Plastic solubility testing is a laboratory evaluation that determines how polymeric materials interact with solvents, fuels, oils, and other liquid chemicals, providing quantitative data on dissolution, swelling, extractables, and chemical resistance. Guided by internationally recognized standards such as ISO 175, ASTM D543, and GB/T 11547, this testing supports material selection, quality control, and failure analysis across the automotive, packaging, medical, and chemical processing industries. This article explains the technical principles, sample types, test items, methods, and typical applications of plastic solubility testing.
Technical Overview
Plastic solubility testing evaluates the physical and chemical interaction between polymeric materials and liquid media, including organic solvents, fuels, oils, cleaning agents, acids, alkalis, and aqueous solutions. When a plastic component contacts an aggressive liquid, several outcomes are possible: complete dissolution of the polymer, partial extraction of additives such as plasticizers and stabilizers, swelling caused by solvent uptake, surface crazing and cracking, or no measurable effect. Quantifying these behaviors is essential for safe material selection and product qualification.
The underlying principle is often summarized as "like dissolves like." A polymer tends to dissolve in a solvent when the cohesive energy densities of the two materials are closely matched. This relationship is expressed through the Hildebrand solubility parameter, and more precisely through Hansen solubility parameters, which separate the total interaction energy into dispersive, polar, and hydrogen-bonding components. Amorphous thermoplastics such as polystyrene and PMMA dissolve readily in matching solvents, while semi-crystalline polymers such as polyethylene and polypropylene generally only swell at temperatures well below their melting points. Crosslinked thermosets and rubbers cannot dissolve and can only absorb solvent to a finite swelling equilibrium.
Solubility testing therefore serves two complementary purposes: determining fundamental solubility parameters for formulation work, and assessing practical chemical resistance by measuring changes in mass, dimensions, appearance, and mechanical properties after controlled immersion, as specified in standards such as ISO 175, ASTM D543, and GB/T 11547. Accredited third-party laboratories routinely perform these evaluations for designers, compounders, and end users.
Testing Samples
Thermoplastic Resins and Pellets
Virgin resin pellets of materials such as PP, PE, PVC, PS, ABS, PC, PMMA, PA, PET, POM, and PEEK can be screened for solubility by observing dissolution, swelling, or sedimentation behavior in candidate solvents. This form of qualitative solubility screening is widely used in polymer identification and recycling research.
Molded Specimens and Test Coupons
Standardized specimens, including tensile bars, rectangular coupons, and discs cut from compression-molded or injection-molded plaques, are the preferred format for immersion testing. Coupon-based testing allows gravimetric, dimensional, and mechanical data to be collected on the same material lot under controlled conditions.
Films, Sheets, and Coatings
Packaging films, geomembranes, adhesive layers, and coated surfaces can be evaluated for solvent attack, delamination, extraction of coating additives, and loss of barrier or adhesion performance after liquid exposure.
Finished Products and Components
Fuel system parts, pump housings, seals, laboratory ware, medical device components, and chemical storage containers can be tested as received. Testing finished parts captures the influence of processing-induced residual stress, which often accelerates solvent-induced crazing and environmental stress cracking.
Foams and Elastomers
Flexible and rigid foams, together with thermoplastic elastomers, may be assessed for solvent absorption, volume swell, compression set changes, and structural collapse in contact with oils and hydrocarbon fluids.
Testing Items
Depending on the objective of the evaluation, plastic solubility testing typically covers the following measurable parameters:
- Mass change after immersion, expressed as percentage mass gain from solvent absorption or mass loss from dissolution and additive extraction
- Dimensional change and swelling ratio in length, width, and thickness
- Soluble matter content and total extractables, quantifying the fraction of the material that dissolves in a given solvent under defined conditions
- Solubility parameter determination, including Hansen parameter components for formulation and compatibility studies
- Retention of mechanical properties, such as tensile strength, elongation at break, and flexural strength, measured before and after exposure
- Hardness change and surface property changes, including gloss loss and texture alteration
- Visual assessment of crazing, cracking, blistering, chalking, softening, and discoloration
- Migration and extractable substance analysis for food contact and medical applications, often linked to regulatory compliance frameworks
- Environmental stress cracking resistance, evaluating susceptibility to cracking under combined mechanical stress and chemical contact
Testing Methods
Immersion Testing to ISO 175, ASTM D543, and GB/T 11547
Specimens are fully immersed in the test liquid at a standard temperature of 23°C or at elevated temperatures such as 40°C, 50°C, or 70°C for defined periods, commonly 24 hours, 7 days, or 28 days. After exposure, specimens are wiped, reconditioned, and re-weighed, re-measured, visually rated, and mechanically tested. Percentage change in each property is calculated and compared with unexposed control specimens. The standards define procedures for volatile liquids, constant-boiling mixtures, and test liquids that require special handling.
Water Absorption Testing to ISO 62 and ASTM D570
Water is treated as a specific immersion medium, with immersion in distilled water at 23°C and in boiling water, followed by gravimetric determination of absorbed water. This method is fundamental for hygroscopic materials such as polyamides.
Solubility Parameter Determination
Hansen solubility parameters can be determined experimentally by equilibrium swelling, in which the degree of swelling of the polymer in a series of solvents indicates proximity of solvent and polymer parameters; by inverse gas chromatography, which measures polymer-solvent interactions directly; by turbidimetric titration, which identifies the boundary of solubility in solvent-nonsolvent mixtures; or by group contribution calculation from the polymer repeat-unit structure.
Extraction and Soluble Fraction Analysis
Reflux and Soxhlet extraction isolate the soluble fraction of a compound, including plasticizers, oligomers, and additives, which can then be weighed and analyzed by techniques such as infrared spectroscopy and chromatography. Two-sided liquid extraction methods, such as ASTM D4754, are used for extractables testing of food contact materials.
Stress Cracking and Combined Loading Tests
Environmental stress cracking is evaluated using bent-strip methods to ASTM D1693, constant-tensile-load tests, and the multi-part ISO 22088 series, which apply defined stress levels while the specimen is in contact with stress-cracking agents such as detergents, alcohols, and oils. UL 746A also specifies short-term chemical immersion procedures for polymeric materials used in electrical equipment.
Applications
Plastic solubility testing data is applied throughout the product life cycle, from resin development to field failure investigation. Typical application areas include:
- Material selection for chemical processing equipment, piping, valves, pump components, and storage tanks exposed to aggressive media
- Automotive fuel and fluid system development, where resistance to gasoline, diesel, ethanol blends, brake fluid, and oils is critical
- Packaging qualification, including solvent resistance of containers, closures, and printed layers, and migration compliance for food contact materials
- Formulation of coatings, adhesives, inks, and polymer solutions, where solubility parameters guide solvent blend selection
- Polymer recycling and identification, using selective dissolution to separate mixed plastic streams
- Quality control and incoming inspection of resins and molded parts against specification limits
- Failure analysis of cracked, swollen, or degraded components recovered from service
- Medical device and pharmaceutical packaging compatibility studies for disinfectants, drug formulations, and infusion fluids
FAQ
What is the difference between solubility testing and chemical resistance testing?
Solubility testing focuses on whether and to what extent a polymer dissolves in a solvent, including fundamental parameters such as solubility coefficients and extractable fractions. Chemical resistance testing is broader, evaluating swelling, mechanical property retention, and appearance changes after exposure to chemicals, and it may be performed even when the polymer does not dissolve at all. In practice, immersion-based methods such as ISO 175 and ASTM D543 deliver data relevant to both objectives.
Which standards are most commonly used for plastic solubility testing?
The most widely applied standards are ISO 175 and ASTM D543 for liquid chemical immersion, GB/T 11547 as the corresponding Chinese method, ISO 62 and ASTM D570 for water absorption, ASTM D1693 and the ISO 22088 series for environmental stress cracking, and ASTM D4754 for two-sided liquid extraction. UL 746A covers chemical exposure requirements for polymeric materials in electrical applications.
What sample size and quantity are required?
For immersion testing, standard tensile bars or coupons of approximately 50 mm by 50 mm, or discs of about 50 mm diameter, are typical, with a minimum of three to five replicate specimens per condition to ensure statistical validity. Solubility screening of pellets and powders requires only a few grams of material. Finished parts can usually be tested as received or by cutting coupons from them.
How long does a typical test program take?
Short-term immersion tests at 24 hours can be completed within a few days including conditioning and measurement. Standard 7-day and 28-day exposure programs, combined with post-exposure mechanical testing, typically require several weeks. Solubility parameter determination by swelling or chromatography generally requires one to two weeks depending on the number of solvents involved.
Can crosslinked plastics and rubbers be tested for solubility?
Crosslinked thermosets and vulcanized rubbers do not dissolve in solvents because of their network structure. Their solvent behavior is instead characterized by swelling tests, in which the equilibrium swelling ratio provides information about crosslink density and chemical compatibility, and this data is fully supported by the immersion methods described above.