Mechanical Properties Testing of Recycled Materials: Standards, Methods, and Applications

Mechanical properties testing of recycled materials is essential for verifying that recycled plastics, films, and metals can reliably replace virgin feedstock in demanding applications. By evaluating tensile strength, flexural modulus, impact resistance, and hardness against recognized standards such as ISO 527, ASTM D638, and GB/T 1040, manufacturers can quantify material degradation, control batch consistency, and support recycled-content claims. This guide explains the sample types, testing items, testing methods, and practical applications involved in recycled material mechanical performance evaluation.

Technical Overview

Recycled materials—commonly known as regrind, regranulate, post-industrial recycled (PIR) resin, or post-consumer recycled (PCR) resin—are subjected to thermal, oxidative, and mechanical stress during both their original service life and the reprocessing stage. These stresses cause polymer chain scission, crosslinking, hydrolysis, and contamination-induced defects, all of which directly reduce mechanical performance. For recycled metals, repeated remelting can alter grain structure and introduce tramp elements that affect strength, hardness, and ductility.

Mechanical properties testing therefore serves three key purposes: quantifying property retention relative to virgin materials, screening batch-to-batch consistency of heterogeneous recyclate streams, and generating reliable data for design allowances when recyclate is substituted into new products. Testing is typically performed by ISO/IEC 17025 accredited third-party laboratories using standardized specimens prepared from pellets or cut directly from finished parts.

Testing Samples

Recycled Plastic Pellets and Granules

Recycled PP, HDPE, LDPE, PET, ABS, PS, PVC, PA, and PC pellets are the most common sample type. Because mechanical tests require shaped specimens, pellets are first converted into standardized test pieces by injection molding (ISO 294-1, ASTM D3641) or compression molding (ISO 293, ASTM D4703). Hygroscopic resins such as PET, PA, and PC must be dried before molding to prevent hydrolytic degradation that would distort the results.

Molded Specimens and Finished Parts

Existing recycled-content products, such as crates, pallets, pipe fittings, and automotive trim, can be evaluated directly. Dog-bone and bar specimens are machined or stamped from parts in accordance with ISO 2818, allowing verification of the actual delivered performance of recycled components rather than laboratory-molded material alone.

Recycled Films, Sheets, and Tapes

PCR polyethylene films used in packaging are tested in strip or specimen form for tensile behavior (ISO 527-3, ASTM D882), tear propagation (ISO 6383, ASTM D1922, ASTM D1004), and dynamic puncture resistance (ISO 7765-1, ASTM D1709, GB/T 9639). These tests are critical because film performance is highly sensitive to degradation and contamination from recyclate feedstock.

Recycled Engineering Plastics and Polymer Blends

Engineering recyclates such as PA6, PA66, PC/ABS, PBT, and glass-filled compounds are tested to confirm retention of stiffness, strength, and notched impact toughness. Blends of recyclate with virgin resin at defined ratios (for example, 30% PCR) are also characterized to establish the performance envelope for each formulation.

Recycled Metals

Remelted aluminum, steel, and copper scrap streams are evaluated through tensile testing (ISO 6892-1, ASTM E8/E8M, GB/T 228.1) and hardness testing using Brinell, Vickers, or Rockwell methods (ISO 6506, ISO 6507, ISO 6508, ASTM E18). These results reveal the effects of alloying-element accumulation and refining quality on strength and ductility.

Testing Items

The core mechanical testing portfolio for recycled materials covers the following properties, each mapped to internationally recognized standards:

• Tensile strength, tensile modulus, yield stress, and elongation at break — ISO 527-1/-2, ASTM D638, GB/T 1040.2, JIS K 7161
• Flexural strength and flexural modulus — ISO 178, ASTM D790, GB/T 9341
• Charpy impact strength (notched and unnotched) — ISO 179-1, ASTM D6110, GB/T 1043.1
• Izod impact strength — ISO 180, ASTM D256, GB/T 1843
• Compressive strength and compression modulus — ISO 604, ASTM D695, GB/T 1041
• Shore hardness (Shore A/D) — ISO 868, ASTM D2240, GB/T 2411
• Rockwell hardness for rigid recyclates — ISO 2039-2, ASTM D785
• Tear resistance of recycled films — ISO 6383, ASTM D1922, ASTM D1004
• Falling dart and puncture impact of films — ISO 7765-1, ASTM D1709, GB/T 9639
• Supporting indicators that reveal molecular degradation: melt mass-flow rate (ISO 1133-1, ASTM D1238, GB/T 3682.1), density (ISO 1183-1, ASTM D792), and heat deflection temperature (ISO 75-2, ASTM D648, GB/T 1634.2)

Testing Methods

Specimen Preparation and Conditioning

Multi-purpose Type 1A or 1B dumbbells and flexure bars are injection molded to ISO 20753 dimensions. Specimens are conditioned at 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 24 hours (typically 40 hours) in accordance with ISO 291 or ASTM D618 before testing. Consistent specimen preparation is especially important for recyclates, since processing history strongly influences measured values.

Tensile Testing Procedure

A universal testing machine fitted with a calibrated load cell and extensometer applies controlled deformation. Per ISO 527-2, a test speed of 1 mm/min is used for modulus determination, while speeds of 5, 50, or 500 mm/min are selected for strength and elongation depending on the material type. Elongation at break is one of the most sensitive indicators of degradation in recycled plastics.

Flexural Testing Procedure

Three-point bending is performed with a support span equal to 16 times the specimen thickness. The test speed is calculated from the specimen depth as specified in ISO 178. Flexural modulus is a key acceptance parameter for recycled PP, PE, and PET used in rigid applications such as pallets, crates, and automotive components.

Impact Testing Procedure

Notched and unnotched specimens are fractured on calibrated pendulum impact testers using Charpy (ISO 179-1) or Izod (ISO 180, ASTM D256) configurations. Notches are machined with controlled radius cutters, and hammer capacity is selected so that the fracture energy falls between 10% and 80% of the pendulum capacity. Impact strength typically shows the largest drop in degraded recyclate and is therefore a primary quality gate.

Hardness Testing Procedure

Shore A or Shore D durometers are applied to elastomeric and rigid recycled materials respectively, with readings taken after a defined dwell time per ISO 868. Rockwell hardness is used for harder, glass-filled engineering recyclates. Hardness changes can signal crosslinking, filler drift, or contamination in recyclate streams.

Data Evaluation and Reporting

Results are reported as mean values with standard deviations. A widely used metric is property retention, calculated as the recycled material value divided by the virgin material value multiplied by 100%. Because recyclate feedstock is heterogeneous, accredited third-party laboratories recommend testing multiple production batches to establish realistic statistical confidence intervals. Test reports issued under ISO/IEC 17025 include full specimen preparation details, conditioning history, and test parameters to ensure traceability and comparability.

Applications

Mechanical properties testing of recycled materials supports decision-making across the entire value chain:

• Incoming quality control and supplier qualification for purchased recyclate pellets and regrind
• Verification of recycled-content claims for eco-labeling, sustainability reporting, and customer specifications
• Automotive engineering, including interior trim, wheel-arch liners, underbody shields, and battery enclosure components
• Packaging development, where PCR films and bottles must retain tensile, tear, and puncture performance at defined recyclate percentages
• Construction products such as recycled PP pipes, drainage systems, and wood-plastic composite decking
• Electrical and electronic equipment housings, where short-term mechanical characterization per UL 746A complements thermal and electrical assessments
• Research and development programs optimizing blend ratios, compatibilizers, and stabilization packages for recyclate formulations

FAQ

What is mechanical properties testing of recycled materials?

It is the standardized laboratory evaluation of tensile, flexural, impact, compressive, and hardness characteristics of recycled plastics, films, or metals, performed to confirm that the material meets the performance requirements of its intended application and to quantify degradation relative to virgin material.

Why do recycled materials show lower mechanical properties than virgin materials?

Repeated processing and prior service exposure cause polymer chain scission, oxidation, crosslinking, hydrolysis, and the introduction of foreign particles or mixed-polymer contamination. In recycled metals, repeated remelting can shift alloy composition and grain structure. These factors typically reduce elongation at break and impact strength most significantly.

Which standards are most commonly applied?

The most frequently used standards are ISO 527 and ASTM D638 for tensile properties, ISO 178 and ASTM D790 for flexural properties, ISO 179 and ISO 180 with ASTM D256 for impact resistance, ISO 868 and ASTM D2240 for Shore hardness, and the corresponding GB/T and JIS national standards for regional compliance.

How much sample material is required for a full mechanical test suite?

For pellet samples, approximately 2 to 5 kilograms of recyclate is usually sufficient to injection mold a complete set of tensile, flexural, and impact specimens in duplicate, including allowance for mold start-up and possible retests. Finished parts or film rolls require fewer raw kilograms but must be large enough to yield full-size specimens.

Can recycled material test data be compared directly with virgin resin datasheets?

Only if specimen preparation, conditioning, and test conditions match those used for the datasheet values. Differences in molding method, specimen geometry, and test speed can shift results noticeably, so comparisons should always reference the exact standard conditions stated in the laboratory report.

How long does mechanical testing of recycled materials take?

Standard turnarounds at accredited third-party laboratories range from 5 to 10 working days, including specimen molding and conditioning. Complex programs involving multiple blend ratios, multiple batches, or extended conditioning may require two to four weeks.