Coal in the Manufacture of Synthetic Rubbers - Industrial

Publication Date: November 1942. ACS Legacy Archive. Cite this:Ind. Eng. Chem. 34, 11, 1382-1386. Note: In lieu of an abstract, this is the article's ...
1 downloads 0 Views 480KB Size
Download PDF
Coal in the Manufact Synthetic Rubbers J

Vulcanized rubber is found in coal, since fossil threads of vulcanized rubber have been discovered in certain German lignite mines. The rubber hydrocarbon, which is the chief constituent of natural rubber, has not yet been synthesized, but the term “synthetic rubber” is justified since the word “rubber” has come to mean rubberlike substances. The. synthetic rubbers in practical use today are outlined, and several charts are given to show how they are or can be produced from the following coal products-benzene, phenol, ethylene, and calcium carbide. The manufacture of the chief synthetic rubbers by the method of emulsifying the ingredients is described, and the substances from coal used in the processing and compounding of synthetic rubbers are discussed. These substances are prepared from coal tar, benzene, and naphthalene.

R

UBBER from coal! This statement does not sound

right, and yet rubber has actually been found in coal. In certain German lignite mines the workmen are familiar with rubbery fibers called “monkey hairs”. These are now known to consist of fossil vulcanized rubber and are the remains of rubber originally in the twigs of a type of rubber-bearing shrub now found only in Africa. The surrounding lignite contains a small proportion of sulfur, and in the course of thousands of years, the sulfur migrated and caused the formation of prehistoric vulcanizates. The commercial conversion of coal into rubber is no more fantastic than its conversion into beautiful dyestuffs, pharmaceutical chemicals, explosives, and plastics. There are several different kinds of synthetic rubbers, and i t is possible to make them all from coal. I n the present paper the possible and practical methods of preparing them will be discussed first, and then the important materials used in the manufacture of goods from synthetic rubbers. Petroleum is the great competitor of coal as a source of energy and also as a source of such fundamental substances as benzene, acetylene, and ethylene, These compounds are starting products for the chemicals used to make synthetic rubbers. Accordingly, it is entirely a question of economics, transportation, and availability of structural materials as to which sources will be used to the greatest extent in the present government program for the production of a million tons of synthetic rubbers. Synthetic rubber has been a fascinating subject ever since a few years before the last war when valuable work was done

HARRY L. FISHER U. S. Industrial Chemicals, Inc., Stamford, Conn.

on the constitution and synthesis of natural rubber. In those early years of the present century the term “synthetic rubber” meant just that-namely, the product formed by the actual synthesis of the rubber hydrocarbon which is the chief constituent of natural rubber. Now, however, the term “rubber” means not only natural rubber itself but also substances Kith rubberlike properties-that is, substances which can be stretched to a t least twice their original length and, after having been stretched and released, will return to approximately their original positions in a reasonably short time. It is worthy of note that, so far as we know, the natural rubber hydrocarbon has not yet been synthesized. The interest in the synthesis of rubber has been strong with organic chemists for a long time because of its scientific value, but in the first decade of this century the interest became strong economically as the demand for rubber increased for tires and the price rose from $0.98 a pound in 1900 to the all-time high of $3.12 in 1910. Similarly the interest became strong again around 1926 when the Stevenson restriction scheme forced the price up to $1.21 a pound from the low average of $0.36 in 1920.

Types of Synthetic Rubber The synthetic rubbers or elastomers are of several types when classified according to the chemicals from which they are manufactured. The chief one from the standpoint of use is Buna S since i t is used for tire treads, and tires take the largest proportion of rubber (approximately 75 per cent). Buna S is a copolymer of 1,3-butadiene and styrene in the proportions of about 75 to 25. This and the other important synthetic rubbers are included in Table I. I n Table I the monomer is the starting material or basic substance, and the unit of polymer represents a section of the polymer. The structure of the unit as given has been determined by chemical and physical means but does not show the exact relation of these units to one another in the chain; in other words, they do not always occur in regular order as indicated. I n natural rubber there is little doubt that the methyl groups occur regularly, but since isoprene is unsymmetrical, synthetic isoprene rubber has the isoprene groups joined in hit-or-miss fashion with the result that the methyl groups occur irregularly. Furthermore, even in synthetic butadiene rubber which begins, so to speak, with a simple symmetrical molecule, there are units as given, but the polymer is not symmetrical because the structure is complicated probably by the presence of cross linkings and irregular unions.

Coal as Source Material How does coal fit into the synthetic rubber picture? Of the monomers mentioned in Table I, butadiene, isoprene, isobutylene, and styrene are found in coal gas and coal tar; but the proportions are small and at present i t is uneconomic 1382

INDUSTRIAL A N D ENGINEERING CHEMISTRY

November, 1942

TABLE I CHEMICAL STRUCTURES OF DIFFERENT VARIETIES OF SYNTHETIC RUBBERS Unit of Polymer

Monomer SIMPLEPOLYMHIRB H H H H

c=c-c=c

H

H 1,a-Butadiene

H

The numbered Bunas, SI& Xer

CHI I H H

CHI I H H

H

-c-c=c-c-

c=c-c==c

H H Natural rubber

H H Isoprene (2-methyl-l,3-butadiene) H& H I

SKB, and

CHs I H

HsC

CHa H

H I I -c-c=c-c-

c=c-c=c

H 1% Methyl rubber

H H 2,3-Dimethyl-1,3-butadiene Cl

H

T - H H

c=c-c=c

H H __ Chloroprene (2-chloro-1.3-butadiene)

Neoprene, Sovprene, Mustone CHa

CHs I H

I H

c=c

-C--c7 I H CHI Vistanex, Oppanol

I H CH1 Isobutylene (2-methylpropene) H

H H -C-CI H

H

c=c

1 H c1 Vinyl chloride

__

Cl

Plasticized polyvinyl chloride materials-Koroseal, Korogel, Flamenol COPOLYMERS

H H H H

H H

H

H

c=c-c=c + c=c H

I

n

-3

v or Buna SS

Buna H H H H

c=c-c=c H

Butadiene

+

H

H H

+ c=c

-+-

H

H

H

H

-c-c=c-c-c-c-

H

I

H

L N acrylonitrile (vinyl cyanide)

H

C=N Perbunan or Perbunan Extra

ELASTOTHIOMERS

s s

8 6

H H CI-4-C-Cl H H Ethylene dichloride (12-dichloroethane)

+

H H H H Cl--CC-0-C-C-CI H H H H Dichloroethyl ether

II II Na-S-S-Na

--f

Sodium tetrasulfide

s s

II II

+ Na-S-S-Na Sodium tetrasulfide

e

H H I1 II -C-C-S-SH H -Thiokol A

+ 2NaCI

H H H H I i -C-C-0-C-C-S-SH H H H Thiokol B, Perduren G

+ 2NaCl

OTHERCOPOLYMERS Butyl rubber is a copolymer of isobutylene with a small amount of butadiene or isoprene. Chemigum and Hycar OR are copolymers of butadiene End other substances, the names of which have not been made public, probably acrylonitrile.

.

1383

to isolate them, although styrene is being recovered on a small scale. However, there are other products from coal which are abundant and can be used . . in various ways in the manufacture of several of the monomers required for synthetic rubbers. These are benzene, phenol, ethylene, and calcium carbide. They can be used as the starting products for the preparation of the most important synthetic rubbers as outlined in Figures 1 to 3, but actually only benzene and calcium carbide are used in the present program. Methyl rubber and isoprene rubber are not being manufactured but all the others are. Butadiene is obtained from petroleum, natural gas, alcohol, and, in Germany, also from acetylene. I n addition, butadiene can be obtained by hydrogenating benzene to cyclohexane and cracking the latter. Butadiene and ethylene are formed a t the same time. The ethylene coupled with more benzene gives ethylbenzene which is dehydrogenated to give styrene. This method for butadiene has been considered but cannot be used since the benzene is needed for other purposes. The ethylene for the production of styrene is obtained from petroleum. Perbunan, Chemigum, a n d Hycar all require butadiene and acrylonitrile, which is prepared by a series of reactions from ethylene and sodium cyanide. These rubbers are oil resistant and are used for gasoline hose, 'oil hose, and gaskets. Two of them are also used for tires. Butyl rubber is a copolymer of isobutylene and a small proportion of isoprene. Isoprene is a simple derivative of butadiene and can be obtained by cracking turpentine. It is an important substance in the chemistry of natural rubber because it is found among the products of the dry distillation of rubber. Isobutylene comes from petroleum. It is also polymerized togive the rubberlike product Vistanex and is used in making 100-octane gasoline. Neoprene is made from acetyle n e w h i c h comes from calcium carbide and therefore stems directly from c o a l . A c e t y l ene is converted into vinylacetylene, a n d h y d r o c h l o r i c a c i d gas is added to produce a chlorine derivative of butadiene,

INDUSTRIAL AND ENGINEERING CHEMISTRY

1384

Vol. 34, No. 11

CaCz Calcium carbide

J. IIlO HCsCH Acetylene

- 4

CuaClz

CH=C-CH=CHz Vinylacetylene

.1

E C h CHFCCl--CH=CHz Chloroprene (2-Chloro-1,3-butadiene) _

4I

+ NHiCl

HzO(HgS03 +HCl

7

CHFCHC1 CHaCHO Vinyl chloride Acetaldehyde Tricresylphosphate $ 0 2 KOROSEAL CHsCOOH Acetic acid

NaOH

+

CHICHOHCHZCHO Aldol

--+

4

H2

CHaCHOHCH2CHzOH 1,3-Butylene glycol

./,-2H10

I

I + CHa-C=CHz

.r

Hz(Mg)

1

J.

Hac CHE

I I

CHs

I I

CHsC-CCHs

r-----i

9

BUNAS

BUTYL

+ CH-CHCS

CHa-C-CECH

I

OH OH Pinacone

OH 2-Methyl-3-butyn-2-01

4

$ -2Hz0

HZ

Y

PERBUNAN, CHEMIGUM, HYCAR OR

CHa

I I

Hac CHs

I

+

CHP-C-CH=CHI

1

CHFC-C=CH* 2,3-Dimethyl-1 ,a-butadiene

OH 2-iMethyl-3-buten-2-01

J.

.1

CH:

I

METHYL RUBBER

CHVC-CH=CHI Isoprene

+ CH-CH-CH=CHz

-

BUTYL

J.

ISOPRENE RUBBER SHEET FIGURE 1. FLOW

O F SYKTHETIC

RUBBERS PREPARED

called “chloroprene”. Neoprene is a polymer of chloroprene. It is very much like natural rubber in its properties and in addition is more resistant to the action of oils, air, sunlight, heat, and coronal discharge. Koroseal is a plasticized polyvinyl chloride and also stems directly from coal. Vinyl chloride is prepared by the addition of hydrochloric acid gas to acetylene and also by the removal of the elements of hydrochloric acid gas from ethylene dichloride. One of the plasticizers, tricresyl phosphate, is also obtained in part from coal. Finally, Thiokol is obtained by the action of sodium tetrasulfide on ethylene dichloride. The ethylene for ethylene dichloride can come from coal but a t present it is derived from petroleum and natural gas. Thiokol is the most oil-resistant of the rubbers. Satisfactory tire treads for emergency use have also been made with it. The relative importance of the different synthetic rubbers from the standpoint of the amounts proposed in the present government plan can be given, but the actual amounts are

WITH CALCIUM CARBIDE A S THB STARTING

MATERIAL

not known definitely. The order is approximately as follows: Buna S, Butyl, neoprene, Thiokol, Perbunan, Hycar OR, and Chemigum, but the order may change even before this article is published.

Manufacture of Synthetic Rubbers The methods of manufacturing the synthetic butadiene rubbers are approximately the same-namely, by the polymerization of the constituents in the form of an emulsion. For example, butadiene and styrene are emulsified in water in the presence of an emulsifying agent, such as ammonium oleate or sodium isopropylnaphthalenesulfonate; a polymerizing agent which usually is a peroxide (benzoyl peroxide) is added; and the mixture is heated with proper stirring to about 60’ C. for 10-15 hours. The product is a synthetic rubber latex which is coagulated very much as natural rubber latex is coagulated by the addition of dilute acetic acid or a solution of a salt. The coagulum is washed on a mill and

1385

INDUSTRIAL AND ENGINEERING CHEMISTRY

November, 1942

3

Phenol I /

I I

OCH~CH.

I 1.

HdNi)

I I

I I

'1

I

I

I I i

fiCH=CHz

I

Hz Cyclohexanol (Hexahydrophenol)

I

I

'I I

-2H

I

I I I

I

I

1

Cyclohexane (Hexahydrobenzene)

1 I

I

Ethylbenzene

I I

!Heat

.1 H~ + CH~=CH--CH=CH~ Butadiene

+

I

I

Styrene (Vinylbenzene)

CHz=CHz Ethylene

I

I

I

I I I

I

Cyclohexene (Tetrahydrobenzene)

I

I I

Heat

I I

BUNA S C-

+ Styrene

CHFCH~ Ethylene

+

CH-CH-CH=CHz Butadiene

SHEETOF SYNTHETIC RUBBERS PREPARED WITH BENZENE AND PHENOL AS THE STARTING MATERIALS FIGURE 2. FLOW

dried. Since there is a tendency for the polymerizing reaction to continue, with the consequent formation of harder and stiffer products, an antipolymerizing agent, phenyl+ naphthylamine, in the proportion of 2 parts to 100 parts of rubber, is milled into the rubber before it is ready for use. The ammonia in ammonium oleate, the sodium isopropylnaphthalenesulfonate, the benzoyl peroxide, and the phenyl/?-naphthylamine are derived from coal. Neoprene is prepared similarly from chloroprene and treated in the same general manner. Thiokol is also obtained as an artificial latex in the reaction of ethylene dichloride and sodium tetrasufide. The latex is coagulated, and the coagulum washed and dried. Koroseal is made by mixing approximately equal proportions of polyvinyl chloride and tricresyl phopphate. The cresol used in the preparation of tricresyl ohosphate comes from coal tar.

Coal Products in Manufacture of Synthetic Rubber Goods Like natural rubber, the chief synthetic rubbers must be vulcanized in order to convert them into products of practical use. The butadiene rubbers are vulcanized with sulfur. Other compounding ingredients are added along with the sulfur to give the rubber the proper working qualities during fabrication and to give the finished products good aging and other necessary or desirable properties. The other compounding ingredients can be classified as accelerators of vulcanization, antioxidants or aging retarders,

plasticizers and softeners, reinforcing and inert filling agents, pigments and coloring agents, odorants, etc. The reinforcing and inert filling agents and some of the pigments are inorganic substances but the others are organic, and most of them are products from coal. They are based on benzene, toluene, naphthalene, phenol, and cresols. The relative amounts required in the present gigantic synthetic rubber program with its total of a million tons a year can be calculated from the proportions ordinarily used, based on 100 parts of rubber; but actual figures cannot be given until the total amounts of each type are known. Furthermore, it must be remembered that the synthetic rubbers vary in their requirements. Neoprene is vulcanized without sulfur and needs no accelerator, and Thiokol is vulcanized with zinc oxide and sulfur acts as an accelerator. Koroseal and Vistanex cannot even be vulcanized. The rubbers used for mechanical goods require much larger proportions of plasticizers and softening agents than those in tread stocks; most of the rubbers show very good aging, and yet antioxidants are added since it happens that they act as antipolymerizing agents or stabilizers. Sulfur is used in the ratio of 1-3 parts to 100 of synthetic rubber; accelerators, 1 part; antioxidants, 1-2 parts; plasticizers and softeners, 3-5 parts for treads and up to 25-50 parts for mechanical goods, etc.; and coloring agents in amounts depending upon tinctorial power. Coloring agents are limited in their use since most of the synthetic rubbers require carbon blacks to bring out their optimum properties.

I N D U S T R I A L A N D -._ ENGINEERING CHEMISTRY

1386

7

THIOKOL A

c

CHz=CHCl Vinyl chloride Trioresyl phosphate KOROSEAL

4

+

Heat

-2H

VULCANIZINQ AGENTS. Sulfur obtained by the Thylox process should be of use in the vulcanization of synthetic rubbers. There are also organic sulfur-containing vulcanizing agents which can be used in some cases. Tetramethylthiuram disulfide, (CH&NCS-SS-CSN(CH&, comes indirectly from a coal product since some of the dimethylamine used in its preparation is obtained from p-nitrosodimethylaniline by treatment with alkali. ACCELERATORS. Mercaptobenzothiazole (Captax) and its oxidation product, benzothiazyl disulfide (Altax), are the most used accelerators. The method of their preparation is as follows:

Aniline

-

Mercaptobenzothiazole

4 7 \ Clz

0,

,c-S.S-C,

S

/\() / S

Benxothiazyl disulfide

Tetramethylthiuram disulfide and zinc N,N-dimethyldithiocarbamate are also used. ANTIOXIDANTS. Phenyl-8-naphthylamine is generally present in the synthetic rubbers as an antipolymerizing agent or stabilizer, and ordinarily nothing more is needed to make them age well. Polymerized trimethyldihydroquinoline (Agerite resin D) is sometimes added for resistance t o heat aging.

NaCN

---+

+CHz=CHz Cla CICHzCHzOCHzCHzCl bis (2-Ch1oroethyl)ether

+

+ catalyst

1

Vol. 34, No. 11

HOCHzCHzCN Ethylene cyanohydrin

4

-Hz0

CHFCH-CN Acrylonitrile (Vinyl cyanide)

SOFTENERS AND PLASTICIZERS. The synthetic rubbers are more difficult to process than natural rubber, and therefore larger proportions of softeners and plasticizers are used: soft coal tar, Bardols, tributylphthalate, triphenyl phosphate, amylnaphthalene, and aldol-a-naphthylamine resin. Benzothiazyl disulfide and diphenylguanidine (D. P. G.) are plasticizers for Thiokol FA and di-o-tolylguanidine (D. 0. T. G.) is a plasticizer for Neoprene GN. The cumarone resins are softeners and are especially good tack producers. COLORIKC AGEKTS. The organic coloring agents must be able to withstand the high temperatures and the action of sulfur during vulcanization. The monastral pigments are very stable and are coming into use. As already mentioned, the use of coloring agents is limited since the compounds generally contain carbon black. SOLVENTS.Synthetic rubbers are generally resistant to the action of organic solvents. However, to make solutions or cements some of them can be dissolved or dispersed in toluene, chlorobenzene, and cyclohexanone. All these are coal tar products, the cyclohexanone being obtained by the oxidation of cyclohexanol and this, in turn, by the hydrogenation of phenol.

Bibliography (1) Davis and Blake, “Chemistry and Technology of Rubber”, A. C. S. Monograph 74, New York, Reinhold Pub. Corp., 1937. (2) Fisher, Proc. A m . SOC.Testing Muterials, 41,521 (1941). (3) Fisher, “Rubber and Its Use”, Brooklyn, Chemical Pub. Co., 1941. (4) Gollmar, IND. ENC.CHEM.,26,130 (1934). (5) Hoffmann, Fritz, Rubber Age (N. Y,), 24,322 (1928). (6) Kindscher, Ber., 57, 1152 (1924). (7) Memmler, “Science of Rubber”, tr. by Dunbrook and Morris, New York, Reinhold Pub. Corp., 1934. (8) Vanderbilt Co., R. T., 1942 Rubber Handbook, New York. PRESENTED as part of the Symposium on Uses of Coal by Various Industrie 8 before the Division of Gas and Fuel Chemistry at the 104th Meeting of the AMERICAN CHEMICAL SOCIETY, Buffalo, N. Y.