Degradation of Elastomers 4. Continuous Chemical Stress Relaxation of Natural Rubber Vulcanizates

1966 ◽  
Vol 39 (5) ◽  
pp. 1640-1655 ◽  
Author(s):  
Walter Scheele ◽  
Karl-Heinz Hillmer
Keyword(s):  
Activation Energy ◽  
Stress Relaxation ◽  
Natural Rubber ◽  
Sulfur Content ◽  
Chemical Stress ◽  
Relative Importance ◽  
Relaxation Velocity ◽  
Natural Rubber Vulcanizates ◽  
Continuous Chemical ◽  
Random Scission

Abstract Temperature dependence of the chemical stress relaxation in oxygen of a series of natural rubber vulcanizates, extracted and unextracted, with the same pretreatment was investigated. Decrease of tension to about 50 per cent of the original was followed, using the half lives as a measure of velocity. The following observations were made: 1) Relaxation velocity of unextracted thiuram vulcanizates increases with increasing sulfur content; the activation energy decreases. 2) The relaxation velocities of extracted thiuram vulcanizates are independent of sulfur content of the mixtures and significantly greater than those of unextracted samples. This is caused by the loss of dithiocarbamate on extraction. 3) Other natural rubber vulcanizates (dithiocarbamate, MBT and peroxide vulcanizates) give the same relaxation half lives after extraction. 4) The relative importance of random scission and crosslink scission was discussed.

Aging of Natural Rubber Vulcanizates in the Presence of Dithiocarbamates

1959 ◽  
Vol 32 (3) ◽  
pp. 739-747 ◽  
Author(s):  
J. R. Dunn ◽  
J. Scanlan
Keyword(s):  
Stress Relaxation ◽  
Natural Rubber ◽  
Protective Action ◽  
Dicumyl Peroxide ◽  
Vulcanized Rubber ◽  
Aging Properties ◽  
Photochemical Aging ◽  
Natural Rubber Vulcanizates

Abstract The thermal and photochemical aging of extracted dicumyl peroxide-, TMTD (sulfurless)- and santocure-vulcanized rubber, in presence of a number of metal and alkylammonium dithiocarbamates, has been investigated by measurements of stress relaxation. The dithiocarbamates have a considerable protective action upon the degradation of peroxide- and TMTD-vulcanizates, but they accelerate stress decay in santocure-accelerated vulcanizates. The reasons for this behavior are discussed. It is suggested that the excellent aging properties of unextracted TMTD vulcanizates are due to the presence of zinc dimethyldithiocarbamate formed during vulcanization.

Longitudinal Cracking in a Carbon Black Filled Natural Rubber Vulcanizate during Chemical Stress Relaxation

1998 ◽  
Vol 71 (2) ◽  
pp. 157-167 ◽  
Author(s):  
G. R. Hamed ◽  
J. Zhao
Keyword(s):  
Stress Relaxation ◽  
Carbon Black ◽  
Natural Rubber ◽  
Longitudinal Crack ◽  
Chemical Stress ◽  
Loading Direction ◽  
Rubber Vulcanizate ◽  
Edge Cracks ◽  
Forced Air ◽  
Oxidative Attack

Abstract Thin specimens of a black-filled, natural rubber vulcanizate have been held in uniaxial tension at 72°C and 200% elongation in a forced air oven. After substantial oxidative attack (inferred from stress relaxation), small edge cracks formed. Initially, these cracks grew perpendicular to the loading direction, but, upon reaching about 0.1 mm in depth, longitudinal crack growth commenced and fracture progressed by a kind of 0°-peel process with “splitting-off” of successive strands of rubber. This phenomenon is attributed to anisotropy in strength caused both by straining and by oxidative attack.

The Determination of Network Chain Density and the Chemical Stress Relaxation of Crosslinked Polymers

1973 ◽  
Vol 46 (2) ◽  
pp. 477-482
Author(s):  
Saburo Tamura ◽  
Kenkichi Murakami
Keyword(s):  
Stress Relaxation ◽  
Natural Rubber ◽  
Main Chain ◽  
Chemical Stress ◽  
Dicumyl Peroxide ◽  
Constant Independent ◽  
Crosslinked Polymers ◽  
Network Chain ◽  
The Difference ◽  
Entanglement Network

Abstract Both initial network chain densities nM(0) and nS(0) of dicumyl peroxide- cured natural rubbers were determined from the tensile stress and swelling method, respectively. The difference between nM(0) and nS(0) was usually constant, independent of the magnitude of network chain density. That is, it was found that the number of entanglement network chains in the crosslinked natural rubber was usually constant, independent of network chain density. The entanglement network chain density nII(0) was 0.7×10−4 mole/cc. This led to the supposition that the molecular weight between entanglement points Me would be about 9000. Although this value is far from exact, it does not differ too greatly from the value found for noncrosslinked natural rubber. Next, in order to calculate the number of main-chain scissions of crosslinked polymers from their chemical stress relaxation, we proposed our modification of Tobolsky's equation. Using our equation, it was found that the scission of dicumyl peroxide-cured natural rubber occurred in the main chain only. Furthermore, this value agreed with the one obtained from the oxidation of toluene solution of noncrosslinked rubber under the same conditions.

The Degradation of Elastomers. 2. Oxidative Degradation of Natural Rubber Vulcanizates at Different Elongations and Temperatures. II

1959 ◽  
Vol 32 (3) ◽  
pp. 759-769 ◽  
Author(s):  
Karl-Heinz Hillmer ◽  
Walter Scheele
Keyword(s):  
Activation Energy ◽  
Natural Rubber ◽  
Oxidative Degradation ◽  
Zero Order ◽  
Relaxation Rates ◽  
Good Qualitative Agreement ◽  
Kcal Mole ◽  
Pure Nitrogen ◽  
Natural Rubber Vulcanizates ◽  
Constant Activation Energy

Abstract The results of measurements of stress relaxation in natural rubber vulcanizates are reported. In the earlier work, the conditions were denned under which the drop in tension of a vulcanizate—after a pre-relaxation in pure nitrogen—follows a zero order law when tested in oxygen. The temperature dependence of the rate of relative tension relaxation was determined for 8 different vulcanizates. With a constant activation energy of about 29 kcal/mole, the vulcanizates differed in their relative relaxation rates at most by two powers of ten. Several general observations on the course of tension relaxation under different conditions were made on the grounds of experimental evidence. Good qualitative agreement was found with relations reported by Berry and Watson.

Stress Relaxation during the Photoxidation of Peroxide Crosslinked Rubber

1960 ◽  
Vol 33 (2) ◽  
pp. 433-444
Author(s):  
J. R. Dunn ◽  
J. Scanlan ◽  
W. F. Watson
Keyword(s):  
Stress Relaxation ◽  
Natural Rubber ◽  
Physical Properties ◽  
Chemical Reactions ◽  
Small Extent ◽  
Organic Peroxides ◽  
Physical Measurement ◽  
Crosslinked Networks ◽  
Photochemical Aging ◽  
Natural Rubber Vulcanizates

Abstract The chemical reactions involved in the thermal and photochemical aging of natural rubber vulcanizates are largely unknown. Experimental difficulties have precluded direct chemical investigation owing to the insolubility of the crosslinked networks and the small extent of reaction required for a great deterioration in physical properties. Accordingly recourse has been made to physical measurement. Tobolsky et al. (e.g. Ref. (1)) have shown that the relaxation in stress on holding a rubber strip at constant extension during aging is a convenient experimental measure capable of interpretation in terms of network breakdown. In photochemical studies the opacity of conventional sulfur vulcanizates provides a further complication. In the present work, the stress relaxation technique has been used in a study of the degradation by 365 mµ radiation of the comparatively transparent and chemically simple rubber networks obtained after crosslinking by organic peroxides.

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