Page 91 - Handbook of Adhesion Promoters

84 Typical Primer Formulations and Applications

Figure 5.1. Different mechanisms of adhesion between a substrate (typically a silicone), a coating (here a primer)
and a metal. [Adapted, by permission, from Picard, L; Phalip, P; Fleury, E; Ganachaud, F, Prog. Org. Coat., 80,
120-41, 2015.]
Part B
18.5 wt% phosphoric acid
16.2 wt% isopropanol
65.3 wt% water
Mixing ratio: 4 parts A+1 part B

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Primer for silicone elastomer/metal
Figure 5.1 illustrates mechanisms of adhesion between silicone coating and primed metal.
The adhesive strength of the interface can be improved by the diffusion of chains between
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the primer film and elastomer surface. Both components must be mutually miscible and
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compatible. Polydimethylsiloxane has the advantage of easily spreading on most sur-
-1
faces because of its low surface tension (20.4 mN m ) which is less than the critical sur-
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-1
face tension for most substrates (between 500 and 1800 mN m for metals). To favor
-1
adhesion, a PDMS of molar mass of 30,000 g mol is used in the elastomer matrix of
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primer to create entanglements by the highly mobile PDMS chains. Primer solutions
(xylene, toluene, naphtha) generally contain between 1 and 25 wt% of active content and
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generate films of 0.1-100 µm thick. Theoretically, a monolayer of silanes on the surface
of a substrate would be enough to obtain a cohesive film primer and a good adhesion with
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the matrix to assemble. Primers containing aminosilanes form stable adhesive bonds
between epoxy adhesive and steel substrate, glycidyloxysilanes are preferred for applica-
tions involving aluminum substrate, mercaptosilanes are particularly useful for enhancing
adhesion to noble metals such as silver or gold, which are inert to most other chemical
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functionalities. For free-radical and addition cure silicone elastomers, vinyl-, allyl- or
methacryloxysilanes are preferred. For condensation cure silicone elastomers, tetraalkox-
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ysilane or its partially hydrolyzed and condensed products are favored. Glycidyloxysi-
lane or glycidyloxy functionalized compounds improve the adhesive strength of the
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primer film on the metal surface, especially with aluminum.