EP0013355B1 - Process for making spun nonwoven sheets - Google Patents

Description

The present invention relates to a method for producing spunbonded nonwovens made of thermoplastic materials according to the preamble of claim 1.

From DE-A-1 900 265 and US-A-3 772 417, a method has become known in which spunbonded fabrics consist of practically endless, approximately tangled yarns which have a higher tensile strength in one direction than in that vertical direction, to improve the properties at temperatures just below the melting temperature in one or more directions. For polypropylene, the stretching temperature is e.g. at 110 to 124 ° C. It is assumed that such fleeces are used, the threads of which are welded or glued at the crossing points, and the stretching is carried out to such an extent that the surface area increases by a factor of up to approximately 15. Since the crossing points in the nonwovens used for this method are rigidly fixed, this causes the individual threads, which fluctuate greatly in the individual titer, to be stretched, with titre fluctuations of the individual threads being achieved by a power of ten. In cases where e.g. For the end use of the nonwovens a greater longitudinal strength than transverse strength is required, the stretching is preferably carried out in one direction only. Stretching in several directions, e.g. two or three directions is used when essentially the same tensile strength is to be achieved in several directions along the fiber plane. The stretching takes place over a heated brake shoe.

DE-A-2 639 466 describes that the properties of staple fiber nonwovens, the individual fibers of which are oriented in the transverse web direction, can be improved by first stretching in the longitudinal direction, then needling, then again in the longitudinal direction and finally in the transverse direction will. This increases the dimensional stability and strength of these nonwovens.

Furthermore, it is known from DE-A-2 239 058 that in the case of non-consolidated, non-consolidated staple fiber nonwovens with relatively short fibers, which are provided with a regular pattern by means of mechanical or fluid forces, by stretching in the transverse direction with simultaneous shrinkage in the In the longitudinal direction, the transverse tensile strength can be improved without the pattern consisting of regular thick and thin places being destroyed. Rather, it can be fully restored again by post-treatment with fluid forces that cause the relatively short fibers to reorient.

Finally, according to DE-A-1 635 634, it is proposed that nonwovens which have a strong orientation in the transverse direction due to paneling be improved in the tear resistance in the longitudinal direction by stretching in the longitudinal direction during needling. This stretching, which at the same time results in an uncontrollable transverse entry, is intended to cause the fiber web, which is laid at an angle of 10 to 15 ° to one another in the panel, to be distorted during the first needling in such a way that the fibers are finally at an angle of 45 'come to rest and are thus fixed.

This process, which can only be carried out during the needling process by disassembling it into many individual small stretching steps, requires a great deal of equipment, since, for example, the needling machine has to work at a low puncture speed but with a high exit speed and should also oscillate, otherwise stripes will form in the fleece . In this DE-A it is also pointed out that a simple stretching of the paneled fleece is not possible, since thin spots form which tear during further stretching.

Spunbonded nonwovens, which are made up of practically endless threads made of thermoplastic materials and deposited in an approximately tangled position, can be produced according to a known method by depositing freshly spun threads which are stretched before being deposited, predominantly by means of air. The degree of presence of residues of parallel bundles of threads also varies with the filing method used. An ideal, completely unoriented tangle is usually not achieved, so that such nonwovens almost always have a higher tensile strength in one direction than in the direction perpendicular to them.

In a number of applications, e.g. in civil engineering, it is not all about the strength in one direction, but in all directions. This means that the lowest tensile strength is decisive for the application, so that the thickness of the fleece must also be selected according to the lowest tensile strength. However, this means an increase in the cost of fleece use, which stands in the way of many large-scale use.

The present invention is based on the object of equalizing the tensile strengths of such spunbonded fabrics in mutually perpendicular directions, the lower tensile strength being increased considerably in one of the two directions, but without the threads themselves being stretched and the thread titer becoming uneven. This object is achieved in that the nonwoven is needled prior to stretching and the stretching is carried out in the direction of the lower tensile strength by 20 to 200% of the relevant original dimension, while in the perpendicular direction the dimension is either retained or previously or simultaneously in the area is changed from ± 10% of the original dimension.

The drawing is carried out in a known manner at a temperature of 85 to 25 ° C. below the crystalline melting point of the thread material.

This measure increases this tear strength, although at the same time the area of the fleece is increased at the cost of the weight / m ² . The fact that a higher minimum tensile strength is nevertheless achieved now opens up the possibility of a considerably more economical use of nonwovens, especially in earthworks, such as road, tunnel, embankment and hydraulic engineering, since it is practically only based on the force-elongation behavior, but not depends on the weight of the fleece per m ² , and thus larger areas can be covered with the same weight of a fleece material.

The fact that a needled fleece, in which the crossing points are not so firmly consolidated that they do not open, can be reinforced, is surprising, since it was to be expected that any thin spots that might be present would be tightened or even holes would appear. However, this is not the case; on the contrary, a more even distribution of the threads in the tangled position is achieved, with threads lying in loops passing into the stretched position with increasing degree of stretching and thus giving the fleece greater strength. However, all of this can only be accomplished if the stretching is carried out in a certain temperature range depending on the crystallite melting point.

A prerequisite for the success of the method according to the invention is that it is assumed that the fleece has been consolidated by needling. In order to achieve good properties, especially with higher elongation ratios, it is advisable not to choose needles that are too light. Nonwovens are preferred which are needled to such an extent that their increase in strength through needling is at least 50% of the optimally achievable increase in strength through needling. This is the case, for example, when using 15x18x34 / 3 inch needles with about 100 punctures / cm ² or with 15 _X 18X36 / 3 inch needles with 120 punctures / cm ² . Particularly favorable results are obtained if nonwovens are used which have been processed with the needle types mentioned with about 180 to 200 punctures / cm ² .

Continuous thread nonwovens of the type mentioned above usually have a lower tensile strength in the transverse direction. According to the present invention, these nonwovens are stretched in the transverse direction to the extent according to the invention, which e.g. is possible in a known stenter.

However, stretching apparatuses can also be used in which the fleece is picked up by discs provided with teeth on the circumference, the plane of which is approximately perpendicular to the fleece plane and which are arranged at an acute angle to the running direction of the fleece such that the fleece when passing through the circumference of the discs is pulled apart. Such a device is e.g. described in DE-OS 2 401 614.

However, if the continuous filament nonwoven is brought to a certain fleece thickness by paneling before needling, it is usually the longitudinal direction which has the lower tear strength. In this case, the nonwoven must then be stretched in the longitudinal direction, which can be done particularly favorably, for example, by a roll stretching method known per se with a short roll gap. However, it is also possible to use any other known longitudinal stretching method, in which case the fleece has to be broken in too much in order to comply with the limits according to the invention. This can be done, for example, by interrupting longitudinal stretching zones with zones in which the fleece is brought back to the width specified in the invention in a cross-tensioning device, which should be within ± 10% of the original width. Even in the case of the paneled nonwoven, the tangle of the continuous filaments is influenced in the stretching process according to the invention. With a reorientation of individual fibers that are at a certain angle from the paneling, which is changed, as is achieved according to DE-A-1 635 634 when stretching staple fiber nonwovens during needling, the stretching process according to the invention has, even if. it is used in paneled continuous filament nonwovens to do nothing.

The choice of the degree of stretching within the range according to the invention depends on the values that are to be achieved. Do you want e.g. the tensile strength in the weaker direction e.g. to increase by 15-20% without wanting to lose longitudinal strength, it is advisable to choose a slight stretch of 20-30%. The higher the degree of stretching in the weak direction is chosen, the more the tensile strength in the stronger direction is reduced, so that e.g. with stretching of 60-100% with regard to tear strength, approximately isotropic nonwovens are obtained, the tensile strength of which lies in the middle range between the original longitudinal and transverse strength. Since the lowest tear strength is decisive for the intended use, the nonwoven can be exposed to a greater load than the starting nonwoven after the treatment according to the invention.

The method according to the invention is applicable to continuous filament nonwovens made of all thermoplastic materials such as polyamide, polyester, polyolefin. Nonwovens made from propylene homo- and copolymers and polyester are particularly preferred. The method according to the invention is to be explained in more detail with the aid of the present examples. The tensile strength and elongation at break values specified therein are determined in accordance with DIN 53857.

example 1

wird ohne Längsverzug in einen Spannrahmen eingespannt, bei einer Temperatur von 130 °C in kontinuierlicher Fahrweise in der Querrichtung um 20% gedehnt. Nach Verlassen des Heissluftofens wird das Vlies aus dem Spannrahmen herausgenommen und kontinuierlich aufgewickelt. Es besitzt folgende Kennzahlen:

A needled continuous thread fleece made of polypropylene with the following key figures:

is clamped in a tenter frame without longitudinal distortion, stretched in the transverse direction by 20% at a temperature of 130 ° C. After leaving the hot air oven, the fleece is removed from the stenter and wound up continuously. It has the following key figures:

The fleece therefore has a transverse strength increased by 50 N while the longitudinal tensile strength remains approximately the same.

In contrast, a non-stretched nonwoven fabric with a weight per unit area of 220 g / m ² produced by the usual spinning process has the following key figures:

The fleece produced according to the invention is therefore superior in terms of tear strength.

Example 2

The same fleece as described in Example 1 is introduced into a stenter and pulled off at such a speed that it is stretched by 10% in the longitudinal direction in the longitudinal direction at room temperature before the side edges are gripped. It is then stretched 20% at 130 ° C. The fleece obtained after stretching and cooling has the following key figures:

In contrast, a continuous filament nonwoven made of polypropylene with a weight per unit area of 200 g / m ² produced by spinning and laying has only a tensile strength of 570 N and 230 N across, and the elongation at break of 90% lengthways and 135% crosswise.

Example 3

wird im Spannrahmen ohne vorherigen Längsverzug bei 135 °C um 40% querverstreckt. Nach Abkühlen hat das Vlies folgende Kennzahlen:

A strongly needled continuous thread fleece made of polypropylene with the following key figures:

is stretched 40% in the stenter without prior longitudinal distortion at 135 ° C. After cooling, the fleece has the following key figures:

In contrast, a fleece with 230 g / m ² , produced like the fleece used as the starting material, has a tensile strength lengthways of 650 N and crossways of only 290 N, and an elongation at break lengthways of 85%, crosswise of 125%.

Example 4

wird in einem Spannrahmen ohne vorherigen Längsverzug bei 135 °C um 60% quer verstreckt.A polypropylene fleece with the following key figures:

is stretched transversely by 60% in a stenter frame without prior longitudinal distortion at 135 ° C.

The fleece thus obtained has the following characteristics:

In comparison, a nonwoven fabric, which is produced by the same method as the starting nonwoven fabric, but has a basis weight of 180 g / m ^2, has a longitudinal strength of 530 N and a transverse strength of 200 N, and an elongation at break of 95% and across 150%.

Example 5

The fleece described in Example 4 is stretched 60% at 140 ° C., at the same time being allowed to shrink 10% in the longitudinal direction. This gives a fleece with the following key figures:

In comparison, a nonwoven fabric, which is produced according to the same process as the starting nonwoven fabric, but has a basis weight of 200 g / m ^2, has a tensile strength along 570 N, tensile strength across 230 N and an elongation at break along 90%, as well an elongation at break of 135%.

Example 6

wird im Spannrahmen ohne vorherigen Längsverzug bei 135 °C in kontinuierlicher Fahrweise um 100% quer verstreckt. Nach dem Verlassen des Heissluftofens hat das Vlies folgende Kennzahlen:

der ursprüngliche hohe Unterschied in der Reissfestigkeit von längs:quer=2,2:1 konnte durch das Streckverfahren auf das Verhältnis längs:quer= 1,2:1 egalisiert werden, wobei die Querreissfestigkeit nach dem Strecken des um 44 Gew.% leichteren Vlieses um 6% von 514 N auf 545 N zugenommen hatte.A needled continuous thread fleece made of polypropylene with the following key figures:

is stretched 100% crosswise in the stenter without prior longitudinal distortion at 135 ° C in a continuous mode of operation. After leaving the convection oven, the fleece has the following key figures:

the original high difference in tensile strength from longitudinal: transverse = 2.2: 1 could be equalized by the stretching process to the ratio longitudinal: transverse = 1.2: 1, the transverse tensile strength after stretching the 44% by weight lighter fleece had increased by 6% from 514 N to 545 N.

Example 7

A needled continuous thread fleece made of polypropylene with the following key figures:

When the weight per unit area decreased by 53%, the longitudinal tensile strength was reduced by only 33%, the transverse tensile strength only by 25%, but the ratio of longitudinal: transverse tensile strength was adjusted from 1.87 to 1.66: 1.

Example 8

A needled fleece according to Example 1 with the following key figures:

When the weight per unit area decreased by 53%, the longitudinal tensile strength decreased by 43%, the transverse tensile strength only by 25% due to the stretching process, whereas the ratio of the longitudinal: transverse tensile strength was adjusted from 2.2: 1 to 1.66: 1.

Example 9

A needled fleece according to Example 1 with the following key figures:

With a decrease in the weight per unit area by 62%, the longitudinal tensile strength only decreased by 33%, the transverse tensile strength after the stretching process only by 36%, the ratio of the longitudinal: transverse tensile strength was slightly adjusted from 1.93: 1 to 1.84: 1.

Example 10

A needled fleece according to Example 1 with the following key figures:

When the weight per unit area decreased by 66%, the longitudinal tensile strength only decreased by 42%, the transverse tensile strength by the stretching process only by 25%, whereas the ratio of the longitudinal: transverse tensile strength was adjusted from 2.25: 1 to 1.72: 1.

Example 11

A needled fleece according to Example 1 with the following key figures:

With a decrease in the weight per unit area of 51%, the longitudinal tensile strength only decreased by 44%, the transverse tensile strength after the stretching process even only by 22%, whereas the ratio of the longitudinal: transverse tensile strength was adjusted from 2.25: 1 to 1.60: 1 .

Example 12

wird ohne Längsverzug in einen Spannrahmen eingespannt, bei einer Temperatur von 135 °C in kontinuierlicher Fahrweise in der Querrichtung um 80% gedehnt. Nach Verlassen des Heissluftofens wird das Vlies aus dem Spannrahmen herausgenommen und kontinuierlich aufgewickelt. Es besitzt folgende Kennzahlen:

A needled filament thread from Polypro ^p y-len with the following key figures:

is clamped in a tenter frame without longitudinal distortion, stretched in the transverse direction by 80% at a temperature of 135 ° C. After leaving the hot air oven, the fleece is removed from the stenter and wound up continuously. It has the following key figures:

In contrast, a non-stretched nonwoven fabric with a weight per unit area of 150 g / m ² produced by the usual spinning process has the following characteristics:

If the same fleece with a basis weight of 230 g / m ^{2 is} stretched only 80% in the transverse direction at 136 ° C, after it had previously been stretched in the longitudinal direction by 10% at room temperature, a fleece with the following characteristics is obtained:

Claims (4)

1. Process for the manufacture of spunbonded webs from thermoplastics, wherein spunbonded webs of virtually continuous filaments in approximately random arrangement, which webs have a higher tensile strength in one direction than in the direction at right angles thereto, are stretched in one direction at temperatures which are 85 to 25 °C below the crystallite melting point of the fibre material, characterised in that the web is needle-punched before being stretched and the stretching is carried out in the direction of the lower tensile strength, by 20 to 200% of the particular original length, whilst either maintaining the length in the direction at right angles to the stretching direction or changing it, beforehand or simultaneously, by an amount within the range of ±10% of the original length.

2. Process according to Claim 1, characterised in that the starting material is a continuous filament web which has been needle-punched to the point that its increase in strength as a result of this needle-punching is more than 50% of the optimum increase in strength achievable by needle-punching.

3. Process according to Claims 1 and 2, characterised in that the starting material is a continuous filament web which has a higher tensile strength in the longitudinal direction than in the transverse direction, and the weg is stretched by 20 to 200% in the transverse direction.

4. Process according to Claims 1 and 2, characterised in that the starting material is a plaited- down continuous filament web which has a higher transverse strength than longitudinal strength, and the web is stretched by 20 to 200% in the longitudinal direction.