PROCESS FOR PRODUCING A NON-WOVEN FABRIC OF HOT-MELT-ADHERED COMPOSITE FIBERS

Abstract: A process for producing a non-woven fabric of hot-melt-­ adhered composite fibres having a high strength in a small weight thereof and sort feel includes forming a web of fibre aggregate consisting of sheath and core type composite fibres with the core component of the composite fibres being composed of a first component which is a fibre-formable polymer and as the sheath component, a second component which is one or more polymers each having a melting point lower than that of the first component by 30°C or more and which has an average thickness of 1.0 to 4.0 microns, or mixed fibres of the composite fibres with other fibres containing the composite fibres in an amount of at least 20% by weight based on the total amount of the mixed fibres; and subjecting the web of fibre aggregate to a heat treatment at a temperature which is lower than the melting point of the first component and equal to or higher than the melting point of the second component, and which affords to the second component an apparent viscosity of 1 × 10³ to 5 × 10⁴ poise as measured at a shear rate of 10 to 100 sec¹, thereby to stabilize the form of web of fibre aggregate by way of the hot-melt adhesion of the second component.

Description: This invention relates to a process for producing a non-woven fabric. More particularly it relates to a process for producing a non-woven fabric of hot-melt-adhered composite fibres. Non-woven fabrics obtained by using composite fibres consisting of composite components of fibre-formable polymers having different melting points are known from Japanese patent publication Sho 42-21,318/1967, Sho 44-22,547/1969, Sho 52-12,830/1974, etc. In recent years, with more variety in the application fields for non-woven fabrics, the properties required for non-woven fabrics have been raised and it has been basically required for the fabrics to retain a high strength for as small a weight of the fabrics as possible, and also to have as soft a feeling as possible. Using the above-mentioned known processes employing composite fibres composed merely of composite components having different melting points, it has been impossible to satisfy the above-mentioned requirements. The present inventors have made strenuous studies on a process for producing a non-woven fabric which retains a high strength in as small a weight of the fabric as possible and also is provided with as soft a feeling as possible, and have attained the present invention. The present invention resides in: A process for producing a non-woven fabric of hot-melt-adhered composite fibres, which includes forming a web of fibre aggregate consisting of sheath and core type composite fibres with the core component of the composite fibres being composed of a first component which is a fibre-formable polymer and as the sheath component, a second component which is one or more polymers each having a melting point lower than that of the first component by 30°C or more, or mixed fibres of the composite fibres with other fibres containing the composite fibres in an amount of at least 20% by weight based on the total amount of the mixed fibres; and subjecting the web and fibre aggregate to a heat treatment at a temperature which is lower than the melting point of the first component and higher than the melting point of the second component, thereby to stabilize the form of web of fibre aggregate by way of the hot-melt adhesion of the second component, characterised in that the sheath component has an average thickness of 1.0 to 4.0 µm and that the heat treatment is carried out at a temperature (i) 20°C or more lower than the melting point of the first component, (ii) 10°C or more higher than the melting point of the second component, and (iii) at which an apparent viscosity of the sheath component of 1 × 10³ to 5 × 10⁴ poises, as measured at a shear rate of 10 to 100 sec⁻¹, is obtained. The difference between the respective melting points of the two components of the composite fibres is set at 30°C or more. The heat treatment has to be carried out at a temperature at which the desired apparent viscosity of the second component (1 × 10³ to 5 × 10⁴ poises as measured at a shear rate of 10 to 100 sec⁻¹) is obtained, and it seems to be impossible to attain such a viscosity unless the temperature is at least 10°C higher than the melting point of the second component. Furthermore, if the difference between the temperature at the time of the heat treatment and the melting point of the first component is 20°C or lower, undesirable results occur with deformation due to heat shrinkage, etc, in the composite fibres thereby to inhibit the dimensional stability of the resulting non-woven fabric. The second component is arranged at the sheath part of the composite fibres, and the average thickness of the component is limited within a range of 1.0 to 4.0 microns, on the following basis: In the case where the average thickness of the second component is less than 1.0 micron, even if the composite fibres are subjected to hot-melt adhesion under heat treatment conditions where an adequate melt viscosity is exhibited, drawbacks occur such that the area of the part where the hot-melt adhesion is effected is so small that the resulting non-woven fabric has a low strength. Even when the web of fibre aggregate is formed during the step in advance of the heat treatment, the second component is liable to be peeled off due to mechanical shock, friction, etc, which the composite fibres incur, and generation of such peeling-off reduced the strength of the non-woven fabric to an extremely large extent. On the other hand, in the case where the average thickness of the second component exceeds 4.0 microns, drawbacks occur such that during the temperature-raising for the heat treatment, a shrinking force acts on the second component in the vicinity of the softening point to the melting point of the second component to form projections and depressions on the surface of the composite fibres. Even when the temperature is thereafter raised to an adequate one and the apparent viscosity of the second component is reduced, the projections and depressions are insufficiently levelled so that the second component exists in the form of a drop or sphere on the surface of the first component, resulting in a reduced adhesive force, a non-woven fabric having a hard feeling, etc. The average thickness of the second component can be readily calculated from the composite ratio of the first component to the second component at the time of spinning on a conventional sheath and core type composite spinning machine, and the fineness (denier) of the resulting composite fibres. The heat treatment temperature for the production of the non-woven fabric is defined as a temperature which is lower than the melting point of the first component and equal to or higher than the melting point of the second component, and affords to the second component, an apparent viscosity of 1 × 10³ to 5 × 10⁴ poises as measured at a shear rate of 10 to 100 sec⁻¹, for the following reasons: In the case where the apparent viscosity is as high as above 5 × 10⁴ (that is, the temperature is low), the area of heat-melt adhesion of the second component at the contact parts between the respective composite fibres is so small that the resulting non-woven fabric has a reduced strength. If the area of the hot-melt-adhesion part is increased by mechanically compressing the web of fibre aggregate at the low heat treatment temperature, the feeling of the resulting non-woven fabric is hard and hence such a case is undesirable. On the other hand, in the case where the apparent viscosity is as low as below 1 × 10³ (that is, the temperature is high), hot-melt-adhesion of the second component at the contact parts between the respective composite fibres is too easy and hence the area of hot-melt-adhesion is so large that the resulting non-woven fabric is paper-like and deficient in softness and has a hard feeling; hence such a case is also undesirable. Further, at such a heat treatment temperature, even if the average thickness of the second component is within the range of 1 to 4 microns, the second component is liable to exist in the form of a drop or sphere on the first component; hence such a case is also undesirable. The composite fibres employed in the present invention must be those having composite components arranged so that the second component has a temperature range affording an apparent viscosity of 1 × 10³ to 5 × 10⁴ poises as measured at a shear rate of 10 to 100 sec⁻¹ and the first component has a melting point higher than the above-mentioned temperature range. The apparent viscosity of the second component referred to herein means the apparent viscosity of the second component after passing through the spinning process. Such a viscosity can be determined by measuring a sample obtained by spinning the second component alone under the same conditions as those on the second component side at the time of composite spinning, according to a known method (eg JIS K7210: a method employing Kohka type flow tester). Examples of the web of fibre aggregate from which a non-woven fabric is produced by heat treatment in the present invention include not only a web of fibre aggregate consisting singly of composite fibres having the above-mentioned specific features, but also a web of fibre aggregate consisting of a mixture of the composite fibres with other fibres containing the composite fibres in an amount of at least 20% by weight in the mixture, and this web of fibre aggregate is also preferably employed. As the other fibres, any may be used which cause neither melting nor large heat shrinkage at the time of heat treatment for producing the non-woven fabric. For example, one or more kinds of fibres suitably chosen from natural fibres such as cotton, wool, etc; semi-synthetic fibres such as viscose rayon, cellulose acetate fibres, etc; synthetic fibres such as polyolefin fibres, polyamide fibres, polyester fibres, acrylic fibres, etc; and inorganic fibres such as glass fibres, asbestos, etc may be used. The amount used is in a proportion of 80% by weight or less based on the total weight of these fibres and the composite fibres. If the proportion of the composite fibres in the web of fibre aggregate is less than 20% by weight, the strength of the resulting non-woven fabric is reduced; hence such proportions are undesirable. For forming the web of fibre aggregate from the composite fibres alone or a mixture thereof with other fibres, any known processes generally employed for producing non-woven fabrics may be employed. Examples of such processes are the carding process, air-laying process, dry pulping process, wet paper-making process, etc. For the heat treatment process for converting the web of fiber aggregate into a non-woven fabric by heat-melt-adhesion of the lower melting component of the composite fibers, any of dryers such as hot-air dryer, suction drum dryer, Yankee dryer, etc. and heating rolls such as flat calender rolls, embossing rolls, etc, may be employed. The present invention will be further described by way of Examples. In addition, methods for measuring values of physical properties shown in the Examples or definitions thereof are collectively shown below. Strength of non-woven fabric: According to JIS L1096, a sample piece of 5 cm wide was measured at an initial distance between grips, of 10 cm and at a rate of stretching per minute of 100%. Feeling of non-woven fabric: Evaluation was made by functional tests by 5 panellers. ○ : case where all the panellers judged the fabric to be soft. Δ: case where three or more panellers judged it to be soft. x: case where three or more panellers judged it to be deficient in soft feeling. Apparent viscosity: According to flow test method of JIS K7210 (reference test), Q value was measured by means of Kohka type flow tester and the viscosity was calculated from the Q value according to the following conversion equations: wherein Q represents an efflux amount (cm³/sec), r represents radius of nozzle (= 0.05 cm) and I represents a length of nozzle (= 1.00 cm), and as the pressure P to be measured, the respective values of 10, 15, 25, 50 and 100 Kg/cm² were employed. Example 1 Melt-spinning was carried out at 265°C, using a polypropylene having a melt flow rate of 15 (m.p. 165°C) as the first component (core component) and an ethylene-vinyl acetate copolymer having a melt index of 20 (vinyl acetate content 15%, m.p. 96°C) as the second component (sheath component), and also employing a spinneret of 50 holes each having a hole diameter of 0.5 mm, to obtain unstretched filaments having various composite ratios shown in Table 1. Further, a gear pump on the first component side was stopped and the second component alone was taken up to prepare a sample for measuring the apparent viscosity. These unstretched filaments were all stretched to 4.0 times the original lengths at 50°C, crimped in a stuffer box and cut to a fiber length of 51 mm to obtain composite fibers of 3 deniers having average thicknesses of the sheath part shown in Table 1. From these composite fibers were prepared webs of about 100 g/m² according to air-laying process, followed by heat treatment at definite temperatures each for 30 seconds by means of an air-suction type dryer to obtain non-woven fabrics. Evaluations of the strength and feeling of the non-woven fabrics thus obtained are shown in Table 1. Example 2 Melt-spinning was carried out at 295°C in the same manner as in Example 1, using a polyethylene terephthalate having an intrinsic viscosity of 0.65 (m.p. 258°C) as the first component and a high density polyethylene having a melt index of 23 (m.p. 130°C) as the second component. The resulting unstretched filaments were stretched to 2.5 times the original length at 110°C, crimped in a stuffer box and cut to a fiber length of 64 mm to obtain composite fibers of 3 deniers having an average thickness of the sheath part shown in Table 2. From these composite fibers were prepared webs of about 20 g/m² according to carding process, followed by heat treatment by means of calender rolls consisting of a combination of a metal flat roll kept at a definite temperature with a cotton roll, under a pressure of 5 Kg/cm² to obtain non-woven fabrics. Evaluations of the strength and feeling of these non-woven fabrics are shown in Table 2 in contrast to the production conditions. From the experiment results of Examples 1 and 2, it is seen that when a web of fiber aggregate consisting of composite fibers the second component (sheath part) of which has an average thickness of 1 to 4 microns is subjected to heat treatment at a temperature which is lower than the melting point of the first component, equal to or higher than the melting point of the second component and affords an apparent viscosity of the second component of 1 × 10³ to 5 × 10⁴ as measured at a shear rate of 10 to 100 sec⁻¹, it is possible to obtain a non-woven fabric having a high strength and also good feeling. Example 3 From mixtures of composite fibers used in Example 1 (Test Nos. 1-3) (20% by weight) with polyester fibers (6d × 64 mm, m.p. 258°C) (80% by weight) were prepared webs of about 200 g/m² according to carding process, followed by heat treatment at 135°C for 30 seconds by means of an air suction type dryer to obtain non-woven fabrics. These non-woven fabrics had a sufficient strength (7.4 Kg) for kilt products and few fluffs on the surface and a soft feeling.

Claims: 1. A process for producing a non-woven fabric of hot-melt-adhered composite fibres, which includes forming a web of fibre aggregate consisting of sheath and core type composite fibres with the core component of the composite fibres being composed of a first component which is a fibre-formable polymer and as the sheath component, a second component which is one or more polymers each having a melting point lower than that of the first component by 30°C or more, or mixed fibres of the composite fibres with other fibres containing the composite fibres in an amount of at least 20% by weight based on the total amount of the mixed fibres; and subjecting the web and fibre aggregate to a heat treatment at a temperature which is lower than the melting point of the first component and higher than the melting point of the second component, thereby to stabilize the form of web of fibre aggregate by way of the hot-melt adhesion of the second component, characterised in that the sheath component has an average thickness of 1.0 to 4.0 µm and that the heat treatment is carried out at a temperature (i) 20°C or more lower than the melting point of the first component, (ii) 10°C or more higher than the melting point of the second component, and (iii) at which an apparent viscosity of the sheath component of 1 × 10³ to 5 × 10⁴ poises, as measured at a shear rate of 10 to 100 sec⁻¹, is obtained.

Inventors: TANIGUCHI, MASAHIKO, TOMIOKA, SUSUMU
Applicants: CHISSO CORPORATION
Application year: 1983
Main classification: D04H1
IPC codes: D04H1, D04H3