How to understand and apply functional coatings

A coating is a functional layer

Applied to add barrier properties, control friction, release contents or improve printing and protection.

Performance depends on the full system

Substrate, applied coat weight, drying, adhesive, printing, and application method radically change the result.

It is validated with tests, not intuition

Choose tests based on end use: water absorption, vapor or oxygen transmission, grease resistance, abrasion, gluing or sealing.

What a coating does

Changes how the surface interacts with water, grease, vapor, ink or other materials. Depending on chemistry and substrate, it may form a film or partially penetrate the paper.

What changes the result

Porosity, absorption, dry coat weight and drying affect coverage. Penetration can be desirable in impregnation treatments, while film barriers need surface continuity.

Where it is used

It is not limited to paper. The same selection logic applies to corrugated, paperboard, preprinted liners, industrial packaging, and some flexible structures when primers, overprints, or functional layers are involved.

Substrate

Key question

What material will you apply to?

Why it matters

Smoothness, porosity, and absorbency change leveling, consumption, and final barrier.

Target property

Key question

Do you need water, vapor, grease, release, skid control, abrasion resistance, or a combination?

Why it matters

Not every property coexists efficiently, so priorities must be clear.

Available equipment

Key question

Will you use rod, blade, roll, spray, flexography or gravure? Inline or offline?

Why it matters

The method defines how much coating you can lay down and how uniform it will be.

Heat and drying

Key question

Do you have enough energy to remove the water without overheating the substrate?

Why it matters

Without proper drying there is no real barrier, and too much heat can also damage the film.

Printing

Key question

Will the sheet be printed before or after coating?

Why it matters

Many coatings reduce ink receptivity or require adjustments with the ink supplier.

Gluing and closing

Key question

Will you use water-based adhesive, hot melt or heat sealing?

Why it matters

The coated surface must be compatible with the adhesive or the opposite sealing surface.

Food contact and regulation

Key question

Will there be direct or indirect food contact? Is the pack exported?

Why it matters

FDA, BfR, and other references depend on food type and conditions of use.

End of life

Key question

Are repulpability and recyclability part of the value proposition?

Why it matters

Functional performance should not break the recycling objective or fiber recovery efficiency.

Wet end / before combining

Best for

Long runs, liner application before combination, good productivity, and the option to coat stripes or specific zones.

Watch out for

Part of the coating can be affected by absorption and process heat; not every chemistry works well here.

Dry end / on combined board

Best for

Application on combined board when the grade and equipment support this stage.

Watch out for

Only certain faces are accessible and drying space is usually more limited.

Post-print flexo

Best for

Flexographic application after printing or in a coating station, using a grade formulated for this method.

Watch out for

Anilox volume, transfer, solids and drying limit coat weight. Trials determine whether one or more layers are needed; flexography is not limited to light barriers.

Off-line / precoat

Best for

Coating the substrate before converting, with the option to control multiple layers and their drying separately.

Watch out for

Uniformity depends on the applicator, substrate and operation. Check reel handling, storage, blocking and total cost.

Water resistance

What it solves

Reduces direct or surface water absorption. It matters in boxes exposed to condensation, refrigeration, or humid environments.

How to validate it

It is commonly validated with Cobb plus visual beading or penetration observation.

What to watch

If the barrier increases sharply, review the impact on gluing and drying.

Vapor resistance / MVTR

What it solves

Controls how much moisture passes through over time. It is different from liquid-water resistance.

How to validate it

It is measured with MVTR under controlled temperature and humidity conditions.

What to watch

High levels usually need excellent uniformity, enough coat weight, and sometimes primer plus topcoat.

Oil and grease resistance

What it solves

Reduces oil and grease penetration and resulting stains. This property alone does not demonstrate food contact compliance.

How to validate it

Tests with the intended food or oil, temperature and contact time. Use Kit only if its applicability to the coating has been validated.

What to watch

Food grease and mineral oil are not the same challenge; contaminant viscosity changes the demand.

Release / anti-stick

What it solves

Helps the packed product detach from the surface: bakery, meat, sticky parts, rubber, or asphalt.

How to validate it

It is validated with functional trials on the real product and at the real packing temperature.

What to watch

Required release changes a lot between fresh, frozen, bake-in, and industrial uses.

Rub resistance / non-scuff

What it solves

Protects print and surface against friction, handling, transport, or stacking.

How to validate it

It is typically checked with rub or Sutherland methods plus visual evaluation.

What to watch

Recycled fiber, undercured inks, or rough surfaces make the problem much more severe.

Non-skid / slip control

What it solves

Increases friction to prevent boxes or sheets from sliding during stacking and logistics.

How to validate it

It is measured with slide angle or coefficient of friction.

What to watch

Too much grip can create problems in automated handling or destacking.

Specialty functions

What it solves

There are also coatings for heat resistance, corrosion inhibition, static dissipation, color, whiteness, or skin-pack applications.

How to validate it

Each family has its own method: gloss, resistivity, thermal adhesion, corrosion, box compression, and others.

What to watch

These applications usually need functional tests that are very close to the customer's final process.

Cobb

Objective

Measures water mass absorbed per unit area (g/m²) over a defined time. It does not measure vapor transmission or demonstrate package leak tightness.

Reference

TAPPI T 441. Record duration (for example, Cobb60), tested side and conditioning; compare equivalent conditions only.

MVTR / WVTR: water vapor

Objective

Measures water vapor transmission, commonly in g/(m²·day). Report temperature, relative humidity on both sides and method; results do not transfer directly to other environments.

Reference

ASTM E96/E96M: gravimetric method, where its scope and sensitivity suit the structure. Agree the protocol with the laboratory.

Kit Test / grease

Objective

Kit ranks surface repellency against test mixtures. It was designed for fluorochemical-treated paper; its result alone does not predict the barrier of a water-based film coating.

Reference

TAPPI T 559: verify applicability to non-fluorochemical treatments and film barriers. Supplement with actual food exposure, including folds.

Rub / abrasion

Objective

Evaluates rub resistance of ink or paper and board surfaces. Record load, cycles, dry or wet conditions and damage criterion.

Reference

TAPPI T 830 for containerboard and corrugated board. Distinguish print resistance from protection of the product against the board.

COF / slide angle

Objective

Confirms whether the sheet or box will grip or slip inside the expected range.

Reference

Define method, surface pair, orientation and conditioning. Distinguish static from dynamic friction; do not directly compare angles with coefficients from other methods.

Gluing and closure

Objective

Checks performance on the folder-gluer, flap sealing, and the end customer's case sealer.

Reference

Real line trials with the correct adhesive, pressure, and compression time.

OTR: oxygen transmission

Objective

Measure oxygen transmission through the complete structure. Record temperature, humidity, gas conditions and units; the value changes with environment and composition.

Reference

ASTM D3985 for films and sheets within its scope. The laboratory should confirm the method for other structures.

Heat sealing

Objective

Evaluate seal strength, leaks and opening behavior with both sealing surfaces. Define temperature, pressure and dwell time.

Reference

Agreed testing on specimens and formed packages; repeat after the intended conditioning and handling.

Pinholes and craters (fisheyes)

Likely causes

Pinholes are small discontinuities associated, for example, with trapped air or insufficient coverage. Fisheyes are coating retraction areas, often caused by contamination or poor wetting.

Recommended action

Identify the defect before adjusting. Check cleanliness, foam, wetting and uniformity; agree any additive or formulation change with the supplier.

Blocking or surface sticking

Likely causes

Insufficient drying, high coat weight, stacking too early, or excessive surface heat without real internal drying.

Recommended action

Adjust energy, speed, ventilation, pre-stack timing, and applied coat weight.

High Cobb or poor barrier

Likely causes

Very open substrate, insufficient coverage, uneven application, or an underdried film.

Recommended action

Check porosity, consider primer or basecoat, raise deposit uniformly, and confirm drying with testing rather than touch alone.

Glue failure

Likely causes

Wrong adhesive, insufficient compression, coating that is too water or oil resistant, or silicone defoamers.

Recommended action

Try a different adhesive, increase compression or dwell time, keep the glue flap free if it makes sense, and avoid additives that kill adhesion.

Printing problems over the coating

Likely causes

Low ink receptivity, insufficient surface energy, or ink not adjusted for a barrier surface.

Recommended action

Align the trial with the ink supplier, confirm whether printing goes over or under the coating, and do not assume that "printable" means universally printable.

Slow drying or overheating damage

Likely causes

Not enough energy to evaporate water, excessive speed, poor airflow, or too much exposure to IR or preheaters.

Recommended action

Find the balance: enough drying to form the film, but not so much that the coating cooks or the substrate distorts.