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unit's of vitamin -1per gram. Potencies of 150,000 units of vitamin A per gram have been obtained by lowering the yield of extract. These concentrates have also had removed much of the fishy odor and taste associated with the original oil. Tall oil has been fractionated to yield a fraction v,ith only 1.3% rosin acid and 2.9% unsaponifiable matter, the remainder being fatty acid ester. This process involves t,hc preferential esterification of the fatt,y acids folloived by fractionation with a mixture of furfural and naphtha. ACKNOWLEDGMENT
The author gratefully aclrnoivledges the assistance of many eo-workers of the Pittsburgh Plate Glass Company, Paint Division Research Laboratories, and especially W, H. Lycan, S. E. Freeman, E.&I. Christenson, and H. A. Vogel.
Vol. 40, No. 2
LITERATURE CITED
(1) D a m , \I J., " and . Evelyn, K. A., Biochem. J . , 32,1008 (1938). 12) Emmerie. 8..and Enele. C.. Rcc. trav. chim.. 57. 1351 (1938). (3) Freeman,' S.'E. (to Pittsburgh Plate Glass Co.), U.'S. Patent 2,200,390 (May 14, 1940). (4) I b i d . , 2,200,391 (May 14, 1940). (5) I b i d . , 2,313,636 (March 9, 1943). (6) I b i d . , 2,278,309 (March 31, 1942). ( 7 ) Freeman, S. E., and Gloyer, S. W. (to Pittsbuigh Plate Glass Co.), U. S. Patent 2,423,232 (July 1, 1947). (8) Hiesen, I$., Fette ZL. Seijen, 44, 426 (1937). (9) Painter, E. P., and Nesbitt, L. L., IND. CNG.CHEM.,ANAL. ED., 15, 123-7 (1943). (10) Parker, W. E., and McFarlane, W.D., Can. J . Research, 18, 406 (1940). (11) Rawlinga, H. W., Oil & S o a p , 21, 257 (1944). RECEI\EDOctober 9, 1947.
FURAN RES1 A R T H U R J. N O R T O N ,
Z S I ~ F I R S T A V E N U E S O U T H , S E A T T L WASH E.
A review of the advantages of the furans as base chemicals for synthetic resin and high polymer work is given. The five basic reactions now utilized industrially in resin work are reviewed, and the advantages accruing from these are discussed in some detail. Particular emphasis is given to the methods of evaluation of plastic materials by tests on rate of flow. In addition to the utilization of the furans by the standard reactions now used, the possibilities of the newer and less understood reactions are emphasized.
I
N ORDER to qualify for an extensive market in the resin and plastics industry, a chemical or seiie? of chemicals must first of all be potentially available in large quantities and of a consistent quality. Second, for direct participation in the foimation of synthetic resins and polymers, the chemical must have a high degree of reactivity and should preferably be,able to enter into a wide diversity of reactions. Third, a high degree of compatability and good solvent poiveis extend the field of usage in this industry, and fourth, the product should be marketable a t a reasonable price. How well the furan chemicals qualify in all these respects is shown by their rapidly increasing rate of usage. I n 1947 the amount of furan chemicals used in the resin and plastics field will be about sixteen times that used in 1937. In the same tenyear period phenol usage in the same industry increased only about five times. The development of a complete background of furan chemistry coupled with the ever-increasing understanding of high polymer chemistry has contributed a great deal to the rapidly increasing rate of usage. At the present time there are five basic reactions of the furans that are commonly utilized in the resin and plastics industry: (a) direct aldehyde condensations, such as in phenol-furfural resins; (b) formation of high polymers through ether linkages, as in the reactions of furfuryl alcohol with dimethyl01 urea; (c) methylene bridging, as in the formation of furfuryl alcohol resins; (d) addition polymerization through the conjugated ring structure of the furan molecule, as illustrated by the final stages of the resinification of furfuryl alcohol in the presence of acid; and ( e ) chemical modifications, as in the production of diamines for polyamide resins. This last phase will not be elaborated on here, but is injected as a reminder of the versatility of the furans as a source of chemicals for resin formation as well as by their more direct utilization in condensation and polymerization reactions.
I n addition t o the classified reactions there are many special and mixed types of reactions that may become of increasing importance as they become better understood. DIRECT ALDEHYDE CONDENSATIONS
Fuifural reacts in general as do all alpha substituted aldehydes. With phenol it condenses in the presence of either alkali or acid to form synthetic resins in a reaction that is quite analogous t o that of phenol with formaldehyde or acetaldehyde. The reaction is exothermic and requires the usual control methods t o prevent the resinification froin going beyond the workable stage. When one mole of phenol is reacted with less than one mole of furfural in the presence of an acid catalyst, the initial reaction appears to be the formation of dihydroxy diphenyl furan methane by the condensation of one mole of furfural with two moles of phenol. Continued reaction links more phenol groups together to form a Novolak or permanently fusible type of hard brittle resin. This resin is largely zt linear polymer of relatively low molecular weight-perhaps eight to twelve phenol groups per molecule. Since only two of the active positions in the phenol ring have been substituted, this product is capable of further reacting with condensing agents or more furfural to form crosslinked or thermoset resins. It is a typical two-step phenolic resin of commerce. Khen one mole or more of furfural is reacted with one of phenol in the presence of catalytic amounts of alkali, the condensation is more rapid, and, without control of the heat of reaction, i t will go to a completely thermoset inert mass. By control it can be stopped a t a point where the condensation product is a hard brittle mass, fusible a t about 100" C. and soluble in spirit solvents. With further heating this intermediate resin will condense through the methylol groups to give a cross-linked thermoset product typical of the one-step phenol aldehyde resins. The condensation products of this general type are usually compounded or mixed with modifying ingredients and used for molding compounds, impregnating solutions, bonding agents, coating materials, or adhesives. In all uses, heat or heat and pressure are required to convert the products t o the inert thermoset stage. Consequently, in every use the ratio of the rate of flow of the material under the application of heat or heat and pressure to the time of cure or setting is the governing factor in the usability of the produet. This relation of the physical property of flow t o the time required
February 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
to convert the product to a completely inert and unworkable material is just as important in bonding or coating work as in molding. If the material does not go through a long enough flowable stage, it will not coat the product t o be bonded properly before curing, or will not level enough during heating t o give a smooth decorative coat. A common method of expressing this ratio of flow to cure is by plotting the rate of flow of the product through an orifice a t a constant temperature and pressure against time. The ratio is generally expressed as grams per second per second. With standard phenolic molding material, when the rate of flow curve is measured at about 150' C. and 2000 pounds per square inch pressure, the first part of the curve rises slowly a t an accelerating rate as the heat of the mold softens the solid cool powder. The first part of the curve, therefore, represents the rate of heat transfer into the material. When all of the material has reached the temperature of the mold, the curve becomes a straight line representing the inherent plasticity or viscosity of the material under the given conditions. Cross linking by further condensation then sets in, and the curve drops to a point of cure-or where the material no longer flows under the conditions. It is a fairly general rule that resins and molding compounds made from the reaction products of phenol with an aldehyde show different flow-cure ratios depending on the molecular weight of the aldehyde. Other things being equal-and they seldom are in polymerization work-the higher the molecular weight of the aldehyde, the faster the rate of flow at a given temperature and pressure, and the longer the flow period because of the slower curing or cross-linking rate. Furfural compounds give more flow than acetaldehyde, which, in turn is greater than formaldehyde. When viewed only from the angle of slower cure time or setting rate, this characteristic used t o be looked on as a disadvantage, for speed of production is very important t o the economic use of plastic materials. Today it is well understood that there is a n optimum flow-cure ratio or rate of flow curve for each type of molded piece and for each condition of resin usage. For example, in molding deep draw pieces such as radio cabinets, the longer flow period lessens the tendency t o precure during molding; it increases the rate of heat transfer and actually, when well balanced, may give a faster cure time for a specific article than a compound that sets too fast and insulates itself during the
Resinoid-bonded grinding wheels
231
molding operation. The correct flow-cure characteristic will also give the optimum hppearance and strength in a given article. The characteristic flow-cure curves of phenol furfural condensation products are taken advantage of in several different ways. First, there is the conventional one-step reaction where one or more moles of furfural are reacted with one mole of phenol with an alkaline catalyst. The compound resulting from this reaction gives products that are inherently thermosetting and are fast flowing with a long period of flow. Second, less than one mole of furfural is reacted with one mole of phenol t o give a Novolak or permanently fusible resin. This can then be compounded with a setting agent such as hexamethylene amine and will result in a product with the fast rate of flow of the furfural types, but with a shorter period of flow because of fast action of the formaldehyde derivative. Third, and obviously, a phenol formaldehyde Novolak can be compounded with hydrofuramide t o combine the slower flow rate of the formaldehyde derivative with the longer flow period of the furfural setting agent. The advantages of this flexibility of control of the all-important flow-cure ratio of resins and their compounds are apparent t o all familiar with the wide diversity of the requirements of the molding and resin using trades. With other phenols the reactions of furfural are about as would be predicted from this discussion. With cresols the reaction rate of the cure cycle is slower than with pure phenol except in the case of rn-cresol. With resorcin, the furfural reaction products will slow down the very fast cures that are sometimes difficult t o control. The fact that furfural is shipped anhydrous offers some advantages in both shipment and storage as well as in the manufacturing operation. There is no water of solution to be removed during the reaction. Another advantage that is sometimes overlooked, or, rather, is not taken full advantage of, is the great solvent power of furfural. It will not only react more readily with some of the substituted amines and phenols that are difficult t o react with aqueous formalin, but, by being used in excess that can later be removed by distillation, it will keep a well advanced resin in solution, and higher molecular weight products can be obtained without getting masses so viscous that stirring and temperature control are difficult. The intermediate one- and two-step furfural phenol resins do not differ markedly in physical and chemical characteristics from those made with formaldehyde and phenol, except for this longer and more plastic flow pe,riod which also helps compounding. Alcoholic solutions of the resins from furfural do tend t o have greater hydrocarbon solvent tolerance, which is often an advantage in the preparation of resin solutions for the impregnation of paper or cloth for laminating. The cured moldings, laminates, and bonds, when properly made, compare favorably with the products from the more conventional formaldehyde-phenol types in general strength and other physical and chemical properties. Less work has been published on the reactions of furfural with ureas, melamines, etc., but the same general principles apply as t o the condensations with phenols. The increased water resistance together with the good flow characteristics t o be expected from furfural resins made with these products opens u p many new avenues of utilization. Ketones, aromatic amines, and other reactive materials condense with furfural in a manner analogous to the reactions with other aldehydes. The physical characteristics of all resins can be varied over a wide range by reaction controls that vary the molecular distribution ratios within the polymer. Just as in the linear addition polymers, these condensation resins consist of a mixture of polymers ranging all the way from traces of the monomeric materials and dimers t o small amounts of thermoset particles.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
This molecular distribution ratio affects the ratios of flow, solubilities, etc., but is no different in principle with the condensation products of furfural than in resins made with other aldehydes. The dark color generally associated with the furfural resins results from side reactions and possibly oxidation. Although this cannot be eliminated entirely, it can be kept to a minimum by good control. The possibility of utilizing the furan ring polymerization for the cross linking of resins made from furfural and phenols or ureas is obvious, but has not been exploited to any great degree under controlled conditions. The high solvent power of furfural, and the fact that the solvent is a reactive one, is taken advantage of in many resin uses, such as the bonding of grinding wheels and cold molding, Here the furfural is used t o soften the iesins, to permit cold forming, and later may be reacted with the resin bond. While resins and the compounds made from them are sold primarily for their specific properties and qualities, the old Q/C or quality over cost formula still holds. With equimolar ratios of phenol and furfural, the material cost of the resulting resin with 9.5-cent furfural is less than that of a corresponding resin made from 37% aqueous formaldehyde a t the old price of 3.2 cents a pound. The increased yield of resin due t o the higher combining weight gives furfural this advantage. FORMATION OF H I G H POLYMERS THROUGH ETHER LINKAGES
Furfuryl alcohol forms ether linkages readily with other active hydroxyl groups. The best examples of this reaction are the resins formed by reacting furfuryl alcohol n i t h dimethylol urea. Two moles of dimethylol urea axe mixed with one mole of furfuryl alcohol in aqueous solution and reacted at 100' C. and then dehydrated to about 607, solids. -4viscous solution of a light amber colored heat hardening resin results. This sets iapidly a t 150" C., and a t a lower pH-about 3.5 t o 4-xill cure a t 80100' C. These products give strong paper-based laminates and can be used for high frequency or lo^ temperature curing assembly adhesives in the woodworking industry. The presence of the furan ring structure minimized crazing, checking, and shrinking of the cured products, and increases their water resistance over that of conventional urea-formaldehyde products. The possibilities of this reaction are obvious, not only with ureas, but with the methylol melamines, methylol phenols, and other polyfunctional methylol types. Here again the flow cure ratio adjustment is quite flexible, this time in the more watersoluble types of intermediate products. By care in minimizing the free furfuryl alcohol content, of this type of ether-linked material, very light-colored products result. Unquestionablv the exploitation of this reaction has just begun. METHYLENE BRIDGING FROM FURFURYL ALCOHOL REACTION
When furfuryl alcohol is mixed with acid in aqueous solution, a highly exothermic reaction almost explosive in nature takes place and results in a dark, inert thermoset product that is unusually alkaline and acid resistant. I t has been shown that by adequate heat transfer facilities, by careful pH control, and by neutralizing a t the desired point, this reaction can be controlled t o give polymers of almost any desired viscosity. I n fact, it has been demonstrated in pilot plant operation that this reaction can be controlled and carried to the desired intermediate stage in a continuous operation. This continuous process eliminates many of the hazards of batch operation and also eliminates the necessity of storage of large batches of the resin. Several reactions are involved in this condensation reaction, but in forming the viscous polymers with the widest ranges of commercial utility, the first and dominating reaction is the build-
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Vol. 40, No. 2
ing up of polymers of furan rings linked by a methylene group. The reaction consists essentially of the elimination of water by the reaction of the alcohol group of one molecule of furfuryl alcohol with an active hydrogen from the ring of an adjacent molecule. The final cross linking or setting reaction probably involves the polymerization of the unsaturated furan rings in one chain with those of another chain-a typical addition polymerization. By stopping the reaction a t the desired point, viscous resins capable of further setting within themselves or of reacting with other reactive resins are obtained. These products have found use in making of cements, mortaring acidproof brick, mixing with asbestos and other fillers for the production of chemically resistant equipment, etc. The reaction can also be run in situ. For example, wood or asbestos board is impregnated with furfuryl alcohol or with a solution of the resins and, on acid treatment, goes t o an inert thermoset r e h . This treatment is often used for laboratory furniture, bar tops, etc., where chemical resistance is desirable. There are many obvious modifications of this reaction and even more use possibilities that are scarcely more than explored. The reaction can be made to run with almost any group containing a reactive hydrogen. For example, furfuryl alcohol can be condensed with formaldehyde to give a series of thermosetting resins which probably have more methylene bridges than are obtained by the direct resinification of the furfuryl alcohol. These dark viscous resins are heat hardening and have been used in laminating paper, bonding wood, and alkali-iesistant finishes. Xany resins such as this are relatively new. The reaction has been only recently explained and understood. While the obvious uses have been evaluated and suggested, it has been the history of the plastics industry that each new compound finds it. own special field enhancing and enlarging the value of all plastic\. ADDITION POLYMERIZATION THROUGH CONJUGATED FURAN RING
Polymerization involving the nuclear double bonds probably occurs to a greater or lesser extent in almost every resin-forming reaction of furan compounds. However, little information is a t hand as t o the mode of such a reaction. Unlike the vinyl types, furans do not appear t o undergo peroxide-catalyzed additionpolymerization; it is probable that the peroxide is used up in oxidation of the furan. Evidence for addition-polymerization is almost solely deduced from the behavior of furfuryl alcohol polymers during the later stages of reaction. I n this case it is indicated that the reaction is induced by high temperature and acidic catalysts. I n addition there are the possibilities of Diels-Alder addition products. The chief commercial utilization of this reaction is in the setting or cross linking of the furfuryl alcohol polymers a t present, but the possibilities of utilizing this reaction more fully are enormous and intriguing. Low pressure laminating resins and phenol-aldehyde resins of a more flexible type using molecular plasticization by copolymerization are among the possibilities that come t o mind. The great reactivity of the furan chemicals and the necessity for careful control to direct the particular type of reaction desired has probably been a retarding influence on their use in plastics, as well as their greatest asset. It is easy t o forget the many types of reactions that may be going on simultaneously with the one desired. As these reactions and their control become more thoroughly understood and better recognized, the possibilities of usage of these unique chemicals, made from a perpetuating source of raw materials, spiral upward at a breath-taking rat?. RECEIVEDOctober 8 , 1947.
O F SYMPOSIUM
