Chemorheology of Irradiation-Cured Natural Rubber. I. Stress Relaxation Mechanisms for Various Curing Systems in Natural Rubber at High Temperature

1975 ◽  
Vol 48 (2) ◽  
pp. 141-153 ◽  
Author(s):  
S. Tamura ◽  
K. Murakami
Keyword(s):  
Stress Relaxation ◽  
Natural Rubber ◽  
Main Chain ◽  
Chain Scission ◽  
Chemical Structures ◽  
Carbon Carbon Bonds ◽  
Interchange Reaction ◽  
Thermal Scission ◽  
Physical And Chemical ◽  
Random Scission

Abstract 1. There was no difference of stress relaxation, either in air or in nitrogen, between DCP cures (Sample 1) and irradiation cures (Sample 2). This suggests that these vulcanizates have the same physical and chemical structures. In air Samples 1 and 2 underwent random scission of only the main chain. 2. In the case of irradiation-TMTD cures (Samples 4 and 5), the stress decay was also based on oxidative scission of the main chain. The number of moles of main chain scission, qm(t), was independent of the ratio ρ (of Nc(0)) based on the carbon—carbon bonds to Nm(0) based on the mono- and disulfide links). However, qm(t) was larger than that of Sample 2. The oxidative scission of the main chain seemed to be accelerated by mono- and disulfide. It was found from comparison of Samples 4 and 5 that TMTD cures (Sample 3) underwent random scission on the main chain. The stress relaxation in nitrogen for Samples 3, 4, and 5 was due to thermal scission of the crosslink. 3. The stress relaxation, either in air or in nitrogen, of accelerated-sulfur-cures (Sample 6) and irradiation-sulfur cures (Samples 7 and 8) was expressed by the sum of two exponential terms. The stress relaxation in air of Samples 6, 7, and 8 could be explained by the interchange reaction of polysulfide links and the random scission on the main chain. The stress decay in nitrogen of these vulcanizates was based on both interchange of polysulfide links and thermal scission of crosslinks. The rate of the interchange reaction in air was very closely consistent with that in nitrogen. 4. The apparent activation energy of oxidative scission of the main chain was about 21 kcal/mol for Samples 2, 6, 7, and 8 and 27 kcal/mol for Samples 3, 4, and 5.

Stress Relaxation of Natural Rubber During Irradiation

1970 ◽  
Vol 43 (2) ◽  
pp. 262-269
Author(s):  
D. Evans ◽  
J. T. Morgan ◽  
R. Sheldon ◽  
G. B. Stapleton
Keyword(s):  
Stress Relaxation ◽  
Natural Rubber ◽  
Crosslink Density ◽  
Chain Scission ◽  
Crosslinking Reaction ◽  
Small Contribution ◽  
Relaxation Data ◽  
Strain Measurements ◽  
Random Scission ◽  
Crosslink Densities

Abstract The site of chain scission and crosslinking in vulcanized natural rubber irradiated with 4 MeV electrons has been determined by analysis of stress relaxation data. Sulfur and peroxide vulcanizates of different crosslink densities were prepared and the crosslink densities determined from stress—strain measurements. Stress relaxation was measured during irradiation using modified commercial relaxometers. The specimens were maintained in an atmosphere of nitrogen to minimize oxidative side effects. Scission is deduced to take place in the vicinity of crosslinks, since the rate of continuous stress relaxation is independent of crosslink density. Scission may be associated both with crosslinks initially present and with those subsequently introduced by irradiation. Crosslinking by radiation is largely a random process. However, there is a crosslinking reaction dependent to a slight extent on crosslink density as well as a small contribution from random scission reactions. G values for the random reactions are given.

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.

Mechanisms Involved in the Recycling of NR and EPDM

1999 ◽  
Vol 72 (4) ◽  
pp. 731-740 ◽  
Author(s):  
M. A. L. Verbruggen ◽  
L. van der Does ◽  
J. W. M. Noordermeer ◽  
M. van Duin ◽  
H. J. Manuel
Keyword(s):  
Natural Rubber ◽  
Crosslink Density ◽  
Main Chain ◽  
Chain Scission ◽  
Ethylene Propylene ◽  
Temperature Range ◽  
Lower Reactivity ◽  
Ethylene Propylene Diene Rubber ◽  
Diene Rubber ◽  
Crosslink Densities

Abstract The thermochemical recycling of natural rubber (NR) and ethylene-propylene-diene rubber (EPDM) vulcanizates with disulfides was studied. NR sulfur vulcanizates were completely plasticized when heated with diphenyldisulfide at 200 °C. It could be concluded that both main chain scission and crosslink scission caused the network breakdown. NR peroxide vulcanizates were less reactive towards disulfide at 200 °C, and only reacted through main chain scission. For EPDM a temperature range of 200–275 °C was studied. In the presence of diphenyldisulfide at 200 °C there was almost no devulcanization of EPDM sulfur vulcanizates, and at 225 and 250 °C there was only slightly more devulcanization. A decrease in crosslink density of 90% was found when 2×10−4 mol diphenyldisulfide/cm3 vulcanizate was added and the EPDM sulfur vulcanizates were heated to 275 °C. EPDM peroxide vulcanizates showed a decrease in crosslink density of ca. 40% under the same conditions. The lower reactivity of EPDM towards disulfide compared with NR is the result of higher crosslink densities, the presence of a higher percentage of more stable monosulfidic crosslinks and the fact that EPDM is less apt to main chain scission relative to NR.

Effect of Curing Systems on Fatigue of Natural Rubber Vulcanizates

1966 ◽  
Vol 39 (3) ◽  
pp. 785-797 ◽  
Author(s):  
W. L. Cox ◽  
C. R. Parks
Keyword(s):  
Fatigue Life ◽  
Natural Rubber ◽  
Main Chain ◽  
Accelerated Aging ◽  
Chain Scission ◽  
Rapid Loss ◽  
Effective Antioxidant ◽  
Natural Rubber Vulcanizates ◽  
Aging In Air ◽  
Oxidative Effect

Abstract The fatigue life of natural rubber-HAF black vulcanizates showed maxima when plotted as a function of crosslink concentration as did other properties related to a tearing process such as tensile strength, crack growth, and tear strength. Accelerated-sulfur vulcanizates were superior to peroxide and nonelemental-sulfur cures; this can be attributed to an exchange of polysulfide crosslinks under stress. An effective antioxidant was essential for maximum fatigue resistance. Accelerated-sulfur systems, although having a higher original fatigue life than peroxide or nonelemental-sulfur cures, showed a rapid loss on accelerated aging in air. This would indicate that an oxidative effect was involved. Sulfur group analyses of the flexed samples showed an increase in the concentration of RSSxSR linkages but a decrease in the total polysulfide sulfur, Sx, with no change in the crosslink densities. This suggests that the polysulfide linkages not only underwent exchange during the fatigue process but also homolytic cleavage to polythiyl radicals. These radicals can add to double bonds and in the presence of oxygen initiate oxidation chains which would lead to main chain scission.

OXIDATIVE STRESS RELAXATION OF NATURAL RUBBER GUM VULCANIZATES CONTAINING VARIOUS ANTIOXIDANTS. I: EFFECT OF SAMPLE THICKNESS

2017 ◽  
Vol 90 (4) ◽  
pp. 633-641
Author(s):  
Junling Zhao ◽  
G. R. Hamed
Keyword(s):  
Oxidative Stress ◽  
Stress Relaxation ◽  
Natural Rubber ◽  
Aging Time ◽  
Oxygen Concentration ◽  
Chain Scission ◽  
Sample Thickness ◽  
Oxygen Absorption ◽  
Square Root ◽  
Diffusion Controlled

ABSTRACT Conventional sulfur-vulcanized natural rubber gums containing various antioxidants have been subjected to oxidative stress relaxation at 72 °C and 25% strain. Oxidation is diffusion controlled, even for samples as thin as 0.15 mm. Assuming that the rate of chain scission is proportional to the rate of oxygen absorption, which previously has been shown to depend on the square root of the oxygen concentration, an equation is derived predicting that stress decay is proportional to aging time to the three-quarters power. Moreover, slopes of these plots are predicted to depend inversely on thickness. Experimental results are in reasonable accord with these predictions.

Helical Spring Stress Relaxometer

1957 ◽  
Vol 30 (3) ◽  
pp. 889-894
Author(s):  
J. P. Berry
Keyword(s):  
Stress Relaxation ◽  
Practical Importance ◽  
Chain Scission ◽  
Routine Test ◽  
Test Procedures ◽  
First Order ◽  
Time Graph ◽  
Aging Conditions ◽  
Force Time ◽  
Random Scission

Abstract Changes in the tensile properties of rubber are obviously of great practical importance. Consequently, measurements of the change in force required to maintain or produce a given extension under controlled conditions of temperature and atmosphere have frequently been made and have advanced our understanding of the physical process of crystallization and of the chemical reactions responsible for thermal aging to the extent that they may become routine test procedures. The decrease in the force required to maintain a constant extension during the period of aging (stress relaxation) has been the most common measurement, in part because rubber elasticity theory predicts an equality between the fractional decrease in force and the fraction of the network chains originally supporting the stress which have become ineffective. Stress-relaxation behavior may characterize the type of scission reaction occurring; for example, the stress relaxation of most vulcanizates can be interpreted as a first-order scission of crosslinks, and not as a random scission of monomeric units in the chains between crosslinks. Stress-relaxation results, however, do not provide all the necessary information on network changes—the final properties of the rubber depend not only on crosslink or chain scission but to a comparable degree on crosslink formation during aging, which does not affect the force at constant extension. To examine this second effect requires measurements on the intermittent stretching of an unstrained sample to a constant extension. Parallel measurements of force at constant and intermittent extension under identical aging conditions are therefore required. The several designs of apparatus already described have usually been rather complicated and require practice and skill in use. A simple apparatus developed in these laboratories, which can readily be used for routine operations, is described in the following paragraphs. The fractional change in force is read directly from a vernier scale on the instrument, and may be immediately plotted on a force-time graph, preferably as log10 (force/forcet=0) vs. time (cf. Reference 3).

Aging with Applied Strain of A Black-Filled Natural Rubber Vulcanizate. Part I: Network Changes

2008 ◽  
Vol 81 (4) ◽  
pp. 650-670 ◽  
Author(s):  
Crittenden J. Ohlemacher ◽  
Gary R. Hamed
Keyword(s):  
Natural Rubber ◽  
Crosslink Density ◽  
Network Formation ◽  
Main Chain ◽  
Chain Scission ◽  
Tensile Behavior ◽  
Applied Strain ◽  
Chain Orientation ◽  
Double Network ◽  
Double Networks

Abstract Black-filled natural rubber, with an inefficient sulfur cure, was aged at 90 °C and 110 °C under nitrogen, with and without applied strain. Samples aged under strain became “double networks” and retained a residual extension ratio. The crosslink density of samples passed through a maximum with increasing severity of aging. Presumably this arises because the thermally labile, polysulfidic crosslinks break, and new crosslinks of lower rank form, resulting in increased crosslink density; but, when aged at 110 °C, this is offset by chain scission and other main-chain modifications. For double networks, it is proposed that a second network, which tends to keep samples extended, is formed at the expense of crosslinks in the original, first network. Unaged and single network samples were isotropic in tensile behavior and only slightly anisotropic in swelling behavior. For double networks, swelling and tensile properties were anisotropic, and there was some evidence that parallel specimens have increased ability to strain-crystallize. The observed anisotropies in double networks are proposed to arise from the chain orientation that persisted after double network formation.

Structural Characterization of Natural Rubber Vulcanizates

1963 ◽  
Vol 36 (2) ◽  
pp. 547-557 ◽  
Author(s):  
C. G. Moore ◽  
M. Porter
Keyword(s):  
Natural Rubber ◽  
Main Chain ◽  
Structural Complexity ◽  
Chain Scission ◽  
Reaction Products ◽  
Chemical Reagents ◽  
Pendent Groups ◽  
Cure Time ◽  
Conjugated Triene ◽  
Natural Rubber Vulcanizates

Abstract The three principal approaches used in the structural resolution of sulfur vulcanizates of natural rubber are described and exemplified by their application to three systems: an unaccelerated sulfur system and two mercaptobenzothiazole (MBT)-accelerated sulfur systems, one of which makes very efficient use and the other inefficient use of the sulfur crosslinking. The ultimate aim of such studies is to relate vulcanizate structure to the physical properties and aging behavior of the vulcanizates during service. The first approach, which gives an overall measure of the structural complexity of the network, involves the determination of the crosslinking efficiency, (E), that is, the number of sulfur atoms combined in the network for each physically-effective, chemical crosslink formed. Crosslinking efficiencies of networks vary with increasing cure time from ca. 6-1.16 for the efficient-MBT system to 55-40 for the unaccelerated sulfur system. Between these extremes, it is found that variations of reactant concentrations and of temperature and time of cure lead to large variations in crosslinking efficiency and therefore of network complexity. Related work not reported here indicates that the latter features are also dependent on the type of accelerator used. More detailed information on network structure is obtained from analysis of reaction products of low molecular weight analogs of natural rubber with the vulcanizing agent and ancillary ingredients. Such studies reveal that the poor efficiency of the unaccelerated sulfur system is due to sulfur being combined wastefully in (i) long polysulfidic crosslinks of alkenyl t-alkyl structure, (ii) vicinal crosslinks which behave physically as one crosslink, and (iii) a large proportion of cyclic monosulfide groups which constitute an important modification of the main rubber chains. Conjugated triene groups, cis, trans-isomerized isoprene units, and main chain scission represent other possible types of modification. In contrast with this complex network, efficiently-cured MBT-accelerated sulfur vulcanizates contain, at early stages of cure, polysulfidic crosslinks of dialkenyl type which decrease with time to mainly monosulfide crosslinks; conjugated triene units are also present but little or no cyclic monosulfides, vicinal crosslinks, or main chain scission. Other accelerated sulfur vulcanizates possess structures intermediate between these two extremes, except that the crosslinks are generally of dialkenyl type and pendent groups terminated by accelerator residues constitute an additional possible type of modification. Knowledge from model olefin studies of the different types of sulfurated groups present in vulcanizates has led to the development of the third experimental approach, i.e. use of chemical reagents (‘chemical probes’) to determine specific groups. This approach is exemplified by reaction with actual vulcanizates of triphenylphosphine which removes sulfur atoms in excess of one or two from polysulfides; as cure proceeds in unaccelerated sulfur vulcanization, the proportion of combined sulfur present in cyclic sulfide groups rises from 75% to 95% while the average number of sulfur atoms in each crosslink unit falls from 12–13 to 2–4. Comparable data for the efficient MBT system confirm the essential simplicity of the network in this case.

Solid-State C-13 NMR Studies of Vulcanized Elastomers XIII. TBBS Accelerated, Sulfur-Vulcanization of Carbon Black Filled Natural Rubber

1995 ◽  
Vol 68 (4) ◽  
pp. 551-562 ◽  
Author(s):  
Makio Mori ◽  
Jack L. Koenig
Keyword(s):  
Double Bond ◽  
Solid State ◽  
Carbon Black ◽  
Natural Rubber ◽  
Main Chain ◽  
Chain Scission ◽  
Nmr Studies ◽  
Double Bond Migration ◽  
Sulfur Vulcanization ◽  
Basic Network

Abstract Solid state C-13 NMR has been used to compare the chemistry of accelerated vulcanization of natural rubber using N-t-butyl-2-benzothiazole sulfenamide in the presence and absence of carbon black. The carbon black filler has an influence on the vulcanization chemistry in addition to its accepted role in reinforcment and stabilization. The basic network structure formation is modified to the extent that crosslink desulfurization from poly to monosulflde structures occurs earlier in the cure in the presence of carbon black. No main chain scission or double bond migration was detected.

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.

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