Vulcanization of elastomers.

(Editors note - this is the conclusion of a five part series that began in August, 1993. Other installments appeared in October and December, 1993 and February, 1994.)

A general strategy for cure system design

statistical techniques)

It is apparent from the above discussions that achieving the proper balance of properties by cure system design can be a tedious process. For this reason, compounders today often use statistically designed experiments to optimize curative levels (ref. 25).

It is beyond the scope of this article to discuss these techniques in detail, however, we would be remiss in not covering the basics. By using properly designed experiments, i.e. balanced or orthogonal arrays of curative levels, regression analysis can be performed which unequivocally quantify the effect of each ingredient, and combination of ingredients, on the properties of interest. From this, an optimum balance of properties is obtained much more efficiently than the old fashioned "vary one ingredient at a time" approach. The reader is encouraged to learn this technique and use if for solving problems in cure system design, or for that matter, any problem involving optimizing combinations of materials.

Effect of other compounding ingredients of vulcanization

The discussion so far has focused only on how cure system design can influence rate and state of cure. Unfortunately (or fortunately from a compounder's job security point of view), the situation is not this simple. One must be aware of how other ingredients besides the elastomer itself, and the cure system might influence cure and scorch behavior.

Usually the effect of a material on cure is pH dependent. Ingredients which are basic in nature will tend to accelerate the rate of both scorch and cure while acidic materials exhibit the opposite effect.

Antidegradants

Amine type antioxidants or antiozonants such as the popular paraphenylene diamines (ppds) can significantly decrease scorch time. This is particularly true in metal oxide curing of polychloroprene or in cases where the ppd had suffered premature degradation prior to cure.

In one experiment the effect of ppd assay was correlated to scorch safety. As the ppd degrades to liberate free amine, scorch time decreases and cure rate is faster. It is obvious that the decomposition products of the ppd were acting as accelerators (ref. 26).

The degradation products apparently serve to activate the cure, since both the induction time (t2) and cure time (t90) decrease with decreasing ppd assay as shown in table 13. However, the effect on unaged properties is minimal (table 14).

    Table 13 - cure and scorch behavior as a
      function of ppd (2 phr antidegradant

              Rheometer @ 153 [degrees] C

% Assay          [t.sub.2]   [t.sub.90]   Max. torque

Blank (no ppd)   9.5         16.5             73
95               8.0         13.2             68
88               7.8         12.5             67
83               5.6          9.8             68
78               5.2          9.3             67

    Table 14 - physical properties (2 phr ppd)

                      300%       Ult.       Ult.
% Assay   Hardness   modulus   tensile   Elongation

Blank        59        1185      2845       525
95           58        1085      2780       545
88           58        1075      2770       555
83           58        1080      2795       550
78           57        1150      2540       505

In contrast, antioxidants can have an opposite effect when peroxide curing. Since peroxide crosslinking involves a free radical mechanism, and antioxidants are designed to scavenge free radicals, it is obvious that peroxide efficiency can be compromised by the addition of antioxidants.

Fillers

Materials used as fillers in rubber can also be classified as acidic, basic or neutral. Furnace blacks, i.e. HAF, FEF or SRF, are somewhat basic. As such, they can have an activating effect on sulfur cure rates. Furthermore, carbon blacks have been found to promote formation of mono/disulfide crosslinks thereby somewhat helping to minimize reversion and enhance aging properties.

Channel blacks such as the old EPC and MPC grades are acidic to neutral and can vary from having little effect to having a slight retardation in cure rate. The pH neutral, large particle size MT thermal black generally has little effect on cure rate.

Nonblack fillers such as the precipitated silicas can reduce both rate and state of cure. The mechanism appears to be one of a competitive reaction between the rubber and the filler for the zinc oxide activator. Use of materials such as diethylene glycol or triethanolamine prevent this competition thereby maintaining the desired cure characteristics.

Neutral fillers such as calcium carbonate (whiting) and clays have little or no effect on the cure properties.

Process oils and plasticizers

Petroleum based rubber process oils generally contain a mixture of paraffinic, naphthenic and aromatic components. These oils vary in composition from grade to grade, but most contain some unsaturated moieties and this unsaturation can compete with the polymer for the curatives. Therefore, state of cure can be decreased. This is not easily detected because oil addition softens the compound and this softening masks the loss of state of cure.

Unsaturation present in process oils can also decrease peroxide cure efficiency. Therefore, paraffinic oils are recommended for peroxide cure compatibility.

Plasticizers can range in composition from the ester types, epoxies, phosphate, amides, etc. The effect of any one of these on curing is usually pH dependent. However, it is prudent to investigate each on a case by case basis.

Some current topics

Environmental issues

These issues can evoke strong emotions. While some feel that current environmental and toxicological restraints have progressed to the extreme, others feel that even more should be done to insure safety in the workplace.

The rubber industry, like many other industries, is involved in this debate, and much of the controversy centers on vulcanization and vulcanization chemicals. Our industry has responded to his issue by developing new product forms for accelerators and other chemicals which improve industrial hygiene and minimize worker exposure to these materials by eliminating dust exposure and improving handling ease. Another important benefit for the new product forms is better, more consistent quality rubber products resulting from the improved factory practices.

Commercial product forms for rubber chemicals are many and varied. Each was developed to do a specific job. To highlight the range of products available today, examples of eleven different, commercially available product forms are (ref. 27):

* Oil coated powder (-2% oil)

* Oil coated powder (4-5% oil)

* Microgranules

* Oil coated microgranules (1-2% oil)

* Oil coated microgranules (4-5% oil)

* Extrusion granules

* Oil coated extrusion granules (1-2% oil)

* Lentil shaped melt granules

* Extruded melt granules

* 80% active wax/stearic acid bound rods

* Crumbs or paste in oil (70-75% active)

The nitrosamine issue

While questions of rubber accelerator toxicity have been with us since the use of aniline in 1906, the rubber industry has generally responded to alleviate these issues. However, the relatively recent findings that secondary amines, so common in rubber accelerators, can react with [NO.sub.x] species to form the suspected human carcinogens, nitrosamines, has prompted active programs to develop alternative accelerators.

Stable, persistent nitrosamines result from the reaction:

[R.sub.2]NH (secondary amine) + [NO.sub.x] (nitrosating agent) [arrow right] [R.sub.2]N-N=O (nitrosamine)

Neither primary nor tertiary amines form stable nitrosamines so they are generally considered to be "safe" materials. The ability of each type of common rubber accelerator class to form nitrosamines is summarized in table 15 (ref. 28).

                     Table - 15

Product class                      Possible formation of
                                   suspect nitrosamines

Sulfenamides (primary amines)               No
Sulfenamides (secondary amines)            Yes
Thiazoles                                   No
Guanidines                                  No
Dithiocarbamates                           Yes
Thiurams                                   Yes
Specialty amines                   No (in most cases)
Dithiophosphates                            No
Sulfur donors (secondary amines)           Yes

The [NO.sub.x] nitrosating agent ipresent in the atmosphere is often due to air pollution. High surface area fillers such as carbon black will absorb [NO.sub.x] and liberate it during the vulcanization process. Of course, this is the process where [NO.sub.x] is most likely to be in contact with the various accelerators. Consequently, unacceptably high levels of nitrosamines have been detected near curing presses, near continuous curing lines and in tire storage warehouses. In West Germany, regulation TRGS 522 calls for a maximum concentration of atmospheric nitros-amines of 1[mu]g/[m.sup.3] air in general areas and 2.5 [mu]g/[m.sup.3] maximum in "problem areas."

Nitrosamines from vulcanizing agents can be controlled or eliminated by using only primary amine based accelerators or accelerators which form stable fragments during cure. This is an active area of research today and some potential new accelerators include those listed in figure 21 (ref. 28).

None of these materials is a drop-in replacement for existing accelerators, and each requires redevelopment of the cure system to be successfully used.

Another way to minimize nitrosamine exposure is to use scavengers. They are reported to function by reacting rapidly with nitrosamines to render them harmless, but the relative effectiveness of each scavenger remains to be better quantified. Materials which have been investigated include those listed in table 17 (ref. 28).

Table 17 - nitrosamine scavengers

* Polyisocyanates
   (Desmodur TT and VK-Bayer)
* Hydroxyl ammonium
   sulfate
* Diethyl hydroxylamine
* Hydrazine sulfate
* Poly carbodiimide
   (Stabaxol - Rhein-Chemie)
* Propyl gallate
* [alpha]-Tocopherol/vitamin E
   (Ronotec 200-Hoffmann-LaRoche)
* Ascorbic acid/vitamin C
* Amide/amines (Rhenofit
   NC - Rhein-Chemie)
* Calcium oxide (BF
   Goodrich)

Quality issues

One final note regarding rubber accelerators and their manufacture and use is the continuing effort for improved product quality. Probably the most active area of research by rubber chemicals producers today is in process improvement leading to consistent, high quality, environmentally acceptable products and product forms produced in enviromentally responsible plants.

Today's increasing demands for TQM and other quality practices dictate this level of research, and clearly the results will be better, safer and more consistent products.

    Table 16 - example of cure system redesign to
                 minimize nitrosamines

Formulation (phr)             ZDEC   ZDBeC

Natural rubber (SMR 5)         100
Stearic acid                   2.5
Zinc oxide                       5
N339 black                      55
Aromatic oil                     3
Ozone protective wax             1
IPPD                           2.5
TMQ                            1.5
Sulfur                         2.3
ZDEC                           0.5        -
ZDGBeC                           -    172.3
Total                        171.8

Vulkameter @ 140 [degrees] C
t110 (min)                     2.1      3.7
t90 (min)                      7.5      6.9
MH (N)                          57       67

Physical properties (cure: 20 min. @ 140 [degrees] C

Tensile strength (MPa)        23.4   21.3
Elongation (%)                 495    450
300% modulus (MPa)            14.5   14.3
Hardness (Shore A)              67     67
Rebound resilience (%)          43     41
C.S. DIN 53517 B (%)
  22 hrs @ 70 [degrees] C       20     19
  70 hrs @ 100 [degrees] C      46     47

Bibliography

1. Coran, A. Y., "Vulcanization," Internal Technical Review, Monsanto Chemical Company, 1988. 2. Thompson, R.S., "Rubber in America before 1492," Godfrey Cabot, Inc., Boston, 1950. 3. Shaw, D., "People who made it possible," European Rubber Journal, 25, June 1989. 4. Warrach, W., Miles, Inc., personal communication, Akron, OH, 1991. 5. International Institute of Synthetic Rubber Producers, 1990. 6. Personal communication, Miles, Inc., 1992. 7. Chemical Market Reporter, October, 1991. 8. Rader, C.P., "Vulcanization," Intermediate Correspondence Course, Rubber Division, ACS, Akron, OH, 1985. 9. Layer, R. W., Elastomerics, May, 1988. 10. Fath, MA. and Lederer, D.A., "Precautions in using insoluble sulfur," Rubber World, 3, 1979. 11. Helt, W.F., To, B.H. and Paris, W. W, "Post vulcanization stabilization of NR," Rubber World, 8, 1991. 12. Brydson, J.A., "Rubber chemistry," Applied Science Publishers, Ltd. London, 1978. 13. Paris, W. W, "Vulcanization, its activation and acceleration," Educational Symposium, Rubber Division, ACS, Fall 1982. 14. Studabaker, M., "Vulcanization of hydrocarbon rubbers," Phillips Chemical Company, Circa 1970. 15. "Vulkalent E retarder," Product information bulletin, Miles, Inc., 1987. 16. "General catalog for peroxides and specialty chemicals," Pennwalt Chemicals Company 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, RE, "Neoprene, the first high performance elastomers," Paper 22, ACS Rubber Division Meeting, Fall 1981. 20. "Crosslinking structures and elastomer 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 ORC-107B, 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 stabilty," 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. Errata: Some of the reference numbers were wrong in the December and October. 1993 installments. The following indicates what was printed with the correct reference following in parenthesis, October. 14(12) and 16(13). December: 18(15); 19(16); 20(16); 2](18). 22(19).