Cellulose Ether

Cellulose ethers comprise methylcellulose, ethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, and their derivatives.

From: Encyclopedia of Materials: Science and Technology, 2001

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Michalina Falkiewicz-Dulik, ... George Wypych, in Handbook of Material Biodegradation, Biodeterioration, and Biostablization (Second Edition), 2015


Cellulose ether polymers are used by the paint industry as thickening agents for waterborne paints. Reduction in their degree of polymerization results in the loss of paint viscosity (paint thinning).9 Endoglucanases hydrolyze at random the 1,4-linkage in the cellulose ether chains. These enzymes cause a rapid drop in substrate viscosity because they lower degree of polymerization in the cellulose ether polymers and cause loss of paint viscosity.9

Many emulsion raw materials are susceptible to attack by bacteria, yeast, and/or fungi.2 These include surfactants, wetting agents, defoamers, and thickeners.2 Production of microbial cellulase may result in viscosity changes.2

Cellulosic components can act as nutrients for microorganisms and organic materials leaching from the paint, such as phosphates can aid algal growth on the surface.39

Cellulose based thickeners undergo fast biodeterioration, but tosylic ester of carboxymethyl cellulose and p-toluensulfonic acid is stable.40

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Characteristics of Adhesive Materials

Sina Ebnesajjad PhD, Arthur H. Landrock, in Adhesives Technology Handbook (Third Edition), 2015

5.9 Cellulose Ether Adhesives

Cellulose ethers are water-soluble polymers derived from cellulose that is the most abundant natural polymer. For more than 60 years, these products have played a significant role in a host of applications, from construction products, ceramics, and paints to foods, cosmetics, and pharmaceuticals [32].

For construction products, cellulose ethers act as thickeners, binders, film formers, and water-retention agents. They also function as suspension aids, surfactants, lubricants, protective colloids, and emulsifiers. In addition, aqueous solutions of certain cellulose ethers thermally gel, a unique property that plays a key role in a variety of applications.

These include ethyl cellulose, hydroxy ethyl cellulose, methyl cellulose, sodium carboxy methyl cellulose, and benzyl cellulose. Ethyl and benzyl cellulose can be used as hot-melt adhesives. Methyl cellulose is a tough material, completely nontoxic, tasteless, and odorless, which makes it a suitable adhesive for food packages. It is capable of forming high-viscosity solutions at very low concentrations, so it is useful as a thickening agent in water-soluble adhesives. Hydroxy ethyl cellulose and sodium carboxy methyl cellulose can also be used as thickeners. The cellulose ethers have fair to good resistance to dry heat. Water resistance varies from excellent for benzyl cellulose to poor for methyl cellulose [7,8,31]. Properties and advantages of cellulose ether adhesives are summarized in Table 5.9.

Table 5.9. Properties and Advantages of Cellulose Ether Adhesives [32]

Property Details Advantages
Binding Used as high-performance binders for extruded fiber-cement materials Green strength
Emulsification Stabilize emulsions by reducing surface and interfacial tensions and by thickening the aqueous phase Stability
Film formation Form clear, tough, flexible water-soluble films

Excellent barriers to oils and greases

Films can be made water-insoluble via cross-linking

Lubrication Reduce friction in cement extrusion; improve hand-tool workability

Improved pumpability of concrete, machine grouts, and spray plasters

Improved workability of trowel-applied mortars and pastes

Nonionic Products have no ionic charge

Will not complex with metallic salts or other ionic species to form insoluble properties

Robust formulation compatibility

Solubility (organic) Soluble in binary organic and organic solvent/water systems for select types and grades Unique combination of organic solubility and water solubility
Solubility (water)

Surface-treated/granular products can be added directly to aqueous systems

Untreated products must first be thoroughly dispersed to prevent lumping

Ease of dispersion and dissolution

Control of solubilization rate

pH stability Stable over a pH range of 2.0–13.0

Viscosity stability

Greater versatility

Surface activity

Act as surfactants in aqueous solution

Surface tensions range from 42 to 64 mN/m


Protective colloid action

Phase stabilization

Suspension Control settling of solid particles in aqueous systems

Antisettling of aggregate or pigments

In-can stability

Thermal gelation Occurs to aqueous solutions of methyl cellulose ethers when heated above a particular temperature

Controllable quick-set properties

Gel go back into solution upon cooling

Thickening Wide range of molecular weights for thickening water-based systems

Range of rheological profiles

Pseudoplastic shear thinning rheology approaching Newtonian


Water retention Powerful water-retention agents; keep water in formulated systems and prevent loss of water to atmosphere or substrate

Highly efficient

Improved workability and open time of dispersion-based systems such as tape joint compounds and aqueous coatings, as well as mineral-bound building systems such as cement-based mortars and gypsum-based plasters

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Cellulose: Chemistry and Technology

D.N.-S. Hon, in Encyclopedia of Materials: Science and Technology, 2001

1.2 Ethers

Cellulose ethers are made by the reaction of cellulose with aqueous sodium hydroxide and then with an alkyl halide, such as methyl or ethyl chloride. The general reaction scheme for making methyl- and ethylcellulose is shown in Scheme Scheme 3.

Scheme 3.

Cellulose ethers comprise methylcellulose, ethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, and their derivatives. Most of the cellulose ethers with low DS are soluble in water, and those with higher DS are soluble in alkaline solution or in organic solvents (see Table 4). Cellulose ethers have gained their position on the market due to their multifunctionality. They exhibit useful properties of thickening, thermal gelation, surfactancy, film formation, and adhesion. Those characteristics earn them applications in areas such as foods, cosmetics, paints, construction, pharmaceuticals, tobacco products, agriculture, adhesives, textiles, and paper. In pharmaceutical applications cellulose ethers such as methylcellulose and ethylcellulose have been widely used as a bulking agent for treating various intestinal ailments such as in diabetic diets, as ingredients for ointments and lotions, and as a binder for tablets. In food applications, cellulose ethers have been used in bakery products, as thickener for canned fruit pie fillings, and in frozen foods. They also have been used in adhesive and cement formulations.

Table 4. Solubility of cellulose ethers.

Cellulose ether Solubility/degree of substitution
Cold 4–8% NaOH 4–8% NaOH Cold water Organic solvent
Methylcellulose 0·1–0·4 0·4–0·6 1·3–2·6 2·5–3
Ethylcellulose 0·5–0·7 0·8–1·3 2·3–2·6
Hydroxyethylcellulose 0·1–0·4 0·5 0·5–1·0
Ethylmethylcellulose 1·0–1·3
Hydroxyethylmethyl cellulose 1·5–2·0
Na-carboxymethyl cellulose 0·1–0·4 0·5 0·5–1·2
Cyanoethylcellulose 2·0
Benzylcellulose 1·8–2·0

The reaction of cellulose with sodium monochloroacetate can produce sodium carboxymethylcellulose (CMC), a water-soluble anionic linear polymer. Sodium CMC is used in probably more varied applications worldwide than any other water-soluble polymer known today. A major application of CMC is in detergent systems to prevent soil redeposition after it has been removed from fabrics by synthetic detergent. It is widely used in the petroleum industry as a water-soluble colloid in drilling fluid systems. It is also used as an additive for the paper industry, a sizing agent for the textile industry, and has many applications in cosmetics and pharmaceuticals.

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Building and Construction Applications

Michael Niaounakis, in Biopolymers: Applications and Trends, 2015

10.1.4 Cellulose Ethers

Cellulose ethers are a major class of commercially important water-soluble polymers for the construction and building industries. Cellulose ethers are capable of increasing the viscosity of aqueous media. The viscosifying ability of a cellulose ether is primarily controlled by its molecular weight, chemical substituents attached to it, and conformational characteristics of the polymer chain. Methyl cellulose (MC), methylhydroxyethyl cellulose (MHEC), ethylhydroxyethyl cellulose (EHEC), methylhydroxypropyl cellulose (MHPC), hydroxyethyl cellulose (HEC), hydrophobically modified hydroxyethyl cellulose (HMHEC) either alone or in combination are among the most widely used cellulose ethers in mortar formulations (see Table 10.2).

Table 10.2. List of Cellulose Ethers Used or with a Potential to be Used in the Construction and Building Industries

Cellulose ether Abbreviation
Methyl cellulose MC
Ethyl cellulose EC
Methylhydroxyethyl cellulose MHEC
Methylhydroxyethylhydroxypropyl cellulose MHEHPC
Methylhydroxypropyl cellulose MHPC
Ethylhydroxyethyl cellulose EHEC
Ethylhydroxypropyl cellulose EHPC
Ethylmethylhydroxyethyl cellulose EMHEC
Ethylmethylhydroxypropyl cellulose EMHPC
Hydroxyethyl cellulose HEC
Hydroxymethylethyl cellulose HMEC
Hydroxyethylmethyl cellulose HEMC
Hydroxyethylpropyl cellulose HEPC
Hydroxypropyl cellulose HPC
Hydroxypropylmethyl cellulose HPMC
Hydroxypropylhydroxyethyl cellulose HPHEC
Carboxymethyl cellulose CMC
Carboxymethylhydroxyethyl cellulose CMHEC
Carboxymethylhydroxypropyl cellulose CMHPC
Hydrophobically modified hydroxyethyl cellulose HMHEC
Sulfoethyl cellulose SEC
Sulfopropyl cellulose SPC
Carboxymethylsulfoethyl cellulose CMSEC
Carboxymethylsulfopropyl cellulose CMSPC
Hydroxyethylsulfoethyl cellulose HESEC
Hydroxypropylsulfoethyl cellulose HPSEC
Hydroxyethylhydroxypropylsulfoethyl cellulose HEHPSEC
Methylhydroxyethylsulfoethyl cellulose MHESEC
Methylhydroxypropylsulfoethyl cellulose MHPSEC
Methylhydroxyethylhydroxypropylsulfoethyl cellulose MHEHPSEC
Allyl cellulose AC
Allylmethyl cellulose AMC
Allylethyl cellulose AEC
Carboxymethylallyl cellulose CMAC
N,N-dimethylaminoethyl cellulose DMAEC
N,N-diethylaminoethyl cellulose DEACC
N,N-dimethylaminoethylhydroxyethyl cellulose DMAEHEC
N,N-dimethylaminoethylhydroxypropyl cellulose DMAEHPC
Benzyl cellulose BC
Methylbenzyl cellulose MBC
Benzylhydroxyethyl cellulose BHEC
Sodium carboxymethyl cellulose ether Na-CMCE

Cellulose ethers are used in building material systems, such as manual and machine plasters, filling compounds, tile adhesives, air-placed concrete materials, flowable floorings, cement extrudates, emulsion paints, thickeners, and water retention agents. The properties of these building material systems, in particular the consistency and the setting behavior can be greatly influenced by the choice of the cellulose ether. Particularly, in gypsum-bound building material systems, i.e., gypsum-containing base mixes to which water has been added, lumps or nodules are often observed, which in the most unfavorable case can lead to irregularities and furrows, and at least result in delays due to intensive reworking (2008, DE102007016726 A1; 2008, DE102007016783 A1, DOW WOLFF CELLULOSICS GMBH). Attempts have been made to eliminate some of these problems by combinations of admixtures.

DE3920025 A1 (1999, AQUALON GMBH) discloses an admixture for gypsum-based mortars, plasters, and/or putties comprising a major amount of a cellulose ether and a small amount of (1) a thickener selected from polyacrylamides and/or starch ethers and of (2) a certain plasticizer containing sulfonic acid groups or sulfonate groups. However, the plasticizer (2) containing sulfonic acid groups or sulfonate groups does not have a high thermal stability if it is subjected to elevated processing temperatures and tends to release substantial amounts of sulfur and sulfur dioxides (1999, WO9964368 A1, DOW CHEMICAL CO).

WO9964368 A1 (1999, DOW CHEMICAL CO) discloses an admixture for gypsum-based plasters comprising a major amount of cellulose ether (e.g., MHPC) and small amounts of a polymerized carboxylic acid and of a (meth)acrylate homo- or interpolymer. However, the preparation of this additive mixture is complicated, requires additional mixing units, and does not always lead to a reduction of the lumps. In addition, the use of aqueous carboxylic acid solutions can lead to pH-induced chain degradation of the cellulose ether (2008, DE102007016726 A1; 2008, DE102007016783 A1, DOW WOLFF CELLULOSICS GMBH).

DE102007016783 A1 (2008, DOW WOLFF CELLULOSICS GMBH) discloses the preparation of mill dried MHPC, and the use thereof in gypsum-bound building material systems, preferably in gypsum machine plaster. The mill dried MHPC is distinguished from MHPCs of the prior art, in that it leads to improved processing properties of gypsum-bound building material systems, in particular little formation of lumps in gypsum machine plasters. DE102007016726 A1 (2008, DOW WOLFF CELLULOSICS GMBH) uses, for the same purpose, methylhydroxyethylhydroxypropyl cellulose (MHEHPC).

Table 10.2 presents a list of cellulose ethers used or with a potential to be used in the construction and building industries.

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Structure and Engineering of Celluloses

Serge PÉrez, Daniel Samain, in Advances in Carbohydrate Chemistry and Biochemistry, 2010

3 Etherification Reactions of Cellulose

Cellulose ethers can be prepared by various methods, as by using the common Williamson ether synthesis, with alkyl halides in the presence of a strong base (Fig. 32). This procedure is most often used to introduce carboxyl functions [O-carboxymethylcellulose (CMC)] or hydroxyl groups [3-hydroxypropylcellulose (HPC) and 2-hydroxyethylcellulose (HEC)].

Fig. 32. Route for the preparation of cellulose ethers from alkyl halides.

Addition of acrylic derivative or related unsaturated compounds such as acrylonitrile (Michael addition) is also employed for the synthesis of cellulose ethers (Fig. 33); moreover, cellulose ethers can be obtained from alkylene oxides reacting in a weakly basic medium (Fig. 34).

Fig. 33. Scheme of cellulose grafting with acrylonitrile.

Fig. 34. Route for the preparation of cellulose ethers from alkylene oxides.

Adsorption of alkoxysilanes onto cellulose fibers also holds promise for modification of the cellulose surface. The basic formula of the silane-coupling agents used has an organofunctional group on one side of the chain and an alkoxy group on the other.176–178 Abdelmouleh179 studied the adsorption of several prehydrolyzed alkoxysilanes onto the surface of cellulosic fibers in ethanol–water mixtures. This investigation offers the possibility for incorporating cellulosic fibers within a polyalkene matrix for the elaboration of composite materials.

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Polymer Reactions

Jett C. ArthurJr., in Comprehensive Polymer Science and Supplements, 1989

2.6.2 Ethers

Cellulose ethers are prepared by nucleophilic reactions, Michael condensation reactions and epoxidations. The most commonly used nucleophilic reaction is the reaction of alkali cellulose with alkyl halide (equation 4)


where X represents halide, usually chloride.6, 37, 38

Michael condensation reactions (base-catalyzed addition of active methylene compounds to activated unsaturated systems) were reported in 1887.40 This reaction has been used to prepare cyanoethylcellulose (equation 5).


Other cellulose ethers of related unsaturated nitriles have been prepared: α-methyleneglutaronitrile, trans-crotonitrile, methacrylonitrile and fumaronitrile. Also, reversible reactions of alkali cellulose with acrylamide and reactions of alkali cellulose with vinyl sulfones have been reported.6, 41

The reactions of ethylene and propylene oxides yield hydroxyethylcellulose and hydroxypropyl-cellulose (equation 6). Hydroxyethyl celluloses have been used in latex paints and in paper.6


Some cellulose esters and ethers that have been prepared and marketed commercially in the 1980s are listed in Table 2.

Table 2. Some currently Commercially Marketed Cellulose Esters and Ethers1, 6, 7, 11, 45

Useful productsa
Cellulose derivative Reagent DS rangeb Solubilityb Product uses
Cellulose esters
Nitrate Nitric acid 1.5–3.0 Sol: MeOH, PhNO2, ethanol–ether Collodion, films, fibers, explosives
Insol: H2O, ethanol, ether, C6H6
Acetate Acetic anhydride 1.0–3.0 Sol:acetone Films, fibers, coatings, fabrics with heat and rot resistances
Cellulose ethers
Methyl Methyl chloride 1.5–2.4 Sol: hot H2O; Insol: cold H2O, forms gel in water Food additives, films, cosmetics, greaseproof paper
Carboxymethyl Chloroacetic acid or Na salt 0.5–1.2 Sol: H2O Food additives, fibers, coatings, oil-well drilling muds, paper size, paints, detergents
Ethyl Ethyl chloride 2.3–2.6 Sol: organic solvents Plastics, lacquers
Hydroxyethyl Ethylene oxide low DS Sol: H2O Films
Hydroxypropyl Propylene oxide 1.5–2.0 Sol: H2O Paints
Hydroxypropylmethyl Propylene oxide/methyl chloride 1.5–2.0 Sol: H2O Paints
Cyanoethyl Acrylonitrile 2.0 Sol: organic solvents Products with high dielectric constants, fabrics with heat and rot resistances
Usually marketed in powder form.
DS = degree of substitution; Sol = soluble, Insol = insoluble.
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Chemistry of chemical admixtures

G. Gelardi, ... R.J. Flatt, in Science and Technology of Concrete Admixtures, 2016 Cellulose-ether derivatives

Cellulose-ether (CE) derivatives are the most widely used and effective water-retaining admixtures. It has been estimated that around 100,000 tons of cellulose derivatives are annually consumed by the construction industry (Plank, 2005), mainly for producing rendering mortars.

Cellulose is a uniform, linear glucose polymer and the most abundant of all natural substances (Khayat and Mikanovic, 2012). Cellulose consists of several hundreds to many thousands of β-1,4-linked d-glucose units, as shown in Figure 9.24. It is insoluble in water: to make it soluble, it is normally modified by attachment of small substituents in the hydroxyl groups of C2, C3 and C6.

Figure 9.24. Chemical structure of cellulose.

In the case of CE derivatives, etherification occurs in alkaline conditions. The most widely used CE derivatives in building materials are hydroxypropyl methyl cellulose, hydroxyethyl methyl cellulose and hydroxyethyl cellulose (Figure 9.25). These admixtures have proved to be stable in the highly alkaline conditions of cementitious systems (Pourchez et al., 2006).

Figure 9.25. Chemical structure of cellulose ethers: (a) hydroxypropyl methyl cellulose; (b) hydroxyethyl methyl cellulose; (c) hydroxyethyl cellulose.

The behaviour of CE derivatives depends on its degree of substitution (DS), the nature of the substituent group and its structural parameters, such as molecular mass and amount of substitution groups (Brumaud et al., 2013). In general, the molecular weight is between 105 and 106 g/mol.

The DS is the number of substituted hydroxyl groups per glucose molecule, and ranges between zero and three. The solubility of cellulose derivatives will depend on the DS. Derivatives of cellulose with a DS lower than 0.1 are generally insoluble. Those with a DS between 0.2 and 0.5 are soluble in aqueous alkaline solutions. Cellulose derivatives with a DS between 1.2 and 2.4 are soluble in cold water (Richardson and Gorton, 2003; Brumaud, 2011; Khayat and Mikanovic, 2012). The substitution of hydroxyl groups in glucose molecules by chemical groups with additional free hydroxyl groups for further substitution is quantified by the molar substitution and has no theoretical upper limit.

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Cellulose Plastics

J.A. Brydson, in Plastics Materials (Seventh Edition), 1999

22.3.2 Miscellaneous Ethers

Only one other cellulose ether has been marketed for moulding and extrusion applications, benzyl cellulose. This material provides a rare example of a polymer which although available in the past is no longer commercially marketed. The material had a low softening point and was unstable to both heat and light and has thus been unable to compete with the many alternative materials now available.

A number of water-soluble cellulose ethers are marketed.4 Methyl cellulose is prepared by a method similar to that used for ethyl cellulose. A degree of substitution of 1.6–1.8 is usual since the resultant ether is soluble in cold water but not in hot. It is used as a thickening agent and emulsifier in cosmetics, as a paper size, in pharmaceuticals, in ceramics and in leather tanning operations.

Hydroxyethyl cellulose, produced by reacting alkali cellulose with ethylene oxide, is employed for similar purposes.

Hydroxypropyl cellulose, like methyl cellulose, is soluble in cold water but not in hot, precipitating above 38°C. It was introduced by Hercules in 1968 (Klucel) for such uses as adhesive thickeners, binders, cosmetics and as protective colloids for suspension polymerisation. The Dow company market the related hydroxypropylmethyl cellulose (Methocel) and also produce in small quantities a hydroxyethylmethyl cellulose.

Reaction of alkali cellulose with the sodium salt of chloracetic acid yields sodium carboxmethyl cellulose, (SCMC). Commercial grades usually have a degree of substitution between 0.50 and 0.85. The material, which appears to be physiologically inert, is very widely used. Its principal application is as a soil-suspending agent in synthetic detergents. It is also the basis of a well-known proprietary wallpaper adhesive. Miscellaneous uses include fabric sizing and as a surface active agent and viscosity modifier in emulsions and suspensions. Purified grades of SCMC are employed in ice cream to provide a smooth texture and in a number of pharmaceutical and cosmetic products.

Schematic equations for the production of fully substituted varieties of the above three ethers are given below (R represents the cellulose skeleton).

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Polymers for a Sustainable Environment and Green Energy

T. Heinze, T. Liebert, in Polymer Science: A Comprehensive Reference, 2012 Carboxymethyl cellulose

The most important ionic cellulose ether is the sodium salt of CMC. In 2006, about 355 000 t were produced. It is an anionic polyelectrolyte and is therefore soluble in water or aqueous alkali solutions. The specific end-use segments are food and beverages, oil-field drilling fluids, drugs and cosmetics, paper-processing aids, detergents, textile processing, and related industries such as ceramic industry, paint and lacquer industry, civil engineering, tobacco industry, and glue and adhesives industry. CMC is applied in different purities ranging from technical grade (with about 25% sodium chloride and sodium glycolate) to high purity (99.5%) suitable for food applications. Major producers are Akzo Nobel, Dow Wolff Cellulosics, Kelco/Noviant, Ashland/Aqualon, Amtex, and a number of Chinese producers such as Chongqing Qiaofeng Industrial.

CMC is prepared by conversion of cellulose with sodium monochloroacetate in the presence of aqueous NaOH mainly with methanol and ethanol of isopropanol as suspending medium (diluent-mediated process, Figure 84). The DS values of commercial CMC are in the range of 0.6–0.9.528

Figure 84. Reaction scheme for the carboxymethylation of cellulose.

It was shown that the change of concentrations or the prolongation of reaction times can influence only the overall DSCMC but not the distribution of the substituents within the AGU or along the polymer backbone.529 The conclusion is that a treatment of the cellulose with aqueous NaOH is a very efficient activation method. An evidence is that totally homogeneous carboxymethylation carried out in Ni(tren)(OH)2 yields polymers with the same pattern of functionalization and properties.530

In order to control the distribution of the functional groups, new synthesis paths have to be developed. Fink et al.531 established the so-called “concept of reacting structural features” with includes stepwise etherification using aqueous NaOH solutions of comparatively low concentrations. Carboxymethylation is thereby mainly achieved in the noncrystalline regions of the cellulose structure.532

A different concept for the synthesis of unconventional CMC is treatment of cellulose dissolved in DMAc/LiCl or cellulose intermediates such as CTFA,126 CF, CA, and TMSC in DMSO with solid water-free NaOH particles and conversion of the resulting gel with sodium monochloroacetate. It was observed that during this treatment, regeneration of cellulose II on the particle–solution interface occurs.141,533,534 This process is called induced phase separation, leading to reactive microstructures (Figure 85).

Figure 85. Schematic drawing representing a ‘reactive microstructure’ formed by treatment of a homogeneous solution of cellulose with a solid reagent such as NaOH.

The reaction yields CMC with DS values as high as 2.2 in a one-step procedure. These CMCs start to dissolve in water at DS values of 1.5. 13C and 1H NMR spectroscopy after hydrolysis with aqueous D2SO4 showed much higher etherification at positions 6 and 3 for the new CMC.535 Furthermore, complete depolymerization of the polymer backbone was achieved by hydrolysis with perchloric acid and the basic repeating units of the CMC chain (2,3,6-tri-, 2,3-; 2,6-; 3,6-di-, 2-; 3-; 6-mono-O-carboxymethyl glucose and glc) were separated by means of high-performance liquid chromatography (HPLC).536 The mole fractions of the repeating units could be determined. A comparison of the mole fractions measured with values calculated by statistics537 showed significant differences between the data sets.

The comparably high amount of glc and 2,3,6-tri-O-carboxymethyl glc units is an evidence for a gradient-like distribution of ether functions along the backbone. Furthermore, endoglucanase fragmentation of the unconventional CMC samples was performed followed by analytical and preparative SEC. It was clearly shown that the highly carboxymethylated fragments were dominated by 2,3,6-tri-O-carboxymethyl glc units, which is a further evidence for a block-like distribution.115 These unconventional cellulose ethers exhibit new superstructural architectures, that is, a network-like system as determined by atomic force microscopy (AFM) (Figure 86) leading to unconventional properties such as a much higher tendency to cover surfaces.538 In addition synthesis of 2,3-O-CMC via the 6-O-triphenylmethyl cellulose or more efficiently via the 4-monomethoxytrityl derivative (see Section was established.539–541

Figure 86. AFM image of CMC with a nonstatistical, block-like structure prepared via CTFA.

Reproduced from Liebert, T.; Heinze, T. Biomacromolecules 2001, 2, 1124,542 with permission.
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Working mechanism of viscosity-modifying admixtures

M. Palacios, R.J. Flatt, in Science and Technology of Concrete Admixtures, 2016

20.3 Working mechanisms of water retention agents

In building materials, cellulose ether (CE) and hydroxypropylguar (HPG)-derived polymers are normally used to improve the ability of a render or plaster to retain its constitutive water. In particular, they reduce the water loss due to capillary absorption into the porous substrate, onto which an overlay material is applied. This makes it possible for cement to hydrate and for adhesive and mechanical properties to develop (Brumaud et al., 2013; Marliere et al., 2012).

The working mechanism of both types of polymers has been proven to be similar. Firstly, CE and HPG admixtures increase the viscosity of cement pore solution. Brumaud et al. (2013) measured the influence of different CEs on the viscosity of distilled water and synthetic cement pore solutions. They confirmed that hydroxyethoxy methoxy cellulose (HEMC) and hydroxypropoxy methoxy cellulose (HPMC) increase the viscosity of these solutions; this increase depends on the molar mass, nature of the ether(s), and its dosage. However, as shown in Figure 20.7, at dosages lower than 1%, the viscosity increase does not depend on the molecular parameters, such as degree of substitution (DS) or mass substitution (MS). Only at very high CE dosages (higher than 1%) is there a slight dependency on these molecular parameters and viscosity of the aqueous solution.

Figure 20.7. Apparent viscosity at 2.5 s−1 for various dosages of hydroxyethoxy methoxy cellulose (HEMC) in distilled water. HEMC B has a molar mass that is 2.5 times higher than the molar mass of HEMCs A and C. HEMCs A and C have an equivalent mass substitution (MS) and a different degree of substitution (DS) whereas HEMCs A and B have a different MS and similar DS.

Reproduced from Brumaud et al. (2013) with permission.

Water retention of cementitious systems increases with the molecular weight and dosage of CE admixtures. Several studies (Brumaud et al., 2013; Bülichen et al., 2012) have confirmed that the mechanism controlling the water retention of CE polymers depends on the concentration used. Below a critical concentration corresponding to the overlap of the polymer coils, water retention of mortars is dictated by the increase of the viscosity of the interstitial solution, no matter the polymer structural parameters (Brumaud et al., 2013). Most of the studies conclude that the increase of the viscosity induced by CE admixtures may reduce the mobility of the interstitial solution and increase water retention.

In contrast, Patural et al. (2012) showed by nuclear magnetic resonance dispersion measurements that the surface diffusion coefficient of water is not modified by the presence of CE, despite the high increase of the solution viscosity induced by these polymers. However, CEs transiently increase the fraction of mobile water molecules present at the solid hydrate surfaces.

At dosages above the critical concentration, the associative nature of the CE molecules mainly governs their water retention ability. There are two reasons for this. The first is that CEs form aggregates in solution, increasing the viscosity of the interstitial fluid. The second is that these polymer aggregates are a few micrometers in size (see Figure 20.8), so that they may jam and/or plug the porosity of the fresh paste or mortar (Sonebi, 2006; Patural et al., 2011; Brumaud et al., 2013). In addition, Jenni et al. (2003, 2006) found that methyl hydroxyethyl cellulose (MHEC) is transported through the capillary pores of mortars and accumulated at the interface between the layer in contact with the substrate and the substrate surface, reducing the porosity.

Figure 20.8. Hydrodynamic diameter of HEMC in aqueous solution below and above the overlapping concentration.

Reproduced from Brumaud et al. (2013) with permission.

Similar results as those presented above for CE have been obtained by Poinot et al. (2014) for HPG. They confirmed that water retention of cement mortars depends on HPG concentration. They also found that at concentrations higher than the overlapping concentration, a high increase of the water retention is induced due to the formation of polymer aggregates. However, at dosages below the critical concentration, almost no effect of HPG on water retention is observed. This last result contrasts with the case of CE polymers (see Figure 20.9) (Poinot et al., 2014). The reasons for this are not clear but may be to be found in the lower intrinsic viscosity of the selected polymers.

Figure 20.9. Effect of polymer agglomerates formation on water retention for (a) hydroxypropylguar (HPG) and (b) hydroxypropoxy methoxy cellulose (HPMC).

Reproduced from Poinot et al. (2014) with permission.

In cement pastes containing CE polymers, Brumaud (2011) determined that the viscosity of the interstitial solution was lower than expected from the dosage used. This difference was explained by a partial adsorption of these polymers onto the surface of cement particles, causing a decrease of polymer concentration in the pore solution and therefore also of the viscosity. Other authors have also confirmed that CE polymers can adsorb on cement particles (Bülichen et al., 2012; Khayat and Mikanovic, 2012; Brumaud et al., 2013) and that the extent of this depends on DS, as shown in Figure 20.10.

Figure 20.10. Relative adsorption of CE as a function of DS for a CE dosage of 0.2% (Roussel et al., 2010; Muller, 2006; Brumaud et al. (2013)).

Reproduced from Brumaud et al. (2013) with permission.

However, because CEs are nonionic polymers, the origin of this adsorption is unclear. According to Bülichen et al. (2012), the adsorbed polymer may not correspond to the cellulose derivative but to byproducts present in commercial MHEC that are able to adsorb onto cement particles. In any case, authors agree that the water retention properties of CE are not related to CE adsorption onto cement particles.

At this point, it is worth mentioning that adsorption measurements of CEs are not easy to perform by the depletion method without introducing artifacts, specifically at dosages above the critical concentration where CE aggregates could be trapped within the porosity of the cement pastes (Brumaud et al., 2013; Bülichen et al., 2012).

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