Plastic resistance to liquid chemicals testing evaluates how polymer materials and products perform when exposed to aggressive liquids such as acids, alkalis, solvents, oils, and fuels. Conducted in accordance with internationally recognized standards such as ISO 175, ASTM D543, and GB/T 11547, this testing quantifies changes in mass, dimensions, mechanical properties, and appearance after controlled immersion. This technical guide explains the test principle, sample requirements, evaluation parameters, procedures, and typical industrial applications of chemical resistance testing for plastics.
Technical Overview
Plastic resistance to liquid chemicals testing, also known as chemical compatibility or liquid chemical resistance testing, is a laboratory procedure used to determine how plastic materials respond to contact with liquid chemical reagents. During service, plastic components such as fuel lines, chemical storage containers, seals, and pipes are frequently exposed to substances that can be absorbed by the polymer, causing swelling, plasticization, extraction of additives, hydrolysis, stress cracking, or permanent degradation of mechanical performance.
The core principle of the test involves immersing standardized specimens in a selected test liquid under controlled temperature and duration conditions, then comparing key properties before and after exposure. The resulting data allows engineers to rank candidate materials, verify suitability for a specific chemical environment, and support quality control during production.
The principal standards governing this testing include:
- ISO 175:2010 – Plastics – Methods of test for the determination of the effects of immersion in liquid chemicals
- ASTM D543 – Standard Practices for Evaluating the Resistance of Plastics to Chemical Reagents
- GB/T 11547 – Plastics – Determination of resistance to liquid chemicals (Chinese national standard, technically aligned with ISO 175)
- ISO 291 – Plastics – Standard atmospheres for conditioning and testing (for specimen conditioning)
Testing Samples
Molded Specimens
Compression-molded or injection-molded plaques, bars, and dumbbell-shaped tensile specimens are the most common sample format. Tensile test pieces are typically prepared according to ISO 20753 or ASTM D638 so that mechanical property changes can be measured on the same geometry used for baseline testing.
Film and Sheet Materials
Flexible films, membranes, and sheets used in packaging, liners, and protective applications can be cut into squares or strips of defined dimensions. Thin samples reach absorption equilibrium faster and reveal effects such as plasticization and embrittlement more quickly.
Finished Products and Components
Actual parts, including pipe segments, fittings, seals, gaskets, housings, containers, and medical device components, may be tested in their end-use configuration to evaluate real-world chemical compatibility, including the influence of processing stresses and wall thickness.
Sample Preparation Requirements
- Specimens should be free from defects, flash, surface contamination, and molded-in stress where possible
- Recommended minimum sample quantity: at least three specimens per condition per test liquid, plus a control set
- Specimens are conditioned in a standard atmosphere (typically 23 °C ± 2 °C and 50% ± 10% relative humidity) before baseline measurement
- Liquids suspected of causing stress cracking are often tested on specimens held under a defined strain or with induced bending stress to evaluate environmental stress cracking behavior
Testing Items
The scope of evaluation depends on the material type and end-use requirements. Commonly measured parameters include:
- Change in mass – percentage mass gain (absorption or swelling) or mass loss (extraction of additives, dissolution)
- Change in dimensions – swelling of length, width, and thickness, expressed as a percentage
- Change in mechanical properties – tensile strength, yield stress, elongation at break, and flexural properties before and after exposure
- Change in hardness – Shore hardness (Shore A or Shore D) measured on the exposed surface
- Visual and surface assessment – discoloration, staining, blistering, crazing, cracking, softening, tackiness, or loss of gloss
- Optional property changes – impact strength, bursting strength of pipes, dielectric properties, or specific gravity, depending on application needs
Testing Methods
Selection of Test Liquids
Two categories of liquids are used: standard reference reagents and service liquids. Standard reagents defined in ASTM D543 and ISO 175 include distilled water, aqueous solutions of acids (such as sulfuric acid, hydrochloric acid, nitric acid, and acetic acid), alkalis (such as sodium hydroxide and sodium carbonate), salts (such as sodium chloride), oxidizing agents, organic solvents (such as ethanol, acetone, toluene, and heptane), and standard oils and reference fuels. Service liquids are the actual chemicals, fuel formulations, cleaning agents, or process fluids encountered in the intended application.
Immersion Procedure
Baseline measurements of mass, dimensions, mechanical properties, and appearance are recorded for the conditioned specimens. The specimens are then fully immersed in the test liquid, ensuring that all surfaces are freely wetted and that specimens do not contact each other or the container walls. The ratio of liquid volume to specimen surface area is maintained to limit depletion of the chemical medium, and the liquid is replaced periodically for prolonged exposures.
Test Conditions
- Temperature: standard conditioning temperature of 23 °C, or elevated temperatures such as 40 °C, 50 °C, 60 °C, 70 °C, or higher to accelerate the effects, provided the liquid remains stable and does not boil
- Duration: common exposure periods include 24 h, 48 h, 72 h, 7 days, 14 days, 28 days, or longer intervals selected to approach equilibrium absorption
- Atmosphere: tests are performed in closed vessels, with condensing vapor exposure or wipe exposure variants available where full immersion is not representative of service
Post-Exposure Evaluation
After immersion, specimens are removed, gently blotted to remove surface liquid, and promptly weighed and measured. For properties affected by ongoing evaporation of absorbed liquid, measurements are taken within a defined short time window after removal. Mechanical and hardness tests are conducted either immediately or after drying and reconditioning, according to the specified procedure. Results are typically reported as percentage change relative to the unexposed control specimens, and appearance changes are documented photographically.
Applications
Chemical resistance testing of plastics supports material qualification and product reliability across many industries:
- Automotive – fuel system components, tanks, hoses, coolant reservoirs, underhood parts exposed to oils, fuels, brake fluid, and cleaning agents
- Chemical processing and storage – tanks, vessels, valves, pumps, and linings in continuous contact with acids, alkalis, and solvents
- Packaging – containers, drums, jerrycans, and closures for household chemicals, agrochemicals, and industrial liquids
- Plumbing and construction – pipes, fittings, and seals exposed to water treatment chemicals and aggressive service environments
- Medical and laboratory – devices, consumables, and equipment requiring resistance to disinfectants, alcohol-based cleaners, and pharmaceutical formulations
- Electronics and consumer goods – housings, gaskets, and coatings evaluated against solvents, cosmetics, sweat, and cleaning products
Test data are also used in research and development for comparing resin grades, validating new formulations or recycled content, investigating field failures, and demonstrating compliance with customer specifications and regulatory requirements through accredited third-party laboratories.
FAQ
What is the difference between ISO 175 and ASTM D543?
Both standards use immersion of plastic specimens in liquid chemicals followed by measurement of property changes. ISO 175 emphasizes the determination of changes in physical properties with flexible exposure conditions, while ASTM D543 specifies a defined list of approximately 50 reference reagents along with recommended exposure conditions. The two standards are broadly equivalent in principle, but the specific reagents, temperatures, and durations should be confirmed before testing begins.
How are test results interpreted?
Results are reported as percentage changes in mass, dimensions, and mechanical properties, together with a visual rating of the specimen surface. A material is generally considered compatible with a liquid when changes in key properties remain within specified limits, for example a mass change below a few percent and retention of most of its original tensile strength, with no cracking, dissolution, or severe softening.
What sample size and quantity are required?
For immersion testing, specimens of standardized geometry such as 50 mm × 50 mm squares, tensile dumbbells, or representative product sections are typical. At least three specimens per liquid per time point are recommended, plus an unexposed control group, so that statistically meaningful comparisons can be made.
Can a mixture or a proprietary liquid be tested?
Yes. In addition to standard reference reagents, actual service fluids, commercial formulations, and multi-component mixtures can be evaluated. When testing volatile, flammable, or hazardous liquids, specialized vessels, safety enclosures, and controlled-temperature baths are used by accredited third-party laboratories to ensure operator safety and result validity.
How long does the test take?
The total project duration depends on the selected exposure period and the properties being measured. A standard 24-hour or 7-day immersion with mass, dimensional, and visual evaluation can be completed within one to two weeks including conditioning, while multi-temperature, multi-liquid programs with mechanical testing at several time points may require four to eight weeks.