Low-Valence Transition Metal Additive

The polymer will produce a lot of smoke during the combustion process, and the smoke produced by the combustion of some polymers is toxic. Alfa Chemistry provides low-valence transition metal additive as smoke suppressant, which is one of the ways to solve the problem of smoke suppression of polymer materials.

Typical Product

Some low-valent transition metal-containing additives can promote polymer crosslinking through the reductive coupling mechanism of polymer chains and play a role in smoke suppression. Typical products include phosphites and other monovalent copper clathrates, a range of low-valent transition metal carbonyls, formates and oxalates, monovalent copper halides, etc.

Copper is one of the most effective additives for PVC smoke suppression using a reductive coupling mechanism. In addition, cuprous and copper compounds can be used as weak acid catalysts to facilitate the alkylation of Friedel-Crafts reactions.

Smoke Suppression Mechanism

Take the smoke suppression mechanism of a series of transition metals such as oxalate and formate as smoke suppressants for PVC as an example. First, zero-valent metals are formed through reduction reactions. Cross-linking can occur in solid-state PVC below 200 °C when additives with transition metals suppress fumes through a reductive coupling mechanism of polymer chains that promote cross-linking. The equations for the reduction and coupling reactions of simple allyl halides and benzyl halides promoted by low-valent organometallic clathrates are shown in the figure below. [1]

Advantages of Reductive Coupling

• Each reductive coupling crosslink can simultaneously terminate the growth of both polyene chains, which can slow down the degradation of the polymer. Especially for the reaction at the allylic position, the reductive coupling crosslinking can suppress the early thermal degradation.
• To a certain extent, coupling may occur at non-allylic positions, which can delay dehydrochlorination of PVC. The relatively short polyene chains formed by allylic coupling can limit the amount of benzene and other aromatics produced during the pyrolysis of PVC.
• Easily reducible metal additives are not only reducing coupling agents, but also typical weak Lewis acids. Therefore, these metal additives can also catalyze some Friedel-Crafts cross-linking that is beneficial to flame retardancy and smoke suppression, but cannot promote carbon layer cracking.

Research Information

Smoke suppression effect of transition metal oxides

Zhoumei Xu et al. developed a series of carbon-coated transition metal oxide particles, including Cu@C, Fe₂O₃@C, and Cu₂O@Fe₂O₃@C, and studied their related smoke suppression mechanism for RPUF. In this work, the smoke suppression mechanism of metal oxides can be mainly attributed to the reductive coupling mechanism: the catalytic effect of Cu₂O or Fe₂O₃on the coupling reaction of polyurethane molecular chains. Under this action, many degradation products are transformed into a dense and continuous carbon layer, which inhibits the heat and mass exchange process, thereby preventing the further decomposition of the internal foam structure. [2]

Schematic illustration for the mechanisms of smoke suppression [2]

References

1. Jianjun Li, et al. (2019). "7. The mechanism of smoke suppression". Theory of Flame Retardation of Polymeric Materials, pp. 195-210.
2. Zhoumei Xu, et al. (2020). "The influence of carbon-encapsulated transition metal oxide microparticles on reducing toxic gases release and smoke suppression of rigid polyurethane foam composites," Composites Part A: Applied Science and Manufacturing 131, 105815.