Last Update: 6 April 2011
Kevlar® is the registered trademark of E. I. du Pont de Nemours and Co. for its aramid fiber. These fibers, naturally yellow in color, have high strength, high modulus [stiffness], and light weight. They are made in several types but Kevlar® 49 is the one most frequently used in reinforced plastics applications. Twaron® is the registered trademark of Teijin Twaron BV for its aramid fiber, which is very similar to Kevlar. There may be slight differences in properties but from a practical viewpoint they can be considered interchangeable for routine construction of canoes, auto parts and the like. We will use the generic term “aramid” as much as possible in this discussion once we get past a couple of specific points. Kevlar darkens from its original bright yellow color to tan or brown with prolonged exposure to light. I was told that Twaron does not darken quite as much. I ran a quick experiment by placing strips of both materials on a window sill with an opaque object covering part of each strip for reference. After several days the Kevlar had darkened noticeably, the Twaron hardly at all. I had hoped to post a photo showing the difference but it did not show up as clearly to the camera as it did to the eye. Most of our aramid materials are Kevlar. Our bias tapes, except for 2", are manufactured in Germany using Twaron yarn; this manufacturer claims to have had better luck with Twaron.
Kevlar 49 is produced in a number of different yarn sizes. 195 and 380 denier are very fine yarns used in the lightest fabrics. 1140 and 1420 denier are the standard yarns used in all common 5-oz materials. (Denier is a rather bizarre measure of yarn size. It is the weight in grams of nine kilometers of yarn!) Kevlar 49 is produced only in its natural yellow color. Only Kevlar 29, a lower-modulus material, is dyed by DuPont, though it is possible that after-market dying of 49 is being done. If you are considering the use of a dyed fabric, be very careful about what you are getting. If the yarn is listed as 1500 denier, that is a dead giveaway that it is 29, an inferior fiber for most composite applications.
Aramid yarn is woven into fabrics in exactly the same manner as fiberglass. Please review the section on fiberglass fabrics elsewhere on this page for a discussion of fabric and loom terminology. The construction details for all of the aramid fabrics we stock are listed on the Kevlar page of our catalog. Unlike glass, aramid does not require a chemical finish to make it compatible with resins. A cleaning process called scouring is all that is required. This “finish” is called 618 by BGF and CS-800 by Hexcel-Schwebel. The only deterioration of the scoured finish is the slow absorption of moisture over time. If stored for a number of years, or exposed to high humidity, it would be advisable to heat the fabric to redry it before use.
Because of its toughness, aramid is difficult to cut. It requires a high-quality pair of Scissors that are dedicated to cutting aramid and other soft, non-abrasive, materials. Using these scissors to cut fiberglass will ruin them quickly. Since fiberglass cuts easily, even with very cheap scissors, this is a silly and costly error to make. Commercial aramid-cutting scissors are often made with carbide or ceramic blades and are very expensive. Good sewing scissors, properly sharpened, will cut aramid easily and for a fraction of the cost. When dulled, unless dulled by misuse, they can be easily resharpened without special tools.
Aramids should be used in conjunction with fiberglass in most laminates because their properties complement one another. The weakest aspect of aramid is its compressive strength and that is one of the strong points of fiberglass. Combining the two combines the tensile strength, toughness, and light weight of aramid with the compressive strength and rigidity of glass. How to do that most effectively is one way the composites engineer or designer earns his pay. For a simple example, consider the hull of a canoe. One typically lays up glass layers on the outside and uses aramid for the inner layers. When the boat strikes an obstacle in the water the hull is deflected inward. This puts the inner skin of the laminate in tension and the outer skin in compression, exactly the forces that this laminate sequence is designed to resist.
This is an introduction to fiberglass yarn terminology, just in case anyone is interested. (This is just for background — we sell only finished laminating fabrics. We do not sell yarn.) First, we’ll look at the chemical composition of the glass and the difference between E-glass and S-glass. E-glass, which stands for Electrical, is the common, all-purpose type. When someone justs says, “fiberglass,” (s)he almost invariably means E-glass. S-glass, which stands for high-Strength, is stronger and stiffer and more expensive. What is commonly called S-glass nowadays is actually S2-glass. The original S-glass, sometimes called military S-glass because it was developed for military applications, is somewhat stronger than S2 glass but it is extremely expensive, due to stringent testing and certification requirements of the military. There is no chemical difference between S and S2. The stringent mil-spec testing will pass only the highest quality material, while the commercial S2 material may be slightly less strong and still pass its test. I’m told that military S-glass is now called lot glass because it is made in a lot or batch, which is then certified as to the processing parameters. It has few commercial applications due to the high cost. S2 glass was developed in the 1960s by Owens-Corning to bridge the gap between E-glass and mil-spec S-glass. When I speak of S-glass I am really speaking of S2-glass and that is generally true in the industry today. Compared to E-glass, S-glass provides about 40% higher tensile and flexural strengths, about 10 to 20% higher compressive strength and flexural modulus, and greater abrasion resistance.
|Glass Composition, % by weight|
|Silicon dioxide||52 - 56%||64 - 66%|
|Calcium oxide||16 - 25%||Trace|
|Aluminum oxide||12 - 16%||24 - 26%|
|Boron oxide||5 - 10%||—|
|Sodium & Potassium oxides||0 - 2%||Trace|
|Magnesium oxide||0 - 5%||9 - 11%|
|Other oxides & fluorides in trace amounts to make 100%.|
Glass yarns are designated by a series of letters and numbers to indicate the type of glass, type and size of filament, and the size and construction of the yarn itself. A typical designation is ECG 150-1/2. The first letter is either E or S to indicate the type of glass. The second letter is C to indicate continuous strands. (There are other possibilities for these letters but these are the only ones that concern us for laminating fabrics.) The third letter, of pair of letters, indicate the diameter of the filaments, from B (the smallest) to K (the largest). The first number indicates the number of yards of yarn in 0.01 lb, so the smaller the number the larger the yarn. It takes 15,000 yards of 150 yarn to make a pound. The fractional number is not truly a fraction. It lists the number of basic strands making up the complete yarn, in this case one strand made up of two of the 150 yarns plied [twisted] together. If the “fraction” is 1/0 it means a single strand of untwisted yarn. A 150-1/2 yarn and a 75-1/0 are for most purposes the same thing, the former having a twist and the latter not. Fabrics made with untwisted yarns should produce a slightly higher glass content in a hand-laid laminate, while fabrics made with twisted yarns may conform to compound curves slightly more easily. The following table lists only some of the most common yarns used in laminating fabrics.
|Fiberglass Yarn Strands|
|Filament Diameter||Strand Weight|
|microns||Yards/0.01 lb||No. of Filaments|
With all these yarns on hand, now we must weave them into fabrics. When a loom is set up, the warp yarns are those running the length of the fabric and the fill yarns are those going across it. When you count up the warp yarns you are counting the number of ends. When you count the fill you are counting picks. The contruction of the fabric gives the number of ends/inch and the number of picks/inch, usually expressed as W x F, or 24 x 22 for example. The size of the yarns and the construction determine the weight of a fabric.
Once the construction is determined, the weave pattern is the next consideration. Most fabrics are plain weave, which means each yarn passes over one then under one of the crossing yarns in the simplest possible pattern. The most common type of twill weave is the two-by-two twill, where each yarn passes over two and under two, resulting in a diagonal pattern that is quite striking, especially when different color yarns alternate, as in our style 94905 Carbon/Kevlar hybrid fabric. Twill weave permits the fabric to be somewhat more conformable without losing a great deal of fabric stability. In a satin weave, each yarn floats over three or more cross yarns and then passes under one. In four-harness satin, a yarn passes over three and under one, while in an 8H-satin, a yarn passes over seven and under one. One particular type of 4H-satin is called crowfoot weave. Satin weaves are the most conformable but are loose and easily distorted during handling. The following table lists only a few of the hundreds of available fiberglass styles with a complete description of each. Although some fabrics have different warp and fill yarns, none of these do, so the yarn shown is used in both.
|Fiberglass Fabric Construction|
|Style||Yarn Description||Warp x Fill||Weight, oz/yd2|
|112||ECD 450-1/2||40 x 39||2.1|
|2112||ECE 225-1/0||40 x 39||2.1|
|1522||ECG 150-1/2||24 x 22||3.7|
|3733||ECG 37-1/0||18 x 18||5.8|
|1800||ECK 18-1/0||16 x 14||9.8|
|6522||SCG 150-1/2||24 x 22||3.7|
|6533||SCG 75-1/2||18 x 18||5.9|
|6580||SCG 150-1/0||73 x 70||5.6|
|6581||SCG 150-1/2||58 x 54||8.9|
|6781||SCG 75-1/0||58 x 54||8.9|
|7781||ECDE 75-1/0||57 x 54||8.8|
Note that 112 and 2112 differ only in that the former has a twisted yarn. The same applies to 6581 and 6781. Style 6522 is the S-glass equivalent of 1522, 6533 is very close to 3733, and 6781 is very close to 7781.
When fiberglass cloth comes off the loom it is said to be “in the greige” [pronounced grey]. In order to be compatible with resin systems it must first be heat cleaned to remove oils and sizing needed for weaving and then treated in a chemical bath, called “finishing.” Finishing formulations are proprietary recipes that enhance bonding with the type of resin to be used. One of the earliest fomulations that is still in use is called Volan, which employs compounds of chromium. We do not recommend the use of Volan finishes because of the chromium content. Chromium is a toxic heavy metal that is considered a strategic material by the military and much of it is imported from Africa. Potential problems are evident! It also imparts a green cast to the fiberglass and the resulting laminate is generally rather grey and dirty looking. Silane finishes produce a clearer laminate and avoid the chrome problems.
Our preferred silane finish from BGF is called 497A. It was developed for critical aerospace applications and is compatible with epoxy, vinylester, and polyester resins. We occasionally have fabrics with 627 finish, also a general-purpose silane. Our fabrics that are made by Hexcel-Schwebel have a CS-767 finish, which we believe to be comparable to the BGF finishes. Very occasionally we will have some material that is finished specifically for one resin system, usually epoxy. In those instances we are careful to assure that a potential buyer is using the correct resin before making a sale.
Fiberglass finishes are degraded by environmental conditions such as moisture or even humidity and they deteriorate to some degree just with time. The 497A finish has especially good longevity, which is one of the reasons we prefer it. While deterioration begins as soon as finishing is completed, there is no significant loss of physical properties over the first year or two, provided the fabric has been wrapped in plastic and protected from high heat and humidity. I would be careful about using fiberglass more than three years old and would probably not use it after five years for any critical application without running some tests first.
Graphite or carbon-fiber is a space-age material that is stiffer and lighter than glass but has virtually no toughness. That means that it can be used to construct a rigid product but when that product fails it does so suddenly and catastrophically, like a light bulb. Kevlar, on the other hand, has high toughness, meaning that it can be bashed around quite a bit, becoming weaker and weaker but not failing completely. Graphite fabric is easily cut with any sort of scissor and does not dull the scissor quickly as does fiberglass.
Graphite yarns are not as complicated as glass yarns. Essentially all carbon fibers are made in the same size. The difference comes in the number of fibers used to make up the yarn or tow. [Tow is heavier than yarn.] The sizes are indicated simply by the number of thousands of fibers in the yarn. Yarns are 1K, 3K, 6K, and 12K, and tows are generally 50K or 250K. Graphite yarn can be woven into fabrics in all of the weave patterns described above under Fiberglass Fabrics. We generally stock only 3K and 6K materials.
The cost per pound of graphite yarn drops as the size of the yarn increases. Thus, even though it is possible to weave an 11-oz fabric with 3K yarn, the 6K version is less expensive and should be chosen unless there is an overriding need to use the smaller yarn. For example, we stock style 94900, which is a 12x12 construction using 6K yarn in 5-harness satin weave. The equivalent 3K material, style 94907, is a 24x24 construction in 8-harness satin. The latter will give a slightly more compact, therefore lighter, laminate, but at a substantially higher cost.
On the other hand, what about lighter fabrics? Standard 3K fabrics, woven with 12 or 12½ ends and picks, weigh about 5.7 oz/yd². It is possible to go down to as few as 8½ ends and picks as in style 94919, weighing 4 oz/yd², but this is a very loose weave with lots of open space between the yarns so it is difficult to lay up without major distortions, which is not only ugly but also introduces stronger and weaker areas in a random fashion. The thickness of this fabric is little different from the heavier one, so the open spaces are filled up with resin, resulting in very little weight savings.
Fabrics much lighter than 5 oz must be made from 1K yarn, which is a very serious undertaking costwise. 1K yarn costs over $100 per pound, compared to perhaps $25 for 3K yarn. The reason for this large discrepancy in price is partly due to the fact that global production of 1K yarn is a fraction that of 3K or any of the larger yarns, so “economy of scale” has an effect. Also, for a given yarn mill, the output is limited by the flow rate in yards much more than it is by yield in pounds, so it takes three times as long to produce a pound of 1K yarn as it does for 3K. Lack of competition may also contribute to the cost discrepancy. I understand that there are now only two factories in the world which produce 1K yarn. An increase in competition could lower the price of 1K yarn. The factors listed so far influence only the yarn price. In addition, the woven fabric price is further increased by the fact that looms can produce fewer yards per hour when using the finer 1K yarn. The net result is that 1K fabrics are very expensive and hence rarely used.
Another class of materials is hybrid fabrics. This simply means that yarns from two different materials are woven into the same cloth. We carry only one hybrid at this time. Style 94905 has 3K graphite and 1420 denier Kevlar alternating in both warp and fill. Style 1016 graphite tape might be thought of as a hybrid but it is more nearly a pure unidirectional material in that there is only a very light glass fill, just enough to hold the warp together.
Please contact us for more information.