Coil coating makes use of a simple, but effective

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1 Novel surface treatment of metal strip in the coil coating process Granocoat 2840, a new coating chemical from Henkel, combines pretreatment and priming in a single operation, thus ensuring both high performance and a shorter, simplified and cheaper coil coating process. The process is more environmentally friendly as the new chemical does not contain toxic heavy metals, cures at low temperatures and is solvent-free. Author: Jörg Sander Henkel AG & Co KGaA Coil coating makes use of a simple, but effective principle: to clean, pretreat and coat flat coils or sheets of steel or aluminium in a continuous operation, before other stages of industrial manufacture. A conventional process sequence in coil coating consists of: ` Cleaning ` Conversion treatment (including optional post-rinse) ` Drying ` Primer coating ` Top coating ` Foil lamination (optional) r Fig 1 Coil coating line schematic Source: voestalpine Stahl Linz, Austria r Fig 2 A chrome-free coil coated aluminium V-groove sheet performing well after 10 years service, even at its most sensitive point the dripping edge A schematic of a steel process line is shown in Figure 1. After thorough cleaning that produces a hydrophilic, highly reactive metallic surface, a corrosion-preventive coating is applied immediately. This is achieved by chemical conversion treatment of the metal surfaces that simultaneously provides an adherent base for the subsequent paint coat. Conversion treatments have been based on chromate chemistry or, more recently, chromium-free, titanium or zirconium containing formulations. Chromium-free, no-rinse processes have become particularly important and commercial products like Bonderite 1455 (formerly Granodine 1455) that have been in industrial use since 1998 provide reliable corrosion protection and adhesion for subsequent topcoats (see Figure 2). Apart from outdoor and accelerated corrosion testing, the inhibiting effect of chromium-free pretreatment has been studied by high-resolution Scanning Kelvin Probe (SKP) experiments (see Figure 3). Electro-galvanised panel samples were cleaned and etched following a standard laboratory process, covered with a protective tape across one end (forming the y-axis) then, perpendicular to it, along the x-axis, treated on half of the remaining bare surface with Bonderite The entire specimen was then coated with a standard clearcoat. Upon removal of the protective tape, the specimen displayed adjacent treated and untreated areas, both 146

2 r Fig 3 Bonderite 1455 electrochemical delamination studies coated with the clearcoat and having a sharp border to a bare metal surface. One part of the bare surface (see left area in schematic of Figure 3) was encased with small stripes of resin, to form a small container which was filled with 0.5M sodium chloride solution. The SKP device was used to scan the area 10mm along the x-axis into the coating. It monitors the electric potential underneath the paint film and its propagation from the defect with time, thus delivering a vivid picture of the advancing corrosion. It was shown that the potential above the untreated zinc surface exhibits the typical Zn/Zn 2+ potential and the difference in potential between the actively corroding defect area and the intact clearcoat/metal interface is around 500mV. Contrary to this, the potential of the Bonderite 1455 pretreated area is shifted to more cathodic values with a potential step of only around 150mV. The delamination front remains stable over a long period, indicating almost no propagation of the corrosion. Despite technical acceptance of chromium-free pretreatments they have only recently gained limited share in the European coil coating market, rising from 0.3% in 1998 to about 4% today. However, the discussion on environmental and health risks has encouraged the coating industry s search for cleaner solutions. In particular, the implementation of the new chemical policy REACH (Registration, Evaluation, Authorisation and Restriction of Chemicals) will boost the replacement of the chromate processes that are proven, but hazardous to human health and the environment. REACH rules the use of all chemical substances manufactured in or imported into the EU and is governed by EC Regulation 1907/2006. Under its provisions, all chemicals that are commercially used above certain threshold volumes require official authorisation and therefore may be subject to assessments of their exposure risks to health and environment. Chemicals of very high concern, eg, CMR-labelled (carcinogenic, mutagenic and toxic to reproduction), and highly toxic substances like chromium (VI) compounds are likely to be phased out wherever possible or effectively banned. Additional economic factors are also driving the need for further coating developments. COMBINED PROCESSES FOR PRIMER PRETREATMENT Steel producers and processors are constantly thinking of how to add valuable new properties to their products, to reduce manufacturing and overall process costs, to make savings and efficient use of resources, and to look for business opportunities. For precoated coil, an evolution is taking place that requires coatings to provide features beyond corrosion protection and aesthetic looks. Additional features, like anti-fingerprint and soil-repellant properties, lubricity a 147

3 sists of 100% consumable ingredients in a single-package flowable blend, and a product based on a waterborne, thermally drying resin dispersion. The latter has passed its experimental stage and is currently being launched. r Fig 4 Granocoat 2840 master coils to support in mechanical forming, weldability (in some applications) and general performance improvements, combine with other factors for the development of surface treatments. The idea of combining pretreatment and primer functions in one single stage in the coil coating process has been around for a number of years. By employing this principle, workflow changes are rendered possible, like doing away with separate lubrication in the press shop, or the use of in-line application of additional coatings and protective lacquers. It would render obsolete one of the currently separate treatment steps, or make the respective treatment station available for other purposes, eg, multilayer coatings. In particular, when using a solvent-free formulation, together with one of the new curing techniques like near-infra-red (NIR) or ultra-violet (UV) that allow very fast curing, the combined primer pretreatment offers an opportunity for compact coating lines for coils and sheet. A modern concept of a primer pretreatment incorporates up to five different technology steps with their related economic and environmental benefits at once. The primer pretreatment products considered here: ` are chromium-free; ` make obsolete the separate conventional priming step, and can even replace backing coats; ` allow in-line priming in galvanising installations; ` save energy through low temperature thermal, or UV curing; and ` do away with organic solvents and also maximise the yield that is obtained with the coating chemical. Two technical approaches have been followed to obtain suitable coating products: a UV-curable chemical that con- THERMALLY CURING AQUEOUS COATING The thermal primer pretreatment Granocoat 2840 combines conventional chromium-free pretreatment chemistry and an acrylic resin dispersion in an aqueous single-package formulation. Its physical features resemble a commercial paint for wall and façade coating. In the coil coating process, the product is applied, as-delivered, by means of a roll coater with a liquid film of 8-10µm thickness. To open up a wider operational range the product can be diluted with small amounts of de-ionised water. The coating is dried in a hot air furnace or an infra-red dryer with a metal temperature of C maximum. The primer coating thus obtained has a thickness of 4-5µm, which gives a performance to match standard industrial requirements for indoor and outdoor architectural uses when combined with commercial polyester topcoats. Figure 4 illustrates Granocoat 2840 master coils thus produced. The process has passed extensive trials in industrial coating lines and is ready for commercial rollout. Pigmented versions are under development for the application as a single-coat backer. The photographs shown in Figure 5 illustrate the performance of Granocoat 2840 in important industrial quality tests where typical industry specifications are well met. Adhesion is tested using T-bend, impact and cupping tests. The coating passes 0.5T bending (ie, the panel being bent back on itself with an inner radius of 0.5 times the panel thickness) without paint de-lamination, and with only a few cracks showing on the surface of the bent shoulder. Impact tests show full paint adhesion at different impact energies, given in impact heights of up to 160mm with a ball-tip weight of 1kg. Erichsen cupping, combined with a crosshatch and 15-min immersion in boiling de-ionised water, is passed at 8mm dome height without flaking. At this elongation, the metal already breaks. Also, corrosion testing in the neutral salt spray test and at outdoor exposure testing in the Hook of Holland for three years has shown positive results. Good performance is also obtained when coating with commercial powder paints. REDUCED PROCESS COMPLEXITY IN COATING LINES In Europe the entire surface finishing of steel coils including rolling, mechanical finishing, cleaning, galvanising and coating is usually done in fully integrated plants. For such an integrated operation, the primer pretreatment concept gains a particular edge as it allows the priming process 148

4 r Fig 5 Granocoat 2840 test results to be integrated at the exit of a coil galvanising line. By combining the pretreatment and priming steps, even the cleaning that is normally the mandatory first stage in a conventional coil coating line can be omitted. Further economic benefit can be generated by an integrated finish coating stage, thereby making storage and transport between galvanising and coating lines obsolete. The effect on the process route is shown in Figure 6. The new primer pretreatment, combined with fast curing techniques, enables process speeds in galvanising and coating to be matched and, for the first time, these two operations have become compatible in in-line operations. A line speed of up to 160m/min has been realised in industrial trials. Solvent-free coating products, moreover, do away with expensive installations for the combustion and thermal destruction of the exhausts from the curing a 149

5 r Fig 6 Process streamlining by primer pretreatment ovens, and substantially reduce the fuel consumption and VOC and CO 2 emissions. Granocoat 2840 also provides a solution for compact coating lines that are especially suited to coat small batch sizes a typical job for steel service centres. High performance applications like outdoor siding and roofing generally necessitate the use of expensive pre-primed feedstock to match the required performance specifications. With Granocoat 2840 primer pretreatment and a powder topcoat, solvent-free coating becomes feasible. CONCLUSIONS Granocoat 2840, a new coating chemical currently being introduced into the coil coating market, combines pretreatment and priming in a single operation, thus ensuring high performance and a shorter, simplified coil coating process. At the same time, the environmental compatibility of the coil coating process is substantially improved as the new chemical does not contain toxic heavy metals (in particular no chromium (VI)) it cures at low temperature and comes solvent-free. It also renders savings in installation cost, inventory, handling and controls, CO 2 and VOC emission fees or quota sales, hazardous substances management (Seveso II) and insurance. Granocoat 2840 supports the sustainable use of resources and economic process adaptations. MS Jörg Sander is Marketing Manager, European steel and coil business, at Henkel AG & Co KGaA, Düsseldorf, Germany. CONTACT: joerg.sander@henkel.com 150