Verify grease resistance with matched specimens representing the dried coating, folding, heat sealing and the finished paper package. Use the intended food or test medium, contact temperature and duration, and assess through-sheet penetration separately from seam leakage. A flat-sheet Kit rating cannot establish performance at a crease or seal. Base the decision on whether each required location meets its defined criterion. When specimens disagree or breakthrough timing straddles the required exposure duration, retain the individual results and hold approval until a documented review and controlled retest resolve the uncertainty.

This framework covers film-forming, nonfluorinated polymer coatings and polymer–mineral coatings on paper and paperboard. Its purpose is to verify grease-barrier retention through converting. It does not establish the analytical basis of a PFAS-free claim or food-contact compliance.

Define the Construction and the Failure Endpoint

Identify the base paper, food-contact side, primer, barrier layer and any separate seal layer. Record dry coat weight in g/m², drying history and conditioning. A resin name alone does not identify the tested construction, and a heat-sealable topcoat may have a different function from the underlying grease barrier.

Continuity deserves its own inspection. In Marinelli and colleagues’ study of dispersion-coated paper, diffuse surface cracks were linked to exposure of underlying fibres and poor barrier performance in a tested coating. For qualification, this supports checking local coverage alongside average coat weight. Treat a coat-weight increase as a proposed correction until the original defect locations have been retested.

Specify the challenge around the intended use: the food or test medium, contact temperature, duration, package orientation and any moisture exposure. Where filling starts hot and the package subsequently cools, document that sequence. An ambient oil test leaves that thermal history untested.

Agree what constitutes failure. Surface discoloration, oil reaching the reverse face and leakage through a seam are different endpoints. State which are unacceptable, where they will be inspected and when observations will be made. Include folds, corners and seams that actually contact the contents during use or handling.

For projects still choosing a coating platform, the separate PFAS-free barrier coating selection guide addresses that earlier decision.

Match the Test Method to the Question

Use the Kit Test Within Its Demonstrated Scope

The published scope of TAPPI T 559 cm-22 describes a surface-repellency and antiwicking test developed for fluorochemical-treated paper. It explicitly calls for evaluating its applicability to film-like barriers and nonfluorochemical treatments. A Kit rating can support screening when its relevance has been established for the coating system; it cannot independently demonstrate service performance after converting.

For an existing Kit-based specification, compare its results with grease penetration under representative use conditions before treating it as a release criterion. Keep any established screening requirement distinct from the converted-package requirement.

Add a Time-Dependent Penetration Measurement

ISO 16532-1:2008 covers grease-resistance testing of creased or uncreased paper and board by measuring breakthrough time using palm kernel oil. Its published scope makes it a relevant method to assess when selecting a penetration test for paper packaging.

Use the full applicable standard for procedural details. If the laboratory departs from its prescribed medium, temperature, crease preparation or specimen arrangement, identify the procedure as modified and document the departures. Give a finished-package exposure test its own protocol identifier. Keep results from different procedures separately reported.

Compare Matched Specimens Across the Converting Sequence

Use adjacent, comparable specimens for each condition. Allocate separate copies to destructive grease testing, microscopy and seal-strength testing; peeling a seal before checking its barrier changes the specimen.

The following is a proposed experimental comparison, rather than a standardized test sequence:

Include an uncoated substrate as a method-response check and an accepted construction as a comparator where available. Neither control replaces the acceptance criterion for the candidate.

For initial comparisons, hold the paper lot, coating batch and conditioning constant. Later production qualification should deliberately sample independent coating runs and relevant reel positions. Repeated drops on one small laboratory sheet provide limited evidence about production variability.

Preserve the Fold Geometry That the Package Actually Uses

Record the fold line relative to the paper machine direction and whether the coating faces inward or outward. For scored board, retain the scoring tool, channel geometry and subsequent folding conditions. A hand fold may be useful for screening while leaving the commercial score unqualified.

Material composition can change the response. In the kaolin-filled styrene-butadiene study, higher mineral loading reduced the binder fraction and was associated with greater coating brittleness and fold cracking. It also changed heat-sealing requirements within the tested formulations. The authors assessed fold direction and coating orientation separately. Their post-fold barrier measurements included water-vapour transmission; those results do not directly establish post-fold grease resistance.

The practical implication is to retest converted specimens after changing the binder–mineral balance, even when the flat-sheet grease result improves. Inspect the relevant folds before exposure and compare them with the first penetration locations afterward. A visible crack is a candidate pathway; matching it to repeatable grease penetration strengthens the diagnosis.

Verify Barrier Retention and Seal Integrity Separately

Use the intended sealing pair: coating-to-coating, coating-to-paper or coating-to-lid, as applicable. Record jaw temperature, dwell time, applied pressure, jaw geometry and cooling interval. Where interface temperature is measured, report it separately from the equipment setpoint. Preserve the package’s actual process order; a fold made before sealing may respond differently from one made afterward.

Test conditions spanning the proposed operating range, including combinations that production is expected to encounter. One successful setting establishes feasibility at that setting. It leaves the surrounding operating range unresolved.

Use separate matched specimens to assess:

• Seal strength and failure mode: how the joint separates under the selected mechanical test.
• Barrier retention: whether oil penetrates the coated material at the seal footprint or adjacent transition zone.
• Seam leakage: whether contents escape along an incompletely sealed channel or another route through the joint.

The published scope of ASTM F88/F88M-23 covers seal-strength measurement in flexible barrier materials, including seals to rigid or semi-rigid mating materials. It measures the force required to separate a sealed test strip and identifies the failure mode. Because that endpoint is mechanical separation, a fibre-tear result alone cannot establish resistance to oil escape from an unopened seam. Evaluate leakage separately. Likewise, record local through-sheet penetration beside a seal separately from opening of the seal interface.

Treat the unfolded, heat-sealed specimen as a test of the combined sealing operation. It includes heat, pressure and interfacial contact; it does not isolate thermal damage.

Report What Failed, Where It Failed and When

Record individual specimen results alongside the tested condition and exposure history. Map the first detected stain to the field coating, fold, score shoulder, seal footprint, seam channel or exposed edge. Check whether apparent field penetration could have originated from an edge and spread laterally.

If an edge is excluded or masked in a laboratory permeability test, state that limitation. Evaluate any edge that encounters the contents in the real package during the finished-package trial.

Keep observation intervals visible in the report. When breakthrough is first seen at an inspection, its onset lies between that inspection and the previous clear observation. When exposure ends without breakthrough, report “no breakthrough during the stated test duration.” Do not convert that observation into an exact failure time or an unlimited service-life claim.

Report the number tested and the number meeting the criterion for each condition. Preserve failures at individual corners or seams instead of combining them into an overall average rating.

Resolve Mixed or Borderline Results Before Approval

The following review sequence is a proposed qualification procedure. It does not replace the selected method’s validity rules or the agreed sampling plan.

When nominally identical specimens give mixed pass/fail results, retain their identities and original observations. Check reel position, coating and paper source, conditioning, exposure records, preparation and inspection history for a documented difference. Review controls against their pre-established expectations. An abnormal control response calls for a test-validity investigation before interpreting the candidate result as a material difference.

Repeat the original condition with fresh matched specimens and the appropriate controls, using a resampling rule defined before the repeat. Keep these results linked to the original series. Passing repeats alone do not cancel valid initial failures. Exclude a result from the qualification decision only when a documented validity review establishes a preparation, equipment or execution problem. Preserve the original record and the reason for exclusion. A crack produced by the intended scoring or folding process remains relevant performance evidence.

A timing ambiguity needs a different correction. If the last clear inspection precedes the required exposure duration and the next inspection detects breakthrough afterward, the record does not establish whether the specimen survived the requirement. Repeat with an observation at the decision time, where the selected method permits. If inspection disturbs the exposure, use separately allocated specimens under a documented protocol instead of repeatedly opening the same package.

Keep the decision on Hold while a relevant failure remains unexplained or the timing interval still crosses the acceptance point. Apply the pre-agreed acceptance rule to the retained evidence; do not tighten the specification or blame the material source merely because the first series is inconsistent.

Base Approval on the Converted Construction

Set the sampling plan and acceptance criteria before reviewing the results. Their severity should reflect the intended package duty and consequences of leakage; this framework does not prescribe a universal Kit rating, exposure duration or allowable failure rate. A candidate can perform worse than an accepted comparator and still meet the agreed requirement; a candidate that outperforms it can still fail that requirement.

Laboratory success can justify a pilot trial. Commercial qualification requires evidence from representative production material within the proposed coating and converting range, followed by the relevant finished-package exposure. Hold approval when a required location fails or when the test leaves an essential use condition unrepresented.

Maintain the PFAS-claim and food-contact evidence as separate approval workstreams. Grease testing does not measure PFAS content; ChemicalCell’s PFAS verification article for EU food-contact packaging addresses that distinct evidence question.

The next step is to complete one matched verification series through the intended coating, folding and sealing sequence, including the defined review of mixed or borderline results. Record the location-specific evidence against the package requirement. Any approval should identify the construction, exposure conditions and operating range it covers. An unresolved failure at a required location keeps the affected construction and condition on Hold.