464 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 32. NO. 4

analysis has been completed and a bottling line is available. Hose from storage tanks are led to the filling machine on the floors below through 8-inch steel ducts. In the manufacture of creams and ointments, the most im-

portant greases are petrolatum and lanolin. Both of these substances are melted, several tanks being used for segrega- tion of various types, before they are pumped to different mixing units. Greasy materials are kept in a fluid state while drugs and ingredients are added and during the packaging operation. In this department the need for flexibility is less than in liquid processing since the basic ingredients for many creams and ointments are less varied. Processing equipment is standard and stationary. Mixers are Abb6-Lenart, vac- uum type (Figure 2 ) . They have a capacity of 65 gallons and are constructed of stainless steel. The AbbA-Lenart mixer utilizes the centrifugal force developed by a ribbed mixing disk located in the bottom of the tank. Enclosing it is a vertical slotted cylinder, open a t the top end, with a nar- row slit on the bottom adjacent to the ribbed mixing disk. Material is forced outward from the mixing vanes through the bottom opening of the stationary vertical cylinder. The material rises along the walls of the tank and is then sucked over the top of the vertical cylinder or through the side vanes of the cylinder, down into the rapidly revolving mixing disk and again outward through the bottom slit. Each mixer has an underdrive unit, powered by a 7.5-horsepower motor, and operates at speeds ranging from 400 to 600 r. p. m. When mixing is complete, the mixed products are discharged through a bottom cock. Cleaning of the mixers is relatively easy as the internal parts are removable. Cleaning in this depart- ment, however, is more of a problem than in the liquid prepa- ration since the heavy mixers and homogenizers cannot be mounted on wheels, Equipment in this department has di- rect connection to waste lines for washing; after that opera- tion they are steamed and dried with an air blast.

Milk of magnesia was formerly manufactured by a batch operation. Solutions of magnesium sulfate and sodium hp-

droxide were mixed together in a precipitation tank, and the resultant milk of magnesia was washed in order to reduce the sodium sulfate content of the product below the specified limit. It was necessary to decant and wash each batch twenty-six to thirty times, and one hundred four wash tanks were required. Solutions also had to be boiled and then cooled during settlement. Furthermore, after washing was complete, it was necessary to remove water by evaporation in order that the finished product would not settle on standing. Modernization demanded a doubled production of milk of magnesia, and in order to do this two hundred washing tanks would have been necessary. Instead, a Dorr countercurrent decantation system was installed, and four thickeners now accomplish the same result as would two hundred tanks.

The new plant, using countercurrent decantation methods, produces 1000 gallons of milk of magnesia every 12 hours. I t employs two 20 X 7 foot and two 16 X 7 foot Dorr cy- press tank thickeners with special gluten-type thickener mecha- nisms made of iron and steel.

As Figure 3 shows, thickeners 1 and 4 are 20 feet in diame- ter. The fourth thickener is large in order to give some stor- age or retention of product and to ensure a suitable final density. The fourth thickener is discharging a milk of mag- nesia with a density of 38 grains per fluid ounce. This den- sity allows direct bottling and thus eliminates the evaporation formerly necessary. Also eliminated are the alternate boiling and cooling cycles practiced in the former batch process. Ad- vantages of the continuous method, aside from space and equipment saved, are impressive. Had the batch method been extended to cover the increased production, it would be necessary to use 7 2 per cent more steam, 75 per cent more water, and 80 per cent more labor than is now necessary.

’ Acknowledgment The assistance of Francis Chilton in the preparation of

this paper is appreciated.

BUNARUBBERS ALBERT KOCH, I. G. Farbenindustrie Akt.-Ges., Leverkusen, Germany

HE chemical and physical knowledge gained during work with polymeric materials for the past fifteen years has brought a valuable insight into the structure of

such natural products. This knowledge has led to the syn- thesis of a large number of high-molecular-weight materials. Of them, the rubberlike elastic products are of primary interest to the rubber industry. These rubberlike elastic materials may conveniently be divided into two classes- vulcanizing and nonvulcanizing.

In the narrower sense products whose basic components are dienes (hydrocarbons closely related to isoprene), can be termed “synthetic rubber”. Such synthetic materials, through a vulcanizing process, can be irreversibly converted from the plastic state into an elastic form. The works of Staudinger and others have made it seem probable that these highly polymerized materials consist of long-chain molecules. The individual molecules are joined to one another by normal chemical linkages in accordance with the theory of KekulB. Based on the measurement of viscosity and through other investigations, it appears nearly certain that in natural rubber there are approximately 2000 isoprene molecules linked or joined to one another. This corresponds to a molecu-

T lar weight of 136,000. Although these findings have been verified only during the past fen- years through the work of Staudinger and his associates, this possible structure was mentioned as early as 1900 by C. 0. Weber and in 1906 by Pickles (2) who stated: “It is suggested that these un- saturated isoprene nuclei unite to form link chains and that the number of isoprene complexes may vary in different kinds of rubber”. Figure 1 shows the chain molecules of the Buna polymers, compared with the structure of natural rubber.

It is certain that when butadiene is polymerized, not only does the normal 1,4 addition occur to form long chains, but also 2,3 and 3,4 additions take place. Through this abnormal process of addition a transverse connection occurs with other molecules and results in cyclization (Figure 2 ) .

These molecular structural differences between natural rubber and Buna synthetic rubber seem to be the explana- tion for the minor deficiencies of Buna, such as its poor mastication characteristics and its inferior tackiness. How- ever, these same structural differences can also explain such advantages of the Buna rubbers as their superior heat re- sistance and greater abrasion resistance and heat stability as compared to natural rubber. By introducing a second

APRIL, 1940 INDUSTRIAL AND ENGINEERING CHEMISTRY 465

polymerizing component into the butadiene chain, it has be- come possible to give Buna certain desirable properties such as greatly improved oil resistance and workability. Since it is still difficult to compound highly polymeric butadiene rubber on account of its hardness, further possibilities still remain for the manufacture of special types with improved properties.

The I. G. Farbenindustrie Aktiengesellschaft manufactures the following Buna types a t the present time:

Buna 85 is a pure butadiene polymer of relatively low molecu- lar weight. The polymerization is accomplished with the help of sodium metal. From this type of product the name “Buna” originates, since i t is a combination of bu for butadiene and na for Natrium, the German name for sodium.

Buna S is a mixed polymer of butadiene and styrene. The polymerization is accomplished in aqueous emulsion.

Perbunan is a mixed polymer of butadiene and acrylic nitrile. The polymerization takes place in aqueous emulsion.

Perbunan Extra is a mixed polymer of butadiene and acrylic nitrile containing a higher proportion of nitrile. The poly- merization is carried out in aqueous emulsion form.

Buna 85 Buna 85 is manufactured only on a small scale. ?From the

technical standpoint this type is less valuable than the mixed polymers. Although Buna 85 is soft, plastic, and easily workable, the tensile strength and elasticity of its soft vul- canizates are considerably lower than those of either natural rubber or the mixed polymers. On the other hand, hard rubber made from Buna 85 has the advantage of high therms1 softening point and excellent chemical resistance.

Buna S This type of material is produced in large quantities in

Germany a t present, principally for use in tires and mechani- cal rubber goods. Buna S represents such an improvement over natural rubber in heat resistance and abrasion resistance that it is by no means to be considered a mere substitute for natural rubber. Road tests with Buna S tires have demon- strated beyond doubt their superior wearing qualities as com- pared to natural rubber tires. For many technical purposes the superior heat resistance of Buna S is a valuable factor.

The swelling characteristics of Buna S vulcanizates in con- tact with gasoline and mineral oil are only a little better than those of natural rubber.

To the electrical cable industry Buna S is of real interest. The excellent heat resistance and superior aging properties of Buna S assures that cables covered with it will last much

longer in service, particularly a t high operating temperatures. The water absorption of Buna S is only about 65 per cent of that of natural rubber; this smaller figure corresponds closely to the water absorption of deproteinized natural rub- ber.

Buna S can be plasticized by carefully controlled hot air treatment. In this manner its workability may be greatly improved, even to the point where i t is about equal to the working properties of natural rubber. Buna S is plasticized by treating the crude product with hot circulating air a t 110” to 140’ C. The pressure may be only normal atmos- pheric, but it is more advantageous to use an air pressure of

The paper presents, in a general way, a de- scription of Buna 85, Buna S, Perbunan, and Perbunan Extra. They are identified and their swelling resistance, plasticization, processing, vulcanization, and some uses are given. For those interested in the processing and handling of these products, the various techniques are described which may be employed for accomplishing proc- esses known and in use in the handling of natural rubber.

Principles to be followed to obtain results with various vulcanizates from Buna-type synthetic are presented.

3 to 5 atmospheres in treating Buna S by this method. The speed and degree of plasticizing accomplished will depend upon the temperature, the air stream speed, and the pressure.

This plasticizing process can be carried out in any of several types of equipment. A hot tunnel dryer through which Buna S passes on a conveyor belt is usable. Shelf- drying ovens and autoclaves, either of which can have hot air circulated through them, will also suffice. In any type of equipment for this purpose it is necessary that the crude Buna S present as great a surface as possible to the action of air; therefore the Buna S is cut into small pieces previous to

Natural rubber - C H ? - F = C H ~ H 2 ~ H 2 - C = C H - C ~ CH z-h3= c CH - c f x c%-; = ~ H - c H ~ ~ c H ~ - ~ = c H - c H ~ - CH3 I CHJ cy3 4 CH3

Numbered Bunas

I , Buna S -CHz-CH PCH -CH2+Hz-CH-CH-CH =CH-CH-kH - C H p C H -CH2-

I 2 , 2 I

6 6

I Perbunan

-CH2-CH =CH-CH$H2-CH -CH-CHiCH, -CH+CHFCH PCH-CH2+CH2-CH =CH-CHz-

I C r N I I I t

FIGITRE 1. CHAIN MOLECULES OF BUNA POLYMERS COMPARED WITH THE STRUCTURE OF NATURAL RUBBER

the hot air treatment: Furthermore, in this plasticizing process for Buna S it is absolutely essential that constant tem- peratures and even circulation of the hot air are obtainable so that a uni- form final product results.

This hot air plasticizing process is accompanied by an exothermic reac- tion. Therefore care must be taken that no uncontrolled decomposition occurs during the softening process. This plasticizing of Buna S by hot air t r e a t m e n t i s p a r t i a l ly reversible. Therefore prolonged storage of the softened material before compounding should be avoided.

This softening process is depend- ent upon the presence of oxygen. Treatment of Buna S under identi- cal conditions with an inert gas such as nitrogen does not result in a plasticiz-

466 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 32, NO. 4

ing action. Although oxygen is necessary to this process, the quantity absorbed is so small that it cannot be analytically determined (1).

The further processing of thermoplasticized Buna S is carried on just as if one were handling natural rubber. For vulcanization, sulfur and accelerators are necessary. Physi- cal test values which correspond closely to those of natural rubber compounds are obtained through the use of reinforcing fillers, particularly channel black, but also by the use of the common softer blacks. The sulfur and accelerator content as well as the quantity of fillers should be in relation to the degree of plasticity of Buna S. That is, the more plastic the Buna S, the more sulfur, accelerator, and filler may properly be used.

Perbunan and Perbunan Extra

Both these mixed polymers are commercially available in the United States. The Perbunan types are those dis- tinguished by their characteristic resistance to swelling by gasoline, lubricating oil, and many other organic solvents.

Although this swelling resistance is the primary property of these types, they possess other desirable physical properties in the vulcanized form. As mentioned before, Perbunan Extra contains more mrylic nitrile in its composition than does Perbunan; likewise Perbunan Extra exhibits better solvent resistance. Perbunan and Perbunan Extra are similar so far as most properties are concerned. Perbunan Extra mills more easily, yields less elastic vulcanizates, and has better oil resistance. Both types of Perbunan are su- perior to natural rubber in their aging qualities, in their higher heat resistance, and in better resistance to abrasion.

In the raw state the Perbunans are insoluble in gasoline and similar aliphatic hydrocarbons. However, the unvulcanized materials are soluble in aromatic hydrocarbons and chlori- nated hydrocarbons; unlike natural rubber and Buna S, they are also soluble in such ketones as acetone and methyl ethyl ketone.

Perbunan and Perbunan Extra, unlike Buna S, cannot be plasticized by treatment with hot air to produce a worth- while improvement in their working properties. It is there- fore necessary to mill Perbunan to plasticize it before com- pounding. This preliminary milling or breakdown of the Perbunans does not produce the same visible effect that mill- ing of natural rubber does. Tievertheless this breakdown of Perbunan is most important for its further processing. The preliminary milling causes softeners to be more readily taken up, extruding properties are improved, the material becomes more easily soluble, and the tackiness of the raw compound is definitely increased.

Mastication of Perbunan should be done on cold, closely set rolls. Depending upon the size of the rolls, their speed ratio, their temperature, and finally upon the size batch of Perbunan, the mastication of the raw stock should be ac- complished in 10 to 25 minutes. Perbunan Extra breaks down more readily and more rapidly than Perbunan.

The relatively poor breakdown and tackiness of Perbunan usually must be improved upon through the use of softeners. With the aid of suitable softeners it is possible in practice to approximate the compounding characteristics of natural rubber. Softeners differ in their specific action with Per- bunan and can conveniently be divided into different classi- fications as follows:

1. Softeners which improve elasticity-for example, dibenzyl

2. Tackifying softeners such as rosin, aldol-a-naphthyl-

3. Extruding softeners such as degras, soft brown substi-

ether, phthalic acid esters, and triphenyl phosphate.

amine resin, liquid coal tar, soft coumarone resins.

tute, and certain alkyd resins.

When resinous softeners are used, it is desirable that they be melted with the liquid softeners prior to being milled into the compound. Certain softeners, such as triphenyl phos- phate and aldol-a-naphthylamine resin are insoluble in gaso- line and mineral oil; consequently they are to be preferred where a maximum of oil resistance is desired.

In a number of fabricating processes such as frictioning, volatile softeners such as tetrahydronaphthalene or cyclo- hexanone may be used to advantage. The quantity of softener used in any instance normally varies between 5 and 30 per cent, based on the Perbunan content. In general, the quantity of softener should be kept as low as possible. Higher softener contents make desirable more filler in the com- pound in order to avoid lowering the physical tests of the vulcanized product.

Natural rubber can also be used as a softener for Perbunan, provided the rubber itself is well broken down before being milled into the Perbunan. Natural rubber as a softener for Perbunan is particularly recommended in such cases as require an odorless and tasteless finished product-for instance, in food packing, drug sundries, etc. The addition of 10 to 20 per cent of natural rubber to Perbunan affects the swell- ing chara&eristics comparatively little but greatly improves the working qualities of the mix.

Fillers in Perbunan have much the same action as pre- viously described in the case of Buna s. High tensile strength and good abrasion resistance can best be attained by using reinforcing carbon blacks such as channel blacks. The semireinforcing softer blacks produce lower tensile strength compounds which have better elasticity and permanent set qualities. The inorganic fillers are of less value in Perbunan compounding. For light colored compounds, such materials as fine zinc oxide, magnesium carbonate, and fine-particle- size clay may be used. Whiting, lithopone, and barytes act merely as inert fillers since they do not improve the physical properties of the vulcanizates.

~ ~ - ~ - ~ ~ - ~ - ~ - - 4 - ~ ~ - - ~ - ~ - - ~ ~

FIGURE 2. BUTADIENE POLYMERIZATES (ACCORDING TO STAVDINGER)

Vulcanization of Perbunan compounds is usually accom- plished through the use of the regular dosages of sulfur, zinc oxide, and accelerators. As accelerators many materials are useful. The mercapto type accelerators, alone or com- bined with basic assistant such as diphenylguanidine, di-o- tolylguanidine, or dibutylamine are satisfactory. Thiuram type accelerators are useful in heatrresistant compounds. The dithiocarbamates are particularly useful for quick or self-vulcanizing products.

Furthermore, satisfactory vulcanizates of Perbunan can be obtained with diphenylguanidine or di-o-tolylguanidine with- out any zinc oxide, provided the curing temperature is about 150' C. These unwlcanized stocks are free from scorching

APRIL, 1940 INDUSTRIAL AND ENGINEERING CHEMISTRY 467

difficulties. Litharge and sulfur, 5 parts each on 100 parts of Perbunan, also produce good results.

In any case the proportion of sulfur and accelerator in a Perbunan compound mill preferably be related to the softener and filler content present. Generally more sulfur and ac- celerator are re4uired when the compound contains higher quantities of softener and filler.

Vulcanizing Perbunan with 15 parts of sulfur t o 100 parts of Perbunan produces a semihard, flexible tulcanizate. T i t h 30 parts or more of sulfur to 100 parts of Perbunan, a vul- canizate of hard-rubber consistency results.

The hard compounds have high softening points and su- perior solvent resistance as compared to those produced from natural rubber.

Antioxidants are for the most part unnecessary in normal compounding of Perbunan. The raw Perbunan itself con- tains enough phenyl-@-naphthylamine to stabilize it suffi- ciently for most classes of service. If heat resistance is re- quired to Perbunan compounds, it is recommended that Antioxidant blB be added particularly in combination with thiuram accelerators, as well as a small amount of sulfur.

Perbunan vulcanizates which will be exposed to light or subjected to tension should have incorporated in them a t least 1.5 parts of ozocerite or paraffin wax, based on 100 parts of Perbunan.

The vulcanization of Perbunan compounds takes place under the same conditions as are commonly used to cure natural rubber. Vulcanization time and temperature will depend upon the type and amount of accelerator used.

Overcuring of Perbunan compounds produces only a slight lowering of the tensile strength and but a slight decrease in the elongation a t the breaking point. However, overcuring considerably decreases tear and fatigue resistance. There- fore, for optimum cure, a time and temperature should be selected which produce not only high tensile strength and elongation values but also good tear resistance.

Vulcanizates produced by hot-air cure of forms dipped in cements or solutions of thiuram-accelerated Perbunan com- pounds attain higher physical test values than when vul- canization of similar dry compounds is carried on in the press.

The rebound elasticity values of Perbunan compounds are at room temperatures below the values of similar Buna S and natural rubber compounds. At higher temperatures, such as 70" C., the rebound elasticity values improve con- siderably and eventually approach those of natural rubber.

The compounding of Perbunan mixtures is usually carried on with the two-roll rubber mill. The preliminary break- down or mastication is as described before, using a cold mill with tightly set rolls and a suitable size batch of Perbunan. For 60-inch mill the best charge is about 30 pounds of Per- bunan. It is desirable that during compounding the tem- perature of the Perbunan mix should not exceed 70" C.

When the Perbunan has heen broken down by mastica- tion, such softeners as are soluble in Perbunan are first added. These would be dibenzyl ether, coal tars, aldol-a- naphthylamine resin, and alkyd resins. After the softeners have been absorbed or worked in, the reinforcing carbon black should be added slowly. Fatlike softeners, such as the soft substitutes, degras, or stearic acid, are mixed alternately with the semireinforcing blacks or inert fillers. Zinc oxide and sulfur should preferably be added, either together with the inorganic fillers or after the Perbunan-soluble softeners have been added to the batch.

Good practice is to let such a milled Perbunan compound age for a day and then to give it a thorough kneading or re- fining and a t the same time add the accelerator. RIixes containing ultra-accelerators should not be stored for any length of time.

Tubing or extruding of Perbunan goes easily when the compound is plasticized n ith those softeners particularly recommended for this purpose. In general, the barrel and worm of the tuber should be thoroughly cold, and the die should be only sufficiently hot to assure that the stock ex- trudes smoothly. If the die has a sharp lip, the sharp edge should be inside toward the 1% orm to secure smoothest ex- trusion. This is in distinct contrast to natural rubber tech- nique. Furthermore, in tubing hose, the pin forming the bore of the tube should terminate just outside or beyond the sharp inner edge of the die.

Clearance betm-een the barrel and worm of the tuber should be normal and not so great as to cause undue churning and heating of the stock. Before the Perbunan is extruded, it should preferably be warmed on a mill, sheeted out, and cut into narrow strips; then it may be dusted with zinc stearate before i t is charged to the tuber.

For calendering Perbunan the same plasticizers are recom- mended for use with Perbunan as in the extrusion process. Compounds which are low in softeners can be sheeted out only into thin sheets 0.5 to 0.75 mm. thick. Extruding qofteners greatly facilitate the working properties of Perbunan compounds on the calender.

Frictioning compounds based on Perbunan should be made quite soft through the use of relatively high softener con- tents. On 100 parts of Perbunan it is desirable to use either 50 parts of dibenzyl ether or 25 parts of dibenzyl ether plus 25 parts of liquid coal tars. The roll temperatures for calen- dering should be between 40" and 50' C., and for frictioning the rolls should be within the temperature range of from 30" to 40" C.

Cements, dipping solutions, and spreading compounds of Perbunan may readily be prepared by dissolving a properly mixed Perbunan batch in such solvents as benzene, toluene, dichloroethylene, or trichloroethylene. It is essential in making such liquid products that the Perbunan be properly masticated to attain good solubility.

For satisfactory tackiness in building up unvulcanized articles, the surface of the plies may be washed with a mixture of toluene and light coal-tar distillates, or with a 5 to 10 per cent rosin solution in toluene or methyl ethyl ketone. The rosin solutions to induce tackiness should, however, be used only for fabricating articles which in service will not be ex- posed to high temperatures.

The properties of the Buna rubbers just described are the result of ten years of extensive research in the laboratories of the I. G. Farbenindustrie Aktiengesellschaft. The first work on the development of synthetic rubber was begun in 1906. This research, owing to scarcity of natural rubber dur- ing the World War, led to the manufacture of the so-called methyl rubber for practical use. This product was a deriva- tive of dimethylbutadiene. The development lapsed for some years after the war and was again taken up in 1927 when ways and means were found to manufacture butadiene on a large scale.

It appears certain that Perbunan and Buna S are to be regarded as only the first steps in the field of mixed polymers, in view of the short time taken to develop these products. It is to be expected that in this field great progress will be made, particularly in regard to better workability and also in the development of products possessing special properties.

Literature Cited (1) Hagen, H., Kautschuk, 14, 203 (1938). (2) Pickles, S. S., J. Chem. SOC., 87, 1088 (1910).

PRESENTED as part of the Symposium on Synthetic Rubber and Elastic Polymers before the Division of Rubber Chemistry a t the 97th Meeting of the American Chemical Society, Baltimore, \Id.