Continuous Stress Relaxation Measurements Research Articles

Oxidative ageing of elastomers: experiment and modelling

During an extensive test programme at the Bundesanstalt für Materialforschung und prüfung, material property changes of EPDM O-rings were investigated at different ageing times and two ageing temperatures of 125∘C\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$125\\,^{\\circ }\\,\\hbox {C}$$\\end{document} and 150∘C\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$150\\,^{\\circ }\\, \\hbox {C}$$\\end{document}. To exclude possible diffusion-limited oxidation (DLO) effects that can distort the data, IRHD microhardness measurements were taken over the cross section of compressed O-rings. Continuous stress relaxation measurements were taken on samples free of DLO effects. The additional effect of physical processes to irreversible chemical ones during a long-term thermal exposure is quantified by the analysis of compression set measurements under various test conditions. By combining the different experimental methods, characteristic times relative to the degradation processes were determined. On the basis of experimental data, a microphysically motivated model that takes into account reversible and irreversible processes was developed. The parameter identification strategy of the material model is based on our experimental investigations on homogeneously aged elastomer O-rings. The simulated results are in good agreement with the experiments.

Chemorheology of carboxyl terminated polybutadienes. Effect of ammonium perchlorate

AbstractAn investigation has been made on chemorheological properties of carboxyl terminated polybutadienes (CTPB) with addition of small amounts of ammonium perchlorate (AP). Curing of the polymers has been done with the imine tris‐[1‐(2‐methyl)‐aziridinyl]phosphine oxide (MAPO), and with mixtures of MAPO and epoxides. AP has been dissolved in MAPO before addition to CTPB. Continuous stress relaxation measurements and intermittent measurements of stress have been made at 110°C on CTPB cured with MAPO and at 80, 88, 100, and 110°C on CTPB cured with MAPO and epoxides. The results show that AP increases the rate of scission reactions in all compositions. Mixtures of MAPO and epoxides as curing agents give better properties to CTPB than only MAPO as curing agent.

Chemorheology of carboxyl terminated polybutadienes with different antioxidants

AbstractAn investigation of chemorheological properties has been made on two types of carboxyl terminated polybutadienes (CTPB), HC‐434, and Butarez CTL. Curing of CTPB has been done with an imine, MAPO. Films have been cast and cured and then cut into strips. Some of these strips have been extracted with acetone and then two types of antioxidants were added, phenyl‐beta‐naphthylamine (PBN) and methylenebis‐(4‐methyl‐6‐tert‐butyl)‐phenol (BKF). Continuous stress relaxation measurements and intermittent measurements of stress have been made at a temperature of 110°C. The results show that Butarez CTL seems to have better ageing properties than HC‐434 even when the same type of antioxidant has been added to the polymers. The most effective antioxidant of the two tested is BKF, which balances scission reactions and crosslinking reactions in a better way than PBN.

Oxidative degradation of polydiene vulcanizates

Intermittent and continuous stress relaxation measurements have been utilized to follow the oxidative degradation of vulcanizates of natural rubber, and of synthetically prepared polybutadienes. Vulcanizates were prepared by exposing samples to high-energy electrons. For natural rubber, only oxidative scission reactions were detected. Emulsion polymerized polybutadiene and all cis 1,4-polybutadiene exhibited both oxidative scission and oxidative crosslinking. With sulfur vulcanizates of these elastomers, there is superposed on the oxidation reactions a continued vulcanization caused by remaining curing agent.

Chain Scission Efficiency in the Oxidation of Natural Rubber Vulcanizates

Abstract Natural rubber when it degrades under the action of oxygen does so primarily by a chain scission reaction. A technique has been developed by Tobolsky and coworkers for assessing the magnitude of this chain scission reaction in vulcanizates by means of continuous stress relaxation measurements. Since the result of the oxygen attack on the rubber is a chain scission reaction, the question of the efficiency of the reaction comes to mind. The influence of antioxidants is of importance in this regard as is the type of vulcanizate or network structure. This paper describes some measurements of the chain scission efficiency of a simple benzothiazolyl disulfide (MBTS) vulcanizate and the effect of several antioxidants on this chain scission reaction. Some of the complications inherent in this type of measurement are discussed. The first published report on the efficiency of the chain scission reaction in vulcanizates was given by Tobolsky, Metz, and Mesrobian in 1950. A more recent publication is that of Baxter, Potts, and Vodden in 1955. Tobolsky has interpreted the stress decay of gum vulcanizates at elevated temperatures as a direct consequence of the cutting of polymer chains of the network. The reduced stress is postulated as being equal to the chain density ratio:

Stress-Time-Temperature Relations in Polysulfide Rubbers

Abstract Polysulfide rubbers of various internal structures have been investigated by measurements of continuous and intermittent relaxation of stress and by creep under constant load at temperatures between 35° C and 120° C. Continuous stress relaxation measurements indicate that these rubbers obey approximately the simple Maxwellian law of relaxation of stress, which indicates that one definite type of bond in the network structure is responsible for stress decay. The activation energy for the relaxation process in each of the polysulfide rubbers is nearly the same, indicating that the same type of bond is responsible for the relaxation behavior of all the polysulfides investigated. In contrast to hydrocarbon rubbers, oxygen is not the cause of high temperature relaxation in polysulfide rubbers, nor does heating in air at moderate temperatures for times comparable to the relaxation time produce changes in physical properties as determined by modulus or by appearance of the samples. Several possibilities regarding the mechanism of the relaxation process and the type of bond involved are considered in the light of the experimental results.

Stress-Time-Temperature Relations in Polysulfide Rubbers

Polysulfide rubbers of various internal structures have been investigated by measurements of continuous and intermittent relaxation of stress and by creep under constant load at temperatures between 35°C and 120°C. Continuous stress relaxation measurements indicate that these rubbers approximately obey the simple Maxwellian law of relaxation of stress, which indicates that one definite type of bond in the network structure is responsible for stress decay. The activation energy for the relaxation process in each of the polysulfide rubbers is nearly the same, indicating that the same type of bond is responsible for the relaxation behavior of all the polysulfides investigated. In contrast to the hydrocarbon rubbers, oxygen is not the cause of high temperature relaxation in polysulfide rubbers, nor does heating in air at moderate temperatures for times comparable to the relaxation time produce changes in physical properties as determined by modulus or by appearance of the samples. Several possibilities regarding the mechanism of the relaxation process and the type of bond involved are considered in the light of the experimental results.