Recycled Polyester Testing

Recycled polyester inspection mainly conducts scientific testing and analysis on fibre composition and content analysis, physical and mechanical property testing, colour fastness testing, thermal property and thermal stability analysis, hazardous substance and restricted substance screening, and intrinsic viscosity, 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

Fibre composition and content analysis: This is the basic item of the recycled polyester inspection, aiming to accurately determine the content of polyethylene terephthalate (PET) in the sample, and distinguish the proportion of the virgin polyester and the recycled polyester in it. For the blended products, the other fibre components (such as cotton and viscose) also need to be quantitatively analysed, to ensure the authenticity of the product label claims. It is the primary basis for judging the "recycled" attribute of the product.

Physical and mechanical property testing: Evaluates the durability of the recycled polyester in application. The core indicators include the breaking strength and elongation at break, abrasion resistance and anti-pilling performance. Since the molecular chains of the recycled materials may degrade during the melt spinning process, their strength is usually slightly lower than that of the virgin fibres, but they must meet the application requirements of the downstream textiles.

Colour fastness testing: The recycled polyester mostly originates from the coloured waste textiles, so its colour fastness is an important monitoring indicator. It mainly includes the colour fastness to washing, colour fastness to rubbing (dry rubbing and wet rubbing), colour fastness to perspiration and colour fastness to light. These tests simulate the daily use environment, ensuring the firm attachment of the dye molecules and avoiding the problems of fading or staining.

Thermal property and thermal stability analysis: The glass transition temperature (Tg), crystallization temperature (Tc) and melting temperature (Tm) of the recycled polyester are determined by the differential scanning calorimetry (DSC). Compared with the data of the virgin polyester, its crystallinity change and thermal history can be analysed. The thermogravimetric analysis (TGA) is used to evaluate its thermal decomposition temperature, judging the thermal stability of the material in processing and use.

Hazardous substance and restricted substance screening: To ensure the ecological safety of the product, the hazardous chemicals in the recycled polyester must be detected. This includes the detection of the heavy metals (lead and cadmium), prohibited azo dyes, phthalate plasticizers and alkylphenol ethoxylates (APEO) in accordance with the relevant regulations (such as OEKO-TEX® Standard 100 and the REACH regulation), preventing the cross-contamination in the recycling process.

Intrinsic viscosity (IV value) determination: The intrinsic viscosity is an important indicator characterizing the molecular weight and degree of polymerization of the recycled polyester. It is measured with a Ubbelohde viscometer in a specific solvent (such as the phenol/tetrachloroethane mixture). The IV value directly affects the spinnability, tensile strength and subsequent processing performance of the fibre, and is the key parameter for the quality grading and suitability judgement of the recycled materials.

Appearance quality and defect inspection: The appearance inspection of the recycled polyester filaments or staple fibres is carried out manually or by instruments (such as yarn fault graders). The main observation contents include the colour uniformity, oil stains, fused filaments, hard lumps, stiff filaments and neps and other defects of the fibre. The appearance quality directly affects the finished product rate and the final appearance of the fabric.

Testing Scope

Recycled polyester staple fibres: The main testing objects, usually made by the physical or chemical regeneration of the waste PET bottle flakes or textiles. The testing covers their linear density, length, crimping performance, oil content and spinnability evaluation, widely used in the fields of spinning, nonwovens and filling materials.

Recycled polyester filaments: Including the fully drawn yarns (FDY), pre-oriented yarns (POY) and draw textured yarns (DTY). The testing focus is on the uniformity of their physical and mechanical properties, dyeing uniformity, interlace degree and the package formation quality, used for making garment fabrics, bag fabrics and decorative fabrics.

Recycled polyester yarns and fabrics: The yarns, woven fabrics and knitted fabrics made of the pure or blended recycled polyester are tested. In addition to the fibre composition, the twist and strength of the yarns, and the wearing properties such as the grammage, dimensional stability, drape and air and moisture permeability of the fabrics also need to be tested.

Recycled polyester nonwoven materials: The recycled polyester nonwoven fabrics made by the processes such as thermal bonding, needle punching and spunlacing are tested. The key items include the mass per unit area, thickness, longitudinal and transverse strengths, bursting strength, filtration efficiency (such as for the filter media) and the liquid penetration time (such as for the wiping materials).

Recycled polyester filling materials: Such as the recycled polyester cotton and hollow recycled polyester staple fibres. The testing focus is on the bulkiness, compression resilience, warmth retention rate, odour and the migratability of the specific hazardous substances, ensuring their safety and comfort in the fields of garment filling, home textiles and toy filling.

Recycled polyester industrial yarns and products: Including the high-strength industrial yarns, conveyor belt yarns and ropes. The testing requirements are extremely high, focusing on the special properties such as the high strength, high modulus, low creep, fatigue resistance and aging resistance (such as UV resistance), to meet the harsh working conditions of the industrial fields.

Recycled polyester raw materials (bottle flakes and foam materials): Before entering the spinning process, the recycled PET bottle flakes and foam materials are tested. The main items include the intrinsic viscosity, melting point, moisture content, ash content, acetaldehyde content, colour (L, a, b values) and the impurity contents (metals and PVC), controlling the final fibre quality from the source.

Testing Methods

Quantitative analysis of the fibre composition (chemical method): In accordance with the standard GB/T 2910.11 (identical to ISO 1833-11), a 75% sulfuric acid solution is used to dissolve the non-fibrous substances or other specific fibres (such as viscose) in the recycled polyester, and the net dry mass percentage of the polyester is calculated by the mass difference subtraction method. It is the benchmark method for the composition arbitration.

Qualitative identification of the fibre composition (microscope method and combustion method): A biological microscope is used to observe the longitudinal surface morphology (smooth, with spots) and the cross-section shape (circular or polygonal) of the recycled polyester. Combined with the combustion method to observe the characteristics such as the melt shrinkage near the flame, black smoke and a special sweet smell during combustion, a rapid preliminary identification is carried out.

Infrared spectroscopy (FTIR) analysis method: The infrared absorption spectrum of the recycled polyester is obtained through a Fourier transform infrared spectrometer, and compared with the standard PET spectrum. Its chemical structure main body can be quickly and non-destructively confirmed as the polyester. This method can also assist in identifying whether other polymers are mixed in.

Differential scanning calorimetry (DSC): Under the nitrogen atmosphere, the heat flow change of the recycled polyester sample is measured by heating and cooling at a constant rate. By analysing the temperature and enthalpy of the obtained melting peak and crystallization peak, its crystallinity can be precisely calculated, and compared with the virgin material, evaluating its thermal history and degradation degree.

Intrinsic viscosity (IV value) determination method: Strictly in accordance with the GB/T 14190 or ASTM D4603 standard, the recycled polyester sample is dissolved in a mixed solvent of phenol and tetrachloroethane (1:1 by weight), and the outflow times of the solution and the pure solvent are measured with a Ubbelohde viscometer in a constant temperature bath at 25±0.1 °C, calculating its intrinsic viscosity value by the formula.

Standard physical property test methods: The breaking strength and elongation are tested in accordance with GB/T 14344 (single fibre) or GB/T 3923.1 (strip method); the abrasion resistance is tested in accordance with GB/T 21196 (Martindale method); and the colour fastness series testing follows the corresponding national standards such as GB/T 3921 (colour fastness to washing) and GB/T 3920 (colour fastness to rubbing).

Hazardous substance test methods: The heavy metal contents are usually determined by the inductively coupled plasma mass spectrometry (ICP-MS) or the atomic absorption spectrometry (AAS); the prohibited azo dyes are detected with a gas chromatography-mass spectrometer (GC-MS) for the carcinogenic aromatic amines reductively decomposed from them; and the phthalates are mostly analysed by the liquid chromatography-mass spectrometry (LC-MS/MS).

Testing Standards

Domestic Standards:

1. GB/T 3923.1, Textiles — Tensile properties of fabrics — Part 1: Determination of breaking force and elongation at breaking force using the strip method

2. GB/T 3920-2024, Textiles — Tests for colour fastness — Colour fastness to rubbing

3. GB/T 3921-2008, Textiles — Tests for colour fastness — Colour fastness to washing with soap or soap and soda

4. GB/T 3922-2013, Textiles — Tests for colour fastness — Colour fastness to perspiration

5. GB 18401-2010, National general safety technical code for textile products

6. GB/T 14190, Testing method for intrinsic viscosity of fibre grade polyester chips

International Standards:

1. ISO 105, Textiles — Tests for colour fastness

2. ISO 13934-1, Textiles — Tensile properties of fabrics — Part 1: Determination of maximum force and elongation at maximum force using the strip method

3. ISO 1833-11, Textiles — Quantitative chemical analysis — Part 11: Mixtures of polyester and certain other fibres (method using sulfuric acid)

4. ASTM D4603, Standard Test Method for Determining Inherent Viscosity of Poly(Ethylene Terephthalate) (PET) by Glass Capillary Viscometer

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

Fibre composition analysis system: The core equipment includes the Soxhlet extractor (for removing the oils), constant temperature shaking water bath (for the chemical dissolution), analytical balance (with an accuracy of 0.0001 g) and vacuum oven. Through the precise chemical treatment and weighing, the system completes the quantitative analysis of the fibre composition.

Electronic single-yarn strength tester: Used to test the breaking strength and elongation at break of a single recycled polyester fibre or yarn. The instrument has a constant tensile speed, precision force sensor and elongation measuring device, and can automatically record and calculate the various mechanical indicators. It is the key equipment for evaluating the spinnability and durability of the fibre.

Colour fastness tester series: Including the washing fastness testing machine (such as the SW-12), rubbing fastness tester, perspiration fastness tester and xenon arc lamp light fastness tester. These equipment can simulate and intensify the various colour fastness test conditions, and the objective rating is carried out with the standard grey scales or a colour measurement instrument.

Thermal analysers: The differential scanning calorimeter (DSC) and thermogravimetric analyser (TGA) are the core instruments for studying the thermal properties of the recycled polyester. The DSC is used to measure the phase transition temperature and enthalpy, and the TGA is used to analyse the thermal weight loss process. The combination of the two can comprehensively evaluate the thermal stability, decomposition behaviour and the possibly contained impurities of the material.

Ubbelohde viscometer and constant temperature water bath: Composed of a precision Ubbelohde viscometer, high-precision constant temperature water bath (with a temperature control accuracy of ±0.1 °C), timer and matching glassware. This is the standard configuration for determining the intrinsic viscosity (IV value) of the recycled polyester, and the accuracy of its measurement results is directly related to the judgement of the polymer molecular weight.

Spectroscopy and chromatography-mass spectrometry instruments: The Fourier transform infrared spectrometer (FTIR) is used for the rapid structure identification; the inductively coupled plasma mass spectrometer (ICP-MS) and the atomic absorption spectrometer (AAS) are used for the trace heavy metal analysis; and the gas/liquid chromatography-mass spectrometry (GC/LC-MS/MS) is the tool for the qualitative and quantitative analysis of the organic hazardous substances.

General pre-treatment equipment of the laboratory: Including the precision pH meter (for preparing the test solutions), sample shredder, Soxhlet extraction device, ultrasonic cleaner and the pipettes, volumetric flasks and other glassware of various specifications. The standardized sample pre-treatment is the prerequisite for ensuring the accurate and reliable results of the subsequent instrumental analysis.