Oxidation of LDPE

Catalyst used for oxidation of LDPE Carboxyl present [20] (%) Hydroperoxide present [16] (%) Carbonyl present [17] (%) Epoxy present [16] (%)... 

Hydroperoxide and hydroxide formation by slight thermal oxidation of LDPE can be confirmed by IR spectroscopy, showing bands at 3555 cm for OOH and about 3400 cm for OH. The band at 3555 cm is very weak or absent for the oxidation products of LLDPE . C FT-NMR is also useful to investigate this process (see Section V.C.4). 

In the early 1970s, surface modification of most polymers was achieved using redox initiators. Ce+4-induced initiation was employed to achieve surface grafting of acrylamide onto LDPE film [117]. The film was first oxidised by chromic acid and then reduced with diborane to form a hydroxyl-rich surface which was then used to initiate graft polymerisation of acrylamide using Ce+4/HN03. The mechanism of chromic-acid-facilitated surface oxidation of LDPE surface is shown in Scheme 6a and that of free-radical generation is represented in Scheme 6b. 

Scheme 6 a The mechanism of chromic acid facilitated surface oxidation of LDPE. b Free-radical generation on surface-oxidised LDPE... 

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). 

EfFect of trcinsition metal acetylacetonates on the oxidation of LDPE during processing and on UV irradiation... 

It seems unlikely then that carbonyl (ketone and aldehyde) formed by thermal breakdown of hydroperoxides are important sensitisers for photo-oxidation of LDPE in normally processed polymers. The evidence is consistent with the theory that allylic hydrx>perx>xlde derived from vlnylidene is the importeuit photo-initiator initially present under these conditions. Vlnylidene disappears as a concomitant of hydroperx>xide photolysis, initiating photo-oxidation in a manner analogous to its function in thermal oxidation. 

The reaction sequence (Scheme 1.60) shows the competition between chain scission and crosshnking reactions as well as the participation of unsaturation in the thermo-oxidation of LDPE in the melt (Al-Malaika, 1989). The process of formation of H and Y crosslinks involving terminal unsaturation (Scheme 1.45) and the formation of terminal unsaturation during p-scission (Scheme 1.49) in the absence of oxygen have been discussed previously. 

Table I. Induction Periods to Onset of Carbonyl Formation in the Thermal Oxidation of LDPE Films (110°C)°...

The progress in the oxidation of LDPE modified with maleic anhydride and alumina proceeds somewhat similar at various doses, because oxygen diffusion is hindered by filler nanoparticles [102]. The noticeable difference between pristine and modified LDPE consists of the presence of maleic anhydride, which interacts with molecular chains due to the electronegativity of oxygen atoms. The same radiation dose affects differently the dielectric behavior of the nanocomposites depending on the filler content. The dose of 50 kGy applied on LDPE-g-AM filled with 5 wt% nano-Al203 leads to a relative permittivity smaller than unfilled LDPE. y-Radiation can lead to a decrease in the dielectric losses of LDPE AI2O3 nanocomposites for properly chosen combination dose-fiUer content. 

The blend preparation was produced by mechanical mixing of PP and LDPE in a laboratory mixer at a temperature of lOO C for 5 minutes in the nitrogen atmosphere to prevent oxidation of LDPE. Isotropic films were fabricated by pressing with cellophane substrate at a temperature of 190°C and pressure of 7.8 MPa, followed by quenching in water (25°C). The thickness of the PP/LDPE films was 130 10 pm. 

Dobrescu V, Andre C, Andrei G. The anfioxi-(Uzing effect of steiically HALS in thermal oxidation of LDPE. Fur Polym J 1988 24 289-94. 

Table 1 Contact Angles (0) and Surface Energies of LDPEs Oxidized Using Purple Hydrocarbon as Oxidant...

LDPE/EVA blend was irradiated using gamma-irradiation and then expanded by heat as a foamed material. The EVA content in the blend was optimised to form a gel. The effects of atmospheres and of irradiation dose rate were studied. The ETIR spectra of the foam revealed the oxidation level. The relations between gel fraction of LDPE/EVA blend, expansion ratio, apparent density, average cell diameter and tensile properties of the foam are discussed. 8 refs. 

LDPE, PP, and TPX are more susceptible to oxidation in comparison with HDPE because of the presence of tertiary carbon atoms in the former. The degradation of LDPE, PP, and TPX is retarded through the use of antioxidants. 

Surface modification of LDPE film can also be brought about by chemical treatment [118] with an aqueous solution of ammoniacal ammonium persulphate in the presence of Ni+2 ions under variable reaction conditions. The investigation of treated surface showed the presence of polar groups (viz. carbonyl and hydroxyl) in the infrared (IR) spectroscopy, with characteristic bands at 1700, 1622 and 3450 cm-1. It is known that the persulphate ion attacks the double-bond-producing epoxy or diol group. However, the destructive oxidation of saturated hydrocarbons does not occur with persulphate alone, but requires the presence of the nickel (II) ion. The authors have proposed the following mechanism of chemical treatment ... 

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