Phenalkamine curing agents, derived from the cashew nutshell liquid, provide outstanding anti-corrosion properties to epoxy coatings. This technology allows the paint formulator to provide the marine paint industry with solvent-free and edge-retentive coatings.

Introduction

Enlarge this picture Ship-building market.

The ship building industry has already moved to Asia. While South Korea and Japan still dominate the ship building market, China is getting involved and will be a major player in the future. The market share for U.S. and European shipyards has shrunk tremendously (Figure 1).

The only way for a western company to maintain or possibly gain some market share is to be innovative, through simplifying the application process, and to participate in a more stringent environmental friendly program while keeping quality high.

Quality can not be ignored, as it increasingly becomes a more important driver for the industry. New regulations on environmental issues1 and for safer ships2 will become increasingly stringent, thus requiring new technologies and skilled workers. Countries that lead in these new technologies will likely remain in the market and increase their market share.

Marine Paint Market

For a ship owner, the principal purpose of a ship is to navigate and not be held up in dry-dock for maintenance. Sturdy and cheap steel is the material of choice to build ships, but it has one drawback — steel corrodes. Steel therefore has to be protected by a coating, which ideally should last as long as the ship’s life.

Another very important feature is fuel consumption, which represents about 50% of the operating cost of a big ship. Fouling of the ship’s hull by barnacles, mollusks and algae roughen the hull surface and may cause a fuel consumption increase of 45% after 6 months.3

Ship owners are therefore requiring two types of protection from a coating: anti-corrosion for all surfaces and anti-fouling for the immerged surfaces. This paper will only deal with protection against corrosion.

The most critical parts of the ship in terms of corrosion are the ballast tanks. Moreover, inspection for ballast tank defects on new double hull tankers and their maintenance is more difficult than for single- hull tankers.4 The anti-corrosive property of the coating therefore has to be outstanding in order to avoid maintenance when in service. Rusty ships give an impression of bad maintenance and, therefore, of being unsafe for navigation.5 A ship owner needs to improve his operational image by keeping his ship rust free.1

Consequently, a ship builder has to provide high-quality ships, keeping in mind that he has to reduce his cost. At first sight, improving quality while decreasing cost could appear to be conflicting goals.

The total paint cost of a ship includes the application cost and the raw material (paint) cost. Depending on the labor cost in a country, the raw material cost represents 20 – 30% of the total paint cost.6 Saving on application cost is therefore the best way to proceed. Application costs can be reduced through:

· increasing productivity (faster curing);

· all-year painting (low temperature curing);

· simplify application process (universal primer);

· reduce inventory and scrap (universal primer);

· EHS monitoring (low VOC to solvent free); and

· easier application.

Major paint manufacturers for the marine industry agree on these needs and are working actively on developing new paints.1,6-7 Epoxy-based coatings are the best technology for anti-corrosion and have been used by ship builders for a long time.

Phenalkamine Chemistry – Cashew Nutshell Liquid Technology

Enlarge this picture The cashew nut.

The cashew tree, Anacardium occidentale L., is native from Eastern Brazil. The Portuguese introduced cashew to east Africa and west India in the 16th century. Today, cashew is the number-one nut crop in the world. World production is exceeding 2 billion tons of nuts. The top 10 countries growing cashew trees are Vietnam, India, Brazil, Nigeria, Tanzania, Indonesia, Guinea-Bissau, Ivory Cost, Mozambique and Benin.8-10 The nut is attached to the cashew apple (Figure 2), which is used in beverages, jams and jellies. The nut has two shells, the smooth outer shell and the harder inner shell. In between there is a honeycomb structure, which contains an oil known as cashew nutshell liquid or CNSL. CNSL constitutes about 20 to 25% of the total weight of the cashew nut. About 500 thousand tons of CNSL are theoretically available each year worldwide.

To extract the oil, the cashew nuts are passed for a few minutes through heated CNSL (187 – 194 °C). The nuts are first wetted to reach an excess of moisture of about 7 – 10% of the weight of the nut. This excess causes the cells to burst. The oil oozes out of the nut into the CNSL bath. Another process consists of exposing the nuts to a rapid change of temperature, from about ambient to a temperature roughly necessary for charring. This causes an explosive pressure in the cellular structure, and the oil is forced to ooze out of the shell.8 CNSL has a reddish-brown color and has versatile industrial applications like brake linings, epoxy curing agents, cement, varnish, etc.

Enlarge this picture CNSL composition.

This paper intends to demonstrate the outstanding properties that curing agents derived from CNSL can provide to protective coatings for the marine industry. CNSL contains mainly four chemicals, as shown in Figure 3.

Enlarge this picture Phenalkamine synthesis.

The main constituent is anacardic acid, which transforms to cardanol through heating by decarboxylation. Robert A. Gardiner invented the first epoxy curing agent derived from CNSL in 1975 and gave it the generic name of phenalkamine. CNSL is distilled to get cardanol. Cardanol is then chemically treated through polymerization and amination to produce the phenalkamines (Figure 4).

Enlarge this picture Cardanol properties.

Renewable natural raw materials are an endless source of chemicals, but their composition fluctuates throughout the year, requiring skill and experience from companies having chosen this route. Cardanol is the core molecule of all phenalkamines. This particular compound will confer outstanding properties to the coatings derived thereof, as shown in Figure 5.

Fast and Low-Temperature Curing

Enlarge this picture Fast curing.

Epoxy 2K-based coatings cured with polyamides are slow curing and require a curing temperature above 10 °C (50 °F). The consequence of this poor curing efficiency is low productivity and an inability to coat outside during the winter season in areas where the climate is cold. This inconvenience can be avoided by using phenalkamine curing agents.

Phenalkamines even cure faster than polyamide adducts, as illustrated in Figure 6.

Enlarge this picture Curing properties of phenalkamines.

Liquid, semi-solid or solid epoxy resins cure quickly even at relatively cold temperature, while keeping a reasonable pot life, as illustrated in Figure 7. Cure time is determined by means of an ICI apparatus (ASTM D 1640) and corresponds to the time when the needle no longer pene-trates the coating. The pot life is the time required to double the Brookfield viscosity after mixing the components of the paint (ASTM D 2196).

It is therefore obvious that using phenalkamine technology will enhance productivity. These coatings can also be used in all seasons, even in countries with cold climates.

Anti-Corrosion Properties

Enlarge this picture Anti-corrosion properties.

Phenalkamines for many years have been widely used in the marine industry for ballast tank coatings, which is evidence of their outstanding anti-corrosion properties. When compared to polyamide curing agents, phenalkamines exhibit a similar excellent corrosion resistance, as illustrated by salt spray tests (ASTM B 117) (Figure 8).

Enlarge this picture Water resistance of epoxy curing agents.

Phenalkamine’s particular structure confers water repellence to coatings based on this type of curing agent. Figure 9 illustrates the outperforming properties of phenalkamines compared to other curing agents. The weight gain by immersing coatings in salty water at different temperatures is by far reduced when phenalkamines are used as curing agents. Ballast tanks are the most sensitive parts of the ship in terms of corrosion.

Another way to illustrate an excellent anti-corrosion property is to perform cathodic disbondment experiments, generally used for pipe coatings. It is widely recognized that cathodic disbondment can be linked to water repellence and hence to corrosion resistance. A cathodic disbondment test was conducted by using a modified ASTM method (G8-90), where the pipe is replaced by a 6” x 12” steel panel.

Enlarge this picture Cathodic disbondment.

Figure 10 illustrates the cathodic disbondment test results obtained by using a 100% solid formulation (Table 1). After a 90-day immersion test no paint could be pried up by means of a screwdriver or a sharp object.

EHS Monitoring (Solvent Free)

Enlarge this picture Edge-retentive, solvent-free formulation.

VOC emissions will become increasingly restricted in the future, and the use of 100% solids will be preferred. Phenalkamine curing agents can be seen as a technology of the future because they allow the formulation of solvent-free coatings and, furthermore, are based on a renewable raw material (Table 1).

Edge-Retentive Coating

Enlarge this picture Edge-retention test.

A marine paint is usually sprayed on the substrate. Edge covering can be inadequate due to paint sagging. A thinner dry film thickness will eventually corrode earlier than the other parts of the coated surface. To prevent such corrosion, some shipyard workers use a brush to overcoat the edges. This practice however can reduce productivity and is not free from human error. Consequently, it is worth coating a ship’s surface with a satisfactory edge-retentive coating.

Edge retention is measured by coating an Aluminium structural angle (90-degree), and evaluating the ratio between the dry film thickness on both sides of the angle and the one on top of the sharp edge (Figure 11). The ratio must not be lower than 70%. The formulation shown in Table 1 was applied on three specimens accordingly, and the ratio between the coated sides and the edge reached 75 to 80% on average.

Conclusion

Phenalkamines are curing agents of choice for the future of the marine paint industry. This technology offers many advantages such as: fast and low-temperature curing; outstanding anti-corrosion properties; solvent-free formulations; and edge-retentive coatings.