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Chemical Migration Testing for Bags

What chemical migration actually means

Migration is the transfer of a substance from a material into something else: skin, sweat, saliva, food or a simulant liquid standing in for one of those. It is a rate and an amount, not a composition. A substance locked into a polymer matrix at 5% by weight may migrate at effectively zero if it is chemically bound, while a substance at 0.01% that sits free on the surface can migrate quickly.

That distinction is why regulators increasingly test migration rather than content. Content testing answers the question what is in this material, which is useful for screening and for substance bans. Migration testing answers the question what will a person actually be exposed to, which is the question that predicts harm. For a bag that is handled daily, sweated on and sometimes chewed by a child, exposure is what matters.

Migration is also conditional. It changes with temperature, contact time, the nature of the contacting medium and the surface area in contact. A bag that passes a 2-hour test at 40 °C can fail a 10-day test at the same temperature, which is why the conditions printed on a report are as important as the result itself.

Why waterproof bags are a special case

A waterproof bag is a laminate: a polymer film bonded to a woven face, plus coatings, inks, adhesives, hardware and sometimes a foam or a window. Each layer can contribute a migrating substance, and the film that makes the bag waterproof is also the layer most likely to hold additives. Plasticisers, heat stabilisers, slip agents, UV absorbers and residual solvent all live there.

Contact is unusually close and unusually long. A dry bag is pressed against wet skin for hours, often at an elevated temperature inside a wetsuit or against a back panel. Warm, damp skin is a far better extraction medium than dry contact, and the sweat that collects under a backpack strap is mildly saline and slightly acidic, which increases extraction further.

Then there is the food-contact case. Cooler-lined dry bags, picnic bags and any bag used to carry unpackaged food are subject to food-contact rules, and the simulants used there are aggressive. This is where most general-purpose bags fail, not because they are unsafe to carry but because nobody specified the food-contact condition.

The regulatory frame you have to design around

Four regimes come up repeatedly for bags: EU REACH with its candidate list of substances of very high concern, California Proposition 65 with its warning obligation, the US Consumer Product Safety Improvement Act with its lead and phthalate limits, and EN 71-3 where a product is marketed to children. Each has a different trigger, a different limit and a different consequence for non-compliance.

The important structural point is that these regimes overlap and do not agree. A bag can be fully REACH compliant and still need a Prop 65 warning, because California uses safe-harbour levels derived from a different methodology. Compliance is therefore a market-specific exercise, and a single certificate rarely covers a global programme.

Enforcement is also moving towards documentation rather than testing alone. A retailer increasingly wants a declaration, a technical file and traceable test reports dated within the last one or two years, not a single sample certificate issued years ago against a different production batch. Our overview of REACH compliance covers the documentation side in more detail.

Migration testing versus content testing

Content testing asks whether a restricted substance is present above a threshold, usually by extracting aggressively with a solvent and analysing the extract. It is fast, cheap and good for screening a long substance list. Its weakness is that it can flag substances that are chemically bound and pose no exposure, and it can miss a surface residue that is present below a content threshold but migrates readily.

Migration testing places the article, or a representative sample, in contact with a simulant under defined conditions and then analyses the simulant. It is slower and more expensive, typically two to ten days of contact plus analysis, and it needs more sample area. Its strength is that it measures exposure directly, which is why food-contact and children's product rules rely on it.

A sensible programme uses both. Screen by content testing to catch obvious problems across a wide list, then confirm by migration testing on the substances that matter for the intended use. Running only content testing leaves you exposed on exposure; running only migration testing on a short list leaves you exposed on substances you never thought to look for.

Simulants and conditions

A simulant is a stand-in liquid chosen to behave like the real contacting medium. Distilled water stands in for aqueous food and sweat, 3% acetic acid for acidic food, 10% or 50% ethanol for a range of foods, and a fatty simulant such as olive oil or isooctane for fatty food. For skin contact, artificial sweat at pH 5.5 and artificial saliva are the standard media.

Conditions are chosen to represent the worst realistic case in a compressed time. Ten days at 40 °C is the standard accelerated stand-in for long-term storage at room temperature; two hours at 70 °C or 100 °C represents brief hot contact. Choosing a milder condition than the real use is the most common way a report ends up misleading.

Surface area matters to the arithmetic. Results are normalised per square decimetre of contact area or per kilogram of simulant, and the convention in the EU is to assume 1 kilogram of food contacts 6 square decimetres of material. A small sample in a large volume of simulant dilutes the result, so check that the report states the area-to-volume ratio it used.

How a migration test is run, step by step

1. Cut a representative sample and record its exact contact area, edge sealing included.
2. Condition the sample; many standards require pre-conditioning at 23 °C and 50% relative humidity.
3. Place it in contact with the simulant by immersion or by a single-sided cell.
4. Hold at the specified temperature for the specified duration, in the dark if required.
5. Separate the sample, and analyse the simulant by ICP-MS, GC-MS or LC-MS depending on the analyte.
6. Normalise against contact area or simulant mass and compare with the limit.

The detail that catches people out is representativeness. A sample cut from the middle of a flat panel ignores the printed logo, the welded seam, the edge trim and the hardware, all of which are far more likely to be the source of a migrating substance. Test the whole article where the standard allows it, or test each component separately and add the contributions.

Blanks matter too. Every run needs a reagent blank and, ideally, a spiked recovery sample, because a result near the detection limit is only meaningful if you know the method recovered what it should have. A report that gives results without a recovery figure or a stated detection limit is not a report you can defend.

Detection limits and what a pass really means

A pass means below the limit under the stated conditions, and nothing more. If the laboratory detection limit is 10 mg per kilogram and the legal limit is 30 mg per kilogram, a reported pass at below 10 tells you the material is compliant with margin. If the detection limit is 25 and the limit is 30, the same word pass carries almost no information.

This is why the detection limit column on a report deserves as much attention as the result column. Non-detect is not zero; it is below whatever the instrument could see. Ask for the limit of quantification for every analyte, and compare it with the regulatory threshold before you accept the certificate.

Batch variation is the other caveat. A test on one sample from one production run says nothing about the run made three months later from a different resin lot. Reasonable programmes test per colourway and per material, at least annually, and whenever a raw material source changes. Testing once at the start of a programme and never again is a common and expensive shortcut.

Substance classes that matter in bag materials

Some substance classes recur in bag testing because they are used for a purpose that is hard to replace. Plasticisers make PVC flexible. Heat stabilisers stop it degrading during processing. Flame retardants and water repellents are added for performance. Aromatic amines can appear as breakdown products of certain azo dyes. Metals arrive in pigments, in hardware plating and in some catalysts.

The good news is that a well-specified TPU laminate avoids most of this list by construction. TPU is inherently flexible, so it needs no plasticiser; it is not made with phthalate plasticisers at all, which is the subject of phthalate-free TPU. That single material decision removes the largest single migration risk in soft waterproof goods.

Hardware is the second priority, because plated metal and some plastic mouldings carry lead, cadmium and nickel. RoHS-style screening catches most of it, and the same logic applies to bags as to electronics; see RoHS and dry bag hardware for the practical approach to specifying buckles, D-rings and zips.

Recycled content deserves particular care in this table. Post-consumer recycled polymer can carry legacy additives from its previous life, which is exactly why recycled streams intended for food contact are strictly controlled. Ask for the source and the screening history on any recycled resin, and test the finished article rather than assuming a virgin-resin result carries across.

Food contact and cooler bags

If a bag is sold or used to carry unpackaged food, food-contact rules apply and the testing gets stricter. The EU plastics regulation sets an overall migration limit of 10 milligrams per square decimetre, or 60 milligrams per kilogram of food, plus specific migration limits for individual substances that are often far lower. A declaration of compliance with traceability to the materials used is mandatory.

The failure mode here is usually the liner rather than the shell. A cooler-lined roll-top may have a compliant outer TPU and a liner with an adhesive, a print or a foam that migrates freely into a fatty simulant. Test the liner, and test it against the fatty simulant rather than water, because plasticisers and antioxidants are fat-soluble and water will not show them.

Temperature changes the answer sharply. A cooler bag holding food at 4 °C is a mild case, but the same bag left in a 35 °C car is a demanding one, and accelerated testing at 40 °C for ten days is the standard representation of that. Full guidance on liner selection is in food-safe dry bag materials.

Reading a report and spotting weak ones

A defensible report names the laboratory and its accreditation, the sample description and date, the standard and condition used, the analytes with their limits of quantification, and the results with units. Missing any one of those and the report is decorative. In particular, an undated report or one with no sample description cannot be tied to a production batch.

Watch for scope language. Reports often state that results relate only to the items tested, which is honest and important: a certificate for a 420D black panel says nothing about a printed orange panel on the same bag. Ask for per-colourway testing, or for a worst-case justification that names the colour or component selected and why it is the worst case.

Also check who commissioned the report and when. A report commissioned by a raw material mill three years ago describes a resin, not a finished bag, and it predates any printing, welding, adhesive or hardware. Finished-article testing is more expensive and worth it; the United States Environmental Protection Agency publishes reference material on chemical assessment that is useful context when you are setting your own thresholds.

Setting a chemical specification for a programme

Write the requirement before sourcing rather than after. A compact specification states the markets, the applicable regimes, the substance list with limits, the test standard and condition per use case, the detection limits you require, the sampling frequency, and who holds the technical file. Six lines of text that remove months of argument later.

Specify worst-case selection explicitly. Name the colourway and component to be tested, usually the darkest print or the softest plastic, and require that a pass on that item covers the range. This avoids the common situation where a compliant sample is submitted and production ships something different.

Require change control. Any change of resin, coating, ink, adhesive or hardware source should trigger re-testing, and that obligation belongs in the specification rather than in goodwill. Dimensional and chemical measurement conventions are maintained by the ISO, and naming the standard in the document removes ambiguity about method.

Our production programme has run waterproof bags since 2014 in an SGS-verified 4,950 m2 facility with 137 staff, and restricted-substance control is built into that flow: resin lots are screened on arrival, inks and adhesives are declared against the target market, and finished bags are tested per colourway. Minimum order is 500 pieces with sampling in 6 to 10 working days and bulk in 35 to 50 days FOB Xiamen.Compliance documentation travels with the shipment rather than following it.

Reducing migration risk at the design stage

Material selection does most of the work. Choose TPU over PVC and you remove the plasticiser question entirely. Choose solution-dyed face fabric and you remove most of the azo dye risk. Choose water-based ink over solvent-based and you remove residual solvent, which is covered in water-based printing ink. Each choice is cheaper than testing for a problem you have designed in.

Curing and processing matter as much as formulation. An adhesive or an ink that has not been fully cured will migrate for weeks after production, and a bag shipped immediately after printing can fail a test it would have passed a fortnight later. Build a post-production hold into the schedule for heavily printed programmes.

Finally, reduce contact where you can. A barrier layer between a printed panel and the food or skin side of a bag costs very little and removes an exposure route entirely. Design-level thinking like this is why independent textile certification schemes exist, and OEKO-TEX and textile safety explains what those labels do and do not cover.

Chemical compliance and sustainability reporting should still be kept separate. A bag can pass every migration test and carry a large carbon footprint, and a bag with high recycled content can fail a migration test if its feedstock is not controlled. Report both honestly, and never let one stand in for the other.

Chemical compliance checklist

• Name the markets first; REACH, Prop 65, CPSIA and EN 71 do not agree on limits.
• Screen by content testing, then confirm by migration testing on the use-relevant substances.
• Match the simulant and condition to real use: fatty simulant for food, sweat at 37 °C for skin.
• Require limits of quantification well below the regulatory limits, not just a pass.
• Test the worst-case colourway and the liner, not a plain panel from the middle of a roll.
• Build change control so a new resin, ink or hardware source triggers re-testing.

Worked example: a European resort ordered 4,000 printed dry bags and assumed an existing REACH declaration covered them. The declaration was for the base fabric, issued four years earlier, and did not cover the new four-colour print or the plated D-rings. Finished-article testing found lead at 180 parts per million in the D-ring plating, above the 100 ppm limit for the market. The hardware was re-specified and re-tested before shipment, at far less cost than a recall.

The wider compliance picture is summarised in REACH compliance for dry bags, and the material choices that avoid most of these problems are listed under eco-friendly dry bag options. Current material declarations for each model are available on the product pages, and printed-panel care, which affects migration over time, is covered in caring for printed dry bags.