Binders electrodeposition paints

Electrodeposition. Electrodeposition paints are suspensions of binders and pigments in fully demineralized water with low concentrations (ca. 3 %) of organic solvents (see Section 3.8). Electrodeposition coating may be either anodic or cathodic. 

One of the main differences of electrodeposition paints with conventional water soluble paints is their lower solids and thus solvent content. A typical binder content is around 10%w, the amount of solvent approximately 5%. The rest, apart from pigmentation, is water. The influence of solvent in the early stages of binder/paint formulation is very similar to the effects described for conventional aqueous paints which is also started from an approx. 70% solids binder solution in coupling solvent(s). The choice of the solvent (blend) is, however, less influenced by its evaporation characteristics as the deposited paint film does not contain much water and is stoved after application. Of more importance are paint stability and electrical properties (conductivity, rupture voltage). 

The binders for cathodic electrodeposition are epoxy resin combinations dispersed in water (see Section 3.8). Advantages of anticorrosive electrocoatings include excellent corrosion resistance at a dry film thickness of ca. 20-30 pm. Electrocoats are stoved at 165 -185 °C to obtain films with the desired properties. The paint industry is now developing electrocoats that can be cured at lower temperatures (140 150 "C). Electrocoating produces a homogeneous film that covers the entire car body surface, including recesses and cavities. 

Solvent selection for electrodeposition (ED) paints will therefore include coupling efficiency considerations as described above and also the partition coefficient of the solvent over the aqueous/micellar organic phase is of importance. The partition coefficient will for instance influence the amount of solvent which is deposited with the binder film and hence the final binder film formation/flow characteristics (Figure 3.7). Practice has shown that an equal distribution over water/organic phase as e.g. encountered by butoxyethanol results in a very satisfactory ED behaviour. 

The binder properties are useful in other pigmented coatings such as clay-based paints (119). The zinc-lithium silicate coatings can be applied by anodic electrodeposition on steel (120). In another-typc of use, lithium polysilicate provides an intermediate bond between the fluorocarbon polymer coating and metal on antisticking cook ware (121). 

Whilst there are some thermoset acrylic emulsions cormnerdally available, the bulk of the thermoset resins, used as the main binder system, are produced in solution. Some may then be made waterborne by neutralisation and inversion (dispersion) into a water phase. Lower molecular weights favour this qrproach. The rate of conversion from solvent based to waterborne industrial thermoset coating systems has been, and is, much slower than the conversion from architectural alkyd paints to emulsion altonatives. There are two principle reasons for this. Firstly there are problems of application and substrate wetting of many waterborne systems. Secondly, the modifications frequently required to induce water dispersibility reduce one or more of the essential performance properties required from the cured film, compared to a solvent based system. Water resistance, with many films having an increased tendency for blushing is one example. However, for some applications, such as electrodeposition, only waterborne systems will work. 

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