I N D U S T R I A L AN,D E N G I N E E R I N G C H E M I S T R Y
January 1955
ACKNOW-LEDGMENT
The authors wish to express their appreciation to H.J. Osterhof and the Goodyear Tire and Rubber Co. for permission to publish this paper; to A. RI. Clifford and J. 0. Cole for valuable criticisms and suggestions; and to C. R.€'arks, Coe Wadelin, J. J. Hoeshr, and F. V. Galati for certain data used in this paper. LITERATURE CITED
Albert, H. E., Smith, G. E. P., Jr., and Gottschalk, G. W., IND.ENG.CHEM.,40, 482 -7 (1948). .lm. SOC. Testing Alaterlals, Philadelphia, "-4ST11 Stalldards," Part 6, pp. 516-18, Method D 1206-52T, 1952. Banes. F. W.. and Ebv. . . " , L. T.. IND.ENG.CHEM..ANAL.ED.. 18, 535 (1946).
171
(6) Gehman, S. D., J . A p p l . Phgs., 19, 456-63 (1948); RubbeT Chem. and TechnoZ., 22, 105-17 (1949). ( 7 ) llooney, M., Wolstenholme, W. E., and Villars, D. S., J . A p p l . P h y s . , 15, 324-37 (1944). ( 8 ) Neal, ~4. h1.Y and Ottenhoff, P.,ISD. E X G . CHEhi., 36, 702 (1944). (9) pedersen, H, L., and xielsen, R , , J . PozynLer Sei,, 7 , 97-103 (1961). (10) Shelton, J. R., and Winn, H., IND.ENG.CHEhr., 39, 1133-6 (1947). (11) Throdahl, 31. C., Zbid., 40, 2180-4 (1948); ASTM Spec. T e c h . Pub., 89, 35-46 (1949); R u b b e r Chem. and Technol., 22, 699-711 (1949). (12) Tobolaky, A. Prettyman, I. B., and Dillon, J. H., J . AppZ. Phys., 15, 380-95 (1944); R u b b e i Chem. and Technol., 17, 551-75 (1944).
v.,
RECEIVED for review April 21, 1954.
ACCEPTED August 16, 1954. Presented before the Division of Rubber Chemistry, . 4 M E R I C h V CHEMICAL SOCIETY. Louisville. Kv.. - Ami1 . 14 to 16. 1954. Contribution 186. Research Laboratory, Goodyear Tire & Rubber Co
Cole, J. O., and Field, J. E., IND.ENG.CHEM.,39, 174 (1947). Eccher, S., and Oberto, S., Trans. I n s t . R u b b e r 1 4 27, 3 2 - 3 7 (1951).
Aging Stability of .Neoprene Latex RELATION BETWEEN CROSSLINKING AND HYDROLYSIS OF ALLYLIC CHLORINE D. E. ANDERSEN
AND
PETER KOVACIC
Jackson Laboratory, E . I . d u Pont de Nemours & Co., Inc., Wilmington, Del.
E
LASTIC films prepared from neoprene latex are strengthened and made more resilient by curing. The curing temperature must be high enough to activate certain crosslinking reactions which connect the polymer molecules in a three-dimenBional network. Empirical evidence suggests that these crosslinking reactions are affected by the age of the latex from which the films are made, as without accelerators the tensile strength of the films declines as the latex ages. The reactions differ from crosslinking in natural rubber, since technically acceptable cures are obtained without using sulfur as a compounding material ( 4 ) . The present paper considers both of these effects by describing the changes in the chemical structure of the polymer in neoprene latex particles which occur as the latex ages. These changes are then related to the decline in tensile strength and to the mechanism of crosslinking. The tensik strength of films from old latices ran be maintained a t optimum levels by the proper choice of compounding materials. MATERIALS AND METHODS OF TEST
Technical grade 2-chIoro-1,3-butadiene (chloroprene) freshly distilled from a 50-50 chloroprene-toluene mixture was used to prepare the latices. The chloroprene was 99% pure and contained less than 0.3% dichlorobutene and 0.12% methyl vinyl ketone (3-buten-2-one). Other materials included commercial N-wood rosin (about 90% abietic acid) and 97% sodium hydroxide. Compounding materials were hard clay (Suprex clay having the composition 44% SiOz, 38% A1203, 1.6% FetOa, and 0.9% LizO). antioxidant (Neozone-D, N-phenyl-2-naphthylamine, phenyl-&naphthylamine), rubber grade zinc oxide, accelerators [Thiuram E, bis(dieth lthiocarbamyl) disulfide, tetraethylthiuram disulfide], and (qepidone, sodium dibutyldithiocarbamate). Unless otherwise noted, the latex used in all experiments was a 100% conversion emulsion polymer dispersed in water to a total solids content of 50y0 by weight. The polymer was sulfurmodified-that is, its molecular weight and plasticity were controlled by the addition of sulfur before polymerization. Aging cycles of the latex were accelerated by heating a t 90" f I" C. Each sample, contained in a tightly capped flask which had been flushed mith nitrogen before filling, was exposed to air only during transfer from reaction vessel to aging flask. Oxida-
tion effects were shown to be negligible by comparing infrared carbonyl oxidation bands before and after aging. Samples were aged for 0, 4, 8, 16, 24, and 48 hours. The concentration of chloride ions in the water phase and the quantity of active chlorine in the polymer phase were determined by methods previously described ( 2 ) . Thin films were prepared by drying the latex on a glass plate for several minutes a t 50" C. The films were leached overnight t o remove soap, catalyst, and other water-soluble materials, and then vacuum-dried. Spectra were obtained on a Perkin-Elmer Model 21 double-beam infrared spectrometer. Differences in film thickness were adjusted by using the 8.15-micron band as a relative measure of thickness. Latices were compounded with zinc oxide, clay, and phengl-2naphthylamine in the ratio of 100 parts of dry polymer to 5 parts of zinc oxide, 10 parts of clay, and 2 parts of antioxidant. These materials were introduced as 50% by weight dispersions. Before compounding, the p H of all latices was adjusted with 2% sodium hydroxide, so that it required 3.5 meq. of hydrochloric acid to lower the p H to 10.5 for each 100 grams of latex. After dipping, the films were leached for 2 hours in ion-free water and dried for 24 hours a t room temperature and finally for 4 hours a t '70" C. This process is known as zero cure. Some crosslinks are formed during the drying cycle, so that zero-cured films are partially vulcanized. Selected films were cured fdr 30, 60, and 120 minutes a t 141 C. in addition to drying. Tensile strengths were obtained from films about 18 mils thick using Die C according to the ASTM D 41241T specifications. Statistical tests made a t the Rubber Laboratory of E. I. du Pont de Nemours & Co. showed that these measurements are reproducible to about 5 3 0 0 pounds per square inch based on the unstretched cross-sectional area of the films. Swelling measurements were made according to Whitby's technique (30). The initial volume of an unswelled sample was taken as the volume of neoprene in the sample calculated from the compounding formula. SWELLING THEORY
The swelling measurements were interpreted in terms of FloryRehner theory (10,l l ) , using the solubility parameters and a polarity factor determined for neoprene by Scott and Magat ( $ 7 ) . The basic equation is
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
172
A inore recent derivatioii of the relation betxveeii welling volume and >1Tc' ( 9 ) has not been used, .iiice the entropy of elasticity, 011 15-hich this relatioii is Iixued, is in dispute ( 1 8 , 29). I .5
I
1
I
Vol. 47, No. I
In the absence of any data on q, it v a s ashunied t o be 1- that is, entanglenienta which act to prcvcri t sn-elling are counted :ttruc cuvalen t croeslinks. CHAXGES IK S'rHUCTLHK 1)UKING AGING
* i t 40" C. pol)-chloroprene po1ymc:r chains grow niairily IJJtlii: 1,4 addition (I) of chlorobutadieiie free radicals (3,as),although ahout one monomer unit in 67 (1.5 mole %) enters the growing chain by 1,2 addition (11)(?, ,?Oj.
I
(3 1
-E
1,1 addition I
'f.
--CI12-&=CH--CH2-C1
CHEMICAL ANALYSIS
/
0
I
1
IO
20
30 AT S O ' C )
L
40
50
Chloride Ions i n X aLer Phase of Veoprene J'aiex
The value for p, applies to most rubber-solvent S\ hte~iisand ii cqual to 1/a where a is the "coordination nuiiibrt "--that is, t h e numbei of neighbor sites around each site in tlic 1:itic.e art u p t o calculate the entropy of mixing. Alpha has brcn estimated to be 3 or 4 (26, as), $0 that aonie sinall uncertainty is introduced in the magnitude of .\I:. h v:tlue of 0.44 has h ~ e nfound for u for natural rubber swollen in lieiizriie ( I ) , while for iieoprcricl in tolucme I* = 0.322 x-hcn calculiited according to bkliiation 2 .
-$-
cI 11 CR,
t
I
LATEX AGE (HR
Figure 1.
1,2 addition I1
(WATER PHASE)
The chlorine in the 1,2 Ytructurc 1s v c ~ yreactive and, i n kt late.;, reitdily undergoes alkaline hydrolysis with the elimiwitioii of chloride ion. This halogen i b eventually found in the watei p1i:isc of the emulsion as hydrochloric acid or ;odiuni chloritlr, in the quantities shown in Figure I . I .O
1
IO
20
LATEX AGE (HR
Figure
3.
I
I
30 AT 90.C
Swelling of
40
50
)
Lnclxrrd
Nen11 r e n r V i I n 1 s
1
0.It
I l r l l l l l
0
2 4 6 B 1 0 1 2 LATEX AGE (DAYS AT 50OC.)
Figure 2.
Chloi*ide Ions in Neo-
prene L a t e x as Function of pII
It ha< Imxi pointed out 1)) f'lorj ( 7 , 8 ) and Flrtcher and ahsociatcs ( 6 )t h a t ,?.I is onl? the appai'cnt niolecwial weight betweeri ciomlinhs. Coiiertionb inurt be npl)liecl to allow for niechanical cntaiiglctineIits and for chain (sricl. 15 hich lie outiide the network anti therefore do iiot coiitiibutc to the c~l:i+t~capropcrtwr. Tliwe rorrectionr h r c the iorni
3lochel (21) has shown that 1007, convcwioii, ,~uli'ur-iiiodifietl neoprene polymers are esscriti:iIly a,ll gcl at, low csonversions (ahout 40a/0), which malic,; it expcrinicntallg difficult to detcrmine t,he primary molecular weight, J f , From tJhe rmults on sol polymere ( 2 8 )it c a n 1)e efitiiiiatcxttthat the imcroraliiiked niolecular n-eight, is at Iwqt 200,000. .I[utene ( 2 , 2 0 ) . S o t only do the infrared data COILf i i n1 the aisociation of the rclPit>cd chlorinc with the structui (' ft om 1,2 addition, but they albo shon that the double bond z h i f t y during thr reaction, as such n. Imnd renirxngrmcnt i5 iicc to iiinl\e tlic absorption tliniiniilr 111 intcn.it5 as t h r h>ti 111o c w d ~ .
Tlw I ate of halogen evolutioii 1. i n i l r p c ~ i i t l e i i tof the iriiti,tl c o ~ i rriitiation of chloride u p to 0 215 I / a i ~ ~ as 1of 1the initial m t i centlation of hydroxyl ionb, Iiox n iri Vigurc 2. Sc.ithcr t h i x S,I nor t h e S,2 nierliitnisni pi opo-t tl E o 1 ttic nurleopliili( -1111-
January 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
stitution of alkyl halides would predict such behavior (16, 16). C7nfortunately, the observed rate of the reaction provides no iriformation on the mechanism, as it fits neither first- nor secondorder kinetics. Presumably the rate is affected by the diffusion of ions into and out of the Iatcx particles. The data, however, are in accord with an intramolecular rearrangement accompanied by hydrolysis, similar to the mechanism suggestcd for the solvolysis of cu,a-diinethylalIyl chloride (3-chloro-3(89). nirth~-I-l-l)utene)
173
be 2 / 2 ( 5 ~ ~ / 5 f p )This . fraction W ~ Sdetermined by the appliration of the Flory-ltehner theory to the equilibrium swelling of zero-cured films in toluene. I t increases with time, as shoir n in Figure 3, indicating that crosslinks arc formed within tiir h t c \ p:irtirles as the latex ages.
K
o
WATER PHASE POLYMER PHASE
S U M OF CURVES IN FIG 5
'0
IO 20 30 40 LATEX AGE (HR. AT 9 0 . C )
50
Figure 4. Active Ttalogen in Neoprene Latex with Excess Piperidine
1
LATEX AGE (HR.
When allyl halides hydrolyze they ordinarily produce alcohols. Hon-ever, further reaction may occur, especially a t high pH, to give diallyl ethers (6, 31). In neoprene tho formation of an ether would result in a crosslink l y x mechanism of the type
9O'C.)
Swelling of Neoprene i n Toluene a t 25" C.
Cure minutes at 141' C.
a0
0 30
v
I;
/I
x
CII
CIL
AT
50
Figure 6 .
-c--
-e-
40
30
20
I
I
I
I
I
10
0
60 120
I
.\lthough the niajor c t i wtur nl modificatiori ic crobdifik f o i nixtion, a second type of rcactive cahlorine (R&Cl), appawlltly tc.1 tiary, is also formed during aging. Thib qecond type i.r found whon piperidine is added to thc Irtteu, if tlic amount a d d d i- in c'rccz~of that necessary t6 react with the allylic chlorine. The rate of chloride ion evolution and i h e quantity of ion eliminatrd, a i shown in Figure 4, are grmter than for agings cairied out nithout piperidine. The quantity of artive rhlorine remaining in the polymer decreases a t a rate about equal to the rate of appealanre of chloride ion in the v-ater phase. I n addition, infrared analyses indicate that the allylic structure disappear6 very rapidly, within 4 hours a t 90" C. It iu concluded from thew experiments that three reactions occur eimultaneoudy,
u IO
20
40 AT 90.C.)
30
LATEX AGE (HR.
50
+ ck12=cII--C:--cI
% .
LTNH
''
D
c
I BCI-I*=CI-I--C-C'l
N
I
H
+ R&Cl
----i,
R2C=C-R'
Figure 5. Active JLalogen in Neoprene Latex Polymer phare
0 Wnter phase
The proposed crosslinking mechanism indicates that t\vo clilorine atoms are released as ionic chlorine, and two chains of niolecular weight M i are formed, for each crosslink introduced by hydrolysis. As each monomer unit in neoprene contains one chlorine atom, the number of chlorine atoms in, n chain is found by dividing the molecular weight of a chain, iM,,by the molecular weight of one monomer unit, ; I f f i . The fraction of chloriric atoms separated from t h e polymer by crosslink formation will
1
I
.
1
+
--+
C=CH--CHZ--O--CEI1-~II=C:
!
+
'I + 1A
=CII-CII,--OI€
with the relative rates A > B > > C. In contrast t o this behavior is the nonpiperidine aging show II in Figure 5. In this case the rntc of appearance of chloritic ion in the water phase is much more rapid than the rate a t which thc polymer loses chlorine. ThiF observation indicatec that tllv
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
174
TABLE I.
VLJILXXIZ.4TIOS O F
70%
300% Modulus, Lb./Sq. I n c h (Cure, C.) 10/353 20/153 *0,'123 2500 3150 20*50
Tc Cl Renioi-ed
Polymel
0
Untreated
Piperidine treated ((8OoC./14 hours)
C O S V E R S I O N hTEOPREXE
370
1.5
Polymer SRF carbon black RIgO ZnO Ethylene tliiouiea
350
100 30 4
5 1 I F
N-Phensl-l-naphtli3.1aniine
I
I
IO00
Figure 7 .
2000
450
I
3000
4000 AVERAGE TENSILE STRENGTH (FS1.)
5000
Relation of Crosslinks t o Tensile Strength 0 No ZnO or clay
0 ZnO V ZnO and clay
active chlorine content of the polyrner is being augmented by the formation of the second type of active chlorine during the aging process. Further evidence t h a t the polymer contains two types of active chlorine is the fact t h a t the halogen in the water phase plus the halogen remaining in the pol? mer equals the halogen removed by piperidine, as shown in Figure 1. VULCANIZ4TIOh 4 Y D TENSILE STREhGTH
I n order to determine the effect of these aging changes on the physical properties of dipped films, it is necessary to know what structural changes occur during vulcanization. It is thought that crosslinking takes place between active chlorine chain poqitions during cure, as well as during aging. Evidence for this I
I
e
I
0 CURE
Vol. 47, No. 1
is provided by the vulcanization of a low conversion neoprene treated with piperidine t o remove most of its labile chlorine. The data in Table I apply to the polymer treat,ed with a large excess of piperidine a t 80" C. and subsequently compounded on a mill. The reeult,s show t h a t when the active chlorine eontent of the polynier is significantly reduced, the polymer cannot be cured satisfactorily by normal neoprene vulcanization techniques. T o study the changes in crosslinking during cure, f i l r n ~werc dipped from laticee of various ages and various compounding formulas. These films were cured and tested for both st,ressstrain relationships and the extent of crosslinking as measured by sT\-ell. The swelling results, shown in Figure 6, lead t o the important conclusion that only a limited number of crosslinks arc possible in neoprene films. This limit is reached after a 30-minute cure a t 141' C. for films not cont,aining compounding mat,erials. Further heating for 1 or 2 hours int,roduces no additional crosslinks. The limit is raised and is reached only after a 60-minute cure for films compounded with zinc oxide. The limiting Q values correspond to a release of 0.70% of the total chlorine in the first case, and l.lyOof the total chlorine in the second case, calculated according to the Flory-Rehner theory and the procedure described previously. An empirical relation between the ratio Mp*j.Tfc' X 100 and 1 similar relation the tensile strength is graphed in Figure 7 . ' has been reported for natural rubber (lg-14, 24). The tensile st,rength of cured neoprene samples, for a given compounding formula, is a linear function of t,he percentage of monomer units engaged in crosslinking v,-ithin the range studied. When thc number of crosslinks remains constant during an increase in curing time, the tensile strength also remains constant. The addition of a clay filler causes a large over-all rise in the t,ensile strength without a corresponding rise in the number of crosslinks measured by snelling. Such a result would be produced through the destruction of bonding forces between polymer and (-lay by the spreading action of the absorbed solvent (33). The tensile strength is also an inverse function of the latex age, as shown in Figure 8. When these tensile strengths are plotted against crosslinking, a linear relation, such as that graphed in Figure 7, is not found. For example, the total number of crosslinks in the films cured 120 minutes is independent of the latex age, whereas the tensile strength of these same filnis declines continuously. The level of crosslinking aft,cr a 120minute cure apparently corresponds to a plurality of tensile etrengths, in contradiction to the data of Figure 7. The contradiction can be removed by making a distinction bet\veen the tot,al number of crosslinks in the film and the "effective number." The total, X , may be expressed as a function
I
4 - .
30'/141'C
0
607 141'C.
3
120'/141"C.
X
E
Fi
3-
A
I
2O
3000 m u v) c -I
I
IO
I
I
20
30
I 40
(HR
8
0
SOT)
5
(HR
8
90'Cl
L A T E X AGE
a later as well as during cure, and the decline in tensile strength with latex age ma:- kw in\ correlated with the anioiint of cw~sslinkingduring aging. T h e decline in tensile strength caii be prevcntcd by compounding with bis(diethylthiocarbainy1) disulfide. T h e calculations of the swelling data are not as coriipletc~:is might be desired, primarily Iicciiwe of difficulty of measuring aud interpreting the molecular weight of high conversion polymcrs and t o some uncertainty in determiiiing p . It is felt that, thc qualitative observations are justified, anti that thry scrre to define both the technically iinportant effect of film structurcx o i i knsile strength and thc nierh:inisni of cur(%.
.I. .Im. C/~om.SOC.,7 1 , 1136 (1949).
LIorrell, S..EI., and Stcrii. .J.. Trajts. I m f . Rubbe? I n d . , 28, 2(\9 (1952).
Scott, R. L.,
,r. c / ) ~~ i ~~ !~13, ~ .. ~ 178 . .(1945).
Scott, 1%. L., and IIaeat, AI.. /bid.,13, 173 (1945). Scott, ti. L., and Magat, :\I.,J . I'oZy/?ar~ Sci., 4, 555 (19.19). Walker, I€. W., and l l o c h e l , '177. E., PTOC. RubhPr Technol.C o ~ j . I?td. C0n.f. ( L o n d o n ) , 69 -79 (1948). \Tall. 1'. 'I?., aiid Flory, P. J., J . Chem.. Phil,?.. 19, 1135 (1951). Whitby, G. S., Evans, A, B. A, and Paiternsek, D. S., T r a n s . Faraday SOC.,38, 269 (1942). \lXiams, E. C., Trans. Am. Inst. Chena. Engrs., 3 7 , 157 (1041). Young, ZIT. G., Winstein, 8..and Goering, 1%.L., J . Am. L'/t(,m. Six., 73, 1958 (1951). % a p p 12. L., and Guth, E., ISD. ENG.CHEX,43. 435 (1 95 I ) .
