T H E EFFECT OF NETWORK STRUCTURE ON AGING OF NATURAL RUBBER VULCANIZATES C A R L R .
P A R K S A N D OTTO
L O R E N Z
The Goodyear Tire C Y Rubber Co., Research Dioision, Akron, Ohio
The effect of different network structures on retarded aging was investigated using natural rubber vulcanizates. The aging was primarily affected by the concentration of cyclic sulfides and polysulfides while other sulfur moieties had only a minor effect. The results suggested that polysulfides acted as oxidation initiators. Also, the oxidation of networks not containing cyclic sulfides occurred randomly along the chain. If cyclic sulfides were present, the oxidation occurred preferentially a t the cyclic sulfides and was accompanied by a deterioration of the physical properties. A study of the effect of varying amounts of cyclic sulfides on the tensile, flex, and tear properties of a vulcanizate containing carbon-carbon cross links indicated that cyclic sulfides did not significantly affect these physical properties.
a t 75" C. a t 12 mm. Totally bound sulfur was determined in the extracted samples as barium sulfate after oxidation with nitric acid-bromine. Zinc sulfide was determined by the liberation of hydrogen sulfide with hydrochloric acid in ether and absorbing the hydrogen sulfide in cadmium chloride solution ( 7 ) . Swelling measurements were carried out in benzenr, the swelling valuc! Q being defined as grams of benzene per gram of rubber gel at swelling equilibrium. The number of chemical cross-links in the vulcanizates were estimated by a method which is based on an empirical relation between the equilibrium svelling value Q and the amount of the quantitative crosslinking agent, dicumyl peroxide, used to vulcanize purified natural rubber (74). The viscosity average molecular weight of the rubber used in this investigation was very similar to that previously used. Since the value of Q will be larger for a gel network cross-linked in the presence of a soluble diluent than for a network cross-linked to the same extent but in the absence of a diluent, a corrected swelling value Q, was calculated using the following empirical relation (20):
RUBBER differs in its aging properties from that of raw rubber because of specific network structures and extra network materials. T h e specific network structure is determined by the choice of vulcanizing agent, the type and concentration of accelerator, and often the curing conditions. Extra network materials added before vulcanization or formed during the cure include antioxidants, accelerators, fillers, etc. 'This investigation is concerned with the effect of the network structure on retarded aging. T h e rate of oxidation depends on the amount of combined sulfur ( g ) , but no attempt has previously been made to relate aging properties to specific sulfur moieties formed during the vulcanization. n'atural rubber was used as the network structures of natural rubber vulcanizates are much better understood than those of other elastomers. ULCASIZED
Experimental
Vulcanizates were prepared according to 'Table I. Samples were approximately 1.0 mm. thick for physical property test, and 0.5 mm. for oxygen absorption measurements. Variable amounts of ingredients were used where a range is indicated. T h e relative amounts- e.g., sulfur-accelerator-zinc oxidewere held constant for any given curing system. After vulcanization. the samples were extracted n i t h acetone for 1 week a t room temperature to remove unreacted sulfur and vulcanization residues ; the acetone was changed every other day. For the tetramethylthiuram disulfide ( T M T D ) samples, a n acetone-chloroform mixture (3 to 1 volume) was used to remove the zinc dimethyldithiocarbamate formed during vulcanization. The samples were dried for 4 hours
Table 1. Density, G. /MI
Natural rubber (smoked sheet) HAF carbon black Zinc oxide Lauric acid Stearic acid Dicumyl peroxide Tetramethylthiuram disulfide Sulfur Mercaptobenzot hiazole Diphenylguanidine Curing, min./' C.
0.912
-
Qc
=
oo(Q
+ 1)
-
1
where u, is the ratio of the volume of the rubber gel network to the total volume of rubber plus the volume of all soluble materials. Unreacted zinc oxide was considered to be an insoluble, separate phase a t vulcanization temperatures and hence not included in the calculation of u,. Densities used for the calculation of u , are given in Table I. A density value of 1.05 was used for zinc stearate and 1.09 for zinc laurate assuming a stoichiometric reaction between zinc oxide and the fatty acid to form zinc stearate or laurate and water.
Compounding and Curing ~
A
100
B 100
1.5-13.5
-~
c
Compound _ _ _ ~ ~ _ _ D
100
100
1.5-9.0 3.0-18.0
F 100 50
100
-
5.0 2.0-6 0
1.02 1.29 2.00 1.42 1.13
1 .O-4.0
1.25
0.75
1 .O-9.0
120 '/l5O '
120'/135
0.5-3. O 0.5-3.0
2.0-6.0
0.1-5.0
0.1-5,0
144 hr/100 '
1 .O-3.0 90 '/135'
60'/150
60 '/150 a
VOL. 2
NO. 4
+ 24 hr/140
+
O
24 hr/140"
DECEMBER 1963
279
.V-Phenyl-2-naphthylamine (PBNA) was added by imbibition from a benzene solution of known concentration of the additive for 96 hours to give samples containing approximately 1 p.h.r. of PBXA. The samples were again dried in vacuo and stored under nitrogen. The amount of PBNA added was determined after extraction with ethanol by ultraviolet absorption measurements and found to be 0.90 + 0.10 p.h.r. for all samples. Vulcanizates containing various amounts of cyclic sulfides Lvere prepared in the following manner: First, natural rubber \cas cured in a press with dicumyl peroxide to give a network itructure containing only carbon-carbon cross links. The samples were then extracted with acetone for 1 week at room temperature. changing the acetone every other day. Various amounts of sulfur were added by imbibition from a benzene solution of known concentration for 96 hours a t room temperature. PBNA was also added along with the sulfur, usually 0.5 p.h.r. The air-dried samples were placed in glass ampoules and after being evacuated overnight a t 10-6 mm., the ampoules were sealed and then heated in a n oven for 24 hours at 140' C. Moore and Trego (76) have shown that when natural rubber and sulfur are heated under similar conditions, 93 to 95% of the combined sulfur was in the form of cyclic sulfides. For the preparation of larger samples for physical property tests, the rubber samples were heated in nitrogen instead of in vacuo. The samples were placed in a vacuum oven and, after evacuating for 4 hours at 0.2 inin., were heated under a positive nitrogen pressure for 24 hours at 140' C. Heating in vacuo or in nitrogen gave similar oxidation rates, provided the rubber was adequately protected by an antioxidant. When 0.1 p.h.r.-were only small amounts of antioxidant-e.g., added and the rubber was heated under nitrogen, abnormally high oxidation rates were obtained probably because oxygen was not completely excluded. Samples for physical property measurements were heated with sulfur under nitrogen in the presence of 0.5 p.h.r. of PBNA. Stress-strain measurements were made using the Instron machine and tear tests using ASTM method D624-54. Flex tests were made by adapting the DeMattia machine to take rubber strips 6 X 1 inches with a 5/16-inch hole in the middle. The samples were stretched 66%, relaxed, and then flexed a t 360 cycles per minute. The flex life was determined by the time to break. For oxygen absorption measurements. the samples were extracted with acetone for 1 week at room temperature to remove any uncombined sulfur and antioxidant. PBNA was again added by imbibition from a benzene solution to give samples containing 1 p.h.r. of the antioxidant (0.95 f 0.05).
Oxygen absorption measurements were made in a multiple unit apparatus a t constant temperature of 100' j= 0.2" C. and a constant oxygen pressure of 740 mm. The oxygen absorbed was plotted against time, and the rate determined from the slope of the initial straight line portion of the curve. Vulcanizates containing cyclic sulfides were also aged in the oxygen absorption apparatus a t 100' C. to give samples having an oxygen content of approximately 0.576 for stress-strain measurements. Results and Discussion
The oxidizability of some natural rubber networks, whose structures have been reasonably well defined, was studied by means of oxygen absorption. Vulcanizates containing varying amounts of specific network structures were prepared to study the effect of their concentration on the oxidation rate. .411 vulcanizates were purified by extraction after curing and contained 0.90 + 0.10 p.h.r. of PBNA xvhich was added by swelling. 280
I&EC
PRODUCT RESEARCH A N D DEVELOPMENT
DICUMYL
PEROXIDE
y H3
- C H2-C=CH-CH-
- CHp-Y=CH-CH- I CH3
TMTD and EFFICIENT
p
MBT
7%
+ SULFUR s R is
- c - N c C HCH, 3
-~H-C=CH-FH-
OR
-s-c,
CONVENTIONAL SULFUR MONO-, DI-.
- ACCELERATOR
SYSTEMS
AND POLYSULFIDES (DIALKENYL)
CYCLl C SULFl DES
CYCLIC
SULFIDES
-CH2- CH- CH -S'
C-CH2-CH2I CH,
CHI
Figure 1
Various network structures of natural rubber
Considerable effort has been made in recent years to develop methods for characterizing the structure of rubber vulcanizates? particularly those from natural rubber. Various network structures are shown in Figure 1. The simplest network structure for natural rubber is obtained by the use of certain peroxides, such as dicumyl peroxide? as the vulcanizing agent. Carbon-carbon cross-links are formed by abstraction of amethylenic hydrogen atoms and the combination of the carbon radicals that are formed ( 7 7). With dicumyl peroxide. one molecule of decomposed peroxide yields one cross link (77). In addition, no significant side chain modifications such as chain scission (78) are known to occur. Tetramethylthiurain disulfide (TMTD) in the absence of free sulfur leads. after long curing times, to monosulfides and some disulfides (23), possibly of the dialkenyl type. Chain modifications probably occur and the attachment of R*N-CS-groups to the rubber chains has been postulated (8). .4 very efficient mercaptobenzothiazole(MBT)--sulfur system employing high amounts of fatty acid and lo\v curing temperatures for long times leads to a network containing mainly dialkenyl monosulfides and to a lesser extent disulfides and small amounts of main chain modifications (3, 15). Conventionally accelerated sulfur vulcanizates are more complicated than the relatively simple systems described above. They contain mainly dialkenyl sulfides, disulfides, and a considerable amount of polysulfides ( 7 ) , especially when diphenylguanidine (DPG) is used as the accelerator (23). In addition, cyclic sulfides are formed which are usually the main reaction products. Moore and Trego (76) have recently shown that cyclic sulfides are predominantly formed when rubber is heated with sulfur for a prolonged time. Polysulfidic cross links, chain scission, and the formation of conjugated double bonds are other features
w
l
W
m
(1:
1.0
0 v, m U
5
0.5
c3
> X
0
n 0
10
20
30
40
50
OXIDATION TIME, HOURS AT 100°C.
Figure 2. Oxygen absorption curves for different vulcanization systems at the same concentration of organically combined sulfur DPG-sulfur, 1.47 p.h.r. sulfur Cyclic sulfides, 1.50 p.h.r. sulfur Efficient MET-sulfur, 1.48 p.h.r. sulfur
ep
t i W
m
U 0 v, m U Z
W
c3
> X
0
0
25
50
75
IO0
I25
OXIDATION TIME, HOURS AT 100" C.
Figure 3. Effect of cyclic sulfide concentration (0 to 6.5 p.h.r.1 on oxygen absorption
of this reaction. After extended times of heating, however, up to 9576 of the combined sulfur may be present in the form of cyclic monosulfides. This method was used to add cyclic sulfides to a carbon-carbon network previously formed by curing rubber with dicuinyl peroxide. In this way, it vias possible to study the effect of cyclic sulfides in a network structure containing practically no sulfur cross links. Swelling measurements indicated that there was little formation of sulfur cross links in the gum vulcanizates u p to about 3 to 4 p.h.r. of bound sulfur when the heating was carried out in vacuo. At higher concentrations of sulfur, appreciable crosslinking evidently did occur. Sulfuration of the isoprene dimer, 2,6-dimethyl-2,6-octadiene has revealed that the cyclic sulfides are five- and six-membered rings which in some cases contain double bonds in the side chains adjacent to the sulfur atom (6). The relative abundance of these compounds depends on the reaction conditions and may also be affected by the accelerator used. T o obtain a n idea of the complexity of the network. the average number of sulfur atoms combined to the network per chemical cross link was determined. This value is given by the ratio of organically combined sulfur to the numbrr of chemical cross links introduced. Table I1 gives the average number of sulfur atoms required to form one cross link for the T M T D , the efficient MBT-sulfur and the DPG-sulfur curing
systems. This number is not identical with the actual number of sulfur atoms in the cross link since the organically bound sulfur also includes sulfur in the form of cyclic sulfides. These values indicate that monosulfidic cross-links are not the only reaction products in the T M T D and the highly efficient MBTsulfur systems. Apparently these networks include sulfurcontaining main chain modifications as well as small amounts of di- and polysulfidic cross-links. The formation of adjacent cross-links may also occur. The number of sulfur atoms per cross-link in the DPG-sulfur system was near 11. I n this network, the actual average number of sulfur atoms in the crosslinks has been determined by Studebaker (23). Cnder the authors' experimental conditions, this figure will be about four. This indicates that most of the organically bound sulfur was in the form of cyclic sulfides provided the amount of adjacent cross links was small. The last column in Table I1 gives the amount of sulfur in the form of zinc sulfide as per cent of the totally combined sulfur. Only small amounts of zinc sulfide were formed in the T M T D samples. in contrast to the efficient MBT-sulfur system \vhich was about 3870 and the DPG-sulfur system, about 23%. Oxygen absorption at 100' C . was used to determine the oxidizability of these different network 0 I
x
0.04
SULFUR
0
I
2
3
4
ORGANICALLY BOUND SULFUR, PHR (DICUMYL PEROXIDE CONSUMED, PHR)
Figure 4. Effect of organically combined sulfur on rate of oxidation for different vulcanization systems
R--S-S,--S-R
’ e R--Sa--S CHB
Cyclic sulfide samples heated: 0 In vacuo 0 In nitrogen
R--S,--S
’
+ R ’-&-s
R”
+ -cH2-C=CH-bH-
-, R”
CH3
0.00097 to 0.00338. The type of linkages introduced by these three systems had only a slight effect on the oxidizability. In the DPG-sulfur system, and also to a certain extent in the carbon-carbon network containing cyclic sulfides, the oxidation rate was much more dependent on the amount of organically combined sulfur-e.g., vulcanizates containing 3 p.h.r. of cyclic-bound sulfur oxidized about 11 times faster than the control \vithout any sulfur while DPG-sulfur vulcanizates with 3 p.h.r. of organically combined sulfur showed a 45-fold increase in the rate. The dependence of the oxidation rate on certain sulfurcontaining structures is probably associated with an activation
’
R--Sa-SH
I
+ -CH2-C=CH-C-
I
etc.
where R ” is H or -S-R”’ The carbon radicals that are produced will readily react with oxygen generating peroxy radicals. In such a mechanism, the polysulfide may act as an oxidation initiator which would explain their adverse effect on the oxidation. The action of polysulfides is still further complicated by the possibility of a decomposition of the perthiyl radical into a thiyl radical and sulfur which is a known retarder of oxidation. Therefore, initiation and retardation may occur simultaneously depending on the fate of the perthiyl radicals.
Table It. Combined Sulfur and Swelling Data
Q 5.02 4.365 3.77 3.37 2,925
4.305 3.22 2.72 2.435 2.22 2.05
5.075 4.49 4.10 3.61 3.36 3.15
QC
4.94 4.26 3.64 3.19 2.69
4.11 2.91 2.34 1.97 1.695 1.43
4.85 4.24 3.83 3.28 2,98 2.72
Chemical Cross-links, hifole/7O4 G. Ru bber
0.252 0,352 0.515 0,662 0.885
0.394 0,776 1.10 1.41 1.71 2.14
0,249 0.364 0.462 0.629 0,746 0.870
Sulfur or TM TD Added, PHR 2 0. -
3.0 4.0 6.0 9.0
Totally Bound Sulfur,
Organically Bound Sulfur,
PHR
PHR
TMTD 0.257 0.332 0.430 0.559 0.641
0 243 0.318 0.407 0.528 0.605
0.5 1.0 1.5 2.0 2.5 3.0
Efficient MBT-Sulfur 0.50 1 .oo 1.50 2.00 2.50 3.00
2.0 2.5 3.0 4.0 5.0 6.0
DPG-Sulfur 1.35 1.67 1.93 2.54 3.34 3.92
0.335 0.619 0.902 1.21 1.48 1.87
1.02 1.28 1.47 1.96 2.59 3.00
Organically Bound
Sulfur Atoms per Crosslink
Sulfur, G. AtornlIOi G. Rubber 0.76
Av.
3.0 2.8 2.5 2.5 2.1 2.6
Av.
2.7 2.5 2.6 2.7 2.7 2.7 2.65
0.99
1.27 1.65 1.89
1.Os 1.93 2.82 3.78 4.62 5.85
3.19 4.00 4.59 6.12 8.10 9.37 Av.
282
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PRODUCT RESEARCH A N D DEVELOPMENT
12.8 11 . o 9.9 9.7 10.9 10.8 10.9
Sulfur as ZnS, % of Totally Bound Sulfur
5.6
33.0 38,l 39.9 39.5 40.8 37.7
24.4 23.1 23.8 22.8 22.3 23.4
Physical Properties of Gum Vulcanizates Containing Cyclic Sulfides, Sample E Cyclic Bound Sulfur, P.H.R. 0 0.77 0.37 0.68 7.57 3.05 4.32 4.12 4.41 4.27 4.24 Swelling (Q), g. benzene/g. rubber 4.23 2260 2650 2770 2990 2700 Tensile stren th, p.s.i. 2620 113 104 100 101 95 96 Stress at 100% /c elongation, p.s.i. 257 223 233 230 210 208 Stress at 300% elongation, p.s.i. 695 720 710 720 690 Ultimate elongation, yo 685 99 97 100 86 Tear strength, p.s.i. 92 94 Table 111.
Oxygen absorbed, yc Tensile retention, yo 10056 Modulus retention, 300% Modulus retention, Elongation retention, yo
0.57 10 49 51 70
72
Table IV.
3.40 Tensile strength. D.s.i. 2440 .~~~~~~ .~~ Stress at lOO$i
