Compositions: coating or plastic – Coating or plastic compositions – Marking
Reexamination Certificate
2000-10-18
2003-03-25
Klemanski, Helene (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Marking
C106S031640, C252S500000, C252S510000, C252S512000, C252S514000
Reexamination Certificate
active
06537359
ABSTRACT:
The invention relates to a conductive ink or paint and to the use thereof for applying an electromagnetic shielding layer to a substrate.
Electrically conductive inks or paints are used in different branches of industry. Some examples of utilization can be found in membrane switches, printed circuit boards, medical sensors, telephone equipment, printed switches, computer housings, heating elements and the like. The function of the ink or paint is to provide a functional conduction, with an electric current being conducted from one location to another, or to provide electromagnetic shielding. Electromagnetic shielding is in particular important in housings of electronic and electric equipment, such as transmitting and receiving equipment and mobile telephones.
In Polym.-Plast. Technol. Eng., 34(2), 271-346 (1995), Mottahed and Manoochehri have published a review of materials used, factors playing a part in the design of electromagnetic shielding layers, and test results in respect of electromagnetic interference shielding or EMI. On page 279, Table 4, a survey is given of different forms of use for realizing EMI. Important examples are conductive plastics, conductive paints, flexible laminates, metal foils and metal layers deposited under vacuum or chemically. The conductive paints are described as a particularly attractive form as far as costs are concerned.
An alternative to an electrically conductive paint is an electrically conductive ink with which, by means of a printing process, electromagnetic shielding layers can selectively be applied to plastic substrates. An electrically conductive ink generally consists of a dispersion of metal or carbon particles and suitable resins in organic solvents. As metal particles, copper, nickel, silver or silver-plated copper particles are usually used. The resin is generally a synthetic resin.
To save costs, the conductive ink or paint is usually selectively applied to the desired moulded parts in the form of a grid or another functional pattern. This requires less ink or paint than when the entire surface of a substrate is to be covered. A drawback of this is that the effectiveness of the shielding decreases substantially when a number of the connections present in such patterns are broken under the influence of temperature or mechanical forces during the production or the use of the equipment. For that reason, it is desired that the ink or the paint, in particular in the form of an applied, dried layer, be deformable.
The wish to have a deformable conductive ink or paint has only increased since the development of the so-called ‘in-mould’ and ‘in-mould decoration’ technologies. According to the in-mould technology, the ink or paint is provided in a die or mould, after which the material of the eventual moulded part is injection-moulded against the ink or paint layer in the form of a polymeric melt. In this respect, it is important that the ink bonds better to the material of the moulded part than to the mould.
According to a variant of the in-mould technique, the ink is first applied to a flat sheet (for instance a polycarbonate sheet), for instance by means of screen-printing. Next, the printed sheet is shaped into the desired, three-dimensional form in a die or mold (under the influence of temperature or mechanical forces) with the layer of ink or paint facing inside or outside, depending on the intended use, whereupon the material of the eventual moulded part is injection-moulded against the layer of ink or paint in the form of a polymeric melt. For a detailed description of this variant, the so-called ‘in-mould decoration’, reference is made to European Plastics News, February 1997, pp. 22-24. Further, it is advantageous that the screen-printing of the desired pattern can be performed on a flat plane. The traditional pad-printing technique, where the ink is applied to the finished, hence already formed, moulded part, is particularly labor-intensive and yields poorer results from an aesthetic viewpoint.
As stated, according to the in-mould decoration technology, a sheet, printed in flat form, is deformed into a desired shape. This deformation can be effected in a mould, but it is also possible to deform the printed sheet into the proper shape before the sheet is introduced into the mould. In both manners of deforming, use is made of the deformability of the material of the sheet. As the ink or paint pattern has already been applied to the sheet, the dried layer of ink or paint must likewise be deformable, because otherwise the pattern is damaged. In addition, the ink or paint used must have an excellent bonding capacity to both the material of the sheet and the material of the moulded part. Because an ink or paint that is sufficiently deformable as well as sufficiently electrically conductive is not available, the in-mould decoration technology is not yet used for manufacturing electromagnetic shielding layers or deformable conductive functional paths.
In international patent application WO-A-96/06438, a deformable, transparent shielding layer is described which is intended for discharging static electricity. The shielding layer consists for 30-70 wt. % of electrically conductive particles smaller than 20 &mgr;m and a thermoplastic resin. Examples of materials of which the conductive particles may consist, include antimony, or tin oxide doped with antimony, tantalum, niobium and phosphorus. As resins, saturated copolyesters are mainly mentioned. The electric conductivity of the shielding layer is so low, namely to the order of 10
5
to 10
13
ohms per square, that the layer cannot be referred to as electrically conductive and cannot be used as electromagnetic shielding layer.
It is an object of the invention to provide an ink or paint which combines an excellent electric conductivity with a good deformability. The object is to provide an ink or paint having such properties that a layer of the ink or paint applied can be deformed, after which the layer is still sufficiently electrically conductive. It is another object of the invention to provide an ink or paint which can be used for applying an electromagnetic shielding layer or a functionally conductive path to a moulded part utilizing the in-mould decoration technology.
Thus, the invention relates to a deformable, electrically conductive ink or paint.
Surprisingly, it has been found that it is possible to prepare an ink or paint which has such properties that after it has been applied to a substrate, it can be deformed to a considerable degree of elongation while it maintains a high electric conductivity. Owing to these properties, the ink or paint is extremely suitable for being used for forming electromagnetic shielding layers.
An ink or paint according to the invention is electrically conductive. This implies that the ink or paint, after being applied to a substrate, hence in dry form, has electrically conductive properties. According to the invention, a layer of ink or paint is electrically conductive when the surface resistance of the layer, at a layer thickness of 25 &mgr;m, is lower than 10,000 &OHgr;/□. In this text, all surface resistances mentioned are based on a layer thickness of 25 &mgr;m. Inks or paints having a surface resistance higher than the surface resistance of air, which under standard conditions is 377 &OHgr;/□, are not suitable for manufacturing electromagnetic shielding layers. For a functionally conductive path, a still lower surface resistance may be needed. For that reason, the ink or paint preferably has a surface resistance of 10 &OHgr;/□ at most. The lower limit of the surface resistance is not particularly critical and will generally be determined by the nature of the materials of which the ink or paint consists. For practical reasons, the surface resistance will usually not be lower than 0.001 &OHgr;/□.
Another important property of an ink or paint according to the invention is the deformability. By this term is meant the possibility of giving the ink or paint in dry condition, that is to say after having been applied
Almer Charles W.
Klemanski Helene
National Starch and Chemical Investment Holding Corporation
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