RU2854485C1 - Method for identifying fibres - Google Patents

Abstract

FIELD: analytical equipment.

SUBSTANCE: invention relates to a method for identifying fibres. Method comprises placing the fibre on a substrate and thermally impacting it and determining the type of fibre by comparison with reference data. Thermal impact is carried out by irradiating a section of the fibre with focused continuous laser radiation with a wavelength of 380-450 nm and a power of not more than 5 mW for 0.1-2 s.

EFFECT: simplifying the process of fibre identification, reducing time and increasing the safety of analysis, as well as ensuring the possibility of reliable identification of microquantities of fibrous material.

5 cl, 3 dwg, 5 tbl

Description

The invention relates to the field of analytical technology, in particular to methods for identifying materials, and can be used to identify various types of fibers, for example, in forensic science when analyzing trace amounts of material evidence, in the textile industry for monitoring the composition of raw materials and finished products, in materials science for studying the properties of fibrous materials, as well as in express analysis and quality control.

A number of methods for identifying fibers are known from the prior art:

1. GOST R 56561-2015. The standard provides for the identification of fibers using the open flame combustion method (visual and organoleptic methods of odor analysis, flame appearance, and residues are used for identification).
Flaws:

• Danger: Risk of fire and burns when working with open flame.

• Sample destruction: requires macro quantities of material (more than 0.1 g), which is not applicable for microtraces.

• Limitations: Does not differentiate between synthetic fibers with similar combustion characteristics (e.g., polyester and nylon).

2. Patent CN 101187635 (Raman spectroscopy). Fiber composition is determined using Raman spectra without sample destruction.
Flaws:

• Complexity of mixture analysis: overlapping spectra of components makes interpretation difficult.

• Sensitivity to contamination: dust or dyes distort the signal.

• Limited depth of analysis: does not reveal the internal structure of multilayer fibers.

3. US Patent 9719853 uses laser-induced emission spectroscopy for elemental analysis of fibers from plasma radiation.
Flaws:

• Destructiveness: high laser power (more than 100 mW) damages the sample.

• Complex calibration: requires reference samples for each fiber type.

• Equipment: bulky systems with vacuum chambers.

• Limitations: No data on the identification of organic fibers.

4. Patent RU 2312325 (Laser microanalyzer). Performs fiber analysis with a picosecond laser (5–5000 ps) and plasma spectrum recording.
Flaws:

• Expensive: Picosecond lasers and high-resolution spectrometers add to the cost.

• Technical complexity: requires precise beam focusing and vibration protection.

• Preparation time: time-consuming adjustment of parameters for different materials.

• Risk of overheating: Even short pulses can char fine fibres.

5. Patent RU 2085914 (Pulse lamp). Identification of fibers by luminescence excited by a pulsed xenon lamp.
Flaws:

• Low sensitivity: detection limit ≥1% synthetic fibers in the mixture.

• Photobleaching: Multiple pulses destroy the dyes.

• Interference: background luminescence from impurities (e.g. bleaches).

• Power consumption: the lamp requires a powerful power source (more than 500 W).

The method for identifying textile fibers described in patent CN 114993873 A (published September 9, 2022) was selected as the closest analogue (prototype). The prototype method involves detecting a sample using a coupled method of thermogravimetry and Fourier transform infrared spectroscopy (TGA-FTIR). During analysis, the sample is heated in the oven of a thermogravimetric analyzer according to a predetermined temperature program, recording its mass change (TG curve) and the first derivative of the mass change (DTG curve). Simultaneously, gaseous products released during thermal decomposition of the sample are fed through a heated interface into the gas cell of a FTIR spectrometer, where their IR absorption spectra are recorded. The fiber type is determined through a comprehensive analysis of the obtained data: characteristic mass loss temperatures on the TG/DTG curves and the identification of the released gases based on their IR spectra.

Disadvantages of the prototype:

Despite the ability to identify a wide range of fibers, the prototype method has a number of disadvantages that limit its application, especially for rapid analysis or micro-quantity analysis:

• Complexity and high cost of equipment: implementation of the method requires expensive and sophisticated equipment – a coupled TGA-FTIR system, including a high-precision thermogravimetric analyzer, a Fourier transform infrared spectrometer, and a special thermostatted interface for gas transfer. This makes the method accessible mainly to specialized laboratories.

• Sample size requirement: TGA methods typically require sample masses in the order of milligrams to obtain reliable data. This is unacceptable when working with trace amounts of fibres (e.g., single fibres in forensics), which may be in the microgram or fractional microgram range.

• Duration of analysis: carrying out a full analysis cycle using the TGA-FTIR method, including heating the sample according to the program and recording data, takes a significant amount of time (from tens of minutes to an hour), which is not suitable for rapid identification tasks.

• Stationarity: the bulkiness and complexity of TGA-FTIR equipment precludes the possibility of creating a simple, compact and portable device for analysis outside the laboratory (for example, at the scene of an incident or during incoming inspection of raw materials).

The above mentioned disadvantages prevent the use of the prototype method for solving problems of fast, safe, economical and simple identification of micro-quantities (trace quantities) of fibers.

The following essential features have been identified for the claimed invention in common with the prototype: a method for identifying fibers, including placing the fiber on a substrate and thermal exposure to it, followed by an evaluation of the result of the interaction to determine the type of fiber.

The technical problem that the claimed invention is aimed at solving is the creation of a simple, safe, economical and fast method for identifying microquantities (weighing from 0.00002 g to 0.1 mg) of various types of fibers that does not require complex stationary equipment and allows for rapid analysis, including outside the laboratory.

The technical result achieved by using the invention is a simplification of the fiber identification process, an increase in the safety of analysis (due to the elimination of open flame and the use of low-energy laser radiation, which is safe for the operator if basic safety precautions are observed), a reduction in equipment costs (the possibility of using widely available and inexpensive diode lasers), a reduction in analysis time to several seconds, and the provision of the possibility of reliable identification of microquantities of fibrous material (weighing from 0.00002 g).

The stated technical problem is solved, and the specified technical result is achieved due to the fact that in the method for identifying fibers, including the placement of the fiber and thermal action on it, followed by an evaluation of the result of the interaction to determine the type of fiber, according to the invention, short-term (0.1-2 seconds) irradiation of a section of the fiber with focused continuous laser radiation with a wavelength in the visible range of 380-450 nm and a power not exceeding 5 mW is used as a thermal action, and the type of fiber is determined by direct visual observation (using magnifying devices, such as a magnifying glass or microscope) and analysis of the nature of the interaction of the fiber with the said laser radiation, including at least one of the following features: the nature of melting, color change, smoke formation, the nature of combustion, and a comparison of the observed nature of the interaction with reference data for known types of fibers, and the type of fiber is determined by sampling gaseous products of the interaction and analyzing their composition using instrumental methods (for example, gas chromatography or mass spectrometry).

The study analyzed 12 fiber types and their combinations. Examples include polyester, nylon, silk, wool, and acrylic.

Detailed description of the method:

The method is carried out as follows (see Fig. 1).

1. Sample preparation and placement: the fiber 2 to be tested (or a fragment thereof) weighing approximately 0.00002 g or more (up to approximately 0.1 mg) is placed on an inert (non-flammable) substrate 1 (e.g., a glass slide, metal, or ceramic plate) or secured in a holder (e.g., microtweezers) . No special sample preparation is required.

2. Focusing the laser beam and irradiating the sample: Laser 3 is turned on, and its beam 4 is directed and focused on the fiber 2 being studied. Depending on the fiber's nature, its behavior varies. A continuous laser source in the visible range is used, preferably a diode laser 3 with a wavelength in the range of 380-450 nm, with a preferred optimum wavelength of approximately 405 nm.

Justification for the wavelength range of 380–450 nm

Table 1. Experimental data on radiation absorption by different fibers.

Fiber type Peak absorption (nm) Absorption coefficient at 405 nm (%) Polyester 400–420 85 Nylon 390–410 78 Cotton 380–400 65 Wool 410–430 70

Conclusion: The 380–450 nm range was chosen because it exhibits maximum absorption for most organic and synthetic fibers. For example:

• Polyester: The absorption peak at 405 nm (85%) is due to the aromatic groups in the composition.

• Wool: absorption at 420 nm (70%) is due to disulfide bonds in keratin.

• Cotton: cellulose absorbs in the UV region (380 nm), but at 405 nm the absorption coefficient is sufficient for heating (65%).

Experiment: When irradiating polyester with a 450 nm laser (5 mW power), the melting time increases to 1.5 seconds (versus 0.5 seconds at 405 nm). For wool, charring begins after 3 seconds at 380 nm (versus 2 seconds at 405 nm). This confirms the optimal range of 380–450 nm.

Justification for choosing the wavelength of 405 nm.

The choice of the optimal wavelength of 405 nm is due to the good absorption of radiation by many colored and some uncolored organic polymers, in contrast to longer-wavelength IR radiation, which can pass through some fibers without interaction at low powers, the availability and low cost of such lasers, as well as the possibility of visual control of the beam position 4.

Reasons for the effectiveness of 405 nm:

• High absorption by organic polymers (absorption coefficient for polyester: ~85%, for nylon: ~78%).

• Minimal reflectivity in the UV-visible range (less than 10%).

• Low photon energy (3.06 eV) eliminates photochemical degradation of fibers.

Inefficiency 765 nm:

• Absorption coefficient for polyester: ~5%.

• To achieve a power density of 255 W/cm² at 765 nm, a power of 25.5 W would be required, which is unacceptable for safety reasons.

• Polyester: Heating to 60°C (not hot enough to melt). Fig. 3 shows a micrograph of polyester fibers irradiated with a 765 nm laser.

• Nylon: Temperature in the impact area is 55°C.

• Cotton/Wool: No visible changes.

Conclusion: The 765 nm wavelength does not provide sufficient absorption for thermal effects at safe powers (no more than 5 mW).

Table 2. Experimental data on the efficiency of interaction with different fibers:

Wavelength (nm) Polyester Nylon Cotton Wool 405 Melting in 0.5 sec Melting in 0.3 sec Charring in 1 sec Charring in 2 seconds 532 Partial melting Slight heating No effect No effect 650 No effect No effect No effect No effect 765 No effect No effect No effect No effect

Laser beam (405 nm, less than 5 mW) is focused using a simple lens (e.g., a short-focus single lens or a low-magnification objective) to a spot of up to 50 µm on the selected section of fiber 2. This ensures safe localized heating of the fiber for visual analysis of melting/charring. The focusing spot size is selected to ensure sufficient power density for thermal action. Focusing the beam to 50 µm allows for the analysis of individual fibers weighing 0.00002–0.1 mg.

Laser beam focusing parameters:

• Lens/Objective Type: Uses a 10mm achromatic lens (e.g. Thorlabs AC254-010-A-ML).

Rationale: Achromatic lens minimizes chromatic aberration, providing sharp focusing in the visible range.

• Focus spot size: The spot diameter is calculated using the formula:

d=2⋅λ⋅f / π⋅D,

where: λ = 405 nm, f = 10 mm, D = 3 mm (diameter of the collimated beam at the lens entrance).

Substituting the values: d=2⋅405⋅10 ⁻⁹ ⋅0.01 / π⋅0.003≈8.6 μm.

Actual spot diameter taking into account diffraction and errors: 10–50 µm.

• Power density in the spot: for laser power P=5 mW and spot diameter d=50 µm

Spot area = π⋅ (50⋅10 ⁻⁶ / 2) ² ≈1.96⋅10 ⁻⁹ m ² = 1.96⋅10 ⁻⁵ cm ² .

Power density = 5⋅10 ⁻³ / 1.96⋅10 ⁻⁵ ≈255 W/cm ² .

This value is sufficient for local heating and melting of most polymer fibers.

• Power control less than 5 mW:

The laser module is equipped with a built-in photodiode and feedback circuitry, ensuring power stability in the 3–5 mW range (with an accuracy of ±2%). Calibration is performed using a reference power meter (e.g., Ophir Vega).

Irradiation is performed for a short time, typically 0.1 to 2 seconds. The exact duration depends on the fiber type and thickness, as well as the degree of focusing, and is selected to allow time for the characteristic signs of interaction to appear.

For example, a single 0.00003 g polyamide fiber (20 µm diameter) melts completely in 0.5 sec, forming a ball with a diameter of 50 µm.

Methodology for selecting the exposure time:

• Fixed intervals: For the initial analysis, a time of 1 sec is used. If no effects are observed, the time is increased to 2 sec.

• Adaptive algorithm: when working with unknown samples, a microscope with a camera is used. The operator stops irradiation at the first signs of interaction (melting, smoke).

• Example for polyamide:

- 0.1 sec: start of fiber softening.

- 0.3 sec: formation of a melt ball.

- 0.5 sec: complete melting.

3. Observation and analysis of the interaction result: During and immediately after irradiation, observe (visually, using a magnifying glass or microscope if necessary) the nature of the fiber's interaction with the laser radiation. For example, a ×5 magnifying glass (e.g., Carson MicroBrite) or an Olympus CX23 microscope (×40 magnification) provide visualization of the exposure zone. Illumination of the exposure zone with white light (5000 K LED backlight) is mandatory. Observation and analysis are performed using known methods, including those described in GOST R 56561-2015.

The following signs (one or more) are recorded:

• Nature of melting: whether melting occurs; melting speed; shape of the resulting melt (ball, drop, uneven mass); color of the melt; hardness of the solidified melt; whether sintering occurs without obvious melting. Example: nylon forms a light ball, polyester - dark with black smoke.

• Color change: does the color of the fiber in the irradiation zone change (darkening, discoloration, appearance of a specific color).

• Smoke formation: presence or absence of smoke; smoke color; intensity of smoke formation.

• Odor (organoleptic control): if smoke is present, its odor can be assessed (requires operator caution and experience).

• Combustion: whether the fibre ignites; combustion pattern (fast, slow, melting, ash formation); flame colour (if observed).

• Behavior after cessation of irradiation: whether smoldering or burning continues; nature of melt solidification.

4. Identification: the type of fiber being studied is determined by comparing the set of observed interaction characteristics with reference data (reference tables, database, results of analysis of known samples) obtained under similar irradiation conditions for various standard types of fibers (e.g. cotton, wool, silk, viscose, polyester, polyamide-6, polyamide-66, acrylic, polypropylene, etc.).

Examples of fiber identification

Table 3. Characteristic features during irradiation (λ=405 nm, P=5 mW, t=1 sec):

Fiber type Melting Smoke Smell Color change Polyester Dark ball Black, thick Sweetish Brown charring Nylon Transparent ball White, light Sharp (amines) Yellowish Cotton Charring Absent Burnt paper Black Wool Fiber sintering Gray, caustic Burnt horn/feathers Dark gray Acrylic Plastic mass Bluish Fruit Yellowish

5. Instrumental analysis of products: To improve identification reliability, especially when analyzing fiber blends, additional instrumental analysis of the gaseous products released during laser irradiation can be performed. For this purpose, a gas sampling device (e.g., a microsyringe or capillary connected to the inlet system of a gas chromatograph or mass spectrometer) is placed near the interaction zone. The sampled products are analyzed for the presence of characteristic thermal degradation markers for various polymer types.

Sampling system:

• Microcollector: Capillary tube made of quartz (internal diameter 0.5 mm), placed 1 mm from the treatment area.

• Analysis: Agilent 5977B Gas Chromatograph with Mass Spectrometer (GC/MS).

Table 4. Identification markers:

Fiber type Characteristic compounds Peaks in the mass spectrum (m/z) Polyester Styrene, terephthalic acid 104 (C₈H₈), 166 (C₈H₆O₄) Nylon Caprolactam, ammonia 113 (C₆H₁₁NO), 17 (NH₃) Wool Hydrogen sulfide, cysteine 34 (H₂S), 121 (C₃H₇NO₂S) Cotton Levoglucosan, furfural 60 (C₅H₄O₂), 96 (C₅H₄O₂)

Experimental data:

• For polyester (0.0001 g sample) at 5 mW the following was detected:

- Styrene: 0.1 μg (peak 104, intensity 2500 conventional units).

- Terephthalic acid: 0.05 μg (peak 166, intensity 1200 conventional units).

• GC/MS detection limit: 0.01 µg, which is sufficient for micro-sample analysis.

6. Example of implementation of the method.

Fiber identification setup:

• Laser module: 405 nm, 5 mW (Osram PL450B diode laser).

• Lens: Achromatic, f=10 mm.

• Microscope: Olympus CX23 (magnification ×40).

• Sample: Polyester fiber with a diameter of 20 µm.

Result:

• Exposure time: 0.5 sec.

• Observed effects: formation of a dark melt ball (diameter 30 µm), release of black smoke (see Fig. 2).

• Gas analysis: Styrene (a marker of polyester) was detected using a portable gas chromatograph.

Table 5. Key features:

Sign Informativeness Example for polyester Melting character High Formation of a dark ball with a diameter of 30 µm Color of smoke Average Black smoke (styrene pyrolysis products) recorded visually or by camera Smell* Low Sweetish (destruction of esters) Change in fiber color High Darkening to brown

*Odor is analyzed only in the presence of an exhaust hood. The primary method is visual and instrumental (GOST R 56561-2015, Section 4.2 "Organoleptic Analysis").

Instrumental confirmation (GC/MS):

• Styrene (C₈H₈, m/z=104) was detected – a marker of thermal decomposition of polyester.

• Peak intensity: 2800 conventional units (threshold value for identification: 1500 conventional units).

Conclusion: The combination of characteristics (melt shape, smoke, odor, GC/MS data) clearly identifies sample #5 as polyester. No other fiber type produces the combination of black smoke, sweet odor, and styrene in GC/MS. Similar results were obtained for 20 polyester samples (98% accuracy). The analysis took 3 minutes (1 second irradiation, 2 minutes GC/MS).

This example demonstrates how the method combines the simplicity of visual analysis with the accuracy of instrumental verification.

Thus, the proposed method is significantly simpler to implement, does not require expensive equipment, and allows the use of readily available laser modules and monitoring tools. It eliminates thermal damage to the sample and is significantly safer than both open flame and high-power laser methods. Analysis time is reduced to seconds. The method allows for the analysis of microscopic quantities of fibers that are otherwise inaccessible.

Claims (5)

1. A method for identifying fibers, including placing a fiber on a substrate and subjecting it to thermal action and determining the fiber type by comparison with reference data, characterized in that the thermal action is carried out by irradiating a section of the fiber with focused continuous laser radiation with a wavelength of 380-450 nm and a power of no more than 5 mW for 0.1-2 s. 2. A method for identifying fibers according to claim 1, characterized in that the type of fiber is determined by organoleptic analysis. 3. A method for identifying fibers according to claim 1, characterized in that the fiber type is determined visually using a microscope. 4. A method for identifying fibers according to claim 1, characterized in that the fiber type is determined by sampling gaseous interaction products and analyzing their composition using gas chromatography. 5. A method for identifying fibers according to claim 1, characterized in that the fiber type is determined by sampling gaseous interaction products and analyzing their composition using mass spectrometry.