Prospects for the application of a new plasticiser dibutoxyethyl adipinate for the production of coldresistant

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1 Kauchuk i Rezina, No. 1, 2008, pp Prospects for the application of a new plasticiser dibutoxyethyl adipinate for the production of coldresistant sealing rubbers N. N. Petrova 1, V. V. Portnyagina 2, and E. S. Fedotova 3 1 Yakutsk State University, Yakutsk 2 Institute of Petroleum and Gas Problems, Siberian Department, Russian Academy of Sciences, Yakutsk 3 OOO (LLC) Ream-RTI, Moscow Selected from International Polymer Science and Technology, 35, No. 7, 2008, reference KR 08/01/18; transl. serial no Translated by P. Curtis A wide range of plasticisers [1, 2] is currently being used in Russian industry, and work on finding new plasticisers is ongoing [3 8]. The development of new products intended to replace traditional plasticisers dibutyl phthalate (DBP), dibutyl sebacinate (DBS), and ester LZ-7 is governed by the need to reduce the volatility, cost, and toxicity of existing plasticisers, expand the feedstock base, and utilise the byproducts of many production processes. A promising plasticiser is dibutoxyethyl adipinate (DBEA) [6], the production of which was assimilated in 2002 by the Urals Chemical Co. (Nizhnii Tagil). The main characteristics of DBEA [6] are given below: Appearance Transparent liquid without impurities Density at 20 C, kg/m Acid number, mg KOH/g 0.08 Saponifi cation number, mg KOH/g Flash point, C 185 Solidifi cation point, C 58 Dynamic viscosity at C, Pa s Investigations conducted in the OAO (Open Joint Stock Company) NIIEMI [9] and in the OAO Urals Mechanical Rubber Goods Works [10] showed that DBEA is a high-performance plasticiser that is superior in properties to ester LZ-7 and can replace it in rubber mixes based on polar rubbers. In a number of rubber formulations, complete or partial replacement with DBEA of the more cold-resistant DBS and also its combinations with DBP is possible. It was shown that dibutoxyethyl adipinate possesses good compatibility with butadiene acrylonitrile and chloroprene rubbers and has a lower volatility than the principal standardproduced plasticisers. Thus, in the case of heating at 150 C for 6 h, the weight loss of the specimens is as follows: 5.8% when using LZ-7, 15.2% when using DBP, 6.9% when using DBS, and 1.03% when using DBEA [9]. It can be assumed that, when the new plasticiser is used, the washing out of DBEA during the service of rubber compounds in hydrocarbon media will be less intensive, which will make it possible to retain a higher level of cold resistance of materials. This is particularly pressing when rubber compounds and mechanical rubber goods operate under extreme conditions, for example in the Sakha Republic (Yakutiya), the climate of which is noted for extremely low temperatures (down to 64 C) in the winter and sudden fluctuations (of up to 40 K) in daytime temperature in the spring autumn period. Dibutoxyethyl adipinate was supplied by the OAO Urals Chemical Co. (the authors are grateful to the marketing department of the company, in particular to Yu. V. Leonova) and was tested in the composition of a model rubber mix based on BNKS-18 butadiene acrylonitrile rubber, with subsequent full-scale exposure in Talakansk field petroleum for a year. The rubber mix formulation (parts) is given below: 2009 Smithers Rapra Technology T/17

2 BNKS-18 butadiene-acrylonitrile rubber Stearic acid 1.5 ZnO Mercaptobenzthiazole 1.5 Phenyl-β-naphthylamine 1.5 P-803 carbon black 50.0 Sulphur 2.0 Dibutoxyethyl adipinate 20.0 The data obtained were compared with the results of previous climatic tests of industrial rubber compounds of the following grades: V-14 and 7V-14-1 (OST ) based on BNKS-18 (containing 30 parts DBP); (TU ) based on BNKS-18 (containing 50 parts DBP + DBS); K (TU ) based on BNKS-40 (25 parts DBS). To investigate the combined action of low temperatures and a physically corrosive hydrocarbon medium on the properties of rubber compounds, specimens placed in petroleum were held in an unheated storeroom for 2 years (1 year) under the climatic conditions of Yakutsk. Periodically, rubber specimens (cylinders, sheets) were taken out of the petroleum and subjected to tests: the physicomechanical properties (GOST ), the development of compression set (GOST ), the coefficient of cold resistance K c (GOST ), and the degree of swelling in the hydrocarbon medium (GOST ) were determined, as well as the amount of plasticiser in the rubber (using IR spectroscopy). This made it possible to characterise the state of the material and to assess the degradative changes occurring in the material as a result of ageing under the action of climatic factors and the physically corrosive medium, and also on account of the occurrence of diffusion processes. Studies were conducted in the period , and the monthly-average air temperatures within this period were similar to the average perennial values (Table 1) [11]. As shown by the studies carried out, the influence of the diffusion processes occurring when butadiene acrylonitrile rubber compounds come into contact with petroleum on the properties of the rubbers is fairly uniform and reduces to the following: the penetration of hydrocarbons initiates the washing out of the plasticiser, producing a change in the volume and weight of the specimens, and in the composition of the rubber compounds. Fluctuations in the degree of swelling of the rubber compounds lead to a reduction in strength and to an increase in compression set. The degree of change in the properties for different rubber compounds is different the reduction in strength can amount to 40%, while the value of the compression set after exposure for 2 years is doubled for a number of materials. However, this deterioration in properties does not reach a critical level: the nominal tensile strength and the compression set stay within the limits of the permissible standard values for sealing rubbers. The main adverse effect of diffusion processes concerns the cold resistance and degree of swelling of the rubber compounds. Most of the standard rubbers investigated are characterised by a reduction in the degree of swelling in the initial period of exposure, which is connected with the predominance of the process of washing out of ingredients from the elastomer matrix (Figure 1). As shown by full-scale tests, in the first months of contact of the investigated rubbers with the medium, intense washing out of the plasticiser occurs, and, after exposure for 2 years, the materials contain no more than 20 30% of the initial amount of plasticiser introduced (Figure 2). Here, both dibutyl phthalate and less volatile plasticisers, for example dibutyl sebacinate, which is specially designated for the development of cold-resistant rubber compounds, are washed out. The differences in the rate at which they are washed out are negligible and hardly affect the overall durability of the rubber compounds, since, on full-scale exposure, the major part of the plasticiser is extracted from the rubber compounds in the first 2 4 months. After this time of exposure in petroleum, most of the standard rubber compounds based on BNKS-18 that were investigated are unable to operate at a temperature of 50 C, and their coefficient of cold resistance amounts to less than 0.2 (Figure 3), i.e. brittle failure of the material can be expected when large impact and shear effects are applied [12]. This seems to explain the numerous failures of rubber parts Table 1. Characteristics of the Yakutsk climate in the course of a year [1] Parameter Month I II III IV V VI VII VIII IX X XI XII Average air temperature, C Number of days with subzero temperature throughout 29 Number of days with temperature passing through 0 C Intensity of solar radiation on surface, kj/(m 2 s) T/18 International Polymer Science and Technology, Vol. 36, No. 3, 2009

3 Figure 1. Dependence of the change in weight on the exposure of rubber compounds (1), K (2), and 7-V-14-1 (3) (from August 2002 to August 2004) the instant the machine or mechanism is moved. While the sealing material is in the rubbery state, some of the energy of impact is expended on strain of the material, but, on transition to the glassy state, the probability of failure becomes much greater. Since the main processes connected with washing out of the plasticiser are completed in the first year of holding of the rubber compounds in petroleum, the time of exposure of the model rubber compound containing the new plasticiser dibutoxyethyl adipinate was shortened to 1 year. The model rubber, just like the standard rubber compounds investigated earlier, is characterised by a reduction in weight of specimens in the first 2 months of exposure (Figure 4a), i.e., in spite of the lower volatility of DBEA, fairly intense washing out of the plasticiser occurs. Whereas, before exposure, the model rubber compound was slightly inferior to the standard rubbers based on BNKS-18 in its coefficient of cold resistance at 50 C (which is entirely understandable, since the rubber compound contains 20 parts DBEA, while, for example, rubber of grade contains 25 parts DBP and 25 parts DBS), after washing out of the major part of the plasticiser, the cold resistance of the rubber compounds is practically identical and the values of K c are close to zero (Figure 4b). The coefficient of cold Figure 2. Dependence of the coefficient of cold resistance under elongation at 50 C (1) of grade V-14 rubber based on BNKS-18 and its content of plasticiser DBP (2) on the exposure (3 variation in ambient temperature) Figure 3. Dependence of the coefficient of cold resistance under elongation at 50 C on the holding time in petroleum under conditions of full-scale exposure of rubber compounds (1), K (2), and 7-V-14 (3) (from August 2002 to August 2004) Figure 4. Dependence of (a) change in weight and (b) coefficient of cold resistance under elongation at 50 C of the model rubber compound containing DBEA on the exposure (from March 2005 to March 2006) 2009 Smithers Rapra Technology T/19

4 resistance at 20 C for all rubber compounds investigated undergoes negligible seasonal fluctuations for the entire time of exposure and has stable high values (Figures 5 and 6). Rubber containing DBEA is characterised by fairly high strength, which, like the compression set of this material, hardly changes after full-scale exposure for 1 year. Thus, the diffusion processes, including both the washing out of the plasticiser and the penetration of the medium into the material, have a considerable effect on the entire combination of properties of the rubber compounds. Under conditions of a cold, distinctly continental climate, cyclic changes in temperature with passage through 0 C twice a day in the spring autumn period lead to intensification of these processes and to higher instability of the main service parameters of the rubber compounds. Figure 5. Dependence of the coefficient of cold resistance under elongation at 20 C on the holding time in petroleum under conditions of full-scale exposure of rubber compounds (1), K (2), and 7-V-14 (3) (from August 2002 to August 2004) Figure 6. Dependence of the coefficient of cold resistance under elongation at 20 C of the model rubber containing DBEA on the holding time in petroleum under conditions of full-scale exposure (from March 2005 to March 2006) The time lag of the processes occurring in the system and the temperature dependence of the coefficients of diffusion result in an increase in the degree of swelling of the rubber compounds and its frequent fluctuations. Under these conditions, the plasticisers are washed out more easily, which leads not only to a reduction in cold resistance but also to an increase in the rigidity of the system, and to a change in the mechanisms of degradation and structure formation in the rubbers. The diffusion processes have a direct infl uence on the degree of swelling of the rubber compounds and on their low-temperature characteristics, and consequently on all other properties of the elastomeric material. The change in the degree of swelling of the rubbers on account of extraction of the plasticiser by the medium may also lead to considerable shrinkage of articles and to deterioration in their sealing capacity. It seems that, with the ageing times investigated, washing out of the plasticisers must be regarded as the principal negative factor leading to deterioration in the service properties of the rubber compounds and of articles manufactured from them. The experimental data obtained provide convincing evidence that the introduction of large amounts of plasticisers cannot be the main method for creating sealing rubbers intended for operation under conditions of the Far North. This is in good agreement with the position taken by the authors back in the mid-1980s [13]. For articles manufactured from butadiene acrylonitrile rubbers and operating as seals in contact with petroleum products under conditions of low and elevated temperatures, we suggested that rubbers with a minimum plasticiser content be used [13], since the presence of plasticisers does not increase the cold resistance of articles, but only lengthens the process of achieving equilibrium with media and pollutes them with the impurities washed out. Of course, a plasticiser has a positive effect on the rheological properties of rubber compounds during their processing, but the introduction into rubber mixes of expensive components that rapidly migrate from articles when they come into contact with hydrocarbon media in a quantity exceeding 20 parts per 100 parts rubber is an extremely wasteful practice. The results of investigating the climatic stability of a rubber compound containing dibutoxyethyl adipinate showed the promise of using it as a cold-resistant plasticiser for mechanical rubber goods. It was established that DBEA is an effective plasticiser ensuring high cold resistance of rubber compounds based on reliable and irreplaceable (for present engineering in the North) butadiene acrylonitrile rubber. Rubbers containing DBEA are practically on a par with standard DBS-containing materials in terms of their combination of properties. The effectiveness of DBEA is due to the structure of its molecules, its compatibility with BNKS- 18, and its low volatility. The main service properties of the model rubber compound studied (its strength and T/20 International Polymer Science and Technology, Vol. 36, No. 3, 2009

5 compression set) after exposure in petroleum for 1 year at the ambient temperatures of Yakutsk do not undergo any significant changes and remain within the standard limits. Being introduced into rubber mixes in considerably smaller quantities (20 parts) by comparison with other plasticisers, DBEA ensures a high level of low-temperature properties at temperatures above 40 C. However, the cold resistance of rubbers containing DBEA, especially at critical service temperatures ranging from 45 C to 50 C, cannot be acknowledged as satisfactory. Its application will solve the problem of retaining the low-temperature properties of rubber compounds when replacing ester LZ-7, which has been withdrawn from production, or DBS, in whose production a valuable raw material (castor oil) is used. Rubbers containing DBEA can be used successfully for service in hydrocarbon media in zones of a moderate climate, but it seems that, to create rubber compounds for operation in such extreme conditions as those of the Sakha Republic, fundamentally different approaches must be taken. REFERENCES 1. F. A. Makhlis and D. L. Fedyukin, Terminological Handbook for Rubbers. Khimiya, Moscow, 1989, 400 pp. 2. A. E. Kornev et al., Technology of Elastomeric Materials. Eksim, Moscow, 2000, 288 pp. 3. G. E. Novikova et al., Preprints of International Rubber Conference Rubber-94 (Moscow, 1994), Vol. 2, p K. F. Krasil nikova et al., Kauch. i Rezina, No. 2, 2000, p V. A. Pankratov et al., Kauch. i Rezina, No. 6, 2002, p TU specifications. Plasticiser DBEA. Ekaterinburg, 10 pp. 7. O. E. Maskalyunaite et al., Abstracts of Papers of International Rubber Conference (Moscow, 2004), p V. A. Bubenev et al., Abstracts of Papers of International Rubber Conference (Moscow, 2004), p Development of recommendations for the use of plasticiser DBEA in the production of generaland special-purpose mechanical rubber goods. Report. OAO NIIEMI, Moscow, Study of the plasticiser in standard and technical rubber mixes of the Urals Mechanical Rubber Goods Works. Report No. SF Ekaterinburg, 2003, 85 pp. 11. M. K. Gavrilova, Climate of Central Yakutiya. Yakutsk Book Publ., Yakutsk, 1973, 120 pp. 12. M. F. Bukhina and S. K. Kurlyand, Cold Resistance of Elastomers. Khimiya, Moscow, 1989, 176 pp. 13. V. P. Petrova and N. R. Pashina, Proizvodstvo Shin, RTI, ATI, No. 12, 1983, p. 27. Received Smithers Rapra Technology T/21