Ozone-resistant natural rubber blends.

Ozone-resistant natural rubber blends

It has been an accepted practice for many years to blend high levels of EPDM with natural rubber (NR) to achieve better ozone resistance (refs. 1-3). Published reports (refs. 4-7) describe various methods of improving ozone and other properties of natural rubber blends with high diene hydrocarbon elastomers.

In tire applications, both static and dynamic ozone resistance are required. This article describes the use of several new liquid EPDMs[refs. 8-10] to improve the static ozone resistance of natural rubber in blends with high molecular weight EPDM in non-tire applications. The objective of this article is to provide selected data which illustrates how the use of liquid EPDM will allow the use of much higher levels of natural rubber and reduce the undesirable properties of vulcanizates normally obtained with high levels of EPDM with natural rubber.

In products such as waterproof footwear, the use of higher levels of natural rubber will give better building tack, adhesion and physical properties which are desirable and important. Building tack is a feature which has been marginal when using the higher levels of high molecular weight EPDM with natural rubber.

This combination of liquid EPDM, conventional EPDM and natural rubber has been found to maintain many of the desirable properties of natural rubber and still provide superb static ozone resistance [ref. 11]. Vulcanized rubber products in which static ozone resistance is desirable include such items as waterproof footwear, mounts, gaskets, rolls, gas masks, wiper blades, etc.

A great deal has been published on the mechanism by which high molecular weight elastomers protect natural rubber from ozone cracking [refs. 12 and 13]. Briefly, the EPDM is present in particles or regions dispersed in the NR phase. Ozone cracks initiate in the NR and continue until the crack tip encounters an EPDM region. There exists a critical crack length below which the ozone cracking will be stopped. Above this critical length the crack may bypass the EPDM particle and continue to grow to a visible ozone crack. Therefore, the lower the concentration of the high molecular weight EPDM in the natural rubber, the more critical it becomes to have the high molecular weight EPDM well dispersed into the continuous natural rubber matrix. The addition of liquid EPDM appears to aid in the dispersion of high molecular weight EPDM into finite particles which improves the ozone resistance through better dispersion.

The liquid EPDM also aids in the dispersion of the other ingredients in the formulation such as antioxidants, curatives, etc. Overall, the liquid EPDM acts as a dispersant and a reactive plasticizer which can lower the processing viscosity of the compound, after which it will co-cure and essentially become a bound portion of the vulcanizate.

The liquid EPDM reduces mill and calendar shrinkage and improves extrusion and molding during processing. Finished or molded rubber articles containing the reactive plasticizer or covulcanizable liquid EPDM do not require oil in many cases and therefore will not exude. The liquid EPDM becomes essentially nonextractable. This is important in auto parts where nonfogging parts are required or where nonextractable or low extractables are specified.

Many papers have been published where the use of modified EPDM to improve ozone resistance of diene rubber blends with EPDM have been described[ref. 14 and 15]. It is suggested that the modified EPDMs provide better dispersion in the NR matrixes or act as cure rate inhibitors to give a more uniform cure rate. These problems are essentially overcome by the better dispersion of EPDM in NR by liquid EPDM.

Experimental

Mixing of liquid EPDM with natural rubber

The data presented in this article are based on stocks compounded in laboratory internal mixers such as a double "O" or "B" internal mixer or in Brabender mixers.

Since good dispersion of all ingredients is essential to improved ozone resistance, the recommended mixing procedure described in table 1 is critical and very essential to successful compounding of an ozone resistant stock. The liquid EPDM should not be added together with the natural rubber at the beginning of the mix cycle. If the liquid EPDM is added along with the high molecular weight polymers, the liquid EPDM acts as a lubricant and very little or very low shear is developed which will result in poor dispersion and consequently a poor static ozone resistant stock. Also, better results are obtained when the AO is added to the natural rubber at the initiation of the mixing cycle.

Discussion

Trilene is the trademark for Uniroyal Chemical's new family of ethylene and propylene low molecular weight terpolymers designed for a variety of uses in the rubber industry. We will refer to these materials as "liquid EPDMs." These low molecular weight polymers not only provide many of the desirable properties of conventional high molecular weight EPDM elastomers, but also have the versatility of liquids, particularly the ability to improve processing and dispersion as well as static ozone resistance in natural rubber blends.

This article specifically discusses liquid terpolymer products based on ethylene/propylene/dicyclopentadiene and ethylene/propylene/5-ethylidene-2-norbornene in blends with natural rubber and high molecular weight EPDM. The materials are identified and their characteristics are described in table 2. A saturated polymer backbone is resistant to many of the oxidative reactions that ordinarily are deleterious to polymer stability. This makes the liquid EPDMs outstanding candidates for applications which require resistance to heat, ultraviolet light and ozone aging. The combination of liquid EPDM, conventional EPDM and an antioxidant (2,2,4-trimethy1-1,2-dihydroquinoline) blended with natural rubber resulted in a synergistic effect on static ozone resistance. The reduction of any one of the three essential ingredients would lead to lower ozone resistance.

In addition, an added plus for the use of liquid EPDM is the improved processing obtained due to lowering of the Mooney viscosity of the compound. In all the examples shown, processing oils were not employed in order to illustrate the effect of liquid EPDM.

Normally, much lower levels of natural rubber have to be combined with at least 30 to 40 parts of a high molecular weight EPDM, along with antiozonants, to give excellent dynamic and static ozone resistance, but the tensile, modulus and tack would decrease.

Liquid EPDM combined with conventional EPDM and the antioxidant gives the opportunity to use higher levels of natural rubber retaining many of the good properties of natural rubber with a minimal loss of physical properties and, in addition, providing excellent static ozone resistance.

As the level of natural rubber increases from 80 parts to 88 or 90 parts, the ozone resistance begins to diminish. The reduction of any one of the three essential components, liquid EPDM, conventional EPDM or antioxidant, will also reduce the static ozone resistance. It appears that at least 5 parts of liquid EPDM are necessary in order to impart good dispersion, etc., and hence, good ozone resistance. The level of EPDM is also critical and data to date indicate that generally at least 5 parts are required to impart some static ozone resistance.

The level of antioxidant is also critical. We have reported previously[ref. 8] that levels in the order of 5 parts are required to improve the ozone resistance. At the level of 80 parts of NR, 10 parts liquid EPDM, 10 parts EPDM and 5 parts of antioxidant, very good ozone resistance results. In this article we have reduced the level to 4 parts successfully. As the level of antioxidant drops, so does the level of ozone resistance.

In effect, each critical ingredient has a concentration at which it remains an effective synergistic antiozonant component; but all three components are essential for a dramatic improvement in static ozone resistance, at a minimal level in the formulation.

In a previous paper[ref. 8], we reported several results with 80 parts of NR and showed that the combination of liquid EPDM, conventional EPDM and AO resulted in a compound with excellent static ozone resistance. Our recent results indicate that equally satisfactory ozone results can be obtained using 85 parts of NR. As the NR level is increased to 88 or 90 parts, the static ozone resistance decreases to a point where samples will crack in 50 to 200 hours versus the NR control which normally cracks in less than 4 hours. Some compounds made with 85 or 80 parts of NR were removed from the ozone test chamber uncracked after 2,000 hours.

The type of high molecular weight EPDM does not appear to be critical. All varieties appear to act synergistically with the liquid EPDM and AO. The oil extended EPDMs gave less satisfactory results but this may be simply a concentration effect.

The antioxidant which is an important ingredient in these formulations is the partially polymerized 2,2,4-trimethyl-1,2-dihydroquinoline. Thus far, this AO appears to be the most effective to date. An evaluation of other antioxidants and antiozonants is in progress and will be completed soon.

Summary

Liquid EPDM when combined with an antioxidant and a high molecular weight EPDM produces a synergistic effect which protects the NR from static ozone cracking.

When 85 parts of NR are combined with a total of 15 parts of liquid EPDM and conventional EPDM along with an antioxidant, the vulcanizate produced withstands more than 2,000 hours in a static ozone test chamber without cracking. Ordinarily 30-40 parts of EPDM are required to impart this level of ozone resistance. When high levels of EPDM are blended with NR, tensile, modulus and tack properties are diminished, but when the higher levels of NR are compounded as described, tensile, modulus and tack are improved substantially and the vulcanizate properties approach that of nonblended NR.

References [1]A.V. Gentile, U.S. Patent 3,419,639 (1968). [2]E.H. Andrews, Rubber Chemistry Technology 40,635 (1967). [3]L. Spenadel and R.L. Sutphin, Rubber Age 102 (12) 55 (1970). [4]A.Y. Coran, Rubber Chemistry and Technology 61,281 (1988). [5]A.Y. Coran, U.S. Patent 4,687,810 (1987). [6]K.H. Wirth, U.S. Patent 3,492,370 (1970). [7]G. Shinoda, Japanese Patent 61-293,240 (1986) C.A. Vol. 107, No. 08, Sec. 139. [8]F.C. Cesare, R.D. Allen and A.U. Paeglis, "Use of liquid EPDM as a reactive plasticizer," Meeting of the Rubber Division, American Chemical Society, Cleveland, Ohio, October 1987. [9]A.U. Paeglis, F.C. Cesare and D.N. Matthews, "Liquid EPDM elastomers," 132nd Meeting of the Rubber Division, American Chemical Society, Cleveland, Ohio, October 1987. [10]A.U. Paeglis, F.C. Cesare and D.N. Matthews, "Flussige EPDM - Elastomere," Gummi/Faser/Kunstsloffe, Vol. 41, June 1988. [11]S.D. Tobing, Rubber World, Vol. 197, No. 5, 33 (1988). [12]E.H. Andrews, J. Appl. Polymer Sci. 10, 47 (1966). [13]A. Draxler, Kaut. Gummi Kunstat. 12, 628 (1970). [14]H.J. Leibu, S.W. Caywood and L.H. Knabeschuh, "A new EPDM for improved ozone resistance in blends with SBR and natural rubber," presented at the Rubber Division, American Chemical Society, Miami Beach, Florida, April 28, 1971. [15]R.T. Morrisey, Rubber Chemistry and Technology, 44, 1025 (1971). [16]K.C. Baranwal and P.N. Son, "Co-curing of EPDM and diene rubbers by grafting accelerators on EPDM," presented at the Rubber Division, American Chemical Society, Denver Colorado, October 9-12, 1973. [17]P.N. Son and K.C. Baranwal, U.S. Patent 3,821,134 (1974). [18]R.T. Hopper, "New types of premature vulcanization inhibitors," presented at the Rubber Division, American Chemical Society, Denver, Colorado, October 9-12, 1973. [19]R.J. Hopper, "Improved co-cure of EPDM-polydiene blends by conversion of EPDMs to macromolecular cure retarders," presented at the Rubber Division, American Chemical Society, New Orleans, Louisiana, October 7-10, 1975. [Tabular Data 1 and 2 Omitted] [Figure 1 to 3 Omitted]