e
re in Therm
EUGENE BARR Bakelife Co., A Division o f Union Carbide and Carbon Corp., Bloomfield, N. J.
Since the inception of the thermosetting resin industry some 45 years ago a wide variety of resins have been introduced and accepted commercially. The more important types include the phenolics, ureas, melamines, alkyds, unsaturated polyesters, silicones, epoxys, and polyester rubbers. The reasons for the success of each of these types and how they differ in cure are explained. Definitions of cure are presented, along with the presently known methods of measuring the degree of cure. These techniques leave much to be desired. The importance of knowing how to measure precisely the degree of cure is stressed. Some progress has been made along this line in the past few years, but it i s a slow and difficult proposition.
H E use of resins dates back to ancient times. Abundant TIndia, evidence exists in the literature of Egypt, Babylon, Persia, Greece, and Rome to indicate that naturally occurring resins R-ere used in incense, medicine, coating compositions, paints, and dyes. Fr. Theophilus, an 11th century monk, made an oil modified varnish from a resin (probably amber) and linseed oil. The Renaissance painters used amber varnishes and the notes of da Vinci bear aitness to the fact that he experimented with oils, varnishes, and pigments. The art of lacquering, based on the use of native natural resins, originated in China and reached its highest development in Japan, where the records of its use there go back to the fourth century ($8, SO). The peoples of India and the Far East have long used the products of the lac insect (mainly shellac) for the fashioning of molded ornaments and the manufacture of varnishes. A red dye extract of shellac, known as lac dye, was a valued article of commerce before the advent of synthetic coloring materials, Shellac mas first introduced to Europe in the 17th century by the Ea& India Co. and thereafter became a very important and valued world commodity (29). Eventually, through England, it came to be used in America and its use greir- n-ith the economy until about 1900. -4t that time, it was generally recognized that shellac was too slovc- a thermosetting resin for the many and varied mass production applications then contemplated. Consequently, numerous people n-ere xorking on shellac substitutes. L. H. Baekeland v a 8 among that group. He was concentrating on the resinous reaction products of phenol and formaldehyde. hIany had tried this approach v-ithout success (IO). Baekeland’s contribution was finding out how to control the cure of phenolic resins. He demonstrated between 1907-10 that through the use of proper catalysts, pressure, and temperature, one could mold a resin into an infusible and insoluble shape in a short period of time (6). His unique discovery marked the beginning of the synthetic resin industry. In fact, it can be said that the very rapid cure of phenolic resins was the biggest single factor in gaining commercial acceptance for these new materials (28). The ramifications of Baekeland’s discovery were so great that his original goal of replacing shellac in phonograph records was not achieved: it aviaited the development of vinyl and polystyrene resins. Another monumental discovery of this period-and one incidentally that preceded Baekeland’s by quite a fen. yearswas Goodyear’s finding that rubber could be cured or vulcanized by the addition of sulfur. The technical value of this contribution and its subsequent effect on the economy of the world is m-ell known. 72
During the 45 years following the Baekeland discovery, a wide variety of new thermosetting resins were introduced and accepted commercially. A representative but not necessarily all inclusive list is Urea-formaldehyde resins Oil modified alkyds Melamine-formaldehyde resins Unsaturated polyesters Silicones Epoxy resins Polyester rubbers A11 these resin types exhibit good physical properties under a variety of conditions, both at room and elevated temperatures. This retention of physical properties at elevated temperature is one of the main characteristics of a thermoset product and it is attributable to the degree of cure. The cure in the various resin types is achieved through different mechanisms and for different reasons. How and why these types differ are reviem-ed. Thermosetting Resins
Urea-Formaldehyde Resins. The commercial development of urea-formaldehyde resins began about 1920 with their use as an adhesive (11, $0). Like phenolics, the very rapid cure of the urea-formaldehyde resins under acid conditions at low temperature was the main factor in gaining acceptance for this type of a material at the expense of naturally occurring glues. I n addition, the potentially lower costs and lighter color were further factors ensuring their commercial future. Oil-Modified Alkyds. It was about 1912 that the first concentrated effort was made to prepare useful alkyd resins that might have commercial value. This work was carried out a t the General Electric Co. and again the goal was the replacement of shellac in electrical insulation. Other than a few patents, very little of practical value was derived from this work. Ho\\ever, in the next 15 years a tremendous amount of experimentation was conducted, as reflected in the scientific and patent literature of that period (18). For a variety of economic and technical reasons, very few of these results found any extensive application, In 1927 a broad patent was issued to Kienle on fattv acid oil-modified alkyd resins, These resins of Kienle ( 1 7 ) were truly varnishes in themselves; they dried to hard coatings in much shorter periods of time than comparable oleoresinous varnishes, and the films were snperior to those of the natural resin vehirles. This discovery revealed the full potentialities of
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THERMOSETTING RESINS alkyd resins, It created a real interest in and a demand for superior fast drying varnishes, which has continued undiminished until the present time. Besides color and gloss retention, one of the factors which gained a major role for oil-modified alkyd resins in t h e paint and varnish field was rapid cure. Melamine-Formaldehyde Resins. The introduction of melamine-formaldehyde resins in 1940 marked the first time that the over-all objective was not the displacement of a naturally occurring product. Rather the purpose was to improve the properties of existing synthetic resins, mainly, the urea-formaldehyde resin type. The melamine-formaldehyde resins did this to a considerable degree. Besides retaining the excellent color of the ureas, the melamine-formaldehyde type had superior stability, as well as better moisture, chemical, and abrasion resistance ($1, 32’). These improvements can be traced to the unique triazine structure in the melamine molecule, which has some of the stability of the benzene nucleus, Brittleness is a factor with this type of a resin, This is due to the high degree of cross linking possible with the melamine molecule (32). Unsaturated Polyesters. The unsaturated polyesters, which are cross linked through the polymerization of vinyl compounds, were introduced during World War I1 (about 1942). The novel feature of this type of thermosetting resin was that no volatiles were liberated during the cure. A series of circumstances gained commercial acceptance for this sort of a material. The most important factor was the ability of this type of resin to be molded in combination with glass cloth a t very low or contact pressures and still give excellent electrical and good mechanical properties. This permitted the use of inexpensive molding equipment at a time and under circumstances when high pressure equipment was neither available nor desirable. The ultimate result of this whole development has been the ability of industry to fabricate large and intricate shapes, which otherwise would not have been possible (19). Silicones. The silicone resins were, likewise, introduced during World War I1 (about 1944). These materials were readily accepted by industry because they imparted a degree of heat resistance to coatings, laminates, moldings, and castings that was previously unknown. Even though the mechanical properties are not very high initially, these properties are retained at high temperatures (200” C.) for extended periods of time. Water repellency, good dielectric properties, nonyellowing, and weather resistance are some of the other desirable qualities that ensure the silicone resins a place in industry (2’4). High cost is a factor which tends to limit the scope of the market at the moment. Epoxy Resins. The epoxy resins were first introduced commercially in 1947. Like the polyesters, this type of a resin does not liberate any volatiles on curing. For polymerization and cross linking, it depends on the opening of the oxirane or epoxy ring with active hydrogen atoms. This type of resin is a fundamental improvement on phenolics and it brings to industry some extraordinarily fine properties of chemical resistance, flexibility, adhesiveness, castability, and dimensional stability ($1). Polyester Rubbers. The polyester rubbers have only recently come on the market, They are combinations of hydroxy- or carboxy-ended saturated polyesters and reactive cross-linking agents, such as diepoxy or diisocyanate compounds. Due to the restricted number of cross links the resulting rubbers have the attractive properties of unusually high tear and abrasion resistance together with an outstanding combination of high modulus and snappy recovery ($6). The foregoing variety of thermosetting resins can be conveniently classified into two main groups. Type A Phenolics Ureas Melamines Silicones lanuary 1956
Alkyds Polyesters Epoxys Polyester rubbers
Type A represents condensation polymers, which liberate volatiles during cure. Type B is indicative of an addition polymer wherein no volatiles are generated during the cross-linking reaction. This breakdown is similar to classification suggested some time ago by Carothers ( 9 ) . The volatiles liberated during the cure of Type A are predominantly water, along with traces of formaldehyde and volatile catalysts (27). When hexamethylenetetramine is used as the hardener with phenolic resin, some ammonia is liberated (34). With silicones the volatile material is generally water.
Measurement of Cure The term “cure” in the chemical sense means complete or total cure. That is to say, all of the reactive groups, such as methylol and epoxy, or all of the reactive sites, such as unsaturation and hydroxyl, in a resin molecule are consumed during the hardening step. However, in the plastics industry this term “cure” has come to mean “optimum cure,” which is that degree of cure necessary to give the best physical properties for the application on hand, The cure requirements in the field vary widely from flexible coatings to molded parts to castings to laminated sheet stock. What has happened over the years is that the proper degree of cure was worked out empirically for each application, generally by the user and sometimes in cooperation with the resin manufacturers. For the most part, therefore, the knowledge of the degree of cure of all thermosetting resins is known only indirectly. This is why reliable, accurate, and if possible nondestructive methods of measuring cure are so badly needed. This is desired not only for the inspection and process control of the resin themselves, but for the establishment of improved cure cycles from the standpoint of both quality and economics. This question of cure-and how to measure it-was recognized as a problem early in the industry. Such tests as, gel time (no agreement on a standard test procedure) acetone extraction ( 1 ), moisture resistance (a), hot flexural strength ( 4 ) , hardness ( 6 ) , resistance to film cracking in acetone, and blister free time were developed over the years and in many cases are still being used as standard ASTM tests. Gel time, which is a measure of the potential reactivity in a resin, is probably the most widely used and controversial test in the industry. I t is somewhat informative in manufacturing and processing all types of resins but is of little use in determining cure. As far as molded, cast, and laminated products are concerned, the hot rigidity or hot flexural test is one of the most useful of all the empirical tests. In coatings, solvent resistance, hardness, and scratch tests are widely used to measure completeness of cure ( 1 6 ) . The conical mandrel test gives a measure of flexibility (2’). I n recent years the subject of the cured state has received considerable attention, The contributions of Carothers ($6) and Flory (13, 1 4 ) on polycondensations, mechanisms, and the gel state have done a great deal to clarify the picture of the infusible state. A variety of workers in Europe such as, Zinke, Hultzsch, Ziegler, and von Euler, ( 2 7 ) have done a fine job of determining the chemical structure of cured phenolic resins, A similar approach, but not as complete a one, has been attempted on the structure of melamine- and urea-formaldehyde resins by a variety of investigators (26, 33). As might be expected the story on the structure of the newest thermosetting polymers is not as complete. The curing cycle of thermosetting resins can arbitrarily be divided into two stages-up to the gel point and from the gel to the set point. This last area is of greatest interest to industry. For example, some resins attain their final cure very quickly after reaching the gel point, whereas other resins will remain fluid for some time before the final set is achieved. This results in quite different properties, due to a reduction in the strain level in the latter case. The reduction in shrinkage of an epoxy casting over a polyester casting illustrates this point.
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The presently known chemical methods, with the possible exception of methylol determinations (29) (also known as labile or latent formaldehyde determinations) are unsatisfactory in the gel to set area because of solubility limitations. There is no doubt that if measurements are made in this area, they nil1 take the form of calibrated physical tests. An effort along this line has been made by a number of investigators in the last few years. Bennett and Avenell have applied the Schmidt-Bisterfeld hot needle test method to phenolic molded parts with some success (7). Dietz and his coworkers have used ultrasonic wave propagation to follow the cure of phenolic molding resins (31). Electrical conductivity measurements ( I % ) , density ( 1 2 ) , and infrared radiation ( 8 ) have been reported as methods of following the cure of molding materials, resins and coatings. These methods, with the exception of the hot needle test, are nondestructive test procedures which give a relative measure of the degree or extent of cure of thermosetting resins. The ultrasonic wave propagation technique of Dietz is probably the best example of an approach whereby data on the extent of cure would be collected during the actual molding operation. The data of Dietz (expressed in the form of parameters) is only relative. The long range goal is to have information absolute. This will come when it is possible to correlate the physical information Kith the chemical structural data of individual polymers. The attractiveness of these test procedures lies in the fact that they can be handled automatically to give reproducible data in a relatiiyely short period of time. This will be a big step forward from the present state of the art which requires empirical timetemperature cycles to achieve questionable optimum properties on finished moldings, castings, laminates, or coatings. In summary then, one of the main selling points of thermosetting resins is their good physical properties under a variety of conditions at both room and elevated temperature. Cure makes it possible to obtain these properties at elevated temperatures. This highlights the necessity of knowing a great deal more about the cured state, about cure, and how to measure it precisely. Some progress is being made but it is a slow and difficult proposition. I t is expected that as the thermosetting industry grows, a great deal more information will be uncovered concerning the nature of the primary and secondary valance bonds in the cure state (33). This in turn, will provide the right atmosphere and additional incentive for the development of fast precise tests for the measurement of cure. Acknowledgment
The author takes pleasure in recognizing the help and valuable assistance of Jane Ulrey and of his associates H. L. Bender, Ivey Allen, L. Shechter, J. Wynstra I-.Auerbach, C. S. Myers, K. D. Hanson, P. A. Thomas. C. F. Pitt, and E. W. Krummel in preparing this paper.
Liferature Cited A.S.T.M. Standards on Paint, Varnish, Lacquer and Related Products, Specification D 49446, Am. Soc. Testing Materials, Phila. 1941. Ibid., D 5 2 2 4 1 , 1946. Ibid., D 5 7 0 4 2 , 1942. Ibid., D 648-45 T, 1945. Ibid., D 785-51, 1951. Baekeland, L. H., Chem. Ztg. 33, 317, 326, 347, 353, 1268 11909); 36, 1245 (1912). Bennett, J. H., and Avenell, C. E., Chemistiy & I n d u s t r y 1952, p. 936. Brugel, W., and Farbe, V., Luch 58, 475, 523 (1952). Carothers, IT. H., J . Am. Chem. SOC.51, 2548 (1929). Ellis, C., “Chemistry of Synthetic Resins,” p. 277, Reinhold, Xew York, 1935. Ibid., p. 564. Fineman, AI. K’.,Puddington, Can. J . Research 25B, 101-7 (1947). Flory, P. J., J . Ant. Chem. SOC.63, 3083 (1941); Chern. Revs.39, 137 (1946). Flory, P. J., “Principles of Polymer Chemistry,” Cornel1 University Press, Ithaca, N. Y., 1953. Gams, A , Widmer, G., and Fisch, W., Hah. Chim.Acta. 24, 302 E(1941); Marvel, C. S.,Elliot. J. R., Roettner, F. E., and Yusha, H., J . Am. Chem. SOC.68, 1681 (1946). Gardner, H. A., and Sward. G. G., “Physical and Chemical Examination of Paints, Tarnishes, Lacquers and Colors,” p. 158, Washington, 1950. Kienle, R. H., hleulen, P. A. van der, and Petke, F. E., J . Am. Chem. SOC.61, 2258, 2268 (1939); .Kienle, R. H.. and Petke, F. E., Ibid.,62, 1053 (1940); 63, 481 (1941). Kirk. R. E., and Othmer, D. F., “Encyclopedia of Chemical Technology,” vol. 1, p. 520-1, Interscience Encyclopedia, Inc., Ii. Y., 1947. Ibid.,p. 590. Ibid., p. 742. Ibid., p. 867. Ibid., vol. 11, p. 667. Ibid., vol. 12, p. 244. Ibid.,vol. 12, p. 393. Mark, H. F., Chem. Eng. S e w s 32, 3122 (1954). Mark, H., and Whitby, “Collected Papers of V.€3. Carothers on High Polymeric Substances,” Interscience Publijhers, Iiew York, 1940. Martin, R. W., “Phenol-Aldehyde Chemistry,” unpublished. RIorell, R. S., “Synthetic Resins and =Illied Plastics,” Oxford Press, 1937. Muller and AIuIler, Kzinstofle 41, 186 (1951); 42, 57 (1952). Scheiber, J., and Sandig, K., “Artificial Resins.” p. 1-7, Pitman, London, 1931. Sofer, G. -4., Dietz. A. G. H., and Ilauser, E. A., IND.ESG. CHEM,45, 2743 (1953). Wohnsiedler, H. P., Ibid., 44, 2679 (1952). Ibid.,45, 2307 (1953). Zinke, A , J . Applied Chem. 1, 256 (1951).
RECEIVED for review June 30, 1955.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
.ACCEPTED September 2 , 1955.
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