Magnesium hydroxide: a new and versatile rubber chemical.

Magnesium hydroxide: A new and versatile rubber chemical

The first significant reference to the use of magnesium hydroxide as a rubber chemical is a paper presented at the 1985 Rubber Division meeting in Los Angeles [ref. 1]. On the basis of this work, magnesium hydroxide was recommended as the inorganic base for the thiadiazole cure system in CPE. This reference is considered "significant" in that it is connected to the first commercial uses of magnesium hydroxide as a rubber vulcanizing agent. In this case, it replaced the traditionally-used magnesium oxide. Improvements derived from the use of magnesium hydroxide included greater scorch safety and shelf-life and improved cure rates for this system (table 1).

Since that initial reference, the use of magnesium hydroxide has been extended to chlorosulfonated polyethylene, specifically in its use in single-ply roofing membranes. Here, too, magnesium hydroxide is recommended instead of the traditional stabilizer, magnesia. Magnesium hydroxide offers the same properties as magnesia, in addition to effective improvement in water resistance [ref. 2], (table 2). Furthermore, substitution of MgO with magnesium hydroxide greatly enhances the bin stability of practical formulations [ref. 3], (table 3).

The magnesium hydroxides studied and recommended as rubber chemicals in these references are precipitated reaction products. Although there are naturally occurring deposits of magnesium hydroxide, called bruxite, use of this chemical in rubber requires a controlled degree of purity, reactivity and particle size that is attainable only from the synthesized product. There are several synthetic routes to the production of magnesium hydroxide, one being the reaction of lime and brine, both natural products. Another is the reaction of magnesium chloride with sodium hydroxide. Both routes have been and are used to produce commercial quantities. The choice of starting materials has obvious implications on the cost of the magnesium hydroxide product and on the nature of the non-magnesium hydroxide components. Trace amounts of Ca, Fe and water soluble salts are present in all commercially available hydroxides, but the absolute amount can vary depending upon processing conditions. The highest purity standard is described by the U.S. Pharmacopeia Monograph for magnesium hydroxide [ref. 4]. This standard is not normally used in technical applications, but is, rather, almost exclusively reserved for use with hydroxides in pharmaceutical applications. For the vast majority of rubber applications, the 96%+ purity level provided by standard technical grades of precipitated magnesium hydroxides is all that is required.

Beyond the chemical purity, the attributes that meaningfully differentiate hydroxides used as rubber additives are particle size and reactivity. Now, magnesium hydroxide is an acid acceptor. Indeed, its classic and wide spread use in both liquid and tablet antacid formulations is based on its ability to neutralize acids. However, the rate of reaction with acids varies from hydroxide to hydroxide and depends upon the reactivity, or activity, of a given hydroxide. In this context, magnesium hydroxides exhibit a range of activity analogous to magnesium oxides, i.e., low to moderate to high.

Magnesium hydroxide activities generally average 3-10X lower than rubber grade magnesium oxides. The activity of both oxides and hydroxides is related to surface area. In the case of magnesium oxide, the surface area or activity is measured by iodine absorption, or by BET nitrogen absorption, the more modern method (table 4).

In polychloroprene, the activity of magnesium oxide correlates with performance - the higher the magnesia activity, the greater the processing safety and the better the vulcanizate properties [ref. 5]. The detailed correlation between the activity of magnesium hydroxide and its performance in rubbers has yet to appear in print and may not even yet have been the focus of study. Some conclusions about this relationship can be drawn from the existing literature, however. In the case of the CPE work cited earlier [ref. 1], the inorganic base, e.g., magnesium oxide or hydroxide, functions as an HC1 acceptor. Given this fact, the activity of the inorganic base could measurably influence and therefore correlate to the cross-linking reaction, although no evidence is presented to either support or contradict this.

In the case of chlorosulfonated polyethylene, clearly some factor other than activity is involved. Again, from a reference cited earlier [ref. 3], it is apparent that high activity magnesia produces virtually the same set of physical properties as low activity magnesium hydroxide. With respect to vulcanizate properties, then, magnesium hydroxide produces results consistent with rubber grade magnesium oxide, a surprising finding. With respect to uncured properties, specifically bin stability as indicated by Mooney viscosity of aged, uncured samples, magnesium hydroxide is superior to all tested oxides, regardless of activity. It is, perhaps, in the uncured state where the low rate of reaction for hydroxide with HC1 can dominate and accounts for the relatively slow formation of cross-links. In this case, where moisture is a curative, the positive effects of low activity could be augmented by the non-hygroscopic nature of magnesium hydroxide. In other words, substituting magnesium hydroxide for magnesium oxide not only reduces the rate of cross-linking by acid acceptance by an inorganic base but could also measurably reduce the formation rate of [H.sub.2] O induced cross-links.

An exploratory study of various magnesias and magnesium hydroxide in carboxylated nitrile rubber showed that, in this system, magnesium hydroxide appeared to function as a very low activity magnesium oxide with respect to vulcanizate properties [ref. 6]. However, the rate of ionic cross-link formation with hydroxide-containing formulations were similar to the zinc oxide-containing control. Since magnesium hydroxide showed promise in reducing scorch relative to the ZnO control, further work should be done with this system to establish the combination of metallic oxides/hydroxides which would optimize both uncured and cured properties (tables 5a, b and c).

If one generalization can be made with respect to the functioning of magnesium hydroxide as a rubber chemical, it is that analogies to MgO or other metal oxides are of limited usefulness as predictive tools. Clearly, magnesium hydroxide can now be classed as a full-fledged vulcanizing agent/stabilizer, to be considered whenever acid acceptance/ionic cross-linking is part of the vulcanization or aging mechanism.

The fact that magnesium hydroxide has performed uniquely and unexpectedly to solve some important problems related to CPE and CSM stability opens up some interesting possibilities for future work. Already noted is the non-hygroscopic nature of magnesium hydroxide coupled with behavior similar to conventional metal oxides. The possibility exists for magnesium hydroxide to replace lead oxide in at least some of the less demanding low water swell applications. There is also the possibility for formulations of carboxylated nitrile rubber with improved scorch safety, already cited. Given similarities between magnesium hydroxide and low activity magnesias, it would be interesting to discover how the performance of halobutyls and fluoroelastomers would be affected by substitution of magnesium hydroxide for the low-to-moderate activity magnesias sometimes recommended in recipes of these polymers.

By far the largest volume future potential for magnesium hydroxide in rubber lies in an area totally distinct from the additive applications mentioned previously - the use of magnesium hydroxide as a smoke suppressant filler. Magnesium compounds have been shown to reduce the amount of smoke generated in the burning mode. They have also been shown to promote char formation [ref. 7]. The range of elastomers evaluated is already broad. EPDM, EVA, PVC sheet, PVC foam and PVC/nitrile blends are among those which show increased values of limiting oxygen index and/or decreased smoke density with the addition of Mg-based compounds (ref. 8). V-O ratings in the UL94 test have been achieved as have LOI ratings of 34, both in non-halogenated polymer systems.

Used as a flame retardant/smoke suppressant (FR/SS) filler in rubbers, magnesium hydroxide is also relatively new, judging from its absence from a 1986 review article on advances in flammability [ref. 9]. The driving force for the increased use of FR/SS fillers such as magnesium hydroxide is the increasing demand for non-halogen-containing compounds. Although traditionally used and very effective, halogen-containing compounds liberate halogen acids upon smoldering and burning, with attendant potential toxicity and corrosion problems to humans and equipment within the immediate environment. Magnesium hydroxide offers a non-halogen alternative to halogenated flame retardants. It functions as a flame retardant by its endothermic decomposition, which draws heat from the smoldering/flaming compound, and release of water vapor, a flammable flame diluent (figure 1). In this regard, magnesium hydroxide is similar to aluminum trihydrate, another more commonly used hydrated filler. With respect to physical properties such as vicosity, cure rate, stress-strain and durometer, magnesium hydroxide is virtually indistinguishable from alumina trihydrate. However, using EPDM as the test case, it has been established that combinations of alumina trihydrate and magnesium hydroxide produce less smoke than compounds filled with the same total loading of either by itself [ref. 7], (figure 2). Furthermore, the compounds with such mixed filler systems exhibit greater ignition resistance in the UL-94 horizontal burn tests and higher limiting oxygen index.

Clearly, there exist some possibilities for compounds based on non-halogen elastomers and fillers to successfully compete with halogenated materials in terms of flame retardancy, plus provide the added potential benefit of reduced smoke. Magnesium hydroxide, because of its acid scavenging properties, can also play a useful role in halogenated compounds by reducing acid gas emissions. Although some references exists within the rubber industry documenting such effects, it has been an area of intense interest and discussion in the plastics industry, where the ubiquitous use of PVC is being challenged because of HC1 emissions [refs. 9-11]. There are a myriad of options available to compounders facing the task of producing flame retardant materials with concurrent required processing and physical properties. The options range from intrinsically flame retardant polymers, new and old, to additives to fillers both halogenated and non-halogenated. Synergistic effects among ingredients multiply the possibilities. Complicating the task are variations in FR performance depending upon test methods, test scale and test conditions.

Successfully negotiating this maze will likely lead to stronger supplier/compounder relationships as each can add specific expertise to the effort. It is also likely supplier alliances, either formal or informal, will develop as synergists or "FR systems" look increasingly effective.

Rubber chemicals came into existence and stay in use because they fill needs in cost-effective ways for rubber producers, fabricators or end-users. Magnesium hydroxide has joined the ranks of these "problem-solvers" and has found niche uses in a number of different systems. Its versatility as an additive and functional filler holds promise for other practical applications yet to be developed. [Tabular Data 1 to 5c Omitted] [Figure 1 & 2 Omitted]

References [1]Flynn, J.H., and Davis, W.H., A.C.S. Rubber Division Spring Meeting, paper #24 (1985). [2]Doherty, F.W., Akron Rubber Group Winter Meeting (1987). [3]Beekman, G.F., and Hastbacka, M.A., A.C.S. Rubber Division Spring Meeting, paper #10 (1986). [4]United States Pharmacopeia, Edition XXI, U.S. Government Printing Office. [5]Murray, R.M., and Thompson, D.C., The Neoprenes, E.I. duPont de Nemours and Company, 1963. [6]Beekman, G.F., and Hastbacka, M.A., op. cit., paper #9. [7]Beekman, G.F., and Hastbacka, M.A. Rubber & Plastics News, Technical Report, Dec. 14, 1987. [8]Fenwick, R.E., Scandinavian Rubber Conference, SRC 80 (1980). [9]Lawson, D.F., Rubber Chemistry and Technology, 59(3), 455 (1986). [10]Andrews, C.R., and Tarquini, M.E., FRCA Meeting (1989). [11]Kroenke, W.J., Journal of Applied Polymer Science, 26, 1167-1190.