WO2013083294A2

WO2013083294A2 - Organic coated steel substrate

Info

Publication number: WO2013083294A2
Application number: PCT/EP2012/005103
Authority: WO
Inventors: Tapan Kumar Rout; Anil Vilas Gaikwad; Theo Dingemans; Mikhail Zheludkevich
Original assignee: Tata Steel Nederland Technology Bv; Tata Steel Limited
Priority date: 2011-12-08
Filing date: 2012-12-10
Publication date: 2013-06-13
Also published as: WO2013083294A3

Abstract

The invention relates to an organic coated steel substrate provided with an organic coating system, the organic coating system comprising an organic primer layer and an organic topcoat layer thereon, wherein the organic primer layer comprises a polyetherimide.

Description

ORGANIC COATED STEEL SUBSTRATE

FIELD OF THE INVENTION The invention relates to an organic coated steel substrate, a method for producing the same and to the use of the organic coated steel substrate in construction, automotive, ship building or stock holding applications.

BACKGROUND

Organic coated steel substrates typically comprise a steel substrate, optionally a corrosion protective layer thereon, for instance a zinc or zinc alloy coating, an organic primer on the steel substrate or the optional zinc or zinc alloy coating and a topcoat on the organic primer. Such systems are designed specifically to prolong the working lifetime of the steel substrate to be protected.

Although conventional zinc alloys (electro zinc (EZ), galvanised (Gl), galvannealed (GA), Galvalloy ^® (zinc with 5% Al), Galfan ^® (zinc with about 5% Al) or Galvalum ^® (zinc with about 55% Al)) afford the underlying steel substrate galvanic corrosion protection, thereby extending the lifetime of the organic coated steel substrate, the market trend, driven by both cost and environmental considerations, is towards zinc or zinc alloy layers having reduced layer thickness with equal or better performance. This trend has led to the development of zinc alloy coatings that comprise magnesium, e.g. Zn-Mg-AI or Zn-Mg-AI-X where X is a further alloying element. While organic coated steel substrates comprising Zn-Mg-X alloys offer several advantages in respect of corrosion protection, weight, cost and environmental acceptance, Zn-Mg-X alloys may nevertheless be characterised by increased sensitivity to blister formation relative to other zinc alloys, which can lead to "filiform" corrosion. Filiform corrosion is a type of corrosion that occurs on metallic surfaces coated with organic films. A number of solutions have been proposed to avoid or at least reduce the effects of filiform corrosion, which include paying special attention to surface cleanliness and providing organic primers that contain zinc or chromate pigments. However, more robust and environmentally acceptable solutions are required.

It is an object of the invention to provide an organic coated steel substrate comprising a chromium-free organic primer having improved corrosion protection, flexibility and adhesion properties.

It is another object of the invention to provide a chromium-free organic primer which has good compatibility with the topcoat. It is a further object of the invention to provide an organic coated steel substrate which is less susceptible to blister formation and filiform corrosion. DESCRIPTION OF THE INVENTION

The first aspect of the invention relates to an organic coated steel substrate provided with an organic coating system, the organic coating system comprising an organic primer layer and an organic topcoat layer, wherein the organic primer layer comprises a polyetherimide.

Organic coated steel substrates which are coated with a conventional organic primer and a top coat are susceptible to blister formation and filiform corrosion. Filiform corrosion is a type of corrosion that occurs on metallic surfaces coated with organic films. The inventors have found that replacing conventional organic primer layers with polyetherimide primer layers significantly reduces the occurrence of blisters and the detrimental effects of filiform corrosion. The polyetherimide primer layer is also flexible and highly adhesive to the topcoat and to the underlying steel substrate.

In a preferred embodiment the steel substrate is selected from the group consisting of sheet, section, tube, rod, bar, beam, plate, column or wire substrates. These steel substrates are typically used in environments where corrosion resistance is of high importance e.g. in construction, automotive, ship building or stock holding applications.

In a preferred embodiment of the invention the steel substrate is provided with a zinc or zinc alloy protective coating. Preferably the coated steel substrate comprises a zinc or zinc alloy coating selected from electro zinc (EZ), galvanised (Gl), galvannealed (GA), Galvalloy * (zinc with 5% Al) or Galfan * (zinc with about 5% Al) which may be applied by hot-dip galvanising, electro- galvanising, galvannealing or by physical vapour deposition (PVD). Galvalum ^β which is an alloy containing zinc and aluminium (55%) may also be used as the zinc alloy.

In a preferred embodiment the zinc alloy comprises Zn as the main constituent, i.e. the alloy comprises more than 50% zinc, and one or more of Mg, Al, Si, Mn, Cu, Fe and Cr. Zinc alloys selected from the group consisting of Zn-Mg, Zn-Mn, Zn-Fe, Zn-AI, Zn-Cu, Zn-Cr, Zn-Mg-AI and Zn-Mg-AI-Si are preferred and afford additional corrosion protection to the underlying steel substrate. The polyetherimide primer layer is also flexible and highly adhesive to both the underlying zinc or zinc alloy and the topcoat, meaning polyetherimide primer layers may be used in lieu of conventional organic primer layers to improve the overall properties of organic coated steel substrates comprising galvanised, galvannealed, Galvalloy ^®, Galfan ^® or Galvalum ^® or Zn- Mg-X coatings where X is an additional alloying element such as Al and/or Si.

Zn-Mg-X alloys exhibit equal or better corrosion resistance relative to more conventional zinc alloy coatings (galvanised, galvannealed, Galvalloy Galfan or Galvalum), even at reduced layer thicknesses. However, until now the use Zn-Mg-X alloy coatings in organic coated steel substrates was restricted/less preferred since Zn-Mg-X coatings facilitate blister formation and/or filiform corrosion. Since filiform corrosion is reduced when polyetherimides of the invention are used as primers on Zn-Mg-X coatings, the manufacturer can now take full advantage of weight reductions afforded to him when using Zn-Mg-X alloy coatings without a reduction in overall corrosion protection.

Preferably the zinc alloy coating is a Zn-Mg-X or Zn-Mg-AI-X alloy consisting of:

0.3-5.0 weight % magnesium;

0.6-5.0 weight% aluminium;

optional < 0.2 weight % of one or more additional elements;

unavoidable impurities;

the remainder being zinc;

preferably the zinc alloy coating layer has a thickness between 3 -12 μιη.

A coating thickness above 12 μιη was deemed not necessary because the zinc alloy exhibits improved corrosion protection properties relative to conventional zinc or zinc alloy coatings consisting of zinc and aluminium. A zinc alloy coating having a thickness of 3-10 μιη is preferred because very good corrosion protection is possible even at reduced coating thicknesses, thereby reducing the overall cost of the organic coated steel substrate. More preferably, the zinc alloy has a coating thickness of 3-8 μιη since this further reduces manufacturing costs without a significant reduction in corrosion resistance.

In a preferred embodiment the zinc alloy coating contains 0.3-2.3 weight% magnesium and 0.6- 2.3 weight % aluminium The magnesium level of 0.3-2.3 weight% is high enough to obtain a corrosion protection against red rust that is far greater than the corrosion protection of conventional hot-dip galvanised coatings consisting of zinc or zinc and aluminium. A minimum magnesium content of 0.3 weight % is necessary to have sufficient corrosion resistance. The magnesium content has been restricted to 2.3 weight% since magnesium is known to facilitate filiform corrosion and could result in brittle coatings being formed. An aluminium level of 0.6-2.3 weight % results in a zinc alloy coating having improved formability and adhesion to the underlying steel substrate. Moreover, when aluminium is combined with magnesium the corrosion resistance properties of the zinc alloy are further improved. Suitable additional elements that may be provided comprise Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi. Pb, Sn, Bi and Sb are usually provided to form spangles. In a preferred embodiment the zinc alloy coating contains 1.6-2.3 weight% magnesium and 1.6- 2.3 weight % aluminium. At these values the corrosion protection of the zinc alloy coating is maximised.

Preferably the zinc alloy coating contains 0.3-1.3 weight% magnesium and 0.6-1.3 weight % aluminium. These amounts of magnesium and aluminium further improve the corrosion protective properties of the zinc alloy. Moreover, such coatings exhibit a reduction in filiform corrosion because of the reduced magnesium content in the zinc alloy. For zinc alloy coatings comprising more than 0.5 weight %, aluminium in the amounts specified above are required to prevent oxidic dross forming on the bath.

Preferably the zinc alloy contains 0.8-1.2 weight% magnesium and/or 0.8-1.2 weight% aluminium, which results in the zinc alloy coating exhibiting improved corrosion resistance, surface quality and formability relative to conventional hot-dip galvanised zinc coatings.

In a preferred embodiment the organic primer layer has a dry film thickness between 1 and 100 μιη, preferably between 3 and 40 pm and more preferably between 10 and 20 pm. The dry film thickness of the organic primer layer largely depends on the end application of the organic coated steel substrate. In highly corrosive environments thicker organic primer layers may be used whereas thinner organic primer layers are more appropriate if the organic coated steel substrates is for indoor use. The inventors found that organic coated steel substrates comprising a polyetherimide primer layer having a dry film thickness between 4 and 10 pm exhibited very good corrosion resistance making them suitable for both indoor and outdoor applications.

In a preferred embodiment the steel substrate or zinc or zinc alloy coating is provided with a chromium-free conversion layer. Typically a conversion layer is provided to improve adhesion between the organic primer layer and the underlying zinc or zinc alloy coating or the steel substrate. However, the polyetherimide organic primer layer exhibits very good adhesion properties, which has the advantage that a conversion layer is not strictly necessary for organic coated steel substrate products comprising polyetherimide primer layers. This offers the manufacturer a significant advantage both in terms of cost and processing. Nevertheless, the polyetherimide organic primer layer also shows very good adhesion towards the conversion layer when a conversion layer is used. A preferred conversion layer dry film thickness is between 300 nm and 50 pm, preferably between 1 and 20 pm and more preferably between 6 and 10μιη with suitable corrosion layers comprising phosphates and zirconates.

In a preferred embodiment the topcoat layer comprises plastisol, polyester, polyurethane or polyfluorocarbons. Plastisol is a generic name for a PVC based paint coating that is applied in liquid form. Plastisols may be provided in a variety of colours and finishes and may be used in both internal and external construction applications. Plastisols are a preferred topcoat material for organic coated steel substrates that have to have very high corrosion protection properties. The plastisol topcoat may be a single layer or a multilayer. Polyesters, polyurethanes and polyfluorocarbons also offer very good corrosion protection properties and are compatible with the polyetherimide primer of the invention thereby reducing the possibility of topcoat delamination and/or blistering. Preferably the polyester comprises a silicone polyester and the polyfluorocarbon comprises polyvinyldifluoride (PVDF), In a preferred embodiment the organic coating system has a dry film thickness of at least 20 pm, preferably 20-500 pm, more preferably 50-200 pm A lower limit of 20 pm is preferred otherwise the organic coating system will not provide sufficient corrosion protection. On the other hand an upper limit of 500 pm is preferred since thicker layers may delaminate from the zinc or zinc alloy coating.

In preferred embodiment the organic primer layer contains a further component comprising any one of:

chromium-free corrosion inhibitors

chromium-free corrosion inhibitor loaded nanocontainers

- infrared absorbing components

metal oxide nanoparticles

Chromium-free corrosion inhibitors may be provided to further improve the corrosion protection properties of the organic primer layer. They may be provided as independent components or preferably they may be loaded into nanocontainers for active corrosion protection. Halloysites, layered double hydroxides, CaC0₃, polymeric containers or mixtures thereof are particularly preferred as nanocontainers. Loading the corrosion inhibitors in nanocontainers has two advantages 1) the corrosion inhibitors are prevented from chemically interacting with the polyetherimide intermediate which could reduce the barrier properties of the polyetherimide primer layer and 2) the corrosion inhibitors are released controllably in response to stimuli such as abrasion, a change in pH, a change in ionic strength and/or the presence of certain ions in a corrosive solution.

Preferred anionic and/or cationic corrosion inhibitors, all of which are suitable for loading into nanocontainers, include aluminium phosphate, sodium gluconate, sodium molybdate Na₂Mo0₄, cerium molybdate Ce2(Mo0 )₃, cerium nitrate Ce(N0₃)₃, calcium nitrate Ca(N0₃)₂, zinc sulfate ZnS0₄, sodium tungstate NaW0₃, sodium phosphomolybdate hydrate Na₃Mo₁₂0₄oP, sodium phosphate Na₃P0₄, sodium hydrophosphate Na₂HP0₄, sodium dihydrophosphate NaH₂P0₄, sodium carbonate Na₂C0₃, sodium polyphosphate NaP0₃x, sodium gluconate, 2- mercaptobenzothiazole, benzimidazole, quinaldic acid, sodium citrate, glycine, 8- hydroxyquinoline, sodium salycilate, sodium benzoate, 1-Hydroxyethylidenediphosphonic acid (etidronic acid) , nitrilo-tris-phosphonic acid, N,N dimethyl amine, di-azo compounds, Cu- thalocyanine, dyes tartrazine (TZ)). Metal oxide nanoparticles were used to improve the corrosion resistance, barrier resistance and the conductivity of the polyetherimide coating. Silica, titania, magnesia or alumina metal oxide nanoparticles have been particularly effective in this respect and are therefore preferred.

The second aspect of the invention relates to a method of manufacturing an organic coated steel substrate according to the first aspect of the invention, which comprises the steps of: i. providing a steel substrate;

ii. optionally providing a zinc or zinc alloy coating on the steel substrate;

iii. applying a solution comprising a polyetherimide intermediate on the steel substrate or the optional zinc or zinc alloy coating and at least partly curing said solution to form an organic primer layer comprising polyetherimide;

iv. applying a topcoat on the organic primer layer comprising polyetherimide;

v. subjecting the coated steel substrate of step (iv) to a heat treatment.

Preferably the second aspect of the invention relates to a method of manufacturing an organic coated steel for building and construction, automotive, ship building or stock holding applications.

In a preferred embodiment the zinc or zinc alloy coating is provided, preferably by hot-dip galvanising, hot-dip galvannealing, electrodeposition or cladding.

In a preferred embodiment the solution comprising the polyetherimide intermediate is water or water based solution, which has the advantage that the method does not make use of organic solvents, some of which are harmful, toxic and difficult to dispose of and handle. One preferred approach comprises the steps of preparing the polyetherimide intermediate in an organic solvent, precipitating the prepared organic curable component from the organic solvent, filtering and drying the precipitate and providing the dried precipitate in water to form a water or water based solution comprising the polyetherimide intermediate. The advantage of preparing the polyetherimide intermediate in an organic solvent is that higher molecular weight polyetherimide intermediates can be obtained, which once cured, result in coatings that exhibit increased formability and corrosion resistance relative to coatings comprising organic curable components that were prepared in water.

In a preferred embodiment the at least partly cured polyetherimide primer layer is optionally subjected to a to an activation treatment to surface modify the at least partly cured organic primer layer comprising polyetherimide. Suitable activation treatments include a plasma (flame or corona) surface treatment or a chemical surface treatment in which the surface is subjected to an acidic or alkaline etch. The improved adhesion properties of the polyetherimide means that it is not always necessary to activate the primer surface before applying the topcoat on the primer, which is not the case if conventional organic (non-polyetherimide primers are provided in the manufacture of organic coated steel substrates.

In preferred embodiment the polyetherimide intermediate comprises an aromatic dianhydride and an aromatic diamine wherein the aromatic diamine comprises an aromatic polyetherdiamine and/or a monoaromatic diamine. Preferably the polyetherimide intermediate comprises m-phenylenediamine (MPA), diaminobenzoic acid (DABA), 2,6-diaminopyridine (DAPY), 3,5-diaminophenol (DAPH) or a mixture thereof as monoaromatic diamine. Polyetherimide intermediates comprising DABA, DAPY or DAPH as monoaromatic diamine comprise carboxylic acid, pyridine and hydroxyl functional groups respectively, which chemically interact with the zinc alloy surface through acid-base interactions and/or H-bonding to increase the adhesion between the two layers.

The copolymerisation of the dianhydride, MPA and an aromatic polyetherdiamine is particularly preferred since MPA introduces irregularities into the resulting polyetherimide primer layer making it amorphous (flexible) instead of crystalline (rigid). The aromatic groups of the aromatic polyetherdiamine contribute to improving corrosion resistance, whereas the ether groups contribute to improving adhesion and the formability of the polyetherimide. Adhesion is improved by the ether groups acting as electron donating Lewis base sites. A preferred aromatic polyetherdiamine is 4,4'-(1 ,3-Phenylenedioxy)dianiline. In a preferred embodiment of the invention the polyetherimide intermediate comprises an aromatic dianhydride and an aliphatic polyetherdiamine, preferably a Jeff amine, which may be defined as a polyetherdiamine comprising at least one primary amino group attached to the terminus of a polyether backbone, wherein the polyether backbone is based either on propylene oxide (PO), ethylene oxide (EO), or mixed EO/PO. The flexibility of the polyetherimide primer layer may be increased by selecting Jeff amines having an increased number of ether groups. The selection of Jeff amines reduces the glass transition temperature { Tg) of the polyetherimide intermediate, which enables lower temperatures to be used when the solution comprising polyetherimide intermediate is at least partly cured. Jeff amines which have been used in accordance with the invention include 0, 0'-Bis(2-aminopropyl) polypropylene glycol-b/oc/ -polyethylene glycol-6/oc/c-polypropylene glycol (J1 ), 4,7, 10- trioxa-1 , 13- tridecanediamine (J2), Poly(propylene glycol) bis(2-aminopropyl ether having a molecular weight 230 (J3), Poly(propylene glycol) bis(2-aminopropyl ether having a molecular weight of 400 (J4) and 1 ,2-bis(2-aminoethoxyethane) (J5). In a preferred embodiment of the invention the solution comprising the polyetherimide intermediate comprises a second polyetherimide intermediate. Advantageously, the solution comprising two different polyetherimide intermediates allows the properties (corrosion resistance, adhesion and flexibility) of the polyetherimide primer to be tailored to suit the needs of a particular building or construction application.

In a preferred embodiment of the invention the polyetherimide comprises an end-capping component, which has the advantage that the overall efficiency of the curing process is improved because such end-capped polyetherimide intermediates exhibit improved melt flow characteristics at relatively low temperatures. End-capping polyetherimide intermediates with aryl amine derivatives comprising carboxylic acid, ester, amine, or hydroxyl functional groups further improves the adhesion of the polyetherimide primer to the topcoat and to the underlying zinc or zinc alloy coating. Other preferred end-capping components comprise phenol and silanes with organofunctional silanes such as 3-glycidoxypropyltrimethoxysilane being particularly preferred since the resulting polyetherimide primer layer exhibits excellent corrosion resistance, flexibility and adhesion properties.

In a preferred embodiment of the invention the solution comprising a polyetherimide intermediate is at least partly cured using induction heating or electromagnetic radiation, preferably infrared or near infrared electromagnetic radiation. In the manufacture of organic coated steel substrates it is typical to use a temperature between 150 and 275^°C to cure and form an organic primer layer on a zinc or zinc alloy surface. The inventors found that polyetherimide primer layers may be formed on the zinc or zinc alloy surface by curing the solution within the same temperature range. However, it is preferred to use a curing temperature of at least 300°C to reduce the amount of time the coated substrate is in the primer furnace. Since polyetherimide primer layers are thermally stable up to 500 ^°C, curing soak temperatures of up to 400°C may be employed without significant degradation of the layer. Preferably a temperature ramp-up is used to prevent degradation at the early stages of curing. The primer furnace comprises a conventional heat convection oven, an Infrared (IR) oven, an induction oven (i.e. heating the substrate directly) or a combination thereof. Advantageously, IR or induction ovens may be retrofitted to existing industrial primer ovens. The retrofitting enables the higher temperature curing range to be used. IR curing is most preferred and works on the premise that an IR source directly transfers electromagnetic radiation to the solution of polyetherimide intermediate that has been applied on the zinc or zinc alloy. However, a proportion of the electromagnetic radiation will be selectively transmitted to the zinc or zinc alloy because IR sources generally emit a broad range of wavelengths, not just a single or narrow band wavelength. Short wavelength IR has a wavelength between 0.8 and 2 pm and is mostly transmitted through the coating and absorbed by the zinc or zinc alloy. However, the absorbed energy causes the underlying zinc or zinc alloy coating to heat up and transfer the thermal energy to the applied solution comprising the polyetherimide intermediate, thereby curing it indirectly. Medium wavelength IR has a wavelength above 2 μνη and at most 5 μιη. The use of medium wavelength radiation has the advantage that a large proportion of the emitted electromagnetic radiation is absorbed by the applied solution comprising the polyetherimide intermediate. Long wavelength IR has a wavelength of above 5μηι and up to 1 mm and is not very effective for curing nor regarding energy efficiency.

By using an IR source that has a peak intensity wavelength coinciding with the IR absorption spectrum of the polyetherimide intermediate, it has been possible to maximise the total amount of energy absorbed by the polyetherimide intermediate and/or polyetherimide. In a preferred embodiment the solution comprising the polyetherimide intermediate comprises infrared absorbing components in the form of pigments, IR absorbing additives or mixtures thereof. The provision of such IR absorbing additives in the solution broadens the absorption spectrum, thereby increasing the total amount of energy absorbed by the solution comprising the polyetherimide intermediate and minimising the amount of energy that is absorbed by the zinc or zinc alloy coating.

The third aspect of the invention relates to the use of the organic coated substrate of the first aspect of the invention in construction, automotive, ship building or stock holding applications.

EXAMPLES

Embodiments of the present invention will now be described by way of example. These examples are intended to enable those skilled in the art to practice the invention and do not in anyway limit the scope of the invention as defined by the claims.

Preparation of polyetherimide primer layer (A)

10 mmol (3.032g) of 4,4'-Biphthalic Anhydride (97%) and de-ionised water (80 ml) are charged into a 200ml one neck flask having a nitrogen inlet. To this solution 0, 0'-Bis(2- aminopropyl)polypropyleneglycol-Woc/c-polyethylene glycol-Woc/c polypropylene glycol (J1 ) (10 mmol) is added to form a white suspension. The white suspension is stirred under N₂ at 60 °C for 4 hours until the aromatic dianhydride and J 1 are solubilised. This solution is stirred for a further 8 hours so as to form the corresponding polyetherimide intermediate. 3- glycidoxypropyltrimethoxysilane (2mmol, 0.472g) is added to the water based solution comprising the polyetherimide intermediate and this solution is stirred for a further four hours to end-cap the polyetherimide intermediate with 3-glycidoxypropyltrimethoxysilane. The water based solution comprising the end-capped polyetherimide intermediate is then provided on a zinc alloy coated steel substrate by any suitable method such as dipping or spray coating. The applied solution of end-capped polyetherimide is dried at a temperature of 80°C for a period of 5 minutes before being subjected to a curing treatment of 200^°C for 5 minutes to cure the end- capped polyetherimide intermediate and form the corresponding polyetherimide primer . Preparation of polyetherimide primer layer (B)

A 100 ml one necked vessel equipped with a nitrogen inlet is charged with 2,2' - (Ethylenedioxy)bis(ethylamine) J5 (3.5 mmol, 0.5187 g), m-phenylenediamine (1.5 mmol, 0.16 g) and NMP (23g). 4,4-Biphthalic anhydride (5 mmol, 1.51 g) is added and this solution is stirred under inert conditions for 8hrs to form a polyetherimide intermediate. N-butyldiethanol amine (5 mmol, 0.8g) is added to this stirred solution, which is stirred for an additional hour. This stirred solution is then added to acetone or an acetone/methanol mixture under mechanical stirring causing the polyetherimide intermediate to precipitate. The precipitate is dried at 50°C. A 10 % wt solution of the dried precipitate is prepared in water; if necessary 1 wt % of N-butyldiethanol amine is added to ease the dissolution. This solution is applied on a zinc alloy coated steel substrate by any suitable method such as dipping or spray coating. The applied solution of end-capped polyetherimide is dried at a temperature of 80°C for a period of 5 minutes before being subjected to a curing treatment of 200°C for 5 minutes to cure the end- capped polyetherimide intermediate and form the corresponding polyetherimide primer.

Providing the topcoat

A plastisol having a viscosity between 0.2 to 0.8 Pa.s is applied on the polyetherimide primer layer by any suitable method e.g. lamination, spraying or dipping. The applied plastisol coating is then cured between 160 and 230°C using a convection oven or by IR.

Experiments: Flexibility

The flexibility of the polyetherimide primer layer was assessed using an Erichsen cupping test (ISO 20482), which is a ductility test that is employed to evaluate the ability of metallic sheets and strips to undergo plastic deformation in stretch forming. Cups were made using 5KN pressure. Following the cupping, no cracks were observed in polyetherimide A and polyetherimide B and therefore both polyetherimides were deemed to have excellent flexibility making them suitable as primers for organic coated steel substrates.

Experiment: Adhesion

Adhesion was evaluated by a scratch tape test (ASTM D 3359), which is a method for assessing the adhesion of coating films to metallic substrates by applying and removing pressure sensitive tape over cuts made in the film. If 5% or less of the coating was removed by the adhesive tape then the adhesion of the coating to the steel substrate is excellent. If 6-15 % of the coating was removed by the adhesive tape then coating adhesion is good, and if the adhesive tape removed greater than 15% of the coating then coating adhesion was bad. Both polyetherimide A and polyetherimide B exhibited excellent adhesion properties to the underlying zinc alloy coating.

Experiment: Corrosion resistance,

The Salt spray test (ASTM B117 standard) is used to measure the corrosion resistance of coated and uncoated metallic specimens, when exposed to a salt spray at elevated temperature. Polyetherimide coated steel substrates were placed in an enclosed chamber at 35 °C and exposed to a continuous indirect spray (fogging) of 5% salt solution (pH 6.5 to 7.2), which falls-out on to the coated steel substrate at a rate of 1.0 to 2.0 ml/80cm²/hour. The fogging of 5% salt solution is at the specified rate and the fog collection rate is determined by placing a minimum of two 80 sq. cm. funnels inserted into measuring cylinders graduated in ml. inside the chamber. This climate is maintained under constant steady state conditions. The samples are placed at a 15-30 degree angle from vertical. The test duration is variable. The sample size is 76 x 127 x 0.8 mm, are cleaned, weighed, and placed in the chamber in the proximity of the collector funnels. After exposure the panels are critically observed for blisters, red rust spots and delaminations.

Polyetherimide coated steel substrates were deemed to have excellent corrosion resistance if 10% or less of the substrate surface was covered by red rust and/or blisters. Polyetherimide coated steel substrates were deemed to have good corrosion resistance if 11 -15% of the substrate surface was covered by red rust and/or blisters. Polyetherimide coated steel substrates were deemed to have bad corrosion resistance if greater than 15% of the substrate surface is covered by red rust and/or blisters. Both polyetherimide A and polyetherimide B exhibited excellent corrosion protection properties.

Claims

1. Organic coated steel substrate provided with an organic coating system, the organic coating system comprising an organic primer layer and an organic topcoat layer thereon, wherein the organic primer layer comprises a polyetherimide.

2. Organic coated steel substrate according to claim 1 wherein the steel substrate is selected from the group consisting of sheet, section, tube, rod, bar, beam, plate, column or wire substrates.

3. Organic coated steel substrate according to claim 1 or claim 2 wherein the steel substrate is provided with a zinc or zinc alloy protective coating.

4. Organic coated steel substrate according to claim 3 wherein the zinc alloy comprises more than 50 wt% Zn and one or more of Mg, Al, Si, Mn, Cu, Fe or Cr, preferably the zinc alloy comprises alloys of Zn-AI, Zn-Fe, Zn-Mg, Zn-Mg-AI or Zn-Mg-AI-Si.

5. Organic coated steel substrate according to any one of the preceding claims wherein the organic primer layer has a dry film thickness between 1 and 100 μπι, preferably between 3 and 40 pm and more preferably between 10 and 20 pm.

6. Organic coated steel substrate according to any one of the preceding claims wherein the topcoat layer comprises plastisol, polyester, polyurethane or polyfluorocarbon.

7. Organic coated steel substrate according to any one of the preceding claims wherein the organic coating system has a dry film thickness of at least 20 pm, preferably 20-500 pm, more preferably 50-200 pm.

8. Organic coated steel substrate according to any one of the preceding claims wherein the organic primer layer contains a further component comprising any one of:

chromium-free corrosion inhibitors

chromium-free corrosion inhibitor loaded nanocontainers

infrared absorbing components

metal oxide fillers

9. Method of manufacturing an organic coated steel substrate according to any one of claims 1-8, which comprises the steps of:

i. providing a steel substrate;

ii. optionally providing a zinc or zinc alloy coating on the steel substrate; iii. applying a solution comprising a polyetherimide intermediate on the steel substrate or the optional zinc or zinc alloy coating and at least partly curing said solution to form an organic primer layer comprising polyetherimide;

iv. applying a topcoat on the organic primer layer comprising polyetherimide;

v. subjecting the coated steel substrate of step (iv) to a heat treatment.

10. Method of manufacturing an organic coated steel substrate according to claim 9 wherein a zinc or zinc alloy coating is provided, preferably by hot-dip galvanising, hot-dip galvannealing, electrodeposition or cladding.

11. Method of manufacturing an organic coated steel substrate according to claim 9 or claim 10 wherein the polyetherimide intermediate comprises an aromatic dianhydride and an aromatic diamine wherein the aromatic diamine comprises an aromatic polyetherdiamine and/or a monoaromatic diamine.

12. Method of manufacturing an organic coated steel substrate according to claim 9 or claim 10 wherein the polyetherimide intermediate comprises an aromatic dianhydride and an aliphatic polyetherdiamine, preferably a polyetherdiamine comprising at least one primary amino group attached to the terminus of a polyether backbone, wherein the polyether backbone is based either on propylene oxide (PO), ethylene oxide (EO), or mixed EO/PO.

13. Method of manufacturing an organic coated steel substrate according to any one of claims 10-13 wherein the diamine comprises an end-capping component.

14. Method of manufacturing an organic coated steel substrate according to claim 11 or claim 12 wherein the solution comprising the polyetherimide intermediate is water or a water based solution.

15. Use of the organic coated steel substrate according to claims 1-8 in construction, automotive, ship building or stock holding applications.