Vulcanization of D. B. FORMAS, R. R. RADCLlFF, AND L. R . 1IAYO Rubber Laboratory, E . T. d u Pont de Nemours & Compnny, Znc., Wilmington, Del.
u i decomposing to yield either free sulfur or a vulcanization,
T h e vulcanization of a new chloroprenc polymer (Neoprene W) having improved stability, processing characteristics, and vulcanizate properties, particularly low compression set, is described. In addition to metallic oxides, such as those of zinc and magnesium, acceleration is required for the maximum development of properties in this polymer. Combinations of sulfur and rubber-type accelerators, as well as the conventional neoprene-type accelerators, may be used to permit a high degree of flexibilitj in compounding. The effects of various accelerator combinations are discussed.
accelerat'or. In addition to possessing improved stability, this neoprene hm been observed to differ markedly from previous chloroprene polymers in many important respects. Perhaps the most striking difference is its unique response to various vulcanization techniques. This difference is believed t'o be due at least in part to w fundameiitai difference in polymer structure as well as the complete absence of thiuram disulfide and sulfur from the polymer. The elastomer compounds discussed herein were prepared and tested according to procedures established by the Americm Societ,y for Testing Materials. The following designations are applicable to the various tests.
Type
HE first general purpose type of chioroprene (2-chloro-1,3butadiene) polymer (Duprene D) was manufactuied in 1932 by means of a mass polymerization method (1, 8). The development of emulsion polymeiization techniques made possible the introduction In 1935 of a second type of chloroprene polymer (Xeoprene Type E), in 1938 a third fundamental change in polymerization methods resulted in a faster curing elastomer, Neoprene Type G . Since that time, several modifications of this latter class of polymer have been produced commercially, notably Neoprene Types GN, CK-A, and RT- -none of which represent a fundamental departure from Type G. More recently, considerable research effort on chloroprene polymerization has been directed toward the development of new polymers showing EL minimum of change in plasticity, processability, or rate of vulcanization during long periods of storage. A consequence of this research has been a fourth basic class of chloroprene polymer represent& by Ifeoprene Type W. This improved polymer has a much more uniform molecular weight distribution than any of the other neoprenes, and in addition contains no sulfur, thiuram disulfide, or other compound capable
Tests Modulus. tensile streneth. and strewstrain Compression set Hardness Heat build-up
.
NeoDrene TvDe W Xeoprene Type G N Phenyl-a-naphthylamine E x t r a light calcined magnesium oxids SRF carbon black Stearic acid Zinc oxide
?din. Cure a t 153' C.
10 20 40
Renilience
~..
100
.,.
i 00
2 4 29
\There indicated, the tendency to scorch of the compounds was measured by means of the Mooney shearing disk viscometea operating a t a temperature of 121.1 C. (250" F.) using the small rotor. The time to scorch was selected as the time for the hlooney viscosity to increase 20 units from the minimum value. When the viscosity had not increased by 20 units within a 45minut,e period, the test was terminat,ed. The chloroprene polymers such as Xeoprene Types E, GS,CG, and RT will vulcanize satisfactorily with suit,nble combinations of metailic oxides (3-6). The curing syst'em most widely employed consists of a mixture of 5 parts of zinc oxide and 4 parts of extra light, calcined magnesium oxide. Table I and Figure 1 show a coniparison of the physical properties of a Neoprene Type W a n d a. conventional Xeoprene Type Gh- compound using this curing system, and clearly demonst>rateits impracticability for this new polymer. The slow rate of cure and low state of cure obtained with the Type W compound, even after vulcanizing for 40 minutes a t 153" C. (307"Fa),show the need for a more active form of acceleration. One of the ways to adjust rate of cure in Neoprene Type QN conipounds is to vary the ratio of me,gnesium oxide to zinc oxide.
0 5 5
0.5 *5
I
Stress a t 500$70 Elongation, Lb./Rq. Inch 250 2275 GOO 2550 1750 2800
2
eooo-
-1
40
10 20 40
$
::
2850
800
1550 2750
3075 3225
B v)
Elongation at Break, % 1060 760 850
6
680
740
600
Hardness
10 20 40
51 52
58
D 398-471' (Method B) D 676-47T (Shore A Durometer, D 623-41T (Method A, a/,,-inoh stroke) D 945-487'
2 4 29
Tensile Strength at Break I,b./Sq. Inch
10 20
A.S.T.M. Designation
L> 412- 41
59 89 61
686
1000
/
TYPE
w
(&"I)
INDUSTRIAL AND ENGINEERING CHEMISTRY
688
Vol. 42, No. 4
are shown. As might be anticipated, the state of cure inComcreases proportionately to the Min. Cured a t 153O C. pound 10 20 40 10 20 40 10 20 40 10 20 40 amount of accelerator added, IdentifiStress a t 500%. Tensile a t Break, Elongation a t Hardalthough the processing safety cation Accelerator Lb./Sq. Inch Lb./Sq. Inch Break, % ' ness becomes questionable when A-4 2-Mercaptothiazoline 1925 2200 2525 3450 3625 8560 770 690 670 55 56 57 B-4 Tetramethylthiuram monosulmore than 0.5 part is used. fide 1800 2075 2100 3400 3575 3250 800 780 700 52 53 53 It is well known t h a t mateC-4 Condensation product of butyraldehyde and aniline 540 1650 2525 1380 2675 3275 1000 710 600 49 52 54 rials such as the di-o-tolylguaniD-4 Diphenylguanidine 420 750 1700 1060 1625 2875 980 940 700 49 0 1 53 dine salt of dicatechol borate and 4,4'-di a m i n o d i p h e n y 1methane exert a powerful accelerating effect on neoprenes such as Types GN or RT and are several accelerators in the absence of sulfur was investigated. usually used whex extremely fast cures are /required. An inThis was of particular interest since the accelerating effects of vestigation showed that these accelerators are also very effective aldehyde, amine, and guanidine accelerators in Neoprene Type GN in Neoprene Type W. The data are summarized in Table VI compounds containing magnesium and zinc oxides are welland conventional Neoprene Type GN vulcanizates with and known. Table I V shows such a comparison in the following without the di-o-tolylguanidine salt of dicatechol borate are inbase compound which was vulcanized at 153" C. (307' F.) for cluded for purposes of comparison. the times indicated: Since the data in Table I1 showed the strong retarding effects Base Compound of magnesium oxide in the absence of sulfur or organic acceleraNeoprene Type W 100 Phenyl-a-naphthylamine 2 tors, a similar comparison was made in the presence of the di-o0 5 Stearic acid
TABLE IV. EFFECT OF VARIOUS ACCELERATORS IN THE ABSENCEOF SULFUR 7
7
Extra light, calcined magnesium oxide Zinc oxide SRF carbon black Accelerator
2 5 29 0.5
esoo
The data in Table IV reveal several interesting and somewhat unexpected results. For example, diphenylguanidine and the condensation product of butyraldehyde and aniline are essentially ineffective in the absence of sulfur, while 2-mercaptothiaaoline and tetramethylthiuram monosulfide are very effective under these conditions. The latter observation is of considerable importance because i t affords an opportunity t o produce satisfactory Neoprene Type W vulcanizates in those applications where the inclusion of even a small amount of sulfur would be undesirable. The data discussed thus far have involved a single concentration (0.5 part) of organic accelerator with or without sulfur. Most vulcanization reactions involving natural rubber are influenced by the concentrations of accelerator. This is also true of Neoprene Type W a s illustrated by the data in Table V and Figure 3 in which the properties of a series of vulcanizates containing varying amounts of 2-mercaptothiazoline in the absence of sulfur
-
i
. 0
2I
2400-
z
-
u7 0
IVI
3
2000-
a
z
16001
Figure 3.
I
I
10
IS
I I I t 20 25 30 35 MINUTES CURE AT 153'C.f307pF.1
I 40
Vulcanization of Neoprene Type W with 2-Mercaptothiazoline
TABLE VI. NONSULFUS VULCANIZATION OF NEOPRENE TYPES W AND GN TABLEv. VULCANIZATION
WITH 2-MERCAPTOTHIAZOLINE Base Compound A-5 B-5 C-5 D-5 Neoprene Type W 100 100 100 100 Phenyl-a-naphthylamine 2 2 2 2 0.5 0.5 0.5 0.5 Stearic acid Extra li h t calcined magnesium oxide 2 2 2 2 SRF oa&on black 29 29 29 29 5 6 5 5 Zinc oxide I ... 0.25 0.5 1 2-Mercaptothiazoline 45+ 45+ 25 13 Mooney scorch, min.
Min. Cure a t 153" C.
10 20 40
Stress a t 500% Elongation, Lb./Sq. Inch 1800 1925 2000 300 1525 2025 2200 2400 1650 2225 2525 2825 Tensile Strength a t Break, Inch 825 3250 3450 2775 3325 3825 2825 3425 3650
10 20
1020 770 730
10
48 62 53
40
20 40
Base Compound
A-6 Neoprene Type W Neoprene Type G N Phenyl-a-naphthylamine Stearic acid SRF carbon black Extra light calcined magnesium oxide Zine oxide Di-o-tolylguanidine salt of dicatechol borate
4,4'-diaminodiphenylmethane Mocney scorch, min.
M h Cure at 153O C.
Lb./Sq.
0.5 29
...2
C-6
D-6
io$
ibo'
0.5 29
0.5 29
2 5
2 5
4
0.5
... 0.5
... 20
24
5
... ... 29
2 0.5 29 4 5
0.5
... 16
Stress at 500% EIonpation, Lb./Sq. Inch 1725 2275 2650 3176 2650 2900 3350 2800 3050 Tensile Strength at Break, Lb./Sq. Inoh 2825 2850 3025 3275 3075 3025 3425 2225 3050
3350 3475 3575
55 56 58
...2
B-6 100
2700 2775 2900
3300 3200 3200
Elongation a t Break, % 730 770 760 725 690 720 680 670 670 Hardness 53 55 53 56 54 57
100
600 560 530 10 20 40
57 57 58
Elongation a t Break, % 740 760 600 520 680 530 620 600 500 Hardness 58 59 57 59 61 67
62 63 63
April 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE11. EFFECT OF MAGNESIA CONCENTRATION UL VULCANIZATION OF NEOPREA-E TYPEVV Base Compound Neoprene T y p e W Phenyl-a-naphthylamine SRF carbon black Stearic acid Zinc oxide 21-2 R-2 % ,Extra light calcined magnesium oxide added t o base compound 0 1 Mooney scorch, niin. 10 34
100 2 29 0.5 5 c-2
1)-2
2 45+
4 35+
b;-2 10 ,1Ri-
Stress at 50070 Elongation, Lb./Sq. Inch 1650 300 250 250 1350 1675 1550 1525 900 500 1750 1650 1650 1750 950
Min. Cure a t 153' C. 10 20 40
Tensile 3150 3150 3376
io
20
40
10 20
820 790 780
10 20
52
40
52 52
40
Strength a t Break, Lb./Sq. Incb 2400 825 800 850 1550 1175 2425 2775 2760 1675 2550 2825 Elongation a t Break, yo 740 1020 1060 1080 850 1000 720 770 680 730 740 840
51 51 52
Hardness 48 51 52 52 53 53
the states of cure are lower than desirable. I n addition, this stock is extremely scorchy and the resistance of the vulcanisate to aging in the air oven is impaired seriously. With 1 part magnesium oxide, the resistance to scorch of the stock is substantially improved; and with 2 parts magnesium oxide, the Mooney scorch value becomes greater than 45 minutes. Larger amounts ?f magnesium oxide markedly decrease the state of cure. .hordingly, the r h o of 2 parts of magnesium oxide t o 5 parts of zinc oxide has been considered as optimum, and was used throughout the compounding studies which follow. Since the state of cure obtained with magnesium and zino oxides is undesirably low, the organic accelerators normally used hy the rubber industry for natural rubber and GR-S were investigated next. The studies were made by applying conventional rubber compounding practices-i.e., sulfur was used in conjunction with the accelerators. It, was soon found that with Xeoprene Type W, the amount of sulfur and a~celerat~or required for vulcanization was generally less than the amounts needed to vulcanize natural rubber. Table 111 shows the stress-strain properties of vulcanizates containing various conventional accelerators in the following base compound which was vulcanized in all cases a t 153" C. (307" F.) for the times indicated.
54 54 58
Base Compound Keoprene Type W Phenyl-a-naphthylamine Stearic acid Extra light calcined magnesium oxide Zinc oxide SRF carbon black Sulfur Accelerator
Properties of Vulcanizates Cured for 20 Min. a t 153" C . and Aged for 7 Day6 in 121" C , Air Oven Tensile strength a t break, Ib./ Brittle 1200 1725 1850 1475 sq. inch Elongation at, break, 7' Brittle 90 180 180 120 loo+ 85 72 Hardness 72 73
NONE I _ -
-----------
0
1
10
15
I 20
I e5
I
I
30
35
40
MINUTES CURE 153°C.(307'F.)
Figure 2. Effect of Magnesium Oxide Concentration on Vulcanization of Neoprene Type W i n Presence of Zinc Oxide
Ordinarily, the lower the ratio, the more rapid the rate of cure ( 5 , 5 ) . Various concentrations of magnesium oxide were added to a Neoprene Type W base compound containing 5 parts of zinc oxide. The results are shown in Table I1 and Figure 2. The data show that with Neoprene Type W, as well as with Neoprene Type GN compounds, varying the amount of magnesium oxide alters the rate of cure. When magnesium oxide is omitted altogether,
TABLE: 111. VKJLc 4NIZATION Conipound Identifiaation -1-3 B-3
C-3 D-3 E-3 F-3 G-3 H-3
1-3 5-3 K-3
681
100,
0,; 5
29 1 0.5
For reference, two cont.rols are included, one containing sulfur in the absence of accelerator and one containing neither sulfur nor accelerator. The data in Table I11 show t'hat a combination of sulfur with many of the conventional organic accelerators vulcanizes Neoprene Type W very e f f e c t i ~ l y . I n several cases, the rates of cure among the various accelerators show differences similar to those shown by the same accelerators in natural rubber vulcanizates. Notable exceptions arc mercaptobenzotbiazole and benzothiaxyl disulfide which are relat'ively weak vulcanization accelerators in Neoprene Type 55' compounds, while they are reasonably strong in natural rubber vulcanization. It is noteworthy, ho.rvever, that both of these chemicals act as accelerators in Xeoprene Type Wp because both are retarders in Neoprene Type GIN. The effect 0% sulfur alone on the vulcanization of Neoprene Type W is of interest because it acts as a mild curing agent under these conditions. The solubility of sulfur in Seoprene Type W vulcanizates is low (less than 1%) and visual observations have indicated that its rate of combination is slow; therefore, blooming may occur in unaccelerated stocks except on extended cures. Since sulfur in the absence of organic accelerators has been shom-n to affect the state of cure of Seoprene Type W, the effect of
WITH I'ARIOUS
Accelerator 2-Mercaptothiazoline Piperidinium pentamethylenedithiocarbamate Butyraldehyde-aniline condeneation product Diphenylguanidine Zinc dimethyldithiocarbamate Mercaptobenzothiazole Beneothiazyl disulfide Teppmethyltbiuram rnonosdtide Tetramethylthiuram disulfide Control (sulfur only) Control (oxides only)
.kCCELERATORS IS THE PRESENCE O F SULFUR
10 20 40 Stress a t 600'%1 I b /Sq Inch 2175 2400 2850 2100
__ - h l l n Cured a t 153' C . -___ 10 20 40 Tensile a t Break, Lb /Sq Inch _______ 3676 3600 3560
2750
3050
3200
3500
1150 2850 1650 2525 2050 2150 1370 1500 1240 1625
3025 2976 2350 1875 2000
2900 2725 3150 2550 2675
3725 3500 3475 3425 3375 3200 2675 3100 3150 3200
3275
1926 2200 2425 3100 1975 2200 2376 3200 830 1825 2200 1825 300 1525 1650 825
3400 3475 3200 3425 2950 3025 2775 2825
10 20 40 Elongation a t Break, 70 .______ 760 670 610 760
640
52
56
58
780 640 560 780 660 600 780 760 700 860 800 800 860 880 780
53 54 52 49 52
68
58 59
53 53 52 48
53 53 52 52
780 720 880 1020
620
20 40 Hardness 58 68 58 10
650 780 740 770
700 680 700 730
58 53 51 52
53 52: 53 58
54 54 35
April 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY VII.
T o m
689
VULCANIZATION WITH DI-+TOLYLGUANIDINE SALTOF DICATECHOL BORATE
(Effect of magnesia concentration) A-7 B-7 Neoprene Type W 100 100 Phenyl-a-naphthylamine 2 2 Steario acid 0.5 0.5 SRF carbon bIrtck 29 29 Extra light oalcined magnesium oxide 2 Zinc oxide 5 5 Di-o-tolylguanidine salt of dicatechol borate 0.5 0.5 Mooney scorch, min. 2 20
C-7 100 2 0.5 29
D-7 100 2 0.5 29
4 5
10 5
...
'
0
I
2 PARTS
6 0 Mto. 108*C.l227'F.)
I
I
I
4
6
a
10
OF MAGNESIUM OXIDE I 1 0 0 PARTS O F NEOPRENE
Figure 4. Effect of Magnesium Oxide Concentration o n Vulcanization of a Neoprene Type W Compound in Presence of Di-o-Tolylguanidine Salt of Dicatechol Borate
tolylguanidine salt of dicatechol borate, a strong accelerator. Table VI1 and Figure 4 show the results of this comparison. The data show clearly that magnesium oxide also is very effective in the presence of an active accelerator. The addition of two parts magnesium oxide markedly increases the processing safety of the stock, as judged by either the Mooney scorch data or the physical properties of the vulcanizate cured a t 108' C. (227" F.), Additional amounts further increase the processing safety of the stocks. The use of 10 parts magnesium oxide results in a decided undercure in the vulcanizates cured a t 153" C., but this effect is not shown in the stocks containing either 2 or 4 parts. Since the processing safety of the stock containing 4 parts magnesium oxide appears to be great enough to ensure suitable factory operations, this amount appears to be the most desirable to use in Neoprene Type W stocks accelerated by the di-o-tolylguanidine salt of dicatechol borate. A complete comparison of the physical properties of vulcanizates of Neoprene Type W with those of Neoprene Type GN or natural rubber is beyond the scope of this paper. However, the various methods of vulcanization of Neoprene Type W have so pronounced an effect on certain properties that comparisons with these elastomers are of interest. Two properties which are considered especially noteworthy in Neoprene Type W vulcanizates are the shape of the stress-strain curve and compression set, particularly a t elevated temperatures. Table VI11 shows comparisons of these properties of the 3 elastomers. With each elastomer 2 compounds were selected; first, a compound which might be considered a representative general purpose or Conventional type, and, secondly, a compound designed to enhance particuIarly the properties of compression set and resilience. Thus, in Table VI11 compounds A-8, C-8, and E 8 represent the fmt type while B-8, D-8, and F-8 represent the second type. With either type of compound a comparison of the stress-strain properties of Neoprene Type W with those of Neoprene Type GN or natural rubber shows that they are intermediate between these elastomers. Both of the Neoprene Type W compounds show lower moduli a t lower elongations and higher moduli a t the higher elongations than the comparable Neoprene Type GN compounds. In this respect, the stress-strain curves of the Neoprene Type W compounds approach those of natural rubber. Figure 5 illustrates these differences for compounds A-8, C-8, E 8 and B-8, D-8, F-8, respectively. The Neoprene Type W vulcanizates do not show the decrease in the ratio of stress to strain a t high elongations which has hitherto been characteristic of chloroprene polymers. The compression set data in Table VI11 illustrate some of the most striking differences between vulcanizates of Neoprene Type W, and those of Neoprene Type GN or natural rubber. Compound A-8, for example, a conventional Neoprene Type W compound, shows set values after exposure for 22 hours a t 70 O C. which
60-Min. Cure at 108O C . Stress at 500% elongation, lb./sq. inch 2550 1110 Tensile strength at break, lb./sq. 2725 1875 inch Elongation a t break, % 540 800 Hardness 56 52
Min. Cure at 153O C.
0.5 28
0.5 45
550
280
1280 860 52
900 1300 53
10 20 40
Stress at 500% Elongation, Lb./Sq. Inch 2625 2850 2600 1400 2600 2950 2960 1950 2550 3025 3250 2625
10 20 40
Tensile Strength a t Break, Lb./Sq. Inch 2625 3325 3500 2450 2875 3150 3050 2750 2550 3350 3250 3000
10 20 40
500 560 500
10
54 54 53
20 40
Elongation at Break # % 580 640 800 580 520 660 540 500 560 Hardn ese 58 58 58 58 58 59
,TYPE
3000
57 59 61
W Id-8)
/ T Y P E QN I C - 8 )
eo00
5
IO00
2 I
2
I
O
I
I
4000
I
I
RUBBER (F-8) TYPE W
18-81
3000 TYPE ON ( 0 - 8 )
eo00
IO00
0 eo0
400
STRAIN
-
600
BOO
%
Figure 5. Comparison of Stress-Strain Curves of Natural Rubber, Neoprene Type W, and Neoprene Type GN Cured 28 min. at 153O C. (307O F.) Above. General purpose compounds Below. Low aompression s e t compounds
INDUSTRIAL AND ENGINEERING CHEMISTRY
690
Vol. 42, No.
4
rooC AND
PROPERTIES OF SEOPRENE TYPE W VULCANIZATES COhiPARIsOH WITH NEOPRENE TYPEGN AND N A T U R A L RUBBER
TYPE
TABLE VIII.
Neoprene T y p e W Neoprene. TypLG N Bmoked sheets Phenyl-a-naphthylamine Stearic acid Extra light calcined magnesium oxide SRF carbon black Zinc oxide 2-mercaptothiaeoline Di-o-tolylguanidine salt of dicatechol t>orate Tetramethylthiuram disulfide h3UlfUT
Base Compound A-8 8-8 C-8 100 100 .. .. . 1'00 ... ... ..i 2 0.5 2 29
2 0.5 2 29 5
4
1
...
... ...
1
Min. Cured a t 1-12' C.
n . j
2 0.5
,..
101)
2 3
2 I
..
4 29 5
~
100
39 5 1
.
,
;.. ...
2.75
0
Stress a t 500% Elongation. Lb./Sq. I n c h 2825 1960 2525 24.50 3325 3300 2125 2625 2850 3350 3300 2375 2850 3250 3550 . . . 2600 ,. , 3400 3350
10 20 40 80
3100 3250 3250 3400
Tensile Strength at Break. Lb./Sq. Inch 3550 2550 2750 4200 4250 3325 2800 2850 4600 4000 3300 3175 2850 4530 4400 3150 3025 2675 4050 4000
10
860 700 680 580
80
Elongation at Break, 840 600 800 580 780 500 G40 400
680 640 900
080
58 69
Hardness 58 62 59 62 61 62 61 63
62 58 58 "58
76 76 77 77
Resilience, % 76 81 76 81 77 81 79 82
82 84 84 83
600 560 580 560
57
57 87 R7
10 20 40 80
67 69 71 76
10 20 40 80
20 13 9 7
Compremon Set,22 Hr. at ?On C., % 13 54 29 29 9 46 26 24 10 36 23 22 10 30 20 18
25
Compression bet 70 i l r a t 70' C , 10 57 40 36
20
10 20 40 80
82 77 72 61
Comprcssion Set, 70 Hi- a t 100° C., 26 101 90 85 21 99 86 78 19 88 73 18 83 64
33 30 28 25
40
Cornpiession Set, 70 Hi at 1 2 1 O C . yc 85 27 93 80 81 38
10
10 20 40 80
82 72 56 47
%
50 48 47 4b
Heat Build-up, C . Soft 47 Soft 51 60 90 61 43
39 31 29 30
I_ -
85 86
85 85 15 14 14 13
-
/
I
/
4
-
TYPE W ( B - E )
-_I1____l-
70
2
1725 2100 2550 2950
58 68
-
100
0.5
10 20 40 80
52 53 53 57
40
201
Stress a t 300y0 Elongation, bb./Sq. Inch 1060 1625 960 14W 1400 1750 1210 1475 1700 1260 1875 1425 1550 1800 1410 1875 1600 2025 1550 1473
10 20 40
-
39 5
690 890 1180 1410
20 40 80
60
~.
10 20 40 80
520 520 500 460
TYPE O N ( 0 - 8 1
~
, ,
1
.".
F'-8
...
ioo
0.5
...
-
TYPE W I C - 2 1
E-8
0-8
2 0.5 29 5
80
GN (C-81
TEMPERATURE
F'igure 6.
121
- 'C.
Comparison of Neoprene Type W with Veoprerie Type GN
70-hour
CXDOSUC'C:
rured 40 min. at L42O C.
This is illustrated in Figures 6 and 7 in which the percentage c i m pression set resulting from 70 hours' compression is plotted against the temperature of the exposure. In Figure 6 a compariaori i s made between Neoprene Types W and GX' stocks in the presence and absence of the di-o-tolylguanidine salt of dicatechol borate. This figure includes a curve on compound (2-2 from Table 11, in addition to the compounds from Table VIII. The inherent low set of Neoprene Type W compounds is illustrated vividly by the unaccelerated Neoprene Type R stock containing only zinc oxide snd magnesium oxide. The set,s of this stock a t various temperathrcs arc lese than t,hose of a n accelerated Neoprene Type GN stock. I t will be seen from Figure 7 that over this temperature range the conventional Neoprene Type 11' compound (A-8) cured with 2irirrcaptothiazoline and sulfur is comparable in set properties to that of the conventional rubber compound (E-@, and that the Neoprene Type W compound accelerated Kith the di-o-to ylgwnidine salt of dicatechol borate (B-8) is not only greatly superioi. to both of these stocks but shows a decided advantage over a Iov set rubber stock (F-8)for the entire temperahre range. SUMMARY
Keoprene Type IT, as a polymer, represents a fundamental departure from Yeoprene Type GN in t,he manufact,ure of chloroprene polymers. This new neoprene is more stable than any of its predecessors and contains no sulfur or chemicals which may de-
30 30 31 30
are significantly lower than those obtained with either a X eoprene Type GN compound or a natural rubber stock compounded specifically for low set (compounds D-8 and F-8). Although under more severe conditions of time and temperature of exposure a Neoprene Type W stock such as compound A-8 continues t o show excellent resistance to permanent deformation under compression, the advantage of a nonsulfur cure such as that obtained with the di-c-tolylguanidine salt of dicatechol borate or 4,4'-diaminodiphenylmethane is readily apparent,.
,-
70
Figure 7 .
100 TEMPERATURE
let
-'e.
Comparison of Neoprene Type W and Natural Rubber
70-hour exposure; cured 40 min. at 142' C.
April 1950
INDUSTRIAL AND ENGINEERING CHEMISTRY
compose to give free sulfur or a vulcanization accelerator. Although it,s compounds should include both magnesium oxide and zinc oxide, Type W also requires acceleration. Many convent,ional neoprene and rubber accelerators can be used. Certain of the rubber accelerators, such as diphenylguanidine or the butyraldehyde aniline condensation products require sulfur. Others, such as 2-mercaptothiazoline and tetramethylthiuram monosulfide, as well as the common neoprene accelerators, such as the di-o-tolylguanidine salt of dicatechol borate, may be used without sulfur. Neoprene Type W vulcanizates differ from those of Type GN by ha,ving a stress-strain curve more nearly approaching that of natural rubber and by having much greater resistance
691
to compression set. The resistance to compression set of Neoprene Type W vulcanizates is greater than that of the best natural rubber stock. LITERATURE CITED
( I ) Bridgwater, E. R., IND. ENC.C H E M . ,26, 33 ( 1 9 3 4 ) . (2) Bridgwater, E. R.,a n d K r i s m a n n , E. H . , Zbid., 25, 280 ( 1 9 3 3 ) . (3) C a t t o n , N. L., Fraser, D. F., a n d F o r m a n , D. B., du P o n t C o . , Rubber C h e m . Div., Rept. 40-2, 15 ( 1 9 4 0 ) . (4) M a y o , L. R . , IND. ENG.C H E M .42, , 696 ( 1 9 5 0 ) . (5) S t a r k w e a t h e r , H. W., a n d W a l k e r , H. W., Zbid., 29, 872 ( 1 9 3 7 ) . (6) T o r r a n c e , M. F., a n d Fraser, D. l;.,Zbid., 31, 939 ( 1 9 3 9 ) . RECEIVED Septemher 26, 19-29.
Stability of Burner Flames with Propane-Hydrogen Mixtures T h e Pennsylvania S t a t e College, S t a t e CoEleqe. I'u.
Blowoff and flash-back data are reported for propane, hydrogen, and mixtures of the two gases. The results obtained i n the region of laminar flow using air-cooled burner tubes may be correlated satisfactorily in terms of velocity gradients at the burner wall. The effect upon blowoff and flash back of increasing concentrations of hydrogen in hydrogen-propane mixtures is shown to be nonlinear for both blowoff and flash back.
T
HE stability and structure of burner flames is a matter of
considerable theoretical interest and technical importance. In recent years significant progress has been made toward an understanding of this subject a5 a result of contributions by Lewis and von Elbe ( 3 )and von Elbe and Mentser ( 2 ) . Using n a t u r d gas, hydrogen, and acetylene, these investigators satisfactorily correlated flame stability in terms of fuel-air ratio and velocity gradient a t the burner wall, independent of burner tube diameter or construction materials. The correlations hold in the region of laminar flow where most of the experimental data were obtained. Bollinger and Williams (I), using propane, have extended the correlation into the region of turbulent flow by the use of appropriate velocity gradient calculations and have shown that for blowoff, the correlation holds satisfactorily u p to a velocity gradient of about 20,000 seconds-'. Because of the interest in the behavior of mixed fuel gases, the present investigation was undertaken to determine whether the velocity gradient concept was applicable to binary mixtures and also to furnish the basis for later studies on multicomponent gases. Mixtures of hydrogen and propane were chosen for the initial study because data were available on these gases separately, and because of the very marked difference in their burning velocity and flame stability properties. APPARATUS AND PROCEDURE
Air, a t the desired pressure, was dried by passing it through anhydrous calcium chloride, and its flow was measured by using a rotameter. Technical grade propane, of about 95% purity, in which the impurities were principally ethane and isobutane in approximately equal proportions, and water-pumped hydrogen of 99.7y0 purity, were used directly from commercial cylinders. 1
Piesent address, Princeton University, Pilnoeton, N. J.
The flow of hydrogen and of propane was measured by means of calibrated capillary flowmeters. The two fuel gases and the primary air mere suitably mixed in the desired proportions and delivered to the base of burner tubes of sufficient length (35 to 100 diameters) to ensure laminar flow at the burner discharge. All tests were conducted using air-cooled burner tubes discharging to the atmosphere. The burning end of the burner tubes were lapped with No. 600 grit to ensure a smooth, flat surface perpendicular to the tube axis. For the majority of the tests, Pyrex glass No. 744 tubes 1.273, 1.093, 0.950, 0.770, and 0.573 cm. in inside diameter were used to secure the desired range of velocity gradient. For special tests on materials of construction, a stainless steel (type 304) tube 1.340 cm. in inside diameter and a transparent vitreous silica tube 1.270 cm. in inside diameter were used. With the exception of the silica tube, all had smooth, polished inner walls. The latter had the slightly wavy inner u-dl characteristic of fused silica tubing. I n order to ensure reproduribility, the following general procedure was employed:
The desired fuel gas flow was established and checked to ensure a steady state condition; primary air was introduced so that the total flow was in the region of stable flame; and the gas was ignited with a small pilot flame. The air supply was gradually altered until the desired phenomenon occurred; the flame was extinguished; and the air flow was readjusted into the region of stability. The air flow was altered in small increments toward instability, and the pilot flame applied at each setting. If flash back did not occur, the flame was extinguished and the procejs repeated until i t did. The air flow was increased in small increments until the flame began t o lift and then by an increment sufficient t o produce comIete blowoff. The blowoff was calculated from the mean of the ast two rates.
f
The foregoing procedure ensures that any instability observed is not due to a previous mixture which has failed to clear the system nor to undue overheating of the burner rim because of prolonged operation near the flash-back point where the flame is drawn into close proximity with the rim. Similarly the influence of tilted flash back or blowoff which unduly prolongs the flame existence is reduced to a minimum. In all cases, a number of determinations were made a t each set of conditions and the re-
