Tensile Testing of Plastics in Humid Environments

Tensile testing of plastics in humid environments evaluates how absorbed moisture influences the mechanical performance of polymer materials, providing critical data for material selection and product design. By conditioning specimens to defined moisture levels and testing them according to recognized standards such as ISO 527 and ASTM D638, manufacturers can quantify changes in tensile strength, modulus, and elongation under real-world wet service conditions. This article explains the principles, sample requirements, test methods, and typical applications of humidity-conditioned tensile testing for plastics.

Technical Overview

Many thermoplastics are hygroscopic, meaning they absorb moisture from their surrounding environment until they reach an equilibrium water content that depends on temperature and relative humidity. Water molecules penetrate the polymer matrix and act as a plasticizer, which typically reduces tensile strength and stiffness while increasing ductility and elongation at break. For highly moisture-sensitive materials such as polyamide 6 (PA6) and polyamide 66 (PA66), the difference between the dry-as-molded state and the fully conditioned (saturated) state can exceed 40–50% in tensile strength, making it essential to characterize materials in the moisture state that matches actual service conditions.

Humid-environment tensile testing combines two procedures: a moisture conditioning stage governed by standards such as ISO 62, ASTM D570, ISO 291, ASTM D618, and ISO 1110, followed by a tensile test performed according to ISO 527 (Parts 1 and 2) or ASTM D638. The resulting data allow engineers to compare dry and wet properties, validate long-term performance in damp or immersed applications, and support quality control, material certification, and failure analysis. Accredited third-party laboratories routinely perform these tests to provide internationally recognized, standards-based reports.

Testing Samples

ISO Multipurpose Dumbbell Specimens

Testing to ISO 527-2 typically uses the Type 1A (thickness 4 mm) or Type 1B multipurpose test specimens, usually produced by injection molding in accordance with ISO 294 or ISO 20753. These specimens have a defined gauge length of 50 mm (1A) or 50 mm with alternative dimensions for 1B, and they are the reference geometry for most European and international data sheets.

ASTM D638 Specimens

Testing to ASTM D638 uses dumbbell specimens of Types I through V, with Type I (thickness 3.2 mm or 4 mm, gauge length 50 mm) being the most common for rigid plastics. Type V miniaturized specimens are used when material is limited, such as when sampling from finished parts or performing screening studies on compound formulations.

Specimens Machined from Finished Products

When validating an actual component, specimens can be die-cut or machined from molded parts, extruded sheets, films, or pipe sections. Because processing history, orientation, and skin-core morphology strongly influence both moisture uptake and tensile behavior, cut specimens should be prepared in consistent directions (parallel and perpendicular to flow where applicable) and their origin documented.

Conditioning Requirements

Before moisture exposure, specimens are often dried to establish a reference dry-as-molded state, for example by conditioning in a desiccator or drying oven. They are then subjected to the specified humidity regime. Specimen thickness significantly affects diffusion time—water absorption follows Fickian diffusion kinetics in most cases—so thick sections require substantially longer conditioning than thin films to reach equilibrium.

Testing Items

A humid-environment tensile test program typically reports the following parameters for each moisture condition evaluated:

• Tensile strength at yield and at break
• Tensile stress at 50% strain (for materials that do not fracture within the standard range)
• Tensile strain at yield and nominal strain at break
• Tensile modulus (Young's modulus, measured between 0.05% and 0.25% strain) and, where required, the 1% secant modulus
• Poisson's ratio, when biaxial strain measurement is performed
• Moisture content of specimens at the time of testing, determined gravimetrically
• Retention percentages comparing conditioned properties against the dry baseline

Optional supplementary measurements include water absorption saturation value and rate (ISO 62 / ASTM D570), dimensional changes after conditioning, and visual assessment of surface whitening or crazing induced by moisture.

Testing Methods

Step 1: Baseline Drying

Specimens are dried in a ventilated oven (commonly 80–110 °C depending on the polymer, always below the glass transition or heat deflection limits) or over desiccant until mass stabilization is achieved. This establishes the dry reference condition and removes residual process moisture that could otherwise bias results.

Step 2: Humidity Conditioning

Specimens are then exposed to the specified humid environment using one of the following approaches:

• Standard atmosphere conditioning at 23 °C and 50% relative humidity per ISO 291 or ASTM D618, representing typical indoor service
• Elevated humidity storage at 23 °C with 90% or higher relative humidity in climate chambers
• Immersion in water at 23 °C per ISO 62 Method 1 or ASTM D570, or immersion at elevated temperatures (for example 50 °C or 70 °C) to accelerate saturation
• Accelerated conditioning in saturated steam environments per ISO 1110, frequently used for polyamides to reach the equilibrium moisture content corresponding to 50% RH more rapidly

Periodic weighing tracks mass gain until equilibrium is reached, defined by a change in mass below a specified threshold over a defined interval.

Step 3: Tensile Testing

Conditioned specimens are mounted in a universal testing machine fitted with appropriate grips and, for modulus determination, a contact extensometer or non-contact video extensometer on the gauge length. Testing is conducted at 23 °C and 50% RH unless the specification requires otherwise, using the test speed prescribed by the standard and material type—commonly 1 mm/min for modulus and 5–50 mm/min for strength and elongation measurements per ISO 527 and ASTM D638. Specimens should be tested promptly after removal from the conditioning environment to prevent moisture loss, and a sufficient number of replicates (typically at least five per condition) is tested for statistical validity.

Step 4: Data Evaluation and Reporting

The test report documents the material designation, specimen type and source, conditioning method, duration, equilibrium moisture content, test speed, and all measured tensile properties with their statistical spread. Comparing the wet-condition results against dry baseline values yields property retention data, which forms the basis for design allowables, material datasheet "conditioned" values, and pass/fail decisions against specification limits.

Applications

Humid-environment tensile testing supports a wide range of industries in which plastic components encounter moisture during manufacturing, storage, or service:

• Automotive engineering: under-hood connectors, cooling system components, air intake manifolds, and structural clips molded from polyamide, polyester, and their glass-filled grades
• Electrical and electronics: connectors, housings, and cable accessories exposed to humid climates or washing processes
• Pipes, valves, and fittings: pressure and drainage systems made from PVC, PE, PP, and POM operating in permanently wet conditions
• Appliances and plumbing: water-contact components in washing machines, dishwashers, kettles, and sanitary fittings
• Medical devices: instruments and housings subjected to steam sterilization, autoclaving, or aqueous cleaning cycles
• Outdoor and construction products: geogrids, cable ties, garden equipment, façade elements, and sports goods exposed to rain and ambient humidity
• Materials research and development: evaluating the effectiveness of impact modifiers, stabilizers, hydrolysis-resistant grades, and bio-based polymers whose properties are strongly moisture-dependent

FAQ

Why does moisture reduce the tensile strength of plastics? Absorbed water molecules occupy free volume between polymer chains and disrupt hydrogen bonding, effectively plasticizing the material. This lowers tensile strength and modulus while usually increasing ductility; in polyamides, the effect is especially pronounced because water hydrogen-bonds with the amide groups.

How long does humidity conditioning take? Conditioning time depends on specimen thickness, diffusivity of the polymer, and the target humidity level. Thin films may reach equilibrium within hours, whereas 4 mm dumbbells of polyamide immersed at 23 °C can require several weeks. Accelerated methods, such as elevated-temperature immersion or ISO 1110 steam conditioning, are often used to shorten the schedule.

Which standards govern this testing? Tensile properties are measured to ISO 527-1/-2 or ASTM D638; conditioning atmospheres are defined by ISO 291, ISO 1110, and ASTM D618; and water absorption behavior is characterized by ISO 62 and ASTM D570. Composite materials may additionally reference ASTM D5229 for moisture absorption characterization.

Should specimens be tested while wet or after removal from the humid environment? The specification dictates the approach, but in most cases specimens are tested immediately after conditioning so that the measured properties reflect the conditioned moisture state. Delays between removal and testing should be minimized and recorded, since moisture can evaporate from thin sections within minutes.

Which plastics most require humid-environment tensile data? Hygroscopic engineering polymers are the primary candidates, including PA6, PA66, PA12, PA46, PBT, PET, TPU, PC, ABS, and cellulose-based or bio-based polymers. Non-hygroscopic polymers such as PE, PP, and PTFE show minimal moisture sensitivity, and their tensile data from standard atmospheres remain representative.