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1. WO1999055942 - POLYOLEFIN STAPLE FIBER FOR THE PRODUCTION OF THERMALLY BONDED NONWOVEN WEB

Note: Text based on automatic Optical Character Recognition processes. Please use the PDF version for legal matters

[ EN ]

POLYOLEFIN STAPLE FIBER FOR THE PRODUCTION OF THERMALLY
BONDED NONWOVEN WEB

SPECIFICATION

The present invention concerns polyolefin staple fibers for the production of thermally bonded nonwoven web. More specifically, the invention concerns polypropylene-based fibers obtained as staple fibers, of the kind suitable for the production of nonwoven web according to the technology known as "thermal bonding", wherein the composition of the polymer blend is such as to give improved properties of mechanical isotropy and softness to the resulting nonwoven fabric. The invention also concerns the nonwoven material obtained from the staple fibers and laminate composite materials comprising a layer of such nonwoven web coupled to a polyolefin film.
As it is known, polypropylene fibers are largely employed not only for textile applications (for instance in yarns for carpets, for upholstery fabric and for industrial uses) and for the production of ropes, but also, and specially, for the production of nonwoven materials for several applications in different fields, such as, e.g., the medical field, the house-care field, the geotextile field, the clothing field and, most importantly, the field of personal care. In the latter field of applications, particularly important is the manufacture of lightweight nonwoven web for diapers, adult incontinence products and feminine sanitary napkins, in particular for the production of the upper layer of such products, intended to come into direct contact with the skin. The said layer, which is sometimes referred to by the English term "top-sheet" (while the corresponding material is commonly referred to in the field by the generic name "cover- stock") is characterized by the particular softness requirements, and by the ability of rapidly transferring any moisture to the inner absorbing layer, thus leaving a "dry" feeling on the skin. The said requirements are additional to the ordinary tenacity requirement (i.e., tensile strength at break) that in all cases a nonwoven material must have to be suitable for being processed and used with no problems.

Another application, which is specific for lightweight polypropylene nonwoven web in the field of diaper and sanitary napkin manufacture, is the production of the corresponding impermeable outer layer (i.e., the "back sheet"). Conventionally, such layer used to be made of a polyethylene film. At present, in order to give a "textile" - and thus more pleasant - appearance to the diaper while this is being worn, the polyethylene (or, in general, polyolefin) film is usually coupled to a layer of nonwoven polypropylene fabric, which is intended to be placed on the external side of the finished diaper. In general, the coupling to give the laminate composite material may be obtained by means of adhesives, glues or "hot-melts", or by heat-sealing in a calender the nonwoven web and the film separately produced, or else by co-extruding the polyolefin resin directly on the nonwoven web.
Polyolefin fibers, for which polypropylene (PP) is by far the most widespread starting material, are normally produced by melt spinning, exploiting the thermoplasticity features of the material. The solution spinning technique is quite less common, and limited to particular cases. One of such particular cases is that of ultra-high molecular weight polyethylene (UHMWPE), which is used for manufacturing high-performance technical fibers, namely fibers with high tenacity and high modulus of elasticity, suitable for use in ropes and high strength fabric, or as a reinforcing fiber in other materials. Such polymer is not spun from the melt, rather it is spun by "gel spinning" from a solution with a suitable solvent. After cooling and partial removal of the solvent, in order to obtain the desired levels of mechanical properties the fibers are drawn at high draw ratios (about 10-100 - the draw ratio being defined as the ratio of the speed of the downstream roll to that of the upstream roll in the operation of drawing the fiber by pulling it).
Coming back to the most usual case of melt spinning, the polyolefin fibers (both intended for yarns and for nonwovens) may be obtained according to several different techniques, two of which, in particular, are suitable for the production of staple fibers: the one known as "long spinning" and the one known as "short spinning".
In both processes the melt polymer, together with suitable additives, is fed by an extruder to a series of spinnerets by means of as many positive-displacement pumps, and the bundles of filaments emerging from the spinnerets are cooled by cold air jets, and become solid. The two processes differ from each other in that in the long spinning process (also referred to as "conventional spinning") the number of holes present in each spinneret is comparatively low (normally 500-3000), the spinning speed is high (of the order of 500-2500 m/min), and the distance between the spinneret and the filament collecting system (which distance is necessary to allow the cooling of the fibers running at high speed) is wide (3-<9 m - resulting in the definition "long spinning"). The short spinning, on the contrary, is characterized by spinnerets with a high number of holes (normally 15,000-60,000), by a low spinning speed (< 100 m/min) and by a short distance between the spinneret and the filament collecting system (1-2 m - resulting in the definition "short spinning").
In the long spinning process, the bundles of cooled filaments collected from each spinneret, after being treated with an antistatic and lubricating agent (in an operation referred to as "finish"), are usually placed in containers in order to be fed to the subsequent processing operations in a separate production stage. This is due to the different speed at which the fiber production operations and the subsequent fiber processing operations may be carried out.
In the second processing stage of the long spinning process, the fibers are drawn by means of systems of rollers running at different speed. After the drawing operation, the fibers are "crimped" by passing them, under pressure, through a slot, thereby giving them a wavy deformation. Thereafter, the fibers are cut into lengths of a few centimeters, to give the staple fibers. The crimping operation is necessary to achieve a suitable cohesion between the staple fibers in the subsequent working steps.
In the case of the short spinning process, the same operations of drawing, crimping and cutting are carried out directly in the spinning line, in a continuous process. This is made possible by the low speed which characterizes the fiber production in the short spinning process, and allows to balance the speed of the different operations, thereby resulting, obviously, in a saving both in the initial capital expenditure and in the labor costs. The latter brings about, clearly, a lower cost of the staple fibers produced by short spinning in comparison with the staple fibers produced by the long spinning process.
In the most common production technique for obtaining nonwoven web from polypropylene staple fibers, the web of nonwoven material is obtained by "carding", i.e. by passing the staple fibers through a machine known as "carding machine" or "card machine". The latter "combs" the staple fibers by means of a series of toothed rolls, and orients the fibers mainly in the ma-chine direction (i.e., M.D.), thus forming the so-called "card web". The latter is then heat-bonded (thermal bonding), normally by passing it through a heated calender, whereby the fibers superficially melt and bond with each other as a result of the heat and pressure applied. In order to avoid an excessive stiffness and a "paper-like" appearance of the nonwoven fabric thus obtained, the thermal bonding is obtained on one portion only of the web surface, by using an embossed calender cylinder, wherein the protruding zones cover about 20-30 % of the cylinder surface.
The lower production costs of the staple fibers obtained by short spinning, referred to above, are balanced in the finished nonwoven web by a lower mechanical performance, especially as concerns the tenacity. The latter parameter has, for nonwovens obtained from the two above-mentioned techniques, values quite different from each other depending on the direction along which it is measured. This is due to the fact that in the carding operation the staple fibers tend to become oriented in the machine direction. Thus, the nonwoven web tenacity tends to be high in the latter direction and low in the direction transverse to the machine direction (i.e., CD., cross-machine direction). In order to take this difference into account, the characterization of the mechanical resistance of nonwoven webs is more correctly measured by giving not only the M.D. tenacity, but also the CD. tenacity. An overall pa- rameter which is normally employed in this connection is the so-called thermal bonding index (T.B.I.), which is given by the geometric mean of the tenacity values in M.D. and in CD. (i.e., the square root of the product of the said values). Evaluated in terms of such parameter, a thermally bonded nonwoven web having a basis weight of 20 g/m2 obtained from staple fibers produced by long spinning may have a T.B.I, of about 20-30 N/5 cm, while a nonwoven web obtained from staple fibers produced by short spinning may have a T.B.I. ≡_ 15-20 N/5 cm.
Other techniques of melt spinning, different from long spinning and short spinning, may result in the production of nonwovens directly form the spinning stage, without passing through the production of staple fibers, by means of an integrated process. The most widespread of such processes is known as the "spunbonded" (or "spun laid") technology, while another more recent method is the one known as "melt blown" (or "melt blowing") process. In the "spunbonded" technology the continuous fiber emerging from the spinneret is taken at very high speed (e.g., about 3500-4000 m/min) with the aid of an aerodynamic system, and is directly laid on a conveyor belt, with a random distribution of the strands, to give the nonwoven web. The latter is generally compacted by thermal bonding, by passing it through a calender and in oven. As a result of the non-oriented distribution of the fibers, the material thus obtained is characterized by a greater mechanical isotropy, and in this case the marked difference between tenacity in M.D. and tenacity in CD. which is typi- cal of the nonwovens obtained from staple fibers is not present. On the other hand, the "spunbonded" nonwoven materials have other shortcomings, including the inhomogeneous appearance due to a non-uniform coverage and a remarkably less soft and more paper-like hand, more similar to the hand of a film.
In the production technology of "melt blown" nonwovens, which is similar to the previous one, two converging jets of compressed hot air are directed on the fiber leaving the spinneret, thus causing the thinning and breaking thereof. The fibrils thus obtained are sucked and deposited on an underlying porous conveyor belt, thus forming the nonwoven web. The mate- rials obtained by this technique are mainly used in the production of bandage and filtering materials for medical use, and in the production of clothing, in particular sports clothing and work wear, and they are not suitable, alone, to be applied as cover layers (upper and outer cover) in diapers and sanitary napkins.
In order to improve the performance of the resulting nonwovens, or to obtain particular properties useful for specific applications, the features and the composition of the starting material to be spun are generally adjusted so as to obtain fibers having a modified nature with respect to the simple polypropylene homopolymer.
In particular, one known technique is the production of the so-called "bicomponent" fibers. Such fibers are made of two different polymers (for instance, polypropylene and polyethylene) that are simultaneously extruded in the spinning step without having been previously admixed with each other, thus resulting in a fiber with two completely separate zones. According to the shape of the spinning head there may be obtained, e.g., bicomponent fibers of the "sheath-core" type (i.e., with a core made of one of the two polymers and an outer sheath made of the other, normally with a cross-section having an annular shape) or bicomponent fibers of the "side-by-side" type (i.e., with one side made of one polymer and the other side made of the other polymer, and a cross-section made of two complementary semicircles). Such fibers are produced by a spinning line comprising at least two extruders (one for each polymeric material) and spinnerets having a complex structure, wherein the two materials are kept separate up to the die head, from which the said materials emerge with the desired geometrical shape.
An example of bicomponent fiber of the sheath-core type, disclosed in JP-A-2139469 (Unitika), consists in a polypropylene core and in a sheath of linear low density polyethylene (LLDPE, obtained by copolymerization of ethylene with a limited amount of an α-olefin with 3-12 carbon atoms, preferably 1-octene - the α-olefin introduces some short side branches in the linear polymer, thus resulting in a lower density). According to the document, the resulting nonwoven fabric, obtained with the "melt-blown" technique described above, has good features of softness and tenacity, and might be used as a cover material in items as diapers. It is evident from the foregoing, however, that bicomponent fibers involve too high production costs (about twice the cost of an ordinary thermobonbable polypropylene fiber) to be used as the base constituent of the "top-sheet" in diapers and the like. As a matter of fact, in the field of nonwovens, bicomponent PP/PE fibers of the sheath-core type, with the external sheath made of PE, are usually employed, admixed with other fibers and in limited amount, with the function of binder material, in order to exploit the lower melting point of PE in comparison with polypropylene.
Besides the "bicomponent" type fibers, another possible modification of the constitution of the base polymer material consists in the production of fibers referred to as "biconstituent". These are produced from a blend of two polymers that are admixed with each other in a phase prior to spinning or, in the alternative, directly in the extruder of the spinning line. In this case the fiber structure is not made of two separate zones each consisting of one of the two polymers, rather it is made of a random dispersion of small zones of one polymer into a matrix of the other polymer. The size and the dispersion of such zones depend on the relative concentration and on the compatibility of the two polymers. It is clear that no special spinning lines or spinnerets are required to produce biconstituent fibers, rather the ordinary equipment used for the single component fibers may be used.
EP-A-0663695 (Moplefan) discloses the production of a kind of such biconsituent fibers, wherein the base ingredient consists of polypropylene homopolymer or copolymer of the kind normally used for the production of thermobondable fibers. To this material, an amount not exceeding 20% of a second modifying polymeric material is added, having the function of increasing the softness of the resulting nonwoven web. The modifying composition consists of a heterophasic polymer (wherein by "heterophasic" polymer it is meant a particular kind of copolymer wherein one polymeric phase is intimately interspersed in the other, which polymer is normally obtained by two or three subsequent polymerization steps - the first one of such steps results in a porous, thinly layered matrix of a first polymer, within which, in the second step, a second polymer is grown) comprising an amount of a random ethyl- ene-propylene copolymer with low ethylene contents and an amount of a propylene-based rubbery copolymer, preferably an ethylene-propylene rubber or an ethylene-propylene-diene rubber. The polymeric composition is used for producing biconstituent fibers by the preferred long-spinning technique, and from the staple fibers thus obtained lightweight thermally bonded nonwovens are made, which are suitable for the production of the top-sheet and the back-sheet of diapers and sanitary towels (in the case of the back-sheet, upon coupling the nonwoven with a polyolefin film). In comparison with nonwovens made of propylene polymer only, obtained by the same technology, such materials have a much softer hand and an only slightly reduced mechanical resistance.
Besides being slightly lower, the tenacity values of the thermobonded nonwoven web obtained from the staple fiber according to the document mentioned above are also quite different if measured in M.D. or in CD., and the document does not take into account the problem of obtaining acceptable values of the strength also in the direction transverse to the working direction. As a matter of fact, such problem has a major importance, since the nonwoven web is subjected, in the subsequent working steps bringing to the finished product (e.g., to the packaged diaper), to mechanical stresses in all directions, and not only in the machine direction. Particularly in the field of adult incontinence products, in addition, the products undergo remarkable stresses also in use, for instance when the diapers are applied to patients who are not able to move.
The need for a better mechanical isotropy in the nonwoven obtained from polypropylene staple fibers has further increased as a result of the development of new industrial high-speed carding machines (200-300 m/min, subject to further increase). Such new carding machines tend more markedly to orient the staple fibers in the machine direction. In order to reduce the above problem, there is a tendency to produce the polypropylene fibers with a lower draw ratio, which makes the staple fibers less orientable in the carding machine. However, this brings about other drawbacks. As a first point, a re- duction of the draw ratio while maintaining unchanged the final titre (i.e., the fineness of the filaments, which for a lightweight nonwoven for "coverstock" should be about 2.2-2.3 dtex) requires the production of a fiber, upstream of the drawing operation, with a lower titre, and this may be obtained only by reducing the productivity, or else by increasing the spinning speed. Secondly, a lower draw ratio may bring about problems of insufficient permanency and fixability of the crimped fibers, with adverse consequences on the processabil-ity of the same fibers.
Another practical aspect making an acceptable mechanical isotropy in the nonwoven web from polypropylene staple fibers highly desirable is connected to the production of materials coupled with polyolefin film for the manufacture of the diapers "back-sheet". In this case it is known that the com-posite layer must be "perspiring", and the polyolefin film is normally made such by incorporating in the extrusion thereof a mineral filler (normally calcium carbonate at a concentration of 20-40% by weight) and then drawing the film thus produced to obtain some microporosities. If the coupled article is produced by thermosealing a nonwoven web and a separately produced film, such drawing operation on the film may be carried out previously. However, when the coupled article is produced by direct co-extrusion of the polyolefin resin on the nonwoven, the drawing must be carried out on the final laminate. Since this product has a high tenacity in the M.D. and is not expected to give, in this direction, the desired elongation, the drawing to render the product "perspiring" should be applied in the direction transverse to the working direction. Unfortunately, the nonwoven web is quite weak in this direction.
Therefore, it is an object of the present invention to provide a formulation of biconstituent fiber material suitable for the production of polypropylene staple fibers, which material may be spun with the "long spin" and the "short spin" techniques, for the production of thermally bonded nonwoven webs which show, in addition to the softness requirement, necessary in the field of personal care products, also an increased mechanical isotropy. Such increased mechanical isotropy corresponds to more balanced values of tenacity in M.D. and in CD., albeit with the same values of thermal bonding index (T.B.I. ).
In this connection, none of the prior art solutions proposing, for various purposes and with various applicative modalities, biconstituent polypro- pylene/polyethylene fibers, i.e. fibers with the same two base materials as the fibers proposed herein, appears to be directed to the problem of mechanical anisotropy of the thermally bonded nonwovens produced from carded staple fibers.
Specifically, EP-A-0260974 (Dow Chemical) concerns biconstituent

PP/PE fibers with improved features of tenacity and softness. Such fibers contain, together with polypropylene, an amount from 20 to 45% by weight of linear low density polyethylene (LLDPE). The fibers, that are produced by short spinning (as it is inferred from the number of apertures of the spinneret referred to in the examples, i.e. 20,500), are also proposed for the production of lightweight nonwoven fabric. However, only the increase in the M.D. tensile strength is experimentally measured for such products, while the CD. tensile strength is not mentioned. Amounts of PE lower than 20% are excluded from any consideration, on the grounds that such low amounts do not increase the tenacity. It is to be noted that a PP/PE blend with a polyethylene concentration of 20% or more is not suitable, unless in particular conditions, for being spun by the long spinning technique (with a high spinning speed) since the high polyethylene contents brings about, at such spinning speeds, a frequent breakage of the filaments.
WO-A-90 10672 (Dow Chemical) discloses articles of thermally bonded nonwoven fabric obtained from fibers similar to those of the previous document, wherein the PP/PE ratio is comprised between 0.6 and 1.5 (≥ 40- 62% PE). Also in this case, the mechanical tests on the nonwoven material, performed in the machine direction only, show an increase of the M.D. tenac- ity.
A biconstituent fiber substantially similar to that of the two previous documents, specifically studied for being produced with high speed spinning techniques (not only long spinning, but also "spunbonded") is described in US- A-4874666 (Kubo et al., Unitika). In this case, the polymer mixture contains polyethylene as the major constituent (50-99% LLDPE and 1-50% PP), and is proposed for the same uses typical of the PP bicomponent fiber with external PE sheath that have been mentioned before, i.e. for use as a "binder" fiber. In order to solve the problems connected with the difficulty of fast spinning of fibers with a high percentage of PE, the document poses precise limitations on the viscosity features of the two constituent polymers, fixing a higher limit of 20 g/10 min. for the "melt index" (or "M.F.I.", melt flow index, or melt flow rate) of polypropylene, as well as lower and higher limits of 25 and 100 g/10 min. respectively for the melt index of polyethylene (LLDPE). According to the disclosure, in addition, a mixture containing more crystalline polypropylene than LLDPE is hardly spinnable at high speed. Also in this case, the tensile resistance of the nonwoven fabric produced from the proposed fibers is evalu-ated in M.D. only.
WO-A-9606210 (Kimberly-Clark) discloses a polypropylene-based (50-95% by weight) bicomponent fibers wherein the second material is a (random block) copolymer of ethylene and propylene, having the purpose of improving the softness of the resulting nonwoven fabric. The fiber, however, does not fall in the field of staple fibers, but is intended to the production of "spunbonded" nonwoven web, possibly laminated with an intermediate layer of "melt-blown" nonwoven, and with another layer of "spunbonded" nonwoven (SMS laminates). The addition of the ethylene-propylene copolymer is aimed at obtaining a higher softness than that obtainable in a "spunbonded" non-woven web of polypropylene only.
JP-A-4214405 (Mitsui), lastly, concerns the use, for the production of high mechanical performance fibers, of a polypropylene/UHMWPE mixture with 85-99.5% of polypropylene and 0.5-15% of polyethylene. Not only polyethylene, but also PP has a very high molecular weight (being specified an intrinsic viscosity of at least 5 dl/g, corresponding to an average molecular weight of about 1 ,300,000 g/mole, while the PP employed for melt spinning has an intrinsic viscosity of about 1.5 dl/g and an average molecular weight of 250,000 g/mole or less), and the fiber production is carried out not by melt spinning, rather by "gel spinning", as previously mentioned, using decalin as the solvent. The fibers thus obtained undergo draw ratios in the range of 10- 100 and the resulting product may be used to produce plates, sheets, film, tape and also nonwoven fabric. In view of their characteristics of stiffness and high mechanical stability, such materials are used not in the filed of personal care products, rather in industrial applications, such as reinforcing and supporting structures for composite materials.
In the frame of the present invention it has been found that by adding to a polypropylene resin of the kind normally used for the production of polypropylene nonwoven web from staple fibers an extremely low amount of polyethylene, and by spinning the resulting mixture, staple fibers much less ori-entable in the carding machine are obtained. Such staple fibers result, in the finished product, in a much more balanced mechanical behavior between M.D. resistance and CD. resistance, in comparison with the behavior shown, under equal conditions, by staple fibers made of polypropylene only. In practice, the nonwoven fabric produced by thermal bonding from the biconstituent PP/PE fiber has lower values of resistance in M.D. and higher values of resistance in CD. than those obtainable with a similar nonwoven web lacking the polyethylene component, while the geometric average of such values (i.e. the T.B.I.) does not change, or even increases. The respective values of elongation at break in each direction undergo similar modifications, and this turns to be extremely advantageous, as it will be clearer further on, for the production of coupled articles nonwoven web/polyolefin film.
Accordingly, the present invention specifically provides staple fibers for the production of thermally bonded nonwoven web obtained from a polymer mixture consisting essentially of:
A) 85-99.9% by weight of polypropylene homopolymer or of polypropylene copolymer, either random o heterophasic, or mixtures thereof;
B) 0.1-15% by weight of polyethylene.
The polypropylene making the main fiber component is preferably isotactic or mainly isotactic PP, with an isotactic index (i.e. the percentage by weight of the fraction insoluble in boiling n-heptane) higher than 85%. However, the invention might also be applied, e.g., to polymer blends based on syndiotactic polypropylene.
In both cases component A) of the mixture may either be polypropylene homopolymer or a random copolymer of propylene with ethylene and/or other α-olefins, wherein the overall contents of comonomers is in the range of from 0.01 to 25% by weight. According to other embodiments, component A) may have the same composition, with modifying heterophasic polymeric component, which is the subject matter of the above-mentioned EP-A-0663965. In such case, component A) has the following composition:
• from 80 to 99.9% by weight of polypropylene homopolymer or of a random copolymer of propylene with ethylene and/or other α-olefins, wherein the overall contents of comonomers is in the range of from 0.01 to 25% by weight; and
• from 0.1 to 20% by weight of a heterophasic polymer consisting of:
a) 10-60% by weight of polypropylene homopolymer or copolymer with an isotactic index higher than 80%;
b) 3-25% by weight of an ethylene-propylene copolymer insoluble in xylene at 23°C;
c) 15-87 by weight of a rubber copolymer ethylene-propylene, or ethylene- propylene-diene, soluble in xylene at 23°C
The polyethylene component B) is preferably present in the mixture in an amount in the range of from 0.5 to 5% by weight, and can be chosen from high density polyethylene (HDPE), low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and mixtures thereof, the preferred one being

HDPE.
The melt index of component A), when this is made of PP homopolymer or of a random copolymer, is preferably in the range of from 1.5 and 60 g/10 min. (measured according to ASTM D-1238, as grams of polymer extruded in 10 min. through a standard opening under a load of 2.16 kg at 230°C), which corresponds to an intrinsic viscosity of 2.5-0.9 dl/g. When the component A) is made of the heterophasic mixture according to the prior art mentioned above, the overall melt index of the heterophasic modifying component is preferably in the range of from 5 and 25 g/10 min.. On the other hand, the flow values of the polyethylene component B) are not relevant, in view of the low concentration of such component in the polymer blend.
As usual, the polymer products for the production of the fibers ac- cording to the invention may comprise one or more stabilizing additives, such as antioxidants and light stabilizers, in particular organic phosphites or phos-phonites or disulphides, phenolic antioxidants, hindered amine light stabilizers (HALS), in a concentration of from 0.01 to 1 % by weight. Other additives may be included, if desired, in the spinning stage, such as, e.g., one or more organic or inorganic pigments and/or one or more opacifiers.
As noted in connection with the general technology of biconstituent fibers, the blending step of the two base polymers may be carried out in a suitable device, or it may be obtained by contacting the two polymers in the extruder immediately upstream the spinning step. Advantageously, polyethylene is directly added in the extruder of the spinning line by using the conventional dosing systems, for instance a continuous gravimetic dosing device.
The invention also concerns the nonwoven webs obtained by thermal bonding from the staple fibers described above, in particular nonwovens of the lightweight type, suitable for use in disposable personal care products such as diapers for babies, adult incontinence products and feminine sanitary napkins. A further aspect of the invention is the use of a polymer composition as defined above for the production of staple fibers, preferably through the "long spinning" technique, for the production of thermally bonded nonwoven fabric. As noted before, the nonwoven according to the invention may be advantageously used for the production of a nonwoven web/polyolefin film coupled material, in particular a material wherein the said polyolefin film is a polyethylene film. In this application, the enhanced tenacity and the higher elongation at break of the nonwoven in the direction transverse to the machine direction allow, in the production of the coupled material by co-extrusion of the film directly on the nonwoven, to obtain a more easily stretchable coupled material and, moreover, to reduce the thickness of the film and/or of the nonwoven web. Furthermore, the presence of polyethylene dispersed within the fiber enhances the adhesion with the polyolefin film, without the need of hav- ing recourse to the use of adhesives and "hot melts", or to the addition, to the polypropylene staple fibers, of bicomponent fibers with a "sheath-core" structure with outer sheath of PE. Actually, the latter give an unsatisfactory bonding with the staple fibers in the calendering step and lower the overall tenacity of the nonwoven web (both in the CD. and in the M.D.).
It is important to note that the nonwovens produced from the staple fibers according to the invention also show a higher softness in comparison with products with the same features but lacking the addition of polyethylene. The advantageous aspects of the products according to the invention will be clearer with reference to the specific embodiments thereof referred to in the following examples, which are not to be intended as limiting in any way the scope of the invention.
EXAMPLE 1
A pilot line of long spinning and finishing for polypropylene was used to produce staple fibers from the following mixture:
• 97% by weight of polypropylene homopolymer with isotactic index 96.5% and melt index 12 dg/min.
• 3% by weight of high density polyethylene (HDPE) with melt index 2 dg/min.
The spinning and finishing conditions were the following:
spinneret with 2400 holes
extruder temperature 267 °C
spinning head temperature 272 °C
collecting speed 700 m/min
spinning count 4.1 dtex
drawing ratio 2.4
final titre 2.2-2.3 dtex
finish 0.4-0.55%
cut 40 mm
The carding-calendering conditions for the production of the nonwoven web were the following:
carding-calendering speed 40 m/min
pressure of calender cylinders 50 kg/linear cm temperature of calender cylinders 145/150 °C
nonwoven web substance 20 g/m2 The nonwoven web obtained was evaluated as to its features of tenacity and elongation, in accordance with UNI 8639 standard, both in M.D. and in CD.. In addition, the softness of said nonwoven web was evaluated, expressed as the average of the judgements given by a test panel, on the basis of a scale of reference vaiues from 1 (stiff) to 5 (very soft). The results obtained were the following:
M.D. tenacity 54.0 N/5 cm
CD. tenacity 13.8 N/5 cm
T.B.I. 27.3 N/5 cm
M.D. elongation 48%
CD. elongation 106%
softness 4.0
Comparative Example 1a
In the same spinning and finishing conditions for the fiber production and in the same carding-calendering conditions for the nonwoven web production as in Example 1 , the following starting material was used:
• 100% of polypropylene homopolymer with isotactic index 96.5 % and melt index 12 dg/min.
The features of the nonwoven web, evaluated with the same methods described above, were the following:
M.D. tenacity 66 N/5 cm
CD. tenacity 9.2 N/5 cm
T.B.I. 24.6 N/5 cm
M.D. elongation 51%
CD. elongation 73%
softness 2.0
It is clear from the above data that a minimum addition (i.e. 3% by weight) of polyethylene resulted in a remarkable enhancement in the mechanical isotropy, with no detriment to the thermal bondability index, rather with an enhancement of the said parameter. Equally remarkable is the enhancement of the product performance in terms of softness.

EXAMPLE 2
In the same spinning and finishing conditions for the fiber production and in the same carding-calendering conditions for the nonwoven web production as in Example 1 , the following mixture was used:
• 80% by weight of polypropylene homopolymer with isotactic index 96.5 % and melt index 12 dg/min.
• 10% by weight of a crystalline copolymer of propylene with ethylene (3.5 % by weight) with melt index 6 dg/min.
. 10% by weight of LLDPE with melt index 9,
The features of the nonwoven web, evaluated with the same methods as in Example 1 , were the following:
M.D. tenacity 50 N/5 cm
CD. tenacity 12.2 N/ 5 cm
T.B.I. 24.7 N/5 cm
M.D. elongation 42%
CD. elongation 98%
softness 4.5
Comparative Example 2a
In the same spinning and finishing conditions for the fiber production and in the same carding-calendering conditions for the nonwoven web production as in Example 1 , the following mixture was used:
• 90% by weight of polypropylene homopolymer with isotactic index 96.5% and melt index 12 dg/min.
• 10% by weight of crystalline copolymer of propylene with ethylene (3.5 % by weight) with melt index 6 dg/min.
The features of the nonwoven web were the following:
M.D. tenacity 61 N/5 cm
CD. tenacity 8.6 N/5 cm
T.B.I. 22.9 N/5 cm
M.D. elongation 48%
CD. elongation 71%

softness 2.5 EXAMPLE 3
In the same spinning and finishing conditions for the fiber production and in the same carding-calendering conditions for the nonwoven web production as in Example 1 , the following mixture was used:
• 80% by weight of polypropylene homopolymer with isotactic index 96.5 % and melt index 12 dg/min.
• 15% by weight of heterophasic copolymer consisting of:
a) 20% by weight of polypropylene homopolymer with isotactic index 96.5% b) 15% by weight of ethylene-propylene copolymer with 3.5% ethylene c) 65% by weight of ethylene-propylene rubber (30% ethylene)
having an overall melt index of 8 dg/min.
. 5% by weight of LDPE with melt index 20.
The features of the nonwoven web, evaluated with the same methods as in Example 1 , were the following:
M.D. tenacity 46 N/5 cm
CD. tenacity 11.1 N/5 cm
T.B.I. 22.6 N/5 cm
M.D. elongation 40%
CD. elongation 87%
softness 5.0
Comparative Example 3a
In the same spinning and finishing conditions and in the same carding-calendering conditions as in Example 1 , the following mixture was used:
• 85% by weight of polypropylene homopolymer with isotactic index 96.5% and melt index 12 dg/min.
• 15% by weight of heterophasic copolymer consisting of:
a) 20% by weight of polypropylene homopolymer with isotactic index 96.5% b) 15% by weight of ethylene-propylene copolymer with 3.5% ethylene c) 65% by weight of ethylene-propylene rubber (30% ethylene)
having an overall melt index of 8 dg/min.
The features of the nonwoven web were the following:
M.D. tenacity 53 N/5 cm C.D. tenacity 7.4 N/5 cm
T.B.I. 19.8 N/5 cm
M.D. elongation 45 %
CD. elongation 61 %
softness 4.0
EXAMPLE 4
A sample of nonwoven web from Example 1 was coupled with a polyethylene film of 20 μm thickness.
The coupling was carried out by welding together in a calendering ma-chine the nonwoven web and the film, at a temperature of the calender cylinders of 125°C and at a cylinders speed of 40 m/min.
The features of the coupled material thus obtained were the following: M.D. tenacity 61 N/5 cm
CD. tenacity 16.7 N/5 cm
M.D. elongation 54%
CD. elongation 101%
adhesion nonwoven/film very good
Comparative Example 4a
A sample of nonwoven web from Comparative Example 1 was coupled with a polyethylene film of 20 μm thickness in the same conditions as in Example 4.
The features of the coupled material thus obtained were the following: M.D. tenacity 69 N/5 cm
CD. tenacity 13.1 N/5 cm
M.D. elongation 58%
CD. elongation 71 %
adhesion nonwoven/film poor
Besides producing a markedly enhanced adhesion between nonwoven and film, the addition of 3% only of polyethylene to the polypropylene fiber allows to markedly increase the tenacity and the elongation in CD., in comparison with a similar nonwoven web lacking the polyethylene constituent.

EXAMPLE 5
A sample of nonwoven web from Example 1 was coupled with a polyethylene film of 15 μm thickness. The coupling was obtained by the same procedure as in the previous example.
The features of the coupled material obtained were the following:
M.D. tenacity 55 N/5 cm
CD. tenacity 14.1 N/5 cm
M.D. elongation 42%
CD. elongation 98%
adhesion nonwoven/film very good
Comparative Example 5a
A sample of nonwoven web from Comparative Example 1 was coupled with a polyethylene film of 15 μm thickness in the same conditions as in Example 4.
The features of the coupled material obtained were the following:
M.D. tenacity 65 N/5 cm
CD. tenacity 12.9 N/5 cm
M.D. elongation 53%
CD. elongation 65%
adhesion nonwoven/film poor
EXAMPLE 6
A nonwoven fabric produced in the same way as in Example 1 , but having a weight of 18 g/m2, was coupled with a polyethylene film of 20 μm thickness in the same conditions as in Example 4.
The features of the coupled material obtained were the following:
M.D. tenacity 51 N/5 cm
CD. tenacity 14.7 N/5 cm
M.D. elongation 50%
CD. elongation 95%
adhesion nonwoven/film very good
Comparative Example 6a
A nonwoven fabric produced in the same way as in Comparative Ex- ample 1a, but having a weight of 18 g/m2, was coupled with a polyethylene film of 20 μm thickness in the same conditions as in Example 4.
The features of the coupled material obtained were the following:
M.D. tenacity 64 N/5 cm
CD. tenacity 11.2 N/5 cm
M.D. elongation 53%
CD. elongation 66%
adhesion nonwoven/film poor
The three preceding examples, together with the corresponding com-parative examples, show that the use of the PP/PE staple fibers according to the invention allows, in the production of laminated material film/nonwoven fabric, to reduce the thickness of the film or, in the alternative, to reduce the substance of the nonwoven, while still obtaining advantageous mechanical properties.
While not wishing to be bound by any theoretical interpretation, it is believed that the reason for the remarkable increase in the mechanical isotropy of the nonwovens obtained from staple fibers as a result of the addition of even very small amounts of polyethylene to the polypropylene resins currently used is to be ascribed to a lower orientability of the staple fibers upon the carding operation. Said reduced orientability results in a reduced parallelism in the fibers of the thermally bonded nonwoven fabric. Such lower orientability is due, at least partly, to the lower stiffness of the fibers containing a small amount of polyethylene, in comparison with the fibers based on polypropylene resins only, and to the resulting lower reactivity of the said fibers to deforma- tion.
In addition, it is also possible that such effect is enhanced by the phenomenon of the waving density increase in the crimped fibers, that has been observed as a consequence of the addition of small amounts of PE to PP. It has been ascertained, actually, that even a small addition of polyethylene to the polypropylene resin, under the same conditions, causes a remarkable increase in the number of waves per unitary fiber length that are formed as a result of the crimping operation (all of the other operation conditions being the same). This results in an increased extensibility of the crimped fibers.
The present invention has been disclosed with particular reference to some specific embodiments thereof, but it should be understood that modifications and changes may be made by the persons skilled in the art without departing from the scope of the invention as defined in the appended claims.