What does “mercerized” cotton mean?

5 12 2012

fullsizeMercerization is a process applied to cellulosic  fibers  – typically cotton (or cotton-covered thread with a polyester core)  but hemp and linen can be mercerized also – to increase luster.  It is done after weaving (in the case of fabrics) or spinning (for yarns or threads).  But early on it was found that the process also had secondary benefits:  the mercerized fibers were able to absorb more water, and therefore absorb more dye, making the color of the dyed cloth brighter and deeper.  The difference is dramatic:  mercerization increases the absorption of dyestuffs by as much as 25%.[1]  unmercerized-101mercerized-101Not only is the color brighter, it also gives the cloth a better resistance to multiple washings,  keeping the colors bright and unchanged over time.   In addition to increasing luster and affinity to dyestuffs, the  treatment increases strength, smoothness, resistance to mildew, and also reduces lint.  So higher quality yarns and fabrics,  for example, are always mercerized.

The process goes back to the 1880’s.   John Mercer was granted a British Patent for his discovery that cotton and other fibers changed character when subjected to caustic soda (NaOh, also known as sodium hydroxide or lye), sulfuric acid, and/or other chemicals.   One of the changes was that caustic soda caused the fiber to swell, become round and straighten out.  But so what – these changes didn’t impart any luster to the fibers, so his patent was largely ignored.  Then in 1890 Horace Lowe found that by applying Mercer’s caustic soda process to cotton yarn or fabric under tension, the fabric gained a  high luster  as a result of the light reflection off the smooth, round surface created by the NaOH. It became an overnight success and revolutionized the cotton industry. The rest is history.[2]

Later testing proved that cotton fiber in its roving state (no twist in the yarns) would absorb more NaOH than fiber in a twisted state and as a result would absorb more water or dye.  Since fine, long stapled fiber gives the best absorption with the lowest twist, ( some twist is required for treating under tension to gain luster) it is usually the long fiber types of cotton (Sea Island, Egyptian, Pima) that are selected for yarn to be mercerized.   So mercerized cotton fabric starts with a better quality cotton fiber.

How is it done?

To get the desired luster and tensile strength,  cotton is held under specified tension for about ten minutes with an application of between 21%-23% caustic soda (NaOH) and wetting agents (used to facilitate the transfer of the NaOH into the fibers), at room temperature.  Then the fabric is neutralized in an acid bath.

Luster is a result of light reflection off a surface. The more glass like the surface, the better the luster. Yarn in its spun, treated state still has a very fine covering of tiny fiber ends (fuzz). This fuzz is removed by passing the yarn (or fabric) through a controlled heated atmosphere termed singeing (gas fired in the past, electric more currently) resulting in a cleaner surface.  (Luster is a result of light reflection off a surface. The more glass like the surface, the better the luster.)
You knew I’d have to look at the toxicity profile of sodium hydroxide, which is considered one of the building blocks of chemistry.  It’s a very powerful alkali.   It’s used in industry in a broad range of categories: chemical manufacturing; pulp and paper manufacturing; cleaning products such as drains, pipe lines and oven cleaners ; petroleum and natural gas; cellulose film;  and water treatment as well as textiles. The US Food and Drug Administration (FDA) considers sodium hydroxide to be generally safe, and recognizes it as not being found to pose unacceptable dietary risks, though it is generally only used on food contact surfaces rather than in foodstuffs.

The chemical is toxic to wildlife, and the EPA requires that effluent containing NaOH not be discharged into groundwater.  Because sodium hydroxide falls in the group of chemicals (salts) which are by far the most often used in textile processing, the sheer volume of NaOH used by the textile industry is important to recognize.  Usual salt concentrations in cotton mill wastewater can be 2,000 – 3,000 ppm[3], far in excess of Federal guidelines for in-stream salt concentrations of 230 ppm.  So treatment of effluent is very important, as prevention is the only reasonable alternative to solve the environmental problems associated with this hard-to-treat, high volume waste.  I have read that electrochemical cell treatment might be a substitute for using NaOH to mercerize.  This process occurs in a low voltage electrochemical cell that mercerizes, sours, and optionally bleaches without effluents and without the purchase of bulk caustic, neutralizing acids, or bleaches.

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When is recycled polyester NOT recycled polyester?

23 03 2011

Fabric might be the only product I can think of which is known by its component parts, like cotton, silk, wool.  These words usually refer to the fabric rather than the fiber used to make the fabric.  We’ve all done it: talked about silk draperies, cotton sheets.  There seems to be a disassociation between the fibers used and the final product, and people don’t think about the process of turning cotton bolls or silkworm cocoons or flax plants into luxurious fabrics.

There is a very long, involved and complex process needed to turn raw fibers into finished fabrics.  Universities award degrees in textile engineering,  color chemistry or any of a number of textile related fields.  One can get a PhD in fiber and polymer science,  or study the design, synthesis and analysis of organic dyes and pigments.  Then there is the American Association of Textile Chemists and Colorists (AATCC) which has thousands of members in 60 different countries.  My point is that we need to start focusing on the process of turning raw textile fiber into a finished fabric – because therein lies all the difference!

And that brings me to recycled polyester, which has achieved pride of place as a green textile option in interiors.  We have already posted blogs about plastics (especially recycled plastics) last year (on 4.28.10, 5.05.10 and 5.12.10) so you know where we stand on the use of plastics in fabrics.  But the reality is that polyester bottles exist,  and recycling some of them  into fiber seems to be a better use for the bottles than landfilling them.

But today the supply chains for recycled polyester are not transparent, and if we are told that the resin chips we’re using to spin fibers are made from bottles – or from any kind of  polyester  –  we have no way to verify that.  Once the polymers are at the melt stage, it’s impossible to tell where they came from, because the molecules are the same.  So the yarn/fabric  could be virgin polyester or  it could be recycled.   Many so called “recycled” polyester yarns may not really be from recycled sources at all because – you guessed it! – the process of recycling is much more expensive than using virgin polyester.   And unfortunately not all companies are willing to pay the price to offer a real green product, but they sure do want to take advantage of the perception of green.   So when you see a label that says a fabric is made from 50% polyester and 50% recycled polyester – well, there is absolutely no way to tell if that’s true.

Some companies are trying to differentiate their brands by confirming that what they say is recycled REALLY is from recycled sources.  Unifi, which supplies lots of recycled resins and yarns, has an agreement with Scientific Certification Systems to certify that their Repreve yarns are made from 100% recycled content.  Then Unifi’s  “fiberprint” technology audits orders across the supply chain to verify that if Repreve is in a product , that it’s present in the right amounts.  But with this proprietary information there are still many questions Unifi doesn’t answer – the process is not transparent.  And it applies only to Unifi’s branded yarns.

Along with the fact that whether what you’re buying is really made from recycled yarns – or not – most people don’t pay any attention to the processing of the fibers.  Let’s just assume, for argument’s sake, that the fabric (which is identified as being made of 100% recycled polyester) is really made from recycled polyester.  But unless they tell you specifically otherwise, it is processed conventionally.  That means that the chemicals used during processing – the optical brighteners, texturizers, dyes, softeners, detergents, bleaches and all others – probably contain some of the chemicals which have been found to be harmful to living things.  The processing uses the same amount of water (about 500 gallons to produce 25 yards of upholstery weight fabric) – so the wastewater is probably expelled without treatment, adding to our pollution burden.  And there is no guarantee that the workers who produce the fabric are being paid a fair wage – or even that they are working in safe conditions.

One solution, suggested by Ecotextile News, is to create a tracking system that follows the raw material through to the final product.  They assumed that this would be very labor intensive and would require a lot of monitoring (all of which adds to the cost of production – and don’t forget, recycled polyester now is fashion’s darling because it’s so cheap!).

But now, Ecotextile News‘ suggestion has become a reality.   There is a new, third party certification which is addressing these issues.  The Global Recycle Standard (GRS), issued by Control Union, is intended to establish independently verified claims as to the amount of recycled content in a yarn. The GRS provides a track and trace certification system that ensures that the claims you make about a product can be officially backed up. It consists of a three-tiered system with the Gold standard requiring products to contain between 95 percent to 100 percent recycled material; the Silver standard requires products to be made of between 70 percent to 95 percent recycled product; and the Bronze standard requires products to have a minimum of 30 percent recycled content.

And – we think this is even more important –  in addition to the certification of the recycled content, the GRS looks at the critical issues of processing and workers rights.  This new standard holds the weaver to similar standards as found in the Global Organic Textile Standard:

  • companies must keep full records of the use of chemicals, energy, water consumption and waste water treatment including the disposal of sludge;
  • all prohibitied chemicals listed in GOTS are also prohibited in the GRS;
  • all wastewater must be treated for pH, temperature, COD and BOD before disposal;
  • there is an extensive section related to worker’s health and safety.