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What is Kevlar?

  • 26 May 2019 6:05 AM
    Reply # 7430834 on 6962053
    Anonymous
    Torey wrote:


    How does Kevlar work?

    and Why is Kevlar so strong?

    Search

    Piece of Kevlar damaged by a missile

    Kevlar® 

    is a register trademark of Dupont.

    Photo: A piece of Kevlar after being hit by a projectile. You can see a dent (coming up toward the camera)—but you can't see a hole. You might be bruised by this impact (or suffer what's called a "blunt trauma" injury), but you wouldn't die. Picture courtesy of US Army.

    What exactly is Kevlar?

    Kevlar is simply a super-strong plastic. Kevlar's amazing properties are partly due to its internal structure (how its molecules are naturally arranged in regular, parallel lines) and partly due to the way it's made into fibers that are knitted tightly together.

    A ballistic test of braided Kevlar

    Photo: Kevlar textiles get their properties partly from the inherent strength of the polymer from which the fibers are made and partly from the way the fibers are knitted tightly together, as shown here in a NASA ballistics test. Picture courtesy of NASA Glenn Research Center (NASA-GRC).

    Kevlar is not like cotton—it's not something anyone can make from the right raw materials. It's a proprietary material made only by the DuPont™ chemical company and it comes in two main varieties called Kevlar 29 and Kevlar 49 (other varieties are made for special applications). In its chemical structure, it's very similar to another versatile protective material called Nomex. Kevlar and Nomex are examples of chemicals called synthetic aromatic polyamides or aramids for short. Calling Kevlar a synthetic aromatic polyamide polymer makes it sound unnecessarily complex. Things start to make more sense if you consider that description one word at a time: 

    • Synthetic materials are made in a chemical laboratory (unlike natural textiles such as cotton, which grows on plants, and wool, which comes from animals). 
    • Aromatic means Kevlar's molecules have a strong, ring-like structure like that of benzene.
    • Polyamide means the ring-like aromatic molecules connect together to form long chains. These run inside (and parallel to) the fibers of Kevlar a bit like the steel bars ("rebar") in reinforced concrete.
    • Polymer means that Kevlar is made from many identical molecules bonded together (each one of which is called a monomer). Plastics are the most familiar polymers in our world. As we've seen, the monomers in Kevlar are based on a modified, benzene-like ring structure. 


    What's so good about Kevlar?

    Kevlar body armor helmet

    Photo: Super-strong Kevlar is best known for its use in body armor. Picture by Lcpl Joseph A. Stephens courtesy of United States Marine Corps.

    These are some of Kevlar's properties:

    • It's strong but relatively light. The specific tensile strength (stretching or pulling strength) of both Kevlar 29 and Kevlar 49 is over eight times greater than that of steel wire.
    • Unlike most plastics it does not melt: it's reasonably good at withstanding temperatures and decomposes only at about 450°C (850°F).
    • Unlike its sister material, Nomex, Kevlar can be ignited but burning usually stops when the heat source is removed.
    • Very low temperatures have no effect on Kevlar: DuPont found "no embrittlement or degradation" down to −196°C (−320°F).
    • Like other plastics, long exposure to ultraviolet light (in sunlight, for example) causes discoloration and some degradation of the fibers in Kevlar.
    • Kevlar can resist attacks from many different chemicals, though long exposure to strong acids or bases will degrade it over time.
    • In DuPont's tests, Kevlar remained "virtually unchanged" after exposure to hot water for more than 200 days and its super-strong properties are "virtually unaffected" by moisture.

    And what's bad?

    It's worth noting that Kevlar also has its drawbacks. In particular, although it has very high tensile (pulling) strength, it has very poor compressive strength (resistance to squashing or squeezing). 

    How is Kevlar made?

    There are two main stages involved in making Kevlar. First you have to produce the basic plastic from which Kevlar is made (a chemical called poly-para-phenylene terephthalamide—no wonder they call it Kevlar). Second, you have to turn it into strong fibers. So the first step is all about chemistry; the second one is about turning your chemical product into a more useful, practical material.

    Polyamides like Kevlar are polymers (huge molecules made of many identical parts joined together in long chains) made by repeating amides over and over again. Amides are simply chemical compounds in which part of an organic (carbon-based) acid replaces one of the hydrogen atoms in ammonia (NH3). So the basic way of making a polyamide is to take an ammonia-like chemical and react it with an organic acid. This is an example of what chemists call a condensation reaction because two substances fuse together into one.

    Chemical structure of kevlar showing chemical bonds in its monomer

    Artwork: Kevlar's monomer: C=carbon, H=hydrogen, O=oyxgen, N=nitrogen, — is a single chemical bond, and = is a double bond. This basic building block is repeated over and over again in the very long chains that make up the Kevlar polymer. Source: "US Patent: 3287323: Process for the production of a highly orientable, crystallizable, filament" by Stephanie Kwolek et al.

    Kevlar's chemical structure naturally makes it form in tiny straight rods that pack closely together, like lots of stiff new pencils stuffed tightly into a box (only without the box). These rods form extra bonds between one another (known as hydrogen bonds) giving extra strength—as though you'd glued the pencils together as well. This bonded rod structure is essentially what gives Kevlar its amazing properties. (More technically speaking, we can say the Kevlar rods are showing what's callednematic behavior (lining up in the same direction), which is also what happens in the liquid crystals used in LCDs (liquid crystal displays).)

    You probably know that natural materials such as wool and cotton have to be spun into fibers before they can turned into useful textile products—and the same is true of artificial fibers such as nylon, Kevlar, and Nomex. The basic aramid is turned into fibers by a process called wet spinning, which involves forcing a hot, concentrated, and very viscous solution of poly-para-phenylene terephthalamide through a spinneret (a metal former a bit like a sieve) to make long, thin, strong, and stiff fibers that are wound onto drums. The fibers are then cut to length and woven into a tough mat to make the super-strong, super-stiff finished material we know as Kevlar.

    Three stages in making Kevlar: 1) Start with a dilute solution; 2) Make the solution more concentrated; 3) Spin to create highly oriented fibers.

    Artwork: How Kevlar is made. 1) The rodlike Kevlar molecules start off in dilute solution. 2) Increasing the concentration increases the number of molecules but doesn't make them align. At this stage, the molecules are still tangled up and not extended into straight, parallel chains. 3) The wet-spinning process causes the rods to straighten out fully and align so they're all oriented in the same direction—forming what's called a nematic structure—and this is what gives Kevlar its exceptionally high strength. Image based on an original artwork from DuPont's Kevlar Technical Guide (see references below).

    What's Kevlar used for?

    Kevlar can be used by itself or as part of a composite material (one material combined with others) to give added strength. It's probably best known for its use in bulletproof vests and knifeproof body armor, but it has dozens of other applications as well. It's used as reinforcement in car tires, in car brakes, in the strings of archery bows, and in car, boat, and even aircraft bodies. It's even used in buildings and structures, although not (because of its relatively low compressive strength) as the primary structural material.

    What makes Kevlar such a good antiballistic material?

    A pair of medieval knights dressed in armor fight with swords.

    Photo: Think of Kevlar as a lightweight modern alternative to heavy, cumbersome, medieval suits of armor! Photo by Staff Sgt. Nate Hauser courtesy of US Marine Corps.

    If you've read our article on bullets, you'll know that they damage things—and people—because they travel at high speeds with huge amounts of kinetic energy. Although there's no such thing as completely "bulletproof," materials like bulletproof glass do a good job at protecting us by absorbing (soaking up) and dissipating (spreading out) the energy of a bullet.

    Kevlar is an excellent antiballistic (bullet- and knife-resistant) material because it takes a great deal of energy to make a knife or a bullet pass through it. The tightly woven fibers of highly oriented (lined-up) polymer molecules are extremely hard to move apart: it takes energy to separate them. A bullet (or a knife pushed hard by an attacker) has its energy "stolen" from it as it tries to fight its way through. If it does manage to penetrate the material, it's considerably slowed down and does far less damage.

    Although Kevlar is stronger than steel, it's about 5.5 times less dense (the density of Kevlar is about 1.44 grams per cubic centimeter, compared to steel, which is round about 7.8–8 grams per cubic centimeter). That means a certain volume of Kevlar will weigh 5–6 times less than the same volume of steel. Think back to medieval knights with their cumbersome suits of armor: in theory, modern Kevlar gives just as much protection—but it's light and flexible enough to wear for much longer periods.

    More layers = more protection

    More Kevlar gives more protection. A chart showing how bullets need to be fired faster to penetrate increasing thicknesses of Kevlar armor.

    Chart: You need a greater thickness of Kevlar body armor to stop higher-speed bullets. If you fire a bullet faster than the penetration speed, it will pierce through the armor but exit with less speed than it entered, because the armor will absorb some of its kinetic energy. Please note that this chart is simply a very rough illustration; it's not based on any particular bullet type.

    If you think of Kevlar "soaking up" the energy of a bullet, it's fairly obvious that a greater thickness of Kevlar—more layers of the material bonded together—will give more protection. As you can see from this chart, the more layers you have, the faster you need to fire a bullet to get it to penetrate through Kevlar armor. In other words, if you want to protect soldiers against high-velocity rifle bullets, you're going to need much thicker armor than if you simply want to protect police officers against handgun bullets, which have lower velocity and less kinetic energy.

    You can see this clearly in the official US National Institute of Justice Body Armor Classification, which ranks bulletproof vests and other body protection (made of Kevlar and other materials) on a scale from I to IV for its ability to protect against bullets fired from weapons of different power. At the low end of the scale, type IIA armor has to protect against smaller handgun bullets (typically 9mm full metal jacketed bullets weighing 8.0g or 0.3 oz and fired at about 373 m/s or 834 mph); you need at least 16 layers of Kevlar for that. Higher up the scale, type IIIA armor has to resist more powerful handheld bullets (such as .44 Magnum bullets weighing 15.6 g or 0.6 oz and fired at 436 m/s or 975 mph); that needs twice as much Kelvar—at least 30 layers. It's important to note that even Kevlar has its limits. For protection against rifle bullets (ordinary ones or armor-piercing ones), which travel much faster (850–900 m/s or 1900–2000 mph) with considerably higher kinetic energy, Kevlar isn't enough: you need body armor made from steel or ceramic plates (classified as type III and IV).


  • 16 Dec 2018 6:47 PM
    Message # 6962053
    Torey (Administrator)


    How does Kevlar work?

    and Why is Kevlar so strong?

    Search

    Piece of Kevlar damaged by a missile

    Kevlar® 

    is a register trademark of Dupont.

    Photo: A piece of Kevlar after being hit by a projectile. You can see a dent (coming up toward the camera)—but you can't see a hole. You might be bruised by this impact (or suffer what's called a "blunt trauma" injury), but you wouldn't die. Picture courtesy of US Army.

    What exactly is Kevlar?

    Kevlar is simply a super-strong plastic. Kevlar's amazing properties are partly due to its internal structure (how its molecules are naturally arranged in regular, parallel lines) and partly due to the way it's made into fibers that are knitted tightly together.

    A ballistic test of braided Kevlar

    Photo: Kevlar textiles get their properties partly from the inherent strength of the polymer from which the fibers are made and partly from the way the fibers are knitted tightly together, as shown here in a NASA ballistics test. Picture courtesy of NASA Glenn Research Center (NASA-GRC).

    Kevlar is not like cotton—it's not something anyone can make from the right raw materials. It's a proprietary material made only by the DuPont™ chemical company and it comes in two main varieties called Kevlar 29 and Kevlar 49 (other varieties are made for special applications). In its chemical structure, it's very similar to another versatile protective material called Nomex. Kevlar and Nomex are examples of chemicals called synthetic aromatic polyamides or aramids for short. Calling Kevlar a synthetic aromatic polyamide polymer makes it sound unnecessarily complex. Things start to make more sense if you consider that description one word at a time: 

    • Synthetic materials are made in a chemical laboratory (unlike natural textiles such as cotton, which grows on plants, and wool, which comes from animals). 
    • Aromatic means Kevlar's molecules have a strong, ring-like structure like that of benzene.
    • Polyamide means the ring-like aromatic molecules connect together to form long chains. These run inside (and parallel to) the fibers of Kevlar a bit like the steel bars ("rebar") in reinforced concrete.
    • Polymer means that Kevlar is made from many identical molecules bonded together (each one of which is called a monomer). Plastics are the most familiar polymers in our world. As we've seen, the monomers in Kevlar are based on a modified, benzene-like ring structure. 


    What's so good about Kevlar?

    Kevlar body armor helmet

    Photo: Super-strong Kevlar is best known for its use in body armor. Picture by Lcpl Joseph A. Stephens courtesy of United States Marine Corps.

    These are some of Kevlar's properties:

    • It's strong but relatively light. The specific tensile strength (stretching or pulling strength) of both Kevlar 29 and Kevlar 49 is over eight times greater than that of steel wire.
    • Unlike most plastics it does not melt: it's reasonably good at withstanding temperatures and decomposes only at about 450°C (850°F).
    • Unlike its sister material, Nomex, Kevlar can be ignited but burning usually stops when the heat source is removed.
    • Very low temperatures have no effect on Kevlar: DuPont found "no embrittlement or degradation" down to −196°C (−320°F).
    • Like other plastics, long exposure to ultraviolet light (in sunlight, for example) causes discoloration and some degradation of the fibers in Kevlar.
    • Kevlar can resist attacks from many different chemicals, though long exposure to strong acids or bases will degrade it over time.
    • In DuPont's tests, Kevlar remained "virtually unchanged" after exposure to hot water for more than 200 days and its super-strong properties are "virtually unaffected" by moisture.

    And what's bad?

    It's worth noting that Kevlar also has its drawbacks. In particular, although it has very high tensile (pulling) strength, it has very poor compressive strength (resistance to squashing or squeezing). 

    How is Kevlar made?

    There are two main stages involved in making Kevlar. First you have to produce the basic plastic from which Kevlar is made (a chemical called poly-para-phenylene terephthalamide—no wonder they call it Kevlar). Second, you have to turn it into strong fibers. So the first step is all about chemistry; the second one is about turning your chemical product into a more useful, practical material.

    Polyamides like Kevlar are polymers (huge molecules made of many identical parts joined together in long chains) made by repeating amides over and over again. Amides are simply chemical compounds in which part of an organic (carbon-based) acid replaces one of the hydrogen atoms in ammonia (NH3). So the basic way of making a polyamide is to take an ammonia-like chemical and react it with an organic acid. This is an example of what chemists call a condensation reaction because two substances fuse together into one.

    Chemical structure of kevlar showing chemical bonds in its monomer

    Artwork: Kevlar's monomer: C=carbon, H=hydrogen, O=oyxgen, N=nitrogen, — is a single chemical bond, and = is a double bond. This basic building block is repeated over and over again in the very long chains that make up the Kevlar polymer. Source: "US Patent: 3287323: Process for the production of a highly orientable, crystallizable, filament" by Stephanie Kwolek et al.

    Kevlar's chemical structure naturally makes it form in tiny straight rods that pack closely together, like lots of stiff new pencils stuffed tightly into a box (only without the box). These rods form extra bonds between one another (known as hydrogen bonds) giving extra strength—as though you'd glued the pencils together as well. This bonded rod structure is essentially what gives Kevlar its amazing properties. (More technically speaking, we can say the Kevlar rods are showing what's callednematic behavior (lining up in the same direction), which is also what happens in the liquid crystals used in LCDs (liquid crystal displays).)

    You probably know that natural materials such as wool and cotton have to be spun into fibers before they can turned into useful textile products—and the same is true of artificial fibers such as nylon, Kevlar, and Nomex. The basic aramid is turned into fibers by a process called wet spinning, which involves forcing a hot, concentrated, and very viscous solution of poly-para-phenylene terephthalamide through a spinneret (a metal former a bit like a sieve) to make long, thin, strong, and stiff fibers that are wound onto drums. The fibers are then cut to length and woven into a tough mat to make the super-strong, super-stiff finished material we know as Kevlar.

    Three stages in making Kevlar: 1) Start with a dilute solution; 2) Make the solution more concentrated; 3) Spin to create highly oriented fibers.

    Artwork: How Kevlar is made. 1) The rodlike Kevlar molecules start off in dilute solution. 2) Increasing the concentration increases the number of molecules but doesn't make them align. At this stage, the molecules are still tangled up and not extended into straight, parallel chains. 3) The wet-spinning process causes the rods to straighten out fully and align so they're all oriented in the same direction—forming what's called a nematic structure—and this is what gives Kevlar its exceptionally high strength. Image based on an original artwork from DuPont's Kevlar Technical Guide (see references below).

    What's Kevlar used for?

    Kevlar can be used by itself or as part of a composite material (one material combined with others) to give added strength. It's probably best known for its use in bulletproof vests and knifeproof body armor, but it has dozens of other applications as well. It's used as reinforcement in car tires, in car brakes, in the strings of archery bows, and in car, boat, and even aircraft bodies. It's even used in buildings and structures, although not (because of its relatively low compressive strength) as the primary structural material.

    What makes Kevlar such a good antiballistic material?

    A pair of medieval knights dressed in armor fight with swords.

    Photo: Think of Kevlar as a lightweight modern alternative to heavy, cumbersome, medieval suits of armor! Photo by Staff Sgt. Nate Hauser courtesy of US Marine Corps.

    If you've read our article on bullets, you'll know that they damage things—and people—because they travel at high speeds with huge amounts of kinetic energy. Although there's no such thing as completely "bulletproof," materials like bulletproof glass do a good job at protecting us by absorbing (soaking up) and dissipating (spreading out) the energy of a bullet.

    Kevlar is an excellent antiballistic (bullet- and knife-resistant) material because it takes a great deal of energy to make a knife or a bullet pass through it. The tightly woven fibers of highly oriented (lined-up) polymer molecules are extremely hard to move apart: it takes energy to separate them. A bullet (or a knife pushed hard by an attacker) has its energy "stolen" from it as it tries to fight its way through. If it does manage to penetrate the material, it's considerably slowed down and does far less damage.

    Although Kevlar is stronger than steel, it's about 5.5 times less dense (the density of Kevlar is about 1.44 grams per cubic centimeter, compared to steel, which is round about 7.8–8 grams per cubic centimeter). That means a certain volume of Kevlar will weigh 5–6 times less than the same volume of steel. Think back to medieval knights with their cumbersome suits of armor: in theory, modern Kevlar gives just as much protection—but it's light and flexible enough to wear for much longer periods.

    More layers = more protection

    More Kevlar gives more protection. A chart showing how bullets need to be fired faster to penetrate increasing thicknesses of Kevlar armor.

    Chart: You need a greater thickness of Kevlar body armor to stop higher-speed bullets. If you fire a bullet faster than the penetration speed, it will pierce through the armor but exit with less speed than it entered, because the armor will absorb some of its kinetic energy. Please note that this chart is simply a very rough illustration; it's not based on any particular bullet type.

    If you think of Kevlar "soaking up" the energy of a bullet, it's fairly obvious that a greater thickness of Kevlar—more layers of the material bonded together—will give more protection. As you can see from this chart, the more layers you have, the faster you need to fire a bullet to get it to penetrate through Kevlar armor. In other words, if you want to protect soldiers against high-velocity rifle bullets, you're going to need much thicker armor than if you simply want to protect police officers against handgun bullets, which have lower velocity and less kinetic energy.

    You can see this clearly in the official US National Institute of Justice Body Armor Classification, which ranks bulletproof vests and other body protection (made of Kevlar and other materials) on a scale from I to IV for its ability to protect against bullets fired from weapons of different power. At the low end of the scale, type IIA armor has to protect against smaller handgun bullets (typically 9mm full metal jacketed bullets weighing 8.0g or 0.3 oz and fired at about 373 m/s or 834 mph); you need at least 16 layers of Kevlar for that. Higher up the scale, type IIIA armor has to resist more powerful handheld bullets (such as .44 Magnum bullets weighing 15.6 g or 0.6 oz and fired at 436 m/s or 975 mph); that needs twice as much Kelvar—at least 30 layers. It's important to note that even Kevlar has its limits. For protection against rifle bullets (ordinary ones or armor-piercing ones), which travel much faster (850–900 m/s or 1900–2000 mph) with considerably higher kinetic energy, Kevlar isn't enough: you need body armor made from steel or ceramic plates (classified as type III and IV).

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