Color fastness to rubbing is a key quality indicator for polypropylene (PP) weft yarn used in carpets, webbing, and woven goods, measuring how much color transfers from the yarn surface to a standard cotton cloth under dry and wet abrasion. This article outlines the sample preparation requirements for yarn forms, the applicable test standards such as ISO 105-X12, AATCC 8, and GB/T 3920, the step-by-step crockmeter procedure, and how grey scale ratings are interpreted for quality control and buyer compliance.
Technical Overview
Polypropylene (PP) weft yarn is a synthetic filament or staple yarn widely used as the filling (weft) component in carpet backing, rugs, webbing, ribbons, and industrial woven fabrics. Unlike conventionally dyed yarns, most PP yarns are dope-dyed (spun-dyed): pigment masterbatch is homogeneously dispersed into the polymer melt before extrusion, so the color is physically locked inside the fiber rather than held by dye-fiber bonds on the surface.
Because of this coloration method, PP weft yarn generally achieves excellent rubbing fastness. However, color transfer can still occur when pigment particles are poorly dispersed, when excessive spin finish or lubricating oils migrate to the yarn surface, or when low-quality masterbatch is used. The color fastness to rubbing test (also called the crocking test) quantifies this risk by rubbing the yarn surface against an undyed cotton cloth under controlled pressure, stroke length, and cycle count, then rating the degree of staining on a 1 to 5 grey scale.
- Dry rubbing: evaluates color transfer under ambient, dry contact conditions.
- Wet rubbing: evaluates color transfer when the cotton cloth is wetted, which accelerates pigment and finish migration.
- Typical commercial acceptance for PP weft yarn: grade 4 or higher (dry) and grade 3 or higher (wet), depending on the end use and buyer specification.
Testing Samples
Specimen Preparation for Yarn Forms
Because weft yarn is not a flat fabric, it must be converted into a testable surface before testing. Standard practice is to wind parallel lengths of yarn tightly and evenly onto a rigid card, template, or suitable frame so that the yarns are side by side, with no gaps or overlaps between adjacent strands. The prepared specimen must fully cover the rubbing area, which is at least 50 mm × 140 mm for a standard crockmeter stroke. Alternatively, a small knitted or woven mini-panel can be produced from the yarn to simulate a fabric surface, provided this is agreed upon between parties.
- Wind the yarn under consistent, moderate tension so the surface is flat and compressible but stable.
- Prepare at least two specimens: one for dry rubbing and one for wet rubbing.
- Avoid handling the test area directly, as skin oils can affect color transfer.
Conditioning Requirements
Specimens and rubbing cloths must be conditioned in a standard atmosphere before testing. For ISO and GB methods, condition at 20 ± 2 °C and 65 ± 4 % relative humidity until moisture equilibrium is reached (typically a minimum of 4 hours, or as specified). For AATCC methods, condition at 21 ± 1 °C and 65 ± 2 % relative humidity. Proper conditioning is essential because moisture content influences both the yarn surface behavior and the staining result, particularly for wet rubbing.
Rubbing Cloth Specifications
The rubbing cloth must be a standard undyed, bleached, desized, plain-weave cotton fabric free from fluorescent whitening agents, starches, and finishes, cut into squares of approximately 50 mm × 50 mm. Any deviation from the standard rubbing cloth can significantly alter the apparent staining grade and invalidate comparison between laboratories.
Testing Items
The core testing scope for polypropylene weft yarn includes the following items, each reported separately:
- Dry rubbing color fastness grade (grey scale for staining, 1 to 5).
- Wet rubbing color fastness grade (grey scale for staining, 1 to 5).
- Wet pick-up verification of the rubbing cloth where wet testing is performed.
- Optional instrumental color-transfer assessment using a spectrophotometer for objective, auditable results.
Testing Methods
ISO 105-X12 and GB/T 3920 (Crockmeter Method)
ISO 105-X12 is the internationally recognized rubbing fastness method, and GB/T 3920 is the technically equivalent Chinese national standard. A crockmeter rubs a white cotton cloth, mounted on a finger of 16 mm diameter, against the yarn specimen along a 104 mm track with a downward force of 9 N. The specimen is rubbed for 10 complete back-and-forth cycles in approximately 10 seconds. For wet testing, the cotton cloth is wetted with distilled or deionized water to 100 ± 5 % pick-up by weight immediately before mounting.
AATCC Test Method 8
AATCC 8 is the North American counterpart, using the same crockmeter geometry (16 mm finger, 9 N force, 10 cycles, 104 mm stroke). The principal difference is the wet pick-up requirement: AATCC 8 specifies 65 ± 5 % wet pick-up, which is lower than the ISO requirement. Consequently, wet rubbing results obtained under the two standards may not be directly interchangeable, and the applicable standard must be stated clearly on test reports.
Test Procedure
The general sequence of operations is as follows:
- Condition the wound yarn specimens and cotton rubbing cloths in the standard atmosphere.
- For the dry test: fix the specimen on the crockmeter base, mount a dry cotton square on the finger, and run 10 rubbing cycles.
- For the wet test: weigh a dry cotton square, wet it to the specified pick-up (100 ± 5 % for ISO/GB, 65 ± 5 % for AATCC), verify by reweighing, mount it, and run 10 rubbing cycles.
- Remove the rubbed cloths, allow wet cloths to air dry at room temperature or at a maximum of 60 °C without pressing, and recondition all cloths before evaluation.
- Rate the color transfer on each cloth against the grey scale, or measure instrumentally.
Rating and Evaluation
Staining is assessed with the grey scale for staining, where grade 5 means no visible color transfer and grade 1 means heavy transfer; intermediate half-step grades (4–5, 4, 3–4, 3, 2–3, 2, 1–2) may be assigned. For objective evaluation, the colorimetric method in ISO 105-A05 (or AATCC Evaluation Procedures) converts measured color difference into a staining grade, reducing operator subjectivity. For filament PP weft yarn with well-dispersed pigments, dry grades of 4–5 and wet grades of 4 are commonly achieved.
Factors That Influence Results
- Masterbatch quality: coarse pigment dispersion or incompatible carriers increase surface transfer.
- Spin finish content: excess oils and waxes can carry pigment to the surface and depress wet grades.
- Winding quality: loose, uneven, or overlapping yarn layers create non-uniform contact pressure.
- Yarn construction: twist level, crimp, and filament fineness change the real contact area of the rubbing finger.
- Wet pick-up accuracy: inaccurate wetting of the rubbing cloth is a leading cause of inter-lab discrepancies.
Applications
Rubbing fastness testing of PP weft yarn supports quality control and compliance across many sectors:
- Carpet and rug manufacturing: verifying that weft and backing yarns will not stain face fibers, adhesives, or flooring during tufting, coating, and use.
- Webbing, ribbon, and narrow fabric production: confirming color stability during high-speed weaving over guides, reeds, and take-up rollers.
- Outdoor furniture and upholstery: demonstrating resistance to color transfer onto light-colored clothing and cushion covers.
- Automotive interiors: meeting OEM specifications that typically demand grade 4 minimum for both dry and wet crocking.
- Sacks, FIBC, and geotextile fabrics: ensuring pigment systems remain stable under abrasive handling and field service.
- Trade compliance: providing third-party test reports for incoming inspection, supplier qualification, and export documentation.
FAQ
What grade is considered a pass for PP weft yarn?
Most commercial contracts require a minimum of grade 4 for dry rubbing and grade 3 for wet rubbing. Premium applications such as automotive interiors or light-colored floor coverings often require grade 4 in both conditions. The acceptance criterion should always be confirmed against the buyer's specification sheet.
Why test the yarn instead of the finished fabric?
Testing the yarn at the incoming or production stage detects masterbatch or finishing problems before weaving, coating, or shipment. Early detection avoids costly rework, claims, and downstream contamination of finished goods, and it allows spinning mills to qualify pigment lots before extrusion.
Why does polypropylene yarn usually show high rubbing fastness?
Because PP is dope-dyed, the pigment is distributed throughout the polymer matrix rather than sitting on the fiber surface. There is no dyestuff to bleed or wash off, so color transfer under rubbing is minimal unless poorly dispersed pigment or surface oils are present.
What should be done if the wet rubbing grade fails?
Investigate the spin finish application rate, verify pigment dispersion quality in the masterbatch, check masterbatch carrier compatibility with the PP resin, and confirm that the rubbing cloth wet pick-up was controlled correctly. Correcting finish levels and masterbatch quality usually restores wet grades to 4 or higher.
Are ISO, AATCC, and GB test results directly comparable?
Dry rubbing results are generally comparable across ISO 105-X12, GB/T 3920, and AATCC 8 because the apparatus and conditions are aligned. Wet results may differ systematically because ISO and GB require 100 % pick-up while AATCC requires 65 % pick-up, so the test standard used must always accompany the reported grade.