Features of processes of adhesion of polymer solutions to metallic substrates

Transcription

1 Plasticheskie Massy, No. 8, 200, pp Features of processes of adhesion of polymer solutions to metallic substrates M. Yu. Dolomatov and M. Yu. Timofeeva Ufa Technological Service Institute Selected from International Polymer Science and Technology, 0, No. 12, 200, reference PM 0/08/19; transl. serial no Translation submitted by P. Curtis Processes of adhesion play the most important role in processes of bonding of plastics and in production technologies of composite, building, textile, and other materials. The mechanism of adhesion includes different types of interphase intermolecular interaction of molecules of the contacting phases [1 ]. A particularly important role is played by adhesive interaction in metallic and polymeric substrates. The aim of the present investigation is to study features of the adhesion of polymer solutions to metals within the framework of a semi-empirical thermodynamic model of adhesion. Existing thermodynamic theories of adhesion are based on the results of investigations of the energy of interphase surface tension, wetting angles at the substrate/ adhesive boundary, and spreading of the adhesive on interphase boundaries with account taken of the viscosity and different contributions of intermolecular forces [, 4]. For theoretical calculation of adhesion it is necessary to know the interphase potentials of paired interactions of molecules of the surface of the substrate and adhesive, which in most cases are unknown or cannot be measured accurately for a real surface containing different structural defects. It is therefore expedient to develop a phenomenological approach. In a model proposed earlier [5 8], the surface of the adhesive (solution) was regarded as a two-dimensional gas of polymer molecules, and the process of adhesion as the isobaric isothermic expansion of this gas in the field of intermolecular forces of the substrate. It is assumed that, with such expansion of the two-dimensional surface gas, the pores and surface defects of the substrate are filled. This gas subsequently interacts with its active centres. With account taken of the volume of expansion at constant pressure, to describe such systems, the isobaric adhesion equation can be used: A = VTR( aβc + a βc + a βc + a βc ) where A is the work of adhesion (N/m 2 ), V is the effective volume of the molecular layer (m ), T is the temperature (K), R is the universal gas constant 8.14 J/K mol, a 1 a 4 are the virial coefficients characterising the non-ideality of the system, β is a coefficient taking into account the effective proportion of the polymer taking part in adhesive substrate interaction, or an analogue of the coefficient of activity of the solutions, and C s is the molar concentration of the polymer at the surface of the substrate (mol/m ). The investigations were carried out on adhesives solutions of polyvinyl acetate (PVA) and polymethylcellulose (PMC) in distilled water with a concentration of mol/m and mol/ m respectively. The upper boundary of concentrations was governed by the zone of strong gelation, and above this concentration a gel is formed, the measurement of adhesion in which is uncertain without loss of integrity. The viscosity-average molecular weight of PVA and PMC according to data of capillary viscometry in toluene is mol/g and mol/g respectively. To clarify the influence of the nature of the substrate on adhesion, an investigation was made of a series of metals of different nature: bronze of grade BrOTs12S5, carbon steel of grade St, chemically pure technicalgrade titanium of grade VT1-00, and aluminium of grade A85. T/8 International Polymer Science and Technology, Vol. 1, No. 5, 2004

2 An experiment to determine the work of adhesion of concentrated solutions of polymers to metals was carried out on a special laboratory unit under isothermal conditions. The work of adhesion was measured from the force of separation of a clean metal disc from the surface of a gel-like solution of polymer. The surface of the metal disc was thoroughly ground on a grinding mill. In preparation for testing, the disc was degreased. The metallic disc was cleaned with cotton material wetted with a solvent, and dried. The experiment was carried out in a thermostatically controlled cell filled with a sample of the adhesive to be investigated. Measurements were carried out at temperatures ranging from 298 to 5 K (for PVA), from 298 to K (for PMC), and from 298 to 2 K (for PAA). The lower temperature boundary corresponds to the state of the solution at room temperature, and the upper boundary is the temperature of the start of degradation of polymers. The surface of the gel-like solution should be even. The disc was lowered onto the surface of the solution and, for a period of 1 min, the adhesive was in contact with the substrate. The time of the experiment was compatible with the time of diffusion of macromolecules to the metal. The force of separation was measured periodically as the system was heated, at intervals of 10 K. The adhesives investigated were gel-like solutions of PVA and PMC. The results of measurements are given in Tables 1 and 2. The results of the experiment (Table 2) were processed by the least-squares method in the Microsoft Excel system. To prove the adequacy of the thermodynamic model, different degrees of non-ideality of the model were examined within the framework of approximation by polynomials of the third, fourth, and fifth order. The results of approximation for the PMC bronze system are given in Table. An assessment was made of the reproducibility of the experiment from the data of five parallel tests. For reliability, the 0.95 confidence interval of the work of adhesion was equal to ±7.19 N/m 2, and the coefficient of variation was 2.85%. The value of the confidence interval was much lower than the spread of points. The absolute and relative errors of the experiment were calculated and are given in Table. From the results obtained (Table ) it follows that, in most cases, in the examined range of adhesive concentrations, the work of adhesion is most adequately described by a fifth-order polynomial. The results of investigating the work of adhesion as a function of the volume concentration of polymer solutions are given in Figure 1. Irrespective of the types of adhesive and substrate, a polyextremal non-linear dependence of the work of adhesion on the PVA and PMC concentration in the solution at a constant temperature of 298 K is observed. Furthermore, a study was made of the temperature dependence of the work of adhesion for fibre polymer and metal polymer systems (Figure 2). From the given data it follows that the dependence is almost linear. To determine the virial coefficients, it is necessary to determine the effective volume of the molecular layer, and this can be estimated approximately from data of approximation of the temperature dependence of the work of adhesion (Figure 2) by means of the equation V = tgα / F( A) R where tg α is the angular coefficient of the temperature dependence of adhesion, and V = mol, from which it follows that the average thickness of the monomolecular adhesion layer is of the order of 10 9 m, which corresponds to the theory of adhesion []. The virial Table 1. Work of adhesion for PVA bronze system (T = 298 K) Concentration, mol/m 2 Work of adhesion of parallel experiments, N/m Average International Polymer Science and Technology, Vol. 1, No. 5, 2004 T/9

3 Table 2. Work of adhesion on different substrates (T = 298 K) Adhesive Polyvinyl acetate Polymethyl cellulose Polymer concentration, mol/m 2 Work of adhesion, N/m steel Substrate Figure 1. Dependence of work of adhesion on concentration (substrate carbon steel; adhesive PVA) Figure 2. Dependence of work of adhesion on temperature (substrate: 1 steel; 2 bronze; titanium; 4 aluminium; adhesive PMC) coefficients obtained from data of approximation of the experimental results are given in Table 4. We will estimate the value of β: β=a id / a exp The coefficients a exp are determined through approximation by the corresponding polynomial of the experimental dependence of the work of adhesion on concentration. For the ideal system (Θ solvent) it is possible to calculate the value of a id on the basis of the T/40 International Polymer Science and Technology, Vol. 1, No. 5, 2004

4 Table. Results of approximation of the dependence of the work of adhesion on concentration (T = 298 K) Concentration, mol/m Total Order of polynomial A exp A c alc bsolute error, A A Relative error, % known relationship between the first virial coefficient and the molecular weight of the polymer: a id = a 1 = 1/ M, where M is the molecular weight of the polymer [4]. Subsequently, all coefficients can be corrected with account taken of β. The corrected virial coefficients, with account taken of the corresponding coefficients β, are given in Table 5. Thus, the results of investigating water-soluble polymers on metallic substrates indicate the non-linear dependence of the work of adhesion on concentration and the near-linear dependence of the work of adhesion on temperature, which confirms the adequacy of the model of isobaric expansion of a two-dimensional real surface polymer gas. It must be pointed out that the proposed model also describes extremely non-ideal systems, for example the adhesion of solutions of polyolefins in multicomponent, high-boiling hydrocarbon fractions [5 8]. International Polymer Science and Technology, Vol. 1, No. 5, 2004 T/41

5 Table 4. Virial coefficients of isobaric work of adhesion Virial coefficients Substrate 2 4 a 1 a 2 a a 4 a 5 1/mol a 0 Adhesive PVA, steel Adhesive PMC steel Table 5. "Corrected" virial coefficients of isobaric work of adhesion S ubstrate β 0 7 Virial coefficients, a 0 a 1 a 2 1/mol Adhesive PVA 2 4, a a 4 a 5 1/mol, steel Adhesive PMC steel CONCLUSIONS A semi-empirical thermodynamic model is proposed that describes the interaction of adhesive with substrate within the framework of concepts of adhesion as the isobaric expansion of a two-dimensional non-ideal polymer gas in the field of molecular forces of the substrate. The adequacy of the model is borne out by the linear dependence of adhesion on temperature and the non-linear dependence on the concentration of the polymer solution on metallic substrates, with aqueous polymer solutions used as the adhesives. REFERENCES 1. Chemical encyclopaedia, Khimiya, Moscow, 1998, Vol. 1, pp L. M. Pritykin et al., Theory of physicochemical description of adhesion properties of organic compounds (cyanoacrylate adhesives), PGASA- Basilian Press, Dnepropetrovsk, 1999, 172 pp.. A. A. Berlin and V. E. Basin, Principles of adhesion of polymers, 2nd edition, Khimiya, Moscow, 1974, 91 pp. 4. Yu. G. Frolov, Colloidal chemistry course. Surface effects and dispersed systems. College textbook, 2nd edition, Khimiya, Moscow, 1988, 464 pp. 5. M. Yu. Dolomatov et al., Adhesion and phase transitions in complex high molecular weight systems. Textbook, Ufa Technological Service Institute, Ufa, 2001, 41 pp. 6. M. Yu. Timofeeva and M. Yu. Dolomatov, Laws governing adhesion of multicomponent systems to fibrous substrates. Plast. Massy, No. 2, T/42 International Polymer Science and Technology, Vol. 1, No. 5, 2004

6 7. M. Yu. Timofeeva et al., Laws governing adhesion of petroleum polymer systems to metals. Abstracts of Papers of rd Congress of Russian Oil and Gas Industry Industrialists on Oil Refining and Petroleum Chemistry, Ufa, 2 May 2001, pp M. Yu. Dolomatov and M. Yu. Timofeeva, Generalised thermodynamic model of adhesion of multicomponent polymer solutions. Symposium of scientific proceedings. Institute of Petrochemical Refining Problems (IP NKhP- BashNII NP), Ufa, 2001, No., pp (No date given) International Polymer Science and Technology, Vol. 1, No. 5, 2004 T/4