Multilayer Pipe Coextrusion: Barrier, Adhesive and the Layer Nobody Sees

Why put layers in a pipe

A single-material pipe has to compromise. Polyethylene is tough, weldable and cheap, but it lets oxygen through. EVOH blocks oxygen almost completely, but it is brittle, moisture-sensitive and cannot be fusion welded. A multilayer pipe uses each where it is strong.

The application that drives the volume is underfloor and radiator heating. Water circulating in a PE-RT or PEX pipe diffuses oxygen through the wall, and that oxygen corrodes the steel boiler, the pump and the radiators. An EVOH layer 0.1 to 0.2 mm thick reduces the oxygen transmission by a factor of a hundred or more.

Other applications are chemical resistance (a thin PVDF or PP inner layer inside a cheap structural outer), UV protection (a carbon-black outer layer over a natural inner), and colour coding without a separate printing step.

The layer structure

A five-layer barrier pipe has an outer structural layer, a tie layer, the barrier, another tie layer, and an inner structural layer. The tie layers are maleic-anhydride-grafted polyolefins that bond to both the polyethylene and the EVOH, which will not bond to each other directly.

The proportions are counter-intuitive. In a 2.8 mm wall, the two PE-RT layers account for about 2.2 mm and the EVOH for 0.15 to 0.2. The tie layers are 0.1 to 0.15 mm each. The functional material is less than 10 percent of the wall.

The tie layer thickness is not a place to save material. Below about 0.08 mm the bond strength falls off sharply, and the pipe delaminates under thermal cycling — not at installation, but after a few heating seasons.

Table 1 — Layer structure of a 20 mm SDR 6 five-layer barrier pipe. The barrier is under 7 percent of the wall and does all of the oxygen work.

Tie layers: the part that decides whether it works

A tie layer is a polyolefin grafted with maleic anhydride. The anhydride groups react with the hydroxyl groups on the EVOH surface, and the polyolefin backbone entangles with the adjacent polyethylene. Without it, EVOH and PE form two layers that touch but do not bond.

Peel strength rises steeply with tie thickness up to about 0.1 mm and then flattens. At 0.04 mm the bond is 3.5 N/mm, which will fail in service; at 0.12 mm it is 12.5 N/mm, which meets the usual 12 N/mm specification; at 0.26 mm it is 14.8, which is no better than 0.12 for twice the material.

The tie layer is also the most sensitive to process. It has a different viscosity from the structural layers, and if the temperature or the layer ratio drifts, the tie becomes discontinuous. A discontinuous tie layer is invisible in the finished pipe and produces delamination months later.

Die design and layer control

A multilayer die distributes five melts from five extruders into a common flow channel without mixing them. The usual design is a spiral mandrel with separate feed ports and a combining section where the layers merge.

The difficulty is that the layers have different viscosities and different flow behaviour. A low-viscosity layer in a high-viscosity stream will migrate toward the wall, and an unstable interface produces waviness or encapsulation. The die and the layer ratios must be designed together, and the temperature profile must keep the viscosities close.

Layer thickness control is by metering pump speed on each extruder. A gravimetric or volumetric feedback loop is standard on a barrier line because the barrier thickness cannot be measured inline easily, and an undetected drift in the EVOH pump produces pipe with no barrier.

Total wall and layer distribution are measured by taking a cross-section, dyeing it, and measuring under a microscope. This is a destructive offline check, done per run or per shift, not continuously.

Delamination: the failure that hides

Delamination is the characteristic failure of a multilayer pipe and it is almost always a tie layer problem. The causes, in order of frequency, are a tie layer that is too thin, a tie layer that has been contaminated with the wrong resin through a shared hopper or a poor purge, and a temperature profile that makes the tie too viscous to wet the EVOH.

The failure appears under thermal cycling. A pipe that holds pressure at installation will show separation after a few hundred heating and cooling cycles, because the two materials have different thermal expansion coefficients and the bond is loaded every cycle.

The test is a peel test on a ring section: cut a strip through the wall, grip the layers and pull. The force at which the EVOH separates from the tie is the peel strength, and it should be above 10 to 12 N/mm. A pipe that peels at 4 N/mm will fail in service even if every other test passes.

Quality checks

Table 2 — Multilayer pipe quality checks. Layer thickness and peel strength are the two that predict service life.

The check that is most often skipped is barrier continuity. A pipe can have the right total wall, the right outside diameter and a good peel strength and still have a gap in the EVOH where the layer was interrupted. A dye penetration test on a section, or a section under a microscope, is the only way to see it.