Crosslinking polysulfide

Figure 3 Distribution of mono-, di- and poly-sulfide crosslinks as a function of reaction time for whole-latex NR vulcanized at 100 °C. Cure system in parts by wt. % of rubber sulfur, 2.5 zinc oxide, 3.0 zinc diethyldithiocarbamate, 1.0 zinc benzothiazole-2-thiolate, 1.0., Total crosslinks , polysulfide A, disulfide , monosulfide crosslinks...

The synthesis of polysulfide elastomers involves the use of a small amount of trichloroalkane in addition to dichloroalkane and sodium sulfide in order to form a branched polymer. The prepolymer is treated with a mixture of sodium hydrosulfide and sodium sulfite followed by acidification to convert all end-groups to thiol groups. Further polymerization and crosslinking is achieved by oxidative coupling of the thiol end-groups by treatment with lead dioxide, p-quinone dioxime, or other oxidizing agent... 

The influence of ZnCFO concentration (3,0 5,0 7,0 phr) on formation of properties complex of the unfilled rubber mixes and their vulcanizates on the basis of isoprene rubber of the following recipe, phr isoprene rubber - 100,0 sulfur - 1,0 di - (2-benzothiazolyl) -disulfide - 0,6 N, N -diphenylguanidine - 3,0 stearic acid - 1,0, was carried out in comparison with the known activator - zinc oxide (5,0 phr). The analysis of Rheometer data of sulfur vulcanization process of elastomeric compositions at 155°C (fig. 5) shows, that on crosslink density and cure rate, about what the constants of speed in the main period (k2) testify, they surpass the control composition with 5,0 phr of zinc oxide. Improvement of the complex of elastic - strong parameters of rubbers with ZnCFO as at normal test conditions, and after thermal air aging (tab. 1), probably, is caused by influence of the new activator on vulcanization network character. So, the percent part of polysulfide bonds (C-Sx-C) and amount of sulfur atoms appropriating to one crosslink (S atoms/crosslink) in vulcanizates with ZnCFO are decreased, the percent part of disulfide bonds (C-S2-C) is increased (fig. 62). 

Scission of Polysulfide Crosslinks in Scrap Rubber Particles. Throughout this study, we used a single lot of scrap rubber peelings having the average composition described in the Experimental Section. We began our studies with Aliquat 336 as the phase transfer catalyst because of its proven effectiveness in simple systems and its commercial availability. When devulcanization is performed in a refluxing benzene/aqueous NaOH mixture, the chemical crosslink density (M... 

Scission of Polysulfide Crosslinks in a Model Crosslinked Polybuta-diene. Figure 2 compares the molecular weight distribution of two... 

Figure 1. Scission of polysulfide crosslinks in rubber peelings with refluxing mixture of 2.1 N NaOH and Aliquat 336 in benzene.

Vulcanization by heating with sulfur alone is a very inefficient process with approximately 40-50 sulfur atoms incorporated into the polymer per crosslink. Sulfur is wasted by the formation of long polysulfide crosslinks (i.e., high values of m in XHI), vicinal crosslinks (XIV), and intramolecular cyclic sulfide structures (XV). (Structures XIV and XV do not contribute significantly to the physical properties of the polymer.)... 

The action of zinc in increasing the efficiency and rate of crosslinking is thought to involve chelation of zinc with the accelerator as well as species XVIII and XIX. Zinc polysulfide compounds such as XX are also likely intermediates. Zinc chelated to sulfur or as zinc sulfide bonds probably facilitate cleavage of sulfur-sulfur bonds in the concerted reactions described by Eqs. 9-20 and 9-21. 

The most widely used dihalide is 1,2-dichloroethane. The use of polyhalides (e.g., 2% 1,2,3-trichloropropane) results in the formation of branched or crosslinked products. Sodium tetrasulfide (Na2S4) is generally used as the polysulfide since it contains scarcely any of the monosulfide which reacts with dihalides to form cyclic by-products with unpleasant odors. 

Chemistry of Polysulfide Polymers. Propellant chemistry based on chemically crosslinked binders had its beginning at the Jet Propulsion Laboratory in the winter of 1946 when potassium and/or ammonium perchlorate were mixed into Thiokol LP-3 polysulfide liquid polymer, to which had been added an oxidative curative, p-quinone dioxime. This polysulfide polymer, as described by Jorczak and Fettes (13), is prepared... 

The accelerator first reacts with sulfur to produce a polysulfidic intermediate (33) which can then react with a rubber polymer (PH) to give a crosslink precursor (34 Scheme 4) (74MI11501). The reactions are probably free radical in nature, although ionic mechanisms cannot be excluded. Zinc salts and fatty acids appear to be necessary to initiate vulcanization which becomes autocatalytic. 

With an unaccelerated sulfur-natural-rubber system, the poor crosslinking efficiency results m sulfur being incorporated into the rubber network as long polysulfide crosslinks, cyclic monosulfides, and vicinal crosslinks, which are very close together and act physically as a single cross-link (see Fig. 2). 

Figure 9.2 Stack plots of 10% sulphur-cured NR for different times of cure. B butadiene-like species, P polysulfidic crosslinks Carbon numbers in 1,4-isoprene unit in NR have been designated as follows -Cl-C2(-C5)=C3-C4- or -Cy-Ca(-Ce)=C 3-C8-...

The chemical shifts due to the monosulfidic crosslinks are influenced not only by the position on the monomeric unit to which it belongs, but also by the position of the carbon atom of the monomeric unit on the other side of the bridge. The shielding parameters of monosulfide substitution on the individual carbons of the isoprene unit have also been determined. It is shown that resolvable polysulfidic crosslink resonances exist in all positions of the backbone carbons while monosulfidic crosslinks appear only between C-l and C-4 carbons with detectable intensity [18]. 

The 13C chemical shifts were assigned in more detail for monosulfidic and polysulfidic crosslinks occurring in the accelerated sulfur vulcanisation of NR [18]. The NR was cured with a pure thiuram formulation (TMTD alone) in order to predominantly prepare monosulfidic bridges in the network. The distortionless enhancement by polarisation transfer (DEPT) experiments, in which the carbons with different level of protonation can be distinguished [22-24], were performed for the NR cured with extended levels of sulfur. Based on the DEPT results and previously reported model compound results [20], the chemical shifts of the resonances occurring in the spectra were assigned. 

In the DEPT experiments, both peaks around 50 and 58 ppm are divided into three components and the levels of the protonation for these six individual resonances are evaluated. The peaks at 57.4, 58.0 and 58.6 ppm are assigned to the polysulfidic crosslinks in the Al, B1 and B2-type structures, respectively. The peaks at 37.2 and 50.7 ppm are due to Al-type polysulfidic crosslinks. There was no apparent structural match for the quaternary peak at 50.2 ppm. 

The peak at 33 ppm is assigned to the trans structure of 1,4-BR. An increasing intensity at 33 ppm peak with cure in both sulfur-cured and accelerated sulfur-cured BR postulates the occurrence of cis-to-trans chain isomerisation in these systems. The resonances at 38 and 50 ppm are assigned to cyclic monosulfide and polysulfidic crosslink structures. The expected monosulfidic junctions are not detected in this study possibly due to the low concentration of these species [33]. 

Silanes have been used in sealants since the early 1960s when they were introduced successfully in polysulfide formulations. It was found that only small amounts of silane were needed in the formulation to improve adhesion to surfaces such as glass, metals, and, in some cases, concrete without using a primer. More recently, hybrid sealants (made of an organic polymer backbone and a silane crosslinker) have been widely commercialized. Many of these hybrid sealants have superior properties and will bond to a variety of substrates. 

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