Лако-красочные материалы — производство

The polysulfide sealant must be specifically formulated to meet the desired requirements and to obtain optimum properties. Pot life, working properties, and sealant properties should be properly adjusted. Suitable fillers should be dispersed and suitable additives should be incorporated into the formulation. Curing agent, curing modifier, filler, plasti­cizer, and adhesion additive are discussed briefly below.

Mercaptan-terminated liquid polysulfide polymers are polymerized to rubbery solids by oxidizing agents (e. g., lead dioxide, activated manganese dioxide, calcium peroxide, cumene hydroperoxide, alkaline dichromates, and p-quinonedioxime). The curing process involves the oxidation of the terminal mercaptan groups in the polysulfide polymers to form the corresponding disulfide.

Mn+ + H + SCH2CH2OCH2OCH2CH2S

n

The process can be written simply as

Mn+2RSH—>• M(n-2) + RSSR + 2H+

and it involves the net transfer of two electrons per molecule of disulfide. Dichromate oxidation of mercaptans in an aqueous medium may proceed via the following pathway:

CrVI + RSH )———————— RSCrVI

RSCrVI RSH CrVI RSSR

Mercaptan oxidation can proceed through coordination of the mercaptan to manganese dioxide, with the manganite ion providing a base for deprotonation of the mercaptan.

In general, the pH of the system governs the curing rate. Acidic materials retard, whereas alkaline materials accelerate the cure. Thus examples of retarders are stearic acid and metallic stearates. Typical accelerators are amines, inorganic bases, water, dinitrobenzene, and sulfur.

Fillers increase the strength, impart needed rheological properties, and reduce the cost of sealants. Tensile properties are increased significantly, depending on the type of filler, its particle size, and the type of cure. Improper filler selection can ruin the perfor­mance of a polysulfide sealant. Calcium carbonates (wet or dry ground limestone, pre­cipitated), carbon blacks (furnace, thermal), calcined clays, silica and silicate fillers, and rutile titanium dioxide are typical fillers used in polysulfide sealants. Generally, combina­tions of fillers are used in formulation. Plasticizers improve the working properties while

lowering the modulus of the sealant. The plasticizer must be compatible with the cured sealant, should have low volatility, and must be safe. Polymeric and esteric types are commonly used plasticizers.

Sealant adhesion is improved by the incorporation of additives. Typical phenolic resin additives are Methylon AP-108, Durez 16674, Bakelite BRL 2741, and Resinox 468. Epoxies are also good adhesion promoters. Silanes (e. g., A-187 and A-189) are known to increase adhesion. Table 1 lists five types of sealant formulations suggested by Panek that are useful in several end applications [10]. Generally, the integral fuel tank polysulfide sealant consists of two parts: 9 parts by weight of sealant base components mixed with 1 part by weight of accelerator. A typical composition is shown in Table 2.

Most sealants, especially those used in building, applications, contain adhesion additives. Occasionally, to obtain good bonding, primers are used. For metals, a dilute solution of silanes in organic solvents has been used. A film-forming primer is required for porous surfaces. Masonary primers generally contain a chlorinated rubber or a modified phenolic resin either alone or in combination with an additional plasticizer. Thin layers of silanes give good polysulfide adhesion to metals, glass, and ceramic substrates.

Production of polysulfide sealant basically involves mixing and dispersion. Therfore, the equipment used in coating and ink manufacture is applicable to sealant manufacture. Viscosities of up to 60,000 P are common in polysulfide sealant, and therefore heavy equipment is generally used in its manufacture. Typical useful equipment includes sigma blade mixers, kneader-extruders, and high-speed dispersators. A three-roll paint mill gives

excellent processing. Transferring and packaging of sealants requires heavy-duty displace­ment pumps.