A common undesirable effect of degradation is discoloration. Both natural polymer
materials and synthetic polymers in adhesives and sealants undergo induced discoloration
as an effect of degradation. Typically observed by an increase in yellowness or
darkening, discoloration often appears prior to any measurable loss in physical
properties. It is usually the first critical indicator of damage that is observed,
which determines the useful service life of the product.
Although color development occurs at widely differing degrees in various adhesives
and sealants and may not directly correspond to a decrease in the mechanical properties,
a change in appearance is often unacceptable in many adhesive applications.
Adhesive without AO
Adhesive with AO-1
Adhesive with AO-2
The photo above illustrates the positive effect of antioxidant systems on the color
stability of an adhesive after heat aging at 170°C.
Although not illustrated, additional benefits of antioxidants may include improved
initial color of the adhesive prior to thermal aging.