Rubbers sample presentation method

It is possible that the lower than required values of D2 reflect a problem with incorrect values of Q, which if too large would result in smaller values of D2. In an interferometric study of the diffusion of toluene in an uncrosslinked natural rubber sample, Mozisek (15) reported results for the mutual diffusion coefficient which were similar to the results of Hayes and Park. In the absence of thermodynamic data from Mozisek s work, correction factors calculated for the present work were applied to his data. The results are shown in Figure 7, which reproduces Mozisek s data along with the values for D2. The extrapolated value at 1, would exceed the self diffusion coefficient for toluene by about two orders of magnitude, similar to the discrepancy seen with Hayes and Park s data. This indicates that the fault with the results in the present case is not due to overly high values of the correction factors. Moreover, the method of calculating D from D12 has been confirmed experimentally by Duda and Vrentas (16) in a comparison of vapor sorption results for toluene diffusion in molten polystyrene with the values of D1 obtained directly using radio-labeled toluene. 

Gee ° has applied this method to the determination of the interaction parameters xi for natural rubber in various solvents. Several rubber vulcanizates were used. The effective value of VelV for each was determined by measuring its extension under a fixed load when swollen in petroleum ether. Samples were then swollen to equilibrium in other solvents, and xi was calculated from the swelling ratio in each. The mean values of xi for the several vulcanizates in each solvent are presented in Table XXXVI, where they are compared with the xi s calculated (Eq. XII-30) from vapor pressure measurements on solutions of unvulcanized rubber in some of the same solvents. The agreement is by no means spectacular, though perhaps no worse than the experimental error in the vapor pressure method. 

Thus, at the present state of our knowledge, rubber elasticity can only be considered as a relatively rough method for crosslink density determinations, but it is practically the unique possible method for thermosets. It is valid for relative determinations in sample series where there are no big structural changes (for instance aliphatic or aromatic) other than crosslink... 

Even with careful conditioning, however, the results produced from specimens manufactured by different methods may vary, and if there is to be a controlled comparison it is important that the test pieces be prepared in exactly the same way. This is particularly important for figures being presented in databases. For example, laboratory samples of a rubber prepared on a mill may differ considerably from factory materials prepared in an internal mixer, and often these differences are not sufficiently emphasized in tables of data. 

A wide range of plastics test methods are available and routinely used to characterize plastics and rubber. Even a basic introduction to the common tests used is beyond the scope of this chapter. The following introductory discussion is on several selected test methods that will allow the reader to better understand the test data presented in other chapters. With most of these tests, there are an extensive theoretical foundation and numerous practical details on how to carry them out as well as the relevant standards that have been left out. With most tests described here, the experience of the operator in sample preparation, carrying out the tests, and analysis of data is critical to obtain rqtroducible results. 

Negri et al. [301] have applied TG-DTA to the characterisation of different types of carbon-black in NR vulcanisates. The method allowed determination of the overall CB content, but where combinations of different blacks were present it was not possible to determine the proportion of each type. TG-DTA has also been used to correlate TGA in airflow and N2 gas flow and some other micro-scale flammability tests i.e., oxygen index, hot-plate ignition and drum friction tests) on covers of different flame-resistant and non flame-resistant rubber conveyer belts [302]. The minimum temperatures at which rapid weight loss of each sample began to appear were determined and compared with the results from the micro-scale flammability tests. 

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