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title: Why Does Greaseproof Paper Actually Resist Oil? A Capillary-Mechanism Take on Picking Packaging Paper
lang: en
source: https://mindsprt.dev/en/knowledge/research-brief-grease-resistant-paper-capillary-transport/
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# Why Does Greaseproof Paper Actually Resist Oil? A Capillary-Mechanism Take on Picking Packaging Paper

*Mai Strategy Lab · 7 min read · 2026-08-04*

> Two sheets of paper, both labeled "paper"—one holds up under fried chicken, the other lets a single drop of oil bleed through. The difference hides in how the microscopic pores inside the sheet behave around fat. This piece walks through one empirical study from a food engineering journal and upgrades your decision-making from "match the spec sheet" to "read the mechanism," so the next time you send a job to press, you already know what to ask and how to verify

**Quick answer:** Why does one sheet of paper hold up to fried food while another lets oil bleed through on contact?

## Overview

You've probably been there: a client sends in a greaseproof paper proof, the spec sheet says "passed COBB testing," then you wrap a burger and some fries and still end up with an oil blot the shape of Texas. Or the reverse, two papers with nearly identical specs, and the production line finds paper A noticeably less greasy than paper B, with no one able to explain why.

The problem usually sits here: we've been comparing specs, not mechanisms. The reason a greaseproof paper works isn't hiding in the chemical name on the surface coating; it lives in how that internal mesh of capillary pores interacts with oil. A study by Westbrook, Ramadan, and Datta in the *Journal of Food Engineering* uses a multi-domain capillary transport model to break down how fried food gets absorbed into or held back by the paper during post-frying storage [1]. What they found: oil migration in paper isn't simply "blocked"—it's a capillary-mechanics play shaped jointly by pore-size distribution, surface energy, and fiber arrangement.

What follows is five questions that unpack this mechanism and bring back judgment methods you can actually use on the print floor and in procurement.

## Does greaseproof paper really "block" oil? Or does it actually "absorb" it?

The real job of greaseproof paper, most of the time, is to "steer where the oil goes," not to seal it out completely. Westbrook et al.'s multi-domain capillary transport analysis shows that the paper's absorption and resistance of oil come from the capillary pressure gradient generated by the pore structure; different pore-size bands each form their own transport paths for oil, water, and air [1]. Put differently, the micro-pore network inside the sheet decides where oil goes, how fast it moves, and where it stops.

What procurement can adjust: stop reading only the grease-resistance grade, and start asking what happens to the oil after it enters the sheet. If the pore-size distribution skews toward larger pores, oil punches through fast; if it skews toward small, evenly distributed pores, oil gets dispersed and locked near the surface, which is where that "feels less greasy" impression in real use actually comes from.

## What are COBB value and KIT rating on the spec sheet actually measuring?

Most paper buyers only look at COBB value (in g/m², the amount of water the paper surface absorbs on contact) and KIT rating (a grading of surface penetration resistance using inks and solvents). Both lean toward macroscopic outcomes, without unpacking the underlying mechanism. Westbrook et al. point out that capillary transport behavior is shaped simultaneously by three variables, pore-size distribution, surface energy, and fiber orientation, so a single spec number can't describe it fully [1].

So how can procurement and print adjust? Flip the reading order of the spec sheet from "look at the number" to "ask about the process": is this paper's fiber source long-fiber or short-fiber? Has it been internally sized at the wet end or surface-coated? Are the coating thickness and porosity published anywhere? These questions tend to predict real grease resistance far better than a single COBB figure.

## Same "greaseproof" label, why such wildly different performance?

This is where most of us in the trade hit the wall. The word "greaseproof" doesn't map to a single mechanism. Westbrook et al.'s model splits capillary transport into multiple physical domains, each tied to a different pore-size range and fluid behavior [1]. In practice, one paper might rely on a hydrophobic surface coating to block initial contact (the kind of performance that earns a high KIT rating), but its internal fiber packing isn't dense enough, so oil still seeps through slowly over a few hours of storage. Another paper with middling surface oil resistance could have an evenly distributed network of small internal pores, giving it steadier overall oil uptake and retention.

Two sheets both stamped "KIT 12" performing worlds apart, before blaming the supplier for being shady, it's worth admitting that the spec was only ever capturing one slice of the mechanism.

## Reading the mechanism backward: what should procurement and pre-press actually ask?

Translating the mechanism above into a runnable pre-press checklist gives you at least three questions worth asking:

・Ask about pore-size distribution and fiber orientation. These are the root variables driving capillary transport [1], yet most mills won't volunteer them, if you can get an SEM image or porosity data, that's a stronger basis for judgment than COBB alone.

・Ask about coating type and heat tolerance. Surface oil resistance often leans on fluorine-based or wax-based coatings, but high-temperature use (think fries straight out of the fryer) changes the coating's surface energy, which then shifts capillary behavior [1].

・Ask about the time axis of the actual use scenario. Greaseproof paper performance is tightly tied to how long the oil sits against the paper; short-window wrapping and long-duration storage call for genuinely different paper structures.

Once those three questions are answered, the fuzzy zones on the spec sheet usually narrow down into something you can actually compare.

## So how can design and print teams use this mechanism?

For design and print teams, the most direct use of this mechanism is "fewer landmines." When a client specifies a "greaseproof lunch box" or "burger wrap paper," it's far better to factor pore structure and coating mechanism into material selection than to discover mid-proof that the thing leaks oil. Westbrook et al. also remind us that capillary transport is a dynamic process, not a verdict sealed at the moment the paper leaves the mill [1]—meaning real-world conditions (temperature, time, oil content) all shift the outcome, and lab data alone won't carry it.

That said, this mechanism has a boundary. When the packaging involves multi-layer composites (say, paper + plastic laminate + aluminum foil), capillary behavior gets driven by the interfaces between layers, and a pure-paper pore model needs adjustment. Same story if the oil is high-viscosity or carries a lot of solids (like the battered-oil mix on fried chicken)—the flow behavior drifts away from the model's assumptions.

Next step: instead of waiting for the supplier to hand you a pretty spec sheet, add "pore-size distribution, surface energy, coating heat tolerance" to your pre-press checklist. That saves more on downstream complaints and reprints than fighting over a cent or two per sheet.

## Mai Strategy Research Lab: One-Line Takeaway

The value of greaseproof paper isn't in how much oil it "blocks"—it's in where it steers the oil. Once you can read the capillary mechanism, picking paper moves from comparing numbers to comparing structure; and the real worth of that pre-press conversation usually lands right here.

## Further Thinking for the Trade

・For print manufacturing: could incoming inspection include a simple pore-size or surface-energy measurement, extending "greaseproof quality" from the acceptance report into process monitoring?

・For brand and design: could the design brief add a "oil contact time and temperature" field, forcing the supply chain to answer with corresponding paper structure?

・For AI and SaaS tooling: is there a way to structure vendor paper data, pore size, coating, COBB, so procurement can compare at the mechanism level rather than the spec level?

・For industry research: should existing greaseproof test methods add a "dynamic capillary transport" dimension, so information beyond KIT rating and COBB value can be standardized?

These questions don't have industry consensus yet, which is exactly why they're worth digging into.

## Key Takeaways

Greaseproof paper's effectiveness comes from the capillary transport behavior of its pore structure, not the surface coating alone.

COBB value and KIT rating describe macroscopic outcomes and can't fully reflect differences in the underlying mechanism.

Two papers both labeled "greaseproof" can perform very differently in practice; the root cause sits in pore-size distribution and fiber structure.

Before sending to press, ask three extra things: pore size, coating type and heat tolerance, and the time axis of actual use.

Composite structures and high-viscosity oils fall outside the pure-paper capillary model and need separate evaluation.

## Further Thoughts

For print manufacturers, this study is a nudge: if incoming inspection stops at COBB value and visual checks, predicting actual greaseproof performance will stay hard. Pulling pore-size distribution or surface energy into supplier evaluation is a direction worth trying. For brand and design teams, writing "oil contact time and temperature" into the design brief forces the supply chain to answer with more concrete paper-structure questions, far more decision-useful than simply specifying "greaseproof paper." For AI and SaaS vendors, a structured paper-spec database that covers capillary-transport parameters could become critical infrastructure for material-selection decisions. Finally, the open questions: is there industry consensus on building a new test standard for "dynamic capillary transport"? And how should capillary models for composite laminates be standardized? These are the directions worth investing in next.

## References

[1] Westbrook, Ramadan, & Datta (2027). [Using paper to reduce oil content during post-frying storage: Mechanistic understanding based on multi-domain capillary transport](https://doi.org/10.1016/j.jfoodeng.2026.113264). *Journal of Food Engineering*. DOI: 10.1016/j.jfoodeng.2026.113264

## FAQ

### Can greaseproof paper fully block oil?

No. Most greaseproof papers work by "steering where the oil goes," using pore structure to direct and lock oil rather than fully sealing it out [1].

### Does a higher COBB value mean worse greaseproof performance?

COBB value is a macroscopic measure of water absorption at the paper surface and doesn't map linearly to greaseproof performance; pore-size distribution and coating structure also need to factor in [1].

### Why do two greaseproof papers both rated KIT 12 perform so differently?

Because KIT rating only captures one slice of surface penetration resistance; differences in internal pore-size distribution and fiber structure still produce noticeably different oil migration behavior [1].

### What three questions should procurement ask most when picking greaseproof paper?

Pore-size distribution and fiber orientation, coating type and heat tolerance, and the time axis of the actual use scenario, these three predict outcomes better than spec numbers alone [1].

### Does this capillary mechanism apply to all greaseproof packaging?

It doesn't apply to multi-layer composites (such as paper + laminate + aluminum foil), nor to high-viscosity or solid-laden oil scenarios, those cases need separate evaluation [1].


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