Accelerator of curing

Eor apphcation temperatures below 10°C or for acceleration of cure rates at room temperature, nonredox systems such as ben2oyl peroxide initiated by tertiary amines such as dimethylaruline (DMA) have been appHed widely. Even more efficient cures can be achieved using dimethyl- -toluidine (DMPT), whereas moderated cures can be achieved with diethylaruline (DEA). 

The advantages of these adducts are low volatility, higher mixing ratios, and faster cure rates. The reason for the faster cure rate is that the adduct is already partially reacted, thus less reaction is required to reach a gel. In addition, the presence of hydroxyl groups causes an acceleration of cure. Faster cure is a distinct advantage in adhesive formulations where thin coatings of adhesive must set rapidly. 

Figure 10 The effect of LP-3 level (in parts LP-3 per 100 parts epoxy resin) on the pot-life of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin for a 100 g mix at 22°C (a) acceleration of cure (DGEBA epoxy resin with Ancamine 1608) (b) retardation of cure (DGEBA epoxy resin with Ancamine MCA). Redrawn with permission from Morton International Ltd, Coventry, UK...

Acceleration of cure rates was reported by Crivello et al., when simultaneous photo initiated eationic polymerizations of epoxides and vinyl ethers are carried out in the presence of diaryliodonium salts. This, however, results in accelerations of ring-openings in epoxide polymerizations but deceleration of vinyl ether polymerizations. The effects are seen both, in mixtures of the two monofunetional monomers as well as in hybrid monomers that bear both functional groups simultaneously, vinyl ether and epirane groups... 

As resoles cure simply by heat activation, no hardener components need to be added to the formulation, which will cure in the region of 140 to 150 °C in 30 to 60 min resoles are stUl active, albeit to a low degree, at ambient temperatures. Simple acceleration of cure cau be achieved by... 

Trade name of Du Pont. This is a ZnCL-MBTS complex used as an accelerator of cure and is reported to be more effective. than using the individual ingredients on their own. 

For acceleration of curing properties, a small amount of water-containing hardener paste, called Booster paste, can be mixed to the main component. Homogeneous mixing of the booster paste is accomplished by means of a static mixer with more mixing elements compared to the PUR Booster described in Section 6.5.6.6. entitled Booster Technology . 

Kalenda, P, Ferrocene and some of its derivatives used as accelerators of curing reactions in unsaturated polyester resins, Eur. Polymer J., 31, 1099, 1995. 

Copolymerization can be carried out with styrene, acetonitrile, vinyl chloride, methyl acrylate, vinylpyridines, 2-vinylfurans, and so forth. The addition of 2-substituted thiazoles to different dienes or mixtures of dienes with other vinyl compounds often increases the rate of polymeriza tion and improves the tensile strength and the rate of cure of the final polymers. This allows vulcanization at lower temperature, or with reduced amounts of accelerators and vulcanizing agents. 

A unique process for chemical stabili2ation of a ceUular elastomer upon extmsion has been shown for ethylene—propylene mbber the expanded mbber obtained by extmsion is exposed to high energy radiation to cross-link or vulcani2e the mbber and give dimensional stabUity (9). EPDM is also made continuously through extmsion and a combination of hot air and microwaves or radio frequency waves which both activate the blow and accelerate the cure. 

Other Accelerators. Amine isophthalate and thiazolidine thione, which are used as alternatives to thioureas for cross-linking polychloroprene (Neoprene) and other chlorine-containing polymers, are also used as accelerators. A few free amines are used as accelerators of sulfur vulcanization these have high molecular weight to minimize volatility and workplace exposure. Several amines and amine salts are used to speed up the dimercapto thiadiazole cure of chlorinated polyethylene and polyacrylates. Phosphonium salts are used as accelerators for the bisphenol cure of fluorocarbon mbbers. 

As a general rule the sulfenamides exhibit faster cure rate than the thiazoles. If secondary accelerators are used, dithiocarbamates are scorchiest and give the fastest cure followed by the thiurams, then the guanidines. Figure 6 summarizes these comparisons to show a series of natural mbber (NR) recipes using either a thiazole (MBTS) or sulfenamide (TBBS) primary accelerator in combination with the various secondary accelerators (21). In this study, the initial primary accelerator levels were selected to produce nearly equivalent modulus or state of cure in the NR. 

A study of the effect of stearic acid and 2iac oxide on a sulfonamide-accelerated, sulfiir-cured natural mbber compound dramatically showed the need for both 2iac and fatty acid activators (Fig. 7) (21). 

Types of Latex Compounds. For comparison with dry-mbber compounds, some examples of various latex compounds and the physical properties of their vulcanizates are given in Table 23. Recipes of natural mbber latex compounds, including one without antioxidant, and data on tensile strength and elongation of sheets made from those, both before and after accelerated aging, are also Hsted. The effects of curing ingredients, accelerator, and antioxidant are also Hsted. Table 24 also includes similar data for an SBR latex compound. A phenoHc antioxidant was used in all cases. 

This is an activator-starved formulation and so is highly sensitive to the presence of nonmbbers that are capable of activating or accelerating vulcanization, and Table 2 illustrates the cure behavior of different grades of SMR (28). Cup lump grades show the highest state of cure and fastest rate of cure, whereas the stabilized grade, SMR CV, shows the lowest state of cure and slowest cure rate. 

Accelerators are chemical compounds that iacrease the rate of cure and improve the physical properties of the compound. As a class, they are as important as the vulcanising agent itself. Without the accelerator, curing requires hours or even days to achieve acceptable levels. Aldehyde amines, thiocarbamates, thiuram sulfides, guanidines, and thiasoles are aU. classified as accelerators. By far, the most widely used are the thiasoles, represented by mercaptobensothiasole (MBT) and bensothiasyl disulfide (MBTS). 

Accelerator activators are chemicals required to initiate the acceleration of the curing process. They also improve the polymer compound quaUty. 

In the presence of the organic siHcate, the heavy-metal salts trigger the chain extension and cross-linking reactions that lead to siHcone mbber and volatile ethanol as a byproduct. Useful metal soaps iaclude stannous octanoate [1912-83-0], ziac octanoate [557-09-5], dibutyltin dilaurate [77-58-7], and dibutyltin diacetate [1067-33-0]. The reactivity of the different salts varies considerably. Stannous octanoate effects a cure ia 0.5—2 min ziac octanoate may require 24—96 h the dibutyltin dilaurate, 10—20 min. Heat and moisture accelerate the curing rate, but to a lesser degree than ia the case of the polysulfide mbbers. 

The earliest study describing vulcanised polymers of esters of acryUc acid was carried out in Germany by Rohm (2) before World War I. The first commercial acryUc elastomers were produced in the United States in the 1940s (3—5). They were homopolymers and copolymers of ethyl acrylate and other alkyl acrylates, with a preference for poly(ethyl acrylate) [9003-32-17, due to its superior balance of properties. The main drawback of these products was the vulcanisation. The fully saturated chemical stmcture of the polymeric backbone in fact is inactive toward the classical accelerators and curing systems. As a consequence they requited the use of aggressive and not versatile compounds such as strong bases, eg, sodium metasiUcate pentahydrate. To overcome this limitation, monomers containing a reactive moiety were incorporated in the polymer backbone by copolymerisation with the usual alkyl acrylates. 

Halobutyl Cures. Halogenated butyls cure faster in sulfur-accelerator systems than butyl bromobutyl is generally faster than chlorobutyl. Zinc oxide-based cure systems result in C—C bonds formed by alkylation through dehydrohalogenation of the halobutyl to form a zinc chloride catalyst (94,95). Cure rate is increased by stearic acid, but there is a competitive reaction of substitution at the halogen site. Because of this, stearic acid can reduce the overall state of cure (number of cross-links). Water is a strong retarder because it forms complexes with the reactive intermediates. Amine cure may be represented as follows ... 

In order to obtain a sufficient rate of cure at moulding temperatures it is usual to add about 0.2-2.0% of a hardener (accelerator). This functions by decomposing at moulding temperatures to give an acidic body that will accelerate the cure rate. A very large number of such latent acid catalysts have been described in the literature, of which some of the more prominent are ammonium sulphamate, ammonium phenoxyacetate, ethylene sulphite and trimethyl phosphate. 

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