Polymer Degradation and Stability (v.75, #1)

Mechanical recycling of post-consumer plastics gives rise, in most cases, to secondary materials with mechanical properties worse than those of the reclaimed polymer. This is mostly due to the thermomechanical treatment undergone during the melt reprocessing operations. In this work, low density polyethylene films have been photo-oxidized in artificial accelerated ageing conditions and then the brittle films have been melt reprocessed. It is shown that the secondary material, after reprocessing, shows mechanical properties, in particular elongation at break, better than those of the photo-oxidized films. This behaviour has been attributed to the fact that the melt reprocessing has the effect of homogenizing the various defects resulting from photo-oxidation. These defects are present in different concentrations varying with the distance from the outer layers of the films as a result of oxygen starvation effects. Because the heterogeneity in the distribution of these various defects is mostly responsible for the failure of the samples, reprocessing causes an improvement of the ultimate properties of the recycled material.
Keywords: Recycling; Ageing; Mechanical properties; Defects homogenisation;

A series of poly (ester-urethane-urea)s containing pendant acrylate and methacrylate functionality has been prepared in the presence of the inhibitors of double bond polymerization and without. Introducing diamine and methacrylic structural units extends the hard urethane segments. The effect of structural segments on the thermal stability of curable poly(ester urethane urea) acrylates was studied. A series of compounds, where one segment of the molecular structure was varied, was tested for thermal stability. Increasing the length of the soft segment increases the thermal stability within a given series of compounds. The thermal oxidative stability of the polyurethane acrylate curing system was measured by TGA under air atmosphere. The decomposition of polyurethane acrylates and polyurethane methacrylates has been compared. Polyol-based urethane urea acrylates were found to be more stable than their amine counterparts. It was verified that the presence of urea groups and longer soft segments promoted an increase in the thermal stability. The oligomer structure and presence or not of the inhibitor of double bond polymerization, all afect the thermal stability of the cured coatings.
Keywords: Polyesters; Polyurethane-acrylate; Thermal stability;

The photochemical and thermal behaviour of bisphenol-A polycarbonate (PC) and trimethylcyclohexane–polycarbonate (TMC–PC) have been compared. The ageing of films, irradiated at short (λ=254 nm) and long (λ>300 nm) wavelengths in the absence and in the presence of oxygen or thermo-oxidised at 170 °C, has been analysed by different spectroscopic and chromatographic methods. A dual photochemistry is shown to account for the photodegradation of TMC–PC, as previously reported for PC. Under excitation at the shortest wavelengths, the mechanism involves photo-Fries rearrangements of the aromatic carbonate units and a photo-induced oxidation of the aliphatic moieties. Under excitation at 254 nm, the second photo-Fries rearrangement is lowered in TMC–PC at the expense of the formation of yellowing structures. Moreover, under short and long wavelengths exposures, the rate of photo-oxidation of TMC–PC was observed to be higher than that of PC. Such effects have been attributed to a reduced mobility of the macromolecules and to a steric effect due to the trimethylcyclohexylidene structure that contains tertiary carbon atoms. Experimental results confirmed that, regarding oxidation which initially involves hydrogen abstraction, tertiary and secondary sites of TMC–PC are more oxidisable than the primary aliphatic ones contained in PC.
Keywords: Bisphenol-A polycarbonate; Trimethylcyclohexane–polycarbonate; Photo-oxidation; Thermo-oxidation; Photolysis;

The thermal degradation and charring of larch lignin and larch lignin treated with boric acid, guanyl urea phosphate, ammonium phosphate and ammonium polyphosphate in the condensed phase, were investigated by a combination of thermogravimetry (TGA) and X-ray photoelectron spectroscopy (XPS). TGA data under nitrogen indicated that larch lignin had a very high char forming ability. Data from both TGA and XPS demonstrated that boric acid, guanyl urea phosphate and ammonium dihydrogen phosphate did not show an obvious char forming effect. This is because larch lignin is highly crosslinked by carbon-carbon bonds between guaiacyl propane units, which could be further proven by data showing low binding energy at room temperature compared with manchurian ash lignin reported in our previous work. In air, larch lignin completely decomposed at 500 °C, however, the flame retardants, especially ammonium polyphosphate, could promote char formation compared with the untreated larch lignin. The experimental results demonstrated that ammonium polyphosphate is a very effective char forming agent under both nitrogen and air for lignin, which is attributed to the catalytic crosslinking effect of polyphosphoric acid produced from ammonium polyphosphate decomposition.
Keywords: Larch lignin; Thermal degradation; Charring flame retardants;

Accelerated degradation of poly( l-lactic acid) (PLLA) fibres was carried out in phosphate buffered saline (PBS) (pH 7.4) and a dilute NaOH solution (pH 11.0) at 80 °C for up to 7–8 days. Characterization of the viscosity-average molecular weight, the mass loss, the thermal behaviours, the morphology and the tensile properties of the PLLA fibres was performed. The results suggested that the PLLA fibres degraded in PBS followed a typical bulk degradation mechanism, but the PLLA fibres degraded in the dilute NaOH solution showed both bulk degradation and surface erosion. The melting temperature of the fibres measured by differential scanning calorimetry decreased notably in both media, while the crystallinity increased. Scanning electron microscopy showed many microcracks on the fibre surfaces across the fibre axis after degradation. Apparently because of these morphological defects and the sharp drop of the molecular weight, the fibres lost their tensile strengths almost completely after 5 days of accelerated degradation. By that time, the mass of the fibres had begun to decrease. The results generated in the accelerated degradation of fibres in PBS as well as in dilute NaOH at 80 °C could have useful implications for the in vitro degradation of PLLA fibres at 37 °C.
Keywords: Poly(l-lactic acid) fibres; Accelerated degradation; Phosphate buffered saline; Dilute alkaline solution;

Polyethylene processing in air yields hydroperoxide groups in a non-homogeneous way. In the initial stages of processing, the hydroperoxide groups were formed in separated clusters. In these clusters the ratio of associated hydroperoxides to total hydroperoxides was constant. In advanced stages of processing, the additional clusters formed were overlapping increasingly with the clusters produced previously. The resulting distribution of hydroperoxide groups was examined from a theoretical and an experimental point of view. The hydroperoxide groups were found to be associated hydroperoxides. The free hydroperoxides detected by IR spectroscopy had to be seen essentially as “statistically free” hydroperoxide groups. There was no significant amount of “isolated” hydroperoxides. The additional associations resulting from cluster overlapping were well accounted for by a Poisson-type equation. The physico-chemical nature of the hydroperoxides is of prime importance for the thermolysis reactions. It was found that the dependency of the amount of hydroperoxides associated with other hydroperoxides on the total amount of hydroperoxides involves both a linear and a quadratic term. It might explain, at least in part, the complexity of the hydroperoxide decomposition reactions.
Keywords: Polyethylene; Thermal oxidation; Hydroperoxides; Geminate termination; Polymer melt; Free hydroperoxides; Associated hydroperoxides; Hydroperoxide distribution;

Free radical degradation of chitosan with potassium persulfate by Shih-Chang Hsu; Trong-Ming Don; Wen-Yen Chiu (73-83).
A thermal dissociation initiator, potassium persulfate (KPS), is added to the chitosan solution at 70 °C; immediately, the solution viscosity and the molecular weight of chitosan decrease in a very short time. Size exclusion chromatography, nuclear magnetic resonance and electron spin resonance were used to study the degradation mechanism. A free radical degradation mechanism of chitosan by KPS is then proposed. When KPS is thermally dissociated into anionic radicals, they are attracted to the cationic amino group in the chitosan ring. Subsequently, the anionic radical attacks the C-4 carbon and transfers the radical to the C-4 carbon by subtracting the hydrogen from it. The presence of free radical at C-4 carbon eventually results in the breakage of the glycosidic C–O–C bond in the chitosan main chain. According to this mechanism, the concentrations of KPS, total free radicals and the degraded chitosan chain at different degradation times are all calculated by solving the rate equations. Finally, the calculated average molecular weights of the degraded chitosan chains at different reaction times agree with the experimental values.
Keywords: Chitosan; Potassium persulfate; Free radical; Degradation;

Investigation of electron irradiation induced-changes in poly(vinylidene fluoride) films by Mohamed Mahmoud Nasef; Hamdani Saidi; Khairul Zaman M. Dahlan (85-92).
The radiation-induced changes taking place in poly(vinylidene fluoride) (PVDF) films exposed to electron irradiation were investigated in correlation with the applied doses. Samples were irradiated in air at room temperature by a universal electron beam accelerator to doses in the range of 100–1200 kGy. Various properties of the irradiated PVDF films were studied using FTIR, differential scanning calorimetry (DSC) and universal mechanical tester. Unirradiated PVDF film was used as a reference. Electron irradiation was found to induce changes in physical, chemical, thermal, structural and mechanical properties of PVDF films and such changes vary depending on the irradiation dose.
Keywords: PVDF; Electron irradiation; Structural changes; Thermal properties; Mechanical properties;

On the action of ozone on polyaniline by Franco Cataldo (93-98).
Polyaniline in the emeraldine-base form (PANI-EB) was ozonized in CHCl3 solution. The reaction with O3 was followed by electronic and FTIR spectroscopy. It is shown that the early stage of O3 attack on PANI causes the formation of radical cations or polarons in the polymer chain. Ozone treatment also causes formation of nitrite and nitrate groups. The chain degradation reaction seems to be directed preferentially toward the quinoneimine units. Quinoneimine units are already present in PANI-EB but under the oxidizing action of the ozone, additional units are formed. Thus, there is evidence for the formation of a pernigraniline-like structure. However, extensive quinoneimine ring cleavage occurs under the continuing action of ozone. The reactivity of O3 with PANI-EB appears to be rather low in comparison to the reaction speed with diene rubbers, like for instance, cis-1,4-polyisoprene.
Keywords: Polyaniline; Ozone; Radical cation; Degradation;

N-Methyl- and N-ethyl-substituted polyaniline (PANI) as well as 2-ethyl- and 3-ethyl-substituted PANI have been ozone treated in CHCl3 solutions. All the polyamines have been studied in their undoped state (emeraldine-base) and doped with camphorsulphonic acid (CSA). The reactions have been followed by electronic spectroscopy and in all cases radical cations (or polarons) were found to be the early reaction intermediate. Alkyl-substituted-PANI show higher reaction rate constants with O3 in comparison to unsubstituted PANI. In CHCl3 solution 3-ethyl-PANI reacts with O3 slower than cis-1,4-polyisoprene.
Keywords: Alkyl-substituted polyaniline; Ozone treatment; Radical cation; Undoped and doped state; Antiozonant activity;

Degradation of acrylic copolymers by Fenton's reagent by Carsten Mai; Andrzej Majcherczyk; Wiebke Schormann; Aloys Hüttermann (107-112).
The degradation of different copolymers of acrylamide and acrylic acid by Fenton's reagent was studied. The polymers tested were either homopolymers or copolymers containing lignin sulfonate, guaiacol or 3,4-dihydroxylbenzoic acid, respectively. Acrylamide copolymers (PAAm) were degraded faster than polymers of acrylic acid (PAA). Among the PAAm, the copolymers of lignin sulfonate and guaiacol were degraded at a significantly higher rate than the corresponding homopolymer, whereas among the PAA, the rate of degradation was highest with copolymers of guaiacol and 3,4-dihydroxylbenzoic acid. The decrease of H2O2, i.e. the rate of hydroxyl radical production in the presence of a certain polymer, did not correlate with the rate of its degradation. It was concluded that the incorporation of lignin and certain phenolic compounds into an acrylic chain may accelerate the decay of these polymers by wood decaying fungi, which reportedly produce hydroxyl radicals extra-cellularly, and through the use of advanced oxidation systems applied in sewage cleaning.
Keywords: Fenton's reagent; Lignin copolymer; Acrylic; Degradation;

Linear biodegradable polyurethanes with varying ratios of the hydrophilic-to-hydrophobic segment were obtained in the study. The hydrophilic segment was based on poly(ethylene oxide-propylene oxide-ethylene oxide) diols (Pluronic®). The hydrophobic segment was based on poly(ϵ-caprolactone) diol. Viscosity-average molecular weights and the polydispersity index of the polyurethanes were in the range of 38.000–85.000 daltons and 1.2–3.2, respectively. The polymers absorbed up 3.9% of water depending on the chemical composition. Tensile strength, Young's modulus and elongation at break of the polymers were in the range of 11–46 MPa, 4.5–91 MPa and 370–960%, and the glass transition and soft segment melting temperatures were −60 to−21.5 °C and 30–55 °C, respectively. Degradation in vitro caused 2% mass loss, 15–80% reduction of molecular weight and slight reduction of polydispersity at 48 weeks. The extent of degradation was dependent on the polymer composition and the hydrophilic segment content. At a comparable degradation time the materials containing Pluronic® were degraded most. Degradation of polyurethanes caused insignificant changes of the pH of the medium.
Keywords: Polyurethanes; Biodegradable; Pluronic®; In vitro degradation;

The oxidation of 100 μm stabilised polypropylene films has been studied at 130 °C in air, using physico-chemical and mechanical characterisation methods. During the induction period, where no carbonyl build-up is observable by IR spectroscopy, both stabiliser consumption (UV spectrophotometry) and molecular weight decrease (steric exclusion chromatography) can be observed. These structural modifications induce a decrease of fracture properties as determined using the essential work of fracture (EWF) method whereas tensile ultimate elongation does not display significant variations. The results clearly indicate the interest of the EWF method to characterise degradation induced embrittlement of initially ductile polymers. The experimental data gathered in this work bring some new elements of discussion about the problem of spatial heterogeneity of the oxidation process.
Keywords: Polypropylene; Oxidation; Induction period; Embrittlement;

Hydroperoxides generated on polyethylene processing in air show complex behavior. The maximum hydroperoxide concentration reached in the experiments decreases significantly with increasing processing temperature. In the temperature range 150–160 °C, the maximum of the hydroperoxide concentration is determined by preferential reaction of the peroxy radicals with the hydroperoxides accumulating in the melt. Abstraction of the tertiary hydrogen atom of the secondary hydroperoxide by a peroxy radical yields a new hydroperoxide group. Simultaneously the original hydroperoxide is transformed into an α-alkyl-hydroperoxy radical. Monomolecular decomposition of the last yields a ketone and a hydroxyl radical continuing the chain reaction. Thus, the hydroperoxide group destroyed is replaced by another one so that the concentration remains practically constant for some time, especially at 150 °C. In the temperature range 170–200 °C, the maximum of the hydroperoxide concentration is determined by bimolecular decomposition of hydroperoxide groups sufficiently close. This is a consequence of initial non-homogeneous distribution of the hydroperoxides in the melt. It is caused by “clusters” of hydroperoxide groups resulting from the reactions initiated by a pair of primary radicals before geminate termination. A concerted reaction involving two hydroperoxide groups as well as the tertiary hydrogen atom of one of the hydroperoxide groups accounts for the data. The mechanisms and kinetics of the reactions are discussed.
Keywords: Polyethylene; Thermal oxidation; Processing; Hydroperoxides; Thermolysis; Reactions; Mechanisms; Kinetics;

Study of the effect of gamma irradiation on polyolefins—low-density polyethylene by João Carlos Miguez Suarez; Elisabeth Ermel da Costa Monteiro; Eloisa Biasotto Mano (143-151).
Many of the physical, chemical and utility properties of polymer materials can be altered by means of high energy radiation such as gamma-rays, neutrons and electron beams. In the present work the exposure to gamma radiation of a bulk virgin low density polyethylene (LDPE) was carried out at various doses up to 2000 kGy. The study of the irradiation effects on the material properties has been make by different methods in an integrated way. The experimental data indicate that the gel content, the hardness and the yellowing increase with gamma radiation dose and that the LDPE gamma irradiation process involves crosslinking at lower doses and chain scission at higher doses. The final results show the reliability of gamma radiation as a practical method for the control of long-term properties.
Keywords: Low-density polyethylene; Gamma radiation; Degradation; Yellowing;

The weathering behaviour of poly(ethylene terephthalate) was investigated. Samples produced by bi-axial extrusion were exposed in the laboratory for periods of up to ∼1100 h. Films produced with different types of stabilizers (a ultraviolet absorber, carbon black and a mixture of TiO2 and BaSO4) were tested for mechanical properties and by UV-visible spectroscopy, titration, size exclusion chromatography and Fourier transform infrared spectroscopy. The results indicated that the unstabilized films are very susceptible to degradation, causing a large deterioration in mechanical properties and reduction in molecular weights. Of the stabilizers used the UV absorber was shown to be the most effective.
Keywords: PET; Weathering; Photodegradation stabilizers;

Conversion of polymers to fuels in a refinery stream by Suat Uçar; Selhan Karagöz; Tamer Karayildirim; Jale Yanik (161-171).
The purpose of this study was to investigate the processability of LDPE, PP, PVC/LDPE and PVC/PP in the hydrocracking unit of a refinery. For this, LDPE or PP has been added to vacuum gas oil (VGO). The blends were hydrocracked over different catalysts using a batch autoclave at 425–450 °C under hydrogen atmosphere. The catalysts used were HZSM-5, Cobalt loaded active carbon (Co-Ac) and DHC-8 (commercial silica-alumina catalyst). Addition of polymer to VGO affected the cracking of VGO, leading to a decrease in the gas yield and an increase in the liquid yield. In hydrocracking over HZSM-5, the product distribution was similar for two blends. HZSM-5 gave higher gas yields than the other catalysts. When using Co-Ac and DHC-8 catalysts, the gas and liquid yields depend on the polymer type as well as temperature. In the case of the PVC-containing blends (PVC/PP/VGO or PVC/PE/VGO), the blends were firstly dechlorinated at 350 °C. Then the dechlorinated mixture was hydrocracked in the 400–450 °C range in the presence of DHC-8. HCl evolved by degradation of PVC during the dechlorination step partially degraded PE and PP and these predegraded polymers were more easily cracked in the hydrocracking step. In addition, we observed the effect of polyene formed from the PVC degradation residue on the thermal hydrocracking. The chlorine compounds in the hydrocracked feed affected the catalytic activity of the catalysts.
Keywords: Hydrocracking; Polyethylene; Polypropylene;

The relationship between char formation and temperature in a range of polymer blends containing acrylonitrile–butadiene–styrene (ABS) and chlorinated poly(vinyl chloride) between 500 and 700 °C has been thoroughly investigated. The results show that for all the ABS/CPVC blends studied, both in the presence and absence of the smoke suppressing compound FeOOH, there is a strong linear relationship between char yield and the temperature at which the char is formed. Having the smoke suppressant present in these systems does not affect the linearity of the char/temperature relationship or reduce the high values of the correlation coefficients. Increasing both the amounts of chlorine and iron compound in the blends increases char yields at all temperatures but the most important factor which controls the yield of char is the actual amount of chlorine (i.e. CPVC) present in both iron-free and iron-containing blends. The authors have been able to explain the very significant smoke suppressing effect which the iron compound has in these blends in terms of iron/chlorine compounds formed in situ and also to show how the maximum smoke suppressing effect occurs at a chlorine content of about 20% in the blends. The research has also demonstrated, for the first time, that the formation of char is directly related to the formation and suppression of smoke in these polymer blends.
Keywords: Flammability; Smoke formation; Char yield; ABS/CPVC blends; Smoke suppressant;

Study on methanolytic depolymerization of PET with supercritical methanol for chemical recycling by Yong Yang; Yijun Lu; Hongwei Xiang; Yuanyuan Xu; Yongwang Li (185-191).
Methanolytic depolymerization of polyethylene terephthalate (PET) was carried out in a stainless stirred autoclave at temperatures of 523–543 K, pressures of 8.5–14.0 MPa, and with a weight ratio of methanol to PET from 3 to 8. The solid products mainly composed of dimethyl terephthalate and small amounts of methyl-(2-hydroxyethyl) terephthalate, bis (hydroxyethyl) terephthalate, dimers and oligomers were analyzed by high performance liquid chromatography (HPLC). The liquid products composed of ethylene glycol and methanol were analyzed by gas chromatography (GC). It was found that both the yield of dimethyl terephthalate and the degree of PET depolymerization were seriously influenced by the temperature, weight ratio of methanol to PET, and reaction time, whilst the pressure has insignificant influence when it is above the critical point of methanol. The optimal depolymerization conditions are temperature of 533–543 K, pressure of 9.0–11.0 MPa, and the weight ratio (methanol to PET) from 6 to 8. The depolymerization of several PET wastes collected from the Chinese market was investigated under the optimal conditions.
Keywords: Polyethylene terephthalate (PET); Chemical recycling; Supercritical methanolysis; Depolymerization;

Relating laboratory and outdoor exposures of acrylic melamine coatings by Jonathan W Martin; Tinh Nguyen; Eric Byrd; Brian Dickens; Ned Embree (193-210).
A cumulative damage model and a laboratory apparatus are described for linking field and laboratory photodegradation results and for predicting the service life of polymeric materials exposed in the laboratory and field. The apparatus was designed to independently and precisely monitor and control in both space and time the three primary weathering factors causing polymeric materials to degrade when exposed in the field. These factors include temperature, relative humidity, and spectral ultraviolet radiation. A model acrylic melamine coating was exposed in the laboratory apparatus to each of 12 different spectral wavebands and four temperature and four relative humidity environments. The spectral dosage and material damage were measured for each exposure treatment and this data input into the cumulative damage model from which estimates of the spectral quantum yield were made. Variables affecting the accuracy of the measurements are discussed.
Keywords: Polymer coating; Cumulative damage model; Degradation; Environmental chamber; Exposure; Photodegradation; Quantitative; UV;