Charged insulator touches a conductor

[lesaid]

I'm suggesting that the molecules may have been polarised whilst warm and then held the molecular distortion when it cooled. It seems that one side of the roll has a different charge from the other side. If it's not that then could it be triboelectric due to distortion of the plastic as it's unrolled and the radius of curvature changed? With a film that thin, the stretching would be very small.
It seems to be a bit of a one-shot effect, though. The stuff sticks to itself very well the first time but re-doing it never sticks so well.

I'm no expert here, but if a molecular distortion was 'locked in' as the film cooled, would it then be capable of causing a discharge? Would it not instead be like an electret material, where the charge itself is 'captive' and the polarisation (and 'stickiness') would remain, until perhaps, the film was warmed again?

I feel a little experimenting coming on!

 

[sophiecentaur]

Yes, it would remain because the potential would not be too high but, once the separation increased (unrolling the film) and the Capacitance Decreased accordingly, the Potential would be greatly increased. The work done in the experiment can easily be disregarded when explaining a number of electrostatic phenomena.

 

[Asymptotic]

We've all seen that to a lesser extent when pulling cling film off the roll at home. It would be interesting to know what the actual charging mechanism is. Is it due to the original heating, stretching and rolling that one face of the film gets one charge and the other face gets the other? Or is it to do with the curvature?
Those long sparks are a great demonstration of Q = CV. The C between faces drops drastically as they separate and so the V has to increase accordingly. The user supplies the energy in this case (and the roll does, in fact, feel sticky due to the attraction)

To set up the scene requires a lot of verbiage.

It is difficult to measure static voltage at the die outlet (>600°F, and not very safe, being at the roll nip), but I never read anything higher than 200V per an older model Simco electrostatic field meter. However, web coming out of the roll stack could hit 150 kV to 200 kV before partially discharging. Surface charge dropped to 10 kV or less after the sheet passed through the next stage - a bath of diluted silicone release agent.

The sheet didn't rub against anything in between the roll stack, and silicone bath, so charge must have been generated within the roll stack itself. The one I have in mind is a conventional 3 roll vertical stack with 30" diameter chromed steel cooling rolls in a "downstack" configuration where melt enters the upstream nip between the middle and bottom roll, and sheet exits the stack from the top roll (moving downward, hence the "downstack" moniker). Haul-offs designed to process PET and PLA are generally in an "upstack" configuration (or a "J" stack variation, or for thin sheet, a horizontal roll arrangement may be used). I think this factors into it, because the only processing lines known for lightning bolts shooting across the sheet were both downstacks.

What makes a downstack problematic in PET/PLA processing is PET freezes very quick upon contacting the first chilled roll it touches. Gravity assures the 'bottom' side of the melt curtain hits the bottom roll first, whereas most of the cooling is done by the middle roll against the 'top' side of the sheet. Another issue is how much material 'bead' is at the roll nip. If there isn't enough bead (or it isn't uniform across the nip) the sheet will have flat spots and other surface defects. Polystyrene, and many other plastics are more forgiving in this regard, but with PET, too much bead demands considerably more drive torque than normal (or can stall the rolls outright), and creates an inordinate amount of roll deflection leading to a different form of surface defect. Finally, because the sheet has been shrinking all the time it has been in contact with the middle roll it adheres to it to a degree, and (with gravity assisting) tends to want to wrap around the middle roll rather than transfer to the upper roll before exiting the stack.

I suspect several mechanisms are at work, but my guess is most of the static generation occurs as the sheet peels away from the center roll. Static discharge appeared to be more severe when operators were running with a heavier-than-normal bead, but I don't know if this was a consequence of squeezing it in the nip, or some other effect.

 

[sophiecentaur]

@Asymptotic : you have definitely 'been there and got the T shirt'. Interest that you measured some very high voltages along the line.
I found this movie about the manufacture. The massive bubble system is interesting. The movie is more about pretty pictures than Physics and the section (around 2minutes +) about how it sticks is not satisfactory for me. The 'added glue' part explains it partly but the commentary is clearly wrong about it not involving Static Electricity because sheets are attracted over distances of 10cm and more. Glue can't do that.

but I never read anything higher than 200V per an older model Simco electrostatic field meter

You don't need many Volts. What you need is Polarisation / Distortion of the molecules - i.e. Charge Imbalance. I think this will be maintained whilst the surfaces are together (it's the lowest energy condition and I don't think leakage is a relevant concept). Pulling the sheets apart is what generates the High Volts.

 

[Asymptotic]

@Asymptotic : you have definitely 'been there and got the T shirt'. Interest that you measured some very high voltages along the line.
I found this movie about the manufacture. The massive bubble system is interesting. The movie is more about pretty pictures than Physics and the section (around 2minutes +) about how it sticks is not satisfactory for me. The 'added glue' part explains it partly but the commentary is clearly wrong about it not involving Static Electricity because sheets are attracted over distances of 10cm and more. Glue can't do that.

You don't need many Volts. What you need is Polarisation / Distortion of the molecules - i.e. Charge Imbalance. I think this will be maintained whilst the surfaces are together (it's the lowest energy condition and I don't think leakage is a relevant concept). Pulling the sheets apart is what generates the High Volts.

I was only briefly and peripherally involved with film manufacturing (helped install an extruder & pelletizer in a film development lab), but you are right - the mention about 'glue' being involved is a head-scratcher. My understanding is that once it has cooled enough the bubble is fed through a slitter and branches off to feed one or more take-up rolls.

If there was a unique mark on the roll it would be possible to estimate it's speed (it appears fast to me), but this video snippet shows film being taken up onto a roll, and generating discharges similar in character to those I had observed with thicker, 12.5 mil sheet at 85 feet per minute.

 

[sophiecentaur]

the mention about 'glue' being involved is a head-scratcher.

Poor script writing by someone who didn't know what it was all about, I expect.

Every process that involves lengths of stuff going over rollers requires static electric charge to be taken into account so there's no surprise it happens here. too. I think it's often dealt with by having metal 'combs' across the surface of the belt / film to neutralise the charges (shock and fire risk). Can you remember such a comb on the machines you saw?

 

[Asymptotic]

Can you remember such a comb on the machines you saw?

At one time it was common practice to stretch copper tinsel across the sheet line, but there isn't a good way to do it without getting in the way of an operator while stringing sheet through at start-up, and such installations didn't survive for long. On the other hand, operators were quick to point out a malfunctioning high voltage deionizer bar at a bagging machine's film roll, because that created a jamming nightmare for them every few minutes :)

 

[sophiecentaur]

a malfunctioning high voltage deionizer bar

That would, presumably be a posh version of a neutralising comb and pretty vital!

 

[darth boozer]

It might be worth reading the article about the electrophorus, which explains how a charged insulator can repeatedly charge a conductor. The induced charge is grounded by touching the conductor then the conductor becomes charged by the work done in removing it from the attraction of the charged insulator, which retains its original charge.
https://en.wikipedia.org/wiki/Electrophorus

 

[sophiecentaur]

It might be worth reading the article about the electrophorus, which explains how a charged insulator can repeatedly charge a conductor. The induced charge is grounded by touching the conductor then the conductor becomes charged by the work done in removing it from the attraction of the charged insulator, which retains its original charge.
https://en.wikipedia.org/wiki/Electrophorus

I already introduced the electrophorus near the top of the thread. Inductive charging is much more efficatious.