Cure package

There are seven principal classes of accelerators and several miscellaneous products that do not fit into these classes. In addition, many proprietary blends of several accelerators are sold which are designed as cure packages for a specific appHcations. Choosing the best cure system is a responsibiUty of the mbber chemist and requites extensive knowledge of each accelerator type and its appHcabiUty in each elastomer. Table 5 shows a rule of thumb comparison of the scorch/cure rate attributes for the five most widely used classes of accelerators used in the high volume diene-based elastomers. 

The cure package is composed of slow accelerator (DCBS or TBSI) with high level of sulfur for improving bond strength between mbber and brass layer. 

Compared to the previous example, the engine mount formulation contains a cure package tending to an SEV system this will provide a greater degree of reversion resistance compared to a CV cure system. Nonetheless, the antireversion agent is still able to provide significant, additional benefit. 

Das V T Manura J J Hartman T Q Volatile organic compounds from electron beam cured and partially electron beam cured packaging using automated short path thermal desorption, PittaCon99 Meeting, Orlando, FL, March 1999. 

The ODR has proved to be a useful tool in the study of rubber cross-linking kinetics. It is readily used to determine cure time. In addition, it is a useful tool in developing vulcanizates by comparing different cure package systems. The ODR should be set at an oscillation rate of 3 to 100 cpm and an arc of 3 . 

Ideal Antiozonants. An ideal antiozonant should be competitively reactive with ozone in the presence of carbon-carbon double bonds in the rubber-molecule backbone. However, it should not too reactive with ozone (or even oxygen) lest it not persist to give long-term protection. It should not react with sulfur accelerators or other ingredients in the cure package. It should be nonvolatile and persist at the surface of the rubber. In addition, the ideal antiozonant should not discolor the rubber. Unfortunately, an ideally active nonstaining chemical antiozonant has not yet been found. 

In the presence of compatibilizers, a similar process might be followed. Kumari et alP mixed poly(ethylene-co-vinyl acetate) (EVA) with NR and NBR (34% ACN content). Blending took place in a two-roll mill at a nip gap of 1.3 mm and at a friction ratio of 1 1.4. The crosslink agent used was DCP at 2 phr and the EVA amount in the mixture did not exceed 6 phr. Kader et alP used an internal mixer to compatibilize with /raw -polyoctylene rubber (TOR) a Standard Malaysian Rubber with NBR (34% ACN content) at a 50/50 weight ratio. The TOR amount reached up to 40 wt% of the blend where the cure package was based on sulfur. ZnO and stearic acid were introduced in the internal mixer while sulfur and N-tert-butyl-2-benzothiazole sulfenamide (TBBS) were added on a two-roll mill. 

The choice of vulcanisation system for the rubber can have a dramatic effect on adhesion. Typically sulphur cured rubbers are easier to bond to than sulphur-free or peroxide cured rubbers. This is believed to be due to the interaction of sulphur with key curative materials in the adhesive. The more sulphur that is present, the more interactions that are available, and hence the better the chance of getting good adhesion. SEV (semiefficient vulcanisation) and EV (efficient vulcanisation) cure packages are typically more difficult to bond because of their lower free sulphur contents. EV refers to cure systems which give predominantly monosulphidic or disulphidic crosslinks whereas conventional sulphur cure systems produce mostly polysulphidic crosslinks. SEV systems fall somewhere between EV and conventional systems in the type of crosslinks produced. Vulcanisation proceeds at different rates and with different efficiencies in different types of polymers, so the amount of sulphur needed to produce an EV cure system will also vary. For example, in NR, an EV system will generally contain between 0.4 and 0.8 phr of sulphur, while in NBR the sulphur level will generally be less than 0.3 phr of free elemental sulphur. 

TMTM is used as both a primary and secondary accelerator for various cure packages for natural and synthetic rubber, especially polychloroprene. Sometimes TMTM imparts better scorch resistance at processing temperatures than TMTD. 

CTP as an inhibitor is rather unique. Conventional retarders will extend scorch safety time but also hurt the state of cure and the cure rate of the rubber compound. If rubber is processed at lower temperatures or subject to less heat history, it is possible to avoid scorch-related problems. However, this is not always feasible. Sometimes adjustments in the cure package, such as changes in the ratio of secondary to primary accelerator, can also lengthen the scorch safety time but possibly hurt cure rate. 

ACM polymers have lower viscosities hence dispersion of ingredients may require more care than with other elastomers. For this reason, it is highly recommended that dispersed forms of various curatives or cure packages be used. In addition, soaps may vary in moisture content, making them difticult to disperse if the water content is low. The moisture content will also influence the cure rate, another reason to use dispersed forms. Relative humidity, thus moisture content of the compound, will influence cure rate, thus changes may occur with different seasons of the year. 

The HyTemp SC-75 cure package also will provide very fast cure rates with excellent shelf stability and fair scorch safety, but does require a post-cure to obtain desired physical properties. This cure system is recommended for injection and transfer as well as compression molding. The physical properties are good with a higher elongation and lower hardness than with other mechanisms. The heat aging and compression set are excellent, making this a very usefid product if process conditions are hard to control and a post-cure can be tolerated. 

A Commercial Colorable Bearing Seal compound based on HyTemp AR-72LF is shown in Table 5.19. The low compression set is obtained by using Zisnet F-PT cure package HyTemp ZC-50. 

With these cure packages it is possible to formulate bin stable AEM compounds with greater than 2X the rate of cure as the standard 1.5 phr HMDC system. The compression sets of the press-cured materials, using the fast HMDC/peroxide cure system, are about 30% lower (better) than the equivalent standard HMDC press-cured vulcanizates. 

The key performance properties of conveyor belts, particularly belt cover compounds, are flex resistance, abrasion resistance and low heat build up. In order to achieve better flex resistance, a conventional cure package is recoimnended. However, such a system suffers from the adverse effects of heat generation. In a recent study, the addition of Perkalink 900 was recommended, the effect is shown in a typical, NR based belt cover formulation in Table 51. 

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