Polyester mesh for screen printing and method of preparation thereof

Abstract

 An improved polyester mesh for screen printing is given. The polyester mesh is composed of polyester fibers having conca­vities and protrusions having a diameter of 0.01 to 0.1 µm in a density of at least 200 per µm² of the surface area thereof. A more preferred polyester mesh for screen printing has finely dispersed concavities and protrusions having a depth or height not exceeding 0.05 µm. Such a polyester mesh is obtained by subjecting a base polyester mesh to a treatment with low-temperature plasma in an atmosphere of a non-oxidizing inorganic gas. An improvement for printing screens is proposed in which a surface active agent is ad­ded to the photosensitive resin.

Description

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a polyester mesh for screen printing or, more particularly, to a polyester mesh for screen print­ing having increased cohesiveness to the layer of photosensitive resin and not accompanied by decrease of strength. The invention also relates to a method of preparation thereof.

[0002] Polyester meshes for screen printing have generally low co­hesiveness to the layer of the photosensitive resin and some times cause inconvenience in several respects such as faling off of the photosensitive resin layer, so that the precision in the printed im­ages and durability of the mesh screen in the prior art are not quite satisfactory.

[0003] A sufficient effect of improving the cohesiveness of polyester meshes to the resin layer is not obtained by the conventional re­medial methods including various chemical treatments and flame processing because of the accompanying rather adverse effects of, e.g. decrease of the strength or extensibility which tends to induce bursting of the mesh in the course of spreading on the frame or printing. At any rate, no methods of substantial improvement of the polyester meshes for screen printing have been established here­tofore.

[0004] Meshes for screen printing in the prior art are sometimes subjected to a treatment using a surface active agent to be impart­ed with antistatic property with an object of preventing blur of the printing ink due to the static electricity but the treatment is usu­ally accompanied by another disadvantage of decrease in the cohe­siveness to the layer of the photosensitive resin.

SUMMARY OF THE INVENTION

[0005] The present invention provides a polyester mesh for screen printing composed of polyester fibers having microscopical conca­vities and protrusions each having a diameter in the range from 0.01 µm to 0.1 µm on the surface in a density of at least 200 per µm² of the surface area. The concavities and protrusions should preferably have a depth or height snot exceeding 0.05 µm with a preferred proviso that the density thereof is at least 250 per µm².

[0006] The invention also provides a method for the preparation of such a polyester mesh comprising a step of low-temperature plas­ma treatment of the base mesh. The low-temperature plasma treatment is carried out preferably in an atmosphere of an inorga­nic gas or gases comprising at least 50% by volume of a non-oxid­izing inorganic gas such as helium, neon, argon, hydrogen and nitrogen under a pressure in the range from 0.005 torr to 5 torr by impressing a high frequency electric power between the electrodes with a power density not exceeding 150 kW/m² of the surface area of the power electrode, i.e. non-grounded electrode.

[0007] The polyester mesh of the invention has an advantage of improved cohesiveness to the photosensitive resin to give greatly upgraded workability and durability in the printing work and, in particular, prevention of blur of the printing ink due to the static electricity by adding a surface active agent to the photosensitive resin without being accompanied by the drawback of decrease in the cohesiveness to the polyester meshes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The inventors have completed the present invention as a re­sult of the extensive studies to solve all of the above described problems in polyester meshes for screen printing by establishing a method of forming an uneven or roughened surface conforming with the specific requirement by the treatment of the polyester meshes with low temperature plasma.

[0009] That is, the present invention provides a polyester mesh for screen printing composed of polyester fibers having an uneven sur­face having microscopically fine concavities and protrusions each having a dimension of the diameter in the range from 0.01 to 0.1 µm in a density of at least 200 per µm² formed by the treatment of the polyester mesh with low temperature plasma.

[0010] The polyester mesh for screen printing is fabricated by weav­ing polyester filaments having a diameter from 20 to 100 µm to gossamers of 50 to 500 meshes per inch and finished up by means of refining, heat-setting and the like. Then, the mesh is treated, prior to coating with a photosensitive resin, by exposing to an at­mosphere of low temperature plasma under specified conditions.

[0011] The apparatus for generating low temperature plasma used in the invention is most preferably of the internal electrode type but in some cases external electrode type apparatuses may also be suitably used. Furthermore, coil-type apparatuses of both the ca­pacitive coupling and inductive coupling are also suitable. Be­sides such a wide possibility in the selection of the types of the plasma treatment apparatus, however, care must be taken to avoid thermal deterioration of the surface of the article under treatment by the heat of electric discharge.

[0012] As is mentioned in the above, the method of plasma treat­ment in the invention is preferably carried out using an apparatus of the internal electrode type though with no particular limita­tions in the form of the electrodes. For example, the powert and grounded elctrodes may not be of the same form but these elec­trodes may be formed in any different shapes including the forms of plate, ring, rod, cylinder and the like. Furthermore, an appa­ratus having a grounded inner wall made of a metal as the coun­terelectrode may also be used for the plasma treatment in the invention. The power electrodes are usually made of a metal such as copper, iron, aluminum and the like and preferably they should have an insulating coating of glass, porcelain, ceramic and the like having a high dielectric strength of, e.g., 10,000 volts or higher in order to ensure stability of the electric discharge. In particular, a rod-formed electrode having an insulating coating is suitable in order to obtain localized generation of low temperature plasma effectively.

[0013] The electric power impressed between the electrodes for dis­charge should be in a frequency band in the range of low-frequency waves, microwaves, direct current and the like in addition to high-­frequency waves which are the most preferable. The frequency of the high-frequency electric power should preferably be in the range from a few kHz to several hundreds MHz.

[0014] The electric power impressed between the electrodes should preferably be controlled not to exceed 150 kW/m² of the power den­sity on the surface of the power electrode in order to avoid de­crease in the strength of the mesh by the thermal decomposition or deterioration due to the heat generated during the plasma treat­ment by impressing an excessively large electric power.

[0015] The atmospheric gas in the plasma treatment apparatus is selected from oxidizing and non-oxiding inorganic gases. Typical examples of the non-oxidizing inorganic gas are helium, neon, argon, hydrogen and nitrogen, and they are used alone or as a mixture. The content of an oxidizing inorganic gas such as oxygen and air should preferably be controlled not to exceed 50% by volume in the plasma atmosphere in consideration of preventing decrease in the strength of meshes. The low temperature plasma treratment is carried out by passing the inorganic gas or gaseous mixture through the plasma chamber suitble for evacuation in which the meshes are held and exposed to low temperature plasma generated by impressing an electric power between the electrodes. The gaseous pressure in the plasma treatment chamber should preferably be kept in the range from 0.005 to 5 torr or, more preferably, from 0.01 to 1 torr since the meshes are susceptible to surface denaturation by the heat or excessive etching by the ano­malous discharge to cause decrease of the mechanical strength thereof when the gaseous pressure in the plasma chamber is out­side the above mentioned preferred range. The exposure time to the plasma atmosphere should preferably be controlled within 100 seconds since the meshes tend to undergo an excessive etching or surface denaturation when the exposure time is extended beyond the above mentioned upper limit.

[0016] The polyester meshes processed with the low temperature plasma under the above-specified conditions have an uneven or roughened surface with microscopically tiny concavities and pro­trusions. The diameter of each of the concavities and protrusions should have a dimension in the range from 0.01 to 0.1µm. The distribution density of such concavities and protrusions on the surfaces of polyester fibers should be at least 200 per µm² or, pre­ferably, at least 250 per µm² of the surface area. The depth or height of each of the concavities and protrusions is not limitative but usually concavities and protrusions having a depth or height of 0.05 µm or smaller are preferred. In such a case, a dense distri­bution of the concavities and protrusions on the surfaces of the polyester fibers is not detrimental provided that the density there­of is at least 250 per µm² of the surface area. When a substantial portion of the concavities and protrusions have a depth or height not smaller than 0.05 µm, the density of them on the surface of the polyester fibers should not preferably exceed 1000 per µm² or, more preferably, 700 per µm². Concavities and protrusions having a diameter exceeding 0.1 µm tend to cause decrease of extensi­bility or strength of the meshes. Similarly, concavities and protru­sions having a depth or height not smaller than 0.05 µm distri­buted in a density exceeding 1000 per µm² of the surface area cause the same kinds of disadvantages. Such polyester meshes would frequently be subject to bursting in the course of handling such as spreading or printing.

[0017] Polyester meshes having minute concavities and protrusions each having a depth or height of 0.05 µm or larger distributed in a density of 250 per µm² or higher exhibit particularly high cohe­siveness to the layer of the photosensitive resin and high wet­tability without the disadvantage of deterioration in the physical properties as described before. The wettability characteristic of the polyester meshes of the invention usually is quite satisfactory as is evidenced by the wettability index of at least 40 dyne/cm and they exhibit sufficient ink-permeability when they are used as a printing screen in the work of screen printing. Polyester meshes having a wettability index lower than 40 dyne/cm would not give such an advantage.

[0018] Printing screens prepared using a polyester mesh provided with minute concavities and protrusions having a depth or height smaller than 0,05 µm on the surfaces of the filaments thereof ex­hibit an excellent workability in printing such as precision of the printed images and the ink-releasability. The polyester meshes of the invention are laminated with a layer of a photosensitive resin by way of the direct method, the direct-indirect method, the indi­rect method or the like after being spread on a suitable screen frame. Patterned screens for printing work are prepared using the resin-coated screen by exposure to light followed by development. The printing screens are excellent in workability in printing due to the high degree of cohesiveness between the mesh and the layer of the photosensitive resin and have high durability in printing works because of the high solvent resistance. The printing screens of the invention provide printed matters with high precision and clearness in any field of printing works including the pattern printing, letter printing, nameplate printing, color printing and the like by virtue of the excellent wettability characteristics of the polyester meshes of the invention. Furthermore, the inventive polyester meshes for screen printing can be easily treated with a surface active agent to be rendered anti-static since the meshes do not suffer the decrease of the cohesiveness to the photosensitive resin by the treatment with a surface active agent. Therefore the meshes are promising as a material in other fields of use owing to the upgraded anti-static property.

[0019] In the following, examples are given to illustrate the inven­tion in more detail but not to limit the scope of the invention in any way.

Example 1.

[0020] A 300 mesh polyester gossamer mesh (Super Strong T300S, a product by Nippon Tokushu Orimono Co.) was set in a low-tem­perature plasma generating apparatus and the pressure inside was reduced to 0.005 torr by evacuation. Plasma treatment of the mesh was carried out for about one second by the impression of an electric power of 45 kW/m² at a frequency of 110 kHz between the electrodes while argon gas was passed through the plasma cham­ber and the gaseous pressure therein was controlled and kept at 0.06 torr. The surface of the mesh thus treated had a wettability index of 46 dyne/cm. The presence of 500 to 600 concavities and protrusions per µm² each having a diameter of 0.01 to 0.03 µm was observed in the electron microphotographs of the surface. The thus plasma-treated mesh was spread on a frame in a conventional manner and subjected to testing for strength to make a comparison with a mesh without the plasma treatment. The results were as shown in Table 1. Frames of 56 cm square were used for spreading the meshes and the tensioning condition for each of the specimens was 1.00 mm as measured using a tension gage (Model STG-75B, a product by Sun Giken Co.). The wettability index of the mesh be­fore the plasma treatment was also measured to give the value shown in Table 2.

[0021] Each of the meshes was then coated with a photosensitive resin (Encosol 2, a product by Naz-Dar Co.) to form a resin layer having a thickness of 12 µm and spread on the frame. Exposure to ultraviolet light was carried out for each screen using a high-pres­sure mercury lamp (a product of Oak Manufacturing Co.) for 3 and a half minutes to form a checkerboard pattern of 1500 squares of 0.2 mm by 0.2 mm each. A pressure-sensitive adhesive tape (Pacron Tape Y683, a product by Sumitomo MMM Co.) was applied and pressed on to the checkerboard pattern by hand and rubbed with a finger tip followed by quick peeling off. The peeling tests were repeated 3 times for each of the specimens and the number of the released squares was recorded to evaluate the cohesiveness of the meshes to the photosensitive resin to give the results shown in Table 2.

[0022] Test printing works were carried out using the thus obtained printing screens to prepare printed circuit boards in which the printing screen using the polyester mesh of the invention could withstand 8000 times of repeated printing while the printing screen without the plasma treatment could withstand only 2000 times of printing due to falling of the resin layer or other defici­encies.

[0023] Following are the details of the testing conditions.

(A) Strength test of the mesh spread on the frame

[0024] The mesh spread on the frame was pierced along the diago­nal line thereof with intervals of 3.5 cm using a bundle of five nee­dles each having a diameter of 0.56 mm to find whether or not bursting of the mesh took place.

(B) Testing methods for the tensile strength and extensibility

[0025] Testings were carried out according to the method specified in Japanese Industrial Standard JIS L 1069-79.

(C) Testing method for wettability characteristics

[0026] The mesh to be tested was coated with a series of mixtures of ethyleneglycol monoethyl ether and formamide (standard testing solutions for wettiability test, prepared by Wako Pure Chemicals Co.) with a prescribed value of the surface tension, and the highest value of the surface tension of the liquid capable of wetting the mesh was recorded as the wettability index. This method is in con­formity with Japanese Industrial Standard JIS K 6786.

Comparative Example 1.

[0027] A comparative test was carried out in the same manner as in Example 1 except that the atmospheric gas in the plasma generat­ing apparatus was oxygen under a pressure of 2 torr, the electric power was 85 kW/m² and the treatment time was 20 seconds. The strength data of the plasma-treated mesh are shown in Table 3. The presence of 50 to 80 concanvities and protrusions was observ­ed per µm² by the electron microscopic examination.

[0028] The printing screen prepared in the same manner as in Ex­ample 1 bursted already after 500 times of printing works for the preparation of printed circuit boards.

Example 2.

[0029] A 250 mesh polyester gossamer mesh (Super Strong T250T, a product by Nippon Tokushu Orimono Co.) was set in a low-tem­perature plasma generating apparatus and the pressure inside was reduced to 0.005 torr by evacuation. Plasma treatment of the mesh was carried out for about 5 seconds by impressing 40 kW/m² of elsctric power at a the frequency of 110 kHz between the elec­trodes while argon gas was passed through the plasma chamber and the gaseous pressure was controlled and kept at 0.05 torr. The surface of the mesh thus treated had a wettability index of 46 dyne/cm. The presence of 500 to 600 of concavities and protrusions per µm² each having a diameter of 0.01 to 0.05 µm and a depth or height of 0.01 to 0.02 µm was observed in the electron microphoto­graphs of the surface.

[0030] The thus treated mesh was spread on a frame in a conven­tional manner and subjected to testing of the strength making comparison with a mesh before the plasma treatment. The results were as shown in Table 4. The conditions in spreading were the same as in Example 1.

[0031] Similar peeling off tests using an adhesive tape were carried out as in Example 1 for the plasma-treated and untreated meshes to give the results shown in Table 5.

[0032] Test printing works were carried out using the above ob­tained printing screens to prepare printed circuit boards in which the printing screen using the polyester mesh of the invention could withstand 7000 times of repeated printing in the printing while the control without the plasma treatment could withstand only 2000 times of printing due to falling of the resin layer or other deficiencies. The testing methods and conditions were the same as in Example 1.

Comparative Example 2.

[0033] A comparative test was carried out in a similar manner to Example 2 except that the atmospheric gas in the plasma generat­ing apparatus was oxygen under a pressure of 0.5 torr, the electric power was 80 kW/m² and the treatment time was 120 seconds. The strength data of the treated mesh are shown in Table 6. The pres­ence of 10 to 20 concavities and protrusions per µm² each having a diameter of 0.2 to 0.3 µm and a depth or height of 0.06 to 0.08 µm was observed electron microscopically.

[0034] The printing screen prepared in the same manner as in Example 1 bursted already after 300 times of printing works for preparing printed circuit boards.

Example 3.

[0035] An experiment was carried out using a polyester mesh (Super Strong T300S) in the same manner as in Example 1 except that the atmospheric gas in the plasma treatment apparatus was helium under a pressure of 0.7 torr, the electric power was 40 kW/m² and the treatment time was 3 seconds. The surface of the mesh thus treated had a wettability index of 46 dyne/cm and the presence of 300 to 400 concavities and protrusions per µm² each having a diameter of 0.03 to 0.07 µm at the surface of the filament members thereof was observed electron microscopically.

[0036] Testings of the strength and cohesiveness were carried out for the plasma-treated polyester mesh similarly to Example 1 to give the results shown in Tables 7 and 8.

[0037] The printing screen prepared using the plasma-treated poly­ester mesh had a durability of 8000 times or more in the printing work to prepare printed circuit boards undertaken in the same manner as in Example 1.

Example 4.

[0038] An experiment was carried out using a polyester mesh (Super Strong T250T) in the same manner as in Example 1 except that the atmospheric gas in the plasma treatment apparatus was helium under a pressure of 0.7 torr, the electric power was 60 kW/m² and the treatment time was 5 seconds. The surface of the mesh thus plasma-treated had a wettability index of 46 dyne/cm and the presence of 500 to 600 concavities and protrusions per µm² each having a diameter of 0.03 to 0.05 µm and a depth or height of 0.01 to 0.3 µm at the surface of the filament members thereof was observed electron microscopically.

[0039] Testings of the strength and cohesiveness were carried out for the plasma-treated polyester mesh similarly to Example 1 to give the results shown in Tables 9 and 10.

[0040] The printing screen prepared using the plasma-treated poly­ester mesh had a durability of 8000 times or more in the printing test to prepare printed circuit boards undertaken in the same man­ner as in Example 1.

Example 5.

[0041] A low-temperature plasma treatment was carried out using a tanned polyester mesh (Super Strong T300S) in the same manner as in Example 1 except that the atmospheric gas in the plasma treatment apparatus was helium under a pressure of 0.1 torr, the electric power was 40kW/m² and the treatment time was 5 sec­onds. The surface of the mesh thus plasma-treated had a wetta­bility index of 46 dyne/cm and the presence of 300 to 350 conca­vities and protrusions per µm² each having a diameter of 0.01 to 0.03 µm on the surface of the filament members thereof was ob­served electron microscopically.

[0042] A printing screen was prepared using the plasma-treated mesh in the same manner as in Example 1 except that a cationic surface active agent of an aliphatic amine quaternary ammonium salt type (Fcall 70, a product by Matsumoto Yushi Seiyaku Co.) was added to the photosensitive resin (Encosol 2) in an amount of 1% by weight by kneading. Formation of a checkerboard pattern of 0.2 mm by 0.2 mm squares was carried out in the same manner as in Example 1.

[0043] The thus prepared printing screen was compared with two kinds of printing screens prepared in a similar manner but (a) us­ing a mesh without plasma treatment or (b) using a mesh without plasma treatment and without the surface active agent added to the photosensitive resin. The results of the tests were as shown in Table 11.

Example 6.

[0044] A low-temperature plasma treatment was carried out using a tanned polyester mesh (Super Strong T300S) in the same manner as in Example 1 except that the atmospheric gas in the plasma treatment apparatus was argon under a pressure of 0.1 torr, the electric power was 50 kW/m² and the treatment time was 5 sec­onds. The surface of the polyester mesh thus plasma-treated had a wettability index of 46 dyne/cm and the presence of 400 to 500 concavities and protrusions per µm² each having a diameter of 0.03 to 0.07 µm and a depth or height of 0.02 to 0.03 µm was ob­served electron microscopically.

[0045] A printing screen was prepared using the plasma-treated mesh similarly to Example 1 but using a different photosensitive resin (One Pot Sol 50M, a product by Murakami Screen Co.) admixed with 1% by weight of the same cationic surface active agent as used in Example 5 by kneading. Formation of a checker­board pattern and comparative testing were carried out in the same manner as in Example 5 to give the results shown in Table 12.

Example 7

[0046] A low-temperature plasma treatment was carried out using a 180 mesh polyester gossamer mesh (Super Strong T180S, a pro­duct by Nippon Tokushu Orimono Co.) in the same manner as in Example 1 except that the atmospheric gas in the plasma treat­ment apparatus was argon under a pressure of 0.1 torr, the elec­tric power was 20 kW/m² and the treatment time was one second. The surface of the polyester mesh thus plasma-treated had a wettability index of 44 dyne/cm and the presence of 400 to 500 concavities and protrusions per µm² each having a diameter of 0.01 to 0.03 µm was observed electron microscopically.

[0047] Preparation of the printing screen using the plasma-treated mesh and formation of a checkerboard pattern were carried out similarly to Example 1.

[0048] A comparative test was carried out for the printing screens made of the plasma-treated and untreated meshes to give the re­sults shown in Table 13.

Example 8.

[0049] A low-temperature plasma treatment was carried out using a 200 mesh polyester gossamer mesh (Super Strong T200S, a pro­duct by Nippon Tokushu Orimono Co.) in the same conditions as in Example 2 except that the atmospheric gas in the plasma treat­ment apparatus was argon under a pressure of 0.8 torr, the elec­tric power was 40 kW/m² and the treatment time was 3 seconds. The surface of the plasma-treated mesh had a wettability index of 46 dyne/cm and the presence of 500 to 600 concavities and protru­sions per µm² each having a diameter of 0.03 to 0.07 µm and a depth or height of 0.02 to 0.04 µm was observed electron micro­scopically.

[0050] The plasma-treated mesh was then soaked with a 1% aque­ous solution of a non-ionic surface active agent of tetraethylene­oxide lauryl ether type (K204, a product by Nippon Oils and Fats Co.) and dried. Preparation of a printing screen using the thus treated polyester mesh and formation of a checkerboard pattern were carried out in the same manner as in Exmple 1. The results of the tape-peeling test and the values of the surface resistivity ob­tained in the comparative tests were as shown in Table 14.

Claims

1. A polyester mesh for screen printing composed of polyester fibers having microscopic concavities and protrusions each having a diameter in the range from 0.01 µm to 0.1 µm on the surface dis­tributed in a density of at least 200 per µm² of the surface area.

2. The polyester mesh for screen printing composed of polyester fibers as claimed in Claim 1, in which the density is not larger than 1000 per µm².

3. The polyester mesh for screen printing composed of polyester fibers as claimed in Claim 1, in which each of the concavities has a depth not exceeding 0.05 µm.

4. The polyester mesh for screen printing composed of polyester fibers as claimed in Claim 3, in which density of the concavities is at least 250 per µm².

5. The polyester mesh for screen printing composed of polyester fibers as claimed in Claim 1, in which each of the protrusions has a height not exceeding 0.05 µm.

6. The polyester mesh for screen printing composed of polyester fibers as claimed in Claim 5, in which the density of the protru­sions is at least 250 per µm².

7. The polyester mesh for screen printing composed of polyester fibers as claimed in Claim 1, in which the fibers have concavities each having a depth not exceeding 0.05 µm and protrusions each having a height not exceeding 0.05 µm in a density of at least 250 per µm² as the sum thereof.

8. A method for the preparation of a polyester mesh for screen printing composed of polyester fibers which comprises a step of low-temperature plasma treatment of the mesh.

9. The method for the preparatiion of a polyester mesh for screen printing composed of polyester fibers as claimed in Claim 8, in which the low temperature plasma treatment is carried out in an atmosphere of a non-oxidizing inorganic gas or an gaseous mix­ture containing a non-oxidizing inorganic gas in a concentration of at least 50% by volume.

10. The method for the preparation of a polyester mesh for screen printing composed of polyester fibers as claimed in Claim 9, in which the non-oxidizing inorganic gas is selected from the group consisting of helium, neon, argon, hydrogen and nitrogen.

11. The method for the preparation of a polyester mesh for screen printing composed of polyester fibers as claimed in Claim 8, in which the pressure of the atmosphere of the low-temperature plasma treatment is in the range from 0.005 torr to 5 torr.

12. The method for the preparation of a polyester mesh for screen printing composed of polyester fibers as claimed in Claim 11, in which the pressure is in the range from 0.01 torr to 1 torr.

13. The method for the preparation of a polyester mesh for screen printing composed of polyester fibers as claimed in Claim 8, in which the low-temperature plasma treatment is carried out by im­pressing an electric power in a density not exceeding 150 kW/m² of the surface area of the power electrode between the electrodes.

14. A printing screen comprising a polyester mesh and a layer of a photosensitive resin in which the polyester mesh is prepared by a method comprising a step of low-temperature plasma treatment.

15. The printing screen comprising a polyester mesh and a layer of a photosensitive resin as claimed in Claim 14, in which the photosensitive resin is further admixed with a surface active agent.

16. The printing screen comprising a polyester mesh and a layer of a photosensitive resin as claimed in Claim 15, in which the sur­face active agent is selected from the group consisting of cationic surface active agents and non-ionic surface active agents.