1. Antioxidants for plastics
Antioxidants are used in a variety of resins to prevent oxidative degradation. Degradation is initiated by the action of highly reactive free radicals caused by heat, radiation, mechanical shear, or metallic impurities. The initiation of free radicals may occur during polymerization, processing, or fabrication.
Once the first step of initiation occurs, propagation follows. Propagation is the reaction of the free radical with an oxygen molecule, yielding a peroxy radical. The peroxy radical then reacts with an available hydrogen atom within the polymer to form an unstable hydroperoxide and another free radical.
In the absence of an antioxidant, this reaction continues and leads to polymer degradation. Degradation is manifested either by cross-linking or chain scissoring. Cross-linking causes the polymer to increase in molecular weight, leading to brittleness, gellation, and decreased elongation. Chain scissoring decreases molecular weight, leading to increased melt flow and reduced tensile strength.
The function of an antioxidant is to prevent the propagation steps of oxidation. Products are classified as primary or secondary antioxidants depending on the method by which they prevent oxidation.
2. What are primary and secondary antioxidants?
Primary antioxidants, usually sterically hindered phenols, function by donating their reactive hydrogen to the peroxy free radical so that the propagation of subsequent free radicals does not occur. The antioxidant free radical is rendered stable by electron delocalization.
Secondary antioxidants retard oxidation by preventing the proliferation of alkoxy and hydroxyl radicals by decomposing hydroperoxides to yield nonreactive products. These materials are typically used in a synergistic combination with primary antioxidants.
The table below lists the chemical types of primary and secondary antioxidants and their major resin applications. Through the remainder of this chapter, antioxidants will be addressed by type based on overall chemistry. The class of antioxidants merely describes its mode of stabilization.
2.1. Primary antioxidants
Amines
Amines, normally arylamines, function as primary antioxidants by donating hydrogen. Amines are the most effective type of primary antioxidant, having the ability to act as chain terminators and peroxide decomposers.
However, they tend to discolor, causing staining, and, for the most part, lack FDA approval. For this reason, amines are found in pigmented plastics in nonfood applications. Amines are commonly used in the rubber industry but also found in minor use in plastics such as black wire and cable formulations and in polyurethane polyols.
Phenolics
The most widely used antioxidants in plastics are phenolics. The products generally resist staining or discoloration.
However, they may form quinoid (colored) structures upon oxidation. Phenolic antioxidants include simple phenolics, bisphenols, polyphenolics, and thiobis phenolics.
The most common simple phenolic is butylated hydroxytoluene (BHT) or 2,6-di-t-butyl-4-methylphenol. BHT possesses broad FDA approval and is widely used as an antioxidant in a variety of polymers. It is commonly called the “workhorse” of the industry but is losing ground to the higher molecular weight antioxidants which resist migration.
The disadvantage of BHT is that it is highly volatile and can cause discoloration. Other simple phenolics include BHA (2- and 3-t-butyl-4- hydroxyanisole) which is frequently used in food applications.
Polyphenolics and bisphenolics are higher in molecular weight than simple phenolics and both types are generally non-staining. The increased molecular weight provides lower volatility but is generally more costly.
However, the loading of polyphenolics is much less than that of simple phenolics. The most commonly used polyphenolic is tetrakis methylene-(3,5-di-t-butyl-4-hydroxyhydrocinnamate)
methane or IRGANOX1010 from Ciba.
Other important bisphenols include Cytec Industries’ CYANOX 2246 and 425…
Thiobis Phenols are less effective than hindered phenols in termi- nating peroxy radicals. They also function as peroxide decomposers (secondary antioxidants) at temperatures above 100°C. Typically, thio- bisphenols are chosen for use in high-temperature resin applications. Users generally prefer hindered phenols over bisphenols where high-temperature service is not involved.
2.2. Secondary antioxidants
Organophosphates
Acting as secondary antioxidants, organophosphates reduce hydroperoxides to alcohols, converting themselves to phosphonates. They also provide color stability, inhibiting the discoloration caused by the formation of quinoid reaction products which are formed upon oxidation of phenolics.
Tris-nonylphenyl phosphite (TNPP) is the most commonly used organophosphate followed by tris(2,4-di-tert-butylphenyl)phosphite (for example, Ciba’s IRGAFOS 168).
The disadvantage of phosphites is their hygroscopic tendency. Hydrolysis of phosphites can ultimately lead to the formation of phosphoric acid, which can corrode processing equipment.
Thioesters
Derived from aliphatic esters of B-thio dipropionic acid, thioesters act as secondary antioxidants and also provide high heat stability to a variety of polymers.
Thioesters function as secondary antioxidants by destroying hydroperoxides to form stable hexavalent sulfur derivatives.
Thioesters act as synergists when combined with phenolic antioxidants in polyolefins.
The major disadvantage of thioester antioxidants is their inherent odor which is transferred to the host polymer.
Deactivators
Metal deactivators combine with metal ions to limit the potential for chain propagation. Metal deactivators are commonly used in polyolefin inner coverings in wire and cable applications where the plastic comes in contact with the metal.
In effect, the deactivator acts as a chelating agent to form a stable complex at the metal interface, thereby preventing catalytic activity. The most common deactivators contain an oxamide moiety that complexes with and deactivates the metal ions. A typical product is Ciba’s IRGANOX MD-1024.