The Plastic Bending of Beams Considering Die Friction Effects

This paper describes a theoretical and experimental analysis of the behavior of simple beams bent by a central load while freely supported at each end. Both plane strain and plane stress conditions and the effect of friction at the beam supports are considered. Materials tested are SAE 4130 steel and titanium 6Al-4V alloy. It is concluded that in beams, whose length between supports is at least 25 times their thickness, the load required to effect deflection is increased by the amount required to overcome friction, but does not affect the circumferential strain at midspan. Theoretical methods of analysis correlate well with experimental results and allow the computation of stresses and strains from the beam geometry and true tensile curves of the materials.

The Preparation and Properties of Highly Purified Rubber

A method is described for obtaining highly purified Hevea rubber, 99.9 per cent pure hydrocarbon, by addition of soap to ammonia-preserved latex to displace the protein from the rubber particle surfaces, followed by repeated creaming, solubilization in hexane, centrifuging, precipitation, and extraction with acetone. Prevention or limitation of oxidation of the pure hydrocarbon was obtained by work in darkness, or by treatment of the latex with pyrogallol and extraction with pyrogallol in acetone. Results are given on the vulcanization behavior of highly purified rubber, showing marked improvement in resilience, while there are remarkable differences in the mechanical properties of vulcaniz zates from sol and gel fractions in some mixes.

Funtumia and Ceara Vs. Hevea

It has to be admitted that both Ceara and Funtumia have some advantages over Hevea. Neither is so exacting regarding climate, and Ceara will thrive on soil unsuitable for Hevea. Ceara has the advantage of quick growth, and Funtumia yields an appreciable amount of rubber per tapping per tree. On the other hand, both of them are difficult to tap, and the trees are easily damaged. The yield per acre is much less than that given by Hevea and the quality of the rubber produced under the best conditions is from 5 to 8 per cent inferior. The crux of the problem is to increase the yield per acre, with a corresponding reduction in tapping costs. Until this has been achieved it is difficult to believe that either tree will receive the serious attention of planters after the present emergency has disappeared.

Rubber from Latex Coagulated with Sulfuric Acid

The results of tests carried out in the laboratories of the London Advisory Committee show that when sheet rubber is prepared by coagulation of latex with sulfuric acid: (1) Only a trace of free sulfuric acid remains in the dry rubber. (2) The rubber is somewhat easier to manipulate in rubber manufacturing operations than that prepared by coagulating latex with acetic acid. (3) The rubber has satisfactory mechanical properties when vulcanized in rubbers sulfur or accelerator mixings. (4) The rubber ages satisfactorily in rubber-sulfur and accelerator mixings. (5) The use of sulfuric acid would tend to increase the variability in vulcanization of first-grade rubber unless its use became general.

The Cause of Variability in the Plasticity of Plantation Rubber after Storage

There are two reasons why a study of the plasticity of rubber is of importance to the planter, viz.: (1) the variable plasticity of plantation rubber leads to many difficulties in manufacturing operations; and (2) first-grade plantation rubber is so hard that it is frequently mixed with inferior grades and other materials to enable it to pass more easily through the different processes. The results of the investigations recorded in this report suggest a definite reason for the hardness and variability of first-grade rubber. Tests carried out by De Vries in Java showed: (1) that freshly prepared plantation rubber is uniform in plasticity, but that many samples gradually become hard or soft on keeping and so develop differences; and (2) that rubber containing more serum substances than usual becomes hard, and that containing less than usual becomes soft. On the other hand, tests carried out by the Scheme on a large number of samples for over three years show that in most cases rubber becomes harder on keeping for six months at 15° C. The hardening of the rubber at European temperatures has been confirmed by Griffiths of the Dunlop Rubber Co., Ltd., who stated that large-scale experiments on rubber from different estates showed that in all cases, after storage for three years, it was necessary to increase the time of milling by about 30 per cent in order to obtain the required plasticity.

The Influence of Certain Accelerators on the Aging of Rubber

Among the accelerators which are at present on the market two are very much used, viz., diphenylguanidine as an accelerator of medium activity, and tetramethylthiouram disulfide as a rapid accelerator. Diphenylguanidine is used most of the time in proportions between 0.5 to 1 per cent, based on the rubber, and with about 3 per cent of sulfur. Tetramethylthiouram disulfide is used in proportions of between 0.2 and 0.5 per cent, based on the rubber, with about 1.8 per cent of sulfur. In this way “nervy” mixtures are obtained which have excellent physical properties; but it must be recognized that these two accelerators are, each in its class, the ones which impart to mixtures the poorest aging qualities. For example, the mixture: Pale crepe 100 Zinc oxide 5 Sulfur 3 Palm oil 1 Stearic acid 0.5 Diphenylguanidine 0.7 vulcanized 1 hour at 3 kilograms pressure, i. e., under conditions which lead to the best physical properties for the proportions used, was investigated.