EP0193798A1

EP0193798A1 - Paper-like polyester fiber sheet

Info

Publication number: EP0193798A1
Application number: EP86102191A
Authority: EP
Inventors: Mutsuo Katsu; Mikio Tashiro; Nobuo Sakashita; Tadashi Hirakawa; Kunio Nishimura
Original assignee: Teijin Ltd
Current assignee: Teijin Ltd
Priority date: 1985-02-26
Filing date: 1986-02-20
Publication date: 1986-09-10

Abstract

A paper-like polyester fiber sheet having enhanced mechanical strength, opaqueness, and resistance to permeation of liquids, gases, and bacteria and a soft touch, including polyester short fibers each having a thickness of 0.1 to 15 denier and a length of 3 to 25 mm and provided with a flat cross-sectional profile having a degree of flatness (a ratio in length of a major axis to a minor axis of the profile) of 2.5 or more. The polyester short fibers preferably exhibited an irreversible spontaneous elongation in hot water at 90°C.

Description

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to a paper-like polyester fiber sheet. More particularly, the present invention relates to a polyester fiber sheet having a paper-like appearance and tough and comprising polyester fibers provided with a specific flat cross-sectional profile and having satisfactory mechanical properties, degree of opaqueness, soft touch, and resistance against permeation of liquids, gases, and bacteria therethrough.
The paper-like polyester fiber sheet of the present invention is useful for surgical operating gowns, medical supplies, and sanitary supplies.

(2) Description of Related Art

Due to recent changes in mode of living and significant development in industrial technology, demand has arisen for paper and paper-like sheet materials with improved properties, for example, mechanical properties and touch, and enhanced performance, for instance, enhanced opaqueness and resistance to permeation of liquid, gas, and bacteria therethrough.
For the above-mentioned requirements, various types of synthetic fibers, for instance, polyamide fibers (nylon 6 fibers or nylon 66 fibers), water-insolubilized polyvinyl alcohol fibers, polyacrylonitrile fibers, and polyester fibers have'been utilized in place of conventional wood pulp to obtain paper-like sheet materials having improved qualities and efficiency.
Generally, paper-like sheet materials produced from synthetic fibers exhibit excellent resistance to moisture and weathering and a high electric insulating property and, therefore, are highly useful for various industrial purposes.
Among the above-mentioned synthetic fibers, water-insolubilized polyvinyl alcohol fibers and polyacrylonitrile fibers have been utilized in a large quantity in various industrial fields.
The above-mentioned synthetic fibers do not, however, always satisfy the requirements for the paper-like sheet materials from newly developed industries. Therefore, it has been attempted to utilize polyester fibers having excellent mechanical, chemical, thermal, and electrical properties in various manners for the new requirements.
However, conventional polyester fiber paper-like sheet materials are disadvantageous in that the resultant paper-like sheet materials have a large weight, a large thickness, a high stiffness, and a high cost derived from the high price of the polyester fibers.
In conventional polyester fiber paper-like sheet materials, decreases in weight and thickness thereof result in a desirable soft touch of the resultant sheet material, but, cause undesirable decreases in mechanical strength and opaqueness thereof. Therefore, the resultant sheet materials are not adequate for use as cloths. Furthermore, the light, thin conventional paper-like sheets exhibit poor resistance against permeation of liquids, gases, and bacteria therethrough and therefore are useless for surgical operating gowns, medical supplies, and sanitary supplies.
Japanese Unexamined Patent Publication (Kokai) No. 55-110545 for Murakami et al discloses a paper-like polyester fiber sheet material wherein the polyester fibers have an irregular cross-sectional profile, for example, a T-shaped, Y-shaped, or star-shaped cross-sectional profile. These irregular cross-sectional profiles aim to improve bulkiness and soft touch of the resultant paper-like sheet material. However, the use of the polyester fibers having the irregular cross-sectional profiles does not completely eliminate the afore-mentioned disadvantages of the conventional paper-like polyester fiber sheet materials.
Japanese Unexamined Patent Publication (Kokai) No. 57-29700 for Yamamoto et al discloses utilization of polyester fibers which exhibit spontaneous irreversible elongation in hot water at a temperature of 90°C for the purpose of producing a paper-like sheet material having an improved soft touch. However, the improvement in the touch of the resultant paper-like polyester fiber sheet material is not satisfactory. Also, the other disadvantages of the conventional paper-like polyester fiber sheet materials are not removed by this type of polyester fiber.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a paper-like polyester fiber sheet having satisfactory mechanical strength, degree of opaqueness, and soft touch for clothing use.
Another object of the present invention is to provide a paper-like polyester fiber sheet having excellent resistance to permeation of liquids, gases, and bacteria therethrough, even when the sheet has a relatively small weight per unit area and thickness.
The above-mentioned objects are attained by the paper-like polyester fiber sheet of the present invention, which comprises polyester short fibers each having a denier of from 0.1 to 15 and a length of from 3 to 25 mm and provided with a flat cross-sectional profile having a degree of flatness, which refers to a ratio in length of a major axis to a minor axis of the profile, of 2.5 or more.
The polyester short fibers preferably exhibit a spontaneous, irreversible elongation of at least 0.1% in hot water at a temperature of 90°C.

BRIEF DESCRIPTION OF THE DRAWINGS

Figures 1 to 10 show cross-sectional profiles of polyester short fibers usable for the paper-like polyester fiber sheet of the present invention;
Fig. 11 is a plane view of an opening of a spinning orifice for producing a polyester short fiber having the cross-sectional profile shown in Fig. 4;
Fig 12 is a plane view of an opening of a spinning orifice for producing a polyester short fiber having the cross-sectional profile shown in Fig. 1; and
Fig. 13 is a plane view of an opening of a spinning orifice for producing a polyester short fiber having the cross-sectional profile shown in Fig. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The paper-like polyester fiber sheet of the present invention comprises polyester short fibers each having a thickness of from 0.1 to 15 denier (0.ll to 16.7 dtex), preferably from 2 to 8 denier (2.2 to 8.9 dtex), and a length of 3 to 25 mm, preferably 5 to 15 mm, and each provided with a flat cross-sectional profile.
When the thickness of the polyester short fibers is less than 0.1 denier (0.11 dtex), the resultant polyester short fibers do not easily disperse uniformly in water in a paper-making process. Therefore, the paper-like sheet-forming property is poor.
Also, when the thickness of the polyester short fibers is more than 15 denier (16.7 dtex), the resultant paper-like polyester fiber sheet exhibits decreased mechanical strength and increased stiffness.
When the length of the polyester short fibers is less than 3 mm, the resultant paper-like polyester fiber sheet exhibits an unsatisfactory decreased mechanical strength.
Also, when the length of the polyester short fibers is more than 25 mm, the resultant short fibers exhibit a poor dispersing property in water. Therefore, the resultant paper-like polyester fiber sheet exhibits poor uniformity of distribution of the short fibers in the sheet.
In the paper-like polyester fiber sheet, it is important that the polyester short fibers from which the paper-like sheet is formed have a flat cross-sectional profile having a degree of flatness of at least 2.5, more preferably 3.5 to 7.0. The term "degree of flatness" refers to a ratio in length of a major axis to a minor axis of the flat cross-sectional profile.
Referring to Fig. 3, a flat cross-sectional profile 3 has a major axis having a length A and a minor axis having a length B. The degree of flatness of the profile 3 corresponds to a ratio A/B.
If the degree of flatness is less than 2.5, the resultant paper-like polyester fiber sheet exhibits unsatisfactory mechanical strength and degree of opaqueness and a poor resistance to permeation of liquids, gases, and bacteria therethrough.
As shown in Figs. 1 to 10, the cross-sectional profiles of the polyester short fibers usable for the present invention are flat. Among those cross-sectional profiles, it is preferable that, as shown in Figs. 2 to 10, the cross-sectional profiles have at least one convexity or concavity formed thereon.
For example, referring to Fig. 1, a flat cross-sectional profile 1 has no convexity and no concavity, while referring to Fig. 2, a flat cross-sectional profile 2 has four convexities 2a or, from another view, has two concavities 2b.
In Figs. 3, 4, and 5, flat cross-sectional profiles 3, 4, 5 respectively have a plurality of convexities 3a, 4a, and 5a, or, from another view, have a plurality of concavities 3b, 4b, and 5b.
Referring to Fig. 6, a flat cross-sectional profile 6 has six convexities 6a, or, from another view, has four concavities 6b, and is provided with three hollows 6d.
Referring to Fig. 7, a flat cross-sectional profile 7 has four convexities 7a. Referring to Fig. 8, a flat cross-sectional profile 8 has two large convexities 8a and four small convexities 8c, or, from another view, has two large convexities 8a and four concavities 8b.
In Fig. 9, a flat cross-sectional profile 9 has three convexities 9a.
In Fig. 10, a flat cross-sectional profile 10 has 10 small convexities 10a and four large convexities 10c or, from another view, has four large convexities 10c and four concavities lOb, four concavities 10f, and four concavities 10g.
The convexities or concavities formed on fiber peripheries, as shown in Figs. 2 to 10, are effective for enhancing the degree of opaqueness of the resultant paper-like polyester fiber sheet.
Also, the hollows formed in the fiber, as shown in Fig. 6, are effective for enhancing the degree of opaqueness, opacifying effect, and degree of bulkiness of the resultant paper-like polyester fiber sheet.
The polyester resins usable for the flat polyester short fibers of the present invention are preferably selected from polyethylene terephthalate homopolymers and copolymers, polybutylene terephthalate homopolymer and copolymers, and polypropylene terephthalate homopolymer and copolymers, more preferably from polyethylene terephthalate homopolymer and copolymers.
The above-mentioned polyethylene terephthalte copolymers may contain at least one comonomer selected from, for example, isophthalic acid, sebacic acid, and 5-sodium sulfoisophthalic acid, as an additional dicarboxylic acid component, in place of a portion of terephthalic acid. Also, the polyethylene terephthalate copolymers may contain, as an additional diol component in place of a portion of ethylene glycol, at least one comonomer selected from, for example, diethylene glycol, 1,4-butanediol, propylene glycol and polyethylene glycol.
The polyester resin to be converted to the polyester short fibers may contain one or more additives, for example, a delustering agent, optical brightening agent, antistatic agent, flame-retarding agent, and coloring agent.
The polyester short fibers usable for the present invention preferably exhibit a spontaneous irreversible elongation of at least 0.1%, more preferably from 1% to 15%, in hot water at a temperature of 90°C.
The spontaneously, irreversibly elongating polyester short fibers are highly effective for enhancing the soft touch and the resistance to permeation of liquids, gases, and bacteria through the resultant paper-like polyester fiber sheet.
The elongating polyester short fibers can be produced by drawing melt-spun polyester filaments at a relatively low temperature of, for example, 55°C to 65°C and then by relaxing the drawn filaments in hot water at a temperature of from 85°C to 95°C.
If the spontaneous elongation of the polyester short fibers is less than 0.1%, the improvement in the soft touch and in the resistance to permeation of liquids, gases, and bacteria of the resultant paper-like polyester fiber sheet is sometimes unsatisfactory.
The paper-like polyester fiber sheet of the present invention may consist of polyester short fibers alone. Otherwise, the polyester fiber sheet contains, in addition to the polyester short fibers, at least one type of additional fibrous material selected from other synthetic fibers, for instance, water-insolubilized polyvinyl alcohol fibers, nylon 6 fibers and nylon 66 fibers; regenerated fibers, for example, viscose rayon fibers and cupra fibers; inorganic fibers, for example, glass fibers, asbesto fibers, and carbon fibers; and wood pulp.
The viscose rayon fibers are very useful as an additional fibrous material to be mixed with the polyester short fibers, for enhancing the mechanical strength of the resultant paper-like polyester fiber sheet.
When the paper-like polyester fiber sheet contains the additional fibrous material, it is preferable that the content of the flat polyester short fibers be at least 5% by weight, more preferably at least 10% by weight.
Also, polyester copolymer fibers containing, as an additional dicarboxylic acid comonomer, 5-sodium sulfoisophthalic acid, are highly compatible with wood pulp or viscose rayon fibers, enhancing the bonding property of the polyester short fibers to the wood pulp particles and viscose rayon fibers.
The polyester short fibers and, optionally, the additional fibrous material may be bonded with each other by means of a binder consisting of, for example, a water-soluble polyvinyl alcohol, a polyacrylic ester resin, or a fiber-shaped polypropylene binder.
The flat polyester short fibers usable for the present invention are either drawn or undrawn fibers. Also, the polyester short fibers usable for the present invention may be a blend of polyester short fibers having a flat cross-sectional profile having a degree of flatness of 2.5 or more and other polyester short fibers having a cross-sectional profile with a degree of flatness of less than 2.5, for example, a circular cross-sectional profile having a degree of flatness of 1.0.
The polyester short fibers usable for the present invention may be selected from the types of blends shown below.

A) A blend of drawn polyester short fibers having a degree of flatness of 2.5 or more with undrawn polyester short fibers having a degree of flatness of 2.5 or more.
B) A blend of drawn polyester short fibers having a degree of flatness of 2.5 or more with undrawn polyester short fibers having a degree of flatness of less than 2.5.
C) A blend of drawn polyester short fibers having a degree of flatness of less than 2.5 with undrawn polyester short fibers having a degree of flatness of 2.5 or more.

In any blends, the content of polyester short fibers having a degree of flatness of 2.5 or more, which short fibers are either drawn or undrawn fibers, is preferably 10% by weight or more, more preferably, 20% by weight or more.
The undrawn polyester short fibers are effective as a binder for the drawn polyester fibers.
The paper-like polyester fiber sheet of the present invention is formed by a conventional wet paper-forming method, dry paper-forming method, or spun bond sheet-forming method.
When the paper-like polyester fiber sheet of the present invention is provided from specific polyester short fibers having a specific thickness, length, and flat cross-sectional profile by means of a paper-making method, the flat polyester short fibers are arranged in such a manner that the major axes of the cross-sectional profile of the polyester short fibers are substantially parallel to the surfaces of the resultant paper-like polyester fiber sheet. That is, in the resultant sheet, gaps formed between the flat polyester short fibers are small. Therefore, the resultant paper-like polyester fiber sheet exhibits as enhanced opacifying effect and degree of opaqueness and an excellent resistance to permeation of liquids, gases, and bacteria therethrough.
Also, in the paper-like sheet, the flat polyester short fibers contact each other at a large contact area. This feature causes the frictional resistance to a tearing force applied to the paper-like sheet to be large and results in an enhanced tear strength thereof.
Furthermore, where the flat polyester short fibers having a spontaneous irreversible elongating property in hot water, which fibers exhibit a relatively low degrees of orientation and crystallinity, are used and the resultant paper-like sheet is heat-treated, for example, during a drying procedure, the heat-treated polyester short fibers elongate and the resultant heat-treated paper-like sheet exhibits a desirable soft touch.
The paper-like polyester fiber sheets of the present invention have a high mechanical strength, a high opaqueness (hiding or opacifying property), a desirable soft touch, and an excellent resistance to permeation of liquids, gases, and bacteria therethrough and, therefore, are useful for clothing, for example, underwear and medical clothing such as surgical operating gowns, for sanitary materials, for example, bottom sheets of sanitary napkins, and for electric industrial materials, for example, resin-impregnated sheets for honeycombs.
The present invention will be further illustrated by the following examples.
In the examples, the spontaneous elongation and specific gravity (crystallinity) of polyester short fibers and the tensile strength, resistance to permeation of liquid (water-repellent property), resistance to permeation of gas (pressure loss), degree of opaqueness, and stiffness (softness) of paper-like polyester fiber sheet were determined as follows.

(1) Spontaneous elongation

The length (ℓ₀) of a fiber to be tested was determined. Then, the fiber was treated in hot water at a temperature of 90°C for 20 minutes. The length (i) of the hot water-treated fiber was determined.
The spontaneous elongation (EL) of the fiber was calculated from the equation:

(2) Specific gravity

A sample of fibers to be tested was immersed in a medium consisting of a mixture of 21.5 parts by weight of n-heptane with 78.5 parts by weight of tetrachloromethane contained in a density gradient tube at a temperature of 25°C for 6 hours. The specific gravity of the fibers was determined from the equilibrated location of the fibers in the tube.

(3) Tensile strength

The tensile strength of a paper-like sheet was determined in accordance with Japanese Industrial Standard (JIS) P 8113 by means of a constant speed tension type tensile tester.

(4) Resistance to permeation of a liquid

A drop of water of a volume of 0.1 ml was placed on a surface of a paper-like sheet to be tested. The time necessary for the complete penetration of the entire amount of water into the paper-like sheet was measured.
The resistance of the paper-like sheet to permeation of liquid is represented by the measured penetration time.

(5) Resistance to permeating of a gas

Air was permeated through a paper-like sheet to be tested at a rate of 4 cm³/sec·cm² under a predetermined pressure, and the pressure loss due to the permeation was measured.
The resistance of the paper-like sheet to permeation of air was represented by the measured pressure loss.

(6) Degree of opaqueness (opacifying (hiding) property)

The degree of opaqueness of a paper-like sheet was determined in accordance with JIS P 8138.

(7) Stiffness

The stiffness of a paper-like sheet to be tested was determined by a cantilever method.

Examples 1 to 12 and Comparison Examples 1 to 5

In each of Examples I to 12 and Comparison Examples 1 to 5, chips of polyethylene terephthalate having an intrinsic viscosity of 0.65 were melted at a temperature of 300°C, the resultant melt was extruded at through a spinneret having 1200 spinning orifices each having an opening in the shape as shown in Fig. 11 at a temperature of 285°C, and the extruded filamentary streams of the melt were solidified and taken up at a speed of 1000 m/min. The size of the openings of the spinning orifices and the extruding rate of the melt were altered so as to obtain undrawn flat polyester filaments different in thickness and degree of flatness and having the flat cross-sectional profile as indicated in Fig. 4.
The resultant consisting of the flat undrawn polyester filaments and having a thickness of 400,000 denier (444,444 dtex) was drawn at a draw ratio of 2.8 at a temperature of 70°C. The resultant drawn filament tow was relaxed under no tension in a relaxing atmosphere at a temperature of 140°C.
The relaxed filaments were cut into the length as indicated in Table 1. The resultant polyester short fibers had the thickness and the degree of flatness as indicated in Table 1.
The polyester short fibers were dispersed in a concentration of 0.5% by weight in water. In this dispersing procedure, the polyester short fibers were mixed with wood pulp and a binder consisting of a polyacrylic acid ester resin in the proportion as shown below.
The resultant aqueous dispersion was subjected to a usual paper-like sheet-forming procedure by means of a cylinder paper-making machine.
A paper-like polyester fiber sheet having a weight of 50 glm² was obtained.
The sheet had the tensile strength, degree of opaqueness, pressure loss, and stiffness as indicated in Table 1. The paper-like sheet-forming property of the polyester short fibers was also evaluated, as indicated in Table 1.
As Table 1 clearly shows, the flat polyester short fibers used in Examples 1 to 12 exhibited a satisfactory or excellent paper-like sheet-forming property in the paper-making method and the paper-like polyester fiber sheets of Examples 1 to 12 in accordance with the present invention exhibited a high tensile strength, a satisfactory opaqueness (opacifying property), a pressure loss (resistance to permeation of air), and a low stiffness (satisfactory soft touch).
In Comparative Example 1, polyester short fibers having a very small thickness of 0.05 denier, which was less than 0.1 denier, could not be uniformly dispersed in water and, therefore, exhibited a poor paper-like sheet-forming property and could not be converted to a satisfactory paper-like sheet.
In Comparative Example 2, polyester short fibers having a large thickness of more than 15 denier, i.e., 16 denier, caused the resultant paper-like sheet to exhibit a low tensile strength, an undesirably large stiffness (unsatisfactory stiff touch), and an unsatisfactory pressure loss (resistance to permeation of gas).
In Comparative Example 3, the polyester short fibers having a length of less than 3 mm, i.e., 2 mm, caused the resultant paper-like sheet to exhibit a poor tensile strength.
In Comparative Example 4, polyester short fibers having a length of more than 25 mm, i.e., 26 mm, were unevenly dispersed in water and therefore exhibited a poor paper-like sheet-forming property. The resultant paper-like sheet was uneven in thickness and useless.
In Comparative Example 5, polyester short fibers having a poor degree of flatness of less than 2.5, i.e., 2.3, resulted in an unsatisfactory tensile strength, a low degree of opaqueness (unsatisfactory opacifying property), and a low pressure loss (poor resistance to permeation of air).

Comparative Example 6

The same procedures as those described in Example 9 were carried out except that each spinning orifice had a circular opening and, therefore, the resultant polyester fibers had a circular cross-sectional profile.
The resultant paper-like sheet exhibited a low tensile strength of 4.5 kg/5 cm, a low degree of opaqueness of 58% (a poor opacifying (hiding) property), a low pressure loss of 10 mmH₂0 (a poor resistance to permeation of air), and a relatively high stiffness of 6.5 cm (a poor softness).

Examples 13 to 20 and Comparative Example 7

In each of Examples 13 to 20 and Comparative Example 7, the same procedures as those carried out in Example 9 were conducted, except that the openings of the spinning orifices had the shape as shown in Table 2.
A portion of the extruded, undrawn polyester filaments was cut into a length of 10 mm to provide undrawn polyester shot fibers having a thickness of 4 denier and a cross-sectional profile as shown in Table 2.
The remaining portion of the extruded, undrawn polyester filaments was drawn in the same manner as that described in Example 9 and the drawn filaments were cut into a length of 10 mm to provide drawn polyester short fibers having a thickness of 2 denier and a cross-sectional profile as indicated in Table 2.
A blend of the above-mentioned drawn and undrawn polyester short fibers was dispersed in a concentration of 0.5% by weight and further mixed with a fibrous polypropylene binder in the proportion shown below:
The resultant aqueous dispersion was subjected to a paper-like sheet-forming process by means of a cylinder paper-making machine.
The resultant paper-like polyester fiber sheets had a weight of 50 g/m2 and exhibited the tensile strength, degree of opaqueness, resistance to permeation of water, and stiffness as indicated in Table 2.
As Table 2 clearly illustrates, in Examples 13 to 20 in accordance with the present invention, the blanks of the drawn and undrawn polyester short fibers exhibited an excellent paper-like sheet forming property and the resultant paper-like sheets had a high tensile strength, a satisfactory opaqueness (hiding property), an excellent resistance to permeation of water, and a low stiffness (high softness).
In Comparative Example 7, the blend of the drawn and undrawn polyester fibers having a circular cross-sectional profile caused the resultant paper-like sheet to exhibit a poor tensile strength, unsatisfactory degree of opaqueness (hiding property) and resistance to permeation of water, and a high stiffness (low softness).
In Examples 13, 16, and 19, the drawn polyester short fibers had a circular cross-sectional profile, and the undrawn polyester short fibers had a flat cross-sectional profile. The resultant paper-like sheets were satisfactory.
In Examples 14 and 18, the drawn polyester short fibers had a flat cross-sectional profile and the undrawn polyester short fibers were of a circular cross-sectional profile. The resulting paper-like sheets were satisfactory.
In Examples 13, 14, 15, 18, 19, and 20, the cross-sectional profiles of the flat polyester short fibers having a plurality of convexities or concavities resulted in an increased degree of opaqueness of the resultant paper-like sheets.

Examples 21 to 32 and Comparative Examples 8 to 12

In each of Examples 12 to 32 and Comparative Examples 8 to 12, the same procedures as those described in Example 1 were carried out with the following exception.
The undrawn flat polyester filaments in the form of a tow having a thickness of 400,000 denier were drawn at a draw ratio of 2.8 at a temperature of 62°C. The drawn filaments were relaxed in hot water at a temperature of 95°C for 2 seconds so as to allow the filaments to shrink at a shrinkage of 40%.
The relaxed filaments were cut into the length as indicated in Table 3.
The resultant short fibers had the thickness spontaneous elongation, specific gravity, and degree of flatness as indicated in Table 3.
The polyester short fibers were converted to a paper-like sheet having a weight of 50 g/m2 by the same procedures as those described in Example 1.
The resultant paper-like sheet had the properties as shown in Table 3.
As Table 3 clearly shows, the flat, spontaneous elongating polyester short fibers described in Examples 21 to 32 exhibited a satisfactory or excellent paper-like sheet-forming property in the usual paper-making method and the resultant paper-like sheets exhibited a high tensile strength, a satisfactory opaqueness (hiding property), resistance to permeation of air, and stiffness/softness).
In Comparative Examples 8 to 12, the polyester short fibers which fell outside of the scope of the present invention exhibited an unsatisfactory paper-like sheet-forming property or resulted in unsatisfactory paper-like sheets.

Comparative Example 13

The same procedures as those described in Example 29 were carried out, except that the spinning orifices had a circular opening and, therefore, the resultant polyester fibers had a circular cross-sectional profile.
The resultant paper-like sheet had a low tensile strength of 2.4 kg/5 cm, a low degree of opaqueness of 58%, a low pressure loss of 10 mmH₂0, and a large stiffness of 9.5 cm and, therefore, exhibited a poor hiding property and a stiff touch.

Examples 33 to 37 and Comparative Example 14

In each of Examples 33 to 37 and Comparative Example 14, a polyethylene terephthalate copolymer containing 2.6 molar% of 5-sodium sulfoisophthalic acid and having an intrinsic viscosity of 0.48 was melted at a temperature of 305°C. The melt was extruded through a spinneret having 500 spinning orifices with openings in the shape as shown in Table 4 at a temperature of 290°C. The extruded filamentary streams of the melt were solidified and taken up at a speed of 1100 m/min.
The resultant undrawn filaments had a thickness of 4 denier and the cross-sectional profile and the degree of flatness as indicated in Table 4.
A portion of the resultant undrawn filaments was bundled into a tow having a thickness of 350,000 denier and drawn at a draw ratio of 3.0 in hot water at a temperature of 65°C. The drawn filaments were relaxed in hot water at a temperature of 90°C for 2 seconds so as to allow shrinkage of 40%. The relaxed filaments were dried in dry atmosphere at a temperature of 105°C for 10 minutes. The dried filaments had a thickness of 2 denier and were cut into a length of 10 mm.
The remaining portion of the undrawn filaments was cut to provide undrawn polyester short fibers having a length of 10 mm.
The drawn short fibers and undrawn short fibers respectively having a cross-sectional profile as shown in Table 4 were dispersed in a concentration of 0.5% by weight in water and mixed with fibrous polypropylene binder in the same proportion as described in Example 13.
The resultant aqueous dispersion as subjected to a paper-like short-forming process by means of a cylinder paper-making machine.
The paper-like sheet-forming property of the polyester short fibers is shown in Table 4. Also, the properties of the resultant paper-like sheet having a weight of 50 g/m² are shown in Table 4.
In view of Table 4, it is clear that the flat polyester short fibers in Examples 33 to 37 caused the polyester short fiber blends to exhibit an excellent paper-like sheet-forming property in the cylinder paper-making machine, and the resultant paper-like sheets of the present invention had satisfactory tensile strength, degree of opaqueness, resistance to permeation of water, and stiffness and exhibited an enhanced hiding property and a soft touch.
However, the resultant paper-like sheet of Comparative Example 14 produced from polyester short fibers having a circular cross-sectional profile had a low tensile strength, degree of opaqueness, resistance to permeation of water, and a high stiffness and exhibited an unsatisfactory hiding property and an undesirable stiff touch.

Claims

1. A paper-like polyester fiber sheet, comprising polyester short fibers each having a thickness of from 0.1 to 15 denier and a length of from 3 to 25 mm and provided with a flat cross-sectional profile having a degree of flatness, which refers to a ratio in length of a major axis to a minor axis of the profile, of 2.5 or more.

2. The paper-like polyester fiber sheet as claimed in claim 1, wherein the polyester short fibers exhibit a spontaneous irreversible elongation of at least 0.1% in hot water at a temperature of 90°C.

3. The polyester fiber sheet as claimed in claim 1 or 2, wherein the flat cross-sectional profile has at least one convexity and/or at least one concavity formed thereon.

4. The paper-like polyester fiber sheet as claimed in claim 1 or 2, wherein at least a portion of the polyester short fibers consists of undrawn polyester short fibers.

5. The paper-like polyester fiber sheet as claimed in claim 1, or 2, wherein at least a portion of the polyester short fibers has a flat hollow cross-sectional profile.

6. The paper-like polyester fiber sheet as claimed in claim 1 or 2, wherein the polyester short fibers comprise at least one member selected from polyethylene terephthalate homopolymer and copolymers, polybutylene terephthalate homopolymer and copolymers and polypropylene terephthalate homopolymer and copolymers.

7. The paper-like polyester fiber sheet as claimed in claim 6, wherein the polyethylene terephthalate copolymer contains copolymerized 5-sodium sulfoisophthalic acid.

8. The paper-like polyester fiber sheet as claimed in claim 1 or 2, wherein the polyester short fibers are mixed with an additional short-forming material consisting of at least one member selected from water-insolubilized polyvinyl alcohol fibers, nylon 6 fibers, nylon 66 fibers, viscose rayon fibers, cupra rayon fibers, glass fibers, asbesto fibers, and wood pulp.