EP3754082B1

EP3754082B1 - Fiber deposition tool for production of non-woven fabrics by spunbonding

Info

Publication number: EP3754082B1
Application number: EP19180828.6A
Authority: EP
Inventors: Stefano Maria ZANARDI; Ivano Sergio DE PELLEGRIN
Original assignee: Societa Italiana Costruzione Aeromeccaniche SICAM SRL
Current assignee: Societa Italiana Costruzione Aeromeccaniche SICAM SRL
Priority date: 2019-06-18
Filing date: 2019-06-18
Publication date: 2022-04-06
Anticipated expiration: 2039-06-18
Also published as: EP3754082A1

Description

FIELD OF THE INVENTION

The present invention relates to an improvement in an apparatus for the production of non-woven fabrics, in particular in the section of the apparatus dedicated to the production of the web of loose fibers that are subsequently bound with different techniques to produce a non-woven fabric.

STATE OF THE ART

Non-woven fabrics have replaced traditional fabrics in a number of applications and products, among which, to cite but a few, surgical masks and caps, disposable clothing, tea bags, vacuum bags, diapers, carpet backing and filters for gasoline, oil and air.
These fabrics may be produced following different techniques, and depending on the production technique are called melt-blown, flashspun or spunbond (the latter also referred to in the field as spunlaid).
The technique of production of spunbond non-woven fabrics is widely adopted in the field because of its relatively high productivity, and because the resulting fabrics have good longitudinal and transverse tensile strength, together with good homogeneity and visual quality.
These fabrics are made in one continuous process, in which the fibers are spun from a reservoir of the melt polymer (generally polypropylene, polyesters, polyamides or similar), cooled by a transverse air flow and, when still in a soft state (that is, not fully consolidated yet) aerodynamically stretched inside ejectors by a high-speed airflow parallel to the fibers; before entering the ejectors for stretching, the fibers are electrostatically charged by suitable devices operating in direct current; the surface charge of the fibers prevent them from sticking to each other in the ejectors. Stretching in the ejectors produces a bundle of loose fibers, which get entangled by air turbulence beneath the ejectors; this way, a web is formed and then collected on a collecting surface, which is usually a running belt. The collecting surface is usually perforated to prevent the air stream from deflecting and carrying the fibers in an uncontrolled manner. The thus formed web of fibers is later on consolidated by methods well known in the field, such as thermal bonding, hydro-entanglement, ultrasonic pattern bonding, needlepunching/needlefelting, or chemical bonding.
Fig. 1 schematically shows a common arrangement of a spunbond production machine; the figure only represents the phases of stretching in ejectors of the fibers and their deposition over a collecting belt. Fig. 1.a is a top view of the ejectors and collecting belt (as well as of a frame carrying cylinders that are described later on), while Fig. 1.b is a side view of a portion of the machine which comprises a single ejector.
In this usual arrangement, a series of ejectors (E) are positioned in parallel over the collector of fibers, which in this case is a running belt (B). The ejectors are aligned along a line perpendicular to the traveling direction of the belt (indicated by the arrow in the figure). Positioned between the outlet of the ejectors and the belt there are a series of equidistant cylinders (C) fixed to a frame (F). The axes of the cylinders lay in a plane parallel to the plane of the belt; these axes may be at an angle with the traveling direction of the belt, but generally and preferably they are parallel to said direction. The frame allows changing the setting of the machine, in particular as to the distance between the cylinders (actually, the plane in which the axes of the cylinders lay) from the outlet of the ejectors and the distance between the (plane of the axes of the) cylinders and the traveling belt, or the mutual distance between two next cylinders. In the position at rest of the machine, the projection of the axis of every ejector passes between, and is equidistant from, a pair of next cylinders. A pair of next cylinders, between which the fibers released from the ejector pass before reaching the collector, will be referred to in this text and the annexed claims "lapper". In operation, the frame is moved back-and-forth of a preset distance by a reciprocating motor; this movement is in the plane of the axes of the cylinders and in a direction perpendicular to said axes, between two limit points; the frequency of the reciprocating movement may vary between about 250 and about 1000 cycles per minute; the amplitude of the reciprocating movement may vary between about 4 and about 20 mm (that is, leftward and rightward strokes between about 2 mm and about 10 mm).
The fibers leaving the ejectors are stretched by the high-speed airflow between two next cylinders, and the rapid horizontal oscillation of the cylinders around their average position, interacting with this high-speed airflow, creates a turbulence that "opens" the bundle of fibers; the turbulence generated by the airflow passing between the cylinders is known in the field of aerodynamics as "Coandǎ effect", and has the result of spreading the fibers and increasing the width of the deposition area. During the phase of stretching and deposition on the belt, the fibers have sufficient time and space to interlace and form a homogeneous web (W).
Patent US 3,293,718 discloses a fibers deposition apparatus comprising a plurality of jets for ejecting filaments of the kind described above, in which below each jet is present a pair of rotating devices each mounted on a drive shaft rotatably mounted on a support structure. Each rotating device consists essentially of a disk-like element having a thickness corresponding to the width of the fluid stream and a peripheral circumferentially extending groove, and portions cut away from the periphery of the disk-like element. The residual groove portion functions as a Coandǎ device causing filament deflection upon rotation of the rotating devices.
Patent US 3,903,569 , aimed at improving air guidance, discloses a carriage with back and forth movement on a frame and calender rollers for cross-lapping.
Two deposition parameters, which directly influence the structure and mechanical strength of the final fabric, are the width of the deposition zone and the speed of the airflow causing deposition of the fibers on the belt. In a classic stretching ejector the deposition width varies from about 150 to about 250 mm. As to airflow, practical experience has shown that the speed of the deposition air should not be lower than 50 m/s, and preferably around 55 m/s, to avoid that the fibers fall on the collection/transport belt too slowly, which has been observed to result in poor homogeneity of the web (areas with thick overlaps of fibers and empty areas).
Several approaches have been tried to avoid these problems, in particular in order to enlarge the deposition width (which would give rise to wider fabrics and higher productivity); these approaches are electrostatically charging the fibers downstream the ejectors, and the adoption of deflectors or aeromechanical agitators; these approaches have however given poor results, and even created side-problems.
Both the electrostatic charges and the diffusers in fact manage to increase the width of the deposition zone by at most 15 to 30 mm; besides, in the case of the diffusers, the undesired effect is also obtained of slowing the air speed well below the critical value of 50-55 m/s, leading to a worsening of the mechanical resistance of the finished product.
As to the aeromechanical agitators, these on one hand increase more the deposition area and the mechanical resistance of the fabric but, on the other hand, due to the strong turbulence and irregular flows that they introduce in the formation area, considerably degrade the visual quality and the homogeneity of the veil.
It is an object of the present invention to improve the productivity of plants for the production of non-woven fabrics, by increasing the width of the deposition zone without however impairing visual quality, softness and homogeneity of the fabric.

SUMMARY OF THE INVENTION

This and other objects are obtained with the present invention, which consists in a deposition tool for webs of non-woven fabrics, comprising:

at least one vertical ejector having an outlet from which a bundle of polymeric fibers is released;
in front of the outlet of the at least one vertical ejector, a collector of said polymeric fibers;
at least two parallel cylinders fixed to a frame, having their axes laying in a plane p₁ perpendicular to the axis of the at least one vertical ejector, said plane p₁ parallel to the plane p₂ of the collector and being positioned between said outlet and said collector, with a distance d₁ between 40 and 120 mm from said outlet and a distance d₂ between 100 and 600 mm from said collector, and a minimum distance d₃ between the cylinders in the range from 10 to 20 mm;

said frame connected to a motor suitable to apply to said frame a reciprocating motion in said plane p₁ and in the direction perpendicular to the axes of the cylinders, with strokes of amplitude between 2 mm and 10 mm around an average position in which said two parallel cylinders are equidistant from the projection of the axis of the ejector;

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described in the following with reference to the Figures, in which:

Fig. 1 schematically represents, in a front view (Fig. 1.a) and in a top view (Fig. 1.b) a deposition tool according to the prior art of an apparatus for producing non-woven fabrics;
Fig. 2 represents, in a front view, a deposition tool of the invention for the production of non-woven fabrics;
Fig. 3 represents a perspective view of a possible cylinder according to the invention;
Fig. 4 represents a section view of another possible, preferred, cylinder according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention consists in a modification of the deposition section of a kind of known apparatuses for the production of non-woven fabrics. The basic unit of the apparatus is the combination of an ejector and a pair of cylinders, positioned as described above over the collector, generally a running belt; each pair of cylinders associated to an ejector forms a lapper of the fibers released by the outlet of the ejector. The modification consists in the fact that, different from the systems of the prior art in which the cylinders making up a lapper are smooth, in the present invention each of the two cylinders of the lapper present at least a part of the lateral surface (as better described in the following) zones presenting indentations.
This condition is represented in Figs. 2 and 3, in which details of the basic unit of the deposition tool of the invention are shown; the part of the apparatus upstream said unit, comprising tools for charging a solid polymer, means for melting the polymer, spinnerets for extruding filaments of liquid polymers and means for cooling said filaments to form solid polymers fibers to be stretched in the ejectors, are the same as in installations of the prior art and are not represented in the drawings.
A basic unit for stretching and depositing fibers comprises an ejector, 20, and two modified cylinders, 21 and 21', fixed to a frame 22; the pair of cylinders 21 and 21' forms the lapper of the invention. Upstream the ejector, just before the inlet point of the fibers, is positioned an electrostatically charging device 23 for charging the surface of the fibers in order to avoid that these stick to each other during the stretching in the ejector. Stretching of the fibers is caused by a high-speed airflow in the ejector, schematically represented by the arrows in Fig. 2. Stretched fibers, 24, leave the ejector from outlet 25, and are projected in the space between cylinders 21 and 21' by the speed acquired in the ejector due to said high-speed airflow; another contribution to the projection of the fibers in said space is the parallel, secondary flow of air from around the ejector, pulled in said space by the low pressure created beneath the outlet by the primary airflow. Beneath the lapper is present the collector, in Fig. 2 represented by transporting belt 26 moving to the left, in the direction of the arrow.
Belt 26 is air permeable and an air intake system, schematically represented as element 27, is placed below it to retain the forming web of fibers on the belt itself.
The dimensional characteristics of basic deposition unit of the invention, and its position with respect to the collector, are the following: minimum distance between two cylinders of a lapper (that is, the distance between the nearest points on the surfaces of the two cylindrical) between 10 and 20 mm, preferably between 15 and 16 mm; distance between the outlet of the ejector and the inlet and plane p₁ of the axes of the cylinders between 40 and 120 mm, preferably between 55 and 90 mm; distance between the plane p₁ of the axes of the cylinders and the plane p₂ of the collector between 100 and 600 mm, preferably between 330 and 580 mm. These dimensional characteristics can be modified, in the given ranges, by lifting or lowering frame 22, or regulating the distance d₃ between cylinders 21 and 21'. Means on the frame for regulating distance d₃ are not shown in the drawing, but will be evident to the skilled person; for instance, horizontal slots can be present on the frame in which supports of the cylinders, coaxial with the same, may slide, and means (for instance bolts) may be provided on the outer surface of the frame for firmly fix the cylinders in the selected position.
By controlling these geometrical parameters, it is possible to control the flow rate of the secondary air flows pulled in the space between the cylinders from around the ejector; in this way, it is possible to regulate this flow rate between 15 and 70% of that of the primary flow (the flow inside the ejector), and therefore to control the air speed in the deposition zone, or at the exit of the lapper; in particular, it is possible to maintain the overall air flowrate at a value above 50 m/s even when distance d₃ is increased.
The cylinders that constitute the lapper(s) of the invention may be completely made of metal, for instance stainless steel or aluminum; of alloys, such as the alloys known as Anticorodal (aluminum-based alloys of the 6000 series, containing 0.4-1.7 wt% magnesium, 0.5-1 wt% manganese and 1-5 wt% silicon); of other materials, such as carbon fibers, polymers or copolymers; or they can have the inner, supporting part, of metal, and the surface covered with a layer of polymer. When the cylinders are made of, or coated with, polymers, the latter may be loaded with conductive particles (e.g., of metals or graphite). The cylinders are preferably as light as possible, in order to facilitate in particular the reciprocating movement of the frame holding them during operation of the system.
The indented zone on the surface of a possible embodiment of cylinders of the invention is shown in detail in Fig. 3. In a zone, 30, defined by generator segments A and B, of the lateral surface of the cylinder of the invention (21 or 21'), are present a series of indentations 31 of circular shape, positioned in said zone according to an rhomboidal array; in the most general embodiment of the invention shown in Fig. 3, the remainder part of the lateral surface of the cylinder, 32, is smooth. Segments A and B are spaced apart along the circumference of the cylinder at least 1/6 of said circumference. Indentations 31 have circular shape, a diameter ranging between 4 and 6 mm, a depth between 1 and 2 mm, and a distance between the centers of two next indentations between 5.5 and 7.5 mm; the depth of the indentations is preferably higher, in said range, the higher their diameter.
A preferred embodiment of the invention is represented in Fig. 4, which shows a section view of a preferred cylinder (21 or 21'). In this embodiment, the lateral surface of the cylinder is subdivided into four zones, separated by generator segments C, D, E and F, spaced apart along the circumference of the cylinder 1/4 of said circumference. This way, four zones 40, 41, 42 and 43, of same area, are defined on the lateral surface of the cylinder; two opposed (non-adjacent) zones, 40 and 42, present indentations, while the other two opposed zones 41 and 43 are smooth; zone 40 present a series of indentations 44, and zone 42 presents indentations 45. Indentations 44 and 45 are different in size, distance and depth. For instance, indentations 44 may have diameter 6 mm, depth 2 mm and distance between centers of two next indentations 7.5 mm, while indentations 45 may have diameter 4 mm, depth 1 mm and distance between centers of two next indentations 5.5 mm.
This arrangement with two zones on the surface of cylinders 21 and 21' presenting indentations with different size and distance, allows selecting which actual zones (whether zones 40 or zones 42 on both cylinders) are positioned facing each other in the settings of the apparatus before starting a deposition run of the web of fibers 24; different zones with different size of indentations give rise to different turbulence effects in the space between the cylinders and thus to different intensity and results of fibers entanglement, making available a further control on apparatus operation that is not possible with lappers of the prior art, having only smooth surfaces.
Of course, other structures of the cylinders of the lapper are possible; for instance, the cylinder shown in Fig. 4 may be modified with a further series of indentations on surface 43, with size and distance different from indentations 44 and 45, to further increase the possible operating configurations of the apparatus; other possible modifications, for instance defining more zones on the surface of the cylinders, said zones smooth or with different indentations, will be apparent to the skilled person. The maximum number of zones on the surface of each cylinder of the lapper is 6: a higher number would give rise to too narrow zones, with loss of control of the indentations actually involved in the turbulence effect in the space between the cylinders.
Frame 22 is connected to a reciprocating motor (not shown in the figures) adapted to apply to the frame a motion with strokes of amplitude between 2 and 10 mm in plane p₁ in a direction perpendicular to the axes of cylinders 21 and 21'.
In operation, stretched fibers 24 are ejected in the space between cylinders 21 and 21'; these are reciprocated between two limit positions, which correspond to strokes between 2 and 10 mm, while the reciprocating motion frequency can vary from 250 to 800 cycles per minute; the combined effect of air coming from inside the ejector, air pulled between the cylinders from outside the ejector, reciprocating motion of the lapper and the enhanced turbulence caused by the indentations on the cylinders give rise a particularly intense Coandǎ effect which dramatically increase the width of the area over belt 26 on which the fibers are distributed. The inventors have observed that the distribution unit of the invention is capable of increasing the deposition area of fibers from values of about 150-250 mm that are typical of systems of the prior art, up to about 300-350 mm. This result is achieved without slowing down (indeed doubling) the air speed in the deposition zone as with the diffusers of the prior art, which led to inhomogeneities in the forming web of fibers; the deposition unit of the invention also operates without introducing turbulence or other disturbances to the air flow in the deposition area, thus being able to form a veil of improved mechanical strength and at the same time high visual quality, softness and homogeneity.
In a preferred embodiment, another modification is applied to the basic unit of the invention. In this preferred embodiment the current applied to charging device 23 is not direct current, but rather a pulsed current that generates a voltage periodic and constant over time.
The frequency of oscillation of the voltage is modulable and between 5 and 60 cycles per second, the value of maximum voltage being between 15 and 60 KV, preferably between 15 and 45 KV.

Claims

Fibers deposition tool for webs of non-woven fabrics, comprising:
- at least one vertical ejector (20) having an outlet (25) from which a bundle of polymeric fibers (24) is released;

- in front of the outlet of the at least one vertical ejector, a collector (26) of said polymeric fibers;

- at least two parallel cylinders (21, 21') fixed to a frame (22), having their axes laying in a plane p₁ perpendicular to the axis of the at least one vertical ejector, said plane p₁ parallel to the plane p₂ of the collector and being positioned between said outlet and said collector, with a distance d₁ between 40 and 120 mm from said outlet and a distance d₂ between 100 and 600 mm from said collector, and a minimum distance d₃ between the cylinders in the range from 10 to 20 mm;

- said frame connected to a motor suitable to apply to said frame a reciprocating motion in said plane p₁ and in the direction perpendicular to the axes of the cylinders, with strokes of amplitude between 2 mm and 10 mm around an average position in which said two parallel cylinders are equidistant from the projection of the axis of the ejector;
characterized in that:
at least a zone (30; 40, 42) on the lateral surface of each of said parallel cylinders, defined by two segments (A, B; C, D, E, F) parallel to the generator of each cylinder spaced apart along the circumference of the cylinder at least 1/6 of said circumference, presents indentations (31; 44, 45) arranged according to a rhomboidal array, having circular shape, diameter between 4 and 6 mm, depth between 1 and 2 mm, and distance between the centers of two next indentations between 5.5 and 7.5 mm.
Fibers deposition tool according to claim 1, further comprising upstream said ejector (20), at the inlet point of the fibers, an electrostatically charging device (23) adapted to apply a pulsed current for charging the surface of the fibers.
Fibers deposition tool according to claim 1 or 2, further comprising an air intake system (27) below said collector of fibers (26).
Fibers deposition tool according to any one of the preceding claims, wherein said distance d₁ is between 55 and 90 mm.
Fibers deposition tool according to any one of the preceding claims, wherein said distance d₂ is between 330 and 580 mm.
Fibers deposition tool according to any one of the preceding claims, wherein said distance d₃ is between 15 and 16 mm.
Fibers deposition tool according to any one of the preceding claims, wherein said cylinders (21, 21') are made of metal, metallic alloys, carbon fibers, polymers, copolymers, or have an inner part made of metal and the outer part made of a polymer.
Fibers deposition tool according to any one of the preceding claims, wherein the lateral surface of each of said cylinders (21, 21') has only one zone (30) defined by parallel segments (A, B) where are present a series of indentations (31).
Fibers deposition tool according to any one of claims 1-7, wherein the lateral surface of each of said cylinders (21, 21') is subdivided into four zones (40, 41, 42, 43) defined by parallel segments (B, C, D, F) spaced apart along the circumference of the cylinder 1/4 of said circumference, two non-adjacent zones (41, 43) being smooth, and the other two non-adjacent zones (40, 42) presenting indentations (44, 45) which are different in size, distance and depth in the latter two zones.
Fibers deposition tool according to claim 2, wherein said electrostatically charging device (23) applies a pulsed current with a frequency between 5 and 60 Hz.
Fibers deposition tool according to claim 10, wherein said electrostatically charging device (23) applies to the fibers a voltage between 15 and 60 kV.
Fibers deposition tool according to claim 11, wherein said electrostatically charging device (23) applies to the fibers a voltage between 15 and 45 kV.