Polymer Degradation and Stability (v.79, #3)

Mechanical, thermal and microstructure evaluation of HDPE after weathering in Rio de Janeiro City by L.C. Mendes; E.S. Rufino; Filipe O.C. de Paula; A.C. Torres Jr. (371-383).
The weathering of high-density polyethylene (HDPE) specimens, with and without additives, submitted to natural atmosphere of Rio de Janeiro City, was studied and mechanical, thermal and structural changes were evaluated. The results indicated an impressive environmental oxidative degradation of non-stabilized HDPE. The impact resistance showed an abrupt drop up to 2520 h of aging and continued to decrease slowly after this time. The elongation at break also presented a pronounced drop and after 2520 h of exposure the value was close to zero revealing the loss of ductility up to this exposure time. We also observed a progressive increase in Young's modulus. The reduction of molecular weight and to some extent the increase of crystallinity were the main factors which caused the changes in properties. The microstructural FT-IR absorptions at 888, 909–991, 964, 1715–1740 cm−1 were detected and compared using a reference band (1368 cm−1). For non-stabilized HDPE, all these absorbances showed a progressive increase and after 3000 h of exposure, we observed an abrupt increase which could represent an auto-acceleration process of reaction of the photo-oxidation. The properties of stabilized HDPE remained practically constant at the same exposure time, revealing the effectiveness of the additives present in this sample.
Keywords: Outdoor weathering; High density polyethylene; Oxidation; Thermal and mechanical properties; Microstructure;

Degradation profile of polyethylene after artificial accelerated weathering by J.V. Gulmine; P.R. Janissek; H.M. Heise; L. Akcelrud (385-397).
Elucidation of the chemical changes that take place during the aging of polyethylene sheared electrical cables, and the correlation of these changes with physical properties need experiments with accelerated aging assays. These were carried out using the most representative polyethylene grades found in today's market: low density (LDPE), linear low density (LLDPE) and high density (HDPE). The samples were exposed to UV- and xenon arc radiation using a varying exposure time and different temperature cycles. The changes in the material structure and properties were analyzed by differential scanning calorimetry (DSC), scanning electron microscopy (SEM), FTIR-spectroscopy, density and hardness measurements. During degradation polar groups in the polyethylene are generated, as well as chain scission and cross-linking. As net effects of these processes respective increases in crystallinity, density and hardness and surface cracking at late stages of degradation are observed. By varying the angle of incidence in attenuated total reflection (ATR) experiments using FTIR-spectroscopy it was possible to detect variations in composition of distinct layers from the surface down to a depth of 1.2 μm. Deeper layers were reached by microtoming the samples in layers down to 60 μm. The main chemical modifications were carbonyl formation of various kinds which were identified in the FTIR–ATR spectra of degraded polyethylene samples.
Keywords: Polyethylene; Degradation; FTIR–ATR;

Photoetching and modification of poly(tetrafluoroethylene-co-hexafluoropropylene) polymer surfaces with vacuum UV radiation by J.X. Chen; D. Tracy; S. Zheng; L. Xiaolu; S. Brown; W. VanDerveer; A. Entenberg; V. Vukanovic; G.A. Takacs; F.D. Egitto; L.J. Matienzo; F. Emmi (399-404).
Films of poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) were exposed to radiation from 6.7 × 104 Pa He and Ar dc arc plasmas that were made to rotate inside of a graphite tube by the application of an auxiliary magnetic field. The films were covered with optical filters having different cut-off wavelengths to vary the vacuum ultraviolet (VUV) radiation that modified the fluoropolymer surface. With arc currents of 7 A and sample temperatures up to 110 °C, the photoetch rate of FEP was ca. 50% higher than for PTFE. Consistent with results for PTFE, treated FEP showed the following: (1) contact angles that started to decrease in the wavelength region between 173 and 160 nm and continued to decrease with shorter wavelengths; (2) surface roughening; (3) defluorination of the surface with a slight increase in the atomic%C and formation of C–C bonds in the top 3–5 nm of the surface as detected by XPS analysis; and (4) incorporation of oxygen, presumably from reaction with oxygen in air.
Keywords: Photodegradation; FEP; UV; VUV; Surface modification; Radiation;

Biodegradation of starch/polylactic acid/poly(hydroxyester-ether) composite bars in soil by R.L. Shogren; W.M. Doane; D. Garlotta; J.W. Lawton; J.L. Willett (405-411).
Injection molded tensile bars composed of native corn starch (0–70%), poly(d,l-lactic acid) (95% L) (PLA, 13–100%) and poly(hydroxyester-ether) (PHEE, 0–27%) were buried in soil for 1 year in order to study the effects of starch and PHEE on rates of biodegradation. Rates of weight loss increased in the order pure PLA (∼0%/year)<starch/PLA (0–15%/year)<starch/PHEE/ PLA (4–50%/year) and increased with increasing starch and PHEE contents. Weight losses were due to starch only with the degradation proceeding from outside to inside along a narrow zone. Tensile strength did not change with time for pure PLA and, after an initial decline, did not change much for the other compositions. Some formulations containing PHEE and lower (40%) starch levels had higher tensile strengths after initial exposure to soil than those without PHEE.
Keywords: Starch; Poly(lactic acid); Epoxy; Biodegradable;

Solvent effects on the lipase catalyzed biodegradation of poly (ε-caprolactone) in solution by G Sivalingam; Sujay Chattopadhyay; Giridhar Madras (413-418).
The effect of various solvents on the biodegradation of poly (ε-caprolactone) (PCL) was investigated at 45 °C using two different lipases: Novozyme 435 and Lipolase. The rate coefficients of polymer degradation and enzyme deactivation in acetone, toluene, benzene, benzyl alcohol, and acetone-water mixture were determined. Continuous distribution kinetics was proposed to model the deactivation of enzyme and polymer degradation and determine the rate coefficients. The effect of polarity and viscosity of the solvents on the degradation of the polymer was investigated. While the degradation rate decreased with viscosity, it increased with polarity for the solvents. The loss of enzymatic activity was less in Novozyme 435 compared to lipolase in non-aqueous solvents. Both the enzymes were sensitive to the amount of water present in the system and showed the maximum degradation of the polymers at an optimum value concentration of 8.7 wt.% of water in acetone.
Keywords: Enzymatic degradation; Poly (ε-caprolactone); Specific chain end scission; Solvent effects; Novozyme-435; Lipolase;

The effect of carbon dioxide (pH 4) on polyamide 11 hydrolysis was studied at 130, 140 and 160 °C in the vapour phase using a viscometric method to characterise the extent of reaction. The main effect of CO2 is to shift the hydrolysis equilibrium towards high conversions (low molar masses), but no significant accelerating effect was observed in the early period of exposure, indicating that catalysis of the reaction by H+ ions is negligible. A theoretical explanation was provided to explain why certain weak acids such as those resulting from hydrolysis of the polymer or CO2 have no accelerating effect on hydrolysis whereas others such as strong inorganic acids in high concentration accelerate hydrolysis. A kinetic model is proposed for the case of CO2 (and other similar weak acids) where it was assumed that the acids act only by amine scavenging, thus shifting the hydrolysis equilibrium. This kinetic model is based on a system of two differential (non linear) equations which needs numerical resolution. A method was proposed for this resolution. The agreement between experimental and theoretical curves is excellent.
Keywords: Polyamide 11; Hydrolysis; Kinetics; Acids; Carbon dioxide;

The mobility of small molecules in a glassy polymer is largely determined by the amount of free volume present in the material. The amount of free volume can be altered by changing the physical state of the polymer. Physical ageing reduces this amount, whereas the thermal rejuvenation increases it. The change in free volume was monitored by oxygen permeation and antioxidant sorption experiments. A clear correlation was found between the physical ageing on the one hand and oxygen permeability on the other. Since the mobility of antioxidants and oxygen are important parameters for the stabilisation of a polymer against oxidation, the physical state of the polymer can have a significant influence on the service life of the product.
Keywords: Antioxidant; Diffusion; Free volume; Oxygen permeability; Physical ageing;

PVC floorings as post-consumer products for mechanical recycling and energy recovery by Nazdaneh Yarahmadi; Ignacy Jakubowicz; Lars Martinsson (439-448).
Old PVC flooring materials obtained from three apartment blocks built in 1964, 1971, and 1974, respectively, have been examined in parallel with newly manufactured PVC floorings. The focus of this study was to investigate how the important properties of PVC floorings change during their service life owing to ageing processes, and how these can influence their suitability as post-consumer products for recycling methods such as mechanical recycling and energy recovery. One important objective was to obtain general information regarding the degradation processes in PVC floorings that could influence these recycling methods. Our study has shown that PVC floorings as plastic waste can be mechanically recycled in the form in which they were recovered without upgrading, and without the addition of new plasticizer. It is also shows that the high alkalinity of moist concrete can lead to the decomposition of the plasticizer when PVC flooring is glued onto it. However, the degree of decomposition of plasticizer is very small relative to the mass loss by evaporation, and consequently should not cause any problems for mechanical recycling. Nevertheless, decomposition products such as butanol and octanol can cause indoor environmental problems sometimes designated as “sick building syndrome”. For this reason, gluing directly onto fresh concrete should be avoided. Gluing also makes mechanical recycling less favourable owing to troublesome dismantling, and the high degree of contamination from the glue. The heat content in PVC floorings is dependent on the proportions of PVC and plasticizer used. Consequently, changes in the heat content caused by long-term use of PVC floorings should be insignificant.
Keywords: Plasticized PVC; Mechanical recycling; Accelerated ageing; Lifetime; Energy recovery; Emissions; Flooring;

This paper reports the results of studies on the thermal and ablative properties of the vulcanizates based on EPDM, maleated EPDM and nitrile rubber reinforced with melamine fibre, one of the recent generation high performance fibres. The thermogravimetric studies of the composites show that the presence of melamine fibre in the vulcanizates reduces the rate of decomposition and the effect is pronounced in the presence of the dry bonding system consisting of resorcinol, hexamethylene tetramine and silica. The first degradation step of the vulcanizate is controlled by the fibre, whereas the fibre as well as the matrix contributes to the second degradation step. An increase in fibre loading decreases the rate of degradation and weight loss in the second degradation step. The rate of decomposition of NBR vulcanizates is lower than those based on EPDM and maleated EPDM rubbers. The activation energy of decomposition of the vulcanizates is increased, if fibre is properly adhered to the matrix in the presence of the dry bonding system. The crosslinking system also affects the activation energy of decomposition, especially for the second degradation step. Melamine fibre causes significant reduction in the thermal erosion rate of the vulcanizates. The fibre filled composites, in the presence of the dry bonding system, display a lower thermal erosion rate compared to those containing no dry bonding system, showing that proper adhesion between the fibre and the matrix is important to achieve improved ablative properties. Among the three matrices, the vulcanizates based on nitrile rubber display the lowest thermal erosion rate.
Keywords: Melamine fiber; EPDM rubber; Maleated EPDM rubber; Nitrile rubber; Thermal degradation; Dry bonding system;

Experimental biodegradable medical polyurethanes with varying hydrophilic-to-hydrophobic segment ratios based on hydrophilic poly(ethylene oxide) MW=600 (PEO) and hydrophobic poly(ε-caprolactone) diol MW=530 and 2000 (PCL), were exposed to gamma radiation at the standard dose of 25 kGy used for sterilization. Irradiated polymers degraded to a various extent, this being associated with a reduction of mechanical properties. For the more hydrophobic polyurethanes based exclusively on polycaprolactone diol the decrease of molecular weight was in the range of 12–30% and the decrease of tensile strength was 12%. For the more hydrophilic polyurethanes based on mixtures of polycaprolactone diol and polyethylene oxide the decrease of molecular weight was in the range of 30–50% and the decrease of tensile strength was 50%. Gamma radiation insignificantly affected surface roughness of hydrophobic polycaprolactone-based polyurethanes and caused a slight increase of contact angle. For the hydrophilic polymers based on polycaprolactone diol and polyethylene oxide, gamma radiation increased by 36–76% surface roughness and decreased by 20–45% contact angle. There was also an evident change in thermal properties of the irradiated materials.
Keywords: Biodegradable polyurethanes; Gamma radiation; Molecular weight; Mechanical properties;

Biodegradable medical polyurethanes with varying hydrophilic-to-hydrophobic segment ratios based on hydrophilic poly(ethylene oxide) and hydrophobic poly(ϵ-caprolactone) diol, were treated with low-temperature, low-pressure plasmas of hydrogen peroxide, oxygen, carbon dioxide and ammonia. All samples treated with hydrogen peroxide plasma were sterile, while samples treated with oxygen, carbon dioxide and ammonia plasmas were nonsterile. The treatment caused a 7% drop in molecular weight and a 15% reduction in tensile strength for polyurethanes with a higher content of the hydrophobic segment; and a 27% reduction of molecular weight and a 20% decrease in tensile strength for the most hydrophilic materials. The hydrogen peroxide plasma extensively etched the surface of the materials, this being accompanied by a significant, 100–200% increase in the surface roughness. In contrast, the oxygen, carbon dioxide and ammonia plasma reduced the surface roughness of most hydrophilic PEO–PCL70–30 material, but minimally affected the surfaces of the remaining polymers. The hydrogen peroxide and oxygen plasmas increased the surface content of oxygen and decreased the content of nitrogen and carbon. The carbon dioxide plasma increased the carbon content, but decreased the oxygen and nitrogen content. The ammonia plasma decreased the nitrogen content, increased the carbon content and decreased the oxygen content. The surface contact angles slightly decreased for polyurethanes treated with oxygen and hydrogen peroxide plasmas, and increased for the samples treated with ammonia and carbon dioxide plasmas. Plasma treatment minimally affected the thermal characteristics of polyurethanes.
Keywords: Biodegradable polyurethanes; Plasma treatments; Surface modification;

The effect of mixing p-chloro-N-phenylphthalimide (p-Cl-NØPI) with the reference stabilizers: dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), barium–cadmium–zinc stearate (Ba–Cd–Zn stearate), n-octyltin mercaptide (n-OTM) or dibutyltin maleate (DBTM) on the stabilizing efficiency in thermally degradation of rigid PVC at 180 °C, in air, has been investigated. Mixing was effected in the range of 0–100 wt.% of p-Cl-NØPI relative to the reference stabilizers. The stabilizing efficiency is evaluated by measuring the length of the induction period (T s), the period during which no detectable amounts of hydrogen chloride gas could be observed, and also from the rate of dehydrochlorination as measured by continuous potentiometric determination on one hand, and the extent of discoloration of the degraded polymer samples on the other. The results reveal that mixing of the stabilizers improves the Ts values, decreases the rate of dehydrochlorination and lowers the extent of discoloration of the polymer. This improvement attains its maximum when the p-Cl-NØPI is mixed with any of DBLC, DBLS, Ba–Cd–Zn stearate or DBTM reference stabilizers in weight ratio of 25/75%, respectively. Mixing of p-Cl-NØPI with n-OTM is an exception, as maximum synergism was attained at equivalent weight ratio (50/50%).
Keywords: Poly(vinyl chloride); Thermal degradation; Discoloration; Potentiometric determination; Mixed stabilizers; Synergistic effect;

The effect of mixing 2-benzimidazolyl-ω-phenylpropenylidineacetonitrile (BPAN) with each of the reference stabilizers: dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), barium–cadmium–zinc stearate (Ba–Cd–Zn stearate), or dibutyltin maleate (DBTM) on the stabilizing efficiency in thermal degradation of rigid PVC at 180 °C, in air, has been investigated. Mixing was effected in the range of 0–100 wt.% of BPAN relative to each of the reference stabilizers. The stabilizing efficiency was evaluated by measuring the length of the induction period (T s), the period during which no detectable amounts of hydrogen chloride gas could be observed, and also from the rate of dehydrochlorination as measured by continuous potentiometric determination on the one hand, and the extent of discoloration of the degraded polymer samples on the other. The results show a true synergistic effect from the combination of BPAN with any of the reference stabilizers. Mixing of the stabilizers improves the T s values, decreases the rate of dehydrochlorination, and lowers the extent of discoloration of the polymer. The synergism reaches its maximum when the BPAN is mixed with either DBLS or Ba–Cd–Zn stearate in weight ratio of 25/75%, respectively. In case of DBLC or DBTM, the maximum synergism was attained when the stabilizers mixtures were in equivalent weight ratio (50/50%). The observed synergism may be attributed to the different mechanisms by which the investigated and the reference stabilizers work.
Keywords: Poly(vinyl chloride); Thermal degradation; Discoloration; Potentiometric determination; Mixed stabilizers; Synergistic effect;

Synergism between polymer antioxidants—kinetic modelling by J. Verdu; J. Rychly; L. Audouin (503-509).
A classical kinetic model, without the stationary state hypothesis, has been developed to simulate the behaviour of a hydrocarbon polymer of polypropylene type, stabilised by a mixture of a chain breaking antioxidant of hindered phenol type with a hydroperoxide decomposer of the organic sulphide or phosphite type. From a numerical solution of the system of differential equations representing the rate variations of the reactive species, one obtains kinetic curves of hydroperoxide build-up from which induction time can be determined. The variations of the induction time (at 140 °C in O2 excess) have been studied with the composition of the stabiliser mixture, the nature of the hydroperoxide decomposition process (unimolecular or bimolecular), the reactivity of the chain breaking antioxidant, and the rate of an “extrinsic” initiation process added to the POOH decomposition. Two main results were obtained: (i) The chosen model is able to simulate strong synergistic effects only due to action of both stabilisers at distinct levels of the radical chain (initiation for hydroperoxide decomposer and termination for the chain breaking antioxidant). (ii) The high stabilising efficiency of these systems is strongly linked to the “closed loop” character of oxidation, i.e. to the fact that the radical chain process generates its own initiator (POOH). When an “extrinsic” initiation process is added, this stabilising efficiency and the magnitude of synergistic effects decrease sharply.
Keywords: Oxidation; Kinetics; Antioxidants; Synergism;

Assessing biodegradability of plastics based on poly(vinyl alcohol) and protein wastes by Jaromı́r Hoffmann; Iveta Řeznı́čková; Jarmila Kozáková; Jan Růžička; Pavol Alexy; Dušan Bakoš; Lenka Precnerová (511-519).
Research was conducted into biodegradability of mixed polymer films based on poly(vinyl alcohol), protein hydrolyzate (collagen hydrolyzate from wastes after chrome tanning) and glycerol in an aqueous aerobic environment. Evaluation of biodegradation was based on carbon dioxide produced in the gas phase. Pure PVAL was degraded by a current mixed culture for water-treatment (unadapted) only after an approx. 10-day lag phase; during breakdown of mixed film the protein component and glycerol were broken down first and PVAL degradation occurred in the second stage. Biodegradation could be well described by 1st-order formal chemical kinetics. Repeated degradation by an adapted culture proceeded in a single stage with considerably shorter lag phase (<30 h) at a simultaneously approx. 1.5-fold greater breakdown rate (rate constants). During degradation of substrates containing PVAL, microbiological tests proved an approx. 100-fold increase in numbers of PVAL-degrading bacteria. Added protein hydrolyzate + glycerol in PVAL contributed to increasing biodegradability more than followed from proportional representation of individual components.
Keywords: Polyvinyl alcohol; PVAL; Protein; Blends; Extruded films; Plastics; Water environment; Biodegradability; Testing; Assessing;

Hygrothermal stability of dicyanate-novolac epoxy resin blends by Baochun Guo; Demin Jia; Weiwen Fu; Qinghua Qiu (521-528).
The chemical nature of the changes in a cyanate ester-novolac epoxy resin blend caused by hygrothermal ageing and the effects of the residue reaction in the blends on the hygrothermal ageing resistance were studied. The blends with epoxy molar fraction less than 0.5 absorb much less moisture in boiling water than does the homopolymer of the cyanate ester. The moisture uptake of the blend decreases with the extent of cure. The results from dynamic mechanical thermal analysis (DMTA) and Fourier transform infrared spectroscopy (FTIR) indicate that the blends possess superior hygrothermal stability compared to homopolycyanate. The results also indicate that the long-term hygrothermal ageing may cause substantial changes in the chemical nature of the blend when the cure extent is not sufficiently high.
Keywords: Cyanate ester; Novolac epoxy; Hygrothermal stability;

Aluminium triisopropoxide (AIP) promoted the methanolysis of polyethylene terephthalate (PET) to form monomers, dimethyl terephthalate (DMT) and ethylene glycol (EG), in an equimolar ratio. The methanolysis at 200 °C in methanol with an AIP catalyst gave DMT and EG in 64% and 63% yields, respectively. The yields were increased by using a toluene/methanol mixed solvent containing 20–50 vol.% toluene; maximum yields, 88% for DMT and 87% for EG, were obtained at 20 vol.% toluene. These results indicate that the rate of methanolysis strongly depends on the solubility of PET. The results of GPC analysis suggest that the methanolysis of PET in the absence of the catalyst includes three steps. In the first step, the depolymerisation occurred at a tie molecule connecting PET crystals and the chain length was shortened to about 1/3. The shortened chain was depolymerized to oligomers in the second step. The GPC curve of the oligomers tailed to low molecular weight, clearly indicating that the depolymerization took place at random positions on the polymer chain. The third step, the depolymerisation from the oligomers to the monomers, was promoted only in the presence of the AIP catalyst.
Keywords: PET; Methanolysis; Aluminium triisopropoxide; Chemical recycling; Catalyst;

Phase-separation enhanced enzymatic degradation of atactic poly(R,S-3-hydroxybutyrate) in the blends with poly(methyl methacrylate) by Yang-Ho Na; Yong He; Tetsuo Nishiwaki; Yasuhide Inagawa; Yasushi Osanai; Shuichi Matsumura; Terumi Saito; Yoshiharu Doi; Yoshio Inoue (535-545).
The dependences of the phase structure and biodegradability on the thermal histories were investigated for amorphous atactic poly(R,S-3-hydroxybutyrate) (aPHB) in amorphous blends with atactic poly(methyl methacrylate) (PMMA).Poly(R-3-bydroxybutyrate)(PHB) depolymerase isolated from Ralstonia pickettii T1 was used as an enzyme. Pure aPHB in a rubbery state could not be degraded by PHB depolymerase. The results of DSC and TEM observations revealed that aPHB/PMMA blends were partially miscible with compositional heterogeneity. When the blends have a proper phase distribution of both the hydrolyzable rubbery phase and the depolymerase-adsorbable glassy phase, resulting from compositional heterogeneity, the enzymatic degradation of aPHB can be induced by blending with amorphous PMMA. Through the thermal treatment, the weight loss due to enzymatic degradation much increased for some blends.The reason is because such blends formed “micro-separated” structure, that is, the blends had “optimal” phase distributions consisting of enzyme-adsorbable PMMA-rich domains dispersed in the aPHB-rich matrix compositionally closer to pure aPHB. It was suggested that the aPHB component should be degraded faster in the aPHB/PMMA blends with compositional heterogeneity than in the miscible blends.
Keywords: Biodegradable polymer; Polymer blend; Polyester; Poly(3-hydroxybutyrate); Poly(methyl methacrylate);

Pyrolysis kinetics of poly(l-lactide) with carboxyl and calcium salt end structures by Yujiang Fan; Haruo Nishida; Shinya Hoshihara; Yoshihito Shirai; Yutaka Tokiwa; Takeshi Endo (547-562).
To clarify the pyrolysis mechanism of poly(l-lactide), which has been reported as complex, the thermal decomposition of carboxyl type and calcium ion end capped PLLA (PLLA-H and PLLA-Ca, respectively) was investigated by means of thermogravimetric analysis (TG), and pyrolysis–gas chromatography–mass spectrometry (Py–GC–MS). The TG data revealed that PLLA-Ca has a lower pyrolysis temperature (220–360 °C) than that of carboxyl type PLLA-H (280–370 °C). The apparent activation energy of the decomposition reaction was estimated from TG curves at different heating rates by plural methods to be 176 and 98 kJ mol−1 for PLLA-H and PLLA-Ca, respectively. Further kinetic studies indicated that PLLA-H degraded mainly through a random reaction with a pre-exponential factor A=2.0×1012 s−1, whereas PLLA-Ca degraded by way of a 1st-order reaction with A=8.4×105 s−1. Pyrolysis products of PLLA-H were composed of lactides and other cyclic oligomers, while the degradation products of PLLA-Ca were principally lactides. The main reaction pathway for PLLA-H pyrolysis was regarded as the random transesterification, whereas for PLLA-Ca pyrolysis the unzipping depolymerization process was dominant.
Keywords: Kinetics; Pyrolysis; Thermal degradation; Poly(l-lactide); Poly(l-lactic acid); Depolymerization; Random degradation; Simulation; Thermogravimetric analysis;

Calendar of events (563-564).