3rd class/polymer dep.
Lect. 8/ Kevlar fibers
Kevlar
From Wikipedia, the free encyclopedia
Kevlar
Identifiers
CAS number
Properties
24938-64-5
Molecular formula
[-CO-C6H4-CO-NH-C6H4NH-]n
Kevlar is the registered trademark for a para-aramid synthetic fiber,
related to other aramids such as Nomex and Technora. Developed by
Stephanie Kwolek at DuPont in 1965,[1][2][3] this high-strength material
was first commercially used in the early 1970s as a replacement for
steel in racing tires. Typically it is spun into ropes or fabric sheets that
can be used as such or as an ingredient in composite material
components.
Currently, Kevlar has many applications, ranging from bicycle tires and
racing sails to body armor because of its high tensile strength-to-weight
ratio; by this measure it is 5 times stronger than steel.[2] It is also used
to make modern drumheads that withstand high impact. When used as
a woven material, it is suitable for mooring lines and other underwater
applications.
A similar fiber called Twaron with roughly the same chemical structure
was developed by Akzo in the 1970s; commercial production started in
1986, and Twaron is now manufactured by Teijin.[4][5]
Production
Kevlar does not dissolve easily. And it does not melt. (it decompose at
500 oC , while spinning the fibers required the product to be in a liquid
state.
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Lect. 8/ Kevlar fibers
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Then this problem is solved by Stephaine Kwolek and Herbert Blades by
addition of lithium chloride or calcium chloride to the reactant solution
as a catalyst.
Kevlar is synthesized in solution from the monomers 1,4-phenylenediamine (para-phenylenediamine) and terephthaloyl chloride in a
condensation reaction yielding hydrochloric acid as a byproduct. The
result has liquid-crystalline behavior, and mechanical drawing orients
the polymer chains in the fiber's direction. Hexamethylphosphoramide
(HMPA) was the solvent initially used for the polymerization, but for
safety reasons, DuPont replaced it by a solution of N-methyl-pyrrolidone
and calcium chloride. As this process had been patented by Akzo (see
above) in the production of Twaron, a patent war ensued.[9]
The reaction of 1,4-phenylene-diamine (para-phenylenediamine) with
terephthaloyl chloride yielding kevlar
Kevlar (poly paraphenylene terephthalamide) production is expensive
because of the difficulties arising from using concentrated sulfuric acid,
needed to keep the water-insoluble polymer in solution during its
synthesis and spinning.
Several grades of Kevlar are available:
1. Kevlar K-29 – in industrial applications, such as cables, asbestos
replacement, brake linings, and body/vehicle armor.
2. Kevlar K49 – high modulus used in cable and rope products.
3. Kevlar K100 – colored version of Kevlar
4. Kevlar K119 – higher-elongation, flexible and more fatigue
resistant
5. Kevlar K129 – higher tenacity for ballistic applications
6. Kevlar AP – 15% higher tensile strength than K-29[10]
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7. Kevlar XP – lighter weight resin and KM2 plus fiber
combination[11]
8. Kevlar KM2 – enhanced ballistic resistance for armor
applications[12]
The ultraviolet component of sunlight degrades and decomposes Kevlar,
a problem known as UV degradation, and so it is rarely used outdoors
without protection against sunlight.[citation needed]
Structure and properties
Molecular structure of Kevlar: bold represents a monomer unit,
dashed lines indicate hydrogen bonds.
When Kevlar is spun, the resulting fiber has a tensile strength of about
3,620 MPa,[13] and a relative density of 1.44. The polymer owes its high
strength to the many inter-chain bonds. These inter-molecular
hydrogen bonds form between the carbonyl groups and NH centers.
Additional strength is derived from aromatic stacking interactions
between adjacent strands. These interactions have a greater influence
on Kevlar than the van der Waals interactions and chain length that
typically influence the properties of other synthetic polymers and fibers
such as Dyneema. The presence of salts and certain other impurities,
especially calcium, could interfere with the strand interactions and care
is taken to avoid inclusion in its production. Kevlar's structure consists
of relatively rigid molecules which tend to form mostly planar sheet-like
structures rather like silk protein.[14]
Thermal properties
Kevlar maintains its strength and resilience down to cryogenic
temperatures (−196 °C); in fact, it is slightly stronger at low
temperatures. At higher temperatures the tensile strength is
immediately reduced by about 10–20%, and after some hours the
strength progressively reduces further. For example at 160 °C (320 °F)
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about 10% reduction in strength occurs after 500 hours. At 260 °C (500
°F) 50% strength reduction occurs after 70 hours.[15]
Glass wool
This article is about the thermal insulation material composed of glass
fibers bonded loosely in a way to trap air. For the plastic composite of
glass fiber and polymer plastic used as structural reinforcement, see
fiberglass. For the glass fiber itself, also sometimes called fiberglass,
see glass fiber.
Fiberglass insulation from a ceiling tile
Glass wool batt insulation
Fiberglass pipe insulation with ASJ (All Service Jacket) penetrating
concrete slab opening about to be firestopped. Intumescent wrap strip
is used to seal off where the fiberglass will be consumed by fire.
Glass wool is an insulating material made from fibres of glass arranged
using a binder into a texture similar to wool. The process traps many
small pockets of air between the glass, and these small air pockets
result in the thermal insulation properties.
Glass wool is produced in rolls or in slabs, with different thermal and
mechanical properties. It may also be produced as a material that can
be sprayed or applied in place, on the surface to be insulated.
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Manufacturing process
After the mixture of natural sand and recycled glass at 1,450 °C, the
glass that is produced is converted into fibers. It is typically produced in
a method similar to making cotton candy, forced through a fine mesh
by centripetal force, cooling on contact with the air. The cohesion and
mechanical strength of the product is obtained by the presence of a
binder that “cements” the fibers together. Ideally, a drop of bonder is
placed at each fiber intersection. This fiber mat is then heated to
around 200 °C to polymerize the resin and is calendered to give it
strength and stability. The final stage involves cutting the wool and
packing it in rolls or panels under very high pressure before palletizing
the finished product in order to facilitate transport and storage.
Uses
Glass wool is a thermal insulation that consists of intertwined and
flexible glass fibers, which causes it to "package" air, resulting in a low
density that can be varied through compression and binder content (as
noted above, these air cells are the actual insulator). Glass wool can be
a loose fill material, blown into attics, or, together with an active binder
sprayed on the underside of structures, sheets and panels that can be
used to insulate flat surfaces such as cavity wall insulation, ceiling tiles,
curtain walls as well as ducting. It is also used to insulate piping and for
soundproofing.
Health problems
In the US, the National Toxicology Program ("NTP"), in June 2011,
removed from its Report on Carcinogens all biosoluble glass wool used
in home and building insulation and for non-insulation products.[1]
Similarly, California's Office of Environmental Health Hazard Assessment
("OEHHA"), in November 2011, published a modification to its
Proposition 65 listing to include only "Glass wool fibers (inhalable and
biopersistent)."[2] The U.S. NTP and California's OEHHA action means
that a cancer warning label for biosoluble fiber glass home and building
insulation is no longer required under Federal or California law. All fiber
glass wools commonly used for thermal and acoustical insulation were
reclassified by the International Agency for Research on Cancer
("IARC") in October 2001 as Not Classifiable as to carcinogenicity to
humans (Group 3).[3]
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Fiberglass will irritate the eyes, skin, and the respiratory system.
Potential symptoms include irritation of eyes, skin, nose, throat,
dyspnea (breathing difficulty); sore throat, hoarseness and cough.[4]
Scientific evidence demonstrates that fiber glass is safe to manufacture,
install and use when recommended work practices are followed to
reduce temporary mechanical irritation.[5]
Fiberglass is resistant to mold but growth can occur if fiberglass
becomes wet and contaminated with organic material. Fiberglass
insulation that has become wet should be inspected for evidence of
residual moisture and contamination. Contaminated fiberglass insulation
should be promptly removed
Wool fibers:
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