Experience with dredging boats proves thermoplastic coatings such as Abcite are a cost-effective alternative in marine applications, write Corinne Dziekan and Raf van Os
Corrosion protection has long relied on a number of wet or dry coating technologies including PVC, Polyurethane, thermosetting polymer and[ds_preview] thermoplastic coatings. However, each of these has drawbacks, either in the level of protection provided, durability, costs, or ease of application. Abcite thermoplastic coatings represent a major breakthrough as they offer a unique combination of properties with many advantages for a wide range of applications.
According to the World Corrosion Organization, the direct costs of corrosion represent 2.2 trillion $ or 3 % of the world’s GDP. The potential to reduce this cost is significant, and Abcite coatings can make an important contribution to this. In addition to traditional corrosion protection systems such as liquid paint or thermoset powders, thermoplastic powder coatings offer many advantages for the applicator as well as the operator, in particular in the most extreme environments such as the marine industry.
This article describes the features and benefits of these coatings in comparison to common corrosion protection systems, and presents some reference cases.
Historical development
In the 1950s the pressure on the market to find more ecological coating processes led to several developments with thermoplastic powder coatings (based on polyethylene). In 1953 the powder fluidisation process was first introduced by the industry.
Even though the arrival of electrostatic spraying methods 20 years later allowed a broader application field for thermoplastic coatings, fluid bed application still occupies an important place in the global thermoplastic coatings market, estimated at approximately 139.000 t in 2010.
Definition of a »thermoplastic« coating
A thermoplastic coating is based on a polymer, whose chemical nature defines the properties and behavior of the coating. Contrary to thermoset coatings, a thermoplastic coating does not require any curing: After cooling the coated pieces are ready to be used.
The process is reversible. For this reason, thermoplastic coatings can be repaired without any loss of integrity or performance. In comparison, liquid paints or thermoset powders are rigid systems and their repair is not without risk. Another feature of thermoplastic coatings is the non-porosity of the film, thus creating a highly effective barrier against many chemicals.
Advantages in detail
Thermoplastic coatings allow to build a continuous, relatively thick, non-porous film with excellent impact and chemical resistance in a very short time. These coatings are made of a broad range of polymers with different functionalities including PVC, polyethylene, fluoro-polymers, polyamide, and ethylene copolymers. Abcite coatings fall in the last category and offer several advantages:
1. Highest degree of corrosion protection.
2. Excellent adhesion and resistance to abrasion and mechanical shocks.
3. Cost-effectiveness as no primer is required at high film thickness in one layer.
4. Maintenance-free throughout the service life of the coated part or structure.
5. Environmentally friendly.
These advantages can be described more explicitly:
1. Highest degree of corrosion protection
In the marine industry, efficient and lasting corrosion protection is a must. This degree of protection can be achieved with a three- to five-layer liquid paint system or a single layer of Abcite. Indeed, laboratory tests (ILF laboratory performance test methods according to ISO 12944-6) confirm that a mono-layer Abcite coating meets the requirements of the category C5-M High, as well as the Im3 High category. In particular, Abcite passes a 1,440 hour salt spray test and a 3,000 hour immersion test with excellent results, while internal testing showed excellent results up to a 3,600 hour salt spray test. This is a unique unmatched performance compared to other, existing coating solutions.
2. Excellent adhesion, resistance to abrasion and mechanical shocks
Abcite is characterized by its excellent adhesion to steel, without the use of a primer, adhesion promoter or other specific surface treatment. The example of a coated pipe (Fig. 1) illustrates its flexibility and mechanical resistance: When hit with a hammer, the pipe will deform but the coating remains undamaged and shows neither failure nor cracks.
3. Cost-effective: no primer required, high film thickness in one layer
With a three-layer coating system, each layer contributes to the overall performance of the system. The effectiveness of the system depends on the application conditions, the properties of each single layer as well as on the cohesion of the total system. The use of thermoplastic coatings allows a reduction in total material consumption (compared to a solvent-based three-layer system) of more than 40 %.
4. Maintenance-free throughout the service life of the coated part/structure
In the marine industry as with any other investment, the life time of an asset is critical, and maintenance costs can easily exceed the budget. For these reasons, operators may specify reliable but expensive materials such as stainless steel in order to avoid the maintenance issue. For an efficient long- term corrosion protection, multiple solutions can be considered:
• stainless steel,
• standard coating (such as epoxy),
• thermoplastic coating.
Fig. 2 illustrates how thermoplastic coatings offer the most effective and economical long-term alternative, taking the cost of the initial installation/application into account as well as ongoing maintenance.
5. Environmentally friendly
An important consideration for both professional coaters and operators is the environmental aspect of each coating. In this respect, the increasingly strict VOC regulations do not affect thermoplastic coatings as they do not contain any volatile components or halogens. While they contribute to the functionality of a coating, certain chemicals such as plasticizers or Bisphenol A have a strong impact on the environment. Consequently, their use is increasingly regulated. Abcite does not contain those substances and is consequently not affected by the tightening regulations.
Thermoplastic coatings in practice
I. Fluidised bed
Even though spraying methods are becoming more and more popular, fluidized bed remains the most common, quickest and economical application method for thermoplastic coatings. The application procedure is as follows: After careful surface preparation (mechanical surface preparation such as sand blasting Sa 2.5 or chemical surface preparation is recommended) the substrate must be clean and degreased. It is then preheated, dipped in a fluidized bed for a few seconds, agitated, pulled out of the bed and hung up to cool. The heat stored in the metal causes the powder to melt and build a smooth coating layer. After cooling, the parts are ready for further processing or use, and post heating is not mandatory.
The film thickness depends on substrate temperature, dipping time, agitation in the bed and part geometry. Generally, a film thickness between 300 and 600 µm is obtained in one pass. Finally, it is worth to note that, compared to other thermoplastic coatings such as polyamide or fluoropolymers, Abcite has a lower melting point and requires less energy.
II. Spraying or hot flocking
As with thermosetting powders, Abcite can be applied by using a common spray gun. The powder is sprayed on the pre-heated surface, and post-heating is often not required. The advantages of the spray method are the possibility to coat big parts or parts with difficult geometry, and to avoid an investment in a fluidised bed. Compared to thermosetting powders, the application of Abcite is much more efficient by lower overspray effect.
III. Overcoating
Abcite is compatible with liquid paint or thermoset powder systems and can be overcoated (note: activation of the surface is necessary in case the top coat is applied after cooling).
Application in the marine industry
Abcite has been in use for more than 20 years on dredging boats worldwide (Fig. 3). Those dredgers are used in subtropical and tropical climates, pumping sand for the building of artificial islands. The »jetwater pipes«, with a diameter up to DN 1000, of three chatters, and designed to pump sea water containing sand, are coated inside and outside with Abcite. More recently, the cooling water pipes of these boats have been protected with the product as well.
The advantages of thermoplastic coating in this application are multiple:
• Long-term corrosion protection under extreme conditions and climate.
• Excellent abrasion resistance.
• Weight saving (allowing the use of thinner pipes).
• Allows the use of standard steel instead of expensive »Cunifer« alloys.
• No maintenance or repair costs despite extreme service conditions.
Summary
Thermoplastic coatings prove advantageous in many ways compared to other solutions. Abcite, however, offers very interesting properties and performance, in particular in extreme conditions of use compared to other thermoplastic coatings. Abcite is the trade name for thermoplastic coatings of Axalta Polymer Powders for enhanced corrosion protection. Axalta Polymer Powders belongs to the Axalta Coating Systems group and is a leading global coatings company dedicated solely to the development, manufacture, and sale of liquid and powder coatings. Van Os-Duracoat is a Belgian company specialised in technical coatings, in particular for the marine and water industry.
Authors: Corinne Dziekan, Axalta
Polymer Powders Switzerland Sarl,
Raf van Os, Van Os-Duracoat NV
Corinne Dziekan, Raf van Os
