by Bob Stenzel
It is not rocket science to understand that a clean, dry and sound bonding
surface is critical to achieve satisfactory adhesion. This statement is
true in our industry as it is in nearly every other industry where adhesive
bonding occurs. “Dry” and “sound” glass parts have generally not been
an issue in the automotive aftermarket. However, the “clean” portion of
this equation has recently come under increased scrutiny as glass part
designs have slowly evolved throughout the industry.
So how would technicians in the field know if a glass part they are
installing is clean enough?
One of the increasing technical challenges in the AGRR market is the identification
and removal of contamination on the bonding surface of automotive aftermarket
glass parts. Historically, industry accepted glass cleaners, applied by
spraying onto the surface and removing with a clean towel, has been the
primary pretreatment recommendation for glass parts in AGR. However, there
is evidence to indicate this pretreatment may not be enough in some instances
where non-traditional contamination (NTC) is present. As you may recall,
traditional contaminants typically include dirt, dust and fingerprint
oils. Non-traditional contaminants, on the other hand, are primarily identified
as silicone residue or mold release agents which are introduced during
the glass manufacturing process. Left untreated or improperly treated,
NTCs can significantly limit or compromise the available area for acceptable
bonding between the glass part and the adhesive and may result in a low
quality or unsafe installation.
But how do technicians know if the glass part has been contaminated
Dirt, dust and fingerprints are fairly obvious to observe prior to installation
when the part is being inspected and cleaned. To confirm contamination
from NTCs the technician can use a simple industry accepted field test
to verify that an NTC is present. One item that works in our favor is
that known NTCs, present in sufficient quantities, will affect the surface
tension of the frit. This characteristic allows glass cleaner to work
as a type of litmus test to detect their existence in the field. To determine
the presence of NTCs, it is recommended that prior to wiping off the glass
cleaner the technician should allow it to rest on the bonding surface
for five to 10 seconds and to look for uneven migration of the cleaner
(See photo at left).Glass cleaner will tend to migrate away from contamination.
If NTCs are detected it is recommended to follow your adhesive manufacturer’s
instruction for treatment. Note that with this method, foaming glass cleaners
tend to display non-uniform surface tension in a manner that is easier
to recognize, but non-foaming cleaners can also work quite well.
What have adhesive manufacturers done to address these concerns with
The relationship between glass and urethane manufacturers has been fortified
in recent years resulting in better understanding and increased discussion
of the interaction that promotes acceptable adhesion. Glass manufacturers
appreciate the feedback provided through results of extensive adhesion
testing of their parts, while adhesive manufacturers welcome a forum to
exchange ideas that will improve the quality of the final assembly. This
progressive synergy has yielded positive benefits and with continued cooperation,
will provide much needed value for the industry at large. The bottom line
is that information sharing helps all involved parties to examine and
acknowledge the critical interface of the bond line that advances the
quality and safety for the entire AGRR market.
Although a key objective of all urethane manufacturers is to reduce the
complexity of their adhesive systems, the primary goal has to remain focused
on a quality installation and ultimately, account for the safety of the
vehicles occupants. Most urethane manufacturers have acknowledged the
presence of NTCs and have addressed the treatment in their written application
procedures. Typically, the initial recommendation by nearly all urethane
manufacturers has been to “wet scrub” the bonding surface with glass cleaner
and a woven abrasive pad. While this method has been proven to be satisfactory
for most cases of NTCs, several polyurethane manufacturers have developed
additional products or techniques to address this issue when wet scrubbing
when glass cleaner is insufficient (See photo above).
The additional investment in time and materials to effectively treat
NTCs is relatively inconsequential when compared to the potential risks
of adhesion failure. Even putting the total amount associated with a catastrophic
failure aside, performing a repair or repair and replacement to fix a
leaker will not only cost your company valuable resources, but your reputation
and future business may be affected by even one dissatisfied customer.
In fact, it could be argued that the entire investment of properly treating
bonding surfaces contaminated with NTCs throughout the year would be justified
by just one less call back.
Although experience dictates that occurrence of NTCs is relatively low
as a percentage of all glass parts installed in the aftermarket, technicians
need to be trained properly to identify and treat the issue when it is
encountered. Regardless of which urethane system you use, improved quality
of an adhesive bond begins with proper treatment for encountered contamination.
Without a solid foundation even the highest quality adhesive installed
by the best technician can be rendered ineffective.
Bob Stenzel is a senior AGR application engineer for Sika Corp.
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