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Contaminated Windscreens: the Missing Link - Glasstec Paul


With each windscreen replacement, a key component of the vehicle’s structural system is put into place. The crucial part of this assembly is the bond between two contact surfaces: the glass and the car (specifically the aperture, or pinchweld). This critical area can develop air and water leaks, and at worst, can compromise the overall safety of the occupants (in the event of a collision or rollover scenario) if contaminants are not removed as part of the preparation process.

Preparing bonded windscreens used to be a bit of a grey area.

Most modern car windscreens have a black band, or ceramic ‘frit’ around the perimeter of the glass. This essentially provides a suitable surface for the Polyurethane (PUR) to adhere to, as well as acting as UV block which, can break down the bond.  When preparing for installation, the glass must be decontaminated to rid this substrate of any substances which may have been introduced during handling, such as grease, oily fingerprints and other substances. The performance of the adhesive can be adversely affected if the glass – and aperture – is not prepared correctly by following the adhesive manufacturer’s process instructions.

New products are now readily available after the recent discovery of one particular – invisible – substance which is deposited on the glass during manufacture. Silicone residue interferes with the adhesive bonding and following extensive research, has now been identified as a cause for bond failure. Previously, some windscreens would reveal areas of wax-like deposits which installers would try and scrub away with an abrasive cloth. But back then, nobody really knew what that substance was, and most installers would take a leap of faith relying on the adhesive manufacturer’s bonding ‘kit’ to do the trick.  Research has proven that standard glass cleaner, or abrasive cloths will not remove silicone contamination.

Today, leading PUR manufacturers recommend that all windscreens are decontaminated with an appropriate agent and neutralised of silicone interference. Whilst some windscreens, especially OEM branded versions, show little or no signs of decontamination, the wisest approach is to go through the removal process as a matter of course.

But there still are grey areas.

Presently there is no official word on whether or not OEM windscreens are decontaminated before being fitted to cars during assembly. But there is evidence to suggest that these ‘genuine’ screens go through a different manufacturing process to their aftermarket counterparts. One of these differences may well be a ‘washing’ process so that glass units arriving at car assembly lines are ready, and fit for purpose. This – if true – would make sense evidently in the interests of a time-saving exercise for car manufacturers to benefit from. One thing however is very clear: most windscreens will show some degree of contamination.

Detecting silicone deposits on glass is very simple: if the glass surface (two-three inch wide area around the edge  of the glass) appears to repel (the adhesive manufacturer’s) glass cleaner, the glass is contaminated.

But what if the glass manufacturers have pre-applied material to that area?

Pre-applied primer on windscreen

Pre-applied primer on windscreen









Windscreens, such as for the Porsche 996 and Golf Mark 4 are produced with a pre-applied bead of PUR. Are these screens decontaminated before this ‘pre-encapsulating’ process? How about other screens, where a primer line is applied to the frit? Many windscreens will also have extruded trims, or mouldings bonded to the glass during manufacture. Installers are taking the same leap of faith when they are applying bonding material to glaze these units directly to the vehicle and are reluctantly relying on someone else’s work for the integrity of that bond. It’s an almost impossible task to remove cured primer and PUR. This Fiat Bravo screen is produced with a formed PUR buffer at the top and bottom of the screen. This is bonded to the glass sandwiching a coating of black adhesion promoter, or primer, between it and the glass surface. A test concluded  that the sides of the windscreen revealed heavy deposits of silicone which, makes a strong claim that this windscreen was not decontaminated before these additional items were introduced.

This windscreen is contaminated

Contaminated Fiat Bravo Windscreen

PUR adhesive manufacturers and windscreen installers have identified a problem and this partnership has brought about a better awareness in how to achieve the best results for correctly bonded – and safe – windscreens. But the industry needs better communication from glass manufacturers. Glass production is a huge global concern with thousands of units being manufactured every day; there are windscreen plants all over the world and UK suppliers are busy doing what they know best; windscreen replacement companies are striving to do the best they can by investing in training and using the best available products. Technological advancement in PUR formulation has given us the best adhesive systems the trade has ever seen, and with glass being the chief character in this plot, the industry deserves better communication from the glass manufacturers.