Secondary coating

While this may appear to be self-explanatory, the need to change color schemes or textures over the lifetime of the installation will need to be reviewed. Self-colored materials do not typically take secondary coatings such as paint or stain. 

Multi-layer insulation constructions are used. With the high-voltage cable, three separate layers may cover the center conductor. The innermost layer will function as an electric screen, the intermediate layer is usually a PE insulation material, and the outer layer serves as an electric screen. The processing technique can be used for primary and secondary coating where the covering is on metallic conductors or coating previously insulated wires. 

In general two types of secondary coatings are used in conjunction with the primary coatings described above. UV-curable epoxy acrylates or urethane acrylates are commonly used for in-line application, as the fiber is drawn. These same materials are also used extensively as single coatings. Extruded thermoplastics, such as nylon or polyester elastomers, are frequently used for off-line application processes. 

Whether these secondary coatings represent effective barriers to the transport of reactants and products from the reactive silicate surface is still actively debated. 

This discussion of chemical stability refers to bare (uncoated) glass fibers, i.e., fibers having neither a specific acid or alkali resistant finish nor a secondary coating. Accordingly, the relationship between fiber composition and chemical stability in water, acids, and bases is complex. It depends on the interaction between (1) the chemical agent [27] to which the glass fiber surface is exposed, (2) foe pH of the glass composition [33] in the fiber surface, and (3) the internal microstructure of foe fiber [27]. 

In summary, the effect on the pH of the bare fiber surface and the effect of the interaction between a chemical agent and a bare fiber surface are predictable. ZrC>2 seems to increase both acid and base resistance. The effect of the internal microstructure [27] of a fiber is highly process dependent and not predictable without a thorough prior investigation of its microstructure. Importantly however, all fibers, except experimental single fibers, have a primary finish some have an additional secondary coating. These modifications further reduce the predictability of their chemical resistance from their compositional make-up alone. 

River terrace deposits are similar in character to those found in river channels. The pebbles of terrace deposits may possess secondary coatings due to leaching and precipitation. These are frequently of calcium carbonate that does not impair the value of the deposit, but if they are siliceous, then this could react with alkalies in high-alkali cements and therefore could be detrimental to concrete. The longer the period of post-depositional weathering to which a terrace deposit is subjected, the greater is the likelihood of its quality being impaired. 

One of our designer materials is a CaP04-sheUed retrievable nanoreactor based on a DOPA-stabilized perfluorocarbon emulsion (see Table 15.1). Others include protein, polysaccharide, and polyethylene glycol-coated retrievable nanoreactors. Each contains a perfluoroocytlbromide core surrounded by one or more lipid stabilizers and a secondary coating (Table 15.1). 

Once this condition is reached, the primary coating s glassy expansion curve from -50 °C to -30 °C and the secondary coating s rubbery expansion curve from 125 °C to 90 °C can be extrapolated upward and downward, respectively, until they meet the linear line extending from the expansion curve between 0°C and 75 °C (dashed line. Fig. 4.36). This process yields glass transition temperatures near -20 °C and 87 °C. DSC measurements at 15°C/min on this same optical fiber sample indicated that glass transition temperatures occur at -31 °C and 78 °C. In normal practice with the probe seated, the final stable heating run would be selected and isolated in a separate plot for determination of Tg and CLTE values. 

Two 0.4-mm-thick free films from the primary and secondary coating materials were run individually in the expansion mode in the TMA (Fig. 4.37). The in situ Tg of both coatings was taken from the intersection of the linear extrapolations of the expansion behavior below and above each glass transition. Note the dashed lines in Fig. 4.37, which help to delineate the beginning... 

Figure 4.37. Contraction of 0.4-mm-thick primary and secondary coatings in film form during cooling from 100 °C at 4 °C/min in helium (compression mode, 0.1 g on a 3-mm-diameter probe (141 Pa pressure) in helium arrows depict the onset and end of each glass transition (Bair, unpublished, 1998).

The results show that the coating is constrained in its longitudinal and radial directions by adhesion of the primary coating to the silica glass and cohesion of the primary and secondary coatings. Last, and perhaps most important, as the temperature is increased, the CLTE ratio decreases, indicating a reduction in adhesive constraints. 

Over the next two decades extruded PVC with a lacquered, lapped rayon secondary coating for protection against soldering temperatures and for identification was the standard wire insulation. The jumper wire over this period was PVC-coated tinned copper wire with a cellulose lacquered rayon braid secondary coating containing a nonflammability additive. 

---