Decolorization testing is a critical safety and quality evaluation that determines whether colorants, pigments, or dyes can migrate out of plastic bottles into contacting media such as food, beverages, cosmetics, or pharmaceutical products. This technical article explains the principles of plastic bottle decolorization testing, typical sample types, test items, standardized methodologies, evaluation criteria, and the international standards that accredited third-party laboratories apply when verifying compliance.
Technical Overview
Decolorization testing, also referred to as color migration or color fastness testing, is a laboratory evaluation designed to determine whether colorants incorporated into plastic bottles—organic dyes, inorganic pigments, and masterbatch carriers—can migrate from the polymer matrix to external surfaces or into contacting liquids. The test simulates real-world contact scenarios by exposing the bottle surface or the complete article to food simulants, oils, or solvents, and then assessing whether any visible or measurable color transfer occurs.
Color migration can occur through several mechanisms: solubility-driven bleeding of soluble dyestuffs into fatty or alcoholic media, poor pigment dispersion or inadequate pigment fixation at the polymer surface, blooming of low-molecular-weight colorant components over time, and cross-contamination introduced during molding, printing, or mechanical recycling. From a regulatory standpoint, migrating colorants raise two principal concerns: the potential ingestion of substances that are not approved for food contact, and the co-migration of impurities such as heavy metals or primary aromatic amines associated with certain pigments. For this reason, food contact regulations worldwide—including the GB 4806 series in China, the European plastics framework, and the colorant provisions of the U.S. Code of Federal Regulations (21 CFR 178.3297)—require that colorants remain firmly bound within the polymer and that no visible transfer occur under standardized test conditions.
Testing Samples
PET Beverage Bottles
Polyethylene terephthalate (PET) bottles for water, carbonated soft drinks, juices, and edible oils are among the most frequently tested sample types. Testing covers both clear and colored containers, including amber, green, and blue bottles, and may be performed on finished bottles, preforms, or cut specimens. Because PET processing employs antimony-based catalysts, decolorization programs for PET often include complementary migration checks for residual catalyst elements.
HDPE and LDPE Bottles
High-density and low-density polyethylene bottles used for dairy products, detergents, and cosmetic formulations are commonly pigmented with inorganic colorants. The softer polyolefin surface and the relatively high loading of masterbatch in white or colored bottles make these materials a standard target for wipe-based decolorization assessments.
PP Bottles and Closures
Polypropylene bottles and matched closures are tested when the package is intended for hot-filling, sterilization, or repeated contact with oils and surfactants. Elevated service temperatures accelerate pigment bleed, so test conditions are typically selected to match or exceed the intended thermal exposure.
Recycled (rPET / PCR) Bottles
Bottles incorporating recycled content carry an elevated risk of color migration because post-consumer flake may retain printing inks, adhesives, and non-food-grade colorants. Decolorization testing of recycled-content bottles is used both for lot release and for validating decontamination and sorting processes within recycling operations.
Testing Items
A comprehensive decolorization program for plastic bottles typically encompasses the following items, with the exact scope determined by the end use and target market:
- Wipe (rubbing) decolorization test using vegetable oil and aqueous ethanol as contacting media
- Immersion color migration test using aqueous, acidic, alcoholic, and fatty food simulants
- Migration of heavy elements associated with pigments, including lead, cadmium, mercury, hexavalent chromium, arsenic, antimony, barium, and selenium
- Migration of primary aromatic amines relevant to azo-type colorants
- Overall migration into food simulants in accordance with EN 1186 and corresponding GB 31604 methods
- Specific migration of listed colorant-related substances against the migration limits of the applicable plastics framework
- Instrumental and visual assessment of color transfer, including gray scale staining evaluation in accordance with ISO 105-A03 and GB/T 251
Testing Methods
Wipe (Rubbing) Decolorization Test
In the wipe method, a piece of white cotton cloth or absorbent cotton is saturated with a standard contacting liquid—commonly soybean or olive oil and 65% aqueous ethanol—and rubbed against the external surface of the bottle for a defined number of back-and-forth strokes under standardized pressure. The cotton is subsequently examined in a standardized light environment, such as a D65 light box, for any visible pickup of color. The test is particularly effective at detecting surface blooming, poorly fixed pigments, and residues from printing or coating operations.
Immersion Migration Test
In the immersion method, specimens cut from the bottle body, closure, or complete small containers are fully immersed in food simulants selected according to the intended use of the package. Typical simulants include water, 4% acetic acid for acidic foods, aqueous ethanol solutions at concentrations such as 20%, 50%, or 65% for alcoholic products, n-hexane or isooctane for fatty foods, and vegetable oil or a modified polyphenylene oxide adsorbent as a fat simulant. Time and temperature conditions mirror the foreseeable use of the bottle—for example, 40 °C for 24 hours for long-term room-temperature storage, 60 °C for 10 days for extended shelf-life simulation, and elevated conditions such as 70 °C for 2 hours for hot-fill applications.
Evaluation and Pass Criteria
Following exposure, the simulant and the wiping medium are evaluated for coloration. Visual assessment is performed by trained inspectors under controlled illumination, and quantification can be carried out by ultraviolet-visible spectrophotometry when objective measurement is required. A passing result generally requires that no visible color transfer be observed on the cotton and that the simulant remains free of tinting; where a gray scale for staining is applied, a grade of 4 or higher is typically specified. Heavy element migration results must remain below the category-specific limits defined in standards such as EN 71-3 and ISO 8124-3 or below the specific migration limits of the applicable food contact framework, and the sum of non-listed primary aromatic amines is restricted to very low thresholds in the European regulatory context.
Applications
Decolorization testing supports compliance and quality assurance across the full life cycle of plastic bottle production and trade:
- Lot-release quality control at blow molding and injection molding plants for food and beverage packaging
- Compliance verification of cosmetic and personal care packaging that contacts creams, oils, and surfactant-based formulations
- Qualification of pharmaceutical oral liquid bottles against Chinese pharmaceutical packaging (YBB) standard requirements
- Assessment of household chemical packaging that contacts detergents, solvents, and aggressive formulations
- Export certification support for bottles shipped to markets governed by GB, EN, and U.S. regulatory frameworks, conducted through accredited third-party laboratories
- Incoming inspection of resins and color masterbatches, validation of new tooling, and qualification of recycled-content feedstocks
FAQ
What is the purpose of the decolorization test for plastic bottles?
The test verifies that colorants remain securely bound within the plastic and cannot transfer to foods, beverages, skin-contact products, or wiping media. It protects consumers from exposure to unapproved colorant substances and helps manufacturers demonstrate compliance with food contact and packaging safety regulations.
Which liquids are used in the test?
The most common media are vegetable oil (such as soybean or olive oil) and aqueous ethanol, typically at 65% concentration, for the wipe method. The immersion method uses food simulants such as water, 4% acetic acid, various ethanol-water mixtures, hydrocarbon solvents, and vegetable oil or an equivalent fat simulant, selected according to the product the bottle will contain.
What result constitutes a pass?
A bottle passes when no visible color transfer is detected on the cotton wiping medium and no tinting is observed in the food simulant after the specified time-temperature exposure. When instrumental grading is applied, a staining grade of 4 or better on the gray scale is generally required, and any heavy element or aromatic amine migration must remain below the applicable regulatory limits.
Do clear or natural bottles need decolorization testing?
Natural bottles without added colorants are usually exempt from the classic wipe decolorization test, since there is no dye or pigment to migrate. However, they may still require migration testing for other substances, and any printed, coated, or labeled areas should be evaluated separately.
How does recycled content affect test outcomes?
Recycled material can introduce residual inks, adhesives, and legacy colorants from post-consumer sources, increasing the probability of detectable color migration. Bottles containing recycled content are therefore tested more rigorously, and repeated decolorization testing is often used to validate the consistency of the recycling feedstock.
Which standards govern the test for exported bottles?
Depending on the destination market, the relevant frameworks include the GB 4806 series and GB 31604 method standards for China, the European plastics regulation with EN 1186 and EN 13130 method standards, EN 71-3 and ISO 8124-3 where toy contact applies, and the colorant provisions of 21 CFR 178.3297 for the United States. Accredited third-party laboratories select the appropriate standard combination based on the product type and target market.