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Chemistry of cellulose ethers

Cellulose
Cellulose is a polysaccharide composed of individual anhydroglucose units which are linked through a 1.4 glycosidic bond (Figure 1). The number “n” of anhydroglucose units in the polymer chain is defined as the degree of polymerisation (DP).

Each anhydroglucose ring carries three OH-groups at positions 2,3 and 6, which are chemically active. The distribution of the substituents introduced onto the polymer chain is largely determined by the relative reactivity of these three OH- groups.

Table 1 Relative reactivity constants of the three cellulose OH-groups
Reactant	Position			New OH group
	2	3	6
Chloroacetic acid Methylchloride Ethylene oxide	2,00 1,45 3,00	1,00 1,00 1,00	2,50 1,50 10,00	20
The starting material for the manufacture of cellulose ethers is highly purified and bleached cellulose in rolls or sheets.

Figure 1
Structure of cellulose

Table 2 Degree of polymerisation of cellulose of different origin (Cuoxam method)
Origin	DP
Cotton Linters Spruce Beech	2000 - 2400 600 - 1700 800 - 1100

Cellulose ethers
lndustrial cellulose ethers are alkyl, alkyihydroxyalkyl, hydroxyalkyl, and carboxyalkyl ethers derived from cellulose. Ethers are formed by substituting some of the cellulose hydroxyl groups. The use of one etherification agent in the substitution process results in a simple cellulose ether, whereas using different kinds of agents leads to mixed ethers.

Classification
Cellulose ethers are divided into ionic and nonionic types. The ionic cellulose ethers, e.g. BLANOSE sodium carboxymethylcellulose, contain substituents which are electrically charged. Nonionic cellulose ethers like CULMINAL methylcellulose and NATROSOL hydroxyethylcellulose carry electrically neutral substituents. Mixed ethers with ionic and nonionic substituents are classified according to their predominant features.

Figure 2
Nonionic cellulose ethers are further subdivided according to their solubility characteristics; however, this is not a sharp division.

Nomenclature
The chemical names of the most important products abbreviations have been introduced as shown below. They will also be used in this brochure. Products manufactured by Aqualon are listed as well.

Manufacture of CULMINAL
The manufacturing process is carried out in one of several steps. The reactions taking place are alkalization and alkylation. These reactions can be carried out in the presence or absence of solvents.

Pure cellulose has large crystalline regions due to hydrogen-bonded hydroxyl groups and is thus insoluble in water and in most organic solvents. In the first step cellulose is activated with sodium hydroxide to become alkali cellulose. Alkali cellulose is degraded oxidatively much more rapidly than cellulose. Controlled reduction of the degree of polymerisation allows for adjustment of the solution viscosities of the final CULMINAL products.

The maximum solution viscosity of the final cellulose ether that can be obtained, can only correspond to the DP of the original cellulose.

By the selection of the substituent the type of CULMINAL is determined. Mixed ethers can be produced with a mixture of reactants or by stepwise addition of the substituents. This reaction step is carried out under pressure and at increased temperature. Alkylation is carried out with alkylhalides, with epoxides the process is called alkoxylation. The molar ratio of reagents to alkali cellulose determines the degree of substitution.

The number of substituted hydroxyl groups per anhydroglucose unit is expressed as DS or degree of substitution. The DS can vary between 0 and 3. As with all polymer reactions, this reaction does not occur uniformly along the polymer chain. The degree of substitution is therefore determined as mean over the whole polymer chain and expressed as average degree of substitution (DS).

Table 3
Chemical description	Abbreviation	Aqualon
Methylcellulose Methylhydroxyethylcellulose Methylhydroxypropylcellulose Hydroxyethylcellulose Hydrophobically modified hydroxyethylcellulose Hydroxypropylcellulose Sodium carboxymethylcellulose Ethylcellulose	MC MHEC MHPC HEC hmHEC HPC CMC EC	CULMINAL CULMINAL CULMINAL NATROSOL NEXTON KLUCEL BLANOSE AQUALON Ethylcellulose

With the alkoxylation, side chains are generated. In this case the molar ratio of alkoxy groups in the side chains to cellulose is specified and expressed as the average molecular substitution (MS).

Instead of DS and MS the weight percent of the substituents in the cellulose ether is often quoted. The relationship between these factors is given in the equations below and in figure 3, in which EOOH stands for OC2H4OH, and POOH for OC3H6OH.

Degree of substitution MHEC

%OCH3 162
DS(OCH3) = ––––––––– X ––––––––––––––––––––––––––––––––––––
31 100 - (%OC2H4OH/1,39 + %OCH3 x 0,45)

%OC2 H4 OH 162
MS(OC2 H4 OH) = ––––––––––– X ––––––––––––––––––––––––––––––––––––
61 100 - (%OC2H4OH/1,39 + %OCH3 x 0,45)

Degree of substitution MHPC

%OCH3 162
DS(OCH3) = ––––––––– X ––––––––––––––––––––––––––––––––––––
31 100 - (%OC3H6OH/1,29 + %OCH3 x 0,45)

%OC3H6OH 162
MS(OC3 H6 OH) = ––––––––––– X ––––––––––––––––––––––––––––––––––––
75 100 - (%OC3H6OH/1,29 + %OCH3 x 0,45)

For mixed ethers the weight % of the second substituent can be expressed as % alkylene oxide (% C2H4O;% C3H6O) or, as in table 2, above, as % alkylene glycol ether (% OC2H4OH;% OC3H6OH).

The use of Figure 3 can be explained with a few examples:

For an MHEC containing 25% methoxyl and 10% EOOH. the DS and MS have to be determined.

In the grid of curves for the percentage of substituents, the straight lines cross each other at point 1, the straight lines being 25% Methoxyl and 10% EOOH. A vertical line through this point intersects the DS axis at a DS of 1.61: the horizontal line running from point 1 to the MS axis gives a valid of 0,33 EOOH.
In example 2, details on how to find the percentage of substituents for an MHIPC with DS 1,34 and MS 0, 16 are given.

The two lines perpendicular to the DS and MS axis cross each other at point 2 in the diagram. This point is located on the straight lines for 22% Methoxyl and 6% POOH

Purification
CULMINAL crude products are generally purified by washing with hot water, then they are dried. ground and screened to the desired particle size.

Surface treatment
In some applications it is necessary to time the beginning of the hydration of the cellulose ethers accurately. This is achieved with the CULMINAL types marked “R” by reversible crosslinking with glyoxal. In the acid pH range these products show a reduced tendency to hydrate. At higher pH values the cross linking reaction is reversed and the CULMINAL cellulose ethers dissolve instantaneously.

Figure 3
Degree of substitution for cellulose ethers in DS, MS, and as a percentage

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