In order to produce a protective coating, the epoxy reacts as it melts and flows, and therefore the powder has to have the correct balance of reactivity and viscosity development in order to be an effective pipe coating. To achieve good flow and wetting of the substrate, temperatures are generally in excess of 200°C for three-layer systems and 230°C for stand-alone.
Developments in epoxy resin technology now allow formulators to develop powders that can achieve a good balance between these properties at lower application temperatures. Through laboratory work, complemented by full scale plant trials, lower energy consumption (LEC) products have been developed for application in the temperature range of 150 to 200°C.
In a laboratory evaluation, LEC FBE was coated onto steel panels that had been grit blasted to a surface cleanliness of SA2½. The panels were heated prior to application. Third party tests against CSA Z245.20-02 were performed by ITI Laboratories of Houston, Texas, on panels that were coated at 190ºC.
Plant trials were performed using new LEC FBE as part of a three-layer polyethylene coating system. Application conditions were identical to the conventional coating except that the preheat temperature ranged from 150 to 160ºC. These trials particularly evaluated the reduction in over thickness of polyethylene due to thinning on the weld bead.
Results
Table 1 outlines the acceptance criteria and results from an independent laboratory testing of steel panels coated at 190ºC using LEC FBE.
Additional modifications of the formulation were undertaken in order to improve adhesion. Results of this modified formulation are presented in Table 2. In this case, application was carried out at various temperatures in order to identify the critical temperature limit.
Figure 1 shows panels of plant-applied three-layer coated at 150ºC and 160ºC after 24 hours CDT showing 4 and 5mm respectively.
Test results obtained for application of the LEC FBE in plant trials where it was used as the primer layer for a three-layer polyethylene system are presented in Table 3.
Discussion
Successful coating with FBE is dependent on many factors relating to time and temperature. The FBE powder sprayed onto the heated pipe has to melt and flow into the anchor profile of the cleaned pipe surface. At the same time the epoxy is reacting and cross-linking to form a stable film. The ability to wet the steel surface is a combination of effects that are related to the melt viscosity of the epoxy resin as well as the reactivity of the powder. These effects are depicted in Figure 2.
Conventional FBE has a relatively high melt viscosity at temperatures below 200ºC and it is difficult for such epoxies to wet the steel surface at low application temperatures. In contrast, the newly developed formulations have much lower melt viscosity.
The results of third party evaluation tests using the new LEC FBE at 190ºC showed that it met or exceeded the requirements of CSA Z245.20-02 for stand-alone FBE coatings (Table 1).
The obvious benefit of using LEC FBE coating is the achievement of the performance of a conventional FBE coating with almost 50ºC lower application temperature without a loss of productivity.
The results presented in Table 3 indicate that the LEC FBE is compatible with the adhesive in a three-layer side extruded coating. In addition, application temperatures as low as 150ºC can be used. Cathodic disbondment testing also exceeded the requirements of CSA Z 245.21-02 as shown in Figure 1.
The polyethylene usage at these lower application temperatures indicated that savings in excess of 10 per cent of the polyethylene could be achieved. In addition the LEC FBE also reduces energy consumption by about 10 per cent.
LEC FBE coatings also offer numerous advantages for the coating of girth welds, including faster cycle times due to the need for a lower peak temperature; less stress on the parent pipe coating during the heating stage, and lower energy requirement in the field situation.
Conclusions
An LEC FBE has been developed that meets or exceeds the requirements of international specifications. This technology can be used to coat pipelines as either a stand-alone product down to 180ºC or as a part of a three-layer coating system down to 150ºC. The LEC coating enables savings in both energy and in material.
Faster cycle times and reduced stress on the parent coating also provide benefits for girth weld coating.