ISO 3781 wet tensile strength test for paper and board | UnitedTest Paper & Board Wet Tensile Testing Machine

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ISO 3781 Paper and Board Wet Tensile Strength Tester | UnitedTest

UnitedTest manufactures ISO 3781 compliant testing machines for paper and board wet tensile strength after water immersion. Measure wet tensile strength and optional wet tensile retention for paperboard packaging and paper product labs.

ISO 3781 Paper and board defines the laboratory method to determine tensile strength after immersion in water. The test outputs wet tensile strength in kN/m and optional wet tensile strength retention percentage. It is applicable to most paper and board grades, with soaking duration adjustable as agreed by testing parties. UnitedTest supplies precision wet tensile test systems for paper mills, packaging manufacturers and third-party material testing laboratories.

Test Principle

A test piece of the paper or board is immersed in water for an agreed period, then pulled to rupture in a tensile tester. The maximum tensile force per unit width measured on the saturated specimen is the wet tensile strength. If the dry, conditioned tensile strength of the same sample is also measured, the ratio of wet to dry is reported as wet tensile strength retention.

The underlying physics: dry paper strength comes largely from hydrogen bonds between cellulose fibres, which water destroys. Wet tensile strength therefore measures how much strength survives wetting — i.e. how effective the fibre bonding, refining, or wet‑strength resin system (UF, MF, PAE, etc.) actually is.

Key definitions

Wet tensile strength (σ_wet) — maximum tensile force per unit width a water‑soaked test piece withstands before breaking; expressed in kN/m.
Wet tensile strength retention (σ_R) — ratio of wet tensile strength to the tensile strength of another test piece from the same sample in the dry, conditioned state; expressed in %.

Test equipment required for ISO 3781 wet tensile strength test for paper and board

Tensile Testing Machine	Recommend UnitedTest tensile testing machine with a constant Elongation Rate. Must comply with ISO 1924‑2 (20 mm/min constant elongation rate) or ISO 1924‑3 (100 mm/min constant elongation rate) for paper tensile measurement.
Soaking water	Distilled or deionized water following ISO 14487, held at the conditioning temperature defined in ISO 187.
Clamps/grips	As specified in ISO 1924 — grip firmly across the full width without slip or damage, aligned with the force axis. (Pneumatic grips with adjustable pressure are the industry‑preferred solution for wet strips, because clamp pressure strongly affects results.)
Specimen cutter	For preparing strips of the specified width (15 mm per ISO 1924).

Test Specimen Information:

Specimen width: dry‑state width of 15 mm, prepared following ISO 1924‑2 or ISO 1924‑3 according to the chosen elongation speed.
Quantity:

Minimum 10 valid replicates for machine direction (MD) and 10 valid replicates for cross direction (CD) for wet tensile measurement.
Extra 10 MD and 10 CD dry specimens are required if calculating wet tensile strength retention.

Key Test Parameters & Stipulations

Extension rate	20 mm/min (ISO 1924-2) or 100 mm/min (ISO 1924-3) — one must be chosen and recorded
Specimen width b	15 mm (= 0.015 m) — the dry width is used in the calculation
Soaking liquid	Distilled/deionised water (ISO 14487) at ISO 187 conditioning temperature
Immersion method	Suitable shallow dish; soak until saturated
Default soaking time	1 h (in the absence of any other agreement)
Extended soaking	Board and hard‑sized papers may need 24 h or longer; time may be chosen by agreement to simulate a particular end‑use
Highly absorbent papers	Only the centre portion is wetted; the parts held by the clamps stay dry (see ISO 12625‑5)
Post‑soaking handling	Remove from dish, lightly blot surplus water, test immediately
Result reporting	Wet tensile strength to three significant figures, in kN/m

Test Procedure of ISO 3781 wet tensile strength test for paper and board

Select/prepare specimens per ISO 1924‑2 or ‑3 and condition if required (ISO 187).
Choose and agree the soaking time — normally 1 h; up to 24 h+ for boards/hard‑sized papers, or a time simulating the intended end use.
Immerse the test pieces in the shallow dish filled with distilled/deionised water at the specified temperature, until saturated.

For very absorbent papers, wet only the central zone and keep the clamped ends dry.

Remove one test piece at a time; lightly blot off surplus surface water.
Immediately clamp and test it in the tensile tester at the chosen constant rate of elongation (20 or 100 mm/min), following ISO 1924‑2 or ‑3. Handling speed matters — a wet strip begins to dry unevenly within seconds.
Repeat for the remaining pieces until 10 valid results in MD and 10 in CD are obtained. (Invalid results — e.g. break at/in the clamp or slip — must be discarded and replaced.)
If retention is required, test 10 additional dry, conditioned specimens in MD and 10 in CD under the same ISO 1924 part.
Calculate mean forces, wet tensile strength, and (optional) retention.

Industry Fields / Applications

Any grade that must survive contact with water or high humidity, either in use or during processing:

Packaging — multiwall paper sacks, carrier bags, liquid/milk cartons, deep‑freeze and moist‑food packaging, meat and fruit wraps, corrugated components for humid or cold‑chain logistics.
Tissue and hygiene — hand towels, wipes, facial tissue, napkins (tested via ISO 12625‑5 rather than ISO 3781, but the same property).
Specialty papers — banknote/currency paper, map paper, poster paper, labels (incl. bottle and horticultural labels), laundry tags, filter paper, tea‑bag and coffee‑filter paper, photographic/blueprint paper, wallpaper base, abrasive and overlay papers, electrical insulating paper.
Decorative and industrial laminates — decorative base paper, impregnation/saturating papers (impregnation itself is a wet process, so the sheet must not disintegrate).
Process control at the mill — verifying wet‑strength resin (UF, MF, PAE, glyoxal) dosage, cure state and uniformity; checking that the resin has fully developed before shipment.
R&D / sustainability — benchmarking new wet‑strength chemistries, and balancing wet strength against repulpability/recyclability, since highly cross‑linked wet‑strength papers are harder to re‑pulped.

Related standard:

ISO 1924-2	Paper and board - Determination of tensile properties - Part 2: Constant rate of elongation method (20 mm/min)
TCVN 1862-2	Paper and board. Determination of tensile properties.Part2:Constant rate of elongation method(20mm/min)
BS 4415-1	Determination of the tensile properties of paper and board.-Constant rate of loading method
ISO 1924-3	Paper and board - Determination of tensile properties - Part 3: Constant rate of elongation method (100 mm/min)
GB/T 22898	Paper and board. Determination of tensile properties. Constant rate of elongation method(100 mm/min)
ISO 12625-4	Tissue paper and tissue products — Part 4: Determination of tensile strength, stretch at maximum force and tensile energy absorption
ASTM D828	Standard Test Method for Tensile Properties of Paper and Paperboard Using Constant-Rate-of-Elongation Apparatus
ASTM D829	Wet tensile test for paper and board (saturated with water) – companion to D828 for moisture‑exposed applications.
ISO 3781	"Paper and board — Determination of tensile strength after immersion in water"
ISO 12625-5	"Tissue paper and tissue products — Part 5: Determination of wet tensile strength"
GB/T 465.2	Paper and board - Determination of tensile strength after immersion in water
ANSI/TAPPI T494	Tensile properties of paper and paperboard (using constant rate of elongation apparatus)
AS/NZS 1301.448	Methods of test for pulp and paper - Tensile strength of paper and paperboard (constant rate of elongation method)

Importance of the Test for Paper / Board Materials

Most untreated paper loses most tensile strength once wetted because water weakens hydrogen‑bonding between cellulose fibres. Adding wet‑strength chemicals improves water‑resistance, yet performance varies with formulation, soaking duration and material grade.

Quantify real‑world wet‑state mechanical performance: Simulate service conditions where paper/board encounters water or high humidity.
Evaluate wet‑strength additive effectiveness: Wet tensile retention percentage is a key metric to judge how well wet‑strength treatments preserve mechanical integrity under water immersion.
Support product specification and quality control: Provides objective numerical data for material comparison, batch‑to‑batch consistency check, and contract‑quality acceptance between suppliers and buyers.
Predict failure risk in humid service environments: Avoid premature tearing or collapse of packaging and functional paper‑board products exposed to moisture or liquid contact.

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Tensile test fixture for plastics is based on the test standatd ASTM D638, determination of the tensile properties of unreinforced and reinforced plastics in the form of standard dumbbell-shaped test specimens when tested under defined conditions of pretreatment, temperature, speed.

Q1. What is ISO 3781?

ISO 3781 is an international test method published by ISO (TC 6/SC 2) that specifies how to determine the wet tensile strength of paper and board after immersion in water for a defined period. The current edition referenced here is ISO 3781:2011 (3rd edition, 2011-09-01), which replaced ISO 3781:1983 and added precision data in Annex A.

Q2. What exactly does it measure?

Two values:

Wet tensile strength (σ_wet) — the maximum tensile force per unit width that a water-saturated test piece withstands before breaking, expressed in kN/m.
Wet tensile strength retention (σ_R) — optional; the ratio of wet tensile strength to the dry, conditioned tensile strength of the same sample, expressed in %.

Q3. Why is it called a "wet strength" test if it is just a tensile test?

Because it is a standard tensile test performed on a saturated specimen. Dry paper strength comes mainly from hydrogen bonds between cellulose fibres; water breaks those bonds, so a normal sheet retains only about 3–10 % of its dry strength when soaked. Measuring tensile strength after soaking is therefore the direct way to quantify how much strength the sheet actually retains — i.e. its wet strength.

Q4. How is ISO 3781 different from a normal dry tensile test (ISO 1924-2 / -3)?

The tensile step is identical (same apparatus, same 15 mm strip, same constant rate of elongation). ISO 3781 adds the immersion step, the blotting step and the "test immediately" requirement, and adds the wet-strength / retention calculation. It also changes the conditioning rule: conditioning is not necessary when soaking is 1 h or longer.

Q5. What is the difference between wet strength and sizing?

They are often confused. Sizing controls how fast and how much water is absorbed (e.g. Cobb, Hercules). Wet strength is the mechanical strength the sheet still has once it is wet. A heavily sized sheet can still fall apart when saturated, and an unsized sheet can have excellent wet strength if it contains wet-strength resin. Only a wet tensile test separates the two.

Q6. Which materials can be tested to ISO 3781?

In principle all paper and board, provided an appropriate soaking time is agreed between the interested parties. Typical users: paper sacks and carrier bags, liquid/food packaging board, labels, map and poster paper, filter paper, tea-bag and coffee-filter paper, banknote paper, wallpaper base, overlay/decorative base paper, photographic and blueprint paper, electrical insulating paper, abrasive paper.

Q7. Can I test tissue paper or kitchen towel to ISO 3781?

No. ISO 3781:2011 explicitly excludes tissue paper and tissue products, plus other lightweight, highly absorbent papers that are difficult to handle or very weak when wet. Use ISO 12625-5 instead. (Note that ISO 3781's own definitions of wet tensile strength and retention are adapted from ISO 12625-5.)

Q8. Can corrugated board be tested?

ISO 3781 itself does not exclude it, but as with ISO 1924-2/-3 the practical approach is to test the components (liner, fluting) rather than the combined board. The soaking time for such materials is usually extended.

Q9. What size is the test specimen?

Prepared per ISO 1924-2 (20 mm/min) or ISO 1924-3 (100 mm/min) — a strip of 15 mm width, with the length/test span defined by that standard (ISO 1924-2 uses an 180 mm span; ISO 1924-3 a 100 mm span). The dry width (15 mm = 0.015 m) is used in the calculation.

Q10. How many test pieces do I need?

10 valid wet results in machine direction (MD) and 10 in cross direction (CD) — minimum 20.
If wet tensile strength retention is also required, add 10 dry specimens in MD and 10 in CD — 40 in total.
Invalid results (slip, break in/at the clamp) must be discarded and replaced.

Q11. What about very absorbent papers?

The standard recommends wetting only the centre portion of the test piece and keeping the parts held by the clamps dry (see ISO 12625-5). This prevents the specimen tearing at the clamp line.

Q12. Can one machine cover ISO 3781, ISO 1924-2/-3, TAPPI T 494 and ISO 12625-5?

Yes — a single constant-rate-of-elongation universal testing machine with the right load cell, pneumatic grips, software methods and (optionally) a Finch-type device can cover the whole family. The key is pre-programmed test methods that lock the correct speed, span, specimen width and calculation, so operators cannot mix them up.

Q13. My wet tensile results are much lower than expected. Why?

Common causes: soaking not complete (but note: incomplete wetting usually gives higher results); slip in the grips; specimen damage at the cut edge (a tiny nick acts as a stress concentrator); testing too slowly after blotting so the sheet partially dries or, conversely, over-blotting; resin not yet cured; wrong extension rate; specimens not representative of MD/CD.

Q14. What wet strength retention value is "good"?

There is no universal pass/fail in ISO 3781 — the value depends on grade and end use. As industry rules of thumb: untreated paper retains only about 3–5 % (up to ~10 %) of its dry strength; paper is generally considered a "wet-strength paper" above about 15 %; commercial wet-strength resin systems (UF, MF, PAE) typically deliver 20–50 %; some speciality grades exceed 50 %. Always set the limit in the product specification, not in the test method.

Q15. How does ISO 3781 relate to TAPPI T 456?

TAPPI/ANSI T 456 "Tensile breaking strength of water-saturated paper and paperboard (wet tensile strength)" is the North American equivalent. Same soak–pull–record principle, but it includes tissue, permits the Finch cup partial-wetting device, and uses TAPPI T 494 for the tensile step. Results are not automatically interchangeable with ISO 3781.

Q16. Why does wet strength matter commercially?

Because it is a direct predictor of field performance: a towel that disintegrates in the hand, a sack that bursts in the rain, a label that slides off a chilled bottle, a banknote destroyed in a washing machine — all are wet-strength failures. The test also drives mill process control (resin dosage, cure, formation), new grade development, and the trade-off between wet strength and repulpability/recyclability.

Q17. Why choose ISO 3781 Wet Tensile Strength Tester for Paper and Board from UnitedTest?

UnitedTest manufactures ISO 3781‑compliant wet tensile strength tester for paper and board. Measure wet tensile strength & wet tensile retention according ISO 3781:2011, ISO 1924‑2, ISO 1924‑3 for paper‑making lab, packaging quality control.

UnitedTest is a professional manufacturer of paper & board material testing instruments, supplying complete testing solutions that fully comply with ISO 3781:2011 Paper and board‑Determination of tensile strength after immersion in water standard.

Our paper wet tensile strength tester supports both 20 mm/min and 100 mm/min constant elongation rate as defined in ISO 1924‑2 and ISO 1924‑3, to measure wet tensile strength and calculate wet tensile strength retention for various paper and board samples.

This ISO 3781 test machine is widely used in paper mills, packaging board factories, R&D laboratories, third‑party QC inspection institutes, for evaluating wet‑strength additive performance, raw‑material incoming inspection, finished‑product quality verification of water‑resistant paper, packaging board, special industrial paper.

Note: ISO 3781 excludes tissue paper and tissue products; UnitedTest also supplies compatible testing solutions for ISO 12625‑5 tissue wet tensile test upon customer request.

Key machine features for ISO 3781 test

Adjustable test speed: 20 mm/min /100 mm/min for ISO 1924‑2 / ISO 1924‑3 compliance
High‑precision force sensor, outputs wet tensile strength in kN/m, auto‑calculates wet tensile retention percentage
Supports MD & CD direction test data recording separately
Compatible with standard 15 mm width paper‑board test strips
Stores test data, generates full test report matching ISO 3781 report requirements
Can be combined with external soaking dish for specimen water‑immersion procedure

UnitedTest provides one‑stop service: instrument supply, operation guidance for ISO 3781 test procedure, after‑sales support for paper‑board quality‑control labs worldwide. Contact us for quotation and technical specification sheet for ISO 3781 wet tensile testing machine.

We offer customization to meet your specific needs. Our expert team will collaborate with you to develop the perfect product for you