proposal - "mechanism of drier action"

Mechanism of Drier Actio

A Proposal and Panel Discussion presented before the Division of Paint, Plastics, and Printing Ink Chemistry at tlie 124th Meeting of the ACS, Chicago, Ill., John K. Wise, presiding

THIS discussion departs from the usual presentation of cxperi- era1 mechanism is fairly well accepted. The prinripal arti- mental evidence, the results of original research, and subordi- cle in this discussion considers those chemical reactions iiivolwtl nates experimental evidence to theory and perhaps to speeu- in the drying of oils and proposes a mechanism to explain ho~v lation. oil-soluble metal compounds affect) these reactions. I2our ~ x -

The subject is a very controversial one in the field of paint and perts in the field comment on and evaluate this proposal. Thi.: varnish chemistry-how do oil-soluble metal salts accelerate discussion is not expected to eliminate all cont,ro the oxidation and polymerization of unsat’urated triglycerides? cerning drier action, but, it is designed to spotlight arm.. of UTI-

In the last 10 years, much good Twrk has been done on tlie chem- certainty a,nd to point the way for future research. JOHN K. WISE ical reactions involved in the drying of oils. Today, the geri-

E. R. 3IUELLER Organic Coatings Division, Battelle Memorial Insti tute, Columbus, Ohio

NY investigators have studied the drying of oils and R1’ air-drying finishes. The effect, of factors--e.g., heat, oxygen, light, a.nd catalysts-on the conversion of an oil film to R

“set” condition, has been ext,ensively investigated. Most of t,hk work has dealt with the effects of heating the oils or est,ers of fatty acids t o relatively high temperatures (260’ to 320’ C.) or osidiz- ing them a t temperatures ranging from room tempcraturc or slightly above to 130’ C.

Investigations concerning the efYect of catalysts, for the most part, have been almost entirely empirical. JVhile a lot of ill- formation is now available on the effect of these “accelerators,” mch as the common drier metals, on the air-drying process, the precise manner in Thich these materials function is still a subject of considerable controversy. I n the organic coatings industry, great strides have been made during the last 25 years in modifying the common vegetable oils to improve their over-all properties, including drying rate. Of particular interest are the quick- drying varnishes of various types made from these oils. Jl‘hile most, varnishes dry much faster than the oils, the process of air drying is generally considered to be affected very little, if a t all, by the resin component, regardless of the type of resin (4bj. For the sake of simplicity, this paper, therefore, nil1 deal only with the role of the drier metals in the oxidative-polymerization of oil films.

In order to better understand how catalysts or accelerators aid in the drying process, it is essential to consider first the processes of polymerization and drying in the absence of these accelerators.

PROCESSES IIVVOLVED DURING DRYING

The physical and chemical processes operative in film formation have been extensively investigated by many workers, both in this country and abroad. A good historical review has been

given by Elm ( 1 9 ; . However, little real progress Tq-as made until after the first \Yorltl Il’ar. Most useful information has appeared since about 1930. Since then, a large number of investigators have added greatly to our knowledge of the thickening, gelling, and set,ting of oils, Colloid chemists made valuable contributions which helped t,o explain the process of film format’ion. It remained for the high polymer chemists, however, to shed the most useful light on this highly intricat,e and complex subject during the last 15 to 20 years.

POI.YlfERIZATIOS BY HEAT. In order t o understand more fully the oxidative polynierizat,ion process and the role of t,lie driers in it, the heat-induced polymerization of oils should be considered. Recently, there have appeared several excellent rcviem on this subject, (IO, 12, 67, 68). It has been rather clearly demonstrated by these and other investigators (15) that, three- dimensional polymers must be formed before gelation can occur. Therefore, it must be assumed that this is what happens when films are baked or force dried. It has been shown by many Lvorkers that such a three-dimensional structure may be derived in several a-ays. hIost applicable in this discussion arc mtcr interchange of higher and lower molecular weight polymers arid cross linking a t the unsaturat,ed centers.

Figure 1 depicts two hypothetical synthetic oil molecules which simply show the structure of several fatty acids attached to polyols. By cross linking of two fat’ty acids, one from each of tn-o such niolecules or similar molecules, one dimer fatty acid already forms a double-eized molecule. Ester interchange occurring in h o such or similar molecules would form a four-unit structure. By cross linking a t the unsaturated centers, it is generally believed that more or less cyclic polymers are formed, depending on the degree of unsaturation of the fatty acids involved.

This work was started almost a centu

562

March 1954 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 563

Olsic, cis - Eleostearic, trans.CIS,trons

0 H H H H

A A A A A l i A Polnilic Ltnolenlc, cis,cis, trans

Penfoeryfhritai (tetrafunctional) ester of four types of neturol oclds

-

RcinOieiZ,cis Liconic, cis,Irons.cis

Llneor (nondrying) QIYCOI ester of two exceptional naturoi oclds

Figure 1. Exemplary Types of Natural Fatty acid Esters

Figure 2 is a diagram from Wheeler (58). It shows a dimer formed by the action of heat upon a normal linoleate and a thermally conjugated linoleate to form a substituted cyclohexene ring. It should be emphasized that the work of Bradley (13) and others has clearly demonstrated that, regardless of the degree of unsaturation of the fatty acids involved, the polybasic acids formed during heat polymerization are largely dimers, with very small amounts of trimer, and practically no tetramer. Thus, the molecular weights of the drying oil polymers are not a t all large when compared to high molecular weight polymers, such as polyethj-lene. Wheeler (57) has elegantly summarized the work of Bradley and Johnston (11 ) on the polymerized products of methyl esters of various fatty acids. I n this connection, these facts concerning the heat bodying of oils seem pertinent to this discussion:

Isomerization to the conjugated form occurs in unconjugated oil> prior to polymerization.

Side reactions occur and create esters of low molecular weight which may combine with monomers to form polymers of intermediate molecular weight.

Functionality of the various fatty acids during heat polymerization will vary with the amount of more highly unsaturated fatty acid. present. Wheeler, Elm, and others have shown that the functionality of oleate will vary from about zero to something l e v than one in a 1 : 1 mixture of oleate and linoleate.

OXYGEN-IYDUCED POLYMERIZ~TIOK. Several excellent re- views on the oxidative polymerization of drying oils have been recently published (19, 46, 82, 57) . Khile the manner by which oils polymerize in the presence of oxygen a t room and slightly elevated temperatures is apparently considerably more complex than theii polymerization by heat, the two processes have many things in common. Many of the more important reactions involved in oxidative polymerization are now well established, although the exact mechanism of oxidative polymerization is still rather controversial. Bn abundance of literature on the subject dates back much further than the literature on heat polymerization. Much of i t is speculative and often contradictory, making it difficult to obtain a clear-cut picture of the process. Still in all, good agreement has been reached in recent years by vaiious investigators on the subject, particularly with respect to the posi- tion and manner of the entry of oxygen into an unsaturated oil or hydrocarbon molecule in the initial stages of oxidation (6, 8, 9, 14, 16, 20, 21, 23-80, 34, 36, 37,41 , 49,64,66, 69). This is a very important consideration when attempting to explain the mechanism of diier action.

In 1942 Farmer and his associates worked primarily with rubber hydrocarbons and methyl esters of unsaturated fatty acids. They theorized that oxygen enters the unsaturated hydrocarbon a t a carbon atom a t o the double bond in unconjugated systems. They observed that, in the initial stages of oxidation,

there was apparently no reduction in unsaturation. They demonstrated, also, that oxygen was taken up in molecular form to produce the monohydroperoxide. As oxidation continued, unsaturation decreased, viscosity increased, and peroxide values fell to a low level. Relatively high molecular weight oxygenated polymers, as well as intermediate molecular weight polymers, were formed. Moreover, compounds lower in molecular weight than the starting materials, as well AS considerable unchanged monomer, were detected in the distilled products. It thus appeared that Oxidation proceeds down an already oxidized molecule and that chain scission occurs.

I n the case of conjugated hydrocarbons or fatty acids, oxygen is conceded by these authors to enter a t the double bond. A number of possible structures were postulated, all of which, together with their breakdown products, could affect the completely gelled or oxidized oil.

Apparently, several types of reactions occur simultaneously and consecutively, depending on such factors as heat, light, and possibly catalysts. This seems to be particularly true with re- spert to changes in temperature (35) . Such factors as viscosity and surface exposed would seem to affect primarily the rate of the reactions occurring, although it has been shown (IS) that thick films gel a t a lower percentage of oxygen than do thinner ones. This does not necessarily indicate that the mechanisms of polymerization are significantly different, however.

Ccl.

(CH$7 COOCH3 COOCH3

LINOLEATE 10,12, CONJUGATED, + 9, 12, NORMAL, - DIMER

LINOLEATE

Figure 2. Heat Dimerization of Ethyl Linoleate

Wheeler (58)

It is possible that the mechanism of polymerization may vary somewhat, depending on the type of coating being studied. It has been shown that the same drier metals may induce crystalliza- tion in some types of coatings and apparently retard i t in others ( 4 ) .

While the exact compositions of all polymers formed under varying conditions of oxidation of unsaturated oils have not been positively identified, many compounds have been isolated, and they shed considerable light on the course of the reactions. The various fatty acids show the same relative degree of functionality in oxidation polymeriea- tion that they do in heat polymerization. However, their functionality in the former will usually be slightly greater. -4s in heat polymerization, the induced conjugation of unconjugated groups usually precedes polymerization (2, 47, 57). Again, the functionality of the oleate is considerably enhanced by small amounts of more highly unsaturated fatty acid esters, such as linoleate (36, 88). It is postulated that the autoxidation of oleate is catalyzed by the hydroperoxide resulting from the union of oxygen with the linoleate.

Significantly, i t has been demonstrated that the alcohol attached to the fatty acid chains has no apparent effect upon the oxidation mechanism of the fatty acids (15,51).

Numerous investigators have shown that a large number of oxygenated compounds or derivatives are formed during autoxidation. These products will vary both with respect to the starting substances and the conditions prevailing. Some of these products of the

TYPES OF POLYMERS.

OXYGENATED COMPOUNDS AND DERIVATIVES.

564 I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY Vol. 46, No. 3

TABLE I. ORGANIC O X Y G E ~ %TED STRCCTURCS 0

' I

0 I /

Aldehydes R-C-H

Organic acids

Cyclic peroxides

Epoxides

Esters

Ketones

Peracids

~ - 6 - 0 ~

I 1 0-0

R-C-C-R

or

R-C-C-R

/ I 'd 0

R-C-CH:,

'4 K-C-0-R

I1

'I

II

0

0

R-C-R

0

R-C-0-OH

OH

Hydroxyls R-C-R

OOH

Hydroperoxides R&=C--F:

oxjdative process are shown in Table I. Certain of these products, part,icularly t,he aldehydes formed during the autoxidation of cottonseed oil, have been identified and t,heir source established (55). 811 of these structural products are capable of entering into further reactions. The course of these subsequent reactions, may depend, in a large measure, on t,he reaction conditions and mat,e- rials present-e.g., wat,er and driers.

OXIWATIOK PROCESS

PoTvers (52 ) arbitrarily divides the &ages in the oxidation of drying ails into four main steps-viz., inhibition, peroxide forina- tion, peroxide decomposition, and polymerization. Figure 3, taken from this article, while admittedly purely arbitrary and varying with conditions of materials employed, forms the basis for the following discussion, including the role of the driers.

IKHIBITION. During the induction period, no significant, amount of oxygen is absorbed by the oil. Coincident with the first detectable absorption of oxygen, peroxides begin t80 form Inhibition is generally at,tributed to the presence of antioxidants, such as the tocopherols in natural oils, although this is often hard to verify (43). Moreover, other factors, not too well understood, may also be responsible. The general observations on recon- stituted oils shoTy no induction period. Evidence exists, however, that highly purified, unsaturated fatty acids may have a relatively long induction period at temperat,ures somewhat above room temperature, while both rosin and purified a b i d e acid have a comparatively short induction period. Catalysts, such as the drier metals, particularly cobalt, greatly shorten this induction period and often eliminate i t entirely.

PEROXIDE FORMATION AND DECOXPOSITION. The peroxide format,ion and decomposition stages in the oxidative process are

best discussed together. IVhen comparing ox\.~en-indiic~:.rl polymerization n-ith heat polymerizat,ion, reference was matlr the fact that most workers are novi in agreement with the theoi,. that molecular oxygen first adds to the oil a t the douhle bond iii

conjugated systems to form products, such as cyclic perosidc..

--C-C--. I t is now generally agreed that oxygen first adds at :I

0-0

I 1 ! I

double bond in uiiconjugated systems as ell as in conjugateil ones. However, after the oxygen adds a t a double bond, thc double bond shifts to produce an a-methylenic hydroperoxide,, as shown in Figure 4. Gunstone and Hilditch (M), while ~vorkiiig with emall amounts of methyl linoleate in methyl olmte, were perhaps the first t,o propose this mechanism of oxygeij addition iri unconjugated systems. In the same year, Farmer ( 2 2 ) a i v accepted this vien., which is contrary to his earlier views (20: t'hat perosidation begins a t an e-methylenic carbon in UII-.

conjugated systems. Gibson (34) also presents evidence thxt t,he nen- concept of oxygen initially adding a t the double bond is valid. He substantiates this with a good theoretical discussion that hydroperoxides are formed by a combined mechanism be- tween the -C=C- double bonds and e-methglenic gi'oiips (-CH$-) to initiate a chain reaction.

OXYGEN IN OIL

TIME-

Figure 3. Stages of Oxidative Polymerization Powers (52)

Farmer's earlier view (20) (hydroperoxidat'ion) a t an 01-'

niethylenic carbon, while once fairly well substantiated b y the kinetic studies of Bolland and Gee ( 9 ) , seems t o have been fairly well disproved. Hotvever, very recently, Allen and Xummerow (3) show evidence t'hat this original mechanism may be valid in some cases. It seems entirely possible that bot'h mechanisms may operate, depending on conditions of oxidation and the starting materials, including driers.

The rate of peroxide development is generally much faster \ ~ i t h catalysts present. Hov-ever, m-ithout catalysts, much higher peroxide levels occur. With oils that have an induction period, the peroxide level usually builds up faster initially with driers. Undoubtedly, this happens because driers shorten the induction period, and peroxides are about the first products of oxidat'ion.

However, in oils or fatty acids having no perceptible induction period, the peroxide levels are initially about the same with or without driers, and then the peroxide level of the samples with drier falls off sharply apparently as polymerization sets in far in advance of final total oxygen uptake. Jackson and Rummeroiv (40) studied che effect of zinc, manganese, and cobalt driers on t,he autoxidation of unconjugated linoleic acid a t 35", 6 j o , and 90" C. They measured the peroxide values obtained as a func-


H I

I R-CH.CH-C-CH*CH-R ' + 02 -

n

tiori of time, as well as the specific ultraviolet absorption coefficient.

Figure 5, taken from this work of Jackson and Kummerow shows the peroxide levels obtained in both conjugated and unconjugated linoleic acids with and without cobalt drier a t 65" C. At this temperature, the peroxide values rise a t about the same rate initially with or without drier This was even more pronounced at 90" c.

W I

R-CH-CH-C-CHsCH-R' I * { oo%-- H

- R-CH-CHaCH-CH-CH-R' I

DOH

R-CH-CH-CH-CHsCH-R' . . I OOH

In Figure 6,. the corresponding data are given for these same materials oxidized a t 30" C.--about room temperature. Here the peroxide values rise much more slowly without drier, but much more rapidly attain a higher level both in the conjugated and unconjugated acids with drier. In the case of the conjugated acids, the curves cross after about 70 hours and level off after about 120 hours. It would be interesting to know what the "set" time of such oil films would be.

800 I I I I I I I I

TIME OF OXIDATION IN HOURS

Figure 5 . Effect of Cobalt Naphthenate Drier on Peroxide Value of Unconjugated and Conjugated

Linoleic Acids Oxidized at 65' C.

Unconjugated linoleic acid with cobalt drier

10,12-Linoleic acid with cobalt drier

1. Unconjugated linoleic acid 2. 3. 10,12-Linoleic acid 4.

In the case of the unconjugated acids, the peroxide level remains much higher after 120 hours for the sample without drier. Interestingly enough, the sample with drier breaks off sharply in peroxide value a t about 80 hours and falls below that of the conjugated acid samples. P1esumab1yJ such a film also would set much before the peroxide level finally drops off.

It would appear significant that, with the unconjugated acids, maximum peroxide values obtained are about the same (800) with drier a t 30" C. and without drier at 65" C. Thus, the drier at room temperature does about the same as heat a t force-dry temperature. The peroxide values obtained are just about half nf those obtained with no drier a t 30" C. Almost exactly the

same correlation is noted with the conjugated acids. However. the total peroxide value attained with the conjugated acids is only about half as large (350). Presumably, in all cases where lower peroxide peaks were realized, peroxide decay and subsequent polymerization had set in much earlier. It is unfortunate that several other pertinent measurements were not made-e.g., viscosity increase and gelation time-so that advancement of polymerization could be better correlated with peroxide development and decay.

An examination of these authors' ultraviolet data a t 65" C. (Figure 7 ) shows that the highest degree of conjugation attained in the unconjugated acids corresponded very closely to the highest level of peroxide at approximately the same time, whether or not driers were used. Thus, i t would appear that a t a high level of peroxides, the peroxides must be in the form of hydroperoxides and predominantly conjugated. This is in agreement with the findings of Lundberg and Chipault ( 4 7 ) that in the early stages of autoxidation of methyl linoleate, diene conjugation occurs, and these dienes are predominantly in the form of hydroperoxides.

W e g 800

L 400

0 20 40 60 80 100 120


Figure 6. Effect of Cobalt Naphthenate Drier on Peroxide Value of Unconjugated and Conjugated

Linoleic Acids Oxidized at 30" C. 1. Linoleic acid 2. Linoleic acid with drier 3. Conjugated linoleic acid 4. Conjugated linoleic acid with drier

At 30' C., Figure 8, the same general observations hold, escept that the degree of conjugation has dropped somewhat prior to the attainment of maximum peroxide. This would indicate peroxide formation a t a faster rate than peroxide decomposition and piobably the beginning of polymerization ( l7 ,18)

In another experiment carried out a t 90" C., using only unconjugated linoleic acid in conjunction with 0.1% each of either cobalt, manganese, or zinc metals in the form of naphthenates or no drier over a period of 20 hours, the peroxide values rose to their highest levels in approximately the same period of time, 2l/2 to 3 l / 2 hours. The highest level attained with cobalt was about 500, while with zinc i t was about 800, and with manganese i t was almost as high as with zinc a t almost exactly the same time. The peroxide level attained without drier was very low even a t the end of the experiment. Thus, efficiency of a drier metal-e.g., zinc-commonly thought to be of little value by itself in the air drying of oil films is demonstrated a t 90' C. I t would have been interesting to have observed the efficiency of zinc alone a t 30' C. or with varying minute amount8 of cobalt.

When the ultraviolet data are observed, it is clear that, even without drier, the highest level of conjugation obtained is quite high and increases in the following order: no drier <zinc <manganese <cobalt. Again, i t would have been interesting to observe these relative values a t lower temperatures to help explain observations in air-dry films with single and so-called auxiliary driers.

566 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 46, No. 3

d l 0

N f t W 3

a

a u Y 0 W LL v)

00

80

6 0

4 0

20

'0 3 6 9 I 2 15 18 21 24 27 29


Figure 7 . Effect of Cobalt Naphthenate Drier on Specific Absorption Coefficient of Unconjupatecl and

Conjugated Linoleic Acid Oxidized at 65" C. 1. Unconjugated linoleic acid 2. 3. 10 12-Linoleic acid 4.

Unconjugated linoleic acid with cobalt drier

10:12-LinoIcic acid with cobalt drier

POLIMERIZATION. The reactions in autoxidative po1ymerie:t- tion closely parallel those of heat polymerization. The same general degree of functionalitj- of the ethenoic fatty acids is observed. The proress of polymerization is non- generally corn ceded to be responsible for the drying of oil films and air-dryiiig varnishes. The mechariisni of Oxidative polymerization has been well reviewed by Powers (53 ) and others. It is now generally accepted to be an oxidative-induced t,ype involving primarily thr decay of the hydroperoxides first formed and very likely continuing so during the whole autoxidative process. While vin3.1 polymerization is no longer believed to be significantly opcrativc ( 5 I ) , it was once believed that it could well be responsible for some of the observations on oxidized oil films (53) .


Figure 8. Effect of Cobalt Kaphthenate Drier on Specific Absorption Coefficient of Cnconjugated and Conjugated Linoleic Acid Oxidized at 30" C.

1. Unconjugated linoleic acid 2. 3. 4. Conjugated linoleic acid

Unconjugated linoleic acid with cobalt-lead drier Conjugated linoleic acid with cobalt-lead drier

There is almost universal agreement that free radical chain- reaction mechanisms are involved in the autoxidative polymerida- tion of drying oils. Lundberg (46) presents a free radical chain mechanism (Figure 9) which is believed to be the only generalized mechanism that appears t o be consistent with all of the currently available chemical evidence, the kinetic data, and the thermo- chemical data.

Other reactions which do not involve the chain propagation of free radicals definitely also occur during oxidative polymerization. Esterification of alcohols and acids formed during the oxidation of oils contribute to increased polymerization. These alcohols and acids are undoubtedlJ- formed hy several processes. When metal

salts-e.g., those of lead-:ire presentj they may be fornicd l)y i i

hydrolysis of the original esters present ( 5 ) . Other$ niay lie formed by oxidation of scission products. Dehydration reactioirs (involving the possible formation of ether-linked poljmers, srid probably also dioxane rings) are adequately described in the literature (20-30? 52, 57 ) and others. Hydrogen bonding of the f1.e~ hydroxyl groups, knoivn to be produced in t,he drying of oils, i,q

also recognized to cont,rilbute to gelation and hardening or oil films (17, 43).

Initiation: IIH + 0, + It.', or other free radicals which 1)y or xIlOOH + ]reaction with RH will give 11."

Propagation: I t w + O2 ---f RO1* RO,' + RII + ROOIl + R*

Termination: 1:" + R" + 1 I t 4 + ROIL -+ &table end product- ItO2* + IiO.* -

Figure 9. Free Radical Chain Alechanism in Oxidative Polymerization

Lundhcrg ($6)

That chain scission occurs rather early in the proccss 0 1 autoxidation has been shown by Gibson (34) and Farmer arid Sundralingam ( 2 7 ) . This has been fairly well substaiitiatcd bj- infrared studies conduct,ed in the Battelle Memoria1 Institute laboratories where infrared spectra of films of a tall oil ester and :I

maleic-modified tall oil ester estended to 16'/~ gallon;: with soy- bean oil 13-ere studied as t'hey dried on sodium chloride cryst:ils. This st>udy was undertaken because earlier work (501, designed 10

find a good pra~t~ical drier level for tall oil vehicles, indicated 1~3ther definitely that a drier level of 0.1% cobalt plus 1,0('; 1 e : ~ t l was about the optiinurn to achieve fast drying in a wide raiige of soi'i oil vehicles, although doubling the cobalt helped ~ o i n c . 111 another study ( S I ) , designed to find a low cobalt drier coiiil)inn-. tion during the recent cobalt shortage, it' was shown from a carefully planned series varying cobalt, manganese, lead, and c.alriump that a drier consisting of 0.03cZ cobalt', 0.06% mangancac, 0.3'; lead, and 0.5% calcium resulted in approximately the wnie dq- - ing times Tvith the same vehicles. A condensation of :he origiri:tl data is shown in Table 11.

TABLE 11. EFFECT O F DRIER LEVEL ON BARNISH ?' \CK.l'Rl.l' TIME

Zapon Tack- Free Time,

Hr. Drier Combination, %

c o 11 n Pb ( a ~

It was decided to remake these vehicles, carefully spray them 011

st,eel panels and sodium chloride cryst,als to a thickness of 1 mil, and compare the dust-free and tack-free times with the infrared absorption bands at these intervals. A11 samples weye cured under comparable conditions. Varnishes containing n o clricr and 0.03%, cobalt were also included, since the varnish cont,airiing cohalt dried appreciablj- slor~er than those with the higher drier levels, and the one containing no drier finally set in about 15 days.

Since several references (1, 52) to the value of using infr:xetl spectroscopy as a tool to elucidate the reactions occurring during the drying of oil films appeared promising, particularly wil h respect to estimating total hydroxyl content (SQ), it WLS believc~d


that peroudation, a t least in the initial stages, could be followed fairly accurately by this technique. Further, if peroxidation continues as polymerization proceeds and levels off when the film becomes immobile, the slower rate of oxygen transfer should be evident from the 2.9-micron band. This band is known to be primarily attributable to -OH and -0OH. The leveling o f f of OH absorption by a varnish after it sets is confirmed with data obtained in the Battelle M e m o d Institute laboratories. Figure 10 shows the rate of change of structural groups for the 16l/2- gdlon maleic-modified varnish with the effective low cobalt drier. For this particular varnish, the tack-free time was 10 hours. The C=O and -OH absorption leveled off sharply a t about that time. The other absorption data plotted in Figure 10 are representative of terminal unsubstituted chains of five or more carbon atoms. It decreases duiing drying and the rate of change p:iraliels closely the rate of change of OH and C=O; this gives support to reported data that chain srission occurs.

100

90

80

70

w 60

$ 5 0 0

8 40

30

20

IO

'0 2 4 6 8 10 12 14 16 I8 20 22 24

SAMPLE PIC

HOURS

Pigiire 10. Rate of Structural Group Change with Time in Maleic-Modified, Soya-Extended

Varnish Per Cent

CO n . n:j M n 0.06 Pb 0 . 5 Ca 0.5

Figure 11 shows the optical density of the 2.9-micron absorpt,ion band for this same material, with and without driers. It is observed that a t tack-free and cotton-free time there is considerably less OH in the sample without drier than in the one with drier. This same observation holds for other oxygen groups. Spectral regions characteristic of C=O and C-0--C show stronger absorption for varnishes with drier added than for varnishes qithout drier. This oxygen content difference exists a t tack-free time and persists through the last test, which was made a t a film age of 5 months.

This comparison of oxygen content in varnish films dried with and without driers is contradictory to several literature reports on oils (17, 16) as well as the conclusions drawn from Jackson and Kummerow's work (40) on fatty acids (dis-ussed previously). The basic factor of oxygen uptake is worthy of additional experimental effort.

Additional spectral data that looked very interesting were obtained in these studies. However, their full significance probably cannot be determined until further experiments are con- ducted. Any further work, however, to elucidate the drying properties of films should initially be done on varnishes made with one resin-e.g., glycerol phthalate modified with a single drying oil fatty acid. This work, a t least, should be correlated with ultmviolet. spectral data, peroxide value determinations and total oxygen pickup and total weight pickup of the dried films, since it is known t8hat the latter two do not nocessarily agree (38).

\\-hen compared with the bulk oxidation studies of Jackson and Kummerow (do), it does appear that the general course of oxidative polymerization at relatively low temperatures is very similar, regardless of the catalysts used, and that any differences noted are primarily rate differences. The role of auxiliary metals-i.e., calcium-remains to be explained.

20 DArS(T4CK o- F R E E )

27 DAYS 20- 5HRlCOTTCN FREE)

10 HR nACK FREE)

-

W 1 ' 1 SAMPLE MODIFIED VARNISU WITH NO MOO!F!ED V4RNISH WITH C q M n

5o Pb,Co CRIER

Figure 11. Traces of 2.9-Micron Absorption Band Demonstrating Effect of Drier on OH Level

This discussion of the processes involved in the drying of oil films certainly is incomplete. It is hoped, horn-ever, that it forms a reasonable basis for discussion of t,he role of the catalysts and the merhanism by which they funrtion.

DRIER ACTION

Before any reasonable theory can be evolved as to the mechanism of drier action, it is necessary to consider the observed effect of driers when used to aid in the air drying of paint films of various types under varying conditions. In addition, it must be recognized that different situations prevail during bulk oxidation at elevated t,emperatures. It is beyond the scope of this paper tmo attempt to consider all conceivable cases.

This discussion has shown that the drier metals play an active part in all of the recognizable stages of autoxidation of oils or air- drying paint films, as follows:

Shorten or completely eliminate the induction period Hasten the rate of peroxide formation

Hasten polymerization to the dry or set condition

1. 2. 3 . Hasten peroxide decay 4.

The exact manner in which changes are brought, about by t,he drier metals is still a subject for considerable speculation. Recog- nized authorities on driers ( 7 , 17 , 18, 39, 43) have published in- forniatiori on the history of driers and the probable role they play in the air drying of oils and paint films. While various explanations are offered for the effectiveness of certain drier met'als under varying conditions, the explanations are usually rather vague. Very recently, Mills (48), in his book on drying oil technology, stat,es:

The mechanism of drier action is still a matter of controversy. but general agreement has been reached as to the properties to be looked for in metal compounds that may be suitable for the purpose.

This would imply that anyone familiar with the use of driers for certain purposes with known materials or classes of materials has learned from experience which drier combinations are safe to use in a particular application.

Paint driers, primarily oil-soluble metallic soaps, have been the subject of much investi ation which continues steadily. Most such studies deal with &e productZion of usable materials with practically no knowledge yet of their actual mechanism of action.

Klebsattel (48) states:

Theories were proposed in the paper (48).

568 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 46, No. 3

Popular iiiechaiiisms for drying (@), which are still generally accepted, may be briefly outlined as follom:

1. Driers speed u11 or eliminate the induction pei,iod by acting upon t'he natural antioxidants in natural oils suppo.-edl$ responsible for hindering oxygen absorption.

xygen arter the inductioti

3. Driers actually enter into the oxidation reaction. This is more widely accepted than the simple catalytic theory. The possibility of the drier reacting with oxygen to form a new compound which reacts n-ith oil? at the double bond is suggested. Oxygen is given up to the oil. and the drier is again free t o react, ivith more oxygen.

Drier compound may enter into some actual chemical combination vith the drying oil at the double honds, forming a nev cornpound which coinlines ivvith oxygen more easily than the original oil itself.

4.

PROPOSED XJECHAYISXI

The foregoing would seem to indicate that, no one has cared to venture a plausible explanation for t,he real manner in vhich driere function. Actuall>., this is not t,he case, since the literature aliounds in references to tile manner in which catalysts may affect the autoxidation process. Rorrevc~, it is equally brue tlmt no simple mechanism has been advanced to arcouiit for all observed phenomena attributable t o drier metals under all con- reivable conditions, Nearly all of the plausible explaiiations ileal, in oiie maiiner or another, with the drier metal?, or catalysts in general, facilitating the generation of free radicals. Conditions prevailing, such as light, presence of water, valences of the drier metals, percentage of drier metals present, presence of perosides, type and amount of solvents, and particularly temperature, all affect, the initial atid contiiiuenl production of diffrrent typt'9 of radicals conceivably produced during the autoxidation of various materials under varying conditions. I t is believed that even the reason for the observed action of lead and possiblb- zinc anti calcium can be logically rsplained on this basis.

Since free radical mechanisms are now generally accepted to account, t o a large degree, for the oxidative polymerization of drying oil films, it is proposed t,hat thc mechanism of drier action is primarily one of initjallv producing free radicals and continuing to help do so during thp entire course of the whole autoxidative process. The types of lree radicals produced as the autoxidative polymerization procpcrls will depend on the conditions prevailing.

The often denioiistraied effect 01 mised driers. inciutling so- called ausiliary driers, can be explained by the fact that the 11101'~

active metal is effective in promoting the initial formation of free radicals to start peroxidation. As the temperature rises, auto- catalytically, the ausiliaq. driers also become effective and hclp promote the ensuing rettclions. It, seems entirely possible that the niorc activc drier uch as cobalt, arc more effective in promoting peroxidation! while the so-called auxiliary driers-i.c.. zinc, calcium, and zirconium-are nearly as effective in pvomoting peroxide decay. Furthermore, since t,he auxiliary driers arc> sloil-er in promoting peroxide decay, they niay actually present, a more favorable situation fL>r the copolymerization of rlifferriit, monomers rather than the homopoly1nerization of the more active monomers (thus resulting in low molecular \wight materials being merely dispersed in more highly polymerized polymers rrhen the film sets).

This could actually explain the rrmon for the observation (33) that rare earth naphthenates in combination with inow active driers produce startling results in white refrigerator enamels. The role of the hydroperoxides in copolymerization is now generally recognized (44). Sctually, there is con~iderable evidcnce that, in order to obtain copoipmers of tlvo or more autoxidizable suh- stances in admixture, the individual materials present should somehoiy be simultaneously brought to a relatively high 11ydro- pe~oxide level so that cop~lymerization ran occur.

The aftertack, so often noted in some dehydrated castor oil finishes, might possibly be attributed to rapid polymerization of the conjugated 10,12-linoleic acid fractions, leaving the un('on- jugated fraction largely unpolymerized.

Without going into any great detail on the types of free radicals that may be produced during the autoxidative process, it does seein pertinent to discuss a fen, in a little more detail. particuli~r.ly with respect to their possible sequence.

ISD~CTIOK PERIOD Ant,ioxidants oxidized by oxidation-reduction mechanism.

Metals capable of existing in several valency states are pro1)ably necessary-ie., cobalt or manganese

Co+++ 71 co++

a.

b. 11eial salt dissociated to produce free carboxyl ra<lim!

0 /

R-C---O 0 p

It.--C--O 0 \\,

o'\

c. Carhoxyl i y r ? i.alt1ic.i carhon at,om and .tart rhn

0 ?

H

R-C-C-=C-~- It f It-( e---c=c--R 11.- -(

:I11 * I ;

H k H '( :< Ir H I l j

GI. Double bond ai,tivatetl -C--C-

e. Oxygen zctirated 0-0 i /

\/ ISITIAL PEROXIDATIOX

a. b.

Activated oxygen attachos at double bond Oxygen attack!: at act,ivatetl double bond

X-e--C-1% $- 0 2 -.+ R-C-C--P, ! I 0.-0

<\ /

then .diifta

H H I

H 0-0

I

I{.-C --c----c:--lJ -+ ]t-C-C=C-R j

OOH I l l

HOOI-1 +- Co++ --* KO" + OH- + Co-' -

riom + ROO- + H +

Co--+ -C ROO- -+ Co-+ $- ROO" (Wibaut and Stratig, .iOj

b. Chain scisqion takes place (production of shorr-i~li:iin

e . Peracids formed (not detectable)

a . Ether linkages formed b. Yew carbon-to-carbon bonds-formed c. Esterification takes place d. Other linkages formed

It' v,-ould be int>cresting to carry out a detailed study o l the lilin formation of several typical fatty acid est,ers, singly and in admixture, and parallel such studies with controlled hulk oxidation procedures under a selectrd number of pract,ical conditions. 10 driere and drier metals, singly or in admixture with other. c

aldehydes and acids)

I'OLYMERIZ.4TION


metals or catalysts, such as organic peroxides, should be used a t room and elevated temperatures.

Suitable experiments to elucidate the chemical changes taking place during observed changes in physical properties should be carried out. The following information is believed to be most needed:

1. Information about the amount of oxygen that enters the sample, and the form in which it exists a t critical point3 in the drying process. Progress along this line could be made by com- bining data from weight gain, ultimate analysis (including direct oxygen) determinations, chemical determination of peroxide and acid, and infrared measurement of total OH and C=O. There is a good possibility tha t several other infrared absorption maxima observed in drying oils could be inter- preted in terms of functional groups responsible for the absorption.

2. Determinations of rates of development and decay of different oxygen groups so that postulated theories about con- secutive and simultaneous reactions could be evaluated.

Extensive double bond analyses, including chemical, ultraviolet, and infrared data, to provide a double check on methods and minimize analyt’ical errors which result from use of only one method.

4. Investigation of the role of chain scission by a combination of infrared measurement of terminal chain decrease, methyl group change, plus simultaneous analysis of volatile compounds formed.

3.

5. Structural examination of products for interpolymers.

The study outlined, carried out on a carefully controlled sample series, would require considerable cooperation among several specialized drying-oil chemists and analysts. When simultaneous data are obtained on identically treated samples, it may be possible to predict with some degree of certainty the mechanism of drier action under the conditions prevailing. Where autoxidation processes are employed, this information could be utilized to produce better products.

ACKNOWLEDGMENT

The author wishes~ to thank Clara D. Smith, who did the spectrographic work, and his associates, who assisted in carrying out the experimental program.

~~

LITERATURE CITED

Adams, K., Auxier, R. Tlr., and Wilson, C. E., Ofic. Dig. Federa- t ion Pa in t & Varnish Production Clubs, No. 322, 669-81 (1951).

dllen, R. R., Jackson, $., and Kummerow, F. A,, J . Am. Oil Chemists’ Soc., 26, 395-9 (1949).

Allen, R. R., and Kummerow, F. A., Ibid. , 28, 101-5 (1951). Austin, A. E., Brand, B. G., A’fueller, E. R., and Schwarta,

C. RI., presented before the Division of Paint, Varnish, and Plastics Chemistry at the 118th Meeting, AM, CHEM. Soc., Chicago, Ill,, 1950.

Balfe, A I . P., and Chatfield, H. W., J . Sac. Cheni. I n d . (London) , 59, 31-4 (1940).

Bawi, C. E. H., J . Oil & Colour Chemists’ Assoc., 36, 443-79 (1953).

Bennett, E. F., “Driers and Drying,” London, Chemical Pub- lishing Co., 1941.

Bloomfield, G. F., J . Chenz. Soc., 1943, 356-60. Bolland, J. L., and Gee, G., Rubber Chem. and Technol., 20, 609-

17 (1947). Bardley, T. F., in hIattiello, J. J., “Protective and Decorative

Coatings,” Vol. 3 , Chap. 4, pp. 87-113, Kew York, John Wiley & Sons, Inc., 1943.

Bradley, T. F., and Johnston, W, B., IND. EXG. CHI:M., 32,

Bardley, T. F., and Tess, R. W., Ibid. , 41, 310-19 (1949). Carrick, L. L., and Snoddon, W. J., Ofic. Dig. Federation Pain t

& Varnish Production Clubs, No. 322, 682-91 (1951). Carriere, &I., Ann. fac . se i . Marseille, 19, 11-154 (1947). Chipault, J. R., Nickell, E. C., and Lundberg, W. 0.. Ofic. Dig.

Federation Pa in t & Varnish Production Clubs. No. 322, 740-

802-9 (1940).

(16) Criegee. R., Pilz, H., and Flygale, H., Ber., 72, 1799-1804 (1939).

(17) Elliott, S. B., presented as part of the Joint Symposium on Drying Oils of the University of Minnesota and Minnesota Section, AM. CHEM. Soc., Minneapolis, Minn., March 27-29, 1947.

(18) Elm, A. C., IXD. ENG, CHEM., 26, 386-8 (1934).

(20) Farmer, E. H., Trans . Faraday Soc., 33, 340-8 (1942). (19) [bid., 41, 319-24 (1949).

(21) Ibid., pp. 356-61. (22) Ibid., 42, 228-36 (1946). (23) Farmer, E. H., Bloomfield, G. F., Sundralingam, A,, and Sutton,

(24) Farmer, E. H., Koch, H. P., and Sutton, D. A., J . Chma. Soc.,

(25) Farmer, E. H., and Michael, S. E., Ib id . , 1942, 513-19. (26) Farmer, E. H., and Sundralingam, A., Ibid., pp. 121-30.

(28) Farmer, E. H., and Sutton, D. A, , Ibid. , 1942, 139-48.

D. A., Ibid. , 38, 348-56 (1942).

1943, 541-7.

(27) Ibid., 1943, 125-33.

(29) Ibid. , 1943, 119-22. (30) Ibid. , pp. 122-5. (31) Fiscella, C. T., and Zacharakis, L. G., Am. Pain t J., 37, 60-5

(32) Gamble, D. L., Barnett, C. E., IND. ENG. CHEX, 32, 375-8

(33) Gardner, C., Am. Pain t J . , 37, 18 (1953). (34) Gibson, G. P., J . Ckem. SOC., 1948, 2275-90. (35) Gunstone, F. D., and Hilditch, T. P., Ibid. , 1945, 836-41.

(37) Hargreaves, K. R., and XIcGookin, A. , J . Soc. Chem. I d .

(88) Hess, P. S., and O’Ilare, G. A, ISD. ENG. CHEX, 44, 2424-8

(39) Honn, F. J., Beaman, I. I., and Daubert, B. F., J . Am. Ciiem.

(-20) Jackson, A. H., Kummerow, F. A, J . Am. Oil Chemists’ Sue.,

(1953).

(1940).

(36) Ibid., 1946, 1022-5.

(London) , 69, 186-91 (1950).

(1952).

SOC., 71, 812-16 (1949).

26, 460-5 (1949), (41) Jordan, L. A., J . Oil & Colour Chemists’ Assoc., 35, 577-95

(1952). ( 42) Klebsattel, C. A . , in Mattiello, J. J., “Protective and Decorative

Coatings,” Vol. 1, Chap. 22, pp. 499-534, New Tork, John Wiley & Sons, 1941.

(43) Klebsattel, C. A., presented before the Division of Paint, Varnish, and Plastics Chemistry a t the 1lGth RIeeting, -%sf. CHEM. SOC., Atlantic City, N. J., 1949.

(44) Konen, J. C., Hanseh, L. I., and Formo, M. W., presented before the Division of Paint, Varnish, and Plastics Chemistry a t the 116th Meeting, L4M. CHEM. SOC., Atlantic City, S. .J., 1949.

(45) Krumbhaar, W. H., Foreword to Bennett, E. F., “Driers and Drying,” London, Chemical Publishing Co., 1941.

(46) Lundberg, W. O., presented before the Division of Paint, TTar- nish, and Plastics Chemistry a t the 116th Meeting, -AM CHEM. Soc., Atlantic City, N. J., 1949.

(47) Lundberg, W. O., and Chipault, J. R., J . Am. Chem. Soc., 69,

(4.8) Mills, hl. R., “Introduction to Drying Oil Technology,” Lon- don, Pergamon Press, 1982.

(49) RIorrell, R. S., Rolam, T. R., Davis, W. R., Marks, S., Phillips, E. O., and Sim, W. E., Trans . Faraday Soc., 38,362-72 (1942).

(50) Mueller, E. R., and XIcSweeney, E. E., Am. Paint J . , 34, 26,

(51) Overholt, J. L., and Elm, A. c., IND. ENG. CHEM., 32, 378 8 3

(52) Powers, P. 0.; Ib id . , 41, 304-9 (1949). (53) Powers, P. O., Overholt, J. L., and Elm, A. C., Ibid. , 33, 1257-

(54) Sutton, D. A, , J . Chem. Soc., 1944, 242-3. (55) Swift, C. E., O’Conner, R. T., Brown, L. E., and Dollear,

F. G., J . Am. Oil Chemists’ Soc., 26,297-300 (1949). (56) Waters, W. A , , J . Chem. Soc., 1946, 409-15. (57) Wheeler, D. H., IND. EYG. CHmr., 41, 252-8 (1949). (58) Wheeler, D. H., Ofic. Dig. Federation Pa in t & Varnish Produc-

(59) Wibaut, J. P., and Strang, A,, Konink l . Ned . Alcad. Wetenschap.,

833-6 (1947).

28, 66-8 (1949).

(1940).

63 (1941).

t ion Clubs, NO. 322, 661-3 (1951).

Proc., 54B, 102-9, 229-35 (1951).

50 (1951). RECEIYED for review December IR, 1953. ACCEPTED January 2.5, 19.54.

proposal - "mechanism of drier action"

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