Лако-красочные материалы — производство

This polymer does not contain S-S or forrmal linkages, and these are the weak links in conventional polysulfides. The polythioether polymer has excellent resistance to fuel and organic solvents and has better thermal stability than that of conventional polysulfides. Polythioethers can be terminated with mercaptan, hydroxyl, silyl, and nonreactive end groups. The curing chemistry can thus be varied based on the terminal groups. A typical structure of this class of polymer is

(och2ch2sch2ch2— ochch2sch2ch2)

CH3

polythioether

Liquid polysulfide polymers are available in a series of viscosities and cross-link densities. In general, polymers in the range 400 to 500 P are used in sealants and adhesives, while lower-viscosity polymers are used for coatings and casting compounds. The tensile properties of unfilled polysulfide polymers are poor but are improved by suitable reinfor­cement with pigments and fillers. Both the molecular weight of the liquid polysulfide polymer and oxidative curing influence the physical properties. Higher tensile strength is obtained with higher-molecular-weight materials.

Cured liquid polysulfide compositions have excellent resistance to many oils and solvents (e. g., hydrocarbons, esters, ketones, dilute acids and alkalis). Systems must be properly formulated and cured to obtain maximum solvent resistance. Swelling tests on cured, filled polysulfides have been reported by Usmani et al. by measuring weight gain versus immersion time [8]. With jet reference fuel (JFR), an equilibrium was quickly reached. During early immersion in water, the weight gain was linear with time. Later, a square-root weight gain versus immersion time was found to exist.

The glass transition temperature (Tg) of polysulfides depends on the hydrocar­bon moiety and the length of the polysulfide chain. The amount of cross-linking monomer is small, and therefore it does not influence Tg. Generally, the greater the hydrocarbon content, the lower the Tg. Higher-ranking polysulfides have higher Tg. The thermal stability of polysulfide polymers depends on the polymer backbone and the curative used to vulcanize the polymer. Commercially available polysulfides have an ethyl formal disulfide backbone, and this regulates the upper temperature limits. In an acid-catalyzed hydrolytic attack, formaldehyde is released, which in turn reduces the disulfide bond to mercaptan. The formic acid so generated catalyzes hydrolysis of the formal group. The terminal mercaptan group can react with a hydroxyl group to give a monosulfide bond. The degradation results in weight loss and loss of flexibility due to the monosulfide structure formation. Disulfide and formal groups provide a flexibilizing effect due to free rotation. Calcium oxide can neutralize formic acid and absorb water and is therefore an effective stabilizer. Practical cure rates cannot be achieved in anhydrous formulations by metal dioxide curing agents. Thermal instability can also arise when the mercaptan group reacts with the metal oxide. Sulfur mitigates formation of the mercaptide groups. Polysulfide sealants cured using manganese dioxide and chromate salts provide continuous service at 250° F.

Tobolsky has studied extensively the viscoelastic properties of polysulfide polymers

[9] . Polysulfide polymers have the unique ability to relieve internal stress or stress between mercaptan and disulfide linkages. The stress decay of cross-linked polysulfide elastomer follows the equation

F(t) = F (0) + e-t/T

where F(t) is the final stress, F(0) the initial stress, t the time, and T the relaxation time. The relaxation times (in hours) for polysulfide polymers at 80° C for some curing agents are 0.68 for lead oxide at 7.3 parts by weight of resin (phr), 32 for manganese dioxide at 18.9 phr, and 200 for 2,4-toluene diisocyanate plus N-methyl-2-pyrolidone at 7.0 phr. The ability of polysulfide polymers to relieve stress is extremely valuable in maintaining adhe­sion in joints subjected to joint movement.