Marine coating technologies

Malcolm Latarche
Malcolm Latarche
ShipInsight

11 December 2017


The move away from wood to iron and steel ships was done for many reasons not least a need to build bigger ships and to accommodate engines. While some of the problems unique to wooden vessels such as ship worms were removed they were replaced by a new problem in that any ferrous metals exposed to the marine environment will inevitably suffer from corrosion.

While this process cannot be entirely halted, it is considerably slowed by the use of protective coatings. Almost any form of paint will confer some delaying effect against corrosion but the diverse areas of a ship and the carrying conditions they are exposed to require specialist products for some areas

Developing coatings is a science and since conditions experienced in the various locations can be very different it is important that the coatings are matched to the expected conditions. The majority of coating manufacturers have a wide range of diverse products each intended for a particular purpose and care should be taken to ensure that when it is necessary to use products from different suppliers that they should be compatible.

As previously mentioned, all coatings products contain substances known as volatile organic compounds (VOCs). These compounds are there for a variety of reasons such as reducing viscosity to aid application, controlling sag resistance, aiding flow and levelling, improving curing time and bonding with substrates and other coats.

VOCs evaporate into the atmosphere where they combine with pollutants such as NOx and SOx to produce smog and low level ozone which is considered a health hazard. Because of these characteristics, VOC content in coatings has been the subject of regulation on a local scale. The US, EU, China and South Korea have been most active and as a consequence of local regulation, coatings manufacturers have been obliged to develop products that comply with them.

Low VOC coatings some of which are water-based have different properties from older products and application methods may be different. Since the regulations were mostly made up to 10 years ago, most products now available can be considered as complying with them. However, in anticipation of further regulation, manufacturers continue to develop new products that can be considered as future proof.

Layers of protection

While some coatings can be used as a one-coat application, coating systems usually consist of a primer, possibly a tie coat and one or more coats of the chosen product. All the coats have a role to play with the primer being the first barrier against corrosion, a tie coat if necessary will bond the primer and the top coat and the top coat will provide the protection the coating system is designed for.

Products from different manufacturers may employ similar technologies but usually a manufacturer develops its own products to work best together. The paint store on a top end ship will usually reflect this but further down the scale of good management this will not always be the case. The vast majority of coatings are applied when the ship is being constructed or during routine drydockings. Application by a ship’s crew is usually restricted to repair and maintenance work or for aesthetic reasons.

Except in a very few cases, the quality of work by the crew is unlikely to match that of coatings contractors. If insufficient attention is given to surface preparation, mixing and method of application, the result will inevitably be inferior. All leading makers supply the necessary information required for both health and safety reasons and for technical advice. For ships with internet access, most of this material can be accessed directly from the maker’s website.

On any coating system, the base coat will be the foundation which all subsequent coats are reliant upon. It must adhere to the steel substrate and will offer a degree of protection against corrosion. Destined never to be seen except when first applied, primers need to be matched to both the substrate and the tie or top coat if the system is to function as intended. There are a small number of offerings in this category that are one-coat products but most are two-part products. Most makers only offer guarantees on systems if the primer used is confirmed as compatible which generally means that all layers must be from their own product ranges.

At the point in time when coating contractors begin work on a newbuilding the structure of the ship will almost certainly have some shop primer covering. Shop primers are used during fabrication and will need to be mechanically prepared or removed before the application of the system primer. They should not be confused with the main primer coat which forms the basis of the coating system.

Today it is common practice to use a universal primer in newbuilding coatings rather than the range of different products that were used until quite recently. A universal primer may seem to be a compromise solution but in fact it would seem to have many benefits. From the shipyards’ point of view, a universal primer may cost more but it does reduce waste and allows for economies of scale in bulk purchases. For the paint manufacturers, product lines can be optimised and R&D more focused on whole systems rather than individual products. Shipowners might not appear to benefit directly but assuming the yards’ cost savings are passed on then there is a financial benefit and in all probability the coatings will be of a higher standard as workers will be more familiar with the product.

For newbuilding work, almost all primers will contain zinc because of its highly effective anti-corrosion properties. There are two basic types of zinc primers – zinc silicate and zinc epoxy. The former is considered better for initial coating at the shipyard while the epoxy is considered as the best option for repair and maintenance work because there is less of a requirement for a high standard of surface preparation.

Epoxy Products

This group of products are widely used on ships and account for most coating systems other than those below the waterline for biofouling management. They come as either pure epoxies or modified epoxies. The modified epoxies will have added ingredients designed for a specific purpose such as reducing curing time or application on areas.

Epoxy coatings need to harden or cure to form the outer protective coating. This can take a considerable time but with the use of a hardening agent the time is much reduced. For this reason most epoxy coatings are two-pack products; one being the resin and the other the hardener. The two parts have to be mixed in precise ratios for the declared properties of the final coating to be achieved. The hardening process is affected by a number of factors with temperature being particularly important. Coating thickness and humidity also affect drying times and quality of finish.

Even when a coating is applied in full accordance with the manufacturer’s instructions the curing process can take several days or weeks before the maximum protective effect is achieved. During this period, care should be taken to avoid damage to the coating. Attempting to speed up a repair job by adding more hardener or adding solvents to extend the working time of a properly prepared mix will also have detrimental effects on the final finish.
Epoxy coatings are used extensively for ballast tanks, cargo tanks and cargo holds.

In some instances it will be possible for a coating to react with particular cargoes. Operators of chemical tankers should be well aware of the compatibility of coatings and cargoes and will doubtless have procedures in place to prevent loading of cargoes in inappropriate tanks. Some regulatory authorities such as the EU and the FDA prohibit the carriage of some foodstuffs in tanks and holds with a coating that can react with or be absorbed into the cargo. Coatings makers will publicise the products which have been given approval by such bodies.

Several of the products used on the superstructure and side shell of the ship above the waterline will be chosen for their cosmetic effect as well as their durability. Often these will not be epoxy coatings but will be alkyds. Alkyds comprise a polyester modified by the addition of fatty acids and other components, Alkyd coatings are typically one component (no hardener added) and will be slower to fully dry. They use the oxygen in the air to complete a reaction that changes the coating from wet-flowing to hard and durable.

Moisture Cure Urethane

Moisture curing urethane or polyurethane coatings are single component coatings in which the curing process is a chemical reaction with atmospheric water. They are an effective and economical coating and can be applied independent of the weather (temperature, humidity and dew point), dry quickly and are surface tolerant. Because this type of coating does not have to be mixed in the same way as two-component coatings there is no risk of error in mixing components. In addition, the coatings remain elastic and resistant to UV radiation to provide long term durability.

The drying rate of one-component coatings is dependent on the relative atmospheric humidity and the temperature. Low temperature and low atmospheric humidity may slow down the drying considerably.

Coating alternatives

In 2016, at SMM and other marine exhibitions, a new anti-fouling products was displayed that while applied to the outer hull is not a coating in the accepted sense. Micanti Antifouling film is a physical barrier against fouling and therefore movement of the vessel is not needed for it to be effective. The product comes on a roll and is applied to the vessel in strips such as wallpaper is applied to walls.

It is described by its Netherlands-based developer as a self-adhesive foil consisting of backing paper, pressure sensitive modified acrylic adhesive, a 12μm polyester carrier foil and antifouling layer or cured acrylic adhesive with embedded nylon fibres. The product feels like a coarse velvet but the maker says this texture prevents settlement of organisms and the total weight as applied is comparable with a typical anti-fouling coating system. Micanti has been tested for more than 10 years in cooperation with institutions like MARIN, TNO and Delft University of Technology as well as in practice on a variety of vessel types.

A new type product not yet available on the market but announced as in development in early 2016 combines the concepts of traditional coatings and application from a roll. PPG which makes Sigma products is working on the eco-friendly Ship Hull film system with fouling Release and fuel saving properties (eSHaRk) project drawing on the work of a project development group including PPG, MACtac, Meyer Werft/ND Coatings, VertiDrive and Hamburg Ship Model Basin HSVA.

The project which has secured EU funding through to November 2018, aims to establish an automatic application process which enables a self-adhesive, fouling release film to be used on commercial ocean-going vessels. This process will allow shipowners and operators to enjoy the fouling release properties and drag reduction capabilities of the PPG SIGMAGLIDE self-adhesive film.

According to PPG the new film is superior to existing paint-based solutions in terms of eco friendliness, ease of application, robustness and drag reduction effects. eSHaRk is expected to have superior drag reduction properties compared with existing anti-fouling and fouling release technologies, up to 10% drag reduction as compared to currently available maximum 5%. The system incorporates a fine-tuned fouling release system, based on PPG’s premium 100% silicone binder technology and a self-adhesive film specially designed by MACtac for underwater use.

As part of the eSHaRk project a robot application technology is being developed by VertiDrive which will be used to apply the film automatically on large commercial vessels. The surface morphology of the film will be optimised to enhance drag reduction, increase fuel savings and reduce emissions to previously unattainable levels. A number of trial applications are underway. The SIGMAGLIDE fouling-release film solution under development within the eSHaRk project has a targeted launch date of 2018.

Ultrasonic anti-fouling systems are not commonly found for use on ships hulls but they are used on some smaller vessels and in specific areas of larger vessels notably sea chests, shafts and in pipework. An ultrasonic system uses a transducer to produce bursts of energy in specific pulse frequencies. This produces microscopic bubbles which when they implode destroy any algae that may have attached to the treated surface removing a food source for other organisms and also making attachment by them more difficult.