Mixed-curing system

Most accelerators used in the accelerated sulfur vulcanization of other high diene rubbers are not applicable to the metal oxide vulcanization of CR. An exception is the use of so-called mixed-curing system for CR, in which metal oxide and accelerated sulfur vulcanization are combined. Along with the metal oxides, TMTD, DOTG, and sulfur are used. This is a good method to obtain high resilience and dimensional stability. 

Mixed aziridine or mixed epoxide-MAPO cures have been the practical solution to problems encountered with single curing agents. The mechanical properties of mixed aziridine or mixed epoxide—aziridine-cured propellants show less change on aging than those of propellants cured with MAPO or BITA alone. It appears that the BITA or the multifunctional epoxide (ERLA-0510 or Epon X-801) in the mixed curing systems provides stable crosslinks and a slight amount of postcure, which results... 

C, depending on the blend ratio. The blends were compounded in a two-roll laboratory miU, with three cure systems, the mixtures and their ratios being blend (100.0), zinc oxide (5.0), stearic acid (1.5), styrenated phenol (1.0), diben-zothiazyl disulfide (0.8, only in the sulfur and the mixed cure systems) DCP (4.0, 40% active ingredient, only in the DCP and the mixed cure systems) and sulfur (2.5, only in the sulfur and the mixed cure systems). The compounded blends were molded to optimum cure in a steam-heated hydraulic press at 160°C to obtain the test samples. The blends containing 0 wt% EVAc could not be fully cured with sulfur. The samples were irradiated with a Co-60 source at a dose rate of 3.21 kGy/h to doses of 100-500 kGy. 

Most accelerators for accelerated-sulfur vulcanization do not work for the metal oxide vulcanization of neoprene rubbers. An exception to this is in the use of the so-called mixed curing system for CR, in which metal oxide vulcanization is combined with accelerated-sulfur vulcanization. In this case, along with the metal oxides, accelerators such as tetram-ethylthiuram disulfide(TMTD) or N,N -di-o-tolylguanidine (DOTG) are used with sulfur. This may be desirable for high resihence or for good dimensional stability. 

Blends of NR/EVA are conventionally cured after intermix mixing. The blends are cured with a mixed cure system of S + DCP (sulfur + dicumyl peroxide). While sulfur cures only the NR phase of the blend, DCP is expected to cure both the phases. The possible network structure is represented in Figure 1.15. The morphology of the blend is predicted based on uncured blend SEM studies and is schematically represented in Figure 1.16. 

When crosslinked using a mixed cure system (DCP and sulfur), good agreement for 30/70 and 70/30 NR/EVA blends in the case of discrete particle model (with EVA as matrix in the former and NR in the composition) is obtained (Figures 1.17, 1.18 and 1.19). 

The polymei latex is then coagulated by addition of salt oi acid, a combination of both, oi by a fiee2e—thaw process. The cmmb is washed, dewatered, and dried. Since most fluorocarbon elastomer gums are sold with incorporated cure systems, the final step in the process involves incorporation of the curatives. This can be done on a two-roU mill, in an internal mixer, or in a mixing extmder. 

Internal mixing is widely used with fluorocarbon elastomers. Gumstocks and compounds that are particularly successful fall in the viscosity ranges discussed earlier, and use both incorporated bisphenol-type and peroxide cure systems. A typical internal mix cycle mns 6—8 min with a drop temperature of 90—120°C. The typical formulations in Tables 4 and 7 are readily mixed in an internal mixer. 

Progressive replacement of amine hardener by a low-viscosity flexibiliser will reduce mix viscosity, increase pot life and reduce the heat distortion temperature of the cured system. Higher impaet strengths are achieved using approximately equivalent amounts of hardener and flexibiliser. 

The resins are hardened in situ by mixing with an acidic substance just before application. A typical curing system would be four parts of toluene-p-sulphonic acid per 100 parts resin. The curing may take place at room temperature if the resin is in a bulk form but elevated temperature cures will often be necessary when the material is being used in thin films or coatings. 

Cure systems, however, paint only part of the picmre with regard to the manufacture of mbber articles polymers and fillers are equally important in meeting performance requirements. In addition, compound mixing and processing play an important role in achieving final vulcanizate properties. 

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. 

The comparative estimation of efficiency of zinc oxide and ZnCFO similar concentrations (3,0 5,0 7,0 phr) as the agents of metaloxide vulcanization system was carried out on example of modelling unfilled elastomeric compositions from chloroprene rubber of recipe, phr chloroprene rubber - 100,0 magnesium oxide - 7,0. Kinetic curves of rubber mixes curing process at 155°C are shown on fig. 8. The analysis of the submitted data testifies, that at increase of zinc oxide contents vulcanization kinetics is changed as follows the scorch time and optimum cure time are decreased, the cure rate is increase. Vulcanization... 

The problems associated with the multifunctional curing agents for CTPB and the resultant aging behavior of the cured polymers have led to a practical solution for curing binders and propellants—i.e., using mixed aziridines or a mixture of an aziridine and an epoxide. Such mixtures, when appropriately balanced, usually provide satisfactory mechanical behavior and high temperature stability. In dual curing systems such as MAPO and BITA or MAPO and a suitable multifunctional epoxide,... 

Aziridine. Propellants cured with MAPO have excellent processing characteristics and satisfactory uniaxial tensile properties over a wide range of temperatures. However, the problems associated with the aging behavior of these propellants have led to the use of other types of curing systems which do not contain the P—N bond. These latter materials are di- and trifunctional aziridines, such as those shown in Table IV, and provide satisfactory propellants in which the uniaxial tensile properties can be tailored to a desired modulus. Such mixed aziridine-cured systems give satisfactory initial properties, reduce the postcure behavior, and improve the storage characteristics of CTPB propellants. 

The basic compounding formulation specifies the minimum requirement of fillers, vulcanizing agents, and other substances that must be added to the rubber compound to achieve the desired properties. After the rubber, cure system and reinforcing filers have been selected it will be necessary to make several adjustments before all requirements are satisfied. It is generally sensible to start with the simplest mix formula for meeting the requirements. The recipe or the formula is usually written on the basis of hundred parts of rubber. For example if 5 parts of zinc oxide is to be added it is denoted as 5 phr (five parts per hundred rubber). Elementary compounding... 

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