What is the difference between these two thread types?
Known as viscose in Europe, rayon is a semi-synthetic fiber made from cellulose that has been treated with alkali and carbon disulfide. As a fabric, rayon is somewhat breathable and does not trap body heat. It can simulate a variety of fabric textures, including linen and silk, and is often blended with more expensive fibers, such as cotton or silk, to reduce cost.
In embroidery, rayon thread has a high luster, giving the embroidery a nice, shiny quality. The glossy appearance alone is enough to convince many embroiderers, both commercial and hobbyist alike, to use rayon thread. High quality rayon thread is also relatively easy to find. For example, national fabric chains such as JoAnn's and Hancock's carry small spools of Sulky brand rayon thread.
Polyester is a synthetic fiber made from polymers. As a fabric, polyester cloth is noted for being relatively uncomfortable. This is because the fiber is not porous: it doesn’t absorb water well and therefore doesn’t wick moisture well either. Garments made of polyester feel far warmer than other, natural fibers. However, polyester also resists wear, tear, wrinkles, shrinking, and fading better than natural fibers. It is often blended with other fibers to combine comfort with durability.
For embroidery, you’re only embellishing the garment, so there’s generally not enough polyester thread to significantly reduce comfort – assuming the garment is made from a breathable fabric to begin with. High quality polyester embroidery thread also has a much higher sheen to it than you normally see in either fabric or general sewing thread. It’s not as shiny as rayon embroidery thread, but it’s still got enough luster that I’ve had an experienced quilter mistake it for rayon.
Which one do I use?
I possess both rayon and polyester embroidery thread, but I generally work with just polyester. Why? Because although rayon thread is very pretty, I think the following three practical factors outweigh its cosmetic appeal:
(1) Rayon is not as color-fast as polyester. It’s a porous fiber, so it will both absorb and bleed color relatively easily. Rayon can be dry cleaned, but it should not be bleached or subjected to whitening agents. Polyester is non-porous. It’s difficult to dye, but once the dye is there, it generally stays put. A good quality polyester thread, such as Isacord brand, can be boiled!
(2) Rayon burns, while polyester melts. Neither is totally ideal, but if you brush by them with an iron on the hottest setting, the polyester may fuse, but the rayon will singe. I'd, personally, rather see the texture change than have the color char to brown or black. (Always iron embroidery from the back of the garment.) Also, polyester can be microwaved without melting, so it takes quite a bit of heat before it can get damaged.
(3) Rayon fibers are weaker than polyester fibers. This means that the thread is more likely to break on the machine – particularly if your machine is capable of running at high speeds (industrial machines can get up to 1000 stitches per minute). Most combination sewing/embroidery machines can only go a maximum of 500 stitches per minute, so this is less of a problem for the hobbyist.* Rayon’s weaker fibers also mean that it is not as resistant to abrasion as polyester thread. If the embroidery is on something that will be washed and dried multiple times, it’s going to look better longer if it is stitched with polyester thread.
As a general rule, once I embroider something, it is out of my hands. I want what I produce to look as good as possible for as long as possible, so I that’s why I use high quality polyester thread. To be exact, I use a combination of Isacord brand thread, which is made in Germany, and Robinson-Anton, which is made here in the USA.**
*If you’re working with rayon thread and find that the thread is shredding during application, consider loosening the tension, using a needle with a larger eye, cleaning the tension disks, running the machine at a lower speed, and/or applying a small amount of sewer’s aid (a silicon-based product) to both the needle and the thread. Or, you could switch to polyester – it’s literally twice as strong as rayon, and some brands (like Isacord) are even stronger.
**There are a lot of good brands out there. These are just the ones that I use. Sticking to just one or two manufacturers makes it easier to acquire a broad pallet of colors without any accidental duplication.
I will also note that thread manufacturers have made significant progress in thread technology over the past few years. In the 1990s, all embroiderers used rayon thread because the polyester thread would stretch, making it difficult to run through the machines. Thread producers now twist the fibers differently, eliminating this problem.