Chain Reactions in the Paint and Varnish Industry H. N. STEPHENS, I n i k e r d t y of Minnesoba, Minneapolis, M n n .
F
ttOM o m point of view, the clieinistry of paints and varnislres is an extremely involved subject because of the coiqilexit,y of tiie substances involved. However, there are sonic: Eiiiidainental problenis of the industry in wliicli w(! may neglect the specific cornplex reactions that t.ake place arid direct our attcntiorr toward the general nature of tliesc reactions, particularly from tlie viewpoint oE their kinetics. In the drying of oils and of paints aiid variiisiies, it IIUS been well established that two main types of reactions occor-autoxidation arid polymerization. At first glanee it iriiglit be assumed t.hnt these two types of reaction, being quite different, would be albogetlier indopendent. However, recent work has offered a continually increasing body of evidence that would indicate a very intimate connection between oxidation and polyinerization in the drying and bodying of oils. Altliough this fact has been recognized for some time, no satisfactory explanation lias been offered up to the present. The most commonly accepted view is that the first. stage in the drying process is oxidation and that this is followed by polymerization or condcrisation of the oxidation products. This theory, liowevc!r, offers no explanation for tlic drying of China wood oil with very little or no accompanying oxidation. It sill bo obvious to anyone acquainted with the phenomena of bodying and drying of oils that any gencral theory must derelop some logical relationship between oxidation and the attendant increase in average moleculnr weight, and in tile q i n i o n of the %writertlie most important. chic tu this relationship is the known sensitivity of both typcs of reactions to traces of impurities. For example, it has long been known that minute quantities of seleiriuni exert a marked retarding effect on tile bodying of oils, particularly China wood oil. It i s also a matter of conxiion experience that scleniuai inliibits the uxidatiun process. Otlier substances also are known whicli exert similar effects although iii a less striking maimer.
CHAISI~EACTIONTHEORY The remarkable effect of traces of inipiirities in retarding the initial rate of a reaction has attracted a great deal of attention during the past few years, and the two outstan(ling
tlicories \rliich attempt to account fur these plienomena are those of Moureu and Uiifritisse (12j and of Christiaiisen (4). The f m n e r deals exclusi\-ely with tlie inhibition of autoxidations, ahereas the latter is rriore griicral ami is known as the chain reaction theory. In vicw uf the increasing body of evidence supporting tlie latter, it id the only one wliicli will Lie considered here. The concept of reactiun ellains, iii rvhicii tlie energy of socrillcd hut molecules of reaction prorioct niay i i e imparted tu nomlal rnolecriles of reactant, thus furnishing the iieccssary energy of activation, lias become familiar during tiie past few years. However, it might not be out of place to indicate briefly why such small concentrations of inhihitors are capable of producing sucli a large effect on tlie initial rate of a reaction. Let us consider a reaction whose iiorrnal chain length is 10,000, such as the photoclieiriical oxidation of berisaldeliyde (3). If we introduce into this substance ail iiihibitor such as hydroquinone in the proportion of 1 rriolecule per 1000, then the probability of each hot nioleciile encountering an inliibitor niolecule on thc first collision will be 0.001. The probability of this event taking place aft,er 1000 collisions will be approximately 1. In other words, the average nuniber of energy transfers that, will take place (assuming tire same efficiency of t,ransfer frrr benzaldehyde and inhibitor molecules) before an inhibitor molecule intervenes, will be 1000. This, t.lien, will represent the initial chain length for tlie inliibiteil reaction if each inhibitor molecule breaks a cliain.' I t i s thus seen that an inhibitor concentration of 0.1 mole per cent may be capable of reducing the averagc chain length and, therefore, the reaction rate, to 0.1 of the normal. If the energy is transferred more efficientlg to imliibitor than to berrealdeliyde molecules, the effect may be iiiucli greater. I t has been rncntioiieii that both autoxidation aiid polymerization of drying uils show the reinarkable sensitivity to inlribitors tliat lias come to be regarded as one of the out8 The sssumptim that oiriy one inhibitor inolecule i s required to bresb chain ~ n s ynot be justified, as it has been shown [4Iyea and Bdekatiom. J . Am. Cham. Soe.. 51, Wl l1929)1 tliat 2 molecoles of so alcohol _e required to brosk a chain in the oxidation o i eudium sulfite. This, however. dnea not a i k t the main arpurnent sboue. N
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of glycerol would be present in butter fat, esterification of this impurity might account for the increase in rate of oxidation.2 Therefore, evidence which has previously been interpreted as favoring the autocatalytic theory may be interpreted just as satisfactorily from the viewpoint of destruction of inhibitors. If the conception of autocatalysis is eliminated from the theory of oxidation of drying oils, t,here is another fundamental practical problem in paint and varnish chemistry which will require reexamination-namely, the mechanism of the action of driers. The acceleration of the process of drying by means of metallic compounds inevitably associates itself with the familiar phenomenon of catalysis, and it is not a t all surprising that the action of driers has been commonly interpreted in this light. Recently, however, a number of workers have classified driers as pseudo rather than true catalysts (13). Rideal and Taylor and Rhodes and Van AUTOCATALYTIC THEORY Wirt express the view that the drier promotes the formation The common interpretation of the early increase in rate of, or stabilizes (Rideal and Taylor) the peroxide, which with time is based on the assumption that the reaction is is considered to be a n autocatalyst. I n the light of evidence autocatalytic, and that the peroxides, which are formed as already presented, the viewpoint is open to question, and it primary products, are the autocatalysts. This idea is ap- appears that experimental facts support equally well the parently given experimental support by the fact that the idea that the drier hastens the destruction of inhibitors addition of partially oxidized linseed oil (18) or partially initially present. Examination of the oxygen absorption curves for linseed oxidized turpentine ( 6 ) to fresh linseed oil shortens the induction period. Similar observations have been made oil alone and in the presence of driers ( 5 , 16, 17') or inon other materials. Another viewpoint, however, is expressed hibitors (14, 1.9) reveals the interesting fact that the maxiby Greenbank and Holm (6) whose study of the oxidation mum slope of the curve is approximately, if not exactly, of butter fat led them to the conclusion that acids (and constant for a given temperature. The presence of driers possibly other products) formed in the oxidation process displaces this region of maximum slope toward the origin along the time axis, while inhibitors cause a displacement were responsible for the increase in rate with time. I n the oxidation of cyclohexene (paper to be published in the opposite direction. If a drier is added in addishortly) studied in this laboratory, it has been found that the tion to a n inhibitor (9, I,$), the displacement due to the ininduction period is greatly shortened by exhaustive purifica- hibitor is reduced. Attainment of maximum slope in the tion. Although it has been impossible to realize experi- oxygen absorption curve in this latter case must be caused, mentally its complete removal, it can safely be said that the a t least partially, by destruction of inhibitor. As the ingreater part of the induction period, even for material which hibitor is destroyed more rapidly in the presence than in has been carefully fractionated, is due to srnall traces of the absence of drier, it seems inevitable that the drier is in impurities which are extremely d f i c u l t to remove. If a some way associated with its destruction. If this is true simple substance such as cyclohexene presents such a tre- when inhibitor is added purposely, it should also be true in mendous problem of purification from inhibitors, it would the case of inhibitors initially present. There seems no be surprising if natural products such as drying oils and fats reason then for doubting that the region preceding maximum were free from, or could readily be freed from, natural in- slope in oxygen absorption curves is caused essentially3by inhibitors. I n fact, it seems obvious from a comparison of the hibition of oxidation. If additional inhibitor is added, the rates of oxidation of crude and refined linseed oils (1) that induction period is lengthened; if drier is added, the destructhe latter does contain natural inhibitors, which are partially tion of inhibitor is accelerated and the induction period removed in the refining process. It is also recorded that in shortened. There are so many different ways in which the drier might the oxidation of the ethyl esters of the drying oil acids ( 8 ) no induction period Kas observed, although the correspond- accelerate the destruction of inhibitor that it would be pure ing glycerides always showed this phenomenon. A logical conjecture to propose at this time a precise mechanism for explanation of this fact would be that the ethyl esters, pre- the process. If we consider the possibility of direct reaction sumably purified by fractional distillation, would be much of drier (in the oxidized state, presumably as the peroxide more apt t o be free from inhibiting impurities than the of the metallic compound) and inhibitor, we are immediately confronted with the fact that increasing the concentration of glycerides, which cannot be distilled without decomposition To return briefly to evidence which seems to support the drier above a small optimum value has only a slight effect autocatalytic theory, the increased rate observed on the on its efficiency. If we regard the drier as a true catalyst addition of partially oxidized to fresh material may be due, of the main reaction, which assists in the process of activation not to the catalytic effect of the peroxide, but to direct oxida- and increases the rate a t which chains are started, thus imtion of inhibitor by the peroxide. Nearly all inhibitors of posing a greater requirement of inhibitor for the breaking oxidation are easily oxidizable substances (7), and, as organic ?Since this was written, a paper by Briggs [J. Dairy Rcnaarch, 3, 61 peroxides are moderately active oxidizing agents, there seems (1931)l has come t o t h e attention of t h e writer, in which i t is shown t h a t no reason to doubt the possibility of such a direct oxidation glycerol in small concentration acts a s a n inhibitor in t h e oxidation of butter I n addition, curd and its pyrolytic products esert a marked retarding taking place, even though the efficiency was much lower fat. effect. than in the destruction of inhibitor in the process of breaking 3 I n this eonnection, it must be recognized t h a t the initiation of new chains. chains by the reaction between peroxide and unchanged molecules may conAgain, with reference to the observed effect of acids in tribute t o the acceleration of rate. However, as Backstrom ( 2 ) has shown, accelerating the oxidation of butter fat, it is quite possible the chain length of this secondary reaction in the oxidation of benzaldehyde has a maximum value oi 17, whereas the main reaction has a chain length of that the acid destroys inhibitors normally present. Al- approximately 10,000. The effect of the secondary reartion will, therefore, cohols are notorious inhibitors of oxidation, and, as traces be very small.
standing characteristics of chain reactions. The other important characteristic of chain mechanisms is the high quantum yield obtained when light is used as the means of activation. I n this latter connection, the oxidation of drying oils has already been studied by Rogers and Taylor ( I d ) , and, although they did not actually establish that the reaction gave a high quantum yield, they consider that their evidence points to a chain mechanism. The earliest accurate study of reaction velocity in the oxidation of drying oils was made by Genthe (S),who noted that the curves obtained showed the peculiar S shape which has been found to be characteristic of autoxidations in general. An examination of curves of this type shows that the only abnormality is in the part preceding maximum slope. From the latter point on, the curves are similar in form to those of normal reactions.
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of these chains, we encounter the opposing fact that the maximum rate of reaction is not appreciably greater in the presence than in the absence of drier. It is difficult to understand why the drier would not continue to exert its catalytic effect after the inhibitor was destroyed and impart a greater maximum slope to the curve than is found in its absence. I n studies on the oxidation of films, we might, of course, be justified in suspecting that the diffusion factor imposed a practical limit on the rate of absorption of oxygen. However, in the work of Wagner and Brier (18, 19) this factor is entirely eliminated, yet nowhere is the constancy of maximum slope more clearly shown.4 The precise mechanism of the destruction of inhibitor in the presence of drier, then, is a problem which will be left for further experimental work to decide, as other explanations which suggest themselves involve assumptions which the writer would hesitate to sponsor a t present. To summarize briefly the writer's conception of the induction period, there are, in the absence of driers, two factors to consider-the destruction of inhibitor in the process of breaking chains and the destruction by normal peroxide molecules. The efficiency of the latter will, of course, be less than that of the hot molecules at the moment of formation, but, as the reaction proceeds, the attendant increase in concentration of peroxide will cause the latter to assume a n increasingly important role. I n the presence of driers there will be a third contributing, or superimposed, effect.
is obvious that oxidat,ion chains may be initiated during the polymerization process. Furthermore, if the energies of activation were of the same order of magnitude, or if the heat of reaction in each case were relatively large in comparison with the energy of activation, there seems a t least the possibility that both of the above conditions could be fulfilled. This possibility immediately opens up the prospect that a single reaction chain might involve a purely chance alternation of oxidation and polymerization, the individual links in the chain being governed by local conditions such as the availability of oxygen (21). However, even though this dual condition was not fulfilled, it seems clear that activation energy may be supplied by one type of reaction to start the other type. Thus oxidation and polymerization in the drying and bodying of oils are inextricably bound up with each other and cannot be dealt with as separate problems. It might be mentioned a t this point that the writer does not wish to imply that the only factor involved in molecular weight increase during the drying of paints and varnishes is necessarily the polymerization of the original glyceride molecules. Some of the products of oxidation, particularly the peroxides, probably do undergo polymerization or condensation with the formation of products of high molecular weight. However, in the case of China wood oil especially, it is felt that the greatest importance should be attached to the two typical reactions-oxidation and polymerization of the original glyceride molecules. Finally, let us consider the recent searches for antioxidants RELATIOXSHIP BETWEEN OXIDATIOXASD POLYMERIZATION which might serve the paint and varnish industry as some have served the rubber industry. As has already been Returning to the relationship between oxidation and pointed out (19), a useful inhibitor must retard the degradapolymerization, it has already been mentioned that selenium tion of the film once it has dried but must not retard the dryinhibits both the oxidation and polymerization of China ing process. The assumption that a reaction will be sensiwood oil. It is also well known that driers, which accelerate tive to traces of inhibitors implies that the reaction possesses oxidation, also accelerate polymerization. Therefore, if the a chain mechanism. However, there is no evidence that oxidation reaction is to be considered a chain reaction, poly- would suggest that the slow degradation of paint and varnish merization must be considered in a similar light. In this films is due to chain reactions, although the reactions inconnection it is of interest to note that, since this material volved in drying undoubtedly are. Hence, it seems extravawas presented, quantum yield measurements have been pub- gant to hope that there is any possibility of inhibitors being lished for the photopolymerization of styrene and vinyl found which are inoperative in reactions which are obviously acetate, which indicate in each case a chain mechanism (16). chain reactions but still efficient in later reactions which The association of polymerization and oxidation reactions probably are not. is by no means confined to drying oils, and many cases have been recorded in which oxygen accelerates tremendously the LITERATURE CITED rate of polymerization. Among notable cases of such be(1) Andes, Farben-Ztg., 16, 1937 (1911). havior are the oxidation and polymerization of acrolein ( I O ) , (2) Backstrom, Medd. Vetenskapsakad. Yobelinst., 6 (16), 33 (1 927). styrene ( 1 2 ) , isoprene (20),and cyclopentadiene (15). Back(3) Backstrom, J . Am. Chem. Soc., 49, 1460 (1927). (4) Christiansen, J . Phiis. Chem., 28, 145 (1924). strom ( 2 ) discusses such cases of induced reactions in the ( 5 ) Genthe, 2. angew. Chem., 19, 2087 (1906). light of the chain reaction theory and concludes that molecules (6) Greenbank and Holm, IND. ENG.CHEM.,16, 598 (1923). activated during propagation of chains in the oxidation (7) Milas, Proc. Nut. Acad. Sci., 15, 596 (1929). process may possess sufficient energy to polymerize. There(8) Morrell, I n d . Chem., 1, 64 (1925). (9) Morrell, J. Oil Colour Chem. Assoc., 10, 278 (1927). fore, there is an inherent possibility of a polymerization chain (10) Moureu and Dufraisse, Bull. .SOC. c h i m . , 31, 1152 (1922). being started at any point in the oxidation chain. (11) Moureu and Dufraisse, Compt. rend., 174, 258 (1922). The conditions necessary for such a coupling of reactions (12) Moureu and Dufraisse, Chem. Rev., 3, 151 (1926). (13) Rideal and Taylor, "Catalysis in Theory and Practice," p. may be outlined very simply and briefly if we adopt some 224, Macmillan, 1926; Rhodes and Van Wirt, ISD. Esc; auch notation as the following: E , = energy of activation for oxidation; &. = heat of oxidation; E, = energy of activation for polymerization; &, = heat of polymerization
-1molecule of drying oil which has been activated and has bubsequently reacted with oxygen will yield a product which has associated with it, at the moment of formation, an energy equal to E, 0,. Then if E, Qo SJ E,, a collision with a normal molecule may provide the latter with sufficient energy to cause it to polymerize. Thus, a polymerization chain Q p 5 E,, it may be started. On the other hand, if E,
+
+
+
4 Rriggs (loc. cit.) has also shown t h a t the maximum elope in the oxidation of butter fat is n o greater in the presence of metallic salts t h a n in their absence.
CHEhf., 15, 1135 (1923); Coffey, J . Chem. Soc., 1922, 18. Rogers and Taylor, J . Phus. Chem., 30, 1334 (1926). Stobbe and Dunnhaupt, Be?., 52, 1436 (1919). Taylor and Vernon, J . Am. Chem. Soc., 53, 2527 (1931). TVaele, de, J . SOC.Chem. Ind.. 39, 49T(1920); Coffey. J . Chem. Soc., 1922, 17. (18) Wagner and Brier, IND. ESG. CHEM.,23, 40 (1931). (19) Wagner and Brier, I b i d . , 23, 662 (1931). (20) Wallach, Ann.. 227, 295 (1885). ( z i j W ~ M ,Farben-Ztg., 26, 2851 (1921).
(14) (1.3) (16) (17)
RECEIVED April 9, 1931. This paper consists of material presented before the Division of Paint and Varnish Chemistry under t h e titles "Chain Reactions in the Paint and Varnish Industry" a t the 74th Meeting of the dmeriean Chemical Society, Minneapolis, Minn.. September 9 t o 13, 1929, and "Induction Period i n Autoxidation of Drying Oils" a t the 81st Meeting of the Society, Indianapolis, Ind., March 30 to hpril 3, 1931.
