Evaporation residue determination is a fundamental safety and quality test for neutral PE-coated paper, a composite material widely used in food contact packaging such as cups, bowls, boxes, and wrapping sheets. This technical article explains the testing principle, food simulants, standard procedures, calculation methods, and acceptance criteria for measuring evaporation residue in neutral coated paper, providing a practical reference for laboratories, manufacturers, and quality engineers.
Technical Overview
Neutral PE-coated paper is a composite material produced by extrusion-laminating a layer of polyethylene (PE) onto one or both sides of a paper substrate. The term "neutral" indicates that the base paper is manufactured under pH-neutral conditions without acidic or alkaline chemical treatments, which makes the material particularly suitable for direct contact with food, pharmaceuticals, and other sensitive products. The PE layer provides heat-sealability, water resistance, and liquid barrier performance, while the paper layer contributes stiffness, printability, and formability.
Evaporation residue, also referred to as total migration or overall migration, is defined as the mass of non-volatile substances that transfer from the coated paper into food simulants under specified time and temperature conditions. These substances may include plastic additives, processing aids, coating chemicals, sizing agents, fillers, and low-molecular-weight polymer fractions. An excessive evaporation residue value indicates that an unacceptably large quantity of migrateable matter could contaminate packaged food, so this parameter serves as a decisive pass/fail criterion in food contact material regulations.
In China, the determination is carried out in accordance with GB 31604.8 for the measurement of total migration quantity, supported by GB 5009.156 and GB 31604.1 for the selection of food simulants and migration conditions, while the compliance limits for paper and board materials are established in GB 4806.8. International equivalents include the overall migration methods of the EN 1186 series under EU Regulation (EU) No 10/2011 and the total extractives tests described in FDA 21 CFR Sections 175 through 177.
Testing Samples
Raw Material Rolls
Unprinted neutral PE-coated paper rolls are sampled from representative positions across the web width. Specimens are cut into rectangular pieces, the exposed area is measured precisely, and both single-side coated and double-side coated structures are tested to characterize the base material before conversion.
Converted Finished Products
Finished articles such as paper cups, soup bowls, lunch boxes, trays, sandwich wraps, and packaging bags are tested in their final commercial form. When possible, these articles are tested by the fill-and-test method, in which the food simulant is placed directly into the article so that the real contact area, sealed seams, and lidding interfaces are all evaluated.
Laboratory-Prepared Specimens
When articles cannot be filled, specimens are cut into strips of approximately 2 cm by 5 cm, ensuring that the total contact area is known and that the area-to-volume ratio is fixed. Printed, inked, or laminated zones are included in the test whenever they are in contact with food in actual use, because printing components can contribute significantly to the migration result.
Testing Items
A complete evaporation residue evaluation of neutral PE-coated paper typically covers the following migration scenarios, each using a different food simulant selected according to the intended end use of the material:
- Evaporation residue in water, simulating neutral aqueous foods and beverages such as drinking water, milk, and tea.
- Evaporation residue in 4% aqueous acetic acid, simulating acidic foods such as juice, vinegar-based sauces, and fermented products.
- Evaporation residue in ethanol solutions, simulating alcoholic beverages and water-based foods containing ethanol; common concentrations include 10%, 20%, 50%, and 95%.
- Evaporation residue in n-hexane or isooctane, simulating fatty foods and oil-containing products such as fried snacks, salad dressings, and edible oils.
- Supporting indicators frequently tested alongside evaporation residue, including potassium permanganate consumption, heavy metals expressed as lead, formaldehyde, fluorescent whitening agents, and pH of the aqueous extract.
Testing Methods
Principle
The test specimen is immersed in a food simulant under controlled time and temperature conditions that reproduce or deliberately exaggerate the intended use of the material. After the migration period, a measured aliquot of the simulant is transferred to a pre-conditioned evaporating dish, the solvent is evaporated, and the dish is dried to constant mass in an oven. The mass of the dry residue, corrected by a blank determination performed on the pure simulant, represents the total quantity of non-volatile migrateable substances.
Apparatus and Reagents
- Analytical balance with a readability of 0.1 mg.
- Constant-temperature water bath or incubator capable of maintaining the selected migration temperature within plus or minus 2 degrees Celsius.
- Quartz or borosilicate glass evaporating dishes, pre-dried and weighed to constant mass.
- Electric drying oven operating at 105 degrees Celsius, with temperature uniformity control.
- Desiccator with fresh silica gel for cooling dishes before weighing.
- Food simulants prepared from analytical-grade glacial acetic acid, anhydrous ethanol, and n-hexane, diluted with laboratory-grade water.
- Glassware: graduated cylinders, pipettes, beakers, and stoppered glass migration cells.
Specimen Preparation and Migration Conditions
For cut specimens, the total contact area is measured and the simulant volume is added at a fixed ratio, commonly 2 mL of simulant per square centimeter of specimen area, ensuring complete immersion without overlapping surfaces. For fillable articles, the simulant is filled to at least four fifths of the usable capacity. Typical migration conditions selected according to the declared use of the material include immersion at 60 degrees Celsius for 30 minutes for general room-temperature and warm applications, immersion at 95 to 100 degrees Celsius for 30 minutes for hot-fill or hot-beverage applications, immersion at 70 degrees Celsius for 2 hours for long-term warm storage, and immersion in n-hexane at room temperature for 2 hours for fatty-food contact. All migration vessels are covered to prevent solvent loss and environmental contamination.
Evaporation, Drying, and Weighing
After migration, the simulant is cooled and mixed thoroughly. A measured aliquot, commonly 200 mL, is transferred to the tared evaporating dish and concentrated gently on a water bath until near dryness. The dish is then dried in the oven at 105 plus or minus 5 degrees Celsius for 2 hours, cooled in the desiccator for 30 minutes, and weighed to 0.1 mg. The drying, cooling, and weighing cycle is repeated until the difference between two consecutive weighings is no greater than 0.5 mg. A blank determination using the same volume of pure simulant, processed identically, is performed in parallel with every batch, and the blank residue is subtracted from the sample residue.
Calculation
The evaporation residue expressed per unit contact area is calculated as X = (m1 - m2) × V / (V1 × S), where m1 is the residue mass of the sample aliquot in milligrams, m2 is the residue mass of the corresponding blank in milligrams, V is the total volume of simulant used in the migration test, V1 is the aliquot volume taken for evaporation, and S is the total contact area in square decimeters. The result is expressed in milligrams per square decimeter. For articles tested by the fill-and-test method, the result may alternatively be expressed in milligrams per liter using the fill volume, which is then converted per unit area or per kilogram of food according to the applicable product standard. Results are reported as the mean of at least two parallel determinations, and the relative deviation between parallels should not exceed 20%.
Quality Control and Precautions
- All glassware must be scrupulously cleaned and rinsed with simulant to eliminate background contamination that can inflate residue values.
- n-Hexane migration must be performed below its boiling point in a well-ventilated fume hood due to flammability.
- Specimen area measurement must include both faces and cut edges exposed to the simulant.
- Excessive drying temperatures or prolonged drying can volatilize low-boiling residues and bias results low.
- The blank value must be small and stable; a high or variable blank indicates reagent or glassware contamination.
- Conditioning of the paper before testing, including temperature and humidity equilibration, should follow the standard to ensure reproducible moisture content.
Applications
Evaporation residue determination for neutral PE-coated paper supports multiple stages of the product life cycle, from material development to market surveillance:
- Regulatory compliance testing and certification for food contact materials intended for the Chinese market, the European Union, the United States, and other jurisdictions.
- Incoming material inspection and routine quality control by converters producing paper cups, bowls, boxes, and flexible packaging.
- Import and export inspection by customs laboratories and third-party testing organizations.
- Research and development, including comparison of different PE grades, coating weights, adhesion promoters, and base paper formulations.
- Failure analysis when off-odor, taste transfer, or visible contamination of packaged food is reported by end users.
- Validation of production changes such as switching resin suppliers, adjusting extrusion temperature, or introducing water-based coatings.
FAQ
What is the acceptance limit for evaporation residue of neutral PE-coated paper?
Under GB 4806.8, the total migration from paper and board intended for food contact shall not exceed 10 mg per square decimeter of contact area. Some legacy product standards based on the older GB/T 5009.60 method apply a limit of 30 mg per liter for water, 4% acetic acid, and ethanol simulants, and 30 mg per liter for n-hexane. The applicable limit depends on the governing product standard and the declared end use, so laboratories should always confirm the regulatory basis before reporting.
Why must several different food simulants be tested?
Real foods differ greatly in polarity, acidity, alcohol content, and fat content, and the migration behavior of substances in the coated paper varies accordingly. Water, acetic acid, ethanol, and n-hexane each represent a worst-case food category, so testing multiple simulants ensures that the material is safe for its entire declared range of applications.
Does the "neutral" character of the paper influence the test result?
Yes. Because neutral paper avoids acidic and alkaline process chemicals, its aqueous extract residues and pH-related interactions with acidic or alkaline simulants are typically lower. However, the PE coating, printing inks, and adhesives can still dominate the evaporation residue, so full multi-simulant testing remains necessary regardless of the neutral classification.
How are materials intended for hot-fill or microwave use tested?
The migration conditions must match or exceed the most severe intended use. For hot-beverage and hot-fill applications, aqueous simulants are typically held at 95 to 100 degrees Celsius for 30 minutes, and the same conditions are applied to all simulants relevant to the declared food types. Testing at milder conditions than actual use is not acceptable for compliance purposes.
How long does a typical determination take?
A single determination, including specimen preparation, migration, evaporation, drying to constant mass, and cooling, generally requires one to two working days. Multi-simulant programs with parallel determinations and blanks are usually completed within three to five working days, allowing additional time for repeat tests if any parallel deviation exceeds the permitted tolerance.