COMPATIBILITY STUDY OF NBR/PVC BLEND WITH GASOLINES AND ETHANOL FUEL
Abstract
An NBR/polyvinyl chloride (PVC) blend changed by aging (20 weeks in different fuels at room temperature) was evaluated for weight, hardness, mechanical properties, and microstructure. The exposure tests were made with premium gasoline, regular gasoline, regular gasoline doped with a rubber solvent, and an oxygenated renewable biofuel (ethanol fuel). After the aging tests, all NBR/PVC blend samples increased in both hardness and elastic modulus, whereas both elongation at break and tension at break decreased, but in different proportions. As the NBR/PVC blends aged, they became less elastomeric and more rigid. The regular gasoline doped with a rubber solvent was the most aggressive of the fuels tested because it promoted the extraction of a large quantity of the blend constituents, thus making the blend harder. In general, NBR/PVC samples immersed in the fuels showed similar mechanical behaviors, except in the case of immersion in ethanol. The values of parameters τ3 and I3 were obtained by positron annihilation lifetime spectroscopy. The changes in the mechanical properties and the reduction of the values of parameters τ3 and I3 were related to extraction of the plasticizer, which was confirmed by thermogravimetric analysis.
INTRODUCTION
NBR/PVC (butadiene–acrylonitrile copolymer/polyvinyl chloride) is a miscible, physical mixture of commercial importance.1 The NBR/PVC blend is perhaps the oldest commercial blend, having been introduced about 60 years ago.2 Notably, NBR acts as a permanent plasticizer for PVC in applications such as wire and cable insulation, in which PVC improves the chemical resistance, thermal aging, and abrasion resistance of NBR.3 This blend is commonly used by the automotive industry in applications such as fuel tanks, fuel filters, and other compartments in the fuel-induction system.4
The knowledge regarding how the NBR/PVC blend will perform in the presence of automotive fuel is important because if the fuel attacks an elastomeric material, the performance of the material in service will be adversely affected. Changes in the fuel composition and in special gasoline, as well as the introduction of biofuels (alcohol and biodiesel), often create many problems in the elastomeric components of the fuel system. Currently, studies about fuel and polymer compatibility are very important because of the different fuel blends being used to avoid environmental problems and to reduce petroleum consumption.
Gasoline is a petroleum-derived product, constituted by a complex mixture of liquid aliphatic and aromatic hydrocarbons (ranging from C4 to C12 carbon atoms), with a boiling point ranging from 30 to 220 °C.5 A typical gasoline is predominantly a mixture of paraffins (alkanes), naphthenes (cycloalkanes), aromatics, and olefins (alkenes), which can also contain some additives (e.g., aliphatic alcohols and methyl ethers to improve its octane number).6
Brazilian regular gasoline (gasoline C) is a mixture of anhydrous ethanol and gasoline with the proportion of anhydrous ethanol ranging from 20 to 25% (v/v). The content of ethanol added to Brazilian gasoline is defined and regulated by the Ministry of Agriculture and Ministry of Mines and Energy of Brazil.7
In Brazil, hydrous ethanol (ethanol fuel) is also widely used as a fuel in cars, mainly as a biofuel alternative to gasoline. The ethanol (92.5 to 93.8% w/w) is used pure or in various proportions in the mixtures with regular gasoline in flexible-fuel vehicles.7 Brazil has the largest and most successful biofuel program in the world, involving the production of ethanol from sugar cane, and it has the world's first sustainable biofuel economy.8
Cross-linked polymers brought into contact with different solvents during service applications usually swell. Depending on the degree of polymer–solvent interaction, the swelling process can reduce the modulus and tensile strength of the polymer and leach additives from it.9 Several works have been published regarding the compatibility of polymers with fuels.
Abu-Isa10 investigated the effect of methanol–gasoline mixtures on the tensile elongation, tensile strength, hardness, and volume change of several elastomers, exposing the fuels for 72 hours at room temperature. The properties of most fuel-resistant elastomers were degraded to a larger extent by mixtures with methanol and gasoline than they were by the pure components. The results obtained on all elastomers, except fluorocarbons, were explained in terms of the solubility parameter. The same researcher in another study11 reported the effects of ethanol/gasoline and methyl-t-butyl ether/gasoline mixtures on the swelling and tensile properties of elastomers and compared with those of methanol/gasoline mixtures. According to the results, the elastomers were more severely affected by the ethanol/gasoline mixture than they were by the pure components. The presence of a higher aromatic content (30% and 50%) in the gasoline resulted in increased swelling and, hence, deterioration of the tensile properties of the elastomers exposed to the gasoline and its mixtures. The ethanol/gasoline mixtures were less severe that the methanol/gasoline mixtures in their effects on most fuel-resistant elastomers.
Recently, Berlanga-Labari et al.12 evaluated the changes in the mechanical and physicochemical properties of high-density polyethylene (HDPE) before and after immersion tests in bioethanol–gasoline blends. Although a slight variation in mechanical properties was observed after exposure to fuels, no effects on the chemical structure and physical properties of that polymer were detected.
The objective of this work was to study the changes in weight, hardness, mechanical properties, and microstructure of NBR/PVC blends that occur after prolonged exposure to petroleum-based fuels (gasolines) and oxygenated, renewable biofuel (ethanol).
EXPERIMENTAL
MATERIALS
NBR/PVC Blend
NBR/PVC blend samples were kindly provided by Produflex Minas Ind. Borrachas LTDA (Minas Gerais, Brazil). Table I shows the rough composition of the blend that was composed of 30% PVC and 70% NBR; note that the NBR contains 33% acrylonitrile. The test specimens (50 mm × 4 mm × 2 mm) of the NBR/PVC blend were prepared for the fuel exposures according to ASTM Standard D 412.13
Fuels Used in the Aging Tests
Regular gasoline, premium gasoline (high-octane gasoline), and regular gasoline doped with 20% rubber solvent were used in the aging tests of the NBR/PVC blend. All gasolines have anhydrous ethanol in their composition (about 25% v/v). Rubber solvent is composed of a mixture rich in aliphatic and naphthenic hydrocarbons with a low content of aromatics. The addition of rubber solvent is one of the most common adulteration practices in Brazil because of the difference in taxation for gasoline versus solvent. It is, however, a criminal practice, which has been curtailed by government action on the quality control of fuels. This kind of adulteration causes environmental pollution, poor engine performance, and tax revenue losses.14 Another fuel tested was ethanol fuel, which is a biorenewable oxygenated fuel. The physicochemical properties of the gasolines and the ethanol fuel studied are presented in the Tables II and III, respectively. All the fuels were purchased at a gasoline station; the rubber solvent was supplied by Petrobrás (Petróleo Brasileiro S/A, Minas Gerais, Brazil).
AGING TESTS—SORPTION AND DESORPTION PROCESSES
Ten test specimens were completely immersed in a beaker containing the specific fuel to be tested. Once a week, five specimens were randomly chosen, removed from the beaker, dried, weighed, and put back into the beaker. Subsequently, the beaker was filled with fuel and covered with aluminum foil. This procedure was followed separately with the four types of fuels studied. The swelling test was carried out according to ASTM Standard D 471,15 at room temperature (23 ± 2 °C), for a period of 20 weeks (approximately 5 month). That period was a strict time limit; a longer period would allow for the formation of a high content of gum because of the oxidation of the gasoline, thus rendering it unsuitable for use.5
To evaluate the actual amount of fuel absorbed by the immersed samples and to obtain the required amount of extractables, five test specimens from each system were submitted to a desorption process, in an oven (FANEM, model Orion 515; São Paulo, Brazil) at 110 °C, for 10 hours and at a reduced pressure to dry the test specimens and evaporate the fuel completely. For further study, five specimens of NBR/PVC blend were subjected to thermal treatment in the oven at 110 °C for 10 hours to evaluate the effect of temperature during the drying of samples.
Several codes (Table IV) systematically connecting the NBR/PVC blend with the fuel used for immersion were created to simplify the identification of samples.
MECHANICAL PROPERTIES
The changes in the mechanical properties of the NBR/PVC blend samples after exposure to fuels and thermal treatment were carried out with an EMIC model DL3000 (São Paulo, Brazil) equipment, in accordance with ASTM Standard D 412,13 at room temperature and a loading cell of 30 kN. The specimens were tested at a rate of 500 mm/min. A digital durometer (PCWI Precision Instruments mod TH-200) was used to measure hardness with a Shore A scale, following ASTM Standard D 2240.16 The hardness values of the samples were determined at five different locations on the samples' surfaces, each measurement lasting approximately 5 s at each location; average results were recorded.
THERMAL ANALYSIS
Thermal treatments were performed by determining the thermogravimetry (TG) of the NBR/PVC blend samples before and after the aging tests. The TG and differential thermal analysis (DTG) curves were obtained using a Shimadzu (Kyoto, Japan) TGA-50H thermobalance in a nitrogen dynamic atmosphere (50 mL/min), at a heating rate of 10 °C/min, with approximately 5 mg of mass. The samples were submitted to gradual heating of the ambient temperature up to about 750–800 °C. Pyrolysis of the pure components of the NBR/PVC blend, such as NBR, NBR–PVC mixture, dioctyl phthalate (DOP) and PVC, were performed in a thermobalance (model 204, NETZSCH, Selb, Germany). Unaged and aged samples of the NBR/PVC blend were subjected to the same experimental conditions.
POSITRON ANNIHILATION LIFETIME SPECTROSCOPY
The positron annihilation lifetime spectroscopy (PALS) measurements were performed at (21 ± 1 °C) using a conventional fast–fast coincidence system (ORTEC, Atlanta, GA, USA), with a time resolution of 280 ps as given by the 60Co prompt curve. The 22Na (formerly Amersham, now GE Healthcare, Piscataway Township, NJ, USA) positron source, with approximately 15 μCi of activity, was sandwiched between two thick foils of Kapton. The lifetime spectra (minimum of three spectra for each sample) were satisfactorily resolved into three components by the Positronfit-extended program,17 leading to the relative intensities Ii and lifetimes τi, where the i = 1, 2 and 3 refer to p-Ps, e+, and o-Ps, respectively: positronium (Ps), the bound state between a positron (e+) and an electron (e−), can be formed with two different spin states: para-positronium (p-Ps, singlet state) and ortho-positronium (o-Ps, triplet state), with intrinsic lifetimes of 0.125 and 142 ns, respectively, in vacuum. The long o-Ps lifetime decreases to a few nanoseconds in a material medium because of pick-off annihilation of the positron by the surrounding electrons.18 According to the free-volume model,19,20 τ3 is related to the free-volume radius of materials, whereas I3 is related to the relative free-volume hole concentration.
The microstructural study of the NBR/PVC samples, before and after the aging tests, was carried out by analyzing parameters τ3 (lifetime) and I3 (relative intensity).
SCANNING ELECTRON MICROSCOPY
The NBR/PVC blends samples, before and after the aging tests, were fractured after immersion in liquid nitrogen, and the fractured surfaces were sputtered with atomized gold using an Edwards Scancoat Six. The morphological analyses were carried out using a scanning electron microscope (JEOL JSM-840A; Tokyo, Japan) with an acceleration voltage of 25 kV.
RESULTS AND DISCUSSION
The effect of immersion time of the NBR/PVC blend in different fuels (Table II) is shown in Figure 1. Initially, the regular gasoline (NRG), gasoline doped with rubber solvent (NRSG), and the premium gasoline (NPG; Table IV) systems gained mass, corresponding to the fuel diffusion (sorption or swelling) in the polymeric matrix as well as in the relaxation of polymer chains, with an increase in weight of 9, 7, and 11%, respectively. The increase in weight (swelling) can be attributed to the extent of higher liquid absorption when compared with the extraction of soluble components from the blend. The ethanol fuel exposure test showed different results from the others: instead of a weight increase, the blend had a weight reduction of approximately 11%, compared with its original weight. This effect was attributed to the absorption of ethanol fuel by the blend, which caused the extraction of soluble components, such as plasticizer, stabilizers, or additives, from the blend. The extraction is a consequence of the polarity of the ethanol molecule. Similar effect was observed for Hasseb et al.21,22 in studies on the compatibility of different elastomers in the presence of biodiesel (methyl esters).
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88974
After the sorption tests (swelling), NBR/PVC blend specimens were dried in an oven at 110 °C, to remove the fuel completely, until a constant weight was obtained. Figure 2 shows the desorption curves obtained for these samples. The weight gain of the samples immersed in fuels, shown in Figure 1, proved to be only “apparent” values because the extraction of the blend components was not taken into consideration during the swelling process. According to Figure 2, this fact is real because, after the drying step, the final weights did not coincide with the initial ones for all samples. Negative values for weight loss indicate the partial extraction of the components of the blend. Therefore, the “real” weight gain was considered to be the amount of fuel absorbed during the immersion tests, which is equal to the mass of the fuel evaporated during the desorption tests (Figure 2). The results listed in the Table V show that, compared with the other gasoline samples studied (NRG and NPG), the gasoline doped with the rubber solvent was more aggressive to the blend (NRSG system), causing greater solubilization (20%). Premium gasoline (the NPG system) caused a similar effect to that observed for regular gasoline (the NRG system) and for ethanol fuel, with about 14% of the blend components extracted. As consequence of its polar character, the ethanol did not promote the swelling of the blend, caused only insignificant fuel absorption (about 3%).
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88974
The changes in the mechanical properties of the NBR/PVC blend after thermal treatment are illustrated in Figures 3a–d. The mechanical properties of the NBR/PVC blend were directly affected by temperature as compared with unaged samples (NU). Thermally aged samples of NBR/PVC blend (NTT) showed an increase in tension at the break and a decrease in elongation at the break; the hardness increased as well. According to Manoj,23 this effect is due to the degradative cross-linking reactions in PVC. However, the difference in elastic modulus was not significant.
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88974
Figure 3a,b presents the elongation at break of the NBR/PVC blend samples, before and after the aging tests in fuels, as well as the relative hardness. After undergoing aging tests at room temperature, all samples presented an increase in the hardness and a decrease in the elongation at break. However, no significant changes were found in the fuels tested.
The results shown in the Figure 3c–d supports the conclusion that the NBR/PVC blend, when exposed to regular gasoline (NRG), presents the highest values of elastic modulus, whereas the opposite trend was observed for its tension at break compared with the unaged samples (NU).
In general, all fuels tested caused significant changes in the mechanical properties of the NBR/PVC blend after 5 months of exposure; the blend was ultimately less elastomeric and more rigid. These changes occurred because of the thermal effect that promoted cross-link reactions, besides the DOP plasticizer extraction by the fuel. Thus, the blends became more rigid because of the increase of the cross-link and the reduction in DOP content. In a recent publication,21 it was shown that the hardness of the NBR decreased slightly upon exposure to palm biodiesel. Normally, biodiesel increases the lubricity of the fuels, and most likely, it acted as a plasticizer. However, the effect in our study was different because the ethanol acted as an efficient solvent for DOP plasticizer.
The thermal degradation of the NBR/PVC blend after aging in fuels was studied to evaluate its thermal stability and to find evidence for the extraction of rubber components during the immersion tests. Figure 4 shows the mass loss during the heat treatment of the NBR/PVC blend, before and after exposure to fuels. The TG curves (Figure 4) of the aged samples, regardless of the fuel in which the sample was immersed, indicate an increase in the thermal stability as compared with the untreated sample (NU). In all NBR/PVC samples (unaged and aged), the first mass loss occurred between 200 and 300 °C, and it can be attributed to the DOP plasticizer or to other low-temperature, volatile components in the rubber.24 The temperature range relative to the thermal degradation of the pure DOP plasticizer and the partial degradation of the rubber can be confirmed by the TG curves shown in Figure 5. During the first event of the thermal degradation of aged samples, the mass loss was less than that of the untreated sample. This suggests that during the immersion tests part of the plasticizer and the small rubber chains with low average molecular weights were extracted from the rubber for fuel. The second event of the thermal decomposition of the aged samples (Figure 4), in the temperature range of 300–600 °C, corresponds to the degradation of the components of the NBR copolymer, as well as the partial degradation of the PVC.24 This result is confirmed by the TG curves for pure PVC and NBR copolymer illustrated in the Figure 5.
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88974
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88974
The experimental results, obtained using the PALS technique, are listed in Table VI. As can be seen, after submitting samples to the aging test, all four systems studied showed a decrease in the value of the I3 parameter, going from 11.7 ± 0.3% in the untreated sample to about 8.0% following the aging test. This observation indicates a significant reduction in the relative concentration of the free-volume holes of the blend following the aging test. The same behavior is observed for the τ3 parameter, indicating a decrease in the average size of the remaining free volumes of the blend after the aging test. It is known that the plasticizer creates space between the polymer chains, keeping them apart from each other. After its extraction, the polymer chains move closer to each other, and the size of the empty space is reduced; as a consequence, the concentration of the free volumes also diminish. The results obtained by PALS confirmed the mechanical tests; the smaller the amount and size of the free volume, the more rigid was the material.
The morphologies for the polymeric matrix, carbon black, blend untreated, and after exposure to different fuels are shown in Figure 6. The micrographs obtained for the NBR/PVC blend, in Figure 6c, shows that it is composed of two phases: spherical carbon black particles and a rubber matrix. These phases can be separately identified in Figure 6a,b. Figure 6d–f shows the changes in the surface of the NBR/PVC blend samples after immersion tests. According to the micrographs, the carbon black particles are more prominent in the aged NBR/PVC blend samples than they are in the untreated samples (Figure 6c) because of the partial extraction of the plasticizer and the polymeric matrix during the aging tests for all fuels tested.
Citation: Rubber Chemistry and Technology 85, 2; 10.5254/rct.12.88974
CONCLUSIONS
The study suggested the following conclusions:
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During the immersion tests, samples of the NBR/PVC blend exposed to different gasolines increased in weight (swelling), whereas the samples exposed to ethanol fuel showed a reduction in mass.
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Regular gasoline doped with rubber solvent promoted the extraction of a large quantity of the blend constituents as compared with the different fuels studied because of the polar character of the ethanol present in the Brazilian gasolines and the apolar behavior of the rubber solvent.
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After immersion tests and drying at 110 °C, all NBR/PVC blend samples increased in hardness and decreased in elongation at break, to different degrees. Additionally, the highest values of the elastic modulus were obtained, whereas the opposite trend was observed for its tension at break. All aged blends became less elastomeric and more rigid.
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Thermogravimetry analysis showed that primarily the DOP plasticizer was extracted by the fuels during immersion tests of the blend.
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The results obtained by PALS showed that the values of the parameters τ3 and I3 decreased after the immersion tests. Such results indicate that the free volume was reduced for the aged blends, and that behavior is consistent with the mechanical tests.
Sorption curves obtained from NBR/PVC blend samples exposed to regular gasoline (NRG), premium gasoline (NPG), regular gasoline doped with a rubber solvent (NRSG), and ethanol fuel (NE) for 20 weeks at room temperature.
Desorption curves obtained at 110 °C for the NBR/PVC blend samples exposed to regular gasoline (NRG), premium gasoline (NPG), regular gasoline doped with rubber solvent (NRSG), and ethanol fuel (NE) for 20 weeks at room temperature.
Variation of the (a) elongation at break, (b) hardness, (c) elastic modulus, and (d) tension at break in the NBR/PVC blend samples unaged (NU), after thermal treatment (NTT), and after exposure to regular gasoline (NRG), premium gasoline (NPG), regular gasoline with rubber solvent (NRSG), and ethanol fuel (NE) for 20 weeks at room temperature.
TG curves of the NBR/PVC blend samples before and after exposure to regular gasoline (NRG), premium gasoline (NPG), regular gasoline doped with rubber solvent (NRSG), and ethanol fuel (NE) for 20 weeks at room temperature (N2, 50 mL/min, 10 °C/min).
TG curves of butadiene-acrylonitrile copolymer (NBR), dioctyl phthalate (DOP), poly(vinylchlorite) (PVC), and NBR/PVC blend samples obtained in a dynamic nitrogen atmosphere (50 mL/min) at a heating rate of 10 °C/min.
SEM micrographs corresponding to (a) NBR/PVC copolymer, (b) carbon black, (c) untreated NBR/PVC blend, and aged samples in (d) regular gasoline, (e) regular gasoline doped with rubber solvent, and (f) ethanol fuel.
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