Designing for recycling

A pack is designed for performance on the press, on the filling line and on the shelf, and its recycling behaviour is usually settled by the same decisions, without anyone having examined them from that side. The coating is a small part of the mass and a large part of the outcome: it governs how the structure comes apart in water, whether printed fibre can be brightened again, what a sorting line reads at the surface, and whether a label leaves the container cleanly. None of that is visible in a gloss figure or a rub-resistance result, so it has to be looked at separately and early, while the structure can still be changed.

Fibre recovery and the stickies problem

Repulping is mechanical work in water. Low-consistency systems typically run at a few per cent stock consistency and high-consistency systems at roughly 12 to 18 per cent, with temperatures commonly between ambient and about 50 degrees Celsius. Those are industry-typical windows rather than a specification, and every mill runs its own.

Fibre separates; a coating does not have to. A film that resists defibring survives as fragments, and what happens next depends on what those fragments are made of. Tacky, deformable fragments, which come mainly from adhesives, hot melts and soft polymer films, extrude through slotted screens, redeposit on press fabrics and dryer surfaces and appear in the finished sheet as specks and holes. These are the stickies, and they cost a mill runnability, sheet quality and reject yield. A hard, non-tacky coating fragment is still a reject to be removed, but it behaves differently on a screen and is the easier of the two problems. The properties that matter are screened reject content and macro-stickies area, by methods such as TAPPI T 275 for pulp screening and TAPPI T 277 for macro stickies, which counts what a laboratory slotted screen at around 100 micrometres retains, with handsheets prepared to ISO 5269-2 and dirt counted by TAPPI T 437 or the ISO 5350 series.

A tougher film is not a better film here

The intuition that a better coating is a harder one runs the wrong way in a repulper. What helps recovery is fragmentation behaviour: a coating that breaks into coarse pieces a screen can remove, or that disperses finely enough to leave with the rejects, is easier to handle than one that shears into small, soft, adhesive particles. A highly cross-linked film with low elongation tends to fracture coarsely. A soft, elastomeric film at the same coating weight tends to produce exactly the particle size a screen cannot catch. Within the Arclear varnishes and the Arcbar barrier coatings these are formulating choices, and they are made against the recovery route as much as against the end-use requirement.

• Coating weight, which sets how much material reaches the repulper at all.
• Cross-link density and glass transition, which set whether the film fractures or shears at repulping temperature.
• Tack at repulping temperature, which decides whether a fragment is a screenable reject or a stickie.
• Adhesion to the fibre surface, which decides whether the coating leaves as a free fragment or drags fibre with it.
• Particle size after fragmentation, measured against the slot width the receiving mill actually runs.
• Whether the coating disperses, and if it does, where the dispersed material ends up.

Deinkability is a different question

Ink removal is not the stickies question restated. In flotation deinking the ink first has to detach from the fibre, and the detached particle then has to be hydrophobic enough to attach to an air bubble and be in the size range the cells collect, a working window of roughly tens of micrometres. Too large and the particle is a screening or cleaning job instead. Too fine and it stays in the stock, where the loss shows up as brightness and whiteness that cannot be recovered: brightness by ISO 2470-2, CIE whiteness by ISO 11475, with residual specks counted alongside by dirt-count methods.

Energy-cured and water-based films do not behave the same way. A fully cured film tends to release as large, flat, non-dispersing flakes, which flotation collects poorly and which show as visible specks wherever screening misses them. A water-based film may soften or partly redisperse, depending on its binder and on whether it has been over-coated. A varnish over the ink changes the detachment step as much as the ink chemistry does. For printed fibre, board and corrugated the Arcfibre range is developed with detachment in mind. Deinkability is a result of the ink, the varnish, the substrate and the deinking line together, and it is measured on the printed sample rather than inferred from the ink.

Mono-material films and the stream a pack ends up in

Replacing an aluminium or polyester layer with a coating does two things at once, and they are worth keeping apart.

The first is a sorting decision. Near-infrared sorting reads the first fraction of a millimetre of the surface presented to the detector, not the whole cross-section, and that cuts both ways. A metallised or foil layer reflects and scatters the beam and can push a pack out of a polyolefin stream while contributing very little to its mass, the effect depending on metal thickness, coverage and which face the sorter sees. Heavy ink coverage or a thick pigmented coating on the outside can weaken or distort the spectrum in the same way. A thin, unpigmented functional coating at a few grams per square metre will normally leave the polymer underneath readable, which is the reason the substitution is attempted.

The second is a barrier trade, and it has to be measured rather than assumed. A coating at a few grams per square metre does not reproduce the oxygen and moisture barrier of aluminium foil, so the size of the gap is a number, taken on the finished laminate: oxygen transmission rate by ASTM D3985 or the ISO 15105 series, water vapour transmission rate by ASTM F1249 or the ISO 15106 series, at the temperature and humidity the pack will actually see, and after flex and crease conditioning if the pack is handled.

There is a third constraint at the far end. A polyester layer inside a polyethylene structure does not melt at polyethylene reprocessing temperature, so it survives as solid inclusions and gels in the regranulate. How much of such a material a structure may carry is set by published design-for-recycling guidelines and by what a reprocessor will accept, which differ between schemes and between markets and are criteria rather than regulation. The relevant checks sit on the regranulate, not on the coating: melt flow rate to ISO 1133, thermal behaviour by differential scanning calorimetry to ISO 11357, gel and inclusion counts on film made from the regranulate, and colour or yellowness on moulded plaques. Arcbar barrier coatings and Arcseal sealing layers are formulated with this substitution in view. Whether a given coating closes the barrier gap for a given pack is a measured result on that structure, not a property of the coating.

Recyclable in principle, and recyclable where the pack is collected

A structure can be technically recoverable and still have nowhere to go. Recyclability in practice requires that the material is collected in that market, that the sorting equipment present can identify it, that a reprocessor accepts the format, and that the recovered material has a buyer. Those four conditions differ between countries, and within India they differ between cities and between the formal and informal collection routes. A flexible pack that is a model mono-material structure in a market with film collection may not be separated at all where film is not collected, and a structure a manual sorting route handles easily can be the one an automated line misreads. This is why we describe structures in terms of the stream they are compatible with, and the conditions under which that holds, rather than attaching a single word to a product.

Labels, adhesives and inks in bottle recycling

Bottle recycling is a wash process, and the label system either survives it or leaves cleanly. Caustic washing is commonly run at around one to two per cent sodium hydroxide, at roughly 75 to 90 degrees Celsius, for something in the order of ten to fifteen minutes, with surfactant. Industrial washes vary, and those figures are typical industry conditions rather than a fixed recipe. Three failures are usual: the adhesive does not release and leaves residue on the flake; the label polymer sits on the same side of the density of water as the flake, so float-sink separation does not remove it; and the ink bleeds and stains either the flake or the wash water, which then has to be treated. Colour carried into the flake is the expensive one, because it caps what the recyclate can be used for. Within the Arcweb label and shrink sleeve range the wash behaviour of the ink and the coating is a design property in its own right, and colour on washed flake is assessed on moulded specimens using established colour and yellowness index methods such as ASTM E313.

Test the structure, not the coating

We do not put a recyclability claim on a coating. A claim attached to a coating alone should be read with care whoever makes it, because the coating is not the thing that gets recycled. What can be done is to state the mechanism, name the method for each property, and then run the recognised laboratory protocols on the actual structure, at the actual coating weight, against the recovery route the pack will meet. That work is done in our technical centre and laboratory in Greater Noida, with the converter’s own substrate and, where possible, with the receiving mill or reprocessor involved. Where a result is favourable we will say what was tested and under what conditions. Where it is not, the useful answer is which layer to change, and that is cheaper to act on before the structure is committed to tooling and artwork.