Polyisoprene Vulcanizates Research Articles

Strategy for improving high-temperature properties of polyisoprene vulcanizates by addition of polymorphic TBIR

Polymer network structures providing necessary elasticity and strength play crucial roles in rubber composites. However, the undesirable high-temperature properties of the rubber composites serving under extreme conditions such as 70–90 °C resulted in the limited serving life of the products. In this work, a novel kind of trans-1,4-poly(butadiene-co-isoprene) (D-TBIR), composed of trans-1,4-polyisoprene (TPI) segments and trans-1,4-polybutadiene (TPB) segments, was synthesized and incorporated into cis-1,4-polyisoprene (IR) to construct rubber materials featuring multiple polymer network structures. Three kinds of D-TBIR samples with varied crystallinity (Xc) and melting temperature (Tm) were synthesized. The aggregation structure, crystal-crystal transition behaviors and mechanical properties of the IR/trans rubber (85/15) blends were thoroughly investigated. Blending IR with D-TBIR2 with relatively high butadiene unit content (62.9 mol%), 12.7 % Xc from TPB-I (Tm = 44–64 °C), 1.8 % Xc from TPI (Tm = 29 °C) and 1.6 % Xc from TPB-II (Tm = 130 °C) constructed a unique phase morphology with uniform-size nanoscale lamellae. The multiple polymer networks, including sulfur bonds, TPI and TPB lamellar stacks have demonstrated diverse endothermic temperatures ranging from 4–104 °C. Compared to IR vulcanizate, the IR/D-TBIR2 (85/15) vulcanizate presented outstanding dynamic mechanical properties under both room temperature and high temperature. Specifically, it demonstrated a 152 % enhancement in tensile fatigue life and an 80 % increase in tensile strength at 90 °C, coupled with a substantial improvement in its comprehensive properties at room temperature. The correlations between structure and properties of the IR/D-TBIR2 vulcanizate were discussed. The D-TBIR copolymers are expected to apply in long-serving-life engineering tires and damping elements under harsh conditions such as high temperature and complex dynamic forces.

异戊橡胶微观序列结构与结晶性能研究

It is widely accepted that the outstanding mechanical properties of nature rubber (NR) are partly attributed to the strain-induced crystallization. The strain-induced crystallization of isoprene rubber should also attract our attention because it is the only alternative for NR. The microstructure and crystallization of three grades of polyisoprene rubber has been studied by 13 C and 1 H NMR. There exists slightly difference of cis -1,4 configuration among the three polyisoprene samples. According to the sequential structure of 1,4-configuration, the cis -1,4 units of SKI-5 and YS-IR adopt only head-to-tail arrangement. However, the 1,4-units of SKI-3 consist of head-to-head (about 0.4%~0.5%), tail-to-tail (about 0.3%~0.6%) and head-to-tail arrangement. According to the quantitative measurements, the regularity of the three samples are similar. The crystalizability of polyisoprene rubber have been studied through X-ray diffraction (XRD) method. The NR vulcanizates filled with carbon black crystallize above 400% elongation. But the crystallization of polyisoprene vulcanizates occurs when it is stretched above 500%. According to the mechanical tests, SKI-3, SKI-5 and YS-IR show comparative mechanical properties. The strain-induced crystallization performance of isoprene rubber is inferior to nature rubber because of the differences in the structure and composition.

Surface morphology, mechanical and tribological properties of blended deproteinized natural and polyisoprene rubbers

The morphology, and mechanical and tribological properties of deproteinized natural rubber with constant viscosity (DPNR) and synthetic cis-1, 4 polyisoprene vulcanizates (IR2200) using ISAF220 carbon black (CB) blends were studied in various ratios: 100/0, 75/25, 50/50, 25/75 and 0/100. Results for different operating test conditions under applied normal load, sliding speed and sliding distance were analyzed. The experimental results showed that the addition of CB (30 phr.) has significantly affected the wear and frictional performance of both DPNR-CV and IR2200 by decreasing the abrasion weight loss by more than 80% compared with the unfilled DPNR and IR2200. Adding 30 phr CB decreases the abrasion weight loss to more than 70%. The surface topography of micro-scales abrasion was characterized by scanning electron microscopy (SEM). Using SEM micrographics, the influence of blend ratios on wear mechanisms and spacing imprinted abrasion patterns was highlighted.

Mechanical properties of deproteinized natural rubber in comparison with synthetic cis‐1, 4 polyisoprene vulcanizates: Gum and black‐filled vulcanizates

AbstractGum and black‐filled vulcanizates having various crosslink densities were prepared from 2 types of rubber, namely, deproteinized natural rubber (DPNR) and synthetic cis‐1, 4 polyisoprene vulcanizates (IR). Their mechanical properties, such as tensile strength, tear strength, abrasion loss, and heat buildup resistance, at various crosslink densities as well as at similar optimum crosslink density were compared. For both gum and black‐filled systems, IR possessed a higher crosslink density than that of DPNR at a fixed curative content. Tensile and tear strength of all vulcanizates passed through a maximum with increasing crosslink density. For gum vulcanizates, tensile and tear strengths of DPNR and IR below the maximum were not much different. However, IR had a narrower tear strength peak relative to DPNR. At a comparable optimum crosslink density, DPNR exhibited higher tensile strength and crack growth resistance than IR. For black‐filled vulcanizates, tensile and tear strengths, and heat buildup resistance of DPNR and IR at a given crosslink density were similar. The results revealed that the properties of gum samples were more dependent upon crosslink density than the black‐filled ones because the reinforcement by carbon black overshadowed the intrinsic properties of the rubbers. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 1139–1144, 2005

Positron Annihilation Lifetime Spectroscopy (PAL) as a Tool to Study Rubber (Polyisoprene) Vulcanizate Network Structures

Positron Annihilation Lifetime Spectroscopy (PAL) as a Tool to Study Rubber (Polyisoprene) Vulcanizate Network Structures

Swelling behavior and mechanical properties of endlinked poly (dimethylsiloxane) networks and randomly crosslinked polyisoprene networks

Measurements of the equilibrium degree of swelling and of the equilibrium modulus were performed on poly(dimethylsiloxane) networks (PDMS) and on polyisoprene vulcanizates. The results support the concept that topological interactions between network chains, e.g. entanglements or the like, have a large influence on the rubber elastic behavior, at least within a certain range of network densities. PDMS networks having network chains of different lengths and varying functionlities of the crosslinks were prepared in bulk by endlinking fractionated α,ω-divinyl PDMS via multifunctional hydrogen-siloxanes (f=3 to 22). Natural rubber (NR) and synthetic liquid polyisoprene (IR) were cured in bulk with various amounts of dicumyl peroxide to give randomly crosslinked samples. The experimentally determined moduli and degrees of swelling were compared with theoretical predictions based on the phantom network theory and affine network theory, taking into account only chemical crosslinks. The observed discrepancies can be traced back to a contribution of topological interactions (trapped entanglements) to the total effective network density. The modulus and swelling data are consistent, thus ruling out non-equilibrium effects.

Kinetics and mechanism of stress relaxation of polyisoprene vulcanizates under ozone ageing

The present paper deals with the kinetics of the decrease in stress ( Δ p ) in polyisoprene vulcanizates at 22°C and in an ozone concentration of (1·8 – 7·3) × 10 −8 mol litre −1 and the kinetics of degradation of polyisoprene macromolecules in solution. It has been found that the number of ruptured macromolecules increases linearly with time, whereas the time dependence of Δ p is more complex. A mechanism has been proposed, which links the stress relaxation with the chemical processes which occur during the action of ozone on vulcanizates. It has been shown that Δ p involves two components possessing a common chemical nature, but different localization; namely, weakening of the sample by growing cracks and chain degradation on the surface of uncracked sites on the sample.

Comparative study of the kinetics of creep and oxidation of a polyisoprene vulcanizate in a mechanical force field

The dependence of creep of SKI-3 sulphur vulcanizates, first stretched by 350%, on the size of the eternal load at different temperatures has been investigated. The existence has been found of a region in which inhibition of the creep process is observed. IR spectroscopy revealed the existence of a region of inhibition of the formation of the C = O groups with rise in the load.

Kinetics and mechanism of stress relaxation in ozone ageing of polyisoprene vulcanizates

Kinetics of stress reduction were examined in polyisoprene vulcanizates at 22°C and [ O 3] o =(1·8−7·3)×10 −8 mole/l. and kinetics of macromolecular breakdown of polyisoprene were studied in solution. The number of macromolecules terminated was found to show a linear increase over a period of time, while the dependence of stress reduction on time is more complex. A mechanism was proposed which links stress relaxation with chemical processes, taking place by the action of ozone on vulcanizates. Stress reduction was found to include two components with general chemical structure but different localization: weakening of sample cross-section by propagating crack and chain termination in the surface layer of the uncracked part of the sample.

Double‐step retraction of a polyisoprene vulcanizate: Comparison with molecular models

AbstractLarge stress peaks have been observed by subjecting a weakly crosslinked polyisoprene vulcanizate to double‐step extensions in opposite directions. The resulting retractions are well described by a BKZ constitutive equation with a strain‐dependent term identical to that for deviations from the statistical elasticity theory at equilibrium. This unexpected nonlinear behavior suggests that disengagement of pendant chains and attenuation of junction constraints both contribute to the stress relaxation of random networks.

Kinetics of the growth of cracks on polyisoprene vulcanizates in ozone

The kinetics of growth of cracks on synthetic polyisoprene SKI-3 rubber vulcanizates in ozone has been studied by optical microscopy. The mean value of crack surface is used as a measure of the degree of cracking. The growth of crack surface comprises two stages; namely, a relatively slow one at the beginning of the process and a faster one which proceeds at a constant rate. Empirical dependences which describe this process at various ozone concentrations are suggested. A physical interpretation of the constants in these expressions is presented.

Kinetics of crack growth on polyisoprene vulcanizates in ozone atmosphere

Abstract The kinetics of crack growth in vulcanizates of synthetic isoprene rubber SKI-3 in ozone atmosphere was studied by optical microscopy. The mean value of crack surface was used as a measure of degree of cracking. The growth of crack surface exhibits two stages: a relatively slow one in the beginning, and a more rapid one proceeding at constant rate. Empirical relations are presented describing this process at various ozone concentrations. A physical interpretation of the constants appearing in the empirical relations is presented.

Changes in the Dynamic Modulus during Thermal Degradation of Polyisoprene Vulcanizates

Abstract Thermal degradation of IR is investigated in air and in N2 by dynamic mechanical measurements. In this dynamic method, the change in the relative dynamic modulus is found to be frequency dependent, but the data at various frequencies can be superimposed upon each other by shifting along the time axis. Changes in relative modulus as a function of time in air and in N2 can be expressed by Equation (1), which contains two exponential terms. The activation energies for k1 and k2 are 84 and 57 kj/mol in air, and 61 and 57 kJ/mol in N2, respectively.

Effect of static deformation on kinetics of oxidation of polyisoprene vulcanizate containing sulphur bonds

A calorimetric study was made of the effect of static deformation on oxidation kinetics of a peroxide vulcanizate of polyisoprene containing a small proportion of combined sulphur. The effect of static deformation varies at different stages of oxidation. At the initial stage sulphides are oxidized and the process is considerably accelerated by the stress formed under static deformations. The effect of static deformation at the stage of auto-catylysis hinders oxidation, reducing the rate constant and increasing the activation energy of this process as a result of a reduction in the conformation mobility of macromolecules.

Identification of Natural and Synthetic Polyisoprene Vulcanizates by the Carbon-14 Dating Technique

Abstract The idea of using the radiocarbon dating technique for the differentiation of natural rubber from synthetic rubber is novel and has never been tested before experimentally. The experiments were designed to demonstrate the feasibility of the approach rather than to perfect the technique. With the limited data gathered here, it was shown that the radiocarbon dating technique furnishes results with accuracy quite satisfactory for the analysis of gum rubbers. For the cured samples, the accuracy is reduced mainly by the errors introduced from steps other than that of the radioactivity measurement. A further reduction of the accuracy results if the age of the natural rubber in the stock is not known. We conclude that the technique is feasible and yields results of an accuracy which is probably the best ever achieved by any known technique.

Identification of Natural and Synthetic Polyisoprene Vulcanizates by Thermal Analysis

Abstract 1. NR and IR vulcanizates can be distinguished from one another by their DSC and DTG curves. 2. Thermal curves remain more or less unaffected by recipe changes. 3. Identification of blends of NR and IR is complicated since different IR's have different exotherm areas. 4. Natural and synthetic trans-polyisoprenes can be distinguished from one another, as well as from their cis counterparts, by their DSC and DTG curves. 5. Differences observed in the natural and synthetic polyisoprenes seem to be due to a higher amount of cyclized product, formed as a result of catalysis by the residual inorganic material in the synthetic product.

The stress birefringence of vulcanizates of polyisoprene–polyethylene blends

AbstractAs a model for the vulcanizates of block copolymers, mixtures of polyisoprene and polyethylene vulcanized both with a peroxide and with sulfur were investigated by stress birefringence. It was found that the polyethylene dispersion showed a reinforcing effect only in the peroxide vulcanizates. On the other hand, the stress birefringence was decreased with increased polyethylene content except at high polyethylene content. In the latter case, the slope of the line in the birefringence–stress plot was almost equal to that for the pure polyisoprene vulcanizate at high stress levels. However, at lower stresses significant optical creep was observed, i.e., the stress increased without bire‐fringence. Such an optical creep as this exists also in styrene–butadiene block copolymer. These facts are interpreted by the assumption that polyethylene dispersion, when it is linked chemically with polyisoprene matrix, acts as a reinforcing agent by forming physical crosslinks similar to the hard domains in block copolymers. Such physical crosslinks can slip during elongation, resulting in the observed optical creep. These phenomena disappear at the elevated temperature.

Transient Changes in Topology and Energy on Extension of Polybutadiene Networks

Abstract The theory of elasticity of polymer networks has been developed along two lines. The phenomenological approach leads to the Mooney-Rivlin relation between stress and extension ratio for uniaxial extension. The statistical theory of elasticity, based on a model for polymer molecules, predicts a similar relation with one of the constants zero. Actual elastic properties of rubbers do not agree fully with either theory. Experimental results are reported obtained with quantitatively cured polybutadiene and polyisoprene vulcanizates. These data are near-equilibrium results through the use of a cyclic stress sequence which largely eliminates the influence of long-time creep. The dependence of the initial modulus and the parameters of the Mooney-Rivlin relation on the chemical nature and the degree of branching of the polymer, the type of cross-links, and temperature has been investigated. A possible relation between the energy component of the elastic force and one of the parameters is discussed. These ...

Infrared study of the reaction of polyisoprene and polybutadiene with sulfur by use of deuterated polymers

AbstractAn infrared study of the sulfur vulcanization process was carried out with very high cis polymers of isoprene‐d8, isoprene‐3‐d1 and butadiene‐2,3‐d2 as well as the corresponding perprotio polymers. The use of deuterated polymers permitted more definitive interpretations of the spectral changes associated with vulcanization than those previously reported or were possible with the undeuterated polyisoprene or polybutadiene. It was concluded that the 10.4 μ band which develops in the spectrum of cis‐polyisoprene on extended reaction with sulfur at 140°C., and which has been the subject of some controversy, arises largely from a combination of saturated ring structures, such as cyclic sulfides, and conjugated double bonds, and to a minor extent from shifted double bonds. In addition to the processes leading to these various structures, crosslinking also occurs, but it apparently produces no important changes in the spectra of the polyisoprene vulcanizate other than that associated with a decrease in the original unsaturation. Polybutadiene, in contrast to polyisoprene, undergoes extensive cis—trans isomerization when extensively vulcanized with sulfur. Apparently, ring formation and concurrent production of conjugated double bonds also occur, but there is only a very slight amount of double‐bond shifting in polybutadiene.