Page 639 - Carrahers_Polymer_Chemistry,_Eighth_Edition

602 Carraher’s Polymer Chemistry

In blade coating, Figure 18.16(b) flexible blade helps control the coating thickness. The blade
may be located after the bath or within the bath as part of the roller system. The blade is fl exible and
adjustable so that the amount of coating material can be controlled by application of force on the
blade and/or distance the blade is from the web or roller. Both of these processes are forms of calen-
dering. The third main coating process involves direct application of the coating material forming a
so-called fl uid curtain, Figure 18.16(c). This process, curtain coating, is usually used in conjunction
with a curing process. The coating thickness is controlled by web speed, polymer concentration, and
rate of application from the extruder.
Coatings can be divided according to formulation, drying mechanism or other system of catego-
rizing. Table 18.8 contains a summary of coatings according to drying mechanism.
A coating is normally a mixture of various components. For instance, the label on a latex emul-
sion-type paint might have as major components poly(ethyl acrylate), titanium dioxide as the white
pigment, and water. It could also have china clay and calcium carbonate as extenders, carboxy-
methylcellulose as a colloid thickener, a defoaming agent, a plasticizer, a surfactant dispersing aid,
additional coloring agents, and an added fungicide.

The hiding power is a measure of the ability of the coating to achieve a specified degree of “hid-
ing” or obliteration. Industrially, it is often tested by comparing the reflectance of the coated surface

overpainting a black surface (that is, the tested paint applied over a black surface) with white panels.
The ability to cover or hide is related to the scattering of incident light hitting the surface and return-
ing to the observer or light meter. As the film surface increases, the ability of light to penetrate the

surface coating and be scattered from the (black for tests) undercoating lessens. For a simple white
latex paint, no absorption occurs and we can consider the scattering occurring at the interfaces of
the transparent polymer matrix and the dispersed pigment particles. The scattering coeffi cient of

polymers can be obtained from reflectance measurements. While the refractive indices for most
polymers do not widely vary (generally about 1.5), the scattering coeffi cients can vary widely. For
good scattering, the refractive index of the polymer should differ from that of the pigment. For

TABLE 18.8
Major Coatings Systems as a Function of Drying Mechanism
A. Film formation through chemical reaction
(a) Through reaction of unsaturated double-bond sites with oxygen
Alkyds
Drying oils—fatty acid and related
(b) Cold curing and thermosetting by reaction with vehicle components
Epoxies
Polyurethanes
Unsaturated polyesters
Urea and melamine–formaldehyde
B. Film formation through evaporation
(a) Solution types
Bituminous coatings
Cellulose derived-acetate, acetate butyrate, ethyl, nitrate
Chlorinated rubbers
Poly(acrylic esters) such as poly(methyl methacrylate)
Poly(vinyl chloride) copolymers
(b) Dispersion types
Poly(acrylic esters) such as poly(methyl methacrylate)
Poly(vinyl acetate) and copolymers
Poly(vinyl chloride) organosols
Poly(styrene–cobutadiene)

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