ACRYLIC RESINS
HARRY T.NEHER Rohm & Haas Company, Bristol, Pa.
NE of the outstanding chemical developments of recent years in the industrial field is the rapid rise of synthetic resins to a position of considerable importance. Among the more recent of the commercially available resins are the polymers of the acrylic acid derivatives, known commercially as Acryloids.’ Their commercial exploitation is another interesting example of the rather belated realization of the importance and value of substances knowu for many years. The credit for the recognition and industrial development of these substances belongs, in large measure, to Otto Rohm of Damstadt, Germany, one of the early investigators in this field.
Historical Basis Acrylic acid itself has been known for almost a hundred years. In 1843 Redtenbacher (19) reported the preparation of a new acid by the oxidation of acrolein with air and named it acrylic acid. The silver, sodium, and ba,rium salts of this new acid were prepared by him. On the basis of later work (8) it appears likely that he also prepared the ethyl ester in an 8 For the sake of simplicity tire term Acrrioids haa km s p d i e d to the variow ~ e b i n s made from derivatives of B E I Y ~ ~ Oneid, CH*:CH.COOR. and methacrylic arid. CHz:C.COOW. They are inanufertured snd sold
CHz in this country by R6hm 8 Hsse Company.Phiiadelphia. Aoryioid ie registered i n the U. S. Patent O f k c .
The trade name
267
irripure form. A solid modification (polymer) of acrylic acid, which gradually swelled in water and alcohol and “finally dissolved to a rubber-lie acid” and formed Salk with metals, was described by Linnemann in 1872 (15). In 1873 Caspary and Tollens (7) prepared the methyl, ethyl, and allyl esters and observed that the allyl ester polymerized a t ordinary temperatures, especially in the sunlight, to give a “clear hard transparent mass.” The polymer of methyl acrylate was iirst described by Kahlbaum in 1880 (14). The perfect transparency of the polymer and its remarkable physical properties led him to investigate it further. He established that its enipirical formula %vas the same as that for methyl acrylate, and determined the solubility, specific gravity, and refractive index of methyl acrylate, its polymer, and a liquid product obtained by destructive distillation of the polymer in vacuum. Weger (40)reported the preparation of the methyl, ethyl, and propyl esters of acrylic acid in 1883, and commented on his ability to obtain only a limited amount of polymer from the methyl ester after long storage or persistent heating. The ethyl and propyl esters polymerized readily, and he described the ethyl ester polymer as being indistinguishable in appearance from the polymer ohtainablc from styrene. The preparation of acrylic acid chloride, anhydride, amide, and nitrile was described by Moureu in 1893 (16). €le observed that the amide and anhydride polymerized readily.
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268
In 1901 Rohm published the results of his researches with von Pechmann on acrylic acid derivatives (18,20,23). Rohm subjected methyl and ethyl acrylates to the action of sodium alcoholate in ether and isolated from the reaction mixtures a dimeric form, subsequently identified as the ester of a-methylene glutaric acid, and a trimeric form which was not further identified. I n every case a considerable amount of solid polymer was formed, the methyl ester appearing to polymerize more readily than the ethyl ester. Rohm described the methyl acrylate polymer (93) as a colorless, transparent, very elastic mass practically free from any odor of monomer; insoluble in water, alcohol, and ligroin; but swelling in acetic acid, phenol, chloroform, acetoacetic ester, acetic anhydride, ethyl benzoate, and nitrobenzene. Boiling ether swelled the mass to six to eight times its volume and dissolved a small portion. Cold mineral acids had little action on the polymer, which was likewise resistant to the action of aqueous acid or alkali. Destructive distillation in vacuum gave a liquid distillate which was shown to contain a-methylene glutarate and a trimeric form, not identical with that obtained by the action of sodium alcoholate on methyl acrylate. Its structure appeared to be COOCH&H : CHCH2CH(COOCH3).CH2. CHvCOOCHs. I n his thesis Rohm offered an interesting comment on the nature of the acrylate polymers. He suggested that these substances may be considered “higher molecular compounds but without assuming a carbon linkage between the individual molecules.” He proposed to call these and similar substances “pseudopolymers” to distinguish them from higher molecular substances in which the monomeric molecules are directly united by carbon linkages. He suggested that reactions such as the conversion of acetylene to benzene should be designated as a “polymerization,” whereas the formation of the solid modification of methyl acrylate from the liquid form should be called a “pseudopolymerization.” These pseudopolymers were further compared with the allotropic forms of the elements, such as phosphorus, sulfur, arsenic. It was suggested that rosin, rubber, and gutta-percha also belong to this class of substances. These comments are interesting as an example of one of the earliest speculations concerning the nature of these polymeric substances. The most exhaustive recently published investigation into the nature of the polyacrylates appears to be that of Staudinger and his co-workers (34,36,36). There can be little doubt but that the polyacrylates, in common with polymers of other compounds containing a vinyl group, are composed of single molecules of high molecular weight, and that the monomeric units are joined by direct carbon-to-carbon linkages.
Industrial Development The remarkable properties of the polymerized acrylates made a lasting impression on Rohm’s mind. During the early part of his industrial career, the pressure of other duties prevented him from taking active steps toward any extensive commercial exploitation of the acrylate field. However, in the laboratory he worked indefatigably on a study of the many interesting properties of the polymers, and he was always on the lookout for possible uses for them. Eventually in 1912 he secured a German patent (21) in which it was claimed that bv their vulcanization with sulfur tge polyacrylates could be used as rubber substitutes. In the same year, work on the preparation of acrylic acid esters was started as part of a general program of investigation in the field of artificial resins. The war interrupted this work in 1914, and an opportunity for its resumption did not present
m r ym
VOL. 28, NO. 3
itself for a number of years. This early work did serve to emphasize, however, the possibilities of the polyacrylates, and R o b became conviuced more than ever of their commercial value. In 1915 he secured a German patent (22) in which the polyacrylates were recommended as substitutes for drying oils in paints and lacquers. The early commercial exploitation of the acrylic acid derivatives was seriously hampered by the lack of a satisfactory method of preparing them in quantity. The method (7, 20, 37) usually employed for their laboratory preparation consisted in adding bromine to allyl alcohol, oxidizing the resulting dibromo alcohol to cy, P-dibromopropionic acid, esterifying the acid with the desired alcohol, and finally dehalogenating it to the acrylic ester by means of zinc. The large-scale preparation of acrylates by this method was obviously out of the question, both because of the number of reactions involved with a low over-all yield and the cost of the raw materials. Consequently, the discovery of a cheap and convenient method of preparing acrylates in quantity represented an important step in the development of this field, The rapid development of chemical warfare and the manufacture of large quantities of mustard gas during the war emphasized the importance of ethylene as a raw material for the synthesis of many aliphatic substances. An ethylene derivative became the starting point of a new synthesis of acrylic acid esters, which made possible their manufacture on a large scale. This synthesis was worked out by Bauer (30, 31) in Rohm’s laboratory, and by 1927 the work had progressed to the point where plant production of a limited quant,ity of methyl acrylate could be commenced. The synthesis is based on the use of ethylene chlorohydrin as a starting material and is effected in essentially two major operations, although a number of reactions are involved in the second operation. The reactions involved are as follows:
++
CH20HCH2Cl NaCN CHz0H.CHzCN ROH
+CH20HCH2CN+ NaCl + HzS04 + CH2:CHCOOR
+ NHiHSOi
(1) (2)
In 1931 the production of acrylic esters was begun in this country by the Rohm & Haas Company. The methods of operation were based on the syntheqis discovered by Bauer but were modified considerably in many details in order to increase their efficiency. This method is best suited for the production of the esters of the lower alcohols. For the manufacture of the acrylates of the higher alcohols it is sometimes preferable to resort to direct esterification of the acrylic acid or to a transesterification between a lower acrylate and the higher alcohol. The acrylates can also be prepared from acrylyl chloride and alcohol (6),or by the catalytic dehydration of hydracrylic esters (2). The ease and violence with which the acrylates can polymerize make it generally undesirable either to store or transport them in the monomeric form. The addition of polymerization inhibitors increases their stability but cannot be relied upon always.
Homologs of the Acrylates Of the homologs of acrylic acid esters, the beta-substituted ones polymerize slowly, if a t all, and consequently are of little interest as a source of polymeric materials. Although esters of the alpha homologs have been known for many years (6),only those of the methyl homolog can be prepared with sufficient ease to be of commercial interest. Ethyl a-methylacrylate seems to have been prepared for the first time by Frankland and Duppa in 1865 (IO). Some years later its tendency to polymerize was commented upon by Fittig and Paul ( 9 ) .
INDUSTRIAL AND ENGINEERING CHEMISTRY
MARCII, 1936
269
A number of methods have been reported for the preparation of methacrylates.% The method first used and most frequently mentioned is that of treating an a-hydroxyisobutyric acid ester with phosphorus trichloride (9,10, 33). Phosphorus oxychloride, which hes been obsemed to dehydrate hydroxy esters (38),can be used in place of the trichloride to give good yields of the methacrylate. Thionyl chloride likewise dehydrates the hydroxy ester to methacrylate. Both methyl and ethyl methacrylates have been formed by the action of sodium nitrite on a-aminoisobutyrste ( I ) . Dehydrohalogenation of an a-bromoisohutyrate to R methacrylate bv means of diethvlaniline or auinoline (11, iZ, 39) has also'been recommended. @-Chloroisobutyric esters are readily dehydrohalogenated by means of hasic ferric chloride or alcoholic caustic ($6). Estersof or-hydroxyisobutyric acid are easily and economically prepared from acetone cyanohydrin. For this reason they are convenient starting materials for the manufacture of various methacrylic acid esters. The manufacture of methacrylates in quantity offers no particular difficulties other than the avoidance of loss through polymerization. Similar to the acrylates, tlie monomeric niethacrylates cannot be stored or transported with safety.
Polymerization and Polymeric
Forms The esters of both acrylic and methacrylic acids polymerize readily under the influence of heat, light, oxygen, and oxygen-yielding substances such as sodium peroxide, hydrogen peroxide, and benzoyl peroxide (25). In general, the acrylates polymerize much Eaten of the aoid CWa:C.COOH are ieleried to
CN, hereafter 81s "msthaerylates" rather than ''a-methyl a e i y l a t d ' in order t@avoid their conitmion with methyl aorylatc. the methyl eater of acrylic mid.
AcRYLoln IIoon FOR A PISSENGEE PLANE, FOFXED o r ONE PIECEAND BENT O X THREE SIDEs, PERMITTING AN UNIM-
Tor:
PAIRED VIEU-
CENTER: BWmOM:
MADE FROX ACHYLOID GROW OF OBJECTS LIADE FROM ACRXLOID VIOLIN
INDUSTRIAL AND ENGINEERING CHEMISTRY
270
RACINGCAREQUIPPED WITH ACRYLOID WINDOWS
its pliability and is a hard,Ftough solid, almost brittle at ordinary temperatures. The tert-butyl acrylate polymer seems to be the hardest of all the aliphatic acrylate polymers. The relationship between the various isomeric polyamyl acrylates is the same as that of the butyl esters, except that the corresponding polymer in each case is slightly softer. A carbon ring increases decidedly the hardness of the polymer. The polymer of cyclohexyl acrylate is quite hard and tough, whereas the n-hexyl ester polymer is soft and tacky. Ethylene glycol diacrylate polymer is extremely hard and practically insoluble in organic solvents. The general influence of variations in the structure of the alcohol group is the same for the polymethacrylates as for the polyacrylates. However, as a class, the polymethacrylates are considerably harder than the polyacrylates. Whereas polymethyl acrylate is a rather soft, elastic, rubber-like substance, polymethyl methacrylate is a very hard, tough mass which can be sawed, carved, or worked on a lathe with ease. It is only when the %-amyl ester is reached that the softness and pliability of the polymer approach those of polymethyl acrylate, The higher homologs of acrylic acid, such as ethacrylic and propacrylic acid, are difficult and expensive to prepare, and their esters polymerize extremely slowly to give soft, semi-liquid polymers. By modifying the conditions under which polymerization is caused to take place, it is possible to obtain from the same monomeric ester, polymers which vary from quite tough, almost insoluble forms to elastic, rather soft, tacky, and highly soluble varieties. The viscosity of a given polymer may be influenced a t will by changes in the catalyst, the solvent, and the temperature used during polymerization. The presence of impurities has also been shown to have a definite influence on the properties.8 In general, mixtures of different polymers do not show the desirable properties to be expected. However, this difficulty may be entirely overcome and additional advantages obtained when the monomeric forms are mixed in desirable proportions before polymerization is effected (17). Both acrylic and methacrylic acids are polymerized bg a Staudinger and Trommadorff [Ann , 502,201-23 (1933)] give an interestina: discussion of the influence of the degree of polymerization on the properties of polyethyl acrylate.
VOL. 28, NO. 3
much the same agents as are used for the polymerization of the esters. The polyacids can be obtained either as hard, rather brittle, transparent, colorless m a s s e s o r a s white powders. Polyacrylic acid gradually swells and dissolves in water to give highly viscous solutions which are useful as thickeners for emulsions, as are also the solutions of the alkali salts of the polyacid. Acrylonitrile (vinyl cyanide) and methacrylonitrile polymerize r a t h e r slowly t o g i v e w h i t e powders, insoluble in practically all common organic solvents. The nitriles are useful, chiefly as components of joint polymers with acrylates or other polymerizable compounds. Because of the considerable influence of the conditions of polymerization on the character of the polymer and of the violence with which the polymerization can sometimes take place, the polymerization of large quantities of acrylates and methacrylates must be carried out under carefully controlled conditions. With the exception of the lower esters of methacrylic acid, the polymers find their greatest usefulness in the form of solutions in organic solvents or as aqueous emulsions. The solutions of the polymers in organic solvents are waterwhite and vary in viscosity from thin liquids to semi-solids, depending on the type and amount of polymer present. Emulsions of the polyacrylates (13) greatly resemble rubber latex in appearance and in many of their properties. In certain processes they can be substituted for rubber latex. Solid polymers in granular or powder form are obtained by polymerizing in a liquid which is a solvent for monomer but in which the polymer is insoluble (98).
Properties and Uses4 The Polymers of the acrylates and methacrylates are capable of being Produced in a wide Variety of f o m b but as a class they are distinguishable from other resins by their colorless transparency, adhesive qualities, great elasticity, and resistance to many reagents. The brilliant water-white color of the Polymers makes it Possible to Secure masses of high light transmission and great optical clarity- Because of the remarkable stability of the polymers to the action of heat and light, these Properties are Permanent. In regard to hardness, toughness, and elasticity, they range from the very hard, tough Polymers of methyl methacrylate to the very soft, sticky semi-liquid P o b e r s of the higher aliphatic acrylates. The water absorption varies from almost 0 UP to 2 O r 3 Per cent, depending on the composition and type of Polymer. As a class they have good electrical res i s t ~ ~Polymethyl ~ . acrylate, for example, has a dielectric Constant Of 5-6, a Surface resistivity a t 1000 Volts Of 4-5 x lo6 megohms, and a specific electrical resistance of 4 x Ohms. The extensibility of the PolYacrYlates is generally greater than that of the PolPethacrYlates. The Polymers are generally insoluble in water, alcohoh and aliphatic hydrocarbons. Most of them are only slightly swelled by ethers. On the other hand, they are either completely dissolved or swelled %osoft gels by coal-tar hydrocar4 Patent applications for a variety of usee have been made by Rtihm & Haas Company, Philadelphia, and Rahm & Haas, A.-G., Darmstadt.
MARCH, 1936
INDUSTRIAL AND ENGINEERING CHEMISTRY
bons, chlorinated hydrocarbons, ketones, esters, ether alcohols, and ether esters. They show excellent resistance to dilute acids, and their resistance to dilute alkali is good. The lower methacrylate polymers appear to be quite resistant to hydrofluoric acid fumes. Both the polyacrylates and polymethacrylates are permanently thermoplastic. The acrylate and methacrylate polymers are characterized by their excellent adhesion to most surfaces. In the case of metal surfaces, baking further improves the adhesion. The properties described make the polyacrylates and polymethacrylates especially suitable for a variety of uses. One of the earliest practical uses for the polyacrylates was as an intermediate layer in laminated glass (%@. Because of their optical clarity, stability to light and heat, toughness, and excellent adhesion, these polymers were particularly suited for this purpose. Furthermore, they can be readily applied directly to the glass sheets in the form of their solutions (27). However, the polyacrylates have the disadvantage, stated previously and common to most plastics used in laminated safety glass, of being influenced to a marked degree by temperature changes. This defect has been largely overcome by using a joint polymer made by polymerizing a mixture of several different monomeric forms (17). Laminated safety glass made from this type of polymer is being manufactured and sold in this country under the trade name of Plexite. In Europe laminated glass containing polyacrylic acid derivatives is known as Luglas and Sigla. The electrical resistance of the polymers is sufficient to make them of value in electrical insulators. The polyacrylates are especially useful (24) where a pliable insulating medium is desired. In cases where tough, rigid insulators are required, the polymethacrylates, especially polymethyl methacrylate (32),are recommended. The adhesive nature of the polymers opens up a wide field of application (3). Their solutions in organic solvents are well suited for use in clear lacquers and varnishes, for coatings on metals, wood, paper, etc., and for undercoatings and finishes on textiles. Their resistance to mineral oils makes them useful as coatings for gasoline and lubricating oil storage tanks. By the proper choice of solvents they can be made suitable for application by either spraying or brushing. The water emulsions offer a convenient form for their application in certain types of problems-for example, as undercoatings in leather finishes. Their uses are increased by adding pigments and fillers. Their compatibility with other resins and cellulose derivatives varies considerably with different polymers and even with different types of the same kind of polymer. In general, the cellulose nitrates are compatible with more forms than are the cellulose acetates. The cellulose ethers are compatible in some cases. Sheets, blocks, rods, and molded, sawed, or turned objects of methyl or ethyl methacrylate polymers are of a brilliantly clear, water-white transparency. Their hardness, toughness, and light weight (specific gravity 1.18)add to their attractiveness. These polymers readily lend themselves to use as substitutes for glass in optical lenses, high ultraviolet-light transmitting sheets, objets d’art. Many striking and beautiful effects can be produced by the incorporation of dyes and pigments. Large quantities of clear sheets of these polymers are being used in airplane windows in Europe.6
271
Chlorination of the polymers of the acrylic and methacrylic acid esters has a marked influence on their properties (4). For example, chlorinated polymethyl acrylate is insoluble in chloroform, ethyl acetate, and many other solvents in which it was soluble before chlorination. Chlorination also increases the hardness and raises the softening point of the polymers. It is apparent from this discussion that the acrylic resins show many unique physical, chemical, and mechanical properties. I n view of the practically inexhaustible supply of raw materials and the many and varied uses to which all of these products have been already put, there is every reason to believe that in the future we shall see a rapid increase in the number of fields in which the acrylic resins will be used to advantage.
Literature Cited Barker and Skinner, J . Am. Chem. SOC., 46, 406-7 (1924). Bauer, U. S. Patent 1,890,277 (Dec. 6, 1932). Zbid., 1,982,946 (Dec. 4, 1934). Ibid., 2,021,763 (Nov. 19, 1935). Bauer and Lauth, Ibid., 1,951,782 (March 20, 1934). Blake and Luttringer, Bull. BOO. chim., [3] 33, 635-52, 760-83 (1905). (7) Caspary and Tollens, Ann., 167, 247-52 (1873). ( 8 ) Ibid., 167, 250 (1873). (9) Fittig and Paul, Zbid., 188, 54-5 (1877). (IO) Frankland and Duppa, Zbid., 136, 12-13 (1865). (11) Hoae and Perkin. J . Chem. Soc.. 99, 773 (1911). Hokells, Thorpe, and Udall, Ibid., 77, 947(1900). I. G. Farbenindustrie, British Patent 358,534 (Oct. 6, 1931). Kahlbaum, Ber., 13, 2348-51 (1880); 18, 2108 (1885). Linnemann, Ann., 163, 369-70 (1872). Moureu, Bull. soc. chim., [3] 9, 386-92, 413-15, 417-19, 424-7 (1893). Neher and Hollander, U. S. Patent 1,937,323 (Nov. 28, 1933). Pechmann, von, and Rohm, Ber., 34,427-9 (1901). Redtenbacher, Ann., 47, 113-48 (1843). Rohm, Ber., 34, 573-4 (1901). Rohm, German Patent 262,707 (Jan. 31, 1912); U. S. Patent 1,121,134 (Dec. 15, 1914). Rohm, German Patent 295,340 (June 5, 1915). Rohm, “‘ijber Polymerisationsprodukte der Akrylaiiure,” thesis, Tubingen, 1901. Rohm and Bauer, U. S. Patent 1,982,831 (Dec. 4 , 1934). Rohm and Haas, A.-G., British Patent 304,681 (Feb. 3,1930). Ibid., 316,547 (Jan. 23, 1930); Bauer, U. 8. Patent 1,864,884 (June 28, 1932). Rohm and Haas, A.-G., British Patent 396,097 (July 31, 1935). Ibid., 404,504 (Jan. 18, 1934). Rohm and Haas, A.-G., French Patent 654,357 (May 16, 1928). Rohm and Haas, A.-G., German Patent 365,530 (Sept. 3. 1919) ; Bauer, U. S. Patent 1,388,016 (Aug. 16,’1921). Rohm and Haas, A.-G., German Patent 571,123 (June 18, 1928); Bauer, U. S. Patent 1,829,208 (Oct. 27, 1931). Rohm and Haas, A.-G., Swiss Patent 146,563 (July 1, 1931). Schryver, J . Chem. SOC., 73, 69 (1898). Staudinger and Kohlschutter, Ber., 64, 2091-8 (1931). Staudinger and Trommsdorff, Ann., 502, 201-23 (1933). Staudinger and Urech, Helv. C h i m . Acta, 12, 1107-33 (1929). Vorlander and Knctzsch, Ann., 294, 317 (1897). Wagner-Jauregg, HeEv. Chim.Acta, 12, 61 (1929). Walden, 2. phusik. Chem.,20, 574 (1896). Weger, Ann., 221, 79-82 (1883). (1) (2) (3) (4) (5) (6)
RECEIVED January
2 , 1936.
6 Manufacturedin Germany by Rohm & Haas, A -G , and sold under the trade name of Plexiglas.
