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Chemical Stress Relaxation Measurements Of Polyurethanes

Report Number: WADD TR 61-71
Author(s): S. J Chlystak, E. A. Peterson, L. A. Harrah, and J. A. Parker
Corporate Author: Armstrong Cork Company
Laboratory: Directorate of Materials and Processes
Date of Publication: 1962-05
Pages: 104
Contract: AF 33(616)-7088
Project: 7360
Task: 73615
AD Number: AD0282837

Three cast solid polyurethane elastomers, designated AF-1, AF-2, and AF-3, constituted with different numbers and kinds of cross-links were prepared from precisely characterized prepolymers and cross-linking processes.

These three polymers were designed to include the various types of linkages that normally occur in the conventional solid propellant binders formulated from polyurethanes. They were adjusted with respect to physical properties to make them more amenable to a preliminary investigation. In addition to special characterizations performed, the conventional physical properties of these elastomers were determined.

These polymers were used to investigate chemorheological stress relaxation and to evaluate its efficiency in characterizing the reliability of polyurethane materials with regard to thermal and mechanical stability.

A complete literature search and annotated bibliography of stress relaxation phenomena was prepared.

The equipment necessary to perform reproducible and reliable continuous and intermittent stress relaxation measurements on high modulus polyurethane was constructed and tested. As a result of preliminary observations made, some modifications and engineering changes were necessary to obtain these reliable data.

Both continuous and discontinuous stress relaxation measurements in standard 1/4" tensile specimens at temperatures of 100°C., 125°C., and 150°C. were made.

The rates of scission of these three different kinds of cross-linked polyurethanes were measured and the activation energies, assuming normal Maxwellian decay for these processes, was determined. These correlate qualitatively with the different polymer structures investigated.

Preliminary high-speed stress-strain analysis of these polymers was performed to observe their behavior at shock loading conditions. An unusual inversion of the relative ranking of the modulus as a function of testing rate and total strain was observed.

Block specimens of these polyurethane polymers were prepared and conditioned at the stress relaxation conditions. These were provided for the parallel contract on ultrasonic testing techniques being conducted by Sperry Products Company, Danbury, Connecticut.

Specimens of loaded prototype solid propellants were obtained from the Elkton Division of the Thiokol Chemical Company for similar tests.

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