Constant Temperature and Humidity Testing of Plastic Materials: Standards, Methods, and Technical Gu

Constant temperature and humidity testing is a core environmental reliability evaluation method used to assess how plastic materials perform when exposed to controlled heat and moisture conditions over extended periods. By simulating real-world humid environments, this test reveals changes in dimensions, appearance, mechanical strength, and electrical performance of polymer materials. This guide covers the applicable standards, sample types, test items, methods, and typical applications of hygrothermal testing for plastics.

Technical Overview

Constant temperature and humidity testing, also known as steady-state damp heat testing or hygrothermal exposure testing, is a widely used environmental reliability test for plastic materials. During the test, plastic specimens are placed inside a climate chamber that maintains a fixed combination of temperature and relative humidity for a defined duration. The purpose is to evaluate how hygroscopic absorption, thermal expansion, and hydrolytic degradation affect the material's physical, mechanical, and electrical properties.

Plastic materials are inherently sensitive to moisture and temperature. Polar polymers such as polyamide (nylon), polycarbonate, and polyurethane absorb water readily, which can cause swelling, plasticization, reduced strength, and dimensional change. Elevated temperature accelerates these effects and may promote hydrolysis, oxidation, and residual stress relief, leading to warpage, blistering, discoloration, or embrittlement. Constant temperature and humidity testing allows manufacturers to predict long-term material behavior in tropical, coastal, or industrial humid environments within a compressed laboratory timeframe.

The test is typically performed in accredited third-party laboratories or in-house reliability laboratories following internationally recognized standards, including:

• IEC 60068-2-78 – Environmental testing, Part 2-78: Tests, Test Cab: Damp heat, steady state
• IEC 60068-2-30 – Damp heat, cyclic test (12 + 12 hour cycle)
• ISO 4611 – Plastics: Determination of the effects of exposure to damp heat, water spray and salt mist
• ISO 291 – Plastics: Standard atmospheres for conditioning and testing
• ASTM D618 – Standard Practice for Conditioning Plastics for Testing
• ASTM D570 – Standard Test Method for Water Absorption of Plastics
• GB/T 2423.3 and GB/T 2423.4 – Chinese national standards for steady-state and cyclic damp heat testing
• JIS Z 2371 and related JIS environmental test methods

Testing Samples

Thermoplastic Resins and Molded Specimens

Common test samples include injection-molded test bars and plaques of polypropylene (PP), polyethylene (PE), acrylonitrile butadiene styrene (ABS), polycarbonate (PC), polyamide (PA6, PA66), polyoxymethylene (POM), polymethyl methacrylate (PMMA), polyethylene terephthalate (PET), and polyvinyl chloride (PVC). These are typically prepared as standardized dumbbell specimens for tensile testing or flat plates for dimensional and appearance evaluation.

Thermosetting Plastics

Epoxy resins, phenolic molding compounds, unsaturated polyesters, and melamine-formaldehyde materials are tested to evaluate moisture-induced degradation, surface tracking, and loss of rigidity, which are critical for electrical insulation components and encapsulants.

Filled and Reinforced Plastic Composites

Glass-fiber reinforced, carbon-fiber reinforced, and mineral-filled plastic compounds are tested to assess fiber-matrix interface degradation under hygrothermal conditions, which can significantly reduce mechanical performance in structural applications.

Finished Plastic Products

Complete components such as electronic enclosures, connectors, battery housings, automotive interior trims, films, and medical device parts can be tested in their end-use configuration to evaluate real product behavior rather than raw material behavior.

Testing Items

Depending on the product specification and applicable standard, the following properties are commonly evaluated before and after constant temperature and humidity exposure:

• Dimensional stability: measurement of length, width, thickness, and flatness changes to detect swelling, warpage, and shrinkage
• Appearance evaluation: visual and instrumental inspection for blistering, whitening, discoloration, staining, surface blooming, cracking, and loss of gloss
• Mass change and moisture absorption: gravimetric determination of water uptake, typically referenced to ISO 62 or ASTM D570
• Mechanical property retention: tensile strength, flexural strength, impact strength, and hardness measured before and after exposure to calculate retention rates
• Electrical properties: insulation resistance, surface resistance, volume resistivity, dielectric strength, and comparative tracking index for insulating plastics
• Functional performance: assembly fit, sealing integrity, snap-fit retention, and mating force of finished plastic components after conditioning

Testing Methods

Steady-State Damp Heat Testing

The specimen is exposed to a constant temperature and constant relative humidity for a continuous period. Commonly used conditions include:

• 23°C / 50% RH – standard conditioning atmosphere per ISO 291
• 40°C / 93% RH – classic damp heat condition widely used for consumer and industrial products
• 40°C / 95% RH – alternative severity level per IEC 60068-2-78 and GB/T 2423.3
• 85°C / 85% RH – accelerated damp heat condition used for electronics, encapsulants, and high-reliability components
• 60°C / 95% RH or 55°C / 95% RH – intermediate accelerated conditions for automotive and electrical equipment

Typical test durations are 24 h, 48 h, 96 h, 168 h (1 week), 500 h, or 1000 h, depending on the product specification or customer requirement.

Cyclic Damp Heat Testing

For products exposed to fluctuating humidity, cyclic tests such as IEC 60068-2-30 and GB/T 2423.4 apply a 12-hour cycle in which temperature varies between 25°C and an upper value (commonly 55°C or 40°C) at high relative humidity. The repeated condensation and drying cycles evaluate resistance to moisture ingress, delamination, and corrosion at plastic-metal interfaces.

Test Procedure

A typical test sequence includes the following steps:

• Initial conditioning of specimens in a standard atmosphere (usually 23°C / 50% RH for at least 24 h)
• Baseline measurement of dimensions, mass, appearance, and functional or mechanical properties
• Placement of specimens in the climate chamber with adequate air circulation and spacing to ensure uniform exposure
• Exposure at the specified temperature, humidity, and duration, with continuous monitoring and recording of chamber parameters
• Recovery of specimens under standard atmosphere or immediate evaluation, as defined by the specification
• Post-test measurement and comparison with baseline values, followed by calculation of change rates and property retention percentages
• Preparation of a test report documenting conditions, equipment, observations, and results

Equipment Requirements

The test is performed in a programmable temperature and humidity chamber capable of maintaining temperature uniformity within ±2°C and humidity uniformity within ±3% RH. Equipment should be calibrated and traceable to national or international metrology standards to ensure result validity and reproducibility.

Applications

Constant temperature and humidity testing of plastics supports quality control, material qualification, and reliability demonstration across many industries:

• Automotive: validation of interior trims, connectors, lamp housings, under-hood components, and battery enclosures against humid and high-temperature service environments
• Electronics and electrical: evaluation of insulating plastics, switch housings, cable accessories, PCB laminates, and encapsulation materials for moisture-related electrical failure
• Medical devices: assessment of sterilizable and non-sterilizable plastic components, packaging materials, and drug-delivery device parts for stability in storage and use conditions
• Consumer goods: qualification of appliance housings, cosmetic packaging, toys, and sporting goods for tropical and coastal market climates
• Packaging and films: determination of moisture barrier behavior and dimensional stability of plastic films, containers, and laminates
• Research and development: comparison of new resin formulations, additives, and processing parameters for hygrothermal durability

FAQ

What is the most common test condition for plastic materials?

The 40°C / 93% RH steady-state condition defined in IEC 60068-2-78 and GB/T 2423.3 is the most widely applied condition for general plastic materials. For electronics-related plastics, 85°C / 85% RH is frequently specified as an accelerated condition.

How long does a constant temperature and humidity test take?

Common durations range from 24 hours to 1000 hours. The required duration depends on the product standard, intended market, and whether the test is used for routine quality control or long-term reliability demonstration.

How many samples are required for testing?

Typically, at least three to five specimens per condition are recommended for statistical validity of mechanical property measurements, plus reference specimens retained under standard atmosphere for baseline comparison.

Can finished plastic products be tested instead of material specimens?

Yes. Testing complete components provides direct evidence of product-level performance, including fit, function, and appearance, and is often required by automotive and electronics specifications before product release.

What results indicate a failure?

Failure criteria are defined by the product specification and may include dimensional change beyond tolerance, visible cracking or blistering, mechanical property retention below a specified percentage, or electrical insulation values falling outside acceptance limits after exposure.

Why is third-party testing recommended?

Accredited third-party laboratories provide calibrated equipment, standardized procedures, and impartial test reports that are widely accepted by buyers, regulators, and certification bodies, supporting market access and customer confidence in the material or product.