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== Kevlar® ==
== Kevlar® ==
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- Filtration at high temperatures
- Filtration at high temperatures
== References ==
== References ==
Revision as of 06:43, 12 January 2009
SeungYeon Kang Applied Physics, G1
Fun facts on soft matter
Good references: Polyelectrolytes in Solution and the Stabilization of Colloids : 
Final Project: Fire Retardant Polymers
- “Flame-retardant”/ “Flame resistance”: understood to be significantly resistant to catching fire, but level of resistance varies depending on the application of the material
- Implied meaning: self-extinguishing
- “Flame-retardant” does NOT mean noncombustible
- Risk is still present in materials but is lower than in other materials¹
- Clothing: fire-retardant for a certain length of time. Usually a short length of time in casual clothing. Higher level of resistance required in fire fighter uniforms.
- examples include: children’s sleepwear / Casual shirts, pants, etc. / Fire fighting overalls
- such as in construction: “Fire doors” and other materials in buildings to retard fire.
General Material Properties
- There are different ways to describe material behavior, among other tests, you can test for:
- Ease of ignition / Flame spread / Fire endurance / Fuel contribution / Smoke evolution
- Can get a different perspective of material from different tests on small, medium and large scales depending on which property is most important for the application.
Theory & Mechanism
- Mechanism of fire: heat generated from flame → touches the polymer surface → the surface produces volatile fragments, and combust → these are feeded to the heat → develops and creates worse situation
- Two general ways to achieve flame resistance: "Solid phase inhibition" method & "Vapor phase inhibition" method
Solid phase inhibition: by extensive crosslinking at the surface when in presence of heat (form a char) → this can insulate the underlying polymer from flame and prevent further developing
- by Endothermic degradation: Some compounds break down endothermically when subjected to high temperatures. Magnesium and aluminium hydroxides are an example, together with various hydrates. This reaction removes heat from the surrounding, thus cooling the material. The use of hydroxides and hydrates is limited by their relatively low decomposition temperature, which limits the maximum processing temperature of the polymers.
- by Dilution of Fuel: Inert fillers, eg. talc or calcium carbonate, act as diluents, lowering the combustible portion of the material, thus lowering the amount of heat per volume of material it can produce while burning.
- by Thermal shielding: A way to stop spreading of the flame over the material is to create a thermal insulation barrier between the burning and unburned parts. Intumescent additives are often employed; their role is to turn the polymer into a carbonized foam, which separates the flame from the material and slows the heat transfer to the unburned fuel.
Vapor phase inhibition: incorporate materials that when released into flame inhibit the flame (quench the flame) → this means that flame requires increased energy to stay lit since its initial radical reactions are inhibited
- by Dilution of gas phase: Inert gases (most often carbon dioxide and water) produced by thermal degradation of some materials act as diluents of the combustible gases, lowering their partial pressures and the partial pressure of oxygen, and slowing the reaction rate.
- by Gas phase radical quenching: Chlorinated and brominated materials undergo thermal degradation and release hydrogen chloride and hydrogen bromide. These react with the highly reactive H* and OH* radicals in the flame, resulting in an inactive molecule and a Cl* or Br* radical. The halogen radical has much lower energy than H* or OH*, and therefore has much lower potential to propagate the radical oxidation reactions of combustion. Antimony compounds tend to act in synergy with halogenated flame retardants. The HCl and HBr released during burning are highly corrosive, which has reliability implications for objects (especially fine electronics) subjected to the released smoke.
- Both methods of inhibition used in typical flame-resistant materials<math>^1</math>
Making Fire-Retardant Fabrics
- There are two ways to make<math>^1</math> fire-retardant fabrics: one is by utilizing "Additive incorporation" and the other is by using "Intrinsically fire-retardant" materials
Additive incorporation - Less expensive method
- Use commodity high volume plastics and add components that improve fire-retardant characteristics
- Example: INDURA® (Engineered fabric<math>^3</math>)
INDURA Engineered fabric is an application of a flame retardant chemical. A high quality phosphonium salt precondensate flame retardant chemical is applied and polymerized with gaseous ammonia forming a long-chain flame retardant polymer impregnated into the core of each cotton fiber. In INDURA engineered fabrics, the flame retardant chemical impregnated in the core of the cotton fiber acts as a catalyst promoting the charring of the fabric. This accelerated charring prohibits the support of combustion by reducing the fuel source. The flame retardant chemical acts in the solid phase to produce this char. The mechanism of action is not based on a gaseous process of extinguishing or "snuffing out" the flame.
Summary: - composed of: cotton + phosphorous additive - contains phosphorous additives:Phosphonium salt polymerized with gaseous ammonia, works as a solid phase inhibition since the additive acts as catalyst to produce charring. - Possible additives include: * Phosphorous -Promotes charring * Halogen (chlorine & bromine) - Quench chain-carrying free radicals in flame (O2-,H+, OH-) - Catalyze charring in polyolefins * Halogen & antimony oxide - Chain rxns produce antimony halides & oxyhalides which are better at free-radical quenching than phosphorous or halogens * In smaller quantities: Nitrogen, boron, alkali metal salts, hydrates of metal oxides
Intrinsically fire-retardant - More expensive method
- Used for polymers needed in: Extreme temperatures (600-1000oC) for a few minutes/ Moderate temperatures (200-300oC) in air for longer periods of time
- Example: Nomex® (more info below)
- Limitations in this method
1. Gas phase radical quenching: The HCl and HBr released during burning are highly corrosive, which has reliability implications for objects (especially fine electronics) subjected to the released smoke.
2. Some fire-retardant polymers, such as polyimides have very low solubility in all solvents → makes polymer intractable and difficult to process (This is due to its linear sequence of cyclic structures that is thermally stable up to about 800 C when it begins to char.)
4. Potential health hazards?
- There have been isolated incidents with Brominate fire retardants<math>^5</math>
a. In 1970’s, one type of Brominated Fire Retardant (Tris-BP), used in clothing at the time, was found to be mutagenic and nephrotoxic. It’s production was quickly stopped.
b. In another incident in the 1970s , polybrominated biphenyls were removed from the market because of poisonings in Michigan attributed to the inadvertent mixing of a bag of Firemaster FF-1, a commercial PBB mixture, into animal feed. The contamination of animal feed resulted in loss of livestock and long-term impacts on the health of farm families in Michigan.
- Some health hazards exist but for the most part, the safety of the polymers are tested before they are incorporated in garments.
- Developed during seeking a fiber that would add thermal resistance to the physical properties of nylon.
- Structure & Properties of Nomex
- Aramid (aromatic polyamide) - Tg = 273 <math>^o</math>C, Tm = 390 <math>^o</math>C - Light density & High strength - Strong resistant to chemcals - High resistance to thermal degradation
- Understanding the Properties
- Conjugated amide bonds and rigid benzene rings stiffens the polymer chain, leading to increased crystalline character.
- Also, ring structures of adjacent chains stack on top of each other very easily and neatly, which makes the polymer even more crystalline.
- How Nomex works
- Fiber itself absorbs heat energy during the carbonization process- Fiber swells and seals opening in the fabric, decreasing air movement and the associated convective heat transfer.
- As fiber and fabric thicken, increases the insulative barrier and reduces conductive heat transfer.
- Synthesized by: Condensation Reaction
- TGA and DSC of NOMEX® in Nitrogen and air
- Shows rapid weight loss above ~427<math>^o</math>C → this means no degradation up to a very high temperature, is very stable compared to other materials
- Inflection curve at 255.3<math>^o</math>C
- DuPont recommends a maximum continuous operating temperature of 204<math>^o</math>C (for continuous several months to years of exposure applications)
- Physical and Rheological Studies of Nomex solution in DMAc/LiCl
Ex) Average molecular weight of Nomex fiber with an [η] value of 2.52 is on the order of 178,200
- Shear viscosity is little affected by shear rate but strongly affected by polymer concentration. Shows similar behavior to conventional chain polymers
Related interesting material: Kevlar®
- Aramid (aromatic polyamide) - Tm = 500 <math>^o</math>C - Condensation Polymerization - Similar properties as Nomex - Greater strength!!!
- Comparison to Nomex fibers
Nomex and Kevlar are fabricated in a same way using the same components. However, compared to the para structure of Kevlar, Nomex molecules have meta oriented phenylene forms bends that reduces rigidity. This results in a more flexible chain which gives a more textile-like qualities while maintaining high temperature properties. Nomex fibers also have easier processing for spinning either wet/dry or melt. This is why Nomex is used more in garments while Kevlar is used in applications that need stronger protection such as in brake linings, and body armor (bullet proof). Furthermore, Kevlar has the greatest tensile strength of all the aramids, and is used in plastic reinforcement for boat hulls, planes, and bicycles. The tendency to exist in "trans" forms allow denser packing of the polymer chains which also adds strength to the material.
- Firefighting equipments, clothing
- Military pilots and aircrew wear one-piece coveralls (flight suits)
- Race car drivers’ multi-layer Nomex driving suit & gloves, socks and shoes
- Industrial materials
- Filtration at high temperatures
1.“Fire Retardant Polymers”. Nelson, Kinson, Quinn. <www.arjournals.annualreviews.org>
9.Handbook of Fiber Science and Technology Vol III, 1993, Jack Preston , Menachem Lewin