Tensile strength of natural rubber

Fig. 107.—Tensile strengths of natural rubber plotted against the degree of cross-linking with bis-azo vulcanizing agent (O), expressed as equivalent percent (pXlOO). Upper curve ( ) sample prepared using one equivalent percent of bis-azo compound plus monoreactive ethyl azodi-carboxylate for the total degrees of modification of the units indicated on the ordinate scale. (Flory, Rabjohn, and Shaffer.

Calculations of the elastic properties, the main tensions and tensile strength of natural rubber carried out without using the empirical adjusting parameters are in good agreement with the experimental data. 

Epoxidized natural rubber is still a strain crystallizing mbber and therefore retains the high tensile strength of natural rubber. However, as can be seen from Table 5, in other respects they have very little in common. The epoxidation renders a much higher damping mbber, a much-improved resistance to oil swelling (insofar as a 50 mol % modified natural mbber has similar oil resistance to a 34% nitrile mbber), and much-reduced air permeability. This latest form of modified natural mbber therefore widens the applications base of the natural material and enables it to seek markets hitherto the sole province of some specialty synthetic mbbers. 

Fig. 2. The influence of polystyrene PS contents on the tensile strength of natural rubber (NR) vulcanizates.

FIGURE 10.26 Tensile strength of natural rubber crosslinked with dicumyl peroxide (DCP) versus temperature. (From Thomas and Whittle (1970).)... 

The Bueche-Halpin theory accounts well for the principal features of the tensile strength of unfilled rubbers. Because of the direct connection between ab and a viscoelastic function, time-temperature superposition of the strength follows naturally. Halpin (216) also found experimentally that ab was apparently the same function of the reduced time to break, tb/aT, whether the rupture experiment was carried out at constant stress, constant extension or constant rate of extension. 

In the preparation of most rubber articles so-called fillers are used. These serve to add body to the product and to increase the tensile strength of the rubber and the ability to resist abrasion. They also decrease the time required for vulcanization. Among the substances commonly used are zinc oxide, barium sulphate, gypsum, whiting, magnesium carbonate, silica, etc. Gas-black, which is obtained as the result of the incomplete combustion of natural gas, is employed in making the rubber used in certain types of automobile tires. For some unknown reason it is superior for this purpose to ordinary lamp-black. 

Compatibility and various other properties such as morphology, crystalline behavior, structure, mechanical properties of natural rubber-polyethylene blends were investigated by Qin et al. [39]. Polyethylene-b-polyiso-prene acts as a successful compatibilizer here. Mechanical properties of the blends were improved upon the addition of the block copolymer (Table 12). The copolymer locates at the interface, and, thus, reduces the interfacial tension that is reflected in the mechanical properties. As the amount of graft copolymer increases, tensile strength and elongation at break increase and reach a leveling off. 

Carbon black increases the tensile strength of an SBR vulcanisate to its 10 to 20-fold (which would, otherwise, be very low), as well as its abrasion resistance. Natural rubber can, because of its stereospecific (cis) chain structure crystallise under strain, and, therefore, reach higher values of its tensile strength for a good abrasion resistance carbon black is also of importance with NR. 

Types of Latex Compounds. For comparison with dry-rubber compounds, some examples of various latex compounds and the physical properties of their vulcanizates are given in Table 23. Recipes of natural rubber 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 listed. The effects of curing ingredients, accelerator, and antioxidant are also listed. Table 24 also includes similar data for an SBR latex compound. A phenolic antioxidant was used in all cases. 

The most commonly reported physical properties of radiation cross-linked natural rubber and compounds made from it are modulus and tensile strength, obtained from stress-strain measurements. Figure 5.5 illustrates some of the results obtained from gum rubber and from a natural rubber compound reinforced by HAF carbon black. In Figure 5.6 the tensile strength of radiation cured gum is compared to that of vulcanizates cured by sulfur and by peroxide. ... 

Tensile strength of radiation cured purified natural rubber, o, gum , compound (50 phr N330 carbon black). (Bohm, G. G. A., and Tveekrem, J. O., Rubb. Chem. Technol., 55 3, p. 619. Reprinted with permission from Rubber Division, ACS.)... 

The prevulcanization of natural rubber in latex form has also been a subject of much investigation. The cross-linking mechanism is not yet fully understood, but the water apparently plays a major role in it. Irradiation results in the cross-linking of the rubber molecules and in coarsening of the latex particles. A process of cross-linking of natural rubber latex has been developed to the point that it can be used for an industrial-scale application. The irradiation is performed in aqueous media by electron beam without a prorad (sensitizer) at a dose of 200 kGy (20 Mrad) or in the presence of n-butyl acrylate at considerably lower doses, typically 15 kGy. The cross-linked film exhibits physical properties comparable to those obtained from sulfur cured (vulcanized) film. As an alternative, the addition of a variety of chloroal-kanes makes it possible to achieve a maximum tensile strength with radiation doses of less than 5 Mrad (50 kGy). ... 

Tensile strength of radiation-cured purified natural rubber Tensile strength of radiation-cured purified natural rubber Examples of industrially important acrylated prepolymers The arrangement for direct roll coating Different types of cells on the surface of a gravure roll Reverse roll coating... 

FIGURE 5.7 Tensile strength of radiation-cured purified natural rubber. Legend = sulfur T = peroxide = EB irradiation. (Bohm, G.G.A. and Tveekram, J.ORubber Chem. Technol. Vol. 55, No. 3 (1982). With permission.)... 

Butyl rubber is produced by a process in which isobutylene is copolymerized with a small amount of isoprene using aluminum chloride catalyst at temperatures around — 150° F. (20). The isoprene is used to provide some unsaturation, yielding a product that can be vulcanized (43). Vulcanized Butyl rubber is characterized by high tensile strength and excellent flex resistance furthermore, as a result of its low residual unsaturation (only 1 to 2% of that of natural rubber) it has outstanding resistance to oxidative aging and low air permeability. These properties combine to make it an ideal material for automobile inner tubes (3), and Butyl rubber has continued to be preferred over natural rubber for this application, even when the latter has been available in adequate supply. 

The physical properties of natural rubber and synthetic rubber compounds are affected greatly by the type and amount of fillers used. Carbon black is the most commonly used filler. Increasing amounts of carbon black increases the hardness and modulus of the vulcanizates. Resilience and resistance to impinging type abrasion decrease along with elongation. Tensile strength and tear strength... 

---