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Cure system design

Up to this point we have discussed the individual chemicals employed in vulcanization as well as postulated some mechanisms for effective crosslinking. This article will discuss the technology of combining curatives and accelerators to achieve desired performance properties through cure system design.

There are three generally recognized classifications for sulfur vulcanization: conventional, EV or efficient cures, and semi-EV or semi-efficient cures. These differ primarily in the type of sulfur crosslinks that form, which in turn significantly influences the vulcanizate properties (ref. 13, fig. 13)

The term "efficient" refers to the number of sulfur atoms per crosslink. An efficiency factor (E) has been proposed by Moore and Drago (ref. 12):

g atoms of combined sulfur/g network E=

g molecules of chemical crosslinks/g network

Examples of "E" values attained in a natural robber compound cured with various cure systems are shown in table 7.

[TABLE OMITTED FROM ORIGINAL PUBLICATION]

Conventional cure systems use relatively high levels (2.5+ phr) of sulfur combined with lower levels of accelerator(s). These typically provide high initial physical properties; tensile and tear strengths, good initial fatigue life, etc., but with a greater tendency to lose these properties after heat aging.

In contrast, the EV cure systems employ much lower levels of free sulfur (0.1-1.0 phr) or they use sulfur donors such as TMTD or DTDM combined with higher accelerator levels. The short mono and disulfide crosslinks which form often do not exhibit the excellent physical properties of the conventional systems but do retain their properties much better after aging.

Semi-EV cures represent a compromise between conventional and EV cures. Although semi-EV cures do yield polysulfide crosslinks, they tend to minimize formation of inefficient moieties such as sulfur bridging, accelerator terminated sulfur linkages, etc. This "cleaner" usage of sulfur is the reason for their compromise properties between conventional and EV cures.

The reason for the greater loss in properties with conventional cure systems can be understood by examining crosslink bond strengths. Polysulfide bond strengths, from conventional cures, are significantly lower than bond strengths from the shorter crosslinks obtained with EV cure systems. The relatively unstable Sx bonds break and rearrange to form mono and disulfide linkages plus noncrosslinking cyclic and/or accelerator terminated fragments. We observe these chemical reactions as changes in properties with increased aging severity.

However, as mentioned before, conventional systems in natural robber do provide better flex life than EV cures, and this is one of the major limitations of EV curing. The short monosulfide bonds are less able to rearrange to relieve localized stresses which build during flexing, whereas the longer Sx bonds can. This ability for stress relief is thought to be the mechanism for the superior flex life of conventional cures.

If the natural robber compounds are subjected to thermal aging plus fatigue, the conventional systems will decrease to the EV systems in performance. The compromise obtained by using semi-EV systems involves the balance between heat aging and flex fatigue life.

Examples of cure systems in NR, SBR and nitrile A key responsibility of the rubber compounder is to design cure systems capable of maximizing performance requirements while minimizing detrimental side effects. Since compromise is the role of the day in compounding, proper choice of cure system and curing conditions is critical. This section illustrates how these compromises occur in natural robber, SBR and nitrile robber. Table 8 illustrates examples of recipes for conventional, semi-EV and EV cure systems in a simple, carbon black filled natural robber compound cured to optimum or 190 cure. The distribution of crosslinks obtained is found in figure 14 (ref. 20).

Clearly, the EV system exhibits the greatest number of thermally stable S1 and S2 crosslinks. As expected, the conventional and semi-EV look similar in crosslink distribution, but again, the semi-EV system has fewer extraneous moieties. Physical properties of these compounds are summarized in figure 15.

All three systems give similar tensile, elongation and hardness properties. Hysteresis, or heat build-up measured by Tan show an advantage for conventional and semi-EV systems and unaged fatigue follows the pattern expected.

Figure 16 summarizes properties obtained after heat aging at [70 degrees C]. Both the EV and semi-EV systems are superior in tensile and hardness retention, and the EV system is inferior in aged flex life. Note, however, in comparing these figures, that loss of fatigue is greatest for the conventional cure.

These observations all illustrate the importance of achieving the necessary crosslink structures for an optimum balance of properties. It is also important to remember that these illustrations are for compounds subjected to "ideal" or 190 cure conditions at a relatively mild [140 degrees C] temperature. Extremely high cure temperatures, such as those used in injection molding, can be expected to alter these results, but not the overall trends.

To further expand this point, figure 17 (ref. 14) shows how crosslink structure of a conventionally cured compound changes as a function of cure time. Note that total crosslinks decrease rapidly past optimum cure (reversion) and that most of the crosslinks lost are the Sx type. In other words, a conventionally cured natural rubber tends to progress towards semi-EV or EV types as cure progresses.

As mentioned earlier, cure temperature affects crosslink density. Figure 18 (ref. 14) shows a dramatic decrease in crosslink density as cure temperature increases. As one might imagine, this is a major concern for the compounder who is charged with increasing productivity by going to higher temperature, shorter cure cycles. While plant efficiencies might improve, compound performance is likely to suffer.

SBR and nitrile rubber

EV and semi-EV cure systems can also be used effectively in other diene type elastomers to improve aging properties. SBR, in particular, can take advantage of EV curing because it does not exhibit the fatigue loss observed in natural robber. The reason for this is that SBR develops a higher level of polysulfide crosslinks with EV cure systems thereby retaining good fatigue properties. Also, the average number of sulfur atoms per crosslink is significantly shorter in SBR than in natural rubber, and this results in a somewhat more thermally stable, conventionally cured network.

One analysis of crosslink types in SBR showed little correlation between type of cure system and crosslink structure. However, the number of sulfur atoms per crosslink, or Sx length, will have an influence, and this was not reported (ref. 14).

Conventional Semi-EV EV

Sulfur level 2.0 1.0 0.5

Sulfenamide level 1.25 2.25 3.0

% Sx 45 42 42

% S1 55 57 57

Although SBR does not exhibit the care system sensitivity of natural rubber, use of EV and semi-EV systems is still advantageous as shown by Pads in figure 19 (mf. 13).

Superior heat aging, measured by elongation retention and resistance to stiffening upon over cam, is apparent for the EV cure. Interestingly, fatigue life before and after aging also favors the EV system.

Pads also demonstrated the effectiveness of EV curing in nitrile rubber (NBR), but he also cautioned on the potential for unsightly bloom which can occur with the high sulfur or accelerator levels (see top of next column).

The improved heat resistance obtained with EV cures is

Conventional EV

MC sulfur 1.5 0.5 0.25

TMTD 0.1 1.0 2.5 4.0

MBTS  1.0 0.5 1.5 1.0

Heat resistance: (70 hours @ 100*C)

% Retained 53 58 74 70

Elongation

Bloom tendency None Slight Medium Heavy

useful for severe service areas such as under the hood automotive and down hole oil well applications. In general, NBR can be successfully cured with accelerator systems similar to those used in SBR. Care must be taken, however, to insure proper sulfur dispersion. Elemental sulfur is incompatible with NBR and this can lead to poor sulfur dispersion and inconsistent performance. Magnesium carbonated treated sulfur (MC sulfur) is usually used to facilitate dispersion. Use of sulfur donors such as DTDM can eliminate. the dispersion problem and provide superior heat aging. Strategies for developing NBR cure systems for specific attributes were published by B.F. Goodrich in the 1960s and are shown in table 9. Although these would require further refinement for a specific application, the strategies have withstood the test of time and are still useful (ref. 21).

Cure systems of butyl 

rubber and EPDM 

Butyl rubber

Nonhalogenated butyl rubber is a copolymer of isobutylene with a small percentage of isoprene to provide a crosslinking site. Since the level of unsaturation is low relative to natural rubber or SBR, cure system design generally requires higher levels of "fast" accelerators such as the dithiocarbamates. Examples of typical butyl rubber cure systems and their attributes and major applications were reviewed by Fisher and are published in the Vanderbilt Handbook (ref. 23). Use of conventional and semi-EV techniques can be used as shown in table 10 (ref. 13).

EPDM

EPDM is a terpolymer of ethylene, propylene and a small amount (<10%) of an unsaturated diene third monomer to provide a cure site. Unlike the elastomers previously discussed, the unsaturation in EPDM is not in the main chain, but it is pendant to the chain.

Strategies for EPDM cure systems center on overcoming the problem of using enough curative/accelerator to adequately crosslink the relatively few diene sites while avoiding a "bloom" problem from using too much accelerator or sulfur.

Nearly every conceivable combination of accelerator has been tried with varying degrees of success. Over the years, a number of basic cure systems have evolved. As in the case with NBR. B.F. Goodrich published a strategy for EPDM cure development, with useful starting points shown in figure 20 (ref. 23).

Peroxide cure systems The advantages for using peroxides and their crosslinking mechanism have been previously discussed. From a practical viewpoint. table 11 gives some guidelines to use as starting points for developing peroxide based cure systems for general purpose elastomers.

If an existing formulation is to be modified to include a peroxide. remove all ingredients associated with the previous cure system and add peroxide.

Basic formulation

NR. IR. BR or SBR 100

Filler variable

Zinc oxide 5.0

Anti-oxidant 1.0

Peroxide:

NR, IR, SBR - DiCup

40KE 2.5-6.0: VulCup

40KE 1.5-3.0;

BR - DiCup 40KE 1.2

2.5, VulCup 40KE

0.75-1.5

Although these guidelines are general in nature, they do illustrate typical peroxide concentrations used and the level of performance properties to expect. A further comparison of a sulfur based cure to two different peroxides in EPDM is shown in table 12 (ref. 17).

Initial properties for these three compounds are reasonably close, however after air aging, the advantages of peroxide curing are apparent. Most dramatic is the improved compression set obtained by the highly stable C-C crosslinks developed with peroxides. Other advantages include ability to achieve high transparency or nondiscoloring and very low creep or stress relaxation. Disadvantages limiting the use of peroxides include poor hot tensile and tear strengths, slow cure rate, a tendency for scorch problems and somewhat more difficult storage and handling problems in the plant.

References

12. Brydson, J.A., Rubber Chemisty," Applied Science Publishers, Ltd., London, 1978.

13. Pads, W. W., "Vulcanization, its activation and acceleration," Educational Symposium Rubber Division, ACS, Fall 1982.

14. Studabaker, M., "Vulcanization of hydrocarbon rubbers," Phillips Chemical Co., circa 1970.

15. "Vulkalent E retarder," product information bulletin, Miles, Inc., 1987.

16. "General catalog for peroxides and specialty chemicals," Pennwalt Chemicals Co. Buffalo, NY

17. "Vulcanizing ethylene-propylene elastomers," Technical Bulletin, ORC-104C, Hercules Inc., Wilmington, DE.

18. Thelamon, C., "Vulcanization of rubber by means of resins," Rubber Chemistry and Technology, 36, 268, 1963.

19. Knox, R.E., "Neoprene, the first high performance elastomers," Paper 22, ACS Rubber Division Meeting, Fall 1981.

20. "Crosslinking structures and elastomers properties," technical bulletin and presentation, Miles, Inc., Akron, OH.

21. "Vulcanizing nitrile rubber," technical bulletin HM-9, B.F. Goodrich Company, Independence, OH.

22. "The Vanderbilt Rubber Handbook," 13th edition, R.T. Vanderbilt Company, Norwalk, CT, 1990.

23. "Epcar elastomers," technical bulletin, B.F. Goodrich Chemicals Company, Independence, OH.

24. "Vulcanizing NR, IR, BR and SBR," technical bulletin 0RC-1073, Hercules, Inc., Wilmington, DE.

25. Box, G.P., Hunter, W.G. and Hunger, J.S., "Statistics for experimenters," John Wiley and Sons.

26. Luecken J.J. and Fath, M.A., "Rubber chemicals stability," Kautschuk and Gummi Kunststoffe, 35, No. 6, 1982.

27. Magg, H., Kempermann, T. and Warrach, W., "Modern supply forms of rubber chemicals," technical bulletin, Miles, Inc., Akron, OH.

28. "Nitrosamines in the rubber industry, " technical bulletin, Miles, Inc.

Note: The first 11 references appeared in the August and October, 1993 issues.