Coatings World - June 2017

Coatings, Silicon-based Technologies and Corrosion

interfacial (Van der Waal’s force) and hydrogen bonds.

The most common solution to the wetting challenge is reducing the paint viscosity by dilution with carrier liquids such as solvents, or water. Effective solvent selection often requires a binary or tertiary blend with an initial rapid flash-off to fix the coating in place, strong solvency to reduce polymer viscosity, and a slow final evaporation rate to allow surface profile penetration. To obtain optimum adhesion, wetting of the surface profile must occur before the film’s glass transition temperature (Tg) exceeds the ambient temperature. This remedy has its limitations in that dilution reduces volume solids, which can increase sagging and dry film permeability (more on that issue later). As emission regulations tighten, the solvent approach to wetting becomes a less attractive option.

Modern coatings rely on non-volatile surfactants, such as silicone-polyether copolymers instead of solvents, to improve coating wet-out. Unlike pure polydimeth-yl siloxanes (silicone oils), silicone-polyethers incorporate organic substituents, providing compatibility with organic paint components, and low (~ 21 dynes per cm) surface tension. The silicone backbone provides low viscosity and mobility within the coating film to deliver excellent surface wetting and reduced surface defects. Significant improvement is often seen at addition rates of 0.1 to 0.5% of coating solids. This versatile technology allows significant copolymer tailoring to achieve the proper balance of compatibility and surface activity with a wide range of coating formulations.

Having achieved surface wetting,
adhesion can be further enhanced be-
yond weak intermolecular attraction
by forming covalent bonds between the
coating and the substrate. Metal sur-
face preparations for adhesive bonding
historically consisted of anodization or
etching processes that used strong acids
and hexavalent chromium. That surface
treatment was often followed by the ap-
plication of a corrosion-inhibiting primer,
typically containing high VOC levels and
more hexavalent chromium. In 1983, it
was found that a primer composed of
an acrylic copolymer, an epoxy resin, a
silica-sol and a trialkoxysilane (Figure 4)
provided superior paintability, degrease
resistance and corrosion resistance af-
ter coating.1 Twelve years later, it was
found that a wash primer, based solely on
silanes could provide similar benefits.2

Alkoxy silanes provide numerous benefits to coatings such as:

• moisture resistance;

• adhesion promotion;

• crosslinking;

• anti-static/anti-microbial;

• pigment dispersion; and • durable polymer synthesis.

The inorganic functionality (typically methoxy or ethoxy) reacts with any hydroxyl group (water, metal hydroxide, carbinol). The organo-functional (either reactive or non-reactive) provides compatibility to organic resin chemistries. Together these functionalities bridge interfaces, forming covalent bonds between organic polymers and mineral surfaces (e.g., pigments, fillers and glass and metal substrates), resulting in improvement in adhesion, water, chemical, abrasion and UV resistance, flow, and pigment and filler dispersion.

Widely known as adhesion promot-
ers, alkoxy silane primers also offer con-
trolled hydrophobicity, excellent thermal
stability, surface activity, chemical resis-
tance and corrosion protection. As early
as 1962, partial hydrolyzates of alkoxy
silanes (e.g., tetra-ethoxysilane), or alkali
silicates, combined with zinc metal pow-
der, were found to provide galvanic pro-
tection of ferrous substrates above that
imparted by organic resin-based zinc
primers. 3 This technology is available as
either two or single-pack systems and has
been the dominant galvanic primer used
in the paint industry.

Treatment of mineral pigments and fillers (e.g., silica, titanium dioxide, etc.,) with alkoxy silanes is well known in the coatings industry. The result is better integration of the inorganic pigment/filler particle into the binder matrix, improving dispersion and physical properties. While pigment or filler suppliers often perform particle treatment with silanes, similar benefits can be observed by incorporating the alkoxy silane directly into a coating formulation to create a more tightly crosslinked, hydrophobic film much less susceptible to moisture attack.

Conclusions

Hydrolytic attack and corrosion are inevitable. Coatings formulators can only hope to slow the process. The permeability and adhesion of a coating are critical to long term performance. Silicon-based technologies, such as silicone surfactants, polymers and alkoxy silanes can be incorporated into coatings to improve the wetting, adhesion and durability of many types of high performance coatings. CW