Effect of ZnO Particle Size on the Curing of Carboxylated NBR and Carboxylated SBR

8/13/2019 Effect of ZnO Particle Size on the Curing of Carboxylated NBR and Carboxylated SBR

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EFFECT OF ZnO PARTICLE SIZE ON THE CURING OF

CARBOXYLATED NBR AND CARBOXYLATED SBR

G. R. HAMED,* K.-C. HUA

RUBBER RESEARCH CENTER, THE UNIVERSITY OF AKRON

AKRON, OH 44325-3909

ABSTRACT

A carboxylated nitrile rubber (XNBR) and a carboxylated SBR (XSBR) were mixed with zinc oxide particles of

different specific surface areas (S, 35 m2/g; M, 3.5m2/g; L, 0.5 m2/g) and cure behavior at 165 C studied using

oscillating disc rheometry. Without added zinc oxide, both raw rubbers slowly stiffen over many hours of heating. This

is probably due to condensation of carboxyl groups to form anhydride crosslinks. XNBR compositions containing the

finely divided S crosslink much more rapidly. Full cure is reached after about 10 minutes of heating. Cure rate decreas-es markedly as the specific surface area of the ZnO decreases. A composition containing M at twice stoichiometry

requires about an hour to cure well, while with L, about 10 hours are required. In contrast, curing of the XSBR depends

little on the specific surface area of the ZnO, either with S or L, curing is essentially complete after 30 minutes. After

simply mixing ZnO into either rubber, it remains as a dispersed particulate. With XNBR, curing appears to be controlled

by the rate of dissolution and diffusion of ZnO, while, with XSBR, reaction is not diffusion limited and may be confined

to regions near particle surfaces.

INTRODUCTION

Pioneering studies of carboxylated rubbers by H. P. Brown1-3 led to commercialization of

carboxylated nitrile rubber (XNBR) and carboxylated styrene-butadiene rubber (XSBR).

Presently, XNBR is sold as a bulk, dry rubber and is used in engineering applications requir-ing high resistance to fracture, while XSBR is supplied as a latex and is employed in less severe

service, such as in carpet backing and paper coating.

Carboxylated rubbers are commonly cured with ZnO, which neutralizes acid groups there-

by producing ionomeric elastomers, whose properties have been studied extensively.4-19

Complete neutralization consists of two steps:

ZnO + RCOOH RCOOZnOH (1)

RCOOZnOH + RCOOH RCOOZnOOCR + H2O (2)

Upon fully reacting, each mole of ZnO neutralizes two moles of acid and a mole of water is pro-

duced. In this paper, we examine the effect of ZnO particle size on the cure behavior of an XNBR

and an XSBR.

EXPERIMENTAL

The XNBR was Nipol 1072 (Zeon Chemical), and the XSBR was Dow CP615NA (Dow

Chemical), which was supplied as a latex. Prior to use, the latex was dried by pouring it onto a

large flat plate in a hood at room temperature. After 48 hours, the latex was essentially dry. As

seen in Table I, the XNBR contained 0.075 0.005 ephr (equivalents of acid per hundred parts

by weight of rubber), while the XSBR contained 0.12 0.01 ephr, i.e., about 50% more acid

functionality than the XNBR. Also shown in Table 1 are properties of Nipol DN2850, a regular

NBR.

214

* Corresponding author. Ph: 330-972-6831; Fax: 300-972-5290; email: [email protected]


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Zinc oxides of different particle size were used (Table II). These are designated S, M

and L for small, medium and large particles, respectively. Gum rubbers are designated

XNBR-0 or XSBR-0, while compositions containing ZnO have a suffix indicating ZnO type and

level, relative to stoichiometry for complete neutralization. For example, XNBR-Sl contains the

stoichiometric amount (3 phr) of the small Zinc Oxide 35; XNBR-M5 contains the medium ZincOxide FP-Z at five times (15 phr) stoichiometry; XSBR-L3 contains the large Cerox 506 at three

times (14.6 phr) stoichiometry.

EFFECT OF ZnO PARTICLE SIZE ON THE CURING OF CARBOXYLATED NBR 215

TABLE I

PROPERTIES OF XNBR (NIPOL 1072), NBR (NIPOL DN2850), AND XSBR (DOW CP 615NA)

TABLE II

ZINC OXIDE TYPES


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MIXING

Compositions were prepared in a Haake Rheomix Model 600 Mixer using a rotor speed of

40 rpm and a fill factor of 0.85. The temperature of the mixer wall was about 35 C before rub-

ber was added. No external heating or cooling of the mixer was used. A thermocouple sticking

into the rubber mass monitored melt temperature during mixing. It took about one minute to add

the raw XNBR or XSBR to the mixer. At three minutes of mixing, ZnO was added and mixing

was continued for an additional four minutes. Compositions were removed and (sausage roll)

passed 10 times through the nip (~ 2 mm) of a two-roll mill (19 and 23 rpm).

CURE RHEOMETRY

Cure behavior was determined with a Monsanto Model R-100 oscillating disk rheometer

(ODR) (165 C, 3 arc) using about 12 g of sample. (Prior to determining the ODR response of

XNBR-0 and XSBR-0 (no ZnO), they were masticated in the internal mixer, and on the mill, in

order to provide the same processing history as compositions containing ZnO.)

RESULTS AND DISCUSSION

INTERNAL MIXING

Torques during the mixing of XNBR-S5 and XSBR-S3 are shown in Figures 1 and 2. Torque

increases as rubber is added to the mixer and then decreases. For XNBR-S5, upon addition of 15

phr of the small ZnO, the torque stays about the same for about one minute before continuing

to decrease. XSBR-S3 behaves differently. After ZnO addition, mixing torque rises, an indica-

tion of the onset of scorch. ODR results, given later, are consistent with the scorchiness of XSBR

relative to the XNBR.

216 RUBBER CHEMISTRY AND TECHNOLOGY VOL. 77

FIG. 1 Torque during internal mixing of XNBR-S5.


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The raw XNBR-0 and XSBR-0 are transparent, before and after mixing. Compositions con-taining ZnO are opaque and white becoming brighter as ZnO content increases. After mixing,

ZnO is a dispersed particulate in each elastomer.

XNBR CURE BEHAVIOR

Results of cure rheometry at 165 C for the XNBR containing the different zinc oxides are

shown in Figures 3-5. In addition, results for XNBR-0 and the raw NBR are given. Figure 3 (S-

series) shows that XNBR-0 undergoes some crosslinking. Upon heating at 165 C for 2 hours,

torque rises from a minimum value of 10 dN-m to about 23 dN-m. The XNBR self-crosslinks

without added curative, probably due to bimolecular condensation of acid groups to form anhy-

dride crosslinks. (In contrast, the raw NBR shows no increase in torque upon heating.) Up toabout 1.5 times (4.5 phr) the stoichiometric amount of ZnO, cure rate and maximum torque

increase rapidly with increased ZnO. Cure behavior is little different for compositions contain-

ing more than two times the stoichiometric amount of ZnO. (The slightly increased torques of S3

and S5 probably result from excess ZnO acting as particulate filler.) Thus, about twice the stoi-

chiometric amount of S is needed for full cure.


FIG. 2 Torque during internal mixing of XSBR-S3.


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Cure results with the large L ZnO are given in Figure 4. Note the much longer time scalecompared to Figure 3. Results for the raw XNBR-0 are the same as those shown in Figure 3, but

now the behavior at longer times is shown. XNBR-0 continues to self-crosslink up to 20 hours

of heating. In contrast to the normal one-step cure seen with the small ZnO, two-step curing

occurs with the large ZnO. In early heating, all compositions (0.5-5x stoichiometry) containing

L have reduced torque relative to XNBR-0. The large ZnO particles slow thermal crosslinking

of the raw XNBR-0. Apparently, some carboxyl groups have reacted with the ZnO (Reaction 1),

thereby reducing self-crosslinking. The first neutralization step does not produce elastically

effective network chains (at least not at sufficiently low conversions where ionic aggregation is

expected to be absent), but, it decreases the concentration of acid groups available to participate

in anhydride formation. For compositions with up to the stoichiometric amount of L, torque

remains less than that of XNBR-0 over the whole range of heating time. At higher levels (1.5-5x

stochiometry) of L, a second curing step takes place, as torque increases rapidly and becomes

higher than that of the raw XNBR-0. The second stage occurs earlier as ZnO concentration

increases and may be due to:

(a) Reaction 2 producing intermolecular links. A further torque increase could result from

aggregation of crosslink sites.

(b) Sufficient Reaction 1 products to result in ionomeric aggregation. This mechanism is feasi-

ble since monovalent cations have been shown to be capable of crosslinking carboxylated

rubbers. (Of course, ionomeric aggregation could involve the products of both Reactions 1

and 2.)


FIG. 3. Cure rheometry of XNBR containing small (S) ZnO.


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The finding that cure rate is strongly dependent on ZnO particle size indicates diffusion-con-trolled reaction(s). With the smaller ZnO, (Figure 3) neutralization is rapid. Indeed, the first step

seems to occur so quickly that it cannot be determined by ODR; only the second stage of cure is

seen. Moreover, neutralization is so rapid with S that at all levels (0.5-5.0x stoichiometry), and

for all cure times, torque exceeds that of XNBR-0. Neutralization occurs before much anhydride

linking has taken place.

Figure 5 compares (on a log-log plot) the cure behavior of compositions containing S,

M, or L at twice stoichiometry. The medium sized ZnO shows intermediate behavior

between S and L. With M, curing is much more rapid than with L, but a two-step cure

is still apparent.


FIG. 4. Cure rheometry of XNBR containing large (L) ZnO.


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Next, diffusion-controlled curing will be considered semi-quantitatively. We begin by defin-ing a time tc when curing is essentially complete. This was done in each case as depicted, for

example, in Figure 6. ODR curves were fitted with two straight lines one in the plateau

region where torque changes little and another corresponding to the maximum slope. The inter-

section of the two lines defines tc. Further, we define a characteristic length scale for diffusion

as the volume (V) of rubber in a composition divided by the total surface area (A) of the ZnO

contained therein. Based on the basic diffusion equation (D = x2/2t), we assume that tc

depends

directly on (V/A)2 and also inversely on the concentration of ZnO,

tc

= f ((V/A)2/phr ZnO)


FIG. 5. Cure rheometry of XNBR containing different zinc oxides at twice stoichiometry.


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Figure 7 shows values of tc for compositions containing various phr of the three types ofZnO. Cure time decreases as ZnO concentration increases and is strongly dependent on particle

size (or specific surface area). Replotting the data in Figure 7 with (V/A)2/phr ZnO as the abscis-

sa, the mastercurve shown in Figure 8 is obtained. This validates the assumptions made to model

the diffusion-controlled neutralization of the XNBR.


FIG. 6. Method of tc

determination, shown for XNBR-M series.


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FIG. 7. Values of tc

for XNBR containing different zinc oxides.

FIG. 8. Mastercurve of tc

for XNBR cured with zinc oxide at 165 C.


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XSBR CURE BEHAVIOR

Figures 9 and 10 show ODR curves for the XSBR containing the S and L zinc oxides,

respectively. Like XNBR-0, XSBR-0 undergoes self-crosslinking. We again attribute this to

intermolecular anhydride formation. However, in contrast to the XNBR, the effect of particle size

is small and the XSBR is scorchy, especially with the S ZnO. XSBR-S3, prior to heating in the

ODR, has undergone so much pre-reaction that its minimum torque is about 20 dN-m above that

of the XSBR-0, and its maximum torque rise above the minimum is only about 5 dN-m. With

L, XSBR compositions are less prone to pre-vulcanization, but they are still scorchier than the

XNBR with S (Figure 3)! In addition, although the XSBR contains 50% more carboxyl groups

than the XNBR, the maximum torques for the XSBR are much less than those for the XNBR.

XSBR begins to neutralize quickly, but does not seem to develop a uniformly crosslinked struc-

ture throughout. Rather, the ZnO in XSBR behaves more like a reactive-filler, with neutraliza-

tion taking place near the surface of ZnO particles, but not effectively throughout the elastomer.

In this case, neutralization would, in effect, be like increasing the volume fraction of filler in

the composition. (Previously, we have shown that the XNBR becomes much stiffer and strongerafter neutralization, while the XSBR shows only a small increase in these properties.) In the next

section, we directly compare the cure behavior of the two elastomers.


FIG. 9. Cure rheometry of XSBR containing small (S) ZnO.


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XNBR COMPARED TO XSBR

Figure 11 gives log-log ODR curves of XNBR and XSBR with S and L at twice stoi-

chiometry. The scorchiness and low maximum torque of XSBR are apparent. (This behavior con-

trasts to normal curing of rubber (e.g., with sulfur or peroxide), where a rapid onset of

crosslinking is followed by a rapid increase in torque.)

Finally, Figure 12 shows tc versus level of ZnO (relative to stoichiometry) for the two elas-

tomers containing S or L. For XNBR tc is very much longer with L, while tc is similar for

XSBR containing either zinc oxide.


FIG. 10. Cure rheometry of XSBR containing large (L) ZnO.


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FIG. 11. Comparison of cure rheometry for XNBR and XSBR.

FIG. 12. Comparison of tc

for XNBR and XSBR.


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ACKNOWLEDGEMENT

These studies were carried out in the Rubber Research Center at The University of Akron.

The Center is supported by a consortium of companies and the DIanni Research Endowment.

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[Received February 2003, revised October 2003 ]


Effect of ZnO Particle Size on the Curing of Carboxylated NBR and Carboxylated SBR

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