Coatings, solids content

A wide range of polyurethane-type products has become available in recent years for coating applications. These include simple solutions of linear polyurethanes, two-pot alkyd-isocyanate and polyether-isocyanate systems and a variety of prepolymer and adduct systems. The coatings can vary considerably in hardness and flexibility and find use mainly because of their toughness, abrasion resistance and flexibility. Uses include metal finishes in chemical plant, wood finishes for boats and sports equipment, finishes for rubber goods and rain-erosion-resistant coatings for aircraft. One type of coating is potentially competitive with PVC leathercloth. Both alkyd-di-isocyanate and adduct-diisocyanate compositions may be coated on to fabrics from solutions of controlled viscosity and solids content. Such coated fabrics are soft, flexible and, unlike PVC leathercloth, free from plasticisers. 

Common to all non-convertible, resins is their very low solid content in solution, typically 10-20%, necessitating the application of a number of coats to give an acceptable film thickness unless one is able to use a dispersion rather than a solution of the resin. 

These are the most important components of paint. Coatings can be either clear or opaque (containing pigments) and either solvent- or water-based. With solvent-based paints, after application, most of the solvent is lost through evaporation. These solvent-based coatings are mostly alkyds (name derived from alkyl/acid) or modified alkyd resins. Normally they contain 30% polymer solids higher solids content is limited by the increasing viscosity of the system. 

The film thickness depends upon many parameters such as the withdrawal speed, the solid content, and the viscosity of the liquid. If the withdrawal speed is chosen such that the shear rates keep the system in the Newtonian regime, then the coating thickness depends upon the aforementioned parameters by the Landau-Levich equation52 ... 

With emulsion polymerization it is possible to prepare very high-molecular-weight polymers at high rates of polymerization. The required reaction temperatures are low and can even be below 20 °C when redox systems are used for initiation (see Examples 3-11). Polymer emulsions with solid contents of 50% and higher can be very stable. In many cases, e.g., poly(vinyl acetate), they are directly used as paints (paint latices), coatings, or adhesives (see Sect. 2.5.4). 

Gel lacquer formulations used for dip coating. The early catapult propellants were inhibited by dipping in a lacquer consisting of toluene 80p, acetone 20p and a total solids content of 15-18%. The grains were dipped twice and had a cured inhibitor thickness of 20 to 30 mils. As the size of experimental slotted tube catapults increased, coatings of 100 mils were required. This necessitated an increase of the solid content to 25-27%. As this soln was too viscous some methylol was incorporated in solvent formulation. In order to improve the room temp gelling characteristics an aliphatic hydrocarbon was added... 

Once an appropriate laboratory-scale process has been established, many of the key elements of the process should have been determined. Some operating parameters (such as inlet-air temperature, coating formulation to be used, and solids content of the coating solution/suspension) can be directly translated to the larger-scale process. Others, however, will have to change, and these include ... 

Figure 7 Example of how the type of spray gun used, gun-to-bed distance, and solids content of the coating fluid can influence the size of droplets generated.

Solids content of coating suspension (based on Sureteric YAE-6-18108) (% w/w) 10-25... 

The data represented by the stressed enteric results (SET), however, tell a slightly different story (Fig. 12A). Clearly, the process operating ranges where acceptable performance can be achieved are quite limited. However, if the level of applied enteric coating is fixed at a minimum of 10% w/w, and the coating suspension solids content is reduced to 15% w/w, then the process operating ranges become quite broad (Fig. 12B). 

Figure 11 Influence of inlet-air temperature and solids content of the coating suspension on enteric test performance of enteric-coated tablets.

COATING FORMULATION Volatilit/. TochiriBss. Viscosity. Surface tension. Solids content. Mechanical properties. 

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