Oxidation curing

Water Dispersions. Polysulftde products are offered as aqueous dispersions (Thiokol WD-6). These are useful for applyiag protective coatings to line fuel tanks, and for concrete, wood, and ia some cases fabrics, felt, leather (qv), and paper (qv). It has been found that a stable emulsion can be made that contains both LP and manganese oxide curing agent. The emulsion can be thinned and appHed as a spray coating. After it is appHed, water evaporates and the LP cures to form a soHd mbber (13). 

Antidegradants. Amine-type antioxidants (qv) or antiozonants (qv) such as the phenylenediamines (ppd) can significantly decrease scorch time. This is particulady tme in metal oxide curing of polychloroprene or in cases where the ppd had suffered premature degradation prior to cure. 

Polyisobutylene and isobutylene—isoprene copolymers are considered to have no chronic hazard associated with exposure under normal industrial use. Some grades can be used in chewing-gum base, and are regulated by the PDA in 21 CPR 172.615. Vulcanized products prepared from butyl mbber or halogenated butyl mbber contain small amounts of toxic materials as a result of the particular vulcanization chemistry. Although many vulcanizates are inert, eg, zinc oxide cured chlorobutyl is used extensively in pharmaceutical stoppers, specific recommendations should be sought from suppHers. 

The accelerator that has been widely used with metal oxide cures is ethylene thiurea (ETU) or 2-mercaptoimidazoline. Further extensive use of ETU in vulcanization of CR is restricted because of suspected carcinogen. The related compound, thiocarbanahde, used formerly as an accelerator for sulfur vulcanization, has been revived for CR vulcanization other substitute for ETU has been proposed [29,30]. 

The first useful organosilicon preceramic polymer, a silicon carbide fiber precursor, was developed by S. Yajima and his coworkers at Tohoku University in Japan [5]. As might be expected on the basis of the 2 C/l Si ratio of the (CH3)2SiCl2 starting material used in this process, the ceramic fibers contain free carbon as well as silicon carbide. A typical analysis [5] showed a composition 1 SiC/0.78 C/0.22 Si02- (The latter is introduced in the oxidative cure step of the polycarbosilane fiber). 

Nitrile rubber can be cured by sulphur, sulphur donor systems and peroxides. However, the solubility of sulphur in nitrile rubber is much lower than in NR, and a magnesium carbonate coated grade (sulphur MC) is normally used this is added as early in the mixing cycle as possible. Less sulphur and more accelerator than is commonly used for curing natural rubber is required. A cadmium oxide/magnesium oxide cure system gives improved heat resistance, but the use of cadmium, a heavy metal, will increasingly be restricted. 

Aqueous polynitrile oxide curing compositions, with good storage stability, have been patented (525). The compositions comprise aqueous dispersions containing nitrile oxides and are useful for coating systems that are cured at room temperature without the release of byproducts. Latexes are cured by mixing a polymer latex with a stable polynitrile oxide, for example, 2,4,6-triethylbenzene -1,3-dicarbonitrile oxide, and removing water from the mixture. 

Nicotine-N -oxide Cured Lf smoke Nicotinic acid, 6-hydroxy P1NT562 Nicotinic acid, methyl ester Lf, Rt, St, Bk,... 

Although not reported here the silanes used in conjunction with the epoxide and polyurethane paints were largely ineffective with the oxidative cure systems. 

High temperature stability of these nonoxide fibers in air is another critical problem. Thermal stability of ceramic fibers derived from polymeric precursors is of special concern mainly because, as mentioned above, they frequently have undesirable phases present in them. Polycarbosilane-derived SiC fibers, such as Nicalon or Tyranno, involve a thermal oxidation curing process as described above and can contain as much as 10 wt % oxygen (Okamura and Seguchi, 1992). Such fibers decompose at temperatures above 1200 C in a nitrogen or argon atmosphere with SiO and CO gas evolution ... 

FIGURE 19.2 Gas evolution from oxidation-cured PCS during thermal decomposition. 

Use Primary accelerator in natural and nitrile rubber and SBR, plasticizer and vulcanization retarder in neoprene type G, cure modifier in neoprene type W, oxidation cure activator in butyl. For extruded and molded goods, tires and tubes, wire and cable, sponge. 

Experiments were run to evaluate the effects of low level static (stretch) and dynamic (ultrasonic Irradiation) stress on water permeation of a WRT Neoprene. At the same time, the contribution of a partial silica filler In the place of black was determined. The materials were Identical zinc oxlde-magne-slum oxide-cured formulations except that one contained 30 phr black and the other contained 15 phr black and 15 phr silica. These were molded Into cups with 0.030 In. thick faces and 0.070 In. thick side walls which were then fitted over 3.5X salt water-filled glass cups and sealed with hose clamps. 

Zinc dithiocarbamate can be used in small quantities (0.25-0.75 phr) with zinc oxide cures used in heat resistant and improve compression set. 

Figure 5-16. Physical stress relaxation at 25 °C in a crosslinked network made by a zinc oxide cure of a halobutyl rubber. The elastomer has been soaked in oil to speed the relaxation. The line is the fit to the data using the Chasset-Thirion equation with E =0.018 MPa, t= 1.04 s and m = 0.31. The nature of the polymer and the cure makes for a chemically stable structure otherwise, the plot would begin to curve downward at long time.

Taki, T., Ohamura, K., and Sato, M., A study of the oxidation curing mechanism of polycarbosilane fibre by solid state high resolution nuclear magnetic resonance, J. Mater. Sci.. 24, 1263 (1989). 

---