Cellulose Acetate and Related Esters - ACS Symposium Series (ACS

43 Cellulose Acetate and Related Esters

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 21, 2015 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch043

LARRY G. CURTIS and JAMES D. CROWLEY1 Eastman Chemical Products, Inc., Kingsport, TN 37662 Reactivity Esterification and Hydrolysis Importance of Hydroxyl Functionality Viscosity Blending Cellulose Acetate (CA) Cellulose Acetate Propionate (CAP) Cellulose Acetate Butyrate (CAB) Future of Organic Acid Esters of Cellulose

'

Cellulose ranks high among natures more abundant products. Because it is the primary structural material of plant life, billions of tons of cellulose are created annually through photosynthesis. Over one million tons of cellulose are used annually by chemical industries in the manufacture of textile fibers, photographic products, inks, adhesives, explosives, plastics, and coatings. The chemical structure of cellulose is relatively simple (Figure 1). The simplicity l i e s in repetitive utilization of the anhydroglucose unit (C6H10O5) as the building block for the chain structure. The cellulose structure contains 31.48% by weight of hydroxyl groups: one primary and two secondary hydroxyl groups per anhydroglucose unit. These functional groups play the important role in chemical modification of cellulose in reactions with organic acids and acid anhydrides to produce organic esters of cellulose. The first organic ester of cellulose was cellulose acetate, prepared by Schutzenberger in 1865 by heating cotton and acetic anhydride to about 180 °C in a sealed tube until the cotton dissolved (1). In 1879, Franchimont acetylated cotton at lower temperatures with the aid of a sulfuric acid catalyst (2). Miles, in 1903, described the preparation of partially hydrolyzed cellulose acetate, which was easily distinguished from the fully acetylated 1Current address: 3800 Hemlock Park Dr., Kingsport, TN 37663. 0097-6156/85/0285-1053S06.00/0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; 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

Figure 1.

Molecular representation of cellulose.



43. CURTIS AND CROWLEY Cellulose Acetate and Related Esters 1055 product by its solubility in common inexpensive solvents (3). This development led almost immediately to commercial applications, particularly in areas where flame resistance was important. Eastman Kodak Co. cast photographic film base from cellulose acetate as early as 1908; however, commercial prominence was not achieved until World War I when it replaced flammable cellulose nitrate in the manufacture of airplane dopes. In the era following the war, cellulose acetate lacquers found many uses, but in most applications its performance was something less than satisfactory. Although cellulose acetate lacquers were excellent in some properties such as flame resistance, high melting point, toughness, and clarity, they did not have the level of water resistance or the desired range of solubility and compatibility with modifying resins for more widespread coatings applications. The general idea of using mixed esters such as acetonitrate, acetopropionate, and acetobutyrate to overcome the deficiencies of cellulose acetate is quite old (4), and as a result of a systematic study of the aliphatic series of cellulose esters and mixed esters by Malm and co-workers, it was learned that the cellulose acetate propionates and the cellulose acetate butyrates provided the desirable properties of the cellulose acetates plus improved solubility and compatibility, water resistance, flexibility, and resistance to weathering (5, 6). The butyrates were commercialized in 1933 for plastic molding, and the propionates were marketed in 1939 for the same purpose. Lacquer applications for the butyrates and propionates were slow to develop primarily because the supplier did not market lowviscosity products attractive to the coatings industry. Both esters had poor compatibility with alkyd resins and would not tolerate resin modification without sacrificing many of their desirable characteristics. At the same time, cellulose nitrate was available in several viscosity grades attractive to the coatings industry and could be modified to a large degree with alkyd resins or natural resins without appreciably altering the performance of the cellulose nitrate polymer. Applications of the mixed organic esters of cellulose were those requiring the high performance of the unmodified ester such as toughness, resistance to ultraviolet light, chemical resistance, etc. A typical use for cellulose acetate butyrate (CAB) requiring a l l these attributes was airplane dopes. Kline, Malberg, and Reinhart compared several cellulose derivatives in many solvent compositions in tests on the tautness and durability of coated aircraft fabrics (7,

-

s

tu

5.0

t

\ 111

2.0 2.0 1.0 A 100 90 80 70 60 50 40 30 20 10 100 A Β 0 10 20 30 40 50 60 70 80 90 0 1.0Β Blending Ratios to Produce Desired Intermediate Viscosity

Figure 5.

Cellulose ester viscosity blending chart.



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ultraviolet rays, have led to extensive use of these film formers in the coating of wire screening for the production of windows for greenhouses and similar structures. The good physical strength of a c e l l u l o s e acetate coating y i e l d s a tough, tear-resistant window. Solution grades of cellulose acetate are used to make membranes for reverse osmosis a p p l i c a t i o n s such as a r t i f i c i a l kidneys, desalination of water, or purification of i n d u s t r i a l waste. A typical formulation for cellulose acetate lacquer is given in Table V. C e l l u l o s e acetate i s compatible with c e l l u l o s e nitrate, but l i t t l e benefit has resulted from using the two together in a coating composition. Limited use has been made of c e l l u l o s e acetate in reactive compositions even though i t i s compatible with ureaformaldehyde resins. Cellulose Acetate Propionate (CAP) During the manufacture of CAP, the acetyl/propionyl ratio can be varied widely, but i f the acetyl content is more than a few percent, the product loses s o l u b i l i t y and compatibility and performs more like cellulose acetate. Thus, there are only two commercial grades of CAP: a general-purpose coating and ink grade, CAP-482-0.5, and an alcohol-soluble product, CAP-504-0.2, which i s useful in flexographic inks and overprints. The cellulose acetate propionates are intermediate in properties between the cellulose acetates and the cellulose acetate butyrates, resembling the c e l l u l o s e acetate butyrates in s o l u b i l i t y and compatibility. Like the acetates, the propionates have practically no odor and thus can be used in applications where low odor i s a requirement. These properties make the propionates especially useful in inks, overprints, plastic, and paper coatings, and various reprographic processes. CAP-482-0.5 requires moderately strong solvents to effect solution (Table VI). Both the propionates may be cross-linked with reactive resins to form thermoset systems that offer the benefits of shelf stability, low-temperature cure, and resistance to high heat, chemicals, and solvents. CAP-504-0.2 is especially useful in this type of system because of its high hydroxyl content—5.0% by weight. This permits i t to be dissolved in solvent systems composed primarily of alcohols, thereby increasing the s t a b i l i t y or shelf l i f e of the solutions. CAP-504-0.2 is a low-viscosity ester soluble in a wide range of solvents but i s p a r t i c u l a r l y interesting because i t i s soluble in blends of ethyl alcohol, isopropyl alcohol, or n-propyl alcohol with water (Figure 6). CAP-504-0.2 forms clear, hard, glossy films from these solvent mixtures. Methyl alcohol i s the only alcohol that w i l l dissolve this ester without the addition of a small amount of water. Paper coated with thermoplastic coatings incorporating CAP-504-0.2 may be repulped under mild conditions. Both CAP-482-0.5 and CAP-504-0.2 form films that have excellent resistance to penetration by o i l s and grease. Overprints of these unmodified f i l m formers were applied to paper at 0.1-mil dry f i l m thickness and allowed to dry overnight. Penetration test materials containing a red dye (corn o i l , castor o i l , hot lard, mineral o i l , peanut o i l , and water) were placed on the overprints and allowed to remain in contact for 24 h. None of the test materials stained the



43.

CURTIS A N D C R O W L E Y

Table IV.

Cellulose Acetate and Related Esters

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Solubility as Indicated by the Solution Viscosity of CA-398-3

Solvent Acetone Methyl acetate Methyl ethyl ketone Ethylene glycolraonomethylether acetate Isophorone Cyclohexanone Diacetone alcohol Ethyl acetate n-Butyl acetate

Solution Viscosity at 15% concn [cP (mPa s)] 189 335 750 2830 4880 6620 9650 Borderline Insoluble

Table V. Cellulose Acetate Paper Lacquer

Components CA-398-3 Acetyl triethyl citrate Acetone Ethyl alcohol Diacetone alcohol Toluene

Quantity (wt %) 18.0 4.5 52.0 9.5 7.0 9.0 100.0


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Table VI. Solubility as Indicated by the Solution Viscosity of CAP-482-0.5 Solution Viscosity at 15% concn [cP (mPa s)]

Solvent Acetone Ethyl acetate Nitromethane Isopropyl acetate (95%) Ethylene glycol monobutyl ether acetate 2-Nitropropane Ethylene glycol monoethyl ether acetate Isobutyl isobutyrate

46 132 282 338 442 720 1350 Insoluble

Insoluble

0

10

20

30

40

50

Wt % Water on Volatile

Figure 6. The effect of water content on v i s c o s i t y and s o l u b i l i t y of CAP-504-0.2 solutions (15 wt % s o l i d s ) .



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paper through the propionate films, whereas, when control films of 0.5-s c e l l u l o s e nitrate, v i n y l chloride-acetate copolymer, and polyamide resins were tested in the same manner for only 2 h, the test material penetrated the films and stained the paper. When the thermosetting overprint lacquer No. 1 (Table VII) was applied to aluminum f o i l and cured at 275 °F (135 °C) for 10 s, heat resistance of the coating was in excess of 500 ° F (260 ° C ) . The thermoplastic overprint lacquer No. 2, when dissolved in an alcoholwater blend, may be applied directly to the wet lithographic ink in order to improve gloss and permit the use of less expensive inks, eliminate starch sprays ordinarily used to prevent setoff, improve scuff resistance, and eliminate hydrocarbon effluents by replacing heat-set inks with oxidizing inks. A very important property of polymers used in printing inks and overprints, especially for plastic substrates, is their ability to release solvents. Some polymers retain a small percent of solvent, thereby causing poor f i l m characteristics or an objectionable residual odor. The propionates have been found to have excellent solvent-release characteristics and are used in laminating inks for plastic because of this property. Figure 7 compares solvent release of CAP-504-0.2 with that of other polymers used in plastic coatings. Cellulose Acetate Butyrate (CAB) Because there i s a wide range of butyryl, a c e t y l , and hydroxyl l e v e l s a v a i l a b l e in the butyrate, there i s also a wide range of properties. Low-butyryl ester, CAB-171, is very similar to cellulose acetate in solubility, compatibility, and performance and i s used where toughness, d u r a b i l i t y , and grease resistance are required. An ester of medium butyryl l e v e l such as CAB-381 i s widely soluble and compatible with p l a s t i c i z e r s and resins. It serves in many coating applications including wood finishes, automotive topcoats, rubber and plastic coatings, cloth coatings, and glass coatings and i s also used in inks, hot melts, and adhesives. It i s frequently employed as a medium in which to disperse pigments on d i f f e r e n t i a l speed two-roll m i l l s . This product has found use in so many different types of coatings that i t is commonly referred to as the "general-purpose" butyrate. An ester with the highest practical butyryl l e v e l , CAB-551, was f i r s t prepared in 1967 for use as an additive in thermosetting acrylics and polyester enamels to improve dry-to-touch time, reduce cratering, provide a better pigment dispersion medium, improve intercoat adhesion, and provide reflow capabilities to thermosetting automotive enamels. It i s also useful in thermosetting powder coatings to provide a pigment dispersion medium, flow control, and anticaking properties to the powder. Although the commercial product, CAB-551-0.2, i s not f u l l y soluble in styrene, lower viscosity grades on the order of 0.01-s viscosity are soluble and provide flow control and pigment control to UV-cured coatings and inks. This very low viscosity ester performs a useful function in powder coatings where i t reduces cratering of the f i n i s h and improves the caking r e s i s t a n c e of the powder at e l e v a t e d temperatures. Because of i t s low T g , 80-100 ° C , and low melting point, 110-130 °C, i t contributes to good flow out during fusion of the powder.


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Table VII.

Overprints Based on CAP-504-0.2 Quantity (wt %) 1 2

Components CAP-504-0.2 Urea-formaldehyde resin (60%) p-Toluenesulfonic acid Ethyl alcohol (anhydrous) Ethyl acetate (99%)

12.5 20.8 0.6 59.5 6.6 100.0

72.0 8.0 100.0

Viscosity [cP (mPa s)] % nonvolatile

82 25.58

163 20

Note:

20.0

Formula 2 may be prepared in blends of 80 parts of alcohol to 20 parts of water.

220 Time, seconds

F i g u r e 7. Solvent r e l e a s e o f r e s i n s d i s s o l v e d i n an e t h y l alcohol (anhyd)/ethyl acetate blend (70/30) (equal f i l m t h i c k n e s s - f i l m s c a s t on aluminum f o i l ) .



43.



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CAB-500-1, with only 0.5% hydroxyl, was designed for hot melt strippable coatings where moisture resistance was one of the primary goals. CAB-553, with a 4.3% hydroxyl, i s alcohol soluble and e s p e c i a l l y useful in converting finishes requiring hydroxyl functionality. When used alone or modified with low l e v e l s of p l a s t i c i z e r and/or resin, CAB may perform well in certain applications such as wire coatings or airplane dopes, but f i l m shrinkage and poor adhesion are problems that prevent its use in many general-purpose applications. Modification with higher levels of resin w i l l not solve the shrinkage or adhesion problems and may compromise the desirable properties of the butyrate; for example, the films reflect the properties of the modifiers more than the butyrate. Thus, p r a c t i c a l l y a l l the butyrates are used in combination with other film formers to impart some desirable properties to the primary film former. Depending on the polymer, the butyrates may be added for one or more of the following reasons: improved flow c o n t r o l ; reduced cratering; quicker-dust free time; better viscosity control; increased intercoat adhesion; improved sprayability; better pigment control; increased cold crack resistance; reduced blocking; pigment dispersing medium; reduced solvent crazing; improved holdout; improved color stability; increased stain resistance; improved s l i p characteristics; and improved toughness. The l e v e l of CAB used with the polymer to obtain improved performance w i l l vary from less than 1% to 50%. In some cases, onetenth of 1% CAB w i l l eliminate craters and improve flow out, whereas 15-25% is required to give good solvent release from a thermoplastic acrylic coating and up to 50% butyrate on the resin may be required to obtain good performance with an oil-free polyester resin. As an i l l u s t r a t i o n , coating formulators blend up to 10% cellulose acetate butyrate ester with urethane elastomers to improve the s l i p characteristics of the coating, raise the blocking or printing temperature, give the coating a more pleasant feel, reduce the d i r t pickup, and reduce the c l i n g i n g tendency of the f i l m . While aiding in these properties, the use of CAB with urethane elastomers generally has some detrimental effect on other properties such as low-temperature f l e x i b i l i t y and/or drape and softness of the coating. The importance of these properties w i l l determine the amount of CAB to be used with the urethane elastomer. An example of use of CAB in a thermoplastic urethane c l o t h coating i s shown in Table VIII. In a thermoplastic acrylic automotive coating, a CAB is used as the pigment dispersion medium and at 15% concentration based on polymer weight provides better sprayability, increased cold crack resistance, better pigment c o n t r o l , better solvent release, and improved exterior durability. (See Table IX.) CAB is added to thermosetting acrylic enamels for improved flow control, quicker dust-free time, increased intercoat adhesion, and better pigment control (Table X). Future of Organic Acid Esters of Cellulose With the coating industry coming under federal and state regulations l i m i t i n g the type and amount of solvent that can be released into the air, with regulations being written that require new technology,


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Table VIII.

Thermoplastic Urethane Cloth Coating Quantity (wt %)


Components Urethane elastomer (25%)a CAB-381-0.5 Slip additive Pigmentb Toluene Ethyl alcohol % nonvolatile Viscosity [cP (mPa s)] Urethane elastomer/CAB ratio a

87.4 2.4 2.9 5.8 1.5 100.0 27.1 10,400 9/1

Such as QI-10 from K. J. Quinn Co.

^Concentration will vary with color.

Table IX.

Acrylic/CAB Automotive Lacquer

Components Acrylic resin (40% in toluene)3 CAB-381-2b Butyl benzyl phthalate Ethylene glycol monoethyl ether acetate Acetone Isopropyl alcohol Toluene Xylene Methyl isobutyl ketone % nonvolatile % CAB on binder Acrylic/CAB/plasticizer a

Quantity (wt %) 52.5 4.5 4.5 8.0 6.5 8.0 7.0 3.0 6.0 100.0 30.0 15.0 70/15/15

Such as Acryloid B-66 from Rohm and Haas.

b

0ther viscosity grades of this ester type may be used.



43.


Table X.


Thermosetting Acrylic/CAB Automotive Enamel

Components Thermosetting acrylic resin (50%)a Melamine resin (55%)° CAB-551-0.2C Aluminum pigment (65%) Ethylene glycol monoethyl ether acetate Toluene n-Butyl alcohol Spray solids a

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Quantity (wt %) 46.4 18.2 7.2 1.6 9.4 10.7 6.5 100.0 36

Such as Acryloid AT-56 from Rohm and Haas. 'Such as Cargill 3382. In this application, the CAB is dissolved at 25% solids in an 80/20 toluene/ethyl alcohol blend before addition to the enamel.



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APPLIED POLYMER SCIENCE

with raw material shortages, and with an energy crisis that may limit the amount of energy that can be used to dry or to cure coatings, i t is difficult to foresee the future of any product designed primarily for use in the coating industry. Yet, it may be these very regulations and material and energy shortages that make the future of the organic acid esters of cellulosic materials promising. A few years ago, these materials were used only where their toughness, grease resistance, and UV resistance were important. Today they are being used extensively as additives to other polymer systems to solve problems such as cratering, solvent retention, picture framing, pigment agglomerations, blocking, solvent crazing, poor intercoat adhesion, etc.—problems that are common within the coatings industry. The organic acid esters of cellulose are being used where exempt solvents are required. They can be readily formulated to meet environmental requirements. When formulated as lacquers, they can be rapidly air-dried. As converting finishes, they can be cured at rather low temperatures or even at ambient temperatures. Thus, l i t t l e or no energy is required to form the film. When used as an additive in a converting system, the cellulose ester does not have to be a high-performance film former but can be very low viscosity and s t i l l perform the function expected of i t . Upon application, the presence of CAB in the coating gives i t certain lacquerlike properties including rapid dry-to-touch time. During conversion, the cellulose ester performs as a polyol and converts with the balance of the reactive film former. We expect the important new development in the organic acid esters of cellulose to be a process improvement and further modification of derivatives now known rather than an introduction of radically new types of cellulose esters. We expect further legislation and emerging technology in the coatings field to make the organic acid esters of cellulose more attractive to the coating chemist. Literature Cited 1. Schutzenberger, P. C. R. Hebd. Seances Acad. Sci. 1865, 61, 485. 2 Franchimont, A. C. R. Hebd. Seances Acad. Sci. 1879, 89, 711. 3. Miles, G. W. U.S. Patent 838 350, Dec. 11, 1906; Chem. Abstr. 1907, 1, 653. 4. Cross, C. F.; Bevans, E. J. "Research on Cellulose"; Longmans, Green and Co.: New York, 1906; Ser. II, p. 90. 5. Malm, C. J.; Hiatt, G. H. "Cellulose and Cellulose Derivatives," 2nd ed.; Interscience: New York, 1955; Vol. 5, Part 2, p. 798. 6. "Cellulose and Cellulose Derivatives," 2nd ed.; Ott, E.; Spurlin, H. M.; Graff l i n , Mildred W., Eds.; Interscience: New York, 1955; Vol. 5, Parts 1-3. 7. Kline, G. M.; Malberg, C. G. Ind. Eng. Chem. 1938, 30, 542. 8. Reinhart, F. W.; Kline, G. M. Film Forming Plast. 1939, 31, 1522. 9. Cyrot, J. Bull. Inst. Textile Fr. 1959, 85, 29-56. 10. Salo, M. Off. Dig. 1959, 31 (Sept).


Cellulose Acetate and Related Esters - ACS Symposium Series (ACS

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