It has been shown, by means of continuous and intermittent stress relaxation measurements and oven aging studies, that polyester-based urethans are more resistant to thermal degradation than the corresponding polyether-based urethans at temperatures in the range of 50° to 150° C. It has also been shown that this difference in thermal stability is due to irreversible, oxidative cleavage of polyether chain segments. Addition of representative antioxidants of the hindered phenolic and amine types to the polyether-based prepolymers prior to curing was found to be ineffective in improving the thermal stability of the resultant polyurethan elastomer. And, finally, the practical significance of these thermal stability studies has been demonstrated through heat buildup measurements.
As part of a study on the factors determining the lightfastness of dyes on cellulosic materials, a means of examining the physical state of direct dyes in viscose was developed which makes use of dyed ultrathin films as primary specimens in the electron microscope.When this method was used to examine the physical state of two direct dyes in viscose, their physical state was found to differ markedly. Aggregates of one dye could not be detected in the viscose films at a resolution limit of about 30 Å, whereas aggregates of the other dye were plainly visible in viscose films dyed under the same conditions. The apparently unaggregated dye exhibited considerably poorer lightfastness on rayon than the one which aggregated in viscose.The results suggest the use of this method of study to investigate the influence of the physical state of direct dyes in viscose on their lightfastness.
Dye rates were measured for unsteamed and steamed solvent-extracted wool. These meas urements indicate that the dye rates for wool that has not been alkali-damaged, or wool in which the alkali damage has been restricted to the fiber surface, are not influenced by steaming. The dyeing time for wool treated with an aqueous alkaline solution is increased considerably by steaming. This would indicate that the effect of steaming is due to modifications in the bulk of the wool fiber.
The continually widening range of thermal stresses imposed on elastomeric parts used in the automotive and aeronautical fields intensifies both the need for more thermally versatile elastomers and the need for effective laboratory methods for evaluating elastomer performance. Because stress relaxation provides a means of gauging product performance under both high temperature and external stress that approximate conditions of use, this technique gives a good estimation of an elastomer's commercial potential and its adaptability to a particular application. To demonstrate the value of stress relaxation, the thermal stability of a polyacrylate elastomer crosslinked with various curatives and the effect of certain additives on thermal stability were studied. Polyacrylates are of major interest for applications that require a high degree of thermal stability because of their ability to perform well over wide temperature ranges (−25° to 400° F). These elastomers are mainly copolymers of ethyl acrylate and a chlorine containing monomer such as vinyl chloroethyl ether. Some newer types are based on ethyl and butyl acrylates and chlorine free monomers such as epoxides and substituted amines. Generally, polyacrylates are compounded and prepared for curing according to standard techniques and they can be crosslinked with a variety of curative systems. Curing is accomplished at between 300° and 400° F in times ranging from 45 seconds to 45 minutes, depending on the elastomer and curing recipe used. The thermal stability of these elastomers is frequently determined by measuring the percentage changes in physical properties, e.g., tensile strength, elongation, modulus, after oven or oil aging or both at elevated temperatures. However, heat aging is carried out with no external stresses applied to the test specimens and the physical measurements before or after aging are made at room temperature. Although this type of test has some value, the results do not necessarily reflect the behavior of the elastomers in practice, as is the case with stress relaxation.
A series of clean, well‐defined polyurethane networks was synthesized from polyester glycols, 2,4‐tolylene diisocyanate, and 1,1′,1″‐trimethylolpropane by means that afforded precise control over the content of urethane groups per network chain. The thermal cleavage of these networks was studied using the technique of stress relaxation. Analysis of the stress‐relaxation data on each network structure revealed two exponential decay processes differing in rate by about an order of magnitude. The rate of the slower process, which dominates the overall stress decay, was shown to be directly dependent on the content of urethane groups per network chain. Positive identification of this process with urethane cleavage was thereby established. The kinetic and thermodynamic constants associated with urethane cleavage were then calculated from data on this process at different temperatures. The more transient stress‐decay process was not uniquely definable, but probably originated from the cleavage of one or more types of weak linkages found in small but variable proportions in the different polyurethane networks. The nature and origin of these weak linkages was discussed.
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