Knowledge of the solubility of coatings is very important for several reasons. These include protection of existing coatings, deliberate removal or cleaning of existing coatings, application of new coatings, and health and safety considerations. A polymer that is to be used as a surface coating must be soluble in a solvent that does not adversely affect the underlying surface. It is also important that the coating should remain soluble in such a solvent.
A true solution is one from which the solute (the dissolved material) can be recovered unchanged by simple physical process. Two substances will only pass spontaneously into true solution if the free energy change for the solution process is negative. Whether or not this will occur depends on the various forces of attraction between molecules, the degree of randomness of the system and on the temperature. Solubility is favoured where the forces of attraction between polymer molecules and solvent molecules are greater than those between adjacent polymer molecules and where polymer and solvent molecules have similar polarity (Johnstone and Webb, 1977).
The inner cohesion of the polymer solid increases with its degree of crystallinity as the efficiency of molecular packing assists intermolecular interaction. Crystalline polymers are therefore less easily dissolved than amorphous polymers. They typically require conversion to a more amorphous form by warming (e.g. waxes). Crosslinked polymers cannot dissolve in the true sense. They may, however, show extensive swelling and softening when they come into contact with compatible solvents. A good introduction to solubility and solvents for conservation problems is provided by Torraca (1988) and by Book 2 in the Science for Conservators series (Moncrieff and Weaver, 1992).
Attempts have been made to relate the most important aspects of the compatibility of polymers and solvents through their solubility parameter. Its significance is that polymers and solvents having similar solubility parameters are likely to form solutions (Horie, 1987). A graphical representation of solubility (see Figure 11.10), developed by J.P. Teas of the Ashland Chemical Company and known as a Teas Diagram, is explained by Torraca (1988). For polymers used in conservation coatings, a more empirical alternative which has been used by some conservators is the concept of solubility grade. This gives a convenient measure of the relative tendencies of polymers to dissolve in hydrocarbon solvents (Feller et al., 1985). The issue of solvents and solubility is discussed in Chapter 11.
Many organic solvents present serious health and safety problems through action on human physiology, through risk of fire and through damage to the environment. The best way to control the risk from organic solvents is to eliminate their use entirely. While this may not yet be practicable the hazard posed by the need to use organic solvents is playing an increasingly important part in materials selection. An alternative approach to that of solvent delivery is to apply surface coatings as emulsions. This has advantage over simple solutions that a much higher solids content can be achieved without making the preparation too viscous and is usually safer. Various acrylic and vinyl polymers can be manufactured in emulsion form. Thermoplastic resin emulsions contain resin particles of colloidal size (0.001—1 pm in diameter) dispersed in water. The process by which emulsions form films involves loss of water followed by coalescing of individual droplets of polymer to form a continuous film. For a given polymer there is a minimum film formation temperature (MFFT) below which the droplets do not flow together and therefore do not form a satisfactory film. The MFFT is important in relation to working properties, hardness and susceptibility to creep (especially important where emulsions are employed as adhesives). Emulsions can be prepared with varying sizes of dispersed particles. Small particles give good penetration and binding of pigment particles whereas large particles tend to remain more on the surface. This can be an advantage where consolidation which minimizes darkening of a chalky friable surface is required. The observations made about coatings in general apply equally to films formed from emulsions.
Before using emulsions as conservation coatings it is wise to investigate their composition since they are prepared by commercial emulsion polymerization processes and may contain undesirable additives. For an emulsion to form a satisfactory film it is necessary that the Tg of the polymer should lie some degrees below the ambient temperature. One way to achieve this is by use of a fugitive plasticizer in the formulation. Such plasticizer is slowly lost by evaporation in the final stages of drying causing Tg to rise and improving hardness. The nature of such plasticizers and other materials such as dispersing and coalescing agents should be known before use. Once dry, these films cannot normally be re-dispersed in water but require a polar organic solvent for removal. The nature of solvents needed for removal should be known before the material is applied. Emulsions are more thoroughly treated in the section on consolidation in Chapter 12.
Each time a varnish is removed there is a risk of solvent action on the underlying paint, including leaching, swelling and partial dissolution of the paint film. All coatings change chemically as they age, to an extent that is largely predictable based on the known chemistry of the coating. It is therefore important to consider carefully at the time of application of a coating, the likely requirements for its removal and the effect this will have on the paint. A resin that is prone to chain scission, or the breaking apart of the molecule into smaller units, will cause the resin to loose some of its cohesive strength. A polymer that crosslinks as the coating cures and ages becomes insoluble making it difficult to remove. Polymers that are chemically stable, undergoing minimal chain scission or crosslinking over time, are the most likely to be long-lived coatings. It is easier to predict the behaviour of a pure substance than of a formulation or mixture containing several materials.
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