Alkylphenol Sulfides as Vulcanizing Agents onipoundinp - i r r \ e ? of the. d h ~ l p h e n o l sulfideed that representati\es of this class of rompouiidare effective vulcaiiiiiiig agents for the butadiene co1~01) mers. A number o f phenol sulfides repre-entiug il cwnsiderable variation in end products were el aluated. \I1 wlfides tested functioned as tackifiers for GR-S but v e r c less effecti\r for this purpose when used in thc Iliciia U tlpes. p - t t , r t - ini?lpheuol disulfide of approximately 23Yo sulfur conteiit wai found to be particitlad? effectiTe as a tulcatii&ig agent for GR-S. The rates 01 \ IilcaniLation of the nlhylphenol sulfides are of the sainc order of magnitudc, : I - thnee realized with +iilfnr. Wheli
t hc alhylpheuol 9ulfides are used, the v ulcaiiiAa te- I+ ith GR-S and Buna Y types are generally superior i n tensile strength, modidus, elongation, tear resistance, resistance to flex-crark growth, and retention of original properties on aging. Coniparati\e compounding tests were run on the p-tert-aniylphetiol clisulfide of 23% sulfur content .and sulfur in G K - S stocks loaded with a range of Tolumes of EPC, FT, and SKF carbon hlarhs. This sulfide was also c,aliiated in a reprcscntatite (;K-S tread stock. Fnrther, the 11Le of alky lphenol sulfich improves the dispersion of >arious niineral filler- in -CR-S, as (,*irlenced by i n c reasc- in t e n eile e t r e n g t h . t n t I elon pat ion.
I
*tiow
T
H H E E of the major problcmi involved in tlic ulilizatiun I I ~ the butadiene copolymers of styrene or acrylonitrile are thc (icficiency of tack in tliesc pol~-mers,the exacting control of rul(.:inizntion that is ustially required, find the retention of original ])ropertics on aging. Chnsiderable time and effort !lave been (.?cpendedin trying t o iinprovc the tack qualiticli of these polymers. Particular cniph has heen placed on the ifi"'1ct of addition of organic material these synthetics for the irriprovcmelit t . ~ f tlwir tack characterisiivi. An examination of :I large nuiiilwr (it' organic agents reri':ilecl that, thc alkylphenol siilfides are ?Ii't,ctive materials for inip:irting h i l d i n g lack tri ilrc. hiitadimi mpolymer types of sptli(!ticz rubber. \Vlien properly reinforced and cured vit,li sulfiir :iiicl organic, :tccelerators, the b~~ta,.livnc copolyiiiers of styreiw GR-S) o r :icrylonitrile (Buna pi w i physical propertic. approaching t hove of natural rubber ~-ulca~iizatei.Vila ( I O ) stair? that t h i , I)henonienon of reversioii a s applied to natural rubber does no1 \ ~ e r c20, ' 45, 60, ailti '30 iriiiiutes a t 280' 1;. Since the p i i r p o . ~of~ this phase of the iiivc-tigation \v:ts to dc1 criiiiiic the effect of varj-ing sulfur ci>iit(,iitill p-tert-aniylphenol -ulfide oii tlic rate of cure, there ,dfitles ~ Y Wcoinpounded on thc: basis of equivalent, sulfur. The nionoaulfides n'ere compoundeil agent,. both as a primary vulcanizing agent and ac ci~~iilcanizing with elenicntal sulfur. Figure 2 cornpar('* !j-fe,t-arii\.li,lit,iiol .dfidtis atid eleiiiei1t:ii sulfur nitli respect t o the tcnsile *t length and clongation inip a r t t d t o a GR-S tread-type stock. The nioiiosulfide jtocks containing 110 ekmental sulfur are definitely slower curing than the stocks vulcanized with sulfur. This fact is clearly sho\\-n in Figiirr 2 by ttic high oli~ngationv:iliic~i ~ v < 'after n %Ringfor t h r t v day5
r
3500
P-TAP-DISULFIDE
SULFUR
6
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tbluvated temperatun-. HUKCVCI~, s t o c k contairiiiig bur11 nioiio~iilfideand elemental sulfur are all faster curing than t l i c control vulcanized with elemerital sulfur aloiic,. TotaiFigure 2 also shoivs the detrimental effect of this :*$Isulfur ilitional sulfur by t h c sharp drop in elongation aii*tig:ttioii. Vulcanizatci- having lour different total sulfur ~ ~ o i i ~ iwi'i, riif~ .tiidied. The formula rmploytd was the 3anic :tl: t t i a i u c ( s i l i i i t t i c preceding .(.ries, i ~ s w i i tthat in the case of tlic t~1cniiwI:il,iiliur stocks, 2.0 parts ( ~ L:f rosin-type softcnw were :idtlt~l. 'Thc tilkylphenol * i i l f i ( l t ~ \ Y ( ~ I Y > chrclied also for tlicii, tack-lJi,oiliir.irig qualities, :iiirl this rosin type s0fteni.r ~ v i i . ndtlccl t ii t lit, el~~mciital sulfur >t (IC!+ for comparativc purpose-. Figurc :3 ,.lioivs the effect r j i i strers-st rail1 proptarties of varying aniouiits C J F vulcanizing ageut before nricl after aging 7 2 lioiir. i l l :iii air ~ Y ( T :it I 312' F. lyitli one exreption the p-terf-arnyII)lieiir,l I lisulficlt~vulcanizates are siiperii~ri i i tcwsile strength and iAlony:itioii I O t lic corresponding elemental sulfur \-ulcanizatcs:. 'l'111, t:loiig:rt i t r i i ~-alue.? of the 1.0-part cleniental sulfur stock aftor agiiig are d u ( , t o t h e fact that the original stock \va$ undercurcd to : I great ( s i ' castcLnt than n'a3 the ccirrespoiidiiig .sulfide stocsk. TIII, ttliisilc data cicarly indicuti. that p-(ert-amylphenol disulfidcb (I! 23:; sulful. cwntciit protiucw :I Eaatcr rate of (*iir~' in GR-S t1i:iii do(+ : I I I oquivalcnt ammini of clcmciit:il s i i i f i i r .
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MINUTES CURE AT 2 8 O o F
Figure 3. Tensile Strength and Elongation CS. Tinie of Ciirr for p - t p r t - 4niylpheiiol Disiilfide a i d Sulfiir F-111caiiizates ,,-'I' i1'-disullidr = p - tert-amylphenol disulfide
snv
( : ~ H B O SBLACKS
'l'lic prlJ[M type and aniount of carbon black are iriiportuiit iwnsiderationh for the reinforccmmt of GR-F. Garvey ani I Frecr;c (.?I investigated ten types of carbon blacks in variouloadings in GR-S using sulfur vulcanization. Inasmuch as thi, previous x o i k indicated that the alkylphenol sulfides improve, I the vulcanizatcs containing 40-50 parts of EPC (easy procchssing channel) black, this investigation \vas extended for other IIJR(Iings. FT (fine thermal) aiicl SRF (semireinforcing furrincc' blacks ~ w r calso included in tlic reriep. p-te,,t-Arriylpherii)l diau!fide of 23:: aulfur content iva,+u w l tliroughout. The forinul:~ employc.tl \vas the first one li ed in the Pection of this paper 011 variatioii of alkyl g r o u p . ( rt's were 30, 60, 90, 120, and 160 niiiiutc.7 :it 280" F. Data on the curves for tt:nsile strength anti eloiigst,ion iig:iinst loading W I W selected from the considercr I opt iniiuii c i m ~of each black loading. These properties were c~valriati~il :it riwni teniperaturc, a t 21'2' F., and after accelcrateil n i t (JVCII aging for 72 hours at 212" F. I.'l'c 13IAC.K. Figure 4 ~Iiowsthe eft'ect of different loudiiigs ot EI'C t)lai-l; C J I ~the tensile el ri~ngtliof GR-S when vulcanized wit11 p-lr,,l-:tm?-lljhciiol disulfide o r sulfur. In the lou-er range (it loadings t h e sulfide vulcanizatq. i u . Tensile strength. Ih. f s q . in. Elongation, % Shore hardnese Tear resietancr, It)., 1 1 ) .
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270 840 GO0
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yroivtli was d ~ r c . : i ~ ~iiue v I t i J tire Iiiglicr atate of cur^', Ciitler the conditioiis of t v r t at' 11 of 1111, sperirncms broke at ttic: start of the flex-crack growth test. These improvenicrits in resistaiict, i o t w r and lies-ct,ack growth are accompanied hy :in increase in hysteresis, :is sho~viiby the higher ltcat build-up values listctl for stork3 A and C. Some sacrifice in retJnuntl rcdience, wmpresqion set, and permanent set resulted from thc use of p-fert-amylphenol I-iisulfick. However, improvement in these physical pi'operties \vas ~iiadcby the me of the copper co-a,ccelerator, wn in st,ock C. Tlicse ahortconiiugs also (XII I Jcompeii ~ or, to iome tlegrey, by the additinn i i f snffcwisr- aiitl nhqt .\IIXKR.\L FILLERS
Preliiiiiriary ivi~rk .?:)iowedt l u i t : i m y l p h e ~ i o l - t i i ~ u lilia!. ~ ~ e ~be :iclaptrd for t hf, vitI(*anizatir~ii I J ~ ' GR-S sto(iks loaded with the mineral fillers to prixluce iniproved phy4cal propcri ics similar to those dho\ni liy 1he carbon black stocks. Five mincmd fillers were compounded i n ii GR-S stock on a bask of 40 parts t)y \wliune. 'Typical proivrtic*. conferred I)y p - t e ~ ~ t - a i i i y l ~ ~ hdisulficle t~~~~il ~I : I I Y , illii ivitli the v:irioii.s ~ i i i i i t ~ r :fillc~r~ .*tack.$ in Table Il-. All o l the stuckx e ~ i i i aiiiiiig t riiiiicral fillers \\'ere grcatly improved in nioduln,~,tc:n.ile strength, elongation, and tear resist,ance through vulcanization with p-tert-smylphenol disulfide. Later work dio\~cclthat niucli greater improverncnts in stress-train propr,rtic.s can be obtained by the use of mincral fillers of much finer particle size. The optimuni cures selected for each mineral filler were the lame for both types of vulcanizates. These cures were chosen 'T.4BLE v. P l I Y , - I C . i L PROPERTIES O F RCS.4 1 T7t?LC,4SIZ.kTES from modulus-time of cure curves with the seTrnsile Strength, Elungation, Shore lected curing time being that point n t n-hich the Lb./Fq. In. % Hardness __ Y l O 0 Y., OrigOrigOneOrigcure began to have a constant, slope. .-t,ck llin. lna. .\ged" inal .Lgeil" inn1 Aged" inal Aged,' Aliter a long curing time stocks reinforced Kith 75 80 Tic1 400 1890 3300 2390 11 20 8511 liard clay possessed the highest tensile strength 350 77 80 3730 320 3410 1791, 3190 45 320 8rl 84 4390 460 4000 4'280 75 2421 €or both types of vulcanizing agents. In these 210 76 80 3.520 431) 2431 .. 3790 1' 20 same clay stocks sulfur produced slight 1y higher 210 79 80 370 3200 300f .. 3860 30 210 7Q 80 310 3880 3200 45 3331 elongation than p-tert-amylphenol disulfide, but 70 74 820 450 1510 4330 2780 ted in Table Y. The disulfide imparted lower modulus, higher tensile strength, and higher elongation to the Buna ?; vulcanizate both before and after airoven aging for 72 hours at 212’ F. T17hen tlic disulfide v-ai used in this amount, i t was a slower curing vulcanizing agent than sulfur. The rate of cure can be increased to some extent by tlic use of a thiuram type of accelerator (stock Q) in place of the
Vol. 38, No. 11
PRESES~L before D the meeting of t h e Division of Rubber Chemistry, AIERICHEXIC.AL SOCIETY, in New York, N. Y . , April, 1944.
CAS
Structure of Alkylphenol Resin Tackifiers for GR-S G. E. P. S I I T H , JR., J. C. -KVIBEL_ING,i N D G. W. GOTTSCH-iLK Firestone Tire & Rubber Company, A k r o n , Ohio
F
RESHLY milled natural rubber, alone or compoundcd, possesses the property of tack in a high degree, whereas GR-S polymer is almost completely lacking in this property. In this discussion the terms “tack” and “tackiness” are defined as the tendency of two surfaces of the same substance to adhere nhen brought together with slight force. The scope of this rvork is limited to this property of tackiness or tack as we h a r c defined it. This is the “autohesion” of Zhukov and Talmud (24) mentioned by Busse, Lambert,, and Verdery ( 2 ) and distinguished by us from the tendency of the surface of a substance (such as rubber) to adhere to t,he surface of some other material (such as glass, wood, steel mill rolls, living skin, etc.). For this latt’er property we have reserved the term “stickiness”, and this is also an important property-as, for instance, in the pressure-sensitive adhesive industry. Very early in the attempts to find materials which would promote tackiness in GR-S, i t was found that the polymer could be made sticky without imparting to it any appreciable amount of tack. Conversely, later on it was found possible to make GR-S very tacky n-ithout producing any appreciable amount of stickiness, a very desirable result in the rubber indus-
try. 3Iany of the important factory operations in building tires or other. products from nat,ural rubber utilize this adhesive characteristic or tackiness; for example, in building a tire the plies readily adhere t o each other if cut from freshly calendered fabric. The tackiness of rubber stocks sufficient to hold the various parts together nithout any separation during the building and subsequent handling operations is frequently termed “building tack” by rubber compounders. The factory practices used for natural rubber stocks cannot ordinarily be used with GR-S, because the tacky characteristics of natural rubber arc not present in this type of synthetic. This lack of tack necessitated the additional operation of spreading the plies with natural rubber cement before tire building could be attempted. Satural rubber cement is used because cements made from GR-S lack the adhesive strength of cements made from the natural product. Thus the absence of tack in milled sheets of GR-S is paralleled by the relatively poor quality of GR-S cements. hlaterials Rhich have been widely used in the rubber industry for increasing the tackiness of natural rubber sheets and cements are rosin, pitch, ‘pine tar, cumarone-indene resins, etc. Early
