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In a PE (polyethylene) foam factory there have been some incidents with static electricity that have created minor fires. Fire is a concern in such a factory, as PE is highly flammable, and even more as PE foam uses propane or butane gas in its manufacturing process (that's what creates the bubbles in the foam).

The issues we've been having are mostly related to the thinnest of the foams we produce, which is 1 mm thick. It's formed by extrusion into a flat fabric of about 1m width and we form rolls of 350 linear meters. The machine's speed is about 78 meters / minute.

The foam is rolled over cardboard cores in a torque machine (the shaft is chrome-coated steel). The plant is located in a very dry, cold place (humidity is often around 40-60% and temp ranges between 10-28°C).

I've gathered the following observations while analysing this problem:

  • While the roll is below 45cm in diameter, the static electricity is negligible (I don't have an instrument to measure it, I just get my arm near the roll and see if the hair is drawn towards the foam).
  • Once the diameter gets beyond the 45cm diameter the static is noticeable, to the point of firing little sparks (when it reaches the max diameter).
  • The static builds up first near the sides of the roll, even though it can be felt in the center too. All of the incidents result from a spark igniting the roll in any of the sides, never in the center.
  • The foam goes through a series of rolls (puller, thkcness measurment) before being formed into a roll. Most of those rolls are metallic (steel) and in direct contact with both the fabric and the chasis of the machine (grounded). I can't notice static in the fabric as it leaves this series of puller rolls.
  • There's static buildup no matter if the foam is in contact with the shaft or not (sometimes it moves sideways, leaving the cardboard core and "touching" the shaft).
  • I suspect some slipping between each layer of foam while the roll is formed (due to the shaft rotating slightly faster than the foam is fed), but I can't confirm (the mechanism is synchronized by a PLC).

My hypothesis is that the static buildup is being caused by friction of the foam with the surrounding air, at a rate that is faster than its ability to release it through the shaft (its only contact with ground).

PE foam roll

Questions:

  1. Shall I discard the idea that the static buildup is being caused by friction between layers of the foam in the roll (as they're the same material, theoretically they're not apart in the triboelectric series)?

  2. Other than spraying the area (or even the foam) with any watery solution to increase the conductivity of the air, what would you suggest to keep the sparks from jolting and causing fires?

2A. If a watery solution spray is the best solution there is, what makes a good mixture?

2B. Would a metallic side plate in contact with the shaft and the side of the roll be a good, safe solution? If so, what material would you use?

  1. How could I measure the static electricity built up into the roll to ascertain a "safe limit"?
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Since asking the question I went to study the subject a little bit more. I want to share my conclusions, hoping that they could be of help to someone else.

Static buildup can't be caused by friction (or contact) between layers of the same material. They don't make a triboelectric pair. Most likely, the static is being generated by the foam slipping in the surrounding air.

This brings us to the second point: try to humidify the air. That could be accomplished by spraying the area, but more technically by installing humidifiers.

In addition to installing the side plate you mentioned (a good conductor, such as copper is the obvious pick, as it doesn't require any mechanical properties), it's a good idea to ensure that the chassis of the rolling machine is properly grounded (as well as those structures in which the material slips).

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