of advance which takes place in a linear zone 12, whereby the width is increased. The zone 12 can be established by a separate set of pinch rollers which are able to drive the film further forward at an angle, as shown. Such rosehip rolls are described in British patent no. 1,078,732. Each of these rollers consists of a rotating core part and several peripheral section parts which can slide to and from the core part synchronously with the rotation. ■ The forward force in the nip will thus be the resultant of the rotation and sliding of a section at the time when it is in the position where it forms part of the nip. In the drawing of the roller device in the above-mentioned British patent, a rubber conveyor belt is also shown, but this is not necessary in connection with the present invention.

The cleaving of the oriented intermediate parts now takes place through a straight pull away from the slanted rosette. - A set of ordinary rosette rollers 13 is used for this pulling. It is not intended that any parts of the sheet should be oriented outwards. liger by this drawing, but only that the trunks (strips) should be displaced relatively while the oriented intermediate parts are split. In the drawing, the inclined nip zone 12 is shown parallel, with the strips of film that are fed to the nip (arrow 10). In principle this is the best way to carry out the splitting, but due to the complications of constructing the special rollers for the rosehip 12 at the small angle that the strips will usually form, it may be preferable to use :a larger angle of -rosehip 12, e.g. 45°, and the transformation can in this case be repeated.

In any case, the opening of the net structure by deflection of the direction of travel should preferably be carried out to a point where the direction of the trunk fibres, represented by the arrow 14, again forms an angle of approx. 45° with the direction of travel. It turns out that the direction of the branch fibres, represented by the arrow 15, then with a good approximation becomes vertical 14. The stem fibers are then stretched in an oven (not shown) using a stretching frame. If this tensile frame is symmetrical, the stem fibers cannot essentially be stretched more than the branch fibers which are already strongly oriented, or in other words, the branch fibers will "lock" the orientation of the stem fibers. For this reason, the stretching is preferably arranged asymmetrically as shown. In zones 16 to 18 and 17 to 20, the sheet is gripped by the edge holders which, as usual, are attached to tension chains or the like, and the devices are built together in such a way that 16 to 17 and 18 to 19 are both parallel to the trunk fibers direction 14. Furthermore, the speeds of the chains are adapted so that the movement from 16 to 18 takes as long as the movement from 17 to 19. Hereby, the trunk fiber direction, see arrow 20, will remain parallel to what it was before stretching. Furthermore, the direction of the chains is adapted to hold the direction of the branch fibres, as shown by arrow 21.

It is noted that it is a simplification to show the process line in one plane, as is done in fig. 7. In reality, the film must follow the rollers and rods over a certain space.

However, rollers or bars can be provided to bring them back to practically the same plane, e.g. after stretching at 8, a cooled rod (not shown) parallel to 8 can be arranged for this purpose.

As mentioned above, the branch fibers become predominantly vertical stem fibers after cleavage when the stem fibers have been displaced one another in the longitudinal direction to a position where they form the same angle with the edge of the sheet as they did at the stage immediately after embossing. Regardless of how such angles are chosen from a few degrees up to and including 90°, said vertical relationship can be proved mathematically under the condition that the orientation ratio within each thin zone is relatively high.

In order to be split, the thin zones must generally be oriented at least in a ratio of 3.5:1, and this value is sufficiently high to result in a practically vertical relationship between the trunk fibers and the branch fibers. In this connection, it is also noted that in order to achieve high cleavability, the orientation ratio in each thin intermediate portion must usually be chosen higher than 3.5:1, e.g. in the case of polypropylene and a high-density polyethylene, the ratio may be between 5:1 and 9:1 or even higher.

In the case when the strips are embossed perpendicular to the edges of the film, the branch fibers will thus become practically parallel to the edges of the sheet. In this case, a normal symmetrical tension frame can be used to stretch the stem fibers. This will be further described below in connection with fig. 14.

In fig. 8, the film 22 comes directly from the extrusion die and is still melted when it meets the smooth cooling roll 23 which also functions as a counterpart for the embossing roll 24. The latter is provided with very fine teeth 25 in the form of generatrices or spirals. The cross-section of each tooth is shown rectangular, but the edges can preferably be somewhat rounded and a gap should preferably be designed for easy release of the film after embossing. Furthermore, a device is provided for supplying a release agent to the embossing roll (not shown).

Before the start of the process line, the two rollers 23 and 24 are regulated so that they barely touch each other. During the process, the surface portion of the film 22 which is close to the roll 23 will solidify, while the cooling zone from the contact line 26 in the nip is regulated (by means not shown) to allow the other surface portion to remain melted or semi-melted until it enters the nip. The solidified part of the film controls the embossing depth, and it is thereby possible to achieve relatively accurate thickness control of the thin parts even when the average thickness is as small as e.g. 50 micromillimeters. The melted to semi-melted film portion preferably solidifies before the film leaves the roll 23.

The film 22 is now provided with stripes 27 generally between 0.2 and 5 mm wide. For most purposes, it is appropriate to design the strips and the intermediate parts with approximately the same width or the intermediate parts somewhat narrower.

Fig. 9 shows a realistic form of the transverse or obliquely embossed film where the strips 28 have been given a "sausage-shaped" cross-section which is usual when the film is embossed, and the intermediate parts 29 are considerably thinner.

In fig. 10 which shows in greater detail the process at 8

on fig. 7, the film is fed in in the direction 30 towards the linear stretching zone 31 and pulled away in the other direction 32.

The drawing covers only a small part of the film width,

and does not include the edges, but to show the stretching better, two dashed lines 33 and 34 are shown which before the stretching are parallel to the edges and the feeding direction 30, but become zigzag-shaped after the stretching. The stretch zone is shown as a true line 31, but will usually include several stripes and intermediate portions. The stripes 35 are marked by parallel hatching, while the intermediate parts 36 are not hatched. In principle, the strips maintain both length and thickness constant, but in practice it is difficult to avoid a minor extension, e.g. 10%.

While the strips are offset from one another in the longitudinal direction, they will also, but to a lesser extent, be pulled apart, with the result that the intermediate parts become uniaxially oriented in the direction shown by arrow 37.

In the following, more quantitative information will be given

tion with regard to the stretching.

Two points A and B define the width of the strips before stretching. The same points after stretching are A^ and B^. To a good approximation, the line A^B^ will be parallel to the direction of the orientation produced 37, and the stretching ratio will be approximately equal to

The projection N of the point A^ is marked, A^N defines the width of the strips after stretching. The following expressions are defined: ^

relative shear S„. = a^

<A>1<B>1

the relative dispersion d =

r AB

The change in direction from 30 to 32 is adapted to the stretching ratio in such a way that the orientation becomes uniaxial, that is, orientation is avoided. Under a true uniaxial orientation, the distances along the secondary axis of the stretch will always be contracted in the same ratio as the thickness, that is, both are contracted in the ratio \/n, where n is the strain ratio along the primary axis. Thus, the surface tension ratio will be = Vn. ^n

With the above-mentioned definitions and taking into account that the strips keep their length constant during stretching, the surface stretch ratio for each intermediate part will be equal to the relative spread, that is d r= Æ. Compared to it

approximate equation

and if one considers the triangle A^NB^, one sees that dr constitutes a rather small part of the stretch and that the following equation is a good approximation <NB>1,

"AB

that is, S = n.

r

For each selected set of parameters, i.e. the stripe angle, the ratio between stripe widths and intermediate parts and the stretching ratio in the direction of orientation, it is now possible to calculate the preferred angle between 30 and 32.

In fig. 11, two films of different materials 38 and 39 are extruded from two different extrusion dies or ten slots in one and the same die. They can also be co-extruded from one and the same slot. The embossing takes place as in fig.

8, but the regulation is simplified if the film 38 has a higher melting point than the film 39. The resulting embossed two-layer film 40 is shown in more detail in fig. 12 and as can be seen, both components have been squeezed into the strips. However, the layer 42 formed by the film 39 has been squeezed far more than the film 41 formed by the film 38. For various reasons, the rest of the layer 42 is completely or partially dissolved and removed from the thin intermediate portions.

It is possible to achieve a very fine cleavage if the team

1, that is, the film 38, is formed from a mixture of polymers which are relatively incompatible with each other, especially when one of said polymers dissolves out of the layer 41 or swells before the splitting and/or stretching of the stems. This is shown in fig. 13. In the intermediate parts, team 41 has been split here

a porous fabric consisting of very fine fibrils in branching relationships with each other. Fabrics of this type are treated in British patent no. 1,148,382, from which it also appears that each fine fibril can be a crystal formation with a diameter of a few micromillimetres, or even less than 1 micromillimetre. However, the cleaved fibrils or crystal threads can also partially join together in bundles. For reasons of simplicity in the drawing, the individual fibrils are not shown in fig. 13, but is symbolized by crosshatch and straight lines. In the zones 43 near the trunks, the structure can be said to exhibit a more biaxial character than in the middle zone 44. Thus, there will generally be more "layers" of crossing fibers in the zones 4 3 and a smaller amount of crossing fibers in the zones 44. This crossing effect naturally exhibits no definite pattern, but is the result of the arbitrary deflection of each fine fibril as a result of splitting and stretching of the stem. There is of course no sharp boundary between zones 43 and 44, but the entire film gradually changes from a more to a less clearly biaxial character.

In this connection, the "uniaxial" and "biaxial or biaxial" character do not necessarily refer to strong molecular orientation, as it is known from the above-mentioned British Patent No. 1-148,382 that the formation of fibrils takes place partly in the melt by extraction of the submicroscopically fine polymer parts that are dispersed in each other and partially by a segregation while the polymer with the highest melting point crystallizes.

In order to reduce the costs of dissolving and washing out or swelling of a component, the layer 41 is preferably designed very thin, and the intermediate parts can be down to approx. 2 g/m 2 in the cleaved product. Even in such a small proportion, the fabric is of great importance as it gives the sheet the character and appearance of a textile. When the sheet is used e.g. as a layer in disposable clothes, the fine fibrils in layer 41 should form a surface of the cloth.

As can be seen from the above, the step involving orientation of the intermediate parts after embossing can be omitted in this embodiment of the invention, as the formation of the fibrillar microstructure which takes place during extrusion and cooling can replace this step.

The line in fig. 14 deviates from what is shown in fig. 7, first at the embossing station where a set of rollers 45 produces a vertical strip formation in the film 4 as indicated by arrow 46. The holding for the following stretch is by means of a set of heated pinch rollers 47 and no further edge holding is necessary. As in fig. 7, the direction of travel is then deflected and the film is pulled by means of another set of rollers 9. The direction of the strips 48 will in principle remain parallel to the first strip direction 46, while the intermediate film sections are oriented in the longitudinal direction as shown by arrow 49. The holding device for the splitting is a set of section rollers 50 which work as 12 in fig. 7, but at a less acute angle. As in fig. 7, the pulling which in turn produces the splitting is produced by means of a set of ordinary pinch rollers 13. Hereby, the stripe direction again becomes perpendicular to the direction of progress, as shown by arrow 51. At the same time, the direction of orientation, i.e. the branch fiber direction, remains practically parallel to the longitudinal direction of the sheet as shown by the arrow 52. The trunk fibers can therefore be drawn in a normal symmetrical tension frame, the entrance to and the exit from this being shown by the dashed lines 53 and 54, whereby the vertical direction of the trunk fibers 55 and the longitudinal

running direction of the branch fibers 56 is maintained.

Example 1

A film of polypropylene with a melt index of 7 according to ASTM D1238-62T, state L, is embossed in a semi-melted state at an angle of 45°. The width and thickness of the strips are respectively 0.68 and 0.35 mm, and the width and thickness of the intermediate portions are respectively 0.38 and 0.058 mm.

The film is then stretched over a heated bar, placed at an angle of 45°, and pulled off at an average stretch ratio of 3.5:1 under deflection of 20° from the feed direction, thereby reducing the strip direction to 25°. The rod is heated to 150°C, and the contact time is regulated so that the thin sections are heated to 125°C.

After cooling, the film is stretched transversely from zone to zone in zones that follow the direction of the strip and thereby split. Finally, the stems are oriented at 125°C in a ratio of 5:1.

Example 2

From a double-slit die, a two-layer film is extruded which makes up 75% of the product, consists of the same polypropylene as in example 1. The other film, which makes up 25% of the product, consists of 80% polypropylene and 20% polystyrene which has been mixed in hot condition in a planetary screw extruder. The mixture has a melt index of 5 according to the specification mentioned in Example 1.

The film is heated as in example 1, with the pure polypropylene layer forming the surface which is cooled before embossing (see fig. 11). In this way, it will mainly be pure polypropylene that is pressed away and into the strips. This will result in finer branch fibers being produced.

Example 3

The same double-slotted matrix is used as in Example 2, but the components are: 1) a copolymer of nylon 6 and nylon 66 (commonly called nylon 6A) with a melting point of 180°C and a melt index of 3.5 according to ASTM D1238-62T, condition K 2) 35% polyethylene and 65% nylon 6, carefully mixed in a planetary screw extruder and with a melt index of 1.5 under the same conditions as 1).

The two films are extruded in equal proportions, the dual thickness being 0.3 mm and then embossed as in example 1, the embossing taking place on the side formed by nylon 6A. During the embossing, nylon 6A is held at around the melting point, whereby nylon 6 will be in a solid state.

No cold stretching is performed to orient the thin zones. The film of nylon 6A is partially removed, and the rest made brittle, by dipping the embossed sheet for a few seconds in methyl alcohol and immediately afterwards in water. The sheet is then processed approx. 10 sec. in boiling xylene to partially remove and partially swell the polyethylene.

When it is then exposed to shear to spread the trunk fibers, a very soft cloth of fine branch fibers is formed.

Claims (8)

1. Method for the production of a web with a net structure that has trunk fibers and between these thinner and shorter branch fibers that connect the trunk fibers, where a film with offset-length thick ribs and thin bands is split into the bands to form the branch fibers, characterized in that the large set non-oriented films are first uniaxially oriented in the thin ribbons obliquely to the longitudinal direction of the thick ribs and so that the ribs are kept in an unoriented state, and that cleavage of the oriented film ribbons is then carried out, followed by orientation of the ribs by stretching them in their longitudinal direction for the formation of the stem fibers. 2. Method as stated in claim 1, characterized in that the thin film ribbons during their orientation are kept at a temperature that is higher than the temperature of the thick rib. 3. Method as stated in claim 1, characterized in that when orientation of the thin film bands without simultaneous orientation of the thick ribs, the film is guided from the feed direction and stretched obliquely in the longitudinal direction of the ribs, whereby a displacement of the ribs towards each other in the longitudinal direction is permitted. 4. The method as set forth in claim 3, characterized in that the film, when splitting the oriented thin film strips during pulling, is guided out of the feed direction in such a way that the displacement of the thick ribs towards each other is brought back. 5. Method as stated in claim 1, characterized in that the foil is formed from at least two interconnected layers of different polymer materials which, to form the thick ribs and thin film bands, are embossed, and that the material in one layer is thus squeezed out of the thin film to the side into the thick ribs. 6. Method as stated in claim 5, characterized in that the material that is squeezed out in the thick ribs in one layer is a material that is relatively resistant to splitting, and that the material in the second layer is material that is easily cleavable in the oriented state. 7. Method as stated in claim 6, characterized in that a mixture of polymer materials which are not compatible with each other is used as easily fissile material. 8. Method as stated in claim 7, characterized in that a polymer material is added to the mixture which is removed either during the splitting or during the tensile orientation of the stem fibers.