The mechanism which leads to the formation of gels is still a matter of considerable controversy. A critical review of the literature leaves the impression that the lack of a satisfactory and general explanation for this so common

Only in recent years has it been possible to produce for the purpose of further studies well-defined monodisperse fractions of colloidal clays by rational supercentrifugal separation (2, 3).The lack of data concerning, primarily, clay sols of low concentration and varying particle sizes made highly desirable a closer study of freshly prepared natural suspensions of the clay mineral montmorillonite (the major constituent of bentonite) in concentrations from about 0.06 to 2 per cent by weight.Of special interest was the question whether any signs of structure could be detected in such low concentrations and, if so, how these change with particle size of the suspension and also what effect certain pretreatments of the original suspensions, as, for instance, thermal concentration at different temperatures, would have on their properties. Changes due to the addition of electrolyte have not been considered in this investigation, but will be discussed separately in a later publication.The particular bentonite used in this work was of the Wyoming type, mined by the American Colloid Co., Chicago, Illinois. (For further details as to location of the deposit, see reference 3.)The analysis of the finest fraction of this bentonite gave the values in table 1.The different particle size fractions were produced according to a method described by Hauser and Reed (2). Six fractions were obtained. The gels produced by supercentrifuging had about 10 per cent dry weight content. They were stored in the condition in which they were scraped from the liner of the centrifuge. The overflow of the finest and final fraction was collected and stored as a sol of 0.46 per cent dry weight. It will be referred to as fraction 7.The following experiments were carried out with fraction 6 containing particles of an average apparent diameter of 14 µ. Comparative studies

Developments in the rubber reclaiming industry are closely related to those in the rubber industry in general. The vulcanized rubber produced by the latter becomes in time the raw material used by the former. Although not superficially obvious, there has been considerable change in the reclaiming industry in the past two decades, required by the introduction of large proportions of synthetic rubber. Since this occurs both alone and in blends with natural rubber, reclaiming of SBR had to be studied in detail so that processes could be adjusted to give approximately the same viscosity from synthetic rubber and from natural, retaining existing procedures for fiber removal and mechanical handling as much as possible. It would have been economically impossible for reclaimers to use any process which required segregation and separate disposal of a substantial part of their raw material. The machinery used in the production of reclaim, and the reclaiming processes used today, are for the major part still the same as used before. The object of reclaiming vulcanized scrap is still the same, i.e., the breakdown (depolymerization) of the scrap to a plastic state which will permit reuse of it in the current rubber processing machinery for the manufacture of new goods. This breakdown is achieved by the application of energy. The type of energy is fundamentally irrelevant, but economics today dictate that it be heat, with partial exclusion of the oxidizing atmosphere, and therefore most of today's reclaim production is carried out in steam. One new continuous reclaiming process was developed during the last fifteen years which relies on electric energy to provide the necessary heat and working of the vulcanized scrap. A fundamental change in requirements of reclaim was also brought about once the synthetic rubber production had proceeded to the point where it was commercially possible to assign more detailed specifications which described the polymer processing behavior limits—a feat not previously achieved for commercial natural rubber. This type of specification was carred over to a considerable extent into the production and sale of today's reclaim. All in all, the extensive research and polymer knowledge which were acquired for the successful production of synthetic rubber have resulted in a much greater control in the production of reclaim and a much greater understanding of the reactions occurring in polymers during reclaiming. Because vulcanized scrap usually contains extraneous material (fiber) which must be removed during reclaiming the process conditions have in the past been selected primarily to accomplish this removal and were not those best suited for the actual reclaiming reaction. Progress has been made during these last years in removing the fibrous materials before reclamation, thereby permitting conditions in the devulcanization cycle to be determined by the actual needs of the vulcanized scrap.

No abstract

In a previous publication (5) it was demonstrated that self-supporting, coherent films could be obtained from bentonite gels3 ****by spreading the gels on an appropriate support and drying them in this condition. The films so obtained have become generally known as Alsifilm (aZuminum silicate film). Since they are entirely composed of pure clay, they offer a new and interesting condition of matter for colloidal studies.