New Alternatives for Environmentally Friendly Heat Stabilizers

In recent years, considerable effort has been expended toward the development of new stabilization systems for PVC processing, driven by a desire to move away from stabilizers based on heavy metals. Although there are now numerous alternatives to lead and cadmium stabilizers--including calcium/zinc, organotin, and barium/zinc systems--each has its drawbacks for the flexible vinyl formulator.

For example, Ca/Zn stabilizers are preferred for use in critical applications because of their low toxicity, but they are generally the least effective option in terms of thermal performance. Organotin mercaptides are widely accepted as the most efficient thermal stabilizers. However, the odor properties of the sulfur-based ligands prevent their use in most flexible PVC processes. Although Ba/Zn products have proven to be a suitable compromise between Ca/Zn and organotins from a thermal perspective, many believe that barium could be the next metal targeted by regulators.

Morton International, previously a supplier of stabilizers only for rigid PVC, recently launched a new all-organic-based heat-stabilizer technology for flexible calendering, extrusion, plastisols, injection molding, and rotomolding. Called Advaflex, these stabilizer systems address the limitations of currently used stabilizers. Presently being field tested, these systems are also cost-effective and free of cadmium, lead, barium, phosphorus, phenolics, and other components that raise environmental and health concerns. End-use applications initially targeted include pond liners, flooring, roofing, footwear, calendered film, wire, and cable.

The impetus for the development of this novel technology is the identification of new activated organosulfide compounds. Such compounds are chemically prepared as blocked or protected thiols, which are designed to decompose under vinyl processing conditions and to generate highly active stabilizing mercaptide species. These "latent mercaptides" offer unique performance attributes relative to conventional mixed-metal systems or organotin mercaptides.

Because the sulfur-bearing compounds are bound to organic protecting groups, they do not exhibit any adverse interactions that would occur with conventional sulfur ligands. As a result, the Advaflex systems have very low odor and volatility. They are hydrolytically and oxidatively stable, and are compatible with a broad range of additives for PVC. Compared with Ba/Cd, Ba/Zn, and Ca/Zn, the stabilizers also show equivalent or even superior performance in both clear and pigmented systems.

Very low metal content

A totally organic product, Advaflex LM-300 can be utilized in various ways. Most significantly, it can be combined with low levels of conventional metal carboxylates to provide highly efficient stabilizer systems.

The simplicity of these two-component systems makes it convenient for processors of flexible PVC to design their own unique stabilizers depending on their varied processing needs. This approach can potentially reduce material inventories and give the user complete control over stabilizer composition and performance, making it especially desirable for those processors and compounders who currently use multiple stabilizers for different processes and applications.

Formulation of the two-component stabilizers simply entails combining Advaflex LM-300 with any of several metallic species, most preferably zinc salts.

Although any number of zinc salts can serve as the metal source, zinc octoate or zinc stearate are the most convenient. Advaflex LM-300 allows very low levels of metal to be employed. In fact, certain systems have been developed that utilize as little as 0.5% zinc as their sole source of metal (versus 10-12% Zn with standard mixed-metal stabilizers).

Ultimately, the optimal ratio of metal to Advaflex LM-300 depends on the PVC formulation, application, and heat-stabilization requirement. As demonstrated in Figs. 1 & 2, systems can be optimized to meet or exceed the performance of current Ba/Zn/P and Ca/Zn technologies at a competitive cost. These thermal stability trends have been observed in both dynamic and static testing.

The formulation used for the dynamic tests shown here is as follows:

Dioctyl phthalate, 55.0 phr
CaCO3, 10.0 phr
TiO2, 5.0 phr
Epoxy soybean oil, 3.0 phr
Stearic acid, 0.2 phr.

Alternative uses

For processors that do not require multiple stabilization systems or do not care to utilize the two-component approach, Morton also offers a drop-in replacement. Available as the Advaflex LM-2000 series, these are uniquely formulated stabilizers containing Advaflex LM-300 and the desired amount of zinc component.

Another potential way to use Advaflex LM-300 is as an adjunct to the primary metal stabilizer. Under this scenario, reductions of up to 75% in the use level of the primary stabilizer have been observed. This approach is effective with both Ba/Zn/P and Ca/Zn. The performance of Advaflex LM-300 with a reduced level of Ca/Zn is shown in Fig. 3.

This new approach to PVC stabilization is consistent with many laws in the U.S. mandating reduced use of toxic heavy metals. Toxicity screenings on this new chemistry are now complete. Based on the results, Morton International is optimistic about the future use of Advaflex in FDA-regulated applications.