Barrier without plastic

What a barrier actually has to stop

“Barrier” is five separate jobs, and most structures need only two or three of them. Naming which ones apply is the first step in any substitution work, because a coating that is strong on one is often weak on another, and a headline number means nothing without the test method and the condition beside it.

• Water vapour. Moisture moving in or out. Measured as water vapour transmission rate by ASTM E96/E96M or ISO 2528, always quoted with its temperature and relative humidity, because the value moves with both. Surface water uptake is a separate and quicker check, by the Cobb test to ISO 535, and it is not a substitute for a transmission rate.
• Oxygen. A main driver of rancidity in fats and of colour loss in some pigments and dyes. Measured as oxygen transmission rate by ASTM D3985, or by ISO 15105-2 where the structure is outside the range of the film method. ASTM D3985 is run dry, so a figure quoted at a given relative humidity has to come from a humidity-controlled method such as ASTM F1927. For a hydrophilic coating the difference between the dry and the humid result can be large, which is why the condition has to travel with the number.
• Grease and oil. Liquid fat wicking into fibre and showing through. Screened by the TAPPI T559 KIT test, which is a rapid solvent wetting proxy and not a measure of sustained fat contact. Hot or fatty product in long contact needs a timed oil permeability method such as the ISO 16532 series.
• Aroma. Flavour leaving the pack and taint entering it. It tracks oxygen barrier loosely, but it is a different property and often fails first. There is no single routine transmission standard for it; it is assessed by sensory panel or by headspace analysis on the packed product, which means it has to be planned into the shelf life trial rather than inferred from an oxygen figure.
• Light. Visible and ultraviolet. Opacity of the base sheet can be measured to ISO 2471, but this is the demand where foil and metallised film are hardest to displace with a coating.

Why a dispersion coating is not extruded polyethylene

Extrusion coating lays a melt curtain of a hydrophobic polymer onto the sheet, at coat weights that are typically in the region of 12 to 25 g/m². A water-based dispersion is a suspension of polymer particles in water. It is applied wet, typically at 6 to 15 g/m² dry and often in two passes, and it then has to lose the water and coalesce those particles into one continuous film, which is why the minimum film forming temperature of the dispersion and the drying profile matter as much as the polymer chemistry. Anywhere coalescence is incomplete, the film has a route through it. Those coat weights are industry-typical ranges rather than our measurement; the mechanism is the point.

On the same board, a single-pass dispersion barrier tested to ASTM E96/E96M or ISO 2528 at a stated condition, for example 38 °C and 90 % relative humidity, gives a higher water vapour transmission rate, meaning less barrier, than an extruded polyethylene layer of comparable weight. The gap is usually substantial. A second pass, a smoother base sheet and a tighter drying profile close part of it and rarely all of it. Extrusion is not automatically defect free either: at low coat weights, at edges and over defects in the base sheet it pinholes as well. The difference is one of degree, and the only figure worth quoting is one measured on the structure in question.

The substitution is a trade, not an equivalence

The reason to make this substitution is not barrier performance. It is that a coated board may be accepted into a fibre recovery stream from which a polyethylene-laminated or foil-lined board is rejected, or from which it is recovered only with difficulty and yield loss. The converter gives up some water vapour and oxygen barrier and gets a simpler structure and a possible route to fibre recovery in exchange. Whether the trade is acceptable is a decision about the product inside, not about the coating. Whether the fibre is actually recoverable is decided by repulping and recyclability testing on the finished structure, against the criteria of the mill that has to take it back. Some barrier coatings fail that testing. It is a result to be obtained, not a property to be assumed from the absence of a film.

Pinholes and coat weight uniformity

Most disappointing barrier results are coverage failures rather than formulation failures. Board is rough, absorbent and compressible, and a barrier layer has to bridge fibre ends, creases and score lines. A pinhole is a direct path, so barrier does not degrade gracefully with defect count: a few defects can cost more than a moderate shortfall in average coat weight. That is why a barrier coating is usually built as two thinner passes rather than one heavy one, the first acting as a levelling and hold-out layer and the second as the barrier proper, and why a primer or a closed base sheet earns its cost. It is also why coat weight uniformity across the web matters more than the average. The mean can be on target while the light lanes fail. On creased and folded packs, test the barrier after creasing and folding, because that is where it has to work.

Tortuous path and platelet fillers

The second route to barrier is geometry rather than chemistry. Disperse high-aspect-ratio platelets, such as layered silicates or other plate-shaped minerals, into the film and orient them parallel to the surface, and a permeant molecule can no longer travel straight through. It has to go around each plate, so the effective path length rises well above the physical film thickness and transmission falls without a proportional increase in that thickness. The gain depends on aspect ratio, on exfoliation, meaning the plates separating rather than staying stacked, and on orientation in the dried film, which is set by rheology and by how the coating is applied and dried. The limits are equally practical: platelet loading raises viscosity, adds inorganic solids, can cost gloss and flexibility, and an overloaded film cracks at the crease, which returns you to the pinhole problem. Within Arcbar, the moisture, oxygen and aroma grades are development work in our Greater Noida laboratory rather than catalogue items.

Shelf life decides whether substitution is possible at all

Every one of these questions collapses into one: how long does the product have to last, and in what climate. A three-week ambient pack for a dry, low-fat product is a different problem from a twelve-month pack for a fat-containing product in a hot and humid distribution chain, and the same coating can be the right answer for the first and an obvious wrong answer for the second. Work backwards from the product’s tolerance, meaning the moisture gain or oxygen uptake it can absorb before it becomes unacceptable, divided across the required shelf life and the pack’s surface area. That gives the transmission rate the structure has to meet. It is an estimate rather than a result: it ignores creasing, seal integrity, headspace and temperature cycling, so a storage trial still has to confirm it. What it does do is tell you early whether a dispersion barrier is in range, whether a hybrid structure is needed, or whether foil or metallised film has to stay. Where the polyethylene was also carrying the seal, removing it removes the seal layer, so a sealable barrier grade has to do both. Typical jaw temperatures for heat seal coatings on board are in the region of 110 to 150 °C at short dwell; that is an industry-typical range, and the window for a given structure has to be established on the converter’s own machine.

How we report barrier figures

We state the method, the condition and the substrate for every barrier figure we issue, because a transmission rate without those is not data. We run the comparison on the converter’s own board rather than on a laboratory sheet, since hold-out on the real base sheet is usually the deciding variable. We do not offer a food contact, migration or recyclability declaration on the strength of a barrier result: those are separate assessments, made against the specific structure, product and end use, and they belong in their own documents. The ranges this work sits in are Arcbar, Arcfibre for paper, board and corrugated, Arcseal where the seal has to come with the barrier, and Arcfoil where foil stays in the structure. Where the shelf life calculation says the plastic cannot come out, that is what the report will say.