Sol and Gel in Hevea Latex and Crude Rubber. Influence of Oxidation on Gel–Sol Transformation.

Abstract

From the work of Caspar! (1), Feuchter (2), and others it is now well known that a large proportion of the hydrocarbon in unmasticated crude Hevea rubber diffuses into and is therefore soluble in petroleum ether or ethyl ether when the rubber is covered with these solvents and allowed to stand at room temperature. This soluble form of rubber has been termed "sol," and the residue from a prolonged and repeated solvent extraction is known as the gel skeleton, which contains practically all of the protein and ash of the original rubber associated with the tough and gristly rubber hydrocarbon termed "gel." It is apparent that the terms "sol" and "gel" rubber are somewhat indefinite, as they do not refer to definite fractions or to rubber having definite physical or chemical characteristics.From a purely colloidal viewpoint and by definition all rubber soluble by diffusion into a solvent is sol. However, we must recognize the indistinct borderline between difficultly soluble sol and almost insoluble gel.The so-called sol rubber carefully prepared from acetone-extracted crepe by diffusion into petroleum ether forms a clear, colorless, and highly elastic substance upon evaporation of the solvent. It is almost pure hy-drocarbon2, since it contains less than 0.005 per cent nitrogen and 0.01 per cent ash. Its unsaturation, as determined from its iodine value (3), corresponds closely to the theoretical value for (CiíH^=)".Up to the present time studies of sol and gel rubber have been largely qualitative and confined to crude rubber and to highly purified rubber from latex. This paper will describe a definite procedure designed to determine the proportion of sol and gel in crude rubber and in dried latex films by the use of a petroleum ether diffusion process. The influence of oxidation on the transformation of gel to sol has been studied by the use of this method. A search of the literature indicates that this field has not been previously investigated.1 Presented before the Division of Rubber Chemistry at the Ninety-seventh Meeting of the American Chemical Society, held in Baltimore, Maryland, April 3-7, 1939. * Midgley (5) found from 0.02 to 0.04 per cent of combined oxygen in sol rubber.

Molecular Weight of Sol and Gel in Crude Hevea Rubber.

“…In this case it is shown that gel can be fractionated in hexane which contains 10 per cent of absolute alcohol and that the solubility diminishes as extraction proceeds. This evidence, together with that recently obtained (1) showing the high resistance of a portion of latex gel to oxidation, leads to the conclusion that the rubber gel in latex is present in variable molecular sizes.…”

“…MOLECULAR WEIGHT OF MILLED CREPE FRACTIONS Fractionation of milled crepe was carried out by the procedure already referred to (1). The molecular weight data on these fractions are given in table 5.…”

Fractionation and Molecular Weight of Rubber and Gutta-Percha

“…Since these authors did not determine the molecular weight of the different fractions, an attempt was made to repeat this work on the R. C. . A. crepe employed in this and previous studies (5,6). It was found that this crepe did not dissolve completely in the mixed solvent at 50°C ., and when cooled to 42°the viscosity was so high that the gel could not be readily separated from the solution.…”

“…A. (Rubber Culture Maatschappij Amsterdam) crepe previously studied (5,6) or smoked sheet to avoid differences due to sample variations. The amount of rubber in solution was determined by drying 25 cc.…”

Natural And Synthetic Rubber Fibres. Electron Microscope Studies.

“…Pummerer (17,18) termed the ether-soluble part the "sol" fraction and the ether-insoluble part, the "gel" fraction. More recently Kemp and Peters (9,10,11) and Bloomfield and portions depends on the type of solvent used and other conditions of extraction. Although the method of Pummerer produces only a rough separation, it was chosen as a convenient starting point for the electron microscope examination of rubber fractions, and the terms "sol" and "gel" will refer here to fractions obtained by this method.…”