Curative sulfur

Although these curative systems may also be used with the polyepichlorohydrin elastomers containing AGE, the polymers were developed to be cured with conventional mbber curatives, sulfur, and peroxides. These polymers containing the pendent aHyl group are readily cured with a typical sulfur cure system such as zinc oxide, and sulfur along with the activators, tetramethylthiuram mono sulfide [97-74-5] (TMTM) and... 

Curatives (sulfur, accelerators, peroxides, sulfur-donor systems, etc.)... 

Refers to moisture, poljrmer, diluent, oil, plasticizer, emulsifiers (eg, in stjrrene-butadiene rubbers), curatives (sulfur, accelerator), antioxidants, antiozonants, and other low boiling components (approx. 300°C or lower)... 

Natural-mbber-based pressure-sensitive adhesives can be cured by standard mbber curatives, eg, sulfur plus an accelerator (see Rubber, natural) ... 

Amine Cross-Linking. Two commercially important, high performance elastomers which are not normally sulfur-cured are the fluoroelastomers (FKM) and the polyacrylates (ACM). Polyacrylates typically contain a small percent of a reactive monomer designed to react with amine curatives such as hexamethylene-diamine carbamate (Diak 1). Because the type and level of reactive monomer varies with ACM type, it is important to match the curative type to the particular ACM ia questioa. Sulfur and sulfur-beating materials can be used as cure retarders they also serve as age resistors (22). Fluoroelastomer cure systems typically utilize amines as the primary cross-linking agent and metal oxides as acid acceptors. 

The productive stock, ie, the curable compound, is made up by mixing the nonproductive stock in the Banbury once more with the curative package (sulfur, accelerators, etc). This time the drop temperature is lower, in the range of 95—112°C. The productive stock is then sheeted or pelletized and coated with the dip coat, cooled, and finally stored, ready for further processing for final fabrication. 

Sulfur has long been known for its properties as a pesticide and a curative agent. Homer spoke of the pest-averting sulfur as far back as 800—1000 BC, Hippocrates (400 Bc) considered sulfur sa an antidote against plague, and Dioscorides (100 ad) used sulfur ointment in dermatology (244). In 1803, the use of a lime—sulfur protective treatment for fmit trees was reported, and in 1850 sulfur dust was used to protect foHage (245). In 1891 sulfur dust was used on soil to control onion smut (246). 

Neoprene Type TW was shown to have low oral toxicity in rats. The LD q was found to be in excess of 20,000 mg/kg. Human patch tests with Types GN, W, WRT, and WHV showed no skin reactions (169). The FDA status of Du Pont Neoprene polymers is described (172). Although polychloroprene itself has not been shown to have potential health problems, it should be understood that many mbber chemicals that may be used with CR can be dangerous if not handled properly. This is particularly tme of ethylenethiourea curatives and, perhaps, secondary amine precursors often contained in sulfur modified polychloroprene types. Material safety data sheets should be consulted for specific information on products to be handled. 

Some of the terpolymers containing high levels of AGE give superior sour gasoline and ozone resistance, particularly dynamic ozone resistance. Since the unsaturation is not in the polymer backbone, it can be, and apparentiy is, sacrificed under sour gasoline or ozone aging. This protection scheme is limited with the peroxide and sulfur cure systems as they involve the aHyl functionaUty of the polymer. The protection is maximized when a dinucleophilic curative, such as trithiocyanurate, is used. 

In mbber-mbber blends, maldistribution of curatives such as sulfur, accelerator, and activator due to their difference in solubility and diffusivity leads to uneven distribution. Blending strategies such as adding the accelerators to each mbber followed by blending the two mixed batches has been found to be more effective than blending the curatives into both mbbers in a single step. Selection of... 

FIGURE 20.6 Phase images of ethylene-propylene-diene terpol3mier (EPDM) samples at different scales. Images of the unvulcanized sample are shown in (a, d) and images of samples, which were cross-linked with different amount of sulfur curative—1 phr—in (b, e) and 2 phr—in (c, f). White arrows in (f) most likely indicate locations with small sulfur crystals. 

FIGURE 20.7 Phase images of ethylene-propylene-diene terpolymer (EPDM) samples loaded with oil (50 wt%). Image in (a) was obtained on the unvulcanized sample and images in (b,c,d) on samples cross-linked with different amounts of sulfur curative 0.5, 1.0, 1.5 phr, respectively. 

FIGURE 20.8 Load-versus-penetration LvP) curves obtained during nanoindentation of ethylene-propylene-diene terpol3nner (EPDM) samples. Approach curves are shown as solid line and retract curves as broken lines. The curves in (a, b) were obtained respectively on the unvulcanized and cross-linked (amount of sulfur curative was 1.0 phr) samples of neat EPDM. The curves in (c, d) were obtained respectively on the unvulcanized and cross-linked (amount of sulfur curative was 1.0 phr) samples of EPDM loaded with oil (50 wt%). 

An extensive series of thiosemicarbazones obtained from 2-acetylpyridine was tested by Klayman et al. [4, 85] for antimalarial activity against Plasmodium berghei in mice. The molecular features essential for activity were found to be a 2-pyridylethylidene moiety, the presence of the thiocarbonyl sulfur, and certain, bulky or cyclic substituents at the terminal AT-atom. The most active 2-acetylpyridine thiosemicarbazones were " N-phenyl- and those with azacycUc substituents. For example, iV-substituents of 4-methylpiperidine, piperazine, and azabicyclo[3.2.2.]nonyl-, 4, were curative at a dose level as low as 20 mg/kg. 

Unreacted free sulfur can be determined to ISO 7269 1995 — Rubber — Determination of free sulfur. Three methods for the determination of free sulfur in vulcanized rubber are detailed two versions of the copper spiral method and the sodium sulfite method. The copper spiral methods are also applicable, subject to limitations, to unvulcanised rubber. The technique of DSC can detect non-reacted rubber curatives such as residual peroxides. 

Nonsulfur Vulcanizing Agents. Many high performance specialty elastomers do not contain diene moieties in their molecular structure and therefore cannot be sulfur-cured. These elastomers require cross-linking agents capable of reacting with the specific functional group(s) contained by the specific elastomer. Some common nonsulfur curatives include peroxides, difunctional resins, and metal oxides. 

Similar observations were made by Bhattacharya and Bhowmick [91] for the NR matrix. They found that entrapment of the curatives in close proximity to the nanoclay not only altered the cure and rheological behavior favorably, but also catalyzed the formation of a supramolecular structure by helping the formation of sulfur-amine complexes involved in sulfur curing. 

Fig. 32 WAXD patterns of NBR compounds reinforced with 5 phr organoclay and cured by sulfur (a), peroxide (b), and NBR master batch (c) (without curatives and uncrosslinked)...

The curative packages contained MgO 4 phr, ZnO 5 phr, stearic acid 2 phr, zinc dithiocarbamate 6 phr, sulfur 1 phr, and ethylene thiourea 1 phr... 

Abstract Plasma polymerization is a technique for modifying the surface characteristics of fillers and curatives for rubber from essentially polar to nonpolar. Acetylene, thiophene, and pyrrole are employed to modify silica and carbon black reinforcing fillers. Silica is easy to modify because its surface contains siloxane and silanol species. On carbon black, only a limited amount of plasma deposition takes place, due to its nonreactive nature. Oxidized gas blacks, with larger oxygen functionality, and particularly carbon black left over from fullerene production, show substantial plasma deposition. Also, carbon/silica dual-phase fillers react well because the silica content is reactive. Elemental sulfur, the well-known vulcanization agent for rubbers, can also be modified reasonably well. 

Coated materials are evaluated in S-SBR and in 50 50 blends of S-SBR and EPDM rubbers. In blends, the partitioning of fillers and curatives over the phases depends on differences in surface polarity. In S-SBR, polythiophene-modified silica has a strong positive effect on the mechanical properties because of a synergistic reaction of the sulfur-moieties in the polythiophene coating with the sulfur cure system. In S-SBR/EPDM blends, a coating of polyacetylene is most effective because of the chemical similarity of polyacetylene with EPDM. The effect of... 

Polyacetylene-modified sulfur is evaluated as a curative in a 50 50 blend of S-SBR/EPDM. In pure S-SBR, the mechanical properties decrease with the polyacetylene coating due to a reduced release rate of the sulfur out of its shell. The cure and mechanical properties of the S-SBR/EPDM blend are nearly doubled because of improved compatibility. 

Sulfur has a striking ability to catenate, or form chains of atoms. Oxygen s ability to form chains is very limited, with H2Oz, 03, and the anions 02,022-, and 03 the only examples. Sulfur s ability is much more pronounced. It appears, for instance, in the existence of S8 rings, their fragments, and the long strands of plastic sulfur that form when sulfur is heated to about 200°C and suddenly cooled. The — S—S— links that connect different parts of the chains of amino acids in proteins are another example of catenation. These disulfide links contribute to the shapes of proteins, including the keratin of our hair thus, sulfur helps to keep us alive and, perhaps, curly haired. 

The system Cl-butyl-cis-polybutadiene has been studied in some detail because it was suitable for the developed differential swelling technique and because this system of blends vulcanized with zinc oxide, sulfur, and thiuram disulfide first revealed the presence of interfacial bonds. This curative system has the feature of a flat cure —i.e.y the two homophases are vulcanized rapidly, and the crosslinked density does not increase radically as vulcanization time is prolonged. This is observed in Table IV by swelling and extractable levels of a series of crosslinked networks cured at increasing times and swollen in a common solvent, cyclohexane. 

The SEM is also used to do X-ray/elemental analysis. This technique is qualitative. X-ray analysis and mapping of the particular elements present is useful for the identification of inorganic fillers and their dispersion in compounds as well as inorganic impurities in gels or on surfaces and curatives, e.g., aluminum, silicon, or sulfur in rubber compounds and Cl and Br in halobutyl blends. (Figure 9)... 

Certain curatives such as those containing either chlorine or sulfur can be tested using x-ray fluorescence (XRF) techniques. The principle of XRF is that... 

An interaction of the curatives occurs to form the active sulfurating agent, Ac-Sx- Ac by a reaction of accelerator (Ac) and activator with sulfur ... 

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