Knit Quality Testing: What to Measure in Polyester
A practical look at pilling, spirality, colour fastness, dimensional stability and bursting strength testing for polyester knits.
A polyester knit's behaviour in the lab matters as much as its weight and composition on paper. For a B2B buyer, quality tests are a shared language that predicts how a fabric will perform through the garment's life. The tests below are the ones most often requested for performance knits; results are usually graded against ISO or AATCC standards.
Pilling
Pilling is the way short fibre ends ball up under abrasion into small surface fuzz. The test is especially important for polyester because the fibre's high strength can keep pills anchored on the surface rather than shedding. Abrasion is applied for a set number of cycles, typically by Martindale or the ICI pilling box, and the result is graded on a photographic scale from 5 (no pilling) to 1 (severe pilling).
Spirality / torque
In single-bed (single jersey) structures, yarn twist tends to relax, causing seam lines to skew sideways after laundering; this is spirality. After the sample is washed and dried, the percentage of seam displacement is measured. Balanced twist and correct heat-setting are the main ways to keep spirality under control.
Colour fastness (wash / rub / light)
- Wash fastness: the sample is laundered with standard detergent; colour change and staining of an adjacent white are rated on the grey scale (1-5).
- Rub fastness (crocking): the transfer of colour to another surface is measured dry and wet; critical for dark shades and digital prints.
- Light fastness: the sample is exposed to controlled light and fading is rated on the blue wool scale (1-8); important for outdoor and curtain-type products.
Dimensional stability and shrinkage
The fabric is marked, washed and dried, and the change in width and length is calculated as a percentage. Knit structures are more prone to shrinkage because they lack a warp-weft balance. Correct heat-setting and relaxation keep shrinkage within tolerance; most performance products target a narrow percentage band.
Bursting strength
Knits are tested for bursting rather than tensile breaking strength: the fabric is inflated by a diaphragm or pushed by a steel ball until it ruptures, and the pressure (kPa) is recorded. This predicts the durability of multidirectionally stressed zones such as elbows and knees. A natural difference is expected between a light single jersey and a heavy interlock.
A practical tip for buyers
Before approving bulk production, define the target tests and acceptance criteria in the technical specification; the standard number (ISO/AATCC), method and pass level should be written together. This clarity reduces both lab cost and downstream disputes.
In depth: the standard test matrix
Knowing the names of the core tests is not the same as making a technical specification unarguable. The matrix below maps each property to its actual standard number, measuring principle and reported unit, collapsing a lab request into a single line and making comparison objective. The key point for knits is that, although most standards cover both woven and knitted fabrics, the method parameters (load, test area, end-of-test criterion) must be chosen for the knitted structure.
| Property | Standard | Method / end-of-test | Reported unit |
|---|---|---|---|
| Abrasion | ISO 12947-2 | Martindale, Lissajous figure; for knits end-point = first hole; load 9 kPa (apparel) or 12 kPa (upholstery/workwear) | Number of cycles |
| Pilling | ISO 12945-2 | Modified Martindale; 125/500/1000/2000/5000/7000 rub stages; photo scale | Grade 5 (none) → 1 (severe) |
| Bursting strength | ISO 13938-1 (hydraulic) / -2 (pneumatic) | Diaphragm inflation; standard knit area 50 cm² (Ø79.8 mm), alt. 7.3/10/100 cm² | Pressure (kPa) + distension (mm) |
| Dimensional change | ISO 6330 + ISO 3759 + ISO 5077 | 6330 wash/dry programme, 3759 marking, 5077 measurement and calculation | Width/length % change |
| Wash fastness | ISO 105-C06 | Standard detergent + steel balls, multifibre reference strip; grey scale | Colour change & staining 1–5 |
| Rub fastness | ISO 105-X12 | Crockmeter, dry and wet rub; staining grey scale | Dry/wet 1–5 |
| Light fastness | ISO 105-B02 | Xenon-arc, exposed alongside blue wool references | Blue wool grade 1–8 |
| Mass (GSM) | EN 12127 | Conditioned small-sample mass/area; ISO 139 atmosphere | g/m² |
| Spirality | ISO 16322-2 | Post-wash diagonal / inverted-T marking of skew angle | % spirality / skew |
The most frequently missed detail in abrasion is the load: ISO 12947-2 prescribes 9 kPa for apparel and 12 kPa for upholstery/workwear, so the same fabric tested under the wrong load returns a misleadingly different cycle count. Because knits are more open than wovens, the end-point is taken as the appearance of the first hole rather than thread rupture; for that reason 20,000 cycles on a knit cannot be graded the same way as 50,000 cycles on a tight woven.
In bursting, the choice of test area directly drives the result: measuring the same fabric over 7.3 cm² instead of 50 cm² yields a higher kPa, simply because the smaller area contains fewer weak points. Writing only 'burst ≥ X kPa' is therefore insufficient; the test area (preferably 50 cm² for knits) and the method (hydraulic 13938-1 or pneumatic 13938-2) must be fixed together.
Dimensional stability rests on a trio rather than one standard: the specimen is marked to ISO 3759, processed through a selected ISO 6330 wash-and-dry programme (a defined combination of water temperature, mechanical action and drying type), and the result is calculated to ISO 5077. Because the same fabric can give markedly different percentages between 40°C line-dry and 60°C tumble-dry, the target percentage must always be written together with the programme number.
In the fastness series, C06 offers several wash-severity variants (A1S, B2S and so on); the variant code defines the temperature (A=40°C, B=50°C, C=60°C …), the number of steel balls (1 = none, 2 = one ball) and whether bleach is present (S = standard, M = with bleach) — so 'C06 ≥ 4' alone is an incomplete criterion and the variant code must be stated too. Colour change is graded on the grey scale and staining of the adjacent multifibre strip separately; for dark, bright disperse shades the wet rub (X12 wet) is usually the hardest line item. A specification written to this depth also pins down mass (EN 12127, a more correct choice for knits than ISO 3801's woven scope) and spirality (ISO 16322-2) with the same discipline.