Importance of sequestering agent in
textile processing
by Dr.
S.M.Imtiazuddin, M. Sohail Tiki, and Azeema Kanwal, AVM Chemical
Industries.Background
of sequestering agent
The present invention relates to a novel composition of
matter having utility in the sequestering or complexing of metal
ions particularly calcium and / or magnesium ions. More
particularly, the present invention is unique combination of
water – soluble aminopolyacetate demonstrating improved
chelating properties. Sequestering agent are employed in a
number of applications such as builders in detergent formation
to eliminate interfering effects caused by dissolved metal
salts, particularly calcium or magnesium salts. Sequestering
agents are also employed in other applications requiring control
of metal ion content in aqueous solution such as waste water
treatment, potable water conditions, micro-nutrient
solubilization, gas conditioning, photographic development
processing catalysis of chemical processes, finishing and
conditioning of textile, etc.
Invention
The present invention is an improved sequestering agent
comprising of a mixture of from about 2% to 20% by weight of
iminodiacetic acid or water soluble salt thereof (IDA) and from
about 80% to 98% by weight of nitrilotriacetic acid or water
soluble salt thereof (NTA). Iminodiacetic acid, HN(CH2CO2H)2,
often abbreviated to IDA, is an polyamino carboxylic acid.
It is discovered that the sequestering agent corresponding to
the above defined range of composition provides improved
sequestration or chelation, particularly of calcium and
magnesium ions in aqueous solution, over the performance of each
component of the composition individually, thereby resulting in
a synergistic addition of chelating properties. Equally
surprising is the fact that addition to a composition of large
amounts of IDA than called for by the present invention
deleteriously affects the performance of the sequestering agent.
A preferred application of the present sequestering agent is
as a detergent builder in combination with additional components
such as surfactants, thereby providing an improved detergent,
particularly suited for use in water containing dissolved
calcium and/ or magnesium ions.(¹) Chelating and sequestering
agents have had considerable applications for many years. These
materials usually form complexes or coordination compounds by
reaction of their negatively charged donor groups with
polyvalent metal ions such as iron, calcium, copper and
magnesium. There are several classes of chelating agents
including the EDTA (ethylenediamine tetra acetic acid) type and
the aldonic (sugar) acid group. An aldonic acid is any of a
family of sugar acids obtained by oxidation of the aldehyde
functional group of an aldose to form a carboxylic acid
functional group. Gluconic and citric acid are examples of the
latter. The choice of chelating agent depends to a great extent
upon the conditions under which it is applied for example, the
EDTA family is effective in acid, neutral, or alkaline media
whereas the sugar acids function best with which this note
concerned(²).
One of the most important sugar based chelating agent is
sodium gluconate, which has found an extensive use as a
chelating agent in the dairy, detergent, brewing, leather and
textile industries. Medicinally it is used as a carrier for
calcium and iron in treatment of deficiencies of these elements.
Gluconates are prepared commercially by enzymatic transformation
of D – glucose with glucose oxidase (EC 1.1.3.4 - D – glucose:
oxygen oxidoreductase) or by catalytic air oxidation of
glucose.
Higher members of the series such as sodium glucohaptonate
can be prepared by addition of sodium cyanide to glucose
followed by hydrolysis. Lactobionic acid and its amides were
less effective as chelater for iron then were corresponding
gluconic acid compounds.
The EDTA and Lactosyl urea were ineffective under this
condition. The presence of an additional carboxyl groups, as in
saccharic acid or sodium mucate, afforded no benefits over
gluconic acid in the amount of ferric chloride complexes. Amides
formed by reaction of glucono or Lactobiono – Lactone with 2 –
methyl – 2 aminopropanol were slightly superior in chelating
power to their parent acids(3-4).
Main classes of sequestrant
The five main classes of sequestrant used in the textile
industry are:
- Polyphosphonic acid.
- Amino polycarboxylic acid.
- Polyphosphates.
- Hydro carboxylic acid.
- Polymeric carboxylic acid.
While comparing a cross range of commercially available
sequestrants it is possible to see the following features:
- Different sequestering power. (i.e, the strength of the
sequester - metal complex)
- Different specific sequestering power for individual
metal.
- Sequestering power not only depends on the specific
sequestrant / metal pairing, but also on pH and temperature.
- Different sequestering capacities.
- Sequestering capacities which are metal ion specific.
- Difference in the spread of pH over which a particular
sequestrant will combine with a particular metal ion.
- That not all sequestrants are stable to high temperatures.
- That not all sequestrants are stable to oxidation or
hydrolysis.
- That some sequestrants will de-metallise/pre-metallise
dyes.
- That some sequestrants can affect the shade
reproducibility of some dyestuffs (the effects can be very
dye-sequestrant specific).
- That some sequestrant does not satisfy the environmental
and toxicological requirements of every market.
- If calcium and magnesium are not sequestered
- If calcium and magnesium are not sequestered, there is the
strong possibility of their combining with natural “soaps”
which have been generated during the alkaline scouring
process, to form waxy substance. These have been referred to
as “Lime soap deposits” they can deposit not only on the
substrate it self but also on the surface of machinery.
- Calcium and magnesium ions reduce the solubility of
anionic dyes causing them to aggregate or even precipitate on
the fiber.
- Aggregated and precipitated dyes cannot migrate or diffuse
they remain on the fiber surface as particular deposits.
A chemical compound which possesses two (or more) chemical
groups, which can surround a metal ion, resulting into a complex
is called chelate. These chelates are more stable then the
simple salts formed with acid (i.e acetic or formic acid) such
simple salts formed with acids are usually highly soluble and
useful way of demineralising cotton before dyeing.
|
Features
of Polyphosphonate sequestrants: |
|
Feature |
Comments |
|
Very high sequestering capacity for
Ca and Mg from pH 5 – 13; And for Cu / Fe from pH 1 – 13. |
The resultant sequestrant – metal
complexes are generally highly soluble, and not easily
precipitated (even in the presence of Rock salts and Sea
salts which are known to contain very high levels of
alkaline earth metal impurities). By contrast,
aminopolycarboxylic acids are more easily precipitated;
and have higher limits of effectiveness over a narrower pH
range. |
|
Improve dyestuff solubility, by
deflocculating and disaggregating effects. |
Avoid dye spotting problem. |
|
Polyphosphonic acids (rather like
aminopolycarboxylic acid) do not readily biodegrade in a
laboratory activated sludge test. |
However, in more realistic die –
away river tests, these substances are rapidly degraded in
the presence of sunlight and ferric ions.
These are also degraded in the soil. They do not
bio-accumulate on aquatic species and are readily
eliminated by fish. |
Polyphosphonates can be eliminated
from water by classical treatment with aluminum sulphate
or lye treatment. Due to their low concentration and
strong adsorption on sewage sludge and on sediments,
mobilization of heavy metals into the aquatic environment
is low.
|
|
However, they are not always resistant to precipitating
condition and also often easily stripped of the metal ion by a
dyestuff molecule, obviously, this is the opposite of the effect
required(5) One of the main class of sequestrant is
polyphosphate.
Another most commonly used sequestrant of textile industry is
EDTA, which is considered as most stable complex, because the
metal atom is enclosed in a 5 or 6 member ring. (see the Figure)
|
Features of
EDTA |
|
Features |
Comments |
|
One of the oldest known, and most
widely used “textile sequestrant “ |
EDTA is cheap and forms strong
complexes with a broad spectrum of metal ions. Weight for
weight comparison shows significantly inferior
sequestering capacity to that of polyphosphonic acid. |
Good sequestering agent for Ca / Mg
at high pH values( but only in the absence of oxidizing
agents).
But for the ferric ion (Fe+++). The sequestering action is
excellent at low pH values but begins to decrease at pH 6.
At higher pH values, EDTA becomes completely ineffective
at controlling Fe+++ |
Never likely to provide protection
for localized “pinhole damage” in peroxide bleaching.
The failure of EDTA to sequester iron above pH 8 renders
it useless as a peroxide bath sequestrant / stabilizer –
unless it is used in combination with a second sequestrant
which can bind iron in the pH range 11 – 13. |
|
EDTA can demetalise many metal
containing dyes, usually resulting in a drastic shade
change. |
EDTA is not
suitable as a dyebath sequestrant. Beware of red, Bordeaux
and burgundy shades turning a very dull blue (almost
purple) |
|
EDTA is hardly soluble in acidic pH,
and is often used, therefore, in the form of a sodium or
ammonium salt. |
|
Advantages of sequestering agent
- Prevention of pinhole formation due to catalytic oxidation
caused by iron during bleaching (chelates iron / heavy
metals).
- During dyeing, it prevents spots, shade change and
unevenness by chelating heavy metals and dispersing
impurities.
- Improved rinsing due to the scale inhibition (effective at
low concentration such as its presence in rinse bath).
- During fabric preparation sequester also provides iron
chelating to prevents fabric / yarn damage and provides
additional stabilizing effect on the hydrogen peroxide.
References
- Richard G. young, Midland, Mich.”Sequestering agent”, The
Dow Chemical Company, Midland, Mich, 219 , 405. August, 4.
(1982).
- F.SCHOLNICK and P.E.PFEFFER - Iron chelating capacity of
Gluconamides and Lactobionamides, Technical notes, J. Dairy
science: 63, 471 – 473. (1980).
- Davis, P.S., and D.L.Deller. Predictions and demonstration
of iron chelating ability of sugars. Nature 212: 404.(1966).
- Mehltretter, C.L. B.H. Alexander, and C.E. Rist.
Sequestration by Sugar acids. Ind. and Eng.Chem.45:2782
(1953).
- Spiro, T.G., and T.Saltman. Polynuclear complexes of iron
and their biological implications. Structure and Bonding 6:116
(1969).
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