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Fundamentals of Windshield Repair Adhesives

"The strength of the adhesive joint will be the strength of it's weakest member."

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1. Adhesive strength - the interaction that develops between the resin and the glass causing a subsequent bond to the glass.
A. Adhesion - resin properties that causes a bond to the glass.
A-1. Absorption - the property of the interface between the resin and the glass.
A-2. Interface - the molecular linking of the resin to the glass.
A-3. Interaction - the atoms of the resin interact with the atoms of the glass causing the molecules to link together.
A-4. Wetting - the filling of the surface crevices and irregularities; a must for good absorption and adhesive strength.
2. Cohesive strength - the internal strength of the adhesive mass or it's resistance to creep under stress; the ability of the resin to resist splitting. To increase cohesive strength it is necessary to increase the molecular weight and the consequent chain entanglement contributions or to provide cross linking.
A. Viscosity (thickness of resin) - the property of resins that causes them to resist flowing. It is caused by internal friction from the resin's molecules moving against each other. One way to increase a liquid's viscosity is to dissolve polymers (long-chain molecules) in it. These molecules become entangled and resist flowing. Putting solid particles in suspension in a resin also increases viscosity as will adding strengthening fillers. Temperature determines how strongly molecules interact; hot resins have lower viscosity than cold. To increase an adhesive's cohesive strength will also increase it's viscosity.
B. Molecular weight - the number and size of the resin's molecules. As the molecular weight is increased, the cohesive strength rises as well as the viscosity. Low molecular weight (viscosity) resins have lower cohesive strength.
C. Creep - describes the slow deformation of a polymer (windshield repair resin) under constant stress.
D. Structural adhesives - able to carry sizable loads without yielding, will support stress as well as the weight of the surfaces bonded. Structural adhesives are higher in viscosity -- over 100 cps.

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References

   FAQ
   Before/After Photos
   Incorrect Resin Photos
   Correct Resin Photos
   Fundamentals of Windshield Repair Adhesives
   Repairable Windshield Damage
   Windshields are Made to Crack
   Windshields Repair Standard (ROLAGS)

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