Acid scavengers in polyolefins
Polyolefins, also termed polyalkene, are polymers produced from simple olefins, such as polyethylene (produced by polymerizing ethylene) or polypropylene. The history of polyolefins starts with the accidental discovery of polyethylene by Hans von Pechmann in 1898. The German chemist synthesized it by heating diazomethane, which was also discovered by Von Pechmann in 1894. The white, waxy substance that he had created, was first named polymethylene. Due to the instability of diazomethane, however, no industrial relevant synthesis method could be developed based on the discovery of Von Pechmann. This would take three more decades, until Michael Perrin, a chemist at Imperial Chemical Industries (ICI) developed a reproducible synthesis in 1939 that was based on applying extremely high pressure to a mixture of ethylene and benzaldehyde. The foundation of this method was established in 1933 by two other ICI chemists: Eric Fawcett and Reginald Gibson. The method of Perrin made the first industrial production of Low Density Polyethylene (LDPE) possible in 1939, but the real breakthrough was the introduction of catalysts that allowed the polymerization at much lower temperatures and pressure. The first step for this was taken by Robert Banks and J. Paul Hogen of Phillips Petroleum, who developed a chromium trioxide based catalyst in 1951. Karl Ziegler’s catalyst, based on titanium halides and organoaluminium compounds, resulted in a synthesis method at even milder conditions. When Giulio Natta became aware of the Ziegler catalyst, he noticed the potential of the catalyst to polymerize α-olefins such as propylene stereoregularly. This resulted in the development of highly crystalline, stereoregular polymers that were previously not possible. In the 1970’s, the catalyst of Ziegler was improved further by the incorporation of magnesium chloride. Homogeneuous catalyst systems, metallocenes, have been reported in 1976 by Walter Kaminsky and Hansjörg Sinn. In 1963, Ziegler and Natta were both awarded the Nobel Prize in Chemistry for their groundbreaking work. From the mid-1950's onward, the Ziegler-Natta catalyst have been used to produce various polyolefins.
Today, the worldwide production volume of plastics, elastomers and rubers produced from olefins with Ziegler-Natta or related catalysts exceeds 100 million tons per year. Polyolefins plastics are used in an extremely wide variety of applications, including packaging, cable insulation, clothing and medical goods. Before a polyolefin material from the reactor is suitable for its intended application, it needs to undergo several processing steps. The elevated temperatures, shear and exposure to oxygen can cause degradation processes that have a major impact on the polymer melt as well as the mechanical and aesthetic properties of the final article. By selecting the right stabilizing additives, these negatives effects can be largely prevented. The basis of a typical additive package for polyolefins include primary (phenolic-) and secondary antioxidants (phosphites or phosphonites) and acid scavengers. A common acid scavenger used in polyolefins is synthetic hydrotalcite, a material that is being used as stabilizer in polyolefins under the brandname DHT-4A since the late 1970's due to the pioneering work of Kyowa Chemical Industry. The key functionality of hydrotalcites is the irreversible adsorption of acidic catalytic residues. By doing so, hydrotalcites prevent many damaging side effects, most notably corrosion of processing equipment and degradation of the polymer itself. Furthermore, hydrotalcites, and especially high quality materials such as DHT-4A, can synergisticaly improve the performance of other additives and pigments in the formulation. A good example of this effect is the improved performance of Hindered Amine Light Stabilizers (HALS) in the presence of hydrotalcite, resulting in increased weatherability of the polyolefin article.
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