Cellulose Ether

6.3.2 ESSENTIAL PRODUCT COMPONENTS

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

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.

1.2 Ethers

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.

10.1.4 Cellulose Ethers

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.

3 Etherification Reactions of Cellulose

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.

2.6.2 Ethers

9.4.3.1 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.

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.

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).

10.05.7.2.1 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.

It was shown that the change of concentrations or the prolongation of reaction times can influence only the overall DS_CMC 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

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.

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).

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.

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).