Role of a coalescing solvent

4 Role of a coalescing solvent Most eoatings based on emulsion polymers are used in environments where they will be expeeted to form a coherent film at temperatures as low as 0°C. However, other physical properties besides film forming capability are required from the polymer. These include abrasion resistance, hardness, chemical resistance, impact performance, etc. These can often be impossible to achieve with a polymer of low Tg. The polymers, which most clearly meet these criteria arc acrylics or copolymers of vinyl acetate or styrene. These would, without additions of coalescing species, be brittle, forming incoherent films with little adhesion to the substrate at normal application temperatures. 

Coalescing solvents allow these polymeric systems to form films at ambient or sub-ambient temperatures. The presence of the coalescing solvent has the following effects. 

An emulsion polymer consists of a dispersion of polymeric particles varying in size from 0.05-1 (Xm, dispersed in an aqueous environment. The coalescing solvent may be found in several different locations depending on their nature. They are classified according to their preferred location in the aqueous system. 

Hydrophobic substances such as hydrocarbons will prefer to be within the polymer particle these are described as A Group coalescents. These tend to be inefficient coalescing agents. Molecules, which are more hydrophilic than hydrocarbons tend to be preferentially sited at the particle - aqueous interface, along with the surfactant system. These are described as AB Group coalescents. These exhibit the best efficiency as coalescing solvents. 

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In the prepolymer-ionomer process , the compound which contains the moieties which are the precursors of ionic groups is incorporated in the polymer chain already at the stage of urethane prepolymer formation. Then they are neutralised, which results in the formation of a prepolymer-ionomer which is emulsified in water and eventually crosslinked. In this process, the prepolymer-ionomer is usually dissolved in a small amount of water-miscible solvent of high boiling point, e.g., N-methylpyrolidone, which plays the role of coalescing agent in the process of film formation. It is then possible to obtain DPUR which contain either cationic DPUR with a pH of less than 7 (cationic moieties are attached to the polyurethane or polyurethane-urea chain) or anionic DPUR with a pH of greater than 7 (anionic moieties are attached to the polyurethane or polyurethane-urea chain). If non ionic hydrophilic moieties are attached to or incorporated in the polyurethane or polyurethane-urea chain, then a nonionic DPUR may be obtained. 

Additionally, the sequestering ability of polyphosphoric acid and phosphonic acid derivatives as well is a valuable property for their use as dispersants. The main task of dispersants is to suspend solid particles in water or organic solvents and avoid their coalescence and precipitation. This complex plays an important role in many processes and as a result dispersants are used in many technical fields. 

In the 1970s, the EPA assumed a leading role in pollution control. With the realization that practically all solvents exhibited some level of photochemical activity, new rules—state and national—were focused on the total solvent content of paints as applied. The index used—VOC (volatile organic compound)—did not differentiate between solvents or coalescents but merely set limits in pounds of emissions per gallon of paint (excluding water) for varying industrial applications. 

The mobility of the solvent-vapour interface may play a significant role in the bubble coalescence process. A mobile interface is expected for a... 

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