LDPE, additives Antioxidants

Polyethylene and polypropylene blended with iron carboxylate complexes, for example, acetylacetonate (FeAcAc) and stearates (FeSt), and irradiated by UV light under accelerated aging conditions were shown to act as effective phtoactivators giving rise to rapid photoxidation as shown from the rapid rate of carbonyl formation without any induction period (see Fig. 16.4a for FeAcAc in HDPE) and with a reduction in molar mass (see Fig. 16.2a for FeSt in LDPE). However, these complexes have been shown to cause considerable oxidation to both PE and PP during processing reflected in a sharp increase in the polymer s melt flow index (reflecting chain scission and drop in molar mass) (Fig 16.4b) and act, therefore, as thermal prooxidants and cannot be used without the use of additional antioxidants in the system [2,3,17-19,48,49]. 

Analysts in industry prefer in many cases to maintain consistent methods for their analyses. Recommended ASTM analytical procedures are quite well developed in the rubber and polymer industry. As an example, we mention the standard test method for determination of phenolic antioxidants and erucamide slip additives in LDPE using liquid chromatography [76]. However, the current industry standard test methods (ASTM, AOAC, IUPAC, etc.) use a large number of solvents in vast... 

Hinman et al. [492] have compared SFE and ASE in the extraction of antioxidants from LDPE. Comparable extraction yields were obtained with both techniques. However, sample clean-up was necessary after ASE , while with SFE the extract could be analysed directly without any post-extraction clean-up. Supercritical fluid extraction of 15 polymer additives (AOs, UVAs, process lubricants, flame retardants and antistatic agents) from eight PS formulations was compared to dissolu-tion/precipitation extractions [557], Additive recoveries were comparable. Numerous additional comparisons can be found under the specific headings of the extraction techniques (Sections 3.3 and 3.4). 

ASTM STP D 1996, Test Method for Determination of Phenolic Antioxidants and Erucamide Slip Additives in LDPE Using Liquid Chromatography, Annual Book of ASTM Standards (D.L. Dolmage, ed.), American Society for Testing and Materials, Philadelphia, PA (1993), Vol. 08.01. 

Tomboulian et al. (2002) has reported that butylated hydroxytoluene (BHT) can impart a "burnt plastic" odor and is an additive in HDPE pipes. Quinone may be derived from BHT due to interactions with residual chlorine in pipes (Anselme et al., 1985). Yam et al. (1996) reported that antioxidants, such as vitamin E, Irganox 1010, and BHT, contributed to off-flavors in water. Vitamin E yielded less off-flavor, possibly due to lower aldehyde and ketone concentrations. Extrusion temperatures over 280 °C and exposure time for melt contributed to more oxidation of LDPE films and higher intensities of off-flavors in water in contact with LDPE with different antioxidants (Andersson et al., 2005). 

Crosslinked polyethylene. For additional investigations, LDPE samples were crosslinked by e-beam radiation with a dose of 250 kGy. In order to remove residual soluble components from the cross-linked LDPE samples, a suitable extraction process was developed [16], After being clamped in a frame, the polyethylene foils were extracted in m-xylene for 24 hours at a temperature of 80 °C. The addition of a small amount of BHT (2,6-di-tert-butyl-4-methylphenol), an antioxidant, prevented unwanted oxidation of the LDPE foils. Afterwards the samples were dried carefully at room temperature for 20 hours, and then dried in vacuo (20 mbar, 48 hours at 40 °C) to constant weight. 

Polyethylenes are susceptible to oxidative degradation at processing temperatures, therefore antioxidants are added. Silica is sometimes added to LDPE as an antiblock to prevent films sticking and N,N-bis(2-hydroxyethyl)alkyl(C8-C18) amine (BEA) as an additive to reduce a build up of static charge (antistat). Table 10.2 gives commonly used substances in polyethylenes. 

Selection of the compound ingredients fractions of LDPEs and HDPEs and specific additives such as stabilizers, antioxidants, flame-retardants, and so forth. 

Titanium dioxide is a known catalyst of thermooxidation and photooxidation of polymers. At 0.5% amount of it in LDPE, durability of the latter drops to about 50% of the initial. There are some data that titanium dioxide destabilizes polypropylene against oxidation by 30 times [3]. These effects can be stopped by addition of antioxidants. 

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 amounts of stabilizer are necessary for improving the stability of polymers [06C1]. The most used polymer, polyethylene, needs antioxidants in its product formulation, even though it is saturate polymer and the oxidation in any conditions starts more slowly than in many other polymer materials. A comparative study on the effect of two additives (hydrolysis-conditioned phosphate and IRGANOX 1076) on the thermal stability of medium density polyethylene (MDPE) and low density polyethylene (LDPE) (Fig. 71) illustrates the performances of material modified with carbon black (CB) and dicumyl peroxide (DCP), respectively [02F1]. This figure demonstrates that the same antioxidants promote differently thermal protection in the direct relation with material formulation. 

The production of polymeric materials with high resistance against damaging runs by the addition of antioxidants and the crosslinking. LDPE subjected to photocrosslinking generates the chemiluminescence curves presented in Fig. 72 [93Z1]. 

Figure 3.12. Mass loss of degradation products from LDPE as the polymer is held under isothermal conditions at elevated temperatures in nitrogen and in oxygen there are no antioxidant additives in these LDPE samples [from Bair (1997) reproduced with permission from Elsevier Ltd.]...

ESR studies of free radicals formed under UV-irradiation were reported for hindered piperidine photostabilisers and antioxidants [820]. Kelen et al. [819] reported an ESR study of hindered piperidine derivatives in a chalk filled PP matrix in the presence of other additives (frganox 1010, Tmuvin 770/622), with particular emphasis on concentration changes of Ai-oxyl radicals and interaction between a HALS compound and a hindered phenol. Other additives present in the polymer influence the concentration of the /V-oxyl radicals. Lattimer et al. [821] studied oxidation of the partially hindered bicyclic amine 3,3-dialkyldecahydroquinoxalin-2-ones (excellent UV stabiliser and thermal antioxidant) with w-chloroperbenzoic acid by means of ESR and reported some extremely stable radical derivatives (over 231 days of stability). ESR was also used to measure the piperidinoxyl radical concentration, and hence the HALS content in LDPE/(Chimassorb 944, Tmuvin 622) agricultural film during use. Evidence was reported for polymer-bound radicals [117]. 

The application of derivative spectroscopy to the determination of polymer additives has also been reported, cfr. also Table 6.37. A typical case is that of the phenolic antioxidants 2,6-di-/ert-butyl-4-methylphenol (AO-4K) and 4-substituted 2,6-xylenol (Chemantox AO-49), which exhibit virtually identical UV spectra [130]. However, the antioxidants can be distinguished in alkaline medium due to a bathochromic phenol-phenolate shift. The use of derivative spectroscopy reduces light scattering and matrix interferences when extracts from PP samples are measured. The use of derivative spectroscopy eliminates those interference phenomena which cause inaccuracies when evaluating direct absorption spectra. Shlyapnikov et al. [147] have used derivative ( = 2) UV spectrophotometry to determine antioxidants (in 0.2-2.0% concentrations) extracted fi om 0.02-0.1 g PE samples by distillation in vacuo at different temperatures with an accuracy of 1-2%. Pump et al. [146] used UV derivative spectroscopy for the quantitative determination of phenolic AOs in LDPE and Talsky et al. [148] determined the polymer/bound azo-content in PC (Fig. 6.4) by means of derivative UV/VIS. 

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