Polymer Degradation and Stability (v.85, #3)

by N.C Billingham (925).

Degradation and stabilisation of polymers and coatings: nano versus pigmentary titania particles by Norman S. Allen; Michele Edge; Amaya Ortega; Gonzalo Sandoval; Christopher M. Liauw; J. Verran; John Stratton; Robert B. McIntyre (927-946).
A series of nano and micron particle size anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. Their thermal and photochemical activites have been determined in a metallocene polyethylene with a monomodal molar mass distribution and alkyd/acrylic based paint films. Their performance has been assessed by FTIR and hydroperoxide analysis, chalking, colour, gloss change and weight loss following artificial weathering. The pigments have also been examined by rapid assessment methodologies using photodielectric microwave spectroscopy, 2-propanol oxidation and hydroxyl analysis. The microwave response under light and dark cycles provides an extended timescale probe of charge carrier dynamics in the pigments. Photooxidation studies on polyethylene containing nano-particle and pigmentary grade titanium dioxide pigments show that in general the former are more photoactive with anatase and rutile forms exhibiting high activity. Pigment particle size and surface area clearly play an important role in dispersion and any polymer-pigment interactions. This relationship is also extrapolated to the thermal sensitisation activities of the pigments on processing and oven ageing. Nano-particle pigments induce oxidation of the polymer during processing and long-term thermal ageing accelerating the formation of hydroperoxide and carbonyl groups. This behaviour can play a major role in controlling the photoactivity of the pigments in a polymer matrix. Photooxidation studies on paint films show a clear demarcation between nano-particle and pigmentary grade titanium dioxide with the former being more active. Here nano and micron particle size anatase and rutile titanium dioxide pigments have been prepared with various densities of surface treatments, particle size and surface area. The photosensitivity of titanium dioxide is considered to arise from localised sites on the crystal surface, and occupation of these sites by surface treatments inhibits photo-reduction of the pigment by ultraviolet radiation and hence the destructive oxidation of the binder is inhibited. Coatings containing 2–5% w/w alumina or alumina and silica are satisfactory for general-purpose paints. If greater resistance to weathering is desired, the pigments are coated more heavily to about 7–10% w/w. The coating can consist of a combination of several materials, e.g. alumina, silica, zirconia, aluminium phosphates of other metals. For example, the presence of hydrous aluminium oxide particles lowers Van der Waals forces between pigments particles by several orders of magnitude, decreasing particle-particle attractions. Hydrous aluminium oxide phases appear to improve dispersibility more effectively than most of the other hydroxides and oxides. Coated nanoparticles are shown to exhibit effective light stabilisation in various water and oil based paint media in comparison with conventional organic stabilisers. Hindered piperidine stabilisers are shown to provide no additional benefits in this regard often exhibiting strong antagonism. The use of photocatalytic titania nanoparticles in the development of self-cleaning paints and microbiological surfaces is also demonstrated.
Keywords: Titanium dioxide; Pigments; Nanoparticles; Degradation; Oxidation; Acrylic paints; Coatings; Stabilisation; Photocatalytic; Absorbers;

Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) with hydroxyvalerate content of 12 mol % is a natural aliphatic polyester. At temperatures above its melting point, thermal decomposition occurs leading to shorter chains with carboxyl end groups. In blends with epoxidized natural rubber (ENR), 50% epoxidation level, these carboxyl chain ends may trigger reactions between the constituents since the epoxidized groups provide reactive sites. Glass transition temperature studies reveal immiscibility of the polymers over the entire composition range. Hence, any reaction between the constituents is restricted to the interfacial region between the components. We report here on studies on melt reactions between the components at elevated temperatures. DSC traces display an exothermic peak when the blends are annealed at temperatures ranging from 220 up to 234 °C. The kinetics of the reaction are discussed in terms of a serial reaction scheme. It turns out that the reaction rate increases with annealing temperature (T a) and with descending PHBV content in the blends when PHBV is the minor component. Morphological studies by optical microscopy reveal that PHBV is still capable of crystallizing at 50 °C after melt reaction at 234 °C. Either ring-banded or fibrillar spherulites can be seen in blends with PHBV in excess. We note, however, that the rate of crystallization is dramatically reduced after melt reaction at T a=234 °C.
Keywords: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate); Epoxidized natural rubber; Morphology; Crystallization;

Some aspects of thermo- and photodegradation and stabilisation of flexible poly(vinyl chloride) (PVC) compounds have been studied by chemiluminescence (CL). The oxidation of PVC is shown to be chemiluminescent and the CL behaviour is sufficiently similar to that of a model polyene, β-carotene, to suggest that the CL arises from the oxidation of polyenes formed by dehydrochlorination and that its intensity can be a measure of polyene formation. CL monitoring is shown to be useful in differentiating functionalities of different stabilisers in terms of their ability to retard polyene formation. Ba/Zn carboxylates which yield good initial or short-term colour stability at high temperatures or good low/moderate temperature stability by blocking of the propagating polyenes can easily be distinguished from stabilisers which do not retard polyene formation so well. The applicability of the CL technique in assistance with additive selection (e.g. HCl absorbers, phosphites) for providing a formulated compound with the required heat stability is demonstrated. The usefulness of CL analysis is also demonstrated in building correlations with long-term weathering tests and thus minimising time for development of new formulations.
Keywords: PVC; Degradation; Stabilisation; Chemiluminescence;

Date palm leaves were compounded with polypropylene (PP) and UV stabilizers to form composite materials. The stability of the composites in natural weathering conditions of Saudi Arabia and in accelerated weathering conditions was investigated. The composites were found to be much more stable than PP under the severe natural weathering conditions of Saudi Arabia and in accelerated weathering trials. Compatibilized samples are generally less stable than uncompatibilized ones as a result of the lower stability of the maleated polypropylene. Irgastab and Tinuvin are found to be efficient stabilizers for PP/cellulose fibre composites. In addition to enhanced stability imparted by the presence of the fibres in the composites, enhanced interfacial adhesion resulting from oxidation of the polymer matrix can be the source of retention of mechanical strength.
Keywords: Degradation; Date palm fibre/polypropylene composites; Natural and artificial weathering;

An efficient method of material recycling of municipal plastic waste by I Fortelný; D Michálková; Z Kruliš (975-979).
The recycling of municipal plastic waste after separation of poly(ethylene terephthalate) (PET) bottles, containing various grades of polyethylene (PE), polypropylene (PP), polystyrene (PS) and a small amount of PET, was studied. It was found that a mixture of ethylene-propylene elastomer EP(D)M with styrene-butadiene (SB) block copolymer is a very efficient compatibiliser for PE/PP/PS blends. Surprisingly, EP(D)M/SB is also a more efficient compatibiliser than neat SB for some binary polyolefin/PS blends, probably due to correlation between locations of SB and EP(D)M in the blend. For achievement of acceptable mechanical properties, blends containing an admixture of PET must be mixed at a high temperature. A strong synergism between the effects of SB/EP(D)M compatibilisers and substituted diamine stabilisers on toughness of recyclates of municipal plastic waste was observed and the recyclate with mechanical properties comparable with virgin polyolefins was prepared. Experiments on model systems led to the conclusion that the synergism is a consequence of the grafting of pre-aged polyolefins to SB copolymers.
Keywords: Recycling; Municipal plastic waste; Compatibilisation; Stabilisers; Synergism; Styrene-butadiene copolymers; Ethylene-propylene elastomers;

Feedstock recycling of plastics in Japan by Akitsugu Okuwaki (981-988).
Several commercial plants for feedstock recycling of waste plastics have started up in Japan since the Niigata liquefaction plant was opened in 1997, utilizing processes such as blast furnace application, coke oven application, gasification, and alcoholysis of post-use polyethylene terephthalate (PET) bottles. This has been brought about by the practical implementation of the Containers and Packaging Recycling Law in 1997 for PET bottles, and for plastic containers and packaging in 2000. This law allows the assessment of feedstock recycling fees only for waste plastics separated from municipal solid waste. Although the recycling fees decrease year-by-year, total recycling costs, including collection and baling costs paid by municipalities, are still high. The key to further cost reduction is the further development of feedstock recycling systems.
Keywords: Feedstock recycling; Plastics; Japan;

Guest editorial by Béla Iván (989).

Laurolactam as a stabilizer against the thermo-oxidative degradation of poly(ether-esters) by G Botelho; P Gijsman; A.V Machado; A Queirós (991-995).
In this study we examined the influence of laurolactam on the thermo-oxidative degradation of a copolymer of poly(butylene terephthalate) and polybutylene glycol and of di(ethyleneglycol) dibenzoate, used as model compound. The degradation was followed by measurement of the oxygen uptake, FTIR and 1H-NMR spectroscopy and measurement of the intrinsic viscosity changes during degradation. A stabilizing effect of laurolactam on the thermo-oxidative degradation could be observed, as an induction time was defined in which hydroperoxides were not formed and molecular weight was kept constant. After this period the oxygen uptake, hydroperoxide formation and polymer conversion increased more slowly.
Keywords: Poly(ether-esters); Thermo-oxidative degradation; Stabilization; Laurolactam;

The effect of some thiosemicarbazide compounds on thermal oxidation of polypropylene by Radu Setnescu; Corina Barcutean; Silviu Jipa; Tanta Setnescu; Maria Negoiu; Ion Mihalcea; Madalina Dumitru; Traian Zaharescu (997-1001).
The effect of some 1–4 disubstituted thiosemicarbazide compounds (TSC), including metal complexes, on thermal oxidation of isotactic polypropylene, has been studied, based on the knowledge that TSC contain various chemical groups such as CS, –NH– that act as radical scavengers. All compounds exhibit antioxidant properties that represent around half of the effectiveness of Irganox 1076 (a strong phenolic antioxidant) during the induction period and around a quarter of this during the propagation period. The influence of X- atom (as a substituent in the 1st position) decreases in the following order: H > Cl > Br. R-group (as a substituent in 4th position) seems to exhibit lower influence than X (R = cyclohexyl, or phenyl ethyl). TSC complexes with Ni are more effective antioxidants as compared to TSC ligands. The activation energy of oxidation of isotactic polypropylene (i-PP) in the presence of Ni-complexes is very low as compared to the values found in the case of those free of additive i-PP or with other metal complexes. A compensation effect seems to be involved in these cases. A synergistic effect has been observed in the case of Ni(A2)2–Irganox 1076 couples, while with similar A5 and A6 couples, additive and antagonistic effects were observed, respectively.
Keywords: Chemiluminescence; Thiosemicarbazide compounds; Thermal oxidation; Polypropylene stabilisation;

It is well known that antioxidants are widely used to prevent thermal degradation of high density and linear low density polyethylene. Antioxidants are not always present in low density polyethylene and only small amounts are usually added to these resins. In this work the effect of an antioxidant system on a low density resin having MFI (190 °C/2.16 kg)=2 g/10′ and density 0.9230 g/cm3 has been studied. Its effect on melt viscosity has been studied by means of a batch mixer and the torque vs time behaviour has been analysed. The results show that a maximum in the torque vs time curve can be seen for the material containing antioxidant. In contrast, the material without antioxidant does not show any maximum in the torque vs time curve and, after a certain time, directly undergoes chain scission. The presence of the antioxidant in a low density polyethylene seems to change the kinetics of two competitive phenomena: long chain branching formation/crosslinking and chain scissions. The antioxidant works essentially by stopping the peroxides formation. This effect slows the molecular weight decrease but does not influence long chain branching formation or crosslinking.
Keywords: Low density polyethylene; Long chain branching; Chain scission; Antioxidant;

Films of a new linear low-density polyethylene (LLDPE) were exposed to natural weathering in southern Italy, for increasing times. Infrared analysis was used to detect the degradation products and the carbonyl index was related to the photooxidation products. The changes in mechanical properties due to the chemical degradation were followed by determining the drawing curve and the elastic modulus. All the mechanical parameters were correlated to the exposure time and to the carbonyl index, determined on the same samples. The change in the carbonyl index and the mechanical parameters with the exposure time allowed the determination of the point beyond which the material becomes useless. Furthermore, it was possible to calculate the ratio between natural and artificial time of degradation, which was found to be about 30.
Keywords: LLDPE; Spherilene; Photooxidation; Weathering; Mechanical properties;

Effect of chain structure on the processing stability of high-density polyethylene by Eszter Pock; Csaba Kiss; Ákos Janecska; Edina Epacher; Béla Pukánszky (1015-1021).
Ethylene/1-hexene copolymers were prepared with various 1-hexene contents to introduce short-chain branches into the polymer chain for the study of their effect on processing stability. The polymer powder was characterised by various techniques. Its density, molecular weight, MFI and melting characteristics were determined by routine techniques, and its functional group content by DRIFT. Besides short-chain branching, the number of other functional groups also varied in the polymerisation reaction, but the vinyl group content of all samples was approximately the same. The inherent properties of the polymer were determined by its chain structure, the existence of branched structure was also proved by the decrease of melting temperature and crystallinity. The chemical structure of the polymer was further modified during processing, the dominating reaction being the addition of radicals to the vinyl groups, which resulted in long-chain branching. This proved that the most active weak site of the chain is the vinyl group; vinylidene and vinylene groups as well as short-chain branches do not take part to the same extent in degradation reactions during processing. The experiments revealed that the residual stability of the polymer is influenced, in fact determined, by another factor, or factors, which have not been investigated in this study. Preliminary experiments and some considerations indicate that traces of oxygen containing groups might determine stability, but further study is needed to verify this tentative explanation.
Keywords: High-density polyethylene; Processing stability; Weak sites; Vinyl group content; Short and long chain branching;

Micro Raman and atomic force microscopy analysis of naturally aged polyethylene by A Szép; P Anna; I Csontos; Gy Marosi; Sz Matkó; Gy Bertalan (1023-1027).
The deterioration of naturally aged low-density polyethylene (LDPE) film was observed by elongation properties, DSC melting enthalpies and surface geometric characteristics by use of atomic force microscopy (AFM). The decrease of elongation at break under the influence of ageing was in good accordance with the increase of melting enthalpies and both phenomena could be explained by the gradual decrease of molecular mass of the polymer. On the wavy surface with a wavelength range of 10–15 μm, valleys with amplitudes from 160 to 600 nm could be observed by AFM depending on the time of ageing. The increase of deepness of valleys was explained not by the gradual erosion of the degraded surface but by the gradual increase of crystallinity, consequently the gradual shrinking on the surface of the polymer film. The gradual increase of crystallinity in the surface region of the film could be demonstrated by confocal Raman microscopy.
Keywords: Natural ageing of LDPE; DSC; confocal Raman microscopy; Atomic force microscopy;

Poly(vinyl chloride) (PVC) and PVC with increased concentrations of reactive tertiary chlorines (PVC(T)) were obtained by free radical polymerisations. PVC(T) was prepared by copolymerisation of vinyl chloride with 2-chloropropene. Thermal degradation of these polymers indicates the significant effect of tertiary chlorine defects on the thermal stability of PVC, i.e. increasing tertiary chlorine content decreases the stability of this polymer, on the one hand. However, efficient chemical modification, introduction of allyl groups by carbocationic derivatisation, is possible in the case of PVC(T) in contrast to the PVC homopolymer. It was found that allylation improves the thermal stability of both PVC and PVC(T). These investigations also indicate that about 50% allylation of thermally labile tertiary chlorines occurs in these polymers under the conditions used.
Keywords: Poly(vinyl chloride); PVC; Poly(vinyl chloride-co-2-chloropropene); Thermal degradation; Cationic allylation;

Epoxidation of thermally degraded poly(vinyl chloride) by Tibor Szakács; Béla Iván (1035-1039).
Epoxidation of conjugated polyenes formed by thermal degradation of poly(vinyl chloride) (PVC) was carried out in cyclohexanone and tetrahydrofuran solution with m-chloroperoxybenzoic acid (mCPBA). PVC was thermally degraded in the solid state under continuous nitrogen flow at 200 °C for 30 min leading to 0.6 mol% double bonds in the polymer chain as determined from the UV-visible spectrum of the degraded polymer. Cyclohexanone proved to be an inappropriate solvent for the epoxidation reaction probably due to its competitive reaction with the peroxide. UV-visible and FTIR spectroscopic analyses indicate that the epoxidation of polyenes by mCPBA in THF occurs in high yields leading to polyepoxy sequences in the PVC chain, and there are no signs of side reactions, such as oxidation of double bonds in polyenes to carbonyl groups. To our knowledge this is the first report on epoxidation of polymers containing conjugated double bond sequences. The resulting new epoxidised PVC might be useful in a variety of applications, such as fully miscible secondary epoxy stabiliser for PVC itself without plasticising effect, starting material for epoxy curing and co-curing for reactive blends, etc.
Keywords: Poly(vinyl chloride); PVC; Thermal degradation; Epoxidation; m-Chloroperoxybenzoic acid;

Photo-re-stabilisation of recycled post-consumer films from greenhouses by N Tzankova Dintcheva; F.P La Mantia (1041-1044).
The photo-oxidation behaviour of recycled post-consumer films from greenhouses has been studied in order to evaluate the environmental resistance of these secondary materials and the effect of some stabilising agents. The presence of the UV absorber slows down the weathering of the post-consumer materials. After about 1000 h of accelerated weathering the unstabilised post-consumer material becomes brittle, while the re-stabilised post-consumer film becomes fragile only after about 1500 h. A synergistic effect is observed when an antioxidant and an UV stabiliser are added. Indeed, in this case, the recycled material is not brittle even after more than 1700 h. These last results can be probably correlated to the protective action of the antioxidant that avoids the formation of oxygenated compounds during the processing.
Keywords: Re-stabilisation; Post-consumer greenhouse film; UV-absorber; Accelerated weathering; Mechanical properties;

Feedstock recycling of polymers by pyrolysis in a fluidised bed by W Kaminsky; M Predel; A Sadiki (1045-1050).
Feedstock recycling of polymers is an attractive option to recycle the increasing amount of plastic wastes and respond to restrictions for waste disposal in a lot of countries. Different processes are investigated such as degradation of plastics to monomers, pyrolysis into monomers and oil, gasification into syngas. Pyrolysis of mixed plastic wastes and elastomers is a cost-effective process to recover feedstocks for the petrochemical industry. The Hamburg process, using an indirectly heated fluidised bed, can be varied to produce mainly monomers, aliphatic hydrocarbons, or aromatics. At temperatures of 450 °C, poly(methyl methacrylate) (PMMA) is depolymerised to more than 98% of the monomer. The influence of fillers on the monomer yield has been studied. Polystyrene as feed gives up to 75% of styrene and 10% of oligomers. First demonstration plants are running for feedstock recycling of PMMA in a fluidised bed.
Keywords: Pyrolysis; Fluidised bed; Polymer recycling; Monomer recovery;

Bed defluidisation during the fluidised bed pyrolysis of plastic waste mixtures by Maria Laura Mastellone; Umberto Arena (1051-1058).
The risk of occurrence of phenomena inducing worsening of fluidisation quality and eventually leading to bed defluidisation is one of the major constraints to an easier utilisation of fluidised beds for the low-temperature pyrolysis of plastic wastes. In order to investigate these phenomena, different mixtures of plastic wastes, obtained by combination of three commercially-available recycled polymers, were fed to a laboratory-scale bubbling fluidised bed reactor. Two mechanisms of defluidisation were identified and the time at which defluidisation occurred was measured and correlated to a key variable of the process. The observed phenomenology was also simulated by means of room-temperature experiments carried out with a selected mineral oil. The results allow us to demonstrate the existence of a synergistic effect between the observed mechanisms of defluidisation.
Keywords: Defluidisation; Plastic waste; Pyrolysis; Fluidised bed;

Light-induced free radicals in crown ether—reactive dyeing systems by A. Vı́g; P. Bakó; A. Rockenbauer; L. Korecz; K. Sirbiladze; I. Rusznák; L. Tőke (1059-1064).
The effect of three different crown ethers on photodegradation and stable free radical concentration generated by light exposure has been studied on two cotton cellulose-reactive azo dye systems, with and without covalent bond between dye and fibre, respectively. A crown ether which decelerated the photofading of the dyed fabric increased the stable free radical content of the system under exposure to light, whereas those crown ethers which accelerated the photofading decreased the stable free radical content.
Keywords: Photofading; Reactive azo dyes; Crown ethers; Stable free radicals; Cellulose;

This work describes the ageing and artificial weathering behaviour of commercial polymer films with thickness of approximately 50 μm for transparent insulation wall applications. The investigated polymers are transparent in the solar range and strongly absorbing in the infrared range. Sufficient absorption in the infrared range is reached by using polymers with carbon-oxygen single bonds. The paper focuses on the characterisation of the ageing behaviour by thermoanalytical and mechanical methods. Useful details on overall ageing phenomena are obtained by thermal analysis. Mechanical characterisation of both, unnotched and notched specimens provides additional information on the nature of ageing, specifically with regard to the occurrence of local ageing and the development of defects.
Keywords: Ageing; Film; PC; PMMA; CTA; PET; Solar energy; DSC; Mechanical properties;