Man-made synthetic fibers

7 07 2010

For millennia mankind depended on the natural world to supply its fiber needs.  But scientists, as a result of extensive research, were able to replicate naturally occurring animal and plant fibers by creating fibers from synthetic chemicals. In the literature, it is often noted that there are three kinds of man-made fibers: those made by “transformation of natural polymers” (also called regenerated cellulosics), those made from synthetic polymers and those made from inorganic materials (These include the fibers made of glass, metal, ceramics and carbon.) But by far the largest group of man made synthetic fibers being produced today are made from synthetic polymers, so we’ll concentrate on those in this post.

Man made  fibers from synthetic polymers  are created using polymerization of various chemical inputs to create polymers.  Polymerization is the process of combining many small molecules into a large molecule – a polymer.    Polymers are simply large molecules composed of repeating structural units.  Polymers used for synthetic fibers are produced from intermediates (which in turn have been produced from crude oil) and applying a catalyst.  Key intermediates are p-Xylene, teraphtalic acid, ethylene glycol and acrylonitrile;  catalysts – manganese, cobalt and antimony oxide –  are used to control the processes.

Polymers are the building blocks of synthetic fibers – and of many other things.   They are the basis of life and play an essential and ubiquitous role in our everyday life, ranging from familiar synthetic plastics to natural biopolymers such as DNA and proteins that are essential for life.  Natural polymeric materials such as shellac, amber and natural rubber have been in use for centuries. Biopolymers, such as proteins and nucleic acids, play crucial roles in biological processes. A variety of other natural polymers exist, such as cellulose, which is the main constituent of cotton and wood.

Synthetic polymers include vulcanized rubber, Bakelite and neoprene (and many more) in addition to polymers used in fibers:   polyester, nylon, polystyrene, polyethylene, PVC, and polyacrylonitrile (known as acrylic).

Synthetic fibers account for about half of all fiber usage, with applications in every field of fiber and textile technology.  Four synthetic fibers – nylon, polyester, acrylic and polyolefin – dominate the market. These four account for approximately 98% by volume of global synthetic fiber production.  But make no mistake, polyester is king:   polyester alone accounted for around 80% of the global market share of man made fibers.[1] Polyester has become the fiber of choice (sometimes blended with cotton) in garment production.  As recently as 1990, world polyester production totaled 20 billion pounds.  In 2002, production had more than doubled to 46 billion pounds – and was 61.5 billion pounds in 2009. [2] Polyester fiber consumption increased at an annual growth rate of 6.2% between 1998 and 2008,[3] although demand has recently moderated as a result of the global economic slowdown.

The raw materials used in synthetic production are mainly produced by large chemical companies which are sometimes integrated down to the crude oil refinery where p-Xylene is the base material used to produce other intermediaries.  For example p-Xylene is used to produce teraphtalic acid.   Major producers of teraphtalic acid include:

  • BP
  • Reliance (India-based Reliance just bought Trevira GmbH&Co)
  • Eastman Chemicals
  • Mitsui
  • Sinopec
  • SK-Chemicals

Among the world’s largest polyester producers are the following companies:

  • DuPont
  • Eastman
  • Invista
  • Wellman
  • M&G Group
  • Mitsui
  • Mitsubishi
  • Reliance
  • Teijin
  • Toray
  • Hyosung
  • Huvis
  • Jiangsu Hengli Chemical Fiber
  • Jiangsu Sanfangxian Industry
  • NanYa Plastics

Synthetic fiber production has definitely moved to Asian countries.   As stated in Textile World :   “the critical mass of fiber manufacturing from the industrialized West to the developing East is a study in comparative economics and social realities.”  In 1990, China represented barely 8% of total man-made production; by 2002 it produced almost 30% (almost a tie with the US and Europe).   India is also making a pitch to be a player: From a virtually nonexistent position in 1990 to currently 5%, with programs in place to expand this further.   According to Textile World, the world of polyester production has begun to resemble a monopoly, led by China.  “The speed with which Asia has dominated fiber production is astounding. The commitment is complete, and the world man-made fiber industry will never be the same – and that’s not necessarily a bad thing. It is obvious that production asset investments of the recent decade are world-class in efficiency and quality – with the world consumer receiving the benefits. The industrialized world must move on to a higher-return economy and let the developing world be satisfied by lower returns on investment, either through lower labor or local funds costs; or government-subsidized manufacturing aimed at employment, and/or accumulation of strong currencies to be used for continued economic development. Either way, the new nexus of the man-made fiber business is Asia.”

Since polyester is the king of synthetics (and because the data is available!) let’s look at how polyester is formed.

 

POLYMERIZATION:
First, the polymer is created; in the case of polyester, the polymer is made by heating either dimethyl teraphthalate (DMT)  or terephthalic acid (TPA) with ethylene glycol in the presence of a catalyst (ususally antimony) at 536 º F for 30 minutes at atmospheric pressure and then for 10 hours under vacuum. The excess of ethylene glycol is distilled off.  The resulting chemical, a monomer (single, non-repeating molecule) alcohol, is combined with terephthalic acid and raised to a temperature of 472°F (280°C). Newly-formed polyester, which is clear and molten, is extruded through a slot to form long ribbons.

DRYING: After the polyester emerges from polymerization, the long molten ribbons are allowed to cool until they become brittle. The material is cut into tiny chips and completely dried to prevent irregularities in consistency.

SPINNING:  Fibers are classified according to the type of spinning that the polymer undergoes: this can be melt spinning, dry spinning or wet spinning:

  1. Melt spinning is the simplest of these three methods:   In melt spinning, the polymer chips are melted at 500 – 518ºF to form a syrup-like solution.  The solution is put in a metal container called a spinneret and forced through its tiny holes. The number of holes in the spinneret determines the size of the yarn, as the emerging fibers are brought together to form a single strand. Melt spinning is used with polymers such as nylon, polyethylene, polyvinyl chloride, cellulose triacetate, and polyethylene terephthalate, and in the multifilament extrusion of polypropylene.
  2. Dry spinning:  the polymer is first dissolved in a solvent. The polymer solution  is then extruded through the spinnerets. The solvent is evaporated with hot air and collected for reuse. The fiber then passes over rollers, and is stretched to orient the molecules and increase the fiber strength. Cellulose acetate, cellulose triacetate, acrylic, modacrylic, aromatic nylon, and polyvinyl chloride are made by dry spinning.
  3. In wet spinning, the polymer solution (i.e., polymer dissolved in a solvent as in dry spinning) is spun into a coagulating solution to precipitate the polymer. This process has been used with acrylic, modacrylic, aromatic nylon, and polyvinyl chloride fibers.

For each pound of fiber produced with solvent spinning processes (dry or wet), a pound of polymer is dissolved in about 3 pounds of solvent.  So the capture and recovery of these solvents is an integral part of the solvent spinning process.  At present, most solvents are recovered, however emissions from the spinning operation are a significant consideration.  But air pollution emissions from polyester fiber production also include polymer dust from drying operations, volatilized residual monomer, fiber lubricants (in the form of fume or oil smoke) and the burned polymer and combustion products from cleaning the spinning equipment.

At the spinning stage, other chemicals may be added to the solution to achieve various effects such as making the material flame retardant, antistatic, or colorful (by adding dye chemicals).  Because these fibers are created from crude oil, they’re highly flammable (in fact, they’re considered an accelerant)  and pose a great threat for fire injury.  The development of a durable flame retardant for synthetics was key in the safe consumer use of synthetic fibers.

It is at the spinning stage that the two varieties of polyester fibers are created: filament and staple fibers:

  • FILAMENT:  When polyester emerges from the spinneret, it is soft and easily elongated up to five times its original length.  To create filament, the fibers are stretched.  The stretching forces the random polyester molecules to align in a parallel formation. This increases the strength, tenacity, and resilience of the fiber. This time, when the filaments dry, the fibers become solid and strong instead of brittle.   Stretched, or drawn,  fibers may vary greatly in diameter and length, depending on the characteristics desired of the finished material. Also, as the fibers are drawn, they may be textured or twisted to create softer or duller fabrics. After the polyester yarn is drawn, it is wound on large bobbins or flat-wound packages, ready to be woven into material.
  • STAPLE:  To create staple fiber, the spinneret has many more holes than when the production is filament fiber.  The rope like bundles of polyester that emerge are called tow.
    • Newly-formed tow is quickly cooled in cans that gather the thick fibers. Several lengths of tow are gathered and then drawn on heated rollers to three or four times their original length.
    • CRIMPING: Drawn tow is then fed into compression boxes, which force the fibers to fold like an accordion, at a rate of 9-15 crimps per inch (3-6 per cm). This process helps the fiber hold together during the later manufacturing stages.
    • SETTING: After the tow is crimped, it is heated at 212-302°F (100-150°C) to completely dry the fibers and set the crimp. Some of the crimp will unavoidably be pulled out of the fibers during the following processes.
    • CUTTING:  Following heat setting, tow is cut into shorter lengths. Polyester that will be blended with cotton is cut in 1.25-1.50 inch (3.2-3.8 cm) pieces; for rayon blends, 2 inch (5 cm) lengths are cut. For heavier fabrics, such as carpet, polyester filaments are cut into 6 inch (15 cm) lengths.

By and large, synthetic fibers are used for their utility in specific markets.

Polyester is difficult and expensive to dye, but has attributes that make it ideal for blending with cotton and other natural fibers.  Easy care of the permanent press fabric made polyester doubleknits extremely popular in the late 1960s.


However, polyester has suffered an “image problem” since that time, and clothes made out of polyester were often devalued and even ridiculed.  Polyester has the advantage of being very cheap to produce, but it is a much less attractive fiber to live with when compared to the inherent breathability, moisture absorption capabilities and heat moderation inherent in natural fibers.  Polyesters have the advantage in wash-and-wear properties, wrinkle resistance – and in durability.   Manufacturers tried to make polyester easier to use in garments by blending polyester with cotton, wool or other natural fibers.   Several new forms of polyester introduced in the early 1990s may help revitalize the image of polyester. A new form of polyester fiber, called microfiber, was introduced to the public in 1991.  Microfibers have diameters that are less than typical fibers; they are about half the diameter of fine silk fiber, one-quarter the diameter of fine wool, and one hundred times finer than human hair. Microfibers allow a fabric to be woven that is lightweight and strong. They can be tightly woven so that wind, rain, and cold do not easily penetrate. Rainwear manufactures use microfibers for this reason. They also have the ability to allow perspiration to pass through them. In addition, microfibers are very flexible because their small fibers can easily slide back and forth on one another. The first fabric made from microfiber was Ultrasuade, in which short polyester microfibers were imbedded into a polyurethane base. Today, microfibers are manufactured primarily from polyesters, nylon, and acrylic fibers. They are used under various trade names to make a variety of products, such as clothing, hosiery, bedding, and scarves.

Textile researchers at North Carolina State University are developing a form of polyester that may be as strong as Kevlar, a superfiber material used to make bulletproof vests.

Nylon, the granddaddy of man-made fibers, seems to be losing share to polyester, overwhelmed by sheer volume if not performance. In carpets, staple nylon gradually is being replaced by filament; tires increasingly use polyester over nylon; and many woven industrial and apparel fabrics seem to favor polyester. Nylon’s dyeability is an advantage, but not sufficiently so to overcome the supply and variants available in polyester.

Acrylic gradually is losing the price battle with polyester and increasingly is relegated to bulk and wool-substitute end-uses.


[1]http://www.officialwire.com/main.php?action=posted_news&rid=137418

[2] Luke, John; Fiber World: A Polyester Saga Geography and All; Textile World; http://www.textileworld.com/Articles/2004/September/Fiber_World/A_Polyester_Saga_Geography_And_All.html

[3] http://www.officialwire.com/main.php?action=posted_news&rid=137418





Will the antimony in polyester fabric hurt me?

17 02 2010

Synthetic fibers are the most popular fibers in the world with 65% of world production of fibers being synthetic and  35%  natural fibers. (1)  Fully  70% of that synthetic fiber production is polyester. There are many different types of polyester, but the type most often produced for use in textiles is polyethylene terephthalate, abbreviated PET.   Used in a fabric, it’s most often referred to as “polyester” or “poly”.  It is very cheap to produce, and that’s a primary driver for its use in the textile industry.

The majority of the world’s PET production – about 60% – is used to make fibers for textiles; and about  30% is used to make bottles.   Annual PET production requires 104 million barrels of oil  – that’s 70 million barrels just to produce the virgin polyester used in fabrics.(2)  That means most polyester – 70 million barrels worth -  is manufactured specifically to be made into fibers, NOT bottles, as many people think.  Of the 30% of PET which is used to make bottles, only a tiny fraction is recycled into fibers.  But the idea of using recycled bottles – “diverting waste from landfills” – and turning it into fibers has caught the public’s imagination.  There are many reasons why using recycled polyester (often called rPET) is not a good choice given our climate crisis, but today’s post is concentrating on only one aspect of polyester: the fact that antimony is used as a catalyst to create PET.  We will explore what that means.

Antimony is present in 80 – 85% of all virgin PET.  Antimony is a carcinogen, and toxic to the heart, lungs, liver and skin.  Long term inhalation causes chronic bronchitis and emphysema.  The industry will say that  although antimony is used as a catalyst in the production process, it  is “locked” into the finished polymer, and not a concern to human health.  And that’s correct:   antimony used in the production of  PET fibers becomes chemically bound to the PET polymer  so your PET fabric does contain antimony but it isn’t available to your living system. (2)

But wait!  Antimony is leached from the fibers during the high temperature dyeing process.  The antimony that leaches from the fibers  is expelled with the wastewater into our rivers (unless the fabric is woven at a mill which treats its wastewater).  In fact, as much as 175ppm of antimony can be leached from the fiber during the dyeing process. This seemingly insignificant amount translates into a burden on water treatment facilities when multiplied by 19 million lbs each year -  and it’s still a hazardous waste when precipitated out during treatment. Countries that can afford technologies that precipitate the metals out of the solution are left with a hazardous sludge that must then be disposed of in a properly managed landfill or incinerator operations. Countries who cannot or who are unwilling to employ these end-of-pipe treatments release antimony along with a host of other dangerous substances to open waters.

But what about the antimony that remains in the PET fabric?  We do know that antimony leaches from PET bottles into the water or soda inside the bottles.  The US Agency for Toxic Substances and Disease Registry says that the antimony in fabric is very tightly bound and does not expose people to antimony, (3) as I mentioned earlier.    So if you want to take the government’s word for it,  antimony in  PET  is not a problem for human health  -  at least directly in terms of exposure from fabrics which contain antimony.  (Toxics crusader William McDonough has been on antimony’s case for years, however, and takes a much less sanguine view of antimony. (4) )

Antimony is just not a nice thing to be eating or drinking, and wearing it probably won’t hurt you, but the problem comes up during the production process  – is it released into our environment?  Recycling PET is a high temperature process, which creates wastewater tainted with antimony trioxide – and  the dyeing process for recycled PET is problematic as I mentioned in an earlier post.   Another problem occurs when the PET (recycled or virgin) is finally incinerated at the landfill – because then the antimony is released as a gas (antimony trioxide).  Antimony trioxide  has been classified as a carcinogen in the state of California since 1990, by various agencies in the U.S. (such as OSHA, ACGIH and IARC)  and in the European Union.  And the sludge produced during PET production (40 million pounds in the U.S. alone) when incinerated creates 800,000 lbs of fly ash which contains antimony, arsenic and other metals used during production.(5)

Designers are in love with polyesters because they’re so durable – and cheap (don’t forget cheap!).  So they’re used a lot for public spaces.  Abrasion results are a function not only of the fiber but also the construction of the fabric, and cotton and hemp can be designed to be very durable, but they will never achieve the same abrasion results that some polyesters have achieved – like 1,000,000 rubs.  In the residential market, I would think most people wouldn’t want a fabric to last that long – I’ve noticed sofas which people leave on the streets with “free” signs on them, and never once did I notice that the sofa was suffering from fabric degredation!  The “free” sofa just had to go because it was out of style, or stained, or something – I mean, have you even replaced a piece of furniture because the fabric had actually worn out?  Hemp linens have been known to last for generations.

But I digress.   Synthetic fibers can do many things that make our lives easier, and in many ways they’re the true miracle fibers.  I think there will always be a place for (organic) natural fibers, which are comfortable and soothing next to human skin.  And they certainly have that cachet: doesn’t  silk damask sound better than Ultrasuede? The versatile synthetics have a place in our textile set – but I think the current crop of synthetics must be changed so the toxic inputs are removed and the nonsustainable feedstock (oil) is replaced.  I have great hope for the biobased polymer research going on, because the next generation of miracle fibers just might come from sustainable sources.

(1) “New Approach of Synthetic Fibers Industry”, Textile Exchange,  http://www.teonline.com/articles/2009/01/new-approach-of-synthetic-fibe.html

(2) Polyester, Absolute Astronomy.com: http://www.absoluteastronomy.com/topics/Polyester and Pacific Institute, Energy Implications of Bottled Water, Gleick and Cooley, Feb 2009, http://www.pacinst.org/reports/bottled_water/index.htm)

(3)  Shotyk, William, et al, “Contamination of Canadian and European Bottled waters with antimony from PET containers”, Journal of Environmental Monitoring, 2006.   http://www.rsc.org/delivery/_ArticleLinking/DisplayHTMLArticleforfree.cfm?JournalCode=EM&Year=2006&ManuscriptID=b517844b&Iss=2

(4)   http://www.atsdr.cdc.gov/toxprofiles/phs23.html

(5)  http://www.victor-innovatex.com/doc/sustainability.pdf

(3) http://www.greenatworkmag.com/gwsubaccess/02mayjun/eco.html








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