Lyocell UV Protection vs Cotton and Viscose: Why Lyocell Often Tests Lower

A buyer sends us two summer knit swatches. Same yarn count, same 18G knitting machine, same stitch length, same light grey shade. The cotton swatch passes the UPF test, and the lyocell one fails. We get this question a lot, and the answer usually sits in the fiber. Lyocell UV protection is weaker than cotton and viscose when the fabric construction is identical. The gap comes from how each fiber reflects, absorbs and scatters ultraviolet light.

When the topic comes up in a development meeting, we start with the fabric, move down to the fiber, and then talk about what to change before bulk.

Two Ways UV Light Gets Through a Knitted Fabric

UV radiation passes through a fabric along two paths. Some of it goes straight through the gaps between yarns and loops. Fabric density and cover factor decide how much. A tighter knit, a heavier GSM or a finer gauge leaves smaller openings.

The rest has to travel through the fibers themselves. Here the fiber’s own transparency matters. A clearer fiber lets more light reach the skin, so it blocks less UV.

Once yarn count and fabric structure are fixed, the fiber is the variable left. That is exactly the situation when a brand asks whether lyocell, cotton or viscose will give better UV protection in the same knit.

How UV Light Is Lost Inside a Single Fiber

When a beam of light hits one fiber, three things reduce what comes out the other side:

• Reflection (R): light bounced back off the fiber surface.
• Absorption (f_ab): light taken up by the fiber material and turned into heat.
• Scattering (f_sc): light redirected inside the fiber by its internal structure.

Whatever is left after these losses is the fiber’s transmittance, τ_i:

τ_i = 1 − R − f_sc − f_ab

A higher τ_i means weaker UV blocking. So to compare the UV protection of lyocell, cotton and viscose, we look at the three terms one by one.

How Fiber Fineness and Cross-Section Shape Affect Reflection

Reflectance depends mainly on specific surface area, the amount of surface per unit of fiber. Two things drive it: linear density and how regular the cross-section is.

For a regular round fiber, specific surface area drops as the fiber gets coarser. The relationship looks roughly like this:

Specific surface ≈ 303.5 × √(1 / Tt) (Tt in dtex)

A finer fiber has more surface, reflects more light and blocks more UV. If lyocell, cotton and viscose are all spun at the same dtex, fineness gives none of them an edge.

Cross-section shape is where they differ. At the same linear density, a perfectly round fiber has the smallest surface area. Lyocell is very close to round. Cotton has a kidney shape, and viscose has a serrated outline. Both carry more surface than lyocell, so both reflect more light. On reflection alone, lyocell UV protection already starts behind.

Reflectance Is Not the Same as Luster

Buyers sometimes mix these two up, and it leads to wrong guesses at the swatch stage. Luster describes how consistently light bounces off in one direction. Reflectance is the total amount of light sent back, in any direction.

A rough, irregular surface creates a lot of diffuse reflection. The result can be high reflectance and poor luster at the same time. So a dull-looking fiber may still block UV well, and a shiny one may block less than it looks.

Why Wet Fabric Lets More UV Through

Plenty of summer tops look opaque when dry and turn see-through once wet. The same physics affects UPF.

Take cotton as an example. Its refractive index is about 1.53, air is 1.00 and water is 1.33. Water sits much closer to cotton than air does. Once the fabric is wet, each fiber surface reflects less light, and R drops.

Water also flattens surface hairiness and makes the fabric structure more even. It soaks into the pores inside the fiber too and pushes the air out. A medium with a close refractive index replaces one with a big gap, so the fiber becomes optically more uniform. Scattering falls sharply, transmittance rises, and the fabric becomes more transparent to the eye and to UV.

That is why moisture regain matters here. Cotton has a standard moisture regain of about 8.5%. Lyocell sits around 11% and viscose around 13%. More water held inside the fiber works against both reflection and scattering, so on this factor lyocell and viscose fall behind cotton. It also explains why lyocell UV protection drops further in sweaty or wet use.

Why Shade Often Matters More Than Fiber for Absorption

Absorption depends on two things. The first is color, and anyone who has run UPF tests has seen it. Black absorbs more than 90% of incoming light, while white absorbs almost none. Most UPF failures we see on trial rolls are white, ecru or pastel. A black version of the same fabric almost never fails.

The second is material uniformity, which controls both absorption and scattering. An uneven internal structure scatters light more, so light travels a longer path inside the fiber. A longer path gives it more chances to be absorbed. In this sense, a less uniform fiber tends to absorb more. Lyocell loses out here again, because its structure is so even.

Why Scattering Is the Main Reason Lyocell UV Protection Falls Behind

In theory, if scattering only bent the light a little, the fiber would look hazier, yet a UV test would measure the same transmittance. The light would still get through, just at a different angle.

Real fibers behave differently. After many scattering events inside the fiber, part of the light turns around completely and leaves from the side it entered. That share never reaches the skin, so the formula needs a small correction:

UV transmittance ≈ 1 − R − f_ab − f_back

Here f_back covers only the scattered light that turns back. This is where the three fibers separate clearly:

• Lyocell has the most uniform internal structure of the three. It scatters the least and sends the least light back, which is why lyocell UV protection is the weakest of the group.
• Cotton has a skin layer, a central lumen and daily growth rings that form as the fiber develops through alternating day and night in the field. Every one of those boundaries scatters light.
• Viscose has a thick skin layer, poor internal uniformity and low orientation. It scatters strongly as well.

A knit fabric customer once told us that the same construction passed the UV test in modal and failed in lyocell. To be honest, we think scattering explains most of that gap. Modal has a thick skin and a less uniform core than lyocell, a difference that also shows up when we run modal vs lyocell fiber identification under the microscope.

Titanium dioxide added to the fiber raises scattering too and improves UV blocking. That is the idea behind many anti-UV fibers, but it sits outside a straight fiber-to-fiber comparison, so we leave it aside here.

Lyocell vs Cotton vs Viscose: UV Protection Compared

Fiber	Reflection: cross-section	Reflection: moisture regain	Absorption	Back-scattering
Lyocell	Round, smallest specific surface, lowest reflectance	Higher (about 11%), lowers reflectance	Uniform structure, low absorption	Uniform, little back-scattering
Cotton	Kidney-shaped, larger surface, higher reflectance	Lower (about 8.5%), keeps reflectance higher	Skin, lumen and growth rings, higher absorption	Less uniform, more back-scattering
Viscose	Serrated, larger surface, higher reflectance	Higher (about 13%), lowers reflectance	Thick skin, low uniformity and orientation, higher absorption	Less uniform, more back-scattering

Putting the factors together, the general ranking for UV protection at the same yarn count, structure and shade puts lyocell at the bottom:

Cotton > Viscose > Lyocell, with modal performing roughly like viscose.

That ranking is a rule of thumb. The lab result on the finished fabric is what counts, because shade, finishing, GSM and knit structure can easily outweigh fiber choice. A dark navy lyocell knit, for example, can beat a white cotton knit on UV protection.

What We Adjust When a UPF Target Is on the Spec Sheet

From our factory view, fiber is only one lever. Many customers pick lyocell for its soft, cool hand in summer knits, for example our 30S lyocell and spun silk cooling yarn. Once a UPF claim is added, improving lyocell UV protection usually starts with these points:

1. Test the right sample. Yarn alone tells you very little. We knit a trial panel at the target gauge and stitch length, wash it, then send the finished fabric to the lab. Common methods such as AATCC TM183 in the US and AS 4399 in Australia rate the fabric itself.
2. Check the wet result. For swimwear, sportswear or anything worn with sweat, ask the lab for a wet test as well. Wet testing is where lyocell UV protection is most likely to drop, because of its higher moisture regain.
3. Leave margin on light shades. White and pastels are the risk zone. A pale lab dip that is borderline when dry will probably fail when wet.
4. Consider a blend. Mixing cotton into lyocell adds surface reflection and internal scattering while keeping much of the smooth hand. That is often easier than redesigning the knit.
5. Tighten the construction where the product allows. A finer gauge, a shorter stitch or a slightly higher GSM closes the gaps between loops.
6. Look at delustered fiber with titanium dioxide when the target is UPF 50+ in a light color.

In our sample room, the quick check before anything goes to the lab is simple. We hold a dry swatch and a wetted swatch over the same lightbox. If the wet one looks clearly more open, we already know where the UPF result is heading, and we adjust the blend or the stitch before paying for a formal test.

Common Questions About Lyocell, Cotton and Viscose UV Protection

Does lyocell fabric block UV rays?

Yes, every fabric blocks some UV. At the same construction and shade, though, lyocell blocks less than cotton or viscose. Its round cross-section and very uniform internal structure reflect and scatter less light. Dark shades and tighter knits can still reach a good UPF rating.

Is cotton better than lyocell for sun protection?

Fiber for fiber, yes. Cotton beats lyocell on UV protection because it has a less regular shape, lower moisture regain and more internal boundaries that send light back. In a finished garment, shade and knit density often matter even more.

Why does a fabric lose UPF when it gets wet?

Water replaces air around and inside the fibers. Its refractive index is much closer to cellulose than air’s, so surface reflection and internal scattering both drop, and more UV passes through.

Can lyocell UV protection reach UPF 50+?

Yes, though it usually needs help. A dark shade, a tight construction, a cotton blend or delustered fiber with titanium dioxide can each move the result a long way. In a light shade and an open summer knit, plan on using at least one of these.

Is modal better than lyocell for UV protection?

Usually a little better. Modal behaves roughly like viscose. Its thicker skin layer and less uniform core scatter more light than lyocell does.