Viscosity , Relaxation and Stability of Natural Rubber
Mooney viscosity and Mooney relaxation of unaged and oven aged SMR L natural rubber samples were studied. The viscosity was characterized by ML (1+4) 100°C and the rate of stress relaxation by the exponent a in a power law M = k (t) a . With the increasing time of oven aging at 140°C the decrease of both viscosity and stress relaxation rate exponent were observed. Aging of raw natural rubber can therefore be studied by Mooney viscosity and Mooney relaxation tests and obtained results are sensitive enough to be used for characterization of raw natural rubber stability.
Physical Testing of Rubber
General considerations standards and standards organizations preparation of test pieces conditioning and test atmosphere tests on unvulcanized rubbers density and dimensions short-term stress and strain properties tensile stress/strain dynamic stress and strain friction and wear creep, relaxation and set fatigue electrical tests thermal properties effect of temperature environment resistance permeability adhesion, corrosion and staining. Appendices: national standards bodies thermal equilibrium times for non-ambient testing.
Firmness and force relaxation characteristics of tomatoes stored intact or as slices
Firmness is an important aspect of the quality of fresh tomatoes. Its measurement is necessary for quality control, as well as for postharvest studies to develop procedures for preparation and handling of fresh-cut tomato slices. This study had two objectives, to determine how initial ripeness and cutting affect the textural changes in tomato tissue during storage and to develop a method for measuring textural attributes of tomato slices. Firmness and force relaxation characteristics were compared for tomato flesh harvested at six maturity stages and stored intact or as slices for 0–20 days. Measurements were made at three sites in the outer pericarp and three in columellar tissue on 7-mm thick transverse slices. Following loading, stress relaxation was recorded for 10 s. A 4-mm cylindrical probe provided more consistent firmness measurements (lower% CV) than a 6.4-mm spherical probe at harvest, and distances of 3 mm provided more consistent results than 1 mm. Therefore, stored tomato samples were tested only with the 4-mm cylindrical probe and distance of 3 mm. Fmax (maximum force) of freshly harvested tomatoes ranged from 15 to 2 N for mature green and red tomatoes, respectively. Fmax was closely related to loading energy (area), slope, and yield force; i.e. all four parameters measured elastic behavior. Fmax also was related (r 2 = 0.77–0.97) to forces at specific relaxation times. Tomatoes stored intact generally had lower CVs than those sliced before storage, although there was little difference in average firmness (Fmax) between the two at a given storage time for a given initial ripeness. A three-parameter model was developed to fit the relaxation curves: Y = 1− A ln(1 + t) − Bt/(C + t) where t is relaxation time. There was little correlation between Fmax and the three relaxation parameters (r 2 = 0–0.33), i.e. the relaxation parameters measured a different physical property, presumably viscoelasticity. The responses of the relaxation parameters over storage time were dependent on the initial maturity of the tomatoes; but values usually differed significantly between tomatoes stored intact or sliced, with tomatoes sliced before storage generally having higher values, which we interpret as having lower viscoelasticity. Published by Elsevier Science B.V.