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Title:
ORGANIC COATED STEEL STRIP
Document Type and Number:
WIPO Patent Application WO/2013/083292
Kind Code:
A1
Abstract:
The present invention relates to an organic coated steel strip for building and construction, which comprises a zinc or zinc alloy coated carbon steel strip having a first surface and a second surface and an organic coating system on at least one said surface, the organic coating system comprising an organic primer layer and an organic topcoat layer thereon, wherein the organic primer layer comprises a polyetherimide.

Inventors:
ROUT TAPAN KUMAR (IN)
GAIKWAD ANIL VILAS (NL)
DINGEMANS THEO (NL)
ZHELUDKEVICH MIKHAIL (PT)
Application Number:
PCT/EP2012/005101
Publication Date:
June 13, 2013
Filing Date:
December 10, 2012
Export Citation:
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Assignee:
TATA STEEL NEDERLAND TECHNOLOGY BV (NL)
TATA STEEL LTD (IN)
International Classes:
C23C2/26; B05D7/14; B05D7/16; C23C2/28; C23C26/00; C23C28/00
Domestic Patent References:
WO2011035920A12011-03-31
WO2010112605A12010-10-07
Foreign References:
US20090324938A12009-12-31
US20100247947A12010-09-30
Other References:
None
Attorney, Agent or Firm:
HERMAN DE GROOT, Johan, Willem (Group Intellectual Property Services - 3G37P.O. Box 10000, CA Ijmuiden, NL)
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Claims:
CLAIMS

Organic coated steel strip for building and construction, which comprises a zinc or zinc alloy coated carbon steel strip having a first major surface and a second major surface and an organic coating system on at least one said surface, the organic coating system comprising an organic primer layer and an organic topcoat layer thereon, wherein the organic primer layer comprises a polyetherimide.

Organic coated steel strip according to claim 1 wherein the organic primer layer has a dry film thickness between 1 and 40 pm, preferably between 3 and 20 pm and more preferably between 4 and 10 pm.

Organic coated steel strip according to claim 1 or claim 2 wherein the zinc alloy comprises alloys of Zn-AI, Zn-Fe, Zn-Mg, Zn-Mg-AI or Zn-Mg-AI-Si.

Organic coated steel strip according to any one of the preceding claims comprising a chromium-free conversion layer between the zinc or zinc alloy coating and the organic primer layer.

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

Organic coated steel strip according to any one of the preceding claims wherein the organic coating system has a dry film thickness of at least 20 pm and at most 200 pm, preferably between 50 pm and 200 pm.

Organic coated steel strip 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

Method of manufacturing an organic coated steel strip for building and construction according to any one of claims 1-7, which comprises the steps of:

i. providing a carbon steel strip;

ii. providing a zinc or zinc alloy coating on the carbon steel strip;

in. applying a solution comprising a polyetherimide intermediate on the 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 carbon steel strip of step (iv) to a heat treatment.

9. Method of manufacturing an organic coated steel strip according to claim 8 wherein the zinc or zinc alloy coating is provided by hot-dip galvanising, hot-dip galvannealing, electrodeposition or cladding.

10. Method of manufacturing an organic coated steel strip according to claim 8 or claim 9 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.

11. Method of manufacturing an organic coated steel strip according to claim 8 or claim 9 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.

12. Method of manufacturing an organic coated steel strip according to any one of claims 8-11 wherein the solution comprising the polyetherimide intermediate comprises a second polyetherimide intermediate.

13. Method of manufacturing an organic coated steel strip according to any one of claims 8-12 wherein the polyetherimide is end-capped with an end-capping component.

14. Method of manufacturing an organic coated steel strip according to any one of claims 8-13 wherein the solution comprising a polyetherimide intermediate is at least partly cured using induction heating or electromagnetic radiation, preferably infrared or near infrared electromagnetic radiation.

15. Method of manufacturing an organic coated strip according to any one of claims 8-14 wherein the solution comprising the polyetherimide intermediate is a water based solution or an aqueous solution.

16. Use of a polyetherimide as an organic primer layer in an organic coated steel strip for building and construction.

Description:
ORGANIC COATED STEEL STRIP

FIELD OF THE INVENTION The invention relates to an organic coated carbon steel strip, a method for producing the same and to the use of a polyetherimide as an organic primer layer in an organic coated steel strip.

BACKGROUND OF THE INVENTION Organic coated strip (OCS) products typically comprise a zinc or zinc alloy coated carbon steel strip substrate provided with an organic coating, which when used for outdoor exposure, comprises an organic primer layer and a topcoat layer on the organic primer layer. Conventional organic primer layers comprise polyesters, epoxies and oil alkyds. OCS products are used in building and construction applications, for instance for roofs and facades, and should have corrosion resistance, cut edge corrosion resistance and flexibility for post-forming operations prior to installation. Weldability is not a requirement in this market.

Conventional OCS products comprise zinc or zinc alloy layers selected from 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). Although these zinc and zinc alloys afford the underlying carbon steel substrate galvanic corrosion protection, thereby extending the lifetime of the OCS product, 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 OCS products 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 that are 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 engineered organic primers that contain zinc or chromate pigments. For the most demanding OCS steel products, highly corrosion resistant Zn-5%AI alloys are used, for instance Galvalloy or Galvan, in combination with an organic coating comprising an organic primer and a thick plastisol coating. A further "high-end" OCS product is based on a Zn- 5%AI coated steel with an organic coating comprising an organic primer and a polyurethane topcoat. One current disadvantage of these high-end OCS systems is that they contain chromium compounds that are toxic and environmentally less acceptable. In view thereof, the market demand is moving towards Cr-free alternatives.

It is an object of the invention to provide an OCS steel product comprising a chromium-free organic primer having improved corrosion protection, flexibility and adhesion to the Zn or Zn-alloy layer.

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 OCS steel product which is less susceptible to blister formation and filiform corrosion when the OCS product comprises Zn-Mg-X or Zn-Mg-AI-X zinc alloy coatings. DESCRIPTION OF THE INVENTION

The first aspect of the invention relates to an organic coated steel strip for building and construction, which comprises a zinc or zinc alloy coated carbon steel strip having a first major surface and a second major surface and an organic coating system on at least one said surface, the organic coating system comprising an organic primer layer and an organic topcoat layer thereon, wherein the organic primer layer comprises a polyetherimide.

Organic coated steel strips comprising a zinc alloy such as Zn-Mg-X and an organic coating system comprising a polyetherimide organic primer layer and a topcoat thereon exhibited a significant reduction in blister formation and filiform corrosion relative organic coated steel strips which comprise an organic primer layer other than polyetherimide. Due to the polyetherimide exhibiting improved filiform corrosion resistance the overall weight of the organic coated steel strip is reduced since Zn-Mg-X alloys can be used, which exhibit equal or better corrosion resistance relative to more conventional zinc alloy coatings (galvanised, galvannealed, Galvalloy Galfan or Galvalum), even at reduced zinc alloy layer thicknesses. The polyetherimide primer layer is also flexible and highly adhesive to the topcoat and underlying zinc or zinc alloy, meaning polyetherimide primer layers may be used in lieu of conventional organic primer layers to improve the overall properties of organic coated steel strips comprising galvanised, galvannealed, Galvalloy ® , Galfan ® or Galvalum ® zinc or zinc alloy coatings.

In a preferred embodiment the organic primer layer has a dry film thickness between 1 and 40 μιη, preferably between 3 and 20 pm and more preferably between 4 and 10 pm. The dry film thickness of the organic primer layer largely depends on the end application of the organic coated steel strip. In highly corrosive environments thicker organic primer layers may be used whereas thinner organic primer layers are more appropriate if the organic coated steel strip is for indoor use. The inventors found that organic coated steel strips comprising a polyetherimide primer layer having a dry film thickness between 4 and 10 μιη exhibited improved corrosion resistance making them suitable for both indoor and outdoor applications. However, in the market place long guarantees (20-40 years) are given for "high-end" organic coated steel strip products typically comprising up to 200 μηι thick organic coating systems. For certain applications it may be necessary to provide an organic coating system having a dry film thickness of more than 200 μιτι. For 'high end' organic coated steel strips comprising plastisol topcoats the preferred dry film thickness of the polyetherimide primer layer is at least 8 μηη, preferably 10 μιτι. For organic coated steel strips comprising polyurethane topcoats the preferred dry film thickness of the polyetherimide primer layer is at least 15 μηι, preferably 25

In a preferred embodiment the carbon steel strip contains in weight %: 0.04 - 0.30 % C, 1.0 - 3.5 % Mn, 0 - 1.0 % Si, 0 - 1.0 % Al and 0 - 1.0 % Cr, preferably 0 - 0.2% C, 0 - 2.0% Mn, 0 - 0.6% Si, 0 - 1.0% Al and 0 - 0.6 % Cr, the remainder being iron and unavoidable impurities. Other alloying elements such as Mo, P, Ti, V, Ni, Nb and Ta can be present but only in small amounts.

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. Conventional OCS products comprise zinc or zinc alloy layers 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 ® which is an alloy containing zinc and aluminium (55%) may also be used.

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 μι η .

A coating thickness above 12 μηη 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 μηι is preferred because very good corrosion protection is possible even at reduced coating thicknesses, thereby reducing the overall cost of the organic coated steel strip. More preferably, the zinc alloy has a coating thickness of 3-8 Mm 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 better 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 carbon steel strip. 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 coated steel strip comprises a chromium-free conversion layer between the zinc or zinc alloy coating and the organic primer layer. Typically a conversion layer is provided to improve adhesion between the organic primer layer and the underlying zinc or zinc alloy coating. 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 strip products comprising polyetherimide primer layers of the invention. 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 10 μιτι, preferably between 3 and 6 μηη, with suitable corrosion layers comprising phosphates and zirconates.

In a preferred embodiment the topcoat layer comprises plastisoi, polyester, polyurethane or polyfluorocarbons. Plastisoi 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 the preferred topcoat material for organic coated steel strips that have to have very high corrosion protection properties. The plastisoi topcoat may be a single layer or a multilayer. Polyesters, polypolyurethanes and fluorocarbon also offer improved corrosion protection properties and are compatible with the polyetherimide primer of the invention thereby reducing the possibility of topcoat delamination/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 20-200 μηι, preferably 50-200 μιη. A lower limit of 20 μιτι is necessary otherwise the organic coating system will not provide sufficient corrosion protection. On the other hand an upper limit of 200 μιτι is preferred since thicker layers may delaminate from the zinc or zinc alloy coating.

Preferably the organic coated steel strip comprises a zinc alloy coating (Zn-5%AI or Zn-Mg-X) and an organic coating system comprising a polyetherimide primer layer and a plastisoi topcoat thereon wherein the organic coating system has a dry film thickness of at least 20 μιτι and up to 200 μιτι. In another embodiment the plastisoi topcoat is replaced by a polyurethane topcoat having a dry film thickness of at least 20 μιτι and up to 50 μπι.

As a consequence of the high corrosion protection properties of the polyetherimide primer, it is possible to reduce the thickness of the topcoat without reducing the overall corrosion protection properties of the organic coating system.

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 polyetherimide primer layer in the organic coated steel strip. They may be provided as independent components or preferably they may be loaded into nanocontainers for active corrosion protection. Halloysites, layered double hydroxides, CaC0 3 , 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 Na 2 Mo0 4 , cerium molybdate Ce2(Mo0 4 ) 3 , cerium nitrate Ce(N0 3 ) 3 , calcium nitrate Ca(N0 3 ) 2 , zinc sulfate ZnS0 4 , sodium tungstate NaW0 3 , sodium phosphomolybdate hydrate Na 3 Moi 2 0 4 oP, sodium phosphate Na 3 P0 4 , sodium hydrophosphate Na 2 HP0 4 , sodium dihydrophosphate NaH 2 P0 4 , sodium carbonate Na 2 C0 3 , sodium polyphosphate NaP0 3 x, 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 strip for building and construction according to the first aspect of the invention, which comprises the steps of:

i. providing a carbon steel strip;

ii. providing a zinc or zinc alloy coating on the carbon steel strip;

iii. applying a solution comprising a polyetherimide intermediate on the 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 carbon steel strip of step (iv) to a heat treatment.

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

In a preferred embodiment of the invention the solution comprising the polyetherimide solution is prepared in a water based solution. Preferably, the polyetherimide solution is prepared in an aqueous solution, which further avoids the problems associated with the handling and transport of solutions containing organic solvents. It is preferred to apply an aqueous polyetherimide intermediate solution on the zinc alloy so that problems associated with the handling and transport of solutions containing organic solvents. Thus, if the polyetherimide intermediate is prepared in a water based solution, it is preferred to transfer the polyetherimide intermediate to an aqueous solution before applying the aqueous polyetherimide intermediate on the zinc or zinc alloy.

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-Woc/(-polyethylene glycol-J /oc/(-polypropylene glycol (J1 ), 4,7, 10- trioxa-1 , 13- tridecanediamine (J2), Poly(propylene glycol) bis(2-aminopropyl ether having a molecular weight 230 (J3), Polypropylene glycol) bis(2-aminopropyl ether having a molecular weight of 400 (J4) and bis(2-aminoethoxyethane) (J5). ln 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 intermediate is end-capped with 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 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 strips.

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 strip 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 least 300 ° C to reduce the amount of time the coated strip 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 even 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 strip 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. 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 pm and at most 5 pm. 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 5pm 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 a polyetherimide as an organic primer layer in an organic coated steel strip for building and construction.

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.

Zinc alloy coating treatment

A carbon steel strip containing 0.022 C wt%, 0.173 wt% Mn, 0.003 wt% Si, 0.056 wt% Al, < 5ppm B and 0.001 Ti was provided, degreased and subsequently provided with a zinc alloy coating in accordance with the following treatment: Step 1 : in 1 1 seconds, heating the carbon steel strip from room temperature to 250 ° C in an atmosphere 85.5% N 2 , 2% H 2 , 11% C0 2 and 1.5% CO;

Step 2: in 11 seconds, heating the carbon steel strip from 250 °C to 670 °C in the same atmosphere as in step 1 ;

Step 3: in 46 seconds, heating the carbon steel strip from 670 "C to 800 ° C in an atmosphere of 85% N 2 and 15% H 2 ;

Step 4: in 68 seconds, cooling the carbon steel strip from 800 °C to 670 C in the same

atmosphere as step 3;

Step 5: in 21 seconds, cooling the carbon steel strip from 670 °C to the strip entry

temperature, usually 475 "C, in the same atmosphere as step 3;

Step 6: dipping the carbon steel strip in liquid zinc alloy containing 1.6 - 2.3 wt% Al and 1.6 -

2.3 wt% Mg, usually at 460 °C for 2 seconds, and wiping the zinc layer on the carbon steel strip with 100% N 2 to regulate the coating weight;

Step 7: in 60 seconds, cooling the carbon steel strip to 80 °C in 100 ° C N 2 .

Preparation of polyetherimide primer layer (A¾

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-fa/oc/ -polyethylene glycol-Woc polypropylene glycol (J1 ) (10 mmol) is added to form a white suspension. The white suspension is stirred under N 2 at 60 °C for 4 hours until the aromatic dianhydride and J1 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 roll coated on the zinc alloy coated carbon steel strip and dried at a temperature of 80 ° C for a period of 5 minutes. Finally, the coated carbon steel strip is subjected to a curing treatment of 200 ° C for 5 minutes to cure the end-capped polyetherimide intermediate and form the corresponding polyetherimide primer having a dry film thickness of 5-6 μιτι on the zinc alloy.

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 the zinc alloy coated carbon steel strip, dried at a temperature of 80 ° C for 5 minutes and cured between 200°C and 250 °C for 5 minutes to form the corresponding polyetherimide having a dry film thickness of 5-6 pm.

Providing the topcoat A plastisol is applied on the polyetherimide primer layer by any suitable method e.g. lamination, spraying, dipping or roller coating. When roller coating, the plastisol, having a viscosity between 0.2 to 0.8 Pa.s, is applied in a coating line having a coating line speed between 60 and 200 m/min. If the viscocity is greater than 0.8 Pa.s then flow and levelling of the applied layer is hindered. The applied plastisol coating is then cured between 160 and 230 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 strip products.

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 B 7 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/80cm 2 /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.