PP fiber cloth (polypropylene fabric) is widely used in chemical filtration, corrosion protection, and industrial applications where exposure to acids and alkalis is unavoidable. Acid and alkali resistance performance testing evaluates how PP woven and nonwoven fabrics retain their tensile strength, mass, and structural integrity after prolonged chemical exposure. This article provides a complete technical guide covering test samples, testing items, standard methods, evaluation criteria, and typical applications.
Technical Overview
Polypropylene (PP) fiber cloth is a hydrocarbon-based synthetic textile composed of polymerized propylene monomers with the repeating unit (C3H6)n. Its molecular backbone contains no hydrolyzable bonds, such as ester or amide linkages, which gives the material inherently strong resistance to most inorganic acids, alkalis, and salt solutions. This chemical inertness distinguishes PP from polyester (PET) fabrics, which are vulnerable to alkaline hydrolysis, and from polyamide fabrics, which degrade in strong mineral acids.
Acid and alkali resistance performance testing is designed to quantify how PP fiber cloth behaves when exposed to aggressive chemical environments over defined periods and temperatures. The test results determine whether a specific PP fabric grade is suitable for applications such as filter press cloths, chemical storage liners, electrolysis separators, and geotextiles in acidic or alkaline soil conditions. Testing is typically conducted in accordance with internationally recognized standards, including ISO 175, ASTM D543, and relevant national textile and geosynthetic testing standards.
Testing Samples
Woven PP Filter Cloth
Woven polypropylene filter cloth, produced on rapier or air-jet looms with yarn linear densities commonly ranging from 500 dtex to 2000 dtex, is the primary sample type for chemical resistance evaluation. Samples are cut in both warp and weft directions to assess directional strength retention, and specimen dimensions typically follow the strip method, for example 50 mm × 300 mm strips, with a minimum of five replicates per direction.
Nonwoven PP Needle-Punched Fabric
Nonwoven PP fabrics used in geotextiles and adsorption media are tested using specimens with standardized mass per unit area, generally between 100 g/m2 and 800 g/m2. Because nonwoven structures rely on fiber entanglement rather than yarn interlacing, both tensile strength and elongation behavior are recorded before and after chemical conditioning.
Sample Conditioning Requirements
Prior to testing, all specimens must be conditioned in a standard atmosphere of 20 ± 2 °C and 65 ± 4% relative humidity for at least 24 hours, in accordance with ISO 139 or ASTM D1776. This ensures that measured changes are attributable to chemical attack rather than variations in ambient moisture content.
Testing Items
The acid and alkali resistance evaluation covers the following measurable performance indicators:
- Tensile strength retention rate in warp and weft directions after acid immersion
- Tensile strength retention rate in warp and weft directions after alkali immersion
- Breaking elongation change before and after chemical exposure
- Mass change rate (mass loss or mass gain, expressed as a percentage)
- Dimensional stability (shrinkage or expansion in length and width)
- Bursting strength retention for nonwoven structures
- Visual appearance assessment, including color change, embrittlement, hardening, surface cracking, and fiber fracture
- Microscopic examination of fiber surface morphology after exposure
Testing Methods
Chemical Reagents and Exposure Conditions
Standardized test solutions are prepared to simulate typical industrial service environments. Commonly used reagents include sulfuric acid solution at 5% to 10% concentration, hydrochloric acid at 5%, and sodium hydroxide solution at 5% to 10% concentration. Exposure temperature is controlled at 20 ± 2 °C for room-temperature tests, while accelerated tests may use elevated temperatures of 60 °C or 80 °C in sealed containers to simulate long-term service in a compressed timeframe. Immersion durations range from 24 hours and 72 hours to extended periods of 7 days, 28 days, or even 90 days for long-term qualification programs.
Immersion Procedure
Specimens are fully immersed in the prepared acid or alkali solution at a liquor ratio of at least 50:1 (solution volume to specimen mass) to ensure complete contact and to prevent localized depletion of the reagent. After the specified exposure period, specimens are removed, rinsed thoroughly with distilled water until the rinse water reaches neutral pH, gently blotted to remove surface moisture, and reconditioned in the standard atmosphere for 24 hours before physical measurements.
Tensile Strength Measurement
Tensile testing is performed on both control (unexposed) and chemically treated specimens using a constant-rate-of-extension tensile tester with a gauge length of 200 mm and a stretching speed of 20 mm/min to 100 mm/min, following the strip method described in ISO 13934-1 or ASTM D5035. The tensile strength retention rate is calculated using the formula: retention rate (%) = (average breaking strength after exposure ÷ average breaking strength before exposure) × 100.
Mass and Dimensional Change Measurement
Specimens are weighed to an accuracy of 0.01 g before and after chemical treatment, and the mass change rate is calculated as the difference divided by the original mass, expressed as a percentage. Dimensional changes are measured along marked reference lines in both directions, with shrinkage or growth recorded to the nearest 1 mm and converted to a percentage of the original dimension.
Evaluation Criteria
Typical acceptance criteria for high-performance PP filter cloth require a tensile strength retention rate of no less than 90% after immersion in 10% sulfuric acid and 10% sodium hydroxide for 72 hours at room temperature, with no visible embrittlement, cracking, or delamination. For geosynthetic applications in aggressive soil environments, retention thresholds of 80% to 85% after extended exposure are commonly specified, supplemented by microscopic verification that fiber surfaces remain smooth and free from pitting or longitudinal splitting.
Applications
PP fiber cloth that passes rigorous acid and alkali resistance testing serves critical roles across multiple industries:
- Filter press cloths for acidic and alkaline slurry dewatering in mining, metallurgy, and chemical plants
- Liquid filtration bags for electroplating wastewater, pickling baths, and caustic soda processing
- Geotextiles for reinforcement and separation in acidic soils, saline-alkali land, and landfill liners
- Battery separator backing fabrics and electrolyte-resistant support layers
- Corrosion protection wrapping fabrics for pipelines and concrete structures exposed to chemical splashes
- Protective clothing base fabrics for workers handling dilute acids and alkalis
- Concrete curing covers and alkali-resistant shuttering fabrics in construction
FAQ
What concentrations of acid and alkali can PP fiber cloth withstand?
PP fiber cloth resists most concentrations of hydrochloric acid, sulfuric acid below approximately 70%, phosphoric acid, acetic acid, and sodium hydroxide at any practical concentration at room temperature. However, it is attacked by strong oxidizing agents such as concentrated nitric acid, fuming sulfuric acid, and chlorosulfonic acid, which cause chain scission in the polypropylene backbone.
Why is PP more alkali-resistant than polyester fabric?
Polypropylene has a purely hydrocarbon molecular chain with no ester bonds, whereas polyester (PET) contains ester linkages that undergo hydrolysis in alkaline environments. As a result, PET fabrics lose strength progressively in caustic solutions, while PP fabrics retain nearly all of their original tensile strength under the same conditions.
How does temperature affect the chemical resistance test results?
Elevated temperatures accelerate both chemical attack and polymer relaxation. Tests conducted at 60 °C to 80 °C can reveal degradation mechanisms that appear only after years of service at ambient temperature, making accelerated testing essential for qualifying fabrics used in hot acid or hot alkali filtration processes.
What strength retention value is considered acceptable?
For most industrial filtration applications, a tensile strength retention rate of 90% or higher after 72-hour immersion in standard test solutions is considered excellent. Values between 80% and 90% indicate acceptable resistance for moderate exposure, while retention below 80% generally disqualifies the fabric for aggressive chemical service.
Which standards govern PP fiber cloth chemical resistance testing?
Commonly referenced standards include ISO 175 for the determination of the effects of liquid chemicals on plastics, ASTM D543 for evaluating the resistance of plastics to chemical reagents, ISO 13934-1 and ASTM D5035 for textile tensile testing, and various national geotextile and industrial fabric specifications that define immersion solutions, temperatures, and pass criteria.