Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D9/00—Electrolytic coating other than with metals
  • C25D9/04—Electrolytic coating other than with metals with inorganic materials
  • C25D9/06—Electrolytic coating other than with metals with inorganic materials by anodic processes
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D11/00—Electrolytic coating by surface reaction, i.e. forming conversion layers
  • C25D11/02—Anodisation
  • C25D11/34—Anodisation of metals or alloys not provided for in groups C25D11/04 - C25D11/32

Definitions

• the present invention relates to a method for corrosion-protective pretreatment of tinplate, in which already in a single process step, a corrosion-protective
• the tinplate is anodically polarized in an electrolyte containing silicates of the composition M 2 O nSiO 2 , where M is an alkali metal ion or quaternary ammonium ion and n is a natural number between 0.8 and 7.
• a subsequent conventional passivation of the tinplate surface can additionally permanently preserve the metallic appearance of the tinplate surface, wherein passivation with an acidic aqueous composition based on water-soluble compound of zirconium and titanium is particularly effective and offers additional advantages.
• Tinplate pretreated in accordance with the invention can be used in particular for the production of food-safe packaging such as beverage cans or cans.
• Tinplate strip is considered in the food industry as a suitable material for the production of packaging units for holding liquids or preserved foods, since tinplate due to the electrochemically noble tin layer even over a prolonged period only small amounts of potentially harmful tin salts in contact with the tin surface Delivers food product. Tinplate strip is therefore an important
• tinplate for can production, which is already provided with an organic topcoat, in order to further minimize the entry of iron salts, which can enter the product if the protective tin layer is damaged and negatively affect the taste of the food.
• lacquered tinplate strip it is necessary to pretreat the tin surface, on the one hand to ensure the adhesion of the paint on the metal surface and on the other hand to build up an additional protection against corrosive infiltration of the paint.
• a suitable pretreatment is the chromating of the tin surface which is still widespread in the state of the art.
• pretreatments of tinplate are known, which include the electrochemical modification of the tin surface and subsequent passivation.
• the aim of this pretreatment methods described in the prior art is in addition to the provision of a suitable Lackhaft groundes to protect against corrosion, in particular the guarantee of color fidelity of pretreated and painted tinplate products in contact with food, the
• the GB 479,746 already describes the problem of discoloration of the inner surfaces of containers made of tinplate, which are in contact with proteinaceous foods, and proposes the
• Tin plate in an ammoniacal electrolyte to impart an anodic current that makes the tin surface insensitive to discoloration by sulfur-containing compounds.
• the anodically oxidized tinplates according to GB 479,746 are then provided with an organic topcoat.
• No. 3,491,001 describes a process for the passivation of tinplates, in which, after an anodic pretreatment in an alkaline electrolyte, a cathodic treatment of the tinplate in an alkaline chromate-containing electrolyte follows.
• An electrolytic process chain as described in US Pat. No. 3,491,001, protects the tin surface against corrosion and before
• the electrolytic chromium-containing passivation also serves as a primer for subsequently applied organic topcoats.
• the object of the present invention is, in particular, to pretreat tinplate products for the production of food packaging in one process step and with as little staining as possible of tin so that an excellent adhesion of organic topcoats to the tinplate with simultaneously durable resistance of the pretreated and painted tin surfaces to discoloration by sulfidic Connections is guaranteed.
• This object is achieved in a process for the electrolytic passivation of tinplate by anodic polarization in an alkaline aqueous electrolyte, wherein the electrolyte contains at least one water-soluble silicate of the composition M 2 0 nSi0 2 , where M is an alkali metal ion or quaternary ammonium ion and n a natural number is between 0.8 and 7.
• Salts which are constituents of the electrolyte in the process according to the invention are considered to be water-soluble in the context of the present invention if their solubility in water at a pH of 8 and a temperature of 20 ° C. is at least 50 g / l, based on the particular salt ,
• water-soluble silicates are compounds of the general empirical formula M 2 O.sub.SiO 2 with M as the alkali metal ion or quaternary ammonium ion and n as the natural number between 0.8 and 7, which at a pH of 8 and a temperature of 20 ° C have a solubility of at least 1 g / l based on Si0 2 .
• the alkali metal ions M of the water-soluble silicates are preferably selected from Li, Na and K.
• quaternary ammonium ions with aliphatic radicals, each having not more than 10 carbon atoms, are equally preferred in electrolytes of the process according to the invention.
• Suitable water-soluble silicates are in particular the so-called water glasses, which are produced by melting Si0 2 with the respective oxide M 2 Q.
• Preferred are such Water glasses whose content of Si0 2 is in the range of 20-40 wt .-%. There are such
• Water glasses are particularly preferred, the molar ratio of Si0 2 : M 2 0 in the range of 2 to 5, in particular in the range of 3 to 4.
• the presence of at least one water-soluble silicate in the electrolyte of the process according to the invention causes a thin silicate layer to be produced on the tinplate during the anodic polarization, which already constitutes a good primer for subsequently applied organic topcoats.
• a thin silicate layer to be produced on the tinplate during the anodic polarization, which already constitutes a good primer for subsequently applied organic topcoats.
• the inventive method thus provides a pretreatment of
• Tinplate products ready, which takes place in a process step and modified tin surfaces suitable to use tinplate as packaging material for food.
• the proportion of water-soluble silicates in the electrolyte of the process according to the invention is preferably at least 0.1% by weight, more preferably at least 1% by weight, particularly preferably at least 2% by weight, but preferably less than 30% by weight preferably less than 20 wt .-% in each case based on the proportion of Si0 second Below a share of
• the layer support based on the element Si, which can be deposited at anodic polarization on the tinplate surfaces is too low to exert a positive effect on the adhesion to subsequently applied organic coating systems.
• the pH of the electrolyte is preferably in a range from 8 to 13, particularly preferably in a range from 10 to 12.
• the tin layer of the tinplates is corroded, while at pH values below 8 the electrolyte is corroded Water solubility of silicates decreases sharply and increasingly Si0 2 is precipitated.
• the electrolyte may additionally contain in the process according to the invention at least one organosilane which, as such, effects improved silicatization of the tinplate surfaces and, moreover, improves adhesion to subsequently applied organic lacquer systems via suitable functionality in the non-hydrolyzable organic radical. It is in the
• organosilanes as constituents of the electrolyte which have at least one hydrolyzable substituent, which is cleaved on hydrolysis as an alcohol having a boiling point below 100 ° C, and at least one non-hydrolyzable substituent, said non-hydrolyzable substituent preferably at least partially having primary amino functions.
• Organosilane selected from compounds of the following general structural formula (I):
• n and n are each independently integers between 1 and 4 and y is an integer between 0 and 4.
• the proportion of organosilanes in the electrolyte of the process according to the invention is preferably in the range of 0.01 to 5 wt .-%.
• the electrolyte of the process according to the invention may be admixed with water-soluble aluminum salts which do not contain halides, preferably in an amount of at least 0.001% by weight, but preferably not more than 1% by weight in total. on aluminum salts.
• the electrolyte may additionally comprise at least one organic dicarboxylic acid having not more than 6 carbon atoms and / or its water-soluble metal salt, which is preferably selected from succinic acid, malonic acid, oxalic acid, glutaric acid, adipic acid and / or their alkali metal salts, more preferably selected from oxalic acid and / or their alkali metal salts.
• organic dicarboxylic acid having not more than 6 carbon atoms and / or its water-soluble metal salt, which is preferably selected from succinic acid, malonic acid, oxalic acid, glutaric acid, adipic acid and / or their alkali metal salts, more preferably selected from oxalic acid and / or their alkali metal salts.
• the proportion of organic dicarboxylic acids in the electrolyte of the process of the invention is preferably in the range of 0.01 to 2 wt .-%.
• additives which may be present in the electrolyte of the process according to the invention are inert water-soluble salts which ensure adequate conductivity and thus ensure the viability of the electrolytic pretreatment process.
• inert salts do not participate in the electrode processes (tinplate, cathode) in aqueous solution, i. they are not involved in heterogeneous electron transfer reactions, and are used exclusively for the
• Suitable inert water-soluble salts are, for example, carbonates, phosphates, sulfates, nitrates and hydroxides of the alkali metals. Inert salts are the electrolyte in the
• the anodic polarization in the method according to the invention is preferably carried out at a
• anodic current densities above 6 A / dm 2 in the context of the present invention are disadvantageous, since at these current densities due to the semiconducting properties of the tin oxide layer, a large portion of the amount of electricity is applied for the evolution of oxygen.
• this oxygen evolution necessitates a strong reduction of the pH in front of the tinplate surface, resulting in increased corrosive removal of the tin oxide layer and, on the other hand, inhomogeneous oxide layers with local defects, which represent a less suitable primer for organic topcoats, due to the intensive evolution of gas bubbles.
• the duration of the anodic polarization in the process according to the invention is preferably at least 0.2 seconds, more preferably at least one second, since at lower
• the tin surface is mainly capacitively reloaded without a sufficient Faraday current flows, which is able to chemically modify the tin surface.
• a polarization time of more than 300 seconds, even at low current densities, does not improve the properties of the oxide covering layer as a coating adhesion base. Rather, with increasing polarization time, the amorphousness of the oxide layer seems to increase due to a constant repassivation of the surface, so that in processes with long-lasting polarization the
• Lacquer adhesion to the pretreated tinplate thus deteriorated.
• anodic polarization can be chosen freely in the method according to the invention and, for example, be potentiostatic, potentiodynamic, galvanostatic or galvanodynamic.
• the galvanostatic embossing of a current is preferred.
• a galvanostatic method according to the invention is therefore also preferred since fluctuations in the conductivity of the electrolyte or slight changes in the spatial orientation of the tinplate to the cathode do not affect the electrochemical modification of the Tin surface effect. If the method according to the invention is potentiostatic or
• a pulse method in which anodic current or voltage pulses are impressed is also suitable in the method according to the invention, wherein the individual pulse preferably lasts at least 0.2 seconds and the total anodic polarization time, that is to say summed over all anodic pulses, preferably not 300 seconds exceeds.
• cathodic polarization should preferably be avoided in the method according to the invention.
• the contacting of the electrolyte with the tinplate is preferably carried out for anodic polarization
• tinplate strip Insofar as only tinplate strip is treated according to the invention, which originates directly from the electrolytic production process for tin-plated strip material and which has not been oiled for transport purposes or for later forming, cleaning of the tinplate surface prior to carrying out the method according to the invention is not required. However, if the tinplate strip has already been stored and in particular wetted with anticorrosion or forming oils, then a cleaning step for removing organic soiling and salt residues is necessary in most cases before the tinplate can be anodically pretreated according to the invention. Surfactant cleaners known in the art can be used for this purpose.
• Process step can be pretreated so that a tin surface is present with good adhesion to subsequently applied organic topcoats and the tin surface at the same time shows a good resistance to sulfur-related black discoloration on contact with protein-containing foods.
• Both lacquer adhesion and resistance to discoloration may optionally be further improved by conventional post-passivation known to the person skilled in the art following a pretreatment according to the invention, but the advantage of a one-step process is given up in such a process.
• the application of an organic lacquer system immediately after the anodic pretreatment therefore takes place with or without intervening water rinsing and / or drying step.
• a method is preferred in which the anodic pretreatment with or without intervening Wasser Hughes- and / or drying step immediately followed by a Nachpassivitation, according to the anodically polarized tinplate is brought into contact with an acidic aqueous composition, the water-soluble inorganic compounds of the elements Zr, Ti, Hf and / or Si, particularly preferably of the elements Zr, Ti and / or Si, in particular of the elements Zr and / or Contains Ti.
• Suitable water-soluble inorganic compounds of these elements are in particular the respective fluorocomplex salts, fluoro acids and / or salts of the fluoro acids, particularly preferably the respective fluoro acids and / or salts of the fluoro acids.
• the acidic aqueous post-passivation composition comprises at least one water-soluble inorganic compound of the element titanium, which is preferably selected from the respective fluorocomplex salts, fluoro acids and / or salts of the fluoro acids of titanium.
• the proportion of the water-soluble inorganic compounds of the elements Zr, Ti, Hf and / or Si in the acidic aqueous composition of the post-passivation is preferably at least 0.001 wt .-%, particularly preferably at least 0.01 wt .-%, but preferably no more altogether as 0.5 wt .-% based on the respective element Zr, Ti, Hf and / or Si, wherein it is further preferred if at least 0.001 wt .-%, particularly preferably at least 0.01 wt .-% based on the acid composition of water-soluble compounds of the element titanium are included.
• the acidic aqueous post-passivation composition contains phosphate ions, preferably at least 0.01% by weight, more preferably at least 0.1% by weight, of the post-passivation acidic aqueous composition preferably not more than 3% by weight, based on PO 4 .
• the acidic aqueous composition for post-passivation of the anodically pretreated tinplate may contain water-soluble and / or water-dispersible organic polymers such as, for example, polyacrylates, polyisocyanates, polyepoxides, polyalkylamines, polyalkyleneimines or amino-substituted polyvinylphenol derivatives.
• water-soluble and / or water-dispersible organic polymers such as, for example, polyacrylates, polyisocyanates, polyepoxides, polyalkylamines, polyalkyleneimines or amino-substituted polyvinylphenol derivatives.
• the electrolyte additionally contains amino-functionalized organosilanes in the anodic pretreatment of the tinplate, preference is given to those water-soluble and / or water-dispersible organic polymers which are present under
• Condensation reactions can be further crosslinked, ie polyisocyanates, polyepoxides and / or mixtures thereof.
• the total proportion of water-soluble and water-dispersible organic polymers in the acidic aqueous composition of the post-passivation in a process according to the invention is preferably in the range from 0.05 to 10% by weight, particularly preferably in the range from 2 to 5% by weight.
• the pH of the acidic aqueous composition which is brought into contact with the anodically pretreated tinplate according to the invention is preferably in the range from 2.5 to 5.5.
• the post-passivation of the anodically pretreated tinplate be free of external current, i. without imprinting a current.
• Composition preferably takes place in the so-called "dry-in-place" process, in which a wet film of the acidic aqueous composition is applied to the tinplate surface, which is dried immediately after application Steel strip material is to be passivated.
• the acidic aqueous composition is applied in Nachpassivi fürsön in the so-called coil coating process.
• running metal strip is continuously coated.
• the acidic aqueous composition can be applied by different methods which are familiar in the prior art. For example, applicator rolls can be used to directly adjust the desired wet film thickness.
• the metal tape may be immersed in the acidic aqueous composition or sprayed with the acidic aqueous composition, after which the desired wet film thickness is adjusted by means of squeeze rolls.
• the thus coated tinplate is heated to the required drying temperature.
• the heating of the coated substrate to the required substrate temperature in the range from 120 to 260 ° C., preferably in the range from 150 to 170 ° C., can take place in a heated continuous furnace Nachpassivitation, however, by infrared radiation, in particular by near infrared radiation, to the corresponding drying or
• a process according to the invention which comprises both the anodic pretreatment in the silicate-containing electrolyte and the subsequent passivation in the acidic aqueous composition is characterized in that, owing to the silicatization of the tin surface already carried out in the anodic pretreatment step, a comparatively lower layer contact based on the elements Zr, Ti, Hf and / or Si must be applied in the post passivation to
• Tinplate in the course of Nachpassivitation a coating layer of at least 0.3 mg / m 2 , more preferably of at least 1 mg / m 2 , but not more than 20 mg / m 2 , more preferably not more than 10 mg / m 2 based on the Elements Zr, Ti, Hf and / or Si results.
• the invention relates to the use of tinplate treated in the method according to the invention for the production of packaging, in particular cans, for
• purified tinplate (tin coating 2.8 g / m 2 ) was first pretreated electrolytically, then rinsed with distilled water and subsequently applied, if necessary, a wet film of a passivating agent by means of Chemcoater ® and dried at 50 ° C for 1 min.
• the corresponding test series are listed in Table 1.
• the whitewares treated in this manner were immersed in a potassium sulfide solution (5 g / LK 2 S + 5 g / L NaOH in water) at 90 ° C. for half a minute without topcoat, rinsed with water and dried.
• a potassium sulfide solution (5 g / LK 2 S + 5 g / L NaOH in water) at 90 ° C. for half a minute without topcoat, rinsed with water and dried.
• Coating layer of titanium 3 mg / m 2 measured by means of X-ray fluorescence analysis (Axios Advanced, Panalytical) corresponds in addition to about 2 mg / m 2 layer coating of zirconium
• Chromating (0.12% by weight CrO 3 );
• Coating chromium 3 mg / m 2 measured by X-ray fluorescence analysis (Axios Advanced, Fa. Panalytical)

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• 2011
  • 2011-01-18 DE DE102011002836A patent/DE102011002836A1/de not_active Withdrawn
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