1524
INDUSTRIAL AND ENGINEERING CHEMISTRY
may produce porosity and disrupted carbon-carbon bonds, and thus prevent the development of graphite from these materials. These conclusions are, in general, eiinilar t o those of Franklin (3). However, her classification of carbons into t v o classes, graphitizing and nongraphitizing, is not borne out for materials used in this n-ork. The curves of Franklin for the t v o classes of carbons are drawn in Figures 5 and 6. It is evident that very fern of the authors' specimens lie on the lower curve for graphitizable carbons. There was no difference in crystallite sizes of the lowtemperature specimens of cokes which graphitized n-ell, as compared with cokes which did not graphitize a t 4650' F. The curve shovn by Franklin for graphitizable carbon would indicate a greater increase of the C dimension than of the A dimension. This, however, has not been observed with either the cokes or the carbon blacks used in these studies. Franklin advances the concept of graphitization by movement of whole layers of carbon
Vol. 46, No. 7
atoms, with a higher probability of their being oriented parallel to the basal plane, rather than becoming attached t o the cdgeatoms of a layer. However, the greater growth of the layer dimension x-ould indicate that in graphitization the layers join edgewise more frequently than by alignment of lagcrs in stacking. LITERATURE CITED
(1) Biscoe, J., and JTarren, B. E., J . A p p l . Phys., 13,364 (1942). ( 2 ) Franklin, R. E., Acta Cryst., 4 , 253 (1951) (3) Franklin, R. E., Proc. Roy. Soc. London, 209A, 196 (1951).
(4) Jones, F.W., Ibid., 166A, 16 (1938). ( 5 ) Ross, P.A . , Phus Rev.,28, 425 (1926). B. E. 9, 693 (1941). (6) (7) JVilson,A. J. C., "X-Ray Optics," London, RIethuen BC Co., 1949. R E C E ~ V E Dior
revlew September 23, 1963 A C C Work peifoimed under AEC contract WS045 eng 92.
E ~ ~J,,,,,~ E ~
11, 1954.
ondiscoloring Stabilizers for B. A. HUNTER, R. R. BARNHART, .INID R. L. PROVOST .Vuugutuck Chemical Division, United States Rubber Co., Yaugutzwk. Conn.
T IS &ellk n o w that Helea rubbw contains small quantities of naturally occurring substances which function as antioxidants and which effectively retard the deterioration of the rubber hydrorarbon. Removal of these substances by acetone extraction is reported to leave the rubber in a state in which it is readily susceptible to oxidation and the formation of tacky substances (8). These naturally occurring antioxidants are considered adequate for the protection of Hevea rubber during the storage period preceding the processing and compounding of the uncured rubber, but the addition of conventional antioxidants is required for the preservation of vulcanizates against the deleterious effects of heat, oxygen, and mechanical stress. Similarly, it has been demonstrated that uncured synthetic rubber polymer of the butadiene-styrene class (GR-S) is highly unstable tonard oxygen unless a preservative is present (8). This susceptibility to o3idation is particularly critical a t elevated temperatures. Indeed, unless unusual precautions are taken, GR-S polymer containing no stabilize1 \\ill undergo iesinification nhen exposed to the heat of the drying oven during manufacture. Inadequately stabilized GR-S has been kno\Yn to ignite when exposed to normal drying temperatures (180" to 220" F.). The destructive effects of heat on GR-S polymer are particularly noticeable in processing manipulations involving elevated temperatures. The benefits of stabilizers in the polymer are readily demonstrated in tests involving exposure to heat. T o be effective in protecting the polymer duiing drying, the stabilizer must be added to the GR-S latex prior to flocculation and washing. This procedure imposes several important requirements in relation to the physical and chemical properties of the stabilizer. The stabilizer must first of all be insoluble in ~ a t e in r order to allow complete retention in the polymer duiing the flocculation and washing operations. The stabilizer must be Eufficientlv resistant to chemical change to prevent undue loss during incorporation and also to avoid the formation of undesirable pioducts in the polymer. Preferably, the stabilizer should be an easily emulsifiable liquid to permit easy and thorough incorporation in the latex. For utilization in the widest range of applications the stabilizer must cause a minimum of discoloration and should not produce staining of lacquers or other materials with which the polymer may come in contact.
HISTORICAL
The chemicals which \Yere adopted in the United States as GR-S stabilizers early in the government synthetic rubber program (1940-1941) mere compounds of the aromatic amine type and included phenyl-2-naphthylamine and a diphenylamineacetone reaction product. Both materials are strongly discoloring. The effects of variations in concentration on the oxygenabsorption propertie- of GIZ-S polymers and vulcanizates have been studied (18). Subsequently, an alkylated diphenylamine was introduced which, though somewhat less effective as an antioxidant, is considerably less discoloring and staining. Recognition of the need for completely nonstaining stabilizers for GR-S polymers intended for the manufacture of white or lightr colored goods or products for use in contact with surfaces subject to stain necessitated a search for chemicals to mcet these applications. Departure from the classical aromatic amine type was dictated by the inherent staining properties of this class. "Oxycresyl camphene" (reaction product of cresol and camphene) v a s an early nondiscoloring stabilizer considered useful by German technologists ( 4 ) . Triphenyl phosphite (IO) was introduced into the GR-S program in 1945 as the first commercially successful nonstaining Stabilizer for GR-Spolymer (GR-S X-178; later GR-S 25). This material is extraordinarily effective in preventing heat deterioration of GR-S and was widely used in polymers employed in light-colored products. Although adequate for many applications, triphenyl phosphite suffered from the disadvantage of undergoing slow- hydrolysis in the presence of water, nhich resulted in the development of a phenolic odor in the rubber and other difficulties. Cresol sulfides were found to be moderately effective as stabilizers and ere developed to overcome some of the objectionable featuies of triphenyl phosphite. The cresol sulfides were used in considerable amount in spite of imperfect nondiecoloring properties. Tests on numerous phenolic compounds indicated this class of materials to be of interest where minimization of discoloration is important and where high heat resistance is not recognized as LL critical factor. Styrenated phenols are completely nondiscoloring and have been used in large volume as GR-S stabilizers. A comparison of alkyl phenols as nondiscoloring antioxidants for GR-S has been published (11 1.
July 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
1525
The extraordinary effectivvness of the aryl phosphite class of stabilizers led to renewed consideration of this class. A very substantial improvement in the hydrolysis resistance of these materials was accomphhed in the development of higher alkylated aryl phosphites. The improved characteristics of the new materials permit their addition to the GR-S blend tanks by conventional emulsion addition tcchniques with no sacrifice in stabilizing quality. Moreover, the hydrolysis products, if any are produced, are low in odor arid exhibit a measure of stabilizing capacity comparable to the previously described phenolic stabilizers. One of the alkylated aryl phosphites has been selected on the basis of optimum balance of properties for successful commercial application. This product is liquid under ordinary conditions and is readily emulsifiable.
I
FUUCTIOY OF STARIZIZERS
The primary function of a GR-S stabilizer is to protect the unvulcanized polymer during manufacture, storage, and processing operations prior to vulcanization. In regard to the protection of GR-S vulcanizates against aging, the benefits of antioxidants are generally lees marked than is the case with natural rubber vulcanizates. However, it has been shown in the present work that under the severe aging conditions provided in the oxygen bomb test it is possible to demonstrate differences in the effectiveness of GR-S stabilizers as antioxidants in cured GR-S vulcanizates. The effect of inadequate stabilization of GR-S in the polymer manufacturing plant, usually becomes evident in several ways. Softening of the polymer in the dryer may cause fluctuations in Mooney viscosity control. Variations in gel content may occur. Partial resinification in the dryer may produce melted particles which can remain to foul the dryer screens or to contaminate the polymer. The presence of hard particles in the polymer bale is usually traceable to improper stabilization. Inadequate stabilization of GR-S polymer may also become evident during the processing of the polymer. Improperly stabilized GR-S is especially susceptible to degradation during processing operations such as hot mill breakdown, high temperature Banbury mixing, and high temperature extrusion. The benefits of an effective stabilizer show up as improved Mooney viscosity control and retardation of gel build-up in the raw polymer. The ultimate result of the effects of inadequate stabilization during either manufacture of the polymer or subsequent fabrication is an inferior finished article.
OO
4
8 12 HOURS AT 130.C.
I
16
20
24
Figure 1. Effect of Heat (130’ C.) on Hevea and GR-S Polymers
DISCUSSION
Although the selection of GR-S stabilizers from classes of chemicals known to have antioxidant properties in natural rubber vulcanizates has had some measure of success, it is interesting to consider the fact that uncured Hevea behaves very diflerently from GR-S polymer in certain important respects. This behavior difference is exemplified in a comparison of the Mooney viscosity changes of the two rubbers during oven aging (Figure l), Heating of natural rubber smoked sheet in the 130” C. oven results in a gradual softening of the polymer, whereas unstabilized GR-S undergoes an opposite and hardening process under the same conditions. The shape of the Mooney viscosity curve of the synthetic oolymer is considerably altered by the presence of a stabilizer such as the widely used acetone-diphenylamine reaction product. Piper and Scott ( I S ) have pointed out that the role of oxygen in the breakdown of GR-S polymer appears to differ, a t least in 8ome degree, from the role of oxygen in the breakdown of natural rubber. Although the natural polymer may be softened by heating in the absence of oxygen, GR-S heated in the absence of air becomes increasingly harder ( I S ) . This fact has been recognized by Hagen (9),who has described the German “thermal softening” of Buna S by heating the polymer for a limited period
r
I
0
4
Figure 2.
8
12 16 20 24 28 RESINIFICATION TIME- HOURS AT 130°C.
32
Effect of Stabilizer on Resinification of Polymer Stabilizer added to GR-S 1503 latex
INDUSTRIAL AND ENGINEERING CHEMISTRY
1526
140
I20
p 0
E
s2
80
&
H
60
40 0
I
2
3
4
5
6
7
Vol. 46, No. 7
mination on polymer subjected to oven aging or hot milling. The 30-minute 300" F. milling is a severe test which provides an indication of the limits of the protective function of the stabilizer under severe conditions of processing and use of the polymer. Degradation of GR-S polymer subjected to heat is also reflected in lowered quality of resulting vulcanixates as indicated in tensile, modulus, elongation, and cut growth properties. The standard vulcanixate aging tests demonstrate the antioxidant properties of stabilizers in GR-S vulcanizates. Experience in the evaluation of GR-Sstabilizers has shown that the degree of variation in test results is indicative of significant differences only when the comparative differences are of considerable magnitude. Unstabilized GR-Scontrols are susceptible t o great variations, depending on conditions of drying and storage. Ease of resinification in improperly stabilized polymer increases the possibility of inhomogeneity in samples and makes for variable test results.
DAYS AT IOO'C
Figure 3.
Effect of Heat Aging (100' C.) on Mooney Viscosity
Thermal decomposition of these hydroperoxides may directly or indirectly produce free radicals. The molecule containing the hydrocarbon free radicals can either split (depolymerization or chain scission), link with another molecule (cross linking), or link with itself (cyclization). The depolymerization reaction results in polymer breakdown and softening, \Thereas t,he cross-linking and cyclization reactions cause gelation and hardening of the polymer. In the case of isoprene polymers such as natural rubber, the chain scission reaction is the predominant process. I n the case of butadiene polymers such as GR-S, the chain scission reaction is soon overtaken by the cross-linking reaction and the hardening effect becomes predominant (3). The differences in behavior between the isoprene and butadiene polymers have been attributed t,o the influence of the methyl group in the structure of the former ( 3 ) . The cross-linking and cyclization processes which occur in GR-S polymer subjected to heat lead to the formation of a benzene-insoluble gel with a high molecular might. This insoluble gel fraction swells in benzene and tends to break down under shear, either to soluble molecules or to dispersible units. White and coworkers (17 ) have examined gel as a definitive property in GR-S technology and have concluded that the portion of the high molecular weight mat'erial which exhibits low swelling volume in benzene (tight gel) markedly affects processing behavior and quality of product. Build-up of tight gel adversely affects tensile and elongation and is particularly deleterious to cut growth resistance of vulcanizates. It has been recognized that for highest quality GR-S vulcanizates, conditions which result in gel build-up during manufacture or processing of the raw polymer must be avoided (16, 17). The comparative effects of several GR-S stabilizers on gel build-up during hot milling of the polymer are described later in the experimental section (see Figure 9). The differences in the behavior of natural rubber and GR-S polymers when subjected t o heat suggests that the search for the most effective stabilizers for the synthetic polymer might be directed toward compounds not commonly used as antioxidants for Hevea.
' involves the formation of a hydroperoxide.
STABILIZER EVALUATIOS T E S T S
The tests which meye employed in the present work were designed, for the most part, to measure the effectiveness of GR-S stabilizers in protecting the unvulcanized polymer from the deteriorating effects of heat. The 130' C. resinification test is a simple and rapid method of selecting chemicals which have stabilizing properties. More precise measures of the extent of heat degradation can be had in Nooney viscosity and gel deter-
EXPERIMENTAL MEIHODS A S D R E S U L T S
Three of the nondiscoloring stabilizers which have attained commercial use were evaluated in the present comparison. d moderately discoloring and a standard discoloring stabilizcr were included in the tests: diphenylaminr-acetone reaction product (discoloring); heptylated diphenylamine (moderately discoloring); cresol monosulfide (practically nondiscoloring); styrcnated phenol (nondiscoloring); and alkylated aryl phosphite (nondiscoloring). r
, 2 4
Figure 4.
4
8
I 12 16 HOURS AT 130'C
I
20
24
Effect of Heat Aging (130" C.) on 3Iooney Viscosity
The stabilizers were added in emulsion form to freshly prepared unstabilized GR-S latex prior to flocculation. GR-S 1503 (without the usual stabilizer) was chosen as the base polymer. (GR-S 1503 is a cold butadiene-styrene copolymer containing 23.5% bound styrene and polymerized a t 41" F. with fatty acid soap emulsifier. Potassium dimethyl dithiocarbamate is used as a shortstop.) The usual salt-acid flocculation technique Tvas employed and the polymers were dried in an oven a t 65' to 75" C. until completely dry (12 to 16 hours). A portion of the unstabilized latex was flocculated and the resulting polymer is referred to as the blank in certain tests described below. This blank was dried very carefully a t room temperature t o avoid deterioration encountered under normal conditions of drying. In spite of these precautions, tests on the unstabilized polymer were erratic in some cases. The addition of GR-S stabilizers to dry unstabilized polymer does not afford a consistently reproducible method of comparison and was, therefore, avoided in this work. Discoloration Tests. The polymers prepared with each of these stabilizers were compounded with the ingredients of a
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1954
5
7
6
6 8 HOURS AT 130'C
DAYS AT IW"C
Figure 5.
Effect of Heat Aging (100' C.) on Gel Build-up
'j
ACETCN~-
DIPHENYLAMINE PIP--.. I
5
Figure 7 .
I
12
Effect of Heat Aging (130" C.) on Gel Build-up
1 25
30
Effect of Hot Milling on Mooney Viscosity
typical white rubber composition as shown in the following table. P a r t s b y Weight
100.0 2.0 10.0 10.0, 1.75
Polymer Sulfur Zinc oxide Titanium dioxide Benzothiazyl disulfide
The stocks were mixed on a rubber mill and cured a t 292" F. for 50 minutes. Samples of the cured stocks were dipped in a white lacquer to one third of their length and mounted for light exposure tests. A strip of cotton fabric placed in contact with the unlacquered portion permitted an examination of the staining characteristics of t,he stabilizers. Cardboard masking of a section of the remaining unlacquered strip allowed a basis of comparison with the unexposed vulcanizate. Samples were exposed to a General Electric sunlamp for 24 hours. Results of the tests are shown in Table I. The severe discoloration and staining features of the acetonediphenylamine reaction product, are evident. Considerably less
TABLE I. DISCOLORATION AND STAINING TESTS (Sunlamp exposure) Discoloration Unexposed Exposed Nil Nil Severe Severe Nil Slight Nil Nil
IO
~
I
IO 15 20 MINUTES MILLING AT 300°F
Blank Acetone-diphenylamine reaction product Heptylated diphenylamine Cresol sulfide Styrenated phenol Alkylated aryl phosphite
Figure 6.
1527
Slight Slight Nil Nil
Cloth Nil Severe
Staining Lacquer Nil Severe
Slight Nil Nil Nil
Moderate Slight Nil Nil
MINUTES MILLING AT 300'F:
Figure 8.
Effect of Hot Milling on Gel Build-up
discoloration and staining are achieved with the heptylated diphenylamine. The slight pigmenting effect of the cresol sulfide stabilizer is evident. Styrenated phenol and the alkylated aryl phosphite are nondiscoloring and nonstaining. Resinification Test. Portions of the polymer crumb were sheeted on an open mill to a thickness of inch and circular samples (1.5 inches in diameter) were cut and placed in a 130' C. circulating air oven. The samples were examined each hour and the time was noted for the formation of a resinified surface film which cracked easily when the cooled sample was stretched. The resinification times on the polymers containing the various stabilizers are shown graphically in Figure 2. The unstabilized polymer is shown for comparison. The unusual effectiveness of the phosphite in preventing heat resinification is noteworthy, Effect of Heat on Mooney Viscosity of Raw Polymer. 100' C. OVEN.AGING. Regular Mooney viscosity samples (seven from each polymer) were cut and placed in the 100" C. oven allowing one sample for each of 7 days' aging. Mooney viscosities taken after the designated aging periods are plotted in Figure 3. The polymers containing cresol monosulfide and styrenated phenol exhibit extreme changes in Mooney viscosity, whereas the alkylated aryl phosphite controls the Mooney viscosity similarly to the more discoloring stabilizers. 130" C. OVENAGING. Similar tests a t 130" C. gave Mooney viscosity curves as shown in Figure 4. The aging periods mere from 2 t o 10 hours a t this temperature. At the shorter aging period a definite initial drop in viscosity is evident for the polymer stabilized with the diphenylamine-acetone reaction product, Here again, greater Mooney viscosity fluctuation for the polymers containing cresol sulfide and styrenated phenol is observed as compared to those containing the alkylated aryl phosphite.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1528
Vol. 46, No. 7
sl
iT =lF
3600
--
3200 I
I
gtl
2800
w
d 2400
Ec
2033
MINLTES MILLNG AT .?COT
Figure 9.
Effect of Hot Milling on Swelling Index
% G E L * 23 SWELLING* INDEX 63 NORMAL MILLING
28
54
46 17 -75'MILLING AT 300'F
525 28
24
69
62 13 30' MILLING* AT 300'F
[Figure 10. Effect of 300' F. Milling of Polymer on Tensile of Vulcanizate Correlation with gel and swelling index
NORMAL MILLING W'MILLING AT 3oo.F
Figure 11. Effect of 300' F. Rlilling of Polymer on 300% Modulus of Vulcanizate Effect of Heat on Gel Build-up Polymers. AT 100" C. Portions of the polymers heated in the 100" C. oven for periods from 1 to 7 days mere subjected t o gel determination using the method of Baker and Muller as described by White and coLTorkers ( 1 7 ) . The data shown in Figure 5 indicat? considerable variation in the effects of the stabilizers on the gel build-up with heating. The alkylated aryl phosphite and the diphenylamine-acetone reaction product are most effective in retarding gel formation. AT 130" C. Similar gel determinations on the 130' C. ovenaged samples gave the data plotted in Figure 6. Here again the diphenylamine-acetone reaction product and the alkylated aryl phosphite appear most effective in controlling gel build-up. The erratic behavior of the blank illustrates a difficulty in sampling resinified unstabilized polymer. Effects of Hot Milling of Polymers on Mooney Viscosity and Gel. I n order to investigate the effects of various stabilizers on Mooney viscosity and gel variation in GR-S polymer subjected to hot milling, portions of each polymer were hot-milled on an open mill a t 300" F. for 5 , 10, 15, 20, and 30 minutes and Mooney viscosity and gel tests were conducted on the r e d t i n g stocks. MOONEYVISCOSITYVARIATIONS.Changes in Mooney viscosity values with hot-milling treatment for the polymers containing the various stabilizers are shown in Figure 7. An initial Mooney inereasp is characteristic of all polymers and is particularly great in the case of the styienated phenol and the cresol sulfide stabilizers. The effective stabilizing action of the alkylated aryl phosphite through the hot-milling treatment is noteworthy.
NORMAL MJLLING
33'MILLING AT 300'F.
Figure 12. Effect of 300' F . Milling of Polymer on Per Cent Elongation of Vulcanizate
GEL BUILD-CP. Gel determinations on the hot-milled polymers gave the data plotted in Figure 8. Comparatively low values are obtained for the phosphite polymer and the two arylamine-stabilized polymers, whereas the polymers containing the styrenated phenol or the cresol sulfide show the build-up of considerable gel. SWELLING INDICES. The gel deteimination method used ( 1 7 ) permits the determination of the smelling index of the gel fraction and affords a classification of the gel as a loose or tight gel as described by White and coworkers ( 1 7 ) . These investigators have shown that swelling indices below 50 indicate tight gel and indices above 70 indicate only loose gel. As tight gel has a marked effect on the quality of the vulcanizates prepared from the polymer, it was of interest t o examine the effect of 300" F. milling on the swelling indices of the polymers. The data are plotted in Figure 9. Examination of the curves shom loiv swelling indices (tight gel) for the cresol sulfide- and styrenated phenol-stabilized polymers after oiily 10 minutes of hot milling, u hereas the alkylated aryl phosphite and the diarylamine stabilizers do not permit the formation of appreciable tight gel until after 30 minutes of hot milling. Effects of Hot Milling of Polymers on Physical Properties of Cured Vulcanizates. Separate portions of each of the polymers
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1954
D
b '
HEPTYLATEO DPA
Oo2
?
'
HENOL
,
1
2
52.5
28
34
17
8
46
Om4 Ome CUT GROWTH (INCHES PER KILOCYCLE)
1529
showed a four- to sixfold increase in cut growth, while the alkylated aryl phosphite and the arylamine stabilizers prevented deterioration of the cut growth properties. The gel figures and the corresponding swelling indices of the 30-minute hotmilled polymers again correlated well with the cut growth results. High gel with low swelling index is characteristic of the polymers with poor cut growth properties (see Figure 13).
X'MlLLlNG AT 3OO'F NORMAL MILUNG
Figure 13. Effect of Hot Milling on C u t Growth Correlation with gel and swelling index
were milled on an open mill a t 300' F. for 7.5 or 30 minutes. These hot-milled polymers, as well as portions of each polymer milled on a cold mill in the normal manner, were compounded in the tread recipe shown in the following table. GR-S polymer EPC black
Zinc oxide Sulfur Benzothiaayl disulfide
UNAGED
UAGED n mms AT roo"c.
Parts b y Weight 100.0 40.0 5.0 2.0 1.75
m M E D 96 H W S IN O X I F E N SCPC.)
Figure 14. Tensile Properties of Aged Vulcanizates
The compounded stocks wwe cured for 30, BO, and 90 minutes Aging Test on Vulcanizates from Normally Milled Polymers. a t 292' F. The tensile properties were considered optimum on The vulcanizates prepared from normally milled polymers were the 60-minute vulcanizate and the data plotted in Figures 10, 11, subjected to oxygen bomb aging (96 hours at 80" C.) and to oven 12, and 13 are based on this cure. The tensile data on the three aging [72 hours a t 100' C. (212' F . ) ] . The tensile data are shown cures are shown in Table 11. in Table 111. The 60-minute cure appeared optimum and the The alight drop in tensile strength after 7.5 minutes of hot unaged and aged values for these vulcanizates are plotted in milling (Figure 10) shows little difference between the stabilizers Figure 14. I t ia evident in comparing the unstabilized stock with in this short heat treatment. The more extended 30-minute the other stocks that each of the stabilizers offers some aging hot-milling treatment, howevctr, brings about a marked deterioraprotection to the vulcanizates. With the exception of the acetion of tensile properties of the unstabilized polymer and of the tone-diphenylamine stock. the 100' C. (212' F.) oven aging of polymers containing cresol sulfide and styrenated phenol. The the vulcanizates does not bring out substantial differences bealkylated aryl phosphite is exceptionally effective in maintaining tween the stabilizers. I n the oxygen bomb test, however, the the tensile strength a t a high level. unstabilized stock and the stock containing styrenated phenol The relationship between tensile properties and gel and swellaged more poorly than the vulcanizates containing the alkylated ing index of 30-minute hot-milled polymer is also indicated in aryl phosphite and the cresol sulfide. None of the nondiscolorFigure 10. High gel with low swelling index correlates well with ing stocks aged as well as the arylamine-stabilized vulcanizates. poor tensile properties and vice versa. This is in agreement with the observations of White and coworkers (17). SUMMARY The 300% mo3ulus properties (Figure 11) and elongation Chemical stabilizers vary widely in their effectiveness in the properties (Figure 12) of the vulcanizates from the 30-minute protection of unvulcanized GR-S polymer. These differences are hot-milled polymers show differences between stabilizers correparticularly apparent in heat exposure tests. sponding to the differences noted in the tensile data. The cresol In protection of GR-S polymer against heat resinification, sulfide polymer gave particularly poor results in these tests. &looney viscosity changes, and gel build-up, the nondiscoloring The advantages of the aryl phosphite over the other nondisalkylated aryl phosphite is comparable to the discoloring arylcoloring stabilizers are evident. amine types and superior to the other nondiscoloring stabilizers. Cut growth tests were conducted on these vulcanizates with the U. S. Rubber bending- machine (14). The cracks were initiated b y means of a modified Precision Penetrometer usOF HOTMILLIXCON TEXSILE PROPERTIES TABLE 11. EFFECT ing a 0.045-inch diameter needle. The Minutes Cure a t 292' F. cut growth data (Figure 13) showed 30 60 90 30 60 90 30 60 90 little differences in the vulcanizates from Stabilizer Not Hot-hIilled 7 5-Min Hot-Milled 30-Min Hot-Milled polyrners not subjected to hot After a 30-minute hot milling of the polymers, however, the vulcanizates from unstabilized polymer " as those from the -polymers containing cresol sulfide and styrenated phenols
Acetone-diphenylamine 2470 reaction product Heptylateddiphenylamine 2590 Cresol suifide 2560 ~$~%da$?$x~hite Blank
2320 2890 2450
3330
337;
2480
3100
3220
1710
26iO
2630
3470 3200 3390 3360 3340
3270 2660 3230 3150 3340
2880 2350 2440 2930 2300
3330 3010 3080 3300 3040
2930 3070 2990 2700 2800
1750 1430 1550 2320 1360
2550 2050 2080 3000 1960
2490 2180 2180 2730 1990
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
1530
REFERENCES (1) Bolland, J L., and Gee, G., Trans.
TABLE 111 TSY,ILEP X O P E R T I E S O F NORXrlL VULCANIZATES 30
Stabiliier
60
-
90
Unagcd
Acetone-diphenylamine reaction product Heptylated diphenylamine Cresol sulfide Styrenated phenol Alkylated aryl phosphite Blank
Minutes Cure a t 292> F 30 00 90 72 Houis at 100’ C (212’ F ) .
2470
3330
3370
2610
2570
2530
2590
3170 3200 3390 3360 3340
3270 2660 3230 3160 3340
2000 2000 1930 1890 1370
1920 1820 1620 2010 1470
1630 2020 1910 1890 1430
25fiO
2320 2890 2450
Deleterious changes which occur in raw GR-S polymer subjected t o heat are reflected in the physical properties of the resulting vulcanizates. Differences show up markedly in tensile, modulus, elongation, and cut growth. The alkylated aryl phosphite shows advantages over the other nondiscoloring stabilizers in these tests. izll stabilizers tested act as antioxidants in the GR-Svulcanizates to some degree. The differences between the nondiscoloring stabilizers were not appreciable in the 100’ C. heat aging test. The aryl phosphite affords best protection among the completeIy nondiscoloring stabilizers in the oxygen aging test. ACKNOWLEDGMENT
Appreciation is expressed to C. D. McCleary, F. L. Holbrook. W, F. Tuley. and L. H. Homland for advice and encouragement and to J. A . Reynolds for assistance in the preparation of the charts.
Vol. 46, No. 7
Faraday Soc., 42, 236 (1946); Rubber Chem. and TechnoZ., 20,609 (1947). (2) Bruson, H. il, Sebrell, L. B., and Vogt, W. W., IND. ENG.CHEM.,19, 1187 (1927). 1170 2400 2600 (3) Cole, J. 0 , and Field, J. E., Ibid., 39, 174 (1947). 2000 2690 2600 (4) D a m o n , T. R., Brit. Intelligence Ob1500 2060 2060 620 1230 1330 j e c t i v e 9 Subcommittee, London , 1600 2050 2230 “The Rubber Industry in Germany 680 650 650 during the Period 1939-1945,” Overall Rept. 7, p. 48. ( 5 ) Farmer, E. H., Trans. Faraday Soc., 42, 228 (1946). (6) Farmer, E. €and I., Associates, Ibid., 38,348 (1942). (7) Farmer, E. H., and Sundralingam, A., J . Chem. Soc., 1943, 125. ( 8 ) Glazer, E. J., Parks, C. R., Cole. J. 0.. and D’Ianni. J. D.. ISD. ESG. CHEM..41.2270 (1949). (9) Hagen, H., Kautschuk, ‘14, 203-10 (Sovember 1938) ; India Rubber World, 108, 45 (1943) (10) Howland, L. H., and Hunter, B. &\., U. S. Patent 2,419,:%4 (April 22, 1947). (11) Kitchen, L. J., Albert, H. E., and Smith, G. E. P.. Jr.. I w . ENC.CHEM.,42, 675 (1950). (12) Moses, F. L., and Rodde, A. L., India Rubber World, 119, 201 (1948). (13) Piper, G. H., and Scott, J. R., .J. Rubber Research, 16,151 (1947). (14) Rainier, E. T . , and Gerke, R. I I . , IND.EXG.CHEM.,ANAL.ED., 7, 368 (1935). (1.5) Taylor, H . S., and Tobolsky, A, J . A m . Chem. S o c . , 67, 2063 (1945). (IG) Vila, G. R., ISD. EXG.CHEM., 36, 1113 (1944). (17) White, L. M., Ebers, E. 6 . , Shriver, G. E., and Breck, S., Ibid., 37, 770 (1945). (18) Winn, H., and Shelton, J. R., Ibid.,40, 2081 (1948). RECEIVED for review September 14, 1953. ACCEPTED February 26, 1954.
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30 BO 90 96 Houis Oxygen Bomb (80’ C )
Presented before the Division of Rubber Chemistry a t the 124th Meeting ef t h e AXERICAN CHEMICAL SOCIETY, Chicago, Ill., 1953.
JO§EPH A. TALALAY The Sponge Rubber Products Co., Shelton, Conn.
F
OAM rubber as an elastic load-carrying space filler is the result of both material development and product design. The basic material consists of a great number of freely interconnecting air cells of a spectrum of diameters ranging from 0.005 to 0.040 inch, bounded by perforated and frequently microcellular elastomeric filme. Latex foam was developed in the early thirties. Almost concurrently with its development it had been found that great benefits are derived from incorporating “cores” into the design of a foam rubber article. Cores are oblong voids of a defined geometric shape, extending on a predetermined space lattice into the body of the foam normally from one of its broadsides and terminating short of the opposite broadside. Consequently, a conventional foam molding possesses one smooth and one cored major surface. These cores, n-hich are produced by attaching metallic pins to the mold cover, not only facilitate t8he manufacture of foam rubber articles of substantial thickness by solving otherwise difficult heat-transfer problems, but they also yield a foam molding of considerably lesser weight for a given load-bearing capacity (hardness) compared with a solid space filled with homogeneous foam rubber. Despite its significant effect, core design seems to have re-
ceived only cursory attention in this country. Somewhat greater attention has been given to this point in Great Britain. MEASUREMENT OF FOAM HARDNESS
Specifications for foam rubber usually include a required hardness. Load-carrying capacity or hardneqs of a foam rubber article is measured in the ‘C’nited States by the load required to produce a 25% compressive deflection in accordance with .4STM specification ( 1 ) . The standard test is performed by determining the height of an article under a preload of 1 pound per 50 square inches. Then a round flat disk 50 square inches in area, while traveling a t 25 inches per minute, is forced into the material to a height equal t,o 7570 of the original (preloaded) height. The force required to do so is read in pounds. I n view of the importance of this test to the industry, automatic machinery to perform the test rapidly has been developed. I n addition to machines designed and built by the major foam manufacturers for their own use, commercial units are available from Ferry Machine Co., from Scott Testers, Inc., and others. The Scott-Toledo tester (Figure l), the most widely used machine, is essentially a scale with an inverted beam on which is mounted a hydraulic cylinder. At the l o m r end of the cylinder, on a ball joint, is mounted a circular flat plate 50 square inches in area. The article to be tested is placed on a stationary perforated table
