EVOH: how the oxygen barrier works
EVOH is a copolymer of ethylene and vinyl alcohol. The vinyl alcohol units carry hydroxyl groups. Those groups form hydrogen bonds between neighbouring chains, and the bonds pull the chains into a dense, crystalline solid. Oxygen dissolves into that structure only in very small amounts, and what does not dissolve cannot travel through. This is why a layer of EVOH a few microns thick outperforms a polyolefin layer many times its thickness.
The same hydrogen bonding creates the material’s one weakness. Water is polar, hydroxyl groups attract it, and absorbed water breaks the bonds that hold the barrier together. EVOH is therefore specified in two dimensions at once. The first is how much barrier it gives when dry. The second is how much of that barrier survives where the product actually sits.
This page covers EVOH as a polymer: what creates the barrier, what removes it, and what the material demands of the structure around it. The comparison with a metallised layer, and the 5 per cent design threshold, sit under barrier options and PPWR recyclability.
EVOH properties at a glance
The properties below are published values for the commercial EVOH family. They describe the material, not a finished structure, and every one of them shifts with ethylene content.
| Property | Typical behaviour |
| Oxygen barrier, dry | Among the highest of any transparent polymer used in packaging |
| Oxygen barrier, humid | Falls as relative humidity rises, and returns as the layer dries |
| Water vapour barrier | Poor, so a polyolefin layer carries that job |
| Density | Roughly 1.12 to 1.21 g/cm³, well above every polyolefin |
| Melting range | About 156 to 191 °C, depending on ethylene content |
| Ethylene content of commercial grades | Roughly 24 to 48 mol per cent |
| Optical | Transparent, so it adds no light barrier |
| Typical role | A thin oxygen barrier layer, buried inside the structure |
One distinction worth making early. EVOH is a passive barrier: it slows oxygen down. It does not absorb oxygen or react with it, which is what an oxygen scavenger does. The two are sometimes described as if they were the same thing, and they are not.
EVOH ethylene content: how it controls barrier and moisture resistance
Ethylene content is the dial that sets everything. EVOH is not one material but a family, and the family is indexed by that single number, expressed in mol per cent. Commercial grades run from roughly 24 to 48 mol per cent ethylene, and every property follows from where a grade sits in that range.
Ethylene units carry no hydroxyl group. Adding ethylene therefore dilutes the hydrogen bonding that creates the barrier, while adding the flexibility and water resistance that polyethylene has. More ethylene means a weaker barrier, better moisture tolerance and easier processing. Less ethylene means the reverse, on all three.
What moves across the range
| Attribute | Low ethylene grade, near 27 mol per cent | High ethylene grade, near 44 mol per cent |
| Oxygen barrier when dry | The strongest of the family | Still strong, but clearly lower |
| Loss of barrier at high humidity | Steep, because more hydroxyl groups absorb more water | Shallow, so the grade holds up in wet conditions |
| Flexibility and resistance to flex cracking | Lower, and the layer is more prone to pinholing | Higher, which matters on packs that are handled |
| Coextrusion behaviour | Harder to run alongside polyolefins | Melt behaviour closer to the layers around it |
Resin producers publish an oxygen transmission rate, OTR, for each grade, measured at a stated temperature, humidity and thickness. Those figures compare grades against each other and nothing else. The value that governs a real pack is measured on the finished structure. The layers around the EVOH decide how much water reaches it.
How humidity affects the EVOH oxygen barrier
Water does not damage EVOH. It plasticises it, and the distinction matters commercially, because plasticisation reverses and damage does not.
The mechanism runs in three steps. Water molecules are attracted to the hydroxyl groups. Once between the chains, they occupy the sites where chains were bonded to each other, so the hydrogen bond network loosens. A loosened network gives the chains room to move, and mobility opens paths. Oxygen that barely dissolved into a rigid solid now travels through a softened one.
Two consequences follow, and they are the ones that get missed in specification.
- The loss is not linear. Barrier holds well up to moderate humidity, then falls away more sharply above it. A structure validated in a dry warehouse can behave differently in a humid one, at the same nominal specification.
- The loss reverses. As the layer dries, the hydrogen bonds re-form and the barrier returns. A pack that has been through a wet process is not permanently compromised, provided it is given time to dry.
This is also why the low ethylene grades, the strongest barriers on paper, are not automatically the right choice. The grade with the most hydroxyl groups has the most sites for water to occupy. So the best dry performer can be the worst wet performer. Grade selection is a judgement about where the pack will live, not a search for the highest published number.
EVOH tie layers: why EVOH does not bond directly to polyethylene
EVOH is polar and polyethylene is not, so the two will not adhere to each other. A tie layer solves it, and it does so chemically rather than by sticking.
The tie resin is a functionalised polyolefin, most often grafted with maleic anhydride. One end of that molecule is polyethylene, so it co-crystallises with the polyethylene layer and becomes part of it. The anhydride end is designed to react with the hydroxyl groups of the EVOH, so the join is chemical rather than mechanical. The exact chemistry depends on the resins, the process conditions and the supplier.
When a tie layer is wrong for the grade, the failure is easy to recognise. The structure delaminates, usually after flexing or after a thermal step. The barrier goes with it, because the EVOH layer is no longer held flat and continuous. Tie layer selection is therefore part of the barrier specification, not a separate detail.
Why EVOH is not used as an outer layer
In flexible packaging EVOH sits inside the structure, encapsulated between protective layers. Three separate properties put it there, and each one is enough on its own.
- It absorbs atmospheric moisture. An exposed EVOH surface would lose barrier to the room before the pack reached the customer.
- It is comparatively rigid and notch sensitive, so an exposed layer would crack where the pack is creased or handled.
- It does not seal to itself at the temperatures a packaging line uses, so it cannot serve as the sealing surface.
The polyolefin layers on either side are not packaging around the barrier. They are part of the barrier system, because they keep water away from the layer that does the work. How those surrounding layers are built, and what that means for the recycling stream, is covered on all-PE structures.
EVOH processing window, drying and gels
EVOH melts well above polyethylene and degrades not far above where it melts. The usable window between the two is narrow. It narrows further at low ethylene content, the same end of the family that gives the best barrier.
Outside the window, the polymer cross links and forms gels. Gels are visible as small lenses in the film. At each one the barrier layer is not continuous. A gel is therefore a defect in function, not only in appearance. Long residence time produces the same result even at correct temperature, so stops, purging and restarts are controlled rather than improvised.
The resin is also hygroscopic before it is processed, so it is dried and stored sealed. Drying temperature and time are grade specific and follow the resin producer’s processing specification, since there is no single correct figure across the family. Moisture carried into the extruder produces bubbles and streaks. The layer is only a few microns thick, so there is no margin to hide them.
EVOH in retort packaging and barrier recovery
A retort cycle exposes the pack to pressurised steam. Water reaches the EVOH layer through the polyolefin around it, the layer plasticises, and oxygen barrier falls during and immediately after the cycle. Measured at that moment, the structure looks like a failure.
It is not. The absorbed water leaves again through the same layers, the hydrogen bond network re-forms, and barrier recovers over the following days. A retort pack is therefore not measured straight out of the autoclave. The number obtained describes a transient state, not the shelf life.
Structures intended for retort are built for that transient. Higher ethylene grades absorb less water, and a thicker moisture layer on the outside slows how much steam reaches the barrier at all.
How an EVOH grade is actually specified
Four questions settle the grade, and none of them is answered by an oxygen transmission rate on its own.
| Question | What it decides |
| How humid is the product, and how humid is the storage | Where in the 24 to 48 mol per cent range the grade should sit |
| Does the pack go through a thermal or wet process | Whether the structure has to be built for recovery, and when it is tested |
| Is the pack flexed, creased or handled repeatedly | How much flex crack resistance the grade needs, which pushes ethylene content up |
| Which polymer family surrounds the barrier | Which tie chemistry is used, and which recycling stream the pack belongs to |
The last question is the one that has changed most in recent years, because the answer now carries a compliance consequence as well as a technical one. That side of the decision is set out under design for recycling criteria.
Frequently asked questions
Is EVOH the same as EVA?
No, and the two are confused constantly because the abbreviations are close. EVOH is ethylene vinyl alcohol, a crystalline barrier copolymer. EVA is ethylene vinyl acetate, a soft, rubbery copolymer used for sealing layers, hot melt adhesives and foam. EVA is a poor oxygen barrier and is not selected as a barrier layer. If a specification says EVA where a barrier is required, it is an error rather than an alternative.
Does EVOH lose its barrier permanently after retort?
No. The loss during a retort cycle is caused by absorbed water plasticising the polymer, and it reverses as the structure dries over the following days. The barrier that matters for shelf life is the recovered one, not the one measured coming out of the autoclave. Grade and structure are chosen so that the transient is survivable, and the test point is set after recovery.
Matching the barrier to the product, not to a datasheet
Send the product, the process it goes through and the shelf life you need. We come back with a structure and the documentation that goes with it.
Related resources
Barrier options and PPWR recyclability
EVOH set against a metallised layer and a ceramic coating, with the design threshold that decides which structure stays in a recycling stream.
Polyethylene as a polymer
The layers that surround an EVOH barrier: what density, comonomer and additives control, and why polyethylene keeps water out but lets oxygen through.
All-PE structures
How a barrier layer is carried inside a single polymer family, and what the structure gives up to stay there.
Multilayer structures
Coextrusion and lamination as two ways of joining layers, and what each one allows a barrier layer to do.