Cellulose Ethers - ACS Symposium Series (ACS Publications)

Chapter 15

Cellulose Ethers Self-Cross-linking Mixed Ether Silyl Derivatives Arjun C. Sau and Thomas G. Majewicz

Downloaded by CORNELL UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0476.ch015

Research and Development Center, Aqualon Company, Little Falls Centre One, 2711 Centerville Road, P.O. Box 15417, Wilmington, DE 19850-5417

-

Functionalization of polysaccharides with silanolate groups, -Si-O , leads to the formation of a new class of silated polysaccharide derivatives. These polymers are alkali-soluble and self-crosslink upon drying to form water-resistant films. They also exhibit good adhesion to hydroxylic surfaces and crosslink in solution with polyvalent metal ions to form viscoelastic gels. The synthesis and properties of silated hydroxyethylcellulose (SIL-HEC) are described in this article. Cellulose is the most abundant and renewable natural material. Structurally, it is a syndiotactic polymer of anhydroglucose units which are connected by (1—>4) β-glycosidic linkages (Figure 1). It is characterized by a

CH OH 2

-Jn-2

C

"20H

Figure 1. The structure of cellulose. high degree of crystallinity and is water-insoluble — the result of inter- and intra molecular hydrogen bonding. Derivatization of the cellulose hydroxyl groups under appropriate conditions disrupts the hydrogen bonding network and leads to watersolubility. Typically, water-soluble derivatives result from etherification of cellulose in the presence of an alkali (1-2). Examples of etherifying agents used to manufacture several commercially available water-soluble cellulose ethers are shown below.

0097-6156/92/0476-0265$06.00/0 © 1992 American Chemical Society

In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

266

MATERIALS AND CHEMICALS FROM BIOMASS Cellulose Ether

Etherifying Agent

Substituent Group

Methylcellulose (MC)

CH C1

-CH

Carboxymethylcellulose (CMC)

C1CH C0 N a

Hydroxyethylcellulose (HEC)

CH -CH \ /

3

2

2

2

+

3

-CH C0 - Na 2

+

2

-(CH -CH -0) -H

2

2

2

n


Ο

Hydroxypropylcellulose (HPC)

CH ~CH-CH 2

\ / ο

3

-(CH -CH-0) -H 2

n

I

CH

3

The water-soluble cellulose ethers (CE's) shown above are widely used as thickeners to control the rheology of water-based formulations, such as latex paints, drilling muds, cosmetics, pharmaceuticals and building materials. Further modification of mono-substituted derivatives with another functional group can enhance or produce novel properties. For example, modification of methylcellulose with low levels of hydroxypropyl or hydroxyethyl groups increases its gelation and flocculation temperatures in water (1). Carboxymethylation of hydroxyethylcellulose produces a product which exhibits excellent tolerance to mono- and di-valent metal ions in solution, but readily crosslinks with tri- or tetra-valent ions to give highly viscoelastic gels (1). Cellulose ethers containing more than one type of functional group are commonly referred to as mixed cellulose ethers. Examples of commercial mixed cellulose ethers include carboxymethylhydroxyethylcellulose ( C M H E C ) , methylhydroxypropylcellulose ( M H P C ) , methylhydroxyethylcellulose ( M H E C ) and cationic hydroxyethylcellulose (1-2). A more recently commercialized mixed cellulose ether is hydrophobically modified hydroxyethylcellulose ( H M - H E C ) (3-4) - a polymeric surfactant. H M H E C is made by modifying H E C with a low level of a hydrophobic group, such as a long hydrocarbon alkyl chain. The hydrophobic moiety imparts associative properties to hydroxyethylcellulose. H M - H E C s exhibit enhanced low-shear solution viscosity and surface activity. Detailed synthesis and solution/rheological properties of H M - H E C have recently been reported (5-6). Its primary application is in latex paints where it functions as an associative thickener (4). It is known that silicon plays a central role in determining the properties of polymers containing silyl functionality (7). Several polysaccharides including cellulose (8-9) have been modified with trimethylsilyl (-SiMe ) substituents to achieve organosolubility. We now have discovered a new class of silated polysaccharides which are soluble in dilute aqueous alkali and self-crosslink on drying to afford water-resistant films (10-11). This paper describes the synthesis and properties of silated H E C (SIL-HEC) — a new mixed cellulose ether containing reactive silyl substituents. 3


267

Cellulose Ethers

15. SAU&MAJEWICZ Experimental

Preparation of SIL-HEC. The S I L - H E C samples were prepared by reacting H E C (hydroxyethyl molar substitution ( H E MS) ~ 2.5-3.3) with various amounts of (3glycidoxypropyl)trimethoxysilane (GPTMS) (Aldrich) in the presence of an alkali at 95-115°C (11). Results and Discussion


1. Preparation of S I L - H E C . H E C reacts with (3-glycidoxypropyl)trimethoxysilane ( G P T M S ) (1) in the presence of an alkali to form a silanolate derivative (2) as shown below (10). OMe / NaOH H E C - O H + CH -CH-CH -0-(CH,) -Si-OMe > 2

2

\

3

I O

\ OMe

(1)

yONa

+

HEC-0-CH -CH-CH -0-(CH ) Si-ONa 2

2

2

+

r

OH

\ > Na

+

(2) The silanolate groups in (2) results from the alkaline hydrolysis of methoxysilyl groups (-Si-OMe) of (1). -Si-OMe + N a O H

— > -Si-O N a

+

+ MeOH

Alkoxysilyl moieties are known to react with alkali metal hydroxides to form alkali-metal silanolates (12-14). We speculate that the glycidoxysilanolate (3), generated in situ, reacts with H E C to form the S I L - H E C (2). OMe

O- N a

/ NaOH / CH -CH-CH O-(CH ) Si-OMe 5» C H - C H - C H - 0 - ( C H ) - S i - 0 - N a 2

r

\ /

2

r

2

\

Ο

\ I OMe

(I)

2

2

+

3

\

Ο

O Na

(3)


+

+

268

MATERIALS AND CHEMICALS FROM BIOMASS


2. Properties of S I L - H E C a) Solution Properties. S I L - H E C , isolated in a neutralized state, is waterinsoluble due to crosslinking of the silanol groups with each other and with the hydroxyls of the H E C (see discussion below on the crosslinking mechanism). These crosslinks, however, are labile in the presence of an alkali or ammonia; hence, S I L - H E C readily dissolves at solution pHs > ~ 11. Once in solution, S I L - H E C does not precipitate out of solution upon acidification: however, viscosity enhancement or gelation may occur. This behavior is controlled by the composition of the S I L - H E C (molecular weight ( M W ) and silyl M S ) , solution concentration, and solution p H . For example, at moderately high concentration (~ 10%), the viscosity of a S I L - H E C with M W ~ 80,000 and silyl M S ~0.1 remains unchanged over a p H range from 7-12. However, a product with M W ~ 300,000 and silyl M S ~0.04 exhibits progressive viscosity enhancement below p H 11.2 and eventually gels at p H 9.5 at 2% polymer concentration (see Figure 2). This process is reversible, i.e., the gel can be broken down by raising its p H . The viscosity enhancement or gelation of a S I L - H E C solution at lower pHs is due to intermolecular crosslinking. A s the solution p H is lowered, the silanolate groups (-Si-O) are converted into silanol groups (-Si-OH) (eqn. 1) which condense and/or hydrogen bond with other silanol or carbinol groups to form a three-dimensional network (see discussion below). H Q+ 3

HEC~Si-0 Na

+

HEC~6i-OH

t

(1)

NaOH b) Self-Crosslinking Properties. Besides being able to crosslink in solution in the presence of an acid, S I L - H E C undergoes self-crosslinking when an aqueous solution (alkaline) is air-dried. This behavior is a unique property of S I L - H E C . Solution-cast films of SIL-HECs hydrate but remain insoluble in water. The hydrated films are transparent and somewhat elastic. These crosslinked films, however, reversibly dissolve in alkalies. 1,1001

3001 9.5

1

1

10

1

10.5

1

11

1

PH

11.5

1

12

1

12.5

1 13

Figure 2. p H Versus 2% Brookfield (BF) viscosity of S I L - H E C ( H E M S ~ 3.2; silyl M S ~ 0.04; M W ~ 300,000). In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

15. SAU & MAJEWICZ

269

Cellulose Ethers

Mechanism of Self-Crosslinking. Sodium methyltrisilanolate (4), which has the same silanolate functionality of S I L - H E C (2), crosslinks when its alkaline

/

Ο Na

+

+

Me-Si-ONa \ Ο Na

+


(4) solution is air dried. It has been reported (15-16) that the silanolate groups of (4) are neutralized with atmospheric C 0 to form the silanetriol intermediate, MeSi(OH)j (5), which in turn condenses to form methylpolysiloxane (6) as shown below. 2

+

MeSi(ONa )

3

+ C0 + H 0 2

2

> [MeSi(OH) ] + N a C 0 3

(4)

2

3

(5) n[MeSi(OH) ] 3

> ( M e S i O ) „ + 3/2nH 0 w

(5)

2

(6)

A similar mechanism is proposed to explain crosslinking of the S I L - H E C . In the presence of atmospheric C O * silanolate groups of the S I L - H E C are converted into silanol groups. Crosslinking occurs via condensation of silanol groups and/or condensation of silanol groups with carbinol (C-OH) groups. These reactions are shown below. S i - O H + HO-Si

> Si-O-Si + H 0

Si-OH + H O - C

> Si-O-C

2

+ H 0 2

(2) (3)

The crosslinked structure of a room temperature cured S I L - H E C ( H E M S ~3.2; silyl M S ~0.15; M W ~ 300,000) is schematically shown in Figure 3. This proposed structure is consistent with the three silicon signals discernible in the ^ i N M R spectrum of the crosslinked S I L - H E C (Figure 4). Based on the ^ i N M R study by Clayden and Palasz (17) of the crosslinked product resulting from silyl condensation of trimethoxysilane functionalized isocyanurate, we assign these signals to the following silicon environments (I-III).


270

MATERIALS AND CHEMICALS FROM BIOMASS

I ο


—Ο—Si—Ο—

Ο Figure 3. Schematic diagram showing the proposed structure for a room temperature cured S I L - H E C . (*\/\Γ= H E C backbone; - O H = hydroxyl group of the H E C ; ~ ^ = - C H C H ( O H ) C H 0 ( C H ) - and Si-O- = Si-O-C bond formed by the reaction of the S i - O H with the hydroxyl (C-OH) of the SIL-HEC). 2

2

2

3

3. Other Properties of S I L - H E C . Besides self-crosslinking, S I L - H E C can also crosslink other hydroxyl containing water-soluble polysaccharides to form waterand acid-resistant composite films. Silanol functions are known to react with hydroxylic polymers (18) to form Si-O-C linkages; however, their reactivity depends on the nature of the hydroxyl group. It has been reported (19) that primary hydroxyls are ten times more reactive than secondary hydroxyls to silanols. In solution, S I L - H E C crosslinks with polyvalent metal ions, such as T i , to form viscoelastic gels. SIL-HECs, in general, exhibit good adhesive properties. Their aqueous solutions can be used as an adhesive to glue various substrates containing surface hydroxyls or other active hydrogens. + 4

Conclusions Functionalization of water-soluble cellulose ethers, e.g., H E C , with silanolate functionality (Si-O) affords a new class of alkali-soluble polymers. They are self-crosslinkable and exhibit adhesion to hydroxylic surfaces. Their solution behavior (viscosity enhancement, gelation, etc.) is dictated by their composition ( M W and silyl MS), solution concentration, and solution p H . In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


15. SAU & MAJEWICZ

271

Cellulose Ethers

'

'

-20

-40

·

'

I

-60

-80

1 1 -100

·

1 -120

•

1— -140

PPM Figure 4. 39 M H z ^ i C P M A S N M R spectrum of a crosslinked SILH E C (silyl M S ~ 0.15). The unique properties of S I L - H E C could be exploited in a number of industrial applications that rely on: a) water- and acid-resistant coatings; b) metal-crosslinked gels; c) adhesion to various substrates. The silation chemistry described here can be extended to prepare analogous derivatives from other polysaccharides, such as guar, starch and their ether derivatives. Literature Cited 1. 2. 3. 4.

5.

Just, E . K . ; Majewicz, T. G . In Encyclopedia of Polymer Science and Engineering; John Wiley & Sons, Inc., Second Edition, 1985, Vol. 3; p. 226. Donges, R. Brit. Polym. J. 1990, 23, 315. Shaw, K . G . ; Leipold, D . P. J. Coatings Technol. 1985, 57(727), 63. NATROSOL PLUS - Modified Hydroxyethylcellulose, The Aqualon Company (a Hercules Incorporated Company), Wilmington, Delaware, 1988. Sau, A . C.; Landoll, L . M . Polymers in Aqueous Media: Performance Through Association; Glass, J. E . Ed.; Advances in Chemistry Series, American Chemical Society, Washington, D.C., 1989, 223, p. 343. In Emerging Technologies for Materials and Chemicals from Biomass; Rowell, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

272 6.

7.

8.


9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

MATERIALS A N D CHEMICALS F R O M BIOMASS

Goodwin, J. W.; Hughes, R. W.; Lam, C. K.; Miles. J. Α.; Warren, B . C. H . In Polymers in Aqueous Media: Performance Through Association; Glass, J. E . Ed.; Advances in Chemistry Series, American Chemical Society, Washington, D.C., 1989, 223, p. 365. Silcon-Based Polymer Science: A Comprehensive Resource; Zeigler, J. M . ; Gordon Fearson, F. W. Eds.; Advances in Chemistry Series, American Chemical Society, D.C., 1990, Vol. 224. Herman, R. E . ; De, K. K.; Gupta, S. K . Carbohydr. Res. 1973, 31, 407 and references cited therein. Klebe, J. F.; Finkbeiner, H . L . J. Polym. Sci. 1969, 7, 1947. Sau, A . C. Polymer Preprints, A C S Meeting in Boston, April, 1990, Vol. 31(1), 636. Sau, A . C. U.S. Patent 4,992,538, February 12, 1991. Noll, W. Chemistry and Technology of Silicones, Academic Press, New York, 1968, p. 86. Hyde, J. F.; Johannson, O. K.; Daudt, W. H.; Fleming, R. F.; Laudenslager H . B.; Roche, M. P. J. Amer. Chem. Soc. 1953, 75, 5615. Plueddemann, E . P. Silane Coupling Agents, Plenum Press, New York, 1982, p. 67. Ref. 11, p. 607. Ref. 13, p. 209. Clayden, N . J.; Palasz, P. J. Chem. Research (S) 1990, 68. West, R.; Burton, T. J. J. Chem. Educ. 1980, 57, 165. Brown, L. H . Film Forming Compositions, Myers, R. R.; Long, J. S. Eds.; Marcel Dekker, New York, 1972; Vol. 1, Part III, Chapter 13, p. 548.

RECEIVED June 8, 1991


Cellulose Ethers - ACS Symposium Series (ACS Publications)

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