Polyester Staple Fiber Inspection

Polyester staple fibre inspection mainly conducts scientific testing and analysis on linear density, length and length deviation, breaking strength and elongation at break, crimping performance, defects and impurity content, moisture regain and oil content, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Linear density: Refers to the mass of the fibre per unit length, usually expressed in decitex (dtex) or denier (D). This is the core indicator for measuring the fineness specification of the polyester staple fibre, directly affecting the count of the subsequent yarn and the style of the fabric. During the testing, it needs to be carried out under standard atmospheric conditions, and calculated by determining the mass of a fixed-length fibre bundle.

Length and length deviation: Including the average length, length deviation rate, overlength fibre percentage and double-length fibre content. The fibre length and its uniformity are the key factors affecting the spinnability, yarn strength and evenness. It is usually measured with a roller-type or photoelectric length analyser.

Breaking strength and elongation at break: Reflects the maximum load borne by the fibre during the tensile fracture process (strength, unit: cN/dtex) and the elongation deformation ability at fracture (elongation, %). These two indicators are directly related to the durability and processing performance of the final textile, and are the basis for evaluating the mechanical properties of the fibre.

Crimping performance: Including the crimp number, crimp ratio, crimp recovery and crimp elasticity. The crimp gives the cohesion between the staple fibres, and is crucial for the smooth progress of the spinning process and the structural stability of the yarn. It needs to be tested with a crimp elasticity tester under specific light and heavy load conditions.

Defects and impurity content: Refers to the abnormal fibres, fused fibres, hard lumps, and the foreign substances such as oil stains and powder in the fibre. The defects and impurities will seriously affect the spinning efficiency and the fabric surface quality, and need to be quantitatively detected by manual visual inspection or a dedicated defect analyser.

Moisture regain and oil content: The moisture regain refers to the mass percentage of the moisture contained in the fibre, affecting the weight and electrical properties of the fibre. The oil content (finish content) refers to the mass percentage of the oil agent carried on the fibre surface. An appropriate oil content is conducive to improving the smoothness, antistatic property and cohesion of the fibre.

Specific resistance: An indicator for measuring the conductive property of the fibre. Polyester is a hydrophobic fibre and is prone to static electricity, so its mass specific resistance or volume specific resistance is an important parameter for evaluating its spinnability and wearing comfort, usually determined with a high-resistance meter.

Testing Scope

Conventional cotton-type and medium-length staple fibres: Applicable to the polyester staple fibres with a length between 30 mm and 51 mm and a linear density between 1.2 dtex and 2.2 dtex. This type of fibre is mainly used for blending with cotton and viscose to produce various woven and knitted fabrics, and is the main object of the inspection.

Wool-type staple fibres: Applicable to the coarser fibres with a length between 60 mm and 120 mm and a linear density between 3.0 dtex and 6.0 dtex. They are mainly used for blending with wool to produce wool-like suiting and blankets, and their inspection focuses more on the length, crimp and friction properties.

Staple fibres for sewing thread and filling: The fibres for sewing thread usually require high strength, high modulus and good abrasion resistance. The filling fibres (such as down-like cotton) pay more attention to the inspection of the linear density, compression resilience and special cross-section morphology.

Dope-dyed polyester staple fibres: Including the staple fibres coloured by the dope dyeing (pre-spinning colouring) and post-spinning dyeing. In addition to the conventional physical indicators, the colour difference, colour fastness (to light, washing and rubbing) and the fibre strength loss possibly caused by the addition of the colour masterbatch need to be key-tested.

Recycled polyester staple fibres: The staple fibres produced from recycled PET bottle flakes or waste textiles as the raw materials. During the inspection, special attention needs to be paid to the intrinsic viscosity (IV value), ash content, heavy metal residues, and the off-colour fibres and impurities that may be introduced due to the complex raw material sources.

Functional polyester staple fibres: Including the modified fibres such as antibacterial, flame retardant, UV resistant, far-infrared and cationic dyeable. In addition to the basic items, the effectiveness and durability of their functional indicators must be detected in accordance with the corresponding standards according to their claimed functions.

Fibres of different production processes: Covering the polyester staple fibres produced by the conventional melt direct spinning, chip spinning and different spinning speeds (conventional spinning, medium-speed spinning and high-speed spinning). Different processes may lead to the differences in the fibre structure (such as the crystallinity and orientation), which in turn affect their physical properties.

Testing Methods

Standard atmospheric conditioning and pre-conditioning: Before all the physical property tests, the samples must be fully conditioned under standard atmospheric conditions (such as temperature 20±2 ℃ and relative humidity 65±4%) to reach the moisture equilibrium. This is the primary prerequisite for ensuring the comparability and accuracy of the test results, executed in accordance with GB/T 6529.

Linear density determination (gravimetric method): The standard method is to cut a fixed-length (usually 10 mm or 20 mm) middle-section fibre bundle, count the number of fibres, weigh it on a precision balance, and calculate the average linear density according to the length, number and mass. A vibroscope fineness tester can also be used for rapid determination.

Length testing (roller-type/photoelectric length analyser method): The roller-type length analyser method is the classic method, combing the fibre bundle through the roller jaws, grouping and weighing by length, and drawing the length-weight distribution diagram. The photoelectric length analyser method uses the photoelectric scanning and computer processing to quickly obtain the various length indicators with higher efficiency.

Single fibre strength and elongation testing (constant-rate-of-extension strength tester method): An electronic single-fibre strength tester is used, and under the specified clamping distance and tensile speed, a single fibre is stretched until fracture, with the instrument automatically recording and calculating the breaking force, breaking strength, elongation at break and initial modulus and other data.

Crimping performance testing (crimp elasticity tester method): A single fibre is randomly drawn from the fibre bundle, hung on the instrument, and the light load (such as 0.002 cN/dtex) and heavy load (such as 0.1 cN/dtex) are successively applied, and then restored to the light load, measuring the length change of the fibre, and calculating the various crimp indicators accordingly.

Defects and oil content testing (Soxhlet extraction method): The defects are usually detected by the manual hand-picking method or the instrumental analysis method. For the oil content determination, the Soxhlet extraction method is mainly adopted, using an organic solvent (such as ether) to circularly extract the oil agent on the fibre surface, and calculating the oil content through the mass difference of the specimen before and after extraction. The result is accurate and reliable.

Specific resistance testing (high-resistance meter method): A fibre specimen of a certain mass is placed between the electrodes of a dedicated test box, a specified DC voltage is applied, the resistance value is measured with a high-resistance meter, and then converted into the mass specific resistance or volume specific resistance according to the specimen mass and packing density.

Testing Standards

Domestic Standards:

1. GB/T 14335, Testing method for linear density of man-made fibres — Staple fibres

2. GB/T 14336, Testing method for length of man-made fibres — Staple fibres

3. GB/T 14337, Testing method for breaking strength and elongation of man-made fibres — Staple fibres

4. GB/T 14338, Testing method for crimping performance of man-made fibres — Staple fibres

5. GB/T 14339, Testing method for defects of man-made fibres — Staple fibres

6. GB/T 14341, Testing method for oil content of man-made fibres — Staple fibres

7. GB/T 6503, Testing method for moisture regain of man-made fibres

International Standards:

1. ISO 5079, Textiles — Man-made fibres — Determination of breaking force and elongation at break of individual fibres

2. ISO 1973, Textile fibres — Determination of linear density — Gravimetric method and vibroscope method

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Electronic balance: Used for weighing the fibres and specimens, requiring high accuracy (at least 0.01 mg) and good stability. It is the basic equipment for almost all the items involving mass weighing, such as the linear density, moisture regain and oil content, and must be regularly calibrated.

Standard atmospheric constant temperature and humidity laboratory: Provides the temperature and humidity environment meeting the standard requirements, and is the necessary condition for carrying out the physical property tests of the fibres. The laboratory needs to be equipped with high-precision air conditioning, humidifying and dehumidifying systems and continuous monitoring devices, ensuring the constant environmental parameters.

Fibre length analyser: Including the mechanical roller-type length analyser and the advanced photoelectric/capacitive automatic length analyser (such as the USTER® AFIS). The latter can quickly and accurately provide multi-dimensional information such as the fibre length distribution, short fibre content and maturity, with a high degree of automation.

Single-fibre electronic strength tester: The core mechanical property testing equipment, adopting the constant-rate-of-extension (CRE) principle, equipped with a high-precision force sensor and elongation measuring device, with a high data acquisition frequency, and can carry out the tests in multiple modes such as the constant elongation and constant load.

Crimp elasticity tester and fibre specific resistance tester: The crimp elasticity tester is used to precisely measure the length change of the fibre under different loads. The fibre specific resistance tester (high-resistance meter) is equipped with a dedicated fibre test electrode box, and is used to evaluate the antistatic performance of the fibre.

Soxhlet extraction device and oven: The Soxhlet extractor is composed of a condenser, extraction tube and receiving flask, and is used for the oil content determination together with a constant temperature water bath. The forced-air oven is used to determine the moisture regain of the fibre (drying method) and to dry the utensils after the experiments.

Defect analyser and microscope: The automatic defect analyser can quickly classify and count the various defects in the fibre sample through the image recognition technology. The optical microscope (including the biological microscope and projection microscope) is used to observe the fibre morphology, cross-section, surface structure and identify the defect types.