Robert Wilt on wed 22 sep 99
I've always wondered why it's so hard to find a chemical formula for CMC.
Now I know. I did a little research into this question and thought I would
share the results. Apologies in advance to any real chemists, and please
let me know if you notice any errors.
CMC is usually said to stand for "carboxymethylcellulose", but as I looked
around I started seeing the names "sodium carboxymethylcellulose" and
"carboxymethylcellulose sodium" pop up a lot. It appears that CMC is in fact
sodium carboxymethylcellulose. For example, from the web site of Akzo Nobel
Functional Chemicals, "CMC (sodium carboxymethylcellulose or cellulose gum)
is a very versatile product, available in many brands and types."
(Web site: http://www.cmc-salts.com/home.htm) I found a German site where
they were listed as synonyms. The chemist I corresponded with later seemed
to imply the same.
As a potter, the first question you ask is "How MUCH sodium?"
Well, you'll see.
First, what is cellulose? It is an organic polymer. The chemical formula
is (C6 H10 O5)n - the "n" meaning that the basic unit repeats. Below is
a diagram to help you visualize how these atoms fit together (although
keep in mind that the actual molecule is three-dimensional and its
geometry is only vaguely similar to this two-dimensional representation):
(p.s. you need a fixed-width font to see this correctly)
H
|
(from previous unit) *--------C-------O +--* (to next unit)
| | |
H--C--O-H | |
| | |
H-O--C--H | |
| | |
H--C------------O
| |
H--C-------+
|
H--C--O-H
|
H
C = Carbon
H = Hydrogen
O = Oxygen
- = single bond
* = connection between monomers
A single unit like this is called a "glucose monomer." It is
very similar to a plain glucose molecule.
As it turns out, cellulose has a few weak spots. These weak
spots are at the three OH sites you see above - the H atoms at these
sites are loosely held. To make CMC, you remove some of these loose H
atoms and replace them with "sodium carboxymethyl."
So what's that? "Carboxy" means -COOH, "methyl" means -CH3, and "sodium"
of course means -Na. The minus sign in front of each implies that they
are radicals - things that attach to other things. When you put these
three radicals together, you get -CH2COONa (a chain of replacements).
This radical can replace the H in the OH's in cellulose.
Here's another diagram where the glucose monomer has been partially
"carboxymethylated" (this is what they say - they often don't
even mention the sodium).
H
|
(from previous unit) *--------C-------O +--* (to next unit)
| | |
H--C--O-H | |
| | |
H-O--C--H | |
| | |
H--C------------O
| |
H--C-------+
|
| H
| |
H--C--O--C--C==O
| | |
H H O--Na
C = Carbon
H = Hydrogen
O = Oxygen
Na = Sodium
- = single bond
= = double bond
* = connection between polymer units
Okay, now here's the tricky part. In CMC, not every H of every
OH is replaced by CH2COONa. In fact, different brands and types
of CMC have different "ds" values (degree of substitution).
Thus the molecular weight and the amount of sodium depend on this
ds value, and are not the same for all CMC.
Philip Raatjes of Akzo Nobel explained it to me this way:
The Sodium concentration depends on the degree of carboxymethylation
(substitution) of the cellulose. This ds varies typically between
0.2 and 1.5 depending on the application. In theory each glucose
monomer of the cellulose has 3 substitution places, which would give
a ds of 3. In practice a ds of 1.5 is the highest one can obtain. At
ds 0.2 the CMC does not really dissolve, only swells. CMC dissolves
(and hence gives viscosifying properties) at ds 0.5 and higher. For
food applications the ds varies between 0.6 and max. 0.95. This
maximum is determined by the Food Chemicals Codex allowing a max.
Na concentration of 9.5 %. Here you have the relation between
carboxymethylation and Na concentration, as a rule of thumb one
may say that each ds 0.1 introduces 1 % Na.
Now of course potters usually work in terms of oxides, i.e. Na2O.
So what is the percentage of Na2O in CMC? To get the answer, I plugged
some formulas into my glaze software, which grudgingly calculated the
answers for me. The generic formula looks like this:
C6 H[10-ds] O5 . (CH2COONa)[ds]
where the "[ds]" and "[10-ds]" are subscripts. Here are some results for
a few values of ds:
ds = 0 0% Na2O (by weight)
ds = 0.2 3.5% Na2O
ds = 0.4 6.4% Na2O
ds = 0.5 7.7% Na2O
ds = 0.6 8.8% Na2O
ds = 0.8 11.0% Na2O
ds = 1.0 12.8% Na2O
ds = 1.5 16.5% Na2O
The final question is whether this small sodium concentration makes
any difference to potters. Most of us would probably guess "no."
If anyone can think of a situation where it might make a difference,
please let me know.
bob wilt
rjw@studiopotter.org
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Copyright (C) 1999 Robert J. Wilt
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