Decentber, 1923
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
LS Rubber-A
1267
New Crude Rubber'
By Ernest Hopkinson U N ~ E STATES D RUBBERCo., New YORK,N. Y.
T
HE methods of obThis paper gioes a general description of the methods of making goods. This rubber taining from latex the standard crudes-fine Para, pale crepe, and smoked sheet. narily cures in about half the The apparatus f o r making sprayed rubber and the method of time required for Pale crepe the standard crude in the 10 per cent sulfur rubbers which appear on operation i s described. The merits of sprayed rubber as brought out during a period compound. The Wick-curthe market today are gening characteristics are due erally known and need of three years' testing are gioen and tables are shown which gioe analytical data and physical tests of sprayed rubber in comparison in part to the maturation to be only briefly described here* Curiously enough, with fine para, pale crepe, and smoked sheet. or decomposition of the fine Para, which has genSprayed rubber is uniform in qualify, ages well i n cured stocks, proteins, but the exact gioes high tensile strength, cures quickly, and is of broad application identity of the natural acerally been considered one celerator has not been deof t h ~best crude rubbers, i n the manufacturing of rubber goods. termined. is obtained by a very elemental method. The naLS RUBBER-SPRAYED RUBBER tives accomplish this by turning a stick or paddle which has been (lipped in latex over a smoky fire until coagulation and In studying the methods by which crudes in common use have been prepared, the writer was impressed with the imevaporation take place, producing a film of rubber on the stick on which successivelayers are added until the mass is portance of giving very careful consideration t o the solid of suoh size as to become unwieldy, when it is removed from constituents of latex not utilized by the usual methods of the and sentto the market. some of the nonrubber p.reparation. The study of the possibility of utilizing materials which, if of value, should very appreciably increase the constituents of the latex are lost. In the manufacture of crepe rubber on the plantations in World's rubber SUPPlY led to an investigation in the Cornthe East, the latex as collected is brought to approximately pany's plantations in Sumatra and in its general laboratories a stalldard dilution of 20 per cent dry rubber content. If it at New York. The results of these investigations very soon is des,ired to make pale crepe, a solution of sodium bisulfite established the fact that when prepared by certain methods is added in the proportion of 1 part of bisulfite to 100 of latex, rubber containing constituents not usually obtained by the Acetic acid is employed as the coagulant and is usually added CoagUlatiOn methods, possesses qualities superior to the as a per cent solution in such quantities as to give 0.1 per The apparatus for producing LS rubber which has so far cent on the latex. Thorough agitation of the latex during coagulation is essential. The coagulum is washed to remove given the best results and which is now being used comthe acid by passing a few times through corrugated rolls and mercially is known as the disk spray. In this process the finally through smooth rolls. The number of passes through latex is allowed to flow onto a rapidly revolving, horizontally mounted disk, from which it is thrown in an umbrella-shaped the creping rolls varies from five to ten,depending on whether spray or mist into a heated current of air or inert gas. The or not the rubber is to be vacuum-dried or air-dried and also disk is located a t the top of a tower, generally of the shape of upon the thickness desired. In making smoked sheet rubber, the latex, preferably a hollo.i?i pyramid, and the fine spray Of rubber globules bulkt:d in lots of 50 gallons or more, is diluted to approx- falling through the hot air or gas settles on the bottom of the imatc:ly a standard concentration of 15 per cent dry rubber unit as a dry, creamy white, spongy mass, which has approxicontent. NO sodium bisulfite is added. Coagulation is mately one-third the apparent density of ordinary crepe effec1,edby adding about 0.5 per cent on the latex of acetic rubber. A current of heated air is forced in a t the top of acid as a 1 per cent solution. The coagulation is carried the tower and escapes a t the outer edge of the bottom of out in rectangular pans and the slabs of coagulum are passed the unit, carrying the moisture with it. The rubber globules a fern,timesthrough smooth rolls and Once through marking are dri'ed almost instantly and the rubber shows no harmful is continued until the sheets are dry, the effect whatever from heat required for evaporation. The rolls, smoking procc:ss usually requiring from 9 to 10 days a t a temperature heat is supplied from an oil-burning furnace and the temperature at the entrance and exit of the spray unit is easily of l l o Oto 120" F. Blanket rubber is made in the same general way 8s crepe, and carefully controlled. The capacity of the spray unit and 1s milled thick prior to drying or it may be plied up from is from 600 to 800 pounds of dry evaporated rubber per hour, varying with the total solid content of the latex used. This thin, dry crepe. process of making rubber is continuous since the bottom of TABLE I-ANALYTICALDATAOF CRUDES, PER CENT the unit consists of a movable platform by means of which Acetone Water the dry, spongy rubber is easily removed from the floor to the Ash Extract Extract Sugars Proteins Fine Para 0.50 3.30 0.50 0.81 3.62 baler. The rubber, which is creamy white when deposited, 0 . 3 0 2.94 0.40 Pale crepe 0.30 3.20 0.85 0.30 2.40 takes on the appearance of commercial light brown crepe 0.31 3.10 Smokedsheet 1.40 4.20 6.50 1.10 4.25 LS rubber when creped or compressed. Slab rubber is made by either natural or acetic acid coCHARACTERISTICS OF LS RUBBER agulation and is not strictly a market grade. The slabs of coagulum are simply pressed or sheeted from 0.5 to 1 inch in CHEMICAL-I~ general, L s rubber has higher ash, water thiclmess, and may be entirely dried or only surface-dried, extract, acetone extract, a n d nitrogen Content than either Much trouble results from sliming because of the putrefaction, Para O r Plantation nhbers. (Table 1) A Portion of the and an offensive odor is developed which may persist in cured water-solubk matter is also &ractable by acetone, and corrections must therefore be made in determining these 1 Presented before the Division of Rubber Chemistry a t the 65th Meeting of the American Chemical Society, New Haven, Conn., April 2 to 7, 1923. amounts. The water-soluble material Consists Of nitroge-
Vol. 15, No. 12
I N 0 USTRIAL A N D ENGINEERING CHEMISTRY
1268
nous matter, sugars, and inorganic salt,s. The nitrogenous matter is largely colloidal. The sugars, which are of the inositol series, make up about 1 per cent of the rubber. The sugars dried during evaporation in the spray process are responsible for giving LS rubber its characteristic sweet odor. The inorganic salts are chiefly potassium phosphate, with traces of magnesium and calcium salts. This waterextractive material is hygroscopic, and because of its presence LS rubber contains normally about 1 per cent of moisture.
3
4
B
-3
&
$
-k 2
t%
75 80
I20
I50
180
210.
240
Cure hmjnufes at40 pounds in skip mold
FIG.~ - A V E R A Q E TENSILE STRENGTH CURVSFOR TWENTY DIFFERENT LOTSOF EACH CRUDEIN 10 PERCENT SULRUR STOCK
PHYSICAL-A noticeable characteristic of LS rubber is its tough, nervy quality. This is attributed to two causesthe presence and nature of the nonrubber constituents, and the fact that the finished rubber is produced almost instantaneously from clean latex and therefore need not be subjected to washing, creping, smoking, drying, and various manipulations required for Para and plantation crudes. The color varies from light brown to dark brown, depending upon the amount of compression to which it is subjected in the baling process. The extra nervy quality of LS rubber is illustrated by comparing it with pale crepe on a standard mill under identical conditions, and the results of this test showed an increase for the LS rubber of approximately 15 per cent both in time required to break down and power consumption. The higher power consumption of LS rubber during milling was confirmed when. the factories carried out works tests with this rubber. It is of interest, however, to note that when the factories worked with quantities of LS rubber made from regular production runs on the plantation in Sumatra, the difference in power consumption between this rubber and the standard plantation rubbers was smaller than that obtained in earlier experiments from rubber made in the experimental unit, and the harder milling qualities of the rubber have ceased to be of concern with the factories using it. The ability of LS rubber to absorb mineral fillers has not been found to be appreciabIy different from that of standard plantation rubbers. However, the LS rubber takes up oil during mixing appreciably faster than do the other rubbers. I n cured stocks LS rubber gives a higher tensile strength than any other crude, a slightly lower stretch, and a higher breaking set. I n many cases a tensile strength in excess of 4000 pounds per square inch is obtained in the 10 per cent sulfur compound. Results obtained from such typical com-
pound of LX rubber, Para, pale crepe and smoked sheet are given in Table 11. In Fig. 1 is shown the average tensile strength curves of LS rubber, smoked sheet, pale crepe, and fine Para compiled from tests of twenty samples of each rubber. It may be noted that the LS rubber gives a tensile strength about 10 per cent greater than any of the others. The effectivecuring range of this rubber as a crude with 10 per cent sulfur covers a period of about 1 hour (good cures being obtained from 90 to 150 minutes at 40 pounds in a strip mold). In Figs. 2, 3, and 4, the curves of the tensile strength obtained for a number of tests made on LS rubber, smoked sheet, and pale crepe, respectively, are shown. Each curve for smoked sheet and pale crepe represents duplicate control tests on a regular shipment of thcse rubbers. Each LS rubber curve represents a test on a different lot of LS rubber made in the experimental unit. When it is considered that these lots of LS rubber were made from latex in which the percentage of preservative was not, constant, which was shipped in different kinds of containers and which had been handled in different ways, the uniformity of the tests is considered t o be remarkable. Moreover, from the fact that in no case have poor lots of LS rubber been obtained such as have been obtained in smoked sheet, pale crepe, and fine Para shipments, it is thought that it can safely be concluded that LS rubber is of more uniform quality than any other crude rubber which has been produced on a commercial scale. This is of great importance to the users of rubber, since it is a longsought quality in crude rubber which has not been previously obtained by the producers of plantation rubber. The abrasion qualities of stocks containing LS rubber are equal and in many cases superior to the same type of stocks made with standard rubbers. Inner tubes seem to have greater resistance to tearing if made with LS rubber. RATEOF CURE-bother remarkable quality is the rapidcuring characteristic of LS rubber. For example, the cure for a few of the standard crudes with 10 per cent sulfur in a strip mold at 40 pounds steam pressure is for fine Para '/2 t o 4 hours, for pale crepe 3 to 3ll2hours, smoked sheet 23/4 t o 31/2hours, and for LS rubber 1*/* to 2 hours. The accelerating quality of this rubber as a crude is maintained in compounded stocks-a fact which is not true for certain quick-curing plantation rubbers of the matured slab type. This accelerating property permits either an economy in compounding cost by virtue of a reduction in the amount of accelerator required, or a saving in curing cost if a shorter cure is desired.
Cure in minutes af4Opounds in strrip mold FIQ. %-TENSILE
STRENGTH C U R V E S CONTROL
TESTSO N
SPRAYED
RUBBER
INDUSTRIAL A N D ENGINEERING CHEMISTRY
December, 1923
The natural accelerator of vulcanization in LS rubber is present in the water-soluble nonrubber constituents. The , exact identity of the various ingredients making up the nonrubber constituents has not been determined, but it is believed that some of these ingredients accelerate the rate of cure, some are neutral, and some may possibly have a retarding action on vulcanization. 16/0 3560 14 OD
9300 Q
.$
3290 jtoo
4 30% 2 2330
*k.
2820
2790
2 ::
2530 s% p 23190 2 2200
.:
211J0
"?
2000
B I990 I800
60
FIG.3-TENSILE
90
180 210 Cure in minutes ot 4Opounds in strip mold 120
150
240
STRENGTH CURVES CONTROL TESTS O N SMOKED S H E E T
From extensive tests made on LS rubber from which the water-soluble constituents were carefully removed, it was determined that the qualities of such prepared rubbers were inferior to the regular unwashed rubber, and for most purposes it is not considered desirable to remove these nonrubber constituents from commercial LS rubbers. A tire-tread stock containing standard plantation crudes which cures in 105 minutes a t 40 pounds in a mold will cure in about 75 minutes if LS rubber is used. I n the case of an inner-tube compound the cure on the standard tube was reduced from 24 minutes to from 12 to 15 minutes by substituting LS rubber for the standard plantation crudes. Proportionate reductions in curing time have been noted in all cases where this rubber has been used. This quick-curing characteristic has not caused trouble with mill burning of stocks in the factories where a great many tons of LS rubber haw: been used in the production of standard products. TABLE II-TuPrcar, CURINGRANGE
Time in Minutes Tensile Stretch Set Timt in Minutes Tensile Stretch Set Tim? in Minutes Tensile Stretch Set
OF
EACHCRUDE 10 PER
COMPOUND (At 40 pounds pressure) Pale Cvepe 150 165 180 2580 2970 3420 10.4 10.1 10.4 0.16 0.16 0.13 Smoked Sheet 120 150 165 2680 3020 3365 10.5 10.3 10.2 0.14 0.17 0.17 LS Rubber 90 105 120 3740 3875 3355 10.3 10.2 10.0 0.22 0.20 0.21 Fin
'
CENT
SULFUR
195 3310 9.8 O,l5
210 2380 8.6 0.12
180 2890 9.8 0.16
210 1140 7.3 0.07
135 3570 9.8 0.22
150 3380 9.5 0.20
AGING&uALITIEs-Another valuable property of LS rubber is its good aging quality, it being very much superior to pale crepe and smoked sheet, and equal a t least, if not superior, to fine Para. This improved aging quality is clearly apparent when accelerated aging tests are carried out on the crude compound with 10 per cent sulfur, and Table I11 gives data for such a test in comparison with crepe, sheet, and para.
1269 111-AGING DATA
TABLE
---TENSILE--
--STRETCH-
----SET-
OptiOptiOptiUnder mum Over Under mum Over Under mum Over Pale crepe 2750 3430 3080 1 0 . 4 10.1 8 . 6 0.13 0.16 0.12 Smokedsheet 2755 3350 3005 9 . 9 9 . 9 9 . 3 0.14 0.20 0 . 1 3 LS rubber 3335 3570 3265 9 . 7 9 . 8 9 . 3 0 . 1 6 0.20 0 . 2 3 Fine Para 2730 3475 3475 9 . 9 9.8 9 . 8 0.16 0.17 0.22 1 Week's Heat ARinR at l5Oo F. Pale crepe 3260 3215 2510 9.2 '9.1 8.1 0.16 0.18 0.14 Smokedsheet 3080 3115 2605 8 . 8 8 . 6 8 . 2 0.14 0.18 0.15 LS rubber 3300 3145 3150 8 . 6 8 . 2 8 . 3 0.24 0.23 0.28 Fine Para 2765 2930 3215 1 0 . 1 9 . 7 9 . 2 0.15 0.15 0.18 2 Weeks' Heat A d n p ut 150' F . Pale crepe 2140 1835 215 S.-l -7.3 3 . 8 0 . 1 3 0 . 1 2 0.00 Smokedsheet 1725 1230 330 7 . 5 6 . 3 3.8 0 . 1 0 0.07 0.00 LS rubber 2660 2660 2495 8 . 1 7.6 7 . 3 0.16 0 . 2 2 0 . 2 1 Fine Para 2030 2685 2335 8 . 9 8 . 7 7 . 9 0 . 1 4 0.16 0 . 1 5 3 Weeks' Heat A p i n g at 150' F . Pale crepe 160 135 104 2 . j -2.1 2 . 0 0.00 0.00 0.00 Smokedsheet 135 230 135 2 3 2 . 1 2 . 1 0.00 0.00 0.00 LS rubber 1260 450 390 6 . 0 3 . 5 3 . 5 0 . 0 8 0.02 0 . 0 0 Fine Para 162 225 155 3 . 8 3 . 7 2 . 4 0 . 0 0 0.00 0.00 Each stock was given a range of three cures the first being under the second being the optimum and the third being o&r. The optimum tensile was taken as the optimum cure. The cures are as follows: TIMEIN MINUTES~ r l 4 0POUNDSPRESSURE Under Optimum Over Pale crepe 150 180 210 Smoked sheet 150 180 210 LS rubber 105 120 135 Fine Para 180 210 240 All rubbers were cured with 10 parts of sulfur on 100 parts of rubber.
Shelf aging tests covering a period of three years have checked the accelerated aging tests, and demonstrate beyond all doubt that this rubber has unusual aging qualities in cured goods. This quality may be due to both the protective action of the nonrubber constituents present and to the method of preparation. 3700 3600
3500 3400
< 3300
,$ 3200 3100 & 300G
& 2900 3 2800
2700 c, 2600
$ 2500 $2400 2300
3' 2700 $2/00
+ 2000
*$ *
",
BO0
/BOO /700 1600
/500 /40G
/300 /ZOO1
I
1
1
60
FIG 4-TENSILE
1
1
90
I
I
I
I
I
I
I
I
I
I
l
l
1
I
1
I
120 /so J80 210 Cure in minufes of 4Opounds in strip mold
I
I
I
I
240
STRENGTH CURVES CONTROL TESTSO N PALE CREPE
APPLICATION LS rubber has been used in tire treads, carcass compounds, inner tubes, soles, heels, belt covers, cements, and rubber goods generally, and because of its superior qualities of uniformity, aging, high strength, and quick cure, it has a wide and rapidly growing application. Annual inspection of t h e mineral wells and springs in Kentucky, Tennessee, Mississippi, Texas, Arkansas, and Missouri t h a t ship water into interstate commerce has been completed by t h e Bureau of Chemistry, over 100 commercial wells having been visited. Improvement in the sanitary conditions about t h e wells and springs and in methods of bottling the water was noted in various places.
