Abstract
Extensive tests on unmasticated and masticated smoked sheet samples in the shearing-cone and Williams plastometers are reported. These constitute a preliminary survey of the behavior of these materials during shearing at various rates and for various periods at elevated temperatures. Masticated rubber shows a temporary stiffening when kept at normal or elevated temperature; to eliminate this, and so obtain reproducible plasticity measurements, the rubber must be sheared at a high rate. Since normal Mooney and Williams tests do not involve high shear rates, the influence of this stiffening as a possible source of error needs examining. In tests at 100° C, the logarithmic stress/strain-rate curve is convex to the log stress axis. The most important difference observed between samples prepared by masticating one batch of smoked sheet in various ways was in the slope of the curve (log stress plotted horizontally), the curves for cold-milled samples being steeper than for samples prepared by hot milling or by mastication in an internal mixer. Curves for samples plasticized in the same way for different periods do not cross. There is evidence that the results of other types of plasticity test can be predicted from the stress/strain-rate curve for the sample. These last two observations, if confirmed by more extensive tests, indicate that for control of normal mastication processes a compression test such as the Williams can be satisfactory, provided the mastication method remains basically the same, that is, using the same type of machine (mill or internal mixer) and similar temperature conditions. The method of mastication appears to be more important than the properties of the raw rubber used as starting material in determining the properties of masticated rubber. Tests on unmasticated smoked sheet showed that many samples stiffen during shearing or during very light mastication; this effect persists longer, under continued shearing, when stiffness is measured at a high strain rate than when measured at a low rate; in other words, the stress/strain-rate curve becomes much less steep and crosses the curve for very lightly sheared rubber. This latter behavior, apparently not previously observed, is shown to be consistent with certain other observations that have been reported. The complex changes produced by short milling treatments, and which vary from one rubber to another, may be the cause of inconsistencies in Mooney tests on unmasticated rubbers, which are usually prepared for testing by a few passages through a mill. The fact that such treatments may alter the stiffness in different directions according as this is measured at high or low strain rate is doubtless one reason why Mooney and Williams tests on unmasticated rubbers do not correlate closely. In contrast to this complex and variable behavior, rubbers that have been subjected to a normal mastication treatment give stress/strain-rate curves that do not cross, so tests at one strain rate give a valid indication of their relative behavior at other strain rates. This observation supports the view already held in some quarters that in classifying natural rubbers by a plasticity test they should first be given a standard mastication or shearing treatment. Many of these conclusions must be regarded as tentative until confirmed on a larger number of rubber samples; with this reservation they form a more complete picture than has hitherto been available of the plastic behavior of unvulcanized raw rubber, which must form the basis for improving control test methods.
