The cellulose ether and cellulose derivative are a large category of additives, usually powdery (or granular), and a few slurry (the suspensions formed when cellulose esters do not dissolve). Despite the competition against synthetic rheological modifiers, the cellulose derivative is still the main force of “thickening agents”, and mainly used for the production of various types of aqueous coatings. It can be broadly categorized as follows:
(1) Carboxymethyl cellulose (CMC)
(2) Hydroxyethyl cellulose (HEC)
(3) Hydrophobically modified HEC (HMHEC)
(4) Methyl cellulose (MC)
(5) Methyl hydroxyethyl cellulose (MHEC)
(6) Methyl hydroxypropyl cellulose (MHPC)
(7) Ethyl hydroxyethyl cellulose (EHEC)
(8) Hydrophobically modified cellulose ethers (EHEC, HM-EHEC)
All cellulose products have an indicator for the degree of substitution (DS), referring to the number of substituted hydroxyl groups in each glucose unit (the maximum theoretical value is 3; the lower the degree of substitution is, the better the water solubility will be). The non-associative cellulose (pure cellulose) products produced and sold are mostly powders, fine granules, or of standard size, a few flakes, some after surface treatment. Since the 1950s, it has been the first rheology modifier to enter the coatings market. Meanwhile, all cellulose ethers are based on the cellulose raw materials. This natural polymer has the widest source in the environment, such as wood pulp or chemical absorbent cotton, and the absorbent cotton has the highest molecular weight. Natural cellulose is insoluble in water and must be modified in order to become water-soluble.
Cellulose also has a plurality of different indicators such as the degree of polymerization (directly related to the viscosity of solution), substituent groups and uniformity of substitution. In general, according to different criteria, cellulose products can be divided into different categories as follows: ionic type such as CMC and non-ionic type, such as HEC, EHEC, MC, HPC, etc.; soluble in cold or hot water such as CMC and HEC, only soluble in cold water such as MC and MHPC, and soluble in water and some solvents such as HPC. When the general cellulose products are dissolved in water, the solution is clear or slightly smoke-like, with a certain viscosity (related to the variety and concentration) and pseudoplastic property. The higher the temperature is, the lower the viscosity will be (MC is an exception and will form a gel when heated).
- Carboxymethyl Cellulose (CMC)
CMC is an anionic and hydrophilic cellulose, usually in the form of powder or granular (easy to disperse and avoid blocking), and can achieve thickening and special rheological property without further treatment. In the late 1940s, CMC products entered the market and more pure CMC is used in food, cosmetics and medicine. Like the degree of substitution, the higher the purity is, the higher the price will be. It can be uniformly dispersed thickened under stirring and provides with excellent film-forming properties and adhesion.
- Hydroxyethyl Cellulose (HEC)
HEC is a kind of well-known non-ionic cellulose. Due to a number of reasons, it is currently the most widely used in the industry. For example, with other components, in particularly the coloring agent, it is highly compatible and not affected by multivalent ions; easy to disperse and dissolve in cold or hot water, slow in dissolution when neutral and fast in dissolution after adding the addition of alkali; it has highly thickening ability (depending on the molecular weight), helps suspension, retains moisture, and can be highly stable within a wider range of PH value; highly water-resistant and easy to mix with polar solvents. In cold water or hot water, the solution can be clear and colorless. Generally, the standard HE can form hydrogen bond bridges to obtain with water molecules to obtain a high viscosity and pseudoplastic property (related to its types, the groups introduced and the molecular weight).
Cellulose of this type can be produced in the range of viscosity from the lowest to the highest value, and the surface treatment of it (adding a controlled amount of glyoxal) can avoid agglomeration when dispersed in water.
- Methyl Hydroxyethyl Cellulose (MHEC)
MHEC can have the basic functions of HEC products, as well as good sag resistance, leveling property and higher viscosity. Since the HEC molecule is added into the hydrophobic groups, its reactivity with most commercialized emulsion should be first considered. The reason is that this reaction can obtain higher viscosity and enables the coatings composed of it to have the flow characteristics of Newtonian fluid. The smaller the latex particles are, the stronger the reactivity between both will be.
Methyl Cellulose (MC)
MC is a special cellulose derivative. Since the thermally reversible flocculation effect may occur and the thermal gel point may exist in the solution, it can only be dissolved in cold water. The solution of pure methyl cellulose gets flocculated at about 45-60 ℃. The gelling temperature and organic solubility are related to the type and volume of substituents. The type of substitution effect determines the surface activity and organic miscibility of MC.