The short answer: soft PVC gets its flexibility from added plasticisers, which can migrate over time and leave it stiffer, sticky or yellowed. TPU, or thermoplastic polyurethane, is flexible because of its very structure, stays supple better in the cold and resists abrasion well. Neither lasts forever: TPU can also darken under prolonged UV exposure, depending on its formulation. Here is the detail, point by point, with our sources at the end of the article.
Article updated on 15 September 2026.
Two plastics, two ways of being flexible
Soft PVC: a rigid plastic made flexible
In its pure state, PVC (polyvinyl chloride) is rigid: it is the material of pipes and window frames. To turn it into a soft, translucent sheet, plasticisers are added, often in large proportions. These molecules are not chemically bound to the polymer. They can therefore migrate to the surface and gradually escape under the effect of heat, oxygen or UV light. This loss is why a soft vinyl object can, over the years, become harder, tacky to the touch or brittle at the folds.
TPU: an elastomer by nature
TPU is a block copolymer: it alternates rigid segments, which act as anchor points, and soft segments, which provide elasticity. Its flexibility comes from this architecture, not from an additive that could leave. It is also a thermoplastic: it softens with heat and sets as it cools, which allows it to be moulded into clean shapes. You find it in objects that need to bend without marking, such as phone cases or some sports equipment.
The comparison in one table
| Criterion | TPU | Soft PVC |
|---|---|---|
| Source of flexibility | The structure of the polymer (soft segments) | Added plasticisers |
| Over time | Stays flexible; can darken under prolonged UV depending on the formulation | Can harden, turn sticky or yellow as plasticisers migrate and the material degrades |
| In the cold | Stays flexible, especially so-called polyether TPUs | Stiffens |
| Friction | Good abrasion resistance | Varies with thickness and formulation |
| Smell | Generally discreet | May smell of "new plastic" when unpacked |
| Rain and splashes | Waterproof | Waterproof |
| EU regulation | Its flexibility does not rely on a plasticiser | Four phthalate plasticisers limited to 0.1% in articles since July 2020 |
Yellowing: what can honestly be said
This is the most sensitive point, and the one where promises go furthest. Here is what the technical sources say.
PVC yellows as it degrades. Under heat and UV light, it releases hydrochloric acid (a process called dehydrochlorination), which creates structures in the material that absorb light and shift towards yellow. Plasticiser migration often accompanies this ageing. It is the typical patina of clear vinyl objects left in the sun.
TPU is not immune either. There are two main families. So-called "aromatic" TPUs, which cost less, gradually darken under UV; so-called "aliphatic" TPUs are formulated to keep a stable colour in light. Anti-UV stabilisers slow the process down without preventing it entirely.
The practical conclusion: a TPU bag does not age like a PVC bag. It has no plasticiser to lose, so it does not harden for that reason. But nobody can honestly promise you that a translucent plastic will never yellow. The best ally of any jelly bag remains the same: do not store it in full sun or near a source of heat.
In the cold, in the rain, every day
In the cold, soft PVC stiffens: a bag folded or squeezed into a locker in winter can keep the mark of the fold. TPU, and polyether TPU in particular, keeps its flexibility at low temperatures. That is a real criterion if you carry your bag in autumn and winter, not just at the beach.
In the rain, both materials are waterproof: water runs off and wipes away. The real difference lies in drying and cleaning, not in water resistance. For beach use, read The jelly bag at the beach: why it really works.
Every day, TPU handles friction well: keys, zips, table corners. No translucent material escapes marks entirely, but a bag that resists abrasion keeps its clean look for longer.
How to tell what a jelly bag is made of
- Read the product page. The material should be named: TPU or PVC. "Jelly material" or "premium plastic" are not compositions.
- Look for the composition label on delivery, inside the bag or on its packaging.
- Smell it when you open the parcel. A strong chemical smell is a clue worth noting, not proof.
- Test it in the cool. Soft PVC taken out of a cold room is noticeably stiffer than at room temperature.
- Ask in writing. A serious seller will answer your question about the composition by email, and that written answer protects you.
For the full checklist to assess a bag before paying: How to choose a jelly bag: the criteria that matter.
What we use at La Firkin
All our translucent bags are made of TPU tinted through the material: the Jelly Firkin, the Mini Jelly Firkin, the Baguette Firkin, the Jelly Croco, the Croco Glacé, the Baguette Croco, the Confetti, the Confetti Cabas and the Riviera. Two separate product pages carry the same line: Le Firkin, in full-grain cowhide leather lined with leather (€112), and Le Firkin PU, in PU (faux leather), a synthetic material with no leather in it (€69.90). The Héritage, for its part, is made of premium-quality PU (polyurethane) with an embossed reptile grain: it is not leather.
We will not tell you that our bags will never yellow. We tell you how the material ages, and how to look after it so that it lasts. If you are still hesitating between models, our Jelly Firkin buying guide 2026 covers every criterion, from size to price.
Care, whatever the material
A soft, slightly damp cloth, a diluted mild soap if needed, a rinse, then drying in the open air, in the shade. Avoid solvents (alcohol, acetone), which can dull or damage the surface of both materials, as well as the washing machine and any source of heat. The full step-by-step: How to clean a TPU jelly bag. And to understand why the jelly bag disappeared before coming back: The jelly bag: the story of a comeback.
Frequently asked questions
Is TPU better than PVC for a bag?
For a bag carried every day, it has three concrete advantages: flexibility that does not depend on a plasticiser, better behaviour in the cold and good abrasion resistance.
Can a TPU bag turn yellow?
Yes, under prolonged UV exposure and depending on its formulation: aromatic TPUs darken, aliphatic ones much less. Store your bag out of the sun.
Is PVC banned in the European Union?
No. Certain plasticisers are restricted: DEHP, DBP, BBP and DIBP may not exceed 0.1% of the plasticised material of an article since July 2020 (REACH regulation, Annex XVII, entry 51).
How can you recognise a PVC jelly bag?
By the composition stated on the product page or the label. Smell and stiffness in the cold are clues; if in doubt, ask the seller for the composition in writing.
Is a TPU bag waterproof?
Yes. TPU is not bothered by rain or splashes; wipe it, then let it dry in the open air.
What are La Firkin bags made of?
Our nine translucent models are made of TPU tinted through the material. Le Firkin is in full-grain cowhide leather lined with leather, €112. The same line in PU (faux leather), a synthetic material with no leather in it, has its own page: Le Firkin PU, €69.90. The Héritage is made of premium-quality PU (polyurethane), not leather.
Sources: Thermoplastic polyurethane and Polyvinyl chloride (Wikipedia) · Plasticiser loss in heritage collections (npj Heritage Science, 2020) · Optical properties (Covestro, aromatic and aliphatic TPUs) · Regulation (EU) 2018/2005 (phthalates, REACH). To go further: the complete jelly firkin guide · the House of La Firkin.