Izod Notched Impact Testing of Polypropylene (PP) Materials: Standards, Methods, and Applications

The cantilever beam notched impact test, commonly known as the Izod notched impact test, is the industry-standard method for evaluating the impact toughness and notch sensitivity of polypropylene (PP) materials. This technical guide explains the testing principles, specimen preparation, applicable standards such as ISO 180 and ASTM D256, calculation methods, and typical application scenarios for PP plastics. Understanding this test helps material engineers, quality inspectors, and product designers select and validate PP grades for demanding impact-loaded applications.

Technical Overview

The cantilever beam notched impact test, internationally known as the Izod notched impact test, is a standardized pendulum-based method used to measure the impact resistance of plastics when a sharp stress concentrator is present. In this test, a notched specimen is clamped as a vertical cantilever beam, and a swinging pendulum hammer strikes the free end on the notched side at a defined height. The energy absorbed by the specimen during fracture is recorded and converted into an impact strength value.

Polypropylene (PP) is one of the most widely used semi-crystalline thermoplastics in the world, valued for its excellent chemical resistance, low density, good fatigue performance, and favorable cost-performance ratio. However, the notch sensitivity of PP is pronounced, particularly at low temperatures or for highly filled grades. Because real-world plastic parts almost always contain molded-in notches, sharp corners, gate marks, weld lines, or scratches, notched impact testing provides a conservative and highly practical measure of a material's resistance to brittle fracture.

The principal standards governing this test include ISO 180 (Plastics — Determination of Izod impact strength), ASTM D256 (Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics), and the Chinese national standard GB/T 1843, which is technically equivalent to ISO 180. Typical reported properties include notched Izod impact strength in kJ/m² (ISO) or J/m (ft·lb/in in ASTM), along with fracture type classification such as complete break, hinge break, partial break, or non-break.

Testing Samples

ISO 180 Standard Specimens

According to ISO 180, the preferred specimen is a rectangular bar of 80 mm × 10 mm × 4 mm, injection molded or machined from compression-molded or extruded sheet. Specimens may be taken directly from finished parts by machining, provided the required geometry can be maintained. Multiple specimens, typically at least five and preferably ten, are tested for each direction and condition to account for material anisotropy and data scatter.

ASTM D256 Standard Specimens

ASTM D256 specifies a specimen of 63.5 mm × 12.7 mm × 3.2 mm (2.5 in × 0.5 in × 0.125 in), with a preferred thickness of 3.2 mm and an alternative of 6.4 mm for materials that exhibit excessive deformation or are too brittle in thin sections. Notching is performed with a specialized cutter producing a defined radius at the notch root.

Notch Geometry and Types

Because notch geometry strongly influences results, the standards define several notch types:

• Type A notch: radius of 0.25 mm with a remaining width of 8.0 mm — the default and most commonly used notch for ISO testing.
• Type B notch: radius of 1.0 mm with a remaining width of 8.0 mm — used for materials that are too brittle with a Type A notch.
• Type C notch: radius of 0.1 mm with a remaining width of 8.0 mm — used for very tough materials requiring a sharper stress concentrator.
• ASTM Type A notch: 45° included angle with a 0.25 mm radius and 10.2 mm remaining width, machined to a depth of 2.54 mm from the 12.7 mm face.

Notches must be machined cleanly without melting, smearing, or rough surfaces, since artifacts at the notch root can cause artificially low or inconsistent results. Conditioning of specimens at 23 °C and 50% relative humidity for a minimum of 40 hours after molding is standard practice, and additional annealing or reconditioning may be specified to relieve molding stresses.

Testing Items

The core parameters evaluated in the PP cantilever beam notched impact test include:

• Notched Izod impact strength (kJ/m² per ISO 180; J/m or ft·lb/in per ASTM D256), calculated from absorbed energy divided by the cross-sectional area (ISO) or specimen width (ASTM) beneath the notch.
• Absorbed impact energy in joules, representing the total energy consumed to initiate and propagate fracture.
• Fracture mode classification: C = complete break, H = hinge break, P = partial break, NB = non-break, as defined by ISO 180.
• Temperature-dependent impact behavior, often tested at 23 °C and at sub-ambient temperatures such as -20 °C or -30 °C to evaluate the ductile-to-brittle transition of PP.
• Comparative notch sensitivity, obtained by testing both notched and unnotched specimens to quantify toughness retention under stress concentration.

For typical PP grades, notched Izod values at 23 °C can range from below 3 kJ/m² for talc-filled homopolymers, to 5–15 kJ/m² for standard homopolymers, and to 30 kJ/m² or higher, or even non-break, for high-impact block copolymers modified with ethylene-propylene rubber or POE elastomers.

Testing Methods

Apparatus

The test is performed on a pendulum impact tester equipped with interchangeable hammers of defined energy capacity, typically ranging from 1 J to 25 J or higher for tough PP grades. The instrument features a rigid clamping vice that grips the specimen as a vertical cantilever, an energy indicating device, and a striking edge with defined geometry that contacts the specimen at the notch plane. Modern machines incorporate electronic encoders or optical sensors for energy measurement and software for automatic data reduction.

Test Procedure

• Measure and record the width and thickness of each specimen at the notch plane to the nearest 0.02 mm.
• Select a pendulum capacity such that the absorbed energy falls between approximately 10% and 80% of the hammer range, ensuring measurement accuracy.
• Frisch the system by performing an idle swing to quantify friction and windage losses, which are corrected in the final calculation.
• Clamp the specimen vertically with the notch facing the pendulum, aligning the notch plane level with the top of the clamping jaws using a gauge.
• Release the pendulum from its locked height, allowing the striking edge to hit the notched face of the specimen 22 mm above the clamp for ISO geometry.
• Record the absorbed energy, classify the fracture type, and repeat for the required number of specimens.
• Calculate the mean, standard deviation, and, if required, report individual values together with test temperature, hammer capacity, and notch type.

Calculation of Results

Under ISO 180, the notched impact strength a

Influencing Factors

• Temperature: PP exhibits a strong ductile-to-brittle transition; impact strength drops significantly as the test temperature approaches or falls below 0 °C.
• Polymer architecture: homopolymer, random copolymer, and impact (block) copolymer PP grades show progressively higher notched toughness.
• Fillers and reinforcements: talc, calcium carbonate, and short glass fibers generally reduce notched impact strength of PP, although proper coupling and particle morphology can mitigate losses.
• Processing conditions: mold temperature, melt temperature, hold pressure, and weld line position alter crystallinity, orientation, and residual stress, all of which influence results.
• Notch quality and specimen conditioning, as described above, remain among the largest sources of variability between laboratories.

Applications

Notched Izod impact data for PP materials directly support material selection, quality control, and failure analysis across many industries:

• Automotive: bumpers, body panels, battery housings, interior trim, and dashboard structures require validated low-temperature impact performance, often specified as notched Izod at -30 °C.
• Home appliances: washing machine tubs, refrigerator liners, air conditioner housings, and power tool cases rely on impact-resistant PP copolymers.
• Packaging: thin-wall containers, closures, and transport crates benefit from toughness data to prevent cracking during filling, stacking, and distribution.
• Consumer goods: furniture, luggage, housewares, and garden equipment use PP grades selected through notched impact comparison.
• Electrical and electronics: connector housings, enclosures, and fan blades must resist impact during assembly and drop events.
• Medical and laboratory products: rigid packaging, trays, and device components made from PP are verified for fracture resistance.

Manufacturers also use the test for incoming resin inspection, comparing supplier lots, evaluating regrind content effects, qualifying colorants and additives, and benchmarking recycled PP against virgin material.

FAQ

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

Both are pendulum impact methods, but the Izod test clamps the specimen vertically as a cantilever and strikes the notched side, whereas the Charpy test supports the specimen horizontally as a simple beam and strikes it opposite the notch. The Izod configuration corresponds to the cantilever beam notched impact test described in this article, and the two tests produce numerically different results that are not interchangeable.

Why is the specimen notched at all?

The notch creates a controlled triaxial stress state and a sharp stress concentrator that promotes brittle fracture. This makes the test highly sensitive to differences in material toughness and notch sensitivity, closely simulating the effect of sharp corners, scratches, and defects found in real plastic parts.

What is a typical acceptance value for PP notched impact strength?

Acceptance limits depend entirely on the application. As a reference, unfilled PP homopolymer typically shows 4–10 kJ/m² at 23 °C with a Type A notch, while high-impact copolymers can exceed 30 kJ/m² or even register non-break. Automotive specifications frequently demand minimum values measured at -30 °C.

Can results from ISO 180 and ASTM D256 be compared directly?

No. Differences in specimen dimensions, notch depth, support geometry, and units of reporting mean the two methods produce different numbers. Data should only be compared within the same standard, specimen type, notch type, and test temperature.

How many specimens should be tested?

ISO 180 recommends at least five specimens unless otherwise agreed, while ten specimens improve statistical confidence, especially for materials showing partial or hinge fractures. Outliers should be investigated for notch machining defects before being excluded, and any exclusion must be documented.