Effect of carbon black on the thermal properties and flammability of cis-1,4-polyisoprene vulcanizates

The effect of carbon black with various particle sizes and specific surfaces on the thermal properties of the sulphur vulcanizates ofcis-1,4-polyisoprene was studied by means of thermal analysis. It was found that the addition of carbon black does not change the nature of the thermal processes in isoprene rubber vulcanizates essentially, but it distincly affects the positions of the peaks recorded in the DTA curves. The specific surface of carbon black affects the temperature of the first exothermic process and the temperature and rate of destruction of cross-linked polyisoprene. The presence of carbon black brings about a reduction in the flammability ofcis-1,4-polyisoprene vulcanizates. The specific surface of carbon black influences the value of the oxygen index OI.

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Zusammenfassung Mittels Thermoanalyse wurde der Einfluß von Ruß mit unterschiedlicher Partikelgröße und unterschiedlicher spezifischer Oberfläche auf die thermischen Eigenschaften von Schwefelvulkanisaten von cis-1,4-Polyisopren untersucht. Es wurde festgestellt, daß die Zugabe von Ruß die thermischen Vorgänge in Isoprengummivulkanisaten nicht wesentlich verändert, daß jedoch die Lage der in den DTA-Kurven aufgezeichneten Peaks eindeutig beeinflußt wird. Die spezifische Oberfläche von Ruß beeinflußt die Temperatur des ersten exothermen Vorganges sowie Temperatur und Geschwindigkeit von Abbau des vernetzten Polyisoprenes. Die Gegenwart von Ruß führt eine Verminderung der Entflammbarkeit voncis-1,4-Polyisopren-Vulkanisaten herbei. Die spezifische Oberfläche von Ruß beeinflußt den Wert der Sauerstoffzahl.

     

Oxidative stress relaxation and the measurement of sol fraction in rubber vulcanizates

A simple method to determine the chain scission mechanism of the oxidative degradation of rubber vulcanizates is proposed. The method involves the measurement of oxidative stress decay and the change in sol fraction, which allow us to distinguish whether scission occurs randomly along the main chain, near crosslinks or of crosslinks. The applicability of this method was well established using natural rubber vulcanizates as the reference samples. The chain scission of cis-1,4-polyisoprene vulcanizaties was proved to take place randomly along the main chain irrespective of their crosslink structure. On the other hand, the chain scission of dicumyl peroxide cured cis-1,4-polybutadiene takes place selectively near crosslinks. It is suggested that the unusual behavior of cis-1,4-polybutadiene vulcanizates is due to the characteristic structure of the crosslinks.     

Influence of cryogenic modification of silica on thermal properties and flammability of cross-linked nitrile rubber

The article presents the results of testing thermal properties and combustibility of butadieneacrylonitrile rubber with 18% contents of bounded acrylonitrile, NBR 18. Two types of silica, Zeosil 175C and Ultrasil VN-3, with different specific surfaces were used as filler. Zeosil 175C and Ultrasil VN-3 were modified via cryogenic dezaggregation method. The activity of unmodified and cryogenic modified silica toward butadiene-acrylonitrile rubber were investigated. The sulphur and peroxide vulcanizates contained 20, 30, 40, and 50 phr. of the filler were studied. The article discusses also the test results of thermal stability and flammability of NBR 18 containing silica prepared "in situ" from alkoxysilane precursor. The test results were obtained with the use of derivatograph, measurements of flammability by the method of oxygen index, and in air. The effect of the silica modification on the SEM and AFM was also examined. The method of cryogenic modification enables to achieve increase of mineral fillers activity towards elastomer and reduction in the flammability of NBR 18 vulcanizates. It has been found that the modification of the vulcanizates of NBR 18 with tetraethoxysilane that makes it possible to form silica "in situ" reduces the flammability of cross-linked rubbers.     

1,4 polymerization of isoprene by titanate catalyst systems

Titanates are versatile in the 1,4 polymerization of isoprene. The (R′O)₄Ti/RAlCl₂ catalyst gives either cis- or trans-1,4-polyisoprene, depending on the nature of both the titanate and the solvent. Primary titanates give cis-1,4-polyisoprene in both aliphatic and aromatic solvents. Secondary titanates give cis-polyisoprene in aliphatic solvents, and trans-1,4-polyisoprene in aromatic solvents. Tertiary titanates give trans-polyisoprene in both aliphatic and aromatic solvents. A mechanism is postulated which takes into consideration the role of the solvent. ESR studies of the various titanate–RAlCl₂ catalysts were made; the paramagnetic structures are related to polymerization mechanisms.     

Effects of Binding Energy of Bioinspired Sacrificial Bond on Mechanical Performance of <Emphasis Type="Italic">cis</Emphasis>-1,4-Polyisoprene with Dual-crosslink

Although bioinspired sacrificial bonds have been demonstrated to be efficient in improving the mechanical properties of polymer materials, the effect of binding energy of a specific dynamic bond on the ultimate mechanical performance of a polymer network with dual-crosslink remains unclear. In this contribution, diamine and sulfur curing package are introduced simultaneously into a sulfonated cis-1,4-polyisoprene to create dually-crosslinked cis-1,4-polyisoprene network with sulfonate-aminium ionic bonds as the sacrificial bonds. Three diamines (primary, secondary and tertiary) with the same spacer between the two nitrogen atoms are used to create the ionic bonds with different binding energies. Although the binding energy of ionic bond does not affect the glass transition temperature of cis-1,4-polyisoprene (IR), it exerts definite influences on strain-induced crystallization and mechanical performance. The capabilities of diamine in dissipating energy, promoting strain-induced crystallization and enhancing the mechanical performance are in the same order of secondary diamine > primary diamine > tertiary diamine. The variations in mechanical performances are correlated to the binding energy of the ionic bond, which is determined by pK_a values.     

Thermo-oxidative aging in surface layer of cis-1,4-polybutadiene vulcanizates

A study of the microstructural changes occurring in the surface layer of cis-1,4-polybutadiene (BR) vulcanizates during thermo-oxidative aging in the temperature range of 120–180°C was carried out by using optical microscope, SEM, MIR–IR, XPS, and swelling. The physical properties, structural changes, and interreaction between rubber and filler (clay) in the surface layer comparing with the center bulk of BR vulcanizates are observed.     

Effect of polymer structure on ease of hydrogen abstraction by cumyloxy radicals

Vulcanizates of five elastomers having different chemical structures were prepared with recipes comprising dicumyl peroxide. Cumyl alcohol/acetophenone molar ratios were determined by gas–liquid chromatography (GLC) on acetone extracts of vulcanizates. Diphenyl ether was used as the internal standard. The ease of hydrogen abstraction by cumyloxy radicals, reflected by the cumyl alcohol/acetophenone ratio, decreases in the following order: cis-1,4-polyisoprene > poly(propylene oxide) > poly(vinyl n-butyl ether) > poly-1-heptene ? EPR. This order is in qualitative agreement with the order based on calculated relative rates of hydrogen abstraction by methyl and tert-butoxy radicals and literature data on oxygen absorption of some of these polymers as a result of hydrogen abstraction by peroxy radicals.     

NMR and DSC investigations of the miscibility of blends of cis-1,4-polyisoprene with polybutadienes of different microstructures

Differential scanning calorimetry and torsional braid analysis investigations of the phase diagram of cis-1,4-polyisoprene/polybutadiene blends as a function of the polybutadiene microstructure were reported by several authors. Polybutadienes containing a high vinyl content were shown to be miscible with cis-1,4-polyisoprene on the DSC spatial scale, whereas polybutadienes containing a low vinyl content were immiscible. In this article, we used variable-temperature determinations of ¹H NMR free induction decays and low-temperature, high-resolution solid-state ¹³C NMR measurements of proton spin-lattice relaxation times in the rotating frame to probe the phase behavior of the cis-1,4-polyisoprene/polybutadiene blends at the smaller spatial scale of the NMR technique. Blends of cis-1,4-polyisoprene with a polybutadiene having a large number of vinyl 1,2 linkages appeared to be miscible on the molecular scale, in spite of small regions in which the polybutadiene component is not uniformily dispersed in the other polymer. On the contrary, blends in which the polybutadiene has a low content of vinyl 1,2 sequences were phase separated over the whole temperature range considered and no intermixed regions could be detected. The limiting case was observed with the polybutadiene containing 33 wt % vinyl 1,2 units, for which miscibility on a molecular scale is highly dependent on the blend composition. © 1995 John Wiley & Sons, Inc.     

Effects of diffusion in the oxidative degradation of vulcanized rubbers. I. Rate of chain scission in the steady state

The effects of diffusion in the oxidation of cis-1,4-polyisoprene vulcanizates were investigated by means of the stress relaxation method. It was assumed that the diffusion of oxygen is coupled with first-order oxygen consumption and that the rate of chain scission is proportional to the rate of oxygen consumption. The diffusion equation of this process was solved under the steady-state condition to give a simple relation between the rate of chain scission and the film thickness. The experimental results were in good agreement with the theoretical treatment. The true activation energy as well as the ratio of the rate of oxidation k to the diffusion constant D could be estimated.     

Diffusion effects in the oxidative degradation of vulcanized rubbers. II. Transient rate of chain scission

The effect of diffusion in the oxidative degradation of cis-1,4-polyisoprene vulcanizates was estimated by the analysis of the transient state of diffusion and oxidative degradation. An expression for the number of network chain scissions was obtained by solving the diffusion equation, which takes into account first-order oxygen consumption as well as constant scission efficiency. A practical method was proposed to analyze the transient process, which makes it possible to estimate diffusion coefficient and first-order rate constant independently. It was shown that the same analytical procedure would be applicable to the concentration jump method. The diffusion coefficient thus obtained was somewhat lower than the extrapolated value from the low-temperature data which was obtained in the absence of oxidation. Possible reasons responsible for this discrepancy were discussed.     

Preparation and modification of low-viscous polymeric composites in rotary-pulsatory apparatus in the self-oscillating mode

Preparation and modification of low-viscous polymeric composites in rotary-pulsatory apparatus operating in the self-oscillating mode are described. These are synthesis of antioxidant dispersions, preparation of artificial latexes from an isoprene rubber, modification of epoxy resins with low-molecular-weight carboxyl-containing rubbers, modification of cis-1,4-polyisoprene with a salt of n-nitrosodiphenylamine. The possibility of substantial intensification of these processes is analyzed.     

Filler-polymer interactions in filled polybutadiene compounds

Bound rubber in a filled rubber compound is formed by physical adsorption and chemisorption between the rubber and filler. Polybutadiene (PB) is composed of three components of 1,2-, cis-1,4-, and trans-1,4-units. Filler-polymer interactions in PB compounds filled with carbon black or silica were studied by analyzing microstructures of the bound rubbers with pyrolysis-gas chromatography. Differences in the filler-polymer interactions of the 1,2-, cis-1,4-, and trans-1,4-units were investigated. The filler-polymer interaction of the 1,2-unit is stronger than those of the cis-1,4- and trans-1,4-units. The interaction of the 1,2-unit with silica is stronger than with carbon black. Bound rubber content is decreased by treatment with ammonia. Change of the bound rubber composition after the ammonia treatment was also studied.     

Activity of fillers in elastomer networks of different structure

The paper deals with interactions between low molecular weight substances and silica. Activity of SiO₂ towards ethylene-propylene-diene rubber (EPDM) also depends on the kind of crosslinking system used. Calcium carbonate, the filler exhibiting low interactions with elastomers does not show any changes in activity, due to application of different crosslinking systems. In our opinion, the observed increase of the fillers activity results from the modification of elastomer chains by crosslinking agents, influence of the latter on the formation of its own network (the so called “structure”) in the elastomer matrix, and above all from nanoheterogeneous distribution of network crosslinks in vulcanizates. The non Gaussian probably bimodal distribution of crosslinks is likely to influence positively properties of elastomers. A similar effect is observed in peroxide vulcanizates of EPM produced with the use of the crosslinking coagents of polar nature. Behaviour of different kinds of carbon black in natural rubber (NR) crosslinked with dicumyl peroxide (DCP), sulphur donors or sulphur with accelerators is discussed. Reinforcing action of carbon black, particularly the active ones, showed to be more effective in sulphur than in peroxide vulcanizates.     

Radiation effects on polymers—IV: Straight radiation vulcanization of styrene-butadiene rubber mixes

Vulcanization of styrene-butadiene rubbers was carried out by gamma radiation in air. For unloaded unextracted rubbers, the dose required for optimum cross-linking was about 40 megaroentgens (MR). Elimination of foreign materials and stabilizers and also incorporation of filler affected the vulcanization. Loaded samples gave vulcanizates with best complex physico-mechanical properties for filler content of 50 (phr) and a dose of 30 MR. Evaluation of various fillers has shown that the best properties were obtained with HAF among the black fillers and silica in the case of white fillers.     

Photorearrangements of hydrocarbon polymers with pendant double bonds

A study was made of the microstructural changes that occur in ultraviolet irradiation under vacuum of thin films of 1,2-poly(cis-1,4-hexadiene) (CHD), 1,2-poly(trans-1,4-hexadiene) (THD), 1,2-poly(trans-1,3-pentadiene) (TPD), equibinary (1,2,-1,4) polybutadiene (EB), and equibinary (3,4-1,4) polyisoprene (EI). These polymers—all containing pendant double bonds—undergo important photoinduced loss of unsaturation, presumably through cyclization of the double bonds, by analogy to the previously reported photocyclization of 1,2-polybutadiene (VB) and 3,4-polyisoprene (VI)films. For the equibinary polymers, which contain internal as well as external (or pendant) double bonds, the loss of unsaturation is considered to involve photocyclization of 1,2-1,4 and 1,2-1,2 dyads in EB and of 3,4-1,4 and 3,4-3,4 dyads in EI. Accompanying thecyclization process in CHD, THD, and TPD is a direct photochemical cis-trans isomerization of ? CH?CH? double bonds analogous to that originally noted for 1,4-polybutadiene. The photorearrangements in the above polymers with pendant double bonds were compared to the corresponding thermally induced rearrangements reported previoulsy;for VB and VI, in particular, the thermal, photo-and radiation-induced cycli-zations were found to be very similar, possibly having a common nonradical, nonionic mechanism involving excited double bonds.     

Activating Mixing in Modification of Rubbers in Solution

The applicability of the previously developed principles of activating mixing to modification of rubbers in solutions was tested, and a process was developed for mechanochemical modification of cis-1,4-polyisoprene in rotary pulsation apparatus.     

Effect of metal with variable valence on the thermal properties of polybutadiene

The effect of mixed synthetic saturated acids and of cobalt salt of these acids on the thermal properties of peroxide vulcanizates ofcis-1,4-polybutadiene, Europrene cis, has been studied by derivatography.The thermal curves of the samples under investigation have been interpreted on the basis of complementary examinations. It has been found that the introduction of a metal with variable valence into the rubber compound alters the optimum vulcanization time and brings about degradation of the elastomer during its crosslinking.The presence of the cobalt salt facilitates the crosslinking of polybutadiene heated to the temperature of the second exothermic transition, resulting at the same time in an increase in the thermal decomposition rate and a decrease in the temperature of this process.

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Zusammenfassung Mittels Derivatographie wurde der Einfluß von synthetischen gesättigten Säuregemischen und der Kobaltsalze dieser Säuren auf die thermischen Eigenschaften von peroxidvulkanisiertemcis-1,4-Polybutadien (Europrencis) untersucht.Die thermischen Kurven der untersuchten Proben wurden auf der Basis ergänzender Betrachtungen interpretiert. Man fand, da die Einbringung eines Metalles mit verschiedenen Bindigkeiten in die Kautschukmischung die optimale Vulkanisierungsheit erhöht und eine Zerlegung des Elastomers während der Vernetzung verursacht.Die Gegenwart von Kobaltsalzen erleichtert bei der Temperatur der zweiten exothermen Umwandlung die Vernetzung von Polybutadien und hat gleichzeitig eine gesteigerte thermische Zersetzungsgeschwindigkeit sowie eine Abnahme der Temperatur dieses Prozesses zur Folge.

     

Recent progress in bio-based elastomers with intrinsic self-healing mechanisms - part I: Natural rubber modifications

This review is focused on recent achievements in bio-based self-healable elastomeric materials with special regard to elastomers made of natural rubber (NR) and its modified forms, including epoxidized natural rubber (ENR), oxidized natural rubber (oNR) or carbonated natural rubber (CNR). Besides natural rubber, also Eucommia ulmoides gum (EUG), which is an isomer of cis-1,4-polyisoprene, is a material of great interest to obtain bio-based self-healable elastomers. The paper attempts to describe the main classification of the most important intrinsic self-repairing mechanisms, including different types of reversible non-covalent interactions (such as hydrogen bonding, ionic interactions, metal–ligand coordination, shape-memory ability), dynamic covalent bonds (for instance Diels-Alder reversible bonds, dynamic sulfur-sulfur bonds, dynamic ester bonds, boronic-ester exchangeable bonds, dynamic imine bonds, silyl ether exchangeable bonds) and their combinations, which are found in self-healing elastomers. Furthermore, examples of NR-based elastomeric materials are provided and the potential applications proposed by researchers are also presented.     

Thermal Properties of Cis-1,4-Poly(butadiene)

Thermal properties of cis-1,4-poly(butadiene), Europrene cis, were investigated by means of thermal analysis and complementary methods. Thermal analysis of polymer was carried out both in air and inert atmosphere with a derivatgraph, DSC and internal TG-FTIR coupling system as well as internal TG, DTA-MS coupling system. It was found that investigations in air atmosphere the method of the sample preparation ofcis-1,4-poly(butadiene) influences the results of thermal analysis, which is connected with the rate oxygen diffusion into the reaction zone. Taking into consideration both the method of the sample preparing and atmosphere of thermal studies, the values of activation energy of destruction of cis-1,4-poly(butadiene) were determined. Using TG-FTIR and TG-MS methods, some products of thermal destruction of elastomer were determined.This revised version was published online in November 2005 with corrections to the Cover Date.     

Improved expression of the mean-square radius of gyration, 3. 1,4-polydienes

The mean-square radii of gyration of cis- and trans-1,4-polybutadiene and corresponding 1,4-polyisoprene were derived using Abe's and Flory's rotational isomeric scheme. Calculations performed using available experimental data showed that the dependence of the mean-square radius of gyration on the molecular weight for these stereoregular polydienes can be expressed as where a and b are constants characteristic of the polymer. This behaviour is analogous to that of vinyl polymers and poly-1,1-disubstituted ethylenes discussed in the preceding papers of the present series. The mean-square radius of gyration of 1,4-polyisoprene with predominantly trans units is more sensitive to stereoirregularity than with predominantly cis units. However, 1,4-polybutadiene chains are comparatively insensitive to the stereochemical composition.