PET plastic haze testing is an essential quality control procedure for measuring the optical transparency and light-scattering properties of polyethylene terephthalate materials used in packaging, films, and consumer products. This comprehensive guide covers haze testing standards, sample preparation, measurement methods, and practical applications for manufacturers and testing laboratories worldwide.
Technical Overview
Polyethylene terephthalate (PET) is a transparent, thermoplastic polyester widely used in beverage bottles, food containers, protective films, and thermoformed packaging. Because the commercial value of many PET products depends heavily on their optical clarity, haze measurement has become one of the most important quality control tests in the plastics industry.
Haze is defined as the percentage of transmitted light that, when passing through a specimen, deviates from the incident parallel beam by a forward angle of more than 2.5 degrees, as specified in ASTM D1003. In practical terms, haze quantifies the milky or foggy appearance of a transparent material caused by light scattering from surface roughness, internal crystallinity, additives, or processing defects. A higher haze value indicates lower transparency and greater light diffusion.
For PET products, haze is influenced by several material and processing factors, including:
- Degree of crystallinity, since spherulites formed during slow cooling or heat exposure scatter visible light
- Surface roughness resulting from mold finish, roller surfaces, or thermoforming conditions
- Specimen thickness, because thicker samples scatter more light along the optical path
- Additives such as slip agents, antiblock particles, colorants, and nucleating agents
- Processing defects including gels, fish-eyes, degradation products, and contamination
By quantifying haze in a standardized manner, manufacturers can verify incoming resin quality, optimize processing parameters, compare supplier materials, and confirm that finished goods meet customer specifications.
Testing Samples
PET Films
Biaxially oriented PET film (BOPET) and cast PET film are the most common film samples submitted for haze testing. Samples are typically cut into squares or circles of at least 50 mm diameter to fully cover the transmission port of a haze meter. Optical-grade films used in displays or laminating applications often require haze values below 1 percent, while industrial packaging films may range from 2 to 6 percent depending on thickness and formulation.
PET Sheets and Thermoformed Containers
Amorphous PET (APET) sheet used for thermoformed trays, blisters, and clamshells is tested in flat sheet form at the specified production thickness, commonly 0.2 to 1.0 mm. Because thermoforming can induce stress whitening and crystallization, both flat sheets and formed sections may be evaluated to assess the optical impact of the converting process.
PET Bottles and Preforms
For bottles and preforms, flat specimens are cut from the sidewall and conditioned before testing. Injection-molded preforms reveal the base resin clarity, while bottle wall sections reflect the optical consequences of stretch blow molding, including stress-induced whitening in highly oriented areas such as base and gate regions.
PETG and Copolyester Materials
PETG and other glycol-modified polyesters are frequently submitted for haze testing in sheet form. These amorphous copolyesters generally exhibit excellent clarity, and haze data are used to verify quality between production lots and to compare competing materials for medical and retail packaging applications.
Testing Items
A complete optical characterization of PET materials typically includes the following measured and calculated parameters:
Total luminous transmittance (Tt): the ratio of transmitted light to incident light, expressed as a percentage, representing overall light transmission through the specimen
Diffuse transmittance (Td): the portion of transmitted light scattered away from the regular (specular) direction by more than the specified angular limit
Haze value: calculated as (Td / Tt) × 100 percent, representing the scattered fraction of all transmitted light
Regular (specular) transmittance: the unscattered component that passes through the specimen without deviation, related to image clarity
Spectral transmittance distribution: transmittance data across the visible wavelength range when a spectrophotometer method is used
Testing Methods
ASTM D1003
ASTM D1003, Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, is the most widely referenced standard in North America. The current edition offers Method A, which uses a dedicated hazemeter with an integrating sphere, and Method B, which uses a spectrophotometer equipped with an integrating sphere. Measurements are performed using illuminant C or D65 with either the 2-degree or 10-degree standard observer.
ISO 14782
ISO 14782, Plastics — Determination of Haze for Transparent Materials, is the international standard commonly required by European and Asian customers. It defines haze using a nearly collimated light beam and specifies the geometric tolerances of the optical system, including the light trap aperture that separates regular and diffuse transmitted components. Results from ISO 14782 and ASTM D1003 are generally comparable, but small differences can arise from beam geometry.
GB/T 2410
GB/T 2410, the Chinese national standard for determining luminous transmittance and haze of transparent plastics, is technically aligned with ASTM D1003 and is routinely specified for PET materials produced or sold in the Chinese market. Laboratories serving global customers often report results under multiple standards simultaneously.
Instrumentation and Measurement Procedure
Testing is performed with a haze meter or a UV-Vis spectrophotometer fitted with an integrating sphere. The general procedure is as follows:
- Condition the specimens at 23 ± 2 °C and 50 ± 5 percent relative humidity for a minimum of 40 hours prior to testing
- Verify instrument calibration using certified haze and transmittance reference standards
- Record the baseline and total transmittance readings with the specimen placed against the transmission port and the light trap open
- Measure diffuse transmittance with the light trap closed so that regularly transmitted light is excluded from the sphere
- Calculate haze as the ratio of diffuse to total transmittance, and report the average of at least three specimens along with thickness data
Specimen surfaces must be clean and free of fingerprints, scratches, dust, and anti-static residues, since surface contamination can add several percentage points of apparent haze. Each specimen should be measured at multiple positions to account for gauge variation across a film or sheet web.
Applications
PET haze testing supports quality assurance and product development across many industries:
Beverage and food packaging: bottle producers verify that resin, preforms, and blown containers meet typical commercial haze limits of 2 to 5 percent for water-grade and CSD-grade applications
Thermoformed packaging: tray and blister manufacturers use haze data to ensure product visibility at retail and to monitor stress whitening after forming
Optical and display films: producers of protective films, touch panel substrates, and window films rely on low-haze certification, often below 1 percent, for consumer electronics supply chains
Pharmaceutical packaging: blister films and sterile barrier materials require documented optical properties for regulatory submissions and inspection of contents
Solar and photovoltaic applications: PET-based backsheets and encapsulant films are qualified by transmittance and haze performance affecting module efficiency
Material development: researchers use haze measurements to evaluate new nucleating agents, recycled content levels, and process windows for injection molding, extrusion, and orientation
FAQ
What haze value is considered acceptable for PET products?
Acceptance limits depend on the application. Optical-grade PET film typically requires haze below 1 percent, bottle-grade amorphous PET usually falls between 1 and 3 percent, and general thermoformed packaging may allow haze of 3 to 8 percent. Customer specifications should always be consulted before establishing internal limits.
What is the difference between haze and transmittance?
Transmittance measures how much light passes through the specimen regardless of direction, while haze measures only the scattered portion of that transmitted light. A material can have high transmittance yet high haze, meaning it lets light through but produces a blurred, milky appearance rather than a clear image.
Does sample thickness affect haze results?
Yes. Haze generally increases with thickness because light travels a longer path and encounters more scattering sites. For this reason, test reports must always state specimen thickness, and comparisons between materials are valid only at matched thickness.
Should I choose ASTM D1003 or ISO 14782?
The choice is usually dictated by the customer, end market, or purchasing specification. ASTM D1003 dominates in North America, ISO 14782 is preferred for international trade, and GB/T 2410 is specified in China. A laboratory equipped with a modern hazemeter can typically satisfy all three standards.
Why does PET packaging become hazy after processing or sterilization?
Post-molding heat exposure, inadequate cooling, or contact with hot filling or retort conditions can induce crystallization in PET, creating spherulites that scatter light and produce a white, hazy appearance. Haze testing before and after such exposures is a valuable way to qualify heat-resistant grades and process settings.