Brief introduction to conductive pigment and its application on coating and printing ink

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In many industry fields, plastics, paint and coatings that are conductive and anti-static or can shield the electromagnetic wave, for example, antistatic coating that used in the electronic equipments, electromagnetic shielding coating that used in the plastic box cover, antistatic floor coating, antistatic facility that used in the explosion-proof buildings and the conductive primer that used in the plastic electrostatic spraying all have close relations with conductive pigment.

Conductive pigment (filler) generally includes conductive mica powder, conductive titanium dioxide, conductive barium sulfate, conductive ATO powder, conductive polyaniline, conductive graphite powder, conductive carbon black, conductive silver powder, conductive glass powder and other various conductive powder materials.

The applications of conductive pigment on coatings mainly include conductive coating, antistatic paint, electromagnetic shielding material and electrothermal coating.

Conductive coating

Conductive coating that made by conductive pigment (filler) is a kind of additive type material, which is the mixture of conductive pigment (filler) as well as insulated polymer, assistant and solvent. Its conductivity is not only influenced by the selected conductive materials, but also is influenced by polymer, assistant, solvent as well as the manufacturing process. In the coating that is filled in the conductive pigment (filler), only when the loading level of conducting particles reaches a certain specific value (the bulk density of the insulated polymer reaches a certain value), the coating will have electrical conductivity, which can also be called the conductive effect that is generated by the mutual contact of conducting particles. According to the electrical property, those coatings that have the specific resistance in the range of 10-3-10-5Ω can be called conductive coating.

Conductive coating is mainly used as anticorrosive paint and floor coating, of which the main function is cathodic protection. As it uses zinc powder as the conductive pigment, it can protect the substrate (steel) by using the conductive effect of zinc powder (sometimes it will replace partial zinc powder with ferrophosphorus powder, aluminum powder and alloy powder). This kind of cathodic protection coating is one of the most important varieties in the anticorrosive paint. In the shop primer of marine paint, its dosage will occupy 1/3 of the total marine paint.

The application of conductive pigment is very wide. Zinc-rich primer that used in heavy anti-corrosion coating is closely related with it, besides, many large steel structure facilities, for example, the high bridges, railroad bridges as well as Three Gorges Reservoir, Bird’s Nest and Hangzhou Bay Bridge in China all use this kind of heavy anti-corrosion coating system that includes zinc-rich primer.

The general development direction of conductive coating will be nanocrystallization technology, in which nano conductive coating will be used. This kind of material will be used in electron, electrical equipment, aviation, chemical industry, printing and many other fields.

Anti-static coating

In the floor coatings, the anti-static floor coating is most expensive. This kind of coating uses conductive graphite, conductive carbon black (deep color) or conductive barium sulfate, conductive titanium dioxide and conductive talcum powder as the conductive pigment (filler).

The specific resistance of anti-static coating is in the range of 105-109Ω, which can be used in anti-static products surface coating, anti-explosion cable as well as can be used as the anti-static products in the oil tank. The anticorrosive paint of oil tank also uses zinc-rich paint, among which the light color oil tank static conductive anticorrosive paint uses metal conductive filler, metal oxide filler, conductive mica powder, carbon nano tube filler and so on; carbon nano tube can increase the coating conductivity to several order of magnitudes, which can be regarded as the most high-end product in oil tank anticorrosive coating up to now; there is also another way that uses conductive polyaniline as the anticorrosive coating. This kind of polyaniline will mix in the formula in the powder form, which will have potential development prospect.

Electromagnetic shielding coating

Electromagnetic shielding coating is prepared by synthetic resin, conductive filler and solvent. Currently, the electromagnetic shielding coating mainly exist in the form of conductive coating that usually added with gold, silver, copper, nickel and other metal powders as well as carbon black, graphite and other non-metallic powders as the conductive filler. Although gold powder has highest conductivity and good chemical stability, it is very expensive and is limited for using. Silver powder also has good conductivity and is cheaper than gold powder, although it will be easy to precipitate and will migrate after glue preparation, it is still commonly used.

Although copper and nickel have similar performances with silver and are much cheaper than silver, they are easy to oxidize, have poor conductivity as well as have poor glue preparation durability. Carbon black and graphite, being served as the conductive filler, have good dispersity and are very cheap, but they have poor conductivity and can be used as the ideal material in electromagnetic shielding.

Silver conductive coating was firstly used as the electromagnetic shielding coating in 1960’s, which is very stable and has good shielding effect, but it is very expensive; as nickel coating has moderate cost, good shielding effect and stronger oxidation resistance than copper, it has become the mainstream of electromagnetic shielding coating in Europe, America and other countries at present; copper coating has good conductivity, but it has poor oxidation resistance. However, with the development of oxidation resistance technology in recent years, the development and application of copper coating is gradually increasing. For example, as copper/acrylic resin (the shop sign is Copalex100) that produced by Showa Denko has special handling on copper, the conductivity is stable and the dosage is only half of nickel coating. As copper has lower volume resistivity than nickel, the surface resistivity of copper will be lower than nickel under same coating thickness.

Regarding carbon black, graphite and other carbon series conductive coatings, the volume resistance will be reduced to less than 10Ω•cm (the lowest volume resistance can reach about 0-2Ω•cm) only when carbon black that has high conductivity and high structure is used as the conductive filler. As carbon coating has poor conductivity, it will have poorer shielding effect than other metal fillers. However, they have good environmental resistance, low density, low cost and other advantages.

Currently, in the research on electromagnetic shielding coating, it is the key to find better ways to solve the oxidation resistance of copper powder and nickel powder as well as the metal filler sedimentation problem that exists in the storage process of coatings. In addition, the other important trend in this field could be the development of water-based paint. Many excellent formulas that use nickel as the filler have occurred in this aspect, this is because this kind of metal will still have good corrosion resistance in a very wide range of PH value.

The resistivity of electromagnetic shielding coating is in the range of 10-2-10-3Ω. In the future, it will develop towards light color, water-based paint, wide effective band and multi-spectral shielding.

Electrothermal coating

Electrothermal coating is a kind of liquid coating that is mixed with organic matter and inorganic matter, of which the mechanisms are as follows: smear a layer of silver and nickel on the surface of inorganic matter, thus it will change to conductive filler; afterwards, disperse this kind of filler into resins that have been cured with heat and make sure that the filler can release heat; the combination of the conductive filler and coating type thermosetting will constitute the base frame of electrothermal coating; when the base frame is charged with electricity, it will form complete electrothermal coating.

Electrothermal coating can be used in metals, plastics, ceramics, glasses, sidings and many other objects. Its advantages are as follows: after coating, it will become completely dry under room temperature in 20mins and the power consumption is very little. For example, heating equipments that are coated with electrothermal coating can save more than doubled power in each square meter. As it will generate far infrared radiation, the surface radiation heat can reach 90 percents. Besides, it has high thermal efficiency and is easy to use, which can become effective immediately as long as the tools and equipments are charged with power.

Other fields

Except for being used in the coating industry, conductive pigment can also be used in other fields. Such as conductive ink, conductive adhesive, conductive rubber, conductive plastic, conductive cloth, conductive glass fibre, conductive ceramic and so on.

Conductive ink is a kind of conductive composite material that is formed after the metal conductive particle is dispersed in the connection material, which will have the same effect as wire, antenna and resistance after being printed on the printing stock. This kind of printing ink can be made into plated circuit when being printed on flexible and hard substrates (such as paper, PVC, PE and so on). After the conductive ink becomes dry, as the distance between conductive particles is shortened, the free electrons will move towards applied electric field and will form electric current, thus it will have good conductivity and can receive exclusive radio frequency signal of RFID.

Conductive ink that used in the printed circuit board includes carbon paste printing ink, copper paste printing ink, silver paste printing ink and gold paste printing ink.

Carbon paste printing ink

Carbon paste printing ink is a kind of liquid and thermosetting printing ink, which will have good feature after forming film. After being cured, it can protect copper foil and transmit current as well as have good conductivity and low impedance property. The ink film layer will not be easy to oxidize and have stable property, besides, after curing, it will become resistant to acid, alkali and chemical solvent. Printing ink has strong adhesive force and good stripping resistance, which can be combined well with copper foil or glass cloth plate. Ink film layer has high hardness, of which the hardness can reach more than 6H of pencil hardness after curing. Besides, it has strong wear resistance. The contact number of keyboard that made by this kind of ink film can reach more than 1million. Patterns that are printed by it will have good thermal shock resistance. In the solder heat resistance experiment that is conducted at 260 degrees Celsius, the ink film will not be influenced after being soldered (50×5) times and there will be no tin pick-up in hot air leveling procedure.

Silver paste printing ink

Silver paste printing ink is a kind of liquid printing ink that is mainly composed by superfine silver powder and thermoplastic resin. It is mainly used in flexible printed circuit, membrane switch, electromagnetic shielding, heating element coating as well as can be printed on PET, PC, and PVC sheet. It has very strong adhesive force and covering power as well as controllable conductivity and low sheet resistance value.

Copper paste printing ink

Copper paste printing ink is a kind of liquid printing ink that is mainly composed by alloys that contain aluminum, copper and zinc or superfine copper powder and thermoplastic resin, which can be printed on microelectronic devices. It has high adhesive force and covering power as well as can be cured under low temperature. Besides, it has good conductivity and low resistance.

Gold paste printing ink

Gold paste printing ink is a kind of liquid printing ink that is mainly composed by superfine gold powder and thermoplastic resin. It has stable chemical property and good conductivity, but it is very expensive and can only be used in thick film integrated circuit printing. According to the patent literature, it can also inhibit the mobility resistance of silver and improve the storage stability of copper powder, which will overcome the above mentioned defects to some extent.

In addition, conductive pigment (filler) can also be used in electroconductive cement, conductive fabric, conductive carpet, anti-static terrazzo, antistatic recording paper and many other fields, which will also have good application in military, industry and civilian field.