Borealis HDPE ME0420 Grafted is a maleic anhydride functionalised high-density polyethylene supplied as free-flowing granules for co-extrusion tie-layer duty. The grafted designation distinguishes the product from unmodified Borealis HDPE ME0420 through the introduction of pendant anhydride functionality on the polyethylene backbone. This functionality provides polar bonding sites at the interface between a fusion-bonded epoxy primer and an unmodified high-density polyethylene topcoat. In a three-layer polyolefin system, the grafted layer is extruded between the cured epoxy film and the topcoat at a dry film thickness typically specified from 120 µm to 250 µm, while the outer HDPE layer is commonly specified at 2.0 mm to 3.2 mm for buried pipe. Grade-specific melt mass-flow rate, density, tensile yield stress, and thermal oxidative stability are controlled by the certificate of analysis released for each production lot. Published data for this specific grafted configuration is limited; therefore the values in the certificate of analysis must be treated as the controlling dataset, not secondary summaries of the unmodified ME0420 baseline.

What Limits Extrusion Output and Melt Stability in Maleic Anhydride Grafted HDPE Tie Layers?

The functionalised resin is sensitive to residual moisture because anhydride groups in the melt can hydrolyse to carboxylic acid. Incoming moisture content is checked according to ISO 15512:2019; values above 0.02 wt% require pre-drying in a desiccant dryer at 80 °C for 2 h with a dew point below -40 °C. At ambient relative humidity above 60%, opened packaging is consumed within 4 h or re-sealed under nitrogen. Extrusion is carried out on single-screw or twin-screw lines with L/D ratios between 30:1 and 40:1. Barrel temperature settings start at 185 °C in the feed section and rise to 225 °C at the die. The melt temperature measured at the breaker plate must not exceed 240 °C. Above 240 °C, the grafted chains undergo simultaneous chain scission and anhydride-induced cross-linking; the result is gel-particle formation, pressure fluctuation at the screen changer, and melt fracture at the crosshead die. The processing window between the upper temperature limit of the grafted tie layer and the minimum die temperature required for the HDPE topcoat is therefore narrow. On a 75 mm grooved-feed extruder with L/D 33:1 and a gear pump, a melt pressure range of 12 MPa to 18 MPa is typically maintained to damp viscosity differences between the tie layer and the topcoat.

Combination with amine-based additives is avoided because primary and secondary amines react with the anhydride functionality at extrusion temperatures, causing premature cross-linking, gel particles, and loss of peel adhesion. Purge with unmodified HDPE is performed at start-up and shutdown. Screws with low-shear barrier sections are preferred over high-shear kneading blocks because the grafted grade is more shear-sensitive than unmodified HDPE. Published capillary rheometry for the grafted grade is limited; extrusion operators should not assume that the pressure profile of unmodified ME0420 applies to the functionalised version. The melt strength of the grafted HDPE is lower than the unmodified baseline when the graft level is increased beyond the supplier tolerance band. This is observed as draw-down instability in the crosshead die at high line speed. To compensate, the die gap is opened by 0.2 mm to 0.4 mm relative to unmodified HDPE settings, and the melt cushion in the gear pump is increased. Die-lip deposition is removed at intervals because anhydride-rich low-molecular-weight fractions accumulate at the exit. The purge cycle uses unmodified HDPE and a reduction in screw speed to 30% of nominal capacity for 10 min before shutdown.

In external three-layer polyethylene pipe coating, the grafted tie layer is co-extruded over a fusion-bonded epoxy primer that has been applied to blast-cleaned steel at a dry film thickness of 60 µm to 80 µm. The steel surface is prepared to Sa 2½ to ISO 8501-1, with an angular profile of 50 µm to 90 µm. The topcoat and tie layer are co-extruded through a crosshead die at a line speed of 1.5 m/min to 4.0 m/min for pipe diameters of 219 mm to 1219 mm; the tie-layer melt temperature is held below 230 °C while the topcoat is delivered from a separate machine at its own melt-temperature setpoint. The steel pipe is preheated to 170 °C to 190 °C before epoxy application. The tie layer must enter the crosshead within the epoxy gel window; on lines running above 3.0 m/min, the interval from epoxy spray to tie-layer contact is below 8 s. A gap in the tie layer creates a heat-shrink void during cooling, which can be detected by holiday testing at 25 kV to 30 kV according to ISO 29601:2011 or NACE SP 0188. The test voltage is selected from the coating thickness and not applied without calibration.

The water-quenched cooling water is controlled at 15 °C to 25 °C because rapid cooling locks in an amorphous surface layer in the topcoat and influences tie-layer residual stresses. A cooling water temperature that is too low can induce microcracking at the tie/topcoat interface. The coating is inspected for thickness and adhesion after 24 h of conditioning at 23 °C; peel values obtained immediately after water quenching may be lower than values obtained after full recrystallisation.

Adhesion Testing and Failure-Mode Interpretation on Blast-Cleaned Steel

The qualification of the tie layer under ISO 21809-1:2018 includes peel adhesion, peel at elevated temperature, indentation resistance, and cathodic disbondment. Peel specimens are tested using a floating roller fixture at a peel speed of 10 mm/min and a test temperature of 23 °C. The results are recorded as force per unit width in N/cm. Where the failure mode is cohesive within the grafted tie layer, the measured value reflects the tensile tearing resistance of the functionalised HDPE itself. Where failure occurs at the FBE/tie interface, the result is controlled by both the epoxy cure and the anhydride concentration at the surface. Where failure occurs at the epoxy/steel interface, the result is not attributable to the tie layer and requires correction of steel blasting or epoxy application. Pull-off adhesion on flat steel panels is performed according to ASTM D4541-17 using a Type V tester before topcoat application. Published production peel data for ME0420 Grafted is limited; therefore first-article testing on the actual coating line is required to set local release limits. The peel fixture radius and specimen width follow the dimensions of ISO 21809-1:2018; typical specimen width is 25 mm. The peel angle is maintained constant by a calibrated winding drum, and the gauge length is excluded from integration. Data are reported as the average of at least 3 specimens from the same pipe quadrant. Interfacial adhesion below the system-qualified release limit is investigated for root cause, but product-specific acceptance limits are set by the coating system qualification and not by a universal threshold.

For cathodic disbondment testing according to ISO 15711:2015, a holiday is created through the total coating thickness and the panel is polarised for 30 days at 23 °C at a potential of -1.5 V versus a reference electrode. The resulting disbondment radius is measured after the test. The grafted tie layer is evaluated for adhesion loss at the FBE interface rather than for topcoat blistering alone; interfacial disbondment at the tie layer is an early marker of functionalisation loss. These measurements provide a systematic comparison to unmodified HDPE, which shows extensive interfacial delamination at the FBE interface when the same fixture is used.

When Grafted HDPE Replaces EVA or Non-Functionalised HDPE Tie Layers

The primary difference between ME0420 Grafted and an unmodified HDPE topcoat is the presence of reactive anhydride groups. Unmodified high-density polyethylene is non-polar and does not form a stable interface with partially cured epoxy; adhesion loss in water immersion at 60 °C occurs more rapidly unless an adhesion promoter is used. EVA-based tie resins provide low-melt-temperature processing and wet-out, but they are softer, have higher oxygen permeability, and may exhibit lower long-term resistance under cathodic disbondment testing at elevated temperatures. Maleic anhydride grafted LLDPE grades offer higher melt flow and lower processing temperature, but their modulus is lower than an HDPE-based tie resin and they may not match the stiffness of an HDPE topcoat. The HDPE backbone of ME0420 Grafted maintains a density and thermal expansion coefficient closer to the unmodified HDPE topcoat, which reduces interfacial stress during thermal cycling. The product is therefore specified when the coating system must combine high stiffness, high upper service temperature, and adhesion to FBE without introducing a highly flexible low-density interlayer. In comparison with solvent-based or co-extruded adhesive films, the grafted HDPE is processed as a thermoplastic; no post-cure is required after cooling.

Unlike a non-functionalised HDPE reprocessed from mixed scrap, the grafted grade is not intended for use as a topcoat. Its polarity increases surface energy, which can alter printability and surface conductivity but does not replace the need for a UV-stabilised outer HDPE layer in above-ground applications. The addition of reground grafted tie layer to the topcoat stream is limited because free anhydride-reactive species introduced with the regrind can create gel slugs at the die lip. Maximum regrind addition is established by trial on the specific crosshead die and not by a universal percentage.

The grade is stored indoors at temperatures below 50 °C and protected from ultraviolet exposure. Bags are kept on pallets away from direct sunlight. The product is not intended for potable water contact unless the applied system has been separately approved under the applicable local hygiene regulation. The polymer should not be blended with incompatible tie resins or with ammonia-based masterbatches; nitrogen purge and dry-air handling are recommended. Incoming-lot application tests should include a melt-flow ratio, moisture analysis, and a peel sample processed on the actual co-extrusion line because laboratory pressed plaques may not reproduce the cooling history of a water-quenched pipe coating. Storage areas with high airborne salinity or high ammonia concentrations are unsuitable because the anhydride functionality is reactive toward basic contaminants. Opened bulk containers should be closed under nitrogen, not compressed air, to limit hydrolysis during extended shutdowns. Thermal oxidative stability of the grafted tie layer is assessed by oxidation induction time according to ISO 11357-6:2018 on a differential scanning calorimeter at 200 °C under oxygen. A reduction in oxidation induction time indicates antioxidant depletion during processing. The grafted grade typically contains a phenolic and phosphite stabiliser package; the exact formulation is proprietary but must be preserved by avoiding melt temperatures above the stated limit. Compared with a maleic anhydride grafted polypropylene tie layer, the HDPE version has lower upper service temperature but better compatibility with HDPE topcoats and lower stiffness mismatch in buried pipe. The grafted HDPE should not be used as a direct-bonded outer layer without topcoat because its polarity increases moisture uptake and may reduce surface hardness.