The below was an Exclusive for Steel Times Internationl.
Paint Systems for Steel.
Bryn Roberts, Laboratory Manager for HMG Paints, reviews the current coatings scene.
Coatings are amongst the most looked at, yet overlooked commodities in the modern world. From the proverbial paint on the Forth Bridge and topcoats on a million motor cars, to camouflage paint and the scratch-proof coatings on spectacles, they help to protect, preserve and enhance everyday products and it is estimated that half the nation’s workforce would have to go home if their coatings supplies didn’t turn up. Because many feature solvent-based technology, they are seen as pollutants, yet coatings play an important environmental role in saving natural resources by preventing manufactured items and artefacts being eaten up by corrosion and decay.
The most common use of industrial coatings is for the corrosion control and surface protection of structural steelwork, steel components, steel fabrications and machinery. Coatings are applied to improve their surface properties such as appearance and colour, to improve their durability and resistance to wear, and prevent corrosion, adding value and greatly extending product life cycles. In fact, the value and performance added to steel products is quite disproportionate to the cost of buying and applying the coatings.
Leaving aside more specialised processes like coil coating and thermal spraying, I shall focus on the two technologies that form the backbone of industrial coatings, liquid coatings or wet paint systems, which have been around for several centuries, and powder coatings, which were introduced to the industrial scene in the 1960’s. Having been involved in the coatings industry since the 80’s and now working at the cutting edge in an R&D role, I have a good overview of the technology. Also, my current employer, HMG Paints of Manchester, is the UK’s leading independent industrial coatings producer, covering everything from single-pack stoving enamels, to waterborne paint systems and powder coatings.
Back in the 60’s, the liquid coatings sector, and the market generally, was dominated by single pack alkyds and stoving enamels, which were air-drying products that produced relatively low film thicknesses, with limited solvent resistance. Regular industrial paint systems then consisted of primer, undercoat and finish coats, with the intermediate coat applied to build the total film thickness and provide a colour base for the topcoat, since the primer was typically red oxide.
Nowadays, the majority of conventional coatings are two-pack, or 2K, solvent-borne paints, which account for around 80% of industrial coatings technology. When the solvent evaporates, this produces a densely cross-linked film, with a hard finish and good chemical resistance, although as solvent vapours are released, they emit volatile organic compounds (VOCs) and other pollutants into the atmosphere. Nevertheless, the development of low VOC compliant coatings, pollution prevention measures in the paintshop, effective solvent recycling and hazardous waste management, and modern paint booths equipped with downdraft airflow and air-fed spray masks, have together ensured the continuing popularity of 2K products.
So, for steel protection applications, two-component topcoats and primers remain the coatings of choice, albeit in lower VOC and high solids formulations, the latter with more than 75% paint solids content by weight. For finishing, 2K polyurethanes are the top of the range choice, giving good durability, gloss levels and colour retention, coupled with excellent chemical and corrosion resistance. The other mainstream topcoats, 2K epoxies, offer outstanding water and chemical resistance and are ideal for very aggressive environments, like oil rigs and chemical plants, but lose their gloss over time due to UV exposure. Alkyds and acrylics are considered general purpose finishes and provide good corrosion resistance at a reasonable cost; while chlorinated rubber withstands atmospheric pollution and immersion in salt and fresh water, although has now been largely superseded by 2K epoxies.
A more recent development has been non-isocyanate topcoats, such as our own Nisocoat 2, which offers similar technical performance to 2-pack polyurethane finishes, without the health and safety issues associated with the isocyanates found in most industrial coatings. Nevertheless, the use of air-fed respiratory equipment is still required when spray painting, so any operational advantage over polyurethanes is somewhat negated.
Modern primers contain much the same range of binding agents as topcoats, with 2K epoxy providing the best value in chemical and corrosion resistance, 1K alkyd being the affordable choice, and 2K polyurethanes ideal for when the primer coat is left unpainted for a period, such as when components are shipped to an OEM. For difficult-to-coat substrates like galvanised steel and aluminium, there are single- and two-pack etch primers; while for highly corrosive applications, such as gritting lorries, there are zinc-rich primers.
The wide choice of primer colours has brought about the gradual disappearance of the undercoat and the proliferation of 2-coat paint systems. Manufacturers of construction equipment and machinery, for example, choose colour-coded primers so that minor scratches and chips in the topcoat are not visible, although the slight shade difference enables spray painters to still see the difference between primer and finishing coats, ensuring full coverage.
The elimination of the undercoating stage is significant, since industrial coatings users mostly regard the paintshop as a bottleneck in the manufacturing process, with throughput dictated by the amount that can be painted per shift, so taking out one complete application and drying process saves valuable time. Moreover, recently developed 2K polyurethane ‘direct’ topcoats, like our own Monothane, moves this one step further, by dispensing with the primer coat altogether and spraying direct onto mild steel, aluminium, galvanised steel and many difficult-to-coat substrates. With force drying times down to as little as 20 minutes, paintshop productivity is dramatically increased and the only real drawback is the need to adopt a slightly revised spraying technique, beginning at the edges, to achieve full coverage, normally assured by the combination of primer and topcoat.
What about waterborne coatings, which use water as the main liquid component to disperse the resin content? Well, across industrial coatings customers, the waterborne revolution has yet to happen, with the rail industry one of the few European market sectors to fully embrace this technology. Manufacturers look at the higher costs and lower solids content of water-based paints and prefer to specify high solids solvent-borne coatings to ensure VOC compliance and reduce the cost-per-litre of equivalent dried paint film. Besides, the conversion to paint spraying plant suitable for applying waterborne products, particularly the use of non-corrosive materials and spray guns, is not on most manufacturers’ investment agenda and I would estimate that three-quarters of customers are not yet capable of handling this technology.
That brings us to powder coatings, where the resin is applied as a free-flowing dry powder and the final cured coating has much the same properties as a 2-pack liquid paint. It does not require a solvent to keep the binding agent and pigmentation in a liquid suspension and is typically applied electrostatically, so the substrates must be electrically conductive and are generally metal. The technology has a number of inherent advantages, notably that the coatings emit zero or near-zero VOC’s, they produce thicker films without running or sagging, and that any overspray can be removed and recycled, achieving around 95% utilisation efficiency and minimising waste disposal costs.
However, since coated parts have to be heated to very high temperatures exceeding 280˚C, allowing the powder to flow and bond to the substrate, large items take much longer to cure. Although I have known big excavators to occupy two expensive hours in a curing oven and huge quarry plant castings half a day, powder coating is mostly applied to smaller items like handrails, shelving and racking, white goods cabinets, lawn mower decks and filing cabinets, where curing times are quicker and energy costs more realistic.
Although for most applications powder coatings can be substituted by liquid coatings, the reverse is only true when objects are not too large to pass through the electrostatic process and curing oven, or where coatings are not applied on-site. The cost of the application equipment precludes most liquid coating users from employing powder coating, other than through sub-contractors, while a recent European Commission report found that most powder coating customers only possess the equipment to apply powders, meaning that there is actually limited switching between the two technologies.
So as long as liquid coatings remain the dominant method of corrosion protection and surface enhancement for steel components and machinery, and with waterborne formulations unlikely to be widely adopted until the commercial picture changes or environmental legislation dictates, then two-component solvent-based formulations are still the way to go for most. All that remains is for suppliers like ourselves to provide the technical support and advice need for you to make the most cost-effective choice, depending on the type of product and surface, the method of application, the intended environment, the desired durability and the required service life. Simples!