Oxygen absorption and drying studies on blends of fatty acids for development of alkyd resin paint binders

Thesis Overview

<p> INTRODUCTION<br>1 COMPOSITION OF A PAINT<br>A paint can be defined a s l a r g e l y organic coating<br>applied t o surfaces t o provide both p r o t e c t i v e and<br>decorative functions.’ It is usually a suspension of a<br>s o l i d o r s o l i d s in a l i q u i d which is applied wet t o a<br>surface but, eventually, it d r i e s t o a more or l e s s<br>opaque adhering s o l i d film. 2<br>The basic components of a paint are:<br>( i ) binders – these are resins, drying (highly 1<br>unsaturated) o i l s , o r r e s i n s modified by such o i l s .<br>Originally l i q u i d s o r semi-solids i n nature, the binders<br>convert t o s o l i d s through the p a i n t ‘ s drying process and<br>thereby provide t h e surface films with the necessary<br>a t t r i b u t e s of adhesion, f l e x i b i l i t y , toughness and<br>d u r a b i l i t y . In the transformation process, binders bind<br>up together the other i n g r e d i e n t s , with the exception of<br>the v o l a t i l e components, of the paint,<br>( i i ) pigments – these a r e f i n e l y dispersed s o l i d<br>materials t h a t determine the colour and opacity of the<br>p a i n t f i l m , and hence of t h e surface t o which it is<br>applied. C e r t a i n t y p e s of pigments s p e c i f i c a l l y a c t by<br>improving film d u r a b i l i t y o r providing corrosion r e s i s t a n c e<br>f o r metal substrates.<br>( i i i ) v o l a t i l e solvents – these enable the application<br>of the paint. Being v o l a t i l e , these evaporate a f t e r a<br>l i q u i d f i l m is deposited, and t h e evaporation causes<br>s o l i d i f i c a t i o n of t h e film,<br>( i v ) other components, which may be termed a n c i l l i a r y<br>t o both binders and pigments, include extenders, d r i e r s<br>and fungicides etc. M e n d e r s a r e cost-reducing<br>ingredients t h a t help to control gloss, t e x t u r e ,<br>suspension, v i s c o s i t y , e t c . Fungicides i n h i b i t mo$d<br>growth on the film’s surface during service exposure.<br>Driers, as the name suggests, control the drying o r<br>curing process of the liquid paint.<br>1.2 THE CHEMICAL NATURE OF VEGETABLE OILS<br>1.2.1 General Definition 394<br>Vegetable o i l s are water-insoluble substances of<br>p l a n t o r i g i n which c o n s i s t predominantly of glyceryl<br>e s t e r s of long-chain f a t t y acids. They are most commonly<br>c a l l e d trielycerides. Common usage considers as o i l s<br>t r i g l y c e r i d e s t h a t a r e l i q u i d a t room temperature and<br>a s ‘fats1 those t h a t are solid o r semi-solid under t h e<br>same conditions. This difference i n t h e i r physical<br>s t a t e a r i s e s from their chemical composition:<br>Fats are composed of high-melting f a t t y a c i d s (mostly<br>saturated) while o i l s are formed from low-melting f a t t y<br>~id(smo st ly uns a tur a t ed) . At hi pher temp~ratures,<br>however, t h i s difference disappears because the f a t s<br>nelt t o become liquid. For this reason, the word I’oll”<br>in t h e expression ”fats and o i l s ” i s understood t o mean<br>the same kind of material as f a t .<br>The chief importance of vegetable o i l s l i e s i n t h e i r<br>food value. They arc v i t a l ingredients of a balanced<br>d i e t ; they can y i e l d approximately k~o/f ~b i o l o g i c a l<br>1<br>energy compared wi th 77’~ JfJo r~ car bohydrate and<br>protein. Pesides t h e i r use as foods, vegetable o i l s are<br>raw materials f o r making soaps and detergents, paints,<br>varnishes, l u b r i c a n t s and p l a s t i c s .<br>1.2.2 Structure of Vegetable C i l s 4-8<br>A vegetable o i l molecule a s a t r i g l y c e r i d e may be<br>considered a s r e s u l t i n g from the reaction of a molecule<br>of glycerol with three molecules of f a t t y acids whereby<br>three molecules of water are l i b e r a t e d a s by-products.<br>The f a t t y acids have the geceral formula CnH2n02 or<br>C n-1 H2n-l COOH (where n is an even number varying<br>between 4 and 24). Glycerol is a t r i h y d r i c alcohol<br>having t h e s t r u c t u r a l formula (1).<br>1<br>H – C -OH<br>The formation of a t r i g l y c e r i d c is represented by<br>the general equation:<br>CHO iH + HO~OCR~ -) B CHOOCR2 + 3H20 1<br>! ! 1<br>R1, R2 and R3 stand f o r hydrocarton chains of f a t t y acids.<br>They a r e designated by d i f f e r e n t numbers t o i n d i c a t e t h a t<br>usually there is more than one kind of f a t t y acid chain<br>i n an o i l molecule. A t r i g l y c e r i d e is known a s simple<br>t r i g l y c e r i d e i f a l l the f a t t y acids are i d e n t i c a l cog.<br>t r i s t e a r i n (2) and as mixed t r i g l y c e r i d t a s i n<br>distearin (3).<br>Generally, n a t u r a l l y occuring t r i g l y c e r i d e s are mixed and<br>contain only small percentage of simple triglycerid es.<br>It i s believed t h a t the f a t t y acids are d i s t r i b u t e d among<br>t h e d i f f e r e n t glyceride molecules i n accordance with the<br>p r i n c i p l e of even d i s t r i b u t i o n which requires t h a t each<br>f a t t y acid should be d i s t r i b u t e d i n a s many t r i g l y c e r i d e s<br>a s possible. For example, i f an o i l contains one-third<br>oleic and two-third s t e a r i c acid the o i l molecule may<br>have the s t r u c t u r e (3). A glyceride molecule such a s (3)<br>1<br>in which only the p o r second f a t t y acid r a d i c a l is<br>d i f f e r e n t is regarded a s symmetrical. If a l l the f a t t y<br>acid r a d i c a l s are d i f f e r e n t , t h e glyceride i s said t o be<br>assymetrical.<br>In r e a l i t y t h e s t r u c t u r a l representation of a<br>t r i g l y c e r i d e molecule as given above is impossible<br>because it implies t h a t the e n t i r e molecule is i n the plme<br>of the paper. I f t h a t i s t h e c a s e a considerable s t r a i n<br>would r e s u l t in the o i l molecule. Instead a t r i g l y c e r i d e<br>molecule has a three-dimensional (or t h r e e d i r e c t i o n a l )<br>the<br>s t r u c t u r e due to&amp;ossibility of f r e e r o t a t i o n along the<br>carbon axis of the glycerol residue. Furthermore the<br>f a t t y acids are thought to be highly zig-zag chains (4)<br>with the carbon-carbon bond forming a 109′ bond angle.<br>The s t r a i g h t l i n e s represent the glycerol group.<br>Since there are three o r more fatty-acid r a d i c a l s<br>occuring i n a p a r t i c u l a r f a t o r o i l , the p o s s i b i l i t i e s<br>of isomerism are numerous. The number of possible<br>t r i g l y c e r i d e s , N, which can be formed from X d i f f e r e n t<br>f a t t y acids i s given by equation 1.2. 4<br>1.3 COMPCSITIQN OF VZGTTAl3,E OILS 5.7-15<br>Although f a t s and o i l s are predominantly triglycerides<br>(which c o n s t i t u t e 95 t o 9%), t h e r e a r e a number of minor<br>components which a r e present i n the n a t u r a l l y occuring<br>f a t s and o i l s . These include phospholipids (or<br>phosphatides) (1 t o yh), s t e r o l s , antioxidants, vitamins,<br>pigments, f r e e f a t t y acids and some impurities. These<br>components a f f e c t t h e colour, odour, and other q u a l i t i e s<br>of the o i l .<br>1 . 3 The Phospholipids (or ~hosphatides)<br>Phospholipids also hown as “gums” are f a t t y<br>substances in o i l s containing phosphorus. There are two<br>types:<br>(a) qly~erophospho1ipids:- these are compounds which<br>are derived from triglycerides in which one f a t t y acid<br>has been replaced by a phosphoric acid or phosphoric acid<br>derivative. Examples are l e c i t h i n (4) and cephalin ( 5 ) .<br>I n l e c i t h i n s , t h e base i s c h o l i n e (HOCH~CaHnd~ f~o r~ ~)<br>cep h a l i n s , ethanolamine (HOCH~CH~NHCr~ud)e. soyabcan<br>o i l contains 2 t o 3% l e c i t h i n s . Lecithin and cephalin<br>are frequently associated with membranes.<br>( b) sphingomyelins: – these are phospholipids which are<br>derived from an alcohol other than glycerol, c a l l e d<br>sphingenine (formerly sphingosine) whose s t r u c t u r e is<br>shown (6). This alcohol contains nitrogen and forms<br>bonds with other compounds which are unlike those formed<br>between glycerol and the fatty acids. Sphingenine can be<br>bound to:<br>– a f a t t y acid and phosphoric acid which is i n turn<br>combined with choline t o form sphingomyelin (7).<br>– a f a t t y acid (usually a very long one, 24 carbon atoms)<br>and one carbohydrate molecule such as galactose, glucose<br>or amino sugar, to form cerebroside (8). The cerebrosides<br>can be e s t e r i f i e d with sulphuric acid t o form sulphatides.<br>Phospholipids have been termed amphipathic compounds since<br>they possess both polar and nonpolar functions.<br>RCONHCH<br>It +<br>H~C-O-P-OCH~CH~N(HC ~)<br>t<br>1.3.2 The Sterols ( Steroid alcohols) 1<br>These are colourless, odourless and generally i n e r t<br>substances found in vegetable o i l s and f a t s . They are<br>c r y s t a l l i n e alcohols possessing 26 – 30 carbon atoms.<br>They are based on phenanthrene s t r u c t u r e (9).<br>The s t e r o l s account for 0.5 – 1.5% nonsaponifiable<br>materials in both vegetable and animal f a t s . An example<br>of s t e r o l which occurs in vegetable o i l is stigmasterol (10)<br>which d i f f e r s from c h o l e s t e r o l (which occurs i n a l l<br>animal t i s s u e s ) only i n having a double bond<br>between carbons 22 and 23,<br>1<br>I , 3, 3 Antioxidants<br>Most vegetable o i l s contain minor prnportions<br>(0.05 – [email protected]&amp;) of antioxidants which serve t o i n h i b i t o r<br>delay atmospheric oxidation a s well a s peroxide formation<br>which causes r a n c i d i t y i n f a t s , Rancidity is marked by<br>presence of v o l a t i l e , bad-smellinp acids and aldehydes i n<br>t h e o i l . The antioxidants i n vegetable o i l s have been<br>i d e n t i f i e d mostly as tocopherols (1 1 ).<br>1.3.4 Vitamins<br>A number of vitamins, namely vitamins A, K, D and E<br>a r e f a t soluble and because some of them a r e found i n<br>f a t s and o i l s , they a r e included i n t h e l i p i d c l a s s of<br>macromole~ules. The vitamin E owes its a c t i v i t y t o i t s<br>tocopherol c o n t e n t . Vitamin 11 ( 12) is produced by t h e<br>a c t i o n of water on t h e c a r o t e n e s ( t h e p r e c u r s o r s of<br>vitamin A) which occur i n unbleached palm o i l and i n<br>t r a c e s i n o t h e r o i l s . It i s l o s t i n t h e r e f i n e d cooking<br>1<br>o i l due t o bleaching.<br>1.3.5 Pigments<br>These substances a r e r e s p o n s i b l e f o r t h e<br>c h a r a c t e r i s t i c c o l o u r s of o i l s . The deep red colqur of<br>palm o i l is due t o presence of 0.1 t o 0.2% of g-carotene<br>(13). The c a r o t e n e s a r e highly unsaturated and owe t h e i r<br>colour t o a long conjugated system of double bmds.<br>Olive o i l and soyabean o i l may c o n t a i n s u f f i c i e n t<br>chlorophyll or r e l a t e d compounds t o produce a greenish<br>tinge.<br>I. 3.6 Free Fatty Acids<br>The f r e e f a t t y acid content of a crude o i l is<br>lsually dependent upon the degree t o which t h e o i l has<br>Deen subjected t o enzymatic hydrolysis i n t h e parent o i l –<br>~earings eed be for e e x t r a c t i o n . Rubberseed o i l i s high’<br>In f r e e f a t t y acids. This has been r e l a t e d to the action<br>)f t h e lipolytic enzyme present i n the seeds.<br>1.4 THE NATURE OF THZ FATTY ACIDS PRESENT I N VEGETAELE<br>-01~ p9 s,I~1<br>With only a few exceptions, the f a t t y acids a r e a l l<br>straight-chain compounds, ranging from three t o eichteen<br>carbons and except f o r the C? and Cg compounds, only acids<br>containing an even number of carbons a r e present i n<br>s u b s t a n t i a l amounts, Those with sixteen and eighteen<br>carbon atoms a r e the most abundant. In addition t o<br>v a r i a t i o n i n chain length the f a t t y acids present i n<br>vegetable o i l s can vary i n the number of C=C double bonds<br>i f any (degree of unsaturation), t h e r e l a t i v e position<br>of the double bonds (degree of con jugation) and the<br>presence of polar groups such as hydroxyl o r keto group<br>a s well as methyl-group branches on the carbon backbone.<br>I n general, unsaturated f a t t y acids (the ones which<br>contain the C=C double bonds) are twice as abundant a s<br>saturated f a t t y acids i n f a t s arid o i l s from both p l a n t s<br>and animals.<br>The properties of a p a r t i c u l a r o i l can be d i r e c t l y<br>related to the f a t t y acid composition and t o a l e s s e r<br>extent properties depend upor. the M, g or V position of<br>1<br>the attachment. Since the amount of glycerol is the same<br>in a l l vegetable o i l s , it follows t h a t the differences in<br>properties encountered w i t h the d i f f e r e n t o i l s a r e l a r g e l y<br>determined by the variations i n the f a t t y acid structure.<br>The structure and physic21 p r o p e r t i e s of some of the more<br>important f a t t y acids present i n vegetable o i l s are given<br>i n Tables 1.1 and 7.2 respectively.<br>‘able 1.1 Structures of Some Fatty Acids Found in<br>Vegetable ~ils~~~~-~~<br>Double<br>EiF<br>,auric acid 0<br>lyri s t i c acid 0<br>‘almitic acid 0<br>Xearic acid 0<br>Jnsaturated<br>Xeic<br>Jnoleic<br>&gt;inolenic<br>iicinoleic<br>Licanic<br>I samic<br>Structure<br>3 C 11 -C li -C H=C H-C H -C H=C H-C H2-C H=C 11-<br>3 2 2<br>3 CI-! 3 -(CH ) -CH=CH-CH-CH-CH=CH- 2 3<br>(cH~)?- CCOH<br>3 CH 3 -(CH ) -CH=CH-CH=CH-CH=CH-(CH~)~- 2 3<br>CO-(CH~)~-COOH<br>1 CH2=CH(C~2)4C~C-C~C2( )C f~ OOH<br>Table 1.2 Physical P r o p e r t i e s of Some Pure Fatty Acids 3 —<br>Molecular Nolecular Iodine<br>Acid formula weight 2:;?T0C ) value<br>Lauric I 2H2402 200.3 44.2 0<br>Palmitic I 6H3202 256.4 67.1 0<br>S t e a r i c C18H3602 284.5 69.6 0<br>Oleic 18~34’2 282.5 16.0 89.9<br>Linoleic<br>Linolenic 1 8H3002 278.4 -11.3 273.5<br>Ricinoleic 1 6H3402 298.5 5.0 85.0<br>obEleostearic 1 8H3002 278.4 49 273.5<br>The p o s i t i o n of double bonds and s u b s t i t u e n t s i n a<br>f a t t y acid chain i s defined by numbering t h e chain from<br>t h e carbonyl carbon. I n most of the unsaturated f a t t y<br>a c i d s t h e r e i s a d o u b l e bond ( d e s i g n a t e d A 9 ) between<br>carbon atoms 9 and 10. I f t h e r e a r e a d d i t i o n a l double bonds,<br>they usually occur between a9 double bond and t h e methylterminal<br>end of t h e chain, The double bonds of n e a r l y a l l<br>t h e n a t u r a l l y occuring unsaturated f a t t y a c i d s a r e i n t h e<br>c i s geometrical c o n f i g u r a t i o n , which produces a r i g i d bend<br>i n the a l i p h a t i c chain. By c i s configuration it is<br>meant t h a t the two hydrogen atoms adjacent t o t h e bond l i e<br>on the same side (14). The unusual trans-acids have t h e<br>opposite configuration (15) :<br>1<br>Cleic acid is t h e most widespread and abundant of a l l<br>f a t t y acids accounting f o r some 40% of the t o t a l<br>accumulation i n a l l n a t u r a l f a t s . It i s a cis-monounsaturated<br>CIR acid w i t h the double bond i n the mid<br>(9, 70) position. Linolenic and elensteari c acids contain<br>the same number of double bonds, but those of e l e o s t e a r i c<br>acid are i n the con jugsted position and are much more<br>reactive. Both oleic and r i c i n o l e i c acids contain a<br>single double bond each but t h e l a t t e r has a hydroxy<br>group, a s a r e s u l t of whick it can under c e r t a i n<br>conditions undergo dehydration ( t h e removal of OH and an<br>adjacent H) giving approximately 25% conjugated and 75%<br>noncon jugated double bonds. Licanic a c i d has a keto<br>Rroup in the chain but is otherwise the same as<br>e l e o s t e a r i c acid i n t h a t it contai.ns three conjugated<br>louble bonds. Isamic acid is an example of f a t t y acid of<br>musual structure: it contains conjugated t r i p l e bonds.<br>The melting point of a p a r t i c u l a r f a t t y acid is<br>lependent upon i t s molecular weight a s well as number and<br>:onfiguration of the double bonds: stearic acid (mol. wt.,<br>284.5) has a higher melting point than palmitic acid<br>:mole w t . , 256.4) but the l a t t e r has a higher melting<br>~ointth an o l e i c (mol. w t . , 282.5).<br>Aspreviously s t a t e d , t h e c h a r a c t e r i s t i c s o f a ,<br>)articular o i l or f a t w i l l depend on the amount of each<br>3f the acids present. Typical f a t t y acid compositions of<br>some important vegetable o i l s and the e f f e c t s of t h e i r<br>&gt;omposition and dryin? characteri stics are l i s t e d in<br>Table 1.3. The c l a s s i f i c a t i o n of the o i l s as drying,<br>semi-drying or nondrying i s dependent on the percentage<br>of unsaturated f a t t y acids in the respective o i l . The<br>r a t e a t which drying occurs tends t o increase as degree<br>of unsaturation increases, Hence, o i l s of mainly linolenic<br>acid dry f a s t e r than those of l i n o l e i c .acid. However,<br>the geometry of unsaturation as determined by ccrn jugation<br>or non-con jugation of the carbon-carbon double bonds also<br>contributes t o the drying c h a r a c t e r i s t i c s of an o i l .<br>Consequently, tung o i l containing mainly e l e o s t e a r i c acid<br>d r i e s f a s t e r than linseed o i l in which linnlenic acid<br>predominates, It w i l l be noted from Table 1.3 t h a t<br>c a s t o r o i l c o n s i s t s l a r g e l y of r i c i n o l e i c acid, It is<br>thus one of the few f a t t y o i l s approaching a pure<br>compound i n character. Dehydrated c a s t o r o i l , i n<br>c o n t r a s t t o c a s t o r o i l , i s a valuable drying o i l ,<br>o i l s<br>The dryingLare so called because when t h i n f i l m s a r e<br>exposed t o a i r , as in painting, they undergo autoxidation<br>followed by polymerization to a hard, resinous coating.<br>Table 1.3 Fatty Acid Composition of Some Vegetable O i l s <br></p>