Optical brighteners

14 07 2010

I got a call awhile ago from Harmony Susalla, founder and chief designer for Harmony Art  (if you haven’t seen her glorious fabrics go right now to www.harmonyart.com).  She was wondering about optical brighteners, and I discovered I couldn’t tell her much except to say that some are derived from benzene, which is a chemical nobody wants to live with.  GOTS allows the use of optical brighteners – with caveats (see below) – but they are supposed to reevaluate them “in two years from date of adoption” of version 2.0, which puts the reevaluation right about now.

So let’s explore optical brighteners, which are used extensively in:

  • Laundry detergents (to replace whitening agents removed during washing and to make the clothes appear cleaner.) – detergents may contain up to 0.2% whitening agents,
  • Paper, especially high brightness papers, resulting in their strongly fluorescent appearance under UV illumination. Paper brightness is typically measured at 457nm, well within the fluorescent activity range of brighteners. Paper used for banknotes does not contain optical brighteners, so a common method for detecting counterfeit notes is to check for fluorescence.
  • Cosmetics: One application is in formulas for washing and conditioning grey or blonde hair, where the brightener can not only increase the luminance and sparkle of the hair, but can also correct dull, yellowish discoloration without darkening the hair).  Some advanced face and eye powders contain optical brightener microspheres that brighten shadowed or dark areas of the skin, such as “tired eyes”.
  • as well as fabrics, which may contain 0.5% OBAs. A side effect of textile optical whitening is to make the treated fabrics more visible with Night Vision Devices than non-treated ones (the fluorescence caused by optical brighteners can easily be seen with an ordinary black light). This may or may not be desirable for military or other applications

You can still buy “bluing” – which is advertised to “whiten whites and brighten colors”.  Bluing works by removing yellow light to lessen the yellow tinge.   Optical brighteners – also called optical brightening agents (OBAs), fluorescent brightening agents (FBAs), and/or fluorescent whitening agents (FWAs) or “synthetic fluorescent dyes” –  work a bit differently.  Optical brighteners are chemicals similar to dyes which absorb ultraviolet light and emit it back as visible blue light – in other words, they fluoresce the ultraviolet light into visible light. The blue light emitted by the brightener compensates for the diminished blue of the treated material and changes the hue away from yellow or brown and toward white.

They are designed to mask yellow or brown tones in the fibers and make the fabric look cleaner and brighter than it would otherwise appear to the naked eye.   In other words, the undesirable color is made invisible to the eye in an “optical manner”.  Optical brighteners are used both on natural fibers (cotton, linen, hemp, silk) as well as in polymer melts for polyester and other synthetic fiber production.

Optical brighteners aren’t effective unless they remain in the fabric, and persist after washing.  They only last so long, until the point when they actually burn out and no longer do anything. They are also subject to fading when exposed long term to UV.

Brighteners can be “boosted” by the addition of certain polyols like high molecular weight polyethylene glycol or polyvinyl alcohol. These additives increase the visible blue light emissions significantly. Brighteners can also be “quenched”. Too much use of brightener will often cause a greening effect as emissions start to show above the blue region in the visible spectrum.

Optical brighteners are synthesized from various chemicals.  The group of chemicals which are called “optical brighteners” consists of approximately 400 different types listed in the Color Index, but less than 90 are produced commercially. (To get more information about the Color Index click here .)

Basic classes of chemicals used in OBAs  include:

  • Triazine-stilbenes (di-, tetra- or hexa-sulfonated)
  • Coumarins
  • Imidazolines
  • Diazoles
  • Triazoles
  • Benzoxazolines
  • Biphenyl-stilbenes

Using these chemicals, many companies compose their own chemical versions of an optical brightener, and sell it under a branded name, such as:

  • Blankophar R
  • Calcofluor
  • Uvitex
  • Bluton
  • CBS
  • DMS E=416
  • Kolorcron 2B

To find out what is in the optical brightener in any fabric, you must know the name of the optical brightener, and also the C.I. number (such as Brightener 24 or 220).  Then you can look up the chemical composition of the substance – but  only if you’re a subscriber to the Color Index database.  So it’s pretty difficult to confirm what is actually in an optical brightener.

In exploring some of the chemicals used in formulating optical brighteners,  I found one called cyanuric chloride, a derivative of 1,3,5 triazine.  Cyanuric chloride is used as a precursor and crosslinking agent in sulfonated triazine-stilbene based optical brighterners.   It is also classified as “very toxic”, “harmful” and “corrosive” by the EU and has several risk phrases identified with it – including R26 (“very toxic by inhalation”).  R26 is a substance which is specifically prohibited by GOTS.  So how can optical brighteners be allowed under GOTS?

The short answer is:  some are allowed, some are not – it depends on the chemical composition of each individual optical brightener.   Like dyestuffs, GOTS allows optical brighteners if they “meet all criteria for the selection of dyes and auxiliaries as defined in chapter 2.4.6, Dyeing.”  Those criteria include the prohibition of all chemicals listed in 2.3.1 and substances which are assigned certain risk phrases “or combinations thereof”.   But in order to know if a particular optical brightener meets these criteria, it’s necessary to know the chemical formula for that brightener.   And that takes a bit of detective work – and even so you might not be able to get final answers.  Don’t you begin to feel like a hamster in one of those wheels going round and round?

What are the problems associated with optical brighteners?
Some brighteners have been proven to cause allergic skin reactions or eye irritation in sensitive people.   The German Textiles Working Group conducted a health assessment of various optical brightening agents  following concerns of potential health risks to the public. It was found that there is a general lack of information on toxicity and a need for studies into dermal  absorption and the release of these substances from clothes.  While it has not been shown to negatively affect health, it has also not been proven safe.

They are known to be toxic to fish and other animal and plant life and have been found to cause mutations in bacteria.

Most OBAs are not readily biodegradable, so chemicals remain in wastewater for long periods of time, negatively affecting water quality and animal and plant life.  It is assumed that the substances accumulate in sediment or sludge, leading to high concentrations.
In wastewater, OBAs can also leach into groundwater, streams, and lakes. Since fluorescence is easy to detect,  optical brightener monitoring is an emerging technique to quickly and cost-effectivley detect the contamination of stormwater by sanitary wastewater.

REACH is the new European Union regulation which aims to  improve human health and the environment through better and earlier identification of the properties of chemical substances.  REACH stands for Registration, Evaluation, Authorisation and Restriction of Chemical substances.   REACH contains provisions to reduce the use of what are called “high volume production” chemicals.  These are defined as chemicals having annual production and/or importation volumes above 1 million pounds.  It is assumed that high volume production is a proxy for high exposure; in addition, large releases of low toxicity substances such as salts do cause environmental harm due to the sheer volume of the substance.
Much of the impact from optical brighteners comes in the form of large releases of low toxicity substances.  A number of these optical brighteners are listed as high and low production volume substances and so will be subject to REACH.   For example, C.I. Fluorescent Brightener 220 is listed as a high production volume chemical.