Haze Testing of Plastic Multi-Layer Co-Extruded Films

Haze testing quantifies the light-scattering behavior of plastic multi-layer co-extruded films and serves as a key optical quality indicator for packaging, medical, and industrial applications. This article explains haze measurement principles, applicable standards such as ASTM D1003, ISO 14782, and GB/T 2410, sample preparation, test procedures, calculation methods, and practical guidance for interpreting results in film manufacturing and quality control.

Technical Overview

Multi-layer co-extruded films are manufactured by simultaneously extruding several polymer melt streams through a single die, producing a film that combines three to eleven functional layers in one processing step. Common structures combine polyolefins such as PE and PP for sealing and toughness, barrier resins such as EVOH and PA for oxygen and aroma resistance, and maleic-anhydride-grafted tie layers for interlayer adhesion. Because these films frequently serve as transparent windows for food, medical, and consumer packages, optical clarity is a critical acceptance attribute alongside mechanical and barrier properties.

Definition of Haze

Haze is defined as the percentage of transmitted light that is scattered by forward scattering, deviating from the direction of the incident beam by more than approximately 2.5 degrees, expressed relative to total transmitted light. A film with high haze appears cloudy or milky and blurs the view of packaged contents even when its total light transmittance remains high. For this reason, haze is always reported together with total luminous transmittance to give a complete picture of optical performance.

Sources of Haze in Co-Extruded Structures

• Refractive index mismatch and interfacial roughness between adjacent polymer layers
• Spherulites and overall crystallinity in semicrystalline resins such as PA6, PP, and PE
• Post-crystallization of EVOH layers, particularly after retort or autoclave processing
• Gels, carbon specks, unmelted resin, and crosslinked polymer particles
• Die lines, melt fracture, and gauge (thickness) variation across the web
• Chill roll surface finish and cooling rate, which control crystallite size and density
• Surface blooming of slip and anti-block additives during storage

Quantifying haze with a standardized method allows manufacturers to distinguish between material-related and process-related scattering sources and to set objective optical specifications for incoming and outgoing inspection.

Testing Samples

Barrier Films for Thermoforming and Vacuum Packaging

Typical PA/Tie/PE and PE/Tie/EVOH/Tie/PA structures used for meat, cheese, and seafood packaging. These samples are tested in the unprinted, unlaminated state to isolate the optical contribution of the co-extruded film itself.

Lamination-Grade and Printing Base Films

Transparent co-extruded films intended for lamination to foils, metallized films, or printed webs. Haze data support both incoming inspection and the optical design of finished laminates.

Medical and Pharmaceutical Packaging Films

Films for sterilizable pouches, lidding, and device packaging where visual inspection of contents is required. Haze is often measured before and after ethylene oxide, steam, or gamma sterilization to assess optical stability.

Transparent Overwrap and General Packaging Films

PP- and PE-based co-extruded films for display packaging, where high clarity directly influences shelf presentation and consumer perception.

Specimen Preparation Requirements

• Cut flat specimens free of wrinkles, gels, scratches, fingerprints, and printed areas; specimen size is typically at least 50 mm by 50 mm or sized to fully cover the instrument transmission port
• Condition specimens at 23 ± 2 °C and 50 ± 5 % relative humidity per ISO 291, ASTM D618, or GB/T 2918 for a minimum of 40 hours before measurement
• Test at least three, preferably five, specimens taken from different positions across the web width to capture gauge-related variation
• Record the full layer structure, total thickness, machine direction orientation, production date, and any thermal history such as retort exposure

Testing Items

Standard haze testing of multi-layer co-extruded films includes the following optical parameters:

• Haze (%): the ratio of diffuse luminous transmittance to total luminous transmittance
• Total luminous transmittance (%): the proportion of incident light transmitted through the specimen in all directions
• Diffuse transmittance (%): the proportion of transmitted light scattered away from the incident beam direction
• Regular (specular) transmittance (%): optionally derived as total minus diffuse transmittance, indicating see-through clarity

Testing Methods

Applicable Standards

• ASTM D1003: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics, Method A (hazemeter) or Method B (spectrophotometer)
• ISO 14782: Plastics — Determination of haze for transparent materials
• GB/T 2410: Determination of light transmittance and haze of transparent plastics
• JIS K 7136 for haze and JIS K 7361 for luminous transmittance, technically aligned with the corresponding ISO methods

The methods are conceptually equivalent, but illuminants, observer angles, and instrument geometry differ slightly between them, so results obtained under different standards should not be compared directly without documented correlation.

Instrumentation

Testing is performed with an integrating sphere hazemeter or a spectrophotometer equipped with a haze analysis package. Key configuration points include the light source (illuminant A is commonly specified in ASTM D1003, while D65 is typical for ISO and GB procedures), the 2-degree or 10-degree standard observer, a calibrated white reference and zero standard, and a light trap that excludes the regular unscattered beam when diffuse transmittance is measured.

Test Procedure

• Warm up and calibrate the instrument using certified haze and transmittance reference standards, and verify performance with a middle-range working standard
• Measure the incident light flux (T1) with the empty transmission port to establish the baseline
• Mount the specimen flat against the transmission port without air gaps, wrinkles, or surface contamination, and record the total transmitted flux (T2)
• Engage the light trap and record the instrument scatter flux (T3) with the port empty and the combined specimen and instrument scatter flux (T4) with the specimen mounted
• Repeat the measurement for each specimen and each position, then calculate mean values and variability

Calculation

Total luminous transmittance is calculated as Tt = (T2 / T1) × 100%. Diffuse transmittance is calculated as Td = [T4 - T3 × (T2 / T1)] / T1 × 100%, which corrects for instrument-induced scatter. Haze is the ratio of the two: Haze = (Td / Tt) × 100%, commonly expressed as (T4/T2 - T3/T1) × 100%. Modern hazemeters perform these calculations automatically and report results to a resolution of 0.1 percentage point.

Reporting

A complete test report should identify the standard used, the instrument and illuminant, specimen structure and thickness, conditioning details, individual and mean values for haze and transmittance, and any observations such as visible gels or thickness variation. Because inter-laboratory reproducibility for clear films is typically within a few tenths of one percentage point absolute, comparisons should ideally be made within a single laboratory or against established correlation data.

Applications

• Incoming inspection of purchased film and optical comparison between suppliers or resin lots
• Extrusion process optimization, including melt temperature, die gap, layer-ratio, chill roll temperature, and line speed studies
• Quality control and batch release against internal optical specifications on certificates of analysis
• Retort and sterilization validation, comparing haze before and after thermal exposure
• Failure analysis of cloudy or milky film complaints, distinguishing bulk scattering from surface blooming
• Research and development of new structures, downgauging studies, and clarity improvement programs

Manufacturers, converters, and brand owners frequently engage accredited third-party laboratories for independent haze verification when disputes arise or when customer specifications require documented, traceable optical data.

FAQ

What is the difference between haze and light transmittance?

Transmittance measures how much light passes through the film, while haze measures how much of that transmitted light is scattered away from the straight-through direction. A film can transmit 90% of incident light yet still look hazy if a large share of that light is diffusely scattered.

Which standard should be chosen: ASTM D1003, ISO 14782, or GB/T 2410?

The choice usually follows the market and customer specification: ASTM D1003 is widely referenced in North America, ISO 14782 in Europe and international trade, and GB/T 2410 in China. All three measure the same physical quantity with slightly different illuminant and geometry settings, so a single standard should be consistently applied within a specification.

How many specimens should be tested?

At least three specimens are recommended, and five or more is preferable for co-extruded film, because thickness variation and localized gels can cause significant point-to-point differences across a web.

What haze level is acceptable for co-extruded barrier films?

Acceptance limits depend on structure, thickness, and end use. High-clarity sealing and overwrap films are commonly specified below 3% to 5% haze, while PA/EVOH barrier structures may naturally fall between roughly 5% and 12% depending on thickness and crystallinity. Specifications should be established from application requirements rather than universal limits.

Why does haze increase after retort or steam sterilization?

Thermal exposure accelerates crystallization of EVOH and PA layers and can cause microvoiding at poorly bonded interfaces, both of which increase light scattering. Measuring haze before and after processing is a practical way to verify retort-grade material selection and process control.

Can haze be measured on laminated or printed films?

Yes, haze can be measured on any substantially transparent laminate, but the result reflects the whole structure and is not directly attributable to the co-extruded layer. To characterize the film alone, test it before lamination or printing.

How can haze in co-extruded films be reduced?

Effective measures include selecting low-crystallinity or clarified resin grades, optimizing chill roll temperature to reduce spherulite size, improving interlayer adhesion and surface matching, removing gels through melt filtration and thorough purging, and balancing slip and anti-block additive levels to limit surface blooming. Applying a refractive-index-matching liquid during development testing can help separate surface haze from internal bulk haze.