Polyester fabrics aka the PUL on the outside of your nappy...is this different to TPU? This is a particularly prominent topic as we're almost halfway through plastic free July!
Polyester (PET or polyethylene terephthalate - think number 1 plastic bottles) is essentially plastic and whilst there are a few naturally occurring polyesters which can decompose, most are synthetic and don't breakdown.
Polyester fabrics are robust. They can withstand higher washing temperatures, higher wind and environmental stress and often give natural fibres like bamboo or cotton the strength to last (think bamboo terry insert fabric). If you've ever seen a well loved natural fibre night nappy you'll notice the cotton/bamboo fabric often has wear holes even though the stitching (usually 100% polyester) is still holding it altogether.
In the case of nappies, this external polyester fabric is bonded to a plastic polyurethane film which is what gives the shell it's water-resistant properties!
The bonding can be chemical (using solvents) or heat and both produce the final product of polyester fabric + laminate = PUL (polyurethane laminate fabric).
Some confusion exists around the use of the word TPU (thermoplastic polyurethane). This is just one of the the types of polyurethane films that can be used to make a PUL fabric. Remember, PUL is the end fabric once bonded regardless of how it's bonded (for ease of remembering).
Nowadays, almost all nappy brands use heat bonding (thermo) and polyurethane (plastic) to make their water-resistant PUL fabric. So when you see TPU on a nappy and PUL on another, they are essentially the same thing - it's just the TPU nappy is specifically stating they used the heat bonding process vs solvent. In a perfect world, all nappies would say PUL and possibly in brackets TPU.
Prior to 2010 (approx.) polyurethane for nappies were most commonly solvent bonded (PUR) as it was a much stronger bond that enabled the fabric to be boiled - great for the medical industry. The downside to solvent bonding is they released a lot of VOC (volatile organic compounds), waste products and chemical fumes into the environment. This fabric was usually more rigid, regardless of what polyester fabric was used on the outside as the polyurethane film and solvents used were firmer after cooling.
Nowadays, PUL used on nappies are almost entirely heat bonded and using heat bonded polyurethane film (TPU) which is much softer, stretchier and of course, more environmentally friendly. The downside is they can delaminate more easily by washing/drying incorrectly or using the wrong washing products.
Like anything though there is variation in quality and you will notice some nappy brands are super soft and stretchy, whilst others which feel rougher/firmer. This can be due to things like the thickness and quality of the polyester fabric used externally, the thickness and quality of the TPU laminate bonded to the other side and the exact process for bonding.
Polyester fabric + polyurethane laminate + heat = PUL
(TPU being the type of polyurethane film as it was heat "thermo" bonded)
Polyester fabric + polyurethane laminate + solvents = PUL
(PUR being the type of polyurethane film bonded with chemicals)
As always, any questions - please feel free to email me.