Coating in Bag fabric: Range, Tolerance and How to Test It

Two rolls of bag fabric can carry the same coating and behave differently, because the figure that matters is not the nominal value but where the process sits inside the tolerance.

What coating actually controls in bag fabric

Coating weight is the real water and abrasion story, not the base denier.

The number is easy to quote and hard to hold, and the difference between those two things is what you are actually buying.

For Bag fabric, the working range is PU 1-3 coats, 20-80 gsm total. Inside that band the choice is commercial; outside it, the product changes category and so does the price.

What PU 1-3 coats, 20-80 gsm total does not tell you

Specifying the ceiling is only justified when the end product carries a warranty that would be expensive to honour. Otherwise it is cost without a customer-visible benefit.

The figure that decides the outcome is not coating itself but the tolerance around it. A supplier who quotes PU 1-3 coats, 20-80 gsm total without a tolerance has not quoted a specification, and the first lot will define the tolerance for you.

How to measure coating

The reference method here is Tear strength, run to ASTM D2261, with an acceptance of >= 40 N for 1680D. The method is the easy part; sampling and conditioning are where reports diverge.

Condition the sample before measuring. Results in this family move by 10-20 % between a dry laboratory and a humid one, and two labs can disagree on the same material without either being wrong.

Record the measured value rather than pass or fail. A marginal pass on coating tells you the process is running at the edge, which is the information you need before the next lot rather than after it.

Coating does not stand alone

Coating moves with abrasion (>= 25 000 Martindale) and with tear strength (>= 40 N (1680D)). Tightening one usually costs money in another, and a supplier who accepts every tolerance without comment has either not understood the interaction or is planning to substitute.

If a quotation is significantly cheaper, the difference is usually in one of these neighbours rather than in the figure being discussed.

The clause to write for coating

The line below is the version we can quote against without inventing anything. Coating appears with a tolerance and a test, not as a standalone number.

1680D ballistic polyester, 2-pass PU with primer, 45 gsm coating, 1500 mm hydrostatic head, 25 000 Martindale, C0 DWR, wet rub >= grade 3, Pantone 19-4005 TCX

A specification without a test method is a preference. It becomes a requirement when the method and the acceptance value are both named, because then a substitution is visible.

When coating goes wrong

Coating crack at folds. White lines at the gusset fold after a few months of use.

The cause is usually too heavy a single coating pass, or a coating with low elongation.

The test that catches it: Flex test 5000 cycles at the fold, then inspect and re-test hydrostatic head.

What to specify instead: Two lighter passes, elongation-compatible coating, and a fold design that avoids a sharp crease.

Note that the fix is a specification change, not a negotiation. By the time the claim is made, the only remaining lever is who pays for it.

Four questions about coating, before you order

1. What tolerance do you hold on coating, and what did your last three lots measure against it?
2. Which method do you use for tear strength, and which laboratory signs the report?
3. When coating falls outside PU 1-3 coats, 20-80 gsm total, do you rework, downgrade, or ship and hope?
4. Which of abrasion or tear strength would you move first to hit a lower price?

The answers are more useful than a certificate, because they show whether the process is controlled or merely inspected. Full ranges for bag fabric are on the product page, and the measurement regime is described on the quality control page.

Failure modes worth a clause in the purchase order

Claims rarely come from a batch that was obviously wrong. They come from a base construction figure that sat inside the quoted band and outside what the product needed.

The other two, with acceptance numbers attached, are in the full failure analysis for bag fabric. A supplier who cannot name the mode he sees most often is describing a catalogue, not a process.

Seven lines that turn a quotation into a specification

Most post-delivery arguments are not about quality. They are about a document that never said which figure would be measured, on which instrument, at whose cost.

1. Coating: PU 1-3 coats, 20-80 gsm total is the market band. Fix the figure you will test at goods-in, and the instrument you will test it with.
2. Abrasion (>= 25 000 Martindale): fix the value per shipment, and state who pays for the re-test if a lot lands outside it.
3. Tear strength: >= 40 N (1680D) is not an answer on its own. Ask for the last three lots’ actual readings and the spread behind them.
4. Water repellency: agree the test method with the number. Two labs, two methods, two answers, one argument.
5. Deviation rule: anything outside the agreed bands is reported before shipment, not after arrival.
6. Packing and marking: shipping mark, roll or carton count and the tolerance on quantity, since a short shipment is a claim too.
7. Who inspects: third-party, in-house or none, decided in advance, with the AQL level written down.

Related reading

• Abrasion in Bag fabric
• Tear strength in Bag fabric
• Bag Fabric for Outdoor and tactical