Volume 12, Issue 2 - March/April 2010

Ask the Doctor
pros who know

Curing Under Pressure
by Richard Campfield

One of the fundamental rules to achieving a structurally sound, long-lasting and visually superior windshield repair comes at the very end of the process. This also is when you can ruin what would have been a perfect repair.

Windshield repair is a chemical process—period. The only thing you leave the customer with is the resin/adhesive. A properly designed windshield repair tool and curing light should assist in maximizing your adhesive properties to the substrate(s).

Knowing the adhesive’s properties and the stresses of the substrate is the first fundamental rule of any adhesive application; match the adhesive to the substrate(s) and its stresses.

We have both glass and polyvinyl butyral (PVB) as substrates and both aesthetics and structural restoration are important. Stone breaks/chips will be subject to thermal stress caused by expansion and contraction of the glass from heat and cold. The most extreme stress condition is the defroster on a cold, un-garaged vehicle—more so than the shock of a car wash on a hot day, because resin is brittle when cold and elastic when hot. The PVB also turns soft, almost liquid, in the summer heat, to hard in the winter cold. If either of these bonds gives way, the repair will become visible or fail.

Windshield repair resins are acrylic—ultraviolet (UV) radical curing acrylics, to be exact. Low-viscosity acrylic resins, with which stone breaks are repaired, wet the glass and NOT the PVB. This creates surface tension. There is no chemical bond between the PVB and the resin. Acrylic resin pulls back and off of the PVB upon curing, and this can create an unsightly repair because the void between the resin and the PVB will be black and/or refracting and will increase as the PVB turns from hard (from cold winter weather) to soft (from hot weather). Even if you do not see this immediately after the repair, the separation is there and it will be visible afterwards from weathering. How do you easily avoid this? By proper curing.

Not curing under pressure compromises both the mechanical and chemical bond to the glass and leaves no bond to the PVB.

“Not curing under pressure compromises both the mechanical and chemical bond
to the glass and leaves no bond to the PVB.”

Curing Under Pressure
Curing under pressure has several benefits, as follows:

• It creates a mechanical bond to the PVB, so it does not separate upon curing or over time, creating an unsightly repair.

• Curing under pressure increases both the mechanical and chemical bond to the glass because the pressure keeps the resin in place while it cures.

• It compensates for shrinkage. The rule of thumb to address adhesive shrinkage is to overfill, and the only way to overfill a stone break is to cure under pressure. Shrinkage is caused by the tight-knit structure of the free-radical double-bonding; the UV cure being exothermic, a chemical reaction that releases heat; and surface tension caused by the wetting of the glass and not the PVB. Shrinkage causes the cross linkage to be stressed instead of relaxed, creating residual stress within the structure. Relaxed cross linkage gives the repair more elasticity to expand and contract without crosslinks snapping/breaking, creating a better bond and preventing deterioration. Slow cure also controls shrinkage, as too fast of a cure increases shrinkage and stress.

• Curing under pressure improves the cohesive strength, which is the internal bond of the resin, to itself by allowing more monomers to join with other molecules to form polymers. This is needed to bridge and fill any void/gap in the break, legs and bullseye.

In summary, do not remove your tool before you cure. Curing with your repair tool in the pressure mode forces the resin to stay put while curing, instead of shrinking and pulling off of the PVB

Richard Campfield is the founder and president of Ultra Bond Inc. in Grand Junction, Colo. Mr. Campfield’s opinions are solely his own and not necessarily those of this magazine.

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