Sahar Al-Malaika

Photooxidation and biodegradation of commercial photodegradable polyethylenes

Abstract Representative samples of commercial photodegradable polyethylenes have been examined with respect to rate and extent of oxidation as measured by carbonyl (carboxylic acid and ester) formation, molar mass reduction and ability to support microbial growth when used as the only source of carbon. An ethylene-carbon monoxide (E/CO) copolymer was found to photodegrade most rapidly but to biodegrade most slowly. An antioxidant iron dithiocarbamate photodegradable polyethylene (Scott-Gilead) and a starch filled iron catalysed polyethylene were shown to produce more carboxylic acids during photooxidation than did the E/CO polymer, resulting in more rapid microbial growth. After removing the microorganisms, the surface of the oxidised polyethylene was found to be eroded with substantial reduction in sample thickness, while the molar mass of the polymer remained unchanged. It is shown that the microbial exo enzymes are able to recognise relatively high molar mass carboxylic acids and remove them from the surface of the polymer under conditions where water is not able to remove them by leaching. From this it is concluded that the oxidation products of oxidised polyethylene are unlikely to present a threat to the environment and that by conversion to biomass they contribute to the fertility of the soil. Abiotic iron catalysed photo- or thermooxidation is the rate-limiting step in the bioassimilation process. It is concluded that abiotic oxidation must precede the onset of biotic degradation, which is shown to occur readily at Mw as high as 40 000. Bioassimilation involves further oxidation catalysed by transition metal ions and probably by exo enzymes from the microorganisms.

The antioxidant role of α-tocopherol in polymers II. Melt stabilising effect in polypropylene

The antioxidant efficiency of the biological antioxidant α-tocopherol during melt extrusion of polypropylene (PP) was investigated. The melt stabilising effect of α-tocopherol was found to be much higher than that of the commercial synthetic hindered phenol antioxidant Irganox 1010 especially at very low concentrations. The excellent stabilising performance of α-tocopherol was maintained even under severe extrusion conditions. The colour stability of PP processed in the presence of α-tocopherol was shown to be as good as that of Irganox 1010 when the former was used at low concentrations. However, higher concentrations of α-tocopherol imparted greater discoloration to the polymer than Irganox 1010 examined at the same concentrations. The incorporation of a phosphite antioxidant or a polyhydric alcohol (non-antioxidant) was shown to reduce drastically polymer discolouration concomitant with higher levels of retention of the tocopherol in the polymer. This was attributed partly to the reduction of the overall concentration of transformation products, formed from tocopherol as a result of its antioxidant function, and partly to interactions of the co-additives with coloured transformation products and regeneration of tocopherol. The transformation products were shown to be more coloured than the parent antioxidant. A mechanism is proposed for the co-operative interactions between α-tocopherol and phosphite U-626 which result in the improvement in melt stability and reduction in colour of PP.

The antioxidant role of α-tocopherol in polymers. III. Nature of transformation products during polyolefins extrusion

Abstract The nature of oxidative transformation products of the biological antioxidant α-tocopherol formed during melt extrusion of polyethylene (PE) and polypropylene (PP) was investigated. The distribution of the different products in the polymers, when processed under different conditions and in the presence of co-additives such as a phosphite and a polyhydric alcohol, was also examined. It is shown that similar transformation products were formed in both PP and PE and these were found to be mainly based on quinonoid-type structures including different dimers and trimers as well as aldehydes. The reduction in discolouration of PP observed, when using combinations of the biological antioxidant with either a phosphite antioxidant or a polyhydric alcohol (non-antioxidant), was attributed to a reduction in the overall concentration of transformation products formed, as well as to a more favourable distribution of these products in the polymer leading to much lower concentrations of the most highly coloured products, e.g. aldehydes. Based on the nature of the products formed a mechanism is proposed for the antioxidant action of α-tocopherol in PP melt. ©

Antioxidant interaction between organic phosphites and hindered amine light stabilizers: effects during photoxidation of polypropylene—II

Abstract The behaviour of mixtures of hindered amine light stabilizers (HALS), their nitroxyl radical, and alkylamine analogues, with aromatic and aliphatic phosphites during photoxidation of polypropylene (PP) has been examined. The photostabilizing performance of the above mixtures of stabilizers was compared with that of the individual antioxidants and with the performance of a number of synthesized bifunctional stabilizers containing aromatic phosphite or phosphonite and either of the above HALS or related functions in the same molecule (referred to as ‘HALS-Phosphites’ or ‘HALS-Phosphonite’). Combinations of the hindered aryl phosphite Irgafos 168, with the hindered amine Tinuvin 770, the tertiary amine Tinuvin 292 and the bis-nitroxyl-analogue of Tinuvin 770, gave antagonistic effects at most molar ratios examined. Furthermore, in all cases, the extent of antagonism was reduced with increasing the amount of Irgafos 168 in the mixtures. Combination of the aliphatic phosphite, trilaurylphosphite, TLP, with Tinuvin 770 in PP, on the other hand, has shown synergistic effect under photoxidative conditions. The HALS-Phosphites and HALS-Phosphonite examined have shown better stabilizing efficiency than the corresponding mixtures of the individual stabilizers.

Reactive processing of polymers : mechanisms of grafting reactions of functional antioxidants on polyolefins in the presence of a coagent

The antioxidant 3,5-di-tert-butyl-4-hydroxy benzyl acrylate (DBBA) reacts with the trifunctional coagent trimethylol propane triacrylate (Tris) in the presence of a small concentration of free radical initiator in PP melt during processing to give very highly grafted DBBA. The nature of the grafting achieved by this special processing method (referred to as novel reactive processing, NRP) has been examined. The nature and extent of the competing reactions was investigated. Very high extent of antioxidant homopolymerisation, about 60% (the major competing reaction) was observed in the case of reactive processing carried out in the absence of Tris led to low grafting levels. The use of NRP method in the presence of Tris, however, has drastically reduced the level of competing reactions (mainly homopolymerisation, to less than 10%) giving rise to over 90% grafting efficiency of DBBA on the polymer. A mechanism of the grafting reaction is proposed based on spectroscopic (solid-state and solution NMR and FTIR) investigations of high temperature reactions of DBBA with PP and with decalin, used as a hydrocarbon model, in the presence and absence of Tris. The mechanism comprises three main steps involving finally the grafting of DBBA-Tris units on the polymer chains without causing adverse modification to the physical characteristics of the polymer, in terms of its molar mass and solubility.

The antioxidant role of vitamin E in polymers V. Separation of stereoisomers and characterisation of other oxidation products of dl-α-tocopherol formed in polyolefins during melt processing

Abstract The nature of specific transformation products based on isolated isomeric forms of oxidative coupling products, aldehydes and a quinone, formed when the antioxidant vitamin E (dl-α-tocopherol) is incorporated in polyethylene (PE) and polypropylene (PP) during melt processing, are investigated. These products were separated and isolated by high performance liquid chromatography (HPLC) and characterised by spectroscopy. It was shown that although a large number of HPLC product peaks are formed, these corresponded to different stereoisomeric forms of only a small number of oxidative coupling products, in addition to a quinone and aldehydes. The observed sixteen HPLC peaks were shown to correspond to three main oxidative coupling products of tocopherol namely, trimers, TRI, dihydroxydimers, DHD, and spirodimers, SPD, formed in both polymers under all processing conditions. The identity of nine of these HPLC product peaks was confirmed to be the different stereoisomeric forms of the trimeric (TRIs) and dimeric (DHDs and SPDs) products of tocopherol. This confirmation was based on a comparative study of their spectral and chromatographic characteristics (FTIR, UV, NMR, MS and HPLC retention times) and authentic isomeric forms of the corresponding products which were synthesised previously in our laboratory from solution oxidation reactions of dl-α-tocopherol. Furthermore, up to five tocopherol-derived aldehydes were found to form at low concentrations under ‘normal’ processing conditions, three of which were fully characterised from extracts of PP-vitamin E stabilised samples. The changes in the concentration of individual aldehydes with processing parameters was examined in both polymers and it is shown that the initially low aldehydes concentration increases dramatically with severe processing, including multi-pass extrusions and presence of oxygen, with the 5-formyl-γ-tocopherol (ALD-1) showing the highest relative concentration (with respect to the other aldehydes) under all conditions. Tocoquinone (TQ) which was formed in both polymers was also isolated and fully characterised.

Thermoplastic elastomers: Part III—Ageing and mechanical properties of natural rubber-reclaimed rubber/polypropylene systems and their role as solid phase dispersants in polypropylene/polyethylene blends

Abstract The effect of partial replacement of natural rubber (NR) by reclaimed rubber (RR) in natural rubber/polypropylene (NR/PP) thermoplastic elastomer blends was investigated. It is shown that replacement of up to half of the NR by RR is possible without altering the mechanical properties of the blend to any significant extent. Furthermore, the behaviour of RR-NR/PP blends was similar to that of NR/PP on remoulding; both systems retained their mechanical properties for up to four remoulding cycles at 180°C. The role of reclaimed rubber (RR) as part of a solid phase dispersant (SPD) system, based on NR/PP, in the incompatible binary blend of polypropylene/low density polyethylene (PP/LDPE) was also examined. The performance of the SPD system containing RR was compared with that of NR/PP (containing no RR), which was shown previously to be a good system for PP/LDPE blends. It is shown that a RR-NR/PP system based on a 1:1 ratio of RR to NR gives similar performance to the control which contains no RR when examined as SPD for PP/LDPE (1:1) blends. However, when compared to other SPD systems for PP/LDPE, e.g. EPDM or the control (NR/PP) which contains no RR, the former system has the advantage of exhibiting much less deterioration in mechanical properties when the binary blend (PP/LDPE) is exposed to UV light. This was attributed to the presence of antioxidants and stabilisers in the RR which contribute to stabilisation of the SPD and the thermoplastic binary blend.

Reactive processing of polymers: melt grafting of glycidyl methacrylate on ethylene propylene copolymer in the presence of a coagent

Glycidyl methacrylate (GMA) was grafted on ethylene-propylene copolymer during melt processing with peroxide initiation in the presence and absence of a more reactive comonomer (coagent), trimethylolpropane triacrylate (Tris). The characteristics of the grafting systems in terms of the grafting reaction yield and the nature and extent of the competing side reactions were examined. The homopolymers of GMA (Poly-GMA) and Tris (Poly-Tris) and the GMA-Tris copolymer (GMA-co-Tris) were synthesized and characterized. In the absence of the coagent, high levels of poly-GMA, which constituted the major competing reaction, was formed, giving rise to low GMA grafting levels. Further, this grafting system resulted in a high extent of gel formation and polymer crosslinking due to the high levels of peroxide needed to achieve optimum GMA grafting and a consequent large drop in the melt index (increased viscosity) of the polymer. In the presence of the coagent, however, the grafting system required much lower peroxide concentration, by almost an order of magnitude, to achieve the optimum grafting yield. The coagent-containing GMA-grafting system has also resulted in a drastic reduction in the extent of all competing reactions, and in particular, the GMA homopolymerization, leading to improved GMA grafting efficiency with no detectable gel or crosslinking. The mechanisms of the grafting reactions, in the presence and absence of Tris, are proposed.

The antioxidant role of vitamin E in polymers. IV. Reaction products of dl-α-tocopherol with lead dioxide and with polyolefins

Abstract The reaction products of lead dioxide oxidation of the synthetic biological antioxidant dl -α-tocopherol in hexane were investigated at different molar ratios. Products were separated by normal phase high-performance liquid chromatography (HPLC) and isolated using semi-preparative HPLC. Four isomeric trimers and three isomeric spirodimers were obtained as the major reaction products in the presence of 10 and 40 molar excess of the oxidant PbO 2 , respectively, whereas the aldehyde, 5-formyl-γ-tocopherol, was obtained as a minor product at 40 molar excess of PbO 2 . Dihydroxydimers were formed as minor products at the stoichiometric ratio of tocopherol to PbO 2 , and were also obtained as major products from a reaction of the spirodimer with LiAlH 4 ; three isomeric dihydroxydimers were obtained from both reactions. The structures of the different products were characterised by FTIR, UV–vis, 1 H-NMR, 13 C-NMR, EI-MS and FAB-MS, and based on these results possible structures of the isomeric forms of the products are suggested. Characterisation of products obtained from the oxidation of dl -α-tocopherol in solution reported here has enabled the identification of a large number of oxidation products, including isomeric forms, detected and isolated from high-temperature melt reactions of the antioxidant dl -α-tocopherol in polyethylene and polypropylene.

Migration of 4-substituted 2-hydroxy benzophenones in low density polyethylene: Part I—Diffusion characteristics

Intrinsic diffusion coefficients and equilibrium solubilities of a series of 4-substituted 2-hydroxy benzophenone (HBP) compounds were determined in low density polyethylene over a wide temperature range (5–100°C) using a polymer stack technique (diffusion coefficients of stabilisers have been determined in the literature but in general only over a limited temperature range). The experimental values are in good agreement with the literature values for analogous compounds when compared at the same temperature of measurements but these are very different if compared with extrapolated values from the literature. The dependence of diffusion coefficient on temperature followed an Arrhenious-type relationship only over a narrow temperature range and extrapolation from high temperatures to ambient is seen to be invalid. The effect of varying the chemical structure of the substituents on the 4-position of the HBP stabilisers on diffusion characteristics has also been investigated. It is shown that diffusion coefficients in a homologous series depends primarily on the chemical structure and shape of the diffusant rather than on molar mass.