Adhesives performance of 3-layer PE pipe coatings: effects of MAH loading, PE particles size, coating interval time and service temperature

Polyethylene (PE) adhesives used in combination with a PE topcoat and a fusion bonded epoxy (FBE) are one of the most common pipeline corrosion-resistant coating systems used in the oil and gas industry. The role of the adhesive in binding the topcoat to the steel surface is critical to ensuring long term protection against corrosion attack to the pipeline. Consistent with this, it is important to understand how adhesion is optimised by controlling the physical and chemical properties of the adhesive. In this work, adhesives were produced by grafting maleic anhydride (MAH) on to the polymer backbone of an inert polyethylene via free radical chemistries during reactive processing then investigating adhesive performance as function of grafting. The effects of particle size on adhesion performance were also studied for two mean particles sizes of 300 μm and 125 μm. Overall, increasing MAH (along with increasing DBP) concentration enhanced the degree of grafting, as characterised by FTIR spectroscopy, which in turn resulted in enhanced interfacial interaction between the non-polar PE topcoat and polar FBE. However, an intermediate level of MAH was found to produce the greatest adhesion due to a subsequent increase in viscosity, as shown by changes in the melt flow index (MFI) and reduced wetting of the adhesive to the primer at higher MAH concentrations. Thermal analyses using Differential Scanning Calorimetry (DSC) confirmed that functionalization of the PE adhesive disrupted its semi-crystalline structure by reducing melting temperature (Tm) and degree of crystallinity (Xc). Investigation of the effects of particle size and coating interval time revealed that finer particle sizes of ≤300 μm produced further improvement in adhesion strength compared to the coarser particles size ≥300 μm, while longer interval times degraded the adhesion performance of the coating system. This finding suggests that, apart from grafting efficiency, selection of particle size and coating times are important considerations for achieving optimum coating adhesion performance in three-layer PE coating systems.