COAL BY-PRODUCTS The Yellowing of Indene-Coumarone Resins WILLIAM H. CARMODY Carmody Research Laboratories, Inc., Springfield, Ohio
HE synthetic resin industry has witnessed spectacular development due to modern demand for improved products to fill new uses, and to considerable competition between the various classes of resins within the industry. All resins have not enjoyed the full benefit of technical advance and in a relatively backward state are Courtesy, C o m p r e s s e d Industrial Gases, I n c . the natural resins and those derived FRACTIONA4TIOSCOLUMNS I N A COMVERCIAL HYDROGEN PLANT from coal by-products. The well Filtered air is compressed i n t h e multistage compressor i n t h e foreground a n d passed throuqll t h e known indene-coumarone resins have absorbers in t h e right background, which remove moisture and carbon dioxide. T h e dry a1r is expanded, and t h e liquefied portion flows t o t h e columns where separation is carried out. Electrolytic h y existed in this dormant state until drogen f r o m accumulators is compressed a n d purified in similar t y p e dryers and p u m p e d direct intu cylinders f o r transportation t o t h e point of consumption. recently. They are quite old as resins and a t present are produced a t an annual rate of close td 20,000,000 pounds by essentially the development has occurred, but efforts have mainly been same methods as ahen first commercially introduced. Some directed to improvement in color, odor, yield, melting point. and solubility in various petroleum solvents. These are physical rather than chemical improvements. The work has adhered to traditional paths and no serious attempt has been made to study the basic materials from a fundamental viewpoint to determine the causes of the more serious detrimental The detrimental after-yellowing of conicharacteristics. mercial indene-coumarone resins when exposed to air and light has prevented their Major Defects universal use in coating compositions. The The yellowing of indene-coumarone resins wab early recogfundamental cause for the discoloration is nized as an insurmountable barrier to widespread use in explained on the basis that indene is not a products requiring permanent color stability. The other derivative of benzene, as has been previously admirable properties of this type of resin have been overassumed, but is a cyclopentadiene derivashadowed by the one major weakness of discoloration. Poor tive. Indene and all its polymerization solubility in petroleum hydrocarbons has also limited the usefulness of these resins in a number of fields. stages retain the activity attributed to Statistics indicate that 85 per cent of indene-coumarone cyclopentadiene and can interreact with production is used in the manufacture of such articles as floor ketones, aldehydes, and oxidized products, coverings and in the rubber industry. Kot over 10 per cent with the development of intensely colored is consumed in paint, varnish, or other coating materials. bodies. Such colored bodies have been The remaining portion enters into the manufacture of such specialties as oilcloth, chewing gum, and adhesives. Color termed “fulvenes” by Thiele and have is of minor importance in the fields that consume the major been recognized as the underlying cause of percentage of this resin. defective characteristics in coal tar resins. Careful analysis of the problem indicated that the discoloraThe slow continued formation of fulvene tion was inherently due to chemical reactivity centering structure in resin during use is the direct around a t least one of the units comprising the polymer molecule. This definite chemical clue, combined with the tremencause of after-yellowing. dous amount of raw material potentially available from coke ovens, made the problem attractive as a research project.
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S o pertinent work prior to the present investigation has been reported in this field. The Neville Company of Pittsburgh initiated a program of resin study in which the author was actively engaged for a time. The newness of the field from a theoretical viewpoint and the general lack of detailed information was encouraging, and promised early solution and numerous discoveries. Approach to the Problem Prior to this time most information had been obtained by the method of trial and error, with no apparent plan or logical attack in evidence. No basic or exact knowledge of the polymerizing reactions or structure of resulting polymers could be found. A number of speculative explanations were offered by different workers (2,3,6), but they were more in the nature of possibilities and did not lead to progress in the industry. Two possible configurations existed for indene, but from lack of incentive no investigation was made as to whether the two different forms might lead to distinct paths of reaction. No published work indicates that the recognition and acceptance of only one of the theoretical structures offered a means of explaining all that had previously been observed, and permitted the prediction of reactions and properties of indene and its polymers. The first step was that of examining the properties of the resulting polymers and explaining these properties in terms of the original structure as closely as the facts would permit. Unsaturation values of the polymers indicated that only one double bond existed per resin molecule, that such unsaturation resided in the terminal unit, and that this terminal unit resembled the parent indene or coumarone, with slight modification. Early attempts by varnish makers to remedy the situation were futile because an incorrect approach did not allow of solution, and were made solely on arbitrary premises. It was believed that impurities were the cause, and that the yellowing varied from year to year, from source to source, and with the process involved in carbonizing coal from which the crude indene-coumarone cut is made. By coating pieces of solid resin with a protective material to exclude contact with air, yellowing was eliminated; preservation in the absence of air maintained constancy of color. This was the only definite recognition of the role played by air (oxygen). All previous research was directed to finding a substance which when physically mixed with the resin would inhibit the reaction by its presence. Failure resulted in all cases.
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ISOMERIC FORMSOF FIGURE1. Two POSSIBLE INDENE A . Only structure fully explanatory of all its
characteristic reactions.
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No evidence indicating its existence.
An approach to the problem from the reverse position brought success; that is, if air cannot be kept from the resin in use, the resin molecuIes should be examined to determine just where the oxygen may enter and to learn what simple or complicated line of reactions its entry may initiate. By adhering to simple assumptions, the polymerizing mechanism
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was satisfactorily worked out, and the reaction between sulfuric acid, indene, and coumarone became fully known.
Indene Structure Two structures present themselves; both appear to be derivatives of benzene. These two forms arise by virtue of the positions a t which the cyclic side chain may be attached to the aromatic rings, with the resulting distribution of linkages. These structure possibilities differ only in the linkages, since the relative positions of the elements of the indene molecules are identical in both forms. They are shown in Figure 1. Literature references overlook the fact that indene map logically be considered a derivative of cyclopentadiene, shown in Figure 2. Credence is given this view since units of five carbon atoms are widespread in nature. In addition, coal is formed mainly from conifer types which are related to present-day pines; and pine is an abundant source of terpeneswhicharemultiplesof a five-carbon unit. This new conception of indene structure was explanatory of a number of phenomena. Tests are well known which are characteristic 5 of the cyclopentadiene structure. Thiele (4) FIGURE 2 described the reaction between ketones and CYCLOPESTA- cyclopentadiene as being due to the reactivity DIENE of the methylene group, with a splitting out of water during condensation, to give highly unsaturated bodies of intense coloration termed “fulvenes”. The term “fulvenation” signifies this type of reaction which indene and a11 its poIymers give with facility, and which bear out the dominance of the cyclopentadiene structure in indene. Color reactions carried out with indene polymers indicate that the structure of Figure 1A is the correct one, since fulvenation is not possible with B. Cyclopentadiene, indene, and their polymers will react with ketones or aldehydes in the presence of alkali t o generate a fulvene structure according to the generalized mechanism of Figure 3 . The threedoubIe bonds in the formula of Figure 3 are in crossed or doubly conjugated position; they impose a structural .$-. strain on the mole“2, cule and in mass I40 I 2 appear colored be.O* cause of the abi?--t--i?‘ R-G-R’ sorption of light of FIGURE 3. FORMATION O F 4 FULVENEd ef i nit e wave TYPECOMPOUND BY T H E REMOVAL OF length, suchfacts ONE MOLECULE OF WATERIN THE can occur accordPRESEXCE OF ALKALI ing to structure A , Figure 1. The results are substantiated by direct experiment and are here offered as fact. Explanation of yellowing or discoloration in terms of the fulvene reaction is simple, yet its importance escaped earlier workers. The entire phenomenon can be explained as follows: Indene and all its polymerization stages contain only one double bond. Like all double bonds or points of unsaturation, this is an inherent weakness and the molecule is particularly open to attack by a number of reagents. Air containing oxygen is one of them; it proceeds to attack and break down the molecule with which it unites. Oxides and peroxides exist as short-lived reaction intermediates which continuously break down with the formation of aldehyde or ketone compounds. These readily react with unoxidized
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INDUSTRIAL AND ENGINEERIKG CHEMISTRY
indene resin polymer molecules to form the fulvene type compound, and the development of such a structure in commercial indene-coumarone resin is the direct cause of afteryellowing. Water is eliminated during this chemical reaction, and the colored hydrocarbon structure does not of necessity contain oxygen. Oxygen i? temporarily added to the resin and later is expelled as fulvrnation spreads through the mass. The simple reactions responsible for the discoloration are here unfolded to show the continuity and completeness of the present conception of resin chemistry from the color viewpoint. Fulvenation has been accelerated by alkali, ultraviolet light, heat, or exposure to oxidizing conditions. \Then such resins are used in films, etc., and exposed to air and sunlight, two reactions set in simultaneously. One causes the aldehyde formation, the other initiates fulvene development. The rate and extent of yellowing depends on the rate of formation, quantity of aldehyde present, molecular magnitude involved, and environment favoring fulvenation. Figure 4 indicates the formation of aldehyde from indene, with the structure of the resulting compound resembling that found in the parent indene. Oxidation may also occur in other ways and further degrade the molecule, or ketones may be formed. Identical reaction possibilities are attributed to polymers of indene, regardless of molecular size.
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cy clo pen t ad i ene - p r e d o m i n a ti n g structure in all indene polymers. A s result of the experimental work it is proposed to rewrite the structure of indene polymers in simplified form; only those components will be shown in their true relationship which are necessary to explain the yellowing phenomenon, and to develop nieans c\c/cH of preventing after-yellowing in availHz able commercial resins. FIGURE 5 . GESFigure 5 , the generalized and ERALIZED FORMULA simplified formula, is constructed FOR AN INDENE along the same plan as the indene POLYMER monomer, with all its reaction possibilities. It can oxidize and fulvenate. R is a multiple of indenemitsin linear This spontaneous and unpreventable arrangement. oxidation and fulrenation is the sole reason why such resins have not been adopted on a broader scale of use by the coatings industry. The double bond outside of the aromatic ring is susceptible to oxidation, and is that center about which the formation of acid, aldehyde, or ketone does take place. Both indene and coumarone and their polymers have parallel reactions at this point, and either can Serve as the source of intermediate oxidation product. The methylene group in the adjacent position, which is the complementary factor in the yellowing reactions, can lose its two hydrogen atoms singly or together. Spontaneous yellowing of a mass of resin is indicated in Figure 6, where an oxidized portion chemically combines with an unoxidized portion. By such a reaction the molecular weight and melting point is increased, while the solubility in petroleum solvents CH is materially diminished. The structure shown is the most recent conception of that exirting in discolored or fulvenated mass of resin. Its existence is synonymous with color: its G>cH2 increase is accompanied by a c darkening of the resin mass. /"= The way to prevent this type CH of discoloration is to prevent the initial reaction between F~~~~~ 6. c ~OF osygen and polymer molecules. This is theoretically accomAN OXIDIZED INDENE plished by any means which POLYMER WITH AN UNsaturates the solitary double R:,',"~~o~f~ bond in the terminal unit outTYPECOMPOCND side the aromatic ring shown in the generalized formula in Figure 5 . This saturation blocks the entire line of reaction steps, and the polymer will remain indefinitely in its initial color state without further change. Color stabilization has been introduced commercially, and has given rise to a new line of resins and oils whose properties are vast improvements over those of the earlier known indenecoumarone resins.
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Since the fulvenes represent a class of compounds, a large number of members are possible, and the color of each individual is different; it depends entirely on the distribution and magnitude of the components adjacent to the doubly conjugated system. The source of aldehyde may be any organic material. Should the resin be cooked in a varnish, the oils themselves serve as the source of aldehyde. Under the influence of heat the oil-derived aldehydes react with the terminal indene unit of the resin to form the dark fulvenated body. This characteristic darkening of indene-coumarone resin has been noted since the first varnish was prepared with this resin as one of the components. It is also one of the reasons why such resins are not widely accepted in varnishes, since variation in color a t this point is often a serious matter. Proof of the present fulvenation theory as applied to resins does not depend only upon the monomeric form of indene. When this form is treated with catalysts, a family of polymers results which may range from the dimer to one containing close to thirty units. Commercially useful polymers will average close to a molecular weight of 800, with an average of seven to eight units in the molecule. The most probable structure of indene polymers was indicated in a prior article; it is the result of the formation of acid-indene complexes, liberation of the acid to form polymers, and liberation of the last trace of acid from the terminal units to regenerate a substituted indene structure (1). The monomer, dimer, tetramer, and octamer have been obtained in reasonable purity, and all have been found to undergo reaction with acetone in the presence of alcoholic sodium hydroxide. These tests show retention and possession of the
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Literature Cited (1) (2) (3) (4) (5)
Carmody, W. H., IND. ENG.CHEM., 29, 576 (1937). Risi and Gauvin, C a n . J. Research, 13B,228 (1935). Staudinger, H., Ber., 53, 1073 (1920) Thiele, Ibid., 33, 666 (1900). Whitby and Kats. J . Am. Chem. Soc., 50, 1162 (1928).
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