Vulcanizate tensile

Difference in vulcanizate tensile strength of EPDMs prepared with various catalyst systems, especially the high strength of EPDM prepared with vanadium carboxylates (V g)>is examined against various structural parameters. Effect of L r l ... 

Among the main molecular structural variables in EPDMs that are stipulated by catalyst systems and that affect the vulcanizate tensile properties we may mention molecular weight (MW) and MWD, degree of unsaturation (LG=C 1) and its distribution in the polymer, composition (C S) and monomer sequence length distribution along molecular chains, and long-chain branching if present. Effect of... 

Comparison of vulcanizate tensile properties of the six sets of comparable samples (Figures 7A-F) gave results (Table I, last column) pointing to the ability of the systems in giving decreasing tensile strength in the following orders... 

Vj g-EPDM and VOCI3-EPDM vulcanizate tensile strength Is not simply explainable by network quality resulting from difference In the third monomer distribution in the terpolymers. 

In Figure 8 are also reproduced data (curves A and C) of the other two systems previously given Q). It is clear that although addition of ETCA to the V g-EtyV Clo system makes the DCPD distribution in the terpolymer much more heterogeneous (curve A, compared to B) and yields lower vulcanizate tensile strength (see above), the much more even distribution of DCPD in V(acac) -EPDM... 

High MW fraction in EPDM ought to contribute favorably to the vulcanizate tensile properties ( ). The following preliminary observations were gathered to examine whether difference in. tensile strength of samples prepared with various catalysts studied was due primarily to the difference in MWD of the terpolymers. ... 

Furthermore, it has to be noted that both VOCI3- and V3.-9-EPDMs have multimodal GPC profiles signifying the presence of high MW fractions, yet they have the greatest disparity in vulcanizate tensile properties, again suggesting MWD to be not a major determinant in defining the observed tensile differences. 

A second important factor is the density of cross-linking. It is a common observation that, with both amorphous and crystallizing vulcanizates, tensile strength passes through a maximum value with increasing cross-link density (Fig. 8.11). Various theories to explain... 

Fig. 16. Effect on vulcanizate tensile strength of peptization time, and type and dosage of peptizer. E, Dispergum 24 F, Struktol A82 G, Struktol A60/Renacit VII 0 1 phr H, Struktol A60/Renacit VII 0-15 phr , unaged i, aged three days at 100°C.

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). 

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). 

Increase in the molecular weight of the polychloroprene, increasing viscosity and tensile strength of vulcanizates. 

A VDF unit also follows FIFP m the structurally much more complex terpo-lymers [30] Dipolymers contain 60 wt % VDF (66% F) and terpolymers contain 33-50 wt % VDF (66-69 5% F) and generally less than 28 wt % TFE Molecular weights range from 1 5 x 10 to 10 Their respective continuous service temperature ranges are -18 to 210 °C and -12 to 230 °C Dipolymer vulcanizates retam over 50% of their tensile strength for more than 1 year at 200 °C or for 2 months at 260 C... 

Neoprene vulcanizates have a high tensile strength, excellent oil resistance (better than natural rubber), and heat resistance. 

Butyl ruhher vulcanizates have tensile strengths up to 2,000 psi, and are characterized hy low permeahility to air and a high resistance to many chemicals and to oxidation. These properties make it a suitable rubber for the production of tire inner tubes and inner liners of tubeless tires. The major use of butyl rubber is for inner tubes. Other uses include wire and cable insulation, steam hoses, mechanical goods, and adhesives. Chlorinated butyl is a low molecular weight polymer used as an adhesive and a sealant. 

Better cross-linking with the latter also improves post Tg viscoelastic responses of the rubber vulcanizates. Similar effect has also been observed with polychloroprene as investigated by Sahoo and Bhowmick [41]. Figure 4.8 represents the comparative tensile stress-strain behavior of polychloroprene rubber (CR) vulcanizates, highlighting superiority of the nanosized ZnO over conventional rubber grade ZnO [41]. 

FIGURE 11.9 Tensile fractured samples show the texture of XNBR vulcanizates (a) 75 25 EPDM/XNBR (b) 75 25 EPDM/XNBR (c) 50 50 EPDM/XNBR (one stage) (d) 50 50 EPDM/XNBR (two stage) (e) 25 75 EPDM/XNBR (one stage) (f) 25 75 EPDM/XNBR (two stage). (From Naskar, M., Debnath, S.C., and Basu, D.K. Rubber Chem. TechnoL, 75, 309, 2002.)... 

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