Nitrocellulose, Ethylcellulose, and Water-Soluble Cellulose Ethers

44 Nitrocellulose, Ethylcellulose, and Water-Soluble Cellulose Ethers 3,1

2

RUFUS F. WINT and KATHRYN G. SHAW 1

Hercules, Inc., 910 Market Street, Wilmington, DE 19899 Hercules, Inc., Research Center, Wilmington, DE 19899

Downloaded by CORNELL UNIV on July 19, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch044

2

Chronology of Developments in Cellulose Chemistry Organosoluble Esters and Ethers Chemistry and Chronology of Nitrocellulose Technology of Ethylcellulose and Ethyl(hydroxyethyl) Cellulose Water-Soluble Cellulose Ethers Basic Functions in Water-Based Coatings Chronology of Water-Dispersible Cellulose Ethers Chemistry of the Major Ethers Chronology of the Major Cellulose Ethers in Latex Paints Technology of the Cellulose Ethers in Latex Paints Methods of Incorporation Rheological Properties Future Outlook for Cellulose Ethers Chronology of Developments in Cellulose Chemistry From a brief study of the important developments in the history of cellulose chemistry, it becomes apparent that very little progress was made in elucidating the structure of cellulose from its initial isolation by Payen in 1840 until 1920 (Table I), largely because of the lack of analytical instrumentation capable of defining molecular structure. However, beginning with the application of X-ray analysis to this problem in 1920 by Scherrer (Table II), great strides were made in determining the basic structure of cellulose. Scherrers studies "showed that cellulose fibers were a composite of ordered and disordered regions" and initiated the extensive studies of crystallinity. These studies were successful in delineating the polymeric structure, in explaining the mechanical properties of fibers, and in revealing what physical attributes of polymeric systems are responsible for fiber-forming properties. "The development of the primary-valence chain theory of cellulose polymers was the next and possibly the most important guidepost to complete understanding of cellulose chemistry and to the development of polymer theory in general. This development 1Current address: 800 Princeton Road, Greenville, DE 19807 '

0097-6156/85/0285-1073$07.75/0 © 1985 American Chemical Society

Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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A P P L I E D P O L Y M E R SCIENCE

Table I.

Scientific

Developments i n C e l l u l o s e C h e m i s t r y 1940-1920

Year


1840 1850

Development

Payen—cellulose

isolated

and

described

F. S c h u l t z Pelouze 1860 1870

J.

Erdman—constitution of

cellulose

Franchimont C r o s s and Bevan 1880 1890 1900

E. S c h u l t z — c e l l u l o s e nomenclature; defined h e m i c e l l u l o s e F r a n c h i m o n t — a c e t y l o s i s of c e l l u l o s e to c e l l o b i o s e Nishikawa—first applied

Table II.

Scientific

Year

to

cellulose

Developments i n C e l l u l o s e C h e m i s t r y 1920-40

Development

Ambronn—beginning o f m i c e l l a r 1920

Scherrer—X-ray

theory

analysis

Herzog—dimensions of c r y s t a l l i t e s F r e u d e n b e r g — c e l l o b i o s e y i e l d as c e l l u l o s e s t r u c t u r e S t a u d i n g e r — r e l a t i o n s h i p o f v i s c o s i t y and D . P . Haworth—proof o f s t r u c t u r e o f c e l l o b i o s e

proof

1930 F r e u d e n b e r g and K u h n — k i n e t i c p r o o f o f c e l l u l o s e structure H e r z o g , H e s s , F r e u d e n b e r g , and S t a u d i n g e r — D . P . measurements F r e y - V / y s s l i n g and K r a t k y — f i b r i l l a r t h e o r y D a v i d s o n — c e l l u l o s e o x i d a t i o n and concept o f a c c e s s i b l e regions



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WINT A N D SHAW

Celluloses and Cellulose Ethers

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was not the result of the individual effort of any one research school, but rather the summation of the knowledge of many. Freudenberg*s degradation of cellulose to cellobiose in the early 1920s, Haworth1s elucidation of the chemical structure of cellobiose, and Staudinger1s studies [on the relationship between viscosity and degree of polymerization (D.P.)] led to the postulation that cellulose was composed of polymer. The concurrent determination of the unit cell dimension by Meyer and Mark when amalgamated v/ith earlier X-ray information led to the enunciation of the modern f i b r i l l a r theory of fiber structure by Frey-V/yssling and Krakty in the early 1930s. It was the concept of crystallinity in linear polymers and its relationship to strength and elongation that guided Carothers in his i n i t i a l polymer investigations, which culminated in the development of the largely polymer investigations, which culminated in the development of the so-called "miracle fibers" of today" (_1). Research in the period 1930-40 was largely directed toward testing, proving, and modifying the i n i t i a l hypothesis growing out of the two major advances made during the previous decade. However, as Table III indicates, the development and use of the electron microscope in 1940 permitted investigators to determine the dimensions and average number of chains that are associated in the clearly defined crystalline regions of the fiber. Since the structure and reactivity of cellulose were reasonably well defined by the early 1950s, most of the research over the next two and a half decades was directed toward its further derivation for specific commercial applications. Today we know that the cellulose fiber is composed of 35-45% amorphous and 55-65% crystalline regions, the crystalline regions consisting of very ordered chains associated through strong hydrogen bonding and linked or hinged together by the amorphous sections (J2). A representation of this structure is depicted in Figure 1. Modern-day methods of molecular weight (MW) determination such as measurement of intrinsic viscosity, osmotic pressure, sedimentation by ultracentrifuge, and light scattering have produced the source-dependent MW and corresponding D.P. values shown in Table IV (3). Qrganosoluble Esters and Ethers Chemistry and Chronology of Nitrocellulose. The charted history of currently known cellulose derivatives begins in 1832 with the nitration of cellulose via concentrated nitric acid by Braconnot (Table V). "However, in 1945 Schonbein nitrated cellulose with a mixture of nitric and sulfuric acids, and he is generally credited with the discovery of nitrocellulose" (à). The early history of this particular polymer is associated largely with the militaries of the European nations. Its eventual use in other applications was advanced when, in 1866 Abel demonstrated that its stability could be enormously improved by removing the acid retained from manufacture. While military uses and raw material sources were developing, nitrocellulose was finding a place in the plastics industry. The key to the development of the first plastic, a nitrocellulose plastic, was the trial-and-error discovery of camphor as a plasticizer in 1872. In 1884 Wilson


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Table I I I .

Scientific


Year

Developments

i n C e l l u l o s e C h e m i s t r y 1940-80

Development

1940

E l e c t r o n microscope i n v e s t i g a t i o n s initiated Hess—mechanical degradation s t u d i e s Husemann and S c h u l z — w e a k - l i n k t h e o r y H i c k e r s o n and Mark—measurements o f c r y s t a l l i n i t y Pacsu—weak-link theory Frey-Wyssling—shov/ed e x i s t e n c e o f i n d i v i d u a l f i b r i l s D e g r a d a t i o n o f c e l l u l o s e and d e r i v a t i v e s

1950

R e a c t i o n u n i f o r m i t y i n h e t e r o g e n e o u s and reactions

1950-80

Further d e r i v a t i z a t i o n

for

commercial

homogeneous

applications

CH OH

CH OH

2

2

TV2

—'n-2 F i gure

1.

(a_) Structural representation of cellulose. (b) Structural f o r m u l a of e t h y l c e l l u l o s e w i t h complete (54.88%) e t h o x y l s u b s t i t u t i o n .



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WINT A N D SHAW


and Green obtained their patent on pyroxylin solutions, the precursors of today*s lacquers. During World War I, nitrocellulose was made from cotton linters in the United States. The unavailability of cotton linters in Germany during this period and a cotton crop shortage in the United States in 1918 instigated research on the use of cellulose from wood pulp, which was plentiful. The researchers were partly successful. However, completely satisfactory and economically feasible processes were not developed until the 1930-40 period. By the end of World War I, a combination of economic and technological situations led to the application of nitrocellulose in lacquers. The war economy left the United States with an overabundant supply of chemical cellulose, a large supply of nitrocellulose no longer needed in munitions, and a great supply of butyl alcohol and acetone. The rapidly expanding automobile industry needed an easily applied, quick-drying, protective coating. The need was satisfied by 1923, and an automobile with a nitrocellulose lacquer finish first appeared (Table VI). This result received a great assist from tv/o process developments; in 1922 the first actual low-viscosity nitrocellulose was produced by batch digestion (_5), and in 1928 the Hilliken digester made this operation continuous (6). Contrary to many people1s beliefs, nitrocellulose lacquer technology has been dynamic during the past forty years (7). A series of innovative formulation design changes have resulted in nitrocellulose lacquers with widely different properties. For the most part, these changes have derived from problems that developed in the use of conventional lacquers. Most conventional lacquers are completely thermoplastic in nature, although some of the new lacquer types do cure chemically. Others impart markedly different protective or decorative properties to the applied film. A l i s t of the key innovations that have occurred in the lacquer industry over the past 40 years is shown in Table VII. During the 1930s a surge of interest developed in minimizing or eliminating the cost of the organic solvents required to apply a lacquer. At this time i t required 4 or 5 lb of volatile solvents/lb of applied lacquer film. This resulted in a substant i a l effort to produce lacquer emulsions (8). Emulsion lacquers have a number of advantages. They can be formulated with a higher nonvolatile content, do not penetrate porous substrates, and are dilutable with water, application equipment can be cleaned with water, and finally, i t is easier to make high-flashpoint lacquer emulsions than high-flash-point lacquers. On the negative side, they dry somewhat more slowly and have somewhat lower gloss than conventional lacquers. Since their development, the leather finishing industry has used both clear and pigmented lacquer emulsions extensively. Because protective coatings for automobiles was a tremendous market, other competitive finishes challenged nitrocellulose. The advent of alkyd resins in the 1930s was prophesied to sound the death knell of nitrocellulose finishes because of the advantages alkyd-amine finishes showed in higher nonvolatile content improved exterior durability, and lower solvent costs. The type of thinking that produced this prophecy led to two


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Table

A P P L I E D POLYMER SCIENCE

IV.

Cellulose:

MW and D . P . V a l u e s V e r s u s


Source

MW Range

Native c e l l u l o s e Chemical cottons Wood p u l p s

Table V.

C h r o n o l o g y of N i t r o c e l l u l o s e

Researcher

1832 1846 1858 1866 1844

Braconnot Maynard Parkes Abel W i l s o n and Green

Table V I .

Year

1905 1915 1923 1928 1935 1956

D.P.

600,000-1,500,000 80,000-730,000 80,000-340,000

Year

Source

Range

3,500-10,000 500-4,500 500-2,100

and E t h y l c e l l u l o s e

1830-1900

Development

N i t r a t i o n of c e l l u l o s e Nitrocellulose solution N i t r o c e l l u l o s e lacquers S t a b i l i z a t i o n of n i t r o c e l l u l o s e Pyroxylin solution

Chronology of N i t r o c e l l u l o s e

and E t h y l c e l l u l o s e

1900-70

Development

S u i d a — e t h e r i f i c a t i o n of c e l l u l o s e N i t r o c e l l u l o s e from wood p u l p Automotive i n d u s t r y a d o p t s n i t r o c e l l u l o s e l a c q u e r Continuous d i g e s t i o n of n i t r o c e l l u l o s e E t h y l c e l l u l o s e — f i r s t commercial p r o d u c t i o n Continuous n i t r a t i o n of c e l l u l o s e



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WINT A N D SHAW

Table VII.

Time

1935-40 1940-45 1945-50 1950-55

Innovations

in Nitrocellulose

Problem or Need

1960-65

Solvents undesirable Low n o n v o l a t i l e c o n t e n t Cost o f v o l a t i l e p o r t i o n S e n s i t i v i t y to a l c o h o l and c o s m e t i c s Single application, multicolor finish Lack o f e l o n g a t i o n

1965-70

S o l v e n t and mar

1970-75

Desire

1975-80

Low-volatile

1955-60

for

resistance

aqueous organic

compound (VOC) l a c q u e r s

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Lacquers

Solution

Emulsions High-solids lacquers Hot-spray lacquers Catalyzed lacquers

(from

18-26°)

Multicolor l a c q u e r enamel Nitrocellulose-ethylene-vinyl acetate Copolymer l a c q u e r s N i t r o c e l l u l o s e - u r e t h a n e super lacquers Waterborne n i t r o c e l l u l o s e coatings Compliance s o l v e n t 1 , 1 , 1 trichloroethane



1080


countermoves i n the l a c q u e r i n d u s t r y i n the 1940s t h a t resulted i n the development o f " h i g h - s o l i d l a c q u e r s . " In a comprehensive study on t h i s subject, Hercules, Inc., reported that the following f i v e f a c t o r s o u t s t a n d i n g l y i n f l u e n c e d the permissible nonvolatile content of nitrocellulose lacquers at selected viscosity levels: (1) molecular weight (viscosity of nitrocellulose); (2) solvent blend; (3) ratio of resin to nitrocellulose; (4) choice of resin; (5) temperature of application. Consequently, high-performance formulations appeared containing 21-30% n o n v o l a t i l e content for clear compositions and 28-36% f o r pigmented c o m p o s i t i o n s (_9). While their n o n v o l a t i l e c o n t e n t s remained below those of alkyd-amine f i n i s h e s , the r e d u c e d tendency f o r n i t r o c e l l u l o s e l a c q u e r s t o sag when spray-applied to vertical surfaces made possible the a p p l i c a t i o n of f i l m s of e q u i v a l e n t t h i c k n e s s . Still h i g h e r s o l i d s l a c q u e r s were produced i n the l a t e 1940s by hot s p r a y i n g . The p r i n c i p l e b e h i n d the s p r a y i n g o f lacquers at elevated temperatures i s a simple one—that of reducing the v i s c o s i t y of the l a c q u e r by h e a t i n g i t i n s t e a d o f by t h i n n i n g it with solvents, so t h a t the n o n v o l a t i l e c o n t e n t i s not reduced. By this means, up to 50% h i g h e r solids were attained, or alternatively, the economic use o f h i g h e r v i s c o s i t y grades of nitrocellulose was made p o s s i b l e . Other advantages a l s o became evident, such as the e l i m i n a t i o n of b l u s h i n g , an improvement i n flowout or l e v e l i n g , and a r e d u c t i o n i n o v e r s p r a y . A l l of t h i s progress at i n c r e a s i n g t h e i r s o l i d s content s t i l l l e f t n i t r o c e l l u l o s e l a c q u e r s i n f e r i o r to alkyd-amine c o a t i n g s in a l c o h o l and c o s m e t i c r e s i s t a n c e , two p r o p e r t i e s v e r y i m p o r t a n t i n furniture finishes. In the e a r l y 1950s, a f t e r s e v e r a l years of effort, these problems were l a r g e l y r e s o l v e d by the development of a new t y p e of lacquer called "catalyzed lacquer" (10). Catalyzed l a c q u e r s were so named because o f t h e i r r e l i a n c e upon c a t a l y s t s t o e f f e c t a s e l e c t i v e p o l y m e r i z a t i o n o f the n o n v o l a t i l e components i n t he system t h a t t y p i c a l l y c o n t a i n e d n i t r o c e l l u l o s e , plasticizer, and an a l k y d - a m i n e r e s i n . N i t r o c e l l u l o s e polymers characteristically played the role of skeletal reinforcing members i n the t h r e e - d i m e n s i o n a l f i l m m a t r i x , thereby g i v i n g i t ample e a r l y toughness and r i g i d i t y . During the e a r l y 1950s, a new n i t r o c e l l u l o s e - b a s e d finish having a unique d e c o r a t i v e e f f e c t appeared on both factoryfinished products and the interior and exterior walls of buildings. I t became known as " m u l t i c o l o r l a c q u e r e n a m e l " ( 1 1 ) . Simply d e s c r i b e d , a m u l t i c o l o r l a c q u e r i s two or more pigmented nitrocellulose lacquers suspended i n water and stabilized to p r e v e n t b l e n d i n g o f the d i f f e r e n t c o l o r e d p a r t i c l e s ( 1 2 ) . These p a r t i c l e s of v a r y i n g shape remain s e p a r a t e d u r i n g a p p l i c a t i o n and drying, and they are l a r g e enough t o form a distinct color pattern (13). The a d v a n t a g e s o f u s i n g m u l t i c o l o r l a c q u e r enamels i n c l u d e the f o l l o w i n g ( 1 4 ) : (1) the a b i l i t y i n a single, safe a p p l i c a t i o n to apply a t h i c k , d u r a b l e f i n i s h o f two, three, or more d i f f e r e n t c o l o r s , i n a v a r i e t y o f t e x t u r e s ; (2) the a b i l i t y to f i n i s h a porous or rough s u r f a c e w i t h a c o a t i n g t h a t appears smoother than the o r i g i n a l s u r f a c e ; (3) the a b i l i t y t o finish adjacent surfaces of d i f f e r e n t absorbent properties; applied



44.

WINT A N D SHAW


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heavily, the l a c q u e r tends t o o b s c u r e the d i f f e r e n c e between the o r i g i n a l surfaces. By 1963 the i n n o v a t i o n s i n l a c q u e r d e s i g n d e s c r i b e d above had i n c r e a s e d l a c q u e r 1 s a c c e p t a n c e so t h a t i t ranked second among a l l industrial finishes, trailing o n l y a l k y d f i n i s h e s and leading p h e n o l i c , amine, a c r y l i c , v i n y l , and epoxy f i n i s h e s . During t h i s same p e r i o d , new c o a t i n g s based on v a r i o u s v i n y l a c e t a t e c o p o l y mers began r e p l a c i n g n i t r o c e l l u l o s e l a c q u e r s on c e r t a i n flexible substrates. The i n h e r e n t high rate of elongation of vinyl polymers made t h i s p o s s i b l e even though f i l m s produced from t h e s e polymers had much lower t e n s i l e s t r e n g t h v a l u e s . The i n c o m p a t i bility between b i n a r y b l e n d s o f n i t r o c e l l u l o s e and commercially available vinyl polymers made mixed c o a t i n g s with a better balance of p r o p e r t i e s i m p o s s i b l e . Then, i n 1964, a p a t e n t was f i l e d t h a t d i s c l o s e d t h a t l a c q u e r s based on b l e n d s of n i t r o c e l l u l o s e and e t h y l e n e - v i n y l a c e t a t e copolymers h a v i n g a v i n y l a c e t a t e content of 37.0% or h i g h e r produced lacquers with vinyl-type flexibility and e l o n g a t i o n f o r use on p a p e r , foil, and other f l e x i b l e substrates (15). When m o d i f i e d w i t h e s t e r r e s i n s such as sucrose i s o b u t y r a t e and s u c r o s e benzoate, high-performance c o a t i n g s f o r wood and m e t a l s u b s t r a t e s a l s o r e s u l t e d . Late in the 1960s, it became apparent that catalyzed lacquers, the hardest, most durable nitrocellulose lacquers c o m m e r c i a l l y a v a i l a b l e f o r c o a t i n g wood, had l e s s r e s i s t a n c e to alcohol, cosmetics, and chemicals than d i d the best quality alkyd-amine compositions or high-pressure melamine overlays. This s i t u a t i o n was soon remedied by the development o f n i t r o c e l lulose-urethane "super lacquers" based on nitrocelluloseisocyanate prepolymers (16). Nitrocellulose as manufactured contains free h y d r o x y l groups t h a t can p r o v i d e s i t e s for true cross-linking with an isocyanate-bonded homogeneous coating. L a b o r a t o r y s t u d i e s c o n f i r m e d the t h e o r e t i c a l r e a c t i o n : 0 2 HOR

OCN-R'NCO

+ nitrocellulose

catalyst

0

RO-C-NH-R 1 -NH-C-OR

—> polyisocyanate

nitrocellulose urethane

Compatibility of nitrocellulose with a wide variety of commercial u r e t h a n e r e s i n s was r e a d i l y e s t a b l i s h e d . Laboratory work was needed, however, to develop u s e f u l formulations with excellent properties. The a l i p h a t i c - t y p e i s o c y a n a t e r e s i n s now a v a i l a b l e produce i n d u s t r i a l c o a t i n g s w i t h good i n i t i a l c o l o r and good c o l o r s t a b i l i t y when combined w i t h n i t r o c e l l u l o s e . Nitrocellulose was commercially a v a i l a b l e only in forms dampened or wet with a l c o h o l s or w a t e r . With the type of chemical reaction involved, i t was o b v i o u s t h a t such hydroxylcontaining solvents would be unsuitable for the practical a p p l i c a t i o n o f t h i s new t e c h n o l o g y . As r e f i n e m e n t s o f the b a s i c chemically reacted coatings were f o u n d , a new t e c h n i q u e was d e v e l o p e d whereby n i t r o c e l l u l o s e c o u l d be produced dampened with isocyanate-grade toluene.



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N i t r o c e l l u l o s e - u r e t h a n e super l a c q u e r s f o r f u r n i t u r e and wood s u b s t r a t e s combine the f a s t speed of dry c o n t r i b u t e d by n i t r o c e l lulose with the toughness and d u r a b i l i t y of urethane. They appeal for school, hospital, kitchen, casual, and contract furniture. The n i t r o c e l l u l o s e - u r e t h a n e l a c q u e r s a p p l i e d o v e r the proper wood s e a l e r s dry r a p i d l y and cross-link when a i r - or force-dried. They develop hardness, abrasion, and impact r e s i s t a n c e e q u i v a l e n t to or b e t t e r than most commercial f i n i s h i n g systems. They have excellent resistance to penetration by solvents and s t a i n i n g by n a i l p o l i s h and n a i l polish removers, dry-cleaning solvents, writing inks, waxes, crayons, l i p s t i c k , m u s t a r d , and Mercurochrome. They are u n a f f e c t e d by s o a p s , a l k a l i detergents, and household c l e a n e r s (17). Because o f the time required by the c r o s s - l i n k i n g r e a c t i o n , recoatability can be a c h i e v e d commensurate v/ith t y p i c a l f a c t o r y c o n d i t i o n s . The durability and solvent resistance of nitrocelluloseurethane finishes have prompted s t u d i e s o f t h e s e coatings for metal s u b s t r a t e s . E x c e l l e n t a d h e s i o n t o s t e e l and aluminum has been demonstrated. Pigmented systems dry r a p i d l y and have h i g h i n i t i a l g l o s s and s c r a t c h r e s i s t a n c e . Thus, a new v i s t a had been c r e a t e d for nitrocellulose, a workhorse i n the c o a t i n g s i n d u s t r y . Tough, r e s i s t a n t n i t r o c e l l u lose-urethane coatings probably v / i l l not be used to replace conventional lacquers. Instead, new o p p o r t u n i t i e s have been created t o f o r m u l a t e p l a s t i c - l i k e m a t e r i a l s t h a t can be spraya p p l i e d and a i r - or f o r c e - d r i e d . T h i s new c h e m i c a l t e c h n i q u e has been created to help solve the needs that are constantly r e c u r r i n g i n our e v e r - c h a n g i n g w o r l d . By t h e e a r l y 1970s, e c o l o g i c a l p r e s s u r e s on the protective coatings industry were t h r e a t e n i n g major coating formulation changes. Los A n g e l e s A i r P o l l u t i o n R e g u l a t i o n 442 came and then spread across the c o u n t r y i n the same or m o d i f i e d forms (18). Lacquers r e a d i l y adapted. However, the p o s s i b l e r e s t r i c t i o n o f effluent s o l v e n t s from a c o a t i n g o p e r a t i o n posed a more serious threat. After powder c o a t i n g s had been t h o r o u g h l y tested in research laboratories and had been r e l e g a t e d t o the role of specialty coatings, e n t h u s i a s m f o r waterborne c o a t i n g s soared. Again lacquers adapted. The l a c q u e r e m u l s i o n t e c h n o l o g y o f the 1935-40 e r a t h a t r e s u l t e d i n both c l e a r and pigmented oil in water e m u l s i o n s used to p r o t e c t and d e c o r a t e l e a t h e r was r e v i v e d and refined (19). Recent d a t a indicate that nitrocellulose coatings or i n k s wherein the i n t e r n a l phase c o n s i s t s o f d i s c r e t e particles, 0.2-0.3 in size: (1) can be produced i n c o n v e n tional high-speed equipment, (2) have good application properties, (3) can be f o r m u l a t e d f o r f a s t a i r - d r y o r f o r lowtemperature o v e n s , (4) can produce f i l m s w i t h p r o p e r t i e s comparable to those of solvent-based lacquers, and (5) can reduce solvent needs by 50% or more. In s p i t e o f t h e s e technological advantages, lacquer e m u l s i o n s proved l e s s f o o l p r o o f than their solvent applied counterparts. While t h e i r performance under laboratory conditions was s u c c e s s f u l , they have not yet been adopted on a p l a n t s c a l e . The p r i n c i p a l d i s a d v a n t a g e was slower dry t i m e , which meant lower p r o d u c t i o n per u n i t o f t i m e . This, in turn, caused a c o s t d i f f e r e n t i a l t h a t the c o m p e t i t i v e market environment c o u l d not t o l e r a t e .



44.

WINT A N D SHAW


The f a i l u r e o f waterborne l a c q u e r s t o match both the c o s t and performance of conventional lacquers caused users to seek alternative methods o f l o w e r i n g v o l a t i l e o r g a n i c compound (VOC) emissions. When the U . S . E n v i r o n m e n t a l P r o t e c t i o n Agency (EPA) included methylene c h l o r i d e and 1,1,1-trichloroethane in a l i m i t e d l i s t o f n o n t o x i c compounds, immediate i n t e r e s t d e v e l o p e d in e v a l u a t i n g t h e s e p r o d u c t s as l a c q u e r s o l v e n t s . In a short time, l a c q u e r f o r m u l a s c o n t a i n i n g 1 , 1 , 1 - t r i c h l o r o e t h a n e appeared in the t r a d e , and soon t h e r e a f t e r r e p o r t s o f lower VOC c o n t e n t lacquers a t t a i n i n g commercial use were h e a r d . Some literature shows 35-46% r e d u c t i o n s i n VOC ( 2 0 ) . Continuing e f f o r t s t o lower the VOC o f l a c q u e r s and i m p r o v e ments i n a p p l i c a t i o n equipment caused both c o a t i n g manufacturers and users of lacquers to evaluate e l e c t r o s t a t i c spray application. On the b a s i s o f e x p e r i e n c e w i t h o t h e r c o a t e d p r o d u c t s , the EPA r e q u e s t e d a 50% t r a n s f e r e f f i c i e n c y from u s e r s o f coatings. T h e i r s t u d i e s showed t h a t c o n v e n t i o n a l s p r a y a p p l i c a t i o n results in 30% t r a n s f e r efficiency. Thus, they f e e l t h a t 50% i s a reasonable, significant, and a t t a i n a b l e improvement. Some studies of e l e c t r o s t a t i c a p p l i c a t i o n of lacquers suggest that this unsupported judgment by EPA may be achievable, and some l i t e r a t u r e (21) tends to s u p p o r t t h i s judgment. T e c h n o l o g y of E t h y l c e l l u l o s e and E t h y l ( h y d r o x y e t h y l ) c e l l u l o s e . The two major commercial o r g a n o s o l u b l e c e l l u l o s e ethers, ethylcellulose and e t h y l ( h y d r o x y e t h y l ) c e l l u l o s e (EHEC), are filmforming polymers d i s t i n g u i s h e d by u n u s u a l p r o p e r t i e s and v e r s a t i lity (22). They contribute t o the b a s i c f i l m properties of s p e c i a l types of i n k s , coatings, and a d h e s i v e s . Their special utility r e s u l t s from the f o l l o w i n g performance p r o p e r t i e s (23): (1) h i g h impact resistance, flexibility, toughness, and the retention o f t h e s e p r o p e r t i e s a t extreme ranges o f temperature; (2) solubility i n a wide range o f s o l v e n t s a l l o w s for economy through s o l v e n t c h o i c e ; EHEC, i n p a r t i c u l a r , has good s o l u b i l i t y i n a l i p h a t i c - r i c h solvent mixtures; (3) broad c o m p a t i b i l i t y w i t h resins, plasticizers, oils, waxes, and tars; (4) excellent electrical properties; (5) thermoplastic characteristics e s s e n t i a l for i n j e c t i o n , extrusion, l a m i n a t i o n , and c a l e n d e r i n g operations, f o r compounding and a p p l i c a t i o n o f hot m e l t s and f o r the application and heat s e a l i n g o f paper coatings; (6) FDA acceptability as food a d d i t i v e s ; (7) good initial color and r e s i s t a n c e t o UV d i s c o l o r a t i o n ; (8) low d e n s i t i e s t h a t a l l o w f o r more economic a p p l i c a t i o n c o v e r a g e ; (9) good r e s i s t a n c e t o s t r o n g alkalis, s a l t s o l u t i o n s , and o z o n e ; good r e s i s t a n c e t o o x i d a t i o n at t e m p e r a t u r e s below the s o f t e n i n g p o i n t ; and (10) no t a s t e or odor. Of the p r o p e r t i e s p r e v i o u s l y l i s t e d , s o l u b i l i t y i n l e a n o r low ( K a u r i - b u t a n o l ) KB s o l v e n t s such as heptane p r o v i d e s i n k and coating manufacturers with best cost/performance balance. This i s i l l u s t r a t e d by the d a t a t h a t f o l l o w . The effect o f l e a n s o l v e n t (heptane) s o l u b i l i t y o f EHEC as w e l l as the c o s o l v e n t e f f i c i e n c y o f s m a l l a d d i t i o n s o f isopropyl a l c o h o l i s i l l u s t r a t e d i n Table V I I I . About 4% i s o p r o p y l a l c o h o l is sufficient t o make heptane d i s s o l v e EHEC, yielding uniform c l e a r s o l u t i o n s a t a n o n v o l a t i l e c o n c e n t r a t i o n as h i g h as 20%. The v i s c o s i t y data i n F i g u r e 2 i l l u s t r a t e graphically the


1083

1084



Table V I I I .

Effect

of

2 - P r o p a n o l Used as

a Cosolvent

for

EHEC

2-Propanol, %

None

1

2

3

4

5

10

Heptane, %

100

99

98

97

96

95

90

Appearance

swollen particles

partially soluble, dispersed

hazy

slightly hazy

clear

clear

clear

Viscosity at 25 ° C 5% concn (cps)

30

150

m

340

300

265

220

Figure

2.

In solvent formulas for ethylcellulose coating compositions, p a r t o f the t o l u e n e (up to 33 1 /3%, but not as much as 66 2 /3%) can be replaced by heptane w i t h o u t c a u s i n g any a p p r e c i a b l e i n c r e a s e i n v i s c o s i t y ; s i m i l a r replacement o f the a l c o h o l by heptane c a u s e s a much g r e a t e r v i s c o s i t y i n c r e a s e (Curves 1, 4, and 5 ) .


44.

WINT A N D SHAW


tolerance of an ethylcellulose toluene:ethanol blend for an aliphatic heptane. A more complete listing of N-type ethylcellulose appears in Table

solution in a 60:40 hydrocarbon, for example, the physical properties of IX.


Water-Soluble Cellulose Ethers Basic Functions in Water-Based Coatings. In contrast to the organosoluble derivatives just described, the water-soluble derivatives of cellulose are not used in coatings primarily as film formers or as additives to augment either the drying rate or the durability of the applied coating. Most of the advantages of the water-soluble derivatives are realized during the manufacture, storage, and application of the aqueous coating. They function as thickeners, rheological control agents, and protective colloids (24). That is, as thickeners they impart to the coating the body and consistency necessary for solid suspension during storage and transfer efficiency prior to application. By increasing the coating's viscosity, they inherently contribute to the development and control of other desirable applied properties such as ease of brushing, rolling, or spraying. As rheological or flow control agents, they govern postapplication properties such as adequate flow and leveling. As protective colloids, they increase storage stability through partial adsorption on the surfaces of the various solids in the system such as the pigments and binder (25). This helps to minimize particle-particle interactions that might otherwise result in problems such as syneresis, hard settling of the pigment, gelation, color drift, or premature coalescence of the binder due to excessive heat or cold during storage. Their superior combination of performance properties, along with their good economy of use relative to that of other natural or wholly synthetic polymers, has enabled the cellulose ethers to become the largest group of water-soluble polymers used today for thickening trade-sale, water-based coatings. The bulk of the discussion concerning their role in aqueous coatings will be directed toward latex trade-sales paints. From this discussion, i t is hoped that the reader will gain a clearer understanding of cellulose ethers and their potential uses in other water-based coatings. Chronology of Water Dispersible Cellulose Ethers. Table V earlier revealed that most of the work before 1900 on derivatizing cellulose was concerned with its nitrate esters. A flurry of activity on structure determination was begun in earnest in 1920. During this same time period, the reactivity of cellulose toward various etherifying agents was initiated (Table X) (26). As a result of Suidafs preliminary though inconclusive work in 1905 of methylating cellulose, Denham and Woodhouse and Lilienfield separately prepared and isolated methylcellulose in 1912. The first hydroxyalkyl derivative, (hydroxyethyl)cellulose or HEC, was proposed by Hubert in 1920. The next year, Jansen realized the first carboxyalkyl derivative, (carboxymethyl)cellulose, more commonly known as CMC (27). Commercialization of these three derivatives in the United States did not begin, however, for another 17 years, when, in 1937, HEC was first


1085

1086


Table IX.

Physical Properties of E t h y l c e l l u l o s e (43.0-49.5% Ethoxyl)


Physical Properties

Bulking value, l b / g a l . , i n granular form Bulking value, g a l . / l b , i n solution Color, Hazen, i n solution, ASTM D 365 Discoloration by sunlight E l e c t r i c a l properties D i e l e c t r i c constant at 25 °C, 1 Mc D i e l e c t r i c constant at 25 °C, 1 kc D i e l e c t r i c constant at 25 °C, 60 c D i e l e c t r i c strength, V/mil, 10-mil f i l m , ASTM D 149-64, step by step Power factor at 25 °C, 1 kc Power factor at 25 °C, 60 c Volume r e s i s t i v i t y , ohm/cm Elongation at rupture, %, 3-mil film, conditioned at 77 °F, and 50% r e l a t i v e humidity F l e x i b i l i t y , folding endurance, MIT double folds, 3-mil film Hardness index, Sward, 3-mil f i l m Light transmission, p r a c t i c a l l y complete, A Light transmission, better than 50% complete, A Moisture absorption, by f i l m i n 24 h at 80% r e l a t i v e humidity, % Odor, flake Refractive index, cast f i l m Softening point, °C (°F) Specific gravity Specific volume, i n . / l b (M /kg) in solution Taste Tensile strength, l b / i n . (M/Pa), 3-mil f i l m , dry 3

Value

2.602.8 0.099-0.104 2-5 very s l i g h t 2.8-3.9 3.0-4.1 2.5-4.0 1500 0.002-0.02 0.005-0.02 10 -10 12

14

7-30 160-2000 52-61 3100-4000A 2000-3100Â β

2 slight 1.47 152-162 (305-323) 1.14

3

23.9 (0.0008604) none

2

Tensile strength, wet (% of dry strength) Water vapor transmission, g/(m 24 h), 3-mil f i l m , ASTM Ε 96-66, Procedure Ε

6800-10,500 (46.9-72.4) 80-85

2

890



44.

WINT A N D SHAW


1087

marketed by the Union C a r b i d e C o r p . Methylcellulose followed i n 1939 when the Dow C h e m i c a l C o . began i t s m a n u f a c t u r e . D u r i n g the next 35 y e a r s , Dow c o m m e r c i a l l y i n t r o d u c e d s e v e r a l h y d r o x y p r o p y l and h y d r o x y e t h y l mixed e t h e r s of methylcellulose. Hercules entered the a r e a o f w a t e r - s o l u b l e c e l l u l o s e e t h e r s i n 1943 with the f i r s t commercial p r o d u c t i o n o f CMC ( 2 8 ) . Next to f o l l o w from Hercules were HEC i n 1962 and ( h y d r o x y p r o p y l ) c e l l u l o s e or HPC i n 1969. Today one o f t h e s e d e r i v a t i v e s , HEC, dominates the w a t e r based coating industry. CMC, HPMC, and HPC a r e a l s o used in these c o a t i n g s , but l e s s f r e q u e n t l y due t o l i m i t a t i o n s . CMC has poor tolerance for organic solvents. Both HPMC and HPC have limited solubility i n water at elevated temperatures. Each d e r i v a t i v e has a c o m b i n a t i o n o f advantages and d i s a d v a n t a g e s t h a t make i t b e s t s u i t e d f o r p a r t i c u l a r a p p l i c a t i o n s . C h e m i s t r y o f the Major E t h e r s . The b a s i c commercial method p r e p a r a t i o n f o r each o f t h e s e e t h e r s i s shown: cellulose

+ a l k a l i + water

> alkali

> alkyl

cellulose

of

cellulose

1.

R-X

ether

2.

|| R'-CH-CH2

> (hydroxyalkyl)cellulose

ether

3.

X-R-COOH

> (carboxyalkyl)cellulose

ether

0 alkali cellulose

+

R = a l k y l , R f = a l k y l or h y d r o g e n , and λ = h a l o g e n . It consists of s w e l l i n g the c e l l u l o s e , o b t a i n e d from b l e a c h e d wood p u l p or cotton linters, w i t h a c o n c e n t r a t e d aqueous solution, usually sodium hydroxide. The a p p r o p r i a t e r e a g e n t then r e a c t s w i t h the a l k a l i c e l l u l o s e under v a r y i n g c o n d i t i o n s o f time, temperature, and p r e s s u r e , depending upon the r e a c t i v i t y and p h y s i c a l form o f the r e a g e n t . Thus, m e t h y l c h l o r i d e i s used f o r the p r e p a r a t i o n of m e t h y l c e l l u l o s e ; propylene oxide for ( h y d r o x y p r o p y l ) c e l l u l o s e ; and ethylene oxide for (hydroxyethyl)cellulose. (Carboxymethyl)cellulose i s formed by the r e a c t i o n of monochloroacetic acid or i t s sodium s a l t w i t h a l k a l i c e l l u l o s e . The effective u t i l i z a t i o n of these v a r i o u s r e a c t i o n c o n d i t i o n s allows products of v a r y i n g degrees o f s u b s t i t u t i o n (DS) t o be p r o d u c e d . The DS is defined as the average number o f hydroxyl groups on the a n h y d r o g l u c o s e u n i t t h a t have been s u b s t i t u t e d . The range o f DS for each d e r i v a t i v e y i e l d i n g water s o l u b i l i t y i s shown i n Table XI. F o r s u b s t i t u e n t s t h a t can c h a i n out such as e t h y l e n e o x i d e , there i s a l s o a n o t h e r measurement, moles o f s u b s t i t u t i o n (MS). T h i s i s d e f i n e d f o r ( h y d r o x y e t h y l ) c e l l u l o s e as the a v e r a g e number of moles o f e t h y l e n e o x i d e per a n h y d r o g l u c o s e u n i t . Those HECs having MS l e v e l s of at l e a s t 1.8-3.0 are commercially most s i g n i f i c a n t i n water-based c o a t i n g s , especially trade-sale latex paints. The d u a l ranges o f commercial s u b s t i t u t i o n f o r the HPMCs i n the same a p p l i c a t i o n a r e g r e a t e r , the methoxyl DS v a r y i n g from 0.4 to 2.0, and the h y d r o x y p r o p y l MS from 0.07 t o 1.4 depending on the p a r t i c u l a r p r o d u c t s e l e c t e d . Although Hercules manufac t u r e s CMC i n f o u r DS t y p e s ( t y p i c a l l y 0 . 4 , 0.7, 0.9, and 1 . 2 ) ,



1088


Table X.

Chronology of Water-Soluble

Cellulose

Ether

Ethers

Synthesis ;and I s o l a t i o n

(Hydroxyethyl)cellulose Methylcellulose Sodium (carboxymethyl)cellulose (Hydroxypropyl)methylcellulose (Hydroxypropyl)cellulose

Table XI.

Cellulose

Cellulose Ethers:

U.S. Commercialization

1920 1912 1921 1927 1960

1937 1939 1943 1948 1969

DS Pvange f o r C o l d - W a t e r S o l u b i l i t y

DS Ranges Cellulose

Ether

Methylcellulose (Hydroxypropyl)methylcellulose (Hydroxyethyl)cellulose (Hydroxypropyl)cellulose Sodium (carboxymethyl)cellulose

Cold-Water Solubility

1.3-2.6 0.2-2.1 M 0 . 0 5 - 1 . 6 HP (MS) 1.3 t o >5 (MS) 2 . 0 t o >5 (MS) >0.4

Commercial Production

1.6-2.0 0.4-2.0 M 0 . 0 7 - 1 . 4 HP (MS) 1 . 5 - 3 . 0 (MS) 3 . 2 - 4 . 5 (MS) 0.4-1.4



44.

WINT A N D SHAW


1089

the highest DS type seems to contribute most to improved flow and leveling in latex semigloss paints. Hercules1 HPC, also of interest for flow improvement in latex semigloss systems, is commercially produced at an MS level greater than 3.0, the most typical range being between 3.2 and 4.5 (29). Each of these ethers is available in a number of viscosity types of varying thickening efficiency as Figure 3 illustrates. The ultra-high viscosity types, usually using chemical cotton as a furnish, may have a degree of polymerization (D.P.) as high as 4500 and achieve viscosities of 5000 cps at 1% solids in water. The moderate- and low-viscosity types may be produced by beginning with a lower D.P. furnish such as wood pulp and/or oxidatively cleaving the glucosidic linkages in the polymer backbone with a reagent like hydrogen peroxide. These types may have D.P.s as low as 150 and yield aqueous solution viscosities of only 100 cps at 10% solids. Both the HECs and the HPMCs are available in rapid-dispersing grades that may be added to water at neutral pH without lumping. Thickening then begins typically in 6-20 min depending upon the amount of glyoxal or other temporary cross-linking agent that was applied to the surface (Figure 4). More rapid thickening may be obtained by raising the pH to 9 or higher by the addition of a small amount of alkali, preferably a fugitive base like ammonium hydroxide. Methylcellulose, HPC, and CMC are not readily surface treatable. These, however, may be added to water without lumping either by preslurrying them in a small amount of an organic, water-miscible, nonsolvent like ethylene glycol or by sifting them slowly into the vortex of vigorously stirred water. Alternatively, methylcellulose and HPC may also be dissolved by preslurrying the ether in hot water (55-60 °C) and then allowing the liquid to cool to room temperature. An order of their relative hydrophilic-hydrophobic properties in aqueous solution may be seen in Table XII where a typical DS or MS type of each derivative is compared (30). The extremely hydrophilic nature of anionic CMC relative to the four nonionic gums is readily apparent from the high percentage of moisture absorbed by its dried films after exposure at 50% relative humidity and 25 °C for 1 week. A l l five derivatives display cold-water solubility but vary in their behavior at elevated temperatures. The more hydrophilic CMC and HEC remain soluble even above boiling, while the particular HPMC and methylcellulose shown gel 65 and 56 °C, respectively. HPC differs from a l l of these by not merely gelling but precipitating from solution as a highly swollen floe at 40 ° C . This behavior is a relative measure of how much more weakly v/ater is bound to these more hydrophobic derivatives. Their tolerances to various salts also differ according to their ability to bind water. A CMC solution is the most tolerant of salts of mono- and divalent nonheavy metals but precipitates in the presence of trivalent cations such as aluminum. Of the nonionic ethers, only the more hydrophilic HEC is soluble in a 26% salt solution. Its compatibility in a 10% Al2(S04)3 solution illustrates the advantage of achieving solubility via nonionic hydrogen-bonding groups as opposed to anionic groups in the case of CMC. HPC, because of its high DS and relatively high hydrophobic functionality, has the lowest


1090



F i g u r e 4.

E f f e c t o f pH and temperature on h y d r a t i o n time o f Natrosol 250 HR.


44.

WINT A N D SHAW

Table

XII.


Cellulose Ethers

Solution


Properties

Ν a CMC

Degree o f S u b s t i t u t i o n 0.82 M.S. E q u i l i b r i u m M o i s t u r e (%) 16.1 Solubility C o l d Water S Hot W a t e r 100°C 5% NaCL S 1 0 % NaCL S 2 6 % NaCL S 1 0 % Na.COS I 10% A l ^ S O , k 70% E t h a n o l I 100% E t h a n o l I Polar Organics none Surface tension (dynes/cm) 71

—

Note:

of Cellulose

HEC

1.3 2.80 5.9 S 100°C S S S I S S I V.few 64

1091

Ethers

HPC

MC

HPMC

1.22 M 0.16 HP 6.0

1.68

—

5.2

S S G e l s , 6 5 °C Gels,65°C S S S S I I I I I I S S I I few few 44-50

( 1 % s o l u t i o n s ) . Key: I = i n s o l u b l e , S =

47-53

soluble.


2 4.0 2.5 S ppts,40°i S I I I I S S Many 45

1092

APPLIED POLYMER SCIENCE


salt tolerance and lowest hot-water solubility. However, these molecular characteristics work to its advantage in giving i t the best solubility in polar organic solvents like ethanol and methanol, propylene glycol, ethylene glycol monoethyl ether, morpholine, and cyclohexanone. Like its methylated counterparts, HPC possesses appreciable surface activity in aqueous solution, reducing the surface tension of water to 45 dyn/cm. In contrast, the more hydrophilic HECs and CMCs exhibit only slight and no surface activity, respectively. Chronology of the Major Cellulosic Ethers in Latex Paints. Today's aqueous emulsion or latex paints find their origin in the water-based casein paste paints first commercially produced around the turn of the century (31). Casein is a phosphatecontaining, alkali-soluble protein. It was widely used as a binder but also functioned as an emulsifying agent and as a water-soluble thickener. In this sense, casein can be considered the forerunner of today's water-soluble cellulose ethers. Gradually, the amount of drying o i l in the casein paint was increased, and the transition from casein paint to the emulsion paint took place. From their introduction in 1934 until the present, oil-based alkyd emulsion paints have played a major role in the protective coating industry. Originally developed for application on interior walls and exterior masonry, these coatings were comparatively low in cost and gave excellent durability, flexibility, gloss retention, solvent resistance, heat resistance, and color retention (32). A number of disadvantages associated with them, however, greatly influenced coating chemists to try to achieve the above advantages with aqueous emulsions. Proper credit for the first large-scale use of aqueous synthetic-resin emulsions must go to the Germans, who prepared poly(vinyl acetate) latexes just prior to World War II to replace solvent-based emulsion paints made scarce by a shortage of drying oils (33). Latex paints based on styrene and butadiene were introduced domestically in 1948 for use primarily on interior surfaces such as walls, wallboard, plaster, and cinderblock (34). Casein was the preferred thickener for styrene-butadiene latex paints; i t thickened efficiently and gave alkyd-like flow properties. Casein began losing the market to cellulosics with the introduction of poly(vinyl acetate) paints in the 1950s (35). One hundred percent acrylic latexes also appeared on the market in the early 1950s. From this point on, the use of both poly(vinyl acetate) and acrylic emulsion paints for exterior masonry and interior surfaces climbed steadily. Concurrently, due to their high thickening efficiency, cellulose ethers became the predominant thickener. Today, the latex paint industry produces primarily acrylic and vinyl-acrylic (vinyl acetate-acrylic copolymer) paints. The vinyl-acrylic latexes are used most commonly in interior flat paints, while the acrylic latexes are used most frequently with exterior paints and interior gloss or semigloss paints. In 1981, approximately $30-40 million worth of cellulose ethers was sold to the paint industry to produce over 1 billion gallons of paint in the United States (36).


44.

WINT AND SHAW



Technology of the Cellulose Ethers in Latex Paints. Earlier in the discussion, the basic functions of these polymers in waterbased coatings were briefly described. This section will describe some of the structural parameters of these ethers, and their influence on several important latex paint properties will be illustrated with Natrosol (hydroxyethyl)cellulose. Methods of Incorporation. The cellulose ether can be incorporated into a latex paint at several stages of the manufacturing process. The method varies among manufacturers, depending on the type of equipment and process used, as well as the paint formulation. Many add a l l or part of the thickener to the premix. This technique increases the viscosity of the grind and makes i t a more efficient dispersing medium. Normally for flat paints, onethird to half of the thickener is added to the premix with the remainder being postadded to the letdown to assist in final viscosity adjustments. The addition ratios depend primarily on the amount of viscosity and shear resistance contributed to the grind by the shape, quantity, and water demand of the pigments in the grind as well as the MW of the thickener. The grind should be viscous enough so that the shear stresses developed during the grinding produce a sufficiently fine dispersion to achieve desirable properties such as good hiding power. Another option is to add the entire thickener during the letdown phase, as is typically done with semigloss and gloss paints. During the paint manufacture and subsequent paint storage, the thickener acts as a protective colloid by adsorbing on or "coating" the particles of pigment and latex. This reduces their mobility and imparts essential storage properties to the paint such as good mechanical, heat, freeze-thaw, and chemical stability. Cellulose ethers can be added during the paint manufacture as a dry powder, stock solution, or slurry. As with most watersoluble polymers, correct incorporation of the cellulosics is important in determining the production time and product quality. The rate of dissolution of the cellulosic is directly related to the degree of agglomeration that occurs during mixing. The key is to be able to disperse the particles before they begin hydrating. The surface-treated grades were developed to temporarily delay hydration until the particles have been wetted and well dispersed. Several other precautions can be taken to assure trouble-free usage of the cellulose ethers. The particles may be sifted into a well-agitated grind. The pH at the point of addition should be 7 or lower. Alkaline ingredients such as pigment dispersants should be added after the cellulosic is dispersed. Sufficient time should be allowed for hydration before the latex is added because the latex can coat the particles and prevent complete hydration. Increasing usage of pigment slurries has made i t difficult for some paint manufacturers to add sufficient amounts of thickener when using a typical 2-3% solution. When pigment slurries are used, large amounts of water, 50% or greater by weight, are added to the paint as part of the slurry. There may not be enough water remaining in the paint formula to make a dilute solution of the cellulosic. An alternative is to add the cellulosic as a slurry, which allows incorporation at a higher


1093

1094



solids concentration. Surface-treated grades may be slurried in water but must be added to the paint immediately so that the cellulose ether does not have time to hydrate and thicken the slurry to an unusable mixture. Glycols and coalescing agents are commonly used to slurry cellulosics for the manufacture of latex paints. The cellulosic may swell, particularly in propylene glycol or ethylene glycol, and hence should not be stored for prolonged periods. Rheological Properties. The prime advantage of using a binder in emulsion form, namely, the attainment of a high MW resin at moderately high solids having a low viscosity in water, is a disadvantage from an applicator's viewpoint. Since the resin is present in a dispersed form, the aqueous or continuous phase controls the viscosity of the emulsion (37). The primary purpose of a thickener in a latex paint system is to control the rheology, or flow properties. Cellulosic thickeners influence the rheology primarily by thickening the aqueous phase. Interactions do occur, however, between the thickener, the pigment, the latex, and possibly the surfactants, which have considerable bearing on the performance of the paint. This section describes the effect of the molecular v/eight or viscosity type of Natrosol on the paint properties such as leveling, sag resistance, film build, and spatter resistance (38). It is necessary to review some basic solution and suspension rheology in order to understand the reasons for the effect of the molecular weight of Natrosol on paint rheology. Solutions of cellulosic ethers exhibit pseudoplastic behavior, i . e . , the apparent viscosity decreases with increasing shear. The viscosity returns to its original value when the shear is removed. The amount of pseudoplasticity is very molecular weight dependent; the lower the cellulosic molecular weight, the less the viscosity varies under increasing shear, as depicted in Figure 5. Suspension rheology can be used to study systems such as latex paints. In paints, bridging networks can be formed when the shear rate is low enough. These interactions dominate the rheology and cause the viscosity to be higher than i t would otherwise be. When shear is applied to this network system, however, these bonds are broken. The entanglements of the thickener polymer in the free liquid then dominate the rheology and contribute to its viscosity and elasticity. The solids that are present increase the viscosity only by a volume-filling effect. Figures 5 and 6 compare the effect of the cellulosic molecular weight on solution viscosity curves and paint viscosity curves (prepared to a constant Stormer viscosity). These figures indicate that the solution viscosity curves correspond to the paint viscosity curves in the high-shear-rate region. This occurs because under high shear, the interaction between paint components is reduced so that solution rheology dominates. Film build is directly related to viscosity during the high-shear processes of brush and r o l l application. Clearly, just as in water alone, the thickener having the lowest MW produces the highest applied viscosity or greatest degree of brush drag.


WINT A N D SHAW

~

10,000 L

to

*

Natrosol 250H4R. 0.6%

& 1,000

I Downloaded by CORNELL UNIV on July 19, 2016 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch044

*

1095


Natrosol 250MR, 0.9% Natrosol

" G R " "

100

3 10 0.001

-L 0.01

0.1

Particle Suspension at Rest, Film Sag Under Gravity, Brushmark Leveling Figure 5.

I 0.

1.0 10 Shear Rate (reciprocal sec) Brookfield Viscometer

100

1,000

10,000

Brushing Spraying

Tumbling or Pouring

Roll Application

Influence of molecular weight and shear rate on viscosity of solutions of Natrosol.

ι 0.01

ι 0.1

ι 1

ι 10

ι 100

1 1,000

—I

Shear Rate (sec1) Figure 6.

Influence of molecular weight of Natrosol rate on paint viscosities.

and


shear

1096



Greater f i l m b u i l d promotes b e t t e r h i d i n g . Spatter resistance, also a high-shear application property, does not depend on thickener viscosity but does depend on the elasticity of the aqueous phase. Higher molecular weight t h i c k e n e r s are more e l a s t i c ; t h e r e f o r e , lower m o l e c u l a r w e i g h t s a r e more r e s i s t a n t t o spatter during r o l l a p p l i c a t i o n . At low shear r a t e s , the p a i n t v i s c o s i t y c u r v e s d i v e r g e from the b e h a v i o r o f the s o l u t i o n viscosity curves. Interactions, such as those between the l a t e x and t h i c k e n e r , dominate the p a i n t rheology under l o w - s h e a r c o n d i t i o n s . The i n t e r a c t i o n s i n c r e a s e as the thickener concentration increases. Therefore, lower m o l e c u l a r weight t h i c k e n e r s , which must be used a t h i g h c o n c e n trations, will have more i n t e r a c t i o n s w i t h the l a t e x and cause increased v i s c o s i t y . The l e v e l i n g p r o c e s s o f a p a i n t f i l m o c c u r s at v e r y low shear r a t e s . The p r e f e r r e d v i s c o s i t y type f o r good l e v e l i n g are those having high molecular weights. Improvements in l e v e l i n g w i l l g e n e r a l l y d e c r e a s e sag r e s i s t a n c e . Therefore, low m o l e c u l a r weight polymers o f f e r the best sag resistance properties. Other f a c t o r s , such as the l e v e l i n g p r o p e r t i e s of the latex and the r a t e o f water a b s o r p t i o n i n t o the substrate, a r e e q u a l l y i m p o r t a n t i n d e t e r m i n i n g the l e v e l i n g / s a g resistance p r o p e r t i e s of a p a i n t . The m o l e c u l a r weight e f f e c t on r h e o l o g i c a l p r o p e r t i e s i s summarized i n T a b l e XIII. Cellulose ethers can a l s o have a profound effect on the dispersion s t a t e o f both the h i d i n g and t i n t pigments i n a l a t e x paint. T h e i r r e l a t i v e d e g r e e s o f a d s o r p t i o n onto the s u r f a c e o f a s e m i g l o s s - g r a d e t i t a n i u m d i o x i d e i n an aqueous paste (50:20, titanium dioxide:water) can be q u a n t i f i e d by d e t e r m i n i n g the amount of a n i o n i c d i s p e r s a n t needed t o d i s p l a c e each of them. Table XIV shows t h e s e v a l u e s f o r an HPMC (Me DS = 1.5, HP DS = 0 . 3 ) and N a t r o s o l a t two d i f f e r e n t MS l e v e l s . A p p r o x i m a t e l y 50% more dispersant was required to displace the methylated derivative, w h i l e the lower MS N a t r o s o l t o o k 100% more, r e l a t i v e to the same D . P . N a t r o s o l o f MS = 2 . 5 a t the same solution concentration. H i g h D . P . CMCs o f DS = 0 . 7 a r e known t o r e q u i r e even greater amounts o f d i s p e r s a n t , whereas CMCs o f both h i g h D.P. and h i g h DS ( 1 . 2 ) have been found t o be d i s p e r s a n t s . Thus, the choice o f both t h i c k e n e r and pigments necessitates close examination of the formula's d i s p e r s a n t demand i n order to a c h i e v e optimum h i d i n g and c o l o r development. Future Outlook for C e l l u l o s e E t h e r s . Cellulose ethers fulfill many o f the r e q u i r e m e n t s demanded o f r h e o l o g i c a l modifiers in latex paints. Cellulosics, particularly (hydroxyethyl)cellulose, have the advantage of being , f a l l - p u r p o s e f f thickeners. HEC thickens a l l t y p e s o f l a t e x e s e f f i c i e n t l y and w i l l work w e l l in an e n t i r e p a i n t l i n e . With the wide v e r s a t i l i t y and good o v e r a l l balance of p r o p e r t i e s , (hydroxyethyl)cellulose w i l l continue to be used by p a i n t m a n u f a c t u r e r s t o ensure q u a l i t y p a i n t performance. The s a l e s o f a r c h i t e c t u r a l c o a t i n g s a r e e x p e c t e d t o grow a t a r a t e o f 2-4% per year ( 3 9 ) . The volume of c e l l u l o s e e t h e r s s o l d t o the l a t e x p a i n t i n d u s t r y i s e x p e c t e d to d e c r e a s e s l i g h t l y o r remain c o n s t a n t . In r e c e n t y e a r s , synthetic thickeners that offer good r h e o l o g i c a l p r o p e r t i e s have been i n t r o d u c e d to the paint industry. However, t h e s e have many f o r m u l a t i o n c o n s t r a i n t s


44.

WINT AND SHAW

Table XIII.

Effect of Natrosol Molecular Weight on Paint Properties

Property


1097


Best MW Thickener

Reason

Spatter resistance

low

Low elasticity

Film build

low

High high-shear viscosity due to solution rheology

Leveling

high

Less polymer creates few interactions

Thickening efficiency

high

Less polymer is needed to achieve desired KU

Table XIV. Effect of Dispersant and Thickener on Pigment Dispersion Using the Fluidity Titration Technique

wt % of Thickener in Test Solution 8

wt % of Tamol° 731 in Test Solution**

None (control) 0.6% Natrosol 250HR 0.6% Natrosol 250KR 0.6% Natrosol 250LR 0.6% (hydroxypropyl)methylcellulose (medium-high viscosity type) 0.6% (hydroxypropyl)methylcellulose (medium-high viscosity type) 0.6% Natrosol 180IIR 0.6% Natrosol 180HR

Titrant vol to Fluidity, mLc

0.6 0.6 0.6 0.6

13.0 13.0 13.0 13.0

0.6

19.0

0.8 0.6 1.1

13.0 24.0 13.0

Added to a standard paste consisting of 50 g of Ti-Pure R900 (Ε. I. du Pont de Nemours & Co.) titanium dioxide and 20 g of distilled water.

a

^Registered trademark of Rohm & Haas Co. c

Titrant volumes greater than 13.0 cc indicate pigment flocculation due to the thickener. The greater the volume, the greater the quantity of dispersant necessary to displace the thickener from the pigment surface and fluidize the paste.


1098



and production difficulties. As of the end of 1982, the synthe tic thickeners are primarily used as speciality items. Signifi cant breakthroughs in technology v/ill be required to expand their usage. Cellulosics should continue to play a major role as rheological modifiers in latex paints. Literature Cited 1. Skolnik, Herman; McBurney, Lane F. Tappi 1954, 37(9), 190A193A. 2. Savage, A. B. "Encyclopedia of Polymer Science and Technology"; Interscience: New York, 1965; Vol. 3, p. 459. 3. Hamilton, J. Kelvin; Mitchell, R. L. "Encyclopedia of Chemical Technology," 2nd ed.; Interscience: New York, 1964; Vol. 4, p. 595. 4. Barsha, J. "Cellulose and Cellulose Derivatives"; Interscience: New York, 1954; Vol. 5, Part II, Chap. ΙλΒ, p. 713. 5. Yeager, J. R. paper presented to the Technical Committee of the National Paint and Coatings Association, Hershey, Pa., Sept 15, 1960. 6. Yeager, J. R. Hercules Chem. 1961, No. 41, 2. 7. Arne, Frances, Ed. Chem. Eng. 1963, 70(4), 90. 8. Creasy, J. Prod. Finish. 1956, 9(7), 81-86. 9. "High-Solids Lacquers," Form 500-497, Hercules, Wilmington, Del., p. 5. 10. "Catalyzed Lacquers for Wood Finishing," CSL-148, Hercules, Wilmington, Del., p. 1. 11. "Lacquer, Multicolored Dispersion Type (For Spray Application)," Federal Specification TT-L-45, Jan. 17, 1958. 12. Zola, John C. U.S. Patent 2 591 904, 1952. 13. "Multicolor Lacquer—Less Flammable than Conventional Types," CSL-101A, Hercules, Wilmington, Del., p. 1. 14. Campbell, John R. Materials Methods 1954, 40, 86-89. 15. Unger, James G. U.S. Patent 3 321 420, 1967. 16. Hercules Chem. 1969, No. 59, 31. 17. "Nitrocellulose-Urethane Super Lacquers," CSL-202, Hercules, Wilmington, Del., p. 1. 18. "Nitrocellulose Lacquers Acceptable Under Rule 66," CSL196, Hercules, Wilmington, Del., p. 1. 19. "Preparation Procedures for Nitrocellulose Waterborne Coatings or Inks," CSL-225, Hercules, Wilmington, Del., p. 1. 20. "Formulating Fast-Dry Lacquers for Electrostatic Coatings," CSL-188, Hercules, Wilmington, Del., p. 1. 21. "Low-VOC (Volatile Organic Compound) Lacquers Formulated with Chlorinated Solvents," CSL-198, Hercules, Wilmington, Del., p. 1. 22. Hamilton, Eugene C.; Early, Lawrence W. Fed. Ser. Coat. Technol. 1972, Unit 21. 23. Klug, E. D. "Encyclopedia of Chemical Technology," 2nd ed.; Interscience: New York, 1964; Vol. 4, pp. 616-52. 24. "Natrosol Controls the Flow Properties of Paints," Bulletin VC800-10, Hercules, Wilmington, Del., 1980; p. 1. Tess and Poehlein; Applied Polymer Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


44. WINT AND SHAW Celluloses and Cellulose Ethers 25. Lindenfors, D. Farbe Lack 1973, No. 3, 44-56. 26. Skolnik, Herman; McBurney, Lane F. Tappi 1954, 37(9), 192A. 27. Savage, A. B. "Encyclopedia of Polymer Science and Technology"; Interscience: New York, 1965; Vol. 3, p. 459. 28. Batdorf, J. B.; Rossman, J. M. In "Industrial Gums"; Whistler, R. L., Ed.; Academic: New York, 1973; Chap. 31, p. 696. 29. Ibid., p. 650. 30. Klug, E. D. J. Polym. Sci.C.,Polym. Symp. 1971, No. 36, 493. 31. Nylen, Paul; Sunderland, Edward, "Modern Surface Coatings"; Interscience: New York, 1965; p. 663. 32. Martens, Charles R. "Technology of Paints, Varnishes, and Lacquers"; Reinhold: New York, 1968; Chap. 4, p. 33. 33. Nylen, Paul; Sunderland, Edward "Modern Surface Coatings"; Interscience: New York, 1965; Chap. 18, p. 664. 34. Allyn, Gerould "Emulsions and Emulsion Technology"; Lissant, Kenneth J., Ed.; Dekker: New York, 1974; Part I, Chap. 7, p. 353. 35. Salzberg, H. K. Am. Paint J. 1964, 48(45), 101. 36. Levine, Ralph M. Am. Paint Coat. J. 1981, 66(14), 122-35. 37. Martens, Charles R. "Technology of Paints, Varnishes;" Reinhold: New York, 1968; Chap. 28, p. 513. 38. "Natrosol Controls the Flow Properties of Paints," Bulletin VC800-10, Hercules, Wilmington, Del., 1930, pp. 14-13. 39. Am. Paint Coat. J. 1932, 12-16.


1099

Nitrocellulose, Ethylcellulose, and Water-Soluble Cellulose Ethers

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