Recycled Material Haze Determination Testing: Principles, Standards, and Procedures for Recycled Pla

Haze determination testing quantifies the light-scattering behavior of recycled plastic materials, providing an objective index of optical clarity that is directly influenced by contamination, polymer degradation, and processing history. This technical article explains the sample types, tested parameters, measurement standards such as ASTM D1003, ISO 14782, and GB/T 2410, and the instrumentation and calculation procedures used by accredited third-party laboratories to evaluate the optical quality of recycled polymers. The resulting data support quality control, recycled content qualification, and specification compliance for transparent and translucent recycled materials.

Technical Overview

Haze is defined as the percentage of transmitted light that, in passing through a specimen, deviates from the incident beam by more than 2.5 degrees as a result of forward scattering. It is expressed as the ratio of diffuse transmittance to total luminous transmittance and is one of the most sensitive optical indicators of material purity in recycled polymers. In virgin resins, haze is governed primarily by crystallinity, additive dispersion, and surface quality; in recycled materials, it is additionally affected by contamination, unmelted particles, gels, black specks, pigmented flakes, and thermo-oxidative degradation accumulated across previous service lives and reprocessing cycles.

A haze determination experiment therefore serves two purposes: it verifies whether a recycled material meets the optical requirements of a target application, and it acts as a diagnostic signal for the effectiveness of sorting, washing, decontamination, melt filtration, and pelletizing operations. Because haze increases measurably with relatively small levels of impurity, the method is widely used in the recycling industry as a routine quality control tool.

• Haze characterizes wide-angle light scattering, not light absorption, so a material can have high transmittance yet unacceptable haze.
• Results depend strongly on specimen thickness, surface condition, and melt history, which must be reported with the data.
• Common drivers of elevated haze in recycled resins include cross-linked gels, dispersed foreign polymers, residual inorganic fillers, and moisture-induced hydrolytic degradation.

Testing Samples

Recycled PET (rPET)

Bottle-grade and thermoforming-grade rPET flakes or pellets are the most frequently tested recycled materials for haze, because end uses such as bottle-to-bottle applications and transparent thermoformed trays impose strict optical limits. Test plaques are typically produced by drying the resin to a specified moisture level, followed by injection molding or compression molding into flat panels of defined thickness.

Recycled Polyolefins (rPP, rHDPE, rLDPE)

Recycled polypropylene and polyethylene are commonly evaluated in film or sheet form for applications such as flexible packaging, carrier bags, and translucent containers. Blown or cast films prepared from recycled pellets are tested to determine whether contamination loads scatter light beyond specification limits.

Recycled Engineering Resins (rPC, rPMMA, rPS, rABS, rPVC)

Optical-grade recycled polycarbonate and acrylic materials are tested when reused in lighting diffusers, glazing, display components, and housings where clarity must be verified after reprocessing. Recycled polystyrene and PVC are similarly evaluated for transparent packaging and card applications.

Specimen Forms and Geometry

• Flat films or sheets cut to the instrument aperture size, commonly 50 mm × 50 mm.
• Compression-molded or injection-molded plaques prepared directly from recycled pellets, typically 1 mm to 3 mm thick.
• Specimens must be free of scratches, fingerprints, dust, sink marks, and surface textures, since surface roughness contributes artifact scattering unrelated to bulk material quality.
• A minimum of three to five specimens per material lot is recommended to characterize lot variability.

Testing Items

The core deliverable of the experiment is the haze value, but a complete optical characterization of recycled material normally includes the following measured and derived quantities:

• Total luminous transmittance, expressed as a percentage of incident light transmitted through the specimen.
• Diffuse transmittance, the scattered component of transmitted light.
• Regular (specular) transmittance, obtained by subtraction or by direct measurement with a light trap in position.
• Haze percentage, calculated as diffuse transmittance divided by total transmittance multiplied by 100.
• Corrected haze, in which instrument and reference beam scattering contributions are subtracted before calculation.
• Thickness-normalized haze comparisons, when materials of different gauge must be benchmarked against one another.

Testing Methods

Reference Standards

• ASTM D1003, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, providing Method A using a hazemeter and Method B using a spectrophotometer.
• ISO 14782, Plastics — Determination of haze for transparent materials, which specifies the optical geometry and the computation of haze with glare correction.
• GB/T 2410, the Chinese national standard for light transmittance and haze of transparent plastics, widely applied to recycled materials in domestic supply chains.
• ISO 291 or ASTM D618 for specimen conditioning, and ISO 293 or ASTM D4703 for compression molding of test plaques from pellets.

Specimen Preparation and Conditioning

Recycled pellets are first dried to the moisture level specified for the polymer family, for example a low residual moisture content for rPET to suppress hydrolytic haze during melting. Pellets are then converted into flat plaques by compression molding under controlled temperature, pressure, and cooling rate, or into films by blown or cast extrusion using documented process settings. After preparation, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a defined period, typically not less than 40 hours, before measurement.

Instrumentation

Measurements are performed with an integrating-sphere hazemeter or a suitably equipped double-beam spectrophotometer. The optical system uses a light source approximating Standard Illuminant D65 or C combined with a photopic detector response. The integrating sphere collects both regularly transmitted and diffusely scattered light; a specular exclusion port or light trap allows the diffuse component to be isolated. The instrument is verified before use with reference standards traceable to national metrology institutes, and baseline readings quantify inherent instrument scatter for subsequent correction.

Measurement Procedure

Four sequential readings are acquired in the classic arrangement: the incident light reference without specimen, the total transmitted light with specimen at the sphere entry, the instrument scatter with a light trap and no specimen, and the diffuse transmitted light with both specimen and light trap in position. Specimens are mounted perpendicular to the beam with the measurement area centered on the port, and multiple positions across each plaque are measured to capture local defects such as gels or unmelted particles.

Calculation and Data Treatment

Total transmittance is calculated as the ratio of the total transmitted reading to the incident reading. Haze is calculated as the ratio of the corrected diffuse transmittance to the total transmittance, multiplied by 100, with instrument background scattering subtracted from both numerator and denominator terms in accordance with the governing standard. Results are reported to 0.1% resolution together with the standard used, specimen thickness, preparation method, conditioning history, illuminant, and the number of specimens and measurement positions. If any reading yields a haze value far above the replicate average, the specimen is inspected for local defects and the observation is documented rather than discarded, because such defects are frequently the quality-relevant finding in recycled materials.

Precision and Uncertainty

Repeatability within a laboratory is typically better than a few tenths of a haze percentage point for homogeneous specimens, while reproducibility between laboratories is governed by differences in plaque preparation, thickness control, and instrument geometry. For recycled materials with heterogeneous contamination, between-specimen scatter is often the dominant uncertainty component, which is why multi-position and multi-specimen protocols are strongly recommended.

Applications

• Incoming inspection of recycled pellets by converters producing bottles, thermoformed trays, films, and transparent housings.
• Qualification of recyclate grades for direct food-contact and non-food optical applications where clarity limits are specified.
• Supporting data for recycled content certification schemes and recyclability design guidelines that reference optical quality criteria.
• Process optimization in recycling plants, where haze trends reveal the effectiveness of flake sorting, hot washing, melt filtration fineness, and vacuum or degassing steps.
• Comparison of mechanical, chemical, and solvent-based recycling routes for the same waste feedstock.
• Failure analysis of hazy or milky parts, distinguishing bulk contamination from surface texture, additive bloom, and crystallinity effects.
• Research and development of clarification additives, compatibilizers, and decontamination technologies for recycled polymers.

FAQ

What is the difference between haze and transmittance?

Transmittance measures how much light passes through the specimen regardless of direction, while haze measures how much of that transmitted light is scattered away from the straight-through path. A recycled material can transmit over 90% of incident light yet remain visibly cloudy, which is why both parameters are reported together.

Which standards are most commonly applied to recycled material haze testing?

ASTM D1003 and ISO 14782 are the dominant international methods, with GB/T 2410 routinely referenced in Chinese supply chains. All three define similar optical geometry, and the standard used must be stated in the report because calculated haze values are not always directly interchangeable between methods.

How are recycled pellets tested if only granules are available?

Granules cannot be measured directly in a hazemeter; they are first dried and molded into flat plaques by compression or injection molding under controlled conditions, or extruded into film. The preparation route must be documented because melt history and cooling rate influence crystallinity and therefore the measured haze.

Does specimen thickness affect the result?

Yes, haze generally increases with thickness because scattering sites accumulate along a longer optical path. Results are only comparable between specimens of the same nominal thickness, and thickness must always accompany the reported haze value.

What haze levels are typically specified for recycled PET applications?

Requirements vary by application, but transparent bottle-to-bottle grades are commonly restricted to single-digit haze percentages, while thermoformed packaging and fiber applications tolerate higher values. Limit values should be agreed between supplier and buyer relative to a defined thickness and test standard rather than quoted generically.

Can haze testing detect contamination in recycled material?

Elevated or erratic haze is a strong indicator of gels, unmelted particles, foreign polymer inclusions, or pigment carryover. Haze measurement is a rapid screening method, and abnormal readings are typically followed by defect mapping or microscopic inspection to identify the scattering sources.

How should laboratories report haze results for recycled materials?

Reports should include the applied standard, instrument and illuminant, specimen form, preparation method, thickness, conditioning, number of specimens and measurement points, individual and mean haze values, total transmittance, and any observations of localized defects or abnormal scatter among replicates.