Low-temperature impact testing is a critical evaluation method used to determine the toughness, brittleness resistance, and cold-crack performance of modified plastics in sub-zero service environments. This technical article explains the applicable standards, specimen requirements, test procedures, and typical applications for low-temperature impact testing of modified plastic materials such as toughened PP, glass-fiber reinforced PA, PC/ABS alloys, and filled thermoplastics.
Technical Overview
Modified plastics—including mineral-filled polypropylene (PP), elastomer-toughened PP and PA, glass-fiber reinforced PA6/PA66, PC/ABS alloys, filled PBT, and POM—are widely used in components that must withstand impact loads in cold climates, outdoor installations, refrigerated environments, and electric vehicle applications. As temperature decreases, the molecular mobility of thermoplastic polymers is reduced, rubber phase toughening mechanisms lose effectiveness, and materials that behave in a ductile manner at room temperature can transition to brittle fracture. Low-temperature impact testing quantifies the energy a material absorbs when fractured at a specified sub-zero temperature, allowing engineers to verify cold-climate durability, compare toughener systems, and qualify materials against automotive, appliance, and electrical equipment specifications.
The most common approach combines pendulum impact methods (Charpy and Izod) with a low-temperature conditioning stage, governed by standards such as ISO 179-1, ISO 180, ASTM D256, ASTM D6110, GB/T 1043.1, and GB/T 1843. For full components and plaques, instrumented falling-weight impact (ISO 6603-2) and drop-impact methods at low temperature are also widely applied. Typical test temperatures include 0 °C, -20 °C, -30 °C, and -40 °C, with some automotive and battery-housing specifications extending to -50 °C or -60 °C.
Testing Samples
Standard Notched Specimens
For ISO-based Charpy and Izod testing, specimens are machined or molded to 80 mm × 10 mm × 4 mm bars. A stress-concentrating notch is milled into the specimen in accordance with ISO 2818, most commonly a Type A notch with a 0.25 mm tip radius and 2 mm remaining depth beneath the notch. Notched results reflect a material's sensitivity to sharp defects, scratches, and weld lines, which is essential information for real-world cold-service parts.
Unnotched Specimens
Unnotched specimens of the same geometry are tested when a specification requires it, or when highly filled or fiber-reinforced compounds exhibit extreme notch sensitivity that makes notched data difficult to interpret. Unnotched values generally measure the bulk toughness of the compound and are substantially higher than notched values.
ASTM-Format Specimens
For ASTM D256 (Izod) and ASTM D6110 (Charpy), specimens are typically 63.5 mm × 12.7 mm × 3.2 mm (thicknesses up to 12.7 mm may be used), with a machined notch depth of 2.54 mm. Results are reported in J/m rather than the kJ/m² used in the ISO system, so the test basis must always be stated when comparing data.
Finished Parts and Plaques
Complete components—such as bumper fascias, battery housings, enclosures, and pipe fittings—may be tested directly using low-temperature drop impact or instrumented impact methods. In these cases, the part, its mounting orientation, the impact point, and the drop energy are defined by the product specification or end-user requirement.
Testing Items
Typical test items evaluated for modified plastics at low temperature include:
- Charpy notched impact strength at specified low temperature (ISO 179-1, GB/T 1043.1, ASTM D6110)
- Charpy unnotched impact strength at specified low temperature
- Izod notched impact strength at specified low temperature (ISO 180, GB/T 1843, ASTM D256)
- Izod unnotched impact strength at specified low temperature
- Instrumented falling-weight impact properties: peak force, deflection at peak force, and total energy to failure (ISO 6603-2)
- Low-temperature drop impact or cold-drop pass/fail testing on finished parts
- Ductile-to-brittle transition behavior determined by testing across a temperature ladder (e.g., 23 °C, 0 °C, -20 °C, -30 °C, -40 °C)
- Failure mode classification: complete break, hinged break, partial break, or non-break
Testing Methods
Specimen Conditioning and Temperature Control
Specimens are first conditioned in a standard laboratory atmosphere (e.g., 23 °C / 50% relative humidity per ISO 291 or ASTM D618) and then transferred to an environmental chamber set to the target test temperature. Bars of 4 mm thickness are typically held at temperature long enough to reach full thermal equilibrium—commonly one to four hours depending on the medium (air or liquid bath). To prevent warming, specimens should be fractured within seconds of removal from the chamber; best practice is to fit the pendulum tester with a temperature-controlled enclosure so that conditioning and fracture occur at the same temperature.
Charpy Impact Procedure
In the Charpy configuration, the horizontally supported specimen (support span 62 mm for ISO bars) is struck at mid-span by a pendulum hammer. Hammer capacities of 1 J, 2.75 J, 5.5 J, 11 J, and 22 J are selected so that the energy absorbed is between 10% and 80% of the capacity. At least 10 specimens are tested per temperature and condition, and impact strength is calculated as absorbed energy divided by the cross-sectional area beneath the notch, reported in kJ/m².
Izod Impact Procedure
In the Izod configuration, the specimen is clamped as a vertical cantilever and struck on the notched face above the clamp. The absorbed energy is corrected for windage and friction, and results are reported in kJ/m² (ISO/GB) or J/m (ASTM). Izod testing is frequently specified in North American automotive and electronics material specifications.
Instrumented and Drop Impact Methods
For molded plaques and finished parts, ISO 6603-2 instrumented puncture testing uses a falling dart with a 20 mm diameter striker to record a full force–deflection curve at low temperature. This distinguishes ductile failures (high deflection, high total energy) from brittle failures (sharp force peak, low energy), which is valuable for ranking toughened PP, TPO, and PC/ABS grades. Component-level cold-drop tests, often performed in accredited third-party laboratories, subject actual parts to a defined impact energy at target temperature and evaluate cracking, fracture, or functional failure.
Data Interpretation
Reported results should include the mean, standard deviation, and failure classification for each temperature. A sharp drop in impact strength between two temperatures indicates the ductile-to-brittle transition region; design engineers typically select materials whose transition temperature lies well below the lowest expected service temperature, with a suitable safety margin.
Applications
Low-temperature impact testing of modified plastics supports material qualification and quality control across many industries:
- Automotive exterior and interior components: bumper fascias, body cladding, grilles, mirror housings, and wheel arch liners made from talc-filled and elastomer-toughened PP
- Electric vehicle components: battery pack housings, cell holders, high-voltage connectors, and charging inlets frequently specified at -40 °C
- Home appliances: washing machine components, air conditioner outdoor units, refrigerator liners, and power tool housings
- Electrical and infrastructure: outdoor distribution boxes, cable management products, conduit, and enclosures installed in cold regions
- Pipes and fittings: PE and PVC systems for gas and water distribution requiring cold-climate impact resistance
- Outdoor and sporting goods, logistics pallets, reusable transit packaging, and cold-chain containers
FAQ
What test temperature should be selected for modified plastics?
The temperature is dictated by the end-use specification. Common selections are -20 °C for general outdoor use, -30 °C for automotive exterior parts, and -40 °C for EV battery components, commercial refrigeration, and arctic-climate applications. A room-temperature (23 °C) reference test is normally performed alongside the low-temperature test for comparison.
Should notched or unnotched specimens be used?
Notched specimens are preferred for most specifications because they reproduce stress concentrations present in real parts and better discriminate between toughened and untoughened compounds. Unnotched testing is useful for highly filled or fiber-reinforced grades and for evaluating overall material toughness.
Why does impact strength decrease at low temperature?
As temperature falls, polymer chain segmental mobility decreases and the glass-transition-related relaxation of the rubber toughening phase becomes less effective, so energy cannot be dissipated through shear yielding and cavitation. The material responds by crazing and cracking, shifting from ductile to brittle failure.
How are results judged as pass or fail?
Acceptance criteria are defined by the product standard or customer specification, typically as a minimum impact strength value (for example, notched Charpy impact strength at -30 °C) or as no cracking of a defined fraction of specimens in a component drop test. Failure mode classification per the test standard should accompany any reported value.
Can low-temperature impact testing be performed on finished parts?
Yes. Finished components can be conditioned in a cold chamber and subjected to drop impact, pendulum impact, or instrumented impact at the specified temperature and energy. Such testing is commonly carried out by accredited third-party laboratories under automotive or appliance validation protocols.