Silicones as a general class of products have an unearned reputation in the industry for poor recoatability and intercoat adhesion. However, through proper choice and application, this does not have to be the case. Due to their surface activity, silicone additives migrate onto the coating surface and, as they usually have no reactive groups, they are not incorporated in the cross-linking mechanism of the binder, i.e. they remain mobile even after baking. This can be demonstrated, for example, by the fact that solvent can be used to wipe them off the surface again. When recoating a silicone-containing coating layer, the silicones then do not remain in the first layer (i.e. the interface between the two coating films); due to their mobility and surface activity they migrate to the new surface, i.e. that of the second coating film. No silicone remains between the two coating films, and for this reason the intercoat adhesion is not negatively influenced, either.
In connection with silicones, there are two factors, however, which can negatively influence intercoat adhesion:
(1) Dosage of the silicone additive,
(2) Baking temperature of the first coating.
For each binder/silicone combination there is an optimum quantity of silicone; higher dosages will not provide greater benefits (more wetting, anti-cratering effect, slip, etc.), but can cause undesirable side effects, such as impaired intercoat adhesion.
In the case of an overdose, some of the silicone molecules remain in the interface between the two coating films and adversely affect adhesion. When using silicones, it is therefore particularly important to use a series of tests to ascertain the optimum dosage and then not exceed it.
In addition, the baking temperature also has an influence on intercoat adhesion; when the first (silicone-containing) coating layer is overbaked, the adhesion of the second layer can be considerably reduced. This is due to the fact that at higher temperatures (140 °C-150 °C) the polyether chains of the additives are degraded by oxidation.
During these degradation processes, reactive groups are created, so that the silicone additive is now incorporated in the binder matrix and therefore loses its ability to migrate.
When overcoating, the decomposition products remaining between the two layers adversely affect adhesion by acting as a release layer. Since the thermal instability mentioned above is caused by the presence of polyether chains, it is therefore possible (by exchanging the polyether chains with more thermally stable groups) to avoid this effect. For example, silicone additives with polyester or aralkyl modifications remain stable at temperatures of up to 220 °C – 250 °C.
Foam can be caused by silicones, but silicones can also be used as defoamers. The critical factor here is the polarity and compatibility of the silicone additive. A highly compatible silicone additive with lower surface tension tends to stabilize foam. If this leads to problems with foam, products with a higher surface tension should be used instead. These products are not defoamers, yet they contribute defoaming properties in addition to the typical silicone properties. Silicones which can be used as defoamers must be adjusted to become even more incompatible. Polyacrylates can also have a defoaming effect at a sufficiently high incompatibility.
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