Chemical Stress Relaxation Research Articles

Crosslinking of partially neutralized carboxyl‐terminated liquid rubbers with bisepoxide and anhydride

AbstractCrosslinking of partially neutralized carboxyl‐terminated liquid rubbers (PNCTLR) with bisepoxide and anhydride was investigated by the PNCTLR–bisepoxide–anhydride reactions. The PNCTLR were prepared, by the partial neutralization with MgO and CaO, from a polymer of butadiene (Hycar CTB 2000X162) and a copolymer of butadiene–acrylonitrile (Hycaṙ CTBN 1300X8). As the bisepoxide, bisphenol‐A diglycidyl ether was used, and hexahydrophthalic anhydride was the anhydride used. It was found that the metal carboxylate groups of PNCTLR catalyze the crosslinking reactions, and, further, the Mg carboxylate group showed higher catalytic activities than the Ca carboxylate group. As for the physical properties of the cured rubbers obtained, the rubbers containing Mg showed higher tensile strength and Shore A hardness than those containing Ca. In addition, the CTBN‐series cured rubbers showed better physical properties than those of the CTB‐series, because of a polarity effect of nitrile groups in the former rubbers. Resistances to water and chemical attack, thermal behavior, and stress relaxation are also discussed.

Copolymerization of liquid sulfur with certain olefinic systems and structure‐property studies on the polymeric materials

AbstractLiquid sulfur–olefinic reactions at 140°C forming crosslinked polysulfide polymers have been investigated. A systematic approach to the characterization and some structure–property studies on these new polymeric materials has been presented. The olefinic systems of interest are endo‐and exo‐dicyclopentadiene and an oligomeric alkenyl polysulfide. Equimolar S8–olefinic system copolymers are amorphous and they show no tendency for sulfur crystallization. A correlation has been drawn between chemical structure and glass transition temperature of the copolymers and terpolymers. Mechanical properties and, in particular, chemical stress relaxation of the crosslinked polysulfide polymers have been investigated.

Chemical stress relaxation of ethylene-propylene copolymer rubber by heat and radiation

In an attempt to shorten the evaluation time for the deterioration under various conditions caused by chemical reactions, we tried to extend the time-temperature superposition principle for the stress relaxation of rubber. In the case of deterioration by radiation instead of by heat, a time-dose rate reduction is proposed and the master curves obtained for chemical-stress relaxation of rubber. A new method which contains a numerical analysis of stress decay curves is proposed to obtain the rate of crosslinking and scission under irradiation for already crosslinked samples.

Structure formation in SKF-26 fluorine-containing rubber using pentaerythritol derivatives containing phosphorus

It was established that hexafluoropropylene-vinylidenefluoride copolymers (SKF-26 fluorine-containing rubber) form structures with pentaerythritol derivatives containing phosphorus. A polymer network is formed as a result of alkylation of phosphites with C-F rubber fragments according to the Arbuzov reaction mechanism. Physical and mechanical properties, heat and chemical stability of vulcanisates were examined. Derivative thermogravimetry and high temperature continuous chemical stress relaxation were used to study thermal oxidative breakdown of the structures obtained, which showed separate stages in the degradation mechanism.

Physical and Chemical Stress Relaxation of Elastomers

Abstract In this paper we have developed a method whereby physical and chemical relaxation processes can be distinguished, using stress relaxation experiments as a function of temperature. We assumed that there exists some temperature range above the glass transition temperature over which the chemical effects can be neglected for the time scale of the experiments. The data in this low temperature range were then used to determine the WLF constants and other physical relaxation parameters. The physical component of the stress relaxation could then be subtracted from high temperature experiments in order to extract chemical kinetic information. Based on certain reasonable assumptions, an equation was developed for the relaxation modulus of a chemically reacting system. This equation could be used to determine the time dependence of the crosslink density, or conversely could be used to predict the long-term relaxation modulus from an assumed kinetic mechanism. These calculations were demonstrated for ethylene-propylene and butyl elastomers.

Studies of Viscoelastic Relaxation of Some Nitrile Rubbers under Compressive Stress

Abstract Stress relaxation in rubbers is usually supposed to be due to chemical aging phenomena of a mainly elastic material. Considerable physical viscoelastic processes can, however, be observed in the rubbery region, depending upon the type of rubber, crosslink density, type of crosslink, filler, and so on. Thus, in chemical stress relaxation experiments chemical and physical mechanisms are superimposed and can seldom be distinguished. Stress relaxation curves registered at different temperatures contain contributions from both types of mechanism. From a practical, as well as a theoretical, point of view it is therefore essential to find methods of distinguishing between the two relaxation processes. This would be possible if the relaxation curves were obtained during periods of time so short that the chemical relaxation can be neglected. The long-term physical relaxation is then obtained by shifting the curves by, for instance, the method of reduced variables. This technique has been utilized by Curro and Salazar. We now present an alternative procedure by which the physical viscoelastic stress relaxation behavior is determined from dynamic data. The physical relaxation curves are calculated from values of E′(ω) and E″(ω) obtained at different frequencies and temperatures. In this method of determining the physical relaxation, no change of sample is needed, nor is it necessary to allow the sample to relax between measurements. The stress relaxation behavior under compression of three nitrile rubbers has been studied, and it is shown that physical mechanisms dominate just above room temperature, while chemical mechanisms dominate at higher temperatures.

The stability and chemical stress relaxation of peroxide cured polyurethane elastomers

AbstractThe chemical stress relaxation of two polyurethane elastomers Genthane S and Adiprene CM has been studied in air and in nitrogen in the temperature range 60 to 150°C. Both continuous and intermittent stress relaxation measurements were made on well characterized samples covering a range of crosslink densities. Scission and crosslinking reactions were observed for both the polyester based Genthane S and the polyether based Adiprene CM. The degradation of Genthane S is largely non oxidative and the scission reaction involves the main chain. The behaviour of Adiprene CM under nitrogen broadly resembles that of Genthane S; it differs from Genthane S in that it also undergoes an oxidative reaction. Activation energies for the scission and crosslinking processes were evaluated from kinetic analysis. Pretreatment of the elastomers with isocyanates inhibits both of these reactions. A mechanism involving the urethane group is proposed to account for the degradation behaviour and the role of water in this respect is discussed briefly.

Physical and chemical stress relaxation of a fluoroelastomer

Stress relaxation measurements were made at various temperatures on V-747-7, a commercial high-temperature rubber formulation from the Parker Seal Company. The data were analyzed by separating the chemical and physical relaxation processes by a method described in an earlier publication. The chemical relaxation process was found to be Arrhenius with an activation energy of 35.7 kcal/mole. The results allow us to predict the relative useful lifetimes of this material up to approximately 320°C.

Accelerated Heat-Aging Studies on Fluororubber in Various Media

Abstract The nature, extent and, rate of chemical changes that take place in certain elastomers at long times is being studied. The chemical degradation of styrene-butadiene rubber (SBR) and fluorosilicone rubber (FVMQ) were reported previously. Investigations carried out with fluororubber (FKM) are discussed in this paper. Like fluorosilicone, this fluorocarbon elastomer is regarded as a high-temperature- and hydrocarbon-fuel-resistant rubber. The designation FKM is given to the copolymers of vinylidene fluoride (VF) and perfluoropropylene (PFP). The one discussed here is reported to contain also small quantities of a third component, tetrafluoroethylene. The detailed structure, assigned to the FKM polymer as a result of nuclear magnetic resonance studies, shows that it consists of blocks of VF homopolymer, 95% of which is head-to-tail, and blocks of VF-PFP copolymer, 93% of which is head-to-tail. The ratio of the copolymer to the VF homopolymer is dependent on the mole fraction of VF used. Vulcanization of FKM with derivatives of diamines is the commercially preferred method, although peroxides and high-energy radiation can be used for the same purpose. In the present study, N, N-dicinnamylidene-1,6-hexanediamine a Schiff 's base of 1,6-hexanediamine, was used with magnesium oxide as the acid acceptor. Chemical stress relaxation was the method most extensively used to determine the network changes taking place in the heat-aged fluororubber. Swelling, gel permeation chromatography (gpc), and infrared (ir) spectroscopy were also used to obtain additional information.

Chemorheology of linear polymers. II

AbstractThe chemical stress relaxation of crosslinked polymers had long been studied. The chemorheology of linear polymers is difficult to study because molecular flow by diffusion is generally much more rapid than relaxation or flow caused by chemical reaction. Our previous paper dealt with the chemorheology of linear polymers on taking such physical flow into consideration. By using another modified expression suggested by Tobolsky, good consistency between the theoretical chemical stress relaxation curve and an experimental one was obtained for polydisperse polystyrene and the commercial polystyrene used.

Effect of macromolecular conformation on the auto-acceleration of oxidation in oriented polypropylene

Transition from equilibrium compact conformations of macromolecular segments in the amorphous phase of isotropic polymers to elongated conformations in oriented samples markedly reduces the yield of hydroperoxide per mole absorbed oxygen in oxidation. Since hydroperoxide is an agent branching kinetics chains of oxidation, a reduction of yield per mole absorbed oxygen reduces auto-acceleration and produces a considerable increase in the induction period of oxidation in the experiment. Chemical stress relaxation in oriented samples during oxidation, which is related to rupture of stressed macromolecules, increases the yield of hydroperoxide to values typical of isotropic samples.

Accelerated Heat-Aging Studies on Fluorosilicone Rubber

Abstract The accelerated aging of a vulcanizate of fluorosilicone rubber was studied by four methods: infrared spectroscopy, GPC, weight loss, and stress relaxation, with the object of gathering information pertinent to understanding the basic mechanism of its aging. No changes were observed in infrared spectra of the rubber aged in air up to 315°C although severe degradation took place, as evidenced by high weight loss and drastic change in the physical condition of the test samples. It was concluded that although the polymer degrades, there is no change detectable within the precision of the analytical methods in the structure of the repeat unit of the elastomer. The gel-permeation chromatogram of unheated rubber showed peaks at MW 450 000 (95%) and 630 (5%). The low-molecular-weight component is very likely the cyclic tetramer of λ,λ′,λ″-trifluoropropylmethylsiloxane, the principal monomer of the FVMQ used. Heat aging increased the concentration of the 630 MW component, but rate studies were prevented by complications resulting from solvent-induced rubber degradation. Rapid decrease in the MW of FVMQ, as found by GPC, indicated random scission in the polymer backbone, as opposed to ordered unzipping of the repeat unit. The same conclusion was reached from the effects of crosslink density on stress relaxation. Activation energies obtained from weight-loss measurements (104 kJ/mol) were lower than those obtained from stress-relaxation measurements, (150–155 kJ/mol), suggesting that the processes being measured in each case may not be the same. Chemical stress relaxation takes place more rapidly in the presence of air than in an inert atmosphere, and intermittent stress-relaxation measurements indicate that crosslinks form during thermal aging in air, their rate first increasing and then decreasing.

Physical and chemical stress relaxation of elastomers

In this paper we have developed a method whereby physical and chemical relaxation processes can be distinguished, using stress relaxation experiments as a function of temperature. We assumed that there exists some temperature range above the glass transition temperature over which the chemical effects can be neglected for the time scale of the experiments. The data in this low temperature range were then used to determine the WLF constants and other physical relaxation parameters. The physical component of the stress relaxation could then be subtracted from high temperature experiments in order to extract chemical kinetic information. Based on certain reasonable assumptions, an equation was developed for the relaxation modulus of a chemically reacting system. This equation could be used to determine the time dependence of the crosslink density, or conversely could be used to predict the long term relaxation modulus from an assumed kinetic mechanism. These calculations were demonstrated for ethylene propylene and butyl elastomers.

Chemical Stress-Relaxation and Other Studies on Styrene-Butadiene Rubber

Abstract The aging of styrene—butadiene rubber (SBR) was studied by three methods: stress relaxation, infrared spectroscopy, and swelling measurements, with the purpose of supplying information pertinent to understanding the basic mechanism of its aging. Stress-relaxation measurements in air and nitrogen at elevated temperatures indicated that atmospheric oxygen is the principal cause of chemical stress relaxation of SBR, rather than heat. Intermittent stress-relaxation measurements showed scission and crosslinking occurring simultaneously during network breakdown, and it was concluded that random scission in the backbone is indicated to take place in preference to scission in the crosslinks. Activation energies obtained from relaxation rates at several temperatures was 28 ± 0.5 kcal, comparable to literature values of 30 ± 2 kcals. The rates of carbonyl and hydroxyl group formation in SBR in air at various temperatures were determined by ir spectroscopy, both induction and maximum rates, νm, being measured. Activation energies calculated from these rates showed lower values compared to those obtained from stress-relaxation measurements. This may be due to the possibility that the processes being measured are not the same in each case. The three peaks appearing in the carbonyl region were ascribed to carboxyl, ketone, or aldehyde, and perester. The presence of these groups was confirmed by microanalytical methods. The absorption centered at 3450 cm−1 was attributed to hydrogen-bonded OH groups, i.e., alcohols and hydroperoxides. Positive chemical tests were obtained for hydroperoxide. The number of new network chains formed, νe, obtained from swelling measurements agreed well with those obtained from stress-relaxation measurements. It was found that the rate of the number of new network chains formed increased rapidly during the latter states of oxidation.

Characterization of polymers in large deformation by repeated chemical stress relaxation of vulcanized rubbers

In this investigation, the repeated chemical stress relaxation measurements were carried out to observe the relaxation behaviors at large deformation. It was found that the repeated chemical stress relaxation curves were affected by both temperature measured and extension ratio of rubber. It was suggested from the results obtained that temperature and mechanical stimulus have a similar effect on the stress relaxation curves. Thus we propose the followingArrhenius type eq. [1] for high extension ratios. $${{f(t)} \mathord{\left/ {\vphantom {{f(t)} {f(0)}}} \right. \kern-\nulldelimiterspace} {f(0)}} = \exp (A \cdot e^{\frac{{E_f }}{\alpha }} \cdot t).[1]$$ Where,α is the extension ratio, andA, andE f are the constants determined experimentally. On the other hand, from eq. [1] and usualArrhenius equation, the universal eq. [2] for the extension ratio and the temperature in large deformation was derived as follows, $$\begin{gathered} \ln \{ (\ln f(0)/f(t))/t\} - E/R(1/T_0 - 1/T) \hfill \\ = \ln A - E_f /\alpha [2] \hfill \\ \end{gathered} $$ where,T 0 is adequate temperature. The curves obtained for different temperatures and extensions were very well consistent with those by eq. [2] in large deformation.

加硫天然ゴムの化学応力緩和機構

種々の橋カケ構造を有する天然ゴム加硫物, 特にイオウあるいはテトラメチルチウラムジスルフィド (TMTD) を含むゴムに放射線を照射することによって作製した混合橋カケ系の空気中および窒素中の応力緩和機構を検討した。空気中においてはあらゆる橋カケ系において主鎖のランダム切断を受ける。しかしその切断量は橋カケ系により異なり, モノスルフィドやポリスルフィドが存在することにより増加する。またポリスルフィドを有する橋カケ系においては交換反応が生じ, 空気中および窒素中の交換反応速度は一致する。主鎖の酸化切断の活性化エネルギーはイオウを含む橋カケ系および炭素-炭素結合を有する橋カケ系に対して21kca1/mole, モノスルフィド橋カケ系に対しては27kcal/moleを得た。

Chemorheology of irradiation-cured natural rubbers: 1. Stress relaxation mechanisms for various curing systems in natural rubber at high temperature

Stress relaxation mechanisms were investigated for various curing systems in natural rubber, especially the ‘complex’ crosslinking irradiation cure systems containing sulphur or tetramethylthiuram disulphide (TMTD) in both air and nitrogen mainly at 100°C. With the systems, the chemical stress relaxation in air showed that in the main oxidative scission was the major cause of stress decay. However, the number of moles of main chain scission, q m ( t) seemed to be dependent on the chemical structure of the crosslink. In the TMTD curing system and irradiation-TMTD curing systems, q m ( t) is independent of the ratio, ρ, of the effective chain density N c (0) based on the carbon-carbon crosslinkages to N m (0) based on mono- and di-sulphide linkages. Stress decay in air of both accelerated-sulphur cures and irradiation-sulphur cures was shown to be due to both the interchange reaction of polysulphide linkages and random scission of the main chain. On the other hand, in nitrogen there are two simultaneous reactions at the crosslink:interchange and thermal scission.

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

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.

Characterization of Polymers under Large Deformation. I. Repeated Chemical Stress Relaxation of Vulcanized Rubbers

Abstract In this investigation, repeated chemical stress relaxation measurements were carried out to observe the relaxation behavior at large deformation. It was found that the repeated chemical stress relaxation curves were affected by both measurement temperature and the extension ratio of rubber. It was suggested on the basis of the results that temperature and mechanical stimulus have a similar effect on the stress relaxation curves.

Effect of new crosslinkages on continuous chemical stress relaxation of polymers

Effect of new crosslinkages on continuous chemical stress relaxation of polymers