Static Myth: That Roll Charged on the Truck

Transcription

1 Static Myth: That Charged on the Truck Kelly Robinson, PE, PhD Electrostatic Answers ABSTRACT Wound rolls of polymers web such as polyethylene, polypropylene and polyethylene terephthalate often seem to charge spontaneously during transit from one facility to another. For example, a wide roll that is seemingly free of static is unloaded from the extruder winder, transported to the coating facility, and unwound only to have high levels of static. A hot roll has enough static to cause sparks and shocks to operators. The hot roll did not charge spontaneously during transit. The root cause of the high static is the large amount of static stored in the roll when wound. An analysis of several ineffective static control systems shows how a roll that seems to have little static is, in fact, a hot roll. Eliminate hot rolls by winding charge free web. 1. INTRODUCTION -to-roll (RtR) manufacturing processes have been used to print on long, continuous paper webs since at least 1847 [1]. Insulating webs have run in high speed processes since at least 1931 [2]. Recent improvements in RtR process technologies have enabled the production of a wide variety of new products including flexible electronic circuits, electronic displays, solar cells, and biologically active sensors [3]. These products each have carefully designed s that provide high value to the end product. s that accumulate during production processes on insulating polymer web risk attracting dust and causing sparks that damage the carefully engineered s, resetting machine control systems, igniting flammable solvent vapors, and injury operators. Static control in RtR processes using highly insulating polymers such as polyethylene (PE), polypropylene (PP), and polyethylene terephthalate (PET) is challenging because these polymers are on the negative side of the triboelectric series [4] and they are prone to becoming highly charged. Controlling static in RtR operations has been a challenge at least since the 1906 when a static bar using pins that are electrically connected was claimed for use in a printing press [5]. A thin web that is many thousands of feet long can be wound into a single roll where large amounts of static charge can be stored. The need to control static on winding rolls has been recognized at least since 1968 when insulating PE, PP, and PET webs were claimed to be wound into rolls substantially free of electrostatic charges by dangling strands of tinsel on the winding roll [6]. Large amounts of charge can be stored even when winding an electrically neutral web. The polymer web is so insulating that the static charges will persist on the web for months and years. When the web is later unwound in the next operation, this stored charge causes static discharges and sparks characteristic of hot rolls. 2. ANALYSIS 2.1 a Charged Web An uncharged web enters the tension control nip in Fig. 1 just prior to being wound. of the web. Tribocharging between the metal roller and the web is very small so that the K. Robinson Static Myth That Charged on the Truck 1/8

2 top of the web exits the nip free of static charge. The charge on the bottom is wound into the roll and stored on the of the insulating web. The of the winding roll is charge free. An electric fieldmeter measures the electric field near the of the winding roll has the same polarity as the charge on the web from the polymer nip roller. The electric field is sufficiently large to cause sparks and shocks to operators. This is a hot roll. roll top Polymer covered nip roll on bottom ~ 15 Fig. 1: Charges on the bottom of the web from the polymer nip roller are stored in the wound roll. Imagine that we unload the roll wound in Fig. 1 from the winder and unwinding it in Fig. 2 across the floor. The charges stored when the roll was wound remain on the insulating web. The of the roll is charge free, so the of the web touching the floor is charge free. The static charges from the polymer covered nip roller stored on the inside of the roll are on the top of the web. An electrostatic fieldmeter measures the electric field near the unwinding roll to have the same polarity as the charge on the web from the polymer covered roller. The electric field is comparable to the field measured when winding the roll. The field is sufficiently large to cause sparks and shock operators. The unwinding roll is a hot roll. ~ 15 Unroll on floor on inside Floor Fig. 2: When the roll is unwound, charge from the polymer covered nip roller remains on the inside of the web. K. Robinson Static Myth That Charged on the Truck 2/8

3 2.2 an Electrically Neutralized Web roller Polymer covered nip roller top on bottom ~ 10 Charged Fig. 3: Charges from the polymer covered nip roller and from the static are stored in the winding roll. The web being wound is electrically neutral. An uncharged web enters the tension control nip in Fig. 3 just prior to being wound. of the web. Static SD1 is located above the web to neutralize the web prior to winding. The static responds to the charge on the bottom of the web and deposits an equal amount of oppositely charged ions on the top of the web forming polar charge [7]. Polar charge is a pattern of charge where one web has positive charge and the other has an equal amount of negative charge. The charges on the web s are wound into the roll. The of the winding roll has negative charge from the static. The interior of the winding roll is approximately charge neutral. It is not charge free! At the winding nip, the negative charge density on the of the roll is approximately equal to the positive charge density on the entering web. An electric fieldmeter measures the electric field near the of the winding roll has the same polarity as the ions from the static. The electric field is sufficiently large to cause sparks and shocks to operators. ~ 10 Unroll on floor Static SD1 Neutralizing charge from Fig. 4: When the roll is unwound, charges from the polymer covered nip roller and from the static remain on the web s. (Note, this charge pattern is the same as tribocharging!) Floor Neutralizing charge from static from polymer roller K. Robinson Static Myth That Charged on the Truck 3/8

4 Imagine that we unload the roll wound in Fig. 3 from the winder and unwinding it in Fig. 4 across the floor. The charges stored when the roll was wound remain on the insulating web. The of the roll has negative charge from the static, so the of the web touching the floor has negative charge. The static charges from the polymer covered nip roller stored on the inside of the roll are on the top of the web. The web on the floor is electrically neutral. However, an electrostatic fieldmeter measures the electric field near the unwinding roll to have the same polarity as the charge on the web from the static. The electric field is comparable to the field measured when winding the roll. The field is sufficiently large to cause sparks and shock operators. Even though the web exiting the unwinding roll is electrically neutral (not charge free), the unwinding roll is a hot roll. Note that the charge pattern on the unwinding roll is identical to tribocharging occurring at the unwinding nip. A roll wound with polar charge is indistinguishable from unwinding a roll where tribocharging occurs at the unwinding nip. The only difference is that polar charge can be 100X larger that the charge resulting from tribocharging! 2.3 with a Dissipater on the roller top Polymer covered nip roller Static SD1 on bottom Neutralizing charge from Charged Static SD2 ~ 0 Fig. 5: Charges from the polymer covered nip roller and from the two static s are stored in the winding roll. The web being wound is electrically neutral. An uncharged web enters the tension control nip in Fig. 5 just prior to being wound. of the web. Static SD1 is located above the web to neutralize the web prior to winding. SD1 responds to the charge on the bottom of the web and deposits an equal amount of oppositely charged ions on the top of the web forming polar charge. The charges on the web s are wound into the roll. Initially, the of the winding roll has negative charge from static SD1. Static SD2 neutralizes a portion of this charge so that the electric field near the of the winding roll is zero. With a portion of the negative charge neutralized, the interior of the winding roll stores an abundance of positive charge. Consequently, SD2 deposits negative charge on the of the winding roll to maintain a zero electric field at the of the winding roll. K. Robinson Static Myth That Charged on the Truck 4/8

5 Shocks to operators from the wound roll are no longer a threat. The result is that the interior of the roll has an abundance of positive charge and the of the roll has an abundance of negative charge. The roll is certainly not charge free. ~ 0 Unroll on floor Fig. 6: When the roll is unwound, charges from the polymer covered nip roller and from both static remain on the web s. Imagine that we unload the roll wound in Fig. 5 from the winder and unwinding it in Fig. 6 across the floor. The charges stored when the roll was wound remain on the insulating web. The of the roll has negative charge from the static s, so the of the web touching the floor has negative charge. The static charges from the polymer covered nip roller stored on the inside of the roll are on the top of the web. The web on the floor has an abundance of positive charge. When we pull the end of the web and hold it above the floor, the electric field near the web has the polarity as charge from the polymer roller. And the field is sufficient to attract dust and cause jams in sheeting operation. The electric field near the unwinding roll is zero because SD2 constrained the field to be zero on the winding roll. 2.4 Use a Static Wand on the Wound roll Neutralizing charge from static s Floor top on bottom Polymer covered nip roll from polymer roller Fig. 7: Charge on the bottom of the web from the polymer covered nip roller is stored in the wound roll. K. Robinson Static Myth That Charged on the Truck 5/8 Hold web off the floor to measure E SPAN ~ 4 ~ 15

6 An uncharged web enters the tension control nip in Fig. 7 just prior to being wound. of the web. The charge on the bottom is wound into the roll and stored on the of the insulating web. The of the winding roll is charge free. An electric fieldmeter measures the electric field near the of the winding roll has the same polarity as the charge on the web from the polymer nip roller. The electric field is sufficiently large to cause sparks and shocks to operators. Pass a passive over the of the wound roll Fig. 8: On the of the wound roll, negative charge from the passive balances the positive charge stored inside the roll from the polymer covered nip roller. A static wand is a handheld passive static often used to treat wound rolls before they are unloaded from the winder. Static wands effectively remove the shock risk to operators who handle the rolls. The static wand in Fig. 8 is a passive tinsel, brush, or ionizing cord that is passed over the of the wound roll while it is still on the winder spindle. After treatment, the electric field near the of the wound roll is nearly zero. ~ 15 Unroll on floor Wound on inside Floor Negative charge band from static wand Fig. 9: When the roll is unwound, charge from the polymer covered nip roller remains on the inside of the web. The band of negative charge has a length equal to the circumference of the wound roll. Charged ~ 0 K. Robinson Static Myth That Charged on the Truck 6/8

7 Imagine that we unload the roll wound in Fig. 8 treated by the static wand and unwind it in Fig. 9 across the floor. The charges stored when the roll was wound remain on the insulating web. After the first lap is unwound, the of the roll is charge free. The static charges from the polymer covered nip roller stored on the inside of the roll are on the top of the web. An electrostatic fieldmeter measures the electric field near the unwinding roll to have the same polarity as the charge on the web from the polymer covered roller. The electric field is comparable to the field measured when winding the roll. The field is sufficiently large to cause sparks and shock operators. The unwinding roll is a hot roll. The charge from the static wand forms a band of negative charge with a length equal to the circumference of the wound roll. The patch is highly negative because there is one negative charge in the patch for every positive charge on the thousands of feet of web in the wound roll. While treatment with a static wand removes the risk of static shocks to operators who handle the roll, high levels of static remain stored within the roll. The roll will be a hot roll when unwound. 2.5 GOAL: Wind a Charge Free! roller Polymer covered nip roller top on bottom Static SD1 Charged free ~ 0 Fig. 10: Charge on the bottom of the web from the polymer covered nip roller is neutralized by static SD1. An uncharged web enters the tension control nip in Fig. 10 just prior to being wound. of the web. The charge on the bottom is neutralized by static SD1 facing the same side of the web that touched the polymer roller. Now, both s of the web entering the winding roll are charge free. An electric fieldmeter measures the electric field near the of the winding roll and confirms that the electric field is nearly zero. When unwound, this roll will have little static. K. Robinson Static Myth That Charged on the Truck 7/8

8 SUMMARY Wound rolls with many thousands of feet of highly insulating web can store large amounts of static. s wound into a roll of highly insulating web will be present when the roll is unwound in the next operation. Wound rolls of highly insulating web do not spontaneously charge while in storage or while in transit. To eliminate hot rolls, wind web that is charge free. REFERENCES 1. Hoe, R. M., Improvement in Rotary Printing-Presses, U.S. Patent No. 5,199, issued 7/24/ Sheppard, S. E. and J. G. McNally, Cellulose Film, U.S. Patent No. 1,808,998, issued 6/9/ Robinson, K. S. and W. Durkin, Electrostatic Issues in -to- Manufacturing Operations, IEEE Transactions on Industry Applications, Vol. 46, No. 6, pg , November / December Robinson, K. S., Triboelectric Series by Electrostatic Answers, 9/01/ Hardwicke, W. L., Static Discharger, U. S. Patent No. 836,576, issued 11/20/ Goetemann, G. B., Method for Forming a Pellicle into Form Substantially Free of Electrostatic Charges on the Surface Thereof, U. S. Patent No. 3,392,311, issued 7/9/ Gibbons, C. B., Method of Altering Electrostatic Charge on an Insulating Material, U.S. Patent No. 3,470,417, issued 9/30/1969. K. Robinson Static Myth That Charged on the Truck 8/8