Sulfur-containing Curatives

Fast cure is often a property needed in two-part structural adhesives. Mercaptan-terminated curatives, when catalyzed with amines, cure rapidly in thin bondlines even on metallic substrates. Useful life after mixing can be in the 2-10-min range. By contrast, most catalyzed amine-containing curatives cure much slower in thin bondlines where the exotherm of curing is removed. 

Capcure 3-800 LC (9) is a very useful material with polyether in the backbone and with three beta hydroxy mercapto end groups. Amine catalysts are usually added to control the work life increasing the concentration of amine leads to a shorter work life. These are the Five Minute Epoxies familiar to the home user. After cure these adhesives tend to be brittle peel strengths are only two to five pounds with catastrophic failure over the length of the bond. 

A need exists for a mercaptan-terminated curative which cures very rapidly to a flexibilized or toughened adhesive there are few materials available. 

The Thiokol mercaptan-terminated polysulfides fill some of this need and are used, alone or in combination, to improve flexibility and adhesion 

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Ultrasonic devulcanization also alters revulcanization kinetics of rubbers. It was shown (Isayev et al., 1996b) that the revulcanization process of devulcanized SBR was essentially different from those of the virgin SBR. The induction period is shorter or absent for revulcanization of the devulcanized SBR. This is also true for other unfilled and carbon black-filled mbbers such as GRT, SBR, NR, EPDM, and BR cured by sulfur containing curative systems, but not for silicone rubber cured by peroxide. It was suggested that a decrease or disappearance of the induction period in case of the sulfur-cured rubbers is due to an interaction between the rubber molecules chemically modified in the course of devulcanization and unmodified rubber molecules resulting in crosslinking. It was shown that approximately 85% of the accelerator remained in the ultrasonically devulcanized SBR rubber (Levin et al., 1997a). 

A butadiene-styrene copolymer (Firestone Duradene 760) was used as a base polymer for experiments. Sulfur and accelerators were introduced as curatives into the rubber. The curatives were used to preserve the shape of substances in Table 1. Rubber containing curatives was added to the internal mixer with various volume fractions from 0.5 to 0.9. 

When rubber containing sulfur and curatives is pressed into contact with brass (typical alloy 70% copper, 30% zinc) and then vulcanized, copper ions diffuse to the brass surface where they combine with sulfiir to form a cuprous sulfide interphase. Joint strengths are controlled by the characteristics of this layer. This bonding is of critical importance in tires reinforced with brass-plated steel cords, and has been extensively studied. 

Polymer compounds often contain curatives or crosslinking agents. This is especially true for rubber compounds. The most important curatives are sulfur and sulfur compoimds. Elementary sulfur exists in a number of aUotropic forms [ 55 ]. Ordinary sulfur is a yellow solid substance, which forms crystals with orthorhombic symmetry. It is called orthorhombic (or rhombic, a-) sulfur. At 112.8 °C orthorhombic sulfur melts to form a straw-colored liquid, which consists of staggered ring Sg molecules and has a low viscosity. If this melt is cooled down, it crystallizes into a second polymorphic form monoclinic sulfur (y-sulfur). Orthorhombic sulfur (a-sulfur) is the most stable form at low temperatures, but above 95.5 C the most stable form is monoclinic sulfur (P-sulfur), which is stable between 94.5 and 120 C. This melts at 120 °C to form an Sg straw-colored melt. When molten sulfur is heated, it becomes more viscous because of its polymerization. It becomes darker in color and turns red. 

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. 

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. 

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... 

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. 

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. 

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. 

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... 

Curatives. The function of curatives is to cross-link the polymer chains into a network the most common ones are the sulfur type for unsaturated rubber and peroxides for saturated polymers. Chemicals called accelerators may be added to control the cure rate in the sulfur system these materials generally are complex organic chemicals containing sulfur and nitrogen... 

Two important types of elastomeric polyolefin blends are reactor-made iPP/ EPR blends and postreactor blend iPP/EPDM. The latter is called thermoplastic vulcanizates (TPVs), produced by dynamic vulcanization of blends containing a thermoplastic and an elastomer. To make iPP/EPDM TPV, the two polymers PP and EPDM are mixed with curatives, such as peroxides, phenolic resins, or sulfur with accelerators, and dynamically cured in an extmder resulting in a blend consisting of micrometer-sized elastomer particles dispersed in the PP matrix (20-24). Paraffinic oils are added in the melt mixing process for viscosity control and cost. In iPP/ EPDM TPV, the crystalline iPP resin is normally the minor phase. Recently, polyolefin plastomers have been added to the class of elastomeric polyolefin blends. Polyolefin plastomers are ultralow molecular weight linear low density polyethylenes (ULMW-LLDPE). Nonelastomeric polyolefin blends are blends of polyolefins with mostly nonpolyolefin (other thermoplastic) matrices as mentioned earlier. 

Kassenbeck [83], however, has shown that cortical cells adjacent to the cuticle are more flat and contain less sulfur than the remaining cortical cells that comprise the bulk of the cortex. Kassenbeck calls these heterotype cortical cells. Swift recently provided evidence for a higher percentage of orthocortical cells in curly hair than in straight hair see the section entitled Curliness or Crimp, described later in this chapter. 

A large number of investigations in acid media have led to the conclusion that the inhibition effect caused by relatively small and simple molecules is due to their adsorption on the metal surface. Compounds of this nature usually contain sulfur and nitrogen, or are of the groups of higher alkyl-alcohols and fatty acids. Typical compounds to be discussed here in more detail are quinoline and thiourea derivatives. Fig. 5 shows a comparison of the effectiveness of several such compounds determined by means of weight loss measurements on carbon steel in 5% sulfuric acid at 40° C. as a function of the inhibitor concentration. A cur-... 

In this chapter we consider types of ternary compounds other than those discussed in Chapter 6. The compoimds we discuss here contain polymers, particle fillers, and additives including processing aids and curatives. Additives mixed into polymer-filler compounds have a strong influence on the interaction of particles with each other and with the polymer matrix. The treatment of reinforcing fillers with additives is very important in the processing industry. Sections 7.2 and 7.3 may be considered a continuation to Chapter 2. Curatives, notably sulfur and its compounds, are usually used with organic accelerators to control the rate of curing step. Section 7.5 continues section 4.10. 

Accelerators and curatives are not needed Minimize or eliminate Pentaerythritol, sulfur and sulfur-bearing accelerators Systems-containing sulfur or sulfur donors. 

ATM— factice contains between 0.3% and 4.0% free sulfur. This needs to be recognized as a significant increase of curative in a sulfur compound. In a peroxide cure situation, only low levels (<10 phr) of sulfur factice can be tolerated unless a sulfurless grade is used (TDI cross-linked castor type). 

Brass/Rubber. When rubber containing sulfur curatives is pressed into contact with brass (typical alloy 65% copper, 35% zinc) and then vulcanized, copper diffuses to the brass surface and reacts with sulfur to form cuprous sulfide (51). This interlayer grows outward from the brass surface, strongly interlocking into the rubber phase (52,53). Again, diffusion to an interface and in situ reaction... 

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