Reversion resistance of engine mounts.

Reversion is defined as the softening and weakening of natural rubber vulcanizate when the curing operation has been continued too long (ref. 1). In some literature, reversion also includes the deterioration of physical properties after aging at elevated temperatures (ref. 2). Experience has linked this phenomenon especially with NR, but it can also occur in all diene rubbers and blends of NR with other diene rubbers. Reversion narrows the cure window of processing, thus increasing the chances for vulcanizates out of tolerance and greatly limiting cure time and temperature for thick sections. Obvious physical manifestations of reversion include an increase of tackiness on the surface of the vulcanizate and a decrease of physical properties such as modulus, compression set and tear resistance. The simplistic reason for this phenomenon is a deterioration of the crosslink network in the vulcanizate because of two very different mechanisms. The first is by splitting the polymer chains themselves, an aerobic aging process, which can be greatly reduced by adding antidegradants. The second is the breaking of the sulfur crosslink, an anaerobic process, that is simply a function of temperature and time and unaffected by antidegradants (ref. 3), but catalyzed by compound ingredients and/or reaction products of the vulcanization (ref. 4).

Most NR vulcanizates contain longer sulfur chains between the crosslinked rubber molecules. The average length of a sulfur chain is usually 4 to 5 sulfur linkages (refs. 5, 6 and 11). The shorter the sulfur chains, the higher the temperature stability. To estimate the temperature stability of such sulfidic sulfur bridges, we have to exclude any other mechanism of cleavage. The reason for this is that there is no method to investigate the cleavage of the sulfur bridges alone (figure 1). Using the swelling method (Flory-Rehner Equation) or the stress-strain experiments (Flory Equation), one is not able to decide if either the polymer backbone or the sulfur bridge is cut (ref. 18). Both will have the same effect on the [M.sub.c] value, calculated according to the above mentioned equations.

[Figure 1 ILLUSTRATION OMITTED]

To be able to make an assumption about the true stability of the sulfur crosslink, one has to exclude the polymer breakdown in such an aging experiment. EPDM, for example, with no double bonds in its polymer backbone, undergoes no chain scission under such experimental conditions. Of course, one has to consider that the sulfur bridge structure in NR vulcanizates is different from EPDM vulcanizates. While NR vulcanizates contain mostly oligo- and poly-sulfidic crosslinks, EPDM (ref. 17) has almost mono- and very little di-sulfidic crosslinks. In a rheometer experiment, reversion in EPDM can be observed at temperatures sometimes higher than 230 [degrees] C (ref. 7). The question is allowed, if we intend to underestimate the stability of the sulfur bridges because of the superposition of polymer chain cleavages, when is a similar experiment performed with NR? There is dependence, of course, from the accelerator system used, which can be shown in EPDM as well as in NR compounds. However, we may conclude that the stability of sulfur crosslinked rubbers could be much higher even if diene rubbers are used, but chain scission of the polymer can be excluded.

Stability of carbon sulfur bridge

It has been found empirically that some accelerators influence the rate of reversion of natural rubber stocks more than others do (refs. 8 and 9). In his work, Kempermann showed in dynamic experiments the dependence of reversion on accelerators of different kinds. He correlated the results to the chemical structure of the investigated accelerators. Kempermann postulated a resultant post-crosslinking compensation entirely, or at least to a large extent, of the reduction of the crosslink density by anaerobic aging. As a screening test, he compared the moduli obtained at vulcanization temperatures of 150 [degrees] C and 180 [degrees] C. In all experiments, thiazole derivatives and dithiophosphates showed superior behaviors than all other accelerators. In a similar work, Tisler performed experiments using a rheometer, which contains a sealed chamber like a closed mold, where oxygen is somehow excluded. He pointed out that sulfenamides used as accelerators showed different influence on reversion (refs. 9 and 10). It is theored that this order of influence on reversion depends on the base strength and quantity of the amines that are split off from the various accelerators during vulcanization. The degree of the polarity, and thus the reactivity of the amine, can explain the arrangement of the accelerators according to their effect on reversion. The examples presented in the Tisler paper definitely show that a specific order exists, and the mixing of two accelerators will cause a degree of reversion determined by the contribution of each individually, i.e., the effects are additive for each accelerator used. The actual network structure depends on the two competing reactions during vulcanization: the crosslink reaction forming crosslinks and the desulfurization reaction destroying networks (ref. 12). Then it might be believed that the basic mechanism of reversion could be summarized as a desulfurization reaction catalyzed by the liberated amine (ref. 16). The observed differences in reversion caused by various accelerators result in the theory that the degree of reversion is related to the amount and basicity of the amines released during vulcanization. It becomes obvious that the rate of reversion can be greatly reduced by using accelerator systems that do not allow the release of amines during vulcanization.

Stability of polymer backbone of diene rubber

All diene-based polymers with main chain unsaturation are sensitive to heat, radiation, and especially oxygen ([O.sub.2]) and ozone ([O.sub.3]). The carbon-carbon double bonds (C=C) are prone to attack by the minute quantities (0-6 pphm) of ozone present in the atmosphere, causing chain scission which results in a deterioration of the rubber network. Even when present in the atmosphere at only a few parts per hundred million (pphm), ozone readily cleaves such "activated" double bonds in elastomers. An unsaturated vulcanizate that has been strained and exposed to ozone quickly develops cracks. Even when ozone cracks are very small, they cause a serious reduction in strength and fatigue life and may lead to premature failure in service. The solubility of [O.sub.2] in elastomers increases with temperature (ref. 13). Today, the car industry requires service temperatures of 120 [degrees] C (250 [degrees] F), sometimes 150 [degrees] C (300 [degrees] F) in some hot points close to an exhaust manifold, for example, so the resistance of [O.sub.2] is much desired for high temperature service rubber.

The mechanism of [O.sub.2] attack is generally considered as a reaction of [O.sub.2] with the unsaturation (C=C) in the rubber to form ozonides (ref. 3). Under strain, cracks are apparent because these ozonides easily decompose and cleave the double bonds. The decomposition products include ozonides, polymeric peroxides, hydroperoxides, esters, aldehydes and lactones.

Antioxidants and antiozonants, which can function physically or chemically, have been developed to inhibit the action of these degradants. Chemical protectants are capable of reacting with the degradants or interfering with the chain of the reactions that otherwise would culminate in degradation. The most common types are amines, phenols and phosphites. These antidegradants are used in conjunction with paraffin waxes of limited solubility in most diene rubbers. Poor solubility means that only small amounts can be added without causing bloom. Various waxes that bloom to the surface are extensively used for this purpose and perform well in static applications, building a barrier on the surface of the rubber.

However, in dynamic applications, the wax film has the tendency to break and the rubber is exposed to attack which is concentrated in the areas of rupture of the wax film. Under continuous dynamic conditions, waxes may become ineffective and even harmful, considering adhesion failures due to wax level. All rubber containing an unsaturation in the main chain like NR and SBR are quite susceptible to [O.sub.2] attack, as well as nitrile, polybutadiene and polyisoprene polymers. Butyl and polychloroprene have much lower unsaturation that slows the reaction with [O.sub.2]. Although butyl rubber contains unsaturation, it only constitutes 2-3% of the butyl rubber molecules. The structures of polychloroprene and NR are similar in that they contain nearly the same number of double bonds for a given molecular weight, and the only structure difference is that the chlorine atom in polychloroprene replaces the methyl group in NR. It is widely believed that the reaction of ozone with the double bonds in polychloroprene is severely inhibited by a combination of stearic hindrance and the deactivation of the double bonds by electron-attracting chlorine atoms. Polymers with a saturated main chain like chlorosulfonated polyethylene and EPDM are much more resistant to [O.sub.2].

Effect of a butyl rubber coating on accelerated aging of natural rubber was reported before (ref. 14). In this article, the development of a heat resistant, protective rubber coating for high temperature engine mount application is reported building a physical barrier to prevent oxygen from being resolved in the rubber during service. A protective coating may prevent thermal oxidative aging and ozone deterioration of the natural rubber engine mount, especially as vehicle under-the-hood temperature increases.

New, innovative synthetic rubber coatings have been developed (ref. 15). The two-coating system is comprised of a polychloroprene base coating which acts as an adhesive to bind the bromo-butyl top coating. The coating system offers the following key features: excellent adhesion and abrasion resistance, ozone resistance and heat resistance. The combination of the reversion resistance accelerator systems where no aminic decomposition is observable with a coating that builds a physical barrier of high elasticity to oxygen is under further investigation.

Experimental

A natural rubber engine mount (99 WJ 16) was selected for this work. The formulation of this compound is listed in table 1. All rubber surfaces of the engine mounts were cleaned using isopropyl alcohol or ethyl alcohol to remove any surface blooming. All coating rubber compounds were mixed using a lab-scale mixer with intermeshing rotors. A measured amount of the resulting rubber slab was milled into thin sheets, chopped into small pieces (about 1 cm x 1 cm), and then added to a blender containing xylene. The rubber solution was mixed at low speed for one hour, then transferred to a metal container equipped with an overhead mechanical stirrer and allowed to stir at high speed for four hours. The rubber solution was adjusted using xylene to contain 15% total solids. The resulting elastomer solution was stored under refrigeration until use. The elastomer solution was removed from the refrigerator and stirred using an overhead mechanical stirrer for four hours prior to coating.

Table 1 - formulation of engine mount part

Ingredients                   phr

NR SMR CV                     100
ZnO/St. Ac.                     7
C.B.                           50
Process oil                     6
Process aid/stabilizer        4.2
Sulfur                        0.7
Accelerator system            1.8

Polychloroprene coating

Initial exploratory work involved preparing polychloroprene coatings and testing the coating on natural rubber mounts for adhesion. A simple peel test was used. Various grades of polychloroprene were selected for possible use. A total of 15 different polychloroprene formulations were compounded, tested (rheological and physical tests), and analyzed using statistical design of experiment using CADChem software. An optimized formulation is listed in table 2.

Table 2 - polychloroprene under coat

Chloroprene                   100
Polyoctenamer                  14
MgO/ZnO/StAc                   10
Antidegradants                  5
C.B./Whiting                   63
Process oil                     8
Accelerator system            2.3

Halo-butyl coating

Halo-butyl coatings were compounded once a satisfactory polychloroprene coating was developed. All halo-butyl compounds passed the stringent GM ozone test (GM 4486P). An optimized halo-butyl coating formulation is listed in table 3. Both polychloroprene and halo-butyl coatings were heat-aged for 70 hours at 127 [degrees] C.

Table 3 - halobutyl coating

Hal-IIR/CR/EPDM blend                100
Polyoctenamer                         10
MgO/ZnO/StAc                           7
Antidegradants                         5
C.B./Whiting                          60
Process oil                           10
Accelerator system                   4.8

The polychloroprene and halo-butyl coatings all contained N330 and N774 carbon black and various amounts of nonblack fillers, including: X50S and X230S silane coupling agents; Aktisil MM; HiSil 243 and HiSil 532; and Nucap Clay 100G, 190 and 200.

Dip coating process

The coating procedure is as follows. All engine mounts were cleaned with isopropyl alcohol or ethyl alcohol and air-dried for five minutes. The mounts were then dipped into the polychloroprene coating, air dried at 70 [degrees] C for four minutes for solvent removal, cooled to room temperature then dipped into the halo-butyl solution. The halo-butyl coating was air-dried at 70 [degrees] C for four minutes, then cured at 170 [degrees] C for 10 minutes. The coating solutions were kept at 15% solids content. Part performance evaluation was tested using an 810 MTS instrument. This instrument is used to perform dynamic and static rates.

Results and discussion

The current elastomer coating system exhibits outstanding tear strength and very good adhesion to natural rubber. The coating retains its elasticity after heat aging (up to 150 [degrees] C or 300 [degrees] F) and has excellent flex fatigue life. Table 4 shows the physical test results of the two coatings. Tearing of the coating occurs once the engine mount itself begins to tear.

Table 4 - physical properties of the coatings

Physical properties                         Chloroprene   Halo-butyl
                                              coating      coating
Modulus 300 - Mpa                              13.1           6.6
Tensile strength - Mpa                           19            12
Elongation-%                                    505           485
Hardness-ShA                                     72            55
Tear strength - ppi                             395           224
C. set (100hrs/70 [degrees] C) - %               37            27
Tensile set (120hrs./127 [degrees] C) - %      22.4          26.1

Figure 2 shows the effect of the coating on the engine mounts for static rate and dynamic performance parameters. It can be seen that the application of heat resistant coatings does not significantly alter the dynamic and static rates of the engine mounts. The coating procedure with its addition of heat during solvent removal and coating vulcanization is kept to a minimum, leaving the natural rubber compound unchanged. There is no influence of the coating on the dynamic performance of the part.

[Figure 2 ILLUSTRATION OMITTED]

Coated and uncoated engine mounts were simultaneously heat aged in a forced air oven for 10 days at 260 [degrees] F for seven days and five days at 300 [degrees] F. Engine mounts that were ozone aged were placed in an ozone chamber (100 pphm, 40 [degrees] C, 1.5 1pm airflow) for one day. The aged engine mounts were placed in 90 mm compression rings and tested using the MTS. The MTS testing procedure involved initially heat soaking the parts for two hours at 300 [degrees] F, then performing fatigue cycle testing (-2,600 N preload, [+ or -] 2G load, 3 Hz) and measuring the number of cycles to 25% tear on any leg. Figures 3 and 4 show the effect of coating on static heat and ozone aging on the number of cycles to failure. Coated engine mounts had a significantly higher number of cycles to failure than uncoated parts.

[Figures 3 and 4 ILLUSTRATION OMITTED]

Figure 3 reveals that the coated engine mounts performed significantly better than uncoated mounts. For example, the coated seven-day static heat aged parts averaged 9,000 cycles to failure, whereas the uncoated parts averaged less than 1,000 cycles. A similar pattern was observed for the parts aged at 260 [degrees] F. The coated parts aged at 260 [degrees] F had approximately twice the number of cycles to fatigue than the parts aged at 300 [degrees] F, thereby indicating the significant effect that heat has on the life of engine mounts.

Ozone aging, combined with heat aging, further accelerates engine mount degradation. Figure 4 shows that the number of cycles to failure for seven-day static heat and one-day ozone- aged mounts decreased compared to heat aged mounts. However, the coated mounts performed much more superior than the uncoated mounts.

Figure 5 shows the effect of fatigue cycles on rates for coated and uncoated mounts. Coated engine mounts had lower dynamic rate increases and higher number of cycles to failure than uncoated mounts. Coating remained elastic throughout the duration of dynamic heat aging. Research remains active in the area of dynamic aging and creep test.

[Figure 5 ILLUSTRATION OMITTED]

Conclusion

From these results it is possible to draw the following conclusions: The two-layer coating can provide the protection against thermal-oxidation and ozone attack for natural rubber based engine mounts. The coating works mainly by stopping diffusion of oxygen and ozone from the surrounding air, thus preventing the cleavage of the polymer backbone. The coated engine mounts have much better life results than mounts without coating.

References

(1.) N.I. Sax, R.J. Lewis, Sr., Hawley's Condensed Chemical Dictionary, 11th edition, New York, Van Nostrand Reinhold Company, 1987, p. 1,007.

(2.) C.M. Kok, "The effects of compounding variables on the reversion process in the sulfur vulcanization of natural Rubber," European Polymer Journal, 23 (1987), 8, p. 611-615.

(3.) J.R. Shelton, R.I. Pecsok, J.L. Koenig, "Fourier transform studies of the uninhibited auto-oxidation of elastomers in durability of macromolecular materials," ed. R.K. Eby, ACS Symposium Series No. 95, Washington, DC, 1979.

(4.) H.J. Graf and A. Johansson, "Discussion on the resistance to reversion of natural rubber with the help of thiophosphate cure systems," paper presented at International Rubber Conference IRC'96, Manchester, UK, 17-21 June, 1996.

(5.) K.H. Nordsiek and J. Wolpers, "Elastomer-network structure and stability of material," Kautschuk Gummi and Kunststoffe, 45, 10 (1992), p. 791-800.

(6.) R. Mukhopadhyay and S.K. De, "Effect of vulcanization temperature and different fillers on the properties of efficiently vulcanized natural rubber," Rubber Chemistry and Technology, 52 (1979), p. 263-277.

(7.) H.J. Graf private communication.

(8.) T. Kempermann, U. Eholzer, "Physical and chemical aspects of reversion," Kautschuk Gummi Kunststoffe, 34, 9 (1981), p. 722-734.

(9.) A.L. Tisler, "Review of sulfenamide acceleration systems for improved environmental desirability and maintenance of compounding flexibility," Paper 88 presented at the ACS Rubber Division Meeting, Nashville, TN, Nov. 1992.

(10.) D.A. Lederer, M. Zaper, Evaluation of N-t-butylbenzothiazole sulfenamide," Paper 59 presented at the ACS Rubber Division Meeting, Detroit, MI, October 1991.

(11.) J. Buehring, Network Density and Network Structure of N-cyclohexyl-2-benzothiazylsulfenamide/Sulfur Cross-Linked Poly-Diene Rubbers, Thesis 1996, University Hannover, G.

(12.) C.H. Chen, J.L. Lienig, J.R. Shelton and E.A. Collins, "Characterization of the reversion process in accelerated sulfur curing of natural rubber," Rubber Chemistry & Technology, 54, 4 (1981), p. 735-750.

(13.) J.D. Hoffman, "Materials failure and materials research policy in durability of macromolecular materials," ACS Symposium Series No. 95, Washington, DC, 1979.

(14.) B. Stenberg, L.-O. Peterson, P. Flink and F. Bjoerk, "Effect of butyl rubber coating on accelerated aging of natural rubber," Rubber Chemistry & Technology, 59, 1 (1986), p. 70-76.

(15.) Patent pending.

(16.) B.H. To, et al, Plast. Rubb. Proc. and Appl., Vol. 12, No. 2 (1989) 89.

(17.) M.A.L. Verbruggen, L. van der Does, J.W.M. Nordermeer, M. van Duin and H.J. Manuel, "Mechanisms involved in the recycling of NR and EPDM," Paper 57 presented at the Rubber Division of ACS Meeting, Nashville, Tennessee, 29. Sept. - 2. Oct., 1998.

(18.) W. Kleemann and K. Weber, Formulas and Tables for Processing of Elastomers, Dr. Gupta Verlag, 1994.

Hans-Joachim Graf and Ed Sayej, Cooper Standard Automotive