Determination of Printing Surface Strength of Neutral Extrusion-Coated Paper

Printing surface strength, also known as pick resistance, is one of the most decisive quality parameters for neutral extrusion-coated (PE-coated) paper, because it determines whether the material can run cleanly on high-speed offset and flexographic presses without picking, linting, or delamination. This technical article explains the definition, reference standards, sample preparation, instrumentation, and step-by-step determination method used by paper mills, converters, and third-party laboratories to measure the critical pick velocity of neutral extrusion-coated paper, together with typical acceptance levels and practical applications.

Technical Overview

Neutral extrusion-coated paper is a composite packaging material produced by laminating one or both sides of a base sheet with a thin polyethylene (PE) film through a melt-extrusion coating process. The term "neutral" indicates that the base paper is manufactured under neutral to slightly alkaline wet-end conditions, typically using reactive neutral sizing agents such as AKD or ASA together with calcium carbonate filler. The resulting stock exhibits a pH close to 7, very low residual acidity or alkalinity, minimal aging degradation, and neutral odor characteristics, which makes it particularly suitable for direct food contact packaging, pharmaceutical wrappers, and archival applications.

Printing surface strength, commonly called pick resistance, is the capacity of the paper surface to withstand the combined pulling forces generated by ink film splitting and blanket-to-paper separation during printing. When these forces exceed the internal bonding strength of the surface layer, characteristic defects appear: coating pick, fiber pick, fluffing or linting, and in severe cases blistering or ply delamination. Because neutral sizing chemistry and relatively high calcium carbonate loading can reduce inter-fiber bonding compared with traditional acid-sized grades, neutral extrusion-coated paper requires systematic verification of its printing surface strength before release for offset printing, where ink tack forces are highest.

The internationally recognized determination is performed on an IGT-type printability tester using the accelerated speed method described in ISO 3783. A test strip is printed with a tacky pick-test ink at continuously increasing velocity; the velocity at which the first picking damage appears, expressed in meters per second, is reported as the critical pick velocity. This value provides a direct, quantitative index of print runnability on commercial web and sheet-fed presses.

Testing Samples

Sample Forms and Dimensions

Test pieces are cut from representative production lots delivered as sheets, rolls, or die-cut packaging blanks. For a standard IGT AIC2-5-type instrument, strips of approximately 55 mm width and 250 mm to 340 mm length are prepared. Each strip must be clearly labeled with the machine direction or cross direction, the felt (top) side or wire side, the lot number, and the production date. In most converting operations the printed image is applied to the uncoated base paper side, so this side is the primary test surface; the reverse side is evaluated where both surfaces may be printed.

Sampling and Conditioning

Sampling is carried out according to statistical sampling standards for paper and board, such as ISO 186 or the corresponding national standard. Samples must remain in their original moisture-proof packaging until testing to preserve their moisture history. All specimens are conditioned in the standard atmosphere of ISO 187 at (23 ± 1) °C and (50 ± 2) % relative humidity until equilibrium is reached, normally not less than 24 hours for composite structures. The test area must be free of wrinkles, water marks, coating defects, and fingerprints, and operators should wear lint-free gloves. Because the PE layer induces curl, strips are flattened gently without creasing before mounting.

Testing Items

The determination typically covers the following items, each reported separately by side and fiber orientation:

• Critical pick velocity of the printing surface in machine direction and cross direction, expressed in m/s
• Identification of the failure mode: coating pick, fiber pick, fluffing or linting, or delamination
• Comparison of felt side versus wire side performance where both sides are printable
• Documentation of test conditions: pick-test ink grade, printing force, speed range or acceleration setting, temperature, and relative humidity
• Optional supplementary determinations: wax pick number by the legacy Denniston method, low-viscosity oil pick resistance per ISO 3782 for weakly bonded grades, and wet pick resistance after dampening simulation

Testing Methods

Reference Standards

The primary reference method is ISO 3783, which specifies the accelerated speed determination of picking resistance using an IGT-type electric printability tester. Supporting standards include ISO 3782 for the low-viscosity oil method, TAPPI T 514 for the rotating press method, and TAPPI T 459 for the wax pick method. Sampling and conditioning follow ISO 186 and ISO 187 or their national equivalents. When contractual specifications exist, the purchaser and supplier should agree in advance on the standard, ink grade, and acceptance level applied.

Test Principle

The force required to split an ink film increases with the separation velocity between the printing disc and the paper surface. In the accelerated method, the tester prints a short strip in a single impression during which the printing speed rises continuously from a low starting value to a high final value. As long as the surface bonding strength exceeds the ink tack force, the print remains clean; at the point where tack force surpasses surface bonding, fibers or coating particles begin to lift. The operator locates the first visible damage on the printed strip and reads the corresponding velocity from a calibrated scale or software overlay. This velocity is the critical pick velocity and represents the practical speed limit of the material under the chosen ink tack conditions.

Apparatus and Materials

• IGT-type printability tester, such as the AIC2-5 series, with a motor-driven accelerating sector and removable printing disc
• Standard inking unit with distribution rollers for applying a defined and repeatable ink film
• Standard pick-test ink of defined tack and viscosity, freshly conditioned to 23 °C
• Magnifier of approximately 10x magnification with a directional light source for identifying the first pick point
• Calibrated velocity scale or instrument software overlay for converting the pick position into velocity
• Cleaning solvents and lint-free wipes for disc and roller maintenance between determinations

Test Procedure

• After conditioning, cut at least five valid strips for each side and each fiber direction and label them completely.
• Apply the specified volume of pick-test ink to the distribution rollers, distribute for the prescribed time to stabilize tack, and transfer the ink film to the printing disc.
• Mount the strip on the sector with the printing surface facing the disc and set the instrument parameters, commonly a printing force of about 800 N, a speed range of approximately 0 to 5 m/s, and a constant acceleration setting, following the requirements of the selected standard.
• Release the sector and produce the accelerated print; immediately remove and label the printed strip.
• Inspect the print under the magnifier in suitable illumination, mark the position of the first picking damage, classify the failure mode, and read the corresponding velocity from the calibrated overlay.
• Repeat the determination on the remaining strips, cleaning and re-inking the disc as required; discard prints showing slippage, uneven ink transfer, or damage within the edge margins.
• Record all results together with the instrument settings and ambient conditions.

Calculation and Expression of Results

The result for each configuration is the arithmetic mean of the valid critical pick velocities, reported to the nearest 0.1 m/s, together with the maximum and minimum values, the standard deviation, and the number of determinations. The report must state the tested side, fiber direction, identified failure mode, ink grade, and test atmosphere. If no picking occurs at the maximum attainable velocity, the result is reported as greater than that maximum velocity. Results obtained with different inks, forces, or instruments are not directly comparable and must be interpreted as internal relative values.

Precision and Typical Acceptance Levels

Within-laboratory repeatability is generally good when ink conditioning, force, and atmosphere are tightly controlled; inter-laboratory comparisons should use the same standard and ink. Acceptance limits are contractual, but general industry guidance indicates that grades intended for modern offset printing achieve pick velocities of 2.0 m/s or higher, while neutral extrusion-coated food packaging grades are commonly specified in the range of 1.0 to 2.5 m/s depending on press speed, ink system, and dampening conditions. Lower values may be acceptable for flexographic or letterpress work where ink tack is reduced. Mills are advised to establish an internal correlation between laboratory pick velocity and actual press performance through controlled printing trials.

Applications

• Mill quality control and lot release of neutral extrusion-coated paper for direct food contact products such as paper cups, soup bowls, lunch boxes, sandwich wraps, and quick-frozen food packaging
• Converter and printer incoming inspection prior to offset, letterpress, or flexographic printing operations
• Failure analysis and complaint resolution for picking, linting, hickies, blanket piling, and coating delamination encountered on press
• Research and development support for optimizing pulp refining, neutral sizing dosage, calcium carbonate filler content and particle shape, surface sizing, calendering, primer application, and PE extrusion parameters
• Qualification of ink, coating, and press consumable suppliers, and pre-press runnability risk assessment when introducing new grades or higher press speeds

By quantifying the printing surface strength at the laboratory scale, producers of neutral extrusion-coated paper can predict pressroom behavior, reduce downtime caused by blanket washing, and demonstrate compliance with the printability requirements demanded by packaging buyers.

FAQ

Why is surface strength tested on the paper side rather than the PE side?

In most converting operations, printing is performed on the uncoated base paper side, which is the surface vulnerable to picking. The polyethylene film contains no fibers or pigment coating that can be lifted by ink tack, so the PE side is instead evaluated through surface energy, typically by dyne testing, and through ink adhesion assessments after corona treatment.

What is the difference between picking, linting, and fluffing?

Picking is the localized removal of coating or fiber bundles from the surface when ink tack force exceeds surface bonding, and it is the phenomenon quantified by the accelerated pick test. Linting and fluffing describe the gradual accumulation of loosely bonded fibers and fine debris on the blanket over many impressions. A high critical pick velocity generally indicates a surface that resists both failure modes.

Which method is preferred, the wax test or the IGT accelerated method?

The IGT accelerated method is preferred because it produces a numerical critical velocity with good reproducibility and simulates the dynamic speed conditions of a real press. The wax pick method remains useful as a fast legacy check, but its results are more subjective, and the warm wax can thermally disturb the polyethylene layer of coated grades, so caution is required with extrusion-coated samples.

How do neutral sizing and calcium carbonate filler influence the result?

Neutral to alkaline papermaking with high calcium carbonate loading tends to reduce inter-fiber bonding at the surface compared with acid-sized sheets. Producers compensate through appropriate refining, surface sizing with starch or polymer, and calendering strategy. The pick test verifies that this balance delivers sufficient surface strength for the intended printing process.

Why is conditioning so important for this material?

Moisture content directly affects fiber bonding, and the PE barrier layer slows equilibration of the composite structure. Deviations from the standard atmosphere of 23 °C and 50 % relative humidity can shift results and increase scatter, so full conditioning in the original packaging is essential for comparable data.

How frequently should the determination be performed?

Testing is normally performed on every production lot or shift sample, and additionally whenever the furnish, chemical system, base sheet formation, or extrusion line conditions are changed. Any pressroom picking complaint should trigger retesting of the retained sample to confirm whether the lot met the specified pick velocity.