Plastic Hot Tensile Properties Testing: Standards, Methods, and Applications

Plastic hot tensile properties testing measures the tensile strength, modulus, and elongation of plastic materials at elevated temperatures, providing critical data for applications exposed to thermal stress. This article explains the applicable standards such as ISO 527 and ASTM D638, specimen requirements, test procedures, and typical industrial applications. It serves as a practical reference for engineers, quality managers, and material developers evaluating polymer performance under heat.

Technical Overview

Hot tensile testing, also referred to as elevated-temperature tensile testing, is a mechanical evaluation method used to determine how plastic materials behave when subjected to tensile loading at temperatures above standard laboratory conditions. Because polymers are viscoelastic materials, their stiffness, strength, and ductility change significantly as temperature rises and molecular mobility increases. Testing at ambient temperature alone is therefore insufficient for components that operate in hot environments.

The test is performed on a universal testing machine equipped with an environmental chamber that maintains a stable, uniform temperature around the specimen during loading. Key results include tensile strength at temperature, tensile modulus, yield stress, and elongation at break, all of which are essential inputs for design, material selection, and failure analysis.

The principal standards governing this test include ISO 527-1 and ISO 527-2, ASTM D638, GB/T 1040 series, and JIS K 7113. Hot tensile testing is routinely carried out by accredited third-party laboratories as well as in-house quality and R&D facilities for industries such as automotive, aerospace, electrical and electronic equipment, piping systems, and home appliances.

Testing Samples

Rigid and Semi-Rigid Molded Plastics

For injection-molded or machined rigid plastics, the most common specimens are the ISO 527-2 Type 1A and Type 1B dumbbell-shaped bars, or the equivalent ASTM D638 Type I through Type V specimens depending on material thickness and available material quantity. Specimens cut from finished parts may also be tested when the objective is to verify the properties of an actual component, provided the geometry and freedom from defects are confirmed.

Films and Sheets

Thin plastic films are tested using strip or dumbbell specimens in accordance with ISO 527-3 or ASTM D882. Film testing at elevated temperature is particularly relevant for packaging laminates, capacitor films, and adhesive films that undergo thermal processing or service in warm environments.

Specimen Preparation and Conditioning

Specimens should be prepared in a consistent manner, typically by injection molding in line with ISO 294-1 or by machining from compression-molded or extruded plaques. Multipurpose test specimens conforming to ISO 3167 are recommended when results need to be comparable across different test methods. Prior to testing, specimens are conditioned at 23°C and 50% relative humidity in accordance with ISO 291 or ASTM D618 unless a material-specific conditioning regime applies. For hot tensile testing, the specimen is additionally soaked inside the chamber until it reaches thermal equilibrium, commonly five to ten minutes depending on thickness.

Testing Items

The following properties are typically reported from a hot tensile test program:

• Tensile strength at elevated temperature
• Tensile stress at yield
• Tensile stress at break
• Tensile modulus (Young's modulus) at elevated temperature
• Elongation at yield
• Elongation at break
• Nominal elongation at maximum force
• Complete stress-strain curves at each test temperature

Retained strength and retained modulus percentages, calculated by comparing elevated-temperature results with baseline 23°C results, are also widely used to quantify thermal sensitivity of a material.

Testing Methods

Equipment and Temperature Control

Testing is conducted on a calibrated universal testing machine fitted with a temperature-controlled environmental chamber, typically covering a range from sub-ambient conditions up to 250°C or 300°C, with some systems reaching 350°C for high-performance engineering plastics. The chamber is equipped with resistant heating elements, a circulating fan for uniformity, and a programmable controller. Load cells and grips located inside or thermally isolated from the chamber must be suitable for operation at the test temperature, and strain measurement is performed either with a high-temperature contact extensometer or a non-contact video extensometer viewing the specimen through an inspection window.

Test Procedure

The general procedure follows the base tensile standards with additional thermal steps:

• Measure and record the width, thickness, and gauge length of each specimen at room temperature.
• Set the chamber to the target temperature and allow it to stabilize within the tolerance specified by the standard, commonly ±2°C or better.
• Mount the specimen in the grips, close the chamber, and soak the specimen until thermal equilibrium is reached.
• Apply the specified crosshead speed; for ISO 527-2 this is typically 1 mm/min for modulus determination and 50 mm/min for strength determination, while ASTM D638 specifies speeds from 5 to 500 mm/min depending on specimen type.
• Record force and extension continuously until rupture, or until the yield and break criteria defined by the standard are met.
• Repeat the test on a statistically valid number of specimens, with a minimum of five per condition recommended by ISO 527-1.

Data Reduction and Calculation

Engineering stress is calculated by dividing the measured force by the original minimum cross-sectional area, and strain is derived from the extensometer reading over the original gauge length. The tensile modulus is determined as a secant modulus between two specified strain points, 0.05% and 0.25% strain according to ISO 527-1, taken from the steepest linear portion of the stress-strain curve. Testing at multiple temperatures, for example 40°C, 85°C, 100°C, 125°C, 150°C, and 180°C, allows construction of a strength-versus-temperature curve that supports safe design near the material's softening region.

Applications

Hot tensile testing supports material qualification, design validation, and failure investigation across many sectors:

• Automotive: under-hood components, cooling system parts, connectors, and interior parts exposed to solar heat loading.
• Electrical and electronics: connectors, coil bobbins, insulating parts, and structural components that experience heat from solder reflow processes or continuous operating currents.
• Piping and plumbing: hot-water pipes, fittings, and valve bodies made from PP-R, PEX, PB, and engineering thermoplastics.
• Aerospace and rail: lightweight structural and interior components requiring verified performance across wide temperature envelopes.
• Home appliances and HVAC: housings, fan impellers, and duct components located near heat sources.
• Materials R&D: comparison of resin grades, filler and reinforcement effects, and validation of simulation input data at service temperatures.

FAQ

What is the difference between hot tensile testing and HDT or Vicat softening testing?

Heat deflection temperature (HDT, ISO 75 or ASTM D648) and Vicat softening temperature (ISO 306 or ASTM D1525) determine single temperature threshold points under a fixed load or penetration condition. Hot tensile testing instead provides the full set of tensile properties at a chosen temperature, giving designers quantitative strength and stiffness values for calculation purposes rather than only a comparative softening point.

What temperature range can be tested?

Most laboratories offer testing from approximately -40°C or -70°C up to 250°C or 300°C using standard environmental chambers, with extended ranges available for high-temperature polymers such as PEEK, PI, and PBI. The practical upper limit depends on both the chamber capability and the grip and extensometer hardware.

How many specimens are required for each temperature?

ISO 527-1 recommends testing at least five specimens for each condition, while ASTM D638 also recommends a minimum of five representative specimens. Additional specimens may be needed if results show excessive scatter, which is common near the glass transition or melting region.

Why does tensile strength decrease while elongation sometimes increases at higher temperatures?

As temperature rises, polymer chain mobility increases, which lowers stiffness and strength. Ductility often improves because the material can deform more readily before fracture, so elongation at break may increase substantially, especially for toughened or semi-crystalline grades tested above their glass transition temperature.

Which standards should be specified when ordering a test?

The commonly specified standards are ISO 527-1 with ISO 527-2 for molded plastics, ISO 527-3 for films, ASTM D638 for rigid plastics, ASTM D882 for thin films, and the GB/T 1040 series for projects following Chinese national standards. The requested test temperature, conditioning duration, and crosshead speed should always be clearly defined in the test plan to ensure reproducible, comparable results.