Antioxidants

All polymeric materials undergo oxidation reactions at any stage of their lifetime (manufacturing, storage, processing and service life). Typical effects of oxidative degradation of polymers and paints can be listed as yellowing, fading and loss of mechanical properties (namely flexibility and adhesion). Depending on high surface-to-volume ratios that accelerate oxygen diffusion and oxidation reactions, oxidation reactions are pronounced in thin-layer applications such as surface coatings. When such coatings are exposed to excessively high temperatures during baking and/or throughout their service life, the use of antioxidants becomes a requirement. Particularly in coil and powder coatings in white and pastel shades, the reduction of color loss resulting from baking is a direct indicator of how effectively the selected antioxidants can provide protection against thermally generated oxidation/yellowing reactions. During oxidation, polymers undergo processes such as chain scission, oxygen uptake, peroxide radical and hydroperoxide formation, ultimately resulting in colored byproducts. Antioxidants prevent oxidation caused by thermal effects in polymers during heat exposure, processing, baking or curing at high temperatures, as well as in coatings during their service life due to thermal causes. Irganox® and Irgafos® products are, respectively, different primary sterically hindered phenolic antioxidants and secondary thioether or phosphite antioxidants.

Why is Stabilization Important?

Stabilization is important • To be able to process a polymer product, • To meet the end-material conditions, some of which are specified below. • Mechanical properties retained throughout their lifetime • End-product color retained after processing and throughout service life • Aesthetic properties retained after processing and throughout service life Unlike HALS, antioxidants have a non-regenerative reaction mechanism. This means that both primary and secondary antioxidants are essentially consumed and rendered ineffective after the reaction. Although HALS is generally not effective in preventing thermal degradation, it has the advantage of enabling antioxidants to function over longer periods due to its regenerative nature. Despite some oligofunctional HALS being effective thermal stabilizers at prolonged moderate low temperatures, HALS are quite suitable for use as mild stabilizers due to the described properties. Primary antioxidants, including sterically hindered phenols, inactivate free radicals (RO•) formed during thermal oxidation and prevent degradation reactions through a kinetic mechanism. Primary antioxidants preferably function by bonding hydrogen atoms to oxygen-centered radicals (RO•, ROO•) and thus prevent the auto-oxidation process. Phenolic antioxidants are multipurpose products that can be used over a wide temperature range and are mostly used to enhance the long-term thermal stability of various coatings and plastics. Secondary phosphite antioxidants decompose hydroperoxides (ROOH) formed during the auto-oxidation process and enhance the performance of primary antioxidants through synergistic effects. Today, hydrolytically stable phosphites and divalent sulfur compounds are the most commonly used secondary antioxidants. Among applications are heat balancing during synthesis, processing, mixing, extrusion and curing of coatings baked or cured at relatively high temperatures (for example, coil and powder coatings).

Hindered Phenolic Antioxidants

• Show activity over a wide temperature range (from room temperature to melting). • Increase long-term thermal stability. • Non-regenerative mechanism, they are consumed during the reaction.

Phosphite Antioxidants

• Are process stabilizers. • Stable against hydrolysis. • Do not form colored reaction products.

Thioether Antioxidants

• Thiosynergistics are "heat lovers". • ROOH decomposition rate increases with temperature change. • Designed to provide protection against elevated temperature aging.

Oligomeric HALS

• HALS with antioxidant properties. • Function up to 120°C. • Stable against light, migration and extraction. • Regenerative mechanism unlike antioxidants. Berna Noyan Sales Manager, Paints Resins and Additives BASF     References: 1) BASF Resins&Additives Brochure 2) Documents by Dr. André C. Kastler (BASF)