Recycled Material Hardness Testing: Experimental Methods, Standards and Applications

Hardness testing is one of the most widely used experimental methods for evaluating the mechanical quality of recycled materials, particularly recycled plastics and rubber compounds. This technical article explains the principles, sample requirements, test items, and internationally recognized standards—such as ISO 868, ISO 2039, ASTM D2240, and ASTM D785—used in recycled material hardness determination experiments.

Technical Overview

A hardness determination experiment measures the resistance of a material to localized plastic deformation, indentation, or penetration under a defined force. For recycled materials (commonly referred to as recyclate or regrind), hardness is a critical quality indicator because repeated processing, thermal degradation, contamination, and the presence of fillers or additives can significantly alter the mechanical behavior of the polymer compared with its virgin counterpart.

In a typical recycled material hardness testing experiment, standardized indenters are pressed into conditioned specimens under controlled loads and dwell times, and the indentation response is converted into a dimensionless hardness value. Commonly applied methods include Shore durometer hardness, Rockwell hardness, and ball indentation hardness. These experiments are routinely performed by accredited third-party laboratories, in-house quality control departments, and research institutions to support incoming material inspection, product development, and conformity assessment.

Because hardness correlates with stiffness, crosslink density, crystallinity, and degree of degradation, the results of a hardness determination experiment provide fast, low-cost, and reproducible insight into the overall consistency and usability of recycled resins such as recycled PP, PE, ABS, PC, PET, PVC, and PA.

Testing Samples

Recycled Thermoplastic Pellets and Regrind

Post-consumer or post-industrial recyclate is most frequently supplied as pellets, flakes, or regrind. Since the particle size of loose regrind is too small for direct indentation, the material is normally first melt-processed into standardized test plaques by compression molding or injection molding before hardness measurement.

Molded Test Plaques and Standardized Specimens

For a valid hardness determination, specimens must have flat, parallel surfaces, be free of voids and sink marks, and meet minimum thickness requirements (typically at least 6 mm for Shore measurements). Where thinner sheets must be evaluated, several layers may be stacked, although stacked results should be interpreted with caution.

Finished and Semi-Finished Recycled Products

Pipes, sheets, profiles, containers, automotive trim parts, and elastomeric components manufactured from recycled compounds can be tested directly, provided the test area is flat, smooth, and sufficiently supported to prevent background deflection.

Typical Recycled Material Families

• Recycled polypropylene (rPP) and recycled polyethylene (rHDPE, rLDPE)
• Recycled ABS, PS, and HIPS from electronic and packaging waste streams
• Recycled polycarbonate (rPC) and PC/ABS blends
• Recycled PET from bottle streams
• Recycled PVC, both rigid and flexible (plasticized) compounds
• Recycled polyamide (rPA) and other engineering resins
• Recycled thermoplastic elastomers (rTPE/rTPU) and reclaimed rubber

Testing Items

A comprehensive recycled material hardness determination experiment may include the following test items:

• Shore A hardness — for soft, flexible, and elastomeric recycled materials such as plasticized PVC and TPE
• Shore D hardness — for rigid recycled thermoplastics such as rPP, rPE, rABS, and rPS
• Rockwell hardness (commonly scales R, L, M, or E) — for harder, rigid plastics where greater differentiation is required
• Ball indentation hardness — suitable for rigid plastics and molded recyclate plaques, reported in N/mm²
• Barcol hardness — occasionally used for reinforced and hard polymer compounds
• Hardness retention testing — comparing hardness before and after heat aging, UV exposure, or accelerated weathering of recyclate compounds
• Virgin-versus-recycled comparison studies — quantifying the effect of recycled content percentage on hardness values

Testing Methods

Shore Durometer Hardness (ISO 868, ASTM D2240, GB/T 2411)

A Shore durometer presses a calibrated spring-loaded indenter against the specimen. Shore A uses a truncated 35° cone indenter for softer materials, while Shore D uses a sharper 30° cone for harder plastics. The displayed value (0–100) is read after a defined dwell time, typically 3 seconds for instantaneous readings or 15 seconds for standard readings. Multiple indentation points are measured, with adequate spacing between indentations and from specimen edges, and the median value is reported.

Rockwell Hardness (ASTM D785, ISO 2039-2)

A steel ball indenter is first applied with a minor load to zero the system, after which the major load is applied for a specified dwell time. The hardness number is derived from the difference in indentation depth between the minor and major load states. Scale R (12.7 mm ball with a 60 kgf major load) is frequently selected for rigid recycled plastics.

Ball Indentation Hardness (ISO 2039-1)

A hardened 5 mm steel ball is pressed into the specimen surface at one of the standardized test loads, and the indentation depth is measured after the specified loading time. The result is expressed as ball indentation hardness in N/mm², which provides a more fundamental, load-dependent measure particularly useful for engineering-grade recyclates.

Specimen Conditioning and Test Environment

Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or the equivalent conditioning clauses of the applicable test standards. Consistent conditioning is essential for recycled materials, as moisture absorption (for example in recycled PA or PET) and temperature strongly influence indentation behavior.

Quality Assurance of Results

• Verify durometer calibration against certified reference blocks at regular intervals
• Confirm specimen thickness, surface flatness, and freedom from defects at the test point
• Ensure rigid backing of the specimen so that only the material, not the support, is deformed
• Apply the specified indenter dwell time consistently across all readings
• Record the number of readings, median value, test standard, scale, and dwell time in the report

Applications

Hardness determination experiments for recycled materials serve a wide range of practical purposes across the plastics and rubber value chain:

• Incoming quality control of purchased recyclate pellets, flakes, and baled regrind before production
• Batch-to-batch consistency verification for recycled compound suppliers
• Research and development of recycled-content blends, filled compounds, and compatibilized mixtures
• Assessment of thermal or oxidative degradation in reprocessed polymers, since degradation often shifts hardness values
• Verification of conformity to customer specifications, data sheets, and regulatory or eco-label requirements
• Comparative benchmarking of recycled materials against virgin resins during material substitution studies
• Support for recycled content certification programs and technical documentation prepared by accredited third-party laboratories
• Failure analysis of recycled plastic parts, for example to detect contamination, insufficient fusion, or excessive filler loading

FAQ

Which hardness scale should be selected for a recycled plastic?

Shore A is appropriate for flexible and elastomeric recycled materials, Shore D for rigid thermoplastics, Rockwell for hard and rigid plastics requiring finer differentiation, and ball indentation hardness when an engineering value in N/mm² is needed. A useful rule of thumb is that Shore A values above about 90 justify switching to Shore D, and Shore D values approaching 100 suggest moving to a Rockwell or ball indentation method.

Why do hardness results vary between different batches of recycled material?

Recycled materials are inherently heterogeneous. Variations in the waste stream source, degree of polymer degradation, residual contaminants, filler or pigment content, moisture level, and reprocessing conditions can all shift hardness values. This variability is precisely why hardness determination experiments are valuable as a routine quality control tool for recyclate.

What specimen thickness is required?

For Shore hardness methods, a minimum total thickness of approximately 6 mm is generally specified. Thinner materials may be tested by stacking a limited number of layers, provided no air gaps are present, although such results should be marked as non-standard. For ball indentation and Rockwell methods, the specimen must be thick enough to prevent the indentation from being influenced by the supporting surface.

How does conditioning affect the test result?

Temperature and moisture have a measurable effect on polymer hardness. Conditioning at 23 °C and 50 % relative humidity per ISO 291 before testing ensures comparability between laboratories and between successive batches of recycled material.

Can finished products made from recyclate be tested directly?

Yes, provided the test surface is flat, smooth, adequately supported, and representative of the material rather than of a thin wall or rib. Curved, textured, or very thin sections may produce non-standard results and should be evaluated carefully or replaced with molded plaques of the same compound.

Is a single hardness reading sufficient?

No. Standard methods require multiple indentation points on one or more specimens, and the median of the readings is reported. Repeating measurements across different locations helps account for the local heterogeneity typical of recycled materials.