Panel Discussion on Mechanism of Drier Action W. 0 . LUKDBERG
FRASCIS SCOFIELBIB
Hormel Institute, L'niversity of Minnesota, Austin, *VTinn.
Yational Paint, Varnish, and Larquer Assor., Washington, D . C .
F. AI. GREESAFALD 'iuodex Products Co., Elizabeth,
E. B. FITZGERALD
\-. J .
E. I . d u Pont d e Xemours &: Go., Philadelphia, Pa.
A s Mr. Mueller has pointed out, any discussion of the mechanism of drier action is necessarily quite epeculative. Its speculative character seems to arise more from a lack of pertinent data on the mechanism of the reaction than from a lack of data on the eflect of driers. The entire panel will probably agree that there is no single simple explanation of the niechanism of drier action. The niechanieni appears to vary Il-ith the nat,ure of the drier, and there is evidence to indicate that, one drier may perham operate in several different fashions. Our understanding of thesc various mechanisms varies--of some we have a modest understanding: of others our knowledge is very incomplete. The following: remarks are confined to just one mechanism with just one drier, the mechanism that seems to bc primarily involved in the effect of cobalt drier on oxidation and polymerization. I n the oxidative polymerization of oils, the first step is the addition of oxygen t o form peroxides. These peroxides decompose to form free radicals. Chain termination (and polgmerizat,ion) occurs hl- reaction of two of these free radicals. The over-all rate of oxidation is proportional to the first power of peroxide concentration, although t,hc chain initiation reaction appears to be proportional to the square of peroxide concentration. This: of course, means that the chain length must be inversely proportional to the peroxide concentration. More peroxide provides more free radicals and more chance for these free radicals to combine and terminatr the chain. From this mechanism of autoxidation, some statements can he made about what cobalt does and what it does not do. It does not appear to oxidize t,he oil directly, nor does it function by direct catalysis of the addition of oxygen; complex formation with oxygen is a t best an outside pomibility. but i t should be mentioned because of thc fact that cobalt does readily form coordination complexes. These statements have been negativc. On the positive side, there is definite evidence that cobalt accelerates peroxide decomposition; the increased rat,e of free radical formation accounts for the acceleration of polymerization or drying. Although it seems clear that, colmlt catalyzes peroxide decomposition, the method by vr-hich it accomplishes this acceleration is not a t all clear. The formation of coordination complexes with the peroxides themselves is a distinct possibility. A comparison of the rate of oxidation in the presence and absence of drier shows that the lat,ter is definitely autocatalytic, whereas the former is not. There are two possible explanations for this behavior. One is that the reaction rate becomes so rapid that, diffusion of oxygen into the film becomes thc limiting factor.
The second, and more probable explanation, is that tlie pcroxidp content does not rise to very high levek, and there seeme to I)c u relatively constant rate of production of free radicals. This view is supported by evidence presented by hIra ;\Iuc.llei showing that, as temperature is increased, t,he rate of oxidation teiids t o become the same with or without driers. This would indicate that a st,eady-state concentration of peroxides had been reached. Further evidence for this explanation seems to lie in the fact that t,he efficiency of cobalt is not increased by formation of a complex n-ith organic amines, although this is effective with neaker driers than cobalt. ,111 these bits of evidence point to the fact that peroxidcq dccompose so rapidly in the presence of cobalt that thc ovcr-all rate loses all appearance of being autocatalytic. This iiirchanism is perfectly consistent with several othw olxervations. Cobalt accelerates the build-up of peroxide to :I re1ativel)- lox- maximum level. much l o m r than that, obtaincil in its absence. In the presence of cobalt the polymrrization i n taking place a t a lower level of oxidat,ion, which again icwnsistent ?Tit11 the free radical mechanism. A third olwrvation consistent 13-ith this mechanism is that in the prcscnce CJf :I drier there i. a greater discrepancy between oxygen uptake a n t i peroxide coilcentration than in its absence. This m a y I,e due. t o chain termination reactions which convert oxygen-containing radicals into polymers in which the 0x1-gen is not found as :I peroxide group. I n summary, there appears to be a great deal of evidence to support the proposition that cobalt accelerates drying by catalyzing peroxide decomposition and free radical formation. thuaccelerating polymerization. Ot,her mechanisms and ot1it.i. driers require further study before statements of even this dcg~c.c~ of crrtainty can be made. W. 0. LUSI)BF:R(:
Despitc the common acceptance of the hypothesis that driw nietale act to initiate peroxidation so that autoxidation can proceed, it has been conclusively established that this is not what dricrs do-rather, they are destroyers of peroxides and h\droperoxides. Although it is clear that this is what driers do, the mechanism by which they do it ie a good deal less clear. Some rathcr recent work does, hon.ever. throw some light on this problem. 1Ieyer and Ing reported [Farbe u.Lack (Slay 1952)] that the addition of a sniall amount of drier to a blown linseed oil containing 1.2 grams of active oxygen per hundred grams of oil caused 3 tern570
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INDUSTRIAL AND ENGINEERING CHEMISTRY
perature rise from 18" to 40" C. in 10 minutes. Analysis showed the peroxide content to decrease with time. Coupled with the fact that similar tests with unoxidized oil showed no temperature rise and no change in peroxide content, these results strongly suggest that the driers enter into a secondary exchange reaction with the oil peroxides. A similar effect is the behavior of a mixture of tetralin hydroperoxide and diphenylamine. With the addition of a trare of drier, the colorless amine is oxidized to a red-brown color. Similar results are secured when oil peroxides are substituted for tetralin hydroperoxide. These and other results make it reasonable to conclude that driers reduce the energy required for peroxide decomposition. Evidence which tends to disprove earlier hypotheses that driers are transmitters of molecular oxygen, by means of a valence change, is found in the behavior of cobalt with peroxides. The green compound formed when the normal cobalt drier is added to oils containing peroxides was studied by polarographic methods in comparison with suitable standards. From this work, it was concluded that the valence change of cobalt was negligible. Analytical work on simpler analogs of this compound has showed two structures to be possible. One is a peroxide in which two oxygen atoms and one carbon chain are attached to cobalt. This presupposes the existence of trivalent uncomplexed cobalt, a debatable assumption. The second, and more probable structure, is one in which two cobalt atoms, each with an alkyl chain attached, are linked through two oxygen atoms. I t is significant that either of these structures can give rise to free radicals, and either can be formed without any necessity that the metal participate in the oxidation reaction. Further evidence relating to valence change in driers is found in polarographic studies of complexes or chelates. Although both manganeee and cobalt form complexes with o-phenanthroline, the drying activity of manganese is enhanced thereby, while that of cobalt is reduced. Study revealed that the manganese was present in the manganous state, whereas cobaltic cobalt was found. From this it could be inferred that driers do not change valence and, further, that they are active only in the reduced state. These studies showed coordinrttion numbers of 2, 4, and 8 for manganese and 4 and 6 for cobalt. In view of the fact that driers are active in the absence of added chelating agent, it is not unreasonable to suppose that the oil molecule is the chelide. In the case of manganese, the complex is probably similar to that formed with o-phenanthroline, while vc-ith cobalt i t seems more probable that the metal is present in the reduced state. The work presented in this discussion represents a challenge for future workers and may offer some leads for further elucidation of the subject of drying oil oxidation. F. M. GREENAWALD
Ai question should be raised concerning the assumption, implicit in this entire discussion, that there is a single mechanism of drier action. I t seems fundamental in any discussion of this problem that it should be recognized that there is not one hut several metals which act as driers. Their mechanisms of action may differ, not only quantitatively, but qualitatively as well. One strong reason supporting this point of view is the often observed phenomenon that mixtures of driers frequently afford shorter drying times than can be achieved by any amount of a single drier. This indicates that the second, or auxiliary, drier is furthering the polymerization by some mechanism beyond the power of the first one. I t is well established that there is not just one, but a t least two, and probably several, major reactions occurring simultaneously in the drying of an oil film. The fact that the system is so extremely complicated may well explain the difficulties which
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have been encountered in attempts to extrapolate from simpler compounds to oils, from heat polymerization to room temperature polymerization, from thin films to thick films, and so on. It may be emphasized that films dried under different conditions are qualitatively different. They differ in hardness, in density, in chemical resistance. and other properties. For example, a force-dried film is not a t all just an air-dried film produced more quickly. It sums from this that any tenable mechanism proposed must be a hypothesis which takes into account the competing reactions in the drying of oils and the differing effects of the various drier metals on them. FRANCIS SCOFIELD
T h e physical effects of driers are a t least as important as the chemical effects. Many aspects of drier action which are of importance to the paint industry actually comprise a subdivision of a much broader topic-film formation. Many of the observed phenomena and properties of films can only be explained in terms of physical processes. For example, diffusion of oxygen into the film, migration of unpolymerized material throughout the film, and other similar processes must exert a major influence on such properties as drying time, hardness, wrinkling, etc. It seems likely that very often these physical processes are the rate controlling steps which actually determine the course which the organic reactions will take, and it is within the framework of this picture that the action of driers is discussed. One of the best known phenomena in the drying of oils is that drying-that is, solidification-usually occurs first a t the surface with the formation of a skin which then thickens until the film has become apparently solid throughout its depth. However, if such an apparently solid film is stripped from its substrate and extracted with an organic solvent, i t is found that considerable unpolymerized material remains. The quantity of extractable material decreases as the drying time is prolonged beyond that just sufficient to produce an apparently solid film. These observations lead to the concept of a gradient of polymer distribution through the film, from surface to substrate. The sequence of events which gives rise to such a gradient merits further discussion. The first step must be the diffusion of oxygen from the air into the freshly deposited layer of oil. Since it is known that diffusion of oxygen into organic liquids is a slow process, conditions thus exist for formation of a gradient of oxygen concentration within the layer of oil. The greatest concentration will, of course, be a t the surface. The second step is the reaction of the dissolved oxygen with monomer to form a hydroperoxide. Since the rate of formation of hydroperoxide is proportional to oxygen pressure a t the low concentrations which must exist within the film, it follows that a distribution gradient will exist for hydroperoxide analogous to that for oxygen. The final step is the decomposition of hydroperoxide to form polymer, bringing about, in this fashion, the gradient of polymer concentration. The chemical reactions which will control the shape of the gradient curve are hydroperoxide formation and hydroperoxide decomposition. The ratio of the rates of these two reactions \Till determine the nature of the distribution. If the rate of peroxide decomposition is negligible, the concentration of hydroperoxide will increase steadily with time and ultimately level off with the disappearance of oxidizable groups. Where the ratio of the two rates is near unity, a steep distribution gradient should result. The physical properties and behavior of the film during the drying period must inevitably depend on the nature of the distribution which exists during this time. A steep gradient may be expected to result in a film which is tack free while a large quantity of low molecular weight material still remains between the surface skin and the substrate. On the other hand, a flatter gradient. should give a film which is well hardened throughout by the time it is tack free.
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INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
For the purposes of this discussion, a very significant property of drying oil films is their ability to mrinlrle under certain conditions. It has been postulated that wrinkling is caused by the s\\-elling of a polymer layer which results from the s l o migration ~ into it of monomer from beloiv the surface. The concentration of polymer at the surface must bc great enough t,hat the surface mechanically can behave like a solid, and there must be enough monomer beneath the surface to s \ d l the polymer to a physically observable extent. I t is thus clear th:Lt there murt be a direct relation between wrinkling and the polymer distribution gradient. From these considerations it is evident that a film with a very steep gradient should wrinkle inore readily than one xr.ith a flatter gradient and, thu:, should wrinkle a t a lower film thickness.
Vol. 46, No. 3
Since t,he film thickness required for wrinkling is easily measured, this property is at present one of the best available methods cor estimating the nature of the polymer distribution gradient. Turning to the effect of driers, one which produces a vei'y fast reaction, and therefore a rate ratio approaching unit)., would be more likelj- to produce a wrinkled film than a drier type (or c-oncentration) which acts more sloivly. The anomalous effccts produced hy combinations of driers are probably related to this distribution pattern. I t seems that a very fertile field for future research on dricrs and drying oils lies in the exploration of the effect of the physical processe8 on t,he thermodynamics and kiuetics of these reactions. E. B. F'IT%GER.ICI[)
structure of
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1
E. G. BOBALEIL L. R. LEBRIS. A . S. POWELL, AUD WILLIAM vox FISCHEK Case InstitiLte of Technology, Cleceland 6 , Ohio
HEORETICAL research on organic coatings has provided a wealth of quantit'ative data regarding the physical chemistry of paint constituents, the sedimentation and rheological characteristics of solid-liquid dispersions, t,he permeability and deterioration charact,eristicsof films, and optical properties of pigments. In most such studies, either the experimental variables have been so controlled, or else such simplifying assumpt,ions have been made, that the end results cannot be applied to r e d paint, films with any degree of certainty. i\lucxh of this research \vas motivated by the hope that such data n-ould provide practical predictions regarding the structure, durability, and appearance of organic coatings. This hope has been realized only in part, and a considerable cause of this disappointment has been uncertainties regarding film structure. It has long been apparent that film properties cannot be explained solely in terms of compoqitional variables. Differences in film structure, as caused by variatioiis in the mechanism of film formation, can affect durability and appearance to an important degree. Qualitative data regarding equivalence or nonequivaIence of film s h c t u r e are essential for proper interpret'ation of other data regarding compositional variables, at least to the extent t,hat it must be knorvn whether reproducible films can be obtained from coatings of even coiist ant, composition. The most direct, but, very difficult approach to such essential qualitative information is through microscopy. Ih is verjdiscouraging, because either no results can be ohtaiiied. or ( what seem like good results are later proved to contain artii'ac,t? t,hat obscure the true picture. At the present time, we have confidence in two techniques, each of which has its recognized limitiitions. SILVER-SII.ICA TECHXIQCE (6). The paint specimen war coated with a thin film of silver by condenmtion of silver vapor in a vacuum chamber. The coated specimen was recovered, and a film of polyvinyl alcohol (the polyvinyl alcoliol used in this n.ork \vas obtained from E. I. du Pont, de Seniours &- Co. arid is dwignated as Elvanol, grade 51-05) was applied to the silver filni and allon-ed to dry, and the silver film with its polyviiiyl alcohol haclting was stripped mechanically from the paint filml exposing the silver negative. The eilver negative was then coated with a thin film of 4 i e a in the vacuum chamber. The composite polyvini-1 alcohol-silver-silica film was then placed in dilute nitric acid, n-hich dissolved away the polyvinyl alcohol and vilver, leaving t,he clean, transparent silica replica. The silica posit'ive v,m shadowed by evaporation of chromium in vacuum at a fixed angle. The shadowed silica replica )vas then observed in the cloctron microscope. Every detail of the procedure must be executed with great care. Alter successful replicas are obtained, a great number must' be examined over a wide area to ascertain what represents the best average area of the original surface. The silver-silica bechnique gives the finest resolution-for cxample. it can distinguish differences in binder structure as illustrated by two samples of baked enamel containing different alkyd resins (Figure 1). The method, holvever, ha? a critical fault.
