Haze determination for recycled materials is a critical optical performance test that measures the degree of light scattering in recycled plastics, directly impacting their suitability for transparent packaging, films, and consumer products. This article provides a comprehensive technical guide covering testing principles, applicable standards such as ASTM D1003 and ISO 14782, sample preparation requirements, instrumentation, and practical applications of haze measurement in recycled material quality control. Understanding haze behavior in recycled resins helps manufacturers balance sustainability goals with optical quality requirements.
Technical Overview
Haze is defined as the percentage of transmitted light that, when passing through a transparent or translucent specimen, deviates from the incident beam by more than 2.5 degrees through forward scattering. For recycled plastic materials, haze determination is particularly significant because the recycling process introduces variables that can substantially degrade optical clarity compared to virgin resins.
Recycled materials typically exhibit higher haze values than their virgin counterparts due to several inherent factors: polymer thermal degradation during reprocessing, contamination from foreign polymers and additives, incomplete removal of inks and labels, the presence of microscopic particulates, and variations in melt history. As global demand for post-consumer recycled (PCR) and post-industrial recycled (PIR) content grows, haze measurement has become an essential quality control parameter for determining whether recycled resins can be used in optically demanding applications.
Haze determination quantifies the ratio of diffuse transmittance to total transmittance, expressed as a percentage. A perfectly clear material would have a haze value approaching zero, while highly scattering materials may exceed 30 percent. The test is performed using dedicated hazemeters or spectrophotometers equipped with integrating spheres, following internationally recognized test methods that ensure consistency and comparability of results across laboratories.
Testing Samples
Recycled Pellets and Granules
Recycled resin pellets in their raw form cannot be measured directly for haze. They must first be converted into standardized plaques or films through compression molding or extrusion processes. The sample preparation conditions, including melt temperature, pressure, and cooling rate, must be carefully controlled and documented, as processing history significantly influences the optical properties of the final specimen.
Molded Plaques and Flat Sheets
Compression-molded or injection-molded plaques with defined thicknesses (typically 1.0 mm, 2.0 mm, or 3.0 mm) are the most common specimen format for rigid recycled materials such as recycled PET (rPET), recycled PP (rPP), recycled HDPE (rHDPE), and recycled PC (rPC). Specimens must be free from surface defects, bubbles, sink marks, and internal stress birefringence that could artificially inflate haze readings.
Blown and Cast Films
Thin films produced from recycled polyethylene and recycled polypropylene are frequently tested for haze, especially for packaging applications. Film specimens should be representative of production output, with thickness measured at multiple points to verify uniformity. Multi-layer films containing recycled content layers can also be evaluated, though the interface effects between layers must be considered during interpretation.
Finished Products and Components
Transparent or translucent end products such as bottles, thermoformed trays, containers, and display components may be sectioned into flat test pieces for haze determination. Curved surfaces should be avoided or flattened using controlled heating, as curvature introduces additional light scattering artifacts.
Testing Items
A comprehensive haze evaluation program for recycled materials typically encompasses the following measurable parameters:
- Total transmittance (Tt): the total light passing through the specimen, including both direct and diffuse components
- Diffuse transmittance (Td): the scattered light component deviating more than 2.5 degrees from the incident beam axis
- Haze percentage: calculated as the ratio of diffuse transmittance to total transmittance (Td / Tt × 100%)
- Regular transmittance: the unscattered light component, useful for characterizing clarity versus haze
- Clarity (see-through quality): the ability to resolve fine detail through the specimen, distinct from haze
- Yellowness index and color coordinates: often measured simultaneously to assess overall optical quality of recycled content
- Haze-thickness relationships: comparative measurements at multiple thicknesses to characterize intrinsic material scattering behavior
Testing Methods
ASTM D1003 Standard Test Method
ASTM D1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics) is the most widely referenced method in North America. The procedure uses either a hazemeter or a spectrophotometer with an integrating sphere. Illuminant A and the CIE 1932 standard observer (2-degree) are specified, with unscattered light exiting within a cone of 2.5 degrees. Method A describes the traditional hazemeter approach using four successive readings (without specimen, with specimen and light trap, without specimen and light trap, and with specimen), while Method B permits spectrophotometric measurement with equivalent geometry.
ISO 14782 International Standard
ISO 14782 (Plastics — Determination of haze for transparent materials) defines haze measurement using a double-beam spectrophotometer with an integrating sphere. The standard specifies test specimen requirements, conditioning parameters (23 ± 2 °C and 50 ± 10% relative humidity), and the calculation methodology for haze. It is the preferred reference in European and international markets and is technically harmonized with the principles of ASTM D1003.
GB/T 2410 Chinese National Standard
GB/T 2410 (Determination of the luminous transmittance and haze of transparent plastics) is the Chinese national standard widely applied for recycled material qualification in the Asian supply chain. The method mirrors the integrating-sphere optical geometry of ASTM D1003 and is commonly specified for incoming resin inspection and recycled content verification programs.
Instrumentation and Calibration
Testing is conducted using a calibrated hazemeter or UV-visible spectrophotometer fitted with an integrating sphere. Calibration verification requires certified reference standards traceable to national metrology institutes. Key instrument parameters include spectral range (typically 380–780 nm for photopic weighting), beam geometry, sphere wall reflectance consistency, and the precision of the light trap or specular exclusion port. Regular verification against calibrated haze standards of known values (for example, approximately 1% and 10% haze) ensures measurement integrity.
Specimen Conditioning and Measurement Procedure
Specimens must be conditioned at standard laboratory atmosphere prior to testing to stabilize surface properties and dimension. Both surfaces are cleaned with lint-free wipes and isopropyl alcohol or an appropriate solvent to remove fingerprints and dust. A minimum of three specimens per sample lot is recommended, with each specimen measured at multiple positions. Mean values and standard deviations are reported along with specimen thickness, because haze values increase with thickness and direct comparison requires thickness normalization.
Applications
Haze determination for recycled materials supports decision-making across the entire plastics value chain:
- Recycled resin supplier qualification: verifying that recycled pellet lots meet optical specifications before shipment to converters
- Packaging development: assessing whether recycled PET, PP, or PE can achieve the clarity required for bottles, films, and thermoformed containers, particularly for food and beverage applications
- Blend ratio optimization: determining the maximum recycled content percentage that can be incorporated while maintaining acceptable optical performance
- Incoming material inspection: converters performing acceptance testing on recycled resin deliveries against agreed haze tolerance limits
- Process improvement: evaluating the effectiveness of decontamination technologies, melt filtration upgrades, and extrusion parameter changes on optical quality
- Certification and compliance support: documenting recycled material quality for eco-labeling programs, recycled content claims, and regulatory submissions
- Research and development: characterizing optical degradation mechanisms in mechanically and chemically recycled polymers across multiple reprocessing cycles
FAQ
Why do recycled materials generally show higher haze than virgin materials?
Recycled plastics accumulate thermal degradation products, cross-contamination from other polymer types, residual pigments, ink particles, additive incompatibilities, and microscopic gels or unmelted particles. Each of these creates refractive index discontinuities within the polymer matrix that scatter light and increase haze. Multiple processing histories also broaden the molecular weight distribution, which can affect crystallization behavior and further contribute to light scattering.
What is an acceptable haze value for recycled materials?
Acceptance criteria depend entirely on the end-use application. For premium transparent applications such as bottle-grade rPET, haze values below 5% to 8% are typically required. General packaging applications may tolerate 10% to 20% haze, while opaque or non-critical applications have effectively no haze limitation. Specifications should always be agreed between supplier and customer with reference to specimen thickness and test standard.
Can haze results from different standards be compared directly?
While ASTM D1003, ISO 14782, and GB/T 2410 are based on the same 2.5-degree scattering geometry, minor differences in illuminant specification, calculation details, and instrument design can produce small systematic deviations. For contractual purposes, results should always be reported with the specific method used, and cross-method correlations should be established when comparing data from different laboratories.
How does specimen thickness affect haze measurements?
Haze increases with specimen thickness because light travels a longer path through the scattering medium. A recycled material that measures 3% haze at 1 mm thickness may measure significantly higher at 3 mm. Therefore, haze values are only comparable at equal thickness, and specifications must always state the test thickness. When characterizing intrinsic material quality, measuring a thickness series and extrapolating to zero thickness can separate surface scattering from bulk scattering contributions.
What steps can reduce haze in recycled materials?
Practical haze reduction strategies include improving feedstock sorting purity (especially near-infrared and color-based separation), employing finer melt filtration, optimizing extrusion temperatures to minimize degradation while ensuring complete melting, vacuum degassing to remove volatiles, compatibilizer addition for mixed-polymer streams, and blending recycled resin with optical-grade virgin material. Chemical recycling routes can produce recycled monomers and polymers with optical properties approaching virgin quality.
Should haze testing be performed by an in-house lab or an accredited third-party laboratory?
In-house hazemeters are suitable for routine process control and lot screening. For supplier qualification, dispute resolution, customer certification, and regulatory submissions, testing at accredited third-party laboratories operating under ISO/IEC 17025 quality systems provides independent verification and internationally recognized traceability of results.