DWRs are surface treatments that help a fabric shed water. DWR stands for ‘durable water repellence’. But how do they work, are DWRs actually that durable, and why are they changing?
DWRs are chemical treatments which are not so much a coating as a layer: an extremely thin layer which covers a fabric. They make no difference to the weight of a garment, no difference to appearance, and most people will never know if a fabric has a DWR until the fabric comes into contact with a liquid. This is where DWRs earn their keep: they repel water and - sometimes - oils. Any absorbency (‘wetting out’) of a shell garment can be detrimental to performance, making it heavier, less breathable, and feel cold. It will be a familiar experience to us all at some point and whilst the correct aftercare will reactivate and prolong their effectiveness, DWR’s do not last indefinitely.
A surface’s water repellence depends on its chemical properties (i.e. what it’s made of) and its physical properties (in particular, its roughness). The chemical properties determine whether a surface is hydrophilic and therefore water attracting, or hydrophobic and so water repellent; the roughness acts as an ‘amplifier’, increasing the surface’s hydrophilicity or hydrophobicity. These combined properties are often referred to as the fabrics ‘water pick-up rate’ and play a key role in optimising the performance of any DWR – ideally a highly effective DWR would have strongly hydrophobic chemistry and be applied to a surface with a roughness of just the right amount that would help shed water.
The problem is, DWRs don’t just have to repel water. They also need to be easy to apply, durable enough to be beneficial to the user, and they can’t cost too much. They also need to be as environmentally benign as possible, and this final point has become a serious stumbling block.
There are various different chemistries that achieve water repellence, and all of them rely on having a low surface energy, meaning that the surface is difficult to interact with and therefore stick to. The chemistries with lowest surface energy and thus those with greatest repellence are those which contain fluorine atoms. Various different molecules, all with a carbon backbone and with fluorine atoms attached to them, have been developed. The length of the carbon backbone or chain is referred to by number, so for example an eight-carbon chain is notated as C8, often written C8. These ‘C8’ molecules have come under widespread scrutiny in recent years because of their persistence in the environment and their ability to accumulate inside organisms.
As a result of their environmental and toxicity shortfalls C8 chemistries are now highly restricted in their use, and are no longer used in any of our products. We and many others however do continue to use C6-based fluorocarbon chemicals in our products. But concerns remain even with these and whilst there is greater doubt over their toxicity, a precautionary approach is leading us to phase out even these on a product by product basis where appropriate. So why do we continue to use any fluorocarbon based treatments at all?
We use fluorocarbon based treatments where we think that they provide a net benefit and where there is a clear and unequivocal performance need not yet met by other alternatives. They remain the most effective and longest lasting DWRs as well as having a unique ability to repel oils. These factors contribute to products which make use of them staying cleaner for longer (and therefore in the case of membranes, working) as well as significantly reducing the need for laundering and reproofing, both of which require vast sums of water and energy over the life of a product.
Ultimately, from a sustainability perspective it is a balancing act between chemistry, energy and water use, garment lifespan and of course, our expectations as to how a garment can and should perform and the slightly awkward truth is that as yet there is perhaps no correct answer. For now though, whilst we continue to reduce our use of fluorocarbon based chemistries they will remain part of the mix whilst we and the partners we work with seek to find effective alternatives.
Nature is incredible because it has evolved numerous different structures that repel water, from the wings of an insect to the leaves of plants. Nature may provide us with inspiration about the direction the industry should take in our development, use, and care, for DWRs. We covet birds’ effortless flight but their feathers, too, have water repellency that puts ours to shame. A bird’s preen gland secretes oils that repel water, and the feathers themselves are spaced so as to provide roughness ideal for maximising water repellence. However, as we’ve seen with the images of seabirds caught up in oil spillages, a feather’s repellence can be overcome by oils and when a bird’s water repellence falters they can lose buoyancy and sink.
Nature has evolved these chemistries and structures to be water repellent, but they rely on input to remain functioning: a bird will not remain water repellent if it does not preen its feathers and keep itself clean. With mankind’s increasing reluctance to use certain chemistries comes greater need for us to care for our products, to keep them clean and repellent, otherwise their effectiveness will decrease.