Curatives, rubber

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. 

Rapra has undertaken extensive research on the nature of the breakdown products from rubber curatives. This has included studies for the U.K. Food... 

Keywords Layered double hydroxides Layered silicates Nanocomposites Organic modification Reinforcement Rubber Rubber curatives... 

Cross-linking agents Organic peroxides, rubber curatives... 

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

Filler dispersion is a property that determines how well the filler partciles in a given rubber compound are dispersed as a result of the mixing process. This relates to carbon black dispersion as well as the dispersion of nonblack fillers such as silica, clay, calcium carbonate, titanium dioxide, etc. Also rubber curatives such as sulfur and accelerators can be poorly dispersed (commonly these ingredients are added late in the mixing cycle). Poor dispersion makes a mixed stock less uniform, and commonly the cured ultimate tensile strength will have more variability. Poor dispersion can affect other important cured physical properties such as abrasion, tear, and fatigue resistance, flexometer heat buildup, and other dynamic properties. 

Both of these curatives are added in the lower temperature, final mixing stage. HMTA must be isolated from the other rubber curatives during storage and batch preparation since its basicity can cause premature decomposition of the rubber cure accelerators and can accelerate the conversion of insoluble sulfur into the soluble form. The structure of HMTA and the reaction with resorcinol are illustrated in Scheme 4.1. Classical chemical studies indicate that as much as 75% of nitrogen remains chemically bonded to the rubber though some ammonia is released during the cure of the resin and the rubber, which can have detrimental effects on rubber composites reinforced with brass coated steel cords. 

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)... 

These TPVs are dependent on the availability and pricing of polypropylene, nitrile rubber, and specific rubber curatives. 

Inhibitors and retarders are sometimes used in the rubber curative package to extend scorch safety time for a rubber compound. 

BBPIB (Figure 10.65) is the second highest volume organic peroxide (next to DCP) that is used to cure rubber. However, since DCP represents 85% of the total peroxide consumption by the rubber industry, BBPIB s consumption is a distant second as a peroxide rubber curative. BBPIB is sometimes selected over DCP because it has greater reactivity per molecule and fewer odors when compared to DCP. On the other hand, BBPIB costs more and is more likely to bloom or exude from an EPDM compound than is DCP. 

A rubber curative based on adducts of quinone dioxime and toluene diisocyanate has been developed that offers improved reversion resistance and heat ageing properties compared to conventional cure systems. This comprehensive article supplies a detailed examination of the use of these adducts as curatives for natural rubber and their effect on bondability. 

Because nitrile rubber is an unsaturated copolymer it is sensitive to oxidative attack and addition of an antioxidant is necessary. The most common practice is to add an emulsion or dispersion of antioxidant or stabilizer to the latex before coagulation. This is sometimes done batchwise to the latex in the blend tank, and sometimes is added continuously to the latex as it is pumped toward further processing. PhenoHc, amine, and organic phosphite materials are used. Examples are di-Z fZ-butylcatechol, octylated diphenylamine, and tris(nonylphenyl) phosphite [26523-78-4]. All are meant to protect the product from oxidation during drying at elevated temperature and during storage until final use. Most mbber processors add additional antioxidant to their compounds when the NBR is mixed with fillers and curatives in order to extend the life of the final mbber part. 

When the compound is dropped out of the mixer, it is essential to remove the heat from the batch of rubber as quickly as possible this is particularly important when the curatives are in the compound. It is just as important to ensure that the batch is fully blended before it is finally cooled and stored prior to use. 

Rubber blends with cure rate mismatch is a burning issue for elastomer sandwich products. For example, in a conveyor belt composite structure there is always a combination of two to three special purpose rubbers and, depending on the rubber composition, the curatives are different. Hence, those composite rubber formulations need special processing and formulation to avoid a gross dissimilarity in their cure rate. Recent research in this area indicated that the modification of one or more rubbers with the same cure sites would be a possible solution. Thus, chlorosulfonated polyethylene (CSP) rubber was modified in laboratory scale with 10 wt% of 93% active meta-phenylene bismaleimide (BMI) and 0.5 wt% of dimethyl-di-(/ r/-butyl-peroxy) hexane (catalyst). Mixing was carried out in an oil heated Banbury-type mixer at 150-160°C. The addition of a catalyst was very critical. After 2 min high-shear dispersive melt mix-... 

ZnO nanoparticles possess greater surface/volume ratio. When used in carboxylated nitrile rubber as curative, ZnO nanoparticles show excellent mechanical and dynamic mechanical properties [41]. The ultimate tensile strength increases from 6.8 MPa in ordinary rabber grade ZnO-carboxylated nitrile rubber system to 14.9 MPa in nanosized ZnO-carboxylated nitrile mbber without sacrificing the elongation at failure values. Table 4.1 compares these mechanical properties of ordinary and nano-ZnO-carboxylated nitrile rubbers, where the latter system is superior due to more rubber-ZnO interaction at the nanolevel. 

Figure 9.15 shows typical force curves for a chlorobutyl mbber (CUR) and a namral rubber (NR) sample. It is immediately obvious that the CIIR sample is softer and, as expected, shows much greater hysteresis and hence poorer resilience than the NR sample. 

Unlike a plastic blend where the properties largely depend on the properties of the individual component and the compatibUizer used, those of a rubber blend depend on the solubility and diffusivity of the curatives, reaction rates, scorch time, etc. Figure 11.16 gives relative cure rate and scorch time for a number of accelerators. Hence, in designing a rubber blend, aU these parameters have to be taken into consideration in order to obtain good properties along with good processability. 

Another vulcanizing agent for diene rubbers is m-phenylenebismaleimide. A catalytic free-radical source such as dicumyl peroxide or benzothiazyldisulfide (MBTS) is commonly used to initiate the reaction [61]. Phenolic curatives, benzoquinonedioxime, and m-phenylenebismaleimide are particularly useful where thermal stability is required. 

A chromatogram obtained for the examination of acetone extractables from an ethylene propylene-based rubber is shown in Figure 3. Excellent separation of the main hydrocarbon components from polar compounding ingredients and curative breakdown products is observed. 

For identification of other organic rubber compounding ingredients such as waxes, and most antioxidants/antiozonants, sample extraction with diethyl ether, followed by GC-MS of the resulting extract is commonly employed. Examination of volatile species can also provide information on the nature of curatives employed. 

A rubber compound containing elastomer, curatives and the minimum of fillers. Also called a pure gum compound. 

Maximum productivity is given by the shortest possible time of cure, provided the selected time and temperature do not produce an unacceptable level of defects or early failures in service. Sufficient cure must be given to prevent porosity, since badly undercured rubber will sponge at pressure release. It is better to risk a slight overcure rather than an undercure, since if the curatives are correctly chosen (in type and quantity) the compound should show a plateau effect, which means that a reasonable overcure will not have any marked effect on the physical properties of the product. 

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