Composite fibres and filter elements formed therefrom

Abstract: Polyolefin/polyester composite fibres having a low apparent Young's modulus are produced by composite spinning a polyolefin as sheath component and a polyester as core component; stretching the resulting unstretched filaments to two or more times their original length at a temperature 20°C or more above the secondary transition point of the polyester and below the melting point of the polyolefin; and annealing the resulting stretched filaments at a temperature 20°C or more above the secondary transition point of the polyester and below the melting point of the polyolefin for 30 seconds or more, to give composite fibres having an apparent Young's modulus of 210 Kg/cm² or less. The fibres are used to make filter elements by winding them on to a core while heating them to the melt-adhesion temperatue of their sheath component; adjusting the void content to a desired rate; and cooling the remaining core.

Description: This invention relates to polyolefin/polyester composite fibres having a low apparent Young's modulus and to filter elements formed from such composite fibres. A process for producing a hollow cylindrical filter element by winding a web composed of polyolefin composite fibres onto a core while heating the web is disclosed in Japanese patent publication No. 56-43139/1981. According to that process, it is possible to obtain a filter element having a desired porosity by controlling the pressure at the time of winding, but since the raw material of the element is formed of polyolefin composite fibres, there is the disadvantage that the upper limit of the service temperature is as low as 80 to 90°C, and the filter element may swell depending on the nature of the liquid passed through the element. In order to obtain a filter element capable of being used at high temperatures, it has been proposed to replace the polyolefin composite fibres by polyester fibres comprising a low melting non-crystalline polyester as sheath component and a high melting crystalline polyester as core component. However, since the low melting polyester is vitreous, the low melting polyester may peel off in powder form during the spinning or stretching step or during the production and use of the filter element. When polyolefin/polyester composite fibres comprising polyethylene as sheath component and a crystalline polyester as core component are used, there is the disadvantage that since the apparent Young's modulus of the polyester is high, the resulting filter element is bulky and of low density. Hence, the element is not good at filtering out fine particles, and also has low hardness and is soft and hence does not have a long life in practical use. According to a first embodiment of the invention there is provided a process for producing polyolefin/polyester composite fibres, which process comprises composite spinning a polyolefin as sheath component and a fibre grade polyester having a melting point of 200°C or more and comprising 50% or more of ethylene terephthalate units as core component; stretching the resulting unstretched filaments to two or more times their original length at a temperature 20°C or more above the secondary transition point of the polyester and below the melting point of the polyolefin, and annealing the resulting stretched filaments at a temperature 20°C or more above the secondary transition point of the polyester and below the melting pointof the polyolefin, for 30 seconds or more, to give composite fibres having an apparent Young's modulus of 20.58 MPa (210 Kg/cm 2 ) or less. The invention further provides a process for producing a filter element by winding polyolefin/polyester composite fibres, obtained in accordance with the first embodiment of the invention, on to a core while heating the composite fibres to the melt-adhesion temperature of their sheath component; adjusting the porosity, or void content, thereof to a desired value, and cooling the resulting material and taking out the core. The filter element may be prepared according to the process disclosed in Japanese patent publication No. 56-43139/1981. The filter element of the present invention has a suitable void content, a high hardness, an upper limit of service temperature as high as 140°C and small swelling by liquids. The polyolefin used in the present invention may be so far known polyolefins such as high density polyethylene, low density polyethylene, crystalline polypropylene, etc., and in the case of polyethylene, those having a melt index of 5 or higher, particularly 10 or higher are preferred, while in the case of crystalline polypropylene, those having a melt flow rate of 10 or more is preferably used. The polyester used in the present invention are those polyesters having a melting point of 200°C or higher with ethylene terephthalate units occupying 50% by weight or more of the molecule and being of fiber grade. The secondary transition point of the polyester is in the range of 65 - 85 °C. Such polyolefin and polyester are subjected to composite spinning so as to give sheath and core type composite fibers according to a known process (see, e.g., document DE-A-3315360). The resulting unstretched filaments are stretched to twice or more the original length at a temperature higher than the secondary transition point of the polyester by 20°C or more, preferably 30°C or more and lower than the melt-adhesion temperature of the polyolefin and then annealed at a temperature higher than the secondary transition point of the polyester by 20°C or more and lower than the melting point of the polyolefin for 30 seconds or longer. If the stretching temperature is lower than the above-mentioned temperature, the resulting composite fibres have a high apparent Young's modulus and if a filter element is produced using the fibers, only a product having drawbacks like the above-mentioned conventional techniques is obtained. Further, if the stretching temperature is the secondary transition point or lower, when the resulting stretched fibres are used for producing filter elements, the fibers result in a high percentage of heat shrinkage at the processing temperature thereof, to yield a filter element having projection and depressions on its surface and a non-uniform distribution of pore therein. If the annealing temperature is lower than the above-mentioned range, or the annealing time is less than 30 seconds, the resulting composite fibers have a high apparent Young's modulus so that filter elements using such composite fibers have a high porosity and hence are unsuitable for precise filtration. The present invention will be described in more detail by way of Examples and Comparative examples. In addition, measurement methods employed in these examples are described below. Apparent Young's modulus: This value was obtained by firstly measuring an initial tensile stress according to item 7-10 of JIS L 1013 (testing methods for man-made filament yarns) and then calculating the value from the above initial tensile stress according to the following equation: Apparent Young's modulus (Kg/mm 2 ) = 9 × ρ × (initial degree of tensile resistance) (g/d) wherein as ρ, 1.15 was used in the case of PP (polypropylene)/PET (polyester) and 1.17, in the case of PE/PET. Filtration test: A suspension of fine abrasive powder (FO #1200, particle diameter 5 to 15 µm, 90%) (20 g) in water (50 ℓ) was passed from the outer surface of a filter element into the hollow portion thereof at a rate of 2 m 3 /hr in a recycle manner. After 2 minutes, particles in 100 mℓ of collected filtrate water were collected on a membrane filter capable of collecting particles down to 1 µm in particle diameter, followed by measuring the particle diameters of 3,000 particles by means of "Luzex 450" (trademark) manufactured by Nileco Co., Ltd. Swelling test: The volume (V') of a filter element treated under a condition of boiling water (100°C, 2 hours), toluene (20°C, 5 days) or hot air (120°C, 5 days) was calculated from the value of outer diameter (d') and lenth (t'). The volume (V) of the filter element before the treatment was calculated in the same manner. The percentage of swelling was calculated according to the following equation: V'- V V × 100 = percentage swelling Example 1 Composite spinning was carried out using a polyester (PET) (intrinsic viscosity 0.65, m.p. 255°C, secondary transition point 70°C) as a core component and a high density polyethylene (melt index 20, m.p. 130°C) as a sheath component in a composite ratio of 1/1 to prepare unstretched filaments (single filament fineness: 10 d), followed by stretching the filaments to 3 times the original length at 110°C and then annealing at 90°C for 3 minutes to obtain stretched filaments (single filament fineness: 3.5 d). The stretched filaments were cut to a fiber length of 51 mm to obtain staple fibers, which were then made up into a web having a basis weight of 20 g/m 2 by means of a carding machine, followed by heating the web successively from its end, to 140 - 150°C for 2 seconds by means of a far infrared ray heater, while sending the web horizontally, to form a web in a state where the high density polyethylene alone was melted, winding up the web in a definite amount onto a stainless steel core (outer diameter: 30 mm, 4 Kg/m) under pressure by its self-weight, cooling the web, drawing out the central core and cutting the web to obtain a filter element having an outer diameter of 70 mm, an inner diameter of 30 mm, a length of 250 mm and a weight of 240 g. The thus obtained filter element was very hard and had neither wrinkle nor warp on both the inner and the outer surfaces, and when its cross section was observed, pores are uniformly distributed. The stretching conditions and apparent Young's modulus of the above stretched filaments and the results of the filtration test and swelling test of the filter element are shown in Table 1. Example 2 Composite spinning was carried out in the same manner as in Example 1 except that a crystalline polypropylene (melt flow rate 15, m.p. 160°C) was used as the sheath component, to obtain unstretched filaments (single filament fineness: 10 d), which were then stretched to 3.5 times the original length at 140°C, followed by annealing at 135°C for 5 minutes and cutting to prepare staple fibers having a single filament fineness of 3 d and a fiber length of 51 mm, which were processed in the same manner as in Example 1 except that the heating temperature was changed to 170-180°C, to obtain a filter element. This element was hard and homogeneous as in the case of that of Example 1. The evaluation results thereof are shown in Table 1. Comparative examples 1 and 2 The unstretched filaments obtained in Example 1 were stretched in the same manner as in Example 1 except that the stretching temperature was changed to 70°C (Comparative example 1) or 90°C (Comparative example 2), followed by winding up the resulting stretched filaments onto a stainless steel core in a definite quantity, cooling and cutting in the same manner as in Example 1 to obtain filter elements. The elements were both soft and not compact and had a non-uniform pore content. The evaluation results are shown in Table 1. Reference example Composite spinning was carried out using a crystalline polypropylene (melt flow rate 8, m.p. 160°C) as a core component and a high density polyethylene (melt index 20, m.p. 130°C) as a sheath component in a composite ratio of 1/1, to prepare unstretched filaments, followed by stretching the filaments to 3.5 times the original length at 90°C and then annealing at 100°C for 3 minutes and cutting to obtain staple fibers (single filament fineness 3 d, fiber length 51 mm). A filter element was obtained using the staple fibers in the same manner as in Example 1 except that the heating temperature was changed to 130-140°C. This element was of a good quality but inferior in the heat resistance and solvent resistance to that of Example 1. P.S.: To convert "denier"[=d] to "tex", multiply by 0.1111.

Claims: A process for producing polyolefin/polyester composite fibres which comprises: composite spinning a polyolefin as sheath component and a fibre grade polyester having a melting point of 200°C or more and comprising 50% or more of ethylene terephthalate units as core component; stretching the resulting unstretched filaments to two or more times their original length at a temperature 20°C or more above the secondary transition point of the polyester and below the melting point of the polyolefin; and annealing the resulting stretched filaments at a temperature 20°C or more above the secondary transition point of the polyester and below the melting point of the polyolefin for 30 seconds or more, to give composite fibres having an apparent Young's modulus of 2058 MPa (210 Kg/mm 2 ) or less. A process according to claim 1 in which the polyolefin is high density polyethylene, low density polyethylene, or crystalline polypropylene. Polyolefin/polyester composite fibres produced according the process of claim 1 or 2. A process for producing a filter element which comprises winding polyolefin/polyester composite fibres according to claim 3 onto a core while heating the fibres to the melt-adhesion temperature of their sheath component to adjust the void content thereof to a desired one; and cooling the resulting material and removing the core.

Inventors: GOI SHIGERU, MATSUDA HIDEO, NAKAZAWA KAZUYOSHI, GOI, SHIGERU, MATSUDA, HIDEO, NAKAZAWA, KAZUYOSHI
Applicants: CHISSO CORP, CHISSO CORPORATION
Application year: 1989
Main classification: D01F8
IPC codes: D04H3, B01D39, D01F8