Two-pack coating

The isocyanate group is more reactive than the epoxy group in that it will react at room temperature with water and hydroxyl groups as well as with amine groups. However, the latter reaction is too fast to be practicable so the standard two-pack coatings are based on isocyanate and polyhydroxyl prepolymers such as hydroxyl terminated polyesters or polyethers as in the last example given in the section on epoxy resins. 

Solid adducts Isolated amine products, in particular, offer advantages such as low colour, low free amine content and low irritation potential they also provide non-yellowing, bloom-free films with good chemical and solvent resistance Amine adducts solvent-based, two-pack coatings, e.g. primers, finishes and coal tar epoxy coatings... 

In drying by polyaddition, low molecular mass reactive polymers such as alkyd resins, saturated polyesters, or polyacrylates react with polyisocyanates or epoxy resins to form cross-linked macromolecules. Because this reaction can take place at room temperature, the binder components must be mixed shortly before application. The period of time during which a coating of this type remains usable after mixing of the components is known as the pot life. These are known as two-pack coatings, differing from the one-pack systems, which can be stored for months or even years. 

Chem. Descrip. Acrylic resin (50%) in xylene Uses Acrylic for fast-drying two-pack coatings, air-drying and forced-drying two-component industrial lacquers Features Hydroxy-functional crosslinkable with polyisocyanates Properties Hazen < 200 color dilutable with toluene, xylene, acetone, MEK, MIBK, methoxypropyl acetate, ethyl acetate, butyl acetate dens. = 0.98 g/cm (20 C) dynamic vise. 600-1200 mPa s hyd. no. 60-70 flash pt. = 26 C 48-52% NV... 

Uses Acrylic for fast drying two-pack coatings, industrial lacquers Features Hydroxy-functional crosslinkable with polyisocyanates Properties Hazen < 80 color dens. = 1.00 g/cm (20 C) dynamic vise. 

Uses Catalyst for PU two-pack coatings heat stabilizer for PVC antioxidant metal cleaning or protective agent... 

Dioctyltin diisooctylthioglycollate catalyst, PU two-pack coatings Dibutyltin bis (laurylmercaptide) catalyst, PU elastomers Ferric acetylacetonate N-Hydroxyethy I pi perazi ne catalyst, purified terephthalic acid polyesters Manganese acetate (ous) catalyst, PVC suspension polymerization Lauroyl peroxide catalyst, pyridine synthesis Cobaltocene... 

An interesting variation of urethane technology is that of the blocked polyisocyanates. These are used when chemical-cure urethane chemistry is desired but, for technical or economical reasons, a two-pack coating is not an option. Heat is needed for deblocking the isocyanate, so these coatings are suitable for use in workshops... 

The acrylic two pack coating produces coatings with similar aesthetics and performance characteristics to the original OEM coating, albeit cured at room temperature or undo low bake conditions. This has become increasingly important in the automotive refinish market with the extensive use of plastic components. A typical air drying refinish clearcoat formulation is as follows ... 

Vinyl tars These are a combination of vinyl resins and selected coal tars. They are claimed to be similar to the two-pack coal tar epoxies but with the advantages that they are a single pack, not dependent on temperature for curing, and are easy to re-coat at any stage. 

Coal tar epoxies These are a combination of epoxy resins and selected coal tars. Properties can vary, depending on the coal tar-to-epoxy ratio. The ideal compromise appears to be approximately 50/50. Coal tar epoxies are only available in black or dark brown. They cost less than straight epoxies and generally have better wetting properties, so they can be used on slightly less than perfect surface preparation. There are similar re-coating problems as for the two-pack epoxies. 

Two-pack polyurethanes Also called urethanes, these materials are similar to two-pack epoxies in that they can be formulated to provide different properties. They can be made into foams or soft, rubbery materials, as well as very hard, tough, abrasion-resistant coatings. 

Two-pack epoxy Section 12.3.3. These are widely used to give the maximum protection to concrete surfaces, floors and walls. They can be applied as relatively thin coatings by spray or as thick epoxy surfaces applied by trowel. In all cases, the application must be preceded by adequate surface preparation (see Section 12.6.1). To allow maximum penetration into the concrete the first coat must have a low viscosity. Coal tar epoxies are used where protection is the main requirement. 

T-wash pretreatment This acid-mordant solution turns the surface black when correctly treated. Problems can occur with use of such an acid solution in situ and from its pungent odor. It must be applied to zinc in a bright condition without corrosion products on its surface. The paint manufacturer s advice must be sought before using under thick coats of two-pack epoxy or urethane. 

Sweep blast or abrading This is a suitable method before application of thick two-pack materials but care is required not to remove too much zinc coating. 

Moisture-curable urethane systems (one-pack) can be considered as two-component systems which use atmospheric moisture as the second component. One-pack urethane coatings can be produced that are similar in physical properties to the two-pack systems for almost all applications. These highly complex systems can have a great deal of flexibility. Claimed advantages are a one-pack system, rapid cure, even at low temperatures, excellent chemical and abrasion resistance and good flexibility. Although these systems have been available for some time in other countries of Europe, they are only recently beginning to be of interest in the UK. 

Coating materials may be based on short or medium-oil alkyds (e.g. primers for door and window frames) nitrocellulose or thermoplastic acrylics (e.g. lacquers for paper or furniture finishes) amino resin-alkyd coatings, with or without nitrocellulose inclusions, but with a strong acid catalyst to promote low temperature cure (furniture finishes) two-pack polyurethanes (furniture, flat boards) unsaturated polyester resins in styrene with free-radical cure initiated by peroxides (furniture) or unsaturated acrylic oligomers and monomers cured by u.v. radiation or electron beams (coatings for record sleeves paperback covers, knock-down furniture or flush interior doors). 

FBE-coated pipe requires careful handling from factory to the pipe trench to avoid mechanical damage. Repairs are undertaken with either trowel or brush-applied, liquid two-pack epoxy resin-based paints or by melt sticks of compressed powder. 

The moisture curing types are one-pack coatings, which, like the two-pack types have excellent chemical resistance and gloss but have a thickness limitation owing to the evolution of carbon dioxide during curing. 

Figure 19.60 illustrates a steel surface which should have been blast cleaned to a high standard before being coated at works with a zinc-rich epoxy primer and on site with two-pack intermediate and finishing coats. After exposure for 18 months in a marine environment, flaking millscale from beneath the paint was observed, and a survey showed that the paint... 

Two-pack polyurethane paints exhibit film properties comparable to those of solvent-borne polyurethane coatings (Bittner and Epple, 1996 Mirgel, 1993). 

Table 9.2 summarizes some technological characteristics of typical one-pack and two-pack FBZ formulations, as well as the curing conditions. Viscosity is sufficiently tow and suitable for spray or roll applications. In most cases, high-solid coatings can be obtained. The materials described herein include the bicomponent polyurethanes obtained by addition of Z1030 to Z1072 or Z1073... 

Abstract— The use of organosilanes as adhesion promoters for surface coatings, adhesives and syntactic foams is described and reviewed in the light of published work. Data are presented on the beneficial effect of silanes, when used as pretreatment primers and additives, on the bond strength of two pack epoxide and polyurethane paints applied to aluminium and mild steel. It is shown that silanes when used as additives to structural epoxide and polyurethane adhesives are less effective than when used as pretreatment primers on metals but are highly effective on glass substrates. The compressive properties of glass microballoon/epoxide syntactic foams are shown to be markedly improved by the addition of silanes. 

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