Chemistry and Technology of Alkyd and Saturated Reactive Polyester

49 Chemistry and Technology of Alkyd and Saturated Reactive Polyester Resins

Downloaded by UNIV OF MARYLAND COLL PARK on October 15, 2014 | http://pubs.acs.org Publication Date: September 25, 1985 | doi: 10.1021/bk-1985-0285.ch049

H. J. LANSON LanChem Corporation, East St. Louis, IL 62205

History of Alkyd Resins Functionality Theory and Synthesis Processing of Alkyd Resins Classification and Properties of Alkyd Resins Formulation and Design of Alkyd Resins Uses of Alkyd Resins Saturated Reactive Polyesters Resin Systems for Reduced Solvent Emissions Low-Solvent and Solvent-Free Alkyd Resins

Alkyd resins have been defined as the reaction product of a polybasic acid and a polyhydric alcohol. This definition includes polyester resins of which alkyds are a particular type. The specific definition that has gained wide acceptance is that alkyds are polyesters modified with monobasic fatty acids. In recent years, the term nonoil or o i l - f r e e alkyd has come into use to describe polyesters formed by the reaction of polybasic acids with polyhydric alcohols in non-stoichiometric amounts. These products are best described as functional saturated polyesters containing unreacted OH and/or COOH groups, and they are finding rapidly increasing uses in organic coatings. History of Alkyd Resins Alkyd resins came into commercial use over 50 years ago, and even with the wide array of other polymers for coatings that have appeared in more recent years, they rank as the most important synthetic coating resins and still constitute about 35% of all resins used in organic coatings. A recent survey reported that over 700 million lb of alkyd resins were used in 1980 (Table I) (1). The formation of alkyd resins is a t y p i c a l example of condensation polymerization. In 1847, Berzelius reported a resinous product formed by the reaction of t a r t a r i c acid and g l y c e r o l . In 1901, Watson Smith (England) prepared a brittle resinous polymer by 0097-6156/85/0285-1181$07.00/0 © 1985 American Chemical Society

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

APPLIED POLYMER SCIENCE


1182

the r e a c t i o n of p h t h a l i c anhydride and g l y c e r o l . I n 1910, the search for better e l e c t r i c i n s u l a t i n g materials stimulated extensive i n v e s t i g a t i o n s on the g l y c e r o l - p h t h a l i c anhydride reaction. The work of Callahan, Arsem, Dawson, and Howell showed that when part of the p h t h a l i c anhydride was r e p l a c e d w i t h monobasic a c i d s , such as b u t y r i c and o l e i c a c i d s , the products were more f l e x i b l e and had b e t t e r s o l u b i l i t y than the g l y c e r y l p h t h a l a t e r e a c t i o n product. However, i t was not u n t i l 1929 that the chemical reactions i n v o l v e d were understood. I n t h a t year, C a r o t h e r s (_2) showed the broad r e l a t i o n s h i p of f u n c t i o n a l i t y to polymer formation and extended the idea of polymerization to condensation reactions as w e l l as addition r e a c t i o n s . H i s e a r l y work d e f i n e d the c o n d i t i o n s necessary f o r intermolecular condensation polymerization of b i f u n c t i o n a l monomers to produce l i n e a r , s o l u b l e , f u s i b l e polymers such as the polyesters from g l y c o l s and dibasic acids. In 1929 and 1930, Kienle and Hovey (3) emphasized the fact that, a l t h o u g h a b i f u n c t i o n a l polymer such as the g l y c o l p h t h a l a t e p o l y e s t e r i s f u s i b l e and s o l u b l e even a t the h i g h e s t degree of p o l y m e r i z a t i o n o b t a i n a b l e , the use of a t r i f u n c t i o n a l component, such as the g l y c e r y l p h t h a l a t e p o l y e s t e r , l e a d s to g e l a t i o n l o n g before completion of the e s t e r i f i c a t i o n . When g l y c e r o l and p h t h a l i c anhydride a r e heated together i n equivalent proportions, the acid number drops very r a p i d l y a t f i r s t because of t h e d i b a s i c a c i d reaction with the primary hydroxyls of the g l y c e r o l to form rather short, l i n e a r polyesters. A f t e r s u f f i c i e n t h e a t i n g , the r e a c t i o n product i s of moderate m o l e c u l a r weight; when c o l d , i t i s a c l e a r resinous m a t e r i a l . As the r e a c t i o n proceeds, the secondary h y d r o x y l s of the g l y c e r o l come i n t o p l a y ; t h e i r r e a c t i o n w i t h m o l e c u l e s of p h t h a l i c anhydride connect the short c h a i n t o form a complex branched or network type structure. When approximately 80% e s t e r i f i c a t i o n has occurred, the product becomes i n f u s i b l e and i n s o l u b l e i n common solvents. Functionality Theory and Synthesis The conditions f o r the g e l a t i o n of such condensation polymers have been t r e a t e d m a t h e m a t i c a l l y by C a r o t h e r s and o t h e r s (2); the a p p l i c a t i o n of g e l a t i o n to the g l y c e r o l - p h t h a l i c anhydride reaction has been s t u d i e d by K i e n l e and coworkers. The s i m p l i f i e d form of the C a r o t h e r s equation s t a t e s t h a t as m o l e c u l a r weight becomes i n f i n i t e a t the g e l p o i n t , ρ = 2/f where ρ i s the extent of r e a c t i o n , and f i s the average degree of f u n c t i o n a l i t y or the average number of f u n c t i o n a l groups i n the reacting molecules. Only s t o i c h i o m e t r i c e q u i v a l e n t s of i n t e r a c t i n g f u n c t i o n a l groups a r e considered. The Carothers equation, when a p p l i e d t o b i f u n c t i o n a l r e a c t a n t s , i n d i c a t e s no g e l a t i o n even though 100% e s t e r i f i c a t i o n occurs. When a p p l i e d to the r e a c t i o n of e q u i v a l e n t q u a n t i t i e s of g l y c e r o l and p h t h a l i c anhydride, f = [(2x3)+(3x2)]/5 = 2.4 and t h e r e f o r e , ρ = 2/2.4 = 0.83 or 83%. The experiments of K i e n l e and h i s coworkers (3) i n which equiva l e n t q u a n t i t i e s of g l y c e r o l and p h t h a l i c anhydride were used showed t h a t g e l a t i o n occurred a t 79.5% r e a c t i o n r e g a r d l e s s of r e a c t i o n temperature. The reactions that did occur are represented by Figure 1. Before g e l a t i o n occurs, the r e s i n i s a p a l e , t r a n s p a r e n t , f u s i b l e product s o l u b l e i n s t r o n g s o l v e n t s such as acetone or a


49. LANSON

1183

A Ikyd and Saturated Reactive Polyesters


TABLE I . Consumption of Synthetic Resins In Paints and Coatings i n 1980 Resin Type M i l l i o n s of Pounds 710 Alkyd Vinyl 390 Acrylic 470 Epoxy 145 Urethane 85 Amino 85 60 Cellulosic Phenolic 25 Styrene-butadiene 25 TOTAL selected resins 1995

Y

Y

I

Y

Y

ι

Ι

Linear polyester

I l

I

I

I

I

Y Y Y A A A

I

I

Y A

Cross-linked polyester Dibasic acid

~\

7~ - Trihydric alcohol

Figure 1. Glycerol-phthalic anhydride reactions.



1184


mixture of a l c o h o l and benzene. I t i s i n s o l u b l e i n p e t r o l e u m s o l v e n t s and possesses a high acid number. In the g l y c e r o l - p h t h a l i c anhydride r e a c t i o n , c o n s i d e r the replacement of part of the d i b a s i c acid with a monobasic f a t t y acid. I f the f i r s t reaction that takes place i s assumed to be between the f a t t y acid and one hydroxyl of the g l y c e r o l , the r e s u l t i n g ester or monoglyceride can be regarded as a g l y c o l because i t has o n l y two hydroxyls. I f the p h t h a l i c group reacts with t h i s modified g l y c o l , a l i n e a r polymer s h o u l d r e s u l t because the f u n c t i o n a l i t y of each r e a c t a n t i s now o n l y two, and c r o s s - l i n k i n g w i t h i t s subsequent g e l a t i o n s h o u l d not occur. T h i s may be represented as shown i n F i g u r e 2. The monobasic acid modifies the properties of the r e s i n i n two ways: f i r s t , by i t s c a p a c i t y t o c o n t r o l f u n c t i o n a l i t y and thus a l l o w c o n t r o l of polymer b u i l d i n g , and second, by nature of i t s own inherent p h y s i c a l properties. A l t h o u g h a l k y d r e s i n s a r e based on three fundamental b u i l d i n g b l o c k s — o i l s or f a t t y a c i d s , d i b a s i c acids, and polyhydric a l c o h o l s — t h e permutations and combinations possible with only these three basic components become enormous, and no resin-forming reaction lends i t s e l f to greater u s e f u l v a r i a t i o n and m o d i f i c a t i o n than the f o r m a t i o n of a l k y d r e s i n s . Nature has been generous i n p r o v i d i n g us w i t h a wide a r r a y of o i l s and f a t t y a c i d s o f v a r y i n g d e g r e e s o f u n s a t u r a t i o n and c h a i n l e n g t h . V a r i a t i o n i n t h i s component alone a l l o w s us a wide gradation i n f i l m types from s o f t , c o l o r l e s s , p l a s t i c i z i n g f i l m s to hard, tough, tackf r e e f i l m s . The mechanism by which a l k y d s a r e converted from a l i q u i d to a dry f i l m depends on the alkyd structure and i t s method of use. A l t h o u g h the a l k y d i s e s s e n t i a l l y a p l a s t i c i z e r , as i n n i t r o c e l l u l o s e lacquers, no chemical reactions are i n v o l v e d i n f i l m formation. I n a p l a s t i c i z i n g a l k y d , f a t t y a c i d s t h a t are f u l l y s a t u r a t e d or c o n t a i n o n l y one double bond a r e g e n e r a l l y used. A l t h o u g h the f a t t y a c i d s present i n the a l k y d a r e d e r i v e d from semidrying or d r y i n g o i l s , the a l k y d can undergo a u t o o x i d a t i o n a t ambient temperature with the oxygen attacking the unsaturated area of the f a t t y acid molecule. The general mechanism of f i l m formation i s s i m i l a r to that for g l y c e r i d e drying o i l s . However, t h e m o l e c u l a r weight of the a l k y d i s s i g n i f i c a n t l y higher than t h e m o l e c u l a r weight of a g l y c e r i d e o i l , and t h e r e f o r e , the number of c r o s s - l i n k s needed to give a dry f i l m i s decreased, and the drying time i s reduced. A l s o , because r e l a t i v e l y few c r o s s - l i n k s a r e required to dry an alkyd f i l m , the l e s s unsaturated semidrying o i l s can be used t o g i v e a l k y d s w i t h good d r y i n g p r o p e r t i e s . The p l e n t i f u l supply of low-cost soybean o i l and h i g h l y refined f a t t y acids from t a l l o i l spurred the growth i n the use of alkyd resins. Alkyds or o i l - m o d i f i e d polyesters are made with drying, semid r y i n g , and nondrying o i l s or the f a t t y a c i d s t h e r e o f . The type s e l e c t e d depends on the c o n d i t i o n s under which the f i l m w i l l be used, and the c o l o r r e t e n t i o n and f i l m p r o p e r t i e s r e q u i r e d . The extent and kind of unsaturation i n the drying o i l f a t t y acids have a profound e f f e c t on the properties of the f i n i s h e d alkyd. By using known mixtures of the v a r i o u s f a t t y a c i d s present i n d r y i n g and semidrying o i l s , s c i e n t i s t s have shown a number of i n t e r e s t i n g r e l a t i o n s h i p s between f i l m - f o r m i n g p r o p e r t i e s and the f a t t y a c i d composition i n o i l - m o d i f i e d alkyds (4).



49.

Π85

Alkyd and Saturated Reactive Polyesters

LANSON

Reactants: —1

1-

\^/

wwwhvh0\

«

dibasic acid

β

glycerine

e=

monobasic acid

Step I - Conversion of t r i f u n c t i o n a l polyol to a b i f u n c t i o n a l polyol

I

monoglyceride

Step II - Conversion to a f a t t y acid modified l i n e a r polymer

-C=K--H=i

I Figure 2. Modified glycerol-phthalic anhydride reactions.


etc.

1186



1.

The rate of drying i s a function of polyunsaturated or polyenoic a c i d content. T h i s r a t e i n c r e a s e s r a t h e r r a p i d l y up t o a polyenoic content of about 50%. Above that f i g u r e , a l i m i t i n g value i s g r a d u a l l y approached. 2. The hardness of a f i l m i s p r o p o r t i o n a l to the p o l y u n s a t u r a t e d a c i d content; a t a constant p o l y u n s a t u r a t e d a c i d content, a change i n the l i n o l e i c - l i n o l e n i c r a t i o produces no appreciable change i n hardness. 3. C o l o r development or a f t e r y e l l o w i n g i s p r o p o r t i o n a l t o the polyunsaturated acid content; t r i e n o i c l i n o l e n i c acid i s f i v e times as potent as dienoic l i n o l e i c acid i n producing yellowing. 4. The presence of conjugated unsaturation up t o a l i m i t of about o n e - h a l f of the t o t a l u n s a t u r a t i o n has a b e n e f i c i a l e f f e c t on the d r y i n g time. However, the u l t i m a t e hardness of the a l k y d f i l m i s not appreciably affected. From these r e l a t i o n s h i p s , i t becomes r e a d i l y apparent t h a t a l k y d s based on soybean o i l g i v e good d r y i n g r a t e s and good c o l o r retention because soybean o i l contains 55-58% polyunsaturated f a t t y acids, of which only 4-6% are t r i e n o i c l i n o l e n i c acid. One of the i n t e r e s t i n g f a c t s about o i l - m o d i f i e d alkyd resins i s that there i s not as much d i f f e r e n c e i n t h e d r y i n g r a t e s of soybean-modified alkyds and linseed-modified alkyds as one would expect on the basis of the drying behavior of the o i l s themselves. This i s because the alkyd r e s i n molecules are s u f f i c i e n t l y polymerized such that a s m a l l amount of c r o s s - l i n k i n g by way of oxidation causes the f i l m to set to a g e l l i k e s t r u c t u r e and appear dry even before the f i n a l oxidation reactions have been completed. The advent of lower cost refined f a t t y acids from t a l l o i l has spurred t h e i r use i n a l k y d r e s i n s . These a c i d s c o n t a i n 43-46% d i e n o i c f a t t y a c i d s , and a l t h o u g h they impart somewhat s l o w e r a i r dry i n a l k y d s than soybean f a t t y a c i d s , they g i v e s l i g h t l y better c o l o r retention and are e s p e c i a l l y u s e f u l i n baking alkyds. Nonoxidizing or p l a s t i c i z i n g alkyds are based on coconut or castor o i l or short chain-saturated acids such as pelargonic, isononanoic, and i s o d e c a n o i c a c i d s . Drying and nondrying o i l s used i n t h e formation of alkyd resins and the f a t t y acid composition of drying o i l s are shown i n Table I I . For r e s i n s of moderate d r y i n g r a t e s and good c o l o r r e t e n t i o n , the standard alkyds are the soybean alkyds. For f a s t e r drying and reduced c o l o r retention requirements, alkyds based on linseed o i l are used. Dehydrated castor o i l i s used for c o l o r - r e t e n t i v e baking alkyds. Tung o i l and o i t i c i c a are o c c a s i o n a l l y used with other o i l s to impart f a s t e r d r y i n g and e a r l i e r hardness. Castor o i l and coconut o i l are used for c o l o r - r e t e n t i v e p l a s t i c i z i n g resins because of the nonoxidizing character of the o i l s . Other o i l s that are used f o r s p e c i a l p r o p e r t i e s a r e f i s h , s u n f l o w e r , walnut seed, and s a f f l o w e r . S a f f l o w e r o i l c o n t a i n s 60-65% l i n o l e i c a c i d w i t h no l i n o l e n i c acid and i s an e x c e l l e n t o i l for preparing resins having a combination of e x c e l l e n t drying properties and c o l o r retention. The f o l l o w i n g compounds are most commonly used for the p o l y o l component of a l k y d r e s i n s : g l y c e r o l , e t h y l e n e g l y c o l , pentae r y t h r i t o l , propylene g l y c o l , d i p e n t a e r y t h r i t o l , neopentyl g l y c o l , s o r b i t o l , diethylene g l y c o l , trimethylolethane, dipropylene g l y c o l , t r i m e t h y l o i propane, and cyclohexanedimethanol. The structures of



Soya Oil

Safflower Oil

—

9

—

Castor Oil

Segregated Cottonseed Fatty Acids

91

(

2

—

3

—

86 3

"6

56 2 39

— 5

—

5-12 195-200* 0.5 250-264 176-187 197-203 7-10 81-90 140-145 20-24 2-4 5 23-26 -10 t o - 15 — 924 968 899*

Coconut Oil

1.2 These

52.5

— —

9.9 36.4

902*

—

197-199* 199 125-130* 5

Tall O i l Fatty Acids (1? Rosin)

NOTE: A l l properties were determined on the o i l except f o r those measurements marked with an a s t e r i k . measurements were determined on the f a t t y a c i d s .

5.5 2.5

T.o

20

22.5 61.5

936

— —

2-4 188-196 155-205 19-21 -20 930

Linseed Oil

3-6 188-194 125-140

Dehydrated Castor O i l

Typical Composition of Important Drying and Nondrying O i l s and Fatty Acids

1-4 0.5-6.0 Acid number Saponification no. 189-195 188-194 120-140 140-150 Iodine no. (Wijs) 20-21 15-18 T i t e r ( C)» Melting point ( C) -20 to -23 -13 t o -18 Density (g/L) 923 923 Fatty acid d i s t r i b . ( J ) * conjugated diene — — nonconjugated diene 52.0 73.2 noneonjugated t r i e n e 4.5 0.1 30.5 20.2 monounsaturated hydroxy monounsaturated — — 13.0 saturated acids 6.5

Property

Table I I .



1188


the l e s s commonly used p o l y o l s are shown i n F i g u r e 3. Of these, g l y c e r o l remains the "workhorse" and i s c l o s e l y f o l l o w e d by p e n t a e r y t h r i t o l . P e n t a e r y t h r i t o l , which has four primary hydroxyl groups, forms more complex resins with p h t h a l i c anhydride than does g l y c e r o l , and i t s r e a c t i v i t y can be reduced, e i t h e r by p a r t i a l replacement with various g l y c o l s ( f u n c t i o n a l i t y of 2), or by the use of l a r g e r p r o p o r t i o n s of f a t t y a c i d s . The high f u n c t i o n a l i t y of p e n t a e r y t h r i t o l i s e s p e c i a l l y useful i n long o i l alkyds containing 6 0 % or more of f a t t y a c i d s , because i t imparts f a s t e r d r y i n g , greater hardness, better g l o s s and g l o s s retention, and better water r e s i s t a n c e than a l k y d s based on g l y c e r o l of equal f a t t y a c i d m o d i f i c a t i o n . S o r b i t o l I i s an i n t e r e s t i n g hexahydric p o l y o l d e r i v e d from the c a t a l y t i c hydrogénation of g l u c o s e . Under t h e c o n d i t i o n s of e s t e r i f i c a t i o n , i t undergoes i n t r a m o l e c u l a r e t h e r i f i c a t i o n t o form s o r b i t a n I I , and t o a l e s s e r e x t e n t , sorbides. This p o l y o l and other p o l y o l s such as trimethylolethane, trimethylolpropane, d i p e n t a e r y t h r i t o l , and cyclohexanedimethanol are used to impart s p e c i a l properties to alkyd resins. CH20H

CH2

i CHOH

I CHOH J

H+ heat

CHOH

I CHOH

IH20H Sorbitol

Sorbitan

I II The t h i r d basic component of alkyd resins i s the polybasic acid. By f a r , the most important i s p h t h a l i c anhydride. The i n c r e a s e d a v a i l a b i l i t y of i s o p h t h a l i c acid at an a t t r a c t i v e p r i c e has r e s u l t e d i n the i n c r e a s e d use of t h i s d i b a s i c a c i d . T h i s m a t e r i a l , u n l i k e p h t h a l i c anhydride, w i l l not form intramolecular c y c l i c structures, and i t gives higher molecular weights and higher v i s c o s i t i e s . When 1,2,4-trimethylbenzene i s o x i d i z e d , t r i m e l l i t i c anhydride i s obtained ( a l s o c o m m e r c i a l l y a v a i l a b l e ) . T h i s i s an i n t e r e s t i n g t r i b a s i c acid because the anhydride portion of the molecule can be r e a c t e d t o form a p o l y m e r i c s t r u c t u r e , which l e a v e s one c a r b o x y l a v a i l a b l e for forming an ammonium or amine s a l t that imparts water s o l u b i l i t y . This reaction w i l l be covered i n greater d e t a i l l a t e r i n t h i s chapter. When f i l m f l e x i b i l i t y and p l a s t i c i z a t i o n by an a l k y d a r e r e q u i r e d , d i b a s i c a c i d s such as a d i p i c , a z e l a i c , and s e b a c i c a c i d s and d i m e r i z e d f a t t y a c i d s a r e used. When f i r e retardant properties are required i n coatings, chlorinated polybasic acids, such as t e t r a c h l o r o p h t h a l i c anhydride or chlorendic anhydride with s i x c h l o r i n e atoms i n the molecule, are used. Other polybasic a c i d s f r e q u e n t l y used i n s m a l l e r q u a n t i t i e s i n a l k y d r e s i n s a r e f u m a r i c a c i d and m a l e i c a n h y d r i d e . These a r e d i b a s i c a c i d reactants, but they a l s o contain a double bond that may react with unsaturated molecules such as unsaturated f a t t y acids, r o s i n , and terpenes to form d i - and t r i f u n c t i o n a l reactants as the case may be. In Applied Polymer Science; Tess, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


49.

LANSON

ψ£>Κ «jHOH),

1189

A Ikyd and Saturated Reactive Polyesters

(JHjOH CH(j:CH,OH

CHOH CHCH,H CH.Ό H CH OH sorbitol trimethylolpropane trimethylolethane dipentaerythritol 3

Chemistry and Technology of Alkyd and Saturated Reactive Polyester

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