Fibers suitable for the production of nonwoven fabrics having improved strength and softness characteristics
The present invention relates to nonwoven fabrics prepared by thermobonding which display improved strength and softness characteristics, and the process for producing them. More particularly, the present invention relates to staple fibers produced by a continuous or discontinuous spinning and drawing process, using polymer materials which comprise heterophasic polyolefin compositions, possessing good flexibility and thermobonding strength. 
     The definition of &#34;fibers&#34; includes also products similar to fibers, such as fibrils. 
     Nonwoven fabrics are widely used in various applications, and for some of these applications the softness and strength of the nonwoven fabrics are particularly desired and requested. For example, in the health and medical fields, where these products are used for sanitary napkins, bandages, gowns, etc., softness is very important because the product comes in contact with the skin. Other fields include, for example, the wrapping and packaging of either fragile or easily damageable objects, where the material must not only be soft but also strong in order to help prevent breakage. 
     Polyolefin fibers used for the preparation of nonwoven materials are already known in the art, said fibers being optionally prepared with a heterophasic polymer and possessing thermobonding properties. For example, fibers with the above mentioned properties are described in published European patent application EP-A-391438. In said patent application the fibers prepared in the examples comprise only propylene homopolymer, or ethylene/propylene copolymer having a thermobonding strength, measured with the method described below, of up to 4 Newton (&#34;N&#34;). 
     Another example of polyolefin fibers containing heterophasic polyolefin compositions is given in published European patent application EP-A-0 552 810. Said patent application describes fibers produced from blends comprising up to 30% of a rubber fraction. 
     The heterophasic polyolefin compositions described in the &#39;810 application are suitable for the preparation of fibers having thermoshrinking characteristics. Said fibers, which have a high count value (the values, mentioned only in the examples, range from 15 to 19 dtex), can be used to produce tufted carpets. However, said patent application does not refer to other uses and properties of the fibers, i.e., there is no mention of which compositions are suitable for the production of fibers having good thermobonding and flexibility properties, nor are the spinning parameters given which would allow one to obtain said results. 
     One embodiment of this invention comprises polyolefin fibers which offer high thermobonding indexes, preferably from 4.5 to 9 N, more preferably from 6 to 9 N, and flexibility indexes preferably ranging from 1020 to 1500. Said properties allow one to obtain nonwoven fabrics having good strength and softness properties. 
     A further embodiment of the present invention relates to the process for the preparation of nonwoven fabrics which comprise said fibers and offer both strength and softness properties. 
     Another embodiment of the present invention relates to the process used to prepare said fibers. 
     Yet another embodiment of the present invention concerns the nonwoven fabrics obtained by said process. 
     Accordingly, the present invention provides a fiber for nonwoven fabrics, which fiber comprises a polymer material containing (by weight): 
     1) from 50 to 80 parts of a propylene homopolymer having an isotactic index greater than 90, or a random copolymer thereof with ethylene and/or a C 4  -C 8  α-olefin; and 
     2) from 20 to 50 parts of a heterophasic polymer comprising: 
     a) from 20 to 70 parts of a propylene homopolymer and/or a random copolymer of propylene with ethylene and/or with a C 4  -C 8  α-olefin, containing from 0.5 to 10% of ethylene and/or C 4  -C 8  α-olefin (Fraction I); and 
     b) from 30 to 80 parts of a copolymer of ethylene with propylene and/or a C 4  -C 8  α-olefin soluble in xylene at 25° C., said copolymer containing from 40 to 70% of ethylene, and having an intrinsic viscosity lower than or equal to 1.5 dl/g, or said copolymer containing less than 40% of ethylene and having an intrinsic viscosity lower than or equal to 2.3 dl/g (Fraction II); 
     said fiber being obtained by spinning the above mentioned polymer material and then drawing it out with a draw-ratio from 1.1 to 1.8, preferably from 1.1 to 1.5. 
     The C 4  -C 8  α-olefins to be used for the preparation of the copolymers of subparagraph 1) above and Fractions I and II are linear or branched alkenes, and they are preferably selected from 1-butene, 1-pentene, 1-hexene, 1-octene and 4-methyl-1-pentene. The preferred α-olefin is the 1-butene. 
     The random copolymer of subparagraph 1) above contains a quantity of comonomer preferably ranging from 0.05 to 15% by weight. 
     The heterophasic polymer is present in the polymer material preferably in an amount ranging from 20 to 45 parts by weight. 
     Fraction I is present in the heterophasic polymer preferably in an amount ranging from 30 to 65 parts by weight, while Fraction II preferably in an amount from 35 to 70 parts, by weight. 
     The heterophasic polyolefin compositions can be prepared either by mechanically blending Fractions I and II in the molten state, or using a sequential polymerization process carried out in two or more stages, and using stereospecific Ziegler-Natta catalysts. The heterophasic polymer obtained in the latter case comprises also a third fraction, which is an essentially linear crystalline ethylene copolymer insoluble in xylene at ambient temperature. This fraction is present in an amount ranging from 2 to 40 parts by weight, preferably from 2 to 20, of the total heterophasic polymer. 
     Examples of the above mentioned heterophasic polyolefin compositions, as well as the catalysts and polymerization processes used for their preparation, can be found in published European patent applications 400333 and 472946. 
     The intrinsic viscosity values of Fraction II, within the limits indicated by the present invention, are obtained either directly in polymerization, or after the polymerization by means, for example, of a process of controlled radical visbreaking, or by other means (thermal degradation, for example). The above mentioned process is carried out by using organic peroxides, for example, such as 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane. The compounds used for the degradation of the polymer chains are added by themselves or together with other additives (such as UV stabilizers, flame retardants, etc.) to the heterophasic polymer to be degraded during the extrusion step, as an example. 
     The heterophasic polymer thus obtained is blended with the proper quantities of a crystalline polymer of subparagraph 1) above; the resulting polymer blend is then subjected to spinning according to known techniques and under the operating conditions indicated below. As a way of example, one can use a die with a real or equivalent output hole diameter of less than 0.5 mm. and a hole length/diameter ratio from 3.5 to 5, operating at temperatures from 250° to 320° C. and at an air speed from 0.1 to 0.6 m/sec. The fiber obtained preferably has a count of from 1 to 4 dtex. 
     The real or equivalent output hole diameter is preferably from 0.2 to 0.45 mm for fibers having a count of less than 4 dtex. The ratio between said output hole diameter and the count is or less than 0.06 mm/dtex, preferably less than or equal to 0.05 mm/dtex, for fibers having count equal to or higher than 4 dtex. 
     By &#34;output diameter of the holes&#34; is meant the diameter of the holes measured at the external surface of the die, i.e. on the front face of the die from which the fibers exit. Inside the thickness of the die, the diameter of the holes can be different from the one at the output. Moreover, the &#34;equivalent output diameter&#34; definition applies to those cases where the hole shape is not circular. In these cases, for the purposes of the present invention, one considers the diameter of an ideal circle having an area equal to the area of the output hole, which corresponds to the above mentioned equivalent diameter. 
     One can add peroxides or other additives to the fibers, such as for example dies, opacifiers and fillers, even during spinning. 
     Tests were conducted on the polymer material and fibers of the present invention to evaluate their characteristics and properties; the methods used for said tests are described below. 
     Melt Flow Rate (MFR): according to ASTM-D 1238, condition L. 
     Weight average molecular weight ( Mw): GPC (Gel Permeation Chromatography) in ortho-dichlorobenzol at 150° C. 
     Number average molecular weight ( Mn): GPC (Gel Permeation Chromatography) in ortho-dichlorobenzol at 150° C. 
     Intrinsic viscosity: 
     1 g of polymer is dissolved in a flask in 100 ml of xylene. The solution, in nitrogen atmosphere, is heated to 135° C. for 30 minutes. Then, while under agitation, the solution is cooled first to 90° C., and then to 25° C. by submerging the flask in water. The solution is allowed to rest at that temperature for 30 minutes. Then it is filtered with paper, and acetone in excess and methanol are added to the filtered solution. The precipitate thus obtained is separated with a G4 filter, then dried and weighed. Finally the intrinsic viscosity is measured on a portion of the precipitate using the tetrahydronaphtalene method at 135° C. 
     Thermobonding strength: in order to evaluate the thermobonding of staple fibers, a nonwoven fabric is prepared with the fiber being tested by way of calendering under set conditions. Then one measures the strength needed to tear said nonwoven fabric when the stress is applied in directions which are both parallel and transversal to that of the calendering. 
     The thermobonding index (TBI) is defined as follows: 
     
         TBI=(TM·TC).sup. 1/2 
    
     where TM and TC represent the tear strength of the nonwoven fabric measured according to ASTM 1682, for the parallel and transversal directions respectively, and expressed in Newtons. 
     The value of the strength determined in this fashion is considered a measure of the capability of the fibers to be thermobonded. 
     The result obtained, however, is influenced substantially by the characteristics regarding the finishing of the fibers (crimping, surface finishing, thermosetting, etc.), and the conditions under which the card web fed to the calender is prepared. To avoid these inconveniences and obtain a more direct evaluation of the thermobonding characteristics of the fibers, a method has been perfected which will be described below in detail. 
     Specimens are prepared from a 400 tex roving (method ASTM D 1577-7) 0.4 meter long, made up of continuous fibers. 
     After the roving is twisted eighty times, the two extremities are united, thus obtaining a product where the two halves of the roving are entwined as in a rope. On said specimen one produces one or more thermobonded areas by means of a thermobonding machine commonly used in a laboratory to test the thermobonding of film. 
     A dynamometer is used to measure the average strength required to separate the two halves of the roving at each thermobonded area. The result, expressed in Newtons (&#34;N&#34;), is obtained by averaging out at least eight measurements. The welding machine used is the Brugger HSC-ETK. The clamping force of the welding plates is 800 N; the clamping time is 1 second, and the temperature of the plates is 150° C. 
     Bonding is also tested at various temperatures around 150° C. in order to pinpoint at which temperature one can obtain a bonding capability equal to the one for the propylene homopolymer fibers at 150° C. 
     Flexibility index 
     The softness is evaluated by way of an index which represents the flexibility of the fiber. Said index is defined in the following manner: 
     
         FI=(1/W)·100 
    
     where W is the minimum quantity in grams of a specimen which when tested with the Clarks Softness-Stiffness Tester changes the direction of the flexion when the plane, on which the specimen is fixed in a perpendicular position, rotates alternatively +/-45° with respect to the horizontal plane. 
     The specimen has the same characteristics as the one used to measure thermobonding strength and is prepared using the same process described above. 
     Spinnability test 
     The polymer material blends are spun on a Leonard 25 spinning apparatus at the following spinning conditions: 
     temperature: 290° C.; 
     number of holes in the die: 61; 
     diameter of the holes: 0.4 mm; 
     length of the holes: 2 mm; 
     hole flow-rate: 0.3 g/min; 
     fiber quenching: lateral air flow with temperatures ranging from 18° to 20° C. and speed at 0.45 m/sec. 
     The extruded filaments are then wound on a bobbin by one of the following winding machines: 
     Leesona 967, which gathers and winds at a speed ranging from 150 to 1250 m/min.; 
     Cognesint GRC T661, which gathers and winds at a speed ranging from 1250 to 5500 m/min. 
    
    
     The following examples are given in order to illustrate and not limit the present invention. 
     A) Preparation of the polypropylene resin 
     In a LABO-30 Caccia turbo-mixer, operating at 1400 rpm, the following products are blended for 4 minutes: 
     1) flake polypropylene with controlled particle size; 
     2) 200 ppm of poly{[6-(1,1,3,3-tetramethylpiperidyl)-imine]-1,3,5-triazine-2,4-diyl][2-2,2,6,6-tetramethylpiperidyl)-amine]hexamethylene-[4-(2,2,6,6-tetramethylpiperidyl) imine]} (Chimassorb 944, marketed by CIBA-GEIGY); and 
     3) 350 ppm g of tris(2,4-di-tert-butyl-phenyl)phosphite (Irgafos 168, marketed by CIBA-GEIGY); 
     4) 500 ppm of calcium stearate. Table 1 shows the properties of the polypropylene used. 
     B) Examples 1-6 
     In a LABO-30 Caccia turbo-mixer, operating at 1400 rpm the following components are blended for 4 minutes: 
     1) heterophasic polymer 
     2) 200 ppm of poly{[6-(1,1,3,3-tetramethylpiperidyl)-imine]-1,3,5-triazine-2,4-diyl][2-2,2,6,6-tetramethylpiperidyl)-amine]hexamethylene-[4-(2,2,6,6-tetramethylpiperidyl) imine]} (Chimassorb 944, marketed by CIBA-GEIGY); and 
     3) 350 ppm g of tris(2,4-di-tert-butyl-phenyl)phosphite (Irgafos 168, marketed by CIBA-GEIGY); 
     4) 500 ppm of calcium stearate; 
     5) Luperox 101 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (marketed by Lucidol, Pennwalt Corp. USA). 
     Table 2 shows the data relative to the heterophasic polymers used to prepare the polymer blends. 
     Once pelletized, the compositions of the heterophasic polymers differ in terms of the intrinsic viscosity values (I.V.) of the amorphous fraction (II), soluble in xylene at 25° C., of the heterophasic polymer. In order to obtain heterophasic polymers with said different values, specific quantities of Luperox 101 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane have been added to the polymer (see Table 3). Table 3 also indicates the intrinsic viscosity, before and after visbreaking with the peroxide, of the amorphous fraction (II) of the heterophasic polymers that were used, which is soluble in xylene. 
     The compositions have been pelletized by extrusion at 210°-240° C. in a Bandera 30 extruder equipped with a 30 mm diameter screw whose length is equal to 30 diameters, has a compression ratio of 3.15, and a screen filter with 125 μm mesh. Extrusion conditions were as follows: 
     temperature of head filter: 220° C.; 
     capacity: 3.5 kg/h; 
     hopper atmosphere: N 2 . 
     The pellets of said compositions, and the polypropylene resin, are then put in a LABO-30 Caccia mixer, and mixing for 4 minutes at 1400 rpm, in order to prepare polymer blends comprising: 
     1) polypropylene prepared as indicated in (A); 
     2) heterophasic polymer composition produced as described in (B). 
     The quantity of polymers present in each polymer blend prepared, the type and characteristics of the heterophasic polymer introduced, and the maximum spinning velocity obtained during the spinning carried out according to the method described in the spinnability test are shown in Table 4. 
     COMPARATIVE EXAMPLES 1c AND 2c 
     Two polymer blends are prepared and then subjected to a spinnability test as described in Examples 1-6. The only difference concerns the intrinsic viscosity of the amorphous fraction (II) of the heterophasic polymers B and C (see Table 2) which is soluble in xylene at 25° C. Table 4 shows the data relative to the comparative examples. 
     EXAMPLES 7-12 
     The polymer blends of Examples 1-6 are respectively spun to produce fibers. The spinning velocity is 1000 m/min. Said fibers are then drawn out using a draw-ratio of 1.5. 
     On the fibers thus obtained, having a count of 2 dtex, one evaluates the thermobonding and flexibility indexes following the methods described above. Table 5 shows the data relative to said indexes. 
     COMPARATIVE EXAMPLE 3 (3c) 
     The polypropylene resin as is used in Examples 1-6 is spun under the same conditions and using the same methods described for Examples 7-12. The results are set forth in Table 5. 
     COMPARATIVE EXAMPLES 13 AND 14 (13c AND 14c) 
     A polymer blend equal to the one described in Example 4 is spun under the same spinning conditions described in Example 3c, using the winding speed and draw ratios indicated in Table 6. In the same Table one can also find the values of the thermobonding and flexibility indexes of the fibers thus obtained. 
     
                       TABLE 1                                                     
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Characteristics of the polypropylene used                                 
______________________________________                                    
Average pellet diameter (μm)                                           
                        450                                               
Residue insoluble in xylene at 25° C. (%)                          
                        96                                                
Melt Flow Rate (dg/min) 12.2                                              
Intrinsic viscosity (dl/g)                                                
                        1.5                                               
Number average molecular weight ( Mn)                                     
                        45,000                                            
Weight average molecular weight ( Mw)                                     
                        270,000                                           
Ashes at 800° C. (ppm)                                             
                        160                                               
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                       TABLE 2                                                     
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                     Fraction II                                          
       Fraction I    ethylene/propylene                                   
       ethylene/propylene                                                 
                     rubber                                               
Heterophasic       %                 %                                    
polymer  Fraction.sup.a)                                                  
                   Ethylene  Fraction.sup.a)                              
                                     Ethylene                             
______________________________________                                    
A        50        3.5       50      30                                   
B        35        3.5       65      30                                   
C        60        2.5       40      60                                   
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 .sup.a) parts by weight                                                  
 
    
     
                       TABLE 3                                                     
______________________________________                                    
Heterophasic                                                              
           I.V..sup.a)                                                    
                      Luperox 101                                         
                                 I.V..sup.a)                              
polymers   (dl/g)     added (ppm)                                         
                                 (dl/g)                                   
______________________________________                                    
A          2.25       0          2.25                                     
A1         2.25       100        1.60                                     
B          3.15       0          3.15                                     
B1         3.15       100        2.20                                     
B2         3.15       200        1.90                                     
B3         3.15       1200       1.30                                     
C          2.70       0          2.70                                     
C1         2.70       600        1.50                                     
______________________________________                                    
 .sup.a) I.V.: intrinsic viscosity of the portion of the heterophasic     
 polymer soluble in xylene at 25° C.                               
 
    
     
                                           TABLE 4                                 
__________________________________________________________________________
                            I.V. of                                       
                                  Maximum                                 
Examples &amp;           Heterophasic                                         
                            amorphous                                     
                                  spinning                                
comparative                                                               
       Heterophasic                                                       
              Polymer (1)                                                 
                     polymer                                              
                            fraction                                      
                                  velocity                                
examples                                                                  
       polymer                                                            
              (g)    (g)    (dl/g)                                        
                                  (m/min)                                 
__________________________________________________________________________
1      A      4000   1000   2.25  2700                                    
2      A1     4000   1000   1.60  3000                                    
1c     B      4000   1000   3.15   500                                    
3      B1     3500   1500   2.20  2700                                    
4      B2     4000   1000   1.90  3600                                    
5      B3     4000   1000   1.30  3000                                    
2c     C      4500    500   1.50   500                                    
6      C1     4500    500   1.50  2400                                    
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                       TABLE 5                                                     
______________________________________                                    
            Thermobonding index                                           
        Blend of  in N       T(°C.)                                
                                    Flexibility                           
Example n.                                                                
        example n.                                                        
                  at 150° C.                                       
                             at 5 N index                                 
______________________________________                                    
7       1         6.8        145    1040                                  
8       2         6.6        146    1060                                  
9       3         8.5        140    1300                                  
10      4         8.4        140    1100                                  
11      5         5.0        150    1200                                  
12      6         6.1        145    1120                                  
3c      resin     5.0        --      800                                  
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                       TABLE 6                                                     
______________________________________                                    
               Indexes of                                                 
Compar.                                                                   
       Winding           Thermobonding                                    
example                                                                   
       speed    Draw     in N at                                          
                                T(°C.)                             
n.     (m/min)  ratio    150° C.                                   
                                at 5 N flexibility                        
______________________________________                                    
13c    750      2.0      4.0    155    1010                               
14c    500      3.0      3.0    157     890                               
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     Other features, advantages and embodiments of the invention disclosed herein will be readily apparent to those exercising ordinary skill after reading the foregoing disclosure. In this regard, while specific embodiments of the invention have been described in considerable detail, variations and modifications of these embodiments can be effected without departing from the spirit and scope of the invention as described and claimed.