Effect of mineral fillers on thermal properties and flammability ofcis-1,4-polyisoprene vulcanizates

The effect of mineral fillers such as Ultrasil VN-3, Ze?O?Sil P-45, Aerosil 200, Frantex 8, Kaolin and precipitated calcium carbonate on the thermal properties sulphur vulcanizates ofcis-1,4-polyisoprene were studied by means of thermal analysis. It was found that the addition of a mineral filler did not change the nature of the thermal processes in isoprene vulcanizates essentially, but it distinctly affected the positions of the peaks recorded in the DTA curves. The mineral fillers affected the temperatures and rates of degradation and destruction of polyisoprene cross-linked with sulphur. The presence of a mineral filler brought about a decrease in the flammability ofcis-1,4-polyisoprene sulphur vulcanizates.     

The effect of the network structure of the thermal properties ofcis-1,4-polyisoprene vulcanizates

The thermal properties ofcis-1,4-polyisoprene and its vulcanizates were investigated by means of thermal analysis and complementary methods. It was found that cross-linking generally increases the thermal stability of this elastomer, though distinct differences between peroxide and sulphur vulcanizates were observed. Polysulphide cross-links cleave readily at elevated temperature, and transform to cross-links of lower sulphidity, mainly disulphide. However, some of them are probably oxidized first to sulphoxides, and further to sulphenic or thiosulphoxylic acids. The products formed involve the ionic decomposition of the peroxides and thereby prevent the autocatalytic thermooxidative destruction ofcis-1,4-polyisoprene.

---

Zusammenfassung Es wurden die thermischen Eigenschaften voncis-1,4-Polyisopren und dessen Vulkanisaten untersucht. Dabei wurden die Derivatographie und komplementäre analytische Methoden benutzt. Es wurde festgestellt, dass die Vernetzung im allgemeinen die thermische Stabilität von diesen Elastomeren erhöht, obwohl ein ausgeprägter Unterschied zwischen Peroxid- und Schwefelvulkanisaten beobachtet wurde. Die polysulfidischen Brücken wurden bei erhöhter Temperatur leicht gespalten und verwandeln sich in Brücken, die eine niedrigere Schwefelatomzahl besitzen und zwar hauptsächlich disulfidisch sind. Ein Teil der Schwefelatome wurde jedoch wahrscheínlich unter Bildung von Sulfin- und Thio-Sulfoxysäuren oxydiert. Diese Produkte verursachen ionische Zersetzung von Peroxiden und verhindern auf diese Weise die autokatalytische thermooxidative Zersetzung voncis-1,4-Polyisopren.

---

-1,4- . , , , , . , , , . , , , . , -1,4 .

     

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.

---

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.

     

Crystallization of trans-1,4-polyisoprene

Crystals of fractionated trans-1,4-polyisoprene (TPI) were grown from amyl acetate solution at two weight fractions, 5.7 × 10^?4 and 0.011; for the lower concentration a precooling followed by heating and then crystallization at temperatures in the 10–32°C range was used, while for the higher concentration this method and direct crystallization at a temperature T_C in the 0–32°C range were employed. The precooling method yielded samples crystallized in the α form, while direct crystallization led to formation of β-TPI at low T_C and α at higher T_C. The value for the DSC endotherm, characteristic of α-form melting, increased with increasing T_C, with a shift to lower values with increasing concentration for precooled samples. A β to α transformation was found to occur for synthetic unfractionated TPI when swollen with amyl acetate at 35°C for 17h. Swelling in n-butyl acetate for one day at 25°C or 17 h at 35°C also led to this transformation. From experimental results 74°C is chosen as the temperature at which the α and β forms coexist in the bulk, and this is used to calculate the enthalpy of fusion of β-TPI, yielding a value of 8.6 kJ mol^?1.     

Crystallization of segmented trans-1,4-polyisoprene/epoxidized trans-1,4-polyisoprene block copolymers from solution

The crystallization of segmented block copolymers of trans-1,4-polyisoprene (TPI)/expoxidized TPI from solution was investigated. One preparation with an average TPI block length of 14.5 and an average epoxidized TPI block length of 10 was crystallized from 2-pentanol at 20°C, 2-octanol at 20°C, and 2-pentanone at 0°C. A second preparation with an average TPI block length of 18 and the same average epoxidized TPI block length was crystallized from 2-octanol at 20°C and 2-pentanone at 0°C. The crystallization products were expoxidized in suspension a second time and then characterized by carbon-13 solution NMR to determine the average noncrystalline traverse length and the average crystalline stem length. The crystallized products were also studied by scanning electron microscopy, Fourier transform infrared spectroscopy, and differential scanning calorimetry to determine the morphology, the crystal form and crystallinity, and the melting point, respectively. The average noncrystalline traverse length was found to be highly dependent on the crystallization conditions and the crystalline stem length of the parent TPI lamellas; the composition of these traverses is discussed.     

Properties of crystallized trans 1,4-polyisoprene

Stirrer crystallization of a trans-1,4-polyisoprene fraction(M?_n = 3 × 10⁵) was carried out from n-butyl acetate and from n-heptane solutions (2% w/v). Fibrous crystals in the ß form were obtained at temperatures of 46-48°C in the two solvents, respectively. At 36-46°C from n-butyl acetate and 25-35°C from n-heptane lamellar crystallization took place leading predominantly to the α form. Melting endotherms and densities for various samples were obtained. The maximum TENDO for α was 74°C and for ß 79°C and the maximum weight fraction crystallinity for ß was 0.78. The stability of performed α nuclei in n-butyl acetate and n-heptane using a fraction with M?_n = 2.5 × 10⁵ was monitored as a function of temperature. The dissolution temperature of fibrous ß-TPI with the maximum TENDO was measured in 13 liquids and the results analyzed in terms of the Flory-Huggins parameter. The heats of fusion for the α and ß forms, obtained by extrapolation of heat of fusion versus change in specific volume, were found to be 8.0 and 10 kJ mol^?1, respectively. The pressure coefficients of the melting temperature were calculated to be 38 and 43 K kbar^?1 and the fold-surface free energies recalculated to be 42 ± 1 and 53 ± 1 erg cm^?2 for the α and ß forms, respectively. The number of monomer units per average noncrystalline chain traverse for the most crystalline fibrous ß-TPI was estimated at 36.     

Dilute-solution properties of trans-1,4-polyisoprene

Trans-1,4-polyisoprene was fractionated by both fractional precipitation and preparative gel permeation chromatography to obtain possibly sharp fractions of narrow molecular weight distribution. Selected fractions were characterized by light scattering, viscosity, and gel permeation chromatography. Necessary corrections for molecular heterogeneity were applied. Some of the characteristic relations between [η] and M _w are [η] = 1.81 × 10^?4 M in benzene at 30°C, [η] = 1.38 × 10^?4 M in n-hexane at 30°C, which are found to be in good agreement with literature data when corrected for molecular heterogeneity.     

Cyclization of synthetic cis-1,4-polyisoprene

Cyclization of synthetic cis-1,4-polyisoprene was carried out by using TiCl₄ and p-toluenesulfonic acid catalyst. The average cyclicity and structure of the polymers cyclized to a different degree was studied by infrared and high-temperature NMR spectroscopy. The methods of synthesizing an average mono-, bi-, tri-, or polycyclic polymer that could be distinguished by high-temperature NMR spectroscopy and followed by gel-permeation chromatography are discussed.     

Morphology of solution crystallized trans-1,4-polyisoprene

The film formation method has been successfully used to grow single crystals and other complex morphological features of low melting form (LMF) and high melting form (HMF) of trans-1,4-polyisoprene (TPIP). Below 40 °C dilute amyl acetate solution gave hexagonal shaped LMF crystals. Thick and elongated hexagonal shaped morphology was shown by HMF crystals at temperatures above 40 °C. Straight faces and sharp corners of the single crystals, and also of complex crystals, acquired round shapes when highly polydispersed TPIP was used for crystallization.     

FTIR study of gamma-irradiated cis-1,4-polyisoprene

The effect of γ-irradiation on the structure and oxidation of cis-1,4-polyisoprene is investigated by ATR-FTIR technique. This method provides the valuable insight into the type of oxidation products produced and the extent and nature of intramolecular cyclization and chain scission reactions. The formation of ketones, alcohols and/or ethers, and hydroperoxides is apparent already at small doses of γ-radiation and it increases with the exposure time significantly. At the highest dose of 309 kGy a decrease in the intensity of C=O stretching mode of ketones (1717 cm⁻¹) was observed while the overall area of the band remained the same as in the case of 188 kGy dose. The shoulders observed at 1740 cm⁻¹ and 1772 cm⁻¹ could be assigned to C=O stretching frequency of esters and five-membered-ring lactones, respectively. Higher doses of γ-radiation also cause the formation of two relatively strong bands in the region of conjugated double bonds. These could origin from the aromatic products or cycloenes with one double bond formed by cyclization and chain scission processes.     

Elastic properties and network structure in silicone gum vulcanizates

A model of a rubber network formed from finite, linear polymer chains is treated, using expressions from accepted elastic theory as well as relationships that have been less generally used or only implied. The resultant empirical mathematical expressions conform very closely to the predictions of the simple kinetic theory of rubber elasticity and are mathematically consistent, in that the major properties and measurements are interrelated, and apparent inconsistencies in the literature are reconciled. As a consequence, an empirical equation is developed for the relationship between elastic properties and network structure in silicone gum vulcanizates. This equation is then applied to the estimation of peroxide crosslinking efficiencies.     

Effect of molecular weight on the growth rates of low melting spherulites of trans-1,4-polyisoprene

Growth rates of G of low-melting spherulites in fractions of trans-1,4-polyisoprene have been measured. The data were analyzed by use of an equation, ln G = ln G₀ ? ΔF^*/RT_c, valid at temperatures close to the equilibrium melting point. Plots of ln G against a function of the critical free energy of nucleation ΔF^* result in a family of straight lines having a common intercept, ln G₀, which is independent of molecular weight. The slope of these lines is a measure of the interfacial free energy of the crystallites and increases with the molecular weight, reflecting increasing irregularity in the structure of the semicrystalline mass. Comparison of growth rates of low-melting and high-melting trans-1,4-polyisoprene indicates that G₀ does not, to a first approximation, depend on the nature of the crystals growing from the melt. The temperature at which spherulites of the two crystalline forms grow at equal rates has been calculated.     

Study of crystallization kinetics of trans-1,4-polyisoprene

The concentrations and the growth rates of high- and low-melting type spherulites of trans-1,4-polyisoprene were measured in the temperature range 39–49°C. It was shown that above about 40°C., the crystallization rate of trans-1,4-polyisoprene is determined primarily by the radial growth rate of high-melting form (HMF) spherulites, whereas the predominance of the low-melting form (LMF) crystals below 40°C. can be attributed to the high rate of formation of LMF primary nuclei at lower crystallization temperatures. Temperature-independent rate parameters were calculated from optical and dilatometric measurements and were found to be in good agreement. Both the change in nucleation habit and spherulite growth rate with temperature can be explained on the basis of a lower end surface free energy of LMF crystals of trans-1,4-polyisoprene compared to that of the HMF crystals.     

Synthesis of N,N-diethyldithiocarbamate functionalized 1,4-polyisoprene, from natural rubber and synthetic 1,4-polyisoprene

N,N-Diethyldithiocarbamate functionnalized 1,4-polyisoprenes were prepared from 1,4-polyisoprenes (natural or synthetic). The syntheses were performed by nucleophilic addition of N,N-diethyldithiocarbamate salts upon oxirane rings of epoxidized units according to a SN2 mechanism with ring opening. Studies on model molecules of epoxidized 1,4-polyisoprene units (1,2-epoxy-1-methylcyclohexane and 4,5-epoxy-4-methyloctane) were previously achieved to develop the procedure. The best yields were obtained at low temperature in polar medium, and more especially in water with sodium N,N-diethyldithiocarbamate (DEDT-Na) as reagent. A diastereospecific addition was noted when reaction was performed in water with DEDT-Na. Afterwards, the developed procedure was successfully generalized to epoxidized synthetic polyisoprenes and epoxidized natural rubber (in THF, then in latex medium). Excellent results were obtained in latex medium with epoxidized natural rubber (ENR) latices. As with the models, a diastereospecific addition of sodium N,N-diethyldithiocarbamate trihydrate onto epoxidized 1,4-polyisoprene units of ENR was observed at the condition to bring the latex medium to pH 8 before introduction of DEDT-Na. Influence of temperature, drc, and DEDT-Na concentration were successively examined to determine the best conditions of the addition on ENR latices.     

13C NMR spectroscopic study of epoxidized 1,4-polyisoprene and 1,4-polybutadiene

Partly epoxidized cis- and trans-1,4-polyisoprenes and cis-and trans-1,4-polybutadienes were prepared, and their ¹³C NMR spectra examined. All the prominent resonances in the spectra of the epoxidized polymers were assigned by using lanthanide shift reagent and off-resonance decoupling experiments. A ¹³C NMR method of quantitative assessment of the epoxide content was developed following determination of relative spin-lattice relaxation time (T₁) and nuclear Overhauser effect (NOE) parameters of the various carbons in the epoxidized polyisoprenes and polybutadienes.     

Effect of the structure of butadiene-nitrile elastomers on the ozone resistance of their vulcanizates

A comparative analysis of the ozone resistance of vulcanizates based on butadiene-nitrile elastomers with the same network density but different contents of acrylonitrile units is performed. The role of the orientation of chain fragments of vulcanizate chains during the reaction of double bonds with ozone is demonstrated.     

DSC and FTIR study of the gamma radiation effect on cis-1,4-polyisoprene

The effect of γ-radiation on the cis-1,4-polyisoprene in the presence of oxygen is investigated by ATR-FTIR technique and non-isothermal DSC measurements. FTIR measurements have shown that the formation of hydroperoxides, ketones, alcohols and/or ethers is apparent already at lower, 20–50 kGy, doses of γ-radiation and it increases significantly with the exposure time. Besides, lactones, anhydrides, peresters, carboxylic acids, and esters are formed, too. Spectral changes in the region of C=C conjugated double bonds indicate a formation of shorter polyene structures and aromatic rings. Kinetic parameters describing the temperature dependence of the induction period have been obtained from DSC measurements using the isoconversional method. Residual stabilities have been calculated in order to characterize the gamma radiation effect on polyisoprene thermooxidative stability. Both methods proved that doses lower than 50 kGy do not cause severe changes in polymer properties.     

Sorption and addition of bromine from aqueous solutions on cis-1,4-polyisoprene

The sorption of bromine from bromine water on cis-1,4-polyisoprene film and the initial stages of bromination are studied at concentrations of 0.002–0.1 mol/l and temperatures of 15–35°C. The diffusion coefficient of bromine into natural rubber (NR) is 1.3–2.0 × 10^?6 cm²/sec for the total sorption and 5–13 × 10^?7 cm²/sec for the irreversible sorption. The partition coefficient of bromine between water and rubber increased from 17.3 at 15°C to 37.1 l/kg at 35°C. The chemical potential, enthalpy and change in entropy of partition are, at 25°C, respectively: ?1.9 kcal/mol, 6.6 kcal/mol and 28.4 cal/mol. K. The irreversible sorption is due to a charge-transfer complex between bromine molecules and double bonds of the rubber. The complex is the first stage of the addition reaction, which becomes noticeable at concentrations above 0.012 mol/l. With increasing bromine concentration the concentration of the complex decreases and the added bromine increases. The charge transfer complex appears to change the conformation of the cis-NR chains so that the bromine addition occurs in the trans-conformation, as shown by FT–IR spectra. The bromination is accompanied by a marked crystallization effect as illustrated by thermal analysis and WAXS measurements.