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Phthalate-Free TPU Explained

What phthalates actually do inside a soft plastic

Phthalates are diesters of phthalic acid used as external plasticisers. They sit between polymer chains, push them apart and let them slide, which turns a rigid resin into a flexible sheet. In PVC, that is essential: unplasticised PVC is a stiff pipe material, and the same resin with 30 to 40 parts of plasticiser per hundred parts of resin becomes the soft, drapable film used in low-cost waterproof bags.

The problem is that they are not chemically bonded. An external plasticiser is a physically held additive, so it can migrate to the surface, transfer to whatever the film touches, and volatilise slowly into the air inside a closed bag. That mobility is exactly the property regulators are concerned about, and it is also why the characteristic sweet smell of a new PVC bag fades over a season as the plasticiser leaves the film.

Restricted phthalates are the ortho-phthalates with short to medium side chains: DEHP, DBP, BBP, DINP, DIDP, DNOP and a longer list that now includes DIBP, DIHP and DHNUP in several jurisdictions. Not every phthalate is restricted, and that distinction is where most conflated marketing language originates, so a serious specification names the substances and the limit rather than relying on a single adjective.

Why TPU does not need a plasticiser to be flexible

Thermoplastic polyurethane is a block copolymer. Hard segments formed from a diisocyanate and a short chain extender cluster into physical crosslinks; soft segments formed from a long polyol chain sit between them and give the material its elasticity. Changing the ratio of the two shifts the same polymer family across a wide hardness range, so a 70 Shore A film for a soft roll-top mouth and a 95 Shore A film for an abrasion panel are both achievable without adding anything to the melt.

That is the whole argument for phthalate-free TPU: the softness is structural, not additive. It also means the film is stable in the two ways a plasticised film is not. It does not stiffen as an additive leaves, and it does not transfer an oily residue to the inside of a bag or to the gear stored in it over a summer in a hot car.

The honest caveat is that TPU is not automatically phthalate-free. A TPU compound can be modified with a plasticiser to hit a very soft hand feel or to reduce cost, and a laminated fabric can carry a plasticised adhesive layer between the film and the face textile. Asking whether the compound is plasticiser-free is a real question, and it belongs in the specification rather than in the brochure.

The regulation map and the 0.1% threshold

Three frameworks do most of the work for bags sold in the EU, the UK and the United States. REACH Annex XVII restricts the listed phthalates to 0.1% by weight of the plasticised material in articles; CPSIA sets 0.1% for children's products, permanently for DEHP, DBP and BBP and with an interim rule for DINP, DIDP and DNOP; and California Proposition 65 requires a warning unless exposure stays below a safe harbour level, which for DEHP is a maximum allowable dose level expressed in micrograms per day.

Scope is what catches importers out. A dry bag sold as general outdoor equipment is an adult article in most markets, but the moment it is marketed to children, packed as a school kit or sold with a lunch box, it becomes a children's product and the stricter rule applies. The same physical bag can be compliant in one channel and non-compliant in another purely because of how it is presented.

Enforcement practice matters as much as the text. Retailers and marketplaces increasingly ask for a declaration plus a test report rather than a declaration alone, and a report dated more than 12 months ago is frequently rejected. Background on the substance class and its health assessment is published by the US EPA, which is the reference most buyers cite when they set an internal restricted substances list.

How a phthalate-free claim is verified in a laboratory

Testing is a solvent extraction followed by gas chromatography with mass spectrometry detection. A laboratory cuts a representative sample, extracts it in an organic solvent with or without ultrasonic assistance, and quantifies against calibrated standards. The result is reported per substance in milligrams per kilogram, and the report states a limit of quantification, typically 10 to 50 mg per kilogram depending on the laboratory and the matrix.

Sampling design is where programmes succeed or fail. A single sample from one roll of film says nothing about the print, the zipper, the label or the buckle, so a credible test plan lists every component separately: film, face fabric, print, coating, webbing, hardware, zipper tape and any soft grip. The cost is in the number of components, which is why a full component list is requested before the quotation, not after it.

Standardised methods exist so that two laboratories can produce comparable numbers. Textile methods published by the ISO cover the determination of phthalates by gas chromatography, and the ASTM catalogue covers the related physical testing of coated and laminated fabrics, including the peel and hydrostatic tests that a compliance file usually bundles with the chemical result.

Where plasticisers hide in a waterproof bag

Assume the film is clean and then audit the rest of the bill of materials. Screen-printed and digital-printed inks are the most common hidden source, followed by soft touch coatings applied over a print, flexible PVC zipper pulls and cord ends, soft rubber logos, adhesive layers in a laminate, and coated hang tags or stickers that sit inside the bag.

The weight argument is often used to dismiss trim, and it is only partly valid. A printed logo might be 0.4% of the finished bag by weight, but the restriction applies to the plasticised material, not to the finished article, so a print layer that is 12% plasticiser is non-compliant even though the bag as a whole tests below 0.1%. This is why component-level testing beats finished-article testing for a restricted substance check.

Hardware is usually the cleanest part of a bag. Metal buckles, anodised eyelets and woven webbing are rarely plasticised, but painted or soft-coated hardware can be, and a powder coating on a spring hook has failed phthalate screens in the past. Our own hardware compliance notes are collected in the RoHS hardware guide, and the wider restricted substance picture sits in our REACH compliance guide.

Ester-based versus ether-based TPU

The soft segment chemistry decides how a film ages. Polyester-based TPU offers the best abrasion resistance and oil resistance at the lowest cost, and it is the default for a hard-wearing duffel. Polyether-based TPU resists hydrolysis and microbial attack far better and stays flexible at low temperature, which is why it is specified for bags that live in tropical humidity or that are stored damp.

Hydrolysis is the mechanism to understand. In warm, humid storage, ester linkages in the soft segment can be cleaved by water, and the film loses tensile strength and turns tacky at the surface. The practical trigger is sustained storage above roughly 40 °C with high relative humidity, not a wet weekend on a boat. Polycarbonate-based TPU resists this further and is used where a long warranty is offered.

Non-ortho plasticisers and what they do mean

DINCH, DOTP and ATBC appear constantly in compliance documents and are frequently presented as proof that nothing was substituted. They are non-ortho plasticisers: structurally different, not restricted as ortho-phthalates, and used to soften PVC or a PVC-blend film where a plasticiser is genuinely needed. Their presence is compatible with a phthalate-free claim if the claim is defined as ortho-phthalate-free, and incompatible if the claim was written as plasticiser-free.

That distinction is worth writing down explicitly in any specification. Two claims look the same on a hang tag and differ by an order of magnitude in what they require from the production line. If the programme genuinely needs no plasticiser of any kind, the film has to be a TPU, TPE or silicone system, and the print and coating partners have to confirm the same.

Odour, fogging and the new-bag smell

Odour is the property end users notice first and the one most weakly correlated with compliance. Plasticiser migration from PVC produces a sweet, slightly chemical smell that fades over weeks. Residual monomer, amine catalyst or a solvent-based print produces a sharper smell from a new TPU bag, and it usually clears within days of airing. Neither smell is a reliable indicator of whether a restricted substance is present.

Where odour has to be controlled, test it rather than describe it. A simple protocol is a sealed jar held at 40 °C for 24 hours followed by a panel assessment on a five-point intensity scale, and a documented threshold of three or below. Automotive-style fogging tests measure the mass of condensate collected on a cooled plate, which is a useful proxy for how much volatile material a film will deposit on a camera lens or a phone screen stored inside a sealed bag. Our guide to new bag odour covers the same ground for buyers comparing samples.

Food contact and migration testing

Most dry bags are not food-contact articles and are not tested as ones. The boundary case is a cooler-style roll-top, a lunch bag or a bag marketed with a picnic set, where a regulator or a retailer will expect a migration dataset. The European framework for plastic food-contact materials sets an overall migration limit expressed as milligrams per square decimetre or per kilogram of simulant, with specific limits for individual substances.

Migration tests use food simulants rather than food: an acidic simulant, an aqueous one, an alcoholic one and a fatty one, each at a defined time and temperature. A bag that passes a chemical content test can still fail a migration test because migration depends on contact time, temperature and the fat content of what is stored. This is the subject of our food-safe materials guide and our migration testing guide.

Reading a compliance file: what to request

Five documents turn a claim into a defensible position. Ask for a restricted substances declaration naming the substances and the limit; a test report dated within 12 months with the component list visible; a bill of materials that identifies every plasticised component; a statement of the film polymer family; and, where the bag is sold into a regulated channel, the specific conformity documentation that channel requires.

Then read the report rather than filing it. Check that the sample description matches the production bag, that the components listed cover print and trim, and that the limit of quantification is below the threshold you care about. A report with a quantification limit of 500 mg per kilogram cannot support a claim at 100 mg per kilogram, however clean the result looks at first glance.

• Restricted substances declaration naming each substance and the limit applied.
• Test report dated within 12 months, with the component list shown, not just the bag.
• Bill of materials flagging every plasticised or coated component.
• Statement of the film polymer family: polyester, polyether or polycarbonate TPU.
• Confirmation that print inks, coatings and adhesives meet the same limit as the film.
• Migration data if the bag will be sold as a cooler, lunch or picnic product.
• Sample the eco-friendly dry bag range against the same checklist before choosing a platform.

Performance trade-offs of a phthalate-free film

A phthalate-free TPU film is not simply a softer PVC with the additive removed, and the differences show up in use. TPU generally welds more cleanly under radio frequency energy, gives a stronger peel value at the seam, and holds flexibility at low temperature without the stiffening that a PVC bag shows on a cold morning. It is also more expensive per square metre and, in most compounds, slightly heavier at equal thickness.

Two properties need watching when switching a programme from PVC to TPU. Printability changes, because a film that accepts a solvent-based PVC ink may need a polyurethane or a water-based system, and adhesion has to be re-qualified with a cross-hatch and a wash test. Abrasion behaviour changes too, so a thickness comparison should be run on the new film rather than carried over from the old one. The wider material argument is set out in our TPU versus PVC guide and our PVC-free alternatives guide.

What degrades a TPU film in service

Four agents shorten the life of a TPU bag: sustained heat with humidity, ultraviolet exposure, aggressive chemicals and abrasive grit. Aromatic TPU yellows under UV and then loses surface strength; aliphatic TPU is colour stable and is used where a light colourway has to survive a season of sun. Storage matters more than use: a bag left rolled and damp in a hot garage will hydrolyse faster than one used weekly and dried in shade.

Chemicals deserve a specific warning. DEET-based insect repellent, high-concentration sunscreen and solvent-based cleaners soften and cloud a TPU surface within hours of contact. Rinse with fresh water, avoid bleach entirely, and keep repellent and sunscreen in a separate pouch rather than loose inside the same bag as the film that has to last a decade.

Writing the requirement into a specification

A specification that says phthalate-free will be interpreted differently by every party that reads it. Write the polymer family, the substance list, the numeric limit, the components covered, the test method and the document set, and require a report per component rather than per bag. Add the acceptance criterion in measurable terms and the claim stops being a matter of opinion.

On the supply side, ask when the film is compounded and whether the compounder provides a declaration for the specific lot, because a generic resin statement does not cover what was added downstream. Require confirmation that any print, coating or adhesive applied after the film leaves the compounder meets the same limit, and keep the lot number with the test report so the two can be matched later.

Programmes that need documentation at scale are usually run with a single film platform and a single print system so that one test report covers several sizes and colourways. Waterproof bags have been built on our line since 2014 in an SGS-verified 4,950 m2 facility with seven production lines and an AQL 2.5 inspection regime; sampling runs 6 to 10 working days, bulk runs 35 to 50 days, terms are T/T 30/70 FOB Xiamen and the minimum is 500 pieces. We document the full production walkthrough from sampling to bulk for every programme, and the material options sit in the TPU waterproof backpack range and the wider product line.