PROCESS AND APPARATUS FOR PRODUCING A VOLUMINOUS NONWOVEN FABRIC

Abstract

Provided is a process for producing a nonwoven fabric. The process includes extruding a plurality of filaments from a spinneret, the filaments being at least bicomponent filaments; depositing the filaments to form a nonwoven fabric on an element collecting the filaments; performing a bonding of the nonwoven fabric; increasing the thickness of the nonwoven fabric by crimping at least part of the filaments by heating the nonwoven fabric; and, preferably performing a setting of the nonwoven fabric, wherein the bonding and the increasing the thickness steps are performed substantially simultaneously by means of a heated calendar. Also provided is the nonwoven fabric formed using the process and an apparatus for carrying out the process.

Claims

1. A process for producing a nonwoven fabric (150), comprising the steps of: (a) extruding a plurality of filaments (100) from a spinneret (1a), said filaments being at least bicomponent filaments; (b) depositing said filaments (100) to form a nonwoven fabric (150) on an element collecting the filaments; (c) performing a bonding of said nonwoven fabric (150); (d) increasing the thickness (H2) of said nonwoven fabric (150) by crimping at least part of the filaments (100) by heating said nonwoven fabric (150); and (e) performing a setting of said nonwoven fabric (150); wherein said steps (c) and (d) are performed substantially simultaneously by means of a heated calendar (20).

2. The process according to claim 1, wherein the composition of the nonwoven fabric treated by the calendar in said step (d) is substantially homogeneous.

3. The process according to claim 1, wherein said filaments (100) of the nonwoven fabric have same composition, so that they can be crimped by the heated calendar (20) in said step (d).

4. The process according to claim 1, wherein said calendar is configured to define, on the nonwoven fabric, a bonding area between 5% and 25%.

5. The process according to claim 1, wherein said calendar has a plurality of protrusions (21) comprising between 4 and 50 protrusions per cm.sup.2.

6. The process according to claim 1, wherein said calendar is heated to a temperature higher than 130? C.

7. The process according to claim 1, further comprising a step of cooling said nonwoven fabric (150) following said step (d).

8. The process according to claim 7, wherein said cooling step is performed by cooling means (5, 8) comprising at least one of: a cooling device (5) configured to direct a gas flow (G3) against said nonwoven fabric (150) at a temperature between 30 and 140? C. a suction roller; a cooled conveyor belt (8).

9. The process according to claim 1, wherein said setting step (e) comprises additional calendaring.

10. The process according to claim 1, wherein during said step (a), a plurality of bicomponent filaments (100) comprising two sub-filaments (100a, 100b) adhered to each other are extruded, said two sub-filaments (100a, 100b) being extruded according to a side-by-side configuration, so as to form a contact surface (105) between said two sub-filaments which, in cross-section of the filament, has a substantially wave-like shape, said two sub-filaments (100a, 100b) made of materials having different melting temperatures and/or different viscosities, said temperature difference being at least 10? C. and said viscosity difference being greater than 20%.

11. A calendar (20) for the treatment of nonwoven fabrics, comprising heating elements (200) and configured to define, on a nonwoven fabric, a bonding area between 5% and 25%, said calendar comprising a number between 4 and 50 protrusions per cm.sup.2.

12. A method comprising using the calendar according to claim 11 to crimp the filaments of a nonwoven fabric in which all the filaments have the same composition.

13. Apparatus (10) for producing a nonwoven fabric (150), comprising a device (1) for extruding filaments (100) equipped with a spinneret (la) for extruding a plurality of filaments (100), collecting means (2) to collect the filaments (100) and form a nonwoven fabric (150), a calendar (20) heated according to claim 11, and an additional setting device (7).

14. The apparatus (10) according to claim 13, wherein the heated calendar (20) is arranged immediately downstream of the device (1) for extruding filaments (100), so that the nonwoven fabric does not undergo thermal and/or bonding treatments between the device (1) for extruding filaments (100) and the heated calendar (20).

15. A nonwoven fabric (150) obtainable by a process according to claim 1, wherein said nonwoven fabric (150) comprises a number of constrained areas (2011) between 4 and 50 per cm.sup.2, to define a bonding area between 5% and 25%.

16. The nonwoven fabric (150) according to claim 15, wherein said nonwoven fabric (150) comprises a number of constrained areas (2011) between 5 and 30 per cm.sup.2 to define a bonding area between 7% and 18%.

17. The process according to claim 2, wherein all said filaments (100) of the nonwoven fabric have same composition and said calendar is configured to define, on the nonwoven fabric, a bonding area between 7% and 18%.

18. The process according to claim 2, wherein said calendar has a plurality of protrusions (21), the number of said protrusions (21) being between 4 and 40 protrusions per cm.sup.2, said calendar is heated to a temperature higher than 160? C. and further comprising a step of cooling said nonwoven fabric (150) following said step (d).

19. The process according to claim 4, wherein said calendar has a plurality of protrusions (21), the number of said protrusions (21) being between 4 and 40 protrusions per cm.sup.2, said calendar is heated to a temperature higher than 160? C. and further comprising a step of cooling said nonwoven fabric (150) following said step (d).

20. The process according to claim 8, wherein during said step (a), a plurality of bicomponent filaments (100) comprising two sub-filaments (100a, 100b) adhered to each other are extruded, said two sub-filaments (100a, 100b) being extruded according to a side-by-side configuration to form a contact surface (105) between said two sub-filaments which, in cross-section of the filament, has a substantially wave-like shape, said two sub-filaments (100a, 100b) made of materials having different melting temperatures and/or different viscosities, said temperature difference being at least 10? C. and said viscosity difference being greater than 20%.

Description

[0055] Hereinafter, referring to the appended figures, exemplary and non-limiting embodiments of the present invention will be described, in which:

[0056] FIG. 1 is a schematic view of an apparatus for producing a nonwoven fabric according to a first embodiment;

[0057] FIG. 2 shows a top schematic view of a nonwoven fabric of FIG. 1;

[0058] FIGS. 3A, 3B are sectional views of possible filaments that can be used to form a nonwoven fabric with an apparatus according to the present invention.

[0059] An apparatus 10 for producing a nonwoven fabric 150 comprises, in a known manner, a device 1 for extruding continuous filaments 100 and collecting means 2 for depositing and moving continuous filaments 100 in a forward direction D. Various devices 1 known in the art can be used for the purpose. For example, the devices described in Patent Applications WO2008/072278 and WO2008/075176 can be used.

[0060] In general, such devices have a spinneret 1a for extruding a plurality of filaments 100, typically followed by a drawing unit 1b. Generally, a cooling zone, not shown and known per se in the art, is arranged upstream of the drawing unit to direct air flows toward the filaments 100 after the extrusion from the spinneret 1a, so that they are cooled appropriately. Patent EP1939334, for example, describes a possible cooling chamber that can be used in the present invention; this Patent describes also a device for extruding and collecting filaments which is adapted to be used in the present invention.

[0061] At its outlet (i.e., the portion from which the nonwoven fabric exits the device 1), the device 1 comprises a pair of rollers 9, wherein the rollers are typically provided with a smooth outer surface. Passing the filament layer through the two rollers 9 allows the filaments of the nonwoven fabric to be compacted. At least one of the rollers 9 can be heated, so as to carry out a first step of crimping at least one portion of the filaments 100, thereby allowing an initial increase in the volume (thickness) of the nonwoven fabric 150. Typically, the rollers 9 are configured so as to avoid forming a bonding between the filaments 100. In particular, the heating temperature of the rollers 9 is preferably lower than the heating temperature of the calendar 20, which is better described below. Preferred temperatures for the rollers 9 are between 50? C. and 140? C., typically around 90? C. and in any case chosen according to the nature of the polymers used, i.e., typically lower than at least the melting temperature of the materials forming the filaments 100.

[0062] Moreover, according to a preferred aspect, the coupling between the rollers 9 and the nonwoven fabric preferably prevents, or at least limits, the inflow of ambient air into the device 1 at the collecting means 2.

[0063] The continuous filaments 100 can have different shapes. In a preferred implementation the continuous filaments 100 are bicomponent filaments, i.e. they have two sub-filaments 100a, 100b coupled to one another. The bicomponent filament 100 can take different configurations, such as core-sheath or, more preferably, side-by-side.

[0064] According to an aspect of the present invention shown in the figures, the filaments 100 comprise two sub-filaments 100a, 100b made by coextruding two materials, typically polymers. The sub-filaments 100a, 100b are arranged in side-by-side configuration. A particular configuration of the filaments 100 is described in detail in the co-pending Application EP16198713.

[0065] In particular, the materials for the two sub-filaments 100a, 100b are preferably selected among PP, coPP, PE, CoPE, PET, CoPET, PA, PLA. Preferred combinations are: PP/PE, PP/CoPP, PP/PP, PET/PP, PET/CoPET, PA/PP, PLA/PP, PLA/PE. According to a preferred embodiment, the materials of the sub-filaments 100a, 100b are selected so as to allow them to crimp during a heat treatment. This is preferably achieved by at least one of the following characteristics: the difference between the melting temperature of the sub-filaments 100a and the melting temperature of the sub-filaments 100b is different by at least 10? C., and preferably by at least 20? C.; the two materials of the sub-filaments 100a, 100b have different viscosity, preferably with a difference of more than 20%, when measured by the same method and under the same conditions. For example, the two materials can be tested with the same viscometer (e.g., rotational or capillary viscometer) or, more generally, the viscosity can be determined by a common method defined in a recognized standard (e.g., ASTM D3835). In other words, for the sub-filaments, polymers having different melting point and similar viscosity or polymers with equal or similar melting point but different viscosity, or else two polymers having different melting points and viscosity, can be selected. As mentioned, the preferred configuration of the two sub-filaments 100a, 100b is the side-by-side one in which the two sub-filaments are provided next to each other so that, in section, the two sub-filaments 100a, 100b are divided by a line representing the contact surface 105. According to a preferred aspect of the invention, the contact surface 105 has at least one inflection so as to define a wavy shape. In other words, the contact surface has a shape that shows at least one peak 3, 32 alternating with at least one trough 33. As known, peaks and troughs are the crests 3, 32, 33 formed by the wave, i.e. the maxima and the minima. The peaks 3, 32 are directed in the opposite direction with respect to the troughs 33. It should be noted that, typically, the difference between the troughs 33 and the peaks 3, 32 is given only by the orientation chosen for the section of the filament.

[0066] Preferably, the section of the contact surface 105 forms a wave with at least two crests 3, 32, 33; in particular, in preferred embodiments there are exactly three crests 3, 32, 33. For convenience's sake, two peaks and one trough will be referred to. Preferably, the period T of the wave is between 40% and 100% of the length of the diameter of the multicomponent filament 100. It should be noted that for convenience's sake, reference will be made to the diameter of the multicomponent filament 100. However, the following description can be applied also to the case of a not-circular filament section. In this case, the diameter should be considered as the greatest dimension of the section. If the troughs 33 and the peaks 3, 32 have the same length, then as a result the length of each trough and peak is preferably between 20% and 50% of the diameter (or between ? and ? of the diameter).

[0067] As known, the period T of the wave is the sum of the lengths of a tough and a peak. The period T may also be measured as the distance between two subsequent peaks (or toughs).

[0068] Preferably, the contact surface 105 changes at least once its curvature, i.e. has at least one inflection. Typically, the section of the contact surface covers at least one period of the waveform. More preferably, the contact surface has at least two peaks and one trough, thus covering at least 1.5 periods of the waveform. Preferably, the waveform meets the edge of the filament section at a middle point between trough and peak, i.e. far from the trough and/or the peak adjacent to the edge.

[0069] In a preferred embodiment shown in FIG. 3A, the wave shape is substantially sinusoidal. Note that, given the small size of the filament section, the waveform will actually approximate to a sinusoid. Specifically, the ideal shape of the section of the filament 100, having a length of 1.5 periods and a strictly sinusoidal shape, is shown in FIG. 3A. FIG. 3B shows a possible real pattern of the section of the contact surface 105, with the wavelength of the contact surface slightly longer than the period T, the peaks cut off at the edge of the section and the wave shape approximating a sinusoid without strictly complying with its geometrical parameters.

[0070] Below the device 1 there are collecting means 2, typically in the form of a conveyor belt or the like, that allows the filaments 100 to be transported in a forward direction D. The collecting means 2 are typically perforated or otherwise gas permeable. Appropriate means, not shown in detail and typically in the form of aspirator or similar element, can be provided below the collecting means 2 so that a depression is created at the zone in which the filaments 100 are deposited on the collecting means 2 themselves.

[0071] The apparatus 10 is configured so as to form a substantially homogeneous nonwoven fabric in which the composition of filaments is substantially constant throughout the entire volume of the nonwoven fabric. Typically, the entire volume of the nonwoven fabric is formed by a single type of filament. The nonwoven fabric is therefore preferably single-layered, or otherwise formed by several layers having composition identical to one another.

[0072] The apparatus 10 further comprises a heated calendar 20 downstream of the device 1 for extruding filaments 100.

[0073] Preferably, the heated calendar 20 is typically arranged immediately downstream of the device 1 for extruding filaments 100. Specifically (excluding any cooling performed by the collecting means), the apparatus does not have any devices adapted to thermally treat, specifically heat, and/or bond, the nonwoven fabric arranged between the device 1 for extruding filaments 100 and the heated calendar 20.

[0074] This heated calendar 20 comprises a plurality of rollers 20a, 20b, preferably a pair of rollers 20a, 20b, and heating elements 200. The heating elements 200 preferably comprise a fluidic circuit which is arranged inside the heated calendar 20 and in which a heated liquid, typically diathermal oil, flows. However, alternative heating elements are possible, such as electric means adapted to heat the calendar 20.

[0075] Protrusions 21 extend on the outer surface 60 of at least one of the rollers 20b. Typically, the heated calendar 20 has a pair of rollers 20a, 20b, in which a first roller 20b has protrusions on its surface, while the second roller 20a has a substantially smooth surface. As discussed, the smooth surface acts as a countering element for the protrusions 21 of the other roller. Typically, moreover, the roller with the protrusions 21 is the one arranged, in use, above the nonwoven fabric 150.

[0076] This heated calendar 20 is configured to define a bonding area between 5% and 25%. As discussed, the bonding area is a concept known in the art as the ratio (typically expressed as a percentage) of the sum of the constrained areas 2011 (i.e., the areas of the nonwoven fabric where the filaments are subject to constraints) in a surface unit to the area of the surface unit 202 of the nonwoven fabric 150.

[0077] According to a possible aspect, the bonding area is between 5% and 25%, more preferably between 7% and 18%.

[0078] In a known way, the surface unit can be chosen as any area, preferably square shaped, whose dimensions may contain a significant number of constrained areas 2011. A preferred surface unit for calculating the bonding area is an area on the surface of the nonwoven fabric having dimensions D1 equal to 10 cm and D2 equal to 10 cm.

[0079] In general, the bonding area can be calculated as the ratio of the sum of the constrained areas 2011 in the surface unit to the area of the surface unit 202 of the nonwoven fabric 150 itself, which is calculated, in the case of a square unit, as D1 multiplied by D2.

[0080] The bonding area can be expressed as a percentage by multiplying by a factor of 100 said ratio of the sum of the constrained areas 2011 to the area of the surface unit 202 of the nonwoven fabric 150.

[0081] According to a possible aspect, the heated calendar 20 comprises a plurality of rollers, preferably a pair of rollers 20a, 20b. The outer surface 60 of at least one of the rollers 20a, 20b, is provided with protrusions 21. The ratio of the number of protrusions 21 on the outer surface 60 of a roller 20a, 20b to the outer surface 60 of the same roller defines the amount of protrusions 21 per surface unit. According to a possible aspect, the number of protrusions 21 per cm.sup.2 is between 4 and 50 protrusions per cm.sup.2, preferably between 4 and 40 protrusions per cm.sup.2, more preferably between 5 and 30 protrusions per cm.sup.2.

[0082] The number of protrusions 21 per cm.sup.2, i.e. the density of protrusions 21, contributes to define the bonding area since the greater the number of protrusions 21, the greater the number of constrained areas 2011, i.e. the greater the numerator of the formula discussed above to calculate the bonding area.

[0083] Typically, the number of protrusions 21 per cm.sup.2 of the calendar corresponds to the density of the constrained areas 2011 formed on the nonwoven fabric 150 by means of the heated calendar 20 itself.

[0084] As a result, according to a preferred aspect, the nonwoven fabric 150 comprises a number of constrained areas 2011 between 4 and 50 per cm.sup.2, preferably between 4 and 40 per cm.sup.2, more preferably between 5 and 30 per cm.sup.2.

[0085] Typically, the apparatus 10 comprises a cooling device 5 arranged downstream of the heated calendar 20.

[0086] Various types of cooling devices can be used. For example, according to a possible aspect, a cooling device 5 may be equipped with means 55 to direct a gas flow G3, preferably air, against the nonwoven fabric. Preferably, the temperature of the gas flow G3 is between 30 and 140? C.

[0087] For example, a cooling device may comprise a surface 51, 52, and preferably two surfaces 51, 52, arranged parallel to the forward direction D of the nonwoven fabric 150, and preferably movable. The cooling device 5 can be configured so as to emit or suction gas G3 from at least one of these surfaces.

[0088] The direction of the gas flow G3 can be incident to the nonwoven fabric 150 and preferably substantially perpendicular to the nonwoven fabric 150. Preferably, the gas flow G3 is typically oriented so as to pass through the nonwoven fabric in the opposite direction with respect to the gravity, that is, from bottom to top, although the possibility of directing the gas flow G3 from top to bottom is not excluded.

[0089] Additionally or alternatively, the apparatus 10 comprises a suction roller, not shown in the figures, equipped with an air suction system, so as to simultaneously attract and cool the nonwoven fabric 150.

[0090] Additionally or alternatively, the apparatus 10 may comprise a cooled conveyor belt 8 to cool the nonwoven fabric 150 by means known in the art and not described in detail herein, such as by air suction means.

[0091] Downstream of the heated calendar 20, considering the forward direction D of the nonwoven fabric 150, and possibly also downstream of the at least one cooling device 5, if any, the apparatus 10 typically comprises an additional setting device 7. Various setting devices are known in the art and can be used in the present invention so as to further consolidate the nonwoven fabric 150.

[0092] According to a preferred aspect, the setting device 7 comprises a second calendar. This second calendar may have reliefs so that additional embossing can be imparted to the nonwoven fabric 150 in order to perform further cohesion at different points of the nonwoven fabric 150.

[0093] Some filaments 100, in use, are extruded from the spinneret 1a and deposited on the collecting means 2, typically after being passed through a drawing unit 1b.

[0094] Preferably, the filaments 100 are deposited in a non-crimped condition on the collecting means 2, that is, they are essentially devoid of crimps when deposited on the collecting means 2. Thus, the thickness H1 of the nonwoven fabric 150 deposited on the collecting means 2 is typically comparable to that of standard spunbond nonwoven fabrics made from single-component or bicomponent filaments.

[0095] As described above, the filaments 100 are bicomponent filaments typically comprising two sub-filaments 100a, 100b next to each other in a side-by-side configuration, with the contact surface preferably wave-shaped when viewed in cross section.

[0096] Referring to FIG. 3A, a possible method for obtaining a wave shape is now described in detail. In particular, the first sub-filament 100a is extruded under a constant pressure P1. The extrusion pressure, i.e. the spinning pressure, of the second sub-filament varies, for example in a sinusoidal way, between to values P0 and P2. P0 is less than P1, whereas P2 is greater than P1. The second sub-filament 100b forms a protrusion within the first sub-filament P1, where the second sub-filament is extruded under pressure P2 (or under a pressure higher than the pressure of the first sub-filament 100a). Conversely, the first sub-filament forms a protrusion within the second sub-filament 100b, where the second sub-filament 100b is extruded under a pressure P0 (or a pressure lower than the pressure of the first filament).

[0097] For the sake of simplicity, an embodiment in which only the pressure of one of the two sub-filaments 100b is varied, has been described. However, in order to obtain a desired shape (e.g. wavy), the extrusion pressure can be varied at different areas of both the sub-filaments 100a, 100b. Generally, the second sub-filament forms a protrusion within the first sub-filament and vice versa, where the pressure of the second sub-filament is greater than the pressure of the first sub-filament.

[0098] When deposited on the collecting means, the filaments 100 form a nonwoven fabric 150 with thickness H1.

[0099] In particular, the filaments 100 are deposited on the belt in a random manner that results in a disordered distribution but substantially uniform density of the filaments. Preferably, as the nonwoven fabric exits the device 1, it passes between two rollers 9 typically provided with smooth outer surfaces. This allows the filaments of the nonwoven fabric to be compacted. Furthermore, according to a possible aspect, by heating at least one of the two rollers 9, a first step of crimping at least one portion of the filaments 100 can be further carried out, thereby allowing an initial increase in the volume (thickness) of the nonwoven fabric 150. As discussed, the temperature of the rollers 9 is typically chosen so as to avoid forming a bonding between the filaments 100. As discussed, in a preferred solution, the upper cylinder can be provided with means to heat it to a temperature preferably between 50? C. and 140? C., usually around 90? C. and in any case chosen according to the nature of the polymers used and able to provide a first cohesion of the filaments.

[0100] The coupling between the rollers 9 and the nonwoven fabric preferably prevents, or at least limits, the inflow of ambient air into the device 1 at the collecting means 2.

[0101] The nonwoven fabric 150 is treated so that its volume (thickness) is increased by means of a heated calendar 20 equipped with protrusions 21.

[0102] By passing the nonwoven fabric 150 through the heated calendar 20, a plurality of constrained areas 2011 can be obtained on the nonwoven fabric, i.e. areas in which the filaments 100 are constrained to each other. The spaces between these constrained areas 2011 allow the filaments to crimp in these spaces. As a result of this filament crimping, the nonwoven fabric increases its volume and in particular achieves a thickness H2 that is greater than the thickness H1 of the nonwoven fabric 150 before the calendaring process. The thickness H2 of the nonwoven fabric 150 increases, as described above, at the zones that have not been constrained, i.e., entangled by the protrusions 21 of the heated calendar 20. The increase in thickness H2 is therefore driven by defining the constrained areas 2011.

[0103] It should also be noted that, preferably, the composition of the filaments 100 in the nonwoven fabric is basically uniform. In a preferred embodiment, all filaments 100 of the nonwoven fabric have the same composition, so they can be all crimped by the heated calendar 20.

[0104] The final layout of the nonwoven fabric can be determined by appropriately selecting the distribution of the protrusions 21, as this actually helps to define the bonding area and, accordingly, the possibility of increasing the thickness H2.

[0105] Preferably, the nonwoven fabric 150 is cooled, for example by one or more of the above-described cooling devices 5, vacuum roller and cooled conveyor belt 8. The cooling device 5 is configured to direct a gas flow G3 against the nonwoven fabric 150 in which said gas flow G3 is directed along a direction incident, preferably substantially perpendicular, to the forward direction of the nonwoven fabric 150. A suction roller, not shown in figure, can be equipped with an air suction system, so as to simultaneously attract and cool the nonwoven fabric 150 by suction. A cooled conveyor belt 8 can be cooled by means known in the art and not described in detail herein, for example air suction means can be used.

[0106] The nonwoven fabric 150 exiting the heated calendar 20 can be treated by an additional setting device 7 such as a calendar, where the nonwoven fabric 150 is consolidated.