PP Plastic Low-Temperature Charpy Impact Testing: Standards, Methods, and Applications

PP plastic low-temperature Charpy impact testing measures the impact resistance of polypropylene under sub-zero service conditions using a simply supported beam (Charpy) configuration. Because polypropylene undergoes a ductile-to-brittle transition as temperatures fall, this test is essential for validating materials used in automotive parts, appliances, piping, and cold-chain packaging. This guide explains the specimen requirements, applicable standards, test procedures, and data interpretation involved in low-temperature Charpy impact testing of PP.

Technical Overview

The Charpy impact test, also known as the simply supported beam impact test, is one of the most widely used methods for evaluating the toughness of thermoplastics. A rectangular specimen is laid horizontally across two supports and struck at mid-span by a swinging pendulum. The energy absorbed during fracture is recorded and converted into impact strength, typically expressed in kJ/m².

For polypropylene (PP), low-temperature testing is particularly important. PP has a glass transition region roughly between -10°C and 0°C for its amorphous phase, and below this range the material loses much of its ductility. At room temperature, a standard PP specimen may exhibit hinged or ductile fracture; at -20°C or -40°C, the same material often fails in a completely brittle manner with very low energy absorption. Low-temperature Charpy testing therefore reveals whether a PP grade, especially an impact-modified copolymer, can survive cold-climate service, cold assembly operations, or drop and impact events during winter transport.

The test is governed internationally by ISO 179-1 (Plastics — Determination of Charpy impact properties — Part 1: Non-instrumented impact test), with ASTM D6110 as the corresponding North American method and GB/T 1043.1 as the Chinese national adoption of the ISO standard.

Testing Samples

Standard Specimens

The preferred specimen for ISO 179-1 is a rectangular bar of 80 mm length, 10 mm width, and 4 mm thickness. Specimens may be directly injection molded or machined from sheets, molded plaques, or finished parts. Because processing conditions strongly influence the morphology and therefore the toughness of PP, specimens intended for material comparison should be molded under identical and documented conditions.

Notched Specimens

For notched Charpy testing, a V-notch is machined into the center of one face to a depth of 2 mm, leaving 8 mm of remaining width. The notch root radius defines the notch type:

• Type A notch: radius of 0.25 mm, the most common and most severe geometry, suitable for materials of low to moderate toughness.
• Type B notch: radius of 1.00 mm, used for tougher materials or when a less severe stress concentrator is required.

Notching quality is critical for PP, since a blunt or poorly machined notch can artificially raise the measured impact strength. Notches should be cut with a fresh cutting tool, and notch geometry should be verified microscopically.

Specimen Conditioning

Before low-temperature exposure, specimens are normally conditioned at standard laboratory atmosphere, 23°C and 50% relative humidity per ISO 291, for a minimum of 16 hours unless otherwise specified. Each reported result should be based on at least five, and preferably ten, valid specimens.

Testing Items

A low-temperature Charpy test program for PP typically includes the following measurement items:

• Charpy notched impact strength (a_cN ... reported as kJ/m²): energy absorbed per unit cross-sectional area behind the notch.
• Charpy unnotched impact strength (a_cU): overall toughness of the material without stress concentration.
• Brittle-to-ductile transition assessment: testing across a temperature ladder, for example 23°C, 0°C, -20°C, -30°C, and -40°C, to locate the temperature at which failure mode changes.
• Fracture appearance classification: complete break (C), hinged break (H), partial break (P), or non-break (N), which qualitatively documents the ductility of the failure.
• Comparative evaluation of material grades: homopolymer versus random copolymer versus impact (block) copolymer, or unfilled versus mineral-filled or glass-filled PP.

Testing Methods

Apparatus

The test uses a pendulum impact machine equipped with interchangeable hammers, commonly ranging from 1 J to 25 J for PP specimens. The specimen rests on two supports set 62 mm apart, and the striking edge hits the specimen opposite the notch at a nominal velocity of approximately 2.9 or 3.8 m/s depending on the pendulum. The machine must be calibrated and verified per ISO 13802, and the selected pendulum energy should consume between 10% and 80% of the available energy for valid measurements. Air resistance and friction corrections are applied when required by the standard.

Low-Temperature Conditioning

Specimens are brought to the test temperature using one of two approaches:

• Environmental chamber: specimens are conditioned in a mechanically refrigerated or liquid-nitrogen-cooled chamber, or the entire test station is enclosed in a cooled cabinet so that supports and anvil are also at temperature.
• Liquid bath: specimens are immersed in a temperature-controlled bath, such as a dry-ice alcohol mixture or silicone oil, held at the target temperature.

A conditioning dwell of at least 30 minutes, and commonly up to 1 hour or more depending on the medium, is used to ensure thermal equilibrium through the 4 mm thickness. If specimens are conditioned outside the tester, the interval between removal from the conditioning medium and impact must not exceed 10 seconds to prevent the specimen from warming.

Test Procedure

The specimen is placed with the notch facing away from the pendulum striking edge, so the hammer strikes the face opposite the notch. The pendulum is released from a known height, the energy absorbed is read from the scale or data system, and the fracture mode is recorded visually. The procedure is repeated for the full set of specimens at each temperature, and anomalous results, such as specimens touched twice or broken at a support, are discarded and retested.

Calculation and Reporting

Charpy impact strength is calculated by dividing the corrected absorbed energy by the cross-sectional area of the specimen (or by the area beneath the notch for notched tests). The report should identify the material grade, molding conditions, notch type, test temperature, conditioning medium and duration, pendulum energy, individual and mean impact strengths, standard deviation, and fracture classification for every specimen.

Typical indicative values for PP show the expected cold-embrittlement trend: a homopolymer may register 3 to 5 kJ/m² notched at 23°C and fall to roughly 2 kJ/m² at -30°C with fully brittle fracture, while a well-formulated impact copolymer may retain 5 to 10 kJ/m² or more at -30°C with hinged or partial breaks. Exact values depend heavily on grade, melt flow index, notch sharpness, and processing history.

Applications

Low-temperature Charpy impact data for PP directly support material qualification and design decisions across many industries:

• Automotive: bumper fascias, wheel-arch liners, underbody shields, battery housings and module components, air ducts, and interior trim are routinely specified with minimum impact strength at -30°C or -40°C to satisfy cold-climate durability requirements.
• Home appliances: washing machine tubs and housings, refrigerator liners, and outdoor air-conditioner enclosures must withstand impact in unheated environments.
• Piping systems: PP pipes and fittings for buried water service and cable-protection ducts are evaluated for toughness at the lowest expected ground temperature to prevent brittle cracking during installation.
• Cold-chain and logistics: reusable transport crates, pallets, and containers certified for freezer storage rely on low-temperature impact validation to survive drops at -20°C to -40°C.
• Material development: resin producers use temperature-ladder Charpy testing to benchmark impact modifiers such as EPDM or POE and to optimize copolymer morphology for cold toughness.

FAQ

Why does PP become brittle at low temperatures?

The amorphous phase of polypropylene enters the glassy state near its glass transition region around -10°C to 0°C. Below this range, molecular segments cannot flow or shear-yield to absorb impact energy, so cracks propagate rapidly from stress concentrators such as notches. Impact copolymers with a dispersed rubber phase blunt cracks and shift the brittle transition to lower temperatures.

What is the difference between the Charpy test and the Izod test?

In the Charpy (simply supported beam) configuration the specimen lies horizontally on two supports and is struck at mid-span, while in the Izod test the specimen is clamped as a vertical cantilever and struck above the clamp. The stress states and notch orientations differ, so results from the two methods are not interchangeable.

Should I test notched or unnotched specimens?

Notched specimens provide a severe, stress-concentrated condition that measures notch sensitivity and is preferred for quality control and most specifications. Unnotched tests are useful for assessing overall ductility and for very tough or rubber-modified grades that may not break completely when notched.

Which low temperature should be selected for testing?

The test temperature should reflect the lowest service or handling temperature of the application. Common choices are 0°C for indoor cold exposure, -20°C for general outdoor and cold-chain use, and -30°C or -40°C for automotive and extreme-climate requirements.

How are results reported when specimens do not break?

When a specimen does not separate, the result is recorded as non-break (N) or partial break (P) together with the measured absorbed energy. Such outcomes indicate high residual ductility at the test temperature, and the reported impact strength is treated as a lower-bound value rather than a true material limit.