As owners of industrial chemical facilities know, roughly 10% of accidents that involve storage tanks are caused by the electrostatic charge generated when dissimilar materials are in relative motion to each other. Any product movement when a tank is filled or emptied or while contents are mixed or agitated — particularly when it involves petroleum-based liquids — can create a static charge between the liquid and the tank wall. Fiberglass tank walls act as insulators and can allow static charges to accumulate in the liquid. The “right” atmospheric conditions, then, can produce the incendiary spark that leads to an explosion.
To address this well-documented risk, fiberglass tank manufacturers have historically relied on the use of anti-static fillers in the resin, typically carbon black or conductive mica, or carbon veils in the tank’s inner wall, combined with stainless steel grounding wires, to dissipate any static charge. However, the use of such fillers is problematic. Filler ratios up to 30% might be necessary, which makes fiberglass wetout more difficult and slows the resin cure rate. Also, for best effect, the carbon filler must be evenly dispersed throughout the part to ensure that there are no electrical discontinuities. Cracking of the finished laminate surface also could cause an electrical discontinuity.
OCSiAl (Leudelange, Luxembourg and Columbus, OH, US) offers a different approach. The company’s TUBALL graphene nanotubes, a type of single-wall carbon nanotubes (SWNTs), can provide ESD protection by dissipating electrostatic charge inside and outside a storage tank. One OCSiAl partner customer previously used conductive carbon black at a loading of 15% but has replaced it with just 0.5% of TUBALL MATRIX 204, a pre-dispersed graphene nanotube concentrate. Permanent and stable volume resistivity of less than 10-6 Ω·cm (ohm-centimeters) has been achieved, independent of humidity and without the “hot spots” possible with poorly dispersed fillers. Moreover, the absence of carbon black has enabled the customer to expand its product’s color range.
Another of OCSiAl’s partners has launched a TUBALL-based epoxy coating system with an electrostatic dissipation function that is specially designed for lining storage tanks containing highly flammable substances. This manufacturer has replaced 30% conductive mica, in an epoxy system, with just 0.3% of TUBALL MATRIX 201 concentrate. The result is a stable level of volume resistivity of between 10-6 and 10-7 Ω·cm.
Because the TUBALL SWNTs are efficient at extremely low dosages, the prospect of overall cost cuts resulting from such significant reductions in the additive loading is a benefit to both coating providers and tank manufacturers, says OCSiAl. In addition, the company’s graphene nanotube concentrates, unlike carbon black filler, do not have associated powder or dust and, applied in such small quantities, minimally impact the host resin’s mechanical properties.