Chemical Stress Relaxation Research Articles

Stress relaxation studies of model silicone RTV networks

AbstractThe stress relaxation behavior of model silicone room temperature vulcanizing (RTV) elastomers has been used to examine the chemistry of the cured silicone network. It has been shown that the tin crosslinking catalyst, together with water, produces siloxane bond rearrangement which results in chemical stress relaxation. The measurement of the rate of stress relaxation of unfilled model elastomers at various temperatures gave an apparent activation energy of 10.3 kcal/mole−1 for the relaxation process. In addition, the effects of some of the constituents of the RTV on relaxation behavior have been examined.

Effect of cross-links which occur during continuous chemical stress-relaxation

Effect of cross-links which occur during continuous chemical stress-relaxation

Application of chemorheology to radiation damage of elastomer

The technique of chemical stress relaxation was applied to investigating radiation damage of elastomers. The interests center of this paper are as follows. 1. 1. The efficiency (G value) of chain scission and cross-linking obtained by the measurement of continuous and intermittent chemical stress relaxation. 2. 2. Dose rate dependence of chain scission and cross-linking. 3. 3. A method which expresses the magnitude of synergistic relationship between heat and radiation on the degradation of elastomers. 4. 4. A quantitative expression on the decrease in heat resistant properties of elastomer by pre-irradiation.

Influence of physical relaxation on chemical stress–relaxation of natural rubber vulcanizates

Networks were prepared in the swollen and dry states to investigate the influence of physical relaxation on chemical stress–relaxation. The stress–relaxation behavior of solution-cured samples was different significantly from that of the samples crosslinked conventionally. The same result was also observed in the number of chain scission for both kinds of samples. On the other hand, the number of chain scission estimated by using the swelling method for samples crosslinked conventionally was in good agreement with that by the chemical stress–relaxation for solution-cured samples. It was found that there is little or no influence of the physical relaxation caused by entanglements, and no effect of dangling chains arising from scission in the equilibrium swelling. The relative change of network chain density determined by means of the swelling method was also consistent with that by sol fraction determination. These results indicate that the swelling method can be used as a measure of a degree of degradation on chemorheology. Taking into account the influence of physical relaxation on chemical stress–relaxation, a new relationship between the relative stress decay and the relative network chain density was experimentally proposed.

Chemorheology of crosslinked polyisoprene/poly(butadiene co-styrene) blends

Physical and chemical properties of IR/SBR blend systems crosslinked by dicumyl peroxide were investigated in detail. In order to study physical structure of cross-linked rubber blends, measurement of birefringence of extended samples and observation by scanning electron microscope (SEM) of ones ruptured in liquid nitrogen were also carried out. In the vicinity of IR/SBR = 30/70 (weight ratio) the phase structure of blends was reversed. The size of domain was in about 0.2 to 0.7 μm dependent on the blend ratio. In preparing the cross-linked rubber blends, cross-linking reaction seemed to occur independently in each rubber phase. The birefringence linearly increased with the blend ratio in the range of IR/SBR = 100/0 to 40/60 and then largely increased in IR/SBR < 30/70. Stress relaxation of rubber blends was observed at 373 K in both air and nitrogen. Chemical stress relaxation of these systems could be represented by a combination of rate constant of stress relaxation and volume fraction of each cross-linked rubber, that is, the degradation of the networks of cross-linked rubber blends occurred independently in each rubber phase.

Secondary Stress Relaxation Mechanism in Ion-Containing Polymers I. A New Analytical Method

With a view to isolating the secondary component from the complexed stress relaxation, a method originally proposed by Tobolsky and used for the study of the chemical stress relaxation in covalently bonded networks was applied to ion-containing polymers to which time-temperature superposition had been previously found inapplicable. A few examples were chosen from the two different groups of ionomers to demonstrate the applicability of this method. Although the secondary component in. one of styrene–sodium methacrylate copolymers (PS–NaMA) with 7.7 mol% of ions cannot be separated by this method at low temperatures, the secondary mechanism can be isolated at high temperatures. In other ionomers with more ions, the secondary mechanism is isolated by this method for the entire temperature ranges. The secondary relaxation is due to the yielding of the ionic structure, and these ionomers studied here approximately follow Maxwellian decay of stress. The activation energy obtained for the secondary mechanism of each ionomer, which ranges from 26 to 31 kcal mol−1, may correspond to the energy required to break the ionic interactions in the ionic clusters.

Chemical Stress Relaxation of NR Networks Prepared in the Swollen State

Abstract From the experimental results of the present investigation, it is apparent that two kinds of networks which have a different three-dimensional network structure give quite different behavior of chemical stress relaxation, even if both networks have the same network chain density. The difference in three-dimensional network structure for the two kinds of rubber arises from the degree of entanglement, which changes with the concentration of the polymer chains prior to the crosslinking process. The direct cause of chemical relaxation is due to the scission of network chains by degradation, whereas the total relaxation is caused by the change of geometrical conformation of network chains. This then casts doubt on the basic concept of chemorheology which is represented by Equation 2.

Structural differences exhibited by networks prepared by chemical and photochemical reactions IV chemical relaxation

AbstractChemical stress relaxation has been used for the characterization of elastomeric networks obtained either by peroxide curing or by photocrosslinking. It has been shown that the differences observed between photocrosslinked and peroxide‐cured elastomers are not limited to their structure and physical properties but are also due to the very nature of the created bonds.

Properties of Diamondlike Carbon Films

AbstractHard carbon films have been deposited onto room-temperature silicon substrates in a de plasma of methane and hydrogen. The substrates are placed on the cathode. A stainless mesh is held at the same potential as the cathode and is set above the substrates. Although the deposited films are amorphous and contain 24.4 atomic percent hydrogen, they have the following diamondlike properties: hardness is almost equivalent to that of natural diamond; electrical resistivity is on the order of 1013Ωcm; chemical inertness is excellent to acids; thermal diffusivity is 5.2 cm2 /sec. However, the films have a large compressive stress of 1.3×1010 dyn/cm2.Annealed films exhibit dehydrogenation, graphitization, an increase in chemical reactivity, volume expansion and stress relaxation above 400°C. The activation energy for the transformation from the diamondlikephase to the graphitic phase is 18 kcal/mole.The dependence of the thermal diffusivity and hydrogen content on both the CH4/H2 gas mix ratio and the total pressure have been measured for the films deposited in a dc plasma without the mesh.

Determination of the thermooxidation kinetic parameters and of various pathways of the consumption of amine antioxidants during the ageing of SKI-3 sulphur vulcanizates in air stream

The kinetics of the consumption of antioxidants, derivatives of N,N′- p-phenylenediamine of various volatility, was studied during the ageing of unfilled and carbon-filled rubbers from SKI-3 in a stream of air and of inert gas. The rates of rubber degradation under these conditions were measured by the method of chemical stress relaxation, and the kinetics of oxygen consumption was followed by means of a volumetric apparatus. Based on kinetic analysis of the data obtained, the contributions of evaporation and of chemical antioxidant consumption were established, and the probability of chain initiation at sulphur bond rupture was calculated, as well as the mean number of kinetic chains terminated by one molecule of antioxidant.

Stress Relaxation of NR and NBR: Comparison of Measured Data with Reduced Variables Estimates

Abstract The application of reduced variables has been tested in long-time compressive stress relaxation at 293 K over a time period of 16 years for NR and NBR. The results show that within this time span there is good agreement with predicted data from reduced variables. However, the results indicate that over much longer time periods (approx. 20 to 50 years) disagreement is to be expected due to dominance of chemical stress relaxation for surface to volume ratios in the range of 0.6/mm.

Thermo-equivalent dose rate on the chemorheology of chloroprene rubber under heat and radiation

This paper proposes a method to compare the efficiency of heat and that of radiation on polymer chain scission. Chemical stress relaxation which corresponds to polymer chain scission of chloroprene rubber was measured under the combined environment of heat and radiation. The temperature range was from 80 to 140°C, dose rate was from 0.40 × 10 5 to 3.26 × 10 5 R/h. “Thermo-equivalent dose rate” was introduced to express the increment of dose rate which gives the same effect as the rise in temperature. It was found that to raise temperature 10°C from the reference condition corresponds to an increased dose rate of 4.5 × 10 4 R/h from the previous condition.

On the separation of physical and chemical component of stress relaxation

This paper proposes a new method to separate physical and chemical components of stress relaxation. The stress relaxation measurements of tetrafluoroethylene-propylene rubber were carried out at various temperatures ranging from 200° to 310°C. A physical stress relaxation master curve could be generated from data of early period of time by the time-temperature superposition principle. The rate of physical stress decay at given temperatures was calculated from the master curve. The rate of chemical stress relaxation was given by subtracting the rate of physical decay from experimentally obtained rate at the corresponding temperatures. The activation energy was found to be 8.4 kcal mol −1 for the rates of the calculated chemical stress relaxation, while it was found to be 5.7 kcal mol −1 for the rates which were obtained in air. The results show that the physical component of stress decay should be subtracted from the measured stress relaxation curve to obtain the rate of chemical stress decay, especially at the low temperature.

Crosslinking of hydroxyl‐terminated liquid rubber with divalent metal salts of mono(hydroxyethyl) phthalate, anhydrides, and bisepoxide

AbstractCrosslinking of hydroxyl‐terminated liquid rubber with divalent metal salts of mono(hydroxyethyl) phthalate, anhydrides and bisepoxide was investigated by the diols‐anhydridesbisepoxide, reactions. As the anhydride, succinic anhydride and hexahydrophthalic anhydride were used, and bisphenol A diglycidyl ether was the bisepoxide used. The metal carboxylate groups of these metal salts catalyzed the crosslinking reactions and, further, the Mg salt showed higher catalytic activities than the Ca salt. As for the physical properties of the metal‐containing cured rubbers obtained, tensile strength and Shore A hardness increased with increase in the metal salt content. In addition, the rubbers containing Mg showed better physical properties than those containing Ca. Meanwhile, elongation showed a maximum with increase in the metal salt content. Resistance; to water and chemical attack, thermal behavior, and stress relaxation are also discussed.

The stress relaxation of thermoplastic elastomer–the influence of oxygen on the network structure of SBS during γ‐radiation

Abstract Thermoplastic elastomer (TPR) of the SBS type was irradiated in a vacuum and in air with various gamma dosages of Co‐60 at room temperature, and the stress relaxation behavior of the irradiated TPR was studied in air and in a nitrogen atmosphere. It was found that TPR was crosslinking during irradiation both in air and in a vacuum, and the crosslinking density of TPR irradiated in air was found to be higher than that irradiated in a vacuum. This result may be attributed to the presence of oxygen which could participate in the radiation‐induced reaction. A TPR sample irradiated in a vacuum showed very extensive physical stress relaxation and minor chemical stress relaxation, measured both in air and in nitrogen. The latter could be caused by oxidative chain scission along the main chain. However, the chemical stress relaxation of TPR irradiated in air was found to be different from that irradiated in vacuum. In addition to the oxidative chemical stress relaxation, the former was further caused by the scission of crosslinking site which was formed during Co‐60 irradiation.

Structure and Properties of Extensively Degraded Rubber Vulcanizates

Chemical stress relaxation is a unique technique to study the degradation of vulcanized natural rubber (NR).It is well-known that the relationship between the initial stress, f(0), and the initial network chain density, n(0), is given by eq. (1).f(t)=n(t) RT(α-α-2) at t=0 (1)at t =t (2)Thus far, eq. (2) has been used for degraded vulcanized NR. However, we have assumed that eq. (2) cannot be applied in the case of extensive degradation, and introduced a new method relating the netwark chain density with the number of scission in thermo-oxidative degradation of NR.Then, we have compared the network chain density calculated from relative stress with that calculated from the number of scission based upon the thermo-oxidative degradation of uncured-NR under the same degradative condition for DCP cured-NR.The values of the network chain density determined by the above two methods agreed with each other only in the early stage of degradation. However, it was found that eq. (2) is not valid as the degradation proceeds extensively. Consequently, a correction of eq. (2) has been advocated experimentally which is represented by f(t)-C·exp{-D·f(t)}=n(t)RT(α-α-2)

Chemorheology of rubber under heat and radiation

Chemical stress relaxation of tetrafluoroethylene-propylene elastomer under the combined environment of heat and radiation was studied at various temperatures ranging from 150°C to 280°C under the dose rate of from 1.21x10 5 R hr to 2.2x10 4 R hr . Each decay curve was found to be Maxwellian. The activation energy of the chemical stress relaxation was found to decrease by irradiation. The rate of the chemical stress relaxation by irradiation is proportional to 0.9 power of dose rate at 240°C. A synergistic relationship between heat and radiation was found to exist on the stress decay of rubber. The coefficient of synergism is proposed to express the magnitude of this synergistic relationship.

Application of chemorheology to radiation damage of polymers—III

Abstract Chemical stress relaxation of ethylene-propylene rubber under the combined environment of heat and radiation was studied. The temperature range is from 120 to 170°C, dose rate is from 2.2 × 104 R/h to 1.05 ×1 05 R/h. Decay curve were found to be Maxwellian over a period of time. A synergistic effect is found to exist in the stress decay of rubber under heat and radiation. The following equation expresses the rate of stress decay under heat and radiation K(r+h) = const. × I α exp (−E′/RT) + K(h) In the equation K(h) is the stress decay by heat alone, I is dose rate, α expresses the dose rate dependence (α=1.0 at 170°C), R is the gas constant, T is the temperature and E' is coefficient of synergism which is proposed in this paper.

Influence of oxygen content in the atmosphere on chemical stress relaxation of natural rubber vulcanizates: 1. Relaxation mechanisms of sulphur-cured systems

Stress relaxation of sulphur-cured natural rubbers of different crosslinked structures was investigated in an atmosphere containing variable amounts of oxygen. At high temperature, even very low contents of oxygen strongly affected the degradation of these vulcanizates. The stress relaxation of each vulcanizate was analysed in relation to the sol fraction and the swelling of sample degraded under the same condition. The scission mechanism of the vulcanizates depended on the quantity of oxygen in the atmosphere. Under nitrogen, the scission occured mainly near the crosslink site in all the samples. However, in the case of their being small amounts of oxygen present it took place near the crosslink site in the initial stages of degradation and then along the main chain at the later stages. In the atmosphere of comparative high oxygen content, the degradation seemed to result from the oxidative scission along the main chain. The degradation mechanisms of sulphur-cured natural rubbers were also discussed.

Significance of Network Chain-Length Distribution in Chemical Stress Relaxation Studies

Abstract The theoretically expected network chain-length distribution for networks where the cross-link points are randomly distributed in the bulk of the material would contain a significant fraction of network chains which are much shorter than the average chain length. The contribution of these very short chains to the stress born by a network is an important consideration in chemical stress relaxation studies. It is proposed that by use of a gamma distribution function as a “functional” network chain-length distribution, the effect of such short network chains might be taken into account in deriving stress-time expressions for chemical stress relaxation studies. While this procedure yields a simple expression for the fractional residual stress at a given extent of scission, the error involved in neglecting the effect of the chain-length distribution appears to be small.