KURT L. SELIGMAN Organic Chemicals Department, Jackson Laboratory, E. 1. du Pont d e Nemours & Co., Inc. Wilmington, Del.
Formation of Microgel during Accelerated Aging of Neoprene Latex Aging polychloroprene latex in an alkaline medium causes chains to lengthen, after which the chains are cross linked to form microgel
T H E term “microgel” originated with Baker (3) to apply to cross-linked butadiene-styrene (GR-S) copolymer which was soluble in solvents normally used for linear polymer. Medalia ( 7 7) defined microgel as a portion of polymer which contains tetrafunctional branching units and has the dimensions of a latex par. ticle. He presented evidence that microgel exhibits the solubility characteristics of sol polymer and described the isolation of microgel by selective oxidation of GR-S (72). Freeman (7) and Bloomfield ( 5 ) recently found that natural rubber latex contained 5 to 30% microgel, depending upon the clone. Coagulating the latex and smoking the sheet did not change the microgel content of the rubber. The microgel content of the latex, however, could be increased by treating the latex with a free radical redox system normally used as a polymerization catalyst. Such an increase in microgel was accompanied by an increase in the viscosity of the polymer. The formation of microgel during polymerization of chloroprene has recently been investigated by Morton and Piirma ( 7 3 ) . These workers observed a peak in the intrinsic viscosity-conversion curve of mercaptan-modified polymerizations, and by molecular weight determinations they established that the decrease in intrinsic viscosity with conversion after the peak was caused by microgel formation rather than degradation of the polymer. Andersen and Kovacic ( 2 , g ) have correlated the decrease in tensile strength of neoprene (poly-2-chloro-1,3-butadiene) films isolated from latex aged at 90’ C. with the formation of cross links during aging. Under these conditions cross linking must yield micro- rather than macrogel, since cross linking occurs exclusively in the latex particle. Unless the
latex particles agglomerate, cross linking cannot yield macrogel. The mechanism of cross linking in a n aging neoprene latex is unique for polychloroprene and is different from that postulated to occur during the polymerization of dienic hydrocarbons. Allylic chlorine is present in polychloroprene as a result of 1.2-polymerization-i.e.(Z, 9, 70)
-CHz-C-
I
c1 I I1
it to macrogel by oxidation has given unreliable and nonreproducible results. Polychloroprene latices therefore provide a n excellent opportunity to study the effect of microgel, caused by a n ionic cross-linking mechanism during latex aging, upon the milling and vulcanizate properties of the polymer. The extent of microgel formation as determined by intrinsic viscosity as well as light-scattering and ultracentrifuge methods was correlated with the above properties.
CH
Experimental
CH a
Polymerization. The recipe employed for the polymerizations carried out at 40’ C. was similar to the one described by Walker and Mochel ( 7 4 ) employing 0.25 gram of dodecyl mercaptan per 100 grams of 2-chloro-1,3butadiene as the modifier. The polymerization was stopped a t 70% conversion by the addition of an emulsion containing phenothiazine and p-tert-butylcatechol. The excess monomer was removed under vacuum. For polymerizations employing ammonium hydroxide as the alkali component of the soap system, 4.7 grams of concentrated ammonium hydroxide (28% ammonia) per 100 grams of the monomer was substituted for 0.55 gram of sodium hydroxide. Aging of Latices. The latices were aged in a constant temperature bath a t 50’ =t 0.1’ C. For intrinsic viscosity and light-scattering determinations, samples of latex were withdrawn successively from the same bottle which had
These units vary from 1 to 2’%, depending upon the temperature of polymerization. The highly reactive allylic chlorine makes possible the curing of neoprene with such compounds as ethylene thiourea and p , p ’-diaminodiphenylmethane. I n aqueous alkaline emulsions the allylic chlorine undergoes hydrolysis to yield cross links in a manner similar to that of the formation of diallyl ether from allyl chloride under hydrolytic conditions accompanied by an allylic rearrangement (6, 76). For neoprene the reaction may be pictured as follows: HzO CH2=CH-&-CI I NaOH
-
HOCHz---CH=&
,
---+ NaOH
I
+ NaCl
C=CH-CHZ-O-CHX-
1
Preliminary work to estimate microgel in polychloroprene latices by converting
CH=A
I
+ NaCl
been kept either under a n air or a nitrogen atmosphere. For milling and vulVOL. 49, NO. 10
OCTOBER 1957
1709
after two passes at a nip setting of' 0.025 inch. The sheet is placed on tin or paper coated well with talc and after 20 minutes a 3 X 6 inch piece is plugged out. The plug is weighed, and the nerve number is obtained from the following equation:
2 1
9,
I IC
Weight of plug - 9.1 9.1
51
500
lCOD
nouas
AT 500c
\.
1500
2000
Figure 1. Change in intrinsic viscosity of polymer from sodium hydroxidebased latex with age of latex at 50' C.
---,
_ _ _ _,
Changes in intrinsic viscosity o f polymer from latex containing only sodium hydroxide Change in intrinsic viscosity o f polymer from latex whose pH was adjusted with tetraethanol-ammonium hydroxide after 940 hours at 50' C.
canizate properties the polymer was isolated from the latex by application of heat to films. The final drying was accomplished by heating the film for less than a minute at 110' to 120' C. Intrinsic Viscosity Determinations. The intrinsic viscosity was determined by the Vistex technique developed by Baker ( 3 ) . This method does not require the isolation of the polymer. About 2 grams of latex of known polymer content was dissolved in 100 ml. of a 55 to 43 mixture of benzene-pyridine, a homogeneous solution being formed while shaking. Dust was removed from the solution by centrifuging for 30 minutes at 1900 r.p.m., a speed insufficient for the sedimentation of high molecular weight polymer. Intrinsic viscosities were obtained using an Ostwald-FenskeCannon viscometer at 30" =t0.1' C. Light Scattering. The light scattering of freshly made and ultracentrifuged solutions of polymer was measured over the angular range of 30" to 150" with the angular dissymmetry instrument recently described by Billmeyer and DeThan (4). The solutions were prepared by dissolving the latex in the 55 to 45 mixture of benzene and pyridine to form solutions containing approximately 1.2% polymer. The clear solution was centrifuged for 30 minutes at 140,000 gravity on a Spinco Model L preparative centrifuge. The percentage of high molecular weight polymer separated by centrifuging was determined by measuring the polymer content in the solution before and after centrifugation. Milling and Vulcanizate Properties. NERVE( 8 ) . The polymer (500 grams) is milled at 50" C. (roll temperature) on a 16 X 6 inch mill a t a 0.03 inch nip setting for 2.5 minutes, followed by milling for 3.5 minutes at a nip setting of 0.06 inch. The polymer is rested for 30 seconds, and the stock is sheeted off
x
1710
=
nerve number
where the value 9.1 is the theoretical weight of a 3 X 6 X 0.025 inch piece of neoprene. The nerve number then indicates the number of times the plugged stock is increased in weight over a theoretical value because of nerve. I n essence this nerve determination yields a value indicative of the amount of mill shrinkage of an elastomer. Nerve numbers greater than 6 are meaningless because polymer sheeted from the mill is lacy and correct weights cannot be obtained. PoLYhiER VISCOSITY.The viscosity of the polymer was determined with the Mooney disk viscometer, ASTM Method D 927-53T. Values reported were obtained with the large disk, and readings were taken 2.5 minutes after starting the motor. The term "10-pass Mooney viscosity" denotes that the polymer had only 10 passes through the mill, while "milled Mooney viscosity" refers to a viscosity determined after milling the polymer for the nerve test. VULCANIZATEPROPERTIES. Stressstrain properties were obtained at room temperature on 1 X 5.5 inch dumbbells with a Scott tester, ASTM Method D 412-51T. The vulcanizates comprised 100 grams of polymer, 4 grams of magnesium oxide, 5 grams of zinc oxide, and 0.35 gram of 2-mercaptoimidazoline, and were cured 20 minutes at 153" C. Results and Biseussion
Physicochemical Studies. The Vistex (3) technique for preparing a polymer solution directly from an aqueous latex is extremely well suited when a dilute polymer solution is required for physicochemical polymer studies. This technique obviates the need for coagulating and drying the polymer and eliminates any changes in structure during the isolation step. The intrinsic viscosity of polymer from a latex containing sodium hydroxide increased from 1.38 to approximately 2.00 (Figure 1) on heating the latex for 1200 hours at 50" C. The p H of the latex during this time interval dropped from 11.85 to 6.55. This increase in acidity is caused by the elimination of hydrogen chloride from the polymer (7). Subsequently, the intrinsic viscosity decreased to a value of 1.56 during an additional 740 hours. To prevent coagulation of the latex by neutralization of the soap, the pH of a portion of the latex was increased to 11.2 bv the addition of
INDUSTRIAL AND ENGINEERING CHEMISTRY
2.3r
0 5
L
SA5 c7Jooo3 n o m s AT 51 c.
ZbO
Figure 2. Change in intrinsic viscosity of polymer from ammonium hydroxidebased latex with age of latex at
50" C.
___ . Under nitrogen - _ _.- Under air
atmosphere
tetraethanol-ammonium hydroxide after 941 hours. The intrinsic viscosity following this pH adjustment decreased at a much faster rate and reached a much lower value (0.52) after a total aging time of 1910 hours than that of polymers from the latex containing only sodium hydroxide. To take advantage of the apparently greater rate of polymer cross linking in latices containing amines and to avoid the inherent long-term colloidal instability of neoprene latices based on sodium hydroxide (7), latices made with ammonium rosinate and containing free ammonia were examined. The changes of the polymer intrinsic viscosity with aging of the ammonia based latex at 50' C. in air and in nitrogen are shown in Figure 2. Both of these curves show the peak in intrinsic viscosity at a shorter aging time than the curve obtained from the latex based on sodium rosinate (Figure 1). Cross linking of the polymer protected with nitrogen is assumed to occur only through the allylic chlorine as previously discussed. Formation of ether bonds from allylic chlorines resulting from 1,2-polymerization is a rational explanation for the cross-linking reaction observed in an ionic medium. This work supports the previously advanced hypothesis only in a complementary manner; it could
y eo
10
-__------______-__l____-_--
I
I
zoo
400 HOURS AT 50'C.
600
___-
d
800
Figure 3. Scattering intensity a t 90' of polymer solution from latex aged at 50" C. Polymer concentration normalized to 1 gram/ 100 ml. of solution Solutions prior to centrifuging , Solutions after removal of high molecular weight polymer b y ultracentrifuging
-- . ----
MICROGEL F O R M A T I O N 70 -
4Or
60n
K
w
a
-__--_ _ _ _ _ _ _ _ _ _ _ _ _*____----
------4
0
I
!
400
600
~
200
I
200
BOO
HOURS AT 5OOC.
Figure 4. Dissymmetry ratio at 30"/150" of polymer solutions from latex aged a t 50' C.
.
- - - -.
Table I.
the early stages of aging as cross links form between allylic chlorine sites on different polymer chains. This type of reaction prpduces a high molecular weight polymeric species which cannot be considered true gel; it is rather a linear molecule containing long branches which will cause an increase in the intrinsic viscositv. At the same time some of these high molecular weight branched molecules formed during polymerization will C b S S link to form true microgel, but their relatively small number will make no observable contribution to the intrinsic viscosity. However, the effect of microgel formation on intrinsic viscosity will become noticeable when the supply of original linear macromolecules approacges depletion. This stage is indicated experimentally by a decrease in intrinsic viscosity because now the microgel particles contribute less to the intrinsic ;iscosity than the original, linear uncoiled molecules that linked together to form the microgel.
.
Characterization of
greater rate of viscosity decrease can be attributed to the additive effect of oxidative cross linking. These data show that the molecular weight of the polymer increases during
Gel Content in Neoprene Latices in 55 to 45 BenzenePyridine is0
Normalized
Age of Wt. 70 Latex a t Removal a t 50' C., by UltraHr. centrifuge Original Centrifuged Original Centrifuged Original Centrifuged
toC= 1
Intensity of Scattered Light at 30' 900 150'
0
38.74 13.45 1850.5 6.45 1129.7 13.64
1.2 474 54.6 786 63.7
12.85 9.83 133.89 4.47 131.25 3.67
10.64 8.78 43.37 4.23 31.14 3.54
800
HOURS AT 50°C. Figure 5. Percentage of polymer removed by ultracentrifuging from polymer solutions from latex aged at 50" C.
Solutions prior to centrifuging Solutions after removal of high moleculat weight polymer by ultracentrifuging
support any other cross-linking mechanism based upon elimination of hydrogen chloride. I n the presence of air the occurrence of the peak of the intrinsic viscosity at a shorter aging time and the
I
600
400
Gram per 100 M1. 10.82 8.37 111.28 8.19 105.50 8.14
Ratio iaoiiso
3.64 1*53 38.26 1.53 36.28 3.86
w
2
4.0-
W
z 3.5,r
,
60 5o
3.0 -
/
/
,
-.--r--+--M'A
100
200
A
HOURS AT
I
I
300
400
50'C
I
500
2.5
-
2 .o
.
- - - -. A.
10-Pass Mooney Mill Mooney
Intrinsic viscosity peak
Figure 7. at 50" C.
I
0
I
I
Change in nerve of polymer with age of latex
A
Intrinsic viscosity peak
VOL. 49, NO. 10
OCTOBER 1957
1711
-.-/ *'/.,
600
,,
quate explanation for this phenomenon. Effect of Microgel on Milling and Vulcanizate Properties. Figure 6 shows the changes in Mooney viscosity caused by the accelerated aging of the latex. Small but definite increases in Mooney viscosity suggest that during the initial stage of aging the original molecules are linked together to form linear branched chains. As aging proceeds, cross linking of these branched polymers to form microgel causes a rapid increase in Mooney viscosity. This occurs before the intrinsic viscosity reaches its peak. At the intrinsic viscosity peak, Mooney viscosity has a value of 1.5 times the original, indicating the tremendous effect of microgel upon the flow properties of the polymer. As additional cross links are formed, producing tighter gel, the rate of increase in Mooney viscosity with aging time lessens considerably. An indication of the effect of milling upon polymer breakdown is shown by the difference between the 10-pass and milled Mooney viscosity determinations. The difference between these values (A viscosity) (Figure 6) remains constant with aging to a point beyond the intrinsic viscosity peak and then decreases rapidly. After 420 hours the two viscosity values are equal. Polymer breakdown therefore does not occur once gel of a sufficient degree of cross linking has been formed. The nerve of the polymer as determined by the mill shrinkage test is markedly affected during the early phase of aging. Within 150 hours, a time considerably shorter than that required to reach the peak in the intrinsic viscosity curve, the nerve number has almost doubled, as shown in Figure 7. At much greater microgel content, resulting from prolonged aging, the nerve of the polymer decreases to a value less than that of the unaged sample in spite of the fact that a concomittant decrease in Mooney viscosity has not occurred. These results are in agreement with those reported by White and others (15) on the effect of gel content upon processing properties of GR-S type elastomers. In the course of this study, polymer nerve was the only polymer property which was adversely affected by the slightly cross-linked gel structures formed during the initial phase of the aging process. An investigation of the stress-strain data of vulcanizates of polymer isolated from latex undergoing accelerated aging supports the conclusions suggested by the intrinsic viscosity and light-scattering determinations (Figures 8 and 9). The small increases in modulus and tensile strength and the higher elongation a t break found for the shorter aging periods can be ascribed to the formation of high molecular weight linear polymer before the peak in the intrinsic viscosity. Further aging of the latex, producing
400b /
200
O!
I
200 A
I
400
HOURS AT
I
50'C.
6oo
Figure 8. Change in moduli of vulcanized polymer with age of latex at
50" C.
. - - _ -. A.
Modulus at 300% elongation Modulus a t 600% elongation Intrinsic viscosity peak
Determination of polymer removed by ultracentrifugation and measurement of the intensity of scattered light provide further evidence that microgel is present in the latex after aging (Table I). An increase in the intensity at the 90' angle signifies an increase in molecular weight, while an increase in the dissymmetry ratio a t the 3Oo/15O0 pair of angles signifies a n increase in molecular complexity. The original fresh sample was essentially free of gel or tail of high molecular weight material, as is shown by the small decrease in 90' scattering of the solutions after centrifuging (Figure 3) and by the low dissymmetry of the uncentrifuged solution even a t the 3Oo/15O0 pair of angles (Figure 4 ) . The small amount of high molecular weight material deposited from the original solution on centrifuging for 30 minutes at 140,000 X gravity confirms these conclusions. The sample aged 474 hours has more than half the polymer present as high molecular weight material or gel (Figure 5). However, the centrifuged solution contains polymer having about the same molecular weight as polymer in the original, unaged solution. This is clearly shown by the small changes in the intensity of the light scattered a t 90' and by the equal dissymmetry ratios of the 3Oo/15O0 pair of angles. Further aging increases the amount of polymer deposited by centrifuging, but the intensity of scattered light appears not to be affected. Even after this prolonged aging, removal of gel polymer by centrifuging leaves polymer in solution which does not significantly increase the intensity of scattered light. A comparison of the light-scattering data with intrinsic viscosity determinations supports the postulate that the decrease in the intrinsic viscosity of the polymer with aging is not caused by polymer degradation but by formation of microgel. The ionic cross-linking mechanism which defines the crosslinking site as the allylic chlorine formed by 1,2-poIymerization provides an ade-
1712
INDUSTRIAL A N D ENGINEERING CHEMISTRY
4dO HOURS
6&---
AT 50-C.
ioo
am
Figure 9. Change of tensile and elongation at break of vulcanized polymer with age of latex a t 50" C. A.
Intrinsic viscosity peak
additional and more highly cross-linked microgel, rapidly degrades the stressstrain properties of the elastomer. These results are in accord with those previously reported on neoprene ( 2 ) and GR-S (75). Acknowledgment
The author wishes to acknowledge the assistance of R. M. Murray, Elastomers Laboratory, E. I. du Pont de Semours & Co., in obtaining milling and vulcanizate properties of the elastomer, and Beverly Price, Chemical Department, E. I. du Pont de Nemours and Co., in carrying out the light-scattering determinations. Literature Cited ( 1 ) Andersen, D. E., Arnold, R. G., I N D . ENG.CHEDI. 45,2727 (1953). ( 2 ) Andersen, D. E., Kovacic, P., Zbid., 47, 171 (1955). ( 3 ) Baker, I$'. O., Zbid., 41, 511 (1949). (~, 4 ) Billmever. F. W..DeThan. C. B..' J . Am. Chem. Soc.' 77, 4763 (1955). ( 5 ) Bloomfield, G. F., Rubber Research Inst. Malaya, Commun. 271 (1951). ( 6 ) Fairbairn, A. LV., Cheney, H. A , , Chevansky, A. J., Chem. Eng. f'rogi-. 43, 280 (1947). ( 7 ) Freeman, R.:Proc. Rubber Technol. Conf., 3rd Conf., 1954, p. 3. ( 8 ) Keown, R. W.,Neoprene Plant, Du Pont Co., Louisville, Ky., private communication. ( 9 ) Kovacic, E'., IND.ENG.CHEY.47, 1090 (1955). (10) Maynard, J. T., Mochel, W. E., J . Polymer Sci. 13, 251 (1954). (11) hkdalia, ,4.I., Zbid., 6, 423 (1951). (12) Medalia, A. I., KolthofF, I. ?"I., Zbid., 6, 433 (1951). (13) Morton, M., Piirma, I., Ibid., 19, 563 (1956). (14) Walker, H. M'., Mochel, W. E., Proc. Rubber Technol. Conf., 2nd Conf. 1948, Preprint 11. (15) White, L. M.: Ebers, L. S., Shriver, G. E., Breck, S., IND.ENG.CHFX 3 7 , 7 7 0 (1945). (16) Williams, E. C.? Trans. Am. Inst. Chfm. Engrs. 37, 157 (1 941 ). RECEIVED for review October 18, 1956 ACCEPTED April 5, 1957 Division of Rubber Chemistry, 130th Meeting, ACS, Atlantic City, N. J., September 1956. Contribution No. 22'7, Organic Chemicals Department, Jackson Laboratory, E. I. du Pont de Nemours & Co., Inc., Wilmington, Del.
