Evaluation of Blends
of Guavule and GR-S Rubbers J
FREDERICK E. CLARK1 AND WILFRED F. L. PL4CE U. S .
Guayule Rubber Extraction Research Unit,
Department of Agriculture, Salinas, CaZif.
A study has been made of the advantages derived from blending small quantities of guayule rubbers with the allpurpose synthetic rubber, GR-S. The lower resin guayule rubbers, used in amounts up to 20% of the total rubber,
produce more improvement in the physical properties of the blends than does the guayule of higher resin content. The latter, however, produces a greater improvement iu processing characteristics than does the former.
T
text, and the symbols are used in the charts and tables in preference to the longer abbreviated names.
HE study presented here was performed during the latter part of the war, when natural rubber continued t o be the
most critical of all strategic materials (13) and the question as to how our limited supply of natural rubber could be used to best advantage was of great importance. With the end of the war the importance of this particular question greatly diminished, but the mixing of natural and synthetic rubbers to obtain superior blends is still of present and future interest. The future situation in regard to rubber cannot be accurately predicted, but both synthetic and natural rubbers will be used. Judging from developments in this country, Mexico, Russia, and Argentina, it is likely that guayule will remain in production for its special properties of plasticity and tack. GR-S is particularly inferior to Hevea in processing characteristics, tack, tear resistance, cut-growth resistance, tensile strength at high temperatures, and heat build-up. One method for alleviating this inferiority is to blend small quantities of natural rubber with GR-S t o improve the processing of the GR-S and improve some of the physical properties of its vulcanizates. This RTas first realized by the Bureau of Ships of the Navy Department; early in 1942 this bureau urged full cooperation between rubber manufacturers and interested government agencies to provide information on the compatibility of various types of guayule rubbers with Hevea rubber, Kith various types of reclaimed rubber, and with all of the available synthetic rubbers. When Mexican or domestic resinous and deresinated guayule rubbers n-ere substituted for GR-S to the extent of 20%, a striking improvement in processability, tack, tear resistance, and cut growth was observed by Morris and eo-workers (10, 11,12). Carlton and Reinbold ( 7 ) showed that a blend of 25y0 Hevea and 75% GR-S had considerably better building tack, tensile strength, elongation, and tear resistance, and less heat build-up, than GR-S alone. Cohan and Myerson (9) found that as small an amount as 5% of domestic resinous guayule rubber gave improved processability, tack, tensile strength, and resistance to flex cracking. The rrork reported here was initiated to investigate further the bubject of GR-S-guayule blends in a typical tread stock, and to study some types of guayule rubber which had not been used for this purpose before. All tests xere performed on a comparative basis; the standards for comparison were similar stocks compounded from GR-S blended with a good grade of Hevea smoked sheet.
TABLEI. STASDSRD FOR CONPARISOS A X D TYPE?OF I I r i i n E k L-S E D
Description Hevea smoked sheet from Honduras Latex guayule from 3-year-old cultivated shrub Alkali-alcohol treated guayule from mature Mexican wild shrub Guayule from retted 3-year-old cultivated shrub Resinous guayule from 3-year-old cultivated shrub Resinous guayule from mature Mexican wild shrub
LG
Abbreviated Tame? Smoked sheet Latex guayule
DG
Deresinated guayu!?
RG
Retted guayule
YG
Young guayule
JIG
IIexican guayule
The analyses of the various guayule rubbers and Hevea control are shon-n in Table 11. TABLE11. ASALTSE': OF HETEAASD GUAYULERCBBERC (MOISTURE-FREE BASIS) YG JIG DG RG SS LG Rubber hydrocarbon, % Acetone solubles, % Acetone-benzene insolubles or protein, % Ash, % Copper, p.p.m. Manganese, p.p.m. Iron, p.p.m. Acid number Moisture, %
82.8 15.3
93.0
4.0
89.8 6.1
77.7 11.9
70.5 23.0
66.8 22.1
:.8 2.2 8.8 7.7 10.9 0.76 0.80 0.97 0.93 1.07 3 3 7 6 2 4 1 3 4 9 9 4 35 120 225 110 170 115 220 120 140 200 160 180 0.09 0.26 0.28 0.80 0.39 1.20
3.0 0.13
The propertiee of GR-S in coniparison with the Rubber Reserve Company specifications (14) are given in Table 111.
OF GIt-SG TABLE!111. PROPERTIES METHOD.>
The types of guayule rubber studied and the kind of rubber used as a standard are shown in Table I. The samples are similar to but not identical with those used in a previous compounding study (8). For convenience a symbol and an abbreviated name are assigned to each rubber. The abbreviations are used in the Present address, Battelle Memorial Inatituse, Columbus, Ohio.
1026
DT
STASDARD TEST
Values
Chemical composition .4cetone extract, Yo Heat loss % F a t t y acih (as stearic acid), '% Soap (as sodium stearate), Yo Ash, %
? !:&$ '
Phvsical properties in etandard compd. Tensile strength, cured 60 min. a t 292O F., lb./sq. in. Ultimate elongation, cured 50 min. a t 292' F. % elongation, lb./sq. in. stress a t &IO? Cured 25 min. a t 292O F. Cured 50 min. a t 292' F. Cured 90 min. a t 292' F. a T h e GR-S used in this investigation serve Company through the courteay of oratory, Mare Island, Calif.
TYPES OF RUBBER
I
Symbol SS
Used 6.11 '.81
3.49
R.R.C. Specifications Minimum LIaximuru
..
3 :do
10.00 0.75 6.00 0.75
1.44
.. ..
?goo
2500
...
550
500
...
620 1050 1500
1000
0.72
400
800
1.50
900 1300
1600
was obtained f r o m the Ruhber Re. the U. S. Kavy l a r d Rubber Lab.
October, 1946
1027
INDUSTRIAL AND ENGINEERING CHEMISTRY
FORMULIS
I
The formula used for the 1007, GR-S stock is the standard formula specified by the Rubber Reserve Company (14) for evaluating GR-S samples. The formula selected for the 1007, natural W K rubber stocks was the modim fied A.C.S. formula developed for rubbers 10% in fat'ty acid W VI content ( I ; ) , except that one instead of one-half part of A BREAKDOWN T I M E B OPTIMUM CURES mercaptobenzothiazole (CapI 2 tax) was used in order t o increase the curing rate, and 50 part,s of easy-processing chanI I I I I I I ok ; 1 do do a0 5 b 100 0 10 20 30 40 50 riel black were added to put Yo N A T U R A L RUBBER % NATURAL RUBBER the formula on a comparable hasis n-ith the GR-S formula. Figure 1. Breakdown Time and Optimum Curing Time Medium-processing channel black is commonly used in natural rubber tread stocke, but to have as few variables 4s possible, easy-processing channel black was used for all atocks. The two formulas were combined in the correct proportions to produce the 3500L / blends indicated in Table IT. This method of formulation x a s considered highly superior t o substituting a given amount of GR-S by the same amount of natural rubber and using the GR-S 2500 formula throughout. In one trial 1 0 0 ~ oyoung guayule MG was compounded on the GR-S \ I I I I I I 1 I formula and gave an opti2000 0 10 20 30 40 50 100 0 10 20 30 40 50 100 Mum tensile strength of 1310 % NATURAL RUBBER % ' NATURAL RUBBER pounds per square inch as Figure 2. Unaged Tensile Strength compared to 2210 pounds obtained with the formula used In this study. All formulas are on the 0 t basis of 100 p u t s of crude z rubber and not rubber hydrou2woLL carbon. For example, a 60% 4 A OPTIMUM CURES 8 OVER CURES GR-S40% Mexican guayule etock contains less actual rubber hydrocarbon than a 60% G R - S 4 0 5 smoked sheet stock. A plasticizer is ilsually included in smoked sheet tread stocks, but was left out in this study so that all of the natura< rubbers I I I I I I I rrould be on the same formula 5000 I 10 20 30 40 50 100 0 10 20 30 40 50 I00 and the results more com% NATURAL RUBBER 96 NATURAL RUBBER parable. Deresinated guayule was Figure 3. Hot Tensile Strength 2rotected before drying with '/a7,Agerite Hipar (aniixture .omposed of 50% phenyl-p-naphthylamine, 25Y0X, S'-diphenylmake the smoked sheet, latex guayule, and Mexican guayule p-phenylenediamine, and 2553 isopropoxydiphenylamine). 100% natural rubber stocks and blends comparable to the other Retted guayule and young guayule had 0.25% JZF (L\r,Ar'rubbers, 0.25% JZF was added to each on the basis of 100 parts diphenyl-p-phenylenediamine)added to each before drying. To of natural rubber during the compounding. The GR-8 mas nde-
I =
:I
\ \
' ' '
'
'
1028
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
2500
Vol. 38, No. 10
cordance with the specifications of the Rubber Reserve 0 0 Company (14) for GR-S. The accepted breakdon-n time for 1007, GR-S is 12 minutes. K W The breakdown times for the 4 1007, natural rubber stocks a 0 two were taken 3s the minutes required to form a smooth, glossy sheet on the mill. S o attempt was made to measure the breakdown time of OPTIMUM CURES the blends because the high percentageof GR-S prevented 1 ' 1 the formation of a gloss>. I I l l 500 " I I 1 0 10 20 30 4 0 50 100 0 10 20 30 40 50 sheet. Therefore, the break% NATURAL RUBBER % NATURAL RUBBER down times for the blends Figure 4. Aged Tensile Strength irere obtained from Figure 1.1. All of t,he stocks compounded satisfactorily except 1 0 0 ~ oyoung guayule and A. O P T I M U M CURES B OVER CURES 100% Mexican guayule, which were sticky and dii2630 ficult to sheet. All of the natural rubbers I u aided the GR-S to wet and E disperse the carbon black. I SO0 W After the sheeted stocks n-ere K 4 cooled, the blends and 100$ 2 v) 0 natural rubber compounds 1000 were smoother in appearance K W and showed less retractioii i n P length than did the 100% a v) GR-S stock. The improvez JOo ment in appearance and in rea 0 P % NATURAL RUBBER Y. NATURAL RUBBER 0 tention of dimensions vas P proportional t o the amount Figure 3. RIodulus at 300yc Elongation of natural rubber in the blend and, furthermore, the high resin guayule rubbers were more effective in this respect than were the low resin gua600 yule rubbers and smoked z sheet,. 0 To determine the effect of c 4 natural rubber on the power g 500 requirements for compound0 -1 ing GR-S, power consumpW A. O P T I M U M C U R E S 8 OVER CURES tion was measured for five z 400 typical 300-gram batches 0 IO 20 30 40 5 0 100 0 IO 23 30 40 50 100 (Table V). The 100% GR-S % NATURAL RUBBER &' NATURAL RUBBER requires considerably more Figure 6. Unaged Ultimate Elongation power to compound than does 100% smolied sheet which, in turn, takes more power than quately protected by the manufacturer Kith 1.3;; PI3S-X does 100% retted guayule. !&sing 20y0smoked sheet with GR-S (phenyl-,%naphthylamine) or l.5yo BLE (acetone-diphenylreduces the power requirements of GR-S appreciably, whereas mising 20% of retted guayule w i t h GR-5 results in a further saving c J f amine reaction product). These antioxidants assist the aging of the vulcanizates but do not have any appreciable accelerating effects on the rate of cure.
I
T . ~ B LIt-. E FORWLAS
COMPOUNDING
All stocks were mixed on a 6 x 12 inch laboratory mill having a ratio of speeds of backroll t o frontroll of 1.4 to 1. The nii11ing temperature for all stocks was kept at 50" C. For the blend:, the natural rubber and GR-S were placed on the mill together a t
the start of the mixing cycle. With the exception of breakdown time the mixing operations for all stocks Fere performed in ac-
GR-S Satural rubber ~Iercaptobenzothiazole
gfF:xg:d SulfurNo. 7 BRT
Easy-processing channel black
100.0 , 1, . 5
90.0 110..405
80 0 2 10 . 04
60.0 4 10 . 03
,..
0.4 5.1 4.5
0.8 5 2 4 0 2.3 50.0
1.6
5.0 5.0
2.0
2.15
50.0 50.0
5.4 3.0 2.6 50.0
160'0 1.0 4 0
6.0
. .
3.5 50 0
October, 1946 powcr. The other guayule rubbers, when blended n-ith GR-S, TTould undoubtedly show a reduction in power rcquirements of the GR-S to a greater or lesser degree depending upon the type of guayule rubber and the aniouiit used in the blend.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1029
z
0 + 2000
z
s
A.
OPTIMUM
CURES
NATURAL
RUBBER
OVER
CURES
w
ae
SURFACE TACK %
Although tack is an important property, there arc no satisfactory quantitative methods for measuring this property. The tack of the stocks in this study were determined qualitatively by the hand test. The stocks rvere sheeted to approximately '/$-inch thickness, laid flat on glassine paper, and allowed to stand 24 hours at room temperature. Strips about 1-inch wide were cut from the sheets, the glassine paper was peeled off to expose fresh surfacc, and the tack was determined by bending the rubber on itself and pulling the adhering surfaces apart again. Separate tests were made by two individuals, and a tackiness rating was established for each stock by consensus. The terms used to describe the tackiness are in ascending quality as follows: none, poor, fair, good, excellent. Plus signs are also used in an effort to make closer distinctions. For example, fair indicates that the tack as somewhere bek e e n fair and good. The results are given in Table VI, The addition of as small an amount as 10% natural rubber to the GR-S produced a noticeable improvement in tack. The high-resin guayule rubbers were more effective in this respect than the low-resin guayules which, in turn, were more effective than smoked sheet.
%
Figure 7.
NATURAL
RUBBER
Aged Ultimate Elongation
MG/ 40-
A
OPTIMUM
CURES
30-
T
' 0
10
20
7 .
30
40
50
NATURAL
100
0
10
Figure 8.
20
%
RUBBER
30
40
NATURAL
50
IO00
RUBBER
Permanent Set at 75970 Ultimate Elongation
+
I 0
z
P W
a
ul
a
z
a :
I
I
I
I
I
I
I
VU LCAIYI ZING
All samples xere vulcanized at 287" F. in a hydraulic press. The tensile and tear test specimens were prepared from A.S.T.M. slabs of 6 X 6 x 0.076 inch. The l/,-inch-thick abrasion specimens, the '/2-inch-thick compression set specimens, and the l-inch-thick hardness, rebound, and flexometer specimens were vulcanized 3, 5, and 15 minutes longer, respectively, than the corresponding tensile slabs, to allow for the increased thickness of these specimens.
All tests were conducted on the optimum cure and the 50% overcure of each stock. For this study optimum cure was defined as the time to reach maximum tensile product. The optimum cure for each stock was selected from data on range of cure with a 5-minute interval between cures in the region of optimum cure. The time to reach optimum cure at 287" F. is plotted
1030
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLEV.
POTERREQUIREUESTS FOR COJIPOUSVTSG 3OO-GRAX BATCHES O F RUBBER Power Consumption,
Kilowatt-Hour
Stock 100% GR-S
0.880 0.612 0.392 0.687 0.578
ss
100% 100% R G
8 0 7 GR-S 20% 9s GR-9: 20% R G
SO^
Egz:
in Blend
ss
LG DG RG YG hlG
Tack w t h Following 0% 10% Poor Fair Poor Good Poor Good Poor Good Poor Good Poor Good
+
Amount of Katural Rubber in Blend: 20%
Good Good Good Good Good Good
++ +
40%
Good Excellent Good Excellent Excellent Excellent
+
100%
Good Excellent Excellent Excellent Excellent Excellent
+ ++
against percentage of natural rubber in tlie blend in Figure 1B. I n every case the natural rubbers had a beneficial effect on the curing rate of GR-S. Stocks containing latex guayule, retted guayule, and young guayule reached the optimum cure about 5 minutes before the stocks containing smoked sheet, deresinated guayule, and Mexican guayule. RESULTS O F TESTS
TENSILE STREXGTH. Unaged tensile strengths .are prescrit et1 in Figure 2. T o avoid confusion in interpretation, only tlie points used in plotting the smoked sheet-GR-S blends are show1 in these and all other graphs. However, equally good precision was obtained for the other blends. The tensile strength of thc smoked sheet blends increased rapidly x i t h increasing smoked sheet content. The latex guayule and deresinated guayule blends showed the same trend as did smoked sheet, but to a lesser degree. The retted guayule blends increased in tensile strength up to 40% retted guayule, then decreased slightly. Tlie young guayule and Nexican guayule blends increased in tensile strength up to 20% natural rubber and then decreased. Tlle slight rise before the drop in these last two curves n-as probably due to better carbon black dispersion over the 1007, GR-S stock. The curves fell in the same order for the overcures as for the optimum cures but were compressed somewhat. The hot tensile strengths are shown in Figure 3. The test was performed 13-ith a hot iron similar t o that used by Braendlc and co-workers ( 6 ) . The temperature of the iron mas kept a t 230 O F. and corresponded roughly to a high temperature Scott tester maintained a t 100" C. Loss of strength at high teniperatures is a serious drawback of GR-S; this is emphasized by the fact that GR-S lost 70% of its original tensile strength i n this test. All of the natural rubber blends are better than the 100% GR-S in hot tensile strength, although the 1007, RIexicsn guayute is about the same as the 10070GR-S. The improvement produced by mixing smoked sheet, deresinated guayule, and latex guayule with GR-S is quite striking. The overcures were aboiit, the same as the optimuni cures. Tensile strengths after 4-day aging a t 100" C. in a forced air circulating oven are charted in Figure 4. The 100% GR-S stock and all of the blends had tensile strengths within experimental error of each other and, therefore, were plotted a h a single line a t 2300 pounds per square inch. The Katural Rubber 10070 natural rubber stocks did not age as well as in Blend the blends and 100% GR-S stock. This is due to SS LG the fact that the natural rubbers did not contain DG a8 much antioxidant as the blends and 100% RG YG GR-S, and also that GR-S possesses good resist31G ance to heat aging. This can be observed
Vol. 38, No. 10
by the relatively low drop in tensile strength for GR-S o r blends rich in GR-S, compared to the larger drop for natural rubber compounds. However, the significant fact is that, under the conditions of this experiment, up to 40% natural rubber in the blend did not lower the tensile strength of GR-S after aging. The tensile strengths of the 100% natural rubber overcures were lower than the optimum cures, and the order was changed somewhat; but the tensile strengths of the blends and 100% GR-S overcures were within experimental error of the optimum cures. MODULCS AT 300% ELONGATION. The moduli at 3007, elongation are given in Figure .i. At optimum cure tlie moduli of the smoked sheet blends increased with increasing smoked shect content,; the reverse was true for the young guayule blends The moduli of the other rubber blends first increased and then (lecreased xyitli increasing natural rubber content. At 50Y0 ovcrcure tlie nioduli of the smoked sheet blends increased, and tliv moduli of tlic other I'lItJber blends decreased, with increasing natural rubber content. The overeure as reported for smoked sheet did not progress to the point where reversion set in. Thc nioduli for alt stocks werc greater for the overcures than for the optimum cures, but the increase was proportionally larger for 1 1 1 ~ .I O O ~ , GR-8 stock tlian for any of the other stocks. I'LTIMATX l3r,oxnar10~. The unaged ultimate elongations are cliartctl i n Figure 6 . At optimum cure the ultimate elongation8 tc.nd to iucreasc with increasing natural rubber content for all I )li:nds except smoked sheet and Mexican guayule blends, which tcad t o decrease. At 507, ovcrcure, the ultimate elongations inrrease with increasing natural rubber content for all of the gua).rile stocks, and tlie smoked sheet blends tend to remain the same a , ~tlie 100% GR-S stock. The 100% GR-S stock did not retain its elongation in overcuring as well as did the blends a n t l 100% natural rubber stocks. It is interesting that the ultimate elongation for every 40% natural rubber blend except latex gnayule blend was higher than that of the corresponding 1007, rintnral rubber blend. Figure 7 shows tile ultimate elongations after 4-day aging at 100" C. The elongations of the blends and 100% GR-S stock were within experiniental error of each other and were plotted as a single line a t 2507,. The superior retention of elongation after heat aging fur GR-S or blends rich in GR-S is noteworthy. Latex guayule and deresinated guayule had the highest elongations of the loo$, natural rubber stocks, and smoked sheet and Mexican guayule had the lowest elongations. All, however, were lower than the blends. The elongations of the overcures were about the same as those of the optimum cures after aging. SHOREHARDNESS. These values were determined on l-inchthick blocks 30 seconds after application of the durometer. The results are not shown. The hardness values for the unaged stocks varied from 60 to 65 and, after the stocks had aged 4 days a t 100' C., the values were between 70 and 76. KO consistent differences could be detected between different rubber blends. The overcures were one or two points higher than the optimum cures, but these differences disappeared after aging. PERMANEXT SET. The permanent set measurements were made at, 75% of the nltimate elongation in accordance with the standard method of A.S.T.M. (1). The results are given in
Minutes t o Iilowout with Following Amount 0% 10% 20% Opt. Over- Opt. O ~ e r - Opt. Overcure cure cure cure cure cure 7.9 7.8 3.3 4G 7.2 2.6 2.6 2.6 2.6 2.6 2.6
3 . :i 3,3 3.3 3.3 3.3
I, 2
7.4 8.0 6.G ti.4
7.5 6.5 5.5 7.2 7.9
8 4 9 2 10.0 8.4 5.7
6.0 7.2 7.0
5.7 5.9
of Natural Rubber in Blend: 40% l O O F Opt. OverOpt. Overcure cure cure cure 15.9 9.0 14.1 8.2 9.G 7.9 8.0 7.1 11.4 8.7 9.G 8.0 28.8 21.2 9.2 6.3 8.fi 10.0 5,4 5.9 5.7 6.6 8.5 8.5
INDUSTRIAL AND ENGINEERING CHEMISTRY
October, 1946
1031
Figure 5. When mixed with I GR-S, all of the natural rubbers OPTIMUM
A
CURES
B
OVER
I
CURES
-i60
c-
ui 5 0 -
8.
OVER
CURES YG.MG
)s
30
-
200
A.
Ib
20
OPTIMUM
io
410
% NATURAL
CURES
,b
I
l
l
l
l
l
l
100
0
10
20
30
40
50
%
RUBBER
NATURAL
20 100
RUBBER
Figure 11. Compression Set at Constant Deflection
h
I
A
OPTlMUM
CURES
YO
had an adverse effect on the permanent set of GR-S. Mexican guayule blends had the highest permanent set. Smoked sheet, latex guayule, and deresinated guayule blends had the lowest permanent sets and wer0 within experimental error of each other. The permanent sets of the overcurves were somewhat lox-er than the permanent sets of the optimum cures. T E A RR E S I S T A X C ET. h i s property was determined by the recommended method of the A.S.T.M. (6). The unaged tear resistances are plotted in Figure 9. All of the natural rubbers, when blended with GR-S, markedly improved tear resistance of the latter, particularly a t the lower percentages of natural rubber. The smoked sheet blends had the highest tear resistance, followed by latex guayule, deresinated guayule, retted guayule, young guayule, and Mexican guayule, respectively. The tear resistance values of the overcures mere lower than the optimum cure values. Tear resistance values after 4day aging a t 100" C. are shown in Figure 10. The natural rubber blends had better tear resistance than the 100% GR-S stock after aging. However, the improvement was not so great as with the unaged stocks. The optimum cure 100% Mexican guayule and the overcure 100% Mexican and 100% young guayules were definitely inferior to 100% GR-S as a result of oven aging. CowmssIos SET. The results of compression set tests at constant deflection are given in Figure 11. A.S.T.M. method B was used (4). An 'increase in compression set with increasing natural rubber content was noted in the case of each natural rubber. Smoked sheet had the least effect, whereas young guayule and Mexican guayule blcnds showed the greatest increaPes i n compression wt. The other three guayule rubber blends were intermediate. The overcure compression sets were considerably lower thau the optimum cure compression sets. ABRASIONRESISTANCE. The resistance to abrasion was determined on the Kational Bureau of Standards abrader in ac-
50
Vol. 38, No. 10
INDUSTRIAL AND ENGINEERING CHEMISTRY
1032
0
z
OG
OG
3
RG
E
I 100
@& N A T U R A L
I
I
0
10
RUBBER
Figure 13.
OVER
CURES
I 20
I
I
I
30
40
50
Y.
NATURAL
RUBBER
Rebound
LG,OG, R G
W
m
W W K
A
750
!o
8.
OPTIMUM CURES
20
20
,b
I
20
% NATURAL RUBBER
100
I 0
I 10
OVER
I
CURES
I
PO
30
X
I 40
I 50
NATURAL
RUBBER
Figure 14. Temperature Rise in Goodrich Flexometer
cordance with h.S.T.31. method €3 (2' The reference standard compound was obtained from the Rubber Manufacturers Association, Inc. All results are expressed as percentages of the standard compound. The results are plotted in Figure 12. All of the natural rubber blends and 1007, natural rubber stocks had lower abrasion resistances than the 100% GR-S. Mexican guayule blends were the poorest, followed by young guayule, retted guayule, latex guayule, deresinated guayule, and smoked sheet blends, respectively. All of the overcure abrasion resistances were higher than the optimum-cure abrasion resistances, except those of the 100% Mexican guayule, 100% young guayule, and 100% retted guayule stocks. REBOUND. This property was measured with the GoodyearHealy rebound pendulum at 75' F. and an angle of drop of 15 (Figure 13). The rebounds of all the 10% natural rubber blends tended t o be the same or slightly higher than the 100% GR-S. I n the blends of more than 10% natural rubber, the rebounds of Mexican guayule, young guayule, and retted guayule blends decreased, the rebounds of latex guayule and deresinated guayule blends remained nearly constant, and the rebound of the smoked sheet blends increased. The rebounds of the overcures were practically the same as the rebounds of the optimum cures. GOODRICHFLEXOMETER. The temperature rise over. room temperature after 25 minutes was obtained in a Goodrich flexometer (S), using 0.175-inch stroke under a load of 143 pounds per square inch a t a frequency of 1800 cycles per minute (Figure 14). The mixing of smoked sheet with GR-S effectively reduced the temperature rise. Latex guayule, deresinated guayule, and retted guayule blends remained about the same in temperature rise
as the 100% GR-S; the young guayule and Mexican guayule blends were inferior to the GR-S, the Mexican blende being poorer than the young guayule blends. The temperature rise values of the opercures were slightly lower than those of the optimum cures. The minutes to blowout were determined in the flexometer, using a 0.25-inch stroke under a load of 285 pounds per square inch a t a frequency of 1800 cycles per minute (Table VII). The precision of this tmt is not very good; therefore the data were not plotted. However, all of the blends and 1 0 0 9 natural rubber stocks were markedly superior to the 100% GR-S in this property. Thir was true for the overcures a5 well as for the optimum cures. The 100% retted guayule and 100% young guayule sample. flattened out after about 3 minutes in the flexometer and, consequently, did not receive as severe flexing as did the other samples. This fact probably accounts for the comparatively high blowout resistance of these samples. The 100% Mexican guayule also flattened out in the flexometer, but in this case the blowout resistance was not abnormally increased.
CONCLUSIONS
.\lthough some properties, such as abrasion resistance, suffer when guayule rubber is blended with GR-S, the authors conclude that, from an over-all evaluation of properties, up t o 20% of any of the guayule rubbers studied produces a considerable improvement in GR-S tread stock. The lower-resin guayule rubbers and Hevea smoked sheet, in amounts up to 20% of the total rubber, produce more improvement in the physical properties of GR-S vuleanizates than do the higher-resin guayule rubbers, whereas the higher-resin guayule rubbers produce a greater improvement in processing characteristics and tack of the GR-S unvulcanized stocks. When even a small amount of natural rubber is blended with GR-S, the compounding formulas should be adjusted to meet the compounding requirements of each rubber in the blend; it is not accurate simply to substitute a given amount of natural rubber for the same amount of GR-S in a GR-S formula. It would be expected that a GR-S-natural rubber blend would have properties somewhat intermediate between the properties of the GR-S and the natural rubber under consideration. This has been borne out in the present study, although most,of the properties are not directly proportional to the amount of natural rubber in the blend. ACKNOWLEDGMELYT
The authors wish t o express their appreciation for assistance given in this work by other members of the Guayule Emergency Rubber Project.
October, 1946
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 LITERATURE CITED
Am. SOC.Testing Materials, Standard Methods of Tension Testing of T-ulcaniaed Rubber, Designation D412-41. I b i d . , Standard Methods of Test for Abrasion Resistance of Rubber Compounds, Designation D304-40. Ibid., Tentative Methods of Test for Comparison Fatigue of T’ulcanized Rubber, Designation D623-41T. Ibid , Tentative Methods of Test for Compression Set of Vulcanized Rubber, Designation D395-40T. Ibid., Tentative SIethods of Test for Tear Resistance of Vulcanized Rubber, Designation D624-41T. Braendle, Valden, and Wiegand, I n d i a Rubber World, 110, 645 (1944).
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(7) Carlton and Reinbold, R d h r A g e (N. T ) , 52, 29 (1942). (8) Clark and Place, I n d z a Rubber W o r l d , 112, 67 (1945). (9) Cohan and Myerson, Rubber Age (N. Y.), 54, 235 (1943). (10) Morris, Barrett, Lew, and Werkenthin, I n d i a Rzchber World, 109, 150 (1943). (11) Ibed., 109, 282 (1943 (12) Morris, Barrett, V a r m o n aiid Werkenthin IXD.E i c CHEM., 36, 60 (1944). (13) President’s Rubber Survey Committee (B. AI. Baruch, K. T. Compton, J. B. Conant) and 1944 Year End ReDorts. War Production Board. Rubber Bureau. (14) Rubber Reserve Co., Specification for OR-S.] , ! d i u RziBher E’orld, 109, 375 (1944). (15) Sackett, I b i d . , 110, 295 (1944).
Catalytic Cracking of Pure Hydrocarbons J
Secondary Reactions of Olefins H.H.VOGE, G.M.GOOD,.iXD B. S. GREENSFELDER Shell Development Company, Emeryville, Calif. T h e secondary reactions of olefins-isomerization, hydrogen transfer, polymerization, arid aromatization-were found to be extensive and to influence greatly the nature of the products from catalytic cracking. Isomerization, both double-bond shift and chain-branching, occurred rapidly under conditions of catalytic cracking. Saturation by hl-drogen transfer was shown to be faster for tertiary than for secondary olefins. This, together with the rapid chain-branching isomerization of olefins, affords an ex-
planation of the high ratios of iso- to normal paraffins obserbed in catalytic cracking. Molecular hydrogen had no more influence than nitrogen on the saturation of olefins; both acted as inert diluents. Polymers were readily formed from lower olefins, but were rapidly converted to other products at the usual conditions for catalytic cracking. Such products from the treatment of normal butenes were found to include benzene, toluene, xylenes, and higher hoiling aromatics in important quantities.
T
I n addition t o the terminology previously applied, the following definitions are required: “Hydrogen transfer” is the saturation of an ethylenic double bond by direct catalytic transfer of hydrogcn to the bond from another hydrocarbon, with no essential participation of free molecular hydrogen; “self-saturation” or “autosaturation” is a special case of hydrogen transfer in which a single hydrocarbon provides both acceptor and donor molecules; “hydrogen donor” is any hydrocarbon which provides hydrogcn for the general case of hydrogen transfer; and “hydrogen acccy tor” is any compound containing an ethylenic double bond which becomes saturated via hydrogen transfer. “Coke” is the carbonaceous deposit remaining on the catalyst after the reacbor is purged with nitrogen; it is measured by the carbon dioxide and water in the regeneration gases after conversion of carbon monoride to the dioxide. “Reactor purge” is material collected by colidensation and by combustion of noncondensables over coppcr oxide during a 20-minute purge of the catalyst with nitrogen. Differing slightly in temperature limitations from previous definitions, “gas” is all material boiling below 25“ C., and “liquid product” is any material boiling above 25” C. “LHSY’ (liquid hourly space velocity) is the liquid input per volume of catalyst .space per hour a t room temperature, usually about 25’ C. “Amount cracked” includes gas, liquid boiling below the originsl, and coke, summed on a no-loss basis.
HE four previous papers (6, 7 ) in this series described individually and compared as classes the catalytic cracking of many paraffin, olefin, naphthene, and aromatic hydrocarbons of a wide variety of types and molecular weights. A total of fifty-
six hydrocarbons was tested with a silica-alumina-zirconia catalyst under conditions similar to those used in the commercial cracking of petroleum fractions. In addition to the primary cracking reactions involving the severance of carbon-to-carbon bonds, a number of secondary reactions were shown to participate t o an important extent in the hydrocarbon transformations observed in catalytic cracking systems; they consequently exerted a substantial influence upon the nature of the products obtained. The majority and the most important of these secondary reactions are those characteristic of the ethylenic double bond, either in the original hydrocarbon or in the products resulting from cracking. This consideration led to a series of experiments on the secondary reactions of olefins in the catalytic cracking system, in the following respects: the nature and extent of olefin isomerization; olefin saturation via hydrogen transfer, and the effect on saturation of the variables of temperature; flow rate, and diluents; the effect of olefin structure upon hydrogen transfer reactions; Decalin and Tetralin as hydrogen donors; a comparison of hydrogen transfer and catalytic hydrogenation; the production of polymers and aromatic hydrocarbons from olefins. I n addition to the silica-alumina-zirconia cracking catalyst (U.O.P. type B) used in most of the experiments, commercial synthetic silica-alumina, a silica-alumina-magnesia, and activated natural clay cracking catalysts were used in some cases and gave similar results.
EXPERIlIENTAL PROCEDURE
-1vertical, fixed-bed reactor system was used for the experiments. The hydrocarbon feeds were metered, pumped into the top of the reactor, and passed over the catalyst. Products left the reactor through a condenser and were collected in a still kettle
