Thermal stabilization of polyolefins

S. Al-Malaika and G. Scott "Thermal Stabilization of Polyolefins," iu N. S. AUen, ed.. Degradation and Stabilisation of Polyolefins, AppUed Science Pubhshers Ltd., London, 1983, pp. 247—281. 

Thermal stabilization of polyolefins has been first demonstrated for low-molecular models-normal structure alkanes [29]. It has been shown that metallic sodium and potassium hydroxide with absorbent birch carbon (ABC) as a carrier are efficient retardants of thermal destruction of n-heptane during a contact time of 12-15 s up to the temperature of 800°C [130]. Olefins and nitrous protoxide, previously reported as inhibitors of the hydrocarbon thermal destruction, are ineffective in this conditions. 

Al-Malaika, S. Scott, G. Thermal stabilizers of polyolefins. In Degradation and Stabilisation of Polyolefins, Allen, N.S., Ed. Applied Science London, 1983 247-281. 

It is generally accepted that thermal stability of polymer nanocomposites is higher than that of pristine polymers, and that this gain is explained by the presence of anisotropic clay layers hindering diffusion of volatile products through the nanocomposite material. It is important to note that the exfoliated nanocomposites, prepared and investigated in this work, had much lower gas permeability in comparison with that of pristine unfilled PE [12], Thus, the study of purely thermal degradation process of PE nanocomposite seemed to be of interest in terms of estimation of the nanoclay barrier effects on thermal stability of polyolefin/clay nanocomposites. 

Uses Antioxidant, heat stabilizer for syn. rug-backing, latex paints, rosin, ester gums, in gasoline and aviation fuels, insulating oils, paraffin wax Features Primary antioxidant provides better thermal stabilization of polyolefins and lower vapor pressure than common antioxidants Properties Gardner 12 clear liq. sol. in abs. ethanol, benzene, MEK,... 

Figure 9 Showing dependence of synergism in thermal stability of polyolefins on acid/base interactions between stabilizer and pigment additives.

At present, high-temperature stabilization of polyolefins is still misunderstood besides, this problem presents serious difficulties. Stabilization of thermal oxidation and photoinduced destruction with the use of stabilizers in this case is inefficient, since at high temperatures these stabilizers are easily evaporated out of the polyolefin melt and decomposed with the formation of radicals capable of initiating additional kinetic chains of destruction. 

Organic phosphites POR(OR )OR where R=Ci—C30 represents aliphatic, cycloaliphatic, or aromatic radical, are also able of inhibiting the thermal destruction of polyolefin [19]. Of light stabilizers, benzo-phenone derivatives have the ability for inhibiting thermal destruction of polyolefins, too. 

It should be noted that the aforementioned few compounds behave as stabilizers of thermal destruction of polyolefins only at temperatures from 200-250°C. 

Polyolefins are exposed to the effects of high temperatures initially during processing and fabrication and subsequently during in-service. Thermal stabilization of po-... 

The lower thermal stability of natural fibers, up to 230°C, the thermal stability is only small, which limits the number of thermoplastics to be considered as matrix materials for natural fiber composites. Only those thermoplastics whose processing temperature does not exceed 230°C are usable for natural fiber reinforced composites. These are, most of all, polyolefines, such as polyethylene and polypropylene. Technical thermoplastics, such as poyamides, polyesters, and polycarbonates, require... 

Thermal and Oxidative Stability. In general, polyolefins undergo thermal transitions at much lower temperatures than condensation polymers thus, the thermal and oxidative stability of polyolefin fibers are comparatively poor. Preferred stabilizers are highly substituted phenols such as Cyanox 1790 and lrganox 1010, or phosphites such as Ultranox 626 and Irgafos 168. 

An interesting case of halogenated polyolefin is that of halogenated poly(vinyl chloride). After-chlorination of PVC improves thermal stability of the polymer. The distribution of chlorine atoms in chlorinated PVC has been studi successfully using Py-GC [57]. 

A non-woven web having antimicrobial properties is made from a melt extmdable composition, which contains a polyolefin and an antimicrobial siloxane quaternary ammonium salt, which may be a trisiloxane having a pendent quatemaiy ammonium group and a molec.wt. of from about 600 to 1,700 or an ABA-type siloxane having a polydispersity of up to about 3.0 and a weight-average molec.wt. of from about 800 to 2,000 in which a central siloxane moiety is terminated at each end by a quatemaiy ammonium salt group. The anion can be any anion, which does not adversely affect the thermal stability of the salt. 

Fig. 19. Effect of branching on the thermal stability of various polyolefins [39).

To prepare multilayer membranes, another irradiation method to prepare cross-linked microporous multilayer membranes with enhanced thermal stability has been developed. It is realized by two steps. First, the polymer-blended layers, such as poly(ethylene glycol) diacrylate/poly(ethylene glycol) methyl ether acrylate are coated onto polyolefin microporous membranes. Second, the resultant membranes are irradiated to form chemically cross-linked membranes. They exhibited higher thermal and electrochemical properties compared to conventional separators. TOth the increase of irradiation dose, the thermal stability of the resultant membranes increases accordingly. By using the microporous multilayer membranes, the advantages of each component layers are well combined. 

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. 

Polyolefins In addition to stabilization of polyolefins against thermal oxidation to reduce the sensitivity to light, stabilization against exposure to light is required for articles to be used outdoors as well as those intended for indoor use [93]. Light stabilizers include UV absorbers of the benzotriazole and the benzophenone types (except for thin sections), HALS, and nickel-containing stabilizers. The latter are used for thin sections such as tapes and films and for surface protection. The type of stabilizer is dictated by the type of polyolefin, its thickness, application, and desired lifetime of the article [20]. 

Another example showing the influence of pair interactions in complex polymer systems concerns the effectiveness of thermal stabilizers for polyolefins, when these are pigmented with a variety of rutiles. In the example, we chose polypropylene (PP) and linear low-density polyethylene (LLDPE) as hosts. These are materials without acid/base interaction potential. In order to stabilize them against thermally-initiated changes during processing, a variety of substituted phenolic substances is usually added. In the current example we have chosen two... 

Albano C, Gonzalez J, Ichazo M et al (1999) Thermal stability of blends of polyolefins and sisal fiber. Polym Degrad Stab 66 179-190... 

Shebani et al. [20] noted that removing extractives improved the thermal stability of different wood species. Therefore, using extracted wood for the production of wood-plastic composite (WPCs) would improve the thermal stability of WPCs. Because wood and other bio-fibres easily undergo thermal degradation beyond 200°C, thermoplastic matrix used in the composites is mainly limited to low-melting-temperature commodity thermoplastics like polyethylene (PE) and polypropylene (PP). However, the inherently unfavourable thermomechanical and creep properties of the polyolefin matrix limit some structural applications of the materials. 

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