Filter Cloth Inspection

Filter cloth inspection mainly conducts scientific testing and analysis on physical property testing, filtration performance testing, structural characteristic testing, chemical stability testing, surface and pore size characteristic testing, and safety and environmental protection performance testing, 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

Physical property testing: Mainly including the thickness, mass per unit area (grammage), density, air permeability, breaking strength/elongation and other items of the filter cloth. The thickness and grammage directly affect the filtration precision and dust holding capacity; the air permeability testing evaluates the difficulty of air passing through the filter cloth, and is the core indicator of the filtration efficiency; the breaking strength and elongation reflect the durability of the filter cloth under the filtration pressure and mechanical stretching.

Filtration performance testing: This is the core for evaluating the function of the filter cloth, including the initial filtration efficiency, final filtration efficiency, dust holding capacity, filtration resistance and regeneration performance. By simulating the actual working conditions, the retention ability of the filter cloth for the particles of different sizes, the amount of dust that can be held when reaching a specific pressure difference, and the performance recovery after cleaning are determined.

Structural characteristic testing: The fabric weave structure (such as plain weave, twill weave and satin weave), yarn count, fibre type (polyester, polypropylene, aramid, PPS, etc.) and post-treatment processes (such as heat setting, singeing, calendering and coating) of the filter cloth are identified. These characteristics determine the dimensional stability, surface smoothness and the peelability of the dust of the filter cloth.

Chemical stability testing: Evaluates the tolerance of the filter cloth material to the specific chemical environments. It mainly includes the acid and alkali resistance, oxidation resistance, hydrolysis resistance and temperature resistance tests. According to the intended working conditions of the filter cloth (such as coal-fired boilers, cement kilns and chemical tail gas), it is exposed to the acid and alkali solutions of the corresponding concentrations or high-temperature environments, and its strength loss rate and dimensional change rate are detected.

Surface and pore size characteristic testing: Dedicated instruments (such as pore size analysers and scanning electron microscopes) are used to analyse the surface morphology, porosity, maximum pore size, average pore size and pore size distribution of the filter cloth fibres. The pore size characteristics are directly related to the filtration precision and the particle capture mechanism (surface filtration or depth filtration).

Safety and environmental protection performance testing: For the specific application fields (such as food and pharmaceutical), the extractables, heavy metal content, microbial limits and flame retardant grades (such as EN 45545-2 and DIN 4102) of the filter cloth need to be tested, ensuring that it will not introduce secondary pollution or cause safety risks during use.

Testing Scope

Woven filter cloths: Including the plain, twill and satin fabrics of various fibre materials such as polyester, nylon, polypropylene, aramid, glass fibre, PPS and PTFE. The testing scope covers their basic physical indicators, high-temperature shrinkage, chemical corrosion resistance and the long-term filtration performance evaluation under different dust working conditions.

Nonwoven filter cloths: Mainly including needle-punched felts, spunlaced cloths and meltblown cloths. In addition to the conventional properties, the uniformity of the three-dimensional fibre network structure, the bonding strength between the fibres, and the improvement degree of the surface filtration performance and cleaning effect after the special post-treatments such as PTFE lamination and oil and water repellency are key-tested.

Composite filter media: Refers to the filter cloths compounded by two or more different materials, such as PTFE laminated filter media, gradient structure filter media and waterproof and oil-repellent filter media. The inspection report needs to separately evaluate the performance of the base fabric and the functional layer, and verify the synergistic effect of the compounding process, such as the adhesion firmness of the membrane layer and the overall air permeability and filtration efficiency after compounding.

Filter cloths for industrial bag filter dust collectors: This is the most important application field of the filter cloth. The testing needs to strictly follow the working conditions, covering the performance verification under different temperature ranges such as normal temperature, medium temperature (<180 ℃) and high temperature (>180 ℃), as well as the corresponding acid, alkali and humidity environments, ensuring its stability under the different cleaning methods such as pulse jet cleaning and mechanical shaking.

Filter cloths for liquid filtration: Mainly used for solid-liquid separation, such as the filter presses and centrifuges in the mining, metallurgy, food and pharmaceutical industries. The testing focus is on the retention precision (mesh number or micron level), water permeability rate, anti-clogging ability, cake discharge performance and the service life against detergents and high-temperature cleaning of the filter cloth.

Filter media for air filtration: Including the filter media for the medium-efficiency and high-efficiency air filters. The testing strictly follows the relevant standards (such as EN 779, EN 1822 and GB/T 14295), focusing on evaluating the counting efficiency for the particles of different sizes (such as 0.3 μm and 0.5 μm), initial resistance and dust holding capacity, and testing the additional functions such as flame retardancy and antibacterial property.

Testing Methods

Physical property test methods: The thickness is measured with a thickness gauge in accordance with GB/T 3820; the mass per unit area is determined by cutting and weighing in accordance with GB/T 4669; the air permeability is tested with an air permeability tester in accordance with GB/T 5453; the breaking strength and elongation are determined with a constant-rate-of-extension (CRE) strength tester in accordance with GB/T 3923.1, recording the tensile curve.

Filtration performance test methods: The dust loading method is often adopted for the filtration efficiency and resistance. In accordance with ISO 11057 or similar standards, the dust of the specified median diameter (such as A2 fine dust and SAE fine dust) is generated on a standard test bench, the upstream and downstream dust concentrations are monitored in real time to calculate the efficiency, and the pressure difference change on both sides of the filter media is synchronously recorded during the test until the termination pressure difference is reached, thereby obtaining the dust holding capacity.

Pore size and pore size distribution test methods: The bubble point method and the mercury intrusion method are commonly used. The bubble point method (ASTM F316) calculates the maximum pore size by measuring the minimum pressure required for the gas to penetrate the liquid-wetted filter media; the mercury intrusion method obtains a more comprehensive pore size distribution curve by measuring the volume of mercury intruded into the pores under different pressures, and is suitable for analysing the complex pore structures.

Chemical stability test methods: For the acid and alkali resistance testing, usually in accordance with ISO 105-X, the filter cloth specimen is immersed in the acid and alkali solutions of the specified concentration and temperature for a certain time, then taken out, washed and dried, and its strength retention rate and dimensional change rate are determined. The temperature resistance testing is carried out in a heat aging chamber, evaluating the mechanical property attenuation under long-term high temperature.

Structural analysis and micro-morphology observation methods: The fibre composition can be identified by the combustion method, dissolution method or infrared spectroscopy (FTIR). The fabric weave structure is observed with a fabric density glass. The surface and cross-section morphology, fibre diameter and distribution of the filter cloth need to be observed and photographed at high magnification with a scanning electron microscope (SEM).

Safety and environmental protection performance test methods: The heavy metal contents (such as Pb and Cd) are determined by the atomic absorption spectrometry in accordance with GB/T 17593. The flame retardant performance is tested by the vertical burning method (such as GB/T 5455) or the oxygen index method (GB/T 5454). In the extractables testing, the filter cloth is immersed in the simulated use media (such as water and alcohol), and the composition and content of the extract are analysed.

Testing Standards

Domestic Standards:

1. GB/T 3923.1-2013, Textiles — Tensile properties of fabrics — Part 1: Determination of breaking force and elongation at breaking force using the strip method

2. GB/T 5455-2014, Textiles — Burning behaviour — Determination of damaged length, afterglow time and afterflame time

3. GB/T 5454-1997, Textiles — Burning behaviour — Oxygen index method

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

International Standards:

1. ISO 11057, Air quality — Test method for filtration characterisation of cleanable filter media

2. ISO 105, Textiles — Tests for colour fastness

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

Universal material testing machine: Used to precisely measure the breaking strength, elongation at break, bursting strength and peeling strength (for the composite filter media) of the filter cloth. The equipment is equipped with pneumatic or hydraulic grips, ensuring no slippage of the specimen during the test, and can output the complete stress-strain curve for analysing the toughness of the material.

Fabric air permeability tester: One of the core equipment, measuring the air volume passing through the filter cloth per unit area per unit time according to the predetermined pressure difference (such as 100 Pa and 200 Pa). The instrument needs to comply with the standards such as GB/T 5453, and be equipped with high-precision pressure difference sensors and flow meters. The test result is expressed in mm/s or L/m²·s, and is the key data for evaluating the resistance of the filter cloth.

Automatic filter media performance test bench: An integrated equipment that can automatically complete the dust generation, efficiency testing, resistance monitoring and cleaning processes. It can simulate the pulse jet cleaning, record the filtration efficiency dynamic curve and the increasing resistance curve in real time, and finally automatically calculate and report the initial efficiency, steady-state efficiency, final resistance and dust holding capacity of the filter media.

Scanning electron microscope (SEM): Used to observe the surface morphology, diameter, cross-section structure of the filter cloth fibres and the attachment state of the dust on the filter cloth surface. The high-resolution SEM images are the most intuitive technical means for analysing the filtration mechanism (surface lamination or depth filtration), evaluating the damage and clogging of the filter cloth.

Pore size analyser: Including the bubble point pore size analyser and the mercury intrusion porosimeter. The bubble point instrument is used to quickly determine the maximum pore size and the mean flow pore size of the filter cloth; the mercury intrusion porosimeter can provide more comprehensive data such as the pore size distribution, porosity and pore volume, but is usually used for the rigid or porous materials that can withstand high pressure, with limited applicability to the soft filter cloths.

Constant temperature and humidity chamber and heat aging test chamber: The constant temperature and humidity chamber is used to condition the filter cloth specimens under the standard state (such as temperature 20±2 ℃ and relative humidity 65±4%) before the testing. The heat aging test chamber is used to evaluate the performance change of the filter cloth under long-term high temperature, and can be programmed for temperature cycling to simulate the thermal stress of the actual working conditions.

Chemical analysis instruments: Including the atomic absorption spectrometer (AAS) or inductively coupled plasma mass spectrometer (ICP-MS) for the trace heavy metal detection; the Fourier transform infrared spectrometer (FTIR) for the qualitative identification of the fibre materials; and the pH meter, constant temperature water bath and other pre-treatment equipment for the chemical stability testing.