Notched Impact Testing of Recycled Plastics: Standards, Methods, and Applications

Notched impact testing is the most widely used method for evaluating the fracture toughness and crack sensitivity of recycled plastics, whose molecular chains typically degrade through repeated heat histories and contamination. This technical guide explains the principles of notched Charpy and Izod impact testing for recycled materials in accordance with ISO 179, ISO 180, ASTM D256, and GB/T standards, covering specimen preparation, testing procedures, result evaluation, and typical quality-control applications. Whether you are qualifying recycled pellets, validating recycled content in finished parts, or optimizing virgin/recycled blend ratios, notched impact data provide a reliable indicator of long-term durability and brittleness risk.

Technical Overview

Recycled plastics—including post-industrial regrind and post-consumer recycled (PCR) resins—undergo thermal oxidation, mechanochemical chain scission, and contamination during collection, sorting, washing, and re-pelletizing. These processes reduce average molecular weight, weaken the entanglement network, and introduce foreign particles such as labels, inks, and dissimilar polymers that act as stress concentrators. As a result, recycled materials frequently exhibit lower ductility and a higher tendency toward brittle fracture than their virgin counterparts.

Notched impact testing quantifies this degradation. A V-shaped notch is machined into a standardized bar specimen to create a controlled stress concentration, and a swinging pendulum strikes the specimen to measure the energy required to initiate and propagate a crack. The absorbed energy, normalized by the cross-sectional area beneath the notch, is reported as notched impact strength in kJ/m² (ISO/GB methods) or J/m (ASTM Izod method).

Because the notch amplifies the effect of micro-defects and reduced molecular toughness, notched impact strength is widely regarded as one of the most sensitive single-number indicators of quality loss in recycled resins. It is routinely specified in purchase specifications for recycled pellets, in incoming inspection at molding plants, and in sustainability certifications that require demonstration of fitness-for-purpose of recycled content.

Testing Samples

Injection-Molded Multipurpose Specimens

The preferred sample route is injection molding of ISO 3167 Type A multipurpose specimens (80 mm × 10 mm × 4 mm) directly from recycled pellets. Melt temperature, mold temperature, and drying conditions must follow the material supplier's recommendations and be documented, since processing history strongly influences impact behavior. Recycled hygroscopic resins such as PA and ABS must be dried before molding to avoid moisture-induced embrittlement or silver-streak defects.

Compression-Molded Specimens

For materials that cannot be cleanly injection molded, or when molding-induced orientation must be eliminated, specimens can be compression molded from milled recycled material and then machined to final dimensions. This route is common for research studies comparing multiple reprocessing cycles of the same resin.

Specimens Machined from Finished Parts

To verify the performance of an actual recycled-content product, specimens may be cut directly from molded parts using a saw or milling cutter per ISO 2818. Results from such specimens are only comparable to data obtained on identical geometry and thickness, and the sampling location must be reported because of local variations in orientation, cooling history, and weld lines.

Notch Machining Requirements

• The standard notch is Type A: a 45° V-notch with a tip radius of 0.25 mm ± 0.05 mm and a remaining ligament width of 8.0 mm ± 0.2 mm on a 10 mm wide specimen.
• Notches must be machined with a sharp cutter in a single pass at controlled feed rate; dull tools leave plastic deformation or micro-cracks at the notch root that artificially lower the measured energy.
• The notch root radius should be verified periodically with a profile projector or optical measuring system, as it is the single most influential geometric parameter in the test.
• Molded-in notches are not acceptable for referee testing under ISO and GB methods, because the molding skin at the notch root changes the failure mode.
• All machining should be completed before final conditioning so that the specimen reaches equilibrium at the test temperature.

Testing Items

Typical notched impact test items requested for recycled materials include:

• Charpy notched impact strength (ISO 179-1, ASTM D6110, GB/T 1043.1) in kJ/m².
• Izod notched impact strength (ISO 180, ASTM D256, GB/T 1843) in kJ/m² or J/m.
• Instrumented impact testing (ISO 179-2) to separate crack initiation and propagation energies.
• Low-temperature notched impact strength at −20 °C, −30 °C, or −40 °C to assess cold brittleness using a cooled chamber.
• Comparative testing of virgin resin versus recycled resin, or of blends containing 10 %, 25 %, 30 %, and 50 % recycled content.
• Multi-cycle reprocessing studies that track notched impact strength versus the number of extrusion or injection cycles.
• Break-type classification for each specimen: complete break (C), hinge break (H), partial break (P), or non-break (N).

Testing Methods

Apparatus and Calibration

Testing is performed on a pendulum impact machine with interchangeable hammers, typically 1 J, 2.75 J, 5.5 J, 11 J, and 22 J for plastics. Machine verification follows ISO 13802, covering potential energy, friction and air-drag losses, velocity at strike, and center-of-percussion alignment. At least five, and preferably ten, specimens are tested for each condition.

Conditioning and Test Atmosphere

Specimens are conditioned per ISO 291 at 23 °C ± 2 °C and 50 % ± 10 % relative humidity, normally for a minimum of 24 hours (up to 88 hours for hygroscopic resins such as recycled PA and PC). Testing should take place under the same atmosphere unless a temperature-controlled chamber is deliberately used for low-temperature evaluation.

Test Procedure

• Select a pendulum so that the absorbed energy falls between 10 % and 80 % of its capacity; otherwise the result loses resolution and must be re-measured with a different hammer.
• For Charpy testing, place the specimen horizontally on supports 62 mm apart (40 mm span for ISO specimens) with the notch facing away from the striker; for Izod testing, clamp the specimen vertically as a cantilever with the notch on the tension side facing the striker.
• Release the pendulum, record the absorbed energy with the manufacturer's correction for friction and windage, and inspect the broken halves.
• Measure the remaining width and thickness at the notch for every specimen to the nearest 0.02 mm and use the individual values in the calculation.
• Classify the break type; if specimens do not break (N) or only partially break (P), report the designation and note that arithmetic mean values are of limited comparative value.

Calculation and Reporting

Notched impact strength is calculated as the corrected absorbed energy divided by the initial cross-sectional area beneath the notch: a = 1000 × E / (h × bN), where E is the corrected energy in joules, h is the specimen thickness in millimeters, and bN is the remaining width in millimeters. The test report must identify the standard used, specimen and notch type, conditioning, pendulum capacity, individual values, mean value, standard deviation, and break designation. Note that ASTM D256 results in J/m are not directly convertible to ISO 180 results in kJ/m² because of different specimen geometry and clamping, so comparisons must always stay within one method family.

Applications

• Incoming quality control of recycled pellets at injection molders and compounders, with minimum notched impact thresholds written into purchase agreements.
• Qualification of PCR content in automotive interior and exterior components, where impact performance at low temperature is safety-relevant.
• Validation of recycled ABS, PC/ABS, PP, HDPE, and PA for appliance housings, power-tool enclosures, connectors, and electronic device casings.
• Documentation for environmental labeling, recycled-content declarations, and eco-design programs that require evidence of fitness-for-purpose.
• Formulation development, including the selection of impact modifiers, chain extenders, and restabilization packages that restore toughness lost during recycling.
• Failure analysis, where unusually low notched impact values reveal contamination, insufficient drying, or over-drying of recycled engineering resins.

FAQ

Why is notched impact strength lower for recycled plastics than for virgin resin?

Repeated melt processing shortens polymer chains through oxidative and hydrolytic degradation, and recycled streams contain particulate contaminants and dissimilar polymer inclusions that act as crack initiation sites. Both effects reduce the energy needed to start and grow a crack, which the sharp notch amplifies dramatically.

What is the difference between Charpy and Izod notched impact tests?

In the Charpy configuration the specimen lies horizontally on two supports and is struck at mid-span opposite the notch; in the Izod configuration the specimen is clamped as a vertical cantilever and struck above the clamp on the notched face. The two methods produce numerically different results and must never be directly compared with each other.

How many specimens should be tested?

ISO and GB standards require a minimum of five specimens, but ten specimens are strongly recommended for recycled materials because their heterogeneous composition produces wider scatter than virgin resin.

Which standard should be specified on a drawing or datasheet?

Use the method consistent with your market and existing data: ISO 179-1 or GB/T 1043.1 for Charpy, ISO 180 or GB/T 1843 for Izod, and ASTM D256 when North-American customer specifications apply. Always state the standard number, specimen type, notch type, and test temperature together with the numeric value.

Do results depend on the notch quality?

Yes, critically. Variations of only a few hundredths of a millimeter in the notch tip radius, or residual damage from a worn cutter, can shift notched impact strength by 10–20 %. Accredited third-party laboratories therefore verify notch geometry and replace cutters on a strict schedule.

Can notched impact testing be done at low temperature on recycled materials?

Yes. A temperature-controlled chamber cooled with liquid nitrogen or dry-ice-equipped systems allows testing at −20 °C to −40 °C. This is particularly important for recycled polypropylene and recycled polyamide used in outdoor and automotive applications, where the ductile-to-brittle transition may fall within the service temperature range.