Polyolefins heat stability testing

The specific requirements for organic pigments in terms of heat stability result from the temperature levels at which individual polyolefins are processed. Standards have been developed to test the heat stability of pigments in polyolefins [33] (see also Sec. 1.6.7). 

Various polyesters derived from phosphorous or phosphoric acids were prepared. Efiicient polyphosphites were synthesised in the early 196(. Polyphosphite prepared from 152 and 4,4 -isopropylidenebis(cyclohexanol) was tested as a thermal stabilizer for PC [199] or as secondary AO for radiation sterilized EPM [200]. Built-in phosphites obtained by transesterification of triallcylphosphite with 4,4 -isopro-pylidenebisphenol or 4,4 -thiobisphenol possess antioxidant properties in polyolefins. Stabilizer containing phosphite moiety 153 was prepared from tris(2-hydroxy-ethyl)isocyanate, decyl alcohol and triphenylphosphite [201]. Various phosphites were derived firom polynuclear phenols or dihydric phenols. For example, a polycondensate prepared by reaction of phosphorus trichloride with 2,5-di-rert-butylhydroquinone was tested as heat and light stabilizer for PP [202], A linear polyester with a built-in phenolic moiety was synthesised from (2,6-di-tm-butyl-4-methylphenyl)bis(6-hydroxyhexyl)phosphite and dimethyl terephthalate [203]. 

The properties of commodity and engineering thermoplastics can be boosted by compounding with mineral fibres and a special compatibilizer, report two Dutch companies, Bennet BV and Lapinus Fibres BV. Polyolefins (HDPE and PP copolymer) and polyamide 6 have been successfully upgraded with 35 and 2 5% respective addition of Rockfil fibres and Bennet compatibilizer. Tests in conjunction with injection moulder Heudijk Kunststoffen BV and masterbatch producer Curver Colour Department indicated that mechanical, stability and processability properties are improved, with improved heat distortion and FR performance, good surface finish and recyclability. 

The onset of the oxidative exotherm due to thermal degradation can be used to obtain a measure of the stability of a polymer. An oxidative atmosphere is used and the sample is either heated at a constant rate to a given temperature, or heated isothermally at a specified temperature. In the former case the temperature at the onset point is used as the endpoint, and in the latter the time is used. A common test to determine the stability of plastics is the OIT (oxidation induction time) test. The method to be used for plastics in general is described in ISO 11357-6 (2002), and methods for polyolefins are described in the standards EN 728 (1997) and ASTM D3895 (2003). Schmid and Affolter [82] have reported on the results of an interlaboratory study investigating the use of DSC for the determination of OIT. 

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