Vinyl Ester Resins' CLIFFORD BENTON Cornell College, Mount Vernon, Iowa
T
HE synthetic resins industry which has been startling the world by its astonishing progress has received an added acceleration by the opening of the vast field of vinyl polymers for industrial use. Nearly all of the newest and most talked-about resins such as Du Pont's "lucite," Carbon and Carbide's "vinylite," Shawinigan's "gelva," etc., are derivatives of vinyl alcohol. The new "light-bending" synthetic resins, supersafety glass, colorful "objets d'art," and rubber-like "glass" displayed a t the New York World's Fair in 1939 are all of vinyl composition or a t least owe their polymerization to the presence of the vinyl group. Before considering the details involved in the preparation and properties of the vinyl ester resins, we will first note the general field covered by the resins. Naturally,theht questionaskedis: What is aresin? In answering this we say that a true resin is a solid solution of many chemical compounds, having such closely related chemical properties that i t is almost impossible to separate them, one from the other. The resins are comprised of two main groups or c l a s s e s t h e natural and the synthetic. Best-known examples of the first are rosin, spruce gum, copal, amber, shellac, the asphalts, rubber, and chicle. Synthetic resins are not so easily classified. However, the engineer succeeds in grouping the majority of ' Presented before the Division of Chemical Education of the American Chemical Society, l0lst meeting, St. Louis, Missouri, April 10, 1941.
them under three headings-the thennosetting, the thermoplastic, and the ml-soluble resins. Thermosetting resins are those requiring chemical combination during the final pressing, i. e., the chemical reaction involved in the preparation is not completed in the production of the molding powders. Examples of this type are the phenol formaldehyde and urea formaldehyde resins.
Phenol formaldehyde (after Raschig) NH
CH2-NH
>,,
Urea formaldehyde (after Diuon)
In thermoplastic resins or mvolaks the resin may be formed and reformed many times into definite shapes by applying heat and pressure, provided sufficient plasticizing agent is present to impart the proper
flow. No chemical reaction takes place during the heating; therefore the material remains in the plastic state as long as the temperature is maintained. The last-mentioned class, the oil-soluble resins, has little application to our subject since it finds no place in a study of plastics, being used mostly by the paint, varnish, and lacquer industry. Strictly speaking, they are thermoplastic resins in mode of formation. The vinyl resins belong to the thermosetting class. They have been known as far back as 1838 when Regnault, a French chemist, noticed that a white powder formed when sealed tubes of vinyl chloride were exposed to the sunlight. Considerable research followed the Regnault discovery, but the practical significance of these researches was not recognized until investigators began to prepare the polymers of the halides on a larger scale using heat in the presence of benzoyl peroxide or hydrogen peroxide. In 1912 Klatte discovered vinyl acetate and found its polymer more amenable to scientific investigation than the halides. Hermann and Haehnel camed on the work, discovering that the polyvinyl acetate could react like a typical organic ester without losing its resinous condition. Staudinger came next in the lineof researchers and showed that vinyl groups combine forming an unbranched carbon chain. He and his co-workers also carried out considerable work on the reactions of the vinyl compounds and thus almost doubled the knowledge of the chemistry of the vinyl derivatiyes and their polymers. To date the vinyl polymers of commercial importance in the field of synthetic resins or plastics are the polymers of the following: (1) Styrene (2) Vinyl chloride (3) Vinyl acetate (and other esters) (4) Acrylic acid (5) Methacrylic acid esters
CHFCH-CeHr CHFCH-CI CHFCH-&COCHa
in the preparation of ethylidene diacetate, used in making acetic anhydride. In this preparation acetylene was passed into glacial acetic acid in the presence of a suitable catalyst, usually mercuric sulfate.
The mercuric sulfate catalyst was prepared from acetic acid solution by precipitation with fuming sulfuric acid.
HAc
HgO
+ H2S04solution HgSO, 4 + H1O
In securing the ethylidene diacetate from the solution, the mercuric sulfate was first removed, then sodium or calcium acetate added to destroy the excess sulfuric acid resulting from the formation of the mercury sulfate:
The desired material was separated from the tarry matter and salts in a vacuum still equipped with an agitator, and finally fractionated under vacuum. It was noted in the process that vinyl acetate formed during the first step when the acetylene reacted with the acetic acid. When the market began to open for synthetic vinyl plastics, the question then arose of preparing the monomer using the same process and equipment but changing the conditions of the first reaction. Research showed that increased circulation of acetylene, employment of a modified mercury sulfate catalyst, and careful temperature control (50' to 60°C. during the latter part of the reaction) would form the desired vinyl acetate:
+ CHiCOOH
C2H2
-
CH1=CHOCOCH*
Rapid removal of the vinyl acetate was necessary to prevent the formation of the ethylidene diacetate. Another method of preparing the monomer consists CH,=CH-COOH of the same steps of refining and the same reacting CHs=C-COOR substances as those employed in the above process I but differs in being a vapor phase process. In this, the acetylene together with the vapor of acetic acid (6) Acrolein is passed over zinc and cadmium acetate catalysts (7) Indene dispersed in charcoal. The charcoal is kept a t a temperature of 200°C. After the monomer has been prepared the vinyl acetate is polymerized in a large, jacketed aluminum Each one of these has certain properties which make kettle fitted with a stirrer. Benzene and catalysts it more applicable than other compounds for specific are added to thevinyl acetate and the solution refluxed The purposes; for instance, the excellent transparency, for five hours a t a temperature of 73' to 76'C. absence of color, electrical, and water-resisting prop- resulting thick solution is allowed to run into a tall erties of polystyrene make i t especially suited as a narrow still through which steam is passing. The molding compound while the unique elasticity, fatigue, solvent, unchanged vinyl acetate, and steam are then and water resistance of the vinyl acetate polymers make condensed and separated from the heavier polyvinyl acetate. The polymer coming from the still is picked them a source for synthetic textile fabrics. The commercial preparation of the vinyl acetate up by an extruder which forces the mass into rods. Catalysts play an important role in these reactions. monomer has had a rather unique history in that its discovery was due largely to the fact that the substance As in the case of most unsymmetrical ethylene comwas formed as an undesirable side reaction product pounds, polymerization is accelerated by oxidizing
agents. Those best adapted to assist the polymerizing are organic peroxides or ozonides, organic acid anhydrides, and metallic percarbonates, perborates, and oxides capable of yielding their oxygen (AgpO). Benzoyl and acetyl peroxides are the usual commercial catalyzers. The former is prepared by the action of benzoyl chloride on sodium peroxide: 0
0
by the action of sodium ~h~ acetyl peroxide is perborate on acetic anhydride: NaBOs
-
U
+ CH~-C/
0
CHrC
-
-
-
//
//"
U
CHsC-&&C-CHz
+ NaBO,
20
Distearyl peroxide, 0
//
0 //
CH3(CH2),6C-4-0-C-(CHn)aCHs has been used as a catalyzer especially in the production of substitute rubber from mixtures of vinyl acetate and dibutyl phthalate. The physical properties of polyvinyl acetate are interesting in determining the practical uses of the compound. In the first place it softens at a low temperature (30° to 40°C.). This low softening temperatnre coupled with the fact that i t has a relatively high water absorption (3 to 5 per cent in 16 hours at 60°C.) makes it unsuitable in the pure state for milling or molding purposes. These faults are partly compensated for in its outstanding light and heat stability, unique adhesive properties, and solubility in alcohols, ketones, esters, and chlorinated and aromatic hydrocarbons; thus, the polyvinyl acetate finds application (when diluted in a suitable solvent and mixed with a filler) as a binding medium in floor tiling compositions and as a dipping lacquer for artificial leather and linoleum. Also, it has found some use as a synthetic textile fabric. By far the best vinyl ester suitable for molding purposes is the conjoint polymer of vinyl acetate [CHF CH-0-COCHs] and vinyl chloride [CHz =CH-Cl] known commercially as "Vinylite" or "Gelva." This polymer forms plastics of remarkable strength, tenacity, and durability, combining the desirable qualities of both polyvinyl chloride and polyvinyl acetate. At the time of the discovery of the copolymer chemists thought that if the polyvinyl chloride and polyvinyl acetate could in some manner be mixed together the resulting substance would be an ideal molding mixture. Their idea proved of no use, however, for mixing the two compounds only resulted in a weak, brittle substance. Finally, after considerable research, i t was discovered that polymeric compounds could be made of the monomers of vinyl chloride and vinyl acetate by the addition of certain catalyzers and plasticizers and subjecting the resulting mixture to heat and pressure.
Today, the commercial preparation consists largely of the same steps employed in preparing the vinyl acetate polymers. The main exception in the process is that certain stabilizers and plasticizers are added to the copolymer in the rolling mill. In the reaction kettle the ratio of the ingredients added depends upon the uses for which the final product is intended; for instance, if articles of good fatigue resistance, impact, and tensile strength are desired, 85 per cent vinyl chloride with the balance in vinyl acetate is used, while if the product is intended for use in lacquers only 65 per cent vinyl chloride is added. In considering the uses of the vinyl copolymer, we note that they are suitable for practically all applications for which plastics may serve. They have been employed in making toothbrushes, pipe lining, wall trim, dentures, lacquers for coating food containers, concrete and transite board, and stiffeners for floor tiling. In dentures and other articles requiring good fatigue resistance, polymers of high molecular weight are used. A polymer of somewhat lower molecular weight is used in the manufacture of phonograph records, floor tiling, sheets, and tubes. Now on the theoretical side we shall consider the nature of the polymerization of vinyl esters. Perhaps chemists are more interested in the chemistry of the commercial vinyl esters than in their practical application, so we shall now devote the remaining minutes to a consideration of the various theories on the constitution and formation of these polymers. The chief sources of our information are to be found in the writings of Ostromuislensky, Harries, Staudinger, Irany, and Marvel. Most authors believe that the vinyl compounds polymerize in a modified chain reaction. We say modi$ed, for ordinary chain reactions may be looked upon as a chain of events whereas the vinyl polymers form chains which actually have physical existence; that is, if the usual chain reaction starts with 100 molecules it ends with 100 molecules. In the case of vinyl compound polymerization, 100 molecules of raw materials react in rapid sequence to produce essentially one molecule. The mechanics of this reaction are still in the realm of speculation. However, it is believed that the oxygen of the polymerizing catalyst (benzoyl peroxide, ozone, etc.) possibly reacts with the vinyl compound to form a highly unstable peroxide which, in turn, reacts with another monomer molecule. A free radical might react with a double bond, leaving a free carbon valence. A mechanism frequently offered involves the formation of an intermediate compound which is supposed to give a "hot" or "trigger" molecule capable of activating the reacting molecules by an energy transfer. Some authors favor a combined stepwise and chain type polymerization. By this we mean that the chain reaction is employed in forming a lower homolog, the molecules of which, after a certain temperature is reached, react stepwise to form a higher homolog.
Evidence in support of this theory is to be found in the of structure.) Or (11), they may join in a head-tofact that vinyl acetate first forms polymers soluble head, fail-to-tail fashion of the followine structure. in many organic reagents. These polymers then react together in the steam still to form a higher homolog insoluble in most organic solvents. A mechanism similar 0 o I I to this was shownby Iriny to exist in the formation COCHI COCH, ICOCH.COCHl I of higher homologs of divinyl ether-vinyl acetate polymers. In Irany's reaction the soluble vinyl Or (III), they may join in a random manner to zive a linear polymer &-which some of the substitu&ts acetate4ivinyl ether polymer-is formed: are on adjacent carbonsand some are in the 1,3-positions with respect to each other. This in turn forms an intermediate partly polymerized Ostromuislensky favors the sequence of structure substance of the following structure, on raising the I1 and supports his claim by showing that the vinyl bromide polymer structure is similar to that of the temperature and pressure. perbromide of polybutadiene which definitely has a "head-to-head" structure. Staudinger, Harries, Steinhofer, and others refute the Ostromuislensky structure in favor of the "head-to-tail" structure. Marvel, of the University of Illinois, has given us evidence in I --CHAH&A~CHI-CHOAC favor of the "head-to-tail" structure in certain vinyl Finally the insoluble product is formed: polymers such as in the polyvinyl alcohol and poly-CH~HOA~SH+-[CH~HOAI).--CHAHOA~ methyl vinyl ketone:
-
1
1
Q
-CH~CHOA~CHAH+CH~HOACI~--CH~CB-CHHCHOA~
I I
I
--CHrCH-CHrCHOk-
Considerable research has been carried out on the question of how the vinyl acetate monomers link together to form the initial soluble polymer. There appear three ways in which these units may combine. They may join (I) head-to-tail fashion to produce a linear polymer.
And also the structure of the important copolymer of vinyl acetate and vinyl chloride may be formed according to this plan. H H H H H H H H H H H H H H H
l l I I , I I l I I I I I I -C-c-c-c-c-c-c-c-c-C-c-C-C-C-CI I I I I I I I I I I I I
I I
I I
O H C l H C 1 H C I H O H C I H C l H C I
(Of course, a higher percentage of vinyl acetate would lead to more acetate and less chloride in this same type
Polyvinyl alcohol
CHs Polymethyl vinyl ketone
On the other hand, Marvel points out that the structures of the polysulfones, compounds quite analogous to these vinyl polymers, are unquestionably of the "head-to-head, tail-to-tail" type. And he concludes that i t is logical to assume these other polymerization reactions may take place in a similar manner. From these considerations it may appear reasonable at this time to conclude that perhaps both type I and type I1 structures exist, i . e., there is a chance that either linkage may be formed when the monomer is polymerized. LITERATURE CITED
B r s , "Synthetic Resins and Their Plastics." Chemical Catalog C Company, Inc., New York, 1923, pp. 299506. GROGGINS,"Unit Processes in Organic Synthesis," 2nd ed.. McGraw-Hill Book Company. Ine., New York, 1938, pp. e
09"
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