Curing system activators

Corrqrlete curing system (activated lead oxides plus curing agent) in predispersed form Provides good processing safety and bin stability, moderate vulcanizate properties Acts as a retarder when scorchiness is caused by excess moisture... 

Zinc oxide and stearic acid are used to activate the curing system as well as to preserve cured properties when overcuring, which is curing beyond the point of time and temperature at which maximum properties are obtained. 

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... 

Accelerated sulphur systems also require the use of an activator comprising a metal oxide, usually zinc oxide, and a fatty acid, commonly stearic acid. For some purposes, for example where a high degree of transparency is required, the activator may be a fatty acid salt such as zinc stearate. Thus a basic curing system has four components sulphur vulcanising agent, accelerator (sometimes combinations of accelerators), metal oxide and fatty acid. In addition, in order to improve the resistance to scorching, a prevulcanisation inhibitor such as A -cyclohexylthiophthalimide may be incorporated without adverse effects on either cure rate or physical properties. 

The close structural similarities between polychloroprene and the natural rubber molecule will be noted. However, whilst the methyl group activates the double bond in the polyisoprene molecule the chlorine atom has the opposite effect in polychloroprene. Thus the polymer is less liable to oxygen and ozone attack. At the same time the a-methylene groups are also deactivated so that accelerated sulphur vulcanisation is not a feasible proposition and alternative curing systems, often involving the pendant vinyl groups arising from 1,2-polymerisation modes, are necessary. 

The UV cure system contains an epoxy or a vinyl ether functionalized PDMS polymer and a photo catalyst [36]. This latter, a diaryliodonium salt is photolyti-cally decomposed to form an active acid that polymerizes the epoxy or vinyl ether groups and crosslinks the network. 

Consist of a range of chemicals which promote cross-linking can initiate cure by catalysing ( catalysts , hardeners, initiators), speed up and control cure (activators, promoters) or perform the opposite function (inhibitors) producing thermosetting compounds and specialised thermoplastics (e.g. peroxides in polyesters, or amines in epoxy formulations). The right choice of a cure system is dependent on process, process temperature, application and type of resin. 

Silica fillers also react with the rubber causing an increase in viscosity and dry and unmanageable processing behaviour. Filler activators need to be added to silica-reinforced compounds to overcome these problems. The usual filler activators used are diethylene glycol, polyethylene glycol and amines such as triethanolamine. Some of these activators not only overcome the problems of processing and accelerator absorption, but depending on the cure system used, will also act as vulcanisation activators. 

Although development of a formulation for a specific product and process requires a great deal of knowledge and experience, there are some basic rules typical of FKM compounding. The levels of acid acceptor (MgO) and activator (Ca(OII)2) in the bisphenol cure system strongly affect not only the cross-linked network as reflected by the physical properties of the material, but also the behavior of the compound during vulcanization. Therefore, the curing system must be optimized to achieve the best balance of properties. 

The recent recognition that there cure numerous activating and detoxification systems and that they differ in different species and in different strains of a given species calls for continued comparative studies in various mammals to provide a broader basis for induction and for predicting human effects. 

Instead of applying pressure in a closed mold, the mix of liquid resin and reinforcing fiber may be laid into an open mold, and optionally pressed gently at room temperature until cured. To accelerate low-temperature cure, more active catalyst systems are added. Alternatively, the assembly may be UV or oven cured. This requires less capital investment but more skilled labor, so it is useful for prototype and small production runs. It permits unlimited size, so the largest reinforced thermoset products are made in this way, for example, large tanks and whole boat hulls. 

Peroxidic cure systems are applicable only to fluorocarbon elastomers with cure sites that can generate new stable bonds. Although peroxide-cured fluorocarbon elastomers have inferior heat resistance and compression set, compared with bisphenol cured types they develop excellent physical properties with little or no postcuring. Peroxide cured fluoroelastomers also provide superior resistance to steam, acids, and other aqueous solvents because they do not require metal oxide activators used in bisphenol cure systems. Their difficult processing was an obstacle to their wider use for years, but recent improvements in chemistry and polymerization are offering more opportunities for this class of elastomers [42]. 

Recently, Endo and coworkers have developed novel sulfonium type initiators that can initiate polymerization either upon irradiation or upon thermal treatment. In addition, these photoinitiators are shown to be functional for both cationic and radical polymerizations. This dual activity is particularly important in hybrid curing systems for coatings and adhesions [28]. Scheme 11.6 contains the general steps for the synthesis of these photoinitiators from sulfides. 

EINECS 205-251-1 Hexanoic acid, 2-ethyl-, zinc salt Octoate Z Zinc 2-ethylhexoate Zinc bis(2-ethyl-hexanoate). Rubber activator used in soluble cure systems in place of stearic acid and partial replacement of zinc oxide for natural and synthetic rubbers. Vanderbilt R.T. Co. Inc. 

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. 

---