Sewing thread is made from three broad categories of material: plant fibers like cotton, animal fibers like silk, and synthetic polymers like polyester and nylon. Most thread sold today is polyester, but the best choice depends on the fabric and the job. Each material brings different strengths, and many threads go through chemical treatments and coatings before they ever reach a spool.
Polyester: The Most Common Thread
The polyester used in sewing thread is polyethylene terephthalate, the same polymer found in plastic water bottles. It’s produced by melting the polymer and extruding it through tiny holes to form continuous filaments, which are then twisted together into thread. Polyester thread absorbs very little water, resists shrinking, and has high tensile strength compared to most natural fibers. These properties make it the default for everything from garment construction to home sewing projects.
Polyester does have limitations. It offers low UV resistance without a tinted coating, so outdoor projects that see constant sun exposure may need a specially treated version. It’s also combustible at high temperatures, though it tends to shrink away from flames and self-extinguishes once ignited.
A growing segment of polyester thread is now made from recycled PET plastic bottles. Brands like Gütermann sell 100% recycled polyester thread that performs the same as conventional polyester. The recycled material goes through the same extrusion and finishing processes, so the end product is functionally identical.
Cotton Thread and Mercerization
Cotton thread is spun from the natural cellulose fibers of the cotton plant. On its own, cotton produces a relatively soft, matte thread. It’s a good match for woven cotton fabrics because it shrinks at roughly the same rate, which prevents puckering after washing. Where cotton thread really changes character is through a finishing process called mercerization.
Mercerization involves soaking cotton fiber under tension in a concentrated sodium hydroxide solution (caustic soda) at room temperature. This swells the fibers, smooths out their natural twists, and rearranges the internal crystal structure of the cellulose. The result is a thread with noticeably more luster, better dye absorption, and increased breaking strength. The treatment works because it opens up more pores and reactive sites within the fiber, allowing dyes to bond more deeply and evenly. It also releases internal stresses in the yarn, eliminating weak points that would otherwise cause the thread to snap under load.
Some manufacturers use liquid ammonia at extremely low temperatures (around negative 40°C) instead of sodium hydroxide. This produces a slightly different crystal structure in the cellulose and tends to preserve more of the thread’s stretch, while the sodium hydroxide method yields marginally higher strength. High-quality cotton sewing thread is almost always mercerized.
Nylon for Heavy-Duty Work
Nylon thread comes in two main types. Nylon 6 is made from a single repeating monomer and has a more flexible molecular structure. Nylon 66 uses two different monomers, creating a denser molecular arrangement with stronger hydrogen bonding between chains. In practical terms, Nylon 66 is stronger, stiffer, and more heat-resistant, making it the better pick for upholstery, leather goods, and applications under mechanical stress. Nylon 6 stretches more and handles impact better, which suits applications where some give is useful.
Both types of nylon are stronger than cotton and more elastic than polyester, so nylon thread is common in outdoor gear, luggage, and shoes. Its main weakness is that it absorbs more moisture than polyester, which can cause slight swelling and may affect stitch tension in very humid conditions.
Silk: Filament vs. Spun
Silk thread is a protein fiber produced by silkworms. It comes in two forms that behave quite differently. Filament silk (also called reeled silk) is unwound directly from the cocoon as one long, continuous strand. It’s glossy, strong, and difficult to pull apart. Spun silk, by contrast, is made from shorter broken fibers twisted together, producing a softer, more matte thread that’s noticeably weaker.
Filament silk’s high sheen makes it a favorite for decorative stitching, heirloom sewing, and embroidery meant to last. Spun silk works fine for general hand sewing where a subtle finish is preferred. Silk thread of either type is considerably more expensive than synthetic alternatives, so its use today is mostly limited to specialty projects.
Specialty Threads for Extreme Conditions
Some industrial applications demand thread that can survive heat, abrasion, or chemical exposure far beyond what polyester or nylon can handle. Aramid fiber, best known under the brand name Kevlar, is an aromatic polyamide that doesn’t melt at all. Instead, it decomposes at temperatures between 427°C and 482°C in air. For long-term use, it’s rated up to about 150°C to 177°C. Aramid thread is sewn into firefighter gear, military equipment, automotive airbags, and protective gloves.
Other high-performance options include PTFE (Teflon) thread for chemical resistance and stainless steel thread for conductive or extremely high-temperature applications. These threads are rarely seen outside industrial manufacturing.
Coatings and Lubricants
Raw fiber alone doesn’t make good sewing thread. Before thread is wound onto spools, it’s typically treated with lubricants that reduce friction as it passes through the needle and tension discs of a sewing machine at high speed. Silicone lubricants are the most common, applied as a thin coating over the thread surface. Some manufacturers use a blend of silicone and organic wax to achieve even, low-friction coverage. Polyethylene glycol is another finishing agent that adds lubricity and helps the thread glide smoothly.
These coatings are invisible to the user but make a real difference. Unfinished thread generates excess heat from needle friction, which can cause skipped stitches, thread breakage, or even melting in synthetic fibers. A well-lubricated thread runs cleanly and produces consistent stitch tension.
How Thread Thickness Is Measured
Thread thickness is described using several different numbering systems, which can be confusing because some count up for thicker thread and others count down.
- Weight (wt): Measures how many kilometers of thread weigh one kilogram. A lower number means thicker thread. So 30wt thread is thicker than 40wt, because it takes fewer kilometers to reach a kilogram.
- Tex: The weight in grams of 1,000 meters of thread. Higher numbers mean thicker thread. Tex 25 means 1,000 meters weighs 25 grams.
- Denier: The weight in grams of 9,000 meters of thread. Like Tex, higher numbers mean thicker thread. A 225-denier thread weighs 225 grams per 9,000 meters.
Tex is the most standardized system internationally, but weight numbers appear more often on consumer spools. If you’re comparing thread from different brands, converting to Tex gives you the most reliable apples-to-apples comparison.