Degradation of Oil-Extended Polymers in Presence of Metal Salts J. M. MITCHELL, W. HI. EMBREE, AND R. B. MACFARLANE Research and Development Division, Polymer Corp., L t d . , Sarnia, Ontario, Canada
T
of latex solids. This masterbatch was heated t o 100"t o 120' F. and stirred continuously during the coagulation process. For several experiments, ferrous sulfate heptahydrate and/or complexing agents were added t o the latex before masterbatching. Coagulation was effected a t 110" F. by adding the oil-latex masterbatch with agitation t o 401, sodium chloride serum at pH 5 to 6. The p H was maintained a t 5 t o 6 during the coagulation by the addition of 0.5% sulfuric acid. After the precipitation was essentially completed, the pH of the slurry was reduced t o 4.0 by the addition of more acid, and leaching was continued for 30 minutes at 100' t o 110" F. for soap conversion. Various metal salts were added t o the dilute coagulating acid t o determine their relative effects on the heat softening of the crumb during the drying cycle. After t h e leaching, t h e serum was decanted, replaced with water a t 100' t o 110" F., and stirred for 20 t o 30 minutes. T h e water was then decanted and the washing repeated, using fresh water. Finally, the crumb was spread on a tray and loaded into a circulating air oven dryer. Based on previous experience, Khich showed good drying conditions when the drier temperature was a t 175" F. for the first hour and was reduced t o 150" F. for the balance of the drying cycle, these polymers were dried for 1 hour a t 175' F. and then subjected t o 24 hours of further drying and oven aging a t 150' F. I n almost all polymers the drying was complete in 3 to 5 hours. During the drying cycle the following samples were obtained:
HE ability of heavy metals such as copper, manganese,
and iron t o catalyze the breakdown of natural rubber during storage and processing is well known (2, 3). Use is made of this effect in certain peptizing agents like ferrous naphthenate and in rubber reclaiming agents (6). I n the early days of the "cold rubber" development it was feared t h a t the ferrous pyrophosphate, present as an activator for peroxide-catalyzed polymerization recipes, would have a deleterious effect on the aging of these polymers. Synthetic polymers were proved, however, to be more resistant to degradation during aging (4, 7 ) , and production has been successful for almost 7 years, These polymer types, when produced to high viscosity and extended with oil, are much more susceptible t o raw polymer breakdown during processing in the copolymer plant and in storage than the normal unextended cold rubbers (9). This undesirable degradation has occurred mainly where relatively highiron, sugar-free recipes are used for the polymerization of the parent polymer. When the polymerization recipe is a sulfoxylateactivated type of very low iron content, deterioration of Sundex 53-extended product during drying and subsequent storage, for as long as 12 months, is negligible. The effect of heavy metals, especially iron, in promoting the breakdown of polymer-oil masterbatches was studied in the pilot plant.
Hours 0 1.75 3.5
EXPERIMENTAL
6.5, 10, 15, 20, a n d 25
Standard high Mooney latex was secured from the plant polymerization facilities, stored in 50-gallon steel drums, and checked for polymer viscosity and bound styrene content in the Pilot Plant Control Laboratory. Table I shows the recipe used.
Table I.
Mooney testing of raw polymer a t 212' F. was done in duplicate on all samples obtained from the dryer and was continued a t intervals on samples removed after 3.5-hour drying. Percentage of ethyl alcohol-toluene azeotrope extract was determined on a selection of samples from each experimental batch. Mooney rings were used for analyses. Intrinsic viscosity was measured on the extracted polymer, using a three-dilution technique in 80/20 toluene-isopropyl alcohol solvent. Percentage of gel in toluene was checked on raw polymer samples from each batch. Viscosity and gel measurements were handled as quickly as possible after sampling, t o minimize additional degradation a t room temperature.
Low-Iron Recipe Parts
Butadiene-styrene Water MTMa K F A soapb Daxad 1 1 C KCld FeSOa.7HnO e
SFSf
Wet crumb samples for w e t drying Partially dried c r u m b sample One half crumb removed, baled, a n d wrapped In Holland cloth for room temperature aging C r u m b samples removed
71.5/28.5 200 0.125-0.150 4.70 0.10 0.50 0.004 0.0228 0,0246 0,0024 0.10 IO
EXPERIMENTAL RESULTS AND DISCUSSION
Since the first experimental practice of extending high h'looney rubber with petroleum-based oils in 1950 ( 6 ,81, polymer degradation has been observed in varying degrees in pilot plant drying equipment. This variation was first thought t o be associated with the design of the air-drying ovens, which were somewhat less efficient than the plant dryer in terms of time required to reduce the moisture content to 0.5%. Improvement in crumb
Sequestrene AAg NaOHh Thiostop N i a 60/20/20 blend of tert-dodecyl, tetradecyl, a n d hexadecyl mercaptans, Phillips Petroleum Co. i~ Potassium f a t t y acid soap flakes, Swift & Co. 0 Polymer of sodium salt of alkyl naphthalenesulfonic acid, Dewey & Alm Chemical Co. d $otassium chloride, technical grade, Canada Colours a n d Chemicals, Ltd. e Sugar copperas, technical grade, C a n a d a Colours and Chemicals, Ltd. f Sodium formaldehyde sulfoxylate dihydrate, trade name Formopon, Rohm & Haas Co. Ethylenediaminetetraacetio acid, Alrose Chemical Co. h Sodium hydroxide Canadian Industries L t d . i Sodium dimethyldjthiocarbonate, Nauga6uck Chemical, Ltd.
Table 11. Dryer Softening in Masterbatches (45 parts Sundex 53)
Antioxidant (1.25 Sample Parts) 1 BLE
Sufficient latex for each coagulation was withdrawn from the drum (6.0 and 8.0 pounds of latex solids for oil-extended and unextended samples, respectively) and stabilized with 1.25 parts of antioxidant per 100 parts of latex solids. The oil, made up as a 40% aqueous emulsion using 4.7 parts of Dresinate 214 (loo%) per 100 parts of oil, was added t o the latex, with apitation, in an amount calculated t o give 45 parts per 100 parts
345
2
BLE
8
Stalite
E% TA Extract 36.08
Test ML/4'
37.20
ibk/4'
'36.44
ib]L/4) [? 1
Drying Time, Hours S t o y g e ' 2 5 Weeks 49.!43.5 15 2.m 45.5 2.77 50.0 2.68
2.41 46.0 2.45 48.0 2.62
1.32 15 1.22 15 0.71
Q% el Final
2.4 1 9 2
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
346
shredding, tray loading, and manual stirring partially offset this deficiency but failed to effect a performance equivalent to the plant unit. Early in 1954, the softening encountered in masterbatches processed in the pilot plant became noticeably worse and persisted with severity even during subsequent storage of the samples (Table 11).
tQ
in high Mooney polymer was checked. The data in Tablc IV and Figure 1 are typical of results obtained with 1.25 parts of BLE present. The degradation, as shown by both raw polymer ?*looney and intrinsic viscosity decrease, is dependent on the presence of iiot only the copper but also the oil extender. When the degrading effect of copper was established, other heavy metals mere added t o the coagulant as chlorides or sulfates in molar concentrations equivalent to the level of coppci used earlier-0.48 mole per 25 pounds of coagulating acid. Table V shows the effect with masterbatches of polymer of low iron content with 45 parts of Sundex 53, stabilized with 1.25 parts of
9 100
b)
g
88
e HIGH MQONEY (NO Qlk)
68
A STANDARD (NO O I L ] HIGH MOONEY ( 4 5 PARTS OIL
U
z
8
82
40
BLE. These data, sho\m graphically in Figure 2, indicate that polymer degradation is relatively insignificant with tin, zinc, and nickel but increasingly severe with chromium, cobalt, iron, manganese, or copper. This order is more in accord with that observed for natural rubber (3)than that reported for GR-S ( I ) , which showed iron to affect the polymer aging more seriously a t 110" C. or a t room temperature than copper or manganese. The absence of degradation in samples coagulated in the presence of tin was not unexpected, because of the beneficial effect of organotin complexes as antioxidants. When it was observed that iron added as ferrous sulfate to the coagulant produced masterbatch softening, further experiments were done a t several iron levels, bracketing the amounts normally encouiitered in sugar-free low temperature polymerization recipes.
lal
8-
w a
20
$
0 0
Vol. 48, No. 2
5
10 15 20 25 TIME ( H R AT I50.F. Figure 1. Effect of Sundex on degradation of masterhatch Copper present in coagulant
By a process of elimination, suspicion was eventually directed to the 0.5% sulfuric acid used in preparing these samples. This acid had been stored for several m e k s in a 3Ionel metal tank. hIonel, although shoxing excellent resistance t o attack by 2 to 5% sulfuric acid, was unsatisfactory in contact with the 0.5% acid. Analysis indicated the presence of 63 p,p.m. of copper and 125 p.p.m, of nickel. Elimination of these contaminants lessened the softening markedly, as shown in Table 111. Subsequently it n-as demonstrated t h a t the degradation viae due primarily to the effect of the copper. More thorough water washing of the product did not lessen its effect; in fact, the softening appeared even more pronounced and it is suspected that it was already progressing during the washing operation. In one experiment the inclusion of a sequestering agent with the latex did not eliminate the effect of copper; the failure of this approach may have been due to the choice of sequestering agent. Because similar degradations had not been apparent in proceving polymer samples which did n o t contain oil, the effect of oil
? - I
m6.5 blR
H R,
HI?.
Figure 2.
cu ION. PRESENT Effect of metal ions in coagulant on aging of masterbatch at 1.50" F. I
Table 111. Masterhatch Degradation in Presence of Copper and Nickel (45 parts Sundex 53, 1.25 parts B L E j "I
EA
A g i n g Time at 150°
l\Ietal Present CuKi
Extract 36.78
lvIL/4'
J
CuNi
36.91
iLIL/4'
6
Nil
36.48
l\IL/4'
7
S i1
35.42
y y 4 '
Banipie 4
Table IV. Sample 8 0
10
1.75
I? 1
3.5
F..Hours 10
6.5
~-
15
20
2.41
[? 1
I? 1
51.5
2.56 44.0
2.49 55 0
Effect of Oil Loading on Softening with Copper in Coagulant
Polymer Krylene
sS
Test
Krynol (no oil) Krynol (oil added)
9%
ETA 6.86 7.04
Test 31L/4' [o I 31L/4'
I ML/4' [o I [ri
36.31
1.75 50.0
...
121 2.96
...
2.GO
Aging Time at 150' F,, Hours 3.5 6.5 10 51.5 50.5 52.0 1.61. 1.62 ... 130 136 135 2.97 4b'.0 38.5 18 ... 1.64 1.48 . I .
2 .j 49 0
1.63
134
2.97
...
,..
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1956
Polymer Degradation a t 150" F
Table V. E8A
Extract 29.5a
Test h.IL/C'
Zinc
36.2
ML/4'
Nickel
36.7
ML/4'
lietal Tin
Time a t 150' F., Hours 6.5 10 15 , ,. 64.0 64.5 62.5 2.86 2.96 ... 56.0 57.5 5?:Q 54 0 2.70 2.82 . , . .., 5 5 . 5 50.5 56.0 2.82 , ,, 2.92 ... 80.0 45.0 55.0 50.5 3.32 2.70 ... 7310 63.0 87.0 83.5 ,., 2.83 ... 2.99 44.0 47.5 42.5 39.0 2.03 2.54 . .. ... 56.0 40.0 30.0 10.0 3.10 2.33 2.22 1.48 38.5 18.0 , .. 2:60 1.64 1.48 1.11 1.75
[') 1
..
[')I
1') 1
Chromium
36.0
ML/4'
Cobalt
21.3a
@L/4'
Iron
36.7
ihlL/4/
Manganese
36.5
XL/4'
Copper
36.3
ML/4'
hI 1') 1 [') I
.
25 64.0 2.86 57.5 2.73 53.0 2.56 37.5 2.27 45.5 2,02 32.5 1.82
. .. ... ...
...
Lower oil loadings, indicated b y % ETA, explain higher maximum RIL/4' observed. a
Because three latex samples of slightly different base intrinsic viscosity (2.90 t o 3.20) were used in this work, the results in Tables VI and VI1 are expressed as percentage retention of intrinsic viscosity.
+IO0
w P
1
I
1:=0-00005 3
4
0.010 0*025-0-25 I
a? 5 0 : Figure 3 .
5 TIME
10
1
I
15
20
degradation agent; iron gives a Mooney drop of from 0 to 30% in 3 months, depending on the amount added, compared to a 75% decrease in 1 month in the presence of copper. Attempts to check the take-up of added ferrous sulfate by analyzing the masterbatch samples, using a colorimeter technique, invariably gave high results. About 50% of the apparent error could be accounted for on the basis of iron introduced as impurities in the oil, antioxidant, and emulsifier. Because little softening was apparent in the absence of added iron, it was concluded that the level of iron a t which softening set in was sufficiently high that the iron introduced as impurities did not itself cause softening, or that the iron introduced as impurity was in a less active form than the iron added as ferrous sulfate or ferrous pyrophosphate. Hence all the results are given in terms of iron added. Earlier work with GR-S types has shown t h a t the degradation encountered in the presence of metals was due to oxidative cross linking and resinification of the polymer ( 1 ) . The accelerator responsible for this degradation or gelation of the polymer was believed t o be the metal soap formed with the fatty acid released in the coagulation process. Copper, manganese, and iron stearates or oleates were soluble in the polymer and carried over into the rubber, where they catalyzed the degradation process. I n these studies iron was found t o be a more active degradation catalyst than copper. In the work with high viscosity polymers blended with oil extenders this order is reversed, and is in agreement with the more generally accepted order of activity ( 3 ) . Because the polymer deterioration was evident as a viscosity decreasedecrease as great as 60% was observed-with no detection of gel foxmation, the mechanism is believed t o be a chain scission reaction. With chromium and manganese the viscosity change was indicated first as a slight rise above the level of the base polymer, followed by a rapid decrease. One might interpret this as a breaking of cross links or branches in some of the molecular
Table VI.
Effect of Added Ferrous SulfaLe on Retention of Viscosity
25
( H R ) AT I5O.R
Degradation of masterbatch with added ferrous sulfate
Table VI and Figures 3 and 4 show that the polymer degradation observed during aging a t 150' F. is greater as the quantity of added iron is increased. An increase above 0.050 part of added iron has little additional effect. Table VI shows that the degradation is observed to a similar degree when the iron is added to the latex as a dilute ferrous sulfate solution or as the ferrous pyrophosphate complex which is used as a polymerization activator, The latter technique waa necessary for the larger additions of iron t o avoid coagulum formation in the latex. The data presented in Table VI1 confirm the observation that degradation is more pronounced with Sundex 53 than with Circosol 2 XH. This has be,en attributed to difference in "aromaticity" of the two oi1s (10). Degradation in the presence of dioctyl adipate, an ester-type plasticizer, is appreciably less than with either of the petroleum-based oils used. As in Table IV, where copper was causing polymer breakdown, elimination of the oil extender also minimizes the viscosity change during aging. Although minor variations were observed with the different antioxidants used-Le., BLE, Stalite, and Polygard-no antioxidant appeared superior in all tests. A study of the aging of ram polymer during storage at room temperature shows iron to be far less active than copper as a
341
(45 parts Sundex 53, 1.25 parts B L E ) FeSO4. 7Hz0 No. of Vacuum Added P a r t ' R u n s Dried Nil 0.005 0.010 0.025 0.050 0.080 0.100 0.250
3
1
3
1
2 1 2 3
3.5
6 5
94.0 96.0 96.2 89.6 86.8 85.8 89.7 91.7
94.6 97.3 93.8 86.9 85.8 82.3 87.5 87.2
92.9 92.8 95.6 92.8 93.9
.. ,
94.0 97.4
0,100
Table VII.
Softener (45 Parts) Sundex 53 Sundex 53 Sundex 53 Circosol 2 X H Circosol 2 X H DOA DOA Nil
20
25-
... ...
92.9 93 9 87.1 74.6 73.6
... , . , ... . .. ...
... . .. ,.. ...
87.2 89.8 72.5 61.8 fil.5 59.0 69.2 66.5
...
...
... .., , . .
72.1 75.7
..,
K ~ ~ , -Dried o ~
Hours a t 1503 F.___ 3.5 6.5 15 25 Yo Retention of~Viscosity ____~_
Nil 0.064 Nil 0.127
84.7 88.9 86.7 92.0
Vacuum
0.050 0.050 0 100
Hours a t 150° F. 10 15
% Retention of Intrinsic Viscosity
94.4 93.4
...
94.0
83.7 87.8 85.1 89.9
73.6 73.5 70.0 74.2
_
62.0 60.9 68.6 69.8
Comparative Aging with 0.10 Part of Added Ferrous Sulfate Antioxidant (1.25 Parts) BLE Stalite B Polygard LE
NO.
of
Runs 2
1
1 2 2 1
Eggard Polygard 1 BLE
1
3 5
Hours a t 150' F. A.5 15,
Yo Retention of Viscosity -89.6 86.8 84.6 92.8 91.2 101.1 101.1 97.0
87.5 77.5 78.7 90.9 92.5 97.4 97.4 98.0
72.1 73.1 68.1 86.9 89.9 94.3 94.3 95.6
25
69.2
...
57.5 82.9 88.0 93.6 93.6 91.6
INDUSTRIAL AND ENGINEERING CHEMISTRY
348
c '--I
gloo rn
t.
8 6.5 HR.. AT 150'R 4 IS HR. AT 1 5 0 O f
Vol. 48, No. 2
to the oxygen attack, or acting as a carrier to get more nictal complex into the polymer in a soluble and hence active form. An interesting observation from the data shown in Table IX (compounding recipe in Table X) is that the physical pioperties of vulcanizates made from degraded stock are not greatly inferior to those from unsoftened stock. Retention of these physical properties during vulcanizate aging was almost identical in both cases. Even in pol3 mers degraded to as low as 20 Mooney viscoqity, microtesting has shown remarkably good retcrition of physical properties.
Table IX.
Physical Properties of Softened and Standard Kr ynol
$ 8 O0
Figure 4.
-025 *050 * 075 PARTS ADDED Fe 9 0 4 . 7 H e 0
e100
Effect of added ferrous sulfate on viscosity retention at 150" F.
networks, perhaps 1 to 2% of which are initially insoluble. As they become soluble, they contribute to the viscosity and the viscosity increases. Unfortunately, the limitations of the technique used for pel measurement are greatest in the samples of low gel content and the authors were not able to confirm this trend by such measurements. All the samples prepared and aged a t 150" F. for 25 hours showed less than 3% toluene-insoluble polymer by the Harris cage technique. When subsequently aged a t 140" F., the polymer, softened in the presence of copper, showed onset of gel formation more rapidly than an unsoftened sample (Table 17111). These effects are observed even in the presence of a good antioxidant surh as B1.E.
Table VIII. Days Aged
n
7 14 28 42 Remaiks
hlon ths Aged
Effect of Aging of Softened and Unsoftened Oil Masterbatch 70
R
%
Gel A
Gel C
Gel B
Accelerated Aging, 140' F. 1.4 n 4.8 1.5 (18.7) 0.4 6.2 1 9 19.5 17.9 Standard production
%
Gel A
1.1 7.0 28.9 30.7 57.4
Pilot plant sample not heatsoftened R21J/4',
A
Pilot plant polynier heatsoftened with copper present
%
Gel C
lIL/4', C
Shelf -4ging a t Roo111 Tempel atlire
Preliminary experimental work on the oxygen absorpt,ion of these masterbatches in the presence of iron shon-s very large oxygen uptake per gram of samples, approximat>ely three to four times as great as with standard GR-S types. This is an indication that the degradat,ion is oxidative in type. Degradation in the presence of both copper and iron is much more pronounced when the polymer is extended with oil. This suggests that the oil is facilitating Lhc chitin scission reaction. It may be functioning in either one or both of two ways: simply spreading out the polymer chains, making them more susceptible
Sample hIL/4' Unsoftened When tested Compound Tensile (50-min. cure) a t 292' F. elongation (50-min. cure) 300% modulus (50-min. cure) Shore hardness Extrusion Rate, g./sec. Ratio, g./inch Added FeS01.7H20, part
T-8 i %58
Krynol
56.5 86.5 63.0 3350
54.0 38.5 41.5
Zion
180
550
iwn
1650 49
51
13 28 n 413 Nil
18 13
0 389 0 925
~
Table X.
Recipe (Banbury RIixing)
Masterbatch Philblack 0 S B S stearic acid NBS zinc oxide Santocure NBS Sulfure
Parts 1nn.n
n o
1.38 3.45 0.86 1.38
The significance of this work is that appreciable degradation of inasterbatches has been detected with amounts of added iron no larger than the quantity present in most sugar-free recipes used for low temperature polymerization. This softening can be minimized by using a low iron or iron-free polymerization formula for preparation of high viscosity polymers which are to be oil extended, It also emphasizes the necessity of avoiding metal contamination in the coagulants for these products. 4CKNOWLEDGBIENT
The authors are grateful to the Polymer Gorp., Ltd., for permission t o publish these results. The compounding and testing work was handled under the supervision of E. B. Storey. LITERATURE CITED
Albert, H. E., Smith, G. E. P., and Gottschalk, G . W., IXD. ENG.CHEW 40, 482 (1948). Dabald, A. R., Rubber Age 71, 621 (1952). Dufraisse, C., in "Chemistry and Technology of Itubber," ed. by Davis and Blake, p. 513, Reinhold, New York, 1937. Farmer, E. H., T r a n s . Inst. Rubber Ind. 21, 122 (1954). Harrington, H. D., Weinstock, I
