Design of EPDM for blends with NR/BR for tire sidewalls: Influence of molecular structure and carbon black distribution on properties

Rubber World, Sept, 2000 by Peter M. van de Ven, Jacques W.M. Noordermeer

Ever since the development of EPDM rubber, its use in tires has been a major challenge. In the early days of its existence, the 1960s, this rubber was considered an ozone-resistant alternative for SBR rubber for whole tires. Indeed, complete tires were built based on EPDM. The penetration of EPDM in this application has never happened because of insufficient performance - mainly wear resistance, cut growth and rolling resistance. On top of that, the absence of tack was experienced as a major problem in tire building, although by proper selection of polymer variables this could be overcome to a large extent (ref. 1).

Later, attention was focused on the application of EPDM rubber in blends with NR, SBR and BR, mainly with the intention to improve the ozone resistance of the latter, to avoid the use of antiozonants or waxes. This pertains in particular to that part of a tire which is in continuous direct contact with the outside air and subject to large dynamic deformations - the sidewall. It is a common experience that about 30-40 phr EPDM is required in conventional tire sidewall compounds in order to `repair' for the loss of ozone-resistance by the omission of the antiozonants (ref. 2).

The inclusion of EPDM, in its turn, creates another problem - the fatigue and cut growth resistance of the tire. This is commonly blamed on the low unsaturation level of the EPDM, compared to NR or BR, so that in the blend, either the EPDM is under-cured or the highly unsaturated rubber components are over-cured. Traditionally, solutions out of this dilemma have been sought in selecting EPDM grades which have a high level of unsaturation (ethylidene-nor-bornene content, most commonly) and a molecular weight as high as possible, as reflected by a very high Mooney viscosity. The resulting problems with mixing were then corrected by blending the polymer with extender oil (ref. 3). Extensive work has been done in the past to overcome this cure-incompatibility by the selection of proper curatives, with little success (ref. 4). Further, the selection of the high Mooney viscosity EPDM grade creates again another problem - the mixing of all components. A poor dispersion of the EPDM in the other rubber components negatively influences the beneficial effects of the EPDM in the first place.

Considerations not commonly taken along in finding ways out of this vicious circle are that the addition of EPDM to NR and/or BR creates phases with intrinsic polarity and viscosity differences. This has a strong effect on carbon black distribution, because it is well known that most carbon black tends to end up in the NR/BR phase and little, if any, in the EPDM phase (ref. 5). Consequently, the EPDM phase behaves as if being non-reinforced. Similarly, curatives tend to migrate to the more polar NR/BR phase, which makes the EPDM even more under-cured than it already is.

It is a challenging job to find a way out of this endless list of problems. The purpose of this article is to step away from the traditional approach of seeking solutions via a high unsaturation and high molecular weight of the EPDM. We will determine which molecular parameters of EPDM govern the fatigue and cut growth resistance in tire sidewall blends. This is done on straight compounds containing blends of the various rubbers, as well as on compounds in which the majority of the carbon black has been masterbatched first into the EPDM before this is mixed with the other polymers. This is done in order to enhance the carbon black level of the EPDM phase in the blend. The ultimate aim is then to find a new way out of this long challenging problem.

Experimental

Materials

A series of commercial EPDM grades was taken along in this study, as given in table 1 with their molecular composition and structural characteristics. An experimental grade was added to this list, corresponding to the common perception of an EPDM grade suited for tire-sidewall applications - high unsaturation and very high molecular weight, as represented by a Mooney viscosity ML(1 4) 150 [degrees] C of 75, even though the polymer is extended with 50 phr paraffinic oil.

Table 1 - compositional and structural characteristics of EPDM
polymer samples

EPDM                             A     B     C    D      E

Ethylene content [wt %]         46    53    53    66    65
Propylene content [wt %]        47    41    40    27    28
ENB content [wt %]               5     5     5     5     5
DCPD content [wt %]              2     2     2     2     2
Oil content [phr]
Polymer Mooney
  ML(1 4) at 125 [degrees] C    34    45    64    32    47
  ML(1 4) at 150 [degrees] C
Mn x [10.sup.3]                 37    47    36    38    42
Mw x [10.sup.3]                217   160   214   142   172
Mz x [10.sup.3]                900   450   680   350   470
Mw/Mn                            6     4     6     4     4

EPDM                             F     G     H     I    Exp.

Ethylene content [wt %]         65    51    50    49      67
Propylene content [wt %]        28    38    38    40      26
ENB content [wt %]               5     9     9     9       9
DCPD content [wt %]              2     2     2     2
Oil content [phr]                                         50
Polymer Mooney
  ML(1 4) at 125 [degrees] C    63    35    45    61
  ML(1 4) at 150 [degrees] C                              75
Mn x [10.sup.3]                 37    39    38    33     200
Mw x [10.sup.3]                182   140   160   235     450
Mz x [10.sup.3]                530   420   500   790   1,000
Mw/Mn                            5     4     4     7     2.3