Effect of interlayer adhesive on three-layer coatings

Nowadays, polymeric interlayer adhesives such as polyethylene grafted by maleic anhydride, are used widely for linking polyolefins to polar polymers (e.g., polyamide, epoxy, polyvinyl alcohol and etc.), or metals. These types of adhesives are used as a middle-layer in oil and gas, multi-layer films, multi-layer pipes, pipes junctions, fuel tanks and etc. In this article the application of interlayer adhesives in three-layer coating has been investigated.

Two of the common issues in oil and gas industry are corrosion and rottenness. Fixing and replacing of pipes cause numerous costs. Thus, investigation of producing coatings which can increase resistance to corrosion had always been of interest. Three-layer polyethylene coating is one of the most consumed coatings which has composed from three layers:

1. The bottom layer which is the epoxy primer
2. The middle layer which is grafted polyethylene with maleic anhydride
3. The outer layer which is high density polyethylene (HDPE)

These layers have been showed in Fig 1.

Fig 1. Components of the three-layer anti-corrosion coating of the pipe [1].

How to apply the above three layers with an extruder is shown in Fig 2.

Fig 2. Applying the coating with an extruder [2]

Steps of applying the coating

1. Preparing the surface of steel pipe by grinding (sand blasting)
2. Washing the surface by chromate or phosphoric acid
3. Spraying epoxy powder on the steel surface
4. Apply the outer layer of polyethylene in the form of a strip by extruder

The advantages and disadvantages of polyethylene coating n summary is shown in Table 1.

Table 1. Advantages and disadvantages of polyethylene coating

If the coating suffers from a defect such as a pore or local separation of the coating from the surface, so that corrosive agents such as water and oxygen are allowed to reach the surface, the coating will no longer be able to protect the pipe-line and the pipe will corrode like Fig 3.

Fig 3. A corroded pipe [3]

To reach the maximum protection against corrosion, the coating must have suitable adhesion to the pipe, so it can transfer the stresses which is created from both outside and inside the pipe (such as expansion and contraction due to temperature changes) to the outer layer of polyethylene. So, it can prevent stress on the interface of the pipe and the epoxy layer, followed by separation of the three-layer coating of the pipe.

According to reports which are published at Iran and the other countries, it has been found that most of the times these coatings will be separated from the surface of pipe in a quietly short time (less than five years). That is why it is made concerns about early corrosion of metal pipes.

One of the known issues of polyolefin coatings are low adherence of coating to the surface of metal. Thus, to improve adhesion and protection from corrosion, usually a layer of epoxy resin under the polyolefin coating is used. Although the adhesion of epoxy to metal is appropriate, its adhesion to polyolefins is very weak. To solve the problem one layer of adhesive based on maleic- polyethylene between epoxy layer and outer polyethylene is used. It causes the adhesion of HDPE to metal layer and compatibility of epoxy coating in terms of polar. The adhesive must be compatible with the polymer coating (non-polar) on the one hand and with the epoxy (polar) substrate on the other hand, so a compatibilizer is used in the components of this adhesive. The compatibilizer is polyethylene bonded with maleic anhydride.

The compatible performance of the polyethylene in the interlayer adhesive is remarkably important because the compound links to the bottom layer (epoxy) of its maleic anhydride head due to a chemical reaction between the maleic anhydride and the epoxy group, and the polyethylene head to the outer black layer. The reaction is shown on the Fig. 4.

Fig 4. Chemical reaction between compatibilizer and epoxy layer [4]

The Aria Polymer company is manufacturer of interlayer adhesive for these coatings. For more information call us.

Author: Emad Izadi Vasafi

References:

1. http://m.cnspipes.com/coating-pipe/3pe-steel-pipe/api-5l-gr-b-x42-x60-x70-saw-ssaw-lsaw-er.html

2. https://www.useon.com/3lpe-external-coating-line/

3. https://www.canusacps.com/non_html/reference/TP_01.pdf

4. Ha, Thu Huong, et al. “Maleic anhydride grafted polyethylene powder coated with epoxy resin: A novel reactive hot melt adhesive.” Journal of applied polymer science116.1 (2010): 328-332.