Cellulose Ether

To purchase this item kindly contact us for more details

Cellulose is a naturally occurring polymer and exists in a fibrous form in plants. 
The purest natural cellulose is the cotton lint or staple fiber which on a dry basis consists of about 95 wt % cellulose. However, due to its high cost cotton staple fiber is not used to manufacture cellulose derivatives. Currently, cellulose materials used to manufacture cellulose derivatives are isolated from trees or raw cotton linters. Cellulose fibers obtained by purification of wood are called wood pulps. Due to their low cost these furnishes are the most commonly employed sources of cellulose for the manufacture of cellulose derivatives.

Raw cotton linters have been considered an excellent source of high molecular weight cellulose for over 80 years. Raw cotton linters, commonly referred to as “linters”, are short fiber residues left on the cottonseed after the longer staple (“lint”) fibers are removed by ginning. Linters are shorter, thicker, and more colored fibers than staple fibers. They, also, adhere more strongly to the seed relative to staple fibers. Linters are removed from cottonseeds using a number of technologies including linter saws and abrasive grinding methods, both of which yield suitable materials. Depending on the number of passes used to remove the linters from the cottonseed, they are called “first-cut”, “second-cut” and “third-cut” raw cotton linters. If the linters are removed in one pass or first- and second-cut linters are manually blended in a weight ratio of approximately 1:4, the resulting material is called “mill runs”. Mill runs and first-cut raw cotton linters are used in medical and cosmetic applications as well as to make upholstery, mattresses, etc. while second cut cotton linters are typically used to manufacture purified cotton linters or chemical cotton. In general, first-cut cotton linters contain less non-cellulosic impurities than do second-cut cotton linters. The amount of hemicellulose, lignin or colored impurities and foreign matter in the various types of raw cotton linters increases in the following order: First-cut<second-cut<third-cut.

Purified cellulose obtained from raw cotton linters is called chemical cotton or purified cotton linters. Given the commercial significance of cotton, it is not surprising that many mechanical separation processes have been developed over the past century to separate lint and linters from other contaminants.

Regrettably, during the isolation and purification of cellulose from raw cotton linters or wood chips significant molecular weight loss of the cellulose occurs depending on the process conditions used to isolate the cellulose. In wood, due to the high concentration of other components, the molecular weight loss during purification is especially acute. In addition, due to oxidation caused by a bleaching process during the purification, undesirable functional groups, such as carboxyl or carbonyl groups are formed on the cellulose backbone and the polydispersity of the cellulose chains changes. Another drawback of purifying raw cotton linters to make chemical cotton or converting wood chips to wood pulp is that the crystallinity and morphology of the “virgin” cellulose fibers change leading to changes in chemical reactivity of the hydroxyl groups present in cellulose. Such an alteration of the cellulose microstructure could lead to changes in its reactivity or accessibility to a modifying agent and/or the formation of modified derivatives having different structures and different behavior in an end-use application. Notably, the processing associated with such purification greatly increases the cost of purified cellulose.

To manufacture high quality cellulose ethers, it is critical to control the physical properties of the cellulose furnish, such as the level of impurities, surface area (fiber length), and crystallinity. Since cellulose is a semi-crystalline material one of the key issues in the manufacture of cellulose ethers is to make the cellulose hydroxyl groups equally accessible for reaction with the derivatizing agent.

Cellulose ethers represent an important class of commercially important water-soluble polymers (WSPs). Examples of such WSPs include carboxy-methylcellulose (CMC), hydroxyethylcellulose (HEC), methylcellulose (MC) and hydroxypropylcellulose (HPC). By incorporating additional functional groups into such cellulose ethers, a wide variety of mixed cellulose ether derivatives can be produced. Currently, cellulose ethers are manufactured by reacting cellulose with appropriate etherifying reagents. The cellulose material used to prepare cellulose ethers is referred to as “furnish”. At present both purified cotton linters and purified wood pulp are used to manufacture cellulose ethers.

The ability of a water-soluble cellulose ether to enhance the viscosity of water is primarily controlled by its molecular weight, chemical derivatization, and substitution uniformity of the polymer chain.

Traditional Cellulose Ethers

Associative Cellulose Ethers

CMC

HM-HEC

HEC

HM-EHEC

MHEC or HEMC

 

HPMC or MHPC

 

MC

 

HPC

 

EHEC

 

MEHEC

 

CMHEC

 

Cotton Linters contain approx 90% cellulose while the purity of wood pulp is only 50%. Cotton Linters have much higher avg DP (Degree of polymerization) than wood pulp. Cotton linters are the most important source for the production of high molecular weight cellulose ethers.

Nature of cellulose is crystalline. By treating cellulose with alkaline media ie, NaOH, a negative repulsive charge is produced. This breaks down the crystallinity of cellulose polymer thereby making hydroxyl group accessible to reagents. With NaOH, hydroxyl group are converted into more reactive alkoxide ions.

This reactive cellulose is called alkaline or alkali cellulose, the production of which is one of secrets of making good cellulose ethers.

A proper amount of NaOH must be brought into contact with cellulose sheet in order to ensure uniform distribution of substitution. If proper care is not taken in generation of alkali cellulose, etherification reaction wont give desired results.

*Etherification is less critical process than Alkalixation / Basification.

*Higher molecular weight cellulose polymers function well as thickeners; the lower molecular weight cellulose polymers function well as rheology modifiers.

A manufacturing process that includes the following process steps

  1. Reaction of cellulose (e.g., wood pulp or cotton linters ) with sodium hydroxide (NaOH) to produce alkali cellulose;
  2. Reaction of alkali cellulose with chemical compound(s), such as chloroacetic acid, ethylene oxide, methyl chloride, propylene oxide, ethyl chloride, to produce a particular cellulose ether;
  3. Washing and purification of cellulose ether; and
  4. Drying of the cellulose ether.
  5. Solids Handling steps downstream of the drying process are not considered part of the cellulose ether process.

Alkali cellulose is most commonly produced by slurrying, spraying, or otherwise mixing chemical cellulose chip, pulp, or sheet with aqueous sodium hydroxide (35-60% w/v). Inert organic solvents may be used as slurry media.
This mixture is then held for a pre-determined time at a controlled temperature and pressure to ensure complete complete reaction and to control the viscosity of the final product through an “aging process”.
Alkali cellulose is quite readily degraded by air oxidation; thus the quantity of oxygen present in the alkalization reaction and the type of cellulose used (cotton linter or wood pulp) have a crucial influence on Degree of polymerization (DP) of the final product and hence its viscosity in aqueous solution.
For high viscosity products, cotton linters are utilized under strictly controlled conditions in order to minimize oxidation. These linters have an extremely high DP and a high a-cellulose CONTENT (>99%). Other products with lower viscosity requirements are made from variety of wood pulps.

 

Because byproducts are generated in these reactions; purification is required to meet the food and other premium application standards. Byproducts include alcohols, alkoxides, ethers, and, in the case of CMC production, glycolic acid and its salts. Generally, the thermally gelling and hot water insoluble products such as methyl cellulose (MC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC), respectively, are purified by hot water washing and filteration procedures. Cold and hot water soluble cellulose ether such as carboxymethyl cellulose (CMC), hydroxyethyl cellulose (HEC) are purified by washing with solvent systems such as aqueous ethanol, IPA or acetone.
These purified cellulose ethers are then dried, and their particle sizes are modified by suitable means. Finally, they are analyzed for premium application compliance and are packaged.

Etherification is a top chemical reaction that does not change the gross fibrous structure of the cellulose. However, in the manufacture of granular types Cellulose ether series, the washing and milling operations are conducted in such a way as to destroy almost completely the fibrous structure.

Most of Cellulose ether have tendency to agglomerate while going into water without sufficient care. The time require for product to dissolve depends upon Degree of Agglomeration. It is important that particles should first be well dispersed throughout the aqueous phase before going into solution.
For this reason most of cellulose ethers are treated with GLYOXAL. Hydration time can be adjusted by varying the quantity of GLYOXAL added to the product.

Cellulose Ether

Degree of Substitution (D.S)

Solubility

Methyl Cellulose (MC)

1.5-2.4

Hot water

Carboxymethyl cellulose (CMC)

0.5-1.2

Cold or hot water

Hydroxyethyl cellulose (HEC)

Low D.S

Cold or hot water

EHEC / MHEC /HPMC

1.5-2.0

Cold water

Ethyl cellulose (EC)

2.3-2.6

Solvent

 



Cellulose Ether
( HEC, HPMC, MHEC, MC)

2% Aqueous Solution NDJ-1

2% Aqueous Solution Brookfield

60000

30000

75000

35000

100000

45000

100000

45000

150000

55000

200000

65000