RUBBER RECLAIMING

deprived of sources of natural rubber during World War II, ... the basis of a 6-day work week,24 hours per day, amounts to ... 5-day week of 24-hour o...
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Slab of Finished Stock is Cut from W ~ I - U Q B O U of R e h e r at Midmst Rubber Reclaiming CO., East St. Louis, Ill.

A Staff-RndustrgCollaboraccve Report RODNEY N. HADER Associate Editor

..

D. S . LE BEAU

in eohboration with

Midtoeat Rubber Reclaiming Co., East St. Louis, Ill.

M.

ANY of the thousands of rubber products manufactured In the United . States today owe at least part oftheirprop

established new production records in both its Barberton, Ohio, and East St. Louis, Ill., plants. The event that provided the 6rst great stimulus to the use of “gum elaetic’’ was the same that set the stage for rubber reolaiming. In 1839,Goodyear discovered that the properties of natural rubber could be improved vastly by the incorporation of sulfur, and almost immediately there was rapid expaneion in the manufacture of rubber products to satisfy the penbup demand for practical rubber p o d s (2). Consumer demand for the new, improved articles became 80 great that normal raw rubber supplies were not sufficientto keep pace with production, and the need for a secondary 8ource of supply became urgent. By about 1850, it was apparent that the reclaiming of scrap rubber wa8 the most practical means of filling this need.

erties to an unsung, glamourless constituent, reclaimed rulher. Reclaim, RS a commodity, has enjoyed IL stradily inrrpaaing market throughout the l a d century (Figure 1). When we were

deprived of aources of natural rubber during World War 11, reclaimed rubber bridged the gap in the supplyofrubtler hydmcarbon and maintained the Bow of transportation. Today tlw productive capacity of this country’s reclaiming industry, on the basia of a M a y work week, 21 hours per day, amounta to about 355,000 long tons per year. In the latter half of 1950, whcn prices of new rubber began an uneven and possibly unwarranted upward climb, and the Fedpral Government ordered curtailments in raw rubber usage and increases in government stockpiling, the wclaiming induatry was sufficiently adaptable to absorb Rome of the shock of the situation. Foreramde. the Midwest Rubber Reclaimimp. &.. whose standard ached&’ is a M a y week of %hour operation, expanded almost at once to %hour, 7 d a y operation and

CHEMISTRY O F RECLAIMING RUBBER

The term devulcanization has been used indiscriminately to describe many reclaiming proteases. Originally vulcanization 250

s

February 1951

INDUSTRIAL AND ENGINEERING CHEMISTRY

251

was the term applied to the changes brought about in rubber by the chemical addition of sulfur. However, b y the converse of this definition, the reclaiming of vulcanized rubber should not be called devulcanization; it has repeatedly been shown that, regardless of the process, the removal of sulfur combined by primary valence forces is not achieved (10). Within the last decade, vulcanization procedures have changed radically. The introduction of organic accelerators greatly reduced the amount of combined sulfur required in vulcanizates. Polychloroprenc was found to be vulcanizable by the addition of metallic oxides without any sulfur. The present-day concept of vulcanization is primarily one of polymerization of the elastomer, brought about by b o t h chemical and physical forces within and between molecules. I n accordance with this concept, “devulcanization,” as applied to the reclaiming of natural and synthetic rubbers, is properly defined as depolymerization. Uncombined sulfur is dissolved from the rubber mass by some reclaiming methods, but this is entirely independent of the reclaiming reaction. Reclaiming is strictly a matter of molecular breakdown. Over-Sized Tires, Halved by Splitter, Are Fed to Cracker Rolls The chemistry of rubber reclaiming may be considered either very simple or very complex, depending on the viewpoint chosen. process has never been fully understood, and i t is doubtful whether From the operator’s standpoint, i t is simple: for the most part, all the reaction products have ever been identified, even in the common, technical grade chemicals are used, and the reaction days when scrap rubber was less complex. The reaction products proceeds readily without the necessity for exact techniques. obtained from the hydrolysis of cellulose with metal chlorides are To fundamental chemists, however, the process is anything but equally varied and no less complex. simple, for the composition of the rubber scrap is highly complex Scrap from articles manufactured years ago contained no carbon at the start, and the products of fiber degradation are so numerblack, no vulcanization accelerators, and no antioxidants; only ous and so diversified in nature that analysis of the digester natural rubber was used, and the reclaiming process was simple. liquid is almost impossible. Although i t is known that caustic The development of organic accelerators tightened vulcanization soda hydrolyzes much of the cellulose in fiber-containing scrap to and improved the properties of rubber for many uses but consodium carbonate, the role played by caustic in the reclaiming tributed difficulties to the reclaiming processes. The development and widespread use of reinforcing carbon blacks and other fillers added t o 1000 the problem by introducing strong secondary 800 forces. Finally, the introduction of synthetic 600 polymers and the alterations produced in the 400 reclaiming processes by reactions specific to the molecular structure and configuration of the synthetics further complicated operations. 4 200 I

c 0

z

L?,

100

380 60

840

f z

z 0 $

10 8

Z 6 8

4 2

I

Figure 1. Consumption of New and Reclaimed Rubber-1850-1950

Synthetics

The decision to produce GR-S in large amounts as an all-purpose rubber accelerated efforts toward the development of satisfactory methods for its reclamation. As the a r t of tire compounding progressed, natural rubber and GR-S were used together or interchangeably, and methods for the satisfactory reclaiming of mixtures of the two rubbers had to be established. Inasmuch as GR-S, Butyl, and natural rubber are the three polymers produced and used in the largest quantities in this country they make up the greatest proportion of the scrap available to the reclaiming industry. The amount of scrap available from Hycar and neoprene is comparatively small. However, these two polymers as well as Butyl are now reclaimed with equal success

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I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY

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The reclaiming process causes the predominant scission of the natural rubber hydrocarbon molecule, followed by a smaller amount of recombination of chain fragments as the time of reclaiming progresses. The net result is a considerable increase in plasticity. However, the structure of GR-S causes a t first a conaiderable amount of scission of its molecules which, as the time of reclaiming proceeds, is soon overshadowed by extensive recombination of molecular fragments (7,22, S I ) , resulting in a progressive and extraordinary hardening of this polymer during the reclaiming process (Figure 2). The rates of scission and recombination of chain fragments of both polymers are affected by the presence of defibering agents and by the temperature a t which the reaction is carried out. Inasmuch as it is practically and economically impossible to segregate the natural rubber hydrocarbon from its synthetic partner, this difference in the behavior of natural and synthetic rubber hydrocarbon has caused numerous difficulties during the reclaiming process. To obtain devulcanized stock of an allover uniform plasticity it is possible either to Sealing Digester Preparatory to Admission of Steam to Jacket interrupt reclaiming after a short time (7, ,@2? 50) and make use of the extraordinary increase in plasticity while scission predominates in the GR-S, or to continue reclainling, using reclaiming agents which by conventional methods, with the judicial use of appropriate reclaiming agents. will prevent the heat hardening of the GR-S. Advantages may be claimed for both methods. The breakdown of ole& polymers in reclaiming has been definitely linked to the presence of oxygen in the reaction system Reclaiming Agents (14, 21, 24). The formation of active radicals, caused by the application of thermal energy in the presence of traces of oxygen, The chemical group^ in general use as reclaiming agents today can be considered the first step in the process leading to hydroare alkylated phenol sulfides, aromatic mercaptans, amino comperoxidation of the unsaturated hydrocarbon chain (6, 6, 11, 15pounds, and certain unsaturated compounds present in coke oven 1 7 ) . The location of the peroxide formation within the hydroby-products. The reactions which make these compounds useful carbon polymer molecule is determined by the alkyl substitution are not fully known. Substitutional groups may exert a considerin the chain; in natural rubber it will occur a t the a-methyl able influence on the reclaiming activity of a compound, ahile carbon (15)and not a t the double bond. isomeric differences in the molecular structure account for only The olefin hydroperoxides are highly unstable compounds, minor differences. This was found to hold true for the alkylated and their breakdown accounts for the depolymerization of the rubber hydrocarbon chain. Strong support is given to this hypothesis by the fact that very little if any decrease in the total unsaturation of natural rubber is effected during the reclaiming process. Furthermore, infrared spectrography (9) has shown that reclaiming does not cause an increase in the amount of combined oxygen in the hydrocarbon molecule over and above the traces of oxygen present after compounding and vulcanization but prior to reclaiming. Finally, the presence of alkali or metal chloride used to destroy the fiber was found to affect the rate of breakdown during the reclaimingprocess (8). This is in agreement with the findings of other investigators (15), who showed that I 2 3 4 S the acidity or alkalinity of the medium affected the breakdown HOURS OF STEAMHEATING AT 200 LBS./SO. IN. reaction of olefinic materials. Figure 2. Plasticity of GR-S and Natural The position of double bonds in an olefinic polymer has a strong Rubber during Reclaiming Process influence on the course of the reclaiming reaction; linear polymers having terminal double bonds in the side chain (synthetic elastomers) will not react in the same way under identical reclaiming phenol sulfides (11). The reclaiming reactions of the compounds conditions as will polymers which have double bonds in the main present in coke oven by-products, on the other hand, were found chain only (natural rubber), Infrared spectrography has shown to be connected with their hydroperoxidic activity, and the posi(1, 18) that GR-S consists of a number of possible molecular tion of a substitutional group definitely influenced the reclaiming configurations inherent in the polymerization behavior of the activity of the compound (20). The reason for the retardation of mixed monomers, as well as in the behavior of incipient cothe recombination of molecular chain fragments (heat hardening) polymers that result. Consequently, any molecular breakdown of GR-S polymer, which is most pronounced when amino compounds are used, is not yet known. of GR-S will be a function of its complicated configuration and Most of the reclaiming difficulties arising from the structures of cannot be delineated as straightforwardly as the breakdown of neoprene and Hycar are similar to those encountered with GR-S. natural rubber.

February leSl

INDUSTRIAL A N D ENGINEERING CHEMISTRY

HBBt Jmrdening is extraordinarily pmnound for vulcanized neoprene and Hycar, and reclaiming Wnta which ratard the recombination of moleoular fragment8 are naturally mora effeotiw tban othem PWLAIMING PROC&SSBS

The hidory of the rubber reclsiming industry has been one of major revisions,keyed to ch&.esin the rubber relatively gOaas manutacturig industry which provides reclaimers with their only of d h c t raw msteriala Hence, in addition to

, t

*

normal tschsplogi~ladvancementa, radical changes have been newsitatad& several periods because of the nhiftingof empaasis in tbe mannfacture of new rubber articles. E k l y wlid rubber artichs, such as car -8 Variety of molded .bock absorbers w d as mountin@ in horse-cm-gave way to fabric-mntaining, lightly vulcaniaed rubber footwear; t h became the 6rnt Iarpvohme Wwce of ffirap rubber for d s i m and provided the major wurce from 1860 until about 1916. Toward the end of the nineteenth century, the bicycle inddeveloped a thriving activity because of the i n h d u c t i o w f the pneumatic rubber tire in 1889. Bicycle tire production reached its peak in the United States from 1882 to 1894; thin resulted in heavy demands on the reclaiming industry for ita product and fmvided, in turn,a Lmp supply of scrsp rubber in a new form. From ahouZ 1890 to 1910, wlid rubber tires for carriages and buggiea and later for automotive truck.ale0 provided an important wurce of m p rubber. h t u a l l y , solid tires for automotbiles and trucks were m p l d by pneumatic tireg, and today tbee provide at least 86% of all m a p rubber reclaimed. Oooayear himself was among the fist to perceive the demrability of recavering rubber for muse, and in 1858 scoured one of

a53

the earliest patents in the &Id (19). Hin process was a purely meohnnicsl one, involving a grinding operation to d u c e the w h n i z a d rubber to a finelydivided atate snd then rawtication to rnix the reclaim w i t h raw rubber. This process was suoceadul because of the low degree of vulcankation of the rubber articles of that time and because of the lack of reinfoming 6llem. PPnrrHePtwA complete history of the development of various reokiming processes ha8 been compiled by Ball (8). The first Birpifioant atep toward a~tuslreclaiming wan the development of a pwhich involved restoration of vulcsnised natursl rubber scrap through the appIication.of heat. Tbia process, which in principle in atill uaed to some extent, ww exploited by plscing the finely divided m a p in shallow open pano and subjecting it for neveml hours to the direct action of steam or bot water in a closed vend. Although the atesm-treated waste never &ed all the properties of raw rubber, it wan a t leest p d y devulcanised snd wan amenable to revulcanization and muse. The pmduct of this pmcesa, which wan hrat patented in 1868 by Hall proved a much mom versatile raw material than that obtained fmm the meohanid prooegl and paved the way for more w i d q m a d use of reclaim in rubber p a l a manufacture. seppI.tion of Fibera The earliest reclaiming p-s were successful principally because them wan d l i c i e n t fabric-fm ffirap to astidy the entire demand for reclaim, and no pmblem of fiberremoval or fiber destruction had been encountered. As the demand for reclaim incmwd, however, it became necemry to employ more and morefabriwontsiningita’ y bootssndshoabinorder that the gmwing demand might be met. The product wwnot

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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

Vol. 43, N o 2

iERE

PLANT PROCESS SERIES

I

WA

is

U

n

SCREW CONVEYOR

AIR

LASTlClZERS

Figure 4.

Flow Sheet ofDigester P r o w s for Reclniming Rubber et East St. Louis, Ill., Plant of Midwest Rubber Redaiming CO.

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In the Heater Process Stock for Devulcanization Is Charged in Open Pans

o f the highest quality; the development of methods for air separation of fibers from rubber scrap further improved the quality of footwear reclaim but i t still remained mediocre.

Acid Process

Ground scrap was defibered in this process by the action of boiling sulfuric acid in lead-lined wooden tanks. The rubber was then given a water wash and was subsequently devulcanized b y heating for 24 hours in closed vessels, using steam at about 300” F. An important contribution to the reclaiming a r t a t the time was the discovery by Mitchell ( 2 7 )of the advantages to be gained b y heating rubber wastes and fiber-destroying agents together in a pressurized vessel. Mitchell’s improvement was of such importance that the use of thfh digester process or the omission of i t soon became associated with the difference between firstand second-quality reclaim.

Neutral Process

In 1913, a patent was granted to Cutler (IS) on the use of zinc chloride and pine oil to produce a neutral reclaim by the pan or heater process. The neutral process is widely used in digesters today to produce reclaims that are less tacky and less nervy than comparable alkali rwlaims, and less susceptible to prevulcanization during calendering or extruding operations. The introduction during the last decade of high percentages of synthetic polymers caused grcat difficulties in the performance of the old alkali process. These difficulties were a direct function of the reactions of the synthetic polymers under the conditions of the process. As a consequence, the importance of the alkali process decreased and metal chloride defibering agents such as calcium chloride or zinc chloride were used to a much greater extent. Today fiber removal is accomplished either by air separation or by hydrolysis with a metal chloride or with alkali. Devulcanization may be achieved either simultaneously or in a subsequent operation.

Alkali Process

The patent of 1899 which revealed details of the alkali digester process (26) has been described as probably the most important patent on the reclaiming of scrap rubber ever issued (9). The significance of the alkali process lay in its simplification of reclaiming to a single-step operation. Any type of natural rubber scrap available simultaneously could be defibered, desulfurized, and devulcanized by a single heating with dilute caustic solution under pressure. Furthermore, the alkali process yielded a vastly superior product, with greater tensile strength and elongation, better working properties, and more L‘nerve’’-the quality of natural rubber which gives it a high degree of resilience and causes i t to recover its shape after distortion.

Banbury Process

The decade between 1930 and 1940 saw the introduction of still another process, known today as the Banbury process. Ground vulcanized scrap is introduced into a Banbury mixer where in the presence of air it is worked under pressure and a t high temperature for a very short time, either with or without a reclaiming oil (28). A subsequent patent (12) described a modification of the process, providing for mastication in the presence of controlled quantities of oxygen, air, or other autoxidation promoters and/or reclaiming agents. This modification recognizes the considerable strain placed on the Banbury machine by mastication of vulcanized scrap and attempts to minimize i t by prior

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steam heating of the scrsp, thereby aoftenhg the rubber and reducing the &mn& ofthe 6hpk Filless such as carbon black may be added to help gcmerate heat and friction.

GROUND SCRAP

LIGHT PARTICLES

Figure 5.

Grain Separator for Classifying Scrap

Particles

In 1949 two additional patents were ienued covering the I-+ plasticisation of unvulcani.ed and scorched (partly vulcanized) acrap (4) and the reclaiming of vulcanized acrap (3). These procesaea alm make UBB of meetication in a Banbury mixer, but the mechanical pressure under which the scrap is worked has been augmented to between Ml and 2M) pounds per square inch; the speed of the rotam is two to three times that ordinarily used in this machine. The Bise of the motor used to drive the rotors must, of course, be wneiderably increased. After mastication the .toek, which requires no washing or drying, is ready for refking. T h e Banbury pis used commercially for the reworking of unvulcanized and mrched factory scrap. SXMWpmcSp,

The Banbury process has not found any extensive we in the reclaiming of vulcanbatea, but a modilication of ite principle using a screw-type plasticator has found wmmeroial application. Very finely ground scrap, preferably defibered by air aeparation, is charged to the m w chamber along with reelaiming a g e ~ t a and fillers. Under the effeds of preand heat the mixture is converted to a soft plsStic masa which is cooled and extruded from the plasticator ready for refining by further working. Like the Banbury process this method exploits the extraordinary depolym&tion of G R S (Figure 2) during the early stages of reclaiming (80). By far the largest amount of reclaim is still produced in aut+ daves. The rates of both the chemical defibering process and the devulcanisation reaction depend on the temperature a t which the scrap is h a t e d . In addition, the kind of celluloeic fiber as well as the kind of synthetic or natural vulcanized polymer present rillinfluence the reaction. EAST ST. WUl.9. ILL., PLANT OF MIDWEST PUDBEU PECLUMMG 00.

The East St. Louis plant of the Midwest Rubber Reclaiming Co. was one of the 6rnt reclaiming factories deaignsd to allow a stnaightline flow of acrap rubber through the many steps from scrap s t o m to the 6nal ahipping dock. Midwed’s plant

operaten smoothly and &ciently thmugh careful scheduling and integration of a great variety of activities; it has a rated daily pmduction of 85 long tons of reelaim.

Midweat

‘RBB orpsnised

v0i. a,N ~ a.

in 1928, with a new plant in Eent St.

Louis,and operated as a mbaidiaryof the Akron Rubber Reclaiming Ca., formed 5 years earlier with ita factory a t B a r b n , Ohio. Through a consolidation in W37, the two c o m p i e s !xcame one, and adopted the Midwest appellation for all Bubaequent operations. Through repeated expansions, the company h?e become one of tbb country’s largest manufacturers of whim. The Bow abeete for the East St. Louis plant ahow the course of the major portion of the material handled. Small runs of Bpeoial stocksor of materialsdestined for special end w, may be handled according to other procedures, but by far the greatest portion of all scrap is pmceased as indicated by the flow aheete to mske ”whole-tire” reclaim (Figurea 3 and 4). For convenience and economy of operation, scrap vulcanized rubber, which is purchased on the market tooby like any other commodity, ia fed whenever poasible directly into the plant from trucb or boxcars, eliminating the multiple handling inherent in working from stockpiles. Large stockpilea are maintained and augmented or depleted in accordance with acrsp ahipmente into the plant area, but W i role is analogoua to that of an auxiliary aurge tank rather than an on-atream unit in wntinuous operation. It is obviously economical to keep plant changeovers a t a minimum: varioua feed stocks are scheduled as far ahead of plant operation as possible, therefore, in order to wmbine batches and attain virtually uninterrupted operation. The handling of automobile tire scrap, which comprises at leaat 85% of all scrap reclaimed, is best demribed as semicontinuous. ~

L

i ofm scnp

Whole automobile tires are fed into the “cracker,” which is the h t item of major processing equipment. Truck and bun tires, which are too large for direct feeding into the cracker rolls, are 6rst halved on a semiautomatic aplittar. This machine, which grip the tires and rotates them against the blade of a stationary

knife, is one of the few machines developed specifically for the reclaiming industry. The cracker is a horimontal roll mill with heavy corrugated‘ rolls, having an exposed length of 38 inches between guides. A t its speed of about 21 r.p.m., the larger roll, with a diameter of 28 inches, bas a peripheral a p e d of about 156 feet per minute, and provides a friction ratio of 2.5: 1 with the smaller roll, 20 inchelv in diameter. These rolls, set close together, afford both a cutting and a tearing wtion, and the scrap is rapidly reduced to pieces with a maximum dimension of a few inches. The mill rolla are water cooled to remove the friction-generated heat. Three m k m are used in parallel in the Eaet St. Louie plant; all 818 mounted on a common shaft and driven by a ID00 hp. synchronous motor (8E). The cmcker discharges the shredded scrap onto a 35inch belt conveyer, where an operator picks off the bead w i m which have been stripped of their rubber by the action of the cracker rolls. It was once standard practice to debead all tires as a separate preliminary operation, but a saving in rubber and in operating costs is effected by combining this operation with the cracking step. Elimination of debeading made 94% of the average tire available for reclaiming instead of the 85% previously recovered. From the belt conveyer, the scrap is discbar& to a 5 X 10 foot oscillating aifter. Particles that are small enough to pass through the screen are carried forward to the next step in the process, and the warse pieces 818 recycled to the same cracker for further mbdivision. Depending on the type scrap being handled and the end we for which the reclaim is intended, the aise of the screen openings used may vary from ‘1s X 2 inches to ‘ 1 8 X ‘h inches. It is estimated that about 30% of the cracker discharge is recycled for further subdivision. Each cracker has a capacity of Mloo to 6ooo pounds of nuitably p u n d material per hour. The 3O-inch belt wnveyer which d e s the scrap from the

,

I N D U S T R I A L A N D E N 0 I N E E R IN-G C *H E M I S T R Y

February 1951

oscillating sifter paaaes close beneath a magnetic pulley, where an electromagnet removes ferrous tramp metal, such as aaila and bita of wire picked up during tire service. The fermua particles, attracted by the magnet, adhere to the moving belt interposed between the magnet and the moving scrap rubber below. The pulley belt the metdlic particlee beyond the field of magnetic intluenm and deposits them in a receptable which is p e r i c d i d y emptied by an operator T h e rubber particles which

tees

TREAD IPEELSI

m

about 20 feet in height with a &urn diameter of about 10 feet, separate the rubber from the air atresm and drop it into the bins below. The discharge porta of the bins are at a greatar elevation than either the digestera or the open steam devulcaniaer, allowing gravity feed from the binato theproceaeing unita. As a substitute for, or a complement to the r e a i p m t h g screens, a grain separator (Figure 6) can be used whenever d+ able as a means of classifying scrap partiolek Thin adapted aPDaratUS 8eDarateS fiber-contsinina D d C h from d i d N b k p&clea, thmugh dserencea in the&buoyancy in a Mused air stream. The separation is accomplished on a p o r n deck, 12.6 feet long and 5.25 feet wide, which serves 811 a vibrating conveyer. A syetem of b d e a beneath the deck permita delivery of a wntrolled volume of air from a centrifugal blower. Variable stock feed and the adjustable t i t of the table also help to control the separation. The mixed scrap is fed to the deck at one comer, and air rising througb the material s h t i f i e a it according to ita density. In the fluid mass, the heavy partidea aink to the dwk and are carried forward by ita vibratory motion. The lightar, fiber-containmg particlea Bow by gravity a~m the ulightly tilted deck and are discharged sepsrately. The particlea of intermediate weigbt are recycled eo thst only two frsctione are collected. The separation achieved is not perfect, but i t is very helpful, since d a e r e a t reclsiming technique8 may be requir or the tread and carcaea segments of the tire map (Figure ). The fractions are recombined digestion to yield wh le reclaim atoch, but they may be treated Bepsrately to attain the name 6nd plasticity and yield a realaim that is readily milled and uniform in properties.

T

Digester h a s

Figure 6. Tread and Carcass Segments of Tire Scrap Require Different Reolaiming Techniques

have p

d througb the slfter screen and beneath the magnet conveyer to a set of necondary cracker mills; these operate in a m u e r sinular to that of the main crackers and grind the pieces of scrap into bita of a d l e r and more uniform size. Like the crackers, the secondary mills are mounted on a common shaft, driven to this instance by a 450 hp. synchronous motor Four reciprocating screen8 (lOE), e m b 40 X 84 inches, c l d y the product of the regrinding mills according to size; particles which will not paca thmugb the ecreens and are therefore too large for proper penetratwn of reclaiming agents in subsequent step are returned to the mills. Since particle shape, as well 88 s h , in0uences the course of the digestion reaction and hence the properties of the iinal reclaim, slotted screens are employed to allow pasage of rectangular particles. The scrap particles which have passed througb the scmns are conveyed by air to a continuous automatic scale (4E) where an additional magnetic separatiou of metal is achieved by a magnetic head pulley. From this point the clasai6ed particles are transferred to large storage bins on the upper level of the building; the particles are blown along in a streem of air from which they are retrieved by cyclone collectors on the mf. The cyclones, are

y

'

trnnsterred by air

The major portion of production at the h s t Et. Louis plant is handled by a battery of 28 digesters, with batuhea char& and discharged on a staggered schednJe ta afford a continuous Bow of devulcanmd stack thmugh subsequent departmenta. A normal charge consists of about 5000 pounds of map and about 10,000 pounds of d h t e amc chloride solution (ordinarily 1% or lens in strength) or dilute alkali solution (approximately 4 to 6% in strength). The zinc chloride and caustic are purchased MI W% solutions and diluted in blending tanks MI needed. The dilute solution of fiberdestroying chemicals is warmed to a b u t 120' F. before it is added to the digester batoh, and the reclaiming oils are blended into the solution during the warming process. The digester8 themselves are vertical jacketed autoclaves

equipped with full-length agitator shafta. Each autoclave is 6 feet 3 inches in internal diameter and 10 feet high in the cylindriFal

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Devulcanized Particles after Washing and Dewatering Operation Operator adjusts water flow to continuous washer

portion. Both top and bottom of the autoclaves are dished. Four sets of paddles are spaced along each agitator shaft, and an extra agitator conforming to the dished bottom prevents settling of the solids. Each agitator shaft is connected through a speed reducer to a 10-hp. motoi. The agitators have a rotational speed of 12 r.p.m. The digesters are rated for 250 pounds per square inch internal working pressure. When the charge has been admitted and the handhole is closed, steam is admitted to the jacket until the internal pressure in the digester is about, 185 pounds per square inch. The internal pressure creeps above this level during the digestion cycle because some of the fiber decomposition products are gaseous. The amount of this increase in internal pressure will depend on the amount of fiber present and on the strength of the defibering solution. It has been observed only when metal chlorides are used as defibering agents. Carcass scraps, in which the fiber content constitutes a large part of the total batch (about 30 to 4070), can exhibit a pressure increase of as much as 75 to 100 pounds per square inch. A minor fraction of this increase can also be due to the use of reclaiming agents. The amount and choice of reclaiming oils depend on the kind of scrap used. Tightly vulcanized or highly reinforced elastomer scrap usually requires greater amounts. Crude solvent naphtha obtained as a by-product from coke oven operations, dipentene, coal tars, and pine tar oil have been used for a long time and are still in use today. The introduction of synthetic rubber has caused the use of other chemicals prev:ously described. Petroleum solvent naphtha is not effective as a reclaiming agent, but it and many other solvents of appropriate boiling

Vol. 43, No. 2

point range can be used as diluting carriers for extraordinari1)effective reclaiming agents, which are used in very small amounts only. The inert carriers have the effect of increasing the volumt~ o f the reclaiming agent,s, thereby permitting wetting and conscquently contact of a greater part of the large surface of scraii particles with the oils. The result is a more uniform dist,ribution of the highly effective reclaiming agents throughout the digester hatch. These exceptionally effective reclaiming agents can be used in quantities as small as 0.25% by weight on the sclnp, n-hereas less effective oils, as well as the inert carriers, may l w wed in amounts as high as 20Y0 or more. There is a natural limit t'o the usable amount of such conipounds. Reclaim is evaluated on the basis of its rubber hytli,ocarbon (RHC)cont'ent. First-line tire reclaim contains an average of 50y0 RHC. If large amounts of ineffective reclainiiiig oils or inert carriers are added, the RHC of the finished product IT-ill be reduced. Since average vulcanized tire scrap, beforc i'ct21aiming, contains approximately 58% RHC, any additioir of reclaiming oils or other fillers must be made judiciously. Washing and Drying. After 9 to 12 hours a t elevated lenipcmture and pressure, the digester batch is pressure dischaigcd t,hrough the bottom port, and the now defibered, devulcanizctl stock is transferred to an 8000-gallon magazine tank about half filled with water for diluting the digester batch and washing thc tubber free of soluble impurities. The blowdon-n of the digest,c,is is accomplished through a cyclone, 8 feet in diameter in the scct'ion where the digester charge enters. The cyclone drop8 thc solids into the wash tank and discharges steam and a small quantit,!; of steam-distillable oils and fiber-decomposition products to t8hr atmosphere through a tall stack 5 feet in diameter. Fivc cyclones and five magazine tanks are sufficient to accommodatt, t,he production of the digester department. After preliminary washing in a magazine tank, the rubber is conveyed through a continuous washing machine vhere four sets of spray nozzles complete the removal of soluble residue. Thc charge is carried through the washers by a vibrating 20-mesh screen which alloivs t'he free passage of water but holds passage of fine rubber particles to a maximum of 1 to 2%. From the \rasher the wet devulcanized stock is transferred by screw conveyer to a wringer-type dewatering press @E), where rollris press the stock against a perforated screen, reducing the watcr content to about 40%. Some stocks are more easily dewatered a t this point by a squeezing action, and to provide for this alternative, each washer is additionally equipped with a screw-typc press ($E). A digester batch of 5000 pounds can be washed and den-atered in 15 to 20 minutes. From the dewatering operation, the stock is removed by screw conveyer to one of five continuous dryers (IbE),each capable of handling 2500 pounds per hour. Each dryer is 8 feet wide, with an over-all length of 44 feet, including the feed hopper. The actual drying compartment in each case is 22.5 feet in length and is divided into six sections for air distribution. Rubber moves along on a wide endless screen through the dryer's three flights-top to center, t o bottom, and out. Air is heated by steam coils a t the top of the dryer, and passes downward through the rubber and the perforated belt. This seemingly upside don-n drying technique is used because of the great susceptibility o f rubber polymers to heat. Under the present arrangement the air having the highest temperature contacts that' part of the devulcanized rubber which is highly moist' and therefore protected against oxidation. That part of the devulcanixed rubber which contains the least moisture, and is t,herefore least protected against Oxidation, is exposed only to the cooler air. The final moisturc content of the discharged stock is held between 5 and 15%. Some moisture is left in the st,ock intentionally, because it later assists in temperature control in the milling operations. Large amounts of heat are generated through friction in milling the reclaim, arid tjhe evaporat,ion of the last traces of moisture in the stock helps to dissipate this heat and prcvent damage to t h r

February 1951

INDUSTRIAL AND ENGINEERING CHEMISTRY

259

rubber, The rubber as discharged from the dryer is loaded in 1000-pound charges into movable bins, for transfer by hydraulic or electric hand trucks to the mill room, There the final compounding, blending, and masticating operations are performed. Fines. The digester liquor, to which has been added the wash n ater, contains some rubber and decomposed fiber particles in the form of a fine suspension. For the dual purpose of saving the rubber and clarifying the plant's effluent water stream, these particles are recovered. The dilute suspension is pumped to a tank farm, where in a batch process the solids are allowed to settle out slowly, usually over a period of a t least 6 hours, and the clear liquid is discarded. The sludge from the tanks is further concentrated to about 20 to 22y0 solids, in a continuous traction thickener (6E). The thickener is 30 feet in diameter and 7.5 feet deep a t the outer edge. The sludge from the mechanical thickener is filtered by means of a continuous rotating 5 X 8 foot drum vacuum filter (9E) and dried on a drum dryer, 3.5 X 10 feet, designed to handle 1200 pounds per hour on a dry basis ( S E ) . The resulting product is either packaged or transferred t o the mill room, where i t may be hlended in as a filler low priced reclaims. .E&!

2r

Heater Process

.rX.---t

- ---

*;".p.--";g

. 3

A small but significant portion of Midwest's production, approximately 10 t o 15%, is handled through the open steam process (Figure 7) which produces a reclaim more suitable for some applications-inner tubes, for example-than the product of alkali digestion. The principal end use of heater process reclaim is in mechanical goods. Finely ground scrap is weighed in small lots of 100 to 200 pounds and intimately blended with reclaiming agents by means of a ribbon mixer. This requires only a few minutes. The blended scrap-and-oil mixture is placed in shallow pans, which are in turn stacked on small flatcars on rails. Several of the flatcars can be pushed into the pressure vessel for simultaneous devulcanization of their charges. Depending on the size of the devulcanizer, 7,000 to 12,000 pounds may be reclaimed a t one time. Live steam is admitted to the tightly sealed single-shell heater, and a pressure of 185 pounds per square inchis maintained for 5 to 6 hours. At the end of this period the pressure is released, and the pans of scrap are removed to the mill room for final working and compounding. The process is simple in operation; it has some advantages in that no losses of rubber particles are incurred and no washing or drying steps are required. The Mill Room

Because of its tremendous power consumption and the large amounts of cooling water required, the mill room is by far the costliest in operation of all the departments of the reclaiming plant. If reclaim is to process well in the ultimate manufacture of rubber goods, i t is essential tbat a great amount of mechanical work be expended on it during the final steps of the reclaiming operation. The devulcanized rubber polymers coming from the dryers are transferred to an internal mixer (YE)where they are blended with fillers and plasticizers or any materials needed to impart specific characteristics. Batches of approximately 450 pounds can be handled in this way in only a few minutes. The fillers such as clay, carbon black, and whiting and the plasticizers such as asphaltic materials are standard compounding ingredients in general use in the rubber industry (80). The blending operation can also be carried out on a two-roll mill, having a roll length of GO inches and roll diameters of 20 and 22 inches, respectively. The devulcanized dried stock is first massed together on the mill and then blended for 5 to 6 minutes with the various ingredients. The batch size is about one third that obtainable in the internal mixer, and the operation is slower.

Strainer Removes Any Remaining Foreign Particles Before Rubber Is Conveyed to Finishing Mills

A preliminary refining (breaking) follows the blending operation. The breaker is a two-roll mill with one roll 24 inches in diameter and the other 21 inches. At its rotational speed of 40 r.p.m., the larger roll has a peripheral speed of 254 feet per minute; the friction ratio of the rolls is high, about 2.5: 1, and the quantities of cooling water required to remove the friction-gencrated heat are substantial. The mills exert a high pressure on the rubber stock during the mastication process, and the rolls are short (36 inches) to prevent distortion a t the pressures developed. The rolls are smooth and accurately crowned, and the rubber is converted to a relatively homogeneous sheet approximately 0.015 inch thick. This sheet is discharged into a pit containing R screw conveyer, which feeds it to a strainer (11E) for removal of any remaining foreign particles. Small bits of brass and other nonferrous materials, against which the magnetic separators are ineffective, are removed by this apparatus which resembles a huge meat grinder. The stock is forced along by a screw and is extruded through the openings of a 20-mesh screen backed by a heavy plate with perforations about 0.25 inch in diameter. The rubber leaves the backing plate as a mass of continuous, plastic rods; these are automatically sheared off a t short intervals by rotating knives and conveyed to the finishing mills. The refining or finishing mills are exactly like the preliminary refiners or breakers and are the last major processing equipment in the reclaim sequence. The rolls are set much closer together for the fmishmg operation, and the product is sheeted out to a thickness of only 0.004 to 0.006 inch. Cooling water is supplied to the refiner roles a t a rate sufficient to hold the surface temperalurc of the rolls a t about 170" F. Any partially digested particles

260

INDUSTRIAL AND ENGINEERING CHEMISTRY

Vol. 43, No, 2

East St. Louis, Ill., Plant of Midwest Rubber Reclaiming C o .

which are not sufficiently workable to pass through the narrow opening are gradually worked to the edges of the mill and periodically removed as “tailings.” The small amounts of tailings, averaging about 3% of the total stock processed, are usually returned to the devulcanizers for a second digestion treatment. The thin sheet produced by a finishing mill is allowed to build up on the water-cooled windup roll until a hollow cylinder with a wall thickness of about 1 inch is formed. This cylinder is then cut from the roll lengthwise, forming a slab of reclaim weighing about 30 pounds. The slabs are dusted with talc to prevent sticking and are then carefully stacked on pallets in a manner which allows air to circulate between slabs, releasing any heat which might still remain in the stock. Trucks remove the filled pallets to the warehouse where the reclaim is securely baled with steel bands. From the warehouse, the finished product can be loaded directly into boxcars or trucks for shipment. Control and Development Laboratories Purchasers of reclaim demand complete analyses of the stocks they use, and Midwest’s factory control and development laboratories maintain a close check on composition and properties of the reclaim in process. The general appearance of the reclaim, specific gravity, and physical properties-such as tensile strength, modulus, elongation, and hardness-and processing properties are tested at specified intervals. For this purpose the reclaim is milled, compounded, and vulcanized, according to a simple test formula (Table I), and is tested under controlled humidity and temperature conditions in accordance with A.S.T.M. D-11 test procedures. This test is used by general agreement throughout the industry. Chemical analyses, such as the determination of acetone and ohloroform extracts, ash, carbon black, and cellulose content, alkalinity, and rubber hydrocarbon content by difference as well as by direct analysis are carried out regularly. The direct analyBi8, by chromic acid oxidation, reveals the amount of natural

rubber present in a mixed stock. Records of all these h s t s are kept, and each shipment can be traced to these records. Aside from these routine tests, certain specific tests are conducted in correlation with the ultimate use of the reclaim in the manufacture of rubber goods. These tests are carried out by compounding the reclaim into formulations representative of those used in the rubber industry for the manufacture of tires, battery boxes, adhesives, automotive articles (steering wbe~ls, radiator hose, weather and n indshield stripping, gaskets, clutch and brake covers, bushing8 and mats) and mechanical goods such as garden hose, belting, household goods, baby carriage and toy tires, soles, and heels. Special reclaims are produced to fit ultimate uses, and special equipment is required for testing them. For example, to tesr the suitability of a reclaim for the various uses in tire cornpounding-these constitute almost 50% of the total yearly consumption of reclaim-heat build-up is tested in a Goodrich flexometer. Flex resistance is trsted in a De Mattio flexometer, and restored energy (elasticity) is calculated from the result8 obtained from a Goodyear Healy, or Liipke rebound machine Abrasion is measured by a Grasselli type abrasion machine as well as by the National Bureau of Standards type machine. Finally, to test the reclaim under actual running conditions, tires are built containing this reclaim, and their performance is compared with that of tires of the same compound formulation but

TABLE I. RECLAIM TESTFORMULA (34) (Cure 10, 15, 20 minutes at 287’ F.) Ingredient Reolaim (RHC) Zinc oxide Stearic acid Sulfur Meroaptobensothiazola Diphenylguanidine

Parts

zoo

10 4

6 1 0.4

February 1951

*

INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY‘

without the reclaim under test. These tires are evaluated either on regular test fleets or on cab fleets in the vicinity of Midwest’s factories. Tread inspections and nonskid measurements are made a t regular intervals. Failures as differentiated from tread wear are also recorded, and the performance of each tire is closely observed in varying weather conditions. In other fields of use the speed of extrusion, plasticity, rate of cure, impact strength, indentation resistance, resistance to ultraviolet light and to heat or oxygen aging may be determined. To handle the compounding and vulcanization of the test samples, three laboratory mills with 6 X 12 inch rolls, one fourspeed size B Banbury mixer, and two hydraulic steam heated presses are in continuous use. The mills and the Banbury are connected with a recording wattmeter so that the power requirements during mixing can be observed; these indicate the processing properties of the reclaim under consideration. Finally, a small pilot plant, consisting of digesters, devulcanizers, and washing, drying, and refining equipment, is available for the testing of new reclaiming oils and catalysts.

261

TABLR 11. COMPOSITION AND PROPBRTIBS OF WHOLE-TIRE REICI.&M-1947-50 1947 1.17

Specific gravity

1948 1.17

1949 1.17

1950 1.17

1

17 1

17 1

%l

Nil ‘9

18

g18i1

52

52

62

33

33

34

Per Cent

P

15 1

Acetone extract Cured chloroform extract

Ash

17

11 Cellulose Nil Carbon blaok 19 Rubber hydrooarbon (by difference) 52 Rubber hydrooarbon (by direct analysisa) 38 Indicative of natural rubber content.

18

-

car lots. The storage tanks for the oils and solutions range in size from 4500 to 12,000 gallons each, and a total storage capacity of 130,500 gallons is available. Solid chemicals are stored in bags or drums in the warehouse. PROPERTIES AND USES OF RECLAIMED RUBBER

Utilities and Auxiliary Equipment

-

4

Steam is a most important commodity in the reclaiming of rubber, and the East St. Louis plant is in a strong position in this regard. Two identical watertube boiler units, each with a capacity of 361 hp., were installed when the plant was built in 1928. Improvements effected since that time have boosted the capacities of the two uriits to 500 hp. each. Either boiler is capable of supplying the total requirements of the plant a t current rates of consumption; the other is a stand-by unit for use during maintenance shutdowns of the operating unit or to allow for the possibility of future plant expansions. Steam is supplied to the plant a t a pressure of about 190 pounds per square inch, and peak demands as high as 40,000 pounds per hour have been met by the single boiler, with a pressure loss of less than 5 pounds per square inch. One half pound of coal is consumed for each pound of reclaim produced. Electric power demand a t the East St. Louis plant is normally about 5000 kilowatts, purchased in toto from the Union Electric CO. Power is received a t 13,800 volts, reduced to 2300 volts by Midwest’s fbst bank of transformers, and to 440 volts by a second bank. Each pouiid of reclaim produced requires the expenditure of 0.6 kw.-hr. in electrical energy. A major portion of the plant’s power consumption is attributable to the tremendous synchronous motors (6E) which drive the b a n h of crackers, breaker mills, and refining mills ( l E , 7 E ) . Three crackers, or as many as eight breaker and refining mills are mounted on a single shaft, and the entire bank is driven by a single slow speed motor. These motors operate a t 2300 volts with a rated horsepower of 300 to lo00 each. Twelve of these are in use in the plant. Cooling water in great quantities is required to remove the heat generated during the blending, breaking, straining, and relining steps. Midwest has drilled its own wells, three in number, to ensure an adequate supply of water a t relatively low temperatures. The wells are 110 feet deep and supply water to the plant a t 60” F. Other than the provision of steam, power. and water, the principal side issue in the rubber reclaiming plant is the problem of storage for the large volumes of liquid chemicals required. Both technical caustic and zinc chloride solutions are purchased in tank car quantities, a t a concentration of 50%. I n order to prevent solidification or settling a t this concentration, the solutions must be warm: Midwest has solved this problem by storing these materials in a large tank house, maintained a t about

90°F. Underground tanks are supplied for all flammable materials, such as crude solvent naphtha. As in the case of the caustic and zinc chloride solutions, the reclaiming oils are purchased in tank

Although reclaiming was originated for the specific purpose of providing a substitute for new rubber, technological evolution has allowed reclaim to emerge as a raw material in its own right, with many unique properties’and with advantages over new rubber in several applications, such as adhesives. The extraordinary uniformity of the product is evident from the chemical analysis of a whole-tire reclaim over a period of years (Table 11).

L

:

.

.

:

‘35 ‘36 “37 “38 ‘39

;

.

W”41

:

:

:

a

:

:

:

.

.

I

‘42 ‘43 ‘44’45 r6 ’47 ‘48 b9 “50

Figure 8. Production and Prices of Reclaimed Rubber-1935-50

In general, reclaim allows faster and easier mastication and mixing than new rubber, with attendant savings in time and power consumption during compounding. Neutral process reclaims, especially the newer ones containing GR-5, permit faster extrusion and calendering than either natural or synthetic new rubber, since they are less scorchy-that is, they are less susceptible to premature vulcanization by the heat developed in the extrusion and calendering processes (93,29). As an additional advantage, control over dimensions during these processes is improved. Because it is less thermoplastic than new rubber, reclaim helps molded or extruded articles to retain their shape during vulcanization or curing. Because it is less nervy, it is less susceptible t o shrinkage in the uncured state. This is of particular importance in the manufacture of such rubber goods as automobile mats where calendering and vulcanizing conditions are such that shrinkage can occur. Cements made from reclaim exhibit a comparatively low viscosity for high concentrations of solids, and the films obtained from reclaim cements possessa high modulus. These two phenomr ena make reclaim indispensable in the manufacture of many

INDUSTRIAL AND ENGINEERING CHEMISTRY

262

TABLE 111.

COXPARISON O F RECLAIM PRODUCTION AND CONSUMPTION WITH S E W

RUBBER COh'SUMPTION-193~-50

(3s)

Ratio, Total Reclaim Consumption of Reclaima, Long Tons xewRubber to Total TransporNontransporConsumption, xew Year tation items tation items Total Long Tons Rubber 117,500 491,544 0,239 ... 119,906 1935 ... 575,000 0.246 141,500 ... ... 150,571 1936 0.298 162,000 543,600 ... ... 185,000 1937 120,800 437,031 0.276 122,400 1938 9 8 ,'gZ3 170,000 592,000 0.287 7i,'477 186,000 1939 190,244 651,060 0.292 116,920 73,280 208,971 1940b 167,231 251,231 781,259 0.322 84,000 274,202 1941b 254,820 394,442 90,73G 164,084 0.647 285,114 1942 291,082 0.595 118,792 488,525 172,290 1943 303.991 0.353 76,875 251,083 710,783 174,208 1944 260,607 96,798 241,036 799,009 144,238 0.302 1945 243.309 275,410 167,480 1,030,296 0.265 107,930 295,612 1946 288,395 1,122,207 0.259 116,581 171,814 291,395 1947 261,113 1,069,404 0,244 112,408 148,703 1948 266,861 988.903 0.226 111,038 222,679 111,641 224,029 1949 0,244 303,000 139,000 1,240,000 1960C 144,000 310,000 a Reported t o U. S. Dept. of Commerce-by producers or consumers of reclaim. b Official estimates U. S. Dept. of Commerce. c Unofficial estimaies based on U. S. Dept. of Cominerce figures, January-October 1950. production of Reclaims, Long Tons

.

Vol. 43, No. 2

used, in spite of the fact that first-line tire reclaim was selling for over 4 cents per pound. During the last 15 years, prices of reclaimed rubber have shown a gradual upward trend (Figure 8), but they have been much more stable than the prices of natural rubber. The price of reclaim, in fact, has long served as tt stabilizer for the price of new rubber. Production costs a t Midwest during periods of capacity operation may be broken down approximately as follows: To suppliers, for scrap rubber, manufacturing supplies, and services T o employees for wages, salaries, and employee benefits Depreciation and amortization

55 % 40 %

5%

100%

industrial adhesives. Because reclaim is capable of taking up high filler loadings in a very short time, because its rate of cure is high, and again because it does not scorch easily, its use is advantageous in the manufacture of hard rubber goods such R R battery cases. While the fundamental propprties of reclaim will reflect llir properties of the kind of elastomer hydrocarbon it contains, it,s principal advantage, perhaps, lies in its adaptability. Since it is it manufactured raw material, it can be modified during manufacture to meet individual specifications, becoming a tailor-made rwvmaterial for the user with a specific production problem. U. S. Department of Commerce figures (Table 111)show Ihat, approximately half of all reclaim produced in the United States now goes into transportation items, consisting of tires, tubes, and tire repair materials (33). In general, reclaim is not' used to any large extent in the tread of the tire. However, a tread compound made from cold rubber and reclaim will outwear a tread compound containing natural rubber only ( 6 6 ) . In t,ire production practice, t,he use of reclaim is not limited to any specific gr:ido of tires. Many other items of automotive equipnient, such as battery boxes, floor mats, radiator hose, and windmy channeling, all classified by the U. S. Department of Commerce as nont,ransportation items, are volume consumers of various types of' reclaim; automobiles, therefore, utilize an estimated two third? of all reclaim produced ( 2 ) . h first-quality tire for passenger car use normally contains from 3 to 4 pounds of reclaim; an xutomobile floor mat may contain 5 pounds and a battery case ail equal amount. The principal uses of reclaim aside from those in the aut,omotive field are in the manufacture of adhesives and mechanical and molded goods. Properly used, reclaim is not introduced "where any impairment of quality of the finiahcd product will result. In some products it gives better qua.lity than can be obtained with new rubber, and its use often rcwdts in an equal quality a t lower cost. ECONOMICS 4 N D PRODUCTIOR ( : O S l S

Reclaim is no longer considered primarily a substitute for crud(% elastomers, but it' does find increased markets during periotls when crudes are available only in limited quantities or a t very high prices. When crude rubber prices are unusually low, t,hc reclaiming industry is certain to feel severe price competition , especially from off-grades of natura.1 rubber. Even a t such times, however, reclaim is required for many uses because of its qpecial properties, and demand remains substantial ( 3 2 ) . In 1932, when the price of crude rubber averaged only 3.4 cents per pound, reclaim still accounted for nearly 19% of all rubbcr

These production costs account for 80% of the sales dollar; the remainder is divided among taxes, dividends t o stockholders, and earned surplus retained for use in the business. The interrelationship of these cost factors is sensitive to production volume, and operation a t levels below capacity causes an increase in the ratio of wages and depreciation costs td total sales. FUTURE PROSPECTS

In t h e latter half of 1950, f,he combined effects of high pricchs o f natural rubber, governnient restrictions on its use, and tho physical shortage of available new rubber caused a large incrcrlsc in demand for reclaim. Production \\-as immediately stepped up throughout the reclaiming industry, and predictions for 1951 production of as much as 350,000 long tons have been announced. This mould exceed by a considerable marg'n the peak wart>imc rate of 304,000 long tons established in 1943. The capacity o l t,he reclaiming industry has been increased considerably sin(:(, t hc cud of World War 11, but a period of maximum effort i i p p c ~ 1,) ~s he abed for all reclaimere. It is unquestionably true that synthetic rubberpolymers arehcrc to stay. Inasmuch as research for new a,nd better-performing synthetic rubber polymers can be expected to continue a t full speed, new polymers will continually be developed. Somr will reach production on a large scale. If these polymers are to he rcclainied successfully, eit.her singly or in combination, tho rr-, claimers must, att>ainfamiliarity with their structure and chemic:il make-up and find the proper reclaiming agents. Usually t'hcre is a lag of 1 to 2 years between thc first major production of these polymers and their arrival a t the reclaiming factory; research ca11 niwke effective use of this time. There are, of course, still considerablc improvements to be ni;rde i n the mechanical equipment and its efficient use; a well colitrolled continuous process is thc desirable goal. To just'ify replacement of the long-established digester process, a,ny now tlevelopnient must offer excrptional advantages either in cost, of operation (including equipment maintenance and replacemrnt), in percentage of tailings produced, or in physical propertics of the reclaim produced. The relatively low level of the pricc of rcclaini and the stabilit,y of this price place severe restrictions 011 the int,roduction of new processes. Any economical substitution of processes, continuous or otherwise, or any improvement in t,he phj-sical properties of reclaim, preferably combined with a decrease in manufacturing cost, \youid constitute a major contribution since it would permit greater advantages in the iisc of i d a i m even a t a time when crudc rubber hydrocarbons might be available a t a low pricc.

L

February 1951

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

263

View o f Mill Room Showing Synchronous Motor and Speed Reducer for Operation of Mills on Common Shaft

LITERATURE CITED

.

Alekseeva, E. N., Belitzkayor, R. M., Rubber Chem. and Technol., 15, 693, 698 (1942). Ball, J. M., “Reclaimed Rubber,” New York, Rubber Reclaimers Association, Inc., 1947. Banbury, F. H., Comes, D. A., and Schnuck, C. F., U. S. Patent 2,461,192 (Feb. 8, 1949). Ibid., 2,461,193. le Beau, D. S., in “Colloid Chemistry, Theoretical and Applied,” edited by Alexander, J., Vol. VII, pp. 569-97, New York, Reinhold Publishing Corp., 1950. le Beau, D. S., I n d i a Rubber W o r l d , 118, No. 1, 59-65 (1948). le Beau, D. S., Rubber A g e , 62, No. 1, 51-5 (1947). le Beau, D. S., Rubber Chem. and Technol., 21, No. 4, 895-908 (1948). Ibid., 22, No. 2, 560-71 (1949). Ibid., 24, No. 1 (1951). Cook, W. S., Albert, H. E., Kilbourne, F. L., Smith, G. E. P., IND. ENG.CHEM.,40, 1194-1202 (1948). Cotton, F. H., and Gibbons, P. A., U. S. Patent 2,408,296 (Sept. 24, 1946). Cutler, D. A., U. S. Patent 1,078,056 (Nov. 11, 1913). (14) Essex, W. G., Ibid., 2,154,894 (April 18, 1939). (15) Farmer, E. H., India-Rubber J., 112, 119-24 (1947). (16) Farmer, E. H., Rubber Chem. and Technol., 20, 366-74 (1947). (17) Farmer, E. H., Trans. Faraday Soc., 42, 228-36 (1946). (18) Field, J. E., et al., J. Applied Phys., 17, 386 (1945). (19) Goodyear, Charles, Brit. Patent 2933 (Dec. 16, 1853). (20) I n d i a Rubber W o r l d (editors), “Compounding Ingredients for Rubber,” 2nd ed., New York, Conway Printing Co., 1947. (21) Ioannu, J. P., U. S. Patent 2,069,151 (Jan. 26, 1937). (22) Kilbourne, F. L., U. S. Patent 2,324,980 (July 20, 1943). (23) Kilbourne, F. L., and Miller, G. W., IND.ENQ.CKEM.,22, p. 69 (1930). (24) Kirby, W. G., and Steinle, L. E., U. S. Patent 2,279,047 (April 7, 1942). (25) Marks, A. H., U. S. Patent 635,141 (Oct. 17, 1899).

(26) Midwest Rubber Reclaiming Co., East St. Louis, Ill., Rept. 13 (1949). (27) Mitchell, N. C., U. S. Patent 395,987 (Jan. 8, 1889). (28) Robinson, Thomas, Ibid., 2,221,490 (Nov. 12, 1940). (29) Shepard, N. A., Palmer, H. F., Miller, G. W., IND.ENG.CHEX., 20, 143 (1928). (30) Sverdrup. E. F.. and Elain, ,J. C., U. S. Patent 2.415.449 (Aprii 11, 1947). (31) Tobolsky, A. V., et nl., J . Applied P h w . , 15, 380 (1944). (32) Trimble, G. K., J . Chem. Education, 19, 420-7 (1942). (33) U. S. Dept. Commerce, Washington 25, D. C., Rubber, Industry Report (March 1950 and September 1950). (34) Vanderbilt, R. T. Co., New York, “Vanderbilt Handbook,” 9th ed. (5. S. Rogers, editor), 1948. Processing Equipment (1E) Adamson-United Co., Akron, Ohio, two-roll mills. (2E) Anderson, V. D., Co., Cleveland, Ohio, screw-type expellers. (3E) Buffalo Foundry & Machine Co., Division of Blaw-Knox Co., Buffalo, N. Y., drum dryer. (4E) Builders-Providence, Inc., Providence, R. I., Toledo-Chronoflo automatic conveyer scale, Type 9153. (5E) Dorr Co., New York, N. Y . ,mechanical thickener. (6E) Electric Machinery Manufacturing Co., Minneapolis, Minn., 2300-volt, slow speed synchronous motors. (7E) Farrel-Birmingham Co., Inc., Ansonia, Conn., No. 11 Banbury mixer, Size B Banbury mixer, two-roll mills. (8E) Louisville Drying Machine Co., Louisville, Ky., dewatering presses. (QE) Oliver-United Filters, Ino., New York, N. Y., vacuum drum filter. (10E) Orville-Simpson, Cincinnati, Ohio, Rotex screens. (11E) Royle, John, & Sons, Paterson, N. J., strainers. (12E) Sargents, C. G., Son Corp., continuous belt dryer. R E C ~ I V EDecember D 26, 1950.