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Natural rubber blends with epoxidizednatural rubberSang Yup Lee a , Joon Hyung Kim a & Byung Kyu Kim aa Department of Polymer Science and Engineering, Pusan NationalUniversity, Pusan, 609-735, KoreaVersion of record first published: 19 Aug 2006.

To cite this article: Sang Yup Lee , Joon Hyung Kim & Byung Kyu Kim (1997): Natural rubber blendswith epoxidized natural rubber, Journal of Macromolecular Science, Part B: Physics, 36:5, 579-594

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J . MACROMOL. SCI. -PHYS., B36(5), 579-594 (1997)

Natural Rubber Blends with Epoxidized Natural Rubber

SANG YUP LEE, JOON HYUNG KIM, and BYUNG KYU KIM Department of Polymer Science and Engineering Pusan National University Pusan 609-735, Korea

ABSTRACT

Blends of NR with ENR have been prepared in full composition following the polymer blend technique. Basic properties (mastication behavior and thermal degradation of each rubber and Mooney viscosity, Flory-Huggins interaction parameters, and cure characteristics of the blends) of the un- cured blends were determined, in addition to the reversion, cross-linking density, mechanical and dynamic mechanical properties, rebound, and sol- vent swell of the blend vulcanizates. It was found that the NR/ENR blends are immiscible, showing two glass transition temperatures (T,’s) that showed outward migration in the blends. This was interpreted in terms of preferred migration of the curatives into the ENR phase. Retention of mechanical properties on aging, solvent resistance, and heat buildup were greater in NR-rich blends due probably to the higher cross-link density of the blends.

INTRODUCTION

Natural rubber (NR) has well-balanced physical properties in terms of me- chanical strength, fatigue resistance, and vibration damping [ 11. However, the very flexible chain backbone and weak intermolecular interactions lead to a very low glass transition temperature (T,) of about - 73 OC. Therefore, the modification of NR has been an active topic, both academically and technically, during the last couple of decades [2]. For example, partial epoxidation gives NR a higher T,, approximately 1 OC per mol% [3]. Expoxidized natural rubber (ENR) has oil resis- tance, gas permeability, and damping performance more like some of the specialty

579

Copyright 0 1997 by Marcel Dekker, Inc.

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580 LEE, KIM, AND KIM

rubbers [4-61. For example, 50 mololo ENR is comparable in oil resistance to a medium nitrile rubber and has air permeability similar to butyl rubber.

Elastomer blends pose some additional difficulties over the plastic blends [2,7]. For example, under all conditions, homogeneous blends or molecular solu- tions are not obtained with rubber blends due to the lack of specific interactions. In addition, the formulations of a cure system have been derived empirically without any real knowledge of the distribution of cross-links between the polymers in the final vulcanizates [8]. Though a number of works have been devoted to it, no technique has been established to determine the cross-links in a blend [9].

This article considers the blends of NR with an ENR of 25 mol% epoxidation. Such blends should be of practical importance with regard to the performance/cost aspect and modification of NR during reactive compounding. Blends of ENR with polyvinyl chloride, chlorosulfonated polyethylene [lo], and chloroprene rubber [ 1 11 have recently been reported, and the existence of a T, of these blends has been reported. However, for NR blends with ENR, no contribution has been made in the past, perhaps except for the work by Maiti, De, and Bhowmick, who reported the migration of fillers in the blends [12].

We describe the basic mastication behavior of NR and ENR to decide the optimum mastication time for the finest breakup of the dispersed domains. Mooney viscosity, Flory-Huggins interaction parameters, and cure characteristics of the uncured blends were then determined. Mechanical and dynamic mechanical proper- ties, heat buildup, rebound of the cured rubber, and the retention of mechanical properties on heat aging are subsequently described.

EXPERIMENTAL

Formulations

The formulations of rubber compounds are given in Table 1 , with the base rubber characteristics given in the footnotes. An efficient vulcanization (EV) system was employed.

Solubility Parameter

The solubility parameters (6) of NR and ENR were determined by solvent swell (Eq. l ) , for which a series of solvents having a solubility parameter 7.2-10.0 (cal/cm3)”* were used [ 131.

m - m , 1

m, Ps Q = x -

In this equation, Q is the swelling coefficient, m is the weight of the swollen sample, m, is the dry weight, and ps is the density of the swelling agent. It is seen that the solubility parameters of NR and ENR are 8.22 and 8.37 (cal/cm3)”* (Fig. l ) , respectively. For NR, the experimental value was the same with theoretical predic- tion by cohesive energy density [14]. The higher value of 6 for ENR compared with NR is due to the contribution of epoxy groups, which augment the intermolecular polar interactions, leading to a lower degree of solvent swell.

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BLENDS OF NR WITH ENR 58 1

TABLE 1 Formulation of Blends

1 2 3 4 5 6 7 8 9

N R" 100 90 80 70 60 50 30 20 E N R ~ 10 20 30 40 50 70 80 100 ZnO 5 5 5 5 5 5 5 5 5 Stearic acid 1 I 1 1 1 1 I 1 1 Calcium stearate 0.3 0.6 0.9 1.2 1.5 2.1 2.4 3.0 Carbon black' 40 40 40 40 40 40 40 40 40 T M D Q ~ 1.5 1 .5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 IPPD' 1.5 1.5 1.5 1.5 1 . 5 1.5 1 . 5 1 . 5 1 . 5 Sulfur 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 CBSf 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2 .5 TMTDY 1 I 1 1 1 1 1 1 1

"SMR CV50, M L , , , (IOO°C): 50-60 "Epoxyprene 25, ML, + J (lOO°C): 70, 25 mol% epoxidation 'Semireinforcing furnace black dPolymerized 2,2,4-trimethyl-1,2-dihydroquinoline 'N-Isopropyl-N' -phenyl-p-phenylenediamine IN-Cyclohexyl-2-benzothiazole sulfenamide XTetramethylthiuram disulfide

Mastication

Mastication of NR and ENR was done on a two-roll mill (150 x 330 mm) at room temperature, and the variation of viscosity during mastication was recorded as a function o f time (Fig. 2).

7 8 9 10

6 (cal/crn')'''

FIG. 1. Determination of solubility parameters of NR and ENR.

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582 LEE, KIM, AND KIM

i E

0 2 4 6 8 10

Mastication time (min)

Mastication behavior of NR and ENR at room temperature (4-in open roll). FIG. 2.

Thermal Properties

Thermal degradation of NR and ENR was determined using a thermogravime- tric analyzer (Du Pont-2100) at 10°C/m at 0-800OC.

Cure Characteristics

Optimum cure time, defined as the time to reach 90% of the maximum torque, and cure torque were determined using a rheometer (Monsanto MDR-2000).

Compounding and Curing

The rubber blends were prepared following the polymer blending method, that is, the base rubbers previously masticated to the desired viscosity were milled in a roll with ingredients, except for the curatives. The mill was operated at 50 f 0.5OC at a friction ratio of 1:25 and a nip of 1.5 mm. On completing milling, curatives were added, and vulcanization was done in an electrically heated press at l5OoC for an optimum cure time determined from the rheometer.

Heat Buildup

Heat buildup was measured using a Goodrich Flexometer on a 12.7 (dia) x 25.4 mm (height) rubber cylinder at 5OoC and 8.75% stroke for 30 min.

Mechanical and Dynamic Mechanical Properties

Tensile properties were determined using a tensile tester (Instron 4200) follow- ing the standard procedure described in ASTM D412-51T. Test specimens were compression molded at 1 5OoC and 150 kg/cm2. The tensile data were used to calcu-

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BLENDS OF NR WITH ENR 583

late the cross-linking density of the vulcanized rubbers. Dynamic mechanical prop- erties of the cured blends were measured using a Rheovibron (Orientec, DDV-OlFP) at 11 Hz.

Rebound

The rebound at room temperature was determined using a Goodyear Healay rebound tester.

Aging

To determine the retention of mechanical properties after aging, samples were aged at 100°C for 72 h.

RESULTS AND DISCUSSION

The mastication behavior of NR and ENR is shown in Fig. 2. An approxi- mately linear decrease of viscosity with mastication time is obtained for NR. On the other hand, the initial breakdown is much faster for ENR compared to NR. There- fore, when the viscosity of virgin NR is higher than that of ENR, the case encoun- tered here, a crossover of viscosity between NR and ENR is obtained during masti- cation. Presently, it is approximately 3.5 m.

The average molecular weight (M,) of NR and ENR at the time of viscosity crossover was determined using GPC (gel permeation chromatography), which turned out to be 257,000 (NR) and 193,OO (ENR). The ENR, having a lower M,, has the same viscosity with NR of higher M, due to its stronger intermolecular forces of polar groups, which augment the molecular cohesion, and hence has more resistance to flow and solvent swell (Fig. 1).

The crossover of component viscosity is of great practical significance in poly- mer blending. Following Wu [15], the finest breakup of the dispersed phase in immiscible polymer blends is obtained when the viscosity of the dispersed phase is close to the continuous one. Therefore, in our experiments, mastication of NR and ENR was done to the extent that their viscosities were matched.

The differential thermogravimetric data of NR and ENR are shown in Fig. 3, in which the maximum peaks are obtained at 392OC (NR) and 402OC (ENR). The higher degradation temperature (by 10°C) and T, (by ca. 25OC) of ENR should provide the blends with higher thermal resistance.

Figure 4 shows the Mooney viscosity of the blends, for which the viscosities of NR and ENR are essentially the same, a result of mastication following the behavior shown in Fig. 2. It is seen that the blend viscosity shows a negative deviation with a minimum (around the middle point) from the simple additivity (the dotted line). This indicates that the blends are rheologically immiscible [12]. In immiscible blends, the negative deviation is often obtained when there are little interactions between the dispersed phases, when there are interfacial slips, or when fibrillations of the dispersed phase occur [16]. It seems that the lack of interfacial adhesion is the prime reason for the negative deviation of viscosity for NR/ENR blends.

Figure 5(a) shows the optimum cure time (T,) of the blends, for both carbon black (C/B) filled and unfilled ones. As expected, the T, of ENR is a bit longer

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584

50

LEE, KIM, AND KIM

-

50

4 0

30

20

10

0

30

20

10

_ - 300 330 360 390 420 450 480

-

- -

I I 1 I I I I I I

Temperature ( "C)

FIG. 3. Differential thermogravimetric curve of NR and ENR

than NR due to its relatively lower unsaturation compared to NR. However, ENR also shows a fast cure compared to synthetic rubbers [17]. With regard to the cure system for ENR, a conventional high sulfur/accelerator ratio gives a fast cure, good mechanical and fatigue properties, but poor aging properties [2]. On the other hand, the lower sulfur efficient vulcanization system, which is being adopted in our experiments, provides cross-links with thermal stability, the lowest compression set, and optimum retention of the properties over an extended period of aging at ele- vated temperature.

0 20 40 60 80 I00

ENR(%)

FIG. 4. Mooney viscosity of NR/ENR blends at room temperature.

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BLENDS OF NR WITH ENR 585

I 0 a

- c .- E v

W

E ." cl

m L 2 V

ENR ( % )

b 14.0 l I I 1 1 I I I l

0 .

Y 3

e - f 6

C

c Mooney high Mooney low

c c.

0.0 0 20 40 60 80 100

ENR ( O h )

FIG. 5. Cure characteristics of NR/ENR blends at 155OC: (a) cure time; (b) cure torque.

With the addition of C/B, cure time is decreased with NR and blends, but not with ENR. It seems that C/B accelerates the cure, especially NR-rich blends. In the blends, the optimum cure time increased with the content of ENR, and the behavior is more or less asymptotic. That is, in NR-rich blends, cure time increases over NR, and in ENR-rich blends, it is more o r less maintained at the level of ENR. The increased cure time in NR-rich blends over the linearity is most likely due to the preferred migration of curatives into the dispersed ENR phase, which is expected from the different polarities of the two rubbers [12,18,19]. With preferred migration of the curatives into ENR, the continuous NR phase is cross-linked to a much lesser degree. Apparently, this slows the cure rate since NR forms the continuous phase in NR-rich blends.

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586 LEE, KIM, AND KIM

The torque of the curing process shows a positive deviation from the simple additivity [Fig. 5(b)]. This should not be confused with the negative deviation of Mooney viscosity (Fig. 4), which is a nonreactive simple blend. Positive deviation often occurs when there are strong interactions between the dispersed domains, namely, to form aggregates in three dimensions [16]. However, it is more likely due to the crosscure between NR and ENR, which is possible since both of the rubbers are cured by the same cure system. With crosscure, block or graft copolymers are formed in situ; these are mainly posed at the interfaces to reduce the interfacial tension and augment the adhesion. The scatter of the torque data reflects the insta- bility of interface reactions. However, the amount of interpolymers formed seems insufficient since the positive deviation is small.

The variation of torque during the overcure at 165OC is shown in Fig. 6(a). The ENR shows a rapid decrease of torque compared to NR. The fast reversion of ENR is probably due to the epoxy ring opening by sulfoxide and sulfenic acid formed during the curing reaction [20], followed by main chain scission. In the blends, the reversion is generally improved, that is, the absolute slope of the curve is decreased in NR-rich blends or maintained at the NR level in ENR-rich blends. The retarded reversion of NR-rich blends is seemingly due to the increased thermal resistance by ENR incorporation. Figure 6(b) compares the reversion at two differ- ent temperatures. Reversion at 18OOC is over two times greater compared to that at 165OC. In blends, the reversion is lower than the additivity throughout the composi- tion, especially in NR-rich blends. However, at 18OOC the reversion in ENR-rich blends increases to follow the additivity, while a lower value below the additivity still holds for NR-rich blends. It seems that the preferred migration of the curatives and fillers into the ENR phase contributes to the thermal resistance of ENR only when ENR forms the dispersed phase.

Figure 7 shows the cross-linking density of the blends, determined from the Mooney-Revlin equation (Eq. 2) using the stress-strain data obtained in these ex- periments.

(2)

where F is tensile stress, X the extension ratio, and C , and Cz are constants. For filled vulcanizates, the extension ratio is given by

X = l + X E (3)

X = 1 + 0.67 fcp + 1.62 f2p2

F = 2 ( C , + C 2 X - ' ) ( X -

where

(4)

where f is the shape factor (experientially, it is 6.5), cp is the volume fraction of filler in the filled vulcanizate, and E is the measured strain.

The elastic constant is given by

C, = pRT (2MJ' ( 5 )

where (M,) is the average molecular weight between cross-links. From the Mooney- Revlin plot of the stress-strain data [Fig. 7(a)], the elastic constant C , of each rubber blend was fitted using a standard regressional analysis, and the cross-linking density was obtained from Eq. 5 [Fig. 7(b)]. It should be mentioned that the Mooney-Revlin equation is valid for single-rubber vulcanizates, and it is an approx-

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BLENDS OF NR WITH ENR 587

13

12

1 1

10

9

8

7

6 0 10 20 30 40 50 60

Cure time(min)

h "9 = 165'C " t 0 18O'C

II I 20 40 80

ENR ( X )

80 100

FIG. 6 . Reversion of NR/ENR blends: (a) torque versus time at 165 OC; (b) reversion after 60 min overcure at 165OC and 180OC.

imation to incompatible rubber blends. However, the method should be qualita- tively correct for rubber blends that are cured by the same system. As expected from the optimum cure time and reversion data, the blends show a significantly higher cross-link density over the additivity rule, with a maximum in NR-rich blends.

The dynamic mechanical properties of the cured rubber blends are shown in Fig. 8. The NR shows Tg at approximately -46OC and ENR at about -2OOC. While the T,, of cured rubbers is compared to that of the uncured one (-70°C and -45OC), the Tg is increased by 25-26OC on curing. In blends, the T, of NR is decreased by approximately IOOC, whereas the T, of ENR is increased by 3OC. In polymer blends, a single T,, is obtained when the two polymers are miscible, and an inward migration is obtained when there are specific interactions between the

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588 LEE, KIM, AND KIM

7 a

6

0.0 0.2 0.4 0.6 0.8 1 .o

k- '

0 2 0 40 60 80 100

ENR ( X )

FIG. 7. Determination of the cross-linking density for the N K / b N K blends: (a) MOO-

ney-Revlin plot; (b) cross-linking density.

constituent polymers [21,22]. In rubber blends, a single T, does not necessarily mean miscibility [11,23,24]. Instead, a single T, can be obtained when crosscure occurs between the different base rubbers [25-271. Such interpolymers play an important role in reducing the interface discontinuity and result in a single relax- ation mechanism.

The outward migration of T, in a blend is not unusual in blends with semicrys- talline polymers. For example, in PE/phenoxy and polypropylene (PP)/phenoxy blends, the T, of phenoxy is increased up to 10°C due presumably to the migration of low molar mass phenoxy molecules into the PP [unpublished results]. However, in our cured rubber blends, the outward migration seems to be driven by the differ- ent extent of cross-linking in each rubber phase. In the blends, more of the curatives

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a

h N

E -2 c h a v

Ll

b

I ' I . I . 1 ' C

-20 - - - -

........... 1---- * - - - - . .q , -20

..-_ *--.... -30 -

h

- U

p: z - - -

- 4 0 - 0

M - - - - _ -. c ._

.... - 5 0 - *... - - - .. .. --*.--. .--. .. 8---..-

-60 - - I . I . I . I .

1 0 ' O L . I , I . I , , , I , I , I ' j

I . I . I . I . I . I . I .

-80 -60 - 4 0 - 2 0 0 20 40 6 0 8 0

Temp era ture ('C )

-10

- U

p: z -

-30 0

M CI

-40

-50

FIG. 8. Dynamic mechanical properties of NRIENR blends: (a) storage modulus; (b) tan 6; (c) tan 6 peak versus composition.

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5 9 0 LEE, KIM, AND KIM

=Tensile strength

are distributed in the ENR phase as the ENR content increases, which leads to the depletion of curatives and hence cross-links in the NR phase, resulting in a lowered T, of NR. On the other hand, the concentration of curatives increases in ENR as the content of NR increases, resulting in the increased T, of ENR. Since both NR and ENR are cured by the EV, a certain level of crosscures between NR and ENR should occur. In spite of such an expectation, two discrete T,’s are observed. This may imply that the crosscure between the two rubber components was insufficient to work for dynamic mechanical compatibilization of the rubber blends.

Figure 9 shows the tensile strength of the blend vulcanizates before and after aging. Before aging, the tensile strength of NR is higher than that of ENR. The blends show a negative deviation from linearity, indicating the incompatible nature of the blends. On aging, ENR shows better retention of tensile strength, apparently a contradiction to the reversion data. That is, the ring-opening temperature of the epoxy group in ENR has been reported to be 155OC [28], which is lower than the reversion temperature (165OC and 18OoC), but higher than the aging temperature. Certainly, at the aging condition, the epoxy group is not opened, and better reten- tion is obtained. In blends, retention of tensile strength shows a positive deviation, especially in the NR-rich blends, from the simple additivity. This implies that ENR dispersed in the NR phase contribute to the thermal resistance of NR, while the NR dispersed in ENR does not.

Elongation of the blend vulcanizates shows essentially the same tendency with strength. Unaged samples show a negative deviation from the additivity. On aging, the retention shows a small positive deviation in NR-rich blends and a small negative deviation in ENR-rich blends (Fig. 10). The hardness of the vulcanizate blends (Fig. 11) shows a positive deviation, especially in NR-rich blends regardless of aging. A higher T, and higher thermal resistance of ENR should contribute to hardening in the blends. The heat buildup of NR is significantly lower than that for ENR (Fig. 12) due to the lower hysteresis of NR. The heat buildup of the blend vulcanizates also shows a positive deviation, especially when ENR forms the dispersed phase.

- N$ 300

Y 2

z - f 250

I. 4 m 200

m 5 150

0 - .d

100

-

-

-

-

-

4 1 0 0 ; I I I

0 20 40

I

-r 60

T

60 100

ENR ( X )

FIG. 9. Tensile strength of NR/ENR blends before and after aging for 72 h at 1 0 0 O C .

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BLENDS OF NR WITH ENR 591

O E l o n g a t i o n -Retention 01 elongation after aging

700 i"' 100 :

40 -r 80

. . . 0 x 80 100

ENR ( X )

FIG. 10. Elongation of NRIENR blends before and after aging for 72 h at 100°C.

The positive deviation is, in part, due to the existence of interfaces of the immiscible blends.

Figure 13 shows the room temperature rebound of the blend vulcanizates. It is 61% for NR and 41% for ENR at room temperature, and the relative magnitude should vary with the test temperature. In blends, the rebound shows a negative deviation. The rebound of ENR is not really improved with 50-60% NR, implying that the damping is mostly governed by the high-damping materials.

-Hardness after aging c 80 h

0

v)

m c. d X

20

0 0 20 40

1 60 I 80 100

ENR ( W )

FIG. 11. Hardness of NR/ENR blends before and after aging for 72 h at loO°C.

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592 LEE, KIM, AND KIM

20

15 h

u v

3 10 m 3:

5

0

1 1 1 1 1 1 1 1 1 1 1

'Base temperature SOT. 30min stroke

1 0 111 -L 60 20 40

ENR ( X )

L 80 100

FIG. 12. Heat buildup (HBU) of NRIENR blend vulcanizates for 30 min stroke at base temperature 50°C.

K - 60 a E z n o 40 m

20

n 0 20 40 60 80 100

ENR ( X )

FIG. 13. Rebound of NRIENR blend vulcanizates at 25OC.

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BLENDS OF NR WITH ENR 593

4 e 0 .- .: 3.0 c c e, 0 0

M

2 2 .5 - r” II)

2.0 0 20 40 60 80 100

ENR ( X )

FIG. 14. Equilibrium swell of the blend vulcanizates in hexane.

Figure 14 shows the equilibrium swell of the blend vulcanizates in hexane, which shows a negative deviation, notably in NR-rich blends, a result expected from the cross-linking density given in Fig. 7.

ACKNOWLEDGMENT

This research was supported by the Pusan National University Grant.

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1.

2. 3 .

4. 5 . 6. 7 .

8 . 9.

10. 1 1 . 12. 13.

14.

15.

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Received July 21, 1996 Revised December 14, 1996 Accepted December 20, 1996

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