Tensile strength determination is the most widely performed mechanical test for evaluating the quality, consistency, and fitness-for-purpose of recycled plastic materials. This article provides a comprehensive technical guide to recycled material tensile strength testing in accordance with international standards such as ISO 527, ASTM D638, and GB/T 1040, covering sample preparation, test procedures, data evaluation, and typical industrial applications.
Technical Overview
Recycled materials—commonly referred to as recyclate, regrind, or regranulate—are thermoplastic resins recovered from post-industrial or post-consumer waste streams and reprocessed into pellets, flakes, films, or molded articles. Because repeated washing, extrusion, and thermal histories can cause polymer chain scission, thermo-oxidative degradation, and solid contamination, the mechanical performance of recyclates may differ substantially from that of virgin resins. Tensile strength determination is therefore the primary screening test used by recyclers, compounders, and end users to verify the structural integrity and batch-to-batch consistency of recycled materials.
In a tensile test, a standardized specimen is gripped at both ends of a universal testing machine (UTM) and pulled at a constant crosshead speed until yielding or fracture. The resulting stress–strain curve is used to determine the tensile strength, yield stress, elongation at break, and tensile modulus of the material. Accredited third-party laboratories routinely perform these determinations in accordance with ISO 527, ASTM D638, GB/T 1040, and related national and international standards, providing traceable data for quality control, supplier qualification, and recycled-content compliance programs.
Testing Samples
Tensile strength determination is applicable to virtually all recycled thermoplastic materials, regardless of their physical form. Depending on the incoming material, samples are either tested directly or first converted into standardized test specimens.
Recycled Pellets and Regranulate
Compound-grade recyclates such as recycled PET (rPET), recycled HDPE (rHDPE), recycled LDPE (rLDPE), recycled PP (rPP), recycled ABS, recycled PS, recycled PC, and recycled polymer blends are typically supplied as pellets. Pellets must first be injection molded or compression molded into standardized dumbbell specimens before tensile testing can be performed.
Post-Consumer Flakes and Washed Regrind
Washed PET bottle flakes, packaging film regrind, and other flaked recyclates cannot be gripped and pulled directly. They are densified and molded into test plaques or dumbbells under controlled conditions so that the resulting tensile data represent the intrinsic quality of the recyclate rather than the geometry of the flakes.
Recycled Films and Sheets
Thin films produced from recycled polyethylene or other flexible recyclates are tested as rectangular strips in accordance with ISO 527-3 or ASTM D882, using appropriate grip inserts, extension measurement, and higher crosshead speeds suited to thin gauge materials.
Specimens Cut from Molded Parts and Finished Goods
Recycled crates, pipes, pallets, electrical housings, and automotive components can be evaluated by stamping or machining tensile specimens directly from the finished product. Specimen location and orientation relative to flow direction must be recorded, since processing-induced orientation significantly influences measured strength.
Testing Items
A complete tensile characterization of recycled materials typically reports the following parameters derived from the stress–strain curve:
- Tensile strength (maximum tensile stress, σM) — the maximum load sustained by the specimen divided by its original cross-sectional area
- Tensile stress at yield (σY) — the stress at the first point at which strain increases without a further increase in stress
- Tensile stress at break (σB) — the stress at specimen rupture
- Elongation at yield (εY) and elongation at break (εB) — ductility indicators that are highly sensitive to recyclate degradation and contamination
- Tensile modulus (Et, Young's modulus) — the slope of the initial linear region of the stress–strain curve, indicating material stiffness
- Full stress–strain curve characterization, including the failure mode (brittle, ductile, or semi-ductile)
Testing Methods
Applicable Standards
The determination is primarily performed to the following standards, depending on the target market and customer specification:
- ISO 527-1 and ISO 527-2 — determination of tensile properties of plastics, using ISO 3167 multipurpose specimens (types 1A and 1B)
- ASTM D638 — standard test method for tensile properties of plastics, using Type I through Type V specimens
- GB/T 1040.1 and GB/T 1040.2 — the Chinese national standards, technically aligned with ISO 527
- ISO 527-3 and ASTM D882 — tensile testing of plastic films and thin sheeting
- ASTM D4703 and ISO 293 — compression molding of test plaques; ISO 294 and ASTM D3641 — injection molding of test specimens
ISO 527 and ASTM D638 use different specimen geometries and crosshead speeds, so results obtained by the two methods are not directly interchangeable. The applicable standard must be agreed before testing.
Specimen Preparation and Conditioning
Pellets and flakes are dried as required (for example, rPET is dried to a moisture content below approximately 0.02 percent to prevent hydrolytic degradation during molding) and then molded into dumbbell specimens, typically ISO 527-2 type 1A specimens (170 mm overall length, 10 mm width, 4 mm thickness) or ASTM D638 Type I specimens (165 mm overall length, 13 mm narrow-section width, 3.2 mm thickness). Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10 percent relative humidity, generally for at least 40 hours in accordance with ISO 291 or ASTM D618, unless the material specification states otherwise.
Equipment and Test Procedure
Testing is carried out on a calibrated universal testing machine fitted with a suitable load cell (commonly 1 kN or 5 kN for recyclate specimens), mechanical or pneumatic grips, and a contact or video extensometer for accurate strain measurement. The width and thickness of each specimen are measured at several points within the gauge length, and the minimum cross-sectional area is used for stress calculation. Typical test speeds are 1 mm/min for modulus determination and 50 mm/min for strength and elongation, although film testing may employ significantly higher speeds. At least five valid specimens are tested for each material condition; specimens that fail in the grips or outside the gauge length are rejected and retested.
Data Evaluation
Tensile stress is calculated as the measured force divided by the original cross-sectional area, and strain as the extension divided by the original gauge length. The tensile modulus is determined from the slope of the stress–strain curve between strain points of 0.0005 and 0.0025 (0.05 to 0.25 percent) when an extensometer is used. Reported results include the arithmetic mean, standard deviation, and, where required by the standard, the median of the valid specimen population. The stress–strain curve shape is also assessed, because a shift from ductile to brittle failure behavior is an early and reliable indicator of recyclate degradation.
Applications
Tensile strength data for recycled materials support decisions across the entire recycling and manufacturing value chain:
- Quality control and grading of recyclate at reprocessing plants, ensuring consistency between production batches
- Incoming material inspection by plastic converters and brand owners incorporating recycled content into new products
- Comparative evaluation of recyclate against virgin resin benchmarks during material selection and substitution studies
- Research and development of recycled compounds, including the optimization of compatibilizers, stabilizers, and virgin-carrier blends
- Documentation for recycled-content claims, eco-labeling schemes, and customer or regulatory verification programs
- Product design verification in packaging, automotive, construction, agriculture, and consumer electronics applications where recycled resins must meet defined mechanical requirements
FAQ
Why is the tensile strength of recycled materials usually lower than that of virgin resins?
Repeated processing causes polymer chain scission and thermo-oxidative degradation, while residual contaminants and foreign polymer inclusions act as stress concentrators. Both effects reduce strength and, even more noticeably, elongation at break. Well-sorted, stabilized, and properly reprocessed recyclates can nevertheless achieve tensile properties close to those of virgin materials.
Can recyclate flakes be tested directly?
No. Flakes and regrind cannot be gripped for tensile testing. They must first be melt-processed into standardized dumbbell specimens by injection molding or compression molding under documented conditions, because the molding parameters themselves influence the measured properties.
Should testing follow ISO 527 or ASTM D638?
The choice depends on the specification, customer, and destination market. ISO 527 (and the equivalent GB/T 1040) uses 1A/1B specimens with a 50 mm gauge length, while ASTM D638 uses Type I specimens with a 50 mm gauge length but different geometry. Results from the two standards are similar in trend but should not be compared directly.
How many specimens are required?
Both ISO 527 and ASTM D638 require a minimum of five valid specimens per material condition. Additional specimens are tested if failures occur outside the gauge section or if excessive scatter is observed.
What are typical tensile strength values for common recyclates?
Indicative values for well-processed materials are approximately 45–60 MPa for rPET, 18–26 MPa for rHDPE and rLDPE, and 22–32 MPa for rPP. Actual values depend strongly on feedstock purity, reprocessing conditions, and additive formulation, which is why laboratory verification of each batch is recommended.
How long does a typical determination take?
Excluding specimen molding, testing of a standard five-specimen set at one speed can be completed within one hour. Including conditioning, molding, and reporting, accredited third-party laboratories typically return full results within three to seven working days.