Лако-красочные материалы — производство

There are two major classes of antioxidants and they are differentiated based on their mechanism of inhibition of polymer oxidation: chain-terminating or primary antioxidants and hydroperoxide-decomposing secondary antioxidants [5]. Primary or free-radical scavenging antioxidants inhibit oxidation via very rapid chain-terminating reactions. The majority of primary antioxidants are hindered phenols or secondary aryl amines. Generally, hindered phenols are nonstaining, nondiscoloring, and are available in a wide

AO-1 2,6-di-t-butyl-p-cresol

AO-2 Tetrakis[methylene(3,5-di- t-butyl-4-hydroxylhydrocinnamate)]methane

AO-3 2,4-bis(w-octylthio)-6-(4′-hydroxyl-3′,5′-di-t-butylanilino)-1,3,5-triazine

AO-4 2,4-bis[(octylthio)methyl]-o-cresol

AO-5 Triethyleneglycol-bis-3-(3′-t-butyl-4,-hydroxy-5′-methylphenyl)propionate

AO-6 Butylated reaction product of p-cresol and dicyclopentadiene

AO-7 Hindered phenol

Figure 1 Radical stabilization using hindered phenols.

range of molecular weights and efficiencies. Amine-based primary antioxidants are very effective. However, they tend to interfere with peroxide cross-linking, and they discolor and stain. They are used primarily in situations where their color addition is unimportant or can be hidden.

A typical hindered phenol primary antioxidant is AO-1 (2,6-di-tert-butyl-para — cresol) (Table 2). Stablilization is achieved through proton donation from the — OH of AO-1 to a peroxy or alkoxy radical (Fig. 1). This reaction is in favorable competition with proton donation from a polymer carbon atom. Important to note, however, is that the resulting phenoxy radical is stable and does not abstract a proton from the polymer chain. This would be an undesirable effect, because the antioxidant would then be acting as a chain transfer agent. Due to its low cost, AO-1 is widely used as an antioxidant. In adhesive formulations, where exposure to high temperatures is possible (e. g., HMAs), the high volatility level of AO-1 renders it virtually useless. To solve this problem, state-of-the — art lower-volatility antioxidants are used to achieve the necessary level of stabilization (Table 2). In addition to their lower volatility, these antioxidants show higher activity, compatibility, and a resistance toward the formation of colored byproducts during compounding and application temperatures of HMAs.

The performance of a primary antioxidant can be improved by the use of a second­ary antioxidant. Secondary antioxidants or peroxide decomposers do not act as radical scavengers but undergo redox reactions with hydroperoxides to form nonradical stable products (Fig. 2). This class of antioxidants (Table 3) includes phosphites such as tris(nonylphenyl)phosphite (PS-1) and thiosynergists or thioesters such as dilauryl

Phosphites:

(RO)3P + FfOOH ——— ► (R0)3P=0 + R’OH

Thiosynergists:

R-S-R + R’OOH———— ► R-S=0 + R’OH (+ further oxidation)

R

Figure 2 Radical stabilization with secondary antioxidants.

Table 3 Key to Secondary Antioxidants

PS-1 Tris(nonylphenyl)phosphite

TS-1 Dilauryl thiodipropionate thiodipropionate (TS-1). Phosphites reduce hydroperoxides to alcohols as they are oxi­dized to phosphate. Phosphites are generally highly effective process stabilizers and are nondiscoloring. Thiosynergists or thioesters are also nondiscoloring and are used with primary antioxidants to achieve long-term heat stability. Secondary antioxidants are gen­erally used exclusively in combination with primary antioxidants. Thus secondary anti­oxidants are referred to as “synergists” because the overall level of stability achieved when a primary and a secondary antioxidant are used in combination is much greater than if either were used alone.

A variety of factors, including compounding or processing conditions, end use, and expected performance, should influence the selection of an appropriate antioxidant system. These factors include compatibility with the polymer and other additives or com­ponents, antioxidant mobility and volatility, discoloration, resistance to hydrolysis, extraction resistance, radical trapping efficiency, toxicity, and cost-effectiveness.

As mentioned previously, hot-melt adhesives are primarily a blend of several hydro­carbon based components, each of which is susceptible to thermooxidative degration. The stabilization of adhesive compounds against thermooxidative degradation is complex. Typically used to inhibit or prevent degradation, antioxidants are added to each hydro­carbon component [6] as well as to the final adhesive formulation. To gain a better under­standing of the stabilization of an adhesive formulation, it is beneficial to examine the degradation mechanisms of the individual components.