Recycled Plastic Tensile Strength Testing: Standards, Methods, and Applications

Recycled plastic tensile strength testing is a fundamental mechanical evaluation used to determine whether regenerated plastic materials can meet the performance requirements of virgin polymers in practical applications. Conducted in accordance with internationally recognized standards such as ISO 527 and ASTM D638, this test measures key properties including tensile strength, elongation at break, and tensile modulus. This article provides a comprehensive overview of recycled plastic tensile strength testing, covering sample preparation, testing items, methodologies, typical applications, and frequently asked questions.

Technical Overview

Tensile strength testing is one of the most widely performed mechanical tests for characterizing recycled and regenerated plastics. During reprocessing, polymer materials undergo repeated thermal and mechanical stress from their original service life, collection, sorting, washing, grinding, and re-granulation. These processes can cause chain scission, oxidation, crosslinking, contamination with foreign polymers or non-polymer impurities, and moisture absorption, all of which directly affect the mechanical integrity of the recyclate.

A tensile test quantifies how a recycled plastic material behaves under a gradually applied uniaxial tensile load until failure. The resulting data — tensile strength, stress at yield, stress at break, elongation, and elastic modulus — allow manufacturers to verify material quality, compare recyclates against virgin resins, optimize blend formulations, and demonstrate compliance with product specifications and regulatory requirements. Because recycled feedstock is inherently more variable than virgin polymer, tensile testing is an essential quality control tool at multiple stages of the recycling value chain.

Testing Samples

Moulded Specimens from Regranulated Pellets

The preferred approach is to convert recycled pellets or flakes into standardized test specimens through injection moulding (ISO 294 or ASTM D4703-compliant compression moulding). This method ensures consistent geometry and allows the intrinsic properties of the recyclate to be evaluated. Moisture-sensitive materials such as recycled PET and polyamide must be thoroughly dried before moulding to prevent hydrolytic degradation that would distort the results.

Specimens Machined from Finished Products

When the object of evaluation is a finished or semi-finished recycled product — pipes, sheets, crates, profiles, or injected housings — dog-bone specimens can be die-cut or machined directly from the article. Test engineers must record the sampling position and direction, because processing-induced molecular orientation and weld lines can create anisotropy that significantly influences tensile results.

3D Printed and Extruded Test Pieces

With the growing use of recycled filament and pellet feedstock in additive manufacturing, tensile specimens printed under defined conditions are increasingly tested. Print orientation, layer adhesion, and infill settings must be documented, as these parameters strongly affect measured strength values.

Testing Items

A standard tensile test program for recycled plastics typically reports the following parameters:

• Tensile strength (ultimate tensile stress at maximum force)
• Tensile stress at yield
• Tensile stress at break
• Strain (elongation) at yield
• Strain (elongation) at break
• Tensile modulus (Young's modulus, determined as the slope of the stress-strain curve between defined strain limits)
• Secant modulus, where specified for particular applications
• Statistical indicators such as mean values, standard deviation, and coefficient of variation across the specimen batch

Due to the heterogeneous nature of recycled materials, reporting the scatter of results is as important as reporting the average values, since wide variability often signals inconsistent feedstock or inadequate sorting.

Testing Methods

Applicable Standards

The principal standards governing tensile testing of plastics, including recyclates, are:

• ISO 527-1 and ISO 527-2 — determination of tensile properties for moulding and extrusion plastics
• ASTM D638 — standard test method for tensile properties of plastics
• GB/T 1040.1 and GB/T 1040.2 — the Chinese national standards technically aligned with ISO 527
• JIS K 7161 and JIS K 7162 — the Japanese standards harmonized with ISO 527
• EN 15342 to EN 15347 — European standards for the characterization of plastics recyclates, under which mechanical property verification is a core element

Specimen Preparation and Conditioning

The standard dumbbell-shaped specimen 1A or 1B of ISO 527-2, or the Type I specimen of ASTM D638, is most commonly used. Specimens must be free from voids, notches, and surface defects, and the width and thickness at the gauge section must be measured precisely to calculate the original cross-sectional area. Before testing, specimens are conditioned in a controlled atmosphere, typically 23 °C and 50 % relative humidity in accordance with ISO 291 or ASTM D618, for a minimum of 24 hours (ASTM D618 Procedure A specifies 40 hours) to ensure moisture and temperature equilibrium.

Test Procedure

The conditioned specimen is mounted in the grips of a calibrated universal testing machine equipped with an appropriate load cell and, for modulus determination, a contact or video extensometer. Testing is performed at a constant crosshead speed selected according to the standard and material behavior — commonly 1 mm/min for modulus measurement and 50 mm/min for strength determination under ISO 527-2, while ASTM D638 typically applies 50 mm/min for rigid thermoplastics with Type I specimens. The machine records the force-extension curve until the specimen breaks, and the software calculates stress, strain, and modulus values.

Data Evaluation and Reporting

ISO 527 requires a minimum of five valid specimens per material; for recycled plastics, accredited third-party laboratories often recommend ten or more specimens to capture the natural scatter of the material. Results are discarded and retested if specimens fail outside the gauge length. The final report should document the material source, specimen preparation method, conditioning parameters, test speed, individual and mean results, and any observed failure modes such as brittle fracture, necking, or impurity-initiated breaks.

Applications

Tensile strength testing of recycled plastics supports decision-making across many industries and product categories:

• Recyclate supplier qualification and incoming inspection for plastics converters purchasing regranulated materials
• Packaging applications, including recycled PET for bottles, trays, and thermoformed containers
• Automotive engineering, where recycled polypropylene is used for interior trim, underbody shields, and battery housings
• Construction products such as recycled HDPE pipes, decking, profiles, and geosynthetics
• Consumer electronics housings and household appliances incorporating recycled ABS, PC, and PC/ABS blends
• Agricultural films, crates, pallets, and logistics containers produced from mixed or single-stream recyclates
• Research and development of new recyclate formulations, compatibilizers, and virgin-recycled blend ratios
• Compliance demonstration for eco-design regulations, recycled content declarations, and customer specifications

In many programs, tensile testing is combined with complementary evaluations such as melt flow rate (ISO 1133), impact resistance (ISO 179 or ASTM D256), thermal analysis (DSC), and spectroscopic identification (FTIR) to build a complete picture of recyclate quality.

FAQ

Why is the tensile strength of recycled plastic usually lower than that of virgin material?

Repeated processing causes polymer chain degradation (reduction in molecular weight), while residual contaminants, mixed polymer fractions, and moisture-related hydrolysis create weak points that initiate premature failure. Well-sorted, single-polymer recyclates processed under controlled conditions can approach virgin-grade performance, whereas poorly sorted mixed recyclates show significantly reduced strength and elongation.

Which standard should be chosen: ISO 527 or ASTM D638?

The choice depends on the target market and customer specification. ISO 527 (or the equivalent GB/T 1040 and JIS K 7161/7162) and ASTM D638 use different specimen geometries, thicknesses, and test speeds, so results are not directly interchangeable. Laboratories should confirm the required standard before testing begins.

How many specimens are needed for a reliable result?

A minimum of five valid specimens is required by ISO 527, but because recycled materials are heterogeneous, testing ten or more specimens provides a more reliable statistical basis and better reveals variability within the batch.

Can finished products made of recycled plastic be tested directly?

Yes. Dog-bone specimens can be cut from finished articles, and the measured values reflect the combined influence of material quality, processing conditions, and product geometry. The sampling location and direction must always be reported.

What does a large scatter in tensile results indicate?

High variability between specimens usually points to inconsistent feedstock, insufficient sorting, contamination, non-uniform drying, or unstable processing parameters. It is a strong signal to review the upstream sorting and regranulation process.

How should moisture-sensitive recyclates be handled before testing?

Materials such as recycled PET and polyamide must be dried thoroughly before specimen moulding, and conditioned specimens should be tested promptly, because absorbed moisture causes hydrolytic degradation and drastically reduces tensile strength and elongation.

How long does a typical tensile test take?

Individual tests usually last from a few minutes to about half an hour depending on material ductility and crosshead speed. Including specimen preparation, conditioning, and multi-specimen testing, a complete evaluation is generally completed within one to several working days at an accredited third-party laboratory.