Recycled plastic hardness testing measures the indentation resistance of recycled polymers using standardized methods such as Shore durometer (ISO 868 / ASTM D2240), Rockwell (ASTM D785 / ISO 2039-2), and ball indentation (ISO 2039-1). As post-consumer and post-industrial recyclates are increasingly specified in demanding applications, hardness data helps verify material integrity, detect degradation and contamination, and confirm batch-to-batch consistency. This guide explains the applicable standards, specimen requirements, test procedures, and typical reference values for recycled plastics.
Technical Overview
Hardness is one of the most frequently measured mechanical properties of recycled plastics because it provides a rapid, low-cost, and largely non-destructive indication of surface rigidity, modulus, and overall material condition. For recyclates, hardness carries additional diagnostic value: thermal and oxidative degradation during the first service life and subsequent reprocessing can cause chain scission or partial crosslinking, both of which shift the hardness of the material away from that of the virgin resin. Hardness is therefore widely used as an incoming-material screening parameter and as an outgoing quality-control parameter in recycling plants, compounder facilities, and accredited third-party laboratories.
Why Hardness Matters for Recyclates
- Mixed-polymer contamination (for example, PP in rHDPE or PET in rPP) produces measurable hardness shifts that flag poor sorting.
- Mineral fillers, talc, calcium carbonate, glass fiber, or wood fiber residues from previous formulations generally increase hardness and stiffness.
- Plasticizer loss in recycled flexible PVC raises Shore A hardness, while absorbed moisture in recycled polyamide can lower it.
- Blending ratios of virgin resin and regrind can be monitored statistically through hardness consistency checks.
- Multiple reprocessing cycles progressively embrittle some resins, and hardness tracking supports the validation of "closed-loop" recyclability claims.
Governing Standards
- ISO 868 and ASTM D2240 — Shore A and Shore D durometer hardness.
- ASTM D785 and ISO 2039-2 — Rockwell hardness of plastics (scales R, L, M, E, K).
- ISO 2039-1 — Ball indentation hardness.
- ASTM D2583 — Barcol impressor hardness for rigid and reinforced plastics.
- ISO 293, ISO 294-1, and ASTM D4703 — preparation of compression-molded and injection-molded test specimens.
- ISO 291 and ASTM D618 — conditioning atmospheres for specimen conditioning before test.
- ISO/IEC 17025 — competence framework applied by accredited third-party laboratories when reporting hardness data for certification purposes.
Testing Samples
Recycled Polyolefins (rPP, rHDPE, rLDPE)
Polypropylene recovered from crates, battery casings, and rigid packaging, and polyethylene recovered from bottles, drums, and films, represent the largest recyclate streams. These semi-rigid materials are typically characterized with Shore D durometers or Rockwell R scale, and the results are compared against the virgin resin data sheet to assess degradation and contamination levels.
Recycled Engineering Resins (rABS, rPC, rPA, rPOM, rPPO)
Engineering plastics recovered from electronics housings, appliance parts, and automotive components are commonly evaluated with Rockwell M or R scales and ball indentation hardness. Recycled polyamide should be dry-conditioned before testing because absorbed moisture measurably suppresses hardness and stiffness.
Recycled PET (rPET)
Bottle-flake and pelletized rPET used in fibers, sheets, and thermoformed packaging is usually tested on Shore D or Rockwell R scales. Because PET degrades primarily through hydrolysis, hardness testing is often paired with intrinsic viscosity measurement to fully characterize the reprocessing history.
Recycled PVC and Flexible Compounds
Rigid rPVC from pipes and profiles is tested on the Shore D scale, while flexible rPVC from cable insulation and film is tested on the Shore A scale. In flexible compounds, hardness is one of the most sensitive indicators of plasticizer migration and content variation between recyclate lots.
Specimen Forms
- Injection-molded plaques prepared per ISO 294-1 or ASTM D4703, typically at least 6 mm thick for Shore testing.
- Compression-molded sheets per ISO 293 for thermally sensitive or highly contaminated recyclates.
- Sections cut directly from finished parts, pipes, profiles, or sheets, provided the test surface is flat and the wall thickness is adequate.
- Stacked layers of thin material to reach the minimum required thickness, using smooth, fully contacting surfaces.
Testing Items
Hardness test programs for recycled plastics typically include one or more of the following reported items:
- Shore A hardness — for elastomeric, soft, and flexible recyclates such as plasticized PVC, TPE regrind, and soft polyolefin foam.
- Shore D hardness — for rigid thermoplastics including rPP, rHDPE, rPET, and rigid rPVC.
- Rockwell hardness — scales R, L, M, E, and K per ASTM D785 / ISO 2039-2 for engineering-grade recyclates requiring finer resolution.
- Ball indentation hardness — expressed in N/mm² (MPa) per ISO 2039-1, commonly specified in European supply agreements.
- Barcol hardness — per ASTM D2583 for reinforced and fiber-filled recycled compounds.
- Hardness retention — comparative testing of virgin resin, first-pass regrind, and multi-cycle reprocessed material to quantify degradation.
- Hardness uniformity mapping — multiple indentation points across large parts or sheets to reveal poor dispersion of contaminants or blends.
Testing Methods
Specimen Preparation and Conditioning
Specimens must have flat, parallel, smooth surfaces free of sink marks, flash, and texture, because surface geometry strongly influences indentation readings. Injection-molded plaques are preferred over finished parts whenever possible to reduce skin-core and orientation effects. Before testing, specimens are conditioned in the standard atmosphere of 23 ± 2 °C and 50 ± 10 % relative humidity per ISO 291, or per ASTM D618 Procedure A, for a minimum of 16 to 40 hours depending on the specification invoked.
Shore Durometer Hardness (ISO 868 / ASTM D2240)
A calibrated durometer presses a defined indenter into the specimen under a defined spring force: a truncated 35° cone for type A instruments and a sharp 30° cone for type D instruments. The specimen must be at least 6 mm thick, and the indentation point must be located a defined distance from any edge (at least 9 mm per ISO 868 and 12 mm per ASTM D2240). Readings are taken after a standardized dwell time, commonly 3 seconds for instantaneous values or 15 seconds where creep is significant. At least five readings at well-spaced points are taken, and the median value is reported along with the durometer type and dwell time.
Rockwell Hardness (ASTM D785 / ISO 2039-2)
A ball indenter is first seated with a minor load of 10 kgf, after which a major load is applied for a fixed time and then removed while the minor load is retained. Common scales for plastics include R (12.7 mm ball, 60 kgf), L (6.35 mm ball, 60 kgf), M (6.35 mm ball, 100 kgf), E (3.175 mm ball, 100 kgf), and K (3.175 mm ball, 150 kgf). The scale is selected so that the reading falls within the valid working range of the instrument, and results are reported with the scale designation, for example Rockwell R or Rockwell M.
Ball Indentation Hardness (ISO 2039-1)
A polished hardened steel ball of 5 mm diameter is pressed into the specimen: a preload of 9.8 N establishes contact, and a test force selected from 49 N, 132 N, 358 N, or 961 N drives the indentation. The indentation depth is measured between defined load levels, corrected for instrument compliance, and converted to hardness in N/mm². Specimens must be at least 4 mm thick with a smooth surface, and the thickness must be sufficient to prevent any backing or anvil effect. This method is valued for engineering recyclates because it yields a quantitative engineering unit suitable for specification limits.
Barcol Hardness (ASTM D2583)
A spring-loaded Barcol impressor with a truncated conical steel indenter is pressed manually onto the specimen surface, and hardness is read directly from the 0 to 100 dial scale. The method is fast and portable, making it suitable for on-site checks of recycled composites, wood-plastic composites, and glass-filled regrind parts, provided the surface is flat and the material is sufficiently rigid.
Data Reporting and Acceptance
- Report the median of the individual readings, the method and scale, dwell time, test temperature, and specimen thickness.
- Compare results against the resin data sheet or purchase specification tolerance, typically expressed as a hardness range rather than a single limit.
- Investigate statistically significant deviations, which usually indicate polymer contamination, filler variation, or degradation.
- For certification and regulatory submissions, testing is performed by ISO/IEC 17025-accredited third-party laboratories with traceable, calibrated instruments.
Applications
- Automotive: interior trim, wheel-arch liners, underbody shields, and battery housings made from rPP and rABS compounds, where hardness consistency supports fit, finish, and assembly performance.
- Packaging: crates, pallets, closures, and containers in rHDPE and rPP, where hardness correlates with stackability and long-term load resistance.
- Construction: recycled pipes, fittings, profiles, and wood-plastic composite decking, where hardness complements stiffness and creep data in product qualification.
- Electrical and electronics: switch boxes, enclosure regrind, and connector housings, often tested alongside flame-retardancy and electrical requirements.
- Consumer goods: housewares, furniture components, and toys produced from certified recyclate streams.
- Recycling operations: incoming bale or flake screening, outgoing pellet certification, and virgin-to-regrind blend verification in compounding lines.
FAQ
Which hardness method should be selected for my recycled plastic?
Soft and flexible recyclates below approximately Shore D 40 are tested with a Shore A durometer, while rigid thermoplastics are tested with Shore D. Engineering grades that require finer discrimination are tested with Rockwell scales or ISO 2039-1 ball indentation, and reinforced or filled compounds may be checked with a Barcol impressor. The controlling product standard or customer specification usually dictates the method.
What specimen thickness is required for Shore hardness testing?
Both ISO 868 and ASTM D2240 require a minimum thickness of 6 mm for reliable readings. Thinner material may be tested only if stacked in smooth, fully contacting layers, and finished parts with thin walls should be tested at reinforced locations or backed by a rigid support arrangement where the standard permits.
Can finished recycled parts be tested directly?
Yes, provided the test area is flat, smooth, and free of texture, sink marks, and ribs directly behind the surface. Curved, textured, or insufficiently supported areas produce misleadingly low readings, so molded plaques remain the preferred specimen form for lot release testing.
How does the recycling history affect hardness results?
Moderate thermal degradation often has only a small effect on hardness, which is why hardness is best used together with melt flow rate and tensile testing. However, contamination with dissimilar polymers, filler carryover, moisture in polyamide, and plasticizer loss in flexible PVC can all produce clearly detectable hardness shifts that justify deeper investigation.
What are typical hardness values for common recyclates?
- rPP: Shore D approximately 68 to 75.
- rHDPE: Shore D approximately 60 to 68.
- rPET: Shore D approximately 78 to 82.
- rABS: Rockwell R approximately 95 to 110.
- Rigid rPVC: Shore D approximately 75 to 85.
- Flexible rPVC: Shore A approximately 50 to 90 depending on plasticizer content.
How many readings are taken and how is the result reported?
A minimum of five indentation points are measured at well-spaced locations, and the median value is reported with the method, scale, dwell time, and conditioning atmosphere. For certification projects, accredited third-party laboratories provide full test reports traceable to calibrated reference instruments.