Barrier Co-Extrusion for Packaging Tubes: EVOH, Tie Layers and Layer Control

Why a barrier tube is a layer-thickness problem

Everyone who specifies a barrier tube starts with the resin. They compare EVOH grades, they read the oxygen transmission tables, they choose the one with the best number. Then the tube fails its shelf-life test anyway, and the reason is almost never the resin — it is that the 20 µm barrier layer ended up at 9 µm on one side of the circumference and 26 µm on the other.

Barrier is additive across layers but it is not linear with thickness in a thin film. A barrier layer that is halved does not halve the transmission — it multiplies it by a factor of two to five, because pinholes, incomplete coverage and edge effects start to dominate. This is why layer uniformity matters more than nominal layer thickness, and why a 15 µm layer held to ± 5 % outperforms a 25 µm layer held to ± 40 %.

The layer structures in use

Table 1 — Layer structures. The five-layer version dominates because it protects the EVOH from moisture on both faces.

The reason a five-layer structure exists rather than a three-layer one is entirely about moisture. EVOH is hygroscopic; its oxygen barrier degrades as it absorbs water, and the degradation is severe — a 32 mol% EVOH can lose 60 – 80 % of its dry barrier at 90 % relative humidity. The PE on the inside keeps product moisture away from the barrier, and the PE on the outside keeps ambient humidity away. Both are barrier-protection layers, not just structural ones.

That has a direct design consequence: the inner PE layer has to be thick enough to act as a moisture buffer, not just thick enough to seal. In practice 40 – 80 µm, and the lower end only for products with low water activity.

Choosing the EVOH grade

EVOH is a copolymer of ethylene and vinyl alcohol, and the ethylene content sets the trade-off.

Table 2 — EVOH grades. Higher ethylene means easier processing and better humidity tolerance, at the cost of barrier.

Most tube work uses 32 mol%, because it is the point where the barrier is still good and the processing window is wide enough for a co-extrusion line that also runs PE. Moving to 27 mol% buys maybe 30 % more dry barrier and costs a much narrower temperature window and a dedicated screw — a bad trade unless the product genuinely needs it.

The counter-intuitive part is that for a humid product, a 38 mol% grade can outperform a 27 mol% grade. At 90 % RH the high-ethylene grade retains more of its barrier, and the crossover happens around 80 % RH. If your product is a water-based cream in a tropical market, the "better" grade is the one with the worse dry number.

Tie layers: the part that gets ignored until it delaminates

EVOH will not bond to PE. Nothing bonds to EVOH except a polymer with both polar and non-polar character, which is what a tie layer is: usually a maleic anhydride grafted polyethylene or an EVA/EMA-based adhesive.

Table 3 — Tie layer options. The tie is 20 – 30 % of the material cost on a five-layer tube and it is where delamination starts.

Two things go wrong with tie layers. The first is thickness: below about 8 µm the layer becomes discontinuous and the bond fails in patches, which shows up as local delamination after the tube is squeezed. The second is temperature — tie resins have their own processing window and it is often narrower than the PE around them. Running the whole die at PE temperatures can leave the tie under-fused, and an under-fused tie has almost no bond strength even though it looks fine on the section.

The test that matters is peel strength on a strip cut from the tube, pulled at 90° or 180°. Below 2 N/15 mm is a marginal bond; 4 N/15 mm or more is a tube that will survive handling. Anything that fails by peeling cleanly at the interface — rather than by tearing one of the layers — is an adhesion failure and will get worse with time.

Layer thickness control

On a co-extrusion line each layer comes from its own extruder, and the ratio of the extruder outputs sets the ratio of the layer thicknesses. That sounds simple and it is not, for three reasons.

First, output is not linear with screw speed. Each extruder's output curve flattens at high speed, so a 10 % speed change on two different extruders does not produce a 10 % change in the layer ratio. Second, melt viscosity differences cause interface distortion. If the barrier layer is much less viscous than the skin layers, it can migrate toward the wall of the flow channel and end up thinner at the die lip than it was at the feed block. Third, the die itself distributes unevenly around the circumference, so the barrier layer is thinner where the flow path is longer.

Table 4 — Where layer thickness variation comes from. Circumferential uniformity is the largest single contributor and the hardest to fix.

The gear pump is the single biggest improvement available. A metering pump between the extruder and the die removes the screw's inherent pulsation and makes the layer ratio independent of screw speed drift. On a five-layer line with three gear pumps, layer thickness repeatability goes from about ± 15 % to ± 5 %, which is the difference between passing and failing a shelf-life test.

Measuring a 20 µm layer inside a 500 µm wall

This is the practical difficulty that stops most plants from controlling the barrier properly. You cannot see a 20 µm layer with a caliper, and you cannot infer it from total wall thickness.

Table 5 — Layer measurement. Only the in-line IR gauge gives continuous data; everything else is a sample.

For most tube plants the practical combination is a microtome section at start-up and every four hours, plus an OTR test on the first and last tube of each batch. The section catches geometry drift; the OTR catches everything the section cannot see, including pinholes and incomplete coverage.

The section has to be taken correctly to mean anything. Cut a short length, embed it in epoxy or clamp it in a sectioning jig, and cut perpendicular to the axis with a fresh blade. A hand-cut section smeared with a knife leaves the layers distorted and reads 30 – 50 % wrong. This is worth doing properly — a bad section is worse than no section, because it produces confident wrong numbers.

Delamination and the interface problems

Table 6 — Interface failures. A bond that passes on day one and fails on day thirty is a shrinkage mismatch, not an adhesion problem.

The ageing failure is the one that catches good plants. A tube that peels cleanly at 4 N/15 mm when new can drop to 1.5 N/15 mm after a month, because the layers shrink at different rates as the residual stress relaxes. PE shrinks more than EVOH, so the interface is under shear, and a marginal bond gives way. Testing only at production is testing the best case.

Trim and regrind in a co-extruded tube

Co-extrusion produces trim — the start-up tube, the cut-off ends, the rejected sections. That trim contains EVOH and tie resin, and putting it back into the skin layer is standard practice up to a limit.

The limit is set by the barrier, not by the mechanics. Regrind containing 4 % EVOH blended into a skin layer at 30 % loading puts roughly 1.2 % EVOH into the skin, which is enough to create discontinuities and nucleation sites but not enough to help the barrier. In practice regrind is capped at 20 – 30 % and is kept out of the product-contact layer entirely, which is why six-layer structures with a dedicated regrind core exist.

Writing a co-extrusion specification that can be met

A barrier tube specification needs more than a layer list. These are the items that decide whether the tube can be made repeatably:

• Layer structure with each layer as a percentage of total wall, not as an absolute thickness — absolute thicknesses change when the total wall is adjusted
• Minimum barrier layer thickness, not just nominal
• Layer thickness tolerance, per layer, with the measurement method
• Tie layer type and minimum thickness
• Peel strength requirement, and the ageing point at which it is measured
• OTR and WVTR targets with the test conditions (temperature and RH matter more than the number)
• Regrind policy — permitted layers, maximum ratio
• Resin grades named by manufacturer and grade, because "EVOH" is not a specification

The two that are most often missing are the peel strength at age and the regrind policy. Both of them decide whether the tube that passed the first-article test still works after a month in a warehouse.