Patent Application
20050214490
The present invention relates to an electroconductive composite plastics sheet suitable as a packaging material for integrated circuits (IC) or electronic parts using IC and to an electroconductive plastics container.
IC or electronic parts using IC may be broken upon exposure to static electricity, and thus an electroconductive plastics sheet is used as a packaging material capable of conferring an antistat effect. For example, an embossed carrier tape is known as a packaging material utilizing the electroconductive plastics sheet.
This embossed carrier tape is prescribed under JIS C0806 and used for example in storing or transporting electronic parts, and is produced for example by slitting a sheet of broad width into a sheet of predetermined width, then making sprocket holes in one side edge of the sheet, and forming a part-fitting concave (pocket) by embossing (see, for example, FIGS. 1 and 2 in JP-A 3-87097).
Such an embossed carrier tape is used in storing and transporting an electronic part in a sealed state with cover tape which is heat-sealed on the carrier tape after accommodating the electronic part in its pocket. Before an automatic process of mounting it on a circuit board or the like, the embossed carrier tape accommodating an electronic part therein is set on an automatic mounting machine, the cover tape is peeled, and the electronic part is removed from the pocket and then mounted on a circuit board.
JP-B 3209393 discloses an electroconductive composite plastics sheet and container. JP-B 3305526 discloses an electroconductive resin composition.
The automatic mounting process described above is a very rapid processing step, and thus the state of the cover tape being peeled off, that is, the peel-off strength of the cover tape, should be always consistent, and inconsistent peel-off strength can cause stopping the mounting process.
However, the peel-off strength of the cover tape may undergo the influence of the environment under which the embossed carrier tape before heat-sealing with a cover tape was stored. For example, when a carrier tape stored under a high-temperature and high-humidity environment and a carrier tape stored in a low-temperature and low-humidity environment are used and heat-sealed with cover tapes respectively under the same heat-sealing conditions, the cover tapes may give different peel-off strength, and this difference may exert adverse influence on the mounting process.
The peel-off strength of a cover tape after heat-sealing with an embossed carrier tape is prescribed in JIS C0806-3, as follows:
The peel-off strength of a cover tape upon peeling the cover tape at an angle of 165° to 180° to the peeling direction and at a peeling speed of 300 mm/min. is:
0.1 to 1.0 N when the width of the tape is 8 mm, and
0.1 to 1.3 N when the width of the tape is 12 to 56 mm.
The adequate peel-off strength of the cover tape depends on the shape of a pocket of the carrier tape or a material in the pocket, but is actually often within the range of ±0.10 N relative to an average of 0.4 to 0.6 N. In the step of heat-sealing with a cover tape, the heat sealing conditions are hardly changed in each operation, and therefore, the peel-off strength of a cover tape heat-sealed under the same conditions should be reproducible. This reproducibility is desirably in the range of ±0.10 N, and products showing reproducibility outside the range of ±0.15 N are not practically usable.
A purpose of the invention provides an electroconductive composite plastics sheet capable of maintaining a stable peel strength regardless of a difference in storing environment before heat-sealing with cover tapes and an electroconductive plastics container using the same.
To achieve the purpose, the present invention provides an electroconductive composite plastics sheet comprising a thermoplastic resin-containing sheet substrate and an electroconductive resin layer formed on the surface of the sheet substrate,
wherein the sheet substrate comprises a styrene-based resin, and the electroconductive resin layer is a layer of an electroconductive resin composition comprising:
(A-1) styrene-based resin,
(A-2) carbon black, and
(A-3) one kind of copolymer, or a mixture consisting of a combination of two or more kinds of copolymers, obtained from ethylene and other monomer copolymerizable with ethylene, wherein the content of the other monomer copolymerizable with ethylene is 6 to 14 mass % on the average, and
the ratio in thickness of the sheet substrate and electroconductive resin layer is in the range of 4/1 to 20/1, and the total thickness of the electroconductive composite plastics sheet is in the range of 0.1 to 1.0 mm,
and an electroconductive plastics container using the same.
In the ratio in thickness of the sheet substrate and electroconductive resin, the thickness of the electroconductive resin layer means the total thickness when the electroconductive resin layer is formed on both sides of the sheet substrate.
In the present invention, the electroconductive composite plastics sheet and the electroconductive plastics container are those having a specific surface resistivity of 102 to 1010 Ω.
The electroconductive composite plastics sheet of the present invention has a sheet substrate and an electroconductive resin layer formed on the surface of the sheet substrate.
The sheet substrate contains a thermoplastic resin, and the thermoplastic resin is preferably a styrene-based resin, but may contain another thermoplastic resin in an amount of 30 mass % or less.
The styrene-based resin includes polymers of styrene or styrene derivatives such as those substituted at the α position thereof or on the nucleus thereof. The styrene-based resin also includes copolymers composed of such monomers as main components and other monomers such as vinyl compounds such as acrylonitrile, acrylic acid and methacrylic acid and/or conjugated diene compounds such as butadiene and isoprene. Examples of such copolymers include polystyrene, high impact polystyrene (HIPS) resin, acrylonitrile-butadiene-styrene copolymer (ABS) resin, acrylonitrile-styrene copolymer (AS resin), styrene-methacrylate copolymer (MS resin), etc.
The styrene-based resin may also contain styrene-based copolymers having carboxyl-containing unsaturated compounds copolymerized therein. The styrene-based copolymers having carboxyl-containing unsaturated compounds copolymerized therein are copolymers prepared by polymerizing carboxyl-containing unsaturated compounds and if necessary other monomers copolymerizable therewith, in the presence of a rubber-like polymer. Specific examples of such copolymers are as follows:
(1) Graft polymer obtained by polymerizing monomers consisting essentially of aromatic vinyl monomers or monomers consisting essentially of aromatic vinyls and carboxyl-containing unsaturated compounds in the presence of a rubber-like polymer having carboxyl-containing unsaturated compounds copolymerized therein.
(2) Graft copolymer obtained by copolymerizing monomers consisting essentially of aromatic vinyls and carboxyl-containing unsaturated compounds in the presence of a rubber-like polymer.
(3) Mixture of rubber-reinforced styrene-based resin not copolymerized with carboxyl-containing unsaturated compounds and a copolymer consisting essentially of carboxyl-containing unsaturated compounds and aromatic vinyls.
(4) Mixture of the above-mentioned (1), (2) and a copolymer consisting essentially of carboxyl-containing unsaturated compounds and aromatic vinyls.
(5) Mixture of the above-mentioned (1), (2), (3), (4) and a copolymer consisting essentially of aromatic vinyls.
In the above-mentioned (1) to (5), the aromatic vinyl is preferably styrene, and the monomer copolymerized with the aromatic vinyl is preferably acrylonitrile. The content of the carboxyl-containing unsaturated compound in the styrene-based resin is preferably 0.1 to 8 mass %, more preferably 0.2 to 7 mass %.
An aging inhibitor, an antioxidant, a lubricant, a plasticizer, an impact improver etc. in addition to the thermoplastic resin such as the styrene based resin can be added to the sheet substrate in such a range that the problem of the invention can be solved.
The electroconductive resin layer comprises an electroconductive resin composition comprising (A-1) styrene-based resin, (A-2) carbon black, and (A-3) one kind of copolymer, or a mixture consisting of a combination of two or more kinds of copolymers, obtained from ethylene and other monomers copolymerizable with ethylene, wherein the copolymers contain those wherein the content of the other monomers copolymerizable with ethylene is 6 to 14 mass % on the average.
The styrene-based resin as the component (A-1) includes the same resin as used in the sheet substrate.
The content of the component (A-1) in the composition is preferably 35 to 90 mass %, more preferably 45 to 75 mass %.
Carbon black as the component (A-2) is preferably the one having a high specific surface area (preferably at least 30 m2/g) and called electroconductive carbon black. As the component (A-2), known components such as furnace black, channel black and acetylene black can be used.
The component (A-2) is compounded in such a ratio that the specific surface resistivity of the electroconductive composite plastics sheet and the electroconductive plastics container can be in the range of 102 to 1010 Ω. The content of the component (A-2) in the composition is preferably 5 to 30 mass %, more preferably 10 to 25 mass %.
The component (A-3) is one kind of copolymer, or a mixture consisting of a combination of two or more kinds of copolymers, obtained from ethylene and other monomers copolymerizable with ethylene, wherein the copolymers contain those wherein the content of the other monomers copolymerizable with ethylene is 6 to 14 mass % (preferably 7 to 10 mass %) on the average.
The mass-average content of the other monomers copolymerizable with ethylene is calculated according to the following equation:
wherein n is the number of copolymers contained in the electroconductive resin composition,
αi is the content (mass %) of the other monomers than ethylene contained in copolymer i, and
xi is the content (mass %) of copolymer i contained in the electroconductive resin composition.
When the component (A-3) is a mixture consisting of a combination of two or more kinds of copolymers, the component (A-3) maybe a combination of copolymer (A-3a) wherein the content of the other monomers is preferably 8 mass % or less, more preferably 6 mass % or less (provided that the content shall not be 0 mass %) and copolymer (A-3b) wherein the content of the other monomers is preferably 15 to 30 mass %, more preferably 15 to 25 mass %.
When the component (A-3) is a mixture consisting of a combination of two or more kinds of copolymers, the component (A-3) may be a mixture composed exclusively of the copolymers (A-3a) and (A-3b), and this mixture may contain 5 mass % or less other ethylene copolymers.
The copolymer (A-3a)/copolymer (A-3b) ratio by mass is preferably from 8/1 to 1/1, more preferably from 6/1 to 2/1.
The copolymer as the component (A-3) is at least one member selected from an ethylene/vinyl acetate copolymer (EVA), an ethylene/ethyl acrylate copolymer (EEA), an ethylene/methacrylate copolymer (EMA), an ethylene/butyl acrylate copolymer (EBA) and an ethylene/methyl methacrylate copolymer (EMMA).
Specific examples include EVAFLEX (EEA) manufactured by Mitsui DuPont Polychemical, Nippon Unicar EVA manufactured by Nippon Unicar Company Limited, Lexpearl (EMA) manufactured by Nippon Polyolefin Co., Ltd., LOTRYL (EMA, EBA) manufactured by ATOFINA, and Acryft (EMMA) manufactured by Sumitomo Chemical Co., Ltd.
The content of the component (A-3) in the composition is preferably 5 to 35 mass %, more preferably 15 to 30 mass %.
An antistatic, a plasticizer and various stabilizers in addition to the components (A-1) to (A-3) can be incorporated into the electroconductive resin composition in such a range that the problem of the invention can be solved.
The electroconductive composite plastics sheet of the present invention can be produced by forming the electroconductive resin layer on one side or both sides of a sheet substrate. This electroconductive resin layer can be formed on a part or the whole of one side or both sides of a sheet substrate.
As the method of forming the electroconductive resin layer, it is possible to use a method of co-extruding a resin material for sheet substrate and the electroconductive resin composition, a method of laminating a sheet substrate with a film consisting of the electroconductive resin composition, etc., among which the co-extrusion method is preferable.
In the electroconductive composite plastics sheet of the present invention, the ratio in thickness of the sheet substrate to the electroconductive resin layer (sheet substrate thickness/electroconductive resin layer thickness) is in the range of 4/1 to 20/1, preferably in the range of 5/1 to 15/1.
The total thickness of the electroconductive composite plastics sheet of the present invention is in the range of 0.1 to 1.0 mm, preferably 0.15 to 0.8 mm, more preferably 0.2 to 0.4 mm.
The electroconductive plastics container of the present invention is produced by forming the electroconductive composite plastics sheet in a desired shape. The electroconductive plastics container is preferable as a packaging material (embossed carrier tape) for integrated circuits (IC) or electronic parts using IC.
By virtue of high bending strength, the electroconductive composite plastics sheet and the electroconductive plastics container can be easily subjected to forming such as embossing, and thus easily formed into an embossed carrier tape for transporting electronic parts.
The electroconductive composite plastics sheet and electroconductive plastics container (embossed carrier tape), even when stored in a storage environment at varying temperature and humidity, show less change in peel-off strength upon heat-sealing with another plastic member (cover tape for embossed carrier tape), and the reproducibility of peel-off strength as determined according to JIS C0806-3 is within the range of ±0.10 N.
The respective components and measurement methods used in Examples and Comparative Examples are as follows:
(1) Used Components
(Sheet Substrate)
Arbitrary 10 sites in the surface of a sheet sample or arbitrary 10 sites in the inner bottom of a container sample were measured with a Lorester surface resistivity meter (manufactured by Mitsubishi Chemical Co., Ltd.) wherein the distance between electrodes was 10 mm, and the logarithmic average value was expressed as specific surface resistivity.
(Bending Strength)
The test was conducted according to “Method of Testing Anti-bending Strength with MIT Testing Machine” in JIS P8115. A sample not broken until it was bent 100 times or more is regarded as an excellent carrier tape.
(Peel-off Strength)
The electroconductive composite plastics sheet obtained in each of the Examples and Comparative Examples was slitted into a strip of 24 mm in width and embossed by vacuum forming (a pocket of 10 mm in length, 14 mm in width and 3 mm in depth was formed) to give an embossed carrier tape (having an outward appearance similar to that in FIGS. 1 and 2 in JP-A 3-87097). This carrier tape was stored in a low-temperature, low-humidity environment or in a high-temperature, high-humidity environment, and then examined for peel-off strength in the following method.
1) Storage in a Low-temperature, Low-humidity Environment
The carrier tape was left in an environment of 23° C. and 50% RH for 1 week or longer and then heat-sealed with a cover tape (corresponding to lid 2 in FIG. 2 in the publication supra) by using a sealing machine (VS-120, manufactured by Vanguard System). Thereafter, its peel test was conducted by using a peel testing machine (VG-20, manufactured by Vanguard System) to determine the peel-off strength.
As the cover tape, ST-20 manufactured by Neptco and AST16 manufactured by SUN A KAKEN Co., LTD were used, and the temperature and pressure in the heat sealing were selected such that the average peel-off strength became about 0.4 N.
2) Storage in a High-temperature, High-humidity Environment
The carrier tape was left in an environment of 50° C. and 90% RH for 72 hours or longer, then heat-sealed with a cover tape in the same manner as described above, and examined for peel-off strength.
The peel-off strength was evaluated by using a difference between the peel-off strength of the sample stored in the low-temperature low-humidity environment and the peel-off strength of the sample stored in the high-temperature high-humidity environment. A sample showing a difference in peel-off strength of less than 0.15 N is regarded as an excellent carrier tape.
The sheet substrate components shown in a composition (expressed in mass %) in Table 1 were melted and kneaded by a twin-screw extruder (PCM30/2-28.5-2V, manufactured by Ikegai Co., Ltd.), then extruded into a strand and cut into resin pellets for sheet substrate.
The resin pellets were fed to a φ50 mm extruder in a multilayer extruder, while a resin composition (expressed in mass %) forming a electroconductive resin layer was fed to a φ30 mm extruder, and these were co-extruded to give a composite electroconductive plastics sheet having a electroconductive resin layer on both sides of a sheet substrate. The sheet was examined in each test described above. The results are shown in Table 1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-84385 | Mar 2004 | JP | national |
