Hydrolysis Resistance Testing of Plastics: Determination Methods, Standards, and Applications

Hydrolysis resistance testing of plastics is a critical durability evaluation that measures how polymer materials withstand degradation caused by water, moisture, and humid environments over time. This technical article explains the principle of polymer hydrolysis, common test samples, measured parameters, recognized testing methods based on ISO, ASTM, IEC, and GB standards, and typical industrial applications. Whether you are selecting materials for automotive connectors, electronic components, or medical devices, understanding hydrolytic stability data helps ensure long-term product reliability.

Technical Overview

Hydrolysis resistance, also referred to as hydrolytic stability, describes the ability of a plastic material to retain its chemical structure and physical properties when exposed to water, humid air, or saturated steam. During hydrolysis, water molecules penetrate the polymer matrix and cleave hydrolyzable chemical bonds such as ester, amide, carbonate, urethane, and ether linkages. This bond scission reduces the average molecular weight of the polymer, which in turn leads to embrittlement, loss of tensile strength and elongation, discoloration, surface crazing, and ultimately mechanical failure of the part.

Polyester-based materials such as PET, PBT, and polycarbonate, aliphatic polyamides such as PA6 and PA66, ester-based thermoplastic polyurethanes (TPU), epoxy molding compounds, and biodegradable polyesters such as PLA are among the polymer families most susceptible to hydrolytic degradation. Because hydrolysis is significantly accelerated by elevated temperature and pressure, laboratory tests are designed to simulate years of service exposure within weeks or months by using intensified conditions such as hot water immersion, damp heat chambers, and pressurized saturated steam environments.

Hydrolysis resistance determination is widely used for material qualification, comparative material screening, incoming material inspection, and lifetime prediction in industries where plastic components encounter moisture, condensation, cleaning fluids, or sterilization processes. Accredited third-party laboratories routinely perform these evaluations in accordance with international and national standards, providing manufacturers with defensible data for design validation and quality assurance.

Testing Samples

Injection-Molded Standard Specimens

The most common samples are standardized test pieces such as tensile bars (for example, ISO 527-2 1A/1B specimens or ASTM D638 Type I dogbones), flexural bars, and impact specimens molded directly from the compound under evaluation. Using identical specimen geometry before and after hydrolytic exposure allows accurate calculation of property retention rates.

Films, Sheets, and Coatings

Polyester films, multilayer packaging laminates, protective coatings, and adhesive layers can be tested as received. Film samples are typically cut into strips for tensile testing or small coupons for weight change and molecular weight measurements.

Finished Components and Assemblies

Actual parts such as electrical connectors, sensor housings, cable jackets, encapsulated electronic modules, and medical device components may be subjected to hydrolytic aging followed by functional testing. This approach verifies real-world performance rather than only material-level behavior.

Polymer Pellets and Resins

For research and compound development, raw pellets or granules can be aged and subsequently analyzed for intrinsic viscosity, molecular weight distribution, and melt flow index changes, providing insight into the fundamental degradation kinetics of the resin.

Testing Items

Hydrolysis resistance determination evaluates changes in material characteristics before and after controlled exposure to water or humid environments. The principal measured items include:

• Tensile strength and tensile strength retention rate after hydrolytic aging
• Elongation at break and ductility retention, often the most sensitive early indicator of hydrolytic embrittlement
• Flexural strength and flexural modulus change
• Impact strength (Charpy or Izod) reduction after exposure
• Mass change (water uptake) and dimensional stability
• Molecular weight distribution by gel permeation chromatography (GPC)
• Intrinsic viscosity (IV) for polyesters such as PET and PBT
• Melt flow rate (MFR/MVR) shift indicating chain scission or post-polymerization
• Color change and yellowness index (YI) increase
• Surface morphology assessment for crazing, chalking, blistering, or microcracking
• Glass transition temperature and thermal transitions by DSC or DMA
• Dielectric strength, insulation resistance, and comparative tracking index for electrical applications
• Hardness change (Shore hardness, ball indentation)

Testing Methods

Hot Water Immersion Method

Specimens are fully immersed in distilled or deionized water at controlled temperatures, commonly ranging from 40°C for mild service simulation up to 95°C or boiling at 100°C for accelerated aging. After a defined exposure period, samples are removed, surface water is blotted, and properties are measured in the wet state and/or after controlled drying. Immersion testing directly evaluates the effect of liquid water on the polymer structure. Relevant water-absorption and chemical-resistance frameworks include ISO 62, ASTM D570, ISO 175, ASTM D543, GB/T 1034, GB/T 11547, and JIS K 7114.

Damp Heat (Humidity) Aging Method

Specimens are placed in a climate chamber maintaining a steady-state environment, with the classic condition of 85°C and 85% relative humidity being widely applied for electronics and electrical components. Other common profiles include 40°C/93%RH and 60°C/95%RH. Damp heat testing reproduces the combined effect of temperature and moisture vapor and is standardized in IEC 60068-2-78 for steady-state conditions, while composite temperature-humidity cycling is covered by IEC 60068-2-38. Test durations typically extend to 500, 1000, or 2000 hours.

Pressure Cooker Test (PCT) and Autoclave Method

For highly accelerated evaluation, specimens are exposed to saturated steam under pressure, for example at 121°C and approximately 2 atmospheres, or 132°C at approximately 3 atmospheres, using an autoclave or pressure cooker chamber. These severe conditions rapidly drive water into the polymer and dramatically accelerate hydrolytic bond cleavage, enabling screening of materials intended for humid, high-temperature service or repeated steam sterilization. Autoclave exposure at 121°C is also directly relevant to medical devices that must survive multiple sterilization cycles.

Cyclic Immersion and Condensation Aging

Some specifications combine immersion, drying, and condensation phases in a cyclic profile to simulate outdoor or automotive service where parts experience alternating wet and dry conditions. Cycle definitions are tailored to the product standard or customer specification.

Post-Exposure Evaluation and Data Expression

Following hydrolytic aging, specimens are conditioned and tested according to the relevant base standards, for example ISO 527 or ASTM D630 for tensile properties, ISO 179/ASTM D256 for impact, and ISO 1133/ASTM D1238 for melt flow. The key reported result is the property retention rate, calculated as the ratio of the aged value to the original value expressed as a percentage. A material is commonly considered hydrolytically stable for a given application when critical properties such as tensile strength or elongation at break retain, for example, 50% to 80% of their initial values after the specified exposure, with the exact acceptance threshold defined by the product specification. Complementary chemical analysis by GPC, intrinsic viscosity measurement, or Fourier-transform infrared spectroscopy confirms the molecular-level extent of hydrolysis.

Applications

Hydrolysis resistance determination supports material selection and reliability qualification across a broad range of industries:

• Automotive: engine bay connectors, sensor housings, cable insulation, fuel system components, and structural parts exposed to splash water and condensation, where PBT and PA materials must resist hydrolytic embrittlement for the vehicle lifetime
• Electronics and electrical engineering: PCB substrates, encapsulants, potting compounds, coil bobbins, and insulating parts validated through 85°C/85%RH damp heat testing per IEC 60068-2-78
• Medical devices: instruments, trays, and polymer components that must withstand repeated steam autoclave sterilization at 121°C or 134°C
• Packaging: PET beverage bottles, food containers, and barrier films where moisture exposure during filling, storage, and hot-fill processing can reduce IV and mechanical strength
• Renewable energy: photovoltaic encapsulant films, junction boxes, and connector systems exposed to long-term heat and humidity
• Consumer goods and appliances: washing machine and dishwasher components, shower fixtures, and outdoor equipment subjected to hot water and humid environments
• Polymer compounding and R&D: evaluating hydrolysis stabilizer packages, comparing resin grades, and supporting biodegradable polymer development where controlled hydrolysis is a design feature rather than a failure mode

FAQ

What is the difference between water absorption testing and hydrolysis resistance testing?

Water absorption testing (ISO 62, ASTM D570) only measures how much water a plastic takes up under defined conditions, whereas hydrolysis resistance testing evaluates the chemical and physical degradation that water exposure causes, including loss of molecular weight, strength, ductility, and appearance. Absorption data alone cannot predict whether a polymer will chemically degrade.

Which plastics are most vulnerable to hydrolysis?

Polymers containing ester, amide, carbonate, or urethane linkages are most susceptible, including PET, PBT, PEN, polycarbonate, aliphatic polyamides, ester-based TPU, epoxy resins, and polylactic acid. Fully hydrocarbon-based polymers such as polyethylene and polypropylene are essentially immune to hydrolysis, though they may still be plasticized or weakened by water in other ways.

How long does a typical hydrolysis resistance test take?

Standard damp heat evaluations commonly run 500 to 2000 hours, while hot water immersion tests may range from 24 hours for screening up to several thousand hours for qualification. Highly accelerated autoclave or pressure cooker tests can compress degradation equivalent to years of service into 96 to 500 hours. The required duration depends on the application standard and the confidence level needed for lifetime prediction.

What results indicate that a plastic has failed the hydrolysis test?

Failure criteria are application-specific, but typical red flags include tensile strength or elongation retention falling below the specified acceptance threshold, visible cracking or embrittlement, a significant drop in molecular weight or intrinsic viscosity, and a marked increase in yellowness index. When any critical property falls below the defined limit, the material is judged unsuitable for the intended moisture-exposed service condition.

Can hydrolysis resistance testing be used to predict product lifetime?

Yes. By testing at multiple elevated temperatures and applying Arrhenius-type kinetic models to the degradation rate of a key property, laboratories can extrapolate an acceleration factor and estimate the service life of the material at the actual use temperature and humidity, provided the degradation mechanism remains consistent across the tested temperature range.