Printing film wetting tension testing measures the surface energy of plastic films to determine their printability, coating adhesion, and laminating performance. Conducted in accordance with internationally recognized standards such as ASTM D2578, ISO 8296, and GB/T 14216, this test is essential for verifying corona treatment effectiveness and preventing defects such as ink retreating, poor adhesion, and delamination. This article provides a comprehensive technical guide covering testing principles, sample types, methods, and practical applications for flexible packaging and printing industries.
Technical Overview
Wetting tension, also commonly referred to as surface tension or dyne level, is a critical surface property of plastic films intended for printing, coating, and lamination. Most polyolefin films, such as polyethylene (PE) and polypropylene (PP), are inherently non-polar materials with low surface energy, typically ranging from 29 to 32 mN/m in their untreated state. Because printing inks, adhesives, and coatings are generally formulated with surface tensions between 30 and 36 mN/m, adequate wetting and adhesion require the film surface energy to exceed the liquid surface tension by a sufficient margin.
To raise the surface energy of these films, manufacturers apply corona treatment, a high-frequency electrical discharge process that oxidizes the film surface and introduces polar functional groups. Printing film wetting tension testing quantifies the resulting surface energy, usually expressed in millinewtons per meter (mN/m) or dynes per centimeter (dyn/cm), which are numerically equivalent. The test is performed by applying liquid test mixtures of known surface tension onto the film surface and observing whether the liquid film remains continuous for a defined period. If the film wets continuously, the film surface tension is equal to or greater than that of the test liquid; if the liquid breaks into droplets within two seconds, the film surface tension is lower than that of the test liquid.
As a rule of thumb, a wetting tension of at least 38 mN/m is required for general printing, while 40 to 44 mN/m is often specified for high-speed printing, lamination, and coating applications. Regular testing helps manufacturers control corona treatment levels, monitor surface energy decay during storage, and avoid costly printing failures.
Testing Samples
Polyethylene (PE) Films
Low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), and high-density polyethylene (HDPE) films are among the most frequently tested materials. Untreated PE typically exhibits a wetting tension of approximately 31 mN/m, and corona treatment is almost always required before printing or lamination.
Polypropylene (PP) Films
Biaxially oriented polypropylene (BOPF/BOPP) and cast polypropylene (CPP) films are widely used in flexible packaging. These films are tested to verify treatment levels before gravure or flexographic printing and before extrusion lamination with PE.
Polyester (PET) Films
Biaxially oriented PET films naturally possess relatively high surface energy, typically around 42 to 46 mN/m, but testing is still performed to confirm that surface contamination, slip agents, or storage degradation have not reduced printability.
Polyamide (PA) and Other Specialty Films
Nylon films used in barrier packaging structures, along with polystyrene, PVC, and biodegradable polymer films, are tested to verify treatment uniformity and surface cleanliness prior to conversion processes.
Multilayer and Composite Films
Coextruded, metallized, and pre-laminated film structures are tested on the functional surface intended for printing or coating to ensure compatibility with inks and adhesives.
Testing Items
The primary measurable parameter in this test is the wetting tension of the film surface, reported in mN/m or dyn/cm. The scope of testing typically includes:
- Wetting tension of the treated (outer) surface of the film
- Wetting tension of the untreated (inner or sealing) surface, where applicable
- Verification of minimum dyne level required for specific printing or laminating processes
- Assessment of surface energy uniformity across the film web width and length
- Evaluation of wetting tension decay after aging, storage, or transportation
- Verification of corona treatment effectiveness on both sides of the film (single-sided or double-sided treatment)
Testing Methods
Several recognized standards govern the determination of wetting tension on plastic films, and all are based on the same fundamental principle of applying graded test liquids of known surface tension to the film surface.
ASTM D2578
ASTM D2578, Standard Test Method for Wetting Tension of Polyethylene and Polypropylene Films, specifies the use of formamide and ethoxyethane (cellosolve) mixtures prepared at graded surface tension intervals, typically 30 to 56 mN/m. A cotton-tipped applicator is used to spread approximately 1 cm² of test liquid onto the film surface, and the behavior of the liquid film is observed for two seconds. The reported wetting tension is the value of the highest test liquid that wets the surface continuously without breaking into droplets within the two-second observation window.
ISO 8296
ISO 8296, Plastics — Film and sheeting — Determination of wetting tension, describes a closely related procedure using mixtures of formamide and ethylene glycol monoethyl ether, or alternative liquid systems, applied to conditioned film specimens. The standard covers both the "wetting" determination method and requirements for test liquid calibration, applicator construction, and environmental control, since ambient temperature and humidity significantly influence results.
GB/T 14216
GB/T 14216, the Chinese national standard for the determination of wetting tension of plastic films and sheeting, aligns technically with ISO 8296 and is widely applied in domestic quality inspection, incoming material control, and corona treatment monitoring.
Test Pens (Dyne Pens) Method
For rapid on-line or incoming inspection, dyne pens containing liquids of calibrated surface tension are drawn across the film surface. If the ink line remains intact for approximately two seconds, the film surface tension meets or exceeds the pen rating. While convenient, pen results should be verified periodically against the standard applicator-and-solution method in a controlled laboratory environment, because pen tips can become contaminated and liquids can change concentration with use.
Test Conditions and Precautions
- Specimens should be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity in accordance with standard laboratory atmospheres
- Only the film surface that will actually receive ink, coating, or adhesive should be tested
- Test surfaces must be free of fingerprints, dust, abrasion, and transfer of slip or antiblock agents
- Each test liquid should be applied once to a given area, and fresh applicator tips must be used to avoid cross-contamination
- Test liquids have limited shelf life and must be stored in sealed containers away from light and heat
- Formamide-based mixtures are toxic; appropriate personal protective equipment and ventilation are required
Applications
Printing film wetting tension testing is applied throughout the flexible packaging, label, and converting industries for the following purposes:
- Corona treatment validation on film extrusion and treatment lines, ensuring treatment targets of typically 38 to 44 mN/m are achieved uniformly across the web
- Incoming quality control by printers and converters, verifying that film rolls received from suppliers meet specified dyne levels before production
- Pre-printing qualification for gravure, flexographic, offset, and digital printing processes, where insufficient surface energy causes ink retreating, pinholes, mottled print, and poor ink adhesion
- Lamination and coating process control, since adhesives and lacquers require higher surface energy for complete wetting and durable bonding
- Storage and aging studies, documenting the natural decay of surface energy over time and establishing shelf-life limits for treated films
- Failure analysis, diagnosing the root cause of print defects, ink delamination, and bond failure in finished packages
- Research and development of new film formulations, surface treatment equipment, and functional coatings
FAQ
What wetting tension value is required for printing on plastic films?
For most printing processes, a minimum wetting tension of 38 mN/m is considered acceptable. High-quality printing, fine halftone reproduction, and subsequent lamination typically require 40 to 44 mN/m. The exact specification depends on the ink system, printing speed, and downstream converting steps.
Why does the wetting tension of treated film decrease over time?
After corona treatment, polar functional groups on the film surface can migrate into the bulk polymer, and slip agents, antiblock additives, and plasticizers can migrate to the surface, masking the treated layer. This decay accelerates at elevated temperatures and with high additive content, which is why films should be printed soon after treatment and re-tested if stored for extended periods.
Are dyne pens as accurate as laboratory test solutions?
Dyne pens provide a fast, convenient indication of surface energy and are suitable for routine on-line checks. However, their accuracy is limited by tip contamination, solvent evaporation, and operator technique. Formal determination in accordance with ASTM D2578 or ISO 8296, using calibrated solutions and controlled laboratory conditions, remains the reference method.
What does it mean when the test liquid breaks into droplets?
If the applied liquid film retracts and forms droplets within two seconds, the surface tension of the film is lower than that of the test liquid, indicating insufficient treatment for that liquid level. The wetting tension of the film lies between the highest liquid that wets continuously and the lowest liquid that retracts.
Can wetting tension testing be performed on metallized or coated films?
Yes. The same methodology applies to metallized, coated, and coextruded surfaces, provided the functional surface to be printed or bonded is tested and the surface is clean and free of transfer agents. Testing different liquids may be needed depending on the expected surface energy range.
Which surfaces of a film roll should be tested?
Only the surface that will receive ink, adhesive, or coating needs to meet the wetting tension specification. However, testing both surfaces is recommended when double-sided treatment is specified, or when the roll is unlabeled and the treated side must be identified before loading onto the printing press.