MODIFIED STARCH, AQUEOUS SOLUTION OF A MODIFIED STARCH AND PROCESS FOR PRETREATING STEEL SURFACES

[0001] The invention relates to a process, a compound and a composition for pretreating metal surfaces, in particular steel, for the purpose of enhancing corrosion resistance of the metal surfaces and/or adhesion of organic and inorganic coatings to the metal surfaces.

BACKGROUND

[0002J Metal surfaces need to be pretreated in order to provide sufficient anticorrosive strength. Superior anti-corrosive properties are obtained by pre-treatment with hexavalent chromium compounds. However, their carcinogenic and allergenic properties have led to an almost total ban on hexavalent chromium compounds. Trivalent chromium compounds are currently the most widely applied pre-treatment agents for steel, even though they perform less than hexavalent chromium. Environmental objections, however, also exist for trivalent chromium, and also for alternative agents based on other metals such as zinc, often combined with treatments based on phosphates. Therefore, the art has searched for alternative agents for providing resistance to corrosion, which are not objectionable in view of health of environmental implications.

[0003] US 1,365,760 describes a dispersion of borax in a dextrin solution as an anti- corrosive coating for meal surfaces. EP-B 850988 discloses an aqueous dispersion of highly acetylated, caprolactone-grafted starch, which is applied to aluminium foil, and, after drying and hot-rolling, forms a protective film.

[0004] US 6,194,033 discloses a composition for anticorrosive treatment of steel sheets containing an aromatic dye, colloidal silica, a hydroxylated and carboxylated polymer and an organic solvent such as tetrahydrofuran. The hydroxylated and carboxylated polymer is e.g. bisphenol epoxy resin, poly(vinyl butyral) or another synthetic resin; alginic acid and starch are mentioned among many other possible polymers, but are not preferred. The use of dyes and organic solvents is a disadvantage of this treatment. Unmodified or N- methylated chitosan in acid solution is proposed as an anticorrosive agent for steel are proposed in WO 01/21854. [0005] EP-A 1078963 describes the use of an emulsion of pregelatinised starch or crosslinked starch and a vegetable oil or wax for the treatment of metal surfaces, especially of steel. The resulting coating should facilitates the subsequent moulding process and provide an temporary anticorrosive effect. It is shown that starch alone does not provide corrosion resistance, and even accelerates corrosion.

[0006] US 6,508,958 presents a process for protecting aluminium surfaces against corrosion by applying a solution of an acid-modified chitosan. The chitosan is modified with polyfunctional acids such as aminotris(methylphosphonic acid) or adipic acid, so as to bridge chitosan strands and to hydrophobise the chitosan. [0007] The alternatives summarised above have not shown to be satisfactory in terms of both anticorrosive performance, acceptable cost and health and environmental safety. Hence, there is still a strong need for novel and improved means and methods for anticorrosive treatments of metal sheets.

DESCRIPTION OF THE INVENTION [0008] It has been found now that corrosion resistance of metal surfaces, in particular steel sheets, can be improved by treatment of the surfaces with an aqueous solution of a high-molecular weight modified polysaccharide. The modified polysaccharide contains anionic groups, whereas a part of the hydroxyl groups has been hydrophobised with alkyl or alkanoyl groups. The treatment can be carried under mild conditions and without health risks or emission of noxious or undesired components.

[0009] The polysaccharide to be used according to the invention can be any polysaccharide having repeating anhydroglycose units or their derivatives such as deoxy- anhydroglycose or anhydroglycuronic acid units. Examples are glucans, galactans, mannans, glucomannans, galactomannans, fructans, arabans, xylans, arabinoxylans, arabinogalactans, etc. Preferably, the polysaccharide is a glucan, which may be either an α-glucan (starch family etc.), or a β-glucan or xyloglucan (cellulose and chitin, tamarind kernel, β-1,3 and/or β-1,6 glucans such as curdlan and scleroglucan). Most preferred, for reasons of economic accessibility and solubility, are α-glucans, including starch, amylose, amylopectin, pullulan, nigeran, dextran, mutan, alternan, reuteran, and other α-1,3-, α- 1,4- and/or α-l,6-linked glucans. Especially preferred is starch, which may be from any source, including potato, corn, rice, tapioca, etc. Prior to its oxidation, the starch is preferably gelatinised in a manner known per se, such as heat treatment in water at 60°C, or treatment with alkali. [0010] The modified polysaccharide should have a molecular weight of at least 30 kDa (DP, degree of polymerisation of about 180), and the molecular weight may be as high as 10 MDa (DP about 60,000) or even higher. Preferably, the average molecular weight is at least 100 kDa or even 200 kDa, up to 3 MDa, or especially 1 MDa; the most preferred molecular weight is between 300 and 800 kDa (DP between about 1,800 and 5,000). If necessary, the molecular weight of a polysaccharide can be reduced by chemical

treatment, such as hydrolysis, prior to or after modification, or even during modification by adapting the modification conditions (e.g. the temperature and/or pH during oxidation).

[0011] The modified polysaccharide contains from 0.1 to 0.9 anionic groups per mono- saccharide unit, preferably between 0.15 and 0.5 anionic groups per unit. The anionic groups may be derived from carboxylic, phosphoric, sulphuric acid, and the like groups. The acid groups can be present as a result of substitution or addition of suitable acid- containing reagents. For example, carboxylic groups may result from carboxyalkylation, in particular carboxymethylation, or from reaction with an anhydride such as maleic or succinic anhydride. Phosphonic groups may be present as phosphate groups, resulting e.g. from reaction with phosphorylating agents (see e.g. WO 97/28298), or as phosphonic or phosphinic acid groups, resulting e.g. from reaction with halomethyl phosphonic acids. Sulphonic acids may be present e.g. as sulphate groups or as a result of sulphite addition to polysaccharide aldehydes (see e.g. WO 99/29354) or to maleic anhydride adducts (products with -0-CO-CH-CH(COOH)-SO ₃ H groups). Preferably, the anionic groups are carboxylic groups resulting from oxidation, e.g. of a (primary) hydroxymethyl group (-CH ₂ OH, usually at C6 of a monosaccharide unit, but also optionally of hydroxymethyl groups in substituents, such as hydroxyethyl or hydroxypropyl groups), or of a bis- (hydroxymethylene) group (-CHOH-CHOH-, usually at C2-C3 of a monosaccharide unit). It is preferred that the anionic groups are the ionised (salt) from, e.g. as alkali metal, alkaline earth metal, other metal, or ammonium salts.

[0012] Most preferably, the anionic groups stem from TEMPO-mediated oxidation of hydroxymethyl groups. Such TEMPO-mediated oxidation is well documented in the art, see e.g. De Nooy, Synthesis 1996, 1153-1174, WO 95/07303, WO 00/50388, WO 00/- 50621, WO 01/00681, WO 02/48197 and. EP 1149846. It involves the use of an oxidising agent such as hypochlorite, hypobromite, hydrogen peroxide, or a peracid, or even oxygen, with a catalytic amount of TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) or an analogue or derivative thereof, optionally with further mediators, such as halide ions, metal complexes or oxidative enzymes (e.g. peroxidase or phenol oxidase or laccase), leading to 6-carboxylic acid derivatives (uronic acids) in glucans and other aldohexose polymers. A catalytic amount of nitroxyl is preferably 0.05-5% by weight, based on dry starch, or 0.05-5 mol% with respect to dry starch. Most preferably, these catalytic amounts are between 0.1 and 1 %. [0013] The oxidation can be performed in aqueous solution or dispersion, e.g. by first

adding TEMPO and optionally further catalysts or mediators, and then gradually or at once adding the oxidising agent, such a sodium hypochlorite solution, and reacting at temperatures between 0 and 50°C, preferably between 10 and 30°C, while maintaining neutral to mildly alkaline conditions, especially at a pH between 6 and 11, depending on the particular oxidising agent, preferably between 8 and 9.5. The degree of oxidation determines the proportion of carboxyl groups in the modified polysaccharide, and can be varied by adapting the amount of oxidising agent. Any aldehyde groups remaining after oxidation may be further oxidised to carboxyl groups using additional of other oxidising agents, such as chlorite. [0014] It was found that improved efficacy is achieved when a part of the remaining hydroxyl groups of the anionic polysaccharide are hydrophobised by etherification or esterification. Etherification can be effected in a manner known per se, e.g. with C _I -C _O alkyl groups by reaction of the polysaccharide with the appropriate alkyl halide, sulphate or sulphonate, for example methyl iodide or diethyl sulphate. Esterification can be done with activated Ci-Cs monocarboxylic acids, such as acid halides or anhydrides of Ci -C _O alkanoic, C3-C8 cycloalkanoic or C ₇ -Cs benzoic or substituted benzoic acids. C2-C4 acyl groups are preferred. The total degree of substitution (DS) of alkyl and acyl groups, preferably with acyl groups, is from 0.08 to 2.2 per monosaccharide unit. Preferably this DS is from 0.1 to 2.0 groups per monosaccharide unit, which corresponds to a percentage of substitution of all available hydroxyl groups of between about 4 and 75%. Preferably, the DS is from 0.2 to 1.8 groups, especially 0.4 to 1.7 groups per unit, or between 7.5 and 70%, especially between 15 and 64% of the available groups. In a particular embodiment, the total DS for acyl groups is between 0.8 and 1.6 per monosaccharide unit. [0015] Particularly effective derivatives were found to be those which contain two different types of groups on the hydroxyl positions, at least one of the two being an alkanoyl group. The other may be another alkanoyl or other acyl group or an alkyl group. Such a combination may e.g. be a methyl or ethyl group and an acetyl or propionyl group, an acetyl group and a propionyl or butyryl group, or a propionyl group and a buryryl group. The ratio between the two different groups is preferably between 1:4 and 4:1, each accounting e.g. for 0.04 to 1.1 , especially from 0.2 to 1.0 group per monosaccharide unit. [0016] The etherification and/or esterification can be carried out before or after the introduction of anionic groups. For practical reasons, in particular in case of introduction of anions by oxidation of the polysaccharide, it may be advantageous to first introduce the anionic groups, and then carry out the alkylation or acylation. The alkylation or acylation

can be performed at the same conditions as most oxidation reactions, i.e. ambient temperature, a pH between 7 and 10, and in aqueous solution, preferably at relatively high concentration (e.g. between 5 and 75% wt.% of dry substance of the reaction mixture). [0017] The invention furthermore pertains to modified glucans as such, which are modified in such a manner to contain from 0.1 to 0.9 carboxyl groups per monosaccharide unit, and from 0.1 to 1.0 acetyl groups and from 0.1 to 1.0 C3-C ₄ alkanoyl and/or C ₂ -C ₄ alkyl groups per monosaccharide unit, the modified glucan having an average molecular weight of between 30 kDa and 10 MDa. [0018] The invention also pertains to an aqueous solution of a modified polysaccharide containing from 0.1 to 0.9 anionic groups per monosaccharide unit and from 0.1 to 2.0 C1-C6 alkyl or alkanoyl groups, especially alkanoyl groups per monosaccharide unit, which is obtainable as described above. The aqueous solutions can be used for treating metal surfaces The aqueous solutions preferably have a dry substance content of between 0.3 and 6 wt.%, more preferably between 1 and 3 wt.%. [0019] The pre-treatment solutions according to the invention can contain further components for stabilising the pre-treatment solution and for optimising the anticorrosive effect, such as lubricants, surfactants, other anti-corrosive agents, etc. Although metal-containing anticorrosive agents may additionally be present, it is preferred that the pre-treatment solution of the invention has low levels of heavy metals (heavy meaning atomically heavier than calcium). In particular, the weight ratio between any heavy metals and modified polysaccharide is below 0.1, preferably below 0.02. [0020] The modified polysaccharides and their solutions can be used for pretreating metal sheets so as to increase their corrosion resistance and to enhance the adhesion of subsequently applied coatings. The polysaccharides can furthermore be used as additives for other pre-treatment solutions such as in phosphating conversion coatings.

[0021] The metal sheets may advantageously be steel sheets, or surface-plated steel, such as tin-coated steel, galvanised steel or other metal-treated steel, or non-ferrous alloys such as aluminium. The sheets can have any commercial shape and sizes. The thickness is e.g. between 0.2 mm and 5 mm and the density, in case of steel, will typically be between 7 and 40 kg/m ² . Before the pre-treatment process, the metal surface may undergo several treatments such as alkaline cleaning or acid pickling. The sheets, after rolling and optional prior treatments and connected with a positive electric potential source, can be dip-coated in the solution for several seconds to several minutes, e.g.

between 15 seconds and 2 minutes at ambient temperature to about 90°C, and then dried. The potential is preferably -0.5 to -0.1 V (vs. saturated Ag/AgCl reference electrode) maintained during the coating stage. The amount of modified polysaccharide coated onto the sheets is preferably between 0.1 and 15 kg/m ² , more preferably between 0.5 and 5 kg/m (dry weight). The resulting treated sheets have a protective layer which provides an improved corrosion resistance, which can be measured using conventional anodic polarisation scan in a salt solution, and increased adhesion of subsequently applied coatings or foils. [0022] The invention also concerns metal sheets coated with a layer of polysaccharide as described above, which is modified to contain from 0.1 to 0.9 anionic groups per monosaccharide unit and from 0.1 to 2.0 C _I -C _O alkyl and/or alkanoyl groups per monosaccharide unit, the modified polysaccharide having an average molecular weight of between 30 kDa and 10 MDa. In particular, the metal sheet is a steel sheet. The metals sheets can be further processed and shaped to produce the desired semi-finished or finished products. The metals sheets can e.g. be used in the automotive industry, aerospace industry, apparatus and utensils, building and construction industry etc.

EXAMPLES:

Molecular weight determination

An estimation of the average molecular weight was determined by Size Exclusion Chromatography (HPLC) using a Waters Associates (Etten-Leur, The Netherlands) series liquid chromatography system with a refractive index (RI 410 nm) detector, a guard column (7.5 mm ID * 7.5 cm, particles size 12 μm) and two size-exclusion columns (7.5 mm ID * 30 cm, particles size 17 μm). The temperature of the columns was maintained at 35°C. A phosphate buffer (100 mM sodium phosphate with 0.02% sodium azide, pH 7) was used as a mobile phase (flow 1 mL/min). The columns were calibrated with dextran standards with a molecular weight of 5,000, 10,000 and 500,000 (Pharmacia, Sweden), respectively. Before injection (20 μl), a 100 mg sample was diluted with 10 mL phosphate buffer.

Example 1: 209 gram (1.3 mol) of partly (30%) C6-oxidised starch (obtained by TEMPO- mediated hypochlorite oxidation of gelatinised potato starch) was dissolved in 3 liter of demineralised water. A mixture of 267 gram (2.6 mol) acetic anhydride and 355 gram (2.6 mol) propionic anhydride was added dropwise during 4 hours. During the reaction, the pH was maintained at 8 by using a pH stat and 2 M NaOH, and the temperature was kept between 25 and 30 ⁰ C. A precipitate was formed, which was

separated by filtration. The residue (200 g) was mixed with 1 liter water to form a suspension, which was then cooled and freeze-dried. The degree of substitution was determined by pretreating a sample by dissolving in water for free acid analysis and in 0.5 M NaOH for total acid analysis. The sample was analysed using HPLC (column Aminex HPX-87H 300 mm x 7.8 mm, eluent 0.01 M sulphuric acid, and RI detection) and the analysis showed a degree of acetyl substitution of 0.8 and degree of propionyl substitution of 0.8.

Example 2:

1 g of modified starch prepared according to Example 1 was added to 100 ml demineralised water containing 3 wt% NaCl as a corrosion-promoting agent. A steel plate of 1 cm ² of plain carbon steel containing < 0.1% carbon was suspended in this solution. A potential scan from -1 tot + 0.25 V (vs. sat. Ag/AgCl reference electrode) was carried out on the steel plate according to standard electrochemical procedures. In the potential range -0.5 to -0.1 V (vs. sat. Ag/ AgCl reference electrode) passivation behaviour of the steel sample was found. This was confirmed visually by the occurrence of transpassive pitting starting around -0.1 V (vs. sat. Ag/AgCl reference electrode), as was observed by an increase in the resulting current density following the passivation region. The same experiment was carried out using a 3 wt.% NaCl solution without the presence of the modified starch. The passivation behaviour was not observed in this case. This is shown in Figure 1 by the dotted line (treatment without modified starch) and the solid line (treatment with modified starch).).

Example 3:

Degreased carbon steel plates (10x 10cm ² ) were subjected to a Galvanostatic deposition process in a medium containing 3.0 wt% NaCl and 0.1wt% of the modified starch at a current of 25 mA for 10 minutes. During the process, the open cell potential (OCP) was recorded. This measurement showed a black deposit settling on the surface. The OCP showed an increase from -460 mV (vs. Ag/ AgCl reference electrode) towards -27OmV (vs Ag/ AgCl). This OCP increase is characteristic for a protective layer build-up. Without the addition of modified starch no stable deposit formation and no increase in potential were found.

Example 4:

Samples prepared with the modified starch according to example 3 were roll bar lacquered. The same process was carried out for panels without the treatment according to example 3. Adhesion of the coatings to the flat panels was tested using

Gitterschnitt test both in dry condition and after samples were sterilised in salted

medium for 60 minutes at 121 ⁰ C. No differences were observed between carbon steel with and without the treatment with modified starch, indicating no negative effects on adhesion.

Example 5: Samples prepared with the modified starch according to example 3 were roll bar lacquered. Subsequently, Electrochemical impedance spectroscopy (EIS) in 3 wt% NaCl medium was used to evaluate the relative corrosion resistance. For reference purposes, carbon steel panels were given the same coating treatment and were subjected to the same tests. From the data of the EIS measurements, it was clearly visible that the deposited layer showed active properties whereas this active behaviour was absent for the untreated sample. Furthermore, it was shown that contributions from the corrosion process, i.e. corrosion products and diffusion of corrosion products, were clearly more pronounced in the data for the not treated samples. Example 6:

Samples prepared with the modified starch according to example 3 were roll bar lacquered. For reference purposes, carbon steel panels were given the same coating treatment. Subsequently, an x-scribe was applied to all samples and scratched samples were exposed in 3 wt% NaCl medium. Visual observations showed that a without the treatment with modified starch, corrosion occurs rapidly, starting from the scribe. After 60 days, 90% of the surface showed corrosion or was even detached. For the samples with the modified starch treatment, slight detachment occurred during the same period and corrosion was present at 50% of the surface.