- Schematic of the Isotub Coating system: layer 1 is fusion bonded epoxy (60-100 microns or 250-400 microns); layer 2 is adhesive (min. 300 microns); and layer 3 is high density polypropylene, 5-30mm.
The use of polypropylene as an anti-corrosion coating for pipelines is now internationally accepted for use at elevated temperatures. It can also be adapted for other purposes.
Isotub Coating, based in Dunkerque, was requested to carry out a program for the design, application and testing of a coating that could give stability at high temperatures; thermal insulation properties; good anti-corrosion properties; and stability subsea. This work was performed with a material supplier and a pipeline operator.
Development of high density polypropylene was based on existing three-layer, polypropylene material and application equipment technology. Through the addition of minerals such as barium sulfate to act as a filler it is possible to increase the density of the coating up to 2,000kg/cu m.
This coating system is applied by the side extrusion method to a thickness range of 3-50mm, and can be produced with a roughened surface to increase friction.
The epoxy layer provides excellent adhesion, chemical resistance and resistance to cathodic disbondment. The elevated glass transition temperature of the epoxy allows the coating system to operate at elevated temperatures.
The purpose of the intermediate layer is to provide a good adhesion between the epoxy primer and the high density polypropylene top coat. The intermediate layer provides flexibility and permits movements of the coating to avoid excessive stresses. The adhesive is a specially designed polypropylene-based copolymer modified with grafted monomers: it is soft, with a low vicat softening point of 110C and low tensile strength properties (15MPa).
A high density polypropylene top coat provides anti-corrosion protection, thermal insulation, mechanical protection and increased negative buoyancy. The material is UV stabilized: by adding further stabilizers it has been adapted for use at elevated temperatures. The material grade has been designed for good processability during coating application.
Thickness of this layer will vary with the service conditions and the properties required at a density of 0.9 to 2. Color depends on the choice and amount of barium sulfate (usually gray).
Tests performed were: impact; indentation; thickness survey; adhesion; bending; hardness; density; thermal aging in air; hydrothermal; thermal conductivity; trawler board; shear test.
Results showed that this coating has the flexibility to be used for the reel barge lay method and provides impact resistance superior to concrete. It combines anti-corrosion, mechanical protection, thermal insulation and increased negative buoyancy in the one coating application.
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