LDPE thermal oxidation

J.M. Pena, N.S. Allen, M. Edge, C.M. Liauw, B. Valange. Interactions between carbon black and stabilisers in LDPE thermal oxidation. Polymer Degradation and Stability 72(1), 163-174 (2001). 

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). 

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. 

Decrease In Thermal Oxidative Stability During Outdoor Weathering of LDPE... 

Black low density polyethylenes have been found to behave differently during thermal oxidation at low temperatures (8.0 to 40°C). The black samples oxidize and crosslink but they do not fall mechanically, as does non-black LDPE. Hawkins(17) has shown the greater thermal stability of black polyethylene as compared to non-black polyethylene In the solid state. Other Investigators (18,19) have demonstrated the activity of carbon black as a free radical trap and others (20) have suggested that polymer radicals form carbon-polymer bonds with the carbon black. The exact reason diy the black samples do not fall mechanically even though they oxidize and crosslink Is not clear and any one, or a combination of all the above factors could be the cause. It appears that the lack of mechanical failure after oxidation Indicates very little or no scission during the oxidative process and finally no further oxidation after the development of the 70% gel found In oxidized samples. 

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

In the case of thirteen LDPEs used in order to study the memory effects, the samples with various shearing histories were prepared hy changing shear working time. The shear working was performed with a Brahender plasticorder equipped with No. 5 rotor at 50 rpm rotor speed. Prior to the shearing with the Brahender, 2000 ppm of if,ii -thiobis (3 methyl-6-tert-butylphenol) was added to the materials in order to prevent them from thermal oxidation. 

Fig. 2 Dependency of specific CL intensity on carbonyl concentration for thermally oxidized LDPE in air at 185 °C. The data were taken from [96S1].

The cost comparison, based on variable operational costs for an LDPE homopolymer train, shows that devolatilisation extrusion and purge air treatment through regenerative thermal oxidation lead to similar operational costs. 

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. 

Figures 3.14 and 3.16 show that the maximum rate of initial carbonyl formation in polyethylene (LDPE) observed in Figure 3.15 is associated with a higher initial hydroperoxide concentration (Fig. 3.14) and a higher rate of hydroperoxide formation during subsequent thermal oxidation (Fig. 3.16). Figure 3.14 and 3.16 also show that the hydroperoxide concentration rises to a maximum and then decays with heating time both in the melt and the solid phase, and that the maximum concentration achieved increases with decreasing temperature. In the absence of oxygen, hydroperoxide concentration decayed to zero in less than 20 h at 110°C [414]. The half-life of polyethylene hydroperoxide is 6.4 h at 100 °C [989].

Chiellini, E., Corti, A., Swift, G., Biodegradation of Thermally-Oxidized Fragmented LDPE Samples, Polym. Degr. Stab, in press. 

The effect of blending LDPE with EVA or a styrene-isoprene block copolymer was investigated (178). The properties (thermal expansion coefficient. Young s modulus, thermal conductivity) of the foamed blends usually lie between the limits of the foamed constituents, although the relationship between property and blend content is not always linear. The reasons must he in the microstructure most polymer pairs are immiscible, but some such as PS/polyphenylene oxide (PPO) are miscible. Eor the immiscible blends, the majority phase tends to be continuous, but the form of the minor phase can vary. Blends of EVA and metallocene catalysed ethylene-octene copolymer have different morphologies depending on the EVA content (5). With 25% EVA, the EVA phase appears as fine spherical inclusions in the LDPE matrix. The results of these experiments on polymer films will apply to foams made from the same polymers. 

The inflexion of the carbonyl formation curve for LDPE oxidised during processing illustrated in Figure 2 always occurs at about the same time of photo-oxidation (see Figure 6). In heavily thermally oxidised polyethylene the carbonyl index actually decreases initially before increasing again. Ketone carbonyl is the main product formed in the... 

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]. 

The radiative-oxidative stability of low density PE, LDPE, with PP blends were found to be more stable than pure PP [Gorelik et al., 1992]. This was explained as due to a decrease in crystallite size and possible interfacial crosslinking. The thermal stability after irradiation was considerably impaired in comparison with untreated material [Minkova et al., 1992]. This was due to the presence of free radicals within the PP component (which itself is significantly more susceptible to thermal degradation after irradiation than LDPE). 

Hui, S., Chattopadhyay, S., Chaki, T. K., Thermal and thermo-oxidative degradation study of a model LDPE/EVA based TPE system Effect of nano silica and electron beam irradiation. Polymer Composites 2010,31(8), 1387-1397. 

Due to their better thermal stability, brominated aromatics have proved to be of use for polyolefins, preferably brominated diphenyl oxides, such as decabromo-diphenyl oxide, with the drawback that there is some tendency to migration, especially in LDPE. 

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