Abstract:
The invention relates to a thermoplastic, thermally bondable polyolefin fibre for production of nonwovens as well as a nonwovens obtained by thermal bonding of such polyolefin fibres. The production of nonwovens for applications in hygienic end uses have thermal bonding and softness characteristics dependent on the fibres. For improvement the fibre of the invention shows a whole plastic deformability under calendaring process in the thermobonding dot and a low surface degradation during spinning. Therefore the thermobonding dots of a nonwovens are characterized by the whole close packing of the fibres. The thermal bonding behavior of the fibre will be reach with a spinning process with spinning head temperature set up suitable in order to obtain the specified thermal degradation.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a thermoplastic, thermally bondable polyolefin fibre for production of nonwovens as well as a nonwovens obtained by thermal bonding of such polyolefin fibres.  
       BACKGROUND OF THE INVENTION  
       [0002]     Polyolefin fibres and, more specifically, polypropylene fibre as themselves or in blend with other fibres like wool, cotton, polyester, are widely used for the production of several articles with different morphology.  
         [0003]     This usage has been remarkably improved by several characteristics of these fibres like high chemical inertia and absence of polar groups, no toxicity and cytoxicity, low specific weight, low thermal conductivity and high insulating power, high abrasion resistance, high mildews and bacteria resistance, high colour fastness and, last but not least, easy processability and low cost.  
         [0004]     Textile (underwear, sportswear), floor coverings (carpets), industrial and hygiene are some of the most important applications which have been developed on the basis of one or more of the above mentioned fibre behaviours.  
         [0005]     It is well known that polyolefin fibres and, more specifically, polypropylene fibre, are produced by the melt spinning technology which consists in melting the polymer at high temperature in one extruder. The melted polymer is afterwards forced to pass through a spinneret maintained at controlled high temperature.  
         [0006]     In order to obtain some important additional behaviours on the fibre, specific chemicals are added to the polymer before or during the spinning step:  
         [0007]     stabilizers (process stabilizers, anti-oxidant etc.)  
         [0008]     coloured pigments  
         [0009]     optical brighteners to improve the whiteness  
         [0010]     matting agents to modify the transparency  
         [0011]     After the exit from the spinneret, the hot spun filaments are quenched by air and undergo the subsequent processing steps of drawing, crimping, drying to reach the final cohesion and mechanical characteristics required by the following fibre processing.  
         [0012]     The fibre obtained by the above mentioned steps is afterwards cut and baled.  
         [0013]     Specially tailored spin finish formulations are applied during some steps of the production process to give to the fibre the antistatic, lubricant and cohesion characteristics necessary for the processability. Furthermore the above spin finish formulations must impart to the fibre the additional hydrophilic or hydrophobic behaviours required by the end use.  
         [0014]     Several different morphologies and structural compositions are shown by prior art polyolefin fibres for thermal bonding: 
        Bicomponent fibres like sheath—core o side by side disclosed for instance in U.S. Pat. No. 4,473,677, U.S. Pat. No. 5,985,193, WO9955942 or U.S. Pat. No. 5,460,884.     These fibres are obtained by using two extruders separately feeding two different polymers (i. e. : polypropylene/polyethylene or polypropylene/polyolefin copolymer) to specially designed spinnerets through separate gear pumps.     Structural bicomponent or “bicostituent” constituted by blends of polymers directly obtained inside the spinning extruder as disclosed for instance in U.S. Pat. No. 5,985,193, WO9955942 or U.S. Pat. No. 5,460,884.     “Natural” bicomponent showing a “skin-core” morphology and obtained from single polymers or from blends of polymers by the use of special conditions of the spinning and quenching steps of the production process which lead to the formation of a degraded skin on the fibre as disclosed for instance in U.S. Pat. No. 5,281,378, U.S. Pat. No. 5,318,735, U.S. Pat. No. 5,431,994, U.S. Pat. No. 5,705,119, U.S. Pat. No. 5,882,562, U.S. Pat. No. 5,985,193 or U.S. Pat. No. 6,116,883.        
 
         [0019]     The above mentioned patents and patent applications assert that the achievement of the bonding behaviour of the skin-core fibres is due to the formation of a degraded skin and claim that such a skin is always obtained by the use of suitable processing conditions.  
         [0020]     On the contrary it will prove that, in absence of a specifically tailored polymer stabilization, the thermal bonding behaviour of these fibres may be poor because of an excessive or, on the contrary, limited degradation of the polymer. The main constraints of prior art are for long spinning process the nonwovens limited softness and for short spinning process the nonwovens low tenacity.  
       SUMMARY OF THE INVENTION  
       [0021]     It is therefore an object of the invention to create a thermal bonding fibre for high nonwovens tenacity, which allows a wider process operating window and better control of the quality consistency of the fibre and the nonwovens.  
         [0022]     An other object of the present invention is to solve some main constraints in the thermobonding process versatility and nonwovens quality when standard homo-PP fibres are used, from both long spinning or short spinning process.  
         [0023]     In accordance with the invention, this object is accomplished by the thermoplastic, thermally bondable polyolefin fibre with the features of claim  1 , the spinning process of such a polyolefin fibre with feature of claim  7  and the nonwovens obtained by thermal bonding of such polyolefin fibres with feature of claim  9 .  
         [0024]     The present invention wants to combine the welding effect due to the enhanced plastic behaviour of the fibre together with the effect of the minimal useful thickness of the welding skin. The invented fibre shows a very low surface degradation during spinning and a whole plastic deformabilility after calandering under pressure. Therefore the thermobonding dots in a thermobonding nonwovens are like a thin and homogeneous polymer foil. All the fibres are loosing their single identity and are welded completely together to the thermobonding dot. In the area of forced contact under the calendar compression the fibres show a complete melting and molecular interpenetration of the surfaces.  
         [0025]     For production of such a fibre a spinning process is proposed wherein the spinning head temperature set up suitable in order to obtain the specified thermal degradation of the fibre.  
         [0026]     The invented nonwovens obtained by thermal bonding of said fibres show a higher tenacity in comparison to prior art due of whole close packing of the fibers in the thermally bonded dot.  
         [0027]     By such a technique the following advantages of the invention are realized: 
        the operating window of the spinning process becomes remarkably wider thus improving the quality evenness of the fibre     a tailor made optimization of the non woven characteristics becomes feasible by favouring the tenacity or the softness or intermediate combinations        
 
         [0030]     To realize the above targets it&#39;s necessary to utilize polymeric systems in which it might be possible to enhance the plastic deformability phenomenon during the calendering stage.  
         [0031]     In the second instance the additive formulation of the polymer must allow the formation of the minimal useful thickness of skin.  
         [0032]     With reference to the analytical problems concerning the determination of the depth of the skin really usable in the welding process, the field of the values of the Degradation Index (DI) lays between 1.50 and 3.0 depending on the characteristics which are selected as targets on the nonwovens. The degradation index DI is the value of the ration between the fibre melt flow rate and the resin melt flow rate as will be described in detail later on. Especially good effects during thermobonding good be reach with a degradation index DI in the range of 2.0 and 2.5.  
         [0033]     As previously mentioned, in order to achieve the plastic deformability, it&#39;s necessary to add structural disorder to the crystalline phase to allow an easy trigger of the molecular sliding under stress.  
         [0034]     Among the possible polymeric system combinations, the followings may be mentioned as no limiting example:  
         [0035]     PP+PP/PE+PP/PB  
         [0036]     PP+PP/PB  
         [0037]     PP+PP/PB/PE  
         [0038]     and, also, all the other combinations containing an high crystallinity homo or copolymer as base, one or more components constituted by homopolymer PP or copolymer PP/PE and an additional component constituted by copolymers of PP or PE with α-olefins characterized by a structure with limited crystallinity. The weight proportion of the blend could be in the range between 0% and 90% homopolymer and between 100% and 10% of PP-∝-olefin copolymer.  
         [0039]     All the above components must show a total miscibility among them in order to assure a good processability during the spinning stage.  
         [0040]     An important characteristic of the above copolymers is the melting temperature of their polypropylene crystalline phase which, generally, is inversely proportional to the content of comonomer (“Polypropylene Handbook”, edited by Edward P. Moore, Jr., 1996. Chapt. 6.3.2, FIG. 6.6).  
         [0041]     For better clarity, furthermore, it can be outlined that a lower melting temperature of crystalline PP corresponds to a lower binding energy of the crystallite itself and this fits perfectly with the previously mentioned concept of easier plastic deformability.  
         [0042]     In fact, the invention is concerned with the spinning process of PP fibre by doling out the skin degradation and by using the plastic behaviour of some blends of polymers. The above target is achieved by using specific set up solutions for:  
         [0043]     additive formula  
         [0044]     raw material blend  
         [0045]     process conditions  
         [0046]     More in particular, the dosing of the fibre skin degradation is controlled by the additive formula and by suitable process conditions. A raw material containing primary antioxidant in the range between 150 ppm and 600 ppm leads to good degradation control.  
         [0047]     By using specific raw material blend, the thermoplastic behaviour of the fibre in the calendering plant is optimized in order to achieve the top of tenacity by also controlling temperature and pressure of the rolls.  
         [0048]     Process conditions in the fiber production and in the following calendering.thermalbonding step are driven according to the raw material formula and characteristics. In such way, the tenacity-softness can be taylored according to the final applicative need.  
         [0049]     Taking into account the poor thermal conductivity of polyolefins, together with the very short residence time of the fibre into the calendering treatment, the thermoplastic behaviour of semi-crystalline polyolefins can assume the dominant role during thermal bonding step in calendering machine.  
     
    
     BRIEF DESCRIPTION OF THE TABLES AND FIGURES  
       [0050]      FIG. 1 : Thermal bonding model for skin-core fibres prior art  
         [0051]      FIG. 2 : Nonwovens bonding dot after calendaring prior art fibres  
         [0052]      FIG. 3 : Thermal bonding model for fibres according invention  
         [0053]      FIG. 4 : Nonwovens bonding dot after calendaring fibres according invention 
     
    
       [0054]     Tab. 1: Influence of the spinning temperature on fibre degradation (MFR) and nonwovens  
         [0055]     Tab. 2: Influence of the stabilization formula on PP thermal stability and thermal oxidative stability of polypropylene  
         [0056]     Tab. 3: Influence of the polymer blend composition and degradations index (DI) on fibre thermal bondability  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0057]     The plastic behaviour of the polymer is the capability to withstand large deformations (until 600-700% in some cases) and to retain the deformed shape after removing the deforming stress. In such deformation process, two different steps are recognized.  
         [0058]     In the first step, below 1%, the deformation is elastic and reversible with the applied stress. During the elastic deformation, some temperature decrease can be observed in the body.  
         [0059]     In the second step, over the elastic limit, the deformation become plastic or irreversible and the relative flow of material in the body is observed. The molecular friction due to the above flow can produce increase of the body temperature if the deformation process is fast enough in reference to the heat dispersion effect due to the thermal conductivity of the material.  
         [0060]     In the calendering process of the nonwoven web, the material plastic behaviour can play active role in the thermal bonding result if a wide plastic deformation of the fibre section is carried out in the suitable way. To this purpose, the following main actions are required: 
        in the spinning plant, use of the suitable polyolefin raw material (containing molecular disorder in the crystalline phase)     in the calender plant, increase pressure and, if required, decrease temperature of the rolls.        
 
         [0063]     Concerning the molecular disorder, it has to be considered that such areas can be the starting point for the molecular plastic flow under external stress. In fact, they are areas where the bonding energy of the crystalline building is lower.  
         [0064]     PP homopolymer can be disordered in different ways when crystallinity is high. One of the more straight ways is by blending to PP homopolymer some quantity of compatible polyolefin copolymer between PP and (α-olefin) co-monomer, where the (α-olefin) co-monomer is below 10%. The effectiveness of the above solution is explained by the disorder effect of the (α-olefin) chain segment during crystallization of the PP chain.  
         [0065]     It follows from the above description that the thermal bonding mechanism of polyolefin fibres is the result of the presence of the degraded skin and of the plastic behaviour of the fibre section under mechanical stress.  
         [0066]     The fibre bonding mechanisms like prior art is using skin-core PP-fibres. Such skin-core PP fibre is widely used in thermal bonding as known. The main feature of the above fibre is the difference in melting point between skin and core. More in particular, being the skin degraded in molecular weight, its melting point is lower in comparison to the high molecular weight core section. In more detail, during the calendering action, when the skin layer is quite in molten state, the core of the fibre is still solid. Following the above considerations, the thermal bonding model with skin fibres according prior art can be outlined as in  FIG. 1 , where it is shown: 
        under the hot roll compression, the single fibre aims to keep its original circular section     the roll compression is putting close together all the fibre and the skin layer is molten firstly, so flowing into the residual free volume between the fibre and like a glue.     after very short time (10 milliseconds about) the compression effect is ended and the fibre assembly aims to re-arrange its position under the residual elastic effect, until the solidification of the molten skin layer. During such re-arrangement the “glue” is stretched and tends to form bridges of membrane and/or filaments between neighbouring fibre, as shown in  FIG. 2 . Of course, quantity and size of the bridges are depending from many process variables (thickness and quality of the skin, temperature, pressure, speed, etc).     as for confirmation of the individual core keeping by the single fibre, in spite of the compression stress applied on the dot during calendering, it can be seen ( FIG. 2 ) that the single fibre is visible also in the fibre intersection zone, in spite of the compressive stress applied.        
 
         [0071]     With the invented fibre, object of the present invention, apart the possible presence of degraded skin, the thermal bonding model is outlined as in  FIG. 3 , where it is shown: 
        the single fibre, under the roll compression in calender, is loosing quite completely the original circular section and id deformed in order to allow the fibre close-packing. In such volume arrangement, all the fibre are loosing also their single identity and the welded dot becomes like a thin and homogeneous polymer foil.     as shown in  FIG. 3 , fibres are closely packed and, even if degraded skin is present, number and size of “glue” bridges between neighbouring fibres is very low.     it is crucial to note that, as first result of the high plastic deformability of the fibre section, the strong thermal bonding effect is obtained with the minimum thickness of degraded skin.     as shown in  FIG. 4 , the welding dot is well homogeneous. With the naked eye, the welding dot appears to be transparent due to the optical homogeneity in the polymer bulk.        
 
         [0076]     The presence of crystalline disorder in polymers can be observed by Differential Scanning Calorimetry (DSC) analysis, where it is measured the enthalpy of fusion and the melting temperature.  
         [0077]     In this analysis, a blend made by PP homopolymer and PP-PE random copolymer shows its melting temperature in between the two components and more close to PP, not just in the middle according to a linear low of just blending.  
         [0078]     This effect is well explained by assuming that, in the solidification process of the blend, the two components are included by a unique crystalline phase having a unique melting process. The lower value of the melting temperature of the blend in comparison to the pure homopolymer means a lower binding energy of the crystalline phase, according to the known theories of the polymer physics. Of course, the inclusion of the copolymer into the homopolymer crystalline building, because of the different molecular stereo-regularity, causes the disorder effect during the blend solidification.  
         [0079]     In a different technique, X-ray diffraction (XRD), the crystalline disorder of polymers can be observed in terms of:  
         [0080]     variation of crystal planes distance  
         [0081]     crystal plane completeness  
         [0082]     On molecular scale, crystalline disorder means “displacement/insertion of atoms/chain segment in the crystalline lamella of PP. As a matter of fact, for example, the PP-PE random copolymer with low content of PE can be considered as imperfect PP where the chain segments of PE are forced to stay inside the PP crystalline building during solidification, so creating disorder and reducing number and energy of the molecular bonds in the solid. This is the reason why also pure polyolefin random co-polymers are suitable resins for the plastic thermal bonding effect. On the other hand, polyolefin blend can be more suitable than pure copolymers for the flexibility of the fibre bulk characteristics.  
         [0083]     The production of calendered nonwovens from fibre staple is carried out several days after the fibre spinning. It is a good cost saving tool to test the staple thermal bondability just after the spinning, before packaging.  
         [0084]     To this purpose, it has been developed the lab test W.I. (Weldability Index, by F. Polato, private com, Nov. 30, 1998)  
         [0085]     In the method, few grams of staple are carded. The small web is submitted to compression load at high temperature for a short time. The tenacity of the thermally bonded web is measured.  
         [0086]     By using controlled conditions for all the steps, the test results are closely related with the tenacity of the industrial nonwovens.  
         [0087]     Different spinning technologies can be used for industrial production of polyolefin staple fibres. Today, the most widely used are usually known as “long spinning” and “short spinning”.  
         [0088]     The two technologies are different for both technical and economical factors. The usual trend for plant set up is looking for the skin-core fibre with the following characteristics: 
        the skin is the external layer of polymer degraded by thermal-oxidation (chain scission) where:     the average MW is very much lower than in the starting resin     the MFR is much higher than in the core of the fibre     the melting temperature is clearly lower than in the starting resin     the core of the fibre is the internal remaining section, and is quite unchanged in comparison to the starting polymer.        
 
         [0094]     In fact, after the hole spinneret, the fibre at high temperature is immersed into air and the oxidation process starts immediately from the fibre surface and penetrate the fibre in radial direction. The oxidative degradation of PP, as known, is a chain scission process in which the polymer molecular weight is reduced.  
         [0095]     The target is to achieve the lower melting temperature and the suitable thickness of the skin, in order to obtain the highest tenacity in calender plant with the minor roll temperature.  
         [0096]     As matter of plant experience, the degraded skin having the right quality for the high tenacity of the thermally bonded nonwovens is obtained only in a narrow range of spinning temperature (see Tab. 1). The most important process conditions for quality and thickness of the skin are: 
        polymer temperature out of the hole spinneret (high temperature inside the spinning line are ineffective     air quenching flow, in terms of thermal capacity flow, for the freezing effect of the thermal-oxidative degradation by decreasing the fiber temperature.        
 
         [0099]     The “thickness” of the degraded skin is the result of interaction between the temperature of the fibre leaving the hole spinneret and the time at high temperature available to oxygen for its central diffusion in the fibre itself.  
         [0100]     In other words, the thermal-oxidative process for the formation of the skin is controlled by two minimum threshold: temperature and time  
         [0101]     Concerning time, the two technologies above mentioned allow similar residence time of the fibre at high temperature (10 milliseconds is the time magnitude order). On the other hand, it is well known that the short spinning technology don&#39;t allow the skin degradation of PP in easy way. For this, it must be taken into account that short spinning technology must use high speed quenching flow and very close to the spinneret holes. The final effect is the lower temperature of the fibre in output of the spinneret and the degradation kinetics lower speed.  
         [0102]     In addition, commercial grades of PP for fibres are containing heavier additive formulas, optimized in long spinning technology, where the thermal-oxidation reaction is easier.  
         [0103]     Further on, it must be related the thermal-degradation process for the skin with the final characteristics of the nonwovens.  
         [0104]     In Tab. 1 it is shown the “fibre MFR” and “nonwovens tenacity TBI” versus the spinning head temperature, all the others process conditions kept constant.  
         [0105]     Firstly, polymer degradation (MFR) is growing slowly with the temperature increase, until the “threshold” value of 280° C. Over the threshold, the degradation process is accelerated more and more. At the same time, the nonwovens tenacity starts to improve at 280° C., reach the peak value at 290° and after decreases in spite of the increase of degradation above mentioned. Of course, the relationship is depending quantitatively from plant type and additive formula.  
         [0106]     From Tab. 1 the standard process dynamics can be explained as follows.  
         [0107]     Until 280° C. of spinning head temperature, skin degradation does not take place on the fibre.  
         [0108]     Over this threshold, the degraded skin layer is growing in thickness with exponential law versus temperature. Of course, the increase of skin thickness means that degradation is proceeding versus the middle, so reducing the size of the residual unchanged core and, at the same time, the tenacity of the fibre. For very high spinning temperatures, the fiber thermal bondability would be excellent but, because of the very poor mechanical characteristics of the degraded fibre, the nonwovens tenacity is worst.  
         [0109]     From all the above points, it can be concluded: 
        the skin-core structure can be obtained only over the temperature threshold     the spinning temperature operating window for the highest nonwovens tenacity and by using PP homopolymer and standard spinning technology is narrow (only few degrees)        
 
         [0112]     Moreover, taking into account the interactions of the several variables, some compensating effect can be used for plant set up among: 
        spinning head temperature     quenching flow temperature     quenching flow speed     distance between spinneret surface and upper surface of quenching flow (=quenching distance)        
 
         [0117]     In fact, the above variable are inter-dependent for the skin formation. In particular, for the same additive formula, the set up of the above variables allows the control over the amount of skin quantity and quality.  
         [0118]     Other useful comments are: 
        over its minimum threshold, the spinning head temperature is dominant for the skin control     below, the skin is undetectable     far over the threshold, the nonwovens tenacity is worst     the amount of antioxidant additives in the polymer recipe is dominant for the skin degradation. More in particular, for skin degradation in short spinning lines, the antioxidant level must be low.     optimal thickness and low melting temperature of the skin are required for the high tenacity of the thermally bonded nonwovens obtained from skin-core PP fibre (see model of  FIG. 1 )     for high tenacity of the thermally bonded nonwovens obtained from plastic PP fibre, the skin thickness required is much lower than with skin-core fibre (see model of  FIG. 3 )        
 
         [0125]     For the detection of the skin in PP fibres, some test methods have been considered: 
        optical microscope analysis of the silicon oil ultrasonic extract of the fiber at high temperature (Takeuchi et al. U.S. Pat. No. 5,705,119; Jan. 6, 1998)     TEM analysis of the fibre section previously stained by RuO4 (Trent et al, Ruthenium tetra-oxide staining of polymers for electron microscopy, Macromolecules, vol 16, Nov. 4, 1983).        
 
         [0128]     Unfortunately it was found that the two test methods are unreliable for analytical use because none close relationship was shown among test results and thermal bondability of the PP.  
         [0129]     On the other hand, it is well accepted that the welding skin is formed on the fibre surface during spinning and because of degradation by chain scission.  
         [0130]     Following this concept, it can be shown the close relationship between nonwoven tenacity (TBI) and the Degradation Index (DI) of the polymer during spinning.  
         [0131]     Definitions 
 
 TBI=SQRT ( CD*MD ))*20/ W    (1) 
 
 DI =( MFR  fibre)/( MFR  resin)   (2) 
        where: CD=cross direction tenacity of the non-woven 
            MD=machine direction tenacity of the non-woven     W=weight of the non-woven     MFR=polymer fluidity according to ASTM D-1238-L    
               
 
         [0136]     Of course, the above close relationship can be obtained by keeping constant the calendering process set up and the resin spinning process, being the spinning temperature variable. In such configuration, the degradation effect (DI) is the straight effect of the spinning temperature.  
         [0137]     The following are first references:  
         [0138]     DI=1.0 is the lower limit (theoretical) with lack of any degradation  
         [0139]     1.5&lt;DI&lt;3 is for intermediate degradation and partial skin formation  
         [0140]     3&lt;DI&lt;4 is the range of typical skin core commercial fibres  
         [0141]     DI&gt;4 is for excessive degradation, fragile fibre and worst non-woven tenacity  
         [0142]     With reference to the thermal bonding mechanism ( FIGS. 3,4 ), if the fibre plastic behaviour in calender is suitable, it is found: 
        higher tenacity of the nonwovens for the same DI value in comparison to skin-core homo PP fibre     high tenacity of the nonwovens also for low DI values, corresponding to low skin presence.        
 
         [0145]     The additive formulation of the polymer is an essential feature as it controls, by definition, the polymer degradation mechanism. Such a control becomes particularly effective on the outer layers of the fibre at the exit of the die when the hot polymer gets in touch with the oxygen of the atmosphere.  
         [0146]     The additive formulation of the polypropylene fibre for non wovens in the hygiene applications is generally studied on the basis of the main degradation mechanisms deriving from:  
         [0147]     a) oxygen at high temperature  
         [0148]     b) high processing temperature in absence of oxygen  
         [0149]     c) long storage time (shelf life)  
         [0150]     The protection to oxygen at high temperature is generally carried out by primary anti oxidants like sterically hindered phenols (C.A.S. Nos. 6683-19-8, 27676-62-6, 2082-79-3 and others), afterwards reported as AO1 or by more recently developed additives like lactones (C.A.S. No.181314-48-7 and others) afterwards reported as AO2.  
         [0151]     The protection to the high processing temperature in absence of oxygen is generally carried out by secondary anti oxidants like organic phosphites (CAS Nos. 31570-04-4, 119345-01-6 and others) or organic phosphonites (CAS No. 119345-01-6 and others) in combination with AO1 or AO2.  
         [0152]     The protection to long storage time (shelf life) is assured by both AO1 and sterically hindered amines (polymeric HALS; CAS Nos. 71878-19-8,106990-43-6 and others).  
         [0153]     Among the above mentioned mechanisms, the most important one is that which controls the thermal oxidative degradation of the polymer at high temperature.  
         [0154]     More specifically, the thermal oxidative mechanism must be quantitatively controlled to obtain the required thickness of degraded skin.  
         [0155]     In other terms, as the degraded and low melting point polymer has insufficient mechanical characteristics, it is necessary to dose the thermal oxidative degradation to reach the minimal useful thickness of degraded skin. An excessive degradation leads to an increase of the bonding skin but the mechanical characteristics of the non woven become worse as also the core of the fibre undergoes degradation (see table 1).  
         [0156]     In order to get the properly dosed thermal oxidative degradation, according to the present invention, the concentration of primary anti oxidants must be between 150 ppm (highest degradation) and 600 ppm (lowest degradation).  
         [0157]     T.O.S.I. (Thermal Oxidation Stability Index, “F. Polato: comunicazione privata Nov. 30, 1998”) represents a very effective testing method to separately and jointly evaluate the stability of polypropylene to oxygen at high temperature and to the high processing temperature in absence of oxygen.  
         [0158]     This method assumes that the MFR, as it is well known, is a good indicator of the average Mw and it&#39;s based on the evaluation of the molecular degradation of the polymer as a consequence of: 
        exposure to a constant temperature and for a defined time in a closed cell, in absence of oxygen     exposure to a thermal oxidative action by extruding the polymer at high temperature in presence of oxygen        
 
         [0161]     A common instrument for the measurement of MFR is used for the above trials.  
         [0162]     As it is shown in Table 2, different additive formulations of the polymer lead to a remarkable difference of the degradation at high processing temperature in absence of oxygen and of the thermal oxidative degradation (Formulations 1, 2).  
         [0163]     In the mean time, certain additive formulations may show very similar levels of thermal oxidative degradation and a noticeable difference of the stability to the high processing temperature in absence of oxygen (Formulations 1, 3).  
         [0164]     Polyolefin homopolymers and copolymers like PP and PE are widely used for the production of thermally bondable fibres for non wovens in the hygiene applications.  
         [0165]     PE homopolymer, nevertheless, shows some important limitations as far as price and tenacity of non woven are concerned, even if its relevant contribution to the softness of the non woven is well known  
         [0166]     The use in low concentration of other polymers as ethylene copolymers containing polar monomers like vinyl acetate, methyl-metacrylate and others, blended with polyolefin homopolymers and copolymers, is reported several times in the existing patent documentation.  
         [0167]     The use of such polymers in the real industrial practice is, nevertheless, very limited due to several factors like:  
         [0168]     price of the raw material  
         [0169]     compatibility limits with polyolefins leading to troubles during the spinning process  
         [0170]     PP homopolymer shows, therefore, the major interest for the production of staple fibres for non wovens in the hygiene applications due to the following reasons:  
         [0171]     lower cost of the raw material  
         [0172]     good processability  
         [0173]     satisfactory tenacity behaviour of the non woven  
         [0174]     On the other hand, the thermal weldability of the PP homopolymer fibre is due to the degraded skin which is formed during the spinning according to the process stages previously reported.  
         [0175]     Polymers different from homopolymer PP (with the exclusion of bicomponent sheath—core fibres obtained by feeding the spinneret with two different polymers) are used only in the cases in which there is the will to improve the softness.  
         [0176]     Even in such cases, as well as in the case of the use of homopolymer PP, the spinning process is performed in a way to optimize the formation of the skin to reach the highest tenacity of the non woven.  
         [0177]     The above mentioned limits of this technology are still existing in any case.  
         [0178]     In Table 3 the results obtained by experimental spinning trials done on a NEUMAG spinning line.  
         [0179]     2.2 dtex/40 mm. cut length PP fibres have been produced by adopting several polymeric compositions and by keeping constant all the process parameters with the exception of the spinning head temperatures.  
         [0180]     These temperatures have been specially tailored to reach well defined levels of DI value on the spun fibres.  
         [0181]     The weldability of the fibres has been afterwards measured by the W. I. testing method Results may be summarized as follows: 
        in absence of both the welding mechanisms (fibre plastic behaviour and presence of degraded skin), a 100% homo PP fibre with a DI&lt;1.50 shows a very low value of W I. (test nr. 1)     in presence of the sole plastic behaviour mechanism (obtained by the use of increasing quantities of raco PP in the polymeric formulation), PP fibres with a DI&lt;1.50 show W. I. values which increase accordingly to the concentration of raco PP till reaching high levels of weldability (tests 2-8)        
 
         [0184]     when both welding mechanisms are present in the fibres (plastic behaviour and presence of degraded skin with a DI&gt;1.50), the fibres themselves reach very high values in the W. I. test (tests 9,10).  
                                           TABLE 1                           Influence of the spinning temperature on polymer degradation (MFR)       and non-woven Tenacity (TBI)            spinning temp.   fibre MFR   non-woven TBI       ° C.   g/10 min   N/5 cm                    270   10.1   11.5       275   11.1   12.4       280   14.3   14.3       285   23.4   19.6       290   36.0   24.8       295   49.5   19.6       300   64.0   11.7       305   68.0   10.8       310   73.0   9.8                  
 
         [0185]    
       
         
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                   
               
               
                 Influence of the stabilization formula on PP thermal stability 
               
               
                 and thermal oxidative stability of polypropylene 
               
             
          
           
               
                   
                 AO1 
                 AO2 
                 total additives 
                 MFR 
                   
                   
               
               
                 formula 
                 ppm 
                 ppm 
                 ppm 
                 g/10 min 
                 TSI 
                 OSI 
               
               
                   
               
               
                 1 
                 150 
                   
                 1150 
                 10.2 
                 1.40 
                 12.5 
               
               
                 2 
                 250 
                   
                 1700 
                 10.2 
                 1.14 
                 10.2 
               
               
                 3 
                 150 
                   
                 1250 
                 10.2 
                 1.05 
                 12.2 
               
               
                   
               
               
                   where:    
               
               
                   MFR = starting fluidity of the polymer    
               
               
                   TSI = thermal stability index    
               
               
                   OSI: = oxygen stability index    
               
             
          
         
       
     
         [0186]    
       
         
               
             
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
               
               
                 Influence of the polymer blend composition and degradability (DI) 
               
               
                 on fibre thermal bondability (WI) 
               
             
          
           
               
                   
                 blend composition (%) 
                   
               
             
          
           
               
                 n. test 
                 PP homo 
                 PP/PE raco 
                 DI 
                 WI 
               
               
                   
               
             
          
           
               
                 1 
                 100 
                 0 
                 1.30 
                 370 
               
               
                 2 
                 90 
                 10 
                 1.31 
                 510 
               
               
                 3 
                 80 
                 20 
                 1.32 
                 780 
               
               
                 4 
                 70 
                 30 
                 1.32 
                 900 
               
               
                 5 
                 60 
                 40 
                 1.33 
                 1150 
               
               
                 6 
                 50 
                 50 
                 1.36 
                 2600 
               
               
                 7 
                 40 
                 60 
                 1.37 
                 3900 
               
               
                 8 
                 20 
                 80 
                 1.41 
                 7800 
               
               
                 9 
                 60 
                 40 
                 1.9 
                 2100 
               
               
                 10 
                 60 
                 40 
                 2.3 
                 4050 
               
               
                 11 
                 60 
                 40 
                 3.1 
                 13000 
               
               
                   
               
               
                   where:    
               
               
                   DI = degradation index    
               
               
                   WI = weldability index