Impact Strength (Izod, Charpy): ASTM D256, ISO 180 and ISO 179, and the 7 Additive Families That Change It

Impact strength is the energy a notched plastic bar absorbs when a pendulum breaks it in one swing, reported in J/m per unit width under ASTM D256-26 or in kJ/m2 per unit area under ISO 180 and ISO 179, and 7 additive families are bought to raise it or chosen so they do not lower it. Why can the same compound read tough on one data sheet and brittle on another? The standard, the notch, the test temperature and the failure type all belong to the number, and ASTM itself states that its Izod method resembles ISO 180 in title only.

Impact modifiers add a rubber phase that stops a crack; compatibilizers and coupling agents bond the phases; fine coated mineral fillers can toughen polypropylene while coarse particles start cracks; reinforcing fibres raise unnotched and can lower notched impact; beta nucleating agents buy toughness while alpha nucleation trades it for stiffness; plasticizers move a brittle polymer towards a non-break; and antioxidants and UV stabilizers keep the value through service. Glass-fibre-filled recycled PET, for example, rose from 5.2 to about 8.1 kJ/m2 in notched Izod with an ethylene copolymer (Monti and colleagues, Polymers, 2021, ISO 180, 23 °C).

This page covers the pendulum and the notch, ASTM D256-26 Test Methods A and C, Charpy under ISO 179 and ASTM D6110-26, the 6 Izod-Charpy differences, the 7 families, the 5 data-sheet checks, falling-weight methods, the value that matters per polymer, recyclate, and what a pendulum cannot answer. Seven families of plastic additives are bought partly for what they do to this one number.

Table 1. Impact strength at a glance

Standard editions are the versions current on 24 October 2026. ISO 180 and ISO 179 are given without an edition year because the ISO text could not be read for this page.

What Is Impact Strength in Plastics?#

Impact strength in plastics is the resistance of a notched specimen to breakage by a single blow from a pendulum, measured as absorbed energy under ASTM D256-26 and ISO 180 for the Izod geometry and under ISO 179 and ASTM D6110-26 for the Charpy geometry. ASTM D6110-26 describes the same property as the resistance of plastics to breakage by flexural shock, and both ASTM methods break the bar with one swing of a pendulum-type hammer.

If the test takes seconds per bar, what does a formulator learn from it? Three things: whether one compound absorbs more energy than another under identical conditions, whether a lot meets its material specification, and whether an additive change worked. Testing plastic additives for toughness starts here, and impact strength sits in the mechanical group of the methods for testing plastic additives, next to the stiffness tests it is traded against.

How a pendulum impact test works#

A pendulum impact test works by releasing a hammer of known energy from a fixed height, letting it break a notched bar at the bottom of its swing, and reading the energy the bar absorbed from how much less high the hammer rises on the far side. The procedure runs in 3 steps.

1. Clamp or support the notched specimen in the fixture, as a cantilever for Izod or as a simple beam for Charpy.
2. Release the pendulum from its fixed height so it strikes the specimen once.
3. Read the energy lost by the pendulum, then divide it by the specimen width (J/m) or by the area under the notch (kJ/m2).

Not all of the lost energy is fracture energy. ASTM D256-26 counts fracture initiation and propagation plus specimen toss, bending, vibration, friction and indentation in the reading: initiation and toss dominate for brittle materials, while propagation, vibration and bending become significant for tough ones. The cantilever geometry carries the name of Edwin Gilbert Izod (1903) and the simple-beam geometry that of Georges Charpy (1901).

What is the purpose of an impact test?#

The purpose of an impact test on a plastic is to rank compounds and lots by how much energy they absorb before a notch turns into a crack, which is why ASTM D256-26 describes the test as mainly useful for quality control and material specification. The same standard warns that toughness rankings can reverse when the test conditions change, so an impact value is a comparison under fixed conditions, not a property of the moulded part.

Notched and unnotched impact strength#

Notched impact strength measures how much energy a plastic absorbs once a crack site already exists, unnotched impact strength measures the energy needed to start the crack as well, so the notched value is the one that predicts how a moulded part with a sharp corner behaves.

ASTM D256-26 and ASTM D6110-26 both require a milled notch, which is why "impact strength" on a data sheet means notched impact unless the sheet says otherwise. Why read the unnotched value at all? In glass-fibre-filled recycled PET (Monti and colleagues, 2021), the glycidyl-methacrylate-functional ethylene copolymer gave the largest unnotched gain while the notched value rose from 5.2 to about 8.1 kJ/m2, so only the unnotched value singled out that grade.

Why does a notch make a plastic break more easily?#

A notch makes a plastic break more easily because it concentrates stress at its root and supplies the crack the material would otherwise have to initiate, so only the energy to drive that crack through the bar is left to measure. Width matters for the same reason: ASTM D256-26 notes that width alone can decide whether a break is brittle or ductile, so bars whose widths differ by more than "a few mils" are not compared.

How Is the Izod Impact Test Performed? ASTM D256 and ISO 180#

The Izod impact test is performed by clamping a notched bar upright as a cantilever, with the notch facing the striking edge, and breaking it with one pendulum swing under ASTM D256-26 or ISO 180, two methods that share a name and a geometry but not their content. ASTM D256-26 was last updated on 29 April 2026 and supersedes D256-24 and D256-23e1; a data sheet that quotes D256-23e1 or an older edition is citing a superseded method.

Four pendulum standards cover the impact strength of plastics, two for each geometry.

Table 2. The 4 pendulum impact standards for plastics

ASTM D256-26 and ASTM D6110-26 state that they resemble ISO 180 and ISO 179 respectively in title only. Where a cell reads "not in our source set", the value was not read from the standard and none is given.

ASTM D256-26: Test Methods A and C#

ASTM D256-26 contains 2 test methods, Test Method A for the standard cantilever-beam test and Test Method C for materials whose Izod impact resistance is below 27 J/m (0.5 ft·lbf/in). The two methods split the standard by energy level.

• Test Method A: the general notched cantilever-beam test, Sections 6 to 11.
• Test Method C: the same geometry for low-energy materials below 27 J/m, Sections 12 to 17, where tossing the broken piece takes a large share of the reading.

D256-26 is maintained by ASTM Committee D20 on plastics, subcommittee D20.10, and replaced D256-24 on 29 April 2026. It adds two instrument notes: a load-cell pendulum may not be equivalent to an energy-remaining tester (Note 2), and manual and digital testers can disagree, mainly for thermosets below 54 J/m (Note 8).

Specimen, notch and conditioning#

Four specimen settings decide whether two Izod results can be compared: the specimen size, the notch, the conditioning and the test temperature.

• Specimen size: the ASTM D256 bar measures 63.5 by 12.7 by 3.2 mm, and its width may vary between limits, so the width is reported with every value.
• Notch: one milled notch design per standard, so the notch geometry is fixed by the standard and not by the mould.
• Conditioning: the temperature and humidity history set before the bar is broken; polyamide 6, which absorbs about 2.4 % moisture at standard conditions, breaks differently dry and conditioned.
• Test temperature: 23 °C unless the specification asks for a low-temperature value, and always stated with the result.

What are the dimensions of an Izod impact test sample?#

An ASTM D256 Izod specimen measures 63.5 by 12.7 by 3.2 mm (2.5 by 0.5 by 0.125 in), with a milled notch, and its width is the dimension the J/m result is divided by. The ISO 180 specimen is set by the ISO standard itself, and because ISO 180 divides by the area under the notch rather than by width, the two bars do not give comparable numbers even when the material is the same.

The 4 failure types: complete, hinge, partial and non-break#

ASTM D256-26 records 4 failure types for every broken bar, complete break (C), hinge break (H), partial break (P) and non-break (NB), and an average is only valid when every specimen in it failed the same way. The 4 codes are listed below.

• Complete break (C): the specimen separates into two or more pieces.
• Hinge break (H): an incomplete break in which the pieces stay joined.
• Partial break (P): an incomplete break that does not qualify as a hinge break.
• Non-break (NB): the specimen does not break, and the result is not reported as a standard impact value.

An average should be calculated only from specimens that failed in the same category. A data-sheet "NB" says the bar did not break under that pendulum; it is not a very high number. The Charpy counterpart is stricter: ASTM D6110-26 requires a complete break and states that values for materials with different failure types are not compared directly.

What Is Charpy Impact Strength? ISO 179 and ASTM D6110#

Charpy impact strength is the energy a notched plastic bar absorbs when it is supported at both ends as a simple beam and broken by one pendulum blow behind the notch, reported in kJ/m2 under ISO 179 and as energy per unit width under ASTM D6110-26. It is used, like Izod, to rank compounds, check lots and confirm additive changes.

ISO 179 comes in two parts: ISO 179-1 is the non-instrumented test that reads the energy the pendulum lost, and ISO 179-2 is the instrumented test that records force against time during the break. Charpy values are common in the filler and reinforcement literature, such as the notched Charpy +13 % with 5 wt% MA-g-PP in a cellulose-basalt/PP hybrid.

The ASTM Charpy method for plastics is ASTM D6110-26, last updated on 6 August 2026 and superseding D6110-18. Like D256, it states in Note 4 that it resembles its ISO counterpart in title only. It requires a complete break, so a compound that hinges or does not break has no standard D6110 value, and it notes that specimen width can decide whether a break is brittle or ductile.

Izod vs Charpy: 6 Differences That Change the Number#

Izod and Charpy differ in 6 ways that change the result: how the bar is held, which way the notch faces, where the pendulum strikes, which standards define them, which unit they report and which data sheets quote them. Table 3 sets the 6 differences side by side for plastics.

Table 3. Izod against Charpy for plastics

The geometry rows describe the test arrangement shared by the ASTM and ISO methods. Neither ASTM method is equivalent to its ISO namesake (D256-26 Note 5, D6110-26 Note 4).

Both are pendulum tests in two geometries, cantilever and simple beam, and a value from one is never read as a value from the other. Impact testing of plastics also has a second family of methods, the falling-weight and puncture tests on sheet, pipe and profile, covered further down this page.

Can Izod results in J/m be converted to kJ/m2?#

No: a J/m value from ASTM D256 and a kJ/m2 value from ISO 180 come from methods that ASTM itself says resemble each other in title only, so dividing one by a specimen dimension produces a number in the right unit but not a comparable result. Within ASTM, D256-26 allows the result to be reported per unit width (J/m) or per unit cross-sectional area under the notch, so a D256 value expressed in kJ/m2 is still a D256 result and not an ISO 180 one. A specification and a supplier's data sheet therefore have to name the same standard, not only the same unit.

How Do Additives Change Impact Strength? The 7 Families#

The 7 additive families that change impact strength are impact modifiers, compatibilizers and coupling agents, mineral fillers, reinforcing fibres, nucleating agents, plasticizers, and antioxidants and UV stabilizers, and impact modifiers are the only family whose job is to raise it. Not every additive moves the number: lanthanide-doped oxide tracer markers left the tensile, impact and flexural properties of PP and ABS unchanged up to 250 ppm.

Table 4 lists the impact value each family moves, the direction and a sourced example, in the order of the 7 sections below.

Table 4. How 7 additive families move a notched impact value

Values are from the cited studies and supplier documents at the stated dosage in the stated polymer and method. Percentage gains are relative to the cited control and are not converted into absolute values.

1. Impact modifiers: the rubber phase that stops a crack#

Impact modifiers raise notched impact strength because their dispersed rubber particles cavitate or craze ahead of the crack and let the surrounding matrix yield in shear, which turns the energy a notch would spend on a clean fracture into plastic deformation. Bucknall's 2007 review (Journal of Polymer Science Part B) names the three mechanisms, crazing, rubber-particle cavitation and matrix shear yielding; in pseudo-ductile matrices the particles cavitate first, then the matrix yields in massive shear. Impact modifiers such as MBS, acrylic core-shell grades, CPE and POE all work through this rubber phase.

In high-impact polystyrene, a 2019 study found that 15 % rubber gave 166.2 J/m notched impact, about four times the 5 % grade. In glass-fibre-filled recycled PET, ethylene copolymers raised notched Izod from 5.2 to about 8.1 kJ/m2 (+55 %, ISO 180, 23 °C). In rigid PVC, patents put the effective range at about 3 to 10 phr. Particle spacing matters more than rubber content alone: Wu's criterion puts the switch to tough behaviour in PA66/rubber blends at a matrix ligament below about 0.3 µm.

The same HIPS study saw tensile strength at break fall from about 17 to about 12 MPa, and the rubber phase also lowers modulus and heat deflection temperature, so an impact value is always read next to a stiffness value. Types, dosage and selection for all impact modifiers sit on the family hub.

2. Compatibilizers and coupling agents: impact strength as the interface test#

Compatibilizers and coupling agents show up first in notched impact strength, because modulus follows the volume of each phase whatever the interface does, while a notched bar breaks along any interface that cannot transfer load. The 4 sourced results below run from post-consumer blends to a multi-polymer system.

• SEBS in post-consumer blends: Kraton reports notched Izod +70 % for CirKular+ C1010 at 5 % in HDPE/PET and +50 % for CirKular+ C1000 in a 72/18 PP/PET blend.
• Maleated polypropylene in recycled PP with nylon: POLYBOND 3150 and 3002 at 5 % can raise notched and reverse Izod about threefold (Plastics Technology).
• Maleated polypropylene as a coupling agent: 5 wt% MA-g-PP raised notched Charpy by 13 % in a cellulose-basalt/PP hybrid, and 3 wt% MAPP was the impact optimum in a glass-fibre and wood/PP compound (Wang and co-workers, Materials, 2026).
• Reactive pairs in multi-polymer blends: 10 wt% POE-g-(MAH-co-St) plus 10 wt% SEBS-g-(MAH-co-St) raised the impact strength of a PA6/PP/ABS/SEBS blend by 823 %, a four-component result rather than a typical gain.

A Dow ENGAGE-type POE at 2 to 5 %, typically 3 %, can roughly triple impact strength and elongation in a 70/30 HDPE/PP recyclate while keeping the modulus (Plastics Technology). Which compatibilizers suit which blend is set out pair by pair on the hub.

3. Mineral fillers: particle size and top cut decide the direction#

A mineral filler can raise or lower notched impact strength, and particle size decides which: fine, well-dispersed, stearic-acid-coated calcium carbonate below about 3 µm can toughen polypropylene, while agglomerates and coarse particles start the crack the notch is waiting for. Coated particles of about 0.7 to 3 µm debond under load and the polymer ligaments between the voids yield (Thio, Argon, Cohen and Weinberg, MIT, Polymer, 2002; Zuiderduin, Westzaan, Huétink and Gaymans, University of Twente, Polymer, 2003).

Which particle starts the crack? Fu, Feng, Lauke and Mai's 2008 review in Composites Part B identifies agglomerates and the coarse tail as crack-initiation sites, which is why the d98 top cut predicts impact better than the median d50. Talc, the platy filler bought for stiffness, generally does not toughen, so a talc-filled grade that needs impact retention carries an impact modifier as well.

Exfoliated montmorillonite at 4.7 wt% in nylon 6 raised HDT to 152 °C while impact stayed comparable (Kojima and co-workers, Toyota, 1993), and colloidal antimony pentoxide costs less impact strength than antimony trioxide. In rigid PVC, precipitated and nano calcium carbonate act as impact co-modifiers. Particle size, top cut and coating are compared across all 13 fillers for plastics on the hub.

4. Reinforcing fibres: notched and unnotched impact move apart#

Reinforcing fibres raise unnotched impact strength and can lower notched impact strength, because an unnotched bar has no crack and the fibres simply carry load, while in a notched bar the nearest fibre ends become the path the crack keeps following. Fibre ends act as stress concentrations, which is why notched and unnotched results can move in opposite directions in glass-filled grades.

What changes the notched result in a fiber compound? Three variables: the interface, where a coupling agent or maleated polyolefin bonds fibre to matrix; the residual fibre length after compounding; and, in polar matrices, a reactive toughener such as the GMA-functional ethylene copolymer that gave the best unnotched value in glass-filled rPET.

A fibre compound needs both values, each with its method and fibre content, because either number alone misdescribes it. Fibre type, length and loading are compared across all reinforcing fibers for plastics on the hub, including the glass fiber and glass-filled nylon pages.

5. Nucleating agents: alpha nucleation costs toughness, beta nucleation buys it#

Alpha nucleating agents raise the stiffness of polypropylene and can lower its notched impact strength, while beta nucleating agents produce the trigonal beta-crystal form, which has markedly higher impact strength and toughness than the alpha form. In PP impact copolymers, Milliken reports that nucleation gives a better stiffness-impact balance, a supplier claim rather than a general rule.

PET shows the opposite case: the 2026 review by Hao and co-workers in Molecules reports that 0.6 wt% TMB-5 raised the crystallization temperature of PET by 16.6 °C and its impact strength about ten-fold.

Varga's 2002 review is the reference for the toughness gain of beta-iPP over alpha-iPP; beta-PP pipe extrusion typically combines 0.02 % alpha nucleator with 0.2 % beta nucleator, and calcium pimelate has been studied at 0.2 wt%. The beta form is metastable and converts to alpha on heating or strain, and its fraction is read from the Turner-Jones WAXD peak-area ratio, so the impact test confirms the effect but does not measure the crystal form. The stiffness-toughness balance of each grade is compared on nucleating agents for polypropylene.

6. Plasticizers: from brittle break to non-break#

Plasticizers raise impact strength because they soften the polymer until it yields instead of cracking, and in PLA 10 wt% cardanol lifted impact strength to 9.4 kJ/m2, 2.6 times the neat polymer, with elongation rising to 472 % (Kang and co-workers, RSC Advances, 2018).

Why is flexible PVC compared by hardness rather than by Izod? Flexible PVC holds 5 to 65 wt% plasticizer, and a bar can pass under the pendulum without breaking. ASTM D256-26 does not report a non-break as a standard result, so plasticized compounds are compared by Shore hardness, with efficiency set at Shore A 80 or at 50 phr against DEHP = 1.00.

Plasticizers are not bound to the polymer, and plasticizer loss causes loss of flexibility, embrittlement and cracking, which a falling impact value can detect in an aged part. How fast different plasticizers for plastics migrate is compared on the hub.

7. Antioxidants and UV stabilizers: keeping impact strength through processing and service#

Antioxidants and UV stabilizers do not raise impact strength; they keep it, by stopping the chain scission that turns a tough compound brittle during extrusion, oven ageing and outdoor exposure. Polypropylene undergoes chain degradation above 100 °C, and oxidation at its tertiary carbon causes beta-scission and molecular-weight loss.

ABS loses impact strength as its butadiene phase photo-oxidises, with yellowing as the visible sign, which is also why butadiene-based modifiers such as MBS lose toughness outdoors while acrylate and hydrogenated styrenic rubbers do not.

The measurable end point is days to embrittlement. In recycled PP at 150 °C, a stabilizer with acid scavenger at 0.2 to 0.4 % extended the time to embrittlement from about 25 days unstabilised to about 37 to 42 days in Songwon's 2025 data for XP 2121 (chart values). Which antioxidants for plastics protect the melt and which protect the part is set out on the hub.

How Do You Read an Impact Strength Value on a Data Sheet?#

Read an impact value by checking 5 things before the number: the standard, the unit, the notch, the test temperature and the failure type, because a value missing any one of them cannot be compared with a value from another source. Each check is listed below.

• Standard: ASTM D256-26 or ISO 180 for Izod, ISO 179 or ASTM D6110-26 for Charpy.
• Unit: J/m per unit width (ASTM) or kJ/m2 per unit area under the notch (ISO); a raw energy in joules is a machine reading, not an impact strength.
• Notch: notched unless the sheet says unnotched.
• Temperature: 23 °C unless stated; a low-temperature value is a different number.
• Failure type: C, H, P or NB; a non-break is a statement, not a high value.

Two impact values should only be compared when the standard, the notch, the temperature and the specimen width are the same. A specification therefore names the value and the method, for example notched Izod in J/m to ASTM D256-26, Test Method A, at 23 °C, with the failure type per specimen.

Low-temperature impact and the ductile-brittle transition#

Impact strength falls as the test temperature drops, and the temperature range where the failure changes from ductile to brittle, the ductile-brittle transition, is the number an automotive or outdoor specification is really asking about. A low-temperature value is a separate line on the data sheet and cannot be read from the 23 °C value.

Polypropylene is embrittled below 0 °C, which is why PP impact copolymers carry an in-reactor EPR phase and impact PP compounds add POE, EPDM or SEBS. Brüggemann markets reactive MAH-grafted POE and EPDM grades for toughening polyamide down to -40 °C. For nylon-rubber blends, Borggreve, Gaymans and co-workers at the University of Twente described the brittle-tough transition in 1987, which Wu's critical interparticle distance of about 0.3 µm explains in terms of particle spacing.

Falling-Weight and Instrumented Impact Tests for Sheet, Pipe and Profile#

Five falling-weight and puncture methods test impact on the shape a part actually has rather than on a notched bar: ISO 6603-2 and ASTM D3763 for instrumented puncture of plates, ASTM D5420 for Gardner falling-weight impact on PVC, EN 744 and ISO 3127 for pipe, and EN 12608-1 for PVC-U window profiles. Table 6 lists the 5 methods with what each tests.

Table 6. Falling-weight and puncture impact standards

These standards are named so that a specification can be matched to the right method. Their conditions and pass criteria are set by each standard and are not reproduced here.

Pipe impact is a falling-weight test on the pipe itself, so a pipe compound's impact modifier and filler level are judged on the pipe and not only on a bar. The pendulum reads notch sensitivity, the falling weight reads a part under a striker, and pipe and profile specifications use the second. The full pipe package behind a falling-weight pass is on additives for plastic pipes.

Window profiles are tested by falling mass under EN 12608-1, and the profile formulations in our source set combine an acrylic core-shell impact modifier with coated calcium carbonate at 5 to 15 %, the package covered under additives for PVC window profiles.

Which Impact Values Matter for Each Polymer?#

The impact value that matters depends on how the polymer fails: polypropylene is read on low-temperature notched impact, rigid PVC on notched and falling-weight impact, styrenics on the rubber phase, polyamides and polyesters on notched impact with and without fibre, and PLA on how far a modifier moves it away from brittle. Table 5 lists why impact is tested in each polymer and the sourced data.

Table 5. Impact data by polymer in our source set

Where a cell says "no value in our source set", no verified neat-polymer impact value exists in the sources behind this page and none is invented. Percentage gains are against the cited control.

Polypropylene and polyethylene#

Polypropylene is tested for impact mainly at low temperature, because it is embrittled below 0 °C, which is why impact copolymers carry an in-reactor EPR phase and automotive TPO compounds add POE, EPR or EPDM to the PP and talc. TPO of this kind goes into bumper fascia, trim, dashboards and airbag covers.

Alpha nucleation buys polypropylene stiffness at a cost in impact, and beta nucleation buys toughness. The impact package for each PP grade is part of additives for polypropylene.

In polyethylene the impact story is recycled blends: 5 % PP in recycled HDPE cuts slow-crack-growth resistance by up to 40 %, and an ENGAGE-type POE at about 3 % roughly triples impact strength in a 70/30 HDPE/PP recyclate.

Rigid PVC#

Rigid PVC is brittle at a notch and at low temperature, so almost every pipe, profile and sheet compound carries an impact modifier, effective at about 3 to 10 phr, with no strength effect below 3 phr and little extra gain above 10 phr.

Chlorinated polyethylene is dosed at 1 to 10 phr, 2.5 to 7.0 phr preferred in the patent literature, and precipitated or nano calcium carbonate acts as an impact co-modifier; the grades are compared under impact modifiers for PVC.

Clear PVC needs a modifier whose refractive index matches the PVC, and outdoor profiles use acrylic core-shell modifiers such as PARALOID KM-361, a crosslinked butyl acrylate core with a methyl methacrylate shell, because the butadiene in MBS loses toughness in sunlight. Impact modifier, filler and stabilizer levels are combined in additives for PVC.

Polystyrene, ABS and polycarbonate#

High-impact polystyrene shows what the rubber phase alone does to the number: raising the in-reactor rubber content from 5 % to 15 % lifted notched impact about four times, to 166.2 J/m, in a 2019 study. The polybutadiene is grafted in the reactor and forms "salami" particles of 1 to 3 µm (optimum 2 to 5 µm), so HIPS carries no compounding modifier dosage.

ABS is a rubber-toughened SAN whose butadiene phase photo-oxidises, causing yellowing and loss of impact strength; no ABS impact value is held.

Polycarbonate compounds are toughened with MBS, acrylic and silicone-acrylic core-shell impact modifiers, and no polycarbonate impact value is held either.

Polyamides, PET and PBT#

Polyesters and polyamides are read on notched impact with and without glass fibre: in glass-filled recycled PET, an ethylene copolymer raised notched Izod from 5.2 to about 8.1 kJ/m2 to ISO 180 at 23 °C (Monti and colleagues, Polymers, 2021).

Polyamide is toughened with reactive MAH-grafted POE or EPDM for low-temperature service. Polyamide 6 absorbs about 2.4 % moisture at standard conditions, so dry-as-moulded and conditioned bars are reported separately, and glass-filled grades are notch-sensitive. Tougheners, glass and conditioning are set out together in additives for nylon (polyamide).

PBT is toughened with E-MA-GMA type ethylene copolymers, MBS and core-shell modifiers; no PBT impact value is held.

PLA#

PLA is brittle as a neat polymer, so its impact value is read as the distance a modifier moves it: 10 wt% cardanol raised impact strength to 9.4 kJ/m2, 2.6 times the neat polymer (Kang and co-workers, RSC Advances, 2018).

In PLA blends, Joncryl ADR studied at 0.1 to 0.5 wt% raised toughness at 0.5 wt%, a chain extender helping a toughness number. Plasticizers, chain extenders and impact modifiers are weighed against each other in additives for PLA.

Impact Testing of Recycled Plastics#

In recyclate, impact strength is the property that falls first and the one that shows whether a repair worked, because foreign polymer, chain scission and lost stabilizer all show up as a more brittle notched bar. The 3 causes are listed below with the sourced result for each.

• Foreign polymer: PE-LD recyclate with up to 7 wt% PP was considerably more brittle than virgin material (Scholz and co-workers, 2026).
• Chain scission: in glass-filled rPET a chain extender alone did not restore impact, while the ethylene copolymer did.
• Lost stabilizer: recycled PP at 150 °C embrittles in about 25 days unstabilised against about 37 to 42 days restabilised (Songwon, chart values).

The repairs are the compatibilizer and toughener results named above: SEBS at 5 % in HDPE/PET, PP-g-MAH at 5 % in PP/nylon and POE at about 3 % in HDPE/PP. Restabilizers, compatibilizers and tougheners for each stream of recycled plastics are on additives for recycled plastics.

Restabilization and compatibilization are judged on the same notched bar, so the impact test is the common acceptance check for both repairs.

What an Impact Test Cannot Tell You#

An impact test cannot answer 5 questions that formulators regularly put to it, and each one has a different test behind it, as listed below.

• How the part fails in service: rankings can reverse under other conditions, so a moulded part is tested by falling weight or a part test.
• Whether the compound is stiff enough: impact modifiers lower modulus and heat deflection temperature, which tensile and HDT tests measure.
• Which crystal form a PP contains: beta content comes from the Turner-Jones WAXD ratio.
• How long the toughness lasts: impact retention after oven ageing or weathering is a separate programme.
• Why the value fell: melt flow rate, DSC and foreign-polymer analysis find the cause.

Which Other Tests Are Run Alongside Impact Strength?#

An impact value is read next to 5 other tests, because toughness is always bought against stiffness, heat resistance, elongation, ageing and processing stability, and each of those has its own method. The complete map of testing plastic additives and additive-dependent properties shows which test answers which formulation question.

Heat deflection temperature and Vicat: the stiffness side of the trade-off#

Heat deflection temperature is the number an impact modifier costs: ASTM D648 and ISO 75 load the bar at 0.455 or 1.82 MPa and heat it at 2 °C/min to a 0.25 mm deflection, and every rubber addition moves that temperature down while it moves notched impact up. What a rubber phase costs in heat resistance shows up as heat deflection temperature, whose data ASTM D648 states are not intended for design. The second heat check is the Vicat softening point, to ISO 306 or ASTM D1525-25 at 10 N (A50) or 50 N (B50). Nanoclay is the rare exception: 4.7 wt% montmorillonite in nylon 6 raised HDT by 87 °C while impact stayed comparable.

Tensile elongation at break#

Elongation at break is the tensile counterpart of impact strength: ASTM D638-22, the tensile method for plastics up to 14 mm thick, and ISO 527-1 and -2 measure how far a bar stretches before it breaks, and a compound that toughens usually stretches further. Film below 1 mm is tested to ASTM D882, and ASTM D638 and ISO 527 are similar but not equivalent. In the HIPS study, tensile strength at break fell while notched impact rose, which is why elongation and strength are read separately in tensile testing of plastics.

Accelerated weathering and heat ageing: impact retention#

Impact retention after accelerated weathering or oven ageing is how a stabilizer package is judged: the bars are exposed in a xenon or fluorescent UV chamber (ISO 4892-2, ASTM G155-25, ISO 4892-3, ASTM G154-23) or held in an oven, then broken and compared with unexposed bars. The retention end point after accelerated weathering tests is set by the product specification, not by a general rule. Oven exposure for heat aging uses ASTM D3012 for polypropylene, ISO 188 and UL 746B, covered under long-term heat aging and RTI.

Melt flow rate and DSC: finding the cause of a falling impact value#

A falling impact value has a cause, and two tests usually find it: a rising melt flow rate shows chain scission, and a DSC scan shows a change in crystallization or a second melting peak from a foreign polymer. A rising melt flow rate to ISO 1133-1 or ASTM D1238-26 (230 °C / 2.16 kg for polypropylene) is the first sign of the chain scission that later shows up as brittleness. A second melting peak in DSC testing is the fastest way to find a foreign polymer.

Fibre length and filler content#

A notched impact value on a filled or fibre-reinforced compound only means something next to the filler content and the residual fibre length, because a lower glass content or shorter fibres change the number as much as any additive. Glass content is measured by calcination to ISO 1172, and residual fibre length is measured by fibre length and glass content measurement after ashing.

Filled and reinforced compounds depend on the interface as much as on the loading, and that interface is built by coupling agents for filled and reinforced plastics such as silanes, titanates and maleated polyolefins.

Whether two compared bars carry the same loading is checked by ash content and filler content testing, to ASTM D5630-22 or ISO 3451.

Is Charpy testing of steel and welds covered here?#

No: Charpy testing of steel and welds uses a different standard family, ASTM E23 and ISO 148-1, with metal specimens and requirements that have nothing to do with plastic additives, and this page covers plastics only. Impact testing in welding and the Charpy test for steel sit outside the scope of plasticadditives.net.

Do you need your own impact tester, or a test lab?#

An in-house pendulum tester pays for itself where notched impact is checked on every batch of an impact-modified or recycled compound, and an accredited laboratory is the better route where the value goes into a specification or where instrumented or falling-weight testing is needed. This page quotes no instrument or test price, because no verified figure exists in our sources. ASTM D256-26 notes that a load-cell pendulum may not match an energy-remaining tester, so the instrument type belongs in the report. To start from the toughness target and work back to the additive by polymer and function, use the additive finder by polymer and function.

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