February 2015 Archives

#202 Antistatic Paints

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Disasters and accidents related to static electricity are caused by electrostatic attraction or discharge. Examples of accidents related to electrostatic discharge include: [1] Fire ignited by flammable materials; [2] Electric shock/discomfort to human body; [3] Light exposure to photographic films, etc.; [4] Failure or destruction of electronic devices including IC; [5] Malfunction or fire in chemical equipment, etc.
Unfavorable events by electrostatic attraction include: [1] Attachment of dust, bacteria, or mold onto electronic devices/components and precision machinery components; [2] Clogging of pipes of precision machinery; [3] Irregularity (jamming) of sheets/films, etc.; [4] Entanglement of threads/fibers; [6] Printing failure; [6] Discomfort to human body.

In Japan, these accidents used to occur in the winter time when humidity is low. However, due to the spread of air-conditioning use and the development of precision electronic components, it is now necessary to implement the countermeasures all year round. The areas that require countermeasures against static electricity include indoor locations, floors, electronic devices and components. [Table 1] shows electrical (leakage) resistance based on the hazard and obstacle levels of the work environment.

[Table 1] Work environment and electrical resistance on floor surface
Work environmentElectrical resistanceLocation applicable
A dangerous place where an explosion or a fire may occur108Ω or lessOperating room, storage room for flammable gas or solvent, or process to handle these
A place where electric shock may occur1010Ω or lessPowder packaging process, paper rewinding process, etc.
A place where a production bottleneck may arise1011Ω or lessComputer room, semiconductor handling room, etc.

The following materials are used for the antistatic coatings.

1. Composite1) Metal: Ag, Cu, Ni; 2) Carbon: conductive carbon; 3) Metal-oxide: tin oxide, antimony-oxide treated TiO2 and Sb-doped tin oxide
2. SurfactantQuaternary ammonium salt, sulfonate, phosphate, alkylbetaine type, alkylimidazolium type, alkylamine, alkyl ether, glycerin fatty acid ester, polystyrenesulfonate, etc.
3. Deposition1) Metal: Au-Pd
2) Oxide: indium oxide, tin oxide, cadmium tin oxide, etc.

The combination of carbon conductive paints and coated materials includes the followings: [1] Acrylic coating with ABS resin, PS, or PPO coated materials; [2] Urethane coating with FRP or polyester; [3] Polyolefin coating with PP or FRP; [4] Alkyd coating with metal; [5] Epoxy resin coating with metal, FRP, or cement.

Carbon powders are stable in the heat. Most of them are made with melamine-based materials with room temperature setting or thermosetting properties.

For the clean room specification, acrylic or vinyl materials are used for walls, ceilings, and light-duty floors while urethane or solventless epoxy materials are used for heavy-duty floors.
In addition, highly conductive carbon coatings are applied to plastic cases of electronic equipment as a static protection measure.

#201 Electromagnetic Pulse Shield Paints

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This coating is also called EMI (electromagnetic interference) shielding paint.

This coating uses an electrically-conductive coating as the main ingredient, which prevents electromagnetic interference and provides EMC (electromagnetic compatibility).

A bandwidth between 10 kHz and 500 MHz of electromagnetic waves is particularly important to information technology devices. Since the 1980s, the United States and Germany have strengthened the EMI regulations on digital devices.

One of the reasons for this is the trend of adopting plastic materials for the equipment casing.

Plastic materials are the major choice for digital equipment casing because they are not only lightweight and economical but also have superior productivity, design, and electric insulation.

However, their electrical insulating properties incapable of providing the shielding effect for electromagnetic waves drove the industry to tighten the regulations.

The EMI regulations define the category based on the application: "Class A" for industrial and commercial equipment; "Class B" for residential applications.

In addition, noises generated from these are classified as "conductive noises", which are transmitted from power/signal cables of the equipment and "radiation noises", which are emitted directly from the internal component of the equipment into space.

The source of electromagnetic wave generation is shown in [Table 1].

[Table 1] Source of electromagnetic wave generation
Natural noise Atmospheric noise Lightening discharge, discontinuity line, low pressure, yellow sand, volcanic eruption, snowstorm
Solar noise Black body radiation from the sun
Cosmic noise Galactic noise
Artificial noise Discharge noise
(sparks, arc)
Flasher unit (e.g. neon lighting), relay, thermostat (electric kotatsu*, electric refrigerator, electric iron, etc.), cash register, electric drill, electric vacuum cleaner, electric mixer, automobile, high-voltage cable.*Small table with an electric heater underneath and covered by a quilt or a blanket
Friction/contact noise Electric razor, electric massager
Glow discharge Fluorescent light, neon discharge tube, high-pressure mercury vapor lamp
High-frequency/ultrasonic noise Microwave oven, high-frequency welding machine, electric scalpel, high-frequency medical device, ultrasonic cleaner
Unwanted emission TV/FM/AM receivers

In order to shield electromagnetic waves, apply the electrically-conductive coatings.

The paints are available in Ag, Ni, Ag-Cu, and Cu formulas. All of these are suitable for plastics as they are a low-temperature curing type.

Thermoplastic acrylic, thermosetting acrylic, epoxy, and phenol resins are used for the binders.

The properties of the coatings are shown in [Table 2].

[Table 2] Electrical properties of the electromagnetic shielding paints
Filler Ag Ag-Cu Ni Cu
Color phase Silver gray Red Grayish black Copper
Electric conductivity Ω/cm 1×10-4 5×10-4 or less 5×10-3 or less 2×10-3 or less
Surface resistance Ω/sq 0.1/10µm 0.15/30µm 0.8/30µm 0.4/40µm

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