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Difference Between Viscose & Polyester

Robert Kay

Posted on September 27 2017

Written by Robert Kay
Co-founder and designer

 

How much do you know about man-made fibres? You may well be familiar with the names: Nylon, Polyester, Viscose and Rayon. Do you know how they are similar and how they are different? Do you know which works best for you?

In this article, we take a closer look at two of the most popular man-made fibres: Polyester and Viscose. Both these fibres may be man-made, but don't let that fool you. There is a world of difference in how they feel to wear, and how they work as clothing.

Polyester vs Viscose: how are they used in everyday clothing?

To keep things simple we will look at how Polyester and Viscose are used in everyday clothing, with particular interest in how they react to water. Then, for those who are interested in the science, we will dive down into the atomic and microscopic levels to see what makes Polyester and Viscose so different.

Polyester

Polyester is a synthetic fibre made from a compound called polyethylene terephthalate (PET for short). Though it comes in many forms, we will consider it here as the form that's most used in sportswear: a fabric made from smooth and long filament fibres.

Polyester is a water resistant fibre, which makes it useful when there's a significant amount of water around. That water could be sweat from working out at the gym, or rain when exploring the great outdoors. Polyester fabric either wicks water away or acts as a barrier: this is dependent on how tightly the Polyester yarn is knitted or weaved to make the fabric.

Loose-weave Polyester: great for wicking sweat away

Do you enjoy a sweat-soaked workout in the gym? Most likely your kit is a stretchy jersey made from knitted Polyester fabric. In this looser form, Polyester makes an excellent moisture wicking material, as the water resistant fibres do not hang onto your sweat. In fact, the water resistant fibres encourage the sweat to move through and along the gaps in the fabric until it reaches the outside layer where it evaporates.

Polyester also dries quickly. The ability to move moisture around means the water does not hang about, which is why your gym kit dries quickly after a wash. All well and good, but there is one small problem: for some reason, Polyester encourages the growth of some particularly stinky types of bacteria.

Tight-weave Polyester: a water barrier

Tightly woven Polyester acts as a barrier. The minute gaps in the fabric are small enough to make it difficult for water and air to pass through: the fabric does not breathe. This might make it ideal for a weather resistant camera bag, but it is not so good for clothing. Workplace uniforms are sometimes made from 100% Polyester fabric in a tight weave because the fabric is crease resistant and maintains its colour. However, because the fabric does not breathe, water vapour remains trapped next to the skin and leaves the wearer feeling hot, sticky and uncomfortable. Happily, even cheaper uniforms these days tend to be made from a Polyester-cotton blend to overcome some of these problems.

Viscose

Viscose works very differently to Polyester. Viscose is a semi-synthetic fibre made from a compound known as cellulose – a plant-based material. Like Polyester, it also comes in long smooth filament fibres, but that is where the similarities end.

Viscose: great for absorbing sweat

Unlike Polyester, Viscose is water absorbing; water is absorbed into tiny micropores in the fibres themselves. This makes Viscose highly breathable, which is crucial for comfort.

Breathable fabrics remove water vapour (and heat) from your skin, leaving you feeling cool, dry and comfortable. Because Viscose does this so well, it is ideal for clothing that is worn all day and for clothing that's worn next to your skin.

This is why we use Viscose fabrics (including bamboo, and micro modal) for our undershirts. Normal amounts of sweating will be absorbed into the fabric and locked away, preventing sweat marks on your shirt. Water vapour is spirited away leaving you feeling cool and fresh. Not only that, but Viscose has been shown to inhibit those very same stinky microbes which Polyester encourages . No one really knows why, but the effect has been measured in several studies. Its lasting freshness only adds to its suitability for all-day wear – especially for those 12 hour days in the office.

But wait! It’s not all rosy for Viscose: it has its problems. Overwhelm Viscose with too much water (sweat) and it will become saturated and wet. Moreover, because it likes to absorb water, not repel it, it takes a long time to dry out. Excessive sweating into a Viscose top could leave you feeling cold and clammy for hours afterwards. This is one of the downsides of water absorbing Viscose.

Summary

Whilst both Polyester and Viscose are man-made, Polyester is water repelling and Viscose is water absorbing. Polyester fabric makes a good wicking fabric, which is great for the gym but may pong a bit. Viscose makes a great water-absorbing fabric, which is great for layers worn against the skin because it breathes and stays fresher longer. Polyester is quick drying; Viscose is not. And, finally, if you want a weather resistant camera bag, make sure it is made from Polyester, not Viscose or the camera will end up a soggy mess. However, make sure your comfortable underwear is made from Viscose not Polyester, or it's you who will be soggy!

So, how come Polyester and Viscose are so different? To answer this we will have to take a closer look – starting at the atomic level. The answer lies in their chemistry.

The chemical difference between Viscose and Polyester

If you were to peer inside a single fibre of either Viscose or Polyester, right down to its atoms, you would see a long chain of a repeating molecule. This process of joining long chains of the same molecule is called polymerisation, and allows us to make materials with uniform and precise characteristics. That is why both Polyester and Viscose can be made as long, smooth, silk-like filaments.

However, even though Polyester and Viscose fibres might look outwardly similar, their characteristics (affinity to water, strength, and so on) is determined by their base material. Polyester is a long chain of repeating PET molecules, whereas Viscose is a long chain of cellulose molecules.

PET molecules (polyethylene terephthalate) are made by combining terephthalic acid and ethylene glycol – both of which are products of petrochemical industry. Cellulose, on the other hand, is a naturally occurring plant material. As cotton is 90% cellulose you will already be familiar with its feel. The cellulose used in Viscose is typically extracted from trees.

Both Viscose and Polyester start life as a fluid containing a form of the base molecule. This fluid is then spun into long, smooth filaments – not unlike silk. It is during the spinning process that the polymerisation (chaining) takes place: the filaments only contain endlessly repeating patterns of their base molecule. These filaments are then made into yarns that are later woven into fabrics for clothes.

So far so good. But why are they so different? The answer lies in the balance of electrical charge across the atoms of each base molecule. In Polyester, the electrical charge is distributed evenly across the whole of the PET molecule. In Viscose, however, the electrical charge on the cellulose molecule is polarised: one end of the molecule carries a positive charge, and the other end has a negative charge. This makes all the difference in the world when the cellulose interacts with water. But why is that?

FACT BOX

Did you ever wonder why oil and water do not mix? Polar molecules (water) and non-polar molecules (oil) do not mix to make a solution.

The reason why some molecules are polar and some are non-polar has to do with the arrangement of atoms and how they are bonded together. The underlying chemistry which causes molecules to be polarised, or non-polarised, is too indepth for this article, but here is a good summary to read if you want to know more.

Viscose: water-loving (hydrophilic)

The polarisation of the cellulose molecule makes it attractive to water molecules. Why? Because water is also a polarised molecule. You know that saying, “opposites attract”? Well, that is exactly what happens when these two molecules get together. The positive end of the water molecule is attracted to the negative end of the cellulose molecule, and vice versa. The cellulose molecule is said to be hydrophilic, or water-loving.

This attraction allows each water molecule to get very close to each cellulose molecule, resulting in a high degree of ‘wetting': the water molecules separate from each other and spread out all over the cellulose structure. Here’s the crux: inside the Viscose fibres are many tiny spaces, sometimes called ‘nanopores’. The water molecules fill up all these nanopores inside the fibre. This is how Viscose fibres absorb water, and this in turn has an important impact on how the textile feels against your skin.

Polyester: water-repelling (hydrophobic)

Polyester is the exact opposite to Viscose. The PET molecule has an even charge all over (non-polarised), making it water-repelling, or hydrophobic. The water molecules have nothing to be attached to, apart from each other, and so they stay clumped together in a liquid droplet on the surface of the fibre. At an atomic level, the fibre itself is not ‘wetted’: there is no bonding between the PET molecules and the water molecules, and the water molecules will not therefore be pulled apart. They will not travel across the PET structure and be absorbed by the fibre.

water droplet on polyester weave

This single difference explains why Polyester is water resistant and why Viscose is water absorbing.

‘Wetting’ under the microscope

You can see this 'wetting' principle in action below.

 The picture shows one Polyester fibre on the left and one Tencel® fibre (Tencel® is a brand of Viscose) on the right under a scanning electron microscope. Both fibres are held in the same controlled atmosphere, which is saturated with water. This atmosphere forces the fibres to react to water – just as if you were wearing these fibres on a hot and humid day. Whereas you can clearly see water droplets sitting on the Polyester fibre on the left, on the right there are no such droplets: the Viscose fibre has absorbed the water instead.

These pictures show cross-sections of Tencel®, Modal and Viscose fibres – all of which are cellulose-based. In each, the water has been treated so that it shows up as dark areas under the microscope. You can see the dark spots inside the fibre where the water has been absorbed and is filling the fibre’s nanopores.

We now know how Polyester and Viscose react differently to water. We have visited the atomic level to look at the chemistry, and the microscopic level to look the fibres’ structures. That's probably enough chemistry for now!

I hope you have enjoyed this whistle-stop tour explaining the difference between Viscose and Polyester. For advice on when to wear Polyester and Viscose fabrics, and how they feel, try out this short article.

The Important Bit

At Robert Owen Undershirts we make men's undershirts from Viscose to ensure all-day comfort in the office. Our undershirts feel amazingly smooth and luxurious on the skin. They absorb sweat, keep you cool and fresh, and they protect your shirt from antiperspirant and sweat marks. With either a deep v-neck or a crew-neck design, our undershirts will always be invisible under open and buttoned shirts.

We call it climate control for your skin. We are so sure you will love it we offer a no nonsense guarantee: if you don't love your new undershirt, it’s on us. In fact, we are pretty certain you are going to want more than one!

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