Hindered combination with phenolic

In packaging, three resins account for the majority of the market for antioxidants PP, PE, and HIPS. For PP, a combination of hindered phenol and phosphite antioxidants is commonly used, with the total concentration normally from 0.08 to 1%, depending on formulation and end use. Clba Specialty Chemicals has developed the phenolic antioxidant family of Irganox for use in PP and PE, and also the Irgafos family, which are phosphite stabilizers used in combination with phenolic antioxidants. For LDPE, BHT, a phenolic antioxidant, is normally incorporated at levels of 50 to 500 ppm however there is a tendency to employ less volatile additives to prevent their migration from the resin. For HDPE and LLDPE, antioxidants less volatile than BHT, such as polyphenols, at higher concentrations, are normally used In combination with phosphites. For HIPS, hindered phenols are used in combination with UV absorbers. Alpha-tocopherol (Vitamin E) is sometimes used as an antioxidant for polyolefins. 

The most widely used antioxidants are sterically hindered phenols and bisphe-nols other additives are combined with phenols mostly in synergistic mixtures. The most recommended structures for stabilization of polyolefins against thermal oxidation and degradation are listed in Table 12.1. 

These compounds are used most frequentiy in combination with hindered phenols for a broad range of apphcations in mbber and plastics. They are also able to suppress color development caused by oxidation of the substrate and the phenoHc antioxidant. Unlike phenols and secondary aromatic amines, phosphoms-based stabilizers generally do not develop colored oxidation products. 

Irg 1076, AO-3 (CB), are used in combination with metal dithiolates, e.g., NiDEC, AO-30 (PD), due to the sensitized photoxidation of dithiolates by the oxidation products of phenols, particularly stilbenequinones (SQ, see reaction 9C) (Table 3). Hindered piperidines exhibit a complex behavior when present in combination with other antioxidants and stabilizers they have to be oxidized initially to the corresponding nitroxyl radical before becoming effective. Consequently, both CB-D and PD antioxidants, which remove alkyl peroxyl radicals and hydroperoxides, respectively, antagonise the UV stabilizing action of this class of compounds (e.g.. Table 3, NiDEC 4- Tin 770). However, since the hindered piperidines themselves are neither melt- nor heat-stabilizers for polymers, they have to be used with conventional antioxidants and stabilizers. 

The reaction between acyl halides and alcohols or phenols is the best general method for the preparation of carboxylic esters. It is believed to proceed by a 8 2 mechanism. As with 10-8, the mechanism can be S l or tetrahedral. Pyridine catalyzes the reaction by the nucleophilic catalysis route (see 10-9). The reaction is of wide scope, and many functional groups do not interfere. A base is frequently added to combine with the HX formed. When aqueous alkali is used, this is called the Schotten-Baumann procedure, but pyridine is also frequently used. Both R and R may be primary, secondary, or tertiary alkyl or aryl. Enolic esters can also be prepared by this method, though C-acylation competes in these cases. In difficult cases, especially with hindered acids or tertiary R, the alkoxide can be used instead of the alcohol. Activated alumina has also been used as a catalyst, for tertiary R. Thallium salts of phenols give very high yields of phenolic esters. Phase-transfer catalysis has been used for hindered phenols. Zinc has been used to couple... 

The most common chain extenders are the dianhydrides (also known as tetracar-boxylic dianhydrides). The most common of these is pyromellitic dianhydride (PMDA). The latter can be used in synergistic combinations with hindered phenolic aromatic phosphates such as IRGANOX 1425 manufactured by Ciba Geigy [6-8], The hindered phenolic aromatic phosphate is used at levels of 0.1-2.5 wt%. The hindered phenolic aromatic phosphate is an advantageous co-synergist since it... 

Due to their peroxidolytic activity, phosphites are normally used in combination with hindered phenols in order to deactivate hydroperoxides formed as a consequence of the antioxidant function of hindered phenols, see reaction 1. The enhanced melt stability of tocopherol-containing PP extruded in the presence of a phosphite is clearly illustrated in Fig. 5a with an optimum melt stabilising performance at a 1 2 w/w ratio in favour of the phosphite. 

The Good-rite 3125 was also combined with the Chimassorb 944LD and Cyanox STDP to evaluate possible synergistic effects between combinations of a hindered phenol stabilizer with first a thioester and then a hindered amine light stabilizer (HALS). 

Two groups of anti-oxidants are of practical importance hindered phenol light stabilisers (HPLS) and hindered amine light stabilisers (HALS). Examples are shown in Fig. 13.7 and the mechanism of the radical trapping reaction is shown in Fig. 13.8. They are often combined with UV absorbents (for partly synergistic effects) and mostly used for the fibre protection. 

This structure has many resonance forms that result in a highly stabilized radical. In eq 8.6, a phosphite is oxidized to a phosphate and the hydroperoxide is reduced to an alcohol. Phosphites are often used in combination with hindered phenols. (Please see references 4-6 for details on functioning of antioxidants.)... 

Alkyl and aryl phosphite esters are also effective melt stabilizers. They are often used in combination with hindered phenols to give highly efficient melt stabilizing systems and to reduce discoloration of the polymer because of the oxidation products of the phenols present. Phosphites (particularly those derived from aliphatic alcohols and unhindered phenols) are, however, generally susceptible to hydrolysis. Consequently, moisture-sensitive phosphites affect adversely the handling characteristics (i.e., flow properties) of the additive package and are a source of other problems corrosion of metal surfaces, formation of dark colored spots, and gel formation. In practice, hydrolysis-resistant phosphites based on sterically hindered phenols are used, e.g., AOs 17 and 18, Table 1. 

Figure 1.42. Weight loss from polyacetal during processing due to liberation of formaldehyde and inhibition by an acid scavenger combined with a hindered phenol antioxidant. Adapted from Zweifel (1998).

With all this boron enolate chemistry it is easy to forget some of the older enolate methods that are still available. The use of an ester containing a hindered phenolic group has been seen before in Chapter 21 to control the formation of trans enolates so that anti aldol products may be produced. It is used here to couple two fragments in just that way. Ester 271 reacts with the hindered base LiTMP (in combination with LiBr) to give trans enolate 272 which was combined with aldehyde 273 to give49 the aldol adduct 274. 

Phenols are stronger AO/AR than HAS. Contrary to the transformation of phenols into quinone methides, discoloration-free products are formed from HAS. Combinations HAS/phenol are, therefore, used to obtain protection at minimum discoloration. The efficient protection of PP or of a block copolymer of propylene with 1.5% ethylene irradiated with a dose of 5 Mrad was reached with 0.1% 28, R = H and 0.05% Irganox 1076. Moreover, it was found that hindered phenolic AO prevent radiolysis of the ester type HAS (e.g. 28) [226]. Combinations of HAS with phenolic AO may, therefore, be recommended for commercial stabilization of y-irradiated hydrocarbon polymers [227]. 

Synthetical skill was exploited to obtain systems containing HAS together with other stabilizing moieties. Various combinations with hindered phenols, e.g. 

Besides synergism, some antioxidants (or, more generally, stabilizers) reveal an nnexpected antagonism between each other. For example, carbon black is a weak antioxidant and an efficient UV stabilizer. When combined with some snlfnr-based antioxidants, carbon black works in synergism. However, in combination with many hindered amines and some phenolic antioxidants, carbon black rednces the degree of protection of polyethylene [10]. For example, at 140°C the degradation induction time of polyethylene in combination with the antioxidants is... 

Perhaps the most important practical applications of inhibitors arise from the use of combinations of phenols that give more effective inhibition than where each is used alone [171]. Mahoney [166] notes that the most striking synergistic effects are found with combinations of a hindered phenol, with one f-butyl in the ortho position, and an unhindered phenol. The foregoing kinetic analysis and schemes help in the understanding of this effect at elevated temperatures, conversion of R02 to... 

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