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== Kevlar® ==
== Kevlar® ==
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Revision as of 08:30, 11 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 resistant to catching on fire, but level of resistance varies
- Implied meaning: self-extinguishing
- “Flame-retardant” does NOT mean noncombustible
- Risk still present in materials (but lower than in other materials)¹
- 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.
- children’s sleepwear / Casual shirts, pants, etc. / Fire fighting overalls
- construction: “Fire doors” and other materials in buildings to retard fire.
General Material Properties
- 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
- Flame generated heat → polymer surface → produces volatile fragments, combust → feed to the heat
- Two general ways to achieve flame resistance:
Solid phase inhibition: by extensive crosslinking at the surface when in presence of heat (form a char) → insulates the underlying polymer from flame - Endothermic degradation: Some compounds break down endothermically when subjected to high temperatures. Magnesium and aluminium hydroxides are an example, together with various hydrates. The 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.
- 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.
- 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) → flame requires increased energy to stay lit since its initial radical reactions are inhibited - 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.
- 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
- Two ways to make<math>^1</math>:
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>)
- composed of: cotton + phosphorous additive - 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 * Phosphorous -Promotes dehydration → water vapor -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 (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®
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? - Isolated incidents with Brominate fire retardants<math>^5</math>
- Tested for safety before released to market
- Some health hazards exist but for the most part, the safety of the polymers are tested before they are incorporated in garments.
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.
- 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.
- Condensation Reaction
- TGA and DSC of NOMEX® in Nitrogen and air
- Rapid weight loss above ~427<math>^o</math>C → no degradation, very stable
- 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.
- Similar behavior to conventional chain polymers
- Aramid (aromatic polyamide) - Tm = 500 <math>^o</math>C - Condensation Polymerization - Similar properties as Nomex - Greater strength!!!