Film sagging

Ellis (60) pointed out that high-solids polyester coatings typically contain so little solvent that the Tg of the resin-solvent combination is above the temperature of the deposited film. Therefore, evaporation of solvent occurs very slowly solely by diffusion as the rate-limiting step therefore, the film sags and runs before sufficient solvent evaporates to provide "set-up" of the film. A precise method was provided for determination of the "transition point" of a given resin-solvent combination. This point is the resin concentration at which solvent evaporation changes from surface-controlled to a diffusion-controlled process. 

Associative thickeners are low-molecular-mass water-soluble polymers with at least two hydrophobes such as hydrophobically modified ethylene oxide-urethane block copolymers (HEUR) or hydrophobically modified hydroxyethylcellulose (HMHEC). The hydrophobes can associate with themselves or with hydrophobes on surfactant, cosolvent, latex, and pigment. This sets up a loose network that is sensitive to mechanical disturbance but re-forms quickly. The result is that pigment settling and film sag are reduced because the network structure increases the low shear viscosity, but the formulations show easy... 

NOTE These are thixotropic resins used to reduce film sagging in high solids automotive paints. 

Sealers. Mica is used in all types of sealers for porous surfaces, such as waHboard masonry, and concrete blocks, to reduce penetration and improve holdout (see Sealants). It permits a thicker film to be appHed and at the same time reduces sagging. Cracking is reduced by the reinforcing action of the flakes, and gaps and holes in rough masonry are bridged by the mica flakes. 

Another group of additives are used as thickeners and antisettle agents. They affect flow and reduce sagging of thick films. 

As stated above, high performance coatings based on epoxies, vinyls or chlorinated rubbers are used almost exclusively on all large ships. A general development in these materials has been the introduction of highly thixotropic types that can be airless sprayed at wet film thicknesses of 300 im or more, that do not run or sag on vertical surfaces. This enables the requisite film thickness to be applied in fewer coats, saving time and reducing application costs. 

Thixotropic Paint a paint which while free-flowing and easy to manipulate under a brush, sets to a gel within a short time when it is allowed to remain at rest. Because of these qualities a thixotropic paint is less likely to drip from a brush than other types and can be applied in rather thicker films without running or sagging. 

Shape retention is a factor in almost all articles made from polymeric materials (cf. warping of plastic articles, deformation of films, etc.). In textiles the lack of shape retention is reflected in the sagging of curtains, the bagging of trousers, etc. Shape retention is determined by the viscoelastic properties of the polymer, especially under the influence of moisture plastic deformation and creep are highly undesirable, whereas resilience is favourable. 

Antisag agents minimize sagging or curtaining of wet films during apphcation. 

Non-sag grades are available for vertical applications. The dry film thickness of the system is approximately 1 mm. 

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