Page 56 - Handbook of Adhesion Promoters

3.3 Detachment 49

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efficiency. HALS can delay the loss of adhesion strength and yellowing. On the con-
trary, UV absorbers and phosphites are involved in delamination and yellowing processes
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due to their photodegradation. The decomposition of UV absorbers leads to the forma-
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tion of benzoic acid and a phenol products. The acid catalyzes the adhesion loss at the
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EVA-glass interface whereby the phenol product is the reason for the discoloration.
The above is just one example of the polymer weathering effect on delamination, a
broad review of the photodegradative processes can be found in the recent monographic
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publication on the subject.
3.3 DETACHMENT
Adhesion of mortars to concrete and brick walls is a complex mechanism that is affected
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by several factors. The adhesion loss leads to mortar detachment from the substrate
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which may cause hazardous situations to passers-by. Without mortar, the wall is directly
exposed to the atmosphere and, therefore, its degradation processes most likely increase. 14
Factors related to mortars’ components, renderers characteristics, substrate characteristics,
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and mortar applications process determine adhesion of mortar to substrate. The pull-off
test, described in the European standard EN 1015-12 for rendering and plastering mortars,
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measures the value of the mortar-substrate normal adhesion, when the interface fails.
3.4 DEBONDING

Debonding, in the context of this book, is and example of loss of adhesion between rein-
forcement (particle or fiber) and matrix, due to the action of different stressors, such as
hygrothermal aging, fatigue, or external stress. Below are some examples of these pro-
cesses.
Subcritical debonding is of particular concern for microelectronic packaging, coat-
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ing, and adhesive applications. The subcritical debonding of epoxy/glass interfaces
under the hygrothermal condition is investigated in the presence of organosilane adhesion
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promoters, 3-aminopropyltriethoxysilane and glycidoxypropyltrimethoxysilane. The
subcritical debonding growth rate mechanism was found to be sensitive to the interface
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chemistry. Hygrothermal aging lowers the critical debonding driving energy required for
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debond extension. The silane adhesion promoters decreased debonding growth rate.
A short glass fiber reinforced polyamide (PA-66) was tested under fatigue loading. 16
The fractographic analysis by means of the field emission scanning electron microscope
showed that fibers are mostly covered by a resin layer, indicating that damage does not
occur at the fiber-matrix interface in a form of fiber-matrix debonding but in the resin, at a
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certain distance from the interface in a form of matrix cracking. Clean fibers on the frac-
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ture surface indicate fiber-matrix debonding. Fibers covered by a resin layer suggest that
damage occurs at a certain distance from the interface in a form of matrix cracking (simi-
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lar to fatigue damage). Sizing can affect the resin layer surrounding the fibers leading to
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the formation of the interphase.
In the silica particle-filled polyvinylbutyral, the use of an adhesion promoter
increased the maximum local stress required to cause debonding but had no effect on the
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interparticle stress fields. With the volume fraction of spherical glass beads increase, the