Determination of Grammage of Neutral PE-Coated Paper: Standards, Methods, and Applications

Neutral PE-coated paper combines a neutral-sized paper base with an extruded polyethylene layer, making grammage (basis weight) one of the most critical quality and trading parameters in packaging, labeling, and food-contact applications. Grammage determination is performed with a gravimetric method in accordance with ISO 536, GB/T 451.2, and TAPPI T 410, in which conditioned specimens of precisely known area are weighed on a calibrated analytical balance. This article explains the sampling, conditioning, test items, calculation procedures, coating-weight separation, and typical applications of grammage determination for neutral PE-coated paper.

Technical Overview

Neutral PE-coated paper refers to a paper substrate manufactured under neutral to alkaline papermaking conditions—typically using synthetic sizing agents such as AKD or ASA together with calcium carbonate filler—followed by extrusion coating with low-density polyethylene (PE) on one or both sides. The neutral papermaking process yields a sheet with a pH of approximately 6.5 to 8.5, which resists acid-catalyzed degradation, offers excellent aging stability, and minimizes odor and taste transfer, making it especially suitable for food-contact and long-life packaging applications.

Grammage, also known as basis weight, is defined as the mass per unit area of paper or board and is expressed in grams per square meter (g/m²). It is the single most fundamental specification parameter in the paper industry because it directly governs strength, stiffness, thickness, barrier performance, printing behavior, and cost. For PE-coated products, three grammage values are distinguished: the total grammage of the finished coated product, the grammage of the base paper, and the PE coating weight applied to each side.

Grammage determination for neutral PE-coated paper follows internationally recognized gravimetric test methods, principally ISO 536 (Paper and board — Determination of grammage), the Chinese national standard GB/T 451.2, and TAPPI T 410. These standards define the sampling rules, specimen dimensions, atmospheric conditioning, balance accuracy, and calculation and reporting requirements needed to obtain reproducible results.

Testing Samples

Sampling from Rolls and Sheets

Samples shall be taken in accordance with ISO 186 or GB/T 450 so that they are representative of the entire lot. For roll goods, at least three to five outer wraps are discarded before sampling, and specimens are taken across the full web width to capture cross-direction grammage variation. Damaged areas, wrinkles, streaks, coating defects, and edge zones affected by slitting must be excluded from test specimens. Each sample is identified with roll number, machine direction, and sampling position to allow traceability.

Specimen Preparation

Specimens are cut using a precision guillotine cutter or a circular cutting die so that the area is accurately known to within 0.5 percent. The preferred specimen size is 100 mm × 100 mm (100 cm²) or larger; where sufficient material is available, larger total areas up to 500 cm² or more improve measurement precision. A minimum of ten specimens is recommended, distributed across the full width of the web. Cut edges must be clean, square, and free of fiber pull-out or PE film tearing, as frayed edges introduce area and mass errors.

Atmospheric Conditioning

Because paper mass is highly sensitive to moisture content, specimens must be conditioned before weighing in a standard atmosphere of 23 ± 1 °C and 50 ± 2 % relative humidity in accordance with ISO 187 or GB/T 10739. The polyethylene layer slows moisture equilibration compared with uncoated paper, so a conditioning period of at least 24 hours is recommended for coated grades. Equilibrium is confirmed when the change in mass between successive weighings is negligible. All cutting and weighing operations should be performed in the same conditioned environment whenever possible.

Testing Items

The grammage evaluation of neutral PE-coated paper typically includes the following measurable items:

• Total grammage of the finished PE-coated product, expressed in g/m².
• Base paper grammage, determined on the uncoated substrate or after removal of the PE film.
• PE coating weight on each side, expressed in g/m² per side, obtained by difference or direct film measurement.
• Cross-direction (CD) grammage profile and uniformity, expressed as minimum, maximum, standard deviation, and coefficient of variation across the web.
• Machine-direction (MD) grammage consistency when samples are taken from different positions along the roll.
• Comparison of measured grammage against nominal specification and contractual tolerance bands.

Testing Methods

Total Grammage — Gravimetric Method

The reference method is the gravimetric procedure of ISO 536, GB/T 451.2, and TAPPI T 410. The sequence of operations is as follows:

• Condition the specimens to equilibrium at 23 ± 1 °C and 50 ± 2 % relative humidity.
• Cut at least ten specimens of accurately known area, each at least 100 cm².
• Weigh each specimen individually on a calibrated analytical balance with a readability of 0.001 g or better, ensuring the balance is level, draft-shielded, and free of static interference.
• Calculate the grammage of each specimen and then compute the arithmetic mean, standard deviation, coefficient of variation, and minimum and maximum values.
• Report the results together with the number of specimens, specimen area, conditioning atmosphere, and the standard applied.

The grammage calculation is performed as: grammage (g/m²) = specimen mass in grams divided by specimen area in square meters. For a 100 cm² specimen, which equals 0.01 m², the grammage is obtained by multiplying the mass in grams by 100. For example, a 100 cm² specimen of neutral PE-coated paper weighing 3.05 g corresponds to a total grammage of 305 g/m².

Determination of Base Paper Grammage

Where the uncoated base paper is accessible, its grammage is determined using the identical gravimetric procedure, ideally on specimens taken from the same position across the web as the coated samples. This paired-sampling approach minimizes cross-direction variation errors and provides the most reliable basis for calculating applied coating weight.

Determination of PE Coating Weight

The coating weight on each side is normally obtained by the difference method: PE coating weight (g/m²) = total grammage of the coated specimen minus grammage of the base paper. For two-side-coated products, the individual side weights can be separated by controlled delamination, in which the film is peeled at a heat-activated tab or separated after softening in hot toluene or xylene under a fume hood, after which the paper is dried, reconditioned, and reweighed. Typical PE coating weights for food-grade grades range from approximately 10 to 40 g/m² per side, with cup stock commonly carrying around 18 to 25 g/m² per side.

On-Line and Rapid Verification Methods

In production environments, beta-ray transmission, X-ray, or infrared gauges mounted on the coater continuously measure total grammage and provide machine-direction and cross-direction profiles for closed-loop control. Because these sensors are influenced by furnish composition and moisture, laboratory gravimetric testing remains mandatory for calibration, lot release, contractual settlement, and dispute resolution.

Precision and Sources of Error

The dominant sources of uncertainty are the accuracy of the cut area (targeted at better than 0.5 percent), the calibration and readability of the balance (better than 0.1 percent of specimen mass), and incomplete atmospheric conditioning. With properly conditioned specimens and a precision cutter, within-laboratory coefficients of variation of 0.5 percent or lower are routinely achievable, comfortably resolving typical specification tolerances of ±2 to ±4 percent on total grammage.

Applications

Grammage determination for neutral PE-coated paper underpins quality control, specification compliance, and commercial settlement across a wide range of end uses:

• Food packaging: single and double-side PE-coated paper for cups, bowls, plates, soup containers, fast-food wraps, and bakery liners, where grammage controls stiffness, liquid holdout, and food-contact compliance.
• Liquid and beverage packaging: coated boards for gable-top cartons and pouches requiring tightly controlled total grammage for converting and sealing performance.
• Medical and pharmaceutical packaging: neutral-pH coated papers for sterilizable pouches and overwraps, where long-term chemical stability and consistent grammage are essential.
• Labels and release liners: PE-coated backing papers for die-cutting and silicone coating, where grammage uniformity governs lay-flat and converting yield.
• Industrial and barrier wrapping: moisture-resistant wraps and interleaving papers whose barrier performance is directly proportional to coating weight.
• Purchasing and trade: grammage certificates based on ISO 536 or GB/T 451.2 serve as the contractual basis for invoicing, customs declaration, and incoming inspection.

FAQ

Question: Which standards govern the grammage determination of neutral PE-coated paper? Answer: The primary methods are ISO 536, the Chinese national standard GB/T 451.2, and TAPPI T 410. Sampling is governed by ISO 186 or GB/T 450, and atmospheric conditioning by ISO 187 or GB/T 10739.

Question: Why is atmospheric conditioning necessary before weighing? Answer: Paper rapidly absorbs or releases moisture with changes in humidity, which changes its mass. Conditioning at 23 ± 1 °C and 50 ± 2 % relative humidity ensures that all measurements reflect a defined and comparable moisture state; without it, grammage results can deviate by several percent.

Question: What specimen size and how many specimens are required? Answer: Specimens of at least 100 cm², such as 100 mm × 100 mm squares, are standard, and at least ten specimens should be tested, distributed across the full web width. Larger total specimen areas improve precision.

Question: How is the PE coating weight separated from the total grammage? Answer: The most common approach is the difference method, subtracting the base paper grammage from the total grammage. Individual side weights for two-side-coated products are obtained by controlled film delamination or solvent separation followed by reweighing of the paper component.

Question: What balance accuracy is required? Answer: An analytical balance with a readability of 0.001 g or better, calibrated with traceable weights and accurate to better than 0.1 percent of the specimen mass, is required to keep weighing error negligible relative to area-cutting error.

Question: What are typical grammage levels for neutral PE-coated paper? Answer: Common total grammages range from about 130 to 400 g/m² depending on end use, with base papers of roughly 120 to 350 g/m² and PE coatings of approximately 10 to 40 g/m² per side.

Question: Can on-line gauges replace laboratory testing? Answer: No. On-line beta, X-ray, and infrared gauges provide continuous profile control but require laboratory gravimetric verification for calibration, lot release certification, and resolution of commercial disputes.