Coatings cohesion

X-cut According to ASTM D 3359, method A, an X is cut into the film to the substrate, pressure tape (TESAPACK 4287) is applied over the X and then removed, and adhesion is evaluated by comparison with descriptions and pictures. The method is used to establish whether the adhesion failure of the coating to the substrate may occur between the coats (adhesive break) or in the coating (cohesive break). 

Mesoscopic stresses occur between particle splats inside a lamella and are responsible for reduced coating cohesion. These stresses result principally from frozen-in contraction of the rapidly quenched molten particles at the substrate interface. 

A novel rotating fluidized bed system has been developed for fluidizing, granulating, and coating cohesive fine powders to tailor their properties and functionalities (Figure 21.12). 

Most solid surfaces are marred by small cracks, and it appears clear that it is often because of the presence of such surface imperfections that observed tensile strengths fall below the theoretical ones. For sodium chloride, the theoretical tensile strength is about 200 kg/mm [136], while that calculated from the work of cohesion would be 40 kg/mm [137], and actual breaking stresses are a hundreth or a thousandth of this, depending on the surface condition and crystal size. Coating the salt crystals with a saturated solution, causing surface deposition of small crystals to occur, resulted in a much lower tensile strength but not if the solution contained some urea. 

Fracture mechanics (qv) affect adhesion. Fractures can result from imperfections in a coating film which act to concentrate stresses. In some cases, stress concentration results in the propagation of a crack through the film, leading to cohesive failure with less total stress appHcation. Propagating cracks can proceed to the coating/substrate interface, then the coating may peel off the interface, which may require much less force than a normal force pull would require. 

Spouted beds Wiirster coaters Moderate (layered) 50 ton/hr continuous cohesive powders, good for coating applications detergents Batch pharmaceuticals, agricultural chemicals, nuclear wastes... 

It is for this reason that the discovery by Ulrich was of significant importance to the successful development of acrylic PSAs. He found that by copolymerizing polar monomers, such as acrylic acid, one could greatly increase the cohesive strength of the polymer allowing PSA articles coated with this type of material to sustain a load without premature shear failure. These polar monomers commonly... 

In some cases, plasticization of a PSA may be detrimental to its performance. A well-known example is the deterioration of the performance of an adhesive applied to plasticized PVC. Migration of the plasticizer from the flexible vinyl into the PSA often softens the adhesive to the point where it fails cohesively from the vinyl, leaving sticky residue behind during removal of the adhesive-coated article from the substrate. One way to address this detrimental effect of plasticizer migration is to formulate an already plasticized PSA, perhaps because a better balance exists between the plasticizer in the PVC substrate and the PSA in contact with it [101]. 

Other polymers used in the PSA industry include synthetic polyisoprenes and polybutadienes, styrene-butadiene rubbers, butadiene-acrylonitrile rubbers, polychloroprenes, and some polyisobutylenes. With the exception of pure polyisobutylenes, these polymer backbones retain some unsaturation, which makes them susceptible to oxidation and UV degradation. The rubbers require compounding with tackifiers and, if desired, plasticizers or oils to make them tacky. To improve performance and to make them more processible, diene-based polymers are typically compounded with additional stabilizers, chemical crosslinkers, and solvents for coating. Emulsion polymerized styrene butadiene rubbers (SBRs) are a common basis for PSA formulation [121]. The tackified SBR PSAs show improved cohesive strength as the Mooney viscosity and percent bound styrene in the rubber increases. The peel performance typically is best with 24—40% bound styrene in the rubber. To increase adhesion to polar surfaces, carboxylated SBRs have been used for PSA formulation. Blends of SBR and natural rubber are commonly used to improve long-term stability of the adhesives. 

The thickness of the coating also affects permeability, and in general, there is a correlation between thickness and life. However, with relatively thick coatings as obtained with some modern materials such as epoxies the relationship is not so well established. There may be a limiting thickness above which either little additional protection is obtained or the increase in cohesive strength reduces its adhesive strength. Coatings should always be applied as closely as possible to the manufacturer s recommended thickness. 

Adhesion the degree of attachment between a paint or varnish film and the underlying material with which it is in contact. The latter may be another film of paint (adhesion between one coat and another) or any other material such as wood, metal, plaster, etc. (adhesion between a coat of paint and its substrate). Adhesion should not be confused with cohesion (. v.). 

Abrasion tests In these tests the end point is normally taken as the amount of abrasion required to penetrate the coating. The results thus reflect the strength of the coating, its cohesion, and in some cases its adhesion to the basis metal as well as resistance to abrasion. 

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