Haze testing is a critical optical quality control procedure for plastic blown films used in packaging, agriculture, medical, and consumer applications. By quantifying the percentage of transmitted light that is scattered during passage through the film, manufacturers can verify product clarity and compliance with recognized standards such as ASTM D1003, ISO 14782, and GB/T 2410. This article presents a comprehensive technical overview of haze detection for blow-extruded films, covering sample types, testing items, measurement methods, industrial applications, and frequently asked questions.
Technical Overview
Blown film extrusion is one of the most widely used processes for manufacturing thin plastic films. In this process, molten polymer is extruded through an annular die and inflated into a tubular bubble by internal air pressure. The bubble is cooled at the frost line, collapsed by nip rolls, and wound into rolls of flat film. The process is used extensively for polyethylene, polypropylene, and multilayer co-extruded film structures destined for packaging, lamination, agriculture, and industrial applications.
Haze is defined as the percentage of transmitted light that is scattered by more than 2.5 degrees from the direction of the incident beam. In other words, haze measures wide-angle light scattering within or at the surfaces of a nominally transparent material. High haze gives a film a milky, cloudy, or translucent appearance and reduces the visibility of packaged contents. Haze must not be confused with total luminous transmittance, which quantifies how much light passes through the film in total, or with clarity, which relates to the ability to resolve fine detail viewed through the film.
In blown films, haze originates from several physical sources, including semi-crystalline spherulites and density fluctuations within the polymer, surface roughness introduced by melt fracture or die lines, gels and fish eyes, inorganic antiblock and slip additives, pigment particles, gauge non-uniformity, and the quenching conditions around the frost line. Because the air-cooled blown process cools the melt relatively slowly compared with chill-roll casting, blown films generally exhibit higher haze than cast films of the same resin, which makes accurate haze measurement especially important for process optimization and specification control.
Testing Samples
Haze testing can be performed on virtually any transparent or translucent blown film structure. Typical sample categories submitted to laboratories or tested in-house include the following:
Polyethylene Blown Films
Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), metallocene LLDPE, and high-density polyethylene (HDPE) blown films are the largest sample group. These include general-purpose packaging films, heavy-duty sacks, shrink films, lamination-grade films, and cling films, typically ranging from about 10 micrometers to over 200 micrometers in thickness.
Polypropylene Blown Films
Blown polypropylene (BOPP feedstock, CPP alternatives, and blown PP packaging films) are tested for haze, particularly when used for textiles, food, and stationery packaging where a balance of stiffness and optical quality is required.
Multilayer and Functional Films
Co-extruded structures such as PE/PA, PE/EVOH/PE barrier films, and films incorporating tie layers, antifog agents, or masterbatches are tested to evaluate how each layer and additive package contributes to overall light scattering.
Other Polymer Films
Blown films made from PVC, EVA, ionomers, polyamide, recycled-content resins, and bio-based polymers are also routinely evaluated, especially during formulation development and supplier qualification.
For all sample types, specimens should be representative of production, free from wrinkles, creases, prints, embossing, and visible contamination, and should be handled with gloves to prevent fingerprints, which can artificially raise measured haze.
Testing Items
A standard optical evaluation of blown film typically reports the following items, often measured in a single instrument sequence:
- Haze: the percentage of transmitted light deviating more than 2.5 degrees from the incident beam, calculated as diffuse transmittance divided by total transmittance.
- Total luminous transmittance: the percentage of incident visible light that passes through the specimen in any direction, characterizing overall transparency.
- Diffuse transmittance: the scattered component of transmitted light, which may be reported separately for diagnostic purposes.
- Clarity (optional): narrow-angle light scattering assessment relevant to see-through image sharpness, typically performed with a dedicated clarity method.
- Gloss (optional): surface reflection measurement that is frequently paired with haze to fully characterize the optical appearance of the film.
Testing Methods
Applicable Standards
The most widely applied standards for haze measurement of plastic films are ASTM D1003 (Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics), ISO 14782 (Plastics - Determination of haze for transparent materials), GB/T 2410 (Determination of light transmittance and haze of transparent plastics), and JIS K 7136. ASTM D1003 offers two procedures: Method A using a dedicated hazemeter with a tristimulus green filter, and Method B using a spectrophotometer with an integrating sphere. Test reports should always state which standard and method were followed, since small geometric differences between methods can produce slightly different results.
Instrumentation
Testing is performed with a hazemeter or a spectrophotometer equipped with a integrating sphere, Illuminant A or C, and a defined standard observer (2-degree or 10-degree). An unpolished (light-trapping) port configuration is required for the ISO 14782 geometry, while ASTM D1003 uses a specified sphere geometry with a light trap for the diffuse readings. Instruments must be verified using reference standards traceable to recognized metrology institutes, and baseline readings must be taken with the sample compartment empty.
Specimen Preparation and Conditioning
Specimens are typically cut to at least 50 mm by 50 mm to fully cover the instrument port, with a minimum of three specimens taken from different positions across the film web to capture gauge and profile variation. Surfaces should be cleaned gently with a lint-free cloth and, if necessary, a suitable cleaning fluid compatible with the polymer. Unless otherwise specified, specimens are conditioned at 23 degrees Celsius plus or minus 2 degrees and 50 percent plus or minus 5 percent relative humidity for a defined period prior to testing. Care must be taken to avoid static attraction of dust, surface scratches, and trapped air bubbles between film layers.
Measurement Procedure
The classical four-reading sequence is performed as follows. T1 is the baseline incident light flux with no specimen in place; T2 is the total light transmitted with the specimen mounted and the sphere fully illuminated; T3 is the baseline with the light trap in position; T4 is the transmitted light with the specimen mounted and the light trap open. From these readings:
Total transmittance (percent) = T2 divided by T1, multiplied by 100. Haze (percent) = (T4 divided by T2 minus T3 divided by T1), multiplied by 100. Results for all specimens are reported as individual values, together with the arithmetic mean. Films exhibiting significant curvature or gauge variation benefit from additional replicate measurements at multiple web positions.
Factors Affecting Results
- Specimen thickness: haze generally increases with thickness because scattering sites accumulate along the optical path, so thickness must always be reported alongside haze.
- Crystallinity and cooling conditions: slower bubble cooling promotes larger spherulites and higher haze.
- Additives: antiblock agents, slip additives, pigments, and recycled content typically increase light scattering.
- Surface defects: die lines, melt fracture, gels, and blocking raise measured haze.
- Handling artifacts: fingerprints, dust, scratches, and wrinkles can significantly distort results.
Applications
Haze testing of blown films supports quality control, product development, and specification compliance across many industries:
- Food packaging: verifying the clarity of bread bags, produce bags, vertical form-fill-seal films, and barrier laminations where product visibility drives consumer appeal.
- Consumer and retail packaging: qualification of shrink films, overwraps, and printed laminates against defined optical grade specifications.
- Agricultural films: balancing haze and direct transmittance in greenhouse and mulch films, where controlled light diffusion can actually benefit plant growth.
- Medical and hygiene packaging: confirming visibility requirements for sterile barrier systems and breathable PE films used in hygiene products.
- Process and formulation development: evaluating the effects of frost line height, blow-up ratio, resin blends, masterbatch loading, and cooling rate on film optics.
- Incoming inspection and supplier certification: verifying that purchased film rolls conform to contractual haze and transmittance limits using accredited third-party laboratories where independent confirmation is required.
FAQ
What haze value is considered acceptable for a blown packaging film?
Acceptance limits depend on the end use. Premium clarity films, such as cast or high-clarity blown structures for retail overwraps, are often specified below 5 to 10 percent haze, while general-purpose LDPE packaging films commonly fall between 6 and 20 percent haze. Heavy-duty and industrial films may tolerate haze above 20 percent. The specification should always be agreed between buyer and seller together with a defined thickness and test standard.
Why does haze increase with film thickness?
Light scattering occurs at every crystalline region, additive particle, and internal interface along the optical path. A thicker film presents more scattering sites to the incident beam, so diffuse transmittance rises faster than total transmittance, and calculated haze increases. This is why haze values are only comparable at equal nominal thickness.
What is the difference between haze and clarity?
Haze quantifies wide-angle scattering (greater than 2.5 degrees), which causes a milky or cloudy appearance and reduces contrast. Clarity quantifies narrow-angle scattering, which blurs fine detail seen through the film. A film can show low haze yet poor clarity, or vice versa, so both properties should be considered when see-through performance is critical.
Why do blown films show higher haze than cast films of the same resin?
Cast films are quenched rapidly on a chilled roll, which suppresses crystal growth and produces smoother surfaces, whereas air-cooled blown films cool more slowly around the frost line, allowing larger crystalline structures and slightly rougher surfaces to develop, both of which increase light scattering.
How can haze in blown film production be reduced?
Common measures include selecting low-haze or metallocene resins, increasing cooling air flow or using internal bubble cooling to raise the quench rate, lowering the frost line height, optimizing the blow-up ratio and melt temperature, reducing antiblock loading to the minimum functional level, ensuring clean melt filtration to remove gels, and maintaining die and air ring surfaces in good condition to avoid die lines and melt fracture.
Which standard should be specified on a purchase agreement?
ASTM D1003 and ISO 14782 are the most common choices internationally, while GB/T 2410 is widely referenced in China. Because the standards differ slightly in sphere geometry and illuminant conditions, the specific standard, method, specimen thickness, and conditioning requirements should be explicitly stated in the technical agreement to ensure reproducible, comparable results.