Polyolefins hindered phenolics

Because of the high melt viscosity of polyolefins, normal spinning melt temperatures are 240—310°C, which is 80—150°C above the crystalline melting point. Because of the high melt temperatures used for polyolefin fiber spinning, thermal stabilizers such as substituted hindered phenols are added. In the presence of pigments, the melt temperature must be carefully controlled to prevent color degradation and to obtain uniform color dispersion. 

A large number of hindered phenoHc antioxidants are based on the Michael addition of 2,6-di-/ f2 -butylphenol and methyl acrylate under basic catalysis to yield the hydrocinnamate which is a basic building block used in the production of octadecyl 3-(3,5-di-/ f2 butyl-4-hydroxyphenyl)propionate, [2082-79-3], tetrakis(methylene-3(3,5-di-/ f2 butyl-4-hydroxylphenyl)propionate)methane [6683-19-8], and many others (63,64). These hindered phenolic antioxidants are the most widely used primary stabilizers in the world and are used in polyolefins, synthetic and natural mbber, styrenics, vinyl polymers, and engineering resins. 2,6-Di-/ f2 -butylphenol is converted to a methylene isocyanate which is trimerized to a triazine derivative... 

When used alone at low temperatures, diaLkyl thiodipropionates are rather weak antioxidants. However, synergistic mixtures with hindered phenols are highly effective at elevated temperatures and are used extensively to stabilize polyolefins, ABS, impact polystyrene (IPS), and other plastics. 

Another approach to safer stabilization is to use a biological antioxidant such as vitamin E (a-tocopherol is the active form of vitamin E, AO-9, Table la). It is essentially a hindered phenol which acts as an effective chain breaking donor antioxidant, donating a hydrogen to ROO to yield a very stable tocopheroxyl radical, a-Tocopherol is a very effective melt stabilizer in polyolefins that offers high protection to the polymer at very low concentration [41], (Table 2). 

The evolution of HALS technology to meet the requirements of emerging polyolefin markets and applications is a story of continuous improvement and structure/property optimisation [19-21], Current trends in the industry are toward low-volatility, extraction-resistant, hindered phenolic AOs, such as oligomeric polysiloxane based, high-MW antioxidants... 

VA Roginskii. Oxidation of polyolefines inhibited by sterically hindered phenols, Doctoral Thesis Dissertation, Institute of Chemical Physics, Chernogolovka, 1983. 

Hindered phenols and dialkylhydroxylamines protect plastics against thermal oxidation. Alkylated benzo[b]furane-2(3H)-ones was introduced very recently as a processing stabilizer for polyolefins. 

Use of both primary and secondary antioxidants usually provides a synergistic effect, where the combined effect of two or more stabilisers is greater than the sum of the effects of the individual stabilisers. It is common practice to include both a phosphite, such as tris(t-butylphenyl)phosphite and a hindered phenol, such as octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyljpropionate to provide improved heat stabilisation in polyolefin formulations. 

High molecular weight hindered phenolic antioxidant with photo-and thermo-stability. Provides low volatility and resistance to extraction from polymer compounds. Suitable for PP, PE, polystyrene, ABS, PVC, nylon, and polyurethane. Also used as a radical scavenger providing stabilization for polyolefins. 

The main function of metal deactivators (MD) is to retard efficiently metal-catalyzed oxidation of polymers. Polymer contact with metals occur widely, for example, when certain fillers, reinforcements, and pigments are added to polymers, and, more importantly when polymers, such as polyolefins and PVC, are used as insulation materials for copper wires and power cables (copper is a pro-oxidant since it accelerates the decomposition of hydroperoxides to free radicals, which initiate polymer oxidation). The deactivators are normally poly functional chelating compounds with ligands containing atoms like N, O, S, and P (e.g., see Table 1, AOs 33 and 34) that can chelate with metals and decrease their catalytic activity. Depending on their chemical structures, many metal deactivators also function by other antioxidant mechanisms, e.g., AO 33 contains the hindered phenol moiety and would also function as CB-D antioxidants. 

Klemchuk, P.P. Horng, P.L. Transformation products of hindered phenolic antioxidants and color development in polyolefins. Polym. Deg. Stab. 1991, 34, 333-346. 

It was noted that the efficiency is increased in the presence of amines (Scott, 1993b), so the effectiveness in conjunction with aromatic amine free-radical (CB-D) stabilizers or the amine end groups of nylon would be expected. It is also noted that cuprous salts are often used as the iodide, and the effective alkyl-scavenging ability of iodo compounds has been demonstrated (Henman, 1979). Copper salts are also effective in the melt stabilization of polyesters, but in this case the effectiveness is improved by using a hindered phenol antioxidant. This performance is in contrast to the strong prodegradant effect of copper and other transition metals on the polyolefins as discussed later. 

Cyanuric chloride has three chlorine substituents and a stable aromatic triazine ring. Different substituents can be introduced onto the triazine because the chlorines have different reactivities. Cyanuric chloride is used in pesticides, herbicides, dyes, detergent brighteners, and so on [48], Cyanuric chloride also is used in the manufacture of hindered-phenol triazines as antioxidants for polyolefin resins. Cyanuric chloride production is more than 100,000 ton/yr [49]. American Cyanamid, Nilok, and Geigy are three U.S. producers of cyanuric chloride [50]. 

It is an important fact, that there is an explicit synergistic effect observed by combining primary and secondary antioxidants. A well known example of such synergistic mixtures is the use of thiodipropionates together with sterically hindered phenols for long-term stabilization of polyolefins. Of course, the different polymers and different applications require laborious optimization of each case. 

The solution oxidation technique allows the study of polyolefin autoxi-dation under conditions where the temperature, concentration of reactants, and rates of radical initiation can be controlled. The results should be considered as a useful prelude to any fundamental understanding of the autoxidation processes which occur in neat polymers where the effects of very high viscosity, partial crystallinity, and oxygen diffusion rates are included. The objective of our work was to determine the kinetics and stoichiometry of the inhibited autoxidation of polypropylene in solution. A relatively detailed study of the oxidation of polypropylene inhibited by 2,6-di-terf-butyl-4-methylphenol [butylated hydroxytoluene (BHT)] has been made for comparison with data obtained in polypropylene oxidations inhibited by a variety of other stabilizers which include commercial polyfunctional antioxidants. Singly hindered phenols appeared to be superior in the inhibited-solution oxidation of polypropylene, and the application of this finding to stabilization technology was investigated briefly. 

Commingled polyolefins are re-stabilized using about 0.06 wt% of a hindered phenol antioxidant, and about 0.09 wt% phosphite. Acids present in the mixture should be neutralized by addition of a sufficient amount of CalOH). For other compositions of PCW, other stabilizers, viz., thio-propionic acid, benzophenones, oxalides, benzo-triazoles, or sterically hindered amines may have to be used [Pauquet et al., 1993]. 

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. 

Low-density polyethylene (LDPE) is extensively used for the manufacture of films. During processing, which is carried out at temperatures of approximately 200°C, cross-Unking, and thus formation of gel, can occur through oxidation if the polymer is not stabilized. Such gel particles are visible in the film as agglomerates, known as fish eyes or arrow heads. The processing stabilizers used in LDPE consist of systems commonly used for polypropylene, namely, combinations of a phosphite or phosphonite and a long-term heat stabilizer (hindered phenol) in overall concentrations up to 0.1%. Concentrations seldom exceed 0.1%, since the compatibility of any additive in LDPE is considerably lower than in any other polyolefins. 

The presence of the above-mentioned metal ions increases the decomposition rate of hydroperoxides and the overall oxidation rate in the autoxidation of a hydrocarbon to such an extent that even in the presence of antioxidants, the induction period of oxygen uptake is drastically shortened. In such a case, sterically hindered phenols or aromatic amines even at rather high concentrations, do not retard the oxidation rate satisfactorily. A much more efficient inhibition is then achieved hy using metal deactivators, together with antioxidants. Metal deactivators are also known as copper inhihitors, because, in practice, the copper-catalyzed oxidation of polyolefins is by far of greatest importance. This is due to the fact that polyolefins are the preferred insulation material for communication wire and power cables, which generally contain copper conductors. 

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. 

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