INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1950
4. I n any sulfur burner, the draft or the sulfur feed may fail, as from mechanical causes, and such failure could result in
haeardous overflow from the burner or in overheating of the burner. Safety demands installation of the level device on every rotary burner to warn of excessively high or low level even if not to control the feed. 5. The controller in conjunction with the three other items permits the automatic and safe burning of sulfur, even that containing bituminous matter. A 4 X 10 foot burner containing bituminous matter ran up to 2100 pounds per hour of sulfur with
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these four improvements in comparison to 800 paunds maximum originally. The four improvementa which were discussed have been in succeasful operation a t the Bay Chemical Company plant a t Weeks, La., for 6 years, but prosecution of the patent application on the controller (now U. S. Patent 2,498,089, February 21, 1950) has prevented an earlier presentation. The controller will be available for license. RECErVlD
November 5 , 1919.
POLYSULFIDE POLYMERS E. M. FETTES AND J. S. JORCZAK Thiokol Corporation, Trenton, N. J. T h e chemistry of the condensation polymerization is reviewed briefly. The structures of the polymeric products as well as the effects of copolymerization, mom-linking, end groups, and molecular weight upon polymer properties are discussed. The composition and properties of the commercial crude rubbers, water dispersions, and
S
INCE the commercial introduction in 1929 of the polysulfide polymers they have been utilized for many types of applications in many industries. Their use is not large in volume compared with general purpose synthetic rubbers, but their excellent oil and solvent resistance as well as good aging properties make them valuable for special applications. Although at one time Dow Chemical Company produced material for the Thiokol Corporation, all of the polysulfide polymers sold under the name Thiokol are now manufactured a t Trenton, N. J. During the war, Dunlop Rubber, Ltd., in England.and Naugatuck Chemicals, Ltd., of Canada manufactured Thiokol under license. Polysulfide polymers are manufactured in other countries also, but these products will not be discussed. Other reviews (16,LW,40,47,60,66,66,63,67) have been made on the polysulfide polymers, but these have covered only the older polymers. The purpose of this article is to describe all of the crude rubbers, latices, and liquid polymers now manufactured. The variations that can be introduced to provide new materials with almost any desired characteristics will also be stressed.
POLYMER PREPARATION The preparation of polysulfide polymers (47, 48,66) is an example of condensation polymerization consisting simply of the reaction of an alkaline polysulfide with a suitable dihalide as illustrated by the reaction of ethylene dichloride with aqueous sodium tetrasulfide: NaS,Na
+ CICHZCH~CI+(CHzCH&), + 2NaCI
By using a dispersing agent such as rnagncsium hydroxide the polymer is obtained in the form of a suspension of particles 3 to 15 microns in size. Becauae of this particle sire and the high specific gravity of the polymer, the dispersion settles readily to permit washing free of electrolytes by successive decantations. INORGANIC POLYSULFIDES
There are several methods of preparing solutions of sodium polysulfide of varying ranks. 1. Reaction of sodium hydroxide with sulfur
6NaOH
+ 6s +2N& + 3HgO + NaaSzOa
liquid polymem are presented together with ideas on the mechanism of their vulcanization. The reason for the odor of polysulfide polymers is discussed. Some information i s presented on solvent resistance, stress relaxation, and other charaoteristics of the polymers. Applications for the different types of products are summarized.
2.
Reaction of sodium sulfide with sulfur NrtlS S d N a & 3. Reaction of sodium hydroxide, sodium hydrosulfide, and sulfur NaOH NaSH S -+ N a A HIO 4. Reaction of sodium with sulfur 2 Na 2 S --+ Na&
+
+
+
+
+
Rank is defined as the number of sulfur atoms present in the anion, thus sodium disulfide has a rank of 2.0 and calcium tetra sulfide, a rank of 4.0. A sodium polysulfide of rank 1.5 represents an equimolar mixture of sodivm monosulfideand sodium disulfide. The reaction of sodium hydroxide with sulfur is at present the most economical method for producing sodium polysulfide solutions of rank 2.00to 4.50. The sodium thiosulfate produced by the reaction does not influence the polymerization reaction. I t is known that aliphatic halides will react with sodium thiosulfate to form sodium alkyl thiosulfates. These salts can be converted to disulfides by oxidizing agents such as iodine and hydrogen peroxide (66). If these types of compounds are formed in the reaction, they must be converted to disulfides by the alkaline sodium polysulfide solution present. Methods 2 and 3 are useful in preparing sodium polysulfide solutions of ranks between 1.00and 2.00 which are difficult to prepare from sodium hydroxide and sulfur without relatively long reaction times or without use of temperatures above the boiling point of the solution. Method 4 is useful in preparing pure sodium polysulfides for research investigations. The reaction can be conducted in a solvent such as alcohol or in liquid ammonia (68). DIHALIDES
A large number of organic dihalides can be used in the preparation of polysulfide polymers, although the number of halides commercially available is very small, The suitable halides which are industrially available are: methylene dichloride, ethylene dichloride, propylene dichloride, glycerol dichlorohydrin, dichloroethyl ether, dichloroethyl formal, and triglycol dichloride. Satisfactory polymem and copolymers can be prepared from all of these raw materials. Polymers have also been prepared from di-
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
chloroethyl sulfide (1, 18). This material, although not generally available, has been produced in litrge quantities. A great many dihalides have been used in laboratory investigations with varying success. Some generalizations can be made:
1. The bromides are more reactive than the chlorides and ‘eld the same polymeric products inasmuch as the halogen is e%& nated as sodium chloride or bromide. 2. The halogen has to be aliphatic in character for easy reactivity. Chlorine atoms attached to doublebonded carbon atoms and attached to aromatic rings are relatively unreactive. Udder severe reaction conditions (35) and when activated, as by a nitro group (8), aromatic chlorides are reactive with sodium polysulfide. 3. Primary chlorines are more reactive than secondary which are in turn more reactive than tertiary. A systematic survey of the physical properties of polymers from different dihalides is being made at present in the authors’ laboratory and will be reported on in the future.
POLYMER STRUCTURE The structure of the organic disulfide polymers is represented unequivocally by a repeating segment (R-SS),. The polymers are polymeric disulfides and are capable of undergoing the normal reactions common to simple organic disulfides. Vigorous reduction of polyethylene disulfide yielded ethanedithiol and oxidation of ethanedithiol produced a polymer closely resembling the original polymer (33, 4 7 ) . Oxidation with nitric acid produced ethanedisulfonic acid (9). The original proposal of Patrick ( 4 7 )for a structure of the polysulfide tetrasulfides having two sulfur atoms in the linear chain and two coordinate sulfur atoms on the side of the chain
CH&Hs--S-S II
11
was based upon the easy removal of two of the sulfurs by treatment of a water dispersion of polymer with sodium hydroxide, sulfide, or sulfite and also by the x-ray diffraction data of Katz (31 ). There have been many investigations on the structure of polysulfides since that time. Physical methods such as molecular refraction (6),viscosity (7), parachor (6),x-ray diffraction ( 3 , 1 9 , 80, 86, 68, sa), electron diffraction (.$I), and ultraviolet spectra (4, 32), have been widely used as well as the use of chemical evidence (11, 16,69,87,36,66). While the question of the exact structure cannot be regarded as settled, there is a preponderance of evidence for the linear trisulfide configuration, at least in the trisulfides. The tetrasulfide structure is less certain. In the above references, the tebrasulfideshave been postulated, depending upon the authors, to contain, one, two, three, or four sulfur atoms in the linear chain with the remainder, if any, coordinated on the side. The end groups of the high molecular weight polymers produced by the normal reaction are not definitely known. As an excess of sodium polysulfide is customarily used in the polymerization, it is doubtful that chlorine terminals would be present. Thiol terminals have been postulated ( 4 7 ) ; however, the extensive experimental work carried out in recent years with polymers known to contain thiol terminals makes that postulate probably ineorrect. Slight hydrolysis of the aliphatic dihalides by the high alkalinity of the sodium polysulfide solution to produce hydroxyl terminals now appears to be the most reasonable assumption. In spite of the relatively few commercial dihalides available, a large number of different products can be produced. Variations in the ratio of dihalides, in the amount of cross-linking agent, in the sulfur rank, in the type of chain terminal, and in the molecular weight, can be employed to produce polymers with desired characteristice. Copolymers from ethylene dichloride and dichloroethyl formal have properties intermediate between the two polymers. Polyethylene disulfide is a hard rigid plastic having elastomer charac
