Contents
Complements workflow

TAPPI T 441 Cobb test: water absorptiveness of sized paper and board

TAPPI T 441 is the Cobb test US mills and box plants use: it weighs how many grams of water one square metre of sized paper, board or corrugated board takes up in a set time. The Dropometer does not measure the Cobb value. It complements the test with a fast water contact angle check on sizing, which you correlate with your own Cobb data to flag a weak lot before the full test.

What brings you here?

Who this is for
US paper mills, linerboard and box makers, and packaging test labs who check sizing with the Cobb test.
Compliance tier
Complements workflow: the Dropometer does not measure the Cobb value. It adds a water contact angle pre check on sizing that you correlate with your Cobb data.
Measurement
TAPPI T 441: water absorbed in g/m², by weighing. Dropometer: sessile drop water contact angle, 10° to 175°, ±0.35° accuracy (Dropometer datasheet).
Setup
About 2 minutes. Battery powered, 1 kg, runs at the machine, the corrugator or in the lab.
Last updated
October 18, 2026
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Free one page checklist for running a water contact angle pre check next to the TAPPI T 441 Cobb test: what to prepare, the settings to lock, how to pair results by side and what to record. Made for mill and box plant QC benches.

Talk to an expert

Book a 15 minute demo on your linerboard

Tell us your grade, sizing and Cobb limits. We will measure the water contact angle live and tell you plainly where a pre check fits next to your Cobb tester and where it does not.

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View the official standard
Gurdeep-Saini-Photo
Written by
Gurdeep Singh Saini
Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.
COO at Droplet Lab
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Droplet-Lab logo
Technical Review by
Droplet Lab Team
Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.
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Verified against TAPPI T 441
Gurdeep-Saini-Photo
Written By

Gurdeep Singh Saini

COO at Droplet Lab

Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.

Droplet-Lab logo
Reviewed By

Droplet Lab Team

Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.

Featured Studies & Publications

Research Paper Contact angle measurement with a smartphone
Research Paper Surface tension measurement with a smartphone using a pendant drop

Evidence box

Standard

ANSI/TAPPI T 441 om-24, Water absorptiveness of sized (non-bibulous) paper, paperboard, and corrugated fiberboard (Cobb test), a TAPPI official method. It revises om-20 and builds on studies by Cobb and Lowe (ANSI Webstore listing).

Compliance tier

Complements workflow. The Cobb value is a mass of water per square metre, found by weighing. The Dropometer does not measure it. It measures the water contact angle on the sized surface, a pre check you correlate with your own Cobb results.

Instrument performance

Contact angle range 10° to 175°, accuracy ±0.35°, display resolution 0.01°, from the Dropometer product specification. The camera runs at 10 frames per second, which suits a reading at a fixed time such as 5 s.

Peer reviewed method

The drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers. See our validation publications.

Customer evidence

A university fibre lab used the Dropometer for static water contact angle on electrospun lignin and PLA fibre mats, read within 10 s (Fibers, 2025). The study did not run a Cobb test or use paper. Read our analysis of the paper.

What it does not tell you

A contact angle reads the surface in the first seconds. It does not weigh the water the sheet takes up over 120 s or 30 min, so it cannot replace a reported TAPPI T 441 Cobb value.

Executive summary

What this standard measures, who uses it and how the Dropometer fits

TAPPI T 441 measures the water absorptiveness of sized paper, paperboard and corrugated board by the Cobb test: the grams of water one square metre of the sheet takes up in a set time. US paper mills, linerboard and box makers, and packaging test labs use the Cobb value to check sizing and how a grade will stand up to water. The Dropometer complements this workflow. It does not measure the Cobb value, which needs weighing, but it adds a fast water contact angle check on sizing that you correlate with your own Cobb data.

The current edition is ANSI/TAPPI T 441 om-24, which revised om-20 (source: ANSI Webstore listing). A Cobb tester clamps a ring with a 100 cm² test area over the sheet and wets one face with water for the test period. The piece is then drained, blotted with a 10 kg roller and weighed again, as the standard specifies (source: Thwing-Albert Cobb tester brochure). You report the mass gain in g/m² with its time, for example Cobb120. US labs commonly run 120 s for paper and 30 min for combined board, and test felt and wire sides separately (source: SGS-IPS). TAPPI sends unsized and absorbent grades to TAPPI T 432 and does not recommend the Cobb test as a sizing test for writing paper (source: SGS-IPS).

The Dropometer contact angle and surface energy kit reads the water contact angle on the same face at a fixed time, such as 5 s, with ±0.35° accuracy (Dropometer product specification). A drop that stays high points to good sizing at the surface; a drop that flattens and soaks in points to a weak spot. For the basics, see the contact angle measurement guide and wettability in our glossary. To keep readings comparable between shifts and mills, follow reproducible contact angle measurement, and sense check values against our water contact angle reference values for common surfaces.

Timed water contact angle on paper also drives wicking control for paper microfluidic and lateral flow tests. A shared facility at Cornell University uses the Dropometer for contact angle work, and a fibre lab there used it to read water contact angle on lignin based fibre mats, a fibrous material rather than paper. If you are choosing an instrument for a mill or converting QC bench, see our best contact angle goniometer for industrial QA.

Further reading: the packaging and containers guide.

How the Dropometer fits the workflow

A water contact angle pre check next to the TAPPI T 441 Cobb test, step by step

From one sheet to a paired angle and Cobb value, side by side

These steps show where the Dropometer fits next to the Cobb test. They do not replace TAPPI T 441. Buy the current edition and follow its apparatus, test period, blotting and specimen rules for any Cobb value you report.

±0.35°

Contact angle accuracy

From the Dropometer product specification. Display resolution is 0.01°.

About 2 min

Setup

Base plate, phone holder, LED and syringe. No PC needed.

8 h

Battery time

5 V battery operation, so the unit can sit at the machine or corrugator.

1

Cut and condition both sets of test pieces

From the same sheet, take test pieces for the Cobb test and a strip for the contact angle, and mark the felt and wire sides. For combined board, mark the outer liner. Condition them in the atmosphere the standard specifies, and handle them by the edges, since skin oils change both results.

2

Read the water contact angle on each side

Lay the strip flat on the Dropometer stage, side under test up. Dispense a fixed drop of the same water you use for the Cobb test and read the angle at a fixed time, such as 5 s. Automatic edge and baseline detection gives the same reading from the same image.

3

Repeat and check against the pre check limit

Place several drops on fresh areas of each side and take the mean. Compare it with your pre check limit for that grade. A low mean flags that lot and side for the Cobb test first; it does not fail the lot on its own.

4

Run the Cobb test

On your Cobb tester, weigh the test piece, clamp it under the ring and wet the same face for the test period, such as 120 s for paper. Drain, blot with the roller and weigh it again, as the standard specifies. The mass gain per square metre is the Cobb value.

5

Pair the results and build the correlation

Plot the Cobb value against the angle for each lot and side of one grade. Add lots that passed and lots that failed. Set the pre check limit with a margin from that plot, not from a published table. See TAPPI T 458 for timed angle readings on paper.

6

Record and report each result by its method

Record the angle, reading time, drop volume and room conditions next to the Cobb value, its test period and the side tested. Report the Cobb value from the Cobb tester as the TAPPI T 441 result. The angle stays a pre check.

7

Watch the full measurement

This video shows a complete Dropometer contact angle measurement, from setup to result. It covers the steps above in one run.

Dropometer contact angle measurement, start to finish.

Validated measurement approach

Independent benchmarking and publication-based validation references.

Peer reviewed method

The Dropometer drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers, one in Review of Scientific Instruments and one in Colloids and Surfaces A. Published contact angle accuracy is ±0.35° (Dropometer product specification).

Read the validation papers

Customer evidence

We have no published client study that pairs the Dropometer with TAPPI T 441. A shared facility at Cornell University uses it for contact angle work, and a fibre lab there used it for water contact angle on lignin based fibre mats (Fibers, 2025).

Read the case study

Correlate the angle with your own Cobb data

TAPPI T 441 · Cobb value · paper, linerboard and box QC

TAPPI T 441 is a test method and sets no pass or fail Cobb value (source: ANSI/TAPPI T 441 om-24 listing). Your grade specification or your customer sets the limit. One published US limit applies to fibreboard boxes for hazardous materials: after a 30 min Cobb test, the outer surface may gain no more than 155 g/m² (source: 49 CFR 178.516). That rule names ISO 535 as the Cobb method.

No published table converts a contact angle into a Cobb value. The two do tend to move together. In one mill study on three paper machines, mean water contact angles of 76.3°, 93.2° and 101.7° went with mean Cobb60 values of 33.4, 29.3 and 28.1 g/m², but the link was not a straight line across all samples (source: Lopes et al., Tecnicelpa 2010).

So build your own correlation for each grade and each Cobb time. Take lots that passed and lots that failed, read the water contact angle on the same face at a fixed time, then run TAPPI T 441 as the standard specifies. Set a pre check limit with a margin, so lots near the line still go to the full Cobb test. Expect a weaker link for a 30 min test than for 120 s, since a longer test depends more on water moving through the whole sheet. Repeat the study when you change furnish, sizing agent, surface size or calender settings.

Example output

Example data for illustration only. These are not measured results. Your own grades, sizing and Cobb limits decide the numbers.

Example: kraft linerboard, three lots, felt and wire side, angle pre check and Cobb120

Lot and side Drops (n) Angle at 5 s (°) Pre check Cobb120 from Cobb tester (g/m²) Decision
Lot A, felt side5101.3Pass26.8Release
Lot A, wire side597.5Pass29.1Release
Lot B, felt side593.0Pass32.4Release
Lot B, wire side588.2Flag34.6Release on the Cobb result; close to both limits
Lot C, felt side584.6Flag39.5Hold the lot, check sizing
Lot C, wire side579.9Flag44.2Hold the lot, check sizing

How to read this table

Each angle is the mean of five drops read at 5 s on the Dropometer. Each Cobb120 value is the mean of five tests on a Cobb tester, not the Dropometer. The example pre check limit is 90° or higher at 5 s, set with a margin from this correlation. The example Cobb120 limit is 35 g/m² or lower, from an example customer specification. A lot is released only when both sides pass the Cobb limit.

Example output chart

The same example data as the table: Cobb120 from the Cobb tester for each lot and side, against an example Cobb120 limit.

Bar chart of example Cobb120 values for three kraft linerboard lots on the felt and wire sides, rising from 26.8 to 44.2 g/m², with Lot C over an example 35 g/m² limit

Example data, not measured results.

Business impact

What changes when a contact angle pre check sits next to the Cobb test

Metric Before Dropometer With Dropometer Results
First sign of a sizing problem After the Cobb test, blotting and the second weighing From a water drop read at 5 s on the side under test No published benchmark
Which lots and sides go to the Cobb tester first Tested in the order they arrive Flagged lots and sides tested first No published benchmark
Where the check runs Lab with balance, Cobb ring and roller At the machine, corrugator or converting line, on battery for up to 8 h Dropometer product specification
Reported TAPPI T 441 value Cobb tester Still the Cobb tester TAPPI T 441 scope

See the pre check on your own board

Bring two samples with known Cobb values: one that passed and one that failed. We will measure the water contact angle on both sides live.

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ROI estimate

Estimate the cost of the sizing failures a pre check can catch

Enter your own numbers. The defaults are examples, not Droplet Lab results.

Quality cost snapshot

Estimate what wetting related failures cost you.

Example only. Use what you see in a trial on your own samples.
Your own figure.
Your own estimate.

Result

~0
Estimated annual cost today
~0
Estimated annual cost avoided
~0
Payback period

Annual exposure combines audit/CAPA cost, customer rejections, and recall probability; avoided exposure applies the risk reduction percentage.

Method settings

Lock these before you pair contact angle and Cobb results

Parameter Recommended Setting Technical Rationale
Cobb test period As the standard specifies or as agreed, for example 120 s for paper Name it in every result, such as Cobb120. US labs often run 30 min on combined board (SGS-IPS). Values at different times do not compare.
Side tested Felt and wire side, each read for angle and Cobb Sizing and texture often differ between the two sides, so pair and report them separately.
Test water The same distilled or deionised water for both tests Impurities change the angle and can change uptake.
Drop volume Fixed for every drop A changing volume adds spread that is not about sizing.
Angle reading time Fixed for every drop, for example 5 s On paper the angle falls as water spreads and soaks in, so time must not vary.
Fit model Young-Laplace or polynomial, the same for every run Both models are in the Dropometer software. Do not mix them within one study.
Pre check limit Set per grade, side and Cobb time from your own data No published table converts a contact angle into a Cobb value.
Environment Conditioned room and water temperature as the standard specifies; record both The Dropometer has no temperature or humidity control and runs at 10 to 45 °C.

Is TAPPI T 441 the right test?

Pick the method by the question you need answered

Start: What do you need to know about the sheet and water?

Grams of water taken up per square metre

Signals:

Your specification, customer or a packaging rule asks for a Cobb value in g/m² at a set time.

Rule-out:

Use TAPPI T 441, this page, or ISO 535 on a Cobb tester. The Dropometer does not measure the Cobb value.

A fast sizing check before the Cobb test

Signals:

You want to flag a weak lot or side at the machine and decide which lots to test first.

Rule-out:

Read the water contact angle at a fixed time on the Dropometer and correlate it with your own Cobb data. It complements the Cobb test and does not replace it.

How a drop wets the surface over time

Signals:

You need ruling and writing quality, or the angle and absorbency over seconds.

Rule-out:

Use TAPPI T 458, where the Dropometer is fully compliant, or see TAPPI T 558 and ISO/TS 14778, where it runs part of the method.

Unsized or very absorbent paper

Signals:

Your grade is made to take up water, such as towel, tissue or blotting paper.

Rule-out:

TAPPI points to TAPPI T 432 for bibulous paper (SGS-IPS). Very absorbent paper can also take the angle below 10°, the bottom of the Dropometer range.

Decision tree at a glance

Start from the question you need answered. Blue boxes are measurements the Dropometer runs.

Decision tree for choosing between the TAPPI T 441 or ISO 535 Cobb test, a water contact angle pre check on the Dropometer, TAPPI T 458 timed contact angle and TAPPI T 432 for absorbent paper

Is TAPPI T 441 the right test?

Interpreting results

What the angle tells you about the Cobb value, and what it does not

Higher angle, usually lower Cobb: Good sizing holds the drop up and keeps water out of the sheet, so a high angle tends to go with a low Cobb value.
A trend, not a conversion: A mill study found the two move together but not in a straight line across all samples (Lopes et al., 2010). Correlate per grade and side.
Surface against the whole sheet: The angle reads the surface in seconds. The Cobb value weighs water taken into the sheet over the test period, so internal sizing and pores matter more.
A 30 min Cobb drifts further: The longer the test period, the more the result depends on water moving through the board. Expect a weaker link to a 5 s angle than at 120 s.
Expert perspective

Why a surface reading and a weighed Cobb value answer different questions

Not all contact-angle measurements are created equal: the right method depends on the application. A sessile drop and a tilted plate can give very different, and equally correct, numbers.

Dr. Alidad Amirfazli

Department of Mechanical Engineering, York University, Co-founder and Scientific Advisor, Droplet Lab

Common pitfalls and limits

Honest limits of the pre check and of the Dropometer

The Dropometer does not measure the Cobb value. Report TAPPI T 441 results from a Cobb tester only.
A correlation holds for one grade, side, sizing system and Cobb time. Rebuild it when any of them changes.
Very absorbent paper can take the angle below 10°, the bottom of the Dropometer range.
The camera runs at 10 frames per second. That suits a reading at 5 s, not millisecond absorption studies.
The Dropometer has no temperature or humidity control. Test in a conditioned room and record its conditions.
Rough, fibrous or corrugated surfaces give an apparent angle that reflects texture as well as sizing. Read on a flat area of the liner.
FAQ

Frequently asked questions

What does TAPPI T 441 measure?

It measures the water absorptiveness of sized paper, paperboard and corrugated board: the mass of water one square metre takes up in a set time, found by weighing a test piece before and after one face is wetted. The result is the Cobb value in g/m², written with its time, for example Cobb120.

Can the Dropometer run TAPPI T 441?

No. TAPPI T 441 weighs the water a sheet absorbs, and the Dropometer does not measure mass. It complements the test: it reads the water contact angle on the same side, which tracks sizing at the surface. Correlate that angle with your own Cobb results to flag weak lots early, and keep the Cobb tester for every TAPPI T 441 value you report.

What test time does TAPPI T 441 use?

The standard sets the test period, and longer or shorter periods can be agreed. US labs commonly run 120 s for paper and 30 min for combined board (source: SGS-IPS TAPPI T 441 page). Always write the time into the result, such as Cobb120 or Cobb1800, because values at different times do not compare.

What Cobb value do UN rated fibreboard boxes need?

US rules for fibreboard boxes that carry hazardous materials limit the outer surface to a gain of 155 g/m² in a 30 min Cobb test (source: 49 CFR 178.516). The rule names ISO 535 as the method. Otherwise your customer or grade specification sets the limit.

Is TAPPI T 441 the same as ISO 535?

Both are Cobb tests that weigh the water a sized sheet takes up over a set time, and both report g/m². They are separate documents, so details such as conditioning, test period and apparatus tolerances can differ. Quote the one your customer or regulation names. See ISO 535.

How does water contact angle relate to the Cobb value?

They usually move in opposite directions: better sizing gives a higher angle and a lower Cobb value. A mill study on three paper machines saw that trend, but not a straight line across all samples (source: Lopes et al., 2010). Build your own correlation for each grade, side and Cobb time before you use the angle as a pre check.

Which papers is TAPPI T 441 not meant for?

Unsized and absorbent grades, such as towel or blotting paper, belong to TAPPI T 432. The Cobb test is also not recommended as a sizing test for writing paper (source: SGS-IPS). For ruling and writing quality, see TAPPI T 458, where the Dropometer is fully compliant.

Legal note

This page summarises TAPPI T 441 in our own words and does not reproduce the standard. Buy the current edition from TAPPI or ANSI before you write a procedure. The Dropometer complements the workflow: it measures water contact angle, not the Cobb value. Droplet Lab does not certify compliance. Your own method validation does.

View the official standard Book a 15 minute demo

How this page was created

Sources, review and tools

Transparency 4 checklist items
01

Sources

ANSI/TAPPI T 441 om-24 listing (ANSI Webstore); TAPPI T 441 om-20 listing, superseded (ANSI Webstore); TAPPI T 441 scope and document history (GlobalSpec); TAPPI Standards Subject Index, November 2025, the Dropometer product specification and Droplet Lab validation papers.

02

Drafting assistance

Drafted with Claude (Anthropic) from the sources listed, then reviewed by Droplet Lab.

03

What we did not do

We did not copy text from the standard. Buy the current edition for the full procedure.

04

Last checked

October 2026

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References

1. ANSI/TAPPI T 441 om-24. webstore.ansi.org/standards/tappi/ansitappi441om24
2. TAPPI T 441 om-20 listing, superseded (ANSI Webstore). webstore.ansi.org/standards/tappi/tappi441om20
3. TAPPI T 441 scope and document history (GlobalSpec). standards.globalspec.com/std/14306172/tappi-t-441
4. TAPPI Standards Subject Index, November 2025. tappi.org/contentassets/0985484c9d6f4f29a05cafd59b7e5b31/standards_sub
5. Droplet Lab. Peer reviewed validation papers in Review of Scientific Instruments and Colloids and Surfaces A. dropletlab.com/validation/publications/
6. Droplet Lab. Dropometer product specification, 2026. dropletlab.com/products/dropometer/