A1. Transparent coatings

It is not possible using carbon black compounds to achieve any kind of (semi-)transparent and still antistatic or conductive products or coatings. For electronic products packaging and handling purposes, e.g., however, this is an important need in order to prevent electrostatic discharges while still maintaining the capability of optical inspections.

The industry has met this demand by either vacuum metallization or humidity dependent antistats. Also ITO ceramic powders and other comparable inorganic (transparent) conductors have been proposed as fillers for transparent coatings or as pure thin layers prepared by deposition methods. These techniques are showing several technical disadvantages, like

At least in some areas, ICP coatings have shown and will even more prove in the future to become a well-performing alternative.

PAni blend coatings are commercially made by an extreme dispersion of PAni powder in suitable coating systems [1]. The dispersed phase is around 70 nm small, the critical volume concentration, where the sudden conductivity breakthrough occurs, is around 1%. These are the preconditions under which coatings with a layer thickness of between 1 and 20 µ are transparent (with a transparency of up to 95%). The coatings are green, hence they absorb at around 350 nm and, beginning with around 650 nm, in the near infrared.

Depending on product type and coating thickness, conductivity values between 10-9 to 10-1 S/cm are achievable (in resistance values: 109 to 102 /), cf. table 1 for a PMMA coating:
 

layer thickness 

0,4 µ 

0,6 µ 

0,8 µ 

1 µ 

 resistance 

107- 109 

106 

104 - 106 

103 

 transparency 

95% 

90% 

85% 

75% 
Coating systems for spray, dip, drawbar, gravure and roller coating techniques, as well as for flexo printing on a variety of substrates (like plastics including polyolefines, glass, paper, cardboard and others) have been developed and are in practical use. The various products also offer different hardness, ranging from 2B to 4H. UV and EB curable systems are available, too. Most of the products are actually solvent born (mainly isopropanole, butanol, toluene or xylene based), but also water born systems are becoming available.

Comparable systems could serve for more than just delivering two properties (conductivity + transparency). Also electrochromic, indicator or sensor functions could be envisioned with coatings based on blends.

Another transparent (and in this case even colorless) coating system is offered by Bayer AG based on ethylendioxy-thiophene. The monomer is polymerized directly on the substrate, an Agfa film, in order to serve as antistatic layer for photographic films, which is virtually colorless. Up to now, this coating application is limited to relatively low conductivity levels, but has its unquestionable value due to the very weak absorption in the visible range. The costs of the monomer and the need of polymerization on the substrate to be coated are additional limitations. Bayer intends to offer a latex with a pre-polymerized PEDT coating on the latex particles, which are subject to the same limitations as described above. (The monomer is furthermore becoming used as polymerizable agent in a through-hole-plating process for printed circuit board production, which was previously performed using pyrrole.)

Footnotes

[1] Ormecon Chemie (a subsidiary of Zipperling Kessler & Co.), technical information "Transparent Coatings"

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