Natural rubber experiments

As already said in the Preface, a question arose as to the appropriateness of having a chapter in this book devoted to the cure of rubbers and to their properties concerned with the recovery of scrap rubber obtained from old tires. However, the amount of old tire rubber is so large that it could be considered as a raw material, and reclaiming will become a necessity when crude oil and natural rubber experience shortages. This problem is so vast that only a book in itself could describe all the possibilities of reusing. Nevertheless, a large number of opportunities has been identified, and the reclaiming processes based on the cure of these scrap rubbers have been considered. 

Figure 3.3 Comparison of experiment (points) and theory [Eq. (3.39)] for the entropy elasticity of a sample of cross-linked natural rubber. [From L. R. G. Treloar, Trans. Faraday Soc. 40 59 (1944).]...

Whether or not a polymer is rubbery or glass-like depends on the relative values of t and v. If t is much less than v, the orientation time, then in the time available little deformation occurs and the rubber behaves like a solid. This is the case in tests normally carried out with a material such as polystyrene at room temperature where the orientation time has a large value, much greater than the usual time scale of an experiment. On the other hand if t is much greater than there will be time for deformation and the material will be rubbery, as is normally the case with tests carried out on natural rubber at room temperature. It is, however, vital to note the dependence on the time scale of the experiment. Thus a material which shows rubbery behaviour in normal tensile tests could appear to be quite stiff if it were subjected to very high frequency vibrational stresses. 

FIGURE 16.13 Fourth pass viscosity of a multistage mixing experiment of butadiene rubber-natural rubber (BR-NR) and styrene-butadiene rubber (SBR)-NR blends (60/40) with 50 pbr of N-234 carbon black. 

This discussion of the structures of diene polymers would be incomplete without reference to the important contributions which have accrued from applications of the ozone degradation method. An important feature of the structure which lies beyond the province of spectral measurements, namely, the orientation of successive units in the chain, is amenable to elucidation by identification of the products of ozone cleavage. The early experiments of Harries on the determination of the structures of natural rubber, gutta-percha, and synthetic diene polymers through the use of this method are classics in polymer structure determination. On hydrolysis of the ozonide of natural rubber, perferably in the presence of hydrogen peroxide, carbon atoms which were doubly bonded prior to formation of the ozonide... 

Lord Kelvin s close associate, the expert experimentalist J. P. Joule, set about to test the former s theoretical relationship and in 1859 published an extensive paper on the thermoelastic properties of various solids—metals, woods of different kinds, and, most prominent of all, natural rubber. In the half century between Gough and Joule not only was a suitable theoretical formula made available through establishment of the second law of thermodynamics, but as a result of the discovery of vulcanization (Goodyear, 1839) Joule had at his disposal a more perfectly elastic substance, vulcanized rubber, and most of his experiments were carried out on samples which had been vulcanized. He confirmed Gough s first two observations but contested the third. On stretching vulcanized rubber to twice its initial length. Joule ob-... 

Id. The Ideal Rubber.—The data available at present as summarized above show convincingly that for natural rubber (dE/dL)T,v is equal to zero within experimental error up to extensions where crystalhzation sets in (see Sec. le). The experiments of Meyer and van der Wyk on rubber in shear indicate that this coefficient does not exceed a few percent of the stress even at very small deformations. This implies not only that the energy of intermolecular interaction (van der Waals interaction) is affected negligibly by deformation at constant volume—which is hardly surprising inasmuch as the average intermolecular distance must remain unchanged—but also that con-... 

Plazek (183) carried out very accurate creep experiments on natural rubber as a function of cross-linking. He found that data at different temperatures could be superimposed by the usual WLF shift factors which were developed for non-cross-linkcd poiymers (27). Temperature-superposed... 

Elastomers. Many experiments have been performed with natural rubber, using either synthetic elastomers or plastomers as the second polymer. 

In the case of rubber this cross linking is called vulcanization. Experiments conducted in 1946 at the Clinton Pile (D6) demonstrated a slight curing action of pile irradiation on natural rubber. Charlesby has shown that the degree of cross linking is directly proportional to radiation dose. Approximately 50 X 106 r. produces one cross link/90 isoprene units. 

Rivlin and Saunders11 suggested from their biaxial extension experiments on vulcanized natural rubber that the form of W for rubber-like material would be... 

Numerous manufacturers in the early 70 s experimented with pure gum rubber for absorber and component piping. The overwhelming evidence from these early installations showed that a soft 40 durometer natural rubber is unsuitable for these units and associated component piping. The linings failed rapidly due to blistering which was caused by moisture absorption. 

Fig. 31. Stacked plot of the heteronuclear two-dimensional J-resolved spectrum of cured, carbon black filled, natural rubber. The proton flip experiment was used with high-power proton decoupling during the detection time. The experiment was performed with the sample spinning at the magic angle (reprinted from Ref. 1911 with permission)...

Goodyear was determined to solve the problems inherent in natural rubber. With no formal training in chemistry, his work was based on trial and error, experimenting with different methods of processing and additives such as magnesia. The solution he discovered resulted partly from serendipity and partly from constant work. 

A similar experiment has been performed [68] but under MAS conditions. For a series of crosslinked natural rubber samples (A - FI), 13C edited 1H spinning sidebands have been extracted from the 2D spectrum. These sideband pattern are encoded by the residual dipolar couplings of the corresponding functional groups and are presented in Figure 14.12. 

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