Grease Resistance Testing of Neutral PE-Coated Paper: Standards, Methods, and Evaluation

Neutral PE-coated paper is a widely used food-grade packaging material whose value depends heavily on its ability to resist oil and grease penetration. This article provides a professional overview of grease resistance testing for neutral PE-coated paper, covering sample preparation, test items, standard methods such as the KIT test and turpentine test, acceptance criteria, and typical applications. Whether you are a packaging manufacturer, quality engineer, or procurement specialist, this guide will help you understand how to verify and interpret the oil-proof performance of coated paper products.

Technical Overview

Neutral PE-coated paper is manufactured by extrusion-laminating a thin layer of polyethylene, typically 10 to 40 g/m², onto a pH-neutral base paper such as bleached or unbleached kraft. The term "neutral" indicates that the base stock is produced without acidic rosin-alum sizing, yielding a cold-water extract pH of approximately 6.0 to 8.0, which makes the material suitable for direct food contact and long-term storage without acid degradation of the fibers.

In this structure, the polyethylene layer serves as the primary barrier against moisture, fats, and oils. The grease resistance of the finished product therefore depends on the continuity, uniformity, and grammage of the PE coating rather than on the absorbency of the base sheet. Even microscopic pinholes or thin spots in the extrusion coating can allow lipids to migrate through the paper, causing visible staining, loss of package strength, and contamination of consumers' hands. Grease resistance testing is consequently one of the most critical quality control procedures for this material category, and it is routinely performed during incoming inspection, production batch release, and new product development.

Testing Samples

Test specimens should be representative of the production lot and free from creases, folds, and surface contamination. Prior to testing, samples must be conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours in accordance with ISO 187 or TAPPI T402, because moisture content significantly influences both paper strength and coating barrier behavior.

Single-Side PE-Coated Neutral Paper

This is the most common configuration, in which polyethylene is applied to one side only, usually at 12 to 25 g/m². It is used for wrappers, bags, and interleaving sheets where grease approaches primarily from the coated side. Specimens must be clearly oriented so that the coated face is the tested surface.

Double-Side PE-Coated Neutral Paper

In this configuration, both faces of the paper are laminated, with coating weights typically ranging from 15 to 40 g/m² per side. It is used for trays, bowls, and containers that hold hot, oily foods. Grease resistance is evaluated independently on each coated surface, since the two extrusion layers may exhibit different quality levels.

High-Grammage Coated Board

For rigid packaging such as clamshells and paper trays, base grammage may exceed 250 g/m². Although the thick substrate provides a long penetration path, the PE coating remains the governing barrier, and the same grease resistance test protocols apply.

Testing Items

A complete evaluation of neutral PE-coated paper typically includes the following items, with grease resistance as the core focus:

• KIT grease resistance rating (grade 1 to 12)
• Turpentine penetration resistance time
• Hot oil resistance at elevated temperature
• PE coating weight and distribution uniformity
• Coating continuity and pinhole detection
• Heat seal strength of the PE layer
• Interlayer bond (peel) strength between PE and base paper
• Tensile strength and bursting strength of the laminate
• Surface pH or cold-water extract pH for neutrality verification
• Food contact compliance, including fluorescent whitening agents and heavy metal migration

Testing Methods

KIT Test (TAPPI T559 / ISO 16532-1)

The KIT test is the most widely referenced method for grading grease resistance. Twelve standard test liquids are prepared using different ratios of castor oil, toluene, and n-heptane, where higher numbers correspond to increasingly aggressive solvent action. A drop of each solution, starting from the lowest grade, is placed on the coated surface of a specimen backed by a dark, non-absorbent plate. After a dwell time of 15 to 30 seconds, the drop is wiped away and the surface is examined against light for darkening, transmission, or attack marks. The KIT rating is the highest numbered solution that the specimen resists.

For neutral PE-coated paper used in greasy food applications, a KIT rating of 6 to 8 is generally considered the minimum acceptance threshold, while premium grades for fried food packaging often achieve ratings of 10 to 12. Testers should note that the toluene fraction in high-number KIT solutions can soften polyethylene, so results must be interpreted as a measure of overall barrier performance rather than pure oil resistance.

Turpentine Penetration Test (TAPPI T454 / DIN 53116)

In this method, a specimen is laid flat on a sheet of white blotting paper that acts as an indicator backing. A glass ring is placed on the specimen, and a measured volume of turpentine saturated with a red dye such as Sudan III or Sudan IV is poured inside the ring, often with a small quantity of sand and a standardized weight applied on top to ensure intimate contact. The elapsed time until the red dye becomes visible on the blotting paper is recorded. Longer times indicate better grease resistance; high-performance greaseproof grades commonly exceed 1800 seconds, while coated packaging grades are typically specified to resist penetration for a defined minimum period relevant to the intended food contact duration.

Hot Oil Resistance Test

Because many packaged foods release oil at elevated temperatures, practical testing often supplements the standard methods. A drop of dyed palm oil, rapeseed oil, or the actual food fat is applied to the horizontal specimen maintained at 40 °C, 60 °C, or 80 °C, and the sample is observed for penetration, blistering, or coating delamination over a specified period. This test effectively simulates real service conditions such as freshly fried food placed directly into a wrapper or tray.

Supporting Physical and Chemical Tests

Coating weight is determined by gravimetric difference or solvent delamination and must be uniform across the web, since grease failure almost always originates at thin spots. Pinholes are detected using dyed turpentine or copper sulfate solutions over a standardized exposure area. Heat seal strength is measured per ASTM F88, and interlayer peel strength confirms the integrity of the PE-to-paper bond. Neutrality is verified by surface pH or extract pH testing per TAPPI T529 or ISO 6588, and food contact safety is assessed against regulations such as FDA 21 CFR 176.170 or the applicable national food contact material standards.

Applications

Neutral PE-coated paper with verified grease resistance is used across a broad range of packaging scenarios:

• Fast food wrappers for hamburgers, sandwiches, burritos, and shawarma
• Bakery bags and wrappers for breads, pastries, cookies, and croissants
• Paper trays, bowls, and clamshells for fried chicken, french fries, and snacks
• Airline catering and takeaway food containers
• Microwave-ready and frozen food packaging
• Interleaving sheets for cheese, cured meats, and other fatty products
• Pet food and oily snack pouches and liners

FAQ

What KIT rating is required for food packaging?

For general bakery and sandwich applications, a KIT rating of at least 6 is usually acceptable. For fried, oily, or hot foods, ratings of 8 or higher are recommended, and premium greaseproof grades reach 10 to 12.

What does "neutral" mean in neutral PE-coated paper?

It means the base paper is acid-free, with a pH of approximately 6.0 to 8.0 in water extract. This prevents acid-catalyzed embrittlement over time and supports compliance with direct food contact requirements.

How does PE coating weight affect grease resistance?

Higher coating weight generally improves barrier reliability by reducing the probability of pinholes and thin spots. In practice, single-side coatings below roughly 14 g/m² carry a higher risk of discontinuity, so coating weight should always be verified together with grease resistance results.

Why do KIT and turpentine results sometimes disagree?

The two methods use different challenge media. KIT solutions contain toluene, which attacks polyethylene chemically, while turpentine primarily tests physical penetration. Disagreement often reveals coating defects rather than true oil failure, so both methods together give a more complete picture of quality.

What sample conditioning is required before testing?

Specimens should be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours before any grease resistance or physical testing, as moisture content measurably affects barrier and strength performance.