Determination of Impact Resistance of Plastic Films: Test Methods, Standards, and Applications

Impact resistance is one of the most important mechanical quality indicators for plastic films, determining whether packaging, agricultural, and industrial films can survive drops, shocks, and puncture events during production, filling, storage, and transport. This article provides a comprehensive technical overview of plastic film impact resistance determination, covering the free-falling dart method and instrumented puncture testing in accordance with ASTM D1709, ISO 7765, and GB/T 9639.1, including specimen requirements, calculation of the F50 impact failure mass, typical applications, and frequently asked questions.

Technical Overview

Impact resistance, often referred to as dart impact strength, describes the ability of a plastic film to absorb energy from a suddenly applied dynamic load, such as a free-falling object, a drop during handling, or a puncture event, without tearing, cracking, or perforating. Because most film failures in service are caused by concentrated dynamic stresses rather than slow static loading, impact performance is a decisive acceptance criterion for film producers, converters, and brand owners.

For flexible films and sheeting below approximately 1 mm in thickness, impact resistance is predominantly determined by two complementary techniques. The first is the free-falling dart method (ASTM D1709, ISO 7765-1, and the Chinese equivalent GB/T 9639.1), a statistical staircase procedure that reports the dart mass at which 50 percent of specimens fail, designated F50 and expressed in grams. The second is the instrumented puncture test (ISO 7765-2, ASTM D7192), in which a hemispherical probe is driven through the film at a controlled speed while a force transducer records a complete force-displacement curve, yielding peak force, energy to puncture, and failure-mode information. Pendulum-type procedures (ASTM D3420) have also been applied to specific flexible-film applications.

The data generated by these methods are used for quality control of extrusion lines, incoming resin and film qualification, packaging design validation, comparison of resin grades and impact modifiers, down-gauging studies, and compliance verification. To ensure traceable and comparable results, testing should be performed by ISO/IEC 17025 accredited third-party laboratories or by in-house laboratories using calibrated equipment and documented test procedures.

Testing Samples

Polyolefin Films

Blown and cast films based on LDPE, LLDPE, metallocene LLDPE, HDPE, mPE, and PP, typically ranging from 10 to 250 micrometers in thickness. Typical products include shopping bags, heavy-duty shipping sacks, form-fill-seal packaging webs, stretch and shrink films, liners, and lamination substrates. These materials normally exhibit ductile, puncture-type failures, and dart impact mass is a key specification for gauge reduction programs.

Barrier and Multilayer Composite Films

Coextruded and laminated structures containing PET, BOPP, CPP, PA, EVOH, PVDC coatings, aluminum foil, or metallized layers used in food, pharmaceutical, and medical device packaging. Impact testing verifies the puncture integrity of retort pouches, vacuum skin packs, modified-atmosphere packages, and liquid packaging webs, where a single perforation can compromise product safety and shelf life.

Engineering and Specialty Films

PVC, amorphous PET, cellulose-based, bio-based, and functional coated films, as well as thin separator and protective films, where puncture resistance is a critical functional property. For oriented films, specimens should be assessed in both machine direction (MD) and transverse direction (TD) orientations because impact response can be anisotropic.

Specimen Preparation and Conditioning

Specimens must be free from creases, wrinkles, folds, and visible defects, and should be sampled from multiple positions across the web width to capture thickness variation. Individual specimens are typically cut at least 150 mm by 150 mm so they fully cover the annular clamping aperture of approximately 127 mm diameter. Thickness is measured at several points per ISO 4593 or ASTM D6988, or gravimetrically per ISO 4591 for very thin films. Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 40 hours in accordance with ISO 291, ASTM D618, or GB/T 2918 prior to testing.

Testing Items

Typical test items and reported parameters for plastic film impact resistance determination include:

• F50 impact failure mass in grams, determined by the free-falling dart staircase method
• Impact failure energy in joules, calculated from the F50 dart mass, drop height, and gravitational acceleration
• Puncture peak force in newtons, determined by instrumented high-speed puncture testing
• Energy to peak force and total puncture energy in joules, with displacement at peak in millimeters
• Failure mode classification: brittle, brittle-ductile transition, or fully ductile
• Statistical parameters including standard deviation, coefficient of variation, and confidence interval of F50
• Optional impact testing at chilled or sub-zero temperatures to simulate cold-chain and freezer distribution conditions

Testing Methods

Free-Falling Dart Method (Staircase Method)

The apparatus consists of a vertical guide tube, a dart with a hemispherical head and removable slotted weights, a pneumatic or mechanical ring clamp holding the specimen over a circular opening, and a failure indicator. In ASTM D1709, Method A uses a 38.1 mm diameter dart head dropped from a height of 0.66 m, while Method B uses a 50.8 mm head dropped from 1.52 m for tougher structures. ISO 7765-1 and GB/T 9639.1 specify a 38 mm diameter dart head with drop heights of 0.5 m or 1.0 m. The general test sequence is as follows:

• Condition the specimens, measure and record thickness, and mark the machine direction on each specimen
• Clamp the specimen securely over the annular opening, ensuring the surface is taut and free of wrinkles
• Select a starting dart mass close to the expected failure mass and a mass increment, typically 5 to 15 percent of the expected F50, so that approximately four to six mass levels are used
• Release the dart centrally onto the specimen; failure is defined as any hole or crack through the film, detected visually or by an electrical contact sensor
• If the specimen fails, reduce the dart mass by one increment for the next specimen; if it survives, increase the mass by one increment
• Continue the staircase sequence until at least 20 failures and 20 non-failures have been recorded
• Calculate F50 and its standard deviation using the staircase statistical formula

The impact failure mass is calculated as F50 = x0 + d(A/N − 0.5), where x0 is the lowest dart mass level at which the less frequent event (failure or non-failure) occurs, d is the mass increment, N is the number of specimens at level x0 and above exhibiting that event, and A is the sum of the products of level number and specimen count at each level. For engineering purposes, the corresponding impact failure energy is E = m·g·h, where m is the F50 mass, g is gravitational acceleration, and h is the drop height. The report should state the standard used, dart head diameter, drop height, average thickness, conditioning, F50 with standard deviation, and observed failure mode.

Instrumented High-Speed Puncture Test

Per ISO 7765-2 and ASTM D7192, a hemispherical-tipped probe, typically 20 to 25 mm in diameter, is driven through a clamped specimen at a controlled velocity while load and displacement are recorded continuously at high sampling rates. The resulting force-displacement curve separates the energy required to initiate failure from the energy required to propagate it, and provides a ductility index that quantifies post-yield behavior. This method requires far fewer specimens than the staircase procedure, supports testing at multiple speeds and temperatures, and is particularly valuable for multilayer laminates and films whose performance changes under high strain rates.

Pendulum Impact Test

Pendulum-based procedures historically covered by ASTM D3420 measure the energy absorbed when a puncture head mounted on a falling pendulum arm perforates a clamped film specimen. The absorbed energy is read directly from the pendulum scale in joules. The method is suitable for very thin or soft films where the mass sensitivity of the dart-drop method is limited; laboratories should confirm the current status of the applicable standard when specifying this technique.

Factors Influencing Test Results

• Temperature and humidity: most polyolefin films become measurably more brittle at low temperature, so conditioning and test temperature must be strictly controlled
• Thickness variation: dart impact mass correlates strongly with gauge; thickness variability within the specimen population increases data scatter
• Orientation and processing: degree of orientation, frost line height, draw ratio, and annealing history affect the balance between MD and TD impact response
• Material composition: impact modifiers, slip and antiblock additives, recycled content, and aging or UV exposure can shift F50 significantly
• Apparatus effects: dart concentricity, guide tube friction, clamping pressure, and worn dart heads are common sources of systematic error
• Statistical design: an inappropriately large mass increment or a poor starting mass reduces the precision of the F50 estimate

Applications

Impact resistance determination supports decision-making across the entire film value chain, from resin development to packaged goods distribution. Principal applications include:

• Quality control of blown and cast film extrusion lines, including statistical process control of dart impact data
• Incoming inspection of purchased film and laminate rolls by converters and brand owners
• Package design validation for heavy-duty sacks, form-fill-seal webs, liquid packaging, and retort pouches
• Cold-chain and frozen food packaging qualification, where sub-ambient impact testing reveals low-temperature embrittlement
• Agricultural films such as mulch films, greenhouse films, and silage covers exposed to hail, handling, and installation stresses
• Medical and pharmaceutical barrier packaging validation for sterile barrier integrity
• Construction vapor barriers, protective sheeting, and liner applications requiring documented puncture performance
• Research and development, including evaluation of new resin grades, impact modifiers, recycled content, and down-gauging studies

FAQ

What does F50 mean in dart impact testing?

F50 is the dart mass at which statistically 50 percent of the specimens fail, expressed in grams. It is derived from the staircase (Bruceton) analysis of a pass/fail test sequence and represents the most reproducible single-number indicator of film impact resistance under the selected test conditions.

Are ASTM D1709 and ISO 7765-1 results directly interchangeable?

No. Although both use a similar dart principle, the drop heights differ: ASTM D1709 Method A drops the dart 0.66 m (impact velocity about 3.6 m/s), while ISO 7765-1 uses 0.5 m or 1.0 m (about 3.1 m/s or 4.4 m/s). Because impact response depends on strain rate, the methods correlate but do not convert directly. Purchase specifications should identify one specific standard and method.

How many specimens are required for a dart drop test?

The staircase method requires enough specimens to obtain at least 20 failures and 20 non-failures, typically a minimum of 40 specimens in total. Instrumented puncture tests generally require only 5 to 10 specimens per condition because each test yields continuous force and energy data rather than a binary pass/fail result.

How do I choose between ASTM D1709 Method A and Method B?

Method A, with its 38.1 mm dart head and 0.66 m drop height, covers the majority of commercial packaging films. Method B, with a 50.8 mm dart head and 1.52 m drop height, is intended for tougher structures for which the Method A dart mass becomes impractically large or fails to discriminate between materials.

What does a brittle failure mode indicate?

A brittle failure, characterized by shattering or radial cracking with little deformation, indicates reduced toughness. Common causes include excessive orientation, low-temperature embrittlement, high draw-down ratios, gel and contaminant defects, excessive recycled content, or material aging. A ductile failure with a clean puncture hole and substantial elongation is generally the desired response for packaging films.

Can dart drop results be converted into puncture energy?

An equivalent energy can be calculated from E = m·g·h using the F50 mass, but this value is only strictly valid for the same dart geometry, drop height, and strain rate. Direct comparison with instrumented puncture energies from ISO 7765-2 or ASTM D7192 should be avoided because probe diameter, velocity, and specimen support conditions all differ.

What laboratory qualifications should be considered when outsourcing this testing?

Preferably, testing should be placed with accredited third-party laboratories holding ISO/IEC 17025 accreditation for the specific standards involved, with calibrated dart drop and instrumented impact equipment, documented conditioning facilities, and experience in plastic film mechanical testing, so that results are traceable and mutually recognized across supply chains.