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.

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.

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.

Determination of Crosslink Density and Network Structure of NR Vulcanizates by Means of TSSR

2013 ◽  
Vol 844 ◽  
pp. 482-485 ◽  
Author(s):  
Norbert Vennemann ◽  
Christina Schwarze ◽  
Claudia Kummerlöwe
Keyword(s):  
Stress Relaxation ◽  
Natural Rubber ◽  
Network Structure ◽  
Crosslink Density ◽  
Relaxation Spectrum ◽  
Strong Impact ◽  
Relaxation Spectra ◽  
Peak Separation ◽  
Temperature Scanning

Unfilled vulcanizates based on natural rubber (NR) were investigated by temperature scanning stress relaxation (TSSR) measurements. Different sulfur based cure systems, i.e. conventional (CV), semi-efficient (SEV) and efficient (EV) vulcanization system, were used to prepare the vulcanizates. It was found that sulfur/accelerator - ratio has a strong impact on the shape of the relaxation spectrum, deduced from TSSR measurements. By deconvolution of the relaxation spectra, peak separation was performed and 3 different peaks were found in case of CV - cured samples. In contrast, only a single peak was found, in case of the EV-cured sample. After thiolamine treatment the shape of the relaxation spectra altered significantly in case of the CV-cured sample whereas the spectra of the SEV-and EV-cured sample exhibited only slight differences. Additionally, the crosslink density of the samples decreased after thiolamine treatment. This is due to selective cleavage of polysulfidic crosslinks. It has been concluded, that the significant peak in the relaxation spectrum at about 120 °C can be attributed to the cleavage of polysulfidic crosslinks. Furthermore, a linear relationship between the percentage of polysulfidic crosslinks and the sulfur/accelerator - ratio is assumed.

Applications of FT-NMR to Crosslink Density Determinations in Natural Rubber Blend Vulcanizates

1992 ◽  
Vol 65 (4) ◽  
pp. 744-760 ◽  
Author(s):  
Paul S. Brown ◽  
M. John ◽  
R. Loadman ◽  
Andrew J. Tinker
Keyword(s):  
Nmr Spectroscopy ◽  
Carbon Black ◽  
Natural Rubber ◽  
Small Molecules ◽  
Crosslink Density ◽  
Chemical Shifts ◽  
Reference Position ◽  
Line Widths ◽  
Good Agreement ◽  
Crosslink Densities

Abstract Previously 90 MHz CW-NMR spectroscopy has been used to estimate the crosslink density in individual components of elastomer blends. Transfer of the technique to a 300 MHz FT instrument is not straightforward. Chemical shifts of polymer resonances in spectra of single-polymer vulcanizates are dependent on crosslink densities of the vulcanizates. Additionally, two resonances are observed for small molecules such as TMS and residual protonated solvent. The smaller resonance of each pair changes in shape and position in synchrony with the polymer signals and is considered to originate from solvent within the swollen polymer. The secondary TMS peak is used as the reference position from which to locate polymer signals in the spectrum. The position of these secondary peaks, and thus the polymer spectrum, relative to the “free” TMS is a function of crosslink density and also of carbon-black loading and type in filled vulcanizates. 13C-NMR line widths are observed to increase with crosslink density and this effect was used to study blends of NR with EPDM or a maleic acid modified EPDM. NR crosslink densities determined from 13C line-width analyses were in good agreement with those obtained from 1H-NMR and increased crosslinking in the modified EPDM was confirmed.

EXPERIMENTAL DETERMINATION OF THE QUANTITY AND DISTRIBUTION OF CHEMICAL CROSSLINKS IN UNAGED AND AGED NATURAL RUBBER. II: A SULFUR DONOR SYSTEM

2019 ◽  
Vol 92 (3) ◽  
pp. 513-530 ◽  
Author(s):  
Samantha Howse ◽  
Christopher Porter ◽  
Tesfaldet Mengistu ◽  
Ivan Petrov ◽  
Richard J. Pazur
Keyword(s):  
Natural Rubber ◽  
Crosslink Density ◽  
Heat Stability ◽  
Chain Scission ◽  
Dynamic Mechanical Properties ◽  
Double Quantum ◽  
Chain Entanglement ◽  
Tetramethylthiuram Disulfide ◽  
Entanglement Density ◽  
Chemical Crosslinks

ABSTRACT A series of unfilled and stabilized natural rubber compounds varying in concentration of tetramethylthiuram disulfide (TMTD) was analyzed using rheometry, hardness, dynamic mechanical properties, stress–strain (Mooney–Rivlin), equilibrium solvent swell (Flory–Rhener), and low-field nuclear magnetic resonance (NMR) by the double quantum (DQ) technique. Crosslinking level increased proportionately with TMTD concentration, and the reaction ratio of three TMTD molecules producing one crosslink was generally upheld. Unreacted TMTD acted as a pseudo-plasticizer and lowered the chain entanglement density with increasing TMTD content. DQ NMR confirmed that the elastic network was homogeneous and that the absolute chemical crosslink distributions broaden with increasing curative level. Upon mild heat aging, zinc complexes based on TMTD/ZnO are likely responsible for causing additional crosslinking, explaining the rise in crosslink density by equilibrium solvent swell and DQ NMR. The amine-based antioxidant, the generation of thiocarbamate radicals from TMTD, and the heat stability of the predominant monosulfide crosslinking system helped to limit network breakdown through chain scission. The chain entanglement increase is likely due to reduction of the plasticizing effect caused by unreacted curative. The distribution of crosslinks slightly broadens toward higher total crosslink density because of the generation of additional chemical crosslinks and chain entanglement densification.

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.

Creep and Stress Relaxation of Natural Rubber Vulcanizates. Part I. Effect of Crosslink Density on the Rate of Creep in Different Vulcanizing Systems

1971 ◽  
Vol 44 (3) ◽  
pp. 707-720
Author(s):  
E. D. Fairlie
Keyword(s):  
Creep Rate ◽  
Stress Relaxation ◽  
Natural Rubber ◽  
Chemical Treatment ◽  
Crosslink Density ◽  
Strong Dependence ◽  
Viscoelastic Behavior ◽  
Creep And Stress Relaxation ◽  
Natural Rubber Vulcanizates ◽  
Determining Factor

Abstract The physical creep of unfilled natural rubber vulcanizates, prepared with different vulcanizing systems, has been studied. For each of the three vulcanizing systems chosen there is a strong dependence of creep rate on crosslink density, but the rates for accelerated sulfur vulcanizates are two or three times higher than those of peroxide vulcanizates of similar crosslink density. Supplementary experiments, in which the crosslink structure of sulfur vulcanizates is modified either by chemical treatment or by variations in the vulcanizing conditions, show that the nature of the crosslink itself is not a determining factor in the type of vulcanizate. Other features, such as the type and quantity of extranetwork material arising from the vulcanizing process, contribute significantly to the viscoelastic behavior of accelerated sulfur vulcanizates.

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