Water-Based Barrier Coatings

Executive summary

Paper is renewable, recyclable and accepted by consumers, but on its own it is porous: it lets grease through, absorbs water and offers no resistance to oxygen or water vapour. For a century the answer has been a polyethylene extrusion layer — which makes the pack a composite that most paper mills reject. Water-based barrier coatings applied on the printing or coating machine can now give paper grease, water and moisture-vapour barrier while keeping it repulpable. This paper explains the barrier mechanisms, the coating chemistries, the test methods that define performance, and the direction of Uniform Industries’ barrier-coating development.

1. What “barrier” means, in numbers

Barrier

Typical requirement

Test

Grease and oil (bakery, fast food, pet food)

Kit rating ≥ 8 – 12 (TAPPI T559); no stain in 24 h ASTM F119

Kit test, oil-stain test

Water (cups, trays, sacks)

Cobb₆₀ ≤ 15 – 25 g/m² (ISO 535); no soak-through

Cobb, water hold-out

Water vapour (dry foods, cement sacks)

WVTR 10 – 50 g/m²·day at 38 °C / 90 % RH (ASTM E96)

WVTR cup

Oxygen (coffee, snacks)

OTR < 10 cm³/m²·day (ASTM D3985)

Coulometric OTR

Heat sealability

Seal strength ≥ 300 g/15 mm

ASTM F88

PE extrusion at 15 – 20 g/m² gives Cobb ~ 0, WVTR ~ 15 and no oxygen barrier. A coating has to match the first two at 5 – 10 g/m² and, for high-barrier packs, add oxygen barrier that PE never provided.

2. How coatings create barrier

Grease barrier is a surface-energy effect: a continuous film with low affinity for oils stops wicking through the fibre network. It is the easiest barrier to achieve and the first to be lost through pinholes, creases and cut edges. Water barrier requires a hydrophobic, continuous film — the coating must cover the peaks of the fibres and bridge the pores, which is why coat weight and paper smoothness matter more than chemistry. Water-vapour barrier depends on the polymer’s own permeability and film thickness: crystalline or highly cross-linked polymers with hydrophobic backbones are the most effective. Oxygen barrier is the opposite: it needs polar, hydrogen-bonded, dense polymers — which are, unfortunately, sensitive to humidity. Achieving both moisture and oxygen barrier therefore requires a layered structure: an oxygen-barrier layer protected by a moisture-barrier top coat.

Figure 1 – Two-layer repulpable barrier: platelet-filled base for oxygen, hydrophobic top coat for water, grease and WVTR (schematic).

3. Coating chemistries

Styrene-acrylic and styrene-butadiene dispersions: the workhorses for grease and water hold-out; good film formation at low coat weight, heat-sealable grades available, fully repulpable.

Vinyl acetate-ethylene and EVA dispersions: heat sealability and flexibility for cups and sachets.

Polyolefin dispersions: the closest performance to extruded PE for WVTR and sealing, still applied from water.

PVOH / EVOH and modified starch: oxygen barrier, humidity-sensitive, used as the inner layer under a hydrophobic top coat.

Wax and paraffin dispersions: low-cost water and vapour barrier for wraps and sacks, limited heat resistance.

Platelet pigments (talc, kaolin, clays) in a polymer matrix: create a tortuous path that multiplies barrier at a given coat weight and improves repulpability by diluting the polymer.

Uniform Industries’ barrier-coating programme combines a hydrophobic dispersion top coat with a platelet-filled base coat, applied in one or two passes on a rod, blade, gravure or flexo coater, and formulated within the food-contact frameworks (FDA 21 CFR 176.170 / BfR XXXVI and Regulation (EC) 1935/2004) with no fluorochemicals.

Figure 2 – Cobb and Kit against dry coat weight: barrier reaches its knee at 6 – 8 g/m² (illustrative).

4. The application step is half the product

A barrier coating that is perfect in the laboratory fails on the machine if it is applied badly. Three variables dominate:

Coat weight and uniformity: barrier rises steeply with coat weight up to the point where the film becomes continuous, then flattens; the target is the knee of that curve, typically 6 – 10 g/m² dry in one or two layers. Uniformity — no pinholes, no skips — matters more than average weight.

Drying and film formation: the dispersion must coalesce; under-dried film has micro-cracks that grease finds. Web temperature at the dryer exit should reach the polymer’s minimum film-forming temperature plus a margin.

Converting: creasing, folding and cutting break films. Formulations for cartons and cups include flexibilisers and are tested after creasing, not on flat sheet.

5. Repulpability and recycling

The reason to use a coating instead of PE is recyclability, so the coating must not defeat it. Repulpability is tested by pulping the coated paper under standard mill conditions and measuring fibre yield and the size of the residual coating particles; the coating must break down into particles that screen out cleanly, or redisperse without sticking to machine surfaces (“stickies”). Dispersion coatings pass these tests at typical coat weights; polymer platelet-filled systems pass more easily still. Certification to the recognised repulpability and recyclability protocols (Cepi, PTS, Western Michigan) is the currency in which brand owners now buy barrier paper.

6. Application segments

Quick-service restaurant wraps and clam-shells (grease); paper cups and trays (water, heat seal); flour, sugar and cement sacks (water vapour); dry-food pouches and bags (moisture, oxygen); frozen-food cartons (water, grease, flexibility at −18 °C); and — increasingly — paper-based flexible packaging replacing plastic laminates where full oxygen barrier is not required.

7. Development direction at Uniform Industries

Current work focuses on single-pass high-solids coatings that give Kit 12 and Cobb < 20 g/m² at 6 g/m²; heat-sealable grease-barrier grades for wraps and sachets; and two-layer moisture/oxygen systems for dry-food pouches, all fluorochemical-free, repulpable and formulated for application on existing gravure and flexo coating units so that a converter can add barrier without buying a coater.

Key takeaways

Barrier is four different properties; each needs a different mechanism and test.

Dispersion coatings match PE for grease and water at half the weight and remain repulpable.

Coat weight uniformity and film formation decide barrier on the machine.

Repulpability certification is what turns a barrier coating into a recyclable pack.

References: TAPPI T559 (Kit); ISO 535 (Cobb); ASTM E96 (WVTR); ASTM D3985 (OTR); ASTM F119 (oil stain); Cepi recyclability test method.