Protective coatings based on PVC plastisols

Transcription

1 Plasticheskie Massy, No. 9, 2005, pp Protective coatings based on PVC plastisols E. M. Gotlib, A. A. Gudkov, and Yu. A. Sokolova Elastomer Development Centre, Kazan State Technological University; Essen Production AG Closed Joint Stock Company, Elabuga, INEP, Moscow Selected from International Polymer Science and Technology, 33, No. 1, 2006, reference PM 05/09/40; transl. serial no Translation submitted by P. Curtis In the developing motor industry, the demand for highquality and relatively inexpensive anticorrosion coatings for the protection of the bottom of motor vehicles and other carbody elements is growing perceptibly. Effective materials for anticorrosion coatings are polyvinyl chloride (PVC) plastisols, which are heterogeneous dispersions of PVC grades in a plasticiser and diluent with additions of stabilisers, fillers, adhesives, modifiers, and other components [1]. In formulations of industrial PVC plastisols, toxic and expensive dioctyl phthalate (DOP) is chiefly used as the plasticiser. The diluent is white spirit or other compounds of the kerosene series, which adversely affects the hygiene properties and increases the fire hazard [2]. The adhesives used are mainly imported. This makes it urgent to select more effective and ecologically optimum plasticisers and diluents for PVC plastisols and to replace expensive imported adhesive additives with cheaper home-produced additives. It is also of practical importance to select modifiers to control the rheological properties of PVC plastisols in order to reduce the energy and labour costs of their processing. Therefore, the development of effective anticorrosion coatings based on PVC plastisols with high adhesive characteristics to different types of primer used by the motor industry, and low toxicity and fire hazard, is of great scientific and practical importance. When PVC plastisols are used, a large role in ensuring the required quality of the anticorrosion protection is played by the diluent. This component determines the main processing characteristics of the PVC plastisol, such as its viscosity and extrusion, and has a considerable influence on the wetting of the primed surface and the film-forming capacity [2]. We have suggested [3] that a-olefins be used as diluents instead of the traditional white spirit. The a- olefins are noted for a relatively high flash point and low volatility. A broad series of a-olefins with different molecular weights were studied, and it was established that, from the viewpoint of the optimum combination of processing, service, hygiene, and fire safety properties, it is most promising to use a-olefins with a content of carbon atoms of 14 (Figure 1). For the partial replacement of the traditionally used DOP it has been suggested [3] that plasticisers EDOS (a mixture of 1,3-dioxane derivatives) and KhP-470 grade chloroparaffin be used. They have lower toxicity and are cheaper than DOP [4]. Figure 1. Dependences on type of diluent of (1) viscosity of PVC plastisols and (2) tensile stress causing failure of coatings based on them 2006 Smithers Rapra Limited T/43

2 Furthermore, chloroparaffins have low flammability and their products present less fire and explosion hazard, with a higher flash point and a lower volatility and acidity than in the case of EDOS. Chloroparaffin, EDOS, and DOP interact chemically during the processing of PVC composites plasticised by them. The dissolution of PVC in their mixture and the thermal breakdown of KhP-470 with the release of HCl occur during the high-temperature forming of anticorrosion coatings based on PVC plastisols. Here, breakdown of the 1,3-dioxane rings of EDOS occurs, catalysed in the presence of acidic decomposition products of both PVC and chloroparaffin. EDOS, the principal components of which are formals of dioxane alcohols, has better compatibility with PVC than does DOP, which is evidently due to its greater polarity [5]. Furthermore, its use in a mixed plasticiser in small quantities increases the strength of coatings based on PVC plastisols (Figure 2). Figure 2. Dependence of (1, 2) tensile stress causing failure and (3, 4) breaking elongation of PVC composites on content of (1, 4) KhP-470 and (2, 3) EDOS This effect may be due to the fact that EDOS increases the mobility of the structural elements of PVC and, consequently, their capacity for orientation under different types of strain, in particular, tensile strain. Also possible is a certain increase in the degree of crystallinity of the PVC on account of easier crystallisation when plasticisers are introduced. A certain role is also played by a reduction in the magnitude of the free volume (increase in the placking density of the polymer macromolecules) owing to filling of the pores of the PVC grains with plasticiser molecules [6]. As the concentration of plasticisers increases, the magnitude of the relative elongation increases consistently. This is connected with the formation of physical, in particular, hydrogen bonds between the PVC molecules via the flexible oxymethylene groups of EDOS [4]. Chloroparaffi n, on the other hand, lowers the strength properties of the PVC composites, i.e. acts like a traditional diluent [7]. In this context, for practical application, a ternary mixture of plasticisers DOP, EDOS, and chloroparaffin that is optimum from the viewpoint of processing and economic indices is recommended. In fact, owing to the high temperature of formation of anticorrosion coatings based on PVC plastisols ( C), EDOS is not suitable for the complete replacement of DOP on account of its relatively high volatility. The application of DOP alone is irrational from the economic and hygiene standpoint, while chloroparaffin is an extender and can be used only in a mixture with dissolving plasticisers of PVC [7]. Thus, the optimum composition of the mixed plasticiser is as follows: DOP, EDOS, and KhP-470 in a ratio of 8:1:1. This plasticiser ensures the production of coatings with a combination of service and processing characteristics at the level of anticorrosion materials with DOP. This is in agreement with published data [8] on the effect of mutual activation in a mixture of ester plasticisers with chloroparaffins. In this case, on account of the different migration rate of the described types of plasticiser, the application of their mixture results in an increase in the resistance of the plasticised PVC composites in time. As mineral fillers of the PVC plastisols, use is generally made of calcium-containing additives, chiefly whiting treated with stearic acids [1]. Home-produced whiting M-90T has a lower plasticiser capacity compared with imported analogues (Socal), which is due to the smaller size of its particles (Table 1). As the plasticiser capacity of the filler increases, the viscosity of the filled PVC plastisols increases consistently. According to the obtained experimental data [3], home-produced whiting M-90T can replace up to 20 50% of the more expensive imported whiting without any adverse effect on the processing and service properties. Besides the economic advantages, M-90T whiting ensures a certain increase in the dry residue of PVC plastisol (Table 1). To ensure stability of the viscous characteristics of PVC plastisols and the required thixotropic properties, modifiers are used [2]. Polyhydric alcohols with different contents of hydroxyl groups and different molecular weights have been tested by the present authors as modifiers [3]. It has been established that the optimum processing properties are possessed by a plastisol in which diethylene glycol is present, ensuring a successful combination of required viscosity and extrusion. This may be attributed to the optimum number of OH groups per diethylene glycol molecule and to the length of the molecular chain, and also to the presence of ether bonds. T/44 International Polymer Science and Technology, Vol. 33, No. 5, 2006

3 Table 1. Properties of fillers and PVC plastisols with their application (numerators with EDOS; denominators with DOP) Grade of whiting M-90T Socal Plasticiser capacity, cm 3 /100 g 27/25 49/47 Surface area 10 3, m 2 /kg Viscosity of PVC plastisol, Pa s 70/75 100/108 Extrusion of PVC plastisol, s 35/38 40/45 Dry residue, % Tensile stress causing failure, MPa Breaking elongation of PVC coating, % Table 2. Effect of diethylene glycol on properties of PVC plastisol and coatings based on it Without modifier With 0.2% diethylene glycol Viscosity, Pa s Extrusion, s Adhesion to primer VKCh-0207 Excellent Excellent Breaking elongation, % Tensile stress causing failure, MPa The latter ensure high flexibility of the molecules of this modifier. The small distance between the OH groups in ethylene glycol does not, however, ensure the necessary flexibility of its molecules, on account of which the viscosity of PVC plastisols modified by it is slightly higher. DEG improves considerably the deformation and strength properties (Table 2), with retention of high adhesion properties of the PVC coatings. This can be attributed to the fact that, between DEG and PVC, hydrogen bonds can form which increase the resistance of the coatings to the mechanical loads applied [3]. Here, the effect of high thixotropicity appears, at the same time as greater flow of the material. The high viscosity of PVC plastisols modified with diethylene glycol prevents them from running off metalwork, and their low extrusion results in easy movement along the piping of the system for plastisol application. This last effect is possible owing to the fact that glycols act as a lubricant and ease the movement of PVC macromolecules. One of the main factors determining the efficiency of anticorrosion coatings of the bottom and body of motor vehicles is their adhesion and the retention of this property during service under the action of strong dynamic loads and corrosive media. Therefore, adhesives are introduced into the formulation of PVC plastisols [1]. As adhesion promoters, we investigated amineand amide-containing compounds capable of forming donor acceptor bonds at the coating primed metal surface boundary [9]. The investigations showed that the type of primer used has a considerable influence on the adhesion capacity of PVC plastisols. Thus, the best effect is obtained by using primer VKCh-0207, the film-forming component of which is a partial ether of maleinised 1,4-cis-butadiene rubber containing carbonyl and carboxyl groups and double bonds. This increases the effectiveness of different types of intermolecular interaction at the phase boundaries by comparison with epoxy-containing primers of grades G-1083 and EP Furthermore, VKCh-0207 is looser than epoxycontaining groups. On account of this, greater diffusion of the PVC plastisols into this primer is possible, which increases the adhesion capacity of the polymer coating. At the same time, VKCh-0207 is not the most promising on account of the lower durability of the anticorrosion coating in corrosive media when this primer is applied. This is due to the presence of unsaturated bonds which are unstable under the action of atmospheric factors and chemical media [10]. The best combination of adhesion characteristics of the coating is ensured by the application of amineand amide-containing components as an adhesive additive. This is connected with the ability of NH 2 and N C=O groups to activate each other, i.e. to result in a synergistic effect through the formation, for example, of hydrogen bonds between the amide oxygen and the amine hydrogen [11]. As an adhesion promoter it is effective to use a mixture of polyamide resin (L-20), caprolactam, and polyethylenepolyamine (PEPA) in a 1:2.5:1.5 ratio (Figure 3) Smithers Rapra Limited T/45

4 Figure 3. Dependence of adhesion of PVC coating to primer G-1083 at temperature of 140 C on concentration in coating of (1) polyamide resin, (2) caprolactam, and (3) triethylenetetramine (TETA) It must be pointed out that, with a concentration of aliphatic amines in the adhesive of over 1.5%, when PVC plastisols are applied to epoxy-containing primers, breakdown of the primed layer and the formation of a porous structure of the coating occur. As a result there is a sharp fall in the adhesion strength (Figure 3, curve 3). This is due to the fact that aliphatic amines are thermally unstable compounds [12] that are capable of migrating to the surface and of post-curing the epoxy resin contained in the primer. This lowers the elasticity of the primer and its adhesion to the metal surface. PVC coatings for the bottom of carbodies undergo environmental effects during service, most dangerous of which are corrosion and stone impacts. On this basis, tests of the developed coatings were carried out in a humidity chamber (relative humidity 95 ± 5% and temperature 40 ± 2 C) and in a salt cloud chamber (temperature 35 ± 2 C, 5% NaCl solution). An assessment was also made of the abrasion resistance of anticorrosion materials under the action of iron powder [3]. The magnitude of the change in service properties of PVC coatings as a result of the action of corrosive factors depends [13] on the density and defectiveness of their structure, which determines the coefficient of diffusion of corrosive liquids. As a result of the adsorption of moisture and a saline solution of NaCl, the process of cracking of PVC coatings is accelerated, which leads to a reduction in their deformation and strength properties and their adhesion characteristics. Here, a key role is played by the activity of the chemical media. Thus, water is more polar than an aqueous solution of NaCl, and its molecules, which have a size of the order of 4 6 Å, penetrate into a greater number of defects in the polymer coating. The formation by them of hydrogen bonds with the PVC molecules is possible [13]. Coatings based on PVC plastisols containing imported components (adhesion promoter, whiting, etc.) have a slightly greater coefficient of resistance in corrosive media by comparison with the developed coatings. However, this difference is not significant. It is evidently due to the use in our formulation of the plasticiser EDOS, which lowers the density of PVC coatings [6]. This, in turn, lowers their resistance in corrosive media by easing the diffusion into the pores of the PVC grains. At the same time, owing to its greater strength, the developed PVC coating possesses increased abrasion resistance [3]. Thus, on the whole, the developed coating is not inferior to the imported analogue in terms of its resistance to service factors and the main deformation and strength and processing properties [3]. Here, the developed composition for anticorrosion protection of carbody elements has better economic and hygiene characteristics. It can be used not only by the motor industry but also as protective coatings for different types of metalwork. REFERENCES 1. A. A. Gudkov et al., Main principles of creation of PVC plastisols for anticorrosion protection of metalwork. Reference book. GASIS, Moscow, 2002, 36 pp. 2. Yu. A. Merinov et al., Features of production, structure of particles, and properties of copolymers based on vinyl chloride for plastisols. NIITEKhIM, Moscow, 1990, 77 pp. 3. A. A. Gudkov, Development of PVC plastisol for motor industry. Author s abstract of thesis, SGTU, Saratov, 2005, 19 pp. 4. E. M. Gotlib and Yu. A. Sokolova, New plasticised polyvinyl chloride and polyvinyl acetate materials. Reference book. GASIS, Moscow, 2001, 112 pp. 5. E. M. Gotlib et al., Anticorrosion coatings based on PVC plastisols. Papers of Scientific Proceedings of 2nd Sunday Lectures on Polymers in Building Industry, Kazan, 2004, pp T/46 International Polymer Science and Technology, Vol. 33, No. 5, 2006

5 6. E. M. Gotlib et al., New plasticiser for polymeric building materials. Reference book. Moscow, 1997, 33 pp. 7. A. A. Tager, Physical chemistry of polymers. Khimiya, Moscow, 1968, 536 pp. 8. O. G. Barashkov and R. S. Barshtein, Properties of PVC composites containing mixtures of plasticisers. Plast. Massy, No. 10, 1987, pp A. D. Yakovlev, Chemistry and technology of paint and varnish coatings. Khimiya, Leningrad, 1989, 384 pp. 10. K. S. Minsker and G. E. Zaikov, Achievements and problems of research in field of ageing and stabilisation of polyvinyl chloride. Plast. Massy, No. 4, 1995, pp I. L. Knunyants (Ed.), Large encyclopaedic dictionary. Bol shaya Rossiiskaya Entsiklopediya, Moscow, 2000, 792 pp. 12. I. Z. Chernin et al., Epoxy polymers and composites. Khimiya, Moscow, 1982, 230 pp. 13. A. I. Tynnyi, Strength and breakdown of polymers under action of liquid media. Naukova Dumka, Kiev, 1975, 216 pp. (No date given) 2006 Smithers Rapra Limited T/47