Our coated fabric shows uneven gloss - should we change the formulation?

Not before the variation pattern is recorded. Gloss unevenness is normally a uniformity problem rather than a formulation problem, and the pattern tells you which uniformity problem it is. A difference across the width points at pick-up and the coating head; a drift along the length points at viscosity, solids or line speed; patches point at contamination or the substrate; and a repeating interval points at a roller.

The gloss differs from side to side but the dry film weight looks acceptable - how?

Because gloss is a surface property and film formation depends on the drying profile, not only on how much compound was applied. Where the fabric dries faster or slower across the width, the film forms with a different surface micro-texture and the gloss varies even where the pick-up is acceptable. Check the drying profile alongside the pick-up, and verify mass per unit area under GB/T 4669 as well.

Two rolls of the same article look different and the coating settings were identical - why?

The base fabric is the usual answer. Absorbency, heat history, weave or knit structure and any residual auxiliary from an earlier finishing step all change how evenly the compound wets out, so the same settings produce a different surface. Run the suspect base fabric against a retained sample of known-good base fabric through the same coating condition: if the difference follows the base fabric, the coating line is not the cause.

The difference repeats at regular intervals along the roll - what does that mean?

That it is very likely a machine cycle rather than a random defect. Measure the interval and write it down, then look at anything in the line that turns or advances at that distance — a roller, a drive element or a guide. Chasing a repeating interval as contamination wastes time, because contamination does not return at a regular spacing.

How many spots across the width should we inspect to call a roll acceptable?

Enough to see a gradient rather than a single value, and always at both selvedges plus the centre at a minimum. A single inspection position cannot distinguish an article-wide problem from a side-to-side gradient, and those two verdicts lead to completely different actions on the line.

Coating Gloss Unevenness: Reading the Variation Pattern Before Changing the Recipe

For fabric development, quality control and process teams Prepared by the Technical Department, Suzhou TTK Textile Technology Group Co., Ltd. The process and testing statements below come from TTK's own capability record and commissioned testing scope.

Bottom line

Coating gloss unevenness is usually a uniformity problem, not a recipe problem: read the variation pattern across the width first.

1. Classify the pattern, and the cause shortens to one or two candidates

The variation pattern is the strongest evidence available, and it costs nothing to collect. Cut or mark the fabric at the point of inspection, and record where the difference sits.

Pattern	What it looks like on the roll	Cause axis it points at
Across the width only	One side glossier than the other, with a smooth gradient or a step	Coating pick-up uniformity — coating head gap, roller condition, cross-width tension, selvedge effect
Along the length, drifting	Gloss changes gradually from the start of the roll to the end	Viscosity or solids drift in the pan, solvent evaporation, temperature drift, line speed drift
Random patches	Isolated dull or glossy areas with no repeatable position	Contamination, foam in the compound, substrate absorbency variation, particles or gels in the coating
Repeating at a fixed interval	The same mark returning at a regular distance	A machine-cycle cause — roller damage, an engraved roller pattern, a drive or gear cycle
Edges only	Bands along both selvedges with the centre correct	Selvedge effect — edge drying, edge tension, coating running to the edge, trimmed-edge heat history
Centre only	The middle differs while both edges agree	Roller crown or parallelism, or a drying profile that peaks at the centre

Two patterns with the same appearance have different remedies. A side-to-side gradient is a coating head question; a repeating interval is a roller question. Recording the pattern before adjusting anything is what stops a week of blind changes.

2. Why uneven gloss reads as uneven colour

Gloss is a surface property, and a visual inspector reports it as a shade difference. What is actually happening:

The dry film weight varies across the fabric, so the surface roughness that produces gloss is different in different places.
The film formation happens at a particular point in the drying profile. Where the fabric dries faster or slower, the film forms differently, and the surface ends up with a different micro-texture even where the compound is identical.
Surface migration of a component to the film surface changes the gloss without changing the bulk formulation.
The base fabric itself carries different heat histories and different absorbency, so the same pick-up produces a different surface.

The practical consequence: "the coating looks different" and "the coating is different" are two separate statements. A gloss difference on the surface can exist on fabric where the dry film weight is acceptable, which is why a recipe change is so often the wrong instrument.

3. Check in this order, and only one variable at a time

Step	What to verify	Why it comes here
1	Reproducibility and position of the pattern	If the pattern moves between rolls, the cause is process drift; if it stays in the same place, the cause is more likely mechanical
2	Whether the base fabric lot is the same	A lot change moves absorbency and heat history, and it invalidates every later comparison
3	Dry film weight across the width, with mass per unit area verified under GB/T 4669	Distinguishes a pick-up problem from a drying problem
4	Coating head and roller condition — gap setting, parallelism, surface wear	The clearest mechanical cause of a side-to-side gradient
5	Viscosity and solids content over the run	Explains a lengthwise drift; the pan is a live variable, not a fixed one
6	Drying profile and the point of film formation	Gloss is set when the film forms, so this is where a cross-width gloss difference is usually born
7	Line speed and tension	Both move the drying time and the pick-up at the same time
8	Cooling and winding condition	A roll wound warm continues to change after the inspection point

Change one variable per trial and keep a marked swatch from each trial. A trial that changes temperature and speed together produces a result nobody can act on, even when the gloss improves.

4. When the cause is the substrate rather than the coating

Two mills can run the same recipe and get different gloss, because the coating is a reaction between the compound and the surface it lands on:

Absorbency variation — the same pick-up reads differently on a fabric that wicks faster.
Heat history — fabric that has been through a different number of earlier drying passes at a different temperature behaves differently in the coating drying zone.
Lot-to-lot variation in the base fabric — yarn, weave or knit structure and finishing residue all change how evenly the compound wets out.
Residual auxiliary from an earlier finishing step — a softener or a repellent that has not been fully fixed can interfere with wetting and show up as a patchy gloss.
Edge condition from an earlier process — pin or clip marks that arrive already heat-set produce a gloss difference at the edges that the coating line did not create.

Test this cheaply: run the suspect base fabric against a known-good retained sample of base fabric through the same coating condition. If the gloss difference follows the base fabric, the coating line is not the problem.

5. Containment, so the next roll does not repeat it

Keep a retained swatch of an accepted roll for each article, taken from the same lot, and keep it in the same conditions as the inspection point.
Inspect across the full width, not one spot. A full-width observation is what turns a complaint into a pattern.
Log the process parameters per roll, not per machine, so a drift can be attributed to the roll that carried it.
Hold the lot line. Mixing base fabric from two lots inside one production run guarantees a gloss verdict that cannot be traced.
Record the drying condition with the roll number, because gloss is set in the drying zone and the record is the only way to compare two runs later.

6. Common pitfalls

Pitfall	What it looks like	Cause
Changing the recipe before classifying the pattern	Several formulations tried, and the gloss difference remains	A uniformity problem was treated as a formulation problem
Adjusting two variables at once	The gloss improves and nobody knows why, then it returns	The trial could not be attributed to a single cause
Judging gloss from one spot per roll	A side-to-side gradient is reported as an article-wide problem	The inspection position was not varied across the width
Ignoring a base fabric lot change	One lot of the same article looks different from the previous lot	Absorbency and heat history changed with the lot
Treating a repeating interval as random	Roller damage is chased as a contamination problem	The interval was never measured
Comparing under different lighting	Two inspectors disagree about the same roll	Gloss and shade verdicts depend on the light source and the viewing angle
No retained swatch from the approved roll	Every complaint becomes an argument	There is no same-lot reference to judge against

7. Working order for a gloss complaint

Record the pattern — where it sits across the width and along the length, and whether it repeats.
Confirm the base fabric lot is the same as the approved reference.
Verify the dry film weight across the width before touching the formulation.
Check the coating head and roller condition if the pattern sits across the width.
Check viscosity and solids drift if the pattern drifts along the length.
Check the drying profile if the pattern has no mechanical explanation.
Change one variable per trial, and keep a marked swatch from each trial.
Re-verify mass per unit area under GB/T 4669 after any change that alters pick-up, so the fix does not open a second problem.

Criteria Table: How to Tell Whether a Gloss Complaint Was Actually Diagnosed

Criterion	What Passing Looks Like	Common Failure
The variation pattern is recorded before anything is changed	The record states whether the difference sits across the width, along the length, in patches, at a fixed interval, or at the edges	The machine is adjusted immediately, and the evidence that would have shortened the diagnosis is destroyed
The pattern is classified against a cause axis	The record names the cause axis the pattern points at, such as pick-up uniformity, drying, contamination or a machine cycle	Every gloss difference is treated as the same defect, and the same wrong remedy is applied each time
The position of the pattern is checked across the full width	Several positions across the width are inspected, and a gradient is recorded as a gradient	One inspection spot per roll is used, and a side-to-side difference is reported as article-wide
Whether the pattern repeats is measured	A repeating interval is measured and the interval is written down	A machine-cycle cause is chased as a contamination problem
The base fabric lot is confirmed	The lot of base fabric is recorded and compared with the approved reference	A lot change goes unnoticed, and every later comparison is invalid
Dry film weight is checked across the width	Pick-up is verified across the width, with mass per unit area under GB/T 4669	The formulation is changed while the actual cause is an uneven pick-up
The comparison uses a retained same-lot reference	A retained swatch from an accepted roll of the same lot exists and is used	The complaint is argued from memory, and no verdict can be reproduced
The viewing condition is fixed	Gloss and shade are judged under one light source and one viewing angle for both judging rounds	Two inspectors disagree about the same roll because the conditions differ
One variable is changed per trial	Each trial changes a single variable and keeps a marked swatch from that trial	Several settings move together, and the fix cannot be attributed or repeated
The drying condition is recorded with the roll	The drying profile used for the roll is recorded against the roll number	Gloss is set in the drying zone, and there is no record to compare two runs against
The coating head condition is monitored	Gap setting, parallelism and surface wear are checked and recorded	A mechanical cause is treated as a formulation cause and never found
Mass per unit area is re-verified after the fix	The finished article is re-checked under GB/T 4669 after any change to pick-up	A fix for gloss opens a mass deviation that reaches the buyer

Coating Gloss Unevenness: Reading the Variation Pattern Before Changing the Recipe