EP0399053B1

EP0399053B1 - Polyester composite mono-filament for screen gauze

Info

Publication number: EP0399053B1
Application number: EP89913175A
Authority: EP
Inventors: Yoshimitsu Itou; Mototada Fukuhara; Akira Kishiro
Original assignee: Toray Industries Inc
Current assignee: Toray Industries Inc
Priority date: 1988-12-05
Filing date: 1989-12-04
Publication date: 1996-06-05
Anticipated expiration: 2009-12-04
Also published as: WO1990006384A1; EP0399053A4; DE68926617D1; KR910700366A; EP0399053A1; ATE138983T1; KR950007817B1; DE68926617T2

Abstract

A polyester composite monofilament for screen gauze comprises (a) a sheath part made of a polyester having a glass transition temp (Tg) of 35-73 deg C, and (b) a core part made of a polyester having a higher Tg surface area ratio of (b) to (a) is 70:30-95:5. The strength at break of the monofilament is at least 6g/d and its modulus at elongation of 10% is at elast 3.5 g/d. Pref both sheath and core comprise polyester copolymer formed mainly of polyethylene terephthalate (PET). Pref the sheath component is made by copolymerising at least one of dimer acid, adipic acid, sebacic acid and a cpd of formula R10(CnH2no)mR2 (I) with polyethylene terephthalate. In (I), R1 and R2 = 1-4C alkyl, n = 2-5 and m = 2-250. The monofilament has a concentric core/sheath structure, has a denier less than 9 or it may have at least 5 cores, each core having a strength of at 3d.

Description

The present invention relates to a polyester composite monofilament suitable for a mesh fabric used for screen printing. In more detail, the present invention relates to a polyester composite monofilament suitable for obtaining a bolting cloth with high mesh and high modulus used in fields wherein high accuracy is required such as electronic circuits.
As bolting cloths for printing, in the past, mesh fabrics consisting of a natural fiber such as silk or an inorganic fiber such as stainless steel fiber have been widely used. Recently, mesh fabrics namely bolting cloths consisting of nylon and polyester having flexibility, durability and dimensional stability, have been used in many cases. Above all, bolting cloths consisting of polyester monofilaments have been widely used as they are little influenced by water and are inexpensive in comparison with nylon .
However, in the field of printing of electronic circuits, there has been a requirement, year by year, for greater accuracy.
Up to the present, polyester monofilaments with low elongation have caused a large amount of scum, so that it has been recognized that weaving is impossible by using such monofilaments with low elongation. However, requirements by the printing industry have become more severe than before and a mesh fabric with a fine denier and a high mesh, namely, with a high weave density, is required. Tension applied on a filament during weaving is not necessarily proportional to its denier and a high tenacity per monofilament is needed. The thinner the denier is, a product with higher breaking strength is needed. Therefore, to improve printing accuracy, it is necessary to have a bolting cloth with high strength, high modulus and fine mesh.
Generally speaking, to produce a polyester filament with high strength and high modulus, the draw ratio in the manufacturing process of the original fiber is raised to a high level. Thus the obtained filament has a highly oriented and highly crystallized structure. However, in the manufacturing process for producing the bolting cloth, a fabric with an extremely high density is woven at a high speed. Therefore, the warp filament is repeatedly exposed to a strong friction with a reed etc., and part of the surface of the filament is thereby scraped and beardlike or powdery scum is easily generated. In particular, as orientation and crystallization become higher, this tendency is enhanced and as a result, it becomes necessary to stop weaving temporarily and to clean the weaving machine. This action not only impairs productivity but also causes unevenness of weaving at this part, which results in a defect in the product. Even when it is not necessary to clean, part of the produced scum is woven into the fabrics and this results in a defect in printing, where precise printing is required. It is therefore an extremely important object of this invention for the original filament with high strength to prevent scum from generation.
Up to now, a number of technologies for improvement have been proposed to reduce scum. For example, in JP-A-16948/1980, it was proposed that a high elongation original filament with a breaking elongation of 38-60% was used as a warp. However, using a high elongation filament means that the draw ratio is set lower at the manufacturing process of the original filament and it becomes inevitably difficult to obtain a fabric with a high modulus as the final product. In other words, in this conventional technology, such characteristics as high strength and high modulus are sacrificed to prevent scum from generation. Therefore, according to the above mentioned document, if the denier is reduced in an attempt to obtain a higher mesh fabric, tenacity becomes insufficient and weaving processability is remarkably impaired. and as a result, it is difficult to obtain a bolting cloth with a high mesh for precise printing. As a result, this presents a problem to be solved, namely how a high modulus filament can be woven while occurrence of scum is suppressed.
In addition, JP-A-276048/1987 and its family corresponding member EP-A-0311687 (date of filing: 10.08.87; published on: 19.04.89; priority date: 17.02.87) disclose a composite filament wherein a polymer such as nylon having good adhesiveness with emulsions and resins for pattern making and being durable to scraping is used as a sheath. Development of scum can be prevented in case of nylon in comparison with polyester, because of its toughness for scraping, but there then exists a defect that nylon exhibits higher moisture absorption and poor dimensional stability. In a process for achieving precise printing, dimensional stability after a fabric is set and fixed on a frame and during storage time, plate making (forming of a printing pattern) and during the printing procedure, is extremely important. If nylon is used, even if it is a part of a composite filament, it is easily influenced by temperature and humidity and the tension imparted to the fabric tends to be relaxed. In particular, if the atmosphere in the workshop is not strictly controlled, the influence becomes larger. Therefore, usingthis technology, weaving characteristics can be improved but accuracy of precise printing cannot be improved.
In addition, in JP-A-276048/1987 and EP-A-0311687, there is also a general reference to the use of a polyester with a low viscosity as a sheath component, but no practical example is described. Indeed, a polyester with a low viscosity is not generally used as a raw material for a monofilament for a bolting cloth. A polyester with a low viscosity generally means a polyester with a low degree of polymerization and which is easily crystallized by heat during drawing. Monofilaments thus obtained exhibit poor toughness and are brittle. Therefore, when a sheath-core type composite monofilament wherein a polyester with a low viscosity is used as a sheath, the polyester with a low viscosity is scraped by friction during weaving ofafabric and scum is produced. Therefore, the sheath-core type composite monofilament wherein a polyester with a low viscosity is used as the sheath component is not suitable for bolting cloth for screen printing.
The present invention seeks to provide a polyester composite monofilament suitable for precise printing, with good dimensional stability, good weaving characteristics and reduced scum.
In particular, the present invention seeks to provide a polyester composite monofilament with high tenacity and high modulus suitable for fine denier and high mesh fabrics.
This invention provides a polyester composite monofilament for bolting cloth for screen printing, which composite monofilament consists of a core of a first polyester component and a sheath of a second polyester component, in which composite monofilament

(a) the first, polyester core, component consists of polyethylene terephthalate and the second, polyester sheath, component, is a copolyester a main part of which is polyethylene terephthalate;
(b) the first, polyester core, component has a glass transition temperature of at least 78°C and the second, polyester sheath, component has a glass transition temperature of 45-65°C;
(c) the area ratio of the core to the sheath is in the range 70:30-95:5; and
(d) the breaking strength of the monofilament is 6 g/d or higher and the modulus at 10% elongation thereof is 3.5 g/d or higher.

A feature of the polyester composite monofilament for a bolting cloth of the present invention is high tenacity per monofilament against tension applied in the weaving process.
The tenacity which particular monofilaments require depends upon the type of a weaving machine to be used and the number of rotations during weaving and when the tenacity is about 50 g/filament or higher, problems during weaving are, in practice, solved. Thus, if the breaking strength is 6 g/d or higher, a monofilament with a denier of about 9 d can be provided and furthermore, if it is 7 g/d or higher, it becomes possible to lower the denier down to 7 d to make thinner monofilaments. Therefore, it is necessary to achieve a breaking strength of the monofilament of 6 g/d and preferably 7 g/d or higher. The higher the breaking strength of the monofilament the better is the result. Satisfactory results can be achieved with a monofilament having a polyethylene terephthalate as the core component and a breaking strength of about 10 g/d or lower.
On the other hand, even if a high mesh fabric consisting of a high tenacity monofilament is used, if it is deformed by stress applied by squeezing during printing, a high printing accuracy cannot be maintained. For this purpose, a monofilament with a high modulus is needed. The modulus of the monofilament means the stress produced in the filament at 10% elongation. Namely, it is a value obtained by dividing tenacity at 10% elongation in the S-S curve of the original filament by its denier. It is necessary to provide the monofilament with a modulus 3.5 g/d or larger and more preferably, 3.8 g/d or larger. The higher the modulus of the monofilament, the better is the result. Satisfactory results can be achieved with a monofilament having a polyethylene terephthalate as the core component and a modulus of about 10 g/d or lower.
To achieve such a high breaking strength and high modulus, it is essential for the original filament to be drawn at high draw ratio during the manufacturing process. As a result, a filament with a low elongation is obtained. However, by applying the technology of the present invention, it becomes possible to perform a sufficiently acceptable weaving even when using a filament having an elongation so low that to date it had been impossible to weave such a filament. In particular, it is now possible to weave a monofilament having an elongation lower than 33% . Of course, it is not preferable, having regard to easy handling of the filament, that the elongation be extremely low and an elongation of 10% or larger is preferable and 15% or larger is more preferable.
On the other hand, if one attempts to achieve high breaking strength and high modulus for a polyester, development of scum during weaving is accordingly enhanced. This is because, in polyester fibers highly oriented and crystallized by drawing, the strength increases in the direction along the fiber axis. As a result, on the contrary the fiber becomes accordingly brittle and weak against bending, shearing and scraping. However, since high strength and high modulus are required for the final product, the fiber itself should have such mechanical characteristics. Under these conditions, a most important object of the present invention is to prevent development of scum.
Thus the present invention seeks to achieve good weaving properties for a monofilament with such a high breaking strength, high modulus and low elongation. Using the present invention, this problem has been solved by providing a monofilament of the sheath-core type and having, furthermore, the features recited in claim 1.
We found from our investigations that a copolyester with a low Tg hardly produces scum. Generally speaking, in a monofilament having a low Tg the mobility of molecular chains in amorphous parts is high and the polymer is regarded as assuming a rubbery state, not a glassy state. Namely, the polymer is soft and is hardly scraped by friction.
If suppression of such scum were the only desideratum, then the lower the Tg, the better is the effect. On the other hand, when Tg is close to room temperature, dimensional stability during application is decreased and it is not suitable for application in the field of precise printing, which is also desired. Therefore, in a polyester composite monofilament of the present invention, the Tg of the polyester sheath suitable for precise printing should be 45 - 65°C.
A polymer having such a Tg can be obtained by copolymerizing a crystalline polyester, in particular a polyester having polyethylene terephthalate as a main component, with a monomer increasing flexibility of the molecular chain or a monomer or a polymer with a relatively low molecular weight hardly producing steric hindrance. For example, suitable copolymers are dicarboxylic acids such as isophthalic acid, adipic acid, dimer acid and sebacic acid, a compound of a general formula R₁O(C_nH_2nO)_mR₂ (wherein R₁ and R₂ are each H or an alkyl group having 1 to 4 carbon atoms; n is an integer of 2-5; m is an integer of 2-250), for example, low molecular weight glycols such as diethylene glycol, butane diol and neopentyl glycol, polyalkylene glycols such as polyethylene glycol and polytetramethylene glycol.
The amount of copolymerized moiety is not indiscriminately determined but depends upon the type of selected copolymer. It should be appropriately chosen in such a way that the Tg of the polymer thus obtained is 45 - 65°C.
In addition, generally speaking, a polymer with a low degree of polymerization (a low viscosity) is easily crystallized by heating in comparison with a polymer with a high degree of polymerization (a high viscosity), exhibits poor toughness and a tendency to become brittle. Therefore, as the sheath component of a monofilament of the present invention, it is preferable that a polymer with a high degree of polymerization with an intrinsic viscosity [η] of 0.60 or larger and which is spinnable is used.
A copolyester with a low Tg is generally soft, but on the contrary, it is difficult to achieve a higher breaking strength, modulus and elongation when using such a copolyester and therefore, it is impossible to obtain a filament satisfying the mechanical characteristics of the monofilament of the present invention as it is. In the present invention, this particular problem can be solved by employing a composite structure wherein a polymer with a lower Tg is used for only the sheath component. In other words, the polymer scraped by friction against a reed and a guide bar is the polymer on the surface. Therefore, it suffices to use a polymer with a low Tg on the surface. Mechanical characteristics such as breaking strength, modulus and elongation should be provided by the polymer forming the core.
Based on such a conception, it is necessary to set the ratio of the amount of core component to that of the sheath component at a relatively high level and as a result, it is necessary that the area ratio of the core to the sheath is at least 70:30, or higher, and is preferably 80:20 or more.
In a monofilament of the present invention, both the core and the sheath are made of polyesters, so peeling at the interface of the composite scarcely occurs. However, if the sheath becomes too thin,a so-called composite disarrangement such as to expose the core polymer on part of the fiber surface tends to occurs. This may result in a decrease in the effect of suppressing the development of scum and therefore, the maximum area ratio should be 95:5.
In other words, if the area ratio is in the range of 70:30 - 95:5, a good composite arrangement could be ensured and suppression of the development of scum is possible. It is preferable that the thickness of the sheath is at most 5µ. or thinner. As the polyester constituting the core of the present invention, taking the cost etc. into consideration, polyethylene terephthalates, used in larger quantities in clothing and industrial applications, are used.
In any case, the Tg of such a core polyester is higher than that of the polymer constituting the sheath and in practice, the Tg of the core polyester is 78°C or higher. Within this range, if necessary, a third component can be incorporated or copolymerized.
In addition, the following structure can be used as the polyester composite monofilament of the present invention.
One preferred morphology is a coaxial cylinder structure which in cross-section provides a concentric circle wherein the centre of the core and the centreofthe sheath coincide. It is not preferable that if the core is eccentric, crimps or curls are produced caused by a little difference in thermal and mechanical characteristics of polymers constituting the core and the sheath.
Another preferred morphology is an islands-in-a-sea type structure wherein a plurality of cores (islands) are surrounded with a sheath (a sea). The advantage of such an islands-in-a-sea type structure is exhibited when a polyethylene terephthalate with higher degree of polymerization, for example, ; an intrinsic. viscosity [η] of 0.75 or higher or a liquid crystalline polyester, which easily provides a high modulus filament as cores is used. These polyesters provides high strength and high modulus, but on the other hand, they lack flexibility and hardly follow the bent structure of warp and weft when a high mesh fabric is prepared.
To solve this problem, the denier of the constituted filament is made thinner. Those fibers with thin denier are flexible even if their moduli are high and easily follow the deformation during weaving. However, if a simple assembled body of multi-filaments is used, the fiber bundle becomes flat in a fabric and it is difficult to ensure a large dimension of the opening when a screen fabric is made of such fabric. As a result, it has become necessary to give a twist to the filament for improving the weaving characteristics and this results in making the process more complex and giving an insufficient uniformity of opening. Therefore, in this case, it is preferable to prepare a structure wherein a plurality of cores are surrounded with a sea.
To realize effectively this purpose, the denier of a single filament constituting an island is preferably 3 d or thinner, more preferably 1 d or thinner and it is preferable that thinner filaments are more flexible. It is necessary that the number of filaments constituting the islands is at least 5 or larger. If the number of islands becomes small, it becomes difficult to provide a sufficiently high ratio of the island component to the sea component as described before. Moreover, this results in a large sea volume being necessary in order to realize a uniform composite and in this case the features of high strength and high modulus are hardly exhibited. It is possible to have 100 or more islands but too high a number only results in making the spinning nozzle too complicated and up to about several dozens should be enough for achieving the purpose of the present invention.
It is preferable that the crosssectional shape of the monofilament should be round. This is because, if the shape is a deformed crosssection, halation occurs when a photosensitive emulsion is cured and this has a bad influence on printing accuracy in some cases. Another reason is that in comparison with a round crosssection filament, a filament with a deformed crosssection exhibits poor straightness and does not provide a screen with a particular uniform opening.
In practice, a composite monofilament of the present invention can be obtained by any conventional well-known composite spinning method. Thus, polymers each forming the core and the sheath are independently melted and metered and both are joined together at the rear face of a spinneret so as to form a sheath-core structure and extruded from the same nozzle to obtain the composite monofilament.
Practical methods for obtaining an islands-in-a-sea type composite filament are, for example, a conventionally known spinning method for preparing a laterally-arranged polymer disclosed in Japanese Patent Publication No. 26723/1972. In this method, the islands-in-a-sea type composite filament is obtained by melting and metering separately two polymers constituting respectively a sea component and an island component, feeding them respectively into the same spinning pack, forming independent sheath-core composite flows at a first stage, joining together each sheath-core flow at a second stage and extruding a multi-core islands-in-a-sea type composite flow from a nozzle. In any event, it is preferable that a plurality of ultrafine filaments each independently form an island and a structure wherein the islands are surrounded by a polymer forming a sea is formed. In the present invention, since it is not the purpose to dissolve the sea component and to obtain the bundle of the ultra-fine filaments, it is allowable for the islands to be partly joined together a little. However, too many joinings of flows reduces the effect of making the filament ultrafine and the filament lacks flexibility. Joinings of flows should be avoided as much as possible by taking the design of the nozzle into consideration in accordance with the viscosity of the polymer to be used.
Best embodiments of the invention will now be explained in more detail with reference to practical Examples hereinbelow.
Evaluations in these Examples were performed by the following methods.

Tg:

10 mg of a powder of the polymer were sampled. The measurement was performed using a differential scanning calorimeter (manufactured by Perkin-Elmer Co., Ltd: Type DSC-4) while the temperature was elevated at a speed of 16°C/min. By a peak obtained in the course of temperature elevation, the glass transition temperature Tg (°C) was obtained using the data treating system of Perkin-Elmer Co., Ltd.

Evaluation of scum:

A mesh fabric was woven by means of a Sulzer type weaving machine, the number of rotations of the weaving machine being 350 rpm. Observing the degree of contamination of a reed, when it was judged that continuous weaving became impossible, the weaving machine was stopped and the reed was washed. The weaving period up to this time was defined as a reed washing cycle (m). The shorter this washing cycle was, the more was the development of scum.

Modulus of a fabric:

A tenacity-elongation curve of a fabric was obtained by means of the labeled strip method described in JIS L1068-1964 using a tensile tester with a constant speed stretching under a testing condition with a test sample width of 5 cm, a clamped distance of 20 cm and an extension speed of 20 cm/min and the tenacity (kg) at 10% elongation was defined as the modulus of the fabric.

Example 1

A polyethylene terephthalate (A) with an intrinsic viscosity [η] of 0.80 was prepared as a core component of a composite monofilament. Tg of this polymer was 79°C. On the other hand, as a sheath component, a polymer (B) with an intrinsic viscosity [η] of 0.67 made by incorporating 8 wt.% of polyethylene glycol with a molecular weight of 1,000 when a polyethylene terephthalate was polymerized, was prepared. Tg of this polymer was 58°C.
A composite monofilament wherein the polymer A was the core and the polymer B was the sheath was spun at 1,000 m/min by means of a conventional well-known composite spinning method at a spinning temperature of 295°C. In this case, the composite area ratio of the core to the sheath was 90:10. Drawing of the obtained undrawn monofilament was performed at various draw ratios by using a pair of hot rolls each heated at 90°C and 140°C to obtain drawn monofilaments with a denier of 8 denier. The monofilaments were woven, finished and treated to obtain high mesh bolting cloth with a mesh of 300.
Drawing conditions, characteristics of the original filaments and the results of evaluation on the obtained fabric are shown in Table 1.
In experiment No.1, as the breaking strength of the monofilament was low, filament breakages occurred during weaving. In experiment No.2, as the modulus of the monofilament was low, a fabric with a low modulus was obtained and it exhibited bad printing accuracy. In experiment Nos.3, 4 and 5, the monofilaments exhibited high strength and high modulus, and yet development of scum was only little. High modulus and high mesh bolting cloth were obtained and it was possible to perform highly precise printings with a line width of 80 µm using the cloth.

Example 2

According to the method of Example 1, sheath-core composite filaments were obtained with a monofilament denier of 7 denier. A polyethylene terephthalate (A) with an intrinsic viscosity of 0.75 was used as the core component and copolyesters each with a different Tg were respectively used as the sheath components and the core-sheath area ratios were different from each other.
High mesh fabric with a mesh of 360 mesh were obtained by weaving the monofilaments, finishing and treating them. The results of evaluation on scum on each composite filament are shown in Table 2. In this table, the dimensional stability was determined by observing the degree of distortion of a printed pattern after printing on 3,000 pieces had been completed and by marking less distortion as good and more distortion as poor.
In experiment No.6, as the content of the copolymerized component was small, Tg of the copolyester was high and scum was developed during weaving. In experiment No.10, as the Tg was too low, dimensional change during printing was large and distortionofthe printed pattern of the printed matter was large. As a result, the bolting cloth was found to provide bad printing accuracy. In experiment No.13, as a copolymerized component with a high Tg was used, Tg of the copolyester was accordingly high and scum was generated during weaving. In experiment No.14, as the area ratio of the core component was small, the filament had low elongation and scum was generated. In experiment No.15, as the area ratio of sheath to core component was small, the effect of suppressing the development of scum became little and scum was generated. Experiments Nos. 7, 9 and 11 are not examples embodying the invention, since the Tg of the copolyesters used for the sheath is outside of the range 45-65°C given in claim 1. Experiments Nos. 8 and 12 were examples embodying the present invention and even though the original filaments exhibited high strength and high modulus, no scum was generated. The obtained high mesh fabric were plain gauzes with high modulus and highly precise printing with a line width of 70 µm was possible.

Example 3, Comparative Example 1

Properties of a plain gauze woven with a composite monofilament with a sheath made of nylon and a core made of polyester (Comparative Example 1) and those of a plain gauze woven with a polyester composite monofilament of Experiment No.4 in Example 1 were compared with each other.
A composite monofilament with a denier of 8 denier was obtained in accordance with Example 1 using a nylon polymer with [η] = 1.2 and Tg = 41°C as the sheath and a polyethylene terephthalate with an intrinsic viscosity [η] = 0.80 and Tg = 79°C at the core. Strength, elongation and modulus of the obtained filament were 6.5 g/d, 32% and 3.5 g/d respectively. In the same way as the filament of Experiment No.4 in Example 1, bolting cloth of 330 mesh was prepared. Both samples exhibited good weaving properties.

The fabrics were each set on a making-up frame with a size of 44 cm and tensions of the fabrics were measured with time. The atmosphere was kept normal, ie at 20°C and 65% RH and after 7 days, they were exposed in an high temperature and high humidity atmosphere such as 40°C and 80% RH and stabilities of tensions of the fabrics were evaluated. Tensions of the fabrics were measured in terms of N/cm by using TENSION METER-40D manufactured by HINRICH MANTEL Co., Ltd. The results are shown in Table 3.

Table 3

	Tension of the fabric (N/cm)
	Example 3	Comparative Example 1
At the beginning when the fabric was set	35.2	35.0
1 days	32.6	31.0
3 days	32.0	30.5
7 days	32.0	30.0
7 days (40°C, 80%)	31.8	28.0

The fabric made of the composite monofilament wherein nylon was the sheath component (Comparative Example 1) exhibited large initial change and large dependence on temperature and moisture, which showed that it was unstable. On the other hand, the fabric using the composite monofilament of the present invention (Example 3) exhibited small initial change and small dependence on temperature and moisture, which showed that it was stable.

Example 4

A polyethylene terephthalate with an intrinsic viscosity [η] of 0.8 was prepared by an ordinary method as an island component of a composite monofilament. Tg of this polymer was 79°C. On the other hand, as a sea component, a polymer with an intrinsic viscosity [η] of 0.64 made by incorporating 10 wt.% of polyethylene glycol with a molecular weight of 1,000 when a polyethylene terephthalate was polymerized, was prepared. Tg of this polymer was 56°C.
Composite spinning was performed by means of a well known method for obtaining a laterally-arranged polymer to obtain a monofilament with a ratio of the sea to the island of 10:90, having 16 independent islands and with a total denier of 10 denier. Strength, modulus at 10% elongation and elongation of this monofilament were 6.5 g/d, 5.3 g/d and 32% respectively. This monofilament showed considerable flexibility. A fabric with 315 mesh was woven and according to the result of evaluation of scum, even though this was a high strength polyester monofilament, the washing cycle of the reed was 1,000 m. When continuous weaving was performed, no washing of the weaving machine was necessary for a long time and a bolting cloth with excellent quality was obtained efficiently.

Comparative Example 2

Using the polyethylene terephthalate used in Example 4 for the island component with an intrinsic viscosity [η] = 0.8 a monofilament with a denier of 10 was prepared by a conventional method. The strength the filament was 6.5 g/d, this being achieved by adjusting spinning and drawing conditions so as to be able to compare directly with Example 4. The obtained monofilament had a modulus at 10% elongation of 5.5 g/d and an elongation of 33% respectively. Even though both monofilaments had the same denier, the monofilament of Example 4 exhibited a hard feeling. Evaluation of scum showed that a large amount of scum was accumulated only after 80 m and continuous weaving was impossible.

Example 5

To compare Tg of the sea component with the development of scum, islands-in-a-sea type monofilaments were obtained using polymers shown in Table 4 as sea components in accordance with Example 4. (Experiment No. C is the only one in accordance with the invention, since Tg is within the claimed range 45-65°C.) The composite ratio in this Example was 15:85 and the monofilaments consisted of 24 islands.
Characteristics and evaluation results on the obtained filaments are shown in Table 4. In addition, dimensional stability shown in Table 4 was obtained by comparing printing accuracy after printing 1,000 times.
In experiments A and H, as Tg of the copolymer used as the sea component was too high, a larger amount of scum was produced in evaluation of scum and the reed washing cycle was extremely short and stable weaving for a long time was difficult. In experiment E, as Tg of the copolymer was too low, dimensional stability of the fabric obtained was poor and the printing accuracy was no good. Experiments B, C, D, F and G were high strength and high modulus polyester monofilaments embodying the present invention, which exhibited along reed washing cycle during weaving and the obtained fabrics had high strengths and high moduli and excellent dimensional stabilities.

Example 6

A polymer copolymerized with adipic acid with Tg of 63°C and an intrinsic viscosity [η] = 0.67 was prepared. A composite monofilament was obtained in accordance with Example 1. In this case, for comparison, the composite ratio of the sea to the island and the number of island components were changed as shown in Table 5. The obtained results are shown in Table 5.
In addition, dimensional stability shown in the Table was obtained by comparing printing accuracy after printing 1,000 times.
In Experiment I, the strength of the obtained monofilament was low and filament breakage occurred during weaving. In Experiment J, as the strength was low, when the fabric was set under stretching, it was easily broken. In Experiment M, as the ratio of the sea was 3, which was too small, no suppression effect on occurrence of scum existed and the reed washing cycle was accordingly short. In Experimment O, as the area ratio of the sea component was 35, which was too large, the strength of the obtained monofilament was low and filament breakage occurred during weaving and it was impossible to weave a fabric.
Experiment K, L and N were high strength and high modulus polyester monofilaments embodying the present ivnention. These monofilaments exhibited little generation of scum during weaving and the obtained fabrics had high strengths and high moduli and excellent dimensional stabilities.
In a polyester composite monofilament embodying the present invention, by using on the one hand a copolyester which does not generate scum, has a low glass transition point and is soft as the sheath component suppressing development of scum and on the other hand a polyester exhibiting enhanced mechanical characteristics of the monofilament as the core component, even though the monofilament provides high strength, high modulus and low elongation, the problem of development of scum during weaving can be solved. As a result, a bolting cloth consisting of a monofilament, with a fine denier and having a high mesh, a high tenacity and a high modulus can be obtained and it is possible to perform with good accurracy a highly precise printing without dimensional change during printing and with a line width of 100 µm or smaller.
Note: To convert g/d to cN/tex, multiply by 8.826;
to convert d to tex, multiply by 0.1111.

Claims

A polyester composite monofilament for bolting cloth for screen printing, which composite monofilament consists of a core of a first polyester component and a sheath of a second polyester component, in which composite monofilament
(a) the first, polyester core, component consists of polyethylene terephthalate and the second, polyester sheath, component, is a copolyester a main part of which is polyethylene terephthalate;

(b) the first, polyester core, component has a glass transition temperature of at least 78°C and the second, polyester sheath, component has a glass transition temperature of 45-65°C;

(c) the area ratio of the core to the sheath is in the range 70:30-95:5; and

(d) the breaking strength of the monofilament is 52.956 cN/tex (6 g/d) or higher and the modulus at 10% elongation thereof is 30.891 cN/tex (3. 5 g/d) or higher.
A polyester composite monofilament according to Claim 1, which is substantially a coaxial cylinder type sheath-core structure.
A polyester composite monofilament according to Claim 1 or Claim 2, wherein the sheath component is a copolyester of polyethylene terephthalate with at least one monomer selected from dimer acid, adipic acid, sebacic acid and a compound of the following general formula

R₁O(C_nH_2nO)_mR₂

wherein R₁ and R₂ are each selected from
H or an alkyl group having 1 to 4 carbon atoms,

n is an integer of 2 - 5,

m is an integer of 2 - 250.
A polyester composite monofilament according to any preceding Claim, which has a denier less than 0.9999 tex (9 denier)
A polyester composite monofilament according to Claim 1, 3 or 4, having 5 or more cores, each individual core having a denier of 0.3333 tex (3 d) or smaller.