Heat-adhesive composite fibers and method for making the same

Abstract: A heat-adhesive composite fiber consisting of core portion (1) and sheath portion (2), the core portion being of the side-by-side type composite structure comprising two core components (1a, 1b) of different polypropylene base polymers at a composite ratio of 1:2 to 2:1, one of the core components having a Q value, expressed in terms of the weight-average molecular weight/the number-average molecular weight, equal to or higher than 6 and the other having a Q value equal to or lower than 5, and the sheath portion meeting at least the requirments that it should comprise a sheath component of a polyethylene base polymer having a melting point lower by at least 20°C than the lower one of the melting points of the two core components, and it should cover the core portion in a proportion of 25 to 55 % by weight based on the total weight of it and the core portion.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to heat-adhesive composite fibers which may be processed by heating into a nonwoven fabric or the like to provide a bulky product with a soft touch or feeling, and a method for making the same fibers. In particular, it relates to a heat-adhesive composite fiber comprising a core portion and a sheath portion, said core portion being of the side-by-side type composite structure comprising two core components of different polymers in a composite ratio of 1:2 to 2:1 and said sheath portion comprising a sheath component of a polyethylene base polymer having a melting point lower by at least 20°C than the lower one of the melting points of said two core components, and covering said core portion entirely in a proportion of 25 to 55 % by Weight based on the total weight of it and said core portion, and to a method for making the above heat-adhesive composite fibers by separately subjecting to composite-spinning a polypropylene base polymer for a core component and a polyethylene base polymer for a sheath component, which has a melting point lower by at least 20°C than the melting point of said polypropylene base polymer, thereby obtaining a composite non-stretched yarn, in which a core portion is covered entirely with a sheath portion, comprising said sheath component in a proportion of 25 to 55 % by weight based on the weight of it and said core portion, and stretching said composite non-stretched yarn by a two-stage stretching process. Statement of the Prior Art Heat-adhesive composite fibers comprising a core portion and a sheath portion are already known from DE-A-35 44 523 and EP-A-132 110. DE-A-3 544 523 discloses a bicomponent fibre of which the core consists of polypropylene (PP) and the outer fibre regions have a high concentration of the polyethylene (PE)-phase, and the process of extruding a mixture of PP and PE following a special stretching treatment together with conventional process. It also discloses that the fibre has thermo-adhesive properties, and exhibits three-demensional crimping, and can be used to make soft, bulky products such as non-wovens. EP-A-0 132 110 discloses a process for producing a heat-adhesive composite monofilament consisting of a low melting polyolefin component containing polyethylene on the sheath side and a high melting polypropylene component on the core side. In this process, two components having a specific melting point and melt flow index ratios being in a specific relation are used, and the composite ratio is 30 : 70 to 60 : 40, and magnification of stretching is 6 - 9. By this process, a monofilament which is characteristic of heat adhesiveness and superior strength and has neither curls nor peeling between the layers can be obtained. It has as an object the preparation of heat-adhesive monofilaments having no curls (no crimps). Making use of the heat-adhesive property of sheath-core composite type fibers which are less likely to cause crimps as compared with a side-by-side type composite fibres, melting point and melt flow index ratio are selected to decrease further crimpability thereby to achieve its purpose. Furthermore, side-by-side type or sheath-core type polypropylene base heat-adhesive composite fibers are already known which comprise two components having different melting points, and have a considerable portion, e.g., one half or more portion of their surfaces occupied by the component having a lower melting point. In the meantime, various improvements have been achieved. In the main, such improvements have aimed to improve the shrink properties of a web in processing the fibers into a nonwoven fabric by heating and enhance the strength, bulkiness and like factors of the resulting nonwoven fabric, and appreciable outcomes have been attained, but referring to the bulkiness the outcomes have been not yet satisfactory. Hitherto, any appreciable outcome has not been attained in terms of not only the bulkiness but also the touch or feeling of nonwoven fabrics obtained from the polypropylene base heat-adhesive composite fibers by heating. Improvements in touch or feeling have been attempted as by using fine deniers or increasing the proportion of other fibers to be mixed with the composite fibers, such as rayon or wool, but have not still resulted in any product excelling in softness and bulkiness. The situation being like this, a strong demand for further improvements in the bulkiness and softness of nonwoven fabrics intended for purposes such as paper diapers or sanitary materials is not satisfied. Thus, it is strongly desired to meet such a demand. SUMMARY OF THE INVENTION A main object of the present invention is to provide heat-adhesive composite fibers which can solve the aforesaid problems, and can easily be processed by heating into a nonwoven fabric with their heat adhesiveness, said nonwoven fabric being not only bulky but also having a highly soft touch or feeling. As a result of intensive and extensive studies made to attain the object, it has been found that the nonwoven fabric structure is extremely stabilized and sufficiently bulked and have soft touch or feeling when the composite fibers processed into the nonwoven fabrics are constructed by a core portion which imparts bulkiness to the nonwoven fabrics and a sheath portion which imparts heat adhesiveness to the fiber, and furthermore, in addition to the above-mentioned construction, when a number of nodular aggregates consisting of the sheath component are formed on the surfaces of the fibers except for the portions of the fibers bonded together, the soft touch or feeling is further elevated. According to one (or the first) aspect of the present invention, there is provided a heat-adhesive composite fiber comprising a core portion and a sheath portion, said core portion being of the side-by-side type composite structure comprising two core components of different polymers in a composite ratio of 1:2 to 2:1 and said sheath portion comprising a sheath component of a polyethylene base polymer having a melting point lower by at least 20°C than the lower one of the melting points of said two core components, and covering said core portion entirely in a proportion of 25 to 55 % by weight based on the total weight of it and said core portion, which is characterized in that said different polymers of the core portion consist of different polypropylene base polymers, one of them having a Q value, expressed in terms of the weight-average molecular weight/the number-average molecular weight, equal to or higher than 6 and the other one having a Q value equal to or lower than 5, and said sheath portion has a number of aggregatable portions, at least latently releasable, which form many modular aggregates thereon by a heat treatment at a temperature higher than the melting point of said sheath component but lower than the lower one of the melting points of said two core components, the aggregatable portions being obtainable by separately subjecting to composite-spinning said two polypropylene base polymers for two core components and said polyethylene base polymer for a sheath component, thereby obtaining a composite non-stretched yarn of said structure that a core portion of the side-by-side type composite structure is covered entirely with a sheath portion, then heating said composite non-stretched yarn under no tension at a temperature of 80°C to below the melting point of said sheath component for 10 seconds or longer and cooling it down to 15 to 40°C, and then subjecting the cooled composite non-stretched yarn to the first-stage stretching at a temperature of 15 to 40°C and at a draw ratio of 1.3 to 2, and without letting the yarn loose subsequently subjecting it to the second-stage stretching at a temperature of 80°C to below the melting point of said sheath component and at a draw ratio of at least 90 % of the maximum draw ratio (at which the yarn drawn in the first-stage stretching begins to snap off by increasing the draw ratio gradually in the second-stage stretching). The heat-adhesive composite fibers according to the present invention further improve the bulkiness and touch or feeling of nonwoven fabrics obtained therefrom, which have been problems in the prior art. According to a preferred embodiment of the present invention said polypropylene base polymer of at least one of said two core components is polypropylene. According to another preferrred embodiment of the present invention said polypropylene base polymer of at least one of said two core components is a copolymer of propylene with a small amount of an α-olefin except for propylene. According to a further preferred embodiment of the present invention said polyethylene base polymer of the sheath component is polyethylene. According to another preferred embodiment of the present invention said polyethylene base polymer of the sheath component is an ethylene-vinyl acetate copolymer having an ethylene content of 98 to 60 % by weight. According to another (or the second) aspect of the present invention, there is provided a method for making the heat-adhesive composite fibers of any of claims 1 to 5 by separately subjecting to composite-spinning a polypropylene base polymer for a core component and a polyethylene base polymer for a sheath component, which has a melting point lower by at least 20°C than the melting point of said polypropylene base polymer, thereby obtaining a composite non-stretched yarn, in which a core portion is covered entirely with a sheath portion, comprising said sheath component in a proportion of 25 to 55 % by weight based on the weight of it and said core portion, and stretching said composite non-stretched yarn by a two-stage stretching process, which is characterized in that the two polypropylene base polymers are subjected to composite spinning for two core component to make said core portion in a side-by-side type composite structure consisting of said two core components in a composite ratio of 1:2 to 2:1, one of said, core components having a Q value, expressed in terms of the weight-average molecular weight/the number-average molecular weight, equal to or higher than 6 and the other one having a Q value equal to or lower than 5, and said melting point of said polyethylene base polymer for sheath component is below by said temperature the lower one of the melting points of said two polypropylene base polymers, and at said composite spinning step, at least one member selected from polysiloxanes and fluorine compounds is added to both polypropylene base polymers for the two core components or to the polyethylene base polymer for the sheath component, or to both polymers for the two core components and the sheath component each in an amount of 0.05 to 1.0 % by weight based on said pertinent polymer, followed by composite-spinning, and prior to the stretching of said composite non-stretched yarn, said composite non-stretched yarn is heated under no tension at a temperature of 80°C to below the melting point of said sheath component for 10 seconds or longer and cooled down to 15 to 40°C, and is then subjected to the first-stage stretching at a temperature of 15 to 40°C and at a draw ratio of 1.3 to 2, and without letting the yarn loose it is subsequently subjected to the second-stage stretching at a temperature of 80°C to below the melting point of said sheath component and at a draw ratio of at least 90 % of the maximum draw ratio of the second-stage stretching (at which maximum draw ratio the yarn drawn in the first-stage stretching begins to snap off by increasing the draw ratio gradually in the second-stage stretching). According to a preferred embodiment of the claimed process said composite unstretched yarn is stretched at a stretching temperature of 15°C to 130°C inclusive and at an overall draw ratio of 1.3 to 9. In the claimed process preferably a polypropylene base polymer having a melt flow ratio of 4 to 40 is used as said polypropylene base polymer for said core component having a Q value of 6 or more. According to another preferred embodiment of the claimed process a polypropylene base polymer having a melt flow rate of 4 to 60 is used as said polypropylene base polymer for said core component having a Q value of 5 or less. Explanation of the First Aspect of the Invention The first aspect of the present invention will now be concretely explained with reference to the accompanying drawings, in which: Figures 1, 2 and 3 each are a schematical section showing the sectional structure of the heat-adhesive composite fiber according to the present invention, and Figure 4 is a sketch depicting the sheath portion on which nodular agglomerates are formed. Referring to the drawings, reference numeral 1 is a core portion (hereinafter simply referred to as the core) of the side-by-side type composite structure comprising core-dividing zones 1a and 1b each consisting of a core component of a different polypropylene base polymer. The side-by-side type composite structure of the core 1 may take on various forms. For instance, the, core 1 may be of the sectional structure which is diametrically divided into two identical semi-circles, as illustrated in Figure 1. Alternatively, the core 1 may be of the sectional structure in which one core-dividing zone 1a is mostly surrounded with the other core-dividing zone 1b, except for its slight peripheral portion, as illustrated in Figure 2. In most cases, the core actually assumes a structure lying between the aforesaid extreme structures. Still alternatively, the core 1 may be located off the center in section of the fiber, as illustrated in Figure 3. Polypropylene base polymers, which are represented by crystalline polypropylene, may include copolymers of propylene with a small amount of other alpha-olefins save propylene, such as ethylene, butene-1 or pentene-1. In this case, it is preferred that the comonomer component content is up to 40 % by weight. Such polypropylene base polymers are used as the core components of the respective core-dividing zones 1a and 1b, and are different from each other in the Q value that is a numerical value expressing the molecular weight distribution of polymers and calculated from the following equation: Q = Mw/Mn wherein Mw stands for the weight-average molecular weight, and Mn indicates the number-average molecular weight. The core component of one core-dividing zone 1a (which may hereinafter be simply referred to as the component 1a) has a Q value of at least 6, and to the component 1a the general-purpose polypropylene is applied, while the core component of the other core-dividing zone 1b (which may hereinafter be referred to as the component 1b) has a Q value of up to 5, preferably 3 to 5. The composite ratio of the core components 1a and 1b forming the core 1 is in a range of 1:2 to 2:1. Thus, the side-by-side type composite structure of the core 1 comprising the components 1a and 1b having different Q values imparts to the composite fibers the crimps revealed after fiber-manufacturing process and in addition the crimps developed in processing from latent crimps by heating, resulting in an increase in bulkiness. Reference numeral 2 is a sheath portion (hereinafter simply called the sheath) which is formed of a sheath component of a polyethylene base polymer, the melting point of which is lower by at least 20°C than the lower one of the melting points of the two core components of the core 1, viz., the components 1a and 1b (or the melting point common to the components 1a and 1b, if there is no difference in the melting point therebetween). Such polyethylene, base polymer may include polyethylene or a copolymer of ethylene/vinyl acetate, having an ethylene content of 98 to 60 % by weight. That polyethylene is exemplified by a low-, intermediate- or high-density polyethylene. The sheath-core type composite fibers of the present invention are constituted by covering the core 1 with the sheath 2 in such a manner that the proportion of the sheath 2 is in a range of 25 to 55 % by weight based on the total weight of it and the core 1. When the proportion of the sheath 2 is below 25 % by weight, the strength of the resulting nonwoven fabric decreases to such a low level that some problems arise practically. In a proportion of the sheath 2 exceeding 55 % by weight, on the other hand, the development of crimps due to the core 1 is inhibited so that the composite fibers are insufficiently crimped and the resulting nonwoven fabrics are inferior in bulkiness. As described above, the sheath 2 is of polyethylene base polymer of a low melting point, thus the adhesion portion between fibers can be formed by heat treatment as in the case of the conventional heat-adhesive composite fiber. As long as the sheath 2 meets the aforesaid sheath requirement that it be of the above-mentioned structure, a nonwoven fabric product obtained from the heat-adhesive composite fibers constituted by it together with the core 1 may have a sufficient bulkiness and show an excellent touch or feeling. However, the following structure which is obtained by the method described later may impart a much softer touch or feeling to the nonwoven fabric product. More specifically, the structure is that there are many portions on the sheath 2 which form a number of nodular aggregates 3 consisting of the sheath component by a heat treatment at a temperature between the melting point of the sheath component and the lower one of the melting points of the two core components 1a and 1b (the portions may hereinafter be called the aggregatable portions). In the aggregetable portions, the sheath 2 is released from the core 1, or is latently released from the core 1 due to their feeble interface affinity. The aggregatable portions are distinguishable from the other portion, depending upon whether or not the nodular aggregates 3 consisting of the sheath component are formed by the heat treatment at the aforesaid temperature, as illustrated in Figure 4. In most cases, a diameter (D₂) of the greatest portion of the nodular aggregate 3 is about two times the diameter (D₁) of the thinnest portion adjacent thereto. Per one centimeter of the actual length of fiber, there are formed 0.1 to 0.5 nodular aggregates 3 having such a diameter (D₂). When the proportion of the sheath 2 exceeds 55 % by weight of the total weight of it and the core 1, the formation of the aggregates 3 is not sufficient and, hence, makes no contribution to improvements in the touch or feeling of nonwoven fabrics. Although no specical limitation is imposed upon the fineness of fibers 1.7 to 7.7 dtex (1.5 to 7 deniers) are suitable in applications in which weight is given to touch or feeling. More suitable is a range of a finer value of 0.77 to 7.7 dtex (0.7 to 7 deniers). The heat-adhesive composite fibers according to the present invention are constructed as mentioned above. Explanation of the Second Aspect of the Invention For the production of the heat-adhesive composite fibers according to the present invention, provided are three polymers, i.e., two polypropylene base polymers for the core components and one polyethylene base polymer for the sheath component, as already mentioned in connection with the first aspect of the present invention. Referring to the polypropylene base polymers for the core components, the polypropylene base polymer for the component 1a having a Q value of at least 6 should preferably show a melt flow rate (hereinafter sometimes abbreviated as MFR and measured according to Table 1, Condition 14 provided by JIS K 7210) of 4 to 40, and the polypropylene base polymer for the component 1b having a Q value of 5 or less should preferably show a melt flow rate of 4 to 60. Polypropylene base polymers having a Q value of 5 or less may be prepared by the following methods, using polypropylene base polymers having a Q value of more than 5 as the starting material. According to one method, added to and mixed with the starting polymer is an organic peroxide compound in an amount of 0.01 to 1.0 % by weight based on the starting polymer, said organic peroxide compound releases oxygen by heating at a temperature equal to or higher than the melting point of the starting polymer, such as t-butyl hydroperoxide, cumene hydroperoxide or 2,5-dimethylhexane-2,5-dihydroperoxide etc., and the resulting mixture is subjected to melting extrusion from an extruder for granulation. According to another method, the starting polymer may be subjected to melting extrusion several times at elevated temperatures, with no addition of the aforesaid organic peroxide compound, for repeated granulation. Since the Q value is decreased a little by melting extrusion, the polymer for the component 1a before melt spinning should preferably have a Q value of slightly higher than 6, while the polymer for the component 1b may have a Q value of slightly higher than 5. The polyethylene base polymer should preferably have a melt index (hereinafter sometimes abbrebiated as MI and measured according to Table 1, Condition 4 provided by JIS K 7210) of 2 to 50. After the aforesaid three polymers have been provided, they are separately supplied to the respective three extruders for melting extrusion, and the obtained molten polymers are guided to a known appropriate composite spinning nozzle by way of the respective gear pumps. For instance, such a spinning nozzle as disclosed in Japanese Patent Publication No. 44-29522 may be used as the known composite spinning nozzle capable of spinning out three polymer components into a sectional structure similar to that of the heat-adhesive composite fiber according to the present invention. When the aforesaid three polymers are guided to such a spinning nozzle, the outputs of the respective gear pumps are regulated in such a manner that the ratio of the amounts of the polymers for the core components 1a and 1b is a given composite ratio within the range of 2:1 to 1:2, and the amount of the polymer for the sheath component is a given one within the range of 25 to 55 % by weight based on the total amount of it and the core components. The thus obtained, nonstretched composite yarns of the given sectional shape are stretched in a single or multi-stage manner. To increase the latent crimping properties of the obtained composite yarns, it is generally preferred that the multi-stage stretching be carried out under the condition that the first-stage stretching temperature is lower than the second-stage stretching temperature, and that the single-stage stretching be effected at normal temperature (15 to 40°C) or a relatively low temperature close thereto. Since stretching is usually accompanied by the generation of heat, the single-stage stretching or the first-stage stretching of the multi-stage stretching is preferably carried out while passing the yarns through the water maintained at normal temperature, or in a room maintained at normal temperature by cooling water. The stretching has to be effected by somewhat complicated steps as mentioned below. Prior to stretching, the composite nonstretched yarns are firstly heat-treated under no tension at a temperature ranging from 80°C to below the melting point of the sheath component for 10 seconds or longer, preferably for 12 to 180 seconds. This heat treatment promotes the crystallization of the two core components 1a and 1b, and decreases the interface affinity of the sheath 2 with respect to the core 1. For the heat treatment, for instance, the yarns may be continuously passed through a dry heat oven or hot water, or batchwise treated in a large dryer. The heat-treated nonstretched yarns are cooled down to normal temperature (15 to 40°C), and the first-stage stretching is then carried out at that normal temperature in a draw ratio of 1.3 to 2, preferably 1.5 to 1.8. Synergistically combined with the said heat treatment occurring prior to stretching, the first-stage stretching promotes a reduction in the interface affinity between the sheath 2 and the core 1. In consequence, the sheath 2 is actually or latently released from the core 1 at their interface to produce a number of the aggregatable proportions. A draw ratio exceeding 2 at the first-stretching stage offers problems such as fuzzing, a drop in fiber strength and an increase in the degree of shrinkage of the resulting nonwoven fabric, whilst a draw ratio of less than 1.3 renders it difficult to obtain the effect as contemplated in the present invention. Subsequently following the first-stage stretching, the second-stage streching is carried out, without relaxing the yarn between the first-stage and second-stage stretching, at a temperature of 80°C or higher and below the melting point of the sheath component. In this case, the draw ratio should be equal to or higher than 90 % of the maximum draw ratio (at which the yarn drawn in the first-stage stretching begins to snap off by increasing the draw ratio gradually in the second-stage stretching). As the fibers are stretched at the second stage without letting the fibers loose after the first-stage stretching, as mentioned above, it is possible to prevent the fibers from being entangled together due to the crimps to be developed by fiber releasing and snapping off by the second-stage stretching. In addition, the second-stage stretching carried out at the temperature and draw ratio, as mentioned above, gives rise to three-dimensional crimping, by which the fiber strength is increased, the degree of shrinkage and bulkiness of the resulting nonwoven fabric are decreased and increased, respectively, and the formation of the aforesaid aggregatable portions is further promoted. In case of producing the heat-adhesive composite fibers their touch or feeling is then made by far softer, if the nonstretched yarns prepared in the following manner are used. That is, when composite spinning is carried out with three polymers, a chemical agent for reducing the interface affinity (which may hereinafter be called the affinity-reducing agent) is added to these polymers. More exactly, the affinity-reducing agent is added to both polypropylene base polymers for the two core components, or to the polyethylene base polymer for the sheath component, or to both polymers for two core components and the sheath component. As such affinity-reducing agents, effective use is made of polysiloxanes such as polydimethylsiloxane, phenyl-modified polysiloxane, amino-modified polysiloxane, olefin-modified polysiloxane, hydroxide-modified polysiloxane and epoxy-modified polysiloxane, and fluorine compounds such as perfluroloalkyl group-containing polymers, perfluoroalkylene group-containing polymers and modified products of these polymers. The affinity-reducing agent is added to each pertinent polymer in an amount of 0.05 to 1.0 % by weight based thereon. Thus, if stretching is applied to nonstretched yarns obtained by composite spinning with adding the affinity-reducing agent to both polypropylene base polymers for the two core components, or to the polyethylene base polymer for the sheath component, or to, both polymers for two core components and the sheath component, the heat-adhesive composite fibers can then be made, while further promoting the formation of the aggregatable portions. After the composite nonstretched yarns have been stretched by the single- or multi-stage stretching, the stretched yarns are dried, as the occasion may be, and may immediately be used, or may be cut to a given length for the purpose intended. In view of efficiency, the treatments of nonstretched yarns such as heating, cooling and stretching after spinning should preferably be carried out usually with nonstretched yarn bundles formed into a tow of several ten thousand to several million deniers. It is also preferred that such a tow is subjected to the given treatments such as heating, cooling and stretching, while passing it continuously therethrough or moving it therethrough at a low speed in an assembled state, without cutting-off of said tow to short fibers, if possible. The treatments such as heating may be carried out in a batchwise manner, as already mentioned. The heat-adhesive composite fibers according to the present invention are obtained by carrying out the second aspect of the present invention, as mentioned above. Effects The heat-adhesive composite fibers according to the present invention are of the composite structure wherein the core of the side-by-side type composite structure, for which two polypropylene base polymer having different Q values are used, is covered with the, sheath of the polyethylene base polymer having a melting point lower than those of the polymers forming the core components. Accordingly, although the heat-adhesive composite fibers according to the prersent invention are of the sheath-core structure which is generally recognized to show a reduced or limited development of crimps, the revealed crimps and latent crimps developed by heating are very large and take on a moderate three-dimensional shape, due to the core being of side-by-side structure. And on account of the sheath-core structure of whole section of the composite fiber, the composite fiber possesses sufficient heat adhesiveness of the sheath which makes it easy to prepare bulky nonwoven fabrics of large bulk and stabilized structure by heating. In addition, when the sheath additionally includes many aggregatable portions, such portions are molten and aggregated by heating on the fiber surfaces, and are then solidified to give a number of nodular aggregates 3 consisting of the sheath component, which imparts high softness to the touch or feeling of nonwoven fabrics. The reasons appear to be that the area of contact of the fiber surfaces is reduced to a remarkable degree, since the nodular aggregate 3 come into point contact with the surface of the adjacent fibers. Accordingly, the heat-adhesive composite fibers according to the present invention further improve the bulkiness and touch or feeling of nonwoven fabrics obtained therefrom, which have been problems in the prior art. Examples and Comparative Examples In what follows, the present invention will be explained in further detail with reference to the examples and comparative examples. Examples 1 to 9 & Comparative Examples 1 to 12 Eight polypropylenes a, b, c, d, e, f, g and h and two polyethylene base polymers i and j set forth in Table 1 were used in the combinations set forth in Table 2. In Example 2, however, 0.10 % by weight of dimethylpolysiloxane was mixed with high-density polyethylene i. The composite fibers of the structure, wherein the cores of the side-by-side type composite structure constructed from the core components 1a and 1b of two polypropylenes were covered with the sheathes formed of one polyethylene base polymer were prepared by the following composite-spinning, heating and stretching treatments. The spinning nozzle used had 120 holes each of 1.0 mm in diameter. The components 1a and 1b forming the core were used in a composite ratio of 1:1, whilst the proportion of the sheath to the total amount of the core plus sheath was varied in a range of 33.3 to 66.7 % by weight. Referring to the spinning temperature ( the Polymer temperature just prior to spinning out from the spinning nozzle), the polypropylenes for both components 1a and 1b and the polyethylene base polymer were spinned at 260°C and 220°C, respectively. In this manner, composite nonstretched yarns of 12.1 dtex/f (11 d/f (deniers per filament)] were obtained. The composite nonstretched yarns were bundled into a tow of about 99 000 dtex (90,000 deniers), which was successively treated in the following manner. First of all, the tow was heated by passing it under no tension through a dry heat chamber of 105°C for 30 seconds. (However, any heat treatment was not applied in Comparative Examples 1, 2, 3, 10 and 12.). Thereafter, the tow was allowed to stand in a tow can to completely cool it down to room temperature (22°C). Then, the tow was passed through a bath of 21°C containing 0.2 % of a surface finishing agent, and was subjected to a double-stage stretching. For stretching, three-stage rolls were used. A single-stage stretching is carried out by passing the tow through the first and second stretching rolls if it is done, whilst the double-stage stretching was done by passing the tow through the third stretching roll following the same first-stage stretching as the above-mentioned single stage stretching. Referring to the stretching temperatures, the first-stage stretching temperature (identical with the stretching temperature in the case of the single-stage stretching) is defined as being identical with the temperature of the first stretching roll, whilst the second-stage stretching temperature is defined as being identical with the temperature of the second stretching roll. Polymer Melting Point (°C) Flowability Q value a Polypropylene 162 MFR 8 7.4 b Polypropylene 162 MFR 10.2 6.6 c Polypropylene 162 MFR 10.0 5.7 d Polypropylene 162 MFR 12.2 4.5 e Polypropylene 162 MFR 14.0 5.4 f Polypropylene 162 MFR 22.0 4.9 g Polypropylene 162 MFR 32.5 4.5 h Polypropylene 162 MFR 34.0 3.6 i High-Density Polyethylene 128 MI 19 - j Mixed polymer of 85 wt.% of high-density polyethylene (MP:128°C3, MI:19) with 15 wt. % of ethylene/vinyl acetate copolymer (ethylene content:80 %, MP:94°C, and MI:20) 127 MI 19.4 - The above mentioned tow was subjected to the first-stage stretching between a pair of cold stretching rolls of 26°C (but of 60°C in Comparative Example 7 and of 90°C in Comparative Examples 9 and 10) at a draw ratio of 1.6. The tow stretched at first-stage was transferred successively to the subsequent second-stage stretching process without letting it loose, and the tow was stretched between a pair of stretching rolls heated at 90°C (but at different temperatures in Comparative Examples 5, 6 and 7) at the draw ratios corresponing to various per cents of various maximum draw ratios in the second-stage stretching, as specified in Table 2, and was thereafter cooled down to room temperature. The strength and elongation of each of the thus obtained heat-adhesive composite fibers were measured, whilst the shape of crimps and the degree of the aggregatability of the sheath were observed. Further, each heat-adhesive composite fiber was used in the amount of 100 % and heated into a nonwoven fabric, the bulkiness and touch or feeling of which were then tested. The procedures of these tests are given below. Fiber Strength and Elongation: JIS L 1015 7.7 Crimp Shape: After heating at 145°C for 5 minutes, visual estimation was made of whether each fiber was three-dimensionally or two-dimensionally crimped. Bulkiness of Nonwoven Fabric: A group of fibers are passed twice through a carding machine to make a web of 100 g/m², from which five 25 cm × 25 cm square web pieces were cut. Each web piece was put between craft paper sheets, and the assembly was placed in a hot-air circulation type dryer of 145°C for 5 minutes to make a nonwoven fabric, which was in turn cooled at room temperature. Each nonwoven fabric was cut into 20 cm × 20 cm pieces. Such five pieces were formed into a stack on which a cardboard was placed, and the thickness of one nonwoven fabric was calculated from the overall thickness of the stack to find the value in mm for bulkiness. Degree of Aggregatability: After heating the composite fibers at 145°C for 5 minutes, 100 fibers each of about 3 to 12 cm in length were observed under an optical microscope. Estimation was made in terms of the following reference numerals 1, 2, 3 and 4, based on an average number of the nodular aggregates per one centimeter of actual fiber length, which had a maximum portion having a diameter at least two times as large as the minimum diameter of the thinner portion adjacent to the nodular aggregate. 1: 0.30 or more 2: 0.10 to 0.29 3: 0.01 to 0.09 4: below 0.01 Touch or Feeling of Nonwoven Fabric The touch or feeling of the nonwoven fabrics obtained in accordance with the procedures mentioned in connection with "Bulkiness of Nonwoven Fabric" was examined by a five-man panel, while comparing with that of the reference nonwoven fabric. Estimation was made by majority in terms of the following numerals. 1: Softness was very good 2: Softness was considerably good 3: Softness was substantially identical 4: Softness was hard and poor The aforesaid reference nonwoven fabric for the estimation of touch or feeling was obtained from the composite fibers of Comparative Example 10 wherein the nonstretched yarn was stretched substantially according to the prior art. The results are shown in Table 2. From Examples 1 to 9 and Comparative Examples 1 to 4 specified in Table 2, it is found that when the two core components have the Q values within the range defined by the present invention, the development of three-dimensional crimps are considerably noticeable and the bulkiness of the obtained nonwoven fabrics are excellent, if other requirements satisfy the present invention. From Examples 1 and 2, it is understood that when affinity-reducing agent such as polysiloxane is added to the raw material polymer in the manufacture, composite fibers by far better aggregatability, i.e. formability of nodular aggregates, can be obtained than those obtained otherwise. From the comparison of Examples 7 and 8 with Comparative Examples 5 to 11, it is found that the composite fibers obtained by the present method are excellent in the development of three-dimensional crimps, and high in the bulkiness of the obtained nonwoven fabrics; however, when the proportion of the sheath, the application of the heat treatment of nonstretched yarns, the stretching temperature and draw ratio depart from the presently defined range, the composite fibers are poor in the aforesaid properties even though the same starting polymers are used. From the comparison of Example 7 and 8 with Comparative Example 12 in particular, it is found that, the composite fibers obtained by applying the heat treatment to be effected prior to stretching of the composite nonstretched yarns are more excellent in the aggregatability and then in the touch or feeling of the resulting nonwoven fabrics, than ones obtained without said heat treatment. Accordingly, it is found that the heat treatment of the composite nonstretched yarns takes great part in the aggregatability. Use Tests The heat-adhesive composite fiber obtained in Example 4 was cut to a length of 64 mm, and was mixed with rayon of 2,2 dtex x 51 (2d x 51) mm in the proportions set forth in Table 3. Nonwoven fabrics of about 100 g/m ² were made substantially according to the procedures for testing the aforesaid "Bulkiness of Nonwoven Fabric", and were then tested in respect of the bulkiness and touch or feeling and measured in terms of the strength and elongation. The reference nonwoven fabric for the estimation of touch or feeling was obtained from 30 % by weight of the composite fibers obtained in Comparison Example 10 and 70 % by weight of rayon in a similar manner. Testing Procedures Bulkiness of Nonwoven Fabric: The same procedures as in Examples 1 to 9. Strength and Elongation of Nonwoven Fabric: Five test pieces of 20 cm × 5 cm are cut out of the nonwoven fabric in such a manner that their sides of 20 cm lie along the flow direction on a carding machine. The breaking strength and elongation of the five test pieces are found with a tensile strength tester at a grab space of 100 mm and a drawing speed of 100 mm/min., and the measurements were averaged. Touch or Feeling of Nonwoven Fabric: The same as in Examples 1 to 9. The results are given in Table 3. Use Test No. Mixing Ratio (weight %) Weight (g/m²) Touch or Feeling Bulkiness (mm) Strength (kg/5cm) Elongation (%) Composite Fibers Rayon 1 10 90 102 4 3.8 0.21 180 2 20 80 100 3 3.9 0.32 120 3 30 70 98 2 5.8 1.01 90 4 40 60 100 2 6.3 2.58 90 5 60 40 98 2 6.8 3.04 84 6 80 20 101 1 7.1 5.44 75 7 100 0 100 1 7.7 7.76 68 Standard Reference Nonwoven Fabric 30 70 98 - 3.4 1.08 94 From the comparison of Use Test Nos. 1 to 2 with Nos. 3 to 7 given in Table 3, it is found that the nonwoven fabrics which are formed from the mixtures of the heat-adhesive compsite fibers according to the present invention with other fibers such as rayon are excellent in the touch or feeling, bulkiness and strength, when said composite fibers are used at least 30 % by weight.

Claims: A heat-adhesive composite fiber comprising a core portion and a sheath portion, said core portion being of the side-by-side type composite structure comprising two core components of different polymers in a composite ratio of 1:2 to 2:1 and said sheath portion comprising a sheath component of a polyethylene base polymer having a melting point lower by at least 20°C than the lower one of the melting points of said two core components, and covering said core portion entirely in a proportion of 25 to 55 % by weight based on the total weight of it and said core portion, characterized in that said different polymers of the core portion consist of different polypropylene base polymers, one of them having a Q value, expressed in terms of the weight-average molecular weight/the number-average molecular weight, equal to or higher than 6 and the other one having a Q value equal to or lower than 5, and said sheath portion has a number of aggregatable portions, at least latently releasable, which form many modular aggregates thereon by a heat treatment at a temperature higher than the melting point of said sheath component but lower than the lower one of the melting points of said two core components, the aggregatable portions being obtainable by separately subjecting to composite-spinning said two polypropylene base polymers for two core components and said polyethylene base polymer for a sheath component, thereby obtaining a composite non-stretched yarn of said structure that a core portion of the side-by-side type composite structure is covered entirely with a sheath portion, then heating said composite non-stretched yarn under no tension at a temperature of 80°C to below the melting point of said sheath component for 10 seconds or longer and cooling it down to 15 to 40°C, and then subjecting the cooled composite non-stretched yarn to the first-stage stretching at a temperature of 15 to 40°C and at a draw ratio of 1.3 to 2, and without letting the yarn loose subsequently subjecting it to the second-stage stretching at a temperature of 80°C to below the melting point of said sheath component and at a draw ratio of at least 90 % of the maximum draw ratio (at which the yarn drawn in the first-stage stretching begins to snap off by increasing the draw ratio gradually in the second-stage stretching). The heat-adhesive composite fiber as defined in claim 1, characterized in that said polypropylene base polymer of at least one of said two core components is polypropylene. The heat-adhesive composite fiber as defined in claim 1, characterized in that said polypropylene base polymer of at least one of said two core components is a copolymer of propylene with a small amount of an α-olefin except for propylene. The heat-adhesive composite fiber as defined in any of claims 1 to 3, characterized in that said polyethylene base polymer of the sheath component is polyethylene. The heat-adhesive composite fiber as defined in any of claims 1 to 3, characterized in that said polyethylene base polymer of the sheath component is an ethylene-vinyl acetate copolymer having an ethylene content of 98 to 60 % by weight. A method for making the heat-adhesive composite fibers of any of claims 1 to 5 by separately subjecting to composite-spinning a polypropylene base polymer for a core component and a polyethylene base polymer for a sheath component, which has a melting point lower by at least 20°C than the melting point of said polypropylene base polymer, thereby obtaining a composite non-stretched yarn, in which a core portion is covered entirely with a sheath portion, comprising said sheath component in a proportion of 25 to 55 % by weight based on the weight of it and said core portion, and stretching said composite non-stretched yarn by a two-stage stretching process, characterized in that the two polypropylene base polymers are subjected to composite spinning for two core components to make said core portion in a side-by-side type composite structure consisting of said two core components in a composite ratio of 1:2 to 2:1, one of said core components having a Q value, expressed in terms of the weight-average molecular weight/the number-average molecular weight, equal to or higher than 6 and the other one having a Q value equal to or lower than 5, and said melting point of said polyethylene base polymer for sheath component is below by said temperature the lower one of the melting points of said two polypropylene base polymers, and at said composite spinning step, at least one member selected from polysiloxanes and fluorine compounds is added to both polypropylene base polymers for the two core components or to the polyethylene base polymer for the sheath component, or to both polymers for the two core components and the sheath component each in an amount of 0.05 to 1.0 % by weight based on said pertinent polymer, followed by composite-spinning, and prior to the stretching of said composite non-stretched yarn, said composite non-stretched yarn is heated under no tension at a temperature of 80°C to below the melting point of said sheath component for 10 seconds or longer and cooled down to 15 to 40°C, and is then subjected to the first-stage stretching at a temperature of 15 to 40°C and at a draw ratio of 1.3 to 2, and without letting the yarn loose it is subsequently subjected to the second-stage stretching at a temperature of 80°C to below the melting point of said sheath component and at a draw ratio of at least 90 % of the maximum draw ratio of the second-stage stretching, (at which maximum draw ratio the yarn drawn in the first-stage stretching begins to snap off by increasing the draw ratio gradually in the second-stage stretching). The method as defined in claim 6, characterized in that said composite unstretched yarn is stretched at a stretching temperature of 15°C to 130°C inclusive and at an overall draw ratio of 1.3 to 9. The method as defined in any of claims 6 and 7,characterized in that a polypropylene base polymer having a melt flow rate of 4 to 40 is used as said polypropylene base polymer for said core component having a Q value of 6 or more. The method as defined in any of claims 6 to 8, characterized in that a polypropylene base polymer having a melt flow rate of 4 to 60 is used as said polypropylene base polymer for said core component having a Q value of 5 or less.

Inventors: ABE MORIO, EJIMA SHOZO, SUGIHARA TAIZO, ABE, MORIO, EJIMA, SHOZO, SUGIHARA, TAIZO
Applicants: CHISSO CORP, CHISSO CORPORATION
Application year: 1987
Main classification: D01F8
IPC codes: D01F8, D01D5, D04H1