Spider silk

From Wikipedia, the free encyclopedia

A garden spider spinning its web

A female specimen of Argiope bruennichi wraps her prey in silk.

Indian summer by Józef Chełmoński (1875, National Museum in Warsaw) depicts a peasant woman with a thread of gossamer in her hand.

Spider silk is a protein fiber spun by spiders. Spiders use their silk to make webs or other structures, which function as nets to catch other animals, or as nests or cocoons to protect their offspring. They can also use their silk to suspend themselves.

Many small spiders use silk threads for ballooning, the popular, though technically inaccurate, scientific term for the dynamic kiting^[1][2] spiderlings (mostly) use for dispersal. They extrude several threads into the air and let themselves be carried away by winds. Although most rides will end a few yards later, it seems to be a common way for spiders to invade islands. Many sailors have reported that spiders have been caught in their ship's sails, even when far from land. The extremely fine silk that spiders use for ballooning is known as gossamer.^[3]

In some cases, spiders may even use silk as a source of food.^[4]

Methods have been developed to collect silk from a spider by force.^[5]

§Biodiversity

§Uses

All spiders produce silks, and a single spider can produce up to seven different types of silk for different uses.^[6] This is in contrast to insect silks, where an individual usually only produces one type of silk.^[7] Spider silks may be used in many different ecological ways, each with properties to match the silk's function(see Properties section). As spiders have evolved, so has their silks' complexity and diverse uses, for example from primitive tube webs 300–400 mya to complex orb webs 110 mya.^[8]

Ecological use	Example	Reference
Prey capture	The orb webs produced by the Araneidae (typical orb-weavers); tube webs; tangle webs; sheet webs; lace webs, dome webs; single thread used by the Bolas spiders for "fishing".	[6][8]
Prey immobilization	Silk used as "swathing bands" to wrap up prey. Often combined with immobilising prey using a venom. In species of Scytodes the silk is combined with venom and squirted from the chelicerae.	[6]
Reproduction	Male spiders may produce sperm webs; spider eggs are covered in silk cocoons.	[6][9]
Dispersal	"Ballooning" or "kiting" used by many small spiders for dispersal.	[2]
Source of food	The kleptoparasitic Argyrodes eating the silk of host spider webs. Some daily weavers of temporary webs also eat their own unused silk daily, thus mitigating a heavy metabolic expense.	[10][4]
Nest lining and nest construction	Tube webs used by "primitive" spiders such as the European Tube Web Spider (Segestria florentina). Threads radiate out of nest to provide a sensory link to the outside. Silk is a component of the lids of Trapdoor spiders, and the "Water" or "Diving bell" spider Argyroneta aquatica builds its diving bell of silk. It is in fact difficult to think of any spider that does not use silk in constructing its abode.	[8]
Guide lines	Some spiders that venture from shelter will leave a trail of silk by which to find their way home again.	[10]
Drop lines and anchor lines	Many spiders, such as the Salticidae, that venture from shelter and leave a trail of silk, use that as an emergency line in case of falling from inverted or vertical surfaces. Many others, even web dwellers, will deliberately drop from a web when alarmed, using a silken thread as a drop line by which they can return in due course. Some, such as species of Paramystaria, also will hang from a drop line when feeding.	[10]
Alarm lines	Some spiders that do not spin actual trap webs do lay out alarm webs that the feet of their prey (such as ants) can disturb, cuing the spider to rush out and secure the meal if it is small enough, or to avoid contact if the intruder seems too formidable.	[10]
Pheromonal trails	Some wandering spiders will leave a largely continuous trail of silk impregnated with pheromones that the opposite sex can follow to find a mate.	[10]

§Types

Meeting the specification for all these ecological uses requires different types of silk suited to different broad properties, as either a fiber, a structure of fibers, or a silk-globule. These types include glues and fibers. Some types of fibers are used for structural support, others for constructing protective structures. Some can absorb energy effectively, whereas others transmit vibration efficiently. In a spider, these silk types are produced in different glands; so the silk from a particular gland can be linked to its use by the spider. See the later section for details on the mechanical properties of silk and how the structure of silk can achieve these different properties.

Gland	Silk Use
Ampullate (Major)	Dragline silk—used for the web’s outer rim and spokes and the lifeline.
Ampullate (Minor)	Used for temporary scaffolding during web construction.
Flagelliform	Capture-spiral silk—used for the capturing lines of the web.
Tubuliform	Egg cocoon silk—used for protective egg sacs.
Aciniform	Used to wrap and secure freshly captured prey; used in the male sperm webs; used in stabilimenta.
Aggregate	A silk glue of sticky globules.
Piriform	Used to form bonds between separate threads for attachment points.

§Properties

§Mechanical properties

Each spider and each type of silk has a set of mechanical properties optimised for their biological function.
Most silks, in particular dragline silk, have exceptional mechanical properties. They exhibit a unique combination of high tensile strength and extensibility (ductility). This enables a silk fiber to absorb a lot of energy before breaking (toughness, the area under a stress-strain curve).

An illustration of the differences between toughness, stiffness and strength

A frequent mistake made in the mainstream media is to confuse strength and toughness when comparing silk to other materials. As shown below in detail, weight for weight, silk is stronger than steel, but not as strong as Kevlar. Silk is, however, tougher than both.

§Strength

In detail a dragline silk's tensile strength is comparable to that of high-grade alloy steel (450 - 1970 MPa),^[11][12] and about half as strong as aramid filaments, such as Twaron or Kevlar (3000 MPa).^[13]

§Density

Consisting of mainly protein, silks are about a sixth of the density of steel (1.31 g/cm³). As a result, a strand long enough to circle the Earth would weigh less than 500 grams (18 oz). (Spider dragline silk has a tensile strength of roughly 1.3 GPa. The tensile strength listed for steel might be slightly higher—e.g. 1.65 GPa,^[14][15] but spider silk is a much less dense material, so that a given weight of spider silk is five times as strong as the same weight of steel.)

§Energy density

The energy density of dragline spider silk is 1.2x10⁸J/m³.^[16]

§Extensibility

Silks are also extremely ductile, with some able to stretch up to five times their relaxed length without breaking.

§Toughness

The combination of strength and ductility gives dragline silks a very high toughness (or work to fracture), which "equals that of commercial polyaramid (aromatic nylon) filaments, which themselves are benchmarks of modern polymer fiber technology".^[17][18]

§Temperature

Whilst unlikely to be relevant in nature, dragline silks can hold their strength below −40°C (-40°F) and up to 220°C (428°F).^[19]

§Supercontraction

When exposed to water, dragline silks undergo supercontraction, shrinking up to 50% in length and behaving like a weak rubber under tension.^[20] Many hypotheses have been suggested as to its use in nature, with the most popular being to automatically tension webs built in the night using the morning dew.^{[citation needed]}

§Highest-performance

The toughest known spider silk is produced by the species Darwin's bark spider (Caerostris darwini): "The toughness of forcibly silked fibers averages 350 MJ/m³, with some samples reaching 520 MJ/m³. Thus, C. darwini silk is more than twice as tough as any previously described silk, and over 10 times tougher than Kevlar".^[21]

§Types of silk

Many species of spider have different glands to produce silk with different properties for different purposes, including housing, web construction, defense, capturing and detaining prey, egg protection, and mobility (gossamer for ballooning, or for a strand allowing the spider to drop down as silk is extruded). Different specialized silks have evolved with properties suitable for different uses. For example, Argiope argentata has five different types of silk, each used for a different purpose:^[22][23]

Silk	Use
major-ampullate (dragline) silk	Used for the web's outer rim and spokes and the lifeline. Can be as strong per unit weight as steel, but much tougher.
capture-spiral (flagelliform) silk	Used for the capturing lines of the web. Sticky, extremely stretchy and tough. The capture spiral is sticky due to droplets of aggregate (a spider glue) that is placed on the spiral. The elasticity of flagelliform allows for enough time for the aggregate to adhere to the aerial prey flying into the web.
tubiliform (a.k.a. cylindriform) silk	Used for protective egg sacs. Stiffest silk.
aciniform silk	Used to wrap and secure freshly captured prey. Two to three times as tough as the other silks, including dragline.
minor-ampullate silk	Used for temporary scaffolding during web construction.
Piriform (pyriform)	Piriform serves as the attachment disk to dragline silk. Piriform is used in attaching spider silks together to construct a stable web.

§Structural

§Macroscopic structure down to protein hierarchy

Structure of spider silk. Inside a typical fiber there are crystalline regions separated by amorphous linkages. The crystals are beta-sheets that have assembled together.

Silks, as well as many other biomaterials, have a hierarchical structure (e.g., cellulose, hair). The primary structure is its amino acid sequence, mainly consisting of highly repetitive glycine and alanine blocks,^[24][25] which is why silks are often referred to as a block co-polymer. On a secondary structure level, the short side chained alanine is mainly found in the crystalline domains (beta sheets) of the nanofibril, glycine is mostly found in the so-called amorphous matrix consisting of helical and beta turn structures.^[25][26] It is the interplay between the hard crystalline segments, and the strained elastic semi-amorphous regions, that gives spider silk its extraordinary properties.^[27][28] Various compounds other than protein are used to enhance the fiber's properties. Pyrrolidine has hygroscopic properties which keeps the silk moist furthermore the additive wards off ant invasion. It occurs in especially high concentration in glue threads. Potassium hydrogen phosphate releases protons in aqueous solution, resulting in a pH of about 4, making the silk acidic and thus protecting it from fungi and bacteria that would otherwise digest the protein. Potassium nitrate is believed to prevent the protein from denaturing in the acidic milieu.^[29]

This first very basic model of silk was introduced by Termonia in 1994^[30] suggested crystallites embedded in an amorphous matrix interlinked with hydrogen bonds. This model has refined over the years: Semi-crystalline regions were found^[25] as well as a fibrillar skin core model suggested for spider silk,^[31] later visualized by AFM and TEM.^[32] Sizes of the nanofibrillar structure and the crystalline and semi-crystalline regions were revealed by neutron scattering.^[33]

§Non-protein composition

Various compounds other than protein are found in spider silks, such as sugars, lipids, ions, and pigments that might affect the aggregation behaviour and act as a protection layer in the final fiber.^[16]

§Biosynthesis

The production of silks, including spider silk, differs in an important respect from the production of most other fibrous biological materials: rather than being continuously grown as keratin in hair, cellulose in the cell walls of plants, or even the fibers formed from the compacted faecal matter of beetles,^[16] it is "spun" on demand from liquid silk precursor sometimes referred to as unspun silk dope, out of specialised glands.

The spinning process occurs when a fiber is pulled away from the body of a spider, be that by the spider’s legs, by the spider's falling and using its own weight, or by any other method including being pulled by humans. The name "spinning" is misleading as no rotation of any component occurs, but the name comes from when it was thought that spiders produced their thread in a similar manner to the spinning wheels of old. In fact the process is a pultrusion^[34]—similar to extrusion, with the subtlety that the force is induced by pulling at the finished fiber rather than being squeezed out of a reservoir of some kind.

The unspun silk dope is pulled through silk glands, of which there may be both numerous duplicates and also different types on any one spider species.

§Silk gland

The gland's visible, or external, part is termed the spinneret. Depending on the complexity of the species, spiders will have two to eight sets of spinnerets, usually in pairs. There exist highly different specialised glands in different spiders, ranging from simply a sac with an opening at one end, to the complex, multiple-section Major Ampullate glands of the Nephila golden orb weaving spiders.^[35]

Behind each spinneret visible on the surface of the spider lies a gland, a generalised form of which is shown in the figure to the right, "Schematic of a generalised gland".

Schematic of a generalised gland of a Golden silk orb-weaver. Each differently coloured section highlights a discrete section of the gland^[36][37][38]

The gland described here will be based upon the major ampullate gland from a golden orb weaving spiders as they are the most-studied and presumed to be the most complex.

1. The first section of the gland labelled 1 on Figure 1 is the secretory or tail section of the gland. The walls of this section are lined with cells that secrete proteins Spidroin I and Spidroin II, the main components of this spider’s dragline. These proteins are found in the form of droplets that gradually elongate to form long channels along the length of the final fiber, hypothesized to assist in preventing crack formation or even self-healing of the fiber.^[39]
2. The second section is the storage sac. This stores and maintains the gel-like unspun silk dope until it is required by the spider. In addition to storing the unspun silk gel, it secretes proteins that coat the surface of the final fiber.^[17]
3. The funnel rapidly reduces the large diameter of the storage sac to the small diameter of the tapering duct.
4. The final length is the tapering duct, the site of most of the fiber formation. This consists of a tapering tube with several tight about turns, a valve almost at the end (mentioned in detail at point No. 5 below) ending in a spigot from which the silk fiber emerges. The tube here tapers hyperbolically, therefore the unspun silk is under constant shear stress, which is an important factor in fiber formation. This section of the duct is lined with cells that exchange ions and remove water from the fiber. The spigot at the end has lips that clamp around the fiber, controlling fiber diameter and further retaining water.
5. Almost at the end of the tapering duct is a valve, approximate position marked "5" on figure 1. Though discovered some time ago, the precise purpose of this valve is still under discussion. It is believed to assist in restarting and rejoining broken fibers^[40] acting much in the way of a helical pump, regulating the thickness of the fiber,^[34] and/ or clamping the fiber as a spider falls upon it.^[40][41] There is some discussion on the similarity of the silk worm’s silk press and the roles each of these valves play in the production of silk in these two organisms.

Throughout the process the unspun silk appears to have a nematic texture,^[42] in a similar manner to a liquid crystal. This allows the unspun silk to flow through the duct as a liquid but maintain a molecular order.

As an example of a complex spinning field, the spinneret apparatus of an adult Araneus diadematus (garden cross spider) consists of the following glands:^[29]

• 500 Glandulae piriformes for attachment points
• 4 Glandulae ampullaceae for the web frame
• about 300 Glandulae aciniformes for the outer lining of egg sacs, and for ensnaring prey
• 4 Glandulae tubuliformes for egg sac silk
• 4 Glandulae aggregatae for glue
• 2 Glandulae coronatae for the thread of glue lines.

§Artificial synthesis

In order to artificially synthesize spider silk into fibers, there are two broad areas that must be covered. These are synthesis of the feedstock (the unspun silk dope in spiders), and synthesis of the spinning conditions (the funnel, valve, tapering duct, and spigot). There have been a number of different approaches discussed below.

Single strand of artificial spider silk produced under laboratory conditions.

§Feedstock

As discussed in the Structural section of the article, the molecular structure of unspun silk is both complex and extremely long. Though this endows the silk fibers with their desirable properties, it also makes replication of the fiber somewhat of a challenge. Various organisms have been used as a basis for attempts to replicate some components or all of some or all of the proteins involved. These proteins must then be extracted, purified and then spun before their properties can be tested. The table below shows the results including the true gold standard- actual stress and strain of the fibers as compared to the best spider dragline.

Organism	Details	Average Maximum breaking stress (MPa)	Average Strain (%)	Reference
Gold Standard: Darwin’s bark spider (Caerostris darwini)	Malagasy spider famed for making webs with strands up to 25 m long across rivers. "...C. darwini silk is more than twice as tough as any previously described silk"	1850 ±350	33 ±0.08	[21]
Gold Standard: Nephila clavipes	Typical golden orb weaving spider	710–1200	18–27	[43][44]
Bombyx mori Silkworms	Silkworms were genetically altered to express spider proteins and fibers measured.[45]	660	18.5	[46]
E. coli	Synthesizing such a large and repetitive molecule (250–320 kDa) is complex. Yet, if this is not achieved, the properties will not match those of actual spiders. Here a 285 kDa protein was produced and spun.	508 ±108	15 ±5	[47]
Goats	Goats were genetically modified to secrete silk proteins in their milk, which could then be purified.	285–250	30–40	[48]
Tobacco & potato plants	Spider proteins were inserted into tobacco and potato plants, the rationale being that should this be successful, scaled-up harvesting would be much facilitated. Patents have been granted in this area,[49] but no fibers have yet been described in the literature.	n/a	n/a	[50]

§Geometry

As was shown in the biosynthesis section, spider silks with comparatively simple molecular structure need complex ducts to be able to spin an effective fiber. There have been a number of methods used to produce fibers, of which the main types are briefly discussed below.

§Syringe and needle

Feedstock is simply forced through a hollow needle using a syringe. This method has been shown to make fibers successfully on multiple occasions.^[51][52]

Although very cheap and easy to assemble, the shape and conditions of the gland are very loosely approximated. Fibers created using this method may need encouragement to change from liquid to solid by removing the water from the fiber with such chemicals as the environmentally undesirable methanol^[53] or acetone,^[52] and also may require post-stretching of the fiber to attain fibers with desirable properties.^[47][51]

§Microfluidics

As the field of microfluidics matures, it is likely that more attempts to spin fibers will be made using microfluidics. These have the advantage of being very controllable and able to test spin very small volumes of unspun fiber^[54][55] but setup and development costs are likely to be high. A patent has been granted in this area for spinning fibers in a method mimicking the process found in nature, and fibers are successfully being continuously spun by a commercial company.^[56]

§Electrospinning

Electrospinning is a very old technique whereby a fluid is held in a container in a manner such that it is able to flow out through capillary action. A conducting substrate is positioned below, and a large difference in electrical potential is applied between the fluid and the substrate. The fluid is attracted to the substrate, and tiny fibers jump almost instantly from their point of emission, the Taylor cone, to the substrate, drying as they travel. This method has been shown to create nano-scale fibers from both silk dissected from organisms and regenerated silk fibroin.

§Other artificial shapes formed from silk

Silk can be formed into other shapes and sizes such as spherical capsules for drug delivery, cell scaffolds and wound healing, textiles, cosmetics, coatings, and many others.^[57]

§Research milestones

Due to spider silk being a scientific research field with a long and rich history, there can be unfortunate occurrences of researchers independently rediscovering previously published findings. What follows is a table of the discoveries made in each of the constituent areas, acknowledged by the scientific community as being relevant and significant by using the metric of scientific acceptance, citations. Thus, only papers with 50 or more citations are included.

Table of significant papers (50 or more citations)

Area of contribution	Year	Main researchers [Ref]	Title of paper	Contribution to the field
Chemical Basis	1960	Fischer, F. & Brander, J.[58]	"Eine Analyse der Gespinste der Kreuzspinne" (Amino acid composition analysis of spider silk)
	1960	Lucas, F. & et al.[59][60]	"The Composition of Arthropod Silk Fibrons; Comparative studies of fibroins"
Gene Sequence	1990	Xu, M. & Lewis, R. V.[61]	"Structure of a Protein Superfiber - Spider Dragline Silk"
Mechanical Properties	1964	Lucas, F.[62]	"Spiders and their silks"	First time compared mechanical properties of spider silk with other materials in a scientific paper.
	1989	Vollrath, F. & Edmonds, D. T.[63]	"Modulation of the Mechanical Properties of Spider Silk by Coating with Water"	First important paper suggesting the water interplay with spider silk fibroin modulating the properties of silk.
	2001	Vollrath, F. & Shao, Z.Z.[64]	"The effect of spinning conditions on the mechanics of a spider's dragline silk"
Structural Characterization	1992	Hinman, M.B. & Lewis, R. V[24]	"Isolation of a clone encoding a second dragline silk fibroin. Nephila clavipes dragline silk is a two-protein fiber"
	1994	Simmons, A. & et al.[65]	"Solid-State C-13 Nmr of Nephila-Clavipes Dragline Silk Establishes Structure and Identity of Crystalline Regions"	First NMR study of spider silk.
	1999	Shao, Z., Vollrath, F. & et al.[66]	"Analysis of spider silk in native and supercontracted states using Raman spectroscopy"	First Raman study of spider silk.
	1999	Riekel, C., Muller, M.& et al.[67]	"Aspects of X-ray diffraction on single spider fibers"	First X-ray on single spider silk fibers.
	2000	Knight, D.P., Vollrath, F. & et al.[68]	"Beta transition and stress-induced phase separation in the spinning of spider dragline silk"	Secondary structural transition confirmation during spinning.
	2001	Riekel, C. & Vollrath, F.[69]	"Spider silk fibre extrusion: combined wide- and small-angle X- ray microdiffraction experiments"	First X-ray on spider silk dope.
	2002	Van Beek, J. D. & et al.[26]	"The molecular structure of spider dragline silk: Folding and orientation of the protein backbone"
Structure-Property Relationship	1986	Gosline, G.M. & et al.[70]	"The structure and properties of spider silk"	First attempt to link structure with properties of spider silk
	1994	Termonia, Y[30]	"Molecular Modeling of Spider Silk Elasticity"	X-ray evidence presented in this paper; simple model of crystallites embedded in amorphous regions.
	1996	Simmons, A. & et al.[25]	"Molecular orientation and two-component nature of the crystalline fraction of spider dragline silk"	Two types of alanine-rich crystalline regions were defined.
	2006	Vollrath, F. & Porter, D.[71]	"Spider silk as an archetypal protein elastomer"	New insight and model to spider silk based on Group Interaction Modelling.
Native Spinning	1991	Kerkam, K., Kaplan, D. & et al.[72]	"Liquid Crystallinity of Natural Silk Secretions"
	1999	Knight, D.P. & Vollrath, F.[73]	"Liquid crystals and flow elongation in a spider's silk production line"
	2001	Vollrath, F. & Knight, D.P.[16]	"Liquid crystalline spinning of spider silk"	The most cited paper on spider silk
Reconstituted /Synthetic Spider Silk and Artificial Spinning	1995	Prince, J. T., Kaplan, D. L. & et al.[74]	"Construction, Cloning, and Expression of Synthetic Genes Encoding Spider Dragline Silk"	First successful synthesis of Spider silk by E. coli.
	1998	Arcidiacono, S., Kaplan, D.L. & et al.[75]	"Purification and characterization of recombinant spider silk expressed in Escherichia coli"
	1998	Seidel, A., Jelinski, L.W. & et al.[76]	"Artificial Spinning of Spider Silk"	First controlled wet-spinning of reconstituted spider silk.

§Human uses

A cape made from Madagascar golden orb spider silk.^[77]

Peasants in the southern Carpathian Mountains used to cut up tubes built by Atypus and cover wounds with the inner lining. It reportedly facilitated healing, and even connected with the skin. This is believed to be due to antiseptic properties of spider silk^[78] and because the silk is rich in vitamin K, which can be effective in clotting blood.^[79]

Some fishermen in the Indo-Pacific ocean use the web of Nephila to catch small fish.^[29]

The silk of Nephila clavipes has recently been used to help in mammalian neuronal regeneration.^[80]

At one time, it was common to use spider silk as a thread for crosshairs in optical instruments such as telescopes, microscopes,^[81] and telescopic rifle sights.^[82]

Due to the difficulties in extracting and processing substantial amounts of spider silk, the largest known piece of cloth made of spider silk is an 11-by-4-foot (3.4 by 1.2 m) textile with a golden tint made in Madagascar in 2009.^[83] Eighty-two people worked for four years to collect over one million golden orb spiders and extract silk from them.^[84]

In 2011, spider silk fibers were used in the field of optics to generate very fine diffraction patterns over N-slit interferometric signals utilized in optical communications.^[85] In 2012, spider silk fibers were used to create a set of violin strings.^[86]

Spider silk is used to suspend inertial confinement fusion targets during laser ignition, as it remains considerably elastic and has a high energy to break at temperatures as low as 10-20K. In addition, it is made from "light" atomic number elements that won't emit x-rays during irradiation that could preheat the target so that the pressure differential required for fusion is not achieved.^[87]

§Attempts at producing synthetic spider silk

Replicating the complex conditions required to produce fibers that are comparable to spider silk has proven difficult to accomplish in a laboratory environment. What follows is a miscellaneous list of attempts on this problem.
However, in the absence of hard data accepted by the relevant scientific community, it is difficult to judge whether these attempts have been successful or constructive.

• One approach that does not involve farming spiders is to extract the spider silk gene and use other organisms to produce the spider silk. In 2000, Canadian biotechnology company Nexia successfully produced spider silk protein in transgenic goats that carried the gene for it; the milk produced by the goats contained significant quantities of the protein, 1–2 grams of silk proteins per liter of milk. Attempts to spin the protein into a fiber similar to natural spider silk resulted in fibers with tenacities of 2–3 grams per denier (see BioSteel).^[88][89] Nexia used wet spinning and squeezed the silk protein solution through small extrusion holes in order to simulate the behavior of the spinneret, but this procedure has so far not been sufficient to replicate the properties of native spider silk.^[90]
• Extrusion of protein fibers in an aqueous environment is known as "wet-spinning". This process has so far produced silk fibers of diameters ranging from 10 to 60 μm, compared to diameters of 2.5–4 μm for natural spider silk.
• In March 2010, researchers from the Korea Advanced Institute of Science & Technology (KAIST) succeeded in making spider silk directly using the bacteria E.coli, modified with certain genes of the spider Nephila clavipes. This approach eliminates the need to milk spiders and allows the manufacture the spider silk in a more cost-effective manner.^[91]
• The company Kraig Biocraft Laboratories has used research from the Universities of Wyoming and Notre Dame in a collaborative effort to create a silkworm that has been genetically altered to produce spider silk. In September 2010 it was announced at a press conference at the University of Notre Dame that the effort had been successful.^[92][93]
• The company AMSilk has succeeded in making spidroin using bacteria, and making it into spider silk. They are now focusing on increasing production rate of the spider silk.^[94]

Kevlar

From Wikipedia, the free encyclopedia

Kevlar

Identifiers
CAS Registry Number	24938-64-5 Y
ChemSpider	N
Properties
Molecular formula	[-CO-C6H4-CO-NH-C6H4-NH-]n
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)

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]

History

Stephanie Kwolek, an American chemist of Polish origin, inventor of kevlar

Poly-paraphenylene terephthalamide – branded Kevlar – was invented by Polish-American chemist Stephanie Kwolek while working for DuPont,^[6] in anticipation of a gasoline shortage. In 1964, her group began searching for a new lightweight strong fiber to use for light but strong tires.^[6] The polymers she had been working with at the time, poly-p-phenylene-terephthalate and polybenzamide,^[7] formed liquid crystal while in solution, something unique to those polymers at the time.^[6]

The solution was "cloudy, opalescent upon being stirred, and of low viscosity" and usually was thrown away. However, Kwolek persuaded the technician, Charles Smullen, who ran the "spinneret", to test her solution, and was amazed to find that the fiber did not break, unlike nylon. Her supervisor and her laboratory director understood the significance of her accidental discovery and a new field of polymer chemistry quickly arose. By 1971, modern Kevlar was introduced.^[6] However, Kwolek was not very involved in developing the applications of Kevlar.^[8]

Production

Kevlar is synthesized in solution from the monomers 1,4-phenylene-diamine (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.^{[citation needed]}

Several grades of Kevlar are available:

Kevlar K-29 – in industrial applications, such as cables, asbestos replacement, brake linings, and body/vehicle armor.

Kevlar K49 – high modulus used in cable and rope products.

Kevlar K100 – colored version of Kevlar

Kevlar K119 – higher-elongation, flexible and more fatigue resistant

Kevlar K129 – higher tenacity for ballistic applications

Kevlar AP – 15% higher tensile strength than K-29^[10]

Kevlar XP – lighter weight resin and KM2 plus fiber combination^[11]

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) about 10% reduction in strength occurs after 500 hours. At 260 °C (500 °F) 50% strength reduction occurs after 70 hours.^[15]

Applications

Protection

Cryogenics

Kevlar is often used in the field of cryogenics for its low thermal conductivity and high strength relative to other materials for suspension purposes. It is most often used to suspend a paramagnetic salt enclosure from a superconducting magnet mandrel in order to minimize any heat leaks to the paramagnetic material. It is also used as a thermal standoff or structural support where low heat leaks are desired.

Armor

Pieces of a Kevlar helmet used to help absorb the blast of a grenade

Kevlar is a well-known component of personal armor such as combat helmets, ballistic face masks, and ballistic vests. The PASGT helmet and vest used by United States military forces since the 1980s both have Kevlar as a key component, as do their replacements. Other military uses include bulletproof facemasks used by sentries and spall liners used to protect the crews of armoured fighting vehicles. Even Nimitz-class aircraft carriers include Kevlar armor around vital spaces. Related civilian applications include emergency services' protection gear if it involves high heat (e.g., fire fighting), and Kevlar body armor such as vests for police officers, security, and SWAT.^[16]

Personal protection

Kevlar is used to manufacture gloves, sleeves, jackets, chaps and other articles of clothing^[17] designed to protect users from cuts, abrasions and heat. Kevlar based protective gear is often considerably lighter and thinner than equivalent gear made of more traditional materials.^[16]

Sports

Kevlar is a very popular material for racing canoes.

Personal protection

It is used for motorcycle safety clothing, especially in the areas featuring padding such as shoulders and elbows. In fencing it is used in the protective jackets, breeches, plastrons and the bib of the masks. It is increasingly being used in the peto, the padded covering which protects the picadors' horses in the bullring.

Equipment

In kyudo, or Japanese archery, it may be used as an alternative to more expensive hemp for bow strings. It is one of the main materials used for paraglider suspension lines.^[18] It is used as an inner lining for some bicycle tires to prevent punctures. In table tennis, plies of Kevlar are added to custom ply blades, or paddles, in order to increase bounce and reduce weight. Tennis racquets are sometimes strung with Kevlar. It is used in sails for high performance racing boats.

Shoes

With advancements in technology, Nike used Kevlar in shoes for the first time. It launched the Elite II Series, with enhancements to its earlier version of basketball shoes by using Kevlar in the anterior as well as the shoe laces. This was done to decrease the elasticity of the tip of the shoe in contrast to nylon used conventionally as Kevlar expanded by about 1% against nylon which expanded by about 30%. Shoes in this range included LeBron, HyperDunk and Zoom Kobe VII. However these shoes were launched at a price range much higher than average cost of basketball shoes.

It was also used as speed control patches for certain Soap Shoes models.^{[citation needed]} and the laces for the adidas F50 adiZero Prime football boot.

Music

Audio equipment

Kevlar has also been found to have useful acoustic properties for loudspeaker cones, specifically for bass and midrange drive units.^[19] Additionally, Kevlar has been used as a strength member in fiber optic cables such as the ones used for audio data transmissions.^[20]

Bowed string instruments

Kevlar can be used as an acoustic core on bows for string instruments.^[21] Kevlar's physical properties provide strength, flexibility, and stability for the bow's user. To date, the only manufacturer of this type of bow is CodaBow.^[22]

Kevlar is also presently used as a material for tailcords (a.k.a. tailpiece adjusters), which connect the tailpiece to the endpin of bowed string instruments.^[23]

Drumheads

Kevlar is sometimes used as a material on marching snare drums. It allows for an extremely high amount of tension, resulting in a cleaner sound. There is usually a resin poured onto the Kevlar to make the head airtight, and a nylon top layer to provide a flat striking surface. This is one of the primary types of marching snare drum heads. Remo's "Falam Slam" patch is made with Kevlar and is used to reinforce bass drum heads where the beater strikes.^{[citation needed]}

Woodwind reeds

Kevlar is used in the woodwind reeds of Fibracell. The material of these reeds is a composite of aerospace materials designed to duplicate the way nature constructs cane reed. Very stiff but sound absorbing Kevlar fibers are suspended in a lightweight resin formulation.^[24]

Other uses

Fire dancing

Fire poi on a beach in San Francisco

Wicks for fire dancing props are made of composite materials with Kevlar in them. Kevlar by itself does not absorb fuel very well, so it is blended with other materials such as fiberglass or cotton. Kevlar's high heat resistance allows the wicks to be reused many times.

Frying pans

Kevlar is sometimes used as a substitute for Teflon in some non-stick frying pans.^[25]

Rope, cable, sheath

Kevlar mooring line

The fiber is used in woven rope and in cable, where the fibers are kept parallel within a polyethylene sleeve. The cables have been used in suspension bridges such as the bridge at Aberfeldy in Scotland. They have also been used to stabilize cracking concrete cooling towers by circumferential application followed by tensioning to close the cracks. Kevlar is widely used as a protective outer sheath for optical fiber cable, as its strength protects the cable from damage and kinking. When used in this application it is commonly known by the trademarked name Parafil.^{[citation needed]}

Electricity generation

Kevlar was used by scientists at Georgia Institute of Technology as a base textile for an experiment in electricity-producing clothing. This was done by weaving zinc oxide nanowires into the fabric. If successful, the new fabric will generate about 80 milliwatts per square meter.^[26]

Building construction

A retractable roof of over 60,000 square feet (5,575 square metres) of Kevlar was a key part of the design of Montreal's Olympic stadium for the 1976 Summer Olympics. It was spectacularly unsuccessful, as it was completed ten years late and replaced just ten years later in May 1998 after a series of problems.^[27][28]

Brakes

The chopped fiber has been used as a replacement for asbestos in brake pads. Dust produced from asbestos brakes is toxic, while aramids are a benign substitute.^{[citation needed]}

Expansion joints and hoses

Kevlar can be found as a reinforcing layer in rubber bellows expansion joints and rubber hoses, for use in high temperature applications, and for its high strength. It is also found as a braid layer used on the outside of hose assemblies, to add protection against sharp objects.^{[citation needed]}

Particle physics

A thin Kevlar window has been used by the NA48 experiment at CERN to separate a vacuum vessel from a vessel at nearly atmospheric pressure, both 192 cm in diameter. The window has provided vacuum tightness combined with reasonably small amount of material (only 0.3% to 0.4% of radiation length).^{[citation needed]}

Smartphones

The Motorola RAZR Family and the Motorola Droid Maxx have a Kevlar backplate, chosen over other materials such as carbon fiber due to its resilience and lack of interference with signal transmission.^[29]

Marine Current Turbine and Wind turbine

The Kevlar fiber/epoxy matrix composite materials can be used in marine current turbine (MCT) or wind turbine due to their high specific strength and light weight compared to other fibers.^[30]

Composite materials

Aramid fibers are widely used for reinforcing composite materials, often in combination with carbon fiber and glass fiber. The matrix for high performance composites is usually epoxy resin. Typical applications include monocoque bodies for F1 racing cars, helicopter rotor blades, tennis, table tennis, badminton and squash rackets, kayaks, cricket bats, and field hockey, ice hockey and lacrosse sticks.^{[31][32][33][34]}

Polytetrafluoroethylene (Teflon)

From Wikipedia, the free encyclopedia

 

Polytetrafluoroethylene

Names
IUPAC name poly(1,1,2,2-tetrafluoroethylene)[1]
Other names Syncolon, Fluon, Poly(tetrafluoroethene), Poly(difluoromethylene), Poly(tetrafluoroethylene)
Identifiers
Abbreviations	PTFE
CAS Registry Number	9002-84-0 Y
ChEBI	CHEBI:53251 N
ChemSpider	Y
KEGG	D08974 N=
Properties
Molecular formula	(C2F4)n
Density	2200 kg/m3
Melting point	600 K 327 °C
Thermal conductivity	0.25 W/(m·K)
Hazards
MSDS	External MSDS
NFPA 704	0 1 0
Supplementary data page
Structure and properties	Refractive index (n), Dielectric constant (εr), etc.
Thermodynamic data	Phase behaviour solid–liquid–gas
Spectral data	UV, IR, NMR, MS
Except where noted otherwise, data is given for materials in their standard state (at 25 °C (77 °F), 100 kPa)

Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene that has numerous applications. The best-known brand name of PTFE-based formulas is Teflon by DuPont Co., which discovered the compound.

PTFE is a fluorocarbon solid, as it is a high-molecular-weight compound consisting wholly of carbon and fluorine. PTFE is hydrophobic: neither water nor water-containing substances wet PTFE, as fluorocarbons demonstrate mitigated London dispersion forces due to the high electronegativity of fluorine. PTFE has one of the lowest coefficients of friction against any solid.

PTFE is used as a non-stick coating for pans and other cookware. It is very non-reactive, partly because of the strength of carbon–fluorine bonds and so it is often used in containers and pipework for reactive and corrosive chemicals. Where used as a lubricant, PTFE reduces friction, wear and energy consumption of machinery. It is also commonly used as a graft material in surgical interventions.

§History

Teflon thermal cover showing impact craters, from NASA's Ultra Heavy Cosmic Ray Experiment (UHCRE)

PTFE was accidentally discovered in 1938 by Roy Plunkett while he was working in New Jersey for DuPont. As Plunkett attempted to make a new chlorofluorocarbon refrigerant, the tetrafluoroethylene gas in its pressure bottle stopped flowing before the bottle's weight had dropped to the point signaling "empty." Since Plunkett was measuring the amount of gas used by weighing the bottle, he became curious as to the source of the weight, and finally resorted to sawing the bottle apart. He found the bottle's interior coated with a waxy white material that was oddly slippery. Analysis showed that it was polymerized perfluoroethylene, with the iron from the inside of the container having acted as a catalyst at high pressure. Kinetic Chemicals patented the new fluorinated plastic (analogous to the already known polyethylene) in 1941,^[2] and registered the Teflon trademark in 1945.^[3][4]

By 1948, DuPont, which founded Kinetic Chemicals in partnership with General Motors, was producing over two million pounds (900 tons) of Teflon brand PTFE per year in Parkersburg, West Virginia.^[5] An early use was in the Manhattan Project as a material to coat valves and seals in the pipes holding highly reactive uranium hexafluoride at the vast K-25 uranium enrichment plant in Oak Ridge, Tennessee.^[6]

In 1954, the wife of French engineer Marc Grégoire urged him to try the material he had been using on fishing tackle on her cooking pans. He subsequently created the first Teflon-coated, non-stick pans under the brandname Tefal (combining "Tef" from "Teflon" and "al" from aluminum).^[7] In the United States, Marion A. Trozzolo, who had been using the substance on scientific utensils, marketed the first US-made Teflon-coated pan, "The Happy Pan", in 1961.^[8]

However, Tefal was not the only company to utilize PTFE in nonstick cookware coatings. In subsequent years, many cookware manufacturers developed proprietary PTFE-based formulas, including Swiss Diamond International, which uses a diamond-reinforced PTFE formula,^[9] Scanpan which uses a titanium-reinforced PTFE formula,^[10] and All-Clad^[11] and Newell Rubbermaid's Calphalon which use a non-reinforced PTFE-based nonstick.^[12] Other cookware companies, such as Meyer Corporation's Anolon, use Teflon^[13] nonstick coatings purchased from DuPont.

In the 1990s, it was found that PTFE could be radiation cross-linked above its melting point in an oxygen-free environment.^[14] Electron beam processing is one example of radiation processing. Cross-linked PTFE has improved high-temperature mechanical properties and radiation stability. This was significant because, for many years, irradiation at ambient conditions had been used to break down PTFE for recycling.^[15] The radiation-induced chain scissioning allows it to be more easily reground and reused.

§Production

PTFE is produced by free-radical polymerization of tetrafluoroethylene. The net equation is:

n F₂C=CF₂ → 1/n —{ F₂C—CF₂}_n—

Because tetrafluoroethylene can explosively decompose to tetrafluoromethane and carbon, special apparatus is required for the polymerization to prevent hot spots that might initiate this dangerous side reaction. The process is typically initiated with persulfate, which homolyzes to generate sulfate radicals:

[O₃SO-OSO₃]²⁻⇌ 2 SO₄•⁻

The resulting polymer is terminated with sulfate ester groups, which can be hydrolyzed to give OH-end-groups.^[16]
Because PTFE is poorly soluble in almost all solvents, the polymerization is conducted as an emulsion in water. This process gives a suspension of polymer particles. Alternatively, the polymerization is conducted using a surfactant such as PFOS.

§Properties

PTFE is often used to coat non-stick pans as it is hydrophobic and possesses fairly high heat resistance.

"Amazing New Concept in Cooking"

PTFE is a thermoplastic polymer, which is a white solid at room temperature, with a density of about 2200 kg/m³. According to DuPont, its melting point is 600 K (327 °C; 620 °F).^[17] It maintains high strength, toughness and self-lubrication at low temperatures down to 5 K (−268.15 °C; −450.67 °F), and good flexibility at temperatures above 194 K (−79 °C; −110 °F).^[18] PTFE gains its properties from the aggregate effect of carbon-fluorine bonds, as do all fluorocarbons. The only chemicals known to affect these carbon-fluorine bonds are certain alkali metals and fluorinating agents such as xenon difluoride and cobalt(III) fluoride.^[19]

Property	Value
Density	2200 kg/m3
Melting point	600 K
Thermal expansion	135 · 10−6 K−1 [20]
Thermal diffusivity	0.124 mm²/s [21]
Young's modulus	0.5 GPa
Yield strength	23 MPa
Bulk resistivity	1016 Ω·m [22]
Coefficient of friction	0.05–0.10
Dielectric constant	ε=2.1,tan(δ)<5(-4)
Dielectric constant (60 Hz)	ε=2.1,tan(δ)<2(-4)
Dielectric strength (1 MHz)	60 MV/m

The coefficient of friction of plastics is usually measured against polished steel.^[23] PTFE's coefficient of friction is 0.05 to 0.10,^[17] which is the third-lowest of any known solid material (BAM being the first, with a coefficient of friction of 0.02; diamond-like carbon being second-lowest at 0.05). PTFE's resistance to van der Waals forces means that it is the only known surface to which a gecko cannot stick.^[24] In fact, PTFE can be used to prevent insects climbing up surfaces painted with the material. PTFE is so slippery that insects cannot get a grip and tend to fall off. For example, PTFE is used to prevent ants climbing out of formicaria.

Because of its chemical inertness, PTFE cannot be cross-linked like an elastomer. Therefore, it has no "memory" and is subject to creep. Because of its superior chemical and thermal properties, PTFE is often used as a gasket material. However, because of the propensity to creep, the long-term performance of such seals is worse than for elastomers which exhibit zero, or near-zero, levels of creep. In critical applications, Belleville washers are often used to apply continuous force to PTFE gaskets, ensuring a minimal loss of performance over the lifetime of the gasket.^[25]

§Applications and uses

The major application of PTFE, consuming about 50% of production, is for wiring in aerospace and computer applications (e.g. hookup wire, coaxial cables). This application exploits the fact that PTFE has excellent dielectric properties. This is especially true at high radio frequencies, making it suitable for use as an insulator in cables and connector assemblies and as a material for printed circuit boards used at microwave frequencies. Combined with its high melting temperature, this makes it the material of choice as a high-performance substitute for the weaker and lower-melting-point polyethylene commonly used in low-cost applications.

In industrial applications, owing to its low friction, PTFE is used for applications where sliding action of parts is needed: plain bearings, gears, slide plates, etc. In these applications, it performs significantly better than nylon and acetal; it is comparable to ultra-high-molecular-weight polyethylene (UHMWPE). Although UHMWPE is more resistant to wear than PTFE, for these applications, versions of PTFE with mineral oil or molybdenum disulfide embedded as additional lubricants in its matrix are being manufactured. Its extremely high bulk resistivity makes it an ideal material for fabricating long-life electrets, useful devices that are the electrostatic analogues of magnets.

Gore-Tex is a material incorporating a fluoropolymer membrane with micropores. The roof of the Hubert H. Humphrey Metrodome in *Minneapolis, US, was one of the largest applications of PTFE coatings. 20 acres (81,000 m²) of the material was used in the creation of the white double-layered PTFE-coated fiberglass dome.

§Other

PTFE (Teflon) is best known for its use in coating non-stick frying pans and other cookware, as it is hydrophobic and possesses fairly high heat resistance.

PTFE tapes with pressure-sensitive adhesive backing

§Niche

PTFE is a versatile material that is found in many niche applications:

• It can be stretched to contain small pores of varying sizes and is then placed between fabric layers to make a waterproof, breathable fabric in outdoor apparel.
• It is used widely as a fabric protector to repel stains on formal school-wear, like uniform blazers, in the UK.
• It is used as a film interface patch for sports and medical applications, featuring a pressure-sensitive adhesive backing, which is installed in strategic high friction areas of footwear, insoles, ankle-foot orthosis, and other medical devices to prevent and relieve friction-induced blisters, calluses and foot ulceration.
• Powdered PTFE is used in pyrotechnic compositions as an oxidizer with powdered metals such as aluminium and magnesium. Upon ignition, these mixtures form carbonaceous soot and the corresponding metal fluoride, and release large amounts of heat. They are used in infrared decoy flares and as igniters for solid-fuel rocket propellants.^[26]
• In optical radiometry, sheets of PTFE are used as measuring heads in spectroradiometers and broadband radiometers (e.g., illuminance meters and UV radiometers) due to PTFE's capability to diffuse a transmitting light nearly perfectly. Moreover, optical properties of PTFE stay constant over a wide range of wavelengths, from UV down to near infrared. In this region, the relation of its regular transmittance to diffuse transmittance is negligibly small, so light transmitted through a diffuser (PTFE sheet) radiates like Lambert's cosine law. Thus PTFE enables cosinusoidal angular response for a detector measuring the power of optical radiation at a surface, e.g. in solar irradiance measurements.
• Certain types of hardened, armor-piercing bullets are coated with PTFE to reduce wear on firearms's rifling that harder projectiles would cause. PTFE itself does not give a projectile an armor-piercing property.^[27]
• Its high corrosion resistance makes PTFE useful in laboratory environments, where it is used for lining containers, as a coating for magnetic stirrers, and as tubing for highly corrosive chemicals such as hydrofluoric acid, which will dissolve glass containers. It is used in containers for storing fluoroantimonic acid, a superacid.^{[citation needed]}
• PTFE tubes are used in gas-gas heat exchangers in gas cleaning of waste incinerators. Unit power capacity is typically several megawatts.
• PTFE is widely used as a thread seal tape in plumbing applications, largely replacing paste thread dope.
• PTFE membrane filters are among the most efficient industrial air filters. PTFE-coated filters are often used in dust collection systems to collect particulate matter from air streams in applications involving high temperatures and high particulate loads such as coal-fired power plants, cement production and steel foundries.
• PTFE grafts can be used to bypass stenotic arteries in peripheral vascular disease if a suitable autologous vein graft is not available.
• Many bicycle lubricants contain PTFE and are used on chains and other moving parts.
• PTFE can also be used for dental fillings to isolate the contacts of the anterior tooth so the filling materials will not stick to the adjacent tooth.

§Safety

The pyrolysis of PTFE is detectable at 200 °C (392 °F), and it evolves several fluorocarbon gases and a sublimate. An animal study conducted in 1955 concluded that it is unlikely that these products would be generated in amounts significant to health at temperatures below 250 °C (482 °F).^[28] More recently, however, a study documented birds having been killed by these decomposition products at 202 °C (396 °F), with unconfirmed reports of bird deaths as a result of non-stick cookware heated to as little as 163 °C (325 °F).^[29]

While PTFE is stable and nontoxic at lower temperatures, it begins to deteriorate after the temperature of cookware reaches about 260 °C (500 °F), and decomposes above 350 °C (662 °F).^{[citation needed]} These degradation by-products can be lethal to birds,^[30] and can cause flu-like symptoms^[31] in humans. In May, 2003, the environmental research and advocacy organization Environmental Working Group filed a 14-page brief with the U.S. Consumer Product Safety Commission petitioning for a rule requiring that cookware and heated appliances bearing non-stick coatings carry a label warning of hazards to people and to birds.

Meat is usually fried between 204 and 232 °C (399 and 450 °F), and most oils start to smoke before a temperature of 260 °C (500 °F) is reached, but there are at least two cooking oils (refined safflower oil and avocado oil) that have a higher smoke point than 260 °C (500 °F). Empty cookware can also exceed this temperature when heated.

§PFOA

Perfluorooctanoic acid (PFOA, or C8) has been used as a surfactant in the emulsion polymerization of PTFE, although several manufacturers have entirely discontinued its use. Overall, PTFE cookware is considered an insignificant exposure pathway to PFOA.^[32][33]

§Similar polymers

Teflon is also used as the trade name for a polymer with similar properties, perfluoroalkoxy polymer resin (PFA).

The Teflon trade name is also used for other polymers with similar compositions:

• Perfluoroalkoxy (PFA)
• Fluorinated ethylene propylene (FEP)

These retain the useful PTFE properties of low friction and nonreactivity, but are more easily formable. For example, FEP is softer than PTFE and melts at 533 K (260 °C; 500 °F); it is also highly transparent and resistant to sunlight.^[34]