Polyolefin Stabilisation

Besides polyolefins, the outdoor weathering performance of unstabilised polyformaldehyde, polystyrene, PVC and polyamides is also inadequate, and that of unstabilised polycarbonate, ABS and SAN is not good. 

Cytec scientists have highlighted the performance of their Cyasorb UV-4611 and 6435 stabilisers in PP, HDPE and LLDPE, claiming to improve the UV stability, colom and gloss of thick sections made of polyethylene and polypropylene. 

Crompton offers a benzophenone-based product, MarkScreen 1413, incorporating 2-hydroxy 4- -octyloxybenzophenone for use in polyolefins and PVC when the best protection against light is required. Colormatrix says that its latest stabilisers can protect PET polymers against UV light up to 390 nm instead of the more usual 370 mn. 

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Prevent degradation of polymers by chemical reactions. Stabilisers are mono-, bi- or polyfunctional. Polyolefin stabilisers represent a major proportion of the stabiliser market. 

Neri C, Costanzi S, Riva RM, Farris R, Colombo R. Mechanism of action of phosphites in polyolefin stabilisation. Polym Degrad Stab 1995 49(l) 65-9. 

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. 

Polyolefins such as polyethylene and polypropylene contain only C—C and C—H bonds and may be considered as high molecular weight paraffins. Like the simpler paraffins they are somewhat inert and their major chemical reaction is substitution, e.g. halogenation. In addition the branched polyethylenes and the higher polyolefins contain tertiary carbon atoms which are reactive sites for oxidation. Because of this it is necessary to add antioxidants to stabilise the polymers against oxidation Some polyolefins may be cross-linked by peroxides. 

Phosphites Tris-(p-nonylphenyl) phosphite (X) No Widely used in conjunction with conventional stabilisers (q.v.) in PVC. Some types appear to be useful heat and light stabilisers in polyolefins. Function primarily as peroxide decomposers rather than chain-breaking antioxidants. 

Report 132 Stabilisers for Polyolefins, C. Krohnke and F. Werner, Clariant Huningue SA. 

High-MW hindered amine thermal stabiliser (HATS) formulations are designed for advanced extraction resistant long-term stabilisation, i.e. for use in extractive environments such as polyolefin pipes, fulfilling the stringent requirement of guaranteeing product lifetime of more than 50 years [565], These systems offer much better gas and hot water resistance than the low-MW phenolic antioxidant systems. Ethanox 330... 

Albemarle) has shown unique resistance to extraction from polyolefins. No radioactivity was detected in water run through a MDPE pipe stabilised with 14C-labelled Ethanox 330 for 10 months at ambient conditions, or in an accelerated 3-month test at 80 °C (limit of detection 25ppb). 

Oligomeric hindered amine light stabilisers, such as Tinuvin 622 and Chimassorb 944, resist satisfactory analysis by conventional HPLC and have required direct UV spectroscopic analysis of a polyolefin extract [596], PyGC of an extract [618,648], or SEC of an extract [649]. Freitag et al. [616] determined Tinuvin 622 in LDPE, HDPE and PP by saponification of the polymer dissolution in hot toluene via addition of an... 

GC is extensively used to determine phenolic and amine antioxidants, UV light absorbers, stabilisers and organic peroxide residues, in particular in polyolefins, polystyrene and rubbers (cf. Table 61 of Crompton [158]). Ostromow [159] has described the quantitative determination of stabilisers and AOs in acetone or methanol extracts of rubbers and elastomers by means of GC. The method is restricted to analytes which volatilise between 160 °C and 300 °C without decomposition. A selection of 47 reports on GC analysis of AOs in elastomers (period 1959-1982) has been published... 

Oligomeric additives with broad MWD tend to be a problem in conventional HPLC conditions. In cases where no interest exists in the oligomer distribution it is common practice to solve the problem by creating a uniform structural unit useful for analysis. For example, isocratic (or gradient) LC-UV was used for the determination of the polymeric light stabiliser Tinuvin 622 in polyolefins using dissolution (toluene)/derivatisation (TBAH)-precipitation (alcohol) the diol formed was quantitatively determined by NPLC [653]. 

Applications Quantitative dry ashing (typically at 800 °C to 1200°C for at least 8h), followed by acid dissolution and subsequent measurement of metals in an aqueous solution, is often a difficult task, as such treatment frequently results in loss of analyte (e.g. in the cases of Cd, Zn and P because of their volatility). Nagourney and Madan [20] have compared the ashing/acid dissolution and direct organic solubilisation procedures for stabiliser analysis for the determination of phosphorous in tri-(2,4-di-t-butylphenyl)phosphite. Dry ashing is of limited value for polymer analysis. Crompton [21] has reported the analysis of Li, Na, V and Cu in polyolefins. Similarly, for the determination of A1 and V catalyst residues in polyalkenes and polyalkene copolymers, the sample was ignited and the ash dissolved in acids V5+ was determined photo-absorptiometrically and Al3+ by complexometric titration [22]. 

Intermaterial competition affects the additive business. For example, if metallocene polyolefins displace PVC markets, then the demand for heat stabilisers and plasticisers might decrease, while the need for products such as fluoropolymer processing aids, antiblocks and slip additives might increase. At present, PVC as the main user of additives is under threat as never before. However, green PVC, containing more environmentally friendly FRs and plasticisers as well as non-heavy-metal stabilisers, is now environmentally acceptable. 

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