1090
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 5
acidaffins, second acidaffins, and paraffins, on polymers containing them, is specific when tested in the form of concentrates. When tested in form of a blend, as available in commercial oils, their effects are additive.
TYPE I OILS 50 40
30 20
ACKNOWLEDGMENT
IO 70
t I
50n
The authors are indebted to the Golden Bear Oil Co., Gulf Oil Co., Phillips Petroleum Co., Shell Oil Co., Standard Oil Co. of Indiana, and Sun Oil Co. for preparing and supplying the samples of oil tested in this investigation.
TYPEII OILS
40
LITERATURE CITED
s
t
(1) Am. Soc. Testing Materials, D 47146T, Method B. (2) Am. SOC.Testing Materials, D 1053-52 T.
'u)
0
L
-
TYPE JII OILS
HOURS AQED AT 5 0 0 . 1
Figure 18. Mooney viscosity breakdown with t i m e of h e a t i n g at 300' F. Nonaerated latex
down in the Banbury are concerned. There does, however, seem to be a factor not yet covered by the specifications. The influence of the fractions defined as nitrogen bases, first
(3) Am. SOC.Testing Materials, D 1158-511'. (4) Baker, W. O., and Mullen, J. W., private communication to Office of Synthetic Rubber. (5) Eby, L. T., Anal. Chem., 25, 1057 (1953). (6) Fenson, D. S., Fifth Canadian High Polymer Forum, London, Ontario, Canada, November 1953. (7) Oldham, E. W., Baker, L. M., and Craytor, M. W., IND.ENO. CHEM.,ANAL.ED.,8,41 (1936). (8) Reconstruction Finance Corp., Office of Synthetic Rubber, Specifications for Government Synthetic Rubber, revised ed., Oct. 1, 1952. (9) Rostler, F. S., and Sternberg, H. W., IND.ENG.CHEM.,41, 598-608 (1949). (10) Rostler, F. S., and White, R. M., Ibid., 46, 610-20 (1954). (11) Schade, J. W.,lndia Rubber World, 123,311 (December 1950). (12) Shearer, R., Juve, A. E., and Musch, J. H., Ihid., 117,216 (1947). (13) Taft, W. K., Duke, J., Laundrie, R. W., Snyder, A. D., Prem, D. C., and Mooney, H., IND. ENG.CHEM.,46,396-412 (1954). (14) Taft, W. K., Duke, J., Snyder, A. D., and Laundrie, It. W., Rubber Age, 75, No. 1, 61-4 (1954). (15) Taft, W. K., Laundrie, R. W., Harrison, T. B., and Duke, J., Ihid., 75, NO. 2, 223-6 (1954). (16) Taft, W. K., Snyder, A. D., and Duke, J., Ihid., 75, No. 6, 830-40 (1954). R E C E I V ~for D review .4pril21, 1954. ACCEPTED October 26, 1954. Presented before the Division of Rubber Chemistry, AMERICAX CHEMICAL SOCIETY,Louisville, Ky., April 1954. Work performed as a part of the research project sponsored by the Reconstruction Finance Gorp., Office of Synthetic Rubber, in connection with the government synthetic rubber program.
Bisalkylation Theory of Neoprene Vulcanization PETER ICOVACIC Jackson Laboratory, E. I . du Pont de Nemours & Co., Znc., Wilmington, Del.
MONG the organic compounds that are recommended ( 6 ) for use with magnesia and zinc oxide as vulcanizing agents for neoprene are ethylenethiourea (2-imidazolidinethione), p , p'diaminodiphenylmethane and t,he di-o-tolylguanidine salt of dicatechol borate. I n the absence,of sulfur, these agents in combination with zinc oxide do not vulcanize natural rubber. This points up a marked difference in the way these elastomers vulcanize. Although a major structural difference is the chlorine atom in neoprene in place of the side methyl group in natural rubber, the small amount of tertiary allylic chlorine formed by 1,2polymerization (8) is the important functional difference. The labile chlorine amounts to about 1.5% of the total chlorine in a
general-purpose neoprene made a t 40" C., such as Neoprene Type W used in this work. I n neoprene latex this active chlorine is gradually liberated, and the polymer becomes cross-linked ( 2 ) . This paper demonstrateB the importance of the labile chlorine in the vulcanization of dry neoprene, accounts for the difference in the vulcanization of neoprene and natural rubber, and suggests a bisalkylation theory of neoprene vulcanization. R E S U L T S AND DISCUSSION
Untreated Neoprene. A small number (about 1.5 mole %) of the monomer units in neoprene are incorporated in the chain by 1,2- addition during polymerization (2, 2, 8, 12).
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1955
c1
1
c1 -CH2-C=CH-CH2
1
Table I. Reagenta
1,4- addition
Piperidine $niline
The tertiary allylic chlorine in the 1,2- structure readily reacts with piperidine or aniline with elimination of chloride ion (Table I). Nitrogen from the aniline or piperidine is present in the polymer in the ratio of one nitrogen atom for each chlorine lost. T h e reaction proceeds, therefore, by substitution, and an S,2' mechanism (2, 1 4 ) provides a logical explanation for the observations.
I
+ 2"
Cl--C-CH=CHz
I
a0+
I CH . HS ) I C
(1)
I n contrast, practically no reaction resulted from treatment of neoprene with triethylamine, pyridine, triethylamine-phenol, or pyridine-phenol in xylene for 14 hours a t 100' to 112' C. This agrees with t h e results of Young, Webb, and Goering ( I d ) , who showed t h a t triethylamine reacts much more slowly with a-methylallyl chloride than does diethylamine. T h e small amount of labile chlorine in neoprene is known ( 2 )
Preparation of Amine-Treated Neoprene Solvent Benzene Benzenee Xylene
....
1,2- addition
YemD., C.b 80 2 80 zt 2
c1 Removed,
Polymer Ana1~si5,
%"
*
Trace
100-105
..
c1 39.4 38.2.f 38.5
1.44
%d
N 0.02 0.27~' 0.27
Ratio of reagent to chloroprene unit, moles = 2.6. Time = 14 hours. c Determined on aqueous extract and based on total available chlorine (37.5% of polymer). d Untreated Neoprene Type W'contains 37.5% chlorine and 0.02% nitrogen. Chlorine content is raised by precipitation from solvent because of removal of solvent-soluble, nonpolymerio components. 6 Essentially same results were obtained by treatment in xylene a t 100105". f Values unchanged by repeated precipitation of polymer. a b
t o be necessary for some types of vulcanization. Neoprene Type W usually is cured by an organic vulcanizing agent together with the oxides of zinc and magnesium. Among these organic vulcanizing agents are bifunctional compounds (3, 6, 7, 9, 1 1 ) of the types shown in Table 11. It is suggested that vulcanization of untreated neoprene by these compounds proceeds through bisalkylation of the vulcanizing agent by the polymer chains a t the active chlorine positions. The cross-linking reaction in the case of piperazine is illustrated as follows:
+ H N/--\NH
I
2Cl--C--CH=CH2
I
A=CH--CH2-N NO VULCANIZING AGENT
1091
I
CURE. 40 MIN. AT 141OC.
-+
\J I--\
\-/
X--CH2-CH=h
I
+ 2HC1
(2)
PHENOL ( 1 % )
Table 11. Neoprene Vulcanizing Agents Diamines
N-METHYL ANILINE (1%)
p,p'-Diaminodiphenylmethane
DI-n-BUTYLAMINE ( I %) ETHYLENE THIOUREA (0.5 %) ( c o N ~ R ~ , L )
I
I
Piperazine 2.5-Dimethylpiperazine Dihydric phenols or derivatives Catechol Di-o-tolylguanidjne salt of dicatechol borate Aminophenols 2-Methyl-4,6-bis(dimethylaminomethyl) phenol Thioureas Ethylenethiourea N,N'-Diphenylthiourea Thioamides e-Thiocaprolactam
l
M300 100
Neoprene N-Phenyl-2-naphthylamine SRF carbon black MgO ZnO
Figure 1.
1 50
2
5
Monofunctional compounds as neoprene vulcanizing agents
3000 NEOPRENE PHENYL-2 NAPHTHYLAMINE SRF CARBON BLACK
-
MgO ZnO
0 0
*
AGENT
100
NEOPRENE PHENYL ENAPHTHYLAMINE SRF CARBON BLACK ETHYLENE THIOUREA
(NO ETHYLENE THIOUREA)
I 30 2
5
NONE
z
Mg0(2%)
ZnO (5%)
I
Mg 0 (2%)+ Zn0(5%) 10001
I
1
I
0
I
2
3
Figure 2.
I
I
I
I
4 5 CATECHOL (%)
6
7
B
I
Relation of modulus to vulcanizing agent concentration
i
I
I
I
100 I
50 0.5
INDUSTRIAL AND ENGINEERING CHEMISTRY
1092
Vol. 41, No. 5
AGENT N EOPR E NE 100 PHENYL 2NAPHTHYLAMINE I SRF CARBON BLACK 5 0
NONE
MgO (2%)
ZnO ( 5 % )
I,5-OIBROMOPENTANE
( 4 . 2 %)
1,6 -DlBROMOHEXANE
( 4 . 6 %)
I,4-DICHLORO-2-BUTENE
'
(2.5%)
MgO(2X)t ZnO(5X) BENZYL
CHLORIDE ( 2 . 5 % )
Mg0(2%)t Z n O ( 5 X ) t ETHYLENE THIOUREA ( 0 . 5 % )
2000
1000
1000
0
3000
2000 M 300
Figure 4.
Effect of metal oxides on neoprene vulcanization
M300
Figure 7.
Vulcanization of piperidine-treated neoprene with labile halides
N O VULCANIZING AGENT
E T H Y L E N E THIOUREA ( 0 . 5
+
-
I. 6 -0lBROMOHEXANE
%)I
PIPERIDINE (0.8%) ETHYLENE THIOUREA ( 0 . 5 % )
1.4 -DICHLORO-2-
B U T E N E ( 2 . 5 %)
E 1000
M300
Figure 8. 2000
Effect of labile dihalides on untreated neoprene
M300
Effect of piperidine on neoprene vulcanization
NO HALOGEN COMPOUND
ZINC CHLORIDE ( 2 % )
TRIPHENYLCHLOROMETHANE
( 4 . 6 %)
0
1000
0
Figure 5.
1.5- DIBROMOPENTANE ( 4 . 2 %)
(2% ) 500
0 300
Figure 6. Active halogen compounds with ethylenethiourea (o.5aj0) in piperidine-treated neoprene
T h e hydrogen chloride, which may initially form an amine hydrochloride, would be expected to react with zinc oxide or magnesium oxide, normally present, to form the corresponding chloride. It is significant that the active agents listed in Table 11, commonly referred to as accelerators, possess a structure capable of alkylation a t two positions by an alkyl halide. I n addition, the monofunctional compounds related to the bifunctional ones are inactive-e.@;., phenol, N-methylaniline, and di-n-butylamine (Figure 1). The limited number of cross-linking sites involving labile chlorine should result in a maximum in the cure as the concentration of the vulcanizing agent is increased, since monoalkylation will occur a t high concentrations with formation of polymer side chains instead of cross links. This is the case as shown with catechol in Figure 2. The position of the maximum in the modulus a s the concentration of the vulcanizing agent is increased is in good agreement with the calculated value of 0.9% catechol required for cross linking with the 1.5y0active chlorine. A sim-
ilar effect is noted (IO)in vulcanization with 2-mercaptothiazoline a t varying concentration. According to the proposed mechanism, the appropriate bifunctional molecule alone should cross-link neoprene. Busse and Billmeyer have shown that neoprene dissolved in Tetralin is gelled on heating with tetraethylenepentamine or ethylenethiourea (4), as well as with benzidine or piperazine hexahydrate (6),indicating that such cross linking indeed occurs. However, in dry neoprene in the absence of metal oxide, ethylenethiourea produced poorly cured stocks (Figure 3). This implies that for practical cures the cross-linking reaction requires a catalyst such as metal oxide or metal chloride. An alternative interpretation is that a combination of cross-linking reactions occurs with zinc oxide also present. Figure 4 shows the partial vulcanization of Neoprene Type W achieved with metal oxides alone. It is believed that this partial cure can be attributed to the formation by zinc oxide of ether cross links a t the active chlorine positions, or to an ionic-type polymerization initiated by zinc chloride. Supporting evidence that zinc chloride plays a role, obtained with various types of neoprene, is provided by the nature of cure retarders, which may be classified into two groups.
1. Those which compete with zinc oxide for hydrogen chloride, such as magnesium oxide and sodium acetate. 2. Those which reduce the catalytic activity of zinc chloride by coordination, such as water and aromatic secondary amines. Amine-Treated Neoprene. The substitution of amine groups for the active chlorine in neoprene causes a radical change in the vulcanization characteristics. The piperidine- and aniline-treated polymers described in Table I were not vulcanized by ethylenethiourea (Table 111). Other bifunctional curing agents listed in Table I1 were also inactive in the piperidine-treated polymer. These compounds give MsW values of 2200 to 3200 pounds per square inch in untreated neoprene. Furthermore, small amounts of piperidine compounded with Keoprene Type W similarly render the polymer insensitive to vulcanization by ethylenethiourea
1093
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1955
(Figure 5). Amine-treated neoprene resembles natural rubber in its inertness to vulcanizing agents of this type. Piperidinetreated neoprene is not vulcanized by ethylenethiourea even on addition of zinc chloride or triphenylchloromethane (Figure 6 ) . The importance of active chlorine attached to the polymer chain is thereby indicated.
Table 111.
Vulcanization Behavior of Amine-Treated Neoprene with Ethylenethiourea Ma00
Ethylenethiourea,
N?
vulcanizing agent
0.5-1 7 0 Neoprene 3150 1390 U.ntreated 450 710 Piperidine-treated 156Ob 1600~ Aniline-treated a Extrapolated. T E = 940 Ib./sq. inch at 230% elongation. 6 Extrapolated. T E = 990 Ib./sq. inch a i 220% elongation.
Piperidine-treated neoprene can, however, be given a high state of cure by treatment with bifunctional agents of other types. Thus, it was possible to obtain moduli as high as 2900 pounds per square inch by the use of labile dihalides (Figure 7 ) . These compounds are not vulcanizing agents for untreated neoprene (Figure 8). It is reasonable to believe that cross linking of the piperidine-treated neoprene by dihalides (X-R-x) occurs by a bisquaternization a t the nitrogen-containing sites in the polymer.
SULFUR (I%)
*
ANILINE-TREATED NEOPRENE WITHOUT SULFUR SULFUR ( I %)
Figure 9.
Sulfur vulcanization of untreated and aminetreated neoprene
PIPERIDINE-TREATED
I
UNTREATED
lzza
AGENT NONE
e-DINITROSOBENZENE
7
(1%)
//// / /
CHLORANIL (1.7-2%) 4,4'-METHYLENE
I
C=CHCH2-S-R-N-CHs-CH=C
I
+
X-
+
DI (PHENYLISOCYANATE) (2%)
!
X-
1000
I
T h e low order of activity of labile monohalides indicates the importance of bifunctionality in the active halogen component (Figure 7). A catalytic effect appears to be operative in the cure of piperidine-treated neoprene with 1,4-dichloro-2-butene, as omission of metal oxides results in a substantially lower modulus (M300= 1080 pounds per square inch). This value, however, is higher than the modulus with carbon black alone ( l K 3 o 0 =
