When BorSafe ME3446 is used as the top layer in a three-layer polyethylene external corrosion protection system for longitudinally submerged-arc-welded line pipe, the compound is extruded over a maleic anhydride-grafted polyethylene adhesive tie layer. The adhesion chain begins with a fusion-bonded epoxy primer applied to the steel substrate. The steel surface is prepared to Sa 2.5 per ISO 8501-1:2007. The anchor profile is maintained between 50 µm and 100 µm. The pipe is preheated to 180 °C–220 °C before the FBE powder is sprayed electrostatically. The FBE primer thickness is specified between 60 µm and 150 µm. The adhesive tie layer is extruded at 200 °C–230 °C. The BorSafe ME3446 top coat is then extruded through a crosshead die at a melt temperature of 200 °C–230 °C. The top coat normally contributes 85%–90% of the total dry film thickness. For a buried onshore gas transmission line with a pipe outside diameter of 610 mm, the specified total 3LPE thickness is commonly 2.0 mm–3.2 mm. The finished product is a 3LPE-coated steel line pipe qualified for buried or above-ground service. Compliance is assessed against ISO 21809-1:2018, DIN 30670:2012, and CSA Z245.21-18. The coating is subjected to holiday inspection using a high-voltage detector according to ASTM D5162-19. The top coat is supplied as a ready-to-use pellet compound. No external carbon black masterbatch, antioxidant masterbatch, or lubricant is added by the coater. The compound contains 2.2 wt%–2.5 wt% carbon black tested per ISO 6964:2019. Clean in-house regrind from the same top coat may be used up to 10 wt% if the project specification permits. Higher regrind ratios require revalidation of elongation at break and oxidation induction time. The adhesion of the top coat to the steel system is evaluated by a peel test after the FBE layer is cut to the substrate. The test outcome is governed by the total coating system, not by the raw-grade tensile properties of ME3446 alone.

What Changes When BorSafe ME3446 Is Crosshead-Extruded onto Steel Water Transmission Pipe?

Crosshead extrusion onto steel water transmission pipe differs from hydrocarbon line pipe in surface preparation and end-use hygiene. Under ANSI/AWWA C215-18, the steel substrate is abrasive blast-cleaned to SSPC-SP 10/NACE No. 2 near-white metal. The surface profile is maintained between 50 µm and 75 µm. A primer adhesive is applied to the blast-cleaned steel before the MDPE top coat. BorSafe ME3446 is used at 100 wt% as supplied. No post-addition of carbon black masterbatch or antioxidant is required. The top coat thickness is specified by pipe diameter and service class. For a 600 mm outside diameter transmission main, the extruded polyolefin top layer is typically 1.8 mm–2.2 mm thick. The side extruder barrel size is matched to the crosshead annular die. A single-screw extruder with L/D 30:1 to 33:1 and a grooved feed section is used. Melt temperature is maintained at 200 °C–220 °C. Melt temperature above 230 °C causes viscosity reduction and sag on the underside of large-diameter pipe. Melt temperature below 190 °C risks incomplete fusion at the tie-layer interface. The finished product is a coated steel pipe intended for potable water, raw water, or wastewater force mains. The relevant electrical inspection is performed by holiday testing according to ASTM D5162-19. The top coat functions as the electrical insulation barrier. The compound’s volume resistivity remains above 10¹⁵ Ω·cm when measured per ASTM D257-14. This property supports the dielectric strength required for buried water mains. The pipe coating line operates at a lower output than hydrocarbon line pipe because water pipes frequently exceed 1,200 mm outside diameter. The larger circumference increases the melt residence time in the crosshead die. The die gap is widened to 2.0 mm–4.0 mm to maintain the required coating thickness without excessive drawdown. The result is a factory-applied AWWA C215-compliant coating ready for open-cut, horizontal directional drilling, or pipe jacking installation.

High-Speed Crosshead Extrusion Demands Melt Strength and Anti-Sag Behaviour Beyond Pipe Extrusion Grades

This application boundary is process-oriented. The melt flow rate of 0.6 g/10 min at 190 °C/2.16 kg per ISO 1133-1:2022 does not fully describe the compound’s response in annular crosshead tooling. The melt exits the die lip and is drawn onto a rotating, translating pipe. The unsupported melt curtain between the die lip and the pipe surface is subject to gravitational sag. BorSafe ME3446 is formulated for elevated melt strength at low shear, making it suitable for coating outside diameters above 1,000 mm. Production lines running 1,220 mm outside diameter pipe use a crosshead die with an adjustable die gap of 1.5 mm–3.0 mm. Output rates are typically 800 kg/h–1,500 kg/h depending on top coat thickness and screw size. The die pressure measured at the crosshead is 180 bar–260 bar. At these pressures, moisture in the feedstock becomes the main defect source. If pellets are stored above 60% relative humidity, pre-drying for 2 h at 80 °C is required. Surface pinholes, interfacial voids, and melt fracture are observed when the moisture content exceeds 0.05 wt%. The compound should not be blended with other PE grades without re-qualification. Carbon black dispersion and oxidation induction time can be diluted below the specification floor. The final coating layer is inspected by spark testing at a voltage selected according to NACE SP0188-2006. The failure mode at the high-temperature boundary is oxidative degradation. The oxidation induction time at 210 °C is >20 min per ISO 11357-6:2018. This value does not permit unlimited residence time at the upper melt temperature. The coater must therefore manage screw speed and crosshead holdup time together. A screw with a Maddock mixing section disperses carbon black but also raises shear heating. The terminal coated pipe is produced without post-extrusion annealing. The cooling water temperature is controlled between 15 °C and 35 °C to avoid quenching stress at the adhesive-MDPE interface.

Factory-applied coating of induction-bent steel pipe uses BorSafe ME3446 in an extrusion-lamination process that replaces the continuous cylindrical crosshead with a profiled die and roller consolidation. The bend is pre-blasted to ISO 8501-1 Sa 2.5. The surface receives a fusion-bonded epoxy primer or liquid epoxy barrier coat before the adhesive and MDPE top coat. The geometry of a 90° bend with 5D radius creates an outer-arc thinning tendency. The top coat thickness on the extrados is therefore allowed to fall within −10% of the specified nominal, while the intrados may be up to +15%. The specification references ISO 21809-1:2018 for straight pipe coatings, but the procedure must be qualified on actual bend coupons. The end product is a coated fitting ready for butt welding into a 3LPE-coated pipeline. Cutbacks are left at 100 mm–150 mm from each weld end to avoid damage during field welding. BorSafe ME3446 is used unblended at 100 wt%. No mineral filler is added. The carbon black dispersion of the supplied compound conforms to ISO 18553:2002 grade ≤ 3. This is critical because dispersion defects on the outer arc are more exposed to long-term soil moisture and UV during open-air storage. The fusion quality at the overlap seam is checked by a peel test after each shift. Published data for the exact overlap peel retention on bends is limited. Project coating procedures therefore set a minimum peel value based on straight-pipe data adjusted for the bend radius. The coated fitting is cooled in still air or mist, not water-quenched. Air cooling reduces the risk of interfacial stress cracking at the intrados-extrados transition. The final fitting is part of the same pipeline corrosion protection train as the straight pipe string.

When Concrete Weight Coating Is Applied over the 3LPE Top Coat

For offshore and marshy crossings, a reinforced concrete weight coating is applied over the 3LPE top coat. The mechanical interface between BorSafe ME3446 and the concrete is designed to transfer axial load during laying and burial. The MDPE surface is not chemically bonded to concrete. Mechanical interlocks, wire mesh, and rough surface bands are used. The top coat thickness under the concrete is increased by 0.2 mm–0.5 mm relative to the bare 3LPE design to compensate for point loading from the steel mesh. The concrete is applied by impingement or compression mold wrapping. The peak line speed is 1.5 m/min–3.0 m/min. The compound’s Shore D hardness of 60 per ISO 868:2003 provides resistance to mesh indentation during concrete application. The reinforced concrete weight-coated pipe is used for nearshore, river crossing, and shallow-water installation. Compliance with external concrete coating is assessed under ISO 21809-5:2010. The cutback lengths are maintained from the pipe ends to allow field welding and field joint coating. The top coat beneath the concrete is inspected for indentation damage before concrete placement. The side extrusion process may require a thicker adhesive layer at the cutback transition to avoid edge disbondment during concrete shear transfer. Published data for the exact loss coefficient under concrete mesh indentation is limited. Project-specific load tests are therefore used to validate the top coat thickness compensation. The final assembly is handled by padded rollers, not steel chain clamps, to avoid point indentation of the exposed cutback ends.

Trenchless Installation Abrasion and Repair Compatibility

Trenchless installation subjects the 3LPE top coat to high sliding friction and bending strain. BorSafe ME3446 is used as the original factory top coat for pipe installed by horizontal directional drilling or pipe jacking. The coating must withstand pull force without tearing. The tensile yield stress of 18 MPa per ISO 527-2:2012 is relevant to the pipe’s resistance to deformation under pull load. The elongation at break exceeds 800%, which prevents brittle failure at field bend radii. However, the coating is not a lubricant. The contractor applies a bentonite layer over the top coat. The top coat is expected to lose 0.1 mm–0.3 mm of thickness under abrasive soil drag over 500 m of installation. Post-installation holiday inspection of above-ground ends may be performed. Repairs are made with polyethylene patch systems. The material has no chemical bond to ambient-cured epoxy repair compounds, so mechanical surface activation is required. Abrasion data for this specific configuration is limited, so pullback force calculations should include a conservative friction coefficient for the soil-polymer interface. The final pipeline is installed without replacing the factory coating. The limiting parameter is not the MDPE top coat thickness alone, but the combination of pull force, soil abrasion, and patch adhesion. The compound should not be exposed to direct flame during welding beyond the specified cutback. The cutback area is protected with a field-applied heat-shrinkable sleeve. The factory-applied top coat remains intact outside the cutback, maintaining the original ISO 21809-1:2018 coating qualification for the buried segment.