How Does DIN 4726 Oxygen Diffusion Govern Layer Thickness in PEX-b/EVOH/PEX-b Heating Pipe?
Closed-loop hydronic heating circuits operating at flow temperatures of 40–70 °C routinely include steel panel radiators, cast-iron circulator housings, and brazed plate heat exchangers. Oxygen ingress through non-barrier polymer layers produces pitting corrosion on these ferrous components and shortens operating life. The three-layer PEX-b/EVOH/PEX-b pipe addresses the diffusion path by placing a discrete poly(ethylene-vinyl alcohol) stratum between two silane-crosslinked polyethylene layers. The compliance anchor for this scenario is DIN 4726:2017-10, which specifies an oxygen diffusion limit of 0.1 g/(m³·d) at 40 °C for barrier pipe used in floor heating and radiator connections. Measurement follows ISO 17455:2005, the multilayer pipe oxygen permeability method, under a pure oxygen gradient with liquid water on the internal pipe wall. The EVOH grade selected for this structure has ethylene content between 27 and 32 mol% because the lower ethylene molar fraction reduces the oxygen transmission coefficient at low relative humidity to below 0.5 cm³·mm/(m²·d·atm) at 20 °C, while the buried position between polyolefin layers slows moisture contact that would otherwise reduce barrier performance.
Layer ratio replaces conventional formulation addition because EVOH is not compounded into the PEX-b matrix; instead it is coextruded as an independently extruded discrete stratum. Typical production practice maintains the EVOH layer at 8–12 % of total wall thickness. For a 16 × 2.0 mm finished pipe, this translates to an EVOH thickness of approximately 0.08–0.12 mm. Each maleic anhydride grafted polyethylene tie layer occupies 5–8 % of total wall thickness, equivalent to 0.05–0.08 mm per layer. The inner and outer PEX-b layers comprise the remaining 80–87 % of wall thickness. Because EVOH density is approximately 1.12–1.17 g/cm³ and PEX-b density is approximately 0.93–0.95 g/cm³, the mass fraction of EVOH in the finished composite is slightly higher than the thickness fraction, typically 10–14 wt%. The tie layer mass fraction is typically 6–10 wt%. This structural ratio is verified on-line with ultrasonic wall-thickness scanning and off-line with microscopic cross-section analysis.
On the production floor, a three-layer coextrusion line is configured with a 45 mm groove-fed main extruder for silane-grafted PE-Xb, a 35 mm barrier extruder reserved for EVOH, and a 30 mm adhesive extruder for the tie resin. The main extruder and barrier extruder both use 30:1 L/D screws, and each melt stream is metered through a gear pump into a spiral mandrel die. Melt temperature is 190–210 °C for PE-Xb, 215–225 °C for EVOH, and 210–230 °C for the MAH-g-PE tie. EVOH must be pre-dried in a desiccant dryer at 80 °C for 4–6 h to a moisture content below 0.20 %; moisture remaining above this threshold produces hydrolysis, microbubbles, and gel formation during extrusion. Post-extrusion, silane moisture crosslinking is carried out at 80–95 °C and 90–95 % relative humidity for 12–24 h. The buried EVOH layer restricts direct contact with liquid water during this curing step. Final product types include PEX-b/EVOH/PEX-b coil pipe in 16 × 2.0 mm, 20 × 2.0 mm, and 25 × 2.3 mm dimensions for radiator connection and closed-loop hydronic distribution.
The operational boundary for this structure occurs when continuous fluid temperature exceeds 80 °C. Under sustained high-temperature water, moisture gradually saturates the inner PEX-b layer and migrates to the EVOH interface, raising the oxygen transmission rate compared with the dry-layer value. The DIN 4726 test therefore conditions the pipe with heated water before oxygen permeability measurement. Published data for oxygen permeation on this specific PEX-b/EVOH/PEX-b configuration after long-term saturation are limited, but industrial barrier-pipe qualification reports consistently measure dry-layer oxygen diffusion below 0.02 g/(m³·d) at 40 °C and conditioned values that remain below the 0.1 g/(m³·d) limit if the EVOH layer is at least 0.08 mm thick and fully continuous.
Compliance matrix for EVOH barrier pipe scenarios in hot and cold water supply| Standard designation | Scope | Numerical limit or test condition |
|---|
| DIN 4726:2017-10 | PE-RT/EVOH pipe for floor heating and radiator connection; oxygen diffusion | 0.1 g/(m³·d) at 40 °C |
| ISO 17455:2005 | Multilayer pipe oxygen permeability of barrier layer | Pure oxygen gradient; liquid water conditioning |
| ISO 22391:2009 | PE-RT pipe for hot and cold water installations | Long-term hydrostatic strength per ISO 9080:2022 |
| ISO 21003-2:2008 | Multilayer piping for hot/cold water inside buildings | Pressure classes PN 10 and PN 16 |
| NSF/ANSI/CAN 61 | Drinking water chemical contaminants from pipe | Extraction at 60 °C |
EVOH Barrier Placement in PE-RT Type II Potable Hot and Cold Water Systems
In domestic potable hot and cold water installations, five-layer PE-RT/EVOH pipe enters the plumbing riser, branch distribution line, and recirculation loop segment. The inner PE-RT Type II layer carries water directly, the EVOH layer reduces oxygen diffusion into the water stream and into the annular space that may connect to open expansion vessels, and the outer PE-RT layer protects the EVOH stratum from construction-site abrasion and outdoor storage humidity. Unlike the closed-loop heating case, the potable water segment requires chemical extraction compliance in addition to mechanical and barrier performance. The relevant standards are ISO 22391:2009 for PE-RT piping, ISO 21003-2:2008 for multilayer hot and cold water installation pipes, NSF/ANSI/CAN 61 for potable water contact, and AS/NZS 4020:2018 for the Australian and New Zealand plumbing market. The oxygen barrier performance is validated by ISO 17455:2005.
The five-layer wall consists of PE-RT inner and outer layers, MAH-g-PE adhesive tie layers, and the central EVOH barrier. The EVOH layer is maintained at 8–10 % of total wall thickness, with each tie layer at 3–5 % and the PE-RT layers providing the structural balance. For a 20 × 2.0 mm pipe, the EVOH thickness is approximately 0.08–0.10 mm, and the tie layers are each approximately 0.03–0.05 mm. The EVOH grade for potable water distribution typically uses ethylene content of 32–38 mol%, which offers higher melt strength and lower moisture sensitivity than the 27–32 mol% grades used for corrosive heating loops, while maintaining an oxygen transmission coefficient low enough for plumbing oxygen ingress control. The resin is not blended into the PE-RT matrix; the ratio is a wall-thickness ratio that is monitored by ultrasonic layer gauging after the calibration tank.
The production line for potable water pipe uses a five-layer coextrusion die with separate melt streams for inner PE-RT, tie, EVOH, tie, and outer PE-RT. The main PE-RT extruder is a 60 mm barrier screw extruder with 30:1 L/D, the EVOH extruder is a 40 mm screw with 24:1 L/D, and the adhesive layer is supplied by a 35 mm extruder. Each stream is gear-pump metered to maintain layer thickness stability at line speeds of 20–40 m/min. PE-RT melt temperature is 190–230 °C, EVOH melt temperature is 215–225 °C, and tie melt temperature is 210–230 °C. EVOH pellets are dried with a dew point of −40 °C and a hopper temperature of 80 °C for 4 h minimum. Interfacial delamination is controlled by specifying a tie resin with melt index within 1.0–3.0 g/10 min at 190 °C/2.16 kg to match the EVOH stream viscosity. Finished pipes are hydrostatically tested at 1.5 times the rated pressure and then coiled or cut into straight lengths from 16 × 2.0 mm to 63 × 5.8 mm for potable distribution and recirculation lines.
A specific operational limitation appears when chlorinated water at 60 °C is used for hot water recirculation. The inner PE-RT layer must satisfy oxidative resistance requirements under ASTM F2023-17; the EVOH layer is not intended for direct chlorinated water contact. The design therefore requires a continuous defect-free inner PE-RT layer at least 0.7 mm thick in finished pipe. Publication data for EVOH barrier deterioration in potable plumbing under long-term recycled hot water are limited; qualifying a new pipe typically requires third-party NSF/ANSI/CAN 61 extraction testing at 60 °C and ISO 9080:2022 long-term hydrostatic testing before market release.
Radiant floor heating circuits embedded in cementitious screed or dry construction panels represent a different downstream condition because the pipe functions as a heat emitter rather than a transport line. In these circuits the fluid temperature is typically held at 35–45 °C with return temperatures near 30–35 °C, and oxygen barrier performance matters because the heated screed accelerates oxygen diffusion through the polymer wall. The relevant standard set includes ISO 11855-1:2021 for embedded water-based heating systems and DIN 4726:2017-10 for the pipe itself. The wall structure is PE-RT/EVOH/PE-RT with two tie layers; the EVOH layer is set at 9–12 % of total wall thickness, tie layers at 4–6 % each, and PE-RT layers as the structural remainder. For the most common 16 × 2.0 mm coil product, this yields an EVOH layer of approximately 0.09–0.12 mm. The production process uses the same five-layer coextrusion line but places greater emphasis on on-line diameter control in the vacuum calibration tanks because the pipe is installed in serpentine loops that must connect to manifolds with compression fittings. Calibration sleeves are typically sized to a vacuum of 0.2–0.6 bar to maintain outer diameter tolerance within ±0.05 mm. Final product is supplied as continuous coils of 120–600 m in dimensions 12 × 2.0 mm, 16 × 2.0 mm, 17 × 2.0 mm, and 20 × 2.0 mm.
When Radiant Cooling Panels Operate Below Dew Point, Moisture Uptake Shifts the Oxygen Barrier Mechanism
A chilled water network operating at 14–18 °C in a space with dew point above 14 °C produces condensation on exposed distribution tubing that can remain present for several hours per day. Moisture uptake through the outer PE-RT layer increases the relative humidity around the EVOH layer and reduces barrier performance. The compliance set for this scenario remains ISO 11855-1:2021 and DIN 4726:2017-10, but the barrier layer is specified at 10–12 % of wall thickness rather than the minimum 8 % used in fully dry environments. The outer PE-RT layer is not reduced below 0.5 mm to slow water ingress to the tie–EVOH interface. In the production process, the outer layer thickness is independently controlled by a separate extruder and monitored after the second calibration tank with a dual-axis ultrasonic gauge. This is a deep-dive point because the oxygen transmission coefficient of EVOH at 85 % relative humidity can be more than ten times higher than the dry value, depending on ethylene content and draw ratio. Published multi-test conditioning data under simulated condensation cycling for this specific pipe configuration are limited; qualification therefore uses ISO 17455:2005 after water immersion conditioning at 80 °C for 90 days to confirm that the barrier remains below the 0.1 g/(m³·d) threshold. The terminal product is a five-layer PE-RT/EVOH pipe coil for radiant cooling circuits and chilled beam distribution, typically in 16 × 2.0 mm and 20 × 2.0 mm dimensions.
Within high-rise plumbing shafts, temperature cycling between 60 °C and 20 °C in hot water recirculation loops loads both the PE-RT pressure layer and the EVOH layer in a manner that is not present in single-storey branch plumbing. The pipe must withstand repeated expansion and contraction inside sealed shafts, and the EVOH layer must retain continuity around the circumference to prevent local oxygen leakage paths at points of bending. The governing compliance set includes ISO 21003-5:2008 for fitness for purpose of multilayer piping systems, NSF/ANSI/CAN 61, and ISO 22391:2009 long-term hydrostatic performance. The layer ratio is similar to potable distribution: EVOH 8–10 % of wall thickness, tie layers 3–5 %, PE-RT balance. Larger diameter riser pipe in 32 × 4.4 mm, 40 × 5.5 mm, 50 × 6.9 mm, and 63 × 8.6 mm requires longer residence time in the calibrating sleeve and additional cooling water capacity. The extrusion line is configured with a 75 mm PE-RT extruder for these dimensions, and the spiral mandrel die is sized with separate temperature zones for the inner, barrier, and outer layers to prevent sag in the larger diameter. The final product is supplied in straight lengths of 5–6 m or coils up to 25 m for riser installation.
Coextrusion Viscosity Matching at the Tie–EVOH–Tie Interface Prevents Interfacial Instability in Heat Pump Distribution Pipe
Heat pump distribution loops differ from conventional hydronic circuits because the flow temperature is lower, often 35–50 °C, while the return temperature may fall to 25–30 °C in air-source systems. The circuit still contains stainless-steel brazed heat exchangers and steel buffer cylinders that require oxygen ingress control. Compliance for this segment is supported by ISO 22391:2009 for PE-RT base material, with oxygen barrier performance measured by ISO 17455:2005 at 40 °C. The coextrusion process itself becomes a critical compliance factor when running thin EVOH layers at higher line speeds because interfacial instabilities can produce local barrier thinning. The selected EVOH grade is typically a 32–38 mol% ethylene copolymer with melt flow rate in the range 1.0–14 g/10 min at 190 °C/2.16 kg. The barrier layer occupies 8–12 % of total wall thickness, tie layers each 4–6 %, and PE-RT the remainder.
In full-scale coextrusion, die design must match shear viscosities at the tie–EVOH interfaces, particularly at shear rates between 100 and 1,000 s⁻¹. Viscosity mismatch greater than approximately 1.8:1 under the die temperature profile triggers layer encapsulation and wavy interfaces that can be observed in microscopic cross-section as periodic reductions in EVOH thickness below 0.05 mm. The production line uses gear pumps on all three melt streams, a five-layer spiral mandrel die, and automated layer-thickness feedback based on ultrasonic measurement. Melt temperature is maintained at 215–225 °C for EVOH and 200–230 °C for PE-RT; maximum continuous residence time at 230 °C is limited to 10 minutes before gel formation occurs. EVOH purging between runs is performed with LDPE at melt temperature below 210 °C to avoid residual carbonyl decomposition. Final product types are PE-RT/EVOH pipe coils in 16 × 2.0 mm, 20 × 2.0 mm, 25 × 2.3 mm, and 32 × 3.0 mm for heat pump distribution headers and flow/return lines.
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Ethylene-vinyl alcohol copolymer is incorporated as a buried oxygen diffusion barrier in coextruded multilayer pipes intended for hot and cold potable water distribution and hydronic heating circuits. The product described here is generically identified as EVOH-HW32, a 32 mol% ethylene barrier grade supplied in pellet form for five-layer pipe construction: crosslinked polyethylene or polyethylene of raised temperature resistance as the inner and outer layers, maleic anhydride-grafted polyolefin tie layers, and the EVOH core. Published data for this specific grade are limited; the values presented are representative ranges from supplier technical bulletins for 30 mol% to 34 mol% ethylene EVOH grades used in barrier pipe.
The function of the EVOH layer is to reduce oxygen ingress from ambient air into the water stream to levels that limit corrosion of ferrous pump, valve, and heat exchanger components in closed-loop systems. The layer thickness is typically 0.08 mm to 0.15 mm, depending on pipe diameter and the oxygen diffusion rate required by the system. The product is used in pipe with outside diameters from 16 mm to 32 mm and wall thicknesses from 2.0 mm to 3.0 mm; common pressure ratings are 10 bar at 20°C and 6 bar at 70°C. System-level compliance is evaluated under ISO 21003-1 and ISO 21003-5 for multilayer piping systems in hot and cold water installations, while oxygen diffusion barrier performance is assessed in accordance with DIN 4726. Gas permeability measurements use ASTM D3985 or ISO 15105-2. In hot water service, the EVOH layer is protected from direct water contact by the inner polyolefin layer because moisture plasticization would otherwise reduce oxygen barrier performance.
What oxygen permeation limits apply to barrier pipes in closed-loop hot water systems?
In closed-loop hydronic systems, oxygen ingress through unfilled polyolefin walls is high enough to cause corrosion of ferrous components. The barrier requirement in DIN 4726 for heating system pipes is commonly expressed as an oxygen permeation rate not exceeding 0.1 g/(m³·d) at 40°C. Pipe constructions meeting this requirement are designated as oxygen diffusion-tight. For EVOH-containing pipe, this is achieved by measuring the oxygen transmission rate of the barrier layer under controlled relative humidity and calculating the oxygen diffusion through the multilayer wall geometry.
Test coupons are typically conditioned at 23°C and 0% RH for dry-barrier measurements, but the actual service condition is wet because the water stream humidifies the pipe bore and the outer ambient can vary. This humidity dependence is the main technical trade-off for EVOH: a 32 mol% ethylene grade retains an oxygen transmission rate of approximately 0.3 cm³·20 μm/(m²·day·atm) at 20°C and 0% RH, but the rate can increase by one to two orders of magnitude at 85% RH. Consequently, the inner and outer layers are not only structural; they limit moisture transfer to the EVOH core. Typical polyolefin wall thicknesses of 1.5 mm to 2.0 mm provide sufficient moisture shielding to keep the buried EVOH layer below approximately 60% RH for the expected design life.
In potable cold water lines, oxygen barrier is less critical for corrosion but remains beneficial for maintaining dissolved oxygen levels in oxygen-sensitive distribution loops. The same EVOH grade is used without changing layer thickness because the pipe platform is standardized across hot and cold service.
EVOH melt processing sets the narrowest window in multilayer pipe production
Because the barrier layer degrades above 240°C, the coextrusion window is narrower than that of the adjacent polyolefin layers. Melt temperature is maintained between 190°C and 225°C; excursions above 240°C promote thermal degradation and gel formation, which appear as pin-holing in the barrier layer and are not visible from the outer surface. Single-screw extruders with 24:1 to 30:1 L/D and compression ratios of 3:1 to 4:1 are used for the barrier layer, while multilayer spiral mandrel dies are used to arrange the five layers. The EVOH layer thickness is controlled by gravimetric feeders and melt pumps to maintain a tolerance of ±0.01 mm.
Resin must be pre-dried at 80°C to 90°C for 4 h to 6 h with a desiccant dryer to a dew point of -40°C or lower when ambient relative humidity exceeds 60%, because moisture in the melt causes hydrolytic chain scission and reduces oxygen barrier retention after extrusion. A residual moisture target below 0.3 wt% is commonly specified. The layer structure is commonly reported as percent of total wall thickness: inner polyolefin 40–45%, inner tie 5–7%, EVOH 5–8%, outer tie 5–7%, and outer polyolefin 35–40%. For a pipe with 2.0 mm wall thickness, this places the EVOH layer at 0.10 mm to 0.16 mm.
Adhesion to the tie layer is monitored by peel testing on coextruded tape or pipe ring specimens; minimum peel values are set by the pipe manufacturer, and published data for this specific configuration are limited. Field failure reports from production-scale coextrusion lines indicate that insufficient tie-layer thickness at the inner EVOH interface is a more frequent cause of delamination than EVOH bulk cohesive failure. Dead zones in the die or feedblock are avoided because EVOH forms crosslinked gels that break loose periodically and cause visible bubble defects in the pipe wall.
When a hot/cold water supply pipe is subjected to thermal cycling at 6 bar
When the pipe enters thermal cycling tests at 6 bar and 70°C, delamination at the EVOH interfaces is the primary failure mode monitored. System testing for multilayer barrier pipes follows the fitness-for-purpose procedures in ISO 21003-5, including pressure cycling and high-temperature exposure. In a typical hot-water test condition at 6 bar and 70°C, the pipe is assessed for delamination at the EVOH interfaces after cycling between 20°C and 70°C. The EVOH layer contributes to oxygen barrier but not to pressure resistance; the hydrostatic design stress is carried by the crosslinked polyethylene or PE-RT layers. Delamination at the tie/EVOH interface is a monitored failure mode because thermal expansion differences between EVOH and polyolefin generate interfacial shear.
The coefficient of linear thermal expansion of dry EVOH is generally reported in the range 70 × 10⁻⁶ K⁻¹ to 90 × 10⁻⁶ K⁻¹, while polyethylene-based layers exceed 150 × 10⁻⁶ K⁻¹. Hot/cold cycling is more aggressive than steady-state hydrostatic pressure because moisture absorption by the polyolefin layers transiently increases moisture available to the EVOH core at the same time that the polymer layers are expanding and contracting. This combined loading can reduce oxygen barrier below the dry-room value if the buried layer has been insufficiently shielded.
EVOH is not in direct contact with chlorinated potable water; the inner polyolefin layer provides chlorine resistance. If the EVOH layer is exposed directly to free chlorine above 1 ppm, oxidative degradation is accelerated. The grade is therefore not suitable as a direct wetted surface in potable water service.
A direct comparison of EVOH with other oxygen barrier approaches for hot and cold water supply pipes requires separating dry-barrier capability, moisture sensitivity, processing window, and layer compatibility. Polyamide 6 and polyamide 6/6,6 are used as barrier layers in some pipes; their oxygen transmission rates at 23°C and 0% RH are generally in the range 15–30 cm³·mm/(m²·day·atm), roughly one to two orders of magnitude higher than EVOH at equivalent thickness. PVDC offers oxygen permeability values comparable to EVOH but has a narrow processing window and can release hydrogen chloride at elevated temperatures, which restricts its use in high-temperature hot water pipe. Aluminum barrier layers provide effectively zero oxygen permeation but introduce higher density, lower flexibility, and require welding in butt-welded aluminum multilayer pipe; EVOH remains a monolithic polymer layer that can be coextruded directly.
| Barrier material | Oxygen transmission at 23°C, 0% RH | Moisture sensitivity | Processing profile |
|---|
| EVOH 32 mol% ethylene | ≤0.5 cm³·20 μm/(m²·day·atm) | High; must be buried between polyolefin layers | Coextruded; narrow 190–225°C window |
| PVDC | 0.3–1.0 cm³·20 μm/(m²·day·atm) | Low to moderate | Heat-sensitive; potential HCl release |
| Polyamide 6 | 15–30 cm³·mm/(m²·day·atm) | Moderate | Wider processing window |
| Aluminum | Effectively zero | None | Requires welding; higher density |
Specification boundaries for the 32 mol% EVOH grade
For the EVOH-HW32 barrier grade, the specification is based on dry moulded plaque measurements. The material is supplied with a melt flow rate of 1.6 g/10 min to 2.0 g/10 min at 190°C and 2.16 kg load per ISO 1133-1:2022, a density of 1.17 g/cm³ to 1.19 g/cm³ per ISO 1183-1:2019, and an ethylene content of 30 mol% to 34 mol%. The oxygen transmission rate at 20°C and 0% RH is specified as ≤0.5 cm³·20 μm/(m²·day·atm) under ASTM D3985. The melting peak by differential scanning calorimetry is typically 180°C to 185°C per ISO 11357-3. The grade is not intended for direct water contact and must be encapsulated by polyolefin layers with a minimum total wall thickness sufficient to meet the system pressure rating.
| Property | Test method | Specification or representative range |
|---|
| Ethylene content | Internal FTIR or equivalent | 30–34 mol% |
| Melt flow rate | ISO 1133-1:2022 | 1.6–2.0 g/10 min at 190°C, 2.16 kg |
| Density | ISO 1183-1:2019 | 1.17–1.19 g/cm³ |
| Oxygen transmission rate | ASTM D3985 | ≤0.5 cm³·20 μm/(m²·day·atm) at 20°C, 0% RH |
| Melting peak | ISO 11357-3 | 180–185°C |
During installation, EVOH-containing multilayer pipe is handled with the same fittings used for the corresponding polyolefin pipe, because the barrier layer is buried. Cut ends should be stored dry because exposed EVOH edges absorb moisture and can form a local hazy zone. Production-scale storage of pipe coils in humid conditions exceeding 85% RH has been associated with edge blistering during subsequent hot forming, although published data for this specific configuration are limited.
Because EVOH oxygen barrier is humidity-dependent, specification values obtained at 0% RH do not represent continuous service in a wet pipe wall. Pipe designers should not use the dry oxygen transmission rate directly for whole-pipe calculations without moisture distribution modeling. The grade is also not recommended for direct contact with hot chlorinated water or high pH reject streams.
