Impact strength testing of PP (polypropylene) plastics is a critical quality control procedure that evaluates the material's resistance to sudden impact loads under notched, unnotched, and low-temperature conditions. This article provides a comprehensive technical overview of PP impact testing, covering specimen preparation, Izod and Charpy test methods, applicable international standards such as ASTM D256 and ISO 179, and typical applications across automotive, packaging, and consumer product industries.
Technical Overview
Polypropylene (PP) is a semi-crystalline thermoplastic widely used in automotive components, packaging, appliances, pipes, and consumer goods due to its excellent chemical resistance, low density, good fatigue performance, and favorable cost-to-performance ratio. However, PP is a notch-sensitive material whose impact behavior varies significantly with molecular structure, crystallinity, temperature, filler content, and processing conditions.
Impact strength testing measures the energy absorbed by a PP specimen when fractured by a rapidly applied dynamic load. Because PP homopolymer becomes increasingly brittle at low temperatures, while random and block (impact) copolymers offer substantially improved toughness, impact testing is essential for material qualification, grade selection, incoming resin inspection, and finished part validation.
Key factors influencing PP impact performance include:
- Polymer type: homopolymer versus random copolymer versus impact (block) copolymer with ethylene-propylene rubber phase
- Notch geometry and notch tip radius, which concentrate stress and initiate brittle fracture
- Test temperature, particularly the ductile-to-brittle transition below room temperature
- Melt flow index and molecular weight distribution of the resin
- Filler and reinforcement content, such as talc, glass fiber, or calcium carbonate
- Processing-induced orientation, weld lines, and residual stress in molded parts
Testing Samples
Notched Izod Specimens
Standard Izod bars are typically 63.5 mm × 12.7 mm × 3.2 mm per ASTM D256 or 80 mm × 10 mm × 4 mm per ISO 180. A machined V-notch of defined depth and tip radius (commonly 0.25 mm radius for ISO notch A) is cut into the narrow face to serve as a stress concentrator. Specimens are usually injection molded and conditioned at 23 °C and 50% relative humidity prior to testing.
Unnotched Specimens
Unnotched bars of the same dimensions are tested to evaluate the total energy absorption capability of the material without artificial stress concentration. The unnotched value is particularly useful for highly toughened PP copolymers that may only partially break or ductile-deform in the notched configuration.
Charpy Specimens
Charpy specimens per ISO 179-1 use horizontally supported bars, commonly 80 mm × 10 mm × 4 mm (Type 1). Both notched and unnotched configurations are available, with notch types A, B, and C defined by varying tip radii.
Flat Plaques and Discs for Falling Dart Testing
For sheet, film, and flat molded plaques, circular or square specimens (typically 60 mm to 100 mm in diameter) are clamped and struck by a falling dart or instrumented punch, per ISO 6603-2 or ASTM D5420. This geometry better represents the multiaxial loading experienced by containers, panels, and housings in service.
Finished Parts and Components
Complete products such as storage boxes, pipe fittings, automotive trim, and battery cases may be tested directly under simulated drop or impact conditions to validate real-world performance, including the influence of weld lines, gates, and wall thickness variation.
Testing Items
A comprehensive PP impact evaluation program typically includes the following test items:
- Notched Izod impact strength, expressed in J/m (ASTM D256) or kJ/m² (ISO 180)
- Unnotched Izod impact strength
- Notched and unnotched Charpy impact strength (ISO 179-1, ASTM D6110)
- Low-temperature impact strength at -10 °C, -20 °C, -30 °C, or -40 °C
- Falling dart impact resistance and failure energy (mean failure energy or statistical pass/fail)
- Instrumented puncture impact, providing full force-displacement curves, peak force, and total energy (ISO 6603-2)
- Tensile impact strength for thin sections and films (ASTM D1822)
- Comparative assessment before and after thermal aging, UV exposure, or annealing
Testing Methods
Izod Impact Test (ASTM D256 / ISO 180)
The specimen is clamped vertically as a cantilever beam, and a swinging pendulum hammer strikes the notched side at a fixed height. The energy absorbed during fracture is calculated from the difference between the initial and residual pendulum energy, corrected for friction and windage. ASTM D256 reports results in J/m of notch width, while ISO 180 reports energy divided by the cross-sectional area in kJ/m². A minimum of five valid specimens is recommended, and specimens that do not break completely are reported as "no break" or partial break.
Charpy Impact Test (ISO 179-1 / ASTM D6110)
The specimen is supported horizontally on two anvils and struck at mid-span by the pendulum, analogous to a three-point bending configuration. Charpy testing is preferred in European practice and is well suited to materials that exhibit excessive vibration or whipping in the Izod fixture. Edgewise, flatwise, and instrumented variants allow detailed fracture analysis.
Falling Dart and Falling Weight Test (ASTM D5420 / ISO 6603-2)
A hemispherical-headed dart of defined mass is dropped from increasing heights onto a clamped specimen. The staircase (Bruceton) method statistically determines the mean failure energy. Instrumented versions record force-time and energy-deflection data, distinguishing between brittle fracture, ductile fracture, and puncture-through failure modes.
Tensile Impact Test (ASTM D1822)
A small dumbbell specimen is fractured in tension by a swinging pendulum crosshead. This method is useful for materials too flexible or too thin for valid bending impact results, such as PP films and thin-wall molded parts.
Temperature Control and Conditioning
For sub-ambient testing, specimens are conditioned in an environmental chamber or liquid bath at the target temperature for a minimum dwell time (typically 5 mm plus 300 s per ISO 179-1) and fractured within seconds of removal. Notches are machined with a dedicated cutting tool, and notch radius is verified microscopically, as the tip radius strongly governs the measured result.
Applications
PP impact strength testing supports quality assurance and product development across many industries:
- Automotive: bumpers, fascias, interior trim, door panels, battery casings, and under-hood components requiring low-temperature ductility
- Packaging: thin-wall containers, caps and closures, buckets, and transport crates subject to drop and handling impacts
- Appliances: washing machine tubs, air conditioner housings, and electric tool casings
- Pipes and fittings: PP-R plumbing systems and chemical drainage fittings requiring long-term toughness
- Medical and laboratory: syringe components, sample cups, and sterile packaging trays
- Consumer goods: storage totes, garden furniture, luggage shells, and sports equipment
- Material development: comparing toughened copolymer grades, recycled content effects, and mineral-filled or glass-reinforced formulations
FAQ
What is the difference between notched and unnotched impact strength?
Notched testing measures the material's resistance to crack propagation from a sharp stress concentrator and reflects notch sensitivity, while unnotched testing measures overall energy absorption. For PP, the unnotched value is often several times higher than the notched value, especially for toughened copolymer grades.
Why do Izod and Charpy results differ for the same PP material?
The two methods use different specimen support, striking geometry, and stress states, and results are reported on different bases (energy per unit width versus per unit area). Values are not directly interchangeable, and comparison must always reference the specific standard used.
How does temperature affect PP impact strength?
PP homopolymer becomes significantly more brittle as temperature drops toward and below 0 °C. Impact copolymers retain useful toughness down to approximately -20 °C to -40 °C, which is why cold-climate applications typically specify block copolymer grades and require low-temperature impact verification.
What are typical notched Izod values for PP?
Representative values at 23 °C are approximately 20-60 J/m for homopolymer PP, 60-150 J/m for random copolymers, and 100-600 J/m or higher for impact (block) copolymers. Glass-filled grades generally show lower notched impact values unless specially toughened.
How can the impact strength of PP be improved?
Common strategies include selecting impact copolymer grades, adding elastomeric tougheners such as EPDM or POE, optimizing talc content and particle size, increasing molecular weight, refining molding conditions to reduce orientation and residual stress, and avoiding sharp corners and weld lines in part design.
How many specimens should be tested?
A minimum of five valid specimens per condition is standard practice for pendulum impact testing, with ten or more recommended for statistical confidence in falling dart evaluations, since impact results are inherently scattered.
