EP1448816A1

EP1448816A1 - Propylene-based heat weldable thermoplastic fibers, method for making same and nonwoven obtained by thermobonding of such fibers

Info

Publication number: EP1448816A1
Application number: EP02787568A
Authority: EP
Inventors: Galliano Boscolo
Original assignee: Albis SpA
Current assignee: Albis SpA
Priority date: 2001-11-05
Filing date: 2002-11-05
Publication date: 2004-08-25
Anticipated expiration: 2022-11-05
Also published as: DK1448816T3; ATE401437T1; US7494947B2; US20040248491A1; FR2831895A1; DE60227692D1; EP1448816B1; FR2831895B1; WO2003040443A1

Abstract

The polypropylene-based thermoplastic fibers are uniform in cross-section, not having any degraded surface skin, and they are heat-weldable at a pressure at a temperature which is lower than the melting temperature, by virtue of internal heating due to the effect of said pressure. They are made of a first constituent with high crystallinity and at least one second constituent, compatible with the first constituent, and of crystallinity that is lower than that of the first constituent. They have a quantity of primary oxidation inhibitor lying in the range 350 ppm to 1000 ppm. Such fibers are made by spinning under conditions that avoid molecular degradation by thermal oxidation at the periphery of the fibers, in particular with cooling that is rapid immediately on leaving the extrusion head. The non-woven fabric obtained by heat-bonding such fibers is characterized by weld points that are in the form of a polymer laminate that is uniform and transparent.

Description

POLYPROPYLENE-BASED THERMOPLASTIC THERMOPLASTIC FIBERS, PROCESS FOR PRODUCING SUCH FIBERS AND NON-WOVEN FABRIC OBTAINED BY THERMALIZING SUCH FIBERS

The present invention relates to thermosealable thermoplastic fibers 5 based on polypropylene, usable for the manufacture of nonwoven by thermobonding, especially in the sanitary-sanitary sector. It also relates to a process for manufacturing such fibers and to a nonwoven obtained by thermobonding said fibers.

Polyolefin fibers, in particular polypropylene fibers,

10 have particular characteristics which have contributed to their development: high chemical inertia, absence of polar group, low specific weight, low thermal conductivity, good insulating power, good abrasion resistance, low water absorption power, resistance mold and bacteria, excellent color fastness obtained by

15 inclusion of pigments during spinning. These polyolefin fibers are used pure or mixed with other fibers in the manufacture of products from the textile sector proper, such as lingerie, sportswear and carpets or even the industrial sector such as geotextiles or filters or the hygienic-sanitary sector such as

20 sanitary napkins. In this hygienic and sanitary sector, it is the properties of resistance to molds and bacteria, chemical inertness and the low rate of water absorption which constitute the main advantages of polyolefin fibers and in particular of polypropylene. Indeed these fibers are considered to be non-toxic and non-cytotoxic.

Polypropylene fibers are generally produced according to the technique of melt spinning, which consists in melting the polymer, with different additives, in an extruder at high temperature and in passing the molten material thus obtained through the holes of a die, a metering pump to maintain a constant pressure during

30 this operation. The hot filaments leaving the die are cooled by an air flow and subjected to various operations, in particular drawing and cut to give fibers with desirable mechanical and cohesive characteristics.

The manufacture of nonwovens from polypropylene fibers is done by various techniques including thermobonding. According to this technique, the polypropylene fibers in the form of a sheet are hot pressed so as to obtain inter-fiber bonds thanks to the localized and surface fusion of the fibers in the welding zones.

To date, three types of polypropylene fibers are known, which are specially designed to be used in the manufacture of nonwovens by the thermobonding technique.

According to the first type of fiber, in particular known from the documents US.4,473,677, US.5,985,193 and WO.99 / 55942, these are two-component fibers. They are obtained by the use of two extruders, each being supplied independently with a specific polymer, the two polymers in question having slightly different melting temperatures. During the thermobonding operation, it is the melting of the component having the lowest temperature which makes it possible to obtain the bond between the fibers. The drawback of this first type of fiber comes in particular from the relatively high manufacturing cost due to the special spinning installation o which is made necessary for this co-extrusion.

According to the second type of fiber, notably known from documents US.5.985.193 and WO.99 / 55942, it is also bi-component or bi-component fibers which consist of a mixture of polymer obtained directly in a single extruder. The fiber structure is irregular, one of the components forming islands in the other component, the dimensions of said islands being a function of the miscibility of the two components.

According to the third type of fiber, notably known from documents US.5.281378, US.5.318.735, US.5.431.994, US.5.7O5.119, US.5.882.662, US.5.985.193 and US.116.683 , these are fibers having 0 superficially degraded skin. Such fibers are commonly referred to as skin fibers. They are obtained under special conditions of the spinning and cooling process which lead to thermooxidative molecular degradation at the periphery of the fibers, the skin being formed by the surface layer of polymer thus degraded. During the thermobonding operation, the cohesion of the fibers between them is obtained by the melting of the polymer constituting the degraded skin, the latter having a lower melting temperature than that of the non-degraded polymer constituting the core of the fiber. .

It emerges from the above examination of the three types of polypropylene fibers which have been specially designed to be used in the manufacture of nonwovens by thermobonding that the cohesion of the fibers between them is always obtained by virtue of a portion of polymer, located on the surface of the fiber, which has a lower melting temperature than the polymer constituting the rest of the fiber. The portion of polymer at a lower melting temperature makes it possible to create the inter-fiber bonds in the nonwoven while the remainder of polymer, not directly affected by thermobonding, ensures the maintenance of the mechanical properties of the fibers.

Regarding the manufacturing cost of the three types of fibers mentioned above, it is undoubtedly the degraded skin fibers which are the most efficient since their manufacture does not require any particular installation but only the implementation of adequate conditions. during spinning and cooling.

However, fibers with degraded skin have a certain number of drawbacks. Without this phenomenon being clearly explained, it can be seen that the fibers comprising a degraded skin have a less soft touch, a harder hand. This inconvenience is further increased when it is a question of assessing the softness of the nonwoven obtained with these fibers with degraded skin, because in the welding zones the inter-fiber bonds are themselves rigid, being formed by the degraded polymer.

Furthermore, to obtain degraded skin during spinning, it is, according to the applicant, necessary to work under precise operating conditions, with therefore an operational window (operarjng window) enough restraint. The difficulty in controlling these operating conditions leads to frequent variations in the quality of the nonwoven obtained with such fibers.

Finally, to obtain the thermo-oxidative molecular degradation, it is necessary to work with a high extrusion temperature, which results in a higher energy consumption.

The object of the present invention is to propose new thermosealable thermoplastic fibers based on polypropylene, which can be used for the manufacture of nonwoven by thermobonding, in particular but not exclusively in the hygienic-sanitary field, which overcomes the aforementioned drawbacks of the three types of polypropylene fibers offered so far.

This object is perfectly achieved by the fibers of the present invention, which, in a characteristic manner, have a homogeneous structure in cross section, having no degraded skin on the surface, which are heat-weldable under a given pressure and at a given temperature which is lower than the melting temperature, thanks to internal heating under the effect of said pressure.

Thus, unlike the three types of fibers mentioned above, the cohesion of the fibers between them is not obtained thanks to a portion of polymer which has a lower melting temperature. According to the particular arrangement of the invention, the fibers have a completely homogeneous structure throughout their cross section, comprising neither degraded skin on the surface, nor islands, nor two separate components. Their macromolecular composition is determined so that an internal heating occurs when they are compressed under a given pressure, this internal heating allowing them to be heat sealable under said pressure and at a given pressure which is lower than their melting temperature.

Preferably, to obtain this internal heating under pressure, the fibers of the invention are composed of a first constituent with high criticality and at least a second constituent, compatible with the first, and whose crystallinity is less than that of the first constituent. The presence of the second constituent, with lower crystallinity, constitutes a certain molecular disorder in the crystalline phase, which facilitates the initiation of a molecular slip during the compression of the fibers during the phase known as plastic phase. The plastic behavior of polypropylene allows it to resist high deformations, which can go up to 6OO to 7OO%, and to keep its shape after deformation in a relatively stable manner. During the compression process, polypropylene goes through two phases, an elastic phase and a plastic phase. During the elastic phase which corresponds to deformations less than or equal to 1%, the polypropylene cools slightly. In the plastic phase, molecular sliding occurs which propagates through the mass of the polypropylene. The molecular friction thus created generates heat which raises the temperature of the fiber. Depending on the initial compression and temperature conditions, this increase in temperature can cause the polypropylene to melt.

The presence of the second constituent with lower crystallinity makes it possible to obtain this fusion almost instantaneously under acceptable pressure and temperature conditions for a thermobonding operation. In an alternative embodiment, the first constituent is a crystalline polypropylene homopolymer and the second constituent is a copolymer of propylene and at least one other monomer chosen from ethylene, butene and α-olefins whose carbon chain comprises at least five carbon atoms.

Preferably, the relative proportions, by weight, of the first and second constituents are from 20 to 99% for the first constituent and from 80 to

20% for the second constituent.

It is another object of the invention to propose a process specially designed for the manufacture by spinning of the aforementioned fibers. Typically, this process implements spinning conditions which avoid therπio-oxidative molecular degradation at the periphery of the fibers during spinning. Among these particular conditions, first of all, the presence of a large amount of antioxidant additive and especially a quantity of primary antioxidant of between 350 and 100 ppm is retained. 5 Conventionally, when spinning polypropylene, the amount of primary antioxidant is generally less than 350 ppm.

A primary antioxidant is an additive that protects the polymer against the action of oxygen at high temperatures. It is preferably chosen from stereo-hindered phenols and from ι o lactones. Other additives are also used such as primary antioxidants which are additives intended to protect the polymer at high manufacturing temperatures, in the absence of oxygen. These include organic phosphite.

Preferably, in the fibers according to the invention, both secondary antioxidants and primary antioxidants are used.

Other additives can also be used, such as antacid agents chosen, for example, from stearates, especially calcium or zinc, which are used to avoid corrosion phenomena.

Additives can also be used to protect the polymer at storage temperatures for extended periods of time (shelf life). These additives are generally chosen from phenols and from stereo-hindered amines.

The applicant has found that it is essential to optimize the stability of the polymer to oxygen, at high temperatures, by using a sufficient quantity of primary antioxidant, in order to remain below the threshold d 'initiation of thermo-oxidative degradation during spinning and thus avoiding the formation of degraded skin on the surface of the fiber.

Among the particular conditions avoiding the thermo-oxidative molecular degradation at the periphery of the fibers during spinning, we retain second 3 o ϋen the conditions for cooling the fiber at the outlet of the die.

More particularly, rapid cooling is carried out, immediately at the outlet of the die, for example by blowing an air flow at a temperature between 16 and 26 ° C., this blowing being directed directly onto the spindle or onion of polymer forming at the outlet of the hole in the die. Advantageously, this air flow has a humidity rate which is between 30 and 80%.

It is another object of the invention to provide a process for manufacturing a nonwoven from a sheet of polypropylene-based fibers with a crystalline structure having the characteristics described above. This process consists in thermoliering said sheet according to welding zones by compressing the fibers in these welding zones at a temperature below their melting temperature and at a pressure which is determined so as to obtain, during their plastic deformation, a internal heating causing the fibers to merge in the weld zones. it should be noted that the nonwoven which is obtained by thermobonding the fibers according to the invention is characterized in that, in the weld zones, the weld points are presented as a multi-layered or laminated polymer (polymer laminate). They are formed by the plastic deformation of the fibers in the weld zones and by the interpenetration and intermingling of the polymer molecules on the surface of said fibers during localized melting. From the point of view of their appearance, they are characterized by their homogeneity and their transparency. This configuration differs from that observed when thermobonding fibers with degraded skin on the surface. In this case, the welding points comprise inter-fiber links in the form of filaments or membranes and have a white appearance. It should be noted that the optical transparency of the solder points can only be observed for a homogeneous polymer of very small thickness, less than a micrometer, without elements of the filament or membrane type and without microbubbles of air. This configuration of the multi-layer polymer type soldering points makes it possible to obtain a nonwoven having good resistance. mechanical.

When a nonwoven formed of fibers with degraded skin is subjected to a rupture test, it is found that this rupture occurs around and outside of welding points, by breaking the unit fibers. On the other hand, when a nonwoven formed of fibers of the present invention is subjected to a rupture test, this rupture occurs first inside and secondarily outside the weld points, by separation of the fibers and secondarily by breaking the unit fibers.

In addition, the feel of the nonwoven obtained is particularly soft. i o This is explained by the perfectly smooth surface condition both of the fibers themselves and also of the inter-fiber bonds. It should be noted that the temperature and compression conditions during thermobonding are such that they do not cause any thermo-oxidative degradation at the surface of the inter-fiber bonds. Thus the nonwoven obtained according to the invention has

15 a perfectly homogeneous structure both at the level of the fibers which compose it and of inter-fiber bonds in the weld zones.

The present invention, its characteristics and the various advantages which they provide will appear even more clearly on reading the description which will be given of exemplary embodiments of fibers.

20 heat seals of homogeneous structure, without degraded skin on the surface, illustrated by the appended drawing in which:

FIG. 1 represents a view of the external surface of a fiber of the invention; Figure 2 shows a cross-sectional view of the fiber in Figure

25 1;

FIG. 3 represents a view of the external surface of a fiber having a heart-skin structure;

Figure 4 shows a cross-sectional view of the fiber of Figure 3; FIG. 5 represents a diagram of a device allowing the implementation of the method of the invention; 6 shows an enlargement in front view of the cooling zone of the device of Figure 5; Figure 7 shows a sectional view of Figure 6; and Figures 8 and 9 show in more detail the adjustable fins of the cooling system.

Fx ^p mpl ^p of ompn rhnn fibers of the inventinn

Table 1 below shows the variation in the melting temperature of a mixture of a polypropylene homopolymer (HP554N produced by the company Basell) and a copolymer of polypropylene and polyethylene (RCXP 2130 produced by the company Basell). The melting temperature was determined to be the temperature corresponding to the peak of enthalpy of fusion measured by differential thermal analysis. The device used is the "DSC 700" manufactured by the company Polymer Lab. The heating rate is 3 ° C / min. Table 1

The melting temperature of the mixture is lower than that of the polymer, which is an indication of the formation of a disorder on the molecular scale. This disorder can be observed on X-rays. It can result either from the limited displacement of certain atoms with respect to the crystallographic planes, or from a dilation or compression of the inter-planar distances, or from a decrease or an increase in the concentration electronic on certain crystallographic planes. Without the Applicant being bound by this explanation, it seems that the plastic weldability properties of the fibers are favored by this last factor which comes in the above example from the presence of methyl groups polypropylene which modify the stereographic position inside the crystal structure.

It is also possible according to the invention to use a polypropylene-polyethylene copolymer whose crystal structure has a certain disorder due to the presence of segments of polyethylene chains in the crystal structure of polypropylene. However, the manufacture of this copolymer needs to be strictly controlled in order to give the final material good plasticity properties. This is not the case when using the above mixture. Antioxidant composition

The applicant has demonstrated that the antioxidant composition added to the polymer blend is very important in order to avoid the formation of degraded skin which is harmful to the tactile and mechanical properties of the nonwoven obtained from the fibers. In the context of the present invention, the concentration of antioxidant composition is of the order of 500 ppm-2000 ppm.

Table 2 below gives three examples of compositions which are preferably incorporated into the polymer mixture cited as an example.

The stability to oxygen and to high temperatures of the mixture obtained is measured by the TOSI method (Thermal Oxidation Stability Index) F. Polato: Comunicazione privata, Nov. 30.1998. This method assumes that the MFR is an indicator as a first approximation of the average molecular weight by mass Mw of the polymer. It consists in evaluating the molecular degradation of the polymer maintained at constant temperature in a closed cell for a defined time, in the absence of oxygen, and in extruding it several times at high temperature in the presence of oxygen from the air. Table 2

A: MFR melting flow rate (abbreviated as MFR) originating from the polymer determined according to standard ASTM D-1238 (Condition L; 230/2016);

B: MFR2 melt flow rate of the polymer measured according to the ASTM standard

D 1238 at 290 ° C for 18 min without oxygen;

C: MIL 2 MFR of the polymer determined according to standard ASTM D 1238

(condition L; 230 / 2.16) after two heatings at 290 ° C for 6 min in the presence of oxygen.

Composition 1:

Total concentration: 500 ppm

Prevented stereo phenol chosen from C.A.S. Nos. 6683-19-8, 27676-62-6 and

2082-79-3) / photo stabilizer / organic phosphites chosen from CAS Ns 31570-04-4 and 119345-01-6

Composition 2:

Total concentration: 1,200 ppm

Lactone C.A.S. N ° s 18131-48-7 / photo stabilizer: congested stereo amine / organic phosphites chosen from CAS N ° 31570-04-4, 119345-01-6

Composition 3:

Total concentration: 750 ppm

Prevented stereo phenol chosen from C.A.S. Nos. 6683-19-8, 27676-62-6 and

2082-79-3) / organic phosphites chosen from CAS Ns 31570-04-4 and 119345-01-6. 500 ppm of calα ^' um stearate was added to all of the above compositions. Process ^p manufacturing of the fihrp

To avoid any thermo-oxidative degradation at the outlet of the die, a cooling device is used comprising air flows which are turned towards the beams or segments leaving the die, which has the effect of reducing the hot contact time of fiber with oxygen from the ambient atmosphere and increase the stability of the molten polymer column.

Figure 1 shows that the surface of the fiber obtained by the method of the invention is perfectly smooth and Figure 2 shows that in cross section, this fiber is perfectly homogeneous. Its cross section does not in fact report any ring structure.

By comparison, FIG. 3 shows that the external surface of a fiber with a heart-skin structure is irregular. In cross section (Figure 4) there is an external skin referenced P and a heart referenced C surrounded by this skin.

Figures 5 to 9 show an embodiment of a device for implementing the process for manufacturing a fiber according to the invention.

FIG. 5 represents a die 1 equipped with an air cooling system 2. The air intake is effected by the tube 2a placed below the spinning head. With reference to FIG. 6, the fibers F which are extruded pass in front of fins 3 which will be more fully described with reference to FIGS. 8 and 9. With reference to FIG. 7, the cooling air is conveyed through filters 6 at the level of the extrusion head 4. The air flow is directed via the fins 3. After passage over the fibers F, the heated air is evacuated. With reference to FIG. 8, the air represented by the arrows passes between the fins 3 which are pivotally mounted on a frame 5. Each fin 3 is pivotally mounted by means of a pivot 3a on the frame 5. With reference to FIG. 9, the cooling air represented by the arrows is directed by the fins 3 and comes to cool the onion of polymer O which forms at the level of the extrusion head 4. The heated air is then evacuated.

According to a particular implementation, the cooling air is at a temperature between 16 ° C and 26 ° C and has a humidity level between 30% and 80%. Method of manufacturing a nonwoven

According to the method of the invention, the sheet of fibers is brought to a temperature below the softening temperature of the polymeric material which constitutes said fibers and compressed strongly in certain areas so as to cause in the fibers located in said areas the phenomenon of creep (initiation and propagation of a molecular slip) previously exposed. During the plastic phase of the deformation of the fibers, under the effect of compression, there is an internal heating in the fibers, which raises the temperature thereof to the melting temperature. This fusion combined with compression causes the creation of weld points of a multilayer or laminated, homogeneous and transparent polymer structure. The heating temperature of the fibers is around 140 ° C - 145 ° C for 10 x 10 ^"3 seconds, which corresponds to the contact time of the fibers with the compression rollers of the thermobonding installation. in use is calculated so as to obtain maximum cohesion of the nonwoven obtained.

Claims

1. Thermoweldable thermoplastic fibers based on polypropylene, usable for the manufacture of nonwovens by thermobonding, characterized in that they are homogeneous in cross section, having no degraded skin on the surface, and in that they are heat sealable under a given pressure and at a given temperature which is lower than their melting temperature, thanks to an internal heating under the effect of said pressure.

2. Fibers according to claim 1 characterized in that they are composed of a first constituent with high crystallinity and at least a second constituent, compatible with the first constituent and whose crystallinity is less than that of the first constituent.

3. Fibers according to claim 2 characterized in that the first constituent is a crystalline polypropylene homopolymer and the second constituent is a copolymer of propylene and at least one other co-monomer chosen from ethylene, butene and α -olefins whose carbon chain contains at least five carbon atoms.

4. Fibers according to one of claims 2 to 3 characterized in that the relative proportions, by weight, of the first and second constituents are from 20 to 99% for the first constituent and from 80 to 1% for the second constituent.

5. Fibers according to one of claims 1 to 4 characterized by an amount of primary antioxidant between 350 and 1000 ppm.

6. A method of manufacturing by spinning fibers based on polypropylene according to claim 1 characterized by conditions avoiding thermo-oxidative molecular degradation at the periphery of the fibers during spinning.

7. Method according to claim 5 characterized in that said conditions include the presence of a large amount of antioxidant additive, preferably an amount of primary antioxidant between 350 and 1000 ppm.

8. Method according to one of claims 5 or 6 characterized in that said conditions include rapid cooling, immediately at the outlet of the die.

9. Method according to claim 7 characterized in that the rapid cooling is obtained by blowing an air flow at a temperature between 16 ° C and 26 ° C, directed directly on the bundle (or onion) of polymer forming at the outlet of the die.

10. A method of manufacturing a nonwoven from a sheet of polypropylene-based fibers according to one of claims 1 to 4 characterized in that it consists in thermolier said sheet according to welding zones by compressing said fibers in said zones at a temperature below their melting temperature and at a given pressure, sufficient to obtain, on the occasion of their plastic deformation, an internal heating causing the fusion of the fibers in the weld zones.

11. Nonwoven obtained by thermobonding of polypropylene-based fibers according to one of claims 1 to 4 characterized in that, in the weld zones, the weld points are presented as a homogeneous and transparent multilayer or laminated polymer .

12. A nonwoven according to claim 11 characterized in that, when subjected to a rupture test, the rupture occurs first inside and secondarily outside the weld points.