Curing system magnesium oxide

The most common polyfunctional alcohol used is pentaerythritol, but a base is also required to complete the cure system, magnesium and calcium oxide giving more controlled cure rates than stronger bases. 

Titanium or beryllium oxide also provides a degree of improvement in thermal conductivity to epoxy systems. Magnesium oxide and aluminum oxide have also been commonly used for this purpose, although the degree of improvement is not as great as with the fillers discussed above. The effect of various fillers on the thermal conductivity of cured adhesive is shown in Fig. 9.6. The incorporation of metal fibers with metal powders has been shown to provide synergistic improvement to the thermal conductivity of adhesive systems,... 

Other flame retardants and/or smoke suppressants can also be used such as magnesium hydroxide, magnesium carbonate, magnesium-zinc complexes and some tin-zinc compositions. Zinc oxide is a common ingredient in many rubber base formulations used as part of the curing system. At the same time, the action of zinc oxide is similar to that of antimony trioxide, but less effective. 

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. 

In addition to the use of peroxides for crosslinking, metal oxide, polyfunctional alcohols, amines and epoxide resin cure systems can be used with CSM rubbers. In the metal oxide based cure systems it is usual to add a weak acid, such as stearic acid, and accelerators, such as MBT, MBTS or TMTD magnesium or lead oxides are generally used. 

Diamine, e.g., hexamethylene diamine Magnesium oxide or Calcium oxide General purpose cure system, fairly resistant to scorch. Best heat resistance, but not particularly resistant to acids. 

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... 

Metal Oxides. Zinc oxide, which is formed from the burning of zinc metal, was the first nonblack filler used for reinforcement of ruhher compoimds. Although zinc oxide and magnesium oxide are still used as a reinforcing filler in some specialty compounds, particularly those that require heat resistance, their role in rubber compounding in the last several decades is that of an activator for the sulfur cure system or as curatives for chloroprene ruhher compoimds. 

Several rubbers may be crosslinked using divalent metal oxides, usually zinc oxide. There are a limited number of polymers that utilise this method, which is used with halogenated polymers such as polychloroprene [8], chloro- and bromobutyl, and chlorosulfonated polyethylene and carboxylated nitrile rubbers. The system may utilise the metal oxide alone or in combination with the organic accelerators used with sulfur-curing systems. In the case of halogenated polymers, magnesium oxide may be added to act as an acid scavenger. 

Elimination is thought to be catalyzed by basic materials, such as magnesium oxide, which are commonly included in commercial formulations. Elimination takes place rapidly and probably occurs during milling when a diamine curing system is used.. Evidence for the presence of unsaturation in amine-treated vinylidene fluoride-hexafluoropropylene copolymer comes from the infrared spectrum of the material. 

With the sulphur-modified polymers cure may be brought about by zinc oxide and magnesium oxide in combination either alone or together with an accelerator such as ethylene thiourea. In the case of the homopolymers it has been common practice to support the zinc oxide/magnesium oxide/ethylene thiourea system with a further component. This component consists of a sulphide or a blend of sulphides of the type more commonly used as accelerators for the diene hydrocarbon polymers. These include mercaptobenzothiazole disulphide (MBTS), diorthotolyl guanidine (DOTG) and tetramethyl thiuram monosulphide (TMTM). In the polychloroprene homopolymers these materials appear to act as retarders of cure at processing temperatures but are accelerators at vulcanization temperatures. Their mechanism does not appear to have been fully elucidated. 

Magnesium oxide can function as an activator, scorch retarder, and cure modifier. As an acid acceptor, magnesium oxide activates amine cure and improves heat resistance of thiuram-thiazole cures. It is effective as a scorch retarder with all cure systems (except amine systems). In carbon black filled compounds, magnesium oxide has a marked effect on scorch safety even at low level (0.25 phr), lengthening the available processability time. In mineral filled compounds, magnesium oxide can function in a role... 

HYPALON chlorosulfonated polyethylene (CSM) will react slowly with magnesium oxide in the presence of moisture. A cure system with 4 phr of MgO is used... 

The same study revealed that antioxidants in general provide little if any additional stability to bromobutyl vulcanisates at temperatures below 150°C, that many antioxidants contribute significantly to heat stability at higher temperatures, and that exceptional heat resistance is achieved with MBI/MgO, with diphenylamine/acetone reaction products plus MgO or, particularly, with combinations of these two antioxidant systems. The author states that magnesium oxide was included with each system because, although its role in bromobutyl vulcanisation is not clear, there is no doubt that it contributes to processing safety, shelf life and heat resistance when used in conjunction with curing systems that do not contain elemental sulphur. 

---