Preparation and properties of epoxidized natural rubber network crosslinked by ring opening reaction

Polymer Gels and Networks 2 (1994) 219-227 ~) 1994 Elsevier Science Limited Printed in Northern Ireland. All rights reserved 0966-7822/94/$07.00 ELSEVIER

Preparation and Properties of Epoxidized Natural Rubber Network Crosslinked by Ring Opening Reaction Azanam S. Hashim Department of Materials Science, Kyoto Institute of Technology, Matsugasaki, Kyoto 606, Japan

& S. Kohjiya Kyoto University, Institute for Chemical Research, Uji, Kyoto 611, Japan A B S T R A CT A rubber network is prepared by reacting epoxidized natural rubber (ENR) with p-phenylenediamine. The ring opening crosslinking reaction was found to have an apparent activation energy of 67.5 kJ/mole. Bisphenol A, a catalyst for the reaction, seems to reinforce the stress-strain properties of the network via hydrogen bonding interaction. Dynamic mechanical analysis and stress-strain measurement indicate that, in comparison to a typical ENR-sulfur network, the ENR-amine structure is more rigid and less stretchable at room temperature as demonstrated by its relatively high glass transition temperature and low rupture point. The ENR-amine network also demonstrates significantly lower tensile strength than the ENR-sulfur one although their crosslink densities are not significantly different. This behavior could be attributed to the former's lack of strain-induced crystallization which is a pronounced feature of the latter.

INTRODUCTION

Epoxidized natural rubber (ENR) is commercially available in 25 mole % epoxidization (ENR 25) and 50 mole % epoxidation (ENR 50). The epoxy groups of ENR, which are randomly distributed, provide sites for chemical modifications or crosslinking with suitable compounds. ~ The grafting of aromatic amines onto ENR or epoxidized polyisoproprene by ring opening of 219

220

A. S. Hashim, S. Kohjiya

+

catalyst

Scheme 1. The grafting reaction of ENR with aniline.

the epoxy groups has been reported. 2-4 Both normal and abnormal additions are possible as demonstrated in Scheme 1. Based on the same principle of reactivity, ENR could be crosslinked with multifunctional amines to form a network. In the present study, ENR-amine network is prepared by reacting the rubber with p-phenylenediamine and the apparent activation energy of the reaction is determined. The resultant network is characterized in terms of stress-strain properties, crosslink density, and dynamic mechanical properties. The properties are compared to those of an ENR-sulfur network whose stress-strain properties are well established. 5-6 The effect of bisphenol A, which catalyzes the crosslinking reaction, on the properties of the network is also elucidated. EXPERIMENTAL ENR was obtained from Guthrie Malaysia. Other chemicals used in this experiment were of standard laboratory grades purchased from local chemical companies. The p-phenylenediamine and bisphenol A were ground to finer size to facilitate their incorporation into the rubber. The ingredients were mixed on an open two-roll mill at normal milling temperature and the mixes were heat-pressed in-between a Teflon sheet of thickness 0.1 mm at the specified temperature. Solubility behavior was determined by immersing the heat-pressed samples in toluene for 48 h at room temperature. Swelling and extraction were carried out under the same conditions in tetrahydrofuran chosen for its relatively good ability to dissolve ENR gum. 7 Samples for extraction were subsequently soaked in ethanol for 1 h and dried to constant weight in vacuum at 50°C. Extraction was carried out in order to minimize the effect of particulate reinforcement on the values of crosslink density as determined by the Mooney-Rivlin equation. The torque-time behavior of the mixes was determined by using a Curelastometer II (JSR Co.). Stress-strain measurements were performed at 10mm/min strain rate at room temperature. The dynamic mechanical properties of the rubber networks were determined with a Rheospectoler DVE-4 (Rheology Co.) at a frequency of 10 Hz and a heating rate of 2°C/min. RESULTS AND DISCUSSION During the initial stage of the investigation, it was observed that a catalyst is required for the crosslinking reactions. Table 1 shows ENR-amine formulations which were subjected to heat-pressing at 150°C for 1 h. By comparing the

Preparation and properties of epoxidized natural rubber network

221

TABLE 1 Formulation of E N R - a m i n e Systems and Solubility B e h a v i o r of the Heat-pressed* Products

Formulations (phr)

1

2

3

4

5

6

100 3 10 -10

100 3

100 3

100 3

1,6-Hexanediamine p-Phenylenediamine Phenol Bisphenol A

100 3 10 ----

10 --

10 10

10 -10

100 3 ---10

Solubility behavior b

dc

ga

d

g

g

d

E N R 50

Na2CO 3

a A t 150°C for 1 h. b In toulene for 48 h at R T . c Totally dissolved. d Gelled.

solubility behavior between formulations 3 and 4, and 5 and 6, the catalyzing effect of phenol or bisphenol A is demonstrated. This observation is consistent with a study in which uncatalyzed ENR-amine reaction was not detected. 8 Model reaction carried out using 3,4-epoxymethylheptane and aniline indicates that phenolic hydroxyl catalyzes the reaction via a trimolecular transition state. 9 The bulk of the experiment utilized bisphenol A as the catalyst for reasons of practicality and ease of handling during the mixing. Bisphenol A has a much higher melting point than phenol; at the mixing temperature, the incorporation of bisphenol A into the rubber is easier and more controllable. The curing reaction of the mixes at 180°C was followed by monitoring the torque (Q) against time (t). Representative cure curves are shown in Fig. 1 which also demonstrates the dependence of the curing reaction on the amine concentration. Based on initial rate conditions, the cure curves obtained at 160-190°C are used to determine the apparent activation energy of the network formation. The equations involved are d Q / d t = - C d[Al/dt = Ck[BI'[Elm[A]"

~ -

AmineConCentration(phr) x

..._:.:.~

O'

. - ~" " "

I

-"0""

-

I0

(1)

30

.

,

50

70

90

Time (min) Fig. 1.

T h e curing b e h a v i o r of E N R 50 with 1 - 7 phr p - p h e n y l e n e d i a m i n e ( . . . . ) and with 5 p h r p-nitroaniline (--) at 180°C (base formulation in phr: rubber, 100; Na2CO3, 3; bisphenol A , 10).

A. S. Hashim, S. Kohjiya

222

and the two logarithmic forms of eqn (1)

In(dQ/dt) = In CAf[B]t[E]'[A] n - E a / R T ln(dQ/dt) = In Ck[B]t[E] " + n In [A]

(2) (3)

where C is a proportionality constant, k is the reaction rate constant, Af is the frequency factor, Ea is the activation energy, [B], [E], and [A] are the respective concentration of bisphenol A, epoxide, and amine, and/, m, and n are the respective orders of the reactants. The respective first terms on the right of eqns (2) and (3) could be considered as constants at initial cure time taken to be at dQ/dtmax. Before dQ/dtmax, the torque is at a minimum or relatively low. It takes some time for a sample to attain a certain degree of crosslinking to register a significant or meaningful torque reading. This region corresponds to an induction period in which the torque is usually considered to be insignificant (Fig. 1). From a chemical viewpoint, it is also possible for the bisphenol A to react with the epoxy groups to form crosslinks in the presence of amine. However, as shown in Fig. 1, the curing behavior of the rubber in the presence of 5 phr p-nitroaniline did not show any significant torque reading. This means, under the present experimental conditions, there is basically insignificant or no crosslinking reaction between ENR and bisphenol A. Thus, based on initial rate conditions, the plots of ln(dQ/dt)max vs 1/T for eqn (2) and ln(dQ/dt)max vs ln[A]0 for eqn (3) yield straight lines of slopes - E a / R and n, respectively. The apparent activation energy was calculated to be 67.5 kJ/mol; n was found to be 0.98 indicating a first order reaction with respect to the amine. The results are consistent with other studies involving catalyzed reaction of epoxidized polymers with aromatic amines as shown in Table 2. The activation energy of the ENR-amine crosslinking reaction is quite comparable to that of the epoxy resin curing but roughly twice that of the grafting reaction of the epoxidized polyisoprene. It should be pointed out that the grafting reaction of the epoxidized polyisoprene was carried out in solution phase while the curing reaction of the ENR and epoxy resin was in solid phase. Furthermore, the crosslinking reaction is expected to be more difficult than

TABLE 2 Comparison of Some Epoxy-amine Reaction Systems

Type of epoxy Type of reaction Catalyst Amine Activation energyf • h Reaction order

Present authors

Ohashi et al. s

Jayawardena et al. 9

ENR crosslinking bisphenol A pPDA b 67.5 1

epoxy resin ~ crosslinking hydroxyl group PDM C + m P D A a 61.9 --

epoxidized polyisoprene grafting phenol pADP" 30.3 ~ 1

Modified epoxy resin containing hydroxylmethyl group. b P -Phenylenediamine. c 4,4'-Diaminodiphenylmethane. d m-Phenylenediamine. "p-Aminodiphenylamine. I In kJ/mole. 8 Calculated by present authors based on reaction rate constant. h With respect to amine.

Preparation and properties of epoxidized natural rubber network

223

the grafting one. Hence, a lower activation energy is observed with the epoxidized polyisoprene. An ENR-amine network obtained by curing the rubber with five parts of amine per 100 parts of rubber (phr) was chosen for the determination of the crosslink density. A comparison is made to an ENR-sulfur network prepared by using a typical sulfur-curing formulation. 6 When a crosslinked rubber is subjected to stress-strain measurements, the stress is related to the strain by the Mooney-Rivlin equation F* = F / A o ( a - a - 2 ) = 2C1 + 2C2a -~

(4)

where a is the extension ratio, C~ and C2 are constants, Ao is the undeformed crossectional area, F / A o is the nominal stress, and F* is the reduced stress. The physical crosslink density is given by ~/phy 2C1/RT

(5)

=

where R is the gas constant, T is absolute temperature and C1 and C2 could be obtained by plotting F* against a -~. Mooney-Rivlin plots of the unextracted and extracted versions of the two networks are shown in Fig. 2. Table 3 shows

#,....~to F* = 1.98, ¢x-~ = 0.14 '

0.7

II~toF*

= 2.29, cCt= 0.14

r~ /~

ENR-sulphur

O.E

0.5 ~ 0.4 . 0.7 I~

O.E

ENR-amine Q extracted o unextracted

0.~ ........................

0.~ 0.3

0.2

0.4

0.6

0.8

Fig, 2. The Mooney-Rivlin plots of ENR-amine and ENR-sulfur networks (vertical dotted lines signify rupture point).

224

A. S. Hashim, S.

Kohjiya

TABLE 3 Properties of ENR-amine and ENR-sulfur Networks ENR -amine" Swollen weightc (%) Extracted weight (%)

448 8.1 Unextracted Extracted

Tensile strength (MPa) Elong. at break (%) VphyX 104 (mol/cm 3)

3.20 355 1 '74

1.54 240 1.63

ENR-sulfur b 478 1.9 Unextracted Extracted 16.10 605 2.08

14.10 615 1.76

* Cured at 180°C. b Cured at 150°C. c In tetrahydrofuran for 48 h at rT.

the swelling behavior and the effect of extraction on the properties of the two networks. Although the vphy values of the two networks are not too different, their stress-strain properties, i.e. tensile strength (Tb) and elongation at break (Eb) differ greatly. In the Mooney-Rivlin plots, the upturns, i.e. deviation from a straight line could be attributed to strain-induced crystallization, 1° a common feature of crystallizable rubbers like ENR. The extent of the upturn can be taken as a measure of the extent of strain-induced crystallization, which may explain the difference in stress-strain properties between the two networks. The E N R sulfur network has a pronounced strain-induced crystallization behavior as demonstrated by the upturn which persists up to a relatively high elongation (low a -a) resulting in relatively high stress (F*). The reason for tile network's ability to elongate up to a relatively high rupture point has been proposed. ENR-sulfur network, such as the one prepared in this study, contains disulfide and polysulfide crosslinks which are weaker than the C - - C bonds of the main chains. 11 Under stress, these crosslinks break but can interchange and rearrange themselves thereby relieving some of the stress (Scheme 2). 12 This interchanging mechanism allows the network to undergo greater deformation, and at higher elongation it crystallizes resulting in high tensile strength. From the Mooney-Rivlin plot, it is apparent that the ENR-amine network has some tendency to strain crystallize as demonstrated by the limited upturn but this behavior is not fully realized due to its low rupture point. It is thus obvious that the ENR-amine structure is relatively rigid and not as stretchable as the ENR-sulfur one. The strength of the bonds may explain this behavior. The C - - N bonds of the amine crosslinks are as strong as the C - - C bond of the main chains. When stress is applied, either the bonds in the crosslinks or those

II

I

I

stress

Scheme 2. The rearrangement of polysulfide crosslinks in ENR-sulfur network under stress.

Preparation and properties of epoxidized natural rubber network

=~'~:"~"-.

.

225

• exlractedENR-mn/ne

\:~ ~'~

o tmexlractedENR-amine

~,k~

o unextractedE~NR-sulphur

em

o ,d

-bo ~ g . 3.

6

T (°C)

5b

16o

Storage modulus curves of ENR-amine and ENR-sulfur networks.

in the main chains will break to relieve the stress. As a result, the neighboring bonds experience greater stress and will soon break. Unlike sulfur crosslinks, the amine crosslinks can not rearrange or interchange once broken. Consequently, the whole network ruptures rapidly which is the reason for the relatively low rupture point. The relative rigidity of the network is also supported by the results of dynamic mechanical analysis. Figure 3 shows the storage moduli (E') of the networks as a function of temperature. In the rubbery region where E' reaches a plateau, the values lie in the range of 1.6-1.8× 106N/m 2 indicating that the networks have a very comparable crosslink density. However, their onsets of stiffening temperature, i.e. the temperature at which the transition region is approached followed by an upturn in E', are markedly different. These temperatures are approximately 50 and 40°C for the unextracted and extracted ENR-amine networks, respectively, and 20°C for the ENR-sulfur one. The glass transition temperatures (Tgs) of the ENR-amine networks, which are represented by the peaks of the curves shown in Fig. 4, are also relatively high. It is obvious that both ENR-amine networks are more rigid than the ENR-sulfur one at room temperature. Thus, the difference in behavior of the storage moduli curves and the Tg peaks is probably due to the difference in the nature of crosslinks between the two systems. Conceivably, the amine crosslinks are bulky and rigid in nature while the sulfur crosslinks are small, linear, and more flexible. The reaction between the ENR and bisphenol A can not be entirely ruled 1 t~

• unexlr~'ted F~R-amin¢ • extracted]~IR-amine / ~ , * ' 3 ~ _ ",,

,o

F

n unextractedENR-sulphur

-2-N T (°C) Fig. 4. Tan 8 vs temperature curves of ENR-amine and ENR-sulfur networks.

226

A. S. Hashim, S. Kohjiya

OH

Fig° 5. Network structure of bisphenol A-contained ENR-amine system showing hydrogen bonding interaction.

out. But as discussed earlier, this reaction is insignificant, at least in terms of torque measurement. Therefore, the extraction process is expected to remove most of the bisphenol A from the ENR-amine network and any difference in properties before and after extraction would highlight the role of the catalyst. The extracted network shows significantly lower Tb and Eb than the unextracted one although there is no significant difference in crosslink density between the two. This is thought to be due mainly to the loss of bisphenol A which accounts for 8.5% of the total formulation in agreement with the extracted weight of 8.1%. The structure of the catalyst-contained network is shown in Fig. 5. The phenolic hydroxyls of bisphenol A are believed to form hydrogen bonds with unreacted epoxy groups providing some reinforcement. This kind of interaction has been described to explain the compatibility and the synergistic effect on the Tg observed in ENR/Novolar resin blend. 13 Similarly in our case, the interaction affects the Tg and reinforces the stress-strain properties. Therefore, the Tg and the stress-strain properties are expected to be reduced when the hydrogen bonding interaction is removed by extracting the catalyst from the network. This is indeed the case. Figure 4 shows that, for the ENR-amine system, the extraction causes a drastic shift of the Tg peak from 14 to 2°C accompanied by the reduction in stress-strain properties (Table 3). Figure 3 indicates that the ENR-amine network has a rubbery region starting at a higher temperature than room temperature. Since the C - - N crosslinks are as strong as the C - - C bonds of the main chains, the network is expected to have a better heat-aging resistance, similar to rubber networks containing C - - C crosslinks obtained by peroxide cure. ~4 In these respects, the ENR-amine system may be suitable for a high temperature application. The high temperature behavior and properties of the ENR-amine network are of

Preparation and properties of epoxidized natural rubber network

227

interest and will be a subject of our future investigation. The stress-strain properties of the E N R - a m i n e network could be improved by taking advantage of the reinforcing effect of bisphenol A which is required as catalyst for the network formation. The utilization of bisphenol A for its dual role in the preparation of high tensile-strength E N R - a m i n e system has been studied and the results are to be reported elsewhere.

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