Rubber hydrolysis resistance testing is a critical durability evaluation that measures how elastomeric materials withstand degradation when exposed to water, humidity, and elevated temperatures. This article provides a comprehensive technical overview of hydrolysis testing for rubber materials, covering sample requirements, testing items, standard test methods such as ASTM D471, ISO 1817, and GB/T 1690, and typical industrial applications.
Technical Overview
Hydrolysis is a chemical degradation mechanism in which water molecules attack and cleave hydrolyzable bonds along polymer chains, causing chain scission, loss of molecular weight, and progressive deterioration of mechanical properties. In rubber and elastomeric materials, hydrolysis primarily affects polymers containing ester, urethane, amide, or acetal linkages. Polyester-based polyurethanes are particularly vulnerable, while polyether-based polyurethanes, silicone rubbers, EPDM, and fluorocarbon rubbers (FKM) exhibit significantly better hydrolytic stability.
The rubber hydrolysis resistance test is a laboratory accelerated aging procedure designed to simulate long-term exposure to water, humid environments, or hydrothermal service conditions within a compressed timeframe. Test specimens are exposed to water at elevated temperatures or controlled humidity conditions, after which changes in mass, volume, hardness, tensile strength, and elongation at break are measured and compared against unexposed control specimens. The resulting data allows engineers and material scientists to predict service life, qualify materials for wet environments, and support quality control during production.
Testing is commonly performed in accordance with recognized international and national standards, including ASTM D471 (Standard Test Method for Rubber Property—Effect of Liquids), ISO 1817 (Rubber, vulcanized or thermoplastic—Determination of effect of liquids), GB/T 1690 (Chinese national standard for resistance of vulcanized rubber to liquids), and related conditioning and mechanical testing standards such as ASTM D412, ISO 37, and ISO 23529. Accredited third-party laboratories routinely conduct these evaluations for material certification and product compliance purposes.
Testing Samples
Standard Vulcanized Rubber Specimens
Laboratory-prepared vulcanized sheets are the most common sample format for hydrolysis resistance studies. Dumbbell-shaped tensile specimens are die-cut from cured sheets with a thickness of 2.0 mm ± 0.2 mm in accordance with ASTM D412 or ISO 37. Rectangular strip specimens and button samples for hardness measurement per ASTM D2240 or ISO 48 are also prepared. Using standard specimens ensures that measured property changes reflect the intrinsic hydrolytic stability of the rubber compound rather than geometric or processing variables.
Finished Rubber Components
Actual manufactured parts, including O-rings, gaskets, seals, diaphragms, rubber hoses, vibration isolators, and molded components, may be tested directly to evaluate real-world product performance. Testing finished components provides insight into how processing conditions, part geometry, and surface finishes influence hydrolytic durability in service.
Polyurethane and Specialty Elastomer Samples
Thermoplastic polyurethanes (TPU) and cast polyurethane elastomers are frequently submitted for hydrolysis resistance testing, particularly when comparing polyester-based versus polyether-based formulations. Millable gums, sponge rubbers, and rubber-to-metal bonded assemblies can also be evaluated, provided that adequate control specimens are retained for baseline comparison.
Sample Preparation and Conditioning
All specimens must be conditioned prior to testing in a controlled atmosphere of 23 °C ± 2 °C and 50 % ± 5 % relative humidity for a minimum of 16 to 24 hours, following ISO 23529 or ASTM D618. Specimens should be free from surface contamination, and the number of replicate specimens per test condition is typically at least three to ensure statistical validity.
Testing Items
The rubber hydrolysis resistance experiment evaluates material performance through a combination of physical, mechanical, and morphological measurements taken before and after water exposure:
- Mass change rate: percentage increase or loss in specimen weight after immersion, indicating water absorption or leaching of soluble constituents.
- Volume change rate: percentage swelling or shrinkage determined by displacement or dimensional measurement, reflecting fluid uptake or material extraction.
- Tensile strength retention: the ratio of post-exposure tensile strength to original tensile strength, expressed as a percentage.
- Elongation at break retention: change in the strain at rupture, one of the most sensitive indicators of hydrolytic chain scission.
- Hardness change: variation in Shore A or Shore D hardness, typically measured before and after exposure.
- Modulus change (100 % or 300 %): alteration in stress at a defined elongation, revealing changes in crosslink density.
- Compression set (optional): permanent deformation after sustained compression under wet-heat conditions, relevant for sealing applications.
- Visual and microscopic examination: surface cracking, blistering, tackiness, discoloration, whitening, or the formation of hydrolysis by-products.
Testing Methods
Water Immersion Test at Elevated Temperature
This is the principal method for evaluating hydrolysis resistance. Specimens are fully immersed in distilled or deionized water maintained at a constant elevated temperature, commonly 70 °C, 85 °C, or 95 °C, for exposure durations ranging from 24 hours to 30 days or longer. The procedure follows ASTM D471, ISO 1817, or GB/T 1690, using closed glass or stainless-steel vessels with temperature control of ± 2 °C. Higher temperatures accelerate hydrolysis kinetics in accordance with Arrhenius behavior, enabling extrapolation of long-term service performance.
Damp Heat (Humid Heat) Aging Test
Specimens are suspended in a humidity cabinet at controlled conditions such as 85 °C and 85 % relative humidity, following guidance from IEC 60068-2-78 or ISO 188 for accelerated aging. This method is particularly relevant for electronic enclosures, cable accessories, and components exposed to tropical or condensing environments, as it evaluates hydrolysis under vapor-phase rather than liquid-phase exposure.
Pressurized Hydrothermal (Autoclave) Testing
For highly accelerated evaluation, specimens may be exposed to pressurized hot water or saturated steam at 100 °C to 120 °C in sealed autoclave vessels. This method compresses months or years of service exposure into days and is frequently used in screening studies for polyurethane and polyester-elastomer compounds, though results must be interpreted carefully to avoid unrealistic degradation mechanisms.
Cycling and Combined Exposure Tests
Some test programs combine immersion with drying cycles, thermal cycling, or mechanical loading to replicate service conditions such as automotive underhood environments, where water exposure alternates with heat and vibration.
Post-Exposure Measurement and Calculation
Upon completion of exposure, specimens are removed, quickly blotted dry with lint-free cloth, and weighed and measured within a standardized time window, typically within 30 minutes, to minimize error from evaporation. Tensile, hardness, and elongation tests are then performed. Property change is calculated as follows: mass change (%) = (m2 − m1) / m1 × 100, and tensile strength change (%) = (T2 − T1) / T1 × 100, where subscript 1 denotes the original value and subscript 2 denotes the value after exposure. Acceptance criteria are defined by product specifications or material datasheets, with high-performance compounds typically required to retain at least 70 % to 80 % of their original tensile strength and elongation.
Applications
Rubber hydrolysis resistance testing supports material selection, quality assurance, and failure analysis across a wide range of industries:
- Automotive engineering: cooling system hoses, radiator seals, windshield wiper blades, and underhood components exposed to hot water, coolant, and condensation.
- Marine and offshore engineering: dock fenders, underwater seals, and cable jackets subjected to continuous seawater immersion.
- Medical devices: tubing, seals, and elastomeric components that must tolerate repeated exposure to water, steam sterilization, and autoclave conditions.
- Industrial sealing: O-rings, gaskets, and hydraulic seals in water-based fluid systems, water treatment plants, and food-processing equipment.
- Footwear and consumer goods: TPU outsoles, waterproof membranes, and wearable elastomer products exposed to perspiration and moisture.
- Wire and cable: moisture-resistant insulation and sheathing compounds qualified through long-term damp heat testing.
- Material research and development: comparative evaluation of polyester-based versus polyether-based polyurethane formulations and development of hydrolysis stabilizers.
FAQ
What is rubber hydrolysis resistance testing?
It is a laboratory accelerated aging test in which rubber specimens are exposed to water, hot water, or humid environments at elevated temperatures, and the resulting changes in mass, volume, hardness, tensile strength, and elongation are measured to assess the material's ability to resist water-induced chemical degradation.
Which rubber materials are most susceptible to hydrolysis?
Polyester-based polyurethanes are the most susceptible because their ester linkages are readily attacked by water, especially at elevated temperatures. Polyether-based polyurethanes, EPDM, silicone rubber, and fluorocarbon rubber (FKM) offer substantially better hydrolysis resistance, making them preferred for wet and hydrothermal service environments.
What standards are used for rubber hydrolysis testing?
The primary standards include ASTM D471 and ISO 1817 for liquid immersion testing, GB/T 1690 as the corresponding Chinese national standard, IEC 60068-2-78 for damp heat testing, and ISO 188 for accelerated heat aging. Mechanical evaluation before and after exposure is conducted per ASTM D412 or ISO 37 for tensile properties and ASTM D2240 or ISO 48 for hardness.
How long does a hydrolysis resistance test take?
Exposure durations vary from as short as 24 hours for screening tests to 30 days or longer for qualification programs. Highly accelerated autoclave tests at 100 °C to 120 °C can compress months of simulated service into a few days, while long-term validation testing may extend to several months depending on the product specification.
What results indicate good hydrolysis resistance?
Materials demonstrating good hydrolysis resistance show minimal mass and volume change, retain at least 70 % to 80 % of original tensile strength and elongation at break, exhibit hardness changes typically within ± 10 Shore points, and display no visible cracking, blistering, or surface degradation after exposure.
How is hydrolysis testing different from ordinary water absorption testing?
Water absorption testing, such as ASTM D570 or ISO 62, measures only physical water uptake, whereas hydrolysis resistance testing evaluates chemical degradation by combining water exposure at elevated temperature with mechanical property evaluation. Hydrolysis testing therefore captures both reversible physical absorption and irreversible chemical breakdown of the polymer network.