Degradation of pre-aged polymers exposed to simulated recycling: Properties and thermal stability

Accelerated thermal and photo-aging of four homopolymers, low-density polyethylene (LDPE), high-density polyethylene (HDPE), polypropylene (PP) and high-impact polystyrene (HIPS), was performed and the impact of subsequent reprocessing conditions on their properties studied. Polymer samples oven-aged at 100 °C for varying periods of time or UV irradiated in a Weather-o-meter (WOM) at λ = 340 nm were reprocessed in a Brabender plasticorder at 190 °C/60 rpm for 10 min. Chemical changes and the evolution of rheological and mechanical properties accompanying the gradual degradation of the individual polymers were monitored and evaluated (DSC, FTIR, colorimetric method, MFI, tensile impact strength). LDPE and HIPS were found to be more susceptible to thermo-oxidation than HDPE and PP, whereas HDPE and PP were affected to a greater extent by UV exposure; the crucial role here is being played by the stabilization of the studied resins. In HDPE the scission and crosslinking reactions competed both in thermo-and photo-degradation. In the case of LDPE, scission prevailed over branching during thermo-oxidation, whereas photo-oxidation of the same sample led predominantly to crosslinking. Abrupt deterioration of the LDPE rheological properties after one week of thermal exposure was suppressed by re-stabilization. The scission reaction was also predominant for PP during thermo-oxidation, and it took place even faster during UV exposure. In the case of HIPS a slight photo-degradation of PS matrix is accompanied by simultaneous crosslinking of the polybutadiene component.     

Influence of the chemical structure of polycarbonates on the contribution of crosslinking and chain scissions to the photothermal ageing

The relative importance of crosslinking and chain scissions reactions involved in the photothermal ageing of polycarbonates is shown to depend on the chemical structure of the polymer. The orientation of these reactions is linked to the modifications of the chemical structure resulting from photolysis, photooxidation and thermooxidation. In this paper, the effects of crosslinking and chain scission of tetramethyl bisphenol-A polycarbonate (TMPC) are determined by size exclusion chromatography (SEC) and gel fraction measurements. These evolutions are correlated to the modifications of the chemical structure observed by FTIR spectroscopy.The presence of the four ortho-methyl substituants on the aromatic rings accounts for the difference in the photothermal ageing of TMPC compared to bisphenol-A polycarbonate (PC). Tetramethyl substitution inhibits photo-Fries rearrangements in TMPC, but in contrast these methyl groups involve crosslinks under irradiation and are the source of a higher oxidizability of TMPC compared to PC. A proposal for the various routes accounting for the crosslinking and the formation of the oxidation products in TMPC is reported in this paper.     

Effect of DBPH and vacuum gamma radiation on metallocenic ethylene-1-hexene and ethylene-1-octadecene copolymers

Two different post reactor processes were compared, ⁶⁰Co vacuum gamma irradiation and chemical modification with 2,5-dimethyl-2,5-di(tert-butylperoxy)-hexane (DBPH), on two metallocenic copolymers. These copolymers have a similar molecular weight and crystallinity, but different side chain lengths and concentration of end vinyl groups. The influence of the crosslinking agents on the structure of the samples was studied using gel extraction, size exclusion chromatography (SEC), FTIR spectroscopy and differential scanning calorimetry (DSC). The analysis of the data indicates that crosslinking reactions predominated over scission reactions in all cases. At the same conversion, peroxide modified samples show higher crosslinking levels than irradiated samples. The modified polymers show a complex rheological behavior and an increment in their rheological properties due to crosslinking. FTIR data demonstrated a depletion of vinyl terminal groups with the increment of the absorbed dose and the peroxide concentrations applied. This depletion was more significant in the peroxide crosslinked samples. A mathematical model that accounts for scission and crosslinking reactions fitted well the experimental data.     

Photo-oxidation of polymers—III: Molecular weight changes in the photolysis and photo-oxidation of polystyrene

Changes of molecular weight distribution resulting from vacuum photolysis and photo-oxidation of polystyrene have been determined by gel permeation chromatography. On irradiation at 253.7 nm, crosslinking is predominant in vacuo; in the presence of oxygen, crosslinking and main chain scission occur simultaneously because of light absorption by two or more different chromophores. Main chain scission is more important than crosslinking in the photolysis and photo-oxidation of polystyrene containing cumene hydroperoxide irradiated at 313 nm or containing benzophenone irradiated in the range 320–420 nm. In this last case, main chain scission and crosslinking are both strongly inhibited if the sample contains napthalene which acts as quencher of the excited triplet state of benzophenone. Only moderate inhibition is observed in the presence of 2,6-di-t-butyl-4 methylphenol.     

Preparation of polypropylene (PP)/ethylene octene copolymer (EOC) thermoplastic vulcanizates (TPVs) by high energy electron reactive processing

Thermoplastic vulcanizates (TPVs) based on 50/50 composition of PP/EOC blend were prepared by electron induced reactive processing. To facilitate dynamic crosslinking in the PP/EOC blend, a 1.5 MeV electron accelerator was directly coupled to an internal mixer to induce chemical reactions via high energy electrons under dynamic conditions of melt mixing process. This kind of setup has been conceptualized for the first time in our laboratory and termed as electron induced reactive processing (EIReP) technique. Mechanical, morphological, and rheological properties of PP/EOC TPVs were studied with special reference to the exposure time (16–64 s) keeping absorbed dose (100 kGy) and electron energy (1.5 MeV) invariable. Chain scission dominates over chain crosslinking in both EOC as well as PP phases with the increase in exposure time. The primary factor is found to be the predominance of oxidative degradation during electron induced reactive processing in air atmosphere. The above observation was supported by Fourier Transform Infrared analyses and gel content values. Furthermore, it was found that mechanical properties depend not only on the extent of degradation in the blend system but also on the state and the mode of dispersion of the blend components.     

Stability of filled poly(dimethylsiloxane) and poly(diphenylsiloxane-co-dimethylsiloxane) elastomers to cyclic stress at elevated temperature

The response of aluminum oxide-filled poly(dimethyl siloxane) and poly(diphenylsiloxane-co-dimethylsiloxane) elastomers, containing 3–24 mol % diphenylsiloxane, to cyclic stress at elevated temperatures (dynamic creep) was evaluated. The materials could be divided into two classes, based on their response to the application of cyclic stress: no or low-diphenylsiloxane content elastomers in which substantial creep and a decrease in crosslink density were observed, and high diphenylsiloxane content (16–24 mol %) elastomers that showed decreased creep with increasing diphenylsiloxane content and an increase in crosslink density. It was suggested that the phenyl groups stabilize the siloxane bond in the polymer backbone, decreasing the rate of chain scission reactions as the diphenylsiloxane content increases and stabilizing the elastomer against creep. The balance of chain scission, chemical crosslinking, and cyclic formation reactions varies depending on diphenylsiloxane content, giving rise to the differences in dynamic creep behavior. An activation energy of 12.9 kcal/mol was measured for dynamic creep of poly(16% diphenylsiloxane/84% dimethyl siloxane), suggesting that a catalyzed degradation mechanism was responsible. The primary catalysts of the degradation reactions are postulated to be the filler particles. © 1996 John Wiley & Sons, Inc.     

Radiation aging and chemi-crystallization processes in polyoxymethylene

The radiochemical degradation of a polyoxymethylene homopolymer (POM) was used to study the effects of molar mass changes in the crystalline structure. The dose rate was 20 kGy h⁻¹ with doses of up to 30 kGy used. Both WAXS and SAXS were used to analyse the structures. Results showed that, under irradiation, the polymer undergoes random chain scission. The radiochemical yield was found to be G = 1.6 chain scission events per 100 eV. It was found that no crosslinking occurs and that only one chain scission mechanism, leading to the formation of formate groups, operates. Proof for the existence of chemi-crystallization is evidenced by (i) an increase in the crystallinity ratio as well as (ii) a decrease in the amorphous layer thickness. Simple models, derived from Rault’s theory, are used to predict both (i) and (ii) from molar mass values.     

超支化聚乙烯接枝马来酸酐及其过程中交联和降解反应的抑制

采用后过渡金属催化剂二-(2,6-二异丙基苯基)丁二亚胺溴化镍(IBNB),通过均相聚合法合成了高分子量和高支化度的超支化聚乙烯(HBPE).以二特戊基过氧化物(DTAP)为引发剂,研究了HBPE接枝马来酸酐(MAH)反应,并用N,N-二乙基肉桂酰胺(DECA)作为抑制剂,有效地抑制了接枝反应体系中的交联和降解副反应.接枝反应体系中分别加入相同摩尔量的DECA、N,N-二甲基乙酰胺、苯乙烯、3-(2-呋喃)丙烯酸、肉桂酸乙酯,通过比较接枝产物的凝胶含量和分子量,发现DECA抑制交联和降解副反应的效果最好,同时解释了DECA的作用机理.     

Photodegradation of poly(3-hydroxybutyrate)

The effect of ultraviolet radiation on the properties of poly(3-hydroxybutyrate) (PHB) was studied. The PHB investigated is produced from microbial fermentation using saccharose from sugarcane as the carbon source to the bacteria. The material was exposed to artificial UV-A radiation for 3, 6, 9 and 12 weeks. The photodegradation effect was followed by changes of molecular weight, of chemical and crystalline structures, of thermal, morphological, optical and mechanical properties, as well as of biodegradability. The experimental results showed that PHB undergoes both chain scission and crosslinking reactions, but the continuous decrease in its mechanical properties and the low amount of gel content upon UV exposure indicated that the scission reactions were predominant. Molar mass, melting temperature and crystallinity measurements for two layers of PHB samples with different depth suggested that the material has a strong degradation profile, which was attributed to its dark colour that restricted the transmission of light. Previous photodegradation initially delayed PHB biodegradability, due to the superficial increase in crystallinity seen with UV exposure. The possible reactions taking place during PHB photodegradation were presented and discussed in terms of the infrared and nuclear magnetic resonance spectra. A reference peak (internal standard) in the infrared spectra was proposed for PHB photodegradation.     

Photochemistry of ring substituted polystyrenes—Part III: Photolysis of poly(para-methoxystyrene)

The photodegradation of films (4 × 10^?4 cm thick) of poly(p-methoxystyrene) with 254 nm radiation under high vacuum at 25°C has been studied. The principal gaseous product is hydrogen, but smaller quantities (in decreasing yield) of methane, methyl alcohol and ethane are also formed, indicating that fission of bonds in the para methoxy group is also involved. Ultraviolet and visible spectra of degraded films indicate the presence of unsaturated groups and of coloured species. Solubility data indicate that crosslinking and chain scission occur simultaneously. Rates of chain scission of a number of p-substituted styrenes are compared and a reasonable correlation between these and the electron donating character of the para group is observed, explainable in terms of the stabilising effect of such groups on the radicals formed during chain scission. Rates of crosslinking are greater than those for polystyrene and this is attributable to the participation of the substituted phenoxy radicals (formed by CH₃O fission) in addition reactions. Quantum yields for the gaseous products and for chain scission and crosslinking have been determined and a mechanism has been advanced to account for the experimental data.     

Effect of lubricants on the photo-oxidative degradation of solid poly(vinyl chloride)

Various commercially employed lubricants, such as low molecular weight polyethylene (Type PA-520 Wax), hydroxystearic acid (Loxiol G-21), butyl stearate (Loxiol G-32) and di-stearyl phthalate (Loxiol G-60), have been examined for their active rôle in the photo-oxidative degradation of poly(vinyl chloride) (PVC). The results show that low molecular weight polyethylene has an obvious accelerating effect on the photo-oxidative degradation of PVC, whereas other lubricants reduce the effect of photo-oxidation. None of the lubricants used has an obvious rôle in the chain scission and crosslinking of PVC.     

Polymer morphology and random chain scission of nylon 66

Nylon 66 films with varying spherulite size but almost constant percentages of crystallinity were prepared (melt method). These films were degraded by NO₂ over a range of temperatures from 35 to 65°C. Random chain scission took place except in the initial stages at low temperatures at which some crosslinking occurred. Observation of the films with the extent of degradation under polarized light revealed that dark bands developed around and also inside spherulite boundaries that became wider with the extent of degradation. This indicates that amorphous material is formed during random chain scission; the spherulites remained practically intact, however. The experimental chain scission rate constants did not change essentially with spherulite diameter until small diameters were reached, at which time the rate constants increased noticeably. Degradation can be accounted for by chain scission in amorphous and interfacial regions; in the latter the rate constants increased with this area and in addition main chain links were weaker in fold regions, due to strain energy, than normal ones in amorphous regions. The energy of activation for chain scission was compatible with a predominantly diffusion-controlled process.     

Mechanism of thermal degradation of urea-formaldehyde polycondensates

The thermal degradation to 500°C of urea-formaldehyde polycondensate occurs in four successive steps. In each step, partial volatilisation takes place while the polymer undergoes chemical modification to give progressively more stable structures.Below 200°C methylene ether bridges are transformed into methylene bridges and branching and crosslinking reactions occur with maximum rates at 125°C and 165°C, respectively. Above 200°C radicals formed by chain scission induce the formation of cyclic structures in the polymer which undergoes extensive fragmentation above 300°C. Water, formaldehyde, carbon monoxide and dioxide, methane, ammonia, monomethylamine and trimethylamine are the gaseous products evolved.By combining data on the chemical modifications and gases evolved in each step, reaction mechanisms are proposed.     

Photodegradation of tetramethylpolycarbonate (TMPC): Correlation of properties with chemical modifications

The consequences of tetramethylpolycarbonate (TMPC) photoageing (λ > 300 nm) on the surface modifications of the material have been analysed. Roughness and stiffness measurements were performed using AFM (Atomic Force Microscopy). Gel fraction measurements, DMTA (Dynamic Mechanical Thermal Analysis) measurements and infrared analyses were also performed. The results indicate that the three-dimensional network forms as a result of crosslinking reactions. The modifications of the properties measured using each technique were followed as functions of the irradiation time, and the influence of oxygen was characterised. The surface modifications are explained in light of the modifications of the chemical structure. Quantitative correlations were obtained between the main relevant criteria characterizing the surface degradation from the chemical structure to the mechanical properties.     

Thermal rearrangement study of low molecular weight polybutadiene

Various polybutadienes (PBDs) of low molecular weight were heated below complete crosslinking at 250 °C under anaerobic nonpyrolytic conditions, and the structural changes were investigated. The predominant crosslinking reactions arise from the presence of 1,2-vinyl isomer and the most important one is intermolecular reaction accompanied with methyl group formation. The analysis showed that two crosslinking types as well as two types of methyl groups have been produced in which one was the result of 1,2-vinyl isomer of one chain crosslinked via methylene carbon of another chain of cis or trans isomer, and the second methyl group was the product of the reaction between 1,2-vinyl isomers of two PBD chains. Chain scission also occurred in two pathways due to the presence of 1,2-vinyl isomer, scission at two adjacent 1,2-vinyl isomer and scission at adjacent 1,2-vinyl with cis or trans isomer giving rise to methyl carbons.     

Processing of poly(lactic acid): Characterization of chemical structure, thermal stability and mechanical properties

The processing of poly(lactic acid) (injection and extrusion/injection) as well as annealing of processed materials were studied in order to analyze the variation of its chemical structure, thermal degradation and mechanical properties. Processing of PLA was responsible for a decrease in molecular weight, as determined by GPC, due to chain scission. The degree of crystallinity was evaluated by means of differential scanning calorimetry and X-ray diffraction. It was found that mechanical processing led to the quasi disappearance of crystal structure whereas it was recovered after annealing. These findings were qualitatively corroborated by means of FTIR. By analyzing ¹H NMR and ¹³C NMR chemical shifts and peak areas, it was possible to affirm that the chemical composition of PLA did not change after processing, but the proportion of methyl groups increased, thus indicating the presence of a different molecular environment. The thermal stability of the various materials was established by calculating various characteristic temperatures from thermograms as well as conversion and conversion derivative curves. Finally, the mechanical behaviour was determined by means of tensile testing (Young modulus, yield strength and elongation at break).     

Mechanisms of chain scission and the role of oxidation products in the oxidative radiolysis of ethylenepropylene copolymer

Many works have been devoted to the photo-oxidation of polyolefins but numerous questions remain. The aim of this paper is to constitute a complementary source of informations leading to the radiation oxidation mechanism of these polymers which could explain the chain scission process the basis of the alteration of mechanical and physical properties of polyolefins. It can be seen from our results that the hydroperoxides and the ketones are primary products of the reaction. A new secondary reaction efficiently leading to the alteration of mechanical properties has been proposed.     

Degradation mechanism of poly(ether‐urethane) Estane® induced by high‐energy radiation. II. Oxidation effects

To predict long‐term polymer behavior during a nuclear waste storage time, radiation effects on a segmented aromatic poly(ether‐urethane) induced by high‐energy radiation under oxygen atmosphere were investigated. To obtain a predictive model of polymer radio‐oxidation during several centuries, the first step consists to elaborate the elementary degradation mechanisms. Thus, electron paramagnetic resonance (EPR), Fourier transform infra‐red spectroscopy (FT‐IR), electrospray ionisation‐mass spectrometry (ESI‐MS), and gas mass spectrometry were carried out to identify radicals, chemical modifications, and gases to reach the radio‐oxidative mechanism at doses inferior than 1000 kGy. Degradation mainly occurs at urethane bonds and in polyether soft segments that produces stable oxidative products as formates, alcohols, carboxylic acids and H₂, CO₂ and CO gases. Predominant degradation occurred at polyether soft segments and crosslinking is in competition with scission. On the basis of the results, a mechanism of degradation for aromatic poly(ether‐urethane) is proposed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 861–878, 2008     

Effect of gamma radiation on chlorobutyl rubber vulcanized by three different crosslinking systems

The development of halogenated butyl rubber (chlorobutyl) in the 1950s and 1960s greatly extended the usefulness of butyl. Their properties allowed the development of more durable tubeless tires with the air retaining innerliner, chemically bonded to the body of the tire. Tire innerliners are by far the largest application for halobutyl. When polymers are subjected to high energy radiation, a number of chemical reactions may occur following the initial ionization and excitation events. These reactions lead to changes in the molecular weight of the polymer through scission (S) and crosslinking (X) of the molecules and affect the physical and mechanical properties. In the halobutyl rubbers the chain scission may predominate. This work aims to show effects of gamma radiation in properties of chlorobutyl rubbers vulcanized with sulfur, sulfur donor and phenolic resin. The butyl rubber has been already studied by us previously. The samples were characterized before and after irradiation. Gamma radiation doses used were: 25 kGy, 50 kGy, 100 kGy, 150 kGy and 200 kGy, in order to identify which cure system is the most stable under irradiation. In this study we observed that the properties of all samples were affected irrespective of the vulcanization system.     

Effects of electron beam irradiation on poly(butylene adipate)diol

Effects of electron beam (EB) irradiation on poly(butylene adipate)diol (PBAD) were studied by means of GPC, DSC, and X-ray diffractometry. Below 5 Mrad, chain scission predominantly occurs, while above 10 Mrad, crosslinking and chain scission take place in parallel. Structure of EB-irradiated PBAD is mainly characterized by the main reactions, degradation and crosslinking. Crystallinity of PBAD increased by EB irradiation. This phenomenon was explained by reorganization due to high molecular mobility of EB-irradiated PBAD. But, increment of crystallinity decreased with increasing dose because of formation of crosslinking, excessive degradation and thermal effect of EB. As the result, the crystallinity of EB-irradiated PBAD with a high dose becomes lower than that of original PBAD by thermal treatment.