Applied Polymer Science - ACS Publications - American Chemical

28 Solvents ROY W. TESS

Downloaded by COLUMBIA UNIV on September 26, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch028

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Origin and Growth of the Solvents Industry Methods of Manufacture of Solvents Solvent Properties and Their Effect on Properties of Solutions and Coatings Effect of Choice of Solvent on Ultimate Mechanical Properties of Deposited Films Effect of Solvents on Properties of Latex and Electrodeposition Paints Surface Tension and Coating Properties V i s c o s i t y of Resin Solutions as Related to Solvent Properties Evaporation Rate of Solvents Evaporation of Solvent-Water Blends Flash Point of Solvents A i r Quality Regulations and Solvents Formulation of Solvent Blends Solvents for High-Solids Coatings

For purposes of this discussion solvents are considered to be liquid organic compounds that are used to dissolve and reduce the viscosity of various resins and other materials. Emphasis will be on solvents for use in surface coatings. In noncoating applications solvents serve as extraction agents for grains, oilseeds, stumps, petroleum products, wood products, and minerals; ingredients and manufacturing aids for toiletries, cosmetics, and drugs; cleaning agents; aids to textile processing and dyeing; hydraulic and heat-transfer fluids; reaction solvents and components i n various other end uses and processes. The rise of the synthetic organic chemical industry has required an ever i n c r e a s i n g share of s o l v e n t s for use as chemical intermediates where s o l v e n c y and other t r a d i t i o n a l solvent properties are immaterial. Examples include use of acetone for 0097-6156/ 85/ 0285-0661 $ 11.00/ 0 © 1985 American Chemical Society

In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


662

APPLIED POLYMER SCIENCE

manufacture of methyl methacrylate, butanol for butyl acrylate and butylated urea and melamine r e s i n s , and xylenes for p h t h a l i c anhydride and t e r e p h t h a l i c acid. Therefore, increased volumes of solvent production do not mainly reflect increased solvent usage in coatings. In the 1920s the advent of the synthetic organic chemical industry led to the displacement of many of the older methods for chemical production that made use of fermentation and destructive distillation of natural materials. Chemical technology advanced rapidly in the 1920s. To meet the need for a forum for discussion of coatings, a new section on paint was started in the American Chemical Society. As an i l l u s t r a t i o n of a topic of the time, D. B. Keyes presented a paper e n t i t l e d "Solvents and Automobile Lacquers" before the Section of Paint and Varnish Chemistry at Baltimore i n 1925. The wealth of data and soundness of formulating p r i n c i p l e s i n t h i s paper may come as a surprise to modern lacquer formulators. In t h i s chapter, the wide scope of the t o p i c of s o l v e n t s precludes d e t a i l e d discussion. Several books (5, % 1_3, 18-23, 30) l i s t e d i n the L i t e r a t u r e Cited section w i l l provide much information for the s c i e n t i s t and technologist. Origin and Growth of the Solvents Industry The production of turpentine by d i s t i l l a t i o n of gum from pine trees was the s t a r t of the solvents industry i n the United States e a r l y i n the e i g h t e e n t h c e n t u r y . By 1900 the a n n u a l p r o d u c t i o n was 30,000,000 g a l and averaged t h i s amount through many years. Turpentine was the solvent of choice for the paint industry, and 80% of the volume i n 1922 was used i n the p a i n t and v a r n i s h i n d u s t r y (I). As the second major source of s o l v e n t s i n the e a r l y 1900s, the fermentation of molasses and g r a i n produced e t h y l a l c o h o l . T h i s product had been made i n t h i s manner for thousands of years, but i t s use as a s o l v e n t i n the U n i t e d S t a t e s was d e l a y e d u n t i l 1906 when denatured a l c o h o l became tax-free i f used as a s o l v e n t (2). In the p e r i o d of World War I , Weizman found t h a t by use of a s p e c i a l s t r a i n of b a c i l l u s the fermentation of corn gave a mixture of acetone (3 parts), η - b u t y l a l c o h o l (6 parts), and e t h y l a l c o h o l (1 p a r t ) . D e s t r u c t i v e d i s t i l l a t i o n of wood c o n s t i t u t e d the t h i r d major source of solvents i n t h i s period; the products consisted of methyl a l c o h o l , acetone, and acetic acid. In 1925, hydrocarbon solvents derived from c o a l tar consisted of 5.1 m i l l i o n g a l of t o l u e n e and 4.0 m i l l i o n g a l of s o l v e n t naphtha i n c l u d i n g x y l e n e (_3). These volumes were much l e s s than those of turpentine at the time. The demand for s o l v e n t s was a b i g f a c t o r i n s t i m u l a t i n g the production of synthetic chemicals i n the 1920s. In 1925, 85% of a l l a l i p h a t i c c h e m i c a l s were obtained by f e r m e n t a t i o n , 13% by the d e s t r u c t i v e d i s t i l l a t i o n of wood, 2% from c o a l , and 0.1% from n a t u r a l gas and petroleum (4). The t o t a l p r o d u c t i o n of a l i p h a t i c c h e m i c a l s i n 1925 was 150 m i l l i o n pounds and reached 9 b i l l i o n pounds i n 1945. Volume of aromatics was about 300 m i l l i o n pounds i n 1925. The o n l y a l i p h a t i c c h e m i c a l s made by e n t i r e l y s y n t h e t i c commercial r o u t e s by 1925 were i s o p r o p y l a l c o h o l and t h r e e


663

28. TESS Solvents

c h l o r i n a t e d hydrocarbons a c c o r d i n g to McClure and Bateman (4). Volumes of some major lacquer solvents i n 1925 were e t h y l acetate, 26.7 m i l l i o n pounds; b u t y l a c e t a t e , 16.5 m i l l i o n pounds; and amyl a c e t a t e , 1.7 m i l l i o n pounds (3). By 1945, 50% of a l i p h a t i c c h e m i c a l s were d e r i v e d from n a t u r a l gas and p e t r o l e u m , 21% from c o a l , and 28% from fermentation processes. The tremendous increase i n solvent production since 1925 can be i l l u s t r a t e d by the f o l l o w i n g f i g u r e s i s s u e d by the U.S. T a r i f f Commission/U.S. International Trade Commission where volumes are i n m i l l i o n pounds:


1925 Acetone Methyl e t h y l ketone Isopropyl a l c o h o l Methyl a l c o h o l

13 0 30 5

1973

1981

1982

1989 541 1835 7064

2144 611 1669 8577

1694 468 1380 7554

These f i g u r e s are m i s l e a d i n g as to s o l v e n t usage because an i n c r e a s i n g l y g r e a t e r share of these products has been used to synthesize other chemicals. Moreover, even when used as s o l v e n t s , many of the c h e m i c a l s a r e used i n noncoating a p p l i c a t i o n s . I n c i d e n t a l l y , the sharp drop i n p r o d u c t i o n i n 1982 p r o b a b l y was mainly due to the economic recession at that time. I t has been estimated by D o o l i t t l e and Holden (2) t h a t , excluding petroleum hydrocarbons, the protective coatings industry i n 1933 consumed about 40% of chemicals c l a s s i f i e d as s o l v e n t s . By 1949 o n l y 25% of these s o l v e n t s were used i n c o a t i n g s (.5), and by 1972 i t was estimated by Stewart (6) t h a t o n l y 14% of oxygenated s o l v e n t s (which i n c l u d e ketones, e s t e r s , a l c o h o l s , and g l y c o l e t h e r s ) were consumed by t h e c o a t i n g s i n d u s t r y . The t o t a l consumption of these s o l v e n t s i n c o a t i n g s was estimated at 1.9 billion pounds for 1972. Based on data of SRI I n t e r n a t i o n a l , the N a t i o n a l P a i n t and C o a t i n g s A s s o c i a t i o n (_7) has r e p o r t e d the q u a n t i t i e s of s o l v e n t s used i n m a n u f a c t u r e of c o a t i n g s , i n t h i n n i n g , i n c l e a n - u p o p e r a t i o n s , and i n d i s s o l v i n g r e s i n s s u p p l i e d to the c o a t i n g s i n d u s t r y ( T a b l e I ) . In 1981 the f o l l o w i n g q u a n t i t i e s of s o l v e n t s were used: 955 m i l l i o n pounds of a l i p h a t i c hydrocarbons, 1215 of aromatic hydrocarbons, 1782 of oxygenated s o l v e n t s , 21 of c h l o r i nated s o l v e n t s , and 34 of miscellaneous s o l v e n t s . The decrease i n solvent usage from 1973 to 1981 was mainly the r e s u l t of l i m i t a t i o n s on s o l v e n t s because of a i r p o l l u t i o n regulations. The t o t a l U.S. production of s o l v e n t s i n 1973, 1981, and 1982 as r e p o r t e d by the U.S. T a r i f f Commission/U.S. I n t e r n a t i o n a l Trade Commission i s shown i n T a b l e I I . The sharp decrease i n volume i n 1982 probably was mainly due to the economic recession. Methods of Manufacture of Solvents The methods of manufacture of s o l v e n t s are much too v a r i e d and extensive for discussion at any length i n t h i s paper. E a r l y methods are discussed by F. C. Zeisberg i n the s i l v e r anniversary volume of the American I n s t i t u t e of C h e m i c a l E n g i n e e r s (8). Another


664



Table I .

Estimated Consumption of Solvents i n Paints and Coatings ( m i l l i o n s of l b ) Consumption

Solvent

1973

1981

A l i p h a t i c hydrocarbons Toluene Xylenes Other aromatics Butyl alcohols Isopropyl alcohol Ethyl alcohol Other alcohols Acetone Methyl ethyl ketone Methyl i s o b u t y l ketone Ethyl acetate Butyl acetates Propyl acetates Other ketones and esters Ethylene g l y c o l Propylene g l y c o l Glycol ethers and ether esters Chlorinated solvents Miscellaneous

1455 690 575 210 110 120 200 60 200 380 122 120 115 60 109 57 30 230 15 40

955 580 465 170 122 110 190 58 175 335 100 103 108 50 100 61 35 235 21 34

4898

4007

Note:

Data from "NPCA Data Bank Program 1982"; SRI International, September 1982.


TESS

28.

Table I I .


665

Solvents

U.S. Production of Major Oxygenated Solvents ( m i l l i o n s of l b ) Production 1982 1981

Solvent

1973

Alcohols Methanol Ethanol, synthetic Isopropyl alcohol η-Propyl alcohol η-Butyl alcohol Isobutyl alcohol 2-Ethylhexyl alcohol

7064 1961 1835 93 519 133 402

8577 1317 1669 154 809 142 389

7554 1023 1380 127 730 156 325

Ketones Acetone Methyl ethyl ketone Methyl i s o b u t y l ketone Diacetone a l c o h o l

1989 541 155 51

2144 611 218 24

1694 468 131 20

a

Esters E t h y l acetate Isopropyl acetate η-Propyl acetate η-Butyl acetate Isobutyl acetate

221 45a 35 81 37a

277 NRb 55 124 67

235 NR 57 121 68

Glycol ethers Methoxyethanol Methoxyethoxyethanol Ethoxyethanol Ethoxyethoxyethanol Butoxyethanol Butoxyethoxyethanol

86 14 190 28 138 25

93 29 206 32 227 50

89 28 178 28 217 48

A l l esters of polyhydric alcohols including (mainly) g l y c o l ether esters As a group

220

NR

NR

3278 502 24

4142 473 NR

4309 400 NR

Glycols Ethylene g l y c o l Propylene g l y c o l Hexylene g l y c o l Note:

Data from U . S . T a r i f f Commission/U.S. International Commission Volumes represent sales instead of production. t>NR means no report.

a


Trade


666


discussion of methods of synthesis i s i n the o r i g i n a l and subsequent e d i t i o n s of the book " S o l v e n t s " by Durrans (9.). The "Kirk-Othmer E n c y c l o p e d i a " (10) c o v e r s much i n f o r m a t i o n on the manufacture of s o l v e n t s of various types. One of the most f a s c i n a t i n g s t o r i e s of the c o a t i n g s i n d u s t r y i n v o l v e s the p r o d u c t i o n of acetone, b u t a n o l , and e t h a n o l by the Weizmann process (11, 12). Because the main o b j e c t i v e was to produce acetone for e x p l o s i v e s , the butanol p i l e d up u n t i l i t was found t h a t b u t y l a c e t a t e was an e x c e l l e n t s o l v e n t f o r the new n i t r o c e l l u l o s e lacquers. Commercial S o l v e n t s C o r p o r a t i o n (of Maryland) was formed i n 1919 to take over the f e r m e n t a t i o n p l a n t s o p e r a t i n g at T e r r e Haute to make b u t a n o l and d e r i v a t i v e s . The a v a i l a b i l i t y of b u t y l a l c o h o l and the acetate was of major aid i n the success of n i t r o c e l l u l o s e lacquers i n new automobile paints that permitted a reduction i n the time required for painting automobiles from 23 days i n 1920 to a matter of about 12 h i n 1940 (13). The f i r s t t o t a l l y s y n t h e t i c route to a s o l v e n t i n the U n i t e d S t a t e s was the s y n t h e s i s of i s o p r o p y l a l c o h o l from p r o p y l e n e by M e l c o Chemical C o r p o r a t i o n i n 1917. In 1928 Union Carbide made acetone from i s o p r o p y l a l c o h o l ; the s y n t h e s i s of acetone i n the cumene-to-phenol process came much l a t e r and now i s the source of about 85% of acetone p r o d u c t i o n . In 1927 Du Pont began the s y n t h e s i s of methanol. S y n t h e t i c e t h y l a l c o h o l was made from e t h y l e n e by Union Carbide i n 1929. S p e c i a l i z e d books on e t h y l a l c o h o l (14, 15) and isopropyl a l c o h o l (16) give many d e t a i l s on the manufacture, properties, and uses of these major products. The oxygenated s o l v e n t i n greatest use i n coatings today (about 335 m i l l i o n l b / y r ) i s methyl e t h y l ketone. In 1932 S h e l l Chemical became the f i r s t commercial producer. I t was made from n-butylene by hydration and subsequent dehydrogenation. M e t h y l e t h y l ketone a l s o i s c u r r e n t l y coproduced with a c e t i c acid by the oxidation of n butane by other manufacturers. η - B u t y l a l c o h o l was f i r s t produced c o m m e r c i a l l y by the fermentation process and l a t e r was made from acetaldehyde v i a the a l d o l route to crotonaldehyde and subsequent reduction. In World War I I the Oxo process was developed i n Germany and adapted to U.S. manufacture of n-butanol and isobutanol; propylene plus synthesis gas (CO and H2) g i v e s a m i x t u r e o f n - b u t y r a l d e h y d e and i s o b u t y r a l d e h y d e (3:1 r a t i o ) which upon h y d r o g é n a t i o n y i e l d s the corresponding a l c o h o l s . 2-Ethylhexanol i s made v i a an a l d o l route from n - b u t y r a l d e h y d e . The S h e l l h y d r o f o r m y l a t i o n process f o r n butanol i s b a s i c a l l y an oxo process run under conditions that lead to much higher r a t i o s of n-butanol to isobutanol. The hydrocarbon s o l v e n t s are s t r a i g h t run d i s t i l l a t e s as w e l l as products made by various refinery processes i n c l u d i n g d i s t i l l a t i o n , e x t r a c t i o n , a l k y l a t i o n , hydroforming, hydrogénation, hydrocracking, and hydrotreating. Hydrocarbons are c l a s s i f i e d and characterized by b o i l i n g range and c o m p o s i t i o n , v i z , v a r i o u s percentages of a l i p h a t i c , naphthenic, and aromatic constituents. Solvents that are predominantly a l i p h a t i c may be c l a s s i f i e d as shown i n Table I I I . Petroleum processes a l s o y i e l d aromatic hydrocarbons such as toluene, xylene, and aromatic concentrates or fractions with b o i l i n g ranges of 300-350 °F and 350-400 °F. U n t i l World War I I the major source of aromatic hydrocarbons was c o a l tar (17), but by 1975 only about 5% of aromatic hydrocarbons were derived from that source.


28. TESS

Solvents

667


Solvent Properties and Their Effect on Properties of Solutions and Coatings The power to d i s s o l v e r e s i n s i s the foremost requirement of a s o l v e n t except i n cases i n v o l v i n g dispersions i n nonaqueous s o l v e n t s (NADs) or d i s p e r s i o n s i n w a t e r ( l a t i c e s , e m u l s i o n s , and d i s p e r s i o n s ) . T h e o r i e s of s o l v e n c y and s o l u t i o n are c o v e r e d by Rider i n the preceding chapter. The c l a s s i c books by Hildebrand and S c o t t (18) and H i l d e b r a n d , P r a u s n i t z , and S c o t t (19) d i s c u s s s o l u b i l i t y and s o l u t i o n s i n considerable depth. The monumental book by D o o l i t t l e (5) c o v e r s both t h e o r e t i c a l and a p p l i e d a s p e c t s of s o l v e n t s . S e v e r a l c h a p t e r s i n the M a t t i e l l o series published i n 1941-46 deal with s o l v e n t s ; the chapter on lacquer s o l v e n t s by Bogin (20) contains many e a r l y references on the development of s o l v e n t technology. Reynolds (21) c o v e r s the p h y s i c a l c h e m i s t r y of petroleum s o l v e n t s . P h y s i c a l constants and properties of s o l v e n t s are compiled by M e l l a n (31) as w e l l as by Durrans (9). Patton (22) c o n s i d e r s p r o p e r t i e s of s o l v e n t s as r e l a t e d to p a i n t f l o w . Some developments i n a p p l i e d and t h e o r e t i c a l a s p e c t s of s o l v e n t s are reported by Tess (23). Commercial brochures published by commercial s u p p l i e r s of s o l v e n t s c o n t a i n much u s e f u l i n f o r m a t i o n ; s o l v e n t charts l i s t i n g major p h y s i c a l properties of commercially a v a i l a b l e s o l v e n t s are quite useful (24). S o l v e n t s f o r c o a t i n g s are sometimes regarded as a source of unwanted expense and s i m p l y as a means to permit a p p l i c a t i o n of a p a i n t m a t e r i a l to a s u r f a c e . The a c t u a l , but o f t e n u n r e a l i z e d , facts show that s o l v e n t s go far beyond t h i s simple function. Some d e s i r a b l e properties of solvent-based paints include the f o l l o w i n g : 1. 2. 3. 4. 5. 6.

7. 8. 9. 10. 11. 12.

Low surface tension causes e x c e l l e n t wetting of substrate which i s required for good adhesion of the coating to the surface. Good adhesion promotes good c o r r o s i o n r e s i s t a n c e , impact resistance, and chip resistance. Low v i s c o s i t y permits p e n e t r a t i o n of the m i c r o s c o p i c rough areas of the substrate and aids adhesion. Easy touch-up and repair of defects i n dry f i l m . Low v i s c o s i t y and c o n t r o l l e d r h e o l o g y and f l o w g i v e smooth coatings with good g l o s s . E v a p o r a t i o n r a t e of s o l v e n t and d r y i n g of c o a t i n g can be r e g u l a t e d e a s i l y by c h o i c e of s o l v e n t s i n s o l v e n t b l e n d s . Proper e v a p o r a t i o n r a t e h e l p s to g i v e smooth f i l m s and to minimize sagging, running, and popping of paint. Evaporation of t y p i c a l s o l v e n t blends i s not g r e a t l y affected by changes i n r e l a t i v e humidity. A p p l i c a t i o n of paints by a i r spray, e l e c t r o s t a t i c spray, r o l l e r c o a t , d i p , and other means can be r e a d i l y performed and a l s o can be done i n e x i s t i n g equipment. Good acceptance of aluminum pigments f o r automotive m e t a l l i c paints. In a r c h i t e c t u r a l paints, continuous f i l m s are formed even i n c o l d weather; w a t e r - s t a i n s are s e a l e d e f f e c t i v e l y w i t h no bleed-through; and fresh f i l m s are not removed by r a i n . Liquid paints are not s p o i l e d by bacteria and molds. Solvent paints can produce wide range of thickness of coating.


668 13.


14.


M o l e c u l e s of r e s i n are i n an extended s t a t e to y i e l d maximum f l e x i b i l i t y and maximum density of f i l m s along with good r e s i s tance to permeation by water and gases. G e n e r a l h i g h q u a l i t y of f i l m s minimizes need f o r frequent repainting.

On the other hand, s o l v e n t - b a s e d p a i n t s have the disadvantage of flammability. A l s o , they r e q u i r e c a r e f u l h a n d l i n g because of p o s s i b l e adverse h e a l t h e f f e c t s i n some cases, and may r e q u i r e c o n t r o l of solvent vapors because of a i r p o l l u t i o n regulations. Perhaps the two most important p r o p e r t i e s of s o l v e n t s are e v a p o r a t i o n r a t e and s o l v e n t power. S o l v e n t power i s r e l a t e d to various fundamental s o l u t i o n parameters as discussed i n the chapter on theories of solvency and s o l u t i o n . However, s o l v e n t power a l s o influences v i s c o s i t y and the o r i e n t a t i o n of molecules which i n turn affects many other properties. Effect of Choice of Solvent on Ultimate Mechanical Properties of Deposited F i l m s . Kauppi and Bass (25) c a s t e t h y l c e l l u l o s e f i l m s from combinations of toluene and e t h y l a l c o h o l i n various propor tions. As shown by F i g u r e 1, the best t e n s i l e s t r e n g t h and e l o n g a t i o n occurred when about 70 to 90% t o l u e n e was used i n the s o l v e n t blend. These mechanical properties degraded r a p i d l y with i n c r e a s i n g p r o p o r t i o n s of e t h a n o l . Minimum v i s c o s i t y of the s o l u t i o n s was attained when the s o l v e n t blend contained about 70% toluene. I t seems r e a s o n a b l e to a s c r i b e the s u p e r i o r f i l m properties to the shape and o r i e n t a t i o n of the polymer molecules i n the s o l u t i o n and i n the f i n a l f i l m . I t i s suggested t h a t the superior f i l m s consist of molecules i n r e l a t i v e l y extended random configuration with a high degree of chain entanglement, whereas the weaker f i l m s consist of molecules i n r e l a t i v e l y more c o i l e d states with fewer intermolecular enganglements. By depositing e t h y l c e l l u l o s e f i l m s from e t h a n o l and t o l u e n e , D o o l i t t l e (26) confirmed the o b s e r v a t i o n s of Kauppi and Bass and a l s o measured the d e n s i t y and p e r m e a b i l i t y of the f i l m s to water vapor, oxygen, and nitrogen as shown i n Table IV. The f i l m d e p o s i t e d from the t o l u e n e s o l u t i o n was lower i n density and higher i n gas transmission than the f i l m from ethanol. As i n d i c a t e d by i t s lower d e n s i t y the f i l m from t o l u e n e can be considered to have more free volume than the f i l m from ethanol. The more r a p i d d i f f u s i o n of gases through the l e s s dense f i l m i s consistent with t h i s viewpoint. These differences i n properties can be a t t r i b u t e d to d i f f e r e n c e s i n o r i e n t a t i o n and shape of the molecules as they are deposited from the s o l u t i o n s . I t i s evident from these r e s u l t s , as w e l l as from r e s u l t s c i t e d l a t e r i n t h i s discussion, that the choice of solvent i s of c r i t i c a l importance i n a c h i e v i n g the best f i l m p r o p e r t i e s from a r e s i n system. The cheapest s o l v e n t combination may not and probably w i l l not g i v e best f i l m p r o p e r t i e s . I t i s a l s o e v i d e n t t h a t i n the process of e v a l u a t i n g the m e r i t s of r e s i n s , c o n c l u s i o n s c o u l d be erroneous unless the optimum s o l v e n t blend i s chosen for each r e s i n . Extrapolating these considerations further, i t can be expected that the same r e s i n c o u l d y i e l d e n t i r e l y d i f f e r e n t p r o p e r t i e s i f i t i s applied to a surface i n different forms, for example, as a s o l u t i o n , a d i s p e r s i o n i n s o l v e n t s , a d i s p e r s i o n i n water, or as a powder.


28.

TESS

669

Solvents

Table I I I .

General C l a s s i f i c a t i o n of A l i p h a t i c Hydrocarbon Solvents B o i l i n g Range


Type Rubber and extraction Lacquer diluents VM and Ρ naphthas Mineral s p i r i t s Heavy solvents

140-200 200-250 240-300 300-400 400-600

solvents

°F °F °F °F

150 43 4-»

600 c e Φ υ u . 550 CO

^

500

•H 60 CO

g

450

H

10 Figure

1.

Table IV.

20

30 40 50 60 70 80 % Alcohol by Volume

90 100

E f f e c t of c o m p o s i t i o n of t o l u e n e - e t h a n o l e t h y l c e l l u l o s e properties.

s o l v e n t on

Properties of E t h y l c e l l u l o s e Films

Property 3

Density, g/cm Permeability Water vapor, mg/cm^/h Oxygen, mL/100 i n . / d a y Nitrogen, mL/100 i n . / d a y 2

2

Film from Ethanol

Film from Toluene

1.1454

1.1428

4.2 1101 402

5.06 1322 436


670



Q u i t e o b v i o u s l y , the s o l v e n t has a f u n c t i o n f a r beyond t h a t of aiding the d e l i v e r y of a r e s i n or paint to a surface. Effect of Solvents on Properties of Latex and Electrodeposition Paints. Solvents are u s u a l l y used i n l a t e x paints to enhance f i l m i n t e g r i t y , strength, and f l e x i b i l i t y ; to obtain maximum smoothness of f i l m ; to a i d c o a l e s c e n c e of l a t e x p a r t i c l e s e s p e c i a l l y at low temperatures; to a i d scrub r e s i s t a n c e ; and g e n e r a l l y to enhance p r o p e r t i e s . E x c e s s i v e q u a n t i t i e s of s o l v e n t s can cause pigment f l o c c u l a t i o n upon storage and ruin freeze-thaw s t a b i l i t y of paint. Ester solvents have a tendency to hydrolyze; the generated a c i d i t y r e s u l t s i n pH d r i f t , change i n v i s c o s i t y of paint, and h y d r o l y s i s of p o l y ( v i n y l acetate) polymers whereupon a c e t i c a c i d and g e l a t i n o u s p o l y ( v i n y l a l c o h o l ) may be formed. Various types of s o l v e n t s have been found to be u s e f u l . G l y c o l ethers such as the monobutyl ether of e t h y l e n e g l y c o l , or p r e f e r a b l y of d i e t h y l e n e g l y c o l , are e x c e l l e n t coalescing agents for l a t e x paints. Normally, about 2% weight of solvent based on t o t a l weight of paint i s very e f f e c t i v e . Other coalescents include hexylene g l y c o l and stable esters. The c o a l e s c e n t i s balanced w i t h an e q u a l weight of e t h y l e n e g l y c o l (except when u s i n g hexylene g l y c o l ) to prevent or a m e l o r i a t e degradation of freeze-thaw resistance often caused by coalescents; some c o a l e s c e n t s degrade freeze-thaw s t a b i l i t y to such a g r e a t extent that addition of g l y c o l cannot overcome the problem. Hoy (27) has examined some factors that influence the e f f i c i e n c y of c o a l e s c i n g a i d s . An important f a c t o r i s the d i s t r i b u t i o n of s o l v e n t between aqueous and polymer phases at the c r i t i c a l time during f i l m formation. Water-soluble s o l v e n t s may be l o s t from the coating through wicking action into the substrate. The evaporation of water and solvent from l a t e x f i l m s has been the s u b j e c t of s e v e r a l s t u d i e s . S u l l i v a n (28) i n v e s t i g a t e d the e v a p o r a t i o n of water, e t h y l e n e g l y c o l , and c o a l e s c e n t s o l v e n t s (mostly b u t y l ethers of ethylene g l y c o l and diethylene g l y c o l ) from c a s t f i l m s , at 25 °C and 50% r e l a t i v e h u m i d i t y , of c l e a r and pigmented a c r y l i c l a t e x polymers. P r i o r to f i l m f o r m a t i o n , e v a p o r a t i o n of v o l a t i l e s i s c o n t r o l l e d by s u r f a c e r e s i s t a n c e phenomena. After f i l m formation, evaporation of v o l a t i l e s may or may not be d i f f u s i o n - c o n t r o l l e d , depending upon the s o l v e n t i n v o l v e d and the interactions among the v o l a t i l e materials. The presence of a c o n t i n u o u s h y d r o p h i l i c network throughout the l a t e x f i l m f a c i l i t a t e s the evaporation of s o l v e n t s , e s p e c i a l l y the more polar s o l v e n t s t h a t tend to evaporate more r a p i d l y than the l e s s p o l a r s o l v e n t s t h a t p a r t i t i o n more s t r o n g l y to the polymer phase. Evaporation rates of solvent from pigmented paint f i l m s were minimal for paint at about the c r i t i c a l pigment volume concentration (cpvc); the pigment p a r t i c l e s below the cpvc apparently acted as b a r r i e r s to s o l v e n t passage through the f i l m . The important c o n c l u s i o n was reached t h a t the h i g h - b o i l i n g s o l v e n t s used i n l a t e x p a i n t s evaporated q u i t e r a p i d l y and t h a t remarkably l i t t l e of these s o l v e n t s remained i n the f i l m s for appreciable times. An e a r l y i n v e s t i g a t i o n by Tess and Schmitz (29) on s t y r e n e butadiene paints demonstrated that use of hexylene g l y c o l improved t e n s i l e s t r e n g t h , e l o n g a t i o n , l e v e l i n g , and scrub r e s i s t a n c e of l a t e x p a i n t s . E l e c t r o n micrographs of the f i l m s , obtained by a r e p l i c a t e c h n i q u e , showed t h a t use of the c o a l e s c e n t r e s u l t e d i n



28.

TESS Solvents

671

much smoother f i l m s ( F i g u r e s 2a, 2b, and 2c). These micrographs were made on alkyd-modified s t y r e n e - b u t a d i e n e l a t e x p a i n t s at 45% pigment volume concentration. I t has been shown by May (30) t h a t s o l v e n t s have s e v e r a l b e n e f i c i a l e f f e c t s i n e l e c t r o d e p o s i t i o n c o a t i n g s where they may comprise about 2-3% of the p a i n t bath. One purpose i n u s i n g s o l v e n t s i s to d i s s o l v e the r e s i n to provide ease of handling during the preparation of the aqueous s o l u t i o n s . A second purpose i s the attainment of smoother f i l m s . A t h i r d reason i s t h a t the s o l v e n t a l s o can confer better water s o l u b i l i t y c h a r a c t e r i s t i c s on the r e s i n and h e l p m a i n t a i n bath s t a b i l i t y . The best s o l v e n t f o r any electrodeposition paint depends on the r e s i n used. In general the raonobutyl ethers of ethylene g l y c o l and diethylene g l y c o l were the most v e r s a t i l e of nine s o l v e n t s t e s t e d , but i n some cases good r e s u l t s were a l s o obtained by use of 2 - e t h o x y e t h a n o l , h e x y l e n e g l y c o l , diacetone a l c o h o l , and possibly even s e c - b u t y l a l c o h o l or isopropyl a l c o h o l . Surface Tension and C o a t i n g P r o p e r t i e s . Surface t e n s i o n has a s t r o n g i n f l u e n c e on s e v e r a l important c o a t i n g p r o p e r t i e s such as g l o s s , surface t e x t u r e , f l o a t i n g and f l o o d i n g of pigments, and adhesion of f i l m s . The e f f e c t s of s u r f a c e t e n s i o n are u s u a l l y a consequence of the basic fact that a l i q u i d of lower surface tension w i l l wet and spread over another material of higher surface tension or surface free energy. The o v e r a l l process can occur i f the net change i n free energy of the system i s n e g a t i v e . Water has a h i g h s u r f a c e t e n s i o n of 72.7 dyn/cm, but the a d d i t i o n of s u r f a c t a n t s as may be present i n latex paints w i l l reduce t h i s value considerably. Most oxygenated organic s o l v e n t s have surface tensions of about 2430 dyn/cm, aromatic hydrocarbons of about 28 dyn/cm, and a l i p h a t i c hydrocarbons of about 18-24 dyn/cm. Because r e s i n s have h i g h e r s u r f a c e t e n s i o n s than s o l v e n t s , f o r example, 42.6 dyn/cm f o r p o l y s t y r e n e , 44.6 dyn/cm f o r an epoxy r e s i n , and 36.5 dyn/cm f o r p o l y ( v i n y l acetate), i t i s evident that the d i s s o l u t i o n of resins i n s o l v e n t w i l l reduce surface t e n s i o n and a i d the w e t t i n g of a s u b s t r a t e whether i t be a m e t a l , p l a s t i c , or other s u r f a c e . Increase i n temperature reduces the surface tension of a l i q u i d , and advantage of t h i s s i t u a t i o n can be made i n some instances. Zisman (32) has developed a method of measurement and the concept of the c r i t i c a l s u r f a c e t e n s i o n of a s u b s t r a t e as being e q u a l to the (maximum value of) surface tension of a series of l i q u i d s that w i l l spontaneously spread when applied to the surface. When the solvent from paint f i l m s evaporates, there may develop a c i r c u l a t o r y motion within c e l l s (Benard c e l l s ) i n the body of the l i q u i d f i l m . This vortex action was described by B a r t e l l and Van Loo i n 1925 and has been e x t e n s i v e l y reviewed and i n t e r p r e t e d by Hansen and P i e r c e (33, 34). Each c e l l has an hexagonal shape, and the o v e r a l l pattern of the dried f i l m consists of raised ridges at the outer edges of c l o s e l y packed hexagons. In each c e l l l i q u i d moves upward at the center of the c e l l , spreads outward at the surface to the edges of the hexagon, and then moves downward to the depths of the f i l m and completes the c i r c u l a r motion. As the l i q u i d moves across the surface i t c o o l s and becomes more concentrated as s o l v e n t evaporates. As a r e s u l t of both of these f a c t o r s the surface tension increases from center to edge of each c e l l . Liquid




F i g u r e 2. E l e c t r o n m i c r o g r a p h s o f l a t e x p a i n t c o n t a i n i n g (a) no h e x y l e n e g l y c o l ; (b) 20% h e x y l e n e g l y c o l on polymer c o n t e n t , e q u i v a l e n t t o 0.31 l b HG p e r g a l . ; and ( c ) 40% h e x y l e n e g l y c o l , e q u i v a l e n t t o 0.62 l b HG p e r g a l .



28. TESS Solvents

673

b u i l d s up i n t o a r i d g e at the perimeter of each c e l l and thereby forms a hexagon pattern i n the dried f i l m . The vortex action i n Benard c e l l s can cause the defects known as f l o o d i n g and f l o a t i n g because pigments of d i f f e r e n t s i z e s and weights w i l l move at different v e l o c i t i e s , w i l l separate, and show nonuniformity of c o l o r s i n f i l m s . The pigments a l s o can be concentrated at c e r t a i n l o c a l spots and thus leave pure binder at other spots that are v u l n e r a b l e to f i l m f a i l u r e upon exposure. Other surface i r r e g u l a r i t i e s of f i l m s i n c l u d i n g f r o s t i n g , w r i n k l i n g , and cratering are r e l a t e d to surface tension effects i n f i l m s . Dannenberg, Wagers, and B r a d l e y (35) have shown t h a t some defects are caused i n many cases by a s m a l l p a r t i c l e of extraneous matter such as dust. Hahn (36) has d i s c u s s e d c r a t e r i n g and i t s r e l a t i o n s h i p to surface tension i n some d e t a i l . Although the formation of surface patterns v i a the Benard c e l l may o c c a s i o n a l l y be useful to form s p e c i a l f i n i s h e s , u s u a l l y i t i s d e s i r a b l e to e l i m i n a t e the s i t u a t i o n . Use of h i g h e r b o i l i n g s o l v e n t s w i l l retard evaporation rate and the c o o l i n g effect that changes surface tension and propels the vortex a c t i o n . Increase i n paint v i s c o s i t y w i l l i n h i b i t the a c t i o n , as w i l l decrease i n f i l m t h i c k n e s s . A d d i t i o n of a s u r f a c t a n t w i l l p r o v i d e a more uniform v a l u e of surface t e n s i o n and a l s o r e t a r d e v a p o r a t i o n and thereby help to i n h i b i t the vortex motion i n c e l l s . Surface t e n s i o n f o r c e s are important i n the process of f i l m f o r m a t i o n from l a t i c e s . Brown (37) has developed equations t h a t i n c l u d e surface t e n s i o n as the prime f o r c e i n the c o a l e s c e n c e process. Use of s o l v e n t i n the l a t e x can change the modulus of the polymer, which i s a l s o a c r i t i c a l f a c t o r i n r e g u l a t i o n of coalescence temperature. When c l e a r f i l m s are applied and dried under humid conditions, frequently a w h i t i s h haze c a l l e d blushing appears i n the f i n a l f i l m . An e l e c t r o n micrograph of a blushed f i l m i s shown i n Figure 3 (38). The b l u s h i n g was caused by the condensation of moisture d r o p l e t s upon the f i l m i n humid a i r when the f i l m became c o l d because of s o l v e n t evaporation. Liquid lacquer with a r e l a t i v e l y low surface tension spontaneously flowed over the water droplets with a higher s u r f a c e t e n s i o n , t h e l a c q u e r t h e n formed a f i l m , and t h e i n c o m p a t i b l e e n c a p s u l a t e d water d r o p l e t s e v e n t u a l l y evaporated through a p a r t i a l l y ruptured f i l m . Good adhesion depends on thorough w e t t i n g of the s u b s t r a t e by the coating and can be achieved quite r e a d i l y by use of s o l u t i o n s of low surface tension. Because good adhesion depends on other factors such as the s t r u c t u r e of the polymer m o l e c u l e , good adhesion does not a u t o m a t i c a l l y r e s u l t from good w e t t i n g , but i t i s a r e q u i s i t e for eventual good adhesion. I f a given r e s i n i s applied i n various forms such as a powder, d i s p e r s i o n , or s o l u t i o n , i t s h o u l d not be expected that equal adhesion r e s u l t s regardless of the form of the r e s i n . As a r u l e , best adhesion of a p a i n t may be expected when applied from s o l u t i o n . V i s c o s i t y of R e s i n S o l u t i o n s as Related to Solvent Properties. A p r i n c i p a l f u n c t i o n of s o l v e n t s i n c o a t i n g s i s to reduce the v i s c o s i t y of the resinous binder so that the coating may be applied to the substrate. The proper a p p l i c a t i o n v i s c o s i t y must be attained



674


at p r a c t i c a l n o n v o l a t i l e content, which may vary from 10 to 80% or more by weight. A s o l v e n t f o r a polymer i s one t h a t d i s s o l v e s the polymer; a good s o l v e n t i s one i n which the polymer e x i s t s i n an u n c o i l e d , random c o n f i g u r a t i o n , and t h e r e i s a h i g h degree of a t t r a c t i o n between the polymer and s o l v e n t molecules. The best s o l v e n t for a given r e s i n may or may not g i v e the lowest v i s c o s i t y of the derived polymer s o l u t i o n . V i s c o s i t y of s o l u t i o n s may be considered to have a contribution from the neat (no d i s s o l v e d resin) v i s c o s i t y of the solvent and a contribution from the d i s s o l v e d polymer m o l e c u l e s . The c o n t r i b u t i o n of the polymer depends on i t s s i z e and shape and the degree of a g g r e g a t i o n of the polymer m o l e c u l e s ; i n t u r n , the c o n f i g u r a t i o n and degree of a g g r e g a t i o n of the polymer m o l e c u l e s depend on the nature of the s o l v e n t . Polymer chemists u s u a l l y are concerned with i n t r i n s i c v i s c o s i t y , which i s i n d i c a t i v e of the s i z e and shape of an i s o l a t e d polymer molecule i n a p a r t i c u l a r s o l v e n t , whereas c o a t i n g chemists are more often concerned w i t h neat v i s c o s i t y of s o l v e n t s and measured v i s c o s i t y of polymer s o l u t i o n s at much higher concentrations of polymer. η

= measured v i s c o s i t y as by c a p i l l a r y flow

Π0

= v i s c o s i t y of solvent

η

= η/rjQ = r e l a t i v e v i s c o s i t y or v i s c o s i t y r a t i o

Γ

D

no)/no = η - l = s p e c i f i c v i s c o s i t y

= (η -

s p

Hred

=

Γ

^sp/c = reduced v i s c o s i t y , where c = cone.

[n] = (Hsp/ ) c = 0 C

=

n

c

f(^ n )/ ] c = 0 r

=

intrinsic viscosity

In very d i l u t e s o l u t i o n s i n which no a g g r e g a t i o n of polymer molecules occurs, the polymer chain i s c o i l e d i n a r e l a t i v e l y poor solvent but i s extended i n a random manner i n a good s o l v e n t ; hence, the polymer m o l e c u l e c o n t r i b u t e s g r e a t e r v i s c o s i t y i n the l a t t e r case, and the i n t r i n s i c v i s c o s i t y of the polymer i s higher than i n a poor s o l v e n t . As the concentration of s o l u t i o n s increases, however, aggregation of polymer molecules occurs i n r e l a t i v e l y poor s o l v e n t s , with the r e s u l t that at high concentrations ( t y p i c a l of lacquers and v a r n i s h e s ) the best s o l v e n t u s u a l l y g i v e s the l o w e s t v i s c o s i t y . Because the neat v i s c o s i t y of s o l v e n t s v a r i e s c o n s i d e r a b l y , t h i s f a c t o r must be taken account of i n p r e d i c t i n g v i s c o s i t i e s ; i n the p r a c t i c a l case where a great number of solvents (or solvent blends) are good s o l v e n t s for a given polymer, the measured v i s c o s i t y of a s o l u t i o n i s r o u g h l y p r o p o r t i o n a l to the neat v i s c o s i t y of the solvent. The data of Reynolds and Gebhart (39) on the v i s c o s i t y of an e a s i l y s o l u b l e long o i l alkyd i l l u s t r a t e some of the considerations discussed (Figure 4). In t h i s case s o l u t i o n v i s c o s i t y i s determined by t h e n e a t v i s c o s i t y o f the s o l v e n t o v e r a w i d e r a n g e o f concentrations. Above 20-30% c o n c e n t r a t i o n the s t r o n g s o l v e n t s (toluene, t e t r a l i n ) s t i l l continue to d i s s o l v e the r e s i n e a s i l y , but the other s o l v e n t s begin to permit aggregation of alkyd molecules with accompanying greater increase i n v i s c o s i t y as concentration of



28. TESS

675

Solvents

F i g u r e 3.

E l e c t r o n micrograph a t 13,000 times m a g n i f i c a t i o n showing moisture b l u s h i n n i t r o c e l l u l o s e l a c q u e r f i l m . Key: SBA. 7.0% w; BUOX: 13.0% w; water: 80.0% w; temp, 25 C ; a i r f l o w , 20 1/min. Reproduced w i t h permp e r m i s s i o n from Ref. 38. C o p y r i g h t 1967 S h e l l Chemical Co.

•

η-Hexane

K

Isooctane

Ο Toluene 4. Methylcyclohexane Ο n-Decane •

Cyclohexane

Δ Tetralin

Resin Concentration, g, 100 ml Solution, at 77*F

F i g u r e 4.

S o l u t i o n v i s c o s i t y o f long o i l a l k y d ( A e r o p l a z 1273, 73% soya) i n v a r i o u s s o l v e n t s as a f u n c t i o n o f concen t r a t i o n . Reproduced w i t h p e r m i s s i o n from Ref. 39. C o p y r i g h t 1957 F e d e r a l S o c i e t y o f P a i n t Technology.



676


r e s i n increases. McGuigan (40) has p l o t t e d n i t r o c e l l u l o s e s o l u t i o n v i s c o s i t y v e r s u s neat s o l v e n t v i s c o s i t y ( F i g u r e 5) f o r a l a r g e number of s t r o n g oxygenated s o l v e n t s and has found t h a t these two v i s c o s i t y values can be c o r r e l a t e d quite w e l l for s o l u t i o n s at 8% concentration. I t i s concluded that, i n s o l u t i o n s of resins i n good s o l v e n t s at modest concentrations, s o l u t i o n v i s c o s i t y i s determined by neat s o l v e n t v i s c o s i t y . Other workers have observed more or l e s s s i m i l a r r e s u l t s as r e l a t e d i n Reference 39. Among many other c o r r e l a t i o n s made by Reynolds and Gebhart, Figure 6 shows a p l o t of reduced v i s c o s i t y of three different alkyd resins as a function of the s o l v e n t power ( s o l u b i l i t y parameter) of s o l v e n t s used i n the v i s c o s i t y measurements. Reduced v i s c o s i t y was c a l c u l a t e d f o r a l k y d s at 40% c o n c e n t r a t i o n , which i s a p r a c t i c a l concentration. Reduced v i s c o s i t y i s a measure of the contribution of the r e s i n to the s o l u t i o n v i s c o s i t y . A l l t h r e e r e s i n s had approximately the same molecular weight, and, therefore, the value of the reduced v i s c o s i t y i s e s s e n t i a l l y an i n d i c a t i o n of the degree of a g g r e g a t i o n of m o l e c u l e s of each a l k y d . As the s o l u b i l i t y parameter of the s o l v e n t decreases, i t i s evident that the degree of aggregation increases modestly i n the case of the long o i l alkyds. In the case of the s h o r t soya a l k y d B e c k o s o l 7 (41% o i l ) , the increase i n aggregation becomes much greater as the s o l v e n t becomes l e s s powerful (lower s o l u b i l i t y parameter). Beckosol 70 (Reichhold product) i s a 65% soya o i l a l k y d , and A e r o p l a z 1273 i s a 73% soya o i l alkyd. Because neat s o l v e n t v i s c o s i t y i s so important i n r e g u l a t i n g v i s c o s i t y of r e s i n s o l u t i o n s , the c a l c u l a t i o n of v i s c o s i t y of blends of s o l v e n t s i s a u s e f u l t o o l i n f o r m u l a t i n g s o l v e n t systems f o r coatings. As p a r t of a computer program to f o r m u l a t e s o l v e n t blends, Nelson et a l . (41) c a l c u l a t e d v i s c o s i t y of blends by use of the f o l l o w i n g equation: log η = Σ V i l o g η

Α

where η i s the v i s c o s i t y of the b l e n d , i s the v i s c o s i t y of the i component, and i s the volume f r a c t i o n of the i component. In t h i s equation the v i s c o s i t y of donor-acceptor s o l v e n t s (e.g., a l c o h o l s and g l y c o l e t h e r s ) i s not the v i s c o s i t y of the pure compounds, which have high v i s c o s i t i e s because of hydrogen bonding; the v i s c o s i t y used i s an e f f e c t i v e v i s c o s i t y , which i s d e f i n e d as the v i s c o s i t y t h a t a s o l v e n t would e x h i b i t i f i t d i d not i n v o l v e hydrogen bonding. Rocklin and Edwards (42) elaborated on the work of Nelson et a l . and presented s u b s t a n t i a l data on b l e n d s c o n t a i n i n g i n t e r a c t i n g s o l v e n t s . They a l s o extended the work to aqueous blends containing up to 20% s o l v e n t . T h e r e a f t e r , R o c k l i n and Barnes (43) f u r t h e r modified the method of c a l c u l a t i o n to handle aqueous b l e n d s containing up to 50% organic s o l v e n t . The c a l c u l a t i o n s were based on s e v e r a l assumptions: the v i s c o s i t y of a b l e n d i s the sum of contributions from a l l i t s components; the effect of each component solvent on the blend i s independent of the other s o l v e n t s ; and the contribution of each solvent depends only on i t s i n t e r a c t i o n with water. C o r r e l a t i o n of experimental values with c a l c u l a t e d values of v i s c o s i t y was e x c e l l e n t . t

n

t

n



28.

677

Solvents

TESS

1.0 NEAT S O L V E N T VISCOSITY, cps.

Figure 5,

V i s c o s i t y of n i t r o c e l l u l o s e s o l u t i o n s v s . v i s c o s i t y of neat s o l v e n t .

Solubility Parameter at TI'T

F i g u r e 6.

Reduced v i s c o s i t y o f s e v e r a l a l k y d r e s i n s as a f u n c t i o n of s o l v e n t power ( s o l u b i l i t y parameter) o f s o l v e n t a t c o n c e n t r a t i o n o f 40 g/mL. C o p y r i g h t 1957 F e d e r a l S o c i e t y of P a i n t Technology.


678


Coatings, e s p e c i a l l y h i g h - s o l i d s coatings, often are heated to a l l o w the c o a t i n g s to be a p p l i e d i n o r d i n a r y equipment. The r e l a t i o n s h i p of temperature and v i s c o s i t y i s of c o n s i d e r a b l e p r a c t i c a l importance. H i l l , K o z l o w s k i , and S h o l e s (44) used a s i m p l i f i e d form of the W i l l i a m s - L a n d e l - F e r r y (WLF) e q u a t i o n to analyze the temperature dependence of v i s c o s i t i e s of s o l u t i o n s of polyester resins and e t h e r i f i e d melamine-formaldehyde c r o s s - l i n k i n g resins. The dependence of v i s c o s i t y on temperature was completely described by a parameter designated as the s o l u t i o n g l a s s t r a n s i t i o n temperature (Tg ). Good predictions of v i s c o s i t y at various r e s i n concentrations and temperatures were made. Sherwin, K o l e s k e , and T a l l e r (45), u s i n g s e v e r a l d i f f e r e n t polymers of low molecular weight, found s t r a i g h t - l i n e r e l a t i o n s h i p s when they p l o t t e d the l o g of s o l u t i o n v i s c o s i t y versus r e c i p r o c a l temperature. In another phase of t h e i r i n v e s t i g a t i o n , they concluded t h a t low m o l e c u l a r weight polymers i n s o l u t i o n can be c o n s i d e r e d to assume conformations approaching r i g i d , impermeable spheres or e l l i p s o i d s .


S

Evaporation Rate of Solvents. Evaporation rate and solvency are the two most important properties of s o l v e n t s and provide a good guide for development of useful formulas for various types of coatings. In 1924, Gardner and Parks (46) d e s c r i b e d a s i m p l e method f o r determination of evaporation rates which consisted of p l a c i n g a 1-2 g sample of s o l v e n t i n a can l i d and making p e r i o d i c weighings of the l i d i n s t i l l a i r . Many i n v e s t i g a t o r s made good use of the method over many years. A comprehensive l i s t of evaporation rates was p u b l i s h e d by D o o l i t t l e (47). Bent and Wik (48) arranged the cups holding s o l v e n t on a round t a b l e and placed a r e v o l v i n g fan at the center to speed up the process. A major advance i n technique was made by C u r t i s , S c h e i b l i , and Bradley (49) i n 1950 when they employed a j o l l y balance device (the S h e l l Thin F i l m Evaporometer) to f o l l o w weight l o s s of s o l v e n t when a p p l i e d to a f i l t e r paper cone. The New York P a i n t and V a r n i s h Production Club made a d e t a i l e d study of the Evaporometer (50), made some modifications i n the method, and concluded that the instrument g a v e r e s u l t s t h a t were r e p e a t a b l e w i t h i n one l a b o r a t o r y , reproducible at different l o c a t i o n s , and i n general provided better r e s u l t s than a l t e r n a t i v e methods investigated. The instrument has been improved over the years and now i s produced i n a h i g h l y automated model (51). The p r a c t i c a l measurement and meaning of evaporation rates of s o l v e n t s are much more complicated than they appear to be at f i r s t glance, as R o c k l i n (52) has reported. I t was shown that evaporation rates from f i l t e r paper, the most popular surface, sometimes d i f f e r considerably from evaporation rates from smooth surfaces such as an aluminum d i s k . In p a r t i c u l a r , e v a p o r a t i o n of water and a l c o h o l s from f i l t e r paper substrate i s r e l a t i v e l y slow, probably because of hydrogen bonding forces between solvent and substrate. E v a p o r a t i o n b e h a v i o r often i s reported as shown i n F i g u r e 7 where weight percent solvent evaporated i s p l o t t e d against time. A convenient numerical value of evaporation rate i s taken as the time i n seconds required for 90% of the s o l v e n t to evaporate when tested under standard condition i n the Evaporometer. I t i s a l s o customary to report evaporation rate of a s o l v e n t r e l a t i v e to that of n - b u t y l


28.

679

TESS Solvents

a c e t a t e taken as I . In t h i s method, the 90% e v a p o r a t i o n times f o r the s o l v e n t s are used. Some values for evaporation rates are shown i n Table V. Evaporation of s o l v e n t blends can be quite complicated because of intermolecular i n t e r a c t i o n s that cause them to deviate from i d e a l behavior as expressed by Raoult s Law. The dotted l i n e i n Figure 8 shows i d e a l behavior of an equal volume blend of two s o l v e n t s , and the dotted l i n e i n F i g u r e 9 shows the e v a p o r a t i o n b e h a v i o r of a s o l v e n t blend that has p o s i t i v e d e v i a t i o n from Raoult s Law. The c o n d i t i o n s under w h i c h a z e o t r o p e s may form d u r i n g evaporation of solvent blends have been discussed by Rudd and T y s a l l (53) and by E l l i s and Goff (54), among others. Inspection of Figure 10 shows that highest vapor pressure of a blend of n-butanol and n octane occurs at 30 mol% n - b u t a n o l , the a z e o t r o p i c c o m p o s i t i o n . Above t h i s p r o p o r t i o n of b u t a n o l i n the l i q u i d the vapor w i l l be poorer i n butanol; for example, a l i q u i d containing 80 mol% butanol w i l l be i n e q u i l i b r i u m w i t h a vapor c o n t a i n i n g about 38 mol% b u t a n o l , and e v a p o r a t i o n of the mixture w i l l l e a v e a remaining l i q u i d that becomes progressively r i c h e r i n butanol. On the other hand a mixture c o n t a i n i n g l e s s than 30 mol% b u t a n o l w i l l l e a v e a r e s i d u a l l i q u i d p r o g r e s s i v e l y r i c h e r i n octane as e v a p o r a t i o n proceeds. A blend at the azeotropic composition w i l l evaporate at constant r a t i o of the two s o l v e n t s i n both the vapor and r e s i d u a l liquid. Thus, some c o n t r o l over the r e s i d u a l s o l v e n t composition can be exercised by choice of s o l v e n t s and t h e i r r e l a t i v e amounts i n a blend (Figure 11). Some factors that influence the rate of evaporation of a s o l v e n t include temperature, flow of a i r over sample, vapor pressure of the s o l v e n t , l a t e n t heat, s p e c i f i c heat, and m o l e c u l a r weight (53). Galstaun (55) i n 1950 reported a thorough study of the evaporation of some hydrocarbon solvents and developed equations for evaporation rates as r e l a t e d to s e v e r a l factors i n c l u d i n g temperature drop of l i q u i d as i t evaporates. Sletmoe (56) developed equations for the evaporation of neat solvent blends. The t o t a l rate of evaporation was proposed to be equal to the sum of the rates for the i n d i v i d u a l s o l v e n t components: f


f

Total Rate = C γ x

χ

Rj

+ C Ύ2 1*2° +

0

2

e t c

-

where C i s t h e c o n c e n t r a t i o n , γ i s the escaping ( a c t i v i t y ) c o e f f i c i e n t , and R i s the rate of evaporation of the pure i n d i v i d u a l components. Sletmoe suggested means of c a l c u l a t i n g s o l v e n t balance (uniform evaporation rate of components during evaporation of blend) as an aid to formulating good s o l v e n t blends. Hansen (57) pointed out t h a t e v a p o r a t i o n of a s o l v e n t from a polymer s o l u t i o n faced two b a r r i e r s when c a s t on an impermeable substrate: resistance to solvent l o s s at the a i r - l i q u i d interface and d i f f u s i o n from w i t h i n the f i l m t o the a i r i n t e r f a c e . Evaporation of neat s o l v e n t s as w e l l as moderately d i l u t e s o l u t i o n s i s l i m i t e d by r e s i s t a n c e at the a i r i n t e r f a c e , but as s o l v e n t concentration becomes low (5-10-15%), the r a t e - c o n t r o l l i n g step i s d i f f u s i o n through the f i l m . Hansen pointed out t h a t at the p o i n t when s o l v e n t l o s s changes to a d i f f u s i o n - l i m i t e d p r o c e s s , the c o n c e n t r a t i o n of s o l v e n t i s s u f f i c i e n t to reduce the g l a s s t r a n s i t i o n temperature, T , of the polymer to the f i l m temperature. g


680


Table V.

Evaporation Rates and B o i l i n g Points of Some Common Solvents

Evap. Time, Seconds to 90% Evap.


Solvent

Evap. Rate Relative to n-Butyl Acetate = 1

Acetone Tetrahydrofuran Lacquer diluent

82 97 101

5.59 4.72 4.6

Ethyl Acetate, 99% Methyl Ethyl Ketone Isopropyl Acetate Methanol Toluene Ethyl A l c o h o l , 100% Methyl Isobutyl Ketone Isobutyl Acetate, 90% V Μ & Ρ Naphtha

117 121 134 221 225 278 282 305 314

3.91 3.79 3.42 2.07 2.00 1.60 1.62 1.50 1.5

Isopropyl Alcohol η-Butyl Acetate, 90-92% Methyl η-Butyl Ketone η-Propyl Alcohol s - B u t y l Alcohol Isobutyl Alcohol Xylene EG Monomethyl Ether Methyl n-Amyl Acetate Methyl Isoamyl Ketone π-Butyl Alcohol EG Monoethyl Ether Cyclohexanone Methyl Isobutyl Carbinol Ethyl Amyl Ketone 4-Methoxy-4-methylpentanone- 2 D i i s o b u t y l Ketone EG Monoethyl Ether Acetate Diacetone Alcohol

319 458 482 530 563 740 770 884 1,004 1,016 1,076 1,213 1,566 1,711 1,770 1,880 2,437 2,533 3,840

1.44 1.00 1.05 0.86 0.81 0.62 0.6 0.52 0.46 0.45 0.43 0.38 0.29 0.27 0.26 0.24 0.19 0.18 0.12

Mineral S p i r i t s

4,560

0.1

6,750 7,410

0.07 0.06

20,000 25,700 27,800 150,000

0.02 0.02 0.02 ο

^ - ^ ^

ISO-BUTYL

^ ^ ^ v . \ N

X

\.

n-BUTYL ACETATE

MNBK

CO

\

^5

1

1

0.01 0.1

10 10

Figure 12.

MEK

1\

-8

1

1

100 1000 2

hrs./cm

Solvent retention of various s o l v e n t s i n a v i n y l r e s i n film.

3 σ

< Q

> Ο Ζ < α

30 40 50 %w EVAPORATED F i g u r e 13. E f f e c t of r e l a t i v e h u m i d i t y on s o l v e n t balance upon e v a p o r a t i o n of s e c - b u t y l a l c o h o 1 - 2 - b u t o x y e t h a n o l - w a t e r b l e n d . Reproduced w i t h p e r m i s s i o n from Ref. 62. C o p y r i g h t 1978 F e d e r a t i o n o f S o c i e t i e s f o r C o a t i n g s Technology.


686


3.


A pyrophoric l i q u i d i s any l i q u i d that i g n i t e s spontaneously i n dry or moist a i r at or below 130 °F (54.5 °C).

The test methods s t i p u l a t e d for measuring f l a s h points are the Tag Closed Tester (ASTM D56-70), the Pensky-Martens C l o s e d T e s t e r (ASTM D93-71), and the Setaflash Closed Tester (ASTM D3278-73). Of these methods, the S e t a f l a s h g i v e s the g r e a t e s t p r e c i s i o n . The Golden Gate S o c i e t y for C o a t i n g s Technology (66) found e x c e l l e n t agreement between the S e t a f l a s h and Tag r e s u l t s f o r f l a s h p o i n t s . In comparison with the Pensky-Martens method, the Setaflash method gave f l a s h points s l i g h t l y lower i n the case of some s o l v e n t s and paints, but the general agreement was very good. Brown, Newman, and Dobson (67) have investigated the f i r e hazard of w a t e r - b o r n e c o a t i n g s . C o r r e l a t i o n of f l a s h p o i n t s w i t h c o m b u s t i b i l i t y of p a i n t s i s not very good. Some products t h a t cannot s u s t a i n combustion often have low f l a s h p o i n t s . In a d e t a i l e d study of f l a s h points of water-solvent blends, s o l u t i o n s of r e s i n s i n b l e n d s and of f i n i s h e d p a i n t s and i n k s , W. Hansen (68) observed t h a t some products w i t h f l a s h p o i n t s of 21-55 °C v a r i e d considerably i n c o m b u s t i b i l i t y . In the case of some products, the i n i t i a l flame went out spontaneously. Walsham (61) developed a computer-based method t h a t gave satisfactory predictions of f l a s h points. He defined an i n d i v i d u a l s o l v e n t f l a s h p o i n t index as an i n v e r s e f u n c t i o n of i t s heat of combustion and vapor pressure at the f l a s h point. F l a s h points of m i x t u r e s were computed by t r i a l and e r r o r as the temperature at which the sum of weighted component indexes e q u a l s 1.0. S o l u t i o n n o n i d e a l i t i e s were accounted for by component a c t i v i t y c o e f f i c i e n t s c a l c u l a t e d by a multicomponent extension of the Van Laar equations. A i r Quality Regulations and Solvents The term smog r e f e r s to an atmospheric c o n d i t i o n caused by i n t e r a c t i o n of organic compounds, nitrogen oxides, and UV l i g h t , and manifested by a combination of poor v i s i b i l i t y , eye i r r i t a t i o n , and p l a n t damage. The e x i s t e n c e of a s e r i o u s smog problem l e d Los Angeles County to conduct extensive i n v e s t i g a t i o n s on i t s causes and cure and issue regulations i n attempts to r e c t i f y the s i t u a t i o n (70, 71). R u l e 66 was the p a r t i c u l a r r e g u l a t i o n designed to reduce g e n e r a t i o n of smog from s o l v e n t e m i s s i o n s ; i t was based on smog chamber t e s t s and used eye i r r i t a t i o n as the p r i n c i p a l c r i t e r i o n , but account was a l s o taken of p l a n t damage and the q u a n t i t i e s of v a r i o u s i n d i v i d u a l s o l v e n t s a c t u a l l y used i n the a r e a . Other i n v e s t i g a t o r s (72, 73) a l s o used smog chambers to study the chemical r e a c t i o n s undergone by s o l v e n t vapors upon exposure to UV l i g h t under c o n t r o l l e d conditions. Rule 66 was adopted i n 1966 and became e f f e c t i v e according to a designated s c h e d u l e . The r u l e was based on the premise t h a t the extent of smog produced depended upon the chemical structure of the s o l v e n t s . Deemed to be conducive to smog production were aromatic hydrocarbons except benzene, branched chain ketones, and e s p e c i a l l y unsaturated compounds. The r e g u l a t i o n specified that s o l v e n t blends were considered to be photochemical l y r e a c t i v e or unreactive on the basis of t h e i r composition as defined i n the box on page 687.



TESS Solvents

D e f i n i t i o n of Photochemically Reactive Solvent by Rule 66 Section k For the purpose of t h i s r u l e , a photochemically r e a c t i v e s o l v e n t i s any s o l v e n t w i t h an aggregate of more than 20% of i t s t o t a l volume composed of the c h e m i c a l s c l a s s i f i e d below or which exceeds any of the f o l l o w i n g i n d i v i d u a l percentage composition l i m i t a t i o n s , referred to the t o t a l volume of s o l v e n t : 1. 2. 3.

A combination of hydrocarbons, a l c o h o l s , aldehydes, e s t e r s , e t h e r s , or ketones h a v i n g an o l e f i n i c or c y c l o o l e f i n i c type of unsaturation: 5%; A combination of aromatic compounds w i t h e i g h t or more c a r b o n atoms t o t h e m o l e c u l e except ethylbenzene: 8%; A combination of e t h y l b e n z e n e , ketones h a v i n g branched hydrocarbon structures, t r i c h l o r o e t h y l e n e , or toluene: 20%.

Whenever any o r g a n i c s o l v e n t or any c o n s t i t u e n t of an o r g a n i c s o l v e n t may be c l a s s i f i e d from i t s c h e m i c a l s t r u c t u r e i n t o more than one of the above groups of organic compounds, i t s h a l l be considered as a member of the most r e a c t i v e c h e m i c a l group, t h a t i s , t h a t group h a v i n g the l e a s t a l l o w a b l e percent of the t o t a l volume of s o l v e n t s .



688


R u l e 66 ( l a t e r c a l l e d R u l e 442 of the South Coast A i r Q u a l i t y Management D i s t r i c t ) a l s o l i m i t e d oven e m i s s i o n s from baked c o a t i n g s . I n c i n e r a t i o n , a b s o r p t i o n , or r e c o v e r y of v o l a t i l e s i s needed to meet t h i s r e s t r i c t i o n . In f u r t h e r r e g u l a t i o n s , use of h i g h - s o l i d s , l o w - s o l v e n t , water-borne, and other types of coatings w i t h l o w - v o l a t i l e e m i s s i o n s was encouraged. The e v o l u t i o n of various a i r p o l l u t i o n regulations has been summarized (74, 75). The main thrust of the Rule 66 type of r e g u l a t i o n was to require t h e use of l e s s r e a c t i v e s o l v e n t s i n p l a c e o f t h e more photochemically r e a c t i v e s o l v e n t s . However, more recent regulations are geared to l i m i t a t i o n of the e m i s s i o n s of a l l s o l v e n t s . Transport theory i n v o l v e s the concept that a i r p o l l u t i o n i n a given area may r e s u l t from emissions i n other areas upwind. Accordingly, the more r e a c t i v e s o l v e n t s are deemed to y i e l d p o l l u t a n t s i n the immediate area where they are e m i t t e d , and the l e s s - r e a c t i v e s o l v e n t s are considered to cause p o l l u t i o n l a t e r i n downwind areas of the country. In t h i s view a l l solvent emissions cause p o l l u t i o n , but differences i n r e a c t i v i t y simply determine where the p o l l u t i o n o c c u r s . Thus, the emphasis on c o n t r o l of p o l l u t i o n from c o a t i n g s has s h i f t e d from r e s t r i c t i n g the c o m p o s i t i o n of s o l v e n t s to r e s t r i c t i n g the emissions of a l l s o l v e n t s from coatings. In November 1976, the 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) issued the important document e n t i t l e d "Control Methods for Surface C o a t i n g O p e r a t i o n s " (76). Thereafter, g u i d e l i n e s for s o l v e n t content for various types of paint for s p e c i f i c end uses have been issued p e r i o d i c a l l y as shown i n Table V I . I t i s important to note that the guidelines specify that a given volume of coating i n c l u d i n g s o l v e n t s h o u l d c o n t a i n no more than a g i v e n weight of s o l v e n t . T h e r e f o r e , i n meeting r e g u l a t i o n s , i t i s advantageous to use s o l v e n t s of powerful solvency and low s p e c i f i c g r a v i t y . Tess (75) has summarized and i n t e r p r e t e d these guidelines' and has suggested options a v a i l a b l e to meet the g u i d e l i n e s . Formulation of Solvent Blends Much information on s p e c i f i c formulations of solvent blends has been p u b l i s h e d i n commercial brochures, t e c h n i c a l a r t i c l e s , and books s u c h as t h o s e c i t e d i n t h i s c h a p t e r (_5, 20_, 23, 3_1). Some techniques and a i d s used by f o r m u l a t o r s w i l l be d i s c u s s e d here briefly. Regions of s o l u b i l i t y of resins i n s o l v e n t s can be expressed i n terms of various s o l u t i o n parameters such as s o l u b i l i t y parameter, hydrogen bonding index, f r a c t i o n a l p o l a r i t y , d i p o l e moment, i n t e r n a l p r e s s u r e , or the components of s o l u b i l i t y parameter due to dispersion, p o l a r , and hydrogen bonding forces. An example of the s o l u b i l i t y of n i t r o c e l l u l o s e i n various s o l v e n t s as a function of s o l u b i l i t y parameter and f r a c t i o n a l p o l a r i t y i s shown i n Figure 14 (77). S o l u b i l i t y i n m i x t u r e s of two s o l v e n t s can be p r e d i c t e d by drawing a straight l i n e between two different s o l v e n t s and f i x i n g a point according to the r e l a t i v e amounts of each s o l v e n t . Three and more component systems can be l o c a t e d i n the s o l u b i l i t y map by s i m i l a r p r i n c i p l e s . In the case of some r e s i n s , hydrogen bonding f o r c e s a r e t h e p r e d o m i n a n t s o l u b i l i z i n g f o r c e s , and i t i s advantageous to i n c l u d e some measure of these f o r c e s i n a twodimensional system. Of course, use of three s o l u t i o n parameters can


28.

689

TESS Solvents

Table V I . EPA Guidelines for Maximum V o l a t i l e Organic Content of Coatings


Guidelines, EPA Volume II

II II II II II

III IV V VI VII

Limitation (g/L) ( l b / g a l )

Process

Can coating Sheet basecoat & overvarnish; two-piece can e x t e r i o r Two-, three-piece can i n t e r i o r body spray, two-piece can e x t e r i o r end Side-seam spray End sealing compound C o i l coating Fabric coating V i n y l coating Paper coating Auto and l i g h t - d u t y truck coating Prime Topcoat Repair Metal furniture coating Magnet wire coating Large appliance coating Miscellaneous metal parts Wood paneling Printed i n t e r i o r Natural f i n i s h hardwood Class I I hardboard

340

2.8

510 660 440 310 350 450 350

4.2 5.5 3.7 2.6 2.9 3.8 2.9

230 340 580 360 200 340 50-520

1.9 2.8 4.8 3.0 1.7 2.8 0.4-4.4

200 380 320

1.7 3.2 2.7

L i m i t a t i o n of V o l a t i l e s VIII

Inks Rotogravure or flexographic Water-borne Solvent type

60% NVM 75% v o l . water and 25% volatiles 70% o v e r a l l reduction of v o l a t i l e s or 60% NVM


Ι

-ν


28.

TESS

Solvents

691

e a s i l y be handled by use of a computer as w i l l be discussed l a t e r , but i s hard to project on paper. Areas of s o l u b i l i t y of resins i n ternary s o l v e n t systems often are expressed i n t r i a n g u l a r diagrams as shown i n F i g u r e 15. An a d d i t i o n a l feature of t h i s diagram i s that the area containing up to 20% t o l u e n e i s a complying ("exempt") s o l v e n t under R u l e 66. I t i s often u s e f u l to express v i s c o s i t y and v o l a t i l i t y of r e s i n s i n t e r n a r y s o l v e n t b l e n d s as shown i n F i g u r e 16. In t h i s f i g u r e the time i n seconds r e q u i r e d f o r 90% of the s o l v e n t to e v a p o r a t e i s given as a measure of v o l a t i l i t y , and the v i s c o s i t y i n centipoises i s given as a measure of v i s c o s i t y . Point A describes the composi t i o n of a s o l v e n t b l e n d t h a t has e v a p o r a t i o n time of 160 s and v i s c o s i t y of 85 cP. A wealth of useful data on evaporation times can be expressed i n a diagram as shown i n Figure 17. The v o l a t i l i t y of combinations of m e t h y l e t h y l k e t o n e w i t h v a r i o u s h i g h e r b o i l e r s has been i n v e s t i g a t e d by Dante (78). H o r i z o n t a l l i n e s show v o l a t i l i t y of methyl i s o b u t y l ketone and methyl η - b u t y l ketone; from the v o l a t i l i t y standpoint each of these can be replaced by compositions indicated by intersections of the h o r i z o n t a l l i n e s with each of the curves. The i n t r o d u c t i o n of R u l e 66 caused tremendous a c t i v i t y i n the reformulation of s o l v e n t blends to conform with the r e g u l a t i o n . An example of n i t r o c e l l u l o s e l a c q u e r s o l v e n t s f o r both c o m p l y i n g (exempt from c o n t r o l Rule 66) and noncomplying (nonexerapt) formula tions i s shown i n Table V I I (79). Exempt solvent systems for alkyd resins have been discussed by Fink and Weigel (80), for n i t r o c e l l u lose and v i n y l resins by Crowley (81), for v i n y l resins by Park (82) and by Burns and McKenna (83), and f o r epoxy r e s i n s by S o m e r v i l l e and Lopez (84). In r e f o r m u l a t i n g thousands of s o l v e n t formulations to comply w i t h R u l e 66, the c o a t i n g s i n d u s t r y faced a tremendous t a s k . To meet t h i s problem, a combination of s o l u t i o n t h e o r y , s o l v e n t e x p e r t i s e , and computer t e c h n o l o g y proved to be of g r e a t h e l p . N e l s o n , F i g u r e l l i , Walsham, and Edwards (41) d e s c r i b e d a two-step method of s o l v e n t s e l e c t i o n by computer. The f i r s t program c a l c u l a t e s average properties of an input solvent blend by using a data f i l e t h a t c o n t a i n s b a s i c data on o v e r 100 s o l v e n t s . A f t e r a known s o l v e n t b l e n d expressed i n volume percent i s fed to the computer, i n a matter of seconds the f o l l o w i n g i n f o r m a t i o n i s obtained: 1. 2. 3. 4. 5. 6. 7. 8.

Weight f r a c t i o n of each component s o l v e n t . Mole f r a c t i o n of each s o l v e n t . Volume f r a c t i o n of each of the four c a t e g o r i e s of s o l v e n t segregated according to the classes i n Rule 66. Three s o l u t i o n parameters that are used to describe solvency of the solvent blend i n fundamental terms: s o l u b i l i t y parameter, f r a c t i o n a l p o l a r i t y , and hydrogen bonding index number (85). Neat v i s c o s i t y . Specific gravity. Evaporation time for each 10% increment evaporated. Raw material cost.


692



100% MEK

100% MIBK Figure 16.

IOO%Toluene

I s o v i s c o s i t y and i s o v o l a t i l i t y curves for a v i n y l r e s i n (Union Carbide V i n y l i t e VYHH) at 20% s o l i d s .


693


28. TESS Solvents

Figure 17.

Evaporation behavior of mixtures of methyl e t h y l ketone with slower evaporating s o l v e n t s .



694


Table V I I .

Solvent Systems for N i t r o c e l l u l o s e Furniture Lacquer

Solvent Composition, %v Methyl ethyl ketone Methyl i s o b u t y l ketone η-Butyl acetate Ethyl amyl ketone Methyl amyl acetate 2-Butoxyethanol Isopropyl alcohol ( i n c . IPA ex. cotton) Toluene Xylene ( i n c . xylene ex. modifying resins) Lacquer diluent (9% toluene) Lacquer diluent (23% toluene) Total

Nonexempt Control 7.0 20.0

Rule 66 Exempt Replacements 1 2 8.8

— — — — 17.8

8.8

— — 4.8

— 5.0 — — 16.0

30.4

21.8

21.0

9.3

3.8 17.8 9.3

15.0

8.0 25.7

16.0

—

8.0 25.7

100.0

100.0

100.0

—


—


28. TESS Solvents

695

In the second program, constraints such as compliance with Rule 66, minimum values of the s o l u t i o n parameters, 90% evaporation time, and maximum value of neat v i s c o s i t y are imposed on the s e l e c t i o n of replacement formulas. This second program i s a l i n e a r optimization program that s e l e c t s a s i n g l e solvent blend from a chosen l i s t of 15-20 candidates, bounded by the imposed constraints and optimized i n some f u n c t i o n , u s u a l l y c o s t . The output of the computer i n d i c a t e s a s i n g l e s o l v e n t b l e n d t h a t meets the c o n s t r a i n t s at minimum c o s t . The suggested b l e n d s h o u l d be checked i n the l a b o r a t o r y to ensure t h a t the f o r m u l a t i o n meets the d e s i r e d requirements. Hansen (86) has m o d i f i e d the method of N e l s o n , et a l . by incorporating s o l u b i l i t y parameters for the three component forces i n the s o l u b i l i t y parameter, v i z , dispersion, p o l a r , and hydrogen bonding forces. The solvent s e l e c t i o n procedure was designed f o r use by p l a n t l a b o r a t o r i e s on a time s h a r i n g t e r m i n a l . The enlistment of the computer i n the s e l e c t i o n of solvent blends has been a boon to the formulator, but the use of older methods i s s t i l l very useful. Solvents for High-Solids Coatings Unless emissions of v o l a t i l e organic compounds (VOC) from coatings are c o n t r o l l e d by i n c i n e r a t i o n , absorption, or other means, current EPA regulations require that only very l i m i t e d quantities of s o l v e n t may be used i n i n d u s t r i a l c o a t i n g s as shown i n T a b l e V I I . The regulations sometimes can be met with s u i t a b l e powder paints, waterborne paints, or 100% c o n v e r t i b l e paints, but solvent-based paints o f t e n are p r e f e r r e d because of t h e i r t r a d i t i o n a l good p r o p e r t i e s . Because o n l y l i m i t e d amounts of s o l v e n t are p e r m i t t e d , i t i s d e s i r a b l e t h a t the s o l v e n t s be as powerful as f e a s i b l e . Also, because the regulations specify a maximum weight of solvent per unit volume of bulk paint, the s o l v e n t used should have as low a s p e c i f i c g r a v i t y as f e a s i b l e . Use of a p p l i c a t i o n methods t h a t can use r e l a t i v e l y h i g h v i s c o s i t y p a i n t s i s advantageous. A p p r o p r i a t e methods i n c l u d e r o l l e r c o a t i n g , s p e c i a l e l e c t r o s t a t i c spray equipment, hot-spray techniques, and two-nozzle spray guns. Because ketones have both high solvency power and low s p e c i f i c g r a v i t y , they are a t t r a c t i v e s o l v e n t s f o r h i g h - s o l i d s c o a t i n g systems. Some ketones and esters have been compared as s o l v e n t s for an epoxy r e s i n as shown by the data i n Table V I I I . The quantity of s o l v e n t i n each case i s maintained at 250 g / L of s o l u t i o n . Comparisons between pairs of ketone and ester s o l v e n t s are made i n the c a t e g o r i e s of l o w - b o i l i n g , m e d i u m - b o i l i n g , and h i g h - b o i l i n g s o l v e n t s . In each case the ketone s o l u t i o n s have s u b s t a n t i a l l y lower v i s c o s i t y than the e s t e r s o l u t i o n . T h i s c o n d i t i o n i s due p a r t l y to the more powerful s o l v e n c y of the ketones and p a r t l y to t h e i r lower s p e c i f i c g r a v i t y which permits more of the ketones by volume i n the f i n a l s o l u t i o n s as shown by the data i n T a b l e V I I I . In the cases of the l o w - b o i l i n g and the medium-boiling s o l v e n t s , the ketones have lower v i s c o s i t i e s i n the neat state i n the absence of resin. S p r i n k l e (87) has p o i n t e d out the importance of c o n t r o l l i n g s u r f a c e t e n s i o n i n h i g h - s o l i d s c o a t i n g s . As c o a t i n g s i n c r e a s e i n



29 42

0.799

0.878

0.818

0.970

Methyl i s o b u t y l ketone

η-Butyl acetate

E t h y l isoamyl ketone

2-Ethoxyethyl acetate

Note:

26

0.897

E t h y l acetate

250

250

250

250

250

250

g of Solvent/ L of Solution

Eponex DRH-151 r e s i n i s a product of S h e l l Chemical Company.

68

48

18

Solution V i s e . , cp

0.802

Solvent Sp. G . , 25 °C

25.8

30.6

28.5

31.3

27.9

31.2

% vol. Solvent

V i s c o s i t y of Eponex DRH-151 Resin Solutions at 250 g/L of Solution

Methyl ethyl ketone

Solvent

Table V I I I .

74.2

69.4

71.5

68.7

72.1

68.8

% vol. Solids


23.3

24.5

23.9

24.7

23.7

24.6

% wt. Solvent

76.7

75.5

76.1

75.3

76.3

75.4

% wt. Solids

1.073

1.020

1.044

1.012

1.053

1.016

Sp. G. Soin.

28. TESS Solvents

697


s o l i d s content, the surface tension increases with the deleterious consequences of decreased s p r a y a b i l i t y , w e t t a b i l i t y , and p o s s i b l e i n c r e a s e d presence of d e f e c t s such as c r a t e r i n g . S e l e c t i o n of s o l v e n t s w i t h low surface t e n s i o n can m i t i g a t e the i n c r e a s e i n surface tension of h i g h - s o l i d s coatings. H i g h - s o l i d s coatings of the solvent type have been discussed i n other sections of t h i s chapter. This f i e l d i s a c t i v e , but greatest emphasis appears to be focused on the r e s i n component of h i g h - s o l i d s solvent coatings. N e v e r t h e l e s s , proper s o l v e n t s e l e c t i o n can m a t e r i a l l y h e l p i n the quest f o r h i g h - s o l i d s c o a t i n g s based on solvents.

Literature Cited 1. Mattiello, 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Powers, P. O. In " P r o t e c t i v e and D e c o r a t i v e Coatings"; J. J., Ed.; Wiley and Sons: New York, 1941; V o l . I, Chap. 23. Holden, H. C.; Doolittle, A. K. Ind. Eng. Chem. 1935, 27, 48591. U.S. T a r i f f Commission, Census of Dyes and Other Synthetic Organic Chemicals, 1925. McClure, H. B.; Bateman, R. L. Chem. Eng. News 1947, 25(44), 3208-14; ibid. 1947, 25(45), 3286-89. Doolittle, A. K. "The Technology of Solvents and Plasticizers"; John Wiley and Sons: New York, 1954; p. 2. Stewart, A. C. address at Chemical Marketing Research Association, May 1972, New York; Chem. Marketing Reporter 1972, 201(19). "NPCA Data Bank Program 1982"; SRI International, September 1982. Zeisberg, F. C. In "Twenty-Five Years of Chemical Engineering Progress"; D. Van Nostrand Company: New York, 1933; Chap. IV. Durrans, T. H. "Solvents"; D. Van Nostrand Company: New York, 1930; also later editions. " K i r k - O t h m e r E n c y c l o p e d i a of Chemical Technology"; Interscience: New York. Clayton, E.; Clark, C. O. Proceedings of Journal of the Society of Dyers and Colorists 1931, 47(7), 183. K e l l y , F. C. "One Thing Leads to Another"; Houghton Mifflin Company: Boston, 1936. Weed, F. G. In "Protective and Decorative Coatings"; M a t t i e l l o , J . J . , Ed.; John Wiley and Sons: New York, 1943; V o l . III, Chap. 14. "Ethyl Alcohol"; Enjay Chemical Co., 1962. "Shell Ethyl Alcohol"; Shell Chemical Co., B u l l e t i n IC:69-32, 1969. "Isopropyl Alcohol"; Enjay Chemical Co., 1966. Kenney, J. Α., "Coke-Oven, L i g h t - O i l Distillates i n the Paint and Varnish Industry"; In "Protective and Decorative Coatings"; John Wiley and Sons: New York, 1941; V o l . I, Chap. 26. H i l d e b r a n d , J . H.; S c o t t , R. L. "The Solubility of Nonelectrolytes," 3rd ed.; Dover Publications: New York, 1964; 1st, 2nd, 3rd eds., Reinhold Publishing Corporation: New York, 1924-1950.


698

19. 20. 21. 22. 23.


24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50.


Hildebrand, J . H.; Prausnitz, J . M.; Scott, R. L. "Regular and Related Solutions"; Van Nostrand Reinhold: New York, 1970. Bogin, C. D. In "Protective and Decorative Coatings"; Matiello, J . J., Ed.; Wiley and Sons, 1941; V o l . I, Chap. 27. Reynolds, W. W. "Physical Chemistry of Petroleum Solvents"; Reinhold: New York, 1963. Patton, T. C. "Paint Flow and Pigment D i s p e r s i o n " ; Interscience: New York, 1966. "Solvents Theory and Practice"; Tess, R. W., Ed.; ADVANCES IN CHEMISTRY SERIES No. 124, American Chemical S o c i e t y , Washington, D.C., 1973. "Solvent Properties Chart"; Shell Chemical Co., B u l l e t i n SC:4874, 1974. Kauppi, Τ. Α.; Bass, S. L. Ind. Eng. Chem. 1938, 30, 74. Doolittle, A. K. Ref. 5, p. 855-57. Hoy, K. L. J. Paint Technol. 1973, 45(579), 51. Sullivan, D. A. J. Paint Technol. 1975, 47(610), 60. Tess, R. W.; Schmitz, R. D. Off. Dig. Fed. Socs. Paint Technol. 1957, 29, 1346. May, C. A. Chapter 10 in Ref. 23. Mellan, I. "Industrial Solvents," 2nd ed.; Reinhold: New York, 1950. Zisman, W. A. J. Paint Technol. 1972, 44(564), 41. Hansen, C. M.; Pierce, P. Ε. "XII FATIPEC Congress Book"; 1974; pp. 91-99. Hansen, C. M.; P i e r c e , P. E. Ind. Eng. Chem. Prod. Res. Develop. 1973, 12(1), 67. Dannenberg, H.; Wagers, J . ; Bradley, T. F. Ind. Eng. Chem. 1950, 42, 1594. Hahn, F. J. J. Paint Technol. 1971 43(562), 58. Brown, G. L. J. Polym. S c i . 1956, 22, 423. "Test Methods and Techniques for the Surface Coatings Industry," 2nd ed.; S h e l l Chemical Co., Bulletin IC:67-53, 1967. Reynolds, W. W.; Gebhart, H. J., J r . Off. Dig. Fed. Soc. Paint Technol. 1957, 29(394), 1174. McGuigan, J. P., private communication. Nelson, R. C.; Figurelli, V. F.; Walsham, J . R.; Edwards, G. D. J. Paint Technol. 1970, 42(550), 644. Rocklin, A. L.; Edwards, G. D. J . Coatings Technol. 1976, 48(620), 68. Rocklin, A. L.; Barnes, J . A. J . Coatings Technol. 1980, 52(655), 23. Hill, L. W.; Kozlowski, K.; Sholes, R. L. J. Coatings Technol. 1982, 54(692), 67. Sherwin, Μ. Α.; Koleske, J . V.; Taller, R. A. J . Coatings Technol. 1981, 53(683), 35. Gardner, Η. Α.; Parks, H. C. Paint Manuf. Assoc. U.S. Tech. C i r c . 218; 1924; p. 113. D o o l i t t l e , A. K. Ind. Eng. Chem. 1935, 27. 1169. Bent, F. Α.; Wik, S. N. Ind. Eng. Chem. 1936, 28, 312. C u r t i s , R. J . ; S c h e i b l i , J . R.; Bradley, T. F. Anal. Chem. 1950, 22, 538. New York Paint and Varnish Production Club, Off. Dig. Fed. Soc. Paint Technol. 1956, 28, 1060; 1958, 30, 1230.


28. TESS Solvents

51. 52. 53. 54. 55. 56. 57.


58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71. 72. 73. 74. 75. 76.

77. 78. 79.

699

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