Flexural Modulus Testing of Recycled Plastics: Determination Methods, Standards, and Applications

Flexural modulus determination is a core mechanical test for characterizing the bending stiffness of recycled plastics and is essential for validating recyclate quality in structural and semi-structural applications. This article explains how the flexural modulus of recycled materials is measured in accordance with ISO 178, ASTM D790, and GB/T 9341, covering sample preparation, test procedures, data evaluation, and typical industrial applications.

Technical Overview

Flexural modulus, also referred to as bending modulus, describes the ratio of flexural stress to flexural strain within the linear elastic region of a bending test. It is expressed in megapascals (MPa) or gigapascals (GPa) and serves as the key indicator of a material's resistance to bending deformation. For recycled plastics, this parameter is particularly important because repeated processing, thermal-oxidative degradation, moisture exposure, and contamination from foreign polymers or additives can reduce molecular weight, alter crystallinity, and introduce defects, all of which directly influence stiffness.

Determination of the flexural modulus of recyclates enables manufacturers to compare secondary materials against virgin resins, verify consistency between production batches, and confirm whether a recycled compound meets the stiffness requirements of a target application. Testing is typically performed by accredited third-party laboratories or in-house quality departments using universal testing machines configured for three-point or four-point bending in accordance with ISO 178, ASTM D790, or the equivalent Chinese standard GB/T 9341.

Testing Samples

Flexural modulus testing of recycled materials can be carried out on several categories of samples, depending on the stage of the recycling value chain and the purpose of the evaluation.

Recycled Polyolefins

Post-consumer and post-industrial recycled polypropylene (rPP), high-density polyethylene (rHDPE), and low-density polyethylene (rLDPE) recovered from packaging, bottles, crates, and film. These materials are commonly re-compounded into pellets and injection molded into standardized flexural bars.

Recycled Engineering Plastics

Recycled polyethylene terephthalate (rPET), acrylonitrile butadiene styrene (rABS), polycarbonate (rPC), and polyamide (rPA) derived from bottles, electronics housings, and automotive shredder residue. These resins are frequently evaluated for use in higher-stiffness, load-bearing components where modulus retention is critical.

Recycled Compounds and Blends

Recyclates blended with virgin resin, impact modifiers, or mineral fillers such as talc, calcium carbonate, and glass fiber. Filler loading and dispersion strongly affect flexural modulus, so compounds are tested after full pelletization and molding to reflect the final material state.

Specimens Machined from Finished Products

Strip specimens cut directly from recycled end products such as pallets, decking boards, crates, or panels. This approach verifies the actual stiffness of the delivered article, although results must be interpreted with care because machining direction, surface condition, and internal orientation can influence the measured values.

Testing Items

A complete flexural characterization of recycled plastics typically includes the following measured and calculated parameters:

• Flexural modulus (chord modulus determined between 0.05 % and 0.25 % flexural strain per ISO 178)
• Flexural strength (maximum flexural stress sustained during the test)
• Flexural stress at 3.5 % strain (conventional deflection limit for materials that do not break)
• Flexural stress at break, where applicable
• Flexural strain at maximum stress and at break
• Complete force-deflection and stress-strain curves for engineering analysis

Testing Methods

Three-Point Bending per ISO 178 and GB/T 9341

The most widely applied method uses a rectangular specimen supported on two lower anvils with a concentrated load applied at mid-span by a moving loading nose. The standard specimen measures 80 mm × 10 mm × 4 mm with a support span of 64 mm (span-to-thickness ratio of 16:1) and a test speed of 2 mm/min. The flexural modulus is calculated as the chord modulus between the strain limits of 0.05 % and 0.25 % using the relationship Ef = (ΔF/Δs) × (L³ / 4bh³), where L is the support span, b the specimen width, h the thickness, and ΔF/Δs the slope of the force-deflection curve in the evaluated interval.

ASTM D790 Procedure A

ASTM D790 employs the same three-point bending geometry with a preferred support span-to-depth ratio of 16:1 and a nominal outer-fiber strain rate of 0.01 mm/mm per minute, with crosshead speed calculated from the span and specimen thickness. Specimens are commonly 3.2 mm thick. Modulus is reported as a tangent modulus over a defined strain interval. Because ISO 178 and ASTM D790 differ in specimen dimensions, strain rate, and modulus definition, results obtained under the two standards are not directly interchangeable and the test standard must always be stated in the report.

Four-Point Bending

Where specified, four-point bending per ISO 178 applies the load through two loading noses, creating a region of pure bending without shear between them. This configuration reduces localized compressive stress under the loading points and is advantageous for brittle recyclates and highly filled or fiber-reinforced recycled compounds.

Specimen Conditioning and Equipment Requirements

Specimens are injection molded in accordance with ISO 294-1 or machined from plaques and parts, then conditioned in the standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10 % relative humidity per ISO 291 before testing. Width and thickness are measured at mid-span to within 0.01 mm. A calibrated universal testing machine equipped with a bending fixture with rounded supports and loading noses is used, and a minimum of five specimens is tested for each material condition, with the mean, standard deviation, and individual values reported.

Applications

Flexural modulus data for recycled materials supports design, qualification, and quality control across many industries:

• Automotive: interior trim, underbody shields, battery housings, and wheel-arch liners incorporating recyclates
• Construction: plastic lumber, decking boards, fencing, cable trunking, and drainage products made from mixed recycled plastics
• Packaging and logistics: returnable crates, pallets, and bulk containers produced from rPP and rHDPE
• Electrical and electronic equipment: housings and structural brackets based on rABS, rPC, and rPET
• Material qualification: incoming inspection of recyclate batches, supplier certification, and comparative studies of recyclate content versus stiffness retention
• Sustainability programs: documenting mechanical property retention to support eco-design declarations and recycled-content claims

FAQ

What is flexural modulus and why does it matter for recycled materials?

Flexural modulus is the stiffness of a material under bending load, calculated from the slope of the stress-strain curve in the elastic region. For recycled plastics it matters because degradation and contamination during previous service and reprocessing can change molecular weight and morphology, so modulus must be verified before the material is used in stiffness-sensitive designs.

Which standard should be selected, ISO 178 or ASTM D790?

The choice depends on the market and customer specification. ISO 178 and GB/T 9341 are harmonized and widely used in Europe and Asia, while ASTM D790 dominates in North America. Because the two standards use different specimen sizes, strain rates, and modulus definitions, data generated under one standard cannot be directly compared with the other without clarification.

How does recycling typically affect flexural modulus?

Polymer chain scission tends to slightly reduce modulus and ductility, while increased crystallinity or the presence of mineral fillers and glass fibers from contamination can raise stiffness. In practice, well-sorted single-polymer recyclates often retain most of their original flexural modulus, whereas mixed or heavily degraded streams show greater variability.

How many specimens are required and how are dimensions controlled?

Both ISO 178 and ASTM D790 require a minimum of five specimens per material condition. Specimen dimensions must be measured precisely at mid-span, and the span-to-thickness ratio must be maintained at 16:1, because modulus results are highly sensitive to thickness measurement errors due to the cubic dependence in the calculation formula.

Can test bars be cut from finished recycled products?

Yes, provided the bars are machined flat and parallel with uniform thickness and their position and orientation relative to the part are documented. Processing-induced orientation and skin-core structures in molded parts can cause anisotropy, so results from machined specimens represent the actual article rather than the raw material.

What are typical flexural modulus ranges for common recyclates?

Indicative ranges are approximately 0.8–1.3 GPa for rHDPE, 1.2–1.8 GPa for rPP, 2.0–2.8 GPa for rPET, and 2.0–2.6 GPa for rABS. Filled or glass-reinforced recyclates can reach considerably higher values. Actual results depend on source material quality, contamination level, and reprocessing conditions, which is why laboratory verification is always recommended.