Polyolefin testing antioxidants

Figure 2 - Processing Stabilizers and Antioxidants used in Polyolefin Testing...

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]. 

Poly[styrene-co-(2-hydroxy-4 -vinylbenzophenone)] was less efficient in PS than 2-hydroxy-4-methoxybenzophenone [334]. Similarly, PE films doped with 4-dodecyloxy-2-hydroxybenzophenone (0.1 mol%) were more stable than PE doped with copolymers of ethylene with polymerisable benzophenones having a comparable content of chromophores [54]. The efficiency of a SAN type LS, a terpolymer of 2-hydroxy-4-(4-vinylbenzyloxy)benzophenone with acrylonitrile and styrene did not exceed that of conventional LS [84]. No efficiency loss of 2-hydroxy-4-methacryloyloxybenzophenone in ABS was observed after bonding into a terpolymer with styrene and acrylonitrile. The homopolymer was slightly inferior to both the monomer and terpolymer [84]. A better protection of PP was provided by poly[(2-hydroxy-3-allyl-4-methoxyphenylbenzophenone)-co-dibutyl maleate] than with 2-hydroxy-3-allyl-4-methoxybenzophenone [335] (stabilization tests were performed in the presence of phenolic antioxidants). A comparable or better light stabilizing efficiency of poly[vinyl acetate-co-(5-methylacryloyloxy salicylate)] or poly(2-allylphenyl salicylate-co-dioctyl maleate) than that of alkyl-phenyl salicylates was observed in polyolefins [335]. 

Determination of the residual antioxidant content in polymers by HPLC and MAE is one way to determine the amoimt needed for reasonable stabilization of a material, and also to compare different antioxidants and their individual efficiencies. During ageing and oxidation of PE, carboxyhc acids, dicarboxylic acids, alcohols, ketones, aldehydes, n-alkanes and 1-alkenes are formed [86-89]. The carboxyhc acids are formed as a result of various reactions of alkoxy or peroxy radicals [90]. The oxidation of polyolefins is generally monitored by various analytical techniques. GC-MS analysis in combination with a selective extraction method is used to determine degradation products in plastics. ETIR enables the increase in carbonyls on a polymer chain, from carboxylic acids, dicarboxyhc acids, aldehydes, and ketones, to be monitored. It is regarded as one of the most definite spectroscopic methods for the quantification and identification of oxidation in materials, and it is used to quantify the oxidation of polymers [91-95]. Mechanical testing is a way to determine properties such as strength, stiffness and strain at break of polymeric materials. 

The OIT test measures the intrinsic thermal stability of a material as well as the amount of stabilisers in the material. DSC is a convenient method for measuring concentrations of hindered phenolic antioxidants in polyolefins. The AO concentration-OIT relationship is linear for the most part of nonvolatile AOs [131-134], cfr Fig. 2.3. DSC-OIT has also been used in determination of the oxidative stability of HDPE film (isothermal at 200°C in O2, ac-... 

DSC is specified in USP for the physical testing of PE containers the quality of packaging material is of decisive importance for the protection of raw materials and end products, such as primary packaging material. As shown in Fig. 2.4, unprotected PE samples decompose almost immediately at the test temperature. However, a PE sample containing 0.04% stabiliser remains protected for approximately 16 min at the test temperature, whereas the PE sample containing 0.055% stabiliser is protected for 25 min [151]. The DSC test thus provides a rapid method of screening for the proper AO levels in a polymer. Bharel et al [152] have reported DSC-OIT for performance evaluation of two diamide antioxidants in HDPE and Hakani et al. [153] for the evaluation of oxidative stability of flexible polyolefins (FPO) with the biological y-oryzanol and a-tocopherol antioxidants for food and medical applications. 

The main reason for using induction time data for the determination of antioxidant concentration in polymers is the frequently observed linear relationship between induction time and antioxidant concentration [131]. In view of the aforementioned considerations great caution should be exercised in quantitative estimation of antioxidant levels in polymers. Wight [181] and others [143] have used quantitative differential thermal analysis (QDTA), in particular for determining the degree of oxidative stability of polyolefins for QC purposes in the wire and cable industry in lieu of a direct antioxidant analysis. Application of the basic purpose of a QC test... 

BS 2782 (1975) Methods of testing plastics chemical properties determination of antioxidants in polyolefin compounds by a spectropho-tometric method. 

In the field of polymer/additive analysis a rather limited number of other laboratory performance studies is available. Recently, the Swiss Eederal Laboratories for Materials Testing and Research (EMPA, St. Gallen) has organised a series of interlaboratory tests on polymeric materials, examining the glass transition point by DSC (amorphous thermoplastics), antioxidant content in polyolefins. 

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