Barrier Pipe Grade EVOH

Barrier Pipe Grade EVOH 32 is a semi-crystalline random ethylene-vinyl alcohol copolymer produced by saponification of an ethylene-vinyl acetate precursor. The pipe extrusion version is distinguished from film, sheet, and bottle grades by a higher molecular weight fraction that raises melt strength during annular coextrusion and by a narrower ethylene content window used to balance dry-state oxygen barrier with moisture tolerance. A representative pipe-specific formulation contains 32 mol% ethylene, a melt mass-flow rate of 1.6 g/10 min at 190 °C under 2.16 kg load in accordance with ISO 1133-1:2022. Its solid-state density is 1.19 g/cm³ measured by ISO 1183-1:2019, and the crystalline melting point is 183 °C under ISO 11357-3:2018. Oxygen transmission rate on a 20 µm cast film at 23 °C and 0% RH is below 0.5 cm³/(m²·day·atm) under ASTM D3985, but at 85% RH the same film can exceed 15 cm³/(m²·day·atm). The primary specification difference from general-purpose packaging EVOH is rheological: this pipe grade is engineered for sag resistance, stable barrier-layer distribution in thick annular dies, and reduced edge fracture, not for thin-film cast or blown operations.

What Limits Oxygen Barrier Retention in Humid Multi-Layer Pipe Service?

EVOH derives its oxygen barrier from dense hydrogen-bonded crystalline domains. Water acts as a plasticizer and disrupts the interchain hydrogen bonding; the oxygen transmission rate rises sharply above 70–75% RH. A barrier pipe design therefore cannot rely on the dry-film value alone. In multi-layer pipe, the EVOH layer is encapsulated between polyolefin layers that reduce water vapour ingress. The equilibrium moisture content of the EVOH layer, not the ambient humidity outside the pipe, controls the long-term oxygen permeation. At EVOH moisture contents below 3 wt% the barrier remains high, while moisture contents above 5 wt% degrade the barrier by approximately one order of magnitude. A buried 0.10 mm EVOH layer in a PE-RT composite with outer polyolefin thickness above 1.5 mm can maintain oxygen diffusion below 0.1 mg/L·day under DIN 4726 at 40 °C. By contrast, the same EVOH layer exposed directly to water vapour at 85% RH would not meet that threshold because local moisture uptake would exceed 5 wt%.

Finished composite pipe is tested for oxygen permeability using ISO 17455-1, with acceptance criteria usually set by application standards. For hydronic heating pipes, DIN 4726 requires oxygen diffusion below 0.1 mg/L·day at 40 °C water temperature. The measured permeability depends on layer position, thickness, and the moisture transport properties of the surrounding polyethylene or PE-RT layers. Dry-state EVOH film data are not sufficient for pipe design because the polyolefin layers delay but do not eliminate moisture contact with the barrier layer over a service life exceeding 20 years.

Pre-drying is mandatory because moisture in the melt hydrolyzes the EVOH backbone and produces gels, fisheyes, and edge instability. A desiccant dryer with supply air dew point of −40 °C and a set point of 80–90 °C for 4–6 h reduces residual moisture below 300 ppm; vented hopper dryers without desiccant cannot reliably reach this level when plant relative humidity exceeds 60%. On a single-screw extruder with an L/D ratio between 25:1 and 30:1 and a barrier screw, the barrel profile is typically set from 180 °C at the feed throat to 220 °C at the die adapter, with melt temperature held below 230 °C. Residence time above 240 °C initiates deacetylation, releasing acetic acid and causing amber discolouration. Purging is therefore performed with a low-melt-index polyethylene or a proprietary purge compound. PVC and PVDC must be excluded from the same extrusion system because halogenated polymer degradation products contaminate EVOH and generate crosslinked black specks.

Coextrusion Die Land Ratios and Interlayer Adhesion Failure Modes

In five-layer pipe coextrusion, the EVOH stream is encapsulated between two maleic anhydride grafted tie layers before entering the final die. Flow stability is maintained by matching the apparent shear rates of the EVOH, tie, and PE-RT layers. For a spiral mandrel die with a final land length of 10–15 mm and a land-to-gap ratio between 10:1 and 15:1, the EVOH layer remains stable when its apparent shear rate is below 200 s⁻¹. A viscosity ratio between EVOH and tie resin at 210 °C and 100 s⁻¹ in the range 0.8:1 to 1.2:1 reduces layer-encapsulation defects. Ratios above 1.5:1 produce waviness and local thinning, while ratios below 0.5:1 permit the tie resin to displace the EVOH layer. Production lines running PE-RT/tie/EVOH/tie/PE-RT report that a 5 °C die-temperature imbalance across the EVOH ring can produce thickness variation of ±20% around the circumference, which is sufficient to create a local oxygen path.

Interlayer adhesion is controlled by the tie resin. Maleic anhydride grafted linear low-density polyethylene is commonly specified with a graft level of 0.5–1.0 wt%. The tie-layer thickness is maintained between 0.05 mm and 0.10 mm. Adhesion between EVOH and tie resin is verified on coextruded sheet using a 180° peel fixture; target values are typically ≥60 N/25 mm, though published data for this specific configuration is limited.

Polyamide barriers such as MXD6 have oxygen transmission coefficients one to two orders of magnitude higher than EVOH measured under ASTM D3985 at 0% RH, but they retain a larger fraction of their barrier under humid conditions. Aluminium barrier pipe is effectively impermeable to oxygen but requires a welded longitudinal overlap and cannot be coiled as readily as all-polymer barrier pipe. PVDC offers moisture-independent oxygen barrier but decomposes above 200 °C and liberates hydrogen chloride, limiting it to low-temperature extrusion. Barrier pipe EVOH is therefore specified where the finished pipe must be coilable, heat-fusion weldable, and maintain oxygen diffusion below 0.1 mg/L·day in accordance with DIN 4726.

When the Barrier Layer is Embedded Below the Condensation Front

When the EVOH layer is positioned closer to the inner wall than to the outer surface, the water vapour pressure difference across the pipe determines moisture loading on the barrier. In a hydronic heating pipe operating at 70 °C with a 2.0 mm total wall thickness, the inner PE-RT layer can transmit water vapour toward the EVOH even though no liquid water is present. The EVOH moisture content increases over the first 1,000 h of service and then approaches a steady state controlled by the outer polyolefin thickness. Published data for this specific configuration is limited, but long-term oxygen diffusion measurements on pipe with a 0.10 mm EVOH layer and outer PE-RT thickness above 1.5 mm show that DIN 4726 limits can be met at 40 °C. If the outer layer thickness is reduced below 0.5 mm, moisture ingress can raise the EVOH local moisture content above 5 wt%, causing oxygen diffusion to exceed the specified limit.

Layer thickness is controlled by the extruder speed ratio between the EVOH and outer-layer extruders. Ultrasonic wall-thickness measurement and section microscopy are used for production verification; the EVOH ring is identified visually as a translucent layer. The minimum continuous thickness is 0.05 mm, and any point below 0.02 mm creates a localized oxygen path. Because EVOH has a higher flexural modulus than PE-RT, EVOH thickness above 0.20 mm reduces flexibility and increases the minimum bending radius. The practical design window is therefore 0.05–0.15 mm, with tighter control required for small-diameter pipe below 16 mm outside diameter.

Primary uses include oxygen-barrier layers in PE-RT and PEX composite pipe for hydronic floor heating, radiator connections, heat-pump distribution, and district heating branch lines. Where drinking-water contact is involved, the inner PE-RT or PEX layer is the wetted surface; the buried EVOH is not directly wetted. Where the selected EVOH grade is evaluated for food-contact use, reference is made to FDA 21 CFR 177.1360 and EU Regulation 10/2011 for the specific grade and additive package.