- October 31, 2012, Kelly Robinson
Why does unwinding a roll of homogeneous web generate so much static electricity? After all, tribocharging occurs when two different materials touch and separate. Since a bare PET web has the same material on both sides, unwinding should generate no tribocharging, right?
Right! Unwinding rolls of homogeneous webs will have low tribocharging. However, very high levels of static occur when unwinding rolls of homogeneous web that were wound with polar charge. I call this “process charge” instead of “tribocharge.” Remember, polar charge is positive charge on one surface of the web and an equal amount of negative charge on the other surface.
To see what is going on, let’s go back to the operation where the roll is wound in Figure 1. In this operation, the static control system is working well and the bare PET web entering the winding section of the machine is charge free. The bottom surface of the web touches a polymer nip roller that isolates winding tension from the rest of the machine. The polymer nip roller deposits negative static on the bottom surface. The electrostatic fieldmeter reading E1 of –10 KV/cm indicates that the charge density on the bottom surface is about –9 μC/m2.
Static bar SB1 has been installed to neutralize the charge from the nip roller. However, the static bar is installed improperly to face the top surface when the static charge is on the bottom surface. Nonetheless, SB1 functions as designed and deposits +9 μC/m2 of neutralizing ions on the top surface of the web. Fieldmeter reading E2 of 0 KV/cm indicates that the web is charge neutral. Of course, the web is not charge free. The non-contacting electrostatic voltmeter VTOP measures +10 volts indicating that the charge density on the top surface of the web is +9 μC/m2. Similarly, the winding roll having just one lap has an electric potential of +10 volts.
As the roll continues to wind in Figure 2, the potential increases by +10 volts with each lap. When the roll has 1,000 laps, the potential is +10 KV.
Now that we’ve wound the roll, let’s unwind the roll in Figure 3 and look at pattern of static charges on the web. Polar charge is a pattern of charge with positive charge on one surface and an equal amount of negative charge on the other surface. Polar charge on an insulating PET web persists for years, and it will be there when we unwind the roll. So, the charge pattern on the unwinding roll will be just the same as the pattern when we wound the roll. When there are 1,000 laps left on the roll, the electric potential of the roll will be +10 KV. The electric field measured on the web span exiting the roll will be zero confirming that the web is charge neutral. Of course, we can use electrostatic voltmeters to measure the charge on the web. Voltmeter reading VTOP of +10 volts indicates that the charge density on the top surface of the web is +9 μC/m2 and reading VBOTTOM of –10 volts shows that the charge density on the bottom surface of the web is –9 μC/m2.
The fieldmeter reading and the voltmeter readings are the same as we would measure if tribocharging occurs on the unwinding roll. Unwinding a roll with tribocharging and unwinding a web with polar charge look exactly the same. This is important because the root causes of the static problems are quite different! Tribocharging is a material interaction. Polar charging is a “process” problem that usually is caused by a static dissipater or many static dissipaters in previous processes that are installed to face the wrong side of the web.
Tribocharging occurs when two different surfaces touch and separate. High static on an unwinding roll of homogeneous web like PET usually is caused by polar charge on the web when winding the roll. The static readings on an unwinding roll look exactly the same when the root cause is tribocharging and when it is polar charge. The main difference is that polar charge can be 10X or even 100X bigger than charge from tribocharging. Really high static on an unwinding roll is usually caused by polar charge.
I invite you to ask questions about this column and to suggest future topics. My email address is: Kelly.Robinson@ElectrostaticAnswers.com.
Static control expert Dr. Kelly Robinson, president of Electrostatic Answers, has 27+ years of experience in problem-solving and consulting. Contact him at 585-425-8158; firstname.lastname@example.org; www.electrostaticanswers.com.