Additive Selection for Specialty Paper: Grease-Resistant & Release Grades

A grease-resistant sheet fails for one of two reasons: the fiber network is too open, letting oil wick through the pores, or the surface treatment applied downstream never had a dense enough base to bond to. Both problems start long before the paper reaches a coater or calender stack — they start at the wet end, where fiber, filler, and polymer chemistry either build a tight, uniform sheet or leave gaps that no amount of post-treatment can fully close.

For release paper, the requirement shifts slightly but the root cause is the same. Silicone or adhesive-release coatings need a base sheet with controlled porosity and a smooth, dimensionally stable surface. A sheet with uneven fiber distribution or poor fines retention will telegraph that unevenness straight through the coating, producing pinholes, uneven release force, or coating that simply won't hold.

This is where wet-end additives — retention aids and dispersants in particular, which main contents are Polyacrylamide (PAM) — do work that surface treatments cannot substitute for. Getting their selection right determines whether the base sheet gives a coating something solid to build on, or something to fight against.

▶ Retention Aids: Building a Denser, More Uniform Base Sheet

Retention aids hold fine fibers, fillers, and fiber fragments in the sheet during formation instead of letting them wash out with the white water. For grease-resistant grades, this matters directly: a sheet that retains more fines has fewer open flow channels for oil to travel through, which is the entire mechanism behind mechanically refined greaseproof paper.

Selection comes down to two variables — molecular weight and charge density. High molecular weight retention aids formulated for papermaking wet-end systems bridge fine particles across longer distances, which suits furnishes with high filler loading or significant recycled content. Charge density needs to be matched to the furnish's own charge demand; overdosing a highly cationic product on a low-anionic-demand stock causes redispersion rather than flocculation, and the sheet ends up looser, not tighter.

For release base papers specifically, the goal is slightly different from maximum retention — it's uniform retention. A patchy floc structure creates local variations in porosity that show up later as inconsistent silicone pickup. Lower dosage with a moderate-charge, moderate-molecular-weight product, applied earlier in the approach flow, generally produces a more even formation than a late, high-dose addition aimed purely at maximizing retention percentage.

▶ Dispersants: Solving Fiber Flocculation Before It Becomes a Coating Defect

Retention and dispersion pull in opposite directions, which is exactly why both additives show up in the same wet-end program. Retention aids flocculate fines to keep them in the sheet; dispersants keep the remaining fiber network from clumping into visible flocs that create thin spots and pinholes once the sheet is stretched, calendered, or coated.

In grease-resistant grades made through heavy mechanical refining, fiber flocculation tends to worsen as freeness drops — the more the fibers are beaten down, the more prone they are to clump before the headbox. A properly dosed dispersant designed for fiber distribution in paper production counteracts this at the point of formation, giving the sheet the closed, pinhole-free structure that grease resistance depends on.

For release liners, dispersant selection also affects downstream coating adhesion. A well-dispersed surface gives silicone or adhesive coatings a consistent substrate to key into; localized fiber bundles create high spots that coat unevenly and release inconsistently under peel testing.

▶ Matching Additive Chemistry to Grease Resistance Testing

Additive selection only means something if it's checked against a measurable result. The industry standard for grease resistance is the Kit Test, formalized as TAPPI Test Method T 559, which rates a sheet's resistance against a series of twelve increasingly aggressive oil-based solutions. A higher kit rating means the sheet holds up against more aggressive oils before staining or wicking occurs.

This test gives papermakers a direct feedback loop for wet-end changes. If a furnish adjustment or a switch in retention aid chemistry drops the kit rating by even one or two points, that's a measurable signal that fines retention or fiber closure has weakened — not something to catch only after a customer complaint. Running kit ratings alongside any wet-end trial, rather than relying on visual inspection alone, turns additive selection into a controlled process instead of guesswork.

▶ The Post-PFAS Shift: Why Wet-End Chemistry Matters More Now

Grease resistance in food-contact paper used to lean heavily on fluorochemical surface treatments, which delivered strong oil repellency with relatively little dependence on base sheet quality. That option has largely closed. The FDA's market phase-out announcement covering PFAS-based grease-proofing substances confirmed that these agents are no longer being sold for food contact use in the U.S., with a compliance deadline for exhausting existing stock.

Without fluorochemical treatments carrying the load, base sheet performance has to do more of the work. Non-fluorinated barrier systems, whether mechanical refining, alternative sizing, or bio-based coatings, all perform better on a denser, more uniformly formed sheet — which puts retention and dispersion chemistry directly in the critical path rather than as a secondary consideration behind the surface treatment.

Mills that treated wet-end additives as a minor lever before the phase-out are increasingly finding that choosing the right polyacrylamide chemistry for papermaking applications now has a direct, measurable effect on whether a non-fluorinated grease-resistant grade meets spec at all.

▶ Dosage, Addition Point, and Common Troubleshooting

Retention aids for grease-resistant and release grades typically run in the range of 0.02–0.08% based on dry fiber weight, with the exact figure depending on furnish freeness and filler loading. Dispersants are usually dosed lower, often 0.01–0.05%, since overdosing can redisperse fines that the retention aid is trying to hold. Both should be introduced through dedicated dilution and metering systems rather than batch-dosed, since inconsistent dilution is one of the most common causes of formation variability on the machine.

A few field issues come up repeatedly:

• Falling kit ratings with no furnish change usually trace back to retention aid degradation from shear at the fan pump — check addition point placement first;
• Pinholing that appears intermittently, not uniformly, points to dispersant underdosing rather than a sizing proble;
• Inconsistent silicone release force on the same reel often correlates with formation streaks, which is a dispersion issue, not a coating line issue;
• Sudden retention drop after a recycled fiber ratio increase typically means anionic trash is consuming cationic retention aid before it reaches the fines — a charge demand titration will confirm this quickly.

▶ A Practical Selection Checklist

Before locking in a wet-end additive program for a grease-resistant or release grade, it helps to work through a short set of checks rather than defaulting to whatever chemistry was used on the last grade run.

1. Confirm the target kit rating and test against it during trials, not just at final QC;
2. Match retention aid molecular weight and charge density to actual furnish charge demand, not a standard dosage from a different grade;
3. Verify dispersant dosage against formation uniformity, not just flocculation reduction on paper;
4. Check addition point placement for shear exposure, especially for high molecular weight products;
5. Re-validate the program whenever fiber source, PFAS-alternative sizing, or recycled content ratio changes — none of these are drop-in replacements for each other.

Get in touch with our technical team to review your furnish and grade requirements against our full range of papermaking chemical solutions for retention, drainage, and fiber dispersion.