Plasticheskie Massy, No. 8, 2006, pp. 17-19 Effect of surface fluorination and sulphonation on the adhesion and tribological properties of polymers V. G. Nazarov, V. P. Stolyarov, L. A. Evlampieva, and V. A. Baranov Interdepartmental Centre of Analytical Research in the Field of Physics, Chemistry, and Biology at the Presidium of the Russian Academy of Sciences Military University of Radiation, Chemical, and Biological Protection Selected from International Polymer Science and Technology, 32, No. 8, 2005, reference PM 06/08/17; transl. serial no. 15705 Translation submitted by P. Curtis Adhesion is regarded as the result of interaction between the surfaces of two bodies in contact. To stop this interaction, an effect of certain magnitude is necessary [1]. A key role in adhesive interaction is played by the chemical nature and morphology of the surface layers of the contacting bodies [2]. For thermoplastics, large adhesion interactions are particularly important for bonding. However, in a number of cases it is necessary to lower the adhesion characteristics of the polymer as much as possible. For example, in the manufacture of glass-fibrereinforced plastic tubes based on epoxy composites, use is made of films of polytetrafluoroethylene as separators to eliminate sticking of the tube being cured to the mandrel on which it is being manufactured, which is fairly expensive. The replacement of polytetrafluoroethylene films with polyolefin films is possible provided the sticking of the latter to epoxy resin is lowered. A known method for controlling the adhesion characteristics of thermoplastics is their surface modification [2]. The aim of the present work was to study the effect of surface fluorination on the adhesion characteristics of low-density polyethylene (LDPE) and rubber compounds based on butadiene acrylonitrile rubber, and also the effect of surface sulphonation on the adhesion characteristics of LDPE. The specimens studied were industrially produced extruded LDPE films of 100 µm thickness. The following specimens were chosen as butadiene acrylonitrile rubber compounds: RS-26 (SKN-26 rubber, filler P-803 carbon black), S-26 (SKN-26 rubber, filler BS-50 silica), NO- 68-1 [mixture of SKN-18 and polychloroprene (PCP) rubbers, filler PM15 black]. Surface treatment of LDPE films and rubber compound specimens was carried out with a mixture of 15 vol.% fluorine with helium at 293 K by the procedure described in reference [3], and sulphonation was carried out with sulphur anhydride with a concentration of 0.07 g/l at 323 K by an improved gas-phase procedure [4]. The intensity of action of the reagent on the LDPE film and rubber compound (degree of surface modification, kg/m 2 ) was monitored from the mass of fluorine (in the case of fluorination) or sulpho groups (in the case of sulphonation) introduced into the polymer as a result of the occurrence of reactions of substitution and addition in terms of the surface area of the polymer. The surface tension γ exp was determined experimentally by the Owens Wendt method [5]. Assessment of the adhesion characteristics of LDPE films was carried out by the pulling of two specimens of films bonded with a Moment -grade non-polar adhesive (with a similar surface tension to LDPE, γ calc = 32 mn/m, calculated by the method set out in reference [5]) with a constant thickness of the adhesive of 100 µm. The force of pulling of the two films was determined on an Instron tensile testing machine. Tests were conducted by the scheme given in Figure 1, which was selected as the most acceptable of the methods known [6]. Failure in all cases occurred by the adhesive joint without significant elongation of the films. The results obtained are presented in Table 1. An analysis of Table 1 indicates that the treatment of LDPE films with sulphur anhydride, even with low degrees of sulphonation, with which the physicomechanical properties of the films are retained, leads to a considerable (more than tenfold) increase in the force of failure by comparison with the initial films. In accordance with known theories of adhesion of polymers [2], the 2007 Smithers Rapra Limited T/11
Table 1. Surface tension and force of failure of adhesive joints of initial and surface-modified films of LDPE No. Type of joint Degree of modification of each γ exp, mn/m Force of failure of of two films joined, x adhesive joint, N 10 4 kg/m 2 1 LDPE LDPE 0 0 28 28 0.30 2 LDPE F _LDPE F 0.7 0.7 27 27 0.21 3 LDPE F _LDPE F 1.3 1.3 24 24 0.10 4 LDPE F _LDPE F 2.0 2.0 21 21 0.05 5 LDPE F _LDPE F 4.0 4.0 20 20 0.05 6 LDPE SO3 H _LDPE SO 3 H 1.5 1.5 34 34 2.1 7 LDPE SO3 H _LDPE SO 3 H 2.3 2.3 36 36 3.0 8 LDPE SO3 H_LDPE SO3 H 4.0 4.0 39 39 4.0 9 LDPE SO3 H_LDPE SO3 H 5.2 5.2 38 38 4.0 10 LDPE SO3 H _LDPE SO 3 H 6.7 6.7 39 39 4.7 11 LDPE SO3 H _LDPE SO 3 H 9.3 9.3 40 40 6.2 12 LDPE SO3 H _LDPE SO 3 H 18.4 18.4 42 42 5.3 13 LDPE SO3 H_LDPE SO3 H 65.4 65.4 54 54 9.5 14 LDPE SO3 H_LDPE SO3 H 73.5 73.5 56 56 6.7 Figure 1. Layout for testing strength of adhesive joint of initial, fluorinated (LDPE F ), and sulphonated (LDPE SO3H ) films of LDPE obtained effect of increase in the strength of the film adhesive joint is due both to an increase in the surface tension of the sulphonated LDPE surface (greater than in the case of the most polar polymers cellulose hydrate, polyacrylonitrile, polyvinyl alcohol) and to an increase, especially at high, in the surface roughness on account of the occurrence of degradation processes in the surface layer of the film up to carbonisation. In spite of the use of a non-polar adhesive (based on diene oligomers), the adhesive joint of strongly polar sulphonated LDPE films is characterised by high adhesive strength, which is retained in a wide range of values. In contrast to sulphonation, fluorination naturally leads to a lower strength of the adhesive joint of fluorinated LDPE films 6 times lower by comparison with the initial films, which is due to the low surface tension of the surface layer formed, similar to fluoroplastics. It is significant and important that the start of substantial change in the force of failure of the adhesive joint corresponds to different degrees of modification, which is due to the specific relationship between the structure of the surface layer and the properties. In the case of fluorination, a considerable reduction in the force is observed for degrees of fluorination of over 1.0 10 4 kg/m 2, and with degrees of fluorination of over 2.0 10 4 kg/m 2 it levels out at a stationary value not dependent on the degree of fluorination of the polymer. This seems to be due to the fact that, with high degrees of fluorination, the interaction of fluorine with the polymer develops chiefly deep into the polymer, while fluorination of the surface layers practically reaches saturation, and there is no further change in the surface tension and morphology of the surface layer. In the case of sulphonation, considerable increase in the force of failure of the adhesive joint is observed with minimum degrees of sulphonation, equal to 1.0 2.00 10 4 kg/m 2. Further increase in the degree of sulphonation may lead to an increase in the force of failure of the adhesive joint, but the results obtained have a large spread (especially at maximum degrees of sulphonation), which seems to be due to the occurrence of degradative processes up to carbonisation, which are difficult to monitor. Thus, the two proposed methods of modification make it possible to vary the adhesion characteristics of polyethylene in a wide range. The established laws governing directional control of the strength of adhesive joints were confirmed on other polymers polyolefins, copolymers of ethylene with vinyl acetate and vinyl chloride, polyvinyl chloride, and rubber compounds. In the latter case, of special interest is the possibility of a considerable reduction by surface fluorination T/12 International Polymer Science and Technology, Vol. 34, No. 10, 2007
in the tribological and adhesion characteristics of rubber compounds, which is important for the reliable operation of sealed assemblies used, for example, in space, aviation, or agricultural engineering, and characterised by long periods for which the rubber is in constant stationary loaded contact with the metal surface being sealed. In light of the fact that the force of shear of rubber against metal is generally significant, in this case, in contrast to LDPE films, use was made of a different test layout. The force of adhesion interaction in the rubber metal pair was characterised by the displacement friction coefficient µ, for the determination of which a shear by tensile load test layout was chosen with two-sided contact of an extractable metal indentor with overlapping rubber specimens [6] (Figure 2). With such a test layout, essentially, dry sliding friction of rubber against metal occurs. The indentors were a plate of tool steel (Jorgansen plate) of class 14 surface purity, a plate of tool steel of class 9 surface purity, and a plate of duraluminium of grade D16 with class 10 surface purity. The displacement friction coefficient µ for the maximum force of displacement friction was calculated by means of the formula From Figures 3 5 it can be seen that, even for small degrees of fluorination of the rubber specimens, corresponding to fluorination times of 5 30 min, there is a considerable reduction in the displacement friction coefficient, and the curves subsequently level out into stationary sections on which the displacement friction coefficient hardly depends on the degree of fluorination. Such a shape of the curves seems to be due to the fact that, even in the initial period of fl uorination, as a result of the interaction of fluorine with rubber macromolecules, which, as is known [7, 8], proceeds extremely vigorously, (1) where F m is the maximum displacement friction force (kgf) and N is the normal load (kgf). The normal load was created by a screw clamp in which the test block with the rubber specimens was fixed (see Figure 2). The normal load created was calculated on the basis of the magnitude of the compression of the rubber specimens (from the change in H before and after loading) and the compression modulus. The compression modulus of the specimens was determined on a Williams plastometer. From the test results, the dependences of the displacement friction coefficient on the fluorination time of rubber specimens were determined (Figures 3 5). Figure 3. Dependences of displacement friction coefficient on fluorination time of specimens of RS-26 rubber compound for different indentors: 1 Jorgansen plate; 2 plate of tool steel; 3 plate of grade D16 duraluminium Figure 2. Layout of tests for determining force of shear of rubber specimens against metal surface: 1 rubber specimens; 2 external metal plates; 3 metal indentor; 4 additional metal plate; H thickness of testing block used in subsequent calculations (explanations in text) Figure 4. Dependences of displacement friction coefficient on fluorination time of specimens of S-26 rubber compound for different indentors: 1 Jorgansen plate; 2 tool steel plate; 3 duraluminium D16 plate 2007 Smithers Rapra Limited T/13
Figure 5. Dependences of displacement friction coefficient on fluorination time of specimens of NO-68-1 rubber compound for different indentors: 1 Jorgansen plate; 2 tool steel plate; 3 duraluminium D16 plate a fairly continuous fluorinated surface layer is formed. Subsequently, as the fluorination process develops, it does so chiefly deep into the rubber. On the basis of this shape of the curves, for significant reduction in adhesion interaction between rubber and metal, fluorination for 20 30 min would be sufficient. However, considering that, during service, the upper fluorinated layer may abrade [7], it would appear to be expedient to carry out fluorination for 1 3 h. On the whole, for all indentors, as a result of fluorination a reduction in the displacement friction coefficient by a factor of 3 6 is achieved. This is observed to a greater degree for a Jorgansen plate with a surface purity of 14. It is known that the frictional force of two bodies can be expressed as the sum of adhesion and deformation components [9]. For the case of friction of rubber against metal with a surface purity of 14, the frictional force is practically equal to the adhesion component, i.e. the reduction in the displacement friction coefficient that is observed in this case can be attributed to a reduction in adhesion interaction between rubber and metal after the fluorination of its surface. Such an explanation is entirely well founded, since the layer arising on the surface of the rubber as a result of fluorination should be similar in properties to perfluorinated polymers, which are characterised by high antiadhesion properties and low friction coefficient values [10, 11]. However, with such consideration, no account is taken of possible variation in the area of physical contact of rubber and metal for the fluorinated and non-fluorinated specimens under an identical normal load. As the present authors recently showed for the case of polyethylene films [12], surface fluorination can lead to a considerable morphological change in surface structure. In the case of contact of rubber with plates of tool steel and duraluminium with a surface purity of 9 10, for the fluorinated specimens again there is a considerable reduction in the values of the displacement friction coefficient (Figures 3 5, curves 2 and 3). However, these values are higher than those for a Jorgansen plate, which seems to be due to the appearance of a deformation component of the frictional force with reduction in the surface purity of the indentor. Furthermore, the higher values of the displacement friction coefficient in the case of contact of rubber with duraluminium as opposed to tool steel must be noted. The nature of the metal of the indentor shows itself here, it seems; i.e. the adhesion interaction of rubber with a duraluminium surface is higher than that with the surface of a tool steel plate. Thus, the surface fluorination of rubbers leads to a considerable reduction in the displacement friction coefficient against metal, which is due to a reduction in adhesion interaction between the components. At the same time, with reduction in the purity of surface treatment of the metal, the deformation components of the force of shear of the rubber in relation to the metal surface increases, which can nullify the obtained effect by reducing the adhesion component of the frictional force. This work was carried out with the financial support of the Russian Foundation for Basic Research (project codes 04-03-080035 and 05-03-35062), the Federal Agency for Science and Innovations (contract 01-08-07-05, from 9/4/2005), and also the Moscow government (grants 1.1.177 and 1.2.54, 2005). REFERENCES 1. A. D. Zimon, Adhesion of films and coatings. Khimiya, Moscow, 1977. 2. A. A. Berlin and V. E. Basin, Principles of adhesion of polymers. Khimiya, Moscow, 1969. 3. V. G. Nazarov et al., Plast. Massy, No. 5, 1993, p. 31. 4. V. G. Nazarov et al., Vys. Soed., A36, No. 1, 1994, p. 80. 5. D. V. Van Krevelen, Properties and chemical structure of polymers. Khimiya, Moscow, 1976 (trans. from English, Ed. A. Ya. Malkin). 6. Yu. A. Kharlamov, Zavod. Lab., No. 5, 1986, p. 63. 7. V. G. Nazarov et al., Vys. Soed., A41, No. 11, 1999, p. 1793. 8. W. Shepherd and K. Schartz, Organic chemistry of fluorine. Mir, Moscow, 1972. 9. G. M. Bartenev and V. V. Lavrent ev, Friction and wear of polymers. Khimiya, Leningrad, 1972. T/14 International Polymer Science and Technology, Vol. 34, No. 10, 2007
10. I. V. Pyatov et al., Kauch. i Rezina, No. 5, 1999, p. 28. 11. Fluorine compounds: synthesis and application. Mir, Moscow, 1990 (trans. from Japanese, Ed. N. Isikawa). 12. V. G. Nazarov et al., Vys. Soed., 2006 (in press). 2007 Smithers Rapra Limited T/15