Vulcanizate properties

Properties (Vulcanizate) Light-cream to white excellent resistance to ozone, to high and low temperatures (from-51 to +148C), and to acids and alkalies good electrical resistance, susceptible to attack by oils pelletized forms available. 

Accelerators. During sulfur vulcanization of rubber, accelerators serve to control time to onset of vulcanization, rate of vulcanization, and number and type of sulfur cross-links that form. These factors in turn play a significant role in determining the performance properties of the vulcanizate. 

There are three generally recognized classifications for sulfur vulcanization conventional, efficient (EV) cures, and semiefficient (semi-EV) cures. These differ primarily ki the type of sulfur cross-links that form, which ki turn significantly influences the vulcanizate properties (Eig. 8) (21). The term efficient refers to the number of sulfur atoms per cross-link an efficiency factor (E) has been proposed (20). 

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. 

In general, however, the vulcanizates suffer from poor low temperature crystallization performance compared to a conventional sulfur cure, and also have inferior tensile and tear properties. Urethane cross-linking systems (37), eg, Novor 950 (see Table 3) are also extremely heat resistant, but exhibit inferior tensile and dynamic properties compared to conventional sulfur-cured vulcanizates. One added virtue is that they can be used in conjunction with sulfur systems to produce an exceUent compromise according to the ratios used (38). 

Table 3. Properties of Carbon Black-Filled Natural Rubber Vulcanizates With Various Cure Systems ...

Mamzen Oil Co. has developed various Ziegler-Natta catalysts that can produce poly(butadiene-i //-prop5iene) (PBR) (78). PBR shows tack (self-adhesion) and green (unvulcanized) dynamic properties superior to those of BR and EPDM. Carbon black-loaded vulcanizates can be compounded to give high strength and elongation at break (79,80). PBR can also be covulcanized with SBR, BR, and EPDM. 

Two kinds of monomers are present in acryUc elastomers backbone monomers and cure-site monomers. Backbone monomers are acryUc esters that constitute the majority of the polymer chain (up to 99%), and determine the physical and chemical properties of the polymer and the performance of the vulcanizates. Cure-site monomers simultaneously present a double bond available for polymerization with acrylates and a moiety reactive with specific compounds in order to faciUtate the vulcanization process. 

Polyisobutylene has the chemical properties of a saturated hydrocarbon. The unsaturated end groups undergo reactions typical of a hindered olefin and are used, particularly in the case of low mol wt materials, as a route to modification eg, the introduction of amine groups to produce dispersants for lubricating oils. The in-chain unsaturation in butyl mbber is attacked by atmospheric ozone, and unless protected can lead to cracking of strained vulcanizates. Oxidative degradation, which leads to chain cleavage, is slow, and the polymers are protected by antioxidants (75). 

Curing. Carboxyl cure sites are incorporated in the ethylene—acryhc terpolymer to permit cross-linking with primary diamines (1,7). Guanidines are added to accelerate the cure. Peroxides may also be used as curing agents in the terpolymer, but generally give inferior properties to vulcanizates based on diamine systems (8). Dipolymers are cured only with peroxides. 

The most widely used plasticizers are paraffinic oils. Por appHcations that specify high use temperatures, or for peroxide cures, paraffinic oils of low volatihty are definitely recommended. However, since paraffinic oils exude at low temperatures from EPDM vulcanizates, or from high ethylene EPDMs, they are often blended with naphthenic oils. On the other hand, naphthenic oils interfere with peroxide cures. Aromatic oils reduce the mechanical properties of vulcanizates, and they also interfere with peroxide cures. Therefore, they are not recommended for EPM/EPDM. 

Mechanical properties depend considerably on the stmctural characteristics of the EPM/EPDM and the type and amount of fillers in the compound. A wide range of hardnesses can be obtained with EPM/EPDM vulcanisates. The elastic properties are by far superior to those of many other synthetic mbber vulcanizates, particularly of butyl mbber, but they do not reach the level obtained with NR or SBR vulcanizates. The resistance to compression set is surprisingly good, in particular for EPDM with a high ENB content. 

The electrical-insulating and dielectric properties of the pure EPM/EPDM are excellent, but in compounds they are also strongly dependent on the proper choice of fillers. The electrical properties of vulcanizates are also good at high temperatures and after heat-aging. Because EPM/EPDM vulcanizates absorb Htde moisture, their good electrical properties suffer minimally when they are submerged in water. 

Wire and cable insulation based on vulcanizates of PZ has also been studied. Again, low fire risk was the target property, and this was achieved. The need to vulcanize the coating, somewhat modest tensile properties, tensile strength of 5.2 to 12.2 MPa (760 to 1770 psi), and high dielectric constant (4—5 at 10,000 Hz) limited interest in this appHcation (19). 

General Vuleanizate Properties. Table 4 smmnaiizes the properties of CR vulcanizates that ate impoitant to the designers of rubber parts, and compares these properties to those of two competitive materials. The comparison is general and varies depending on compound design (90). The... 

Compound processibiUty is a key factor in the optimiza tion of new polychloroprene types. As a result, commercial compounds can be mixed, shaped, and cured by virtually all the methods used in the mbber industry. A typical polychloroprene compound includes a variety of additives designed to improve compound rheology, cure rate, and vulcanizate properties. 

Plasticizers. Addition of plasticizers (qv) to polyether elastomers alters physical properties, improves processing, and can improve low temperature flexibiUty. Plasticizers also reduce vulcanizate costs by allowing the use of higher levels of less expensive fillers. 

The polarity of the polyethers makes them incompatible with hydrocarbon-type plasticizers, which tend to bleed. Effective plasticizers are ethers such as di(butoxyethoxyethyl)formal [143-29-3] (Thiokors TP-90B), esters such as di(2-ethylhexyl) phthalate [117-81-7] dioctyl phthalate (DOP), polyesters such as Paraplex G50 (Rohm and Haas), and ether—esters such as di(butoxyethoxyethyl) adipate [114-17-3] (Thiokol s TP-95). The lower mol wt plasticizers, DOP, TP-90B, and TP-95 improve vulcanizate low temperature performance. The polymeric plasticizers maintain higher temperature and long-term aging properties. Epoxidized plasticizers should be avoided because they interfere with vulcanization. 

Neoprene XD. Developed in the 1980s, this polychloroprene family was prepared using special xanthogendisulphides as chain modifiers, offering improved processability and vulcanizate properties. In solution, these polychloroprenes show slow crystallization and high temperature resistance. 

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