Diffusion Phenomena in Solvent Deresination of Guayule Rubber

Abstract

ATURAL rubber of excellent quality has been obtained by acetone deresination of the resinous worms from the pebble milling of freshly harvested comminuted guayule shrub (4).The removal of the resin fraction is so rapid and complete in this instance that it is difficult to study factors which influence the rate and extent of deresination. Since the intelligent design of of these factors, it seemed desirable to prepare the material in a form more adaptable to fundamental study. The present paper reports an investigation of diffusion phenomena occurring during the solvent extraction of resin from thin sheets of resinous guayule rubber impressed on stainless steel screens. * an efficient extraction process, requires a thorough knowledge REVIEW OF PREVIOUS WORKHauser and Le Beau carried out Soxhlet type extractions with thin sheets of resinous guayule rubber for the purpose of comparing the effectiveness of various resin solvents (6). The solvents that they investigated can be arranged in the following order of decreasing efficiency for resin removal at their boiling points: furfural, acetone, nitropropane, ethyl alcohol, and methanol. They noted, however, that furfural was inferior to acetone a t lower temperatures. Nishimura et aE. (9) preferred glacial acetic acid for the deresination of resinous guayule in the form of oven-dried spongy sheets, This solvent was apparently even more effective than acetone for removing the deteriorative substances which they found in guayule rubber obtained in the above fashion.Recently Wood and Fanning (14) carried out a series of diffusion type deresinations with boiling acetone on sheets of resinous guayule milled "as thin as possible and then cut into small pieces." These smaller sheets were supported horizontally on properly spaced pieces of galvanized window screen, in a Soxhlet type extractor. Extractions were conducted for periods of 1,4, 8, and 12 hours, respectively. I n 12 hours the resin content had dropped from 19.4 to 2.7%. Since this corresponded to a slower rate than that obtained by them in another series of experiments with mastication, it appears likely that the extraction rate was being limited by diffusion. Their data have now been plotted (see Figure l), and a diffusion limitation to deresination is indeed evidenced by the close conformity of the experimental extraction curve to the theoretical curve based on Fiok's diffusion law (a). The semilogarithmic plot of the fraction of resin remaining (q/qo) versus time is a straight line after t = 1 hour.Since the sheet thickness is not known, the diffusion coefficient cannot be calculated, and therefore the accompanying theoretical curve is based merely on the experimentally determined extraction rate. Clark et al. ( 4 ) have reported on batch countercurrent deresinations. Warm acetone was employed in the preparation of 4000 pounds of low-resin content, high viscosity guayule rubber suitable for truck carcass stock compound. This work revealed various significant advantages for the deresination of guayule in the form ...

Elastomers

“…Fundamental data on rates of diffusion for a large-scale extraction apparatus for guayule show that the extraction of resin under controlled conditions conforms fairly well with the Fick law of diffusion (10). Pilot plant experiments have shown that rubber of uniform high quality can be prepared from the tiny guayule "worms" by deresination with acetone (54).…”

High-Quality Rubber by Acetone Deresination of Guayule

“…have been replotted in Figure 5 to test conformance with Fick's diffusion law (4), a general equation which applies to nonstationary mass transfer by diffusion. The derivation of the law and a discussion of conditions requisite for its application to this type of extraction are presented elsewhere (2). The law applies in its simplest form only to constant rate processes•-that is, to the straight-line portion of a plot of logic <?/?o versus t where q/qo is the fraction of extractant (resin) remaining at time, t. Although the diffusion coefficients cannot be calculated from the current data, it is valid to show by comparison of the respective slopes that the diffusion coefficients at the two temperatures bear the following relationship:…”