Electrical conductivity of thermoplastic elastomers modified with gaseous fluorine

Transcription

1 Kauchuk i Rezina, No. 3, 2016, pp Electrical conductivity of thermoplastic elastomers modified with gaseous fluorine G.N. Petrova, 1 T.V. Rumyantseva, 1 V.G. Nazarov, 2 and Yu.A. Sapego 1 1 All-Russian Scientific Research Institute of Aviation Materials, Moscow, Russia 2 Ivan Fedorov Moscow State University of Printing, Moscow, Russia Selected from International Polymer Science and Technology, 43, No. 10, 2016, reference KR 16/03/12; transl. serial no Translated by P. Curtis Summary The electrical conductivity of thermoplastic elastomers (TPEs) modified with gaseous chlorine was studied. A TPE based on polyether ( Vitur -grade polyurethane) and the same TPE but with phosphorus-containing fireproofing agent were held in gaseous fluorine for 1 and 3 h, and were also treated with alcohol after fluorination. The surface morphology of specimens was studied using an atomic force microscope (AFM). The changes in morphology were similar to the changes observed after the surface fluorination of rubbers. The volume and surface resistivities were measured using an Agilent 4339B instrument (USA). In all cases, the volume resistivities of modified TPEs were two orders of magnitude lower than those of the initial TPEs. The introduction of a fireproofing agent leads to a 2.5-fold reduction in volume resistivity, and a 100-fold reduction in surface reactivity. The influence of the fireproofing agent is associated with it having an effect similar to that of fillers, and with the formation of a thin layer on the surface of specimens. it was shown that ethanol treatment leads to a reduction in volume resistivity. A promising method for creating new polymeric materials is to modify polymers of standard industrial production. Different methods of chemical or physical modification make it possible to control the properties of polymers in the necessary direction, significantly raising the level of their service characteristics [1 12]. For example, to produce a rubber compound from fluorine-containing rubber that possesses increased crack growth resistance, Kevlar filler is used with a fibre length of about 6 mm, and grafting of substances with hydroxyl groups to fluorine-containing rubber makes it possible to vulcanise coatings at room temperature [13]. Of particular interest is the method of surface modification with gaseous fluorine. Transformation of the structure of the surface and transition layers enables qualitatively different materials with a combination of improved properties to be produced on the basis of known polymers [11, 14]. During surface treatment of polymers with gaseous fluorine or other fluorinating agents, it is possible for hydrogen atoms to be replaced by fluorine atoms to a depth ranging from tens of ängstroms to several tens of micrometres, and to create a surface layer similar in properties to fluoropolymers. Fluoropolymers comprise a class of polymers differing from hydrocarbon analogues in the replacement of some (or all) of the hydrogen atoms by fluorine. The C F bond is one of the strongest, and as the electron density is concentrated on the line of the bond, the fluoropolymer macromolecule is inert to external chemical action. The given polymers possess a high heat resistance and thermal stability, chemical resistance, and reliability when operating under high mechanical loads, and they have good dielectric, antifriction and antiadhesion properties, very low friction coefficients, and a number of other valuable characteristics [13, 15]. The possibility of the fluorination of hydrocarbon polymer articles having an effect on their tribological characteristics was demonstrated graphically by the fluorination of rubber compounds [11, 14, 15 18], and the possibility of fluorination having an effect on abrasion resistance was demonstrated by the fluorination of 2017 Smithers Information Ltd. T/15

2 thermoplastic elastomers [TPEs] polymers possessing the deformability of rubber compounds and the technological effectiveness of plastics [3, 19 21]. This work is devoted to a study of the effect of surface gas-phase fluorination on the electrical conductivity of TPEs. The urgency of the investigations given is due to the need to broaden the range of TPEs and the areas of their application by modifying standard-produced polymers. The investigations were conducted on TPEs based on polyether ( Vitur -grade polyurethane) (composite A) and based on polyether with an inflammable additive (phosphorus-containing fireproofing agent) (composite B). The investigations were conducted on specimens manufactured from the composites given by compression moulding, in the initial state, after surface fluorination for 3 h and after alcohol treatment of specimens fluorinated by the indicated schedules. The surface fluorination of prepared specimens was done at the Ivan Fedorov Moscow State University of Printing. The process of fluorine treatment was carried out in three stages: holding of the article in gaseous reagent in a sealed stainless steel reactor; vacuum treatment to remove reaction products and unreacted fluorine; bringing the pressure in the reactor to atmospheric pressure. A fluorine-containing layer was formed as a result of the occurrence of chemical reactions directly in the surface layer of the TPE, which enables the problem of guaranteeing adhesion to be eliminated [11, 14]. The electrical conductivity of the composites investigated was assessed from the values of the volume resistivity (r v ) and surface resistivity (r s ), in view of the fact that, for application purposes, the electrical conductivity of polymeric materials is characterised as the inverse of conductivity in the physical sense [22 24]. The volume resistivity is an estimate of the resistance of the material when a direct current is passed through its volume, and the surface resistivity is an estimate of the resistance of the material when a direct current is passed over its surface. In a dry atmosphere, and in the absence of impurities, the magnitude of r s may be considerably higher than the magnitude of r v. Volume resistivity and surface resistivity are electrical properties of dielectrics, which, on account of their very high resistivity, is what most plastics are. The purpose of dielectrics is to create between conductive parts having different electrical potentials such a medium that will prevent the passage of current between these parts. Nonetheless, to a lesser or greater extent, electrical conductivity, in the majority of cases of ionic nature, is also observed in all plastics [19, 22]. To determine the electrical characteristics of composites, an Agilent 4339B teraohmmeter (USA) was used, the operating principle of the measuring part of which is based on measuring the charging time of a capacitor by a current proportional to the resistance being measured. To investigate the surface morphology of fluorinated TPE specimens, atomic force microscopy (AFM) was used, which makes it possible to determine the surface relief of the specimen with a resolution ranging from tens of ängstroms to the atomic. Data from visualisation of the surface of the polymer by AFM indicate that the fluorination of TPEs, as in the case of rubber compounds, is accompanied with a change in the morphology of the surface layer (Figure 1). Figure 1. The surface morphology of control (a) and fluorinated (b) TPE specimens (size of scanning region µm). Surface profilograms are presented above T/16 International Polymer Science and Technology, Vol. 44, No. 2, 2017

3 From the figures given and profilograms it can be seen that the surface morphology of the modified specimen (Figure 1b) by comparison with the untreated specimen (Figure 1a) is more developed on account of the appearance of a high-frequency structure ripples. The appearance of such a structure may be attributed to the fact that the elastic regions of the TPE, as the less dense regions, are fluorinated more intensely and to a greater depth with partial degradation. At the same time, the waviness (low-frequency structure) of the surface of the control TPE specimen and the fluorinated TPE specimen, in contrast to rubber compounds, is practically identical, possibly on account of the lower concentration of fluorine introduced into the polymer. The results of investigations of the effect of surface gas-phase fluorination on the electrical conductivity of TPE composites are given in Table 1 and in Figures 2 and 3. It must be pointed out that the main electrophysical characteristic of the material is its volume resistivity, while the surface resistivity is a relative quantity. It is determined largely by the surface state, the degree of its contamination, and the humidity of the surrounding medium. At increased humidity, thin films of moisture can form on the surface of the insulation materials, through which leakage currents pass [19, 22, 25]. It was established that, when a fireproofing agent is introduced into the TPE, the electrical conductivity of the latter increases, i.e. both indices decrease: r v about 2.5-fold, and r s more than 100-fold. Such a change in characteristics is due to the presence of the fireproofing agent, which behaves like a filler: its presence on the surface and in the volume of the TPE leads to a change Figure 2. The dependence of the volume resistivity of TEP composites (A) without fireproofing agent, (B) with fireproofing agent on the fluorination time: initial; fluorinated for 1 h (1) and for 3 h (2); fluorinated for 1 h (1) and for 3 h (2) and treated with alcohol Table 1. The electrical characteristics of TEP composites Conditions Thickness of specimen, mm Treatment r v 10 11, W cm r s 10 11, Ω Composite A Fluorinated for 1 h 2.38 Initial Fluorinated Fluorinated + ethanol treated Fluorinated for 3 h 2.32 Initial Fluorinated Fluorinated + ethanol treated Composite B Fluorinated for 1 h 2.40 Initial Fluorinated Fluorinated + ethanol treated Fluorinated for 3 h 2.34 Initial Fluorinated Fluorinated + ethanol treated Smithers Information Ltd. T/17

4 increases slightly r s = (32 100) Ω on account of the formation of a fluorinated surface layer. Lower r s values were possessed by specimens treated with fluorine for 3 h, i.e. an increase in the fluorination time leads to an increase in the electrical conductivity of the composite. Treatment of the specimen with alcohol promoted a further reduction in surface resistivity. Thus, the results of the conducted investigations showed that the use of surface gas-phase fluorination of composites of TPEs based on polyether made it possible to increase their electrical conductivity, as indicated by a reduction in the volume resistivity by two orders of magnitude. The electrical conductivity of the TPE composites also depends on the fluorination time: with increase in the fluorination time from 1 to 3 h, the volume resistivity decreases. The treatment of fluorinated specimens of composites with ethanol leads to an additional reduction in volume resistivity. The surface resistivity of the TPE composites is determined by the surface state of the material, and also depends on the presence of filler (fireproofing agent) and a fluorinated layer. ACKNOWLEDGEMENTS Figure 3. The dependence of the surface resistivity of TEP composites (A) without fireproofing agent, (B) with fireproofing agent on the fluorination time: initial; fluorinated for 1 h (1) and for 3 h (2); fluorinated for 1 h (1) and for 3 h (2) and treated with alcohol in the surface morphology of the TPE (a thin film of fireproofing agent is formed), and consequently to an increase in electrical conductivity (Table 1). The nature of change in the volume resistivity of the investigated composites during surface gas-phase fluorination is identical: on specimens with and without fireproofing agent, with increase in the fluorination time there is a decrease in the given index. The alcohol treatment of fluorinated specimens also leads to a reduction in r v, i.e. to an increase in conductivity. The effect given can be used in the manufacture of articles on the surface of which an electrical charge must not build up. The behaviour of the second index the surface resistivity r s on specimens of a composite without a fireproofing agent is similar. When a fireproofing agent is introduced into TPE, the surface resistivity r s decreases sharply from ( ) to ( ) Ω. After surface fluorination, it The authors are grateful to senior research fellow V.P. Stolyarov and to chief engineer D.N. Perfilova for their help in the study, which received financial support from the Russian Foundation for Basic Research (project code ofi_m). REFERENCES 1. Kablov E.N., Chemistry in aviation materials science. Ross. Khim. Zh., LIV(1):3 (2010). 2. Kablov E.N., Aerospace materials science. Vse Mater. Entsikloped. Sprav., (3):2 (2008). 3. Petrova G.N. et al., Fire-safe injection-moulded thermoplastics and thermoplastic elastomers. Trudy VIAM, (11):02 (2013). 4. Kablov E.N. et al., Strategic directions in the development of structural materials and technologies for their processing for aviation engines of the present and future. Avtom. Svarka, (10):23 (2013). 5. Krasnov K.V. et al., Certain aspects of the modification of composite materials based on thermoplastic elastomers with organoclays. Uspekhi Khim., 25(3):76 (2011). 6. Kablov E.N., Science as a field of economics. Nauka Zhizn, (10):6 (2009). 7. Petrova G.N., Targeted modification of polysulphones and the creation on their T/18 International Polymer Science and Technology, Vol. 44, No. 2, 2017

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