Kevlar (tm) is one of the strongest and most heat resistant commercially available threads. This buying guide helps you select the right thread by explaining: Kevlar's uses and alternatives to Kevlar, Kevlar properties, sizes, and color; and specifications for standard sizes.
Kevlar Thread Uses Kevlar thread (also called Para-Aramid) is one of the strongest and most heat resistant commercially available threads. It is about 2.5 times stronger than nylon and polyester, has very little stretch, does not melt, and decomposes at 800F. This makes it a great choice for high stress applications including sewing seams on heavy duty clothing, conveyer belts, harnesses and bags, reinforcing plastics, and other fibers, and even providing high-temperature controls for model airplanes and rockets. To put Kevlar thread in perspective, here is a side-by-side comparison of Kevlar and three other middleweight (Size 92) threads:
Stretch before breaking
Kevlar thread is expensive, in most cases it is only available in yellow, and should only be used to sew material that has similar qualities. Consider these Kevlar alternatives:
Polyester Thread - Use a heavier weight nylon or polyester thread instead of a lighter weight Kevlar. For example, a Size 138 nylon or polyester thread has about the same strength as a Size 69 Kevlar thread, costs about 80% less, and comes in colors. But, it is twice as thick, not fire retardant, and has 25% elasticity.
Fire Retardant Sewing Thread -
Spun Kevlar and spun Nomex threads have about the same fire retardance as Kevlar thread and are available in many colors. This type of thread is the usually the best choice for sewing seams and attaching emblems and logos. These threads look and feel like cotton because they are made by wrapping cotton around a Kevlar or Nomex filament core.
Fire Retardant Embroidery Thread -
Use Nomex embroidery thread to embroider logos and emblems for fire retardant gear. The thread looks like top-quality, 40 weight, polyester embroidery thread, but it is just as fire retardant as Kevlar thread. It is not strong enough to sew seams or attach emblems. It is very expensive and should only be used when fire retardance is essential.
Filament Nomex Thread - This thread has about the same fire retardance as filament Kevlar but only 1/3 of its strength. It has a smooth, shiny feel. We are discontinuing this thread because reliable supplies are not reliable because of government priorities.
Kevlar Properties Kevlar thread is stronger, more heat resistant, less stretchy, durable than most other threads. But, thread is usually a very small part of material content. This means that using Kevlar thread with materials that do not have its properties is a waste and could be dangerous.
Decomposition - Kevlar begins to decompose (turn to ash) at about 800F (426C). Much higher than nylon or polyester.
Kevlar thread does not melt. This is important in fire retardant gear.
Stretchiness - Kevlar thread stretches about 2% before it breaks. For example, a 100 foot piece of Kevlar would be 102 feet long just before it breaks. If you sew stretchy fabrics with Kevlar the seams will pucker.
Acids and Alkalis -
Good resistance to dilute acids and bases. Degraded by strong mineral acids and to lesser extent, by strong mineral bases. This is important when Kevlar thread is used in a laboratory setting.
Bleaching and Solvents - Should not be bleached. Excellent solvent resistance. This matters when Kevlar gear is reused.
Mildew, Aging, and Sunlight - Excellent resistance to mildew and aging. Prolonged exposure to sunlight causes deterioration.
Polyester thread works just as well in some cases.
Good abrasion resistance.
Color--Nobody buys filament Kevlar thread for its color. It normally comes a color called Natural (yellow) and cannot be dyed after it is manufactured.
Usually, color is not a problem because it takes a back seat to strength and heat resistance in most applications.
There is some black filament Kevlar on the market. We sell it when available, but do not have a steady source.
Sizes--With Kevlar thread, a higher size number means a thicker thread. As size increases diameter, strength, needle size, and stitch visibility, and sewing machine requirements increase while yards per pound decreases. Let's see how this affects three representative sizes:
As size increases:
Needle size increases
16 to 18
20 to 22
26 to 28
Stitch visibility increases
Sewing machine requirements increase
Yards per pound decrease
Kevlar Terminology--We use several "technical" terms to describe our Kevlar thread. Understanding these terms is important because they affect how you can use the thread:
Bonded / Soft -
Bonded means that the thread has a coating that reduces heat at the tip of the needle and prevents unraveling; soft means that the thread does not have this coating. Here are our recommendations:
Machine Sewing - Use bonded thread for Sizes 69 and higher; bonding does not matter with thinner size thread.
Wrapping and Whipping - Bonding may interfere with adhesives. It can be removed by soaking the thread in alcohol.
Hand Sewing and Hanging - It does not matter if the thread is soft or bonded.
Monocord - Monocord means that the thread is made with a single ply (like fishing line) and has a coating similar to bonding. This gives the thread a flat, ribbon-like shape and makes it usable in double-needle machines. Many of our lighter weight threads use this construction
Left Twist / Right Twist - When threads have more than one ply the plies are twisted to the left or the right. Unless we say otherwise our thread is left twist. This is because single-needle machines made for the U.S. market require left twist or monocord thread.
Use right twist thread with double-needle machines and some machines designed for Asian markets. Twist does not matter in hand sewing and non-sewing applications.
Loose Twist / No Twist / and Yarn - These kinds of threads are frizzy and almost impossible to put through a needle. Consider them for non-sewing applications such as wrapping and hanging.
How to Save--We love selling Kevlar thread. But, it costs about five times more than nylon or polyester. Here are four ways to save:
Do you really need Kevlar?
- Kevlar seems to have a cult following and customers buy it when less expensive nylon or polyester will do the job. Please, ask yourself or ask us if you really need Kevlar thread.
Buy what you need - Our junior spools in 1 to 4 Ounces, coils, and thread packs are a great value for specific, one-time jobs and hobbyists. They generally ship 1st Class to U.S. addresses so shipping costs are low compared to price.
Choose lighter weights You get more yards per ounce with lighter weight thread. But, make sure that the thread has the strength you need.
Consider non-standard sizes and closeouts These threads sell at a 25% to 50% discount compared to our regular thread. Our featured Tex 105 and Tex 300 Kevlar thread is on par with our standard sizes. But, we cannot provide certificates of conformance (CERTS) and will probably not restock them in the future. Our closeouts are old thread that sells at very low prices, but is not recommended for machine sewing.
Nominal Weights--Industry norms allow Kevlar thread spool weights to vary by ±10% from the stated nominal weight. We follow this practice, but have biased our standards so that customers receive slightly more thread than less thread in most cases.
Here is a table that shows our tolerances. When spool weights fall within these tolerances we do not charge for overages or give refunds for underages.
Certificates of Compliance - We can email manufacturer's CERTS for our 16 Ounce, standard size Kevlar. We cannot provide CERTS for any non-standard size or closeout thread.
CERTS apply to specific batch / lots. We wind our smaller spools from the 16 Oz standard size spools and do not keep batch / lot information for these spools. We can send a "representative" CERT for these spools that applies in a general way to the thread purchased. Please call if you must have batch / lot traceable CERTS on small spools and are willing to pay a $2 a spool up-charge.
No Medical Devices - Our Kevlar thread is clean in the ordinary sense. But, it is not sterile and using it in any medical device could put lives at risk. We will not knowingly sell Kevlar thread for use in medical devices.
Kevlar Specifications -
Here are specifications for our standard size Kevlar thread