Haze Determination of Plastic Pellets: Testing Methods, Standards, and Applications

Haze determination of plastic pellets is a key quality control test used to evaluate the optical clarity of transparent and translucent polymers before they are processed into finished products. Because raw pellets cannot be measured directly, they are first molded into standardized test specimens and then analyzed with a hazemeter or integrating-sphere spectrophotometer in accordance with ASTM D1003, ISO 14782, and GB/T 2410. This article explains the testing principle, sample preparation, measurement procedures, and typical applications of haze testing for plastic resins.

Technical Overview

Haze is defined as the percentage of transmitted light that, when passing through a specimen, is scattered by forward scattering through an angle greater than approximately 2.5 degrees from the average direction of the incident beam. A high haze value gives a transparent plastic a cloudy, milky, or frosted appearance and degrades the clarity of objects viewed through the material. Haze should not be confused with luminous transmittance, which quantifies the total amount of light that passes through the specimen regardless of scattering direction.

For plastic pellets (resin granules), haze cannot be measured directly with meaningful results because the granules have irregular geometry, rough surfaces, and undefined optical path lengths. Instead, the pellets are first converted into flat test plaques or films under controlled molding conditions, and the haze and luminous transmittance of these specimens are then quantified. The measured haze reflects both the intrinsic optical quality of the resin and the influence of specimen preparation, so standardized molding procedures are essential for comparable results.

The principal sources of haze in plastic resins include semi-crystalline morphology (for example, in PP, PE, and PET), incompletely melted or gelled particles, contamination and foreign matter, poorly dispersed additives such as slip or antiblock agents, catalyst residues, moisture-induced degradation during melt processing, and micro-voids created by volatiles. Haze testing of incoming pellets therefore serves as a sensitive indicator of resin purity, drying discipline, compounding quality, and process consistency.

The governing test methods are ASTM D1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics), ISO 14782 (Plastics — Determination of Haze for Transparent Materials), and GB/T 2410 (the corresponding Chinese national standard). Specimen preparation is typically controlled by ISO 293, ISO 294-2, or ASTM D4703, and conditioning is performed according to ISO 291 or ASTM D618. Accredited third-party laboratories routinely offer these tests as part of resin qualification and incoming inspection programs.

Testing Samples

Compression-Molded Plaques

Compression molding is the most widely used preparation route for pellet haze testing. Pellets are dried according to the resin supplier's recommendations, pre-melted, and then compression molded at a specified temperature, pressure, and cooling rate into flat plaques. Typical plaque dimensions are 50 mm × 50 mm (or 100 mm × 100 mm) with a thickness of 1 mm, 2 mm, or 3 mm, depending on the specification. The molding surfaces must be polished and free of contamination, and the finished plaque must contain no bubbles, sink marks, gels, or molded-in stress, all of which would artificially inflate the haze value.

Injection-Molded Plaques

For resins destined for injection molding applications, plaques of approximately 60 mm × 60 mm are injection molded in accordance with ISO 294-2 or ASTM D4703. Injection-molded specimens can show molecular orientation and residual stress that differ from compression-molded plaques, so the preparation method must be stated in the test report and kept consistent when comparing data between suppliers or lots.

Melt-Pressed or Extruded Films

Film-grade resins, such as PE and PP intended for cast or blown film, are commonly evaluated by preparing thin films (typically 20 µm to 100 µm) by melt pressing or laboratory extrusion. Film haze is strongly thickness-dependent, so the film gauge must be tightly controlled and reported together with the haze result.

Sample Quantity and Conditioning

A typical submission consists of 100 g to 300 g of pellets per material, which is sufficient to mold several plaques for replicate measurement. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity (per ISO 291 or ASTM D618) for a minimum of 40 hours, unless otherwise specified.

Testing Items

The optical characterization package for molded pellet specimens typically includes the following measurable parameters:

• Haze (total haze, expressed in %) — the primary reported result for plastic pellets
• Total luminous transmittance (Tt, %) — the overall light-passing ability of the specimen
• Diffuse transmittance (Td, %) — the scattered component of transmitted light
• Parallel (regular) transmittance (%) — the unscattered component, useful for clarity assessment
• Specimen attributes: thickness, surface condition, molding method, and conditioning state

Testing Methods

Specimen Preparation

Pellets are dried, molded into plaques or films under documented conditions, and inspected visually for defects. At least three valid specimens per material are recommended so that average values and scatter can be assessed. The molding temperature, pressure, cooling rate, and final thickness are recorded because each of these parameters measurably influences haze, particularly in semi-crystalline resins where cooling rate controls crystallite size and content.

Hazemeter Method (ASTM D1003 / GB/T 2410)

A dedicated hazemeter uses an integrating sphere, a defined light source (illuminant C or D65), and a CIE 2° or 10° standard observer. Four successive luminous readings are taken:

• T1 — incident light, no specimen, light trap closed
• T2 — specimen in place, light trap closed
• T3 — no specimen, light trap open (instrument sphere error)
• T4 — specimen in place, light trap open

Total luminous transmittance is calculated as Tt = T2 / T1, the diffuse transmittance is obtained by correcting T4 for the instrument contribution measured in T3, and haze is computed as the ratio of diffuse to total transmittance. Modern automatic hazemeters perform the entire sequence and the calculation in a single operation.

Spectrophotometer Method (ISO 14782 / ASTM D1003 Method B)

An integrating-sphere UV-visible spectrophotometer may be used as an alternative to a dedicated hazemeter. Spectral transmittance data are acquired across the visible wavelength range with and without a light trap, integrated over the specified illuminant and observer, and converted into total transmittance, diffuse transmittance, and haze. This approach provides full spectral information and is useful when color or tinting of the resin must also be characterized.

Calculation and Expression of Results

The haze value is expressed as:

Haze (%) = (Diffuse transmittance Td / Total transmittance Tt) × 100

Results are reported to 0.1 % precision, together with the specimen thickness, molding method, standard used, illuminant/observer condition, number of specimens, and individual and mean values. Because haze increases with thickness, comparisons between materials are only valid at identical thickness and identical preparation conditions.

Applications

Haze determination of plastic pellets supports quality decisions across many sectors of the plastics industry:

• Optical-grade resins — qualification of PC, PMMA, and COC/COP polymers for lenses, light guides, display windows, and optical films where haze specifications are often below 1 %
• Packaging — incoming inspection of PET for bottles and preforms, PP for thin-wall containers, and PE or PP film resins for flexible packaging clarity
• Lighting — evaluation of polycarbonate and polystyrene-based diffuser materials, where a controlled balance of haze and transmittance is engineered deliberately
• Automotive — release approval of glazing, interior trim, and cover-lens resins where visual clarity requirements apply
• Medical and healthcare — clarity verification of resins for IV components, diagnostic consumables, and transparent housings
• Compounding and masterbatch development — assessment of how additives, colorants, fillers, and processing aids affect optical properties
• Litigation-independent lot release — routine lot-to-lot statistical control of resin clarity within accredited third-party laboratories or in-house QC labs

FAQ

Can haze be measured directly on plastic pellets without molding them first?

No. Standard haze methods require flat specimens with defined thickness and polished surfaces. Irregular pellet geometry scatters light unpredictably and produces meaningless data, so pellets must always be converted into plaques or films under controlled conditions before measurement.

What is the difference between haze and transmittance?

Transmittance measures how much light passes through the specimen in total, while haze measures how much of that transmitted light has been scattered away from the straight-through direction. A material can have high transmittance yet high haze (light passes but images are blurred), or low transmittance with low haze (dim but sharp). Both values are normally reported together.

How does specimen thickness affect the haze result?

Haze generally increases with thickness because the optical path containing scatterers becomes longer. For semi-crystalline resins, thickness also interacts with cooling rate through crystallinity. Specimen thickness must therefore always be specified and held constant when comparing materials or lots.

Which standards govern haze testing of plastic pellets?

The measurement standards are ASTM D1003, ISO 14782, and GB/T 2410. Specimen preparation is governed by ISO 293 or ASTM D4703 for compression molding and ISO 294-2 for injection molding, while conditioning follows ISO 291 or ASTM D618.

What causes abnormally high haze in a resin lot?

Common root causes include excessive or coarse crystallinity, gels and unmelted particles, contamination by foreign matter or dust, inadequate drying leading to hydrolytic degradation, poor dispersion of antiblock or slip additives, catalyst residues, and micro-voids from volatiles. A haze trend that drifts between lots is a strong signal of a raw material or process change that warrants investigation.

What haze levels are typically specified?

Requirements are application-specific. Optical-grade polycarbonate or PMMA is often specified below 1 % haze, packaging PET commonly around 1 % to 3 %, and general-purpose PP or PE packaging grades may be acceptable in the 5 % to 20 % range. For lighting diffusers, deliberately high haze with high transmittance is engineered into the product. Always define the specification together with thickness and preparation method.