Polypropylene weft yarn knot strength testing evaluates the tensile performance of the knot joints that connect yarn ends during winding, spooling, and weaving operations. This test is critical for weaving mills, yarn manufacturers, and quality-control laboratories that need to minimize weft breakage, maximize loom efficiency, and guarantee consistent fabric quality. This article covers the testing principles, sample preparation, applicable standards, test procedures, and practical applications of knot strength evaluation for polypropylene weft yarns.
Technical Overview
Polypropylene (PP) weft yarn is widely used in the production of woven sacks, geotextiles, carpet backing, and industrial fabrics due to its low density, high chemical resistance, and excellent abrasion performance. During package winding and weaving, broken or exhausted yarn ends must be rejoined, and the resulting knot joint becomes the weakest point in the yarn path. A poorly tied or inherently weak knot can slip or rupture under loom tension, causing weft breaks, machine stops, and fabric defects such as missing picks and flaugs.
Knot strength testing measures the breaking force of a yarn specimen containing a standardized knot positioned at the midpoint of the test length. The results are reported as the absolute breaking force (in cN or N) and as knot strength retention, also called knot efficiency, which is the ratio of the knotted specimen breaking force to the breaking force of the same yarn without a knot. Because polypropylene has a relatively smooth, low-friction surface and moderate tensile modulus, knots in PP yarn are more prone to slippage than knots in cotton or polyester spun yarns, making systematic strength verification essential.
Testing Samples
Knot Types
The most common joint configurations evaluated in practice include the weaver's knot (a sheet-bend family knot preferred for its small tail and clean passage through the shed), the fisherman's knot and double fisherman's knot, the square or reef knot, and air-spliced joints used as knot-free alternatives on automatic winders. Each configuration produces a different stress concentration at the joint, so the knot type must be recorded with every test result.
Yarn Forms
Testing can be performed on polypropylene multifilament weft yarn, PP spun staple-fiber yarn, and PP tape or fibrillated yarn used in circular and flat weaving of sacks and geotextiles. Linear densities typically range from 300 denier to 2500 denier for filament weft and from 400 dtex to 1400 dtex for tape yarn, and the test report must state the nominal linear density because knot efficiency is normalized against yarn tenacity.
Sample Preparation
- Condition all specimens at 20 ± 2 °C and 65 ± 4 % relative humidity for at least 24 hours in accordance with ISO 139 or ASTM D1776.
- Draw specimens randomly from at least five packages per lot to obtain a representative sample.
- Tie the designated knot at the geometric center of each specimen using consistent technique and uniform tail length, typically 3 to 5 mm.
- Reject specimens with visibly damaged, loose, or double-struck knots before testing, and record the number of rejections.
- Handle specimens by the ends only to avoid weakening the yarn or disturbing the knot geometry before loading.
Testing Items
A complete knot strength evaluation of polypropylene weft yarn includes the following measured and derived parameters:
- Breaking force of the knotted specimen, expressed in cN or N, recorded at the moment of joint failure.
- Breaking force of the parent yarn without a knot, tested under identical conditions as the reference value.
- Knot strength retention or knot efficiency, calculated as knotted breaking force divided by unknotted breaking force, multiplied by 100 percent.
- Extension at break of the knotted specimen, expressed in millimeters or percent.
- Failure mode classification, distinguishing knot slippage, yarn rupture at the knot, and yarn rupture away from the knot.
- Statistical indicators including mean value, standard deviation, and coefficient of variation across all valid specimens.
Testing Methods
Testing is performed on a constant-rate-of-extension (CRE) electronic single-yarn tensile tester equipped with pneumatic or precision mechanical clamps lined with rubber or cork pads to prevent jaw slippage of the smooth polypropylene filament surface. The reference procedures are ASTM D2256 for tensile properties of yarn by the single-strand method and ISO 2062 for determination of single-end breaking force and elongation at break; the Chinese national standard GB/T 3916 is technically aligned with ISO 2062 and is widely used in domestic laboratories.
- Gauge length: 500 mm as the standard setting, or 250 mm when package length is limited, keeping the length consistent for paired knotted and unknotted tests.
- Pretension: 0.5 cN/tex (0.05 gf/den), applied to remove slack without disturbing knot seating.
- Extension rate: 500 mm/min for a 500 mm gauge, or the equivalent rate specified by the standard for the measured elongation class of the yarn.
- Specimen count: a minimum of 20 valid breaks, with 30 to 50 recommended for reliable statistical confidence.
- Knot positioning: the knot is centered between the clamps so that both yarn legs bear equal load; asymmetric placement increases the risk of pre-mature leg rupture.
After mounting, the specimen is extended to rupture and the peak force, extension, and failure mode are recorded automatically. Knot efficiency is then calculated for each specimen and averaged. Failures by knot slippage must be counted and reported separately, because a high slippage rate indicates that the joint form or yarn finish is unsuitable even when the average breaking force appears acceptable. For air-spliced joints, the same procedure applies and the results are compared against benchmark knot data to quantify the strength advantage of splicing.
Applications
- Incoming quality inspection of polypropylene weft yarn lots at weaving mills, verifying that joints made by supplier winders meet contractual strength retention levels.
- Optimization of knotting technique and operator training in winding and preparation departments, comparing knot types, tail lengths, and tightening forces.
- Feasibility studies for replacing knots with air splicers on automatic winding machines, using measured strength retention and splice failure rates as decision criteria.
- Loom tension setting and shed geometry optimization, since verified knot strength data define the safe upper limit for weft insertion tension.
- Product development of PP yarn finishes, twist levels, and tape geometries, where knot efficiency serves as an indicator of inter-fiber cohesion and frictional behavior.
- Third-party testing and trade dispute resolution, providing standardized evidence of joint quality between yarn suppliers and fabric producers.
FAQ
What level of knot strength retention is acceptable for polypropylene weft yarn? For most weaving applications, a knot efficiency of at least 60 percent is considered workable, and well-tied weaver's knots in PP filament yarn typically achieve 60 to 75 percent. High-speed shuttleless looms with aggressive weft insertion generally demand the upper end of this range.
Why do polypropylene knots often fail by slippage rather than breakage? Polypropylene has a smooth, hydrophobic, low-friction surface, so the normal pressure within the knot generates less frictional grip than in cotton or wool yarns. Insufficient knot tightening, short tails, and slippery spin finishes further reduce grip and promote slippage.
Which knot type gives the highest strength in PP weft yarn? The weaver's knot and the double fisherman's knot generally outperform the square knot because their nipping structure clamps both legs securely. The best choice balances strength retention with the small knot size required to pass cleanly through the shed and reed.
Can the same method be used for air-spliced joints? Yes. Spliced specimens are tested identically, and the splice strength retention, typically 80 to 95 percent of parent yarn strength, is compared against knot benchmarks to justify splicer adoption.
How many specimens should be tested? A minimum of 20 valid breaks is required by the standards, but 30 to 50 specimens per condition are recommended because knot strength data show higher variability than plain yarn tensile results.