Polysulphide Coatings

The properties of polysulphide coatings are given in Table 7.11. A volume swell of less than 150% after 72 h is acceptable (Lowe, 1994a). 

Figure 7.4 Applications for polysulphide coatings (a) filter bed (b) reservoir roof. Reproduced with permission from Morton International Ltd, Coventry, UK...

Figure 103 The weathering resistance of epoxy-polysulphide coatings after one month of outdoor exposure [A] LP-epoxy modified coating [B] unmodified epoxy coating. Substrate = oily, rusty and wet mild steel. Reproduced with permission from Morton International...

Another example of epoxy-polysulphide coating application is for the Indian Molasses Co. Ltd tank farm facilities in Mangalore where the nature of the atmosphere is highly corrosive. Several tanks have been coated (Figure 10.8) (Morton International, undated d). 

Epoxy-polysulphide coatings exhibit excellent adhesion to steel, aluminium and concrete without the use of primers (Morton International,... 

Figure lO Epoxy-polysulphide coating at Indian Molasses Co. Ltd, Mongalore. The tank on the front left-hand side has two coats. The next tank on the right has its seccnd coat almost completed. The third tank has reedved one coat. The tank on the far right-hand side is being prepared to receive its first coat. Reproduced with permission from Morton International Ltd, Coventry, UK... 

The polysulphides are frequently used in casting mixes and to a less extent in coating, laminating and adhesive applications. Their value in casting and encapsulation lies mainly with their low curing shrinkage and flexibility in the cured state. Their tendency to corrode copper and the somewhat inferior electric insulation properties of the blends does lead to certain limitations. 

In this book I have confined discussion to those polymeric materials which are cured by chemical reaction and which have found widespread application in the construction industry. As such, the book covers materials based on epoxies, polyurethanes, silicones, polysulphides, alkyds and polyesters. In addition, there is a chapter on hybrid polymer systems and one on acrylics. It is true that acrylic emulsions are not strictly thermosetting polymer systems, but their widespread use and importance made their exclusion difficult. These materials find use as coatings, sealants, adhesives, grouts, flooring compounds, repair compounds and waterproofing agents. 

Polysulphides. Polysulphides are used as curing agents for epoxies in the presence of tertiary amines which accelerate the cure such cured systems exhibit good flexibility and tensile strength at ambient temperature. In a real sense they are flexibihsers for epoxy systems. Specific applications are for the crack injection system (for moving cracks) and coatings (Chapter 10). 

The potential market for polysulphides is for water-industry sealants, and fuel-resistant sealants and coatings for secondary containments. As a construction, civil engineering and insulating-glass sealant polysulphides have a poor to fair market share. Table 7.1 summarises the specific applications of polysulphide systems in the construction industry. 

The practice of modifying epoxy resins with liquid polysulphides may not be new but the method of doing so has advanced significantly in recent years to the extent that these systems are being given serious consideration in the design of specialist coatings, primers, adhesives and encap-sulants (Morton International, undated a Rees and Wilford, 1994 Rees et al., 1994). 

Adhesion. Owing to improved wetting properties, excellent adhesive characteristics are exhibited for coatings and adhesives with relatively small additions of liquid. Polysulphides are required to produce significant improvements in the lap shear strength of the system (Rees et al., 1994). 

Chemical resistance. Unmodified epoxy resins have excellent resistance to dilute acids, alkalis and certain solvents. The incorporation of an LP component into the epoxy resin extends this resistance to a wide variety of oils, aromatic and aliphatic hydrocarbons, esters and ketones. Coatings and adhesives based on polysulphide-epoxy resins exhibit reduced swell and maintenance of flexibility and adhesion when immersed in these fluids. In view of this, LP-modified epoxies are used on oilrigs, fuel-storage installations and chemical plants (Figure 10.4). 

Ethylene glycol, 5% aq (cutting fluid) Organosilane solution, 0.1-1% aq (glass coating) Phenyl mercuric nitrate, 0.05% pet (preservative) Polyester resin, 2-5% aq (glass coating) Polysulphide polymer, 1% pet (sealant)... 

Coatings and fillers also influence permeability. Waxing diminished the permeability of celluloses, cellulose compounds, and synthetic resins (2). Collodion and casein coatings decreased the air and water permeabilities of leather(20). Sager (23) found that carbon black incorporated in polysulphide rubbers increased their permeability to hydrogen. On the other hand (11), the water permeability of paint films was diminished by incorporating aluminium. 

For a few special applications such as solvent resistant coatings for concrete, the alkyl polysulphide polymer dispersion is used directly without further treatment. 

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