Notched Izod (Cantilever Beam) Impact Testing of Recycled Plastics: Standards, Methods, and Applicat

Notched Izod impact testing, also known as cantilever beam impact testing, is the principal method for quantifying the impact toughness of recycled plastics, delivering essential data for incoming quality control, material qualification, and end-use performance validation. This technical guide explains the test principle, applicable standards such as ISO 180, ASTM D256, and GB/T 1843, specimen preparation and conditioning requirements, test procedures, result interpretation, and typical applications for recycled polymers across the automotive, electronics, packaging, and construction industries.

Technical Overview

The notched Izod impact test measures the energy absorbed by a standardized, notched plastic specimen that is clamped as a vertical cantilever beam and fractured by a single swing of a pendulum striking edge. The absorbed energy, normalized to the specimen dimensions, is reported as the notched Izod impact strength and reflects the material's resistance to brittle fracture and crack propagation from a sharp stress concentration.

For recycled plastics, this test is particularly important. Mechanical recycling exposes polymers to repeated thermal and shear histories, oxidative degradation, hydrolysis (notably for recycled PET, PC, and PA), and contamination from labels, adhesives, inks, pigments, and foreign polymers. These factors reduce average molecular weight, weaken phase interfaces, and introduce defects, all of which typically manifest as reduced notched impact toughness. Because notched impact performance is one of the most sensitive indicators of embrittlement, it is routinely used to benchmark recyclate quality against virgin resin, to set internal release limits, and to validate formulations containing compatibilizers or impact modifiers.

The test is performed by accredited third-party laboratories as well as in-house quality and R&D laboratories worldwide, using pendulum impact testers verified in accordance with recognized plastics testing standards. The most widely applied standards include:

• ISO 180 — Plastics — Determination of Izod impact strength

• ASTM D256 — Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics

• GB/T 1843 — Chinese national standard for Izod impact strength of plastics, technically aligned with ISO 180

• JIS K 7110 — Japanese industrial standard for Izod impact strength of plastics

Testing Samples

Standard Injection-Molded Specimens

The preferred sampling route is to homogenize the recycled plastic pellets (with documented drying, melt temperature, and molding conditions) and injection-mold dedicated test bars. For ISO 180 and GB/T 1843, the Type 1 specimen measures 80 mm × 10 mm × 4 mm; for ASTM D256, the standard specimen is 63.5 mm long and 12.7 mm wide, with a thickness typically between 3.2 mm and 12.7 mm. Injection molding should follow the specimen-preparation requirements of ISO 294-1 or equivalent procedures so that molding-induced orientation and residual stress do not distort the results.

Specimens Machined from Finished Products

When the objective is to verify the actual performance of a manufactured recyclate article, specimens can be cut directly from finished parts. In this case, the report must clearly state the sampling location and orientation, because injection-molded parts exhibit anisotropy and properties that differ between flow and transverse directions. Machining must use sharp tooling and low-heat cutting practices to avoid introducing additional notches, burns, or thermal damage that would artificially lower the measured toughness.

Notch Machining and Notch Geometry

The notch creates a controlled stress concentration so that the test measures resistance to crack initiation and propagation rather than general ductility. The notch is machined in the center of the specimen length using a dedicated profiling cutter. Under ASTM D256, the Type A notch has a radius of 0.25 mm (the standard configuration), while Type B (1.00 mm) and Type C (0.10 mm) radii are used for special evaluations. Under ISO 180, Type A (0.25 mm) and Type B (1.00 mm) radii are specified, with a notch depth of 2 mm in the Type 1 specimen, leaving a remaining ligament of 8 mm × 4 mm. Cutter sharpness and notch surface quality strongly influence results, so cutters must be inspected and replaced at defined intervals.

Conditioning Before Testing

Specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 40 hours in accordance with ISO 291 or ASTM D618 before testing. Hygroscopic recycled resins such as rPET, rPA, and rPC must also be properly dried before molding, since residual moisture causes hydrolytic degradation and markedly lower impact values. For low-temperature evaluations, additional conditioning at the test temperature (for example, −40 °C for automotive applications) is required immediately prior to fracture.

Testing Items

Typical test programs for recycled plastics include the following evaluated items:

• Notched Izod impact strength, expressed in kJ/m² (ISO 180, GB/T 1843, JIS K 7110) or J/m and ft·lb/in (ASTM D256)

• Absorbed impact energy in joules and its statistical distribution across replicate specimens

• Failure mode classification: complete break (C), hinge break (H), partial break (P), or non-break (NB)

• Benchmarking of recyclate against the corresponding virgin resin

• Recycled-content ladder studies, for example comparing 10 %, 25 %, 50 %, and 100 % recyclate in the formulation

• Comparison of post-consumer versus post-industrial recyclate streams

• Effect of repeated reprocessing cycles on toughness retention

• Temperature-dependent impact behavior between −40 °C and elevated temperatures, including ductile-to-brittle transition assessment

• Effectiveness verification of compatibilizers, impact modifiers, and purification upgrades on toughness recovery

Testing Methods

Apparatus and Calibration

Testing is performed on a pendulum impact machine with interchangeable hammers covering the required energy range, verified and calibrated in accordance with ISO 13802 and the requirements of the governing test standard. The pendulum is selected so that the energy absorbed by the specimen falls between approximately 10 % and 80 % of the pendulum capacity, ensuring measurement accuracy. The pendulum velocity at strike is approximately 3.5 m/s. Friction and windage losses, the striking edge geometry, and the clamping fixture dimensions must be verified before testing campaigns.

Specimen Mounting and Test Execution

The notched specimen is clamped vertically as a cantilever beam with the notch facing the direction of the striking edge. The pendulum strikes the specimen at a fixed distance, nominally 22 mm, above the notch, fracturing the ligament in a single impact. The absorbed energy is read from the scale, digital display, or data acquisition system after correction for friction. The entire test, from removal of the specimen from the conditioning atmosphere to fracture, should be completed within a short, standardized time window so that the conditioning state is preserved.

Calculation and Expression of Results

For ISO 180 and GB/T 1843, the notched Izod impact strength is calculated as the corrected absorbed energy divided by the cross-sectional area of the specimen under the notch, that is a_iN = E_c × 1000 / (h × b_N), expressed in kJ/m², where E_c is the corrected energy in joules, h is the specimen thickness, and b_N is the width under the notch. For ASTM D256, the impact resistance is the absorbed energy divided by the width of the specimen, expressed in J/m or ft·lb/in. The test report should include individual and mean values, standard deviation, break classification, notch type and radius, specimen orientation, molding conditions, conditioning details, and test temperature.

Statistical Considerations for Recyclates

Recycled plastics generally exhibit wider property scatter than virgin resins because of feedstock variability. A minimum of five specimens is required by most specifications, but testing ten or more replicates per material lot is strongly recommended. Sampling plans should account for lot homogeneity, and any outlier investigation should consider localized contamination, unmelted particles, or voids at the fracture surface. Photographs of fracture surfaces and documentation of defects provide valuable diagnostic information for interpreting unusually low values.

Applications

Notched Izod impact data for recycled plastics support a wide range of industrial decisions, including:

• Incoming quality control and lot release of recyclate at compounders and converters, ensuring consistency of purchased recycled feedstock

• Development of material datasheets for recycled compounds and demonstration of conformance to automotive, electronics, and appliance material specifications

• Qualification of recycled ABS, PC/ABS, HIPS, and PP blends for electrical and electronic equipment housings where drop and impact resistance are critical

• Validation of recycled HDPE and PP for returnable transport packaging such as crates, pallets, and containers subjected to repeated handling impacts

• Performance verification of recycled PVC profiles, wood-plastic composites, and other construction products incorporating recyclate

• Research and development of compatibilized and impact-modified recyclate formulations, including optimization of recycled content without sacrificing toughness

• Technical documentation and third-party test reports supporting recyclate content claims, eco-labeling programs, and customer audits

FAQ

What is notched Izod impact testing of recycled plastics?

It is a standardized pendulum test in which a notched specimen of recycled plastic is clamped as a vertical cantilever beam and struck above the notch by a swinging pendulum. The energy absorbed during fracture, normalized to specimen geometry, quantifies the material's resistance to brittle failure, which is a key indicator of recyclate quality.

What is the difference between the Izod and Charpy methods?

In the Izod configuration (ISO 180, ASTM D256, GB/T 1843), the specimen is held as a vertical cantilever in a clamp and struck above the notch. In the Charpy configuration (ISO 179, ASTM D6110), the specimen lies horizontally on two supports and is struck at mid-span opposite the notch. The two methods use different specimen geometries and stress states, so their numerical results are not interchangeable.

Why is the specimen notched before testing?

The notch creates a sharp, reproducible stress concentration that promotes crack initiation at a known location. This allows the test to measure resistance to crack propagation, which closely reflects real-world behavior where scratches, gate marks, weld lines, or sharp corners act as failure initiation sites in service.

Why do recycled plastics often show lower notched impact strength than virgin resins?

Repeated processing causes chain scission, oxidation, and hydrolysis that reduce molecular weight. Contamination with foreign polymers, paper fibers, metals, and unmelted particles, as well as poorly bonded blend interfaces, introduces weak points. All of these factors increase notch sensitivity and shift failure toward brittle behavior, which the notched Izod test detects very effectively.

Can ASTM results in J/m be directly converted to ISO results in kJ/m²?

No. The ASTM D256 and ISO 180 specimens differ in dimensions, notch depth, ligament area, and normalization basis. Values should only be compared qualitatively between the two systems, and specification limits should always reference the exact standard used for testing.

How many specimens should be tested?

Most standards require at least five valid specimens per condition. Because recycled plastics show higher lot-to-lot and specimen-to-specimen variability, testing ten or more replicates per lot provides a more reliable estimate of mean toughness and scatter.

How can the impact performance of recycled plastics be improved?

Common strategies include improving feedstock sorting and washing, applying fine melt filtration to remove contaminants, strictly controlling drying for hygroscopic resins, adding compatibilizers for mixed-polymer streams, incorporating impact modifiers such as elastomeric tougheners, and blending controlled percentages of recyclate with virgin resin to meet target toughness levels.