Summary

A PFAS-free paper coating can pass a flat-sheet grease test and still leak after folding or heat sealing because converting can create barrier defects that the original screening condition never produced. The most useful diagnostic signal is usually where grease penetration first appears: directly on a fold, beside a score, across a seal impression, at a seal edge, or randomly across the coated surface. That location helps separate coating fracture, substrate-related weak spots, incomplete film formation, and heat-seal process effects. The first troubleshooting step should be a paired comparison of flat, fold-only, heat-only, and fully converted specimens from the same material set. Increasing coat weight or changing raw materials before this comparison can hide the mechanism without creating a reliable production window.

Start With the Location of the First Barrier Failure

Once the flat coating repeatedly meets the selected grease-resistance requirement, the original grease rating provides limited information about why a converted package fails.

The failure location becomes more useful.

Grease penetration that follows the outer radius of a fold is more consistent with loss of coating continuity under tensile strain.

Failure around a score shoulder, corner, or intersecting crease may indicate concentrated deformation where the coating experiences a more severe local strain than on a simple laboratory fold.

Penetration across the centre of a heat-seal impression raises questions about thermal softening, coating displacement, pressure, or interaction between the barrier layer and the sealable phase.

Failure concentrated along the edge of the seal can point toward a transition zone where pressure, temperature, or local deformation changes rapidly.

Random staining away from folds and seals deserves a different investigation. Coating coverage, pinholes, substrate holdout, foam, contamination, or coat-weight distribution become more plausible variables.

This distinction should be made before changing the formulation.

Why the Flat Grease Test Can Give an Incomplete Answer

Flat-sheet testing evaluates the barrier before the structure experiences much of the deformation that occurs during package conversion.

This matters particularly when the Kit Test is used as the main grease-resistance screen. The official TAPPI T 559 cm-22 method states that the test was developed primarily for paper and paperboard treated with fluorochemicals. TAPPI also notes that application of the method to film-like barriers or nonfluorochemical treatments needs to be evaluated.

A successful result can therefore support a conclusion such as:

The coated flat sheet resisted the selected grease challenge under the test conditions.

It does not, by itself, demonstrate:

• resistance after creasing;
• resistance after repeated folding;
• barrier retention after heat sealing;
• acceptable performance at score intersections;
• production robustness across normal paper and process variation.

The qualification question changes as soon as converting starts.

A PFAS-free composition is a material attribute. It is not a mechanical failure mechanism. Once the flat coating passes, troubleshooting should focus on where and when barrier continuity is lost during converting.

Folding Can Create a Local Pathway Through an Otherwise Intact Barrier

A continuous coating can provide strong grease resistance over nearly the entire sheet and still fail at a very small damaged region.

During folding, the coating and the fibre-based substrate do not necessarily deform equally. The outside of the fold experiences tensile deformation while other regions experience compression. Creasing changes the local board structure before the final fold is made.

If the coating cannot accommodate the imposed strain, several defects may become possible:

• microcracking;
• local delamination;
• coating separation around surface irregularities;
• exposure of fibre at a highly stressed point;
• opening of an already thin region of the coating.

Once exposed fibre provides a continuous path, grease can migrate through a narrow defect even though most of the coated area remains functional.

The useful question is therefore not:

Did the original sheet achieve a high grease rating?

The useful question becomes:

Did converting create a connected pathway through the barrier?

Polymer flexibility, binder balance, pigment or platelet loading, crosslinking, film formation, coating thickness, substrate roughness, moisture condition, and adhesion can all influence this response. Their importance should be demonstrated through controlled comparisons rather than assumed from the formulation alone.

Heat Sealing Introduces a Different Failure Window

Heat sealing exposes the coated structure to temperature, pressure, and dwell time simultaneously.

That creates two performance requirements.

The sealable region must develop enough interfacial contact to form an acceptable seal.

The barrier must retain enough structural integrity to prevent new grease pathways.

Those requirements can interact.

A coating formulation with high rigidity or high inorganic loading may show good flat barrier properties yet tolerate less deformation during converting. A softer or more mobile polymer phase may seal more readily but can respond differently to pressure, thermal exposure, or blocking conditions.

Research on styrene-butadiene dispersion coatings with different kaolin contents illustrates this interaction within the studied formulations: pigment content affected both heat-seal behavior and fold-cracking resistance. The result does not establish a universal pigment limit, but it supports evaluating barrier retention and sealability together rather than treating them as independent properties. See the original study in Coatings.

For troubleshooting, heat-seal strength alone is therefore insufficient.

A seal may appear mechanically acceptable while grease leakage develops near the seal centre, edge, or pressure-transition area.

Use Four Paired Controls Before Reformulating

The most efficient first experiment is a small control matrix using material from the same coating batch and, where possible, the same paper lot.

The sequence is deliberately simple.

If the fold-only specimen fails while the heat-only specimen remains stable, attention should move toward coating flexibility, local coat continuity, paper direction, crease geometry, and substrate mechanics.

If the heat-only specimen fails, a sealing-window investigation becomes more useful than an immediate formulation change.

If both isolated controls pass and only the fully converted specimen fails, interaction between fold geometry, heat, pressure, and local coating thickness becomes a stronger hypothesis.

If even the flat control fails intermittently, converting may not be the primary issue. Coating uniformity, substrate variation, contamination, film formation, or sampling should be checked first.

Problem Diagnosis Table

Possible causes in this table remain hypotheses until the comparison reproduces or eliminates them.

Separate Coating, Paper, and Converting Variables by Failure Location

Troubleshooting becomes less efficient when every converted-package failure is treated as a raw-material problem.

The failure pattern can narrow the investigation much faster.

When the defect follows a fold

Check:

• coating continuity before and after folding;
• outer versus inner fold surface;
• paper machine direction versus cross direction;
• score depth and geometry;
• local coating thickness near the fold;
• moisture conditioning before converting.

A formulation change becomes more defensible only when the same fold geometry and substrate repeatedly distinguish one coating system from an appropriate control.

When the defect follows the seal

Check:

• seal temperature;
• applied pressure;
• dwell time;
• jaw geometry;
• seal-centre versus seal-edge damage;
• coating pickup or visible displacement;
• barrier performance after heat exposure without folding.

The target is a usable process window, not a single condition that produces one successful seal.

If acceptable barrier performance exists only at a very narrow setting, the system may remain difficult to control in production even when a laboratory result can be made to pass.

When the defect appears randomly

Move upstream.

Review:

• coating uniformity;
• substrate roughness and porosity;
• local holdout;
• foam or pinholes;
• drying and film formation;
• contamination;
• coat-weight distribution.

Random failures are less consistent with a single crease or heat-seal mechanism.

Sample, Pilot, and Production Results Answer Different Questions

A laboratory sample can establish whether a coating concept is capable of forming a useful barrier under controlled conditions.

A pilot trial tests whether that barrier can survive a more realistic coating and converting sequence.

Commercial production asks whether the process has enough margin to tolerate normal variation.

These stages should not share the same approval logic.

Sample stage

The priority is mechanism screening.

Confirm that the coating can form a continuous barrier and compare flat, fold-only, and heat-only performance.

A successful sample should not move directly to material approval if fold or seal robustness has not been challenged.

Pilot stage

The priority shifts to process interaction.

Check whether realistic drying, substrate variation, scoring, folding, and sealing reproduce the laboratory result.

If the pilot material fails in a location that the laboratory sample never challenged, the discrepancy should be diagnosed before another scale increase.

Production stage

The priority becomes operating margin.

A system that passes only under a narrow combination of moisture, crease condition, sealing temperature, or pressure may remain vulnerable even if selected samples meet specification.

Commercial approval should therefore depend on repeatability across the relevant process range, not the best individual result.

The Critical Signal Is the First Point Where Continuity Is Lost

Discussions of PFAS-free grease barriers often focus heavily on the initial grease-resistance result.

That metric is useful during screening.

After the flat control already passes, it contributes much less diagnostic information than the first physical location where the converted structure loses barrier continuity.

This leads to a more practical development logic.

When grease penetration repeatedly matches fold cracks, adding more initial barrier performance may not create enough converting margin.

When fold-only specimens remain intact and failure appears only under specific sealing conditions, the formulation may still be workable if a sufficiently broad process window can be demonstrated.

When failure locations move randomly from sheet to sheet, a single mechanical explanation becomes less convincing.

For R&D teams, the development target should include post-converting barrier retention, not only flat-sheet resistance.

For production teams, the relevant output is a repeatable fold and seal window with enough tolerance for normal process variation.

For quality teams, recording the failure location can be more useful than a simple pass/fail entry.

For procurement and supply-chain teams, supplier screening data should be treated as preliminary unless the material has been evaluated on the intended paper and converting sequence.

When Scale-Up Should Stop

Further scale-up should be paused when the failure is reproducible but the responsible variable remains unresolved.

Examples include:

• fold-only specimens repeatedly develop barrier loss;
• seal performance exists only in an impractically narrow operating range;
• pilot-coated material behaves differently from the laboratory sample before package conversion;
• failures appear at multiple commercially relevant scores or seal locations;
• the failure pattern changes between batches without an understood process or substrate explanation.

Changing supplier should not be the first troubleshooting action.

A different material can reproduce the same failure if crease geometry, substrate holdout, drying, or heat-seal conditions remain unchanged.

A material change becomes more justified when controlled testing shows that one formulation repeatedly loses barrier continuity under the same paper, conditioning, and converting conditions while an appropriate comparator remains intact.

Next Action Order

Begin with material from the same coating and paper lot.

Run flat, fold-only, heat-only, and fully converted controls.

Map the first grease breakthrough against the fold apex, score shoulder, seal centre, seal edge, and areas away from converting stresses.

Use that location to decide whether the next experiment should focus on coating mechanics, substrate variation, film formation, or the sealing window.

Only after the dominant failure mode is reproduced should the team adjust coat weight, binder balance, pigment or platelet loading, crosslinking, plasticization, coating chemistry, or material source.

For a material or troubleshooting inquiry, provide the paper grade, coating chemistry, dry coat weight if available, grease-test method, fold or score geometry, observed failure location, conditioning history, heat-seal conditions, pilot or production observations, and required sample or documentation needs. ChemicalCell can use these details to discuss relevant material specifications, available technical documentation, and sample requirements for the intended coating system.