The decolorization test for packaging materials evaluates whether colorants, printing inks, pigments, and coatings can migrate or transfer from packaging into foods, food simulants, or contacting media under simulated real-world conditions. As a core hygienic requirement under food-contact frameworks such as the GB 4806 series, EU regulations, and FDA requirements, it helps manufacturers prevent unsafe dye migration, protect product integrity, and demonstrate regulatory compliance. This article explains the technical principles, sample types, test items, methods, and applications of decolorization testing for packaging materials.
Technical Overview
The decolorization test, also known as a color bleeding or color migration test, is a laboratory procedure used to determine whether colored packaging materials release their colorants into contacting media. Packaging is frequently pigmented, printed, lacquered, or dyed, and if the colorants are not sufficiently locked into the material matrix, they can bleed into food, beverages, pharmaceuticals, cosmetics, or onto skin during handling and storage. Because many legacy dyes and pigments contain unapproved substances or heavy-metal compounds, uncontrolled color migration is treated as both a safety hazard and a quality defect.
From a regulatory standpoint, the test supports compliance with the Chinese GB 4806 series of national food-contact material standards, which prohibit visible color transfer from packaging into food simulants and require colorants to comply with the positive list in GB 9685. Equivalent expectations exist internationally under EU Framework Regulation (EC) No 1935/2004, Plastics Regulation (EU) No 10/2011, and FDA food-contact regulations in 21 CFR. Methodologically, the test draws on classical hygiene analysis methods such as GB/T 5009.60, paper and board color fastness standards such as EN 646, and rub fastness methods such as ISO 105-X12 / GB/T 3920 and ASTM D5264 for printed surfaces.
Testing Samples
Plastic Packaging
Flexible films, rigid containers, bottles, caps, closures, trays, and sachets produced from polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polystyrene (PS), polyvinyl chloride (PVC), and other polymers. Both the base resin pigmentation and any surface printing are evaluated.
Paper and Paperboard
Folding cartons, paper cups, paper tableware, wrapping papers, and molded pulp products. For these substrates, the test also targets water-soluble dyes and fluorescent whitening agents that may leach into aqueous media, as required by the GB 4806 paper and board standard.
Printed Packaging and Labels
Surface-printed and reverse-printed films, self-adhesive labels, shrink sleeves, and in-mold labeled containers. Migration from ink layers, overprint varnishes, and adhesives is a primary concern, particularly for low-migration ink systems used in food packaging.
Laminates, Foils, and Coated Materials
Multilayer laminates, lacquered or printed aluminum foils, metallized films, and internal can coatings, where colorants may be embedded in coating layers that contact the product directly.
Other Flexible and Textile-Based Packaging
Woven polypropylene bags, nonwoven shopping bags, jute or cotton sacks, and dyed secondary packaging where color transfer to hands, garments, or adjacent goods must be controlled.
Testing Items
Typical items assessed within a decolorization test program include:
- Color bleeding into cold water and hot water soaking solutions
- Color bleeding into 4% acetic acid (acidic food simulant)
- Color bleeding into ethanol solutions, commonly 20% to 65% (alcoholic food simulant)
- Color bleeding into n-hexane or vegetable oil (fatty food simulant)
- Color transfer to absorbent cotton during rubbing with vegetable oil or ethanol
- Dry and wet crocking (rub) fastness of printed or dyed surfaces
- Ink rub and abrasion resistance of printed packaging surfaces
- Color migration under accelerated thermal or storage conditions
- Presence of water-soluble dyes and fluorescent whitening agents in paper substrates
Testing Methods
Immersion (Soaking) Test
Specimens with defined surface area are fully immersed in food simulants under standardized time and temperature conditions, for example 60 °C for 2 hours in water or 4% acetic acid, room temperature for 2 hours in ethanol solutions, and room temperature for about 1 hour in n-hexane, with actual conditions selected according to the governing standard and intended end use. After exposure, the soaking solutions are compared against reagent blanks: any visible tint, haze, or colored extract indicates decolorization and constitutes a failure. For quantitative evaluation, the absorbance or color difference of the extract can be measured spectrophotometrically.
Cotton Wiping (Rubbing) Test
In the classical method derived from GB/T 5009.60 and the legacy hygiene standards it supports, a piece of absorbent cotton impregnated with vegetable oil or 65% ethanol is pressed against the colored or printed surface and rubbed back and forth with moderate force, typically 100 strokes. The cotton is then examined for staining; any visible color pickup demonstrates that colorants are not adequately fixed in the material.
Crockmeter (Color Fastness to Rubbing) Test
For dyed and printed packaging where surface fastness is critical, a crockmeter applies a standardized rubbing finger covered with white cotton fabric under a fixed load, in both dry and wet modes, following ISO 105-X12 / GB/T 3920 or the equivalent JIS L 0849 procedure. Staining of the cotton fabric is graded against the grey scale for staining in accordance with GB/T 251 / ISO 105-A03, with grade 4 or higher commonly required for acceptance.
Printed Surface Rub Resistance Test
Printed cartons, labels, and films are subjected to reciprocal rubbing against a counter substrate, either face-to-face or face-to-back, using Sutherland-type or similar rub testers per ASTM D5264. Ink density loss, color difference, and visible transfer are measured before and after the specified number of cycles to quantify abrasion and blocking resistance.
Evaluation and Acceptance Criteria
Visual assessment is performed under controlled D65 illumination by trained examiners, supported where necessary by instrumental color measurement. The generally accepted pass criteria are: no visible coloring of any soaking solution compared with the blank, no visible color transfer to the rubbing cotton or fabric, and, where grey scale grading is applied, a staining grade of at least 4. Quantitative limits, where specified, are defined in the applicable product standard or in customer specifications.
Applications
Decolorization testing is applied across the full packaging value chain:
Food and beverage packaging compliance: demonstrating conformity with GB 4806 requirements for the Chinese market, EU food-contact regulations for European exports, and FDA 21 CFR provisions for the United States.
Pharmaceutical and healthcare packaging: verifying that printed blister foils, labels, and closures do not transfer color to dosage forms or to handling surfaces.
Cosmetics and personal care packaging: confirming that colored bottles, compacts, and pumps resist bleeding into formulations and onto skin.
Printing process control: validating ink selection, curing degree, corona treatment levels, and overprint varnish performance during development and production.
Supplier qualification and incoming inspection: releasing batches of films, inks, and printed materials against agreed fastness specifications.
Recycled and sustainable materials: assessing whether dyes from recycled fibers or post-consumer resins migrate into packaged products.
FAQ
What is the decolorization test for packaging materials?
It is a hygienic and quality test that determines whether colorants in packaging can migrate into food simulants, oils, solvents, or onto rubbing media under simulated use conditions. A positive result, meaning visible color transfer, indicates that the colorant system is not compliant or not adequately fixed.
Which standards govern the test?
Key references include the GB 4806 series for food-contact materials in China, GB 9685 for the colorant positive list, GB/T 5009.60 for classical soaking and wiping methods, EN 646 for color fastness of paper and board, ISO 105-X12 / GB/T 3920 and JIS L 0849 for rub fastness, and ASTM D5264 for rub resistance of printed surfaces. Migration testing frameworks such as GB 31604.1 define the simulant conditions used for exposure.
What simulants and conditions are typically used?
Common simulants are water, 4% acetic acid, 20% to 65% ethanol, and n-hexane or vegetable oil. Exposure conditions, typically combining temperatures from room temperature up to 60 °C or higher with dwell times of one to two hours or more, are selected according to the intended use of the packaging, such as hot-fill, refrigerated, or ambient storage.
What are the pass criteria?
The standard acceptance criteria are that soaking solutions remain visually identical to reagent blanks and that absorbent cotton or fabric used in rubbing tests shows no visible staining. Where grey scale grading applies, a staining grade of at least 4 out of 5 is the usual requirement.
How does decolorization testing differ from overall migration testing?
Overall migration quantifies the total mass of substances released into simulants, expressed in mg per square decimeter, while the decolorization test is a qualitative pass or fail assessment focused specifically on the transfer of visible color. The two are complementary and are often performed together on the same extracts.
Can the test be performed quantitatively?
Yes. Extracts can be analyzed by UV-visible spectrophotometry or by measuring instrumental color difference against blanks, and rub tests can be quantified by color difference or optical density change of the printed surface. Quantitative approaches are useful for research, specification setting, and dispute resolution.
How should testing be arranged?
Manufacturers should submit representative production samples, including both the base material and the printed or colored areas, together with a declaration of the intended use conditions. Accredited third-party laboratories with food-contact and packaging testing capability can then select the applicable standards, conduct the exposure and evaluation, and issue test reports suitable for regulatory submission or customer acceptance.