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Hydrocarbon-based synthetic polymers are versatile and cost-effective materials in packaging and agriculture. Unstabilised polyolefins are known to biodegrade through peroxidation initiated by both abiotic and biotic and mechanisms. Both abiotic and biotic peroxidation can be accelerated and controlled by specially designed antioxidants and this has resulted in their commercial use in packaging, mulching films and baler twines. They are currently being evaluated in controlled release fertilisers.

Two known methods of covalently attaching iron-containing photoprooxidants to polymer chains are examined and have been shown to provide experimental procedures for producing non-extractable photodegradable polymers in contact with extractive food media. One of the adducts, Fe (III) 3–thiopropionate, is a light stabiliser at higher concentrations and provides a potential method for controlling the photodegradation of packaging plastics.

O-Macroalkyl hydroxylamines (>NOP) have been synthesised by reaction of 2,2,6,6-tetramethyl-4-hydroxy piperidinoxyl (>NO•) with ethylene-propylene rubber (EPR) and polypropylene (PP) by reactive processing in the presence of a dialkyl peroxides. Under optimal conditions, 70% of >NOP was formed based on >NO• added to the polymer, the remainder being free hydroxylamine (>NOH). >NOEPR and >NOPP are effective photoantioxidants for EPR and PP respectively. After exhaustive extraction of the films to remove >NOH, a proportion of the activity remained pro-rata to the [>NO ] covalently bound to the polymer. It is concluded that >NOPs are converted to >NO• from the commencement of photooxidation without a significant induction period, possibly through the intermediate >NOH. However, most of the >NO  is present in the polymers as macromolecular >NOP during UV exposure since it is resistant to repeated cycles of solvent extraction/photooxidation. Some possible mechanisms for the regeneration of >NO• from >NOP are considered.

A polymeric system based on LDPE would be qualified as a “photo(bio)degradable synthetic polymer” for use as films or thin systems in plasticulture and later in packaging where severe specific criteria should be respected. The evolution of such a system in environmental conditions should present three phases. In Phase I, corresponding to storage and use, in the presence of physicochemical and biological aggression, chemical evolution should be very limited and resistance to any microorganism should be observed. In Phase II a rapid abiotic degradation should occur until the complete destruction of physical (mechanical) properties and spontaneous fragmentation of the thin systems into more and more divided parts. Phase III corresponds to bioassimilation of heavily transformed (oxidized) solid particles. Phase I should be predicted and controlled on the basis of artificial photoaging or thermoaging experiments. Depending on the desired lifetime of the system, nonaccelerated, accelerated, or ultra-ac...

The nature of transformation products of dithio- and thionophosphoric acids in the presence and absence of hydroperoxides and their role as transformation products formed during oxidation of thiophosphoryl disulfide at high temperatures is investigated. The major transformation product of dithiophosphoric acid was found to be the corresponding disulfide while thiophosphoric acid was the major oxidation product of the reaction of thionophosphoric acid with hydroperoxide. In the case of thiophosphoryl disulfide, it is shown that thionophosphoric acid was found to be one of the major transformation products in the presence of hydroperoxides, whereas no acids were formed in the absence of added hydroperoxides. © 1993 John Wiley & Sons, Inc.

Bis-acryloyl HALS (hindered amine light stabilisers) react with polypropylene under reactive processing conditions to give initially a cross-linked product which rearranges under conditions of high shear and high temperature to give a homogeneous gel-free polymer graft which blends with normal polypropylene to give an effective polymer-bound photoantioxidant. Monoacrylates and acrylamides and bis-methacryloyl HALS bind to polypropylene to a much lower extent under the same conditions and this is rationalised on the basis of the higher tendency of these monomers to homopolymerise.

The nature of transformation products of high-temperature reactions of thiophosphoryl disulphides with hydroperoxides in the absence and presence of oxidisable substrates and the role of the disulphide in the overall antioxidant mechanism was investigated using a range of techniques such as oxygen absorption, 31P nuclear magnetic resonance (NMR), gas-liquid chromatography (GLC), and peroxide determination. The major transformation products of the above oxidation reactions were found to be the thio and thiono-phosphoric acids, in addition to mono and polysulphides. At high molar ratios of peroxide to disulphide (>10), these oxidation products are the main catalysts for peroxide decomposition, while at lower ratios the disulphide itself was found to play a major role in the antioxidant mechanism.

1-Acryloyl-4-acryloyloxy-2,2,6,6-tetramethylpiperidine (AATP) has been covalently attached to polypropylene under conditions of reactive processing to give substantial concentrations of essentially 100% grafted antioxidant (AATP-B). Solution of the concentrates showed that at high concentration of grafted antioxidant some homopolymerized AATP was present and the polymer undergoes transient cross-linking during processing. Concentrates of bound AATP (AATB-B) when used as conventional additives for polypropylene are very effective photoantioxidants due to conversion to the related polymer-bound nitroxyl radicals by photosensitized oxidation. A typical commercial hindered phenol, Irganox 1076, when used at low concentration in combination with -B also acts as a photosensitizer for nitroxyl formation. Unlike low molar mass-hindered piperidinoxyl, polymer-bound nitroxyls are effective thermal antioxidants that synergize very effectively with low concentrations of Irganox 1076.

The term "sustainable" is often assumed to be synonymous with "renewable". However, polymers made from renewable natural resources are not necessarily eco-efficient. Cellulose-based polymers utilize more nonrenewable fossil fuels and are more polluting during manufacture than petro-based polymers. Sustainable polymers also have to be industrially acceptable, and although ultimate biodegradability in the natural environment is important, polymer-based products are required to biodegrade in a controlled way. Service life may be a year or more before the commencement of environmental degradation occurs. Many natural polymers such as rubber, lignin, and humus, like the synthetic polyolefins, biodegrade by an oxidative mechanism (oxo-biodegradation), and consequently much of nature's biological waste cannot satisfy the rapid mineralization criteria currently advocated by standards committees for synthetic polymers. Although biometric tests are more convenient to use than full composting tests, they are meaningless when applied to hydrocarbon polymers, whether natural or synthetic, since oxo-biodegradation is a slower process than hydro-biodegradation at ambient temperatures. Biodegradation standards currently proposed are unrealistic and will need to be modified on the basis of recent scientific evidence.

As environmental performance becomes increasingly important, the development of man-made polymers and their associated benefits has been overshadowed by problems relating to their ultimate disposal. In the light of wider acceptance of polymers for use in high technology applications, Polymers and the Environment aims to redress the balance. The book reviews the properties and industrial applications of polymers and discusses their environmental benefits compared with traditional materials. It also addresses the issues of polymer durability, recycling processes to aid waste minimization and biodegradable polymers. This text is intended to introduce the non-specialist reader to the benefits and limitations of polymeric materials from an environmental viewpoint, and will prove a useful book for both students and professionals.

The oxidation of the hydrocarbon polymers begins during processing (mechanooxidation), and the formation of hydroperoxides during fabrication of polymers affects the rate of thermo- and photooxidation of the polyolefins during subsequent use (aging and weathering). The end products of thermal and photooxidation are low molecular weight carboxylic acids which are involved in the eventual bioassimilation of the polymers in the ecosystem.

1. A Theoretical Approach to the Optimisation of Antioxidant Action.- 2. Mechanisms of Antioxidant Action of Phosphite and Phosphonite Esters.- 3. Antioxidant Mechanisms of Derivatives of Dithiophosphoric Acid.- 4. Polymers and High-Energy Irradiation: Degradation and Stabilization.- 5. Photodegradation and Stabilization of PPO(R) Resin Blends.- 6. Photo-oxidation and Stabilization of Polyethylene.- 7. Analysis of Antioxidants and Light Stabilisers in Polymers by Modern Liquid Chromatography.

Nickel O,O-dialkyldithiophosphates and nickel xanthates stabilize low density polyethylene against both thermal and photo-oxidation. Under thermal oxidative conditions, the initial complexes are rapidly destroyed and the anti-oxidant function is associated with the further oxidation products formed via the corresponding disulphides. Their photostabilizing activity is related primarily to the photostability of the metal complexes themselves which act as light stable reservoirs for the anti-oxidant species.

The powerful antioxidant activity of 3,5-di-tert-butyl-4-hydroxybenzyl sulphides has been shown to be due to the generation of a Lewis acid catalyst which may destroy > 3 × 105 moles of hydroperoxide per mole of sulphur compound. The same catalytic species formed from a variety of benzylic sulphides and is believed to be an inorganic sulphur acid (probably S03). Dibenzyl monosulphide itself is not initially an antioxidant and one function of the hindered phenolic group in the auto-synergists is believed to be the removal of free radicals formed during the early stages of the reaction.

Aldonitrones are effective melt stabilizers for polypropylene, the most effective being benzaldonitrones containing a partially hindered 4-hydroxy group in the aromatic ring. They form nitroxyl radicals in the polymer during processing, and these and the related hydroxylamines are believed to be involved in a catalytic CBA/CBD antioxidant cycle during mechano- and photooxidation. During thermal oxidation (oven aging) they behave as stoichiometric CB-D antioxidants.

The effect of thermal and photo-oxidation on rubber (ethylene-propylenediene terpolymer) modified polypropylene is examined and it is concluded that, although the rate of photo-oxidation increases with the amount of modifier, the rate of loss of impact resistance decreases. Antioxidants and uv stabilisers improve the oxidation resistance of the impact modified polypropylene but conventional stabilisers are removed by solvent leaching. Antioxidants bound to the EPDM through sulphur show a similar stabilising performance to commercial stabilisers before extraction but are much more effective after extraction.

It is shown that the oxidation of nickel dialkyl dithiophosphates by hydroperoxides at room temperature, as measured by 31P NMR, yields thiophosphoryl monosulphide and thionophosphoric acid as major products when the molar ratio of hydroperoxide to the nickel complex is high. Processing of thiophosphoryl disulphide concentrates in polypropylene is also shown to result predominantly in the formation of the sulphur acid and thiophosphoryl esters in various forms. It is also demonstrated that the incorporation of a synthesised sample of thionophosphoric acid in polypropylene affords comparable melt- and UV-stabilisation to that of thiophosphoryl disulphide.

The strategic significance of hydroperoxides as initiators for the photo-oxidation of hydrocarbon polymers has only recently become generally accepted by polymer photochemists. This has in turn led to the recognition that uv stabilisers are essentially antioxidants acting by the same mechanism as thermal antioxidants, but differing from them in their much greater stability under conditions of photo-oxidation. Many of the most effective photo-antioxidants act catalytically and two general classes of catalytic photo-antioxidant have received most attention during the past 10 years: 1. 1. The catalytic chain-breaking antioxidants are redox systems which oxidise alkyl radicals (CB-A) and reduce alkylperoxyl radicals (CB-D) in a cyclical repeating process. Nitroxyl radicals derived from commercial hindered amine light stabilisers (HALS) are the most important CB-A/CB-D antioxidants, and some recent examples of new photo-antioxidants involving this principle are discussed. 2. 2. The light-stable nickel thiolate antioxidants (particularly the dithiocarbamates and dithiophosphates) are slowly converted during photo-oxidation to lower molecular mass sulphur acids which are effective catalysts for peroxidolysis (PD-C). New developments in this field are reviewed.

Bis(diisopropyl)thiophosphoryl disulfide (DIPDIS) is used as a sulfur donor vulcanizing system for cis-1,4-polyisoprene. It is shown that the network structure consists of poly- and disulfidic crosslinks at early stages of cure, simplifying at optimum cure to monosulfidic crosslinks. It is thought that pendent accelerator groups are bound to the rubber molecule at early stages of cure, but are subsequently replaced by cyclic sulfidic groups. The good thermal and thermal oxidative aging behaviour of the vulcanizate is due to the formation of zinc diisopropyldithiophosphate (ZDP) in situ.

It is shown that antioxidants previously reported to be u.v. stabilizers can be distinguished by their mode of action into two categories. These are melt stabilizers, which destroy hydroperoxides during the processing operation, and u.v. stabilizers, which have the additional ability to destroy hydroperoxides during u.v. exposure. The interdependence of the chemical changes occurring during processing and on u.v. exposure are demonstrated.

The rubber damping peak of pure polybutadiene (PB) cannot be measured by a visco-elastomeric technique (Rheovibron). On photo-oxidation. the polymer becomes cross-linked and exhibits a damping peak at −80 similar to that in high impact polystyrene (HIPS). The cross-linking reaction can be followed by measuring gel formation and cross-link density by swelling; it is associated, both in PB and in HIPS, with decay of unsaturation in the rubber phase. Gel formation, like hydroperoxide formation, shows no induction period. Both rise to a maximum but the peak of gel formation is reached earlier than the peak of hydroperoxide formation and both precede the onset of rapid carbonyl and hydroxyl formation. The initially formed gel is peroxidic in nature and subsequent photolysis of peroxy gel leads to scission of the graft between polybutadiene and polystyrene. On more extended photo-oxidation, a more stable ether gel is formed involving only polybutadiene.

A synergistic combination of bis(diisopropyl)thiophosphoryl disulfide, dimorpholyl disulfide, and sulfur is used to produce an efficient vulcanizing system for a range of rubbers. This produces vulcanizates with the exceptional thermal and thermal-oxidative stability characteristic of sulfur donor vulcanizates and the rapid and extensive crosslinking reaction normally associated with a conventional sulfur–accelerator combination. A pronounced induction period is noted. The crosslinks initially produced in cis-1,4-polyisoprene rubbers are predominantly polysulfide but reduce to mono- and disulfides at optimum and extended cures. The crosslinks of the ethylene–propylene terpolymer and the styrene–butadiene vulcanizates are initially mainly disulfide but are rapidly reduced to monosulfides at the high curing temperatures (180°C) used. A comparison with vulcanization systems based on tetramethylthiuram disulfide and bis(diisopropyl)thiophosphoryl tri-and tetrasulfides as sulfur donors and with a conventional cyclohexylbenthazyl-2-sulfenamide–sulfur combination, respectively, shows it to have distinct advantages.

Unstabilized PVC processed at 170 and 210°C was found to undergo immediate mechano-chemically induced thermal oxidation with the formation of unsaturation and peroxides. The peroxides were partially in the form of cross-links between polymer molecules at this stage and were manifested in the form of solvent insoluble gel. Conjugated double bonds and colour formation both increased linearly with time. The evidence suggests that the initial mechano-chemical process sensitizes the polymer to subsequent thermal degradation.

An antioxidant (3,5-di-tert-butyl-4-hydroxybenzyl mercaptan) and a u.v. stabilizer (4-benzoyl-3-hydroxyphenyl-O-ethylthioglycollate) containing the thiol group have been reacted with ABS latex to give polymers with exceptionally high heat and light stability. Both chemical and physical phenomena contribute to the high activity of the bound antioxidants. The concentration of the molecularly dispersed antioxidants in the most oxidatively vulnerable part of the polyblend (the polybutadiene segment) has an important role in determining stabilizer efficiency and, particularly under high temperature conditions, the immobilization of the antioxidants by chemical bonding leads to much lower rates of physical loss. The autosynergistic hindered phenol is primarily responsible for the thermal stabilizing activity of the combination but it also powerfully augments u.v. stabilizing effectiveness of the u.v. stabilizer.

The limitations of conventional antioxidants and stabilisers in polymers are associated with their migration into contacting fluids. This leads to toxicity in packaging and medicinal applications and loss of effectiveness in many engineering uses of polymers. Reactive processing procedures are being increasingly used to chemically attach antioxidants to polymers and the most successful of these involve the synthesis of bound antioxidant concentrates which can be used as macromolecular adducts for normal polymers. Present evidence suggests that a polymer modified by substantial concentration of antioxidants loses its physical identity and can be used in a range of other polymers. Future developments in which the additives are 100% bound to the polymer substrate are discussed.

The antioxidant activities of a number of N-phenyl nitrones containing phenolic functions were compared with N-methyl nitrones and conventional antioxidants in peroxide and TMTD sulphur-less vulcanizates. The N-phenyl nitrones were found to be effectively bound during vulcanization whereas the N-methyl nitrones were not. Unhindered phenols reacted to a higher degree and were relatively more effective than the typical hindered phenol antioxidant structures. A conventional bisphenol was effective in the sulphurless vulcanizate after solvent extraction but this is considered to be due to the formation of an insoluble zinc salt. Synergism was observed in the TMTD sulphurless vulcanizate both before and after extraction.

The chemical reactions involved in the antioxidant behaviour of the thiodipropionate esters have been investigated and three distinct mechanisms have been identified. The first is a radical trapping process involving the sulphenic acid formed by breakdown of the sulphinyldipropionate ester which is an oxidation product of the thiodipropionate. The second is a radical generating reaction resulting from a redox reaction between hydroperoxides and the sulphenic acid. The third is a Lewis acid catalysed destruction of hydroperoxide by a further oxidation product of the sulphenic acid. Sulphur dioxide satisfies some of the requirements of this catalytic species but it seems likely that other products are also involved. Hydroperoxide deactivation by complexing with sulphoxide does not appear to be involved.

A phenolic thiol antioxidant and two thiol amides have been reacted with nitrile-butadiene rubber both in the latex and during melt processing to give masterbatch concentrates. BHBM (I) reacts to give higher bound antioxidant levels at 20% masterbatch concentration than the amides in the latex but MADA (II) gives the highest level of binding by the mechanochemical procedure (80% masterbatch concentration). The primary initiation process is mechanochemical but secondary peroxide initiated adduct formation occurs during the later stages.

1. The present studies have shown that N-nitrosamines derived from the more active diarylamine antifatigue agents are considerably more effective as antifatigue agents than the parent amines when they are predispersed into rubber on an open mill, followed by conventional vulcanization. 2. Some N-nitrosamines are effective retarders of premature vulcanization. 3. All the nitrosamines examined with the exception of nitroso diphenylamine are effective heat stabilizers for rubber in an air oven test. 4. The nitrosamines are partially converted to the parent amine during vulcanization, but other products are also formed which contribute to the antidegradant activity.

The antioxidant mechanism of zinc dialkyl dithiocarbamate is examined in detail in model compounds. The kinetics of the reaction of zinc dialkyl dithiocarbamate with hydroperoxides were studied in inert and oxidizable substrates. A typical electron donor antioxidant and a base (CaCO3) were found to inhibit the peroxide decomposition. The oxidation products of the Zinc II complex appear to be quite different from those of the nickel II and iron III complexes. Two processes are involved in the antioxidant function of the zinc complex: The first is a free radical decomposition of the peroxides, which predominates at molar cumene hydroperoxide (CHP)/zinc diethyl dithiocarbamate (ZnDEC) ratios of 10 or less. At higher molar ratios an ionic reaction predominates, although there is a radical contribution at all ratios. Thiocarbamoyl disulfide does not seem to be formed as an intermediate from ZnDEC during the ionic process.

The mechanical properties of three binary mixtures of incompatible polymers (LDPE/PP, LDPE/PS and LDPE/PVC) have been studied in the presence of potential solid phase dispersants. The results show that it is possible to achieve an acceptable level of toughness in the composite products for the re-use of the materials in demanding applications. It is suggested that the main factor necessary to achieve good impact resistance and elongation at break of the polymer is the presence of a rubbery interphase between the heterophases. Satisfactory tensile strength on the other hand depends more on the dispersant properties of the third component.

The effect of the processing operation on the nature and quantity of transformation products of 2,2,6,6-tetramethyl-4-hydroxypiperidinoxyl (〉NO.) and its corresponding hydroxylamine (〉NOH) was investigated in polypropylene. It was found that both 〉NO. and 〉NOH are actively involved in a regenerative cycle and that this redox couple is responsible for the stability of the polymer whether the initial additive is 〉NO. or 〉NOH. The alkyl hydroxylamine (〉NOR), on the other hand, does not appear to play a significant role in the melt stabilization of the polymer. Quantitative measurements of 〉NOH and 〉NO. concentrations show that, under conditions of both restricted oxygen (CM) and oxygen excess (OM), the concentrations of both species change reciprocally throughout the processing operation. In a closed mixer (CM), the total overall concentration of 〉NO. and 〉NOH remains at a high constant level, whereas in an open mixer (OM), their total concentration decreases continuously to a low level, indicating the irreversible destruction of the redox couple (〉NO. and 〉NOH). The photo-oxidative stability of the polymer was found to depend on the total concentration of the redox couple left in the system after processing in either CM or OM.

Nitro tertiary alkanes are photo-antioxidants for polyolefins. The mechanism is shown to involve photolysis to NO 2 which can subsequently participate in the catalytic deactivation of photochemically formed macroalkyl radicals. The mechanism proposed bears formal resemblance to that of hindered piperidinoxyls in the antioxidant mechanism of the hindered piperidines.

It is shown that the antioxidant 3,5-di-tert-butyl-4-hydroxybenzyl acrylate, may be grafted to the surface of polymer artefacts. The antioxidant activity of the resulting “efficiently dispersed” antioxidant is very much higher than that of low molecular weight antioxidants containing the same functional group. Even after extraction with an effective solvent for the antioxidant-monomer and derived polymer, the oxidative stability of the polymer films is very much greater than that of polymer containing the antioxidant-monomer or antioxidant-polymer incorporated by a conventional melt technique without extraction. Optimum activity is believed to be associated with the formation of frequent short chain grafts in the surface of the polymer and is achieved by the use of a polymerization photo-activator such as benzophenone which also terminates kinetic chains. Surface grafted antioxidants are also found to be effective u.v. stabilizers for polypropylene in contrast to the normal ineffectiveness of chain terminating antioxidants incorporated into the polymer by conventional techniques.

Isopropylaminodiphenylamine (IPPD, I) and a hindered bispiperidine ( IIa ) are effective antifatigue agents in vulcanized rubber. The former is also a powerful thermal antioxidant and antiozonant whereas the latter is not. An investigation of the mechanism of antifatigue action shows that both I and IIa are converted to the corresponding nitroxyl radical during dynamic flexing of the rubber and that the nitroxyl is continuously regenerated in a cyclical mechanism involving the corresponding hydroxylamines.

The antioxidant mechanisms involved in the u.v. stabilising activity of two nickel complexes, nickel dibutyl dithiocarbamate (NiDBC) and nickel acetophenone oxime (NiOx), have been investigated both by oxygen absorption measurements and in peroxide decomposition studies. NiDBC gives rise to a powerful catalyst for non-radical hydroperoxide decomposition; the rate of the reaction is faster in the presence than in the absence of light although the overall mechanisms appear to be very similar. NiOx is decomposed by hydroperoxides in a series of stoichiometric reactions which involve the consumption of at least six molecules of hydroperoxide per molecule of NiOx. Again the reaction is catalysed by light but the mechanism, which does not involve free-radical formation, is the same in the presence and absence of light. NiOx also appears to have weak radical trapping properties. Attempts to show that NiDBC and NiOx might interact sacrificially with photo-excited states of molecules (triplet carbonyl and singlet oxygen) were unsuccessful; the rates of photo-destruction of both complexes were unaffected by the presence of ketones both in the presence and absence of oxygen. This work confirms earlier conclusions that the nickel complex u.v. stabilisers function by auto-synergistic mechanisms involving u.v. screening and conventional chain-breaking and preventive antioxidant processes which operate during thermal processing operations as well as during environmental exposure. Although excited state quenching processes may occur, they appear to be less important in the overall scheme than the well established antioxidant mechanism.

Several important classes of u.v. activator for polymers have been studied in polyethylene. The progress of the oxidation was followed by measuring carbonyl formation in polyethylene films (carbonyl index) and by following the change in dynamic modulus of the polymer with time. The two procedures gave good agreement on the rates and extents of the two degradative reactions occurring. Three different kinds of u.v. oxidative behaviour were observed. The first, typical of the efficient triplet activators such as benzophenone and its derivatives, caused an initial rapid oxidation, increasing with ketone concentration, followed by auto-retardation. The second, typical of transition metal ions, caused an auto-accelerating rate of oxidation increasing with concentration, the third, characteristic of sulphur-containing metal complexes, showed an initial induction period which increased with increasing concentration, followed by rapid autoacceleration to physical disintegration. It is shown that, in each class, the u.v. behaviour is an extension of the function of the additive in the polymer melt during processing.

The effects of commercial antioxidants and u.v. stabilisers are examined for two typical polyethylene based polyblends containing solid phase dispersants. Both were found to be effectively stabilised against the effects of light and heat by nickel dibutyl dithiocarbamate, a u.v. stable catalytic peroxide decomposer.

Stabilisation of polypropylene by benzophenone-substituted xanthates is examined. These systems afford greater melt stabilisation of polypropylene than the corresponding controls, i.e. benzophenone and alkyl xanthates. In contrast to the pro-oxidant behaviour of benzophenone during photo-oxidation of polypropylene, benzophenone-substituted metal xanthates show a three-step behaviour: an initial rapid photo-oxidation, followed by a secondary inhibition period and a third autoacceleration step. The first step is shown to be mainly due to the benzophenone substituent while the length of the inhibition period is directly related to the amount of the sulphur component. During this period xanthates therefore lead to complete inhibition of photo-oxidation both with regard to chemical oxidation and physical change.

The mechanism of the photo-oxidation of both low density and high density polyethylenes has been studied kinetically. The kinetics are consistent with the view that when substantial quantities of carbonyl compounds are present in the polymers by photolysis of thermally formed hydroperoxides, they are intimately involved as initiators in the photo-oxidation by Norrish I photolysis of ketones. The latter process is also responsible for the ultimate end products of photo-oxidation, viz. carboxylic acids. The two polymers differ in that cross-linking occurs initially in LDPE with reduction in vinylidene unsaturation whereas chain scission predominates in HDPE. The effect of iron compounds is in accord with their behaviour in thermal oxidation. Iron carboxylates activate both types of polyethylene to photo-oxidation primarily by decreasing the amount of carbonyl compounds present, although initially they also generate radicals by photolysis. The iron dithiocarbamates on the other hand are initially antioxidants and initially prevent carbonyl formation in the polymer. They function as photo-activators primarily by photolysis to free radicals.

A number of nitrones containing a partially hindered phenolic group are shown to be effective antidegradants for sulfur vulcanized rubbers. They exhibit a high level of antiozonant and antifatigue activity, but unlike commercial antiozonants they do not cause discoloration of light colored formulations. The new antidegradants are less effective than conventional additives as thermal antioxidants, but their performance as antifatigue agents and antioxidants is augmented by their use in combination with nonstaining antioxidants. The mechanism of action of the phenolic nitrones is believed to involve their oxidation to stable phenoxyl/nitroxyl radicals.

The effectiveness of galvinoxyl (G·(I)) as a thermal antioxidant in polypropylene at 140°C is dependent on the prior processing treatment of the polymer. A characteristic fluctuation of antioxidant activity with time of processing is shown to be associated with the extent of conversion of galvinoxyl to hydrogalvinoxyl (GH) in the polymer. It is concluded that the main stabilisation mechanism involved is deactivation of alkylperoxyl radicals (CB-D) by GH but secondary processes (CB-D and CB-A) involving G· may also make a contribution.

The effects of thermal processing on the chemical and physical properties of low density polyethylene are strongly dependent on the amount of oxygen present in the mixer. Subsequent thermal and photo-oxidation show very similar characteristics which appear to depend primarily on the amount of hydroperoxide formed during processing. Removal of hydroperoxide by heat treatment in an inert atmosphere, although increasing ketonic carbonyl concentration, markedly decreased the rate of photooxidation, introducing an induction period similar to that of an unprocessed sample. It is concluded that hydroperoxides are the most important initiators during the early stages of photo-oxidation of normally processed polymers, although carbonyl initiation is important in heavily oxidised polymers in the later stages of photo-oxidation.

Antioxidant activities, both in model substrate oxidation and in polypropylene under technological conditions, have been examined for a series of alkyl (3,5-di-tert-butyl-4-hydroxybenzyl) sulphides (I). They were found to be intrinsically much more active than typical commercial chain-breaking antioxidants in a closed system. This higher activity is attributed to an additional antioxidant function resulting from the presence of the sulphur atom. The latter is the precursor for a highly active peroxidolytic species which leads to autosynergism. In a thermal-oxidative (air-oven) test I (R, C2—C18) increases in activity with decreasing volatility but effectiveness as melt stabilizer during the processing of polypropylene and as u.v. stabilizer appears to depend more on the physical state of the additive in the polymer than on its volatility.

It is shown that nitroso-tert-alkanes are highly active UV stabilizers which become at least partially chemically attached to polypropylene during processing. The aromatic nitroso compounds are less effective as light stabilizers but are more effective than their alkyl analogues as thermal oxidative stabilizers. Mechanisms are proposed to account for the antioxidant activity of the nitroso compounds and their ability to cause crosslinking of the polymer during processing.

Copolymers have been synthesised by processing polyethylene/polystyrene (PE/PS) and polyethylene/poly(vinylchloride) (PE/PVC) blends in an internal mixer in the presence of radical generators. Solid phase dispersants (SPDs) were obtained by this procedure which, when incorporated into physical blends of PE and PS or PE and PVC respectively, improved the strength of the composites. They were found to be inferior to rubber-based copolymers in improving toughness.

Effet de l'operation de mise en œuvre sur HTMPO lui-meme et lien avec son activite antioxydante et stabilisante UV

It is demonstrated by the use of spin traps that during the early stages of thermal processing of PVC considerable concentrations (> 3 × 10−6 mol g−1) of free radicals are produced which are primarily responsible for the initial products (hydroperoxides, unsaturation, and hydrogen chloride) previously shown to be formed in the polymer. From a semiquantitative analysis of these products, it is estimated that more than 50% of the radicals are formed from hydroperoxides and the rest by mechanoscission of the polymer chain. The spin traps are effective processing stabilizers in combination with a tin maleate HC1 scavenger. One of them (2-methyl-2-nitroso propane, MNP) has also been shown to be a photoantioxidant.

The photoxidative degradation of PVC was studied by i.r. and u.v. spectroscopy and gel permeation chromatography. It was found that the photoxidative degradation of unprocessed PVC is an auto-accelerating chain scission process. Carboxylic groups were found to be the main product formed during degradation. The molecular weights M n and M w both decreased, but the molecular weight distribution widened with increasing length of exposure. Single additives, such as calcium stearate. Wax E and a solid organotin stabilizer, altered the rate but not the mode of the degradation. Combination of the three additives changed the mode of the photoxidation from auto-accelerating to constant rate of degradation. Processing at 170° with the combined additives increased the rate constants.