SeDtember. 1930
I N D CSTRIAL A N D ENGIh7EERISG CHEMISTRY
1001
Surface Application of Age Resisters to Cured Rubber vvs. Mill Incorporation Prior to Cure’ W. L. Semon, A. W. Sloan, and David Craig THEB. P. GOODRICH COXPANY, AKROK,OHIO
The protection afforded against accelerated aging alcohol solution of the mawas determined by applying representative hydroterial. I n every case the reoils or chemical solucarbons, aldehydeamines, phenols, primary amines, sults were beneficial. tions may be applied alkarylamines, diarylamines, and thiazines to tread Moureu, Dufraisse, a n d to the surface of rubber to stock. A comparison of surface application vs. mill retard deterioration hris long Lotte (10) have stressed the incorporation was made in a high-grade non-blooming been recognized. Kreussler idea that “antioxygens” are tread, a high zinc oxide blooming tread, a white meextremely irregular in their and Budde ( 5 ) in 1881 menchanical, and a pure-gum stock. tioned and patented t8heapprotective action. Referring For most purposes mill incorporation is preferable plication of paraffin or paraft o the surface method of apto surface application of age resisters to rubber, alfin oils for preserving cured p l y i n g “antioxygens” they though surface application will retard deterioration rubber articles. I n 1898 it ha.ve s t a t e d : ‘‘There is a even after it has once commenced. No inversion of the was mentioned anonymously danger, therefore, if one does protective action has been noted from the results of (1) that the useful life of not take all the necessary many tests with nitrogenous age resisters. It thererubber articles might be proprecautions, of hastening the fore appears that age resisters of the amine types are longed by washing them in loss of rubber objects in trying dependable for assuring protection to rubber articles. soda soap and immersing in to p r o t e c t them.” T h e y ammonium carbonate soluhave tabulated the excessive tion, although better results were to be obtained by applying variation of the protective action of hydroquinone. Varying creosote or carbolic acid solution. Wolfgang and Walter Ost- results from almost perfect protection to fair protection were wald (11) in 1909 recommended the preservation of rubber by obtained when an ether solution was used, but an acetone sopainting, swelling, or spraying with liquids or solutions of lution caused accelerated deterioration. “neutral and basic aromatic nitrogen compounds, neutral or basic heterocyclic substances and alkaloids, either alone or Experimental mixed.” The application of terpineol or emulsions thereof was suggested by Beyer ( 2 ) in 1911 as a means of preserving The results obtained and conclusions drawn by lloureu, the elasticity of rubber. I n 1919 Zertuche (14) applied t o Dufraisse, and Lotte are so a t variance with general observarubber, as a preservative, the juices of certain desert shrubs tions which the writers have made using age resisters typical such as mesquite. Paints made from various oils, waxes, tars, resins, or pitches of classes of materials now being marketed for rubber comhave for many years been used to improve the appearance positions that it has seemed desirable to compare more fully and possibly to retard the deterioration of cured rubber the method of surface application with mill incorporation goods. Reference to the patents granted on these paints prior to cure. The method of surface application does not seem to have shows to what extremes the idea of painting was carried. One example is a composition of tar, pitch, resin, rubber cement, been extensively practiced. The purpose of this article is fish glue, and glycerol suggested by Thierfelder and Schmaelzle to show that this lack of practical application is due, not to any inherent defect in the method, but rather to processing (12) for preserving treads of tires. difficulties and to less effective action. Moureu and Dufraisse (Y) in 1922 patented the surface a p The comparison has been extended and has included a large plication of phenols t o rubber and other oxidizable materials as a means of preventing their deterioration. Martin ( 7 ) number of preservatives.2 Certain typical results are premixed aniline or other amines with cottonseed oil and applied sented because of their general interest. Figure 1 shows in chart form the relative protection against the mixture to tires, hose, etc., for lengthening their useful life. Cadwell ( 3 ) investigated a wide range of organic com- aging afforded to a high zinc oxide, blooming tread stock by pounds which absorb activated oxygen. He reported solu- twenty age resisters when 0.5 per cent was incorporated prior tions of aldehydeamines, p-aminophenol hydrochloride, di- to cure and when test strips of the stock were soaked for 10 phenylnitrosamine, p-nitrosodiphenglamine, and numerous minutes in 5 gram per 100-cc. solutions of the age resisters in others as being of value for preserving rubber when applied to acetone or benzene prior to aging. Accelerated aging tests were run in the Bierer bomb a t 70” C. under 21.1 kg. per sq. the surface of cured articles. Recently Fiedler (4)has patented the preservation of rubber cni. (300 lbs. per sq. in.) of oxygen or in the Geer oven at 70’ articles by painting them with mixtures such as glycerol and C. in a continually renewed current of air. The upper bar of each pair represents the tensile strength of benzaldehyde or glucose and phenylhydrazine, either alone or in rubber cements. A composition consisting of a mixture test pieces in which 0.5 per cent of age resister was incorporated of glycerol and drying oil was patented by Kurtz (6) to be on the mill prior to cure while the lower bar shows the tensile strength of the samples t o which the age resister was applied used for preserving the surface of air bags. The Bureau of Standards (13) carried out tests in which to the surface of the cured stock prior to aging. Figure 2 shows a similar comparison of ten materials in a five different age resisters were applied to the surface of articles such as surgeons’ gloves, by dipping them m a benzene- high-grade non-blooming tread stock.
HE idea that various
T
1 Received April 15, 1930. Presented before the Division of Rubber Chemistry a t the 79th Meeting of the American Chemical Eoclety, Atlanta, Ga., April 7 t o 11, 1930.
* Many of the materials tested in this investigation are the subjects of patents issued or pending and cannot, therefore, be used commercially in rubber except under license from the assignees.
AFTER48 HOURS AT 70' c. 21.1 KG, PER SQ. CM. OXYGEN
ORIGINAL Ultimate tensile
Kg. per
Lbs. p e r
cm.
sg. on.
%
3385 3224 3742 3604
668 603 660 647
sg.
Control 0.5% Pyrogallol 0 . 5 9 Pyrogallol aniline (1:l mol) 0 . 5 2 Aniline
Elong. at break
238 226 263 253
Order of Activity of Age Resisters The results show that the order of activity of the age resisters is essentially the same in both stocks whether they are applied to the surface or milled into the rubber. The results shown in the figures are also typical of, and agree with, the order of effectiveness established by more extensive studies with tread stocks. Ceresin as a representative of a class of waxes which protect stretched rubber samples from checking in the sunlight is without effect in retarding oxygen or heat deterioration. Phenols are comparatively weak in their action, the polyhydroxy compounds being more actil-e than the monohydroxy while the amine salts of either show considerably more activity. Phenols containing two or more ring structures are in general more active than those containing on1,y one. Primary amines show relatively weak protective action in the oxygen bomb yet are fairly efficient when tested in the Geer oven. Monoprimary amines show low activity while the di and poly primary amines are considerably more active. The combination of a hydroxy and a primary amino group in a single molecule gives an effect which is more than the additive effect of the two groups.
?-I
ultimate tensile
Kg.
Be7
Lbs. per
sq.
cm.
sq. in.
52 49 77 69
742 698 1089 977
kq per sq.o n of Uxqyen
After 7Days h Geer Oven ut 70'6
Control (aged)
Ultimate tensile
Kg. Ber
%
sg.
323 277 480 373
cm.
126 114 191 154
atElong. break
Lbs. per sq. in. 1789 1624 2725 2191
70 500 395 542 525
This is of interest in the light of the increased activity of phenolamine salts. Certain aldehydeamines show fair activity, especially toward heat deterioration. Aryl secondary amines as a class are of considerable commercial importance. I n general they are good age resisters and also retard cracking upon repeated flexing. Alkarylamines are of mild activity, although their effect is enhanced by putting tm-o secondary amino groups into the same molecule. Diarylamines show higher activity. The activity increases as substituent groups are introduced into the diphenylamine molecule. Thus, ditolylamine and phenylnaphthylamine have higher activity than the unsubstituted diphenylamine. The introduction of hydroxy or phenylamino groups likewise causes an increase in activity. I n general, the diarylamine type of age resister shows higher activity in the oxygen bomb than in the Control (unaged)
--
AfterdBHoursaI 7 0 ° C 21.1 hq per sq cm ofOryqm
Control (aged) Ceresin
Control (unaged)
21 I
at*long. break
Hydroquinone
Afier 48 Hours af 70-C
AFTER7 D A Y 5 I N GESR O V E N A T 70" C.
After 70ays h Geer Oven at 70.C.
F, F4 ,
m-Phenylenediamine p,$'-Diaminodiphenylmethane Aldol-a-naphthylamine
Ceresin
Phenyl-@-naphthylamine s Di-@-naphthyl9-uhenylenediaminea s Diphenyl$-phenylenediamine Thiodiphenylamine s Diphenylethylenediamine
@-Naphthol Hydroquinone Aniline salt of hydroquinone Benzidine m-Phenylenediamine
p,p'-Diaminodiphenylmethane p-Aminophenol
0
/oo
200 0 100 T e n s i l e k g . per sq. cm.
zoo
Figure 2-Comparison of Mill Incorporation a n d Surface Ap HcaTread t i o n ofStock Various Material, for Retarding Deterioration of High-&ade
Acetaldehydeaniline Aldol-a-naphthylamine
o
See note under Figure 1.
Phenyl-wnaphthylamine
Geer oven. The tetraarylhydrazines (not reported here) are of extremely high activity. Diarylnitrosoamines and thiodiarylamines are of high activity, although for various reasons they are not widely used. The specific properties of certain of the materials modify the generalizations made above. The effect of an age resister only slightly soluble in rubber is, of course, limited by the amount which will dissolve. A surface bloom of age resister affords temporary surface protection but detracts from the appearance of the finished article.
Phenyl-@naphthylamine s Di-@-naphthyl$-phenylenediamine* Di-$-tolylamine a Diphenyl*phenylenediamine s Diphenylethylenediamine s Di-o-tolyl: ethylenediamine Phenyl-&naphthylnitrosoamine Thiodiphenylamine #-Hydroxydiphenylamine
Surface Application vs. Mill Incorporation
200 0 /oo 200 Tensile-kg. per sq. cm. Figure I-Comparison of Mi11 Incorporation a n d Surface Application of Varioue Materials for Retarding Deterioration of a H i g h Zinc Oxide, Blooming Tread Stock This was much more dilute a A saturated solution in acetone was used. than 6 per cent.
0
100
.
As shown on the charts, in nearly every case better protection was afforded by mill incorporation. The data indicate, however, that protection is not necessarily due to products formed during cure by reaction of the age resister with sulfur, rubber, resins, zinc oxide, or accelerator.
INDUSTRIAL .4ND ENGINEERING CHEMISTRY
September, 1930
Results with Pure-Gum Stocks
I n pure-gum stocks a similar order of activity of the age resisters obtained. However, the difference in effect between mill incorporation and surface application was small. Table 11-Effect of?3olvent Employed u p o n Accelerated B o m b AQing of.Test.Pieces.Dipped for 10 M i n u t e s in 5 G r a m per 100 c c . S o l u t i o n s of Phenyl-E-Naphthylamine STOCK EMPLOYED Selected Dale creoe. . 100 = . ...... Lithopone . . . . . . . . . . . . . . . . 100 Waterfloated whiting.. . . . . 75 5 Cured in 2.4-mm. (*/sr-inch) sheets in Zinc oxide (lead-free). ..... press 20 minutes at 142‘ C. (287” F.) Light mineral o i l . . . . . . . . . . 5 Sulfur. . . . . . . . . . . . . . . . . . . 3 Diphenylguanidine . . . . . . . 1 289 ~~~
~
~~
.
~~~~~
-
SOLVENT
Stock not treated
TIMEIN BOMB
Hours 0 24 48 72 96
Acetone
0 24 4s 72 96
A1coho1
0 24 48 72 96 0 24 48 72 96 0 24
Benzene
For comparison, stock containing 0.57, phenyl-&naphthylamine mill-mixed
48
72 96
AFTER AGINGI N BIERERBOMBAT 70’ C. I N 21.1 KG, PER SQ. CK OXYGEN
Ultimate tensile
Kg. per
sq. cm.
146 106 83 34
Melted 146 148 138 125 122 146 143 123 110 125 146 142 143 136 139 155 147 147 118 130
Elong. at
break
Lbs. p e r rq. w. 2075 1514 1184 479
523 500 435 250
2075 210s 1964 1783 1738 2075 2034 1753 1558 1770 2075 2022 2027 1934 1940 2198 2091 2090 1673 1852
523 507 527 493 497 523 500 537 530 505 523 505 520 a17 510 530 497 503 483 493
70
1003
tion gave the best results and alcohol the poorest, since cured rubber is swollen appreciably by benzene and slightly by alcohol. The detrimental effects of a turpentine solution as noted by Moureu, Dufraisse, and Lotte ( I O ) may be ascribed to the well-known catalytic effect which autoxidizable materials exert in initiating the oxidation of rubber. One would also expect drying oils or aldehydes to be poor solvents for the application of age resisters to rubber even though the age resisters do retard the oxidation of these materials. Retardation after Once Started
It was a point of general interest to determine whether the deterioration of rubber could be retarded after the action had once begun. The results of experiments designed to answer the question are represented graphically in Figure 3. Surface application of age resisters will inhibit deterioration even after it has progressed to a considerable extent. However, it would be better to guard against this need by the use of sufficient age resister in the compound prior to cure. Limit of Protective Action
Age resisters do not exert their protective action indefinitely. I n general high concentrations are more effective than low concentrations. This lends support to a theory that age resisters are consumed during the time that they exert their protective action, and does not fit in with a theory that the materials are being constantly regenerated.
Tests for Inversion of Protective Action
Moureu (8) has cautioned that under slightly different circumstances “antioxygens may become prooxygens.” In extensive work with the nitrogenous age resisters an inversion of their protective action has never been observed. With certain phenols variability of action has been secured when non-alkaline rubber compounds were used. Table I shows the results of experiments in which 0.5 per cent of pyrogallol, aniline or their mutual salt was mill-mixed into the high zinc oxide, blooming tread stock previously mentioned. A comparison of the tensiles and especially the elongations after aging shows that pyrogallol has not retarded deterioration of the stock. The addition of aniline alone has slight preservative effect. The increased effect obtained by using pyrogallol and aniline in conjunction is apparent. In general it has been found that certain acids or substances, which readily oxidize tend to accelerate deterioration. Sulfur, turpentine, and aldehydes may be cited as examples. The addition of organic basic materials more or less counteracts this effect and may even convert deterioration accelerators into deterioration retarders. Thus the addition of tx-ethyl+ propylacrolein to a rubber stock accelerated with polybutyraldehyde aniline will cause it to soften and become very weak on aging. The further incorporation of an amine, such as aniline, metaphenylenediamine, or a-naphthylamine, will counteract the effect and impart superior aging properties to the stock. Table I1 shows a comparison in which different solventsacetone, alcohol, and benzene-were used as a means for applying phenyl-P-naphthylamine t o a white stock. The parallel aging results show the small effect that inert solvents have upon deterioration. As one would expect, the benzene solu-
Hours of 7OoC, 2 / / (9
Figure 3-Effect
p r r s 9 cm of Oxyqen
of Applying Phenyl-&Naphthylamine t o Partially Aged Stock
Conclusions
If the results of bomb aging, oven aging, and sunlight exposure are compared, one is led to the conclusion that the deterioration of rubber is a complex phenomenon. The relative effectiveness of the different deterioration inhibitors varies with the test applied. The rubber technologist is interested, not only in the protection afforded against heat deterioration, continued cure, oxidation of the rubber, deterioration caused by oxidized sulfur, or deterioration induced by sunlight; but also in properties such as the activation of accelerators giving rise to definite desirable types of vulcanizates, plasticizing or stiffening of the uncured stock, improved dispersion of the pigments, decreased scorching tendencies, elimination of surface tackiness caused by dry air cure, and prevention of flexure cracking in the cured product. Surface application may be effective with respect to all of the properties listed in the first group, but only mill incorporation can lend the advantages enumerated in both groups. Moreover, wear will remove a large part of the age resister applied to the surface
1004
INDUSTRIAL A X D ENGINEERING CHEMISTRY
which cannot have penetrated by diffusion to a concentration beyond its solubility, whereas abrasive action may expose new portions of age resister milled into the stock. ADVAKTAGES OF MILLINCORPORATION-(1) For high black stocks more effective protection is secured from mill incorporation than from surface application of age resisters. ( 2 ) Larger amounts of preservatives in the compounds may be secured by mill incorporation, thus affording protection for a longer period of time. (3) Owing to the slow diffusion into rubber of age resisters of high molecular weight, mill incorporation may be expected to give better protection to the interior of thick rubber articles than could be obtained by surface application. This holds true especially for protection against heat deterioration. (4) It is evident that slightly soluble materials may be incorporated by mill mixing in amounts in excess of that needed to saturate the rubber, and as the dissolved part reacts and loses its effectiveness for protecting the rubber new portions of the excess may dissolve to sustain the protective action. (5) Mill incorporation is much less laborious, sinre the material may be mixed into batches weighing 100 to 300 kg. (220 to 660 lbs.). Contrast with this the surface application to hundreds of single articles which might be produced from these batches. (6) If the materials are incorporated on the mill, there is
Vol. 22, Yo.19
no loss of costly solvent, with the attendant problems of recovery and of fire and health hazards. ADVANTAGES OF SURFACE APPLICATION-(1) Surface application is the only means by which protection with age resisters can be secured for articles containing none, either new or after deterioration has commenced. ( 2 ) Surface application is R means of applying materials which would materially interfere with the curing or processing of the stock. Literature Cited (1) Anonymous. Gummi-Zlg., 12, 92, 98 (1898). (2) Beyer, German Patent 243,248 (April 20, 1911). (3) Cadwell, U. S. Patent 1,556,415 (October 6, 1925): British Patent 220,321 (July 2 , 1925). (4) Fiedler, U. S. Patent 1,661,887 (March 6, 1928). ( 5 ) Kreussler and Budde, German Patent 18,740 (1851); Ber., 15, 1779 (1882). (6) Kurtz, U. S. Patent 1,694,879 (December 11, 1928). (7) Martin, U. 5. Patent 1,422,115 (July 11, 1922). (8) Moureu, Chimie & induslrie, Special No., 101 (1927). (9) Moureu and Dufraisse, French Patent 548,325 (1922). (10) Moureu, DufrdiSse. and Lotte, IXD. EX. CHEM., 22, 549 (1930). (11) Ostwald, Walter, and Wolfgang, German Patent 243,346 (1909); British Patent 10,361 (1910). (12) Thierfelder and Schmaelzle, U. S. Patent 1,312,007 (August 5, 1919). (13) U. S. Bur. Standards, Tech. News Bull. 140 (December, 1928). (14) Zertuche, U. S. Patent 1,302,266 (April 29, 1919).
Effect of Curing Temperature upon the Properties of a Rubber-Sulfur Mix' C. R. Park2 T H E GOODYEAR TIRE & RUBBERCOMPANY, AKRON,OHIO
T A meeting of the Data are presented Covering a range of equivalent only exception being that cerRubber Division sevcures at six differenttemperatures on a stock consisting tain of the tensile tests were era1 years ago it was of rubber 100, sulfur 6.25. Neither the original propermade on the Goodyear testties nor the aging behavior show any definite trend intimated that any given rubing machine rather than on with increase of curing temperature. Comparison the testing machine generally ber mix would show much improved properties if the coris made of natural aging to artificial aging as conducted used. The die used for this in the Geer oven and the Bierer and Davis bomb. The rect technical cure could be machine is 1 cm. in width. accompl&hed in a s h o r t e r temperature coefficient of vulcanization is determined Care was taken throughout time by any device, even by three methods and found to be 2-50 per 10" C- or thatsampleswhichwerecomt h a t of raising the curing 1.64 per 10" F. pared in any way were tested temperature. For example, under the same conditions. The stock mentioned above was a factory stock and the if a stock reached its optimum properties in 3 hours a t 140" C., its quality would be much improved by curing under the rubber used was always made up from a mixture of first equivalent condition of 27 minutes a t 162" C. It was this quality sheet and crepe. So much time has elapsed bestatement that suggested the present study. tween the start of the experiments and the assembly of the Because of the immense amount of work involved it was data that the exact proportions of these rubbers used in this considered inadvisable to try to cover the whole situation in particular batch have been last from the record. one set of experiments. It seemed simpler to study, as Tabie I-Curing Periods thoroughly as possible, one particular mixing. The formula chosen was rubber 100, sulfur 6.25, since this would eliminate N O . 135.5" c. 141.5' c. 147.5' c. 152.5' c. 157.5' c. 162' c.
A
the variable effects of accelerators and other compounding ingredients and reduce the problem to its simplest terms. It will be understood, therefore, that the conclusions drawn are meant to apply only to the mixture in question. Experimental Procedure
The experimental procedure was made to conform to the recommendations of the Physical Testing Committee ( 5 ); the Received April 15, 1930. Presented before the Division Of Rubber Chemistry at the 79th Meeting of the American Chemical Society, Atlanta, Ga., April 7 to 11, 1930. * Present address,:United Carbon Co., Charleston, W.Va.
Mtnutes 300 4
390
Mznules 150 165 180 195
Mtnutes 89 97 105 113
Mtnulcs 46 52 58 64
Mmutes 34 38 41 45
Minutes 21 24 27 30
Sufficient stock for all the experimental work was taken from a single factory-milled batch. I n order to insure uniformity, this stock was blended, without excessive milling, upon a 20-inch (51-cm.I experimental mill. The whole batch was then sheeted to the proper thickness, for curing and testing. The shown in Table 1were established as being a8 nearly equivalent as it was possible to obtain. Judgment was based upon stress-strain behavior and an examination of the
