Paints and Protective Coatings - Extensive coverage07 December 2004

Just about everything we see is surfaces. And just about every manufactured product has had its surface coated or treated in some way to improve its performance. The benefits can be immense. For instance, the world's shipping fleets would consume an estimated 40% more fuel (70m tonnes) if they did not have anti-fouling coatings on their hulls. The coating on the propulsion screw on a single tanker can save over £300,000 on its annual fuel costs.

If we ignore the communicative role of surfaces - for example, colour-coding different areas of a floor to segregate pedestrian and vehicle traffic, then the plant engineer's interest in surfaces is mainly related to protecting them against mechanical wear, chemical corrosion (not forgetting water), or both. These surfaces could be the building fabric, they could be containers of materials or components (or the contents themselves), or items of plant and equipment that the plant engineer is charged with looking after.

Paint is the form of surface protection with which we are probably most familiar. The importance of choosing the right kind of paint for any given job, and of properly preparing the surface prior to application, cannot be overstated. Fortunately, many specialist industrial paint suppliers are happy to provide advice on both of these aspects.

One example is HMG Paints, an independent Manchester-based company with over 75 years of experience supplying paints and powder coatings to commercial transport, automotive, general industrial and marine businesses. It has set up its own Knowledge Base on the web at www.hmgpaint.com. This includes a list of applicable British and International Standards; advice on spray-gun setup and surface preparation; a chart for calculating paint coverage; a factsheet on overcoating schedules; and an explanation of accelerated weathering tests. There's also comprehensive advice on surface preparation, covering steel, galvanised steel, aluminium, woodwork, masonry, concrete and previously-painted substrates. Guides to painting concrete floors and swimming pools are also available.

Whatever the surface, corrosion and wear pose a far greater threat combined than individually. Discussing industrial processing applications Colin John, managing director of surface coating specialist Poeton Industries, comments: "Our research shows that the effect of corrosion and wear occurring together can cause more than ten times the loss of material from a component than either on acting alone. The phenomenon is caused by the wearing away of normally passive layers that might otherwise slow the corrosive attack, which immediately exposes the substrate to fresh corrosion - which is why, to get the best from a surface coating, it needs to be carefully specified to ensure it is fit for purpose."

For example, Poeton earlier this year launched an anticorrosive surface treatment for storage drums containing low-risk contaminated waste. In this instance, long-term exposure to aggressive environments is the threat to be tackled; the Apticote coating was originally designed to provide a 50-year life for storage drums at British Nuclear Fuels. The treatment consists of a sacrificial 'superzinc' base coating, finished with a two-pack epoxy polymer coating, spray applied.

Poeton's research and development manager John Archer comments that while a conventional 'envelope' coating is prone to corrosion through minute holes in the surface, a sacrificial coating relies on progressive oxidation to provide protection. (Galvanisation with zinc is a common example.)

"The reason the Apticote treatment is so durable is that it protects the sacrificial coating with an extremely tough corrosion-resistant epoxy polymer that has already lasted ten years in our own atmospheric tests with absolutely no signs of deterioration," he says. The coating can also be specified to protect against saltwater or atmospheric corrosion.

In some applications, it's not failure of a structure through corrosion that must be guarded against, but the formation of any rust at all. The food industry is a prime example, as John F Krotzer of power transmission company Boston Gear explains: "In these times of heightened awareness of food-borne illness such as listeria and E coli, the importance of eliminating rust is at an all-time high. The danger with rust is that it provides a rough surface on which bacteria can cling and multiply. Replace the possibility of rust with a nice smooth surface, and that is one less thing the processor needs to worry about."

Boston Gear's latest weapon in the war against rust is the Stainless Bost-Kleen (SBK) washdown protection treatment available on its speed reducers; this offers enhanced protection against aggressive cleaning chemicals used in the food industry.

One satisfied user of SBK is Darrel Kahler, assistant plant manager at the Henningsen Foods egg processing plant in David City, Nebraska, USA. "The typical paint or coating system used on supposed 'washdown duty' gearboxes start showing signs of distress within the first couple of weeks, but Boston Gear's SBK is different," he says. "For us, speed reducers with the new treatment are a great alternative to the high price of stainless steel."

But it's not only caustic washdown chemicals that can lead to the formation of rust in these environments, explains Boston Gear's director of engineering, Ralph Whitley:

"During installation and operation, the gearbox can endure abrasive contact with tools or is scratched or dropped during the process of unpacking and stocking. SBK was designed to both tolerate additional abuse without scratching and to maintain the integrity of the washdown protection if a scratch does occur."

Getting the right surface coating or treatment is not just a concern when buying new equipment - it can also be an important issue to maximise the life of existing assets. This was the case for consumer packaging supplier Rexam. At its factory in Yate near Bristol, the company has developed a proprietary process for moulding thin-wall plastic containers trays out of PET - a popular choice in the food industry because it is transparent.

However, PET has two properties that can make it difficult to process in high-volume moulding machines. First, it is abrasive, causing aluminium moulds to wear quickly; second, it is sticky, which makes it difficult to remove trays from moulds, and causes trays stick to together when stacked. Denesting trays is also difficult.

Rexam had been gritblasting the mould to roughen the surface, and blending an expensive migrating wax additive with the PET. This improved de-nesting but didn't solve the mould wear problem; and as the additive left a residue, the mould also needed regular cleaning.

Paul Hawkins, technical manager at the Yate site, called in Poeton Industries to advise on coatings, primarily to improve the long-term wear characteristics of the moulds.

Poeton's engineers recommended a coating of Apticote 450 to reduce wear in the mould cavity; and beadblasting rather than gritblasting as a more effective pre-treatment.

Initial trials were carried out on 20 out of 120 cavities in a drum mould. After six weeks of 24/7 operation there were no signs of wear in the coated cavities; Rexam agreed to have the whole tool coated. In addition, experimenting with reduced amounts of the de-nesting additive in the PET revealed that de-nesting was significantly improved when only half the amount of the expensive additive was used. The process now produces 1.5 million twincavity trays each week, to feed a production line that operates every day of the week.

As with paints, many surface coating manufacturers have a wealth of information and advice, accumulated over many years of experience, which they can offer to customers. Poeton Industries for example, has created a PC-based surface engineering selection software package called Apticote-Isis. This may be downloaded free of charge by coatings users; they can also download a Screencam demo of the product.

The package uses the expertise of Poeton's engineers to recommend the most appropriate surface treatment for engineering components, based on factors such as materials, heat treatment, environment and temperature, surface hardness and finish, loading and contact geometry, component shape and masking, and, of course, the wear regime, including any abrasives used.

Making paint work

Paints are made up of four main constituents:

Film-forming elements - resins, binders and polymers. Paints are often defined by the type of binder present such as a polyurethane car paint or a polyvinyl acetate latex paint. Other types of binders include bitumen, chlorinated rubber and nitrocellulose.

Solvent - which thins or disperses the resin, making the paint easier to apply and aiding application of the paint film. Together the resin and the solvent provide adhesion of the paint film to the substrate, and provide much of the paint's protection. Environmental concerns in recent years have led to a drive to reduce the amount of volatile organic compounds (VOCs) in paint solvents. They also help to hold together the third constituent:

Pigments - which give the paint its colour and help it to hide the surface it is covering. The most common white pigment is titanium dioxide (TiO2). Secondary pigments or extenders have less of an effect on opacity, but help to control other aspects of the paint such as viscosity, levelling and sheen. In addition to giving the paint its colour and opacity, the pigment ladds to the paint's strength and toughness lgives protection from corrosion, mildew and ultraviolet radiation lprovides stain and weather resistance, and decreases moisture permeability laffects the level of gloss in the paint film lalso helps the paint film to adhere to the substrate.

Additives are only used in small amounts, but have an important effect improving the behaviour of the paint. For example, they can improve drying time, prevent the paint from sagging on application, provide a smoother finished surface, or prevent a skin from forming in the can.

Drying of paint can occur in a number of ways:
- Evaporation. In paints where the binder is dissolved in the thinner, a film is deposited on the painted surface when the thinner evaporates. In emulsion paints, evaporation of water causes latex particles, which were held in suspension, to coalesce into a continuous film.
- Chemical reaction. There are three main possibilities here:
- Oxidation of the binder into a plastic solid (after the solvent has evaporated)
- Components in the binder react together. As this must not happen before the paint is applied, some kind of trigger must be required to promote the reaction - such as heating in an oven, or applying infrared or ultraviolet radiation
- Mixing two reactive components together immediately before application (as is the case with epoxy paints).

Sources of information:

Boston Gear
www.bostongear.com
(UK distributors: 01827 57561, 01926 411544, 01274 870804)

British Coatings Federation
www.coatings.org.uk
01372 360660

HMG Paints
www.hmgpaint.com
0161 205 7631

ICI Paints
www.icipaints.com
01753 550000

Poeton Industries
www.poeton.co.uk
01452 300500

Sterigenics International
www.sterigenics.com
001 630 928 1700

Surface Engineering Association
www.sea.org.uk
0121 237 1123

SOE

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