Solvent-based coatings

Organic solvents are nsed in coating formulations for their preparation and application. When the organic liquid is not able to dissolve the polymer binder it is preferable to call it the diluent. In case of the water-based coatings, water can act as a solvent for some components, but as a non-solvent for the synthetic polymers. 

For many applications, the most effective blends have been based on ketones and aromatic hydrocarbon solvents currently restricted as hazardous air pollutant (FIAP) products. Since new solvent systems will have smaller amounts of HAP, the blend cost is likely to rise. Methyl n-amyl ketone (MNAK) and n-butyl propionate (BuProp) are attractive from the point of view of the environment and their physical properties [68]. 

In the first stage of organic solvent evaporation, its rate is independent on the presence of the polymer. Evaporation rate depends on  

During evaporation, the viscosity of the system and glass transition temperature (Tg) increase, free volume decreases and the rate of loss of solvent from the film becomes dependent not on how fast the solvent evaporation will take place but rather on how rapidly the solvent molecules will diffuse through the film [54]. 

More often the solvent-based coatings are made of alkyds, acrylics, PU or EP polymers. Alkyd-based polyols and unsaturated dibasic acids were the first synthetic polymers used in coating technology. It was successful in chemically combining oil or oil derived fatty acids into a polymer structure, thus enhancing the mechanical properties, drying speed and durability over and above those of the oils themselves and the oleoresins then available [61]. 

---

Fig. 2. Conventional thermally cured, organic solvent base coating system.

Loss of Dry. When the initial dry time of a solvent-based coating becomes substantially longer after aging, it is said to lose dry. The primary cause of this problem has been identified as adsorption of the drier on the pigment surface. Pigments with large surface areas are the worst offenders. 

Numerous alternatives to low-solid, solvent-based coatings traditionally used by, e.g., the wood furniture industry are shown in Table 5.58 with the estimated reduction in volatile organic compound emissions. 

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 powdered polymers from emulsion or suspension polymerizations intended to be used for solvent-based coatings are often milled into finer particle size with higher surface area for easier dissolution when used as coatings for metal and other substrates.58... 

In almost all cases a radiation curable polymer can be manufactured under the same conditions as conventional solvent based coatings except for special handling of possible toxic materials. Thus, differences in total energy requirements for both manufacturer and use (application and cure) are directly relatable to the energy consumed in application and curing. 

Conventional Solvent-Based Coatings Aliphatic- Aromatic Hydrocarbons Melamine Epoxy Blocked isocyanates Polyester Acrylic Epoxy Urethane 40-60% solvent 30-40% polymer 10-20% crosslinking resin... 

The cost of the radiation curable coatings formulation is 5.13/gal, but it covers 2 to 2.5 times more surface area than the solvent-based coating. The cost for equal coverage of radiation curable coating is 18.50 gal/hr 2 or 2.5 times 5.13/g l = 37.96/hr to 47.45/hr. 

There may even be some new opportunities under this law, as plastics processors who now paint or otherwise use a solvent-based coating on their products may be forced to stop coating and use molded-in color. 

Stresses in solvent based coatings arise from the differential shrinkage between the thin film coatings and the corresponding substrates. These stresses are due to volume changes associated with solvent evaporation, chemical reaction (i.e. cyclization in polyimide formation) and differences in thermal expansion coefficients of the coating and substrate (4>5). The level of residual stress depends on the material properties such as modulus, residual solvent content and crosslinking (5) and its thermal-mechanical history. 

Recommended for Water- and solvent based coatings Solvent based coatings ... 

Air-oil Solvent-based coating and leveling aid Foam control in gas scrubbing at petrochemical plants... 

---