Patent Publication Number: US-2013244024-A1

Title: Modified sheet silicates as corrosion protection

Description:
DESCRIPTION 
     The present invention relates to the use of modified sheet silicates in combination with polycationic polymers for protecting metal surfaces against corrosion. The invention further relates to metallic surfaces which are protected against corrosion by means of modified sheet silicates in combination with polycationic polymers. The invention further provides methods of protecting metallic surfaces against corrosion and provides anticorrosion compositions. 
     Further embodiments of the present invention may be found in the claims, the description and the examples. It goes without saying that the abovementioned features of the subject matter of the invention and those still to be explained below can be used not only in the combination specifically indicated in each case but also in other combinations without going outside the scope of the invention. The embodiments of the present invention in which all features have the preferred or very preferred meanings are preferred or very preferred. 
     Protection of metal surfaces against corrosion is a field which has been intensively studied for a very long time since the damage caused by corrosion is of great economic importance and a high outlay for precautions and repair of damage is regularly expended. 
     Corrosion of metals can be attributed essentially to chemical and electrochemical corrosion reactions. Anticorrosion pigments intervene in the corrosion process in various ways. They act physically by extending the diffusion path of water, oxygen and further corrosive substances from the surface of the coating to the metal surface. Electrochemically acting anticorrosion pigments passivate the metal surface. 
     Anticorrosion pigments are frequently based on compounds which are nowadays no longer used or used to only a very restricted extent because of their properties which damage human health and the environment, for example lead-comprising and chromate-comprising anticorrosion pigments. The phosphating of metallic surfaces is also relatively undesirable in this respect for ecological reasons because of the additions of nickel required for good phosphating. 
     It is known that nanoparticles, for example of silicon dioxide, titanium dioxide, iron oxides or manganese oxides, can display a corrosion-inhibiting action. However, this action of the nanoparticles is not lasting: the corrosion process is merely delayed for some time and the nanoparticles used as corrosion protection lose their effect relatively quickly after the corrosion process has commenced. 
     The corrosion-inhibiting action of oxidic nanoparticles having an average particle size of from 2 to 2000 nm in combination with at least one polycationic polymer on metallic surfaces is known from WO 2011/067329. 
     Furthermore, JP 2004315762 A discloses compositions comprising fine particles, dispersion media and corrosion inhibitors. The fine particles can be sheet compounds such as montmorillonites. 
     Thin films comprising montmorillonites and branched polyethylenimines are described in M. A. Priolo et al., Applied Materials &amp; Interfaces, Vol. 2, No. 1, pp. 312-320 (2010). 
     The rheological properties of charged polyelectrolytes on montmorillonite suspensions are described in M. M. Ramos-Tejada et al., J. Rheol. 50(6), 995-1007 (2006). 
     It was therefore an object of the present invention to provide further-improved corrosion protection which does not need compounds which are known to be problematical in terms of environmental toxicity, for example those based on lead, chromium or nickel. 
     These and other objects are, as can be seen from the disclosure content of the present invention, achieved by the various embodiments of the use according to the invention of modified sheet silicates (a) in combination with at least one polycationic polymer (b) for protecting metallic surfaces against corrosion. 
     For the purposes of the present invention, modified sheet silicates (a) are sheet silicates which have been modified by the interaction with polycationic polymers (b). 
     The metallic surfaces which have been protected in accordance with the invention by a combination of modified sheet silicates and at least one polycationic polymer display a higher resistance to corrosion than metallic surfaces which have been protected by oxidic nanoparticles having an average particle size of from 2 to 2000 nm in combination with at least one polycationic polymer. 
     The positive effect of the polycationic polymers on the corrosion protection afforded to metallic surfaces by modified sheet silicates is probably due to the increase in the zeta potentials of the modified sheet silicates as a result of the presence of the polycationic polymers. A possible explanation of this positive effect is that for many substrates the reduction in cationic oxygen caused by incipient corrosion processes often increases the pH to about 9-13. The modified sheet silicates are, as a result of the negative charge, probably less strongly bound to the generally likewise negatively charged metallic substrate surfaces. The polycationic polymers become attached to the surface of the modified sheet silicates and increase the zeta potential thereof. It can be assumed that the adsorption of the modified sheet silicates on the metallic substrate surface is thereby stabilized even when the pH is increased as a result of the corrosion processes taking place. This effect is independent of whether the modified sheet silicates or the polycationic polymers are applied in a polar dispersion medium such as water or a nonpolar dispersion medium such as petroleum spirit as application medium. Significantly improved protection of metallic surfaces against corrosion is also obtained when the active components are used in a binder. 
     A further surprising improvement in corrosion protection has been found for the combination of modified sheet silicates and polycationic polymers compared to the combination of oxidic nanoparticles with polycationic polymers. 
    
    
     The present invention is described in detail below. 
     The modified sheet silicates (a) and the at least one polycationic polymer (b) are usually used together in a formulation comprising (a) and (b). 
     The modified sheet silicates generally have an average particle size of from 50 nm to 50 μm, preferably from 100 nm to 10 μm, and particularly preferably from 150 nm to 5 μm. The average particle size is usually determined by means of AFM and TEM and corresponds to the averaged diameter of the particles. 
     In a preferred embodiment, the modified sheet silicates (a) have a platelet-like particle shape. In particular, the ratio of thickness to diameter of the platelet-like particles is from 1:50 to 1:3, preferably from 1:30 to 1:10. Preferred platelet-like modified sheet silicate particles are obtained by delamination of the silicate sheets (cf., for example, M. Segad, et al., Langmuir 26 (8), pp. 5782-5790, 2010). The platelet diameter is then particularly preferably from 100 to 300 nm, and the layer thickness is frequently from about 1 to 10 nm. The particle shape of the modified sheet silicates (a) is advantageously measured by electron microscopic analysis, for example by means of AFM or TEM. 
     The modified sheet silicates (a) preferably comprise sheet silicates selected from among the natural sheet silicates antigorite, kaolinite, halloysite, saponite, talc, pyrophyllite, hectorite, montmorillonite, bentonite, vermiculite, muscovite, biotite and mixtures thereof. 
     Modified sheet silicates suitable for the purposes of the invention are commercially available, for example under the trade names Ceratosil®, Opazil ®, Perstab®, Printosil®, Pitchbent® from Südchemie, or from Southern Clay Products Inc. under the trade names Bentolite®, Mineral Colloid®. 
     The modified sheet silicates (a) to be used according to the invention are generally compatible with water or polar solvents. 
     For use in nonpolar media such as petroleum spirit, treatment of the modified sheet silicates (a) with a compound which hydrophobicizes the surface can be advantageous. Substances suitable for hydrophobically modifying the modified sheet silicates (a) are known to those skilled in the art. The modification can be effected, for example, by treatment with hexamethylenedisilazane, octamethylcyclotetrasiloxane, stearic acid or polypropylene oxide. Such additionally hydrophobicized sheet silicates (a) are also commercially available. In a preferred embodiment of the invention, additionally hydrophobicized sheet silicates (a) are used. This applies particularly when the modified sheet silicates (a) are used in a nonpolar application medium or dispersion medium. 
     In another preferred embodiment of the present invention, the modified sheet silicates (a) (modified by the cationic polymers) are used without further modification, particularly when the application medium or dispersion medium is polar, in particular when water is used as application medium or dispersion medium. 
     According to the invention, at least one modified sheet silicate (a) is used; thus, it is possible to use either one modified sheet silicate (a) or mixtures of two, three or more modified sheet silicates (a). 
     According to the invention, the modified sheet silicates (a) are used in combination with at least one polycationic polymer. 
     For the purposes of the present invention, polycationic means that the polymer has a minimum charge density greater than 1 mequ/g, preferably from 5 to 25 mequ/g and particularly preferably from 10 to 20 mequ/g, in each case measured at a pH of from 4 to 5. According to the invention, it is possible to use all polymers which either comprise free or alkyl-substituted amino groups or quaternary ammonium groups in the polymer chain or bear secondary or tertiary amino groups or quaternary ammonium groups bound directly or via linkers to the polymer chain. These amino or quaternary ammonium groups can also be members of 5- or 6-membered ring systems, for example morpholine, piperidine, piperazine or imidazole rings. According to the invention, the cationic polymer can be selected from among polyamides, polyimines and polyamines, preferably highly branched polyamines, for example Lupasol® from BASF SE, polydiallyldimethylammonium chloride, polyvinylamine, polyvinylpyridine, polyvinylimidazole and polyvinylpyrrolidone, and also natural and semisynthetic polymers including cationically modified starch and mixtures thereof. 
     The polycationic polymers to be used according to the invention preferably have a number average molecular weight in the range from 500 to 2 000 000 g/mol, preferably from 10 000 g/mol to 1 000 000 g/mol, particularly preferably from 20 000 to 750 000 g/mol. 
     As polycationic polymer (b), preference is given to using polyethylenimine, where the polyethylenimine preferably has a number average molecular weight of from 500 g/mol to 125 000 g/mol and particularly preferably from 750 g/mol to 100 000 g/mol. 
     The polycationic polymers can be linear, branched, hyperbranched or be present as dendrimers and are preferably present as hyperbranched polymers or dendrimers. 
     Particular preference is given, according to the invention, to using polyethylenimine which is present as hyperbranched polymer or dendrimer. Such polyethylenimines can be obtained, for example, under the trade name Lupasol® from BASF SE. A more precise description of such polyimines may be found, for example, in Macromolecules Vol. 2, H.-G. Elias, 2007 Vol. 2, pages 447 to 456. 
     The term dendritic polymer or highly branched polymer is the generic term for a series of different branched molecular structures. They include, for example, dendrimers, star polymers and hyperbranched polymers. 
     Dendrimers are formed by outward growth from a center (generally a small molecule having a plurality of reactive end groups) onto which a branching monomer is attached by means of a continually repeated controlled reaction sequence, generation by generation. Thus, the number of monomer end groups in the dendrimer formed increases exponentially with each reaction step. A characteristic feature of dendrimers is the number of reaction stages (generations) carried out to form them. Due to their uniform structure (in the ideal case, all branches comprise exactly the same number of monomer units), dendrimers are essentially monodisperse, i.e. they generally have a defined molar mass. Molecularly and structurally uniform highly branched polymers will hereinafter also be referred to uniformly as dendrimers. 
     For the purposes of the present invention, “hyperbranched polymers” are highly branched polymers which, in contrast to the abovementioned dendrimers, are both molecularly and structurally nonuniform. Hyperbranched polymers therefore have a nonuniform molar mass distribution (polydispersity). Hyperbranched polymers, are produced by various distinct synthesis strategies. An overview of possible synthetic methods may be found in C. Gao, D. Yan, Prog. Polym. Sci. 29 (2004), 183. 
     For the definition of dendritic and hyperbranched polymers, see also P. J. Flory, J. Am. Chem. Soc. 1952, 74, 2718 and H. Frey et al., Chemistry—A European Journal, 2000, 6, No. 14, 2499. 
     Dendritic polymers can be characterized by their degree of branching. For the definition of the degree of branching, reference may be made to H. Frey et al., Acta Polym. 1997, 48, 30. The degree of branching DB is defined as DB (%)=(T+Z)/(T+Z+L)+100, where 
     T is the average number of terminally bound monomer units,
 
Z is the average number of monomer units which form branches,
 
L is the average number of linearly bound monomer units.
 
     The parameters T, Z and L can be determined, for example, by means of  13 C-NMR in D 2 O. 
     Dendrimers generally have a degree of branching DB of at least 99%, especially from 99.9 to 100%. 
     Hyperbranched polymers preferably have a degree of branching DB of from 10 to 95%, preferably from 25 to 90% and in particular from 30 to 80%. 
     The polycationic polymers used according to the invention, in particular polyethylenimines, preferably have a degree of branching (DB) per molecule of greater than 10%, in particular from 10 to 99%, preferably greater than 20%, in particular from 20 to 90%, particularly preferably greater than 30%, in particular from 30 to 80%, and very particularly preferably greater than 50%, in particular from 50 to 80%. 
     In a particularly preferred embodiment of the invention, the at least one polycationic polymer is polyethylenimine which has a number average molecular weight of from 500 g/mol to 125 000 g/mol, preferably from 750 g/mol to 100 000 g/mol, and is present as dendrimer. 
     According to the invention, at least one polycationic polymer (b) is used; it is thus possible to use either one polycationic polymer or mixtures of two, three or more polycationic polymers. 
     According to the invention, it is also possible to use mixtures of a plurality of modified sheet silicates (a) and polycationic polymers (b). 
     The combination of modified sheet silicates (a) and polycationic polymer (b) provides very effective protection against corrosion even in low concentrations. The at least one polycationic polymer (b) is preferably used in a substoichiometric amount relative to the amount (weight) of modified sheet silicates (a). The weight ratio of the at least one polycationic polymer (b) to modified sheet silicate (a) is, according to the invention, preferably from 1:1000 to 2:1, particularly preferably from 1:100 to 1:1, in particular from 1:100 to 1:2. 
     In general, especially in the case of zinc-plated surfaces or aluminum, the pH increases as a result of the corrosion processes, usually to pH values above 9.5. As a result, the surface of the modified sheet silicates becomes negatively charged, i.e. the zeta potential of the modified sheet silicates becomes negative and desorption of the modified sheet silicates from the likewise negatively polarized (charged) metal surface occurs. As a result of the use of the modified sheet silicates in combination with at least one polycationic polymer, the zeta potential of the modified sheet silicates in the presence of the at least one polycationic polymer in the pH range from 4 to 13, preferably in the pH range from 7 to 11, is increased to at least −2, preferably to at least −1 and particularly preferably to at least 0, measured at 25° C. 
     The modified sheet silicates in combination with the at least one polycationic polymer are usually applied with the aid of an application medium to the surface to be protected. The modified sheet silicates (a) and the at least one polycationic polymer (b) are preferably used in an application medium in a total concentration of at least 0.1% by weight, based on the total amount of application medium, (a) and (b), preferably in a total concentration of at least 0.5% by weight. The modified sheet silicates (a) and the at least one polycationic polymer (b) are usually used in a total concentration of not more than 3% by weight, preferably not more than 2.5% by weight, based on the total amount of application medium, (a) and (b), since no further improvement in the anticorrosion effect can be achieved when the total concentration of (a) and (b) is increased, so that higher concentrations are not normally employed for economic reasons. 
     According to the invention, all metallic surfaces which can usually be damaged by corrosion can be protected by means of the modified sheet silicates (a) in combination with the at least one polycationic polymer (b). These include, for example, steel surfaces and zinc-plated surfaces, surfaces composed of Al and Mg and also alloys, for example alloys based on Zn/Mg. 
     The present invention further provides a method of protecting metallic surfaces against corrosion, which comprises the steps:
         (i) provision of a formulation comprising modified sheet silicates (a) and at least one polycationic polymer (b) as described above and an application medium,   (ii) application of the formulation to the metallic surface to be protected and   (iii) optionally drying and/or heat treatment of the surface.       

     In step (i) of the method of the invention, a formulation comprising modified sheet silicates (a) and at least one polycationic polymer (b), as have been described comprehensively above, is provided. The weight ratio of the at least one polycationic polymer (b) to modified sheet silicates (a) is preferably from 1:1000 to 2:1, particularly preferably from 1:100 to 1:1, in particular from 1:100 to 1:2. The formulation further comprises an application medium. The application medium serves as medium for applying the modified sheet silicates (a) and the at least one polycationic polymer (b) to the surface to be protected. This application medium is preferably flowable. The application medium can be simple solvents such as water, petroleum spirit, alcohols and the like, but coating systems which already comprise binders and optionally further additives which are customary for this purpose can also be used as application medium. 
     The modified sheet silicates and the at least one polycationic polymer are dissolved or dispersed in the application medium. The choice of application medium for producing the formulation is carried out in accordance with the prerequisites of the end use and extends to solvent-based/oleophilic systems and to water-based systems. It is thus possible to use all known solvents such as water, alcohols, glycols, esters, ketones, amides, hydrocarbons such as synthetic oils and waxes, and also natural systems such as linseed oil, modified linseed oils (alkyd resins) and natural waxes. The polycationic polymers become attached, inter alia, to the surface of the sheet silicates modified thereby. When a binder system is used as application medium, it is also possible firstly to produce a mixture of the modified sheet silicates and the at least one polycationic polymer in a solvent and subsequently introduce this mixture into the binder system. 
     In step (ii), the formulation is applied to the metallic surface to be protected. The formulation can be applied by known methods such as dipping, spraying, doctor blade coating, painting, rolling and so forth. 
     This is optionally followed by step (iii) drying and/or heat treatment of the surface. 
     The present invention further provides an anticorrosion composition comprising: 
     from 0.1 to 3% by weight of modified sheet silicates (a) and at least one polycationic polymer (b), where the weight ratio of (b) to (a) is from 1:1000 to 1:1, preferably from 1:100 to 1:2, in particular from 1:100 to 1:2,
 
from 97 to 99.9% by weight of liquid dispersion medium,
 
from 0 to 5% by weight of at least one inorganic salt selected from the among phosphates and fluorides of Li, Na, K, Mg, Ca, Ba, Zn, Mn, Fe, Ti and Zr, based on the total amount of the anticorrosion composition.
 
     In a further embodiment, the anticorrosion composition comprises: 
     from 0.1 to 3% by weight of modified sheet silicates (a) and at least one polycationic polymer (b), where the weight ratio of (b) to (a) is from 1:1000 to 1:1, preferably from 1:100 to 1:2, in particular from 1:100 to 1:2,
 
from 0.1 to 30% by weight of at least one emulsifier,
 
from 5 to 90% by weight of liquid dispersion medium,
 
from 0 to 5% by weight of at least one inorganic salt selected from among phosphates and fluorides of Li, Na, K, Mg, Ca, Ba, Zn, Mn, Fe, Ti and Zr,
 
based on the total amount of the anticorrosion composition.
 
     The modified sheet silicates (a) and polycationic polymers (b) can of course also have the above-described preferred properties, for example in respect of the particle size, the particle shape or the constitution, in the anticorrosion composition. The abovementioned modified sheet silicates (a) and polycationic polymers (b) are thus used for the anticorrosion composition of the invention. 
     If the at least one inorganic salt is comprised in the anticorrosion composition, its minimum concentration is 0.1% by weight, based on the total amount of the anticorrosion composition. 
     The modified sheet silicates (a) comprised in the anticorrosion composition and the at least one polycationic polymer (b) comprised therein have been described above. The anticorrosion composition of the invention preferably comprises antigorite, kaolinite, halloysite, saponite, talc, pyrophyllite, hectorite, montmorillonite, bentonite, vermiculite, muscovite, biotite and mixtures of these as modified sheet silicates and polyethylenimine as polycationic polymer. Particular preference is given to using polyethylenimine having a number average molecular weight of from 500 g/mol to 2 000 000 g/mol, preferably from 10 000 g/mol to 1 000 000 g/mol, particularly preferably from 20 000 to 750 000 g/mol. Very particular preference is given to using polyethylenimine which is present as a dendrimer. 
     As liquid dispersion medium, it is possible to use the compounds and systems mentioned above as application medium. 
     The present invention further provides metallic surfaces which are protected against corrosion using, in accordance with the invention, modified sheet silicates (a) in combination with at least one polycationic polymer (b), as described above. 
     The invention is illustrated below by means of examples without the examples limiting the subject matter of the invention. 
     A) Production of the Silicate Dispersions 
     The dispersions were produced by mixing 980 g of water with 10 g of a polyethylenimine (Lupasol® from BASF) and 10 g of silicate. An appliance from VMA-Getzmann GmbH having the trade name Dispermat® FE was used as stirring apparatus. The stirring speed was set to 6000-7000 rpm and a dispersing disk was used (diameter 40 mm, tooth height 2 mm). 
     While the spherical silicates (silica gels) of the prior art (cf. WO 2011/067329) are present in nanosize form, delamination of the sheet silicates occurs in water and is presumably aided by the cationic polymers described. 
     B) Electrochemical Studies 
     Confirmation of the (corrosion-)inhibiting action is carried out by means of electrochemical measurement methods using Tafel plots (see Stephen Tait; Introduction to Electrochemical Korrosion Testing for Practical Engineers and Scientists, PairODocs Publication 1994, page 55 ff; ISBN 0-9660207-0-7). Here, current-voltage curves in which, in particular, the position of the OCP (open circuit potential) and its change/constancy over time permit a conclusion as to a) the passivation and b) the stability of the corrosion protection are determined. The measurements were carried out using a VFP 600 potentiostat from Gamry. The measurement was carried out on untreated steel plates (Gardobond OC; 10.5×19 cm, from Chemetall) in comparison with metal plates which had been treated with the silicate dispersions of the invention. 
     C) Corrosion Tests 
     In addition to the electrochemical studies, corrosion tests were carried out in accordance with DIN 50017 or DIN 10289. For this purpose, cleaned plates were dipped into the dispersions described at 50° C. for 3 minutes and, after intermediate cleaning by dipping into a rinsing bath filled with deionized water, treated with a cationic dip coating composition (CDCC). Corrosion testing was in each case carried out using 2 plates in the salt spray test in accordance with DIN 50017. The evaluation in respect of undermining at the score mark was carried out as described in DIN EN ISO 4628-8. The method of carrying out cationic dip coating of test plates is described, inter alia, in VDA 230-213, Chapter 5.9.7. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Summary of the results of the tests 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 Undermining 
               
               
                   
                   
                   
                   
                   
                 of cationic dip 
               
               
                   
                   
                 Poly- 
                 OCP, 
                 OCP, 
                 coat at the 
               
               
                 Ex- 
                   
                 ethylenimine 
                 1 min 
                 120 min 
                 score mark * 
               
               
                 ample 
                 Silicate 
                 Mw [g/mol] 
                 [mV] 
                 [mV] 
                 [mm] 
               
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 C1 
                 Aerosil ® 200 
                    800 
                 −430 
                 −550 
                 2.3 
               
               
                   
                 spherical 
                 Lupasol FG 
                   
                   
                   
               
               
                 C2 
                 Aerosil ® 200 
                 750 000 
                 −415 
                 −530 
                 2.1 
               
               
                   
                 spherical 
                 Lupasol P 
                   
                   
                   
               
               
                 3 
                 Opazil ® AO 
                    800 
                 −400 
                 −439 
                 1.8 
               
               
                   
                 sheet silicate 
                 Lupasol FG 
                   
                   
                   
               
               
                 4 
                 Opazil ® AO 
                 750 000 
                 −372 
                 −412 
                 1.4 
               
               
                   
                 sheet silicate 
                 Lupasol P 
                   
                   
                   
               
               
                 5 
                 Ceratosil ® 
                    800 
                 −385 
                 −426 
                 1.3 
               
               
                   
                 XXMG 
                 Lupasol FG 
                   
                   
                   
               
               
                   
                 sheet silicate 
                   
                   
                   
                   
               
               
                 6 
                 Ceratosil ® 
                 750 000 
                 −365 
                 −398 
                 1.0 
               
               
                   
                 XXMG 
                 Lupasol P 
                   
                   
                   
               
               
                   
                 sheet silicate 
                   
                   
                   
                   
               
               
                 7 
                 Ceratosil ® 
                  25 000 
                 −375 
                 −408 
                 1.1 
               
               
                   
                 XXMG 
                 Lupasol HF 
                   
                   
                   
               
               
                   
                 sheet silicate 
                   
                   
                   
                   
               
               
                 8 
                 Ceratosil ® 
                 — 
                 −410 
                 −715 
                 2.9 
               
               
                   
                 XXMG 
                   
                   
                   
                   
               
               
                   
                 sheet silicate 
                   
                   
                   
                   
               
               
                 C9 
                 — 
                 — 
                 −590 
                 −780 
                 5.3 
               
               
                   
               
               
                 C1 and C2 are comparative examples using spherical particles; 
               
               
                 C9 is a comparative example without silicate and polymer (blank); 
               
               
                 * DIN EN ISO 4628-8 
               
               
                 Aerosils ® are pyrogenic silicas, Opazils ® and Ceratosils ® are alkali-activated sheet silicates. 
               
            
           
         
       
     
     It is found that the sheet silicates which have been treated with a cationic polymer offer even better corrosion protection than spherical silica gels treated in a corresponding way. 
     Relatively high molecular weight species display improved properties compared to the low molecular weight species, so that polymers in the molar mass range from 25 000 to 2 000 000 are preferably used. 
     D) Dip Coating 
     The pretreatment of metal sheets increases the protective action of a cationic dip coating composition. Cationically modified sheet silicates can also be used as formulation constituents of a cationic dip coating composition (1-5% based on solids/pigment).