of non-conductive web surfaces, such as pressure-sensitive adhesives (PSA)
tapes, must be vigilant in controlling static electricity.
Converters of non-conductive web surfaces,
such as pressure-sensitive adhesive (PSA) tapes, must be vigilant in
controlling static electricity. High levels of electrostatic charges can be
dangerous; moderate levels can permanently destroy an adhesive system, foul and
jam production processes, and damage finished products.
New technology allowing faster production
speeds exacerbates static-related problems. To reduce static, operators
sometimes will intentionally slow down the production process, which can be a
The Cause: Contact and Separation
Static electricity is an accumulation of electrical charges
on a surface. The polarity of the charge can result from an excess of electrons
(negative charge) or a deficiency of electrons (positive charge). Charges
accumulate on the surface of non-conductive material, such as paper or plastic
film, and can also accumulate on ungrounded conductive materials, such as
machine parts and the human body.
When surfaces are in contract with each
other, a transfer of electrons occurs. Friction, pressure and web speed accelerate
this transfer. When separated, the surface that has gained electrons becomes
negatively charged, and the mating surface giving up the electrons becomes
This process, known as triboelectrification
(or tribocharging), results in the generation of static charges on the surface
of PSA tape as it unwinds from the roll and as it contacts and separates from
surfaces, such as idler rolls, nip rolls, and printing or coating rolls. The
buildup of electrostatic charges is a cumulative process, increasing each time
the web contacts another surface. PSA tape can, therefore, accumulate a high
charge as it passes through transport systems in which it comes in contact with
On non-conductive materials, such as papers
and films used for PSA tape, charges do not build up uniformly across the
surface. A single, contiguous surface may accumulate a positive charge in some
spots and a negative charge, or no charge, in others. The charge’s intensity
can also vary dramatically. For example, a positively charged “island” of 30 kV
might coexist 12 inches from a positively charged island of only 500 volts, and
18 inches from a negatively charged island of 2.5 kV.
Everyone is familiar with the shock or spark caused by
so-called electrostatic discharge (ESD) events. On a small scale, you feel an
ESD in the form of a spark when you touch a doorknob after walking across a
carpet. On a much larger scale, an ESD is observed when lightning occurs. An ESD
takes place when a static electrical field exceeds a certain threshold value,
causing an ionized conductive channel to form in the air as Nature’s desire for
balance plays out.
The following should be considered when ESD
An ESD can be painful and cause burns, cardiac problems, and even death. There
is also the secondary hazard of operators suddenly recoiling from the shock and
injuring themselves by falling against dangerous machinery.
Static discharge is a spark. Areas
with flammable liquids or gases, such as solvents, must remain spark-free at
all times. Electrostatic discharge can have sufficient energy to ignite
hazardous vapors in coating heads and gravure-printing operations. An ESD event
can also disrupt logic in PLCs and sensing equipment, causing processing errors
and costly downtime.
Damage to the
Static buildup and ESD events can
damage the PSA release liner of the product being unwound. When the silicone
coating is disrupted by static, it will no longer function as a release system
in the area affected by the discharge. The adhesive will “split” between two
sides of release liner. In addition, the liner can fail to release where it is
damaged, making it very difficult to remove the liner as designed.
The accumulation of electrostatic charges alone can cause problems, even if ESD
events do not occur. If static charges are not neutralized, sheets of materials
will stick together, creating jams in downstream processes.
Electrostatic fields also attract dust particles, fibers, bugs and hair, which
results in surface contamination. This causes quality problems in printing,
coating and laminating, as well as cleanliness problems with medical PSA
applications. Moreover, static charges can cause uneven coatings and “wicking”
of inks, and pressure-sensitive tape carrying a static charge can damage
sensitive electronic components.
Common Static Control Methods
To control static on coating and converting machines, two
types of ionizers are available:
passive ionizers and active ionizers. Passive ionizers (also called
induction ionizers and non-permanent static control devices) include static tinsel
and static string, which are grounded emitters placed parallel and close to the
charged material. The electrical energy of the charged material will excite the
passive ionizer, causing it to generate air ions of the opposite polarity. When
properly positioned, a passive ionizer can successfully reduce the
Active ionizers (also called permanent
static control devices), such as high-output static neutralizer bars and static
blowers, are powered by an external source of electrical voltage.
Alternatively, radioactive material can be used as an ion source. The
relatively high purchase cost of active ionizers has limited their use in the
Tinsel, also know as garland, is relatively
inexpensive and remains one of the most widely used methods for controlling
static. However, anti-static tinsel is composed of copper, and pieces of this
soft metal can break off and cause contamination.
To address this problem, engineers at Adchem Corp. tested a
variety of passive ionizer products to determine whether any of these methods
could provide equal or better static control compared to tinsel, without
tinsel’s risk of product contamination.
Side-by-side evaluations were performed at
Adchem’s 200,000-square-foot engineering and manufacturing facility in
Riverhead, NY. Five passive ionizer products were tested: anti-static tinsel,
static elastic, static wire and two types of static string. Each anti-static
product was prepared and installed in static “hot-spots” along the production
The static level was measured in a
free-space (an ample distance from any rollers) 10 minutes after the start of a
production run. Precise measurement of the static charge over a free-span web
surface was performed with a hand-held electrostatic field meter.
The testing procedure was repeated for five
different PSA products with non-conductive web surfaces composed of a variety
Superior Performance of "Stop Static" String
The test results showed that alternative passive ionizer
methods can provide better static control than tinsel. In fact, one product,
the Static String from Stop Static, a division of Alpha Innovation, Marblehead,
MA, provided significantly better static control for all five web surfaces
tested. The product consistently maintained static levels below the “5000 volts
rule” established by Albert E. Seaver in 1993, which is generally accepted as
the industry safety standard. The Static String outperformed other passive
ionizer products, including a similar string product from another manufacturer.
The Static String offers the advantage of
effectiveness when installed 0.25-2 inches from the web. Because it does not
need to make contact with the web, it lessens the possibility of web damage,
coating damage or contamination. It was also found to be relatively easy to
install and remove, especially compared to tinsel.
Tests at Adchem show that, when installed correctly, Static
String consistently reduced electric charge below the desired level of 5kV in
all products tested to date. Adchem has replaced tinsel with Static String on
all its coating and converting machines. As a result, the measured static level
on all machines has been reduced. Moreover, the Adchem plant has eliminated the
risk of product contamination by pieces of copper tinsel.
For more information, contact Adchem Corp.,
1852 Old Country Road, Riverhead, NY 11901; phone (631) 727-6000; fax (631)
727-6010; e-mail firstname.lastname@example.org; or visit www.adchem.com.