The demand for high performance in engineering products is ever-increasing. Thus, in response to this, we are seeing the emergence of innovative and advanced materials. These materials are birthed by combining different sub-materials for a higher quality than what one material will give. One such is Kevlar.
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Kevlar is a member of the Aramid family of synthetic fibers, which was invented by Dupont. Kevlar fiber can be found in roving, veil mats, chopped strands, or woven fabrics, and it is used in a variety of products ranging from protective armor to ropes and cables. It has many advantages, but it also has some disadvantages.
Exceptional strength
The para-aramid fiber known as "Kevlar" has a relative density of 1.44 and a tensile strength of about 3,620 MPa. This makes it strong while remaining lightweight.
Although lighter than steel, it is strong. Kevlar's chemical structure is made up of several repeating inter-chain bonds. Due to the hydrogen bonding in these chains, their tensile strength is 10 times greater than steel's for equal weight.
Zero thermal shrinkage
Kevlar retains its strength and resilience down to cryogenic temperatures (196C), implying that there is no embrittlement at such low temperatures.
It can also withstand temperatures of up to 450C. It becomes a little bit stronger at cold temperatures. The tensile strength diminishes by around % right away at higher temperatures and continues to decline over time.
Excellent heat and flame resistance
After more than 200 days of exposure to heat, such as hot water, Kevlar fiber can remain virtually unchanged. Kevlar is naturally flame resistant, withstanding temperatures of up to 800 degrees Fahrenheit.
Kevlar fibers will not melt, drip, or burn. It can be ignited, but when the source is removed, the burning stops.
Resistant to tearing and stretching
Because of their high resistance to impact damage, aramid fibers are frequently used in areas prone to impact.
Kevlar fiber's completely expanded and well-aligned molecular chains act as a powerful barrier to prevent wounds, scratches, and pricks.
It is resistant to almost all types of chemicals, giving it the properties of high cut resistance.
High-velocity defense
Kevlar fibers are so tightly spun that separation is nearly impossible. When a bullet or other high-velocity projectile strikes Kevlar, the fibers absorb and dissipate the energy.
Despite its strength and resilience, Kevlar has some drawbacks that limit its effectiveness.
Absorbing moisture
The quick absorption of moisture by Kevlar fibers makes them more environmentally sensitive than other materials. Although only prolonged exposure to moisture degrades Kevlar, this moisture absorption can render it unsuitable for damp environments.
Minimal compressive strength
Kevlar has a good degree of strength and tensile, but it has a poor tolerance for compression, making it challenging to cut.
Cutting and drilling are difficult
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Dry fabric is usually cut with special scissors, and cured laminates can only be pierced with special drill bits.
Ultraviolet light sensitivity
In comparison to polyester, Kevlar loses strength in sunlight about twice as fast due to its rapid absorption of ultraviolet light. This can cause a decrease in fiber properties over time.
Personal protective equipment (PPE) is highly regulated and required by many trades. With so many types of PPE on the market like Kevlar, which is a common brand of para-aramid materials, and high-performance polyethylene (HPPE) composite yarn, it is difficult to know which material will perform best in your environment for your workers. To make the decision easier, here&#;s a breakdown of the pros and cons of para-aramids and HPPE.
Kevlar (para-aramid materials) was introduced on a commercial scale as PPE in the s and 70s. To this day, it is known for its strength. At the point of creation, para-aramid fibers have a strength of around 2.03/2.06 N/tex. N/tex is the same as GPa/g/cm^3 &#; these equate to one million newton-meters per kilogram. N/tex is a measure of force per linear density.
Para-aramid fibers are synthetic fibers joined through hydrogen bonds in rigid polymer chains. The process to make para-aramid fibers, as well as their crystalline structure, leaves them vulnerable to weakening over time.
Para-aramid fibers are naturally yellow. Any additive, including dyes, weakens these fibers. That being said, washing para-aramid fibers also compromises their hydrogen bonding and leaves the material weaker than when it came out of the factory.
In general, para-aramid fibers lose 15 percent of their strength after eight wash cycles. This percentage could be even larger if bleach is added to the laundry, as that makes the fibers decompose. With many detergents having a small amount of bleach included in the solution, this increased distress to the material is not always completely avoidable.
Ultraviolet rays and moisture also weaken the material. This means para-aramids are not ideal for outdoor work environments where the sun is shining and people are sweating. Any moisture, such as a person&#;s sweat, will elongate the fibers. When the fibers stretch out the molecular makeup of the material is altered. Para-aramid fibers are also heavier than HPPE fibers on average, making it a hotter and less breathable material to work in and increasing the likelihood of a person to perspire.
HPPE has the highest tensile strength to mass ratio of any fiber available. It is 15 times stronger than steel, with a tensile strength of about 3.8-3.9 N/tex. Its molecular makeup is slick and chemically inert, making the material more resistant to abrasion compared to para-aramid fibers. It is also stronger when encountering a strong abrasive force.
HPPE is unaffected by moisture (e.g. sweat) or UV radiation. This means it is able to maintain its molecular integrity despite harsh working environments, such as summer days in the Southeast United States inside non-environmentally controlled plants or in direct sunlight.
Because of its ability to withstand abrasion and its overall strength, HPPE materials are the best products for people in the glass, waste recycling, knife sharpening, food processing, sheet metal production, and automotive manufacturing industries.
Para-aramid fibers do not melt. However, they will begin to degrade at temperatures above 500°C. This means they do not burn until exposed to temperatures of almost 1,000°F. For this reason, para-aramid fibers are often used by military troops. The strength of para-aramids and their ability to withstand extreme temperatures make them great protective equipment in combat zones where hostile attacks can come quickly and without warning.
Para-aramid fibers are also best suited for people who work on or in close proximity to furnaces/fires or with welding equipment. These fibers are frequently the main PPE material used by firefighters.
HPPE is a polyethylene plastic, which means it burns quickly. HPPE begins to dissolve at temperatures of 220°F or greater, which poses a safety risk to people who are working in high temperatures environments.
Rather than burning, polyethylene melts at high temperatures. This is an advantage when you want to recycle the material, but a disadvantage if that hot, liquified plastic comes into contact with a person&#;s skin. HPPE, therefore, should not be used where unexpected high temperatures could occur, since the material could melt onto a person&#;s skin, leaving significant burns. For this reason, HPPE fibers are outlawed from use for military clothing contracts.
As previously stated, Kevlar is a shade of yellow in its naturally produced form. Remember, when the color or breathability of the material is altered, the material becomes weaker. For instance, the military often requests black para-aramid fibers for camouflage. While the difference in strength is not directly visible to the person wearing the clothing, it is apparent and can also reduce the longevity of the product. Para-aramid fibers are best kept as is when they exit the spinnerets from dry jet wet spinning.
HPPE is totally customizable. Its fibers are produced through a gel spinning process and their molecular makeup does not weaken as a result of additives. Therefore, customers can have softeners, dyes, oils, and different protective fibers added to their HPPE without consequence. HPPE can also have ANSI hi-vis polyester and reflection visibility tape added to the product to make it more visible for construction work or work in hazardous areas, such as along highways.
Manufacturers have started to blend CBD oil with HPPE to help with pain maintenance and promote a calm work environment. Manufacturers also frequently add cotton or other soft materials to make the PPE more appealing and comfortable for workers. Another common customization is adding thumb holes, thumb loops, and mesh backs.
The thumb holes and loops prevent the PPE from shifting while workers move their elbows or lift heavy items, and the mesh back increases breathability and promotes cooling. With all of HPPE&#;s customizable features, molecular strength, and flexibility, it typically requires a greater front-end investment compared to para-aramids. However, the durability of HPPE products can lead to cost savings in the long term, as they are replaced much less frequently than para-aramid PPE.
Here are the &#;winners,&#; based on the elements we outlined in this article. The preferred PPE material when considering the following elements are:
Ultimately, there is no right or wrong material. Your choice will depend on your working environment. However, now that you know the strengths and weaknesses of each material and the environments they&#;re best suited for, you can make an informed decision that you feel good about.
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